Activation of phosphatidylinositol-specific phospholipase C by HDL-associated lysosphingolipid. Involvement in mitogenesis but not in cholesterol efflux
Jr. Nofer et al., Activation of phosphatidylinositol-specific phospholipase C by HDL-associated lysosphingolipid. Involvement in mitogenesis but not in cholesterol efflux, BIOCHEM, 39(49), 2000, pp. 15199-15207
Our earlier studies demonstrated that high-density lipoproteins (HDLs) stim
ulate multiple signaling pathways, including activation of phosphatidylchol
ine-specific phospholipases C and D (PC-PLs) and phosphatidylinositol-speci
fic phospholipase C (PI-PLC). However, only activation of PC-PLs was linked
to the HDL-induced cholesterol efflux. In the study presented here, the ro
le 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 protei
n component of HDL, apo A-I, did not induce PtdInsP2 turnover or Ca2+ mobil
ization 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 e
ffect on cholesterol efflux. In fibroblasts from patients with Tangier dise
ase which are characterized by defective cholesterol efflux, neither HDL-in
duced 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 (HPL
C) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) revealed t
hat the PI-PLC-inducing activity is identical with two bioactive lysosphing
olipids, namely, lysosulfatide (LSF) and sphingosylphosphorylcholine (SPC).
Like native HDL, LSF and SPC induced PtdInsP(2) turnover, Ca2+ mobilizatio
n, and fibroblast proliferation. However, both compounds did not promote ch
olesterol efflux. In conclusion, two agonist activities are carried by HDL.
Apo A-I stimulates phosphatidylcholine breakdown and thereby facilitates c
holesterol efflux, whereas LSF and SPC trigger PI-PLC activation and thereb
y stimulate cell proliferation.