Tetraspanin CD151 stimulates adhesion-dependent activation of Ras, Rac, and Cdc42 by facilitating molecular association between β1 integrins and small GTPases

Tetraspanins from the liver fluke Opisthorchis viverrini stimulate cholangiocyte migration and inflammatory cytokine production

The liver fluke Opisthorchis viverrini (Poirier, 1886) (Digenea) secretes extracellular vesicles (EVs) bearing CD63-like tetraspanins on their surface. Fluke EVs are actively internalised by host cholangiocytes in the bile ducts, where they drive pathology and promote neoplasia through induction of cellular proliferation and secretion of inflammatory cytokines. We investigated the effects of tetraspanins of the CD63 superfamily by co-culturing recombinant forms of the large extracellular loop (LEL) of O. viverrini tetraspanin-2 (rLEL-Ov-TSP-2) and tetraspanin-3 (rLEL-Ov-TSP-3) with non-cancerous human bile duct (H69) and cholangiocarcinoma (CCA, M213) cell lines. The results showed that cell lines co-cultured with excretory/secretory products from adult O. viverrini (Ov-ES) underwent significantly increased cell proliferation at 48 hours but not 24 hours compared to untreated control cells (P < 0.05), whereas rLEL-Ov-TSP-3 co-culture resulted in significantly increased cell proliferation at both 24 hours (P < 0.05) and 48 hours (P < 0.01) time points. In like fashion, H69 cholangiocytes co-cultured with both Ov-ES and rLEL-Ov-TSP-3 underwent significantly elevated Il-6 and Il-8 gene expression for at least one of the time points assessed. Finally, both rLEL-Ov-TSP-2 and rLEL-Ov-TSP-3 significantly enhanced migration of both M213 and H69 cell lines. These findings indicated that O. viverrini CD63 family tetraspanins can promote a cancerous microenvironment by enhancing innate immune responses and migration of biliary epithelial cells.

Tetraspanins from Opisthorchis viverrini stimulate cholangiocyte migration and inflammatory cytokine production

The liver fluke Opsithorchis viverrini secretes extracellular vesicles (EVs) bearing CD63-like tetraspanins on their surface. Fluke EVs are actively internalized by host cholangiocytes in the bile ducts, where they drive pathology and promote neoplasia through induction of cellular proliferation and secretion of inflammatory cytokines. We investigated the effects of tetraspanins of the CD63 superfamily by co-culturing recombinant forms of the large extracellular loop (LEL) of O. viverrini tetraspanin-2 (rLEL- Ov -TSP-2) and tetraspanin-3 (rLEL- Ov -TSP-3) with non-cancerous human bile duct (H69) and cholangiocarcinoma (CCA, M213) cell lines. The results showed that cell lines co-cultured with excretory/secretory products from adult O. viverrini ( Ov- ES) underwent significantly increased cell proliferation at 48 hours but not 24 hours compared to untreated control cells ( P <0.05), whereas rLEL- Ov -TSP-3 co-culture resulted in significantly increased cell proliferation at both 24 hr ( P <0.05) and 48 hr ( P <0.01) time points. In like fashion, H69 cholangiocytes co-cultured with both Ov -ES and rLEL- Ov -TSP-3 underwent significantly elevated Il-6 and Il-8 gene expression for at least one of the time points assessed. Finally, both rLEL- Ov -TSP-and rLEL- Ov -TSP-3 significantly enhanced migration of both M213 and H69 cell lines. These findings indicated that O. viverrini CD63 family tetraspanins can promote a cancerous microenvironment by enhancing innate immune responses and migration of biliary epithelial cells.

The versatile roles of testrapanins in cancer from intracellular signaling to cell-cell communication: cell membrane proteins without ligands

The tetraspanins (TSPANs) are a family of four-transmembrane proteins with 33 members in mammals. They are variably expressed on the cell surface, various intracellular organelles and vesicles in nearly all cell types. Different from the majority of cell membrane proteins, TSPANs do not have natural ligands. TSPANs typically organize laterally with other membrane proteins to form tetraspanin-enriched microdomains (TEMs) to influence cell adhesion, migration, invasion, survival and induce downstream signaling. Emerging evidence shows that TSPANs can regulate not only cancer cell growth, metastasis, stemness, drug resistance, but also biogenesis of extracellular vesicles (exosomes and migrasomes), and immunomicroenvironment. This review summarizes recent studies that have shown the versatile function of TSPANs in cancer development and progression, or the molecular mechanism of TSPANs. These findings support the potential of TSPANs as novel therapeutic targets against cancer.

Isolation of a cytolytic subpopulation of extracellular vesicles derived from NK cells containing NKG7 and cytolytic proteins

NK cells can broadly target and kill malignant cells via release of cytolytic proteins. NK cells also release extracellular vesicles (EVs) that contain cytolytic proteins, previously shown to induce apoptosis of a variety of cancer cells in vitro and in vivo. The EVs released by NK cells are likely very heterogeneous, as vesicles can be released from the plasma membrane or from different intracellular compartments. In this study, we undertook a fractionation scheme to enrich for cytolytic NK-EVs. NK-EVs were harvested from culture medium from the human NK-92 cell line or primary human NK cells grown in serum-free conditions. By combining ultracentrifugation with downstream density-gradient ultracentrifugation or size-exclusion chromatography, distinct EV populations were identified. Density-gradient ultracentrifugation led to separation of three subpopulations of EVs. The different EV isolates were characterized by label-free quantitative mass spectrometry and western blotting, and we found that one subpopulation was primarily enriched for plasma membrane proteins and tetraspanins CD37, CD82, and CD151, and likely represents microvesicles. The other major subpopulation was enriched in intracellularly derived markers with high expression of the endosomal tetraspanin CD63 and markers for intracellular organelles. The intracellularly derived EVs were highly enriched in cytolytic proteins, and possessed high apoptotic activity against HCT-116 colon cancer spheroids. To further enrich for cytolytic EVs, immunoaffinity pulldowns led to the isolation of a subset of EVs containing the cytolytic granule marker NKG7 and the majority of vesicular granzyme B content. We therefore propose that EVs containing cytolytic proteins may primarily be released via cytolytic granules.

Fos regulates macrophage infiltration against surrounding tissue resistance by a cortical actin-based mechanism in Drosophila

The infiltration of immune cells into tissues underlies the establishment of tissue-resident macrophages and responses to infections and tumors. Yet the mechanisms immune cells utilize to negotiate tissue barriers in living organisms are not well understood, and a role for cortical actin has not been examined. Here, we find that the tissue invasion of Drosophila macrophages, also known as plasmatocytes or hemocytes, utilizes enhanced cortical F-actin levels stimulated by the Drosophila member of the fos proto oncogene transcription factor family (Dfos, Kayak). RNA sequencing analysis and live imaging show that Dfos enhances F-actin levels around the entire macrophage surface by increasing mRNA levels of the membrane spanning molecular scaffold tetraspanin TM4SF, and the actin cross-linking filamin Cheerio, which are themselves required for invasion. Both the filamin and the tetraspanin enhance the cortical activity of Rho1 and the formin Diaphanous and thus the assembly of cortical actin, which is a critical function since expressing a dominant active form of Diaphanous can rescue the Dfos macrophage invasion defect. In vivo imaging shows that Dfos enhances the efficiency of the initial phases of macrophage tissue entry. Genetic evidence argues that this Dfos-induced program in macrophages counteracts the constraint produced by the tension of surrounding tissues and buffers the properties of the macrophage nucleus from affecting tissue entry. We thus identify strengthening the cortical actin cytoskeleton through Dfos as a key process allowing efficient forward movement of an immune cell into surrounding tissues.

