Accelerated Ageing in Type 1 Diabetes: A Focus on Molecular Mechanisms Underlying Telomere Shortening

Abstract

Type 1 diabetes mellitus (T1D) is increasingly recognised not only as an autoimmune metabolic disorder but also as a condition associated with accelerated biological ageing. Among the hallmarks of ageing, telomere shortening has emerged as a key feature, driven by multiple molecular pathological pathways linked to T1D onset and progression. This review explores the molecular mechanisms contributing to telomere attrition in T1D, including cytokine-induced β-cell damage, ROS-mediated DNA damage, impaired mitochondrial dynamics, and dysregulated DNA damage response pathways. Empirical evidence supports a consistent association between shortened telomeres and T1D, vascular complications, nephropathy, and mortality in T1D patients. Furthermore, the bidirectional relationship between telomere biology and immune-metabolic stress suggests novel directions for intervention. Understanding these pathways may enhance the predictive value of telomere length as a biomarker and inform targeted therapeutic strategies aimed at mitigating premature ageing and disease progression in T1D.

1. Introduction

In diabetes mellitus, a collection of metabolic disorders involving the metabolism of carbohydrates, glucose is both overproduced as a result of improper gluconeogenesis and glycogenolysis and underutilised as an energy source, leading to hyperglycaemia. Both elevated glycated haemoglobin (HbA1c) and elevated glucose concentrations in venous plasma are used as diagnostic criteria for diabetes mellitus, according to current guidelines. Conventionally, diabetes is divided into a number of clinical categories, including type 1 or type 2 diabetes, gestational diabetes mellitus, and other distinct forms that result from various factors such as monogenic diabetes, exocrine pancreatic diseases, and hyperglycaemia-inducing pharmacological agents [1]. In April 2025, the International Diabetes Federation formally recognised type 5 diabetes mellitus, formerly referred to as malnutrition-related diabetes, as a previously disregarded type of the disease. This disorder, which is classified as severe insulin-deficient diabetes, is caused by defective pancreatic development, which is a result of long-term dietary restriction throughout important stages of development, like childhood or adolescence [2].

In 2024, more than 9.4 million individuals worldwide were affected by type 1 diabetes (T1D), alongside over 500,000 new diagnoses reported. One in three individuals passes prematurely from the disease, despite advancements in technological devices and therapeutic options [3].

The infiltration of immune cells into the pancreatic islets, which results in the progressive destruction of insulin-producing β-cells, is the main feature of T1D, which is now recognised as a chronic metabolic autoimmune illness. Insulin insufficiency and eventual hyperglycaemia are the ultimate consequences of this damage. Due to the need for lifelong, intense insulin therapy, T1D patients have a substantial and expensive health burden [4]. Although their role in β-cell destruction is still unknown, seroconversion of islet autoantibodies to insulin, glutamate decarboxylase, insulinoma antigen 2, or zinc transporter 8 is the first noticeable sign of autoimmunity during the development of T1D. Their combined presence in serum continues to be the best predictor for both loss of immune tolerance (i.e., induction of autoimmunity) and clinical manifestation of T1D [5,6]. By increasing the immune system’s exposure to islet antigens presented by HLA class I molecules, immune cells that invade the pancreas and target insulin-producing cells produce an inflammatory environment [6]. This occurs as immune cells, particularly CD8+ T cells, infiltrate the pancreas and target insulin-producing beta cells, leading to elevated levels of pro-inflammatory cytokines such as IFN-γ, TNF-α, and IL-1β. These cytokines, along with the presence of autoreactive CD4+ T cells, infiltrating macrophages, and dendritic cells, contribute to the destruction of β-cells and the development of T1D [7]. In this direction, the autoimmune characteristics were demonstrated by the detection of islet-specific autoreactive CD4+ and CD8+ T cells in peripheral blood, pancreatic draining lymph nodes, and insulitic lesions. The autoreactive T cells are believed to arise, in part, from defective thymic education, which is a failure in central tolerance mechanisms that normally eliminate self-reactive T cells during their development in the thymus [6]. However, as our understanding of the pathophysiology of T1D grows, various theories and mechanisms have been suggested to be employed in T1D onset, which are summarised in

Table 1. Pathophysiological theories in T1D.

Theory	Mechanism	Reference
Autoimmune destruction	Genetic susceptibility + environmental trigger → loss of tolerance → activation of CD4+/CD8+ T cells → islet infiltration → β-cell attack → progressive β-cell destruction	[8]
Cytokine-mediated death	Th1 cells, macrophages → IFN-γ, TNF-α, IL-1β release → oxidative stress + ER stress in β-cells → activation of death pathways → apoptosis	[8,9]
Bystander activation	Local inflammation → cytokines (e.g., IL-2) + innate signals (TLRs) activate APCs and some immune cells → not antigen-independent T cell activation → self-antigens act as drivers of autoimmunity	[10]
Molecular mimicry	Structural similarities between viral/environmental antigens and β-cell proteins → cross-reactive T cell activation → immune attack on β-cells	[11,12]
Defective central tolerance	Thymic deletion failure (e.g., AIRE/INS mutation) → escape of autoreactive T cells → peripheral activation → β-cell targeting	[13]
Neoantigen formation	β-cell stress → post-translational modifications (PTMs) or hybrid insulin peptides (HIPs) → formation of novel epitopes → presented via MHC → activation of autoreactive T cells	[14]
Innate immune activation	Innate cells (macrophages) respond to stress/injury → cytokine release → recruitment + activation of T cells → β-cell destruction → innate-adaptive immune synergy drives autoimmunity	[15]
Dedifferentiation	Inflammatory cytokines → stress pathway activation → repression of β-cell identity genes (PDX1, MAFA) → loss of insulin expression → emergence of progenitor-like state → β-cell failure	[16]

Abbreviations: CD4⁺—Cluster of Differentiation 4 positive (helper) T cell; CD8⁺—Cluster of Differentiation 8 positive (cytotoxic) T cell; Th1—T helper type 1 cell; IFN-γ—Interferon gamma; TNF-α—Tumour Necrosis Factor alpha; IL-1β—Interleukin 1 beta; ER—Endoplasmic Reticulum; IL-2—Interleukin 2; TLRs—Toll-Like Receptors; APCs—Antigen-Presenting Cells; AIRE—Autoimmune Regulator; INS—Insulin gene; PTMs—Post-Translational Modifications; HIPs—Hybrid Insulin Peptides; MHC—Major Histocompatibility Complex; PDX1—Pancreatic and Duodenal Homeobox 1; MAFA—v-Maf Avian Musculoaponeurotic Fibrosarcoma Oncogene Homolog A.

.

In order to develop a controlled immune system insensitivity to self-antigens, tolerance is an important stage in the maturation of the adaptive immune system. Some autoreactive T cells frequently evade central tolerance in healthy people. When this happens, secondary selection and other processes typically eliminate, neutralise, or reduce antigen-specific cells that recognise self-antigens in peripheral tissues, including the spleen and nearby lymph nodes [17]. However, T1D patients lose their ability to tolerate self-antigens because of deficiencies in both central and peripheral tolerance [18]. Accordingly, it is hypothesised that, in T1D, anomalies in central and peripheral tolerance eventually enable antigen-specific self-reactive T cells to evade these mechanisms and engage with self-antigens in the pancreas and other locations, resulting in the development of autoimmunity and autoantibodies [19]. Both acute and long-term consequences are linked to T1D. Diabetes-associated ketoacidosis and chronic hypoglycaemia are examples of acute complications. Long-term issues may be either macrovascular or microvascular. Retinopathy, nephropathy, and neuropathy are examples of microvascular consequences. Microvascular problems have a complicated aetiology. Since hyperglycaemia is a prevalent underlying risk factor, it is imperative to optimise glycaemic control. T1D patients are also more likely to develop vascular disease and coronary artery disease, among other macrovascular problems [20]. Modifying the disease course through pharmacotherapy has been attempted for numerous autoimmune diseases, including T1D. Developing treatments that can create an organ free of immune cell infiltration and parenchymal cell damage must be the ultimate objective. In the context of the pancreas, this indicates renewed and preserved β-cells within the islets, without activated infiltrating immune cells [21]. T1D adults usually receive continuous subcutaneous insulin or several daily basal and prandial insulin doses. Non-insulin therapy has been studied as an insulin adjunct. Pramlintide, a β-cell peptide analogue, lowers A1C and body weight, while metformin lowers insulin and cholesterol levels but has an insignificant long-term effect on glycaemic management. By increasing glucose-dependent insulin secretion and prolonging stomach emptying, GLP-1 receptor agonists lower A1C, reduce body weight, and insulin dosages. SGLT2 inhibitors promote glycosuria to lower A1C, body weight, and blood pressure. However, they increase the risk of diabetic ketoacidosis. These pharmacologic agents improve metabolism, but safety issues, especially diabetic ketoacidosis, limit their use [22].

