Regulation of the Hypoxia-Inducible Factor (HIF) by Pro-Inflammatory Cytokines

Hypoxia and inflammation are frequently co-incidental features of the tissue microenvironment in a wide range of inflammatory diseases. While the impact of hypoxia on inflammatory pathways in immune cells has been well characterized, less is known about how inflammatory stimuli such as cytokines impact upon the canonical hypoxia-inducible factor (HIF) pathway, the master regulator of the cellular response to hypoxia. In this review, we discuss what is known about the impact of two major pro-inflammatory cytokines, tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), on the regulation of HIF-dependent signaling at sites of inflammation. We report extensive evidence for these cytokines directly impacting upon HIF signaling through the regulation of HIF at transcriptional and post-translational levels. We conclude that multi-level crosstalk between inflammatory and hypoxic signaling pathways plays an important role in shaping the nature and degree of inflammation occurring at hypoxic sites.

Bidirectional Crosstalk Between Hypoxia Inducible Factors and Glucocorticoid Signalling in Health and Disease

Glucocorticoid-induced (GC) and hypoxia-induced transcriptional responses play an important role in tissue homeostasis and in the regulation of cellular responses to stress and inflammation. Evidence exists that there is an important crosstalk between both GC and hypoxia effects. Hypoxia is a pathophysiological condition to which cells respond quickly in order to prevent metabolic shutdown and death. The hypoxia inducible factors (HIFs) are the master regulators of oxygen homeostasis and are responsible for the ability of cells to cope with low oxygen levels. Maladaptive responses of HIFs contribute to a variety of pathological conditions including acute mountain sickness (AMS), inflammation and neonatal hypoxia-induced brain injury. Synthetic GCs which are analogous to the naturally occurring steroid hormones (cortisol in humans, corticosterone in rodents), have been used for decades as anti-inflammatory drugs for treating pathological conditions which are linked to hypoxia (i.e. asthma, ischemic injury). In this review, we investigate the crosstalk between the glucocorticoid receptor (GR), and HIFs. We discuss possible mechanisms by which GR and HIF influence one another, in vitro and in vivo, and the therapeutic effects of GCs on HIF-mediated diseases.

Hypoxia-inducible factors and obstructive sleep apnea

Intermittent hypoxia (IH) is a hallmark manifestation of obstructive sleep apnea (OSA), a widespread disorder of breathing. This Review focuses on the role of hypoxia-inducible factors (HIFs) in hypertension, type 2 diabetes (T2D), and cognitive decline in experimental models of IH patterned after O2 profiles seen in OSA. IH increases HIF-1α and decreases HIF-2α protein levels. Dysregulated HIFs increase reactive oxygen species (ROS) through HIF-1-dependent activation of pro-oxidant enzyme genes in addition to reduced transcription of antioxidant genes by HIF-2. ROS in turn activate chemoreflex and suppress baroreflex, thereby stimulating the sympathetic nervous system and causing hypertension. We also discuss how increased ROS generation by HIF-1 contributes to IH-induced insulin resistance and T2D as well as disrupted NMDA receptor signaling in the hippocampus, resulting in cognitive decline.

Current Understanding of the HIF-1-Dependent Metabolism in Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) is the 10th most frequent human malignancy and is thus a global burden. Despite some progress in diagnosis and therapy, patients’ overall survival rate, between 40 and 55%, has stagnated over the last four decades. Since the tumor node metastasis (TNM) system is not precise enough to predict the disease outcome, additive factors for diagnosis, prognosis, prediction and therapy resistance are urgently needed for OSCC. One promising candidate is the hypoxia inducible factor-1 (HIF-1), which functions as an early regulator of tumor aggressiveness and is a key promoter of energy adaptation. Other parameters comprise the composition of the tumor microenvironment, which determines the availability of nutrients and oxygen. In our opinion, these general processes are linked in the pathogenesis of OSCC. Based on this assumption, the review will summarize the major features of the HIF system-induced activities, its target proteins and related pathways of nutrient utilization and metabolism that are essential for the initiation, progression and therapeutic stratification of OSCC.

