Oxygen versus Reactive Oxygen in the Regulation of HIF-1α: The Balance Tips

Mitochondrial metabolism regulation and epigenetics in hypoxia

The complex and dynamic interaction between cellular energy control and gene expression modulation is shown by the intersection between mitochondrial metabolism and epigenetics in hypoxic environments. Poor oxygen delivery to tissues, or hypoxia, is a basic physiological stressor that sets off a series of reactions in cells to adapt and endure oxygen-starved environments. Often called the "powerhouse of the cell," mitochondria are essential to cellular metabolism, especially regarding producing energy through oxidative phosphorylation. The cellular response to hypoxia entails a change in mitochondrial metabolism to improve survival, including epigenetic modifications that control gene expression without altering the underlying genome. By altering the expression of genes involved in angiogenesis, cell survival, and metabolism, these epigenetic modifications help cells adapt to hypoxia. The sophisticated interplay between mitochondrial metabolism and epigenetics in hypoxia is highlighted by several important points, which have been summarized in the current article. Deciphering the relationship between mitochondrial metabolism and epigenetics during hypoxia is essential to understanding the molecular processes that regulate cellular adaptation to reduced oxygen concentrations.

The DUX4-HIF1α Axis in Murine and Human Muscle Cells: A Link More Complex Than Expected

FacioScapuloHumeral muscular Dystrophy (FSHD) is one of the most prevalent inherited muscle disorders and is linked to the inappropriate expression of the DUX4 transcription factor in skeletal muscles. The deregulated molecular network causing FSHD muscle dysfunction and pathology is not well understood. It has been shown that the hypoxia response factor HIF1α is critically disturbed in FSHD and has a major role in DUX4-induced cell death. In this study, we further explored the relationship between DUX4 and HIF1α. We found that the DUX4 and HIF1α link differed according to the stage of myogenic differentiation and was conserved between human and mouse muscle. Furthermore, we found that HIF1α knockdown in a mouse model of DUX4 local expression exacerbated DUX4-mediated muscle fibrosis. Our data indicate that the suggested role of HIF1α in DUX4 toxicity is complex and that targeting HIF1α might be challenging in the context of FSHD therapeutic approaches.

Aerobic Exercise Improves Radiation Therapy Efficacy in Non-Small Cell Lung Cancer: Preclinical Study Using a Xenograft Mouse Model

The "oxygen effect" improves radiation efficacy; thus, tumor cell oxygen concentration is a crucial factor for improving lung cancer treatment. In the current study, we aimed to identify aerobic exercise-induced changes in oxygen concentrations in non-small cell lung cancer (NSCLC) cells. To this end, an NSCLC xenograft mouse model was established using human A549 cells. Animals were subsequently subjected to aerobic exercise and radiation three times per week for 2 weeks. Aerobic exercise was performed at a speed of 8.0 m/m for 30 min, and the tumor was irradiated with 2 Gy of 6 MV X-rays (total radiation dose 12 Gy). Combined aerobic exercise and radiation reduced NSCLC cell growth. In addition, the positive effect of aerobic exercise on radiation efficacy through oxygenation of tumor cells was confirmed based on hypoxia-inducible factor-1 and carbonic anhydrase IX expression. Finally, whole-transcriptome analysis revealed the key factors that induce oxygenation in NSCLC cells when aerobic exercise was combined with radiation. Taken together, these results indicate that aerobic exercise improves the effectiveness of radiation in the treatment of NSCLC. This preclinical study provides a basis for the clinical application of aerobic exercise to patients with NSCLC undergoing radiation therapy.

Targeting Hypoxia-Inducible Factor-1 (HIF-1) in Cancer: Emerging Therapeutic Strategies and Pathway Regulation

Hypoxia-inducible factor-1 (HIF-1) is a key regulator for balancing oxygen in the cells. It is a transcription factor that regulates the expression of target genes involved in oxygen homeostasis in response to hypoxia. Recently, research has demonstrated the multiple roles of HIF-1 in the pathophysiology of various diseases, including cancer. It is a crucial mediator of the hypoxic response and regulator of oxygen metabolism, thus contributing to tumor development and progression. Studies showed that the expression of the HIF-1α subunit is significantly upregulated in cancer cells and promotes tumor survival by multiple mechanisms. In addition, HIF-1 has potential contributing roles in cancer progression, including cell division, survival, proliferation, angiogenesis, and metastasis. Moreover, HIF-1 has a role in regulating cellular metabolic pathways, particularly the anaerobic metabolism of glucose. Given its significant and potential roles in cancer development and progression, it has been an intriguing therapeutic target for cancer research. Several compounds targeting HIF-1-associated processes are now being used to treat different types of cancer. This review outlines emerging therapeutic strategies that target HIF-1 as well as the relevance and regulation of the HIF-1 pathways in cancer. Moreover, it addresses the employment of nanotechnology in developing these promising strategies.

Hypoxia-Mediated Upregulation of Xanthine Oxidoreductase Causes DNA Damage of Colonic Epithelial Cells in Colitis

Xanthine oxidoreductase (XOR) serves as the primary source of hydrogen peroxide and superoxide anions in the intestinal mucosa. However, its specific contribution to the progression of colonic disease remains unclear. In this study, we investigated the role of XOR in ulcerative colitis (UC) and attempted to identify the underlying mechanisms. We used the dextran sulfate sodium (DSS)-induced mouse model to mimic UC and observed that XOR inhibitors, allopurinol and diphenyleneiodonium sulfate (DPI), significantly alleviated UC in mice. In addition, treatment with cobalt chloride (CoCl2) and 1% O2 increased the expression of XOR and induced DNA oxidative damage in colonic epithelial cells. Furthermore, we identified that XOR accumulation in the nucleus may directly cause DNA oxidative damage and regulates HIF1α protein levels. In addition, allopurinol effectively protected colon epithelial cells from CoCl2-induced DNA damage. Altogether, our data provided evidence that XOR could induce DNA damage under hypoxic conditions, indicating a significant role of XOR in the initiation and early development of colitis-associated colorectal cancer (CAC).

Hypoxia and the Hypoxia-Inducible Factors in Lymphocyte Differentiation and Function

Molecular oxygen doubles as a biomolecular building block and an element required for energy generation and metabolism in aerobic organisms. A variety of systems in mammalian cells sense the concentration of oxygen to which they are exposed and are tuned to the range present in our blood and tissues. The ability to respond to insufficient O2 in tissues is central to regulation of erythroid lineage cells, but challenges also are posed for immune cells by a need to adjust to very different oxygen concentrations. Hypoxia-inducible factors (HIFs) provide a major means of making such adjustments. For adaptive immunity, lymphoid lineages are initially defined in bone marrow niches; T lineage cells arise in the thymus, and B cells complete maturation in the spleen. Lymphocytes move from these first stops into microenvironments (bloodstream, lymphatics, and tissues) with distinct oxygenation in each. Herein, evidence pertaining to functions of the HIF transcription factors (TFs) in lymphocyte differentiation and function is reviewed. For the CD4+ and CD8+ subsets of T cells, the case is very strong that hypoxia and HIFs regulate important differentiation events and functions after the naïve lymphocytes emerge from the thymus. In the B lineage, the data indicate that HIF1 contributes to a balanced regulation of B-cell fates after antigen (Ag) activation during immunity. A model synthesized from the aggregate literature is that HIF in lymphocytes generally serves to modulate function in a manner dependent on the molecular context framed by other TFs and signals.

