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 be nzo[a]pyrene (BaP, a polycyclic aryl hydrocarbon)-treated cells under condi tions 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 t hat induction of mdm2 mRNA by BaP is entirely dependent upon aryl-hydrocarb on-induced genotoxicity and does not involve direct aryl-hydrocarbon recept or-mediated transcriptional activation of the mdm2 gene. Heterologous mdm2 promoter-reporter constructs containing p53-response elements were not resp onsive to BaP treatment. Therefore the p53-response elements in the mdm2 pr omoter are insufficient to confer DNA damage-dependent expression of mdm2, Furthermore, mdm2 transcripts were induced by BaP in p53 null cells from tr ansgenic mice (although both basal and BaP-induced mdm2 expression levels w ere reduced in these cells relative to p53(+/+) cultures). These data show that p53-mediated mechanisms cannot account for BaP/DNA damage-induced mdma expression. Mdm2 promoter-reporter gene assays and nuclear run-off analyse s of nascent mdm2 transcripts showed that transcriptional induction was una ble to account for the large changes in mdma transcript levels following Ba P treatment. However, mdm2 mRNA half-life measurements showed stabilization of the mdm2 transcript (from similar to 1 h to >4 h) in response to BaP, T o our knowledge, this is the first report of control of mdma at the post-tr anscriptional level and in a p53-independent manner. Transient ectopic expr ession of mdm2 strongly augmented aryl-hydrocarbon-induced apoptosis, demon strating that mdma levels can have a profound effect on the cellular respon se to DNA damage. Overall, our results suggest a potentially important link between DNA damage signaling and RNA stability that may be relevant to cel l cycle regulation, tumor suppression, and environmental carcinogenesis.