Phosphatidylinositol-3-Kinase Regulates Scavenger Receptor Class B Type I Subcellular Localization and Selective Lipid Uptake in Hepatocytes

Abstract

Objective-The high-density lipoprotein (HDL) receptor scavenger receptor Class B type I (SR-BI) plays a key role in mediating the final step of reverse cholesterol transport. This study examined the possible regulation of hepatic SR-BI by phosphatidylinositol-3-kinase (PI3K), a well known regulator of endocytosis and membrane protein trafficking. Methods and Results-SR-BI-dependent HDL selective cholesterol ester uptake in human HepG2 hepatoma cells was decreased (Ϸ50%) by the PI3K inhibitors wortmannin and LY294002. Insulin increased selective uptake (Ϸ30%), and this increase was blocked by PI3K inhibitors. Changes in SR-BI activity could be accounted for by pronounced changes in the subcellular localization and cell surface expression of SR-BI as determined by HDL cell surface binding, receptor biotinylation studies, and confocal fluorescence microscopy of HepG2 cells expressing green fluorescent protein-tagged SR-BI. Thus, under conditions of PI3K activation by insulin, and to a lesser extent by the SR-BI ligand HDL, cell surface expression of SR-BI was promoted, resulting in increased SR-BI-mediated HDL selective lipid uptake. Conclusion-Our

Regulation of SR-BI-mediated selective lipid uptake in Chinese hamster ovary-derived cells by protein kinase signaling pathways

“…Our findings differ somewhat from the reported wortmannin-dependent reduction in HDL selective lipid uptake in 3T3-L1 adipocytes and HepG2 cells (13,14). It is not clear whether this is due to differences in the cell type and/or experimental protocols used.…”

“…ever, reduce cell surface (biotinylated) mSR-BI in otherwise untreated cells (Fig. 6, lane 9; compared with lane 1), in agreement with its reported effects in 3T3-L1 adipocytes and HepG2 human hepatoma cells (13,14). HDL blocked the wortmannin-induced decrease in cell surface SR-BI (Fig.…”

“…In polarized cells, cholesterol depletion induces SR-BI redistribution from the basal to apical membrane surface in a protein kinase Adependent manner (12). In differentiating 3T3-L1 adipocytes and HepG2 human hepatoma cells, recruitment of SR-BI to the cell surface from internal sites is induced by insulin and/or serum and dependent on the phosphatidyl inositol 3-kinase (PI3K)/Akt (protein kinase B) signaling pathway (13,14). Murine scavenger receptor, class B, type I (mSR-BI)-mediated HDL lipid uptake does not, however, appear to require endocytosis, because purified, reconstituted mSR-BI is active (15) and mSR-BI-mediated HDL lipid uptake in cells is not affected by inhibition of endocytosis (16,17).…”

Aging bone marrow mesenchymal stromal cells have altered membrane glycerophospholipid composition and functionality

“…Thus, changes in the ratio of PI/PS may have profound effects on hBMSC therapeutic functions via the regulation of the activities of kinases, phospholipases, and/or other proteins. We also emphasize that PIPs regulate the scavenger receptor B1 (SR-B1) of lipoprotein traffi cking ( 66 ), and in our study, SR-B1 was expressed at a higher level in the hBMSCs of the young donors. Interestingly, we found that gene expression of several ATPases was altered.…”

MCP-1 impacts RCT by repressing ABCA1, ABCG1, and SR-BI through PI3K/Akt posttranslational regulation in HepG2 cells

“…Many studies have indicated that the subcellular localization of ABCA1, ABCG1, and SR-BI can be posttranslationally modulated by certain substances to allow them to move to the plasma membrane and, in turn, affect RCT (18)(19)(20)27 ). We have shown that treatment with MCP-1 resulted in reductions in the ABCA1 and SR-BI total proteins while leaving the ABCG1 level unchanged.…”