The role of calcium in hypoxia-induced signal transduction and gene expression

The role and regulation of hypoxia-inducible factor-1alpha expression in brain development and neonatal hypoxic-ischemic brain injury

During neonatal hypoxic-ischemic brain injury, activation of transcription of a series of genes is induced to stimulate erythropoiesis, anti-apoptosis, apoptosis, necrosis and angiogenesis. A key factor mediating these gene transcriptions is hypoxia-inducible factor-1alpha (HIF-1alpha). During hypoxia, HIF-1alpha protein is stabilized and heterodimerizes with HIF-1beta to form HIF-1, subsequently regulating the expression of target genes. HIF-1alpha participates in early brain development and proliferation of neuronal precursor cells. Under pathological conditions, HIF-1alpha is known to play an important role in neonatal hypoxic-ischemic brain injury: on the one hand, HIF-1alpha has neuroprotective effects whereas it can also have neurotoxic effects. HIF-1alpha regulates the transcription of erythropoietin (EPO), which induces several pathways associated with neuroprotection. HIF-1alpha also promotes the expression of vascular endothelial cell growth factor (VEGF), which is related to neovascularization in hypoxic-ischemic brain areas. In addition, HIF-1alpha has an anti-apoptotic effect by increasing the expression of anti-apoptotic factors such as EPO during mild hypoxia. The neurotoxic effects of HIF-1alpha are represented by its participation in the apoptotic process by increasing the stability of the tumor suppressor protein p53 during severe hypoxia. Moreover, HIF-1alpha plays a role in cell necrosis, by interacting with calcium and calpain. HIF-1alpha can also exacerbate brain edema via increasing the permeability of the blood-brain barrier (BBB). Given these properties, HIF-1alpha has both neuroprotective and neurotoxic effects after hypoxia-ischemia. These events are cell type specific and related to the severity of hypoxia. Unravelling of the complex functions of HIF-1alpha may be important when designing neuroprotective therapies for hypoxic-ischemic brain injury.

Hypoxia stimulates via separate pathways ERK phosphorylation and NF-κB activation in skeletal muscle cells in primary culture

Mammalian cells sense oxygen levels and respond to hypoxic conditions through the regulation of multiple signaling pathways and transcription factors. Here, we investigated the effects of hypoxia on the activity of two transcriptional regulators, ERK1/2 and NF-κB, in skeletal muscle cells in primary culture. We found that hypoxia significantly enhanced ERK1/2 phosphorylation and that it stimulated NF-κB-dependent gene transcription as well as nuclear translocation of a green fluorescent protein-labeled p65 NF-κB isoform. Phosphorylation of ERK1/2- and NF-κB-dependent transcription by hypoxia required calcium entry through L-type calcium channels. Calcium release from ryanodine-sensitive stores was also necessary for ERK1/2 activation but not for NF-κB-dependent-transcription. N-acetylcysteine, a general scavenger of reactive oxygen species, blocked hypoxia-induced ROS generation but did not affect the stimulation of ERK1/2 phosphorylation induced by hypoxia. In contrast, NF-κB activation was significantly inhibited by N-acetylcysteine and did not depend on ERK1/2 stimulation, as shown by the lack of effect of the upstream ERK inhibitor U-0126. These separate pathways of activation of ERK1/2 and NF-κB by hypoxia may contribute to muscle adaptation in response to hypoxic conditions.

Cellular oxygen sensing, signalling and how to survive translational arrest in hypoxia

