Interaction of discoidin domain receptor 1 isoform b (DDR1b) with collagen activates p38 mitogen-activated protein kinase and promotes differentiation of macrophages

Discoidin domain receptors (DDRs): Potential implications in periodontitis

Periodontitis is a chronic inflammatory disease leading to the destruction of periodontal tissues associated with high prevalence and significant economic burden. As special collagen-binding tyrosine kinase receptors, the discoidin domain receptors (DDRs) can control cell migration, adhesion, proliferation, and extracellular matrix remodeling. DDRs are constitutively expressed and widely distributed in periodontal tissues which are rich in collagen. Ddr1/2 knockout mice showed significant periodontal defects including connective tissue destruction, alveolar bone loss, and even tooth loss. It has been demonstrated that bone homeostasis, inflammation, matrix metalloproteinases, and autophagy are crucial characteristics involved in the pathogenesis of periodontitis. Of note, DDRs have been reported to participate in the above pathophysiological processes, implicating the potential roles of DDRs in periodontitis. In this review article, we aim to illustrate the possible roles of DDRs in periodontitis in an attempt to explore their potential value as therapeutic targets for periodontitis.

Insulin/IGF signaling and discoidin domain receptors: An emerging functional connection

The insulin/insulin-like growth factor system (IIGFs) plays a fundamental role in the regulation of prenatal and postnatal growth, metabolism and homeostasis. As a consequence, dysregulation of this axis is associated with growth disturbance, type 2 diabetes, chronic inflammation and tumor progression. A functional crosstalk between IIGFs and discoidin domain receptors (DDRs) has been recently discovered. DDRs are non-integrin collagen receptors that canonically undergo slow and long-lasting autophosphorylation after binding to fibrillar collagen. While both DDR1 and DDR2 functionally interact with IIGFs, the crosstalk with DDR1 is so far better characterized. Notably, the IIGFs-DDR1 crosstalk presents a feed-forward mechanism, which does not require collagen binding, thus identifying novel non-canonical action of DDR1. Further studies are needed to fully explore the role of this IIGFs-DDRs functional loop as potential target in the treatment of inflammatory and neoplastic disorders.

DDR1 role in fibrosis and its pharmacological targeting

Discoidin domain receptor1 (DDR1) is a collagen activated receptor tyrosine kinase and an attractive anti-fibrotic target. Its expression is mainly limited to epithelial cells located in several organs including skin, kidney, liver and lung. DDR1's biology is elusive, with unknown downstream activation pathways; however, it may act as a mediator of the stromal-epithelial interaction, potentially controlling the activation state of the resident quiescent fibroblasts. Increased expression of DDR1 has been documented in several types of cancer and fibrotic conditions including skin hypertrophic scars, idiopathic pulmonary fibrosis, cirrhotic liver and renal fibrosis. The present review article focuses on: a) detailing the evidence for a role of DDR1 as an anti-fibrotic target in different organs, b) clarifying DDR1 tissue distribution in healthy and diseased tissues as well as c) exploring DDR1 protective mode of action based on literature evidence and co-authors experience; d) detailing pharmacological efforts attempted to drug this subtle anti-fibrotic target to date.

Discoidin Domain Receptors: Potential Actors and Targets in Cancer

The extracellular matrix critically controls cancer cell behavior by inducing several signaling pathways through cell membrane receptors. Besides conferring structural properties to tissues around the tumor, the extracellular matrix is able to regulate cell proliferation, survival, migration, and invasion. Among these receptors, the integrins family constitutes a major class of receptors that mediate cell interactions with extracellular matrix components. Twenty years ago, a new class of extracellular matrix receptors has been discovered. These tyrosine kinase receptors are the two discoidin domain receptors DDR1 and DDR2. DDR1 was first identified in the Dictyostelium discoideum and was shown to mediate cell aggregation. DDR2 shares highly conserved sequences with DDR1. Both receptors are activated upon binding to collagen, one of the most abundant proteins in extracellular matrix. While DDR2 can only be activated by fibrillar collagen, particularly types I and III, DDR1 is mostly activated by type I and IV collagens. In contrast with classical growth factor tyrosine kinase receptors which display a rapid and transient activation, DDR1 and DDR2 are unique in that they exhibit delayed and sustained receptor phosphorylation upon binding to collagen. Recent studies have reported differential expression and mutations of DDR1 and DDR2 in several cancer types and indicate clearly that these receptors have to be taken into account as new players in the different aspects of tumor progression, from non-malignant to highly malignant and invasive stages. This review will discuss the current knowledge on the role of DDR1 and DDR2 in malignant transformation, cell proliferation, epithelial to mesenchymal transition, migratory, and invasive processes, and finally the modulation of the response to chemotherapy. These new insights suggest that DDR1 and DDR2 are new potential targets in cancer therapy.

Searching novel diagnostic markers and targets for therapy of CKD

Over the last decade, identification and characterization of novel markers of progression and targets for therapy of chronic kidney disease (CKD) have been challenging for the research community. Several promising candidates have emerged, mainly from experimental models of CKD that are yet to be investigated in clinical studies. The authors identified two candidate genes: periostin, an extracellular matrix protein involved in bone and dental development, and the discoidin domain receptor 1 (DDR1), a collagen-binding membrane receptor with tyrosine kinase activity. Both genes are inactive in adulthood under normal conditions but have been shown to be highly inducible following injury to glomerular or tubular epithelial cells. The objective of this review is to summarize recent evidence supporting the role of periostin and DDR1 as potential novel biomarkers and therapeutic targets in CKD.

