Retinol-Binding Protein 4 in Obesity-Associated Metabolic Dysfunction: Mechanisms, Controversies, and Therapeutic Opportunities

Abstract

Retinol-binding protein 4 (RBP4) is the primary carrier of retinol in the circulation and is produced mainly by the liver. Interest in RBP4 expanded after reports of elevated circulating levels in insulin-resistant mice and some human populations, leading to its proposal as an adipokine, an immunometabolic mediator, and a biomarker of obesity-related metabolic disease. However, subsequent research has revealed that its biology is considerably more complex. The metabolic effects of RBP4 are influenced by multiple factors, including its tissue of origin, retinol-binding status, association with transthyretin, receptor interactions, renal clearance, and the surrounding inflammatory environment. This review summarizes current evidence linking RBP4 to insulin resistance, adipose tissue dysfunction, and type 2 diabetes, while distinguishing classical retinol-bound holo-RBP4–dependent signaling through STRA6 (stimulated by retinoic acid 6) from retinol- and STRA6-independent mechanisms involved in innate and adaptive immune activation. We also discuss the transcriptional regulation of RBP4, focusing on PPARγ (peroxisome proliferator-activated receptor γ), HMGA1 (high mobility group A1 protein), and HIF-1α (hypoxia-inducible factor 1α)-related hypoxia-responsive pathways. Available evidence suggests that these factors do not operate within a simple HIF-1α-PPARγ-RBP4 signaling axis but rather participate in interconnected regulatory networks that shape adipocyte differentiation, secretory function, and metabolic adaptation. Although higher circulating RBP4 concentrations have been associated with adverse metabolic traits and an increased risk of type 2 diabetes, their clinical interpretation remains challenging because of the influence of renal function, vitamin A status, molecular heterogeneity, and methodological differences between assays. Pharmacological approaches to lower RBP4 are now feasible, but whether systemic reduction is beneficial remains uncertain. Targeting the specific pathogenic actions of RBP4, while preserving its essential role in retinol transport, may offer a more effective therapeutic strategy for metabolic disease.

1. Introduction

Obesity-associated insulin resistance is a complex disorder that cannot be explained by impaired glucose metabolism alone. Rather, it results from coordinated alterations across multiple organs, including adipose tissue, liver, skeletal muscle, pancreatic β-cells, and the immune system. Among these, adipose tissue plays a central role because its expansion is accompanied by profound changes in lipid handling, extracellular matrix remodeling, vascularization, immune-cell infiltration, and endocrine signaling. These alterations reshape systemic metabolic homeostasis and contribute to the development of chronic low-grade inflammation. Within this context, retinol-binding protein 4 (RBP4) has emerged as an important candidate linking vitamin A metabolism with metabolic and inflammatory regulation [1,2,3,4,5]. RBP4 was initially characterized as the major plasma carrier responsible for transporting retinol from hepatic stores to peripheral tissues [6,7]. Interest in its metabolic role increased after reports showing elevated circulating RBP4 levels in insulin-resistant mice and in several cohorts of individuals with obesity or type 2 diabetes, leading to the proposal that RBP4 functions as an adipokine that promotes systemic insulin resistance [1,2]. Subsequent studies, however, have revealed a more nuanced picture. Although adipocytes express RBP4, hepatocytes are now recognized as the principal source of circulating protein, as demonstrated by tissue-specific mouse models [8]. In addition, moderate liver-specific overexpression of RBP4 failed to impair glucose tolerance or insulin sensitivity in one experimental model [9], suggesting that circulating RBP4 concentration alone is insufficient to drive metabolic dysfunction. These findings indicate that the biological effects of RBP4 depend not only on its circulating levels but also on its tissue of origin, molecular form, and mechanisms of action [4,8,9]. Beyond its established role in retinol transport, RBP4 can activate receptor-dependent signaling pathways and modulate inflammatory responses through mechanisms that are either dependent on, or independent of, classical retinoid transport [10,11,12,13]. Interpretation of clinical studies is further complicated by factors such as renal function, vitamin A status, transthyretin binding, the relative proportions of apo- and holo-RBP4, and differences in analytical methods used to quantify the protein [4,14,15,16,17].

This review considers the complex role of RBP4 in metabolic disease. Although RBP4 is often described as a biomarker of metabolic risk or a direct contributor to insulin resistance and inflammation, its effects may depend on where it is produced, the molecular form in which it circulates, and the pathways through which it acts. Drawing on findings from experimental models, human genetic and clinical studies, and therapeutic research, we first describe the development of insulin resistance and adipose tissue dysfunction. We then examine the structure and distribution of RBP4, its molecular forms, and its roles in cell signaling, immune regulation, and gene expression. Particular attention is given to its relationship with peroxisome proliferator-activated receptor γ (PPARγ), high mobility group A1 protein (HMGA1), and hypoxia-inducible factor 1α (HIF-1α). We conclude by considering the clinical relevance of RBP4 and whether it represents a viable therapeutic target in obesity-related metabolic dysfunction.

Literature Search Strategy

A narrative literature search was manually performed using PubMed/MEDLINE, Scopus, Web of Science, and ClinicalTrials.gov to identify studies relevant to RBP4 biology and obesity-associated metabolic dysfunction. Searches included studies published from database inception to July 2026 and combined the terms “retinol-binding protein 4”, “RBP4”, “obesity”, “insulin resistance”, “type 2 diabetes”, “adipose tissue”, “retinol”, “transthyretin”, “STRA6”, “inflammation”, “TLR2”, “TLR4”, “NLRP3”, “PPARγ”, “HMGA1”, “hypoxia”, and “therapeutic targeting”.

English-language original studies, clinical trials, authoritative reviews, meta-analyses, and relevant ClinicalTrials.gov records were prioritized when they informed RBP4 structure and function, metabolic phenotypes, clinical measurement, or therapeutic targeting. Reference lists of key articles were also screened to identify additional relevant studies. Experimental studies were selected to illustrate mechanistic evidence, tissue-specific effects, and conflicting causal findings, whereas human studies were selected to represent different populations, study designs, analytical methods, and major confounders, when available. Relevant findings from the authors’ previous original research were also incorporated when they directly contributed to the mechanistic or interpretative framework of the review.

Before addressing RBP4 biology in detail, the following section briefly outlines the pathophysiological context of obesity-associated insulin resistance and adipose tissue dysfunction.

2. Insulin Resistance and Adipose Tissue Dysfunction

2.1. Molecular Basis and Progression of Insulin Resistance

Insulin resistance is defined as a reduced responsiveness of target tissues to the metabolic actions of insulin and typically develops years before overt type 2 diabetes. In the early stages, pancreatic β-cells compensate by increasing insulin secretion, allowing blood glucose concentrations to remain within the normal range. As insulin resistance worsens, this compensatory response gradually fails, resulting in persistent hyperglycemia and the onset of type 2 diabetes [18,19]. At the cellular level, insulin binding triggers autophosphorylation of the insulin receptor and recruitment of insulin receptor substrates (IRS), leading to activation of the phosphoinositide 3-kinase (PI3K)-Akt signaling pathway. Akt is a central regulator of insulin actions, promoting glucose transporter 4 (GLUT4) translocation and glucose uptake in skeletal muscle and adipose tissue while coordinating glycogen synthesis, lipid metabolism, protein synthesis, and suppression of hepatic glucose production [20,21].

Obesity disrupts this finely regulated signaling cascade through a combination of metabolic and inflammatory stress. Stress-responsive kinases, including c-Jun N-terminal kinase (JNK) and IκB kinase (IKK), phosphorylate IRS proteins at inhibitory sites, weakening PI3K-Akt signaling and reducing insulin-stimulated glucose uptake. As a consequence, insulin becomes less effective at suppressing lipolysis in adipose tissue and glucose production in the liver [22,23,24,25,26]. Additional factors, including lipotoxicity, oxidative stress, mitochondrial dysfunction, and endoplasmic reticulum stress, further impair insulin action and contribute to the development of dyslipidemia, metabolic dysfunction-associated steatotic liver disease, and cardiovascular complications [19,27,28]. Insulin sensitivity is also influenced by transcriptional control of the insulin receptor. HMGA1 promotes assembly of the transcriptional machinery at the insulin receptor (INSR) promoter [29,30]. More specifically, HMGA1 is part of a transcriptionally active multiprotein complex at the INSR promoter, where it cooperates with Sp1 and C/EBPβ. Through direct interactions with these transcription factors, HMGA1 facilitates their binding to the promoter and enhances their transcriptional synergy, thereby supporting the full activation of the INSR and other genes involved in glucose metabolism [31]. Reduced HMGA1 expression or activity decreases INSR abundance, thereby limiting cellular responsiveness to insulin [30,32]. This mechanism is particularly relevant in the context of RBP4, as HMGA1 also directly regulates transcription of the RBP4 gene [33,34].

