Mucosal acidosis elicits a unique molecular signature in epithelia and intestinal tissue mediated by GPR31-induced CREB phosphorylation.

Metabolic changes associated with tissue inflammation result in significant extracellular acidosis (EA). Within mucosal tissues, intestinal epithelial cells (IEC) have evolved adaptive strategies to cope with EA through the up-regulation of SLC26A3 to promote pH homeostasis. We hypothesized that EA significantly alters IEC gene expression as an adaptive mechanism to counteract inflammation. Using an unbiased RNA sequencing approach, we defined the impact of EA on IEC gene expression to define molecular mechanisms by which IEC respond to EA. This approach identified a unique gene signature enriched in cyclic AMP response element-binding protein (CREB)-regulated gene targets. Utilizing loss- and gain-of-function approaches in cultured epithelia and murine colonoids, we demonstrate that EA elicits prominent CREB phosphorylation through cyclic AMP-independent mechanisms that requires elements of the mitogen-activated protein kinase signaling pathway. Further analysis revealed that EA signals through the G protein-coupled receptor GPR31 to promote induction of FosB, NR4A1, and DUSP1. These studies were extended to an in vivo murine model in conjunction with colonization of a pH reporter Escherichia coli strain that demonstrated significant mucosal acidification in the TNFΔARE model of murine ileitis. Herein, we observed a strong correlation between the expression of acidosis-associated genes with bacterial reporter sfGFP intensity in the distal ileum. Finally, the expression of this unique EA-associated gene signature was increased during active inflammation in patients with Crohn's disease but not in the patient control samples. These findings establish a mechanism for EA-induced signals during inflammation-associated acidosis in both murine and human ileitis.

Microbial-derived indoles inhibit neutrophil myeloperoxidase to diminish bystander tissue damage.

During episodes of acute inflammation, polymorphonuclear leukocytes (PMNs) are actively recruited to sites of inflammation or injury where they provide anti-microbial and wound-healing functions. One enzyme crucial for fulfilling these functions is myeloperoxidase (MPO), which generates hypochlorous acid from Cl- and hydrogen peroxide. The potential exists, however, that uncontrolled the extracellular generation of hypochlorous acid by MPO can cause bystander tissue damage and inhibit the healing response. Previous work suggests that the microbiota-derived tryptophan metabolites 1H-indole and related molecules ("indoles") are protective during intestinal inflammation, although their precise mechanism of action is unclear. In the present work, we serendipitously discovered that indoles are potent and selective inhibitors of MPO. Using both primary human PMNs and recombinant human MPO in a cell-free system, we revealed that indoles inhibit MPO at physiologic concentrations. Particularly, indoles block the chlorinating activity of MPO, a reliable marker for MPO-associated tissue damage, as measured by coulometric-coupled HPLC. Further, we observed direct interaction between indoles and MPO using the established biochemical techniques microscale thermophoresis and STD-NMR. Utilizing a murine colitis model, we demonstrate that indoles inhibit bystander tissue damage, reflected in decreased colon 3-chlorotyrosine and pro-inflammatory chemokine expression in vivo. Taken together, these results identify microbiota-derived indoles that acts as endogenous immunomodulatory compounds through their actions on MPO, suggesting a symbiotic association between the gut microbiota and host innate immune system. Such findings offer exciting new targets for future pharmacological intervention.

Microbial-Derived Butyrate Promotes Epithelial Barrier Function through IL-10 Receptor-Dependent Repression of Claudin-2.

