Accumulation of PNPLA3 on lipid droplets is the basis of associated hepatic steatosis

Abstract

Fatty liver disease (FLD) is a disorder in which accumulation of triglycerides (TGs) in the liver can lead to inflammation, fibrosis, and cirrhosis. Previously, we identified a variant (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) that is strongly associated with FLD, but the mechanistic basis for the association remains elusive. Although PNPLA3 has TG hydrolase activity in vitro, inactivation or overexpression of the WT protein in mice does not cause steatosis. In contrast, expression of two catalytically defective forms of PNPLA3 (I148M or S47A) in sucrose-fed mice causes accumulation of both PNPLA3 and TGs on hepatic lipid droplets (LDs). To determine if amassing PNPLA3 on LDs is a cause or consequence of steatosis, we engineered a synthetic isoform of PNPLA3 that uncouples protein accumulation from loss of enzymatic activity. Expression of a ubiquitylation-resistant form of PNPLA3 in mice caused accumulation of PNPLA3 on hepatic LDs and development of FLD. Lowering PNPLA3 levels by either shRNA knockdown or proteolysis-targeting chimera (PROTAC)-mediated degradation reduced liver TG content in mice overexpressing PNPLA3(148M). Taken together, our results show that the steatosis associated with PNPLA3(148M) is caused by accumulation of PNPLA3 on LDs.

Pathogenesis of Nonalcoholic Steatohepatitis: An Overview

“…The most strongly associated genetic variant with NASH is a single‐nucleotide polymorphism (I148M) in the patatin‐like phospholipase domain‐containing protein 3 ( PNPLA3 ) gene, which encodes a lipid droplet protein and is involved at this lipolytic step. I148M variant PNPLA3 is degradation resistant, accumulates on lipid droplets, and is sufficient to induce steatosis . Hydroxysteroid 17β‐dehydrogenase 13 (HSD17B13), another lipid droplet protein, is up‐regulated in NASH .…”

HSD17B13 as a promising therapeutic target against chronic liver disease

“…In brief, the role of HSD17B13 in human physiology and the molecular mechanism behind the genetic protection are far from being clear. The interplay between HSD17B13 and PNPLA3 is a further complicating issue, especially given that PNPLA3 itself is a potential target for therapeutic inhibition in the context of FLD . It will be important to determine what proportion of the protective effect of the HSD17B13 variant is mediated by PNPLA3 downregulation.…”

The Underpinnings of PNPLA3‐Mediated Fatty Liver Emerge

“…Another study published recently highlights the importance of the I148M mutation on PNPLA3 turnover and protein abundance . The authors found that despite similar mRNA levels, the protein expression of the I148M variant is higher than wild type.…”

“…Despite the importance of PNPLA3 I148M, the intrinsic pathogenic mechanisms underlying this variant have been unclear. However, several recent papers have provided new insights into the detailed molecular mechanism through which the I148M variant leads to steatosis …”

“…However, several recent papers have provided new insights into the detailed molecular mechanism through which the I148M variant leads to steatosis. (2)(3)(4)(5)…”