Bidirectional crosstalk between Hypoxia-Inducible Factor and glucocorticoid signalling in zebrafish larvae

In the last decades in vitro studies highlighted the potential for crosstalk between Hypoxia-Inducible Factor-(HIF) and glucocorticoid-(GC) signalling pathways. However, how this interplay precisely occurs in vivo is still debated. Here, we use zebrafish larvae (Danio rerio) to elucidate how and to what degree hypoxic signalling affects the endogenous glucocorticoid pathway and vice versa, in vivo. Firstly, our results demonstrate that in the presence of upregulated HIF signalling, both glucocorticoid receptor (Gr) responsiveness and endogenous cortisol levels are repressed in 5 days post fertilisation larvae. In addition, despite HIF activity being low at normoxia, our data show that it already impedes both glucocorticoid activity and levels. Secondly, we further analysed the in vivo contribution of glucocorticoids to HIF activity. Interestingly, our results show that both glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) play a key role in enhancing it. Finally, we found indications that glucocorticoids promote HIF signalling via multiple routes. Cumulatively, our findings allowed us to suggest a model for how this crosstalk occurs in vivo.

Mitochondrial NIX Promotes Tumor Survival in the Hypoxic Niche of Glioblastoma

Cancer cells rely on mitochondrial functions to regulate key survival and death signals. How cancer cells regulate mitochondrial autophagy (mitophagy) in the tumor microenvironment as well as utilize mitophagy as a survival signal is still not well understood. Here, we elucidate a key survival mechanism of mitochondrial NIX-mediated mitophagy within the hypoxic region of glioblastoma, the most malignant brain tumor. NIX was overexpressed in the pseudopalisading cells that envelop the hypoxic-necrotic regions, and mitochondrial NIX expression was robust in patient-derived glioblastoma tumor tissues and glioblastoma stem cells. NIX was required for hypoxia and oxidative stress-induced mitophagy through NFE2L2/NRF2 transactivation. Silencing NIX impaired mitochondrial reactive oxygen species clearance, cancer stem cell maintenance, and HIF/mTOR/RHEB signaling pathways under hypoxia, resulting in suppression of glioblastoma survival in vitro and in vivo. Clinical significance of these findings was validated by the compelling association between NIX expression and poor outcome for patients with glioblastoma. Taken together, our findings indicate that the NIX-mediated mitophagic pathway may represent a key therapeutic target for solid tumors, including glioblastoma. SIGNIFICANCE: NIX-mediated mitophagy regulates tumor survival in the hypoxic niche of glioblastoma microenvironment, providing a potential therapeutic target for glioblastoma.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/20/5218/F1.large.jpg.

[Regulation mechanism of HIFs, PPARs and AMPK in hypoxic training-induced reduction of body weight]

Hypoxic exposure activates hypoxia inducible factors (HIFs) to up-regulate the expression of its target genes. These genes encode glucose metabolism related proteins, such as glucose transporters (GLUTs) and glycolysis related enzymes, including lactate dehydrogenase A (LDHA) and aldolase A (ALDA). Therefore, HIFs participate in oxygenolysis of glucose and play an important role in mediating hypoxia response and weight loss. Exercise training influences fatty acid metabolism, insulin sensitivity and body energy balance through activating peroxisome proliferator-activated receptors (PPARs), which plays an active role in losing weight. In addition, hypoxic exposure or exercise training can activate energy sensor 5'-AMP activated protein kinase (AMPK) in cells and promote oxidation of glucose and fatty acid and weight loss. It has been shown that hypoxic training exerts a better effects on controlling weight, compared with either hypoxic exposure or exercise training alone. This paper reviewed synergistic interactions among HIFs, PPARs and AMPK under hypoxic training and proposed possible mechanisms of hypoxic training-induced weight loss via AMPK-HIFs axis or AMPK-PPARs axis, thus providing theoretical guidance for application of hypoxic training in weight control.

[Is Oxygen Deficiency Always Harmful?]

Is Oxygen Deficiency Always Harmful? Abstract. The role of the cardiovascular circulation is to supply tissue with oxygen and nutrients. Oxygen deficiency (hypoxia) is considered life-threatening, since cells die, either through apoptotic or necrotic processes. Tissue tries to counteract this by means of evolutionary signalling pathways, such as the nuclear hypoxia-inducible factor, which protects the tissue by promoting cell survival strategies and simultaneously intervening in angiogenesis, haematogenesis and metabolic processes. Recent findings indicate that these conserved signalling pathways can also function as therapeutic approaches in wound healing of bones and skin, as well as in the regeneration of tissues, e.g. in the liver, and in the hematopoietic system.

