PNPLA3-I148M is a Neomorph that Interferes with Two Primary Hepatic Triglyceride Clearance Pathways

Abstract

A common variant of PNPLA3, encoding PNPLA3-I148M, is the most significant genetic determinant of fatty liver disease worldwide. However, it is unclear precisely how PNPLA3-I148M drives disease risk. Here, we show that endogenous human PNPLA3-I148M impairs secretion of Apolipoprotein B (ApoB), the scaffolding protein of very low-density lipoproteins (VLDLs), from hepatocytes. This is not due to a generalized secretory pathway defect, nor is it equivalent to loss of function of PNPLA3. The VLDL secretory defect is conserved in mice expressing human I148M. Untargeted lipidomics reveal that I148M human cells are enriched in polyunsaturated fatty acid (PUFA)-containing triglycerides at the expense of PUFA-containing phosphatidylcholine, causing reduced membrane dynamics, concomitantly hindering biogenesis of secreted VLDLs. ApoB secretion is substantially rescued in I148M cells that overexpress ABHD5/CGI-58, an I148M binding partner that activates lipolysis by ATGL/PNPLA2 when not bound to I148M. Conversely, knocking down CGI-58 or PNPLA2 mimics I148M. We propose that neomorphic PNPLA3-I148M exacerbates fatty liver risk by simultaneously impeding two major CGI-58-dependent pathways for liver triglyceride clearance: lipolysis and secretion.