The TNFSF12/TWEAK Modulates Colonic Inflammatory Fibroblast Differentiation and Promotes Fibroblast-Monocyte Interactions.

Fibroblasts acquire a proinflammatory phenotype in inflammatory bowel disease, but the factors driving this process and how fibroblasts contribute to mucosal immune responses are incompletely understood. TNF superfamily member 12 (TNFSF12, or TNF-like weak inducer of apoptosis [TWEAK]) has gained interest as a mediator of chronic inflammation. In this study, we explore its role as a driver of inflammatory responses in fibroblasts and its contribution to fibroblast-monocyte interaction using human primary colonic fibroblasts, THP-1 and primary monocytes. Recombinant human TWEAK induced the expression of cytokines, chemokines, and immune receptors in primary colonic fibroblasts. The TWEAK upregulated transcriptome shared 29% homology with a previously published transcriptional profile of inflammatory fibroblasts from ulcerative colitis. TWEAK elevated surface expression of activated fibroblast markers and adhesion molecules (podoplanin [PDPN], ICAM-1, and VCAM-1) and secretion of IL-6, CCL2, and CXCL10. In coculture, fibroblasts induced monocyte adhesion and secretion of CXCL1 and IL-8, and they promoted a CD14high/ICAM-1high phenotype in THP-1 cells, which was enhanced when fibroblasts were prestimulated with TWEAK. Primary monocytes in coculture with TWEAK-treated fibroblasts had altered surface expression of CD16 and triggering receptor expressed on myeloid cells-1 (TREM-1) as well as increased CXCL1 and CXCL10 secretion. Conversely, inhibition of the noncanonical NF-κB pathway on colonic fibroblasts with a NF-κB-inducing kinase small molecule inhibitor impaired their ability to induce a CD14high phenotype on monocytes. Our results indicate that TWEAK promotes an inflammatory fibroblast-monocyte crosstalk that may be amenable for therapeutic intervention.

Novel fibroblast phenotypes in homeostasis and chronic inflammation: From functions to potential regulators.

Fibroblasts are essential components of the stroma, sustaining a variety of tissues and being key to the process of tissue repair after injury. Their role in tissue repair has been attributed to their ability to acquire a contractile, extracellular matrix-producing phenotype known as myofibroblasts. This property is primarily dependent on their response to the pleiotropic cytokine transforming growth factor-ß1. Until recently, the potential role of fibroblasts in other homeostatic and disease-related processes was less well understood. Although in vitro studies indicated that fibroblasts are able to respond to and secrete inflammatory mediators, definitive evidence of their contribution to chronic inflammation was limited. However, the emergence of techniques that allow exploration of tissues at the single cell level has challenged the previous paradigms on fibroblast identity and functions, and has led to the discovery of significant diversity, showing the presence of fibroblasts with alternate transcriptional profiles in a variety of tissues. These studies have also suggested potential roles of novel fibroblast subtypes as regulators of epithelial homeostasis and renewal, inflammatory cell infiltration and activation, and antigen presentation. Here, we provide a comprehensive review of the recent literature on fibroblast diversity in the digestive tract, skin, lungs and joints. We also review evidence of their contribution to the regulation of homeostasis and chronic inflammation, as well as their interactions with other cells in various tissue compartments. We discuss evidence of different factors involved in the control of fibroblast function, addressing the role of various cytokines, transcription factors and epigenetic changes, as well as microenvironmental factors, including extracellular matrix stiffness, hypoxia, and metabolic shifts.

Lymphotoxin beta receptor signaling directly controls airway smooth muscle deregulation and asthmatic lung dysfunction.

