A novel synergistic stimulation of Swiss 3T3 cells by extracellular ATP and mitogens with opposite effects on cAMP levels.

In Swiss 3T3 mouse fibroblasts, the mitogenic effect of extracellular ATP depends on stimulation of adenylyl cyclase. Lysophosphatidic acid (LPA) and phosphatidic acid (PA) inhibited adenylyl cyclase but synergized with ATP in mitogenic stimulation. This unusual synergism of two mitogens with opposite effects on cAMP levels was further investigated. LPA and PA inhibited the elevation of cAMP caused by cholera toxin, prostaglandin E2, or forskolin, but not the rise induced by ATP. In fact, ATP overcame the inhibitory effects of LPA or PA on cAMP levels. Indeed, in the presence of ATP and either cholera toxin or prostaglandin E2, LPA became a stimulator of adenylyl cyclase. Stimulation of DNA synthesis and inhibition of cAMP accumulation by LPA were inhibited by pertussis toxin, but with different dose-response characteristics. In addition, a normal mitogenic response to LPA was obtained in transfected mutant cells with a defective regulatory subunit for protein kinase A and in cells whose regulation of cAMP levels was abnormal because of overproduction of cAMP phosphodiesterase. The data support the hypothesis that the mitogenic effect of LPA involves a PTX-sensitive Gi protein but not inhibition of adenylyl cyclase.