The infiltration of immune cells into tissue underlies the establishment of tissue-resident macrophages, and responses to infections and tumors, but how do they overcome tissue barriers? This study shows that macrophages upregulate the proto-oncogene Fos, increasing the density and crosslinking of cortical actin, thereby counteracting the tension of surrounding tissues and protecting the macrophage nucleus.

Role of Tetraspanins in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is characterized by high prevalence, morbidity, and mortality. Liver cancer is the sixth most common cancer worldwide; and its subtype, HCC, accounts for nearly 80% of cases. HCC progresses rapidly, and to date, there is no efficacious treatment for advanced HCC. Tetraspanins belong to a protein family characterized by four transmembrane domains. Thirty-three known tetraspanins are widely expressed on the surface of most nucleated cells and play important roles in different biological processes. In our review, we summarize the functions of tetraspanins and their underlying mechanism in the life cycle of HCC, from its initiation, progression, and finally to treatment. CD9, TSPAN15, and TSPAN31 can promote HCC cell proliferation or suppress apoptosis. CD63, CD151, and TSPAN8 can also facilitate HCC metastasis, while CD82 serves as a suppressor of metastasis. TSPAN1, TSPAN8, and CD151 act as prognosis indicators and are inversely correlated to the overall survival rate of HCC patients. In addition, we discuss the potential of role of the tetraspanin family proteins as novel therapeutic targets and as an approach to overcome drug resistance, and also provide suggestions for further research.

Effects of fluid shear stress on expression of focal adhesion kinase in MG-63 human osteoblast-like cells on different surface modification of titanium

This study aimed to investigate the effect of fluid shear stress (FSS) on cell proliferation and expression of focal adhesion kinase (FAK) in MG-63 cells on different modified titanium surfaces. MG63 cells were cultured on three different surfaces: glass slide, polished treatment (PT) titanium surface and sandblasted/acid-etched surfaces (SLA) titanium surface. The surface topography and roughness were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The cells were subjected to FSS, and the cell appearance before and after the stress was evaluated. MTT assay was applied to estimate cell proliferation. The mRNA and protein levels of FAK were determined by qRT-PCR and western blotting. Titanium plates demonstrated different surface microtopography. Parameter Ra values of SLA group were around 3.4 µm, which was higher than PT group. Exposure to the FSS of 12 dynes/cm² significantly induced positive upregulation of cellular proliferation and the expression of FAK, which were directly correlated with the duration of exposure and surface. Cells in SLA group were able to endurance the longtime of FSS, especially under the FSS of 16 dynes/cm². SLA surface had a positive influence on the expression of FAK. Different surface modifications created different microtopography of titanium plates. Cell proliferation and the mRNA and protein expression of FAK were stimulated by FSS and regulated by a marked synergistic effect of surface topography and the level and duration of FSS.

Exosomal Protease Cargo as Prognostic Biomarker in Colorectal Cancer

Objective:

The aim of the study was to develop a model for predicting cancer risk in colorectal polyps’ patients (CPPs), as well as to reveal additional prognosis factors for Stage III colorectal cancer based on differences in subpopulations of tetraspanins, tetraspanin-associated and tetraspanin-non-associated proteases in blood plasma exosomes of CPPs and colorectal cancer patients (CRCPs).

Methods:

The subpopulations of CD151- and Tspan8-positive exosomes, the subpopulations of metalloproteinase at the surface of СD9-positive exosomes and the level of 20S proteasomes in plasma exosomes in 15 CPPs (tubulovillous adenomas) and 60 CRCPs were evaluated using flow cytometry and Western blotting. Logistic regression analysis was performed to predict cancer risk of CPPs.

Results:

The levels of 20S proteasomes in exosomes, MMP9+, MMP9+/MMP2+/EMMPRIN+ in CD9-positive blood plasma exosomes are associated with the risk of malignant transformation of colorectal tubulovillous adenomas. In patients with Stage III CRC, the levels of 20S proteasomes (less than 2 units) and MMP9+ subpopulations (more than 61%) in plasma exosomes are unfavorable prognostic factors for overall survival. The levels of 20S proteasomes and ADAM10+/ADAM17- subpopulations in CD9-positive blood plasma exosomes are the most significant values for predicting relapse-free survival.

Conclusion:

Protease cargo in CD9-positive blood plasma exosomes is prognostic biomarker for colorectal polyps and colorectal cancer.

Key Role of CD151-integrin Complex in Lung Cancer Metastasis and Mechanisms Involved

Tetraspanin CD151 was found to be upregulated in malignant cell types and has been identified as a tumor metastasis promoter. In this study, we aimed to examine the role of the CD151-integrin complex in lung cancer metastasis and the underlying mechanisms. CD151 QRD^194-196 →AAA^194-196 mutant was generated and used to transfect A549 human lung adenocarcinoma cells. We found that there was no significant difference in CD151 protein expression between CD151 and CD151-AAA mutant groups. In vitro, CD151-AAA mutant delivery abrogated the migration and invasion of A549 cells, which was promoted by CD151 gene transfer. Furthermore, CD151-AAA delivery failed to activate FAK and p130Cas signaling pathways. Western blot and immunohistochemical staining showed strong CD151 expression in lung cancerous tissues but not in adjacent normal tissues. Increased level of CD151 protein was observed in 20 of the patients and the positive rate of CD151 protein in specimens was 62.5% (20/32). In addition, CD151 was co-localized with α3 integrin at the cell-cell contact site in carcinoma tissues. These results suggested that the disruption of the CD151-α3 integrin complex may impair the metastasis-promoting effects and signaling events induced by CD151 in lung cancer. Our findings identified a key role for CD151-α3 integrin complex as a promoter in the lung cancer.

Understanding the role of integrins in breast cancer invasion, metastasis, angiogenesis, and drug resistance

Integrins are cell adhesion receptors, which are typically transmembrane glycoproteins that connect to the extracellular matrix (ECM). The function of integrins regulated by biochemical events within the cells. Understanding the mechanisms of cell growth by integrins is important in elucidating their effects on tumor progression. One of the major events in integrin signaling is integrin binding to extracellular ligands. Another event is distant signaling that gathers chemical signals from outside of the cell and transmit the signals upon cell adhesion to the inside of the cell. In normal breast tissue, integrins function as checkpoints to monitor effects on cell proliferation, while in cancer tissue these functions altered. The combination of tumor microenvironment and its associated components determines the cell fate. Hypoxia can increase the expression of several integrins. The exosomal integrins promote the growth of metastatic cells. Expression of certain integrins is associated with increased metastasis and decreased prognosis in cancers. In addition, integrin-binding proteins promote invasion and metastasis in breast cancer. Targeting specific integrins and integrin-binding proteins may provide new therapeutic approaches for breast cancer therapies. This review will examine the current knowledge of integrins' role in breast cancer.

Integrin α2β1 inhibition attenuates prostate cancer cell proliferation by cell cycle arrest, promoting apoptosis and reducing epithelial-mesenchymal transition

Integrin α2β1 plays an important role in cellular migration and metastasis processes associated with prostate cancer. The aim of this study was to assess whether selective inhibition of integrin α2β1 is an effective strategy to target metastatic prostate cancer cells. In this regard, we examined the effects of the inhibitor BTT-3033, which selectively interferes with the connection between integrin a2b1 and its ligand, on migration, epithelial-mesenchymal transition (EMT), cell cycle arrest, apoptosis, and specific intracellular signaling pathways using LNcap-FGC and DU-145 prostate cancer cell lines. Western blot analysis and immunocytochemistry assays showed that inhibition of integrin a2b1 inhibits EMT, through the increased expression of E-cadherin and decreased expression of N-cadherin and vimentin. Scratch wound healing assays revealed a direct effect on integrin α2β1 in the migration capacity of cells. In addition, treatment with BTT-3033 induced a reduction in cell viability and proliferation, as assessed by MTT and BrdU assays. In addition, the results show that BTT-3033 inhibits cell proliferation by inducing G1 cell cycle arrest. Moreover, inhibition of integrin α2β1 induces apoptosis through the activation of ROS, Bax protein upregulation, caspase-3 activation, and depletion of ΔΨm.  Molecular signaling studies showed that integrin α2β1 was a positive regulator of MKK7 phosphorylation. In conclusion, our results reveal a critical role for integrin a2b1 in the proliferation of prostate cancer cells, as demonstrated by EMT inhibition, cell cycle arrest, and apoptosis induction in response to treatment with its specific inhibitor BT-3033.

