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

Possible Contribution of Inflammation-Associated Hypoxia to Increased K2P5.1 K+ Channel Expression in CD4+ T cells of the Mouse Model for Inflammatory Bowel Disease

Previous studies have reported the up-regulation of the two-pore domain K⁺ channel K_2P5.1 in the CD4⁺ T cells of patients with multiple sclerosis (MS) and rheumatoid arthritis (RA), as well as in a mouse model of inflammatory bowel disease (IBD). However, the mechanisms underlying this up-regulation remain unclear. Inflammation-associated hypoxia is involved in the pathogenesis of autoimmune diseases, such as IBD, MS, and RA, and T cells are exposed to a hypoxic environment during their recruitment from inflamed tissues to secondary lymphoid tissues. We herein investigated whether inflammation-associated hypoxia is attributable to the increased expression and activity of K_2P5.1 in the splenic CD4⁺ T cells of chemically-induced IBD model mice. Significant increases in hypoxia-inducible factor (HIF)-1α transcripts and proteins were found in the splenic CD4⁺ T cells of the IBD model. In the activated splenic CD4⁺ T cells, hypoxia (1.5% O₂) increased K_2P5.1 expression and activity, whereas a treatment with the HIF inhibitor FM19G11 but not the selective HIF-2 inhibitor exerted the opposite effect. Hypoxia-exposed K_2P5.1 up-regulation was also detected in stimulated thymocytes and the mouse T-cell line. The class III histone deacetylase sirtuin-1 (SIRT1) is a downstream molecule of HIF-1α signaling. We examined the effects of the SIRT1 inhibitor NCO-01 on K_2P5.1 transcription in activated CD4⁺ T cells, and we found no significant effects on the K_2P5.1 transcription. No acute compensatory responses of K_2P3.1–K_2P5.1 up-regulation were found in the CD4⁺ T cells of the IBD model and the hypoxia-exposed T cells. Collectively, these results suggest a mechanism for K_2P5.1 up-regulation via HIF-1 in the CD4⁺ T cells of the IBD model.

Dendritic Cells as Sensors, Mediators, and Regulators of Ischemic Injury

Dendritic cells (DCs) are highly specialized, bone marrow (BM)-derived antigen-processing and -presenting cells crucial to the induction, integration and regulation of innate, and adaptive immunity. They are stimulated by damage-associated molecular patterns (DAMPS) via pattern recognition receptors to promote inflammation and initiate immune responses. In addition to residing within the parenchyma of all organs as part of the heterogeneous mononuclear phagocyte system, DCs are an abundant component of the inflammatory cell infiltrate that appears in response to ischemia reperfusion injury (IRI). They can play disparate roles in the pathogenesis of IRI since their selective depletion has been found to be protective, deleterious, or of no benefit in mouse models of IRI. In addition, administration of DC generated and manipulated ex vivo can protect organs from IRI by suppressing inflammatory cytokine production, limiting the capacity of DCs to activate NKT cells, or enhancing regulatory T cell function. Few studies however have investigated specific signal transduction mechanisms underlying DC function and how these affect IRI. Here, we address current knowledge of the role of DCs in regulation of IRI, current gaps in understanding and prospects for innovative therapeutic intervention at the biological and pharmacological levels.

Systematic large-scale meta-analysis identifies miRNA-429/200a/b and miRNA-141/200c clusters as biomarkers for necrotizing enterocolitis in newborn

Necrotizing enterocolitis (NEC) is a critical neonatal disease with a high mortality. The possibility that miRNAs may play an important role in NEC has raised great attention. Hence, the present study identified biomarkers that affected NEC in newborn progression through miRNA and gene expression profile analysis. miRNA chip GSE68054 and gene chip GSE46619 of NEC in newborn were analyzed to screen out differentially expressed miRNA and differentially expressed genes (DEGs). Next, target genes of differentially expressed miRNA were predicted, and differentially expressed miRNA-DEG regulatory network was constructed to select key miRNAs. After gene ontology and kyoto encyclopedia of genes and genomes enrichment analysis on target genes of key miRNAs, the target genes enriched in pathways were extracted to establish differentially expressed miRNA-DEG-disease gene network for gene interaction analysis. Targetting relationship between miRNAs and target genes was verified. A total of 15 miRNAs were differentially expressed in NEC in newborn, amongst which miR-429/200a/b and miR-141/200c clusters were poorly expressed and might play a significant role in NEC in newborn. Besides, target genes of miR-429/200a/b and miR-141/200c clusters were enriched in 11 signaling pathways. Vascular endothelial growth factor (VEGFA), E-selectin (SELE), kinase insert domain receptor (KDR), fms-related tyrosine kinase 1 (FLT1), and hepatocyte growth factor (HGF) were highly expressed in NEC in newborn, which were negatively regulated by miR-429/200a/b and miR-141/200c clusters and shared close association with disease genes. miR-429/200a/b and miR-141/200c clusters are poorly expressed while their target genes (VEGFA, SELE, KDR, FLT1, and HGF) are highly expressed in NEC in newborn, which might be identified as important biomarkers for this disease.

Immunity, Hypoxia, and Metabolism-the Ménage à Trois of Cancer: Implications for Immunotherapy

It is generally accepted that metabolism is able to shape the immune response. Only recently we are gaining awareness that the metabolic crosstalk between different tumor compartments strongly contributes to the harsh tumor microenvironment (TME) and ultimately impairs immune cell fitness and effector functions. The major aims of this review are to provide an overview on the immune system in cancer; to position oxygen shortage and metabolic competition as the ground of a restrictive TME and as important players in the anti-tumor immune response; to define how immunotherapies affect hypoxia/oxygen delivery and the metabolic landscape of the tumor; and vice versa, how oxygen and metabolites within the TME impinge on the success of immunotherapies. By analyzing preclinical and clinical endeavors, we will discuss how a metabolic characterization of the TME can identify novel targets and signatures that could be exploited in combination with standard immunotherapies and can help to predict the benefit of new and traditional immunotherapeutic drugs.

