Interorgan crosstalk in metabolic dysfunction-associated steatotic liver disease

Abstract

Interorgan crosstalk refers to the bidirectional communication and interaction between different organs in the body. It plays a crucial role in maintaining metabolic homeostasis and is essential for proper physiological function. Dysregulation of organ crosstalk has been associated with the development of various metabolic disorders, including metabolic dysfunction-associated steatotic liver disease (MASLD). MASLD is a growing health issue worldwide, affecting approximately 30% of the global population. There has been a growth in literature to address the dysregulated crosstalk between multiple extrahepatic organs and the liver; thus, many striking findings have been published. However, the underlying mechanisms remain largely obscure. In this study, we focused on the perspective of circulating proteins, metabolites, neuroendocrine signals, and extracellular vesicles, summarizing systematically how they affect extrahepatic organs and the liver in the pathogenesis of MASLD. Furthermore, particular attention is placed on the potential novel therapeutic strategies.

Introduction

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a series of diseases featuring excessive lipid deposition in the liver cells and is often accompanied by obesity, diabetes, dyslipidemia, abnormal blood pressure, and other metabolic disorders.^[1–4] To date, up to 30% of patients with MASLD will develop inflammation of the portal vein and hepatic lobules as well as hepatocellular damage, which is emphasized as metabolic dysfunction-associated steatohepatitis (MASH).^[5,6] In 2024, resmetirom, an oral thyroid hormone receptor-beta agonist, was granted accelerated approval by the Food and Drug Administration (FDA) for the management of MASH with moderate-to-advanced fibrosis in adult patients.^[7] Currently, the FDA has granted approval for Wegovy (semaglutide 2.4 mg, Novo Nordisk, Copenhagen, Denmark) to treat MASH in adults with F2–F3 stage fibrosis.^[8] However, the drug may cause side effects such as diarrhea and nausea. With Phase IV trials pending, continued evaluation will be used to comprehensively assess its clinical benefits and risk-benefit profile. Thus, the investigation of innovative targets that reduce fat deposition and simultaneously ameliorate hepatic fibrosis remains imminent.^[9]

The causes and development of MASLD are intricately influenced by a multitude of complex and multifactorial factors. Initially, the “two-hit hypothesis” was developed to explain the pathogenesis of the MASLD spectrum.^[10] According to this most widely regarded theory, the first “hit” involves the accumulation of fat (steatosis) in the liver, primarily due to metabolic disorders such as insulin resistance. The second “hit” encompasses a variety of inflammatory and oxidative stressors that exacerbate liver injury, leading to inflammation and hepatocyte damage, which can promote fibrosis and cirrhosis if not addressed. However, the traditional “two-hit” pathophysiological theory has been subsequently challenged by the fact that hepatic steatosis may be accompanied by many different mechanisms of injury instead of “the first hit”.^[11] For this reason, the “multiple parallel hits” hypothesis was proposed.^[12,13]

The liver is a key integrator for numerous physiological processes, receiving changes in the gradient of exogenous signals.^[14,15] Multiple sources of evidence indicate that ongoing communication between different organs is essential for the progression of MASLD.^[16,17] The adipose tissue, skeletal muscle, pancreas, and gut are highlighted as key players in this interorgan crosstalk that influences the development and severity of the disease. Excessive accumulation of fat, particularly visceral fat, releases free fatty acids (FFAs) and inflammatory factors, inducing insulin resistance and metabolic disorders. Abnormal insulin signaling from the pancreas, imbalanced gut microbiota, and their metabolites interact with the disrupted hepatic lipid synthesis/degradation reciprocally, collectively promoting the pathogenesis of MASLD. Moreover, the reduced muscle mass further exacerbates this pathological cycle. It is worth noting that organ interactions influencing the formation of MASLD are not necessarily limited to the effects of molecules on the liver itself. For instance, the accumulation of visceral fat due to obesity and peripheral insulin resistance leads to elevated levels of FFA and glucose in the body. This state of nutrient excess promotes an increase in hepatic fat and triglycerides, thereby triggering MASLD.^[18,19] However, metabolites produced by the gut microbiota could circulate through the bloodstream to adipose tissue, contributing to the amelioration of obesity. Therefore, considering the broader systemic effects and organ–organ crosstalk in understanding and managing liver diseases is of great importance. In this review, we endeavor to focus on the interorgan crosstalk that transpires between the liver and extrahepatic tissues, summarizing how these contributing factors mediate the pathogenesis of MASLD. Meanwhile, this article will contribute to the development of the therapeutic targets.

Circulating Proteins in MASLD

Proteins play a crucial role in numerous biological functions, including various processes that contribute to metabolic disorders, such as signaling pathways, immune responses, and the regulation of metabolism.^[20–22] Various tissues release circulating proteins that function as endocrine factors in the bloodstream, which can be used to assess the progression and individual disease risk.

Growth and differentiation factors

Growth and differentiation factors (GDFs) are vital members of the transforming growth factor-β (TGF-β) superfamily, playing crucial roles in various biological processes.^[23] These factors are involved in regulating cell proliferation, differentiation, development, and repair, and they play an essential role in the onset of various diseases.^[24–26] Among the at least 11 identified members of the GDF family, GDF11 and GDF15 demonstrate a strong association with MASLD. On the one hand, these molecules released from adipose tissue and muscle tissue act via paracrine and endocrine pathways to orchestrate energy metabolism, insulin sensitivity, and inflammatory responses, thereby linking core metabolic organs. On the other hand, the liver serves not only as a source but also as a target of GDF molecules, establishing a closed-loop circuit in interorgan communication.

Over the past decade, GDF11 has emerged as a pivotal “anti-aging” cytokine, demonstrating remarkable potential in reversing age-related pathologies in elderly mice. It has been shown to rejuvenate cardiac and skeletal muscle function, enhance vascular integrity, and promote neurogenesis in the brain.^[27] Recent studies have further elucidated the mechanistic role of GDF11 in adipogenesis, identifying it as a master regulator of the associated signaling cascades. In mature adipocytes, GDF11 initiates the wingless and int (Wnt) signaling pathway, which modulates adiponectin release and enhances insulin sensitivity.^[28,29] In white adipose tissue (WAT), the administration of GDF11 in obese murine models leads to a reduction in adipocyte size.^[28] In brown adipose tissue (BAT), the transfer of the GDF11 gene in obese and diabetic mice promotes energy expenditure and oxidation by stimulating the expression of uncoupling protein 1 (UCP1) and uncoupling protein 2 (UCP2), thereby increasing the conversion of food energy into heat and preventing weight gain.^[30] Sustained GDF11 activation also stimulates adenosine monophosphate-activated protein kinase (AMPK), a mechanism crucial for alleviating the insulin resistance and glucose intolerance associated with a high-fat diet (HFD) [Table 1].^[31]

Table 1.