Hypoxia is a consequence of inadequate oxygen availability. At the cellular level, lowered oxygen concentration activates signal cascades including numerous receptors, ion channels, second messengers, as well as several protein kinases and phosphatases. This, in turn, activates trans-factors like transcription factors, RNA-binding proteins and miRNAs, mediating an alteration in gene expression control. Each cell type has its unique constellation of oxygen sensors, couplers and effectors that determine the activation and predominance of several independent hypoxia-sensitive pathways. Hence, altered gene expression patterns in hypoxia result from a complex regulatory network with multiple divergences and convergences. Although hundreds of genes are activated by transcriptional control in hypoxia, metabolic rate depression, as a consequence of reduced ATP level, causes inhibition of mRNA translation. In a multi-phase response to hypoxia, global protein synthesis is suppressed, mainly by phosphorylation of eIF2-alpha by PERK and inhibition of mTOR, causing suppression of 5'-cap-dependent mRNA translation. Growing evidence suggests that mRNAs undergo sorting at stress granules, which determines the fate of mRNA as to whether being translated, stored, or degraded. Data indicate that translation is suppressed only at 'free' polysomes, but is active at subsets of membrane-bound ribosomes. The recruitment of specific mRNAs into subcellular compartments seems to be crucial for local mRNA translation in prolonged hypoxia. Furthermore, ribosomes themselves may play a significant role in targeting mRNAs for translation. This review summarizes the multiple facets of the cellular adaptation to hypoxia observed in mammals.

p300 expression is induced by oxygen deficiency and protects neuron cells from damage

Low oxygen level or oxygen deficiency (hypoxia) is a major factor causing neuronal damage in many diseases. Inducing cell adaptation to hypoxia is an effective method for neuroprotection that can be achieved by either inhibiting the death effectors or enhancing the survival factors. Transcription coactivator p300 is necessary for hypoxia-induced transcriptional activation and plays an important role in neuron survival. However, the alteration of p300 expression under hypoxia condition and its role in hypoxia-induced neuronal damage remain unclear. In this study, the distribution of p300 in rat brain and the alteration of its expression in rat hippocampus during hypobaric hypoxia exposure were detected. In addition, the role of p300 in neuronal-like PC12 cell damage induced by oxygen deficiency (3% oxygen) was evaluated. Our results showed that p300 protein was mainly expressed in the cells expressed beta-tubulin III in the cerebral cortex, hippocampus, cerebellum cortex, medulla oblongata and hypothalamus. Less or no positive signal of p300 expression was observed in beta-tubulin III negative cells. This indicated that p300 was predominantly expressed in neurons of rat brain. Furthermore, p300 expression was up-regulated in rat hippocampus during hypoxia exposure and in neuronal-like PC12 cells under 3% oxygen condition. Interestingly, neuronal-like PC12 cell damage induced by oxygen deficiency (3% oxygen) was increased by suppression of p300 expression with short hairpin RNA (shRNA). These data indicate that p300 is an important molecule for neuroprotection under hypoxia.

Hypoxia reprograms calcium signaling and regulates myoglobin expression

Myoglobin is an oxygen storage molecule that is selectively expressed in cardiac and slow-twitch skeletal muscles that have a high oxygen demand. Numerous studies have implicated hypoxia in the regulation of myoglobin expression as an adaptive response to hypoxic stress. However, the details of this relationship remain undefined. In the present study, adult mice exposed to 10% oxygen for periods up to 3 wk exhibited increased myoglobin expression only in the working heart, whereas myoglobin was either diminished or unchanged in skeletal muscle groups. In vitro and in vivo studies revealed that hypoxia in the presence or absence of exercise-induced stimuli reprograms calcium signaling and modulates myoglobin gene expression. Hypoxia alone significantly altered calcium influx in response to cell depolarization or depletion of endoplasmic reticulum calcium stores, which inhibited the expression of myoglobin. In contrast, our whole animal and transcriptional studies indicate that hypoxia in combination with exercise enhanced the release of calcium from the sarcoplasmic reticulum via the ryanodine receptors triggered by caffeine, which increased the translocation of nuclear factor of activated T-cells into the nucleus to transcriptionally activate myoglobin expression. The present study unveils a previously unrecognized mechanism where the hypoxia-mediated regulation of calcium transients from different intracellular pools modulates myoglobin gene expression. In addition, we observed that changes in myoglobin expression, in response to hypoxia, are not dependent on hypoxia-inducible factor-1 or changes in skeletal muscle fiber type. These studies enhance our understanding of hypoxia-mediated gene regulation and will have broad applications for the treatment of myopathic diseases.

