Administration of an AAV vector coding for a P2X7-blocking nanobody-based biologic ameliorates colitis in mice

BACKGROUND

The pro-inflammatory ATP-gated P2X7 receptor is widely expressed by immune and non-immune cells. Nanobodies targeting P2X7, with potentiating or antagonistic effects, have been developed. Adeno-associated virus (AAV)-mediated gene transfer represents an efficient approach to achieve long-term in vivo expression of selected nanobody-based biologics. This approach (AAVnano) was used to validate the relevance of P2X7 as a target in dextran sodium sulfate (DSS)-induced colitis in mice.

RESULTS

Mice received an intramuscular injection of AAV vectors coding for potentiating (14D5-dimHLE) or antagonistic (13A7-Fc) nanobody-based biologics targeting P2X7. Long-term modulation of P2X7 activity was evaluated ex vivo from blood samples. Colitis was induced with DSS in mice injected with AAV vectors coding for nanobody-based biologics. Severity of colitis, colon histopathology and expression of chemokines and cytokines were determined to evaluate the impact of P2X7 modulation. A single injection of an AAV vector coding for 13A7-Fc or 14D5-dimHLE efficiently modulated P2X7 function in vivo from day 15 up to day 120 post-injection in a dose-dependent manner. An AAV vector coding for 13A7-Fc significantly ameliorated DSS-induced colitis and significantly reduced immune cell infiltration and expression of chemokines and proinflammatory cytokines in colonic tissue.

CONCLUSIONS

We have demonstrated the validity of AAVnano methodology to modulate P2X7 functions in vivo. Applying this methodological approach to a DSS-induced colitis model, we have shown that P2X7 blockade reduces inflammation and disease severity. Hence, this study confirms the importance of P2X7 as a pharmacological target and suggests the use of nanobody-based biologics as potential therapeutics in inflammatory bowel disease.

The Role of IL-18 in P2RX7-Mediated Antitumor Immunity

Cancer is the leading cause of death worldwide despite the variety of treatments that are currently used. This is due to an innate or acquired resistance to therapy that encourages the discovery of novel therapeutic strategies to overcome the resistance. This review will focus on the role of the purinergic receptor P2RX7 in the control of tumor growth, through its ability to modulate antitumor immunity by releasing IL-18. In particular, we describe how the ATP-induced receptor activities (cationic exchange, large pore opening and NLRP3 inflammasome activation) modulate immune cell functions. Furthermore, we recapitulate our current knowledge of the production of IL-18 downstream of P2RX7 activation and how IL-18 controls the fate of tumor growth. Finally, the potential of targeting the P2RX7/IL-18 pathway in combination with classical immunotherapies to fight cancer is discussed.

Protocol for Evaluating In Vivo the Activation of the P2RX7 Immunomodulator

BACKGROUND

P2RX7 is a purinergic receptor with pleiotropic activities that is activated by high levels of extracellular ATP that are found in inflamed tissues. P2RX7 has immunomodulatory and anti-tumor proprieties and is therefore a therapeutic target for various diseases. Several compounds are developed to either inhibit or enhance its activation. However, studying their effect on P2RX7's activities is limited to in vitro and ex vivo studies that require the use of unphysiological media that could affect its activation. Up to now, the only way to assess the activity of P2RX7 modulators on the receptor in vivo was in an indirect manner.

RESULTS

We successfully developed a protocol allowing the detection of P2RX7 activation in vivo in lungs of mice, by taking advantage of its unique macropore formation ability. The protocol is based on intranasal delivery of TO-PRO™-3, a non-permeant DNA intercalating dye, and fluorescence measurement by flow cytometry. We show that ATP enhances TO-PRO™-3 fluorescence mainly in lung immune cells of mice in a P2RX7-dependant manner.

CONCLUSIONS

The described approach has allowed the successful analysis of P2RX7 activity directly in the lungs of WT and transgenic C57BL6 mice. The provided detailed guidelines and recommendations will support the use of this protocol to study the potency of pharmacologic or biologic compounds targeting P2RX7.

Abstract 6326: Reshaping immune lung populations to inhibit pulmonary fibrosis and cancer by activation of purinergic receptor P2RX7

Lung cancer is the most prevalent cancer worldwide with high mortality rate despite recent advances in therapies. Not only is fibrosis a major hallmark of lung cancer, it is also a risk factor for lung cancer development (mainly non-small cell lung cancer). In fact, patients diagnosed with idiopathic pulmonary fibrosis (IPF) are very likely to develop lung cancer. However, treatment and outcome of lung cancer patients with IPF is different than those without IPF despite their common genetic, molecular, cellular and immune properties. New therapeutic strategies are needed for improving outcome of patients bearing both diseases. To address this, we proposed that boosting the activation of P2RX7 with a positive modulator (HEI3090) would be a novel strategy to fight lung cancer. P2RX7 is a purinergic receptor activated by high extracellular ATP levels found in inflamed and tumor sites. It has anti-tumor and immunomodulatory properties by its ability to induce cell death and the release of IL-1b and IL-18. We showed previously that HEI3090 inhibits lung tumor growth by triggering an anti-tumor immune response that relies on IL-18. Indeed, HEI3090-release of IL-18 enhanced the cytotoxicity of NK cells and favored a Th1-biased immune response. Given that lung fibrosis is partly driven by inflammation with a dysregulated Th1/Th2/Th17 ratios and given the ability of HEI3090 to favor a Th1-biased immune response, we wondered if HEI3090 was able to inhibit fibrosis. To do so, we used the bleomycin mouse model where we show that activation of P2RX7 with HEI3090 inhibits lung fibrosis. HEI3090 targets P2RX7-expressing immune cells, decreases lung inflammation and favors an anti-fibrotic immune signature. Indeed, HEI3090 promotes the production of IFN-g by T cells and decreases pro-fibrotic IL-17A release by Th17 cells. Moreover, production of TGF-b, the key cytokine driving fibrosis, was greatly reduced in immune cells in HEI3090-treated mice. The antifibrotic effect of HEI3090 seems to rely on an increase of IL-18, known to induce the production of IFN-g by T cells. Overall, we show that reshaping lung immune phenotype by activation of P2RX7 is a promising strategy in lung fibrosis. HEI3090’s effect on cancer-related fibrosis is ongoing on a Kras mouse model. We highlight a new therapeutic strategy for patients with IPF that would ameliorate treatment and outcome of patients with both lung cancer and fibrosis

Citation Format: Serena Janho dit Hreich, Alina Ghinet, Valérie Vouret-Craviari. Reshaping immune lung populations to inhibit pulmonary fibrosis and cancer by activation of purinergic receptor P2RX7 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6326.

The Purinergic Landscape of Non-Small Cell Lung Cancer

Lung cancer is the most common cancer worldwide. Despite recent therapeutic advances, including targeted therapies and immune checkpoint inhibitors, the disease progresses in almost all advanced lung cancers and in up to 50% of early-stage cancers. The purpose of this review is to discuss whether purinergic checkpoints (CD39, CD73, P2RX7, and ADORs), which shape the immune response in the tumor microenvironment, may represent novel therapeutic targets to combat progression of non-small cell lung cancer by enhancing the antitumor immune response.

A small-molecule P2RX7 activator promotes anti-tumor immune responses and sensitizes lung tumor to immunotherapy

Only a subpopulation of non-small cell lung cancer (NSCLC) patients responds to immunotherapies, highlighting the urgent need to develop therapeutic strategies to improve patient outcome. We develop a chemical positive modulator (HEI3090) of the purinergic P2RX7 receptor that potentiates αPD-1 treatment to effectively control the growth of lung tumors in transplantable and oncogene-induced mouse models and triggers long lasting antitumor immune responses. Mechanistically, the molecule stimulates dendritic P2RX7-expressing cells to generate IL-18 which leads to the production of IFN-γ by Natural Killer and CD4+ T cells within tumors. Combined with immune checkpoint inhibitor, the molecule induces a complete tumor regression in 80% of LLC tumor-bearing mice. Cured mice are also protected against tumor re-challenge due to a CD8-dependent protective response. Hence, combination treatment of small-molecule P2RX7 activator followed by immune checkpoint inhibitor represents a strategy that may be active against NSCLC.