In the context of T1D, comorbidities and premature ageing continue to affect a significant number of patients. T1D is associated with accelerated biological ageing, which is facilitated by chronic inflammation, oxidative stress, and immune dysregulation, all of which are significant contributors to telomere shortening, a hallmark of ageing. Understanding the molecular mechanisms that underlie telomere attrition in T1D is a valuable approach to identify new therapeutic targets. The objective of this review is to investigate the relationship between T1D and accelerated ageing, with a particular emphasis on the molecular pathways that contribute to telomere shortening.

2. Telomere Structure and Function

Telomeres, which are repeating DNA sequences found at the ends of linear eukaryotic chromosomes, are essential for preventing the recombination of chromosome ends and ultimately for maintaining genome integrity [23,24,25]. Additionally, they have a significant impact on ageing, and their aberrant shortening can increase the susceptibility of developing age-related diseases [26]. Telomere sequences typically diminish with each cell division due to the “end replication” mechanism, leading to cellular senescence [27]. Notably, multiple morphological, structural, and functional changes, including increased expression and secretion of pro-inflammatory and tissue-remodelling mediators, along with permanent cell cycle arrest, characterise cellular senescence. This senescent state in cells, which arises in response to potentially harmful stress signals, is vital to cell function and disease [28].

Telomeres are repeating DNA sequences of 5′-TTAGGG-3′ that are located at the ends of chromosomes, creating protective caps with the shelterin protein complex. The six subunits of the shelterin protein complex that protect telomeres from the activated DNA damage response are as follows: protection of telomeres 1 (POT1), repressor/activator protein 1 (RAP1), TRF1-interacting nuclear protein 2 (TIN2), TIN2-interacting protein 1 (TPP1), and telomere repeat-binding factors 1 and 2 (TRF1 and TRF2) [29,30].

To prevent chromosomal shortening, a ribonucleoprotein enzyme called telomerase uses a segment of RNA as a template to generate telomeric repeats at the ends of chromosomes. Telomerase is an enzyme active in stem cells and may theoretically be present in all cells. If the activity were to increase, chromosomal shortening could be delayed [31].

In time, telomeres lose their capacity to bind enough telomere-capping proteins as they become shorter. This reveals the last fragments of DNA and triggers the DNA damage response pathways, which stop proliferation by activating the cell cycle inhibitors [26]. Although telomere length can be inherited genetically, several xenobiotics and bioactive compounds have been linked to telomere maintenance. In this direction, research has focused in the last few years on assessing the impact of various pharmacologically active agents on telomere dynamics. These investigations are particularly important in the context of the ongoing efforts to provide personalised pharmacotherapeutic regimens to patients to increase health outcomes [32]. For example, in the context of T1D, the first-line pharmacological agent, exogenous insulin, has been studied for its impact on telomere length. Interestingly, exogenous insulin has been demonstrated to have a negative impact on telomere length maintenance [33], but more research is still needed to draw definitive lines. Another example is metformin, a first-line drug used for the management of type 2 diabetes [34] and sometimes a drug recommended off-label for type 1 diabetes [35]. From a molecular perspective, metformin has been shown to activate AMP-activated protein kinase (AMPK), which in turn inhibits the mechanistic target of rapamycin (mTOR) pathway, central to cell growth and ageing regulation [36]. Metformin also reduces oxidative stress by decreasing mitochondrial reactive oxygen species (ROS) production and enhances autophagy to promote cellular maintenance [37]. Furthermore, it was shown to modulate the senescence-associated secretory phenotype (SASP), lower pro-inflammatory cytokine levels, and upregulate telomerase activity [32,38]. Furthermore, sodium–glucose co-transporter 2 (SGLT2) inhibitors, which are commonly prescribed for type 2 diabetes, improve senescent cell clearance and decrease age-related phenotypic alterations [39].

Interestingly, multiple lines of evidence have demonstrated that even lifestyle factors and diet can influence the rate of telomere shortening [40,41]. However, as a hallmark of the physiological ageing process, telomere shortening is intimately connected to other significant ageing-related factors such as inflammation, oxidative stress, genomic instability, cellular senescence, disruption of epigenetic control, stem cell depletion, mitochondrial instability, and cellular senescence [42,43]. These molecular alterations are exacerbated in chronic disease states, and elucidating their interplay may offer valuable insights for improving patient outcomes and guiding the development of targeted therapeutic strategies.

3. Evidence of Telomere Shortening and Accelerated Ageing in T1D Patients

Accelerated ageing denotes heightened physical morbidity and mortality, potential cognitive deterioration, and elevated levels of ageing biomarkers [44]. Deteriorating health, illness, and mortality are closely associated with chronological age, which is defined as the amount of time since the date of birth. Nonetheless, ageing is a wide-ranging process, and individuals of the same age might have very different health outcomes. Distinctive cells, tissues, and organs age in distinct ways at the individual level [45]. The fact that older people of the same age lead wildly disparate lives—some are fully independent, while others need extensive care for everyday tasks—is indicative of heterogeneous ageing at the population level. Since ageing is a diverse process, intra-individual variation and internal biological processes are not well reflected by chronological age [46]. The term “biological age” refers to a person’s physiological condition rather than a period of time since birth. It can be used to evaluate the effects of therapies aimed at slowing or reversing ageing, as well as the risk of age-related diseases and mortality [47]. In this direction, discovering novel, reliable biomarkers of the ageing process could prove a valuable tool for designing personalised treatment plans or predicting the susceptibility of developing age-related diseases.

In individuals with diabetes mellitus, organ and system damage manifests at earlier ages, particularly when diabetes is inadequately managed. Macrovascular and microvascular changes are the most significant and early indicators of ageing; they also substantially influence the ageing process and serve as factors of longevity. Individuals with diabetes mellitus are susceptible to age-related comorbidities, including frailty, cognitive decline, Alzheimer’s disease, bladder dysfunction, osteoporosis, visual impairment, and renal failure, suggesting that diabetes mellitus constitutes a pro-ageing condition. Telomere attrition, along with cellular, tissue, and organ senescence, constitutes the ultimate outcome [48]. On the other hand, telomere length reduction is directly linked to the development and occurrence of many chronic diseases, including diabetes, cancer, and cardiovascular disease [43]. This underscores the predictive nature of telomeres for preserving cellular health and function. Environmental factors such as stress, pollution, and poor lifestyles are linked to aberrant telomere shortening, providing additional evidence of the influence of lifestyle on cellular health. Therefore, an improved lifestyle may facilitate the maintenance of healthy telomere length and lower the risk of disease [49]. Despite numerous studies suggesting that telomere length is a prognostic indicator for ageing-related disorders and mortality, clinical practice currently underutilises its detection [43]. In this direction, Guanping Wei et al. used data from genome-wide association studies to conduct a bidirectional Mendelian randomisation analysis. According to forward analysis, reduced telomere lengths significantly raise the risk of both nonspecific kinds of diabetes and T1D. Furthermore, telomere length was positively causally associated with a number of diabetes-related diseases, such as diabetic ketoacidosis, diabetic nephropathy, and other comorbidities. The reverse analysis showed that T1D and associated comorbidities had a positive causal influence on telomeres. The authors concluded that shorter telomeres raise the likelihood of T1D, while T1D may set off compensatory mechanisms that impact telomere maintenance [50]. Accordingly,

Table 2. T1D and telomere length in empirical studies.

