Possible involvement of tetraspanin CO-029 in hematogenous intrahepatic metastasis of liver cancer cells

Molecular Regulation and Oncogenic Functions of TSPAN8

Tetraspanins, a superfamily of small integral membrane proteins, are characterized by four transmembrane domains and conserved protein motifs that are configured into a unique molecular topology and structure in the plasma membrane. They act as key organizers of the plasma membrane, orchestrating the formation of specialized microdomains called "tetraspanin-enriched microdomains (TEMs)" or "tetraspanin nanodomains" that are essential for mediating diverse biological processes. TSPAN8 is one of the earliest identified tetraspanin members. It is known to interact with a wide range of molecular partners in different cellular contexts and regulate diverse molecular and cellular events at the plasma membrane, including cell adhesion, migration, invasion, signal transduction, and exosome biogenesis. The functions of cell-surface TSPAN8 are governed by ER targeting, modifications at the Golgi apparatus and dynamic trafficking. Intriguingly, limited evidence shows that TSPAN8 can translocate to the nucleus to act as a transcriptional regulator. The transcription of TSPAN8 is tightly regulated and restricted to defined cell lineages, where it can serve as a molecular marker of stem/progenitor cells in certain normal tissues as well as tumors. Importantly, the oncogenic roles of TSPAN8 in tumor development and cancer metastasis have gained prominence in recent decades. Here, we comprehensively review the current knowledge on the molecular characteristics and regulatory mechanisms defining TSPAN8 functions, and discuss the potential and significance of TSPAN8 as a biomarker and therapeutic target across various epithelial cancers.

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.

Tspan15 Is a New Stemness-Related Marker in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the second cause of cancer-related deaths worldwide. A clearer understanding of the molecular mechanisms underlying tumor growth and invasiveness remains crucial for developing new therapies. Here, the expression of tetraspanins, a family of plasma membrane organizers involved in tumor progression, has been addressed. Integrative approaches combining transcriptomics and bioinformatics allow demonstrating the induced and heterogeneous expression of Tspan15 in HCC. Tspan15 positive tumors exhibit signatures related to hepatic progenitor cells as well as recurrence of cancer. Immunohistochemistry experiments confirm Tspan15 expression in the subset of HCC expressing stemness-related markers such as EpCAM and Cytokeratin-19. Functional networks reveal that most of these genes expressed in correlation to Tspan15 support cell proliferation. Furthermore, Tspan15 overexpression in the hepatoma cell line HepG2 significantly increases cell proliferation. A quantitative proteomic analysis of the secretome reveals a higher abundance of the protein connective tissue growth factor (CTGF), a pleiotropic matricellular signaling protein. Proteomic profiling of Tspan15 complexes allows identifying numerous membrane proteins including several growth factor receptors. Finally, Tspan15 increases ERK1/2 phosphorylation that directly controls CTGF expression and secretion. In conclusion, Tspan15 is a new stemness-related marker in HCC which exhibits high potential of tumor growth and recurrence.

Tspan8-β-catenin positive feedback loop promotes melanoma invasion

Due to its high proclivity to metastasize, and despite the recent development of targeted and immune therapy strategies, melanoma is still the deadliest form of skin cancer. Therefore, understanding the molecular mechanisms underlying melanoma invasion remains crucial. We previously characterized Tspan8 for its ability to prompt melanoma cell detachment from their microenvironment and trigger melanoma cell invasiveness, but the signaling events by which Tspan8 regulates the invasion process still remain unknown. Here, we demonstrated that β-catenin stabilization is a molecular signal subsequent to the onset of Tspan8 expression, and that, in turn, β-catenin triggers the direct transcriptional activation of Tspan8 expression, leading to melanoma invasion. Moreover, we showed that β-catenin activation systematically correlates with a high expression of Tspan8 protein in melanoma lesions from transgenic Nras; bcat* mice, as well as in deep penetrating naevi, a type of human pre-melanoma neoplasm characterized by a combined activation of β-catenin and MAP kinase signaling. Overall, our data suggest that β-catenin and Tspan8 are part of a positive feedback loop, which sustains a high Tspan8 expression level, conferring to melanoma cells the invasive properties required for tumor progression and dissemination.

Tetraspanin-8 promotes hepatocellular carcinoma metastasis by increasing ADAM12m expression

Recent evidence indicates that tetraspanin-8 (TSPAN8) promotes tumor progression and metastasis. In this study, we explored the effects of TSPAN8 and the molecular mechanisms underlying hepatocellular carcinoma (HCC) metastasis using various HCC cell lines, tissues from 149 HCC patients, and animal models of HCC progression. We showed that elevated expression of TSPAN8 promoted HCC invasion in vitro and metastasis in vivo, but did not influence HCC cell proliferation in vitro. Increased TSPAN8 expression in human HCC was predictive of poor survival, and multivariate analyses indicated TSPAN8 expression to be an independent predictor for both postoperative overall survival and relapse-free survival. Importantly, TSPAN8 enhanced HCC invasion and metastasis by increasing ADAM12m expression. We therefore conclude that TSPAN8 and ADAM12m may be useful therapeutic targets for the prevention of HCC progression and metastasis.

