Platelet-activating factor induces cell growth through tyrosine phosphorylation pathway in cultured rat mesangial...

Accumulating evidence suggests that platelet-activating factor (PAF) may play a role in renal pathophysiology. Therefore, in order to investigate this notion further, the effects of PAF on cell growth and tyrosine phosphorylation were analyzed in cultured rat mesangial cells. PAF was found to enhance a time and concentration-dependent increase in phosphotyrosine in several proteins and stimulate 3H-thymidine incorporation. Tyrosine phosphorylation was also enhanced by PAF in protein kinase C (PKC) depleted cells, whereas a tyrosine kinase inhibitor, genistein, inhibited tyrosine phosphorylation of these proteins at the concentration of 1 microgram/ml. PAF stimulated 3H-thymidine incorporation at concentrations below 10(-6) M, but exerted progressive inhibition at concentrations above 10(-6) M. Pre-treatment with phorbol 12-myristate 13-acetate (PMA) did not affect PAF-enhanced incorporation at lower concentrations of PAF, and reversed the inhibitory effects of PAF at higher concentrations. Finally, genistein pre-treatment completely inhibited PAF-induced cell growth at the concentration of 1 microgram/ml. Both tyrosine phosphorylation and 3H-thymidine incorporation induced by PAF were completely inhibited by pre-treatment with the PAF-receptor antagonist, CV-6209, at the concentration of 10(-5)M. These results suggest that PAF enhancement of tyrosine phosphorylation occurred in a PKC-independent manner and that a tyrosine kinase was associated with PAF-induced tyrosine phosphorylation. Moreover, they indicate that the phosphoinositide hydrolysis-PKC pathway is not essential for PAF-induced cell proliferation, and that PKC activation may play an inhibitory rather than a stimulatory role in mitogenesis in response to PAF. Our results indicate that the tyrosine phosphorylation pathway induced by PAF may participate critically in downstream mitogenic signaling through the PAF receptor.