The role of the Ah receptor and p38 in Benzo[a]pyrene-7,8-dihydrodiol and Benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide-induced apoptosis

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants in the environment. Benzo[ a] pyrene ( B[ a] P), a prototypical member of this class of chemicals, affects cellular signal transduction pathways and induces apoptosis. In this study, the proximate carcinogen of B[ a] P metabolism, trans-7,8-dihydroxy-7,8-dihydrobenzo[ a] pyrene ( B[ a] P-7,8-dihydrodiol) and the ultimate carcinogen, B[ a]P-r-7,t-8-dihydrodiol-t-9,10-epoxide(+/-) ( BPDE-2) were found to induce apoptosis in human HepG2 cells. Apoptosis initiated by B[ a] P-7,8-dihydrodiol was linked to activation of the Ah receptor and induction of CYP1A1, an event that can lead to the formation of BPDE-2. With both B[ a] P-7,8-dihydrodiol and BPDE-2 treatment, changes in anti- and pro-apoptotic events in the Bcl-2 family of proteins correlated with the release of mitochondrial cytochrome c and caspase activation. The onset of apoptosis as monitored by caspase activation was linked to mitogen-activated protein ( MAP) kinases. Utilizing mouse hepa1c1c7 cells and the Arnt-deficient BPRc1 cells, activation of MAP kinase p38 by B[a]P-7,8-dihydrodiol was shown to be Ah receptor-dependent, indicating that metabolic activation by CYP1A1 was required. This was in contrast to p38 activation by BPDE-2, an event that was independent of Ah receptor function. Confirmation that MAP kinases play a critical role in BPDE-2-induced apoptosis was shown by inhibiting caspase activation of poly( ADP-ribose) polymerase 1 (PARP-1) by chemical inhibitors of p38 and ERK1/2. Furthermore, mouse embryo p38(-/-) fibroblasts were shown to be resistant to the actions of BPDE-2-induced apoptosis as determined by annexin V analysis, cytochrome c release, and cleavage of PARP-1. These results confirm that the Ah receptor plays a critical role in B[ a] P-7,8-dihydrodiol-induced apoptosis while p38 MAP kinase links the actions of an electrophilic metabolite like BPDE-2 to the regulation of programmed cell death.