The genomic structure of discoidin receptor tyrosine kinase

DDR1-targeted therapies: current limitations and future potential

Discoidin domain receptor (DDR)-1 has a crucial role in regulating vital processes, including cell differentiation, proliferation, adhesion, migration, invasion, and matrix remodeling. Overexpression or activation of DDR1 in various pathological scenarios makes it a potential therapeutic target for the treatment of cancer, fibrosis, atherosclerosis, and neuropsychiatric, psychiatric, and neurodegenerative disorders. In this review, we summarize current therapeutic approaches targeting DDR1 from a medicinal chemistry perspective. Furthermore, we analyze factors other than issues of low selectivity and risk of resistance, contributing to the infrequent success of DDR1 inhibitors. The complex interplay between DDR1 and the extracellular matrix (ECM) necessitates additional validation, given that DDR1 might exhibit complex and synergistic interactions with other signaling molecules during ECM regulation. The mechanisms involved in DDR1 regulation in cancer and inflammation-related diseases also remain unknown.

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.

Expression of DDR1 in the CNS and in myelinating oligodendrocytes

Discoidin domain receptor 1 (DDR1) is a tyrosine kinase receptor that is activated by fibrillar collagens. Here, we review the expression and role of DDR1 in the central nervous system (CNS). In a murine model, DDR1 is expressed in oligodendrocytes in the developing brain and during remyelination. In human adult brain tissue, DDR1 is detected in a similar pattern as other classical myelin proteins such as myelin basic protein (MBP). Up to 50 transcripts of DDR1 have been detected in human tissues, of which 5 isoforms have been identified. In the human brain, all 5 isoforms are detectable, but DDR1b is the most highly expressed, and DDR1c is coexpressed with myelin genes. DDR1 sequence variants have been associated with psychiatric disorders, and upregulation of this gene occurs in gliomas. Moreover, mutations in DDR1 have been found in tumors of Schwann cells, which are the myelinating cells of the peripheral nervous system. All these data suggest that DDR1 plays a role in myelination and is relevant to neuropsychiatric diseases.

Discoidin domain receptor 1: New star in cancer-targeted therapy and its complex role in breast carcinoma

Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase activated by various types of collagens that performs a critical role in cell attachment, migration, survival and proliferation. The functions of DDR1 in various types of tumor have been studied extensively. However, in breast carcinoma, the roles of collagen-evoked DDR1 remain ill defined. Although a number of studies have reported that DDR1 promotes apoptosis and inhibits migration in breast carcinoma, it has also been reported to be associated with tumor cell survival, chemoresistance to genotoxic drugs and the facilitation of invasion. The present review summarizes current progress and the complex effects of DDR1 in the field of breast carcinoma, and presents DDR1 as a promising therapeutic target.

Discoidin domain receptor functions in physiological and pathological conditions

The discoidin domain receptors, DDR1 and DDR2, are nonintegrin collagen receptors that are members of the receptor tyrosine kinase family. Both DDRs bind a number of different collagen types and play important roles in embryo development. Dysregulated DDR function is associated with progression of various human diseases, including fibrosis, arthritis, and cancer. By interacting with key components of the extracellular matrix and displaying distinct activation kinetics, the DDRs form a unique subfamily of receptor tyrosine kinases. DDR-facilitated cellular functions include cell migration, cell survival, proliferation, and differentiation, as well as remodeling of extracellular matrices. This review summarizes the current knowledge of DDR-ligand interactions, DDR-initiated signal pathways and the molecular mechanisms that regulate receptor function. Also discussed are the roles of DDRs in development and disease progression.

Discoidin domain receptor tyrosine kinases: new players in cancer progression

Almost all human cancers display dysregulated expression and/or function of one or more receptor tyrosine kinases (RTKs). The strong causative association between altered RTK function and cancer progression has been translated into novel therapeutic strategies that target these cell surface receptors in cancer. Yet, the full spectrum of RTKs that may alter the oncogenic process is not completely understood. Accumulating evidence suggests that a unique set of RTKs known as the discoidin domain receptors (DDRs) play a key role in cancer progression by regulating the interactions of tumor cells with their surrounding collagen matrix. The DDRs are the only RTKs that specifically bind to and are activated by collagen. DDRs control cell and tissue homeostasis by acting as collagen sensors, transducing signals that regulate cell polarity, tissue morphogenesis, and cell differentiation. In cancer, DDRs are hijacked by tumor cells to disrupt normal cell-matrix communication and initiate pro-migratory and pro-invasive programs. Importantly, several cancer types exhibit DDR mutations, which are thought to alter receptor function and contribute to cancer progression. Other evidence suggests that the actions of DDRs in cancer are complex, either promoting or suppressing tumor cell behavior in a DDR type/isoform specific- and context-dependent manner. Thus, there is still a considerable gap in our knowledge of DDR actions in cancer tissues. This review summarizes and discusses the current knowledge on DDR expression and function in cancer. It is hoped that this effort will encourage more research into these poorly understood but unique RTKs, which have the potential of becoming novel therapeutic targets in cancer.

