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CD36 and scavenger receptor class B, type I (SR-BI) are both class B scavenger receptors that recognize a broad variety of ligands, including oxidized low density lipoprotein (oxLDL), HDL, anionic phospholipids, and apoptotic cells. In this study we investigated the role of mouse CD36 (mCD36) as a physiological lipoprotein receptor. We compared the association of various lipoprotein particles with mCD36 and mSR-BI expressed in COS cells by adenovirus-mediated gene transfer. mCD36 bound human oxLDL and mouse HDL with high affinity. Human LDL bound poorly to mCD36, indicating that mCD36 is unlikely to play a significant role in LDL metabolism. The ability of mCD36 to mediate the selective uptake of cholesteryl esters (CE) from receptor-bound HDL was assessed. In comparison with mSR-BI, mCD36 inefficiently mediated the selective uptake of CE. Hepatic overexpression of mCD36 in C57BL/6 mice by adenovirus-mediated gene transfer did not result in significant alterations in plasma LDL and HDL levels. We conclude that mCD36, while able to bind HDL with high affinity, does not contribute significantly to HDL or LDL metabolism.

Scavenger receptors bind and internalize modified low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Because the expression of scavenger receptors is not down-regulated by cholesterol, macrophages (Mphi) expressing scavenger receptors can internalize substantial quantities of cholesteryl ester from oxidized LDL and HDL, leading to foam cell formation. Mphi express several different classes of the growing scavenger receptor family on their cell surface and their relative contribution to Mphi cholesterol physiology and atherogenesis is the subject of intense investigation. We focus on the potential role of two scavenger receptors, macrosialin and SR-BI/II in Mphi cholesterol metabolism. Macrosialin is a predominantly Mphi-specific oxidized LDL-binding protein and an atherogenic diet markedly up-regulates its hepatic expression in atherosclerosis-susceptible and atherosclerosis-resistant mouse strains. The HDL receptor, SR-BI and its splicing variant SR-BII, colocalize with caveolin in caveolae in Mphi. Caveolae are initial acceptor sites for cholesteryl esters and these findings indicate a possible role for caveolae and SR-BI in Mphi-selective lipid uptake and in regulating Mphi cholesterol flux in the vascular wall.

The scavenger receptor class B, type I (SR-BI), binds high density lipoprotein (HDL) and mediates selective uptake of cholesteryl ester from HDL and HDL-dependent cholesterol efflux from cells. We recently identified a new mRNA variant that differs from the previously characterized form in that the encoded C-terminal cytoplasmic domain is almost completely different. In the present study, we demonstrate that the mRNAs for mouse SR-BI and SR-BII (previously termed SR-BI.2) are the alternatively spliced products of a single gene. The translation products predicted from human, bovine, mouse, hamster, and rat cDNAs exhibit a high degree of sequence similarity within the SR-BII C-terminal domain (62-67% identity when compared with the human sequence), suggesting that this variant is biologically important. SR-BII protein represents approximately 12% of the total immunodetectable SR-BI/II protein in mouse liver. Subcellular fractionation of transfected Chinese hamster ovary cells showed that SR-BII, like SR-BI, is enriched in caveolae, indicating that the altered cytoplasmic tail does not affect targeting of the receptor. SR-BII mediated both selective cellular uptake of cholesteryl ether from HDL as well as HDL-dependent cholesterol efflux from cells, although with approximately 4-fold lower efficiency than SR-BI. In vivo studies using adenoviral vectors showed that SR-BII was relatively less efficient than SR-BI in reducing plasma HDL cholesterol. These studies show that SR-BII, an HDL receptor isoform containing a distinctly different cytoplasmic tail, mediates selective lipid transfer between HDL and cells, but with a lower efficiency than the previously characterized variant.

Mice deficient in both macrophage-colony-stimulating factor (M-CSF, op) and apolipoprotein E (apoE) have elevated cholesterol levels but are protected from atherosclerosis. To assess the contribution of macrophage (Mphi) phenotypic heterogeneity and scavenger receptor (SR-A) expression to this seeming paradox, we characterized the Mphi phenotype by immunohistochemistry in these animals. Lesion size was determined in animals fed a chow or Western-type diet, and lipoprotein clearance studies were performed in vivo. Op0/E0 mice have fourfold smaller aortic root lesions than op2/E0 animals despite 2.5-fold higher total plasma cholesterol levels. Mphis in atherosclerotic lesions of op2/E0 mice constitute a predominantly recruited and M-CSF-dependent population. In addition, Mphis in different locations in plaques show phenotypic heterogeneity. SR-A expression in op0/E0 mice is reduced in proportion to the decrease in Mphi numbers, and M-CSF is thus not an essential requirement for SR-A expression in vivo. M-CSF-deficient mice degrade injected AcLDL , showing an adequate level of SR-A activity present in vivo. In contrast, beta-VLDL clearance in op0/E0 mice is decreased, implicating monocytes/Mphis in its catabolism. There is prominent lipid accumulation in op2/E0 Kupffer cells and hepatocytes but not in M-CSF-independent Kupffer Mphis from op0/E0 mice. SR-A, while abundantly expressed on both Kupffer cells and sinusoidal endothelial cells in op2/E0 mice, remains mainly on sinusoidal endothelial cells in op0/E0 mice. This may explain preservation of SR-A activity in these animals. Our findings clearly illustrate the importance of both M-CSF and M-CSF-dependent monocytes/Mphis in maintaining cholesterol homeostasis and in atherogenesis.

The class B, type I scavenger receptor has been implicated as a receptor for high density lipoprotein (HDL). We have isolated a murine cDNA clone encoding an alternative form of SR-BI that differs in the putative cytoplasmic domain of the receptor. This variant form, likely the result of alternative mRNA splicing, is designated SR-BI.2. SR-BI.2 mRNA was detected in mouse tissues known to express SR-BI and tissue-specific differences in the relative abundance of SR-BI.2 were apparent. In mouse adrenal glands, SR-BI.2 represented approximately one-third of total SR-BI mRNA, whereas in mouse testes, SR-BI.2 represented the major mRNA species (79% of total). SR-BI.2 was also detected in the human cell lines examined, namely HeLa, HepG2, and THP-1 cells. CHO cells transfected with the mouse SR-BI.2 cDNA expressed significant levels of SR-BI.2 protein and acquired the ability to take up fluorescent lipid (DiI) from DiI-HDL. Alternative splicing of SR-BI represents a potentially important process for the regulation of SR-BI expression and function.