Feedback regulators of hypoxia-inducible factors and their role in cancer biology

Malignant tumors are characterized by regions of low oxygen concentration (hypoxia). The hypoxic tumor microenvironment contributes to tumor progression by activating a set of adaptive responses via the key transcriptional regulators HIF-1 alpha and HIF-2 alpha. These factors have been traditionally linked to an aggressive tumor phenotype by promoting processes essential for tumor growth, such as angiogenesis, glycolysis, metastasis and invasion, as well as differentiation and self renewal. Notably, the complex HIF pathway also initiates anti-tumorigenic mechanisms that lead to cell cycle arrest or cell death, indicating the need for a stringent control of the extent and the direction of the hypoxia response. The importance of this control for tumor cell survival is illustrated by the intricate regulation of HIF activity at the mRNA, protein and epigenetic level by a complex network of positive and negative feedback regulators. We propose that these feedback regulators help to flexibly adjust and adapt HIF activated responses to the fluctuating oxygen concentrations within tumors during acute and chronic hypoxia and to curtail the tumor-suppressing components of the HIF pathway. Moreover, feedback regulation of HIF induces a switch from HIF-1 alpha to HIF-2 alpha driven responses under chronic hypoxia which may have essential functions in the regulation of tumor cell differentiation and tumor stem cell maintenance. Given their central role in cancer biology, HIF feedback regulators may represent an attractive and novel anti-tumor therapy target to overcome cell death resistance in tumors.