P2RX7B is a new theranostic marker for lung adenocarcinoma patients

Rationale: The characterization of new theranostic biomarkers is crucial to improving the clinical outcome of patients with advanced lung cancer. Here, we aimed at characterizing the P2RX7 receptor, a positive modulator of the anti-tumor immune response, in patients with lung adenocarcinoma.

Methods: The expression of P2RX7 and its splice variants was analyzed by RT-qPCR using areas of tumor and non-tumor lung adenocarcinoma (LUAD) tissues on both immune and non-immune cells. The biological activity of P2RX7 was studied by flow cytometry using fluorescent dyes. Bi-molecular fluorescence complementation and confocal microscopy were used to assess the oligomerization of P2RX7. Tumor immune infiltrates were characterized by immunohistochemistry.

Results: Fifty-three patients with LUAD were evaluated. P2RX7A, and 3 alternative splice variants were expressed in LUAD tissues and expression was down regulated in tumor versus adjacent non-tumor tissues. The protein retained biological activity only in immune cells. The P2RX7B splice variant was differentially upregulated in immune cells (P < 0.001) of the tumor and strong evidence of oligomerization of P2RX7A and B was observed in the HEK expression model, which correlated with a default in the activity of P2RX7. Finally, LUAD patients with a high level of P2RX7B had non-inflamed tumors (P = 0.001).

Conclusion: Our findings identified P2RX7B as a new theranostic tool to restore functional P2RX7 activity and open alternative therapeutic opportunities to improve LUAD patient outcome.

Pyroglutamide-Based P2X7 Receptor Antagonists Targeting Inflammatory Bowel Disease

This report deals with the design, the synthesis, and the pharmacological evaluation of pyroglutamide-based P2X7 antagonists. A dozen were shown to possess improved properties, among which inhibition of YO-PRO-1/TO-PRO-3 uptake and IL1β release upon BzATP activation of the receptor and dampening signs of DSS-induced colitis on mice, in comparison with reference antagonist GSK1370319A. Docking study and biological evaluation of synthesized compounds has highlighted new SAR, and low toxicity profiles of pyroglutamides herein described are clues for the finding of a usable h-P2X7 antagonist drug. Such a drug would raise the hope for a cure to many P2X7-dependent pathologies, including inflammatory, neurological, and immune diseases.

Alternative splicing of P2RX7 pre-messenger RNA in health and diseases: Myth or reality?

Alternative splicing (AS) tremendously increases the use of genetic information by generating protein isoforms that differ in protein-protein interactions, catalytic activity and/or subcellular localization. This review is not dedicated to AS in general, but rather we focus our attention on AS of P2RX7 pre-mRNA. Whereas P2RX7 mRNA is expressed by virtually all eukaryotic mammalian cells, the expression of this channel receptor is restrained to certain cells. When expressed at the cell membrane, P2RX7 controls downstream events including release of inflammatory molecules, phagocytosis, cell proliferation and death and metabolic events. Therefore, P2RX7 is an important actor of health and diseases. In this review, we summarize the general mechanisms leading to AS. Further, we recapitulate our current knowledge concerning the functional regions in P2RX7, identified at the genetic or exonic levels, and how AS may affect the expression of these regions. Finally, the potential of P2RX7 splice variants to control the fate of cancer cells is discussed.

VEGFC acts as a double-edged sword in renal cell carcinoma aggressiveness

Hypoxic zones are common features of metastatic tumors. Due to inactivation of the von Hippel-Lindau gene (VHL), renal cell carcinomas (RCC) show constitutive stabilization of the alpha subunit of the hypoxia-inducible factor (HIF). Thus, RCC represents a model of chronic hypoxia. Development of the lymphatic network is dependent on vascular endothelial growth factor C (VEGFC) and lies at the front line of metastatic spreading. Here, we addressed the role of VEGFC in RCC aggressiveness and the regulation of its expression in hypoxia. Methods: Transcriptional and post transcriptional regulation of VEGFC expression was evaluated by qPCR and with reporter genes. The involvement of HIF was evaluated using a siRNA approach. Experimental RCC were performed with immuno-competent/deficient mice using human and mouse cells knocked-out for the VEGFC gene by a CRISPR/Cas9 method. The VEGFC axis was analyzed with an online available data base (TCGA) and using an independent cohort of patients. Results: Hypoxia induced VEGFC protein expression but down-regulated VEGFC gene transcription and mRNA stability. Increased proliferation, migration, over-activation of the AKT signaling pathway and enhanced expression of mesenchymal markers characterized VEGFC-/- cells. VEGFC-/- cells did not form tumors in immuno-deficient mice but developed aggressive tumors in immuno-competent mice. These tumors showed down-regulation of markers of activated lymphocytes and M1 macrophages, and up-regulation of M2 macrophages markers and programmed death ligand 1 (PDL1). Over-expression of lymphangiogenic genes including VEGFC was linked to increased disease-free and overall survival in patients with non-metastatic tumors, whereas its over-expression correlated with decreased progression-free and overall survival of metastatic patients. Conclusion: Our study revisited the admitted dogma linking VEGFC to tumor aggressiveness. We conclude that targeting VEGFC for therapy must be considered with caution.

MiR-223-3p inhibits angiogenesis and promotes resistance to cetuximab in head and neck squamous cell carcinoma

MicroRNAs (miRs) participate in tumor growth and dissemination by regulating expression of various target genes. MiR-223-3p is suspected of being involved in head and neck squamous cell carcinoma (HNSCC) growth although its precise role has not been elucidated. In this study, we showed that miR-223-3p is present in biopsies of HNSCC patients and that its presence is correlated with high neutrophil infiltrate. We found that overexpression of miR-223-3p slightly increased proliferation of the CAL27 squamous carcinoma cell line both in vitro and in vivo. Moreover, miR-223-3p induced CAL27 apoptosis in an orthotopic xenograft mouse model, counteracting the proliferative effect and resulting in no impact on overall tumor growth. We analyzed the effect of miR-223-3p overexpression on signaling pathways and showed that it induced pERK2, pAKT and AKT, consistent with an increase in cell proliferation. In addition, we found that miR-223-3p reduced the STAT3 level correlating with increased cell apoptosis and inhibited vasculature formation. In HNSCC tissues, miR-223-3p expression was inversely correlated to CD31, highlighting the relationship between miR-223 and vessel formation. Finally, we studied the effect of miR-223-3p on response to selected anticancer agents and showed that cells expressing miR-223-3p are more resistant to drugs, notably cetuximab.

In conclusion, our study is the first to show the antiangiogenic properties of miR-223-3p in HNSCC patients and to question whether expression levels of miR-223-3p can be evaluated as an indicator of eligibility for non-treatment of HNSCC patients with cetuximab.