Roles of some GDF and FGF family members in metabolic regulation and MASLD.

Circulating protein	Source	Target organ	Mechanism	Phenotype	Reference
GDF11	Unknown	WAT	Ucp1↓ Ucp2↓	Adipogenesis of WAT	[30]
		Islet β cell	Wnt↑ AMPK↑	Preservation of islet β cell	[31]
GDF15	Islet β cell	WAT	RET↑ AMPK↑	Lipolysis	[37,36]
FGF21 (FGF21-KLB-FGFR)	Liver	WAT	AMPK↑ PGC-1↑ UCP-1↑	White-to-brown shift	[50,57]
		Muscle tissue	SANRE↓ PI3K/Akt↓	Insulin resistance	[51,52]
FGF19	Liver	Muscle tissue	CREB-PGC-1α↑	Energy expenditure	[63,188]
FGF23	Kidney	Gut	CYP27B1↓	Intestinal phosphate absorption	[66]
FGF1	Liver	Muscle tissue	AKT↓	Insulin resistance	[68]
		Liver	AMPK↓	Gluconeogenesis	[68,71]
		Adipose tissue	PPARγ↓	Remodel of adipose tissue	[73]

↑: Activation of pathways or upregulation of the expression. ↓: Inhibition of the pathway or downregulation in expression. AMPK: Adenosine monophosphate-activated protein kinase; CYP27B1: Cytochrome P450 family 27 subfamily B member 1; FGF: Fibroblast growth factor; GDF: Growth and differentiation factor; KLB: Klotho beta; MASLD: Metabolic dysfunction-associated steatotic liver disease; PGC-1α: Proliferator-activated receptor γ coactivator-1α; PPAR: Peroxisome proliferator-activated receptor; PI3K: Phosphoinositide 3-kinase; UCP: Uncoupling protein; WAT: White adipose tissue; Wnt: Wingless and int; SANRE: Stress-associated neuroendocrine-immune regulation of epigenetics.

Another growth and differentiation factor, GDF15, is a stress-responsive cytokine that plays a significant role in inflammatory processes.^[32,33] Under physiological conditions, it is primarily expressed in placental tissue, with low levels in the pancreas, liver, and kidneys.^[34] Acute or chronic exercise induces skeletal muscle to upregulate and secrete GDF15, which in turn promotes adipose tissue lipolysis.^[35] In rodent models, GDF15 directly bolsters pancreatic β-cell function and combats insulin resistance.^[36] These actions collectively alleviate systemic inflammation in diabetes and promote hepatic lipolysis in MASLD. The primary receptor of GDF15 is glial cell line-derived neurotrophic factor (GDNF) receptor alpha-like (GFRAL). By forming a trimeric complex with the rearranged during transfection (RET) receptor, these ligands activate downstream signaling pathways including protein kinase B (AKT), extracellular regulated protein kinases (ERK), and phospholipase C-γ (PLC-γ). This cascade modulates the autonomic nervous system to suppress appetite and, through enhanced muscular energy expenditure, ameliorates MASH [Table 1].^[37] Similarly, the GFRAL receptor is expressed in the area postrema (AP) and the nucleus of the solitary tract (NTS) of the brainstem.^[38,39] Hepatocyte-derived GDF15 is secreted into the bloodstream, where it acts as an anorexigenic signal to the brain. This reduces food intake and counteracts energy overload, thereby alleviating the hepatic burden at its source. The liver–brain axis mediates the protective effects of GDF15 as a stress hormone, thereby establishing a critical pathway for hepatic distress signaling to the central nervous system.

Treatment strategies in this field primarily focus on the regulation of glucose and lipid metabolism and anti-inflammatory therapies. Of note, there is a lack of specific therapeutic agents, and relevant safety and efficacy evaluations.

Fibroblast growth factors

The fibroblast growth factor (FGF) family comprises a class of multifunctional polypeptide growth factors, including at least 28 distinct members.^[40] Among these, endocrine FGF19 subfamily members (FGF19, FGF21, and FGF23) are pivotal hormones that regulate the homeostasis of bile acids, lipids, and glucose.^[41–43] They also represent significant therapeutical targets for a range of metabolic disorders, including type 2 diabetes, obesity, MASH, and chronic kidney disease.^[44,45]

FGF21, a key molecule in energy metabolism, was discovered in 2005 for its ability to normalize plasma glucose and triglyceride levels in obese mice.^[46] In the next few years, scientists have found that it plays an essential role in facilitating the adaptive response to starvation by triggering ketogenesis, gluconeogenesis, lipolysis, and lipid β-oxidation.^[47,48] In 2008, FGF21 was classified as an Akt-regulated myokine.^[49] Exercise significantly enhances the expression of FGF21 in skeletal muscle through the sustained activation of the AMPK signaling pathway^[50] and the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway [Table 1].^[51,52] The release of FGF21 from muscle tissue enhances systemic insulin sensitivity and lipid metabolism via the FGF21–adiponectin axis, which in turn ameliorates lipid metabolism disorders in both adipose tissue and the liver.^[53–56] Additionally, an 8-week aerobic exercise intervention in obese mice upregulated browning genes in WAT and thermogenic genes in BAT, such as peroxisome proliferator-activated receptor (PPAR) γ coactivator 1-α (PGC-1α) and UCP-1, further improving lipid metabolism disorders [Table 1].^[57] Inducing the browning of WAT can enhance systemic insulin sensitivity and alleviate hepatic steatosis at its source. The liver responds to nutritional stress signals during fasting, high sugar (particularly fructose), HFDs, and mitochondrial stress by robustly inducing the secretion and synthesis of FGF21 in hepatocytes. This hormone crosses the blood–brain barrier and acts on specific receptor regions within the central nervous system, exerting highly selective suppression of carbohydrate and alcohol intake.^[58,59] Following the integration of FGF21 signaling by the hypothalamus, sympathetic nerve terminals are activated to release norepinephrine (NE), which concurrently promotes lipolysis and induces browning of WAT. FGF19 is predominantly expressed in enterocytes of the small intestine and colon, exhibiting insulin-mimetic properties.^[60,61] It is a representative signaling molecule within the gut–liver axis. A 2017 study identified that the FGF19-Klotho beta (KLB) complex activates an insulin-independent signaling pathway across multiple metabolic tissues, including the liver, adipose tissue, and brain.^[62] In the mouse models of obesity induced by an HFD, FGF19 intervention significantly reduced body weight and decreased fat accumulation.^[63] In patients with type 2 diabetes, elevated FGF19 levels showed therapeutic potential by regulating bile acid homeostasis.^[64] The activated FGF19-KLB-FGFR4 complex negatively regulated the expression of cholesterol 7α-hydroxylase (CYP7A1), the rate-limiting enzyme in bile acid synthesis, thereby modulating the gut–liver axis.^[65] Due to the potential protumorigenic risk associated with native FGF19, engineered FGF19 analogs are being developed for the treatment of metabolic diseases. These therapeutics are specifically designed to target and inhibit the bile acid synthesis pathways.

