COMPARISON OF THE INTRACELLULAR METABOLISM AND TRAFFICKING OF 25-HYDROXYCHOLESTEROL AND CHOLESTEROL IN MACROPHAGES

Oxysterols arising from the diet or through lipid peroxidation may be important in the modulation of cellular cholesterol metabolism. In thi s study, the metabolism of one of the oxysterols, 25-hydroxycholestero l (25OHC), was examined in J774 and mouse peritoneal macrophages. Upta ke of 25OHC from serum was rapid and substantial. Esterification of th e cellular 25OHC was also rapid as was hydrolysis of pre-formed esters . Like cholesterol, 25OHC was removed from cells by an extracellular a cceptor such as high density lipoprotein. Unlike cholesterol, 25OHC wa s also rapidly and extensively removed from cells by serum albumin, bu t not by ovalbumin. The differential removal of oxysterols and cholest erol from cells by albumin allows separation of cellular effects due t o oxysterols and cholesterol. In order to understand more about this d ifferential efflux of sterols, a computer model for sterol mass transp ort in cells was used to compare intracellular trafficking of choleste rol and 25OHC. The rate constants determined by this model for movemen t of sterols between cytoplasm and plasma membrane were similar for bo th cholesterol and 25OHC, whereas those for esterification and ester h ydrolysis as well as those for bidirectional movement between plasma m embrane and extracellular medium were greater for 25OHC than for chole sterol. For both sterols, the rate-limiting step for removal of cellul ar esters appeared to be the rate of cytoplasmic ester hydrolysis. As 25OHC and cholesterol differ significantly in aqueous solubility, the similarity in their rate constants for movement between cytoplasm and plasma membrane is consistent with facilitation of transport between t hese two loci.