Phosphotyrosine mediated protein interactions of the discoidin domain receptor 1

The receptor tyrosine kinase DDR1 has been implicated in multiple human cancers and fibrosis and is targeted by the leukemia drug Gleevec. This suggests that DDR1 might be a new therapeutic target. However, further insight into the DDR1 signaling pathway is required in order to support its further development. Here, we investigated DDR1 proximal signaling by the analysis of protein-protein interactions using proteomic approaches. All known interactors of DDR1 were identified and localized to specific phosphotyrosine residues on the receptor. In addition, we identified numerous signaling proteins as new putative phosphotyrosine mediated interactors including RasGAP, SHIP1, SHIP2, STATs, PI3K and the SRC family kinases. Most of the new proteins contain SH2 and PTB domains and for all interactors we could directly point the site of interaction to specific phosphotyrosine residues on the receptor. The identified proteins have roles in the early steps of the signaling cascade, propagating the signal from the DDR1 receptor into the cell. The map of phosphotyrosine mediated interactors of DDR1 created in this study will serve as a starting point for functional investigations which will enhance our knowledge on the role of the DDR1 receptor in health and disease. This article is part of a Special Section entitled: Understanding genome regulation and genetic diversity by mass spectrometry.

Role of DDR1 in the gelatinases secretion induced by native type IV collagen in MDA-MB-231 breast cancer cells

Discoidin domain receptors (DDRs) are receptor tyrosine kinases that get activated by collagens in its native triple-helical form. In mammalian cells, DDR family consists of two members, namely DDR1 and DDR2, which mediates migration and proliferation of several cell types. DDR1 is activated by native type IV collagen and overexpressed in human breast cancer. Type IV collagen is the main component of basement membrane (BM), and the ability to degrade and penetrate BM is related with an increased potential for invasion and metastasis. Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that collectively are capable of degrading all components of the extracellular matrix, including the BM. In breast cancer cells, denatured type IV collagen induces MMP-9 secretion and invasion. However, the role of DDR1 in the regulation of gelatinases (MMP-2 and -9) secretion and invasion in breast cancer cells remains to be studied. We demonstrate here that native type IV collagen induces MMP-2 and -9 secretions and invasion through a DDR1 and Src-dependent pathway, together with an increase of MMP-2 and -9-cell surface levels. MMP-2 and -9 secretions require PKC kinase activity, epidermal growth factor receptor (EGFR) activation, arachidonic acid (AA) production and AA metabolites in MDA-MB-231 breast cancer cells. In summary, our data demonstrate, for the first time, that DDR1 mediates MMP-2 and -9 secretions and invasion induced by native type IV collagen in MDA-MB-231 breast cancer cells.

DDR1/E-cadherin complex regulates the activation of DDR1 and cell spreading

Discoidin domain receptors (DDRs) 1 and 2, collagen receptors, regulate cell adhesion and a broad range of cell behavior. Their adhesion-dependent regulation of signaling associated with adhesion proteins has not been elucidated. We report a novel mechanism: the cross talk of DDR1 and E-cadherin negatively and adhesion dependently regulated both DDR1 activity and DDR1-suppressed cell spreading. E-cadherin forms complexes with both DDR1 isoforms (a and b). E-cadherin regulates DDR1 activity associated with the cell-junction complexes formed between DDR1 and E-cadherin. These complexes are formed independently of DDR1 activation and of beta-catenin and p120-catenin binding to E-cadherin; they are ubiquitous in epithelial cells. Small interfering RNA-mediated gene silencing of E-cadherin restores both DDR1 activity and DDR1-suppressed cell spreading and increases the apically and basally located DDR1 in E-cadherin-null cells. We conclude that E-cadherin-mediated adhesions decrease DDR1 activity, which subsequently eliminates DDR1-suppressed cell spreading, by sequestering DDR1 to cell junctions, which prevents its contact with collagen ligand.

Collagens in the progression and complications of atherosclerosis

Collagens constitute a major portion of the extracellular matrix in the atherosclerotic plaque, where they contribute to the strength and integrity of the fibrous cap, and also modulate cellular responses via specific receptors and signaling pathways. This review focuses on the diverse roles that collagens play in atherosclerosis; regulating the infiltration and differentiation of smooth muscle cells and macrophages; controlling matrix remodeling through feedback signaling to proteinases; and influencing the development of atherosclerotic complications such as plaque rupture, aneurysm formation and calcification. Expanding our understanding of the pathways involved in cell-matrix interactions will provide new therapeutic targets and strategies for the diagnosis and treatment of atherosclerosis.

