Points of Departure for BPDE-induced cellular senescence and cell death are characterized by altered DNA damage signaling and regulated by unrepaired DNA double-strand breaks.

The existence of thresholds for carcinogenic compounds is an important topic in toxicology and regulatory science. Traditionally, genotoxic carcinogens are thought to exhibit no thresholds. However, cellular defense mechanisms like DNA repair and apoptosis can neutralize low levels of genotoxic stress implying different Points of Departure (PoDs) for different cellular endpoints. Moreover, since cellular PoDs are regulated by the DNA damage response (DDR) and the associated DNA damage signaling cascades, the question arises whether the DDR and its cellular outcome change, depending on the level of DNA damage. Here we analyzed whether PoDs for distinct cellular processes induced by benzo[a]pyrene-9,10-diol-7,8-epoxide (BPDE) are observed at the same or different level of DNA damage and whether these PoDs correlate with activation of different DNA damage signaling routes. BPDE represents the active metabolite of the polycyclic aromatic hydrocarbon benzo[a]pyrene (B[a]P) which is a product of incomplete combustion and therefore ubiquitously present in the natural environment. Our data indicate a PoD with a LOAEL (lowest observed adverse effect level) between 0.1 and 0.25 µM for DNA strand break formation, DDR activation, induction of cell death and cellular senescence. A high amount of cell death was observed at a dose of 1 µM and was accompanied by accumulation of DNA strand breaks and mediated by a switch from the p53Ser15 signaling axis to the p53Ser46 axis of the DDR. Importantly, BPDE-induced mutagenicity was observed predominantly at low BPDE concentration that failed to trigger the DDR and cellular senescence. These results suggest that low BPDE concentrations, which are unable to activate the DDR, are especially harmful in relation to mutation formation and carcinogenesis, eventually even more than DDR-activating concentrations.