Development of Inhibitors Targeting Hypoxia-Inducible Factor 1 and 2 for Cancer Therapy

Hypoxia is frequently observed in solid tumors and also one of the major obstacles for effective cancer therapies. Cancer cells take advantage of their ability to adapt hypoxia to initiate a special transcriptional program that renders them more aggressive biological behaviors. Hypoxia-inducible factors (HIFs) are the key factors that control hypoxia-inducible pathways by regulating the expression of a vast array of genes involved in cancer progression and treatment resistance. HIFs, mainly HIF-1 and -2, have become potential targets for developing novel cancer therapeutics. This article reviews the updated information in tumor HIF pathways, particularly recent advances in the development of HIF inhibitors. These inhibitors interfere with mRNA expression, protein synthesis, protein degradation and dimerization, DNA binding and transcriptional activity of HIF-1 and -2, or both. Despite efforts in the past two decades, no agents directly inhibiting HIFs have been approved for treating cancer patients. By analyzing results of the published reports, we put the perspectives at the end of the article. The therapeutic efficacy of HIF inhibitors may be improved if more efforts are devoted on developing agents that are able to simultaneously target HIF-1 and -2, increasing the penetrating capacity of HIF inhibitors, and selecting suitable patient subpopulations for clinical trials.

Hypoxia-inducible factors: key regulators of myeloid cells during inflammation

Hypoxia is a prominent characteristic of many acute or chronic inflammatory diseases, and exerts significant influence on their progression. Macrophages and neutrophils are major cellular components of innate immunity and contribute not only to O2 deprivation at the site of inflammation, but also alter many of their functions in response to hypoxia to either facilitate or suppress inflammation. Hypoxia stabilizes HIF-αs in macrophages and neutrophils, and these O2-sensitive transcription factors are key regulators of inflammatory responses in myeloid cells. In this review, we will summarize our current understanding of the role of HIF-αs in shaping macrophage and neutrophil functions in the pathogenesis and progression of multiple inflammatory diseases.

Hypoxia-inducible factors: a central link between inflammation and cancer

The tumor immune response is in a dynamic balance between antitumor mechanisms, which serve to decrease cancer growth, and the protumor inflammatory response, which increases immune tolerance, cell survival, and proliferation. Hypoxia and expression of HIF-1α and HIF-2α are characteristic features of all solid tumors. HIF signaling serves as a major adaptive mechanism in tumor growth in a hypoxic microenvironment. HIFs represent a critical signaling node in the switch to protumorigenic inflammatory responses through recruitment of protumor immune cells and altered immune cell effector functions to suppress antitumor immune responses and promote tumor growth through direct growth-promoting cytokine production, angiogenesis, and ROS production. Modulating HIF function will be an important mechanism to dampen the tumor-promoting inflammatory response and inhibit cancer growth.

Hypoxia-dependent regulation of inflammatory pathways in immune cells

Uncontrolled inflammation underpins a diverse range of diseases where effective therapy remains an unmet clinical need. Hypoxia is a prominent feature of the inflammatory microenvironment that regulates key transcription factors including HIF and NF-κB in both innate and adaptive immune cells. In turn, altered activity of the pathways controlled by these factors can affect the course of inflammation through the regulation of immune cell development and function. In this review, we will discuss these pathways and the oxygen sensors that confer hypoxic sensitivity in immune cells. Furthermore, we will describe how hypoxia-dependent pathways contribute to immunity and discuss their potential as therapeutic targets in inflammatory and infectious disease.

HIF-1 at the crossroads of hypoxia, inflammation, and cancer

The complex cross-talk of intricate intercellular signaling networks between the tumor and stromal cells promotes cancer progression. Hypoxia is one of the most common conditions encountered within the tumor microenvironment that drives tumorigenesis. Most responses to hypoxia are elicited by a family of transcription factors called hypoxia-inducible factors (HIFs), which induce expression of a diverse set of genes that assist cells to adapt to hypoxic environments. Among the three HIF protein family members, the role of HIF-1 is well established in cancer progression. HIF-1 functions as a signaling hub to coordinate the activities of many transcription factors and signaling molecules that impact tumorigenesis. This mini review discusses the complex role of HIF-1 and its context-dependent partners under various cancer-promoting events including inflammation and generation of cancer stem cells, which are implicated in tumor metastasis and relapse. In addition, the review highlights the importance of therapeutic targeting of HIF-1 for cancer prevention.

