Tetraspanin CD151 regulates transforming growth factor beta signaling: implication in tumor metastasis

Tetraspanins CD63 and CD81 as potential prognostic biomarkers in breast cancer

Exosome markers, CD63 and CD81, belong to the tetraspanin family and are expressed in solid tumors. It has been reported that these tetraspanin family members are prognostic factors in some cancers. However, the expression of CD63 and CD81 in pathological breast cancer specimens has not been reported. It has been reported that CD63 promotes the proliferation of breast cancer cells in vitro through yes-associated protein (YAP). Therefore, in this study, the expression of tetraspanin family members, particularly CD63, CD81, and YAP were investigated in breast cancer tissue, by immunohistochemistry, to clarify the relationship between clinicopathological factors and prognosis. The number of CD63 and YAP double-positive breast cancer cells was significantly higher in patients with pathological T factor (pT) status (p = 0.030) and tended to be higher in patients with pathological N factor (pN) status (p = 0.054). Furthermore, the number of CD81 and YAP double-positive breast cancer cells was significantly higher in patients with histological grade (p = 0.015), pT status (p = 0.001), and Ki67 expression (p = 0.049), and tended to be higher in patients with pN status (p = 0.062) and TNM stage (p = 0.052). In addition, CD63 and YAP double-positive breast cancers and CD81 and YAP double-positive breast cancers were associated with shorter disease-free and breast cancer-specific survival, respectively. In conclusion, CD63 and YAP, and CD81 and YAP may serve as potential prognostic biomarkers in patients with breast cancer.

Therapeutic Implications of the Drug Resistance Conferred by Extracellular Vesicles Derived from Triple-Negative Breast Cancer Cells

Anticancer drug resistance is a significant impediment in current cancer treatment. Extracellular vesicles (EVs) derived from cancer cells were recently acknowledged as a critical mechanism of drug resistance, tumor progression, and metastasis. EVs are enveloped vesicles comprising a lipid bilayer that transfers various cargo, including proteins, nucleic acids, lipids, and metabolites, from an originating cell to a recipient cell. Investigating the mechanisms whereby EVs confer drug resistance is still in the early stages. In this review, I analyze the roles of EVs derived from triple-negative breast cancer cells (TNBC-EVs) in anticancer drug resistance and discuss strategies to overcome TNBC-EV-mediated drug resistance.

CD63 acts as a functional marker in maintaining hematopoietic stem cell quiescence through supporting TGFβ signaling in mice

Hematopoietic stem cell (HSC) fate is tightly controlled by various regulators, whereas the underlying mechanism has not been fully uncovered due to the high heterogeneity of these populations. In this study, we identify tetraspanin CD63 as a novel functional marker of HSCs in mice. We show that CD63 is unevenly expressed on the cell surface in HSC populations. Importantly, HSCs with high CD63 expression (CD63^hi) are more quiescent and have more robust self-renewal and myeloid differentiation abilities than those with negative/low CD63 expression (CD63^-/lo). On the other hand, using CD63 knockout mice, we find that loss of CD63 leads to reduced HSC numbers in the bone marrow. In addition, CD63-deficient HSCs exhibit impaired quiescence and long-term repopulating capacity, accompanied by increased sensitivity to irradiation and 5-fluorouracil treatment. Further investigations demonstrate that CD63 is required to sustain TGFβ signaling activity through its interaction with TGFβ receptors I and II, thereby playing an important role in regulating the quiescence of HSCs. Collectively, our data not only reveal a previously unrecognized role of CD63 but also provide us with new insights into HSC heterogeneity.

Proteomic Landscape of Exosomes Reveals the Functional Contributions of CD151 in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer. Patients with TNBC have poor overall survival because of limited molecular therapeutic targets. Recently, exosomes have been recognized as key mediators in cancer progression, but the molecular components and function of TNBC-derived exosomes remain unknown. The main goal of this study was to reveal the proteomic landscape of serum exosomes derived from ten patients with TNBC and 17 healthy donors to identify potential therapeutic targets. Using a tandem mass tag–based quantitative proteomics approach, we characterized the proteomes of individual patient-derived serum exosomes, identified exosomal protein signatures specific to patients with TNBC, and filtered out differentially expressed proteins. Most importantly, we found that the tetraspanin CD151 expression levels in TNBC-derived serum exosomes were significantly higher than those exosomes from healthy subjects, and we validated our findings with samples from 16 additional donors. Furthermore, utilizing quantitative proteomics approach to reveal the proteomes of CD151-deleted exosomes and cells, we found that exosomal CD151 facilitated secretion of ribosomal proteins via exosomes while inhibiting exosome secretion of complement proteins. Moreover, we proved that CD151-deleted exosomes significantly decreased the migration and invasion of TNBC cells. This is the first comparative study of the proteomes of TNBC patient–derived and CD151-deleted exosomes. Our findings indicate that profiling of TNBC-derived exosomal proteins is a useful tool to extend our understanding of TNBC, and exosomal CD151 may be a potential therapeutic target for TNBC.

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Quantitative proteomics of TNBC patient serum-derived exosomes.

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CD151 is significantly enriched in the TNBC patient serum-derived exosomes.

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CD151 regulates the secretion of ribosomal and complement proteins via exosomes.

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Exosomal CD151 promotes TNBC cell migration and invasion.

CD151 enrichment in exosomes of luminal androgen receptor breast cancer cell line contributes to cell invasion

Breast cancer is the most common and highly heterogeneous disease in women worldwide. Given the challenges in the treatment of advanced metastatic breast cancer, it is necessary to understand the molecular mechanisms related to disease progression. Exosomes play various roles in the progression of tumors, including promoting the invasion and advancing the distant metastasis. To study the molecular mechanisms related to the progression of luminal androgen receptor (LAR) breast cancer, we first isolated exosomes of MDA-MB-453 cells, a representative cell line of LAR. Through quantitative proteomic analysis, we identified 180 proteins specifically enriched in exosomes after comparing with those in cells, microvesicles, and the 150K supernatant. Among these, CD151, a protein involved in the regulation of cell motility was the most enriched one. CD151-knockdown exosomes reduced the invasion ability of the recipient breast cancer cell and lowered the phosphorylation level of tyrosine-protein kinase Lck, indicating that the invasion of LAR breast cancer may be due to CD151-enriched exosomes. Our work reports for the first time that CD151 was highly abundant in the exosomes of MDA-MB-453 cells and expands the understanding of the development process of LAR subtype, suggesting CD151 may be a potential candidate for the treatment of LAR breast cancer.

