Endothelial PAS domain protein 1 activates the inflammatory response in the intestinal epithelium to promote colitis in mice

Hypoxic Functional Regulation Pathways in the GI Tract: Focus on the HIF-1α and Microbiota's Crosstalk

Hypoxia is an essential gastrointestinal (GI) tract phenomenon that influences both physiologic and pathologic states. Hypoxia-inducible factors (HIFs), the primary drivers of cell adaptation to low-oxygen environments, have been identified as critical regulators of gut homeostasis: directly, through the induction of different proteins linked to intestinal barrier stabilization (ie, adherent proteins, tight junctions, mucins, integrins, intestinal trefoil factor, and adenosine); and indirectly, through the regulation of several immune cell types and the modulation of autophagy and inflammatory processes. Furthermore, hypoxia and HIF-related sensing pathways influence the delicate relationship existing between bacteria and mammalian host cells. In turn, gut commensals establish and maintain the physiologic hypoxia of the GI tract and HIF-α expression. Based on this premise, the goals of this review are to (1) highlight hypoxic molecular pathways in the GI tract, both in physiologic and pathophysiologic settings, such as inflammatory bowel disease; and (2) discuss a potential strategy for ameliorating gut-related disorders, by targeting HIF signaling, which can alleviate inflammatory processes, restore autophagy correct mechanisms, and benefit the host-microbiota equilibrium.

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).

Discovery and Validation of Ferroptosis-Associated Genes of Ulcerative Colitis

Background

Ulcerative colitis (UC) is a long-lasting idiopathic condition, but its precise mechanisms remain unclear. Meanwhile, evidence has demonstrated that ferroptosis seems to interlock with the progress of UC. This research sought to identify hub genes of UC related to ferroptosis.

Methods

First, the relevant profiles for this article were obtained from GEO database. From the FerrDb, 479 genes linked to ferroptosis were retrieved. Using analysis of the difference and WGCNA on colonic samples from GSE73661, the remaining six hub genes linked to ferroptosis and UC were discovered. Through logistic regression analyses, the diagnostic model was constructed and was then evaluated by external validation using dataset GSE92415. Afterwards, the correlation between immune cell filtration in UC and hub genes was examined. Finally, a mice model of colitis was established, and the results were verified using qRT-PCR.

Results

We acquired six hub genes linked to ferroptosis and UC. In order to create a diagnostic model for UC, we used logistic regression analysis to screen three of the six ferroptosis related genes (HIF1A, SLC7A11, and LPIN1). The ROC curve showed that the three hub genes had outstanding potential for disease diagnosis (AUC = 0.976), which was subsequently validated in samples from GSE92415 (AUC = 0.962) and blood samples from GSE3365 (AUC = 0.847) and GSE94648 (AUC = 0.769). These genes might be crucial for UC immunity based upon the results on the immune system. Furthermore, mouse samples examined using qRT-PCR also verified our findings.

Conclusion

In conclusion, the findings have important implications for ferroptosis and UC, and these hub genes may also offer fresh perspectives on the aetiology and therapeutic approaches of UC.

Hypoxia-inducible factor 3α1 increases epithelial-to-mesenchymal transition and iron uptake to drive colorectal cancer liver metastasis

BACKGROUND/OBJECTIVES

Hypoxia-inducible factor (HIF)-3α1's role in colorectal cancer (CRC) cells, especially its effects on epithelial-mesenchymal transition (EMT), zinc finger E-box binding homeobox 2 (ZEB2) gene expression, and iron metabolism, remains largely unstudied. This research sought to elucidate these relationships.

METHODS

RNA-seq was conducted to investigate the impact of HIF-3α1 overexpression in CRC cells. Dual-luciferase reporter assays assessed the direct targeting of ZEB2 by HIF-3α1. Scratch assays measured changes in cell migration following HIF-3α1 overexpression and ZEB2 knockdown. The effects of HIF-3α1 overexpression on colon tumour growth and liver metastasis were examined in vivo. Iron chelation was used to explore the role of iron metabolism in HIF-3α1-mediated EMT and tumour growth.

RESULTS

HIF-3α1 overexpression induced EMT and upregulated ZEB2 expression, enhancing cancer cell migration. ZEB2 knockdown reduced mesenchymal markers and cell migration. HIF-3α1 promoted colon tumour growth and liver metastasis, increased transferrin receptor (TFRC) expression and cellular iron levels, and downregulated HIF-1α, HIF-2α, and NDRG1. Iron chelation mitigated HIF-3α1-mediated EMT, tumour growth, and survival.

CONCLUSIONS

HIF-3α1 plays a critical role in colon cancer progression by promoting EMT, iron accumulation, and metastasis through ZEB2 and TFRC regulation, suggesting potential therapeutic targets in CRC.

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.

The emerging role of hypoxia and environmental factors in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic and debilitating disorder characterized by inflammation of the gastrointestinal tract. Despite extensive research, the exact cause of IBD remains unknown, hampering the development of effective therapies. However, emerging evidence suggests that hypoxia, a condition resulting from inadequate oxygen supply, plays a crucial role in intestinal inflammation and tissue damage in IBD. Hypoxia-inducible factors (HIFs), transcription factors that regulate the cellular response to low oxygen levels, have gained attention for their involvement in modulating inflammatory processes and maintaining tissue homeostasis. The two most studied HIFs, HIF-1α and HIF-2α, have been implicated in the development and progression of IBD. Toxicological factors encompass a wide range of environmental and endogenous agents, including dietary components, microbial metabolites, and pollutants. These factors can profoundly influence the hypoxic microenvironment within the gut, thereby exacerbating the course of IBD and fostering the progression of colitis-associated colorectal cancer. This review explores the regulation of hypoxia signaling at the molecular, microenvironmental, and environmental levels, investigating the intricate interplay between toxicological factors and hypoxic signaling in the context of IBD, focusing on its most concerning outcomes: intestinal fibrosis and colorectal cancer.