Discovery of NKCC1 as a potential therapeutic target to inhibit hepatocellular carcinoma cell growth and metastasis

Metastasis is the essential cause for the high mortality of hepatocellular carcinoma (HCC). In order to investigate the mechanism of metastasis, and to discover therapeutic targets for HCC, the quantitative proteomic technique was applied to characterize the plasma membrane proteins of two HCC cell lines with low (MHCC97L) or high (MHCC97H) metastatic potentials. One of the plasma membrane proteins, sodium-potassium-chloride cotransporter 1 (NKCC1), was upregulated in MHCC97H cell line. Immunohistochemistry result in HCC patients showed that NKCC1 expression was associated with poor differentiation and microvascular invasion. Knockdown of NKCC1 via RNA interference reduced HCC cell proliferation and invasion abilities in vitro and in vivo, whereas over-expression of NKCC1 significantly increased HCC cell proliferation and invasion abilities in vitro and in vivo. Additionally, blocking NKCC1 activity with bumetanide attenuated the proliferation and invasion abilities of HCC cells in vitro and limited the HCC growth in vivo. Further results suggested that NKCC1 promotes the invasion ability via MMP-2 activity, and that the WNK1/OSR1/NKCC1 signal pathway might play roles in HCC metastasis. For the first time, our study demonstrated that NKCC1 plays a role in HCC metastasis, and could be served as a potential target to inhibit HCC cell growth and metastasis.

Tetraspanin 8 is a novel regulator of ILK-driven β1 integrin adhesion and signaling in invasive melanoma cells

Melanoma is well known for its propensity for lethal metastasis and resistance to most current therapies. Tumor progression and drug resistance depend to a large extent on the interplay between tumor cells and the surrounding matrix. We previously identified Tetraspanin 8 (Tspan8) as a critical mediator of melanoma invasion, whose expression is absent in healthy skin. The present study investigated whether Tspan8 may influence cell-matrix anchorage and regulate downstream molecular pathways leading to an aggressive behavior. Using silencing and ectopic expression strategies, we showed that Tspan8-mediated invasion of melanoma cells resulted from defects in cell-matrix anchorage by interacting with β1 integrins and by interfering with their clustering, without affecting their surface or global expression levels. These effects were associated with impaired phosphorylation of integrin-linked kinase (ILK) and its downstream target Akt-S473, but not FAK. Specific blockade of Akt or ILK activity strongly affected cell-matrix adhesion. Moreover, expression of a dominant-negative form of ILK reduced β₁ integrin clustering and cell-matrix adhesion. Finally, we observed a tumor-promoting effect of Tspan8 in vivo and a mutually exclusive expression pattern between Tspan8 and phosphorylated ILK in melanoma xenografts and human melanocytic lesions. Altogether, the in vitro, in vivo and in situ data highlight a novel regulatory role for Tspan8 in melanoma progression by modulating cell-matrix interactions through β1 integrin-ILK axis and establish Tspan8 as a negative regulator of ILK activity. These findings emphasize the importance of targeting Tspan8 as a means of switching from low- to firm-adhesive states, mandatory to prevent tumor dissemination.

Exosomal tetraspanins mediate cancer metastasis by altering host microenvironment

The metastases of malignant tumors develop through a cascade of events. The establishment of a pre-metastatic micro-environment is initiated by communication between tumors and host. Exosomes come into focus as the most potent intercellular communicators playing a pivotal role in this process. Cancer cells release exosomes into the extracellular environment prior to metastasis. Tetraspanin is a type of 4 times transmembrane proteins. It may be involved in cell motility, adhesion, morphogenesis, as well as cell and vesicular membrane fusion. The exosomal tetraspanin network is a molecular scaffold connecting various proteins for signaling transduction. The complex of tetraspanin-integrin determines the recruiting cancer exosomes to pre-metastatic sites. Tetraspanin is a key element for the target cell selection of exosomes uptake that may lead to the reprogramming of target cells. Reprogrammed target cells assist pre-metastatic niche formation. Previous reviews have described the biogenesis, secretion and intercellular interaction of exosomes in various tumors. However, there is a lack of reviews on the topic of exosomal tetraspanin in the context of cancer. In this review, we will describe the main characteristics of exosomal tetraspanin in cancer cells. We will also discuss how the cancer exosomal tetraspanin alters extracellular environment and regulates cancer metastasis.

Tetraspanin 8 mediates AEG-1-induced invasion and metastasis in hepatocellular carcinoma cells

Astrocyte-elevated gene-1 (AEG-1) positively regulates tumor progression and metastasis. Here, we document that AEG-1 upregulates transcription of the membrane protein tetraspanin 8 (TSPAN8). Knocking down TSPAN8 in AEG-1-overexpressing human hepatocellular carcinoma (HCC) cells markedly inhibited invasion and migration without affecting proliferation. TSPAN8 knockdown profoundly abrogated AEG-1-induced primary tumor and intrahepatic metastasis in an orthopic xenograft model in athymic nude mice. Coculture of TSPAN8 knockdown cells with human umbilical vein endothelial cells (HUVEC) markedly inhibited HUVEC tube formation indicating that inhibition of angiogenesis might cause reduction in primary tumor size. TSPAN8 inhibition might be a potential therapeutic strategy for metastatic HCC.

