Promoting MPhi transepithelial migration by stimulating the epithelial cell P2Y(2) receptor

Macrophages and Epithelial Cells Mutually Interact through NLRP3 to Clear Infection and Enhance the Gastrointestinal Barrier

Activation of the nod-like receptor protein 3 (NLRP3) leads to the release of the proinflammatory cytokine IL-1β, which then facilitates pathogen control by macrophages. The role of NLRPs in controlling infection of epithelial cells is not well understood. Our hypothesis was that activation of the NLRP3 inflammasome in colonic epithelial cells would promote macrophage-mediated epithelial recovery after infection with the pathogen Citrobacter rodentium. We devised a co-culture model using mouse colonic epithelial cells (CMT-93) and macrophages (J774A.1) during infection with C. rodentium. Inflammasome was activated using LPS and ATP and inhibited by YVAD. We assessed cytokine secretion (ELISA), macrophage recruitment and pathogen penetration (immunofluorescence), and epithelial barrier integrity (transepithelial electrical resistance). Macrophages were recruited to the apical membrane of epithelial cells, associated with tight junctions, promoted epithelial barrier recovery, and displaced C. rodentium. While NLRP3 was expressed in infected epithelial cells, IL-18 or IL-1β secretion remained unchanged. Supernatants from infected epithelial cells promoted infection clearance by macrophage; while this was inflammasome-independent, ATP significantly improved epithelial barrier recovery. The inflammasome appears to promote epithelial barrier function, independent of IL-18 and IL-1β secretion. Inflammasome activation in macrophages plays a dual role of promoting pathogen clearance and improving epithelial barrier integrity.

Matrix Metalloproteinase MMP-12 Promotes Macrophage Transmigration Across Intestinal Epithelial Tight Junctions and Increases Severity of Experimental Colitis

BACKGROUND AND AIMS

Matrix metalloproteinases [MMPs] play an important role in extracellular matrix regulation during cell growth and wound healing. Increased expression of MMP-12 [human macrophage elastase] has been reported in inflammatory bowel disease [IBD] which is characterised by the loss of epithelial tight junction [TJ] barrier function and an excessive inflammatory response. The aim of this study was to investigate the role of MMP-12 in intestinal TJ barrier function and inflammation.

METHODS

Wild type [WT] and MMP-12-/- mice were subjected to experimental acute or chronic dextran sodium sulphate [DSS] colitis. The mouse colonic permeability was measured in vivo by recycling perfusion of the entire colon and ex vivo by Ussing chamber studies.

RESULTS

DSS administration increased colonic permeability through modulation of TJ proteins and also increased MMP-12 expression in the colonic mucosa of WT mice. The acute as well as chronic DSS-induced increase in colonic TJ permeability and the severity of DSS colitis was found to be markedly attenuated in MMP-12-/- mice. The resistance of MMP-12-/- mice to DSS colitis was characterised by reduced macrophage infiltration and transmigration, and reduced basement membrane laminin degradation. Further in vitro and in vivo studies show that macrophage transmigration across the epithelial layer is MMP-12 dependent and the epithelial TJ barrier is compromised during macrophage transmigration. Conclusions: Together, these data demonstrate that MMP-12 mediated degradation of basement membrane laminin, macrophage transmigration, and associated loss of intestinal TJ barrier are key pathogenic factors for intestinal inflammation.

Control of Gut Inflammation by Modulation of Purinergic Signaling

Inflammatory bowel disease (IBD) is a serious inflammatory condition of the gastrointestinal tract. Crohn's disease (CD) and ulcerative colitis (UC) are two of the most common IBD manifestations and are both associated with unfettered inflammation, often refractory to conventional immunosuppressive treatment. In both conditions, imbalance between effector and regulatory cell immune responses has been documented and is thought to contribute to disease pathogenesis. Purinergic signaling is a known modulator of systemic and local inflammation and growing evidences point to extracellular ATP/adenosine imbalance as a key determinant factor in IBD-associated immune dysregulation. In vitro and pre-clinical studies suggest a role for both ATP (P2) and adenosine (P1) receptors in dictating onset and severity of the disease. Moreover, our experimental data indicate ENTPD1/CD39 and CD73 ectoenzymes as pivotal modulators of intestinal inflammation, with clear translational importance. Here we will provide an updated overview of the current knowledge on the role of the purinergic signaling in modulating immune responses in IBD. We will also review and discuss the most promising findings supporting the use of purinergic-based therapies to correct immune dysregulation in CD and UC.

