HIFs and tumors--causes and consequences

Chronic excessive erythrocytosis induces endothelial activation and damage in mouse brain

Excessive erythrocytosis results in severely increased blood viscosity, which may have significant detrimental effects on endothelial cells and, ultimately, function of the vascular endothelium. Because blood-brain barrier stability is crucial for normal physiological function, we used our previously characterized erythropoietin-overexpressing transgenic (tg6) mouse line (which has a hematocrit of 0.8–0.9) to investigate the effect of excessive erythrocytosis on vessel number, structure, and integrity in vivo. These mice have abnormally high levels of nitric oxide (NO), a potent proinflammatory molecule, suggesting altered vascular permeability and function. In this study, we observed that brain vessel density of tg6 mice was significantly reduced (16%) and vessel diameter was significantly increased (15%) compared with wild-type mice. Although no significant increases in vascular permeability under normoxic or acute hypoxic conditions (8% O2for 4 h) were detected, electron-microscopic analysis revealed altered morphological characteristics of the tg6 endothelium. Tg6 brain vascular endothelial cells appeared to be activated, with increased luminal protrusions reminiscent of ongoing inflammatory processes. Consistent with this observation, we detected increased levels of intercellular adhesion molecule-1 and von Willebrand factor, markers of endothelial activation and damage, in brain tissue. We propose that chronic excessive erythrocytosis and sustained high hematocrit cause endothelial damage, which may, ultimately, increase susceptibility to vascular disease.

Preserved placental oxygenation and development during severe systemic hypoxia

Local tissue oxygenation profoundly influences placental development. To elucidate the impact of hypoxia on cellular and molecular adaptation in vivo, pregnant mice at embryonic days 7.5–11.5 were exposed to reduced environmental oxygen (6–7% O2) for various periods of time. Hypoxia-inducible factor (HIF)-1α mRNA was highly expressed in the placenta, whereas HIF-2α was predominantly found in the decidua, indicating that HIF-1 is a relevant oxygen-dependent factor involved in placental development. During severe hypoxia, HIF-1α protein was strongly induced in the periphery but, however, not in the labyrinth layer of the placenta. Accordingly, no indication for tissue hypoxia in this central area was detected with 2-(2-nitro-1 H-imidazol-1-yl)- N-(2,2,3,3,3-pentafluoropropyl)acetamide staining and VEGF expression as hypoxic markers. The absence of significant tissue hypoxia was reflected by preserved placental architecture and trophoblast differentiation. In the search for mechanisms preventing local hypoxia, we found upregulation of endothelial nitric oxide synthase (NOS) expression in the labyrinth layer. Inhibition of NOS activity by Nω-nitro-l-arginine methyl ester application resulted in ubiquitous placental tissue hypoxia. Our results show that placental oxygenation is preserved even during severe systemic hypoxia and imply that NOS-mediated mechanisms are involved to protect the placenta from maternal hypoxia.

A3 adenosine receptors modulate hypoxia-inducible factor-1alpha expression in human A375 melanoma cells

Hypoxia-inducible factor-1 (HIF-1) is a key regulator of genes crucial to many aspects of cancer biology. The purine nucleoside, adenosine, accumulates within many tissues under hypoxic conditions, including that of tumors. Because the levels of both HIF-1 and adenosine are elevated within the hypoxic environment of solid tumors, we investigated whether adenosine may regulate HIF-1. Here we show that, under hypoxic conditions (< 2% O2), adenosine upregulates HIF-1alpha protein expression in a dose-dependent and time-dependent manner, exclusively through the A3 receptor subtype. The response to adenosine was generated at the cell surface because the inhibition of A3 receptor expression, by using small interfering RNA, abolished nucleoside effects. A3 receptor stimulation in hypoxia also increases angiopoietin-2 (Ang-2) protein accumulation through the induction of HIF-1alpha. In particular, we found that A3 receptor stimulation activates p44/p42 and p38 mitogen-activated protein kinases, which are required for A3-induced increase of HIF-1alpha and Ang-2. Collectively, these results suggest a cooperation between hypoxic and adenosine signals that ultimately may lead to the increase in HIF-1-mediated effects in cancer cells.

Thioredoxin-1 modulates transcription of cyclooxygenase-2 via hypoxia-inducible factor-1alpha in non-small cell lung cancer

Hypoxic induction of gene expression occurs mainly via the hypoxia-inducible factor-1 (HIF-1) transcription factor and is a critical step in tumor growth. Cyclooxygenase-2 (COX-2) is commonly overexpressed in non-small cell lung cancer (NSCLC). In this study, we sought to determine the role of HIF-1 in the induction of COX-2 expression during hypoxia. Through sequence comparison of hypoxia-responsive genes, COX-2 promoter deletion analysis, and site-directed mutagenesis, we identified a hypoxia-responsive element within the COX-2 promoter that interacts with HIF-1alpha and underlies the mechanism of hypoxic activation of COX-2 in lung cancer cells. Proteomic analysis of NSCLC identified thioredoxin-1 as a redox protein overexpressed in NSCLC correlated with poor prognosis. We also show that thioredoxin-1 stabilizes HIF-1alpha to induce hypoxia-responsive genes under normoxic conditions. Our results identify two new mechanisms for regulation of COX-2 expression in NSCLC.

Antioxidant activity in Spalax ehrenbergi: a possible adaptation to underground stress

The blind subterranean mole rat Spalax ehrenbergi superspecies has evolved adaptive strategies to cope with underground stress. Hypoxia is known to stimulate reactive oxygen species generation; however, mechanisms by which Spalax counteracts oxidative damage have not been investigated before. We studied in Spalax the oxidative status of the Harderian gland (HG), an organ which is particularly vulnerable to oxidative stress in many rodents. With regard to the sexual dimorphism found in this gland, differences between males and females were determined and compared to the surface-dwelling Syrian hamster. Our results show, for the first time, that Spalax exhibits remarkably low biomolecular damage, which implies the existence of physiological strategies to avoid oxidative damage under fluctuating O(2) and CO(2) levels existing in the mole rat's subterranean niche. Correspondingly, main antioxidant enzymes, such as superoxide dismutase (SOD), catalase, and glutathione reductase (GR), exhibited high activities in both genders; in particular, remarkably high levels were measured in SOD. SOD and GR activities showed statistically significant differences between sexes. Melatonin, an important circadian agent is also a very important antioxidant molecule and is synthesized in the Harderian glands (HGs) of Spalax. Therefore, the possible interaction between antioxidant enzymes and melatonin is suggested.