Activation of Intestinal HIF2α Ameliorates Iron-Refractory Anemia

In clinics, hepcidin levels are elevated in various anemia-related conditions, particularly in iron-refractory anemia and in high inflammatory states that suppress iron absorption, which remains an urgent unmet medical need. To identify effective treatment options for various types of iron-refractory anemia, the potential effect of hypoxia and pharmacologically-mimetic drug FG-4592 (Roxadustat) are evaluated, a hypoxia-inducible factor (HIF)-prolyl hydroxylase (PHD) inhibitor, on mouse models of iron-refractory iron-deficiency anemia (IRIDA), anemia of inflammation and 5-fluorouracil-induced chemotherapy-related anemia. The potent protective effects of both hypoxia and FG-4592 on IRIDA as well as other 2 tested mouse cohorts are found. Mechanistically, it is demonstrated that hypoxia or FG-4592 could stabilize duodenal Hif2α, leading to the activation of Fpn transcription regardless of hepcidin levels, which in turn results in increased intestinal iron absorption and the amelioration of hepcidin-activated anemias. Moreover, duodenal Hif2α overexpression fully rescues phenotypes of Tmprss6 knockout mice, and Hif2α knockout in the gut significantly delays the recovery from 5-fluorouracil-induced anemia, which can not be rescued by FG-4592 treatment. Taken together, the findings of this study provide compelling evidence that targeting intestinal hypoxia-related pathways can serve as a potential therapeutic strategy for treating a broad spectrum of anemia, especially iron refractory anemia.

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.

Disruption of hypoxia-inducible factor-2α in neutrophils decreases colitis-associated colon cancer

Neutrophils are abundant immune cells in the colon tumor microenvironment. Studies have shown that neutrophils are recruited into hypoxic foci in colon cancer. However, the impact of hypoxia signaling on neutrophil function and its involvement in colon tumorigenesis remain unclear. To address this, we generated mice with a deletion of hypoxia-inducible factor (HIF)-1α or HIF-2α in neutrophils driven by the MRP8Cre (HIF-1αΔNeu) or (HIF-2αΔNeu) and littermate controls. In an azoxymethane (AOM)/dextran sulfate sodium (DSS) model of colon cancer, the disruption of neutrophils-HIF-1α did not result in any significant changes in body weight, colon length, tumor size, proliferation, or burden. However, the disruption of HIF-2α in neutrophils led to a slight increase in body weight, a significant decrease in the number of tumors, and a reduction in tumor size and volume compared with their littermate controls. Histological analysis of colon tissue from mice with HIF-2α-deficient neutrophils revealed notable reductions in proliferation as compared with control mice. In addition, we observed reduced levels of proinflammatory cytokines, such as TNF-α and IL-1β, in neutrophil-specific HIF-2α-deficient mice in both the tumor tissue as well as the neutrophils. Importantly, it is worth noting that the reduced tumorigenesis associated with HIF-2α deficiency in neutrophils was not evident in already established syngeneic tumors or a DSS-induced inflammation model, indicating a potential role of HIF-2α specifically in colon tumorigenesis. In conclusion, we found that the loss of neutrophil-specific HIF-2α slows colon tumor growth and progression by reducing the levels of inflammatory mediators.NEW & NOTEWORTHY Despite the importance of hypoxia and neutrophils in colorectal cancer (CRC), the contribution of neutrophil-specific HIFs to colon tumorigenesis is not known. We describe that neutrophil HIF-1α has no impact on colon cancer, whereas neutrophil HIF-2α loss reduces CRC growth by decreasing proinflammatory and immunosuppressive cytokines. Furthermore, neutrophil HIF-2α does not reduce preestablished tumor growth or inflammation-induced colitis. The present study offers novel potential of neutrophil HIF-2α as a therapeutic target in CRC.

Iron promotes glycolysis to drive colon tumorigenesis

Colorectal cancer (CRC) is the third most common cancer and is also the third leading cause of cancer-related death in the USA. Understanding the mechanisms of growth and progression of CRC is essential to improve treatment. Macronutrients such as glucose are energy source for a cell. Many tumor cells exhibit increased aerobic glycolysis. Increased tissue micronutrient iron levels in both mice and humans are also associated with increased colon tumorigenesis. However, if iron drives colon carcinogenesis via affecting glucose metabolism is still not clear. Here we found the intracellular glucose levels in tumor colonoids were significantly increased after iron treatment. 13C-labeled glucose flux analysis indicated that the levels of several labeled glycolytic products were significantly increased, whereas several tricarboxylic acid cycle intermediates were significantly decreased in colonoids after iron treatment. Mechanistic studies showed that iron upregulated the expression of glucose transporter 1 (GLUT1) and mediated an inhibition of the pyruvate dehydrogenase (PDH) complex function via directly binding with tankyrase and/or pyruvate dehydrogenase kinase (PDHK) 3. Pharmacological inhibition of GLUT1 or PDHK reactivated PDH complex function and reduced high iron diet-enhanced tumor formation. In conclusion, excess iron promotes glycolysis and colon tumor growth at least partly through the inhibition of the PDH complex function.

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.

APE1/Ref-1 as a Therapeutic Target for Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is characterized by chronic relapsing inflammation of the gastrointestinal tract. The prevalence of IBD is increasing with approximately 4.9 million cases reported worldwide. Current therapies are limited due to the severity of side effects and long-term toxicity, therefore, the development of novel IBD treatments is necessitated. Recent findings support apurinic/apyrimidinic endonuclease 1/reduction-oxidation factor 1 (APE1/Ref-1) as a target in many pathological conditions, including inflammatory diseases, where APE1/Ref-1 regulation of crucial transcription factors impacts significant pathways. Thus, a potential target for a novel IBD therapy is the redox activity of the multifunctional protein APE1/Ref-1. This review elaborates on the status of conventional IBD treatments, the role of an APE1/Ref-1 in intestinal inflammation, and the potential of a small molecule inhibitor of APE1/Ref-1 redox activity to modulate inflammation, oxidative stress response, and enteric neuronal damage in IBD.