Tetraspanins: useful multifunction proteins for the possible design and development of small-molecule therapeutic tools

Tetraspanins constitute a well-conserved superfamily of four-span small membrane proteins (TM4SF), with >30 members in humans, with important roles in numerous mechanisms of cell biology. Moreover, tetraspanins associate with either specific partner proteins or another tetraspanin, generating a network of interactions involved in cell and membrane compartmentalization and having a role in cellular development, proliferation, activation, motility, and membrane fusions. Therefore, tetraspanins are considered regulators of cellular signaling and are often depicted as 'molecular facilitators'. In view of these many physiological functions, it is likely that these molecules are important actors in pathological processes. In this review, we present the main characteristics of this superfamily, providing a more detailed description of some significant representatives and discuss their relevance as potential targets for the design and development of small-molecule therapeutics in different pathologies.

The tetraspanin CD151 marks a unique population of activated human T cells

Tetraspanins are a family of proteins with an array of functions that are well studied in cancer biology, but their importance in immunology is underappreciated. Here we establish the tetraspanin CD151 as a unique marker of T-cell activation and, in extension, an indicator of elevated, systemic T-cell activity. Baseline CD151 expression found on a subset of T-cells was indicative of increased activation of the MAPK pathway. Following TCR/CD3 activation, CD151 expression was upregulated on the overall T-cell population, a quintessential feature of an activation marker. CD151+ T-cell frequencies in the spleen, an organ with increased immune activity, were twice as high as in paired peripheral blood samples. This CD151+ T-cell frequency increase was not paralleled by an increase of CD25 or CD38, demonstrating that CD151 expression is regulated independently of other T-cell activation markers. CD151+ T-cells were also more likely to express preformed granzyme B, suggesting that CD151+ T cells are pro-inflammatory. To this end, HIV-1 patients on antiretroviral therapy who are reported to exhibit chronically elevated levels of immune activity, had significantly higher CD4+CD151+ T-cell frequencies than healthy controls, raising the possibility that proinflammatory CD151+ T cells could contribute to the premature immunological aging phenotype observed in these patients.

Crosstalk between Cell Adhesion Complexes in Regulation of Mechanotransduction

Physical forces regulate numerous biological processes during development, physiology, and pathology. Forces between the external environment and intracellular actin cytoskeleton are primarily transmitted through integrin-containing focal adhesions and cadherin-containing adherens junctions. Crosstalk between these complexes is well established and modulates the mechanical landscape of the cell. However, integrins and cadherins constitute large families of adhesion receptors and form multiple complexes by interacting with different ligands, adaptor proteins, and cytoskeletal filaments. Recent findings indicate that integrin-containing hemidesmosomes oppose force transduction and traction force generation by focal adhesions. The cytolinker plectin mediates this crosstalk by coupling intermediate filaments to the actin cytoskeleton. Similarly, cadherins in desmosomes might modulate force generation by adherens junctions. Moreover, mechanotransduction can be influenced by podosomes, clathrin lattices, and tetraspanin-enriched microdomains. This review discusses mechanotransduction by multiple integrin- and cadherin-based cell adhesion complexes, which together with the associated cytoskeleton form an integrated network that allows cells to sense, process, and respond to their physical environment.

HPV caught in the tetraspanin web?

Tetraspanins are master organizers of the cell membrane. Recent evidence suggests that tetraspanins themselves may become crowded by virus particles and that these crowds/aggregates co-internalize with the viral particles. Using microscopy, we studied human papillomavirus (HPV) type 16-dependent aggregates on the cell surface of tetraspanin overexpressing keratinocytes. We find that aggregates are (1) rich in at least two different tetraspanins, (2) three-dimensional architectures extending up to several micrometers into the cell, and (3) decorated intracellularly by filamentous actin. Moreover, in cells not overexpressing tetraspanins, we note that obscurin-like protein 1 (OBSL1), which is thought to be a cytoskeletal adaptor, associates with filamentous actin. We speculate that HPV contact with the cell membrane could trigger the formation of a large tetraspanin web. This web may couple the virus contact site to the intracellular endocytic actin machinery, possibly involving the cytoskeletal adaptor protein OBSL1. Functionally, such a tetraspanin web could serve as a virus entry platform, which is co-internalized with the virus particle.

Tetraspanin 33 (TSPAN33) regulates endocytosis and migration of human B lymphocytes by affecting the tension of the plasma membrane

B lymphocytes are a leukocyte subset capable of developing several functions apart from differentiating into antibody-secreting cells. These processes are triggered by external activation signals that induce changes in the plasma membrane properties, regulated by the formation of different lipid-bilayer subdomains that are associated with the underlying cytoskeleton through different linker molecules, thus allowing the functional specialization of regions within the membrane. Among these, there are tetraspanin-enriched domains. Tetraspanins constitute a superfamily of transmembrane proteins that establish lateral associations with other molecules, determining its activity and localization. In this study, we identified TSPAN33 as an active player during B-lymphocyte cytoskeleton and plasma membrane-related phenomena, including protrusion formation, adhesion, phagocytosis, and cell motility. By using an overexpression model of TSPAN33 in human Raji cells, we detected a specific distribution of this protein that includes membrane microvilli, the Golgi apparatus, and extracellular vesicles. Additionally, we identified diminished phagocytic ability and altered cell adhesion properties due to the aberrant expression of integrins. Accordingly, these cells presented an enhanced migratory phenotype, as shown by its augmented chemotaxis and invasion rates. When we evaluated the mechanic response of cells during fibronectin-induced spreading, we found that TSPAN33 expression inhibited changes in roughness and membrane tension. Contrariwise, TSPAN33 knockdown cells displayed opposite phenotypes to those observed in the overexpression model. Altogether, our data indicate that TSPAN33 represents a regulatory element of the adhesion and migration of B lymphocytes, suggesting a novel implication of this tetraspanin in the control of the mechanical properties of their plasma membrane.

The metastasis suppressor CD82/KAI1 represses the TGF-β 1 and Wnt signalings inducing epithelial-to-mesenchymal transition linked to invasiveness of prostate cancer cells

BACKGROUND

The epithelial-to-mesenchymal transition (EMT) is closely associated with cancer invasion and metastasis. Since the transforming growth factor β (TGF-β) and Wnt signals induce EMT in various epithelial cell types, we examined whether and how the CD82/KAI1 metastasis suppressor affects the TGF-β and Wnt signal-dependent EMT in human prostate cancer cells.

METHODS

The invasiveness of cancer cells was evaluated by examining their ability to pass through the basement membrane matrigel. The subcellular localizations of Smad4 and β-catenin proteins were respectively examined by confocal microscopy following immunofluorescence antibody staining and immunoblotting analysis following subcellular fractionation. The transcriptional activities of the TGF-β₁ -responsive TRE and Wnt-responsive Tcf/Lef promoters were determined by a luciferase reporter assay following transfection of the recombinant reporter vector into the cell.

