Therapeutic ultrasound (ULS) and the resulting cavitation process has been
shown to induce irreversible cell damage. In this study, we wanted to furth
er investigate the mechanism of ULS-induced cell death and to determine whe
ther apoptosis is involved. Nigh intensity focused pulsed ULS sonication at
a frequency of 750 KHz was delivered to HL-60, K562, U937, and M1/2 leukem
ia cell line cultures. ULS exposure used with induction of transient cavita
tion in the focal area was delivered with an intensity level of 103.7 W/cm(
2) and 54.6 W/cm(2) spatial-peak temporal-average intensity, As a control,
ULS of lower intensity was delivered at 22.4 W/cm(2) spatial-peak temporal-
average intensity, presumably without generation of cavitation. Our results
indicated that DNA damage induced by ULS cavitation did not involve genera
tion of free radicals in the culture media. Morphological alterations obser
ved in cells after exposure to ULS included: cell shrinkage, membrane blebb
ing, chromatin condensation, nuclear fragmentation, and apoptotic body form
ation. Apoptotic cells were evaluated by fluorescence microscopy and detect
ed using the terminal deoxynucleotidyl transferase-mediated dUTP nick-end l
abeling assay, which identifies DNA breaks, and by the leakage of phosphati
dylserine from the inner to the outer side of the membrane layer of treated
cells. Some bioeffects induced on sonicated HL-60 cells, such as inhibitio
n of cell proliferation, DNA repair, and cell-dependent apoptosis, were fou
nd to be similar to those produced by gamma-irradiation. Thus, much of the
cell damage induced by therapeutic ULS in leukemia cells surviving ULS expo
sure appears to occur through an apoptotic mechanism.