Low Hepatic CEACAM1 Tethers Metabolic Dysfunction Steatohepatitis to Atherosclerosis

Abstract

Metabolic dysfunction-associated steatohepatitis (MASH) and atherosclerosis are cardiometabolic twin disorders with shared underlying pathophysiological mechanisms such as insulin resistance and chronic inflammation. This review explores the salient role of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) in linking hepatic dysfunction to cardiovascular disease. Findings in mice with genetic modulation of Ceacam1 gene established a critical role for CEACAM1 protein in regulating insulin and lipid metabolism and endothelial integrity and modulating immune response. Loss of CEACAM1 in hepatocytes impairs insulin clearance, causing chronic hyperinsulinemia, a process that ultimately leads to insulin resistance and hepatic and extra-hepatic fat accumulation, which in turn causes inflammatory infiltration. This prompts a paradigm shift that positions impaired hepatic CEACAM1 function as a mechanistic underpinning of the link between insulin resistance, MASH, and atherosclerosis.

1. Introduction

The rampant increase in the prevalence of obesity and insulin resistance worldwide has precipitated an alarming rise in metabolic disorders, including metabolic dysfunction-associated steatohepatitis (MASH) and atherosclerosis. Modern medicine is redefining these abnormalities of the hepatic and vascular systems as twin manifestations of a broader systemic metabolic dysregulation. Atherosclerosis, characterized by intimal plaque accumulation, remains the principal cause of cardiovascular mortality worldwide [1,2]. Concurrently, MASH, an advanced stage of metabolic dysfunction-associated steatotic liver disease (MASLD) afflicts approximately 30% of the adult population and represents a major risk factor for liver cirrhosis and hepatocellular carcinoma [3,4]. Characterization of shared pathophysiological mechanisms is emerging to bridge hepatic dysfunction and vascular injury. These include chronic low-grade inflammation, oxidative stress, and aberrant lipid metabolism [5,6]. Moreover, epidemiological analysis provides evidence of a two-to-threefold increase in cardiovascular risk among patients with MASLD, independent of traditional risk factors such as type 2 diabetes (T2D) and dyslipidemia. In parallel, patients with established atherosclerotic disease also present with comorbid hepatic steatosis [7,8,9]. Furthermore, these analyses frame insulin resistance, a critical hallmark of multisystem metabolic derangements, as a unifying disease driver [10,11]. For instance, insulin resistance promotes de novo lipogenesis and impairs fatty acid β-oxidation (FAO) in the liver, ultimately leading to hepatic steatosis and hepatocellular injury [12]. Similarly, insulin resistance diminishes nitric oxide (NO) bioavailability and compromises endothelial integrity, contributing to the development of an atherogenic milieu in the vasculature [13,14]. Thus, hepatic and cardiovascular pathophysiology resides at the crossroad of hyperinsulinemia, aberrant lipid metabolism, endothelial dysfunction, and chronic proinflammatory signaling.

Recent findings have established a regulatory role for carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) in hepatic metabolism and vascular integrity [15,16]. CEACAM1, expressed predominantly in hepatocytes, promotes insulin clearance via receptor-mediated endocytosis [10,17]. Interestingly, obesity and metabolic syndrome (MetS) are associated with low hepatic CEACAM1 levels and impaired insulin clearance with resultant persistent hyperinsulinemia and insulin resistance [18,19]. Moreover, murine models with global null deletion or hepatocyte-specific deletion of Ceacam1 gene emulate cardinal features of human MetS, including insulin resistance, steatohepatitis, visceral adiposity, and endothelial dysfunction [20,21]. CEACAM1 deficiency promotes hepatic inflammation via the activation of nuclear factor kappa B (NF-κB) and attenuates endothelial NO synthase (eNOS) activity, producing a proinflammatory atherogenic milieu with oxidative stress in mice. CEACAM1 loss serves therefore as a molecular bridge between hepatic insulin resistance endothelial injury and oxidative stress [15,22,23].

It is becoming increasingly apparent that efficient therapeutic management of MetS requires an integrated multisystem molecular approach, targeting shared mediators such as CEACAM1, that is particularly relevant in addressing the role of the liver in systemic metabolic dysfunction. This review aims to discuss the impact of the loss of hepatic CEACAM1 in the broader spectrum of metabolic disorders by providing evidence of its function in hepatic metabolism and vascular physiology. Furthermore, it posits CEACAM1′s potential as a therapeutic target in MetS-associated clinical presentations, namely atherosclerosis and MASH.

2. Pathophysiology of Atherosclerosis: Mechanisms and Cardiovascular Implications

2.1. Atherosclerosis: Prevalence and Clinical Significance

Atherosclerosis is the major indicator of cardiovascular disease (CVD), which represents the most common cause of death worldwide [1]. The World Health Organization (WHO) has estimated that ~18 million people die from CVD annually, accounting for ~32% of all deaths worldwide [2]. The salient rise in CVD incidence is tethered to a global epidemic of obesity and T2D [5]. Although atherosclerotic lesions may begin at an early age, the timeline of clinical manifestations and related complications remains poorly characterized [24,25]. Initially raised as an obstruction process of the arteries, atherosclerosis is mostly caused by accumulation of cholesterol plaques in the intima, the innermost layer of blood vessels, and hardening of elastic (i.e., aorta) and muscular arteries, such as coronary and cerebral arteries [26,27]. The pathogenesis and progression of atherosclerosis are best described as an insidious process slowly unfolding over the span of several decades, often with minimal symptomatic manifestation. Ultimately, this culminates in critical vessel narrowing and plaque accumulation, thereby instigating life-threatening complications such as angina pectoris, acute coronary syndrome, and sudden cardiac death [28]. Atherosclerosis is now recognized as part of MetS, a constellation of metabolic and vascular conditions such as obesity, hypertension, and T2D. Lifestyle modification such as smoking cessation, physical activity, and healthy diet can mitigate atherosclerosis progression, further corroborating the recognition of atherosclerosis as a lifestyle disease [1].

2.2. The Pathophysiological Hallmarks of Atherosclerosis

It is crucial to delineate the fundamental pathophysiological processes in atherosclerosis to gain a better understanding of pathways and molecular events that could constitute therapeutic targets.

2.2.1. Endothelial Dysfunction

The luminal surface of all blood vessels is lined by a layer of endothelium [29]. Together with collagen and elastic fibers, the endothelium comprises the intima, which is wrapped up mostly by the vascular smooth muscle layer of the tunica media, followed by the adventitia, a dense connective tissue matrix, which provides robust structural support [29]. Endothelium is essential in preserving the structural integrity and regulating the physiological function of blood vessels. This includes maintaining the equilibrium between vasodilation and vasoconstriction, as well as orchestrating the migration and multiplication of vascular smooth muscle cells [30,31].

Endothelial dysfunction, an early sign of atherogenesis [32,33], often starts at the branching points and orifices of the arteries. These sites experience disturbed blood flow and shear stress, which leads to the accumulation of low-density lipoproteins (LDL) and lipoprotein buildup [14,34,35,36,37]. Endothelial dysfunction is further exacerbated by altered expression of specific endothelial genes, often as a direct consequence of disturbed blood flow and abnormal hemodynamic forces [38,39]. To date, more than 40 atherogenic genes linked to endothelial dysfunction have been identified [40,41]. In fact, these genes are upregulated in endothelial cells and are implicated in different steps of plaque formation. For example, monocyte chemoattractant protein 1 (MCP-1) that attracts monocytes to the vessel wall [42,43] and platelet-derived growth factor (PDGF) that promote vascular smooth muscle migration [44,45].

Conversely, NO, an endothelium-derived vasodilator, plays a critical athero-protective role in the maintenance of vascular homeostasis and endothelial integrity [46]. NO has multiple beneficial effects: Besides its role in smooth muscle relaxation and vasodilation, it also enhances cardiovascular and metabolic outcomes. NO slows down the development of atherosclerosis by reducing inflammation and oxidative stress and inhibiting platelet aggregation. Furthermore, NO promotes insulin secretion, facilitates glucose clearance, and reduces hepatic steatosis and triglyceride levels, thereby offering protection against the broader metabolic dysfunction [47,48]. However, the protective effects of NO are weakened by cardiovascular risk factors. For example, hyper-cholesterolemic and hypertensive patients often have reduced NO levels [49,50]. In fact, cardiovascular risk factors—namely hypertension, hyperlipidemia, obesity, and diabetes—potentiate NF-κB downstream proinflammatory signaling and cytokine production, in addition to the vasoconstrictor Endothelin-1, which in turn inhibits the activity of the endothelial nitric oxide synthase (eNOS) [51,52]. Even aside from its impact on eNOS, the oxidative stress linked to these risk factors directly contributes to endothelial dysfunction and atherosclerosis [13,14,53].

