Activation of phosphatidylinositol-specific phospholipase C by HDL-associated lysosphingolipid.: Involvement in mitogenesis but not in cholesterol efflux

Our earlier studies demonstrated that high-density lipoproteins (HDLs) stimulate multiple signaling pathways, including activation of phosphatidylcholine-specific phospholipases C and D (PC-PLs) and phosphatidylinositol-specific phospholipase C (PI-PLC). However, only activation of PC-PLs was linked to the HDL-induced cholesterol efflux. In the study presented here, the role of HDL-induced PI-PLC activation was studied. In human skin fibroblasts, HDL potently induced PI-PLC as inferred from enhanced phosphatidylinositol bisphosphate (PtdInsP(2)) turnover and Ca2+ mobilization. The major protein component of HDL, apo A-I, did not induce PtdInsP2 turnover or Ca2+ mobilization in these cells. Both HDL and apo A-I promoted cellular cholesterol efflux, whereas only HDL induced fibroblast proliferation. Inhibition of PI-PLC with U73122 or blocking intracellular Ca2+ elevation with Ni2+ or EGTA markedly reduced the extent of HDL-induced cell proliferation but had no effect on cholesterol efflux. In fibroblasts from patients with Tangier disease which are characterized by defective cholesterol efflux, neither HDL-induced PtdInsP(2) breakdown and Ca2+ mobilization nor cell proliferation was impaired. HDL-induced fibroblast proliferation, PtdInsP2 turnover, and Ca2+ mobilization were fully mimicked by the lipid fraction isolated from HDL. Analysis of this fraction with high-performance liquid chromatography (HPLC) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) revealed that the PI-PLC-inducing activity is identical with two bioactive lysosphingolipids, namely, lysosulfatide (LSF) and sphingosylphosphorylcholine (SPC). Like native HDL, LSF and SPC induced PtdInsP(2) turnover, Ca2+ mobilization, and fibroblast proliferation. However, both compounds did not promote cholesterol efflux. In conclusion, two agonist activities are carried by HDL. Apo A-I stimulates phosphatidylcholine breakdown and thereby facilitates cholesterol efflux, whereas LSF and SPC trigger PI-PLC activation and thereby stimulate cell proliferation.