Gut microbial metabolites in MASLD: Implications of mitochondrial dysfunction in the pathogenesis and treatment

Abstract

With an increasing prevalence, metabolic dysfunction-associated steatotic liver disease (MASLD) has become a major global health problem. MASLD is well-known as a multifactorial disease. Mitochondrial dysfunction and alterations in the gut bacteria are 2 vital events in MASLD. Recent studies have highlighted the cross-talk between microbiota and mitochondria, and mitochondria are recognized as pivotal targets of the gut microbiota to modulate the host's physiological state. Mitochondrial dysfunction plays a vital role in MASLD and is associated with multiple pathological changes, including hepatocyte steatosis, oxidative stress, inflammation, and fibrosis. Metabolites are crucial mediators of the gut microbiota that influence extraintestinal organs. Additionally, regulation of the composition of gut bacteria may serve as a promising therapeutic strategy for MASLD. This study reviewed the potential roles of several common metabolites in MASLD, emphasizing their impact on mitochondrial function. Finally, we discuss the current treatments for MASLD, including probiotics, prebiotics, antibiotics, and fecal microbiota transplantation. These methods concentrate on restoring the gut microbiota to promote host health.

FIGURE 1

(A) MtFAO is a vital way for the liver to reduce fat accumulation. In the classical pathway of mtFAO, FFAs are converted to acyl-CoA in the cytoplasm, which can be gradually metabolized to water, CO₂, and a large number of ATP by way of β-oxidation, tricarboxylic acid cycle and OXPHOS in mitochondria. However, mtFAO function is defective in subjects with MASLD and animal models, which is manifested as decreased expression of β-oxidation and OXPHOS-related genes and protein, including CoA, CPTI, CPT2, and β-HAD, and mitochondrial complex (Ⅰ, Ⅱ, Ⅲ, Ⅳ, and Ⅴ). Deficient ETC function could result in the production of ROS, which could further impair mitochondrial function. (B) Excessive ROS can cause OS and play an important role in mitochondrial dysfunction, inflammation, and lipid peroxidation. ROS will generate oxidative damage to mtDNA and be associated with decreased MMP and increased MP. Increased MP enables mtDAMP release into the cytosol, promoting an inflammatory response. Apparently, ROS can directly activate inflammatory pathways, promoting MASLD. Additionally, OS can contribute to insulin resistance, facilitating fat accumulation in the liver by promoting lipolysis, DNL, and hepatic uptake of FFAs. Besides, elevated ROS levels will lead to increases in LP, which can produce toxic byproducts (such as 4-HNE and MDA), thereby inducing mitochondrial dysfunction, more ROS production and inflammation. In conclusion, mitochondrial dysfunction can lead to increased mitochondrial production of ROS, which causes a vicious cycle of mitochondrial dysfunction, leading to more ROS production. Abbreviations: β-HA, β-hydroxyacyl-CoA-dehydrogenase; CPT1, carnitine palmitoyl transferase 1; CPT2, carnitine palmitoyl transferase 2; DNL, de novo lipogenesis; ETC, electron transport chain; FFA, free fatty acid; 4-HNE, 4-hydroxynonenal; LP, lipid peroxidation; MDA, malondialdehyde; MASLD, metabolic dysfunction–associated steatotic liver disease; MMP, mitochondrial membrane potential; MP, mitochondrial permeability; mtDAMP, mitochondrial damage-associated molecular patterns; mtFAO, mitochondrial fatty acid oxidation; OS, oxidative stress; OXPHOS, oxidative phosphorylation; ROS, reactive oxygen species; TCA cycle, tricarboxylic acid cycle.

