Proteasome-Mediated Degradation of PNPLA3

A missense mutation in PNPLA3 (148M) is the strongest genetic risk factor for fatty liver disease (FLD). PNPLA3 (148M) evades ubiquitylation and proteasomal degradation, and accumulates on hepatic lipid droplets (LDs). E3 Ubiquitin Ligases provides specificity to ubiquitin-mediated proteasomal degradation of proteins in cells. The goal of this project was to identify the E3 Ubiquitin Ligase(s) involved in PNPLA3 degradation. Identification of this ligase will provide a molecular handle on PNPLA3 degradation, and enable us to determine how PNPLA3 (148M) evades degradation by the ubiquitin-proteasome system. We used an RNAi screen in cultured human hepatoma cells and identified Bifunctional Apoptosis Regulator (BFAR) as a candidate E3 Ubiquitin Ligase in the proteasomal degradation of PNPLA3. Further validation of BFAR in liver-derived and non-liver derived cultured cells confirmed its role in promoting the degradation of PNPLA3. We further show that PNPLA3 undergoes polyubiquitination in the presence of BFAR in-vitro and also in cultured cells, establishing it as a direct substrate for BFAR. However, experiments showing PNPLA3 accumulation in BFAR-deficient cells treated with a proteasomal inhibitor also raises the possibility that more than one E3 Ubiquitin Ligase contributes to the degradation of PNPLA3. PNPLA3 (148M) also undergoes proteasomal degradation in the presence of BFAR, which is also consistent with multiple E3 Ubiquitin ligase(s). PNPLA3 is a lipid droplet resident protein, while BFAR localizes in the endoplasmic reticulum (ER). We show that PNPLA3 ubiquitylation is uncoupled from its subcellular localization in LDs. Finally, we observe a modest accumulation of PNPLA3 in hepatic LDs in mice with hepatic depletion of BFAR, which can be due to insufficient reduction in hepatic BFAR proteins or amino acid sequence variation between the species. Future studies with genetically modified murine models will be critical in providing a better mechanistic understanding.