Analysis of hypoxia-associated dendritic cells in colitic mice and effects of probiotics on IL-10 production in inflammatory dendritic-cells under hypoxia

The aim of this study was to analyse hypoxia-associated dendritic cells (DCs) in colitic mice and the effects of probiotics on interleukin (IL)-10 production in inflammatory DCs under hypoxic conditions. Extensive hypoxia was observed in the colonic mucosa of dextran sodium sulphate-induced colitic mice. Flow cytometric analysis demonstrated that hypoxia-inducible factor-1α⁺ DCs in colonic lamina propria (CLP) lymphocytes and mesenteric lymph nodes (MLN) were more abundant in colitic mice than those in controls. Among three subsets of DCs, i.e. plasmacytoid DCs, conventional DCs (cDCs), and monocyte-derived DCs (mDCs), cDCs and mDCs were more abundant in CLP of colitic mice. Bone marrow-derived Flt-3L-induced DCs (Flt-DCs) but not bone marrow-derived GM-CSF-induced DCs (GM-DCs), incubated with 1% O₂ exhibited an inflammatory phenotype, with higher CD86, IL-6, and tumour necrosis factor-α expression, and lower IL-10 levels than those in Flt-DCs incubated with 21% O₂. The hypoxia-induced decrease in IL-10 expression in Flt-DCs was restored by Bifidobacterium bifidum JCM 1255^T promoted IL-10 expression through the p38 pathway under normoxic conditions. The anti-inflammatory effects of B. bifidum JCM 1255^T in Flt-DCs were mediated through different cellular mechanisms under hypoxic and normoxic conditions. B. bifidum JCM 1255^T could be used therapeutically for its anti-inflammatory effects.

Non-canonical HIF-1 stabilization contributes to intestinal tumorigenesis

The hypoxia-inducible transcription factor HIF-1 is appreciated as a promising target for cancer therapy. However, conditional deletion of HIF-1 and HIF-1 target genes in cells of the tumor microenvironment can result in accelerated tumor growth, calling for a detailed characterization of the cellular context to fully comprehend HIF-1's role in tumorigenesis. We dissected cell type-specific functions of HIF-1 for intestinal tumorigenesis by lineage-restricted deletion of the Hif1a locus. Intestinal epithelial cell-specific Hif1a loss reduced activation of Wnt/β-catenin, tumor-specific metabolism and inflammation, significantly inhibiting tumor growth. Deletion of Hif1a in myeloid cells reduced the expression of fibroblast-activating factors in tumor-associated macrophages resulting in decreased abundance of tumor-associated fibroblasts (TAF) and robustly reduced tumor formation. Interestingly, hypoxia was detectable only sparsely and without spatial association with HIF-1α, arguing for an importance of hypoxia-independent, i.e., non-canonical, HIF-1 stabilization for intestinal tumorigenesis that has not been previously appreciated. This adds a further layer of complexity to the regulation of HIF-1 and suggests that hypoxia and HIF-1α stabilization can be uncoupled in cancer. Collectively, our data show that HIF-1 is a pivotal pro-tumorigenic factor for intestinal tumor formation, controlling key oncogenic programs in both the epithelial tumor compartment and the tumor microenvironment.

Regulation of CD11b by HIF-1α and the STAT3 signaling pathway contributes to the immunosuppressive function of B cells in inflammatory bowel disease

B cells have been reported to have a suppressive function in autoimmune diseases, which appears to require an increase of CD11b expression on B cells. However, little is known how CD11b is induced in B cells to play the function. In this study, we found that the high expression of CD11b in B cells occurred not only in the mucosal immune organs, but also in systemically immune organs such as the spleen during dextran sulfate sodium (DSS)-induced colitis. Since the inflammatory lesions in mouse models of inflammatory bowel disease (IBD) were revealed to be significantly hypoxic or even anoxic, the B cells from colitic mice Peyer's patches (PP) were investigated to express higher levels of hypoxia-inducible factor-1α (HIF-1α) than naïve B cells from wildtype (WT) mice. HIF-1α siRNA transfection or HIF-1α protein inhibition led to decreased CD11b expression at both the mRNA and protein levels in vitro. B cells with HIF-1α specific knockdown were then adoptively transferred to Rag-1^-/- mice. The result displayed that CD11b expression was decreased in B cells and an exacerbated colitis occurred. The bio-informatics promoter analysis and ChIP assay showed that HIF-1α was the critical transcription factor for CD11b and cooperatively formed a complex with the p-STAT3 homodimers to bind onto hypoxia-responsive element (HRE) regions, which was guaranteed by MEK/ERK pathway activation and IL-10 secretion. In conclusion, our study demonstrated the key function of the hypoxia-associated transcription factor HIF-1α together with p-STAT3 in driving CD11b transcription in B cells and controlling B cell's protective activity in experimental inflammatory bowel disease (IBD).

