Hypoxic induction of hypoxia-inducible factor-1alpha and oxygen-regulated gene expression in mitochondrial DNA-depleted HeLa cells

Intercellular transfer of mitochondrial DNA carrying metastasis-enhancing pathogenic mutations from high- to low-metastatic tumor cells and stromal cells via extracellular vesicles

Background

Mitochondrial DNA (mtDNA) carrying certain pathogenic mutations or single nucleotide variants (SNVs) enhances the invasion and metastasis of tumor cells, and some of these mutations are homoplasmic in tumor cells and even in tumor tissues. On the other hand, intercellular transfer of mitochondria and cellular components via extracellular vesicles (EVs) and tunneling nanotubes (TNTs) has recently attracted intense attention in terms of cell-to-cell communication in the tumor microenvironment. It remains unclear whether metastasis-enhancing pathogenic mutant mtDNA in tumor cells is intercellularly transferred between tumor cells and stromal cells. In this study, we investigated whether mtDNA with the NADH dehydrogenase subunit 6 (ND6) G13997A pathogenic mutation in highly metastatic cells can be horizontally transferred to low-metastatic cells and stromal cells in the tumor microenvironment.

Results

When MitoTracker Deep Red-labeled high-metastatic Lewis lung carcinoma A11 cells carrying the ND6 G13997A mtDNA mutation were cocultured with CellLight mitochondria-GFP-labeled low-metastatic P29 cells harboring wild-type mtDNA, bidirectional transfer of red- and green-colored vesicles, probably mitochondria-related EVs, was observed in a time-dependent manner. Similarly, intercellular transfer of mitochondria-related EVs occurred between A11 cells and α-smooth muscle actin (α-SMA)-positive cancer-associated fibroblasts (CAFs, WA-mFib), macrophages (RAW264.7) and cytotoxic T cells (CTLL-2). Intercellular transfer was suppressed by inhibitors of EV release. The large and small EV fractions (L-EV and S-EV, respectively) prepared from the conditioned medium by differential ultracentrifugation both were found to contain mtDNA, although only S-EVs were efficiently incorporated into the cells. Several subpopulations had evidence of LC3-II and contained degenerated mitochondrial components in the S-EV fraction, signaling to the existence of autophagy-related S-EVs. Interestingly, the S-EV fraction contained a MitoTracker-positive subpopulation, which was inhibited by the respiration inhibitor antimycin A, indicating the presence of mitochondria with membrane potential. It was also demonstrated that mtDNA was transferred into mtDNA-less ρ⁰ cells after coculture with the S-EV fraction. In syngeneic mouse subcutaneous tumors formed by a mixture of A11 and P29 cells, the mitochondria-related EVs released from A11 cells reached distantly positioned P29 cells and CAFs.

Conclusions

These results suggest that metastasis-enhancing pathogenic mtDNA derived from metastatic tumor cells is transferred to low-metastatic tumor cells and stromal cells via S-EVs in vitro and in the tumor microenvironment, inferring a novel mechanism of enhancement of metastatic potential during tumor progression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-021-00391-5.

Polysulfide inhibits hypoxia-elicited hypoxia-inducible factor activation in a mitochondria-dependent manner

In cellular signaling, the diverse physiological actions of biological gases, including O₂, CO, NO, and H₂S, have attracted much interest. Hypoxia-inducible factors (HIFs), including HIF-1 and HIF-2, are transcription factors that respond to reduced intracellular O₂ availability. Polysulfides are substances containing varying numbers of sulfur atoms (H₂S_n) that are generated endogenously from H₂S by 3-mercaptopyruvate sulfurtransferase in the presence of O₂, and regulate ion channels, specific tumor suppressors, and protein kinases. However, the effect of polysulfides on HIF activation in hypoxic mammalian cells is largely unknown. Here, we have investigated the effect of polysulfide on cells in vitro. In established cell lines, polysulfide donors reversibly reduced cellular O₂ consumption and inhibited hypoxia-induced HIF-1α protein accumulation and the expression of genes downstream of HIFs; however, these effects were not observed in anoxia. In Von Hippel-Lindau tumor suppressor (VHL)- and mitochondria-deficient cells, polysulfides did not affect HIF-1α protein synthesis but destabilized it in a VHL- and mitochondria-dependent manner. For the first time, we show that polysulfides modulate intracellular O₂ homeostasis and regulate HIF activation and subsequent hypoxia-induced gene expression in a VHL- and mitochondria-dependent manner.

mtDNA as a Mediator for Expression of Hypoxia-Inducible Factor 1α and ROS in Hypoxic Neuroblastoma Cells

Mitochondria consume O₂ to produce ATP and are critical for adaption of hypoxia, but the role of mitochondria in HIF-1α pathway is as yet unclear. In this study, mitochondrial DNA (mtDNA) enriched (SK-N-AS) and depleted (ρ⁰) cells of neuroblastoma were cultured in a hypoxic chamber to simulate a hypoxic condition and then the major components involved in mitochondrial related pathways, hypoxia-inducible factor 1α (HIF-1α) and reactive oxygen species (ROS) were measured. The results showed that hypoxia-stimulated exposure elevated expression of HIF-1α, which was additionally influenced by level of generated ROS within the cytosol. Moreover, elevation of HIF-1α also resulted in increases of lactate dehydrogenase A (LDH-A) and pyruvate dehydrogenase kinase 1 (PDK1) in both hypoxic cells. The expression of mitochondrial biogenesis related proteins and metabolic components were noted to increase significantly in hypoxic SK-N-AS cells, indicating that mtDNA was involved in mitochondrial retrograde signaling and metabolic pathways. An analysis of dynamic proteins found elevated levels of HIF-1α causing an increased expression of dynamin-related protein 1 (DRP1) during hypoxia; further, the existence of mtDNA also resulted in higher expression of DRP1 during hypoxia. By using siRNA of HIF-1α or DRP1, expression of DRP1 decreased after suppression of HIF-1α; moreover, the expression of HIF-1α was also affected by the suppression of DRP1. In this study, we demonstrated that mtDNA is a mediator of HIF-1α in eliciting metabolic reprogramming, and mitochondrial biogenesis. Identification of a mutual relationship between HIF-1α and DRP1 may be a critical tool in the future development of clinical applications.