2.2.2. Lipid Accumulation and Plaque Formation

Atherosclerosis arises from an interplay between endothelial dysfunction and lipoprotein accumulation, which promotes a chronic inflammatory environment that drives disease progression [54]. Cholesterol, a major structural component of the cell membrane, is carried in the blood by lipoproteins. Among the five types of plasma lipoproteins, LDL, intermediate density lipoproteins (IDL), and very low-density lipoproteins (VLDL) are most strongly linked to plaque formation [55]. LDL particles cross the endothelium through a process called caveolae-mediated transcytosis, which involves receptors like SR-B1 and ALK1 [56,57,58]. The proatherogenic role of caveolae-facilitated LDL transcytosis is evidenced by the higher levels of the structural protein caveolin 1 in atherosclerotic plaques [59]. Once inside the vessel wall, LDL is oxidized by enzymes like phospholipase and lipoxygenase and by reactive oxygen species (ROS) [60]. Additionally, the depletion of antioxidants, such as alpha-tocopherol and carotenoids, further exacerbates LDL oxidation. Oxidized LDL (oxLDL) triggers proinflammatory signaling cascades in endothelial cells and drives macrophage chemotaxis, constituting key pathogenic events in atherosclerosis [60]. Activation of NF-κB in endothelial cells promotes the expression of its transcriptional targets such as VCAM-1 and ICAM-1 adhesion molecules and MCP-1 and IL-8 chemokines, as well as other prothrombotic factors [61]. After endothelial cells are activated, monocyte recruitment occurs through a multistep process: rolling, adhesion, activation, and transmigration [62]. This entire process is mediated by the interaction between monocyte integrins and endothelial adhesion molecules. Chemokines such as CXCL1, CXCL2, CXCL4, and CCL5 coordinate these events [63]. While monocyte infiltration in atherosclerosis primarily occurs through paracellular migration across endothelial junctions, the transcellular route can also be implicated [64]. The chemokine MCP-1 plays a crucial role in facilitating monocyte trans-endothelial migration [65]. Once in the subintimal space, monocytes differentiate into macrophages, which can either adopt a proinflammatory M1 phenotype or an anti-inflammatory M2 phenotype, dictated by the cytokine milieu [66,67]. M1-polarized macrophages produce NO and inflammatory cytokines, which further increase endothelial permeability and promote inflammation [68]. These macrophages take up oxLDL via different scavenger receptors such as CD36, SRA-l, and LOX-I, which are upregulated in response to high oxLDL levels, promoting further lipid internalization [69,70]. Under homeostatic conditions, the ABCA1 transporter system facilitates cholesterol efflux from macrophages, thereby preventing excessive lipid accumulation [69]. However, when exposed to a continuous inflammatory stimulus, the macrophage’s lipid efflux mechanism is disrupted, leading to the transformation of M1 macrophages into foam cells that play a direct role in atherosclerotic plaque formation [71,72]. This self-perpetuating process continues as the lipid-laden macrophages and oxLDL trigger NF-κB pathway activation, which in turn promotes further monocyte recruitment, differentiation and oxLDL internalization, marking the formation of a fatty streak—an early phase of atherosclerosis [71,72]. Additional pathways, such as NLRP3 inflammasome activation and consequent caspase-1 activation in macrophages, also contributes to atherosclerotic plaque progression and cholesterol crystal formation [73].

2.2.3. Vascular Smooth Muscle Cell Migration and Proliferation

Monocytes are not the only cells that internalize oxLDL and contribute to the foam cell population. In fact, vascular smooth muscle cells (VSMC) constitute up to 50% of the foam cell population in atherosclerotic plaques [74]. These VSMCs, typically located in the tunica media, migrate to the subendothelial space in response to epidermal growth factor (EGF), fibroblast growth factor (FGF), insulin-like growth factor (IGF), PDGF, transforming growth factor-β (TGF-β), and vascular endothelial growth factor (VEGF) secreted by foam cells and endothelial cells. Once in the subendothelial space, VMSCs begin to proliferate and secrete extracellular matrix (ECM) components such as collagen and elastin, contributing to the formation of the fibrous cap that covers the atherosclerotic plaque [75]. Additionally, plaque-residing macrophages produce IL-1, which promotes VSMC proliferation [71,72]. Progressively, microcalcifications form within the plaque driven by the osteogenic trans-differentiation of pericytes and VSMCs in response to the high calcium orthophosphate microenvironment. These cells acquire an osteoblast-like phenotype, like processes taking place in bone tissue formation. This calcification event produces a hardened fibrous atherosclerotic lesion, which promotes plaque stability and reduces the risk of complications such as rupture and thrombus formation.

2.3. Risk Factors and Their Impact on Atherosclerosis Progression

There are several well-recognized risk factors for atherosclerosis, including obesity, hypercholesterolemia, hypertension, diabetes, and smoking. These risk factors share overlapping and interrelated pathophysiological pathways that lead to the development of atherosclerotic disease. In fact, atherosclerosis is now considered a state of chronic vascular inflammation, triggered by the effects of these risk factors on the arterial wall. Identifying the role of these risk factors and implementing targeted lifestyle modifications is an effective way to halt atherosclerosis progression [76]. For instance, toxic chemical constituents of cigarettes are potent dose-dependent risk factors for the development of calcified atherosclerotic plaques. Moreover, smoking cessation is strongly recommended to slow atherosclerosis progression and reduce mortality risk [77]. As mentioned earlier, oxidative stress plays a central role in the development of atherosclerotic lesions. Diabetes, hypertension, obesity, dyslipidemia, and smoking are all known inducers of ROS generation that lower NO bioavailability. Among these factors, hypertension is the most significant contributor to endothelial dysfunction and can be worsened by other comorbidities, such as obesity, diabetes, insulin resistance, hyperinsulinemia, hypercholesterolemia, and renal disease. For instance, obesity stimulates the renin–angiotensin–aldosterone system, promoting sodium retention and hypertension [78,79]. Hypercholesterolemia is strongly linked to atherosclerosis development. In fact, mounting evidence of this association have supported the “lipid hypothesis” of atherosclerosis. The advent of statins (HMG-CoA reductase inhibitors)—the most potent lipid-lowering agents thus far—provided definitive evidence that reducing plasma cholesterol levels can prevent atherosclerosis [80]. Elevated levels of LDL, due to hypercholesterolemia, not only trigger aberrant endothelial activation but also upregulate adhesion molecules on the endothelial cell surface, facilitating monocyte and lymphocyte adherence to the intima [81]. Hyperglycemia also has a strong association with cardiovascular disease, leading to sorbitol accumulation via the aldose reductase pathway, which contributes to endothelial dysfunction [82]. Moreover, elevated glucose levels induces post-translational modifications in myocardial ECM proteins and impairs the expression and function of intramyocellular calcium channels, resulting in systolic and diastolic dysfunction [83]. Additionally, platelet aggregation plays a crucial role in the ensuing micro- and macrovascular complications of diabetes mellitus and atherogenesis [72]. Non-enzymatic glycation leads to the formation of advanced glycation end products (AGEs), which modify LDL and cause damage to the endothelium. AGEs also interact with the receptor for advanced glycation end products (RAGE), present on vascular smooth muscle cells, to accelerate atherosclerosis [84,85]. Another contributing pathway is hyperglycemia-induced polyol pathway activation, which further exacerbates vascular damage in patients with diabetes [86]. Activation of protein kinase C (PKC) and hexosamine flux pathways have also been reported to contribute to atherosclerotic plaque initiation. The complex crosstalk between adipose tissue and the cardiovascular system has been extensively documented. In 1847, an autopsy report on an obese man described the presence of a large, fibrous heart filled with fat. Subsequent studies revealed that severely obese individuals often exhibit increased cardiac output and pulmonary hypertension and eventually develop heart failure [87]. Abdominal obesity subjects the heart to high afterload pressures, leading to increased cardiac output, which leads to left ventricular (LV) remodeling, wall thickening, and left ventricular hypertrophy (LVH). Furthermore, obesity in patients with T2D is strongly associated with heart failure [87,88]. In metabolic abnormal states marked by high levels of fatty acids and carbohydrates, such as obesity and insulin resistance, lipids accumulate in the myocardium, a process known as cardiac steatosis. Lipid deposition in cardiomyocytes is largely due to an imbalance between lipid uptake and fatty acid β-oxidation [89]. Furthermore, the accumulation of visceral adipose tissue in pericardial and epicardial fat depots produces inflammatory cytokines and adipokines, which can affect myocardial contractility. In addition, free fatty acid-mediated potentiate macrophage-associated inflammation, disrupting cardiac electric modeling [90]. The NF-κB signaling pathway also contributes to the pathogenesis of heart failure owing to its involvement in cardiac remodeling [91].