Intestinal expression patterns of transcription factors and markers for interstitial cells in the larval zebrafish

For the digestion of food, it is important for the gut to be differentiated regionally and to have proper motor control. However, the number of transcription factors that regulate its development is still limited. Meanwhile, the interstitial cells of the gastrointestinal (GI) tract are necessary for intestinal motility in addition to the enteric nervous system. There are anoctamine1 (Ano1)-positive and platelet-derived growth factor receptor α (Pdgfra)-positive interstitial cells in mammal, but Pdgfra-positive cells have not been reported in the zebrafish. To identify new transcription factors involved in GI tract development, we used RNA sequencing comparing between larval and adult gut. We isolated 40 transcription factors that were more highly expressed in the larval gut. We demonstrated expression patterns of the 13 genes, 7 of which were newly found to be expressed in the zebrafish larval gut. Six of the 13 genes encode nuclear receptors. The osr2 is expressed in the anterior part, while foxP4 in its distal part. Also, we reported the expression pattern of pdgfra for the first time in the larval zebrafish gut. Our data provide fundamental knowledge for studying vertebrate gut regionalization and motility by live imaging using zebrafish.

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.

Alpha-ketoglutarate alleviates cadmium-induced inflammation by inhibiting the HIF1A-TNFAIP3 pathway in hepatocytes

The threat to public health posed by rapidly increasing levels of cadmium (Cd) in the environment is receiving worldwide attention. Although, Cd is known to be absorbed into the body and causes non-negligible damage to the liver, the detailed mechanisms underlying its hepatoxicity are incompletely understood. In the present study, investigated the effect of TNFAIP3 and α-ketoglutarate (AKG) on Cd-induced liver inflammation and hepatocyte death. Male C57BL/6 mice were exposed to cadmium chloride (1.0 mg/kg) while being fed a diet with 2 % AKG for two weeks. We found that Cd induced hepatocyte injury and inflammatory infiltration. In addition, TNFAIP3 expression was inhibited in the liver tissues and cells of CdCl₂-treated mice. Mouse hepatocyte-specific TNFAIP3 overexpression by tail vein injection of an adeno-associated virus (AAV) vector effectively alleviated Cd-induced hepatic necrosis and inflammation, which was mediated by the NF-κB signaling pathway. Notably, this inhibitory effect of TNFAIP3 on Cd-induced liver injury was dependent on AKG. Exogenous addition of AKG prevented Cd exposure-induced increases in serum ALT, AST and LDH levels, production of pro-inflammatory cytokines, activation of the NF-κB signaling pathway, and even significantly reduced Cd-induced oxidative stress and hepatocyte death. Mechanistically, AKG exerted its anti-inflammatory effect by promoting the hydroxylation and degradation of HIF1A to reduce its Cd-induced overexpression in vivo and in vitro, avoiding the inhibition of the TNFAIP3 promoter by HIF1A. Moreover, the protective effect of AKG was significantly weaker in Cd-treated primary hepatocytes transfected with HIF1A pcDNA. Overall, our results reveal a novel mechanism of Cd-induced hepatotoxicity.

Hypoxia inducible factor-1α is an important regulator of macrophage biology

Hypoxia-inducible factor-1 (HIF-1), a heterodimeric transcription factor composed of the α and β subunits, regulates cellular adaptive responses to hypoxia. Macrophages, which are derived from monocytes, function as antigen-presenting cells that activate various immune responses. HIF-1α regulates the immune response, viability, migration, phenotypic plasticity, and metabolism of macrophages. Specifically, macrophage-derived HIF-1α can prevent excessive pro-inflammatory responses by attenuating the transcriptional activity of nuclear factor-kappa B in vivo and in vitro. HIF-1α modulates macrophage migration by inducing the release of various chemokines and providing necessary energy. HIF-1α promotes macrophage M1 polarization by targeting glucose metabolism. Additionally, HIF-1α induces the upregulation of glycolysis-related enzymes and intermediates of the tricarboxylic acid cycle and pentose phosphate pathway. HIF-1α promotes macrophage apoptosis, necroptosis and reduces autophagy. The current review highlights the mechanisms associated with the regulation of HIF-1α stabilization in macrophages as well as the role of HIF-1α in modulating the physiological functions of macrophages.

Evaluation of genetic variants in ferroptosis-related genes and house dust mite-induced allergic rhinitis risk

BACKGROUND

Ferroptosis-related genes disrupt iron homeostasis and enhance lipid peroxidation to initiate respiratory system diseases. However, the association between genetic variants in the ferroptosis-related genes with house dust mite (HDM)-induced allergic rhinitis (AR) susceptibility remains unclear.

METHODS

A case-control study, involving 222 cases and 237 healthy controls from a Chinese population, was conducted to evaluate the relationship between single nucleotide polymorphisms (SNPs) in ferroptosis-related genes and HDM-induced AR risk. A gene-based analysis was performed by multi-marker analysis of genomic annotation (MAGMA) to identify candidate associated ferroptosis-related genes. A logistic regression model and joint analysis were used to assess the effect of SNPs on HDM-induced AR susceptibility.

