Myxozoans (Cnidaria) do not Retain Key Oxygen-Sensing and Homeostasis Toolkit Genes

For aerobic organisms, both the hypoxia-inducible factor pathway and the mitochondrial genomes are key players in regulating oxygen homeostasis. Recent work has suggested that these mechanisms are not as highly conserved as previously thought, prompting more surveys across animal taxonomic levels, which would permit testing of hypotheses about the ecological conditions facilitating evolutionary loss of such genes. The Phylum Cnidaria is known to harbor wide variation in mitochondrial chromosome morphology, including an extreme example, in the Myxozoa, of mitochondrial genome loss. Because myxozoans are obligate endoparasites, frequently encountering hypoxic environments, we hypothesize that variation in environmental oxygen availability could be a key determinant in the evolution of metabolic gene networks associated with oxygen-sensing, hypoxia-response, and energy production. Here, we surveyed genomes and transcriptomes across 46 cnidarian species for the presence of HIF pathway members, as well as for an assortment of hypoxia, mitochondrial, and stress-response toolkit genes. We find that presence of the HIF pathway, as well as number of genes associated with mitochondria, hypoxia, and stress response, do not vary in parallel to mitochondrial genome morphology. More interestingly, we uncover evidence that myxozoans have lost the canonical HIF pathway repression machinery, potentially altering HIF pathway functionality to work under the specific conditions of their parasitic lifestyles. In addition, relative to other cnidarians, myxozoans show loss of large proportions of genes associated with the mitochondrion and involved in response to hypoxia and general stress. Our results provide additional evidence that the HIF regulatory machinery is evolutionarily labile and that variations in the canonical system have evolved in many animal groups.

Overexpression of hydroxyproline via EGLN/HIF1A is associated with distant metastasis in pancreatic cancer

For pancreatic cancer, the probability of distant metastasis can help choose the best course of treatment. The aim of this study is to establish the efficacy of hydroxyproline as a biomarker for distant metastasis for pancreatic cancer and to clarify the mechanism of EGLN/HIF1A axis that controls the invasion and metastasis. Metabolites (hydroxyproline) and genes (EGLN2 and EGLN3) were identified by metabolome analysis of the serum with pancreatic cancers with and without distant metastasis. The mechanism of EGLN/HIF1A axis including angiogenesis was examined in pancreatic cancer cells. Hydroxyproline associated with these mechanisms was evaluated to suggest the association with overall survival in pancreatic cancer. Decreased expression of EGLN2 and EGLN3 in pancreatic cancer, via the HIF1A and TGF ß1 pathway, was associated with the induction of angiogenic factors, increased vascular invasion, and poor overall patient survival. Hydroxyproline concentrations were regulated via the HIF1A pathway by EGLN2 and EGLN3, and that increased concentrations of hydroxyproline promote the invasion and metastasis of pancreatic cancer cells. These results suggested that the expression of hydroxyproline through the HIF1A pathway induced by EGLN2 and EGLN3 could be a surrogate marker for treatment and might predict distant metastasis in pancreatic cancer.

The EGLN-HIF O2-Sensing System: Multiple Inputs and Feedbacks

The EGLN (also called PHD) prolyl hydroxylase enzymes and their canonical targets, the HIFα subunits, represent the core of an ancient oxygen-monitoring machinery used by metazoans. In this review, we highlight recent progress in understanding the overlapping versus specific roles of EGLN enzymes and HIF isoforms and discuss how feedback loops based on recently identified noncoding RNAs introduce additional layers of complexity to the hypoxic response. Based on novel interactions identified upstream and downstream of EGLNs, an integrated network connecting oxygen-sensing functions to metabolic and signaling pathways is gradually emerging with broad therapeutic implications.

Expression of prolyl hydroxylases 2 and 3 in chick embryos

Hypoxic cellular response is crucial for normal development as well as in pathological conditions in order to tolerate low oxygen. The response is mediated by Hypoxia Inducible Factors (HIFs), where the α-subunit of HIF is stabilised and able to function only in low oxygen. Prolyl hydroxylases (PHDs) are oxygen dependent dioxygenase enzymes that hydroxylate HIF-α leading to HIF degradation. Thus PHDs function as an oxygen sensor for the function of HIFs. Here we describe the mRNA expression pattern of PHDs in chick embryos. Up to embryonic day 2, PHDs are weak without specific localisation, whereas from day 3 localised expression was observed in the eye, branchial arches and dermomyotome. Later in the limb development PHDs were expressed in the perichondral mesenchyme, excluded from the developing limb cartilages.

The 2-oxoglutarate analog 3-oxoglutarate decreases normoxic hypoxia-inducible factor-1α in cancer cells, induces cell death, and reduces tumor xenograft growth

The cellular response to hypoxia is primarily regulated by the hypoxia-inducible factors (HIFs). HIF-1α is also a major mediator of tumor physiology, and its abundance is correlated with therapeutic resistance in a broad range of cancers. Accumulation of HIF-1α under hypoxia is mainly controlled by the oxygen-sensing HIF prolyl 4-hydroxylases (EGLNs, also known as PHDs). Here, we identified a high level of normoxic HIF-1α protein in various cancer cell lines. EGLNs require oxygen and 2-oxoglutarate for enzymatic activity. We tested the ability of several cell-permeable 2-oxoglutarate analogs to regulate the abundance of HIF-1α protein. We identified 3-oxoglutarate as a potent regulator of HIF-1α in normoxic conditions. In contrast to 2-oxoglutarate, 3-oxoglutarate decreased the abundance of HIF-1α protein in several cancer cell lines in normoxia and diminished HIF-1α levels independent of EGLN enzymatic activity. Furthermore, we observed that 3-oxoglutarate was detrimental to cancer cell survival. We show that esterified 3-oxoglutarate, in combination with the cancer chemotherapeutic drug vincristine, induces apoptosis and inhibits tumor growth in vitro and in vivo. Our data imply that a novel treatment strategy targeting HIF-1α in combination with the use of existing cytotoxic agents could serve as potent, future antitumor chemotherapies.

PHD3-SUMO conjugation represses HIF1 transcriptional activity independently of PHD3 catalytic activity

By controlling HIFα hydroxylation and stability, the prolyl hydroxylase domain (PHD)-containing proteins are essential to the maintenance of oxygen homeostasis; therefore these enzymes are tightly regulated. Small ubiquitin-like modifier (SUMO) is a 10-kDa protein readily conjugated to lysine residues of the targeted proteins in a process termed SUMOylation. In this study, we introduce SUMO conjugation as a novel regulator of PHD3 (also known as EGLN3). PHD3 SUMOylation occurs at a cluster of four lysines at the C-terminal end of the protein. Furthermore, PHD3 SUMOylation by SUMO2 or SUMO3 contributes to PHD3-mediated repression of HIF1-dependent transcriptional activity. Interestingly, PHD3-SUMO conjugation does not affect PHD3 hydroxylase activity or HIF1α stability, providing new evidence for a dual role of PHD3 in HIF1 regulation. Moreover, we show that hypoxia modulates PHD3-SUMO conjugation and that this modification inversely correlates with HIF1 activation. PHD3 SUMOylation highlights a new and additional layer of regulation that is likely required to fine-tune HIF function.