Eok Soo Oh

TGF-β1 Represses Activation and Resultant Death of Microglia via Inhibition of Phosphatidylinositol 3-Kinase Activity

Overactivation of microglial cells may cause severe brain tissue damage in various neurodegenerative diseases. Therefore, the overactivation of microglia should be repressed by any means. The present study investigated the potential mechanism and signaling pathway for the repressive effect of TGF-β1, a major anti-inflammatory cytokine, on overactivation and resultant death of microglial cells. A bacterial endotoxin LPS stimulated expression of inducible NO synthase (iNOS) and caused death in cultured microglial cells. TGF-β1 markedly blocked these LPS effects. However, the LPS-evoked death of microglial cells was not solely attributed to excess production of NO. Because phosphatidylinositol 3-kinase (PI3K) was previously shown to play a crucial role in iNOS expression and cell survival signals, we further studied whether PI3K signaling was associated with the suppressive effect of TGF-β1. Like TGF-β1, the PI3K inhibitor LY294002 blocked iNOS expression and death in cultured microglial cells. Both TGF-β1 and LY294002 decreased the activation of caspases 3 and 11 and the mRNA expression of various kinds of inflammatory molecules caused by LPS. TGF-β1 was further found to decrease LPS-induced activation of PI3K and Akt. TGF-β1 and LY294002 suppressed LPS-induced p38 mitogen-activated kinase and c-Jun N-terminal kinase activity. In contrast, TGF-β1 and LY294002 enhanced LPS-induced NF-κB activity. Our data indicate that TGF-β1 protect normal or damaged brain tissue by repressing overactivation of microglial cells via inhibition of PI3K and its downstream signaling molecules.

The matrix metalloproteinase-7 regulates the extracellular shedding of syndecan-2 from colon cancer cells.

The cell surface heparan sulfate proteoglycan syndecan-2 regulates the activation of matrix metalloproteinase-7 (MMP-7) as a docking receptor. Here, we demonstrate the role of MMP-7 on syndecan-2 shedding in colon cancer cells. Western blot analysis showed that shed syndecan-2 was found in the culture media from various colon cancer cells. Overexpression of MMP-7 enhanced syndecan-2 shedding, whereas the opposite was true when MMP-7 levels were knocked-down using small inhibitory RNAs. Consistently, HT29 cells treated with MMP-7, but neither MMP-2 nor MMP-9, showed increased shed syndecan-2 in a time- and concentration-dependent manner. Furthermore, MALDI-TOF MS analysis and N-terminal amino acid sequencing revealed that MMP-7 cleaved both recombinant syndecan-2 and an endogenously glycosylated syndecan-2 ectodomain in the N-terminus at Leu(149) residue in vitro. Taken together, the data suggest that MMP-7 directly mediates shedding of syndecan-2 from colon cancer cells.

Syndecans play dual roles as cell adhesion receptors and docking receptors.

Syndecan are a family of cell surface heparan sulfate proteoglycans that act as cell surface receptors. Most cell surface receptors have a limited number and type of ligand interactions, responding only to the binding of (a) specific ligand(s). In contrast, syndecans can interact with various numbers and types of ligands, and thus play more diverse roles than others. Various syndecan functions have not yet been fully classified and categorized, but we herein review previous studies suggesting that syndecans play dual function as cell surface receptors by acting as both adhesion receptors and docking receptors. Through this dual regulatory function, syndecans are capable of regulating both intra‐ and extracellular activities, potentially altering a variety of cell behaviors.