RESULTS

Two independent SNPs (rs2305128 in ENPP2 and rs1868088 in EPAS1) were significantly associated with HDM-induced AR risk (OR = 1.82, 95% CI = 1.19-2.79, P = 5.98 × 10-3, PFDR = 4.88 × 10-2; OR = 2.14, 95% CI = 1.23-3.72, P = 6.95 × 10-3, PFDR = 4.87 × 10-2, respectively). Moreover, combined analysis of these two SNPs revealed that an increased risk of HDM-induced AR was positively associated with an increasing number of risk genotypes (Ptrend = 8.48 × 10-5). The stratification analysis showed that the cumulative effect of two SNPs on HDM-induced AR risk was more pronounced among patients presenting more serious symptoms and harboring one or two risk genotypes.

CONCLUSIONS

These findings suggest that the genetic variants in ferroptosis-related genes ENPP2 and EPAS1 may increase HDM-induced AR risk and serve as potential predictors of HDM-induced AR susceptibility.

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.

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.

Updated perspective of EPAS1 and the role in pulmonary hypertension

Pulmonary hypertension (PH) is a group of syndromes characterized by irreversible vascular remodeling and persistent elevation of pulmonary vascular resistance and pressure, leading to ultimately right heart failure and even death. Current therapeutic strategies mainly focus on symptoms alleviation by stimulating pulmonary vessel dilation. Unfortunately, the mechanism and interventional management of vascular remodeling are still yet unrevealed. Hypoxia plays a central role in the pathogenesis of PH and numerous studies have shown the relationship between PH and hypoxia-inducible factors family. EPAS1, known as hypoxia-inducible factor-2 alpha (HIF-2α), functions as a transcription factor participating in various cellular pathways. However, the detailed mechanism of EPAS1 has not been fully and systematically described. This article exhibited a comprehensive summary of EPAS1 including the molecular structure, biological function and regulatory network in PH and other relevant cardiovascular diseases, and furthermore, provided theoretical reference for the potential novel target for future PH intervention.

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.

The Role of Inflammatory Mediators in Colorectal Cancer Hepatic Metastasis

Colorectal cancer (CRC) is the second leading cause of death in cancer patients in the USA, whereas the major cause of CRC deaths is hepatic metastases. The liver is the most common site of metastasis in patients with CRC due to hepatic portal veins receiving blood from the digestive tract. Understanding the cellular and molecular mechanisms of hepatic metastases is of dire need for the development of potent targeted therapeutics. Immuno-signaling molecules including cytokines and chemokines play a pivotal role in hepatic metastases from CRC. This brief review discusses the involvement of three representative cytokines (TNF-α, IL-6 and IL-1β), a lipid molecule PGE2 and two chemokines (CXCL1 and CXCL2) in the process of CRC liver metastases.

Hypoxia: molecular pathophysiological mechanisms in human diseases

Hypoxia, a low O2 tension, is a fundamental feature that occurs in physiological events as well as pathophysiological conditions, especially mentioned for its role in the mechanism of angiogenesis, glucose metabolism, and cell proliferation/survival. The hypoxic state through the activation of specific mechanisms is an aggravating circumstance commonly noticed in multiple sclerosis, cancer, heart disease, kidney disease, liver disease, lung disease, and in inflammatory bowel disease. On the other hand, hypoxia could play a key role in tissue regeneration and repair of damaged tissues, especially by acting on specific tissue stem cells, but their features may result as a disadvantage when it is concerned for neoplastic stem cells. Furthermore, hypoxia could also have a potential role in tissue engineering and regenerative medicine due to its capacity to improve the performance of biomaterials. The current review aims to highlight the hypoxic molecular mechanisms reported in different pathological conditions to provide an overview of hypoxia as a therapeutic agent in regenerative and molecular therapy.

Graphical abstract

Epithelial HIF2α expression induces intestinal barrier dysfunction and exacerbation of arthritis

OBJECTIVE

To investigate how the mucosal barrier in the intestine influences the development of arthritis, considering that metabolic changes in the intestinal epithelium influence its barrier function.

METHODS

Intestinal hypoxia inducible factor (HIF)-2α expression was assessed before, at onset and during experimental arthritis and human rheumatoid arthritis (RA). Intestinal epithelial cell-specific HIF2α conditional knock-out mice were generated (HIF2α∆IEC) and subjected to collagen-induced arthritis. Clinical and histological courses of arthritis were recorded; T-cell and B-cell subsets were analysed in the gut and secondary lymphatic organs; and intestinal epithelial cells were subjected to molecular mRNA sequencing in HIF2α∆IEC and littermate control mice. The gut intestinal HIF2α target genes were delineated by chromatin immunoprecipitation and luciferase experiments. Furthermore, pharmacological HIF2α inhibitor PT2977 was used for inhibition of arthritis.

RESULTS

Intestinal HIF2α expression peaked at onset of experimental arthritis and RA. Conditionally, deletion of HIF2α in gut epithelial cells inhibited arthritis and was associated with improved intestinal barrier function and less intestinal and lymphatic Th1 and Th17 activation. Mechanistically, HIF2α induced the transcription of the pore-forming claudin (CLDN)-15, which inhibits intestinal barrier integrity. Furthermore, treatment with HIF2α inhibitor decreased claudin-15 expression in epithelial cells and inhibited arthritis.

CONCLUSION

These findings show that the HIF2α-CLDN15 axis is critical for the breakdown of intestinal barrier function at onset of arthritis, highlighting the functional link between intestinal homeostasis and arthritis.

Small nucleolar RNAs and SNHGs in the intestinal mucosal barrier: Emerging insights and current roles

BACKGROUND

Previous studies have focused on the involvement of small nucleolar RNAs (snoRNAs) and SNHGs in tumor cell proliferation, apoptosis, invasion, and metastasis via multiple pathways, including phosphatidylinositol-3-kinase/protein kinase B (PI3K/AKT), Wnt/β catenin, and mitogen-activated protein kinase (MAPK). These molecular mechanisms affect the integrity of the intestinal mucosal barrier.

