Effects of combinations of interferon-β_ser and interferon-γ on interferon-inducible proteins and on the cell cycle

SKCO 1 human colon carcinoma cells have been shown to be synergistically inhibited in their growth by the combinations of α-interferon (IFN-α) or β-interferon (IFN-β_ser) and γ-interferon (IFN-γ). To determine if a correlation could be established between this synergistic antiproliferative effect and a synergistic induction in IFN-inducible proteins, or a unique perturbation in the cell cycle, we studied the effects of IFN-β_ser and IFN-γ, alone and in combination, on 2′,5′-oligoadenylate (2-5A) synthetase, indoleamine-2,3-dioxygenase (IDO), a human analogue of the murine Mx protein (p78), and the phases of cell cycle. 2-5A synthetase was maximally induced after a 24-h exposure to both IFN-β and IFN-γ. A synergistic enhancement of 2-5A synthetase activity was observed only with low concentration of each IFN (0.05 ng/ml). IDO activity was induced by IFN-γ and the combination of IFN-β_ser and IFN-γ, but not IFN-β_ser alone. The differences in IDO activity between IFN-γ and the combination, however, were not statistically significant. The p78 protein was induced in a dose-dependent manner by IFN-α and IFN-β_ser, IFN-γ enhanced the expression of p78 induction by IFN-α or IFN-β_ser, even at concentrations of IFN-γ that did not induce the protein when administered as a single agent. The combination of IFN-α and IFN-β_ser, which results in an antagonstic antiproliferative effect, also resulted in an antagonistic induction of p78. No changes in the cell cycle were observed following exposure to IFN-β_ser, IFN-γ, or the combination, and treatment with IFN-γ did not inhibit the accumulation of cells in G₂M caused by colchicine. Thus, the synergistic antiproliferative effect produced by IFN-β_ser and IFN-γ in SKCO 1 cells could not be correlated with a synergistic enhancement in 2-5A synthetase or IDO activity, or with a perturbation in the cell cycle. In contrast, the combination of IFN-γ and IFN-α or IFN-β_ser synergistically enhanced the expression of p78 protein in these cells.