RADIOSENSITIZATION OF HL-60 HUMAN LEUKEMIA-CELLS BY BRYOSTATIN-1 IN THE ABSENCE OF INCREASED DNA FRAGMENTATION OR APOPTOTIC CELL-DEATH

Ionizing radiation produced a dose-dependent reduction in the prolifer ative capacity of HL-60 human promyelocytic leukaemia cells. A small p ercentage of the cell population demonstrated morphological evidence o f apoptosis at 24 h following radiation doses of greater than or equal to 5 Gy (i.e. 8% at 5 Gy and 16% at 10 Gy respectively), and produced a laddered oligonucleosomal pattern of DNA fragments by static-field gel electrophoresis. The antiproliferative effects of 1 and 2.5 Gy ion izing radiation were significantly enhanced by preincubating cells wit h bryostatin-1 at a concentration (10 nM) and time frame (24 h) associ ated with down-regulation of total cellular protein kinase C (PKC) act ivity. Potentiation by bryostatin-1 of the radiation effect on prolife ration was not associated with a concomitant increase in internucleoso mal DNA fragmentation, in the fraction of cells exhibiting apoptotic m orphology, or in the extent of radiation-induced single- or double-str and breaks in bulk DNA. Staurosporine, a potent but non-specific inhib itor of PKC, was ineffective in altering the radiosensitivity of HL-60 cells or the degree of DNA fragmentation induced by ionizing radiatio n. These findings indicate that bryostatin 1 increases the sensitivity of human myeloid leukaemic cells to low radiation doses without enhan cing DNA fragmentation or apoptosis, and that this capacity may involv e factors other than, or in addition to, down-modulation of PKC activi ty.