Apoptosis induced by mild hyperthermia in human and murine tumour cell lines: a study using electron microscopy and DNA gel electrophoresis.

Mild hyperthermia is known to enhance apoptosis in a range of normal and neoplastic cell populations. Studies of tumours previously shown to respond to heating in this manner might be expected to provide insights not only into the mechanism of hyperthermic cell killing, but also into the apoptotic process in general. In the present study, cell death induced by 43 degrees C heating for 30 min in two human Burkitt's lymphoma lines, BM 13674 and WW1, and in murine mastocytoma P-815 x 2.1 was found to be exclusively apoptotic in type, identification being based on light and electron microscopic appearances and on the presence of internucleosomal cleavage of DNA into fragments that are multiples of 180-200 base pairs, which was demonstrated by agarose gel electrophoresis. The heat-induced apoptosis was prevented by the presence of zinc sulphate, an inhibitor of the endonuclease considered to be responsible for the DNA cleavage, but was not suppressed by the protein synthesis inhibitor cycloheximide. The findings question the validity of the widely held view that active protein synthesis is an invariable prerequisite for the execution of apoptosis. It is suggested that an inositol triphosphate-mediated increase in cytosolic Ca2+, resulting from limited membrane damage, might be the critical event responsible for activation of apoptosis by mild hyperthermia.