RADIATION-INDUCED APOPTOSIS MEDIATED BY RETINOBLASTOMA PROTEIN

The role of the retinoblastoma gene product, RB, in transmitting the s ignals of apoptosis is unclear, but RB is considered to be antiapoptot ic because RB mediates cell cycle arrest that also can interrupt intra cellular signaling pathways leading to apoptosis, gamma-Radiation can cause apoptosis, the process of programmed cell death, via several mec hanisms including DNA damage, ceramide production and the generation o f free radical oxygen species. We investigated the effect of RB on rad iation-induced apoptosis by restoring normal RB expression in DU-145 p rostate cancer cells that have one deleted and one truncated RB gene. DU-145 cells are highly resistant to apoptosis induced either by radia tion or by the addition of ceramide. Two independently derived RB-posi tive DU-145 derivative cell lines underwent apoptosis after irradiatio n or exposure to the cell permeable C-2-ceramide, Apoptosis in the RB- positive cell lines was not associated with major changes in the cell cycle response to irradiation. RB-mediated apoptosis occurred in the a bsence of expression of caspases 8, 6, 3, and 7 and without detectable cleavage of poly(ADP)ribose polymerase. However, a specific inhibitor of serine proteases, Na-p-Tosyl-L-lysyl- chloromethyl ketone, inhibit ed radiation-induced apoptosis in DU-145 cells expressing RB. Radiatio n-induced apoptosis was preceded by an increase in JUN protein express ion and accompanied by activation of the stress-related JUN kinase, Ou r data show that RB is proapoptotic in DU-145 cells and acts upstream of JUN expression and JNK activation.