Transient Changes in the Radiosensitivity of Mammalian Cells on Transfer from in vivo to in vitro Culture Conditions

Pretreatment growth environments alter the sensitivity of tumor cells to cytotoxic agents

The effects of pretreatment growth conditions on the sensitivity of tumor cells to various cytotoxic agents were investigated using murine Ehrlich ascites tumor cells grown in two different environments. The tumor cells adapted to grow in the peritoneal cavity of mice were found to be more sensitive to ionizing radiation, oxygen toxicity, doxorubicin, and bleomycin than tumor cells adapted to grow in vitro. However, there was no difference in their sensitivity to 5-fluorouracil. One obvious difference between these two growth environments is oxygen tension; it is between 2.6 and 5.2% (20-40 mmHg) for the peritoneal cavity and 21% (159 mmHg) for the regular tissue culture. To investigate the role of oxygen tension, tumor cells from the peritoneal cavity were grown in tissue culture having either 21% O2 or 4% O2 in the gas phase. Within 4 d, tumor cells that were exposed to 21% O2, but not to 4% O2, in vitro gradually became as resistant to cytotoxic agents as the tumor cells continuously cultured in vitro under 21% O2. It appears that the adaptation of tumor cells to different environments having different partial pressure of oxygen alters their sensitivity not only to oxygen toxicity but also to other cytotoxic agents that damage or kill cells by generating free radicals.

Adaptation and cross-resistance: an explanation for the existence of different radiosensitivities among malignant melanoma cells

A hypothesis is presented to explain the apparent difference in the radioresponsiveness of melanoma lesions whether they are located on the skin or in other parts of the body. The hypothesis states that the radiosensitivity of a cell may change when the cell adapts to live and grow in a different environment. The most important environmental factor that affects the radiosensitivity of cutaneous melanoma cells appears to be the partial pressure of oxygen in their immediate environment. By virtue of adapting to grow in an environment having a high partial pressure of oxygen, the melanoma cells located on the skin may have developed a better antioxidant defense mechanism than cells that metastasize to, and grow in, other parts of the body having lower partial pressures of oxygen such as lymph nodes, brain and viscera. Because some of the cell-damaging effects of both oxygen and ionizing radiation are mediated through a similar mechanism, the melanoma cells on the skin become cross-resistant to ionizing radiation because of their higher tolerance to oxygen toxicity.