Pervanadate activation of intracellular kinases leads to tyrosine phosphorylation and shedding of syndecan-1

ABL Kinases Modulate EZH2 Phosphorylation and Signaling in Metastatic Triple Negative Breast Cancer

Triple-negative breast cancer (TNBC) remains a leading cause of cancer associated deaths in women owing to its highly metastatic potential and limited treatment options. Recent studies have shown that expression of proteins associated with epigenetic regulation of gene expression are associated with metastatic relapse, however targeting epigenetic regulatory proteins has not resulted in effective therapies for TNBC in the clinic. The ABL tyrosine kinases promote metastasis of breast cancer cells in mouse models. However, a role of ABL kinases in the regulation of epigenetic processes in solid tumor metastasis remains unexplored. Here we show that inactivation of ABL kinases in bone metastatic TNBC cells led to a significant enrichment in gene signatures associated with the PRC2 protein complex, revealing a functional link between ABL kinases and the PRC2 complex. ABL inactivation promotes EZH2-T487 phosphorylation through the regulation of a FAK-CDK1 signaling axis. We find that phosphorylated EZH2 T487 or a phosphomimic EZH2 T487D mutant exhibit increased binding to non-canonical binding partners of EZH2 including c-MYC and ZMYND8. Notably, we identify a therapeutic vulnerability in TNBC cells whereby combination treatment with ABL allosteric inhibitors and EZH2 inhibitors elicits a synergistic decrease in TNBC cell survival in vitro, and impairs TNBC metastasis, prolonging survival of tumor-bearing mice treated with the combination therapy.

One Sentence Summary

ABL Kinases indirectly impact EZH2 catalytic activity by blocking a signaling cascade that leads to changes in the phosphorylation, protein interactions, and function of the PRC2 catalytic component EZH2 in TNBC.

Mapping proteolytic neo-N termini at the surface of living cells

N terminomics is a powerful strategy for profiling proteolytic neo-N termini, but its application to cell surface proteolysis has been limited by the low relative abundance of plasma membrane proteins. Here we apply plasma membrane-targeted subtiligase variants (subtiligase-TM) to efficiently and specifically capture cell surface N termini in live cells. Using this approach, we sequenced 807 cell surface N termini and quantified changes in their abundance in response to stimuli that induce proteolytic remodeling of the cell surface proteome. To facilitate exploration of our datasets, we developed a web-accessible Atlas of Subtiligase-Captured Extracellular N Termini (ASCENT; http://wellslab.org/ascent). This technology will facilitate greater understanding of extracellular protease biology and reveal neo-N termini biomarkers and targets in disease.

Glycocalyx is critical for blood-brain barrier integrity by suppressing caveolin1-dependent endothelial transcytosis following ischemic stroke

The breakdown of the blood‐brain barrier (BBB) is related to the occurrence and deterioration of neurological dysfunction in ischemic stroke, which leads to the extravasation of blood‐borne substances, resulting in vasogenic edema and increased mortality. However, a limited understanding of the molecular mechanisms that control the restrictive properties of the BBB hinders the manipulation of the BBB in disease and treatment. Here, we found that the glycocalyx (GCX) is a critical factor in the regulation of brain endothelial barrier integrity. First, endothelial GCX displayed a biphasic change pattern, of which the timescale matched well with the biphasic evolution of BBB permeability to tracers within the first week after t‐MCAO. Moreover, GCX destruction with hyaluronidase increased BBB permeability in healthy mice and aggravated BBB leakage in transient middle cerebral artery occlusion (t‐MCAO) mice. Surprisingly, ultrastructural observation showed that GCX destruction was accompanied by increased endothelial transcytosis at the ischemic BBB, while the tight junctions remained morphologically and functionally intact. Knockdown of caveolin1 (Cav1) suppressed endothelial transcytosis, leading to reduced BBB permeability, and brain edema. Lastly, a coimmunoprecipitation assay showed that GCX degradation enhanced the interaction between syndecan1 and Src by promoting the binding of phosphorylated syndecan1 to the Src SH2 domain, which led to rapid modulation of cytoskeletal proteins to promote caveolae‐mediated endocytosis. Overall, these findings demonstrate that the dynamic degradation and reconstruction of GCX may account for the biphasic changes in BBB permeability in ischemic stroke, and reveal an essential role of GCX in suppressing transcellular transport in brain endothelial cells to maintain BBB integrity. Targeting GCX may provide a novel strategy for managing BBB dysfunction and central nervous system drug delivery.

ADAM10 and ADAM17 proteases mediate proinflammatory cytokine-induced and constitutive cleavage of endomucin from the endothelial surface

Contact between inflammatory cells and endothelial cells (ECs) is a crucial step in vascular inflammation. Recently, we demonstrated that the cell-surface level of endomucin (EMCN), a heavily O-glycosylated single-transmembrane sialomucin, interferes with the interactions between inflammatory cells and ECs. We have also shown that, in response to an inflammatory stimulus, EMCN is cleared from the cell surface by an unknown mechanism. In this study, using adenovirus-mediated overexpression of a tagged EMCN in human umbilical vein ECs, we found that treatment with tumor necrosis factor α (TNF-α) or the strong oxidant pervanadate leads to loss of cell-surface EMCN and increases the levels of the C-terminal fragment of EMCN 3- to 4-fold. Furthermore, treatment with the broad-spectrum matrix metalloproteinase inhibitor batimastat (BB94) or inhibition of ADAM metallopeptidase domain 10 (ADAM10) and ADAM17 with two small-molecule inhibitors, GW280264X and GI254023X, or with siRNA significantly reduced basal and TNFα-induced cell-surface EMCN cleavage. Release of the C-terminal fragment of EMCN by TNF-α treatment was blocked by chemical inhibition of ADAM10 alone or in combination with ADAM17. These results indicate that cell-surface EMCN undergoes constitutive cleavage and that TNF-α treatment dramatically increases this cleavage, which is mediated predominantly by ADAM10 and ADAM17. As endothelial cell-surface EMCN attenuates leukocyte-EC interactions during inflammation, we propose that EMCN is a potential therapeutic target to manage vascular inflammation.

Effect of levamisole on expression of CD138 and interleukin-6 in human multiple myeloma cell lines

INTRODUCTION

Multiple myeloma (MM) is a B-cell malignancy accounting for 0.8% of all cancer deaths globally. This malignancy is characterized by lytic bone disease renal insufficiency, anemia, hypercalcemia, and immunodeficiency. The myeloma cells have enhanced expression of CD138. CD138 is a transmembrane heparin sulfate glycoprotein expressed on different types of adherent and nonadherent cells.CD138 is used as a standard marker for identification of tumor cells.

AIMS AND OBJECTIVES

Despite introduction of many therapeutic agents, the management of multiple myeloma (MM) remains a challenge and search for new therapeutic agents is in progress. In this study, we attempted to evaluate the effect of an alkaline phosphatase inhibitor, levamisole on expression of CD138, and level of interleukin-6 (IL-6) in human MM cell lines RPMI 8226 and U266 B1.

MATERIAL AND METHODS

U266B1 and RPMI 8226 cell lines were obtained from the National Centre for Cell Sciences, Pune. Alkaline phosphatase assay, Interleukin-6 assay and CD138 expression on myeloma cells by flow cytometry were investigated when the cells were exposed to Levamisole.

RESULTS

Levamisole-mediated growth inhibition of myeloma cells in vitro is associated with a loss of CD138 and increased IL-6 secretion. The increased secretion of IL-6 by myeloma cells could be an attempt to protect themselves from apoptosis.

CONCLUSION

Levamisole inhibited CD138 expression and affected the levels of IL-6 in a dose-dependent manner. The results of the present study add new dimension to levamisole's mode of action as inhibitor of CD138 and IL-6 and as an antiapoptotic agent.

Dually modified transmembrane proteoglycans in development and disease

Aberrant cell signaling in response to secreted growth factors has been linked to the development of multiple diseases, including cancer. As such, understanding mechanisms that control growth factor availability and receptor-growth factor interaction is vital. Dually modified transmembrane proteoglycans (DMTPs), which are classified as cell surface macromolecules composed of a core protein decorated with covalently linked heparan sulfated (HS) and/or chondroitin sulfated (CS) glycosaminoglycan (GAG) chains, provide one type of regulatory mechanism. Specifically, DMTPs betaglycan and syndecan-1 (SDC1) play crucial roles in modulating key cell signaling pathways, such as Wnt, transforming growth factor-β and fibroblast growth factor signaling, to affect epithelial cell biology and cancer progression. This review outlines current and potential functions for betaglycan and SDC1, with an emphasis on comparing individual roles for HS and CS modified DMTPs. We highlight the mutual dependence of DMTPs' GAG chains and core proteins and provide comprehensive knowledge on how these DMTPs, through regulation of ligand availability and receptor internalization, control cell signaling pathways involved in development and disease.

