Membrane-related oxysterol function: preliminary results on the modification of protein kinase C activity and substrate phosphorylation by 7 beta,25-dihydroxycholesterol.

Oxysterols exhibit a wide variety of biological activities, including potent immunosuppressive effects. 7 beta,25-Dihydroxycholesterol (7,25-OHC), a synthetic oxysterol, has been shown to strongly inhibit the lymphocyte response to different stimuli. This compound has been chosen as a model compound to investigate the mechanisms underlying the immunosuppressive effects of oxysterols. As protein kinase C (PKC) constitutes a key enzyme in the pathways leading to cell activation, we have studied the effect of 7,25-OHC on PKC activity in the cytosolic and particulate fractions of spleen cells. Lymphocytes treated with 7,25-OHC showed a decrease of the relative PKC activity in the particulate fractions compared to control cells. These results are confirmed by the observation that 7,25-OHC also reduces the phosphorylation of the endogenous PKC substrates. Thus oxysterols interfere with two membrane related phenomena, ie the modification of membrane PKC activity and the inhibition of the phosphorylation of the substrates of PKC located in the membrane. Previous results obtained by fluorescence polarisation revealed a modification of the membrane fluidity after oxysterol treatment. Furthermore, it has been demonstrated that oxysterols are incorporated into cell membranes. The alteration of the cell membrane could impair the signal transduction and may explain the immunosuppressive activity of oxysterol. Thus, along with other biological effects previously reported, oxysterols decrease membrane associated PKC activity in immune cells.