Identification of disulfide-linked dimers of the receptor tyrosine kinase DDR1

Discoidin domain receptor 1 (DDR1) is a transmembrane receptor tyrosine kinase activated by triple-helical collagen. So far six different isoforms of DDR1 have been described. Aberrant expression and signaling of DDR1 have been implicated in several human diseases linked to accelerated matrix degradation and remodeling, including tumor invasion, atherosclerosis, and lung fibrosis. Here we show that DDR1 exists as a disulfide-linked dimer in transfected as well as endogenously expressing cells. This dimer formation occurred irrespective of its kinase domain, as dimers were also found for the truncated DDR1d isoform. A deletion analysis of the extracellular domain showed that DDR1 mutants lacking the stalk region failed to form dimers, whereas deletion of the discoidin domain did not prevent dimerization. Point mutagenesis within the stalk region suggested that cysteines 303 and 348 are necessary for dimerization, collagen binding, and activation of kinase function. The identification of DDR1 dimers provides new insights into the molecular structure of receptor tyrosine kinases and suggests distinct signaling mechanisms of each receptor subfamily.

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.

Activation of discoidin domain receptor 1 isoform b with collagen up-regulates chemokine production in human macrophages: role of p38 mitogen-activated protein kinase and NF-kappa B

Macrophages produce an array of proinflammatory mediators at sites of inflammation and contribute to the development of inflammatory responses. Important roles for cytokines, such as IL-1 or TNF-alpha, and bacterial products, such as LPS, in this process have been well documented; however, the role for the extracellular matrix proteins, such as collagen, remains unclear. We previously reported that discoidin domain receptor 1 (DDR1), a nonintegrin collagen receptor, is expressed during differentiation of human monocytes into macrophages, and the interaction of the DDR1b isoform with collagen facilitates their differentiation via the p38 mitogen-activated protein kinase (MAPK) pathway. In this study, we report that the interaction of DDR1b with collagen up-regulates the production of IL-8, macrophage inflammatory protein-1alpha, and monocyte chemoattractant protein-1 in human macrophages in a p38 MAPK- and NF-kappaB-dependent manner. p38 MAPK was critical for DDR1b-mediated, increased NF-kappaB trans-activity, but not for IkappaB degradation or NF-kappaB nuclear translocation, suggesting a role for p38 MAPK in the modification of NF-kappaB. DDR1b-mediated IkappaB degradation was mediated through the recruitment of the adaptor protein Shc to the LXNPXY motif of the receptor and the downstream TNFR-associated factor 6/NF-kappaB activator 1 signaling cascade. Taken together, our study has identified NF-kappaB as a novel target of DDR1b signaling and provided a novel mechanism by which tissue-infiltrating macrophages produce large amounts of chemokines during the development of inflammatory diseases. Intervention of DDR1b signaling may be useful to control inflammatory diseases in which these proteins play an important role.

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

Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase activated by collagen. DDR1 is constitutively expressed in a variety of normal and transformed epithelial cells and plays a role in cell migration and differentiation through as yet unidentified signaling pathways. We previously reported inducible expression of DDR1 in human leukocytes and suggested a role for the DDR1a isoform in leukocyte migration through extracellular matrix. Here, we evaluated the contribution of DDR1 in the differentiation of the human monocytic THP-1 cells overexpressing these isoforms and of primary macrophages. Interestingly, collagen activation of DDR1b, but not DDR1a, further promoted phorbol ester-induced differentiation of THP-1 cells as determined by reduced cell proliferation and up-regulated expression of HLA-DR, CD11c, CD14, and CD40. Collagen activation of DDR1b also induced the recruitment and phosphorylation of Shc and subsequent phosphorylation of p38 mitogen-activated protein (MAP) kinase and its substrate ATF2. A p38 MAP kinase inhibitor, SB203580, completely inhibited DDR1b-mediated HLA-DR expression. Activation of DDR1 endogenously expressed on macrophages also up-regulated their HLA-DR expression in a p38 MAP kinase-dependent manner. Thus, DDR1b in response to collagen transduces signals that promote maturation/differentiation of HLA-DR-positive antigen-presenting cells and contributes to the development of adaptive immunity in a tissue microenvironment.