2.4. Rising Incidence of HFpEF and LV Diastolic Dysfunction in Women Suggests a Potential Link to Hyperinsulinemia

Left ventricular diastolic dysfunction (LVDD) is defined as functional and metabolic instability during myocardial relaxation, leading to insufficient filling of the left ventricle. This condition can result from impaired cardiac relaxation, increased myocardial stiffness, and left atrial dysfunction [92]. LVDD is diagnosed using echocardiographic measurements and can range from mild, asymptomatic condition to progression into heart failure, with symptoms such as dyspnea and chest discomfort. While the prevalence of LVDD in the general population ranges between 3.1% and 35%, contingent on factors such as age, lifestyle, and discrepancies in diagnostic criteria, the global prevalence of heart failure is ~1–2.5% worldwide [93]. Subsequently, management of risk factors and a robust understanding of the preclinical disease stages are essential for prevention. Although there is no significant difference in the prevalence of heart failure between men and women, women are more likely to develop heart failure with preserved ejection fraction (HFpEF). In one study, 67% of women with heart failure exhibited HFpEF, compared to only 42% of men [93]. For non-modifiable risk factors, sex-related differences in cardiac function and structure accounts for such statistical disparities. In fact, women typically have lower left ventricular mass and volume, along with a higher left ventricular ejection fraction and a greater Global Longitudinal Strain compared to men [94,95,96]. As women age, they experience an abrupt increase in left ventricular mass and slower cardiomyocyte compared to men, predisposing them to a greater concentric LV remodeling and evolving HFpEF [97]. When LVDD progresses to HFpEF, clinical symptoms become apparent despite a preserved ejection fraction (EF > 50%) [98]. To date, the pathophysiology of HFpEF remains poorly understood, and accordingly, adequate treatment is lacking. In the context of confounding comorbidities, such as chronic obstructive pulmonary disease (COPD) and atrial fibrillation, the diagnosis of HFpEF becomes challenging, as it can mimic other presentations resembling exercise-induced dyspnea. HFpEF seems to be underdiagnosed in elderly women [99], in particular in patients with diabetes [100]. Failure to diagnose HFpEF is detrimental to patient’s quality of life. Obesity, on the other hand, is an established risk factor for heart failure, with women having 4% to 29% higher prevalence of obesity compared to men [101]. Persistent hypercholesterolemia and insulin resistance adversely affect the cardiovascular system, since insulin receptors are abundantly expressed on myocardial, endothelial, and vascular smooth muscle cells [102]. Moreover, insulin signaling is important for the maintenance of cardiac integrity, mitochondrial metabolism, and substrate uptake for β-oxidation. Impairment of these processes may manifest themselves as left ventricular remodeling to contribute to HFpEF emergence. Downstream insulin signaling stimulates phosphoinositide-3 kinase (PI3K/Akt) and Shc/Ras/mitogen-activated protein kinase (MAPK) pathways [103]. Insulin resistance and hyperinsulinemia overstimulate the MAPK pathway, leading to increased production of the vasoconstrictor Endothelin-1, in turn enhancing the hypertensive effects of the sympathetic nervous system, eventually progressing to cardiac hypertrophy and atherosclerosis [104]. Concurrently, hyperinsulinemia inhibits the PI3K/Akt pathway, which regulates the metabolic effects of insulin and the production of NO by vascular smooth muscle cells and endothelial cells [104,105]. The cumulative effects of disrupted insulin signaling and the growth-promoting effects of chronic hyperinsulinemia on vascular smooth muscle cells lead to increased left ventricular mass with a concentric remodeling pattern that worsens HFpEF. Early efforts in reducing hyperinsulinemia and insulin resistance could mitigate their adverse cardiovascular co-morbidities and halt progression to HFpEF [106].

3. Pathophysiology of MASH

3.1. Overview, Prevalence, and Clinical Significance of MASH

MASLD has been recognized as the hepatic manifestation of MetS. MASLD encompasses a broad spectrum of clinical stages ranging from hepatic steatosis to more severe presentations, such as steatohepatitis, MASH, fibrosis, and cirrhosis, that could eventually culminate in hepatocellular carcinoma (HCC) and end-stage liver disease that necessitates liver transplantation [107,108]. The global prevalence of MASLD exceeds 30% of the adult population, rendering it a worldwide health threat [3,4,109]. Alarmingly, approximately 20 million people are projected to die from complications of steatotic liver disease worldwide. Data from the Global Burden of Disease database indicates that MASLD is the fastest-growing cause of liver cirrhosis, liver failure, and liver cancer. It is important to note that MASLD is not merely a liver-specific disorder but a component of concomitant systemic conditions associated with multi-organ metabolic dysfunction. The diagnosis of MASLD requires the detection of steatosis in more than 5% of hepatocytes, associated with a defined metabolic abnormality, such as obesity, T2D, or dyslipidemia [110,111,112]. Unfortunately, MASLD imposes a growing economic and public health burden on society, highlighting the urgent need for targeted therapies that depend on further exploration of its underlying mechanisms [113,114].

3.2. Key Mechanisms in the Development of MASH

3.2.1. Hepatic Steatosis and Lipid Accumulation

Lipid levels in hepatocytes are tightly regulated via a delicate balance between lipid uptake and de novo synthesis on one hand and FAO and fatty acid export on another. Disruption of hepatic lipid homeostasis results in lipid accumulation, primarily in the form of triglycerides within hepatocytes. Lipid uptake is mediated by various proteins, including fatty acid-binding protein 1 (FABP1), which is predominantly expressed in the liver. FABP1 is an important regulator of lipid metabolism that participates in fatty acid transport, β-oxidation, and incorporation into triglycerides [12]. In fact, higher levels of FABP1 correlate with increased hepatic lipid accumulation and steatosis [115]. However, the utilization of FABP1 serum levels as a diagnostic biomarker of hepatic steatosis remains challenging, as it is co-expressed by renal tubules [116].

Additionally, cluster of differentiation 36 (CD36) and fatty acid transport protein 5 (FATP5) are also implicated in lipotoxicity. For instance, CD36 expression is upregulated in response to a high-fat diet and correlates with liver fat content of MASLD patients [117,118]. Similarly, FATP 5 expression is also increased, particularly in males with MASLD, reflecting a higher degree of hepatic steatosis in these patients [119]. CD36 and FATP5 serve as important therapeutic targets for halting the progression of MASLD into MASH. Aberrant triglyceride synthesis via de novo lipogenesis is also implicated in the development of hepatic steatosis [120]. De novo lipogenesis is mediated by three enzymes: acetyl-CoA carboxylase (ACC), which converts acetyl-CoA to malonyl-CoA; fatty acid synthase (FASN), which transforms malonyl-CoA into palmitate; and stearoyl-CoA desaturase-1 (SCD1), which generates oleate and palmitoleate [121]. In fact, higher levels of oleate and palmitoleate are associated with increased hepatic steatosis [121]. Moreover, activation of the transcription factors sterol regulatory element-binding protein 1c (SREBP-1c) by insulin and carbohydrate regulatory element-binding protein (ChREBP) upregulate lipogenic enzymes, thereby enhancing hepatic lipid accumulation [122,123,124]. Notably, ChREBP knock out mice display reduced expression of ACC and FASN in response to a carbohydrate-rich diet, highlighting its pro-steatotic role [125].

