Phosphatidate and monooleylphosphatidate inhibition of fibroblast adenylate cyclase is mediated by the inhibitory coupling protein, Ni.

It has previously been shown that monooleylphosphatidate (MOPA) and phosphatidate inhibit cAMP accumulation in VA13 and WI-38 fibroblasts. In this study we investigated whether this inhibition might be due to a decrease in adenylate cyclase activity. Our results showed that both MOPA and phosphatidate inhibit prostaglandin E1-stimulated adenylate cyclase in WI-38 membranes in a concentration-dependent manner with half-maximal inhibitions at 0.1 and 0.5 microM, respectively, and maximal inhibitions of 35-55%. A 5 microM concentration of structurally similar lipids caused no significant inhibition. The inhibitory effects of MOPA and phosphatidate on adenylate cyclase were GTP dependent, greater at low concentrations of Mg2+, eliminated following treatment of cells with islet-activating protein, nonadditive with carbachol, and noncompetitive with prostaglandin E1. Collectively these data suggested that MOPA and phosphatidate inhibitions of cAMP accumulation were due at least in part to an Ni-mediated inhibition of adenylate cyclase. Furthermore, the inhibitions showed the same characteristics normally associated with hormonal inhibition of this enzyme.