title: Enhancement of radiation effects in vitro by the estrogen metabolite 2-methoxyestradiol.
abstract: 2-Methoxyestradiol. 2-Methoxyestradiol (2-ME) is an endogenous estradiol metabolite that disrupts microtubule function, suppresses murine tumors, and inhibits angiogenesis. Since some microtubule inhibitors have been shown to alter radiosensitivity, we have evaluated 2-ME as a radiation enhancer in vitro. H460 human lung cancer cells were plated, treated with 2-ME for 24 h, and irradiated; then colony-forming ability was assessed. The radiation dose enhancement ratios (DERs) using this protocol were 1.3, 1.8 and 2.1 for 1, 1.5 and 2 microM 2-ME, respectively. Using a single-cell plating protocol, the respective DERs were 1.2, 1.5 and 1.8. The parent compound of 2-ME, beta-estradiol, did not enhance radiation effects at equally cytotoxic doses. Isobologram analysis showed that 1 microM 2-ME was additive with radiation, but that 1.5 and 2 microM were synergistic. Cell cycle analysis showed a dose-dependent increase in the percentage of cells in the radiosensitive G(2)/M phase after a 24-h treatment with 2-ME; a threefold increase in the percentage of cells in G(2)/M phase was observed using 2 microM 2-ME. Treatment with 2 microM 2-ME almost completely inhibited repair of sublethal damage (SLD) as shown using split-dose recovery. Radiosensitive, repair-deficient murine SCID (severe combined immunodeficient) cells did not show enhancement of radiation effects with 2 microM 2-ME, but enhancement was observed in the wild-type parental cells (CB-17). SCID cells complemented with human DNA-dependent protein kinase restored radioenhancement by 2-ME. In addition, MCF-7 breast cancer cells were also radiosensitized by 2 microM 2-ME (DER = 2.1). These data suggest that 2-ME is a potential radiation sensitizer, in addition to its previously reported antitumor and antiangiogenic properties. We have verified the antiangiogenic activity of 2-ME in vitro using human endothelial cells. Based on these results, we hypothesize that the mechanism of radiation enhancement may involve redistribution of cells into G(2)/M phase by 2-ME, and that the resulting population of cells is repair-deficient and thus radiosensitive.
authors.affiliation: Vanderbilt University Medical Center, Department of Radiation Oncology, Nashville, Tennessee 37232, USA.
authors.forename: G P
authors.initials: GP
authors.lastname: Amorino
authors.forename: M L
authors.initials: ML
authors.lastname: Freeman
authors.forename: H
authors.initials: H
authors.lastname: Choy
mesh_terms: D000077584:2-Methoxyestradiol; D000818:Animals; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects*; D002478:Cells, Cultured; D004958:Estradiol / Q000031:analogs & derivatives* / Q000494:pharmacology; D016195:G2 Phase / Q000528:radiation effects; D006801:Humans; D051379:Mice; D016513:Mice, SCID; D008938:Mitosis / Q000528:radiation effects; D011829:Radiation Dosage; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
chemical_list: D011838:Radiation-Sensitizing Agents; D004958:Estradiol; D000077584:2-Methoxyestradiol