RESULTS

TGF-β₁ and Wnt3a treatments of human prostate cancer cells without CD82 expression resulted in not only increased invasiveness but also EMT involving the development of motile structures, downregulation of E-cadherin, and upregulation of the mesenchymal proteins. However, in the cells with high levels of CD82, the TGF-β₁ and Wnt3a stimulations neither elevated invasiveness nor induced EMT. Furthermore, the TGF-β₁ signaling events occurring in the CD82-deficient cells, such as phosphorylation of Smad2, nuclear translocation of Smad4, and transactivation of the TRE promoter, did not take place in the high CD82-expressing cells. Further, high CD82 expression interfered with the Wnt signal-dependent alterations in the phosphorylation pattern of glycogen synthase kinase 3β (GSK-3β) in prostate cancer cells, which allowed GSK-3β to continue phosphorylating β-catenin, thereby attenuating the Wnt signaling effects on the nuclear translocation of β-catenin and subsequent transactivation of the Tcf/Lef promoter.

CONCLUSIONS

The results of the present study suggest that CD82/KAI1 functions in suppressing TGF-β₁ - and Wnt-induced EMT in prostate cancer cells by inhibiting the TGF-β₁ /Smad and Wnt/β-catenin pathways. Therefore, loss or decrease of CD82 expression is likely to render prostate cancer cells prone to respond to the TGF-β₁ and Wnt signals with EMT, resulting in the development of a motile and invasive mesenchymal phenotype related to the initiation of the metastatic cascade.

[The role of exosomal tetraspanins and proteases in tumor progression]

Major (CD9, CD63, CD81) and others (CD82, CD151, Tspan8) tetraspanins are widely represented in exosomes, where they interact with various proteins and form functional tetraspanin complexes. Tetraspanin complexes include proteases. Tetraspanin-associated exosomal proteases (ADAM proteases, MMPs, EMMPRIN) play an important role in the processes of cell motility, migration, invasion and formation of metastases. Also, a significant contribution to tumor progression is made by proteases that are not associated with tetraspanins. They destabilize intercellular contacts, promote migration and invasion of tumor cells, participate in the regulation of the expression IGF-I, VEGF and transcription factors activation/deactivation. The role of other proteases of exosomes in the processes of tumor progression is being clarified.

Tetraspanin CD82 represses Sp1-mediated Snail expression and the resultant E-cadherin expression interrupts nuclear signaling of β-catenin by increasing its membrane localization

Tetraspanin membrane proteins form physical complexes with signaling molecules and have been suggested to influence the signaling events of associated molecules. Of the tetraspanin proteins, CD82 has been shown to promote homotypic cell-cell adhesion, which partially accounts for its role in suppressing cancer invasion and metastasis. We found here that CD82-induced cell-cell adhesion is attributed to increased E-cadherin expression through CD82-mediated downregulation of the E-cadherin repressor Snail. The Snail repression by CD82 resulted from the reduced binding of the Sp1 transcription factor to the Snail gene promoter. Notably, high CD82 expression did not allow the fibronectin matrix to induce Sp1 phosphorylation, implicating CD82 inhibition of the fibronectin-integrin signaling-dependent Sp1 activation. Meanwhile, E-cadherin upregulated by CD82 pulled β-catenin up to the membrane region, and consequently reduced the amount of cytoplasmic β-catenin that was able to move into to the nucleus. The Wnt signal-induced nuclear translocation of β-catenin was also inhibited by the CD82 function of upregulating E-cadherin. Overall, high CD82 expression was likely to suppress fibronectin adhesion-induced Sp1 activation signaling for Snail expression, resulting in continuous E-cadherin expression, which contributed not only to the maintenance of strong cell-cell adhesion but also to the blockage of nuclear β-catenin signaling.

The tetraspanin CD82 regulates bone marrow homing and engraftment of hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cell (HSPC) transplantation represents a treatment option for patients with malignant and nonmalignant hematological diseases. Initial steps in transplantation involve the bone marrow homing and engraftment of peripheral blood–injected HSPCs. In recent work, we identified the tetraspanin CD82 as a potential regulator of HSPC homing to the bone marrow, although its mechanism remains unclear. In the present study, using a CD82 knockout (CD82KO) mouse model, we determined that CD82 modulates HSPC bone marrow maintenance, homing, and engraftment. Bone marrow characterization identified a significant decrease in the number of long-term hematopoietic stem cells in the CD82KO mice, which we linked to cell cycle activation and reduced stem cell quiescence. Additionally, we demonstrate that CD82 deficiency disrupts bone marrow homing and engraftment, with in vitro analysis identifying further defects in migration and cell spreading. Moreover, we find that the CD82KO HSPC homing defect is due at least in part to the hyperactivation of Rac1, as Rac1 inhibition rescues homing capacity. Together, these data provide evidence that CD82 is an important regulator of HSPC bone marrow maintenance, homing, and engraftment and suggest exploiting the CD82 scaffold as a therapeutic target for improved efficacy of stem cell transplants.

The metastasis suppressor CD82/KAI1 inhibits fibronectin adhesion-induced epithelial-to-mesenchymal transition in prostate cancer cells by repressing the associated integrin signaling

The transmembrane protein CD82/KAI1 suppresses the metastatic potential of various cancer cell types. Moreover, decrease or loss of CD82 expression is closely associated with malignancy and poor prognosis in many human cancers including prostate cancer. Despite intense scrutiny, the mechanisms underlying the metastasis-suppressing role of CD82 are still not fully understood. Here, we found that a fibronectin matrix induced mesenchymal phenotypes in human prostate cancer cells with no or low CD82 expression levels. However, high CD82 expression rendered prostate cancer cells to have intensified epithelial characteristics upon fibronectin engagement, along with decreased cell motility and invasiveness. The CD82 function of inhibiting fibronectin-induced epithelial-to-mesenchymal transition (EMT) was dependent not only on CD82 interactions with fibronectin-binding α₃β₁/α₅β₁ integrins but also on the integrin-mediated intracellular signaling events. Notably, CD82 attenuated the FAK-Src and ILK pathways downstream of the fibronectin-receptor integrins. Immunofluorescence staining of human prostate cancer tissue specimens illustrated a negative association of CD82 with EMT-related gene expression as well as prostate malignancy. Altogether, these results suggest that CD82 suppresses EMT in prostate cancer cells adhered to the fibronectin matrix by repressing adhesion signaling through lateral interactions with the associated α₃β₁ and α₅β₁ integrins, leading to reduced cell migration and invasive capacities.

CD151-mediated adhesion is crucial to osteosarcoma pulmonary metastasis

CD151, a tetraspanin family protein involved in cell-cell and cell-extracellular matrix interaction, is differentially expressed in osteosarcoma cell membranes. Thus, this study aimed to investigate the role of CD151 in osteosarcoma metastasis. We analyzed CD151 expression in patient tissue samples using immunohistochemistry. CD151 expression was also silenced with shRNA in osteosarcoma cells of high metastatic potential, and cell adhesion, migration and invasion were evaluated in vitro and pulmonary metastasis was investigated in vivo. Mediators of cell signaling pathways were also examined following suppression of CD151 expression. Overall survival for patients with low versus high CD151 expression level was 94 vs. 41 months (p=0.0451). CD151 expression in osteosarcoma cells with high metastatic potential was significantly higher than in those with low metastatic potential (p<0.001). shRNA-mediated silencing of CD151 did not influence cell viability or proliferation; however, cell adhesion, migration and invasion were all inhibited (all p<0.001). In mice inoculated with shRNA-transduced osteosarcoma cells, the number and size of lung metastatic lesions were reduced compared to the mice inoculated with control-shRNA transduced cells (p<0.001). In addition, CD151 knockdown significantly reduced Akt, p38, and p65 phosphorylation as well as focal adhesion kinase, integrin β1, p70s6, and p-mTOR levels. Taken together, CD151 induced osteosarcoma metastasis likely by regulating cell function through adhesion signaling. Further studies are necessary to fully explore the diagnostic and prognostic value of determining CD151 expression in osteosarcoma patients.

