title: A Novel Role of Salt-Inducible Kinase 1 (SIK1) in the Post-Translational Regulation of Scavenger Receptor Class B Type 1 Activity.
abstract: Salt-inducible kinase 1 (SIK1) is a serine/threonine kinase that belongs to the stress- and energy-sensing AMPK family of kinases. SIK1 expression is rapidly induced in Y1 adrenal cells in response to ACTH via the cAMP-PKA signaling cascade, and it has been suggested that an increased level of SIK1 expression inhibits adrenal steroidogenesis by repressing the cAMP-dependent transcription of steroidogenic proteins, CYP11A1 and StAR, by attenuating CREB transcriptional activity. Here we show that SIK1 stimulates adrenal steroidogenesis by modulating the selective HDL-CE transport activity of SR-B1. Overexpression of SIK1 increases cAMP-stimulated and SR-B1-mediated selective HDL-BODIPY-CE uptake in cell lines without impacting SR-B1 protein levels, whereas knockdown of SIK1 attenuated cAMP-stimulated selective HDL-BODIPY-CE uptake. SIK1 forms a complex with SR-B1 by interacting with its cytoplasmic C-terminal domain, and in vitro kinase activity measurements indicate that SIK1 can phosphorylate the C-terminal domain of SR-B1. Among potential phosphorylation sites, SIK1-catalyzed phosphorylation of Ser496 is critical for SIK1 stimulation of the selective CE transport activity of SR-B1. Mutational studies further demonstrated that both the intact catalytic activity of SIK1 and its PKA-catalyzed phosphorylation are essential for SIK1 stimulation of SR-B1 activity. Finally, overexpression of SIK1 caused time-dependent increases in SR-B1-mediated and HDL-supported steroid production in Y1 cells; however, these effects were lost with knockdown of SR-B1. Taken together, these studies establish a role for SIK1 in the positive regulation of selective HDL-CE transport function of SR-B1 and steroidogenesis and suggest a potential mechanism for SIK1 signaling in modulating SR-B1-mediated selective CE uptake and associated steroidogenesis.
authors.affiliation: Geriatric Research, Education and Clinical Center, Veterans Affairs Palo Alto Health Care System , Palo Alto, California 94304, United States.
authors.forename: Zhigang
authors.initials: Z
authors.lastname: Hu
authors.affiliation: Geriatric Research, Education and Clinical Center, Veterans Affairs Palo Alto Health Care System , Palo Alto, California 94304, United States.
authors.forename: Jie
authors.initials: J
authors.lastname: Hu
authors.affiliation: Geriatric Research, Education and Clinical Center, Veterans Affairs Palo Alto Health Care System , Palo Alto, California 94304, United States.
authors.forename: Wen-Jun
authors.initials: WJ
authors.lastname: Shen
authors.affiliation: Geriatric Research, Education and Clinical Center, Veterans Affairs Palo Alto Health Care System , Palo Alto, California 94304, United States.
authors.forename: Fredric B
authors.initials: FB
authors.lastname: Kraemer
authors.affiliation: Geriatric Research, Education and Clinical Center, Veterans Affairs Palo Alto Health Care System , Palo Alto, California 94304, United States.
authors.forename: Salman
authors.initials: S
authors.lastname: Azhar
mesh_terms: D000311:Adrenal Glands / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001693:Biological Transport, Active; D002460:Cell Line; D002788:Cholesterol Esters / Q000378:metabolism; D055785:Gene Knockdown Techniques; D008075:Lipoproteins, HDL / Q000378:metabolism; D008297:Male; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D051381:Rats; D017207:Rats, Sprague-Dawley; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051122:Scavenger Receptors, Class B / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D002788:Cholesterol Esters; C059714:HDL cholesteryl ester; D008075:Lipoproteins, HDL; D011993:Recombinant Fusion Proteins; C495125:Scarb1 protein, rat; D051122:Scavenger Receptors, Class B; D017346:Protein Serine-Threonine Kinases; C495376:Sik1 protein, mouse; C120299:Sik1 protein, rat