TM4SF1 Promotes Proliferation, Invasion, and Metastasis in Human Liver Cancer Cells

Transmembrane 4 superfamily member 1 (TM4SF1) is a member of tetraspanin family, which mediates signal transduction events regulating cell development, activation, growth and motility. Our previous studies showed that TM4SF1 is highly expressed in liver cancer. HepG2 cells were transfected with TM4SFl siRNA and TM4SF1-expressing plasmids and their biological functions were analyzed in vitro and in vivo. HepG2 cells overexpressing TM4SF1 showed reduced apoptosis and increased cell migration in vitro and enhanced tumor growth and metastasis in vivo, whereas siRNA-mediated silencing of TM4SF1 had the opposite effect. TM4SF1 exerts its effect by regulating a few apoptosis- and migration-related genes including caspase-3, caspase-9, MMP-2, MMP-9 and VEGF. These results indicate that TM4SF1 is associated with liver tumor growth and progression, suggesting that TM4SF1 may be a potential target for treatment of liver cancer in future.

Therapeutic targeting of tetraspanin8 in epithelial ovarian cancer invasion and metastasis

Epithelial ovarian cancer (EOC) invasion and metastasis are complex phenomena that result from the coordinated action of many metastatic regulators and must be overcome to improve clinical outcomes for patients with these cancers. The identification of novel therapeutic targets is critical because of the limited success of current treatment regimens, particularly in advanced-stage ovarian cancers. In this study, we found that tetraspanin 8 (TSPAN8) is overexpressed in about 52% (14/27) of EOC tissues and correlates with poor survival. Using small interfering RNA-mediated TSPAN8 knockdown and a competition assay with purified TSPAN8 large extracellular loop (TSPAN8-LEL) protein, we identified TSPAN8-LEL as a key regulator of EOC cell invasion. Furthermore, monotherapy with TSPAN8-blocking antibody we developed shows that antibody-based modulation of TSPAN8-LEL can significantly reduce the incidence of EOC metastasis without severe toxicity in vivo. Finally, we demonstrated that the TSPAN8-blocking antibody promotes the internalization and concomitant downregulation of cell surface TSPAN8. Collectively, our data suggest TSPAN8 as a potential novel therapeutic target in EOCs and antibody targeting of TSPAN8 as an effective strategy for inhibiting invasion and metastasis of TSPAN8-expressing EOCs.

Generation of a human antibody that inhibits TSPAN8-mediated invasion of metastatic colorectal cancer cells

Tetraspanin 8 (TSPAN8) is a tumor-associated antigen implicated in tumor progression and metastasis. However, the validation of TSPAN8 as a potential therapeutic target in metastatic colorectal cancer (mCRC) has not yet been studied. In this study, through several in vitro methodologies, we identified a large extracellular loop of TSPAN8 (TSPAN8-LEL) as a key domain for regulating mCRC invasion. Using phage display technology, we developed a novel anti-TSPAN8-LEL human antibody with subnanomolar affinity that specifically recognizes amino acids 140-205 of TSPAN8-LEL in a conformation-dependent manner. Finally, we demonstrated that the antibody specifically reduces invasion in the HCT116 and LoVo mCRC cell lines more potently than in the HCT-8 and SW480 non-mCRC cell lines. Our data suggest that TSPAN8-LEL may play an important role in mCRC cell invasion, and that the antibody we have developed could be a useful tool for inhibiting the invasion of TSPAN8-expressing mCRCs.

TM4SF1 as a prognostic marker of pancreatic ductal adenocarcinoma is involved in migration and invasion of cancer cells

The cell surface protein Transmembrane 4 L6 family member 1 (TM4SF1) has been detected in various tumors, and its expression on tumor cells is implicated in cancer cell metastasis and patient prognosis. The role of TM4SF1 in malignant tumors remains poorly understood, particularly in pancreatic cancer. We performed immunohistochemical staining to analyze the expression of TM4SF1 in resected pancreatic tissues and investigated the correlation between TM4SF1 expression and prognosis. The function of TM4SF1 in the invasion and migration of pancreatic cancer cells was analyzed in vitro using an RNA interference technique. In pancreatic cancer tissues, TM4SF1 expression was detected in cancer cells, and patients with high tumor levels of TM4SF1 showed longer survival times than those with low TM4SF1 levels (P=0.0332). In vitro, reduced TM4SF1 expression enhanced the migration (P<0.05) and invasion (P<0.05) of pancreatic cancer cells partially via decreased E-cadherin expression. TM4SF1 protein levels were also reduced after TGF-β1-induced epithelial-mesenchymal transition (EMT).TM4SF1 expression is associated with better prognosis in pancreatic cancer. Loss of TM4SF1 contributes to the invasion and migration of pancreatic cancer cells.

