Hypoxia and mitochondrial oxidative metabolism

It is now clear that mitochondrial defects are associated with a large variety of clinical phenotypes. This is the result of the mitochondria's central role in energy production, reactive oxygen species homeostasis, and cell death. These processes are interdependent and may occur under various stressing conditions, among which low oxygen levels (hypoxia) are certainly prominent. Cells exposed to hypoxia respond acutely with endogenous metabolites and proteins promptly regulating metabolic pathways, but if low oxygen levels are prolonged, cells activate adapting mechanisms, the master switch being the hypoxia-inducible factor 1 (HIF-1). Activation of this factor is strictly bound to the mitochondrial function, which in turn is related with the oxygen level. Therefore in hypoxia, mitochondria act as [O₂] sensors, convey signals to HIF-1directly or indirectly, and contribute to the cell redox potential, ion homeostasis, and energy production. Although over the last two decades cellular responses to low oxygen tension have been studied extensively, mechanisms underlying these functions are still indefinite. Here we review current knowledge of the mitochondrial role in hypoxia, focusing mainly on their role in cellular energy and reactive oxygen species homeostasis in relation with HIF-1 stabilization. In addition, we address the involvement of HIF-1 and the inhibitor protein of F₁F₀ ATPase in the hypoxia-induced mitochondrial autophagy.     

Hypoxia sensing and pathways of cytosolic Ca2+ increases

Oxygen-sensing and reactivity to changes in the concentration of oxygen is a fundamental property of cellular physiology. This central role is determined, mainly, by, to the fact that oxygen represents the final acceptor of electrons, derived from the normal cellular metabolism, at the end of the mitochondrial respiratory chain. Despite significant advances in molecular characterization of various oxygen-sensitive processes, the nature of the oxygen-sensor molecules and the mechanisms that link sensors to effects remains unclear. One such controversy is about the role and nature of reactive oxygen species (ROS) changes during hypoxia. Irrespective of the mechanisms of oxygen sensing, one of the constant early responses to hypoxia in almost all cell types is an increase in intracellular Ca2+ ([Ca2+]i). In many instances, this increase is mediated by the activation of various plasma membrane Ca2+ conductances. Some of these channels have specific Ca2+ permeability (e.g. voltage-operated Ca2+ channels), whereas others have non-specific cation conductances and are activated by a variety of ligands (ligand-operated channels). In the last decade, a large superfamily of channels with significant Ca2+ permeability has been progressively identified and characterised: the TRP channels. Through their properties, some groups of the TRP channels provide a link to the other hypoxia-activated mechanism of [Ca2+]i increase: the release of Ca2+ from intracellular Ca2+ stores. Since the [Ca2+]i signals, depending on their localization and intensity, are important regulators of the subsequent cellular responses to hypoxia, a deeper understanding of the mechanisms through which hypoxia regulate the activity of these pathways that increase intracellular Ca2+ could point the way towards the development of new therapeutic approaches to reduce or suppress the pathological effects of cellular hypoxia, such as those seen in stroke or myocardial ischemia.     

Moderate Hypoxia Influences Potassium Outward Currents in Adipose-Derived Stem Cells

Moderate hypoxic preconditioning of adipose-derived stem cells (ASCs) enhances properties such as proliferation and secretion of growth factors, representing a valuable strategy to increase the efficiency of cell-based therapies. In a wide variety of cells potassium (K⁺) channels are key elements involved in the cellular responses to hypoxia, suggesting that ASCs cultured under low oxygen conditions may display altered electrophysiological properties. Here, the effects of moderate hypoxic culture on proliferation, whole-cell currents, and ion channel expression were investigated using human ASCs cultured at 5% and 20% oxygen. Although cell proliferation was greatly enhanced, the dose-dependent growth inhibition by the K⁺ channel blocker tetraethylammonium (TEA) was not significantly affected by hypoxia. Under both normoxic and hypoxic conditions, ASCs displayed outward K⁺ currents composed by Ca²⁺-activated, delayed rectifier, and transient components. Hypoxic culture reduced the slope of the current-voltage curves and caused a negative shift in the voltage activation threshold of the whole-cell currents. However, the TEA-mediated shift of voltage activation threshold was not affected by hypoxia. Semiquantitative real-time RT-PCR revealed that expression of genes encoding for various ion channels subunits related to oxygen sensing and proliferation remained unchanged after hypoxic culture. In conclusion, outward currents are influenced by moderate hypoxia in ASCs through a mechanism that is not likely the result of modulation of TEA-sensitive K⁺ channels.     

