Role of Tyrosine Phosphorylation in Radiation-Induced Cell Cycle-Arrest of Leukemic B-Cell Precursors at the G2-M Transition Checkpoint

Here we provide experimental evidence that ionizing radiation induces inhibitory tyrosine phosphorylation of the p34^cdc2 kinase in human leukemic B-cell precursors. Herbimycin A markedly reduced tyrosine phosphorylation of p34^cdc2 in irradiated leukemic B-cell precursors, thereby preventing radiation-induced cell cycle arrest at the G2-M transition checkpoint. Thus, tyrosine phosphorylation is directly responsible for the inactivation of p34^cdc2 in irradiated human leukemic B-cell precursors and activation of protein tyrosine kinases is a proximal and mandatory step in radiation-induced G2-arrest arrest at the G2-M checkpoint. Human WEE1 kinase isolated from unirradiated or irradiated leukemic B-cell precursors had minimal tyrosine kinase activity towards p34^cdc2. We detected no increase of human WEE 1 kinase activity after radiation of leukemic B-cell precursors, as measured by (a) autophosphorylation, (b) tyrosine phosphorylation of a synthetic peptide derived from the p34^cdc2 amino-terminal region or (c) recombinant human p34^cdc2-cyclin B complex. Thus the signaling pathway leading to inhibitory tyrosine phosphorylation of p34^cdc2 and G2-arrest in irradiated human leukemic B-cell precursors functions independent of p49^WEE1Hu and enzymes which augment the tyrosine kinase activity of p49^WEE1Hu.