A Unique Phenylalanine in the Transmembrane Domain Strengthens Homodimerization of the Syndecan-2 Transmembrane Domain and Functionally Regulates Syndecan-2

Background: The transmembrane domains of syndecan family members harbor a GXXXG motif forming non-covalently linked SDS-resistant dimers. Results: A unique phenylalanine in the syndecan-2 transmembrane domain contributes to its transmembrane homointeraction and unique functions. Conclusion: A unique phenylalanine in syndecan-2 endows its transmembrane domain with specific functions. Significance: This report provides insight into the importance of the transmembrane domain for syndecan receptor function. The syndecans are a type of cell surface adhesion receptor that initiates intracellular signaling events through receptor clustering mediated by their highly conserved transmembrane domains (TMDs). However, the exact function of the syndecan TMD is not yet fully understood. Here, we investigated the specific regulatory role of the syndecan-2 TMD. We found that syndecan-2 mutants in which the TMD had been replaced with that of syndecan-4 were defective in syndecan-2-mediated functions, suggesting that the TMD of syndecan-2 plays one or more specific roles. Interestingly, syndecan-2 has a stronger tendency to form sodium dodecyl sulfate (SDS)-resistant homodimers than syndecan-4. Our structural studies showed that a unique phenylalanine residue (Phe167) enables an additional molecular interaction between the TMDs of the syndecan-2 homodimer. The presence of Phe167 was correlated with a higher tendency toward oligomerization, and its replacement with isoleucine significantly reduced the SDS-resistant dimer formation and cellular functions of syndecan-2 (e.g. cell migration). Conversely, replacement of isoleucine with phenylalanine at this position in the syndecan-4 TMD rescued the defects observed in a mutant syndecan-2 harboring the syndecan-4 TMD. Taken together, these data suggest that Phe167 in the TMD of syndecan-2 endows the protein with specific functions. Our work offers new insights into the signaling mediated by the TMD of syndecan family members.

Specific tyrosine phosphorylation of focal adhesion kinase mediated by Fer tyrosine kinase in suspended hepatocytes

Cell adhesion to the extracellular matrix (ECM) can activate signaling via focal adhesion kinase (FAK) leading to dynamic regulation of cellular morphology. Mechanistic basis for the lack of effective intracellular signaling by non-attached epithelial cells is poorly understood. To examine whether signaling in suspended cells is regulated by Fer cytoplasmic tyrosine kinase, we investigated the effect of ectopic Fer expression on signaling in suspended or adherent hepatocytes. We found that ectopic Fer expression in Huh7 hepatocytes in suspension or on non-permissive poly-lysine caused significant phosphorylation of FAK Tyr577, Tyr861, or Tyr925, but not Tyr397 or Tyr576. Fer-mediated FAK phosphorylation in suspended cells was independent of c-Src activity or growth factor stimulation, but dependent of cortactin expression. Consistent with these results, complex formation between FAK, Fer, and cortactin was observed in suspended cells. The Fer-mediated effect correlated with multiple membrane protrusions, even on poly-lysine. Together, these observations suggest that Fer may allow a bypass of anchorage-dependency for intracellular signal transduction in hepatocytes.

Trans-regulation of Syndecan Functions by Hetero-oligomerization

Background: Syndecans form non-covalently linked homodimers through their highly conserved transmembrane domains. Results: Syndecan-2 and -4 exhibit heteromolecular interaction, and this interaction regulates syndecan-mediated cellular functions. Conclusion: Hetero-oligomerization produces distinct syndecan functions. Significance: Our finding offers new insights into the underlying signaling mechanisms of syndecans. Syndecans, a family of transmembrane heparansulfate proteoglycans, are known to interact through their transmembrane domains to form non-covalently linked homodimers, a process essential for their individual functions. Because all syndecan transmembrane domains are highly conserved and thus might mediate interactions between different members of the syndecan family, we investigated syndecan interactions in detail. All recombinant syndecan-2 and -4 protein variants containing the transmembrane domain formed not only sodium dodecyl sulfate (SDS)-resistant homodimers but also SDS-resistant heterodimers. Biochemical and structural data revealed that recombinant syndecan-2 and -4 formed intermolecular interactions in vitro, and the GXXXG motif in transmembrane domain mediated this interaction. When exogenously expressed in rat embryonic fibroblasts, syndecan-2 interacted with syndecan-4 and vice versa. Furthermore, bimolecular fluorescence complementation-based assay demonstrated specific hetero-molecular interactions between syndecan-2 and -4, supporting hetero-oligomer formation of syndecans in vivo. Interestingly, hetero-oligomerization significantly reduced syndecan-4-mediated cellular processes such as protein kinase Cα activation and protein kinase Cα-mediated cell adhesion as well as syndecan-2-mediated tumorigenic activities in colon cancer cells such as migration and anchorage-independent growth. Taken together, these data provide evidence that hetero-oligomerization produces distinct syndecan functions and offer insights into the underlying signaling mechanisms of syndecans.