BACKGROUND: Dysregulation of airway smooth muscle cells (ASM) is central to the severity of asthma. Which molecules dominantly control ASM in asthma is unclear. High levels of the cytokine LIGHT (aka TNFSF14) have been linked to asthma severity and lower baseline predicted FEV1 percentage, implying that signals through its receptors might directly control ASM dysfunction. OBJECTIVE: Our study sought to determine whether signaling via lymphotoxin beta receptor (LTßR) or herpesvirus entry mediator from LIGHT dominantly drives ASM hyperreactivity induced by allergen. METHODS: Conditional knockout mice deficient for LTßR or herpesvirus entry mediator in smooth muscle cells were used to determine their role in ASM deregulation and airway hyperresponsiveness (AHR) in vivo. Human ASM were used to study signals induced by LTßR. RESULTS: LTßR was strongly expressed in ASM from normal and asthmatic subjects compared to several other receptors implicated in smooth muscle deregulation. Correspondingly, conditional deletion of LTßR only in smooth muscle cells in smMHCCreLTßRfl/fl mice minimized changes in their numbers and mass as well as AHR induced by house dust mite allergen in a model of severe asthma. Intratracheal LIGHT administration independently induced ASM hypertrophy and AHR in vivo dependent on direct LTßR signals to ASM. LIGHT promoted contractility, hypertrophy, and hyperplasia of human ASM in vitro. Distinguishing LTßR from the receptors for IL-13, TNF, and IL-17, which have also been implicated in smooth muscle dysregulation, LIGHT promoted NF-κB-inducing kinase-dependent noncanonical nuclear factor kappa-light-chain enhancer of activated B cells in ASM in vitro, leading to sustained accumulation of F-actin, phosphorylation of myosin light chain kinase, and contractile activity. CONCLUSIONS: LTßR signals directly and dominantly drive airway smooth muscle hyperresponsiveness relevant for pathogenesis of airway remodeling in severe asthma.

A Deficiency in the Cytokine TNFSF14/LIGHT Limits Inflammation and Remodeling in Murine Eosinophilic Esophagitis.

Eosinophilic esophagitis (EoE) is a chronic type 2 allergic disease, with esophageal tissue remodeling as the mechanism behind clinical dysphagia and strictures. IL-13 is thought to be a central driver of disease, but other inflammatory factors, such as IFNs and TNF superfamily members, have been hypothesized to play a role in disease pathogenesis. We recently found that the cytokine TNFSF14/LIGHT is upregulated in the esophagus of patients with EoE and that LIGHT promotes inflammatory activity in esophageal fibroblasts. However, the global effects of LIGHT on EoE pathogenesis in vivo remain unknown. We investigated the impact of a LIGHT deficiency in a murine model of EoE driven by house dust mite allergen. Chronic intranasal challenge with house dust mite promoted esophageal eosinophilia and increased CD4+ T cell numbers and IL-13 and CCL11 production in wild-type mice. Esophageal remodeling was reflected by submucosal collagen accumulation, increased muscle density, and greater numbers of fibroblasts. LIGHT-/- mice displayed normal esophageal eosinophilia, but exhibited reduced frequencies of CD4 T cells, IL-13 expression, submucosal collagen, and muscle density and a decrease in esophageal accumulation of fibroblasts. In vitro, LIGHT increased division of human esophageal fibroblasts and selectively enhanced IL-13-mediated expression of a subset of inflammatory and fibrotic genes. These results show that LIGHT contributes to various features of murine EoE, impacting the accumulation of CD4 T cells, IL-13 production, fibroblast proliferation, and esophagus remodeling. These findings suggest that LIGHT may be, to our knowledge, a novel therapeutic target for the treatment of EoE.

A Deficiency in the Cytokine TNFSF14/LIGHT Limits Inflammation and Remodeling in Murine Eosinophilic Esophagitis.

Eosinophilic esophagitis (EoE) is a chronic type 2 allergic disease, with esophageal tissue remodeling as the mechanism behind clinical dysphagia and strictures. IL-13 is thought to be a central driver of disease, but other inflammatory factors, such as IFNs and TNF superfamily members, have been hypothesized to play a role in disease pathogenesis. We recently found that the cytokine TNFSF14/LIGHT is upregulated in the esophagus of patients with EoE and that LIGHT promotes inflammatory activity in esophageal fibroblasts. However, the global effects of LIGHT on EoE pathogenesis in vivo remain unknown. We investigated the impact of a LIGHT deficiency in a murine model of EoE driven by house dust mite allergen. Chronic intranasal challenge with house dust mite promoted esophageal eosinophilia and increased CD4+ T cell numbers and IL-13 and CCL11 production in wild-type mice. Esophageal remodeling was reflected by submucosal collagen accumulation, increased muscle density, and greater numbers of fibroblasts. LIGHT-/- mice displayed normal esophageal eosinophilia, but exhibited reduced frequencies of CD4 T cells, IL-13 expression, submucosal collagen, and muscle density and a decrease in esophageal accumulation of fibroblasts. In vitro, LIGHT increased division of human esophageal fibroblasts and selectively enhanced IL-13-mediated expression of a subset of inflammatory and fibrotic genes. These results show that LIGHT contributes to various features of murine EoE, impacting the accumulation of CD4 T cells, IL-13 production, fibroblast proliferation, and esophagus remodeling. These findings suggest that LIGHT may be, to our knowledge, a novel therapeutic target for the treatment of EoE.

