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

Abstract

Background & Aims

Metabolic dysfunction-associated steatohepatitis (MASH) is a serious chronic liver disease with limited therapies. Although fibroblast growth factor 21 (FGF21) analogs have shown promising therapeutic benefits for MASH in multiple preclinical and clinical studies, the underlying mechanisms remain elusive.

Methods

Liver-specific PPP6C knockout (PPP6C LKO) mice, βKlotho knockout (βKlotho LKO) and their wild-type littermates were fed with Amylin liver NASH (AMLN) diet for 16 weeks or choline-deficient, L-amino acid-defined, high-fat diet (CDA-HFD) for 8 weeks, followed by daily subcutaneous injection of rFGF21 (0.5 mg/kg) or vehicle for 4 weeks. Mass spectrometry assay was used for identification of PPP6C as a βKlotho binding protein. The in vitro phosphatase assay was used to evaluate the effects of FGF21 on PPP6C activity. Human studies shown that deregulation of PPP6C is associated with the progression of MASH.

Results

We identified serine and threonine phosphatase PPP6C as a direct target of FGF21. Hepatic PPP6C deficiency accelerates MASH progression in mice fed with AMLN diet or CDA-HFD, which blocks FGF21 action on MASH. Mechanistically, PPP6C is sufficient to interact with the coreceptor βKlotho upon FGF21 treatment, directly dephosphorylates tuberous sclerosis complex 2 (TSC2) at Ser939 and Thr1462, thereby inhibiting mTORC1 and promoting nuclear entry of TFE3 and Lipin1. In livers of MASH subjects, expression levels of PPP6C are decreased whereas TSC2 phosphorylation is elevated.

Conclusions

These results define a fundamental mechanism underlying FGF21 signals in hepatocytes and demonstrate that targeting PPP6C may have therapeutic potential for treating MASH.

Impact and implications

MASH is a severe chronic liver disease that increases susceptibility to more severe cirrhosis and hepatocellular carcinoma, yet currently lacks effective clinical therapeutic strategies. Here we have identified serine and threonine protein phosphatase PPP6C as a key regulator of MASH progression in mice and humans. PPP6C is directly activated by FGF21 via FGFRs/βKlotho and improves MASH features through dephosphorylation of TSC2 in hepatocytes. This study implies that pharmacological approaches, genetic or metabolic factors targeting PPP6C activity may offer attractive prospects for treating liver fibrosis and MASH.