The role of polycyclic aromatic hydrocarbon metabolism in dimethylbenz[a]anthracene-induced pre-B lymphocyte apoptosis

Previous studies indicated that two prototypic PAH, benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA), suppress the developing immune s ystem by inducing apoptosis in bone marrow pre-B lymphocytes. In bone marro w cultures consisting of pre-B cells growing on bone marrow stromal cell mo nolayers, pre-B cell apoptosis was shown to be dependent on the aryl hydroc arbon receptor/transcription factor (AhR) expressed in stromal cells. Howev er, it was not determined if AhR activation alone is sufficient or if DMBA metabolism is required for induction of a stromal cell-derived apoptosis si gnal. To address these issues we assessed: 1) the ability of poorly metabol ized AhR ligands to induce pre-B cell apoptosis and 2) the capacity for and the mechanism through which an early DMBA metabolite induces pre-B cell ap optosis. Three poorly metabolized AhR ligands, 2,3,7,8-tetrachlorodibenzo-p -dioxin, 3,3',4,4',5-pentachlorobiphenyl, and 3,3',4,4'-tetrachlorobiphenyl failed to induce pre-B cell apoptosis in bone marrow cultures, indicating that AhR activation alone is not sufficient to induce apoptosis and suggest ing a role for PAH metabolism in induction of an apoptosis signal. Consiste nt with this hypothesis, DMBA-3,4-dihydrodiol, an early DMBA metabolite, in duced significant pre-B cell apoptosis. The ability of DMBA-3,4-dihydrodiol to activate the AhR, inhibition of DMBA-3,4-dihydrodiol-induced apoptosis by alpha-naphthoflavone, and the significantly lower levels of DMBA-3,4-dih ydrodiol-induced apoposis in pre-B cell populations maintained on AhR(-) st romal cells strongly support a role for the AhR in DMBA-3,4-dihydrodiol-ind uced apoptosis. Of two DMBA-metabolizing enzymes evaluated, CYP1A1 and CYP1 B1, the latter appeared to be the more likely to play a role in DMBA-induce d apoptosis. These data confirm a role for the AhR in PAM-induced pre-B cel l apoptosis, indicate a role for DMBA metabolism, and suggest a feedback lo op in which at least one product of DMBA metabolism augments AhR signaling, leading to induction of an apoptosis stimulus. (C) 1999 Academic Press.