Targeting G-protein-coupled receptors and gut microbiota: Ge-Lian Qi-Shen decoction elevates GLP-1 to combat non-alcoholic fatty liver disease

Background: Non-alcoholic fatty liver disease (NAFLD), often accompanied by Insulin resistance, obesity, and hyperlipidemia, is a challenging metabolic disorder to treat. Ge-Lian Qi-Shen Decoction, a traditional Chinese herbal formula, has been clinically used to alleviate symptoms associated with NAFLD, but its underlying mechanisms remain unclear.

Methods: A NAFLD model was established in C57BL/6J mice using a high-fat diet (HFD). The effects of 4-week GQD intervention at different doses on NAFLD-related symptoms were assessed using biochemical analyses, pathological sections, and oral glucose tolerance tests. ELISA and qPCR were employed to investigate the impact of GQD on serum GLP-1 levels and intestinal Gcg gene expression in NAFLD mice. The direct stimulatory effects of GQD on GLP-1 secretion were examined in NCI-H716 cells and HFD-fed mice. UPLC-MS/MS was used to analyze the composition of ileal contents in GQD-treated mice, and the regulatory effects of 24 identified compounds on GLP-1 secretion were evaluated. Additionally, 16S rDNA Sequencing, metabolomics and fecal microbiota transplantation were utilized to explore the role of gut microbiota in GQD's anti-NAFLD effect.

Results: GQD improved HFD-induced hepatic steatosis, impaired glucose tolerance, and elevated blood lipid levels in a dose-dependent manner. It increased serum GLP-1 levels, reduced energy intake, and enhanced glucose tolerance in mice. A single dose of GQD directly elevated serum GLP-1 levels in HFD-fed mice and improved glucose tolerance in a GLP-1-dependent manner. In NCI-H716 cells, GQD promoted intracellular calcium influx and GLP-1 release by activating two G-protein-coupled receptors (GPCRs): bitter taste receptors and TGR5. Compounds such as berberine, coptisine, nuciferine, liensinine, higenamine, aurantio-obtusin, and obtusifolin in GQD activated bitter taste receptors, while maslinic acid and cycloastragenol activated TGR5, facilitating GLP-1 secretion. Furthermore, GQD gavage increased the levels of Muribaculaceae and Akkermansia in mouse feces, leading to elevated concentrations of short-chain fatty acids (SCFAs) such as acetate, propionate, butyrate, and valerate. These SCFAs potentially activated fatty acid-related GPCRs, such as GPR41, in the colon, thereby enhancing colonic Gcg expression. FMT experiment showed that gut microbiota can partially mediate the effect of GQD in increasing GLP-1 levels thus alleviating NAFLD.

Conclusion: Some Alkaloids, Anthraquinones, and triterpenoids in GQD can activate GPCRs, including bitter taste receptors and TGR5, in intestinal endocrine cells, promoting GLP-1 secretion. Simultaneously, GQD regulates gut microbiota composition and metabolism, increasing SCFA levels and Gcg gene expression, leading to sustained elevation of GLP-1 levels. These combined effects contribute to the alleviation of NAFLD symptoms.