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.
SR-BI-dependent HDL selective cholesterol ester uptake in human HepG2 hepatoma cells was decreased (approximately 50%) by the PI3K inhibitors wortmannin and LY294002. Insulin increased selective uptake (approximately 30%), 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.
Our data indicate that PI3K activation stimulates hepatic SR-BI function post-translationally by regulating the subcellular localization of SR-BI in a P13K-dependent manner. Decreased hepatocyte PI3K activity in insulin-resistant states, such as type 2 diabetes, obesity, or metabolic syndrome, may impair reverse cholesterol transport by reducing cell surface expression of SR-BI.
"Certain studies have reported that PI3Ks are involved in the metabolism of different lipoproteins. For example, Shetty et al (19) demonstrated that PI3K plays an important role in class B type I scavenger receptor subcellular localization and selective lipid uptake in hepatocytes. Kzhyshkowska et al (20) reported that PI3K activity is required for the transfer of stabilin-1 and its ligand, acetylated low-density lipoprotein, from early endosomes to late endosomes. "
[Show abstract][Hide abstract] ABSTRACT: Individuals carrying mutations at both ataxia telangiectasia mutated (ATM) gene alleles reportedly have increased plasma cholesterol and triglyceride levels. Previous studies have demonstrated that defective ATM function promotes atherosclerosis. We previously demonstrated that ATM facilitates the clearance of plasma apolipoprotein (Apo)E-deficient, ApoB48-containing (E-/B48) lipoproteins in ApoE-deficient mice (ApoE-/- mice). However, to date there is no exact explanation available as to the mechanism(s) through which ATM is involved in the removal of E-/B48 lipoprotein in ApoE-/- mice. In this study, to our knowledge, we demonstrate for the first time that heterozygous ATM mutation reduces the hepatocyte uptake of E-/B48 lipoproteins in ApoE-/- mice; however, heterozygous ATM mutation did not affect hepatocyte binding to E-/B48 lipoproteins. Moreover, our results revealed that ATM proteins were localized in the nucleus, early endosomes and late endosomes, but not in the plasma membrane in the hepatocytes of ApoE-/- mice. In addition, following treatment with the ATM activator, chloroquine, and E-/B48 lipoproteins, ATM interacted with class III phosphatidylinositol-3-kinases (PI3Ks) and the activated ATM protein enhanced class III PI3K activity. Furthermore, treatment with a class III PI3K inhibitor (LY290042 and 3-MA) attenuated the intracellular total cholesterol accumulation induced by ATM activation. These results provide insight into the mechanisms behind the involvment of ATM in the process of endocytosis of E-/B48 lipoprotein in ApoE-/- mice, demonstrating the role of class III PI3K protein.
International Journal of Molecular Medicine 11/2013; 33(2). DOI:10.3892/ijmm.2013.1566 · 2.09 Impact Factor
"In particular, a cholesterol depletion (by an acute MβCD treatment) of SR-BI-transfected MDCK cells induced the transcytosis of SR-BI from basolateral membrane, where it was colocalized with caveolae, to apical domain by a PKA-dependent process (Burgos et al. 2004). Furthermore, intracellular trafficking of SR-BI and its surface exposure, conditioning its activity, were under the dependence of PI-3- kinase (Shetty et al. 2006). Finally, SR-BI is also described to mediate HDL-cholesterol exchange with the cell by an endocytosis/resecretion mechanism, so-called " retroendocytosis " (Pagler et al. 2006; Rhainds et al. 2004). "
[Show abstract][Hide abstract] ABSTRACT: As a key constituent of the cell membranes, cholesterol is an endogenous component of mammalian cells of primary importance, and is thus subjected to highly regulated homeostasis at the cellular level as well as at the level of the whole body. This regulation requires adapted mechanisms favoring the handling of cholesterol in aqueous compartments, as well as its transfer into or out of membranes, involving membrane proteins. A membrane exhibits functional properties largely depending on its lipid composition and on its structural organization, which very often involves cholesterol-rich microdomains. Then there is the appealing possibility that cholesterol may regulate its own transmembrane transport at a purely functional level, independently of any transcriptional regulation based on cholesterol-sensitive nuclear factors controling the expression level of lipid transport proteins. Indeed, the main cholesterol "transporters" presently believed to mediate for instance the intestinal absorption of cholesterol, that are SR-BI, NPC1L1, ABCA1, ABCG1, ABCG5/G8 and even P-glycoprotein, all present privileged functional relationships with membrane cholesterol-containing microdomains. In particular, they all more or less clearly induce membrane disorganization, supposed to facilitate cholesterol exchanges with the close aqueous medium. The actual lipid substrates handled by these transporters are not yet unambiguously determined, but they likely concern the components of membrane microdomains. Conversely, raft alterations may provide specific modulations of the transporter activities, as well as they can induce indirect effects via local perturbations of the membrane. Finally, these cholesterol transporters undergo regulated intracellular trafficking, with presumably some relationships to rafts which remain to be clarified.
European Biophysics Journal 12/2007; 36(8):869-85. DOI:10.1007/s00249-007-0193-8 · 2.22 Impact Factor
"In polarized cells, cholesterol depletion induces SR-BI redistribution from the basal to apical membrane surface in a protein kinase A dependent manner . 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 is dependent on the phosphatidylinositol 3-kinase (PI3K)/Akt (protein kinase B) signaling pathway  . In a variety of cell types, HDL is internalized and undergoes resecretion or retro-endocytosis back to the cell *Y. "
[Show abstract][Hide abstract] ABSTRACT: The scavenger receptor SR-BI plays an important role in the hepatic clearance of HDL cholesterol and other lipids, driving reverse cholesterol transport and contributing to protection against atherosclerosis in mouse models. We characterized the role of endocytosis in lipid uptake from HDL, mediated by the human SR-BI, using a variety of approaches to inhibit endocytosis, including hypertonic shock, potassium or energy depletion and disassembly of the actin cytoskeleton. Our studies revealed that unlike mouse SR-BI, human SR-BI-mediated HDL-lipid uptake was reduced by inhibition of endocytosis. This was not dependent on the cytoplasmic C-terminus of SR-BI. Monitoring the uptake of both the protein and lipid components of HDL revealed that although overall lipid uptake was decreased, the degree of selective lipid uptake was increased. These data suggest that that endocytosis is a dynamic regulator of SR-BI's selective lipid uptake activity.
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