MSF-A interacts with hypoxia-inducible factor-1alpha and augments hypoxia-inducible factor transcriptional activation to affect tumorigenicity and angiogenesis

Hypoxia-inducible factor-1 (HIF-1) is a key transcription factor in the signaling pathway that controls the hypoxic responses of cancer cells. Activation of the HIF system has been observed in carcinogenesis and numerous cancers. We found an interaction between a member of the mammalian septin gene family (MSF-A) and the HIF system. MSF-A is a nuclear protein that interacts with HIF-1alpha protein to prevent its ubiquitination and degradation, thus activating the HIF transcriptome. Cells overexpressing MSF-A protein exhibit increased HIF transcriptional activity and higher proliferation rates in vitro and in vivo. Xenograft-derived human tumors from these cells were larger and more vascular. These findings link a function of a septin protein with angiogenesis through activation of the HIF pathway.

The progress and promise of molecular imaging probes in oncologic drug development

As addressed by the recent Food and Drug Administration Critical Path Initiative, tools are urgently needed to increase the speed, efficiency, and cost-effectiveness of drug development for cancer and other diseases. Molecular imaging probes developed based on recent scientific advances have great potential as oncologic drug development tools. Basic science studies using molecular imaging probes can help to identify and characterize disease-specific targets for oncologic drug therapy. Imaging end points, based on these disease-specific biomarkers, hold great promise to better define, stratify, and enrich study groups and to provide direct biological measures of response. Imaging-based biomarkers also have promise for speeding drug evaluation by supplementing or replacing preclinical and clinical pharmacokinetic and pharmacodynamic evaluations, including target interaction and modulation. Such analyses may be particularly valuable in early comparative studies among candidates designed to interact with the same molecular target. Finally, as response biomarkers, imaging end points that characterize tumor vitality, growth, or apoptosis can also serve as early surrogates of therapy success. This article outlines the scientific basis of oncology imaging probes and presents examples of probes that could facilitate progress. The current regulatory opportunities for new and existing probe development and testing are also reviewed, with a focus on recent Food and Drug Administration guidance to facilitate early clinical development of promising probes.

Analysis of hypoxia-inducible factor-1alpha accumulation and cell cycle in geldanamycin-treated human cervical carcinoma cells by laser scanning cytometry

BACKGROUND

Tumor hypoxia has been linked to increased disease aggressiveness and poorer treatment outcomes, and the transcription factor hypoxia-inducible factor-1 (HIF-1) has been identified as the key molecule mediating the cellular response to hypoxic microenvironments. The alpha-subunit of this factor is accumulated under hypoxia and rapidly degraded during re-oxygenation, rendering the reliable measurement of HIF-1alpha a difficult task. Heat shock protein 90 (Hsp90) is an essential protein that controls the activity, turnover, and trafficking of a variety of other proteins including HIF-1alpha and cell cycle regulators. Hsp90 inhibitors like geldanamycin therefore have the potential to target tumor-cell survival by at least two mechanisms, compromising the accumulation of HIF-1alpha and cell proliferation.

METHODS

We describe here the simultaneous measurement of HIF-1alpha and cell cycle parameters by laser scanning cytometry (LSC) after exposure of two different human cervical carcinoma cell lines to hypoxia and geldanamycin.

RESULTS

Our analysis demonstrates that the cell lines react to hypoxia and drug treatment in a distinct way, with SiHa being more affected by low oxygen concentrations than is ME180, which was more sensitive to geldanamycin treatment. Both cell lines respond to geldanamycin with a G(2)/M-phase arrest and a decrease in HIF-1alpha accumulation. Cell death due to geldanamycin occurs in association with mitosis, presumably through mitotic catastrophe.

CONCLUSION

Our results indicate that LSC can significantly contribute to the evaluation of in vitro drug effects particularly with respect to tumor hypoxia and the measurement of HIF-1alpha.

Endocrine and signalling role of adipose tissue: new perspectives on fat

White adipose tissue (WAT) is now recognized as a major endocrine and secretory organ, releasing a wide range of protein factors and signals termed adipokines - in addition to fatty acids and other lipid moieties. A paradigm shift came with the discovery of leptin, a pleiotropic hormone which is a critical signal to the hypothalamus in the control of appetite and energy balance. A number of adipokines, including adiponectin, tumour necrosis factor-alpha, interleukin (IL)-1beta, IL-6, IL-8, IL-10, monocyte chemoattractant protein-1, macrophage migration inhibitory factor, nerve growth factor, vascular endothelial growth factor, plasminogen activator inhibitor-1 and haptoglobin, are linked to inflammation and the inflammatory response. Obesity is characterized by a state of mild inflammation, and the expression and release of inflammation-related adipokines generally rises as adipose tissue expands; a notable exception is adiponectin, with its anti-inflammatory action, the levels of which fall. WAT may be the main site of inflammation in obesity, increased circulating levels of inflammatory markers reflecting spillover from an 'inflamed' tissue, leading to the obesity-associated pathologies of type 2 diabetes and the metabolic syndrome. From the wide range of adipokines now identified, it is evident that WAT is highly integrated into overall physiological regulation, involving extensive crosstalk with other organs and multiple metabolic systems. Whether major changes in adipokine production in obesity, particularly of those factors linked to inflammation, are unique to this condition, or are a feature of all situations in which there are substantial increases in adipose mass (such as pregnancy, and pre-hibernatory and pre-migratory fattening) requires consideration.

Ca2+-activated K+ channels in human melanoma cells are up-regulated by hypoxia involving hypoxia-inducible factor-1alpha and the von Hippel-Lindau protein

Under chronic hypoxia, tumour cells undergo adaptive changes involving hypoxia-inducible factors (HIFs). Here we report that ion currents mediated by Ca2+-activated K+ (K(Ca)) channels in human melanoma IGR1 cells are increased by chronic hypoxia (3% O2), as well as by hypoxia mimetics. This increase involves the HIF system as confirmed by overexpression of HIF-1alpha or the von Hippel-Lindau tumour suppressor gene. Under normoxic conditions the K(Ca) channels in IGR1 cells showed pharmacological characteristics of intermediate conductance K(Ca) subtype IK channels, whereas the subtype SK2 channels were up-regulated under hypoxia, shown with pharmacological tools and with mRNA analysis. Hypoxia increased cell proliferation, but the K(Ca) channel blockers apamin and charybdotoxin slowed down cell growth, particularly under hypoxic conditions. Similar results were obtained for the cell line IGR39 and for acutely isolated cells from a biopsy of a melanoma metastasis. Thus, up-regulation of K(Ca) channels may be a novel mechanism by which HIFs can contribute to the malignant phenotype of human tumour cells.

