The Prolyl Isomerase PIN1 Impairs Methionine Adenosyltransferase α1 Mitochondrial Targeting in Alcoholic Liver Disease

Abstract

Background Methionine adenosyltransferase alpha1 (MATα1) is responsible for hepatic biosynthesis of S‐adenosylmethionine (SAMe). Lower MATα1 activity and SAMe levels occur in alcoholic liver disease (ALD), a condition characterized by mitochondrial dysfunction. We recently showed MATα1 is in the mitochondrial matrix of hepatocytes and is important for mitochondrial function. PIN1 is a peptidyl‐prolyl cis‐trans isomerase that can alter the activity, stability, interactions and subcellular localization of its targets by binding to a specific phosphorylated motif pSer/pThr‐Pro. PIN1’s role in ALD is unknown and MATα1 has a PIN1 binding motif (Ser114‐Pro115). The aim of this study was to examine whether PIN1 can regulate MATα1 function in ALD. Methods MATα1 phosphorylation was examined using immunoprecipitation (IP) with anti‐phospho‐Ser antibody followed by immunoblotting, phosphoproteomics and mass spectrometry (MS). IP using recombinant proteins, liver and cell lysates studied MATα1‐PIN1 interaction. Mitochondria were isolated from AML12 cells treated with 100mM ethanol for 48 hours (in vitro model), NIAAA livers (in vivo model), liver samples from patients with alcoholic hepatitis, and HepG2 cells where PIN1 expression was varied to evaluate MATα1 mitochondrial content. MATα1 S114A mutation investigated its impact on PIN1 interaction. Results PIN1 expression was unchanged in ALD models and in the livers of ALD patients. We found that MATα1 and PIN1 interact directly, in liver and cell lysates, and that their interaction is enhanced by ethanol in experimental models and in the livers of patients with alcoholic cirrhosis. Ethanol treatment in AML12 cells and in the NIAAA model increased MATα1 phosphorylation. Phosphoproteomics/MS revealed MATα1 is phosphorylated at Ser114 and mutation of Ser114 to alanine significantly reduced its interaction with PIN1. Increased PIN1‐MATα1 interaction reduced MATα1 mitochondrial targeting, as silencing PIN1 raised mitochondrial MATα1 content in AML12 and HepG2 cells, and attenuated ethanol‐induced triglyceride accumulation. Consistently, mitochondrial MATα1 content was preferentially reduced (by 60–80%) as compared to total MATα1 content (by 20–50%) in both in vitro and in vivo models and in livers from alcoholic hepatitis patients. Conclusion We unveiled a novel mechanism of mitochondrial injury in ALD that involves PIN1. In ALD, increased MATα1 phosphorylation and interaction with PIN1 lead to impaired MATα1 mitochondrial targeting. Inhibiting PIN1‐MATα1 interaction may serve as a new therapeutic strategy in ALD. Support or Funding Information This work was supported by NIH grants R01AA026759 (SC Lu and J Van Eyk) and R24AA025017 (Sun Z). In ALD, increased MATα1 phosphorylation and interaction with PIN1 impaired MATα1 mitochondrial targeting.