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 Ca(2+) mobilization. The major protein component of HDL, apo A-I, did not induce PtdInsP(2) turnover or Ca(2+) 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 Ca(2+) elevation with Ni(2+) 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 Ca(2+) mobilization nor cell proliferation was impaired. HDL-induced fibroblast proliferation, PtdInsP(2) turnover, and Ca(2+) 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, Ca(2+) 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.
Nofer
J R
JR
Institut für Klinische Chemie und Laboratoriumsmedizin, Westfälische Wilhelms-Universität, Münster, Germany. nofer@unimuenster.de
Fobker
M
M
Höbbel
G
G
Voss
R
R
Wolinska
I
I
Tepel
M
M
Zidek
W
W
Junker
R
R
Seedorf
U
U
von Eckardstein
A
A
Assmann
G
G
Walter
M
M
D016632:Apolipoprotein A-I / Q000494:pharmacology; D020013:Calcium Signaling; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism; D004247:DNA / Q000096:biosynthesis; D004533:Egtazic Acid / Q000494:pharmacology; D004789:Enzyme Activation; D004963:Estrenes / Q000494:pharmacology; D005347:Fibroblasts; D006801:Humans; D008075:Lipoproteins, HDL / Q000494:pharmacology*; D008246:Lysophospholipids / Q000494:pharmacology*; D008934:Mitogens / Q000494:pharmacology*; D009532:Nickel / Q000494:pharmacology; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000378:metabolism; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D054801:Phosphoinositide Phospholipase C; D010767:Phosphorylcholine / Q000031:analogs & derivatives / Q000494:pharmacology; D011609:Psychosine / Q000031:analogs & derivatives / Q000494:pharmacology; D011760:Pyrrolidinones / Q000494:pharmacology; D013107:Sphingolipids / Q000494:pharmacology*; D013110:Sphingosine / Q000031:analogs & derivatives / Q000494:pharmacology; D013631:Tangier Disease / Q000378:metabolism; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors / Q000187:drug effects*
D016632:Apolipoprotein A-I; D004963:Estrenes; D008075:Lipoproteins, HDL; D008246:Lysophospholipids; D008934:Mitogens; D019269:Phosphatidylinositol 4,5-Diphosphate; D011760:Pyrrolidinones; D013107:Sphingolipids; C005356:sphingosine phosphorylcholine; D010767:Phosphorylcholine; C060229:1-(6-((3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione; D011609:Psychosine; D004533:Egtazic Acid; C019064:psychosine-3'-sulfate ester; D009532:Nickel; D004247:DNA; D002784:Cholesterol; D010738:Type C Phospholipases; D054801:Phosphoinositide Phospholipase C; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D013110:Sphingosine