Up-regulation in vascular endothelial cells of binding sites of high density lipoprotein induced by 25-hydroxycholesterol.

Exposure of bovine vascular endothelial cell cultures to 25-hydroxycholesterol (50--100 microgram/ml) result in a 5--10-fold increase in cell surface binding sites of high density lipoprotein (HDL). This increase in HDL-binding sites was dependent on time and temperature. After a 48-h exposure to the oxygenated sterol, a maximal increase in HDL binding could be observed, and newly binding sites disappeared rapidly once 25-hydroxycholesterol was removed from the medium. No increase in HDL-binding sites was observed when cells were maintained at 4 degrees C. In contrast, cultures maintained at 37 degrees C did show an increase in HDL-binding sites when exposed to 25-hydroxycholesterol. Since simultaneous exposure of the cells to 25-hydroxycholesterol and cycloheximide resulted in an inhibition of HDL binding to the cells, it is suggested that de novo synthesis of HDL-binding sites is induced by 25-hydroxycholesterol. When the abilities of HDL2 and HDL3 to bind to newly synthesized HDL-binding sites were compared, HDL3 was found to bind more efficiently than HDL2. It is therefore unlikely that apoprotein E plays a major role in the binding of HDL to newly synthesized HDL-binding sites. When the properties of newly synthesized HDL-binding sites were analyzed, they were found to have a high affinity for HDL, since half-maximal binding was reached at a concentration as low as 5 microgram HDL protein/ml, and were saturable. Such HDL-binding sites had a relaxed specificity, since they were capable of binding low density liprotein (LDL). However, when LDL bound to newly synthesized HDL binding sites, it was no longer internalized, as reflected by a 90% reduction of LDL degradation, and instead of being cytotoxic it became mitogenic.