Signalling of GPI-anchored CD157 via focal adhesion kinase in MCA102 fibroblasts

CD157: From Myeloid Cell Differentiation Marker to Therapeutic Target in Acute Myeloid Leukemia

Human CD157/BST-1 and CD38 are dual receptor-enzymes derived by gene duplication that belong to the ADP ribosyl cyclase gene family. First identified over 30 years ago as Mo5 myeloid differentiation antigen and 10 years later as Bone Marrow Stromal Cell Antigen 1 (BST-1), CD157 proved not to be restricted to the myeloid compartment and to have a diversified functional repertoire ranging from immunity to cancer and metabolism. Despite being a NAD⁺-metabolizing ectoenzyme anchored to the cell surface through a glycosylphosphatidylinositol moiety, the functional significance of human CD157 as an enzyme remains unclear, while its receptor role emerged from its discovery and has been clearly delineated with the identification of its high affinity binding to fibronectin. The aim of this review is to provide an overview of the immunoregulatory functions of human CD157/BST-1 in physiological and pathological conditions. We then focus on CD157 expression in hematological tumors highlighting its emerging role in the interaction between acute myeloid leukemia and extracellular matrix proteins and its potential utility for monoclonal antibody targeted therapy in this disease.

A pre-steady state and steady state kinetic analysis of the N-ribosyl hydrolase activity of hCD157

hCD157 catalyzes the hydrolysis of nicotinamide riboside (NR) and nicotinic acid riboside (NAR). The release of nicotinamide or nicotinic acid from NR or NAR was confirmed by spectrophotometric, HPLC and NMR analyses. hCD157 is inactivated by a mechanism-based inhibitor, 2'-deoxy-2'-fluoro-nicotinamide arabinoside (fNR). Modification of the enzyme during the catalytic cycle by NR, NAR, or fNR increased the intrinsic protein fluorescence by approximately 50%. Pre-steady state and steady state data were used to derive a minimal kinetic scheme for the hydrolysis of NR. After initial complex formation a reversible step (360 and 30s(-1)) is followed by a slow irreversible step (0.1s(-1)) that defined the rate limiting step, or kcat. The calculated KMapp value for NR in the hydrolytic reaction is 6nM. The values of the kinetic constants suggest that one biological function of cell-surface hCD157 is to bind and slowly hydrolyze NR, possibly converting it to a ligand-activated receptor. Differences in substrate preference between hCD157 and hCD38 were rationalized through a comparison of the crystal structures of the two proteins. This comparison identified several residues in hCD157 (F108 and F173) that can potentially hinder the binding of dinucleotide substrates (NAD+).

Binding of CD157 protein to fibronectin regulates cell adhesion and spreading

CD157/BST-1 behaves both as an ectoenzyme and signaling receptor and is an important regulator of leukocyte trafficking and ovarian cancer progression. However, the molecular interactions underpinning the role of CD157 in these processes remain obscure. The biological functions of CD157 and its partnership with members of the integrin family prompted us to assume the existence of a direct interaction between CD157 and an unknown component of the extracellular matrix. Using solid-phase binding assays and surface plasmon resonance analysis, we demonstrated that CD157 binds fibronectin with high affinity within its heparin-binding domains 1 and 2. Furthermore, we found that CD157 binds to other extracellular matrix proteins containing heparin-binding domains. Finally, we proved that the CD157-fibronectin interaction occurs with living cells, where it elicits CD157-mediated cell responses. Indeed, knockdown of CD157 in Met-5A mesothelial cells changed their morphology and cytoskeleton organization and attenuated the activation of intracellular signaling pathways triggered by fibronectin. This led to impaired cell spreading and adhesion to selected extracellular matrix proteins. Collectively, these findings indicate a central role of CD157 in cell-extracellular matrix interactions and make CD157 an attractive therapeutic target in inflammation and cancer.

Novel SCRG1/BST1 axis regulates self-renewal, migration, and osteogenic differentiation potential in mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) remodel or regenerate various tissues through several mechanisms. Here, we identified the hMSC-secreted protein SCRG1 and its receptor BST1 as a positive regulator of self-renewal, migration, and osteogenic differentiation. SCRG1 and BST1 gene expression decreased during osteogenic differentiation of hMSCs. Intriguingly, SCRG1 maintained stem cell marker expression (Oct-4 and CD271/LNGFR) and the potentials of self-renewal, migration, and osteogenic differentiation, even at high passage numbers. Thus, the novel SCRG1/BST1 axis determines the fate of hMSCs by regulating their kinetic and differentiation potentials. Our findings provide a new perspective on methods for ex vivo expansion of hMSCs that maintain native stem cell potentials for bone-forming cell therapy.

