OVEREXPRESSION OF C-SRC ENHANCES BETA-ADRENERGIC-INDUCED CAMP ACCUMULATION

During our investigations into the physiological role of c-src tyrosine kinase in normal cells, we found that clonal transfectants of C3H10T1/2 murine fibroblasts overexpressing chicken c-src exhibited strikingly elevated levels of cAMP accumulation in response to adrenergic stimulation as compared to control cells. Enhanced cAMP accumulations were detected when cells were treated with the β-agonists, epinephrine, isoproterenol, or terbutaline and were blocked by treatment with the β-specific antagonist propranolol, indicating action through β-adrenergic receptors. The hyperresponsiveness was not observed in cells overexpressing kinase-defective c-src. No differences in basal levels of cAMP, agonist concentration dependence, or kinetics of cAMP accumulation were detected between cells containing elevated levels of wild-type or kinase-defective c-src protein and control cells. To determine if the degree of c-src overexpression could influence the response, multiple clones, transfected with DNA encoding genes for wild-type or kinase-defective c-src plus neomycin resistance or neomycin resistance alone, were derived in parallel and assayed for the amounts of c-src protein produced and the levels of cAMP accumulated in response to epinephrine. Only clones with abundant wild-type c-src protein (>10-fold above endogenous) exhibited enhanced cAMP accumulation, averaging 3.3-fold above control cells. We conclude, therefore, that the enhanced degree of cAMP accumulation in cells overexpressing c-src is dependent upon activation of β-adrenergic receptors and upon a threshold level of pp60(c-src) that retains full tyrosine kinase activity.