Hypoxic Preconditioning and Erythropoietin Protect Retinal Neurons from Degeneration

Reduced tissue oxygenation stabilizes the alpha-subunit of the transcription factor hypoxia-inducible factor-1 (HIF-1). This leads to the induction of a number of hypoxia responsive genes. One of the best known HIF-1 targets is erythropoietin that exerts neuroprotective effects on ischemia-related injury in the brain. Thus, pre-exposure to low environmental oxygen concentrations might be exploited as a preconditioning procedure to protect tissues against a variety of harmful conditions. We present recent work on neuroprotection of retinal photoreceptors induced by hypoxic preconditioning or by systemically elevated levels of Epo in mouse plasma.

Soluble nickel inhibits HIF-prolyl-hydroxylases creating persistent hypoxic signaling in A549 cells

Soluble nickel compounds are carcinogenic to humans although the mechanism by which they cause cancer remains unclear. One major consequence of exposure to nickel is the stabilization of hypoxia inducible factor-1alpha (HIF-1alpha), a protein known to be overexpressed in a variety of cancers. In this study, we report a persistent stabilization of HIF-1alpha by nickel chloride up to 72 h after the removal of nickel from the culture media. In addition, we show that the HIF-prolyl hydroxylases (PHD's) are inhibited when cells are exposed to nickel and that they remain repressed for up to 72 h after nickel is removed. We then show that nickel can inhibit purified HIF-PHD's 2 in vitro, through direct interference with the enzyme. Through theoretical calculations, we also demonstrate that nickel may be able to replace the iron in the active site of this enzyme, providing a plausible mechanism for the persistent inhibition of HIF-PHD's by nickel. The data presented suggest that nickel can interfere with HIF-PHD directly and does not inhibit the enzyme by simply depleting cellular factors, such as iron or ascorbic acid. Understanding the mechanisms by which nickel can inhibit HIF-PHD's and stabilize HIF-1alpha may be important in the treatment of cancer and ischemic diseases.

The metal-responsive transcription factor-1 contributes to HIF-1 activation during hypoxic stress

Hypoxia-inducible factor-1 (HIF-1), the major transcriptional regulator of the mammalian cellular response to low oxygen (hypoxia), is embedded within a complex network of signaling pathways. We have been investigating the importance of another stress-responsive transcription factor, MTF-1, for the adaptation of cells to hypoxia. This article reports that MTF-1 plays a central role in hypoxic cells by contributing to HIF-1 activity. Loss of MTF-1 in transformed Mtf1 null mouse embryonic fibroblasts (MEFs) results in an attenuation of nuclear HIF-1alpha protein accumulation, HIF-1 transcriptional activity, and expression of an established HIF-1 target gene, glucose transporter-1 (Glut1). Mtf1 null (Mtf1 KO) MEFs also have constitutively higher levels of both glutathione (GSH) and the rate-limiting enzyme involved in GSH synthesis--glutamate cysteine ligase catalytic subunit--than wild type cells. The altered cellular redox state arising from increased GSH may perturb oxygen-sensing mechanisms in hypoxic Mtf1 KO cells and decrease the accumulation of HIF-1alpha protein. Together, these novel findings define a role for MTF-1 in the regulation of HIF-1 activity.

Molecular mechanisms of breast cancer metastases to bone

Bone metastases lead to hypercalcemia, bone pain, fractures, and nerve compression. They cause increased morbidity and mortality in patients with advanced breast cancer. Animal models reproduce many of the features seen in patients with breast cancer and permit identification of tumor- and bone-derived factors important in skeletal metastasis. These factors provide novel targets for therapeutic interventions. Specific tumor-bone molecular interactions mediated by these factors drive a vicious cycle that perpetuates skeletal metastases. In breast cancer, osteolytic metastases are most common, but mixed and osteoblastic metastases occur in a significant number of patients. Parathyroid hormone-related protein is a common osteolytic factor, and vascular endothelial growth factor and interleukins 8 and 11 also contribute. Osteoblastic metastases can be caused by tumor-secreted endothelin-1 (ET-1), but there are a variety of other potential osteoblastic factors. Stimulation of osteoblasts can paradoxically increase osteoclast function, as bone-synthesizing osteoblasts are the main regulators of bone-destroying osteoclasts. Coexpression of osteolytic and osteoblastic factors can thus produce mixed metastases or increased osteolysis. Cancer treatments, especially sex steroid deprivation therapies, stimulate bone loss. Bone resorption results in the release of bone growth factors, which may unintentionally increase the formation of bone metastases by activating the vicious cycle. Clinically approved bisphosphonates prevent bone resorption and reduce the release of bone growth factors. Parathyroid hormone-related protein-neutralizing antibody, inhibitors of the receptor activator of nuclear factor-kB ligand pathway, and ET-1 receptor antagonists are in clinical trials. These agents act on bone cells rather than tumor cells. Recent experiments identify new potential targets for prevention of bone metastases.

Regular endurance training reduces the exercise induced HIF-1alpha and HIF-2alpha mRNA expression in human skeletal muscle in normoxic conditions

Regular exercise induces a variety of adaptive responses that enhance the oxidative and metabolic capacity of human skeletal muscle. Although the physiological adjustments of regular exercise have been known for decades, the underlying mechanisms are still unclear. The hypoxia inducible factors 1 and 2 (HIFs) are clearly related heterodimeric transcription factors that consist of an oxygen-depended alpha-subunit and a constitutive beta-subunit. With hypoxic exposure, HIF-1alpha and HIF-2alpha protein are stabilized. Upon heterodimerization, HIFs induce the transcription of a variety of genes including erythropoietin (EPO), transferrin and its receptor, as well as vascular endothelial growth factor (VEGF) and its receptor. Considering that several of these genes are also induced with exercise, we tested the hypothesis that the mRNA level of HIF-1alpha and HIF-2alpha subunits increases with a single exercise bout, and that this response is blunted with training. We obtained muscle biopsies from a trained (5 days/week during 4 weeks) and untrained leg from the same human subject before, immediately after, and during the recovery from a 3 h two-legged knee extensor exercise bout, where the two legs exercised at the same absolute workload. In the untrained leg, the exercise bout induced an increase (P<0.05) in HIF-1alpha fold and HIF-2alpha fold mRNA at 6 h of recovery. In contrast, HIF-1alpha and HIF-2alpha mRNA levels were not altered at any time point in the trained leg. Obviously, HIF-1alpha and HIF-2alpha mRNA levels are transiently increased in untrained human skeletal muscle in response to an acute exercise bout, but this response is blunted after exercise training. We propose that HIFs expression is upregulated with exercise and that it may be an important transcription factor that regulates adaptive gene responses to exercise.

