title: 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 domain-containing 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 approximately 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 muM) 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.
authors.affiliation: Departments of Molecular Genetics and Internal Medicine, The Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
authors.forename: Yongcheng
authors.initials: Y
authors.lastname: Huang
authors.forename: Shaoqing
authors.initials: S
authors.lastname: He
authors.forename: John Zhong
authors.initials: JZ
authors.lastname: Li
authors.forename: Young-Kyo
authors.initials: YK
authors.lastname: Seo
authors.forename: Timothy F
authors.initials: TF
authors.lastname: Osborne
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Cohen
authors.forename: Helen H
authors.initials: HH
authors.lastname: Hobbs
mesh_terms: D000818:Animals; D002460:Cell Line; D047369:Chromatin Immunoprecipitation; D002874:Chromosome Mapping; D004040:Dietary Carbohydrates / Q000494:pharmacology; D024202:Electrophoretic Mobility Shift Assay; D005215:Fasting / Q000502:physiology; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D008049:Lipase / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D009752:Nutritional Status / Q000502:physiology*; D009841:Oligonucleotides / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000217:Acyltransferases; D054512:Phospholipases A2, Calcium-Independent
chemical_list: D004040:Dietary Carbohydrates; D047908:Intracellular Signaling Peptides and Proteins; D008049:Lipase; D008565:Membrane Proteins; D009841:Oligonucleotides; C102196:SREBP cleavage-activating protein; C533682:PNPLA3 protein, human; D000217:Acyltransferases; D054512:Phospholipases A2, Calcium-Independent
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