The negative correlation between
coronary heart disease and plasma levels of HDL has been attributed to the ability of HDL to take up cellular
cholesterol. The HDL3-induced removal of cellular
cholesterol was reported to be impaired in fibroblasts from patients with familial HDL deficiency (
Tangier disease, TD). In addition, we have recently shown that HDL3 stimulates the hydrolysis of
phosphatidylcholine (PC) in
cholesterol-loaded fibroblasts. To investigate whether this cell signaling pathway is involved in
cholesterol efflux mechanisms, we compared the HDL3-induced PC hydrolysis in normal fibroblasts and in fibroblasts from a TD kindred, in whom the HDL3- and
apolipoprotein A-I (
apo A-I)-induced mobilization of cellular
cholesterol was found to be reduced by 50%. The HDL3-induced formation of
phosphatidic acid (PA) via PC-specific
phospholipase D (PC-
PLD) was markedly reduced by 60-80% in these cells, whereas the formation of
diacylglycerol (DG) via PC-specific
phospholipase C (
PC-PLC) was two- to threefold enhanced. Defective 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-independent stimulation of PC hydrolysis by PMA. A Tangier-like PA and DG formation pattern could be induced in normal cells after preincubation with
pertussis toxin, suggesting the involvement of a
G-protein. The impaired mobilization of radiolabeled cellular
cholesterol in TD cells could completely be overcome by increasing the PA levels in the presence of the PA
phosphohydrolase inhibitor
propranolol. Conversely, the inhibition of PA formation in the presence of 0.3%
butanol as well as the inhibition of DG formation in the presence of the
PC-PLC inhibitor
D 609 reduced the mobilization of cellular
cholesterol both in normal and 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 efflux. The molecular defect in this TD kindred appears to affect an upstream effector of
protein kinase C responsible for the
G-protein-dependent regulation of PC-specific
phospholipases.