Strange Bedfellows: Nuclear Factor, Erythroid 2-Like 2 (Nrf2) and Hypoxia-Inducible Factor 1 (HIF-1) in Tumor Hypoxia

The importance of the tumor microenvironment for cancer progression and therapeutic resistance is an emerging focus of cancer biology. Hypoxia, or low oxygen, is a hallmark of solid tumors that promotes metastasis and represents a significant obstacle to successful cancer therapy. In response to hypoxia, cancer cells activate a transcriptional program that allows them to survive and thrive in this harsh microenvironment. Hypoxia-inducible factor 1 (HIF-1) is considered the main effector of the cellular response to hypoxia, stimulating the transcription of genes involved in promoting angiogenesis and altering cellular metabolism. However, growing evidence suggests that the cellular response to hypoxia is much more complex, involving coordinated signaling through stress response pathways. One key signaling molecule that is activated in response to hypoxia is nuclear factor, erythroid 2 like-2 (Nrf2). Nrf2 is a transcription factor that controls the expression of antioxidant-response genes, allowing the cell to regulate reactive oxygen species. Nrf2 is also activated in various cancer types due to genetic and epigenetic alterations, and is associated with poor survival and resistance to therapy. Emerging evidence suggests that coordinated signaling through Nrf2 and HIF-1 is critical for tumor survival and progression. In this review, we discuss the distinct and overlapping roles of HIF-1 and Nrf2 in the cellular response to hypoxia, with a focus on how targeting Nrf2 could provide novel chemotherapeutic modalities for treating solid tumors.

The antioxidant N-acetyl cysteine suppresses lidocaine-induced intracellular reactive oxygen species production and cell death in neuronal SH-SY5Y cells

BACKGROUND

The local anesthetic lidocaine can affect intra- and extra-cellular signaling pathways in both neuronal and non-neuronal cells, resulting in long-term modulation of biological functions, including cell growth and death. Indeed, lidocaine was shown to induce necrosis and apoptosis in vitro. While several studies have suggested that lidocaine-induced apoptosis is mitochondrial pathway-dependent, it remains unclear whether reactive oxygen species (ROS) are involved in this process and whether the observed cell death can be prevented by antioxidant treatment.

METHODS

The effects of lidocaine and antioxidants on cell viability and death were evaluated using SH-SY5Y cells, HeLa cells, and HeLa cell derivatives. Cell viability was examined via MTS/PES ([3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt]/phenazine ethosulfate) assay. Meanwhile, cell apoptosis and necrosis were evaluated using a cell death detection assay with Annexin V-FITC and PI staining, as well as by assaying for caspase-3/7 and caspase-9 activity, and by measuring the release of lactate dehydrogenase, respectively. Mitochondrial transmembrane potential (ΔΨm) was assessed using the fluorescent probe tetramethylrhodamine ethyl ester.

RESULTS

Lidocaine treatment resulted in suppression of the mitochondrial electron transport chain and subsequent attenuation of mitochondrial membrane potential, as well as enhanced ROS production, activation of caspase-3/7 and caspase-9, and induction of apoptosis and necrosis in SH-SY5Y cells in a dose- and time-dependent manner. Likewise, the anesthetics mepivacaine and bupivacaine also induced apoptosis in SH-SY5Y cells. Notably, the antioxidants N-acetyl cysteine (NAC) and Trolox successfully scavenged the mitochondria-derived ROS and suppressed local lidocaine-induced cell death.

CONCLUSIONS

Our findings demonstrate that the local anesthetics lidocaine, mepivacaine, and bupivacaine inhibited the activity of mitochondria and induced apoptosis and necrosis in a dose-dependent manner. Furthermore, they demonstrate that treatment with the antioxidants NAC, Trolox, and GGA resulted in preservation of mitochondrial voltage and inhibition of apoptosis via suppression of caspase activation.

Prolyl hydroxylase domain enzymes and their role in cell signaling and cancer metabolism

The prolyl hydroxylase domain (PHD) enzymes regulate the stability of the hypoxia-inducible factor (HIF) in response to oxygen availability. During oxygen limitation, the inhibition of PHD permits the stabilization of HIF, allowing the cellular adaptation to hypoxia. This adaptation is especially important for solid tumors, which are often exposed to a hypoxic environment. However, and despite their original role as the oxygen sensors of the cell, PHD are currently known to display HIF-independent and hydroxylase-independent functions in the control of different cellular pathways, including mTOR pathway, NF-kB pathway, apoptosis and cellular metabolism. In this review, we summarize the recent advances in the regulation and functions of PHD in cancer signaling and cell metabolism.

Apurinic/apyrimidinic endonuclease/redox factor-1 (APE1/Ref-1) redox function negatively regulates NRF2

Apurinic/apyrimidinic endonuclease/redox factor-1 (APE1/Ref-1) (henceforth referred to as Ref-1) is a multifunctional protein that in addition to its base excision DNA repair activity exerts redox control of multiple transcription factors, including nuclear factor κ-light chain enhancer of activated B cells (NF-κB), STAT3, activator protein-1 (AP-1), hypoxia-inducible factor-1 (HIF-1), and tumor protein 53 (p53). In recent years, Ref-1 has emerged as a promising therapeutic target in cancer, particularly in pancreatic ductal carcinoma. Although a significant amount of research has centered on Ref-1, no wide-ranging approach had been performed on the effects of Ref-1 inhibition and transcription factor activity perturbation. Starting with a broader approach, we identified a previously unsuspected effect on the nuclear factor erythroid-related factor 2 (NRF2), a critical regulator of cellular defenses against oxidative stress. Based on genetic and small molecule inhibitor-based methodologies, we demonstrated that repression of Ref-1 potently activates NRF2 and its downstream targets in a dose-dependent fashion, and that the redox, rather than the DNA repair function of Ref-1 is critical for this effect. Intriguingly, our results also indicate that this pathway does not involve reactive oxygen species. The link between Ref-1 and NRF2 appears to be present in all cells tested in vitro, noncancerous and cancerous, including patient-derived tumor samples. In particular, we focused on understanding the implications of the novel interaction between these two pathways in primary pancreatic ductal adenocarcinoma tumor cells and provide the first evidence that this mechanism has implications for overcoming the resistance against experimental drugs targeting Ref-1 activity, with clear translational implications.

