We tested the possibility that hyperthermia kills HA-1 cells in a manner analogous to growth factor deprivation. HA-1 cells were inactivated by serum starvation when incubated in Eagle's MEM at a density of <tex-math>$40\ \text{cells}/{\rm cm}^{2}$</tex-math> or less. Cells became resistant to the absence of serum when the cell density was greater than <tex-math>$400\ \text{cells}/{\rm cm}^{2}$</tex-math> or when lethally irradiated HA-1 feeder cells were present. The feeder cells exerted their effect through a diffusible factor. In addition, a 1:1 mixture of Eagle's MEM and Ham's F-12 enabled HA-1 cells to remain viable without serum. Ten days growth in Eagle's MEM + Ham's F-12 without serum resulted in the formation of microcolonies of cells. This indicated that growth factor deprivation was not lethal to HA-1 cells, and it suggested that they may have been partially transformed. The presence of the growth factors insulin, transferrin, and fibroblast growth factor (FGF) reduced cell killing by a small amount during conditions of serum starvation. After hyperthermia, the presence of growth factors again diminished cell killing by a modest amount (approximately twofold). Feeder cells also improved cell survival after hyperthermia. The effect of feeder cells was greatest when cells were trypsinized immediately after hyperthermia. When cells were not trypsinized after heating, feeder cells increased survival less than twofold. In summary, the absence of growth factors was not lethal to HA-1 cells, and therefore the cytotoxic effects of hyperthermia could not be explained fully by the failure to bind growth factors. HA-1 feeder cells secreted undefined, growth-promoting substances, but feeder cells exerted only a small positive effect on cell survival after hyperthermia when cells were not trypsinized after heating.

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