High-throughput sequence analysis of the tyrosine kinome in acute myeloid leukemia

To determine whether aberrantly activated tyrosine kinases other than FLT3 and c-KIT contribute to acute myeloid leukemia (AML) pathogenesis, we used high-throughput (HT) DNA sequence ana-lysis to screen exons encoding the activation loop and juxtamembrane domains of 85 tyrosine kinase genes in 188 AML patients without FLT3 or c-KIT mutations. The screen identified 30 nonsynonymous sequence variations in 22 different kinases not previously reported in single-nucleotide polymorphism (SNP) databases. These included a novel FLT3 activating allele and a previously described activating mutation in MET (METT1010I). The majority of novel sequence variants were stably expressed in factor-dependent Ba/F3 cells. Apart from one FLT3 allele, none of the novel variants showed constitutive phosphorylation by immunoblot analysis and none transformed Ba/F3 cells to factor-independent growth. These findings indicate the majority of these alleles are not potent tyrosine kinase activators in this cellular context and that a significant proportion of nonsynonymous sequence variants identified in HT DNA sequencing screens may not have functional significance. Although some sequence variants may represent SNPs, these data are consistent with recent reports that a significant fraction of such sequence variants are "passenger" rather than "driver" alleles and underscore the importance of functional assessment of candidate disease alleles.

Reversal of renal disease: is it enough to inhibit the action of angiotensin II?

Over the last years, evidence emerged demonstrating that the progression of renal fibrosis is reversible in experimental models. The present review summarizes the new insights concerning the mechanisms of progression and regression of renal disease and examines this novel evidence under the light of feasibility and transfer to human nephropathies. The involved mechanisms are discussed with particular emphasis on the fibrotic role of vasoactive peptides such as angiotensin II and endothelin, and growth factors such as transforming growth factor beta (TGFbeta). The possibility of regression is introduced by presenting the in vivo efficiency of anti-hypertensive treatments and of systems that antagonize the fibrogenic action of TGFbeta such as bone morphogenic protein-7 (BMP-7) and hepatocyte growth factor. Finally, we provide a brief description of the promising future directions and clinical considerations about the applications of the experimental data to humans.

Bacterial c-di-GMP is an immunostimulatory molecule

Cyclic diguanylate (c-di-GMP) is a bacterial intracellular signaling molecule. We have shown that treatment with exogenous c-di-GMP inhibits Staphylococcus aureus infection in a mouse model. We now report that c-di-GMP is an immodulator and immunostimulatory molecule. Intramammary treatment of mice with c-di-GMP 12 and 6 h before S. aureus challenge gave a protective effect and a 10,000-fold reduction in CFUs in tissues (p < 0.001). Intramuscular vaccination of mice with c-di-GMP coinjected with S. aureus clumping factor A (ClfA) Ag produced serum with significantly higher anti-ClfA IgG Ab titers (p < 0.001) compared with ClfA alone. Intraperitoneal injection of mice with c-di-GMP activated monocyte and granulocyte recruitment. Human immature dendritic cells (DCs) cultured in the presence of c-di-GMP showed increased expression of costimulatory molecules CD80/CD86 and maturation marker CD83, increased MHC class II and cytokines and chemokines such as IL-12, IFN-gamma, IL-8, MCP-1, IFN-gamma-inducible protein 10, and RANTES, and altered expression of chemokine receptors including CCR1, CCR7, and CXCR4. c-di-GMP-matured DCs demonstrated enhanced T cell stimulatory activity. c-di-GMP activated p38 MAPK in human DCs and ERK phosphorylation in human macrophages. c-di-GMP is stable in human serum. We propose that cyclic dinucleotides like c-di-GMP can be used clinically in humans and animals as an immunomodulator, immune enhancer, immunotherapeutic, immunoprophylactic, or vaccine adjuvant.

Discoidin domain receptor 1 mediates collagen-induced nitric oxide production in J774A.1 murine macrophages

Nitric oxide (NO) is an important regulator of immune responses. Effects of cytokines, such as tumor necrosis factor (TNF)-alpha or IFN-gamma, and bacterial products, such as lipopolysaccharide, on macrophage NO production have been well documented; however, the role of the extracellular matrix proteins, including collagen, in this process remains unclear. We previously reported that discoidin domain receptor 1 (DDR1), a nonintegrin collagen receptor, was expressed in human macrophages, and its activation facilitated their differentiation as well as cytokine/chemokine production. Here, we examined the role for DDR1 in collagen-induced NO production using the murine macrophage cell line J774 cells that endogenously express DDR1. Activation of J774 cells with collagen induced the expression of inducible NO synthase (iNOS) and NO production. Inhibition of DDR1, but not beta1-integrins, abolished collagen-induced iNOS and NO production. Activation of J774 cells with collagen-activated nuclear factor-kappaB, p38 mitogen-activated protein kinase (MAPK), and c-jun N-terminal kinase (JNK) and a pharmacological inhibitor of each signaling molecule significantly reduced collagen-induced NO production. Thus, we have demonstrated, for the first time, that the interaction of DDR1 with collagen induces iNOS expression and subsequent NO synthesis in J774 cells through activation of NF-kappaB, p38 MAPK, and JNK and suggest that intervention of DDR1 signaling in macrophages may be useful in controlling inflammatory diseases in which NO plays a critical role.