The Sign of Nuclear Magnetic Resonance Chemical Shift Difference as a Determinant of the Origin of Binding Selectivity: Elucidation of the Position Dependence of Phosphorylation in Ligands Binding to Scribble PDZ1

The use of nuclear magnetic resonance chemical shift perturbation to monitor changes taking place around the binding site of a ligand-protein interaction is a routine and widely applied methodology in the field of protein biochemistry. Shifts are often acquired by titrating various concentrations of ligand to a fixed concentration of the receptor and may serve the purpose, among others, of determining affinity constants, locating binding surfaces, or differentiating between binding mechanisms. Shifts are quantified by the so-called combined chemical shift difference. Although the directionality of shift changes is often used for detailed analysis of specific cases, the approach has not been adapted in standard chemical shift monitoring. This is surprising as it would not require additional effort. Here, we demonstrate the importance of the sign of the chemical shift difference induced by ligand-protein interaction. We analyze the sign of the ¹⁵N/¹H shift changes of the PDZ1 domain of Scribble upon interaction with two pairs of phosphorylated and unphosphorylated peptides. We find that detailed differences in the molecular basis of this PDZ-ligand interaction can be obtained from our analysis to which the classical method of combined chemical shift perturbation analysis is insensitive. In addition, we find a correlation between affinity and millisecond motions. Application of the methodology to Cyclophilin a, a cis-trans isomerase, reveals molecular details of peptide recognition. We consider our directionality vector chemical shift analysis as a method of choice when distinguishing the molecular origin of binding specificities of a class of similar ligands, which is often done in drug discovery.

Osteoprotegerin induces podosome disassembly in osteoclasts through calcium, ERK, and p38 MAPK signaling pathways

Osteoclasts are critical for bone resorption and use podosomes to attach to bone matrix. Osteoprotegerin (OPG) is a negative regulator of osteoclast function that can affect the formation and function of podosomes. However, the signaling pathways that link OPG to podosome function have not been well characterized. Therefore, this study examined the roles of intracellular calcium and MAPKs in OPG-induced podosome disassembly in osteoclasts. We assessed the effects of the intracellular calcium chelator Bapta-AM, ERK inhibitor U0126, and p38 inhibitor SB202190 on OPG-treated osteoclast differentiation, adhesion structures, intracellular free Ca(2+) concentration and the phosphorylation state of podosome associated proteins (Pyk2 and Src). Mouse monocytic RAW 264.7 cells were differentiated to osteoclasts using RANKL (30ng/mL) and M-CSF (25ng/mL). The cells were pretreated with Bapta-AM (5μM), U0126 (5μM), or SB202190 (10μM) for 30min, followed by 40ng/mL OPG for 3h. Osteoclastogenesis, adhesion structure, viability and morphology, intracellular free Ca(2+) concentration and the phosphorylation state of Pyk2 and Src were measured by TRAP staining, scanning electron microscopy, real-time cell analyzer, flow cytometry and western blotting, respectively. OPG significantly inhibited osteoclastogenesis, the formation of adhesion structures, and reduced the amount of phosphorylated Pyk2 and Src-pY527, but increased phosphorylation of Src-pY416. Bapta-AM, U0126, and SB202190 partially restored osteoclast differentiation and adhesion structures. Both Bapta-AM and U0126, but not SB202190, restored the levels of intracellular free Ca(2+) concentration, phosphorylated Pyk2 and Src-pY527. All three inhibitors blocked OPG-induced phosphorylation at Src-pY416. These results suggest OPG disrupts the attachment structures of osteoclasts and activates Src as an adaptor protein that competes for the reduced amount of phosphorylated Pyk2 through calcium- and ERK-dependent signaling pathways. p38 MAPK signaling may have a different role in OPG-induced osteoclast retraction. Our findings potentially offer novel insights into the signaling mechanisms downstream of OPG that affect osteoclast attachment to the extracellular matrix.

Shed Syndecan-1 is involved in chemotherapy resistance via the EGFR pathway in colorectal cancer

Background:

Syndecan-1 (Sdc-1) shedding induced by matrix metalloproteinase-7 (MMP-7) and additional proteases has an important role in cancer development. However, the impact of Sdc-1 shedding on chemotherapeutic resistance has not been reported.

Methods:

We examined Sdc-1 shedding in colorectal cancer by enzyme-linked immunosorbent assay (ELISA), Dot blot, reverse transcription-PCR (RT-PCR), immunohistochemistry and so on, its impact on chemotherapeutic sensitivity by collagen gel droplet embedded culture-drug sensitivity test (CD-DST) and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide), and potential mechanisms of action by Dot blot, western blot and immunofluorescence.

Results:

Sdc-1 shedding was increased in colorectal cancer patients, Sdc-1 serum levels in postoperative patients were lower than in preoperative patients, but still higher than those observed in healthy adults. Patients with high preoperative Sdc-1 serum levels were less responsive to 5-Fluorouracil, Oxaliplatin, Irintecan, Cisplatin or Paclitaxel chemotherapy. Moreover, the disease-free survival of patients with high preoperative Sdc-1 serum levels was significantly poorer. The possible mechanism of chemotherapy resistance in colorectal cancer can be attributed to Sdc-1 shedding, which enhances EGFR phosphorylation and downstream signalling.

Conclusions:

Shed Sdc-1 is involved in chemotherapy resistance via the EGFR pathway in colorectal cancer, and Sdc-1 serum levels could be a new prognostic marker in colorectal cancer.

Vanadate from air pollutant inhibits hrs-dependent endosome fusion and augments responsiveness to toll-like receptors

There is a well-established association between exposure to air pollutants and pulmonary injuries. For example, metals found in ROFA (residual oil fly ash) increase susceptibility of mice as well as humans to microbial infections. In our research, we have found that vanadate substantially increased the response of several Toll-like receptors (TLRs) to stimulation with their ligands. Although vanadate caused generation of reactive oxygen species (ROS), the addition of ROS scavenger N-acetyl cysteine (NAC) had no effect on augmented lipopolysaccharide (LPS) stimulation. We further showed that vanadate inhibits endosome fusion. This effect was determined by measuring the size of endosomes, NF-κB activity and TLR4 degradation in Hrs (hepatocyte growth factor-regulated tyrosine kinase substrate) overexpressed cells. Moreover, we identified the role of Hrs phosphorylation in these processes. Based on our findings, we can conclude that vanadate potentiates TLR4 activity by increasing Hrs phosphorylation status, reducing the size of Hrs/TLR4-positive endosomes and impacting TLR4 degradation, thus contributing to the detrimental effects of air pollutants on human health.

Chemotherapy stimulates syndecan-1 shedding: a potentially negative effect of treatment that may promote tumor relapse

In patients with multiple myeloma, the heparan sulfate proteoglycan syndecan-1 (CD138) is shed from the surface of tumor cells and accumulates in the serum and within the extracellular matrix of the bone marrow where it promotes tumor growth and metastasis. In the present study we discovered that commonly used anti-myeloma drugs stimulate syndecan-1 shedding both in vitro and in animals bearing myeloma tumors. Enhanced shedding is accompanied by increased syndecan-1 synthesis prior to drug induced tumor cell death. Addition of a caspase inhibitor blocks the drug-induced shedding of syndecan-1 in vitro indicating that shedding is linked to the onset of apoptosis. ADAM inhibitors or siRNA targeting ADAMs blocked drug-induced shedding suggesting that upregulation or activation of ADAMs is responsible for cleaving syndecan-1 from the tumor cell surface. These results reveal that myeloma chemotherapy stimulates synthesis and shedding of syndecan-1, a potentially negative side effect that may lead to the accumulation of high levels of syndecan-1 to establish a microenvironment that nurtures relapse and promotes tumor progression. Interestingly, we also found that chemotherapeutic drugs stimulated syndecan-1 shedding from pancreatic cancer cells as well, indicating that drug-induced shedding of syndecan-1 may occur in many cancer types. Overall, our results indicate that the use of metalloproteinase inhibitors (to inhibit syndecan-1 shedding) in combination with chemotherapy may represent a novel therapeutic strategy to prevent re-establishment of a microenvironment conducive for tumor relapse.