P2Y2 Receptor Functions in Cancer: A Perspective in the Context of Colorectal Cancer

Purinergic signaling has recently emerged as a network of signaling molecules, enzymes and receptors that coordinates the action and behavior of cancerous cells. Extracellular adenosine 5' triphosphate activates a plethora of P2 nucleotide receptors that can putatively modulate cancer cell proliferation, survival and dissemination. In this context, the G protein-coupled P2Y₂ receptor was identified as one of the entities coordinating the cellular and molecular events that characterize cancerous cells. In this chapter, we will look at the contribution of the P2Y₂ receptor in cancer outcomes and use this information to demonstrate that the P2Y₂ receptor represents a drug target of interest in the setting of colorectal cancer, for which the role and function of this receptor is poorly defined. More particularly, we will review how the P2Y₂ receptor modulates cancer cell proliferation and survival, while promoting cell dissemination and formation of metastases. Finally, we will investigate how the P2Y₂ receptor can contribute to the detrimental development of drug resistance that is often observed in cancerous cells.

Transmigration of Leukocytes Across Epithelial Monolayers

Migration of leukocytes through epithelial monolayers represents an essential step in the generation of an inflammatory response and is often seen in inflammatory conditions such as Crohn's disease (Matthews et al., Toxicol Pathol 42:91-98, 2014) and asthma (Lambrecht and Hammad, Nat Med 18:684-692, 2012). Transepithelial migration involves adhesion to the basal surface of the epithelium before migration through the epithelial cell layer to the apical surface. Analyzing this process can present a technical challenge due to complications of using a coculture model and trying to recapitulate an intact monolayer. Here we describe two methods of assessing transepithelial migration based on a Transwell assay, the first of which measures the apical-basal migration of epithelial cells and the second "Inverted" transwell assay that measures basal-apical transmigration of leukocytes and therefore more closely mimics the in vivo process.

Important roles of P2Y receptors in the inflammation and cancer of digestive system

Purinergic signaling is important for many biological processes in humans. Purinoceptors P2Y are widely distributed in human digestive system and different subtypes of P2Y receptors mediate different physiological functions from metabolism, proliferation, differentiation to apoptosis etc. The P2Y receptors are essential in many gastrointestinal functions and also involve in the occurrence of some digestive diseases. Since different subtypes of P2Y receptors are present on the same cell of digestive organs, varying subtypes of P2Y receptors may have opposite or synergetic functions on the same cell. Recently, growing lines of evidence strongly suggest the involvement of P2Y receptors in the pathogenesis of several digestive diseases. In this review, we will focus on their important roles in the development of digestive inflammation and cancer. We anticipate that as the special subtypes of P2Y receptors are studied in depth, specific modulators for them will have good potentials to become promising new drugs to treat human digestive diseases in the near future.

C/EBPβ regulates P2X7 receptor expression in response to glucose challenge in intestinal epithelial cells

Activation of the ATP-dependent P2X7 receptor modulates glucose transport in intestinal epithelial cells through the downregulation of glucose transporter GLUT2. In the present study, we show that an increase in glucose concentration stimulates P2X7 receptor transcription via modulation of CCAAT/enhancer binding proteins (C/EBPs) α and β expression. The described human P2X7 receptor promoter region (GenBank Y12851) was cloned upstream of a luciferase reporter gene in pGL4.10 plasmid and used to determine whether C/EBPs, namely C/EBPα and C/EBPβ, are able to stimulate the transcription of P2X7 receptor. Results show that C/EBPβ was the main regulator of P2X7 receptor expression in response to a glucose challenge. Chromatin immunoprecipitation (ChIP) assays further revealed that C/EBPβ occupied the –213 to +6 nt P2X7 promoter region. Surprisingly, C/EBPα was also able to bind this region as revealed by ChIP assays, but without inducing receptor transcription. In fact, C/EBPα and the C/EBPβ-LIP isoform blocked the C/EBPβ-dependent regulation of P2X7 receptor transcription. These findings suggest that glucose is not only the major source of energy for cell function but may also act as a signaling molecule to stimulate the expression of regulatory proteins.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Extracellular nucleotides inhibit oxalate transport by human intestinal Caco-2-BBe cells through PKC-δ activation