Group	TL Assessment	Results	Reference
102 long-term T1D (≥45 years duration) vs. 55 healthy controls	Singleplex qPCR	TL and SIRT1 mRNA lower in T1D; TL shorter in CHD subgroup; inversely correlated with HbA1c; chronic hyperglycaemia, triglycerides, and inflammation likely contribute to telomere attrition.	[51]
157 T1D with nephropathy vs. 116 normoalbuminuric T1D; 11-year follow-up	Southern blot (terminal restriction fragment analysis)	TL not different between groups; shorter TL independently predicted all-cause mortality; inversely correlated with age, diabetes duration, and systolic BP; suggests TL reflects biological ageing and vascular stress.	[52]
199 T1D (128 without and 71 with vascular complications) vs. 140 non-diabetic controls	qPCR	TL shorter in T1D vs. controls; inversely correlated with age and diabetes duration; not different by complication status; weak negative correlation with pulse pressure; no significant correlation with HbA1c, insulin sensitivity, oxidative stress or most inflammation markers. Independent predictors: age and T1D presence.	[53]
306 T1D: 187 with NDR/NPDR, 119 with PDR/LPC	qPCR	TL longer in PDR/LPC vs. NDR/NPDR (p = 0.036); in NDR/NPDR, TL negatively correlated with age, BMI, waist/hip ratio, LDL, cholesterol; no such correlations in PDR/LPC; suggests altered telomere regulation in advanced DR.	[54]
115 patients: 72 LADA, 13 LADY, 30 T2D (cross-sectional, GADA-based classification)	qPCR	TL shorter in LADA vs. T2D (p = 0.0121); no difference vs. LADY; longer TL in T2D taking metformin + insulin; suggests protective effect of therapy and possible autoimmunity-linked attrition in LADA.	[55]
1119 children with high-risk HLA genotypes; nested case–control for IA (n = 389) and T1D (n = 118)	Whole-genome sequencing (WGS); Computel tool used to estimate TL from FASTQ data	TL not associated with risk for IA or T1D; shorter TL in children from Sweden and Finland; paternal age positively correlated with TL; TL influenced by HLA genotype (DR4/4 or DR4/X = longer TL), but no causal link to T1D development.	[56]
39 AYA-T1D and 40 controls; non-randomised prospective; 18-month COC vs. IM	MM-qPCR on PBMC DNA; validated with Flow-FISH	IM increased TL in AYA-T1D; COC caused TL reduction; hs-CRP negatively correlated with TL in T1D; oestrogen (EE) linked to inflammatory telomere shortening.	[57]
132 T1D (48 normo-, 7 micro-, 77 macroalbuminuria); 44 controls; 6.9-year follow-up	Southern blot	Progressors had shorter TL and higher % of short telomeres; both were independent predictors of nephropathy progression; reflects oxidative stress and inflammatory ageing in T1D.	[58]
83 T1D youth (6–18 years), 1-year longitudinal cohort	MM-qPCR (Cawthon method, T/S ratio from leukocytes)	At baseline, TL positively correlated with 1RM, muscle power, and overall fitness; no associations at 1-year follow-up; suggests muscle strength may support telomere maintenance via anti-inflammatory and antioxidant mechanisms.	[59]
34 T1D, 62 T2D, 40 controls (cross-sectional, Chinese Han)	qPCR	TL significantly shorter in T1D vs. controls; 8-OHdG level elevated in T1D; 8-OHdG was an independent negative predictor of TL; suggests oxidative stress–driven telomere attrition in T1D.	[60]
1147 T1D: 536 with DKD vs. 611 with ≥15 year T1D and no DKD	Monochrome qPCR	TL significantly shorter in DKD vs. controls (p = 6.6 × 10−5; p = 0.028 after adjustment); methylation analysis of 1091 CpGs in 376 telomere genes revealed differential patterns (496 in DKD, 412 in ESKD); top genes: MAD1L1, PFKP, TUBB; Wnt signalling and chromosomal maintenance implicated.	[61]
260 T1D (GENEDIAB cohort) + 767 pooled T1D (GENEDIAB + GENESIS); 12–15 year follow-up	Monochrome multiplex qPCR	Short TL predicted CHD (HR 3.14) and all-cause mortality (HR 1.63); SNPs in TERT, TERC, NAF1, TNKS, MEN1, BICD1 also associated with CHD; suggests telomere shortening and genetic susceptibility contribute to vascular ageing in T1D.	[62]
53 children with newly diagnosed T1D (age 4–14)	Monochrome multiplex qPCR (MM-qPCR, T/S ratio; β-globin reference)	Shorter TL associated with higher BMI-SDS (p = 0.049); vitamin D levels negatively correlated with BMI-SDS; no correlation between ATL and vitamin D, HbA1c, or age at onset; suggests BMI-linked inflammation may drive telomere shortening in T1D youth.	[63]
26 T1D, 20 T2D, 71 GDM pregnancies vs. 127 controls; cord blood samples	Flow-FISH (CBMC telomere length by MESF units); telomerase by PCR-ELISA	No difference in cord blood TL across groups; telomerase activity ↑ in T1D and GDM vs. controls (p < 0.05); suggests upregulated telomerase as compensation for in utero telomere damage from oxidative stress.	[64]
478 long-standing T1D (GENEDIAB cohort); 10-year follow-up	Monochrome qPCR	Short TL independently predicted baseline and incident LEA; HR ~0.25–0.29 for longer vs. shortest tertile; 8-OHdG associated with LEA only in partially adjusted models; TL attrition reflects vascular ageing and oxidative DNA damage.	[65]

Abbreviations: T1D—Type 1 diabetes mellitus; T2D—Type 2 diabetes mellitus; DR—Diabetic retinopathy; NPDR—Non-proliferative diabetic retinopathy; PDR—Proliferative diabetic retinopathy; LPC—Laser photocoagulation; NDR—No diabetic retinopathy; LEA—Lower-extremity amputation; DKD—Diabetic kidney disease; CHD—Coronary heart disease; LADA—Latent autoimmune diabetes in adults; LADY—Latent autoimmune diabetes in youth; AYA—Adolescents and young adults; COC—Combined oral contraceptive; IM—Injectable contraceptive; qPCR—Quantitative polymerase chain reaction; MM-qPCR—Monochrome multiplex qPCR; WGS—Whole-genome sequencing; TL—Telomere length; 8-OHdG—8-hydroxy-2′-deoxyguanosine; PBMCs—Peripheral blood mononuclear cells; SIRT1—Sirtuin 1; SNP—Single nucleotide polymorphism; BMI—Body mass index; HbA1c—Glycated haemoglobin.

summarises evidence from empirical studies that assessed the relationship between telomere maintenance or attrition in T1D patients.

TL is often shorter in T1D patients compared to healthy controls, suggesting an impact of the disease on cellular ageing. TL appears to be inversely correlated with diabetes duration, age, blood pressure, oxidative stress and inflammation. Some studies point to possible protection by specific treatments, while others point to genetic and epigenetic involvement in susceptibility to complications. There is variability in results depending on the method of TL determination, study design, and population investigated, suggesting the need for standardisation and longitudinal investigations.

It is preferable to forecast health outcomes for diabetes patients early on and to use adjunct pharmacotherapeutic regimens to lower the risk of accelerated ageing and associated chronic problems. Further investigations to stratify individual risk are desirable in light of the diabetes epidemic and the residual risk assessments utilising existing risk equations. This valuable biomarker, telomere length, may be helpful in predicting health outcomes, tracking health status, and perhaps serving as a novel therapeutic target in the rapidly developing field of precision medicine for diabetics [66].

4. Molecular Mechanisms Contributing to Telomere Shortening in T1D

4.1. Chronic Inflammation and Immune Activation

Recent advancements in T1D research have introduced cytokines as prospective immunotherapeutic targets due to their direct role in the development of the disease by coordinating multicellular interactions between immune cells and pancreatic β-cells. Pancreatic β-cell destruction and overt hyperglycaemia are the outcomes of immune system breakdown, mainly through T helper 1 (Th1) cells, which in turn trigger immune cell activation and infiltration into the islets [67]. T1D is a disease that is further exacerbated by the inflammation of islet cells, which occurs as a consequence of the infiltration of autoreactive immune cells. Furthermore, β-cells are susceptible to external stimuli, including cytokines, which induce inflammation. High levels of pro-inflammatory cytokines, including interleukins, interferons, transforming growth factor-β (TGF-β), tumour necrosis factors (TNF) [68], and nitric oxide (NO) cascades [69], exacerbate this condition. On the other hand, cytokines that counteract inflammation are linked to protective effects against the survival of β-cells [70]. Insulitis, defined by the infiltration of immune cells within the islets of Langerhans, is closely linked to T1D progression and outcome. Insulitis refers to the inflammation of pancreatic islets, predominantly impacting the insulin-secreting β-cells. T lymphocytes, B lymphocytes, invading macrophages, and NK cells are all involved. CD8 T cells are notably present in post-cadaveric histological evaluations, potentially attributable to the timing of the examination, as CD4+ T cells may have extravasated from the pancreas and migrated to secondary lymphoid organs [8].

Extensive literature exists regarding the effects of tumour necrosis factor-alpha (TNF-α), vascular endothelial growth factor (VEGF), interferon-gamma (IFN-γ), interleukin 1 (IL-1), IL-6, and IL-12 on the destruction of pancreatic β-cells [67,71]. Azharuddin Sajid Syed Khaja et al. recently demonstrated that increased levels of IL-6, IL-8, and IL-10, along with reduced vitamin D levels and smoking among T1D participants, may exacerbate T1D and act as predictive indicators [70]. However, the exact mechanisms underlying the inflammatory responses associated with the onset of T1D complications remain only partially understood [71]. The function of emerging biomarkers is approximately clarified. Several cytokines, including IL-9, IL-17, and IL-33, which are potentially implicated in endothelial dysfunction, are inadequately characterised. IL-9 and IL-17, produced by the Th-9 and Th-17 cells, appear to be significant in the onset of T1D and the later stages of the disease [72,73]. Additionally, various chemokines and growth factors appear to contribute to the development of vascular disturbances. Moreover, dysregulation of monocyte chemoattractant protein-1 (MCP-1) levels may reduce the quantity and impair the function of endothelial progenitor cells (EPCs), which are involved in cardiovascular repair and regeneration processes [74]. Nevertheless, it is unknown how other chemokines, like eotaxin, IL-8, and macrophage-derived chemokine (MDC), function [71].