The heparanase/syndecan-1 axis in cancer: mechanisms and therapies

Heparanase is an endoglucuronidase that cleaves heparan sulfate chains of proteoglycans. In many malignancies, high heparanase expression and activity correlate with an aggressive tumour phenotype. A major consequence of heparanase action in cancer is a robust up-regulation of growth factor expression and increased shedding of syndecan-1 (a transmembrane heparan sulfate proteoglycan). Substantial evidence indicates that heparanase and syndecan-1 work together to drive growth factor signalling and regulate cell behaviours that enhance tumour growth, dissemination, angiogenesis and osteolysis. Preclinical and clinical studies have demonstrated that therapies targeting the heparanase/syndecan-1 axis hold promise for blocking the aggressive behaviour of cancer.

The structure of the Tiam1 PDZ domain/ phospho-syndecan1 complex reveals a ligand conformation that modulates protein dynamics

PDZ (PSD-95/Dlg/ZO-1) domains are protein-protein interaction modules often regulated by ligand phosphorylation. Here, we investigated the specificity, structure, and dynamics of Tiam1 PDZ domain/ligand interactions. We show that the PDZ domain specifically binds syndecan1 (SDC1), phosphorylated SDC1 (pSDC1), and SDC3 but not other syndecan isoforms. The crystal structure of the PDZ/SDC1 complex indicates that syndecan affinity is derived from amino acids beyond the four C-terminal residues. Remarkably, the crystal structure of the PDZ/pSDC1 complex reveals a binding pocket that accommodates the phosphoryl group. Methyl relaxation experiments of PDZ/SCD1 and PDZ/pSDC1 complexes reveal that PDZ-phosphoryl interactions dampen dynamic motions in a distal region of the PDZ domain by decoupling them from the ligand-binding site. Our data are consistent with a selection model by which specificity and phosphorylation regulate PDZ/syndecan interactions and signaling events. Importantly, our relaxation data demonstrate that PDZ/phospho-ligand interactions regulate protein dynamics and their coupling to distal sites.

Ahsg-fetuin blocks the metabolic arm of insulin action through its interaction with the 95-kD β-subunit of the insulin receptor

We previously have shown that Ahsg, a liver glycoprotein, inhibits insulin receptor (InsR) tyrosine kinase (TK) activity and the ERK1/2 mitogenic signaling arm of insulin signaling. Here we show that Ahsg blocks insulin-stimulated GLUT4 translocation and Akt activation in intact cells (mouse myoblasts). Furthermore, Ahsg inhibits InsR autophosphorylation of highly-purified insulin holoreceptors in a cell-free, ATP-dependent system, with an IC50 within the range of single-chain Ahsg concentrations in human serum. Binding of (125)I-insulin to living cells overexpressing the InsR shows a dissociation constant (KD) of 250pM, unaltered in the presence of 300 nM Ahsg. A mutant InsR cDNA encoding the signal peptide, the β-subunit and the furin processing site, but deleting the α-subunit, was stably expressed in HEK293 cells. Treatment with peroxovanadate, but not insulin, dramatically increased the 95 kD β-subunit tyrosine phosphoryation. The level of tyrosine phosphorylation of the 95-kD β-subunit can be driven down sharply by treatment of living HEK293 transfectant cells with physiological doses of Ahsg. Treatment of myogenic cells with Ahsg blunts insulin-stimulated InsR autophosphorylation and AKT phosphorylation. Taken together, we show that Ahsg antagonizes the metabolic functions initiated by InsR activation without interference in insulin binding. The experiments suggest a direct interaction of Ahsg with the InsR ectodomain β-subunit in a mode that does not significantly alter the high-affinity binding of insulin to the holoreceptor's two complementing α-subunits.

PTPIP51 in protein interactions: regulation and in situ interacting partners

This study investigated the regulation of 14-3-3β binding to PTPIP51 by the tyrosine phosphorylation status of PTPIP51. The tyrosine 176 residue is phosphorylated by c-Src. Up to now, nothing is known about the impact of such well-established phosphorylation events on the interaction profile of PTPIP51 with its partners of the mitogen-activated protein kinase (MAPK) pathway. In human keratinocytes the PTPIP51 phosphorylation was varied by inhibiting the phosphatase activity, thus enhancing the phosphorylation of PTPIP51. Differential blocking of Src kinase family members (despite c-Src) by PP2 increased the activity of c-Src and the tyrosine phosphorylation of PTPIP51 at position 176, which is the substrate of c-Src kinase. The amount of PTPIP51 interactions with 14-3-3β, Raf-1, PTP1B and c-Src was evaluated and the resulting data were compared to an untreated control group. The increased phosphorylation level resulted in a sharp drop of the 14-3-3β/PTPIP51 and 14-3-3β/Raf-1 interaction. Besides the 14-3-3 interaction of PTPIP51, the interaction with the two MAPK modulators, protein kinase A (PKA) and diacylglycerol kinase alpha (DAGKα), are also regulated by the tyrosine phosphorylation status of PTPIP51. Additional immunostaining experiments were done investigating the functional implication on these interactions of the phosphorylation in apoptotic processes. In the pervanadate- and PP2-treated HaCaT cells, higher amounts of apoptotic cells were not detected as compared to the control group. The presented data confirms a tyrosine phosphorylation-dependent interaction of PTPIP51 with 14-3-3β and Raf-1 in vivo and a tyrosine-dependent interaction profile with DAGKα and PKA. The non-interaction of PTPIP51 with 14-3-3 is not sufficient for triggering apoptosis.

Syndecan-1 - A new piece in B-cell puzzle

Syndecans are transmembrane proteoglycans, with core proteins mainly decorated with heparan sulfate chains. Syndecan-1 is expressed in a tissue-, cell-and differentiation-specific manner. Its extra-cellular domain can bind via HS chains to matrix elements, to growth factors (especially "heparin-binding" proteins) and to certain biological agents. The ectodomain released by proteolysis can also be functionally active. The cytoplasmic domain can take part in signaling processes as well as in modifying cell shape. In hematopoietic cells syndecan-1 is expressed in normal pre-B-cells and plasma cells, as well as in plasmocytoid and lymphoplasmocytoid malignancies. According to our study syndecan-1 is expressed in B-CLL cells both in tissue environment and in circulation.

Inhibition of PDGF-B induction and cell growth by syndecan-1 involves the ubiquitin and SUMO-1 ligase, Topors

Syndecans are receptors for soluble ligands, including heparin-binding growth factors, and matrix proteins. However, intracellular targets of syndecan-1 (Sdc-1)-mediated signaling are not fully understood. A yeast two-hybrid protein interaction screening of a mouse embryo library identified the ubiquitin and SUMO-1 E3 ligase, Topors, as a novel ligand of the Sdc-1 cytoplasmic domain (S1CD), a finding confirmed by ligand blotting and co-precipitation with Sdc-1 from cell lysates. Deletion mutagenesis identified an 18-amino acid sequence of Topors required for the interaction with the S1CD. By immunohistochemistry, Topors and Sdc-1 co-localized near the cell periphery in normal murine mammary gland (NMuMG) cells in vitro and in mouse embryonic epithelia in vivo. Finally, siRNA-mediated knockdown of Topors demonstrated that Topors is a growth promoter for murine arterial smooth muscle cells and is required for the inhibitory effect of Sdc-1 on cell growth and platelet-derived growth factor-B induction. These data suggest a novel mechanism for the inhibitory effects of Sdc-1 on cell growth that involves the interaction between the cytoplasmic domain of Sdc-1 and the SUMO-1 E3 ligase, Topors.

