The Role of Endocytosis in SR-BI Mediated Lipid Uptake from Lipoproteins

Abstract

<p>Scavenger receptor Class B type I (SR-BI), is a multiligand cell surface receptor with broad binding specificity. It is an atypical member of the scavenger receptor family of cell surface receptors, due to its unique binding properties. SR-BI is mostly studied in the context of cholesterol metabolism, as its ligands include lipoproteins, which are the primary vesicles of cholesterol trafficking in the body. Lipoprotein ligands of SR-BI include chemically modified lipoproteins which are associated with retention and accumulation of lipids in the body (leading to disease conditions), as well as unmodified lipoproteins which are involved in clearance of cholesterol from the body and maintenance of normal homeostasis. SR-BI is the only known physiologically relevant receptor of high density lipoprotein particle (HDL), and has been shown to play an important role in HDL metabolism. The mechanism of SR-BI mediated transfer of lipid from lipoprotein has not yet been elucidated. However, several studies have proposed the involvement of known mechanisms of endocytosis in SR-BI mediated transfer of lipids from HDL. Given the broad spectrum of ligands that bind SR-BI and the high specificity of binding and uptake of ligands, we propose that the SR-BI mediates uptake of lipoproteins in a ligand dependent manner. Using lipoprotein ligands labelled with a fluorescent lipid we demonstrated that blocking cellular endocytosis by potassium depletion, hyperosmolarity and disrupting the structure of micro filaments in cells, has a differential effect on murine SR-BI mediated lipid transfer from modified lipoprotein (Ac-LDL) and native lipoprotein (HDL.) Lipid transfer from Ac-LDL is susceptible to these treatments, whereas lipid transfer from HDL was not affected to the same extent. On the other hand, human SR-BI mediated lipid transfer from both Ac-LDL as well as HDL is affected under these conditions. However, human SR-BI was shown not to colocalize with clathrin, in the presence of HDL and Ac-LDL, dismissing the possibility that clathrin mediated uptake might be involved in lipid uptake by cells. Using a dominant negative mutant of dynamin 1 (Dyn1-K44A) we showed that murine and human SR-BI mediated lipid uptake from HDL and AcLDL is dynamin dependent. Using stable cell lines expressing fluorescent protein tagged human SR-BI, and fluorescent lipid tagged HDL and Ac-LDL, we were able to show the dynamics of human SR-BI mediated lipid uptake in cells. This process was shown to be dependent on microfilament and microtubules.</p>