HIF1A regulates xenophagic degradation of adherent and invasive Escherichia coli (AIEC)

The hypoxia inducible transcription factor HIF1 activates autophagy, a general catabolic pathway involved in the maintenance of cellular homeostasis. Dysfunction in both autophagy and HIF1 has been implicated in an increasing number of human diseases, including inflammatory bowel disease (IBD), such as Crohn disease (CD). Adherent invasive E. coli (AIEC) colonize ileal mucosa of CD patients and strongly promote gastrointestinal inflammatory disorders by activation of HIF-dependent responses. Here, we aim to characterize the contribution of HIF1 in xenophagy, a specialized form of autophagy involved in the degradation of intracellular bacteria. Our results showed that endogenous HIF1A knockdown increased AIEC survival in intestinal epithelial cells. We demonstrate that the increase in survival rate correlates with a dramatic impairment of the autophagic flux at the autolysosomal maturation step. Furthermore, we show that AIEC remained within single-membrane LC3-II-positive vesicles and that they were unable to induce the phosphorylation of ULK1. These results suggested that, in the absence of HIF1A, AIEC were found within LC3-associated phagosomes. Using blocking antibodies against TLR5 and CEACAM6, the 2 well-known AIEC-bound receptors, we showed that downstream receptor signaling was necessary to mediate ULK1 phosphorylation. Finally, we provide evidence that HIF1 mediates CEACAM6 expression and that CEACAM6 is necessary to recruit ULK1 in a bacteria-containing signaling hub. Collectively, these results identify a new function for HIF1 in AIEC-dedicated xenophagy, and suggest that coactivation of autophagy and HIF1A expression may be a potential new therapy to resolve AIEC infection in CD patients.

Diet-induced hypoxia responsive element demethylation increases CEACAM6 expression, favouring Crohn's disease-associated Escherichia coli colonisation

OBJECTIVE

Adherent-invasive Escherichia coli (AIEC) are abnormally predominant on Crohn's disease (CD) ileal mucosa. AIEC strains adhere to enterocytes via interaction between type 1 pili and CEACAM6 receptors abnormally expressed on CD ileal mucosa, leading to gut inflammation. We analysed whether epigenetic mechanisms are involved in the upregulation of CEACAM6 expression in intestinal epithelial cells (IECs).

DESIGN

Methylation of CEACAM6 promoter was analysed using bisulfite sequencing and site-specific methylation by SnapShot. pCpGfree reporter system was used to analyse CEACAM6 promoter activity. Transgenic mice expressing human CEACAM6 fed either standard food or a low-methyl diet (LMD) were orally challenged with 10(9) AIEC LF82. After 3 days, gut-associated AIEC and proinflammatory cytokines were quantified.

RESULTS

Analysis of CEACAM6 gene promoter revealed potentially methylated dinucleotide CpGs within HIF-1-responsive elements (HREs). Methylation levels of CpG within CEACAM6 promoter were inversely correlated with CEACAM6 expression in IEC expressing various levels of CEACAM6. We show the critical role of HRE methylation and transcription factor HIF-1 in the regulation of CEACAM6 gene in IEC. This was confirmed in transgenic mice expressing human CEACAM6 fed a LMD. LMD-dependent HRE demethylation led to abnormal gut expression of CEACAM6, favouring AIEC colonisation and subsequent inflammation.

CONCLUSIONS

HRE hypomethylation in CEACAM6 promoter correlates with high expression in IEC. Our findings suggest that abnormal DNA methylation leading to CEACAM6 increased expression and AIEC-mediated gut inflammation can be related to changes in nutritional habits, such as low intake in methyl donor molecules, leading to abnormal epigenetic marks in mouse model mimicking CD susceptibility.

Genetic and pharmacological inactivation of the purinergic P2RX7 receptor dampens inflammation but increases tumor incidence in a mouse model of colitis-associated cancer

Colitis-associated cancer (CAC) is a complication of inflammatory bowel disease (IBD). Binding of extracellular ATP to the purinergic receptor P2RX7 has emerged as a critical event in controlling intestinal inflammation, acting to limit elevation of proinflammatory mast cells and cytokines and promote survival of regulatory T cells (Treg) and enteric neurons. In this study, we investigated the effect of P2RX7 blockade in an established mouse model of CAC. Using genetic and pharmacologic tools, we found unexpectedly that while P2RX7 mediated inflammatory responses, it also acted at an early time to suppress CAC development. P2RX7 blockade enhanced proliferation of intestinal epithelial cells and protected them from apoptosis. The proliferative effects of P2RX7 blockade were associated with an increased production of TGFβ1 that was sufficient to stimulate the proliferation of intestinal epithelial cells. Finally, P2RX7 blockade also altered immune cell infiltration and promoted Treg accumulation within lesions of the digestive system. Taken together, our findings reveal an unexpected role for P2RX7 in preventing CAC, suggesting cautions in the use of P2RX7 inhibitors to treat IBD given the possibility of increasing risks CAC as a result.

Autophagy plays a critical role in the degradation of active RHOA, the control of cell cytokinesis, and genomic stability

Degradation of signaling proteins is one of the most powerful tumor-suppressive mechanisms by which a cell can control its own growth. Here, we identify RHOA as the molecular target by which autophagy maintains genomic stability. Specifically, inhibition of autophagosome degradation by the loss of the v-ATPase a3 (TCIRG1) subunit is sufficient to induce aneuploidy. Underlying this phenotype, active RHOA is sequestered via p62 (SQSTM1) within autolysosomes and fails to localize to the plasma membrane or to the spindle midbody. Conversely, inhibition of autophagosome formation by ATG5 shRNA dramatically increases localization of active RHOA at the midbody, followed by diffusion to the flanking zones. As a result, all of the approaches we examined that compromise autophagy (irrespective of the defect: autophagosome formation, sequestration, or degradation) drive cytokinesis failure, multinucleation, and aneuploidy, processes that directly have an impact upon cancer progression. Consistently, we report a positive correlation between autophagy defects and the higher expression of RHOA in human lung carcinoma. We therefore propose that autophagy may act, in part, as a safeguard mechanism that degrades and thereby maintains the appropriate level of active RHOA at the midbody for faithful completion of cytokinesis and genome inheritance.

Crohn disease-associated Escherichia coli promote gastrointestinal inflammatory disorders by activation of HIF-dependent responses

Crohn disease (CD) ileal lesions are colonized by adherent-invasive E. coli (AIEC) that locally induce inflammation. Hypoxia inducible factor (HIF)-1alpha protein is expressed in acute and chronically inflamed site; however the molecular basis of this expression is not fully understood. The aim of the study was to access whether AIEC induce HIF-1α expression and to study the consequence of HIF-1α expression on the onset of Crohn disease pathogenesis. We show that HIF-1α is maximally expressed in inflamed ileal epithelium of CD-patients. CEACAM6, a protein that acts as a receptor of AIEC, is expressed in this particular condition. Using CEABAC 10 transgenic mice that express CEACAM6, we show that AIEC bacteria, but not non-pathogenic E. coli K12, induce the production of HIF-1alpha protein and the activation of VEGF/VEGFR signaling. Downstream analyses on human intestinal epithelial cells silenced for hif- 1α, highlight the crucial role of this protein in production of pro-angiogenic factors. This study highlights the crucial role of AIEC bacteria as promoter of inflammatory disorders of the gastrointestinal tract and provides clear evidence that HIF-1α protein plays a major role in mediating this effect.

Microbes-induced EMT at the crossroad of inflammation and cancer

It is noteworthy that bacterial or viral infections, and the resulting chronic inflammation, have been shown to predispose individuals to certain types of cancer. Remarkably, these microbes upregulated some transcription factors involved in the regulation of the epithelial to mesenchymal transition, referred herein as EMT. EMT is a cellular process that consists in the conversion of epithelial cell phenotype to a mesenchymal phenotype. Under physiological conditions EMT is clearly important for embryogenesis, organ development, wound repair and tissue remodeling. However, EMT may also be activated under pathologic conditions, more particularly in carcinogenesis and metastatic progression. In this review, we make a parallel between microbes- and growth factors- induced transcription factors. A unifying EMT model then emerges that may help in understanding the development of microbial pathogenesis and in defining new potential future therapeutic strategy in treating diseases linked to infections.