Acute hypoxia differentially regulates K<Superscript>+</Superscript> channels. Implications with respect to cardiac arrhythmia

The first ion channels demonstrated to be sensitive to changes in oxygen tension were K⁺ channels in glomus cells of the carotid body. Since then a number of hypoxia-sensitive ion channels have been identified. However, not all K⁺ channels respond to hypoxia alike. This has raised some debate about how cells detect changes in oxygen tension. Because ion channels respond rapidly to hypoxia it has been proposed that the channel is itself an oxygen sensor. However, channel function can also be modified by thiol reducing and oxidizing agents, implicating reactive oxygen species as signals in hypoxic events. Cardiac ion channels can also be modified by hypoxia and redox agents. The rapid and slow components of the delayed rectifier K⁺ channel are differentially regulated by hypoxia and -adrenergic receptor stimulation. Mutations in the genes that encode the subunits for the channel are associated with Long QT syndrome and sudden cardiac death. The implications with respect to effects of hypoxia on the channel and triggering of cardiac arrhythmia will be discussed.     

Hypoxia and lipid signaling

Sufficient oxygen supply is crucial for the development and physiology of mammalian cells and tissues. When simple diffusion of oxygen becomes inadequate to provide the necessary flow of substrate, evolution has provided cells with tools to detect and respond to hypoxia by upregulating the expression of specific genes, which allows an adaptation to hypoxia-induced stress conditions. The modulation of cell signaling by hypoxia is an emerging area of research that provides insight into the orchestration of cell adaptation to a changing environment. Cell signaling and adaptation processes are often accompanied by rapid and/or chronic remodeling of membrane lipids by activated lipases. This review highlights the bi-directional relation between hypoxia and lipid signaling mechanisms.     

Hypoxia promotes relaxation of bovine coronary arteries through lowering cytosolic NADPH

Hypoxia relaxes endothelium-denuded bovine coronary arteries (BCA) through mechanisms that do not appear to involve reactive oxygen species, prostaglandins, or nitric oxide. Because of similarities in the relaxation of BCA to hypoxia (Po(2) = 8-10 Torr) and inhibitors of the pentose phosphate pathway (PPP) including 6-aminonicotinamide and epiandrosterone, we measured NADPH and NADP and found that hypoxia caused NADPH oxidation (decreased NADPH/NADP). The relaxation to hypoxia was similar to previously reported properties of relaxation to PPP inhibitors in that both responses were associated with glutathione oxidation and depressed intracellular calcium release and calcium influx-mediated contractile responses. Inhibitors of potassium channels had minimal effects on these relaxation responses. Relaxation to hypoxia and PPP inhibitors were attenuated by a thiol reductant (3 mM dithiothreitol) and by eliciting contraction with an activator of protein kinase C (phorbol 12,13-dibutyrate). In the presence of contraction to U-46619, relaxation to hypoxia and PPP inhibitors were attenuated by the sarco(endo)plasmic reticulum Ca(2+)-ATPase pump inhibitor 200 microM cyclopiazonic acid and by 10 mM pyruvate. Hypoxia decreased BCA levels of glucose-6-phosphate but not ATP. Pyruvate prevented the hypoxia-elicited decrease in glucose-6-phosphate and glutathione oxidation, and it increased NADPH levels under hypoxia to levels observed under normoxia. Thus hypoxia causes a metabolic stress on the PPP that promotes BCA relaxation through processes controlled by lowering the levels of cytosolic NADPH.     

Acid-sensing ion channels under hypoxia

Hypoxia represents the lack of oxygen below the basic level, and the range of known channels related to hypoxia is continually increasing. Since abnormal hypoxia initiates pathological processes in numerous diseases via, to a great degree, producing acidic microenvironment, the significance of these channels in this environment has, until now, remained completely unknown. However, recent discovery of acid-sensing ion channels (ASICs) have enhanced our understanding of the hypoxic channelome. They belong to the degenerin/epithelial Na⁺ channel family and function once extracellular pH decreases to a certain level. So does the ratiocination emerge that ASICs participate in many hypoxia-induced pathological processes, including pain, apoptosis, malignancy, which all appear to involve them. Since evidence suggests that activity of ASICs is altered under pathological hypoxia, future studies are needed to deeply explore the relationship between ASICs and hypoxia, which may provide a progressive understanding of hypoxic effects in cancer, arthritis, intervertebral disc degeneration, ischemic brain injury and so on.     

Hypoxia reduces oxygen consumption of fetal skeletal muscle cells in monolayer culture.