HIF-1 regulation: not so easy come, easy go

The hypoxia-inducible factor-1 (HIF-1) is the master regulator of the cellular response to hypoxia and its expression levels are tightly controlled through synthesis and degradation. It is widely accepted that HIF-1alpha protein accumulation during hypoxia results from inhibition of its oxygen-dependent degradation by the von Hippel Lindau protein (pVHL) pathway. However, recent data describe new pVHL- or oxygen-independent mechanisms for HIF-1alpha degradation. Furthermore, the hypoxia-induced increase in HIF-1alpha levels is facilitated by the continued translation of HIF-1alpha during hypoxia despite the global inhibition of protein translation. Recent work has contributed to an increased understanding of the mechanisms that control the translation and degradation of HIF-1alpha under both normoxic and hypoxic conditions.

Blood-brain barrier tight junction permeability and ischemic stroke

The blood-brain barrier (BBB) is formed by the endothelial cells of cerebral microvessels, providing a dynamic interface between the peripheral circulation and the central nervous system. The tight junctions (TJs) between the endothelial cells serve to restrict blood-borne substances from entering the brain. Under ischemic stroke conditions decreased BBB TJ integrity results in increased paracellular permeability, directly contributing to cerebral vasogenic edema, hemorrhagic transformation, and increased mortality. This loss of TJ integrity occurs in a phasic manner, which is contingent on several interdependent mechanisms (ionic dysregulation, inflammation, oxidative and nitrosative stress, enzymatic activity, and angiogenesis). Understanding the inter-relation of these mechanisms is critical for the development of new therapies. This review focuses on those aspects of ischemic stroke impacting BBB TJ integrity and the principle regulatory pathways, respective to the phases of paracellular permeability.

Chronic hypoxia up-regulates alpha1H T-type channels and low-threshold catecholamine secretion in rat chromaffin cells

alpha(1H) T-type channels recruited by beta(1)-adrenergic stimulation in rat chromaffin cells (RCCs) are coupled to fast exocytosis with the same Ca(2+) dependence of high-threshold Ca(2+) channels. Here we show that RCCs exposed to chronic hypoxia (CH) for 12-18 h in 3% O(2) express comparable densities of functional T-type channels that depolarize the resting cells and contribute to low-voltage exocytosis. Following chronic hypoxia, most RCCs exhibited T-type Ca(2+) channels already available at -50 mV with the same gating, pharmacological and molecular features as the alpha(1H) isoform. Chronic hypoxia had no effects on cell size and high-threshold Ca(2+) current density and was mimicked by overnight incubation with the iron-chelating agent desferrioxamine (DFX), suggesting the involvement of hypoxia-inducible factors (HIFs). T-type channel recruitment occurred independently of PKA activation and the presence of extracellular Ca(2+). Hypoxia-recruited T-type channels were partially open at rest (T-type 'window-current') and contributed to raising the resting potential to more positive values. Their block by 50 microm Ni(2+) caused a 5-8 mV hyperpolarization. The secretory response associated with T-type channels could be detected following mild cell depolarizations, either by capacitance increases induced by step depolarizations or by amperometric current spikes induced by increased [KCl]. In the latter case, exocytotic bursts could be evoked even with 2-4 mm KCl and spike frequency was drastically reduced by 50 microm Ni(2+). Chronic hypoxia did not alter the shape of spikes, suggesting that hypoxia-recruited T-type channels increase the number of secreted vesicles at low voltages, without altering the mechanism of catecholamine release and the quantal content of released molecules.

Hyperoxia-induced signal transduction pathways in pulmonary epithelial cells

Mechanical ventilation with hyperoxia is necessary to treat critically ill patients. However, prolonged exposure to hyperoxia leads to the generation of excessive reactive oxygen species (ROS), which can cause acute inflammatory lung injury. One of the major effects of hyperoxia is the injury and death of pulmonary epithelium, which is accompanied by increased levels of pulmonary proinflammatory cytokines and excessive leukocyte infiltration. A thorough understanding of the signaling pathways leading to pulmonary epithelial cell injury/death may provide some insights into the pathogenesis of hyperoxia-induced acute inflammatory lung injury. This review focuses on epithelial responses to hyperoxia and some of the major factors regulating pathways to epithelial cell injury/death, and proinflammatory responses on exposure to hyperoxia. We discuss in detail some of the most interesting players, such as NF-kappaB, that can modulate both proinflammatory responses and cell injury/death of lung epithelial cells. A better appreciation for the functions of these factors will no doubt help us to delineate the pathways to hyperoxic cell death and proinflammatory responses.