Conversely, impaired FAO and defective export of lipids from hepatocytes also leads to lipid accumulation. FAO occurs mainly in the mitochondria, facilitated by the outer mitochondrial membrane (OMM) enzyme carnitine palmitoyl transferase 1 (CPT1), which is in turn regulated by peroxisome proliferator-activated receptor-α (PPARα) [121,126,127,128,129]. PPARα activation induces transcription of lipolytic genes in the mitochondria (CPT1), peroxisomes (acyl-CoA oxidase, ACOX), and cytochromes (Cyp4A family) [126,130,131]. Reciprocally, hepatic PPARα levels are regulated by glycogen synthase kinase 3β (GSK3β) via phosphorylation at its serine-73 residue (Ser73) [129]. GSK3β-mediated phosphorylation of PPARα promotes its ubiquitination and subsequent degradation, thereby decreasing its lipolytic activity [129]. This is consistent with the observation that patients with MASLD exhibit a diminished activity and expression of PPARα, which accounts for its involvement in diet-induced hepatic steatosis [130,132]. Triglyceride export is another mechanism that may contribute to intrahepatic lipid buildup. Apolipoprotein B100 (apoB100) and microsomal triglyceride transfer protein (MTTP) are key regulators of this process. MTTP facilitates lipid loading onto apoB100, enabling the maturation and secretion of VLDL. VLDL particles are generated in the endoplasmic reticulum (ER) and are subsequently sent to the Golgi apparatus, where they undergo maturation before being released into the bloodstream through an ApoB100-dependent mechanism. However, excessive fatty acid levels overwhelm ApoB100 secretion capacity, inducing endoplasmic reticulum (ER) stress and promoting steatosis. This underscores the implication of genetic defects in MTTP, and the ensuing alteration in triglyceride export, that account for the onset of MASLD and the pathophysiological mechanisms responsible for aberrant intrahepatic lipid accumulation [133]. Studies in the Han Chinese population demonstrated that MTTP polymorphisms influence MASLD development. In a genetic murine MASLD model, high MTTP expression reduces hepatic triglyceride levels via enhancing VLDL secretion. Similarly, diminished ApoB100 levels results in decreased secretion of VLDL and promotes lipid accumulation in hepatocytes. Additionally, patients with MASH exhibit lower ApoB100 synthesis, indicative of a lower lipid transport rate, which could contribute significantly to advanced steatosis and lipotoxicity in these patients [134]. Furthermore, buildup of saturated fatty acids generates lipotoxic intermediates, such as diacylglycerols, which induce ER stress and ROS production, recognized for their direct participation in MASH pathogenesis [135,136,137]. In addition, SFAs bind to Toll-like receptor 4 (TLR4), triggering pro-inflammatory NF-κB signaling and exacerbating mitochondrial dysfunction [136]. Delineating the physiological derangements in pathways of lipid metabolism that occur in MASLD provides mechanistic insights that could be exploited in the context of therapeutic interventions.

3.2.2. Oxidative Stress and Mitochondrial Dysfunction

Excess lipid buildup in hepatocytes enhances mitochondrial lipolysis and β-oxidation, leading to increased ROS production within the respiratory chain. The resulting increase in ROS levels disrupts mitochondrial integrity by increasing membrane permeability and impairing mitochondrial antioxidative defense mechanisms [138]. The ensuing ROS-mediated mitochondrial injury initiates a self-perpetuating cycle of ROS production. In palmitic acid-mediated lipotoxicity, the mitochondrial protein Sab interacts with JNK, causing electron transport chain (ETC) dysfunction, amplified ROS production, and eventually initiation of apoptosis [139]. Accumulated ROS activates the mitochondrial apoptosis pathway by opening the mitochondrial membrane permeability transition pore (MPTP). Subsequently, the uncoupling of the electron transport chain occurs, and ATP production is disrupted. The release of cytochrome c, which combines with apoptosis activator factor-1 (Apaf-1), subsequently activates caspase-9 and caspase-3, thereby inducing cell death [140]. The voltage-dependent anion channel (VDAC) plays a key role in sensing lipotoxicity by altering outer mitochondrial membrane permeability. Reduced GSK3-mediated phosphorylation of VDAC promotes calcium and water influx, leading cytochrome c release and apoptosis. In addition, oxidative stress overwhelms the body’s antioxidant defense system leading to peroxidation of membrane lipids by polyunsaturated fatty acids. This increases the production of ROS and reactive nitrogen species (RNS), which contribute to further tissue damage as they diffuse into the surrounding extracellular space [141,142].

In both mice and humans, mitochondrial respiration is initially augmented in hepatic steatosis. For instance, patients with MASH display a higher degree of mitochondrial uncoupling, proton leakage, structural alterations, and diminished mitochondrial function, leading to disproportionate hepatic oxidative stress [143,144]. Noteworthy, mitochondrial autophagy (mitophagy) ensures the maintenance of metabolic and energy homeostasis by selectively eliminating mitochondria that generate excessive amounts of ROS [145]. This can hold important implications in treating MASLD. Moreover, cAMP-response element-binding protein H (CREBH), a key regulator of hepatic lipid metabolism has emerged as a promising therapeutic target as it enhances mitochondrial resistance to oxidative stress and inflammation in MASH [146].

Damage-associated molecular patterns (DAMPs), such as high mobility group box-1 protein (HMGB), released as a consequence of ROS-induced tissue injury, bind to TLR4 on Kupffer cells (KCs) [147]. This triggers NF-κB signaling, leading to proinflammatory cytokine release, thereby amplifying the inflammatory response. Moreover, DAMPs can also bind to TLR9 on KCs, amplifying TLR9-dependent ROS synthesis, further perpetuating a pro-inflammatory vicious cycle [148]. These findings underscore the contribution of ROS-induced hepatic injury and mitochondrial dysfunction in the metabolic and pathophysiologic impairments observed in patients with MASLD.

3.2.3. Hepatic Inflammation and Immune Response

Lipid accumulation in the liver causes lipotoxicity that in turn fosters a proinflammatory environment and steatohepatitis. The inflammatory response in MASH involves both the activation of liver immune cells and the recruitment of bone-marrow-derived myeloid cells [149]. ER stress can be propagated to neighboring hepatocytes via Connexin 43 channels as demonstrated in mice fed a high-fat diet [150,151]. Other players involved in sensing and controlling this metabolic and inflammatory process include inositol-requiring enzyme 1 alpha (IRE1α) and c-Jun N-terminal kinase (JNK) [152]. ER stress has been shown to drive the inflammatory response through the secretion of ceramide-enriched vesicles in a IRE1α-dependent mechanism [153].

Intracellular pattern-recognition receptors (PPRs), such as the NOD-like receptor protein-3 (NLRP3), play an important role in cytokine production in hepatocytes and immune cells. These receptors are primarily activated by microbial signals and DAMPs [e.g., adenosine triphosphate (ATP)]. Saturated fatty acids also activate the NLRP3 inflammasome, inducing the release of major proinflammatory cytokines, including IL-1β and IL-18 [154]. Inhibition of NLRP3 with sulforaphane improves high-fat diet-induced steatosis in mice, highlighting the role of this pathway in steatosis [155]. In addition, NOD-like receptor protein-6 (NLRP6), caspase activation and recruitment domain (CARD), and TLR signaling are also implicated in this process [156,157]. Moreover, B cells play an important role in the adaptive immune response that mediates hepatic inflammation by secreting profibrotic cytokines such as TNF-α and IL-6 and contributing to the formation of effector memory CD4⁺ and CD8⁺ T-cells in the liver [158,159]. Activation of these inflammatory pathways accelerates steatohepatitis progression, mediated by TNF-α, IL-1, IL-6, and IL-11, as supported by the amelioration of steatohepatitis in mice by inhibiting IL-6 and IL-1 [160,161,162].

Activated liver-resident Kupffer cells aggravate the inflammatory reactions [163]. In response to Kupffer cell-derived TNF-α and chemokine (C–C motif) ligand 2 (CCL2), innate immune cells such as macrophages and neutrophils migrate into the liver [164,165,166] to produce factors that promote steatohepatitis [e.g., neutrophil extracellular trap (NET)] [167], or to protect against it (e.g., macrophage-derived osteopontin) [168]. Antigen presenting cells are tightly associated with the ensuing inflammatory response in MASH, as evidenced by the abundance of dendritic cells and monocytes in hepatocytes of patients with MASH [166,169]. Type 1 conventional dendritic cells (cDC1) are responsible for inflammatory T-cell polarization [170]. The number of XCR1+ conventional type-1 dendritic cells increases in the liver and blood of mice and patients with MASH, and their genetic ablation curtails disease progression in mice [157]. Recent findings indicate that CXCR6+ cytotoxic T cells, activated by IL-15 rather than traditional antigen presentation, become more sensitive to metabolic triggers like acetate and extracellular ATP, thereby aggravating steatohepatitis in mice [171]. In MASLD, diverse metabolic changes lead to the conversion of regulatory T cells into T helper (TH) 17 cells [172]. This process is associated with elevated levels of IL-17, which correlates with specific inflammatory markers, particularly eotaxin, produced by smooth muscle cells and indicative of early atherosclerosis [173]. Conversely, the absence of cytotoxic T cells in mice has been shown to protect against diet-induced steatohepatitis [174]. This body of findings establishes the pathological involvement of innate immune cells and TH17 cells in mediating the inflammatory injury to hepatocytes in MASLD. Targeting these implicated mediators could be judicious in the context of MASLD prevention and treatment.

