Enhancement of radiation effects in vitro by the estrogen metabolite 2-methoxyestradiol

2-Methoxyestradiol (2-ME) is an endogenous estradiol metabolite that disrup ts microtubule function, suppresses murine tumors, and inhibits angiogenesi s. Since some microtubule inhibitors have been shown to alter radiosensitiv ity, we have evaluated 2-ME as a radiation enhancer ill vitro. H460 human l ung cancer cells were plated, treated with 2-ME for 24 h, and irradiated; t hen colony-forming ability was assessed. The radiation dose enhancement rat ios (DERs) using this protocol were 1.3, 1.8 and 2.1 for 1, 1.5 and 2 mu M 2-ME, respectively. Using a single-cell plating protocol, the respective DE Rs 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 analy sis showed that 1 mu M 2-ME was additive with radiation, but that 1.5 and 2 mu M were synergistic. Cell cycle analysis showed a dose-dependent increas e 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 mu M 2-ME. Treatment with 2 mu M 2-ME almo st completely inhibited repair of sublethal damage (SLD) as shown using spl it-dose recovery, Radiosensitive, repair-deficient murine SCID (severe comb ined immunodeficient) cells did not show enhancement of radiation effects w ith 2 mu M 2-ME, but enhancement was observed in the wild-type parental cel ls (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 mu M 2-ME (DER = 2.1). These data suggest th at 2-ME is a potential radiation sensitizer, in addition to its previously reported antitumor and antiangiogenic properties. We have verified the anti angiogenic activity of 2-ME in vitro using human endothelial cells. Based o n 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 th e resulting population of cells is repair-deficient and thus radiosensitive . (C) 2000 by Radiation Research Society.