The Context-Dependent Impact of Integrin-Associated CD151 and Other Tetraspanins on Cancer Development and Progression: A Class of Versatile Mediators of Cellular Function and Signaling, Tumorigenesis and Metastasis

Simple Summary

Tetraspanins are a family of molecules abundantly expressed on the surface of normal or tumor cells. They have been implicated in recruiting or sequestering key molecular regulators of malignancy of a variety of human cancers, including breast and lung cancers, glioblastoma and leukemia. Yet, how their actions take place remains mysterious due to a lack of traditional platform for molecular interactions. The current review digs into this mystery by examining findings from recent studies of multiple tetraspanins, particularly CD151. The molecular basis for differential impact of tetraspanins on tumor development, progression, and spreading to secondary sites is highlighted, and the complexity and plasticity of their control over tumor cell activities and interaction with their surroundings is discussed. Finally, an outlook is provided regarding tetraspanins as candidate biomarkers and targets for the diagnosis and treatment of human cancer.

Abstract

As a family of integral membrane proteins, tetraspanins have been functionally linked to a wide spectrum of human cancers, ranging from breast, colon, lung, ovarian, prostate, and skin carcinomas to glioblastoma. CD151 is one such prominent member of the tetraspanin family recently suggested to mediate tumor development, growth, and progression in oncogenic context- and cell lineage-dependent manners. In the current review, we summarize recent advances in mechanistic understanding of the function and signaling of integrin-associated CD151 and other tetraspanins in multiple cancer types. We also highlight emerging genetic and epigenetic evidence on the intrinsic links between tetraspanins, the epithelial-mesenchymal transition (EMT), cancer stem cells (CSCs), and the Wnt/β-catenin pathway, as well as the dynamics of exosome and cellular metabolism. Finally, we discuss the implications of the highly plastic nature and epigenetic susceptibility of CD151 expression, function, and signaling for clinical diagnosis and therapeutic intervention for human cancer.

CD151 promotes Colorectal Cancer progression by a crosstalk involving CEACAM6, LGR5 and Wnt signaling via TGFβ1

CD151 impacts various signaling pathways in different cancers, and promotes colorectal cancer (CRC) cell malignancy by yet undefined mechanisms. This study aimed to comprehensively assess CD151's function in CRC. CD151 levels were significantly higher in CRC tissues and cells compared with controls in the tissue microarray. Cell viability, migration and invasion were suppressed by CD151 downregulation in CRC cells. Consistently, mouse xenografts were inhibited by CD151 silencing. RNA-seq revealed that multiple genes were significantly altered by CD151 knockdown in cultured CRC cells and xenografts. Particularly, transforming growth factor β1 (TGFβ1), carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) and leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5) alongside CD151 were downregulated both in vitro and in vivo. Co-immunoprecipitation and mass spectrometry results were validated by qRT-PCR and immunoblot. Moreover, pull-down assay and immunofluorescence confirmed the associations of TGFβ1, CEACAM6 and LGR5 with CD151. This study demonstrated CEACAM6, LGR5 and Wnt pathway suppression by CD151 silencing might occur through TGFβ1 regulation, offering a comprehensive view of CD151's roles in colorectal carcinogenesis. Our findings provide an insight into the CD151-involved signaling network in CRC oncogenesis, which could be utilized to design novel targeted therapies against CD151-based signaling in treatment for CRC.

Upregulation of HIF1-α via an NF-κB/COX2 pathway confers proliferative dominance of HER2-negative ductal carcinoma in situ cells in response to inflammatory stimuli

There are data to suggest that some ductal carcinoma in situ (DCIS) may evolve through an evolutionary bottleneck, where minor clones susceptible to the imposed selective pressure drive disease progression. Here, we tested the hypothesis that an impact of the inflammatory environment on DCIS evolution is HER2-dependent, conferring proliferative dominance of HER2-negative cells. In tissue samples, density of tumour-infiltrating immune cells (TIICs) was associated only with high tumour nuclear grade, but in 9% of predominantly HER2-negative cases, the presence of tumoral foci ('hot-spots') of basal-like cells with HIF1-α activity adjacent to the areas of dense stromal infiltration was noted. Results of in vitro analyses further demonstrated that IL-1β and TNF-α as well as macrophage-conditioned medium triggered phosphorylation of NF-κB and subsequent upregulation of COX2 and HIF1-α, exclusively in HER2-negative cells. Treatment with both IL-1β and TNF-α resulted in growth stimulation and inhibition of HER2-negative and HER2-positive cells, respectively. Moreover, ectopic overexpression of HIF1-α rescued HER2-positive cells from the negative effect of IL-1β and TNF-α on cell growth. Our data provide novel insight into the molecular basis of HER2-dependent proliferation of DCIS cells and indicate the NF-κB/COX2 → HIF1-α signalling axis as a dominant mechanism of DCIS evolution induced by inflammatory microenvironment. Presented findings also highlight the clinical significance of heterogeneity of DCIS tumours and suggest that HIF1-α might be considered as a predictive marker of disease progression.

EM2D9, A monoclonal antibody against integrin α5β1, has potent antitumor activity on endometrial cancer in vitro and in vivo

Endometrial cancer, a type of primary epithelial malignant tumor in the endometrium, is one of the three most common malignant tumors of the female reproductive system. While the incidence of endometrial cancer has been recently rising, its etiology remains unclear. In this study we found that EM2D9, an independently developed monoclonal antibody, specifically recognized endometrial cancer cells; we further determined that EM2D9 target protein was α5β1. In vitro and in vivo experiments showed that EM2D9 inhibited the migration of endometrial cancer cells. Real-time quantitative PCR results showed that the expression of CD151 mRNA in endometrial carcinoma cells significantly decreased after EM2D9 treatment. We also found that EM2D9 affected the FAK signaling pathway. Collectively, these results shed light on a new mechanism for the development of endometrial carcinoma.

CD151 in Respiratory Diseases

The tetraspanin, Cluster of Differentiation 151 (CD151), is ubiquitously expressed in adult tissue, especially in the lungs where it has been implicated in lung cancer, asthma, influenza, and idiopathic pulmonary fibrosis (IPF). CD151 interacts with laminin-binding integrins and growth factor receptors, and is reported in cancer-promoting processes such as tumor initiation, metastasis, and angiogenesis. In asthma, CD151 was shown to promote airways hyperresponsiveness through calcium signaling whereas in influenza, CD151 was shown to be a novel host factor for nuclear viral export signaling. Furthermore, CD151 was shown to be associated with increased disease severity and poorer survival outcome in asthma and lung cancer, respectively. In this review, we provide an update on the current understanding of CD151 with regards to its contribution to lung pathophysiology. We also summarize factors that have been shown to regulate CD151 expression and identify key areas that need to be taken into consideration for its utility as a screening or prognostic tool in disease management and/or as a therapeutic target for the treatment of lung diseases.