2.2. Adipose Tissue Expansion and Metabolic Dysfunction

Adipose tissue is now recognized as far more than a passive reservoir for energy storage. It is a highly active metabolic and endocrine organ that regulates lipid handling, insulin sensitivity, and communication with other tissues through a complex secretome composed of proteins, lipids, metabolites, cytokines, and extracellular vesicles [35,36]. Healthy expansion of adipose tissue depends on coordinated adipogenesis, angiogenesis, extracellular matrix remodeling, and sufficient capacity to store triglycerides. During obesity, especially within visceral fat depots, these adaptive processes become progressively impaired. As a result, tissue growth occurs mainly through hypertrophy of existing adipocytes rather than the formation of new ones. Enlarged adipocytes respond poorly to insulin and fail to adequately suppress lipolysis, leading to increased release of non-esterified fatty acids and glycerol. The excess lipid flux promotes ectopic fat deposition in the liver and skeletal muscle, contributing to systemic insulin resistance [35,36,37,38].

As adipocytes become stressed, the surrounding tissue also undergoes marked remodeling. Extracellular matrix deposition increases, damaged adipocytes accumulate, and immune cells are progressively recruited [39]. Macrophages surround stressed or dying adipocytes and engage in continuous crosstalk with them through cytokines, chemokines, fatty acids, and reactive oxygen species [35]. Over time, these interactions shift adipose tissue from an efficient energy-buffering organ to a chronic source of inflammatory and metabolic signals that promote whole-body insulin resistance [26].

2.3. Adipose Tissue Hypoxia and HIF-1-Dependent Remodeling

Among the mechanisms contributing to adipose tissue dysfunction, hypoxia has emerged as an important driver of maladaptive remodeling in obesity. As adipocytes enlarge, oxygen diffusion becomes less efficient, while angiogenesis and tissue perfusion may fail to keep pace with tissue expansion. Local oxygen availability may be further affected by changes in mitochondrial activity and oxygen consumption. Adipose tissue hypoxia should therefore be understood as a dynamic imbalance between oxygen supply and metabolic demand, rather than solely as a consequence of reduced capillary density [39,40,41,42,43]. Reduced oxygen availability stabilizes hypoxia-inducible factor 1-alpha (HIF-1α) by preventing its hydroxylation-dependent degradation, thereby enabling its nuclear accumulation and the activation of hypoxia-responsive genes. Although transient HIF-1 activation supports adaptive responses to reduced oxygen tension, persistent activation promotes extracellular matrix deposition, fibrosis, inflammatory signaling, and impaired adipocyte differentiation, ultimately contributing to adipose tissue dysfunction and insulin resistance [44,45,46,47]. In line with these effects, proteomic studies have shown that hypoxia extensively remodels the adipocyte secretome, altering proteins involved in extracellular matrix organization, lipid metabolism, and insulin sensitivity [48]. Importantly, the metabolic consequences of adipose tissue hypoxia extend beyond canonical HIF-1 signaling. Hypoxia can also reshape adipocyte transcriptional and post-transcriptional programs through HIF-1α-associated nuclear regulators, including HMGA1, as well as through mechanisms such as miR-128-mediated regulation of insulin receptor expression [48,49]. Together, these findings indicate that adipose tissue hypoxia acts through multiple interconnected pathways rather than through a single transcriptional program.

2.4. Potential Links Between Hypoxia and RBP4

The broad effects of hypoxia on adipocyte differentiation, inflammation, and secretory function suggest that it may also influence RBP4 biology. However, direct transcriptional regulation of RBP4 by HIF-1α has not been demonstrated. To date, neither HIF-1α binding to the RBP4 promoter nor a functional hypoxia-response element controlling RBP4 transcription has been conclusively identified. Instead, hypoxia is more likely to influence RBP4 indirectly by modifying adipocyte differentiation, PPARγ-dependent transcriptional programs, inflammatory pathways, or the overall composition of the adipose secretome. Importantly, inflammatory signaling does not necessarily increase RBP4 expression. In human adipocytes, both tumor necrosis factor alpha (TNF-α) and interleukin-1 beta (IL-1β) have been reported to suppress RBP4 production, underscoring the complexity of its regulation [50,51].

HMGA1 represents a possible point of convergence between these pathways. Previous work from our group demonstrated that HMGA1 binds regulatory regions of the RBP4 gene and is required for both basal and cAMP-induced transcription [33,34]. Given that HMGA1 also cooperates with HIF-1 in regulating selected hypoxia-responsive genes, it is plausible that hypoxia alters the transcriptional environment in which RBP4 is expressed. Nevertheless, direct evidence for an HIF-1α-HMGA1 complex controlling RBP4 transcription is currently lacking. Taken together, the available data support a model in which hypoxia shapes a metabolic, inflammatory, and transcriptional milieu capable of influencing RBP4 expression or function through indirect, context-dependent mechanisms, rather than through a direct linear HIF-1α-RBP4 signaling pathway.

3. RBP4: From Retinol Carrier to Metabolic and Immune Signal

The preceding sections highlight how adipose tissue dysfunction is characterized by altered lipid metabolism, chronic inflammation, and remodeling of the adipose secretome. Within this context, RBP4 has attracted considerable attention because it serves not only as the principal carrier of retinol but also as an active regulator of metabolic and immune signaling. These functions are closely interconnected, as the biological activity of RBP4 depends on both its molecular form and its tissue of origin.

3.1. Genetics, Structure, Molecular Forms and Physiological Function

The human RBP4 gene is located on chromosome 10q23.33 [52]. Its transcription is regulated by a complex 5′-flanking promoter region containing multiple cis-regulatory elements that support basal, cell-specific, and signal-responsive expression [53,54]. Functional studies have identified enhancer, negative regulatory, and promoter elements within this region [53,54]. In addition, the RBP4 promoter contains retinoic acid-responsive elements (RAREs) separated by an intervening GC-rich regulatory sequence, which may contribute to the cooperative assembly or stabilization of transcriptional complexes required for retinoid sensitivity [55]. Furthermore, its basal and inducible activity can be modulated by metabolic and hormonal signals, including cAMP-HMGA1-dependent mechanisms [33,34]. Regulatory genetic variation and additional cis-regulatory regions may further influence RBP4 transcriptional activity [56,57]. The encoded protein, RBP4, is synthesized as a 201-amino-acid precursor containing an N-terminal signal peptide. After cleavage of this signal peptide, the mature secreted protein consists of 183 amino acids and has a molecular mass of approximately 21 kDa [4,6]. As a member of the lipocalin family, RBP4 adopts the characteristic eight-stranded antiparallel β-barrel that forms a hydrophobic binding pocket for retinol, allowing efficient transport of this poorly water-soluble vitamin through the circulation [58,59]. In hepatocytes, retinol mobilized from hepatic stores binds RBP4 before being released into the bloodstream [7].

Once in circulation, retinol-bound holo-RBP4 forms a complex with transthyretin [3,14], increasing its effective molecular size and reducing renal filtration [14,58]. Retinol-free apo-RBP4 also circulates but differs from holo-RBP4 in stability, clearance, receptor interactions, and signaling properties [4]. In plasma, retinol-bound holo-RBP4 associates with transthyretin (TTR), increasing the effective molecular size of the complex and limiting renal filtration [3,4,14]. Retinol-free apo-RBP4 also circulates, but it binds TTR less efficiently and is cleared more readily than holo-RBP4 [14]. These molecular differences are biologically relevant because holo-RBP4 is the principal form involved in retinol transport and the best-characterized ligand for STRA6-mediated retinol uptake and JAK2/STAT signaling [10,60]. By contrast, retinol- and STRA6-independent inflammatory effects of RBP4 have been described in macrophage–adipocyte systems [11,13]. Therefore, the biological effects attributed to RBP4 should be interpreted in relation to its molecular form and the experimental system used. Total circulating RBP4 is therefore an incomplete descriptor of RBP4 biology. Renal dysfunction can increase apo-RBP4, holo-RBP4, and C-terminally truncated RBP4 species [61], while circulating RBP4 and TTR concentrations may vary with age and sex even in metabolically healthy individuals [62]. Accordingly, measurement of total RBP4 may not accurately reflect the abundance of the molecular forms most relevant to metabolic signaling. Because molecular-form information is rarely available in metabolic cohorts, this limitation complicates causal interpretation and the development of form-selective therapeutic strategies, as discussed in the following sections.