Interaction between HIF-1 alpha (ODD) and hARD1 does not induce acetylation and destabilization of HIF-1 alpha

Hypoxia inducible factor-1 alpha (HIF-1 alpha) is a central component of the cellular responses to hypoxia. Hypoxic conditions result in stabilization of HIF-1 alpha and formation of the transcriptionally active HIF-1 complex. It was suggested that mammalian ARD1 acetylates HIF-1 alpha and thereby enhances HIF-1 alpha ubiquitination and degradation. Furthermore, ARD1 was proposed to be down-regulated in hypoxia thus facilitating the stabilization of HIF-1 alpha. Here we demonstrate that the level of human ARD1 (hARD1) protein is not decreased in hypoxia. Moreover, hARD1 does not acetylate and destabilize HIF-1 alpha. However, we find that hARD1 specifically binds HIF-1 alpha, suggesting a putative, still unclear, connection between these proteins.

Signalling role of adipose tissue: adipokines and inflammation in obesity

White adipose tissue (WAT) is a major endocrine and secretory organ, which releases a wide range of protein signals and factors termed adipokines. A number of adipokines, including leptin, adiponectin, tumour necrosis factor α, IL-1β (interleukin 1β), IL-6, monocyte chemotactic protein-1, macrophage migration inhibitory factor, nerve growth factor, vascular endothelial growth factor, plasminogen activator inhibitor 1 and haptoglobin, are linked to inflammation and the inflammatory response. Obesity is characterized by a state of chronic mild inflammation, with raised circulating levels of inflammatory markers and the expression and release of inflammation-related adipokines generally rises as adipose tissue expands (adiponectin, which has anti-inflammatory action is an exception). The elevated production of inflammation-related adipokines is increasingly considered to be important in the development of diseases linked to obesity, particularly Type II diabetes and the metabolic syndrome. WAT is involved in extensive cross-talk with other organs and multiple metabolic systems through the various adipokines.

Pericellular oxygen depletion during ordinary tissue culturing, measured with oxygen microsensors

Recent research has found important differences in oxygen tension in proximity to certain mammalian cells when grown in culture. Oxygen has a low diffusion rate through cell culture media, thus, as a result of normal respiration, a decrease in oxygen tension develops close to the cells. Therefore, for the purpose of standardization and optimization, it is important to monitor pericellular oxygen tension and cell oxygen consumption. Here, we describe an integrated oxygen microsensor and recording system that allows measurement of oxygen concentration profiles in vertical transects through a 1.6-mm deep, stagnant, medium layer covering a cell culture. The measurement set-up reveals that, when confluent, a conventional culture of adherent cells, although exposed to the constant oxygen tension of ambient air, may experience pericellular oxygen tensions below the level required to sustain full oxidative metabolism. Depletions reported are even more prominent and potentially aggravating when the cell culture is incubated at reduced oxygen tensions (down to around 4% oxygen). Our results demonstrate that, if the pericellular oxygen tension is not measured, it is impossible to relate in vitro culture results (for example, gene expression to the oxygen tension experienced by the cell), as this concentration may deviate very substantially from the oxygen concentration recorded in the gas phase.

Brain-derived erythropoietin protects from focal cerebral ischemia by dual activation of ERK-1/-2 and Akt pathways

Apart from its hematopoietic function, erythropoietin (Epo) exerts neuroprotective functions in brain hypoxia and ischemia. To examine the mechanisms mediating Epo's neuroprotective activity in vivo, we made use of our transgenic mouse line tg21 that constitutively expresses human Epo in brain without inducing excessive erythrocytosis. We show that human Epo is expressed in tg21 brains and that cortical and striatal neurons carry the Epo receptor. After middle cerebral artery occlusion, human Epo potently protected brains of tg21 mice against ischemic injury, both when severe (90 min) and mild (30 min) ischemia was imposed. Histochemical studies revealed that Epo induced an activation of JAK-2, ERK-1/-2, and Akt pathways in the ischemic brain. This activation was associated with elevated Bcl-XL and decreased NO synthase-1 and -2 levels in neurons. Intracerebroventricular injections of selective inhibitors of ERK-1/-2 (PD98059) or Akt (wortmannin) pathways revealed that both ERK-1/-2 and Akt were required for Epo's neuroprotective function, antagonization of either pathway completely abolishing tissue protection. On the other hand, ERK-1/-2 and Akt blockade did not reverse the neuronal NO synthase-1/-2 inhibition, indicating that Epo down-regulates these NO synthases in an ERK-1/-2 and Akt independent manner. On the basis of our data, the dual activation of ERK-1/-2 and Akt is crucial for Epo's neuroprotective activity.

Cytokines and the regulation of hypoxia-inducible factor (HIF)-1alpha

Hypoxia-inducible factor (HIF)--an oxygen sensor? The HIF-oxygen sensing association type of dogma is, unequivocally, well anchored. But this is only one face of, at least, a double-sided coin. Current concepts charge HIF of taking sides with a yet not well-founded identity--an immunologic sensor and/or regulator. Or, is it really a sensor, put it more correctly, a key player in sensing mechanisms? The evolving association between HIF and immunity emanates from an established linkage that bonds oxidative stress and inflammation--notably the 'biologic response modifiers', or cytokines. HIF is a redox(y)-sensitive transcription factor, and so are cytokines. Recently, cytokines emerged as major regulators of HIF, under physiologic conditions extending the realm of hypoxia. Alternatively, can HIF, like the so infamous inflammatory transcription factor NF-(kappa)B, prove itself as a key player in the regulation of cytokines and, subsequently, the inflammatory process. The targeting of HIF would be, at least theoretically, of therapeutic value, but does it make sense given its intricate role in hypoxia signaling? It is the theme of HIF being an immunologic sensor that will be explored therein--with special emphasis on the regulatory role of cytokines.

Review: hypoxia-inducible factor-1 (HIF-1): a novel transcription factor in immune reactions

Hypoxia-inducible factor-1 (HIF-1) is a dimeric transcriptional complex that has been recognized primarily for its role in the maintenance of oxygen and energy homoeostasis. The HIF-1alpha subunit is O(2) labile and is degraded by the proteasome following prolyl-hydroxylation and ubiquitination in normoxic cells. The present review summarizes evidence that HIF-1 is also involved in immune reactions. Immunomodulatory peptides, including interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), stimulate HIF-1 dependent gene expression even in normoxic cells. Both the hypoxic and the cytokine-induced activation of HIF-1 involve the phosphatidylinositol- 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) signaling pathways. In addition, heat shock proteins (HSP) and other cofactors interact with HIF-1 subunits. HIF-1 increases the transcription of several genes for proteins that promote blood flow and inflammation, including vascular endothelial growth factor (VEGF), heme oxygenase-1, endothelial and inducible nitric oxide synthase (NOS) and cyclooxygenase-2 (COX-2). The pharmacologic activation of the HIF-1 complex can be desirable in ischemic and inflammatory disorders. In contrast, HIF-1 blockade may be beneficial to prevent tumor angiogenesis and tumor growth.