Intestinal Epithelial Cells-Derived Hypoxia-Inducible Factor-1α Is Essential for the Homeostasis of Intestinal Intraepithelial Lymphocytes

Hif-1α is a master regulator which involved in the transcriptional regulation of anti-inflammatory or cellular responding to hypoxia. Previous work shows that the absence of Hif-1α results in the destruction of intestinal epithelial cell (IEC) and abnormalities of intestinal barrier function. However, we know very little about other functions of Hif-1α on intestinal intraepithelial lymphocyte (IEL). Therefore, we generated a transgenic mouse (Hif1-α^ΔIEC mice), which was knocked out Hif1-α specifically in IECs, to study the effect of Hif1-α on IEL. IELs were isolated from the small intestine and colon of mice, respectively, and examined by flow cytometry and quantitative real-time PCR. All the cytokines expression was detected by quantitative real-time PCR. The NSAID enteropathy was induced by gavaged with 5 mg/kg indomethacin and the experimental colitis was induced by administration of 2.5% DSS. We found that the number of IELs is increased in Hif1-α ^ΔIEC mice. It is showed that knockout of Hif1-α in IECs led to significant changes in IEL phenotype, including a marked decline in the CD8αα⁺ and TCRγδ⁺ population. The reduction of CD8αα⁺ IELs is accompanied by increased apoptosis, decreased proliferation and weakened migration in Hif1-α^ΔIEC mice. Moreover, absence of intestinal epithelial Hif1-α markedly changed the population of IELs in NSAID-induced small intestinal injury and increased susceptibility to dextran sulfate sodium-induced colitis. In summary, our results first time demonstrate that IEC-derived Hif1-α is essential for maintaining IELs homeostasis and intestinal microbiota.

Copper-Fructose Interactions: A Novel Mechanism in the Pathogenesis of NAFLD

Compelling epidemiologic data support the critical role of dietary fructose in the epidemic of obesity, metabolic syndrome and nonalcoholic fatty liver disease (NAFLD). The metabolic effects of fructose on the development of metabolic syndrome and NAFLD are not completely understood. High fructose intake impairs copper status, and copper-fructose interactions have been well documented in rats. Altered copper-fructose metabolism leads to exacerbated experimental metabolic syndrome and NAFLD. A growing body of evidence has demonstrated that copper levels are low in NAFLD patients. Moreover, hepatic and serum copper levels are inversely correlated with the severity of NAFLD. Thus, high fructose consumption and low copper availability are considered two important risk factors in NAFLD. However, the causal effect of copper-fructose interactions as well as the effects of fructose intake on copper status remain to be evaluated in humans. The aim of this review is to summarize the role of copper-fructose interactions in the pathogenesis of the metabolic syndrome and discuss the potential underlying mechanisms. This review will shed light on the role of copper homeostasis and high fructose intake and point to copper-fructose interactions as novel mechanisms in the fructose induced NAFLD.

Myeloid Cell Hypoxia-Inducible Factors Promote Resolution of Inflammation in Experimental Colitis

Colonic tissues in Inflammatory Bowel Disease (IBD) patients exhibit oxygen deprivation and activation of hypoxia-inducible factor 1α and 2α (HIF-1α and HIF-2α), which mediate cellular adaptation to hypoxic stress. Notably, macrophages and neutrophils accumulate preferentially in hypoxic regions of the inflamed colon, suggesting that myeloid cell functions in colitis are HIF-dependent. By depleting ARNT (the obligate heterodimeric binding partner for both HIFα subunits) in a murine model, we demonstrate here that myeloid HIF signaling promotes the resolution of acute colitis. Specifically, myeloid pan-HIF deficiency exacerbates infiltration of pro-inflammatory neutrophils and Ly6C⁺ monocytic cells into diseased tissue. Myeloid HIF ablation also hinders macrophage functional conversion to a protective, pro-resolving phenotype, and elevates gut serum amyloid A levels during the resolution phase of colitis. Therefore, myeloid cell HIF signaling is required for efficient resolution of inflammatory damage in colitis, implicating serum amyloid A in this process.

Hypoxia-inducible factor-1α regulation of myeloid cells

Hematopoietic myeloblasts give rise to macrophages, dendritic cells, and neutrophils. Circulating myeloid cells detect invading microbes using pattern recognition receptors and subsequently orchestrate an innate immune response to contain and kill the pathogens. This innate immune response establishes an inflammatory niche characterized by hypoxia due to host and pathogen factors. Hypoxia-inducible factor (HIF) transcription factors are the primary regulators of the myeloid response to hypoxia. In particular, HIF-1α is a critical hub that integrates hypoxic and immunogenic signals during infection or inflammation. Hypoxia induces HIF-1α stabilization, which drives metabolic and phenotypic reprogramming of myeloid cells to maximize antimicrobial potential. HIF-1α activity in myeloid-derived cells enhances the host response to infection, but may also play a role in pathogenic inflammatory processes, such as atherosclerosis. In this review, we summarize recent advances that have elucidated the mechanism by which myeloid cells regulate HIF-1α activity and, in turn, how HIF-1α shapes myeloid cell function.

Vitamin C and immune cell function in inflammation and cancer

Vitamin C (ascorbate) is maintained at high levels in most immune cells and can affect many aspects of the immune response. Intracellular levels generally respond to variations in plasma ascorbate availability, and a combination of inadequate intake and increased turnover during severe stress can result in low plasma ascorbate status. Intracellular ascorbate supports essential functions and, in particular, acts as an enzyme cofactor for Fe- or Cu-containing oxygenases. Newly discovered enzymes in this family regulate cell metabolism and epigenetics, and dysregulation of their activity can affect cell phenotype, growth and survival pathways, and stem cell phenotype. This brief overview details some of the recent advances in our understanding of how ascorbate availability can affect the hydroxylases controlling the hypoxic response and the DNA and histone demethylases. These processes play important roles in the regulation of the immune system, altering cell survival pathways, metabolism and functions.

Intestinal HIF-1α deletion exacerbates alcoholic liver disease by inducing intestinal dysbiosis and barrier dysfunction

BACKGROUND & AIMS

Alcoholic liver disease (ALD) is characterized by gut dysbiosis and increased gut permeability. Hypoxia inducible factor 1α (HIF-1α) has been implicated in transcriptional regulation of intestinal barrier integrity and inflammation. We aimed to test the hypothesis that HIF-1α plays a critical role in gut microbiota homeostasis and the maintenance of intestinal barrier integrity in a mouse model of ALD.

