TRANSIENT TYROSINE PHOSPHORYLATION OF P34(CDC2) IS AN EARLY EVENT IN RADIATION-INDUCED APOPTOSIS OF PROSTATE-CANCER CELLS

BACKGROUND. Previous studies have demonstrated that androgen-independe nt human prostate cancer cells undergo radiation-induced apoptosis. Th e present study investigated the early events that trigger the apoptot ic response of prostate cancer cells after exposure to ionizing irradi ation. METHODS. Human prostate cancer cells (PC-3) were exposed to sin gle doses of ionizing irradiation, and the immediate protein phosphory lation events were temporally correlated with induction of apoptosis. Apoptosis among the irradiated cell populations was evaluated using th e fluorescein-terminal transferase assay. RESULTS. The kinetics of pho sphorylation of a Mr 34,000 substrate followed a transient course: an initial increase was observed after 10 min postirradiation, reaching m aximum levels by 60 min, and the protein subsequently underwent rapid dephosphorylation. Subsequent analysis revealed that the substrate for this tyrosine phosphorylation is the serine/threonine p34(cdc2) prote in kinase, a cell cycle regulatory protein that controls cell entry in to mitosis. This enhanced phosphorylation temporally preceded the radi ation-induced apoptotic DNA fragmentation as detected by the terminal transferase technique. Arresting the cells in G(0)/G(1) phase by pretr eatment with suramin totally abrogated radiation-induced phosphorylati on of p34(cdc2) protein at the tyrosine residue, indicating that this posttranslational modification occurs in cell populations that escape G(2) arrest and undergo apoptosis in response to radiation. CONCLUSION S. These results suggest that a rapid and transient phosphorylation of a protein that controls mitotic progression precedes and potentially triggers radiation-induced apoptosis in prostate cancer cells. (C) 199 7 Wiley-Liss, Inc.