The Mechanisms of Action of Hyperbaric Oxygen in Restoring Host Homeostasis during Sepsis

The perception of sepsis has shifted over time; however, it remains a leading cause of death worldwide. Sepsis is now recognized as an imbalance in host cellular functions triggered by the invading pathogens, both related to immune cells, endothelial function, glucose and oxygen metabolism, tissue repair and restoration. Many of these key mechanisms in sepsis are also targets of hyperbaric oxygen (HBO2) treatment. HBO2 treatment has been shown to improve survival in clinical studies on patients with necrotizing soft tissue infections as well as experimental sepsis models. High tissue oxygen tension during HBO2 treatment may affect oxidative phosphorylation in mitochondria. Oxygen is converted to energy, and, as a natural byproduct, reactive oxygen species are produced. Reactive oxygen species can act as mediators, and both these and the HBO2-mediated increase in oxygen supply have the potential to influence the cellular processes involved in sepsis. The pathophysiology of sepsis can be explained comprehensively through resistance and tolerance to infection. We argue that HBO2 treatment may protect the host from collateral tissue damage during resistance by reducing neutrophil extracellular traps, inhibiting neutrophil adhesion to vascular endothelium, reducing proinflammatory cytokines, and halting the Warburg effect, while also assisting the host in tolerance to infection by reducing iron-mediated injury and upregulating anti-inflammatory measures. Finally, we show how inflammation and oxygen-sensing pathways are connected on the cellular level in a self-reinforcing and detrimental manner in inflammatory conditions, and with support from a substantial body of studies from the literature, we conclude by demonstrating that HBO2 treatment can intervene to maintain homeostasis.

HIF-1 signaling: an emerging mechanism for mitochondrial dynamics

A growing emphasis has been paid to the function of mitochondria in tumors, neurodegenerative disorders (NDs), and cardiovascular diseases. Mitochondria are oxygen-sensitive organelles whose function depends on their structural basis. Mitochondrial dynamics are critical in regulating the structure. Mitochondrial dynamics include fission, fusion, motility, cristae remodeling, and mitophagy. These processes could alter mitochondrial morphology, number, as well as distribution, to regulate complicated cellular signaling processes like metabolism. Meanwhile, they also could modulate cell proliferation and apoptosis. The initiation and progression of several diseases, such as tumors, NDs, cardiovascular disease, were all interrelated with mitochondrial dynamics. HIF-1 is a nuclear protein presented as heterodimers, and its transcriptional activity is triggered by hypoxia. It plays an important role in numerous physiological processes including the development of cardiovascular system, immune system, and cartilage. Additionally, it could evoke compensatory responses in cells during hypoxia through upstream and downstream signaling networks. Moreover, the alteration of oxygen level is a pivotal factor to promote mitochondrial dynamics and HIF-1 activation. HIF-1α might be a promising target for modulating mitochondrial dynamics to develop therapeutic approaches for NDs, immunological diseases, and other related diseases. Here, we reviewed the research progress of mitochondrial dynamics and the potential regulatory mechanism of HIF-1 in mitochondrial dynamics.

Polydatin Ameliorates High Fructose-Induced Podocyte Oxidative Stress via Suppressing HIF-1α/NOX4 Pathway

Long-term high fructose intake drives oxidative stress, causing glomerular podocyte injury. Polydatin, isolated from Chinese herbal medicine Polygonum cuspidatum, is used as an antioxidant agent that protects kidney function. However, it remains unclear how polydatin prevents oxidative stress-driven podocyte damage. In this study, polydatin attenuated high fructose-induced high expression of HIF-1α, inhibited NOX4-mediated stromal cell-derived factor-1α/C-X-C chemokine receptor type 4 (SDF-1α/CXCR4) axis activation, reduced reactive oxygen species (ROS) production in rat glomeruli and cultured podocytes. As a result, polydatin up-regulated nephrin and podocin, down-regulated transient receptor potential cation channel 6 (TRPC6) in these animal and cell models. Moreover, the data from HIF-1α siRNA transfection showed that high fructose increased NOX4 expression and aggravated SDF-1α/CXCR4 axis activation in an HIF-1α-dependent manner, whereas polydatin down-regulated HIF-1α to inhibit NOX4 and suppressed SDF-1α/CXCR4 axis activation, ameliorating high fructose-induced podocyte oxidative stress and injury. These findings demonstrated that high fructose-driven HIF-1α/NOX4 pathway controlled podocyte oxidative stress damage. Intervention of this disturbance by polydatin could help the development of the therapeutic strategy to combat podocyte damage associated with high fructose diet.

Metabolic reprogramming: A novel therapeutic target in diabetic kidney disease

Diabetic kidney disease (DKD) is one of the most common microvascular complications of diabetes mellitus. However, the pathological mechanisms contributing to DKD are multifactorial and poorly understood. Diabetes is characterized by metabolic disorders that can bring about a series of changes in energy metabolism. As the most energy-consuming organs secondary only to the heart, the kidneys must maintain energy homeostasis. Aberrations in energy metabolism can lead to cellular dysfunction or even death. Metabolic reprogramming, a shift from mitochondrial oxidative phosphorylation to glycolysis and its side branches, is thought to play a critical role in the development and progression of DKD. This review focuses on the current knowledge about metabolic reprogramming and the role it plays in DKD development. The underlying etiologies, pathological damages in the involved cells, and potential molecular regulators of metabolic alterations are also discussed. Understanding the role of metabolic reprogramming in DKD may provide novel therapeutic approaches to delay its progression to end-stage renal disease.

Berberine-mediated REDD1 down-regulation ameliorates senescence of retinal pigment epithelium by interrupting the ROS-DDR positive feedback loop

BACKGROUND

Accumulation of age-associated senescent cells accompanied with increased reactive oxygen species (ROS) and inflammatory factors contributes to the progression of age-related macular degeneration (AMD), the main cause of blindness in the elderly. Berberine (BBR) has shown efficacy in the treatment of age-related diseases including diabetes and obesity by decreasing ROS. However, the pharmacological effect of BBR on alleviating retinal aging remains largely unknown.

PURPOSE

Our study aimed to investigate the pharmacological effect of BBR as an anti-aging agent in retinal aging and its further molecular mechanisms.

METHODS

D-galactose (DG)-induced ARPE-19 cell senescence and retinal aging were employed to evaluate the anti-aging effect of BBR in vivo and in vitro. The siRNA transfection, Western-Blot analyses, SA-β-Gal assay and immunofluorescence were performed to investigate the potential mechanisms of BBR on anti-aging of RPE.