Localization and activity of calmodulin is involved in cell–cell adhesion of tumor cells and endothelial cells in response to hypoxic stress

Adhesion of tumor cells to endothelial cells is known to be involved in the hematogenous metastasis of cancer, which is regulated by hypoxia. Hypoxia is able to induce a significant increase in free intracellular Ca2+ levels in both tumor cells and endothelial cells. Here, we investigate the regulatory effects of calmodulin (CaM), an intracellular calcium mediator, on tumor cell-endothelial cell adhesion under hypoxic conditions. Hypoxia facilitates HeLa cell-ECV304 endothelial cell adhesion, and results in actin cytoskeleton rearrangement in both endothelial cells and tumor cells. Suppression of CaM activation by CaM inhibitor W-7 disrupts actin cytoskeleton organization and CaM distribution in the cell-cell contact region, and thus inhibits cell-cell adhesion. CaM inhibitor also downregulates hypoxia-induced HIF-1-dependent gene expression. These results suggest that the Ca2+ -CaM signaling pathway might be involved in tumor cell-endothelial cell adhesion, and that co-localization of CaM and actin at cell-cell contact regions might be essential for this process under hypoxic stress.

Differential expression of oxygen-regulated genes in bovine blastocysts

Low oxygen conditions (2%) during post-compaction culture of bovine blastocysts improve embryo quality, which is associated with a small yet significant increase in the expression of glucose transporter 1 (GLUT-1), suggesting a role of oxygen in embryo development mediated through oxygen-sensitive gene expression. However, bovine embryos to at least the blastocyst stage lack a key regulator of oxygen-sensitive gene expression, hypoxia-inducible factor 1alpha (HIF1alpha). A second, less well-characterized protein (HIF2alpha) is, however, detectable from the 8-cell stage of development. Here we use differential display to determine additional gene targets in bovine embryos in response to low oxygen conditions. While development to the blastocyst stage was unaffected by the oxygen concentration used during post-compaction culture, differential display identified oxygen-regulation of myotrophin and anaphase promoting complex 1 expression, with significantly lower levels observed following culture under 20% oxygen than 2% oxygen. These results further support the hypothesis that the level of gene expression of specific transcripts by bovine embryos alters in response to changes in the oxygen environment post-compaction. Specifically, we have identified two oxygen-sensitive genes that are potentially regulated by HIF2 in the bovine blastocyst.

The role of oxygen in ruminant preimplantation embryo development and metabolism

The long-term effects of in vitro embryo culture on animal health are presently unknown, however, current knowledge directs investigations toward understanding the mechanisms involved in regulating embryo development. In vitro culture is known to have short-term effects, particularly on gene expression and metabolism at the blastocyst stage, while large offspring syndrome is commonly observed following transfer of in vitro produced bovine embryos. Indeed, it is likely that the environment surrounding the early embryo, prior to implantation, may program later development. Regulation of gene expression and metabolism, through gene activation, is mediated by transcription factors, which are themselves controlled by internal and external factors. Alterations in the surrounding environment during preimplantation embryo development, such as that which occurs with inadequate developmental 'support' during in vitro culture, may modify the activation, or inactivation, of several transcription factors, and may therefore have long-term consequences for the developing offspring. In vitro culture deviates from in vivo conditions in many respects, but one of the critical factors that is generally not considered is the oxygen tension under which embryos are cultured. Numerous studies have demonstrated that atmospheric oxygen conditions during culture have detrimental effects on embryo development. While it is generally believed that this arises from the production of reactive oxygen species, this presents an over-simplistic view of the role of oxygen during development. The hypoxia-inducible factor transcription factor family is involved in the responses of cells to alterations in external oxygen concentrations, regulating the expression of numerous genes. Alterations in expression of some of these genes have been highlighted by recent studies in the bovine embryo, implicating oxygen as a regulator of several cellular and metabolic pathways. While it is clear that oxygen plays a role during embryo development, further work to investigate interactions between oxygen and other signaling pathways such as pH and Ca(2+), mitochondria and metabolism is required, as well as exposure of embryos at different time points, to determine the mechanisms that control preimplantation development, the interactions of a range of stimuli and to establish culture procedures that support optimal development and minimize risks to health. This review focuses largely on work undertaken in ruminant models, with brief references to other species.

