CD148, a membrane protein tyrosine phosphatase, is able to induce tyrosine phosphorylation on human lymphocytes

Regulation of Src family kinases involved in T cell receptor signaling by protein-tyrosine phosphatase CD148

CD148 is a receptor-like protein-tyrosine phosphatase known to inhibit transduction of mitogenic signals in non-hematopoietic cells. Similarly, in the hematopoietic lineage, CD148 inhibited signal transduction downstream of T cell receptor. However, it also augmented immunoreceptor signaling in B cells and macrophages via dephosphorylating C-terminal tyrosine of Src family kinases (SFK). Accordingly, endogenous CD148 compensated for the loss of the main SFK activator CD45 in murine B cells and macrophages but not in T cells. Hypothetical explanations for the difference between T cells and other leukocyte lineages include the inability of CD148 to dephosphorylate a specific set of SFKs involved in T cell activation or the lack of CD148 expression during critical stages of T cell development. Here we describe striking differences in CD148 expression between human and murine thymocyte subsets, the only unifying feature being the absence of CD148 during the positive selection when the major developmental block occurs under CD45 deficiency. Moreover, we demonstrate that similar to CD45, CD148 has both activating and inhibitory effects on the SFKs involved in TCR signaling. However, in the absence of CD45, activating effects prevail, resulting in functional complementation of CD45 deficiency in human T cell lines. Importantly, this is independent of the tyrosines in the CD148 C-terminal tail, contradicting the recently proposed phosphotyrosine displacement model as a mechanism of SFK activation by CD148. Collectively, our data suggest that differential effects of CD148 in T cells and other leukocyte subsets cannot be explained by the CD148 inability to activate T cell SFKs but rather by its dual inhibitory/activatory function and specific expression pattern.

Increased chemoattractant induced neutrophil oxidative burst, accelerated apoptosis, and dysregulated tyrosine phosphorylation associated with lifelong bacterial infections

A boy with lifelong recurrent bacterial infection at cutaneous and mucosal sites was investigated. PMN oxidative burst to phorbol myristate acetate (PMA) and zymosan was normal but was increased 20- to 50-fold upon C5a or formyl-met-leu-phe (fMLP) chemoattractant stimulation, accompanied by accelerated PMN apoptosis. His PMNs showed increased constitutive tyrosine phosphorylation of 21-, 25-, and 44-kDa proteins, and of src-family kinases (p59(hck), p58(fgr), and p53/56(lyn)). Phosphorylation was abnormally enhanced following fMLP stimulation. Expression and activity of the major PMN tyrosine phosphatases, i.e., CD45, CD148, and SHP-1 and -2, was normal. However, dephosphorylation of phospho-p58(fgr) and phospho-p53/56(lyn) by lysates of patient's PMNs was enhanced. Thus, another phosphatase may be overactive, perhaps dephosphorylating a regulatory (inhibitory) site on a protein tyrosine kinase, accounting for the abnormal PMN tyrosine phosphorylation and function. With age (now 13 years), T-cell lymphopenia and loss of T-cell responses developed. This appears to be a unique primary immunodeficiency with abnormal PMN oxidative and apoptotic responses to chemoattractants, dysregulated protein tyrosine phosphorylation, serious bacterial infection, and T-lymphocyte attrition.

The tyrosine phosphatase DEP-1 induces cytoskeletal rearrangements, aberrant cell-substratum interactions and a reduction in cell proliferation

The receptor protein tyrosine phosphatase density-enhanced phosphatase-1 (DEP-1) has been implicated in aberrant cancer cell growth and immune cell function, however, its function within cells has yet to be properly elucidated. To investigate the cellular function of DEP-1, stable cell lines inducibly expressing DEP-1 were generated. Induction of DEP-1 expression was found to decrease PDGF-stimulated tyrosine phosphorylation of a number of cellular proteins including the PDGF receptor, and to inhibit growth factor-stimulated phosphorylation of components of the MAPK pathway, indicating that DEP-1 antagonised PDGF receptor signalling. This was supported by data showing that DEP-1 expression resulted in a reduction in cell proliferation. DEP-1-expressing cells had fewer actin-containing microfilament bundles, reduced vinculin and paxillin-containing adhesion plaques, and were defective in interactions with fibronectin. Defective cell-substratum adhesion correlated with lack of activation of FAK in DEP-1-expressing cells. Time-lapse interference reflection microscopy of live cells revealed that although small focal contacts at the leading edge were generated in DEP-1-expressing cells, they failed to mature into stable focal adhesions, as found in control cells. Further motility analysis revealed that DEP-1-expressing cells retained limited random motility, but showed no chemotaxis towards a gradient of PDGF. In addition, cell-cell contacts were disrupted, with a change in the localisation of cadherin from discrete areas of cell-cell contact to large areas of membrane interaction, and there was a parallel redistribution of beta-catenin. These results demonstrate that DEP-1 is a negative regulator of cell proliferation, cell-substratum contacts, motility and chemotaxis in fibroblasts.