CD151 Expression Is Associated with a Hyperproliferative T Cell Phenotype

The tetraspanin CD151 is a marker of aggressive cell proliferation and invasiveness for a variety of cancer types. Given reports of CD151 expression on T cells, we explored whether CD151 would mark T cells in a hyperactivated state. Consistent with the idea that CD151 could mark a phenotypically distinct T cell subset, it was not uniformly expressed on T cells. CD151 expression frequency was a function of the T cell lineage (CD8 > CD4) and a function of the memory differentiation state (naive T cells < central memory T cells < effector memory T cells < T effector memory RA⁺ cells). CD151 and CD57, a senescence marker, defined the same CD28^- T cell populations. However, CD151 also marked a substantial CD28⁺ T cell population that was not marked by CD57. Kinome array analysis demonstrated that CD28⁺CD151⁺ T cells form a subpopulation with a distinct molecular baseline and activation phenotype. Network analysis of these data revealed that cell cycle control and cell death were the most altered process motifs in CD28⁺CD151⁺ T cells. We demonstrate that CD151 in T cells is not a passive marker, but actively changed the cell cycle control and cell death process motifs of T cells. Consistent with these data, long-term T cell culture experiments in the presence of only IL-2 demonstrated that independent of their CD28 expression status, CD151⁺ T cells, but not CD151^- T cells, would exhibit an Ag-independent, hyperresponsive proliferation phenotype. Not unlike its reported function as a tumor aggressiveness marker, CD151 in humans thus marks and enables hyperproliferative T cells.

Molecular interactions shaping the tetraspanin web

To facilitate the myriad of different (signaling) processes that take place at the plasma membrane, cells depend on a high degree of membrane protein organization. Important mediators of this organization are tetraspanin proteins. Tetraspanins interact laterally among themselves and with partner proteins to control the spatial organization of membrane proteins in large networks called the tetraspanin web. The molecular interactions underlying the formation of the tetraspanin web were hitherto mainly described based on their resistance to different detergents, a classification which does not necessarily correlate with functionality in the living cell. To look at these interactions from a more physiological point of view, this review discusses tetraspanin interactions based on their function in the tetraspanin web: (1) intramolecular interactions supporting tetraspanin structure, (2) tetraspanin-tetraspanin interactions supporting web formation, (3) tetraspanin-partner interactions adding functional partners to the web and (4) cytosolic tetraspanin interactions regulating intracellular signaling. The recent publication of the first full-length tetraspanin crystal structure sheds new light on both the intra- and intermolecular tetraspanin interactions that shape the tetraspanin web. Furthermore, recent molecular dynamic modeling studies indicate that the binding strength between tetraspanins and between tetraspanins and their partners is the complex sum of both promiscuous and specific interactions. A deeper insight into this complex mixture of interactions is essential to our fundamental understanding of the tetraspanin web and its dynamics which constitute a basic building block of the cell surface.

Tetraspanins Function as Regulators of Cellular Signaling

Tetraspanins are molecular scaffolds that distribute proteins into highly organized microdomains consisting of adhesion, signaling, and adaptor proteins. Many reports have identified interactions between tetraspanins and signaling molecules, finding unique downstream cellular consequences. In this review, we will explore these interactions as well as the specific cellular responses to signal activation, focusing on tetraspanin regulation of adhesion-mediated (integrins/FAK), receptor-mediated (EGFR, TNF-α, c-Met, c-Kit), and intracellular signaling (PKC, PI4K, β-catenin). Additionally, we will summarize our current understanding for how tetraspanin post-translational modifications (palmitoylation, N-linked glycosylation, and ubiquitination) can regulate signal propagation. Many of the studies outlined in this review suggest that tetraspanins offer a potential therapeutic target to modulate aberrant signal transduction pathways that directly impact a host of cellular behaviors and disease states.

Regulation of Glioblastoma Tumor-Propagating Cells by the Integrin Partner Tetraspanin CD151

Glioblastoma (GBM) stem cells (GSCs) represent tumor-propagating cells with stem-like characteristics (stemness) that contribute disproportionately to GBM drug resistance and tumor recurrence. Understanding the mechanisms supporting GSC stemness is important for developing therapeutic strategies for targeting GSC-dependent oncogenic mechanisms. Using GBM-derived neurospheres, we identified the cell surface tetraspanin family member CD151 as a novel regulator of glioma cell stemness, GSC self-renewal capacity, migration, and tumor growth. CD151 was found to be overexpressed in GBM tumors and GBM neurospheres enriched in GSCs. Silencing CD151 inhibited neurosphere forming capacity, neurosphere cell proliferation, and migration and attenuated the expression of markers and transcriptional drivers of the GSC phenotype. Conversely, forced CD151 expression promoted neurosphere self-renewal, cell migration, and expression of stemness-associated transcription factors. CD151 was found to complex with integrins α3, α6, and β1 in neurosphere cells, and blocking CD151 interactions with integrins α3 and α6 inhibited AKT phosphorylation, a downstream effector of integrin signaling, and impaired sphere formation and neurosphere cell migration. Additionally, targeting CD151 in vivo inhibited the growth of GBM neurosphere-derived xenografts. These findings identify CD151 and its interactions with integrins α3 and α6 as potential therapeutic targets for inhibiting stemness-driving mechanisms and stem cell populations in GBM.

Tetraspanin CD151 Promotes Initial Events in Human Cytomegalovirus Infection

Human cytomegalovirus (HCMV), a betaherpesvirus, can cause life-threatening disease in immunocompromised individuals. Viral envelope glycoproteins that mediate binding to and penetration into target cells have been identified previously. In contrast, cellular proteins supporting HCMV during entry are largely unknown. In order to systematically identify host genes affecting initial steps of HCMV infection, a targeted RNA interference screen of 96 cellular genes was performed in endothelial cells by use of a virus strain expressing the full set of known glycoprotein H and L (gH/gL) complexes. The approach yielded five proviral host factors from different protein families and eight antiviral host factors, mostly growth factor receptors. The tetraspanin CD151 was uncovered as a novel proviral host factor and was analyzed further. Like endothelial cells, fibroblasts were also less susceptible to HCMV infection after CD151 depletion. Virus strains with different sets of gH/gL complexes conferring either broad or narrow cell tropism were equally impaired. Infection of CD151-depleted cells by a fluorescent virus with differentially labeled capsid and envelope proteins revealed a role of CD151 in viral penetration but not in adsorption to the cell. In conclusion, the tetraspanin CD151 has emerged as a novel host factor in HCMV entry and as a putative antiviral target.

IMPORTANCE

At present, the events at the virus-cell interface and the cellular proteins involved during the HCMV entry steps are scarcely understood. In this study, several host factors with putative roles in this process were identified. The tetraspanin CD151 was discovered as a previously unrecognized proviral host factor for HCMV and was found to support viral penetration into the target cells. The findings of this study shed light on the cellular contribution during the initial steps of HCMV infection and open a new direction in HCMV research.

