Metabolic Dysfunction-Associated Steatotic Liver Disease: From Pathogenesis to Current Therapeutic Options

Abstract

The epidemiological burden of liver steatosis associated with metabolic diseases is continuously growing worldwide and in all age classes. This condition generates possible progression of liver damage (i.e., inflammation, fibrosis, cirrhosis, hepatocellular carcinoma) but also independently increases the risk of cardio-metabolic diseases and cancer. In recent years, the terminological evolution from "nonalcoholic fatty liver disease" (NAFLD) to "metabolic dysfunction-associated fatty liver disease" (MAFLD) and, finally, "metabolic dysfunction-associated steatotic liver disease" (MASLD) has been paralleled by increased knowledge of mechanisms linking local (i.e., hepatic) and systemic pathogenic pathways. As a consequence, the need for an appropriate classification of individual phenotypes has been oriented to the investigation of innovative therapeutic tools. Besides the well-known role for lifestyle change, a number of pharmacological approaches have been explored, ranging from antidiabetic drugs to agonists acting on the gut-liver axis and at a systemic level (mainly farnesoid X receptor (FXR) agonists, PPAR agonists, thyroid hormone receptor agonists), anti-fibrotic and anti-inflammatory agents. The intrinsically complex pathophysiological history of MASLD makes the selection of a single effective treatment a major challenge, so far. In this evolving scenario, the cooperation between different stakeholders (including subjects at risk, health professionals, and pharmaceutical industries) could significantly improve the management of disease and the implementation of primary and secondary prevention measures. The high healthcare burden associated with MASLD makes the search for new, effective, and safe drugs a major pressing need, together with an accurate characterization of individual phenotypes. Recent and promising advances indicate that we may soon enter the era of precise and personalized therapy for MASLD/MASH.

Keywords: MAFLD; MASLD; NAFLD; NASH; clinical trials; drug therapy; fatty liver; liver fibrosis.

Figure 1

Genetic background and various metabolic dysfunctions contribute to the advancement of metabolic dysfunction-associated steatotic liver disease (MASLD) [17]. Besides the genetic predisposition, several metabolic dysfunctions, including visceral obesity and type 2 diabetes mellitus (T2DM), are primary risk factors for MASLD progression. Other factors can also contribute to the environmental background and include gut dysbiosis, excess dietary fructose, cholesterol, alcohol intake, environmental pollution, and food contaminants [17,18]. MASLD is characterized by intrahepatic triglyceride accumulation exceeding 5% and follows a complex continuum spectrum of disease [19]. In MAFL, the picture is characterized by steatosis alone, portal inflammation, or hepatocyte ballooning. In MASH, the typical findings include architectural distortion, cellular injury, and inflammation, hepatocyte ballooning degeneration and hepatic lobular inflammation, acidophil apoptotic bodies, mild chronic portal inflammation, perisinusoidal collagen deposition resulting in zone 3 accentuation in a “chicken wire” pattern, portal fibrosis without perisinusoidal or pericellular fibrosis, Mallory-Denk bodies (previously called Mallory bodies or Mallory hyaline), mega-mitochondria, PAS-diastase-resistant Kupffer cells, glycogenated (vacuolated) nuclei in periportal hepatocytes, lobular lipogranulomas, mild hepatic siderosis involving periportal hepatocytes or panacinar reticuloendothelial cells, and macronodular cirrhosis, which is an end-stage result of MASH [15]. About 22% of individuals progress from MASH to cirrhosis, and those with severe cirrhosis may develop hepatocellular carcinoma (HCC). F1: portal fibrosis without septa. F2: portal fibrosis with few septa. F3: numerous septa without cirrhosis. F4: cirrhosis.