The main target organ of FGF23 is the kidney. It primarily interacts with FGFR1 and α-Klotho protein, reducing the synthesis of active vitamin D by inhibiting the activity of cytochrome P450 family 27 subfamily B member 1 (CYP27B1), which further indirectly decreases intestinal phosphate absorption^[66,67] and affects the progression of liver inflammation and fibrosis [Table 1].

Unlike traditional endocrine FGFs, the secreted mitogen FGF1 also possesses the potential for organ interactions. In 2014, researchers discovered that the delivery of recombinant FGF1 (rFGF1) via parenteral routes could effectively lower blood glucose levels and enhance insulin sensitivity in diabetic mice. Subsequent experimental findings indicated that chronic administration of rFGF1 significantly augmented insulin-stimulated AKT signaling in the liver and skeletal muscle of the mice.^[68] This suggests that FGF1 may facilitate glucose uptake and utilization in skeletal muscle while concurrently inhibiting hepatic gluconeogenesis. Further studies also found that variants of FGF1 activate AMPK through FGFR1c on the surface of skeletal muscle and FGFR4 on the surface of hepatocytes, thereby exerting a protective effect on MASLD.^[69–72] FGF1 not only influences peripheral tissues through systemic circulation, facilitating interorgan interactions, but also participates in the remodeling of adipose tissue under the regulation of PPARγ, reducing the process of lipolysis specifically [Table 1].^[73–75] The reduction in lipolytic activity leads to a decrease in circulating FFA levels, which alleviates the lipid burden on the liver and improves the pathological state of MASLD.

Currently, FGF21 and FGF1 analogs have demonstrated the ability to improve liver histology in MASLD patients during clinical trials and have contributed to the reduction of hepatic fat content.^[76,77] Nonetheless, a pivotal challenge in the field lies in enhancing the stability and half-life of FGF family molecules and developing more efficient delivery systems.

Metabolites in MASLD

The regulation of cellular functions is widely recognized as a complex, multitiered process, where gene transcription represents the most upstream level, protein synthesis acts as an intermediary phase, and metabolic and biological activities serve as the downstream outcomes.^[78,79] Cells synthesize a diverse array of metabolites through metabolic processes.^[80] Metabolic diseases encompass a range of conditions characterized by disruptions in metabolic pathways, including energy metabolism, lipid processing, protein synthesis, carbohydrate utilization, hormonal regulation, and signaling molecules.^[81–83]

Lipids

Traditionally, when the triglyceride content in the liver surpasses 5% and there is no history of excessive alcohol consumption, the patient is generally deemed a prospective candidate for MASLD.^[84] The investigation of lipid metabolites follows a highly structured paradigm, where sphingolipid metabolism has long emerged as a facial point of interest. Dysregulation of bioactive sphingolipid metabolism in pathological states can contribute to disease development. Sphingolipids are primarily composed of ceramides, sphingomyelins, and sphingosine-1-phosphate (S1P). These molecules are pivotal structural elements of cellular membranes and act as second messengers, playing a key role in cellular signaling pathways associated with obesity, diabetes, cardiovascular diseases, and certain mental disorders [Table 2].^[85–87]

Table 2.

Several major classes of sphingolipids involved in the organ interactions associated with the pathogenesis of MASLD.

Metabolite	Source	Target organ	Mechanism	Phenotype	Reference
Sphingomyelin	Dietary (milk and dairy products)	Gut	Hydrogen bond↑ NPC1L1↓	Intestinal cholesterol absorption↑	[189,190]
Ceramide	De novo biosynthesis	Adipose tissue	UCP3↓ SOCS-3↑ β-oxidation capacity in brown adipose tissue ↓ Energy expenditure ↓	Weight gain↑	[191–193]
		Skeletal muscle	PKCζ↑ PP2A↑ IRS-1↓ Akt↓	Insulin resistance↑	[194]
	Gut	Liver	Kupffer cell↑ AMPKα1-SMPD3↑	Liver inflammation↑ Hepatic ceramide↑	[93]
S1P	Gut	Liver	S1P-S1PR1/3↑ PI3K-AKT↑ ERK↑	Intestinal epithelial cell apoptosis↓ Inflammatory cell↑ Macrophage M1 polarization↑	[195]
	Brain	Liver	S1P-S1PR2↑ ERK1/2↑	Blood–brain barrier integrity↓ Neurotoxic substances↑	[196,197]
Ganglioside	Gut	Liver	HDLs↓ LDLs↑	Liver sinusoidal endothelial fenestrae↓ Plasma TG levels↑	[198]

↑: Activation of pathways or upregulation of the expression. ↓: Inhibition of the pathway or downregulation in expression. AMPK: Adenosine monophosphate-activated protein kinase; ERK: Extracellular regulated protein kinase; HDL: High-density lipoprotein; IRS: Insulin receptor substrate; LDL: Low-density lipoprotein; MASLD: Metabolic dysfunction-associated steatotic liver disease; NPC1L1: Niemann-pick C1-like 1; PI3K: Phosphoinositide 3-kinase; PKCζ: Protein kinase C ζ; S1P: Sphingosine-1-phosphate; S1PR: Sphingosine‑1‑phosphate receptor; SMPD3: Sphingomyelin phosphodiesterase 3; SOCS: Suppressor of cytokine signaling; TG: Triglyceride; UCP: Uncoupling protein.

