Mechanism of the Combined Antitumor Effect of Natural Human Tumor Necrosis Factor‐α and Natural Human Interferon‐α on Cell Cycle Progression

We have studied the mechanism of the synergistic effect of the combination of tumor necrosis factor‐α (TNF‐α) and interferon‐a (IFN‐α) on cell cycle progression using two‐parameter flow cytometry in vitro and an immunohistochemical staining method in vivo. The cells used were human colon cancer cell line RPMI 4788 in vitro and in vivo, and human breast cancer cell line MX‐1 and human renal cancer cell line NAMKO‐1 in vivo. In the in vitro experiment, the cell cycle progressed normally as time elapsed in the control group. However, in the group treated with TNF‐α and IFN‐α in combination (combination group), it appeared that the transition from the S phase to the G(2)/M phase was blocked, and the cells that accumulated in the S phase died. In the in vivo experiment with male nude mice of a CD‐1 genetic background, the antitumor effect on all three kinds of cancer cells was significantly greater in the combination group than in the control group. The cell labeling index on staining with bromodeoxyuridine in the combination group became markedly larger and the mitotic index smaller than in the other groups. From these results, it was concluded that in the combination group, both in vitro and in vivo, tumor cells markedly accumulated in the S phase and their progression from the S phase to the G(2)/M phase in the cell cycle was inhibited.