METHODS

Wild-type (WT) and intestinal epithelial-specific Hif1a knockout mice (IEhif1α^-/-) were pair-fed modified Lieber-DeCarli liquid diet containing 5% (w/v) alcohol or isocaloric maltose dextrin for 24 days. Serum levels of alanine aminotransferase and endotoxin were determined. Fecal microbiota were assessed. Liver steatosis and injury, and intestinal barrier integrity were evaluated.

RESULTS

Alcohol feeding increased serum levels of alanine aminotransferase and lipopolysaccharide, hepatic triglyceride concentration, and liver injury in the WT mice. These deleterious effects were exaggerated in IEhif1α^-/- mice. Alcohol exposure resulted in greater reduction of the expression of intestinal epithelial tight junction proteins, claudin-1 and occludin, in IEhif1α^-/- mice. In addition, cathelicidin-related antimicrobial peptide and intestinal trefoil factor were further decreased by alcohol in IEhif1α^-/- mice. Metagenomic analysis showed increased gut dysbiosis and significantly decreased Firmicutes/Bacteroidetes ratio in IEhif1α^-/- mice compared to the WT mice exposed to alcohol. An increased abundance of Akkermansia and a decreased level of Lactobacillus in IEhif1α^-/- mice were also observed. Non-absorbable antibiotic treatment reversed the liver steatosis in both WT and IEhif1α^-/- mice.

CONCLUSION

Intestinal HIF-1α is essential for the adaptative response to alcohol-induced changes in intestinal microbiota and barrier function associated with elevated endotoxemia and hepatic steatosis and injury.

LAY SUMMARY

Alcohol consumption alters gut microbiota and multiple intestinal barrier protecting factors that are regulated by intestinal hypoxia-inducible factor 1α (HIF-1α). Absence of intestinal HIF-1α exacerbates gut leakiness leading to an increased translocation of bacteria and bacterial products to the liver, consequently causing alcoholic liver disease. Intestinal specific upregulation of HIF-1α could be developed as a novel approach for the treatment of alcoholic liver disease.

Cellular Stress Responses and Gut Microbiota in Inflammatory Bowel Disease

Progresses in the past two decades have greatly expanded our understanding of inflammatory bowel disease (IBD), an incurable disease with multifaceted and challenging clinical manifestations. The pathogenesis of IBD involves multiple processes on the cellular level, which include the stress response signaling such as endoplasmic reticulum (ER) stress, oxidative stress, and hypoxia. Under physiological conditions, the stress responses play key roles in cell survival, mucosal barrier integrity, and immunomodulation. However, they can also cause energy depletion, trigger cell death and tissue injury, promote inflammatory response, and drive the progression of clinical disease. In recent years, gut microflora has emerged as an essential pathogenic factor and therapeutic target for IBD. Altered compositional and metabolic profiles of gut microbiota, termed dysbiosis, are associated with IBD. Recent studies, although limited, have shed light on how ER stress, oxidative stress, and hypoxic stress interact with gut microorganisms, a potential source of stress in the microenvironment of gastrointestinal tract. Our knowledge of cellular stress responses in intestinal homeostasis as well as their cross-talks with gut microbiome will further our understanding of the pathogenesis of inflammatory bowel disease and probably open avenues for new therapies.

Hypoxia-inducible factor 1α plays a predominantly negative role in regulatory T cell functions

Hypoxia-inducible factor 1α (HIF-1α) regulates cellular responses to hypoxia. However, conflicting roles for HIF-1α in the functions of regulatory T cells (Tregs) have been reported. In this review, we summarize observations on the requirement for HIF-1α for FOXP3 expression and Tregs development, as well as for HIF-1α-mediated downregulation of FOXP3 and Tregs destabilization. We also examine the association of HIF-1α with Tregs under pathogenic conditions. Based on these findings, we suggest that HIF-1α mainly plays a detrimental role in the function and stability of Tregs and that HIF-1α is disposable for the development and suppressive function of Tregs.

Hypoxia, Metabolism and Immune Cell Function

Hypoxia is a hallmark of inflamed, infected or damaged tissue, and the adaptation to inadequate tissue oxygenation is regulated by hypoxia-inducible factors (HIFs). HIFs are key mediators of the cellular response to hypoxia, but they are also associated with pathological stress such as inflammation, bacteriological infection or cancer. In addition, HIFs are central regulators of many innate and adaptive immunological functions, including migration, antigen presentation, production of cytokines and antimicrobial peptides, phagocytosis as well as cellular metabolic reprogramming. A characteristic feature of immune cells is their ability to infiltrate and operate in tissues with low level of nutrients and oxygen. The objective of this article is to discuss the role of HIFs in the function of innate and adaptive immune cells in hypoxia, with a focus on how hypoxia modulates immunometabolism.