IL-1 is a 17 kDa protein that is a necessary mediator of the acute-phase response, fever, and inflammation. IL-1 is an indispensable molecule in the innate immune defence against microorganisms, trauma, and stress, as well as an effector molecule that participates in tissue destruction and fibrosis. Through the NFκB and mitogen-activated protein-kinase pathways, IL-1 signalling contributes to the dysfunction and destruction of β-cells, resulting in endoplasmic reticulum (ER) and mitochondrial stress, and ultimately activating the apoptotic machinery. Furthermore, IL-1 regulates T-lymphocytes [75,76]. In their study, Li et al. assessed the levels of IL-1β in the blood of T1D patients and healthy controls. T1D patients had a noticeably higher concentration of IL-1β than healthy controls, according to their findings [77].

Additionally, IL-6 is a multifunctional cytokine secreted by T cells and macrophages that stimulates the immune system’s reaction during inflammation and infection. This cytokine is involved in the inflammatory response linked to insulin resistance [78]. The interaction between IL-6 and its receptor (IL-6R) results in the dimerisation of gp130, subsequently activating JAK family kinases. Following this, STAT proteins (STAT1 and STAT3) undergo phosphorylation, dimerisation, and translocation to the nucleus, leading to the induction of transcription for target genes. IL-6 signalling occurs through the interaction of gp130 with a complex formed by IL-6 and a soluble form of the IL-6 receptor (sIL-6R). This is produced either by the translation of an IL-6R splice variant or through the proteolytic digestion of the IL-6R from the cell surface, a process known as shedding. ADAM17, also known as TACE, is the primary protease responsible for shedding IL-6R. The role of IL-6 in inflammation is linked to the IL-6R/gp130/STAT3 pathway, which is needed for the differentiation of Th17 cells, the inhibition of regulatory T (Treg) cell development, and the resistance of T effector cells to reduction by Treg cells [79,80]. Various other results corroborate the importance of the IL-6 pathway in T1D, demonstrating that STAT3 activation orchestrates elevated IL-6R expression in CD4+ and CD8+ T cells derived from T1D patients. T1D frequently involves dysregulated IL-6 synthesis and downstream receptor signalling, which is commonly linked to insulitis and β-cell destruction [67]. In the same direction, Hundhausen et al. showed that increased IL-6R surface expression contributes to improved T cell immune responses to IL-6 in T1D. Dysregulated IL-6 response suggests that T1D patients might benefit from IL-6-targeted therapy intervention and may contribute to diabetes through various pathways, particularly altered T cell trafficking [81]. Moreover, Haghnazari et al. conducted an investigation of IL-6, finding that the G allele of SNP rs1042522 in the TP53 gene is associated with IL-6 and raises the likelihood of developing T1D in an Iranian population. Additionally, adult-onset T1D was linked to IL-6R rs2228145 [82].

Monocytes, macrophages, CD4+ and CD8+ T cells, B cells, lymphokine-activated killer cells, NK cells, endothelial cells, a number of non-haematopoietic tumour cell lines, and other sources, including mast cells and neutrophils, produce TNF-α, a potent inflammatory mediator, in response to stimulation [83]. In a wide range of cells and tissues, there is a widespread expression of TNF-α receptors, indicating that TNF-α is engaged in various biological processes. In addition to its role in promoting inflammation, TNF-α helps with T cell proliferation in vitro investigations, prevents the deletion of T cells caused by superantigens, and plays a crucial role in the development of germinal centres after immunisation [84]. In the presence of pathological conditions, each of these characteristics, which have a role in the formation, maintenance, or enhancement of particular immune responses, may result in tissue damage in an abnormal manner [85]. In the nonobese diabetic mouse model, TNF-α was found to improve antigen presentation, which accelerated CD8+ T cell destruction of β-cells. In pancreatic lymph nodes, TNF-α plays a function in DC maturation, especially in the CD11b+CD11c+ subgroup, which has the capacity to activate islet-specific T lymphocytes [86]. Furthermore, it has been shown that TNF-α directly damages pancreatic β-cells and that high levels of this cytokine are linked to the aetiology of T1D [86,87,88]. A recent investigation indicated that the expression of cytokine genes was elevated in patients with T1D. The expression of the IL-10 gene showed a significant increase in patients experiencing ketoacidosis, and a positive correlation was observed with HbA1c levels. A negative correlation was identified between IL-10 expression and both the age of patients with diabetes and the duration since diagnosis of the disease. A positive correlation was observed between TNF-α expression and age. The expression levels of IL-10 and TNF-α genes demonstrated a notable increase in patients with T1D [89]. Importantly, the TNFR1-associated death domain protein (TRADD) is involved in the TNF-induced NF-κB signalling pathway and interacts with the internal structural domain of TNF receptor 1 (TNFR1) [90,91]. Then, TRADD activates NF-κB signalling by directly engaging with TRAF2 and RIP, two ubiquitin ligases involved in protein kinase receptor-interacting serine/threonine-protein kinase. IL-1 receptor-associated kinase (IRAK) quickly attaches to and phosphorylates IL-1RI in T cells. TRAF6 is activated by IL-1 to attach to IRAK and is quickly brought to IL-1R, suggesting that TRAF6 is also employed in the IL-1-NF-κB signalling pathway. TRAF6 is mainly responsible for transmitting signals that activate canonical NF-κB signalling, which is initiated by IL-1 and the Toll-like receptor (TLR) [92,93].

It is hypothesised that the cytokine IFN-γ, the pleiotropic cytokine, has a role in the aetiology of type 1 diabetes. Notable characteristics of the condition include the presence of IFN-γ signalling in the islets, which allows activation of the JAK-STAT pathway and overexpression of MHC class I. The pro-inflammatory function of IFN-γ is important for autoreactive T cell homing into islets and CD8+ T cell direct identification of beta cells. Recent findings indicate that IFN-γ plays a role in regulating the proliferation of autoreactive T cells [94,95]. Furthermore, when IFNs interact with a receptor monomer, the receptor complex dimerises and Janus Kinases (JAK), which are already linked to the cytoplasmic tail of each receptor subunit, come closer to one another [96]. This causes the JAKs to become transphosphorylated and increases their enzymatic activity. Following ligand interaction, JAK activity increases, recruiting STAT transcription factors that interact with receptor phospho-tyrosine residues on the cytoplasmic tail. JAKs then phosphorylate STAT proteins on a transactivation domain tyrosine residue. This causes STATs to dissociate from the receptor complex, dimerise with other STATs in the cytosol, and move to the nucleus to control gene transcription by binding to DNA consensus sequences [97].

In the same direction, these abovementioned cytokines activate NF-κB by attaching to specific receptors, activating the IKK complex (IκB kinase) and recruiting adaptor proteins. The inhibitor of NF-κB, IκBα, is marked for ubiquitination and proteasomal destruction when IKK phosphorylates it. This permits the nuclear translocation of the NF-κB p65/p50 dimer. From within the nucleus, NF-κB attaches to κB sites in DNA, which in turn enhances the transcription of genes associated with inflammation and reparatory response [98]. These processes establish a self-perpetuating feedback loop that sustains the heightened inflammatory state. This is particularly important for understanding telomere attrition under T1D conditions. In the NF-κB1−/− mouse, a rodent model of chronic low-level inflammation, it was discovered that persistent inflammation mediates telomere attrition [99]. Moreover, NADPH-oxidases, COX2, inducible nitric oxide synthetase (iNOS), and 5-lipoxygenases are among the ROS-producing enzymes that are expressed as a result of NFκB nuclear translocation and activation, which increases the oxidative status of the disease [100].

A bidirectional regulatory relationship exists between telomeres and inflammation: inflammation accelerates telomere attrition, leading to telomere dysfunction, while telomeric components themselves modulate the intensity and duration of the inflammatory response, as illustrated in Figure 1.

Chronic inflammatory diseases have also been connected to telomerase dysregulation and telomere shortening. Prolonged inflammation can alter telomeres and expedite ageing in mice. Numerous cytokines associated with inflammation and signalling pathways, such as the NF-κB signalling system, which controls different telomere components, may affect telomere length and telomerase activity [101]. Telomere shortening occurs naturally with ageing, and inflammation can worsen telomere dysfunction by accelerating telomere attrition. This process contributes to cellular senescence and promotes the ageing process [102]. On the other hand, telomere dysfunction is a significant contributor to various inflammatory diseases in older adults, regardless of genetic polymorphisms in telomere-associated genes. Abnormal telomeres in a subset of cells within affected tissues can induce localised inflammation, resulting in heightened generation of reactive oxygen species and subsequent telomere dysfunction in adjacent cells. This intensifies the inflammatory state and plays a role in the progression of multiple age-related conditions [103].

