A Fenton reaction at the endoplasmic reticulum is involved in the redox control of hypoxia-inducible gene expression

Oxidative stress in synapse development and function

Oxidative stress, caused by increased levels of reactive oxidative species (ROS), is considered a major contributor to the aging process. How oxidative stress may bring about changes to structures and function in the aging brain is poorly understood. Oxidative stress activates a number of cellular responses, including activation of the Jun-N-terminal kinase (JNK) pathway and autophagy. In addition to their pathological role, ROS also act as signaling molecules. ROS such as nitric oxide have a well-known role in learning and memory. In addition, activation of JNK and its transcriptional effector AP-1 are well-known mediators of synaptic function and growth. Both are essential mediators of physiological correlates of learning and memory such as long-term potentiation. JNK and AP-1 are potently activated and regulated by oxidative stress and mediate protective cellular responses such as autophagy. Recent work at the Drosophila neuromuscular junction implicates autophagy as a regulator of synaptic growth via activation of the JNK signaling pathway. We here outline a framework predicating oxidative stress as a major regulator of synaptic function and growth by the activation of JNK/AP-1 and autophagy. Such responses, we suggest, may underpin some forms of synaptic growth responses and synaptic aging.

Role of reactive oxygen species in the regulation of HIF-1 by prolyl hydroxylase 2 under mild hypoxia

The function and survival of eukaryotic cells depends on a constant and sufficient oxygen supply. Cells recognize and respond to hypoxia by accumulation of the transcription factor hypoxia-inducible factor 1 (HIF-1), composed of an oxygen-sensitive HIF-1α and a constitutive HIF-1β subunit. Besides physiology, HIF-1 induction is involved in major pathological processes such as cardiovascular disease, inflammation and cancer, which are associated with the formation of reactive oxygen species (ROS). ROS have been reported to affect HIF-1 activity but the role for ROS in regulating HIF-1 has not been definitely settled. In order to shed light on the redox-regulation of HIF-1 by ROS, we studied the impact of exogenous ROS treatment (H(2)O(2)) on HIF-1α and HIF-1 regulatory protein prolyl hydroxylase 2 (PHD2) in the human osteosarcoma cell line U2OS. At early reaction periods, H(2)O(2) induced HIF-1α but at prolonged observation phases the opposite occurred. Herein, modulation of PHD activity appeared to be the key element, because knockdown and inhibition of the PHD2 prevented reduction of HIF-1α. However, H(2)O(2) treatment constantly suppressed HIF-1 transactivation at all time-points. Our data indicate a dual redox regulation of HIF-1α protein amount with a constant suppression of HIF-1 target gene expression by ROS.

Hypoxia inducible factor-1 as a target for neurodegenerative diseases

Hypoxia inducible factor-1 (HIF-1) is a transcriptional factor responsible for cellular and tissue adaption to low oxygen tension. HIF-1, a heterodimer consisting of a constitutively expressed β subunit and an oxygen-regulated α subunit, regulates a series of genes that participate in angiogenesis, iron metabolism, glucose metabolism, and cell proliferation/survival. The activity of HIF-1 is controlled by post-translational modifications on different amino acid residues of its subunits, mainly the alpha subunit. Besides in ischemic stroke (see review [1]), emerging evidence has revealed that HIF-1 activity and expression of its down-stream genes, such as vascular endothelial growth factor and erythropoietin, are altered in a range of neurodegenerative diseases. At the same time, experimental and clinical evidence has demonstrated that regulating HIF-1 might ameliorate the cellular and tissue damage in the neurodegenerative diseases. These new findings suggest HIF-1 as a potential medicinal target for the neurodegenerative diseases. This review focuses on HIF-1α protein modifications and HIF-1's potential neuroprotective roles in Alzheimer's (AD), Parkinson's (PD), Huntington's diseases (HD), and amyotrophic lateral sclerosis (ALS).

HIF-independent regulation of thioredoxin reductase 1 contributes to the high levels of reactive oxygen species induced by hypoxia

Cellular adaptation to hypoxic conditions mainly involves transcriptional changes in which hypoxia inducible factors (HIFs) play a critical role. Under hypoxic conditions, HIF protein is stabilized due to inhibition of the activity of prolyl hydroxylases (EGLNs). Because the reaction carried out by these enzymes uses oxygen as a co-substrate it is generally accepted that the hypoxic inhibition of EGLNs is due to the reduction in oxygen levels. However, several studies have reported that hypoxic generation of mitochondrial reactive oxygen species (ROS) is required for HIF stabilization. Here, we show that hypoxia downregulates thioredoxin reductase 1 (TR1) mRNA and protein levels. This hypoxic TR1 regulation is HIF independent, as HIF stabilization by EGLNs inhibitors does not affect TR1 expression and HIF deficiency does not block TR1 hypoxic-regulation, and it has an effect on TR1 function, as hypoxic conditions also reduce TR1 activity. We found that, when cultured under hypoxic conditions, TR1 deficient cells showed a larger accumulation of ROS compared to control cells, whereas TR1 over-expression was able to block the hypoxic generation of ROS. Furthermore, the changes in ROS levels observed in TR1 deficient or TR1 over-expressing cells did not affect HIF stabilization or function. These results indicate that hypoxic TR1 down-regulation is important in maintaining high levels of ROS under hypoxic conditions and that HIF stabilization and activity do not require hypoxic generation of ROS.

Basic principles and emerging concepts in the redox control of transcription factors

Convincing concepts of redox control of gene transcription have been worked out for prokaryotes and lower eukaryotes, whereas the knowledge on complex mammalian systems still resembles a patchwork of poorly connected findings. The article, therefore, reviews principles of redox regulation with special emphasis on chemical feasibility, kinetic requirements, specificity, and physiological context, taking well investigated mammalian transcription factor systems, nuclear transcription factor of bone marrow-derived lymphocytes (NF-κB), and kelch-like ECH-associated protein-1 (Keap1)/Nrf2, as paradigms. Major conclusions are that (i) direct signaling by free radicals is restricted to O(2)•- and •NO and can be excluded for fast reacting radicals such as •OH, •OR, or Cl•; (ii) oxidant signals are H(2)O(2), enzymatically generated lipid hydroperoxides, and peroxynitrite; (iii) free radical damage is sensed via generation of Michael acceptors; (iv) protein thiol oxidation/alkylation is the prominent mechanism to modulate function; (v) redox sensors must be thiol peroxidases by themselves or proteins with similarly reactive cysteine or selenocysteine (Sec) residues to kinetically compete with glutathione peroxidase (GPx)- and peroxiredoxin (Prx)-type peroxidases or glutathione-S-transferases, respectively, a postulate that still has to be verified for putative mammalian sensors. S-transferases and Prxs are considered for system complementation. The impact of NF-κB and Nrf2 on hormesis, management of inflammatory diseases, and cancer prevention is critically discussed.

Discovery of the DIGIRR gene from teleost fish: a novel Toll-IL-1 receptor family member serving as a negative regulator of IL-1 signaling

Toll-IL-1R (TIR) family members play crucial roles in a variety of defense, inflammatory, injury, and stress responses. Although they have been widely investigated in mammals, little is known about TIRs in ancient vertebrates. In this study, we report a novel double Ig IL-1R related molecule (DIGIRR) from three model fish (Tetraodon nigroviridis, Gasterosteus aculeatus, and Takifugu rubripes), adding a previously unknown homolog to the TIR family. This DIGIRR molecule contains two Ig-like domains in the extracellular region, one Arg-Tyr-mutated TIR domain in the intracellular region, and a unique subcellular distribution within the Golgi apparatus. These characteristics distinguish DIGIRR from other known family members. In vitro injection of DIGIRR into zebrafish embryos dramatically inhibited LPS-induced and IL-1β-induced NF-κB activation. Moreover, in vivo knockdown of DIGIRR by small interfering RNA significantly promoted the expression of IL-1β-stimulated proinflammatory cytokines (IL-6 and IL-1β) in DIGIRR-silenced liver and kidney tissues and in leukocytes. These results strongly suggest that DIGIRR is an important negative regulator of LPS-mediated and IL-1β-mediated signaling pathways and inflammatory responses. The Arg-Tyr-mutated site disrupted the signal transduction ability of DIGIRR TIR. Evolutionally, we propose a hypothesis that DIGIRR and single Ig IL-1R related molecule (SIGIRR) might originate from a common ancient IL-1R-like molecule that lost one (in DIGIRR) or two (in SIGIRR) extracellular Ig-like domains and intracellular Ser and Arg-Tyr amino acids. DIGIRR might be an evolutionary "transitional molecule" between IL-1R and SIGIRR, representing a shift from a potent receptor to a negative regulator. These results help define the evolutionary history of TIR family members and their associated signaling pathways and mechanisms.

