A feed-forward loop amplifies nutritional regulation of PNPLA3

Abstract

The upsurge in prevalence of obesity has spawned an epidemic of nonalcoholic fatty liver disease (NAFLD). Previously, we identified a sequence variant (I148M) in patatin-like phospholipase domaincontaining protein 3 (PNPLA3) that confers susceptibility to both hepatic triglyceride (TG) deposition and liver injury. To glean insights into the biological role of PNPLA3, we examined the molecular mechanisms by which nutrient status controls hepatic expression of PNPLA3. PNPLA3 mRNA levels, which were low in fasting animals, increased ∼90-fold with carbohydrate feeding. The increase was mimicked by treatment with a liver X receptor (LXR) agonist and required the transcription factor SREBP-1c. The site of SREBP-1c binding was mapped to intron 1 of Pnpla3 using chromatin immunoprecipitation and electrophoretic mobility shift assays. SREBP-1c also promotes fatty acid synthesis by activating several genes encoding enzymes in the biosynthetic pathway. Addition of fatty acids (C16:0, C18:1, and C18:2) to the medium of cultured hepatocytes (HuH-7) increased PNPLA3 protein mass without altering mRNA levels. The posttranslational increase in PNPLA3 levels persisted after blocking TG synthesis with triascin C. Oleate (400 μM) treatment prolonged the half-life of PNPLA3 from 2.4 to 6.7 h. These findings are consistent with nutritional control of PNPLA3 being effected by a feed-forward loop; SREBP-1c promotes accumulation of PNPLA3 directly by activating Pnpla3 transcription and indirectly by inhibiting PNPLA3 degradation through the stimulation of fatty acid synthesis.adiponutrin | SREBP-1c | triglycerides | hepatic steatosis

PNPLA3 gene in liver diseases

“…In human liver cells, PNPLA3 mRNA is more highly expressed in hepatic stellate cells than in hepatocytes [72]. This gene is highly influenced by nutritional status [73]. Thus, PNPLA3 is regulated at the transcriptional level by insulin through the induction of sterol regulatory element binding protein-1c…”

Association of PNPLA3 Polymorphism with Hepatocellular Carcinoma Development and Prognosis in Viral and Non-Viral Chronic Liver Diseases

“…This SNP is located in the patatin-like phospholipase domain containing 3 (PNPLA3) or adiponutrin gene encoding for a protein of 53 kDa with 481 amino acids and is induced by lipogenesis steroid regulatory element binding protein-1c (Huang et al, 2010). The sequence variation of rs738409 (C>G) is responsible for an amino acid change at position 148 from isoleucine to methionine (Ile148Met).…”

Circulating Triacylglycerol Signatures in Nonalcoholic Fatty Liver Disease Associated With the I148M Variant in PNPLA3 and With Obesity

“…In humans, PNPLA3 is expressed predominantly in the liver (8). In vitro studies using recombinant purified human PNPLA3 have shown that the wild-type enzyme hydrolyzes triglycerides and that the I148M substitution abolishes this activity (9).…”