Acetylation of mitochondrial trifunctional protein α-subunit enhances its stability to promote fatty acid oxidation and is decreased in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) has become the most common liver disease, and decreased fatty acid oxidation is one of the important contributors to NAFLD. Mitochondrial trifunctional protein α-subunit (MTPα) functions as a critical enzyme for fatty acid β-oxidation, but whether dysregulation of MTPα is pathogenically connected to NAFLD is poorly understood. We show that MTPα is acetylated at lysine residues 350, 383, and 406 (MTPα-3K), which promotes its protein stability by antagonizing its ubiquitylation on the same three lysines (MTPα-3K) and blocking its subsequent degradation. Sirtuin 4 (SIRT4) has been identified as the deacetylase, deacetylating and destabilizing MTPα. Replacement of MTPα-3K with either MTPα-3KR or MTPα-3KQ inhibits cellular lipid accumulation both in free fatty acid (FFA)-treated alpha mouse liver 12 (AML12) cells and primary hepatocytes and in the livers of high-fat/high-sucrose (HF/HS) diet-fed mice. Moreover, knockdown of SIRT4 could phenocopy the effects of MTPα-3K mutant expression in mouse livers, and MTPα-3K mutants more efficiently attenuate SIRT4- mediated hepatic steatosis in HF/HS diet-fed mice. Importantly, acetylation of both MTPα and MTPα-3K is decreased while SIRT4 is increased in the livers of mice and humans with NAFLD. Our study reveals a novel mechanism of MTPα regulation by acetylation and ubiquitylation and a direct functional link of this regulation to NAFLD.