title: Ring finger protein20 regulates hepatic lipid metabolism through protein kinase A-dependent sterol regulatory element binding protein1c degradation.
abstract: Sterol regulatory element binding protein1c (SREBP1c) is a key transcription factor for de novo lipogenesis during the postprandial state. During nutritional deprivation, hepatic SREBP1c is rapidly suppressed by fasting signals to prevent lipogenic pathways. However, the molecular mechanisms that control SREBP1c turnover in response to fasting status are not thoroughly understood. To elucidate which factors are involved in the inactivation of SREBP1c, we attempted to identify SREBP1c-interacting proteins by mass spectrometry analysis. Since we observed that ring finger protein20 (RNF20) ubiquitin ligase was identified as one of SREBP1c-interacting proteins, we hypothesized that fasting signaling would promote SREBP1c degradation in an RNF20-dependent manner. In this work, we demonstrate that RNF20 physically interacts with SREBP1c, leading to degradation of SREBP1c via ubiquitination. In accordance with these findings, RNF20 represses the transcriptional activity of SREBP1c and turns off the expression of lipogenic genes that are targets of SREBP1c. In contrast, knockdown of RNF20 stimulates the expression of SREBP1c and lipogenic genes and induces lipogenic activity in primary hepatocytes. Furthermore, activation of protein kinase A (PKA) with glucagon or forskolin enhances the expression of RNF20 and potentiates the ubiquitination of SREBP1c via RNF20. In wild-type and db/db mice, adenoviral overexpression of RNF20 markedly suppresses FASN promoter activity and reduces the level of hepatic triglycerides, accompanied by a decrease in the hepatic lipogenic program. Here, we reveal that RNF20-induced SREBP1c ubiquitination down-regulates hepatic lipogenic activity upon PKA activation.
  
  
  CONCLUSIONS
  RNF20 acts as a negative regulator of hepatic fatty acid metabolism through degradation of SREBP1c upon PKA activation. Knowledge regarding this process enhances our understanding of how SREBP1c is able to turn off hepatic lipid metabolism during nutritional deprivation.
authors.affiliation: School of Biological Sciences, Institute of Molecular Biology and Genetics, Seoul National University, Seoul, Korea.
authors.forename: Jae Ho
authors.initials: JH
authors.lastname: Lee
authors.forename: Gha Young
authors.initials: GY
authors.lastname: Lee
authors.forename: Hagoon
authors.initials: H
authors.lastname: Jang
authors.forename: Sung Sik
authors.initials: SS
authors.lastname: Choe
authors.forename: Seung-Hoi
authors.initials: SH
authors.lastname: Koo
authors.forename: Jae Bum
authors.initials: JB
authors.lastname: Kim
mesh_terms: D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology*; D005234:Fatty Liver / Q000378:metabolism; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D050356:Lipid Metabolism / Q000502:physiology*; D008099:Liver / Q000737:chemistry* / Q000378:metabolism*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D009752:Nutritional Status; D055550:Protein Stability; D051780:Sterol Regulatory Element Binding Protein 1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000502:physiology*
chemical_list: D051780:Sterol Regulatory Element Binding Protein 1; C571257:RNF20 protein, mouse; D044767:Ubiquitin-Protein Ligases; D017868:Cyclic AMP-Dependent Protein Kinases