Appearance of 50,000- and 52,000-dalton cAMP receptor proteins in the nucleoli of regressing MCF-7 human breast cancer upon estrogen withdrawal.

Affinity purified antibodies to type I and type II regulatory subunits (RI and RII, respectively) of cAMP-dependent protein kinase were utilized to identify and localize the cAMP receptor proteins in growing vs regressing MCF-7 tumors in nude mice. In the nuclear extracts of growing tumors the RI antibody immunoprecipitated cAMP receptor protein of 47,000 daltons, whereas the RII antibody precipitated 44,000- and 34,000-dalton cAMP receptor proteins. Following estrogen withdrawal, new species of cAMP receptor proteins with molecular weights of 50,000 and 52,000 appeared in the nuclei of regressing tumors. The 50,000- and 52,000-dalton proteins were specifically precipitated by the RII antibody but not by RI antibody. Indirect immunofluorescence revealed that during regression of MCF-7 tumors, the intensity of immunofluorescence of RII antibody crossreacting cAMP binding proteins dramatically increased in the nucleoli whereas the immunofluorescence of RI remained the same. These results suggest a regulatory role of RII in mammary cancer regression.