The Acute Kidney Injury to Chronic Kidney Disease Transition: A Potential Opportunity to Improve Care in Acute Kidney Injury

Recent controlled trials, epidemiological analyses and basic research studies offer a comprehensive view of the short and long-term clinical repercussion of de novo acute kidney injury or AKI. While most post-AKI patients recover their baseline renal function, a significant number, approximately ~20% of those affected, will go on to develop long term illness characterized by an increase in late stage CKD, cardiovascular complications, and increased death rates. When AKI occurs in hospitalized patients, selected demographic and laboratory results can be incorporated into risk calculators that identify those at higher risk for long-term complications. This review touches on some of the salient epidemiological studies of the AKI to CKD transition. It also focuses on certain recent advancements in our understanding of the biological and functional impact of AKI on the renal tubule repair mechanism, as well as the important role that genetic, epigenetic, biochemical and inflammatory events, seemingly beneficial to the re-establishment of normal renal function, can be offset by mediators of progressive fibrosis and irreversible structural changes. Characterization of basic processes that mediate the AKI to CKD transition reveals promising pharmacological and biological agents that hopefully will one day be used in the early stages of AKI to prevent its deadly consequences.

Ocular Angiogenesis: Vascular Endothelial Growth Factor and Other Factors

Systematic study of the mechanisms underlying pathological ocular neovascularization has yielded a wealth of knowledge about pro- and anti-angiogenic factors that modulate diseases such as neovascular age-related macular degeneration. The evidence implicating vascular endothelial growth factor (VEGF) in particular has led to the development of a number of approved anti-VEGF therapies. Additional proangiogenic targets that have emerged as potential mediators of ocular neovascularization include hypoxia-inducible factor-1, angiopoietin-2, platelet-derived growth factor-B and components of the alternative complement pathway. As for VEGF, knowledge of these factors has led to a product pipeline of many more novel agents that are in various stages of clinical development in the setting of ocular neovascularization. These agents are represented by a range of drug classes and, in addition to novel small- and large-molecule VEGF inhibitors, include gene therapies, small interfering RNA agents and tyrosine kinase inhibitors. In addition, combination therapy is beginning to emerge as a strategy to improve the efficacy of individual therapies. Thus, a variety of agents, whether administered alone or as adjunctive therapy with agents targeting VEGF, offer the promise of expanding the range of treatments for ocular neovascular diseases.

Hypoxia inducible factors in liver disease and hepatocellular carcinoma: current understanding and future directions

Hypoxia inducible transcription factors (HIFs) activate diverse pathways that regulate cellular metabolism, angiogenesis, proliferation, and migration, enabling a cell to respond to a low oxygen or hypoxic environment. HIFs are regulated by oxygen-dependent and independent signals including: mitochondrial dysfunction, reactive oxygen species, endoplasmic reticular stress, and viral infection. HIFs have been reported to play a role in the pathogenesis of liver disease of diverse aetiologies. This review explores the impact of HIFs on hepatocellular biology and inflammatory responses, highlighting the therapeutic potential of targeting HIFs for an array of liver pathologies.

Oxygen sensing and metabolic homeostasis

Oxygen-sensing mechanisms have evolved to maintain cell and tissue homeostasis since the ability to sense and respond to changes in oxygen is essential for survival. The primary site of oxygen sensing occurs at the level of the carotid body which in response to hypoxia signals increased ventilation without the need for new protein synthesis. Chronic hypoxia activates cellular sensing mechanisms which lead to protein synthesis designed to alter cellular metabolism so cells can adapt to the low oxygen environment without suffering toxicity. The master regulator of the cellular response is hypoxia-inducible factor (HIF). Activation of this system under condition of hypobaric hypoxia leads to weight loss accompanied by increased basal metabolic rate and suppression of appetite. These effects are dose dependent, gender and genetic specific, and results in adverse effects if the exposure is extreme. Hypoxic adipose tissue may represent a unified cellular mechanism for variety of metabolic disorders, and insulin resistance in patients with metabolic syndrome.

Regulation of wound healing and fibrosis by hypoxia and hypoxia-inducible factor-1

Wound healing is a complex multi-step process that requires spatial and temporal orchestration of cellular and non-cellular components. Hypoxia is one of the prominent microenvironmental factors in tissue injury and wound healing. Hypoxic responses, mainly mediated by a master transcription factor of oxygen homeostasis, hypoxia-inducible factor-1 (HIF-1), have been shown to be critically involved in virtually all processes of wound healing and remodeling. Yet, mechanisms underlying hypoxic regulation of wound healing are still poorly understood. Better understanding of how the wound healing process is regulated by the hypoxic microenvironment and HIF-1 signaling pathway will provide insight into the development of a novel therapeutic strategy for impaired wound healing conditions such as diabetic wound and fibrosis. In this review, we will discuss recent studies illuminating the roles of HIF-1 in physiologic and pathologic wound repair and further, the therapeutic potentials of HIF-1 stabilization or inhibition.

