Benzo[a]pyrene represses DNA repair through altered E2F1/E2F4 function marking an early event in DNA damage-induced cellular senescence

Transcriptional regulation of DNA repair is of outmost importance for the restoration of DNA integrity upon genotoxic stress. Here we report that the potent environmental carcinogen benzo[a]pyrene (B[a]P) activates a cellular DNA damage response resulting in transcriptional repression of mismatch repair (MMR) genes (MSH2, MSH6, EXO1) and of RAD51, the central homologous recombination repair (HR) component, ultimately leading to downregulation of MMR and HR. B[a]P-induced gene repression is caused by abrogated E2F1 signalling. This occurs through proteasomal degradation of E2F1 in G2-arrested cells and downregulation of E2F1 mRNA expression in G1-arrested cells. Repression of E2F1-mediated transcription and silencing of repair genes is further mediated by the p21-dependent E2F4/DREAM complex. Notably, repression of DNA repair is also observed following exposure to the active B[a]P metabolite BPDE and upon ionizing radiation and occurs in response to a p53/p21-triggered, irreversible cell cycle arrest marking the onset of cellular senescence. Overall, our results suggest that repression of MMR and HR is an early event during genotoxic-stress induced senescence. We propose that persistent downregulation of DNA repair might play a role in the maintenance of the senescence phenotype, which is associated with an accumulation of unrepairable DNA lesions.