Role of Insulin Receptor Substrate Proteins in Protection from Chemotherapy-induced Cell Death

Abstract

IL-4 has been shown to protect various types of tumor cells, including breast cancer cells, from chemotherapeutic agents. IL-4 activates two signaling pathways, the signal transducers and activator of transcription (STAT)-6 pathway, and the insulin receptor substrate (IRS) pathway. Both STAT6 and the IRS proteins, IRS1 and IRS2, have been implicated in cancer due to their impact on the regulation of apoptosis, motility, and proliferation. The IL-4-induced activation of STAT6 has been shown to suppress chemotherapy-induced apoptosis by inducing expression of c-FLIP and BCL-xL. However, the role of IRS1 or IRS2 in sensitivity to chemotherapy has remained unclear. Therefore, the goal of this study was to test the hypothesis that IRS1 and IRS2 would protect cells from chemotherapy-induced cell death and that the addition of IL-4 would enhance this effect. However, using the myeloid cell line 32D, we found that expression of IRS1, but not IRS2, enhanced sensitivity to chemotherapy-induced, caspase-mediated cell death; IL-4 did not modulate this effect. The IRS1-mediated sensitization was partially due to up-regulation of Annexin A2 and was abrogated by co-expression of IRS2. Furthermore, we found that human MCF7 breast cancer cells expressed substantially higher levels of IRS1 and were more sensitive to taxol-induced death than human MDA-MB-231 breast cancer cells. Decreasing IRS1 expression in MCF7 cells by shRNA-targeting reduced their sensitivity to some, but not all, chemotherapeutic agents. Analysis of human breast tumor microarrays demonstrated varied expression patterns of IRS1, IRS2, STAT6 and tyrosine phosphorylated STAT6 depending on the tumor type and tumor grade. In human tumors, elevated IRS1 was strongly correlated with ERalpha and tyrosine phosphorylated STAT6. In contrast, elevated IRS2 showed a highly significant correlation with increased tumor grade. Taken together, these results suggest that increased expression of IRS1, in the absence of high levels of IRS2, may be a good indicator for enhanced sensitivity to chemotherapy.