Commensal interactions between the enteric microbiota and distal intestine play important roles in regulating human health. Short-chain fatty acids (SCFAs), such as butyrate, produced through anaerobic microbial metabolism represent a major energy source for the host colonic epithelium and enhance epithelial barrier function through unclear mechanisms. Separate studies revealed that the epithelial anti-inflammatory IL-10 receptor α subunit (IL-10RA) is also important for barrier formation. Based on these findings, we examined if SCFAs promote epithelial barrier through IL-10RA-dependent mechanisms. Using human intestinal epithelial cells (IECs), we discovered that SCFAs, particularly butyrate, enhanced IEC barrier formation, induced IL-10RA mRNA, IL-10RA protein, and transactivation through activated Stat3 and HDAC inhibition. Loss and gain of IL-10RA expression directly correlates with IEC barrier formation and butyrate represses permeability-promoting claudin-2 tight-junction protein expression through an IL-10RA-dependent mechanism. Our findings provide a novel mechanism by which microbial-derived butyrate promotes barrier through IL-10RA-dependent repression of claudin-2.

Cytokine responses and epithelial function in the intestinal mucosa.

Inflammatory diseases of mucosal organs are significantly influenced by the microenvironment in which they reside. Cytokines found within this microenvironment contribute significantly to endpoint functions of the mucosa. Studies dating back to the 1990s have revealed that epithelial cells are both a source as well as a target for numerous cytokines and that such signaling can substantially influence the outcome of mucosal disease, such as inflammatory bowel disease. Here, we will review literature regarding intestinal epithelial cells as sources and responders to cytokines found in the intestinal milieu. These studies highlight the dynamic nature of these pathways and lend insight into the complexity of treating mucosal inflammation.

Escherichia coli heme oxygenase modulates host innate immune responses.

Induction of mammalian heme oxygenase (HO)-1 and exposure of animals to carbon monoxide (CO) ameliorates experimental colitis. When enteric bacteria, including Escherichia coli, are exposed to low iron conditions, they express an HO-like enzyme, chuS, and metabolize heme into iron, biliverdin and CO. Given the abundance of enteric bacteria residing in the intestinal lumen, our postulate was that commensal intestinal bacteria may be a significant source of CO and those that express chuS and other Ho-like molecules suppress inflammatory immune responses through release of CO. According to real-time PCR, exposure of mice to CO results in changes in enteric bacterial composition and increases E. coli 16S and chuS DNA. Moreover, the severity of experimental colitis correlates positively with E. coli chuS expression in IL-10 deficient mice. To explore functional roles, E. coli were genetically modified to overexpress chuS or the chuS gene was deleted. Co-culture of chuS-overexpressing E. coli with bone marrow-derived macrophages resulted in less IL-12p40 and greater IL-10 secretion than in wild-type or chuS-deficient E. coli. Mice infected with chuS-overexpressing E. coli have more hepatic CO and less serum IL-12 p40 than mice infected with chuS-deficient E. coli. Thus, CO alters the composition of the commensal intestinal microbiota and expands populations of E. coli that harbor the chuS gene. These bacteria are capable of attenuating innate immune responses through expression of chuS. Bacterial HO-like molecules and bacteria-derived CO may represent novel targets for therapeutic intervention in inflammatory conditions.

Carbon monoxide and heme oxygenase-1 prevent intestinal inflammation in mice by promoting bacterial clearance.

BACKGROUND & AIMS: Heme oxygenase-1 (HO-1) and its metabolic by-product, carbon monoxide (CO), protect against intestinal inflammation in experimental models of colitis, but little is known about their intestinal immune mechanisms. We investigated the interactions among CO, HO-1, and the enteric microbiota in mice and zebrafish. METHODS: Germ-free, wild-type, and interleukin (Il)10(-/-) mice and germ-free zebrafish embryos were colonized with specific pathogen-free (SPF) microbiota. Germ-free or SPF-raised wild-type and Il10(-/-) mice were given intraperitoneal injections of cobalt(III) protoporphyrin IX chloride (CoPP), which up-regulates HO-1, the CO-releasing molecule Alfama-186, or saline (control). Colitis was induced in wild-type mice housed in SPF conditions by infection with Salmonella typhimurium. RESULTS: In colons of germ-free, wild-type mice, SPF microbiota induced production of HO-1 via activation of nuclear factor erythroid 2-related factor 2-, IL-10-, and Toll-like receptor-dependent pathways; similar observations were made in zebrafish. SPF microbiota did not induce HO-1 in colons of germ-free Il10(-/-) mice. Administration of CoPP to Il10(-/-) mice before transition from germ-free to SPF conditions reduced their development of colitis. In Il10(-/-) mice, CO and CoPP reduced levels of enteric bacterial genomic DNA in mesenteric lymph nodes. In mice with S typhimurium-induced enterocolitis, CoPP reduced the numbers of live S typhimurium recovered from the lamina propria, mesenteric lymph nodes, spleen, and liver. Knockdown of HO-1 in mouse macrophages impaired their bactericidal activity against E coli, E faecalis, and S typhimurium, whereas exposure to CO or overexpression of HO-1 increased their bactericidal activity. HO-1 induction and CO increased acidification of phagolysosomes. CONCLUSIONS: Colonic HO-1 prevents colonic inflammation in mice. HO-1 is induced by the enteric microbiota and its homeostatic function is mediated, in part, by promoting bactericidal activities of macrophages.