RESULTS

In RPE-choroid of both natural aged and DG-induced accelerated aged mice, oxidative stress was increased along with the up-regulation of p21 expression, which was ameliorated by BBR treatment. BBR down-regulated the expression of REDD1 to decrease intracellular ROS content, attenuating DG-induced senescence in vitro and in vivo. Furthermore, p53 instead of HIF-1α was identified as the transcriptional regulator of REDD1 in DG-induced premature senescence. Importantly, NAC and BBR reversed the expression of p53 and the content of 8-OHdG, indicating that the positive feedback loop of ROS-DNA damage response (DDR) was formed, and BBR interrupted this feedback loop to alleviate DG-induced premature senescence by reducing REDD1 expression. In addition, BBR restored DG-damaged autophagy flux by up-regulating TFEB-mediated lysosomal biosynthesis by inhibiting REDD1 expression, thereby attenuating cellular senescence.

CONCLUSION

BBR down-regulates REDD1 expression to interrupt the ROS-DDR positive feedback loop and restore autophagic flux, thereby reducing premature senescence of RPE. Our findings elucidate the promising effects of REDD1 on cellular senescence and the great potential of BBR as a therapeutic approach.

The role of the tumor microenvironment in colorectal cancer and the potential therapeutic approaches

BACKGROUND

Colorectal cancer (CRC) with a high prevalence is recognized as the fourth most common cause of cancer-related death globally. Over the past decade, there has been growing interest in the network of tumor cells, stromal cells, immune cells, blood vessel cells, and fibroblasts that comprise the tumor microenvironment (TME) to identify new therapeutic interventions.

METHODS

Databases, such as Google Scholar, PubMed, and Scopus, were searched to provide an overview of the recent research progress related to targeting the TME as a novel therapeutic approach.

RESULTS

Tumor microenvironment as a result of the cross talk between these cells may result in either advantages or disadvantages in tumor development and metastasis, affecting the signals and responses from the surrounding cells. Whilst chemotherapy has led to an improvement in CRC patients' survival, the metastatic aspect of the disease remains difficult to avoid.

CONCLUSIONS

The present review emphasizes the structure and function of the TME, alterations in the TME, its role in the incidence and progression of CRC, the effects on tumor development and metastasis, and also the potential of its alterations as therapeutic targets. It should be noted that providing novel studies in this field of research might help us to achieve practical therapeutic strategies based on their interaction.

Looking for the Holy Grail—Drug Candidates for Glioblastoma Multiforme Chemotherapy

Glioblastoma multiforme (GBM) is the deadliest and the most heterogeneous brain cancer. The median survival time of GBM patients is approximately 8 to 15 months after initial diagnosis. GBM development is determined by numerous signaling pathways and is considered one of the most challenging and complicated-to-treat cancer types. Standard GBM therapy consist of surgery followed by radiotherapy or chemotherapy, and combined treatment. Current standard of care (SOC) does not offer a significant chance for GBM patients to combat cancer, and the selection of available drugs is limited. For almost 20 years, there has been only one drug, Temozolomide (TMZ), approved as a first-line GBM treatment. Due to the limited efficacy of TMZ and the high rate of resistant patients, the implementation of new chemotherapeutics is highly desired. However, due to the unique properties of GBM, many challenges still need to be overcome before reaching a ‘breakthrough’. This review article describes the most recent compounds introduced into clinical trials as drug candidates for GBM chemotherapy.

Dimethyloxalylglycine (DMOG), a Hypoxia Mimetic Agent, Does Not Replicate a Rat Pheochromocytoma (PC12) Cell Biological Response to Reduced Oxygen Culture

Cells respond to reduced oxygen availability predominately by activation of the hypoxia-inducible factor (HIF) pathway. HIF activation upregulates hundreds of genes that help cells survive in the reduced oxygen environment. The aim of this study is to determine whether chemical-induced HIF accumulation mimics all aspects of the hypoxic response of cells. We compared the effects of dimethyloxalylglycine (DMOG) (a HIF stabiliser) on PC12 cells cultured in air oxygen (20.9% O_2, AO) with those cultured in either intermittent 20.9% O₂ to 2% O₂ (IH) or constant 2% O₂ (CN). Cell viability, cell cycle, HIF accumulation, reactive oxygen species (ROS) formation, mitochondrial function and differentiation were used to characterise the PC12 cells and evaluate the impact of DMOG. IH and CN culture reduced the increase in cell numbers after 72 and 96 h and MTT activity after 48 h compared to AO culture. Further, DMOG supplementation in AO induced a dose-dependent reduction in the increase in PC12 cell numbers and MTT activity. IH-cultured PC12 cells displayed increased and sustained HIF-1 expression over 96 h. This was accompanied by increased ROS and mitochondrial burden. PC12 cells in CN displayed little changes in HIF-1 expression or ROS levels. DMOG (0.1 mM) supplementation resulted in an IH-like HIF-1 profile. The mitochondrial burden and action potential of DMOG-supplemented PC12 cells did not mirror those seen in other conditions. DMOG significantly increased S phase cell populations after 72 and 96 h. No significant effect on PC12 cell differentiation was noted with IH and CN culture without induction by nerve growth factor (NGF), while DMOG significantly increased PC12 cell differentiation with and without NGF. In conclusion, DMOG and reduced oxygen levels stabilise HIF and affect mitochondrial activity and cell behaviour. However, DMOG does not provide an accurate replication of the reduced oxygen environments.

Characterization of H2O2-Induced Alterations in Global Transcription of mRNA and lncRNA

Hydrogen peroxide (H₂O₂) is an important reactive oxygen species that plays a major role in redox signaling. Although H₂O₂ is known to regulate gene expression and affect multiple cellular processes, the characteristics and mechanisms of such transcriptional regulation remain to be defined. In this study, we utilized transcriptome sequencing to determine the global changes of mRNA and lncRNA transcripts induced by H₂O₂ in human pancreatic normal epithelial (HPNE) and pancreatic cancer (PANC-1) cells. Promoter analysis using PROMO and TRRUST revealed that mRNAs and lncRNAs largely shared the same sets of transcription factors in response to ROS stress. Interestingly, promoters of the upregulated genes were similar to those of the downregulated transcripts, suggesting that the H₂O₂-responding promoters are conserved but they alone do not determine the levels of transcriptional outputs. We also found that H₂O₂ induced significant changes in molecules involved in the pathways of RNA metabolism, processing, and transport. Detailed analyses further revealed a significant difference between pancreatic cancer and noncancer cells in their response to H₂O₂ stress, especially in the transcription of genes involved in cell-cycle regulation and DNA repair. Our study provides new insights into RNA transcriptional regulation upon ROS stress in cancer and normal cells.