AIM OF REVIEW

Current evidence regarding snoRNAs and SNHGs in the context of the mucosal barrier and modulation of homeostasis is fragmented. In this review, we collate the established information on snoRNAs and SNHGs as well as discuss the major pathways affecting the mucosal barrier.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Intestinal mucosal immunity, microflora, and the physical barrier are altered in non-neoplastic diseases such as inflammatory bowel diseases. Dysregulated snoRNAs and SNHGs may impact the intestinal mucosal barrier to promote the pathogenesis and progression of multiple diseases. SnoRNAs or SNHGs has been shown to be associated with poor disease behaviors, indicating that they may be exploited as prognostic biomarkers. Additionally, clarifying the complicated interactions between snoRNAs or SNHGs and the mucosal barrier may provide novel insights for the therapeutic treatment targeting strengthen the intestinal mucosal barrier.

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.

Disruption of adipocyte HIF-1α improves atherosclerosis through the inhibition of ceramide generation

Atherosclerosis is a chronic multifactorial cardiovascular disease. Western diets have been reported to affect atherosclerosis through regulating adipose function. In high cholesterol diet-fed ApoE^–/– mice, adipocyte HIF-1α deficiency or direct inhibition of HIF-1α by the selective pharmacological HIF-1α inhibitor PX-478 alleviates high cholesterol diet-induced atherosclerosis by reducing adipose ceramide generation, which lowers cholesterol levels and reduces inflammatory responses, resulting in improved dyslipidemia and atherogenesis. Smpd3, the gene encoding neutral sphingomyelinase, is identified as a new target gene directly regulated by HIF-1α that is involved in ceramide generation. Injection of lentivirus-SMPD3 in epididymal adipose tissue reverses the decrease in ceramides in adipocytes and eliminates the improvements on atherosclerosis in the adipocyte HIF-1α-deficient mice. Therefore, HIF-1α inhibition may constitute a novel approach to slow atherosclerotic progression.

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.

Intestinal HIF-2α Regulates GLP-1 Secretion via Lipid Sensing in L-Cells

BACKGROUND & AIMS

Compelling evidence shows that glucagon-like peptide-1 (GLP-1) has a profound effect in restoring normoglycemia in type 2 diabetic patients by increasing pancreatic insulin secretion. Although L-cells are the primary source of circulating GLP-1, the current therapies do not target L-cells to increase GLP-1 levels. Our study aimed to determine the molecular underpinnings of GLP-1 secretion as an impetus to identify new interventions to target endogenous L-cells.

METHODS

We used genetic mouse models of intestine-specific overexpression of hypoxia-inducible factor (HIF)-1α and HIF-2α (Vhl^ΔIE), conditional overexpression of intestinal HIF-2α (Hif-2α^{LSL;Vilin-Cre/ERT2}), and intestine-specific HIF-2α knockout mice (Hif-2α^ΔIE) to show that HIF signaling, especially HIF-2α, regulates GLP-1 secretion.

RESULTS

Our data show that intestinal HIF signaling improved glucose homeostasis in a GLP-1-dependent manner. Intestinal HIF potentiated GLP-1 secretion via the lipid sensor G-protein-coupled receptor (GPR)40 enriched in L-cells. We show that HIF-2α regulates GPR40 in L-cells and potentiates fatty acid-induced GLP-1 secretion via extracellular regulated kinase (ERK). Using a genetic model of intestine-specific overexpression of HIF-2α, we show that HIF-2α is sufficient to increase GLP-1 levels and attenuate diet-induced metabolic perturbations such as visceral adiposity, glucose intolerance, and hepatic steatosis. Lastly, we show that intestinal HIF-2α signaling acts as a priming mechanism crucial for postprandial lipid-mediated GLP-1 secretion. Thus, disruption of intestinal HIF-2α decreases GLP-1 secretion.

CONCLUSIONS

In summary, we show that intestinal HIF signaling, particularly HIF-2α, regulates the lipid sensor GPR40, which is crucial for the lipid-mediated GLP-1 secretion, and suggest that HIF-2α is a potential target to induce endogenous GLP-1 secretion.

HIF-Dependent NFATC1 Activation Upregulates ITGA5 and PLAUR in Intestinal Epithelium in Inflammatory Bowel Disease

Intestinal epithelial cells exist in physiological hypoxia, leading to hypoxia-inducible factor (HIF) activation and supporting barrier function and cell metabolism of the intestinal epithelium. In contrast, pathological hypoxia is a common feature of some chronic disorders, including inflammatory bowel disease (IBD). This work was aimed at studying HIF-associated changes in the intestinal epithelium in IBD. In the first step, a list of genes responding to chemical activation of hypoxia was obtained in an in vitro intestinal cell model with RNA sequencing. Cobalt (II) chloride and oxyquinoline treatment of both undifferentiated and differentiated Caco-2 cells activate the HIF-signaling pathway according to gene set enrichment analysis. The core gene set responding to chemical hypoxia stimulation in the intestinal model included 115 upregulated and 69 downregulated genes. Of this set, protein product was detected for 32 genes, and fold changes in proteome and RNA sequencing significantly correlate. Analysis of publicly available RNA sequencing set of the intestinal epithelial cells of patients with IBD confirmed HIF-1 signaling pathway activation in sigmoid colon of patients with ulcerative colitis and terminal ileum of patients with Crohn's disease. Of the core gene set from the gut hypoxia model, expression activation of ITGA5 and PLAUR genes encoding integrin α5 and urokinase-type plasminogen activator receptor (uPAR) was detected in IBD specimens. The interaction of these molecules can activate cell migration and regenerative processes in the epithelium. Transcription factor analysis with the previously developed miRGTF tool revealed the possible role of HIF1A and NFATC1 in the regulation of ITGA5 and PLAUR gene expression. Detected genes can serve as markers of IBD progression and intestinal hypoxia.