Tetraspanin 8-rictor-integrin α3 complex is required for glioma cell migration

The malignant glioma remains one of the most aggressive human malignancies with extremely poor prognosis. Glioma cell invasion and migration are the main causes of death. In the current study, we studied the expression and the potential functions of tetraspanin 8 (Tspan8) in malignant gliomas. We found that Tspan8 expression level is high in both malignant glioma tissues and in several human glioma cell lines, where it formed a complex integrin α3 and rictor, the latter is a key component of mammalian target of rapamycin (mTOR) complex 2 (mTORC2). Disruption of this complex, through siRNA-mediated knockdown of anyone of these three proteins, inhibited U251MG glioma cell migration in vitro. We further showed that Tspan8-rictor association appeared required for mTORC2 activation. Knockdown of Tspan8 by the targeted siRNAs prevented mTOR-rictor (mTORC2) assembly as well as phosphorylation of AKT (Ser-473) and protein kinase C α (PKCα) in U251MG cells. Together, these results demonstrate that over-expressed Tspan8 in malignant glioma forms a complex with rictor and integrin α3 to mediate mTORC2 activation and glioma cell migration. Therefore, targeting Tspan8-rictor-integrin α3 complex may provide a potential therapeutic intervention for malignant glioma.

Tetraspanin 7 (TSPAN7) expression is upregulated in multiple myeloma patients and inhibits myeloma tumour development in vivo

BACKGROUND

Increased expression of the tetraspanin TSPAN7 has been observed in a number of cancers; however, it is unclear how TSPAN7 plays a role in cancer progression.

METHODS

We investigated the expression of TSPAN7 in the haematological malignancy multiple myleoma (MM) and assessed the consequences of TSPAN7 expression in the adhesion, migration and growth of MM plasma cells (PC) in vitro and in bone marrow (BM) homing and tumour growth in vivo. Finally, we characterised the association of TSPAN7 with cell surface partner molecules in vitro.

RESULTS

TSPAN7 was found to be highly expressed at the RNA and protein level in CD138(+) MM PC from approximately 50% of MM patients. TSPAN7 overexpression in the murine myeloma cell line 5TGM1 significantly reduced tumour burden in 5TGM1/KaLwRij mice 4 weeks after intravenous adminstration of 5TGM1 cells. While TSPAN7 overexpression did not affect cell proliferation in vitro, TSPAN7 increased 5TGM1 cell adhesion to BM stromal cells and transendothelial migration. In addition, TSPAN7 was found to associate with the molecular chaperone calnexin on the cell surface.

CONCLUSION

These results suggest that elevated TSPAN7 may be associated with better outcomes for up to 50% of MM patients.

Tetraspanin-enriched microdomains and hepatocellular carcinoma progression

As in many tumors, heterogeneity within the cell population is one of the main features of hepatocellular carcinoma (HCC). Heterogeneity results from the ability of tumor to produce multiple subpopulations of cells with diverse genetic, biochemical and immunological characteristics. Little is known about how heterogeneity emerges and how it is maintained. Fluctuations in single cells can be masked or completely misrepresented when cell populations are analyzed. It has become exceedingly apparent that the utility of measurement based on the analysis of bulk specimens is limited by intra-tumor genetic and epigenetic heterogeneity, as characteristics of the most abundant cell type might not necessarily predict the properties of cell populations. Yet, such non-uniformities often unveil molecular patterns that can represent mechanisms of tumor progression. Interestingly, variability among single cells in a population may arise from different responses to intrinsic and extrinsic perturbations mainly mediated by the plasma membrane. The association of certain proteins, including tetraspanins, and lipids in specific location on the plasma membrane constitutes specialized structure called tetraspanin-enriched microdomains (TEMs). TEMs organization in cancer may reveal essential clues for understanding pathogenic mechanisms underlying cancer progression. Along these lines, TEMs and HCC progression represent a valuable paradigm for gaining a deeper understanding of such mechanisms.

Tetraspanin proteins promote multiple cancer stages

An abundance of evidence shows supporting roles for tetraspanin proteins in human cancer. Many studies show that the expression of tetraspanins correlates with tumour stage, tumour type and patient outcome. In addition, perturbations of tetraspanins in tumour cell lines can considerably affect cell growth, morphology, invasion, tumour engraftment and metastasis. This Review emphasizes new studies that have used de novo mouse cancer models to show that select tetraspanin proteins have key roles in tumour initiation, promotion and metastasis. This Review also emphasizes how tetraspanin proteins can sometimes participate in tumour angiogenesis. These recent data build an increasingly strong case for tetraspanins as therapeutic targets.

