title: Potentiation of etoposide-induced cytotoxicity and DNA damage in CCRF-CEM cells by pretreatment with non-cytotoxic concentrations of arabinosyl cytosine.
abstract: Pretreatment of the human lymphoblastoid cell line CCRF-CEM with 0.02 microM arabinosyl cytosine (ara C) enhances both the cytotoxic and the DNA-damaging effects of etoposide. This concentration of ara C is itself non-cytotoxic and results in no detectable DNA damage as measured by alkaline elution. Ara C pretreatment results in the synchronisation of cells, a 24-h pretreatment resulting in the accumulation of cells in the early S phase. The sensitivity of cells to etoposide-induced cytotoxicity was increased 2.5 times and DNA damage was enhanced 1.66 times by this pretreatment. Maximal potentiation of etoposide-induced DNA damage (2.06-fold increase) was observed after 48 h continuous treatment with ara C, but no further enhancement of cytotoxicity occurred. Cell-cycle analysis demonstrated that 48 h ara C treatment resulted in the accumulation of cells in the late S/G2M phase. Cells returned to a normal cell-cycle distribution within 24 h of the removal of ara C, and the potentiation of etoposide activity was then reduced to a 1.3- to 1.4-fold level. DNA damage induced by etoposide following ara C pretreatment was qualitatively identical to that produced by etoposide alone, suggesting a mechanism involving topoisomerase II. To investigate this possibility, we measured topoisomerase II protein levels by immunoblotting. Measurement of topoisomerase II levels in whole-cell lysates of ara C-pretreated cells showed a 3- to 5-fold increase in topoisomerase levels relative to total protein content. This suggests that elevated enzyme levels may be responsible for the increased sensitivity of ara C-pretreated cells to etoposide.
authors.affiliation: Department of Oncology, University College & Middlesex School of Medicine, London, UK.
authors.forename: C M
authors.initials: CM
authors.lastname: Chresta
authors.forename: R
authors.initials: R
authors.lastname: Hicks
authors.forename: J A
authors.initials: JA
authors.lastname: Hartley
authors.forename: R L
authors.initials: RL
authors.lastname: Souhami
mesh_terms: D002453:Cell Cycle / Q000187:drug effects*; D002460:Cell Line; D003561:Cytarabine / Q000494:pharmacology*; D004249:DNA Damage / Q000187:drug effects*; D004250:DNA Topoisomerases, Type II / Q000378:metabolism*; D004277:DNA, Single-Stranded / Q000187:drug effects; D004357:Drug Synergism; D005047:Etoposide / Q000633:toxicity*; D006801:Humans
chemical_list: D004277:DNA, Single-Stranded; D003561:Cytarabine; D005047:Etoposide; D004250:DNA Topoisomerases, Type II