Received for publication June 22, 2007.
Revised September 17, 2007.
Accepted for publication October 9, 2007.

P53 AND ATM/ATR REGULATE 7,12-DIMETHYLBENZ[A]ANTHRACENE (DMBA) INDUCED IMMUNOSUPPRESSION

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 mEH dependent, demonstrating that the DMBA-3,4-dihydrodiol-1,2-epoxide metabolite (DMBA-DE) is likely 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 accumulates in the nuclei of spleen cells in WT and AhR null mice following 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. ATM and 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 mice, p53 null and AhR null mice, but not in CYP1B1 null or mEH null mice. Therefore, ATM and ATR appear 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.

Key words: Cytochrome P450, Ah receptor, DNA damage and repair, Protein targets, Reactive intermediates, Knockout, Tumor suppressors