ATM complexes with HDM2 and promotes its rapid phosphorylation in a p53-independent manner in normal and tumor human cells exposed to ionizing radiation.

To further understand the mechanism(s) by which DNA damage activates p53, we analysed the expression levels of p53 and HDM2 (the human homolog of murine MDM2) in various human diploid fibroblast and tumor cell strains during the period that precedes activation of known downstream effectors of p53. In X-irradiated human cells, HDM2 protein was rapidly phosphorylated in serine/threonine residues in a p53, p14ARF and p73-independent manner. In p53 wild-type cells, HDM2 phosphorylation precedes a detectable increase in the levels of p53 and is not observed in ataxia telangiectasia (AT) fibroblasts. The transfection of AT cells with a vector expressing ATM restored the ability to rapidly phosphorylate HDM2 following X-irradiation, confirming a role for ATM in its phosphorylation. We also show that ATM complexes with HDM2. The DNA lesions signaling the early rapid phosphorylation of HDM2 are a result of X-ray and not UV-type damage. The ATM-promoted early covalent modification of HDM2 in X-irradiated human cells may provide a mechanism to activate p53.