γ-secretase inhibitors abrogate oxaliplatin-induced activation of the Notch-1 signaling pathway in colon cancer cells resulting in enhanced chemosensitivity

Because Notch signaling is implicated in colon cancer tumorigenesis and protects cells from apoptosis by inducing prosurvival targets, it was hypothesized that inhibition of Notch signaling with γ-secretase inhibitors (GSI) may enhance the chemosensitivity of colon cancer cells. We first show that the Notch-1 receptor, as well as its downstream target Hes-1, is up-regulated with colon cancer progression, similar to other genes involved in chemoresistance. We then report that chemotherapy induces Notch-1, as oxaliplatin, 5-fluorouracil (5-FU), or SN-38 (the active metabolite of irinotecan) induced Notch-1 intracellular domain (NICD) protein and activated Hes-1. Induction of NICD by oxaliplatin was caused by an increase in the activity and expression of γ-secretase complex, as suppression of the protein subunit nicastrin with small interfering RNA (siRNA) prevented NICD induction after oxaliplatin. Subsequent inhibition of Notch-1 signaling with a sulfonamide GSI (GSI34) prevented the induction of NICD by chemotherapy and blunted Hes-1 activation. Blocking the activation of Notch signaling with GSI34 sensitized cells to chemotherapy and was synergistic with oxaliplatin, 5-FU, and SN-38. This chemosensitization was mediated by Notch-1, as inhibition of Notch-1 with siRNA enhanced chemosensitivity whereas overexpression of NICD increased chemoresistance. Down-regulation of Notch signaling also prevented the induction of prosurvival pathways, most notably phosphoinositide kinase-3/Akt, after oxaliplatin. In summary, colon cancer cells may up-regulate Notch-1 as a protective mechanism in response to chemotherapy. Therefore, combining GSIs with chemotherapy may represent a novel approach for treating metastatic colon cancers by mitigating the development of chemoresistance.