Ultrasmall Barium Titanate Nanoparticles for Highly Efficient Hypoxic Tumor Therapy via Ultrasound Triggered Piezocatalysis and Water Splitting

Hypoxia in a solid tumor microenvironment (TME) can lead to the overexpression of hypoxia-inducible factor-1α (HIF-1α), which correlates to tumor metastasis. Reactive oxygen species (ROS) induced tumor cell apoptosis is becoming a promising method in tumor treatment. Currently, the ROS generating systems, e.g., photodynamic treatment and sonodynamic treatment, highly depend on oxygen (O₂) in the tumor microenvironment (TME). However, the level of O₂ in TME is too low to produce enough ROS. Herein, we developed an ultrasmall DSPE-PEG₂₀₀₀ coated barium titanate nanoparticle (P-BTO) for tumor treatment based on ultrasound triggered piezocatalysis and water splitting. Interestingly, irradiated by ultrasound, the surface of ultasmall P-BTO nanoparticles produced imbalance charges, which induced a cascade of redox reaction processes to simultaneously generate ROS and O₂, the latter one was hardly generated in large-sized barium titanate nanoparticles. The as-synthesized P-BTO reached the highest accumulation in the tumor site at 4 h after intravenous injection. The results showed that the produced O₂ significantly alleviated the hypoxia of TME to down-regulate the expression of HIF-1α, and the produced ROS can efficiently kill tumor cells. Moreover, the tumor metastasis was also inhibited, providing a different way to treat triple-negative breast cancer, which was easily metastatic and lacked effective treatments in the clinic.

Cyclic Hypoxia: An Update on Its Characteristics, Methods to Measure It and Biological Implications in Cancer

Regions of hypoxia occur in most if not all solid cancers. Although the presence of tumor hypoxia is a common occurrence, the levels of hypoxia and proportion of the tumor that are hypoxic vary significantly. Importantly, even within tumors, oxygen levels fluctuate due to changes in red blood cell flux, vascular remodeling and thermoregulation. Together, this leads to cyclic or intermittent hypoxia. Tumor hypoxia predicts for poor patient outcome, in part due to increased resistance to all standard therapies. However, it is less clear how cyclic hypoxia impacts therapy response. Here, we discuss the causes of cyclic hypoxia and, importantly, which imaging modalities are best suited to detecting cyclic vs. chronic hypoxia. In addition, we provide a comparison of the biological response to chronic and cyclic hypoxia, including how the levels of reactive oxygen species and HIF-1 are likely impacted. Together, we highlight the importance of remembering that tumor hypoxia is not a static condition and that the fluctuations in oxygen levels have significant biological consequences.

ROS-associated immune response and metabolism: a mechanistic approach with implication of various diseases

The immune system plays a pivotal role in maintaining the defense mechanism against external agents and also internal danger signals. Metabolic programming of immune cells is required for functioning of different subsets of immune cells under different physiological conditions. The field of immunometabolism has gained ground because of its immense importance in coordination and balance of immune responses. Metabolism is very much related with production of energy and certain by-products. Reactive oxygen species (ROS) are generated as one of the by-products of various metabolic pathways. The amount, localization of ROS and redox status determine transcription of genes, and also influences the metabolism of immune cells. This review discusses ROS, metabolism of immune cells at different cellular conditions and sheds some light on how ROS might regulate immunometabolism.

Oxygen battle in the gut: Hypoxia and hypoxia-inducible factors in metabolic and inflammatory responses in the intestine

The gastrointestinal tract is a highly proliferative and regenerative tissue. The intestine also harbors a large and diverse microbial population collectively called the gut microbiome (microbiota). The microbiome-intestine cross-talk includes a dynamic exchange of gaseous signaling mediators generated by bacterial and intestinal metabolisms. Moreover, the microbiome initiates and maintains the hypoxic environment of the intestine that is critical for nutrient absorption, intestinal barrier function, and innate and adaptive immune responses in the mucosal cells of the intestine. The response to hypoxia is mediated by hypoxia-inducible factors (HIFs). In hypoxic conditions, the HIF activation regulates the expression of a cohort of genes that promote adaptation to hypoxia. Physiologically, HIF-dependent genes contribute to the aforementioned maintenance of epithelial barrier function, nutrient absorption, and immune regulation. However, chronic HIF activation exacerbates disease conditions, leading to intestinal injury, inflammation, and colorectal cancer. In this review, we aim to outline the major roles of physiological and pathological hypoxic conditions in the maintenance of intestinal homeostasis and in the onset and progression of disease with a major focus on understanding the complex pathophysiology of the intestine.

Metabolic interaction of hydrogen peroxide and hypoxia in zebrafish fibroblasts

Cells require oxygen for aerobic metabolism, which may also result in the production of reactive oxygen species (ROS) as a by-product. Under low oxygen conditions, ROS formation has been reported to either increase or decrease. We addressed this physiological response for the first time in zebrafish embryonic fibroblasts (Z3) and used a hydrogen peroxide (H₂O₂)-specific fluorescent protein (roGFP2-Orp1) either targeted to the mitochondria or expressed in the cytosol. Microfluidic live-cell imaging measurements showed that oxygen deprivation in Z3 cells results in decreased or stable H₂O₂ levels within the mitochondria or the cytosol, respectively, and that the reductive shift recorded in the mitochondrial matrix is directly dependent on oxygen concentration. The response was accompanied by a transient increase in extracellular acidification rate (ECAR) and a lower cellular reducing potential as assessed by the viability stain alamarBlue. Complex I and III inhibition with Rotenone and Antimycin A led to H₂O₂ production under normoxia but these inhibitors were not able to avert the reductive shift under hypoxia. Only by system-wide inhibition of flavin-containing oxidases with Diphenyleneiodonium (DPI) were we able to decrease the reductive shift, while selective inhibition of NADPH oxidases with the inhibitor Apocynin had no effect on the hypoxia response. Since DPI also led to a strong increase in ECAR we found that, in order to keep the cytosolic H₂O₂ levels stable, glycolytic metabolism was of fundamental importance. According to our experiments with the glucose-6-phosphate dehydrogenase inhibitor 6-Aminonicotinamide, this was attributable to the pentose phosphate pathway producing reducing equivalents required for ROS degradation.

Hypoxia Inducible Factor-1α: The Curator of Gut Homeostasis

The human gut microbiome is a stratified and resilient ecosystem co-inhabited by a diverse and dynamic pool of microorganisms. Microbial selection, establishment, and colonization are modulated through a complex molecular network of host-microbial interactions. These molecular bioprocesses ensure the taxonomic composition of the mature human gut microbiome. The human gut microbiome plays a vital role in host health; otherwise, any microbial dysbiosis could predispose to the onset of physiological and metabolic disorder/s. Focussed research are being carried out to identify key molecular agents defining gut homeostasis. These molecules hold the potential to develop effective therapeutic solutions for microbial dysbiosis-associated human disorders. Of these, Hypoxia-inducible factor-1α (HIF-1α) is a central player in host-microbial crosstalk to maintain gut homeostasis. Human gut microbial metabolites regulate its cellular stability, which in turn regulates various cellular processes required for the stable gut microbiome. In the present review, an effort has been made to summarize the key role of HIF-1α to maintain gut homeostasis.

