PNPLA3 mediates hepatocyte triacylglycerol remodeling

The I148M substitution in patatin-like phospholipase domain containing 3 (PNPLA3^I148M) determines a genetic form of nonalcoholic fatty liver disease. To elucidate the mode of PNPLA3 action in human hepatocytes, we studied effects of WT PNPLA3 (PNPLA3^WT) and PNPLA3^I148M on HuH7 cell lipidome after [¹³C]glycerol labeling, cellular turnover of oleic acid labeled with 17 deuterium atoms ([D17]oleic acid) in triacylglycerols (TAGs), and subcellular distribution of the protein variants. PNPLA3^I148M induced a net accumulation of unlabeled TAGs, but not newly synthesized total [¹³C]TAGs. Principal component analysis (PCA) revealed that both PNPLA3^WT and PNPLA3^I148M induced a relative enrichment of TAGs with saturated FAs or MUFAs, with concurrent enrichment of polyunsaturated phosphatidylcholines. PNPLA3^WT associated in PCA with newly synthesized [¹³C]TAGs, particularly 52:1 and 50:1, while PNPLA3^I148M associated with similar preexisting TAGs. PNPLA3^WT overexpression resulted in increased [D17]oleic acid labeling of TAGs during 24 h, and after longer incubations their turnover was accelerated, effects not detected with PNPLA3^I148M. PNPLA3^I148M localized more extensively to lipid droplets (LDs) than PNPLA3^WT, suggesting that the substitution alters distribution of PNPLA3 between LDs and endoplasmic reticulum/cytosol. This study reveals a function of PNPLA3 in FA-selective TAG remodeling, resulting in increased TAG saturation. A defect in TAG remodeling activity likely contributes to the TAG accumulation observed in cells expressing PNPLA3^I148M.