3.2.4. Fibrosis and Progression to Cirrhosis

Unresolved inflammatory responses and the activation of adaptive immune cells in the liver drive fibrosis, tissue dysfunction, and tumorigenesis. Hepatic stellate cells (HSCs), located in the subendothelial space of Disse, play a major role in maintaining liver integrity and vitamin A storage and secretion [175]. Activation of HSCs and their transformation into fibrogenic contractile myofibroblasts lead to scar tissue formation [176]. Excessive levels of fibrotic scar tissues lead to irreversible liver injury. Delineating the pathways governing HSC-mediated fibrinogenesis promises significant therapeutic applications [177]. Transforming growth factor-beta (TGF-β) is a potent pro-fibrogenic cytokine produced by hepatic macrophages. In patients, hepatocytes produce bone morphogenic proteins (BMPs), such as BMP8B and BMP9, which are members of the TGF-β superfamily. A key study demonstrated that exogenous BMP9 administration ameliorates liver disease in a CCl4-induced mouse model of hepatotoxicity, while BMP8B deficiency resulted in a significant reduction in liver fibrosis [178,179]. These findings highlight the importance of these proteins as potential therapeutic targets in liver disease management.

The activation of HSCs is mediated by various immune-dependent signals, including activation (phosphorylation) of the TGF-β/SMAD signaling pathways [180,181,182]. The depletion of retinol coupled with the accumulation of free cholesterol sensitizes HSCs to TGF-β signaling [183]. Moreover, PDGF, IL-1β, IL-6, and TNF-α, secreted by macrophages and T-cells, play a prominent profibrotic role [184,185,186]. Initially, IL-17A was thought to directly activate HSCs based on in vitro stimulation assays. However, more recent findings suggest that IL-17A instead prompts the proliferation of fibrogenic CD9+TREM2+ macrophages, driving hepatic fibrosis [187,188,189,190]. In addition, adipokines, like leptin and adiponectin, are also involved in the pathogenesis of hepatic fibrosis. While adiponectin exhibits robust antifibrotic properties, leptin is chiefly profibrogenic. Conversely, nuclear receptors, such as liver X receptor (LXR), farnesoid X receptor (FXR), and peroxisome proliferator-activated receptors (PPARγ and PPARδ), inhibit the activation of HSCs and fibrosis. Persistent inflammation eventually progresses to irreversible chronic inflammation, impairing liver repair and leading to extensive extracellular matrix deposition and fibrosis. Therefore, targeting these pathways may be an effective approach to slow the pro-fibrotic progression of liver injury.

3.3. Risk Factors and Their Impact on MASH Progression

Metabolic dysfunction is a central hallmark of MASLD pathogenesis, which is often regarded as the hepatic manifestation of metabolic syndrome [191]. The growing global obesity pandemic is paralleled by a remarkable increase in the incidence of MASLD. The concurrent surge in these metabolic diseases is largely attributed to overnutrition and to a sedentary lifestyle [192]. Notably, more than 70% of patients with T2D develop MASLD. Reciprocally, more than 20% of patients with MASLD present with T2DM, suggesting shared pathophysiological mechanisms [193,194]. The establishment of the role of insulin resistance in the pathogenesis of MASLD was originally proposed by Marchesini et al. [195]. Elevated serum insulin levels are strongly associated with hepatic ballooning and lobular inflammation [196]. Insulin resistance and chronic hyperinsulinemia induce intrahepatic fat accumulation by promoting de novo lipogenesis, partly through transcriptional upregulation of lipogenic genes expression by activating SREBP-1c. In addition, insulin resistance increases fatty acid mobilization from adipocytes eventually reaching the hepatocytes through the portal circulation [122]. Other inflammatory pathways are also critical in the development of hepatic insulin resistance. Interestingly, constitutive activation of nuclear factor kappa-B kinase subunit beta (IKK-β) in mice leads to hepatic insulin resistance, whereas hepatocyte-specific IKK-β knockout mice do not develop hepatic insulin resistance in response to sustained high-fat intake [197,198]. IKK-β activation is driven by oxidative stress and proinflammatory cytokines, particularly TNF-α, both of which are remarkably elevated in patients with MASLD. In addition, the activation of both protein kinase C epsilon (PKC-ε) and c-Jun N-terminal kinase 1 (JNK1) inhibit the phosphorylation of insulin receptor substrates (IRS-1 and IRS-2), blunting insulin signaling. Dyslipidemia is another important risk factor for MALSD, affecting ~70% of patients with MASLD and MASH. Dyslipidemia is characterized by a state of elevated triglyceride (TG) levels and decreased high-density lipoprotein cholesterol (HDL-C) [199]. Interestingly, the extent of visceral adipose tissue (VAT) is significantly associated with hepatic steatosis in patients with MASLD [200]. Chronic dyslipidemia upregulates the expression and activity of SREBP-1c, leading to hepatic lipid accumulation [201]. Moreover, insulin resistance in patients with T2D impairs lipid metabolism, dyslipidemia, oxidative stress, and membrane lipid peroxidation, accelerating the progression of MASLD to advanced stages of liver disease. The apolipoprotein B/apolipoprotein AI (ApoB/AI) ratio has also emerged as a valuable predictor for cardiovascular disease risk and is associated with MASLD prevalence [201,202]. Diet, weight gain, and sedentary lifestyle are well-established risk factors for MASLD. High fructose intake, from sweetened beverages and processed foods, is closely associated with MASLD development and progression [203]. In fact, one of the major dietary constituents involved in liver disease progression from MASLD to MASH is fructose as it promotes hepatic fat deposition and fibrosis [204,205]. This stems from its lipogenic role in serving as a substrate that fuels de novo lipogenesis in part by stimulating lipogenic enzymes transcription via SREBP1c and ChREBP [206,207]. The release of non-esterified fatty acids (NEFA) from VAT is a major contributor to hepatic steatosis. Accordingly, obesity is another important risk factor for MALSD and metabolic diseases. In obesity, elevated levels of circulating FFAs from VAT and subcutaneous adipose tissue (SAT) alter hepatic lipid metabolism to contribute to insulin resistance in hepatocytes and skeletal muscle cells and promote dyslipidemia. This highlights the importance of lifestyle modifications (i.e., dietary changes and increased physical activity) as essential components of MASLD therapy [208]. Other non-modifiable risk factors include advanced age (>60 years), which is associated with a more severe disease phenotype, characterized by a higher incidence of fibrosis. Moreover, single-nucleotide polymorphisms (SNPs) in the PNPLA3 gene, which regulate triacylglycerol breakdown in adipocytes, are closely linked to MASLD susceptibility [6,209].