Krebs Cycle Reborn in Macrophage Immunometabolism

A striking change has happened in the field of immunology whereby specific metabolic processes have been shown to be a critical determinant of immune cell activation. Multiple immune receptor types rewire metabolic pathways as a key part of how they promote effector functions. Perhaps surprisingly for immunologists, the Krebs cycle has emerged as the central immunometabolic hub of the macrophage. During proinflammatory macrophage activation, there is an accumulation of the Krebs cycle intermediates succinate and citrate, and the Krebs cycle–derived metabolite itaconate. These metabolites have distinct nonmetabolic signaling roles that influence inflammatory gene expression. A key bioenergetic target for the Krebs cycle, the electron transport chain, also becomes altered, generating reactive oxygen species from Complexes I and III. Similarly, alternatively activated macrophages require α-ketoglutarate-dependent epigenetic reprogramming to elicit anti-inflammatory gene expression. In this review, we discuss these advances and speculate on the possibility of targeting these events therapeutically for inflammatory diseases.

Prenatal hypoxia increases susceptibility to kidney injury

Prenatal hypoxia is a gestational stressor that can result in developmental abnormalities or physiological reprogramming, and often decreases cellular capacity against secondary stress. When a developing fetus is exposed to hypoxia, blood flow is preferentially redirected to vital organs including the brain and heart over other organs including the kidney. Hypoxia-induced injury can lead to structural malformations in the kidney; however, even in the absence of structural lesions, hypoxia can physiologically reprogram the kidney leading to decreased function or increased susceptibility to injury. Our investigation in mice reveals that while prenatal hypoxia does not affect normal development of the kidneys, it primes the kidneys to have an increased susceptibility to kidney injury later in life. We found that our model does not develop structural abnormalities when prenatally exposed to modest 12% O2 as evident by normal histological characterization and gene expression analysis. Further, adult renal structure and function is comparable to mice exposed to ambient oxygen throughout nephrogenesis. However, after induction of kidney injury with a nephrotoxin (cisplatin), the offspring of mice housed in hypoxia exhibit significantly reduced renal function and proximal tubule damage following injury. We conclude that exposure to prenatal hypoxia in utero physiologically reprograms the kidneys leading to increased susceptibility to injury later in life.

The role of myoglobin in epithelial cancers: Insights from transcriptomics

The muscle-associated respiratory protein myoglobin (MB) is expressed in multiple types of cancer, including breast and prostate tumors. In Kaplan-Meier analyses of the two tumor types, MB positivity is associated with favorable prognoses. Despite its well-characterized function in myocytes, the role of MB in cancer remains unclear. To study the impact of endogenous MB expression, small interfering RNA MB-knockdown cells were engineered using breast, prostate and colon cancer cell lines (MDA-MB468, LNCaP, DLD-1), and their transcriptomes were investigated using RNA-Seq at different oxygen levels. In MB-positive cells, increased expression of glycolytic genes was observed, which was possibly mediated by a higher activity of hypoxia-inducible factor 1α. In addition, the results of the gene set enrichment analysis suggested that MB contributed to fatty acid transport and turnover. MB-positive, wild-type-p53 LNCaP cells also exhibited increased expression of p53 target genes involved in cell cycle checkpoint control and prevention of cell migration. MB-positive cells expressing mutant p53 exhibited upregulation of genes associated with prolonged cancer cell viability and motility. Therefore, it was hypothesized that these transcriptomic differences may result from MB-mediated generation of nitric oxide or reactive oxygen species, thus employing established enzymatic activities of the globin. In summary, the transcriptome comparisons identified potential molecular functions of MB in carcinogenesis by highlighting the interaction of MB with key metabolic and regulatory processes.

MicroRNAs in Tumor Cell Metabolism: Roles and Therapeutic Opportunities

Dysregulated metabolism is a common feature of cancer cells and is considered a hallmark of cancer. Altered tumor-metabolism confers an adaptive advantage to cancer cells to fulfill the high energetic requirements for the maintenance of high proliferation rates, similarly, reprogramming metabolism confers the ability to grow at low oxygen concentrations and to use alternative carbon sources. These phenomena result from the dysregulated expression of diverse genes, including those encoding microRNAs (miRNAs) which are involved in several metabolic and tumorigenic pathways through its post-transcriptional-regulatory activity. Further, the identification of key actionable altered miRNA has allowed to propose novel targeted therapies to modulated tumor-metabolism. In this review, we discussed the different roles of miRNAs in cancer cell metabolism and novel miRNA-based strategies designed to target the metabolic machinery in human cancer.

Coupling Krebs cycle metabolites to signalling in immunity and cancer

Metabolic reprogramming has become a key focus for both immunologists and cancer biologists, with exciting advances providing new insights into underlying mechanisms of disease. Metabolites traditionally associated with bioenergetics or biosynthesis have been implicated in immunity and malignancy in transformed cells, with a particular focus on intermediates of the mitochondrial pathway known as the Krebs cycle. Among these, the intermediates succinate, fumarate, itaconate, 2-hydroxyglutarate isomers (D-2-hydroxyglutarate and L-2-hydroxyglutarate) and acetyl-CoA now have extensive evidence for "non-metabolic" signalling functions in both physiological immune contexts and in disease contexts, such as the initiation of carcinogenesis. This review will describe how metabolic reprogramming, with emphasis placed on these metabolites, leads to altered immune cell and transformed cell function. The latest findings are informative for new therapeutic approaches which could be transformative for a range of diseases.

Early redox activities modulate Xenopus tail regeneration

Redox state sustained by reactive oxygen species (ROS) is crucial for regeneration; however, the interplay between oxygen (O2), ROS and hypoxia-inducible factors (HIF) remains elusive. Here we observe, using an optic-based probe (optrode), an elevated and steady O2 influx immediately upon amputation. The spatiotemporal O2 influx profile correlates with the regeneration of Xenopus laevis tadpole tails. Inhibition of ROS production but not ROS scavenging decreases O2 influx. Inhibition of HIF-1α impairs regeneration and stabilization of HIF-1α induces regeneration in the refractory period. In the regeneration bud, hypoxia correlates with O2 influx, ROS production, and HIF-1α stabilization that modulate regeneration. Further analyses reveal that heat shock protein 90 is a putative downstream target of HIF-1α while electric current reversal is a de facto downstream target of HIF-1α. Collectively, the results show a mechanism for regeneration via the orchestration of O2 influx, ROS production, and HIF-1α stabilization.