In a previous study on acute asphyxia in unanesthetized fetal sheep near term we showed that reduced oxygen delivery to peripheral organs reduces total oxygen consumption, suggesting that oxygen itself may be a determinant of oxygen consumption (Jensen, Hohmann & Künzel, 1987). To test this hypothesis we developed an in vitro perfusion model, which enabled us to measure the oxygen consumption of fetal skeletal muscle cells in monolayer culture in a control period (at approximately 145 mmHg) and during various degrees of hypoxia (6-140 mmHg). In 57 experiments on 57 cultures the mean oxygen consumption at a mean 'entry PO2' of 145.3 +/- 10.4 mmHg was 10.3 +/- 9.3 (SD).10(-6) microliters O2 per h per skeletal muscle cell. These measurements were made after an average of 4.2 +/- 2.3 transfers of the cells and at a cell density of 2.0 +/- 1.2.10(5) cells per cm2. In 54 of these experiments hypoxia was induced. There was a close positive correlation between the PO2 of the perfusate entering the Petridish ('entry PO2') and the change of the oxygen consumption of the cells (y = 5.17 - 0.54x + 0.03x2 - 0.00016x3, r = 0.97, p less than 0.0001). When oxygen tension fell, there was a concomitant fall in cellular oxygen consumption. We conclude that oxygen is a determinant of cellular oxygen consumption. Thus, hypoxia may reduce oxygen consumption of skeletal muscle cells, and oxygen may be preserved to maintain oxidative metabolism in central fetal organs.(ABSTRACT TRUNCATED AT 250 WORDS)     

低氧与铁代谢相关蛋白

氧和铁这两种元素对生命活动十分重要. 低氧诱导因子(hypoxia-inducible factors, HIFs)作为转录因子,参与一系列靶基因的表达调控以适应低氧. 铁参与 DNA合成、氧气运输、代谢反应等多种细胞活动,过量游离铁会通过Haber-Weiss或 Fenton反应产生毒性自由基. 细胞通过与铁吸收、存储和利用有关的多种铁代谢相 关蛋白之间的协同作用来维持铁稳态. 与铁稳态相关的一些基因是HIFs的靶基因或 者间接受低氧调控,包括转铁蛋白、转铁蛋白受体、二价金属转运体1、铁调素、膜 铁转运蛋白、血浆铜蓝蛋白、铁蛋白等,而胞内铁浓度的改变能影响HIFs的表达. 本文就低氧与铁代谢相关蛋白的关系,尤其是低氧对铁代谢相关蛋白的调节作一综 述.     

Hypoxia regulates expression and activity of Kv1.3 channels in T lymphocytes: a possible role in T cell proliferation

T lymphocytes are exposed to hypoxia during their development and also when they migrate to hypoxic pathological sites such as tumors and wounds. Although hypoxia can affect T cell development and function, the mechanisms by which immune cells sense and respond to changes in O(2)-availability are poorly understood. K(+) channels encoded by the Kv1.3 subtype of the voltage-dependent Kv1 gene family are highly expressed in lymphocytes and are involved in the control of membrane potential and cell function. In this study, we investigate the sensitivity of Kv1.3 channels to hypoxia in freshly isolated human T lymphocytes and leukemic Jurkat T cells. Acute exposure to hypoxia (20 mmHg, 2 min) inhibits Kv1.3 currents in both cell types by 20%. Prolonged exposure to hypoxia (1% O(2) for 24 h) selectively decreases Kv1.3 protein levels in Jurkat T cells by 47%, but not Kvbeta2 and SK2 Ca-activated K(+) channel subunit levels. The decrease in Kv1.3 protein levels occurs with no change in Kv1.3 mRNA expression and is associated with a significant decrease in K(+) current density. A decrease in Kv1.3 polypeptide levels similar to that obtained during hypoxia is produced by Kv1.3 channel blockage. Our results indicate that hypoxia produces acute and long-term inhibition of Kv1.3 channels in T lymphocytes. This effect could account for the inhibition of lymphocyte proliferation during hypoxia. Indeed, we herein present evidence showing that hypoxia selectively inhibits TCR-mediated proliferation and that this inhibition is associated with a decrease in Kv1.3 proteins.     