3.2. Tissue Sources and Regulation of RBP4

The liver is the principal source of circulating RBP4, with hepatocytes coordinating its synthesis and secretion according to retinol availability [7,8]. Tissue-specific knockout studies in mice indicate that hepatocytes account for the majority of circulating RBP4 under physiological conditions [8]. Adipose tissue also expresses and secretes RBP4, although its contribution to plasma concentrations appears relatively small and may become more relevant under conditions of metabolic stress [8,63,64]. Interest in adipose-derived RBP4 began with the landmark study by Yang et al., which identified RBP4 as an adipokine whose expression increased in insulin-resistant mice and whose manipulation altered systemic insulin sensitivity [1]. Subsequent studies confirmed that adipocytes produce RBP4 [65,66] but also revealed marked variability depending on species, adipose depot, differentiation state, obesity, and overall metabolic health [63,65,67].

Compartmentalization is central to the interpretation of RBP4 biology. Hepatic RBP4 primarily sustains systemic retinol transport, whereas adipocyte-derived RBP4 may act mainly through local autocrine or paracrine mechanisms that influence adipose metabolism and inflammation [63,64]. This tissue specificity is further supported by evidence from brown adipose tissue (BAT), where RBP4 overexpression enhanced adrenergic signaling, thermogenesis, energy expenditure, and glucose homeostasis in mice [68]. The molecular mechanisms regulating RBP4 expression also differ between the liver and adipose tissue (Figure 1). In hepatocytes, retinol availability and fasting-related pathways involving cAMP, glucagon, and forkhead box O1 (FOXO1) play major regulatory roles [33,34,69]. HMGA1 further enhances both basal and cAMP-induced transcription by promoting assembly of a transcriptional complex at the RBP4 promoter [33,34]. In adipocytes, RBP4 expression is influenced by differentiation status, nuclear receptor signaling, insulin sensitivity, and inflammatory stimuli. Importantly, these regulatory relationships are context-dependent. Although circulating RBP4 decreases following PPARγ agonist treatment in some studies [1,5,66], troglitazone increased RBP4 production in primary human adipocytes, whereas TNF-α reduced it [50]. These observations suggest that neither PPARγ activation nor inflammatory signaling exerts a uniform effect on RBP4 expression across different biological settings.

Figure 1. Tissue-specific regulation and actions of RBP4 in hepatocytes and adipocytes. Schematic overview of the tissue-specific biology of retinol-binding protein 4 (RBP4) in hepatocytes and adipocytes. In the liver, retinol released from hepatic retinyl ester stores is loaded onto RBP4, allowing secretion of holo-RBP4, which predominantly circulates in complex with transthyretin (TTR). Under conditions of limited retinol availability, apo-RBP4 is produced, displays reduced TTR binding, and is more readily cleared by the kidney. Hepatic RBP4 expression is stimulated during fasting through glucagon/cAMP signaling and the transcription factors FOXO1 and HMGA1. In adipose tissue, nutrient excess promotes adipocyte hypertrophy, inflammation, hypoxia, cellular stress, impaired differentiation, and altered secretory activity. In this setting, RBP4 expression is regulated by multiple pathways, including HMGA1, PPARγ/RXR-dependent adipogenic programs, hypoxia-responsive signaling, and inflammatory mediators such as TNF-α and IL-1β. Adipocyte-derived RBP4 may exert local autocrine and paracrine effects that contribute to impaired insulin signaling. Solid arrows indicate direct or experimentally supported biological processes, including transcriptional activation, protein synthesis, retinol loading, complex formation, secretion, and cellular responses. Blunt-ended lines indicate inhibitory effects. Dashed arrows indicate indirect, proposed, or context-dependent relationships that may require intermediate molecular pathways or changes in cellular state/tissue microenvironment. Dashed lines in the apo-RBP4 route indicate an alternative pathway favored by limited retinol availability. Dotted arrows indicate downstream functional consequences rather than direct molecular interactions. Overall, the figure emphasizes that RBP4 biology depends on tissue origin, retinol-loading status, TTR association, and the metabolic and inflammatory state of adipose tissue.

3.3. Human Genetics of RBP4 and STRA6

Candidate-gene studies suggest that genetic variation at the RBP4 locus may influence RBP4 expression and metabolic traits, although the evidence remains heterogeneous. Promoter and regulatory variants, including rs3758539, have been associated with adipose RBP4 expression, circulating RBP4 concentrations, insulin resistance, body-mass index, type 2 diabetes, or hypertriglyceridemia in Mongolian, European, and Chinese cohorts [70,71,72,73,74]. These associations are biologically plausible because promoter variation may affect tissue-specific gene expression; however, effect sizes, associated alleles, and metabolic phenotypes have not been consistent across populations.

Replication studies have provided mixed results. In a South Indian case–control study including 2002 participants, individual RBP4 single-nucleotide polymorphisms were not independently associated with type 2 diabetes, although an RBP4 haplotype and selected STRA6 variants showed significant associations [75]. Differences in ancestry, linkage disequilibrium, adipose distribution, renal function, vitamin A status, and sample size may contribute to these discrepancies. Therefore, candidate-gene associations should not be interpreted as genetic proof that increased circulating RBP4 directly causes insulin resistance. Genome-scale analyses further support the genetic regulation of the retinol-transport system. A recent genome-wide study identified loci influencing circulating retinol and highlighted the relevance of the RBP4-TTR complex in human retinoid biology [76]. However, these data do not establish a direct causal pathway from genetically regulated RBP4 or retinol transport to obesity-associated insulin resistance or type 2 diabetes. Overall, human genetics supports population-dependent regulation of RBP4-related pathways, while leaving the metabolic causal role of RBP4 unresolved.

4. Transcriptional Regulation of RBP4

The tissue-specific functions of RBP4 emerge from complex transcriptional networks that govern adipocyte identity, lipid storage, insulin responsiveness, and adaptation to metabolic stress. Among these regulators, PPARγ is the master determinant of the differentiated adipocyte phenotype, while HMGA1 and hypoxia-responsive pathways provide additional layers of regulation that intersect with, but are not fully dependent on, PPARγ signaling.

4.1. PPARγ and Adipocyte Metabolic Competence

PPARγ is a ligand-activated nuclear receptor that plays a central role in adipocyte differentiation and metabolic function. Two major isoforms have been identified: PPARγ1, which is expressed in several tissues, and PPARγ2, which is largely restricted to adipose tissue. Structurally, the receptor contains an N-terminal transactivation domain, a conserved DNA-binding domain, and a C-terminal ligand-binding domain. Upon ligand binding, PPARγ heterodimerizes with retinoid X receptor (RXR) and binds peroxisome proliferator response elements (PPREs) within regulatory regions of target genes [77,78,79].

Rather than controlling a single metabolic pathway, PPARγ orchestrates a broad transcriptional program that supports fatty-acid uptake, triglyceride synthesis, lipid-droplet formation, insulin-stimulated glucose utilization, adipokine secretion, and the differentiation of metabolically competent adipocytes [77,78,80,81,82]. By expanding the lipid-storage capacity of adipose tissue, PPARγ also limits ectopic fatty acid deposition in the liver and skeletal muscle, thereby contributing to whole-body insulin sensitivity.

In obesity, impaired PPARγ function cannot simply be attributed to reduced receptor expression. Its transcriptional activity is influenced by ligand availability, cofactor recruitment, inflammatory signaling, and post-translational modifications. For example, obesity-associated phosphorylation selectively alters the expression of metabolically relevant target genes while preserving much of the receptor’s adipogenic activity [83].

4.2. Hypoxia and Inflammation as Modifiers of PPARγ Signaling

Chronic hypoxia and inflammation can disrupt PPARγ-dependent transcriptional programs within adipose tissue. HIF-1α induces the transcriptional repressor DEC1/Stra13, which suppresses PPARγ2 expression and inhibits adipocyte differentiation [84]. Reduced recruitment of new insulin-sensitive adipocytes may consequently favor hypertrophy of existing cells. In parallel, inflammatory mediators including TNF-α, NF-κB, and JNK antagonize PPARγ activity and further impair adipocyte function [24,85,86]. Together, these processes diminish adipose tissue plasticity, promote lipolysis and fibrosis, and sustain immune-cell infiltration.

However, the interaction between hypoxia and PPARγ is not uniform. The effects of HIF-1 signaling depend on the duration and severity of hypoxia, the stage of adipocyte differentiation, and the adipose depot involved [41,46]. Rather than acting as a universal inhibitor, hypoxia is better viewed as a context-dependent modulator of PPARγ-driven transcription.

4.3. The Relationship Between PPARγ and RBP4 Is Not Uniformly Inverse

Because PPARγ promotes adipocyte insulin sensitivity, reduced PPARγ activity has often been assumed to increase RBP4 expression. Experimental findings, however, indicate that this relationship is considerably more complex.

