Altered regulation of cholesterol and cholesteryl ester synthesis in Chinese-hamster ovary cells overexpressing the oxysterol-binding protein is dependent on the pleckstrin homology domain

Oxysterol-binding protein (OSBP) is a high-affinity receptor for a variety of oxysterols, such as 25-hydroxycholesterol, that down-regulate cholesterol synthesis and stimulate cholesterol esterification. To examine a potential role for OSBP in regulating cholesterol metabolism, we stably overexpressed this protein in Chinese-hamster ovary (CHO)-K1 cells. Compared with mock-transfected controls, several cell lines overexpressing wild-type OSBP (CHO-OSBP) displayed a 50% decrease in cholesteryl ester synthesis when cultured in medium with delipidated serum, 25-hydroxycholesterol or low-density lipoprotein (LDL). CHO-OSBP cells showed a 40-60% decrease in acyl-CoA:cholesterol acyltransferase activity and mRNA, a 50% elevation in mRNA for three sterol-regulated genes [LDL receptor, 3-hydroxy-3-methylgluraryl (HMG)-CoA reductase and HMG-CoA synthase], and an 80% increase in [¹⁴C] acetate incorporation into cholesterol. CHO-K1 cells overexpressing two OSBP mutants with a complete or N-terminal deletion of the pleckstrin homology (PH) domain had cholesterol esterification and synthesis rates that were similar to those shown by mock-transfected controls. Unlike wild-type OSBP, both PH domain mutants displayed diffuse cytoplasmic immunofluorescence staining and did not translocate to the Golgi apparatus in the presence of 25-hydroxycholesterol. CHO-K1 cells overexpressing OSBP have pronounced alterations in cholesterol esterification and synthesis, indicating a potential role for this receptor in cholesterol homoeostasis. The phenotype observed in cells overexpressing OSBP is dependent on the PH domain, which appears to be necessary for ligand-dependent localization of OSBP to the Golgi apparatus.