Reactive Oxygen Species Formation in the Brain at Different Oxygen Levels: The Role of Hypoxia Inducible Factors

Hypoxia inducible factor (HIF) is the master oxygen sensor within cells and is central to the regulation of cell responses to varying oxygen levels. HIF activation during hypoxia ensures optimum ATP production and cell integrity, and is associated both directly and indirectly with reactive oxygen species (ROS) formation. HIF activation can either reduce ROS formation by suppressing the function of mitochondrial tricarboxylic acid cycle (TCA cycle), or increase ROS formation via NADPH oxidase (NOX), a target gene of HIF pathway. ROS is an unavoidable consequence of aerobic metabolism. In normal conditions (i.e., physioxia), ROS is produced at minimal levels and acts as a signaling molecule subject to the dedicated balance between ROS production and scavenging. Changes in oxygen concentrations affect ROS formation. When ROS levels exceed defense mechanisms, ROS causes oxidative stress. Increased ROS levels can also be a contributing factor to HIF stabilization during hypoxia and reoxygenation. In this review, we systemically review HIF activation and ROS formation in the brain during hypoxia and hypoxia/reoxygenation. We will then explore the literature describing how changes in HIF levels might provide pharmacological targets for effective ischaemic stroke treatment. HIF accumulation in the brain via HIF prolyl hydroxylase (PHD) inhibition is proposed as an effective therapy for ischaemia stroke due to its antioxidation and anti-inflammatory properties in addition to HIF pro-survival signaling. PHD is a key regulator of HIF levels in cells. Pharmacological inhibition of PHD increases HIF levels in normoxia (i.e., at 20.9% O₂ level). Preconditioning with HIF PHD inhibitors show a neuroprotective effect in both in vitro and in vivo ischaemia stroke models, but post-stroke treatment with PHD inhibitors remains debatable. HIF PHD inhibition during reperfusion can reduce ROS formation and activate a number of cellular survival pathways. Given agents targeting individual molecules in the ischaemic cascade (e.g., antioxidants) fail to be translated in the clinic setting, thus far, HIF pathway targeting and thereby impacting entire physiological networks is a promising drug target for reducing the adverse effects of ischaemic stroke.

Hypoxia-inducible factor HIF-1α modulates drugs resistance in colon cancer cells

Introduction: Drug resistance mechanisms may be associated with decreased cell death and its induction may depend on the response to oxidative stress caused by hypoxia. The correlation between hypoxia-inducible factor HIF-1α, the number of reactive oxygen species and their effect on cell survival has not yet been evaluated.Objective: The purpose of this study was to evaluate the effect of HIF-1α activity and reactive oxygen species (ROS) accumulation in apoptosis of colon cancer cells.Materials and methods: HT29 colon cancer cells were treated with CoCl2 or doxorubicin and the activity of HIF-1α was determined by ELISA assay. ROS were determined using fluorescence probe carboxy-H2DFFDA. Apoptosis was assessed by caspase-3 activation analysis, and PUMA and BAX mRNA levels by qRT-PCR. The reduction of the antiapoptotic effect due to hypoxia was attenuated by use of the endonuclease APE-1 (E3330) inhibitor. The endonuclease E3330 APE-1 inhibitor allowed evaluating the effect of ROS generated by doxorubicin and CoCl2 on apoptosis.Results: Chemical hypoxia in combination with doxorubicin is an oxidative stressor in HT29 cells and induces a reduction in the apoptotic process in a time-dependent manner.Conclusion: Resistance to hypoxia and doxorubicin-mediated cell death could be controlled by a mechanism related to the activity of HIF-1α and the amount of reactive oxygen species generated.

Regulatory effect of immunosuppressive agents in mice with renal ischemia reperfusion injury

This study was designed to investigate the regulatory effect of rapamycin in mice with renal ischemia-reperfusion injury. A total of 100 mice were randomly divided into normal control, sham operation, model and experimental groups with 25 rats in each group. Mice in the experimental group were subjected to rapamycin gavage. Mice in each group were sacrificed 24 h after operation. Then, blood, spleen and left kidney were collected. PAS staining was used for semi-quantitative analysis of renal pathological injury. Serum creatinine (SCr) and blood urea nitrogen (BUN) levels were measured. TUNEL method was used to detect cell apoptosis. Flow cytometry was used to detect the percentage of NKT cells. The expression of CXC chemokine ligand 10 (CXCL10), hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) mRNA were detected by RT-qPCR. Semi-quantitative scoring of renal pathological injury showed that pathological injury score of the experimental group was significantly lower than that of the model group (p<0.05). Serum levels of SCr and BUN in the experimental group was compared to those in the model group (p<0.05). The number of apoptotic cells in the experimental group was compared to that of the model group (p<0.05). The percentage of NKT cells in the experimental group was compared to that of the model group (p<0.05). The percentage of NKT cells was significantly higher in the kidney and peripheral blood of the experimental group than that in the model group (p<0.05). The expression levels of HIF-1α and VEGF mRNA in the model and experimental groups were significantly lower in the experimental group than those in the model group (p<0.05). The expression levels of CXCL10 mRNA in the model and experimental groups were significantly higher in the experimental group than those in the model group (p<0.05). The results indicated that rapamycin can significantly upregulate the expression level of CXCL9 and promote the accumulation of NKT cells in kidney from spleen through peripheral blood. Rapamycin can also inhibit the HIF-1α expression level and protect renal ischemia-reperfusion injury.

Need (more than) two to Tango: Multiple tools to adapt to changes in oxygen availability

Oxygen is a fundamental element for the life of a large number of living organisms allowing an efficient energetic utilization of substrates. Organisms relying on oxygen evolved complex structures for oxygen delivery and biochemical machineries dealing with its safe utilization and the ability to overcome the potentially harmful consequences of changes in oxygen availability. On fact, cells composing complex Eukaryotic organisms are set to live within an optimum narrow range of oxygen, quite specific for each cell type. Minute modifications of oxygen availability, either positive or negative, induce the expression of specific genes, the major actors of this responses being the transcription factors HIF and Nrf2 that control the attempt to cope with low oxygen (hypoxia) or to either high oxygen or to an oxygen "overflow," respectively. This review describes the interaction between these two transcription factors and their interaction with the transcription factor NF-κB acting as a pivotal determinant of final cell response. © 2018 BioFactors, 44(3):207-218, 2018.

Kv3.1 and Kv3.4, Are Involved in Cancer Cell Migration and Invasion

Voltage-gated potassium (Kv) channels, including Kv3.1 and Kv3.4, are known as oxygen sensors, and their function in hypoxia has been well investigated. However, the relationship between Kv channels and tumor hypoxia has yet to be investigated. This study demonstrates that Kv3.1 and Kv3.4 are tumor hypoxia-related Kv channels involved in cancer cell migration and invasion. Kv3.1 and Kv3.4 protein expression in A549 and MDA-MB-231 cells increased in a cell density-dependent manner, and the pattern was similar to the expression patterns of hypoxia-inducible factor-1α (HIF-1α) and reactive oxygen species (ROS) according to cell density, whereas Kv3.3 protein expression did not change in A549 cells with an increase in cell density. The Kv3.1 and Kv3.4 blocker blood depressing substance (BDS) did not affect cell proliferation; instead, BDS inhibited cell migration and invasion. We found that BDS inhibited intracellular pH regulation and extracellular signal-regulated kinase (ERK) activation in A549 cells cultured at a high density, potentially resulting in BDS-induced inhibition of cell migration and invasion. Our data suggest that Kv3.1 and Kv3.4 might be new therapeutic targets for cancer metastasis.