Discoidin Domain Receptor 1 Null Mice Are Protected against Hypertension-Induced Renal Disease

A frequent complication of hypertension is the development of chronic renal failure. This pathology usually is initiated by inflammatory events and is characterized by the abnormal accumulation of collagens within the renal tissue. The purpose of this study was to investigate the role of discoidin domain receptor 1 (DDR1), a nonintegrin collagen receptor that displays tyrosine-kinase activity, in the development of renal fibrosis. To this end, hypertension was induced with angiotensin in mice that were genetically deficient of DDR1 and in wild-type controls. After 4 or 6 wk of angiotensin II administration, wild-type mice developed hypertension that was associated with perivascular inflammation, glomerular sclerosis, and proteinuria. Systolic pressure increase was similar in the DDR1-deficient mice, but the histologic lesions of glomerular fibrosis and inflammation were significantly blunted and proteinuria was markedly prevented. Immunostaining for lymphocytes, macrophages, and collagens I and IV was prominent in the renal cortex of wild-type mice but substantially reduced in DDR1 null mice. In separate experiments, renal cortical slices of DDR1 null mice showed a blunted response of chemokines to LPS that was accompanied by a considerable protection against the LPS-induced mortality. These results indicate the importance of DDR1 in mediating inflammation and fibrosis. Use of DDR1 inhibitors could provide a completely novel therapeutic approach against diseases that have these combined pathologies.

Activating Transcription Factor 2 Mediates Matrix Metalloproteinase-2 Transcriptional Activation Induced by p38 in Breast Epithelial Cells

Mounting evidence suggests a role for matrix metalloproteinase (MMP)-2 in the malignant progression of breast cancer cells. We showed previously that H-Ras, but not N-Ras, induced invasion of MCF10A human breast epithelial cells through Rac-MKK3/6-p38 pathway resulted in MMP-2 up-regulation. Activation of p38 pathway by MKK6 caused a selective up-regulation of MMP-2. In this study, we aimed to elucidate the transcriptional regulation of MMP-2 by p38 pathway leading to the invasive phenotype of MCF10A cells. By using 5' deletion mutant constructs of MMP-2 promoter, we showed that deletion of the region containing activator protein-1 (AP-1) site caused the greatest reduction of MMP-2 promoter activity both in MKK6- and H-Ras-activated MCF10A cells, suggesting that the AP-1 binding site is critical for the MMP-2 promoter activation. DNA binding and transcriptional activities of AP-1 were increased by MKK6 or H-Ras as evidenced by electrophoretic mobility shift assay and luciferase assay using an AP-1-driven plasmid. By doing immunoinhibition assay and chromatin immunoprecipitation assay, we revealed the activating transcription factor (ATF) 2 as a transcription factor for MMP-2 gene expression through binding to the functional AP-1 site. Activation of ATF2, which depended on p38 activity, was crucial for MMP-2 promoter activity as well as induction of invasive and migrative phenotypes in MCF10A cells. This is the first report revealing ATF2 as an essential transcription factor linking MKK3/6-p38 signaling pathway to MMP-2 up-regulation, providing evidence for a direct role of ATF2 activation in malignant phenotypic changes of human breast epithelial cells.

Discoidin domain receptor 1 contributes to eosinophil survival in an NF-kappaB-dependent manner in Churg-Strauss syndrome

Churg-Strauss syndrome (CSS) is a systemic disease that shows marked eosinophilia along with eosinophil infiltration in the tissue. Prolonged eosinophil survival plays an important role in the pathogenesis of CSS; however, its detailed molecular mechanism remains unclear. Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase, and its ligand is collagen. DDR1 was expressed in human leukocytes and fibroblasts, and it plays an important role in leukocyte cytokine production and fibroblast survival in an NF-kappaB-dependent manner. In this study, we examined in vitro and in vivo eosinophil DDR1 expression and its function in CSS patients. The expression level of DDR1 was significantly higher in the eosinophils of CSS patients, and the predominant isoform was DDR1b. Immunohistochemical findings revealed that the tissue-infiltrating eosinophils expressed endogenous DDR1. In CSS patients, DDR1 activation inhibited Fas agonistic antibody-induced apoptosis and up-regulated Fas agonistic antibody-induced cytokine production of eosinophils in an NF-kappaB-dependent manner. Suppression of DDR1 expression in the eosinophils by using RNA interference and addition of the DDR1-blocking protein abolished these effects. We propose that DDR1 contributes to the eosinophil survival in the tissue microenvironment of CSS and that it might be involved in the development of CSS.

Discoidin Domain Receptor 1–deficient Mice Are Resistant to Bleomycin-induced Lung Fibrosis

RATIONALE

Discoidin domain receptor 1 (DDR1) is a tyrosine kinase activated by native collagens. Based on previous findings showing increased DDR1 expression in bronchoalveolar lavage cells from patients with idiopathic pulmonary fibrosis, we hypothesized that DDR1 mediates disease progression after lung injury.

OBJECTIVES

To investigate the inflammatory and fibrotic responses of DDR1 knockout and wild-type mice to bleomycin-induced lung injury.

METHODS

Age- and sex-matched DDR1 knockout and wild-type C57BL/6 mice received a single intratracheal instillation of 2 U/kg bleomycin or saline, respectively. After 2 wk, lung inflammation and fibrosis were assessed using immunohistochemistry, real-time polymerase chain reaction, TUNEL assay, ELISA, fluorescence-activated cell sorting, and Western blot analysis.

