TRANSIENT FAILURE TO DEPHOSPHORYLATE THE CDC2-CYCLIN B1 COMPLEX ACCOMPANIES RADIATION-INDUCED G(2)-PHASE ARREST IN HELA-CELLS

Ionizing radiation causes a division delay in mammalian cells, dominat ed by a period of G(2)-phase arrest. The G(2)- to M-phase transition i n dividing mammalian cells is dependent on the kinase activity of the cdc2-cyclin B protein complex. In the present investigation we measure d the quantities of these two proteins, the formation of their complex and the kinase activity of the complex as a function of cell age in t he cell cycle for irradiated and control mammalian cell populations. T he human HeLa S3 cells were synchronized at the G(1)/S-phase border by double thymidine block and exposed 3 h after release to 1.75 Gy of X rays. Studies of HeLa cells at other laboratories have shown that, for doses of 5 Gy or more, division delay is associated with a suppressio n of production of cyclin B mRNA, Here we report that, for cells irrad iated with low doses, there is a transient failure of the complex to a ctivate which correlates with the duration of radiation-induced G(2)-p hase arrest, The irradiated cells showed an increase in both cyclin B and phosphorylated cdc2 over the levels in control cells, and both per sisted for a much longer period than in controls, further confirmation of delay in the activation of the catalytic subunit. (C) 1995 by Radi ation Research Society