| Abstract: | We have previously reported that alpha-thrombin induces in resting hamster fibroblasts (CCL39) the formation of inositol phosphates (IP) by activating a GTP-binding protein (G protein) sensitive to pertussis toxin (Paris, S., and Pouysségur, J. (1986) EMBO J. 5, 55-60). Here we show that IP formation in CCL39 cells can also be induced by NaF with AlCl3 and by vanadate. In the presence of Li+, IP accumulation is linear over 30 min with no detectable lag and is concentration-dependent. NaF alone is slightly stimulatory, but a marked potentiation is observed in the presence of AlCl3, by itself without effect. Maximal stimulation is obtained with 10 mM NaF and 3 microM AlCl3, and with vanadate half-maximal effect is achieved at 0.3 mM. Both stimulations are markedly inhibited (up to 80%) by pertussis toxin (half-maximal inhibition at 1-2 ng/ml). We therefore conclude that phospholipase C is stimulated by NaF plus AlCl3 (presumably acting as AlF-4) and by vanadate by direct activation of the regulatory G protein. In addition, NaF inhibits the inositol-1-phosphatase, but this effect is not potentiated by AlCl3. Similarly, vanadate inhibits inositol trisphosphate degradation. Maximal stimulations of phospholipase C by AlF-4 and vanadate are not additive, whereas they are both additive with thrombin effects. Pretreatment of cells for 15 min with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate nearly completely abolishes induction of IP formation by AlF-4 and vanadate, suggesting that protein kinase C exerts a feedback negative control either on the G protein or on phospholipase C itself. An increase in cellular cyclic AMP similarly results in a marked attenuation of AlF-4-induced IP formation, indicating that activation of phospholipase C can be controlled also by cyclic AMP. However, the stimulatory effect of AlF-4 on phospholipase C is clearly dissociated from its effect on the adenylate cyclase system. |