Heparan sulfate chains of syndecan-1 regulate ectodomain shedding

Matrix metalloproteinases release intact syndecan-1 ectodomains from the cell surface giving rise to a soluble, shed form of the proteoglycan. Although it is known that shed syndecan-1 controls diverse pathophysiological responses in cancer, wound healing, inflammation, infection, and immunity, the mechanisms regulating shedding remain unclear. We have discovered that the heparan sulfate chains present on syndecan core proteins suppress shedding of the proteoglycan. Syndecan shedding is dramatically enhanced when the heparan sulfate chains are enzymatically degraded or absent from the core protein. Exogenous heparan sulfate or heparin does not inhibit shedding, indicating that heparan sulfate must be attached to the core protein to suppress shedding. Regulation of shedding by heparan sulfate occurs in multiple cell types, for both syndecan-1 and syndecan-4 and in murine and human syndecans. Mechanistically, the loss of heparan sulfate enhances the susceptibility of the core protein to proteolytic cleavage by matrix metalloproteinases. Enhanced shedding of syndecan-1 following loss of heparan sulfate is accompanied by a dramatic increase in core protein synthesis. This suggests that in response to an increase in the rate of shedding, cells attempt to maintain a significant level of syndecan-1 on the cell surface. Together these data indicate that the amount of heparan sulfate present on syndecan core proteins regulates both the rate of syndecan shedding and core protein synthesis. These findings assign new functions to heparan sulfate chains, thereby broadening our understanding of their physiological importance and implying that therapeutic inhibition of heparan sulfate degradation could impact the progression of some diseases.

Large-scale phosphotyrosine proteomic profiling of rat renal collecting duct epithelium reveals predominance of proteins involved in cell polarity determination

Although extensive phosphoproteomic information is available for renal epithelial cells, previous emphasis has been on phosphorylation of serines and threonines with little focus on tyrosine phosphorylation. Here we have carried out large-scale identification of phosphotyrosine sites in pervanadate-treated native inner medullary collecting ducts of rat, with a view towards identification of physiological processes in epithelial cells that are potentially regulated by tyrosine phosphorylation. The method combined antibody-based affinity purification of tyrosine phosphorylated peptides coupled with immobilized metal ion chromatography to enrich tyrosine phosphopeptides, which were identified by LC-MS/MS. A total of 418 unique tyrosine phosphorylation sites in 273 proteins were identified. A large fraction of these sites have not been previously reported on standard phosphoproteomic databases. All results are accessible via an online database: http://helixweb.nih.gov/ESBL/Database/iPY/. Analysis of surrounding sequences revealed four overrepresented motifs: [D/E]xxY*, Y*xxP, DY*, and Y*E, where the asterisk symbol indicates the site of phosphorylation. These motifs plus contextual information, integrated using the NetworKIN tool, suggest that the protein tyrosine kinases involved include members of the insulin- and ephrin-receptor kinase families. Analysis of the gene ontology (GO) terms and KEGG pathways whose protein elements are overrepresented in our data set point to structures involved in epithelial cell-cell and cell-matrix interactions ("adherens junction," "tight junction," and "focal adhesion") and to components of the actin cytoskeleton as major sites of tyrosine phosphorylation in these cells. In general, these findings mesh well with evidence that tyrosine phosphorylation plays a key role in epithelial polarity determination.

Activated Syndecan-1 shedding contributes to mice colitis induced by dextran sulfate sodium

BACKGROUND

Syndecan-1(Sdc1) plays important roles in many steps of inflammatory responses. In ulcerative colitis patients, decreased Sdc1 expression was observed and Sdc1 analogue heparin could improve the disease course. A better understanding of how Sdc1 functions in colitis will benefit the disease intervention.

AIMS

To evaluate the role of Sdc1 in dextran sulfate sodium (DSS)-induced colitis.

METHODS

BALB/c mice were grouped randomly into control, DSS, and heparin+DSS. The DSS group was given 4% DSS orally and heparin+DSS group was given 4% DSS with heparin (enoxaparin) subcutaneously, while the control was given distilled water orally. All mice were killed at day 7. Disease activities, histopathological changes, membrane-bound and free Sdc1 level and mRNA expression of Sdc1, IL-1, and IL-10 in colon mucosa were detected.

RESULTS

Significant colitis was observed in the DSS group, but disease activity index and histological score showed significant lower in the heparin+DSS group than those in the DSS group. Compared to the control group, decreased Sdc1 protein expression was detected in colon mucosa of DSS-induced colitis while Sdc1 ectodomain level in serum was much higher. Inhibited Sdc1 ectodomain shedding was detected in the heparin+DSS group compared to the DSS group. RT-PCR demonstrated that both IL-1 and IL-10 expression were up-regulated in DSS-induced colitis while heparin lessened the up-regulation extent.

CONCLUSIONS

Sdc1 shedding is activated in DSS-induced colitis and heparin, which mimics Sdc1 functions, relieves colitis severity by inhibiting Sdc1 shedding and down-regulating cytokines expression.

Molecular and cellular mechanisms of ectodomain shedding

The extracellular domain of several membrane-anchored proteins is released from the cell surface as soluble proteins through a regulated proteolytic mechanism called ectodomain shedding. Cells use ectodomain shedding to actively regulate the expression and function of surface molecules, and modulate a wide variety of cellular and physiological processes. Ectodomain shedding rapidly converts membrane-associated proteins into soluble effectors and, at the same time, rapidly reduces the level of cell surface expression. For some proteins, ectodomain shedding is also a prerequisite for intramembrane proteolysis, which liberates the cytoplasmic domain of the affected molecule and associated signaling factors to regulate transcription. Ectodomain shedding is a process that is highly regulated by specific agonists, antagonists, and intracellular signaling pathways. Moreover, only about 2% of cell surface proteins are released from the surface by ectodomain shedding, indicating that cells selectively shed their protein ectodomains. This review will describe the molecular and cellular mechanisms of ectodomain shedding, and discuss its major functions in lung development and disease.

Hepatic heparan sulfate proteoglycans and endocytic clearance of triglyceride-rich lipoproteins

Hypertriglyceridemia, characterized by the accumulation of triglyceride-rich lipoproteins in the blood, affects 10-20% of the population in western countries and increases the risk of atherosclerosis, coronary artery disease, and pancreatitis. The etiology of hypertriglyceridemia is complex, and much interest exists in identifying and characterizing the biological and environmental factors that affect the synthesis and turnover of plasma triglycerides. Genetic studies in mice have recently identified that heparan sulfate proteoglycans are a class of receptors that mediate the clearance of triglyceride-rich lipoproteins in the liver. Heparan sulfate proteoglycans are expressed by endothelial cells that line the hepatic sinusoids and the underlying hepatocytes, and are present in the perisinusoidal space (space of Disse). This chapter discusses the dependence of lipoprotein binding on heparan sulfate structure and the identification of hepatocyte syndecan-1 as the primary proteoglycan that mediates triglyceride-rich lipoprotein clearance.

Tyrosine dephosphorylation of the syndecan-1 PDZ binding domain regulates syntenin-1 recruitment

Heparan sulfate proteoglycan receptor syndecan-1 interacts with the carboxyl-terminal LG4/5 domain in laminin 332 (alpha3LG4/5) and participates in cell adhesion and spreading. To dissect the function of syndecan-1 in these processes, we made use of a cell adhesion model in which syndecan-1 exclusively interacts with a recombinantly expressed alpha3LG4/5 fragment. Plating HT1080 cells on this fragment induces the formation of actin-containing protrusive structures in an integrin-independent manner. Here we show that syndecan-1-mediated formation of membrane protrusions requires dephosphorylation of tyrosine residues in syndecan-1. Accordingly, inhibition of phosphatases with orthovanadate decreases cell adhesion to the alpha3LG4/5 fragment. We demonstrate that the PDZ-containing protein syntenin-1, known to connect cytoskeletal proteins, binds to syndecan-1 in cells plated on the alpha3LG4/5 fragment and participates in the formation of membrane protrusions. We further show that syntenin-1 recruitment depends on the dephosphorylation of Tyr-309 located within syndecan-1 PDZ binding domain EFYA. We propose that tyrosine dephosphorylation of syndecan-1 may regulate its association with cytoskeleton components.

