GROWTH-FACTOR MODULATION OF P53-MEDIATED GROWTH ARREST VERSUS APOPTOSIS

Irradiation of mammalian cells can cause cell cycle perturbations and apoptotic cell death. We have investigated the modulation of these phy siologic end points by growth factor stimulation: irradiation of a mur ine hematopoietic cell line in the presence of interlekin-3 (IL-3) ind uces G(1) arrest, and irradiation in the absence of IL-3 results in ra pid apoptotic cell death. Both of these end points are dependent on p5 3. Transient removal of IL-3 at the time of irradiation results in dec reased clonogenic survival of irradiated cells. The removal of IL-3 re sults in a failure of the irradiated cells to arrest at the G(1) check point, despite induction of p53 and p21(WAF1/CIP1), and then the cells enter S-phase where they undergo apoptosis. There are no cytokine-rel ated changes in Bcl-2, Bax, or Bcl-x protein levels that could account for the modulation of G(1) arrest versus apoptosis by growth factors. In contrast, rapid p53-independent alterations of basal levels of gad d45 and p21(WAF1/CIP1) expression are linked to IL-3 withdrawal, sugge sting a potential mechanism for this modulation. Constitutive activati on of cytokine-like pathways with induced expression of v-Src or activ ated c-Raf inhibits the radiation-induced apoptosis and the alteration s in p21(WAF1/CIP1) and gadd45 expression. These observations suggest additional molecular mechanisms that can contribute to the development of radioresistance and resistance to apoptosis during tumorigenesis a nd provide an explanation for the observed lack of p53 mutations in so me tumor types. In addition, these data suggest that oncogenic changes occurring during multistep tumorigenesis could be classified as those that either enhance or decrease apoptosis tendencies.