title: The translational regulation of lipoprotein lipase by epinephrine involves an RNA binding complex including the catalytic subunit of protein kinase A.
abstract: The balance of lipid flux in adipocytes is controlled by the opposing actions of lipolysis and lipogenesis, which are controlled primarily by hormone-sensitive lipase and lipoprotein lipase (LPL), respectively. Catecholamines stimulate adipocyte lipolysis through reversible phosphorylation of hormone-sensitive lipase, and simultaneously inhibit LPL activity. However, LPL regulation is complex and previous studies have described translational regulation of LPL in response to catecholamines because of an RNA-binding protein that interacts with the 3'-untranslated region of LPL mRNA. In this study, we identified several protein components of an LPL RNA binding complex. Using an LPL RNA affinity column, we identified two of the RNA-binding proteins as the catalytic (C) subunit of cAMP-dependent protein kinase (PKA), and A kinase anchoring protein (AKAP) 121/149, one of the PKA anchoring proteins, which has known RNA binding activity. To determine whether the C subunit was involved in LPL translation inhibition, the C subunit was depleted from the cytoplasmic extract of epinephrine-stimulated adipocytes by immunoprecipitation. This resulted in the loss of LPL translation inhibition activity of the extract, along with decreased RNA binding activity in a gel shift assay. To demonstrate the importance of the AKAPs, inhibition of PKA-AKAP binding with a peptide competitor (HT31) prevented epinephrine-mediated inhibition of LPL translation. C subunit kinase activity was necessary for LPL RNA binding and translation inhibition, suggesting that the phosphorylation of AKAP121/149 or other proteins was an important part of RNA binding complex formation. The hormonal activation of PKA results in the reversible phosphorylation of hormone-sensitive lipase, which is the primary mediator of adipocyte lipolysis. These studies demonstrate a dual role for PKA to simultaneously inhibit LPL-mediated lipogenesis through inhibition of LPL translation.
authors.affiliation: Central Arkansas Veterans HealthCare System, and Department of Medicine, Division of Endocrinology, and the Department of Geriatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA.
authors.forename: Gouri
authors.initials: G
authors.lastname: Ranganathan
authors.forename: Dan
authors.initials: D
authors.lastname: Phan
authors.forename: Irina D
authors.initials: ID
authors.lastname: Pokrovskaya
authors.forename: Joan E
authors.initials: JE
authors.lastname: McEwen
authors.forename: Chunling
authors.initials: C
authors.lastname: Li
authors.forename: Philip A
authors.initials: PA
authors.lastname: Kern
mesh_terms: D020413:3' Untranslated Regions / Q000235:genetics; D016475:3T3 Cells; D054758:A Kinase Anchor Proteins; D048868:Adaptor Proteins, Signal Transducing; D017667:Adipocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002384:Catalysis; D002454:Cell Differentiation; D004837:Epinephrine / Q000494:pharmacology*; D007700:Kinetics; D008071:Lipoprotein Lipase / Q000235:genetics*; D051379:Mice; D014176:Protein Biosynthesis / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism*; D021122:Protein Subunits / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics; D016601:RNA-Binding Proteins / Q000378:metabolism*; D014158:Transcription, Genetic
chemical_list: D020413:3' Untranslated Regions; D054758:A Kinase Anchor Proteins; D048868:Adaptor Proteins, Signal Transducing; C486333:Akap1 protein, mouse; D002352:Carrier Proteins; D021122:Protein Subunits; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D011494:Protein Kinases; D008071:Lipoprotein Lipase; D004837:Epinephrine