Mitochondrial alterations during carcinogenesis: a review of metabolic transformation and targets for anticancer treatments

Mitochondria play important roles in multiple cellular processes including energy metabolism, cell death, and aging. Regulated energy production and utilization are critical in maintaining energy homeostasis in normal cells and functional organs. However, mitochondria go through a series of morphological and functional alterations during carcinogenesis. The metabolic profile in transformed cells is altered to accommodate their fast proliferation, confer resistance to cell death, or facilitate metastasis. These transformations also provide targets for anticancer treatment at different levels. In this review, we discuss the major modifications in cell metabolism during carcinogenesis, including energy metabolism, apoptotic and autophagic cell death, adaptation of tumor microenvironment, and metastasis. We also summarize some of the main metabolic targets for treatments.

Hydrogen sulfide inhibits hypoxia- but not anoxia-induced hypoxia-inducible factor 1 activation in a von hippel-lindau- and mitochondria-dependent manner

AIMS

In addition to nitric oxide and carbon monoxide, hydrogen sulfide (H(2)S) is an endogenously synthesized gaseous molecule that acts as an important signaling molecule in the living body. Transcription factor hypoxia-inducible factor 1 (HIF-1) is known to respond to intracellular reduced oxygen (O(2)) availability, which is regulated by an elaborate balance between O(2) supply and demand. However, the effect of H(2)S on HIF-1 activity under hypoxic conditions is largely unknown in mammalian cells. In this study, we tried to elucidate the effect of H(2)S on hypoxia-induced HIF-1 activation adopting cultured cells and mice.

RESULTS

The H(2)S donors sodium hydrosulfide and sodium sulfide in pharmacological concentrations reversibly reduced cellular O(2) consumption and inhibited hypoxia- but not anoxia-induced HIF-1α protein accumulation and expression of genes downstream of HIF-1 in established cell lines. H(2)S did not affect HIF-1 activation induced by the HIF-α hydroxylases inhibitors desferrioxamine or CoCl(2). Experimental evidence adopting von Hippel-Lindau (VHL)- or mitochondria-deficient cells indicated that H(2)S did not affect neosynthesis of HIF-1α protein but destabilized HIF-1α in a VHL- and mitochondria-dependent manner. We also demonstrate that exogenously administered H(2)S inhibited HIF-1-dependent gene expression in mice.

INNOVATION

For the first time, we show that H(2)S modulates intracellular O(2) homeostasis and regulates activation of HIF-1 and the subsequent gene expression induced by hypoxia by using an in vitro system with established cell lines and an in vivo system in mice.

CONCLUSIONS

We demonstrate that H(2)S inhibits hypoxia-induced HIF-1 activation in a VHL- and mitochondria-dependent manner.

Oxygen Sensing, Cardiac Ischemia, HIF-1α and Some Emerging Concepts

Oxygen plays a critical role in the perpetuation and propagation of almost all forms of life. The primary site of cellular oxygen consumption is the mitochondrial electron transport chain and in addition, oxygen is also used as a substrate for various enzymes involved in cellular homeostasis. Although our knowledge of the biochemistry and physiology of oxygen transport is century old, recent development of sophisticated tools of biophysical chemistry revealed that tissue oxygenation and oxygen sensing is a highly evolved process, especially in mammals. Perturbation of normal oxygen supply is associated with diseases like tumorigenesis, myocardial infarction and stroke. Available information suggests that when tissue oxygen supply is limited, mitochondria emanate signals involving reactive oxygen species generation which in turn stabilizes oxygen sensing transcription factor HIF-1. Upon stabilization, HIF-1 elicits necessary genetic response to cope with the diminished oxygen level. In view of such critical role of HIF-1 in cellular oxygen sensing, recently there has been a heightened interest in understanding the biology of HIF-1 in the context of cardiovascular system. The following review describes some of the recent advances in this regard.

Mitochondrial regulation of oxygen sensing

Hypoxia promotes physiological processes such as energy metabolism, angiogenesis, cell proliferation, and cell viability through the transcription factor Hypoxia Inducible Factor (HIF). Hypoxia also diminishes the activity of ATP consuming processes to promote cell survival. The mechanism(s) by which hypoxia activates HIF and diminishes ATP demand are a subject of intensive research. Here we outline the model in which mitochondrial complex III regulate the activity of HIF and diminish ATP utilization processes through the increased production of ROS during hypoxia.

Targeting the mitochondria for cancer therapy: regulation of hypoxia-inducible factor by mitochondria

As tumors develop, they outgrow the vascular network that supplies cells with oxygen and nutrients needed for survival. In response to decreased oxygen levels, the tumor cells initiate a program of adaptation by inducing the transcription of multiple genes via the activation of the transcription factor hypoxia-inducible factor (HIF). Proteins encoded by a subset of genes induced by HIF promote tumorigenesis by acting directly on both the tumor cells and the microenvironment in which the tumor cells reside. The mechanism(s) by which hypoxia activates HIF is a subject of intensive research. Understanding how hypoxia activates HIF will provide targets for the development of therapies that could specifically target growing tumors by not allowing adequate adaptation to hypoxia, which is necessary for cancer progression. Here we outline how mitochondria regulate the activity of HIF during hypoxia.