The fibrotic microenvironment as a heterogeneity facet of hepatocellular carcinoma

It has long been recognized that hepatocellular carcinoma heterogeneity arises from variation in the microenvironment or from genomic alteration. Only recently it has become clear that non-genetic alterations, such as cytoskeletal rearrangement, protein localization and formation of protein complexes, are also involved in generating phenotype variability. These proteome fluctuations cause genetically identical cells to vary significantly in their responsiveness to microenvironment stimuli. In the cirrhotic liver pre-malignant hepatocytes are continuously exposed to abnormal microenvironments, such as direct contact with activated hepatic stellate cells (HSCs) and extracellular matrix components. These abnormal environments can have pronounced influences on the epigenetic aspects of cells, translating into abnormal phenotypes. Here we discuss non-genetic causes of phenotypic heterogeneity of hepatocellular carcinoma, with an emphasis on variability of membrane protein complexes and transferred functions raising important implications for diagnosis and treatment.

Tetraspanin CD9 promotes the invasive phenotype of human fibrosarcoma cells via upregulation of matrix metalloproteinase-9

Tumor cell metastasis, a process which increases the morbidity and mortality of cancer patients, is highly dependent upon matrix metalloproteinase (MMP) production. Small molecule inhibitors of MMPs have proven unsuccessful at reducing tumor cell invasion in vivo. Therefore, finding an alternative approach to regulate MMP is an important endeavor. Tetraspanins, a family of cell surface organizers, play a major role in cell signaling events and have been implicated in regulating metastasis in numerous cancer cell lines. We stably expressed tetraspanin CD9 in an invasive and metastatic human fibrosarcoma cell line (CD9-HT1080) to investigate its role in regulating tumor cell invasiveness. CD9-HT1080 cells displayed a highly invasive phenotype as demonstrated by matrigel invasion assays. Statistically significant increases in MMP-9 production and activity were attributed to CD9 expression and were not due to any changes in other key tetraspanin complex members or MMP regulators. Increased invasion of CD9-HT1080 cells was reversed upon silencing of MMP-9 using a MMP-9 specific siRNA. Furthermore, we determined that the second extracellular loop of CD9 was responsible for the upregulation of MMP-9 production and subsequent cell invasion. We demonstrated for the first time that tetraspanin CD9 controls HT1080 cell invasion via upregulation of an integral member of the MMP family, MMP-9. Collectively, our studies provide mounting evidence that altered expression of CD9 may be a novel approach to regulate tumor cell progression.

Adenovirus-mediated delivery of siRNA targeting TM4SF4 attenuated liver cancer cell growth in vitro and in vivo

Gene targeting using short interfering RNA (siRNA) has become a common strategy to explore gene function because of its prominent efficacy and specificity. The human transmembrane 4 superfamily member 4 (TM4SF4) was originally identified in intestine and liver as a cell proliferation-related gene. Recently, it showed an increased expression in the hepatocellular carcinoma (HCC) tissues. In this study, we developed an adenoviral vector harboring an effective siRNA targeting TM4SF4 (AdSiTM4SF4) and identified its function in suppression of tumor cell growth. It was confirmed that TM4SF4 was overexpressed in HCC tissues compared with its paired non-tumor tissues by western blot analysis and immunohistochemistry. Remarkably, it was more abundant on the cell surface of HCC cells. The signals of ectopically expressed TM4SF4 in four cell lines dramatically localized in the plasma membrane, slightly in the cytoplasm, and absent in the nucleus, demonstrating that TM4SF4 is a membrane protein. Targeting TM4SF4 by AdSiTM4SF4 successfully exerted a gene knockdown effect. The QGY-7701 and SMMC-7721 HCC cells infected with AdSiTM4SF4 displayed remarkably attenuated growth potential. Moreover, intratumoral injection of AdSiTM4SF4 significantly suppressed tumor growth in a xenograft mouse model using SMMC-7721 hepatoma cells. Our results indicated that targeting TM4SF4 might be a promising modality for inhibition of HCC.

Expression of the tetraspanin family members Tspan3, Tspan4, Tspan5 and Tspan7 during Xenopus laevis embryonic development

Tetraspanins comprise a large family of integral membrane proteins involved in the regulation of cell adhesion, migration and fusion. In humans it consists of 33 members divided in four subfamilies. Here, we examined the spatial and temporal gene expression of four related tetraspanins during the embryonic development of Xenopus laevis by quantitative real-time PCR and in situ hybridization: Tspan3 (encoded by the gene Tm4sf8 gene) Tspan4 (encoded by the gene Tm4sf7), Tspan5 (encoded by the gene Tm4sf9) and Tspan7 (encoded by the gene Tm4sf2). These genes appeared first in the vertebrates during the evolution and are conserved across different species. In humans, they were associated with several diseases such as sclerosis, mental retardation and cancer; however their physiological role remained unclear. This work provides a comprehensive comparative analysis of the expression of these tetraspanins during the development of X. laevis. The more closely related tetraspanins Tspan3, Tspan4 and Tspan7 exhibited very similar spatial expression patterns, albeit differing in their temporal occurrence. The corresponding transcripts were found in the dorsal animal ectoderm at blastula stage. At early tailbud stages (stage 26) the genes were expressed in the migrating cranial neural crest located in the somites, developing eye, brain, and in otic vesicles. In contrast, Tspan5 appeared first at later stages of development and was detected prominently in the notochord. These data support close relatedness of Tspan3, Tspan4 and Tspan7. The expression of these tetraspanins in the cells with a high migratory potential, e.g. neural crest cells, suggests their role in the regulation of migration processes, characteristic for tetraspanin family members, during development. Similarity of the expression profiles might indicate at least partial functional redundancy, which is in concordance with earlier findings of tissue-limited or absent phenotypes in the knock-down studies of tetraspanins family members performed.