Bone marrow-induced Mef2c deficiency delays B-cell development and alters the expression of key B-cell regulatory proteins

The Mef2 family transcriptional regulator Mef2c (myocyte enhancer factor 2c) is highly expressed in maturing bone marrow and peripheral mature B-cells. To evaluate the role of this transcription factor in B-cell development, we generated a B-cell-specific conditional deletion of Mef2c using the Mb-1-Cre transgene that is expressed during the early stages of immunoglobulin rearrangement. Young mice possessing this defect demonstrated a significant impairment in B-cell numbers in bone marrow and spleen. This phenotype was evident in all B-cell subsets; however, as the animals mature, the deficit in the peripheral mature B-cell compartments was overcome. The absence of Mef2c in mature B-cells led to unique CD23+ and CD23- subsets that were evident in Mef2c knockout primary samples as well as Mef2c-deficient cultured, differentiated B-cells. Genome-wide expression analysis of immature and mature B-cells lacking Mef2c indicated altered expression for a number of key regulatory proteins for B-cell function including Ciita, CD23, Cr1/Cr2 and Tnfsf4. Chromatin immunoprecipitation analysis confirmed Mef2c binding to the promoters of these genes indicating a direct link between the presence (or absence) of Mef2c and altered transcriptional control in mature B-cells.

Overexpression of CD157 contributes to epithelial ovarian cancer progression by promoting mesenchymal differentiation

Epithelial ovarian carcinoma (EOC) is an aggressive tumor often diagnosed at an advanced stage, when there is little or no prospect of cure. Despite advances in surgical and chemotherapeutic strategies, only marginal improvements in patient outcome have been obtained. Hence, unraveling the biological mechanisms underpinning EOC progression is critical for improving patients’ survival. Recently, we reported that CD157 (an ectoenzyme regulating leukocyte diapedesis) is expressed in EOC and that high expression of the molecule is negatively correlated with the disease outcome in patients. Here, we demonstrate that forced overexpression of CD157 in OVCAR-3, TOV-21G, A2780 and OV-90 ovarian cancer cell lines promotes morphological and phenotypic changes characterized by disruption of intercellular junctions, downregulation of epithelial markers and upregulation of mesenchymal ones. These changes in cell shape and phenotype bring to reduced sensitivity to anoikis, increased anchorage-independent growth, cell motility and mesothelial invasion. Conversely, knockdown of CD157 in OV-90 and OC314 cells reverts the mesenchymal phenotype and reduces the cells’ migratory potential. Transcriptome profiling analysis highlighted 378 significantly differentially expressed genes, representing the signature of CD157-overexpressing OVCAR-3 and OV-90 cells. The modulation of selected genes translates into alteration of protein expression that give cells a highly malignant phenotype. The overall picture deduced from the analysis of the modulated transcripts is that high expression of CD157 strengthens a number of biological processes favoring tumor progression (including development and cell motility), and weakens several biological processes hindering tumor progression (such as apoptosis, cell death and response to stress). Together, these findings implicate CD157 in the progression of EOC to metastatic disease and suggest that CD157 may represent a valuable therapeutic target.

The CD157-integrin partnership controls transendothelial migration and adhesion of human monocytes

CD157, a member of the CD38 gene family, is an NAD-metabolizing ectoenzyme and a signaling molecule whose role in polarization, migration, and diapedesis of human granulocytes has been documented; however, the molecular events underpinning this role remain to be elucidated. This study focused on the role exerted by CD157 in monocyte migration across the endothelial lining and adhesion to extracellular matrix proteins. The results demonstrated that anti-CD157 antibodies block monocyte transmigration and adhesion to fibronectin and fibrinogen but that CD157 cross-linking is sufficient to overcome the block, suggesting an active signaling role for the molecule. Consistent with this is the observation that CD157 is prevalently located within the detergent-resistant membrane microdomains to which, upon clustering, it promotes the recruitment of β(1) and β(2) integrin, which, in turn, leads to the formation of a multimolecular complex favoring signal transduction. This functional cross-talk with integrins allows CD157 to act as a receptor despite its intrinsic structural inability to do so on its own. Intracellular signals mediated by CD157 rely on the integrin/Src/FAK (focal adhesion kinase) pathway, resulting in increased activity of the MAPK/ERK1/2 and the PI3K/Akt downstream signaling pathways, which are crucial in the control of monocyte transendothelial migration. Collectively, these findings indicate that CD157 acts as a molecular organizer of signaling-competent membrane microdomains and that it forms part of a larger molecular machine ruled by integrins. The CD157-integrin partnership provides optimal adhesion and transmigration of human monocytes.