Syndecan-2 cytoplasmic domain up-regulates matrix metalloproteinase-7 expression via the protein kinase Cγ–mediated FAK/ERK signaling pathway in colon cancer

The syndecan family of heparan sulfate proteoglycans contributes to cell adhesion and communication by serving as co-receptors for cell signaling and extracellular matrix molecules. Syndecan-2 is located at the cell surface, and we previously reported that it induces matrix metalloproteinase-7 (MMP-7) expression in colon cancer cells. However, the underlying regulatory mechanisms are unknown. Here, we report that overexpression of syndecan-2 in HT-29 colon cancer cells increases the phosphorylation of focal adhesion kinase (FAK) and ERK in parallel with up-regulated MMP-7 expression, but a syndecan-2 mutant lacking the cytoplasmic domain showed significant reductions in these effects. Consistent with this observation, FAK inhibition via FAK-related non-kinase expression or inhibition of ERK with the ERK1/2 inhibitor SCH772984 diminished the syndecan-2–mediated up-regulation of MMP-7. Activation of PKC enhanced syndecan-2–mediated MMP-7 expression, whereas inhibition of PKC had the opposite effect. Of note, the exogenous expression of syndecan-2 triggered localization of PKCγ to the membrane. Expression of syndecan-2 harboring a phosphomimetic (S198E) mutation of the variable region of the cytoplasmic domain enhanced MMP-7 expression and FAK phosphorylation. Finally, experimental suppression of shedding of the syndecan-2 extracellular domain did not significantly affect the syndecan-2–mediated up-regulation of MMP-7 in the early period after syndecan-2 overexpression. Taken together, these findings suggest that syndecan-2s cytoplasmic domain up-regulates MMP-7 expression in colon cancer cells via PKCγ-mediated activation of FAK/ERK signaling.

Cell Adhesion in Cancer

During cancer progression, cells lose their original tissue contacts, move through the extracellular matrix (ECM), enter into the lymphatic and/or blood system, extravasate, and ultimately form new tumors. Therefore, tumor cells inevitably experience alterations in cell-cell and cell-ECM adhesion and the transformation activities of tumor cells are highly influenced by cell adhesion via adhesion receptors such as cadherins, integrins, cell surface proteoglycans, and tetraspanins. These adhesion receptors, together with extracellular ligands in the tumor microenvironment, couple the extracellular environment to intracellular signals, thereby enhancing cancer cell migration, invasion, proliferation, and survival. Therefore, knowledge of the role of cell adhesion in cancer is key to understanding the development of cancer, and such knowledge could potentially form the basis for effective approaches to cancer treatment. Therefore, in this issue, we have invited several authors to address such issues. n nThe special issue will be composed of three parts. A set of four papers discuss regulatory mechanisms in the light of interesting issues that have arisen recently, such as the role of cell-ECM interactions in genetic mutations in tumors, as well as the important roles of lipid rafts, turnover of focal adhesions, and anoikis resistance in cancer cell adhesion and migration. All of these characteristics have been shown to be important in cancer cell adhesion, and recent findings provide potential mechanisms of tumor cell interactions and metastatic activities. n nAnother set of five papers of this special issue address the molecular level regulatory role of cell adhesion molecules such as integrins and the immunoglobulin super family and adhesion-related regulatory molecules such as plakoglobin, HIC-5, and prostaglandins. All of these molecules have received considerable attention in cell adhesion research and have distinct role(s) in different aspects of cancer progression, but all are important regulators of human carcinogenesis through their capacity to regulate cancer cell adhesion. However, because their roles in cancer cell adhesion and metastasis are still under investigation, this special issue addresses their functional significance, particularly in the context of cancer cell adhesion. n nA set of two papers address the possibility of suppressing tumors by regulating cell adhesion status using ECM-derived functional peptides or a physiological regulator such as heparin. Research in this area will improve the understanding of cancer progression and may provide the basis for a new strategy for treating cancer. n n nEok-Soo Oh n nMotoharu Seiki n nMartin Gotte n nJun Chung