Chlorination of epithelial tight junction proteins by neutrophil myeloperoxidase promotes barrier dysfunction and mucosal inflammation.

Neutrophils (polymorphonuclear leukocytes, PMNs) comprise a major component of the immune cell infiltrate during acute mucosal inflammation and have an important role in molding the inflammatory tissue environment. While PMNs are essential to clearance of invading microbes, the major PMN antimicrobial enzyme myeloperoxidase (MPO) can also promote bystander tissue damage. We hypothesized that blocking MPO would attenuate acute colitis and prevent the development of chronic colitis by limiting bystander tissue damage. Using the acute and chronic dextran sodium sulfate model of murine colitis, we demonstrated that MPO-deficient mice experienced less inflammation and more rapidly resolved colitis relative to wild-type controls. Mechanistic studies demonstrated that activated MPO disrupted intestinal epithelial barrier function through the dysregulation of the epithelial tight junction proteins. Our findings revealed that activated MPO chlorinates tyrosine within several tight junction proteins, thereby promoting tight junction mislocalization and dysfunction. These observations in cell models and in murine colitis were validated in human intestinal biopsies from individuals with ulcerative colitis and revealed a strong correlation between disease severity (Mayo score) and tissue chlorinated tyrosine levels. In summary, these findings implicate MPO as a viable therapeutic target to limit bystander tissue damage and preserve mucosal barrier function during inflammation.

Epithelial heme oxygenase-1 enhances colonic tumorigenesis by inhibiting ferroptosis.

Colorectal cancer has been linked to chronic colitis and red meat consumption, which can increase colonic iron and heme. Heme oxygenase-1 ( Hmox1 ) metabolizes heme and releases ferrous iron, but its role in colonic tumorigenesis is not well-described. Recent studies suggest that ferroptosis, the iron-dependent form of cell death, protects against colonic tumorigenesis. Ferroptosis culminates in excessive lipid peroxidation that is constrained by the antioxidative glutathione pathway. We observed increased mucosal markers of ferroptosis and glutathione metabolism in the setting of murine and human colitis, as well as murine colonic neoplasia. We obtained similar results in murine and human colonic epithelial organoids exposed to heme and the ferroptosis activator erastin, especially induction of Hmox1 . RNA sequencing of colonic organoids from mice with deletion of intestinal epithelial Hmox1 (Hmox1 ΔIEC ) revealed increased ferroptosis and activated glutathione metabolism after heme exposure. In a colitis-associated cancer model we observed significantly fewer and smaller tumors in Hmox1 ΔIEC mice compared to littermate controls. Transcriptional profiling of Hmox1 ΔIEC tumors and tumor organoids revealed increased ferroptosis and oxidative stress markers in tumor epithelial cells. In total, our findings reveal ferroptosis as an important colitis-associated cancer signature pathway, and Hmox1 as a key regulator in the tumor microenvironment.

An anti-inflammatory role for carbon monoxide and heme oxygenase-1 in chronic Th2-mediated murine colitis.