Evaluating function of transmembrane protein tyrosine phosphatase CD148 in lymphocyte biology

The transmembrane protein tyrosine phosphatase CD148 is expressed on numerous cell types, including most cells of the hematopoietic lineage. CD148 has been shown to regulate density-dependent inhibition of cell growth as well as cellular differentiation in nonhematopoietic cells and has been shown to regulate signal transduction processes in several nonlymphoid hematopoietic cell types. Analysis of CD148 expression on lymphoid cells has demonstrated that CD148 is expressed at low levels on T cells and that it is upregulated in response to activation. Several groups have observed that CD148 negatively regulates T cell activation in response to crosslinking of the T cell antigen receptor, suggesting that it may play a role in feedback inhibition of the T cell immune response. In the B cell compartment, CD 148 expression appears to be restricted to the memory subpopulation, raising the possibility that it serves a unique function in these cells, which has yet to be determined. Recent studies have shown that CD148 interacts with the PDZ domain-containing protein syntenin, raising the possibility that its function or its localization with substrates in T and B cells may be controlled through this or a related interaction with another PDZ domain protein.

Hepatocyte growth factor receptor tyrosine kinase met is a substrate of the receptor protein-tyrosine phosphatase DEP-1

The receptor protein-tyrosine phosphatase (PTP) DEP-1 (CD148/PTP-eta) has been implicated in the regulation of cell growth, differentiation, and transformation, and most recently has been identified as a potential tumor suppressor gene mutated in colon, lung, and breast cancers. We have generated constructs comprising the cytoplasmic segment of DEP-1 fused to the maltose-binding protein to identify potential substrates and thereby suggest a physiological function for DEP-1. We have shown that the substrate-trapping mutant form of DEP-1 interacted with a small subset of tyrosine-phosphorylated proteins from lysates of the human breast tumor cell lines MDA-MB-231, T-47D, and T-47D/Met and have identified the hepatocyte growth factor/scatter factor receptor Met, the adapter protein Gab1, and the junctional component p120 catenin as potential substrates. Following ligand stimulation, phosphorylation of specific tyrosyl residues in Met induces mitogenic, motogenic, and morphogenic responses. When co-expressed in 293 cells, the full-length substrate-trapping mutant form of DEP-1 formed a stable complex with the chimeric receptor colony stimulating factor 1 (CSF)-Met and wild type DEP-1 dephosphorylated CSF-Met. Furthermore, we observed that DEP-1 preferentially dephosphorylated a Gab1 binding site (Tyr(1349)) and a COOH-terminal tyrosine implicated in morphogenesis (Tyr(1365)), whereas tyrosine residues in the activation loop of Met (Tyr(1230), Tyr(1234), and Tyr(1235)) were not preferred targets of the PTP. The ability of DEP-1 preferentially to dephosphorylate particular tyrosine residues that are required for Met-induced signaling suggests that DEP-1 may function in controlling the specificity of signals induced by this PTK, rather than as a simple "off-switch" to counteract PTK activity.

Negative regulation of immunoreceptor signaling

Immune cells are activated as a result of productive interactions between ligands and various receptors known as immunoreceptors. These receptors function by recruiting cytoplasmic protein tyrosine kinases, which trigger a unique phosphorylation signal leading to cell activation. In the recent past, there has been increasing interest in elucidating the processes involved in the negative regulation of immunoreceptor-mediated signal transduction. Evidence is accumulating that immunoreceptor signaling is inhibited by complex and highly regulated mechanisms that involve receptors, protein tyrosine kinases, protein tyrosine phosphatases, lipid phosphatases, ubiquitin ligases, and inhibitory adaptor molecules. Genetic evidence indicates that this inhibitory machinery is crucial for normal immune cell homeostasis.