Deletion of tetraspanin CD151 alters the Wnt oncogene-induced mammary tumorigenesis: A cell type-linked function and signaling

Tetraspanin CD151 is increasingly implicated as a multifaceted mediator of cancer development and progression. Here we investigated the role of CD151 in breast cancer in the context of the Wnt oncogenic activation. Our data showed that removal of one or both of CD151 alleles in the MMTV-Wnt1 model significantly decreased the tumor-free survival of mice from 34 weeks on average to 22 weeks and 18 weeks, respectively. This effect coincided with an accelerated tumor growth and an increased number of Ki-67⁺ proliferative cells. Mechanistically, the CD151-deficient tumors were largely ER⁺, and exhibited hyperactivation of the Wnt pathway as reflected by a marked upregulation in β-catenin and Cyclin D1, and their target genes. In addition, E-cadherin displayed a cytosolic distribution and transcription factor Snail was markedly upregulated. Collectively, this data implies that CD151 suppresses the Wnt1-driven tumorigenesis, at least in part, via counteracting the epithelial-mesenchymal transition (EMT)-like program in luminal epithelial cells. Meanwhile, the proportion of tumor cells expressing CK5 or p63, the biomarkers of myoepithelial/basal cells, markedly decreased in the absence of CD151. This change was accompanied by a decreased invasiveness of tumors and their incompetence to form a long-term cell culture. Consistent with this basal cell-linked role, the CD151 downregulation impairs mammosphere formation in MCF-10A cells and the defect was rescued by re-expression of intact CD151 ORF, but not its integrin binding-defective mutant. Overall, our study suggests that CD151 is a key player in the Wnt oncogene-driven tumorigenesis and impacts breast cancer malignancy in a cell type-dependent manner.

Marine natural compound cyclo(L-leucyl-L-prolyl) peptide inhibits migration of triple negative breast cancer cells by disrupting interaction of CD151 and EGFR signaling

Cyclo (L-Leucyl-L-Prolyl) peptide/CLP is a marine natural metabolite and well recognized as an antimicrobial and antioxidant agent with limited studies on anticancer activity. The current study aims to determine the effect of CLP on migration and growth of triple negative breast cancer cell lines. The anti-growth potential was evaluated by MTT, BrdU and TUNEL assays; DNA damage by γH2AX and Dead green assays; antimigration activity by Boyden chamber invasion and wound healing assays. Interaction of CLP with CD151 was resolved by PatchDock. Effect of CLP on the expression of transmembrane CD151 was evaluated by cell-based ELISA assay. The interaction between CD151 and EGFR was predicted by using FireDoc Web server. Impact of CLP on the interaction of CD151 with EGFR was evaluated by co-immunoprecipitation assay. The effect of CLP on the cell cycle and its controlling proteins was determined by Western blotting. CLP reduced the viability of MDA-MB-231 and MDA-MB-468 TNBC cell lines but not human breast healthy epithelial cell line (MCF-12A) similar to eribulin, standard. CLP also inhibited proliferation; cell cycle and migration. It induced DNA strand breaks, DNA damage, and cell death. It showed the most favorable interactions with CD151 in in silico docking and significantly reduced the expression of membrane-bound CD151 proteins. FireDoc Web study predicted the association between CD151 and EGFR with -29.13 kcal/mol of binding energy. CLP reduced the interaction of CD151 with EGFR along with the expression of cyclin D, CDK4, PAK, RAC1, and P27kiP1. This study concludes that CLP suppresses growth and migration by attenuating cell cycle of TNBC cell lines via EGFR and CD151 signaling. Thus, exploring the EGFR and CD151 signaling pathway targeted by CLP may provide a new approach in the treatment of TNBC.

[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 CD151 impairs heterodimerization of ErbB2/ErbB3 in breast cancer cells

CD151/Tspan24 (SFS-1, PETA3) is one of the best characterized members of the tetraspanin family, whose involvement in breast cancer (BCa) progression was demonstrated both in vitro and in vivo. We have recently reported that in ErbB2-overexpressing BCa cells grown in 3D laminin-rich extracellular matrix, CD151 regulated basal phosphorylation and homodimerization of ErbB2 and sensitized the cells to Herceptin (trastuzumab). Following from these data, we have here analyzed an involvement of CD151 in regulation of ErbB2/ErbB3 heterodimerization and its impact on cell response to Herceptin. CD151 was found to: (1) impair ErbB2/ErbB3 heterodimerization, (2) inhibit heregulin-dependent cell growth in 3D and signaling, and (3) counteract the protective effect of heregulin on Herceptin-mediated growth inhibition. Analysis of tissue samples demonstrated for the first time clinical significance of CD151 in patients with ErbB2-overexpressing BCa undergone trastuzumab-based therapy. Consistent with in vitro results, CD151 impact on disease outcome was ErbB3-dependent. In patients with ErbB3-negative tumors, CD151 significantly improved both overall survival (OS) (hazard ratio [HR] = 0.19, P = 0.034) and progression-free survival (PFS) (HR = 0.36, P = 0.043), while in ErbB3-positive cases it had no significant effect on patient survival (OS: HR = 3.33, P = 0.283; PFS: HR = 2.40, P = 0.208). These results support previous findings and show that CD151 acts as an important component of ErbB2 signaling axis in BCa cells, affecting their sensitivity to ErbB2-targeting therapy.

CD151 regulates expression of FGFR2 in breast cancer cells via PKC-dependent pathways

Expression of the tetraspanin CD151 is frequently upregulated in epithelial malignancies and correlates with poor prognosis. Here, we report that CD151 is involved in regulation of the expression of fibroblast growth factor receptor 2 (FGFR2). Depletion of CD151 in breast cancer cells resulted in an increased level of FGFR2. Accordingly, an inverse correlation between CD151 and FGFR2 was observed in breast cancer tissues. CD151-dependent regulation of the FGFR2 expression relies on post-transcriptional mechanisms involving HuR (also known as ELAVL1), a multifunctional RNA-binding protein, and the assembly of processing bodies (P-bodies). Depletion of CD151 correlated with inhibition of PKC, a well-established downstream target of CD151. Accordingly, the levels of dialcylglycerol species were decreased in CD151-negative cells, and inhibition of PKC resulted in the increased expression of FGFR2. Whereas expression of FGFR2 itself did not correlate with any of the clinicopathological data, we found that FGFR2-/CD151+ patients were more likely to have developed lymph node metastasis. Conversely, FGFR2-/CD151- patients demonstrated better overall survival. These results illustrate functional interdependency between CD151 complexes and FGFR2, and suggest a previously unsuspected role of CD151 in breast tumorigenesis.