Melatonin-Mediated Colonic Microbiota Metabolite Butyrate Prevents Acute Sleep Deprivation-Induced Colitis in Mice

Radical cure colitis is a severe public health threat worldwide. Our previous studies have confirmed that melatonin can effectively improve gut microbiota disorder and mucosal injury caused by sleep deprivation (SD). The present study further explored the mechanism whereby exogenous melatonin prevented SD-induced colitis. 16S rRNA high-throughput sequencing and metabolomics analysis were used to explore the correlation between SD-induced colitis and intestinal microbiota and metabolite composition in mice. Fecal microbiota transplantation (FMT) and melatonin or butyrate supplementation tests verified the core role of gut microbiota in melatonin-alleviating SD-induced colitis. Further, in vitro tests studied the modulatory mechanism of metabolite butyrate. The results demonstrated that SD leads to reductions in plasma melatonin levels and colonic Card9 expression and consequent occurrence of colitis and gut microbiota disorder, especially the downregulation of Faecalibacterium and butyrate levels. The FMT from SD-mice to normal mice could restore SD-like colitis, while butyrate supplementation to SD-mice inhibited the occurrence of colitis, but with no change in the plasma melatonin level in both treatments. However, melatonin supplementation reversed all inductions in SD-mice. In intestinal epithelial cells, the inflammatory ameliorative effect of butyrate was blocked with pretreatments of HDAC3 agonist and HIF-1α antagonist but was mimicked by GSK-3β and p-P65 antagonists. Therefore, the administration of MLT may be a better therapy for SD-induced colitis relative to butyrate. A feasible mechanism would involve that melatonin up-regulated the Faecalibacterium population and production of its metabolite butyrate and MCT1 expression and inhibited HDAC3 in the colon, which would allow p-GSK-3β/β-catenin/HIF-1α activation and NF-κB/NLRP3 suppression to up-regulate Card9 expression and suppress inflammation response.

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.

Macrophage HIF-2α suppresses NLRP3 inflammasome activation and alleviates insulin resistance

The interrelation between hypoxia and immune response has pivotal roles in the pathogenesis of chronic metabolic diseases. However, the role of macrophage HIF-2α in NLRP3 inflammasome activation remains unclear. Here, we show that deficiency of HIF-2α in macrophages results in excessive activation of the NLRP3 inflammasome in a manner dependent on CPT1A-mediated enhancement of fatty acid oxidation (FAO). Mechanistically, HIF-2α binds directly to the Cpt1a promoter and is involved in the regulation of H3K27me3 methylation during NLRP3 inflammasome activation. Myeloid-specific Hif2α knockout mice exhibit exacerbated insulin resistance and increased activation of NLRP3 inflammasome in macrophages. Overexpression of the Hif2α gene or stabilization of the protein by FG-4592 ameliorates insulin resistance and reduces NLRP3 inflammasome activation in macrophages. Taken together, our results suggest that macrophage HIF-2α inhibits FAO-mediated activation of the NLRP3 inflammasome and alleviates insulin resistance.

Charting human development using a multi-endodermal organ atlas and organoid models

Organs are composed of diverse cell types that traverse transient states during organogenesis. To interrogate this diversity during human development, we generate a single-cell transcriptome atlas from multiple developing endodermal organs of the respiratory and gastrointestinal tract. We illuminate cell states, transcription factors, and organ-specific epithelial stem cell and mesenchyme interactions across lineages. We implement the atlas as a high-dimensional search space to benchmark human pluripotent stem cell (hPSC)-derived intestinal organoids (HIOs) under multiple culture conditions. We show that HIOs recapitulate reference cell states and use HIOs to reconstruct the molecular dynamics of intestinal epithelium and mesenchyme emergence. We show that the mesenchyme-derived niche cue NRG1 enhances intestinal stem cell maturation in vitro and that the homeobox transcription factor CDX2 is required for regionalization of intestinal epithelium and mesenchyme in humans. This work combines cell atlases and organoid technologies to understand how human organ development is orchestrated.

The role of mucosal barriers in human gut health

The intestinal mucosa is continuously exposed to a large number of commensal or pathogenic microbiota and foreign food antigens. The intestinal epithelium forms a dynamic physicochemical barrier to maintain immune homeostasis. To efficiently absorb nutrients from food, the epithelium in the small intestine has thin, permeable layers spread over a vast surface area. Epithelial cells are renewed from the crypt toward the villi, accompanying epithelial cell death and shedding, to control bacterial colonization. Tight junction and adherens junction proteins provide epithelial cell-cell integrity. Microbial signals are recognized by epithelial cells via toll-like receptors. Environmental signals from short-chain fatty acids derived from commensal microbiota metabolites, aryl hydrocarbon receptors, and hypoxia-induced factors fortify gut barrier function. Here we summarize recent findings regarding various environmental factors for gut barrier function. Further, we discuss the role of gut barriers in the pathogenesis of human intestinal disease and the challenges of therapeutic strategies targeting gut barrier restoration.

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.

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.

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.

