R. Araki et al., Enhanced phosphorylation of p53 serine 18 following DNA damage in DNA-dependent protein kinase catalytic subunit-deficient cells, CANCER RES, 59(15), 1999, pp. 3543-3546
DNA-dependent protein kinase (DNA-PK) controls signal transduction followin
g DNA damage. However, the molecular mechanism of the signal transduction h
as been elusive. A number of candidates for substrates of DNA-PK have been
reported on the basis of the in vitro assay system. In particular, the Ser-
15 amino acid residue in p53 was one of the first such in vitro substrates
to be described, and it has drawn considerable attention due to its biologi
cal significance. Moreover, p53 Ser-15 is a site that has been shown to be
phosphorylated in response to DNA damage. In addition, crucial evidence ind
icating that DNA-PK controls the transactivation of p53 following DNA damag
e was reported quite recently.
To clarify these important issues, we conducted the experiments with dna-pk
cs null mutant cells, including gene knockout cells. As a result, we detect
ed enhanced phosphorylation of p53 Ser-18, which corresponds to Ser-15 of h
uman p53, and significant expression of p21 and mdm2 following ionizing rad
iation. Furthermore, we identified a missense point mutation in the p53 DNA
-binding motif region in SCGR11 cells, which were established from severe c
ombined immunodeficient (SCID) mice and used for previous study on the role
of DNA-PK in p53 transactivation.
Our observation clearly indicates that DNA-PK catalytic subunit does not ph
osphorylate p53 Ser-18 in vivo or control the transactivation of p53 in res
ponse to DNA damage, and these results further emphasize the different path
ways in which ataxia telangiectasia-mutated (ATM);and DNA-PK operate follow
ing radiation damage.