Mechanism of interaction between interferon-gamma and antineoplastic agent on the differentiation of HL-60 promyelocytic cells.

Exposure of HL-60 promyelocytic leukemia cells to a combination of interferon-gamma (IFN gamma) and 5-fluorouracil (5-FU), at concentrations ineffective by themselves, induced a significant differentiation into monocyte-like cells. This phenomenon was accompanied by a synergistic antiproliferative effect. Further characterization of these two activities of the IFN gamma/5-FU combination on HL-60 cells was carried out. Whereas a brief pretreatment of the cells with IFN gamma followed by 5-FU was sufficient to exert the synergistic antiproliferative action, the effect on differentiation was dependent on a prolonged concomitant exposure to both drugs. In an attempt to gain more insight into the biochemical mechanisms of these phenomena, we have examined the effects of RNA and protein synthesis inhibitors and of cytoskeleton disrupting agents on the actions of IFN gamma. Inhibition of RNA or protein synthesis by actinomycin D or cycloheximide did not prevent the antiproliferative action of IFN gamma nor the induction of monocytic differentiation, yet these two compounds blocked the priming effect of IFN gamma on the potentiation of 5-FU action. Actinomycin D synergistically potentiated the antiproliferative action of IFN gamma. Colchicine, vinblastine, and cytochalasin B, disrupting the microtubular and microfilament structure, did not interfere with the actions of IFN gamma; higher concentrations of the drugs even improved the priming effect. Exogenous thymidine, known to counteract the antiproliferative effect of 5-FU, also blocked the antigrowth action but not the differentiation induced by the IFN gamma/5-FU combination. The results suggest the existence of two different mechanisms of the IFN gamma/5-FU synergism: one governing the antiproliferative action via an effect on thymidine synthetase, inducible by a short-term IFN gamma pretreatment and dependent on de novo RNA and protein synthesis; and the other mediating the induction of differentiation requiring a long-term exposure of the cells to both drugs. From a clinical point of view, drug combinations such as IFN gamma and 5-FU, inducing differentiation as well as inhibiting proliferation, may suggest a new approach to the treatment of leukemia.