In brown adipocytes, activation of PPARγ and PPARα stimulates both RBP4 transcription and protein secretion. A functional PPRE has been identified within the RBP4 promoter, and PGC-1α further enhances RBP4 expression during thermogenic activation [87]. Similarly, PPARγ agonists increase RBP4 expression in differentiated white adipocytes, and troglitazone has been shown to stimulate RBP4 production in primary human adipocytes [50]. In contrast, several studies have reported lower circulating RBP4 concentrations following treatment with thiazolidinediones [1,5,66]. These systemic changes are more likely to reflect improvements in insulin sensitivity, altered lipid partitioning, changes in hepatic RBP4 production, renal clearance, retinol availability, or shifts in the relative contribution of individual adipose depots than direct transcriptional repression by PPARγ.

Accordingly, reduced PPARγ activity during adipose hypoxia or inflammation should not automatically be interpreted as a stimulus for RBP4 expression. The direction and magnitude of this relationship vary according to species, adipose depot, differentiation state, pharmacological intervention, and whether RBP4 is measured locally or in the circulation. Moreover, although RBP4 is the principal transporter of retinol, it is not itself a PPARγ ligand. Any influence of RBP4 on PPARγ-dependent transcription would therefore be expected to occur indirectly through changes in retinoid delivery, intracellular retinoid metabolism, or downstream receptor signaling.

4.4. HMGA1 as a Regulator of Insulin Signaling and RBP4 Transcription

Compared with PPARγ, HMGA1 provides a more direct mechanistic link between RBP4 expression and insulin sensitivity. HMGA1 binds regulatory regions of the RBP4 gene and promotes assembly of the transcriptional complexes required for both basal and cAMP-induced gene expression [33,34]. The cAMP-HMGA1-RBP4 axis therefore represents one of the best-characterized pathways controlling RBP4 transcription. HMGA1 also regulates expression of the insulin receptor. Reduced HMGA1 expression or activity lowers insulin receptor abundance and contributes to impaired insulin signaling in both experimental models and humans [29,30,32]. Through these complementary actions, HMGA1 occupies a strategic position at the interface between insulin responsiveness and RBP4 regulation.

Its metabolic effects, however, extend beyond these individual targets. Adipose-specific overexpression of HMGA1 impairs adipocyte differentiation while simultaneously protecting mice from diet-induced obesity and insulin resistance, highlighting its broader influence on adipose tissue remodeling and systemic metabolism [88]. HMGA1 also cooperates with HIF-1 during the hypoxic transcriptional response, enhancing the expression of VEGF and visfatin in hypoxic adipocytes [48]. At present, however, there is no evidence that HMGA1 and HIF-1 cooperate directly at the RBP4 promoter, nor has a direct physical interaction between HMGA1 and PPARγ been demonstrated.

4.5. An Integrated Model

Current evidence does not support a single, direct HIF-1α-PPARγ-RBP4 signaling pathway. Instead, RBP4 expression appears to be regulated within a broader transcriptional network in which chronic hypoxia and inflammation reshape adipose tissue through coordinated changes in HIF-1-dependent responses, PPARγ-controlled adipocyte programs, HMGA1-mediated transcription, and the overall secretory phenotype. Within this framework, HMGA1 directly regulates both basal and cAMP-induced RBP4 transcription [33,34], whereas PPARγ can promote RBP4 expression in selected adipocyte contexts, particularly during differentiation and thermogenic activation [50,87]. HIF-1α suppresses adipocyte differentiation [84] and cooperates with HMGA1 on a subset of hypoxia-responsive genes [47], but direct HIF-1α-mediated transcriptional regulation of RBP4 has not been demonstrated.

RBP4 should therefore be regarded as one component of an integrated metabolic and transcriptional network rather than the endpoint of a single signaling cascade. Its expression and biological effects are shaped by the interplay among tissue origin, adipocyte differentiation state, retinoid availability, inflammation, hypoxia, and systemic metabolic status. An overview of the tissue-specific regulation and function of RBP4 in hepatocytes and adipocytes is presented in Figure 1.

5. Biological Actions of RBP4

5.1. STRA6-Dependent Signaling and Insulin Action

Beyond its role in retinol transport, RBP4 can directly influence insulin signaling through several complementary mechanisms. In adipocytes, RBP4 exposure has been associated with reduced insulin-stimulated phosphorylation of IRS and Akt, impaired GLUT4 translocation, and diminished glucose uptake [1,5,10]. Many of these effects involve signaling through STRA6, a membrane protein that functions both as a retinol transporter and as a signaling receptor.

Binding of holo-RBP4 to STRA6 activates JAK2 and downstream STAT signaling, leading to increased expression of suppressor of cytokine signaling 3 (SOCS3). SOCS3 subsequently inhibits insulin receptor and IRS signaling, thereby attenuating activation of the PI3K-Akt pathway [10,60,89]. STRA6-mediated signaling has also been implicated in pancreatic β-cell dysfunction associated with RBP4 [90]. However, STRA6 does not account for all biological effects of RBP4, and its expression varies considerably among tissues and metabolic states. In preclinical in vitro and in vivo obesity models, Zemany et al. reported marked downregulation of STRA6 in adipocytes and adipose stromovascular cells, while adipocyte-specific Stra6 knockdown produced only modest effects on insulin sensitivity under high-fat-diet conditions [91]. Moreover, formation of the RBP4-TTR complex may limit receptor accessibility [4], and several pro-inflammatory effects of RBP4 occur independently of both retinol and STRA6 [11], indicating that alternative signaling mechanisms also contribute.

RBP4 has likewise been implicated in hepatic glucose metabolism. Increased systemic RBP4 enhanced phosphoenolpyruvate carboxykinase expression and hepatic glucose production in several experimental models [1]. By contrast, moderate hepatocyte-specific overexpression failed to impair glucose tolerance or insulin sensitivity [9]. Together, these findings suggest that the metabolic consequences of RBP4 depend not simply on circulating concentrations but also on the duration of exposure, tissue source, retinol-loading status, and accompanying inflammatory environment.

5.2. Local Adipose Actions and Inter-Organ Communication

Although adipose tissue contributes relatively little to circulating RBP4, locally produced RBP4 may exert important effects within the adipose microenvironment. Adipocyte-specific overexpression has been linked to increased inflammation, enhanced lipolysis, and greater release of fatty acids from adipose tissue, changes that are followed by ectopic lipid accumulation and hepatic steatosis [64]. In cultured human adipocytes, RBP4 also stimulates lipolysis and promotes communication between adipocytes and macrophages [63]. These observations support the concept that adipose-derived RBP4 acts primarily through autocrine and paracrine mechanisms, amplifying local inflammation and lipid mobilization before indirectly affecting distant organs through altered lipid flux and inflammatory mediators [64]. This model helps explain why adipocyte-derived RBP4 can exert substantial metabolic effects despite contributing only modestly to circulating RBP4 concentrations [8,64]. Whether this mechanism plays an equivalent role in humans remains less clear [63]. Adipose RBP4 expression is not consistently elevated in obesity or insulin resistance and likely varies according to adipose depot, disease stage, retinoid availability, and cellular composition of the tissue.

5.3. Innate and Adaptive Immune Activation

Accumulating evidence indicates that RBP4 also contributes to metabolic dysfunction by modulating immune responses. In macrophages, RBP4 activates JNK and Toll-like receptor-associated signaling pathways, promoting production of inflammatory mediators that impair insulin signaling in neighboring adipocytes. Notably, these effects can occur independently of retinol binding and STRA6 [11]. RBP4 also influences immunity by enhancing antigen presentation and promoting CD4⁺ T-cell activation. In mouse models with elevated RBP4 expression, blockade of T-cell co-stimulation using CTLA4-Ig reduced adipose inflammation and improved insulin sensitivity, supporting a direct contribution of adaptive immune activation of RBP4-mediated metabolic dysfunction [12]. More recently, RBP4 has been shown to prime the NLR family pyrin domain containing 3 (NLRP3) inflammasome through toll-like receptor 2 and 4 (TLR2/TLR4)-dependent pathways, increasing macrophages’ production of interleukin-1β following an appropriate secondary stimulus [13]. These findings place RBP4 within the broader immunometabolic network linking nutrient excess, innate immune activation, and insulin resistance.

