title: Ras/mitogen-activated protein kinase (MAPK) signaling modulates protein stability and cell surface expression of scavenger receptor SR-BI.
abstract: The mitogen-activated protein kinase (MAPK) Erk1/2 has been implicated to modulate the activity of nuclear receptors, including peroxisome proliferator activator receptors (PPARs) and liver X receptor, to alter the ability of cells to export cholesterol. Here, we investigated if the Ras-Raf-Mek-Erk1/2 signaling cascade could affect reverse cholesterol transport via modulation of scavenger receptor class BI (SR-BI) levels. We demonstrate that in Chinese hamster ovary (CHO) and human embryonic kidney (HEK293) cells, Mek1/2 inhibition reduces PPARα-inducible SR-BI protein expression and activity, as judged by reduced efflux onto high density lipoprotein (HDL). Ectopic expression of constitutively active H-Ras and Mek1 increases SR-BI protein levels, which correlates with elevated PPARα Ser-21 phosphorylation and increased cholesterol efflux. In contrast, SR-BI levels are insensitive to Mek1/2 inhibitors in PPARα-depleted cells. Most strikingly, Mek1/2 inhibition promotes SR-BI degradation in SR-BI-overexpressing CHO cells and human HuH7 hepatocytes, which is associated with reduced uptake of radiolabeled and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyane-labeled HDL. Loss of Mek1/2 kinase activity reduces SR-BI expression in the presence of bafilomycin, an inhibitor of lysosomal degradation, indicating down-regulation of SR-BI via proteasomal pathways. In conclusion, Mek1/2 inhibition enhances the PPARα-dependent degradation of SR-BI in hepatocytes.
authors.affiliation: Faculty of Pharmacy, University of Sydney, Sydney, New South Wales 2006, Australia.
authors.forename: Peta
authors.initials: P
authors.lastname: Wood
authors.forename: Vishwaroop
authors.initials: V
authors.lastname: Mulay
authors.forename: Masoud
authors.initials: M
authors.lastname: Darabi
authors.forename: Karen Cecilia
authors.initials: KC
authors.lastname: Chan
authors.forename: Joerg
authors.initials: J
authors.lastname: Heeren
authors.forename: Albert
authors.initials: A
authors.lastname: Pol
authors.forename: Gilles
authors.initials: G
authors.lastname: Lambert
authors.forename: Kerry-Anne
authors.initials: KA
authors.lastname: Rye
authors.forename: Carlos
authors.initials: C
authors.lastname: Enrich
authors.forename: Thomas
authors.initials: T
authors.lastname: Grewal
mesh_terms: D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology*; D057809:HEK293 Cells; D022781:Hepatocytes / Q000378:metabolism*; D006801:Humans; D008247:Lysosomes / Q000235:genetics / Q000378:metabolism*; D048369:MAP Kinase Kinase 1 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D048370:MAP Kinase Kinase 2 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000502:physiology*; D019950:Mitogen-Activated Protein Kinase 1 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D047493:PPAR alpha / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects / Q000502:physiology; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism; D055550:Protein Stability / Q000187:drug effects; D051122:Scavenger Receptors, Class B / Q000096:biosynthesis* / Q000235:genetics; D048490:raf Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; D047493:PPAR alpha; C495124:SCARB1 protein, human; D051122:Scavenger Receptors, Class B; C482128:MAP2K2 protein, human; D048490:raf Kinases; C535150:MAPK1 protein, human; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; C482125:MAP2K1 protein, human; D046988:Proteasome Endopeptidase Complex; D018631:ras Proteins