Tetraspanin CD82 Regulates the Spatiotemporal Dynamics of PKCα in Acute Myeloid Leukemia

Patients with acute myeloid leukemia (AML) have increased myeloid cells within their bone marrow that exhibit aberrant signaling. Therefore, therapeutic targets that modulate disrupted signaling cascades are of significant interest. In this study, we demonstrate that the tetraspanin membrane scaffold, CD82, regulates protein kinase c alpha (PKCα)-mediated signaling critical for AML progression. Utilizing a palmitoylation mutant form of CD82 with disrupted membrane organization, we find that the CD82 scaffold controls PKCα expression and activation. Combining single molecule and ensemble imaging measurements, we determine that CD82 stabilizes PKCα activation at the membrane and regulates the size of PKCα membrane clusters. Further evaluation of downstream effector signaling identified robust and sustained activation of ERK1/2 upon CD82 overexpression that results in enhanced AML colony formation. Together, these data propose a mechanism where CD82 membrane organization regulates sustained PKCα signaling that results in an aggressive leukemia phenotype. These observations suggest that the CD82 scaffold may be a potential therapeutic target for attenuating aberrant signal transduction in AML.

Pancreatic cancer stem cell markers and exosomes - the incentive push

Pancreatic cancer (PaCa) has the highest death rate and incidence is increasing. Poor prognosis is due to late diagnosis and early metastatic spread, which is ascribed to a minor population of so called cancer stem cells (CSC) within the mass of the primary tumor. CSC are defined by biological features, which they share with adult stem cells like longevity, rare cell division, the capacity for self renewal, differentiation, drug resistance and the requirement for a niche. CSC can also be identified by sets of markers, which for pancreatic CSC (Pa-CSC) include CD44v6, c-Met, Tspan8, alpha6beta4, CXCR4, CD133, EpCAM and claudin7. The functional relevance of CSC markers is still disputed. We hypothesize that Pa-CSC markers play a decisive role in tumor progression. This is fostered by the location in glycolipid-enriched membrane domains, which function as signaling platform and support connectivity of the individual Pa-CSC markers. Outside-in signaling supports apoptosis resistance, stem cell gene expression and tumor suppressor gene repression as well as miRNA transcription and silencing. Pa-CSC markers also contribute to motility and invasiveness. By ligand binding host cells are triggered towards creating a milieu supporting Pa-CSC maintenance. Furthermore, CSC markers contribute to the generation, loading and delivery of exosomes, whereby CSC gain the capacity for a cell-cell contact independent crosstalk with the host and neighboring non-CSC. This allows Pa-CSC exosomes (TEX) to reprogram neighboring non-CSC towards epithelial mesenchymal transition and to stimulate host cells towards preparing a niche for metastasizing tumor cells. Finally, TEX communicate with the matrix to support tumor cell motility, invasion and homing. We will discuss the possibility that CSC markers are the initial trigger for these processes and what is the special contribution of CSC-TEX.

Function and Dynamics of Tetraspanins during Antigen Recognition and Immunological Synapse Formation

Tetraspanin-enriched microdomains (TEMs) are specialized membrane platforms driven by protein–protein interactions that integrate membrane receptors and adhesion molecules. Tetraspanins participate in antigen recognition and presentation by antigen-­presenting cells (APCs) through the organization of pattern-recognition receptors (PRRs) and their downstream-induced signaling, as well as the regulation of MHC-II–peptide trafficking. T lymphocyte activation is triggered upon specific recognition of antigens present on the APC surface during immunological synapse (IS) formation. This dynamic process is characterized by a defined spatial organization involving the compartmentalization of receptors and adhesion molecules in specialized membrane domains that are connected to the underlying cytoskeleton and signaling molecules. Tetraspanins contribute to the spatial organization and maturation of the IS by controlling receptor clustering and local accumulation of adhesion receptors and integrins, their downstream signaling, and linkage to the actin cytoskeleton. This review offers a perspective on the important role of TEMs in the regulation of antigen recognition and presentation and in the dynamics of IS architectural organization.

CD151 knockdown inhibits osteosarcoma metastasis through the GSK-3β/β-catenin/MMP9 pathway

Osteosarcoma (OS) is a primary bone malignancy with a high early metastatic propensity. It is crucial to find specific protein targets to develop therapeutic strategies against this lethal disease. Tetraspanin CD151 is involved in facilitating tumor metastasis. However, the role and molecular mechanism of CD151 in promoting OS metastasis remain enigmatic. In the present study, we used small interfering RNA (siRNA) to inhibit CD151 expression in highly metastatic OS cells and the results demonstrated that CD151 knockdown inhibited their migration, invasion and metastasis. We further investigated the molecular mechanism of CD151 by inhibiting genes known to be involved in metastasis in OS cells and found that CD151 modulated matrix metalloproteinase 9 (MMP9) expression through the glycogen synthase kinase 3 (GSK-3β)/β-catenin signaling pathway. We conclude that CD151 knockdown inhibits the expression of MMP9 through the GSK-3β/β‑catenin pathway and also inhibits OS migration and invasion in vitro and metastasis in vivo in highly metastatic OS. This suggests that CD151 may be a useful antimetastatic target for OS.

Tetraspanins as regulators of the tumour microenvironment: implications for metastasis and therapeutic strategies

One of the hallmarks of cancer is the ability to activate invasion and metastasis. Cancer morbidity and mortality are largely related to the spread of the primary, localized tumour to adjacent and distant sites. Appropriate management and treatment decisions based on predicting metastatic disease at the time of diagnosis is thus crucial, which supports better understanding of the metastatic process. There are components of metastasis that are common to all primary tumours: dissociation from the primary tumour mass, reorganization/remodelling of extracellular matrix, cell migration, recognition and movement through endothelial cells and the vascular circulation and lodgement and proliferation within ectopic stroma. One of the key and initial events is the increased ability of cancer cells to move, escaping the regulation of normal physiological control. The cellular cytoskeleton plays an important role in cancer cell motility and active cytoskeletal rearrangement can result in metastatic disease. This active change in cytoskeletal dynamics results in manipulation of plasma membrane and cellular balance between cellular adhesion and motility which in turn determines cancer cell movement. Members of the tetraspanin family of proteins play important roles in regulation of cancer cell migration and cancer-endothelial cell interactions, which are critical for cancer invasion and metastasis. Their involvements in active cytoskeletal dynamics, cancer metastasis and potential clinical application will be discussed in this review. In particular, the tetraspanin member, CD151, is highlighted for its major role in cancer invasion and metastasis.

LINKED ARTICLES

This article is part of a themed section on Cytoskeleton, Extracellular Matrix, Cell Migration, Wound Healing and Related Topics. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-24.

The tetraspanins CD151 and Tspan8 are essential exosome components for the crosstalk between cancer initiating cells and their surrounding

Tspan8 and CD151 are metastasis-promoting tetraspanins and a knockdown (kd) of Tspan8 or CD151 and most pronounced of both tetraspanins affects the metastatic potential of the rat pancreatic adenocarcinoma line ASML. Approaching to elaborate the underlying mechanism, we compared ASMLwt, -CD151kd and/or Tspan8kd clones. We focused on tumor exosomes, as exosomes play a major role in tumor progression and tetraspanins are suggested to be engaged in exosome targeting. ASML-CD151/Tspan8kd cells poorly metastasize, but regain metastatic capacity, when rats are pretreated with ASMLwt, but not ASML-CD151kd and/or -Tspan8kd exosomes. Both exosomal CD151 and Tspan8 contribute to host matrix remodelling due to exosomal tetraspanin-integrin and tetraspanin-protease associations. ASMLwt exosomes also support stroma cell activation with upregulation of cytokines, cytokine receptors and proteases and promote inflammatory cytokine expression in hematopoietic cells. Finally, CD151-/Tspan8-competent exosomes support EMT gene expression in poorly-metastatic ASML-CD151/Tspan8kd cells. These effects are not seen or are weakened using ASML-CD151kd or -Tspan8kd exosomes, which is at least partly due to reduced binding/uptake of CD151- and/or Tspan8-deficient exosomes. Thus, CD151- and Tspan8-competent tumor exosomes support matrix degradation, reprogram stroma and hematopoietic cells and drive non-metastatic ASML-CD151/Tspan8kd cells towards a motile phenotype.