Dysfunction in the mitochondrial Fe-S assembly machinery leads to formation of the chemoresistant truncated VDAC1 isoform without HIF-1α activation

Biogenesis of iron-sulfur clusters (ISC) is essential to almost all forms of life and involves complex protein machineries. This process is initiated within the mitochondrial matrix by the ISC assembly machinery. Cohort and case report studies have linked mutations in ISC assembly machinery to severe mitochondrial diseases. The voltage-dependent anion channel (VDAC) located within the mitochondrial outer membrane regulates both cell metabolism and apoptosis. Recently, the C-terminal truncation of the VDAC1 isoform, termed VDAC1-ΔC, has been observed in chemoresistant late-stage tumor cells grown under hypoxic conditions with activation of the hypoxia-response nuclear factor HIF-1α. These cells harbored atypical enlarged mitochondria. Here, we show for the first time that depletion of several proteins of the mitochondrial ISC machinery in normoxia leads to a similar enlarged mitochondria phenotype associated with accumulation of VDAC1-ΔC. This truncated form of VDAC1 accumulates in the absence of HIF-1α and HIF-2α activations and confers cell resistance to drug-induced apoptosis. Furthermore, we show that when hypoxia and siRNA knock-down of the ISC machinery core components are coupled, the cell phenotype is further accentuated, with greater accumulation of VDAC1-ΔC. Interestingly, we show that hypoxia promotes the downregulation of several proteins (ISCU, NFS1, FXN) involved in the early steps of mitochondrial Fe-S cluster biogenesis. Finally, we have identified the mitochondria-associated membrane (MAM) localized Fe-S protein CISD2 as a link between ISC machinery downregulation and accumulation of anti-apoptotic VDAC1-ΔC. Our results are the first to associate dysfunction in Fe-S cluster biogenesis with cleavage of VDAC1, a form which has previously been shown to promote tumor resistance to chemotherapy, and raise new perspectives for targets in cancer therapy.

A redox-sensitive signaling pathway mediates pro-angiogenic effect of chlordecone via estrogen receptor activation

AIM

Chlordecone is able to induce pro-angiogenic effect through an estrogen receptor (ERα) pathway involving NO release and VEGF. The present study aimed to determine the molecular mechanisms by which chlordecone promotes angiogenesis in human endothelial cells.

RESULTS

High but not low concentration of chlordecone increased mitochondrial respiratory capacity and mitochondrial DNA content in endothelial cells. The ROS scavenger MnTMPyP was able to prevent the increase of both VEGF expression and capillary length induced by chlordecone. A significant increase of cytoplasmic O₂^- production was observed after 1 and 4 h incubation of chlordecone, but not after 2 h. The NADPH oxidase inhibitor apocynin or silencing p47phox prevented angiogenesis and tube formation but also the increase in production of O₂^- at 1 h. In addition, apocynin as well silencing p47phox prevented eNOS activation and the NO synthase inhibitor L-NAME inhibited mitochondrial O₂^-production. All the previous effects of chlordecone were prevented by fulvestrant.

CONCLUSION

Our results indicate that an adaptation of the mitochondrial energy metabolism occurs in the chlordecone angiogenic response. Finally, we showed that chlordecone induces endothelial cells angiogenesis by a cross-talk involving NADPH oxidase and mitochondrial O₂^-via a NO sensitive pathways through activation of ERα. These findings propose that a molecular mechanism may partly explain the epidemiological evidence implicating chlordecone as risk factor carcinogenesis.

LPA Induces Metabolic Reprogramming in Ovarian Cancer via a Pseudohypoxic Response

Although hypoxia has been shown to reprogram cancer cells toward glycolytic shift, the identity of extrinsic stimuli that induce metabolic reprogramming independent of hypoxia, especially in ovarian cancer, is largely unknown. In this study, we use patient-derived ovarian cancer cells and high-grade serous ovarian cancer cell lines to demonstrate that lysophosphatidic acid (LPA), a lipid growth factor and GPCR ligand whose levels are substantially increased in ovarian cancer patients, triggers glycolytic shift in ovarian cancer cells. Inhibition of the G protein α-subunit Gαi2 disrupted LPA-stimulated aerobic glycolysis. LPA stimulated a pseudohypoxic response via Rac-mediated activation of NADPH oxidase and generation of reactive oxygen species, resulting in activation of HIF1α. HIF1α in turn induced expression of glucose transporter-1 and the glycolytic enzyme hexokinase-2 (HKII). Treatment of mice bearing ovarian cancer xenografts with an HKII inhibitor, 3-bromopyruvate, attenuated tumor growth and conferred a concomitant survival advantage. These studies reveal a critical role for LPA in metabolic reprogramming of ovarian cancer cells and identify this node as a promising therapeutic target in ovarian cancer.Significance: These findings establish LPA as a potential therapeutic target in ovarian cancer, revealing its role in the activation of HIF1α-mediated metabolic reprogramming in this disease. Cancer Res; 78(8); 1923-34. ©2018 AACR.

Nrf2 is the key to chemotherapy resistance in MCF7 breast cancer cells under hypoxia

Hypoxia leads to reactive oxygen species (ROS) imbalance, which is proposed to associate with drug resistance and oncogenesis. Inhibition of enzymes of antioxidant balancing system in tumor cells was shown to reduce chemoresistance under hypoxia. However, the underlying mechanism remains unknown. The key regulator of antioxidant balancing system is nuclear factor erythroid 2-related factor 2 (NFE2L2, Nrf2). In this study, we showed that hypoxia induced ROS production and increased the Nrf2 activity. Nrf2 activation increased levels of its downstream target antioxidant enzymes, including GCLC and GCLM. The Nrf2-overexpressing also confers chemo-resistant MCF7 cells under normoxia. The in vivo mouse model also demonstrated that the chemical inhibition of Nrf2 can increase cisplatin (CDDP) cytotoxicity. Together, these results showed that Nrf2 serves as a key regulator in chemotherapeutic resistance under hypoxia through ROS-Nrf2-GCLC-GSH pathway. Therefore, targeting Nrf2 can be a potential treatment for hypoxia-induced drug resistance in breast cancer cells.

Role of Mitochondrial Reactive Oxygen Species in the Activation of Cellular Signals, Molecules, and Function

Mitochondria are a major source of intracellular energy and reactive oxygen species in cells, but are also increasingly being recognized as a controller of cell death. Here, we review evidence of signal transduction control by mitochondrial superoxide generation via the nuclear factor-κB (NF-κB) and GATA signaling pathways. We have also reviewed the effects of ROS on the activation of MMP and HIF. There is significant evidence to support the hypothesis that mitochondrial superoxide can initiate signaling pathways following transport into the cytosol. In this study, we provide evidence of TATA signal transductions by mitochondrial superoxide. Oxidative phosphorylation via the electron transfer chain, glycolysis, and generation of superoxide from mitochondria could be important factors in regulating signal transduction, cellular homeostasis, and cell death.