4.2. Oxidative Stress and Mitochondrial Dysfunction

4.2.1. ROS-Induced Telomeric DNA Damage

Throughout the course of extensive research, oxidative stress (OS) has been employed to depict the imbalance between antioxidants and oxidants. A variety of oxidases, including cyclooxygenases (COX), uncoupled nitric oxide synthases (NOS), lipoxygenases (LPO), xanthine oxidase (XO), nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, and cytochrome P450 enzymes, are responsible for the production of ROS in reaction with alterations in the oxidation–reduction reactions that occur within the cells. ROS are produced as a result of aerobic metabolic processes that involve oxygenation. The action of antioxidants is able to overcome minor amounts of ROS, including superoxide (O₂⁻), hydroxyl radical (OH), hydrogen peroxide (H₂O₂), and peroxynitrite (ONOO⁻), in healthy individuals. Nevertheless, OS occurs when ROS production surpasses the generation and efficacy of antioxidants under pathological conditions [104,105,106]. The immune system is supported by these species at modest levels through their involvement in phagocytosis. Additionally, they facilitate cellular signalling and play a role in vasomodulation. Nevertheless, OS is induced when the body’s antioxidant capacity to neutralise oxidative species is insufficient. Cellular macromolecules, including proteins, DNA, RNA, and lipids, can be damaged by this stress, which leads to the pathogenesis of diseases and the ageing process by progressively deteriorating biological processes and cellular structures. Thus, general health depends on the body’s capacity to control OS and maintain it at ideal levels [107,108].

ROS cause single-strand breaks (SSBs) at telomeres, either directly or as byproducts in lesion repair, resulting in replication fork collapse and subsequent telomere loss. Lesions obstructing telomere replication may lead to the accumulation of unreplicated single-stranded DNA, forming multi-telomeric foci at chromatid ends, known as fragile telomeres. In addition, oxidative lesions become a barrier to the binding of shelterin or the transcription of TERRA transcripts at telomeres [109,110]. The telomeric region’s high guanine content renders it vulnerable to oxidative damage [111]. The molecule 8-oxodG is recognised as a pre-mutagenic lesion that can improperly bind to adenine, resulting in GC-TA transversions. This process may lead to various alterations, including single-strand breaks, improper replication of telomeric DNA, increased telomere shortening [112] and, if DNA damage repair mechanisms are ineffective, potential modifications in cellular physiology, senescence, or apoptosis [113]. OS-induced damage can disrupt the assembly of telomere maintenance proteins, including TRF1, TRF2, and POT1, with DNA; even a solitary 8-oxodG lesion can cause a 50% decrease in the levels of these proteins, resulting in telomeric instability. It has been shown that the levels of 8-oxodG in senescent cells are 35% greater than in control cells due to the induction of cell growth arrest [114]. An additional event connecting OS and accelerated telomere attrition is the depletion of the enzyme peroxiredoxin-1 (PRDX1), which results in the inefficacy of the DNA damage sensor-dependent repair associated with the base excision repair (BER) mechanism, namely poly(ADP-ribose) polymerase-1 (PARP1) [115,116]. This situation leads to the buildup of telomeric SSBs, which may subsequently result in harmful telomeric double-strand breaks (DSBs). Two pathways, high-fidelity template-dependent homologous recombination (HR) and the error-prone non-homologous end joining (NHEJ), are responsible for repairing DSBs [117].

Insulin deficiency and the resulting hyperglycaemia are hallmarks of T1D. It results in complete insulin insufficiency and is caused by autoimmune β-cell death in the pancreas. The rate at which the pancreatic β-cells are destroyed by the autoimmune process determines how T1D develops. However, as insulin insufficiency worsens, individuals develop insulin dependence in addition to severe hyperglycaemia and ketoacidosis. Additionally, the patient becomes totally reliant on insulin therapy for survival due to the rapid development and severity of T1D. Unfortunately, many people cannot achieve adequate blood glucose control with this pharmacotherapeutic regimen [118]. OS was hypothesised to play a significant role in the pathogenesis and complications of T1D. Diabetic patients exhibit elevated ROS production and OS indicators, alongside a reduction in antioxidant levels. Hyperglycaemia may elevate oxidative stress markers, including membrane lipid peroxidation. Lipid peroxidation levels in erythrocytes were found to be directly proportional to both in vitro glucose concentrations and blood glucose levels, as measured by glycosylated haemoglobin, in diabetic patients. Hyperglycaemia, a common hallmark in both T1D and T2D, significantly contributes to OS. Hyperglycaemia-induced OS is hypothesised to lead to the immediate production of ROS and modify the redox balance. This phenomenon is believed to arise through multiple established mechanisms, such as enhanced polyol pathway flux, elevated intracellular synthesis of advanced glycation end-products, stimulation of protein kinase C, or excessive superoxide production by the electron transport chain in mitochondria [119].

Mouse studies suggest that immune cells’ production of nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase 2 (NOX2)-derived superoxide can affect autoimmune reactions, in addition to pancreatic β-cell immunopathogenesis. Studies employing a single alteration in the neutrophil cytosolic factor 1 gene (Ncf1), which encodes the NOX2 p47phox component, to totally ablate its function, was shown. The mutation (Ncf1 m1J) prevents p47phox from being expressed, which is necessary for the functional NOX2 complex to form. NOD.Ncf1 m1J mice were protected from spontaneous autoimmune diabetes by not having an active NOX2 complex. Additionally, NOD.Ncf1 m1J mice lacking active NOX2 were protected from an intensive adoptive transfer model of T1D using diabetogenic CD4 T cells. These findings emphasise the role of immune-derived free radicals in T1D development, as macrophages and neutrophils express NOX2 at the highest levels [120,121]. Additionally, NOX2 facilitates Th1 responses by regulating CD4+ T cell differentiation. Recently, Chen et al. identified an intrinsic role of NOX2 in CD8+ T cells that is necessary for the initiation of autoimmune diabetes. In order to transmit the redox signal in CD8+ T cells, the superoxide produced by NOX2 must be converted to hydrogen peroxide. Additionally, the tumour suppressor complex is deactivated by NOX2-generated oxidants, which subsequently activate RheB and mTOR complex 1. These findings suggested that NOX2 serves a nonredundant function in the effector function of CD8+ T cells that TCRs mediate. NOX2 is necessary for the promotion of effector function by T cells and the enhancement of mTOR1 activity, which in turn leads to the production of Tc1 cytokines and molecules. This exacerbates chronic oxidative stress and inflammation in T1D patients [122]. Moreover, the presence of intact NOX2 in conventional dendritic cells is needed for these cells to process and present antigens to CD4+ T cells during the initiation of experimental autoimmune encephalomyelitis. Dendritic cells from a NOX2-deficient mouse model were found to be incapable of cross-presenting antigens to CD8+ T cells, resulting in a delayed onset of autoimmune T1D [123].

Chronic inflammation, oxidative stress, and telomere dysfunction are interconnected in a self-reinforcing cycle. Transcription factors regulating iNOS expression include NF-κB in response to IL-1, as well as IRF-1 and STAT-1 in response to IFN-γ [124]. NF-κB signalling is upregulated by chronic immune activation, which subsequently enhances the expression of iNOS and COX-2 in immune cells [125]. This increases the concentration of peroxynitrite (ONOO⁻), a potent oxidant that exacerbates telomere damage and nitrates DNA. It has been shown that NF-κB and the cellular redox environment interact in both directions. Oxidative stress can either activate or inhibit NF-κB, and NF-κB activation can work as an antioxidant or pro-oxidant depending on the kind of cell or context [126]. Notably, NO functions as a potent free radical, reacting with superoxide to produce mediators that include nitrite and nitrogen dioxide, which can damage cellular DNA. NOS isoforms exhibit context-dependent roles: eNOS typically mediates vascular protection, while iNOS is primarily pro-inflammatory, contributing to β-cell dysfunction during cytokine exposure in T1D. Low amounts of NO generated by eNOS can activate and control COX2, NF-κB, and pro-inflammatory cytokines. Conversely, elevated eNOS levels in kidney tissue and during renal ischemia–reperfusion may be crucial in lowering oxidative stress, inflammation, and damage to renal tissue. On the other hand, elevated iNOS isoform levels worsen inflammation and damage [127]. Through its influence on pancreatic β-cells, iNOS induces an additional level of insulin inhibition. Nitrosative and oxidative stress have been associated with the development of diabetes and its associated issues. In case of acute pancreatitis, T1D, T2D, and other conditions, iNOS is stimulated in pancreatic islets. Both human β-cells exposed to pro-inflammatory cytokines and β-cells from acutely diabetic NOD mice consistently showed the production of peroxynitrite. Under these circumstances, the presence of excessive levels of NO and/or peroxynitrite could result in highly destructive effects, resulting in the dysfunction or apoptosis of β-cells and the inhibition of insulin secretion [128].

