REGULATION OF RADIATION-INDUCED APOPTOSIS IN ONCOGENE-TRANSFECTED FIBROBLASTS - INFLUENCE OF H-RAS ON THE G2 DELAY

Primary fibroblasts, after serum withdrawal or after irradiation, do n ot undergo apoptosis. Myc-transfected fibroblasts, in contrast, underg o apoptosis upon serum withdrawal and after irradiation. We have studi ed the relationship of apoptosis induction to effects on the G2 phase cell cycle in a series of rat embryo cells transformed by ras(H) plus myc or immortalized by myc alone. In this system, while the presence o f ras(H) had little effect on the extent of apoptosis induction by ser um withdrawal, ras(H) greatly suppressed the apoptotic response of myc -transfected cells to X-rays. The cells into which ras(H) had been int roduced showed a profound G2 arrest associated with suppression of cyc lin B1 mRNA expression. In contrast, cells with myc alone had a minima l G2 delay after irradiation and no suppression of cyclin B1 mRNA expr ession, We hypothesize that ras(H), by influencing the G2 response of cells to X-rays, exerts an anti-apoptotic effect. In support of this h ypothesis; we found that treatment of cells with caffeine, an agent th at relieves the G2 delay after irradiation resulted in increased apopt osis in the irradiated cells, but not in control cells.