Syndecan-1 ectodomain shedding is regulated by the small GTPase Rab5

The ectodomain shedding of syndecan-1, a major cell surface heparan sulfate proteoglycan, modulates molecular and cellular processes central to the pathogenesis of inflammatory diseases. Syndecan-1 shedding is a highly regulated process in which outside-in signaling accelerates the proteolytic cleavage of syndecan-1 ectodomains at the cell surface. Several extracellular agonists that induce syndecan-1 shedding and metalloproteinases that cleave syndecan-1 ectodomains have been identified, but the intracellular mechanisms that regulate syndecan-1 shedding are largely unknown. Here we examined the role of the syndecan-1 cytoplasmic domain in the regulation of agonist-induced syndecan-1 shedding. Our results showed that the syndecan-1 cytoplasmic domain is essential because mutation of invariant cytoplasmic Tyr residues abrogates ectodomain shedding, but not because it is Tyr phosphorylated upon shedding stimulation. Instead, our data showed that the syndecan-1 cytoplasmic domain binds to Rab5, a small GTPase that regulates intracellular trafficking and signaling events, and this interaction controls the onset of syndecan-1 shedding. Syndecan-1 cytoplasmic domain bound specifically to Rab5 and preferentially to inactive GDP-Rab5 over active GTP-Rab5, and shedding stimulation induced the dissociation of Rab5 from the syndecan-1 cytoplasmic domain. Moreover, the expression of dominant-negative Rab5, unable to exchange GDP for GTP, interfered with the agonist-induced dissociation of Rab5 from the syndecan-1 cytoplasmic domain and significantly inhibited syndecan-1 shedding induced by several distinct agonists. Based on these data, we propose that Rab5 is a critical regulator of syndecan-1 shedding that serves as an on-off molecular switch through its alternation between the GDP-bound and GTP-bound forms.

Pervanadate-induced shedding of the intercellular adhesion molecule (ICAM)-1 ectodomain is mediated by membrane type-1 matrix metalloproteinase (MT1-MMP)

In several vascular diseases, the ectodomain of intercellular adhesion molecule (ICAM)-1 is shed by the proteolytic activity of a zinc-dependent endopeptidase, releasing a soluble form of the protein (sICAM-1), a common marker for inflammatory diseases. Since reactive oxygen species (ROS) generated during prolonged inflammation are known to induce shedding or cleavage of several transmembrane proteins, we sought to explore the cleavage and enzymatic effects that the pervanadate, via oxidation and subsequent inactivation of protein tyrosine phosphatase, has on ICAM-1 cleavage. In these studies, we used endothelial cells (ECs) and 293 human embryonic kidney (HEK) cells expressing high-levels of surface ICAM-1. In addition, use of specific tissue inhibitors of metalloproteinases (TIMPs), small interfering (si)RNA designed to knockdown endopeptidase activity, and an immunocolocalization assay were employed to determine the identity of a specific metalloproteinase mediating pervanadate-induced sICAM-1 shedding. Our data indicate that membrane type-1 matrix metalloproteinase (MT1-MMP) is involved in pervanadate-mediated shedding of the sICAM-1 ectodomain in both cell types. Immunostaining and confocal microscopy provide visual evidence that ICAM-1 and MT1-MMP colocalize at the cellular surface following pervanadate treatment, further implicating the involvement of MT1-MMP activity in this mode of ICAM-1 shedding.

Shedding of a soluble form of BMP receptor-IB controls bone cell responses to BMP

Bone morphogenetic proteins (BMP) are members of the transforming growth factor beta (TGF-beta) superfamily and are involved in a wide variety of biological processes, including osteoblast differentiation and bone healing. The activities of the BMP are mediated by signal transduction via three BMP receptors (BMPR-IA, -IB and -II), which are thus essential for the biological actions of the BMP. Although the precise mechanisms which control the BMPR are not yet known, it is possible that post-translational regulation of these cell surface antigens by shedding could modulate their expression and thereby at least partly determine the response of the cells to the BMP. To test this possibility, the present study has examined whether soluble forms of the BMPR are produced by shedding from primary human bone cells in vitro. The results showed that human bone cells expressed both mRNA transcripts and antigens corresponding to BMPR-IA, -IB and -II. Incubation of the cells with phorbol 12-myristate 13-acetate (PMA), a potent inducer of proteolytic shedding, resulted in a pronounced decrease in cell surface expression of all three BMPR and, concurrently, the presence of "soluble" forms of these antigens in culture supernatants. Moreover, PMA treatment significantly reduced the level of BMP-2-induced Smad1/5 phosphorylation, a major early activation step in signal transduction initiated by BMP/BMPR interaction. It is notable that, while treatment of bone cells with interleukin-1beta (IL-1beta) also reduced the level of surface BMPR-IB, this inflammatory cytokine had no effect on BMPR-IA or -II levels, hence only the soluble form of BMPR-IB was detected. Furthermore, in addition to down-regulating BMP-2-induced Smad1/5 phosphorylation, IL-1beta also caused a reduction in the level of BMP-2-induced alkaline phosphatase activity and osteocalcin expression, both closely associated with bone cell differentiation. In conclusion, our study has provided evidence, for the first time, that BMPR can be modulated at the cell surface by the shedding of a soluble form of the antigen, resulting in a markedly diminished response to BMP-2 in vitro.

Mechanical strain regulates syndecan-4 expression and shedding in smooth muscle cells through differential activation of MAP kinase signaling pathways

Syndecan-4 (S4) belongs to a family of transmembrane proteoglycans, acts as a coreceptor for growth factor binding as well as cell-matrix and cell-cell interactions, and is induced in neointimal smooth muscle cells (SMCs) after balloon catheter injury. We investigated S4 expression in SMCs in response to several force profiles and the role of MAP kinase signaling pathways in regulating these responses. S4 mRNA expression increased in response to 5% and 10% cyclic strain (4 h: 200 +/- 34% and 182 +/- 17%, respectively; P < 0.05) before returning to basal levels by 24 h. Notably, the SMC mechanosensor mechanism was reset after an initial 24-h "preconditioning" period, as evident by an increase in S4 gene expression following a change in cyclic stress from 10% to 20% (28 h: 181 +/- 1%; P < 0.05). Mechanical stress induced a late decrease in cell-associated S4 protein levels (24 h: 70 +/- 6%; P < 0.05), with an associated increase in S4 shedding (24 h: 537 +/- 109%; P < 0.05). To examine the role of MAP kinases, cells were treated with U-0126 (ERK1/2 inhibitor), SB-203580 (p38 inhibitor), or JNKI I (JNK/SAPK inhibitor). Late reduction in cell-associated S4 levels was attributed to ERK1/2 and p38 signaling. In contrast, accelerated S4 shedding required both ERK1/2 (5-fold reduction in accelerated shedding; P < 0.05) and JNK/SAPK (4-fold reduction; P < 0.05) signaling. Given the varied functions of S4, stress-induced effects on SMC S4 expression and shedding may represent an additional component of the proinflammatory, growth-stimulating pathways that are activated in response to changes in the mechanical microenvironment of the vascular wall.

Syndecan-1 regulates alphavbeta5 integrin activity in B82L fibroblasts

B82L mouse fibroblasts respond to fibronectin or vitronectin via a syndecan-1-mediated activation of the alphavbeta5 integrin. Cells attached to syndecan-1-specific antibody display only filopodial extension. However, the syndecan-anchored cells extend lamellipodia when the antibody-substratum is supplemented with serum, or low concentrations of adsorbed vitronectin or fibronectin, that are not sufficient to activate the integrin when plated alone. Integrin activation is blocked by treatment with (Arg-Gly-Asp)-containing peptides and function-blocking antibodies that target alphav integrins, as well as by siRNA-mediated silencing of beta5 integrin expression. In addition, alphavbeta5-mediated cell attachment and spreading on high concentrations of vitronectin is blocked by competition with recombinant syndecan-1 ectodomain core protein and by downregulation of mouse syndecan-1 expression by mouse-specific siRNA. Taking advantage of the species-specificity of the siRNA, rescue experiments in which human syndecan-1 constructs are expressed trace the activation site to the syndecan-1 ectodomain. Moreover, both full-length mouse and human syndecan-1 co-immunoprecipitate with the beta5 integrin subunit, but fail to do so if the syndecan is displaced by competition with soluble, recombinant syndecan-1 ectodomain. These results suggest that the ectodomain of the syndecan-1 core protein contains an active site that assembles into a complex with the alphavbeta5 integrin and regulates alphavbeta5 integrin activity.

