Modulation of nitrosourea toxicity in rodent embryonic cells by O6-benzylguanine, a depletor of O6-methylguanine-DNA methyltransferase.

Micromass culture systems of mesencephalon (CNS) and limb bud (LB) regions were used to evaluate the impact of the O6-methylguanine-DNA methyltransferase (MGMT) inhibitor, O6-benzylguanine (BG), on ethylnitrosourea (ENU) and methylnitrosourea (MNU)-induced in vitro toxicity. These primary rat embryo cells were preincubated with BG (2.5 microM for CNS; 10 microM for LB) for 24 hr and then ENU (0-1708 microM) or MNU (0-1600 microM) was added. Five days after culture initiation, cell survival was assessed using neutral red uptake and cell differentiation was assessed using selective staining (Alcian blue in LB cultures and hematoxylin in CNS) and image analysis. In both LB and CNS cultures, preexposure to BG resulted in a significant increase in inhibition of differentiation following MNU exposure but no increase in cytotoxicity. There were no statistically significant changes in the toxicity induced by ENU in these differentiating cells consequent to BG preexposure. Our observations lend support to the idea of a specific role for DNA adducts in defining MNU's toxic effects on cell viability and differentiation.