Immune Targeting of Tetraspanins Involved in Cell Invasion and Metastasis

Metastasis is the ultimate consequence of cancer progression and the cause of patients’ death across different cancer types. Patients with initial diagnosis of distant disease have a worst 5-year survival compared to patients with localized disease. Therapies that target primary tumors fail to eradicate distant dissemination of cancer. Recently, immunotherapies have improved the survival of patients with metastatic disease, such as melanoma and lung cancer. However, only a fraction of patients responds to immunotherapy modalities that target the host immune system. The need to identify new druggable targets that inhibit or prevent metastasis is, therefore, much needed. Tetraspanins have emerged as key players in regulating cell migration, invasion, and metastasis. By serving as molecular adaptors that cluster adhesion receptors, signaling molecules, and cell surface receptors; tetraspanins are involved in all steps of the metastatic cascade. They regulate cell proliferation, participate in EMT transition, modulate integrin-mediated cell adhesion, and participate in angiogenesis and invasion processes. Tetraspanins have also been shown to modulate metastasis indirectly through exosomes and by regulating cellular interactions in the immune system. Importantly, targeting individual tetraspanin with antibodies has impacted tumor progression. This review will focus on the contribution of tetraspanins to the metastatic process and their potential as therapeutic tumor targets.

Antitumor Immunity Is Controlled by Tetraspanin Proteins

Antitumor immunity is shaped by the different types of immune cells that are present in the tumor microenvironment (TME). In particular, environmental signals (for instance, soluble factors or cell–cell contact) transmitted through the plasma membrane determine whether immune cells are activated or inhibited. Tetraspanin proteins are emerging as central building blocks of the plasma membrane by their capacity to cluster immune receptors, enzymes, and signaling molecules into the tetraspanin web. Whereas some tetraspanins (CD81, CD151, CD9) are widely and broadly expressed, others (CD53, CD37, Tssc6) have an expression pattern restricted to hematopoietic cells. Studies using genetic mouse models have identified important immunological functions of these tetraspanins on different leukocyte subsets, and as such, may be involved in the immune response against tumors. While multiple studies have been performed with regards to deciphering the function of tetraspanins on cancer cells, the effect of tetraspanins on immune cells in the antitumor response remains understudied. In this review, we will focus on tetraspanins expressed by immune cells and discuss their potential role in antitumor immunity. New insights in tetraspanin function in the TME and possible prognostic and therapeutic roles of tetraspanins will be discussed.

CD151 promotes cell metastasis via activating TGF-β1/Smad signaling in renal cell carcinoma

Tetraspanin CD151 has been identified as a tumor promoter, which is upregulated in various malignant cell types. However, the function of CD151 and its underlying mechanism in renal cell carcinoma is still unknown. In this study, we detected the expression of CD151 in RCC cells and tissues and explored its regulatory mechanism. We found that CD151 was upregulated in renal cell carcinoma tissues and cells and its expression was significantly associated with tumor stage (p=0.019) and survival (p=0.001) by analyzing tissue microarrays. After silencing of CD151 via lentivirus vector in Caki-1 and Caki-2 cells, reduced ability of migration and invasion were detected with downregulation of CD151. The opposite results were observed in cells with CD151 overexpression. Furthermore, western blotting was performed to investigate the influence of CD151 on epithelial-to-mesenchymal transition, matrix metalloproteinase 9 and TGF-β1/Smad signaling pathway in RCC. Subsequently, upregulating the protein level of transforming growth factor-β1 in cells with silencing of CD151 could rescue the malignant behaviors inhibited, which indicated that CD151 may play its promoting role in RCC partially by stimulating the expression of TGF-β1. Conclusively, CD151 might exhibit a prominent role in migration and invasion of RCC cells via activating TGF-β1/Smad signaling pathway.

FGFR2-Driven Signaling Counteracts Tamoxifen Effect on ERα-Positive Breast Cancer Cells

Signaling mediated by growth factors receptors has long been suggested as one of the key factors responsible for failure of endocrine treatment in breast cancer (BCa). Herein we present that in the presence of tamoxifen, FGFs (Fibroblast Growth Factors) promote BCa cell growth with the strongest effect being produced by FGF7. FGFR2 was identified as a mediator of FGF7 action and the FGFR2-induced signaling was found to underlie cancer-associated fibroblasts-dependent resistance to tamoxifen. FGF7/FGFR2-triggered pathway was shown to induce ER phosphorylation, ubiquitination and subsequent ER proteasomal degradation which counteracted tamoxifen-promoted ER stabilization. We also identified activation of PI3K/AKT signaling targeting ER-Ser167 and regulation of Bcl-2 expression as a mediator of FGFR2-promoted resistance to tamoxifen. Analysis of tissue samples from patients with invasive ductal carcinoma revealed an inversed correlation between expression of FGFR2 and ER, thus supporting our in vitro data. These results unveil the complexity of ER regulation by FGFR2-mediated signaling likely to be associated with BCa resistance to endocrine therapy.

Joint features and complementarities of Tspan8 and CD151 revealed in knockdown and knockout models

Tetraspanins are highly conserved 4-transmembrane proteins which form molecular clusters with a large variety of transmembrane and cytosolic proteins. By these associations tetraspanins are engaged in a multitude of biological processes. Furthermore, tetraspanin complexes are located in specialized microdomains, called tetraspanin-enriched microdomains (TEMs). TEMs provide a signaling platform and are poised for invagination and vesicle formation. These vesicles can be released as exosomes (Exo) and are important in cell contact-independent intercellular communication. Here, we summarize emphasizing knockdown and knockout models' pathophysiological joint and selective activities of CD151 and Tspan8, and discuss the TEM-related engagement of CD151 and Tspan8 in Exo activities.

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.

Inhibition of the integrin/FAK signaling axis and c-Myc synergistically disrupts ovarian cancer malignancy

Integrins, a family of heterodimeric receptors for extracellular matrix, are promising therapeutic targets for ovarian cancer, particularly high-grade serous-type (HGSOC), as they drive tumor cell attachment, migration, proliferation and survival by activating focal adhesion kinase (FAK)-dependent signaling. Owing to the potential off-target effects of FAK inhibitors, disruption of the integrin signaling axis remains to be a challenge. Here, we tackled this barrier by screening for inhibitors being functionally cooperative with small-molecule VS-6063, a phase II FAK inhibitor. From this screening, JQ1, a potent inhibitor of Myc oncogenic network, emerged as the most robust collaborator. Treatment with a combination of VS-6063 and JQ1 synergistically caused an arrest of tumor cells at the G2/M phase and a decrease in the XIAP-linked cell survival. Our subsequent mechanistic analyses indicate that this functional cooperation was strongly associated with the concomitant disruption of activation or expression of FAK and c-Myc as well as their downstream signaling through the PI3K/Akt pathway. In line with these observations, we detected a strong co-amplification or upregulation at genomic or protein level for FAK and c-Myc in a large portion of primary tumors in the TCGA or a local HGSOC patient cohort. Taken together, our results suggest that the integrin–FAK signaling axis and c-Myc synergistically drive cell proliferation, survival and oncogenic potential in HGSOC. As such, our study provides key genetic, functional and signaling bases for the small-molecule-based co-targeting of these two distinct oncogenic drivers as a new line of targeted therapy against human ovarian cancer.

