ESTROGENIC REGULATION OF A NOVEL 34 KDA PROTEIN ASSOCIATED WITH CYCLIN B1 IN MCF-7 BREAST-CANCER CELLS

Recent studies have revealed altered regulation of cyclins in breast c ancer cells. To understand the role of aberrant cyclin B1 expression i n the proliferation of breast cancer cells, we examined cyclin B1-asso ciated proteins in estrogen-responsive MCF-7 cells in a cell cycle-dep endent manner. Immunoprecipitation of cell lysate with a monoclonal an ti-human cyclin B1 antibody, followed by Western blot probing with an anti-human cdc2 (PSTAIR) antibody revealed the presence of a 34 kDa pr otein in estradiol-treated cells at 16 h after initiation of cell cycl e progression. Flow cytometry and [H-3]-thymidine (Thd) incorporation experiments showed a dramatic increase in the percentage of S phase ce lls at this time point. This protein was suppressed by an antiestrogen , 4-hydroxytamoxifen. It was not found in MCF-1OA, a normal breast epi thelial cell line. The 34 kDa protein was not reactive with antibodies raised against other cyclin dependent kinases (CDKs), including p34(c dc2(Carboxy terminal)). This protein was functionally active as determ ined by histone H1 kinase activity. These data suggest that the induct ion of a cyclin B1-associated 34 kDa protein during the G1 --> S trans ition might be a positive regulator of cell cycle progression in estro gen-responsive breast cancer cells.