Irisin alleviates hepatic steatosis by activating the autophagic SIRT3 pathway

Disruption of hepatic lipid homeostasis leads to excessive hepatic triglyceride accumulation and the development of metabolic dysfunction-associated steatotic liver disease (MASLD). Autophagy, a critical process in liver lipid metabolism, is impaired in MASLD pathogenesis. Irisin, a skeletal muscle-driven myokine, regulates lipid metabolism, but its impact on hepatic lipid metabolism is not well understood. Here, we aimed to explore the role of irisin in hepatic steatosis and the underlying mechanisms involved.

A high-fat diet (HFD)-induced MASLD mouse model was used, and the recombinant irisin protein, herein referred to as "Irisin", was intraperitoneally administered for 4 weeks to evaluate the effects of irisin on hepatic lipid accumulation. Liver tissues were stained with Oil red O (ORO), and triglyceride (TG) and total cholesterol (TC) contents were measured in serum and liver homogenates. The expression of the autophagosome marker microtubule-associated protein 1 light chain 3 (LC3), the autophagy receptor protein sequestosome-1 (SQSTM1/p62), autophagy initiation complex unc-51-like kinase 1 (ULK1) and the lysosomal functional protein cathepsin B was measured via Western blotting, and the expression of the transcription factor EB (TFEB) was analyzed via immunofluorescence to explore autophagic changes. The effect of irisin on autophagic flux was further evaluated in palmitic acid-induced HepG2 cells by measuring autophagic degradation with chloroquine (CQ), and analyzing the colocalization of LC3 and lysosome-associated protein 1 (LAMP1). The possible mechanism was examined by measuring the expression of the autophagic sirtuin 3 (SIRT3) pathway and further validated using overexpression of SIRT3 with plasmid transfection or small interfering RNA (siRNA)-mediated knockdown. Student's t -test was utilized for statistical analysis.

Irisin significantly reduces hepatic lipid accumulation in mice fed with HFD, accompanied by enhanced hepatocyte autophagy and upregulation of the SIRT3 pathway. In HepG2 cells, Irisin attenuated palmitic acid-induced lipid accumulation, which was partially dependent on SIRT3 levels. Mechanistically, Irisin treatment upregulated SIRT3 and phosphorylated AMP-activated protein kinase (AMPK), inhibited mammalian target of rapamycin (mTOR) activity, promoted TFEB nucleus translocation, increased cathepsin B expression, enhanced autophagic degradation, and alleviated hepatic steatosis. No significant changes in phosphorylation of ULK1 in the hepatocytes were observed. However, when siRNA was used to knock down SIRT3 , the changes of those protein were partially reversed, and hepatic steatosis was further exacerbated.

Our findings highlight irisin as a potential therapeutic for hepatic steatosis by modulating autophagy and lipid metabolism, potentially providing a novel therapeutic target for the management of MASLD. Further research is needed to elucidate the underlying mechanisms and explore the potential clinical applications of this approach in the treatment of MASLD.