Influences of Growth Conditions and Cell-Cell Contact on Responses of Tumor Cells to Ultrasound

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Abstract
The response of EMT6/Ro [mouse mammary carcinoma] multicell spheroids at early stages of growth (1-3 days) to 1-MHz ultrasonic irradiation was compared to EMT6/Ro cells which were grown under suspension or as attached monolayers. The surviving fraction of cells was evaluated by measuring both direct cell lysis and clonogenicity of cells not lysed during sonication. Irradiation was for 1 min at 0, 1, 3 or 5 W/cm2 (spatial peak, temporal peak) at 37.degree. C. At 3 and 5 W/cm2, there was a 9- to 10-fold enhancement in the surviving fraction of spheroids as compared to that of monolayer cells. Individual spheroid cells do not appear to be physiologically altered with respect to their response to ultrasound since cells from dissociated spheroids exhibit survival parameters which are similar to those of cells grown in monolayers. The basis for the increased survival of whole spheroids appears to be due to a physical protection of the cells in spheroids. Studies examining effects of trypsin treatment indicate that a slight decrease in survival which occurred at 1 W/cm2 was associated with the trypsin treatment. Of the 9- to 10-fold increase in survival of whole spheroids, 2- to 3-fold increase was related to trypsin with a 6- to 7-fold increase related to the muticellular nature of spheroids (termed contact effect). Experiments with increased atmospheric pressure and temperature measurements indicate that cavitation is the main mechanism producing these bioeffects.