Effects of perturbations of pools of deoxyribonucleoside triphosphates on expression of ribonucleotide reductase, a G₁/S transition state enzyme, in p53-mutated cells

Effects of drug treatment with antimetabolites on a human colon cancer cell line, SW480, were studied. Cells were treated with 10 μM of 5- fluorouracil (5FU), an inhibitor of pyrimidine synthesis, or 1000 μM of hydroxyurea (HU), an inhibitor of both purine and pyrimidine syntheses, or the combination. Recombinant α-2a-interferon (IFN), 500 U/mL, also was employed, as this augments the effects of both antimetabolites in vitro and in vivo. The predominant effect of this combination was to block cells in early S phase as measured by 5-bromo-2'-deoxyuridine (BrdUrd) incorporation. By 24 hr, 86% of the cells had accumulated in S phase, but failed to progress to G₂/M. This was accompanied by an early, rapid decline in all four deoxyribonucleoside triphosphates (dNTPs) by 38-86% at 4-24 hr. Despite these effects, expression of the G₁/S transition state enzyme, ribonucleotide reductase (RR), increased at 24 hr as measured by a 3 to 5-fold increase in mRNA levels for the M2 subunit, in the absence of a measurable effect on protein levels. The rise in levels of RR mRNA and the continued progression of cells into S phase were associated with a synergistic inhibition of cell cycle proliferation resulting from treatment with the three-drug combination. This suggests that in the presence of antimetabolite-induced depletion of dNTPs, SW480 cells, which lack a normal p53 gene, will proceed into S phase, and that this is associated with a rise in expression of the G₁/S transition state enzyme, RR. Cells arrested in S phase by a p53-independent mechanism will undergo a synergistic enhancement of cell death.