Cohort studies have demonstrated a positive correlation between serum ceramide concentrations with the pathophysiology of patients with MASH.^[88–90] Dysfunction of adipose tissue is a significant contributor to elevated systemic sphingolipid levels. During the simple steatosis stage, ceramides primarily originate from the de novo synthesis pathway. The substantial release of FFAs, particularly palmitic acid, from adipose tissue is transported to the liver, significantly exacerbating hepatic ceramide accumulation and lipotoxicity. In contrast, in the MASH stage, the main source of ceramides shifts to the sphingomyelin phosphodiesterase 3 (SMPD3)-mediated sphingomyelin hydrolysis pathway triggered by DNA damage signaling.^[91] Distinct from the hepatic intrinsic metabolic processes, another significant source of sphingolipids is Bacteroidetes, the dominant phylum of the gut microbiome.^[92] Researchers have found that the gut symbiont Bacteroides xylanisolvens can efficiently degrade nicotine and inhibit the intestinal epithelial AMPKα1-SMPD3 signaling pathway, leading to a reduction in the secretion of gut-derived ceramides, which in turn suppresses the progression of MASLD to MASH through the gut–liver axis.^[93] Intriguingly, investigators have also discovered that thyroid hormone receptor-beta agonist HSK31679 alleviates the characteristics of diet-induced steatohepatitis through the regulation of sphingolipid metabolism within intestinal microbiota.^[94] Far from being a passive bystander, the gut microbiota functions as a dynamic biosynthetic factory that actively shapes the host’s sphingolipid metabolism. This role is twofold: first, certain microbial taxa (especially Bacteroidetes) possess unique enzymatic pathways enabling de novo synthesis of bacterial-derived sphingolipids. Second, the microbiota modulates host-derived (dietary and endogenous) sphingolipids through enzymes such as microbial sphingomyelinases. The gut–liver axis is a critical pathway for sphingolipid-driven organ crosstalk in MASLD. Microbiota–sphingolipid interactions contribute to disease pathogenesis by activating invariant natural killer T (iNKT) cells, synergizing with lipopolysaccharide (LPS) to disrupt intestinal tight junctions (increasing gut permeability), and interfering with bile acid signaling through the farnesoid X receptor.^[95,96] Collectively, these mechanisms drive hepatic inflammation, metabolic dysfunction, and fibrosis progression.

As we mentioned earlier, the gut–liver axis plays a pivotal role in regulating sphingolipid metabolism, primarily through the influence of gut microbiota and dietary components. Certain gut bacteria, such as the Bacteroidetes phylum, directly synthesize ceramides, which can be absorbed into circulation and impact hepatic insulin signaling and lipid metabolism. Additionally, by metabolizing dietary sphingolipids and transforming primary bile acids into secondary ones, the gut microbiota generates farnesoid X receptor ligands that regulate host sphingolipid metabolism in the liver. Crucially, intestinal barrier dysfunction allows gut-derived LPS and proinflammatory cytokines to reach the liver, where they drive inflammation and promote ceramide generation, thereby creating a vicious cycle that exacerbates MASLD progression. Therefore, the gut–liver axis functions as a central hub integrating microbial, nutritional, and inflammatory signals to control sphingolipid homeostasis.

Mammalian cells have developed biologically active sphingolipid networks that complicate the investigation of the molecular and biochemical mechanisms underlying the regulation of sphingolipid metabolism. A thorough understanding of ceramides and other sphingolipids is crucial to uncover their roles in cellular regulation and biological processes, enabling the effective utilization of their physiological and pathological functions.

Amino acids

Amino acids are not only the fundamental building blocks of protein synthesis, but they also function as signaling molecules that regulate cellular growth, differentiation, and metabolism.^[97] Genetic abnormalities cause deficiencies in the activity of biochemical enzymes, disrupting the synthesis, metabolism, transport, and absorption of amino acid-related products in the organism.^[98,99]

Research indicates that plasma levels of various amino acids undergo significant alterations in patients with MASLD,^[100,101] particularly evidenced by an increase in plasma concentrations of branched-chain amino acids (BCAAs) and aromatic amino acids (AAAs). Adipose tissue and skeletal muscle are the primary source organs. In muscle tissues, branched-chain amino acid transaminase 2 (BCAT2) catalyzes the transamination of BCAAs into branched-chain keto acids (BCKAs).^[102] These BCKAs can then be released into the bloodstream and transported to the core reactive organ (the liver). The activity of the branched-chain alpha-keto acid dehydrogenase (BCKDH) complex exhibits its highest activity in the liver, further facilitating the metabolism of these BCKAs [Figure 1].^[103] The overexpression of BCKDH kinase in the liver then enhances the phosphorylation of adenosine triphosphate (ATP)-citrate lyase, thereby activating de novo lipogenesis (DNL).^[104] In 2022, scientists discovered that the knockout of BCAT2 in WAT reduces the levels of acetyl-CoA generated from the breakdown of BCAAs in adipocytes.^[105] This reduction leads to decreased acetylation of Prdm16, strengthening the Prdm16-PPARγ interaction and upregulating Ucp1 and other thermogenic genes, which promotes the browning of WAT in mouse lean tissue.^[106,107] Given the promoting effect of elevated BCAA levels on the progression of MASLD, researchers have been actively screening for BCAA inhibitors. They ultimately discovered that the angiotensin receptor antagonist telmisartan directly binds to BCAT2, inhibiting its activity and effectively mitigating obesity induced by a HFD.^[105]

Figure 1.

