Abstract
Ionizing radiation stimulates so called DNA damage checkpoint singalling pathway in mammalian cells, whose process is initiated by auto-phosphorylation of ATM protein at serine 1981. Phosphorylated ATM forms discrete foci, and we found, in normal human diploid cells, that the initial phosphorylated ATM foci grew rapidly concurrently with DNA repair, and the foci were colocalized with the foci of phosphorylated histone H2AX, 53BP1 and NBS1. These observation provides a possibility that phosphorylated ATM foci growth plays a role in amplifying the DNA damage signals. To prove this, we examined p53 phosphorylation at serine 15, as a marker for DNA damage signal, in cells defective in non homologous end-joining (NHEJ), which is the major DNA repair pathway in the G1 cells. We used CHO and xrs-5 cells, in which the latter lacks Ku80 involved in NHEJ. One hour after 1 Gy of X-rays, most of the initial foci grow in CHO cells, however, defective growth was observed specifically in G1 cells derived from xrs-5 cells, while cells in other cell cycle phases normally form grown foci. The cells showing defective foci growth also demonstrated weak phosphorylation signal of p53 at serine 15. These results indicate that the growth of phosphorylated ATM foci were coupled with DNA double strand break repair, and that the foci growth is necessary for the efficient transduction of DNA damage checkpoint signals.
