Hypoxia contributes to the pathophysiology of human disease, including myocardial and cerebral ischemia, cancer, pulmonary hypertension, congenital heart disease and chronic obstructive pulmonary disease (1). In cancer and particularly solid tumors, hypoxia plays a critical role in the regulation of genes involved in stem cell renewal, epithelial to mesenchymal transition (EMT), metastasis and angiogenesis. In the tumor microenvironment (TME), hypoxia influences the properties and function of stromal cells (e.g., fibroblasts, endothelial and immune cells) and is a strong determinant of tumor progression (2,3).
HIF-1 or hypoxia inducible factor 1 (predicted molecular weight 93kDa), is a transcription factor commonly referred to as a "master regulator of the hypoxic response" for its central role in the regulation of cellular adaptations to hypoxia. In its active form under hypoxic conditions, HIF-1 is stabilized by the formation of a heterodimer of HIF-1 alpha and ARNT/HIF-1 beta subunits. Nuclear HIF-1 engages p300/CBP for binding to hypoxic response elements (HREs). This process induces transcription and regulation of genes including EPO, VEGF, iNOS2, ANGPT1 and OCT4 (4,5).
Under normoxic conditions, the HIF-1 alpha subunit is rapidly targeted and degraded by the ubiquitin proteasome system. This process is mediated by prolyl hydroxylase domain enzymes (PHDs), which catalyze the hydroxylation of key proline residues (Pro-402 and Pro-564) within the oxygen-dependent degradation domain of HIF-1 alpha. Once hydroxylated, HIF-1 alpha binds the von Hippel-Lindau tumor suppressor protein (pVHL) for subsequent ubiquitination and proteasomal degradation (4). pVHL dependent regulation of HIF-1 alpha plays a role in normal physiology and disease states. Regulation of HIF-1 alpha by pVHL is critical for the suppressive function of Foxp3+ regulatory Tcells (6). Repression of pVHL expression in chronic lymphocytic leukemia (CLL) B cells leads to HIF-1 alpha stabilization and increased VEGF secretion (7).
1. Semenza, G. L., Agani, F., Feldser, D., Iyer, N., Kotch, L., Laughner, E., & Yu, A. (2000). Hypoxia, HIF-1, and the pathophysiology of common human diseases. Advances in Experimental Medicine and Biology.
2. Muz, B., de la Puente, P., Azab, F., & Azab, A. K. (2015). The role of hypoxia in cancer progression, angiogenesis, metastasis, and resistance to therapy. Hypoxia. https://doi.org/10.2147/hp.s93413
3. Huang, Y., Lin, D., & Taniguchi, C. M. (2017). Hypoxia inducible factor (HIF) in the tumor microenvironment: friend or foe? Science China Life Sciences. https://doi.org/10.1007/s11427-017-9178-y
4. Koyasu, S., Kobayashi, M., Goto, Y., Hiraoka, M., & Harada, H. (2018). Regulatory mechanisms of hypoxia-inducible factor 1 activity: Two decades of knowledge. Cancer Science. https://doi.org/10.1111/cas.13483
5. Dengler, V. L., Galbraith, M. D., & Espinosa, J. M. (2014). Transcriptional regulation by hypoxia inducible factors. Critical Reviews in Biochemistry and Molecular Biology. https://doi.org/10.3109/10409238.2013.838205
6. Lee, J. H., Elly, C., Park, Y., & Liu, Y. C. (2015). E3Ubiquitin Ligase VHL Regulates Hypoxia-Inducible Factor-1 alpha to Maintain Regulatory T Cell Stability and Suppressive Capacity. Immunity. https://doi.org/10.1016/j.immuni.2015.05.016
7. Ghosh, A. K., Shanafelt, T. D., Cimmino, A., Taccioli, C., Volinia, S., Liu, C. G.,... Kay, N. E. (2009). Aberrant regulation of pVHL levels by microRNA promotes the HIF/VEGF axis in CLL B cells. Blood. https://doi.org/10.1182/blood-2008-10-185686