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

Exposure of HL-60 promyelocytic leukemia cells to a combination of interferon-γ (IFNγ) 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 IFNγ/5-FU combination on HL-60 cells was carried out. Whereas a brief pretreatment of the cells with IFNγ 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 IFNγ. Inhibition of RNA or protein synthesis by actinomycin D or cycloheximide did not prevent the antiproliferative action of IFNγ nor the induction of monocytic differentiation, yet these two compounds blocked the priming effect of IFNγ on the potentiation of 5-FU action. Actinomycin D synergistically potentiated the antiproliferative action of IFNγ. Colchicine, vinblastine, and cytochalasin B, disrupting the microtubular and microfilament structure, did not interfere with the actions of IFNγ; 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 IFNγ/5-FU combination. The results suggest the existence of two different mechanisms of the IFNγ/5-FU synergism: one governing the antiproliferative action via an effect on thymidine synthetase, inducible by a short-term IFNγ 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 IFNγ and 5-FU, inducing differentiation as well as inhibiting proliferation, may suggest a new approach to the treatment of leukemia.