Defining the role of HIF and its downstream mediators in hypoxic-induced cerebral angiogenesis

Now that some of the basic mechanisms that underlie hypoxia-induced cerebral angiogenesis have been described, it has become clear that the hypoxia-inducible transcription factors, HIF-1 and HIF-2, play an important role in the process by causing the upregulation of vascular endothelial growth factor (VEGF). The heterogeneity of the brain parenchyma means that further progress in understanding capillary pathophysiology requires techniques that allow determination of the roles of individual components of the neurovascular unit. Multi-stain fluorescence co-localization techniques provide one such approach.

Genetically heterogeneous origins of idiopathic erythrocytosis

The idiopathic erythrocytosis (IE) group of disorders is defined by an absolute increase in red cell mass and hematocrit without elevation of the megakaryocytic or granulocytic lineages. It is associated with a wide range of serum erythropoietin (Epo) levels and broadly falls into groups of raised/inappropriately normal or low/undetectable Epo levels. A spectrum of molecular defects has been described in association with IE, which reflects the heterogeneity of this disorder. To date the most common identified cause of IE has been mutations in the von Hippel Landau (VHL) protein, which results in aberrant oxygen sensing and dysregulated Epo production. Studying the molecular basis of IE will provide insights into the control of Epo synthesis and Epo-induced signaling pathways.

Chronic intermittent hypoxia increases β cell mass and activates the mammalian target of rapamycin/hypoxia inducible factor 1/vascular endothelial growth factor A pathway in mice pancreatic islet

BACKGROUND

Growing evidence from population and clinic based studies showed that obstructive sleep apnea (OSA) and its characterizing chronic intermittent hypoxia (IH) were independently associated with the development of type 2 diabetes mellitus. However, the pathogenesis by which OSA induces glucose metabolic disorders is not clear. We determined changes in pancreatic β cell mass and the mammalian target of rapamycin (mTOR)/hypoxia inducible factor 1 (HIF-1)/vascular endothelial growth factor A (VEGF-A) pathway following IH exposure.

METHODS

A controlled gas delivery system regulated the flow of nitrogen and oxygen into a customized cage housing mice during the experiment. Twenty-four male wild C57BL/6J mice were either exposed to IH (n = 12) or intermittent air as a control (n = 12) for 56 days. Mice were anaesthetized and sacrificed after exposure, pancreas samples were dissected for immunofluorescent staining. Insulin and DAPI staining labelled islet β cells. Insulin positive area and β cell number per islet were measured. P-S6, HIF-1α and VEGF-A staining were performed to detect the activation of mTOR/HIF-1/VEGF-A pathway.

RESULTS

After eight weeks of IH exposure, insulin positive area increased by an average of 18.5% (P < 0.05). The β cell number per islet increased (92 vs. 55, respectively for IH and the control groups, P < 0.05) with no change in the size of individual β cells. Islet expression of HIF-1α and VEGF-A were higher in IH group than control group, and percentage of p-S6 positive β cell also increased after IH exposure (16.8% vs. 4.6% respectively for IH and the control groups, P < 0.05).

CONCLUSION

The number of pancreatic β cells increased as did the activity of the mTOR/HIF-1/VEGF-A pathway after exposure to IH.

Cobalt chloride decreases fibroblast growth factor-21 expression dependent on oxidative stress but not hypoxia-inducible factor in Caco-2 cells

Fibroblast growth factor-21 (FGF21) is a potential metabolic regulator with multiple beneficial effects on metabolic diseases. FGF21 is mainly expressed in the liver, but is also found in other tissues including the intestine, which expresses β-klotho abundantly. The intestine is a unique organ that operates in a physiologically hypoxic environment, and is responsible for the fat absorption processes including triglyceride breakdown, re-synthesis and absorption into the portal circulation. In the present study, we investigated the effects of hypoxia and the chemical hypoxia inducer, cobalt chloride (CoCl(2)), on FGF21 expression in Caco-2 cells and the consequence of fat accumulation. Physical hypoxia (1% oxygen) and CoCl(2) treatment decreased both FGF21 mRNA and secreted protein levels. Gene silence and inhibition of hypoxia-inducible factor-α (HIFα) did not affect the reduction of FGF21 mRNA and protein levels by hypoxia. However, CoCl(2) administration caused a significant increase in oxidative stress. The addition of n-acetylcysteine (NAC) suppressed CoCl(2)-induced reactive oxygen species (ROS) formation and completely negated CoCl(2)-induced FGF21 loss. mRNA stability analysis demonstrated that the CoCl(2) administration caused a remarkable reduction in FGF21 mRNA stability. Furthermore, CoCl(2) increased intracellular triglyceride (TG) accumulation, along with a reduction in mRNA levels of lipid lipase, hormone sensitive lipase (HSL) and adipose triglyceride lipase (ATGL), and an increase of sterol regulatory element-binding protein-1c (SREBP1c) and stearoyl-coenzyme A (SCD1). Addition of both NAC and recombinant FGF21 significantly attenuated the CoCl(2)-induced TG accumulation. In conclusion, the decrease of FGF21 in Caco-2 cells by chemical hypoxia is independent of HIFα, but dependent on an oxidative stress-mediated mechanism. The regulation of FGF21 by hypoxia may contribute to intestinal lipid metabolism and absorption.