FIGURE 2

(A) The roles of mitophagy, mitochondrial dynamics, and mitochondrial biogenesis play in MASLD progression. (B) The gut-liver axis is altered in the development of MASLD. When patients with MASLD were compared with healthy subjects, the differences in BA profiles persisted, which can be mainly reflected in the production of both primary and secondary bile acids. Meanwhile, according to the changes in the content of different BA species, it can be inferred that FXR signaling is inhibited in MASLD. Additionally, intestinal epithelial barrier damage is also one of the key factors promoting MASLD, which can result in an increased risk of metabolites of bacterial origin and microbial translocation from the gut to the periphery. (C) The intestinal flora is rich in metabolites, including both beneficial and unfavorable components. From the bacteria and mitochondria interaction perspective, LPS, EE, and TMAO can exert dramatic detrimental effects on mitochondrial function, including inhibiting mitochondrial respiration, increasing oxidative stress, and impairing mitochondrial quality control. Conversely, mitochondrial dysfunction was beneficially impacted by I3A, NAM, and SCFAs. However, their expression was reduced in patients with MASLD. Notably, decreased mitochondrial respiratory function in the gut epithelium affects the structure of the gut microbiota, generating an increase in harmful metabolites, including TMAO. (D) Gut microbiota-involved biosynthesis and metabolism of bile acids. Primary BAs are synthesized in hepatocytes from cholesterolviaclassical or alternative pathways. In the classical pathway, cholesterol is initiated by CYP7A1 and converted into 2 primary BAs, CA, and CDCA. The alternative pathway is initiated by CYP27A1 and forms mostly CDCA. They are further conjugated with taurine (in mice) or glycine (in humans) and transformed into conjugated BAs (eg, TCA, GCA, TCDCA, and GCDCA). Part of BAs is converted to secondary bile acids(eg, UDCA, DCA, LCA, TUDCA, TDCA, TLCA, GUDCA, GDCA, and GLCA) by various organisms by intestinal flora. FXR is activated by unconjugated BAs, thus stimulating the expression of SHP, which further binds to LRH1 to inhibit CYP7A1 transcription. In addition, FGF19/FGF15 released by intestinal epithelial cells also induces JNK1/2 and ERK1/2 signaling by binding to FGFR4/β-klotho complexes. Abbreviations: BA, bile acid; DCA, deoxycholic acid; EE, endogenous ethanol; FGFR4, FGF receptor 4; FXR, farnesoid X-activated receptor; GCA, glycocholic acid; GCDCA, glycochenodeoxycholic acid; I3A, indol-3-acetic acid; LCA, lithocholic acid; LPS, lipopolysaccharide; LRH1, liver receptor homolog 1; MASLD, metabolic dysfunction–associated steatotic liver disease; NAM, nicotinamide; SCFAs, short-chain fatty acids; SHP, small heterodimer partner; TCA, taurocholic acid; TCDCA, taurochenodeoxycholic acid; TMAO, trimethylamine N-oxide; UDCA, ursodeoxycholic acid.

FIGURE 3

Schematic diagram of liver immune changes in MASLD. After excessive accumulation of FFAs, hepatic steatosis causes the release of DAMPs and other substances and stimulates TLRs, which enables hepatocytes, KCS, and other pro-inflammatory cells to activate downstream MAPK and NF-κB signaling pathways, thereby activating NLRP3 and ultimately promotes the release of pro-inflammatory cytokines (including IL-β, TNF-α, IL-6, IL-8, TGF-β, etc.). Immune activation and hepatocyte injury will affect HSC activation by upregulating pro-fibrotic and pro-inflammatory cytokines such as TGF-β1 and IL-β, further aggravating MASLD. In addition, activated DC coordinates the T-cell immune response. Moreover, the increased level of ROS might contribute to the release of mtDAMP release, which can activate KCs and upregulate the NLRP3 expression. Abbreviations: BA, bile acid; DC, dendritic cell; FFA, free fatty acid; MASLD, metabolic dysfunction–associated steatotic liver disease; TLR, toll-like receptors.

FIGURE 4

Microbiota-derived metabolites impact mitochondrial dysfunction in MASLD. Gut microbiota and its metabolites affect the structure and function of hepatocyte mitochondria through enterohepatic circulation, and the interaction between microbiota and mitochondria completes the progression of gut microbiota to promote MASLD. Abbreviations: ADH, alcohol dehydrogenase; AhR, aryl hydrocarbon receptor; ALT, alanine aminotransferase; AST, aspartate aminotransferase; AMPK, AMP-activated protein kinase; BA, bile acids; CPT1, carnitine palmitoyl transferase Ⅰ; CPT2, carnitine palmitoyl transferase Ⅱ; Drp1, dynamin-related protein 1; EE, endogenous ethanol; Fndc5, Fibronectin type III domain-containing protein 5; FXR, farnesoid X-activated receptor; GSH, glutathione; I3A, indol-3-acetic acid; LPS, lipopolysaccharide; MASLD, metabolic dysfunction–associated steatotic liver disease; MCJ, methylation-controlled J protein; MDA, malondialdehyde; NAM, nicotinamide; OXPHOS, oxidative phosphorylation; PGC-1α, proliferator-activated receptorγ coactivator-1α; SCFAs, short-chain fatty acids; SDHB, succinate dehydrogenase complex subunit B; SH-SY5Y, human neuroblastoma cell line; SIRTs, sirtuins; SOD2, superoxide dismutase; TG, triglycerides; TLR4, toll-like receptors 4; TMAO, trimethylamine N-oxide.