The objective of the present study was to measure the sensitizing effect of oxygen following acute as well as protracted hypoxia in cell cultures of maximum homogeneity. Human cells of the established line NHIK 3025 were synchronized by the method of mitotic selection and irradiated while in late G1 (aerobic conditions) or in the oxygen-sensitive restriction point near the G1/S border. Four different conditions of oxygen treatment were used: (1) Cells were irradiated under aerobic conditions 5 h after mitotic selection (G1 lasts for 6.5 h in these cells). (2) Cells were irradiated under extremely hypoxic conditions (i.e. < 4 ppm O2) following an acute (30 min) deoxygenation starting 5 h after mitotic selection. (3) Cells were irradiated under aerobic conditions immediately following 20 h of extreme hypoxia which was started 5 h following mitotic selection. (4) Cells were irradiated under extremely hypoxic conditions after 20 h of extreme hypoxia starting 5 h after mitotic selection. In addition, asynchronous cells were irradiated, either without any pretreatment, or after reoxygenation, following 20 h of extreme hypoxia. The data indicate that the sensitizing effect of oxygen (after protracted hypoxia) is strictly dose-modifying for cells rendered hypoxic while in late G1 and arrested in the oxygen-sensitive restriction point in late G1. There is a non-dose-modifying sensitization when synchronized control cells, irradiated under aerobic conditions, are compared with synchronized cells irradiated during acute hypoxia. In this case, however, one is comparing cells in two different biological states, i.e. cells in an ordinary G1 are compared with cells arrested in the oxygen-sensitive restriction point in G1. The non-dose-modifying nature of the oxygen sensitization as observed in this case may, therefore, reflect biological differences between the cell cultures that are compared rather than differences with respect to the radiochemical sensitization as induced by oxygen at small and large radiation doses. The present data indicate, however, that if cancer cells in human tumours are best represented by a mixture of aerobic cells and chronic hypoxic cells, oxygen may appear to exert no effect, or perhaps even a net protective effect, at low radiation doses.

Quantitative changes in the expression of two tumour-associated surface antigens on human FME melanoma cells were studied by flow cytometry after exposure to hypoxia and acidic pH, either alone or in combination. The expression of the p250 antigen recognized by the monoclonal antibody (MAb) 9.2.27 was reduced immediately after exposure to hypoxia. The magnitude and duration of the reduction increased with increasing exposure time. Twelve to 16 hr after the end of a 6-hr exposure to hypoxia the antigen expression reached the normal level, followed by a temporary increase above this level. The p97a antigen recognized by the 4.1 MAb underwent similar changes after exposure to hypoxia for 6 hr. After exposure to hypoxia in acidic environment, the magnitude and duration of the reduction in the expression of the p250 antigen increased with increasing acidity. The enhancement in antigen expression above the normal level was less after hypoxia at acidic pH than after hypoxia at physiological pH. The combined treatment had an additive effect on the expression of the melanoma-associated antigen but did not enhance hypoxia-induced cell killing. The observed changes in antigen expression might be of importance if hypoxic tumour cells are subjected to MAbs conjugated to radioisotopes or cytotoxic agents for diagnostic or therapeutic purposes.

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