IN-VITRO MODULATION OF RAT ADIPOCYTE GHOST MEMBRANE FLUIDITY BY CHOLESTEROL OXYSTEROLS

The effects of cholesterol and cholesterol-derived oxysterols (cholest anone, cholestenone, coprostanone and epicoprostanol) on adipocyte gho st membrane fluidity were studied using a fluorescence depolarization method. The fluorescence anisotropy of the treated membranes was deter mined using 1,6-diphenyl-1,3,5-hexatriene (DPH) and trimethylammoniump henyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). Cholestanone and cholester ol decreased membrane fluidity at both the concentrations tested (10 a nd 50 mu M) while the rest of the sterols did not exert any significan t effect on membrane fluidity. In the presence of epinephrine, cholest anone partitioned more towards the lipid core but cholesterol partitio ning was not affected. The fusion activation energies (Delta E) obtain ed for membranes preincubated with cholestanone (8.6 kcal/mol) and cho lesterol (8.2 kcal/mol) were not significantly different from that of untreated membranes (8.3 kcal/mol). Membranes preincubated with choles tanone and cholesterol did not exhibit any change in lipid phase throu ghout the temperature range (10-45 degrees C tested. The sterols were found to inhibit fisetin-induced phospholipid methylation in isolated rat adipocytes in the rank order of cholesterol > epicoprostanol > cho lestanone = cholestenone = coprostanone, while basal methylation was u naffected, When adipocytes were preincubated with the sterols before t he addition of fisetin, cholestanone and cholestenone showed 74% and 6 6% inhibition of maximal methylation respectively. These results indic ated that cholesterol oxysterols interact differently with rat adipocy te membranes, with cholestanone interacting more with phospholipids lo cated at the inner lipid bilayer (e.g. phosphatidylethanolamine) while cholesterol interacts more with phosphatidylcholine located at the ou ter lipid bilayer. This differential interaction may cause selective c hanges in membrane fluidity at different depths of the bilayer and thu s may modulate the activities of membrane-bound proteins such as enzym es and receptors.