Lysophosphatidic acid, but not phosphatidic acid, is a potent Ca2(+)-mobilizing stimulus for fibroblasts. Evidence for an extracellular site of action.

Abstract

Lysophosphatidic acid (LPA) is a potent mitogen for quiescent fibroblasts. Among the earliest detectable responses to LPA is GTP-dependent phosphoinositide hydrolysis (van Corven, E. J., Groenink, A., Jalink, K., Eichholtz, T., and Moolenaar, W. H. (1989) Cell 59, 45-54). Here we describe the Ca2(+)-mobilizing properties of LPA in human fibroblasts and present evidence suggesting that previously reported Ca2(+)-mobilizing effects of phosphatidic acid are attributable to contamination with LPA. Addition of LPA (1-oleoyl or 1-palmitoyl) to fibroblasts evokes the formation of inositol 1,4,5-trisphosphate accompanied by an immediate but transient rise in [Ca2+]i which originates primarily from intracellular stores. The Ca2+ response is dose-dependent with a half-maximal effect at LPA concentrations as low as 10 ng/ml, far below the reported half-maximal effect for DNA synthesis (5-10 micrograms/ml). LPA-induced Ca2+ release is also observed in various other cell types, both normal and transformed, but not in Jurkat T cells and neutrophils. The Ca2(+)-mobilizing action of LPA is specific, in that 1,2-dioleoyl-phosphatidic acid (when prepared free of LPA contaminants), other lysophospholipids, monoacylglycerol, and free fatty acids have no effect. Furthermore, LPA, unlike lysophosphatidylcholine, does not cause detectable membrane leakiness, even when added at high concentrations (500 micrograms/ml). The LPA-induced Ca2+ signal is blocked completely by tetradecanoylphorbol acetate, but is not affected by prior stimulation of the cells with Ca2(+)-mobilizing agonists such as bradykinin or histamine. In contrast, pretreating the cells with a low dose of LPA desensitizes the Ca2+ response to subsequent addition of higher doses. This homologous desensitization is not inhibited by staurosporine, nor by down-regulating protein kinase C with tetradecanoylphorbol acetate, suggesting independence of functional protein kinase C activity. Addition of La3+ instantaneously blocks inositol phosphate production and Ca2+ mobilization in response to LPA, but not to bradykinin, most likely due to formation of inactive La3(+)-LPA complexes, suggesting that LPA acts at an extracellular site on the plasma membrane to trigger GTP-dependent phosphoinositide breakdown.