p53 induction and activation of DDR1 kinase counteract p53-mediated apoptosis and influence p53 regulation through a positive feedback loop

DDR1, discoidin domain receptor 1, belongs to a subfamily of tyrosine kinase receptors with an extracellular domain homologous to Dictyostellium discoideum protein discoidin 1. We showed that DDR1 is a direct p53 transcriptional target, and that DNA damage induced a p53-dependent DDR1 response associated with activation of its tyrosine kinase. We further demonstrated that DDR1 activated the MAPK cascade in a Ras-dependent manner. Whereas levels of p53, phosphoserine-15 p53, p21, ARF and Bcl-X(L) were increased in response to exogenous overexpression of activated DDR1, dominant-negative DDR1 inhibited irradiation-induced MAPK activation and p53, phosphoserine-15 p53, as well as induced p21 and DDR1 levels, suggesting that DDR1 functions in a feedforward loop to increase p53 levels and at least some of its effectors. Nonetheless, inhibition of DDR1 function resulted in strikingly increased apoptosis of wild-type p53-containing cells in response to genotoxic stress through a caspase-dependent pathway. These results strongly imply that this p53 response gene must predominately act to alleviate the adverse effects of stress induced by p53 on its target cell.

Characterization of the mouse aortic carboxypeptidase-like protein promoter reveals activity in differentiated and dedifferentiated vascular smooth muscle cells

The dedifferentiation and proliferation of vascular smooth muscle cells (VSMCs) contribute to the formation of vascular lesions. In this study, the regulation of aortic carboxypeptidase-like protein (ACLP) expression in VSMCs was investigated. After mouse carotid injury, the expression of ACLP increases in the dedifferentiated VSMCs of the neointima in a pattern that differs from that of smooth muscle alpha-actin. To better understand the regulation of ACLP in VSMCs, we characterized the 21-exon mouse ACLP gene and 5'-flanking region and examined its promoter activity. In transient transfection assays, 2.5 kb of the ACLP 5'-flanking sequence directed high levels of luciferase reporter activity in primary cultured rat aortic smooth muscle cells, and this activity was not dependent on serum response factor. We identified a positive element between base pairs -156 and -122 by analysis of 5' deletion and mutant constructs. By use of electrophoretic mobility shift assays with rat aortic smooth muscle cell nuclear extracts, Sp1 and Sp3 transcription factors bound to this region, and transfection assays in D.Mel.2 cells revealed that both Sp1 and Sp3 transactivated the ACLP promoter. Transgenic mice harboring the -2.5-kb ACLP promoter upstream from a nuclear-targeted LacZ gene were generated, and expression was detected in the VSMCs of large blood vessels, arterioles, and veins. Interestingly, ACLP promoter-LacZ reporter activity increased within the neointimal VSMCs of injured carotid vessels, consistent with the expression of the endogenous ACLP protein. The ACLP promoter may provide a novel tool to target gene expression to dedifferentiated VSMCs.

Functional analysis of discoidin domain receptor 1: effect of adhesion on DDR1 phosphorylation

Discoidin domain receptor 1 (DDR1), a receptor tyrosine kinase (RTK), has been shown to be activated mainly by soluble fibrillar collagen. Unusually, the kinetics of phosphorylation of the receptor is slow, with maximal phosphorylation observed after 90 min. To understand the reasons for slow phosphorylation of the receptor, we examined several cell lines under different conditions. We confirm that endogenous DDR1 is phosphorylated slowly by collagen in adherent T47D and HCT116 cells. In detached and resuspended cells, collagen induced rapid phosphorylation of DDR1. This was further confirmed with a semiadherent cell line (COLO201) and one that grows as a suspension (K562), both of which express endogenous DDR1. Replating K562 on fibronectin to mimic adherent conditions altered the kinetics of phosphorylation from rapid to slow, similar to those of adherent cells. The slow kinetics of phosphorylation in the adherent state was probably not due to cell-cell contacts because EDTA had no major effect. However, pervanadate in the absence of collagen was able to induce strong DDR1 phosphorylation, indicating that a phosphatase may inhibit or delay the phosphorylation of DDR1. Further, downstream signals after phosphorylation of DDR1 by collagen were not transmitted through the classical mitogen-activated protein kinase pathway. In addition, a chimeric TrkA-DDR1 receptor failed to become phosphorylated on stimulation with nerve growth factor (NGF), although it dimerized normally. This is the first RTK whose kinetics of phosphorylation is dependent on cellular context. The interaction of the cells with the matrix, rather than cell-cell contact, is probably responsible for the inhibition of phosphorylation.