Amplification loop of the inflammatory process is induced by P2X7R activation in intestinal epithelial cells in response to neutrophil transepithelial migration

Inflammatory bowel diseases (IBD) are characterized during their active phase by polymorphonuclear leukocyte (PMNL) transepithelial migration. The efflux of PMNL into the mucosa is associated with the production of proinflammatory cytokines and the release of ATP from damaged and necrotic cells. The expression and function of purinergic P2X(7) receptor (P2X(7)R) in intestinal epithelial cells (IEC) and its potential role in the "cross talk" between IEC and PMNL have not been explored. The aims of the present study were 1) to examine P2X(7)R expression in IEC (T84 cells) and in human intestinal biopsies; 2) to detect any changes in P2X(7)R expression in T84 cells during PMNL transepithelial migration, and during the active and quiescent phases of IBD; and 3) to test whether P2X(7)R stimulation in T84 monolayers can induce caspase-1 activation and IL-1beta release by IEC. We found that a functional ATP-sensitive P2X(7)R is constitutively expressed at the apical surface of IEC T84 cells. PMNL transmigration regulates dynamically P2X(7)R expression and alters its distribution from the apical to basolateral surface of IEC during the early phase of PMNL transepithelial migration in vitro. P2X(7)R expression was weak in intestinal biopsies obtained during the active phase of IBD. We show that activation of epithelial P2X(7)R is mandatory for PMNL-induced caspase-1 activation and IL-1beta release by IEC. Overall, these changes in P2X(7)R function may serve to tailor the intensity of the inflammatory response and to prevent IL-1beta overproduction and inflammatory disease.

Autophagy and Crohn's disease: at the crossroads of infection, inflammation, immunity, and cancer

Inflammatory bowel diseases (IBD) are common inflammatory disorders of the gastrointestinal tract that include ulcerative colitis (UC) and Crohn's disease (CD). The incidences of IBD are high in North America and Europe, affecting as many as one in 500 people. These diseases are associated with high morbidity and mortality. Colorectal cancer risk is also increased in IBD, correlating with inflammation severity and duration. IBD are now recognized as complex multigenetic disorders involving at least 32 different risk loci. In 2007, two different autophagy-related genes, ATG16L1 (autophagy-related gene 16-like 1) and IRGM (immunity-related GTPase M) were shown to be specifically involved in CD susceptibility by three independent genome-wide association studies. Soon afterwards, more than forty studies confirmed the involvement of ATG16L1 and IRGM variants in CD susceptibility and gave new information on the importance of macroautophagy (hereafter referred to as autophagy) in the control of infection, inflammation, immunity and cancer. In this review, we discuss how such findings have undoubtedly changed our understanding of CD pathogenesis. A unifying autophagy model then emerges that may help in understanding the development of CD from bacterial infection, to inflammation and finally cancer. The Pandora's box is now open, releasing a wave of hope for new therapeutic strategies in treating Crohn's disease.

HIF-1alpha mediates the induction of IL-8 and VEGF expression on infection with Afa/Dr diffusely adhering E. coli and promotes EMT-like behaviour

Microbes regulate a large panel of intracellular signalling events that can promote inflammation and/or enhance tumour progression. Indeed, it has been shown that infection of human intestinal cells with the Afa/Dr diffusely adhering E. coli C1845 strain induces expression of pro-angiogenic and pro-inflammatory genes. Here, we demonstrate that exposure of cryptic-like intestinal epithelial cells to C1845 bacteria induces HIF-1alpha protein levels. This effect depends on the binding of F1845 adhesin to the membrane-associated DAF receptor that initiates signalling cascades promoting translational mechanisms. Indeed, inhibition of MAPK and PI-3K decreases HIF-1alpha protein levels and blocks C1845-induced phosphorylation of the ribosomal S6 protein. Using RNA interference we show that bacteria-induced HIF-1alpha regulates the expression of IL-8, VEGF and Twist1, thereby pointing to a role for HIF-1 in angiogenesis and inflammation. In addition, infection correlates with a loss of E-cadherin and cytokeratin 18 and a rise in fibronectin, suggesting that bacteria may induce an epithelial to mesenchymal transition-like phenotype. Since HIF-1alpha silencing results in reversion of bacteria-induced EMT markers, we speculate that HIF-1alpha plays a key role linking bacterial infection to angiogenesis, inflammation and some aspects of cancer initiation.

Differential expression and regulation of ADAM17 and TIMP3 in acute inflamed intestinal epithelia

The acute phase of Crohn's disease (CD) is characterized by a large afflux of polymorphonuclear leukocytes (PMNL) into the mucosa and by the release of TNF-alpha. Conversion of inactive TNF-alpha into an active form requires the cleavage of a transmembrane TNF-alpha precursor by the TNF-alpha-converting enzyme (ADAM17), a protease mainly regulated by the tissue inhibitor of metalloproteinase 3 (TIMP3). The aim of the present study was to investigate in an in vitro model of PMNL transepithelial migration and in the intestinal mucosa of patients with CD the expression and regulation of ADAM17 and TIMP3 in intestinal epithelial cells (IEC). ADAM17 and TIMP3 expression was analyzed by Western blotting, RT-PCR, confocal microscopy, and immunohistochemistry by using the T84 model and digestive biopsies. ADAM17 expression in IEC was increased at a posttranscriptional level during the early phase (from 2 to 4 h) of PMNL transepithelial migration whereas TIMP3 was only increased 24 h later. TNF-alpha induced an early upregulation of ADAM17 in T84 cells, whereas PMNL adhesion, H(2)O(2), or epithelial tight junction opening alone did not affect the amount of ADAM17. Immunohistochemistry of intestinal biopsies revealed that strong expression of ADAM17 was associated with a high activity of CD. In contrast, TIMP3 was very poorly expressed in these biopsies. ADAM17 and TIMP3 profiling did not correlated with the NOD2/CARD15 status. The ADAM17 activity was higher both in the early phase of PMNL transepithelial migration and in active CD. These results showed early posttranscriptional upregulation of ADAM17 in IEC linked to PMNL transepithelial migration and a high activity of CD.

Up-regulation of intestinal vascular endothelial growth factor by Afa/Dr diffusely adhering Escherichia coli

BACKGROUND

Angiogenesis has been recently described as a novel component of inflammatory bowel disease pathogenesis. The level of vascular endothelial growth factor (VEGF) has been found increased in Crohn's disease and ulcerative colitis mucosa. To question whether a pro-inflammatory Escherichia coli could regulate the expression of VEGF in human intestinal epithelial cells, we examine the response of cultured human colonic T84 cells to infection by E. coli strain C1845 that belongs to the typical Afa/Dr diffusely adhering E. coli family (Afa/Dr DAEC).

METHODOLOGY

VEGF mRNA expression was examined by Northern blotting and q-PCR. VEGF protein levels were assayed by ELISA and its bioactivity was analysed in endothelial cells. The bacterial factor involved in VEGF induction was identified using recombinant E. coli expressing Dr adhesin, purified Dr adhesin and lipopolysaccharide. The signaling pathway activated for the up-regulation of VEGF was identified using a blocking monoclonal anti-DAF antibody, Western blot analysis and specific pharmacological inhibitors.

PRINCIPAL FINDINGS

C1845 bacteria induce the production of VEGF protein which is bioactive. VEGF is induced by adhering C1845 in both a time- and bacteria concentration-dependent manner. This phenomenon is not cell line dependent since we reproduced this observation in intestinal LS174, Caco2/TC7 and INT407 cells. Up-regulation of VEGF production requires: (1) the interaction of the bacterial F1845 adhesin with the brush border-associated decay accelerating factor (DAF, CD55) acting as a bacterial receptor, and (2) the activation of a Src protein kinase upstream of the activation of the Erk and Akt signaling pathways.

