Multiple mechanisms of growth inhibition by cyclic AMP derivatives in rat GH1 pituitary cells: Isolation of an adenylate cyclase‐deficient variant

Abstract

GH pituitary cells have been widely utilized for studies of hormone response mechanisms. Studies reported here were motivated by the desirability of isolating characterized GH clones defective in cyclic AMP synthesis or action. Spontaneously occurring GH₁ cell variants resistant to the growthinhibitory effects of cyclic AMP analogs were isolated. Characterization of four variants showed that these were deficient in adenosine kinase and had acquired resistance to the cytotoxic effects of purine nucleoside derivatives formed in the culture medium. A second‐stage selection was undertaken with mutagenized adenosine kinase‐deficient cells. One 8 Br cAMP‐resistant variant was found to have normal cyclic AMP‐dependent protein kinase activity but exhibited altered adenylate cyclase activity. Activation of cyclase activity by fluoride, guanyl nucleotides, cholera toxin, and hormone (VIP) was subnormal in the variant. Mndependent cyclase activity was also subnormal, suggesting that the 8 Br cAMP‐resistant variant may have a deficiency in the catalytic moiety of adenylate cyclase. Surprisingly, adenosine 3′ :5′ ‐monophosphate and 5′ ‐monophosphate derivatives were found to be equally potent in growth‐inhibiting adenosine kinasedeficient cells. Cross‐resistance to 8 Br AMP was observed in the 8 Br cAMP‐resistant variant. We conclude that cyclic AMP derivatives inhibit growth of GH cells by an unanticipated mechanism that is, nonetheless, related to endogenous cyclic AMP synthesis.