Cancer stem cells (CSCs) are a population of self-renewal cells with high tumorigenic potency. CSCs can adopt easily with changes in the nearby milieu, and are more resistant to conventional therapies than other cells within a tumor. CSC resistance can be induced secondary to radio- and chemotherapy, or even after chemotherapy secession. A combination of both intrinsic and extrinsic factors is contributed to CSC-mediated therapy resistance. CSCs represent protective autophagy and efficient cell cycling, along with highly qualified epithelial-mesenchymal transition (EMT) regulators, reactive oxygen species (ROS) scavengers, drug transporters, and anti-apoptotic and DNA repairing systems. In addition, CSCs develop cross-talking and share some characteristics with other cells within the tumor microenvironment (TME) being more intense in higher stage tumors, and thereby sophisticating tumor-targeted therapies. TME, in fact, is a nest for aggravating resistance mechanisms in CSCs. TME is exposed constantly to the nutritional, metabolic and oxygen deprivation; these conditions promote CSC adaptation. This review is aimed to discuss main (intrinsic and extrinsic) mechanisms of CSC resistance and suggest some strategies to revoke this important promoter of therapy failure.
Keywords: Cancer stem cell (CSC); Chemotherapy; Dedifferentiation; Epithelial-mesenchymal transition (EMT); Plasticity; Radiotherapy; Reactive oxygen species (ROS); Resistance; Stemness; Tumor microenvironment (TME).
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