Expression of the membrane protein tyrosine phosphatase CD148 in human tissues

CD148, a receptor-like protein tyrosine phosphatase also known as HPTP-eta/DEP-1, is involved in signal transduction in leucocytes and is thought to contribute to mechanisms of cellular differentiation. We have investigated the in situ expression of CD148 in various fresh-frozen tissues by immunohistology and analyzed its expression on subpopulations of activated peripheral blood leucocytes by flow cytometry. In lymphoid organs, CD148 was found to be widely expressed on B and T cells, granulocytes, macrophages, certain dendritic cells as well as mature thymocytes. The cellular level of CD148 was increased after in vitro activation of peripheral blood leucocytes. Comparative analysis of tissue samples from normal gut and from patients with active Crohn's disease showed that leucocytes expressing CD148 are significantly upregulated in inflamed tissues and that a subset of these cells co-express the activation marker CD25. In non-lymphoid tissues, CD148 was found to be present on many epithelial cell types with glandular and/or endocrine differentiation as well as on fibrocytes, melanocytes and Schwann cells. CD148 expression was maintained also in malignant counterparts of such tissues. However, a marked loss of CD148 immunoreactivity was apparent in some of the investigated high-grade carcinomas. In summary, our results confirm a role of CD148 as a leucocyte activation marker. Among non-hematopoietic cells, CD148 is expressed by characteristic types of epithelial and non-epithelial cells. Downregulation of CD148 might promote dedifferentiation and autonomous growth of such cells in malignant tumors.

CD148, a new membrane tyrosine phosphatase involved in leukocyte function

Protein tyrosine phosphatases play an essential role in the control of leucocyte cell growth an differentiation. Recently a new receptor type membrane tyrosine phosphatase named CD148 has been identified. This molecule is present on the membrane of all the hematopoietic lineages as well as on several other cell types, mainly epithelial cells and its expression increases after cell activation. This molecule is able to act as a transducing molecule. Moreover, CD148 is able to modulate the signal transduction through the TCR/CD3 complex, in a manner similar to CD45. It has also been suggested that CD148 could be involved in mechanisms of differentiation and inhibition of cell growth. In addition, CD148 seems to be associated with a serine/threonine kinase in certain epithelial cell lines and leucocytes. Here, we review recent data on the expression and function of CD148 in both human, mouse and rat.

Endothelial localization of receptor tyrosine phosphatase, ECRTP/DEP-1, in developing and mature renal vasculature

Developmental assembly of the renal microvasculature requires spatially and temporally coordinated migration, assembly, differentiation, and maturation of endothelial cells in the context of adjacent epithelial and mesangial cells. In this study, endothelial expression and distribution of the receptor tyrosine phosphatase ECRTP/DEP-1 were evaluated during and after developmental assembly of the renal microvasculature. Monoclonal antibodies against ECRTP/DEP-1 ectodomain epitopes localize its expression to membrane surfaces of endothelial cells in glomerular, peritubular capillary, and arterial renal sites of mature human and murine kidney. During kidney development, ECRTP/DEP-1 immunostaining is evident on a subpopulation of metanephric mesenchymal cells and on putative progenitors of glomerular capillary endothelial cells early in their recruitment to developing glomeruli. ECRTP/DEP-1 is prominently displayed on luminal membrane surfaces with punctate accumulations at inter-endothelial contacts that overlap with vascular endothelial-cadherin staining. ECRTP/DEP-1 is recruited to inter-endothelial contacts in confluent cultured human renal and dermal microvascular endothelial cells, yet experimental dissociation of vascular endothelial-cadherin from endothelial junctional complexes fails to redistribute ECRTP/DEP-1. These findings indicate that ECRTP/DEP-1 is expressed in anticipation of glomerular capillary endothelial recruitment during development, and suggest that ECRTP/DEP-1 ectodomain interacts with endothelial surface ligands that are engaged by cell-cell contact.