Cancer stem cells (CSCs) are the main culprits involved in therapy resistance and disease recurrence in colorectal carcinoma (CRC). Results using cell culture, animal models and tissues from patients with CRC suggest the indispensable roles of colorectal CSCs in therapeutic failure. Conventional therapies target proliferating and mature cancer cells, while CSCs are mostly quiescent and poorly differentiated, thereby they can easily survive chemotherapeutic insults. The aberrant activation of Wnt/β-catenin, Notch, Hedgehog, Hippo/YAP (Yes-associated protein) and phosphatidylinositol 3-kinase/protein kinase B facilitates CSCs with excessive self-renewal and therapy resistance property in CRC. CSCs survive the chemo-radiotherapies by escaping therapy mediated DNA damage via altering the cell cycle checkpoints, increasing DNA damage repair capacity and by an efficient scavenging of reactive oxygen species. Furthermore, dysregulations of miRNAs e.g., miR-21, miR-93, miR-203, miR-215, miR-497 etc., modulate the therapeutic sensitivity of colorectal CSCs by regulating growth and survival signalling. In addition, a reversible quiescent G0 state and the re-entering cell cycle capacity of colorectal CSCs can accelerate tumour regeneration after treatment. Moreover, switching to favourable metabolic signatures during a therapeutic regimen will add more complexity in therapeutic outcomes against CSCs. Therapeutic strategies targeting these underlying mechanisms of CSCs' therapy resistance could provide a promising outcome, however, deep understanding and concerted research are necessary to design novel therapies targeting CSCs. To conclude, the understanding of these mechanisms of CSC in CRC could lead to the improved management of patients with CRC.
Keywords: DNA damage; cancer stem cells; colorectal carcinoma; microRNAs; signalling pathways; therapy resistance.