Discoidin domain receptor 1 isoform-a (DDR1alpha) promotes migration of leukocytes in three-dimensional collagen lattices

Although integrins are crucial for migration of leukocytes through endothelium, integrin-independent mechanisms appear to take over and mediate the migration of leukocytes through extracellular matrix (ECM) in a three-dimensional tissue microenvironment. Discoidin domain receptor (DDR) 1 is a receptor tyrosine kinase activated by collagen, the most abundant ECM protein. In the present study, we detected that peripheral blood mononuclear cells (PBMC) and polymorphonuclear neutrophils were induced to express DDR1 after incubation in RPMI 1640. The expression level of DDR1 in PBMC was increased further by stimulation with tumor necrosis factor-alpha, interleukin-1beta, granulocyte-macrophage colony-stimulating factor, lipopolysaccharide, or phytohemagglutinin, but not with interferon-gamma. In vivo, DDR1 mRNA was detectable in mononuclear leukocytes infiltrating human renal tumor tissue. Among three DDR1 isoforms, DDR1alpha was the major transcript in leukocytes. Functionally, overexpression of either DDR1alpha or DDR1beta in THP-1 cells resulted in increased adherence to collagen-coated plates in a beta1-integrin independent manner. However, only DDR1alpha-, but not DDR1beta-, overexpressing cells exhibited marked pseudopod extension and migrated successfully through three-dimensional collagen lattices. Consequently, we propose that the interaction of DDR1alpha with collagen of the ECM results in a requisite intracellular signaling that enables leukocytes to migrate in a tissue microenvironment and participate in host defense.

Discoidin domain receptor 1 tyrosine kinase has an essential role in mammary gland development

Various types of collagen have been identified as potential ligands for the two mammalian discoidin domain receptor tyrosine kinases, DDR1 and DDR2. Here, we used a recombinant fusion protein between the extracellular domain of DDR1 and alkaline phosphatase to detect specific receptor binding sites during mouse development. Major sites of DDR1-binding activity, indicative of ligand expression, were found in skeletal bones, the skin, and the urogenital tract. Ligand expression in the uterus during implantation and in the mammary gland during pregnancy colocalized with the expression of the DDR1 receptor. The generation of DDR1-null mice by gene targeting yielded homozygous mutant animals that were viable but smaller in size than control littermates. The majority of mutant females were unable to bear offspring due to a lack of proper blastocyst implantation into the uterine wall. When implantation did occur, the mutant females were unable to lactate. Histological analysis showed that the alveolar epithelium failed to secrete milk proteins into the lumen of the mammary gland. The lactational defect appears to be caused by hyperproliferation and abnormal branching of mammary ducts. These results suggest that DDR1 is a key mediator of the stromal-epithelial interaction during ductal morphogenesis in the mammary gland.

Discoidin domain receptors: structural relations and functional implications

Multicellular life relies on the presence of extracellular matrix to provide scaffolding for cells and tissue compartments. To provide communication between cells and tissues, a multitude of cell surface receptors are triggered by soluble ligands and components of the extracellular matrix. A large family of these receptors transmit signals through the use of an intrinsic tyrosine kinase function. The subgroup of discoidin domain receptors (DDRs) is distinguished from other members of the receptor tyrosine kinase family by a discoidin homology repeat in their extracellular domains that is also found in a variety of other transmembrane and secreted proteins. Sequence comparisons show that non-mammalian orthologs of DDRs exist: three closely related genes in Caenorhabditis and one in the sponge Geodia cydonium. Recently, various types of collagen have been identified as the ligands for the two mammalian discoidin domain receptor tyrosine kinases, DDR1 and DDR2. The binding of collagen to DDRs results in a delayed but sustained tyrosine kinase activation. Both receptors display several potential tyrosine phosphorylation sites that are able to relay the signal by interacting with cytoplasmic effector proteins. The potential cross-talk to other receptors for collagen and the clinical aspects of DDR function are discussed.