Roles of the three major macronutrient metabolisms in MASLD. The gut microbiota-derived sphingolipids contribute to MASLD-to-MASH progression via the gut–liver axis. Exercise-induced muscle BCKAs activate hepatic BCKDH, stimulating DNL while simultaneously promoting white adipose tissue browning. SCFAs enhance fatty acid synthesis but suppress lipogenic genes, ultimately attenuating hepatic steatosis. Methionine improves insulin sensitivity, and its dietary restriction prevents obesity induced by an HFD through the regulation of gut microbiota. Insulin and metabolite-activated ChREBP, synergistically upregulate lipogenic pathways, connecting glucose metabolism with hepatic fat production. BCAA: Branched-chain amino acid; BCKA: Branched-chain keto acid; BCKDH: Branched-chain alpha-keto acid dehydrogenase; ChREBP: Carbohydrate-responsive element binding protein; DNL: De novo lipogenesis; HFD: High-fat diet; MASH: Metabolic dysfunction-associated steatohepatitis; MASLD: Metabolic dysfunction-associated steatotic liver disease; MRD: Methionine-restricted diet; SCFA: Short-chain fatty acid; SREBP-1c: Sterol regulatory element-binding protein-1c; TMAO: Trimethylamine N-oxide.

Another representative metabolite of amino acids that exerts its beneficial effects through the gut–liver axis is short-chain fatty acids (SCFAs).^[108–110] SCFAs are the predominant metabolites produced by intestinal microbiota, primarily comprising acetate, propionate, and butyrate [Figure 1]. They modulate MASLD through multiple mechanisms. Acetate generated in the gut is transported to the liver through the portal vein, where it acts to activate the PPARα and inhibit the excessive phosphorylation of ERK1/2 signaling pathways.^[111,112] This negative feedback regulation can promote lipid catabolism and ameliorate hepatic steatosis. Propionate and butyrate stimulate the activation of AMPK and inhibit the activity of sterol regulatory element-binding protein-1c (SREBP-1c), thereby downregulating adipogenic gene expression.^{[108,109,113,114]} Conversely, the gut microbiota can metabolize choline and l-carnitine to produce trimethylamine N-oxide (TMAO),^[115,116] a deleterious metabolite, enhancing the severity of MASLD.

Recent studies also indicate that a methionine-restricted diet (MRD) can ameliorate the onset and progression of various age-related chronic diseases in multiple model organisms [Figure 1].^[117–119] Previous studies have suggested that MRD may enhance insulin sensitivity by increasing the expression of FGF21.^[120] Its most significant effect is the preferential reduction of WAT, particularly visceral fat, concomitantly promoting its browning into beige adipocytes to enhance thermogenesis. Additionally, MRD may potentiate insulin-dependent glucose uptake and utilization in the gastrocnemius muscle via the H19/IRS-1/Akt pathway.^[121] Furthermore, MRD can lead to an upregulation of adiponectin expression through the formation of hydrogen sulfide (H₂S) and the expression of disulfide bond A oxidoreductase-like protein.^[122] These changes collectively contribute to the observed improvements in metabolic health and energy metabolism in the skeletal muscle, mitigating the detrimental effects of insulin resistance in MASLD. Meanwhile, MRD modulates the gut microbiota composition and metabolic product levels in the colon, thereby regulating systemic metabolism and thus preventing HFD-induced obesity.^[123] Specifically, by restricting substrates for H₂S-producing microbiota, it suppresses the generation of this mitochondrial toxin, thus aiding in the maintenance of intestinal barrier integrity. Short-term trials have shown that an MRD has certain effects on metabolism, but its long-term impact on human health still requires further research.

Carbohydrates

Excessive intake of structurally imbalanced carbohydrates^[124–127] such as highly processed and refined sugars, high-fructose corn syrup, and high-glycemic index foods disrupts normal communication between multiple organs, forming a vicious cycle that promotes hepatic steatosis and inflammation. Patients with MASLD often experience insulin resistance, which indicates a diminished effectiveness of insulin in promoting glucose absorption and utilization, leading to elevated blood sugar levels.^[128,129] The comorbidity risk between type 2 diabetes and MASLD is estimated to be approximately 55–70% globally.^[130–131]

In a hyperglycemic state, pancreatic β-cells secrete insulin, which binds to hepatocyte insulin receptors, activating their tyrosine kinase activity. This leads to receptor substrate phosphorylation, activating the PI3K/Akt and mammalian target of rapamycin (mTOR) pathways and promoting SREBP-1c maturation and nuclear translocation [Figure 1].^[132] Persistently elevated insulin levels lead to the development of insulin resistance in adipose tissue. The normal inhibitory action of insulin on lipolysis is profoundly impaired due to insulin resistance, resulting in dysregulated and excessive hydrolysis of stored triglycerides into FFAs. These liberated FFAs are extensively taken up by the liver via the portal venous system, augmenting hepatic lipid load and directly contributing to hepatocellular steatosis. Furthermore, insulin released from the pancreas prompts liver cells to enhance the conversion of glucose.^[133] Cellular metabolic products can also directly bind to carbohydrate-responsive element binding protein (ChREBP), facilitating its dephosphorylation and activation, thereby enabling its translocation into the nucleus to stimulate the expression of downstream genes associated with lipogenic enzymes [Figure 1].^[134–136]

Notably, the gastrointestinal tract serves as a pivotal origin of proinflammatory and hepatotoxic signaling pathways to the liver. Excessive intake of fructose and sucrose, as well as certain highly processed foods, alters the gut microbiota composition.^[124,137] Significantly, fructose metabolism depletes cellular energy ATP in intestinal epithelial cells, disrupts tight junctions, and compromises the intestinal barrier, resulting in leaky gut.^[138,139] Such dysbiotic diets also promote the expansion of harmful bacteria. Dysbiosis and impaired intestinal barrier function facilitate the translocation of bacterial products, such as LPS and other pathogen-associated molecular patterns (PAMPs), to the liver via the portal vein. Hepatic immune cells, particularly Kupffer cells, recognize these PAMPs through their surface receptor Toll-like receptor 4 (TLR4), triggering the release of large quantities of proinflammatory cytokines (TNF-α, IL-6)^[140] that initiate and exacerbate hepatic inflammation and fibrosis [Figure 2].

Figure 2.