Targeting tetraspanins in cancer

INTRODUCTION

Tetraspanins are a family of small proteins that cross the membrane four times and form complexes by interacting between themselves and with a variety of transmembrane and cytosolic proteins, building a network of interactions referred to as tetraspanin web or tetraspanin enriched microdomains (TEMs). These domains provide a signaling platform involved in many important cellular functions and malignant processes.

AREAS COVERED

The authors describe the methods and the rationale for targeting tetraspanins in the therapy of cancer in this review.

EXPERT OPINION

Targeting tetraspanins in cancer may be a promising therapy due to the importance of tetraspanins in several steps of tumor formation, communication with the environment, dissemination, and metastasis.

Tetraspanin CO-029 inhibits colorectal cancer cell movement by deregulating cell-matrix and cell-cell adhesions

Alterations in tetraspanin CO-029 expression are associated with the progression and metastasis of cancers in the digestive system. However, how CO-029 promotes cancer metastasis is still poorly understood. To determine the mechanism, we silenced CO-029 expression in HT29 colon cancer cells and found that the CO-029 knockdown significantly reduced cell migratory ability. The diminished cell migration was accompanied by the upregulation of both integrin-dependent cell-matrix adhesion on laminin and calcium-dependent cell-cell adhesion. The cell surface levels of laminin-binding integrin α3β1 and fibronectin-integrin α5β1 were increased while the level of CD44 was decreased upon CO-029 silencing. These changes contribute to the altered cell-matrix adhesion. The deregulated cell-cell adhesion results, at least partially, from increased activity of cadherins and reduced level of MelCAM. In conclusion, CO-029 functions as a regulator of both cell-matrix and cell-cell adhesion. During colon cancer progression, CO-029 promotes cancer cell movement by deregulating cell adhesions.

Global gene profiling of spontaneous hepatocellular carcinoma in B6C3F1 mice: similarities in the molecular landscape with human liver cancer

Hepatocellular carcinoma (HCC) is an important cause of morbidity and mortality worldwide. Although the risk factors of human HCC are well known, the molecular pathogenesis of this disease is complex, and in general, treatment options remain poor. The use of rodent models to study human cancer has been extensively pursued, both through genetically engineered rodents and rodent models used in carcinogenicity and toxicology studies. In particular, the B6C3F1 mouse used in the National Toxicology Program (NTP) two-year bioassay has been used to evaluate the carcinogenic effects of environmental and occupational chemicals, and other compounds. The high incidence of spontaneous HCC in the B6C3F1 mouse has challenged its use as a model for chemically induced HCC in terms of relevance to the human disease. Using global gene expression profiling, we identify the dysregulation of several mediators similarly altered in human HCC, including re-expression of fetal oncogenes, upregulation of protooncogenes, downregulation of tumor suppressor genes, and abnormal expression of cell cycle mediators, growth factors, apoptosis regulators, and angiogenesis and extracellular matrix remodeling factors. Although major differences in etiology and pathogenesis remain between human and mouse HCC, there are important similarities in global gene expression and molecular pathways dysregulated in mouse and human HCC. These data provide further support for the use of this model in hazard identification of compounds with potential human carcinogenicity risk, and may help in better understanding the mechanisms of tumorigenesis resulting from chemical exposure in the NTP two-year carcinogenicity bioassay.

Gene expression profiles of human melanoma cells with different invasive potential reveal TSPAN8 as a novel mediator of invasion

Background:

Metastatic melanoma requires early detection, being treatment resistant. However, the earliest events of melanoma metastasis, and especially of dermal invasion, remain ill defined.

Results and methods:

Gene expression profiles of two clonal subpopulations, selected from the same human melanoma cell line, but differing in ability to cross the dermal–epidermal junction in skin reconstructs, were compared by oligonucleotide microarray. Of 26 496 cDNA probes, 461 were differentially expressed (>2-fold; P< 0.001), only 71 genes being upregulated in invasive cells. Among them, TSPAN8, a tetraspanin not yet described in melanoma, was upregulated at mRNA and protein levels in melanoma cells from the invasive clone, as assessed by RT–PCR, flow cytometry and western blot analysis. Interestingly, TSPAN8 was the only tetraspanin in which overexpression correlated with invasive phenotype. Flow cytometry of well-defined melanoma cell lines confirmed that TSPAN8 was exclusively expressed by invasive, but not non-invasive melanoma cells or normal melanocytes. Immunohistochemistry revealed that TSPAN8 was expressed by melanoma cells in primary melanomas and metastases, but not epidermal cells in healthy skin. The functional role of TSPAN8 was demonstrated by silencing endogenous TSPAN8 with siRNA, reducing invasive outgrowth from tumour spheroids within matrigel without affecting cell proliferation or survival.