Co-Stimulation-Impaired Bone Marrow-Derived Dendritic Cells Prevent Dextran Sodium Sulfate-Induced Colitis in Mice

Dendritic cells (DC) are important in the onset and severity of inflammatory bowel disease (IBD). Tolerogenic DC induce T-cells to become therapeutic Foxp3+ regulatory T-cells (Tregs). We therefore asked if experimental IBD could be prevented by administration of bone marrow-derived DC generated under conventional GM-CSF/IL-4 conditions but in the presence of a mixture of antisense DNA oligonucleotides targeting the primary transcripts of CD40, CD80, and CD86. These cell products (which we call AS-ODN BM-DC) have demonstrated tolerogenic activity in preventing type 1 diabetes and preserving beta cell mass in new-onset type 1 diabetes in the NOD mouse strain, in earlier studies. In addition to measuring efficacy in prevention of experimental IBD, we also sought to identify possible mechanism(s) of action. Weight, behavior, stool frequency, and character were observed daily for 7–10 days in experimental colitis in mice exposed to dextran sodium sulfate (DSS) following injection of the AS-ODN BM-DC. After euthanasia, the colons were processed for histology while spleen and mesenteric lymph nodes (MLNs) were made into single cells to measure Foxp3+ Treg as well as IL-10+ regulatory B-cell (Breg) population frequency by flow cytometry. AS-ODN BM-DC prevented DSS-induced colitis development. Recipients of these cells exhibited significant increases in Foxp3+ Treg and IL-10+ Breg in MLN and spleen. Histological examination of colon sections of colitis-free mice remained largely architecturally physiologic and mostly free of leukocyte infiltration when compared with DSS-treated animals. Although DSS colitis is mainly an innate immunity-driven condition, our study adds to the growing body of evidence showing that Foxp3+ Treg and IL-10 Bregs can suppress a mainly innate-driven inflammation. The already-established safety of human DC generated from monocytic progenitors in the presence of the mixture of antisense DNA targeting the primary transcripts of CD40, CD80, and CD86 in humans offers the potential to adapt them for clinical IBD therapy.

Biological and Immunological Aspects of Iron Deficiency Anemia in Cancer Development: A Narrative Review

Iron Deficiency Anemia (IDA) is a universal health problem and a risk factor for the development of cancer. IDA changes the microenvironment of the human body by affecting both the biological and immunological systems. It increases DNA damage and genomic instability by different mechanisms. IDA is one of the leading causes of the imbalance between different antioxidant enzymes as well as enzymes involved in DNA damage and DNA repair systems of the body. It can affect the biogenesis/expression of microRNAs. IDA interrupts the oxidative phosphorylation energy metabolism and intestinal Cytochrome-P450 systems. It also disturbs multicellular signaling pathways involved in cell survival and helps in tumor angiogenesis. Moreover, IDA is also responsible for the functional deterioration of innate and adaptive immune systems that lead to immunological dysfunctions against invading pathogens. Genomic instability and immunological dysfunctions are the hallmarks of cancer development. In this review, we will review the evidence linking IDA to increased cancer risk.

Norisoboldine, a natural AhR agonist, promotes Treg differentiation and attenuates colitis via targeting glycolysis and subsequent NAD+/SIRT1/SUV39H1/H3K9me3 signaling pathway

Norisoboldine (NOR), a natural aryl hydrocarbon receptor (AhR) agonist, has been demonstrated to attenuate ulcerative colitis (UC) and induce the generation of Treg cells. Under UC condition, hypoxia widely exists in colonic mucosa, and secondary changes of microRNAs (miRs) expressions and glycolysis contribute to Treg differentiation. At present, we worked for exploring the deep mechanisms for NOR-promoted Treg differentiation in hypoxia and its subsequent anti-UC action from the angle of AhR/miR or AhR/glycolysis axis. Results showed that NOR promoted Treg differentiation in hypoxia and the effect was stronger relative to normoxia. It activated AhR in CD4⁺ T cells under hypoxic microenvironment; CH223191 (a specific AhR antagonist) and siAhR-3 abolished NOR-promoted Treg differentiation. Furthermore, the progress of glycolysis, levels of Glut1 and HK2, and expression of miR-31 rather than miR-219 and miR-490 in CD4⁺ T cells were downregulated by NOR treatment under hypoxic microenvironment. However, HK2 plasmid but not miR-31 mimic significantly interfered NOR-enhanced Treg polarization. In addition, NOR reduced NAD⁺ and SIRT1 levels, facilitated the ubiquitin-proteasomal degradation of SUV39H1 protein, and inhibited the enrichment of H3K9me3 at -1, 201 to -1,500 region of Foxp3 promoter in CD4⁺ T cells under hypoxic microenvironment, which was weakened by HK2 plasmid, CH223191, and siAhR-3. Finally, the correlation between NOR-mediated activation of AhR, repression of glycolysis, regulation of NAD⁺/SIRT1/SUV39H1/H3K9me3 signals, induction of Treg cells, and remission of colitis was confirmed in mice with DSS-induced colitis by using CH223191 and HK2 plasmid. In conclusion, NOR promoted Treg differentiation and then alleviated the development of colitis by regulating AhR/glycolysis axis and subsequent NAD⁺/SIRT1/SUV39H1/H3K9me3 signaling pathway.

The Hypoxia-Adenosine Link during Intestinal Inflammation

Intestinal inflammation is a key element in inflammatory bowel disease and is related to a combination of factors, including genetics, mucosal barrier dysfunction, bacteria translocation, deleterious host-microbe interactions, and dysregulated immune responses. Over the past decade, it has been appreciated that these inflammatory lesions are associated with profound tissue hypoxia. Interestingly, an endogenous adaptive response under the control of hypoxia signaling is enhancement in adenosine signaling, which impacts these different endpoints, including promoting barrier function and encouraging anti-inflammatory activity. In this review, we discuss the hypoxia-adenosine link in inflammatory bowel disease, intestinal ischemia/reperfusion injury, and colon cancer. In addition, we provide a summary of clinical implications of hypoxia and adenosine signaling in intestinal inflammation and disease.

