Abstract
It is widely believed that cancer stem cells (CSCs) in tumor volume exhibit resistance to radiation therapy. Both normal stem cells and CSCs maintain low levels of reactive oxygen species (ROS) and other free radical species in their cytosol. Conventional radiation therapies such as radiation and chemotherapies eliminate the substantial mass of the tumor, but the surviving tiny amount of CSCs in the tumor mass is found to be radioresistant, which facilitates tumor regrowth and relapse of cancer in the treated patients. Among other factors such as cytokines in the tumor microenvironment, ROS are believed to be an important determinant of CSCs’ radioresistance. Moreover, CSCs proliferate slowly, making radiotherapy less efficient to eliminate them as rapidly dividing cells are more sensitive to radiation action. This review aims to examine the role of ROS in tumorigenesis, DNA damage and repair, cell proliferation, induction of apoptotic death, and cytokines in the tumor microenvironment. The underlying molecular mechanisms are discussed, and strategies are underpinned for promoting the CSCs' sensitivity to radiation. It is emphasized that, in addition to conventional anticancer drugs, the search for new CSC-targeted agents needs to be discovered. The question of well-known cytokine storm generated in COVID-19 infections and their consequences on cancer patient treatment are addressed, especially involving CSCs. Furthermore, the intrinsic molecular mechanisms rendering CSCs resistant to anticancer therapies together with cytokines are re-examined (e.g., interleukins in the tumor microenvironment). The much-desired clinical practice of applying stem cell therapy to COVID-19 patients is discussed. It is stressed that future research must focus on developing antiinflammatory agents, stem cell therapy, and CSC drug toxicity to tackle the challenges of cancer treatment and COVID-19 infection in patients.
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