Neuroprotection by hypoxic preconditioning: HIF-1 and erythropoietin protect from retinal degeneration

Hypoxic exposure of cells or organisms induces expression of a number of hypoxia responsive genes through the activation of the hypoxia-inducible factor-1 (HIF-1). One of the most prominent HIF-1 targets is erythropoietin that has beneficial effects on ischemia-related injury in the brain. Exposure to low environmental oxygen concentrations can be used as a preconditioning paradigm to protect cells or tissues against a variety of harmful conditions. Here, we summarize recent work on neuroprotection of retinal photoreceptors and ganglion cells induced by hypoxic preconditioning or by systemically elevated levels of Epo in mouse plasma.

Hereditary Paraganglioma/Pheochromocytoma and Inherited Succinate Dehydrogenase Deficiency

Mitochondrial complex II, or succinate dehydrogenase, is a key enzymatic complex involved in both the tricarboxylic acid (TCA) cycle and oxidative phosphorylation as part of the mitochondrial respiratory chain. Germline succinate dehydrogenase subunit A (SDHA) mutations have been reported in a few patients with a classical mitochondrial neurodegenerative disease. Mutations in the genes encoding the three other succinate dehydrogenase subunits (SDHB, SDHC and SDHD) have been identified in patients affected by familial or 'apparently sporadic' paraganglioma and/or pheochromocytoma, an autosomal inherited cancer-susceptibility syndrome. These discoveries have dramatically changed the work-up and genetic counseling of patients and families with paragangliomas and/or pheochromocytomas. The subsequent identification of germline mutations in the gene encoding fumarase--another TCA cycle enzyme--in a new hereditary form of susceptibility to renal, uterine and cutaneous tumors has highlighted the potential role of the TCA cycle and, more generally, of the mitochondria in cancer.

The Proinflammatory Cytokine Interleukin 1β and Hypoxia Cooperatively Induce the Expression of Adrenomedullin in Ovarian Carcinoma Cells through Hypoxia Inducible Factor 1 Activation

Adrenomedullin (ADM) is a potent hypotensive peptide produced by macrophages and endothelial cells during ischemia and sepsis. The molecular mechanisms that control ADM gene expression in tumor cells are still poorly defined. It is known, however, that hypoxia potently increases ADM expression by activation of the transcription factor complex hypoxia inducible factor 1 (HIF-1). Proinflammatory cytokines produced by tumor invading macrophages likewise activate expression of ADM. Herein, we show that apart from hypoxia, the proinflammatory cytokine interleukin 1beta (IL-1beta) induced the expression of ADM mRNA through activation of HIF-1 under normoxic conditions and enhanced the hypoxia-induced expression in the human ovarian carcinoma cell line OVCAR-3. IL-1beta significantly increased accumulation and nuclear translocation of HIF-1alpha under normoxic conditions and amplified hypoxic HIF-1 activation. IL-1beta treatment affected neither HIF-1alpha mRNA levels nor the hydroxylation status of HIF-1alpha and, thus, stability of the protein. Instead cycloheximide effectively prevented the increase in HIF-1alpha protein, indicating a stimulatory effect of IL-1beta on HIF-1alpha translation. Finally, treatment of HIF-1alpha with short interfering RNA revealed a significant role for HIF-1 in the IL-1beta-dependent stimulation of ADM expression.

Targeted replacement of hypoxia-inducible factor-1alpha by a hypoxia-inducible factor-2alpha knock-in allele promotes tumor growth

Hypoxia-inducible factors (HIF) are essential transcriptional regulators that mediate adaptation to hypoxic stress in rapidly growing tissues such as tumors. HIF activity is regulated by hypoxic stabilization of the related HIF-1alpha and HIF-2alpha subunits, which are frequently overexpressed in cancer cells. To assess the relative tumor-promoting functions of HIF-1alpha and HIF-2alpha directly, we replaced HIF-1alpha expression with HIF-2alpha by creating a novel "knock-in" allele at the Hif-1alpha locus through homologous recombination in primary murine embryonic stem cells. Compared with controls, s.c. teratomas derived from knock-in embryonic stem cells were larger and more proliferative, had increased microvessel density, and exhibited increased expression of vascular endothelial growth factor, transforming growth factor-alpha, and cyclin D1. These and other data indicate that HIF-2alpha promotes tumor growth more effectively than HIF-1alpha in multiple contexts.

Carbonic anhydrase inhibitors. Inhibition of isozymes I, II, IV, V and IX with complex fluorides, chlorides and cyanides

The inhibition of five human carbonic anhydrase (hCA, EC 4.2.1.1) isozymes, the cytosolic hCA I and II, the membrane-bound hCA IV, the mitochondrial hCA V and the tumour associated, transmembrane hCA IX, with complex anions incorporating fluoride, chloride and cyanide, as well as B(III), Si(IV), P(V), As(V), Al(III), Fe(II), Fe(III), Pd(II), Pt(II), Pt(IV), Cu(I), Ag(I), Au(I) and Nb(V) species has been investigated. Apparently, the most important factors influencing activity of these complexes are the nature of the central metal ion/element, and its charge. Geometry of these compounds appears to be less important, since both linear, tetrahedral, octahedral as well as pentagonal bipyramidal derivatives led to effective inhibitors. However, the five isozymes showed very different affinities for these anion inhibitors. The best hCA I inhibitors were cyanide, dicyanocuprate and dicyanoaurate (K(I)s in the range of 0.5-7.7 microM), whereas the least effective were fluoride and hexafluoroarsenate. The best hCA II inhibitors were cyanide, hexafluoroferrate and tetrachloroplatinate (K(I)s in the range of 0.02-0.51 mM), whereas the most ineffective ones were fluoride, hexafluoroaluminate and chloride. The best hCA IV inhibitors were dicyanocuprate (K(I) of 9.8 microM) and hexacyanoferrate(II) (K(I) of 10.0 microM), whereas the worst ones were tetrafluoroborate and hexafluoroaluminate (K(I)s in the range of 124-126 mM). The most effective hCA V inhibitors were cyanide, heptafluoroniobate and dicyanocuprate (K(I)s in the range of 0.015-0.79 mM), whereas the most ineffective ones were fluoride, chloride and tetrafluoroborate (K(I)s in the range of 143-241 mM). The best hCA IX inhibitors were on the other hand cyanide, heptafluoroniobate and dicyanoargentate (K(I)s in the range of 4 microM-0.33 mM), whereas the worst ones were hexacyanoferrate(III) and hexacyanoferrate(II).

