The influence of cell-cell contact on radiosensitivity of human squamous carcinoma cells

Previous reports have indicated that for a number of cell lines, radiation resistance was greater when cells were grown as small spheroids than when grown as monolayer cultures. Experiments were conducted to study the increased radioresistance relative to recoveries from potentially lethal and sublethal damage (PLD and SLD, respectively) in two human squamous carcinoma cell lines, A431 and CaSki. Exponential-phase monolayer cultures and small spheroids 30-70 microns in diameter were used. No significant difference in the distribution of cells in the stages of the cell cycle was seen between the two cultures. To study PLD repair, cells were irradiated and then allowed to repair, all at 37 degrees C, for 6 h. In split-dose experiments, the two doses were given 6 h apart. After irradiation, the surviving fraction of cells was measured by clonogenic assay. Cells from CaSki spheroids (D0 = 1.25 Gy, Dq = 3.08 Gy) were more radioresistant than cells from the monolayer culture (D0 = 0.92 Gy, Dq = 2.8 Gy). No difference in radiation response was seen between the spheroid and monolayer cultures of A431 cells. The radioresistance of CaSki cells, which was maximum in intact small spheroids and immediately after disaggregation of spheroids, decayed completely by 15 h at 37 degrees C. The decay process was inhibited if cells were incubated at 0 degrees C. Relative PLD and SLD repair capacities for both cell lines were greater for monolayer cultures than for small spheroids. However, no correlation was found between the two cell lines in terms of repair capacity and cellular radiosensitivity.