CD151 Gene and Protein Expression Provides Independent Prognostic Information for Patients with Adenocarcinoma of the Esophagus and Gastroesophageal Junction Treated by Esophagectomy

BACKGROUND

Esophageal and gastroesophageal junctional (GEJ) adenocarcinoma is one of the most fatal cancers and has the fastest rising incidence rate of all cancers. Identification of biomarkers is needed to tailor treatments to each patient's tumor biology and prognosis.

METHODS

Gene expression profiling was performed in a test cohort of 80 chemoradiotherapy (CRTx)-naïve patients with external validation in a separate cohort of 62 CRTx-naïve patients and 169 patients with advanced-stage disease treated with CRTx.

RESULTS

As a novel prognostic biomarker after external validation, CD151 showed promise. Patients exhibiting high levels of CD151 (≥median) had a longer median overall survival than patients with low CD151 tumor levels (median not reached vs. 30.9 months; p = 0.01). This effect persisted in a multivariable Cox-regression model with adjustment for tumor stage [adjusted hazard ratio (aHR), 0.33; 95 % confidence interval (CI), 0.14-0.78; p = 0.01] and was further corroborated through immunohistochemical analysis (aHR, 0.22; 95 % CI, 0.08-0.59; p = 0.003). This effect was not found in the separate cohort of CRTx-exposed patients.

CONCLUSION

Tumoral expression levels of CD151 may provide independent prognostic information not gained by conventional staging of patients with esophageal and GEJ adenocarcinoma treated by esophagectomy alone.

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.

CD151 promotes α3β1 integrin-dependent organization of carcinoma cell junctions and restrains collective cell invasion

Integrins function in collective migration both as major receptors for extracellular matrix and by crosstalk to adherens junctions. Despite extensive research, important questions remain about how integrin signaling mechanisms are integrated into collective migration programs. Tetraspanins form cell surface complexes with a subset of integrins and thus are good candidates for regulating the balance of integrin functional inputs into cell-matrix and cell-cell interactions. For example, tetraspanin CD151 directly associates with α3β1 integrin in carcinoma cells and promotes rapid α3β1-dependent single cell motility, but CD151 also promotes organized adherens junctions and restrains collective carcinoma cell migration on 2D substrates. However, the individual roles of CD151s integrin partners in CD151s pro-junction activity in carcinoma cells were not well understood. Here we find that CD151 promotes organized carcinoma cell junctions via α3β1 integrin, by a mechanism that requires the a3b1 ligand, laminin-332. Loss of CD151 promotes collective 3D invasion and growth in vitro and in vivo, and the enhanced invasion of CD151-silenced cells is α3 integrin dependent, suggesting that CD151 can regulate the balance between α3β1s pro-junction and pro-migratory activities in collective invasion. An analysis of human cancer cases revealed that changes in CD151 expression can be linked to either better or worse clinical outcomes depending on context, including potentially divergent roles for CD151 in different subsets of breast cancer cases. Thus, the role of the CD151-α3β1 complex in carcinoma progression is context dependent, and may depend on the mode of tumor cell invasion.

Expression of CD151/Tspan24 and integrin alpha 3 complex in aid of prognostication of HER2-negative high-grade ductal carcinoma in situ

The pro-tumorigenic and pro-metastatic functions of the tetraspanin protein CD151 (Tspan24) are thought to be dependent on its ability to form complexes with laminin-binding integrin receptors (i.e. alpha6beta1, alpha3beta1, alpha6beta4). We have previously reported that in invasive ductal carcinoma (IDC), CD151/alpha3beta1 complex was of prognostic value in patients with HER2-negative tumors. Extrapolating these findings to the pre-invasive setting, we aimed to make an assessment of a potential relationship between expression of the CD151/alpha3beta1 complex in DCIS and Van Nuys prognostic index (VNPI) in high-grade ductal carcinoma in situ (DCIS) in relation to the HER2 status. Protein distributions were analyzed in 49 samples of pure DCIS using immunohistochemistry. For each case immunoreactivity was assessed in at least 5 ducts (325 ducts in total) and an average score was taken for statistical analyses. When analyzed in the whole cohort, there was no statistical association between the VNPI and any of the proteins scored either separately or in combination. When stratified according to the HER2 status, in the HER2-negative subgroup, CD151 assessed in combination with alpha3beta1 was significantly correlated with VNPI (P = 0.044), while neither protein analyzed individually showed any significant link with the prognostic index. Expression of the CD151/alpha3beta1 complex in HER2-negative DCIS might reflect tumor behavior relevant to the patient outcome and thus might aid prognostication of the disease.

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.

CD151 represses mammary gland development by maintaining the niches of progenitor cells

Tetraspanin CD151 interacts with laminin-binding integrins (i.e., α3β1, α6β1 and α6β4) and other cell surface molecules to control diverse cellular and physiological processes, ranging from cell adhesion, migration and survival to tissue architecture and homeostasis. Here, we report a novel role of CD151 in maintaining the branching morphogenesis and activity of progenitor cells during the pubertal development of mammary glands. In contrast to the disruption of laminin-binding integrins, CD151 removal in mice enhanced the tertiary branching in mammary glands by 2.4-fold and the number of terminal end buds (TEBs) by 30%, while having minimal influence on either primary or secondary ductal branching. Consistent with these morphological changes are the skewed distribution of basal/myoepithelial cells and a 3.2-fold increase in proliferating Ki67-positive cells. These novel observations suggest that CD151 impacts the branching morphogenesis of mammary glands by upregulating the activities of bipotent progenitor cells. Indeed, our subsequent analyses indicate that upon CD151 removal the proportion of CD24(Hi)CD49f(Low) progenitor cells in the mammary gland increased by 34%, and their proliferating and differentiating activities were significantly upregulated. Importantly, fibronectin, a pro-branching extracellular matrix (ECM) protein deposited underlying mammary epithelial or progenitor cells, increased by >7.2-fold. Moreover, there was a concomitant increase in the expression and nuclear distribution of Slug, a transcription factor implicated in the maintenance of mammary progenitor cell activities. Taken together, our studies demonstrate that integrin-associated CD151 represses mammary branching morphogenesis by controlling progenitor cell activities, ECM integrity and transcription program.

Tetraspanins in cell migration

Tetraspanins are a superfamily of small transmembrane proteins that are expressed in almost all eukaryotic cells. Through interacting with one another and with other membrane and intracellular proteins, tetraspanins regulate a wide range of proteins such as integrins, cell surface receptors, and signaling molecules, and thereby engage in diverse cellular processes ranging from cell adhesion and migration to proliferation and differentiation. In particular, tetraspanins modulate the function of proteins involved in all determining factors of cell migration including cell-cell adhesion, cell-ECM adhesion, cytoskeletal protrusion/contraction, and proteolytic ECM remodeling. We herein provide a brief overview of collective in vitro and in vivo studies of tetraspanins to illustrate their regulatory functions in the migration and trafficking of cancer cells, vascular endothelial cells, skin cells (keratinocytes and fibroblasts), and leukocytes. We also discuss the involvement of tetraspanins in various pathologic and remedial processes that rely on cell migration and their potential value as targets for therapeutic intervention.

