Gut microbiota-derived indole compounds attenuate metabolic dysfunction-associated steatotic liver disease by improving fat metabolism and inflammation.

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most common chronic liver disease, and its prevalence has increased worldwide in recent years. Additionally, there is a close relationship between MASLD and gut microbiota-derived metabolites. However, the mechanisms of MASLD and its metabolites are still unclear. We demonstrated decreased indole-3-propionic acid (IPA) and indole-3-acetic acid (IAA) in the feces of patients with hepatic steatosis compared to healthy controls. Here, IPA and IAA administration ameliorated hepatic steatosis and inflammation in an animal model of WD-induced MASLD by suppressing the NF-κB signaling pathway through a reduction in endotoxin levels and inactivation of macrophages. Bifidobacterium bifidum metabolizes tryptophan to produce IAA, and B. bifidum effectively prevents hepatic steatosis and inflammation through the production of IAA. Our study demonstrates that IPA and IAA derived from the gut microbiota have novel preventive or therapeutic potential for MASLD treatment.

De Novo Sphingolipid Biosynthesis in Atherosclerosis.

Atherosclerosis is the formation of fibrofatty lesions in the arterial wall, and this inflammatory state of the artery is the main cause of advanced pathological processes, including myocardial infarction and stroke. Dyslipidemic conditions with excess cholesterol accumulate within the arterial vessel wall and initiate atherogenic processes. Following vascular reaction and lipid accumulation, the vascular wall gradually thickens. Together with the occurrence of local inflammation, early atherosclerotic lesions lead to advanced pathophysiological events, plaque rupture, and thrombosis. Ceramide and sphingomyelin have emerged as major risk factors for atherosclerosis and coronary artery disease. Currently, the clinical association between de novo sphingolipid biosynthesis and coronary artery disease has been established. Furthermore, therapeutic strategies to modulate this pathway, especially those involving serine palmitoyltransferase and sphingomyelin synthase, against atherosclerosis, cancer, type 2 diabetes, and non-alcoholic fatty liver disease are actively under development. In this chapter, we focus on the relationship between de novo sphingolipid biosynthesis and coronary artery disease.

Upregulation of the serine palmitoyltransferase subunit SPTLC2 by endoplasmic reticulum stress inhibits the hepatic insulin response.

Endoplasmic reticulum (ER) stress is induced by various conditions, such as inflammation and the presence of excess nutrients. Abnormal accumulation of unfolded proteins leads to the activation of a collective signaling cascade, termed the unfolded protein response (UPR). ER stress is reported to perturb hepatic insulin response metabolism while promoting insulin resistance. Here, we report that ER stress regulates the de novo biosynthesis of sphingolipids via the activation of serine palmitoyltransferase (SPT), a rate-limiting enzyme involved in the de novo biosynthesis of ceramides. We found that the expression levels of Sptlc1 and Sptlc2, the major SPT subunits, were upregulated and that the cellular concentrations of ceramide and dihydroceramide were elevated by acute ER stress inducers in primary hepatocytes and HepG2 cells. Sptlc2 was upregulated and ceramide levels were elevated by tunicamycin in the livers of C57BL/6J wild-type mice. Analysis of the Sptlc2 promoter demonstrated that the transcriptional activation of Sptlc2 was mediated by the spliced form of X-box binding protein 1 (sXBP1). Liver-specific Sptlc2 transgenic mice exhibited increased ceramide levels in the liver and elevated fasting glucose levels. The insulin response was reduced by the inhibition of the phosphorylation of insulin receptor ß (IRß). Collectively, these results demonstrate that ER stress induces activation of the de novo biosynthesis of ceramide and contributes to the progression of hepatic insulin resistance via the reduced phosphorylation of IRß in hepatocytes.

Annona muricata leaf extract attenuates hepatic lipogenesis and adipogenesis.

Annona muricata (graviola) is a medicinal plant that can be used to alleviate chronic human diseases by providing antioxidants and inducing immunomodulation. In this study, we found that treatment of AML12 hepatocytes with steam (SGE) and ethanol (EGE) extracts of graviola leaf downregulated the expression of fatty acid (FA) oxidation genes, including ACOX1, CPT1, and PPARα, with no change in the expression of FA synthesis genes. However, whereas EGE inhibited the differentiation and lipid accumulation of 3T3-L1 adipocytes and downregulated FA synthesis genes, no similar changes were observed in response to treatment with SGE. In an in vivo experiment using mice fed a high-fat diet (HFD), body weight was reduced in response to treatment with EGE, which also dose-dependently alleviated liver hepatocyte ballooning induced by the consumption of a HFD. However, genes involved in FA oxidation and the secretion of very low density lipoprotein (VLDL) were downregulated. We also found that the size of adipocytes was reduced in response to EGE treatment, and that there was a downregulated expression of genes involved in adipogenesis and FA synthesis. Furthermore, we detected increases in the levels of cholesterol in the plasma, whereas ALT activity was reduced. Collectively, these results indicates that EGE inhibits lipid influx into the liver and adipogenesis in adipose tissues. These bioactive properties of EGE indicate its potential as a natural ingredient that can be used to prevent obesity.