IGPR-1 promotes colorectal cancer tumor cell survival and modifies the response of cancer cells to chemotherapeutics
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Colorectal cancer (CRC) is the third leading cause of cancer-related death in women and fourth in men globally. While expansions in preventative measures have increased the detection of CRC at the early stages of disease, only 40% of CRC patients are diagnosed when the disease is at a local stage. Moreover, many anti-cancer drugs fail to significantly improve the life expectancy of patients due to innate and acquired resistance, underscoring a need for better diagnostic and therapeutic strategies for CRC. Immunoglobulin-containing and proline-rich receptor-1 (IGPR-1) is a novel cell adhesion molecule (CAM) that was recently identified in our laboratory. IGPR-1 is expressed in epithelial and endothelial cells and promotes cell-cell adhesion. Expression of IGPR-1 in endothelial cells regulates angiogenesis; however, its role in epithelial cells, particularly cancer cells with an epithelial origin, remains unknown. The overall goal of this study was to investigate the possible function of IGPR-1 in CRC tumor cell growth and response to chemotherapeutic agents. Specifically, we aimed to test the hypothesis that increased expression of IGPR-1 in CRC tumor cells promotes cell survival and contributes to the resistance of tumor cells to doxorubicin. Human CRC tumor cell lines, HCT116 and HT29, were transduced via a retroviral system to express IGPR-1 or empty retroviral vector pQCXIP. The effect of overexpression of IGPR-1 in HCT116 and HT29 cells was measured by MTT assay in non-adherent 24-well plates. In addition, cells were viewed under a light microscope, and images were taken to assess multicellular aggregation. Results demonstrated that expression of IGPR-1 in HCT116 and HT29 tumor cells promoted CRC tumor cell growth, increased multicellular aggregation, and stimulated resistance to the conventional chemotherapeutic agent doxorubicin in non-adherent cell culture conditions in vitro. Intriguingly, treatment of cells with doxorubicin promoted phosphorylation of IGPR-1 at serine 220 (Ser220), suggesting a critical role for phosphorylation of IGPR-1 in the development of resistance to chemotherapeutics. In addition, non-adherent cell culture conditions promoted activation of the key pro-apoptotic kinase, p38 MAPK in CRC tumor cells. Ectopic expression of IGPR-1 reversed this activation. This data suggests that IGPR-1, by suppressing p38 activity, in part, promotes tumor cell survival and increases the resistance of tumor cells to the killing effects of doxorubicin. Our findings are the first to demonstrate that IGPR-1 promotes CRC tumor cell growth and increases the resistance of CRC tumor cells to the cytotoxic effects of chemotherapeutic agents. The data suggests that IGPR-1 plays an important role in CRC by inhibiting the cellular apoptotic response and promoting chemotherapeutic resistance. Finally, IGPR-1 phosphorylation at Ser220 in response to doxorubicin may account for the IGPR-1-mediated development of resistance to doxorubicin in CRC.