Translational control of collagen prolyl 4-hydroxylase-alpha(I) gene expression under hypoxia

Hypoxia is a pro-fibrotic stimulus, which is associated with enhanced collagen synthesis, as well as with augmented collagen prolyl 4-hydroxylase (C-P4H) activity. C-P4H activity is controlled mainly by regulated expression of the alpha C-P4H subunit. In this study we demonstrate that the increased synthesis of C-P4H-alpha(I) protein in human HT1080 fibroblasts under long term hypoxia (36 h, 1% oxygen) is controlled at the translational level. This is mediated by an interaction of RNA-binding protein nucleolin (approximately 64 kDa form) at the 5'- and 3'-untranslated regions (UTR) of the mRNA. The 5'/3'-UTR-dependent mechanism elevates the C-P4H-alpha(I) expression rate 2.3-fold, and participates in a 5.3-fold increased protein level under long term hypoxia. The interaction of nucleolin at the 5'-UTR occurs directly and depends on the existence of an AU-rich element. Statistical evaluation of the approximately 64-kDa nucleolin/RNA interaction studies revealed a core binding sequence, corresponding to UAAAUC or AAAUCU. At the 3'-UTR, nucleolin assembles indirectly via protein/protein interaction, with the help of another 3'-UTR-binding protein, presumably annexin A2. The increased protein level of the approximately 64-kDa nucleolin under hypoxia can be attributed to an autocatalytic cleavage of a high molecular weight nucleolin form, without alterations in nucleolin mRNA concentration. Thus, the alteration of translational efficiency by nucleolin, which occurs through a hypoxia inducible factor independent pathway, is an important step in C-P4H-alpha(I) regulation under hypoxia.

Sustained stromal stem cell self-renewal and osteoblastic differentiation during aging

We have reported the isolation of a unique subpopulation of human stromal cells from bone marrow termed marrow-isolated adult multilineage inducible (MIAMI) cells. The expression of embryonic stem cell markers SSEA-4, Oct-4, Rex-1, and telomerase reverse transcriptase indicates the developmentally immature status of these cells. They resemble primitive stem cells in their capacity to differentiate, at least in vitro, into mature-like cells from all three germ layers. MIAMI cells are characterized by a unique molecular profile that distinguishes them from other marrow stromal cell populations. Although the frequency of MIAMI cells, among all marrow nucleated cells, decreases from 0.01% at age 3 to 0.0018% at age 45, their numbers remain unchanged after age 45. The level of expression of the markers characteristic of MIAMI cells remains constant independent of age and gender. In long-term in vitro expansion experiments aging increased the population doubling time by about 30%, whereas specific in vitro differentiation of MIAMI cells toward osteoblastic cells was unaffected. Because the oxygen tension in bone marrow ranges from 1% to 7%, we examined the role of oxygen tension in regulating the capacity of MIAMI cells to self-renew and maintain their pluripotentiality during long-term culture. Low oxygen tension upregulated mRNAs for primitive embryonic stem cell markers. Our results suggest that maintaining developmentally primitive human cells in vitro at low oxygen tension is more physiologic and favors stemness. For osteoblastic differentiation, gap-junctional communication mediated by connexin43 is required. Its inhibition not only blocked osteoblastic differentiation but stimulated the adipocytic differentiation.