Patients with MASLD exhibit an increased risk of CVD, as supported by two independent findings. First, metabolic manifestations in MASLD including T2D, dyslipidemia, hypertension, and obesity, all of which are independently linked to increased CVD. Remarkably, patients with MASLD with concurrent T2D are likely to exhibit the worst prognosis, although some studies rule out this association [210]. Other cross-sectional studies demonstrated an existing correlation between MASLD and atherosclerosis independent of T2D [211]. A recent study examining the link between atherosclerotic cardiovascular disease (ASCVD) risk scores and overall and cardiac-specific mortality rates in MASLD patients indicates that ASCVD is associated with a higher risk of both overall and cardiac-specific mortality [212]. Other studies suggest that the heightened risk and mortality rates from CVD may be due to advanced fibrosis (Stage 3 or 4) and T2D, independent of MASLD [213,214,215,216]. On the other hand, individuals with MASLD with viral hepatitis or who consume alcohol moderately exhibit a higher 10-year CVD risk compared to those with MASLD alone. This suggests that the impact of MASLD on CVD risk may be influenced by additional factors related to hepatic injury. Understanding the shared pathophysiological mechanisms between these diseases is crucial, as targeting one disease pathway could provide collateral benefits for treating the other comorbidities. Several interconnected pathophysiological pathways may link MASLD and T2D to an increased CVD risk, including a proatherogenic lipid phenotype, increased prothrombotic factors, insulin resistance, endothelial dysfunction, increased oxidative stress, low-grade inflammation, and intestinal dysbiosis [8,217,218,219,220]. ROS generation, secondary to hepatic inflammation, also plays a key role in the development of atherosclerotic lesions and disease progression. ROS induce endothelial cell dysfunction and vascular smooth muscle cell proliferation [221,222]. Moreover, insulin resistance and chronic hyperglycemia trigger aberrant ROS production, impairing vascular endothelial cells and accelerating smooth muscle cell proliferation, thereby fueling atherogenesis [223]. In addition to the role of inflammation in endothelial dysfunction, it is also responsible for decreased vascular tone and vascular plaque development. The prevalence of lean MASLD is approximately 8.4% of total MASLD reported cases. This diagnosis requires both evidence of hepatic steatosis and the occurrence of metabolic dysregulation, defined by the presence of at least two risk factors (i.e., increased waist circumference, hypertension, low serum HDL levels, high cholesterol levels, hypertriglyceridemia, hyperglycemia, insulin resistance, and chronic subclinical inflammation) [224,225,226]. A noteworthy study evaluated the importance of anthropometric measurements, including height, weight, body mass index (BMI), and circumference of various body parts (e.g., waist and hips), in predicting long-term outcomes of patients with MASLD. Stratifying MASLD patients by BMI and waist circumference has proven to be a more accurate prognostic biomarker. Notably, MASLD patients with lean BMI but obese waist-to-hip ratio have a significantly higher risk of mortality due to hypertension, hyperlipidemia, T2D, and insulin resistance. In addition, CVD mortality risk is nearly three times higher in individuals with lean BMI and obese waist circumference compared to those with overweight BMI and normal waist circumference. These findings reinforce the link between CVD-associated MASLD mortality and highlight the need for personalized therapeutic management based on individual body composition. Moreover, assessment of waist circumference should be incorporated in clinical practice [7].

4. Common Risk Factors and Pathways

Mounting evidence indicates a pathophysiological link between MASLD and atherosclerosis, although the shared mechanism(s) underlying concurrent disease progression is (are) yet to be elucidated. However, several overlapping etiologies have been identified. A strong association between MASLD and endothelial dysfunction, the chief hallmark of atherosclerosis, has been established [227]. Fetuin-A, an insulin signaling inhibitor that serves as a metabolic syndrome diagnostic biomarker, was found to be positively correlated with endothelial dysfunction and carotid artery atherosclerosis in a study involving 115 MASLD patients [228]. A separate report showed a significant decrease in brachial artery flow medial dilatation (FMD) in a sample of 52 patients with MASLD [229]. Moreover, a direct association between reduced NO in endothelial cells, impaired vasodilation, increased oxidative stress, increased inflammation, and endothelial dysfunction was demonstrated in MASLD. At the level of the endothelium, inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS) are responsible for nitric oxide production, a major regulator of vascular tone. In a stressful physiological milieu, NO is produced via eNOS and iNOS [230]. Evidence suggests that iNOS activation is more common in inflammatory conditions and can exacerbate insulin resistance and hyperglycemia, increase oxidative stress, and downregulate eNOS, all of which can lead to endothelial dysfunction. Abnormal eNOS activity was reported in insulin resistance, leading to low NO production [231,232]. Endothelial dysfunction in hepatic sinusoids triggers hepatic stellate and Kupffer cell activation [233,234]. Upon their activation, stellate and Kupffer cells produce several prothrombotic factors and recruit neutrophils and platelets, all of which partake in sinusoidal microthrombus development and parenchymal extinction, hence accelerating the evolution of fibrosis [235,236]. Therefore, endothelial dysfunction is not only a principal contributor to atherosclerosis, but also a key indicator of MASLD severity. In fact, a recent study documented increased levels of VCAM-1, NOX2 activity, endothelial progenitor cells, fetuin-A, and carotid artery intima media thickness (CIMT) in patients with MASLD [237]. In early-stage MASLD, inflammation acts as a critical bridge between liver fibrosis and atherosclerosis. Through oxidative stress and inflammation, neutrophils accelerate atherosclerosis and contribute to plaque instability. Epicardial adipose tissue produces pro-inflammatory molecules, including IL-1β, IL-6, MCP-1, and TNF-α, which contribute to myocardial inflammation in the early stages and to atherosclerosis in the later stages [238]. In a similar manner, disrupting insulin intracellular signaling of inflammatory pathways, mainly IKK/NF-κB and JNK/AP1 pathways, plays a key role in MASLD progression [239]. These pathways induce the production of hepatic CRP that stimulates NF-κB which further disrupts insulin signaling. Moreover, it was found that hepatic lipid accumulation leads to inflammation and activation of NF-κB. The latter induces the transcription of proinflammatory genes and cytokine production to lead to insulin resistance and MASLD [227]. Furthermore, inflammation leads to ROS production that, in turn, mediate the oxidation of polyunsaturated fatty acids to 4-hydroxy-nonenal (HNE), malonyl-aldehyde (MDA), and 4-oxy-2-nonenal (ONE). These markers are not only found in patients with MASLD but are also associated with vascular damage [240]. Caimi et al. investigated the link between metabolic syndrome and oxidative stress by assessing lipid peroxidation, measured as thiobarbituric acid-reactive substances (TBARS), nitric oxide metabolites (nitrite + nitrate) expressed as NOx, and their ratio TBARS/NOx in 106 patients with metabolic syndrome. In comparison to controls, the results revealed higher TRABS and NOx levels as well as lower TRABS/NOx ratio. This implies that redox and inflammatory status are important contributors, as there is a correlation between lipid peroxidation and inflammation in this group of patients [241]. When pro-inflammatory transcription factors like NF-κB and activator protein-1 (AP-1) are activated in adipocytes of obese individuals, inflammatory cytokines like TNF-α, IL-1β, and IL-6 are released. This creates a vicious cycle because the release of these cytokines increases ROS production [242,243].

Many studies focused on uncovering the mechanism by which central obesity causes oxidative stress. Obesity is marked by nutritional excess, adipocyte hypertrophy, and release of inflammatory molecules related to PKC and polyol pathways, which induce NADPH oxidase (NOXs), nitric oxide synthase, uncoupled endothelial NOS (eNOS), and myeloperoxidase activation. The resulting alteration in adipocyte’s function affects the transcription of inflammatory factors, leading to oxidative stress and inflammation that subsequently halt antioxidant defense mechanisms. The product of this functional change is metabolic dysfunction [244].

Insulin resistance is associated with both MASLD and atherosclerosis. Under homeosteatic conditions, insulin binds to its receptor on endothelial cells and maintains vascular tone for proper tissue perfusion by promoting NO-dependent vasodilation and Endothelin-1-dependent vasoconstriction via PI3K and MAPK pathways, respectively. In the insulin-resistance state, insulin favors the MAPK/ET-1 pathway over the PI3K/NO pathway, tipping the balance towards more vasoconstriction with decreased endothelial vasodilation and endothelial dysfunction [11,245]. Likewise, the p38MAPK pathway is implicated in the development and progression of atherosclerosis. Smooth muscle cell proliferation, macrophage activation, and monocyte chemotaxis are among the processes that are activated by p38MAPK in several cell types typically found in atherosclerotic plaques. This pathway contributes to the spread of plaques in atherosclerotic disease [246]. Supporting data to this association was presented by the finding that metformin, an antidiabetic medication used to enhance insulin sensitivity in the liver of patients with insulin resistance, decreases ET-1 and increases NO blood levels. It was also shown that metformin therapy for 3 years reduces major cardiovascular complications significantly in patients with T2D [247]. Furthermore, insulin resistance downregulates lipoprotein lipase (LPL) and hepatic lipase (HL), leading to lipid profile disturbances and culminating in atherogenic dyslipidemia. As previously mentioned, insulin halts the activity of both lipases. These enzymes function in the catabolism of triglyceride rich lipoproteins [248]. This malfunction causes an increase in triglyceride and LDL and a decrease in HDL level, which is often reported in MASLD. Patients with MASLD exhibit a dose-dependent rise in fasting blood triglycerides and a fall of serum HDL, indicating a distinct pathophysiologic relationship between insulin resistance, dyslipidemia, and MASLD [227]. A recent study was carried out on a population of newly diagnosed insulin-sensitive patients with familial combined hyperlipidemia. The prevalence of hepatic steatosis in the study sample was 75%, and a markedly increased risk of atherosclerotic disease was observed in cases presenting with liver fibrosis [199]. Patients with MASLD exhibit high levels of triglycerides and LDL in addition to low HDL levels [249,250]. This lipid profile is associated with increased CETP activity, an enzyme that transfers cholesterol esters from HDL to VLDL and LDL in exchange for triglycerides. In MASLD, the activity of CETP is enhanced, leading to hypertriglyceridemia and the consequent production of larger VLDL particles. These particles are metabolized into atherogenic particles, small dense LDL and small HDL. Due to their small size, they have the capacity to pass through the endothelial layer more easily, initiating plaque formation in arteries [251]. Moreover, TGF-β, a key profibrogenic molecule, is a primary driver of hepatic fibrosis. In fact, it has a direct influence on hepatic stellate cell activation, transforming them into collagen-secreting myofibroblasts, promoting fibrogenesis. TGF-β also has a fibrotic role in the myocardium; when fibrosis develops in the heart, the injured myocardium releases pro-fibrotic substances such as angiotensin II, TGF-β1, and IL-1β, which combined perpetuate the inflammatory cycle [251]. Moreover, patients with atherosclerosis exhibit elevated levels of TGF-β1 and the long non-coding RNA-ATB (lncRNA-ATB), suggesting that these markers could be a useful diagnostic marker for atherosclerosis [252]. Thus, TGF-β1 emerges as a critical pathophysiological link, tethering hepatic fibrosis to atherosclerosis.

