Previous studies have shown that treatment of a rat hepatoma cell line with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) caused concurrent decreases of the steroid-inducible levels of both tyrosine aminotransferase (TAT) enzyme activity and TAT-specific RNA. These studies have been extended and it has now been shown that MNNG treatment had no effect on the RNA levels of the constitutive housekeeping gene alpha-tubulin nor on the steroid- and metal-inducible metallothionein (MT) gene. Although attempts were made to measure the levels of the glucocorticoid receptor by both biochemical and molecular methods, receptor levels were too low (< 100 fmol/mg protein) to quantitate accurately. However, the lack of effect of MNNG on MT RNA levels suggests that the inhibitory effect of the carcinogen is not mediated through alterations in receptor level or affinity. The steroid hormone system has also been employed to examine the effects of an oncogene on expression of a cytoskeletal protein. A polyoma virus middle T gene has been recombined with a steroid-inducible promoter, rendering the oncogene sensitive to induction by steroids. It has been shown in two separately transfected clones that increases in the levels of the middle-T gene cause increases in alpha-tubulin gene expression. Also being studied is the role of drug metabolism in determining susceptibility to chemical carcinogens. The induction kinetics of cytochromes PI- and P3-450 have been determined in fetal mice after exposure to various inducers. The P-450s exhibited both tissue- and inducer-dependent specificity. It was also demonstrated that P3-450 is induced during the fetal period. Finally, a long-term carcinogenicity study has been initiated to determine if alterations in expression of the genes regulating drug metabolic enzymes affect the toxicological and carcinogenic consequences of diethylstilbestrol (DES) administration. The biochemical studies, as well as the chemical treatment of all the animals for the 2-year carcinogenicity study, have been completed.