Induced chronic hypoxia negates the pro-angiogenic effect of surface immobilized heparin in a polyurethane porous scaffold

Porous scaffolds are frequently utilized in tissue regeneration. We have developed a polyurethane (PU) scaffold with a freely interconnecting porosity that can be modified with a covalently linked heparinized surface. The ability of this surface functionality to stimulate vessel and cellular growth into the PU scaffold has been evaluated by subcutaneous implantation of discs in the rat under normoxia and chronic hypoxia (hypobaric chamber) for 10 days. The heparinized surface alone was able to significantly increase vascularization and cellularization under normoxia (p < 0.05), but this response was negated by hypoxia. Addition of vascular endothelial growth factor to heparinized discs resulted in increased vascularity and cellularization under both conditions (p < 0.05). This suggests that endogenous growth factor production was limiting under chronic hypoxia but that an angiogenic response could still occur with exogenous delivery of factors.

Glutathione- and non-glutathione-based oxidant control in the endoplasmic reticulum

The redox-active tripeptide glutathione is an endogenous reducing agent that is found in abundance and throughout the cell. In the endoplasmic reticulum (ER), the ratio of glutathione to glutathione disulfide is lower compared with non-secretory organelles. This relatively oxidizing thiol-disulfide milieu is essential for the oxidative folding of nascent proteins in the ER and, at least in part, maintained by the activity of ER-resident endoplasmic oxidoreductin 1 (Ero1) enzymes that oxidize cysteine side chains at the expense of molecular oxygen. Glutathione disulfide and hydrogen peroxide formed as a consequence of Ero1 activity are widely considered as being inoperative and potentially dangerous by-products of oxidative protein folding in the ER. In contrast to this common view, this Commentary highlights the importance of glutathione- and non glutathione-based homeostatic redox control mechanisms in the ER. Stability in the thiol-disulfide system that prominently includes the protein disulfide isomerases is ensured by the contribution of tightly regulated Ero1 activity, ER-resident peroxidases and the glutathione-glutathione-disulfide redox pair that acts as a potent housekeeper of redox balance. Accordingly, the widely held concept that Ero1-mediated over-oxidation in the ER constitutes a common cause of cellular demise is critically re-evaluated.

Regulation of hypoxia inducible factor-1α expression by the alteration of redox status in HepG2 cells

Hypoxia inducible factor-1 (HIF-1) has been considered as a critical transcriptional factor in response to hypoxia. It can increase P-glycoprotein (P-Gp) thus generating the resistant effect to chemotherapy. At present, the mechanism regulating HIF-1α is still not fully clear in hypoxic tumor cells. Intracellular redox status is closely correlated with hypoxic micro-environment, so we investigate whether alterations in the cellular redox status lead to the changes of HIF-1α expression. HepG2 cells were exposed to Buthionine sulphoximine (BSO) for 12 h prior to hypoxia treatment. The level of HIF-1α expression was measured by Western blot and immunocytochemistry assays. Reduce glutathione (GSH) concentrations in hypoxic cells were determined using glutathione reductase/5,5^'-dithiobis-(2-nitrob-enzoic acid) (DTNB) recycling assay. To further confirm the effect of intracellular redox status on HIF-1α expression, N-acetylcysteine (NAC) was added to culture cells for 8 h before the hypoxia treatment. The levels of multidrug resistance gene-1 (MDR-1) and erythropoietin (EPO) mRNA targeted by HIF-1α in hypoxic cells were further determined with RT-PCR, and then the expression of P-Gp protein was observed by Western blotting. The results showed that BSO pretreatment down-regulated HIF-1α and the effect was concentration-dependent, on the other hand, the increases of intracellular GSH contents by NAC could partly elevate the levels of HIF-1α expression. The levels of P-Gp (MDR-1) and EPO were concomitant with the trend of HIF-1α expression. Therefore, our data indicate that the changes of redox status in hypoxic cells may regulate HIF-1α expression and provide valuable information on tumor chemotherapy.

Angiotensin II type 1 receptor antibodies and increased angiotensin II sensitivity in pregnant rats

Pregnant women who subsequently develop preeclampsia are highly sensitive to infused angiotensin (Ang) II; the sensitivity persists postpartum. Activating autoantibodies against the Ang II type 1 (AT(1)) receptor are present in preeclampsia. In vitro and in vivo data suggest that they could be involved in the disease process. We generated and purified activating antibodies against the AT(1) receptor (AT(1)-AB) by immunizing rabbits against the AFHYESQ epitope of the second extracellular loop, which is the binding epitope of endogenous activating autoantibodies against AT(1) from patients with preeclampsia. We then purified AT(1)-AB using affinity chromatography with the AFHYESQ peptide. We were able to detect AT(1)-AB both by ELISA and a functional bioassay. We then passively transferred AT(1)-AB into pregnant rats, alone or combined with Ang II. AT(1)-AB activated protein kinase C-α and extracellular-related kinase 1/2. Passive transfer of AT(1)-AB alone or Ang II (435 ng/kg per minute) infused alone did not induce a preeclampsia-like syndrome in pregnant rats. However, the combination (AT(1)-AB plus Ang II) induced hypertension, proteinuria, intrauterine growth retardation, and arteriolosclerosis in the uteroplacental unit. We next performed gene-array profiling of the uteroplacental unit and found that hypoxia-inducible factor 1α was upregulated by Ang II plus AT(1)-AB, which we then confirmed by Western blotting in villous explants. Furthermore, endothelin 1 was upregulated in endothelial cells by Ang II plus AT(1)-AB. We show that AT(1)-AB induces Ang II sensitivity. Our mechanistic study supports the existence of an "autoimmune-activating receptor" that could contribute to Ang II sensitivity and possible to preeclampsia.

Redox control of GTPases: from molecular mechanisms to functional significance in health and disease

Small GTPases, including the proto-oncoprotein Ras and Rho GTPases, are involved in various cellular signaling events. Some of these small GTPases are redox sensitive, including Ras, Rho, Ran, Dexras1, and Rhes GTPases. Thus, the redox-mediated regulation of these GTPases often determines the course of their cellular signaling cascades. This article takes into consideration the application of Marcus theory to potential redox-based molecular mechanisms in the regulation of these redox-sensitive GTPases and the relevance of such mechanisms to a specific redox-sensitive motif. The discussion also takes into account various diseases, including cancers, heart, and neuronal disorders, that are often linked with the dysregulation of the redox signaling cascades associated with these redox-sensitive GTPases.