Bone morphogenetic protein receptors and bone morphogenetic protein signaling are controlled by tumor necrosis factor-α in human bone cells

Bone morphogenetic proteins (BMP) stimulate osteoblast differentiation by signal transduction via three BMP receptors (BMPR-IA, -IB and -II), whereas the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) has been shown to suppress osteoblast differentiation. Although the mechanisms which regulate the BMPR are not yet known, it is possible that they may be negatively controlled by TNF-alpha, thereby inhibiting BMP-induced osteoblast differentiation. To test this hypothesis, we have examined the effects of TNF-alpha on BMPR-IA, -IB and -II expression and the functional consequences of this cytokine on BMPR-mediated functions in human bone cells. The results showed that although TNF-alpha down-regulated BMPR-IA and -II transcripts, it increased the level of BMPR-IB mRNA via a MAPK-dependent pathway. In marked contrast, however, TNF-alpha nevertheless caused marked down-regulation of the expression of the BMPR-IB surface antigen specifically. Moreover, the cytokine-induced decrease in BMPR-IB expression was found to be associated with the concurrent presence of a 'soluble' form of this antigen in supernatants of TNF-alpha-treated cultures. Furthermore, the TNF-alpha-induced loss of BMPR-IB was found to ablate BMP-2-stimulated bone cell functions, including phosphorylation of Smad1/5/8, alkaline phosphatase activity and osteocalcin expression. In conclusion, our study has provided evidence, for the first time, that BMPR can be differentially modulated by TNF-alpha at both the post-transcriptional and post-translational levels, with the TNF-alpha-induced shedding of the BMPR-IB antigen associated with a significantly diminished response to BMP-2 in vitro.

Constitutive and accelerated shedding of murine syndecan-1 is mediated by cleavage of its core protein at a specific juxtamembrane site

Syndecan-1 is a developmentally regulated cell surface heparan sulfate proteoglycan (HSPG). It functions as a coreceptor for a variety of soluble and insoluble ligands and is implicated in several biological processes, including differentiation, cell migration, morphogenesis, and recently feeding behavior. The extracellular domain of syndecan-1 is proteolytically cleaved at a juxtamembrane site by tissue inhibitor of metalloprotease-3 (TIMP-3)-sensitive metalloproteinases in response to a variety of physiological stimulators and stress in a process known as shedding. Shedding converts syndecan-1 from a membrane-bound coreceptor into a soluble effector capable of binding the same ligands. We found that replacing syndecan-1 juxtamembrane amino acid residues A243-S-Q-S-L247 with human CD4 amino acid residues can completely block PMA-induced syndecan-1 ectodomain shedding. Furthermore, using liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS), we identified the proteolytic cleavage site of syndecan-1 as amino acids A243 and S244, generated by constitutive and PMA-induced shedding from murine NMuMG cells. Finally, we show that basal cleavage of syndecan-1 utilizes the same in vivo site as the in vitro site. Indeed, as predicted, transgenic mice expressing the syndecan-1/CD4 cDNA do not shed the syndecan-1 ectodomain in vivo. These results suggest that the same cleavage site is utilized for basal syndecan-1 ectodomain shedding both in vitro from NMuMG and CHO cells and in vivo.

Syndecan-4 is a signaling molecule for stromal cell-derived factor-1 (SDF-1)/ CXCL12

Stromal cell-derived factor-1 (SDF-1)/CXCL12, the ligand for CXCR4, induces signal transduction. We previously showed that CXCL12 binds to high- and low-affinity sites expressed by primary cells and cell lines, and forms complexes with CXCR4 as expected and also with a proteoglycan, syndecan-4, but does not form complexes with syndecan-1, syndecan-2, CD44 or beta-glycan. We also demonstrated the occurrence of a CXCL12-independent heteromeric complex between CXCR4 and syndecan-4. However, our data ruled out the glycosaminoglycan-dependent binding of CXCL12 to HeLa cells facilitating the binding of this chemokine to CXCR4. Here, we demonstrate that CXCL12 directly binds to syndecan-4 in a glycosaminoglycan-dependent manner. We show that upon stimulation of HeLa cells by CXCL12, CXCR4 becomes tyrosine phosphorylated as expected, while syndecan-4 (but not syndecan-1, syndecan-2 or beta-glycan) also undergoes such tyrosine phosphorylation. Moreover, tyrosine-phosphorylated syndecan-4 from CXCL12-stimulated HeLa cells physically coassociates with tyrosine phosphorylated CXCR4. Pretreatment of the cells with heparitinases I and III prevented the tyrosine phosphorylation of syndecan-4, which suggests that the heparan sulfate-dependent binding of SDF-1 to this proteoglycan is involved. Finally, by reducing syndecan-4 expression using RNA interference or by pretreating the cells with heparitinase I and III mixture, we suggest the involvement of syndecan-4 and heparan sulfate in p44/p42 mitogen-activated protein kinase and Jun N-terminal/stress-activated protein kinase activation by action of CXCL12 on HeLa cells. However, these treatments did not modify the calcium mobilization induced by CXCL12 in these cells. Therefore, syndecan-4 behaves as a CXCL12 receptor, selectively involved in some transduction pathways induced by SDF-1, and heparan sulfate plays a role in these events.

Role of Tyrosine Phosphorylation in the Regulation of Cleavage Secretion of Angiotensin-converting Enzyme

Both germinal (gACE) and somatic (sACE) isozymes of angiotensin-converting enzyme (ACE) are type I ectoproteins whose enzymatically active ectodomains are cleaved and shed by a membrane-bound protease. Here, we report a role of protein tyrosine phosphorylation in regulating this process. Strong enhancements of ACE cleavage secretion was observed upon enhancing protein Tyr phosphorylation by treating gACE- or sACE-expressing cells with pervanadate, an inhibitor of protein Tyr phosphatases. Secreted gACE, cell-bound mature gACE and its precursors were all Tyr-phosphorylated, as was the endoplasmic reticulum protein, immunoglobulin heavy chain-binding protein, that co-immunoprecipitated with ACE. The enhancement of cleavage secretion by pervanadate did not require the presence of the cytoplasmic domain of ACE, and it was not accomplished by enhancing the rate of intracellular processing of the protein. The observed enhancement of cleavage secretion of ACE in pervanadate-treated cells was specifically blocked by an inhibitor of the p38 mitogen-activated protein (MAP) kinase but not by inhibitors of many other Ser/Thr and Tyr protein kinases, including a specific inhibitor of protein kinase C that, however, could block the enhancement of cleavage secretion elicited by phorbol ester. These results indicate that ACE Tyr phosphorylation, probably in the endoplasmic reticulum, enhances the rate of its cleavage secretion at the plasma membrane using a regulatory pathway that may involve p38 MAP kinase.

Removal of heparan sulfate by heparinase treatment inhibits FGF-2-dependent smooth muscle cell proliferation in injured rat carotid arteries

Smooth muscle cells (SMC) of the rat carotid arterial media proliferate and migrate in response to injury during the formation of a neointima. The interaction of fibroblast growth factor (FGF-2), which is released at the site of injury, with heparan sulfate proteoglycans (HSPGs) is necessary to induce signaling, which elicits an FGF-dependent mitogenic response by arterial smooth muscle cells, and also serves as a mechanism for storage of the growth factor within the extracellular matrix. However, whether these interactions are critical during neointimal formation has not been directly tested. In this study, a model of FGF-2-dependent medial SMC mitogenic response in balloon-injured rat carotid artery was used to test the effect of degradation of vessel wall heparan sulfate on subsequent SMC proliferation. Treatment of balloon-catheterized rat carotid arteries with chondroitin ABC lyase and/or heparin lyases eliminated heparan sulfates in the vessel wall, as determined by immunoperoxidase staining. In contrast, the distribution in the carotid vessel wall of the large core protein of perlecan, a major vessel wall HSPG that binds FGF-2, is not decreased. The effect of glycosaminoglycan digestion in situ on medial SMC proliferation in response to a bolus injection of FGF-2 after injury was determined by measuring the percentage of SMC nuclei that incorporated 5-bromo-2'-deoxyuridine (BrdU) 48 h after injury. Enzymatic removal of heparan sulfate reduced BrdU incorporation into medial SMC by 60-70% (P < 0.001) at 48 h after injury. Moreover, pre-incubation of FGF-2 with heparin prior to injection restored SMC replication to the levels present in injured vessels treated with buffer alone (P < 0.01). These experiments indicate that endogenous HSPGs are essential to promote FGF-2-driven medial SMC proliferation following injury, and that heparinase treatment can abrogate FGF-2-dependent responses in vivo.

Heparan sulfate core proteins in cell-cell signaling

Heparan sulfate (HS) binds numerous extracellular ligands, including cell-cell signaling molecules and their signal-transducing receptors. Ligand binding sites in HS have specific sulfation patterns; and several observations suggest that the HS sulfation pattern is the same for every HS chain that a cell synthesizes, regardless of the core protein to which it is attached. Nonetheless, virtually every Drosophila, zebrafish, Xenopus, and mouse that lacks a specific HS core protein has a mutant phenotype, even though other HS core proteins are expressed in the affected cells. Genetic manipulation of HS core protein genes is beginning to indicate that HS core proteins have functional specificities that are required during distinct stages of development.

