Inhibition of HIF prolyl hydroxylase-2 blocks tumor growth in mice through the antiproliferative activity of TGFβ

Knockdown of prolyl-4-hydroxylase domain 2 inhibits tumor growth of human breast cancer MDA-MB-231 cells by affecting TGF-β1 processing

The prolyl-4-hydroxylase domain 1-3 (PHD1-3) enzymes are regulating the protein stability of the α-subunit of the hypoxia-inducible factor-1 (HIF-1), which mediates oxygen-dependent gene expression. PHD2 is the main isoform regulating HIF-1α hydroxylation and thus stability in normoxia. In human cancers, HIF-1α is overexpressed as a result of intratumoral hypoxia which in turn promotes tumor progression. The role of PHD2 for tumor progression is in contrast far from being thoroughly understood. Therefore, we established PHD2 knockdown clones of MDA-MB-231 breast cancer cells and analyzed their tumor-forming potential in a SCID mouse model. Tumor progression was significantly impaired in the PHD2 knockdown MDA-MB-231 cells, which could be partially rescued by re-establishing PHD2 expression. In a RNA profile screen, we identified the secreted phosphoprotein 1 (SPP1) as one target, which is differentially regulated as a consequence of the PHD2 knockdown. Knockdown of PHD2 drastically reduced the SPP1 expression in MDA-MB-231 cells. A correlation of SPP1 and PHD2 expression was additionally verified in 294 invasive breast cancer biopsies. In subsequent analyses, we identified that PHD2 alters the processing of transforming growth factor (TGF)-β1, which is highly involved in SPP1 expression. The altered processing capacity was associated with a dislocation of the pro-protein convertase furin. Thus, our data demonstrate that in MDA-MB-231 cells PHD2 might affect tumor-relevant TGF-β1 target gene expression by altering the TGF-β1 processing capacity.

HIF-1α is a protective factor in conditional PHD2-deficient mice suffering from severe HIF-2α-induced excessive erythropoiesis

Erythropoiesis must be tightly balanced to guarantee adequate oxygen delivery to all tissues in the body. This process relies predominantly on the hormone erythropoietin (EPO) and its transcription factor hypoxia inducible factor (HIF). Accumulating evidence suggests that oxygen-sensitive prolyl hydroxylases (PHDs) are important regulators of this entire system. Here, we describe a novel mouse line with conditional PHD2 inactivation (cKO P2) in renal EPO producing cells, neurons, and astrocytes that displayed excessive erythrocytosis because of severe overproduction of EPO, exclusively driven by HIF-2α. In contrast, HIF-1α served as a protective factor, ensuring survival of cKO P2 mice with HCT values up to 86%. Using different genetic approaches, we show that simultaneous inactivation of PHD2 and HIF-1α resulted in a drastic PHD3 reduction with consequent overexpression of HIF-2α-related genes, neurodegeneration, and lethality. Taken together, our results demonstrate for the first time that conditional loss of PHD2 in mice leads to HIF-2α-dependent erythrocytosis, whereas HIF-1α protects these mice, providing a platform for developing new treatments of EPO-related disorders, such as anemia.

Emerging novel functions of the oxygen-sensing prolyl hydroxylase domain enzymes

Oxygen-sensing prolyl hydroxylase domain enzymes (PHDs) target hypoxia-inducible factor (HIF)-α subunits for proteasomal degradation in normoxia through hydroxylation. Recently, novel mechanisms of PHD activation and function have been unveiled. Interestingly, PHD3 can unexpectedly amplify HIF signaling through hydroxylation of the glycolytic enzyme pyruvate kinase (PK) muscle isoform 2 (PKM2). Recent studies have also yielded insight into HIF-independent PHD functions, including the control of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor trafficking in synaptic transmission and the activation of transient receptor potential cation channel member A1 (TRPA1) ion channels by oxygen levels in sensory nerves. Finally, PHD activation has been shown to involve the iron chaperoning function of poly(rC) binding protein (PCBP)1 and the (R)-enantiomer of 2-hydroxyglutarate (2-HG). The intersection of these regulatory pathways and interactions highlight the complexity of PHD regulation and function.