Nephrolithiasis remains a major health problem in Western countries. Seventy to 80% of kidney stones are composed of calcium oxalate, and small changes in urinary oxalate affect risk of kidney stone formation. Intestinal oxalate secretion mediated by the anion exchanger SLC26A6 plays an essential role in preventing hyperoxaluria and calcium oxalate nephrolithiasis, indicating that understanding the mechanisms regulating intestinal oxalate transport is critical for management of hyperoxaluria. Purinergic signaling modulates several intestinal processes through pathways including PKC activation, which we previously found to inhibit Slc26a6 activity in mouse duodenal tissue. We therefore examined whether purinergic stimulation with ATP and UTP affects oxalate transport by human intestinal Caco-2-BBe (C2) cells. We measured [¹⁴C]oxalate uptake in the presence of an outward Cl⁻ gradient as an assay of Cl⁻/oxalate exchange activity, ≥50% of which is mediated by SLC26A6. We found that ATP and UTP significantly inhibited oxalate transport by C2 cells, an effect blocked by the PKC inhibitor Gö-6983. Utilizing pharmacological agonists and antagonists, as well as PKC-δ knockdown studies, we observed that ATP inhibits oxalate transport through the P2Y₂ receptor, PLC, and PKC-δ. Biotinylation studies showed that ATP inhibits oxalate transport by lowering SLC26A6 surface expression. These findings are of potential relevance to pathophysiology of inflammatory bowel disease-associated hyperoxaluria, where supraphysiological levels of ATP/UTP are expected and overexpression of the P2Y₂ receptor has been reported. We conclude that ATP and UTP inhibit oxalate transport by lowering SLC26A6 surface expression in C2 cells through signaling pathways including the P2Y₂ purinergic receptor, PLC, and PKC-δ.

P2Y2 receptor promotes intestinal microtubule stabilization and mucosal re-epithelization in experimental colitis

P2Y(2) receptor expression is increased in intestinal epithelial cells (IECs) during inflammatory bowel diseases (IBDs). In this context, P2Y(2) stimulates PGE(2) release by IECs, suggesting a role in wound healing. For this study, we have used the non-cancerous IEC-6 cell line. IEC-6 cell migration was determined using Boyden chambers and the single-edged razor blade model of wounding. The receptor was activated using ATP, UTP, or 2-thioUTP. Pharmacological inhibitors, a blocking peptide, a neutralizing antibody and interfering RNAs were used to characterize the signaling events. Focal adhesions and microtubule (MT) dynamics were determined by immunofluorescence using anti-vinculin and anti-acetylated-α-tubulin antibodies, respectively. In vivo, the dextran sodium sulfate mouse model of colitis was used to characterize the effects of P2Y(2) agonist 2-thioUTP on remission. We showed that P2Y(2) increased cell migration and wound closure by recruiting Go protein with the cooperation of integrin α(v) . Following P2Y(2) activation, we demonstrated that GSK3β activity was inhibited in response to Akt activation. This leads to MT stabilization and increased number of focal adhesions. In vivo, P2Y(2) activation stimulates remission, as illustrated by a reduction in the disease activity index values and histological scores as compared to control mice. These findings highlight a novel function for this receptor in IECs. They also illustrate that P2Y receptors could be targeted for the development of innovative therapies for the treatment of IBDs.

Glucose transporter 2 expression is down regulated following P2X7 activation in enterocytes

With the diabetes epidemic affecting the world population, there is an increasing demand for means to regulate glycemia. Dietary glucose is first absorbed by the intestine before entering the blood stream. Thus, the regulation of glucose absorption by intestinal epithelial cells (IECs) could represent a way to regulate glycemia. Among the molecules involved in glycemia homeostasis, extracellular ATP, a paracrine signaling molecule, was reported to induce insulin secretion from pancreatic β cells by activating P2Y and P2X receptors. In rat's jejunum, P2X7 expression was previously immunolocalized to the apex of villi, where it has been suspected to play a role in apoptosis. However, using an antibody recognizing the receptor extracellular domain and thus most of the P2X7 isoforms, we showed that expression of this receptor is apparent in the top two-thirds of villi. These data suggest a different role for this receptor in IECs. Using the non-cancerous IEC-6 cells and differentiated Caco-2 cells, glucose transport was reduced by more than 30% following P2X7 stimulation. This effect on glucose transport was not due to P2X7-induced cell apoptosis, but rather was the consequence of glucose transporter 2 (Glut2)'s internalization. The signaling pathway leading to P2X7-dependent Glut2 internalization involved the calcium-independent activation of phospholipase Cγ1 (PLCγ1), PKCδ, and PKD1. Although the complete mechanism regulating Glut2 internalization following P2X7 activation is not fully understood, modulation of P2X7 receptor activation could represent an interesting approach to regulate intestinal glucose absorption.

