The PNPLA3 Variant Associated With Fatty Liver Disease (I148M) Accumulates on Lipid Droplets by Evading Ubiquitylation

A sequence variation (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) is strongly associated with fatty liver disease, but the underlying mechanism remains obscure. In this study, we used knock-in (KI) mice (Pnpla3(148M/M)) to examine the mechanism responsible for accumulation of triglyceride (TG) and PNPLA3 in hepatic lipid droplets (LDs). No differences were found between Pnpla3(148M/M) and Pnpla3(+/+) mice in hepatic TG synthesis, utilization, or secretion. These results are consistent with TG accumulation in the Pnpla3(148M/M) mice being caused by impaired TG mobilization from LDs. Sucrose feeding, which is required to elicit fatty liver in KI mice, led to a much larger and more persistent increase in PNPLA3 protein in the KI mice than in wild-type (WT) mice. Inhibition of the proteasome (bortezomib), but not macroautophagy (3-methyladenine), markedly increased PNPLA3 levels in WT mice, coincident with the appearance of ubiquitylated forms of the protein. Bortezomib did not increase PNPLA3 levels in Pnpla3(148M/M) mice, and only trace amounts of ubiquitylated PNPLA3 were seen in these animals. Conclusion: These results are consistent with the notion that the 148M variant disrupts ubiquitylation and proteasomal degradation of PNPLA3, resulting in accumulation of PNPLA3-148M and impaired mobilization of TG from LDs.