CONCLUSIONS

Results demonstrate that a Afa/Dr DAEC strain induces an adhesin-dependent activation of DAF signaling that leads to the up-regulation of bioactive VEGF in cultured human intestinal cells. Thus, these results suggest a link between an entero-adherent, pro-inflammatory E. coli strain and angiogenesis which appeared recently as a novel component of IBD pathogenesis.

ILK is required for the assembly of matrix-forming adhesions and capillary morphogenesis in endothelial cells

Integrins play a key role in regulating endothelial cell survival, migration and differentiated function during angiogenic blood-vessel remodeling. Integrin-linked kinase (ILK) is a multidomain protein that interacts with the cytoplasmic tail of integrin β subunits and is thought to participate in integrin-mediated signal transduction. We report here that attenuation of ILK expression in cultured bovine aortic endothelial cells by RNA interference had marked effects on surface distribution of α5β1 integrin and the organization of cell-matrix adhesions characterized by the disappearance of fibrillar (3D-like) adhesions that are rich in α5β1 and paxillin, and associated fibrillar fibronectin matrix. This defect was not caused by a decrease in fibronectin mRNA levels or by intracellular retention of the protein. Adhesion to surface-adsorbed matrix proteins based on β1 and β3 integrin was enhanced following ILK depletion, whereas cell spreading, migration and multilayer alignment into capillary-like structures on Matrigel were impaired. We conclude that ILK is an important regulator of the endothelial phenotype and vascular network formation by directing the assembly and/or maturation of α5β1-competent matrix-forming adhesions.

Modulation of Rho GTPase activity in endothelial cells by selective proteinase-activated receptor (PAR) agonists

The proteinase-activated receptors (PAR) PAR1 and PAR2 mediate responses to thrombin and trypsin-like proteases, respectively. Both receptors are expressed on endothelial cells where they have been reported to transduce a similar set of intracellular responses. In cultured human umbilical vein endothelial cells (HUVEC), we observed a marked difference in shape changes induced by PAR-activating peptides (PAR-APs); unlike PAR1-AP, PAR2-AP failed to stimulate cell rounding. Objectives were to shed light on the mechanisms underlying PAR-mediated cytoskeletal responses. We examined the activation of the Rho family GTPases in HUVEC using highly selective PAR1- and PAR2-APs to do this. Both peptides induced a robust and transient activation of RhoA, with the time course of activation being more sustained for the PAR1-AP. Interestingly, divergent effects on Rac activity were observed. Addition of PAR1-AP inhibited basal Rac activity as well as the phosphorylation of the Rac effector, p21-activated kinase (PAK). In contrast, PAR2-AP induced a modest activation of Rac, phosphorylation of PAK and translocation of cortactin from the cytosol to membrane ruffles, a Rac-dependent event. In vivo, only PAR1-AP rapidly enhanced vascular permeability in a mouse skin assay. We conclude that the differential regulation of the Rac/PAK pathway by PAR1 and PAR2 agonists in endothelial cells points toward distinct roles for these receptors in the control of vascular permeability and blood vessel remodeling.

EphrinA1 inactivates integrin-mediated vascular smooth muscle cell spreading via the Rac/PAK pathway

Interactions between the Eph receptor tyrosine kinase and ephrin ligands transduce short-range signals regulating axon pathfinding, development of the cardiovascular system, as well as migration and spreading of neuronal and non-neuronal cells. Some of these effects are believed to be mediated by alterations in actin dynamics. The members of the small Rho GTPase family elicit various effects on actin structures and are probably involved in Eph receptor-induced actin modulation. EphrinA1 is proposed to contribute to angiogenesis as it is strongly expressed at sites of neovascularization. Moreover, angiogenic factors induce the expression of ephrinA1 in endothelial cells. In this study, using rat vascular smooth muscle cells (VSMCs), we investigated the contribution of the small Rho GTPases in ephrinA1-induced integrin inactivation. EphrinA1 did not significantly affect early adhesion of VSMCs on purified laminin or fibronectin, but strongly impaired cell spreading. The Rho kinase inhibitor Y-27632 partly reversed the ephrinA1 effect, suggesting involvement of Rho in this model. However, inhibition of RhoA synthesis with short interfering (si)RNA had a modest effect, suggesting that RhoA plays a limited role in ephrinA1-mediated inhibition of spreading in VSMCs. The ephrinA1-mediated morphological alterations correlated with inhibition of Rac1 and p21-activated kinase 1 (PAK1) activity, and were antagonized by the expression of a constitutively active Rac mutant. Moreover, repression of Rac1 synthesis with siRNA amplifies the ephrinA1-induced inhibition of spreading. Finally, sphingosine-1-phosphate (S1P), a lipid mediator known to inhibit Rac activation in VSMCs amplifies the ephrinA1 effect. In conclusion, our results emphasize the role of the Rac/PAK pathway in ephrinA1-mediated inhibition of spreading. In this way, ephrinA1, alone or in synergy with S1P, can participate in blood vessel destabilization, a prerequisite for angiogenesis.

Vasopressin and oxytocin reverse adenosine-induced pituicyte stellation via calcium-dependent activation of Cdc42

In view of the potential impact of pituicyte morphology on neurohypophysial hormone secretion, we have studied the mechanisms involved in the shape changes induced by vasopressin (AVP) and oxytocin (OXT) in cultured rat pituicytes. Pituicytes induced to become stellate in the presence of 10 micro m adenosine revert to their nonstellate shape approximately 20 min after application of AVP or OXT. The IC50 for this effect is 0.1 nm for AVP and 36 nm for OXT. Both agonists induce Ca2+ signals in pituicytes, comprised of a transient peak and a plateau phase that is dependent on the presence of extracellular Ca2+. The EC50 values of AVP for the transient and sustained responses are 4.5 and 0.1 nm, respectively; corresponding values for OXT are 180 and 107 nm. We determined pharmacologically that these hormone-induced Ca2+ signals are mediated by the V1a subtype of vasopressin receptors, similar to what we previously observed for hormone-induced reversal of stellation. Removal of extracellular Ca2+ or chelation of intracellular Ca2+ partially prevented AVP from reversing stellation, suggesting a role for Ca2+ in this event. We previously established that adenosine-induced stellation of pituicytes occurs via RhoA inhibition. However, pharmacological experiments and pull-down assays presented here show that AVP-induced reversal of stellation does not involve RhoA activation. Rather, AVP was found to induce a time-dependent activation of Cdc42, another small GTPase involved in cytoskeletal plasticity. Activation of Cdc42 by AVP is sensitive to intra- and extracellular Ca2+ depletion, similar to AVP-induced reversal of stellation. Furthermore, AVP-induced reversal of stellation is blocked by expression of an NWASP fragment known to inhibit endogenous Cdc42.

Distinct signals via Rho GTPases and Src drive shape changes by thrombin and sphingosine-1-phosphate in endothelial cells

Soluble mediators such as thrombin and sphingosine-1-phosphate regulate morphological changes in endothelial cells that affect vascular permeability and new blood vessel formation. Although these ligands activate a similar set of heterotrimeric G proteins, thrombin causes cell contraction and rounding whereas sphingosine-1-phosphate induces cell spreading and migration. A functional requirement for Rho family GTPases in the cytoskeletal responses to both ligands has been established, yet the dynamics of their regulation and additional signaling mechanisms that lead to such opposite effects remain poorly understood. Using a pull-down assay to monitor the activity of Rho GTPases in human umbilical vein endothelial cells, we find significant temporal and quantitative differences in RhoA and Rac1 activation. High levels of active RhoA rapidly accumulate in cells in response to thrombin whereas Rac1 is inhibited. In contrast, sphingosine-1-phosphate addition leads to comparatively weak and delayed activation of RhoA and it activates Rac1. In addition, we show here that sphingosine-1-phosphate treatment activates a Src family kinase and triggers recruitment of the F-actin-binding protein cortactin to sites of actin polymerization at the rim of membrane ruffles. Both Src and Rac pathways are essential for lamellipodia targeting of cortactin. Further, Src plays a determinant role in sphingosine-1-phosphate-induced cell spreading and migration. Taken together these data demonstrate that the thrombin-induced contractile and immobile phenotype in endothelial cells reflects both robust RhoA activation and Rac inhibition, whereas Src- and Rac-dependent events couple sphingosine-1-phosphate receptors to the actin polymerizing machinery that drives the extension of lamellipodia and cell migration.