Functional role and prognostic significance of CD157 in ovarian carcinoma

BACKGROUND

CD157, an ADP-ribosyl cyclase-related cell surface molecule, regulates leukocyte diapedesis during inflammation. Because CD157 is expressed in mesothelial cells and diapedesis resembles tumor cell migration, we investigated the role of CD157 in ovarian carcinoma.

METHODS

We assayed surgically obtained ovarian cancer and mesothelial cells and both native and engineered ovarian cancer cell lines for CD157 expression using flow cytometry and reverse transcription-polymerase chain reaction (RT-PCR), and for adhesion to extracellular matrices, migration, and invasion using cell-based assays. We investigated invasion of human peritoneal mesothelial cells by serous ovarian cancer cells with a three-dimensional coculture model. Experiments were performed with or without CD157-blocking antibodies. CD157 expression in tissue sections from ovarian cancer patients (n = 88) was examined by immunohistochemistry, quantified by histological score (H score), and categorized as at or above or below the median value of 60, and compared with clinical parameters. Statistical tests were two-sided.

RESULTS

CD157 was expressed by ovarian cancer cells and mesothelium, and it potentiated the adhesion, migration, and invasion of serous ovarian cancer cells through different extracellular matrices. CD157-transfected ovarian cancer cells migrated twice as much as CD157-negative control cells (P = .001). Blockage of CD157 inhibited mesothelial invasion by serous ovarian cancer cells in a three-dimensional model. CD157 was expressed in 82 (93%) of the 88 epithelial ovarian cancer tissue specimens. In serous ovarian cancer, patients with CD157 H scores of 60 or greater had statistically significantly shorter disease-free survival and overall survival than patients with lower CD157 H scores (CD157 H score > or =60 vs <60: median disease-free survival = 18 months, 95% confidence interval [CI] = 5.92 to 30.07 vs unreached, P = .005; CD157 H score > or =60 vs <60: median overall survival = 45 months, 95% CI = 21.21 to 68.79 vs unreached, P = .024). Multivariable Cox regression showed that CD157 is an independent prognostic factor for recurrence (hazard ratio of disease recurrence = 3.01, 95% CI = 1.35 to 6.70, P = .007) and survival (hazard ratio of survival = 3.44, 95% CI = 1.27 to 9.31, P = .015).

CONCLUSIONS

CD157 plays a pivotal role in the control of ovarian cancer cell migration and peritoneal invasion, and it may be clinically useful as a prognostic tool and therapeutic target.

CD66b regulates adhesion and activation of human eosinophils

Eosinophils and their products are likely important in the pathophysiology of allergic diseases, such as bronchial asthma, and in host immunity to parasitic organisms. However, the mechanisms for proinflammatory mediator release by eosinophils are poorly understood. CD66b (CEACAM8, CGM6, NCA-95) is a single chain, GPI-anchored, highly glycosylated protein belonging to the carcinoembryonic Ag supergene family. CD66b is an activation marker for human granulocytes; however, its biological functions are largely unknown in eosinophils. We found that CD66b is highly expressed on the surface of human peripheral blood eosinophils isolated from healthy individuals. Engagement of CD66b, but not CD66a, by mAb or a natural ligand, galectin-3, activated a Src kinase family molecule, hemopoietic cell kinase (Hck), and induced cellular adhesion, superoxide production, and degranulation of eosinophils. CD66b molecules were localized in lipid rafts, and disruption of lipid rafts or removal of the GPI anchor inhibited the adhesion and activation of eosinophils. Importantly, CD66b was constitutively and physically associated with a beta2 integrin, CD11b, and cross-linking of CD66b induced a striking clustering of CD11b molecules. Thus, CD66b molecules are involved in regulating adhesion and activation of eosinophils, possibly through their localization in lipid rafts and interaction with other cell surface molecules, such as CD11b. Binding of exogenous or endogenous carbohydrate ligands(s) to CD66b may be important in the release of proinflammatory mediators by human eosinophils.