Gut–liver axis in MASLD pathophysiology. Under metabolic dysregulation, hepatic lipid homeostasis is disrupted, characterized by enhanced adipose lipolysis, upregulated hepatic lipogenesis and cholesterol synthesis. LPS binding to TLR4 on HSCs activates NF-κB signaling, driving ECM production and fibrosis. Intrahepatic lipid accumulation promotes oxidative stress and release of DAMPs, while gut-derived LPS and other PAMPs activate Kupffer cells, sustaining proinflammatory cytokine release (e.g., MCP1, TNFα, IL-6). Gut microbiota modulate bile acid metabolism and intestinal immunity via pathways such as PRRs-PAMP/DAMP recognition, the α4β7-MAdCAM-1 homing axis, and CCR9-CCL25 signaling. Dysbiosis impairs gut barrier function, accelerating MASLD progression. Conversely, intestinal L-cell-derived GLP-1 and FGF19 activate receptors on intraepithelial lymphocytes and hepatocytes, respectively, eliciting hepatoprotective signaling. α4β7: Alpha-4 beta-7 integrin; CCR9: C-C motif chemokine receptor 9; CCL25: C-C motif chemokine ligand 25; DAMP: Damage-associated molecular pattern; ECM: Extracellular matrix; FGF: Fibroblast growth factor; GLP-1: Glucagon-like peptide-1; MAdCAM-1: Mucosal vascular addressin cell adhesion molecule-1; MCP1: Monocyte chemoattractant protein-1; HSC: Hepatic stellate cell; LPS: Lipopolysaccharide; MASLD: Metabolic dysfunction-associated steatotic liver disease; NF-κB: Nuclear factor kappa-light-chain-enhancer of activated B cell; PAMP: Pathogen-associated molecular; PRP: Pattern recognition receptor; TLR4: Toll-like receptor 4.

The pathogenesis of fatty liver induced by fructose and HFDs is quite distinct. The initial metabolism of high-fructose diets occurs within the enterocytes of the small intestine and is independent of the gut microbiota.^[141] When excessive fructose intake surpasses the absorptive capacity of the small intestine, surplus fructose spills into the colon, where microbial fermentation produces toxic metabolites. The core mechanism of fructose metabolism involves bypassing rate-limiting steps to generate the intermediate metabolite glyceric acid,^[139,142] which activates ChREBP, upregulates lipogenic genes, and directly promotes DNL in the liver. Thus, enhancing intestinal fructose clearance through transgenic techniques or directly inhibiting ketohexokinase with pharmacological agents can confine fructose metabolism to the small intestine, alleviating fatty liver progression.

Neural Regulation in MASLD

In recent years, research on the regulation of metabolism and MASLD through the autonomous nervous system and neural mediators has been increasing rapidly. The gut–liver–brain axis has emerged as a dynamic communication connection that integrates signals from the central nervous system to the gastrointestinal and metabolic systems, impacting a broad range of physiological processes. The gut functions as the initial interface, influencing hepatic and central nervous system functions through microbiota, barrier integrity, and immunomodulation via portal circulation and neuroenteric pathways. The liver, serving as the metabolic hub, processes nutrients, toxins, and microbial metabolites derived from the gut, and modulates systemic signaling to the brain through metabolic reprogramming, immune responses, and bile acid synthesis. The brain, particularly the hypothalamus and limbic system, receives peripheral signals and reciprocally regulates gut motility, permeability, hepatic glucose and lipid metabolism, and immune-inflammatory responses via the nervous system (e.g., sympathetic nervous, vagus nerve), the hypothalamic-pituitary-adrenal (HPA) axis, and neuroendocrine pathways.

Chronic exposure to an HFD or high fructose intake leads to energy surplus. Subsequently, saturated fatty acids and excess nutrients cross the blood—brain barrier, activating microglia—the resident immune cells in the hypothalamus. This activation triggers the release of proinflammatory cytokines and initiates hypothalamic inflammation. Critically, inflammatory signaling pathways, such as inhibitor of kappa B kinase beta (IKKβ)/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and c-Jun N-terminal kinase (JNK), disrupt key leptin and insulin post-receptor signaling cascades, including the janus kinase 2 (JAK2)–signal transducer and activator of transcription 3 (STAT3) and insulin receptor substrate (IRS)–PI3K pathways, respectively.^[143,144] This impairment results in central leptin and insulin resistance, whereby hypothalamic neurons become unable to properly sense circulating levels of these hormones. Consequently, the hypothalamus elicits aberrant responses, such as enhancing the activity of appetite-stimulating neurons to promote hyperphagia, and reducing thermogenesis in BAT to lower basal metabolic rate.^[145] These maladaptive mechanisms collectively exacerbate obesity and peripheral insulin resistance, further promoting hepatic DNL and creating a self-perpetuating vicious cycle that drives the progression of MASLD.

Patients with MASLD often have increased sympathetic activity and decreased parasympathetic activity. The activation of the sympathetic nervous system (SNS) is usually associated with the stress response. In this state, the body prioritizes dealing with stress, thereby suppressing appetite and disrupting circadian rhythms [Figure 3].^[146] Concurrently, under stress conditions, the nervous system activates the HPA axis, leading to the release of cortisol and other hormones [Figure 3]. This process exacerbates hepatic inflammation and lipid accumulation, thereby worsening MASLD.^[147] Meanwhile, the overactivation of the SNS is usually accompanied by the release of neuropeptide Y (NPY)^[148] and NE^[149–151] as well as the development of catecholamine (CA) resistance [Figure 3]. Although NPY is widely recognized for its appetite-stimulating effects in the brain,^[152] its role in adipose tissue is more nuanced, indirectly influencing lipolysis and thermogenesis.^[148] CA resistance, in particular, is a pivotal factor in the progression of insulin resistance and metabolic disorders under conditions of nutritional excess.^[153] In this state, adipose tissue demonstrates a markedly diminished response to β-adrenergic stimulation, resulting in reduced cyclic adenosine monophosphate (cAMP) signaling and compromised lipolysis. Over time, these functional impairments can precipitate chronic inflammatory responses and fibrosis.

Figure 3.

Neural regulation in MASLD pathophysiology. Acute stress triggers rapid SNS activation, increasing cardiovascular activity and stimulating NPY-mediated lipolysis. Chronic stress engages the HPA axis, where CRH induces ACTH release, ultimately elevating cortisol levels that promote hepatic inflammation and lipid accumulation. ACTH: Adrenocorticotropic hormone; CA: Catecholamine; CRH: Corticotropin-releasing hormone; HPA: Hypothalamic-pituitary-adrenal; MASLD: Metabolic dysfunction-associated steatotic liver disease; NE: Norepinephrine; NPY: Neuropeptide Y; SNS: Sympathetic nervous system.