Figure 2

Origin and metabolism of free fatty acids (FFAs) in the liver. FFAs are supplied to hepatocytes from three major sources. (A) About 60% of the total FFA pool derives from the uptake of circulating FFAs that originate from the lipolysis of triglycerides (TGs) in the adipose tissue. FFAs enter hepatocytes across specific transporters, such as (1) fatty acid translocase/cluster of designation 36 (FAT/CD36) transporter, (2) fatty acid binding protein (FABPpm), and (3) caveolin-1. (B) About 15% of the total FFA pool derives from dietary FFAs. In the intestinal lumen, within enterocytes, FFAs are incorporated into TGs of chylomicrons, following ingestion of fat, with the help of conjugated bile acid (BAs) micelles. Chylomicron remnants are taken up by specific receptors in the hepatocyte with a high affinity for ApoE. (C) About 25% of the total FFA pool originates within the hepatocytes from de novo lipogenesis (DNL), utilizing dietary carbohydrates. The hepatocellular FFA pool can undergo peroxisome ω-oxidation, mitochondrial β-oxidation, endoplasmic reticulum β-oxidation, or re-esterification with glycerol to form TGs. TGs can be stored in lipid droplets in small amounts (<5%) or exported into the circulation as very-low-density lipoproteins (VLDL) which are assembled in the endoplasmic reticulum. Right inlet: stars represent BAs. Adapted from Di Ciaula et al. [41].

Figure 3

Lipid metabolism in MASLD. (A) At the gut level, lipase facilitates the breakdown of triacylglycerol (TG) into monoacylglycerol (MAG) and free fatty acids (FFAs) which in the enterocytes are re-synthesized into TG through enzymatic processes mediated by mannoside acetylglucosaminyltransferase (MGAT2) and diglyceride acyltransferase (DGAT1). TGs are transferred to chylomicrons (CMs) via the microsomal triglyceride transfer protein (MTTP), and transported via the lymphatic vessels to the liver, where remnants of CMs are absorbed post-lipolysis [80,81]. (B) In the adipocyte, insulin plays a pivotal role in lipid storage by suppressing lipolysis through the inhibition of adipose triglyceride lipase (ATGL), phosphodiesterase 3B (PDE3B), and hormone-sensitive lipase (HSL) regulated by protein kinase A (PKA) and perilipins (PLINs). However, in insulin-resistant states such as obesity or type 2 diabetes mellitus (T2DM), reduced insulin sensitivity fosters heightened lipolysis, resulting in an increased flux of FFAs to the liver. (C) In the liver, various key enzymes govern the de novo lipogenesis of saturated fatty acids (SFA), monosaturated fatty acids (MUFA), diacylglycerol (DAG), TG, and include including acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), stearoyl-CoA desaturase (SCD1), and DGAT2. Another important pathway includes the transformation of glucose to pyruvate, which then enters the mitochondrial tricarboxylic acid cycle (TCA), with the production of citrate [82].

Figure 4

The interplay between glucose, fructose, insulin, and de novo lipogenesis (DNL) in MASLD. Increased glucose transport into the hepatocyte increases the glycolysis and pyruvate synthesis which contributes to the tricarboxylic acid (TCA) cycle. Increased pyruvate can be converted either to lactate or oxaloacetate via anaplerosis [127]. Conversion of pyruvate to lactate inhibits the histone deacetylase (HDAC) activity, thereby stimulating DNL. Production of oxalacetate is associated with increased gluconeogenesis, glucose production, and DNL. Both oxaloacetate and lactate are enhanced in MASLD [41,128]. Fructose enters the hepatocyte and is rapidly phosphorylated to fructose-1-phosphate (F-1-P) by the ketohexokinase (KHK). Adenosine triphosphate (ATP) hydrolysis to adenosine diphosphate (ADP), to adenosine monophosphate (AMP) and inosine monophosphate (IMP) provides increased uric acid levels which further contributes to DNL. Increased insulin upregulates the liver carbohydrate-responsive element-binding protein (ChREBP) and the sterol regulatory element-binding protein 1 (SREBP-1), which increases DNL with free fatty acids (FFAs) storage as triglycerides (TGs). Insulin also reduces very-low-density lipoprotein (VLDL) production through downregulation of the synthesis of the microsomal triglyceride transfer protein (MTTP) and apolipoprotein B (ApoB) [41]. Production of reactive oxygen species (ROS) which promote inflammation and hepatocellular injury can depend on increased glycolysis and FFA oxidation with acetyl-CoA abundance and enhanced activity of the TCA cycle. At the same time, ketogenesis is reduced. Moreover, the activity of the mitochondrial respiratory chain (MRC) is reduced, increasing the ROS generation. Uncoupling protein (UCP2) expression increases in MASLD. This step is associated with impaired efficiency of ATP synthesis and decreased ATP content [127].