Interactions between nitric oxide and hypoxia-inducible factor signaling pathways in inflammatory disease

Induction and activation of nitric oxide (NO) synthases (NOS) and excessive production of NO are common features of almost all diseases associated with infection and acute or chronic inflammation, although the contribution of NO to the pathophysiology of these diseases is highly multifactorial and often still a matter of controversy. Because of its direct impact on tissue oxygenation and cellular oxygen (O(2)) consumption and re-distribution, the ability of NO to regulate various aspects of hypoxia-induced signaling has received widespread attention. Conditions of tissue hypoxia and the activation of hypoxia-inducible factors (HIF) have been implicated in hypoxia or in cancer biology, but are also being increasingly recognized as important features of acute and chronic inflammation. Thus, the activation of HIF transcription factors has been increasingly implicated in inflammatory diseases, and recent studies have indicated its critical importance in regulating phagocyte function, inflammatory mediator production, and regulation of epithelial integrity and repair processes. Finally, HIF also appears to contribute to important features of tissue fibrosis and epithelial-to-mesenchymal transition, processes that are associated with tissue remodeling in various non-malignant chronic inflammatory disorders. In this review, we briefly summarize the current state of knowledge with respect to the general mechanisms involved in HIF regulation and the impact of NO on HIF activation. Secondly, we will summarize the major recent findings demonstrating a role for HIF signaling in infection, inflammation, and tissue repair and remodeling, and will address the involvement of NO. The growing interest in hypoxia-induced signaling and its relation with NO biology is expected to lead to further insights into the complex roles of NO in acute or chronic inflammatory diseases and may point to the importance of HIF signaling as key feature of NO-mediated events during these disorders.

Redox control of protein-DNA interactions: from molecular mechanisms to significance in signal transduction, gene expression, and DNA replication

Protein-DNA interactions play a key role in the regulation of major cellular metabolic pathways, including gene expression, genome replication, and genomic stability. They are mediated through the interactions of regulatory proteins with their specific DNA-binding sites at promoters, enhancers, and replication origins in the genome. Redox signaling regulates these protein-DNA interactions using reactive oxygen species and reactive nitrogen species that interact with cysteine residues at target proteins and their regulators. This review describes the redox-mediated regulation of several master regulators of gene expression that control the induction and suppression of hundreds of genes in the genome, regulating multiple metabolic pathways, which are involved in cell growth, development, differentiation, and survival, as well as in the function of the immune system and cellular response to intracellular and extracellular stimuli. It also discusses the role of redox signaling in protein-DNA interactions that regulate DNA replication. Specificity of redox regulation is discussed, as well as the mechanisms providing several levels of redox-mediated regulation, from direct control of DNA-binding domains through the indirect control, mediated by release of negative regulators, regulation of redox-sensitive protein kinases, intracellular trafficking, and chromatin remodeling.

Redox activity within the lysosomal compartment: implications for aging and apoptosis

The lysosome is a redox-active compartment containing low-mass iron and copper liberated by autophagic degradation of metalloproteins. The acidic milieu and high concentration of thiols within lysosomes will keep iron in a reduced (ferrous) state, which can react with endogenous or exogenous hydrogen peroxide. Consequent intralysosomal Fenton reactions may give rise to the formation of lipofuscin or "age pigment" that accumulates in long-lived postmitotic cells that cannot dilute it by division. Extensive accumulation of lipofuscin seems to hinder normal autophagy and may be an important factor behind aging and age-related pathologies. Enhanced oxidative stress causes lysosomal membrane permeabilization, with ensuing relocation to the cytosol of iron and lysosomal hydrolytic enzymes, with resulting apoptosis or necrosis. Lysosomal copper is normally not redox active because it will form non-redox-active complexes with various thiols. However, if cells are exposed to lysosomotropic chelators that do not bind all the copper coordinates, highly redox-active complexes may form, with ensuing extensive lysosomal Fenton-type reactions and loss of lysosomal stability. Because many malignancies seem to have increased amounts of copper-containing macromolecules that are turned over by autophagy, it is conceivable that lysosomotropic copper chelators may be used in the future in ROS-based anticancer therapies.

Intracellular redox compartments: mechanisms and significances

With the appearance of oxygen and the development of aerobic life on earth, reactive oxygen species (ROS) became important factors influencing a number of processes within a cell. Although initially considered unwanted and harmful by-products of a number of cellular reactions, the last decade has shown that ROS can also act as signalling molecules mediating changes in O(2) tension, as well as the response to hormones, growth factors, and mechanical or chemical stress. Different ROS-generating and ROS-degrading systems in different intracellular compartments seem to play an important role. In line with this, it appears that proteins already well known for an ROS-unrelated specific function in one compartment participate in the ROS response within another compartment. Thus, it is easy to envision that redox changes in different compartments and resulting changes in ROS levels may represent an important mechanism of intracellular communication between different cellular compartments.

The antioxidant quercetin inhibits cellular proliferation via HIF-1-dependent induction of p21WAF

Flavonoids are dietary antioxidants that may play a role as adjunct nutritional supplements in cancer or during inflammatory disorders. Hypoxia and the transcription factor hypoxia-inducible factor-1alpha also appear to play a key role in many human cancers. In this study, we investigated the role of quercetin in the hypoxia-dependent HIF-1alpha induction. It was shown that quercetin induced HIF-1alpha expression and HIF-1 activity under normoxia and hypoxia in human HepG2 hepatoma cells. By using actinomycin D and cycloheximide, we showed that quercetin acted post-transcriptionally by prolonging the HIF-1alpha protein half-life. Thereby quercetin interfered with the proline hydroxylation-dependent HIF-1alpha protein destabilization in the N-terminal HIF-1alpha transactivation domain. Experiments with quercetin analogues revealed that a flavonol structure and the presence of hydroxyl groups at position 3' and 4' are a prerequisite for the HIF-1alpha stabilizing effects. Further, quercetin inhibited cell proliferation and induced expression of the cell cycle inhibitor p21WAF and knocking-down HIF-1alpha disrupted these effects. These results provide evidence that quercetin inhibits the cell cycle and that induction of the HIF-system contributes to these effects of quercetin.

Redox control of endoplasmic reticulum function

The lumen of the endoplasmic reticulum constitutes a separate intracellular compartment with a special proteome and metabolome. The redox conditions of the organelle are also characteristically different from those of the other subcellular compartments. The luminal environment has been considered more oxidizing than the cytosol due to the presence of oxidative protein folding. However, recent observations suggest that redox systems in reduced and oxidized states are present simultaneously. The concerted action of membrane transporters and oxidoreductase enzymes maintains the oxidized state of the thiol-disulfide and the reduced state of the pyridine nucleotide redox systems, which are prerequisites for the normal redox reactions localized in the organelle. The powerful thiol-oxidizing machinery of oxidative protein folding continuously challenges the local antioxidant defense. Alterations of the luminal redox conditions, either in oxidizing or reducing direction, affect protein processing, are sensed by the accumulation of misfolded/unfolded proteins, and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce programmed cell death if these attempts fail. Recent findings strongly support the involvement of redox-based endoplasmic reticulum stress in a plethora of human diseases, either as causative agents or as complications.

XQ2, a novel TPZ derivative, induced G2/M phase arrest and apoptosis under hypoxia in non-small cell lung cancer cells

Hypoxia is one of the inevitable circumstances of various tumors. It controls various levels of regulation in tumor progression and results in tumor resistance to radiotherapy and chemotherapy. Here we investigated a synthetic TPZ derivative, N-ethoxymethyl-3-amino-1,2,4-benzotriazine-1,4-dioxide (XQ2), a novel compound that induced anti-cancer effects both in normoxia and in hypoxia, cell proliferation assay found that XQ2 exhibited a potent inhibitory effect on the tested cancer cell lines both in normoxia and in hypoxia. Flow cytometry and western blot studies indicated that XQ2 induces G2/M arrest and a caspase-dependent apoptosis in A549 cells. Additionally, intracellular reactive oxygen species (ROS) appear to play a key role in the anticancer effect of XQ2 in hypoxia. Taken together, our data suggest that XQ2 exerted anticancer action by suppressing the ROS level and triggering cell-cycle arrest and the caspase-dependent pathway, which is associated with apoptosis.

