OVEREXPRESSION OF WILD-TYPE P53 OVERRIDES THE MITOGENIC EFFECT OF RI-ALPHA SUBUNIT OF PROTEIN-KINASE-A IN HUMAN BREAST CELLS

Protein kinase A type I (PKAI) and its regulatory subunit RI alpha are overexpressed in cancer cells and are induced by mitogenic hormones a nd growth factors in nontransformed cells. RI alpha/PKAI are directly involved in the G1>S transition and cell proliferation of non-transfor med human breast MCF-10A cells. Retroviral vector-mediated overexpress ion of RI alpha in these cells (MCF-10A RI alpha) confers the ability to grow in serum-free medium. p53 controls a G1 check point before tra nsition to the S phase, playing a key role in the regulation of cell p roliferation and in the preservation of DNA integrity. In this study w e evaluated the interaction of p53 and RI alpha on cell cycle progress ion and cell proliferation of MCF-10A cells. Retroviral vector-mediate d overexpression of wild-type p53 in the MCF-10A neo and MCF-10A RI al pha cells determined a marked inhibition of RI alpha protein expressio n in MCF-10A-p53 cells and induced G0/G1 accumulation, cell gowth arre st and changes in cell morphology not due to apoptosis in both MCF-10A -p53 and MCF-10A RI alpha-p53 cells. On the other hand, in the MCF-10A RI alpha cells we observed an increased expression of the endogenous p53, although these cells were still able to proliferate. These result s suggest that overexpression of wildtype p53 acts in a dominant fashi on to abrogate the RI alpha induction of G1>S transition and cell prol iferation. Moreover, overexpression of RI alpha leads to increased syn thesis of endogenous p53 which, however, is unable to interfere with t he RI alpha-dependent mitogenic signalling.