Ras/Mitogen-activated Protein Kinase (MAPK) Signaling Modulates Protein Stability and Cell Surface Expression of Scavenger Receptor SR-BI

Vishwaroop Mulay,Albert Pol,Peta Wood,Masoud Darabi,Thomas Grewal,Kerry-Anne Rye,Carlos Enrich,Gilles Lambert,Karen Cecilia Chan,Joerg Heeren

doi:10.1074/jbc.m111.236398

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.

Highlights

Anti-atherosclerotic properties of high density lipoprotein (HDL) and the major HDL apolipoprotein, apoA-I, are believed to include their ability to induce signaling events that promote cholesterol export from peripheral cells to the liver for disposal
We determined the effect of PD98059 on scavenger receptor class BI (SR-BI) protein levels in Chinese hamster ovary (CHO) cells incubated with the peroxisome proliferator activator receptors (PPARs)␣ antagonist MK886 (Fig. 1B)
In this study we demonstrate that members of the Ras/ mitogen-activated protein kinase (MAPK) signaling cascade, including H-Ras, K-Ras, Mek1/2, and Erk1/2, regulate the protein levels of SR-BI via PPAR␣-inducible degradation pathways in hepatocytes and fibroblasts

Summary

Introduction

Anti-atherosclerotic properties of HDL and the major HDL apolipoprotein, apoA-I, are believed to include their ability to induce signaling events that promote cholesterol export from peripheral cells to the liver for disposal. Ectopic expression of constitutively active H-Ras and Mek1 increases SR-BI protein levels, which correlates with elevated PPAR␣ Ser-21 phosphorylation and increased cholesterol efflux. We show that MAPK inhibition down-regulates SR-BI in CHO and HEK293 cells treated with PPAR␣ agonists, whereas enhanced Ras/MAPK activity increases PPAR␣ Ser-21 phosphorylation and SR-BI expression levels, respectively.

Results

Conclusion