Abstract 3937: SN-38 attenuates the chemoresistance acquired by colon cancer cells under hypoxic condition, via inhibition of hypoxia inducible factor (HIF) 1

Abstract Introduction: Hypoxia is a common feature of rapidly growing solid tumors. Under hypoxic condition, cancer cells acquire the resistance to chemotherapeutic agents. Accumulation of hypoxia-inducible factor (HIF)-1α in cancer cells plays important physiological roles, including resistance the therapy, through various mechanisms, including anti-apoptotic effects and the induction of G1 cell cycle arrest. Here, we aimed to clarify the mechanisms underlying the hypoxia-induced chemoresistance of colon cancer cells to 5-fluorouraacil (5-FU), oxaliplatin, and SN-38 (active metabolite of irinotecan), the standard agents for the chemotherapy of colorectal cancer. Methods: Human colon cancer cells, namely HT29 and SW480, were cultured under hypoxic condition (1% O2), and treated with the appropriate doses of the chemotherapeutics. The proliferative ability was analyzed by reading the fluorescence intensity of calcein-stained cells, and by counting the trypan blue-stained cells. Apoptosis was quantified by flow-cytometry after double-staining of the cells with AnnexinV/propidium iodide (PI). The cell cycle was evaluated by flow-cytometry after staining of cells with PI. HIF-1α expression was evaluated by Western blot analysis. Results: Under hypoxic condition, the proliferative ability of both HT29 and SW480 cells was significantly decreased compared with cells cultured under normoxia. Cells cultured under hypoxia were less responsive to 5-FU and oxaliplatin, compared to those cultured under normoxic condition, but were inhibited by SN-38 at the similar levels compared normoxia. Under hypoxia, a strong arrest of the cells to the G0/G1 phase was observed and, as a consequence, the typical changes of the cell cycle distribution, i.e., the intra-S arrest induced by 5FU and the G2/M arrest induced by oxaliplatin, were not identified after the treatment with the chemoterapeutics. By the treatment with SN-38, an increase of the fraction of sub-G1 phase cells was observed, which was confirmed to be dependent on the induction of apoptosis by the flow-cytometric analysis of AnnexinV-stained cells. SN-38 induced apoptosis of colon cancer cells at the similar rates under hypoxia and normoxia. Contrarily, the apoptosis rate induced by 5-FU and oxaliplatin was significantly reduced under hypoxia. Interestingly, SN-38 significantly reduced the expression of HIF-1α in colon cancer cells cultured under hypoxia, whereas neither 5-FU nor oxaliplatin affected its expression. Conclusion: Our findings suggest that SN-38 can inhibit the expression of HIF-1α in colon cancer cells under hypoxic condition, consequently overcoming the chemoresistance to this agent. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3937. doi:1538-7445.AM2012-3937