5.4. Tissue Specificity and Unresolved Causality

Collectively, experimental studies support an important role for RBP4 in metabolic regulation and inflammation [11,13], but they do not demonstrate that every increase in circulating RBP4 is inherently pathogenic [9]. Much of the apparent inconsistency across experimental models likely reflects tissue-specific biology. Hepatocyte-derived RBP4 dominates the circulating pool [8], whereas adipocyte-derived RBP4 may act predominantly within the local tissue environment [63,64]. Likewise, apo- and holo-RBP4 differ in receptor interactions, while association with transthyretin influences both protein stability and receptor accessibility [4,14]. The biological effects of RBP4 also vary across adipose depots [64,68]. Recent work has shown that brown adipocyte-specific overexpression enhances adrenergic signaling, fatty-acid oxidation, thermogenesis, energy expenditure, and glucose clearance [68]. Although these findings do not exclude pathogenic actions in white adipose tissue or immune cells, they clearly demonstrate that the effects of RBP4 are highly context-dependent. Taken together, current evidence supports viewing RBP4 not as a uniformly pathogenic adipokine but as a retinoid carrier whose signaling functions are shaped by tissue type, metabolic state, and molecular context. This perspective is particularly important when interpreting circulating RBP4 measurements, investigating its transcriptional regulation, or considering therapeutic strategies aimed at modulating RBP4 activity. Table 1 summarizes the key preclinical studies supporting the tissue- and context-dependent actions of RBP4.

Table 1. Key preclinical evidence on RBP4 in obesity-associated metabolic dysfunction.

Reference	Model/System	Experimental Methods	Key RBP4-Related Findings	Metabolic and/or Mechanistic Interpretation	Interpretation/Evidence
Yang et al., 2005 [1]	Rbp4-/- mice; Rbp4-overexpressing transgenic mice; WT mice infused with recombinant RBP4	Glucose uptake assays, insulin signaling analysis, hepatic gene expression, hyperinsulinemic-euglycemic clamp.	Circulating RBP4 was increased in insulin-resistant mice and reduced in Rbp4-deficient mice.	↑ RBP4 promotes systemic insulin resistance.	Preclinical in vivo evidence of a pathogenic role of circulating RBP4 in insulin resistance.
Sell & Eckel, 2007 [50]	Primary human adipocytes	Cytokine treatment; RBP4 expression analysis	TNF-α reduced RBP4 production, whereas troglitazone increased RBP4 production in primary human adipocytes.	Inflammatory signaling and PPARγ activation do not regulate adipocyte RBP4 in a uniformly predictable or inverse manner.	Ex vivo human adipocyte evidence; inconsistent with a simple pro-inflammatory RBP4 model.
Motani et al., 2009 [92]	Rodent models treated with a non-retinoid RBP4 ligand	Ligand-binding assays; pharmacokinetic analyses; plasma RBP4 and retinol measurements; in vivo metabolic characterization	A non-retinoid RBP4 ligand reduced circulating RBP4 by disrupting retinol-dependent RBP4 stabilization.	↓ Circulating RBP4 → reduced retinol transport and enhanced RBP4 clearance	Preclinical in vivo evidence of successful RBP4 inhibition; reduced RBP4-specific metabolic causality not demonstrated (or absent).
Mody et al., 2008 [14]	ob/ob mice and HFD-fed mice	RBP4-TTR binding studies; renal clearance analysis	TTR binding reduced renal clearance and stabilized circulating RBP4	↑ TTR → ↓ renal RBP4 clearance → ↑ circulating RBP4 (stabilization) → may contribute to obesity-associated insulin resistance	Mechanistic evidence for RBP4 stabilization; metabolic causality of increased holo-RBP4 not demonstrated.
Chiefari et al., 2009 [33]	Hmga1-/- mice; human cell models	Promoter analysis; ChIP; gene expression; metabolic assessment	HMGA1 directly regulated RBP4 transcription.	↓ HMGA1 → ↓ RBP4 expression → altered glucose homeostasis	In vitro and in vivo explanation of RBP4 transcriptional regulation; downstream metabolic causality of RBP4 reduction not demonstrated.
Preitner et al., 2009 [93]	HFD-fed mice treated with fenretinide	Plasma RBP4 quantification; metabolic phenotyping; glucose and insulin tolerance tests; hepatic lipid assessment	Fenretinide reduced circulating RBP4 by disrupting the RBP4-TTR complex.	↓ Circulating RBP4 → ↑ insulin sensitivity → ↓ hepatic steatosis and diet-induced obesity	Preclinical in vivo evidence of successful RBP4 inhibition; reduced RBP4-specific metabolic causality not demonstrated.
Berry et al., 2011 [10]	Wild-type and Stra6-/- mice; STRA6-expressing cellular models	RBP4/retinol stimulation; JAK2-STAT5 signaling; SOCS3 expression; insulin signaling assays	Holo-RBP4 activated STRA6-dependent JAK2/STAT5 signaling.	Holo-RBP4 → STRA6/JAK2-STAT5 → ↑ SOCS3 induction → impaired insulin signaling.	Direct mechanistic evidence for STRA6-dependent signaling; preclinical in vivo and in vitro evidence for metabolic and inflammatory relevance of RBP4.
Koh et al., 2012 [94]	HFD-fed genetically obese (ob/ob) mice treated with fenretinide	Metabolic phenotyping; glucose and insulin tolerance tests; hepatic lipid analysis; assessment of circulating metabolic parameters	Fenretinide reduced circulating RBP4 levels.	↓ Circulating RBP4 → ↑ insulin sensitivity → ↓ hepatic steatosis	Preclinical in vivo evidence of successful RBP4 inhibition; reduced RBP4-specific metabolic causality not demonstrated.
Norseen et al., 2012 [11]	Bone marrow-derived macrophages (BMDMs); 3T3-L1 adipocytes; macrophage-adipocyte co-culture systems	RBP4 stimulation; cytokine assays; insulin signaling; TLR4/JNK inhibition	RBP4 activated macrophages through TLR4/JNK signaling.	↑ RBP4 → macrophage activation (TLR4/JNK) → adipose inflammation → insulin resistance	In vitro mechanistic evidence for RBP4-induced inflammatory activities; direct metabolic relevance of adipose RBP4 not assessed.
Kotnik et al., 2013 [51]	Primary human adipocytes (SGBS cells and ex vivo adipocytes from human donors)	Cytokine stimulation; RBP4 expression analysis; NF-κB inhibition	IL-1β suppressed adipocyte RBP4 expression via NF-κB signaling.	↑ IL-1β → NF-κB activation → ↓ adipocyte RBP4 expression	Ex vivo human adipocyte evidence; inconsistent with a simple pro-inflammatory RBP4 model.
Moraes-Vieira et al., 2016 [12]	Rbp4-overexpressing mice; Rbp4-/- mice; MyD88-/- mice	MHC-II expression analysis; CD4+ T-cell profiling; CTLA4-Ig intervention studies	Elevated RBP4 promoted MHC-II-dependent immune activation	↑ RBP4 → adaptive immune-mediated insulin resistance	Preclinical mechanistic and interventional evidence of insulin resistance and inflammation induction by RBP4; causality demonstrated in multiple preclinical models.
Lee et al., 2016 [64]	Adipocyte-specific Rbp4-overexpressing mice	Hepatic lipid analysis; metabolic phenotyping; adipose lipolysis assessment	Adipocyte-specific RBP4 overexpression increased circulating RBP4 and adipose lipolysis.	↑ Adipocyte-derived RBP4 → ↑ lipolysis → ↑ FFA flux to liver → hepatic steatosis and metabolic dysfunction	Preclinical mechanistic evidence of induced metabolic dysfunction and lipid accumulation by adipose-specific RBP4; causality demonstrated in preclinical models.
Thompson et al., 2017 [8]	Liver-specific and adipocyte-specific Rbp4-/- mice	Tissue-specific knockout; plasma RBP4 quantification	Liver-specific Rbp4 deletion markedly reduced circulating RBP4, whereas adipocyte-specific deletion had minimal effects.	↓ Circulating RBP4 → no major improvement in insulin sensitivity or glucose homeostasis	Preclinical tissue-of-origin evidence identifying hepatocytes as the principal source of circulating RBP4, while indicating a local autocrine/paracrine role for adipocyte-derived RBP4.
Fedders et al., 2018 [9]	Liver-specific Rbp4-overexpressing mice	AAV-mediated hepatic overexpression, metabolic phenotyping	Hepatic overexpression markedly increased circulating RBP4.	↑ Hepatocyte-derived RBP4 did not induce insulin resistance	Preclinical mechanistic evidence that increased liver-derived circulating RBP4 was not sufficient to induce insulin resistance.
Moraes-Vieira et al., 2020 [13]	Rbp4-overexpressing mice and WT mice infused with recombinant human (holo- and apo-) RBP4; immune cell-specific gene knockout (TLR2/4, NLRP3, caspase-1)	Inflammasome activation assays; cytokine profiling (IL-1β, TNF-α); NF-κB signaling analysis; macrophage functional studies	Elevated RBP4 activated TLR2/4 signaling and primed the NLRP3 inflammasome	↑ RBP4 → NLRP3 inflammasome activation → adipose inflammation → systemic insulin resistance	In vivo and in vitro preclinical evidence of metabolic causality and pro-inflammatory effects of RBP4.
Huang et al., 2021 [90]	Pancreatic β-cell models; Rbp4-overexpressing transgenic mice; db/db mice	STRA6 activation assays; β-cell insulin secretion assays; apoptosis and ER stress markers; insulin signaling and gene expression profiling	RBP4 activated STRA6-dependent signaling in pancreatic β-cells	↑ RBP4 → STRA6/JAK2-STAT1 activation → ↓ insulin secretion → β-cell dysfunction	Mechanistic evidence for RBP4-induced β-cell dysfunction; causality demonstrated in multiple preclinical models.
Park et al., 2025 [68]	Brown adipose tissue (BAT)-specific Rbp4-overexpressing mice	Thermogenic and metabolic phenotyping; indirect calorimetry; BAT gene expression analysis; glucose and insulin tolerance tests	BAT-derived RBP4 enhanced adrenergic signaling and thermogenic gene expression.	↑ BAT-derived RBP4 → ↑ thermogenesis and energy expenditure → improved glucose homeostasis and insulin sensitivity.	Preclinical evidence supports a role for BAT-derived RBP4 in thermogenic regulation, with causality in metabolic benefits.