Diametrically opposed effects of hypoxia and oxidative stress on two viral transactivators

Background

Many pathogens exist in multiple physiological niches within the host. Differences between aerobic and anaerobic conditions are known to alter the expression of bacterial virulence factors, typically through the conditional activity of transactivators that modulate their expression. More recently, changes in physiological niches have been shown to affect the expression of viral genes. For many viruses, differences in oxygen tension between hypoxia and normoxia alter gene expression or function. Oxygen tension also affects many mammalian transactivators including AP-1, NFkB, and p53 by affecting the reduced state of critical cysteines in these proteins. We have recently determined that an essential cys-x-x-cys motif in the EBNA1 transactivator of Epstein-Barr virus is redox-regulated, such that transactivation is favoured under reducing conditions. The crucial Tat transactivator of human immunodeficiency virus (HIV) has an essential cysteine-rich region, and is also regulated by redox. Contrary to EBNA1, it is reported that Tat's activity is increased by oxidative stress. Here we have compared the effects of hypoxia, oxidative stress, and cellular redox modulators on EBNA1 and Tat.

Results

Our results indicate that unlike EBNA1, Tat is less active during hypoxia. Agents that generate hydroxyl and superoxide radicals reduce EBNA1's activity but increase transactivation by Tat. The cellular redox modulator, APE1/Ref-1, increases EBNA1's activity, without any effect on Tat. Conversely, thioredoxin reductase 1 (TRR1) reduces Tat's function without any effect on EBNA1.

Conclusions

We conclude that oxygen partial pressure and oxidative stress affects the functions of EBNA1 and Tat in a dramatically opposed fashion. Tat is more active during oxidative stress, whereas EBNA1's activity is compromised under these conditions. The two proteins respond to differing cellular redox modulators, suggesting that the oxidized cysteine adduct is a disulfide bond(s) in Tat, but sulfenic acid in EBNA1. The effect of oxygen partial pressure on transactivator function suggests that changes in redox may underlie differences in virus-infected cells dependent upon the physiological niches they traffic to.

Thioredoxin and Cancer: A Role for Thioredoxin in all States of Tumor Oxygenation

Thioredoxin is a small redox-regulating protein, which plays crucial roles in maintaining cellular redox homeostasis and cell survival and is highly expressed in many cancers. The tumor environment is usually under either oxidative or hypoxic stress and both stresses are known up-regulators of thioredoxin expression. These environments exist in tumors because their abnormal vascular networks result in an unstable oxygen delivery. Therefore, the oxygenation patterns in human tumors are complex, leading to hypoxia/re-oxygenation cycling. During carcinogenesis, tumor cells often become more resistant to hypoxia or oxidative stress-induced cell death and most studies on tumor oxygenation have focused on these two tumor environments. However, recent investigations suggest that the hypoxic cycling occurring within tumors plays a larger role in the contribution to tumor cell survival than either oxidative stress or hypoxia alone. Thioredoxin is known to have important roles in both these cellular responses and several studies implicate thioredoxin as a contributor to cancer progression. However, only a few studies exist that investigate the regulation of thioredoxin in the hypoxic and cycling hypoxic response in cancers. This review focuses on the role of thioredoxin in the various states of tumor oxygenation.

Oxygen-sensing under the influence of nitric oxide

The transcription factor complex Hypoxia inducible factor 1 (HIF-1) controls the expression of most genes involved in adaptation to hypoxic conditions. Oxygen-dependency is maintained by prolyl- and asparagyl-4-hydroxylases (PHDs/FIH-1) belonging to the superfamily of iron(II) and 2-oxoglutarate dependent dioxygenases. Hydroxylation of the HIF-1alpha subunit by PHDs and FIH-1 leads to its degradation and inactivation. By hydroxylating HIF-1alpha in an oxygen-dependent manner PHDs and FIH-1 function as oxygen-sensing enzymes of HIF signalling. Besides molecular oxygen nitric oxide (NO), a mediator of the inflammatory response, can regulate HIF-1alpha accumulation, HIF-1 activity and HIF-1 dependent target gene expression. Recent studies addressing regulation of HIF-1 by NO revealed a complex and paradoxical picture. Acute exposure of cells to high doses of NO increased HIF-1alpha levels irrespective of the residing oxygen concentration whereas prolonged exposure to NO or low doses of this radical reduced HIF-1alpha accumulation even under hypoxic conditions. Several mechanisms were found to contribute to this paradoxical role of NO in regulating HIF-1. More recent studies support the view that NO regulates HIF-1 by modulating the activity of the oxygen-sensor enzymes PHDs and FIH-1. NO dependent HIF-1alpha accumulation under normoxia was due to direct inhibition of PHDs and FIH-1 most likely by competitive binding of NO to the ferrous iron in the catalytically active center of the enzymes. In contrast, reduced HIF-1alpha accumulation by NO under hypoxia was mainly due to enhanced HIF-1alpha degradation by induction of PHD activity. Three major mechanisms are discussed to be involved in enhancing the PHD activity despite the lack of oxygen: (1) NO mediated induction of a HIF-1 dependent feedback loop leading to newly expressed PHD2 and enhanced nuclear localization, (2) O2-redistribution towards PHDs after inhibition of mitochondrial respiration by NO, (3) reactivation of PHD activity by a NO mediated increase of iron and 2-oxoglutarate and/or involvement of reactive oxygen and/or nitrogen species.

Organ preservation solutions attenuate accumulation and nuclear translocation of hypoxia-inducible factor-1alpha in the hepatoma cell line HepG2

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a key transcription factor orchestrating hypoxic and inflammatory reactions. Here, we determined the impact of organ preservation solutions (Celsior; histidine-tryptophan-ketoglutarate solution, HTK; University of Wisconsin solution; UW), oxygen supply, and temperature on HIF-1alpha accumulation, recorded by Western blotting and immunocytochemistry, in the human hepatoma cell line HepG2. Generation of reactive oxygen species (ROS), NO, and cell viability were concomitantly assessed. At 4 degrees C, HIF-1alpha accumulation was not detectable. In normothermic (37 degrees C) cell culture medium (Dulbecco's Modified Eagle's Medium, DMEM), HepG2 cells accumulated HIF-1alpha even in normoxia (21% O(2)) which was not observed in either of the preservation solutions. This correlated to high generation of NO, a normoxic stabilizer of HIF-1alpha, and L-arginine content (substrate for NO synthesis) in DMEM, and low NO production and absence of L-arginine in preservation solutions. In normothermic hypoxia up to 24 h, intracellular HIF-1alpha accumulated in all conditions, but less in preservation solutions compared to DMEM. The inhibitory effect on accumulation and nuclear translocation was most prominent for HTK, the only solution containing the activator of HIF-1alpha degradation, alpha-ketoglutarate. Addition of other intermediates of the tricarbon acid cycle-succinate, fumarate, malate-did not alter HIF-1alpha accumulation, although succinate exhibited a beneficial effect on cell viability in cold storage. In conclusion, preservation solutions attenuate accumulation and nuclear translocation of the transcription factor HIF-1alpha, and this property is seemingly related to their chemical composition (L-arginine, alpha-ketoglutarate). Thus, it appears feasible to design preservation solution specifically to modify HIF-1alpha accumulation and nuclear translocation.

Hypoxia-inducible factors: post-translational crosstalk of signaling pathways

Hypoxia-inducible factor-1 (HIF-1) has a central role in the mammalian program by which cells respond to hypoxia in both physiological and pathological situations. HIF-1 transcriptional activity, protein stabilization, protein-protein interaction, and cellular localization are mainly modulated by Post-translational modifications such as hydroxylation, acetylation, phosphorylation, S-nitrosylation, and SUMOylation. Here, we summarize current knowledge about Post-translational HIF-1 regulation and give additional information about useful methods to determine some of these various modifications.