Hypoxia and Imbibition Injuries to Aging Seeds

The development of hypoxia and primary injuries were examined during the imbibition of aging pea seeds (Pisum sativum L., cv. Nemchinovskii). The distribution of air-dry pea seeds by their room-temperature phosphorescence revealed the presence of two fractions (I and II) in a seed lot with 72% germinability and three fractions (I, II, and III) in a seed lot with 50% germinability. The water uptake during imbibition was slower in the fraction I seeds than in the fraction-II seeds. The fraction-I seeds produced normal seedlings, whereas the fraction-II seeds either produced seedlings with morphological defects (abnormal) or did not germinate at all. The fraction-III seeds were all dead. The phosphorescence of endogenous porphyrins, emitted only at low O₂ content, was measured after 20-h seed imbibition. The fraction-I seeds emitted no discernible phosphorescence. The fraction-II comprised highly phosphorescent seeds incapable of radicle protrusion and moderately phosphorescent seeds producing abnormal seedlings. The fraction-II seeds experienced hypoxia during the imbibition because of rapid oxygen consumption by the embryo and restrictions to O₂ diffusion imposed by the seed coat. In the fraction-I seeds, the rate of oxygen consumption by the embryo was slower and the seed coat resistance to oxygen diffusion was lower than in the fraction-II seeds. Therefore, hypoxia did not arise in the fraction-I seeds. The submergence of seeds in water caused lethal injuries. The imbibition of seeds without any contact with water caused no lethal damages but did not reduce the percentage of seeds dying of hypoxia. A slow imbibition of seeds in the media containing either an osmoticum (PEG) or an inhibitor of aquaporin channels (p-chloromercuribenzoate) prevented the lethal injuries at early stages of seed hydration and retarded the appearance of oxygen deficiency in fraction-II seeds. Different rates of water uptake by fraction-I and fraction-II seeds were controlled by permeability of cell membranes rather than by permeability of seed coat. It is proposed that low permeability of plasma membranes to water in fraction-I seeds results from the predominantly closed aquaporin channels, whereas a higher permeability of weak seeds (fraction II) is due to open channels.     

Hypoxia and its effect on the cellular system

Eukaryotic cells utilize oxygen for different functions of cell organelles owing to cellular survival. A balanced oxygen homeostasis is an essential requirement to maintain the regulation of normal cellular systems. Any changes in the oxygen level are stressful and can alter the expression of different homeostasis regulatory genes and proteins. Lack of oxygen or hypoxia results in oxidative stress and formation of hypoxia inducible factors (HIF) and reactive oxygen species (ROS). Substantial cellular damages due to hypoxia have been reported to play a major role in various pathological conditions. There are different studies which demonstrated that the functions of cellular system are disrupted by hypoxia. Currently, study on cellular effects following hypoxia is an important field of research as it not only helps to decipher different signaling pathway modulation, but also helps to explore novel therapeutic strategies. On the basis of the beneficial effect of hypoxia preconditioning of cellular organelles, many therapeutic investigations are ongoing as a promising disease management strategy in near future. Hence, the present review discusses about the effects of hypoxia on different cellular organelles, mechanisms and their involvement in the progression of different diseases.     

Engineering Tools for Regulating Hypoxia in Tumour Models

Major advances in the field of genomic technologies have led to an improvement in cancer diagnosis, classification and prognostication. However, many cancers remain incurable due to the development of drug resistance, minimal residual disease (MRD) and disease relapse, highlighting an incomplete understanding of the mechanisms underlying these processes. In recent years, the impact of non-genetic factors on neoplastic transformations has increasingly been acknowledged, and growing evidence suggests that low oxygen (O₂) levels (ie hypoxia) in the tumour microenvironment play a critical role in the development and treatment of cancer. As a result, there is a growing need to develop research tools capable of reproducing physiologically relevant O₂ conditions encountered by cancer cells in their natural environments in order to gain in-depth insight into tumour cell metabolism and function. In this review, the authors highlight the importance of hypoxia in the pathogenesis of malignant diseases and provide an overview of novel engineering tools that have the potential to further drive this evolving, yet technically challenging, field of cancer research.     

Human Skin Hypoxia Modulates Cerebrovascular and Autonomic Functions

Because the skin is an oxygen sensor in amphibians and mice, we thought to confirm this function also in humans. The human upright posture, however, introduces additional functional demands for the maintenance of oxygen homeostasis in which cerebral blood flow and autonomic nervous system (ANS) function may also be involved. We examined nine males and three females. While subjects were breathing ambient air, at sea level, we changed gases in a plastic body-bag during two conditions of the experiment such as to induce skin hypoxia (with pure nitrogen) or skin normoxia (with air). The subjects performed a test of hypoxic ventilatory drive during each condition of the experiment. We found no differences in the hypoxic ventilatory drive tests. However, ANS function and cerebral blood flow velocities were modulated by skin hypoxia and the effect was significantly greater on the left than right middle cerebral arteries. We conclude that skin hypoxia modulates ANS function and cerebral blood flow velocities and this might impact life styles and tolerance to ambient hypoxia at altitude. Thus the skin in normal humans, in addition to its numerous other functions, is also an oxygen sensor.