Differences in Tolerance to Hypoxia: Physiological, Biochemical, and Molecular-Biological Characteristics

Hypoxia plays an important role in the development of many infectious, inflammatory, and tumor diseases. The predisposition to such disorders is mostly provided by differences in basic tolerance to oxygen deficiency, which we discuss in this review. Except the direct exposure of different-severity hypoxia in decompression chambers or in highland conditions, there are no alternative methods for determining organism tolerance. Due to the variability of the detection methods, differences in many parameters between tolerant and susceptible organisms are still not well-characterized, but some of them can serve as biomarkers of susceptibility to hypoxia. At the moment, several potential biomarkers in conditions after hypoxic exposure have been identified both in experimental animals and humans. The main potential biomarkers are Hypoxia-Inducible Factor (HIF)-1, Heat-Shock Protein 70 (HSP70), and NO. Due to the different mechanisms of various high-altitude diseases, biomarkers may not be highly specific and universal. Therefore, it is extremely important to conduct research on hypoxia susceptibility biomarkers. Moreover, it is important to develop a method for the evaluation of organisms' basic hypoxia tolerance without the necessity of any oxygen deficiency exposure. This can contribute to new personalized medicine approaches' development for diagnostics and the treatment of inflammatory and tumor diseases, taking into account hypoxia tolerance differences.

Homocysteine promotes hepatic steatosis by activating the adipocyte lipolysis in a HIF1α-ERO1α-dependent oxidative stress manner

Hyperhomocysteinemia (HHcy) is related to liver diseases, such as nonalcoholic fatty liver (NAFL). Although the precise pathogenesis of NAFL is still largely unknown, the links between organs seem to play a vital role. The current study aimed to explore the role of white adipose tissue in homocysteine (Hcy)-induced NAFL. Blood samples from nonhyperhomocysteinemia or hyperhomocysteinemia individuals were collected to assess correlation between Hcy and triglyceride (TG) or free fatty acids (FFAs) levels. C57BL/6 mice were maintained on a high-methionine diet or administered with Hcy (1.8 g/L) in the drinking water to establish an HHcy mouse model. We demonstrated that Hcy activated adipocyte lipolysis and that this change was accompanied by an increased release of FFAs and glycerol. Excessive FFAs were taken up by hepatocyte, which resulted in lipid accumulation in the liver. Treatment with acipimox (0.08 g kg ⁻¹ day ⁻¹), a potent chemical inhibitor of lipolysis, markedly decreased Hcy-induced NAFL. Mechanistically, hypoxia-inducible factor 1α (HIF1α)-endoplasmic reticulum oxidoreductin 1α (ERO1α) mediated pathway promoted H₂O₂ accumulation and induced endoplasmic reticulum (ER) overoxidation, ER stress and more closed ER-lipid droplet interactions, which were responsible for activating the lipolytic response. In conclusion, this study reveals that Hcy activates adipocyte lipolysis and suggests the potential utility of targeted ER redox homeostasis for treating Hcy-induced NAFL.

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Hcy elevates adipocyte lipolysis process.

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Inhibition of adipocyte lipolysis via acipimox improves the Hcy-induced nonalcoholic fatty liver.

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Adipocyte lipolytic response relies on ERO1α-mediated oxidative stress.

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Activation of adipocyte HIF1α mediates ERO1α upregulation.

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Deficiency of adipocyte HIF1α alleviates the Hcy-induced lipolytic response and nonalcoholic fatty liver.

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.

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.

Xenon blunts NF-κB/NLRP3 inflammasome activation and improves acute onset of accelerated and severe lupus nephritis in mice

Xenon, an inert anesthetic gas, is increasingly recognized to possess desirable properties including cytoprotective and anti-inflammatory effects. Here we evaluated the effects of xenon on the progression of lupus nephritis (LN) in a mouse model. A two hour exposure of either 70% xenon or 70% nitrogen balanced with oxygen was administered daily for five weeks to female NZB/W F1 mice that had been induced to develop accelerated and severe LN. Xenon treatment improved kidney function and renal histology, and decreased the renal expression of neutrophil chemoattractants, thereby attenuating glomerular neutrophil infiltration. The effects of xenon were mediated primarily by deceasing serum levels of anti-double stranded DNA autoantibody, inhibiting reactive oxygen species production, NF-κB/NLRP3 inflammasome activation, ICAM-1 expression, glomerular deposition of IgG and C3 and apoptosis, in the kidney; and enhancing renal hypoxia inducible factor 1-α expression. Proteomic analysis revealed that the treatment with xenon downregulated renal NLRP3 inflammasome-mediated cellular signaling. Similarly, xenon was effective in improving renal pathology and function in a spontaneous LN model in female NZB/W F1 mice. Thus, xenon may have a therapeutic role in treating LN but further studies are warranted to determine applicability to patients.

Hypoxia is a key mechanism for regulating inflammation in ulcerative colitis

Intestinal bowel diseases (IBD), including ulcerative colitis (UC), is the group of difficult to diagnose widespread among the population diseases. Pathogenesis of the disease is associated with a complex interaction of the genetic factors, the environment, the microbiome and the unpredicted reaction of the immune system, and the existing treatment methods are not effective enough. It is known, that hypoxia plays a key role in both system and local inflammatory reactions, mainly due to microcirculatory disorders and disseminated intravascular coagulation. Therefore a lot of studies have demonstrated that severity of any inflammatory diseases, including Crohn's disease (CD) and UC depends on hypoxia resistance. In this review we discussed microcirculation of blood and physiological hypoxia in the intestine, the role of hypoxia-inducible factors in the development of IBD and UC, as well as their influence on the severity of the inflammatory process. Authors described the protective effect of various PHD inhibitors and its benefits and disadvantages, so as new approaches of searching of very specific low molecular weight substanses as drugs for the control of IBD and UC.

Hypoxia-inducible factors in metabolic reprogramming during sepsis

Sepsis is a highly heterogeneous syndrome that is caused by an imbalanced host response to infection. Despite huge investments, sepsis remains a contemporary threat with significant burden on health systems. Vascular dysfunction and elevated oxygen consumption by highly metabolically active immune cells result in tissue hypoxia during inflammation. The transcription factor hypoxia-inducible factor-1a (HIF1α), and its family members, plays an important role in cellular metabolism and adaptation to cellular stress caused by hypoxia. In this review, we discuss the role of HIF in sepsis. We show possible mechanisms by which the inflammatory response activated during sepsis affects the HIF pathway. The activated HIF pathway in turn changes the metabolism of both innate and adaptive immune cells. As HIF expression in leukocytes of septic patients can be directly linked with mortality, we discuss multiple ways of interfering with the HIF signaling pathway.

