THE MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY MEDIATES GROWTH ARREST OR E1A-DEPENDENT APOPTOSIS IN SKBR3 HUMAN BREAST-CANCER CELLS

Previously, we have shown that phorbol ester (PMA) induces p21(WAF1/CI P1)-dependent growth arrest in SKBr3 breast cancer and LNCaP prostate cancer cells. Here, I demonstrate that inhibition of Raf-I kinase by d ominant-negative Raf-I or pharmacological depletion of Raf-I prevented PMA-mediated induction of p21(WAF1/CIP1). Similarly, PD98059, a speci fic inhibitor of MEK, abolished p21(WAF1/CIP1) induction and PMA-induc ed growth arrest. Like PMA, the H-ras oncogene, another activator of t he Raf-I/MEK/MAPK pathway, transactivated p21(WAF1/CIP1) in SKBr3 cell s. I further investigated PMA-induced growth arrest following infectio n of SKBr3 cells with 12S EIA expressing adenovirus. Although high lev els of EIA oncoprotein prevented both PMA induced p21(WAF1/CIP1) and g rowth arrest, smaller amounts of EIA abrogated growth arrest without d own-regulation of p21(WAF1/CIP1). Therefore, EIA can stimulate prolife ration downstream of p21(WAF1/CIP1) Albeit less effective than full ac tivity, either Rb- or p300- binding activity of EIA was sufficient for the abrogation of PMA-mediated growth arrest. EIA-driven proliferatio n of PMA-treated SKBr3 cells was accompanied by apoptosis. New therape utic approaches can be envisioned that would utilize stimulation of th e Raf-I/MEK/MAPK pathway to inhibit growth of PMA-sensitive cancer cel ls.