Glucocorticoid regulation of the genes encoding thymidine kinase, thymidylate synthase, and ornithine decarboxylase in P1798 cells

The expression of a number of genes was measured in P1798 cells treated for various periods of time with 0.1 mm dexamethasone. Thymidine kinase (TK) activity decreased under these conditions with 50% inhibition achieved within approximately 8 h. Decreased TK activity was associated with reduced abundance of TK mRNA. Analysis of nuclear tran-scription indicated that this was attributable to a decrease in the number of RNA polymerase II molecules engaged in transcription of the TK gene. With respect to TK, there was an overall correlation between enzyme activity, mRNA, and nuclear transcription. The data are consistent with the hypothesis that glucocorticoid inhibition of expression of TK is primarily due to inhibition of transcription. Transcription of the TK gene was also reduced by greater than 90% after inhibition of protein synthesis for 6 h. This suggests that transcription of this gene requires a protein of short biological half-life. It is proposed that this hypothetical transcription factor is regulated by glucocorticoids. The amount of thymidylate synthase and dihydrofolate reductase remained constant for at least 24 h in dexamethasone-treated P1798 cells. Dihydro folate reductase mRNA likewise remained constant. However, the mRNA encoding thymidylate synthase decreased 80-90% within 24 h. The mRNA encoding ornithine decarboxylase also decreased. In neither case did this appear to be primarily due to inhibition of transcription of the respective genes. The abun-dance of the mRNAs encoding hypozanthine-guanine phosphoribosyl transferase and phosphoglycerate kinase did not decrease in dexamethasone-treated cells. The data indicate that glucocorticoids are regulating gene expression by both transcriptional and posttranscriptional mechanisms.