Protein phosphatase 6 regulates metabolic dysfunction-associated steatohepatitis via the mTORC1 pathway

Background & Aims: Metabolic dysfunction-associated steatohepatitis (MASH) is a serious chronic liver disease for which therapeutic options are limited. Although fibroblast growth factor 21 (FGF21) analogs have shown therapeutic promise for MASH in multiple preclinical and clinical studies, their underlying mechanisms of action remain elusive. Methods: Liver-specific PPP6C and βKlotho knockout mice and their wild-type littermates were fed an AMLN (Amylin liver NASH) diet for 16 weeks or a CDA-HFD (choline-deficient, L-amino acid-defined, high-fat diet) for 8 weeks, followed by daily subcutaneous injection of recombinant FGF21 (0.5 mg/kg) or vehicle for 4 weeks. A mass spectrometry assay identified PPP6C as a βKlotho-binding protein. An in vitro phosphatase assay was used to evaluate the effects of FGF21 on PPP6C activity. PPP6C expression was also analyzed in human samples from patients with MASH. Results: We identified serine and threonine phosphatase PPP6C as a direct target of FGF21. Hepatic PPP6C deficiency accelerates MASH progression in mice fed an AMLN diet or CDA-HFD, which blocks the effect of FGF21 on MASH. Mechanistically, PPP6C is sufficient to interact with the coreceptor βKlotho upon FGF21 treatment and directly dephosphorylates tuberous sclerosis complex 2 (TSC2) at Ser939 and Thr1462, thereby inhibiting mTORC1 and promoting nuclear entry of TFE3 and Lipin1. In the livers of patients with MASH, expression levels of PPP6C are decreased whereas TSC2 phosphorylation is elevated. Conclusions: PPP6C acts as a fundamental downstream mediator essential for FGF21 signaling in hepatocytes and targeting PPP6C by FGF21 may offer therapeutic potential for treating MASH in humans. Impact and implications: Metabolic dysfunction-associated steatohepatitis (MASH) is a severe chronic liver disease that increases susceptibility to more severe cirrhosis and hepatocellular carcinoma. Effective therapeutic strategies for MASH remain an unmet need. Herein, we have identified serine and threonine protein phosphatase PPP6C as a negative regulator of MASH progression in mice and humans. PPP6C activity is increased by FGF21 via an autocrine effect mediated by FGFRs/βKlotho in hepatocytes. Pharmacological administration of FGF21 protects against MASH pathology at least in large through the interaction between βKlotho and PPP6C and PPP6C-mediated dephosphorylation of TSC2 in hepatocytes. This study implies that pharmacological approaches targeting PPP6C activity may offer attractive prospects for treating liver fibrosis and MASH.