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.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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Animals
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CHO Cells
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Cricetinae
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Cricetulus
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Enzyme Inhibitors / pharmacology
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Gene Expression Regulation / drug effects
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Gene Expression Regulation / physiology*
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HEK293 Cells
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Hepatocytes / metabolism*
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Humans
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Lysosomes / genetics
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Lysosomes / metabolism*
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MAP Kinase Kinase 1 / antagonists & inhibitors
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MAP Kinase Kinase 1 / genetics
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MAP Kinase Kinase 1 / metabolism
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MAP Kinase Kinase 2 / antagonists & inhibitors
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MAP Kinase Kinase 2 / genetics
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MAP Kinase Kinase 2 / metabolism
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MAP Kinase Signaling System / drug effects
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MAP Kinase Signaling System / physiology*
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Mitogen-Activated Protein Kinase 1 / antagonists & inhibitors
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Mitogen-Activated Protein Kinase 1 / genetics
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Mitogen-Activated Protein Kinase 1 / metabolism
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Mitogen-Activated Protein Kinase 3 / antagonists & inhibitors
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Mitogen-Activated Protein Kinase 3 / genetics
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Mitogen-Activated Protein Kinase 3 / metabolism
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PPAR alpha / genetics
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PPAR alpha / metabolism
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Phosphorylation / drug effects
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Phosphorylation / physiology
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Proteasome Endopeptidase Complex / genetics
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Proteasome Endopeptidase Complex / metabolism
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Protein Stability / drug effects
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Scavenger Receptors, Class B / biosynthesis*
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Scavenger Receptors, Class B / genetics
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raf Kinases / antagonists & inhibitors
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raf Kinases / genetics
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raf Kinases / metabolism
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ras Proteins / genetics
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ras Proteins / metabolism
Substances
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Enzyme Inhibitors
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PPAR alpha
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SCARB1 protein, human
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Scavenger Receptors, Class B
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MAP2K2 protein, human
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raf Kinases
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MAPK1 protein, human
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Mitogen-Activated Protein Kinase 1
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Mitogen-Activated Protein Kinase 3
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MAP Kinase Kinase 1
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MAP Kinase Kinase 2
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MAP2K1 protein, human
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Proteasome Endopeptidase Complex
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ras Proteins