AAV, adeno-associated virus; BAT, brown adipose tissue; BMDMs, bone marrow-derived macrophages; ChIP, chromatin immunoprecipitation; CTLA4-Ig, cytotoxic T-lymphocyte-associated protein 4 immunoglobulin; FFA, free fatty acids; HFD, high-fat diet; HMGA1, high mobility group A1; JAK2, Janus kinase 2; JNK, c-Jun N-terminal kinase; MHC-II, major histocompatibility complex class II; MyD88, myeloid differentiation primary response 88; NF-κB, nuclear factor kappa B; NLRP3, NOD-, LRR- and pyrin domain-containing protein 3; SGBS, Simpson-Golabi-Behmel syndrome; SOCS3, suppressor of cytokine signaling 3; STAT1, signal transducer and activator of transcription 1; STAT5, signal transducer and activator of transcription 5; STRA6, stimulated by retinoic acid 6; TLR2, Toll-like receptor 2; TLR4, Toll-like receptor 4; TNF-α, tumor necrosis factor-α; TTR, transthyretin; WT, wild type.

6. Human Clinical Evidence on RBP4

6.1. Evidence from Human Metabolic Studies

The discovery of a link between RBP4 and insulin resistance in experimental models prompted considerable interest in its clinical relevance. Early cross-sectional studies consistently reported higher circulating RBP4 concentrations in individuals with obesity, impaired glucose tolerance, and type 2 diabetes, with several showing an inverse relationship between RBP4 levels and insulin-stimulated glucose disposal [1,2]. Elevated RBP4 has also been associated with visceral adiposity, dyslipidemia, hypertension, MASLD, and other features of the metabolic syndrome [63,95,96].

Longitudinal studies have provided further support for a role of RBP4 as a marker of metabolic risk, although the findings have not always been consistent. Higher baseline RBP4 concentrations have been linked to an increased risk of developing type 2 diabetes in several adult cohorts [97,98]. Similar associations have been reported in individuals with prediabetes, although some studies suggest that the relationship may be non-linear rather than simply proportional to circulating RBP4 levels [99]. In younger populations, elevated RBP4 has been associated with the later development of insulin resistance or metabolic syndrome [100]. Nevertheless, these observations require confirmation across larger and more diverse populations.

Taken together, these studies suggest that elevated RBP4 may identify individuals with an unfavorable metabolic profile before overt diabetes becomes clinically apparent. However, they do not establish RBP4 as a causal driver of disease progression. Indeed, several longitudinal studies have reported that changes in circulating RBP4 do not consistently mirror improvements in insulin sensitivity, while others found stronger associations with adiposity, lipid metabolism, or systemic inflammation than with direct measures of insulin action [101,102]. The principal human studies examining circulating or adipose RBP4 in relation to insulin resistance, adiposity, renal function, and diabetes risk are summarized in Table 2.

Table 2. Human studies of RBP4 in obesity-associated metabolic dysfunction. Findings are observational unless explicitly stated.

Reference	Design/Population (n, Sex, Ancestry)	RBP4 Assay and Units	Renal; Retinol/TTR Handling	Main Finding/Effect Estimate	Interpretation/Evidence	Principal Limitation
Graham et al., 2006 [2]	Three human cohorts spanning lean, obese, IGT and T2D adults; both sexes; European/US	Quantitative Western blot and immunoassay; μg/mL	Renal adjustment NR; retinol/TTR not systematically measured	RBP4 correlated inversely with clamp insulin sensitivity; exercise-related decline tracked improved sensitivity in responders.	Association plus intervention concordance; no direct human causality	Heterogeneous cohorts; assay calibration and residual confounding
Janke et al., 2006 [65]	Cross-sectional n = 74 postmenopausal women (n = 15 lean, n = 26 overweight, n = 33 obese); weight-loss subgroup n = 17; European	Competitive ELISA (serum) and adipose RT-qPCR; μg/mL	Renal adjustment NR; retinol/TTR NR	Serum RBP4 was similar across BMI groups; adipose RBP4 expression was not increased in obesity.	Null/discordant human evidence	Women only; subcutaneous tissue; modest intervention subgroup
Vitkova et al., 2007 [101]	Diet intervention in n = 24 obese women with lean comparison; European	ELISA and adipose RT-qPCR; concentration units reported by study	Renal adjustment NR; retinol/TTR NR	Calorie restriction reduced plasma/adipose RBP4 while glucose disposal improved, but changes were not related.	Parallel change without mediation evidence	Small, women-only, uncontrolled physiological co-changes
Raila et al., 2007 [16]	n = 62 T2D (26 with microalbuminuria) plus n = 35 controls; both sexes; European	Plasma/urine RBP4 immunoassay	Albuminuria assessed; plasma/urine retinol and TTR measured	RBP4 was highest with microalbuminuria; no correlation with insulin-resistance indices.	Renal status, not insulin resistance, best explained RBP4 elevation	Cross-sectional; modest subgroup size
Yao-Borengasser et al., 2007 [66]	n = 86 (70 women/16 men; 54 NGT/32 IGT); plasma subset n = 52; mostly White US individuals	ELISA; confirmatory Western blot; μg/mL; tissue RT-qPCR	Renal adjustment NR; retinol/TTR NR	SAT RBP4 mRNA was unrelated to BMI or insulin sensitivity; pioglitazone increased tissue RBP4 expression but not plasma RBP4.	Inflammation association; no support for plasma/tissue RBP4 as insulin-sensitivity mediator	Mostly women; plasma subset; small treatment arms
Klöting et al., 2007 [67]	n = 196 adults with paired visceral/subcutaneous samples; both sexes; European	Serum immunoassay and adipose RT-qPCR	Renal adjustment NR; TTR measured in subsets; retinol NR	Visceral expression exceeded subcutaneous expression; serum RBP4 tracked visceral fat and inversely associated with insulin sensitivity.	Cross-sectional tissue association	Surgical selection; no causal direction; residual renal/retinoid confounding
von Eynatten et al., 2007 [102]	n = 365 men: 126 T2D, 143 CAD, 96 controls; European	ELISA validated by quantitative Western blot; μg/mL	Renal/retinol/TTR handling not central to analysis	No association with HOMA-IR or HbA1c; positive associations with triglyceride-rich/pro-atherogenic lipoproteins.	Lipid association, not an insulin-resistance marker	Men only; HOMA rather than clamp; cross-sectional
Henze et al., 2008 [17]	n = 230 (104 T2D, 126 non-diabetic), stratified by eGFR; both sexes; European	ELISA; μmol/L	eGFR explicitly stratified; retinol/TTR NR	T2D vs. control: 2.65 vs. 2.01 μmol/L (p < 0.001), but difference disappeared within eGFR strata; eGFR r = −0.477 (p < 0.001), HbA1c NS.	Strong evidence for renal confounding	Cross-sectional; no molecular-form resolution
Erikstrup et al., 2009 [15]	n = 233 NGT/IGT/T2D adults; both sexes; European	Circulating RBP and retinol; molar ratio	Retinol measured; TTR and detailed renal adjustment not central	Total RBP4 was not elevated, but the RBP4-to-retinol ratio was higher in IGT/T2D.	TheRBP4-to-retinol ratio may be outperform total RBP4 in detecting IGT/T2D	Cross-sectional; assay does not separate all RBP4 forms
Sun et al., 2014 [97]	Prospective n = 2091; 41.1% men; 507 incident T2D over 6 years; Chinese adults	Plasma total RBP4 immunoassay; μg/mL	Multivariable adjustment; retinol/TTR and molecular forms not measured	Higher baseline RBP4 was associated with greater incident T2D risk.	Prospective association/prediction	Residual confounding; single baseline measure; no data on RBP4 molecular form or retinol
Li et al., 2018 [100]	Prospective; baseline n = 3445 (50.9% boys); at follow up n = 352; Chinese children	R&D Systems ELISA; ng/mL; intra/inter-assay CV 6.2/8.5%	Renal/retinol/TTR not central	Adding RBP4 changed AUC 0.785→0.813 (p = 0.062); NRI 0.376 (p = 0.012) for predicting metabolic syndrome at 10-year follow-up.	Predictive association, not causation	Only 352 longitudinal participants had follow-up data
Kilicarslan et al., 2020 [63]	n = 20 morbidly obese women plus n = 6 lean women; separate human adipocyte donors; ethnicity not reported (probably European ancestry)	Serum/tissue RBP4; tracer two-step clamp; concentration units reported	Renal/retinol/TTR not primary covariates	Serum RBP4 inversely correlated with glucose disposal and insulin suppression of lipolysis/EGP; in vitro RBP4 stimulated lipolysis.	Human association supported by complementary in vitro functional evidence	Very small, women-only cohort; correlative analysis only

