title: N-methyl-N'-nitro-N-nitrosoguanidine and benzo[a]pyrene-7,8-diol-9,10-epoxide inhibit glucocorticoid-inducible polyoma virus middle-T gene expression in rat mT-1 cells by a post-transcriptional mechanism.
abstract: Treatment of rat mT-1 cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE) inhibited the steroid-induced expression of the polyoma virus middle-T (mT) gene. Rat mT-1 cells were derived from normal F111 fibroblasts transfected with a plasmid (pSVM-mT) containing a mT cDNA sequence linked to the mouse mammary tumor virus long terminal repeat, thus rendering the mT gene responsive to glucocorticoid hormones. Following a 1-h treatment period with either 100 ng/ml of MNNG or 100 ng/ml of BPDE, administration of 1 microM dexamethasone (DEX) for 24 h resulted in, respectively, 51 and 33% less induction of phosphorylated mT protein in carcinogen-treated than in solvent-treated cells. MNNG appeared to increase the basal level of phosphorylated mT antigen expression by 44%, whereas BPDE had little or no effect on the level of phosphorylated mT protein in uninduced cells. MNNG and BPDE had little or no effect on the level of total steroid-induced mT RNA following addition of DEX for 4 or 20 h. The concentrations of MNNG and BPDE used in these experiments caused, respectively, an 18 and 30% decline in cell number 24 h following carcinogen treatment and had no effect on cell viability as determined by trypan blue exclusion. Previous studies demonstrating the inhibitory effects of chemical carcinogens on the steroid-mediated induction of endogenous genes suggested that carcinogens mediate their effects via a pretranslational mechanism. The present study, using a recombinant DNA construct inserted into the cell genome, provides evidence that carcinogens can inhibit hormone-induced gene expression by a post-transcriptional mechanism as well.
mesh_terms: D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000494:pharmacology*; D000818:Animals; D000956:Antigens, Viral / Q000378:metabolism; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D003907:Dexamethasone / Q000494:pharmacology*; D004101:Dihydroxydihydrobenzopyrenes / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects*; D005814:Genes, Viral / Q000187:drug effects*; D008769:Methylnitronitrosoguanidine / Q000494:pharmacology*; D011120:Polyomavirus / Q000187:drug effects / Q000235:genetics*; D012323:RNA Processing, Post-Transcriptional; D051381:Rats; D014162:Transfection
chemical_list: D000956:Antigens, Viral; D004101:Dihydroxydihydrobenzopyrenes; D008769:Methylnitronitrosoguanidine; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide; D003907:Dexamethasone