Effects of combinations of interferon-beta ser and interferon-gamma on interferon-inducible proteins and on the cell cycle

Abstract

SKCO 1 human colon carcinoma cells have been shown to be synergistically inhibited in their growth by the combinations of alpha-interferon (IFN-alpha) or beta-interferon (IFN-beta ser) and gamma-interferon (IFN-gamma). 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-beta ser and IFN-gamma, 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-beta and IFN-gamma. A synergistic enhancement of 2-5A synthetase activity was observed only with low concentrations of each IFN (0.05 ng/ml). IDO activity was induced by IFN-gamma and the combination of IFN-beta ser and IFN-gamma, but not IFN-beta ser alone. The differences in IDO activity between IFN-gamma and the combination, however, were not statistically significant. The p78 protein was induced in a dose-dependent manner by IFN-alpha and IFN-beta ser. IFN-gamma enhanced the expression of p78 induction by IFN-alpha or IFN-beta ser, even at concentrations of IFN-gamma that did not induce the protein when administered as a single agent. The combination of IFN-alpha and IFN-beta ser, which results in an antagonistic antiproliferative effect, also resulted in an antagonistic induction of p78. No changes in the cell cycle were observed following exposure to IFN-beta ser, IFN-gamma, or the combination, and treatment with IFN-gamma did not inhibit the accumulation of cells in G2M caused by colchicine. Thus, the synergistic antiproliferative effect produced by IFN-beta ser and IFN-gamma 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-gamma and IFN-alpha or IFN-beta ser synergistically enhanced the expression of p78 protein in these cells.