Promotion of bone morphogenetic protein signaling by tetraspanins and glycosphingolipids

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor β (TGFβ) superfamily of secreted molecules. BMPs play essential roles in multiple developmental and homeostatic processes in metazoans. Malfunction of the BMP pathway can cause a variety of diseases in humans, including cancer, skeletal disorders and cardiovascular diseases. Identification of factors that ensure proper spatiotemporal control of BMP signaling is critical for understanding how this pathway is regulated. We have used a unique and sensitive genetic screen to identify the plasma membrane-localized tetraspanin TSP-21 as a key new factor in the C. elegans BMP-like “Sma/Mab” signaling pathway that controls body size and postembryonic M lineage development. We showed that TSP-21 acts in the signal-receiving cells and genetically functions at the ligand-receptor level. We further showed that TSP-21 can associate with itself and with two additional tetraspanins, TSP-12 and TSP-14, which also promote Sma/Mab signaling. TSP-12 and TSP-14 can also associate with SMA-6, the type I receptor of the Sma/Mab pathway. Finally, we found that glycosphingolipids, major components of the tetraspanin-enriched microdomains, are required for Sma/Mab signaling. Our findings suggest that the tetraspanin-enriched membrane microdomains are important for proper BMP signaling. As tetraspanins have emerged as diagnostic and prognostic markers for tumor progression, and TSP-21, TSP-12 and TSP-14 are all conserved in humans, we speculate that abnormal BMP signaling due to altered expression or function of certain tetraspanins may be a contributing factor to cancer development.

The bone morphogenetic protein (BMP) signaling pathway is required for multiple developmental processes during metazoan development. Various diseases, including cancer, can result from mis-regulation of the BMP pathway. Thus, it is critical to identify factors that ensure proper regulation of BMP signaling. Using the nematode C. elegans, we have devised a highly specific and sensitive genetic screen to identify new modulators in the BMP pathway. Through this screen, we identified three conserved tetraspanin molecules as novel factors that function to promote BMP signaling in a living organism. We further showed that these three tetraspanins likely form a complex and function together with glycosphingolipids to promote BMP signaling. Recent studies have implicated several tetraspanins in cancer initiation, progression and metastasis in mammals. Our findings suggest that the involvement of tetraspanins in cancer may partially be due to their function in modulating the activity of BMP signaling.

CD151-A Striking Marker for Cancer Therapy

Cluster of differentiation 151 (CD151) is a member of the mammalian tetraspanin family, which is involved in diverse functions such as maintaining normal cellular integrity, cell-to-cell communication, wound healing, platelet aggregation, trafficking, cell motility and angiogenesis. CD151 also supports de novo carcinogenesis in human skin squamous cell carcinoma (SCC) and tumor metastasis. CD151 interacts with α3β1 and α6β4 integrins through palmitoylation where cysteine plays an important role in the association of CD151 with integrins and non-integrin proteins. Invasion and metastasis of cancer cells were diminished by decreasing CD151 association with integrins. CD151 functions at various stages of cancer, including metastatic cascade and primary tumor growth, thus reinforcing the importance of CD151 as a target in oncology. The present review highlights the role of CD151 in tumor metastasis and its importance in cancer therapy.

Reduced CD151 expression is related to advanced tumour stage in urothelial bladder cancer

AIMS

CD151 belongs to the group of tetraspanins and is aberrantly expressed in different tumours and differential expression has been associated with prognosis. The aim of this study was to clarify the relationship of CD151 expression with tumour phenotype and clinical outcome in bladder cancer.

METHODS

A bladder cancer tissue microarray containing samples from 686 urothelial bladder cancers was analysed by immunohistochemistry.

RESULTS

Membranous CD151 immunostaining was recorded in 409 (66.0%) of 620 analysable cases. High CD151 expression was seen in normal urothelium and in most non-invasive tumours. Low CD151 expression levels were associated with a more unfavourable tumour phenotype. CD151 staining was seen in 71.5% of 284 pTa, 62.1% of 145 pT1 and 60.4% of 187 pT2-4 cancers (p = 0.0033). CD151 staining was detectable in 77.3% of 75 grade 1, 71.1% of 273 grade 2 and 57.7% of 272 grade 3 cancers (p < 0.0001). CD151 expression status was not associated with overall or tumour-specific survival in muscle-invasive cancers (pT2-4), tumour progression in pT1 and recurrences in pTa tumours.

CONCLUSION

On the basis of our data we conclude that loss of CD151 may contribute to bladder cancer progression through attenuation of cell adhesion. In clinically defined subgroups CD151 expression does not provide additional prognostic information.

TSPAN2 is involved in cell invasion and motility during lung cancer progression

In lung cancer progression, p53 mutations are more often observed in invasive tumors than in noninvasive tumors, suggesting that p53 is involved in tumor invasion and metastasis. To understand the nature of p53 function as a tumor suppressor, it is crucial to elucidate the detailed mechanism of the alteration in epithelial cells that follow oncogenic KRAS activation and p53 inactivation. Here, we report that KRAS activation induces epithelial-mesenchymal transition and that p53 inactivation is required for cell motility and invasiveness. Furthermore, TSPAN2, a transmembrane protein, is responsible for cell motility and invasiveness elicited by p53 inactivation. TSPAN2 is highly expressed in p53-mutated lung cancer cells, and high expression of TSPAN2 is associated with the poor prognosis of lung adenocarinomas. TSPAN2 knockdown suppresses metastasis to the lungs and liver, enabling prolonged survival. TSPAN2 enhances cell motility and invasiveness by assisting CD44 in scavenging intracellular reactive oxygen species.

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.

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.