Regulation of FAK Activity by Tetraspan Proteins: Potential Clinical Implications in Cancer

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that regulates multiple cell signaling pathways in both physiological and pathological conditions. Overexpression and activation of FAK is associated with many advanced stage cancers through promoting cancer cell tumorigenicity and progression as well as by regulating the tumor microenvironment. FAK has multiple binding partners through which FAK exerts its functions including RhoGEF, Src family, talin, cortactin, and paxilin. Over the last few years, it has been proposed that a novel group of four transmembrane proteins can interact with FAK and regulate its activity. These include select tetraspanins such as CD151 and CD9 as well as the GAS3 family members epithelial membrane protein-2 (EMP2) and peripheral myelin protein-22 (PMP22). In this review, we discuss the current knowledge of the interaction between FAK and tetraspan proteins in physiological and pathological conditions, with an emphasis on the potential of tetraspan family members as therapeutic targets in cancer.

Tetraspanin CD9 regulates cell contraction and actin arrangement via RhoA in human vascular smooth muscle cells

The most prevalent cardiovascular diseases arise from alterations in vascular smooth muscle cell (VSMC) morphology and function. Tetraspanin CD9 has been previously implicated in regulating vascular pathologies; however, insight into how CD9 may regulate adverse VSMC phenotypes has not been provided. We utilized a human model of aortic smooth muscle cells to understand the consequences of CD9 deficiency on VSMC phenotypes. Upon knocking down CD9, the cells developed an abnormally small and rounded morphology. We determined that this morphological change was due to a lack of typical parallel actin arrangement. We also found similar total RhoA but decreased GTP-bound (active) RhoA levels in CD9 deficient cells. As a result, cells lacking a full complement of CD9 were less contractile than their control treated counterparts. Upon restoration of RhoA activity in the CD9 deficient cells, the phenotype was reversed and cell contraction was restored. Conversely, inhibition of RhoA activity in the control cells mimicked the CD9-deficient cell phenotype. Thus, alteration in CD9 expression was sufficient to profoundly disrupt cellular actin arrangement and endogenous cell contraction by interfering with RhoA signaling. This study provides insight into how CD9 may regulate previously described vascular smooth muscle cell pathophysiology.

Tetraspanins at a glance

Tetraspanins are a family of proteins with four transmembrane domains that play a role in many aspects of cell biology and physiology; they are also used by several pathogens for infection and regulate cancer progression. Many tetraspanins associate specifically and directly with a limited number of proteins, and also with other tetraspanins, thereby generating a hierarchical network of interactions. Through these interactions, tetraspanins are believed to have a role in cell and membrane compartmentalization. In this Cell Science at a Glance article and the accompanying poster, we describe the basic principles underlying tetraspanin-based assemblies and highlight examples of how tetraspanins regulate the trafficking and function of their partner proteins that are required for the normal development and function of several organs, including, in humans, the eye, the kidney and the immune system.

Correlations between transmembrane 4 L6 family member 5 (TM4SF5), CD151, and CD63 in liver fibrotic phenotypes and hepatic migration and invasive capacities

Transmembrane 4 L6 family member 5 (TM4SF5) is overexpressed during CCl₄-mediated murine liver fibrosis and in human hepatocellular carcinomas. The tetraspanins form tetraspanin-enriched microdomains (TEMs) consisting of large membrane protein complexes on the cell surface. Thus, TM4SF5 may be involved in the signal coordination that controls liver malignancy. We investigated the relationship between TM4SF5-positive TEMs with liver fibrosis and tumorigenesis, using normal Chang hepatocytes that lack TM4SF5 expression and chronically TGFβ1-treated Chang cells that express TM4SF5. TM4SF5 expression is positively correlated with tumorigenic CD151 expression, but is negatively correlated with tumor-suppressive CD63 expression in mouse fibrotic and human hepatic carcinoma tissues, indicating cooperative roles of the tetraspanins in liver malignancies. Although CD151 did not control the expression of TM4SF5, TM4SF5 appeared to control the expression levels of CD151 and CD63. TM4SF5 interacted with CD151, and caused the internalization of CD63 from the cell surface into late lysosomal membranes, presumably leading to terminating the tumor-suppressive functions of CD63. TM4SF5 could overcome the tumorigenic effects of CD151, especially cell migration and extracellular matrix (ECM)-degradation. Taken together, TM4SF5 appears to play a role in liver malignancy by controlling the levels of tetraspanins on the cell surface, and could provide a promising therapeutic target for the treatment of liver malignancies.

Tetraspanins CD9 and CD151 at the immune synapse support T-cell integrin signaling

Understanding how the immune response is activated and amplified requires detailed knowledge of the stages in the formation of the immunological synapse (IS) between T lymphocytes and antigen-presenting cells (APCs). We show that tetraspanins CD9 and CD151 congregate at the T-cell side of the IS. Silencing of CD9 or CD151 blunts the IL-2 secretion and expression of the activation marker CD69 by APC-conjugated T lymphocytes, but does not affect the accumulation of CD3 or actin to the IS, or the translocation of the microtubule-organizing center toward the T-B contact area. CD9 or CD151 silencing diminishes the relocalization of α4β1 integrin to the IS and reduces the accumulation of high-affinity β1 integrins at the cell-cell contact. These changes are accompanied by diminished phosphorylation of the integrin downstream targets FAK and ERK1/2. Our results suggest that CD9 and CD151 support integrin-mediated signaling at the IS.

The tetraspanin CD81 protein increases melanoma cell motility by up-regulating metalloproteinase MT1-MMP expression through the pro-oncogenic Akt-dependent Sp1 activation signaling pathways

Despite the importance of multiple tetraspanin proteins in cancer invasion and metastasis, little is known about the role and significance of tetraspanin CD81 in these processes. In the present study, we examined CD81 effects on melanoma cell invasiveness and metastasis. Transfection of CD81 into melanoma cells lacking endogenous CD81 expression significantly enhanced the migrating, invasive, and metastatic abilities of melanoma cells. Interestingly, membrane type 1 matrix metalloproteinase (MT1-MMP) expression was found in CD81-expressing melanoma cells but not in CD81-deficient cells. siRNA knockdown of CD81 in melanoma cells with endogenous CD81 demonstrated decreased MT1-MMP levels and cell motility. Notably, CD81-induced cell migration was abrogated by antibody blocking and siRNA knockdown of MT1-MMP, indicating that MT1-MMP is responsible for CD81-stimulated melanoma cell migration. Promoter analysis revealed an essential role of the Sp1 transcription factor in CD81-induced MT1-MMP transcription. We also demonstrate that the Sp1-activating Akt pathway is involved in adhesion-dependent CD81 signaling to induce MT1-MMP expression and cell motility. Importantly, human skin cancer tissue specimens displayed a positive correlation of CD81 with MT1-MMP expression levels and a close association of CD81 with malignant melanomas. Taken together, these results strongly suggest that CD81 stimulates melanoma cell motility by inducing MT1-MMP expression through the Akt-dependent Sp1 activation signaling pathway, leading to increased melanoma invasion and metastasis.

