Bone marrow stromal cells constitutively express high levels of cytochromeP4501B1 that metabolize 7,12-dimethylbenz[a]anthracene

The polycyclic aromatic hydrocarbon 7,12-dimethylbenz[a]an anthracene (DMBA ) is a potent carcinogen that produces immunotoxic effects in bone marrow. Here, we show that bone marrow stromal cells metabolize DMBA to such produc ts as 3,4-dihydrodiol, the precursor to the most mutagenic DMBA metabolite. The BMS2 bone marrow stromal cell line constitutively expressed higher lev els of CYP1B1 protein and mRNA than C3H10T1/2 mouse embryo fibroblasts. BMS 2 cells also produced a DMBA metabolite profile that was consistent with CY P1B1 activity. Treatment with the potent aryl hydrocarbon receptor (AhR) li gand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced a similar to 2-fold increase in CYP1B1 mRNA, protein, and activity in BMS2 cells. Two forms of the AhR (97 and 104 kDa) and the AhR nuclear translocator were detected in BMS2 cells. The AhR translocated to the nucleus after treatment with TCDD or DMBA but was similar to 5 times slower with DMBA. Primary bone marrow st romal (BMS) cell cultures established from AhR(-/-) mice showed similar bas al CYP1B1 expression and activity as cell cultures established from heteroz ygous littermates or C57BL/6 mice. However, primary EMS cells from AhR-/- m ice did not exhibit increased CYP1B1 protein expression after incubation wi th TCDD, EMS cells therefore constitutively express functional CYP1B1 that is not dependent on the AhR, This contrasts with embryo fibroblasts from th e same mouse strain, in which basal CYP1B1 expression is AhR dependent. We therefore conclude that bone marrow toxicity may be mediated by CYP1B1-depe ndent DMBA metabolism, which is regulated by factors other than the AhR.