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.
Hsing
A
A
Cancer Research Center, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Faller
D V
DV
Vaziri
C
C
D016475:3T3 Cells; D000818:Animals; D001564:Benzo(a)pyrene / Q000494:pharmacology*; D004249:DNA Damage; D005347:Fibroblasts / Q000378:metabolism; D006207:Half-Life; D051379:Mice; D009687:Nuclear Proteins; D011499:Protein Processing, Post-Translational; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis* / Q000235:genetics; D051736:Proto-Oncogene Proteins c-mdm2; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D014158:Transcription, Genetic; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*
D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D016159:Tumor Suppressor Protein p53; D001564:Benzo(a)pyrene; C497207:Mdm2 protein, mouse; C497208:Mdm2 protein, rat; D051736:Proto-Oncogene Proteins c-mdm2