title: Inhibition of steroid-inducible tyrosine aminotransferase gene expression by N-methyl-N'-nitro-N-nitrosoguanidine in a rat hepatoma cell line.
abstract: 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.
authors.forename: M S
authors.initials: MS
authors.lastname: Miller
authors.forename: G N
authors.initials: GN
authors.lastname: Wogan
mesh_terms: D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002460:Cell Line; D004247:DNA / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology*; D004790:Enzyme Induction / Q000187:drug effects; D005786:Gene Expression Regulation; D008114:Liver Neoplasms, Experimental / Q000201:enzymology; D008745:Methylation; D008769:Methylnitronitrosoguanidine / Q000633:toxicity*; D051381:Rats; D014444:Tyrosine Transaminase / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics*
chemical_list: D008769:Methylnitronitrosoguanidine; D003994:Bucladesine; D003907:Dexamethasone; D004247:DNA; D014444:Tyrosine Transaminase