Sequence-specific oxidative base modifications in hypoxia-inducible genes

Reactive oxygen species associated with hypoxic signaling in pulmonary arterial endothelial cells (PAECs) oxidatively modify specific nucleotides in the hypoxic response element (HRE) of the VEGF gene (FASEB J.19:387-394; 2005). In this study, we determined in PAECs if hypoxia caused genome-wide oxidative modifications or if they were restricted to the promoters of genes differentially regulated by hypoxia. Comet assays indicated that there were no differences between normoxic and hypoxic PAECs in terms of global DNA damage. However, a simple PCR-based method involving DNA amplification before and after treatment with formamidopyrimidine DNA glycosylase (Fpg), a bacterial DNA repair enzyme that cleaves at sites of purine base oxidation, revealed that hypoxia caused modifications in the HREs of the hypoxia-inducible VEGF, HO-1, and ET-1 genes which coincided with accumulation of their respective mRNA transcripts. Promoter sequences not involved with hypoxic induction and coding regions of these genes failed to display Fpg-sensitive sites. Oxidative modifications also were not detected in sequences of the hypoxia down-regulated ornithine decarboxylase and TFAM genes or the constitutively expressed beta-actin gene. These findings show that hypoxia-mediated oxidative DNA modifications cluster in functionally relevant promoter sequences in hypoxia-inducible genes and suggest that such oxidative modifications may be biologically significant.

Mitochondrial oxygen sensing: regulation of hypoxia-inducible factor by mitochondrial generated reactive oxygen species

Decreased oxygen availability (hypoxia) promotes physiological processes such as energy metabolism, angiogenesis, cell proliferation and cell viability through the transcription factor HIF (hypoxia-inducible factor). Activation of HIF can also promote pathophysiological processes such as cancer and pulmonary hypertension. The mechanism(s) by which hypoxia activates HIF are the subject of intensive research. In this chapter we outline the model in which mitochondria regulate the stability of HIF through the increased production of ROS (reactive oxygen species) during hypoxia.

The regulation, control, and consequences of mitochondrial oxygen utilization and disposition in the heart and skeletal muscle during hypoxia

The major oxygen-dependent function of mitochondria partitions molecular oxygen between oxidative phosphorylation and reactive oxygen species generation. When oxygen becomes limiting, the modulation of mitochondrial function plays an important role in overall biologic adaptation. This review focuses on mitochondrial biology in the heart and skeletal muscle during hypoxia. The disparate mitochondrial responses discussed appear to be dependent on the degree of hypoxia, on the age at exposure to hypoxia, and on the duration of exposure. These hypoxia-induced changes include modulation in mitochondrial respiratory capacity; activation of the mitochondrial biogenesis regulatory program; induction of mitochondrial antioxidant defense systems; regulation of antiapoptotic mitochondrial proteins, and modulation of mitochondrial sensitivity to permeability transition. The mitochondria-derived reactive oxygen species signal-transduction events in response to hypoxia also are reviewed. The cardiac and skeletal muscle phenotypic signatures that result from mitochondrial adaptations include an amelioration of resistance to cardiac ischemia and modulations in exercise capacity and oxidative fuel preference. Overall, the data demonstrate the plasticity in mitochondrial regulation and function that facilitates adaptations to a limited oxygen supply. Moreover, data supporting the role of mitochondria as oxygen-sensing organelles, integrated into global cellular signal transduction are discussed.

Oxidative Stress Mediates CoCl2-Induced Prostate Tumour Cell Adhesion: Role of Protein Kinase C and p38 Mitogen-Activated Protein Kinase

Cobalt chloride (CoCl(2)), an agent demonstrated to stabilize hypoxia-inducible factor-1, has been associated with various hypoxic responses, and recently, some reports have linked it to increasing tumour malignancy. In this study, we observed the alteration of cell adhesion after CoCl(2) treatment and analysed the potential mechanisms responsible for such adaptations in a prostate cancer cell line PC-3 M cell. We found that CoCl(2 )increased the tumour cell adhesion in a dose-dependent manner, which correlated with reactive oxygen species (ROS) generation. When cells were incubated with the thiol reductive agent pyrrolidine dithiocarbamate (PDTC), both the ROS generation and the CoCl(2)-induced cell adhesion were abolished. Moreover, p38 mitogen-activated protein kinase (p38 MAPK) was activated in CoCl(2)-treated cells, which could be antagonized by PDTC. And when cells were pre-incubated with specific p38 MAPK inhibitor, SB203580, the cell adhesion induced by CoCl(2 )was diminished. Moreover, the protein kinase C could up-regulate cell adhesion through activating p38 MAPK. In conclusion, CoCl(2) induced ROS generation, thereby placing cells under oxidative stress and up-regulating cell adhesion; p38 MAPK and protein kinase C could be activated in a ROS-dependent fashion, which in turn contributed to cell adhesion induced by CoCl(2).

Redox regulation of the hypoxia-inducible factor

Reactive oxygen species (ROS) have long been considered only as cyto- and genotoxic. However, there is now compelling evidence that ROS also act as second messengers in response to various stimuli, such as growth factors, hormones and cytokines. The hypoxia-inducible transcription factor (HIF) is a master regulator of oxygen-sensitive gene expression. More recently, HIF has also been shown to respond to non-hypoxic stimuli. Interestingly, recent reports indicate that ROS regulate HIF stability and transcriptional activity in well-oxygenated cells, as well as under hypoxic conditions. Consequently, ROS appear to be key players in regulating HIF-dependent pathways under both normal and pathological circumstances. This review summarizes the current understanding of the role of ROS in the regulation of the mammalian HIF system.