Hypoxia-inducible factors as key regulators of tumor inflammation

Low levels of oxygen or hypoxia is often an obstacle in health, particularly in pathological disorders like cancer. The main family of transcription factors responsible for cell survival and adaptation under strenuous conditions of hypoxia are the "hypoxia-inducible factors" (HIFs). Together with prolyl hydroxylase domain enzymes (PHDs), HIFs regulates tumor angiogenesis, proliferation, invasion, metastasis, in addition to resistance to radiation and chemotherapy. Additionally, the entire HIF transcription cascade is involved in the "seventh" hallmark of cancer; inflammation. Studies have shown that hypoxia can influence tumor associated immune cells toward assisting in tumor proliferation, differentiation, vessel growth, distant metastasis and suppression of the immune response via cytokine expression alterations. These changes are not necessarily analogous to HIF's role in non-cancer immune responses, where hypoxia often encourages a strong inflammatory response.

HIF-prolyl hydroxylases and cardiovascular diseases

Prolyl hydroxylases belong to the family of iron- and 2-oxoglutamate-dependent dioxygenase enzyme. Several distinct prolyl hydroxylases have been identified. The hypoxia-inducible factor (HIF) prolyl hydroxylase termed prolyl hydroxylase domain (PHD) enzymes play an important role in oxygen regulation in the physiological network. There are three isoforms that have been identified: PHD1, PHD2 and PHD3. Deletion of PHD enzymes result in stabilization of HIFs and offers potential treatment options for many ischemic disorders such as peripheral arterial occlusive disease, myocardial infarction, and stroke. All three isoforms are oxygen sensors that regulate the stability of HIFs. The degradation of HIF-1α is regulated by hydroxylation of the 402/504 proline residue by PHDs. Under hypoxic conditions, lack of oxygen causes hydroxylation to cease HIF-1α stabilization and subsequent translocation to the nucleus where it heterodimerizes with the constitutively expressed β subunit. Binding of the HIF-heterodimer to specific DNA sequences, named hypoxia-responsive elements, triggers the transactivation of target genes. PHD regulation of HIF-1α-mediated cardioprotection has resulted in considerable interest in these molecules as potential therapeutic targets in cardiovascular and ischemic diseases. In recent years, attention has been directed towards identifying small molecule inhibitors of PHD. It is postulated that such inhibition might lead to a clinically useful strategy for protecting the myocardium against ischemia and reperfusion injury. Recently, it has been reported that the orally absorbed PHD inhibitor GSK360A can modulate HIF-1α signaling and protect the failing heart following myocardial infarction. Furthermore, PHD1 deletion has been found to have beneficial effects through an increase in tolerance to hypoxia of skeletal muscle by reprogramming basal metabolism. In the mouse liver, such deletion has resulted in protection against ischemia and reperfusion. As a result of these preliminary findings, PHDs is attracting increasing interest as potential therapeutic targets in a wide range of diseases.

HIF prolyl hydroxylase-2 inhibition diminishes tumor growth through matrix metalloproteinase-induced TGFβ activation

A right amount of oxygen and nutrients is essential for a tumor to develop. The role of oxygen dependent pathways and their regulators is therefore of utmost importance although little is known about the detailed impact they can have. Recently we have shown that inhibition of the oxygen sensor PHD2 in tumor cells blocks tumor growth due to the anti-proliferative activity of TGFβ. In this study, we refined these results by comparing different shPHD2 sequences in depth in the early phase of tumor growth. Our findings also reveal an intriguing role for MMP2 and MT1MMP in these settings, as these activated proteases display an anti-proliferative characteristic through the activation of downstream TGFβ targets. In conclusion, PHD2 inhibition is essential for the regulation of the anti-tumoral activity in mouse tumor cells and might bring some new insight in our understanding of tumor growth inhibition.

Gene-targeting of Phd2 improves tumor response to chemotherapy and prevents side-toxicity

The success of chemotherapy in cancer treatment is limited by scarce drug delivery to the tumor and severe side-toxicity. Prolyl hydroxylase domain protein 2 (PHD2) is an oxygen/redox-sensitive enzyme that induces cellular adaptations to stress conditions. Reduced activity of PHD2 in endothelial cells normalizes tumor vessels and enhances perfusion. Here, we show that tumor vessel normalization by genetic inactivation of Phd2 increases the delivery of chemotherapeutics to the tumor and, hence, their antitumor and antimetastatic effect, regardless of combined inhibition of Phd2 in cancer cells. In response to chemotherapy-induced oxidative stress, pharmacological inhibition or genetic inactivation of Phd2 enhances a hypoxia-inducible transcription factor (HIF)-mediated detoxification program in healthy organs, which prevents oxidative damage, organ failure, and tissue demise. Altogether, our study discloses alternative strategies for chemotherapy optimization.