Saponins from the roots of Platycodon grandiflorum suppresses TGFβ1-induced epithelial-mesenchymal transition via repression of PI3K/Akt, ERK1/2 and Smad2/3 pathway in human lung carcinoma A549 cells

Transforming growth factor β (TGFβ) is a multifunctional cytokine that induces growth arrest, tissue fibrosis, and epithelial-mesenchymal transition (EMT) through activation of Smad and non-Smad signaling pathways. EMT is the differentiation switch by which polarized epithelial cells differentiate into contractile and motile mesenchymal cells. Our previous studies have shown that saponins from the roots of Platycodon grandiflorum (CKS) have antiinflammatory, antioxidant, antimetastatic, and hepatoprotective effects. In this study, we investigated the inhibitory effect of CKS on TGFβ1-induced alterations characteristic of EMT in human lung carcinoma A549 cells. We found that CKS-treated cells displayed inhibited TGFβ1-mediated E-cadherin downregulation and Vimentin upregulation and also retained epithelial morphology. Furthermore, TGFβ1-increased Snail expression, a repressor of E-cadherin and an inducer of the EMT, was reduced by CKS. CKS inhibited TGFβ1-induced phosphorylation of Akt, ERK1/2, and glycogen synthase kinase-3β (GSK-3β). Inhibition of PI3K/Akt and ERK1/2 also blocked TGFβ1-induced GSK-3β phosphorylation and Snail activation. Furthermore, TGFβ1-increased Snail expression was reduced by selective inhibitors of Akt and ERK1/2. Moreover, CKS treatment attenuated TGFβ1-induced Smad2/3 phosphorylation and upregulated Smad7 expression. These results indicate that pretreatment with the CKS inhibits the TGFβ1-induced EMT through PI3K/Akt, ERK1/2, GSK-3β and Smad2/3 in human lung carcinoma cells.

Tetraspanin CD151 expression associated with prognosis for patients with advanced gastric cancer

PURPOSE

Tetraspanin CD151 is known to be involved in cancer invasion and metastasis, and its overexpression appears to be associated with a poor prognosis for various types of cancer. However, the expression status of CD151 and its prognostic impact in advanced gastric cancer (AGC) has not yet been clarified.

METHODS

Immunohistochemistry was used to investigate the expression of CD151, c-erbB2, and c-Met in 159 cases of AGC. The clinicopathological and prognostic significance of these biomarkers were then evaluated.

RESULTS

The overexpression of CD151 was observed in a subset of advanced gastric adenocarcinomas (25.8 %), and c-erbB2 and c-Met were overexpressed in 15.1 and 35.2 % of the cohort, respectively. CD151 overexpression was more frequently observed in tumors from younger patients (P = 0.028). There were close associations between CD151 and c-erbB2 overexpression (P = 0.033) and between c-erbB2 and c-Met overexpression (P = 0.001). CD151 overexpression was closely correlated with patient' overall survival (OS; P < 0.001) and disease-free survival (DFS; P < 0.001). Furthermore, the expression rate of CD151 seemed to increase gradually according to the depth of invasion (T stage) (χ(2) test for trend; P = 0.101), N stage (P = 0.238), and pathologic stage (P = 0.153), although trends were not statistically significant. In a multivariate analysis, CD151 overexpression was an independent prognostic factor predicting worse OS (P = 0.002) and DFS (P = 0.005), along with the T and N stage.

CONCLUSIONS

CD151 was found to be an independent prognostic marker for patients with AGC.

Reduced susceptibility to two-stage skin carcinogenesis in mice with epidermis-specific deletion of CD151

Altered expression of the tetraspanin CD151 is associated with skin tumorigenesis; however, whether CD151 is causally involved in the tumorigenic process is not known. To evaluate its role in tumor formation, we subjected epidermis-specific Cd151 knockout mice to chemical skin carcinogenesis. Mice lacking epidermal Cd151 developed fewer and smaller tumors than wild-type mice following DMBA/TPA treatment. Furthermore, Cd151-null epidermis showed a reduced hyperproliferative response to short-term treatment with TPA compared to that of wild-type skin, while epidermal turnover was increased. Tumors were formed in equal numbers following DMBA only treatment. We suggest that DMBA-initiated keratinocytes lacking Cd151 leave their niches in the epidermis and hair follicles in response to TPA treatment and subsequently are lost by differentiation. Because genetic ablation of Itga3 also reduced skin tumor formation, we tested whether reduced expression of α3 could further suppress tumor formation in epidermis-specific Cd151 knockout mice. Although the response to DMBA/TPA-induced formation of skin tumors was similar in compound heterozygotes for Cd151 and Itga3 to that in wild-type mice, heterozygosity for Itga3 on a Cd151-null background diminished tumorigenesis suggesting genetic interaction between the two genes. We thus identify CD151 as a critical factor in TPA-dependent skin carcinogenesis.

miR-506 regulates epithelial mesenchymal transition in breast cancer cell lines

Epithelial-mesenchymal transition (EMT) is an important parameter related to breast cancer survival. Among several microRNAs predicted to target EMT-related genes, miR-506 is a novel miRNA found to be significantly related to breast cancer patient survival in a meta-analysis. miR-506 suppressed the expression of mesenchymal genes such as Vimentin, Snai2, and CD151 in MDA-MB-231 human breast cancer cell line. Moreover, NF-κB bound to the upstream promoter region of miR-506 to suppress transcription. Overexpression of miR-506 inhibited TGFβ-induced EMT and suppressed adhesion, invasion, and migration of MDA-MB-231 cells. From these results, we concluded that miR-506 plays a key role in the process of EMT through posttranslational control of EMT-related genes.

Knockout of the tetraspanin Cd9 in the TRAMP model of de novo prostate cancer increases spontaneous metastases in an organ-specific manner

Prostate cancer is an extremely heterogeneous disease; patients that do progress to late-stage metastatic prostate cancer have limited treatment options, mostly palliative. Molecules involved in the metastatic cascade may prove beneficial in stratifying patients to assign appropriate treatment modalities and may also prove to be therapeutic antimetastatic targets. The tetraspanin group of molecules are integral membrane proteins that associate with motility-related proteins such as integrins. Clinical studies have mostly shown that reduced expression levels of the tetraspanin CD9 are correlated with tumour progression in a range of cancers. Furthermore, functional studies have shown CD9 to be involved in cell motility and adhesion and that it may influence metastasis. The effects of endogenous CD9 on prostate cancer initiation and progression were analysed by crossing a Cd9-/- (KO) murine model with a model of de novo developing and spontaneously metastasising prostate cancer, namely the transgenic adenocarcinoma of mouse prostate model. Our study demonstrates for the first time that ablation of Cd9 had no detectable effect on de novo primary tumour onset, but did significantly increase metastasis to the liver but not the lungs.