An important target of energy metabolism is the Nrf2 pathway, a master regulator of cellular defence against oxidative and xenobiotic stressors. Nrf2 helps cells establish a defence response against endogenous and environmental stresses. Nrf2 binds to Cullin 3 ubiquitin E3 ligase and Kelch-like ECH-associated protein 1 (KEAP1) to become sequestered in the cytoplasm under less stressful circumstances. When exposed to oxidative and electrophilic stressors, Nrf2 becomes disassociated from the cytosolic inhibitor KEAP1, enters the nucleus, and, there, it combines with the proteins MAF and JUN to form a complex [129]. It then binds to the antioxidant response element to increase the expression of target proteins, including heme oxygenase-1 (HMOX1) and NAD(P)H quinone oxidoreductase-1 (NQO1). According to recent research, in spontaneous nonobese diabetic mouse models, systemic stimulation of Nrf2 signalling delays the establishment of T1D. As a result, Nrf2 targeting may have promise for both T1D prevention and treatment [130]. Moreover, the beginning and development of T1D have been linked to changes in Nrf2 function. For instance, compared to their NOD:Keap1+/+ counterparts, nonobese diabetic (NOD) mice crossed with Keap1 knockdown mice (NOD:Keap1FA/−) showed enhanced insulin secretion and lower pancreatic T cell infiltration [131]. Additionally, in an alloxan-induced model of T1D, Nrf2−/− mice showed increased hepatic gluconeogenesis, elevated blood glucose, elevated serum triglycerides, and more pancreatic β-cell damage than Nrf2+/+ mice after STZ injection, confirming the protective function of Nrf2 in T1D [132]. Healthy cells maintain redox equilibrium through coordinated Nrf2 and NF-κB activity. Consequently, Li et al. found that mice lacking Nrf2 have higher levels of inflammatory mediators like interleukins, TNF-α, iNOS, and COX-2. Nrf2 deficiency leads to pro-inflammatory reactions mediated by NF-κB [133]. In the same direction, it has been shown that Nrf2 deletion in human monocytes increased the inflammatory cytokines generated by TNF. Mechanistically, autocrine TNF synthesis is caused by TNF-induced persistent induction of Nrf2 and target gene expression via a TNFR1-dependent mechanism. Accordingly, Nrf2 suppression increased the expression of pro-inflammatory genes triggered by TNF-α as well as p50 and p65 DNA-binding, suggesting that Nrf2 negatively regulates NF-kB activation [134]. These findings support the existence of a sustained pathogenic interplay between oxidative stress and inflammation, wherein each process amplifies the other. This reciprocal reinforcement contributes to the chronic inflammatory and oxidative milieu observed in T1D, as illustrated in Figure 2.

Altogether, these findings underscore the central role of oxidative and nitrosative stress in driving telomeric DNA damage and immune-mediated β-cell dysfunction in T1D. Therapeutic targeting of redox-sensitive pathways may offer novel strategies to slow telomere erosion and disease progression.

4.2.2. Mitochondrial ROS and Impaired Antioxidant Defence in T1D

The pathophysiology of T1D may involve immunological and β-cell mitochondrial dysfunction. The primary function of β-cells, which is to secrete insulin in response to glucose, depends on mitochondrial energy production. A significant source of ROS is the mitochondria. Mitochondrial ROS contribute to β-cell damage during the immune attack. Similarly, the physiology, shape, and metabolism of mitochondria strongly control the fate of T cells throughout immunological responses. The signalling involved in antigen-specific T cell activation depends on the production of mitochondrial ROS [135]. The increase in glycolytic flux during T cell activation is well-documented; however, the significance of mitochondrial flux remains ambiguous. Sena et al. demonstrated that mitochondrial metabolism alone, independent of glucose metabolism, is adequate for the induction of IL-2. In a model of T-Uqcrfs(-/-) mice, which exhibit diminished mitochondrial ROS production in T cells, it was demonstrated that mitochondria are required for T cell activation. This process involves the generation of mitochondrial ROS necessary for the activation of nuclear factor of activated T cells (NFAT) and the subsequent induction of IL-2. The mice were unable to induce antigen-specific T cell expansion in vivo; however, Uqcrfs1(-/-) T cells maintained their capacity for proliferation in vivo under lymphopenic conditions. This indicates that Uqcrfs1(-/-) T cells were not deficient in bioenergetics but instead lacked particular ROS-dependent signalling events necessary for antigen-specific expansion. Mitochondrial metabolism is essential for T cell activation via the generation of complex III ROS [136]. In the same direction, in a rat model of hyperglycaemia, it was found that mitochondrial complex I activity decreased and thioredoxin reductase activity increased in rat brain mitochondria, which decreased hydrogen peroxide generation, oxygen consumption, and mitochondria-coupled hexokinase coupled to oxidative phosphorylation activity. Through mitochondrial complex II, T1D raised respiratory parameters and mitochondria-coupled hexokinase activity. The authors demonstrated that early hyperglycaemia in brain tissue modifies glucose phosphorylation coupling by shifting brain mitochondria’s oxidative machinery towards complex II-dependent electron harvest. In addition, T1D boosted H₂O₂ generation by α-ketoglutarate dehydrogenase without inducing oxidative stress. T1D boosted PTEN oxidation and decreased NF-kB activity. Therefore, mitochondrial glucose–oxygen–ROS axis reorganisation may affect glucose turnover, brain amino acid, redox, and inflammatory signalling [137]. The mitochondrial respiratory chain in pancreatic β-cells is a primary source of superoxide production via complexes I and III in the mitochondrial membrane, much like the NOX complex in immune cells. Through the activation of glycolysis and the tricarboxylic acid cycle for ATP synthesis, the pancreatic β-cells transport glucose inside the cell for appropriate insulin secretion. The increase in mitochondrial-derived generation of superoxide follows a rise in glycolytic flux in pancreatic β-cells. Due in part to a naturally weak antioxidant defence system, the constant rise in glycolytic flux under hyperglycaemic circumstances in both T1D and T2D may exacerbate oxidative stress in the β-cell and impair function [138].

As previously stated, T1D is an autoimmune condition characterised by elevated levels of inflammation [139]. Numerous studies have demonstrated the link between mitochondrial dysfunction and the inflammatory process [140]. A vicious cycle of recurring inflammation and oxidative stress may be established when inflammation fosters mitochondrial malfunction, and defective mitochondria contribute to the pathophysiology of inflammation through a number of mechanisms. For example, in apoptotic cells exposed to pro-inflammatory cytokines, MHCI and the cytokines CXCL9, CXCL10, and CCL5 were overexpressed, which caused endoplasmic reticulum (ER) stress and apoptosis. Pro-inflammatory cytokines have been connected to a malfunction in glucose-mediated insulin secretion and a reduction in the insulin-processing enzymes PC1/3, PC2, and CPE, which causes proinsulin to accumulate in cells. Because mitochondria reduce ATP synthesis, raise superoxide levels, and restrict pyruvate utilisation, there is evidence that they are an important variable in cytokine-induced cell death [141].

Increased ROS generation and mitochondrial dysfunction are associated with the onset and progression of a number of inflammatory diseases, including T1D. The excessive production of oxygen-based reactive species, which damages mitochondrial proteins, lipids, and mitochondrial DNA (mtDNA), is a hallmark of both acute and chronic inflammatory disorders [142,143]. Normal dynamics and function of the mitochondria are adversely affected by these changes. Inflammation also increases the activity of iNOS in the mitochondria, which increases the synthesis of reactive nitrogen species (RNS) and NO in the mitochondria. Both RNS and ROS cause mtDNA changes, lower respiratory chain activity and ATP synthesis, and ultimately cause cell damage and death [144].

Moreover, T cells from T1D patients showed signs of mitochondrial inner-membrane hyperpolarisation (MHP). Regardless of previous antigen exposure, elevated MHP was a universal characteristic seen in T cell subsets and was unrelated to subject age, HbA1C levels, or the length of diabetes. Interestingly, T2D patients did not have elevated T cell MHP. Increased activation-induced IFNγ production was linked to T cell MHP, and activation-induced IFN-γ was linked to the formation of ROS that were specific to mitochondria. Moreover, T cells from T1D patients showed reduced intracellular ATP levels. Therefore, the intrinsic mitochondrial dysfunction in T1D modifies the production of mitochondrial ATP and IFN-γ, which is linked to the creation of ROS [145]. From this point, increased ROS levels might cause oxidative base modifications and strand breaks in telomeric regions, impairing replication and accelerating erosion [109]. Numerous studies have demonstrated that one of the leading causes of DNA damage is oxidative stress brought on by excess ROS generation. During oxidative phosphorylation, intracellular ROS are primarily generated in the mitochondrial matrix’s electron transport chain (ETC). Since mtDNA is located close to the ETC and is not shielded by histones, it is more susceptible to oxidative damage than nuclear DNA. Consequently, mtDNA integrity is compromised by mitochondrial oxidative stress, thereby rendering it possible for oxidised mtDNA to enter the cytoplasm and ultimately the extracellular space. This leads to the production of IFN and pro-inflammatory reactions [146]. Moreover, the phenotypic transition of macrophages from regulatory M2 to inflammatory M1 macrophages, which has significance for T1D pathogenesis, is influenced by ROS generated by NADPH oxidase 2. M1 macrophages attract CD4+ T cells and release chemokines. Additionally, macrophages restimulate CD4+ T cells in the islet microenvironment by presenting them with β-cell antigens. In addition to ROS and NO produced by macrophages, the inflammatory cytokines IFNγ, TNFα, and IL-1β produced by CD4+ T cells and M1 macrophages are capable of eliminating or seriously damaging β-cells. The internal and extracellular ROS and NO levels in the β-cells may be the cause of this possible damage, which could lead to mitochondrial failure [135].

