Molecular mechanisms of 7,12-dimethylbenzanthracene-induced bone marrow B cell apoptosis

Polycyclic aromatic hydrocarbons (PAHs) are common environmental pollutants which suppress the immune system in part by inducing bone marrow B cell apoptosis. Previously, we demonstrated that a prototypic PAH, 7,12-dimethylbenzanthracene (DMBA), induced B cell apoptosis in cells of a pro/pre-B cell line when these cells were co-cultured with cloned bone marrow stromal cells (BMS2) and that pro/pre-B cell death was mediated in part by NF-κB down-regulation. The goals of the research described were: (1) to determine if the intracellular signaling pathway leading to DMBA-induced pro/pre-B cell death is distinct from the apoptosis signaling pathway responsible for clonal deletion in immature B cells, (2) to evaluate the contribution of caspase-8 and death receptors capable of activating caspase-8 in PAH-dependent, bone marrow B cell apoptosis, and (3) to investigate the role of a mitochondrial apoptosis signaling pathway, with particular emphasis on the contribution of initiator caspase-2 and caspase-9. These goals were accomplished by: (1) evaluating the contribution of c-Myc, p27, p21 and p53, known regulators of immature B cell clonal deletion, to PAH-induced pro/pre-B cell apoptosis, (2) defining the roles of initiator caspase-8 and/or caspase-8-activating death receptors, and (3) investigating whether a second apoptosis pathway involving caspase-2, mitochondrial activation, and caspase-9 is also invoked. Several strains of mice deficient in apoptosis-related genes were exploited for these studies.; Results. (1) As in clonal deletion models, DMBA-induced death of pro/pre-B cells was at least partially dependent on c-Myc down-regulation. However, in contrast to clonal deletion, DMBA-induced pro/pre-B cell death was not dependent on p27Kip1 or p21WAF1 up-regulation but did coincide with p53 induction. (2) Caspase-8 was shown to be a critical caspase in DMBA-induced pro/pre-B cell apoptosis and that caspase-8 activation is likely to activate effector caspase-3 and Bid. However, obvious caspase-8-activating death receptors such as TNFR-1, TNFR-2, Fas, or DR-6, are not critical for caspase-8 activation in this system. (3) Several markers of mitochondrial activation, including mitochondrial membrane potential loss and cytochrome C release from mitochondrial stores, supported a role for the mitochondria in DMBA-induced, bone marrow B cell apoptosis signaling. Significant contributions of caspase-2, caspase-9, Bid, and Bax to apoptosis induction were consistent with mitochondrial activation.; Conclusion. Collectively, these studies suggest that environment pollutants (PAHs) inappropriately down-regulate c-Myc, activate caspase-2, -3, -8 and -9, and BCL-2 family members Bid and Bax, to effect programmed cell death in developing bone marrow B cells.