Abstract

The tumor suppressor protein p53 is a transcription factor that regulates apoptotic responses produced by genotoxic agents. Previous studies have reported that 7,12-dimethylbenz[a]anthracene (DMBA)-induced bone marrow toxicity is p53-dependent in vivo. Our laboratory has shown that DMBA-induced splenic immunosuppression is CYP1B1- and microsomal epoxide hydrolase (mEH)-dependent, demonstrating that the DMBA-3,4-dihydrodiol-1,2-epoxide metabolite (DMBA-DE) is probably responsible for DMBA-induced immunosuppression. DMBA-DE is known to bind to DNA leading to strand breaks. Therefore, we postulated that a p53 pathway is required for DBMA-induced immunosuppression. In the present studies, our data show that activated p53 accumulated in the nuclei of spleen cells in WT and AhR-null mice after DMBA treatment, but not in CYP1B1-null or mEH-null mice. These results suggest that DMBA activates p53 in a CYP1B1- and mEH-dependent manner in vivo but is not AhR-dependent. Ataxia telangiectasia mutated (ATM) and ATM and Rad3-related protein (ATR) are sensors for DNA damage that signal p53 activation. Increased ATM, phospho-ATM (Ser¹⁹⁸⁷), and ATR levels were observed after DMBA treatment in WT, p53-null, and AhR-null mice but not in CYP1B1-null or mEH-null mice. Therefore, ATM and ATR seem to act upstream of p53 as sensors of DNA damage. Ex vivo immune function studies demonstrated that DMBA-induced splenic immunosuppression is p53-dependent at doses of DMBA that produce immunosuppression in the absence of cytotoxicity. High-dose DMBA cytotoxicity may be associated with p53-independent pathways. This study provides new insights into the requirement of genotoxicity for DMBA-induced immunosuppression in vivo and highlights the roles of ATM/ATR in signaling p53.