MEASUREMENTS AND MAIN RESULTS

Compared with wild-type animals, DDR1-null mice were largely protected against bleomycin-induced injury. Bleomycin-induced increases in collagen protein levels and tenascin-C mRNA levels were abrogated in knockout animals. Furthermore, myofibroblast expansion and apoptosis were much lower in these animals compared with their wild-type counterparts. Absence of inflammation in knockout mice was confirmed by lavage cell count and a cytokine ELISA. Western blot analysis of injured lung tissue revealed that DDR1-null mice failed to respond to the bleomycin insult with p38 MAPK activation, which was readily observed in wild-type mice.

CONCLUSIONS

DDR1 expression is a prerequisite for the development of lung inflammation and fibrosis. Blockade of DDR1 may therefore be a novel therapeutic intervention in patients with pulmonary fibrosis.

Discoidin domain receptor-1a (DDR1a) promotes glioma cell invasion and adhesion in association with matrix metalloproteinase-2

Invasion of glioma cells involves the attachment of invading tumor cells to extracellular matrix (ECM), disruption of ECM components, and subsequent cell penetration into adjacent brain structures. Discoidin domain receptor 1 (DDR1) tyrosine kinases constitute a novel family of receptors characterized by a unique structure in the ectodomain (discoidin-I domain). These cell surface receptors bind to several collagens and facilitate cell adhesion. Little is known about DDR1 expression and function in glioblastoma multiforme. In this study we demonstrate that DDR1 is overexpressed in glioma tissues using cDNA arrays, immunohistochemistry and Western blot analysis. Functional comparison of two splice variants of DDR1 (DDR1a and DDR1b) reveal novel differences in cell based glioma models. Overexpression of either DDR1a or DDR1b caused increased cell attachment. However, glioma cells overexpressing DDR1a display enhanced invasion and migration. We also detect increased levels of matrix metalloproteinase-2 in DDR1a overexpressing cells as measured by zymography. Inhibition of MMP activity using MMP inhibitor suppressed DDR1a stimulated cell-invasion. Similarly, an antibody against DDR1 reduced DDR1a mediated invasion as well as the enhanced adhesion of DDR1a and DDR1b overexpressing cells. These results suggest that DDR1a plays a critical role in inducing tumor cell adhesion and invasion, and this invasive phenotype is caused by activation of matrix metalloproteinase-2.

Collagen type I selectively activates ectodomain shedding of the discoidin domain receptor 1: involvement of Src tyrosine kinase

The discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase that is highly expressed in breast carcinoma cells. Upon binding to collagen, DDR1 undergoes autophosphorylation followed by limited proteolysis to generate a tyrosine phosphorylated C-terminal fragment (CTF). Although it was postulated that this fragment is formed as a result of shedding of the N-terminal ectodomain, collagen-dependent release of the DDR1 extracellular domain has not been demonstrated. We now report that, in conjunction with CTF formation, collagen type I stimulates concentration-dependent, saturable shedding of the DDR1 ectodomain from two carcinoma cell lines, and from transfected cells. In contrast, collagen did not promote cleavage of other transmembrane proteins including the amyloid precursor protein (APP), ErbB2, and E-cadherin. Collagen-dependent tyrosine phosphorylation and proteolysis of DDR1 in carcinoma cells were reduced by a pharmacologic Src inhibitor. Moreover, expression of a dominant negative Src mutant protein in human embryonic kidney cells inhibited collagen-dependent phosphorylation and shedding of co-transfected DDR1. The hydroxamate-based metalloproteinase inhibitor TAPI-1 (tumor necrosis factor-alpha protease inhibitor-1), and tissue inhibitor of metalloproteinase (TIMP)-3, also blocked collagen-evoked DDR1 shedding, but did not reduce levels of the phosphorylated CTF. Neither shedding nor CTF formation were affected by the gamma-secretase inhibitor, L-685,458. The results demonstrate that collagen-evoked ectodomain cleavage of DDR1 is mediated in part by Src-dependent activation or recruitment of a matrix- or disintegrin metalloproteinase, and that CTF formation can occur independently of ectodomain shedding. Delayed shedding of the DDR1 ectodomain may represent a mechanism that limits DDR1-dependent cell adhesion and migration on collagen matrices.

Discoidin domain receptor 1 contributes to the survival of lung fibroblast in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF), characterized by fibroblast proliferation and accumulation of extracellular matrix, including collagen, is a chronic progressive disorder that results in lung remodeling and fibrosis. However, the cellular mechanisms that may make fibroblasts resistant to apoptosis have not been completely elucidated. Discoidin domain receptor 1 (DDR1), a receptor tyrosine kinase whose ligand is collagen, is expressed in vivo and contributes in vitro to leukocyte differentiation and nuclear factor (NF)-kappaB activation, which may play an important role in fibroblast survival. In this study, we examined in vivo and in vitro DDR1 expression and its role in cell survival using fibroblasts obtained from IPF and non-IPF patients. Immunohistochemically, fibroblasts present in fibroblastic foci expressed endogenous DDR1. The DDR1 expression level was significantly higher in fibroblasts from IPF patients, and the predominant isoform was DDR1b. In IPF patients, DDR1 activation in fibroblasts inhibited Fas ligand-induced apoptosis and resulted in NF-kappaB nuclear translocation. Suppression of DDR1 expression in fibroblasts by siRNA abolished these effects, and an NF-kappaB inhibitor abrogated the anti-apoptotic effect of DDR1 activation. We propose that DDR1 contributes to fibroblast survival in the tissue microenvironment of IPF and that DDR1 up-regulation may occur in other fibroproliferative lung diseases as well.