Tumour regulation of fibroblast hyaluronan expression: a mechanism to facilitate tumour growth and invasion

Hyaluronan, a high molecular weight glycosaminoglycan is associated with cellular proliferation and migration. In a number of different tumour types, there is a close correlation between tumour progression and hyaluronan production, either by the tumour cells or the surrounding stromal cells. We have examined the ability of an aggressive melanoma cell line (C8161) to stimulate the synthesis of fibroblast hyaluronan, and the association of cell-surface CD44 receptors and hyaluronan with invasion. Melanoma cell-conditioned medium (CM) prepared in low glucose medium (1 mg/ml) stimulated the synthesis of fibroblast glycosaminoglycan as measured by [3H] glucosamine incorporation, and the synthesis of hyaluronan as measured using a specific hyaluronan-binding plate assay, while tumour cell-CM prepared in high glucose medium (4.5 mg/ml) inhibited the synthesis of fibroblast glycosaminoglycan. High glucose tumour cell-CM contained large amounts of lactate that appeared to inhibit the tumour-derived factor stimulation of fibroblast glycosaminoglycan synthesis, as removal of the lactate restored the stimulating activity. Melanoma cells seeded on contracted collagen lattices and incubated at the air/liquid interface rapidly formed a multilayered cell mass on the surface, with significant invasion of the gel. Hyaluronan staining was apparent within the collagen gel, and strong staining was seen around the invading tumour cells, but not around those cell layers near the surface. CD44 expression on the tumour cells was confined to those invading cells and corresponded to cellular hyaluronan staining. Hyaluronan staining was also apparent around and between tumour cells invading fibroblast-free collagen lattices. Monolayer cultures of C8161 cells stained strongly for CD44, but few cells stained for hyaluronan, while no detectable hyaluronan was released into the medium. In summary, the C8161 melanoma cells stimulated the synthesis of fibroblast hyaluronan, and in collagen lattices, only the invasive tumour cells expressed CD44 and hyaluronan, either in the presence or absence of fibroblasts.

Carbonic anhydrase inhibitors. Interaction of isozymes I, II, IV, V, and IX with carboxylates

A detailed inhibition study of five carbonic anhydrase (CA, EC 4.2.1.1) isozymes with carboxylates including aliphatic (formate, acetate), dicarboxylic (oxalate, malonate), hydroxy/keto acids (l-lactate, l-malate, pyruvate), tricarboxylic (citrate), or aromatic (benzoate, tetrafluorobenzoate) representatives, some of which are important intermediates in the Krebs cycle, is presented. The cytosolic isozyme hCA I was strongly activated by acetate, oxalate, pyruvate, l-lactate, and citrate (K(A) around 0.1 microM), whereas formate, malonate, malate, and benzoate were weaker activators (K(A) in the range 0.1-1mM). The cytosolic isozyme hCA II was weakly inhibited by all the investigated anions, with inhibition constants in the range of 0.03-24 mM. The membrane-associated isozyme hCA IV was the most sensitive to inhibition by carboxylates, showing a K(I) of 99 nM for citrate and oxalate, of 2.8 microM for malonate and of 14.5 microM for pyruvate among others. The mitochondrial isozyme hCA V was weakly inhibited by all these carboxylates (K(I)s in the range of 1.67-25.9 mM), with the best inhibitor being citrate (K(I) of 1.67 mM), whereas this is the most resistant CA isozyme to pyruvate inhibition (K(I) of 5.5mM), which may be another proof that CA V is the isozyme involved in the transfer of acetyl groups from the mitochondrion to the cytosol for the provision of substrate(s) for de novo lipogenesis. Furthermore, the relative resistance of CA V to inhibition by pyruvate may be an evolutionary adaptation of this mitochondrial isozyme to the presence of high concentrations of this anion within this organelle. The transmembrane, tumor-associated isozyme hCA IX was similar to isozyme II in its slight inhibition by all these anions (K(I) in the range of 1.12-7.42 mM), except acetate, lactate, and benzoate, which showed a K(I)>150 mM. The lactate insensitivity of CA IX also represents an interesting finding, since it is presumed that this isozyme evolved in such a way as to show a high catalytic activity in hypoxic tumors rich in lactate, and suggests a possible metabolon in which CA IX participates together with the monocarboxylate/H(+) co-transporter in dealing with the high amounts of lactate/H(+) present in tumors.

Carbonic anhydrase inhibitors: synthesis and inhibition of cytosolic/tumor-associated carbonic anhydrase isozymes I, II, and IX with bis-sulfamates

A series of bis-sulfamates incorporating aliphatic, aromatic, or betulinyl moieties in their molecules was obtained by reaction of the corresponding diols/diphenols with sulfamoyl chloride. The library of bis-sulfamates thus obtained was tested for the inhibition of three physiologically relevant human carbonic anhydrase (hCA, EC 4.2.1.1) isozymes, the cytosolic hCA I and II, and the transmembrane, tumor-associated hCA IX. The new compounds reported here inhibited hCA I with K(I) s in the range of 79 nM-16.45 microM, hCA II with K(I) s in the range of 6-643 nM, and hCA IX with K(I) s in the range of 4-5400 nM. Several low nanomolar hCA IX inhibitors were detected, such as 1,8-octylene-bis-sulfamate or 1,10-decylene-bis-sulfamate (K(I) s in the range of 4-7 nM), which showed good selectivity ratios (in the range of 3.50-3.85) for hCA IX over hCA II inhibition. The most selective hCA IX inhibitor was phenyl-1,4-dimethylene-bis-sulfamate (K(I) of 61.6 nM), which was a 10.43 times better hCA IX than hCA II inhibitor. These derivatives are interesting candidates for the development of novel antitumor therapies targeting hypoxic tumors, since hCA IX is highly overexpressed in such tissues, and its presence is correlated with bad prognosis and unfavorable clinical outcome.

Ontogenetic expression of erythropoietin and hypoxia‐inducible factor‐1 alpha genes in subterranean blind mole rats

Blind subterranean mole rats of the Spalax ehrenbergi superspecies in Israel have evolved multiple adaptive strategies to face underground hypoxia. Hypoxia-inducible factor-1alpha (HIF-1alpha) and erythropoietin (Epo) are key factors in the development of normal erythropoiesis and angiogenesis. Here, we demonstrate via real-time polymerase chain reaction (PCR) quantification that Spalax fetal liver and kidney express higher levels of Epo mRNA than Rattus, generating reinforcement of fetal erythropoiesis underground and adapting it to life underground in an atmosphere of abrupt and sharp fluctuations of O2 supply. In neonates, Rattus liver and kidney express higher Epo levels than Spalax under both normoxia and hypoxia, probably due to Rattus ineffective erythropoiesis during embryonic life and its birth in a poorly ventilated breeding nest under ground. Adult Rattus kidney and liver, and adult Spalax liver express similar levels of Epo mRNA under normoxia and hypoxia. However, adult Spalax hypoxic kidney, the major site of erythropoietin production in adult mammals, shows levels that were twice as high as that of Rattus. Spalax expresses remarkably higher levels of HIF-1alpha mRNA than Rattus at all developmental stages studied, which peaked in neonates, as an adaptation against hypoxia.