Age-related hearing loss in CD/1 mice is associated to ROS formation and HIF target proteins up-regulation in the cochlea

Pathologies of senescence, in particular those of neurosensory organs represent an important health problem. The improvement of the life expectation entails the fast increase of the frequency of the age-related hearing loss (ARHL) in the population. There are numerous factors that contribute to this process, which include altered vascular characteristics, hypoxia/ischemia, genetic mutations and production of reactive oxygen species. We were interested in understanding the mechanisms involved in the cochlear degeneration in a mouse model of ARHL, the cd/1 mice. Since in human, hypoxia/ischemia is an important pathogenetic factor for inner ear disease, the regulation of HIF-1 activity in the cochlea, the presence of radical oxygen species in the cochlea and its subsequent disturbances of cellular signaling cascades were investigated. In this study, we explored auditory function of cd/1 mice at the age of 4, 12 and 24 weeks and correlated it with the presence of oxidative damage in the cochlea, and cochlear HIF-1 responsive target genes regulation, involved in pathways promoting inflammation such as tumor necrosis factor (TNF-alpha), or cell death with the p53 protein, Bax protein and surviving factors with insulin-like growth factor-1 (IGF-1). After implantation of electrodes for auditory nerve acoustic thresholds measurements, we analyzed every cochlea. First, we confirmed that the cd/1 mice presented a characteristic profile of ARHL starting at 12 weeks of age. Then, according to our previous report [Riva, C., Longuet, M., Lucciano, M., Magnan, J., Lavieille, J.P., 2005. Implication of mitochondrial apoptosis in neural degeneration in a murin model for presbyacusis. Rev. Laryngol. Otol. Rhinol. 126 (2), 67-74], we noticed many alterations in the cochlea. Histologically, at 4 weeks, intensive HIF-1alpha expression was detected in the cochlea followed by ROS formation at 12 weeks, which may lead to cochlear degeneration and induction the onset of ARHL in the cd/1 mice model. In the cochlea, while the inner and the outer hair cells remained intact at 4 and 12 weeks, the spiral ganglion was more altered. Moreover, the Schwann cells of the spiral ganglion seemed to be more vulnerable to free radical damage than the neurons and degenerated more rapidly. The mechanisms of degeneration in the spiral ganglion involved a caspase-3 and Bax mediated-apoptosis via p53 protein accumulation. Since oxygen radicals are required for the post-translational stabilization of HIF-1alpha during hypoxia, the tandem " HIF-ROS " induced multiple reactions within the cochlea, like a strong inflammatory response with increased expression of TNF-alpha, and inhibition of neuronal protection mechanisms with repression of IGF-1.

Hypoxia-inducible factor-1 mediates the expression of DNA polymerase iota in human tumor cells

Hypoxia generated in tumors has been shown to contribute to mutations and genetic instability. However, the molecular mechanisms remain incompletely defined. Since reactive oxygen species (ROS) are overproduced immediately after reoxygenation of hypoxic cells and generate oxidized guanine, we assumed that the mechanisms might involve translesion DNA polymerases that can bypass oxidized guanine. We report here that hypoxia as well as hypoxia mimetics, desferrioxamine, and CoCl(2), enhanced the expression of DNA polymerase iota (pol iota) in human tumor cell lines. Searching the consensus sequence of hypoxia response element to which HIF-1 binds revealed that it locates in the intron 1 of the pol iota gene. These results suggest that HIF-1-mediated pol iota gene expression may be involved in the generation of translesion mutations during DNA replication after hypoxia followed by reoxygenation, thereby contributing to the accumulation of genetic changes in tumor cells.

Angiotensin II stimulates hypoxia-inducible factor 1alpha accumulation in glomerular mesangial cells

Hypoxia increases hypoxia-inducible factor 1alpha (HIF-1alpha) protein levels by inhibiting ubiquitination and degradation of HIF-1alpha, which regulates the transcription of many genes. Recent studies have revealed that many ligands can stimulate HIF-1alpha accumulation under nonhypoxic conditions. In this study, we show that angiotensin II (Ang II) increased HIF-1alpha protein levels in a time- and dose-dependent manner under normoxic conditions. Treatment of mesangial cells with Ang II (100 nM) increased production of reactive oxygen species (ROS). Ang II (100 nM) increased the phosphorylation of PDK-1 and Akt/PKB in glomerular mesangial cells. Ang II-stimulated HIF-1alpha accumulation was blocked by the phosphatidylinositol 3-kinase (PI-3K) inhibitors, Ly 294001, and wortmannin, suggesting that PI-3K was involved. Because increased ROS generation by Ang II may activate the PI-3K-PKB/Akt signaling pathway, these results suggest that Ang II may stimulate a ROS-dependent activation of the PI-3K-PKB/Akt pathway, which leads to HIF-1alpha accumulation.

Hypoxia-inducible factors in the kidney

Tissue hypoxia not only occurs under pathological conditions but is also an important microenvironmental factor that is critical for normal embryonic development. Hypoxia-inducible factors HIF-1 and HIF-2 are oxygen-sensitive basic helix-loop-helix transcription factors, which regulate biological processes that facilitate both oxygen delivery and cellular adaptation to oxygen deprivation. HIFs consist of an oxygen-sensitive alpha-subunit, HIF-alpha, and a constitutively expressed beta-subunit, HIF-beta, and regulate the expression of genes that are involved in energy metabolism, angiogenesis, erythropoiesis and iron metabolism, cell proliferation, apoptosis, and other biological processes. Under conditions of normal Po(2), HIF-alpha is hydroxylated and targeted for rapid proteasomal degradation by the von Hippel-Lindau (VHL) E3-ubiquitin ligase. When cells experience hypoxia, HIF-alpha is stabilized and either dimerizes with HIF-beta in the nucleus to form transcriptionally active HIF, executing the canonical hypoxia response, or it physically interacts with unrelated proteins, thereby enabling convergence of HIF oxygen sensing with other signaling pathways. In the normal, fully developed kidney, HIF-1alpha is expressed in most cell types, whereas HIF-2alpha is mainly found in renal interstitial fibroblast-like cells and endothelial cells. This review summarizes some of the most recent advances in the HIF field and discusses their relevance to renal development, normal kidney function and disease.