Hypoxia-inducible factor-1 (HIF-1) activation in myeloid cells accelerates DSS-induced colitis progression in mice

Inflammatory bowel disease (IBD) is a chronic inflammatory disease, in which the intestinal epithelium loses its barrier function. Given the existence of the oxygen gradient in the intestinal epithelium and that inflammation further contributes to the tissue hypoxia, we investigated the role of hypoxia-inducible factor (HIF), a transcription factor activated under hypoxic conditions in myeloid cells, in the progression of IBD. To do this, we utilized myeloid-specific knockout (KO) mice targeting HIF pathways, created by a Cre-loxP system with human MRP8 (hMRP8), an intracellular calcium-binding protein, as the myeloid promoter. By feeding 5% dextran sodium sulfate (DSS) to hMRP8 von Hippel Lindau (Vhl) KO mice, in which HIF-1α and HIF-2α are constitutively activated in myeloid cells, we found that these mice were highly susceptible to DSS-induced colitis, demonstrating greater body weight loss, increased mortality, faster onset of rectal bleeding, shortened colon length, and increased CD11b- or Gr-1-positive myeloid cells in the colon compared with wild-type (WT) mice. These parameters were restored to, if not better than, the WT levels when we examined hMRP8 Hif-1a KO mice upon 5% DSS feeding. hMRP8 Hif-2a KO mice, on the other hand, exhibited a similar degree of DSS-induced colitis to that of WT mice. Lastly, when DSS was given together with azoxymethane to induce tumorigenesis in the colon, we found that hMRP8 Hif-1a KO mice exhibited comparable levels of colorectal tumors to those of WT mice, indicating that HIF-1α in myeloid cells is dispensable for tumorigenesis. Collectively, our results suggest that HIF-1α activation in myeloid cells critically regulates IBD progression.

Summary: We challenged myeloid-specific knockout mice targeting the hypoxia-inducible factor (HIF) pathway to dextran sodium sulfate-induced colitis, demonstrating that HIF-1α, but not HIF-2α, activation in myeloid cells regulates colitis severity in mice.

Knockdown of myeloid cell hypoxia-inducible factor-1α ameliorates the acute pathology in DSS-induced colitis

Inflammation and hypoxia are hallmarks of inflammatory bowel disease. Low oxygen levels activate hypoxia-inducible factors as central transcriptional regulators of cellular responses to hypoxia, particularly in myeloid cells where hypoxia-inducible factors control immune cell function and survival. Still, the role of myeloid hypoxia-inducible factor-1 during inflammatory bowel disease remains poorly defined. We therefore investigated the role of hypoxia-inducible factor-1 for myeloid cell function and immune response during colitis. Experimental colitis was induced by administration of 2.5% dextran sulfate sodium to mice with a conditional knockout of hypoxia-inducible factor-1α in myeloid cells and their wild type siblings. Murine colon tissue was examined by histologic analysis, immunohistochemistry, and quantitative polymerase chain reaction. Induction of experimental colitis increased levels of hypoxia and accumulation of hypoxia-inducible factor-1α positive cells in colon tissue of both treated groups. Myeloid hypoxia-inducible factor-1α knockout reduced weight loss and disease activity index when compared to wild type mice. Knockout mice displayed less infiltration of macrophages into intestinal mucosa and reduced mRNA expression of markers for dendritic cells and interleukin-17 secreting T helper cells. Expression of inflammatory and anti-inflammatory cytokines also showed a reduced and delayed induction in myeloid hypoxia-inducible factor-1α knockout mice. Our results show a disease promoting role of myeloid hypoxia-inducible factor-1 during intestinal inflammation. This might result from a hypoxia-inducible factor-1 dependent increase in pro-inflammatory interleukin-17 secreting T helper cells in the absence of obvious changes in regulatory T cells. In contrast, knockout mice appear to shift the balance to anti-inflammatory signals and cells resulting in milder intestinal inflammation.

Hypoxia-sensitive pathways in intestinal inflammation

Inflammatory bowel disease (IBD) is a common chronic intestinal disorder characterised by a loss of epithelial barrier function leading to the unregulated movement of luminal antigenic material into mucosal tissue with resultant inflammation. In IBD, multiple components of the inflammatory response lead to tissue hypoxia. Mucosal hypoxia leads to the inactivation of prolyl hydroxylase domain-containing (PHD) enzymes, which in turn leads to the stabilisation of the hypoxia-inducible factor (HIF), which induces the expression of barrier protective genes. Furthermore, pharmacological hydroxylase inhibition has been shown to be protective in colitis, at least in part through enhancing intestinal epithelial barrier function through HIF-1-dependent barrier-protective gene expression. Therefore, targeting hypoxia-sensitive pathways represents a new and promising therapeutic approach in IBD.

Intestinal hypoxia and hypoxia-induced signalling as therapeutic targets for IBD

Tissue hypoxia occurs when local oxygen demand exceeds oxygen supply. In chronic inflammatory conditions such as IBD, the increased oxygen demand by resident and gut-infiltrating immune cells coupled with vascular dysfunction brings about a marked reduction in mucosal oxygen concentrations. To counter the hypoxic challenge and ensure their survival, mucosal cells induce adaptive responses, including the activation of hypoxia-inducible factors (HIFs) and modulation of nuclear factor-κB (NF-κB). Both pathways are tightly regulated by oxygen-sensitive prolyl hydroxylases (PHDs), which therefore represent promising therapeutic targets for IBD. In this Review, we discuss the involvement of mucosal hypoxia and hypoxia-induced signalling in the pathogenesis of IBD and elaborate in detail on the role of HIFs, NF-κB and PHDs in different cell types during intestinal inflammation. We also provide an update on the development of PHD inhibitors and discuss their therapeutic potential in IBD.

