Scavenger receptor B1 (SR-B1) profoundly excludes high density lipoprotein (HDL) apolipoprotein AII as it nibbles HDL-cholesteryl ester [Lipids]

Reverse cholesterol transport (RCT) - transfer of macrophage-cholesterol in the subendothelial space of the arterial wall to the liver - is terminated by selective high density lipoprotein (HDL)-cholesteryl ester (CE) uptake, mediated by scavenger receptor class B, type 1 (SR-B1). We tested the validity of two models for this process - gobbling, i.e., one-step transfer of all HDL-CE to the cell and nibbling, multiple successive cycles of SR-B1-HDL association during which a few CE transfer to the cell. Concurrently, we compared cellular uptake of apo AI with that of apo AII, which is more lipophilic than apo AI, using HDL-[3H]CE labeled with [125I]apo AI or [125I]apo AII. The studies were conducted in CHO-K1 and CHO-ldlA7 cells (LDLR-/-) with (CHO-SR-B1) and without SR-B1 over expression, and in human Huh7 hepatocytes. Relative to CE, both apo AI and apo AII were excluded from uptake by all cells. However, apo AII was more highly excluded from uptake (2-4x) than apo AI. To distinguish gobbling vs. nibbling mechanisms, media from incubations of HDL with CHO-SR-B1 cells were analyzed by non-denaturing PAGE, size exclusion chromatography, and the distribution of apo AI, apo AII, cholesterol and phospholipid among HDL species as a function of incubation time. HDL size gradually decreased, i.e., nibbling, with the concurrent release of lipid-free apo AI; apo AII was retained in an HDL remnant. Our data support a SR-B1 nibbling mechanism that is similar to that of streptococcal serum opacity factor, which also selectively removes CE and releases apo AI leaving an apo AII-rich remnant.

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