Reactive oxygen species in the control of hypoxia-inducible factor-mediated gene expression

Reactive oxygen species (ROS) have long been considered as cytotoxic. However, recent evidence indicates a prominent role of ROS as signaling molecules in the response to hormones, growth and coagulation factors, cytokines and other factors as well as to changes in oxygen tension. The hypoxia-inducible transcription factors (HIFs) are key players in the cellular response to changes in oxygen tension. Recently, HIFs have also been shown to respond to the above-mentioned non-hypoxic stimuli. In this article, the role of ROS in the regulation of HIF-1 under hypoxic and non-hypoxic conditions is summarized.

Mitochondrial disorders in renal tumors

As early as 1930, Warburg discovered that metabolic alterations were associated with carcinogenesis and that cancer cells fermented even in the presence of oxygen using glycolysis to fulfill their energy needs, though less efficiently than with respiration. The kidney requiring a very active energy production for its pumping functions has a high mitochondrial activity. Kidney tumors can exist either in relatively benign forms, as for example, in oncocytomas that are crowded with mitochondria or in very aggressive forms such as in clear cell renal carcinomas that exhibit strongly down-regulated mitochondrial activities. These carcinomas can produce metastases that are resistant to anti-mitotic drugs and current treatments only delay the fatal issue. In this review, the mitochondrial alterations observed in various forms of renal tumors will be discussed with the aim of understanding how the knowledge of mitochondrial impairment mechanisms could be helpful to develop new anti-cancer strategies.

Mitochondrial cytochrome oxidase produces nitric oxide under hypoxic conditions: implications for oxygen sensing and hypoxic signaling in eukaryotes

Eukaryotic cells respond to low-oxygen concentrations by upregulating hypoxic nuclear genes (hypoxic signaling). Although it has been shown previously that the mitochondrial respiratory chain is required for hypoxic signaling, its underlying role in this process has been unclear. Here, we find that yeast and rat liver mitochondria produce nitric oxide (NO) at dissolved oxygen concentrations below 20 microM. This NO production is nitrite (NO2-) dependent, requires an electron donor, and is carried out by cytochrome c oxidase in a pH-dependent fashion. Mitochondrial NO production in yeast is influenced by the YHb flavohemoglobin NO oxidoreductase, stimulates expression of the hypoxic nuclear gene CYC7, and is accompanied by an increase in protein tyrosine nitration. These findings demonstrate an alternative role for the mitochondrial respiratory chain under hypoxic or anoxic conditions and suggest that mitochondrially produced NO is involved in hypoxic signaling, possibly via a pathway that involves protein tyrosine nitration.

Mitochondrial regulation of oxygen sensing

Maintenance of oxygen homoeostasis is required for the survival of higher organisms. The transcription factor hypoxia inducible factor (HIF) has a crucial role in oxygen homoeostasis by integrating the decrease in oxygen levels to increase hypoxic gene expression. How cells sense decrease in oxygen levels to elicit an increase in HIF dependent gene expression is not fully understood. In this review, we discuss the role of mitochondria as oxygen sensors regulating HIF.

Mitochondria: Oxygen sinks rather than sensors?

At the cellular level, oxygen partial pressure (pO2) is sensed by a family of protein hydroxylases. These enzymes transmit the information about the current pO2 directly to hypoxia-inducible transcription factors (HIFs) in the form of covalently attached hydroxy groups which regulate abundance and activity of the HIFs. In addition to this highly specific and direct mechanism of oxygen sensing, mitochondria were repeatedly proposed to sense oxygen and to transmit the signal in the form of a side product of the electron transport chain, i.e. the reactive oxygen species (ROS). However, the exact correlation between pO2 and ROS production, the precise downstream targets of ROS, and how ROS regulate these targets at the molecular level, are questions that remain unanswered. Supported by recent novel data, an alternative model is discussed which is based on the redirection of oxygen towards the protein hydroxylase oxygen sensors. Under conditions of changes in oxygen usage, e.g. following changes in mitochondrial function or cellular metabolism, oxygen redirection would provide an elegant explanation for HIF regulation under apparently constant external oxygen concentrations.

Adaptive responses to mitochondrial dysfunction in the rho degrees Namalwa cell

Mitochondrial function depends on the synchronised expression of nuclear and mitochondrial-encoded genes. The expression of many nuclear genes is sensitive to the functional state of mitochondria. Mitochondria containing mutated mtDNA populations have been shown to withstand stress to a high level of tolerance. The prolonged survival of cells that are devoid of mitochondrial DNA is further evidence for the existence of this adaptive response. Yeast cells lacking the mitochondrial genome configure their pattern of nuclear gene expression to accommodate changes in nutrient availability and in response to different stress conditions. Little is known about the signalling pathways that mediate the adaptive response in mammalian cells with mitochondrial dysfunction. To gain a global perspective on adaptive responses in cells with mitochondrial dysfunction, we used Affymetrix microarray technology to compare the whole genome transcription profile of Namalwa cells (rho(+)) to that of a Namalwa cell depleted of mitochondrial DNA (rho degrees ). Our results demonstrate that rho degrees Namalwa induce genes encoding hypoxia target proteins, mitochondrial ribosomal proteins, transport channels, tRNA synthetases and enzymes of the glycolytic pathway and the fatty oxidation pathway suggesting that hypoxia and the ensuing cellular adaptations could play an important adaptive role in cells with mitochondrial dysfunction.

