Requirement for caspase activation in monocytic differentiation of myeloid leukemia cells

Human myeloid leukemia cells respond to 12-tetradecanoylphorbol-13-acetate (TPA) and other activators of protein kinase C (PKC) with the induction of terminal monocytic differentiation, The present studies demonstrate that TPA treatment of U-937 leukemia cells is associated with release of mitochondrial cytochrome c, activation of caspase-3 and induction of internucleosomal DNA fragmentation. By contrast, the TUR cell variant, which is deficient in PKC beta, failed to respond to TPA with release of cytochrome c and induction of the caspase-3 cascade. Moreover, stable overexpression of PKC beta in TUR cells reconstituted sensitivity to TPA-induced cytochrome c release and activation of caspase-3, The results also demonstrate that treatment of cells with the caspase inhibitor z-VAD-fmk blocks both TPA-induced apoptosis and monocytic differentiation, Similar results were obtained in U-937 cells stably expressing the CrmA caspase inhibitor. These findings demonstrate that TPA induces cytochrome c release by a PKC beta-dependent mechanism and that activation of caspase-mediated signaling is required for induction of the differentiated monocytic phenotype.