Cyclic AMP‐Binding Proteins: Inverse Relationship with Estrogen‐Receptors in Hormone‐Dependent Mammary Tumor Regression

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Abstract
Dimethylbenzanthracene‐induced rat mammary carcinomas possess activities binding cyclic adenosine 3′:5′‐monophosphate (cAMP) and estrogen. When dimethylbenzanthracene‐induced tumors regress after ovariectomy of the host, a change in the specific binding of cAMP and estrogen occurs in the tumors. Six days after ovariectomy, cAMP binding increases 5‐fold in the nuclei and 2‐fold in the cytosol of tumors, while nuclear and cytoplasmic estrogen binding decreases by 80% and 50%, respectively. These changes in activities binding cAMP and estrogen are detectable within 1 day after ovariectomy and the changes are reversed when resumption of tumor growth is induced by the injection of 17β‐estradiol. When dimethylbenzanthracene‐induced tumors fail to regress after ovariectomy, the change in activities binding cAMP and estrogen does not occur. Significant increases in the cAMP level as well as in adenylate cyclase and cAMP‐phosphodiesterase activities are also found in the regressing tumors. Concomitant with the increase of cAMP‐binding activity is an increase in histone kinase activity in the regressing tumor. These data suggest the involvement of cAMP in the growth control of a hormone‐dependent mammary tumor.