Figure 5

The interaction between bile acid (BAs) and gut microbiota is shown in the liver, the gallbladder, the terminal ileum, and the colon. Derangement of pathways at several levels can play a role in MASLD (see text for details). (A) Starting from cholesterol in the hepatocyte, the classical pathways use the oxysterol 7α-hydroxylase (CYP7A1), and CYP8B1, resulting in 7α-OH-4-cholesten-3-one (C4) and then “primary” BAs (I BAs) cholic acid (CA) and chenodeoxycholic acid (CDCA). The alternative pathway relies on CYP27A1 and CYP7B1, resulting in small amounts of CDCA [169,170]. Primary BAs are promptly conjugated (symbol O) with taurine and glycine, to increase solubility in bile [102]. The transport of BAs from the hepatocyte includes several pathways. Approximately 5% of BAs are transported to the systemic circulation via the multidrug resistance-associated protein 3 (MRP3), MRP4, and the organic solute transporter (OSTα/β). Basolateral import of BAs is mediated by sodium taurocholate co-transporting polypeptide (NTCP) (Na+-dependent) and organic-anion-transporting polypeptide (OATP) isoforms (Na+-independent). Intracellular BAs contribute to the negative feedback regulation of BA synthesis via the activation of farnesoid X receptor (FXR)-retinoid X receptor (RXR)-dependent pathways. These pathways increase the small heterodimer partner (SHP) expression and inhibit the hepatocyte nuclear factor 4α (HNF4 α) and nuclear receptor liver receptor homolog-1 (LRH1) which, in turn, leads to decreased activity of CYP7A1 and CYP8B1 [171]. Activation of the FXR-SHP pathway also inhibits de novo lipogenesis (DNL), promotes peroxisome proliferator-activated receptor α (PPARα) β-oxidation, and stimulates very-low-density lipoprotein (VLDL) production and TG export [144,145]. The nuclear thyroid hormone receptor β (THRβ) also contributes to DNL and works in concert with the above-mentioned nuclear receptor pathways. Conjugated BAs are secreted in bile canaliculus by the bile salt export pump (BSEP) and multidrug resistance-associated protein 2 (MRP2), and aggregate as micelles and vesicles with secreted cholesterol and phospholipids. (B) Bile enters the gallbladder to be temporarily stored and concentrated during fasting. Upon consumption of a fat-enriched meal, the cholecystokinin release prompts gallbladder contraction and secretion of bile/BAs into the duodenum. (C) In the terminal ileum, approximately 95% of BAs undergo reabsorption by enterocytes through the apical sodium-dependent bile salt transporter (ASBT), transported via the ileal bile acid-binding protein (I-BABP), and subsequently excreted into the portal vein via OSTα/β [172]. In humans, the BA-induced activation of ileal FXR has several consequences, including the activation of SHP with inhibition of ASBT, the RXR-mediated activation of OSTα/β and the fibroblast growth factor 19 (FGF19) production and secretion into the portal blood. Upon reaching the liver, FGF19 binds the liver FGFR4/β-klotho receptor with effects on FXR, with the above-mentioned effects on BA synthesis and DNL [173]. In the ileum, the activation of the membrane BAs receptor G-protein coupled BA receptor-1 (GPBAR-1) increases the cyclic adenosine monophosphate (cAMP) and increases the secretion of glucagon-like peptide-1 (GLP-1), GLP-2 and peptide YY (PYY) leading to a number of systemic metabolic effects. (D) In the colon, small amounts of primary BAs undergo bacterial biotransformation to unconjugated secondary BAs (II BAs) deoxycholic acid (DCA), and lithocholic acid (LCA) which are passively transported back to the liver. Under healthy conditions, undigestible dietary fibers represent the microbiota accessible carbohydrates (MACs). These are fermented by the local microbiota and produce short-chain fatty acids (SCFAs), mainly butyrate, acetate, and propionate. SCFAs are actively transported in the colonocyte to produce local beneficial effects, including anaerobic conditions maintenance through β-oxidation, decreased nitrate production, and balanced metabolic homeostasis in conjunction with peroxisome proliferator-activated receptor gamma (PPARγ). SCFAs also contribute to metabolic stability through the secretion of GLP-1, fasting-induced adipose factor (FIAF), and Yin-Yang 1 (YY1) [174,175]. These mechanisms are highly impaired at the onset and progression of MASLD and gut dysbiosis (red pathways).

