Chloroquine prevents hypoxic accumulation of HIF-1 alpha by inhibiting ATR kinase: implication in chloroquine-mediated chemosensitization of colon carcinoma cells under hypoxia

BackgroundChloroquine (CQ) is an effective and safe antimalarial drug that is also used as a disease-modifying antirheumatic drug. Recent studies have shown that CQ can sensitize cancer cells to anti-cancer therapies. MethodsIn this study, we investigated the molecular mechanisms underlying CQ-mediated chemosensitization in human colon carcinoma cells. ResultsCQ prevented hypoxia-inducible factor (HIF)-1 alpha protein induction in human colon carcinoma cells. CQ also suppressed HIF-1 activity, as represented by CQ inhibition of HIF-1-dependent luciferase activity and reduced induction of vascular endothelial growth factor. Under hypoxia, CQ restricted HIF-1 alpha synthesis but did not affect HIF-1 alpha transcription and protein stability. The hypoxic state activated ataxia telangiectasia and Rad3-related (ATR) kinase and increased the level of phosphorylated checkpoint kinase 1, a substrate of ATR kinase; however, this was prevented by CQ. An ATR kinase inhibitor suppressed the hypoxic induction of HIF-1 alpha protein and was as effective as CQ. The cytotoxicity of 5-fluorouracil (5-FU), the first choice for the treatment of colorectal cancer, was attenuated under hypoxia. CQ enhanced the cytotoxicity of 5-FU treatment, which was mimicked by the transient transfection with HIF-1 alpha siRNA. ConclusionsUnder hypoxia, CQ-mediated sensitization of colon carcinoma HCT116 cells to 5-FU involves HIF-1 inhibition via ATR kinase suppression.