Potentiation of ara-C-induced apoptosis by the protein kinase C activator bryostatin 1 in human leukemia cells (HL-60) involves a process dependent upon c-Myc.
The role of the nuclear phosphoprotein c-Myc has been examined with respect to the regulation of 1-beta-D-arabinofuranosylcytosine (ara-C)-induced apoptosis in human leukemia cells exposed to bryostatin 1 and other pharmacologic protein kinase C (PKC) activators. Pretreatment of HL-60 cells for 24 hr with 10 nM bryostatin 1 significantly potentiated the ability of ara-C (10 microM; 6 hr) to induce apoptosis without reducing the expression of c-Myc protein. In contrast, equivalent exposure to the stage 2 tumor-promoting PKC activator mezerein (10 nM) in conjunction with ara-C reduced c-Myc levels by 87% and failed to potentiate apoptosis. Co-administration of bryostatin 1 with mezerein before ara-C prevented down-regulation of c-Myc and augmented cell death, whereas co-treatment with the calcium ionophore A23187 (250 nM) and bryostatin 1 reduced c-Myc levels by 80% and abrogated the increase in ara-C-induced apoptosis. When cells were exposed for 24 hr to a c-myc antisense oligonucleotide (AS-ODN;10 microM) but not to a scrambled sequence ODN (SS-ODN) prior to ara-C, c-Myc expression was reduced by 81%, and apoptosis and cell viability were unperturbed. However, AS-ODN (but not SS-ODN) reduced c-Myc protein in cells pre-exposed to bryostatin 1 by 74% and abrogated potentiation of ara-C-induced apoptosis. The actions of c-myc AS-ODN did not stem from proximal G1 arrest/differentiation or biochemical events, since they were not associated with a reduction in the S-phase cell fraction, p21(WAF1/CIP1) induction, pRb hypophosphorylation, or alterations in ara-C metabolism. Together, these findings indicate that HL-60 cell apoptosis proceeds by both c-Myc-dependent and -independent pathways, and that only the former are involved in the potentiation of ara-C-mediated cell death by bryostatin 1.
Chelliah
J
J
Department of Medicine, Medical College of Virginia, Richmond 23298, U.S.A.
Freemerman
A J
AJ
Wu-Pong
S
S
Jarvis
W D
WD
Grant
S
S
D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects*; D054713:Bryostatins; D002453:Cell Cycle / Q000187:drug effects*; D003561:Cytarabine / Q000494:pharmacology*; D053938:DNA Fragmentation; D004224:Diterpenes; D004357:Drug Synergism; D004789:Enzyme Activation; D018922:HL-60 Cells / Q000187:drug effects*; D006801:Humans; D007700:Kinetics; D007783:Lactones / Q000494:pharmacology*; D018942:Macrolides; D016376:Oligonucleotides, Antisense / Q000494:pharmacology*; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013873:Thionucleotides
D000970:Antineoplastic Agents; D054713:Bryostatins; D004224:Diterpenes; D007783:Lactones; D018942:Macrolides; D016376:Oligonucleotides, Antisense; D016271:Proto-Oncogene Proteins c-myc; D013729:Terpenes; D013873:Thionucleotides; D003561:Cytarabine; C011309:mezerein; D015240:Phorbol 12,13-Dibutyrate; C046785:bryostatin 1; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate