Hypoxia-inducible factor 1 in dendritic cells is crucial for the activation of protective regulatory T cells in murine colitis

Inflammatory Gene Panel Guiding the Study of Genetics in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a complex disease that develops through a sequence of molecular events that are still poorly defined. This process is driven by a multitude of context-dependent genes that play different roles based on their environment. The complexity and multi-faceted nature of these genes make it difficult to study the genetic basis of IBD. The goal of this article is to review the key genes in the pathophysiology of IBD and highlight new technology that can be used in further research. This paper examines Nanostring RNA probe technology, which uses tissue analyzed without the use of enzymes, transcription, or amplification. Nanostring offers several panels of genes to test, including an inflammation panel of 234 genes. This article analyzes this panel and reviews the literature for each gene's effect in IBD for use as a framework to review the pathophysiology of the disease. The panel was narrowed to 26 genes with significant evidence of mechanistic potential in IBD, which were then categorized into specific areas of pathogenesis. These include gut barrier breakdown, inappropriate recognition of commensal bacteria, immune cell activation, proinflammatory cytokine release, and subsequent impairment of the anti-inflammatory response. The eventual goal of this paper is the creation of a customized panel of IBD genes that can be used to better understand the genetic mechanism of IBD and aid in the development of future therapies in IBD.

The cellular-centered view of hypoxia tumor microenvironment: Molecular mechanisms and therapeutic interventions

Cancer is a profoundly dynamic, heterogeneous and aggressive systemic ailment, with a coordinated evolution of various types of tumor niches. Hypoxia plays an indispensable role in the tumor micro-ecosystem, drastically enhancing the plasticity of cancer cells, fibroblasts and immune cells and orchestrating intercellular communication. Hypoxia-induced signals, particularly hypoxia-inducible factor-1α (HIF-1α), drive the reprogramming of genetic, transcriptional, and proteomic profiles. This leads to a spectrum of interconnected processes, including augmented survival of cancer cells, evasion of immune surveillance, metabolic reprogramming, remodeling of the extracellular matrix, and the development of resistance to conventional therapeutic modalities like radiotherapy and chemotherapy. Here, we summarize the latest research on the multifaceted effects of hypoxia, where a multitude of cellular and non-cellular elements crosstalk with each other and co-evolve in a synergistic manner. Additionally, we investigate therapeutic approaches targeting hypoxic niche, encompassing hypoxia-activated prodrugs, HIF inhibitors, nanomedicines, and combination therapies. Finally, we discuss some of the issues to be addressed and highlight the potential of emerging technologies in the treatment of cancer.

Cellular succinate metabolism and signaling in inflammation: implications for therapeutic intervention

Succinate, traditionally viewed as a mere intermediate of the tricarboxylic acid (TCA) cycle, has emerged as a critical mediator in inflammation. Disruptions within the TCA cycle lead to an accumulation of succinate in the mitochondrial matrix. This excess succinate subsequently diffuses into the cytosol and is released into the extracellular space. Elevated cytosolic succinate levels stabilize hypoxia-inducible factor-1α by inhibiting prolyl hydroxylases, which enhances inflammatory responses. Notably, succinate also acts extracellularly as a signaling molecule by engaging succinate receptor 1 on immune cells, thus modulating their pro-inflammatory or anti-inflammatory activities. Alterations in succinate levels have been associated with various inflammatory disorders, including rheumatoid arthritis, inflammatory bowel disease, obesity, and atherosclerosis. These associations are primarily due to exaggerated immune cell responses. Given its central role in inflammation, targeting succinate pathways offers promising therapeutic avenues for these diseases. This paper provides an extensive review of succinate's involvement in inflammatory processes and highlights potential targets for future research and therapeutic possibilities development.

Exposure to normobaric hypoxia shapes the acute inflammatory response in human whole blood cells in vivo

Cells of the immune defence, especially leukocytes, often have to perform their function in tissue areas that are characterized by oxygen deficiency, so-called hypoxia. Physiological hypoxia significantly affects leukocyte function and controls the innate and adaptive immune response mainly through transcriptional gene regulation via the hypoxia-inducible factors (HIFs). Multiple pathogens including components of bacteria, such as lipopolysaccharides (LPS) trigger the activation of leukocytes. HIF pathway activation enables immune cells to adapt to both hypoxic environments in physiological and inflammatory settings and modulates immune cell responses through metabolism changes and crosstalk with other immune-relevant signalling pathways. To study the mutual influence of both processes in vivo, we used a human endotoxemia model, challenging participants with an intravenous LPS injection post or prior to a 4-h stay in a hypoxic chamber with normobaric hypoxia of 10.5% oxygen. We analysed changes in gene expression in whole blood cells and determined inflammatory markers to unveil the crosstalk between both processes. Our investigations showed differentially altered gene expression patterns of HIF and target genes upon in vivo treatment with LPS and hypoxia. Further, we found evidence for effects of hypoxic priming upon inflammation in combination with immunomodulatory effects in whole blood cells in vivo. Our work elucidates the complex interplay of hypoxic and inflammatory HIF regulation in human immune cells and offers new perspectives for further clinical research.

The Application of Drugs and Nano-Therapies Targeting Immune Cells in Hypoxic Inflammation

Immune cells are pivotal in the dynamic interplay between hypoxia and inflammation. During hypoxic conditions, HIF-1α, a crucial transcription factor, facilitates the adaptation of immune cells to the hypoxic micro-environment. This adaptation includes regulating immune cell metabolism, significantly impacting inflammation development. Strategies for anti-inflammatory and hypoxic relief have been proposed, aiming to disrupt the hypoxia-inflammation nexus. Research extensively focuses on anti-inflammatory agents and materials that target immune cells. These primarily mitigate hypoxic inflammation by encouraging M2-macrophage polarization, restraining neutrophil proliferation and infiltration, and maintaining Treg/TH17 balance. Additionally, oxygen-releasing nano-materials play a significant role. By alleviating hypoxia and clearing reactive oxygen species (ROS), these nano-materials indirectly influence immune cell functions. This paper delves into the response of immune cells under hypoxic conditions and the resultant effects on inflammation. It provides a comprehensive overview of various therapies targeting specific immune cells for anti-inflammatory purposes and explores nano-materials that either carry or generate oxygen to alleviate anoxic micro-environments.

Crosstalk between hypoxia-inducible factor-1α and short-chain fatty acids in inflammatory bowel disease: key clues toward unraveling the mystery

The human intestinal tract constitutes a complex ecosystem, made up of countless gut microbiota, metabolites, and immune cells, with hypoxia being a fundamental environmental characteristic of this ecology. Under normal physiological conditions, a delicate balance exists among these complex "residents", with disruptions potentially leading to inflammatory bowel disease (IBD). The core pathology of IBD features a disrupted intestinal epithelial barrier, alongside evident immune and microecological disturbances. Central to these interconnected networks is hypoxia-inducible factor-1α (HIF-1α), which is a key regulator in gut cells for adapting to hypoxic conditions and maintaining gut homeostasis. Short-chain fatty acids (SCFAs), as pivotal gut metabolites, serve as vital mediators between the host and microbiota, and significantly influence intestinal ecosystem. Recent years have seen a surge in research on the roles and therapeutic potential of HIF-1α and SCFAs in IBD independently, yet reviews on HIF-1α-mediated SCFAs regulation of IBD under hypoxic conditions are scarce. This article summarizes evidence of the interplay and regulatory relationship between SCFAs and HIF-1α in IBD, pivotal for elucidating the disease's pathogenesis and offering promising therapeutic strategies.

Hypoxia-Induced Signaling in Gut and Liver Pathobiology

Oxygen (O2) is essential for cellular metabolism and biochemical reactions. When the demand for O2 exceeds the supply, hypoxia occurs. Hypoxia-inducible factors (HIFs) are essential to activate adaptive and survival responses following hypoxic stress. In the gut (intestines) and liver, the presence of oxygen gradients or physiologic hypoxia is necessary to maintain normal homeostasis. While physiologic hypoxia is beneficial and aids in normal functions, pathological hypoxia is harmful as it exacerbates inflammatory responses and tissue dysfunction and is a hallmark of many cancers. In this review, we discuss the role of gut and liver hypoxia-induced signaling, primarily focusing on HIFs, in the physiology and pathobiology of gut and liver diseases. Additionally, we examine the function of HIFs in various cell types during gut and liver diseases, beyond intestinal epithelial and hepatocyte HIFs. This review highlights the importance of understanding hypoxia-induced signaling in the pathogenesis of gut and liver diseases and emphasizes the potential of HIFs as therapeutic targets.

