DEFECTIVE REGULATION OF PHOSPHATIDYLCHOLINE-SPECIFIC PHOSPHOLIPASE-C AND PHOSPHOLIPASE-D IN A KINDRED WITH TANGIER DISEASE - EVIDENCE FOR THE INVOLVEMENT OF PHOSPHATIDYLCHOLINE BREAKDOWN IN HDL-MEDIATED CHOLESTEROL EFFLUX MECHANISMS

The negative correlation between coronary heart disease and plasma lev els of HDL has been attributed to the ability of HDL to take up cellul ar cholesterol, The HDL(3)-induced removal of cellular cholesterol was reported to be impaired in fibroblasts from patients with familial HD L deficiency (Tangier disease, TD), In addition, we have recently show n that HDL, stimulates the hydrolysis of phosphatidylcholine (PC) in c holesterol-loaded fibroblasts. To investigate whether this cell signal ing pathway is involved in cholesterol efflux mechanisms, we compared the HDL(3)-induced PC hydrolysis in normal fibroblasts and in fibrobla sts from a TD kindred, in whom the HDL(3)- and apolipoprotein A-I (ape A-I)-induced mobilization of cellular cholesterol was found to be red uced by 50%. The HDL(3)-induced formation of phosphatidic acid (PA) vi a PC-specific phospholipase D (PC-PLD) was markedly reduced by 60-80% in these cells, whereas the formation of diacylglycerol (DG) via PC-sp ecific phospholipase C (PC-PLC) was two- to threefold enhanced. Defect ive regulation of PC-PLC and PC-PLD was similarly observed in response to apo A-I and endothelin, but not in response to the receptor-indepe ndent stimulation of PC hydrolysis by PMA. A Tangier-like PA and DG fo rmation pattern could be induced in normal cells after preincubation w ith pertussis toxin, suggesting the involvement of a G-protein, The im paired mobilization of radiolabeled cellular cholesterol in TD cells c ould completely be overcome by increasing the PA levels in the presenc e of the PA phosphohydrolase inhibitor propranolol, Conversely, the in hibition of PA formation in the presence of 0.3% butanol as well as th e inhibition of DG formation in the presence of the PC-PLC inhibitor D 609 reduced the mobilization of cellular cholesterol both in normal a nd in TD cells. Our data indicate that the coordinate formation of PA and DG via PC-PLD and PC-PLC is essential for efficient cholesterol ef flux. The molecular defect in this TD kindred appears to affect an ups tream effector of protein kinase C responsible for the G-protein-depen dent regulation of PC-specific phospholipases.