Rational Effectiveness of an Anti-Mouse IL-6 Receptor Monoclonal Antibody in Chemotherapy Targeting Colon Cancer Stem Cells

Introduction: Recent studies have demonstrated that cancer stem cells (CSCs) can initiate and sustain tumor growth and exhibit resistance to clinical cytotoxic therapies. Therefore, CSCs represent the main target of anticancer therapy. Interleukin-6 (IL-6) promotes cellular proliferation and drug resistance in colorectal cancer. Therefore, IL-6 and its downstream represent potential molecular targets. In the present study, we investigated the effect of IL-6 and its downstream signaling components on stem cell biology, particularly the chemoresistance of CSCs. Methods: The colon cancer cell line WiDr was cultured in serum-free, non-adherent, three-dimensional spheroid-forming conditions to enrich the stem cell population. The expression of stem cell-related genes (Oct4, Klf4, Bmi1, and Lgr5) and IL-6 downstream signaling components (STAT3 phosphorylation (p-STAT3) and Notch3 expression) was investigated. Result: Spheroid-forming cells slowly proliferated and expressed high levels of Oct4, Klf4, Bmi1, Lgr5, IL-6, and Notch3 compared to adherently cultured cells. Treatment with an anti-human IL-6 receptor monoclonal antibody reduced spheroid formation; stem cell-related genes expression, and 5-FU resistance. In addition, IL-6 treatment enhanced the level of p-STAT3 (Tyr705), the expression of Oct4, Klf4, Lgr5, and Notch3, and chemoresistance to 5-FU. siRNA targeting Notch3 suppressed spheroid formation, the expression of Oct4 and Lgr5, and 5-FU chemoresistance, whereas STAT3 inhibition enhanced Oct4, Klf4, Lgr5, and Notch3 expression and 5-FU chemoresistance with reduced spheroid growth. Conclusion: These results indicate that IL-6 functions in dichotomous pathways involving the induction of Notch3 and the activation of STAT3. The former pathway is involved in cancer stem cell biology and enhanced chemoresistance, whereas the latter pathway leads to accelerated proliferation and reduced chemoresistance. Thus, anti-mouse IL-6R monoclonal antibody or Notch3 inhibition may be superior to STAT3 inhibition for CSC-targeting therapies concomitant with anti-cancer drugs.