Suppression of discoidin domain receptor 1 by RNA interference attenuates lung inflammation

Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase whose ligand is collagen. Recently, we have reported the association of DDR1 in the cytokine production of human leukocytes in in vitro and in vivo expression in idiopathic pulmonary fibrosis. However, its role in in vivo inflammation has not been fully elucidated. Small interference RNA (siRNA) can induce specific suppression of in vitro and in vivo gene expression. In this study, using a bleomycin-induced pulmonary fibrosis mouse model, we administered siRNA against DDR1 transnasally and evaluated histological changes, cytokine expression, and signaling molecule activation in the lungs. Histologically, siRNA against DDR1 successfully reduced in vivo DDR1 expression and attenuated bleomycin-induced infiltration of inflammatory cells. Furthermore, it significantly reduced inflammatory cell counts and concentrations of cytokines such as MCP-1, MIP-1alpha, and MIP-2 in bronchoalveolar lavage fluid. Subsequently, bleomycin-induced up-regulation of TGF-beta in bronchoalveolar lavage fluid was significantly inhibited, and collagen deposition in the lungs was reduced. Furthermore, siRNA against DDR1 significantly inhibited bleomycin-induced P38 MAPK activation in the lungs. Considered together, we propose that DDR1 contributes to the development of bleomycin-induced pulmonary inflammation and fibrosis.

Sensing extracellular matrix: an update on discoidin domain receptor function

Discoidin Domain Receptors (DDRs) have recently emerged as non-integrin-type receptors for collagen. The two mammalian gene products Discoidin Domain Receptor 1 and -2 constitute a subfamily of tyrosine kinase receptors that are selectively expressed in a number of different cell types and organs. Upon collagen activation, DDRs regulate cell adhesion, proliferation and extracellular matrix remodeling. Here we review the various signaling pathways and cellular responses evoked by activated DDRs. Additionally, we give an overview of the more recent advances in understanding the role of DDRs in various human diseases, in particular during tumor progression, atherosclerosis, inflammation and tissue fibrosis. Furthermore, we discuss potential roles of genes homologous to mammalian DDRs identified in flies, worms and sponges. We show that the structural organization of these DDR-related genes is highly conserved throughout evolution suggesting that invertebrate DDRs may also function as receptors for collagen. By highlighting current questions about these unusual collagen receptors, we hope to attract new research on DDRs from a variety of different fields.

Discoidin domain receptor 1: a new class of receptor regulating leukocyte-collagen interaction

Previous studies demonstrated that type I collagen, a major component of the extracellular matrix, could influence the differentiation and function of leukocytes; however, it is not clear whether those effects of collagen were based on its interaction with the classic collagen receptors, alpha1beta1 and alpha2beta1 integrins. We recently detected significant upregulation of discoidin domain receptor 1 (DDR1), a new class of collagen receptor, in human leukocytes, including neutrophils, monocytes, and lymphocytes, in vitro, leading to the hypothesis that the leukocyte-activating effects of collagen might be owing to its interaction with DDR1. In this review, we summarize our recent findings demonstrating that DDR1-collagen interaction facilitates the adhesion, migration, differentiation/maturation, and cytokine/chemokine production of leukocytes. We also describe the intracellular signaling pathways activated by DDR1 interaction with collagen.

Pinpointing phosphotyrosine-dependent interactions downstream of the collagen receptor DDR1

Activation of the receptor tyrosine kinase DDR1 by collagen results in robust and sustained phosphorylation, however little is known about its downstream mediators. Using phosphopeptide mapping and site-directed mutagenesis, we here identified multiple tyrosine phosphorylation sites within DDR1. We found that Nck2 and Shp-2, two SH2 domain-containing proteins, bind to DDR1 in a collagen-dependent manner. The binding site of Shp-2 was mapped to tyrosine-740 of DDR1 within an ITIM-consensus sequence. Lastly, ablation of DDR1 in the mouse mammary gland resulted in delocalized expression of Nck2, suggesting that defects observed during alveologenesis are caused by the lack of the DDR1-Nck2 interaction.

