INTERFERON AUGMENTS THE CYTOTOXICITY OF HYDROXYUREA WITHOUT ENHANCINGITS ACTIVITY AGAINST THE M2 SUBUNIT OF RIBONUCLEOTIDE REDUCTASE - EFFECTS IN WILD-TYPE AND RESISTANT HUMAN COLON-CANCER CELLS

The effects of prolonged exposure to the ribonucleotide reductase (RR) inhibitor, hydroxyurea (HU), were assessed in the presence or absence of recombinant interferon alfa-2a (IFN) in wild-type human colon canc er cells (HT-29) and variants expressing low-level resistance to HU (R 200). IFN at nontoxic concentrations decreased the IC50 of HU from 368 mu M to 215 mu M (P < 0.01) in wild-type cells, but not in the resist ant variants. Potential cellular targets for the HU/IFN interaction we re examined. In wild-type, but not resistant cells, treatment with HU at clinically achievable concentrations (1000 mu M) resulted in rapid early inhibition of RR activity between 4 and 24 h after treatment wit h a maximal decrease of 65% at 12 h, decreases in cellular levels of d ATP, dCTP and dGTP by 50-90% over the same time course, and a two- to fourfold increase in the level of mRNA for both the M1 and M2 subunits of RR, at 24, but not between 1 and 4 h, which probably represents a response to the earlier decrease in RR activity. IFN at a clinically a chievable concentration (500 U/ml) failed to augment the effects of HU on RR protein, RR mRNA levels or RR enzyme activity in either the wil d-type or resistant cells, suggesting that the mechanism by which IFN augments the effects of HU in the wild-type cells is independent of th e effects of HU on M2.