The tetraspanin CD151 in papillomavirus infection

Human papillomaviruses (HPV) are non-enveloped DNA tumor viruses that infect skin and mucosa. The most oncogenic subtype, HPV16, causes various types of cancer, including cervical, anal, and head and neck cancers. During the multistep process of infection, numerous host proteins are required for the delivery of virus genetic information into the nucleus of target cells. Over the last two decades, many host-cell proteins such as heparan sulfate proteoglycans, integrins, growth factor receptors, actin and the tetraspanin CD151 have been described to be involved in the process of infectious entry of HPV16. Tetraspanins have the ability to organize membrane microdomains and to directly influence the function of associated molecules, including binding of receptors to their ligands, receptor oligomerization and signal transduction. Here, we summarize the current knowledge on CD151, and CD151-associated partners during HPV infection and discuss the underlying mechanisms.

CD151 in cancer progression and metastasis: a complex scenario

Originally identified as a molecular organizer of interacting proteins into tetraspanin-enriched microdomains, the tetraspanin CD151 has now been shown to be involved in tumour progression. Increasing evidence emerging from in vitro, in vivo and clinical analyses implicates this tetraspanin in supporting growth of various types of tumours at different levels. It affects both cell autonomous behavior and communication with neighboring cells and the microenvironment. CD151 regulates post-adhesion events, that is, cell spreading, migration and invasion including subsequent intravasation and formation of metastasis. Present on both neoplastic and endothelial cells, CD151 is engaged in promotion of tumour neovascularization. The molecular mechanism of CD151 in cancer is based on its ability to organize distribution and function of interacting proteins, ie, laminin-binding integrins (α3β1, α6β1 and α6β4), receptors for growth factors (HGFR, EGFR and TGF-β1R) and matrix metalloproteinases (MMP-7, MMP-2 and MMP-9), which indicates its importance in disease development. Results of clinical analyses of CD151 expression in different types of cancer and a large number of in vivo models demonstrate its impact on tumour growth and invasion and implicate CD151 as a valuable diagnostic and prognostic marker as well as a potential target for anti-cancer therapy.

Signaling mechanisms controlling cranial placode neurogenesis and delamination

The neurogenic cranial placodes are a unique transient epithelial niche of neural progenitor cells that give rise to multiple derivatives of the peripheral nervous system, particularly, the sensory neurons. Placode neurogenesis occurs throughout an extended period of time with epithelial cells continually recruited as neural progenitor cells. Sensory neuron development in the trigeminal, epibranchial, otic, and olfactory placodes coincides with detachment of these neuroblasts from the encompassing epithelial sheet, leading to delamination and ingression into the mesenchyme where they continue to differentiate as neurons. Multiple signaling pathways are known to direct placodal development. This review defines the signaling pathways working at the finite spatiotemporal period when neuronal selection within the placodes occurs, and neuroblasts concomitantly delaminate from the epithelium. Examining neurogenesis and delamination after initial placodal patterning and specification has revealed a common trend throughout the neurogenic placodes, which suggests that both activated FGF and attenuated Notch signaling activities are required for neurogenesis and changes in epithelial cell adhesion leading to delamination. We also address the varying roles of other pathways such as the Wnt and BMP signaling families during sensory neurogenesis and neuroblast delamination in the differing placodes.

Integrin α3β1-CD151 complex regulates dimerization of ErbB2 via RhoA

Integrin α3β1 regulates adhesive interactions of cells with laminins and have a critical role in adhesion-dependent cellular responses. Here, we examined the role of α3β1-integrin in ErbB2-dependent proliferation of breast cancer cells in three-dimensional laminin-rich extracellular matrix (3D lr-ECM). Depletion of α3β1 in ErbB2-overexpressing breast cancer cells suppressed growth and restore cell polarity in 3D lr-ECM. The phenotype of α3β1-depleted cells was reproduced upon depletion of tetraspanin CD151 and mirrored that of the cells treated with Herceptin, an established ErbB2 antagonist. Breast cancer cells expressing the α3β1-CD151 complex have higher steady-state phosphorylation of ErbB2 and show enhanced dimerization of the protein when compared with α3β1-/CD151-depleted cells. Furthermore, Herceptin-dependent dephosphorylation of ErbB2 was only observed in α3β1-CD151-expressing cells. Importantly, the inhibitory activity of Herceptin was more pronounced when cells expressed both α3β1 and CD151. We also found that the level of active RhoA was increased in α3β1- and CD151-depleted cells and that Rho controls dimerization of ErbB2. Expression of α3β1 alone did not have significant prognostic value in patients with invasive ductal carcinoma of the breast. However, expression of α3β1 in combination with CD151 represented a more stringent indicator of poor survival than CD151 alone. Taken together, these results demonstrate that the α3β1-CD151 complex has a critical regulatory role in ErbB2-dependent signalling and thereby may be involved in breast cancer progression.

Plakophilin 2 affects cell migration by modulating focal adhesion dynamics and integrin protein expression

Plakophilin 2 (PKP2), a desmosome component, modulates the activity and localization of the small GTPase RhoA at sites of cell-cell contact. PKP2 regulates cortical actin rearrangement during junction formation, and its loss is accompanied by an increase in actin stress fibers. We hypothesized that PKP2 may regulate focal adhesion dynamics and cell migration. Here we show that PKP2-deficient cells bind efficiently to the extracellular matrix, but upon spreading display total cell areas ≈ 30% smaller than control cells. Focal adhesions in PKP2-deficient cells are ≈ 2 × larger and more stable than in control cells, and vinculin displays an increased time for fluorescence recovery after photobleaching. Furthermore, β4 and β1 integrin protein and mRNA expression is elevated in PKP2-silenced cells. Normal focal adhesion phenotypes can be restored in PKP2-null cells by dampening the RhoA pathway or silencing β1 integrin. However, integrin expression levels are not restored by RhoA signaling inhibition. These data uncover a potential role for PKP2 upstream of β1 integrin and RhoA in integrating cell-cell and cell-substrate contact signaling in basal keratinocytes necessary for the morphogenesis, homeostasis, and reepithelialization of the stratified epidermis.

Tspan8 and CD151 promote metastasis by distinct mechanisms

AIM

CD151 and Tspan8 are metastasis-promoting tetraspanins. To define whether Tspan8 and CD151 fulfil redundant or additive activities, Tspan8 and CD151 were stably knocked-down in highly metastatic rat pancreatic adenocarcinoma BSp73ASML cells (ASML(wt), ASML-Tspan8(kd), ASML-CD151(kd)).

RESULTS

ASML-CD151(kd) and ASML-Tspan8(kd) cells metastasise via the lymphatics to the lung with delay and a 2-3-fold increased survival time compared to ASML(wt) cells. Yet, CD151 and Tspan8 distinctly contribute to metastasis. Pronounced adhesion of ASML-Tspan8(kd) cells is due to CD151 associating with the alpha3 integrin chain, whereas strikingly increased ASML-CD151(kd) cell motility is efficiently inhibited by anti-beta4. These opposing Tspan8 and CD151 activities are due to distinct beta4 recruitment into Tspan8 complexes, accompanied by beta4 phosporylation, src recruitment, focal adhesion kinase (FAK) and Ras activation. On the other hand, CD151 associates more readily with proteases, particularly matrix metalloproteinase (MMP)13 and MMP9, than Tspan8. The stronger CD151-MMP association is accompanied by pronounced collagen I and IV and laminin111 degradation, also seen in metastatic tissue, and strengthens invasiveness.

CONCLUSION

CD151 and Tspan8 coordinately promote metastasis, where Tspan8 overrides the adhesive features of CD151 by recruiting integrins out of adhesion into motility promoting complexes. CD151 more efficiently than Tspan8 recruiting and activating MMP9 and MMP13 creates a path for migrating tumour cells.