Extracellular Vesicles in MASLD

Extracellular vesicles (EVs) are small membrane-bound vesicles released by cells that contain a variety of bioactive molecules, including proteins, lipids, and nucleic acids.^[154–156] In recent years, EVs have emerged as critical mediators of intercellular communication via horizontal transfer of their cargo in various physiological and pathological processes, including MASLD.^[157–159] Released exosomes are not limited to the original tissue; they also circulate through the bloodstream. This section aims to enhance comprehension of the exosome-mediated interactions in the development of MASLD.

Adipocyte-derived exosomes

Recent years have witnessed an increasing focus on exosomal microRNA (miRNA)-mediated communication between metabolic organs. Exosomal miRNAs from white adipocytes have been found to contribute to MASLD development [Table 3]. Exosome-microRNA-122 (EXO-miR-122) is possible to directly combine the 3′UTR (untranslated region) of nicotinamide adenine dinucleotide (NAD)-dependent deacetylase (Sirt1) to inhibit its expression. In patients with MASLD, elevated miR-122 levels promote hepatic lipogenesis by targeting Sirt1 to inhibit the liver kinase B1 /AMPK pathway.^[160] Additionally, miR-122 suppresses FGF21 expression by targeting FGFR1, functioning as a metabolic silencer that exacerbates steatosis.^[161] The combination therapy of miR-22 inhibitors and obeticholic acid has proven to be effective in improving diet-induced MASLD.^[161] Exo-miR-199a-5p enhances liver lipid accumulation by regulating hepatic macrophage stimulating 1 (MST1) expression and influencing fatty acid metabolism.^[162] In addition, knockout of Exo-miR-141-3p in mice impairs insulin response and glucose uptake in AML12 cells.^[163] Additionally, elevated miR-155 in MASH patients exacerbates liver steatosis by regulating hepatic lipid synthesis and inflammation.^[164] Another example of the formation and release of exosomes between the liver and adipose tissue is regulated by geranylgeranyl diphosphate synthase (Ggpps). Increased Ggpps expression promotes the release of liver-derived exosomes (locally modulated by miRNA-let-7e-5p), which infiltrate adipose tissue through a Rab-GTPase-dependent pathway.^[165–168]

Table 3.

The mediators through which exosomes function.

Source	Name	Phenotype	Reference
WAT	Exo-miR-122↑	Insulin resistance↑	[160,199,200]
	Exo-miR-22↑	Lipid accumulation↑	[161]
	Exo-miR-199↑	MST1↑Mitochondrial damage↑	[162]
	Exo-miR-141-3p↓	Insulin resistance↑	[163]
	Exo-CD36↑	AMPKα1↓	[169]
	Exo-Akr1b7↑	ROS↑	[201]
BAT	Exo-miR-99b↓	Hepatic glucose tolerance↓	[202]
	Exo-miR-132-3p↓	Lipogenesis↑	[203]
Muscle tissue	Exo-miR-133a↓	Insulin resistance↑	[171]
	Exo-miR-133b↓	Insulin resistance↑	[171]
	Exo-miR-146a-5p↓	Lipogenesis↑ Fatty acid uptake↑	[172]
	Exo-miR-127-3p↓	Muscle stem cells differentiation↓	[173]
	Exo-miR-206-3p↓	Intermuscular fat infiltration↑	[174]
	Exo-miR-27a/b-3p↓	Intermuscular fat infiltration↑	[175]
Intestinal microbiota	Exo-LPS↑	Systemic inflammatory response↑	[181,204]
	Exo-mtDNA↑	Insulin resistance↑ cGAS/STING↑	[185,187,205]
Intestinal epithelia	Exo-PCs↑	Hepatic insulin response↓	[184]
Pancreatic β cell	Exo-miR-29s↑	Hepatic steatosis↑ Insulin resistance↑	[206]
	Exo-neutral ceramidase↓	ROS↑ Insulin resistance↑	[207]

↑: Activation of pathways or upregulation of the expression. ↓: Inhibition of the pathway or downregulation in expression. AMPK: Adenosine monophosphate-activated protein kinase; BAT: Brown adipose tissue; cGAS/STING: Cyclic GMP-AMP synthase/stimulator of interferon genes; LPS: Lipopolysaccharide; MASLD: Metabolic dysfunction-associated steatotic liver disease; MST1: Macrophage stimulating 1; PC: Phosphatidylcholines; ROS: Reactive oxygen species; WAT: White adipose tissue.

Surprisingly, protein-containing exosomes also contribute to this type of organ interaction, markedly differing from the function of adipokines. For example, the activation of AMPKα1 in WAT mitigates the development of MASLD induced by an HFD.^[169] This kind of activation mainly aims to inhibit hepatic endoplasmic reticulum (ER) stress, which finally alleviates hepatic steatosis.^[170] Meanwhile, the activation of AMPKα1 also plays a significant role in the diminished secretion of adipocyte-derived exosomal scavenger receptor class B (CD36), endocytosed by hepatocytes to facilitate hepatic lipid accumulation and inflammation driven by an HFD [Table 3].^[169]

Myocyte-derived exosomes

There are relatively few reports on the exosome-mediated communication originating from muscle tissue in the liver during MASLD status. After high-intensity interval training (HIIT), the muscle tissue releases exosomes that can deliver miR-133a and miR-133b, regulating insulin sensitivity in the liver of mice [Table 3].^[171] Exosomes derived from skeletal muscle deliver miR-146a-5p to adipose tissues, targeting the growth differentiation factor 5 (GDF5)-PPARγ signaling pathway to inhibit lipogenesis and fatty acid uptake.^[172] And recently, researchers have found that in the early stages of skeletal muscle damage, fibro-adipogenic progenitor cells can secrete exosomes rich in miR-127-3p, which promote the activation and differentiation of muscle stem cells.^[175] As muscle stem cells continue to proliferate and differentiate, the newly formed muscle cells will release exosomes abundant in miR-206-3p and miR-27a/b-3p, which inhibit the adipogenic differentiation of fibro-adipogenic progenitor cells, effectively reducing the infiltration of intermuscular fat [Table 3].^[173–175] Muscle mass is a crucial determinant of systemic insulin-mediated glucose metabolism and influences hepatic fat oxidation and energy homeostasis. The intramuscular fat content decreases with MASH improvement.^[176,177]

