Cell-cycle associated transcriptional regulation of ribonucleotide reductase in L1210 leukemia cells and drug-resistant variants.

Previous studies from this laboratory have shown that the steady-state levels of the mRNA for the non-heme iron (NHI) subunit of ribonucleotide reductase were markedly elevated in hydroxyurea-resistant L1210 cell lines with minimal changes in the mRNA levels for the effector-binding (EB) subunit. In the present study, wild-type L1210 cells and their drug-resistant variants [hydroxyurea-resistant (HU-7); deoxyadenosine-resistant (Y-8); and deoxyadenosine/pyrazoloimidazole-resistant (ED2)] were synchronized by EGTA treatment in the G0/G1-phase of the cell cycle. Upon the addition of CaCl2, the cells reentered the cell cycle. The steady-state levels and the transcriptional rates of the mRNAs for the EB subunit and glyceraldehyde-3-phosphate dehydrogenase were measured and found to be similar in the drug-resistant variants compared to the wild-type cells. While the steady-state level of the mRNA for the NHI subunit was increased 35-fold in the HU-7 cell line, the transcription rate was increased only 7-fold. The increase in the transcription rate did not account for the large increase in the steady-state level. These data indicate that the increased steady-state level of the mRNA for the NHI subunit in the HU-7 L1210 cell line was not due to cell-cycle differences and that post-transcriptional processing and/or stability may play a role as well.