New structural insight of C-terminal region of Syntenin-1, enhancing the molecular dimerization and inhibitory function related on Syndecan-4 signaling

The PDZ domain-containing scaffold protein, syntenin-1, binds to the transmembrane proteoglycan, syndecan-4, but the molecular mechanism/function of this interaction are unknown. Crystal structure analysis of syntenin-1/syndecan-4 cytoplasmic domains revealed that syntenin-1 forms a symmetrical pair of dimers anchored by a syndecan-4 dimer. The syndecan-4 cytoplasmic domain is a compact intertwined dimer with a symmetrical clamp shape and two antiparallel strands forming a cavity within the dimeric twist. The PDZ2 domain of syntenin-1 forms a direct antiparallel interaction with the syndecan-4 cytoplasmic domain, inhibiting the functions of syndecan-4 such as focal adhesion formation. Moreover, C-terminal region of syntenin-1 reveals an essential role for enhancing the molecular homodimerization. Mutation of key syntenin-1 residues involved in the syndecan-4 interaction or homodimer formation abolishes the inhibitory function of syntenin-1, as does deletion of the homodimerization-related syntenin-1 C-terminal domain. Syntenin-1, but not dimer-formation-incompetent mutants, rescued the syndecan-4-mediated inhibition of migration and pulmonary metastasis by B16F10 cells. Therefore, we conclude that syntenin-1 negatively regulates syndecan-4 function via oligomerization and/or syndecan-4 interaction, impacting cytoskeletal organization and cell migration.

Processing of syndecan-2 by matrix metalloproteinase-14 and effect of its cleavage on VEGF-induced tube formation of HUVECs

Syndecans (SDCs) are transmembrane proteoglycans that are involved in cell adhesion and cell communication. Specifically, SDC2 plays a key role in tumorigenesis, metastasis, and angiogenesis. Previously, we found that rat SDC2 is shed by matrix metalloproteinase-7 (MMP-7) in colon cancer cells. Here, we analyzed the susceptibility of rat SDC2 to various MMPs. We found that the rat SDC2 ectodomain (ECD) fused to the C-terminal Fc region, which was expressed in mammalian cells, was cleaved more efficiently by MMP-14 than MMP-7. Likewise, when anchored on the surface of HeLa cells, rat SDC2 was cleaved more efficiently by the treatment of MMP-14 than MMP-7 and was shed more readily by membrane-anchored MMP-14 than soluble MMP-14. Furthermore, MMP-14 cleaved recombinant SDC2-ECD expressed in Escherichia coli into multiple fragments. Using N-terminal amino acid sequencing and the top-down proteomics approach, we determined that the major cleavage sites were S88↓L89, T98↓M99, T100↓L101, D132↓P133, and N148↓L149 for rat SDC2-ECD and S55↓G56, S65↓P66, P75↓K76, N92↓I93 D122↓P123, and S138↓L139 for human SDC2-ECD. Finally, the rat and human SDC2-ECD lost the ability to suppress vascular endothelial growth factor-induced formation of capillary-like tubes by human umbilical vein endothelial cells following cleavage by MMP-14, but its major cleavage-site mutant of rat SDC2-ECD did not. These results suggest that MMP-14 is a novel enzyme responsible for degrading SDC2 and impairing its physiological roles including angiogenesis.