NR, not reported; CAD, coronary artery disease; CV, coefficient of variation; EGP, endogenous glucose production; eGFR, estimated glomerular filtration rate; IGT, impaired glucose tolerance; NGT, normal glucose tolerance; NRI, net reclassification improvement; SAT, subcutaneous adipose tissue; T2D, type 2 diabetes; TTR, transthyretin.

6.2. Assay Limitations and Clinical Confounders

Interpreting clinical findings and associations is further complicated by the biology of circulating RBP4 itself. Most clinical assays quantify total RBP4 and do not distinguish between apo-RBP4 and holo-RBP4, despite evidence that RBP4-to-retinol ratio may correlate more closely with metabolic dysfunction than total RBP4 concentrations alone [4,15]. In addition, circulating levels are influenced by several physiological and methodological factors, including transthyretin binding, vitamin A status, age, sex, liver function, nutritional status, and assay variability. Renal function represents another major source of variability. Because the kidney plays a central role in RBP4 filtration and catabolism, impaired renal function, including reduced glomerular filtration, microalbuminuria, and diabetic kidney disease, can substantially increase circulating RBP4 independently of insulin resistance [16,17,103]. These factors have not been systematically addressed in most clinical studies. Frey et al. showed that chronic kidney disease is associated with increased apo-RBP4, holo-RBP4, and C-terminally truncated RBP4 isoforms, indicating that impaired renal handling affects not only total RBP4 concentrations but also the distribution of circulating molecular forms [61]. More recently, quantitative LC-MS/MS analyses in metabolically healthy individuals showed that serum RBP4 and TTR concentrations vary according to age and sex, while kinetic modeling further emphasized the role of RBP4-TTR-retinol interactions in determining circulating RBP4 levels [62].

Future clinical studies should therefore adjust for estimated glomerular filtration rate and albuminuria and, whenever possible, include measurements of retinol, TTR, and the RBP4-to-retinol ratio. Assessment of the molecular and binding state of RBP4 would also improve interpretation of circulating RBP4 levels and their relationship with metabolic outcomes.

Overall, the available evidence supports RBP4 as a promising candidate biomarker of metabolic risk rather than an established clinical marker. At present, standardized diagnostic thresholds are lacking, and it remains uncertain whether measuring RBP4 provides clinically meaningful information beyond established clinical and laboratory indicators such as adiposity, fasting glucose, HbA1c, triglycerides, and renal function.

7. Therapeutic Targeting of RBP4

7.1. Indirect Modulation of RBP4: Thiazolidinediones

Thiazolidinediones, including pioglitazone and rosiglitazone, improve insulin sensitivity primarily through activation of PPARγ, leading to profound remodeling of adipose tissue. Their metabolic effects include enhanced adipocyte differentiation, greater subcutaneous lipid-storage capacity, suppression of excessive lipolysis, increased adiponectin secretion, and reduced ectopic lipid accumulation [82,83,104].

Several experimental and clinical studies have shown that improvements in insulin sensitivity during thiazolidinedione therapy are accompanied by reductions in circulating or adipose RBP4 [1,5,104]. These observations initially raised the possibility that RBP4 suppression might contribute to their beneficial metabolic effects. Subsequent work, however, has revealed a more complex picture. PPARγ activation can directly stimulate RBP4 transcription in differentiated adipocytes [50,87], indicating that changes in RBP4 are unlikely to represent a simple pharmacological response.

Instead, alterations in circulating RBP4 during treatment probably reflect broader improvements in metabolic homeostasis, including changes in adipose tissue composition, hepatic secretion, renal handling, and retinoid metabolism. Given the broad range of pathways regulated by PPARγ, it is not possible to attribute the insulin-sensitizing effects of thiazolidinediones specifically to modulation of RBP4.

Their clinical use is further limited by well-recognized adverse effects, including weight gain, fluid retention, edema, worsening or new-onset heart failure, and an increased risk of fractures. Pioglitazone should also be used cautiously in patients with active bladder cancer or a relevant clinical history. Consequently, although thiazolidinediones remain valuable for understanding adipose tissue biology, they cannot be regarded as pharmacological validation of RBP4 as a therapeutic target.

7.2. Strategies Aimed at Reducing Circulating RBP4

7.2.1. Fenretinide and Early Attempts to Lower Circulating RBP4

Fenretinide, a synthetic retinoid, was one of the first compounds used to investigate whether lowering circulating RBP4 could improve metabolic dysfunction. By binding to RBP4 and disrupting its interaction with transthyretin, fenretinide enhances renal clearance of the protein, resulting in reduced circulating concentrations of both RBP4 and retinol [1]. In both diet-induced and genetic mouse models of obesity, fenretinide improved glucose tolerance, enhanced insulin sensitivity, reduced adiposity, and attenuated hepatic steatosis [1,93,94]. Clinical evidence is more limited but encouraging. In overweight women enrolled in a breast cancer prevention trial, fenretinide treatment was associated with improvement in a surrogate measure of insulin sensitivity [105].

Despite these findings, the interpretation of fenretinide studies requires caution. The compound is not a selective inhibitor of RBP4 but exerts multiple biological effects, including modulation of retinoid signaling, ceramide metabolism, oxidative stress, and other cellular pathways. Consistent with these broader mechanisms of action, several of its metabolic benefits are preserved in RBP4-deficient mice, indicating that they cannot be explained solely by reductions in circulating RBP4 [93]. Moreover, as a retinoid derivative, fenretinide has the potential to interfere with vitamin A homeostasis during long-term treatment, raising concerns about visual adaptation and other retinoid-dependent physiological processes. For these reasons, the available evidence does not demonstrate that selective inhibition of RBP4 alone is sufficient to improve insulin resistance.

7.2.2. Non-Retinoid RBP4 Antagonists and Evidence of Target Engagement

Non-retinoid RBP4 antagonists were developed to reduce circulating RBP4 without activating classical retinoid receptors. Among the earliest compounds, A1120 binds RBP4 and disrupts its interaction with transthyretin, leading to marked reductions in circulating RBP4 and retinol in mice. Although this clearly demonstrated effective target engagement, treatment with A1120 failed to improve glucose tolerance or insulin sensitivity in high-fat-diet-fed animals [92]. These findings suggest that simply lowering circulating RBP4 is unlikely to be sufficient to reverse insulin resistance. They also raise the possibility that the metabolic actions of fenretinide are mediated, at least in part, through mechanisms independent of RBP4. Another explanation is that the pathogenic effects of RBP4 depend less on its overall circulating concentration than on its tissue-specific production, particular molecular forms, or activation of inflammatory pathways that are not adequately suppressed by reducing the systemic pool.

More recently, non-retinoid RBP4 antagonists, including BPN-14136, STG-001, and tinlarebant, have further demonstrated the feasibility of pharmacological RBP4 modulation. These agents have been developed primarily for inherited retinal diseases and geographic atrophy, where reduction of circulating RBP4 is intended to limit retinol delivery to the retina. BPN-14136 reduced circulating RBP4 and retinal bisretinoid formation in preclinical models [106]. STG-001 has been evaluated in a completed Phase 2a study in Stargardt disease [107]. Tinlarebant has progressed further, with completion of the Phase 3 DRAGON study in adolescent patients with Stargardt disease and an active, not recruiting Phase 3 PHOENIX study in geographic atrophy [108,109]. Together, these clinical trial programs provide human evidence of target engagement and pharmacological control of systemic retinol transport, but they do not establish that RBP4 inhibition improves insulin sensitivity, glycemic control, adipose inflammation, or other metabolic outcomes.

