RADIATION WITH 1 GY PREVENTS THE ACTIVATION OF THE MITOTIC INDUCERS MITOSIS-PROMOTING FACTOR (MPF) AND CDC25-C IN HELA-CELLS

The mechanism of the transient G(2) arrest induced by small doses of i onizing radiation involves the failure to activate the correctly forme d pre-mitosis-promoting factor (MPF) complex of cyclin B and p34(cdc2) by dephosphorylation at Tyr15 of the latter, as recent studies of oth er laboratories have indicated. Similar data were obtained with the G( 2) arrest-inducing agents epidermal growth factor and the phorbol este r 12-O-tetradecanoylphorbol-13-acetate (H. Earth and V. Kinzel, Exp. C ell Res., 212: 383-388, 1994, and H. Earth and V. Kinzel, J. Cell. Phy siol., 162: 44-51, 1995). To differentiate the radiation consequences in synchronized HeLa cells from those of 12-O-tetradecanoylphorbol-13- acetate and epidermal growth factor, experiments with a very small dos e (1 Gy) have been carried out in cells close to the G(2)-M border and , for comparison, in mitotic cells, We show that in addition to the fa ilure of p34(cdc2) dephosphorylation at Tyr15, radiation with 1 Gy als o prevents the activation of the phosphatase cdc25-C, the enzyme catal yzing the MPF activation, Tn contrast, irradiation of mitotic cells wi th 1 Gy did not influence that fraction of either MPF or cdc25-C alrea dy activated. Moreover, the gain in MPM-2 antigenicity of cdc25-C, usu ally indicative of an activating phosphorylation, is shown to be preve nted by 1 Gy, The data indicate that the initiation of the proposed au tocatalytic loop between MPF and cdc25-C becomes interrupted by radiat ion, but they give no hint at which point.