Inhibition of IRE1 RNase activity modulates tumor cell progression and enhances the response to chemotherapy in colorectal cancer

Drug resistance is one of the clinical challenges that limits the effectiveness of chemotherapy. Recent reports suggest that the unfolded protein response (UPR) and endoplasmic reticulum stress-adaptation signalling pathway, along with increased activation of its inositol-requiring enzyme 1a (IRE1a) arm, may be contributors to the pathogenesis of colorectal cancer (CRC). Here, we aimed to target the IRE1a/XBP1 pathway in order to sensitise CRC cells to the effects of chemotherapy. The CT26 colorectal cell line was treated with tunicamycin, and then was exposed to different concentrations of 5-fluorouracil (5-FU), either alone and/or in combination with the IRE1a inhibitor, 4 mu 8C. An MTT assay, flow cytometry and RT-PCR were performed to determine cell growth, apoptosis and IRE1a activity, respectively. In vivo BALB/c syngeneic colorectal mice received chemotherapeutic drugs. Treatment responses, tumour sizes and cytotoxicity were assessed via a range of pathological tests. 4 mu 8C was found to inhibit the growth of CRC, at a concentration of 10 mu g/ml, without detectable cytotoxic effects and also significantly enhanced the cytotoxic potential of 5-FU, in CRC cells. In vivo experiments revealed that 4 mu 8C, at a concentration of 50 mu M/kg prevented tumour growth without any cytotoxic or metastatic effects. Interestingly, the combination of 4 mu 8C with 5-FU remarkably enhanced drug responses, up to 40-60% and also lead to significantly greater inhibition of tumour growth, in comparison to monotherapy, in CRC mice. Targeting the IRE1a/XBP1 axis of the UPR could enhance the effectiveness of chemotherapy in both in vitro and in vivo models of CRC.