The immune effects of naturally occurring and synthetic nanoparticles

Ultrafine particles and engineered nanoparticles have unique aerodynamic and biochemical properties that affect the immune system and human health in ways that are different from or exceed those seen with gases or larger particulates. These effects result from a unique set of physical characteristics and surface moieties, which generate an ability of UFPs to enter tissues and cells, interact with proteins and DNA at a molecular level and directly and indirectly modulate the immune system by novel mechanisms. In recent years, a new field known as nanotechnology has impacted multiple industries by taking advantage of the special qualities of these small "atomic-sized" particles. Nanomedicine has already opened up a new avenue of research in cancer therapy, drug delivery and immune regulation. While the benefits of this new science to human civilization are seemingly immeasurable, it is also important to appreciate that these particles can also lead to harmful effects on human health. In vitro and animal studies are showing that nanoparticles and UFPs are capable of activating proinflammatory cytokines, chemokines and adhesion molecules, with recruitment of inflammatory cells including basophils, macrophages, dendritic cells, T cells, neutrophils and eosinophils. These changes may have an impact on immune defense, but also on the Th1/Th2 balance, and even on non-immunologic function. Resulting immune system derangement can lead to increases in incidence of autoimmune, allergic and even neoplastic diseases. Cardiorespiratory effects have been observed to occur in humans. Much further research is needed to establish safe exposure levels for this important new class of particulates.

FoxO1 and HNF-4 are involved in regulation of hepatic glucokinase gene expression by resveratrol

Resveratrol, a polyphenol derived from grapes, exerts important effects on glucose and lipid metabolism, yet detailed mechanisms mediating these effects remain unknown. The liver plays a central role in energy homeostasis, and glucokinase (GK) is a key enzyme involved in glucose utilization. Resveratrol activates SIRT1 (sirtuin 1), which promotes deacetylation of the forkhead transcription factor FoxO1. Previously, we reported that FoxO1 can suppress and that HNF-4 can stimulate GK expression in the liver. Here, we examined the role of FoxO1 and HNF-4 in mediating resveratrol effects on liver GK expression. Resveratrol suppressed hepatic GK expression in vivo and in isolated hepatocytes, and knocking down FoxO1 with shRNAs disrupted this effect. Reporter gene, gel shift, supershift assay, and chromatin immunoprecipitation studies show that FoxO1 binds to the GK promoter and that the interplay between FoxO1 and HNF-4 within the GK promoter is essential for mediating the effects of resveratrol. Resveratrol promotes deacetylation of FoxO1 and enhances its recruitment to the FoxO-binding element. Conversely, resveratrol suppresses recruitment of HNF-4 to its binding site, and knockdown of FoxO1 blocks this effect of resveratrol. Coprecipitation and chromatin immunoprecipitation studies show that resveratrol enhances interaction between FoxO1 and HNF-4, reduces binding of HNF-4 to its own site, and promotes its recruitment to the FoxO site in a FoxO1-dependent manner. These results provide the first evidence that resveratrol represses GK expression via FoxO1 and that the interaction between FoxO1 and HNF-4 contributes to these effects of resveratrol.

Complex regulation of the transactivation function of hypoxia-inducible factor-1 alpha by direct interaction with two distinct domains of the CREB-binding protein/p300

Activation of transcription in response to low oxygen tension is mediated by the hypoxia-inducible factor-1 (HIF-1). HIF-1 is a heterodimer of two proteins: aryl hydrocarbon receptor nuclear translocator and the oxygen-regulated HIF-1 alpha. The C-terminal activation domain of HIF-1 alpha has been shown to interact with cysteine/histidine-rich region 1 (CH1) of the coactivator CBP/p300 in a hypoxia-dependent manner. However, HIF forms lacking C-terminal activation domain (naturally occurring or genetically engineered) are still able to activate transcription of target genes in hypoxia. Here, we demonstrate that the N-terminal activation domain (N-TAD) of HIF-1 alpha interacts with endogenous CBP and that this interaction facilitates its transactivation function. Our results show that interaction of HIF-1 alpha N-TAD with CBP/p300 is mediated by the CH3 region of CBP known to interact with, among other factors, p53. Using fluorescence resonance energy transfer experiments, we demonstrate that N-TAD interacts with CH3 in vivo. Coimmunoprecipitation assays using endogenous proteins showed that immunoprecipitation of CBP in hypoxia results in the recovery of a larger fraction of HIF-1 alpha than of p53. Chromatin immunoprecipitation demonstrated that at 1% O(2) CBP is recruited to a HIF-1 alpha but not to a p53 target gene. Upon activation of both pathways, lower levels of chromatin-associated CBP were detected at either target gene promoter. These results identify CBP as the coactivator directly interacting with HIF-1 alpha N-TAD and mediating the transactivation function of this domain. Thus, we suggest that in hypoxia HIF-1 alpha is a major CBP-interacting transcription factor that may compete with other CBP-dependent factors, including p53, for limiting amounts of this coactivator, underscoring the complexity in the regulation of gene expression by HIF-1 alpha.

SENP3 is responsible for HIF-1 transactivation under mild oxidative stress via p300 de-SUMOylation

The physiological function of Sentrin/SUMO-specific proteases (SENPs) remains largely unexplored, and little is known about the regulation of SENPs themselves. Here, we show that a modest increase of reactive oxygen species (ROS) regulates SENP3 stability and localization. We found that SENP3 is continuously degraded through the ubiquitin-proteasome pathway under basal condition and that ROS inhibit this degradation. Furthermore, ROS causes SENP3 to redistribute from the nucleoli to the nucleoplasm, allowing it to regulate nuclear events. The stabilization and redistribution of SENP3 correlate with an increase in the transcriptional activity of the hypoxia-inducing factor-1 (HIF-1) under mild oxidative stress. ROS-enhanced HIF-1 transactivation is blocked by SENP3 knockdown. The de-SUMOylating activity of SENP3 is required for ROS-induced increase of HIF-1 transactivation, but the true substrate of SENP3 is the co-activator of HIF-1 alpha, p300, rather than HIF-1 alpha itself. Removing SUMO2/3 from p300 enhances its binding to HIF-1 alpha. In vivo nude mouse xenografts overexpressing SENP3 are more angiogenic. Taken together, our results identify SENP3 as a redox sensor that regulates HIF-1 transcriptional activity under oxidative stress through the de-SUMOylation of p300.

15-Deoxy-Delta(12,14)-prostaglandin-J(2) reveals a new pVHL-independent, lysosomal-dependent mechanism of HIF-1alpha degradation

Hypoxia-inducible factor-1alpha (HIF-1alpha) protein is degraded under normoxia by its association to von Hippel-Lindau protein (pVHL) and further proteasomal digestion. However, human renal cells HK-2 treated with 15-deoxy-Delta(12,14)-prostaglandin-J(2) (15d-PGJ(2)) accumulate HIF-1alpha in normoxic conditions. Thus, we aimed to investigate the mechanism involved in this accumulation. We found that 15d-PGJ(2) induced an over-accumulation of HIF-1alpha in RCC4 cells, which lack pVHL and in HK-2 cells treated with inhibitors of the pVHL-proteasome pathway. These results indicated that pVHL-proteasome-independent mechanisms are involved, and therefore we aimed to ascertain them. We have identified a new lysosomal-dependent mechanism of HIF-1alpha degradation as a target for 15d-PGJ(2) based on: (1) HIF-1alpha colocalized with the specific lysosomal marker Lamp-2a, (2) 15d-PGJ(2) inhibited the activity of cathepsin B, a lysosomal protease, and (3) inhibition of lysosomal activity did not result in over-accumulation of HIF-1alpha in 15d-PGJ(2)-treated cells. Therefore, expression of HIF-1alpha is also modulated by lysosomal degradation.