Targeting HIF-1α alleviates the inflammatory responses and rebuilds the CD4+ T cell subsets balance in the experimental autoimmune myasthenia gravis inflammation model via regulating cellular and humoral immunity

BACKGROUND

Cells and tissues in an inflammatory state are usually hypoxic. The hypoxic environment can affect the differentiation of immune cells and produce Hypoxia-inducible Factor-1α (HIF-1α). Inflammation is also a major contributor to the development and deterioration of Myasthenia Gravis (MG). There are limited studies on the immunopathological mechanism and targeted therapy associated with MG exacerbated with inflammation. This research aimed to explore whether BAY 87-2243 (HIF-1α inhibitor) ameliorates the symptoms of the Experimental Autoimmune Myasthenia Gravis (EAMG) inflammation model and study its regulatory mechanism on cellular immunity and humoral immunity.

METHODS

We first establish the EAMG inflammation model using Lipopolysaccharide (LPS), BAY 87-2243 was applied to the EAMG inflammation model and its therapeutic effects were evaluated in vivo and in vitro experiments.

RESULTS

The proportion of Treg cells was increased whereas Th1, Th17, and Th1/17 cells were decreased in BAY 87-2243-treated EAMG inflammation model. BAY 87-2243 ameliorated the acetylcholine receptors (AChRs) loss and the complement deposited at the neuromuscular junction of the EAMG inflammation model, declined the levels of IFN-γ, IL-17, and IL-6 in serum, and further attenuated responses in the germinal center and reduced the antibody levels by inhibiting the IL-6-dependent STAT3 axis.

CONCLUSION

BAY 87-2243 restored the balance of CD4+T cell subsets and reduced the production of the pro-inflammatory cytokines, thus acting as both an immune imbalance regulator and anti-inflammatory. The current study suggests that HIF-1α might be a potential target for the treatment of MG exacerbated with inflammation.

Hyperbaric oxygen augments susceptibility to C. difficile infection by impairing gut microbiota ability to stimulate the HIF-1α-IL-22 axis in ILC3

Hyperbaric oxygen (HBO) therapy is a well-established method for improving tissue oxygenation and is typically used for the treatment of various inflammatory conditions, including infectious diseases. However, its effect on the intestinal mucosa, a microenvironment known to be physiologically hypoxic, remains unclear. Here, we demonstrated that daily treatment with hyperbaric oxygen affects gut microbiome composition, worsening antibiotic-induced dysbiosis. Accordingly, HBO-treated mice were more susceptible to Clostridioides difficile infection (CDI), an enteric pathogen highly associated with antibiotic-induced colitis. These observations were closely linked with a decline in the level of microbiota-derived short-chain fatty acids (SCFAs). Butyrate, a SCFA produced primarily by anaerobic microbial species, mitigated HBO-induced susceptibility to CDI and increased epithelial barrier integrity by improving group 3 innate lymphoid cell (ILC3) responses. Mice displaying tissue-specific deletion of HIF-1 in RORγt-positive cells exhibited no protective effect of butyrate during CDI. In contrast, the reinforcement of HIF-1 signaling in RORγt-positive cells through the conditional deletion of VHL mitigated disease outcome, even after HBO therapy. Taken together, we conclude that HBO induces intestinal dysbiosis and impairs the production of SCFAs affecting the HIF-1α-IL-22 axis in ILC3 and worsening the response of mice to subsequent C. difficile infection.

RNA m6A modifications regulate crosstalk between tumor metabolism and immunity

In recent years, m6A modifications in RNA transcripts have arisen as a hot topic in cancer research. Indeed, a number of independent studies have elaborated that the m6A modification impacts the behavior of tumor cells and tumor-infiltrating immune cells, altering tumor cell metabolism along with the differentiation and functional activity of immune cells. This review elaborates on the links between RNA m6A modifications, tumor cell metabolism, and immune cell behavior, discussing this topic from the viewpoint of reciprocal regulation through "RNA m6A-tumor cell metabolism-immune cell behavior" and "RNA m6A-immune cell behavior-tumor cell metabolism" axes. In addition, we discuss the various factors affecting RNA m6A modifications in the tumor microenvironment, particularly the effects of hypoxia associated with cancer cell metabolism along with immune cell-secreted cytokines. Our analysis proposes the conclusion that RNA m6A modifications support widespread interactions between tumor metabolism and tumor immunity. With the current viewpoint that long-term cancer control must tackle cancer cell malignant behavior while strengthening anti-tumor immunity, the recognition of RNA m6A modifications as a key factor provides a new direction for the targeted therapy of tumors. This article is categorized under: RNA Processing > RNA Editing and Modification RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Mitochondrial-regulated Tregs: potential therapeutic targets for autoimmune diseases of the central nervous system

Regulatory T cells (Tregs) can eliminate autoreactive lymphocytes, induce self-tolerance, and suppress the inflammatory response. Mitochondria, as the energy factories of cells, are essential for regulating the survival, differentiation, and function of Tregs. Studies have shown that patients with autoimmune diseases of the central nervous system, such as multiple sclerosis, neuromyelitis optica spectrum disorder, and autoimmune encephalitis, have aberrant Tregs and mitochondrial damage. However, the role of mitochondrial-regulated Tregs in autoimmune diseases of the central nervous system remains inconclusive. Therefore, this study reviews the mitochondrial regulation of Tregs in autoimmune diseases of the central nervous system and investigates the possible mitochondrial therapeutic targets.

Interrelation of Hypoxia-Inducible Factor-1 Alpha (HIF-1 α) and the Ratio between the Mean Corpuscular Volume/Lymphocytes (MCVL) and the Cumulative Inflammatory Index (IIC) in Ulcerative Colitis

We intended to investigate the presence and medical application of serum hypoxia-inducible factor-1 alpha (HIF-1α) along with the already known systemic inflammatory markers and the new one's inflammatory indices, the proportion of mean corpuscular volume and lymphocytes (MCVL) and the cumulative inflammatory index (IIC), for patients with ulcerative colitis (UC). We sought to establish correlations that may be present between the serum levels of HIF-1α and these inflammatory indices, as well as their relationship with disease activity and the extent of UC, which can provide us with a more precise understanding of the evolution, prognosis, and future well-being of patients. Serum samples were collected from 46 patients diagnosed with UC and 23 controls. For our assessment of the serum levels of HIF-1α, we used the Enzyme-Linked Immunosorbent Assay (ELISA) technique. Thus, for HIF-1α we detected significantly higher values in more severe and more extensive UC. When it came to MCVL and IIC, we observed statistically significant differences between the three groups being compared (Severe, Moderate, and Mild). Our study highlighted that HIF-1α correlated much better with a disease activity score, MCVL, and IIC. With MCVL and IIC, a strong and very strong correlation had formed between them and well-known inflammation indices. By examining the ROC curves of the analyzed parameters, we recognized that TWI (accuracy of 83.70%) provides the best discrimination of patients with early forms of UC, followed by HIF-1α (73.90% accuracy), MCVL (70.90% accuracy), and PLR (70.40%). In our study, we observed that HIF-1α, MCVL, and PLR had the same sensitivity (73.33%) but HIF-1α had a much better specificity (60.87% vs. 58.70%, and 54.35%). Also, in addition to the PLR, HIF-1α and MCVL can be used as independent predictor factors in the discrimination of patients with early forms of UC.

The signaling pathway of hypoxia inducible factor in regulating gut homeostasis

Hypoxia represent a condition in which an adequate amount of oxygen supply is missing in the body, and it could be caused by a variety of diseases, including gastrointestinal disorders. This review is focused on the role of hypoxia in the maintenance of the gut homeostasis and related treatment of gastrointestinal disorders. The effects of hypoxia on the gut microbiome and its role on the intestinal barrier functionality are also covered, together with the potential role of hypoxia in the development of gastrointestinal disorders, including inflammatory bowel disease and irritable bowel syndrome. Finally, we discussed the potential of hypoxia-targeted interventions as a novel therapeutic approach for gastrointestinal disorders. In this review, we highlighted the importance of hypoxia in the maintenance of the gut homeostasis and the potential implications for the treatment of gastrointestinal disorders.

Role of glycolysis in inflammatory bowel disease and its associated colorectal cancer

Inflammatory bowel disease (IBD) has been referred to as the "green cancer," and its progression to colorectal cancer (CRC) poses a significant challenge for the medical community. A common factor in their development is glycolysis, a crucial metabolic mechanism of living organisms, which is also involved in other diseases. In IBD, glycolysis affects gastrointestinal components such as the intestinal microbiota, mucosal barrier function, and the immune system, including macrophages, dendritic cells, T cells, and neutrophils, while in CRC, it is linked to various pathways, such as phosphatidylinositol-3-kinase (PI3K)/AKT, AMP-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), and transcription factors such as p53, Hypoxia-inducible factor (HIF), and c-Myc. Thus, a comprehensive study of glycolysis is essential for a better understanding of the pathogenesis and therapeutic targets of both IBD and CRC. This paper reviews the role of glycolysis in diseases, particularly IBD and CRC, via its effects on the intestinal microbiota, immunity, barrier integrity, signaling pathways, transcription factors and some therapeutic strategies targeting glycolytic enzymes.

HIF-1α targeted deletion in myeloid cells decreases MDSC accumulation and alters microbiome in neonatal mice

The newborn's immune system is faced with the challenge of having to learn quickly to fight off infectious agents, but tolerating the colonization of the body surfaces with commensals without reacting with an excessive inflammatory response. Myeloid-derived suppressor cells (MDSC) are innate immune cells with suppressive activity on other immune cells that regulate fetal-maternal tolerance during pregnancy and control intestinal inflammation in neonates. Until now, nothing is known about the role of MDSC in microbiome establishment. One of the transcription factors regulating MDSC homeostasis is the hypoxia-inducible factor 1α (HIF-1α). We investigated the impact of HIF-1α on MDSC accumulation and microbiome establishment during the neonatal period in a mouse model with targeted deletion of HIF-1α in myeloid cells (Hif1a loxP/loxP LysMCre+). We show that in contrast to wildtype mice, where an extensive expansion of MDSC was observed, MDSC expansion in neonatal Hif1a loxP/loxP LysMCre+ mice was dramatically reduced both systemically and locally in the intestine. This was accompanied by an altered microbiome composition and intestinal T-cell homeostasis. Our results point toward a role of MDSC in inflammation regulation in the context of microbiome establishment and thus reveal a new aspect of the biological role of MDSC during the neonatal period.

Hypoxia in the Pathophysiology of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is an idiopathic disease of disordered chronic inflammation in the intestines that affects many people across the world. While the disease is still being better characterized, greater progress has been made in understanding the many components that intersect in the disease. Among these components are the many pieces that compose the intestinal epithelial barrier, the various cytokines and immune cells, and the population of microbes that reside in the intestinal lumen. Since their discovery, the hypoxia-inducible factors (HIFs) have been found to play an expansive role in physiology as well as diseases such as inflammation due to their role in oxygen sensing-related gene transcription, and metabolic control. Making use of existing and developing paradigms in the immuno-gastroenterology of IBD, we summarized that hypoxic signaling plays as another component in the status and progression of IBD, which may include possible functions at the origins of inflammatory dysregulation. © 2023 American Physiological Society. Compr Physiol 13:4767-4783, 2023.

Hypoxia and Intestinal Inflammation: Common Molecular Mechanisms and Signaling Pathways

The gastrointestinal tract (GI) has a unique oxygenation profile. It should be noted that the state of hypoxia can be characteristic of both normal and pathological conditions. Hypoxia-inducible factors (HIF) play a key role in mediating the response to hypoxia, and they are tightly regulated by a group of enzymes called HIF prolyl hydroxylases (PHD). In this review, we discuss the involvement of inflammation hypoxia and signaling pathways in the pathogenesis of inflammatory bowel disease (IBD) and elaborate in detail on the role of HIF in multiple immune reactions during intestinal inflammation. We emphasize the critical influence of tissue microenvironment and highlight the existence of overlapping functions and immune responses mediated by the same molecular mechanisms. Finally, we also provide an update on the development of corresponding therapeutic approaches that would be useful for treatment or prophylaxis of inflammatory bowel disease.

Emerging role of hypoxia-inducible factor-1α in inflammatory autoimmune diseases: A comprehensive review

Hypoxia-inducible factor-1α (HIF-1α) is a primary metabolic sensor, and is expressed in different immune cells, such as macrophage, dendritic cell, neutrophil, T cell, and non-immune cells, for instance, synovial fibroblast, and islet β cell. HIF-1α signaling regulates cellular metabolism, triggering the release of inflammatory cytokines and inflammatory cells proliferation. It is known that microenvironment hypoxia, vascular proliferation, and impaired immunological balance are present in autoimmune diseases. To date, HIF-1α is recognized to be overexpressed in several inflammatory autoimmune diseases, such as systemic lupus erythematosus (SLE), rheumatoid arthritis, and function of HIF-1α is dysregulated in these diseases. In this review, we narrate the signaling pathway of HIF-1α and the possible immunopathological roles of HIF-1α in autoimmune diseases. The collected information will provide a theoretical basis for the familiarization and development of new clinical trials and treatment based on HIF-1α and inflammatory autoimmune disorders in the future.

Hypoxia inducible factor prolyl hydroxylases in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic disease that is characterized by intestinal inflammation. Epithelial damage and loss of intestinal barrier function are believed to be the hallmark pathologies of the disease. In IBD, the resident and infiltrating immune cells consume much oxygen, rendering the inflamed intestinal mucosa hypoxic. In hypoxia, the hypoxia-inducible factor (HIF) is induced to cope with the lack of oxygen and protect intestinal barrier. Protein stability of HIF is tightly controlled by prolyl hydroxylases (PHDs). Stabilization of HIF through inhibition of PHDs is appearing as a new strategy of IBD treatment. Studies have shown that PHD-targeting is beneficial to the treatment of IBD. In this Review, we summarize the current understanding of the role of HIF and PHDs in IBD and discuss the therapeutic potential of targeting PHD-HIF pathway for IBD treatment.

Succinate metabolism and its regulation of host-microbe interactions

Succinate is a circulating metabolite, and the relationship between abnormal changes in the physiological concentration of succinate and inflammatory diseases caused by the overreaction of certain immune cells has become a research focus. Recent investigations have shown that succinate produced by the gut microbiota has the potential to regulate host homeostasis and treat diseases such as inflammation. Gut microbes are important for maintaining intestinal homeostasis. Microbial metabolites serve as nutrients in energy metabolism, and act as signal molecules that stimulate host cell and organ function and affect the structural balance between symbiotic gut microorganisms. This review focuses on succinate as a metabolite of both host cells and gut microbes and its involvement in regulating the gut - immune tissue axis by activating intestinal mucosal cells, including macrophages, dendritic cells, and intestinal epithelial cells. We also examined its role as the mediator of microbiota - host crosstalk and its potential function in regulating intestinal microbiota homeostasis. This review explores feasible ways to moderate succinate levels and provides new insights into succinate as a potential target for microbial therapeutics for humans.

Oxygen Sensing in Innate Immune Cells: How Inflammation Broadens Classical Hypoxia-Inducible Factor Regulation in Myeloid Cells

Significance: Oxygen deprivation (hypoxia) is a common feature at sites of inflammation. Immune cells and all other cells present at the inflamed site have to adapt to these conditions. They do so by stabilization and activation of hypoxia-inducible factor subunit α (HIF-1α and HIF-2α, respectively), enabling constant generation of adenosine triphosphate (ATP) under these austere conditions by the induction of, for example, glycolytic pathways. Recent Advances: During recent years, it has become evident that HIFs play a far more important role than initially believed because they shape the inflammatory phenotype of immune cells. They are indispensable for migration, phagocytosis, and the induction of inflammatory cytokines by innate immune cells and thereby enable a crosstalk between innate and adaptive immunity. In short, they ensure the survival and function of immune cells under critical conditions. Critical Issues: Up to now, there are still open questions regarding the individual roles of HIF-1 and HIF-2 for the different cell types. In particular, the loss of both HIF-1 and HIF-2 in myeloid cells led to unexpected and contradictory results in the mouse models analyzed so far. Similarly, the role of HIF-1 in dendritic cell maturation is unclear due to inconsistent results from in vitro experiments. Future Directions: The HIFs are indispensable for immune cell survival and action under inflammatory conditions, but they might also trigger over-activation of immune cells. Therefore, they might be excellent setscrews to adjust the inflammatory response by pharmaceuticals. China and Japan and very recently (August 2021) Europe have approved prolyl hydroxylase inhibitors (PHIs) to stabilize HIF such as roxadustat for clinical use to treat anemia by increasing the production of erythropoietin, the classical HIF target gene. Nonetheless, we need further work regarding the use of PHIs under inflammatory conditions, because HIFs show specific activation and distinct expression patterns in innate immune cells. The extent to which HIF-1 or HIF-2 as a transcription factor regulates the adaptation of immune cells to inflammatory hypoxia differs not only by the cell type but also with the inflammatory challenge and the surrounding tissue. Therefore, we urgently need isoform- and cell type-specific modulators of the HIF pathway. Antioxid. Redox Signal. 37, 956-971.

HIF-1α Inhibition Improves Anti-Tumor Immunity and Promotes the Efficacy of Stereotactic Ablative Radiotherapy (SABR)

Simple Summary

Stereotactic ablative radiotherapy (SABR), which irradiates tumors with high-dose radiation per fraction, promotes anti-tumor immunity by stimulating various immune processes. SABR also induces vascular damage and obstructs blood flow, thereby increasing tumor hypoxia and upregulation of hypoxia-inducible factors HIF-1α and HIF-2α, master transcription factors for the cellular response to hypoxia. HIF-1α and HIF-2α are key players in the upregulation of immune suppression in hypoxia. Therefore, the radiation-induced increase in anti-tumor immunity is masked by the HIF-mediated immune suppression. Pre-clinical experiments show that inhibition of HIF-1α effectively prevents immune suppression and improves anti-tumor immunity. A combination of HIF-1α inhibitors with immunotherapy with checkpoint blocking antibodies may represent a novel approach to boost anti-tumor immunity and enhance the efficacy of SABR.

Abstract

High-dose hypofractionated radiation such as SABR (stereotactic ablative radiotherapy) evokes an anti-tumor immune response by promoting a series of immune-stimulating processes, including the release of tumor-specific antigens from damaged tumor cells and the final effector phase of immune-mediated lysis of target tumor cells. High-dose hypofractionated radiation also causes vascular damage in tumors, thereby increasing tumor hypoxia and upregulation of hypoxia-inducible factors HIF-1α and HIF-2α, the master transcription factors for the cellular response to hypoxia. HIF-1α and HIF-2α are critical factors in the upregulation of immune suppression and are the master regulators of immune evasion of tumors. Consequently, SABR-induced increase in anti-tumor immunity is counterbalanced by the increase in immune suppression mediated by HIFα. Inhibition of HIF-1α with small molecules such as metformin downregulates immunosuppressive pathways, including the expression of immune checkpoints, and it improves or restores the anti-tumor immunity stimulated by irradiation. Combinations of HIFα inhibitors, particularly HIF-1α inhibitors, with immune checkpoint blocking antibodies may represent a novel approach to boost the overall anti-tumor immune profile in patients and thus enhance outcomes after SABR.

DFO treatment protects against depression-like behaviors and cognitive impairment in CUMS mice

Depression has several negative effects on emotion as well as learning and memory abilities. Previous studies showed that depression could exacerbate inflammation, which in turn further aggravated depression. Deferoxamine (DFO) is a chelating agent binding iron and aluminium, and is clinically applied to treat acute ion poisoning and hemochromatosis. Researches showed that it could reduce inflammation via increasing the expression of hypoxia-inducible factor-1alpha (HIF-1α). Here, we established a chronic unpredictable mild stress (CUMS) model to investigate whether DFO exerted a neuroprotective function in depression. The results demonstrated that CUMS (4 weeks) effectively induced depression-like behaviors in mice based on sucrose preference test (SPT), forced swim test (FST), tail suspension test (TST), open field test (OFT), and elevated plus-maze test (EPT). It also brought cognitive deficits based on Morris water maze (MWM) test and the impairment of synaptic plasticity based on in vivo electrophysiological recordings. Additionally, CUMS exposure significantly decreased the expression of hippocampal synapse related proteins and the spine density of neurons in the DG region, accompanied by increasing the expression of hippocampal inflammatory cytokines, and promoted the activation of microglia in the hippocampus. The expression of HIF-1α was down-regulated as expected. However, DFO distinctly reversed the CUMS-induced impairments. The mechanism is associated with the DFO inhibition of inflammation by upregulating HIF-1 expression, thereby alleviating a series of pathology changes. Together, these findings suggest that DFO likely plays a protective role in cognitive impairments and synaptic plasticity deficits resulting from depression.

Eosinophils, Hypoxia-Inducible Factors, and Barrier Dysfunction in Functional Dyspepsia

Functional dyspepsia (FD) is a highly prevalent disorder of gut-brain interaction (DGBI), previously known as a functional gastrointestinal disorder. Characterized by early satiety, postprandial fullness, and/or epigastric pain or burning, diagnosis depends on positive symptomatology and exclusion of obvious structural diseases. A subtle inflammatory phenotype has been identified in FD patients, involving an increase in duodenal mucosal eosinophils, and imbalances in the duodenal gut microbiota. A dysregulated epithelial barrier has also been well described in FD and is thought to be a contributing factor to the low-grade duodenal inflammation observed, however the mechanisms underpinning this are poorly understood. One possible explanation is that alterations in the microbiota and increased immune cells can result in the activation of cellular stress response pathways to perpetuate epithelial barrier dysregulation. One such cellular response pathway involves the stabilization of hypoxia-inducible factors (HIF). HIF, a transcriptional protein involved in the cellular recognition and adaptation to hypoxia, has been identified as a critical component of various pathologies, from cancer to inflammatory bowel disease (IBD). While the contribution of HIF to subtle inflammation, such as that seen in FD, is unknown, HIF has been shown to have roles in regulating the inflammatory response, particularly the recruitment of eosinophils, as well as maintaining epithelial barrier structure and function. As such, we aim to review our present understanding of the involvement of eosinophils, barrier dysfunction, and the changes to the gut microbiota including the potential pathways and mechanisms of HIF in FD. A combination of PubMed searches using the Mesh terms functional dyspepsia, functional gastrointestinal disorders, disorders of gut-brain interaction, duodenal eosinophilia, barrier dysfunction, gut microbiota, gut dysbiosis, low-grade duodenal inflammation, hypoxia-inducible factors (or HIF), and/or intestinal inflammation were undertaken in the writing of this narrative review to ensure relevant literature was included. Given the findings from various sources of literature, we propose a novel hypothesis involving a potential role for HIF in the pathophysiological mechanisms underlying FD.

Knockout of Factor-Inhibiting HIF (Hif1an) in Colon Epithelium Attenuates Chronic Colitis but Does Not Reduce Colorectal Cancer in Mice

Inflammatory bowel disease such as chronic colitis promotes colorectal cancer, which is a common cause of cancer mortality worldwide. Hypoxia is a characteristic of inflammation as well as of solid tumors and enforces a gene expression response controlled by hypoxia-inducible factors (HIFs). Once established, solid tumors are immunosuppressive to escape their abatement through immune cells. Although HIF activity is known to 1) promote cancer development and 2) drive tumor immune suppression through the secretion of adenosine, both prolyl hydroxylases and an asparaginyl hydroxylase termed factor-inhibiting HIF (FIH) negatively regulate HIF. Thus, FIH may act as a tumor suppressor in colorectal cancer development. In this study, we examined the role of colon epithelial FIH in a mouse model of colitis-induced colorectal cancer. We recapitulated colitis-associated colorectal cancer development in mice using the azoxymethane/dextran sodium sulfate model in Vil1-Cre/FIH^+f/+f and wild-type siblings. Colon samples were analyzed regarding RNA and protein expression and histology. Vil1-Cre/FIH^+f/+f mice showed a less severe colitis progress compared with FIH^+f/+f animals and a lower number of infiltrating macrophages in the inflamed tissue. RNA sequencing analyses of colon tissue revealed a lower expression of genes associated with the immune response in Vil1-Cre/FIH^+f/+f mice. However, tumor occurrence did not significantly differ between Vil1-Cre/FIH^+f/+f and wild-type mice. Thus, FIH knockout in colon epithelial cells did not modulate colorectal cancer development but reduced the inflammatory response in chronic colitis.

The transcription factor HIF-1α mediates plasticity of NKp46+ innate lymphoid cells in the gut

Gut innate lymphoid cells (ILCs) show remarkable phenotypic diversity, yet microenvironmental factors that drive this plasticity are incompletely understood. The balance between NKp46+, IL-22-producing, group 3 ILCs (ILC3s) and interferon (IFN)-γ-producing group 1 ILCs (ILC1s) contributes to gut homeostasis. The gut mucosa is characterized by physiological hypoxia, and adaptation to low oxygen is mediated by hypoxia-inducible transcription factors (HIFs). However, the impact of HIFs on ILC phenotype and gut homeostasis is not well understood. Mice lacking the HIF-1α isoform in NKp46+ ILCs show a decrease in IFN-γ-expressing, T-bet+, NKp46+ ILC1s and a concomitant increase in IL-22-expressing, RORγt+, NKp46+ ILC3s in the gut mucosa. Single-cell RNA sequencing revealed HIF-1α as a driver of ILC phenotypes, where HIF-1α promotes the ILC1 phenotype by direct up-regulation of T-bet. Loss of HIF-1α in NKp46+ cells prevents ILC3-to-ILC1 conversion, increases the expression of IL-22-inducible genes, and confers protection against intestinal damage. Taken together, our results suggest that HIF-1α shapes the ILC phenotype in the gut.

HIF-1α modulates sex-specific Th17/Treg responses during hepatic amoebiasis

BACKGROUND & AIMS

An invasive form of intestinal Entamoeba (E.) histolytica infection, which causes amoebic liver abscess, is more common in men than in women. Immunopathological mechanisms are responsible for the more severe outcome in males. Here, we used a mouse model of hepatic amoebiasis to investigate the contribution of hepatic hypoxia-inducible factor (HIF)-1α to T helper 17 (Th17)/regulatory T cell (Treg) responses in the context of the sex-specific outcome of liver damage.

METHODS

C57BL/6J mice were infected intrahepatically with E. histolytica trophozoites. HIF-1α expression was determined by qPCR, flow cytometry and immunohistochemistry. Tregs and Th17 cells were analysed by immunohistochemistry and flow cytometry. Finally, male and female hepatocyte-specific Hif1α knockout mice were generated, and the effect of HIF-1α on abscess development, the cytokine milieu, and Th17/Treg differentiation was examined.

RESULTS

E. histolytica infection increased hepatic HIF-1α levels, along with the elevated frequencies of hepatic Th17 and Treg cells. While the Th17 cell population was larger in male mice, Tregs characterised by increased expression of Foxp3 in female mice. Male mice displayed increased IL-6 expression, contributing to immunopathology; this increase in IL-6 expression declined upon deletion of hepatic HIF-1α. In both sexes, hepatic deletion of HIF-1α reduced the Th17 cell frequency; however, the percentage of Tregs was reduced in female mice only.

CONCLUSIONS

Hepatic HIF-1α modulates the sex-specific outcome of murine E. histolytica infection. Our results suggest that in male mice, Th17 cells can be modulated by hepatic HIF-1α via IL-6, indicating marked involvement in the immunopathology underlying abscess development. Strong expression of Foxp3 by hepatic Tregs from female mice suggests a potent immunosuppressive function, leading to initiation of liver regeneration.

LAY SUMMARY

Infection with the parasite Entamoeba histolytica activates immunopathological mechanisms in male mice, which lead to liver abscesses that are larger than those in female mice. In the absence of the protein HIF-1α in hepatocytes, abscess formation is reduced; moreover, the sex difference in abscess size is abolished. These results suggest that HIF-1α modulates the immune response involved in the induction of immunopathology, resulting in differential disease susceptibility in males and females.

Physicochemical aspects of the tumour microenvironment as drivers of vasculogenic mimicry

Tumour vascularisation is vital for cancer sustainment representing not only the main source of nutrients and oxygen supply but also an escape route for single or clustered cancer cells that, once detached from the primary mass, enter the blood circulation and disseminate to distant organs. Among the mechanisms identified to contribute to tumour vascularisation, vasculogenic mimicry (VM) is gaining increasing interest in the scientific community representing an intriguing target for cancer treatment. VM indeed associates with highly aggressive tumour phenotypes and strongly impairs patient outcomes. Differently from vessels of healthy tissues, tumour vasculature is extremely heterogeneous and tortuous, impeding efficient chemotherapy delivery, and at the meantime hyperpermeable and thus extremely accessible to metastasising cancer cells. Moreover, tumour vessel disorganisation creates a self-reinforcing vicious circle fuelling cancer malignancy and progression. Because of the inefficient oxygen delivery and metabolic waste removal from tumour vessels, many cells within the tumour mass indeed experience hypoxia and acidosis, now considered hallmarks of cancer. Being strong inducers of vascularisation, therapy resistance, inflammation and metastasis, hypoxia and acidosis create a permissive microenvironment for cancer progression and dissemination. Along with these considerations, we decided to focus our attention on the relationship between hypoxia/acidosis and VM. Indeed, besides tumour angiogenesis, VM is strongly influenced by both hypoxia and acidosis, which could potentiate each other and fuel this vicious circle. Thus, targeting hypoxia and acidosis may represent a potential target to treat VM to impair tumour perfusion and cancer cell sustainment.

The role of hypoxia-inducible factor 1-alpha in inflammatory bowel disease

Inflammatory bowel disease (IBD) develops as a result of a combination of genetic predisposition, dysbiosis of the gut microbiota, and environmental influences, which is mainly represented by ulcerative colitis (UC) and Crohn's disease (CD). IBDs can result in inflammatory hypoxia by causing intestinal inflammation and vascular damage. The hypoxia-inducible factor 1-alpha (HIF-1α), as a transcription factor, can regulate the cellular adaptation to low oxygen levels and support the development and function of the gut barrier. HIF-αplays its functions through translocating into the nucleus, dimerizing with HIF-1β, and binding to hypoxia-responsive elements of HIF-1 target genes. So far, most studies have addressed the function of HIF-1α in murine models of IBD. In this review, we aim to outline the major roles of HIF-1α in the IBD.

Regulation of the Hypoxia-Inducible Factor (HIF) by Pro-Inflammatory Cytokines

Hypoxia and inflammation are frequently co-incidental features of the tissue microenvironment in a wide range of inflammatory diseases. While the impact of hypoxia on inflammatory pathways in immune cells has been well characterized, less is known about how inflammatory stimuli such as cytokines impact upon the canonical hypoxia-inducible factor (HIF) pathway, the master regulator of the cellular response to hypoxia. In this review, we discuss what is known about the impact of two major pro-inflammatory cytokines, tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), on the regulation of HIF-dependent signaling at sites of inflammation. We report extensive evidence for these cytokines directly impacting upon HIF signaling through the regulation of HIF at transcriptional and post-translational levels. We conclude that multi-level crosstalk between inflammatory and hypoxic signaling pathways plays an important role in shaping the nature and degree of inflammation occurring at hypoxic sites.

Tolerogenic Dendritic Cells Shape a Transmissible Gut Microbiota That Protects From Metabolic Diseases

Excess chronic contact between microbial motifs and intestinal immune cells is known to trigger a low-grade inflammation involved in many pathologies such as obesity and diabetes. The important skewing of intestinal adaptive immunity in the context of diet-induced obesity (DIO) is well described, but how dendritic cells (DCs) participate in these changes is still poorly documented. To address this question, we challenged transgenic mice with enhanced DC life span and immunogenicity (DC^hBcl-2 mice) with a high-fat diet. Those mice display resistance to DIO and metabolic alterations. The DIO-resistant phenotype is associated with healthier parameters of intestinal barrier function and lower intestinal inflammation. DC^hBcl-2 DIO-resistant mice demonstrate a particular increase in tolerogenic DC numbers and function, which is associated with strong intestinal IgA, T helper 17, and regulatory T-cell immune responses. Microbiota composition and function analyses reveal that the DC^hBcl-2 mice microbiota is characterized by lower immunogenicity and an enhanced butyrate production. Cohousing experiments and fecal microbial transplantations are sufficient to transfer the DIO resistance status to wild-type mice, demonstrating that maintenance of DCs' tolerogenic ability sustains a microbiota able to drive DIO resistance. The tolerogenic function of DCs is revealed as a new potent target in metabolic disease management.

HIF in the heart: development, metabolism, ischemia, and atherosclerosis

The heart forms early in development and delivers oxygenated blood to the rest of the embryo. After birth, the heart requires kilograms of ATP each day to support contractility for the circulation. Cardiac metabolism is omnivorous, utilizing multiple substrates and metabolic pathways to produce this energy. Cardiac development, metabolic tuning, and the response to ischemia are all regulated in part by the hypoxia-inducible factors (HIFs), central components of essential signaling pathways that respond to hypoxia. Here we review the actions of HIF1, HIF2, and HIF3 in the heart, from their roles in development and metabolism to their activity in regeneration and preconditioning strategies. We also discuss recent work on the role of HIFs in atherosclerosis, the precipitating cause of myocardial ischemia and the leading cause of death in the developed world.

Hypoxia-inducible factor-1 drives divergent immunomodulatory functions in the pathogenesis of autoimmune diseases

Hypoxia-inducible factor-1 (HIF-1) is a heterodimeric (HIF-1α/ HIF-1β) transcription factor in which the oxygen-sensitive HIF-1α subunit regulates gene transcription to mediate adaptive tissue responses to hypoxia. HIF-1 is a key mediator in both regulatory and pathogenic immune responses, because ongoing inflammation in localized tissues causes increased oxygen consumption and consequent hypoxia within the inflammatory lesions. In autoimmune diseases, HIF-1 plays complex and divergent roles within localized inflammatory lesions by orchestrating a critical immune interplay sponsoring the pathogenesis of the disease. In this review, we have summarized the role of HIF-1 in lymphoid and myeloid immunomodulation in autoimmune diseases. HIF-1 drives inflammation by controlling the Th17/T_reg /Tr1 balance through the tipping of the differentiation of CD4⁺ T cells in favour of pro-inflammatory Th17 cells while suppressing the development of anti-inflammatory T_reg /Tr1 cells. On the other hand, HIF-1 plays a protective role by facilitating the expression of anti-inflammatory cytokine IL-10 in and expansion of CD1d^hi CD5⁺ B cells, known as regulatory B cells or B10 cells. Apart from lymphoid cells, HIF-1 also controls the activation of macrophages, neutrophils and dendritic cells, thus eventually further influences the activation and development of effector/regulatory T cells by facilitating the creation of a pro/anti-inflammatory microenvironment within the autoinflammatory lesions. Based on the critical immunomodulatory roles that HIF-1 plays, this master transcription factor seems to be a potent druggable target for the treatment of autoimmune diseases.

Hypoxia/HIF Modulates Immune Responses

Oxygen availability varies throughout the human body in health and disease. Under physiological conditions, oxygen availability drops from the lungs over the blood stream towards the different tissues into the cells and the mitochondrial cavities leading to physiological low oxygen conditions or physiological hypoxia in all organs including primary lymphoid organs. Moreover, immune cells travel throughout the body searching for damaged cells and foreign antigens facing a variety of oxygen levels. Consequently, physiological hypoxia impacts immune cell function finally controlling innate and adaptive immune response mainly by transcriptional regulation via hypoxia-inducible factors (HIFs). Under pathophysiological conditions such as found in inflammation, injury, infection, ischemia and cancer, severe hypoxia can alter immune cells leading to dysfunctional immune response finally leading to tissue damage, cancer progression and autoimmunity. Here we summarize the effects of physiological and pathophysiological hypoxia on innate and adaptive immune activity, we provide an overview on the control of immune response by cellular hypoxia-induced pathways with focus on the role of HIFs and discuss the opportunity to target hypoxia-sensitive pathways for the treatment of cancer and autoimmunity.

Cutting Edge: Hypoxia-Induced Ubc9 Promoter Hypermethylation Regulates IL-17 Expression in Ulcerative Colitis

Dysregulated IL-17 expression is central to the pathogenesis of several inflammatory disorders, including ulcerative colitis. We have shown earlier that SUMOylation of ROR-γt, the transcription factor for IL-17, regulates colonic inflammation. In this study, we show that the expression of Ubc9, the E2 enzyme that targets ROR-γt for SUMOylation, is significantly reduced in the colonic mucosa of ulcerative colitis patients. Mechanistically, we demonstrate that hypoxia-inducible factor 1α (HIF-1α) binds to a CpG island within the Ubc9 gene promoter, resulting in its hypermethylation and reduced Ubc9 expression. CRISPR-Cas9-mediated inhibition of HIF-1α normalized Ubc9 and attenuated IL-17 expression in Th17 cells and reduced diseases severity in Rag1 ^-/- mice upon adoptive transfer. Collectively, our study reveals a novel epigenetic mechanism of regulation of ROR-γt that could be exploited in inflammatory diseases.

Local Stabilization of Hypoxia-Inducible Factor-1α Controls Intestinal Inflammation via Enhanced Gut Barrier Function and Immune Regulation

Intestinal epithelial cells are adapted in mucosal hypoxia and hypoxia-inducible factors in these cells can fortify barrier integrity to support mucosal tissue healing. Here we investigated whether hypoxia-related pathways could be proposed as potential therapeutic targets for inflammatory bowel disease. We developed a novel hypoxia-inducible factor (HIF) prolyl hydroxylase inhibitor, CG-598 which stabilized HIF-1α in the gut tissue. Treatment of CG-598 did not affect extra-intestinal organs or cause any significant adverse effects such as erythropoiesis. In the experimental murine colitis model, CG-598 ameliorated intestinal inflammation with reduction of inflammatory lesions and pro-inflammatory cytokines. CG-598 treatment fortified barrier function by increasing the expression of intestinal trefoil factor, CD73, E-cadherin and mucin. Also, IL-10 and IL-22 were induced from lamina propria CD4⁺ T-cells. The effectiveness of CG-598 was comparable to other immunosuppressive therapeutics such as TNF-blockers or JAK inhibitors. These results suggest that CG-598 could be a promising therapeutic candidate to treat inflammatory bowel disease.

Hypoxia-inducible factor 1 alpha limits dendritic cell stimulation of CD8 T cell immunity

Dendritic cells are sentinels of the immune system and represent a key cell in the activation of the adaptive immune response. Hypoxia-inducible factor 1 alpha (HIF-1α)–a crucial oxygen sensor stabilized during hypoxic conditions–has been shown to have both activating and inhibitory effects in immune cells in a context- and cell-dependent manner. Previous studies have demonstrated that in some immune cell types, HIF-1α serves a pro-inflammatory role. Genetic deletion of HIF-1α in macrophages has been reported to reduce their pro-inflammatory function. In contrast, loss of HIF-1α enhanced the pro-inflammatory activity of dendritic cells in a bacterial infection model. In this study, we aimed to further clarify the effects of HIF-1α in dendritic cells. Constitutive expression of HIF-1α resulted in diminished immunostimulatory capacity of dendritic cells in vivo, while conditional deletion of HIF-1α in dendritic cells enhanced their ability to induce a cytotoxic T cell response. HIF-1α-expressing dendritic cells demonstrated increased production of inhibitory mediators including IL-10, iNOS and VEGF, which correlated with their reduced capacity to drive effector CD8⁺ T cell function. Altogether, these data reveal that HIF-1α can promote the anti-inflammatory functions of dendritic cells and provides insight into dysfunctional immune responses in the context of HIF-1α activation.

Role of dendritic cell metabolic reprogramming in tumor immune evasion

The dendritic cell (DC) is recognized as a vital mediator of anti-tumor immunity. More recent studies have also demonstrated the important role of DCs in the generation of effective responses to checkpoint inhibitor immunotherapy. Metabolic programming of DCs dictates their functionality and can determine which DCs become immunostimulatory versus those that develop a tolerized phenotype capable of actively suppressing effector T-cell responses to cancers. As a result, there is great interest in understanding what mechanisms have evolved in cancers to alter these metabolic pathways, thereby allowing for their continued progression and metastasis. The therapeutic strategies developed to reverse these processes of DC tolerization in the tumor microenvironment represent promising candidates for future testing in combination immunotherapy clinical trials.

Functional foods from the tropics to relieve chronic normobaric hypoxia

Functional foods with antioxidant and anti-inflammatory properties are regarded as a complementary therapy to improve chronic diseases such as obesity and inflammatory bowel disease (IBD). Obesity is a chronic low-grade inflammatory state leading to organ damage with increased risk of common diseases including cardiovascular and metabolic disease, non-alcoholic fatty liver disease, osteoarthritis and some cancers. IBD is a chronic intestinal inflammation categorised as Crohn's disease and ulcerative colitis depending on the location of inflammation. These inflammatory states are characterised by normobaric hypoxia in adipose and intestinal tissues, respectively. Tropical foods especially from Australia and South America are discussed in this review to show their potential in attenuation of these chronic diseases. The phytochemicals from these foods have antioxidant and anti-inflammatory activities to reduce chronic normobaric hypoxia in the tissues. These health benefits of the tropical foods are relevant not only for health economy but also in providing a global solution by improving the sustainability of their cultivation and assisting the local economies.

HIF1α activation in dendritic cells under sterile conditions promotes an anti-inflammatory phenotype through accumulation of intracellular lipids

Obesity is among the leading causes of elevated cardiovascular disease mortality and morbidity. Adipose tissue dysfunction, insulin resistance and inflammation are recognized as important risk factors for the development of cardiovascular disorders in obesity. Hypoxia appears to be a key factor in adipose tissue dysfunction affecting not only adipocytes but also immune cell function. Here we examined the effect of hypoxia-induced transcription factor HIF1α activation on classical dendritic cell (cDCs) function during obesity. We found that deletion of Hif1α on cDCs results in enhanced adipose-tissue inflammation and atherosclerotic plaque formation in a mouse model of obesity. This effect is mediated by HIF1α-mediated increased lipid synthesis, accumulation of lipid droplets and alter synthesis of lipid mediators. Our findings demonstrate that HIF1α activation in cDCs is necessary to control vessel wall inflammation.

The Importance of Hypoxia-Inducible Factors (HIF-1 and HIF-2) for the Pathophysiology of Inflammatory Bowel Disease

(1) Background: Hypoxia is a common feature of inflammation when hypoxia inducible factors (HIFs) adapt cells to conditions of low oxygen tension and inflammation. We studied the role of HIF-1 and HIF-2 in cells of the myeloid lineage in a mouse model of acute colitis. (2) Methods: Mice with and without a conditional knockout for either Hif-1a or Hif-2a or Hif-1a and Hif-2a in cells of the myeloid lineage were treated with 2.5% dextran sodium sulfate (DSS) for 6 days to induce an acute colitis. We analyzed the course of inflammation with respect to macroscopic (disease activity index) and microscopic (histology score and immunohistochemical staining of immune cells) parameters and quantified the mRNA expression of cytokines and chemokines in the colon and the mesenteric lymph nodes. (3) Results: A conditional knockout of myeloid Hif-1a ameliorated whereas the knockout of Hif-2a aggravated murine DSS colitis by increased recruitment of neutrophils to deeper layers of the colon. This led to higher expression of Il6, Ifng, Cd11c, Cd4, and Cd8 in the colon but also induced anti-inflammatory mediators such as Foxp3 and Il10. A conditional knockout of Hif-1a and Hif-2a did not show any differences compared to wildtype mice. (4) Conclusions: Myeloid HIF-1α and HIF-2α play opposing roles in acute DSS colitis. Thus, not only a cell type specific, but also the isoform specific modulation of HIFs needs to be addressed in attempts to modify HIF for therapeutic purposes.

Role of hypoxia inducible factor-1 in cancer stem cells (Review)

Cancer stem cells (CSCs) have been found to play a decisive role in cancer recurrence, metastasis, and chemo‑, radio‑ and immuno‑resistance. Understanding the mechanism of CSC self‑renewal and proliferation may help overcome the limitations of clinical treatment. The microenvironment of tumor growth consists of a lack of oxygen, and hypoxia has been confirmed to induce cancer cell invasion, metastasis and epithelial‑mesenchymal transition, and is usually associated with poor prognosis and low survival rates. Hypoxia inducible factor‑1 (HIF‑1) can be stably expressed under hypoxia and act as an important molecule to regulate the development of CSCs, but the specific mechanism remains unclear. The present review attempted to explain the role of HIF‑1 in the generation and maintenance of CSCs from the perspective of epigenetics, metabolic reprogramming, tumor immunity, CSC markers, non‑coding RNA and signaling pathways associated with HIF‑1, in order to provide novel targets with HIF‑1 as the core for clinical treatment, and extend the life of patients.

Hypoxia-inducible factors not only regulate but also are myeloid-cell treatment targets

Hypoxia describes limited oxygen availability at the cellular level. Myeloid cells are exposed to hypoxia at various bodily sites and even contribute to hypoxia by consuming large amounts of oxygen during respiratory burst. Hypoxia-inducible factors (HIFs) are ubiquitously expressed heterodimeric transcription factors, composed of an oxygen-dependent α and a constitutive β subunit. The stability of HIF-1α and HIF-2α is regulated by oxygen-sensing prolyl-hydroxylases (PHD). HIF-1α and HIF-2α modify the innate immune response and are context dependent. We provide a historic perspective of HIF discovery, discuss the molecular components of the HIF pathway, and how HIF-dependent mechanisms modify myeloid cell functions. HIFs enable myeloid-cell adaptation to hypoxia by up-regulating anaerobic glycolysis. In addition to effects on metabolism, HIFs control chemotaxis, phagocytosis, degranulation, oxidative burst, and apoptosis. HIF-1α enables efficient infection defense by myeloid cells. HIF-2α delays inflammation resolution and decreases antitumor effects by promoting tumor-associated myeloid-cell hibernation. PHDs not only control HIF degradation, but also regulate the crosstalk between innate and adaptive immune cells thereby suppressing autoimmunity. HIF-modifying pharmacologic compounds are entering clinical practice. Current indications include renal anemia and certain cancers. Beneficial and adverse effects on myeloid cells should be considered and could possibly lead to drug repurposing for inflammatory disorders.

The Role of HIF in Immunity and Inflammation

HIF is a transcription factor that plays an essential role in the cellular response to low oxygen, orchestrating a metabolic switch that allows cells to survive in this environment. In immunity, infected and inflamed tissues are often hypoxic, and HIF helps immune cells adapt. HIF-α stabilization can also occur under normoxia during immunity and inflammation, where it regulates metabolism but in addition can directly regulate expression of immune genes. Here we review the role of HIF in immunity, including its role in macrophages, dendritic cells, neutrophils, T cells, and B cells. Its role in immunity is as essential for cellular responses as it is in its original role in hypoxia, with HIF being implicated in multiple inflammatory diseases and in immunosuppression in tumors.

An update on T-2 toxin and its modified forms: metabolism, immunotoxicity mechanism, and human exposure assessment

T-2 toxin is the most toxic trichothecene mycotoxin, and it exerts potent toxic effects, including immunotoxicity, neurotoxicity, and reproductive toxicity. Recently, several novel metabolites, including 3',4'-dihydroxy-T-2 toxin and 4',4'-dihydroxy-T-2 toxin, have been uncovered. The enzymes CYP3A4 and carboxylesterase contribute to T-2 toxin metabolism, with 3'-hydroxy-T-2 toxin and HT-2 toxin as the corresponding primary products. Modified forms of T-2 toxin, including T-2-3-glucoside, exert their immunotoxic effects by signaling through JAK/STAT but not MAPK. T-2-3-glucoside results from hydrolyzation of the corresponding parent mycotoxin and other metabolites by the intestinal microbiota, which leads to enhanced toxicity. Increasing evidence has shown that autophagy, hypoxia-inducible factors, and exosomes are involved in T-2 toxin-induced immunotoxicity. Autophagy promotes the immunosuppression induced by T-2 toxin, and a complex crosstalk between apoptosis and autophagy exists. Very recently, "immune evasion" activity was reported to be associated with this toxin; this activity is initiated inside cells and allows pathogens to escape the host immune response. Moreover, T-2 toxin has the potential to trigger hypoxia in cells, which is related to activation of hypoxia-inducible factor and the release of exosomes, leading to immunotoxicity. Based on the data from a series of human exposure studies, free T-2 toxin, HT-2 toxin, and HT-2-4-glucuronide should be considered human T-2 toxin biomarkers in the urine. The present review focuses on novel findings related to the metabolism, immunotoxicity, and human exposure assessment of T-2 toxin and its modified forms. In particular, the immunotoxicity mechanisms of T-2 toxin and the toxicity mechanism of its modified form, as well as human T-2 toxin biomarkers, are discussed. This work will contribute to an improved understanding of the immunotoxicity mechanism of T-2 toxin and its modified forms.

Sishen Pill Treatment of DSS-Induced Colitis via Regulating Interaction With Inflammatory Dendritic Cells and Gut Microbiota

Sishen Pill (SSP) is a typical prescription in the pharmacopeia of traditional Chinese medicine (TCM), and is usually used to treat inflammatory bowel disease (IBD). It is known that inflammatory dendritic cells (DCs) and imbalance of gut microbiota play significant roles in the pathogenesis of IBD. However, it is not clear whether SSP can treat IBD by regulating interaction of DCs and gut microbiota. In the present study, the levels of inflammatory DCs and gut microbiota were analyzed by flow cytometry and 16S rDNA analysis. SSP relieved the pathological damage to the colon of mice with colitis induced by dextran sodium sulfate (DSS). As typical indicators of inflammatory DCs, the levels of CD11c⁺CD103⁺E-cadherin⁺ cells and pro-inflammatory cytokines [interleukin (IL)-1β, -4, -9, and -17A] were decreased in mice with colitis treated by SSP for 10 days. Simultaneously, the gut microbiota composition was regulated, and beneficial bacteria were increased and pathogenic bacteria were reduced. The results indicated that SSP regulated the interaction between inflammatory DCs and gut microbiota to treat DSS-induced colitis.

Barrier integrity and chronic inflammation mediated by HIF-1 impact on intestinal tumorigenesis

Colorectal cancer ranks among the top three most frequent malignancies in the world. While overall incidence and mortality of colorectal cancer has substantially decreased in recent years, tumor subtypes with poor response rates to standard antiproliferative therapies remain particularly challenging. Hypoxia in the microenvironment of solid tumors is associated with malignant progression, e.g. local invasion, systemic spread and therapy resistance. A detailed molecular understanding of hypoxia's role for the pathobiology of colorectal cancer is a prerequisite to design and evaluate the consequences of interference with hypoxic signaling for the progression of this cancer type. Here, we summarize the current knowledge about the role of hypoxia-inducible factor 1, an essential molecular mediator of the hypoxic response, for colorectal cancer pathogenesis. Special attention is given to intestinal microbiota, gut barrier integrity and chronic inflammation as these are of pivotal importance for intestinal tumorigenesis and noticeably associated with hypoxic signaling.

Prolyl hydroxylase domain 2 reduction enhances skeletal muscle tissue regeneration after soft tissue trauma in mice

The transcription factor Hypoxia-inducible factor 1 (HIF-1) plays a pivotal role in tissue regeneration. HIF-1 is negatively controlled by O₂-dependent prolyl hydroxylases with a predominant role of prolyl hydroxylase 2 isoform (Phd2). Transgenic mice, hypomorphic for this isoform, accumulate more HIF-1 under normoxic conditions. Using these mice, we investigated the influence of Phd2 and HIF-1 on the regenerative capability of skeletal muscle tissue after myotrauma. Phd2-hypomorphic and wild type mice (on C57Bl/6 background) were grouped with regeneration times from 6 to 168 hours after closed mechanic muscle trauma to the hind limb. Tissue samples were analysed by immuno-staining and real-time PCR. Bone marrow derived macrophages of wild type and Phd2-hypomorphic mice were isolated and analysed via flow cytometry and quantitative real-time PCR. Phd2 reduction led to a higher regenerative capability due to enhanced activation of myogenic factors accompanied by induction of genes responsible for glucose and lactate metabolism in Phd2-hypomorphic mice. Macrophage infiltration into the trauma areas in hypomorphic mice started earlier and was more pronounced compared to wild type mice. Phd2-hypomorphic mice also showed higher numbers of macrophages in areas with sustained trauma 72 hours after myotrauma application. In conclusion, we postulate that the HIF-1 pathway is activated secondary to a Phd2 reduction which may lead to i) higher activation of myogenic factors, ii) increased number of positive stem cell proliferation markers, and iii) accelerated macrophage recruitment to areas of trauma, resulting in faster muscle tissue regeneration after myotrauma. With the current development of prolyl hydroxylase domain inhibitors, our findings point towards a potential clinical benefit after myotrauma.

Hypoxia and Innate Immunity: Keeping Up with the HIFsters

Recent years have witnessed an emergence of interest in understanding metabolic changes associated with immune responses, termed immunometabolism. As oxygen is central to all aerobic metabolism, hypoxia is now recognized to contribute fundamentally to inflammatory and immune responses. Studies from a number of groups have implicated a prominent role for oxygen metabolism and hypoxia in innate immunity of healthy tissue (physiologic hypoxia) and during active inflammation (inflammatory hypoxia). This inflammatory hypoxia emanates from a combination of recruited inflammatory cells (e.g., neutrophils, eosinophils, and monocytes), high rates of oxidative metabolism, and the activation of multiple oxygen-consuming enzymes during inflammation. These localized shifts toward hypoxia have identified a prominent role for the transcription factor hypoxia-inducible factor (HIF) in the regulation of innate immunity. Such studies have provided new and enlightening insight into our basic understanding of immune mechanisms, and extensions of these findings have identified potential therapeutic targets. In this review, we summarize recent literature around the topic of innate immunity and mucosal hypoxia with a focus on transcriptional responses mediated by HIF.

Mutual regulation between butyrate and hypoxia-inducible factor-1α in epithelial cell promotes expression of tight junction proteins

Inflammatory bowel disease is a kind of multi-aetiological chronic disease that is driven by multidimensional factors. Hypoxia-inducible factor-1α (HIF-1α) plays an important role in anti-inflammatory and cellular responses to hypoxia. Previous studies have found that B or T-cell-specific HIF-1α knock out mice exhibit severe colonic inflammation. However, we know very little about other functions of HIF-1α in intestinal epithelial cells (IECs). In our study, HIF-1α^ΔIEC mice were used to study the function of HIF-1α in IECs. HIF-1α was knocked down in Caco-2 cells by transfection with a small interfering (si) RNA. Immunohistochemical staining and western blotting were used to detect the expression of zonula occluden-1 (ZO-1) and Occludin. The content of colon was harvested for high-performance liquid chromatography analysis to examine the levels of butyrate in the gut. Our research found that HIF-1α played a protective role in dextran sulphate sodium-induced colitis, which was partly due to its regulation of tight junction (TJ) protein expression. Further study revealed that HIF-1α mediated TJ proteins levels by moderating the content of butyrate. Moreover, we found that butyrate regulated TJ protein expression, which is dependent on HIF-1α. These results indicated that there is a mutual regulatory mechanism between butyrate and HIF-1α, which has an important role in the maintenance of barrier function of the gastrointestinal tract.