Potentiation of Radiation Cytotoxicity by Recombinant Interferons, a Phenomenon Associated with Increased Blockage at the G₂-M Phase of the Cell Cycle

Radiation sensitivity of human hypernephroma tumor cells, ACHN, was evaluated in the presence or absence of various recombinant interferons (IFNs): α D (rHuIFN-α D), β_sec(rHuIFN-β_sec), and γ (rHuIFN-7). Exponentially growing monolayers of ACHN cells were incubated with IFNs (10³units/ml) for 2, 6, and 24 h prior to irradiation. The effects of IFNs on the growth inhibition, radiation cell survival, and cell cycle redistribution were determined from growth curves, colony-forming efficiency, and flow cytometric analysis. rHuIFN-α D and rHuIFN-β_secwere associated with a modest inhibition in cell growth and the growth delay was reversible upon the removal of IFNs. However, rHuIFN-γ exhibited the greatest inhibitory effect on both cell growth delay and rate. IFN and radiation have an additive effect on cell growth inhibition. Although IFN treatments alone (0–1000 IU/ml) did not have a significant effect on cell survival, radiation killing was increased by IFN pretreatment. The enhancing factor measured at a survival level of 0.1 for rHuIFN-a D, rHuIFN-β_sec, and rHuIFN-γ was 1.21, 1.28, and 1.44, respectively. In addition, this potentiation effect was dependent on the dose of radiation and exposure time to IFN with the maximal effect observed after 24 h of treatment with IFN at 12 Gy. When the distributions of G₁, S, and G₂-M cells were measured 24 and 48 h after combined IFN and radiation treatment, there was a significant increase in the accumulation of cells at the G₂-M phase of the cell cycle compared to either IFN or radiation treatment alone. We conclude that IFNs and radiation have an additional inhibitory effect on ACHN cell growth, that IFN can potentiate radiation cytotoxicity, and that this phenomenon is associated with an increased blockage at G₂-M phase of the cell cycle.