Constitutive overexpression of human erythropoietin protects the mouse retina against induced but not inherited retinal degeneration

Elevation of erythropoietin (Epo) concentrations by hypoxic preconditioning or application of recombinant human Epo (huEpo) protects the mouse retina against light-induced degeneration by inhibiting photoreceptor cell apoptosis. Because photoreceptor apoptosis is also the common path to cell loss in retinal dystrophies such as retinitis pigmentosa (RP), we tested whether high levels of huEpo would reduce apoptotic cell death in two mouse models of human RP. We combined the two respective mutant mouse lines with a transgenic line (tg6) that constitutively overexpresses huEpo mainly in neural tissues. Transgenic expression of huEpo caused constitutively high levels of Epo in the retina and protected photoreceptors against light-induced degeneration; however, the presence of high levels of huEpo did not affect the course or the extent of retinal degeneration in a light-independent (rd1) and a light-accelerated (VPP) mouse model of RP. Similarly, repetitive intraperitoneal injections of recombinant huEpo did not protect the retina in the rd1 and the VPP mouse. Lack of neuroprotection by Epo in the two models of inherited retinal degeneration was not caused by adaptational downregulation of Epo receptor. Our results suggest that apoptotic mechanisms during acute, light-induced photoreceptor cell death differ from those in genetically based retinal degeneration. Therapeutic intervention with cell death in inherited retinal degeneration may therefore require different drugs and treatments.

L-gamma-Glutamyl-L-cysteinyl-glycine (glutathione; GSH) and GSH-related enzymes in the regulation of pro- and anti-inflammatory cytokines: a signaling transcriptional scenario for redox(y) immunologic sensor(s)?

Of the antioxidant/prooxidant mechanisms mediating the regulation of inflammatory mediators, particularly cytokines, oxidative stress-related pathways remain a cornerstone. It is conspicuous that there is a strong association between free radical accumulation (ROS/RNS; oxidative stress) and the evolution of inflammation and inflammatory-related responses. The scenario that upholds a consensus on the aforementioned is still evolving to unravel, from an immunologic perspective, the molecular mechanisms associated with ROS/RNS-dependent inflammation. Cytokines are keynote players when it comes to defining an intimate relationship among reduction-oxidation (redox) signals, oxidative stress and inflammation. How close we are to identifying the molecular basis of this intricate association should be weighed against the involvement of specific signaling molecules and, potentially, transcription factors. L-gamma-Glutamyl-L-cysteinyl-glycine, or glutathione (GSH), an antioxidant thiol, has shaped, and still is refining, the face of oxidative signaling in terms of regulating the milieu of inflammatory mediators, ostensibly via the modulation (expression/repression) of oxygen- and redox-responsive transcription factors, hence termed redox(y)-sensitive cofactors. When it comes to the arena of oxygen sensing, oxidative stress and inflammation, nuclear factor-kappaB (NF-kappaB) and hypoxia-inducible factor-1alpha (HIF-1alpha) are key players that determine antioxidant/prooxidant responses with oxidative challenge. It is the theme therein to underlie current understanding of the molecular association hanging between oxidative stress and the evolution of inflammation, walked through an elaborate discussion on the role of transcription factors and cofactors. Would that classify glutathione and other redox signaling cofactors as potential anti-inflammatory molecules emphatically remains of particular interest, especially in the light of identifying upstream and downstream molecular pathways for conceiving therapeutic, alleviating strategy for oxidant-mediated, inflammatory-related disease conditions.

Erythropoietin protects from axotomy-induced degeneration of retinal ganglion cells by activating ERK-1/-2

Apart from its hematopoietic function, erythropoietin (Epo) exerts neuroprotective activity upon reduced oxygenation or ischemia of brain, retina, and spinal cord. To examine whether Epo has an impact on the retrograde degeneration of retinal ganglion cells (RGCs) following optic nerve transection in vivo, we made use of our transgenic mouse line tg21 that constitutively expresses human Epo preferentially in neuronal cells without inducing polycythemia. We show that the tg21 retina expresses human Epo and that RGCs in this mouse line carry the Epo receptor. Upon axotomy, the RGCs of Epo transgenic tg21 mice were protected against degeneration, as compared with wild-type control animals. Western blot analysis revealed decreased phosphorylation levels of STAT-5 and reduced expression of Bcl-XL in RGCs of axotomized tg21 animals, suggesting that the corresponding pathways are not crucial for Epo's neuroprotective activity. Increased phosphorylation levels of ERK-1/-2 and Akt, as well as decreased caspase-3 activity, however, were observed in injured tg21 retinae. Injection of selective inhibitors of ERK-1/-2 (PD98059) or Akt (Wortmannin) pathways into the vitreous space revealed that transgenic Epo protected the RGCs by a pathway involving ERK-1/-2 but not Akt. In view that axotomy-induced degeneration of RGC occurs slowly, and considering the earlier data on the safety and efficacy of Epo in human stroke patients, we predict the clinical implementation of recombinant human Epo not only in patients with acute ischemic stroke, but also with more delayed degenerative neurological diseases.

Carbonic anhydrase inhibitors: synthesis and inhibition of cytosolic/tumor-associated carbonic anhydrase isozymes I, II, and IX with sulfonamides incorporating 1,2,4-triazine moieties

A series of benzenesulfonamide derivatives incorporating triazine moieties in their molecules was obtained by reaction of cyanuric chloride with sulfanilamide, homosulfanilamide, or 4-aminoethylbenzenesulfonamide. The dichlorotriazinyl-benzenesulfonamides intermediates were subsequently derivatized by reaction with various nucleophiles, such as water, methylamine, or aliphatic alcohols (methanol and ethanol). The library of sulfonamides incorporating triazinyl moieties was tested for the inhibition of three physiologically relevant carbonic anhydrase (CA, EC 4.2.1.1) isozymes, the cytosolic hCA I and II, and the transmembrane, tumor-associated hCA IX. The new compounds reported here inhibited hCA I with K(I)s in the range of 75-136nM, hCA II with K(I)s in the range of 13-278nM, and hCA IX with K(I)s in the range of 0.12-549nM. The first hCA IX-selective inhibitors were thus detected, as the chlorotriazinyl-sulfanilamide and the bis-ethoxytriazinyl derivatives of sulfanilamide/homosulfanilamide showed selectivity ratios for CA IX over CA II inhibition in the range of 166-706. Furthermore, some of these compounds have subnanomolar affinity for hCA IX, with K(I)s in the range 0.12-0.34nM. These derivatives are interesting candidates for the development of novel unconventional anticancer strategies targeting the hypoxic areas of tumors. Clear renal cell carcinoma, which is the most lethal urologic malignancy and is both characterized by very high CA IX expression and chemotherapy unresponsiveness, could be the leading candidate of such novel therapies.