RhoA inhibition is a key step in pituicyte stellation induced by A(1)-type adenosine receptor activation

Pituicyte stellation in vitro represents a useful model with which to study morphological changes that occur in vivo in these cells during times of high neurohypophysial hormone output. This model has helped us establish the hypothesis of a purinergic regulation of pituicyte morphological plasticity. We first show that ATP induces stellation in 37% of pituicytes, an effect that is secondary to the metabolism of ATP to adenosine. Adenosine-induced stellation of pituicytes appears to be mediated by A(1)-type receptors. The effect is independent of intracellular calcium and does not involve the mitogen-activated protein kinase pathway. The basal (nonstellate) state of pituicytes depends on tonic activation of a Rho GTPase because both C3 transferase (a Rho inhibitor) and Y-27632 (an inhibitor of p160Rho kinase) can induce stellation. Lysophosphatidic acid, a Rho activator, blocks the morphogenic effect of adenosine dose-dependently. Using a specific RhoA pull-down assay, we also show that downregulation of activated RhoA is the key event coupling A(1) receptor activation to pituicyte stellation, via F-actin depolymerization and microtubule reorganization. Finally, both vasopressin and oxytocin can prevent or reverse adenosine-induced stellation. The effects of vasopressin, and those of high concentrations of oxytocin, are mediated through V(1a) receptors. Placed within the context of the relevant literature, our data suggest the possibility of a purinergic regulation of pituicyte morphological plasticity and subsequent modulation of hormone release, with these hormones providing a negative feedback mechanism.

Angiogenesis and G-protein-coupled receptors: signals that bridge the gap

Angiogenesis is a mechanism that has repercussions in a number of physiological and pathological situations. Vascular endothelial growth factor and basic fibroblast growth factor have understandably received enormous research coverage for being the major mediators of new blood vessel growth, often overshadowing other agonist that also have strong angiogenic potential. We wish to put the spotlight on GPCR agonists that undoubtedly have their word to say on the subject of angiogenesis. In this short review, we will discuss our findings along with the work from other groups on the mechanisms by which GPCR agonists, like thrombin and angiotensin II, control a number of angiogenic signals. A complete understanding of these mechanisms could, by the design of new therapeutic strategies, have a strong impact in clinical oncology.

Effects of cytotoxic necrotizing factor 1 and lethal toxin on actin cytoskeleton and VE-cadherin localization in human endothelial cell monolayers

Integrity of the vascular endothelium is largely dependent on endothelial cell shape and establishment of intercellular junctions. Certain pathogenic bacterial toxins alter the cytoskeletal architecture of intoxicated cells by modulating the GTPase activity of p21 Rho family proteins. In the present study we have analyzed the effect of Rho-directed toxins on the actin cytoskeleton and monolayer integrity of endothelial cells. We report here that Escherichia coli cytotoxic necrotizing factor 1 (CNF1) activates Rho in human umbilical vein endothelial cells (HUVEC). In confluent monolayers, CNF1 treatment induces prominent stress fiber formation without significantly modifying peripheral localization of VE-cadherin, a specific marker of vascular endothelial cell adherens junctions. Further, Rho activation with CNF1 blocks thrombin-induced redistribution of VE-cadherin staining and gap formation in HUVEC monolayers. Inhibition of Rho by prolonged treatment of cells with C3 exoenzyme (Clostridium botulinum) eliminates actin stress fibers without disrupting the continuity of VE-cadherin staining, indicating that Rho-dependent stress fibers are not required for maintaining this adhesion receptor at sites of intercellular contact. Lethal toxin (Clostridium sordellii), an inhibitor of Rac as well as Ras and Rap, potently disrupts the actin microfilament system and monolayer integrity in HUVEC cultures.

Regulation of the actin cytoskeleton by thrombin in human endothelial cells: role of Rho proteins in endothelial barrier function

Endothelial barrier function is regulated at the cellular level by cytoskeletal-dependent anchoring and retracting forces. In the present study we have examined the signal transduction pathways underlying agonist-stimulated reorganization of the actin cytoskeleton in human umbilical vein endothelial cells. Receptor activation by thrombin, or the thrombin receptor (proteinase-activated receptor 1) agonist peptide, leads to an early increase in stress fiber formation followed by cortical actin accumulation and cell rounding. Selective inhibition of thrombin-stimulated signaling systems, including Gi/o (pertussis toxin sensitive), p42/p44, and p38 MAP kinase cascades, Src family kinases, PI-3 kinase, or S6 kinase pathways had no effect on the thrombin response. In contrast, staurosporine and KT5926, an inhibitor of myosin light chain kinase, effectively blocked thrombin-induced cell rounding and retraction. The contribution of Rho to these effects was analyzed by using bacterial toxins that either activate or inhibit the GTPase. Escherichia coli cytotoxic necrotizing factor 1, an activator of Rho, induced the appearance of dense actin cables across cells without perturbing monolayer integrity. Accordingly, lysophosphatidic acid, an activator of Rho-dependent stress fiber formation in fibroblasts, led to reorganization of polymerized actin into stress fibers but failed to induce cell rounding. Inhibition of Rho with Clostridium botulinum exoenzyme C3 fused to the B fragment of diphtheria toxin caused loss of stress fibers with only partial attenuation of thrombin-induced cell rounding. The implication of Rac and Cdc42 was analyzed in transient transfection experiments using either constitutively active (V12) or dominant-interfering (N17) mutants. Expression of RacV12 mimicked the effect of thrombin on cell rounding, and RacN17 blocked the response to thrombin, whereas Cdc42 mutants were without effect. These observations suggest that Rho is involved in the maintenance of endothelial barrier function and Rac participates in cytoskeletal remodeling by thrombin in human umbilical vein endothelial cells.

Multimer formation and ligand recognition by the long pentraxin PTX3. Similarities and differences with the short pentraxins C-reactive protein and serum amyloid P component

PTX3 is a prototypic long pentraxin consisting of a C-terminal 203-amino acid pentraxin-like domain coupled with an N-terminal 178-amino acid unrelated portion. The present study was designed to characterize the structure and ligand binding properties of human PTX3, in comparison with the classical pentraxins C-reactive protein and serum amyloid P component. Sequencing of Chinese hamster ovary cell-expressed PTX3 revealed that the mature secreted protein starts at residue 18 (Glu). Lectin binding and treatment with N-glycosidase F showed that PTX3 is N-glycosylated, sugars accounting for 5 kDa of the monomer mass (45 kDa). Circular dichroism analysis indicated that the protein consists predominantly of beta-sheets with a minor alpha-helical component. While in gel filtration the protein is eluted with a molecular mass of congruent with900 kDa, gel electrophoresis using nondenaturing, nonreducing conditions revealed that PTX3 forms multimers predominantly of 440 kDa apparent molecular mass, corresponding to decamers, and that disulfide bonds are required for multimer formation. The ligand binding properties of PTX3 were then examined. As predicted based on modeling, inductive coupled plasma/atomic emission spectroscopy showed that PTX3 does not have coordinated Ca2+. Unlike the classical pentraxins CRP and SAP, PTX3 did not bind phosphoethanolamine, phosphocholine, or high pyruvate agarose. PTX3 in solution, bound to immobilized C1q, but not C1s, and, reciprocally, C1q bound to immobilized PTX3. Binding of PTX3 to C1q is specific and saturable with a Kd 7.4 x 10(-8) M as determined by solid phase binding assay. The Chinese hamster ovary cell-expressed pentraxin domain bound C1q when multimerized. Thus, as predicted on the basis of computer modeling, the prototypic long pentraxin PTX3 forms multimers, which differ from those formed by classical pentraxins in terms of protomer composition and requirement for disulfide bonds, and does not recognize CRP/SAP ligands. The capacity to bind C1q, mediated by the pentraxin domain, is consistent with the view that PTX3, produced in tissues by endothelial cells or macrophages in response to interleukin-1 and tumor necrosis factor, may act as a local regulator of innate immunity.

