Nonconcordance of mutagenicity and carcinogenicity assays results when data from short-term mutation assays do not predict the results of 2 year bioassays. Our studies on those chemicals which produced positive mutagenicity in vitro and failed to produce carcinogenicity in NTP bioassays demonstrated the requirement for cell proliferation in the early stages of chemical exposure for positive carcinogenicity results. Chemicals which fail to cause cell proliferation also fail to cause carcinogenesis, regardless of their activity in mutagenesis assays. We have examined a number of chemicals to date and observed that the ability to cause cell proliferation and carcinogenesis is organ-specific and site-specific within an organ, and may be sex- and species-specific. We are also studying the mechanism(s) whereby chemicals that induce peroxisomes or cytochrome P450 isozymes produce hepatocarcinogenesis, and the relationship of this effect to human risk following exposure to these chemicals. Recent studies have used transgenic (Big Blue) mice to detect in vivo mutagenesis induced by diaminotoluene isomers. Oxazepam, a benzodiazepine compound, is a central nervous system depressant which is widely prescribed for the treatment of anxiety. In NTP rodent bioassays, exposure of male B6C3F1 mice to 0, 125, 2,500 and 5,000 ppm of oxazepam resulted in hepatocellular carcinoma in 47, 38, 100 and 100%, respectively, of the mice. Male B6C3F1 Big Blue transgenic mice were fed 2500 ppm oxazepam or control diet alone for 180 days and sacrificed on the next day. The mutant frequency of lacI in control mice was 5.02 x10-5 whereas the MF in the oxazepam-treated mice was 9.17 x10-5, a significant increase (p<0.05). Clonal expansion of mutations at guanines in 5'-CpG-3' sequencing contexts at three sites was noted. It is postulated that some of the mutations found in the oxazepam-derived spectrum were due to oxidative damage elicited by induction of CYP2B isozymes as the result of chronic oxazepam administration. Results from this study demonstrate that the Big Blue transgenic rodent mutation assay can detect mutations derived from a carcinogen that is nonmutagenic in vitro and can distinguish between the mutation spectrum from treated mice compared to that from control mice. Methyleugenol was found to produce selective toxicity to HCl secreting cells in the glandular stomach resulting in increased stomach pH and serum gastrin levels. Gastrin is thought to mediate the neuroendocrine gastric tumors observed following two year bioassay of methyleugenol. By a different mechanism (alkylation of hepatic DNA) methyleugenol produced highly significant increases in mutation frequency in livers of Big Blue mice that correlated well with the hepatocarcinogenicity of methyleugenol in chronic studies. Future plans in our laboratory are to evaluate the ability of CYP 1A induction to produce gene mutations in vivo by chronic exposure of Big Blue mice to TCDD, and to evaluate the effects of chronic oxidative stress on mutation induction following chronic dietary overload to iron. Additionally, in support of NTP studies on the characterization of the reproductive and developmental effects of tamoxifen, another nongenotoxic carcinogen and reproductive toxicant, we have developed and are using an analytical technique utilizing capillary electrophoresis for the quantification of tamoxifen and metabolites in maternal rodent serum and fetal tissues. The goal of this research is to determine rate constants for transplacental transfer of tamoxifen and metabolites.
Showing the most recent 10 out of 45 publications
