Two subpopulations of tumor cells obtained from a single human colon adenocarcinoma have been examined with regard to their intrinsic sensitivity to X-irradiation and to how this intrinsic radiosensitivity might be altered by growth in the polar solvent, N,N-dimethylformamide (DMF). These subpopulations, designated clones A and D, differ significantly in their radiation sensitivity. Using the single-hit multitarget equation to fit the survival responses, clone A exhibited survival parameters: extrapolation number (n) = 6.3; quasi-threshold dose (Dq) = 2.50 Grays; and mean lethal dose (Do) = 1.36 Grays. These parameters for clone D were: n = 2.3; Dq = 1.27 Grays, and Do = 1.56 Grays. Alteration of intrinsic radiosensitivity by DMF was then studied by adapting colon tumor cells to continuous growth in Roswell Park Memorial Institute Tissue Culture Medium 1640 containing 0.8% DMF. This treatment promotes differentiation of these colon cancer cells and is associated with an increased cell culture doubling time of 50 hr (20 hr for control cultures). It was found that, for cells grown in DMF medium, irradiated in fresh complete medium, and replated into fresh complete medium, survival parameters were: clone A—n = 5.2, Dq = 1.91 Grays, and Do = 1.15 Grays; clone D—n = 2.2, Dq = 0.96 Grays, and Do = 1.21 Grays. For tumor cells grown in DMF medium, irradiated in fresh complete medium, and replated into medium containing 0.8% DMF, survival parameters were: clone A—n = 2.8, Dq = 1.07 Grays, and Do = 1.03 Grays; clone D—n = 2.0, Dq = 0.82 Grays, and Do = 1.19 Grays. Survival responses of cells grown in fresh complete medium, irradiated in fresh complete medium, and replated into medium containing 0.8% DMF were not different from control responses. Using the Dq as an index of the sensitization occurring in the clinically relevant dose range, the pretreatment in DMF medium sensitizes both clone A and D cells to subsequent X-irradiation by a factor of approximately 1.3, whereas pre- and posttreatment in DMF medium increases this factor for clone A to 2.3 and for clone D to 1.5. Therefore, growth of colon cancer cells in DMF medium prior to irradiation, which results in cell differentiation, actually increases the sensitivity of cells to X-rays. Furthermore, subsequent reculturing of irradiated cells in DMF-containing medium compared to regular growth medium results in a further augmentation of the response.

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This investigation was supported by USPHS Grants CA 25687, CA 23225, CA 13943, and CA 20892 awarded by the National Cancer Institute, Department of Health and Human Services.

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