title: Suppression of c-myc expression and c-Myc function in response to sustained DNA damage in MCF-7 breast tumor cells.
abstract: The topoisomerase II inhibitors teniposide (VM-26), doxorubicin, and amsacrine (m-AMSA), as well as ionizing radiation, induce a transient suppression of c-myc mRNA, which correlates with growth inhibition of MCF-7 breast tumor cells. To further assess the involvement of c-mvc in the DNA damage-induced signal transduction pathways of the breast tumor cell, we determined the influence of sustained DNA damage on c-myc expression, c-Myc protein levels and c-Myc function. Continuous exposure of MCF-7 breast tumor cells to VM-26 induced DNA strand breaks that were sustained for at least 9 hr. DNA strand breakage was accompanied by a decline in c-myc transcripts and c-Myc protein levels by >90% after VM-26 exposure for 24 hr. The activity of a transcriptional target of the c-Myc protein, ornithine decarboxylase, was reduced by approximately 75% within 9 hr of DNA damage, in parallel to the declines in c-myc mRNA and protein levels. Extended exposure to VM-26 resulted in an initial loss of approximately 35% of the cell population followed by the death of additional cells such that by 72 hr only 50% of the cells were viable. Although apoptosis was evident 72 hr after initiating drug exposure [based on cell cycle analysis, terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assays, and an assessment of cell morphology], the primary phase of cell killing, which occurred during the first 24 hr was non-apoptotic. These studies indicate that non-apoptotic pathways can also mediate cell death in the breast tumor cell and support the role of c-myc expression, c-Myc protein, and c-Myc function as elements of the DNA damage response pathway in the breast tumor cell.
authors.affiliation: Department of Medicine, Medical College of Virginia at Virginia Commonwealth University, Richmond 23298-0230, USA.
authors.forename: K J
authors.initials: KJ
authors.lastname: Magnet
authors.forename: M S
authors.initials: MS
authors.lastname: Orr
authors.forename: J L
authors.initials: JL
authors.lastname: Cleveland
authors.forename: C
authors.initials: C
authors.lastname: Rodriguez-Galindo
authors.forename: H
authors.initials: H
authors.lastname: Yang
authors.forename: C
authors.initials: C
authors.lastname: Yang
authors.forename: Y M
authors.initials: YM
authors.lastname: Di
authors.forename: P T
authors.initials: PT
authors.lastname: Jain
authors.forename: D A
authors.initials: DA
authors.lastname: Gewirtz
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis; D001943:Breast Neoplasms / Q000235:genetics* / Q000473:pathology; D002453:Cell Cycle / Q000187:drug effects; D016923:Cell Death / Q000187:drug effects; D004249:DNA Damage / Q000187:drug effects / Q000502:physiology*; D016193:G1 Phase / Q000502:physiology; D015870:Gene Expression / Q000187:drug effects; D020868:Gene Silencing; D006801:Humans; D009955:Ornithine Decarboxylase / Q000378:metabolism; D065108:Ornithine Decarboxylase Inhibitors; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D013713:Teniposide / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects; D014407:Tumor Cells, Cultured
chemical_list: D000970:Antineoplastic Agents; D065108:Ornithine Decarboxylase Inhibitors; D016271:Proto-Oncogene Proteins c-myc; D013713:Teniposide; D009955:Ornithine Decarboxylase