HIF-1alpha expression follows microvascular loss in advanced murine adriamycin nephrosis

Cellular hypoxia has been proposed as a major factor in the pathogenesis of chronic renal injury, yet to date there has been no direct evidence to support its importance. Therefore, we examined cortical hypoxia in an animal model of chronic renal injury (murine adriamycin nephrosis; AN) by assessing nuclear localization of the oxygen-dependent alpha-subunit of hypoxia-inducible factor-1 (HIF-1alpha) in animals 7, 14, and 28 days after adriamycin. Results were assessed in conjunction with quantitation of the cortical microvasculature (by CD34 immunostaining) and cortical expression of VEGF. Cortical apoptosis was also examined by terminal deoxynucleotidyl transferase dUTP nick-end labeling staining. A dramatic and significant increase in nuclear localization of HIF-1alpha was seen 28 days after adriamycin in the context of severe glomerular and tubulointerstitial damage. Areas of nuclear HIF-1alpha staining did not colocalize with areas of cellular apoptosis. AN was also associated with a significant attenuation of the peritubular capillaries that was significant at 14 and 28 days after adriamycin. Cortical VEGF expression fell in a stepwise manner from day 7 until day 28 after adriamycin. In conclusion, these data are consistent with a significant increase in cellular hypoxia occurring in the advanced stages of murine AN. Increased cortical hypoxia was preceded by significant reductions in both the number of peritubular capillaries (i.e., oxygen supply) and the angiogenic cytokine VEGF. Apart from providing the first direct evidence for cellular hypoxia in a model of chronic renal disease, these results suggest that a primary dysregulation of angiogenesis may be the cause of increased hypoxia in this model.

Thyroid hormone induces erythropoietin gene expression through augmented accumulation of hypoxia-inducible factor-1

Oxygen is of vital importance for the metabolism and function of all cells in the human body. Hypoxia, the reduction of oxygen supply, results in adaptationally appropriate alterations in gene expression through the activation of hypoxia-inducible factor 1 (HIF-1) to overcome any shortage of oxygen. Thyroid hormones are required for normal function of nearly all tissues, with major effects on oxygen consumption and metabolic rate. Thyroid hormones have been found to augment the oxygen capacity of the blood by increasing the production of erythropoietin (EPO) and to improve perfusion by vasodilation through the augmented expression of adrenomedullin (ADM). Because the hypoxic expression of both genes depends on HIF-1, we studied the influence of thyroid hormone on HIF-1 activation in the human hepatoma cell line HepG2 under normoxic and hypoxic conditions. We found that thyroid hormones increased HIF-1α protein accumulation by increasing HIF-1α protein synthesis rather than attenuating its proteasomal degradation. HIF-1α expression directly correlated with augmented HIF-1 DNA binding and transcriptional activity of luciferase reporter plasmids, whereas HIF-1β levels remained unaffected. Knocking down HIF-1α by short interfering RNA (siRNA) clearly demonstrated that thyroid hormone-induced target gene expression required the presence of HIF-1. Although an increased association of the two known coactivators of HIF-1, p300 and SRC-1, was found, thyroid hormone did not affect the activity of the isolated COOH-terminal transactivating domain of HIF-1α. Increased synthesis of HIF-1α may contribute to the adaptive response of increased oxygen demand under hyperthyroid conditions.

Hypoxic stress tolerance of the blind subterranean mole rat: expression of erythropoietin and hypoxia-inducible factor 1 alpha

Blind subterranean mole rats (Spalax, Spalacidae) evolved adaptive strategies to cope with hypoxia that climaxes during winter floods in their burrows. By using real-time PCR, we compared gene expression of erythropoietin (Epo), a key regulator of circulating erythrocytes, and hypoxia-inducible factor 1 alpha (HIF-1 alpha), Epo expression inducer, in the kidneys of Spalax and white rats, Rattus norvegicus. Our results show significantly higher, quicker, and longer responses to different O(2) levels in Spalax compared with Rattus. (i) In normoxia, both Spalax and Rattus kidneys produce small amounts of Epo. Maximal expression of Rattus Epo is noticed after a 4-h hypoxia at 6% O(2). Under these conditions, Spalax Epo levels are 3-fold higher than in Rattus. After 24 h of 10% O(2), Spalax Epo reaches its maximal expression, remarkably 6-fold higher than the maximum in Rattus; (ii) the HIF-1 alpha level in normoxia is 2-fold higher in Spalax than in Rattus. Spalax HIF-1 alpha achieves maximal expression after 4-h hypoxia at 3% O(2), a 2-fold increase compared with normoxia, whereas no significant change was detected in Rattus HIF-1 alpha at any of the conditions studied; (iii) at 6% O(2) for 10 h, in which Rattus cannot survive, Epo and HIF-1 alpha levels in Spalax galili, living in heavily flooded soils, are higher than in Spalax judaei, residing in light aerated soil. We suggest that this pattern of Epo and HIF-1 alpha expression is a substantial contribution to the adaptive strategy of hypoxia tolerance in Spalax, evolved during 40 million years of evolution to cope with underground hypoxic stress.

Rescue of lethal c-KitW/W mice by erythropoietin

Homozygous natural white-spotted (W) mutations in the gene encoding the receptor tyrosine kinase c-Kit are associated with hypoplastic bone marrow, severe macrocytic anemia, and lethality during early postnatal life. c-Kit(W/W) mice can be rescued by wild-type hematopoietic stem cells (HSCs), but it is not known whether the lethality of c-Kit(W/W) mice is the result of HSC failure or defects specific for erythropoiesis. Here we show that transgenic expression of erythropoietin (EPO) can overcome the lethality caused by the c-Kit(W/W) mutation. In W mutant mice rescued by EPO, termed WEPO, erythrocyte colony-forming units (CFU-Es) are rescued to normal frequencies. Hence, Epo receptor signals can partially bypass the strict requirement for c-Kit signaling in erythropoiesis in the absence of c-Kit in vivo. Using a series of W and rescue mouse strains, we define here the erythropoietic threshold permitting survival in vivo. The lethality of c-Kit(W/W) mice has precluded analysis of this crucial receptor-ligand pair in adult stem/progenitor cells. Our strategy to generate viable c-Kit(W/W) mice will be useful to analyze the role of this important receptor tyrosine kinase in adult life in vivo.