Real-time imaging of HIF-1alpha stabilization and degradation

HIF-1α is overexpressed in many human cancers compared to normal tissues due to the interaction of a multiplicity of factors and pathways that reflect specific genetic alterations and extracellular stimuli. We developed two HIF-1α chimeric reporter systems, HIF-1α/FLuc and HIF-1α(ΔODDD)/FLuc, to investigate the tightly controlled level of HIF-1α protein in normal (NIH3T3 and HEK293) and glioma (U87) cells. These reporter systems provided an opportunity to investigate the degradation of HIF-1α in different cell lines, both in culture and in xenografts. Using immunofluorescence microscopy, we observed different patterns of subcellular localization of HIF-1α/FLuc fusion protein between normal cells and cancer cells; similar differences were observed for HIF-1α in non-transduced, wild-type cells. A dynamic cytoplasmic-nuclear exchange of the fusion protein and HIF-1α was observed in NIH3T3 and HEK293 cells under different conditions (normoxia, CoCl₂ treatment and hypoxia). In contrast, U87 cells showed a more persistent nuclear localization pattern that was less affected by different growing conditions. Employing a kinetic model for protein degradation, we were able to distinguish two components of HIF-1α/FLuc protein degradation and quantify the half-life of HIF-1α fusion proteins. The rapid clearance component (t_1/2 ∼4–6 min) was abolished by the hypoxia-mimetic CoCl_2, MG132 treatment and deletion of ODD domain, and reflects the oxygen/VHL-dependent degradation pathway. The slow clearance component (t_1/2 ∼200 min) is consistent with other unidentified non-oxygen/VHL-dependent degradation pathways. Overall, the continuous bioluminescence readout of HIF-1α/FLuc stabilization in vitro and in vivo will facilitate the development and validation of therapeutics that affect the stability and accumulation of HIF-1α.

Intervention of oxygen-control ability to radiation sensitivity, cell aging and cell transformation

Oxygen is essential for life, and cells have therefore developed numerous adaptive responses to oxygen change. Here, we examined the difference in oxygen-control functions of human (HE), mouse (ME), and Syrian hamster embryo (SHE) cells cultured under different oxygen conditions (0.5%, 2% and 20%), and also examined whether oxygen tensions contributed to cellular lifespan and transformation. HE cells had their replicative lifespan slightly extended under hypoxic (0.5% and 2% oxygen) conditions, but were not immortalized under any of the oxygen concentrations. On the other hand, although ME cells cultured under 20% oxygen tension decreased their proliferation potency temporarily at early stage, all rodent cells were immortalized and acquired anchorage-independency, regardless of oxygen tension. These results suggest that cellular oxygen control function is related to sensitivities cellular immortalization and transformation. To understand intervention of oxygen control ability on cellular immortalization and transformation, we examined the intracellular oxidative level, mitochondria functions and radiation sensitivity. Intracellular oxidative levels of hypoxically cultured rodent cells were significantly enhanced. Mitochondrial membrane potential was altered depend on oxygen tensions, but the change was not parallel to mitochondria number in rodent cells. ME cells were particularly sensitive to oxygen change, and showed a clear oxygen effect on the X-ray survival. However, there was no difference in frequency of radiation-induced micronuclei between HE and ME cells. These results suggest that the response to oxygen change differs markedly in HE and rodent cells.

The redox state of glutathione regulates the hypoxic induction of HIF-1

Hypoxia inducible factor 1 (HIF-1) regulates the transcription of vascular endothelial growth factor (VEGF), which plays important roles in angiogenesis. We investigated the redox effect of glutathione (GSH) on the hypoxic induction of HIF-1 in a human oral squamous cell carcinoma (HSC-2) cell line. The maximal induction of HIF-1 in HSC-2 cells was observed 30 h after hypoxia, and VEGF mRNA was expressed after 36 h under hypoxia. GSH ethyl ester (GSHee, a membrane permeable analog of GSH) and N-acetyl-L-cysteine (NAC, a membrane permeable precursor of GSH) reduced HIF-1 binding activity in a dose-dependent manner. Further, HIF-1 dependent promoter activity was similarly reduced by GSHee and NAC. However, ebselen, which increases glutathione peroxidase activity and oxidizes GSH, negated the effect of GSHee on HIF-1 dependent promoter activity. The inhibitory effect of GSHee and NAC on HIF-1 binding activity was reversed by bis (2-chlorethyl)-nitrosourea, an oxidized glutathione (GSSG) reductase inhibitor which increases the concentration of GSSG. GSSG methyl ester (GSSGme), a membrane permeable analog of GSSG, enhanced HIF-1 dependent promoter activity and exhibited a bell-shaped concentration-dependant activity curve. The increasing effect of GSSGme on HIF-1 induction was also observed under chemically-induced hypoxia obtained using cobalt chloride. These results suggest that changes in the intracellular GSSG/GSH ratio may regulate HIF-1 induction during hypoxia.

The iron/heme regulated genes of Haemophilus influenzae: comparative transcriptional profiling as a tool to define the species core modulon

BACKGROUND

Haemophilus influenzae requires heme for aerobic growth and possesses multiple mechanisms to obtain this essential nutrient. Although an understanding of the heme acquisition mechanisms of H. influenzae is emerging, significant gaps in our knowledge remain. Unresolved issues include the identities of all genes exhibiting altered transcription in response to iron and heme availability, the fraction of such genes functioning in iron/heme acquisition, and the heterogeneity of this gene set among clinical isolates. Previously we utilized H. influenzae strain Rd KW20 to demonstrate the utility of transcriptional profiling in defining the genes exhibiting altered transcription in response to environmental iron and heme levels. The current study expands upon those observations by determining the iron/heme modulons of two clinical isolates, the type b isolate 10810 and the nontypeable isolate R2866. These data are used to begin to define the core iron/heme modulon of the species.

RESULTS

Microarray studies were performed to compare gene expression on transition from iron/heme-restricted to iron/heme-replete conditions for each isolate. Of 1820 ORFs on the array corresponding to R2866 genes, 363 were significantly differentially expressed: 233 were maximally transcribed under iron/heme-replete conditions and 130 under iron/heme-restricted conditions. Of the 1883 ORFs representing genes of strain 10810, 353 were significantly differentially transcribed: 150 were preferentially transcribed under iron/heme-replete conditions and 203 under iron/heme-restricted conditions. Comparison of the data sets indicated that 163 genes exhibited similar regulation in both isolates and that 74 of these exhibited similar patterns of regulation in Rd KW20. These comprise the putative core iron/heme modulon.

CONCLUSION

This study provides evidence for a conserved core of H. influenzae genes the transcription of which is altered by the availability of iron and/or heme in the growth environment. Elucidation of this modulon provides a means to identify genes with unrecognized roles in iron/heme acquisition or homeostasis, unanticipated responsiveness to environmental levels of the micronutrients or potential roles in virulence. Defining these core genes is also of potential importance in identifying targets for therapeutic and vaccine designs since products of these genes are likely to be preferentially expressed during growth in iron/heme restricted sites of the human body.

Hypoxia inducible factor 1 as a therapeutic target in ischemic stroke

In stroke research, a significant focus is to develop therapeutic strategies that prevent neuronal death and improve recovery. Yet, few successful therapeutic strategies have emerged. Hypoxia-inducible factor 1 (HIF-1) is a key regulator in hypoxia. It has been suggested to be an important player in neurological outcomes following ischemic stroke due to the functions of its downstream genes. These include genes that promote glucose metabolism, angiogenesis, erythropoiesis, and cell survival. Many lines of evidence have shown that HIF-1 is induced in ischemic brains. Importantly, it seems that HIF-1 is primarily induced in the salvageable tissue of an ischemic brain, penumbra. However, the effect of HIF-1 on neuronal tissue injuries is still debatable based on evidence from in vitro and preclinical studies. Furthermore, it is of importance to understand the mechanism of HIF-1 degradation after its induction in ischemic brain. This review provides a present understanding of the mechanism of HIF-1 induction in ischemic neurons and the potential effect of HIF-1 on ischemic brain tissue. The author also elaborates on potential therapeutic approaches through understanding of the induction mechanism and of the potential role of HIF-1 in ischemic stroke.

