Cell type-specific mechanisms of regulating expression of the ornithine decarboxylase gene after growth stimulation.

Ornithine decarboxylase (ODC) mRNA is strongly induced by mitogenic activation of resting Swiss 3T3 fibroblasts and T lymphocytes. Nuclear run-on analysis revealed a low level of nascent transcripts in resting fibroblasts that was elevated upon activation. In contrast, there was a high level of transcription across the entire ODC gene in resting T cells, which remained unchanged upon activation. The stability of the mature ODC message was found to be unaffected by mitogenic stimulation. These results indicate that ODC mRNA levels are regulated transcriptionally in Swiss 3T3 cells and posttranscriptionally within the nucleus of T lymphocytes in response to mitogenic stimuli. In this unique situation, the mitogenic induction of a single gene, ODC, is regulated by two very distinct, cell-specific mechanisms.