Cigarette smoking is a significant environmental factor in the human inflammatory bowel diseases, remarkably, conferring protection in ulcerative colitis. We previously demonstrated that a prominent component of cigarette smoke, CO, suppresses Th17-mediated experimental colitis in IL-10(-/-) mice through a heme oxygenase (HO)-1-dependent pathway. In this study, homeostatic and therapeutic effects of CO and HO-1 were determined in chronic colonic inflammation in TCR-α-deficient ((-/-)) mice, in which colitis is mediated by Th2 cytokines, similar to the cytokine milieu described in human ulcerative colitis. TCRα(-/-) mice exposed to CO or treated with the pharmacologic HO-1 inducer cobalt protoporphyrin demonstrated amelioration of active colitis. CO and cobalt protoporphyrin suppressed colonic IL-1ß, TNF, and IL-4 production, whereas IL-10 protein secretion was increased. CO induced IL-10 expression in macrophages and in vivo through an HO-1-dependent pathway. Bacterial products regulate HO-1 expression in macrophages through MyD88- and IL-10-dependent pathways. CO exposure and pharmacologic HO-1 induction in vivo resulted in increased expression of HO-1 and IL-10 in CD11b(+) lamina propria mononuclear cells. Moreover, induction of the IL-10 family member IL-22 was demonstrated in CD11b(-) lamina propria mononuclear cells. In conclusion, CO and HO-1 induction ameliorated active colitis in TCRα(-/-) mice, and therapeutic effects correlated with induction of IL-10. This study provides further evidence that HO-1 mediates an important homeostatic pathway with pleiotropic anti-inflammatory effects in different experimental models of colitis and that targeting HO-1, therefore, is a potential therapeutic strategy in human inflammatory bowel diseases.

Characterization of an interferon-stimulated response element (ISRE) in the Il23a promoter.

We have demonstrated previously that IFN-γ plays a protective role in the initiation of chronic intestinal inflammation through attenuation of Toll-like receptor-mediated IL-23 induction in macrophages. Here, an interferon-stimulated response element (ISRE) is identified in a region of conserved nucleotide sequences in the Il23a promoter. This ISRE mediated, in part, Il23a promoter induction by LPS and inhibition of LPS-induced activity by IFN-γ. LPS and IFN-γ recruit interferon regulatory factors (IRFs) to the Il23a ISRE in murine bone marrow-derived macrophages (BMMs). Functionally, IRF-1 is a negative regulator of Il23a in LPS-stimulated BMMs. IRF-1(-/-) BMMs demonstrated enhanced LPS-induced Il23a expression compared with WT BMMs. Moreover, IRF-1 deficiency resulted in prolonged occupancy of RelA on the Il23a promoter. Consequently, IRF-1(-/-) mice were more susceptible to colonic injury by trinitrobenzenesulfonic acid, and IL-10/IRF-1 double-deficient (IL-10/IRF-1(-/-)) mice demonstrated more severe colonic inflammation compared with IL-10(-/-) mice. The severity of colitis in both models correlated with increased colonic IL-23. CD11b(+) lamina propria mononuclear cells, comprising predominantly macrophages, were identified as the major source of IL-23 in colitis-prone mice. Basal and heat-killed Escherichia coli-stimulated levels of Il23a were increased in IL-10/IRF-1(-/-) compared with WT and IL-10(-/-) colonic CD11b(+) lamina propria mononuclear cells. In conclusion, these experiments characterize IRF-ISRE interactions on the Il23a promoter, which have in vivo relevance as a homeostatic checkpoint in chronic intestinal inflammation.

Use of Ruxolitinib for the Simultaneous Treatment of Severe Refractory Ulcerative Colitis and Polycythemia Vera.

Tofacitinib is the only medicine in the class of Janus kinase (JAK) inhibitors that has been approved for use in moderate-to-severely active ulcerative colitis (UC). The potential of other JAK inhibitors to treat UC has not been fully explored. We present a case describing the successful use of the selective JAK inhibitor, ruxolitinib, to treat a patient with concomitant UC and polycythemia vera.

Disruption of monocyte-macrophage differentiation and trafficking by a heme analog during active inflammation.

