The present studies were undertaken to characterize further the potential role of protein kinase C (PKC) in the regulation of apoptosis in HL-60 promyelocytic leukemia cells. The capacity of acute exposure to specific and nonspecific pharmacological inhibitors of PKC to promote apoptotic DNA fragmentation was examined both quantitatively and qualitatively and correlated with effects on cellular differentiation and proliferation. Incubation of HL-60 cells for 6 h with chelerythrine and calphostin C (highly specific inhibitors that act at the regulatory domain) or H7 and gossypol (nonspecific inhibitors that act at the PKC catalytic domain) produced concentration-dependent increases in DNA fragmentation. Induction of DNA fragmentation by chelerythrine, calphostin C, and gossypol was biphasic, resulting in a sharp decline in effect at concentrations above 5 µm, 0.1 µm, and 100 µm, respectively, whereas maximal and more stable effects were observed in response to H7 (100 µm). A 6-h exposure to staurosporine, a nonspecific but potent PKC inhibitor, failed to induce DNA fragmentation at concentrations generally used to achieve maximal inhibition of enzyme activity (e.g., 50 nm) but promoted fragmentation at considerably higher concentrations (e.g., ≥200 nm). In contrast, 6-h exposures to the nonspecific protein kinase inhibitor hypericin (0.1 to 100 µm) or to the nonspecific inhibitor of protein kinase A, HA1004 (50 µm), were without effect on DNA fragmentation. DNA obtained from cells exposed to chelerythrine (5 µm), calphostin C (100 nm), H7 (50 µm), gossypol (50 µm), and staurosporine (200 nm)—but not hypericin (25 µm)—exhibited clear evidence of internucleosomal DNA cleavage on agarose gel electrophoresis; moreover, these cells exhibited the classical morphological features of apoptosis (cell shrinkage, nuclear condensation, and the formation of apoptotic bodies). All of the PKC inhibitors that induced apoptosis, and one of the inhibitors that did not (hypericin), substantially inhibited HL-60 cell clonogenicity at the concentrations evaluated. None of the agents tested induced cellular maturation as assessed by nonspecific esterase and nitro-blue tetrazolium positivity. DNA fragments obtained from cells exposed to specific and nonspecific PKC inhibitors possessed predominantly 5′-phosphate termini, consistent with the action of a Ca2+-/Mg2+-dependent endonuclease. Finally, Northern blot analysis revealed that exposure to calphostin C at a concentration that induced apoptosis (100 nm) failed to alter expression of bcl-2, an oncogene known to block apoptosis in both lymphoid and myeloid leukemia cells. Together, these observations suggest that certain inhibitors of PKC, administered alone and on a transient basis, are capable of inducing apoptotic DNA fragmentation and cell death in HL-60 promyelocytic leukemia cells in a highly concentration-dependent manner. These findings also suggest a protective involvement of basal PKC activity in the regulation of programmed cell death in myeloid leukemia cells.
This work was supported by Grant CH-523 from the American Cancer Society (S. G.), Grant CA-40615 (L. F. P.) and Cancer Center Support Core Grant CA-16059 from the National Cancer Institute, and the Bone Marrow Transplantation Core Research Laboratory of the Medical College of Virginia. W. D. J. is recipient of a Cancer Biology Fellowship supported by USPHS Training Grant CA-09564-05 and of National Research Service Award CA-09380. Portions of this material were presented in preliminary form at the 82nd Meeting of the American Association for Cancer Research, Inc.