LIGHT controls distinct homeostatic and inflammatory gene expression profiles in esophageal fibroblasts via differential HVEM and LTßR-mediated mechanisms.

Fibroblasts mediate tissue remodeling in eosinophilic esophagitis (EoE), a chronic allergen-driven inflammatory pathology. Diverse fibroblast subtypes with homeostasis-regulating or inflammatory profiles have been recognized in various tissues, but which mediators induce these alternate differentiation states remain largely unknown. We recently identified that TNFSF14/LIGHT promotes an inflammatory esophageal fibroblast in vitro. Herein we used esophageal biopsies and primary fibroblasts to investigate the role of the LIGHT receptors, herpes virus entry mediator (HVEM) and lymphotoxin-beta receptor (LTßR), and their downstream activated pathways, in EoE. In addition to promoting inflammatory gene expression, LIGHT down-regulated homeostatic factors including WNTs, BMPs and type 3 semaphorins. In vivo, WNT2B+ fibroblasts were decreased while ICAM-1+ and IL-34+ fibroblasts were expanded in EoE, suggesting that a LIGHT-driven gene signature was imprinted in EoE versus normal esophageal fibroblasts. HVEM and LTßR overexpression and deficiency experiments demonstrated that HVEM regulates a limited subset of LIGHT targets, whereas LTßR controls all transcriptional effects. Pharmacologic blockade of the non-canonical NIK/p100/p52-mediated NF-κB pathway potently silenced LIGHT's transcriptional effects, with a lesser role found for p65 canonical NF-κB. Collectively, our results show that LIGHT promotes differentiation of esophageal fibroblasts toward an inflammatory phenotype and represses homeostatic gene expression via a LTßR-NIK-p52 NF-κB dominant pathway.

Type 2 Immunity and Age Modify Gene Expression of Coronavirus-induced Disease 2019 Receptors in Eosinophilic Gastrointestinal Disorders.

ABSTRACT: Infection with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) can lead to coronavirus-induced disease 2019 (COVID-19). The gastrointestinal (GI) tract is now an appreciated portal of infection. SARS-CoV-2 enters host cells via angiotensin-converting enzyme-2 (ACE2) and the serine protease TMPRSS2. Eosinophilic gastrointestinal disorders (EGIDs) are inflammatory conditions caused by chronic type 2 (T2) inflammation. the effects of the T2 atopic inflammatory milieu on SARS-COV-2 viral entry gene expression in the GI tract is poorly understood. We analyzed tissue ACE2 and TMPRSS2 gene expression in pediatric eosinophilic esophagitis (EoE), eosinophilic gastritis (EG), and in normal adult esophagi using publicly available RNA-sequencing datasets. Similar to findings evaluating the airway, there was no difference in tissue ACE2/TMPRSS2 expression in EoE or EG when compared with control non-EoE/EG esophagus/stomach. ACE2 gene expression was significantly lower in esophagi from children with or without EoE and from adults with EoE as compared with normal adult esophagi. Type 2 immunity and pediatric age could be protective for infection by SARS-CoV-2 in the gastrointestinal tract because of decreased expression of ACE2.

Animal Models of Contact Dermatitis: 2,4-Dinitrofluorobenzene-Induced Contact Hypersensitivity.

Allergic contact dermatitis (ACD) is a common skin disease with high prevalence in work environments. Human allergic contact dermatitis is triggered by the exposure to haptens that leads to an initial phase known as sensitization. During this phase, hapten-protein complexes presented by antigen-presenting cells activate a T-cell-mediated response, leading to the generation of memory cells against the hapten. Upon re-exposure to the same hapten, the elicitation phase is initiated. This phase is characterized by a quicker acute inflammatory response involving activation and/or infiltration of a variety of immune cell populations. Human ACD can be studied through the use of animal models of contact hypersensitivity (CHS). The 2,4-dinitrofluorobenzene (DNFB)-induced CHS model is a commonly used mouse model that has been helpful in the study of the mechanisms as well as potential therapeutic interventions of ACD. In this chapter I will provide a detailed protocol to develop acute DNFB-induced CHS in mice in a period of 7 days. In addition, I will discuss several key considerations for experimental design including best controls, potential expected outcomes, and sample collection.

