Modulation of carcinogen metabolism and DNA interaction by calcium glucarate in mouse skin

Almost all the polycyclic aromatic hydrocarbons (PAHs) require metabolic activation to exert their carcinogenic activity. Environmental carcinogen [(3)H] benzo[a]pyrene (BP) is carcinogenic only after its metabolic transformation to a reactive intermediate, which can then bind to cellular macromolecules. Inhibition of dimethylbenz anthracene- (DMBA-) DNA binding generally accompanied inhibition of tumor initiation as most inhibitors of initiation interfere with the metabolic activation of the initiator. The importance of carcinogen-DNA interaction and the enzymes involved in the metabolism of carcinogenic polycyclic hydrocarbons has led to a search for inhibitors that would be useful in modifying the cancer-causing effects of the PAHs. We tested the effect of calcium glucarate (Cag), a naturally occurring nontoxic compound, on carcinogen metabolism and DNA interaction. Cag inhibited [(3)H] BP binding to both calf thymus DNA in vitro and to epidermal DNA in vivo. Application of Cag to mouse skin caused a dose-dependent inhibition of [(3)H] BP binding to epidermal DNA. To establish the relevance of the in vivo results to the in vitro situation, we followed the in vitro effect of Cag on [(3)H] BP binding to calf thymus DNA and observed that Cag inhibited the [(3)H] BP binding to calf thymus DNA in the presence of microsomes prepared from animals treated with DMBA. We also studied related events like DNA synthesis and carcinogen metabolism. For assessing the DNA synthesis, thymidine kinase was used as marker. Cag caused a dose-dependent inhibition of DMBA-induced thymidine kinase activity. At the same time, Cag caused a marked inhibition of DMBA-induced aryl hydrocarbon hydroxylase (AHH) activity, an enzyme responsible for the metabolism of PAHs like BP, both in vivo and in vitro. Our study indicates that Cag exerted its antitumor effect possibly by inhibiting the carcinogen-DNA binding, which appears to be due to reduced DNA synthesis and AHH activity.