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Carcinogenesis is a multistep process that is frequently associated with p53 inactivation. The class 1 carcinogen cadmium ([Cd.sup.2+]) causes renal cancer and is known to inactivate p53. [G.sub.2]/mitosis (M) arrest contributes to stabilization of p53-deficient mutated cells, but its role and regulation in [Cd.sup.2+]-exposed p53-deficient renal cells are unknown. In p53-inactivated kidney proximal tubule (PT) cells, comet assay experiments showed that [Cd.sup.2+] (50-100 [mu]M) induced DNA damage within 1-6 h. This was associated with peak formation of reactive oxygen species (ROS) at 1-3 h, measured with dihydrorhodamine 123, and [G.sub.2]/M cell cycle arrest at 6 h, which were abolished by the antioxidant [alpha]-tocopherol (100 p,M). [Cd.sup.+]-induced G2/M arrest was enhanced approximately twofold on release from cell synchronization (double thymidine block or nocodazole) and resulted in approximately twofold increase of apoptosis, indicating that G2/M arrest mirrors DNA damage and toxicity. The Chk1/2 kinase inhibitor UCN-01 (0.3 [mu]M), which relieves [G.sub.2]/M transition block, abolished [Cd.sup.2+] or -induced [G.sub.2] arrest and increased apoptosis. This was accompanied by prevention of [Cd.sup.2+]-induced cyclin-dependent kinase cdc2 phosphorylation at tyrosine 15, as shown by immunofluorescence microscopy and immunoblotting. The data indicate that in p53-inactivated PT cells [Cd.sup.2+]-induced ROS formation and DNA damage trigger signaling of checkpoint activating kinases ataxia telangiectasia-mutated kinase (ATM) and ataxia telangiectasia and Rad3-related kinase (ATR) to cause G2/M arrest. This may promote survival of premalignant PT cells and [Cd.sup.2+] carcinogenesis. 7-hydroxystaurosporine; cyclin-dependent kinase; [p21.sup.WAF1/CIPI]; cyclin; DNA strand breaks doi: 10.1152/ajprenal.00273.2009