Increased Production of LIGHT by T Cells in Eosinophilic Esophagitis Promotes Differentiation of Esophageal Fibroblasts Toward an Inflammatory Phenotype.

BACKGROUND & AIMS: Eosinophilic esophagitis (EoE) is an antigen-mediated eosinophilic disease of the esophagus that involves fibroblast activation and progression to fibrostenosis. Cytokines produced by T-helper type 2 cells and transforming growth factor beta 1 (TGFß1) contribute to the development of EoE, but other cytokines involved in pathogenesis are unknown. We investigate the effects of tumor necrosis factor superfamily member 14 (TNFSF14, also called LIGHT) on fibroblasts in EoE. METHODS: We analyzed publicly available esophageal CD3+ T-cell single-cell sequencing data for expression of LIGHT. Esophageal tissues were obtained from pediatric patients with EoE or control individuals and analyzed by immunostaining. Human primary esophageal fibroblasts were isolated from esophageal biopsy samples of healthy donors or patients with active EoE. Fibroblasts were cultured; incubated with TGFß1 and/or LIGHT; and analyzed by RNA sequencing, flow cytometry, immunoblots, immunofluorescence, or reverse transcription polymerase chain reaction. Eosinophils were purified from peripheral blood of healthy donors, incubated with interleukin 5, cocultured with fibroblasts, and analyzed by immunohistochemistry. RESULTS: LIGHT was up-regulated in the esophageal tissues from patients with EoE, compared with control individuals, and expressed by several T-cell populations, including T-helper type 2 cells. TNF receptor superfamily member 14 (TNFRSF14, also called HVEM) and lymphotoxin beta receptor are receptors for LIGHT that were expressed by fibroblasts from healthy donors or patients with active EoE. Stimulation of esophageal fibroblasts with LIGHT induced inflammatory gene transcription, whereas stimulation with TGFß1 induced transcription of genes associated with a myofibroblast phenotype. Stimulation of fibroblasts with TGFß1 increased expression of HVEM; subsequent stimulation with LIGHT resulted in their differentiation into cells that express markers of myofibroblasts and inflammatory chemokines and cytokines. Eosinophils tethered to esophageal fibroblasts after LIGHT stimulation via intercellular adhesion molecule-1. CONCLUSIONS: T cells in esophageal tissues from patients with EoE express increased levels of LIGHT compared with control individuals, which induces differentiation of fibroblasts into cells with inflammatory characteristics. TGFß1 increases fibroblast expression of HVEM, a receptor for LIGHT. LIGHT mediates interactions between esophageal fibroblasts and eosinophils via ICAM1. This pathway might be targeted for the treatment of EoE.

Pharmacologic inhibition of hypoxia-inducible factor (HIF)-hydroxylases ameliorates allergic contact dermatitis.

BACKGROUND: When an immune cell migrates from the bloodstream to a site of chronic inflammation, it experiences a profound decrease in microenvironmental oxygen levels leading to a state of cellular hypoxia. The hypoxia-inducible factor-1α (HIF-1α) promotes an adaptive transcriptional response to hypoxia and as such is a major regulator of immune cell survival and function. HIF hydroxylases are the family of oxygen-sensing enzymes primarily responsible for conferring oxygen dependence upon the HIF pathway. METHODS: Using a mouse model of allergic contact dermatitis (ACD), we tested the effects of treatment with the pharmacologic hydroxylase inhibitor DMOG, which mimics hypoxia, on disease development. RESULTS: Re-exposure of sensitized mice to 2,4-dinitrofluorobenzene (DNFB) elicited inflammation, edema, chemokine synthesis (including CXCL1 and CCL5) and the recruitment of neutrophils and eosinophils. Intraperitoneal or topical application of the pharmacologic hydroxylase inhibitors dymethyloxalylglycine (DMOG) or JNJ1935 attenuated this inflammatory response. Reduced inflammation was associated with diminished recruitment of neutrophils and eosinophils but not lymphocytes. Finally, hydroxylase inhibition reduced cytokine-induced chemokine production in cultured primary keratinocytes through attenuation of the JNK pathway. CONCLUSION: These data demonstrate that hydroxylase inhibition attenuates the recruitment of neutrophils to inflamed skin through reduction of chemokine production and increased neutrophilic apoptosis. Thus, pharmacologic inhibition of HIF hydroxylases may be an effective new therapeutic approach in allergic skin inflammation.

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.