Gene expression profiling reveals the profound upregulation of hypoxia-responsive genes in primary human astrocytes

Oxygen is vital for the development and survival of mammals. In response to hypoxia, the brain initiates numerous adaptive responses at the organ level as well as at the molecular and cellular levels, including the alteration of gene expression. Astrocytes play critical roles in the proper functioning of the brain; thus the manner in which astrocytes respond to hypoxia is likely important in determining the outcome of brain hypoxia. Here, we used microarray gene expression profiling and data-analysis algorithms to identify and analyze hypoxia-responsive genes in primary human astrocytes. We also compared gene expression patterns in astrocytes with those in human HeLa cells and pulmonary artery endothelial cells (ECs). Remarkably, in astrocytes, five times as many genes were induced as suppressed, whereas in HeLa and pulmonary ECs, as many as or more genes were suppressed than induced. More genes encoding hypoxia-inducible functions, such as glycolytic enzymes and angiogenic growth factors, were strongly induced in astrocytes compared with HeLa cells. Furthermore, gene ontology and computational algorithms revealed that many target genes of the EGF and insulin signaling pathways and the transcriptional regulators Myc, Jun, and p53 were selectively altered by hypoxia in astrocytes. Indeed, Western blot analysis confirmed that two major signal transducers mediating insulin and EGF action, Akt and MEK1/2, were activated by hypoxia in astrocytes. These results provide a global view of the signaling and regulatory network mediating oxygen regulation in human astrocytes.

Calcium signalling: Past, present and future

Ca2+ is a universal second messenger controlling a wide variety of cellular reactions and adaptive responses. The initial appreciation of Ca2+ as a universal signalling molecule was based on the work of Sydney Ringer and Lewis Heilbrunn. More recent developments in this field were critically influenced by the invention of the patch clamp technique and the generation of fluorescent Ca2+ indicators. Currently the molecular Ca2+ signalling mechanisms are being worked out and we are beginning to assemble a reasonably complete picture of overall Ca2+ homeostasis. Furthermore, investigations of organellar Ca2+ homeostasis have added complexity to our understanding of Ca2+ signalling. The future of the Ca2+ signalling field lies with detailed investigations of the integrative function in vivo and clarification of the pathology associated with malfunctions of Ca2+ signalling cascades.

Erythropoietin receptor spliced forms differentially expressed in blind subterranean mole rats

Erythropoietin (Epo) is the primary regulator of erythropoiesis, controlling the proliferation, maturation, and survival of erythroid progenitor cells. The functions of Epo are mediated through its specific receptor (EpoR) expressed mainly on the surface of erythroid progenitor cells, and the expression of both responds to hypoxia. The subterranean mole rat (Spalax) is a unique model system to study the molecular mechanisms for adaptation to hypoxia. Here, we cloned two forms of Spalax EpoR: a complete EpoR cDNA as well as a novel truncated bone marrow specific EpoR form. In the full-length Spalax EpoR (sEpoR), two out of the eight conserved tyrosine- phosphorylation sites were substituted (Y481F and Y499G), suggesting that Spalax Epo signaling pathways may be modulated. The level of the sEpoR mRNA in the spleen and in bone marrow was relatively low and similar in Spalax newborns and adults, with no significant response to hypoxia. The truncated sEpoR was not detected in the spleen and comprised only approximately 1% of the sEpoR expressed in the bone marrow. In Rattus, the truncated EpoR form was approximately 15% of the total expressed receptor. The level of Rattus EpoR in newborn spleens was three- to fourfold higher than in Spalax newborns and decreased toward adulthood. Severe hypoxia induces a significant increase in adult Rattus EpoR. Our data provide further insight into the adaptive mechanisms of Spalax to the extreme conditions of hypoxia in its subterranean environment.

Molecular cloning and characterization of calmodulin genes in young oak seedlings (Quercus petraea L.) during early flooding stress

As part of an integrated study on the molecular response of woody plants to flooding, three CaM genes were isolated from oak seedlings (Quercus petraea Liebl.) and characterized. QpCaM-1 was almost exclusively expressed in roots, whereas QpCaM-2 and -3 were more evenly distributed throughout the plant. The present paper documents the differential expression of these genes during hypoxia.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.