5. Loss of CEACAM1 Links Hepatic Fibrosis to Atherosclerosis

5.1. MASLD and Atherosclerosis: Twin Diseases

MASLD and atherosclerosis are often interconnected, representing different manifestations of the same underlying disease. MASLD involves fat buildup in the liver, while atherosclerosis is marked by fat accumulation in blood vessels. Hepatic inflammation, especially in MASH, is strongly linked to increased atherosclerosis compared to simple hepatic steatosis [253,254,255]. MASLD, a liver-related feature of metabolic syndrome [9,256,257,258], includes conditions like abdominal obesity, dyslipidemia, hypertension, insulin resistance, and glucose intolerance [259,260]. About 90% of MASLD patients exhibit one or more of these metabolic abnormalities, and 33% meet the full criteria for MetS [261,262]. This raises death risk, primarily due to CVD or liver complications [263,264]. The strong connection between MASH and atherosclerosis highlights the role of the liver in cardiovascular health, but the cause–effect relationship between these two health conditions remains elusive [265,266]. Identifying the mechanisms linking these diseases is key to developing better therapeutic strategies. Factors like insulin resistance, inflammation, lipid disorders, oxidative stress, and systemic release of oxidized LDL cholesterol may contribute to both conditions [256,267]. In insulin resistance, the liver and adipose tissue become targets and contributors to systemic inflammation, with increased lipolysis-derived NEFA and pro-inflammatory cytokines such as IL-6 and TNF-α [260]. Due to the heightened cardiovascular risk in MASLD patients, a multidisciplinary approach is critical to manage these interconnected cardiometabolic disorders effectively.

5.2. CEACAM1—Its Loss Links Reduced Insulin Clearance to MASH

In the liver, CEACAM1, a plasma membrane glycoprotein, belongs to a family of proteins containing four immunoglobulin-like (Ig) structures in its extracellular domain. It is ubiquitously expressed in many cell types, most predominantly in hepatocytes, followed by renal proximal tubule cells, the main sites of insulin clearance.

In hepatocytes, CEACAM1 is mainly expressed as two alternative spliced isoforms that differ by the length of their cytoplasmic tail (72–74 vs. 10–12 amino acids depending on species) and in their subcellular localization. In contrast to the short isoform (CEACAM1-4S), the long (CEACAM1-4L) contains two tyrosine sites (Y493 and Y520 in humans) within the well-conserved immunoreceptor tyrosine-based inhibitory motifs (ITIMs). While the expression of the short isoform is restricted to the bile canalicular domain of hepatocytes, CEACAM1-4L is mainly expressed in the sinusoidal domain in the space of Disse [268,269]. This localization of CEACAM1-4L positions it to undergo phosphorylation on Y493 (Y488 in rodents) by the insulin receptor tyrosine kinase upon its activation by insulin that is passively transported through the fenestrae of the capillaries lining the sinusoid [10,17]. CEACAM1-4L phosphorylation causes it to partake in the insulin–receptor complex to increase the rate of insulin targeting to its lysosomal degradation process in hepatocytes. This promotes insulin clearance, which, together with insulin secretion, serves to maintain the physiologic levels of insulin that reach peripheral target tissues and ensure proper insulin action (Figure 1).

The role of CEACAM1-4L in insulin clearance is tightly associated with its role in mediating a suppressive effect on FASN activity by acutely secreted insulin [270]. Following its endocytosis, phosphorylated CEACAM1-4L binds to cytoplasmic FASN, an event that detaches it from the insulin–receptor complex to allow insulin’s dissociation from its receptor to undergo lysosomal degradation in late endosomes, as the receptor is recycled back to the plasma membrane. Binding of CEACAM1-4L to FASN causes suppression of its enzymatic activity, demonstrating that acutely, insulin exerts an anti-lipogenic effect in hepatocytes, as opposed to the well-accepted lipogenic effect of chronic hyperinsulinemia [270,271].

It is commonly believed that reduced insulin clearance plays a compensatory mechanism alongside insulin hypersecretion to prolong the function of pancreatic β-cells. Conversely, reduced insulin clearance has emerged in recent years as a risk factor for MetS and MASLD, particularly among Native Americans, African Americans, and Hispanic individuals of Mexican descent [272,273]. This is based on the notion that reduced insulin clearance drives chronic hyperinsulinemia, which in turn causes hepatic insulin resistance (by downregulating the insulin receptor) and promotes de novo lipogenesis and, subsequently, hepatic steatosis.

Bril et al. [274] and Watada et al. [275] have demonstrated that in patients with MASLD, hyperinsulinemia is mainly due to impaired insulin clearance, rather than aberrant insulin secretion. In a cohort of 1019 subjects from the second Portuguese prevalence study of type 2 diabetes (PREVADIAB2), we recently reported that circulating CEACAM1 levels decreases in parallel with reduced insulin clearance as the disease progresses from normoglycemia to hyperglycemia, concomitant with an increase in fatty liver index [276]. Consistent with a critical role for CEACAM1 in insulin and lipid metabolism, hepatic CEACAM1 levels are lower in obese individuals with insulin resistance and hepatic steatosis than age-matched insulin sensitive subjects, with the extent of fat accumulation being directly correlating with the degree of CEACAM1 reduction [18,19]. Furthermore, CEACAM1 protein levels are significantly lower in primary hepatocytes of obese individuals with steatotic livers compared to age-matched lean controls, indicating an inherent loss of hepatic CEACAM1 in obesity [18]. Hepatic CEACAM1 expression is significantly downregulated in patients with MASH and further declines in hepatocytes and liver sinusoidal endothelial cells as hepatic fibrosis advances [277]. Moreover, hepatic CEACAM1 levels are notably reduced in liver biopsies of obese South Korean subjects presenting with insulin resistance and microvesicular steatosis, even in the absence of T2D [19]. These findings underscore the need to delineate the mechanistic involvement of CEACAM1 in the pathogenesis of insulin resistance driven by impaired insulin clearance and associated comorbidities [278]. Notably, patients with T2D are more likely to present with concomitant hepatic steatosis and reduced insulin clearance compared to their non-diabetic counterparts [279]. Insulin clearance in these patients is rescued via PPARγ agonists, possibly through a CEACAM1-dependent mechanism, given the established role of PPARγ as a transcriptional inducer of CEACAM1 expression in hepatocytes and HSCs [280,281,282]. It is intriguing that the insulin sensitizers PPARγ agonists or secretagogues (GLP-1 receptor agonists) activate CEACAM1 transcription [281,283], thus suggesting that inducing CEACAM1 expression could constitute an effective targeted therapy.

Additionally, recent findings have underscored the interplay between elevated plasma NEFA, hepatic CEACAM1 expression, and impaired insulin clearance in the etiology of metabolic disease [271]. For instance, short-term exposure to a Western-style diet rapidly impaired insulin clearance and induced early hepatic insulin resistance in healthy South Asian men but not in their Caucasian counterparts, highlighting population-specific metabolic response to diet and predisposition to metabolic disease [284]. This reduction in insulin clearance was associated with increased NEFA levels, emphasizing the role of lipolysis-derived NEFA release in mediating CEACAM1 downregulation [271]. Mechanistically, NEFA likely suppress CEACAM1 expression via a PPARα-dependent pathway, initiating a vicious cycle wherein impaired insulin clearance causes hyperinsulinemia, further exacerbating insulin resistance and hepatic steatosis [285].