Accordingly, reduced mitochondrial number and oxidative phosphorylation capacity in T1D have been attributed to disrupted mitochondrial biogenesis. The peroxisome proliferator-activated receptor gamma (PPARγ) coactivator (PGC-1α) is a significant molecular driver of mitochondrial biogenesis. Nuclear transcription factors (NRFs) 1 and 2 and mitochondrial transcription factor A, PPARγ, and PPARα are all coactivated by PGC-1α. These variables control the expression of genes related to oxidative phosphorylation and mitochondrial replication. Additionally, PGC-1α coactivates transcription factors for a number of additional energy homeostasis-related genes [147]. Hyperglycaemia-induced mitochondrial dysfunction has been identified as a common cause of both the decrease in mitochondrial biogenesis and the increase in OS, which are significantly influenced by diabetic nephropathy and renal impairment, and the decrease in PGC-1α expression caused by hyperglycaemia. Even if the precise cause of hyperglycaemia-induced disruption of PGC-1α expression is unknown, reduced PGC-1α expression levels may occur with growing SGLT2-dependent increases in cytoplasmic sodium and protons in diabetic renal cells [148]. Moreover, during inflammation, NF-κB activation reduces PGC-1α production, and low PGC-1α levels promote oxidative stress, which in turn downregulates its antioxidant target genes. Conversely, low PGC-1α levels and associated oxidative stress increase NF-κB activation, which intensifies the inflammatory response [149].

On the other hand, data from multiple clinical trials assessing mitochondrial activity in vivo and ex vivo in T1D patients and controls of similar age were analysed. Adults with T1D had decreased maximal mitochondrial oxidative capacity and mitochondrial efficiency and greater rates of anaerobic glycolysis, according to in vivo data. Even after controlling for age and body fat percentage, T1D patients still had decreased maximum mitochondrial capacity and increased anaerobic glycolysis. There were no discernible changes across the two groups in the ex vivo data. Adults with T1D exhibited mitochondrial malfunction, as demonstrated by the in vivo investigation, which also raised the possibility that substrate or oxygen supply deficiencies contribute to in vivo dysfunction [150].

Important signalling enzymes known as mitogen-activated protein kinases (MAPKs) cause target proteins to be expressed in response to mitogenic stimulation or environmental stress. Environmental stressors or inflammatory cytokines activate p38 MAPKs and c-Jun N-terminal kinase (JNK) [151]. By moving to mitochondria and encouraging Bax translocation, JNK damages mitochondrial integrity, raises mitochondrial outer membrane permeability, and starts cytochrome c release [152]. MtDNA can be released into the cytosol as circular structures or DNA fragments as oxidative damage in the mitochondria builds up. In the cytosol, mtDNA plays a significant part in causing inflammation. The primary mechanism for mtDNA leakage is the opening of the mitochondrial permeability transition pore. After the pro-apoptotic proteins BCL-2 homologous antagonist/killer (BAK) and Bax oligomerise on the outer mitochondrial membrane, stable protein-permeable pores are created. These pores allow cytochrome c to leak, which initiates the intrinsic apoptotic pathway, in addition to mitochondrial rupture of the inner mitochondrial membranes and ejection of cytoplasmic components, including mtDNA [153]. While ROS generation occurs physiologically during oxidative phosphorylation, mitochondrial defects or inflammatory stress increase electron leakage, elevating ROS production beyond homeostatic levels. Defective mitochondria not only produce ROS, but they are also the targets of oxidative stress in the mitochondria, which increases the generation of ROS in the mitochondria. Defective mitochondria produce mitochondrial ROS, which degrade organelle structure and function and ultimately cause premature ageing [154].

4.3. DNA Damage Response and Cellular Senescence Pathways

Increased ROS levels can cause oxidative stress in cells, and prolonged exposure to this stress can irreversibly alter the DNA. It is widely acknowledged that the buildup of oxidative DNA damage may encourage human disease, mutagenesis, and loss of homeostasis. Endogenous factors, including oxygen metabolism, apoptosis, and immune system-mediated inflammation, can cause these oxidative damages [155,156]. These oxidative DNA lesions include deaminated and adducted bases, sugar moiety alterations, abasic sites, and SSBs. The presence of 8-oxo-2′-deoxyguanosine (8-oxo-dG) is one of the more prevalent oxidative DNA lesions [155]. DSBs can occasionally result from oxidative damage. Topoisomerases cleaving next to an SSB on the opposite strand, two SSBs forming near each other on opposing strands, and ROS-induced DNA damage interfering with transcription or DNA replication can all result in a DSB [157]. Furthermore, when two lesions in a cluster are attempted to be repaired simultaneously, or when a changed base is excised next to an unrepaired SSB on the opposite strand, DSBs may be produced. Because it only takes a few of these lesions to trigger gene mutations, chromosomal abnormalities, and cell transformation, DSBs are the most harmful form of DNA damage [155]. At the telomeric level, oxidative DNA damage—specifically, 8-oxoguanine—is the most common type of DNA damage in human cells. An accumulation of oxidative damage in the DNA may impede the replication fork and trigger the DNA damage response. TRF1 and TRF2, which are important in appropriate telomere replication and T-loop formation, respectively, were shown to be reduced following acute oxidative stress at telomeres [158] (Figure 3).

It has been suggested that cytokines generated within and adjacent to pancreatic islets during islet inflammation have a role in the development of diabetes by impairing β-cell function and β-cell apoptosis [159]. Numerous β-cell responses during islet inflammation are regulated by NO, which is generated by β-cells reacting to cytokine exposure. DNA strand breakage, interstrand crosslinks, and base oxidation and deamination are examples of NO-induced DNA damage. Since DNA damage is induced in β-cells treated with cytokines and NO before cell lysis, there is evidence that DNA damage plays a role in β-cell death following IL-1 exposure. Thus, β-cells have a limited ability to repair DNA damage, even though it is a contributing factor to β-cell mortality [160,161]. When DNA ends are recognised as broken, replicative senescence is initiated. This results in the activation of DDR pathways, the creation of telomere-induced DNA damage foci (TIF), and cell cycle arrest, which is mediated by the tumour suppressor pathways p53/p21WAF1/CIP1 and p16INK4A/pRB. Only a small percentage of senescent cells have TIF, and telomere length profiles vary widely, indicating that distinct DDR signalling events at telomeres may play a role in the development of senescence. According to this theory, during senescence, telomeres maintain an adequate amount of shelterin to partially shield against DNA repair processes while also generating DDR signals required for a permanent cell cycle arrest [162,163,164]. The PK family of serine or threonine kinases includes ATM, ATR, and DNA-dependent protein kinases (DNA-PKs). As the most upstream kinases, they phosphorylate the serine/threonine glutamine motifs in response to DNA damage [165]. Additionally, they control a number of checkpoints after DNA damage. The preliminary NHEJ process is activated in part by the DNA-PKs. However, by starting their phosphorylation, ATM and ATR proteins progressively activate the checkpoint kinases, which include ChK1, ChK2, and MK2. In particular, the ATM is triggered by DSBs, while the ATR reacts to a broader range of DNA damage. Consequently, γ-H2AX is a sensitive and widely used marker of DNA double-strand breaks and replication-associated damage [166].

In experimental models, both T1D (via cytokine-driven oxidative damage) and T2D (via metabolic stress and hyperglycaemia) impair DNA repair pathways, though through distinct upstream mechanisms. In vitro and in diabetic mouse models, it was discovered that carbohydrates initiate this cascade by lowering the NAD+/NADH ratio and NHEJ-repair. Restoring DNA repair with nuclear overexpression of phosphomimetic RAGE restored organ function by lowering fibrosis, inflammation, and DNA damage [167]. Moreover, it is interesting to note that oxidised cells, both spontaneous and H₂O₂-induced, were significantly more common in T1D patients than in controls. Patients generally displayed decreased NO production and iNOS expression. Additionally, adolescents with T1D, particularly those who were female, had significantly higher levels of spontaneous nuclear damage, as measured by γ-H2AX foci. Giovannini et al. verified that oxidative stress contributes to the disease by destroying cell membrane lipids and, more significantly, by inducing genetic damage in the circulating white blood cells of teenagers who are affected. This suggests that oxidative damage can impact multiple bodily tissues [168].