Figure 6

Potential progression of metabolic dysfunction-associated steatotic liver disease (MASLD) phenotypes in accord with extrahepatic disorders and therapeutic approaches. (A) Starting from the healthy liver, the hepatic burden of MASLD consists of simple steatosis (metabolic dysfunction-associated steatotic liver, MASL), necro-inflammatory status (metabolic dysfunction-associated steatohepatitis, MASH), fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Fibrosis stages F1–F4 are reported. (B) The extrahepatic disorders are depicted, with the main concerns as cardiovascular disease, extrahepatic tumors, and kidney disease. As soon as MASH is demonstrated, the hepatic burden of the disease moves forward and becomes a main concern because of the potential progression to advanced liver disease, complications, and end-stage disease. (C) The mainstay of therapeutic approaches whenever possible consists of early prevention (lifestyle modification) of both hepatic and extrahepatic disorders. At a later stage, the use of precision medicine consists of personalized drugs targeting metabolism, body weight, and risk of malignancy. With MASH, further therapeutic approaches are aimed at the resolution of MASH, prevention of fibrosis and progression, or reduction of fibrosis. With cirrhosis and HCC, specific chemotherapy, locoregional treatments, liver resection and liver transplant (OLT) must be taken into account.

Figure 7

Potential therapeutic approaches in MASLD. Due to the complex interplay between pathogenesis, pathways, and organs involved, several options are being tested. (A) lifestyle including a healthy, balanced diet and regular physical activity improve insulin sensitivity and liver steatosis. (B) Whenever indicated, metformin can bring beneficial effects. (C) Bariatric surgery can play a role in the subgroup of severe obesity and increased cardiovascular risk. (D) With respect to drugs, effects can target the liver, several organs, and the microbiota, acting on specific pathways (see text for details). Green arrows indicate activation; red lines with dots indicate inhibition; grey arrows indicate interplay between organs. Abbreviations: Acc1/2, acetyl-CoA carboxylase 1, 2; AMPK, AMP-activated protein kinase; ASK1, apoptosis signal-regulating kinase-1; BAs, bile acids; FGF, fibroblast growth factor; FXR, farnesoid X receptor; GLP-1, glucagon-like peptide-1; mGPD, mitochondrial glycerophosphate dehydrogenase; mitochondrial pyruvate carrier (mPC); PPAR, peroxisome proliferator-activated receptor α/β/γ; ROS, reactive oxygen species; SGLT1/2, sodium-dependent glucose transporters 1,2; SCD1, stearoyl-CoA desaturase-1; THRβ, thyroid hormone receptor β.

Figure 8

The process showing the change of nomenclature for liver steatosis (from 1980 to 2023), in relation to the progression of disease and interplay between several professionals and stakeholders. The terminological evolution (i.e., from NAFLD to MAFLD and, finally, to MASLD) has been paralleled by a progressive growth of knowledge about the combined effects of diverse pathogenic factors (i.e., genetic and external factors) in the onset and progression of steatotic liver disease. This evidence, in particular, underscores the association of fat overstorage in the liver not only with the possible progression of hepatic damage but also with systemic metabolic disturbances and cardiovascular risk factors, and the need for a multi-disciplinary and transversal approach to this disease. The cooperation between different stakeholders (including subjects at risk, health professionals, and pharmaceutical industries) could significantly improve either the management of disease and the implementation of primary and secondary prevention measures.