Hypoxia Inducible Factor (HIF) Hydroxylases as Regulators of Intestinal Epithelial Barrier Function

Human health is dependent on the ability of the body to extract nutrients, fluids, and oxygen from the external environment while at the same time maintaining a state of internal sterility. Therefore, the cell layers that cover the surface areas of the body such as the lung, skin, and gastrointestinal mucosa provide vital semipermeable barriers that allow the transport of essential nutrients, fluid, and waste products, while at the same time keeping the internal compartments free of microbial organisms. These epithelial surfaces are highly specialized and differ in their anatomic structure depending on their location to provide appropriate and effective site-specific barrier function. Given this important role, it is not surprising that significant disease often is associated with alterations in epithelial barrier function. Examples of such diseases include inflammatory bowel disease, chronic obstructive pulmonary disease, and atopic dermatitis. These chronic inflammatory disorders often are characterized by diminished tissue oxygen levels (hypoxia). Hypoxia triggers an adaptive transcriptional response governed by hypoxia-inducible factors (HIFs), which are repressed by a family of oxygen-sensing HIF hydroxylases. Here, we review recent evidence suggesting that pharmacologic hydroxylase inhibition may be of therapeutic benefit in inflammatory bowel disease through the promotion of intestinal epithelial barrier function through both HIF-dependent and HIF-independent mechanisms.

Cancer acidity: An ultimate frontier of tumor immune escape and a novel target of immunomodulation

The link between cancer metabolism and immunosuppression, inflammation and immune escape has generated major interest in investigating the effects of low pH on tumor immunity. Indeed, microenvironmental acidity may differentially impact on diverse components of tumor immune surveillance, eventually contributing to immune escape and cancer progression. Although the molecular pathways underlying acidity-related immune dysfunctions are just emerging, initial evidence indicates that antitumor effectors such as T and NK cells tend to lose their function and undergo a state of mostly reversible anergy followed by apoptosis, when exposed to low pH environment. At opposite, immunosuppressive components such as myeloid cells and regulatory T cells are engaged by tumor acidity to sustain tumor growth while blocking antitumor immune responses. Local acidity could also profoundly influence bioactivity and distribution of antibodies, thus potentially interfering with the clinical efficacy of therapeutic antibodies including immune checkpoint inhibitors. Hence tumor acidity is a central regulator of cancer immunity that orchestrates both local and systemic immunosuppression and that may offer a broad panel of therapeutic targets. This review outlines the fundamental pathways of acidity-driven immune dysfunctions and sheds light on the potential strategies that could be envisaged to potentiate immune-mediated tumor control in cancer patients.

Modulation of Inflammatory Response to Implanted Biomaterials Using Natural Compounds

Tissue engineering offers a promising strategy to restore injuries resulting from trauma, infection, tumor resection, or other diseases. In spite of significant progress, the field faces a significant bottleneck; the critical need to understand and exploit the interdependencies of tissue healing, angiogenesis, and inflammation. Inherently, the balance of these interacting processes is affected by a number of injury site conditions that represent a departure from physiological environment, including reduced pH, increased concentration of free radicals, hypoglycemia, and hypoxia. Efforts to harness the potential of immune response as a therapeutic strategy to promote tissue repair have led to identification of natural compounds with significant anti-inflammatory properties. This article provides a concise review of the body's inflammatory response to biomaterials and describes the role of oxygen as a physiological cue in this process. We proceed to highlight the potential of natural compounds to mediate inflammatory response and improve host-graft integration. Herein, we discuss the use of natural compounds to map signaling molecules and checkpoints that regulate the cross-linkage of immune response and skeletal repair.

Quantitative analysis of mucosal oxygenation using ex vivo imaging of healthy and inflamed mammalian colon tissue

Colonic inflammation is associated with decreased tissue oxygenation, significantly affecting gut homeostasis. However, the crosstalk between O2 consumption and supply in the inflamed tissue are not fully understood. Using a murine model of colitis, we analysed O2 in freshly prepared samples of healthy and inflamed colon tissue. We developed protocols for efficient ex vivo staining of mouse distal colon mucosa with a cell-penetrating O2 sensitive probe Pt-Glc and high-resolution imaging of O2 concentration in live tissue by confocal phosphorescence lifetime-imaging microscopy (PLIM). Microscopy analysis revealed that Pt-Glc stained mostly the top 50-60 μm layer of the mucosa, with high phosphorescence intensity in epithelial cells. Measured O2 values in normal mouse tissue ranged between 5 and 35 μM (4-28 Torr), tending to decrease in the deeper tissue areas. Four-day treatment with dextran sulphate sodium (DSS) triggered colon inflammation, as evidenced by an increase in local IL6 and mKC mRNA levels, but did not affect the gross architecture of colonic epithelium. We further observed an increase in oxygenation, partial activation of hypoxia inducible factor (HIF) 1 signalling, and negative trends in pyruvate dehydrogenase activity and O2 consumption rate in the colitis mucosa, suggesting a decrease in mitochondrial respiration, which is known to be regulated via HIF-1 signalling and pyruvate oxidation rate. These results along with efficient staining with Pt-Glc of rat and human colonic mucosa reveal high potential of PLIM platform as a powerful tool for the high-resolution analysis of the intestinal tissue oxygenation in patients with inflammatory bowel disease and other pathologies, affecting tissue respiration.