Novel links between HIFs, type 2 diabetes, and metabolic syndrome

Hypoxia inducible factors (HIFs) are master-regulators of cellular responses to hypoxia, and thus are crucial for survival. HIFs also play a role in regulating cellular processes in β-cells, liver, muscle, and adipose tissue, have effects on the regulation of weight, and play a role in type 2 diabetes (T2D). Indeed, in people with T2D the HIF pathway is dyregulated in major metabolic tissues involved in the pathogenesis of diabetes. This review covers the contrasting, complementary and conflicting effects of decreasing and increasing HIFs in various tissues, and shows that a delicate balance exists between HIF levels and optimal metabolic function. We propose that increasing the activity of HIFs might be a potential therapeutic strategy for treating T2D.

[Hypoxia, inflammation and phenotypic plasticity of macrophages: the central role of HIF-1 and NFkappaB]

Decrease of oxygen concentration, i.e. hypoxia, in organism tissues and cells is an important pathogenetic component in a large number of diseases. In these cases hypoxia is not only an important component of diseases pathogenesis, but can also influence immune reactions determining the outcome of diseases. Thus, concentration of macrophages in hypoxic areas and their reaction to hypoxia are the key moments in understanding the mechanisms of hypoxia influence on immunity. Macrophages are of the utmost importance in the congenital immune startup and define the vector of development of the adaptive response. In this review we present updated data on influence of hypoxia on macrophages phenotype and their plasticity, and we also analyze genetic trait of macrophages reaction to hypoxia. Molecular mechanisms of immune cells reaction on hypoxia and the role of transcription factors, HIF-1 and NF-kappaB, are analyzed. As a whole, it allowed to describe an important biological phenomenon - hypoxia-regulated control of macrophages phenotypic plasticity, and to define ways of search of new effective approaches to the management of diseases with hypoxic disturbances.

Induction of gastrin expression in gastrointestinal cells by hypoxia or cobalt is independent of hypoxia-inducible factor (HIF)

Gastrin and its precursors have been shown to promote mitogenesis and angiogenesis in gastrointestinal tumors. Hypoxia stimulates tumor growth, but its effect on gastrin gene regulation has not been examined in detail. Here we have investigated the effect of hypoxia on the transcription of the gastrin gene in human gastric cancer (AGS) cells. Gastrin mRNA was measured by real-time PCR, gastrin peptides were measured by RIA, and gastrin promoter activity was measured by dual-luciferase reporter assay. Exposure to a low oxygen concentration (1%) increased gastrin mRNA concentrations in wild-type AGS cells (AGS) and in AGS cells overexpressing the gastrin receptor (AGS-cholecystokinin receptor 2) by 2.1 ± 0.4- and 4.1 ± 0.3-fold (P < 0.05), respectively. The hypoxia mimetic, cobalt chloride (300 μM), increased gastrin promoter activity in AGS cells by 2.4 ± 0.3-fold (P < 0.05), and in AGS-cholecystokinin receptor 2 cells by 4.0 ± 0.3-fold (P < 0.05), respectively. The observations that either deletion from the gastrin promoter of the putative binding sites for the transcription factor hypoxia-inducible factor 1 (HIF-1) or knockdown of either the HIF-1α or HIF-1β subunit did not affect gastrin promoter inducibility under hypoxia indicated that the hypoxic activation of the gastrin gene is likely HIF independent. Mutational analysis of previously identified Sp1 regulatory elements in the gastrin promoter also failed to abrogate the induction of promoter activity by hypoxia. The observations that hypoxia up-regulates the gastrin gene in AGS cells by HIF-independent mechanisms, and that this effect is enhanced by the presence of gastrin receptors, provide potential targets for gastrointestinal cancer therapy.