Pathway paradigms revealed from the genetics of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a complex genetic disease that is instigated and amplified by the confluence of multiple genetic and environmental variables that perturb the immune-microbiome axis. The challenge of dissecting pathological mechanisms underlying IBD has led to the development of transformative approaches in human genetics and functional genomics. Here we describe IBD as a model disease in the context of leveraging human genetics to dissect interactions in cellular and molecular pathways that regulate homeostasis of the mucosal immune system. Finally, we synthesize emerging insights from multiple experimental approaches into pathway paradigms and discuss future prospects for disease-subtype classification and therapeutic intervention.

Mucosal inflammation downregulates PHD1 expression promoting a barrier-protective HIF-1α response in ulcerative colitis patients

The HIF hydroxylase enzymes (PHD1-3 and FIH) are cellular oxygen-sensors which confer hypoxic-sensitivity upon the hypoxia-inducible factors HIF-1α and HIF-2α. Microenvironmental hypoxia has a strong influence on the epithelial and immune cell function through HIF-dependent gene expression and consequently impacts upon the course of disease progression in ulcerative colitis (UC), with HIF-1α being protective while HIF-2α promotes disease. However, little is known about how inflammation regulates hypoxia-responsive pathways in UC patients. Here we demonstrate that hypoxia is a prominent microenvironmental feature of the mucosa in UC patients with active inflammatory disease. Furthermore, we found that inflammation drives transcriptional programming of the HIF pathway including downregulation of PHD1 thereby increasing the tissue responsiveness to hypoxia and skewing this response toward protective HIF-1 over detrimental HIF-2 activation. We identified CEBPα as a transcriptional regulator of PHD1 mRNA expression which is downregulated in both inflamed tissue derived from patients and in cultured intestinal epithelial cells treated with inflammatory cytokines. In summary, we propose that PHD1 downregulation skews the hypoxic response toward enhanced protective HIF-1α stabilization in the inflamed mucosa of UC patients.

The Role of Immune Cells and Cytokines in Intestinal Wound Healing

Intestinal wound healing is a complicated process that not only involves epithelial cells but also immune cells. In this brief review, we will focus on discussing the contribution and regulation of four major immune cell types (neutrophils, macrophages, regulatory T cells, and innate lymphoid cells) and four cytokines (interleukin-10, tumor necrosis factor alpha, interleukin-6, and interleukin-22) to the wound repair process in the gut. Better understanding of these immune factors will be important for developing novel targeted therapy.

Adipocyte Hypoxia-Inducible Factor 2α Suppresses Atherosclerosis by Promoting Adipose Ceramide Catabolism

Obesity-induced adipose dysfunction is a major contributor to atherosclerosis. Cold exposure has been reported to affect atherosclerosis through regulation of adipose function, but the mechanism has not been well clarified. Here, adipocyte hypoxia-inducible factor 2α (HIF-2α) was upregulated after mild cold exposure at 16°C and mediated cold-induced thermogenesis. Adipocyte HIF-2α deficiency exacerbated Western-diet-induced atherosclerosis by increasing adipose ceramide levels, which blunted hepatocyte cholesterol elimination and thermogenesis. Mechanistically, Acer2, the gene encoding alkaline ceramidase 2, was identified as a novel target gene of HIF-2α, triggering ceramide catabolism. Adipose overexpression of ACER2 rescued adipocyte HIF-2α-deficiency-induced exacerbation of atherosclerosis. Furthermore, activation of adipose HIF-2α by the HIF prolyl hydroxylase inhibitor FG-4592 had protective effects on atherosclerosis, accompanied by a reduction in adipose and plasma ceramide and plasma cholesterol levels. This study highlights adipocyte HIF-2α as a putative drug target against atherosclerosis.

Macrophage metabolic reprogramming aggravates aortic dissection through the HIF1α-ADAM17 pathway✰

Background

Aortic dissection is a severe inflammatory vascular disease with high mortality and limited therapeutic options. The hallmarks of aortic dissection comprise aortic inflammatory cell infiltration and elastic fiber disruption, highlighting the involvement of macrophage. Here a role for macrophage hypoxia-inducible factor 1-alpha (HIF-1α) in aortic dissection was uncovered.

Methods

Immunochemistry, immunofluorescence, western blot and qPCR were performed to test the change of macrophage HIF-1α in two kinds of aortic dissection models and human tissues. Metabolomics and Seahorse extracellular flux analysis were used to detect the metabolic state of macrophages involved in the development of aortic dissection. Chromatin Immunoprecipitation (ChIP), enzyme-linked immunosorbent assay (ELISA) and cytometric bead array (CBA) were employed for mechanistic studies.

Findings

Macrophages involved underwent distinct metabolic reprogramming, especially fumarate accumulation, thus inducing HIF-1α activation in the development of aortic dissection in human and mouse models. Mechanistic studies revealed that macrophage HIF-1α activation triggered vascular inflammation, extracellular matrix degradation and elastic plate breakage through increased a disintegrin and metallopeptidase domain 17 (ADAM17), identified as a novel target gene of HIF-1α. A HIF-1α specific inhibitor acriflavine elicited protective effects on aortic dissection dependent on macrophage HIF-1α.

Interpretation

This study reveals that macrophage metabolic reprogramming activates HIF-1α and subsequently promotes aortic dissection progression, suggesting that macrophage HIF-1α inhibition might be a potential therapeutic target for treating aortic dissection.