Anti-Inflammatory Effects of Hyperbaric Oxygenation during DSS-Induced Colitis in BALB/c Mice Include Changes in Gene Expression of HIF-1α, Proinflammatory Cytokines, and Antioxidative Enzymes

Reactive oxygen species (ROS) and nitrogen species have an indispensable role in regulating cell signalling pathways, including transcriptional control via hypoxia inducible factor-1α (HIF-1α). Hyperbaric oxygenation treatment (HBO₂) increases tissue oxygen content and leads to enhanced ROS production. In the present study DSS-induced colitis has been employed in BALB/c mice as an experimental model of gut mucosa inflammation to investigate the effects of HBO₂ on HIF-1α, antioxidative enzyme, and proinflammatory cytokine genes during the colonic inflammation. Here we report that HBO₂ significantly reduces severity of DSS-induced colitis, as evidenced by the clinical features, histological assessment, impaired immune cell expansion and mobilization, and reversal of IL-1β, IL-2, and IL-6 gene expression. Gene expression and antioxidative enzyme activity were changed by the HBO₂ and the inflammatory microenvironment in the gut mucosa. Strong correlation of HIF-1α mRNA level to GPx1, SOD1, and IL-6 mRNA expression suggests involvement of HIF-1α in transcriptional regulation of these genes during colonic inflammation and HBO₂. This is further confirmed by a strong correlation of HIF-1α with known target genes VEGF and PGK1. Results demonstrate that HBO₂ has an anti-inflammatory effect in DSS-induced colitis in mice, and this effect is at least partly dependent on expression of HIF-1α and antioxidative genes.

Partial Enteral Nutrition Mitigated Ischemia/Reperfusion-Induced Damage of Rat Small Intestinal Barrier

Background and Aims: This study was designed to investigate a relatively optimum dose of partial enteral nutrition (PEN) which effectively attenuates intestinal barrier dysfunction initiated by ischemia/reperfusion injury (IRI). Methods: In experiment 1, 60 male Sprague-Dawley (SD) rats were subjected to intestinal IRI and assigned to six groups according to the different proportion of EN administrations: namely total parenteral nutrition (TPN or 0%EN), 10%EN, 20%EN, 40%EN, 60%EN, and total enteral nutrition (TEN or 100%) groups, the deficits of intraluminal calorie were supplemented by PN. In experiment 2, 50 male SD rats were subjected to intestinal IRI and divided into five groups based on the results of experiment 1: TPN, TEN, 20%EN, TPN plus pretreatment with NF-κB antagonist 30 min before IRI (TPN+PDTC), and TPN plus pretreatment with HIF-1α antagonist 30 min before IRI (TPN+YC-1) groups. Results: In experiment 1, previous IRI combined with subsequent EN shortage disrupted the structure of intestinal epithelial cell and tight junctions (TJs). While 20% dose of EN had an obviously protective effect on these detrimental consequences. In experiment 2, compared with TPN only, 20%EN exerted a significant protection of barrier function of intestinal epithelium. Analogous results were observed when TPN combined with specific NF-κB/HIF-1α inhibitors (PDTC and YC-1). Meanwhile, the expression of NF-κB/HIF-1α had a similar trend among the groups. Conclusions: Our findings indicate that 20%EN is the minimally effective dosage of EN which promotes the recovery of intestinal barrier function after IRI in a rat model. Furthermore, we discreetly speculate that this benefit is, at least partly, related to NF-κB/HIF-1α pathway expression.

Hypoxia-dependent regulation of inflammatory pathways in immune cells

Uncontrolled inflammation underpins a diverse range of diseases where effective therapy remains an unmet clinical need. Hypoxia is a prominent feature of the inflammatory microenvironment that regulates key transcription factors including HIF and NF-κB in both innate and adaptive immune cells. In turn, altered activity of the pathways controlled by these factors can affect the course of inflammation through the regulation of immune cell development and function. In this review, we will discuss these pathways and the oxygen sensors that confer hypoxic sensitivity in immune cells. Furthermore, we will describe how hypoxia-dependent pathways contribute to immunity and discuss their potential as therapeutic targets in inflammatory and infectious disease.

Dendritic Cells under Hypoxia: How Oxygen Shortage Affects the Linkage between Innate and Adaptive Immunity

Dendritic cells (DCs) are considered as one of the main regulators of immune responses. They collect antigens, process them, and present typical antigenic structures to lymphocytes, thereby inducing an adaptive immune response. All these processes take place under conditions of oxygen shortage (hypoxia) which is often not considered in experimental settings. This review highlights how deeply hypoxia modulates human as well as mouse immature and mature dendritic cell functions. It tries to link in vitro results to actual in vivo studies and outlines how hypoxia-mediated shaping of dendritic cells affects the activation of (innate) immunity.

Oxygen sensing in intestinal mucosal inflammation

Hypoxia is a hallmark of chronically inflamed tissue. Hypoxia develops from vascular dysfunction and increased oxygen consumption by infiltrating leukocytes. With respect to inflammatory bowel disease (IBD), hypoxia is likely to be of particular importance: Impairment of the intestinal barrier during IBD allows anoxia from the lumen of the gut to spread to formerly normoxic tissue. In addition, disturbed perfusion of inflamed tissue and a higher oxygen demand of infiltrating immune cells lead to low oxygen levels in inflamed mucosal tissue. Here, cells become hypoxic and must now adapt to this condition. The hypoxia inducible factor (HIF)-1 complex is a key transcription factor for cellular adaption to low oxygen tension. HIF-1 is a heterodimer formed by two subunits: HIF-α (either HIF-1α or HIF-2α) and HIF-1β. Under normoxic conditions, hydroxylation of the HIF-α subunit by specific oxygen-dependent prolyl hydroxylases (PHDs) leads to ubiquitin proteasome-dependent degradation. Under hypoxic conditions, however, PHD activity is inhibited; thus, HIF-α can translocate into the nucleus, dimerize with HIF-1β, and bind to hypoxia-responsive elements of HIF-1 target genes. So far, most studies have addressed the function of HIF-1α in intestinal epithelial cells and the effect of HIF stabilization by PHD inhibitors in murine models of colitis. Furthermore, the role of HIF-1α in immune cells becomes more and more important as T cells or dendritic cells for which HIF-1 is of critical importance are highly involved in the pathogenesis of IBD. This review will summarize the function of HIF-1α and the therapeutic prospects for targeting the HIF pathway in intestinal mucosal inflammation.