P2Y6 receptor contributes to neutrophil recruitment to inflamed intestinal mucosa by increasing CXC chemokine ligand 8 expression in an AP-1-dependent manner in epithelial cells

BACKGROUND

Inflammatory bowel diseases are characterized by the presence of CXCL8 at the site of lesions resulting in neutrophil recruitment and loss of tissue functions. We report that P2Y(6) receptor activation stimulates CXCL8 expression and release by intestinal epithelial cells (IECs). In this context, we investigated if uridine 5'-diphosphate (UDP) enemas stimulate neutrophil recruitment to the mucosa of mice suffering from colitis-like disease and we characterized the signaling events linking P2Y(6) to CXCL8 expression in IEC.

METHODS

Neutrophil recruitment was monitored by immunofluorescence and FACS analysis. Expression of Cxcl1, a mouse functional homolog of CXCL8, was determined by quantitative real-time polymerase chain reaction (qPCR). Pharmacological inhibitors and interfering RNAs were used to characterize the signaling pathway. The outcomes of these treatments on protein phosphorylation and on CXCL8 expression were characterized by western blots, qPCR, luciferase, and chromatin immunoprecipitation (ChIP) assays.

RESULTS

Mutation of the AP-1 site in the CXCL8 core promoter abolished the UDP-stimulating effect. The c-fos/c-jun dimer was identified as the AP-1 complex regulating CXCL8 in response to UDP stimulation. Regulation of CXCL8 expression by P2Y(6) required PKCδ activation upstream of the signaling pathway composed of MEK1/2-ERK1/2 and c-fos. UDP administration to mice suffering from colitis-like disease increased the number of neutrophil infiltrating the mucosa, correlating with Cxcl1 increased expression in IEC and the severity of inflammation.

CONCLUSIONS

This study not only describes the P2Y(6) signaling mechanism regulating CXCL8 expression in IEC, but it also illustrates the potential of targeting P2Y(6) to reduce intestinal inflammation.

P2Y(2) receptor expression is regulated by C/EBPβ during inflammation in intestinal epithelial cells

Inflammatory bowel diseases are characterized by relapses and remission periods during which numerous factors, including stress factors and nucleotides, are mobilized to re-establish intestinal mucosal homeostasis. We have previously found that expression of the P2Y(2) nucleotide receptor is increased in colonic tissue isolated from inflammatory bowel disease patients as well as in a mouse model of colitis, and that P2Y(2) transcription is regulated in part by nuclear factor κB (NF-κB) p65. Transcription factor DNA-binding site analysis identified three potential CCAAT/enhancer-binding protein β (C/EBPβ) binding sites in the P2Y(2) proximal promoter. We then assessed the role of C/EBP transcription factors in the regulation of P2Y(2) in intestinal epithelial cells (IECs). We identified a region between -229 and -220 bp upstream of the transcription initiation site as a DNA-binding site for C/EBPβ, by electrophoretic mobility and supershift assays. Mutagenesis of this site decreased C/EBPβ-dependent P2Y(2) expression, as assessed by luciferase assays. In vivo, C/EBPβ as well as P2Y(2) expression was increased in colonic IECs isolated from mice with dextran sulfate sodium-induced acute colitis. In contrast, P2Y(2) expression was decreased in C/EBPβ-deficient mice treated with dextran sulfate sodium. Although C/EBPβ was sufficient to induce P2Y(2) transcription, the effect of C/EBPβ and NF-κB p65 on receptor transcription was synergistic. Chromatin immunoprecipitation assays revealed that both proteins simultaneously bind to the P2Y(2) promoter. Thus, we have identified C/EBPβ as a novel regulator of P2Y(2) expression.

