Incubation of cells in high salt/alkali typically leads to denaturation and unwinding of DNA, yet DNA from Chinese hamster V79 cells grown for 1 day as spheroids stops unwinding after only 5-10 min. We previously postulated that this was a result of "constraints" to DNA unwinding present in cells in spheroids but not in monolayers, and that these constraints could be responsible for the increased resistance of spheroids of V79 cells to killing by ionizing radiation (i.e., the contact effect). However, studies reported here indicate that this limited DNA unwinding is correlated with a round cell shape and lack of cell surface fibronectin. In round cells which continue to synthesize fibronectin, demonstration of constraints requires prior exposure to trypsin in order to digest cell surface fibronectin. However, trypsin did not influence cell killing by ionizing radiation. Therefore, the increase in radiation resistance of V79 spheroids and the change in their DNA unwinding kinetics both appear contingent upon a change in cell shape; differences in DNA denaturation rates which are detected in spheroids using the unwinding assay are apparently not directly responsible for the contact effect.

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