5.3. Repositioning CEACAM1 as a Critical Immunometabolic Regulator

In addition to its established regulatory role in insulin and lipid metabolism, CEACAM1 is also implicated in inflammation, particularly under states of nutritional excess. The homeostatic immuno-metabolic network, coupling hepatic lipid accumulation with inflammatory signaling, is orchestrated by the CEACAM1—CD36 axis [286]. This sophisticated molecular bridge governs fatty acid and lipopolysaccharide uptake, thereby influencing lipid droplet formation and modulating inflammatory responses in the liver.

CEACAM1 serves as a crucial immunological signaling mediator. It orchestrates immune and non-immune processes through both homophilic and heterophilic interactions. When CEACAM1-4L undergoes tyrosine phosphorylation by SRC-family kinases, ITIMs recruit SH2 domain-containing phosphatases, transducing inhibitory signaling cascades in T-cells [287]. Conversely, the short isoform (CEACAM1-4S) lacks ITIMs and is therefore associated with non-inhibitory responses to exert a pro-inflammatory response. CEACAM1-4L has been implicated in immunosuppression and cancer immune evasion via interaction with TIM-3, partaking in checkpoint-mediated immune inhibition. Additionally, pathogen–CEACAM1 interaction facilitates microbial host invasion and immune bypass [287]. This corroborates CEACAM1’s function as a key immunoregulatory molecule and a potential therapeutic target in inflammatory diseases [288].

Moreover, CEACAM1-4L expression in neutrophils modulates ischemia-reperfusion injury (IRI) during orthotopic liver transplantation. Specifically, elevated neutrophil CEACAM1-4L expression is associated with reduced NETosis, improved graft function, and lower rates of allograft rejection. Therefore, targeting CEACAM1-related pathways hold promises of mitigating aberrant inflammatory responses in the context of IRI, potentially improving organ transplant outcomes [289]. Aberrant alternative splicing of CEACAM1 disrupts the equilibrium between its L and S isoforms, driving aberrant inflammatory responses, immune evasion, and tumor progression. Antisense oligonucleotides represent a promising strategy to therapeutically tip the splicing balance towards the immunoregulatory CEACAM1-4L isoform [290,291,292].

In the vasculature, CEACAM1 is involved in maintaining endothelial barrier integrity, facilitating nitric oxide signaling, and promoting angiogenesis. The proinflammatory cytokine TNF-α significantly upregulates CEACAM1 in endothelial cells via NF-κB and β-catenin signaling, contributing to endothelial dysfunction and aging-associated inflammation [293]. CEACAM1 emerges therefore as a promising therapeutic target in the context of age-related cardiovascular disease.

In sum, CEACAM1 serves as a critical immune-metabolic regulator, integrating lipid trafficking, endothelial function, and inflammatory response. This underscores the potential contribution of CEACAM1 expression to multiple human disease processes.

5.4. CEACAM1—Its Loss Links MASH to Atherosclerosis

Patients with MASLD/MASH are at higher risk for cardiovascular diseases, such as atherosclerosis [294,295,296,297]. Furthermore, CEACAM1 levels are lower in hepatocytes (and liver sinusoidal endothelial cells) of patients with MASH and as hepatic fibrosis stage advances, in inverse relationship with plasma Endothelin-1 levels [298]. Similarly, rats selectively bred with low aerobic capacity exhibit all of the cardiometabolic features of MetS relative to those with high-aerobic capacity, including MASH [299] and atherosclerosis [300], in parallel to their lower hepatic CEACAM1 expression [299,300]. Moreover, the phenotype of cell-specific and global Cc1^−/− models closely recapitulate the phenotypic presentation and pathophysiologic processes that occur in human MetS.

These observations have led to efforts to uncover potential CEACAM1-dependent common molecular pathways that link these cardiometabolic conditions. Accordingly, global (Cc1^−/− mice) and liver-specific deletion (i.e., AlbCre+Cc1^fl/fl mice) of the Ceacam1 gene cause chronic hyperinsulinemia, mainly due to decreased insulin clearance. This is followed by hepatic insulin resistance, steatohepatitis, visceral obesity, and eventually systemic insulin resistance [20,21]. Additionally, these mutations cause human MASH-characteristic liver histology, including hepatic inflammation and fibrosis even when mice are fed a regular chow diet [191,301]. They also lead to cardiovascular abnormalities such as hypertension and endothelial and cardiac dysfunction [22,23]. Conversely, liver-specific reconstitution of CEACAM1 reverses insulin resistance with other features of metabolic dysfunction and hepatic fibrosis in Cc1^−/− nulls, even when they are fed with a high-fat diet [302]. It also reverses hypertension and cardiac and endothelial dysfunction, at least in part resulting from curbing the hyperinsulinemia-driven release of Endothelin-1 in Cc1^−/− nulls [23]. This underscores the role of the loss of hepatic CEACAM1 in linking metabolic to cardiovascular dysregulation [23,303].

AlbCre+Cc1^fl/fl mice bred on Ldlr^−/− background and fed with an atherogenic diet developed chronic hyperinsulinemia due to impaired insulin clearance, high cholesterol levels, a proinflammatory state, and increased oxidative stress [15]. They also developed MASH (steatohepatitis, apoptosis, and fibrosis) and atherosclerotic plaque lesions. Mechanistically, reduced hepatic insulin clearance caused by the loss of CEACAM1 in hepatocytes led to hyperinsulinemia, which in turn drove the downregulation of the insulin receptor and systemic insulin resistance including in the aorta. This was followed by impaired Akt/eNOS and Shc/MAPK NF-kB pathways downstream of the insulin receptor, the former suppressing NO production, and the latter stimulating that of Endothelin-1 to shift the balance towards vasoconstriction, followed by portal hypertension and oxidative stress [15], key mechanisms underlying hepatic fibrosis. This mouse model provides an in vivo demonstration of how insulin resistance caused by hyperinsulinemia links MASH to atherosclerosis when coupled with hypercholesterolemia (Figure 2). Thus, insulin resistance, caused by impaired CEACAM1-dependent hepatic insulin clearance pathways, plays a key role in linking MASLD/MASH to atherosclerosis.

However, the role of insulin resistance, independent of dyslipidemia, remains unclear. Mice with endothelial-specific deletion of Ceacam1 bred on Ldlr^−/− background and fed with an atherogenic diet developed hypercholesterolemia without insulin resistance. This led to vascular inflammation and hepatic and aortic fibrosis without an increase in atheroma formation [304]. Mechanistically, this was marked by decreased NO bioavailability and increased oxidative stress, caused by an impaired Akt/eNOS pathway downstream of the VEGF receptor. In addition, endothelial loss of CEACAM1 led to the hyperactivation of Shc/MAPK NF-kB signaling, followed by stimulating pro-inflammatory and pro-fibrogenic events (including excessive production and Endothelin-1 secretion) that led to endothelial dysfunction and aortic and hepatic fibrosis [304]. Nonetheless, these mice did not develop hyperinsulinemia, insulin resistance, aortic steatosis, liver steatosis, or atheroma [304,305]. This highlights the pivotal role of hyperinsulinemia-driven insulin resistance in the development of overt atherosclerosis and tissue steatosis. Collectively, these studies emphasize that inflammation can drive fibrosis, but hyperinsulinemia and insulin resistance are central to atherosclerosis progression and steatosis. This validates the use of insulin sensitizers to prevent atherosclerosis progression in patients with insulin resistance and MASLD/MASH. We posit that the efficacy of these drugs could be partly mediated by inducing CEACAM1 transcription [281,283].

6. Conclusions

This review presents CEACAM1 as a master immuno-metabolic regulator, coordinating hepatic insulin and lipid metabolism, immune cell-response, and endothelial/vascular physiology. The impairment of CEACAM1-related pathways contributes to the development of insulin resistance, MASLD/MASH, hepatic and aortic fibrosis, and cardiovascular disease. More specifically, impaired CEACAM1-dependent insulin clearance in hepatocytes promotes hyperinsulinemia, causing insulin resistance and precipitating hepatic steatosis, inflammation, and fibrosis, while also prompting endothelial dysfunction and vascular plaque formation. Although these findings are yet to lead to tangible clinical application, they lay a robust foundation for future interventions designed to target the immuno-metabolic dysregulation characteristic of metabolic liver diseases and atherosclerosis [286]. We posit that research directed toward developing strategies targeting CEACAM1 to mitigate cardiometabolic disease burden will disrupt the vicious cycle of hyperinsulinemia-induced hepatic and endothelial injury.