Proline hydroxylation and gene expression

Hypoxia-inducible factor (HIF) is a master transcriptional regulator of hypoxia-inducible genes and consists of a labile alpha subunit (such as HIF1alpha) and a stable beta subunit (such as HIF1beta or ARNT). In the presence of oxygen, HIFalpha family members are hydroxylated on one of two conserved prolyl residues by members of the egg-laying-defective nine (EGLN) family. Prolyl hydroxylation generates a binding site for a ubiquitin ligase complex containing the von Hippel-Lindau (VHL) tumor suppressor protein, which results in HIFalpha destruction. In addition, the HIFalpha transcriptional activation function is modulated further by asparagine hydroxylation by FIH (factor-inhibiting HIF), which affects recruitment of the coactivators p300 and CBP. These findings provide new mechanistic insights into oxygen sensing by metazoans and are the first examples of protein hydroxylation being used in intracellular signaling. The existence of three human EGLN family members, as well as other putative hydroxylases, raises the possibility that this signal is used in other contexts by other proteins.

Negative and positive regulation of HIF-1: a complex network

Hypoxia inducible factor-1 (HIF-1) is as a key transcriptional mediator of the hypoxic response in eukaryotic cells, regulating the expression of a myriad of genes involved in oxygen transport, glucose uptake and glycolysis and angiogenesis. Deregulation of HIF-1 activity occurs in many human cancers, usually at the level of the HIF-1alpha subunit. HIF-1 is regulated by a variety of mechanisms including transcription, translation post-translational modification, protein-protein interaction and degradation. Our understanding of the key signalling pathways that regulate HIF-1 has significantly progressed in recent years and has highlighted the potential for targeting the HIF-1 pathway as a basis for the development of new cancer therapies.

Oxygen sensing requires mitochondrial ROS but not oxidative phosphorylation

Mammalian cells detect decreases in oxygen concentrations to activate a variety of responses that help cells adapt to low oxygen levels (hypoxia). One such response is stabilization of the protein HIF-1 alpha, a component of the transcription factor HIF-1. Here we show that a small interfering RNA (siRNA) against the Rieske iron-sulfur protein of mitochondrial complex III prevents the hypoxic stabilization of HIF-1 alpha protein. Fibroblasts from a patient with Leigh's syndrome, which display residual levels of electron transport activity and are incompetent in oxidative phosphorylation, stabilize HIF-1 alpha during hypoxia. The expression of glutathione peroxidase or catalase, but not superoxide dismutase 1 or 2, prevents the hypoxic stabilization of HIF-1 alpha. These findings provide genetic evidence that oxygen sensing is dependent on mitochondrial-generated reactive oxygen species (ROS) but independent of oxidative phosphorylation.

Mitochondrial dysfunction resulting from loss of cytochrome c impairs cellular oxygen sensing and hypoxic HIF-alpha activation

While cellular responses to low oxygen (O(2)) or hypoxia have been studied extensively, the precise identity of mammalian cellular O(2) sensors remains controversial. Using murine embryonic cells lacking cytochrome c, and therefore mitochondrial activity, we show that mitochondrial reactive oxygen species (mtROS) are essential for proper O(2) sensing and subsequent HIF-1 alpha and HIF-2 alpha stabilization at 1.5% O(2). In the absence of this signal, HIF-alpha subunits continue to be degraded. Furthermore, exogenous treatment with H(2)O(2) or severe O(2) deprivation is sufficient to stabilize HIF-alpha even in the absence of cytochrome c and functional mitochondria. These results provide genetic evidence indicating that mtROS act upstream of prolyl hydroxylases in regulating HIF-1 alpha and HIF-2 alpha in this O(2)-sensing pathway.

A targeted antioxidant reveals the importance of mitochondrial reactive oxygen species in the hypoxic signaling of HIF-1alpha

Exposure to limiting oxygen in cells and tissues induce the stabilization and transcriptional activation of the hypoxia-inducible factor 1 alpha (HIF-1alpha) protein, a key regulator of the hypoxic response. Reactive oxygen species (ROS) generation has been implicated in the stabilization of HIF-1alpha during this response, but this is still a matter of some debate. In this study we utilize a mitochondria-targeted antioxidant, mitoubiquinone (MitoQ), and examine its effects on the hypoxic stabilization of HIF-1alpha. Our results show that under conditions of reduced oxygen (3% O(2)), MitoQ ablated the hypoxic induction of ROS generation and destabilized HIF-1alpha protein. This in turn led to an abrogation of HIF-1 transcriptional activity. Normoxic stabilization of HIF-1alpha, on the other hand, was unchanged in the presence of MitoQ suggesting that ROS were not involved. This study strongly suggests that mitochondrial ROS contribute to the hypoxic stabilization of HIF-1alpha.

The Drosophila mitochondrial ribosomal protein mRpL12 is required for Cyclin D/Cdk4-driven growth

The Drosophila melanogaster cyclin-dependent protein kinase complex CycD/Cdk4 stimulates both cell cycle progression and cell growth (accumulation of mass). CycD/Cdk4 promotes cell cycle progression via the well-characterized RBF/E2F pathway, but our understanding of how growth is stimulated is still limited. To identify growth regulatory targets of CycD/Cdk4, we performed a loss-of-function screen for modifiers of CycD/Cdk4-induced overgrowth of the Drosophila eye. One mutation that suppressed CycD/Cdk4 was in a gene encoding the mitochondrial ribosomal protein, mRpL12. We show here that mRpL12 is required for CycD/Cdk4-induced cell growth. Cells homozygous mutant for mRpL12 have reduced mitochondrial activity, and exhibit growth defects that are very similar to those of cdk4 null cells. CycD/Cdk4 stimulates mitochondrial activity, and this is mRpL12 dependent. Hif-1 prolyl hydroxylase (Hph), another effector of CycD/Cdk4, regulates growth and is required for inhibition of the hypoxia-inducible transcription factor 1 (Hif-1). Both functions depend on mRpL12 dosage, suggesting that CycD/Cdk4, mRpL12 and Hph function together in a common pathway that controls cell growth via affecting mitochondrial activity.

