Inhibition of steroid-inducible tyrosine aminotransferase gene expression by N-methyl-N'-nitro-N-nitrosoguanidine in a rat hepatoma cell line.

Treatment of rat hepatoma cells (the Fao clone of Reuber H35 cells) with a 500 ng/ml concentration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) caused a marked inhibition of steroid-inducible tyrosine aminotransferase (TAT) gene expression. Following a 1-h treatment period with MNNG, addition of 1 microM dexamethasone (DEX) for 6 and 24 h resulted in, respectively, 78 and 51% less induction of TAT enzyme activity in MNNG-treated than in solvent-treated cells. MNNG had little effect on the basal level of TAT activity 6 h after treatment with the carcinogen, but the uninduced level of TAT activity declined to 36% of the control value 24 h following MNNG treatment. MNNG-treated cells concurrently exhibited 75% less total steroid-induced TAT RNA following addition of 1 microM of DEX for 4 h, but showed little or no change in the basal level of total TAT RNA or in the level of total alpha-tubulin RNA. In addition, MNNG treatment resulted in 45% less induction of TAT enzyme activity following a 4-h treatment period with 0.5 mM N6,O2'-dibutyryl-adenosine 3',5'-cyclic monophosphate. The concentration of MNNG used in these experiments caused only a 14% decline in cell number 24 h following carcinogen treatment and had no effect on cell viability as determined by trypan blue exclusion. The data indicate that the inhibitory effects of MNNG appear to be mediated at a pre-translational step. MNNG inhibits the accumulation of total DEX-inducible TAT RNA either by preventing the steroid-mediated increase of the rate of TAT RNA transcription or decreasing total TAT RNA stability, or both.