CD157 plays a pivotal role in neutrophil transendothelial migration

Paracellular diapedesis, a key step in leukocyte recruitment to the site of inflammation, occurs at endothelial junctions and is regulated by highly coordinated interactions between leukocytes and endothelium. We found that CD157, a glycosylphosphatidylinositol-anchored ectoenzyme belonging to the NADase/ADP-ribosyl cyclase family, plays a crucial role for neutrophil diapedesis, because its ligation with specific monoclonal antibodies (both on neutrophils or endothelial cells) results in altered neutrophil movement on the apical surface of endothelium and, ultimately, in loss of diapedesis. Real-time microscopy revealed that CD157 behaves as a sort of compass during the interaction between neutrophils and endothelial cells; indeed, following CD157 ligation, neutrophils appear disoriented, meandering toward junctions where they eventually stop without transmigrating. These findings are relevant in vivo because CD157-deficient neutrophils obtained from patients with paroxysmal nocturnal hemoglobinuria are characterized by a severely impaired diapedesis.

Thy-1, via its GPI anchor, modulates Src family kinase and focal adhesion kinase phosphorylation and subcellular localization, and fibroblast migration, in response to thrombospondin-1/hep I

Normal fibroblast subpopulations have differential surface expression of the GPI-linked raft protein Thy-1, which correlates with differences in cellular adhesion and migration in vitro. Thrombospondin-1 (TSP-1) induces an intermediate state of adhesion in fibroblasts and other cells which facilitates migration. TSP-1 and the hep I peptide derived from the amino-terminal/heparin-binding domain of TSP-1 induce disassembly of cellular focal adhesions. Our lab previously reported that the induction of focal adhesion disassembly in fibroblasts by TSP-1 or by hep I requires surface expression of Thy-1, as well as lipid raft integrity and Src family kinase (SFK) signaling. We now report that TSP-1/hep I-induced fibroblast migration requires Thy-1 expression and FAK phosphorylation, and that following TSP-1/hep I stimulation, Thy-1 associates with FAK and SFK in a lipid raft-dependent manner. Furthermore, the GPI anchor of Thy-1, which localizes the protein to specific lipid raft microdomains, is necessary for hep I-induced FAK and SFK phosphorylation, focal adhesion disassembly, and migration. This is the first report of an association between Thy-1 and FAK. Thy-1 modulates SFK and FAK phosphorylation and subcellular localization, promoting focal adhesion disassembly and migration in fibroblasts, following exposure to TSP-1/hep I.

The CD38/CD157 mammalian gene family: An evolutionary paradigm for other leukocyte surface enzymes

Human CD38 is the mammalian prototype of a family of phylogenetically conserved proteins which share structural similarities and enzymatic activities involved in the production of an intracellular second messenger with calcium mobilizing effects. Engagement of CD38 by agonistic monoclonal antibodies and the CD31 ligand initiates a cytoplasmic signaling cascade involving tyrosine phosphorylation of the proto-oncogene c-cbl and of the extracellular regulated kinase 1 of 2 complex. Further requirements for signal transduction include a privileged localization within the cholesterol-rich areas of the plasma membrane and physical association with specialized surface receptors. CD38-mediated signals are crucial in heterotypic cell adhesion and migration as well as in the activation of proliferation/survival programs by normal and neoplastic cells. Here we review the most recent literature on this complex topic and attempt to formulate a single model reconciling the enzymatic and receptorial activities of CD38.

CD157 is an important mediator of neutrophil adhesion and migration

CD157, a glycosylphosphatidylinositol (GPI)-anchored protein encoded by a member of the CD38 NADase/ADP-ribosyl cyclase gene family, is expressed on the surface of most human circulating neutrophils. This work demonstrates that CD157 is a receptor that induces reorganization of the cytoskeleton and significant changes in cell shape, and that signals mediated by CD157 act through modulation of cytosolic Ca(2+) concentration. These signals are independent of the products of CD157's enzymatic activities (ie, cyclic adenosine diphosphate [ADP]-ribose and ADP-ribose). Indeed, the enzymatic activities of CD157 in circulating neutrophils as well as in dimethyl sulfoxide (DMSO)-differentiated (CD157(+)/CD38(-)) HL-60 cells, are hardly detectable. This work also shows that the receptorial activity relies on cross-talk between CD157 and beta(2) integrin. CD157 localizes in GM1-enriched lipid rafts and, upon activation, it migrates to the uropod, a structure specialized in motility and adhesive functions. Indeed, CD157 is involved in adhesion to extracellular matrix proteins and in chemotaxis induced in vitro by formyl-methionyl-leucyl-phenylalanine (fMLP). These findings were consistent with the results obtained in neutrophils from patients with paroxysmal nocturnal hemoglobinuria (PNH), in which CD157 is deficient. These neutrophils showed constant defects in adhesion and migration. Our data attribute specific and crucial roles to CD157 in the regulation of innate immunity during inflammation.

