[The effect of estradiol on the activity of cAMP-dependent protein kinase in cells of estradiol-dependent tumors of the rat mammary gland. The role of a thermally stable protein inhibitor of cAMP-dependent protein kinase].

Stimulation of rat mammary tumour growth by estradiol is due to the activation of the adenylate cyclase system and cAMP-dependent protein kinases. A single administration of estradiol to ovariectomized rats causes a rise in the cAMP-dependent protein kinase activity in cell nuclei within the first 4-6 hours after injection. This effect is probably due to the translocation of enzymes into nuclei and an increase of their synthesis. The high level of the cAMP-dependent protein kinase activity in cell nuclei was observed in actively growing intact mammary tumours, in contrast to regressing ones in ovariectomized animals. This phenomenon can be accounted for by the decrease in the content of a thermostable protein inhibitor of cAMP-dependent protein kinases rather than by the high level of cAMP.