Infection of polarized airway epithelial cells by normal and small-colony variant strains of Staphylococcus aureus is increased in cells with abnormal cystic fibrosis transmembrane conductance regulator function and is influenced by NF-κB

The infection of nonphagocytic host cells by Staphylococcus aureus and more particularly by small-colony variants (SCVs) may contribute to the persistence of this pathogen in the lungs of cystic fibrosis (CF) patients. The development of chronic infections is also thought to be facilitated by the proinflammatory status of CF airways induced by an activation of NF-κB. The aim of this study was to compare the infection of non-CF and CF-like airway epithelial cells by S. aureus strains (normal and SCVs) and to determine the impact of the interaction between cystic fibrosis transmembrane conductance regulator (CFTR) and NF-κB on the infection level of these cells by S. aureus. We developed an S. aureus infection model using polarized airway epithelial cells grown at the air-liquid interface and expressing short hairpin RNAs directed against CFTR to mimic the CF condition. A pair of genetically related CF coisolates with the normal and SCV phenotypes was characterized and used. Infection of both cell lines (non-CF and CF-like) was more productive with the SCV strain than with its normal counterpart. However, both normal and SCV strains infected more CF-like than non-CF cells. Accordingly, inhibition of CFTR function by CFTRinh-172 increased the S. aureus infection level. Experimental activation of NF-κB also increased the level of infection of polarized pulmonary epithelial cells by S. aureus, an event that could be associated with that observed when CFTR function is inhibited or impaired. This study supports the hypothesis that the proinflammatory status of CF tissues facilitates the infection of pulmonary epithelial cells by S. aureus.

ATP induces contraction mediated by the P2Y(2) receptor in rat intestinal subepithelial myofibroblasts

Intestinal subepithelial myofibroblasts (IMFs) exist just under the epithelial membrane directly facing the mucosal microvascular capillary surface distributed in the lamina propria. In the gastrointestinal tract, ATP is released from epithelial and endothelial cells in response to mechanical stimuli. Although it has been reported that mechanical stimuli evoke synchronized Ca(2+) waves in cultured IMFs, the contractile responses by ATP stimulation have not been examined. The aim of this study was to clarify the mechanism of the contraction of IMFs in response to ATP. ATP (1-30μM) induced contraction in a concentration-dependent manner. These contractions were inhibited by LaCl(3) (100-300μM) and by Ca(2+)-free solution (0.5mM EGTA). Fura-2/Ca(2+) signals indicated that ATP (1-10μM) elicited transient increases in intracellular Ca(2+) concentration ([Ca(2+)](i)). In addition, αβ-methylene-ATP (10, 30 and 300μM), a broad spectrum P2X agonist at a concentration higher than 100μM, induced neither contraction nor [Ca(2+)](i) rise. UTP (1-30μM), a selective P2Y(2) and P2Y(4) agonist in rodent, induced concentration-dependent contractions and [Ca(2+)](i) increases, whereas ADP and UDP (10μM) did not induce contractions. Pretreatment with suramin (30-100μM), a relatively selective P2Y(2) antagonist, strongly inhibited ATP- and UTP-induced contractions and [Ca(2+)](i) increases. However, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS: 10-30μM), a receptor antagonist for several P2X and P2Y but less effective to P2Y(2) receptor, failed to inhibit ATP- and UTP-induced contractions and [Ca(2+)](i) increases. By RT-PCR, mRNA expressions of the P2Y(1) and P2Y(2) receptors, but not P2Y(4) or P2Y(6), were detected in IMFs. These results suggest that ATP induces [Ca(2+)](i)-dependent contraction in IMFs, which is mediated through the P2Y(2) receptor.

The biliary HCO(3)(-) umbrella: a unifying hypothesis on pathogenetic and therapeutic aspects of fibrosing cholangiopathies

This review focuses on the hypothesis that biliary HCO(3)(-) secretion in humans serves to maintain an alkaline pH near the apical surface of hepatocytes and cholangiocytes to prevent the uncontrolled membrane permeation of protonated glycine-conjugated bile acids. Functional impairment of this biliary HCO(3)(-) umbrella or its regulation may lead to enhanced vulnerability of cholangiocytes and periportal hepatocytes toward the attack of apolar hydrophobic bile acids. An intact interplay of hepatocellular and cholangiocellular adenosine triphosphate (ATP) secretion, ATP/P2Y- and bile salt/TGR5-mediated Cl(-)/ HCO(3)(-) exchange and HCO(3)(-) secretion, and alkaline phosphatase-mediated ATP breakdown may guarantee a stable biliary HCO(3)(-) umbrella under physiological conditions. Genetic and acquired functional defects leading to destabilization of the biliary HCO(3)(-) umbrella may contribute to development and progression of various forms of fibrosing/sclerosing cholangitis.