Myofibroblastic conversion of mesothelial cells

BACKGROUND

The continuous chemical, physical, and inflammatory insults of prolonged continuous ambulatory peritoneal dialysis (CAPD) incite mesothelial cell responses, which may result in peritoneal fibrosis. The transforming growth factor-beta (TGF-beta), especially the isoform TGF-beta 1, has long been known to play crucial role in the fibrogenic process. Although several studies have implicated TGF-beta in peritoneal fibrosis, the underlying mechanism has not been completely elucidated.

METHODS

To test the effects of exogenous TGF-beta 1 on mesothelial cells, we assessed cytoarchitectural changes of human peritoneal mesothelial cells (HPMC) in in vitro culture by light, immunofluorescent, electron and immunoelectron microscopy, and differential gene expression analysis using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and cDNA expression array assays.

RESULTS

The TGF-beta 1-induced myofibroblastic conversion was a transdifferentiation process resulting in characteristic myofibroblastic phenotype that included prominent rough endoplasmic reticuli (rER) with dilated cisternas, conspicuous smooth muscle actin (SMA) myofilaments, frequent intercellular intermediate and gap junctions, and active deposition of extracellular matrix (ECM) and formation of fibronexus. The gene expression array analysis revealed complex modulation of gene expression involving cytoskeletal organization, cell adhesion, ECM production, cell proliferation, innate immunity, cytokine/growth factor signaling, cytoprotection, stress response, and many other essential metabolic processes in mesothelial cells.

CONCLUSION

This report describes myofibroblastic conversion of mesothelial cells, a previously undefined, yet frequently speculated, cell adaptive or pathogenic process. Our study helps to elucidate the complex molecular and cellular events involved in myofibroblastic conversion of mesothelial cells. We propose that differentiated epithelial cells of mesothelium convert or transdifferentiate into myofibroblasts, which implies the recruitment of fibrogenic cells from mesothelium during serosal inflammation and wound healing.

CD157, the Janus of CD38 but with a unique personality

CD157 is a pleiotropic ectoenzyme which belongs to the CD38 family and to the growing number of leukocyte surface molecules known to act independently as both receptors and enzymes. A 45-kDa surface structure with a GPI anchor, the CD157 molecule displays two distinct domains in its extracellular component. The first is implicated in the enzymic activities of the molecule and the second features adhesion/signalling properties. CD157 shares several characteristics with CD38, including a similar amino acid sequence and enzymic functions. Both molecules are involved in the metabolism of NAD(+), and the CD157 gene is synthenic on 4p15 with CD38, with which it also shares a unique genomic organization. Their conservation in phylogeny is striking evidence for their relevance in the life and death cycle of the cell.

CD157 undergoes ligand-independent dimerization and colocalizes with caveolin in CHO and MCA102 fibroblasts

CD157, a glycosylphosphatidylinositol (GPI)-anchored glycoprotein, has recently been shown to induce protein tyrosine phosphorylation in monocytes differentiated from HL-60 cells (mHL-60) in a ligand-dependent manner, but in a ligand-independent manner in stable CD157-transfected CHO (CHO/CD157) and MCA102 (MCA/CD157) fibroblasts [Cell Signal. 11 (1999) 891-897.]. Many GPI-anchored proteins need to be clustered by their ligands or antibodies to induce redistribution to caveolae and a concomitant activation of the associated signal-transducing proteins [Nature 387 (1997) 569-572.]. Here, we demonstrate that CD157, independent of antibody crosslinking, undergoes dimerization with disulfide bond formation and localization in caveolae in CHO/CD157 and MCA/CD157 fibroblasts. However, the native CD157 induced in mHL-60 cells remains a monomer form. The structural integrity of caveolae is required for the association of CD157 with caveolin and CD157-mediated tyrosine kinase signalling in the fibroblasts. We propose that an overexpression of CD157 could lead to its dimerization and relocation to caveolae and to further result in the initiation of signalling processes.