Heme metabolism is a key regulator of inflammatory responses. Cobalt protoporphyrin IX (CoPP) is a heme analog and mimic that potently activates the NRF2/heme oxygenase-1 (HO-1) pathway, especially in monocytes and macrophages. We investigated the influence of CoPP on inflammatory responses using a murine model of colitis. Surprisingly, conditional deletion of myeloid HO-1 did not impact the colonic inflammatory response or the protective influence of CoPP in the setting of dextran sodium sulfate-induced colitis. Rather, we reveal that CoPP elicits a contradictory shift in blood myeloid populations relative to the colon during active intestinal inflammation. Major population changes include markedly diminished trafficking of CCR2+Ly6Chi monocytes to the inflamed colon, despite significant mobilization of this population into circulation. This resulted in significantly diminished colonic expansion of monocyte-derived macrophages and inflammatory cytokine expression. These findings were linked with significant induction of systemic CCL2 leading to a disrupted CCL2 chemoattractant gradient toward the colon and concentration-dependent suppression of circulating monocyte CCR2 expression. Administration of CoPP also induced macrophage differentiation toward a MarcohiHmox1hi anti-inflammatory erythrophagocytic phenotype, contributing to an overall decreased inflammatory profile. Such findings redefine protective influences of heme metabolism during inflammation, and highlight previously unreported immunosuppressive mechanisms of endogenous CCL2 induction.

Markers of Hypoxia Correlate with Histologic and Endoscopic Severity of Colitis in Inflammatory Bowel Disease.

BACKGROUND: Inflammation results in significant shifts in tissue metabolism. Recent studies indicate that inflammation and hypoxia occur concomitantly. We examined whether circulating and tissue markers of hypoxia could serve as surrogate indicators of disease severity in adult and pediatric patients with inflammatory bowel disease (IBD). METHODS: Serum and colonic biopsies were obtained from pediatric subjects with active IBD colitis and adult subjects with active and inactive ulcerative colitis, along with healthy non-colitis controls of all ages. Disease activity was evaluated by endoscopy and histopathology. Levels of serum hypoxia markers (macrophage inflammatory protein-3α [MIP-3α], vascular endothelial growth factor [VEGF], and erythropoietin [EPO]) were measured. RESULTS: Children with active IBD colitis had higher levels of serum MIP-3α and VEGF compared to non-colitis controls (p<0.01 and p<0.05, respectively). In adult subjects with endoscopically active ulcerative colitis, serum MIP-3α and EPO were significantly elevated compared to non-colitis controls (both p<0.01). In parallel, analysis of colon tissue MIP-3α mRNA and protein in pediatric subjects revealed increased expression in those with IBD colitis compared to controls (p<0.05 and p<0.01 for mRNA and protein, respectively). Serum MIP-3α and VEGF significantly increased with histology grade. CONCLUSION: Peripheral blood hypoxia markers may be useful indicators of disease activity for pediatric and adult IBD patients.

Adaptation to inflammatory acidity through neutrophil-derived adenosine regulation of SLC26A3.

Acute intestinal inflammation includes the early accumulation of neutrophils (PMN). Based on recent evidence that PMN infiltration "imprints" changes in the local tissue environment through local oxygen depletion and the release of adenine nucleotides, we hypothesized that the interaction between transmigrating PMN and intestinal epithelial cells (IECs) results in inflammatory acidification of the tissue. Using newly developed tools, we revealed that active PMN transepithelial migration (TEM) significantly acidifies the local microenvironment, a decrease of nearly 2 pH units. Using unbiased approaches, we sought to define acid-adaptive pathways elicited by PMN TEM. Given the significant amount of adenosine (Ado) generated during PMN TEM, we profiled the influence of Ado on IECs gene expression by microarray and identified the induction of SLC26A3, the major apical Cl-/HCO3- exchanger in IECs. Utilizing loss- and gain-of-function approaches, as well as murine and human colonoids, we demonstrate that Ado-induced SLC26A3 promotes an adaptive IECs phenotype that buffers local pH during active inflammation. Extending these studies, chronic murine colitis models were used to demonstrate that SLC26A3 expression rebounds during chronic DSS-induced inflammation. In conclusion, Ado signaling during PMN TEM induces an adaptive tissue response to inflammatory acidification through the induction of SLC26A3 expression, thereby promoting pH homeostasis.