The therapeutic potential of oxygen tension manipulation via hypoxia inducible factors and mimicking agents in guided bone regeneration. A review

Intraoral bone grafting is routinely employed for implant site development prior or simultaneously to implant placement. Bone graft consolidation is a complex biological process depending on the formation of blood vessels into the augmented area. It is highly regulated by the angiogenesis and osteogenesis coupling phenomenon. The vascular system apart from supplying nutrients and oxygen to the developing and regenerating bone, also delivers critical signals which stimulate mesenchymal cell differentiation towards an osteogenic phenotype. Hypoxia inducible factors (HIFs) and mimicking agents (HMAs) (or alternatively HIF stabilizing agents) are considered to act as key stimulators of blood vessel formation. Under normoxia, HIFs are rapidly degraded. However, their degradation is prevented under hypoxia, which in turn, triggers angiogenesis. Hence, the major role of HMAs is to prevent degradation of HIFs under normoxic conditions. Recent studies suggest that HIFs and HMAs trigger the initiation and promotion of angiogenic-osteogenic cascade events. In vitro and animal studies involving genetic manipulation of individual components of the HIFs and HMAs have provided clues to how angiogenic-osteogenic coupling is achieved. Evidence from preclinical studies further suggests that topical application of HMAs enhance angiogenesis in intraoral augmented sites. In this article, we review the current understanding of the cellular and molecular mechanisms responsible for angiogenic-osteogenic coupling. We also discuss the therapeutic manipulation of HIFs and HMAs in intraoral bone repair and regeneration. Such discoveries suggest promising approaches for the development of novel therapies to improve intraoral bone repair and regeneration procedures.

[The role of hypoxia-inducible factor family (HIF) proteins in the regulation of cells physiologic responses to hypoxia]

The review is devoted to the role of hypoxia-inducible factors (HIF) in the regulation of oxygen-dependent gene signalling. Structural features of HIF alpha and beta subunits as well as involvement of hydroxylation in the regulation of HIF stability and activity are described. Special attention is given to the role of phosphorylation and other post-translational modifications in the regulation of HIF expression and activity. The survey considers the involvement of hormones, cytokines and growth factors in HIF stimulation under normoxia. HIF target genes and promotor/enhancer sequences, responsible for oxygen-dependent gene regulation, are also described.

Hypoxia signaling and resistance in C. elegans

In normal development and homeostasis and in many disease states, cells and tissues must overcome the challenge of oxygen deprivation (hypoxia). The nematode C. elegans is emerging as an increasingly powerful system in which to understand how animals adapt to moderate hypoxia and survive extreme hypoxic insults. This review provides an overview of C. elegans responses to hypoxia, ranging from adaptation and arrest to death, and highlights some of the recent studies that have provided important insights into hypoxia signaling and resistance. Many of the key genes and pathways are evolutionarily conserved, and C. elegans hypoxia research promises to inform our understanding of oxygen-sensitive signaling and survival in mammalian development and disease.

Hypoxia and metastasis in breast cancer

In this review we summarize the evidence for a role for hypoxic response in the biology of metastasis, with a particular emphasis on the metastasis of breast cancer and the function of the hypoxia inducible factor (HIF).

[Hypoxia in cancer malignity. Review]

Hypoxia is a common characteristic of solid tumors. It contributes to local and systemic tumor progression, as well as the lack of response to radio and chemotherapy, therefore increasing the probability of tumor recurrence. The HIF-1 transcription factor is the main regulator of tumor adaptation to hypoxia stress, stimulating the expression of many genes that allow cells to survive under these conditions. Products dependent on HIF-1 factor are involved in processes of tumor progression, such as proliferation, glucose metabolism, ph-acidosis, angiogenesis and metastasis. It has become increasingly necessary to gain knowledge on the HIF-1 mechanisms of action, since it is possible to find inhibitors that could be used therapeutically against cancer. In this review, a summary is given on the role that the HIF-1 factor plays in hypoxia, as well as its implications on angiogenesis and metastasis.