Additionally, cellular reactions to stress are significantly influenced by the p53 transcription factor. When it is activated in response to DNA damage, it either causes cellular senescence or apoptosis, which preserves genome integrity, or it causes cell growth arrest, which permits DNA repair. While senescence initially acts as a tumour-suppressive barrier, prolonged SASP activity can paradoxically promote tumourigenesis and tissue dysfunction. Furthermore, by releasing pro-inflammatory cytokines, senescent cells can adversely affect the adjacent tissue microenvironment and nearby cells, ultimately leading to tissue malfunction and/or negative effects [169,170]. Persistent DNA damage is the primary cause of cellular senescence. According to Hara et al., long-term DNA damage significantly reduces the production of the histone dimethylating enzyme G9a, which causes the genome to have more open chromatin, which in turn triggers the expression of the SASP factor gene. Additionally, the same research found that the late stage of cellular senescence accelerates the DNA damage response in senescent cells, resulting in very small DNA fragments through cytokinesis block with subsequent nuclear division. It is believed that these aberrant cytoplasmic segments of DNA will activate DNA sensors and cause innate immune inflammatory reactions [171]. As mentioned before, the pro-inflammatory cytokine production is elevated in senescent cells, a characteristic known as the senescence-associated secretory phenotype (SASP). As a result, the innate and adaptive immune systems become persistently and aggressively activated. SASP is typically thought of as an immune system signal for self-elimination. Immune cell senescence is another consequence of the bilateral and ongoing activation of innate and adaptive immunity. Cell cycle arrest, telomere dysfunction, ER stress, resistance to apoptosis, and metabolic reprogramming are a few of the biological processes that are closely linked to senescence [172,173]. Pro-inflammatory cytokines, chemokines, proteases, growth factors, bioactive lipids (like oxidised lipid mediators), extracellular vesicles, and others are the main categories of SASP. SASP composition varies depending on the cell type, senescence trigger, tissue microenvironment, and time since senescence onset [174]. SASP has the ability to cause tissue remodelling (a positive impact of cellular senescence) as well as chronic inflammation (a negative result of cellular senescence). Nonetheless, tumour suppressor genes, p16^INK4a, p53, p21WAF1/Cip1 and senescence-associated-β-galactosidase, which are regarded as indicators of cellular senescence, are expressed in both types of senescence [175]. While there is no rise in CDKN2A or p16^INK4a, CDKN1A and p21Cip1 are upregulated in T1D donor pancreatic tissue and in a human islet model of DNA damage-induced senescence. This suggests that the stressor determines how β-cells respond to p21Cip1 (preferentially activated by oxidative stress) and p16^INK4a (favoured by chronic replicative stress or DNA damage). According to certain models, these findings help explain how β-cell senescence progresses over time during metabolic stress, with CDKN1A being expressed early on and CDKN2A remaining a sign of established cellular senescence [176].

Moreover, the decreased function of senescent β-cells may be attributed to the increased production of genes that are disallowed. Senescent β-cells in both T1D and T2D develop an SASP, but it is yet unknown how this modifies or interferes with the β-cell’s normal secretory machinery. Over time, ageing leads to decreased transcriptional fidelity and reduced β-cell function, even while overexpression of some age-dependent factors alone does not cause age-related deterioration of β-cells. Changes in mitochondrial activity and reactive oxygen species, which have not yet been fully understood in the context of senescent β-cells, are among the significant consequences of senescence on cellular metabolism [176]. Because β-cells generate insulin peptide fragments into the bloodstream that can activate CD4+ T cells, even at remote locations, there is evidence that interaction between immune cells and β-cells plays a role in the pathophysiology of T1D. IFN-γ and TNF are examples of pro-inflammatory cytokines that can cause β-cells to express class I and II HLA molecules. IFN-α could operate through a TYK2-STAT2-IRF9 axis in the early stages of the disease, while IFN-γ-driven STAT1 activation could eventually contribute to elevated HLA expression. An antigen source for autoreactive T lymphocytes is provided by senescent or apoptotic β-cells [177]. IFN-γ stimulation produces “fragile” regulatory T cells (Tregs), which have a preserved Treg phenotype regarding FOXP3+ but reduced suppressive efficacy. But in addition to the well-established induction of indoleamine-pyrrole 2,3-dioxygenase (IDO1) and programmed death-ligand 1 (PD-L1) expression, chronic exposure and constitutive signalling of IFN-γ also directly inhibit T cell stemness, development, clonal diversity, and preservation, as well as having a pro-apoptotic effect during the contraction phase through FAS and BIM [178]. Interestingly, despite lower PD-1 expression levels on memory T cells, phenotypes linked to accelerated immunological ageing in T1D included higher CXCR3+ and PD-1-positive proportions in naive and memory T cell subsets. After adjusting for age, T1D-related phenotypes were prognostic of T1D condition. Therefore, Shapiro et al. provided disease-associated phenotypes for biomarker evaluation and therapy approaches, as well as accelerated immune ageing in T1D [179].

5. Discussions, Knowledge Gaps and Future Directions

As presented in Table 2, telomere shortening is a consistent feature in individuals with T1D, reflecting accelerated biological ageing driven by chronic inflammation, oxidative stress, and metabolic imbalance. ROS generated by mitochondrial dysfunction, immune cell activity, and hyperglycaemia contribute significantly to telomeric DNA damage, especially via 8-oxoguanine accumulation and shelterin disruption [180]. Moreover, β-cell susceptibility to oxidative and nitrosative stress and impaired DNA repair mechanisms promote telomere instability, cellular senescence, and β-cell dysfunction, further propagating the autoimmune process [181]. Short leukocyte telomere length was associated with microvascular and macrovascular complications, independently predicting mortality and cardiovascular events in T1D [65,182]. All these molecular alterations are interdependent and reinforce one another in a feed-forward loop that accelerates ageing in T1D. In this narrative review, we aimed to provide an integrative perspective on the molecular interactions that occur under the stress caused by the onset and/or progression of T1D. This conceptual framework, in which telomere length is presented as a mediator and as a marker for biological ageing accentuated in T1D, offers a new perspective on understanding telomere dynamics. Thus, the potential of telomeres for developing personalised therapy strategies that could include risk stratification, the development of a complex panel of biomarkers for disease monitoring, and the development of new adjuvant therapies to counteract chronic complications associated with T1D is highlighted.

Regarding potential strategies involving counteracting aberrant shortening in patients with T1D, it is important to note that, at present, the evidence in the literature is rather limited. According to pharmacotherapy guidelines in Europe and the US, first-line therapy in patients with T1D is exogenous insulin, administered in various regimens [183,184]. Although there is ample evidence discussing the negative impact of chronic hyperglycaemia on telomere length [185,186,187], paradoxically, exogenous insulin has been identified as an additional factor accelerating telomere erosion [33]. In this regard, it is necessary to evaluate the effect of insulin therapy in larger, multicentric studies with a long follow-up period. At the same time, it is important to mention that the effect of different types of exogenous insulin, together with other pharmacological agents, on telomere dynamics should also be evaluated, as a polytherapeutic regimen, in this case, could have certain benefits. Still, the common long-term risks associated with complex therapeutic regimens, such as drug–drug interactions, possible potentiation of adverse reactions, and decreased patient adherence, should also be considered [188]. Consequently, research in this area remains emerging, and the potential to comprehend the complex molecular pathways linked to accelerated ageing serves as the starting point for future investigations.

Concerning lifestyle factors, it is well established that a healthy lifestyle can counteract aberrant telomere erosion [43,189]. Regular physical activity has been shown to be beneficial in preserving telomeres by reducing systemic inflammation, oxidative stress, telomerase activation and increasing the expression of proteins in the shelterin complex [190]. However, in patients with T1D, certain physical activities should be avoided, especially high-impact or resistance exercises, particularly if T1D is associated with complications such as retinopathy and neuropathy [191]. Similarly, specific diets have been discussed as having promising effects for T1D patients, such as the Mediterranean diet (reviewed comprehensively in [192]). However, T1D patients should follow a diet that must be adjusted according to several factors, including the pharmacotherapeutic regimen followed by the patient [193]. In this context, efforts to personalise therapy for patients with T1D should focus on integrating all these factors to improve long-term health outcomes, and multidisciplinary teams of doctors, pharmacists, and nutritionists should collaborate constantly.

Although there is ample evidence for the association between T1D and short telomeres, and therefore an increased risk of developing comorbidities, there are still various gaps in our current knowledge. Firstly, most studies are cross-sectional, making it difficult to establish the cause and the effect. Furthermore, the cohorts that have been analysed in numerous studies are relatively small, which may increase the likelihood of type II errors. It is necessary to conduct extensive research that involves particular populations, including children, adolescents, and adults with varying durations of T1D onset and associated comorbidities. Furthermore, the sensitivity and specificity of the methods used to analyse telomere length, such as qPCR, qFISH, flow-FISH, and Southern blot, can result in specific differences and discrepancies. Similarly, the development of methods for the widespread utilisation of telomere length as a biomarker in clinical practice is ongoing, and standardised values must be established to facilitate their translation into clinical practice. Finally, as previously mentioned, the interactions between lifestyle factors, epigenetic modifications, and telomere dynamics in T1D are not entirely understood.

6. Conclusions

In conclusion, the bidirectional relationship between telomere attrition and T1D pathogenesis underscores the potential of telomere length as a biomarker for disease monitoring and a therapeutic target to mitigate ageing-related complications in T1D. However, the wide implementation of telomere length measurements in clinical practice still needs further investigations.