Tyrosine 740 phosphorylation of discoidin domain receptor 2 by Src stimulates intramolecular autophosphorylation and Shc signaling complex formation

DDR2 is a receptor tyrosine kinase whose activating ligands are various collagens. DDR2-mediated cellular signaling has been shown to require Src activity. However, the precise mechanism underlying the Src dependence of DDR2 signaling is unknown. Here, using baculoviral co-expression of the DDR2 cytosolic domain and Src, we show that Src targets three tyrosine residues (Tyr-736, Tyr-740, and Tyr-741) in the activation loop of DDR2 for phosphorylation. This phosphorylation by Src stimulates DDR2 cis-autophosphorylation of additional tyrosine residues. In vitro Shc binding assays demonstrate that phosphotyrosines resulting from DDR2 autophosphorylation are involved in Shc binding to the DDR2 cytosolic domain. Mutating tyrosine 740 of DDR2 to phenylalanine stimulates autophosphorylation of DDR2 to an extent similar to that resulting from Src phosphorylation of DDR2. In addition, the DDR2 Y740F mutant protein displays collagen-independent, constitutively activated signaling. These findings suggest that tyrosine 740 inhibits DDR2 autophosphorylation. Collectively, our findings are consistent with the following mechanism for Src-dependent DDR2 activation and signaling: 1) ligand binding promotes phosphorylation of Tyr-740 in the DDR2 activation loop by Src; 2) Tyr-740 phosphorylation stimulates intramolecular autophosphorylation of DDR2; 3) DDR2 autophosphorylation generates cytosolic domain phosphotyrosines that promote the formation of DDR2 cytosolic domain-Shc signaling complexes.

Involvement of discoidin domain receptor 1 in the deterioration of pulmonary sarcoidosis

The prognosis of sarcoidosis with pulmonary infiltrates differs in each case, and several cytokines are reported to contribute to its deterioration. However, the detailed mechanism has not been fully elucidated. Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase activated by collagen and associated with cytokine production from inflammatory cells. We previously reported the functional expression of DDR1 on CD14-positive bronchoalveolar lavage fluid (BALF) cells in vivo. In this study, we hypothesized that DDR1 might be associated with the deterioration of pulmonary sarcoidosis (PS), and investigated 33 patients with sarcoidosis with pulmonary infiltrates, prospectively. We found that patients with deteriorated PS showed significantly higher DDR1 expression in CD14-positive BALF cells predominant with DDR1b isoforms. Activation of DDR1 induced monocyte chemoattractant protein-1 (MCP-1) and matrix metalloproteinase-9 (MMP-9) production in a p38 mitogen-activated protein kinase-dependent manner from CD14-positive BALF cells of patients with deteriorated sarcoidosis. DDR1 activation also induced NF-kappaB nuclear translocation in CD14-positive BALF cells of patients with deteriorated PS. The inhibitor of NF-kappaB inhibited the production of MCP-1 and MMP-9. We propose that DDR1 is associated with the deterioration of pulmonary sarcoidosis.

Activation of Discoidin Domain Receptor 1 on CD14-Positive Bronchoalveolar Lavage Fluid Cells Induces Chemokine Production in Idiopathic Pulmonary Fibrosis

Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase activated by collagen. We previously reported the functional expression of DDR1 on human monocyte-derived macrophages in vitro; however, information regarding its role in diseases is limited. Idiopathic pulmonary fibrosis (IPF) is a chronic lung disease, and the lesions contain an abundance of collagen. In this study, we examined DDR1 expression on bronchoalveolar lavage fluid (BALF) cells and investigated its functionality using samples obtained from 28 IPF patients, 13 chronic obstructive pulmonary disease patients, and 14 healthy volunteers. The DDR1 expression level in CD14-positive BALF cells was higher in IPF patients than in chronic obstructive pulmonary disease patients or healthy volunteers. The predominant isoform was DDR1b in the IPF group, while DDR1a was predominant in the other two groups. Using immunohistochemical analysis, we also detected DDR1 expression on infiltrating inflammatory cells in the IPF lesion. In IPF patients, DDR1 activation induced the production of MCP-1, IL-8, MIP-1 alpha, and matrix metalloproteinase-9 (MMP-9) from CD14-positive BALF cells in a p38 MAPK-dependent manner. In contrast, DDR1 activation of CD14-positive BALF cells in the other groups did not induce the production of these chemokines or MMP-9. These chemokines and MMP-9 contribute to the development of IPF and, therefore, we suggest that DDR1 might be associated with the pathogenesis of IPF in the tissue microenvironment.

Clinical investigation: increased serum stromal derived factor 1 alpha levels in pulmonary tuberculosis

Pulmonary tuberculosis, a granulomatous disease, has few serological markers for its activity. Recently, an increased plasma level of stromal derived factor 1 alpha (SDF-1alpha), which can induce strong chemotaxis of cells through its receptor CXCR4, was detected in patients with tuberculosis. In this study we investigated serum SDF-1alpha levels and CXCR4 expression on peripheral blood mononuclear cells (PBMCs). Fifty-five active tuberculosis patients, 30 resolved tuberculosis patients, 27 acute bronchitis patients and 8 healthy volunteers were examined. Histological expression of SDF-1alpha in the tuberculosis lesion and CXCR4 expression of PBMCs were also analysed. Serum SDF-1alpha levels in active tuberculosis patients were significantly higher than other groups. The sensitivity and specificity for the diagnosis of active tuberculosis was 88.5% and 85.3% (cutoff value = 650 pg/ml), respectively. CXCR4 expression levels on PBMCs showed a significant negative correlation with serum SDF-1alpha levels. Inflammatory cells including multinuclear giant cells in the lesion expressed SDF-1alpha. Measurement of serum SDF-1alpha could be a useful screening marker for the identification of active pulmonary tuberuculosis. We propose that interaction of SDF-1alpha and CXCR4 might be involved in the pathogenesis of pulmonary tuberculosis.

