γ-Secretase Inhibitors Enhance Taxane-Induced Mitotic Arrest and Apoptosis in Colon Cancer Cells

Background & Aims: Colorectal cancers are resistant to conventional chemotherapeutic treatments, including taxanes. γ-Secretase is a multimeric membrane protein complex responsible for the intramembrane proteolysis of various type I transmembrane proteins, including amyloid β-precursor protein and Notch. γ-Secretase inhibitors have attracted increasing interest as anticancer drugs because of their ability to inhibit Notch signaling. However, the therapeutic usefulness of γ-secretase inhibitors against colorectal cancers remains unclear. Methods: The effects of γ-secretase inhibitors on growth and apoptosis induced by various chemotherapeutic agents in colon cancer cells were evaluated using Hoechst 33342 staining, colony formation assay, and cell cycle analysis. The effect of γ-secretase inhibitors on taxane-induced mitotic arrest was evaluated using the cyclin B1–associated histone H1 kinase assay and MPM-2 reactivity. The involvement of Notch signaling was evaluated by the silencing of Notch/CBF1 signaling by RNA interference. Results: γ-Secretase inhibitors enhanced taxane-induced mitotic arrest and apoptosis of colon cancer cells both in vitro and in vivo, although γ-secretase inhibitors alone did not affect growth and apoptosis of colon cancer cells. We also showed that this effect by γ-secretase inhibitors was restricted to taxanes and colon cancer cells. Silencing of Notch/CBF1 signaling failed to affect paclitaxel-induced mitotic arrest and apoptosis. Conclusions: These data suggest that γ-secretase inhibitors could be a new therapeutic modality for overcoming resistance to taxanes in colorectal cancers.