PNPLA3 overexpression results in reduction of proteins predisposing to fibrosis

Abstract

Liver fibrosis is a pathological scarring response to chronic hepatocellular injury and hepatic stellate cells (HSCs) are key players in this process. PNPLA3 I148M is a common variant robustly associated with liver fibrosis but the mechanisms underlying this association are unknown. We aimed to examine a) the effect of fibrogenic and proliferative stimuli on PNPLA3 levels in HSCs and b) the role of wild type and mutant PNPLA3 overexpression on markers of HSC activation and fibrosis.Here, we show that PNPLA3 is upregulated by the fibrogenic cytokine transforming growth factor-beta (TGF-β), but not by platelet-derived growth factor (PDGF), and is involved in the TGF-β-induced reduction in lipid droplets in primary human HSCs. Furthermore, we show that retinol release from human HSCs ex vivo is lower in cells with the loss-of-function PNPLA3 148M compared with 148I wild type protein. Stable overexpression of PNPLA3 148I wild type, but not 148M mutant, in human HSCs (LX-2 cells) induces a reduction in the secretion of matrix metallopeptidase 2 (MMP2), tissue inhibitor of metalloproteinase 1 and 2 (TIMP1 and TIMP2), which is mediated by retinoid metabolism. In conclusion, we show a role for PNPLA3 in HSC activation in response to fibrogenic stimuli. Moreover, we provide evidence to indicate that PNPLA3-mediated retinol release may protect against liver fibrosis by inducing a specific signature of proteins involved in extracellular matrix remodelling.

Genetic determinants of steatosis and fibrosis progression in paediatric non‐alcoholic fatty liver disease

“…PNPLA3 has been attributed a retinyl‐palmitate lipase activity in hepatic stellate cells (HSC), and stable overexpression of PNPLA3 rs738409[G] was associated with decreased retinol release from human HSC ex vivo and concomitantly impaired reduction in fibrogenic factors . Recently, the rs738409[G] variant has been shown to disrupt ubiquitination and proteasomal degradation of PNPLA3, resulting in accumulation of PNPLA3‐148M and impaired mobilization .…”

Mutant PNPLA3 I148M protein as pharmacological target for liver disease

“…Taken together, these data suggest that accumulation of the 148M, but not wild‐type, PNPLA3 protein drives hepatic fat accumulation and that it may be possible to mitigate the effects of the 148M mutant by preventing its accumulation. Furthermore, because of the association in patients with nonalcoholic fatty liver disease with the 148M variant, we demonstrated that PNPLA3 expression in hepatic stellate cells is increased in the liver of carriers of the mutation, which is important because stellate cells are involved in the regulation of fibrogenesis and carcinogenesis . Paralleling what happens in hepatocytes, overexpression of the mutant I148M PNPLA3 protein in stellate cells in vitro impaired the release of retinol and lipids that results from dynamic remodeling of intracellular lipid droplets, resulting in a more fibrogenic phenotype …”

PNPLA3 and RNF7 Gene Variants are Associated with the Risk of Developing Liver Fibrosis and Cirrhosis in an Eastern European Population

“…It is known that PNPLA3 as a lipase is responsible for retinyl-palmitate hydrolysis in the hepatic stellate cells in humans [24]. Furthermore, the same group has recently shown that PNPLA3-mediated retinol release may protect against liver fibrosis [25]. Other studies have shown that the PNPLA3 I148M variant is relevant for the retinyl-palmitate content in human liver [26] and influences circulating retinol levels in adults [27].…”