Hydroxylase inhibition regulates inflammation-induced intestinal fibrosis through the suppression of ERK-mediated TGF-ß1 signaling. [corrected].

Fibrosis is a complication of chronic inflammatory disorders such as inflammatory bowel disease, a condition which has limited therapeutic options and often requires surgical intervention. Pharmacologic inhibition of oxygen-sensing prolyl hydroxylases, which confer oxygen sensitivity upon the hypoxia-inducible factor pathway, has recently been shown to have therapeutic potential in colitis, although the mechanisms involved remain unclear. Here, we investigated the impact of hydroxylase inhibition on inflammation-driven fibrosis in a murine colitis model. Mice exposed to dextran sodium sulfate, followed by a period of recovery, developed intestinal fibrosis characterized by alterations in the pattern of collagen deposition and infiltration of activated fibroblasts. Treatment with the hydroxylase inhibitor dimethyloxalylglycine ameliorated fibrosis. TGF-ß1 is a key regulator of fibrosis that acts through the activation of fibroblasts. Hydroxylase inhibition reduced TGF-ß1-induced expression of fibrotic markers in cultured fibroblasts, suggesting a direct role for hydroxylases in TGF-ß1 signaling. This was at least in part due to inhibition of noncanonical activation of extracellular signal-regulated kinase (ERK) signaling. In summary, pharmacologic hydroxylase inhibition ameliorates intestinal fibrosis through suppression of TGF-ß1-dependent ERK activation in fibroblasts. We hypothesize that in addition to previously reported immunosupressive effects, hydroxylase inhibitors independently suppress profibrotic pathways.

REST is a hypoxia-responsive transcriptional repressor.

Cellular exposure to hypoxia results in altered gene expression in a range of physiologic and pathophysiologic states. Discrete cohorts of genes can be either up- or down-regulated in response to hypoxia. While the Hypoxia-Inducible Factor (HIF) is the primary driver of hypoxia-induced adaptive gene expression, less is known about the signalling mechanisms regulating hypoxia-dependent gene repression. Using RNA-seq, we demonstrate that equivalent numbers of genes are induced and repressed in human embryonic kidney (HEK293) cells. We demonstrate that nuclear localization of the Repressor Element 1-Silencing Transcription factor (REST) is induced in hypoxia and that REST is responsible for regulating approximately 20% of the hypoxia-repressed genes. Using chromatin immunoprecipitation assays we demonstrate that REST-dependent gene repression is at least in part mediated by direct binding to the promoters of target genes. Based on these data, we propose that REST is a key mediator of gene repression in hypoxia.

Prolyl hydroxylase-1 regulates hepatocyte apoptosis in an NF-κB-dependent manner.

Hepatocyte death is an important contributing factor in a number of diseases of the liver. PHD1 confers hypoxic sensitivity upon transcription factors including the hypoxia inducible factor (HIF) and nuclear factor-kappaB (NF-κB). Reduced PHD1 activity is linked to decreased apoptosis. Here, we investigated the underlying mechanism(s) in hepatocytes. Basal NF-κB activity was elevated in PHD1(-/-) hepatocytes compared to wild type controls. ChIP-seq analysis confirmed enhanced binding of NF-κB to chromatin in regions proximal to the promoters of genes involved in the regulation of apoptosis. Inhibition of NF-κB (but not knock-out of HIF-1 or HIF-2) reversed the anti-apoptotic effects of pharmacologic hydroxylase inhibition. We hypothesize that PHD1 inhibition leads to altered expression of NF-κB-dependent genes resulting in reduced apoptosis. This study provides new information relating to the possible mechanism of therapeutic action of hydroxylase inhibitors that has been reported in pre-clinical models of intestinal and hepatic disease.

REST mediates resolution of HIF-dependent gene expression in prolonged hypoxia.

The hypoxia-inducible factor (HIF) is a key regulator of the cellular response to hypoxia which promotes oxygen delivery and metabolic adaptation to oxygen deprivation. However, the degree and duration of HIF-1α expression in hypoxia must be carefully balanced within cells in order to avoid unwanted side effects associated with excessive activity. The expression of HIF-1α mRNA is suppressed in prolonged hypoxia, suggesting that the control of HIF1A gene transcription is tightly regulated by negative feedback mechanisms. Little is known about the resolution of the HIF-1α protein response and the suppression of HIF-1α mRNA in prolonged hypoxia. Here, we demonstrate that the Repressor Element 1-Silencing Transcription factor (REST) binds to the HIF-1α promoter in a hypoxia-dependent manner. Knockdown of REST using RNAi increases the expression of HIF-1α mRNA, protein and transcriptional activity. Furthermore REST knockdown increases glucose consumption and lactate production in a HIF-1α- (but not HIF-2α-) dependent manner. Finally, REST promotes the resolution of HIF-1α protein expression in prolonged hypoxia. In conclusion, we hypothesize that REST represses transcription of HIF-1α in prolonged hypoxia, thus contributing to the resolution of the HIF-1α response.