Conclusion:

TSPAN8 expression may enable melanoma cells to cross the cutaneous basement membrane, leading to dermal invasion and progression to metastasis. TSPAN8 could be a promising target in early detection and treatment of melanoma.

Tetraspanins: push and pull in suppressing and promoting metastasis

Tumours progress through a cascade of events that enable the formation of metastases. Some of the components that are required for this fatal process are well established. Tetraspanins, however, have only recently received attention as both metastasis suppressors and metastasis promoters. This late appreciation is probably due to their capacity to associate with various molecules, which they recruit into special membrane microdomains, and their abundant presence in tumour-derived small vesicles that aid intercellular communication. It is reasonable to assume that differences in the membrane and vesicular web components that associate with individual tetraspanins account for their differing abilities to promote and suppress metastasis.

TM4SF3 promotes esophageal carcinoma metastasis via upregulating ADAM12m expression

Esophageal cancer is characterized by rapid clinical progression and poor prognosis due to adjacent tissue invasion and distant organs metastasis at a very early stage. TM4SF3 (transmembrane 4 superfamily 3), a member of tetraspanin family, has been reported as a metastasis associated gene in many types of tumors. Herein, we described new properties of TM4SF3 in tumor metastasis, which suggested that this gene might be involved in esophageal carcinoma metastasis. Western blotting revealed that TM4SF3 was overexpressed in 57.1% (8/14) of esophageal carcinomas and esophageal carcinoma cell lines with high-invasive potential. Exogenous expression of TM4SF3 in two low-invasive esophageal carcinoma cell lines, KYSE150 and EC9706, significantly promoted cell migration and invasion. Upregulating TM4SF3 expression in EC9706 cells promoted xenograft tumor invading into surrounding tissues, enhanced lung metastasis, and shortened the lifespan of mice (median survival EC9706-TM4SF3 106.5 days versus EC9706-Vector 169.0 days, P < 0.0001) in a spontaneous metastasis model. Further studies demonstrated that ADAM12m was upregulated by TM4SF3 overexpression in vitro and in vivo. Abrogating up-expression of ADAM12m by siRNA significantly suppressed TM4SF3-mediated invasion. Together, these data from our studies indicated that overexpression of TM4SF3 in esophageal cancer conferred advantage to the invasion and metastasis of this destructive disease. Upregulated expression of ADAM12m by TM4SF3 might play a key role in TM4SF3-mediated invasion and metastasis. TM4SF3 and ADAM12m might be potential targets of esophageal carcinoma for anti-metastasis therapy.

Systemic induction of the angiogenesis switch by the tetraspanin D6.1A/CO-029

Expression of the tetraspanin CO-029 is associated with poor prognosis in patients with gastrointestinal cancer. In a pancreatic tumor line, overexpression of the rat homologue, D6.1A, induces lethally disseminated intravascular coagulation, suggesting D6.1A engagement in angiogenesis. D6.1A-overexpressing tumor cells induce the greatest amount of angiogenesis in vivo, and tumor cells as well as exosomes derived thereof strikingly increase endothelial cell branching in vitro. Tumor cell-derived D6.1A stimulates angiogenic factor transcription, which includes increased matrix metalloproteinase and urokinase-type plasminogen activator secretion, pronounced vascular endothelial growth factor expression in fibroblasts, vascular endothelial growth factor receptor expression, and strong D6.1A up-regulation in sprouting endothelium. Thus, D6.1A initiates an angiogenic loop that, probably due to the abundance of D6.1A in tumor-derived exosomes, reaches organs distant from the tumor. Most importantly, because of the strong D6.1A up-regulation on sprouting capillaries, angiogenesis could be completely inhibited by a D6.1A-specific antibody, irrespective of whether or not the tumor expresses D6.1A. Tetraspanins have been suggested to be involved in morphogenesis. This is the first report that a tetraspanin, CO-029/D6.1A, promotes tumor growth by its capacity to induce systemic angiogenesis that can effectively, and with high selectivity for sprouting endothelium, be blocked by a D6.1A-specific antibody.

Proteomic analysis of the tetraspanin web using LC-ESI-MS/MS and MALDI-FTICR-MS

Tetraspanins are integral membrane proteins involved in a variety of physiological and pathological processes. In cancer, clinical and experimental studies have reported a link between tetraspanin expression levels and metastasis. Tetraspanins play a role as organizers of a molecular network of interactions, the "tetraspanin web". Here, we have performed a proteomic characterization of the tetraspanin web using a model of human colon cancer consisting of two cell lines derived from primary tumor and metastasis from the same patient. The tetraspanin complexes were isolated after immunoaffinity purification and the proteins were identified by MS using LC-ESI-MS/MS and MALDI-FTICR. The high resolution and mass accuracy of FTICR MS allowed reliable identification using mass finger printing with only two peptides. Thus, it could be used to resolve the composition of complex peptide mixtures from membrane proteins. Different types of membrane proteins were identified, including adhesion molecules (integrins, Lu/B-CAM, GA733 proteins), receptors and signaling molecules (BAI2, PKC, G proteins), proteases (ADAM10, TADG15), and membrane fusion proteins (syntaxins) as well as poorly characterized proteins (CDCP1, HEM-1, CTL1, and CTL2). Some components were differentially detected in the tetraspanin web of the two cell lines. These differences may be relevant for tumor progression and metastasis.