Expression of monocyte chemotactic protein-3 in human monocytes exposed to the mycobacterial cell wall component lipoarabinomannan

Monocyte chemotactic protein-3 (MCP-3) is a C-C chemokine which interacts with the CCR1, CCR2 (MCP-1) and CCR3 receptors and has a distinct spectrum of action. The present study was designed to assess whether mycobacterial components were able to induce expression and production of MCP-3 in human monocytes. Mycobacterial lipoarabinomannan (LAM) induced expression of MCP-3 mRNA in human peripheral blood mononuclear cells. The non-mannose-capped version of lipoarabinomannan (AraLAM) was considerably more potent than the mannose-capped version ManLAM or the simpler version phosphatidylinositol mannoside (PLM). Among mononuclear cells, monocytes were responsible for LAM-induced MCP-3 mRNA expression. Whole mycobacteria (Mycobacterium bovis BCG) strongly induced MCP-3 expression. Pretreatment with actinomycin D abolished LAM-induced MCP-3 expression, whereas cycloheximide only partially reduced the expression. LAM-induced MCP-3 expression was associated with the production of immunoreactive PTX3. Interleukin 10 (IL-10) and IL-13 inhibited the induction of MCP-3 by LAM. Thus mycobacterial cell wall components induced expression of MCP-3 in human monocytes. MCP-3, a chemokine active on mononuclear phagocytes, NK cells, T cells and dendritic cells, may be relevant to the induction and expression of immunity against mycobacteria.

Expression of a long pentraxin, PTX3, by monocytes exposed to the mycobacterial cell wall component lipoarabinomannan

PTX3 is a prototypic long pentraxin composed of a C-terminal domain similar to those of classical pentraxins (e.g., C reactive protein) and an unrelated N-terminal portion. PTX3 is expressed in a variety of cell types, notably mononuclear phagocytes and endothelial cells, after exposure to the inflammatory cytokines interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNF-alpha). The present study was designed to assess whether mycobacterial components were able to induce expression and production of PTX3. Mycobacterial lipoarabinomannan (LAM) induced expression of PTX3 mRNA in human peripheral blood mononuclear cells. The non-mannose-capped version of lipoarabinomannan (AraLAM) was considerably more potent than the mannose-capped version ManLAM or the simpler version phosphatidylinositol mannoside. Among mononuclear cells, monocytes were responsible for LAM-induced PTX3 mRNA expression. Whole mycobacteria (Mycobacterium bovis BCG) strongly induced PTX3 expression. Pretreatment with actinomycin D abolished LAM-induced PTX3 expression, whereas cycloheximide only partially reduced the expression. LAM-induced PTX3 expression was associated with the production of immunoreactive PTX3. IL-10 and IL-13 did not inhibit the induction of PTX3 by LAM. Under the same conditions, these anti-inflammatory cytokines inhibited MCP-1 expression. In contrast, gamma interferon inhibited LAM-induced PTX3 expression. Thus, in addition to IL-1, TNF, and lipopolysaccharide, mycobacterial cell wall components also induce expression and production of the long pentraxin PTX3. The significance of PTX3 in the immunobiology of mycobacterial infection and its relevance in relation to clinical involvement remain to be determined.

Post-translational and activation-dependent modifications of the G protein-coupled thrombin receptor

The purpose of the present study was to analyze the post-translational and activation-dependent modifications of the G protein-coupled thrombin receptor. A human receptor cDNA was engineered to encode an epitope tag derived from the vesicular stomatitis virus glycoprotein at the COOH terminus of the receptor and expressed in human embryonic kidney 293 cells. We show here that the mature receptor is a glycosylated protein with an apparent molecular mass ranging from 68 to 80 kDa by SDS-polyacrylamide gel electrophoresis. Removal of asparagine-linked oligosaccharides with N-glycosidase F leads to the appearance of a 36-40-kDa receptor species. The current model for receptor activation by thrombin involves specific hydrolysis of the arginine-41/serine-42 (Arg-41/Ser-42) peptide bond. Cleavage of the receptor by thrombin was demonstrated directly by Western analyses performed on membranes and glycoprotein-enriched lysates from transfected cells. Whereas thrombin treatment of cells results in increased mobility of the receptor in SDS-polyacrylamide gel electrophoresis, we found that their treatment with the thrombin receptor agonist peptide leads to a decrease in thrombin receptor mobility due, in part, to phosphorylation. The serine proteases trypsin and plasmin also cleave and activate the receptor similar to thrombin, whereas chymotrypsin cleaves the receptor at a site distal to Arg-41, thus rendering it unresponsive to thrombin while still responsive to thrombin receptor agonist peptide.

Distinct mechanisms regulate 5-HT2 and thrombin receptor desensitization

We have compared the desensitization of two receptors, the thrombin receptor which displays dual coupling to both pertussis toxin-sensitive (Gi) and -insensitive (Gq) proteins and the serotonin type 2 (5-HT2) receptor which selectively couples to Gq. In the case of the thrombin receptor, cleavage induces activation and irreversible receptor modification followed by rapid (T1/2 = 3 min) and extensive desensitization of the receptor's ability to modulate phospholipase C (Gq). 5-HT-induced desensitization of its receptor is markedly slower (T1/2 = 10 min) and by 60 min only 50% of the phospholipase C response is lost. This effect occurs with a parallel disappearance of 5-HT receptors from the cell surface. Whole cell phosphorylation studies showed that the thrombin receptor is rapidly phosphorylated upon activation. In contrast, the 5-HT2 receptor displays a low basal level of phosphorylation which is not increased upon agonist treatment. The cytoplasmic tail of the 5-HT2 receptor which contains several protein kinase consensus sequences was found not to be involved in receptor activation or desensitization. However, a chimeric receptor having the core of the 5-HT2 receptor and the cytoplasmic tail of the thrombin receptor was able to undergo 5-HT-induced desensitization and phosphorylation. These results indicate that (i) both 5-HT2 and thrombin receptors have unique shut-off mechanisms, and (ii) that sequences in the carboxyl terminus of the thrombin receptor are sufficient to trigger rapid uncoupling of the receptor from its G protein(s) and downstream effector(s).

