Bone marrow stromal cell cytochrome P4501B1 is required for pre-B cell apoptosis induced by 7,12-dimethylbenz[a] anthracene

We previously demonstrated that murine bone marrow stromal cells express hi gh levels of cytochrome P4501B1 (CYP1B1) that metabolizes 7,12-dimethylbenz a[a]anthracene (DMBA), and that DMBA activates the Ah receptor (AhR) in the se cells in vitro. More recently, we reported that CYP1B1 is required for D MBA-induced lymphoblastoma formation in vivo. In this study, we addressed t he hypothesis that bone marrow stromal cell CYP1B1, and not AhR activation, is required for DMBA-induced pre-B-cell apoptosis. Although DMBA did not d irectly cause apoptosis in pre-B cells, dose-dependent apoptosis of pre-B c ells was observed when they were cocultured with a bone marrow stromal cell line. The DMBA 3,4-dihydrodiol metabolite was more potent in effecting pre -B-cell apoptosis than DMBA, whereas the potent AhR agonist 2,3,7,8-tetrach lorod-ibenzo-p-dioxin was inactive. Both pre-B cells and bone marrow stroma l cells contained DMBA-diol-epoxide DNA adducts, indicating that reactive m etabolites were transferred from stromal cells to pre-B cells. DMBA caused apoptosis when cocultured with primary bone marrow stromal cells isolated f rom AhR-null mice but not CYP1B1-null mice. When cocultured with AhR-null p rimary bone marrow stromal cells, DMBA induced approximately 50% of the pre -B-cell apoptosis seen with stromal cells from AhR-heterozygous mice. This reduced level of apoptosis parallels the decreased CYP1B1 expression in AhR -null mouse bone marrow stromal cells. These findings provide convincing ev idence that bone marrow stromal cell CYP1B1 metabolism of DMBA, but not AhR activation, is required for DMBA-induced pre-B-cell apoptosis.