DNA-damaging Aryl Hydrocarbons Induce Mdm2 Expression via p53-independent Post-transcriptional Mechanisms

During previous studies, we found that mdm2 mRNA levels were elevated in benzo[a]pyrene (BaP, a polycyclic aryl hydrocarbon)-treated cells under conditions of DNA damage-induced cell cycle arrest (Vaziri, C., and Faller, D. V. (1997) J. Biol. Chem. 272, 2762-2769). We have identified potential aryl-hydrocarbon receptor-binding sites in the mdm2 promoter. However, we show that induction of mdm2 mRNA by BaP is entirely dependent upon aryl-hydrocarbon-induced genotoxicity and does not involve direct aryl-hydrocarbon receptor-mediated transcriptional activation of the mdm2 gene. Heterologous mdm2 promoter-reporter constructs containing p53-response elements were not responsive to BaP treatment. Therefore the p53-response elements in the mdm2 promoter are insufficient to confer DNA damage-dependent expression of mdm2. Furthermore, mdm2 transcripts were induced by BaP in p53 null cells from transgenic mice (although both basal and BaP-induced mdm2 expression levels were reduced in these cells relative to p53(+/+) cultures). These data show that p53-mediated mechanisms cannot account for BaP/DNA damage-induced mdm2 expression. Mdm2 promoter-reporter gene assays and nuclear run-off analyses of nascent mdm2 transcripts showed that transcriptional induction was unable to account for the large changes in mdm2 transcript levels following BaP treatment. However, mdm2 mRNA half-life measurements showed stabilization of the mdm2 transcript (from approximately 1 h to >4 h) in response to BaP. To our knowledge, this is the first report of control of mdm2 at the post-transcriptional level and in a p53-independent manner. Transient ectopic expression of mdm2 strongly augmented aryl-hydrocarbon-induced apoptosis, demonstrating that mdm2 levels can have a profound effect on the cellular response to DNA damage. Overall, our results suggest a potentially important link between DNA damage signaling and RNA stability that may be relevant to cell cycle regulation, tumor suppression, and environmental carcinogenesis.