Structure-activity analysis of synthetic alpha-thrombin-receptor-activating peptides

alpha-Thrombin stimulates G-protein-coupled effectors leading to secretion and aggregation in human platelets, and to a mitogenic response in CCL39 hamster fibroblasts. alpha-Thrombin receptors can be activated by synthetic peptides corresponding to the receptor sequence starting with serine-42, at the proposed cleavage site. We have previously determined that the agonist domain of receptor-activating peptides resides within the five N-terminal residues [Vouret-Craviari, Van Obberghen-Schilling, Rasmussen, Pavirani, Lecocq and Pouysségur (1992) Mol. Biol. Cell. 3, 95-102], although the 7-residue peptide (SFFLRNP) corresponding to the hamster alpha-thrombin receptor was 10 times more potent than the 5-residue peptide for activation of human platelets. In the present study we have analysed the role of individual amino acids in receptor activation by using a series of modified hexa- or hepta-peptides derived from the human alpha-thrombin-receptor sequence. Cellular events examined here include phospholipase C activation, adenylyl cyclase inhibition and DNA synthesis stimulation in non-transformed CCL39 fibroblasts and a tumorigenic variant of that line (A71 cells). Modification of the peptide sequence had similar functional consequence for each of the assays described, indicating that either a unique receptor or pharmacologically indistinguishable receptor subtypes activate distinct G-protein signalling pathways. Furthermore, we found that: (1) the N-terminal serine can be replaced by small or intermediately sized amino acids (+/- hydroxyl groups) without loss of activity. However, its replacement by an aromatic side-chain or omission of the N-terminal amino group severely reduces activity. (2) An aromatic side-chain on the penultimate N-terminal residue appears to play a critical role since phenylalanine in this position can be substituted by tyrosine without complete loss of activity whereas an alanine in its place is not tolerated. (3) Deletion of the first, second or third N-terminal residue leads to a loss of activity, suggesting that a defined spacing of more than one structural component may be important for ligand-receptor interaction. Finally, we did not observe an antagonistic effect of the inactive peptides on phospholipase C activation or DNA synthesis induced by alpha-thrombin (1 nM) or SFLLRNP (3 microM).

Differential activation of p44mapk (ERK1) by alpha-thrombin and thrombin-receptor peptide agonist

alpha-Thrombin (thrombin), a potent mitogen for CCL39 hamster lung fibroblasts, stimulates phosphoinositide-specific phospholipase C (PI-PLC) and inhibits adenylate cyclase via cleavage of a specific G-protein-coupled receptor (TH-R), recently cloned from human and hamster cells. This action can be entirely mimicked by the synthetic peptide SFFLRNP, referred to here as TMP (thrombin-mimicking peptide). TMP corresponds to the first seven amino acids of the new N-terminus generated by thrombin cleavage of the hamster TH-R. Although thrombin and TMP apparently generate identical early transmembrane signals, only thrombin is mitogenic on its own. TMP needs to be associated with fibroblast growth factor (FGF), a tyrosine kinase-activating growth factor, to induce cell-cycle re-entry. Here, we have examined the early and late phase of p44 MAP kinase (p44mapk) activation in G0-arrested CCL39 cells after stimulation by thrombin, TMP, FGF or TMP+FGF. We found that: (i) both thrombin and TMP rapidly activate p44mapk in a dose-dependent manner with maximum activation at around 5 min, (ii) after the initial burst of activation, a second and long-lasting wave of activation is observed in response to thrombin (10-100 nM) but not to TMP (up to 300 microM), (iii) FGF alone (25 ng/ml), like thrombin, rapidly and persistently activates p44mapk (20-fold at 5 min and about 3-fold after 2 h), (iv) TMP added together with FGF strongly potentiates the second and sustained phase of p44mapk activation. From these results we propose that: (1) thrombin-induced mitogenesis is mediated only in part by the TH-R recently cloned and (2) activation of p44mapk, in particular the long-lasting phase that correlates with DNA synthesis, is an obligatory event for cell-cycle re-entry.

Synthetic alpha-thrombin receptor peptides activate G protein-coupled signaling pathways but are unable to induce mitogenesis

alpha-Thrombin (thrombin) stimulates phospholipase C and modulates the activity of adenylate cyclase in a number of cell types via G protein-coupled receptors. It is also a potent growth factor, notably for a line of hamster fibroblasts (CCL39 cells). Recently, predicted amino acid sequences for human and hamster thrombin receptors have been reported that display a putative thrombin cleavage site in the N-terminal extracellular domain. Synthetic peptides corresponding to 14 residues carboxyl to the presumed thrombin cleavage site of the human receptor have been shown to activate platelets as well as the thrombin receptor expressed in Xenopus oocytes. In the present study we have examined the effects of synthetic peptides corresponding to the same region of the hamster receptor (S-42-L-55) and shorter peptides (2-7 residues) on signal transducing systems in CCL39 cells. Our results indicate that hamster receptor peptides of greater than or equal to 5 residues effectively stimulate phospholipase C in CCL39 cells via the thrombin receptor and induce rapid desensitization of the response. The same peptides also inhibit adenylate cyclase in a pertussis toxin-sensitive manner. Although the peptides are potent agonists of serotonin release in platelets, unlike thrombin, by themselves they are not mitogenic. However, they potentiate DNA synthesis in cooperation with growth factors possessing tyrosine kinase receptors. Hence, we conclude that the potent mitogenic action of thrombin cannot be accounted for solely by the activation of the cloned receptor. We postulate the existence of an additional receptor activated by thrombin, which is required for its full mitogenic potential.

cDNA cloning and expression of a hamster alpha-thrombin receptor coupled to Ca2+ mobilization

The serine protease alpha-thrombin (thrombin) potently stimulates G-protein-coupled signaling pathways and DNA synthesis in CCL39 hamster lung fibroblasts. To clone a thrombin receptor cDNA, selective amplification of mRNA sequences displaying homology to the transmembrane domains of G-protein-coupled receptor genes was performed by polymerase chain reaction. Using reverse transcribed poly(A)+ RNA from CCL39 cells and degenerate primers corresponding to conserved regions of several phospholipase C-coupled receptors, three novel putative receptor sequences were identified. One corresponds to an mRNA transcript of 3.4 kb in CCL39 cells and a relatively abundant cDNA. Microinjection of RNA transcribed in vitro from this cDNA in Xenopus oocytes leads to the expression of a functional thrombin receptor. The hamster thrombin receptor consists of 427 amino acid residues with 8 hydrophobic domains, including one at the extreme N-terminus that is likely to represent a signal peptide. A thrombin consensus cleavage site is present in the N-terminal extracellular region of the receptor sequence followed by a negatively charged cluster of residues present in a number of proteins that interact with the anion-binding exosite of thrombin.

Cloning, functional expression and role in cell growth regulation of a hamster 5-HT2 receptor subtype

We have isolated a hamster fibroblast cDNA clone that encodes a serotoninergic receptor whose deduced amino acid sequence displays 94% identity with the rat brain serotonin (5-HT) type 2 receptor. When expressed in Xenopus oocytes, the hamster receptor efficiently couples to the phosphoinositide second messenger system and leads to intracellular Ca2+ mobilization in response to 5-HT. To determine the pharmacological properties of this receptor, and to evaluate the role of phospholipase C (PLC) activation in growth modulation by 5-HT, we have expressed it in hamster fibroblasts. Transfected cells that express 5-HT receptors were selected using a novel method based on coexpression of the Na+/H+ antiporter gene as a selectable marker. After co-transfection of the 5-HT receptor and Na+/H+ antiporter cDNAs in fibroblasts lacking antiporter activity (variants of the CCL39 line), 50% of the clones resistant to an acute acid load express functional receptors. The pharmacological profile of the transfected receptor is consistent with it being of the 5-HT2 subtype, and the extent of 5-HT-stimulated PLC activation in independent clones correlates with their relative level of cRNA expression. In cells in where addition of 5-HT leads to strong activation of PLC, and inhibition of adenylate cyclase via endogenous 5-HT1b receptors, 5-HT alone has little effect on DNA synthesis stimulation. Thus we conclude that activation of the PLC signalling pathway in these cells is not sufficient to trigger G0/G1 to S phase transition. Strong activation of PLC via 5-HT2 receptors does however contribute to the synergy observed between 5-HT (Gi-coupled pathway) and fibroblast growth factor (tyrosine kinase-activated pathway) on DNA synthesis reinitiation in transfected cells.