A Central Role for Heme Oxygenase-1 in the Control of Intestinal Epithelial Chemokine Expression.

In mucosal inflammatory disorders, the protective influence of heme oxygenase-1 (HO-1) and its metabolic byproducts, carbon monoxide (CO) and biliverdin, is a topic of significant interest. Mechanisms under investigation include the regulation of macrophage function and mucosal cytokine expression. While there is an increasing recognition of the importance of epithelial-derived factors in the maintenance of intestinal mucosal homeostasis, the contribution of intestinal epithelial cell (IEC) HO-1 on inflammatory responses has not previously been investigated. We examined the influence of modulating HO-1 expression on the inflammatory response of human IECs. Engineered deficiency of HO-1 in Caco-2 and T84 IECs led to increased proinflammatory chemokine expression in response to pathogenic bacteria and inflammatory cytokine stimulation. Crosstalk with activated leukocytes also led to increased chemokine expression in HO-1-deficient cells in an IL-1ß dependent manner. Treatment of Caco-2 cells with a pharmacological inducer of HO-1 led to the inhibition of chemokine expression. Mechanistic studies suggest that HO-1 and HO-1-related transcription factors, but not HO-1 metabolic products, are partly responsible for the influence of HO-1 on chemokine expression. In conclusion, our data identify HO-1 as a central regulator of IEC chemokine expression that may contribute to homeo-stasis in the intestinal mucosa.

Perturbation of neddylation-dependent NF-κB responses in the intestinal epithelium drives apoptosis and inhibits resolution of mucosal inflammation.

Recent work has revealed a central role for neddylation (the conjugation of a Nedd8-moiety to Cullin proteins) in the fine tuning of the NF-κB response (via Cullin-1). In the present study, we investigated the contribution of Cullin-1 neddylation and NF-κB signaling to mucosal inflammatory responses in vitro and in vivo. Initial in vitro studies using cultured intestinal epithelial cells revealed that the neddylation inhibitor MLN4924 prominently induces the deneddylation of Cullin-1. Parallel western blot, luciferase reporter and gene target assays identified MLN4924 as a potent inhibitor of intestinal epithelial NF-κB. Subsequent studies revealed that MLN4924 potently induces epithelial apoptosis but only in the presence of additional inflammatory stimuli. In vivo administration of MLN4924 (3 mg/kg/d) in a TNBS-induce colitis model significantly accentuated disease severity. Indeed, MLN4924 resulted in worsened clinical scores and increased mortality early in the inflammatory response. Histologic analysis of the colon revealed that neddylation inhibition results in increased tissue damage and significantly increased mucosal apoptosis as determined by TUNEL and cleaved caspase-3 staining, particularly prominent within the epithelium. Extensions of these studies revealed that ongoing inflammation is associated with significant loss of deneddylase-1 (SENP8) expresssion. These studies reveal that intact Cullin-1 neddylation is central to resolution of acute inflammation.

Heme oxygenase-1 and carbon monoxide regulate intestinal homeostasis and mucosal immune responses to the enteric microbiota.

Heme oxygenase-1 (HO-1) and its enzymatic by-product carbon monoxide (CO) have emerged as important regulators of acute and chronic inflammation. Mechanisms underlying their anti-inflammatory effects are only partially understood. In this addendum, we summarize current understanding of the role of the HO-1/CO pathway in regulation of intestinal inflammation with a focus on innate immune function. In particular, we highlight our recent findings that HO-1 and CO ameliorate intestinal inflammation through promotion of bacterial clearance. Our work and that of many others support further investigation of this global homeostatic pathway in the human inflammatory bowel diseases (IBDs).