Nucleolar accumulation of cyclic adenosine 3':5'-monophosphate receptor proteins during regression of MCF-7 human breast tumor.

Cyclic adenosine 3':5'-monophosphate (cAMP) receptor proteins (high-affinity binding proteins) present in growing and regressing MCF-7 human breast tumors were identified and characterized by the use of the photoaffinity-labeled 8-azido[32P]-cAMP and the affinity-purified antibodies to type I and type II regulatory subunits (RI and RII, respectively) of cAMP-dependent protein kinase. The cytosol fraction of growing MCF-7 tumors contained four major types of the 8-azido[32P]cAMP-binding proteins with molecular weights of 35,000, 47,000, 50,000, and 52,000. Following estrogen withdrawal, the amount of these proteins increased in the cytosol of regressing tumors. RI antibody immunoprecipitated cAMP receptor protein with a molecular weight of 47,000, whereas RII antibody immunoprecipitated Mr 50,000 and 52,000 proteins. The Mr 35,000 protein was not precipitated by either RI or RII antibodies. In the nuclear extracts of the growing tumors, the 8-azido-[32P]cAMP-binding proteins with molecular weights of 34,000, 35,000, 44,000, and 47,000 were detected. Following estrogen withdrawal, the 8-azido[32P]cAMP-binding proteins with molecular weights of 50,000 and 52,000 newly appeared in the nuclei of regressing tumors. The Mr 47,000 protein was immunoprecipitated by RI antibody and the Mr 34,000, 44,000, 50,000, and 52,000 proteins were precipitated by RII antibody. An indirect immunofluorescence revealed that, during regression of MCF-7 tumors, the intensity of immunofluorescence of RII proteins dramatically increased in the nucleoli, whereas immunofluorescence of RI remained the same in the nuclei. These results suggest that, during hormone-induced regression of human breast tumors, the Mr 50,000 and Mr 52,000 RII cAMP-binding proteins are translocated to the nucleoli from cytoplasm. Thus, the accumulation of these cAMP receptor proteins at nucleolar site(s) correlates with the regression of MCF-7 tumors.