Non-electron transfer chain mitochondrial defects differently regulate HIF-1α degradation and transcription

Mitochondria are the main consumers of molecular O2 in a cell as well as an abundant source of reactive oxygen species (ROS). Both, molecular oxygen and ROS are powerful regulators of the hypoxia-inducible factor-1α-subunit (HIF-α). While a number of mechanisms in the oxygen-dependent HIF-α regulation are quite well known, the view with respect to mitochondria is less clear. Several approaches using pharmacological or genetic tools targeting the mitochondrial electron transport chain (ETC) indicated that ROS, mainly formed at the Rieske cluster of complex III of the ETC, are drivers of HIF-1α activation. However, studies investigating non-ETC located mitochondrial defects and their effects on HIF-1α regulation are scarce, if at all existing. Thus, in the present study we examined three cell lines with non-ETC mitochondrial defects and focused on HIF-1α degradation and transcription, target gene expression, as well as ROS levels. We found that cells lacking the key enzyme 2-enoyl thioester reductase/mitochondrial enoyl-CoA reductase (MECR), and cells lacking manganese superoxide dismutase (MnSOD) showed a reduced induction of HIF-1α under long-term (20h) hypoxia. By contrast, cells lacking the mitochondrial DNA depletion syndrome channel protein Mpv17 displayed enhanced levels of HIF-1α already under normoxic conditions. Further, we show that ROS do not exert a uniform pattern when mediating their effects on HIF-1α, although all mitochondrial defects in the used cell types increased ROS formation. Moreover, all defects caused a different HIF-1α regulation via promoting HIF-1α degradation as well as via changes in HIF-1α transcription. Thereby, MECR- and MnSOD-deficient cells showed a reduction in HIF-1α mRNA levels whereas the Mpv17 lacking cells displayed enhanced HIF-1α mRNA levels under normoxia and hypoxia. Altogether, our study shows for the first time that mitochondrial defects which are not related to the ETC and Krebs cycle contribute differently to HIF-1α regulation by affecting HIF-1α degradation and HIF-1α transcription where ROS play not a major role.

Protein trafficking in the mitochondrial intermembrane space: mechanisms and links to human disease

Mitochondria fulfill a diverse range of functions in cells including oxygen metabolism, homeostasis of inorganic ions and execution of apoptosis. Biogenesis of mitochondria relies on protein import pathways that are ensured by dedicated multiprotein translocase complexes localized in all sub-compartments of these organelles. The key components and pathways involved in protein targeting and assembly have been characterized in great detail over the last three decades. This includes the oxidative folding machinery in the intermembrane space, which contributes to the redox-dependent control of proteostasis. Here, we focus on several components of this system and discuss recent evidence suggesting links to human proteopathy.

The cytosolic 5'-nucleotidase cN-II lowers the adaptability to glucose deprivation in human breast cancer cells

The cytosolic 5'-nucleotidase cN-II is a highly conserved enzyme implicated in nucleotide metabolism. Based on recent observations suggesting additional roles not directly associated to its enzymatic activity, we studied human cancer cell models with basal or decreased cN-II expression. We developed cancer cells with stable inhibition of cN-II expression by transfection of shRNA-coding plasmids, and studied their biology. We show that human breast cancer cells MDA-MB-231 with decreased cN-II expression better adapt to the disappearance of glucose in growth medium under normoxic conditions than cells with a baseline expression level. This is associated with enhanced in vivo growth and a lower content of ROS in cells cultivated in absence of glucose due to more efficient mechanisms of elimination of ROS. Conversely, cells with low cN-II expression are more sensitive to glucose deprivation in hypoxic conditions. Overall, our results show that cN-II regulates the cellular response to glucose deprivation through a mechanism related to ROS metabolism and defence.

Metabolic zonation of the liver: The oxygen gradient revisited

The liver has a multitude of functions which are necessary to maintain whole body homeostasis. This requires that various metabolic pathways can run in parallel in the most efficient manner and that futile cycles are kept to a minimum. To a large extent this is achieved due to a functional specialization of the liver parenchyma known as metabolic zonation which is often lost in liver diseases. Although this phenomenon is known for about 40 years, the underlying regulatory pathways are not yet fully elucidated. The physiologically occurring oxygen gradient was considered to be crucial for the appearance of zonation; however, a number of reports during the last decade indicating that β-catenin signaling, and the hedgehog (Hh) pathway contribute to metabolic zonation may have shifted this view. In the current review we connect these new observations with the concept that the oxygen gradient within the liver acinus is a regulator of zonation. This is underlined by a number of facts showing that the β-catenin and the Hh pathway can be modulated by the hypoxia signaling system and the hypoxia-inducible transcription factors (HIFs). Altogether, we provide a view by which the dynamic interplay between all these pathways can drive liver zonation and thus contribute to its physiological function.

Adipose HIF-1α causes obesity by suppressing brown adipose tissue thermogenesis

Hypoxia-inducible factor-1α (HIF-1α) in adipose tissue is known to promote obesity. We hypothesized that HIF-1α interferes with brown fat thermogenesis, thus decreasing energy expenditure. To test this hypothesis, we compared transgenic mice constitutively expressing HIF-1α in adipose tissues (HIF-1α++) at usual temperature (22 °C), where brown fat is somewhat active, or at thermoneutrality (30 °C), where brown fat is minimally active. HIF-1α++ mice or control litter mates were separated into room temperature (22 °C) or thermoneutrality (30 °C) groups. We assessed weight gain, food intake, calorimetry, activity, and oxygen consumption and transcriptional changes in isolated white and brown adipocytes. At 22 °C, HIF-1α++ mice exhibited accelerated weight gain, cold and glucose intolerance, hyperglycemia, and decreased energy expenditure without changes in food intake or activity. These changes were absent or minimal at thermoneutrality. In brown adipocytes of HIF-1α++ mice, oxygen consumption decreased ~50 % in association with reduced mitochondrial content, uncoupling protein 2, and peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1α). In conclusion, adipose HIF-1α overexpression inhibits thermogenesis and cellular respiration in brown adipose tissue, promoting obesity in the setting of reduced ambient temperature.

KEY MESSAGE

Constitutive HIF-1α activation in adipose tissue promotes weight gain in mice. The weight gain is associated with reduced brown adipose tissue function and oxygen consumption. Reduced oxygen consumption may be mediated by reductions in mitochondria.

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.

HIF1α and metabolic reprogramming in inflammation

HIF1α is a common component of pathways involved in the control of cellular metabolism and has a role in regulating immune cell effector functions. Additionally, HIF1α is critical for the maturation of dendritic cells and for the activation of T cells. HIF1α is induced in LPS-activated macrophages, where it is critically involved in glycolysis and the induction of proinflammatory genes, notably Il1b. The mechanism of LPS-stimulated HIF1α induction involves succinate, which inhibits prolyl hydroxylases (PHDs). Pyruvate kinase M2 (PKM2) is also induced and interacts with and promotes the function of HIF1α. In another critical inflammatory cell type, Th17 cells, HIF1α acts via the retinoic acid-related orphan receptor-γt (RORγt) to drive Th17 differentiation. HIF1α is therefore a key reprogrammer of metabolism in inflammatory cells that promotes inflammatory gene expression.

The ever-expanding role of HIF in tumour and stromal biology

Low oxygen tension (hypoxia) is a hallmark of cancer that influences cancer cell function, but is also an important component of the tumour microenvironment as it alters the extracellular matrix, modulates the tumour immune response and increases angiogenesis. Here we discuss the regulation and role of hypoxia and its key transcriptional mediators, the hypoxia-inducible factor (HIF) family of transcription factors, in the tumour microenvironment and stromal compartments.