The HIF-1-active microenvironment: an environmental target for cancer therapy

Solid tumors possess unique microenvironments that are exposed to chronic hypoxic conditions, so-called tumor hypoxia. Although more than half a century has passed since it was suggested that tumor hypoxia correlated with bad treatment outcomes and contributed to the recurrence of cancer, no fundamental solution to this problem has yet been found. Hypoxia-inducible factor HIF-1 is the main transcription factor that regulates the cellular response to hypoxia. It induces various genes, whose function is strongly associated with the malignant alteration of the entire tumor. The cellular changes induced by HIF-1 are extremely important therapeutic targets of cancer therapy, particularly in the therapy against refractory cancers. Therefore targeting strategies to overcome the HIF-1-active microenvironment are important for cancer therapy.

Diabetic cardiomyopathy--a distinct disease?

Diabetic individuals have a significantly increased likelihood of developing cardiovascular disease. Whilst part of this association is explained by the presence of concomitant risk factors, large epidemiological studies have consistently reported diabetes as a strong risk factor for the development of heart failure after adjusting for such covariates. This has resulted in the notion that there is a distinct cardiomyopathy specific to diabetes, termed 'diabetic cardiomyopathy'. The natural history is characterized by a latent subclinical period, during which there is evidence of diastolic dysfunction and left ventricular hypertrophy, before overt clinical deterioration and systolic failure ensue. These clinical findings have been supported by a growing body of experimental data which support the notion that diabetes inflicts a direct insult to the myocardium, with cellular, structural and functional changes manifest as the diabetic myocardial phenotype. Several of these mechanisms appear to work in unison, forming complicated reciprocal pathways of disease. Reactive oxygen species and alterations in intracellular calcium homeostasis appear to play significant roles in many of these mechanisms. Determining the hierarchy of this cascade of disease will allow identification of the pathological trigger most responsible for disease. Translational research in this field is currently hindered by a lack of clinical studies and intervention trials specifically in patients with diabetic cardiomyopathy. Future clinical and experimental studies of accurate models of diabetic cardiomyopathy should help to define the true aetiology and lead to the development of specific pharmacotherapies for this condition, ultimately reducing the increased cardiovascular morbidity and mortality in diabetic patients.

Glutathione depletion in immortalized midbrain-derived dopaminergic neurons results in increases in the labile iron pool: implications for Parkinson's disease

Glutathione depletion is one of the earliest detectable events in the Parkinsonian substantia nigra (SN), but whether it is causative for ensuing molecular events associated with the disease is unknown. Here we report that reduction in levels of glutathione in immortalized midbrain-derived dopaminergic neurons results in increases in the cellular labile iron pool (LIP). This increase is independent of either iron regulatory protein/iron regulatory element (IRP/IRE) or hypoxia inducible factor (HIF) induction but is both H(2)0(2) and protein synthesis-dependent. Our findings suggest a novel mechanistic link between dopaminergic glutathione depletion and increased iron levels based on translational activation of TfR1. This may have important implications for neurodegeneration associated with Parkinson's disease in which both glutathione reduction and iron elevation have been implicated.

Hypoxia-inducible factor-1, a new possible important factor in neoplasia

Hypoxia-inducible factor-1 (HIF-1) is one of the major factors responsible for the activation of compensation processes during cell hypoxia. Hypoxia-inducible factor-1 is a dimeric protein complex and serves as a transcription factor regulator for many target genes. Under normoxic conditions it is constitutively produced and degraded by the ubiquitin-proteasome system. But under hypoxic conditions HIF-1 becomes stabilized. The expression of HIF-1 increases vascularization of the ischaemic area and regulates anaerobic metabolism. The same processes were observed in neoplastic tissues. Hypoxia-inducible factor-1 and its participation in neoplasia, the inhibition and stimulation of its transcriptional activity, attracts the attention of many scientists as the understanding of its function in tumour progression could be used in the future for preparation of new antineoplastic therapies.

Molecular mechanisms of normal iron homeostasis

Humans possess elegant control mechanisms to maintain iron homeostasis by coordinately regulating iron absorption, iron recycling, and mobilization of stored iron. Dietary iron absorption is regulated locally by hypoxia inducible factor (HIF) signaling and iron-regulatory proteins (IRPs) in enterocytes and systematically by hepatic hepcidin, the central iron regulatory hormone. Hepcidin not only controls the rate of iron absorption but also determines iron mobilization from stores through negatively modulating the function of ferroportin, the only identified cellular iron exporter to date. The regulation of hepatic hepcidin is accomplished by the coordinated activity of multiple proteins through different signaling pathways. Recent studies have greatly expanded the knowledge in the understanding of hepcidin expression and regulation by the bone morphogenetic protein (BMP) signaling, the erythroid factors, and inflammation. In this review, we mainly focus on the roles of recently identified proteins in the regulation of iron homeostasis.