Two-photon imaging of cellular activities in oxygen sensing tissues

The cellular oxygen sensing system of the body ensures appropriate adaptation of cellular functions toward hypoxia by regulating gene expression and ion channel activity. Two-photon laser microscopy is an ideal tool to study and prove the relevance of the molecular mechanisms within oxygen sensing pathways on the cellular and complex tissue or organ level. Images of hypoxia inducible factor 1 (HIF-1) subunit nuclear mobility and protein-protein interaction in living cells, of hypoxia-induced changes in membrane potential and intracellular calcium of live ex vivo carotid bodies as well as of rat kidney proximal tubulus function in vivo, will be shown.

Metallothionein is upregulated by hypoxia and stabilizes hypoxia-inducible factor in the kidney

Recent studies underscore that chronic hypoxia in the tubulointerstitium is a final common pathway to progression to end-stage renal failure regardless of etiology. We used microarray analysis of rat kidneys made hypoxic by unilateral renal artery stenosis to measure transcriptomic events and clarify pathophysiological mechanisms of renal injury induced by chronic hypoxia. Many genes were upregulated in the kidney by chronic hypoxia, but we focused on metallothionein due to its antioxidative properties. Using tubular epithelial cells transfected with a reporter construct of luciferase, driven by the hypoxia-responsive elements (HRE), we found that addition of metallothionein to the culture media increased luciferase activity. This was associated with upregulation of the target genes of hypoxia-inducible factor (HIF), such as vascular endothelial growth factor and glucose transporter-1. Stimulation of the HIF-HRE pathway by metallothionein was confirmed by metallothionein overexpression. Hypoxia and exogenous metallothionein increased HIF-1alpha protein without changes in its mRNA levels, suggesting protein stabilization. Upregulation of the HIF-HRE system by metallothionein was associated with phosphorylation of ERK but not Akt. MEK inhibition and rapamycin decreased metallothionein-induced HIF activity. Our study shows that upregulation of metallothionein expression by hypoxia activates the HIF-HRE system through the ERK/mTOR pathway and may be a novel defense against hypoxia.

Nuclear oxygen sensing: induction of endogenous prolyl-hydroxylase 2 activity by hypoxia and nitric oxide

The abundance of the transcription factor hypoxia-inducible factor is regulated through hydroxylation of its alpha-subunits by a family of prolyl-hydroxylases (PHD1-3). Enzymatic activity of these PHDs is O2-dependent, which enables PHDs to act as cellular O2 sensor enzymes. Herein we studied endogenous PHD activity that was induced in cells grown under hypoxia or in the presence of nitric oxide. Under such conditions nuclear extracts contained much higher PHD activity than the respective cytoplasmic extracts. Although PHD1-3 were abundant in both compartments, knockdown experiments for each isoenzyme revealed that nuclear PHD activity was only due to PHD2. Maximal PHD2 activity was found between 120 and 210 microm O2. PHD2 activity was strongly decreased below 100 microm O2 with a half-maximum activity at 53 +/- 13 microm O2 for the cytosolic and 54 +/- 10 microm O2 for nuclear PHD2 matching the physiological O2 concentration within most cells. Our data suggest a role for PHD2 as a decisive oxygen sensor of the hypoxia-inducible factor degradation pathway within the cell nucleus.

Redox regulation of cell survival

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play important roles in regulation of cell survival. In general, moderate levels of ROS/RNS may function as signals to promote cell proliferation and survival, whereas severe increase of ROS/RNS can induce cell death. Under physiologic conditions, the balance between generation and elimination of ROS/RNS maintains the proper function of redox-sensitive signaling proteins. Normally, the redox homeostasis ensures that the cells respond properly to endogenous and exogenous stimuli. However, when the redox homeostasis is disturbed, oxidative stress may lead to aberrant cell death and contribute to disease development. This review focuses on the roles of key transcription factors, signal-transduction pathways, and cell-death regulators in affecting cell survival, and how the redox systems regulate the functions of these molecules. The current understanding of how disturbance in redox homeostasis may affect cell death and contribute to the development of diseases such as cancer and degenerative disorders is reviewed. We also discuss how the basic knowledge on redox regulation of cell survival can be used to develop strategies for the treatment or prevention of those diseases.

Redox-based endoplasmic reticulum dysfunction in neurological diseases

The redox homeostasis of the endoplasmic reticulum lumen is characteristically different from that of the other subcellular compartments. The concerted action of membrane transport processes and oxidoreductase enzymes maintain the oxidized state of the thiol-disulfide and the reducing state of the pyridine nucleotide redox systems, which are prerequisites for the normal functions of the organelle. The powerful thiol-oxidizing machinery allows oxidative protein folding but continuously challenges the local antioxidant defense. Alterations of the cellular redox environment either in oxidizing or reducing direction affect protein processing and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce apoptosis if the attempt fails. Recent findings strongly support the involvement of this mechanism in brain ischemia, neuronal degenerative diseases and traumatic injury. The redox changes in the endoplasmic reticulum are integral parts of the pathomechanism of neurological diseases, either as causative agents, or as complications.

Opposite expression of the antioxidant heme oxygenase-1 in primary cells and tumor cells: regulation by interaction of USF-2 and Fra-1

Heme oxygenase-1 is the rate-limiting enzyme for the degradation of the prooxidant heme. Previously, we showed that an E-box within the HO-1 promoter is crucial for the regulation of HO-1 expression in primary hepatocytes. Further to investigate the importance of this E-box, we determined the regulatory capacity of the E-box-binding factor USF-2 in primary cells in comparison with transformed cell lines. We found that HO-1 expression was inhibited by USF-2 in primary cells, whereas it was induced in tumor cell lines. Mutation of either the E-box or the AP-1 site within the HO-1 promoter only partially affected the USF-dependent regulation. However, this regulation was dramatically reduced in tumor cells and completely abolished in primary cells transfected with an HO-1 promoter construct containing mutations in both the E-box and the AP-1 site, suggesting that AP-1 factors and USF-2 may act in a cooperative manner. Indeed, protein-protein interaction studies revealed that USF proteins interacted with Fra-1. Further, the USF-dependent HO-1 promoter activity was not detectable with an USF-2 mutant lacking residues of the USF-specific region (USR) or the transactivation domain encoded by exon 4. Together, these data suggest that USF-2 has opposite regulatory roles for HO-1 gene expression in primary cells and tumor cell lines.

Comparison between the effects of normoxia and hypoxia on antioxidant enzymes and glutathione redox state in ex vivo culture of CD34(+) cells

Hypoxia maintained biological characteristics of CD34(+) cells through keeping lower intracellular reactive oxygen specials (ROS) levels. The effects of normoxia and hypoxia on antioxidant enzymes and glutathione redox state were compared in this study. Hypoxia decreased the mRNA expression of both catalase (CAT) and glutathione peroxidase (GPX), but not affected mRNAs expression of superoxide dismutase (SOD). While the cellular GPX activities under hypoxia were apparently less than those under normoxia, neither SOD activities nor CAT activities were affected by hypoxia. The analysis of glutathione redox status and ROS products showed the lower oxidized glutathione (GSSG) levels, the higher reduced glutathione (GSH) levels, the higher GSH/GSSG ratios, and the less O(2)- and H(2)O(2) generation under hypoxia (versus normoxia). Meanwhile more primary CD34(+)CD38(-) cells were obtained when cultivation was performed under hypoxia or with N-acetyl cysteine (the precursor of GSH) under normoxia. These results demonstrated the different responses of anti-oxidative mechanism between normoxia and hypoxia. Additionally, the present study suggested that the GSH-GPX antioxidant system played an important role in HSPCs preservation by reducing peroxidation.

n-Propyl gallate activates hypoxia-inducible factor 1 by modulating intracellular oxygen-sensing systems

HIF-1 (hypoxia-inducible factor 1) is a master regulator of cellular adaptive responses to hypoxia. The expression and transcriptional activity of the HIF-1alpha subunit is stringently controlled by intracellular oxygen tension through the action of prolyl and asparaginyl hydroxylases. In the present study we demonstrate that PG (n-propyl gallate) activates HIF-1 and expression of its downstream target genes under normoxic conditions in cultured cells and in mice. The stability and transcriptional activity of HIF-1alpha are increased by PG. PG treatment inhibits the interaction between HIF-1alpha and VHL (von Hippel-Lindau protein) and promotes the interaction between HIF-1alpha and p300, indicating that PG inhibits the activity of both prolyl and asparaginyl HIF-1alpha hydroxylases. We conclude that PG activates HIF-1 and enhances the resultant gene expression by directly affecting the intracellular oxygen sensing system in vitro and in vivo and that PG represents a lead compound for the development of a non-toxic activator of HIF-1.

