Phorbol ester induces apoptosis in HL-60 promyelocytic leukemia cells but not in HL-60 PET mutant

One of the factors regulating the population size of a clone of proliferating cells is the induction of a physiological suicide mechanism known as apoptosis. We studied apoptosis in the HL-60 human promyelocytic leukemia cell line which differentiates when exposed to phorbol ester (S-cell), and in the PET-cell mutant of HL-60 which is defective in its response to phorbol ester. Exposing S-cells to 12-O-tetradecanoylphorbol 13-acetate (TPA) (3 nM and above) induced morphological changes characteristic of apoptosis (visualized by light microscopy), and induced fragmentation of chromatin DNA to oligonucleosomal lengths. These changes were obvious in 48 h. In contrast, 1000 nM TPA for five days did not induce apoptosis in the PET-cell. DNA fragmentation was induced in both cell lines by A23187 (0.25 microM) and etoposide (7 microM). Novobiocin (600 and 900 microM) induced DNA fragmentation in S-cells, but higher concentrations inhibited fragmentation. Novobiocin is believed to induce DNA fragmentation by a direct action on DNA. In the case of PET-cells, novobiocin did not induce DNA fragmentation at any concentration, and prior treatment of PET-cells with novobiocin (300-1200 microM for 30 min) inhibited DNA fragmentation induced by A23187. Novobiocin inhibited cell growth equally in S-cell and PET-cells. It is concluded that the promyelocytes have the capacity to undergo apoptosis in response to agents which activate protein kinase C, and that the PET-cell has a mutation which disables both protein-kinase C-induced and novobiocin-induced DNA fragmentation, leaving intact the ability of novobiocin to protect DNA from calcium-entry-initiated fragmentation. The elucidation of the lesion responsible for the PET phenotype is likely to increase our understanding of this important pathway for regulating cellular proliferation and how it bears on leukemogenesis and chemotherapy.