Understanding the Oxygen-Sensing Pathway and Its Therapeutic Implications in Diseases

Abstract

Maintaining oxygen homeostasis is a most basic cellular process for adapting physiological oxygen variations, and its abnormality typically leads to various disorders in the human body. The key molecules of the oxygen-sensing system include the transcriptional regulator hypoxia-inducible factor (HIF), which controls a wide range of oxygen responsive target genes (eg, EPO and VEGF), certain members of the oxygen/2-oxoglutarate-dependent dioxygenase family, including the HIF proline hydroxylase (PHD, alias EGLN), and an E3 ubiquitin ligase component for HIF destruction called von Hippel-Lindau. In this review, we summarize the physiological role and highlight the pathologic function for each protein of the oxygen-sensing system. A better understanding of their molecular mechanisms of action will help uncover novel therapeutic targets and develop more effective treatment approaches for related human diseases, including cancer.

Figure 1

Schematic review of the hypoxia-inducible factor (HIF) system controlled by oxygen content. Under normoxic conditions, prolyl hydroxylases (PHDs) catalyze the hydroxylation of two proline residues within the oxygen-dependent degradation domain of HIF-α. These hydroxylation modifications allow the von Hippel–Lindau (VHL) E3 ubiquitin ligase complex to bind and catalyzes ubiquitination on HIF-α, which eventually leads to its proteasomal degradation. On the other hand, the factor inhibiting HIF-1 (FIH) catalyzes an additional hydroxylation on the asparagine residue of HIF-α. The asparagine hydroxylation blocks the transcriptional co-activator p300 from binding with HIF-α, thereby inhibiting HIF transcriptional activity. Both PHDs and FIH belong to the 2-oxoglutarate (2-OG)–dependent dioxygenase family that uses oxygen and 2-OG as cosubstrates and Fe(II) as cofactor to catalyze the hydroxylation reaction. Their enzymatic activity can be repressed by some oncogenic metabolic intermediates, such as 2-hydroxyglutarate (2-HG). Under hypoxic conditions, the activity of PHDs and FIH is inhibited because of lack of oxygen. Unhydroxylated HIF-α translocates to the nucleus, forms a complex with HIF-β and p300, and activates transcription of HIF target genes. HRE, hypoxia response element; OH, hydroxide.

Figure 2

The multiple functions of von Hippel–Lindau protein (VHL). VHL suppresses tumor and relies on its E3 ligase-dependent function. Proline hydroxylation modification by the prolyl hydroxylases (PHDs) on the proteins is the prerequisite for VHL to recognize and ubiquitinate its substrates; the ubiquitinated protein is subsequently degraded by the proteasome. Hypoxia suppresses the activity of PHDs and leads to the accumulation of these VHL substrates. On the other hand, VHL can modulate kinase activity, protein stability, and transcriptional activity of some important proteins through an E3 ligase-independent manner. VHL loss or mutations lead to accumulation or dysregulation of its substrates, which causes diseases, such as clear cell renal cell carcinoma (ccRCC). AKT, protein kinase B; ALDH2, aldehyde dehydrogenase 2; CARD9, caspase recruitment domain family member 9; EPOR, erythropoietin receptor; HIF-α, hypoxia-inducible factor-α; NDRG3, N-myc downstream-regulated gene 3; OH, hydroxide; SFMBT1, Scm like with four mbt domains 1; TBK1, TANK binding kinase 1; ZHX2, zinc fingers and homeoboxes 2.

Figure 3

Targeting the hypoxia-inducible factor (HIF) system with multiple therapeutic approaches. The schematic shows several examples for intervening in the HIF pathway in different layers, including modulating the mRNA translation, protein stability, dimerization, and transcriptional activity of HIF. Translation of HIF-α can be activated by the phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway, which makes it possible to modulate HIF-α protein level by using inhibitors targeting either PI3K (wortmannin, LY294002, and GDC-0941) or mTOR (rapamycin and PP242). EZN-2698 is an antisense RNA antagonist that specifically binds and inhibits the translation of HIF-1α mRNA. Another translational inhibitory agent includes PX-478. The histone deacetylase inhibitor vorinostat can block the heat shock protein 90 (Hsp90)–mediated HIF-α protein stabilization and transcriptional activation. Other agents modulate the HIF-α transcriptional activity, including the factor-inhibiting HIF (FIH) inhibitor dimethyloxalylglycine and proteasome inhibitor bortezomib. MG132 can stabilize HIF-α protein by direct blocking proteasomal degradation. PT2385, PT2399, and PT2977 are selective compounds that allosterically disrupt the heterodimerization of HIF-2α and HIF-1β. HRE, hypoxia response element; mTORC, mTOR complex; OH, hydroxide; p300, a transcriptional co-activator; PHD, prolyl-4-hydroxylase; Ub, ubiquitin; VHL, von Hippel–Lindau.