Melatonin modulates the expression of VEGF and HIF-1 alpha induced by CoCl2 in cultured cancer cells

Melatonin is an important natural oncostatic agent. At present there are no data available as to its possible influence on tumor angiogenesis, which is a major biological mechanism responsible for tumor growth and dissemination. It is well known that vascular endothelial growth factor (VEGF) is crucial to a solid tumor's higher vascularization and development. To investigate the possible influence of melatonin on angiogenesis, we studied the effect of melatonin on endogenous VEGF expression in three human cancer cell lines (PANC-1, HeLa and A549 cells). In this study, we report that physiologic concentrations of melatonin have no obvious impact on the VEGF expression, whereas pharmacologic concentrations of melatonin suppress the VEGF mRNA and protein levels induced by hypoxia mimetic cobalt chloride (CoCl(2)). Melatonin also decreases hypoxia-inducible factor (HIF)-1alpha protein levels, suggesting a role for transcription factor HIF-1 in the suppression of VEGF expression. The effect of pharmacologic concentrations of melatonin on VEGF and HIF-1alpha under normoxia is uncertain, which indicates that the regulatory mechanisms of VEGF in the absence or presence of CoCl(2) are different and other or additional transcription factors may be involved. Taken together, our data show that melatonin in high concentrations markedly reduces the expression of endogenous VEGF and HIF-1alpha induced by CoCl(2) in cultured cancer cells.

Superoxide and derived reactive oxygen species in the regulation of hypoxia-inducible factors

Superoxide and its derived reactive oxygen species (ROS) have been considered for a long time to be generated as toxic byproducts of metabolic events. More recently, it has been acknowledged that ROS generated in low amounts are also able to act as signaling molecules in a variety of responses. One of the major pathways regulated by the ambient concentration of oxygen relies on the activity of hypoxia-inducible transcription factors (HIF). Originally described to be only induced and activated under hypoxia, accumulating evidence suggests that HIFs play a more general role in the response to a variety of cellular activators and stressors, many of which use ROS as signal transducers. Indeed, ROS have been found to modulate the levels of HIF not only under hypoxia, but also in response to many factors and under different stress conditions. However, the underlying regulatory mechanisms by which superoxide and derived ROS control HIF are only slowly beginning to be elucidated. We summarize here current knowledge about the mechanisms by which ROS can regulate HIF and give additional information about useful methods to determine ROS under various conditions.

Glucose up-regulates HIF-1 alpha expression in primary cortical neurons in response to hypoxia through maintaining cellular redox status

It has been suggested that hypoxia-inducible factor 1 (HIF-1), a key regulator in cell's adaptation to hypoxia, plays an important role in the fate of neurons during ischemia. However, the mechanism of HIF-1 regulation is still not fully understood in neurons subjected to ischemia. In this study, we demonstrated that glucose up-regulated the expression of HIF-1alpha, the oxygen-dependent subunit of HIF-1, in rat primary cortical neurons exposed to hypoxia. To understand the mechanism of glucose-regulated HIF-1alpha expression, we investigated the relationships between HIF-1alpha expression, reactive oxygen species (ROS), and redox status. Low levels of HIF-1alpha protein expression were observed in the neurons exposed to in vitro ischemic conditions that had high levels of ROS (oxidizing environments), and vice versa. The glutathione (GSH) precursor, N-acetyl cysteine, induced HIF-1alpha protein expression in hypoxic neurons while the GSH synthesis inhibitor, l-buthionine sulfoximine, inhibited the expression. Moreover, (-)-epicatechin gallate, a ROS scavenger, elevated HIF-1alpha expression in the neurons subjected to in vitro ischemia. Furthermore, results from a systemic hypoxia model showed that a reducing environment increased HIF-1alpha expression in rat brains. Taken together, these data presented the first evidence that glucose promoted HIF-1alpha stabilization through regulating redox status in primary neurons exposed to hypoxia. The results imply that hypoxia only may not be sufficient to stabilize HIF-1alpha and that a reducing environment is required to stabilize HIF-1alpha in neurons exposed to hypoxia.

Vesicular localization of the rat ATP-binding cassette half-transporter rAbcb6

The clarification of subcellular localization represents an important basis toward characterization of ATP-binding cassette (ABC) transporters and resolution of their roles in cellular physiology. Rat Abcb6 (rAbcb6) is a membrane-situated half-transporter belonging to the ABC protein superfamily. To investigate rAbcb6 subcellular distribution, the human colon adenocarcinoma line LoVo, which we found to be devoid of endogenous human ABCB6 mRNA, was employed for heterologous expression of rAbcb6 bearing a COOH-terminal epitope tag (rAbcb6-V5). Following subcellular fractionation, rAbcb6-V5 was observed as an N-glycosylated protein in fractions enriched with lysosomal/endosomal membrane proteins. Indirect immunofluorescence analyses of rAbcb6-V5 using antibodies against a rAbcb6-specific peptide or against the V5-tag revealed a punctate pattern that was colocalized with lysosome-associated membrane protein 1 (LAMP1), a marker of lysosomes/late endosomes. Substantial colocalization of tagged rAbcb6 with lysosomal/late endosomal marker was confirmed with living, unfixed LoVo cells coexpressing rAbcb6 fused to enhanced green fluorescent protein. Vesicular distribution in LoVo cells was consistent with localization of endogenous rAbcb6 expressed in rat primary hepatocyte cultures or in liver sections, as revealed by overlap of rat Lamp1 with rAbcb6 in double immunofluorescence analyses. Since several Abcb6-related half-transporters confer heavy metal tolerance, we investigated whether rAbcb6 expression in LoVo cells might affect sensitivity toward transition metal toxicity. Applying MTT viability assays, we found that expression of either rAbcb6-V5 or untagged rAbcb6 conferred tolerance toward copper, but not to cobalt or zinc. In summary, these results demonstrate that rAbcb6 is a glycosylated protein targeted to intracellular vesicular membranes and suggest involvement of rAbcb6 in transition metal homeostasis.

Building and operating an antibody factory: redox control during B to plasma cell terminal differentiation

When small B lymphocytes bind their cognate antigens in the context of suitable signals, a dramatic differentiation program is activated that leads to the formation of plasma cells. These are short-lived specialized elements, each capable of secreting several thousands antibodies per second. The massive increase in Ig synthesis and transport entails a dramatic architectural and functional metamorphosis that involves the development of the endoplasmic reticulum (ER) and secretory organelles. Massive Ig secretion poses novel metabolic requirements, particularly for what concerns aminoacid import, ATP synthesis and redox homeostasis. Ig H and L chains enter the ER in the reduced state, to be rapidly oxidised mainly via protein driven relays based on the resident enzymes PDI and Ero1. How do plasma cells cope with the ensuing metabolic and redox stresses? In this essay, we discuss the physiological implications that increased Ig production could have in the control of plasma cell generation, function and lifespan, with emphasis on the potential role of ROS generation in mitochondria and ER.