TNF-related apoptosis-inducing ligand is involved in neutropenia of systemic lupus erythematosus

Neutropenia is a common laboratory finding in systemic lupus erythematosus (SLE). However, the molecular mechanism of SLE neutropenia has not been fully explained. In this study, we examined whether TNF-related apoptosis-inducing ligand (TRAIL) is involved in the pathogenesis of SLE neutropenia using samples from SLE patients. Serum TRAIL levels in SLE patients with neutropenia were significantly higher than those of SLE patients without neutropenia and healthy volunteers. Serum TRAIL levels showed a significant negative correlation with neutrophil counts in SLE patients. The expression of TRAIL receptor 3 was significantly lower in SLE patients with neutropenia than in patients without neutropenia or in healthy volunteers. Treatment with glucocorticoids negated the decrease of TRAIL receptor 3 expression on neutrophils of SLE patients. TRAIL may accelerate neutrophil apoptosis of neutrophils from SLE patients, and autologous T cells of SLE patients, which express TRAIL on surface, may kill autologous neutrophils. Interferon gamma and glucocorticoid modulated the expression of TRAIL on T cells of SLE patients and also modulated the expression of cellular Fas-associating protein with death domain–like interleukin-1β–converting enzyme (FLICE)–inhibitory protein (cFLIP), an inhibitor of death receptor signaling, in neutrophils. Thus, our results provide a novel insight into the molecular pathogenesis of SLE neutropenia.

Linkage and association studies of discoidin domain receptor 1 (DDR1) single nucleotide polymorphisms (SNPs) in juvenile oligoarthritis

OBJECTIVES

Multiple independent juvenile oligoarthritis susceptibility loci have been identified within the major histocompatibility complex (MHC), including HLA-A, HLA-DRB1 and an as yet unlocalized effect in the centromeric class I region. The discoidin domain receptor 1 (DDR1) gene resides within this region and codes for a receptor tyrosine kinase that plays an important role in regulating cell attachment to collagen, chemotaxis, proliferation and matrix metalloproteinase (MMP) production. DDR1 expression in chondrocytes has not been investigated. The objectives of this study were to investigate expression of DDR1 in healthy chondrocytes and to identify linkage and association of this candidate gene with juvenile oligoarthritis.

METHODS

A set of 135 simplex juvenile idiopathic arthritis families consisting of one affected child and healthy parent(s) and 199 healthy unrelated individuals were genotyped for six single nucleotide polymorphisms (SNPs) within the DDR1 gene using the primer extension SNaPshot trade mark method. Single-point and multipoint transmission disequilibrium tests were carried out with the ETDT and TDTPHASE packages. Allele frequency comparisons between cases and controls were carried out with the chi(2) test. DDR1 expression was investigated in normal articular cartilage by RT-PCR and immunofluorescence methods.

RESULTS

No linkage and association with any of the six SNPs or their haplotypic combinations were observed in the families studied. No significant differences were observed in allele frequencies between patients and controls. DDR1 expression was found in normal articular cartilage by RT-PCR and by immunofluorescence.

CONCLUSIONS

The DDR1 SNPs examined are not involved in susceptibility to juvenile oligoarthritis.

Activation of Discoidin Domain Receptor 1 Facilitates the Maturation of Human Monocyte-Derived Dendritic Cells Through the TNF Receptor Associated Factor 6/TGF-β-Activated Protein Kinase 1 Binding Protein 1β/p38α Mitogen-Activated Protein Kinase Signaling Cascade

Maturation of dendritic cells (DCs) is critical for their ability to stimulate resting naive T cells in primary immune responses. Previous studies demonstrated that collagen, such as type I collagen, could facilitate DC maturation; however, the basis of collagen-mediated DC maturation remains unclear. Discoidin domain receptor 1 (DDR1) is a nonintegrin collagen receptor constitutively expressed in a variety of epithelial cells, including tumor cells, and is inducible in leukocytes. In this study, we evaluated the role of DDR1 in DC maturation using human monocyte-derived DCs. Two DDR1 isoforms, DDR1a and DDR1b, were expressed in both immature and mature DCs. Activation of DDR1 on immature DCs resulted in their partial maturation; however, DDR1 activation markedly amplified TNF-alpha- and LPS-induced phenotypic and functional maturation of DCs through activation of p38 mitogen-activated protein kinase (MAPK), suggesting the involvement of DDR1b in this process. Activation of DDR1b on differentiated DDR1b-overexpressing THP-1 cells or DDR1 on mature DCs induced the formation of TNFR associated factor 6 (TRAF6)/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK complex and p38alpha autophosphorylation. Transfection of differentiated DDR1b-overexpressing THP-1 cells with dominant negative TRAF6 completely abrogated DDR1b-mediated p38 MAPK phosphorylation, indicating a critical role of TRAF6 in DDR1b-mediated p38 MAPK activation. Taken together, our data suggest that DDR1b-collagen interaction augments the maturation of DCs in a tissue microenvironment through a unique TRAF6/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK signaling cascade and contributes to the development of adaptive immune responses.