Signaling defect in the activation of caspase-3 and PKC delta in human TURleukemia cells is associated with resistance to apoptosis

Exposure of the two related human leukemic cell lines U937 and TUR to chemo therapeutic compounds resulted in opposite effects on induction and resista nce to apoptosis. Incubation of U937 cells with 1-beta-D-arabinofuranosylcy tosine or the etoposide VP-16 was accompanied by growth arrest in G(0)/G(1) of the cell cycle and an accumulation of a population in the sub-Gr, phase which exhibited characteristics typical for the apoptotic pathway. In cont rast, human TUR leukemia cells demonstrated no significant effects after a similar treatment with Ara-C and VP-16. Thus, TUR cells continued to prolif erate in the presence of these anti-cancer drugs and the number of apoptoti c cells as evaluated by propidium iodide staining and the detection of inte rnucleosomal DNA fragmentation was significantly reduced when compared to t he parental U937 cells. Similar effects were observed upon serum-starvation demonstrating resistance to apoptosis in TUR cells. Whereas induction of a poptosis is regulated by a network of distinct factors including the activa tion of proteolytically active caspases, we investigated these pathways in both cell lines. U937 cells demonstrated activation of the 32-kDa caspase-3 upon drug treatment by cleavage into the 20-kDa activated form. However, t here was no 20-kDa caspase-3 fragment detectable in TUR cells. Simultaneous ly, the enzymatic activity of caspase-3 was significantly increased in drug -treated U937 cells as measured lit vitro by enhanced metabolization of a f luorescence substrate and in vivo by cleavage of an appropriate substrate f or caspase-3, namely protein kinase C delta. In contrast, there was little if any caspase-3 activation detectable in drug-treated TUR cells. Taken tog ether, these data suggest a signaling defect in the activation of the caspa se-3 proteolytic system in TUR cells upon treatment with chemotherapeutic c ompounds which is associated with resistance to apoptosis in these human le ukemia cells. (C) 1999 Academic Press.