Huachansu (HCS), an extract from dried toad (Bufo gargarizans or B. melanostictus) skin, contains biologically active substances including steroidal cardiac glycosides and indole alkaloids. It possesses antitumor activity and is well tolerated by cancer patients. The present study examined whether HCS enhances radiosensitivity of cancer cells. Three human lung cancer cell lines, H460 and A549 (p53 wild type), and H1299 (p53 null) were used. They exhibited dose-dependent cytostatic and/or cytotoxic responses to 24 h treatment with HCS, with an IC50 of approximately 20 mg/ml assayed by cell viability. IC50 dose was used in subsequent experiments in combination with IR. Treatment endpoint was clonogenic cell survival determined 12-14 days after irradiation with 2-6 Gy, with or without HCS. Cells were treated with HCS for 24 h before IR or was added immediately post exposure for 24 h. A 24 h pretreatment, but not post IR treatment, strongly enhanced radiosensitivity of H460 and A549 cells, with enhancement factors of 1.91 and 1.94, respectively, at 50% survival. Neither HCS treatment scheme affected radiosensitivity of H1299 cells. HCS pretreatment of H460 and A549 cells significantly prolonged the presence of radiation-induced double-strand breaks detected on the basis of γH2AX foci at 30 min, 4, 16 or 24h after 2 Gy IR. This suggests that inhibition of DNA repair may be an underlying mechanism of HCS-induced enhanced radiosensitivity. In addition, H460 and to lesser degree A549 cells treated with HCS were more susceptible to radiation-induced apoptosis. For example, the percentage of apoptotic cells (TUNEL) in H460 exposed both to HCS and 4 Gy (measured 24 h after IR) was 43.4 ± 1.1%, a value significantly higher than additive values of 5.2 ± 1.2% (HCS treatment alone), 10.5 ± 2.7% (IR alone). The percentage of apoptotic cells in untreated control was 4.2 ± 0.9%. This finding was supported molecularly by increased expression of cleaved caspase-3 and decreased expression of Bcl-2 analyzed by Western blot. Thus, increased susceptibility of cells to radiation-induced apoptosis is another mechanism underlying HCS-induced enhancement of cell radiosensitivity. Additional experiments showed that treatment of H460 and A549 cells with HCS induced no significant changes in cell cycle distribution to affect radiosensitivity. In conclusion, our findings demonstrated that HCS strongly enhanced radiosensitivity of human lung cancer cell lines and that the effect may be related to p53 status since only the p53 wt cell lines responded. Major underlying mechanisms of HCS-induced enhanced radiosensitivity include inhibition of DNA damage repair and increase in radiation-induced apoptosis. These data suggest that HCS may have potential to improve the efficacy of radiotherapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5674.