7.3. Limitations of Systemic RBP4 Lowering and Rationale for Selective Targeting

Whether systemic RBP4 inhibition represents an effective treatment for metabolic disease remains uncertain. Several biological considerations suggest that indiscriminate suppression may not be the optimal approach. RBP4 plays a fundamental role in mobilizing hepatic vitamin A stores and delivering retinol to peripheral tissues [3,7]. Although alternative transport pathways can partially compensate under conditions of adequate vitamin A availability, sustained reductions in circulating RBP4 inevitably lower plasma retinol and could compromise physiological processes such as dark adaptation, night vision, and other retinoid-dependent functions [3,4,110]. These considerations underscore a key limitation of broadly lowering circulating RBP4 and support the development of more selective strategies that preserve physiological retinol transport while targeting pathological RBP4 signaling.

7.3.1. Inhibition of RBP4-STRA6 Signaling

Selective inhibition of RBP4-STRA6 signaling may represent one approach to limit pathogenic RBP4 activity without broadly reducing circulating RBP4 levels. RBP4-STRA6 signaling has been linked to insulin resistance through JAK2/STAT activation and SOCS3 induction, which impair INSR and IRS activities [10,60]. In vivo, Stra6 ablation protected mice from RBP4-induced suppression of insulin signaling while producing only modest effects on retinoid homeostasis outside the eye [111]. However, the relevance of this pathway appears context-dependent. STRA6 expression varies across tissues and metabolic states, and adipocyte-specific Stra6 knockdown produced only modest metabolic effects under high-fat-diet conditions [91]. Thus, inhibition of RBP4-STRA6 signaling may offer a more selective alternative to systemic RBP4 lowering, but its therapeutic value in metabolic disease remains to be established.

7.3.2. Inhibition of RBP4-Mediated TLR2/TLR4 Signaling

RBP4-mediated Toll-like receptor signaling represents another potential strategy to limit the pro-inflammatory actions of RBP4. RBP4 can activate macrophages through a retinol-independent TLR4/JNK pathway, increasing pro-inflammatory cytokine production and impairing insulin signaling in neighboring adipocytes [11]. More recent evidence further implicates TLR2 and TLR4 in RBP4-induced inflammatory activation, including upstream priming of the NLRP3 inflammasome [13].

Selective inhibition of RBP4-mediated TLR2/TLR4 signaling could therefore attenuate adipose inflammation and downstream metabolic dysfunction without broadly lowering circulating RBP4. However, whether this approach can improve metabolic outcomes while preserving physiological innate immune responses remains unknown.

7.3.3. Inhibition of RBP4-Associated NLRP3 Inflammasome Activation

As noted above, NLRP3 inflammasome activation appears to be a downstream component of RBP4-associated inflammatory signaling. RBP4 can prime NLRP3 through TLR2- and TLR4-dependent mechanisms, thereby increasing IL-1β production after a secondary inflammatory stimulus. Genetic disruption of TLR2, TLR4, NLRP3, or caspase-1 attenuated RBP4-induced inflammatory responses, whereas pharmacological TLR4 inhibition reduced adipose tissue inflammation and improved insulin sensitivity in RBP4-overexpressing mice [13].

These findings support the potential relevance of the RBP4-TLR2/TLR4-NLRP3 pathway in metabolic inflammation. Consistent with the broader role of NLRP3 in metabolic disease, the selective NLRP3 inhibitor VTX3232 reduced metabolic inflammation, hyperglycemia, and hepatic steatosis in obese mice and improved insulin resistance in primary human hepatocytes and adipocytes [112]. However, these data do not establish that NLRP3 inhibition specifically reverses RBP4-driven metabolic dysfunction. The therapeutic value of targeting the RBP4-associated NLRP3 pathway therefore remains to be determined.

7.3.4. Selective Targeting of Adipose-Derived RBP4 and Future Therapeutic Directions

As discussed in Section 3.2, hepatocytes account for most circulating RBP4 under physiological conditions [8], whereas adipocyte-derived RBP4 appears to exert predominantly local inflammatory and lipolytic effects [63,64]. Systemic RBP4 depletion may disproportionately affect physiological retinol transport while leaving pathogenic signaling within adipose tissue or immune cells only partially inhibited. These observations provide a rationale for exploring strategies that selectively limit pathogenic RBP4 actions within white adipose tissue or adipose-resident immune cells. However, direct evidence that RBP4 can be selectively modulated in white adipose tissue while preserving its functions in other compartments remains limited. This limitation is particularly important because RBP4 activity appears to differ across adipose depots. In brown adipose tissue, RBP4 has been reported to support adrenergic signaling, fatty-acid oxidation, energy expenditure, glucose homeostasis, and adaptive thermogenesis [68]. Therefore, non-selective targeting of adipose-derived RBP4 could theoretically interfere with beneficial BAT-dependent metabolic adaptation. Future therapeutic strategies should therefore distinguish pathogenic RBP4 signaling in dysfunctional white adipose tissue from potentially beneficial RBP4 actions in brown adipose tissue. Rather than complete systemic depletion, partial or pathway-selective inhibition may offer a more favorable therapeutic window by reducing inflammatory or lipolytic RBP4 actions while preserving retinol transport and BAT thermogenic function. Careful patient selection is also likely to influence therapeutic success. Factors such as renal function, vitamin A status, transthyretin concentration, adipose distribution, and the predominant molecular form of circulating RBP4 may help identify individuals most likely to benefit. Patients with disproportionately elevated RBP4 relative to retinol, preserved renal function, and evidence of RBP4-associated inflammatory activation may represent a more biologically homogeneous population than unselected individuals with obesity or type 2 diabetes. Future clinical trials should also move beyond plasma RBP4 reduction as the primary pharmacodynamic endpoint. Demonstrating clinical benefit will require comprehensive assessment of insulin sensitivity, hepatic glucose production, adipose tissue inflammation, lipid metabolism, glycemic control, and hepatic steatosis. Equally important will be careful monitoring of retinol status, retinal function, dark adaptation, and other consequences of impaired vitamin A transport to establish the long-term safety of this therapeutic approach.

8. Conclusions

RBP4 lies at the crossroads of retinoid transport, adipose tissue biology, immune regulation, and glucose metabolism. Experimental evidence indicates that RBP4 can impair insulin signaling, promote lipolysis, activate macrophages and T cells, and enhance NLRP3 inflammasome activity. These findings provide biologically plausible mechanisms linking RBP4 to insulin resistance and adverse metabolic phenotypes.

At the same time, current evidence argues against viewing RBP4 as uniformly harmful. Its biological effects depend on tissue origin, molecular form, receptor interactions, and the surrounding inflammatory milieu. Hepatic and adipose-derived RBP4 appear to have distinct functions, while evidence from brown adipose tissue suggests that RBP4 may support thermogenesis and glucose homeostasis in specific contexts. This context dependence cautions against indiscriminate systemic depletion and favors therapeutic strategies that selectively target pathogenic RBP4 signaling while preserving physiological retinol transport.

The regulatory network involving adipose hypoxia, PPARγ, and HMGA1 is similarly more nuanced than a simple linear pathway. HMGA1 directly controls the transcription of both RBP4 and the insulin receptor, whereas PPARγ can stimulate RBP4 expression under specific adipocyte conditions. Although HIF-1 influences adipocyte differentiation and cooperates with HMGA1 in regulating selected hypoxia-responsive genes, direct transcriptional control of RBP4 by HIF-1 has yet to be convincingly demonstrated. Clinical studies consistently associate circulating RBP4 with markers of metabolic dysfunction, supporting its potential as a biomarker of metabolic risk. However, several factors, including renal function, vitamin A status, molecular heterogeneity, and differences between analytical assays, continue to limit its clinical implementation. Likewise, while pharmacological strategies can successfully reduce circulating RBP4 concentrations, current evidence has not shown that lowering total RBP4 alone translates into meaningful improvements in metabolic disease.

Taken together, the current literature identifies RBP4 as a compelling biological mediator and a promising therapeutic target, though not yet as a clinically validated metabolic target. Future research should distinguish its physiological role in retinol transport from its pathological signaling functions and prioritize interventions that selectively disrupt the tissue-specific or receptor-dependent mechanisms underlying its adverse metabolic effects.