Profiling of the tetraspanin web of human colon cancer cells

Tetraspanins are integral membrane proteins involved in a variety of physiological and pathological processes. In cancer, clinical and experimental studies have reported a link between tetraspanin expression levels and metastasis. Tetraspanins play a role as organizers of multimolecular complexes in the plasma membrane. Indeed each tetraspanin associates specifically with one or a few other membrane proteins forming primary complexes. Thus, tetraspanin-tetraspanin associations lead to a molecular network of interactions, the "tetraspanin web." We performed a proteomic characterization of the tetraspanin web using a model of human colon cancer consisting of three cell lines derived from the primary tumor and two metastases (hepatic and peritoneal) from the same patient. The tetraspanin complexes were isolated after immunoaffinity purification using monoclonal antibodies directed against the tetraspanin CD9, and the associated proteins were separated by SDS-PAGE and identified by mass spectrometry using LC-MS/MS. This allowed the identification of 32 proteins including adhesion molecules (integrins, proteins with Ig domains, CD44, and epithelial cell adhesion molecule) (EpCAM), membrane proteases (ADAM10, TADG-15, and CD26/dipeptidyl peptidase IV), and signaling proteins (heterotrimeric G proteins). Importantly some components were differentially detected in the tetraspanin web of the three cell lines: the laminin receptor Lutheran/B-cell adhesion molecule (Lu/B-CAM) was expressed only on the primary tumor cells, whereas CD26/dipeptidyl peptidase IV and tetraspanin Co-029 were observed only on metastatic cells. Concerning Co-029, immunohistofluorescence showed a high expression of Co-029 on epithelial cells in normal colon and a lower expression in tumors, whereas heterogeneity in terms of expression level was observed on metastasis. Finally we demonstrated that epithelial cell adhesion molecule and CD9 form a new primary complex in the tetraspanin web.

The tetraspanin D6.1A and its molecular partners on rat carcinoma cells

Tetraspanins function as molecular organizers of multi-protein complexes by assembling primary complexes of a relatively low mass into extensive networks involved in cellular signalling. In this paper, we summarize our studies performed on the tetraspanin D6.1A/CO-029/TM4SF3 expressed by rat carcinoma cells. Primary complexes of D6.1A are almost indistinguishable from complexes isolated with anti-CD9 antibody. Indeed, both tetraspanins directly associate with each other and with a third tetraspanin, CD81. Moreover, FPRP (prostaglandin F2alpha receptor-regulatory protein)/EWI-F/CD9P-1), an Ig superfamily member that has been described to interact with CD9 and CD81, is also a prominent element in D6.1A complexes. Primary complexes isolated with D6.1A-specific antibody are clearly different from complexes containing the tetraspanin CD151. CD151 is found to interact only with D6.1A if milder conditions, i.e. lysis with LubrolWX instead of Brij96, are applied to disrupt cellular membranes. CD151 probably mediates the interaction of D6.1A primary complexes with alpha3beta1 integrin. In addition, two other molecules were identified to be part of D6.1A complexes at this higher level of association: type II phosphatidylinositol 4-kinase and EpCAM, an epithelial marker protein overexpressed by many carcinomas. The characterization of the D6.1A core complex and additional more indirect interactions will help to elucidate the role in tumour progression and metastasis attributed to D6.1A.

Identification of FGF10 Targets in the Embryonic Lung Epithelium during Bud Morphogenesis

Genetic studies implicate Fgf10-Fgfr2 signaling as a critical regulator of bud morphogenesis in the embryo. However, little is known about the transcriptional targets of Fgf10 during this process. Here we identified global changes in gene expression in lung epithelial explants undergoing FGF10-mediated budding in the absence of other growth factors and mesenchyme. Targets were confirmed by their localization at sites where endogenous Fgf10 signaling is active in embryonic lungs and by demonstrating their induction in intact lungs in response to local application of FGF10 protein. We show that the initial stages of budding are characterized by marked up-regulation of genes associated with cell rearrangement and cell migration, inflammatory process, and lipid metabolism but not cell proliferation. We also found that some genes implicated in tumor invasion and metastatic behavior are epithelial targets of Fgf10 in the lung and other developing organs that depend on Fgf10-Fgfr2 signaling to properly form. Our approach identifies Fgf10 targets that are common to multiple biological processes and provides insights into potential mechanisms by which Fgf signaling regulates epithelial cell behavior.