Targeted delivery of the hydroxylase inhibitor DMOG provides enhanced efficacy with reduced systemic exposure in a murine model of colitis.

Targeting hypoxia-sensitive pathways has recently been proposed as a new therapeutic approach to the treatment of intestinal inflammation. HIF-hydroxylases are enzymes which confer hypoxic-sensitivity upon the hypoxia-inducible factor (HIF), a major regulator of the adaptive response to hypoxia. Previous studies have shown that systemic (intraperitoneal) administration of hydroxylase inhibitors such as dimethyloxalylglycine (DMOG) is profoundly protective in multiple models of colitis, however the therapeutic potential of this approach is limited due to potential side-effects associated with systemic drug exposure and the fact that orally delivered DMOG is ineffective (likely due to drug inactivation by gastric acid). In order to overcome these issues, we formulated DMOG in a liquid emulsion drug delivery system which, when coated with specific polymer coatings, permits oral delivery of a reduced dose which is released locally throughout the colon. This colon-targeted DMOG formulation demonstrated increased relative colonic bioactivity with reduced systemic exposure and provided a similar degree of protection to systemic (intraperitoneal) administration at a 40-fold lower dose in DSS-induced colitis. In summary, targeted delivery of DMOG to the colon provides local protection resulting in enhanced efficacy with reduced systemic exposure in the treatment of colitis. This novel approach to targeting hydroxylase inhibitors to specific diseased regions of the GI tract may improve it's potential as a new therapeutic in inflammatory bowel diseases such as ulcerative colitis.

Hypoxia-sensitive pathways in inflammation-driven fibrosis.

Tissue injury can occur for a variety of reasons, including physical damage, infection, and ischemia. The ability of tissues to effectively recover from injury is a cornerstone of human health. The healing response in tissues is conserved across organs and typically involves distinct but overlapping inflammatory, proliferative, and maturation/resolution phases. If the inflammatory phase is not successfully controlled and appropriately resolved, an excessive healing response characterized by scar formation can lead to tissue fibrosis, a major clinical complication in disorders such as Crohn's disease (CD). As a result of enhanced metabolic and inflammatory processes during chronic inflammation, profound changes in tissue oxygen levels occur leading to localized tissue hypoxia. Therefore, inflammation, fibrosis, and hypoxia are coincidental events during inflammation-driven fibrosis. Our current understanding of the mechanism(s) underpinning fibrosis is limited as are the therapeutic options available. In this review, we discuss what is known about the cellular and molecular mechanisms underpinning inflammation-driven fibrosis and how hypoxia may play a role in shaping this process.

A dynamic model of the hypoxia-inducible factor 1α (HIF-1α) network.

Activation of the hypoxia-inducible factor (HIF) pathway is a critical step in the transcriptional response to hypoxia. Although many of the key proteins involved have been characterised, the dynamics of their interactions in generating this response remain unclear. In the present study, we have generated a comprehensive mathematical model of the HIF-1α pathway based on core validated components and dynamic experimental data, and confirm the previously described connections within the predicted network topology. Our model confirms previous work demonstrating that the steps leading to optimal HIF-1α transcriptional activity require sequential inhibition of both prolyl- and asparaginyl-hydroxylases. We predict from our model (and confirm experimentally) that there is residual activity of the asparaginyl-hydroxylase FIH (factor inhibiting HIF) at low oxygen tension. Furthermore, silencing FIH under conditions where prolyl-hydroxylases are inhibited results in increased HIF-1α transcriptional activity, but paradoxically decreases HIF-1α stability. Using a core module of the HIF network and mathematical proof supported by experimental data, we propose that asparaginyl hydroxylation confers a degree of resistance upon HIF-1α to proteosomal degradation. Thus, through in vitro experimental data and in silico predictions, we provide a comprehensive model of the dynamic regulation of HIF-1α transcriptional activity by hydroxylases and use its predictive and adaptive properties to explain counter-intuitive biological observations.