Exosomes can also serve as a novel approach for treating metabolic disorders such as MASLD and atherosclerosis. Researchers have encapsulated low-density lipoprotein receptor (LDLR) mRNA within exosomes, and by forcibly expressing LDLR in donor α mouse liver AML12 cells, they have reversed the phenotypes of fatty liver, elevated LDL cholesterol, and atherosclerosis [Table 3].^[178] In this approach, exosomes function as carriers to deliver nucleic acids to recipient cells and successfully translate functional therapeutic proteins within those cells, providing an effective means for drug delivery.

Gut-derived exosomes

Bacterial exosomes cross the intestinal barrier and are transported to various host tissues, with the liver being the primary site of EVs uptake.^[179] Fecal EVs from patients with MASH increase intestinal permeability by inhibiting nonmuscle myosin light chain kinase (nmMLCK) proteins and thereby reducing the expression of the tight junction protein zonula occludens-1 (ZO-1) in intestinal epithelial cells.^[180] Upon reaching the liver, they will increase intrahepatic vascular permeability, elevate inflammatory cytokines and chemokines in hepatic endothelial cells, and activate the LPS/TLR4 pathway [Table 2].^[181] Meanwhile, exosomes produced by the intestinal epithelia in the feces of mice on an HFD or type 2 diabetics are enriched in phosphatidylcholines (PCs). These PCs bind to and activate aromatic hydrocarbon receptors in the liver, suppressing genes linked to the insulin signaling pathway (IRS-2, PI3K/Akt), thereby causing insulin resistance.^[182–184] In addition, exosomes in the feces of HFD mice are also enriched with bacterial DNA (mainly mitochondrial DNA), which can trigger hepatocyte inflammation and hepatic stellate cell fibrosis activation via the activation of cyclic guanosine monophosphate (GMP)–adenosine monophosphate (AMP) synthase/stimulator of interferon genes (cGAS/STING).^[185–187]

Conclusions

The etiology of MASLD and its advancement is a multifaceted progression. Previously, the pathogenesis of MASLD was mainly focused on elucidating end-stage pathogenesis, such as liver fibrosis, insulin resistance, reactive oxygen species, mitochondrial autophagy, and ER stress. However, increasing evidence indicates that organ interactions are essential for the pathogenesis of MASLD. A variety of intricate mechanisms, viewed through the lenses of endocrinology, exosome biology, and metabolism, play a significant role in the development of MASLD and the progression of hepatic fibrosis. The disruption of some signaling pathways, resulting from genetic mutations or inflammatory processes, contributes to the onset of MASLD, obesity, or type 2 diabetes. Therefore, it is important to highlight that our investigation centered on molecules and exosomes, particularly the mechanisms by which, they alter within the bloodstream, their interactions with hepatic tissue, and the pathways leading to metabolic dysregulation.

In adipose–liver crosstalk, WATs mainly release FFAs and proinflammatory exosomes that promote hepatic lipid accumulation and inflammation, alongside reduced protective adiponectin. Circulating proteins can also promote the browning of WAT and increase energy expenditure. In muscle–liver crosstalk, the release of hepatotoxic metabolites including ceramides and BCAAs contributes to the development of hepatic insulin resistance. In gut–liver crosstalk, microbial dysbiosis leads to increased intestinal permeability (“leaky gut”), allowing LPS and other bacterial products to translocate to the liver via the portal vein, triggering inflammation. SCFAs, as beneficial metabolites, exert protective effects on MASLD by regulating gut microbiota balance, reducing hepatic lipid accumulation, and mitigating inflammation. In contrast, trimethylamine N-oxide, a harmful metabolite, exacerbates MASLD progression through promoting hepatic steatosis, triggering oxidative stress, and disrupting lipid metabolism homeostasis. In pancreas–liver crosstalk, hyperinsulinemia directly stimulates hepatic DNL and conversely, while glucagon reduces hepatic fat accumulation and improves insulin resistance. In brain–liver crosstalk, hypothalamic inflammation and resistance to leptin/insulin and its regulation of appetite and circadian rhythm disrupt whole-body energy balance, promoting overnutrition and fat deposition. On one hand, cardiac hormones such as atrial natriuretic peptide promote lipolysis in adipose tissue, thereby increasing the FFA to the liver. On the other hand, excessive levels of FGF23 from cardiac origin not only cross the blood–brain barrier to impair hypothalamic function but also exert direct proinflammatory effects on the liver.

In addition, exosomes influence distant organs and tissues by carrying bioactive cargo. Adipocytes release exosomes containing miRNAs (miR-122, miR-22, miR-199, miR-141-3p and miR-155), inflammatory cytokines (TNF-α, IL-6), and proteins (CD36, Akr1b7). Muscle-derived exosomes have been traditionally found to contain miRNAs (miR-146a-5p, miR-127-3p, miR-27a/b, and miR-206-3p). Exercise-induced muscle exosomes (miR-133a, miR-133b), in particular, have been shown to have a protective role in improving liver steatosis. Furthermore, the gut–liver axis is a critical component of MASLD pathogenesis. Dysbiosis of the gut microbiome, characterized by microbial imbalance, disrupts exosomal cargo and worsens liver inflammation and metabolic dysfunction. Gut-derived exosomes transport proteins (nmMLCK), lipids (PCs), and bacterial DNA (mainly mtDNA) that modulate liver function and inflammation.

Moving forward, a more systematic understanding of the pathogenic crosstalk between the liver and extrahepatic organs will not only help us shift the focus toward a holistic approach that considers the disease’s systemic nature but may also be used as a direction for the development of drug delivery systems.

Acknowledgments

We acknowledge all the researchers whose contributions to the field have laid the foundation for this review.

Funding

This work was supported by grants from National Natural Science Foundation of China (Nos. 82325007, 92468202) and the National Key Research and Development Program of China (No. 2021YFA1100502).