Hypoxia-Inducible Factor-Dependent and Independent Mechanisms Underlying Chemoresistance of Hypoxic Cancer Cells.

Simple Summary: In solid tumors, oxygen concentration varies between different regions. Generally, while oxygen supply is sufficient in the vicinity of blood vessels, the concentration of oxygen gradually drops as the distance from the blood vessel increases. In the regions with low oxygen content (hypoxic regions), cancer cells acquire various malignant properties, like invasiveness, altered metabolism, and therapy resistance, leading to recurrence and poor clinical outcomes of patients. Hypoxia-inducible factor (HIF) is a major regulator of hypoxia responses. Herein, we summarized how tumor hypoxia activates different mechanisms, in both HIF-dependent and HIF-independent manners, and contributes to the acquisition of chemotherapy resistance. We also discussed the involvement of epigenetic regulation in hypoxia-induced chemoresistance, with a specific example of ATAD2 protein degradation inducing drug resistance under hypoxia. Finally, we briefly reviewed some current clinical trials that target HIF and tumor hypoxia for cancer treatment or therapy sensitization. In hypoxic regions of malignant solid tumors, cancer cells acquire resistance to conventional therapies, such as chemotherapy and radiotherapy, causing poor prognosis in patients with cancer. It is widely recognized that some of the key genes behind this are hypoxia-inducible transcription factors, e.g., hypoxia-inducible factor 1 (HIF-1). Since HIF-1 activity is suppressed by two representative 2-oxoglutarate-dependent dioxygenases (2-OGDDs), PHDs (prolyl-4-hydroxylases), and FIH-1 (factor inhibiting hypoxia-inducible factor 1), the inactivation of 2-OGDD has been associated with cancer therapy resistance by the activation of HIF-1. Recent studies have also revealed the importance of hypoxia-responsive mechanisms independent of HIF-1 and its isoforms (collectively, HIFs). In this article, we collate the accumulated knowledge of HIF-1-dependent and independent mechanisms responsible for resistance of hypoxic cancer cells to anticancer drugs and briefly discuss the interplay between hypoxia responses, like EMT and UPR, and chemoresistance. In addition, we introduce a novel HIF-independent mechanism, which is epigenetically mediated by an acetylated histone reader protein, ATAD2, which we recently clarified. [ABSTRACT FROM AUTHOR]