Signals mediating cleavage of intercellular adhesion molecule-1

ICAM-1, a membrane-bound receptor, is released as soluble ICAM-1 in inflammatory diseases. To delineate mechanisms regulating ICAM-1 cleavage, studies were performed in endothelial cells (EC), human embryonic kidney (HEK)-293 cells transfected with wild-type (WT) ICAM-1, and ICAM-1 containing single tyrosine-to-alanine substitutions (Y474A, Y476A, and Y485A) in the cytoplasmic region. Tyrosine residues at 474 and 485 become phosphorylated upon ICAM-1 ligation and associate with signaling modules. Cleavage was assessed by using an antibody against the cytoplasmic tail of ICAM-1, which recognizes intact ICAM-1 and the 7-kDa membrane-bound fragment remaining after cleavage. Cleavage in HEK-293 WT cells was accelerated by phorbol ester PMA, whereas in EC it was induced by tumor necrosis factor-alpha. In both cell types, a 7-kDa ICAM-1 remnant was detected. Tyrosine phosphatase inhibitors dephostatin and sodium orthovanadate augmented cleavage. PD-98059 (MEK kinase inhibitor), geldanamycin and PP2 (Src kinase inhibitors), and wortmannin (phosphatidylinositol 3-kinase inhibitor) dose-dependently inhibited cleavage in both cell types. SB-203580 (p38 inhibitor) was more effective in EC, and D609 (PLC inhibitor) mostly affected cleavage in HEK-293 cells. Cleavage was drastically decreased in Y474A and Y485A, whereas it was marginally reduced in Y476A. Surprisingly, phosphorylation was not detectable on the 7-kDa fragment of ICAM-1. These results implicate distinct pathways in the cleavage process and suggest a preferred signal transmission route for ICAM-1 shedding in the two cell systems tested. Tyrosine residues Y474 and Y485 within the cytoplasmic sequence of ICAM-1 regulate the cleavage process.

Cellular activity and signaling induced by osteoprotegerin in osteoclasts: involvement of receptor activator of nuclear factor κB ligand and MAPK

Osteoprotegerin (OPG) is a decoy receptor for receptor activator of nuclear factor kappaB ligand (RANKL), an inducer of osteoclastogenesis via its receptor RANK. We recently demonstrated that OPG also exerts a direct effect in osteoclasts by regulating protease expression. Herein, we showed that OPG-induced pro-matrix metalloproteinase-9 activity was abolished by ras/MAPK inhibitors in purified osteoclasts. OPG induced the phosphorylation of p38 and ERK1/2 in RAW264.7 cells. Only p38 activation was totally abolished by a blocking anti-RANKL antibody or an excess of RANKL. Surface plasmon resonance experiments revealed that RANK, RANKL and OPG are able to form a tertiary complex. These results suggested a potential formation of a tertiary complex RANK-RANKL-OPG on osteoclasts. Thus, OPG is not only a soluble decoy receptor for RANKL but must be also considered as a direct effector of osteoclast functions.

The shedding of betaglycan is regulated by pervanadate and mediated by membrane type matrix metalloprotease-1

Betaglycan is a membrane-anchored proteoglycan that binds transforming growth factor-beta (TGF-beta) via its core protein. A soluble form of betaglycan can be released by proteolytic cleavage (also known as shedding) of the membrane-bound form, yielding soluble betaglycan. The mechanism leading to the generation of soluble betaglycan is poorly understood. Because the membrane and soluble forms of betaglycan have opposite effects regulating the availability of TGF-beta, it is important to characterize the shedding of betaglycan further. Here we present evidence showing that in certain cell types, pervanadate, a general tyrosine phosphatase inhibitor, induces the release of the previously described fragment that encompasses almost the entire extracellular domain of betaglycan (sBG-120). In addition, treatment with pervanadate unveils the existence of a novel 90-kDa fragment (sBG-90). Noticeably, the cleavage that generates sBG-90 is mediated by a tissue inhibitor of metalloprotease-2-sensitive protease. Overexpression of all membrane type matrix metalloproteases (MT-MMPs) described to date indicates that MT1-MMP and MT3-MMP are endowed with ability to generate sBG-90. Furthermore, the patterns of expression of different MT-MMPs in the cell lines used in this study suggest that MT1-MMP is the protease involved in the shedding of sBG-90. Overexpression of MT1-MMP in COS-1 cells, which do not express detectable levels of this metalloprotease, confirms the feasibility of this hypothesis. Unexpectedly, during the course of these experiments, we observed that MT2-MMP decreases the levels of MT1-MMP and betaglycan. Finally, binding competition experiments indicate that, similar to the wild type membrane betaglycan, sBG-90 binds the TGF-beta2 isoform with greater affinity than TGF-beta1, suggesting that once released, it could affect the cellular availability of TGF-beta.

Syndecan-1 transmembrane and extracellular domains have unique and distinct roles in cell spreading

Raji cells expressing syndecan-1 (Raji-S1) adhere and spread when plated on heparan sulfate-binding extracellular matrix ligands or monoclonal antibody 281.2, an antibody directed against the syndecan-1 extracellular domain. Cells plated on monoclonal antibody 281.2 initially extend a broad lamellipodium, a response accompanied by membrane ruffling at the cell margin. Membrane ruffling then becomes polarized, leading to an elongated cell morphology. Previous work demonstrated that the syndecan-1 cytoplasmic domain is not required for these activities, suggesting important roles for the syndecan-1 transmembrane and/or extracellular domains in the assembly of a signaling complex necessary for spreading. Work described here demonstrates that truncation of the syndecan-1 extracellular domain does not affect the initial lamellipodial extension in the Raji-S1 cells but does inhibit the active membrane ruffling that is necessary for cell polarization. Replacement of the entire syndecan-1 transmembrane domain with leucine residues completely blocks the cell spreading. These data demonstrate that the syndecan-1 transmembrane and extracellular domains have important but distinct roles in Raji-S1 cell spreading; the extracellular domain mediates an interaction that is necessary for dynamic cytoskeletal rearrangements whereas an interaction of the transmembrane domain is required for the initial spreading response.

Syndecan 3 intramembrane proteolysis is presenilin/gamma-secretase-dependent and modulates cytosolic signaling

The syndecans play critical roles in several signal transduction pathways. The core proteins of these heparan sulfate proteoglycans are characterized by highly conserved transmembrane and intracellular domains which are required for signaling across the membrane and for interaction with cytosolic proteins. However, regulatory mechanisms controlling these functions remain largely unknown. Here we show that, upon ligand-induced primary proteolytic cleavage within the ectodomain, the intracellular domain of syndecan 3 is released by regulated intramembrane proteolysis. The cleavage is mediated by presenilin/gamma-secretase complex and negatively regulates the plasma membrane targeting of the transcriptional cofactor CASK.

Syndecan-1 accumulates in lysosomes of poorly differentiated breast carcinoma cells

Expression patterns of syndecan-1, the cell surface heparan sulfate proteoglycan (HSPG) predominant on epithelial cells, were analyzed in tissue samples from 30 infiltrating human breast carcinomas and in 9 human breast carcinoma cell lines. Immunohistochemical staining demonstrates that while a subset of the breast carcinomas lose syndecan-1, this proteoglycan is expressed or overexpressed in a majority of the cases. Interestingly, cells in poor grade tumors contain intracellular syndecan-1, an observation that has not been previously described and was thus further investigated. Examination of cultured breast carcinoma cell lines indicates that they also display the phenotype of the syndecan-1 positive tumors and thereby provide a model system for analysis of intracellular syndecan-1. All cell lines examined express syndecan-1, and poorly differentiated lines such as BT549 cells internalize the proteoglycan from the cell surface where it accumulates as intact HSPG in intracellular vesicles. Colocalization studies using fluorescent markers identify these to be lysosomes. This finding is unexpected, as the accepted mechanism for degradation of syndecan HSPG following endocytosis is fragmentation of the protein core and glycosaminoglycan chains in endosomes, followed by delivery of the fragments to lysosomes. Lysosomal inactivation using ammonium chloride demonstrates that well-differentiated lines such as T47D and MCF-7 cells, which maintain the majority of syndecan-1 on their cell surfaces, also target intact constitutively endocytosed syndecan-1 to lysosomes. Taken together, these results suggest that mammary epithelial cells utilize a previously uncharacterized mechanism for syndecan-1 catabolism. In this pathway the proteoglycan remains intact as it passes through the endosomal system, prior to arriving at its site of intracellular degradation in lysosomes.