Evidence of increased microvessel density and activation of the hypoxia pathway in tumours from the hereditary leiomyomatosis and renal cell cancer syndrome

The Mendelian tumour syndromes hereditary leiomyomatosis and renal cell cancer (HLRCC) and hereditary paragangliomatosis with phaeochromocytomas (HPGL) result from mutations in nuclear genes (FH and SDHB/C/D, respectively) that encode Krebs cycle enzymes. HPGL tumours are highly vascular and there is evidence that inactivation of SDH leads to activation of the hypoxia/angiogenesis pathway. In contrast, uterine leiomyomas are not generally regarded as particularly vascular lesions. In order to test the possibility that activation of the hypoxia/angiogenesis pathway contributes to tumourigenesis in HLRCC, increased vascularity and hypoxia pathway activation were searched for in HLRCC tumours. Microvessel density was markedly higher in uterine leiomyomas from HLRCC than in the surrounding myometrium; it was notable that sporadic uterine leiomyomas were actually less vascular than normal myometrium. In HLRCC tumours, there was increased expression of transcripts from the hypoxia-responsive genes vascular endothelial growth factor (VEGF) and BNIP3; sporadic uterine leiomyomas did not show these changes. All uterine leiomyomas showed decreased expression of thrombospondin 1. Although sporadic and HLRCC uterine leiomyomas appear to have identical morphology, their pathways of tumourigenesis may be fundamentally different. As is the case in HPGL, it is probable that failure of the Krebs cycle in HLRCC tumours causes inappropriate signalling that the cell is in a hypoxic state, leading to angiogenesis and perhaps directly to clonal expansion and tumour growth through some uncharacterized, cell-autonomous effect.

Superoxide dismutase-3 promotes full expression of the EPO response to hypoxia

Extracellular superoxide dismutase (SOD3) is the primary extracellular enzymatic scavenger of superoxide (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(^{{\cdot}}\mathrm{O}_{2}^{-}\) \end{document}). SOD3's expression is highest in the kidney, but its distribution and biologic functions there are unknown. To investigate the function of renal SOD3, we colocalized it with erythropoietin (EPO) to proximal tubules using in situ hybridization and immunohistochemistry. We then exposed wild-type (Wt) and SOD3 knock-out (KO) mice to hypoxia and found a late hematocrit response in the KO strain. EPO mRNA expression was attenuated in KO mice during the first 6 hours of hypoxia preceded at 2 hours by less accumulation of nuclear hypoxia-inducible transcription factor 1 α (HIF-1α) protein. Meanwhile KO mice exposed to hypoxia showed increases in renal mRNA for superoxide-producing nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX4) and early significant increases in glutathione disulfide (GSSG)/glutathione (GSH), a marker of oxidative stress, compared with Wt mice. Plasma nitrite/nitrate and renal 3-nitrotyrosine (3-NTyr), indicating peroxynitrite formation, increased later in hypoxia, and renal endothelial nitric oxide synthase protein induction was similar in both strains. These data show that hypoxic activation of HIF-1α and its target gene EPO in mouse kidney is regulated closely by the oxidant/antioxidant equilibrium involving SOD3, thus identifying renal SOD3 as a regulatory element in the body's innate adaptation to hypoxia.

Heart mitochondria signaling pathways: appraisal of an emerging field

The contribution that mitochondria make to cardiac function extends well beyond their critical bioenergetic role as a supplier of ATP. The organelle plays an integral part in the regulatory and signaling events that occur in response to physiological stresses, including but not limited to myocardial ischemia and reperfusion, hypoxia, oxidative stress, and hormonal and cytokine stimuli. Research on both intact cardiac muscle tissue and cultured cardiomyocytes has just begun to probe the nature and the extent of mitochondrial involvement in interorganelle communication, hypertropic growth, and cell death. This review covers particular aspects of the newly emerging field of mitochondrial medicine offering a critical guide in the assessment of mitochondrial participation at the molecular and biochemical levels in the multiple and interrelated signaling pathways, gauging the effect that mitochondria have as a receiver, integrator, and transmitter of signals on cardiac phenotype. We also discuss future directions that may impact on the treatment of cardiac diseases.

Regulation of hypoxia-inducible factor-1alpha by nitric oxide through mitochondria-dependent and -independent pathways

Nitric oxide (NO) has been reported both to promote and to inhibit the activity of the transcription factor hypoxia-inducible factor-1 (HIF-1). In order to avoid the pitfalls associated with the use of NO donors, we have developed a human cell line (Tet-iNOS 293) that expresses the inducible NO synthase (iNOS) under the control of a tetracycline-inducible promoter. Using this system to generate finely controlled amounts of NO, we have demonstrated that the stability of the alpha-subunit of HIF-1 is regulated by NO through two separate mechanisms, only one of which is dependent on a functional respiratory chain. HIF-1alpha is unstable in cells maintained at 21% O(2), but is progressively stabilized as the O(2) concentration decreases, resulting in augmented HIF-1 DNA-binding activity. High concentrations of NO (>1 microM) stabilize HIF-1alpha at all O(2) concentrations tested. This effect does not involve the respiratory chain, since it is preserved in cells lacking functional mitochondria (rho(0)-cells) and is not reproduced by other inhibitors of the cytochrome c oxidase. By contrast, lower concentrations of NO (<400 nM) cause a rapid decrease in HIF-1alpha stabilized by exposure of the cells to 3% O(2). This effect of NO is dependent on the inhibition of mitochondrial respiration, since it is mimicked by other inhibitors of mitochondrial respiration, including those not acting at cytochrome c oxidase. We suggest that, although stabilization of HIF-1alpha by high concentrations of NO might have implications in pathophysiological processes, the inhibitory effect of lower NO concentrations is likely to be of physiological relevance.