Tyrosine phosphorylation of GSalpha and inhibition of bradykinin-induced activation of the cyclic AMP pathway in A431 cells by epidermal growth factor receptor.

An increasing amount of experimental data suggest that cross-talk exists between pathways involving tyrosine kinases and heterotrimeric G proteins. In a previous study, we demonstrated that bradykinin (BK) increases the intracellular accumulation of cAMP in the human epidermoid carcinoma cell line A431 by stimulating adenylate cyclase activity via a stimulatory G protein (Gsalpha) (Liebmann, C., Graness, A., Ludwig, B., Adomeit, A., Boehmer, A., Boehmer, F.-D., Nürnberg, B., and Wetzker, R. (1996) Biochem. J. 313, 109-118). Here, we present several lines of evidence indicating the ability of epidermal growth factor (EGF) to suppress BK-induced activation of the cAMP pathway in A431 cells via tyrosine phosphorylation of Gsalpha. Gsalpha was specifically immunoprecipitated from A431 cells using the anti-alphas antiserum AS 348. Tyrosine phosphorylation of Gsalpha was detectable in EGF-pretreated cells with monoclonal anti-phosphotyrosine antibodies. Additionally, A431 cells were labeled with [32P]orthophosphate in vivo and treated with EGF, and the resolved immunoprecipitates were subjected to amino acid analysis. The results clearly indicate that EGF induces tyrosine phosphorylation of Gsalpha in A431 cells. Treatment of A431 cells with EGF decreased BK-induced cAMP accumulation in intact cells as well as the stimulation of adenylate cyclase by BK, NaF, and guanyl nucleotides, but not by forskolin. Also, EGF treatment abolished both the BK- and isoprenaline-induced stimulation of guanosine 5'-O-(3-[35S]thiotriphosphate) binding to Gsalpha. In contrast, the BK-evoked, Gq-mediated stimulation of inositol phosphate formation in A431 cells was not affected by EGF pretreatment. Thus, EGF-induced tyrosine phosphorylation of Gsalpha is accompanied by a loss of its susceptibility to G protein-coupled receptors and its ability to stimulate adenylate cyclase via guanyl nucleotide exchange. We propose that Gsalpha may represent a key regulatory protein in the cross-talk between the signal transduction pathways of BK and EGF in A431 cells.