Multiple metalloproteinases process protransforming growth factor-alpha (proTGF-alpha)

Shedding of TNF-alpha requires a single cleavage event, whereas the ectodomain of proTGF-alpha is cleaved at N-proximal (N-terminal) and membrane proximal (C-terminal) sites to release mature TGF-alpha. Tumor necrosis factor-alpha converting enzyme (TACE) was shown to have a central role in the shedding of both factors. Here we show that cleavage of the proTGF-alpha C-terminal site, required for release of mature growth factor, is less sensitive to a panel of hydroxamates than TNF-alpha processing. Recombinant TACE cleaves TNF-alpha and N-terminal TGF-alpha peptides 50-fold more efficiently than the C-terminal TGF-alpha peptide. Moreover, fractionation of rat liver epithelial cell membranes yields two populations: one contains TACE and cleaves peptides corresponding to TNF-alpha and both proTGF-alpha processing sites, while the other lacks detectable TACE and cleaves only the C-terminal proTGF-alpha processing site. Activities in both fractions are inhibited by hydroxamates and EDTA but not by cysteine, aspartate, or serine protease inhibitors. Both membrane fractions also contain ADAM 10. ADAM 10 correctly cleaves peptides and a soluble form of precursor TGF-alpha (proTGFecto) at the N-terminal site but not the C-terminal site. However, the kinetics of N-terminal peptide cleavage by ADAM 10 are 90-fold less efficient than TACE. Our findings indicate that while TACE is an efficient proTGF-alpha N-terminal convertase, a different activity, distinguishable from TACE, exists that can process proTGF-alpha at the C-terminal site. A model that accounts for these findings and the requirement for TACE in TGF-alpha shedding is proposed.

Transmodulation of cell surface regulatory molecules via ectodomain shedding

Cell responses to exogenous stimuli often result in a rapid decrease of cell surface density of a wide range of diverse regulatory proteins, receptor and adhesion molecules in particular. This decrease may occur in a ligand-dependent fashion (down-regulation), following endocytosis and degradation by lysosomal proteases, or by down-modulation, where molecules are targeted by endoproteases directly on cell surface. These proteases are recruited by trans-modulating agents, different from ligand, which act via their own receptors and the related intracellularly-generated signals. Endoproteolytic activity determines the release of large portions (shedding) of substrate proteins, called ectodomains, which are usually not ligand-bound, and therefore represent biologically-active molecules. Ectodomain shedding is involved in a number of pathophysiological processes, such as inflammation, cell degeneration and apoptosis, and oncogenesis. Common features of the process, such as the involvement of protein kinase C and of transmembrane metalloproteases, have been identified. In this review, we summarize basic concepts on down-modulation and ectodomain shedding, and provide an update of the issue with respect to: (i) new entries to the list of molecules found involved in the process; (ii) current views about the upstream control of shedding, i.e. the pathways linking the signals triggered by the trans-modulating agents to the activation of endoproteolytic activity on the cell surface.

Constitutive shedding of the amyloid precursor protein ectodomain is up-regulated by tumour necrosis factor-alpha converting enzyme

The amyloid precursor protein (APP) of Alzheimer's disease is a transmembrane protein that is cleaved within its extracellular domain, liberating a soluble N-terminal fragment (sAPP alpha). Putative mediators of this process include three members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10 and ADAM17/TACE (tumour necrosis factor-alpha converting enzyme). Tumour necrosis factor-alpha protease inhibitor (TAPI-1), an inhibitor of ADAMs, reduced constitutive and muscarinic receptor-stimulated sAPP alpha release in HEK-293 cells stably expressing M3 muscarinic receptors. However, the former was less sensitive to TAPI-1 (IC(50)=8.09 microM) than the latter (IC(50)=3.61 microM), suggesting that these processes may be mediated by different metalloproteases. Constitutive sAPP alpha release was increased several-fold in cells transiently transfected with TACE, and this increase was proportional to TACE expression. In contrast, muscarinic-receptor-activated sAPP alpha release was not altered in TACE transfectants. TACE-dependent constitutive release of co-transfected APP(695) was inhibited by TAPI-1 with an IC(50) of 0.92 microm, a value significantly lower than the IC(50)s for inhibition of either constitutive or receptor-regulated sAPP alpha shedding mediated by endogenous secretases. The results indicate that TACE is capable of catalysing constitutive alpha-secretory cleavage of APP, but it is likely that additional members of the ADAM family mediate endogenous constitutive and receptor-coupled release of sAPP alpha in HEK-293 cells.

Protein kinase C delta and eta isoenzymes control the shedding of the interleukin 6 receptor alpha in myeloma cells

The soluble interleukin 6 receptor alpha is an agonistic molecule of interleukin 6 (IL-6) and is important in the biology of multiple myeloma. More precisely, it potentiates the deleterious effects of IL-6 during tumour progression, facilitating angiogenesis and bone resorption. Because the mechanisms involved in the shedding of the interleukin 6 receptor alpha (IL-6Ralpha) in multiple myeloma are not known, we have investigated them in the XG-6 human myeloma cell line. Here we provide evidence that PMA-induced IL-6Ralpha shedding is controlled by a metalloproteinase and by protein kinase C (PKC) isoenzymes that do not require Ca(2+) for their activation. We show that XG-6 cells express PKC-delta, -eta and -zeta isoenzymes. However, after stimulation with PMA, only PKC-delta and PKC-eta are activated, as shown by their translocation to the membrane. Treatment with PMA induces an increase in PKC-delta phosphorylation in its active loop. In addition, by using rottlerin, a specific inhibitor of PKC-delta, we demonstrate that PKC-delta is involved in the PMA-induced shedding of IL-6Ralpha. With the use of UO126, a specific inhibitor of the mitogen-activated protein kinase (MAPK) pathway, we show that the PMA-induced IL-6Ralpha shedding is mediated in part by the MAPK pathway. Finally, whereas GF109203X, a general PKC inhibitor, inhibits the activation of ERK1/2 (extracellular signal-regulated protein kinase 1/2), rottlerin has no inhibitory effect, indicating that the Ras/MAPK activation is PKC-dependent but PKC-delta-independent. Taken together, these results suggest that the PMA-induced shedding of IL-6Ralpha is mediated by a PKC isoenzyme network.

Constitutive shedding of the amyloid precursor protein ectodomain is up-regulated by tumour necrosis factor-alpha converting enzyme

The amyloid precursor protein (APP) of Alzheimer's disease is a transmembrane protein that is cleaved within its extracellular domain, liberating a soluble N-terminal fragment (sAPP alpha). Putative mediators of this process include three members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10 and ADAM17/TACE (tumour necrosis factor-alpha converting enzyme). Tumour necrosis factor-alpha protease inhibitor (TAPI-1), an inhibitor of ADAMs, reduced constitutive and muscarinic receptor-stimulated sAPP alpha release in HEK-293 cells stably expressing M3 muscarinic receptors. However, the former was less sensitive to TAPI-1 (IC(50)=8.09 microM) than the latter (IC(50)=3.61 microM), suggesting that these processes may be mediated by different metalloproteases. Constitutive sAPP alpha release was increased several-fold in cells transiently transfected with TACE, and this increase was proportional to TACE expression. In contrast, muscarinic-receptor-activated sAPP alpha release was not altered in TACE transfectants. TACE-dependent constitutive release of co-transfected APP(695) was inhibited by TAPI-1 with an IC(50) of 0.92 microm, a value significantly lower than the IC(50)s for inhibition of either constitutive or receptor-regulated sAPP alpha shedding mediated by endogenous secretases. The results indicate that TACE is capable of catalysing constitutive alpha-secretory cleavage of APP, but it is likely that additional members of the ADAM family mediate endogenous constitutive and receptor-coupled release of sAPP alpha in HEK-293 cells.

Heparan sulfate proteoglycans in the nervous system: their diverse roles in neurogenesis, axon guidance, and synaptogenesis

Development of the mammalian nervous system involves generation of neurons from neural stem cells, migration of generated neurons toward genetically determined locations, extension of axons and dendrites, and establishment of neuronal connectivity. Recent progresses revealed diverse role of heparan sulfate proteoglycans in these processes. This article reviews our current knowledge about the functional roles of heparan sulfate proteoglycans in three critical events in mammalian neural development, namely neurogenesis, axon guidance, and synapse development.