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.

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.

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.

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.