A radioresistant variant cell line, NMT-1R, isolated from a radiosensitive rat yolk sac tumour cell line, NMT-1: differences of early radiation-induced morphological changes, especially apoptosis.

A radioresistant variant cell line, NMT-1R, was isolated by repeated radiation exposure of a radiosensitive rat yolk sac tumour cell line, NMT-1, producing alpha-fetoprotein, with the potential for lymphatic metastasis in the inbred Wistar rat. Cultured NMT-1R cells showed more cobblestone-like appearances, although the morphological features were almost the same as radiosensitive NMT-1 cells reported previously. The doubling time of NMT-1R cells was 13.6 h, being shorter than that of NMT-1 cells (16.0 h). For NMT-1R cells, D0 for radiation sensitivity was 165 +/- 3 cGy, 1.7 times as large as for NMT-1 cells. The extrapolation number, n, was 1.48 +/- 0.17 for NMT-1R cells although that for NMT-1 cells was 1.08 +/- 0.15. The surviving fractions at 2 Gy (SF2) were 0.42 for NMT-1R cells and 0.28 for NMT-1 cells. The population of G2-M phase for NMT-1R cells was larger than for NMT-1 cells (32.5 versus 26.8%) in exponentially growing cells. Although a clear G2 delay was observed after irradiation with a dose of 182 cGy for both cell lines, NMT-1R cells had a shorter recovery time from G2 block than NMT-1 cells, G1 arrest was observed in NMT-1 cells. NMT-1 cells showed much higher incidence of early morphological changes, especially apoptosis, after irradiation with a dose > 500 cGy compared with NMT-1R cells.