title: Regulation of SR-BI protein levels by phosphorylation of its associated protein, PDZK1.
abstract: Scavenger receptor class B type I (SR-BI) is a high-density lipoprotein (HDL) receptor that mediates the selective uptake of HDL cholesterol and cholesterol secretion into bile in the liver. Previously, we identified an SR-BI-associated protein, termed PDZK1, from rat liver membrane extracts. PDZK1 contains four PSD-95/Dlg/ZO-1 (PDZ) domains, the first of which in the N-terminal region is responsible for the association with SR-BI. PDZK1 controls hepatic SR-BI expression in a posttranscriptional fashion both in cell culture and in vivo. In this study, we demonstrated that the C-terminal region of PDZK1 is crucial for up-regulating SR-BI protein expression. Metabolic labeling experiments and phosphoamino acid analysis revealed that PDZK1 is phosphorylated at Ser residues within this region. Point-mutation analysis demonstrated that PDZK1 is phosphorylated at Ser-509. Interestingly, a mutant PDZK1, in which Ser-509 was replaced with Ala, lost the ability to up-regulate SR-BI protein. We identified Ser-509 of PDZK1 as the residue that is phosphorylated by the cAMP-dependent PKA in vitro as well as in cell culture. Ser-509 of PDZK1 in rat liver was also phosphorylated, as shown by an Ab that specifically detects phosphorylated Ser-509. Administration of glucagon to Wistar rats increased PDZK1 phosphorylation as well as hepatic SR-BI and PDZK1 expression while it decreased plasma HDL levels, indicating that PDZK1 phosphorylation is hormonally regulated. These findings suggest that phosphorylation of PDZK1 has an important role in the regulation of hepatic SR-BI expression and, thus, influences plasma HDL levels.
authors.affiliation: Department of Health Chemistry, Graduate School of Pharmaceutical Sciences, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Nakamura
authors.forename: Norihito
authors.initials: N
authors.lastname: Shibata
authors.forename: Takako
authors.initials: T
authors.lastname: Nishimoto-Shibata
authors.forename: Dongdong
authors.initials: D
authors.lastname: Feng
authors.forename: Mamoru
authors.initials: M
authors.lastname: Ikemoto
authors.forename: Kiyoto
authors.initials: K
authors.lastname: Motojima
authors.forename: Naoyuki
authors.initials: N
authors.lastname: Iso-O
authors.forename: Kazuhisa
authors.initials: K
authors.lastname: Tsukamoto
authors.forename: Masafumi
authors.initials: M
authors.lastname: Tsujimoto
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Arai
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics*; D000818:Animals; D018955:CD36 Antigens; D016466:CHO Cells; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D008076:Cholesterol, HDL / Q000097:blood; D006224:Cricetinae; D003412:Cricetulus; D003598:Cytoskeletal Proteins; D005934:Glucagon / Q000008:administration & dosage; D008099:Liver / Q000378:metabolism; D008110:Liver Extracts / Q000378:metabolism; D010766:Phosphorylation; D017354:Point Mutation; D051381:Rats; D017208:Rats, Wistar; D011971:Receptors, Immunologic / Q000378:metabolism*; D051116:Receptors, Scavenger; D051122:Scavenger Receptors, Class B; D012694:Serine / Q000235:genetics / Q000378:metabolism; D015854:Up-Regulation / Q000187:drug effects / Q000235:genetics
chemical_list: D018955:CD36 Antigens; D002352:Carrier Proteins; D008076:Cholesterol, HDL; D003598:Cytoskeletal Proteins; D008110:Liver Extracts; C109245:Pdzk1 protein, rat; D011971:Receptors, Immunologic; D051116:Receptors, Scavenger; C495125:Scarb1 protein, rat; D051122:Scavenger Receptors, Class B; D012694:Serine; D005934:Glucagon