Structure and function of the HDL receptor SR-BI : scavenger receptor class B type I
Abstract: Low plasma levels of high-density lipoprotein (HDL) and high levels of low-density lipoprotein (LDL) are well-recognized risk factors for cardiovascular disease. The uptake and degradation of LDL cholesterol have been characterized in detail at the molecular level. Less is known about the metabolism and cellular uptake of HDL cholesterol. In 1996 a novel receptor, scavenger receptor class B type I (SR-BI), was proposed to be an authentic HDL receptor. The aim of this thesis was to study the relevance of SR-BI as an HDL and scavenger receptor.The rat SR-BI cDNA was cloned and characterized. It encodes a protein containing 509 amino acids with two putative transmembrane domains and several motifs, including a peroxisomal targeting sequence, a leucine zipper domain and potential intracellular phosphorylation sites. The predicted double-transmembrane hairpin topology of SR-BI was verified experimentally and the SR-BI gene was localized on rat and human chromosomes.Expression of SR-BI was high in rat ovary, adrenal cortex and liver. In the rat ovary, expression of SR-BI was found in corpus luteum, known to use exogenous cholesterol as substrate for steroid synthesis, and in theca cells of follicles at all stages of development, including atretic follicles. Theca cells and cells overexpressing SR-BI bound apoptotic granulosa cells in vitro, suggesting a scavenger receptor function of SR-BI in the removal of apoptotic cells as part of the remodeling of atretic follicles to secondary interstitial cells. In the gallbladder, SR-BI was present at the apical portion of columnar epithelial cells, suggesting that this receptor may also influence the composition of the bile.The role of the carboxy(C-)terminal intracellular domain of SR-BI was studied with respect to protein interaction, targeting and function of the receptor. At the C-terminus of SR-BI a peroxisomal targeting motif was identified, indicating possible interaction with peroxisomes. SR-BI was shown by coimmunoprecipitation to interact with the peroxisomal targeting import receptor, Pex5p, and overexpression of the C-terminal intracellular domain of SR-BI resulted in translocation to peroxisomes, in vitro. The C-terminal intracellular domain of SR-BI was not required for HDL lipid uptake, but was found important for cellular sorting/targeting of the receptor. The leucine zipper domain was shown to be of potential importance for dimerization of SR-BI and for receptor-mediated HDL binding and lipid transfer.Structural and functional analyses of SR-BI in vitro and in vivo support its proposed role as a physiologically relevant HDL receptor. Further studies of structure and function of SR-BI may increase the understanding of the physiology and pathophysiology relating to HDL cholesterol metabolism and possibly contribute to novel therapeutic strategies.
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