Effect of prolyl hydroxylase domain-2 haplodeficiency on the hepatocarcinogenesis in mice

BACKGROUND & AIMS

The two major primary liver cancers in adults are hepatocellular carcinoma and cholangiocarcinoma. These tumors rapidly outgrow their vascular supply and become hypoxic, resulting in the production of hypoxia inducible factors. Recently, interest has grown in the regulators of these factors. Several reports have been published describing the role of prolyl hydroxylase domains--the key oxygen sensor responsible for the degradation of hypoxia inducible factors--tumor progression and vascularisation. The effect of prolyl hydroxylase domain 2 on the pathogenesis of liver cancer has never been studied.

METHODS

A diethylnitrosamine-induced mouse model was used in this study, allowing primary hepatic tumors to occur as a result of chronic liver damage. Several parameters of prolyl hydroxylase domain 2-haplodeficient mice were compared to those of wild type mice, thereby focussing on the expression of angiogenic factors and on the hepatic progenitor cell activation and differentiation.

RESULTS

This study shows that inhibiting prolyl hydroxylase domain 2 increases the hepatocarcinogenesis and stimulates the development of cholangiocarcinoma. Furthermore, PHD2 deficiency and the accompanying continuous HIF activation, selected for a more metastatic tumor phenotype.

CONCLUSIONS

The effect of prolyl hydroxylase domain 2 deficiency on hepatocarcinogenesis hold a great potential for therapeutic intervention, since hypoxia and the selection for a more aggressive cholangiocarcinoma phenotype might also have a repercussion on patients receiving long-term treatment with anti-angiogenic compounds.

Molecular oxygen sensing: implications for visceral surgery

BACKGROUND

Since mammalian cells rely on the availability of oxygen, they have devised mechanisms to sense environmental oxygen tension, and to efficiently counteract oxygen deprivation (hypoxia). These adaptive responses to hypoxia are essentially mediated by hypoxia inducible transcription factors (HIFs). Three HIF prolyl hydroxylase enzymes (PHD1, PHD2 and PHD3) function as oxygen sensing enzymes, which regulate the activity of HIFs in normoxic and hypoxic conditions. Many of the compensatory functions exerted by the PHD-HIF system are of immediate surgical relevance since they regulate the biological response of ischemic tissues following ligation of blood vessels, of oxygen-deprived inflamed tissues, and of tumors outgrowing their vascular supply.

PURPOSE

Here, we outline specific functions of PHD enzymes in surgically relevant pathological conditions, and discuss how these functions might be exploited in order to support the treatment of surgically relevant diseases.

Involvement of SIRT1 in hypoxic down-regulation of c-Myc and β-catenin and hypoxic preconditioning effect of polyphenols

SIRT1 has been found to function as a Class III deacetylase that affects the acetylation status of histones and other important cellular nonhistone proteins involved in various cellular pathways including stress responses and apoptosis. In this study, we investigated the role of SIRT1 signaling in the hypoxic down-regulations of c-Myc and β-catenin and hypoxic preconditioning effect of the red wine polyphenols such as piceatannol, myricetin, quercetin and resveratrol. We found that the expression of SIRT1 was significantly increased in hypoxia-exposed or hypoxic preconditioned HepG2 cells, which was closely associated with the up-regulation of HIF-1α and down-regulation of c-Myc and β-catenin expression via deacetylation of these proteins. In addition, blockade of SIRT1 activation using siRNA or amurensin G, a new potent SIRT1 inhibitor, abolished hypoxia-induced HIF-1α expression but increased c-Myc and β-catenin expression. SIRT1 was also found to stabilize HIF-1α protein and destabilize c-Myc, β-catenin and PHD2 under hypoxia. We also found that myricetin, quercetin, piceatannol and resveratrol up-regulated HIF-1α and down-regulated c-Myc, PHD2 and β-catenin expressions via SIRT1 activation, in a manner that mimics hypoxic preconditioning. This study provides new insights of the molecular mechanisms of hypoxic preconditioning and suggests that polyphenolic SIRT1 activators could be used to mimic hypoxic/ischemic preconditioning.