The CD9/CD81 tetraspanin complex and tetraspanin CD151 regulate α3β1 integrin-dependent tumor cell behaviors by overlapping but distinct mechanisms

Integrin α3β1 potently promotes cell motility on its ligands, laminin-332 and laminin-511, and this may help to explain why α3β1 has repeatedly been linked to breast carcinoma progression and metastasis. The pro-migratory functions of α3β1 depend strongly on lateral interactions with cell surface tetraspanin proteins. Tetraspanin CD151 interacts directly with the α3 integrin subunit and links α3β1 integrin to other tetraspanins, including CD9 and CD81. Loss of CD151 disrupts α3β1 association with other tetraspanins and impairs α3β1-dependent motility. However, the extent to which tetraspanins other than CD151 are required for specific α3β1 functions is unclear. To begin to clarify which aspects of α3β1 function require which tetraspanins, we created breast carcinoma cells depleted of both CD9 and CD81 by RNA interference. Silencing both of these closely related tetraspanins was required to uncover their contributions to α3β1 function. We then directly compared our CD9/CD81-silenced cells to CD151-silenced cells. Both CD9/CD81-silenced cells and CD151-silenced cells showed delayed α3β1-dependent cell spreading on laminin-332. Surprisingly, however, once fully spread, CD9/CD81-silenced cells, but not CD151-silenced cells, displayed impaired α3β1-dependent directed motility and altered front-rear cell morphology. Also unexpectedly, the CD9/CD81 complex, but not CD151, was required to promote α3β1 association with PKCα in breast carcinoma cells, and a PKC inhibitor mimicked aspects of the CD9/CD81-silenced cell motility defect. Our data reveal overlapping, but surprisingly distinct contributions of specific tetraspanins to α3β1 integrin function. Importantly, some of CD9/CD81's α3β1 regulatory functions may not require CD9/CD81 to be physically linked to α3β1 by CD151.

Prognostic significance of CD151 overexpression in non-small cell lung cancer

The overexpression of tetraspanin CD151 - a transmembrane protein that promotes tumor invasion and metastasis - is associated with poor prognosis in various cancers. However, its clinical significance in non-small cell lung cancers (NSCLCs) has not been fully elucidated. We investigated CD151 expression status by immunohistochemical analysis in paraffin-embedded specimens obtained from 380 patients with surgically resected NSCLCs (245 squamous cell carcinomas [SCCs] and 135 adenocarcinomas [ADCs]) between 1994 and 2001. High CD151 expression was detected in 28.7% NSCLCs (20.8% of SCCs and 42.9% of ADCs) and was significantly associated with male gender, smokers, and ADCs. Moreover, elevated CD151 levels were correlated with reduced overall (OS) and disease-free survival (DFS), and were an independent negative prognostic factor for OS in NSCLC. According to histological type, high CD151 expression was an independent prognostic factor for lower OS in ADC, although not in each subtype, and the elevated CD151 expression levels were more common in solid-predominant tumors (48.3%). In contrast, there was no prognostic correlation in SCC. High CD151 expression appeared to correlate with aggressive behavior in NSCLC, suggesting that it may be a useful prognostic marker for lung ADC patients and a potential molecular target for NSCLC treatment.

Integrin-associated CD151 drives ErbB2-evoked mammary tumor onset and metastasis

ErbB2+ human breast cancer is a major clinical problem. Prior results have suggested that tetraspanin CD151 might contribute to ErbB2-driven breast cancer growth, survival, and metastasis. In other cancer types, CD151 sometimes supports tumor growth and metastasis. However, a definitive test of CD151 effects on de novo breast cancer initiation, growth, and metastasis has not previously been done. We used CD151 gene-deleted mice expressing the MMTV-ErbB2 transgene to show that CD151 strongly supports ErbB2+ mammary tumor initiation and metastasis. Delayed tumor onset (by 70-100 days) in the absence of CD151 was accompanied by reduced survival of mammary epithelial cells and impaired activation of FAK- and MAPK-dependent pathways. Both primary tumors and metastatic nodules showed smooth, regular borders, consistent with a less invasive phenotype. Furthermore, consistent with impaired oncogenesis and decreased metastasis, CD151-targeted MCF-10A/ErbB2 cells showed substantial decreases in three-dimensional colony formation, EGF-stimulated tumor cell motility, invasion, and transendothelial migration. These CD151-dependent functions were largely mediated through α6β4 integrin. Moreover, CD151 ablation substantially prevented PKC- and EGFR/ERK-dependent α6β4 integrin phosphorylation, consistent with retention of epithelial cell polarity and intermediate filament cytoskeletal connections, which helps to explain diminished metastasis. Finally, clinical data analyses revealed a strong correlation between CD151 and ErbB2 expression and metastasis-free survival of breast cancer patients. In conclusion, we provide strong evidence that CD151 collaborates with LB integrins (particularly α6β4 and ErbB2 (and EGFR) receptors to regulate multiple signaling pathways, thereby driving mammary tumor onset, survival, and metastasis. Consequently, CD151 is a useful therapeutic target in malignant ErbB2+ breast cancer.

Tetraspanin CD151 mediates papillomavirus type 16 endocytosis

Human papillomavirus type 16 (HPV16) is the primary etiologic agent for cervical cancer. The infectious entry of HPV16 into cells occurs via a so-far poorly characterized clathrin- and caveolin-independent endocytic pathway, which involves tetraspanin proteins and actin. In this study, we investigated the specific role of the tetraspanin CD151 in the early steps of HPV16 infection. We show that surface-bound HPV16 moves together with CD151 within the plane of the membrane before they cointernalize into endosomes. Depletion of endogenous CD151 did not affect binding of viral particles to cells but resulted in reduction of HPV16 endocytosis. HPV16 uptake is dependent on the C-terminal cytoplasmic region of CD151 but does not require its tyrosine-based sorting motif. Reexpression of the wild-type CD151 but not mutants affecting integrin functions restored virus internalization in CD151-depleted cells. Accordingly, short interfering RNA (siRNA) gene knockdown experiments confirmed that CD151-associated integrins (i.e., α3β1 and α6β1/4) are involved in HPV16 infection. Furthermore, palmitoylation-deficient CD151 did not support HPV16 cell entry. These data show that complex formation of CD151 with laminin-binding integrins and integration of the complex into tetraspanin-enriched microdomains are critical for HPV16 endocytosis.

Genetic ablation of the tetraspanin CD151 reduces spontaneous metastatic spread of prostate cancer in the TRAMP model

Tetraspanins are integral membrane proteins that associate with motility-related molecules such as integrins. Experimental studies have indicated that they may be important regulators of tumor invasion and metastasis, and high expression of the tetraspanin CD151 has been linked to poor prognosis in a number of cancers. Here, we show for the first time that genetic ablation of CD151 inhibits spontaneous metastasis in a transgenic mouse model of de novo tumorigenesis. To evaluate the effects of CD151 on de novo prostate cancer initiation and metastasis, a Cd151(-/-) (KO) murine model was crossed with the Transgenic Adenocarcinoma of Mouse Prostate (TRAMP) model. Mice were analyzed for initiation of prostate tumor by palpation and primary tumors were analyzed by immunohistochemistry. Liver and lungs were examined for incidence and size of spontaneous metastatic lesions by histopathology. Knocking-out Cd151 had no significant effect on prostate cancer initiation or on expression of markers of proliferation, apoptosis, or angiogenesis in primary tumors. However, it did significantly decrease metastasis in a site-specific fashion, notably to the lungs but not the liver. Thus, CD151 acts principally as promoter of metastasis in this model. Prostate cancer is the second highest cause of cancer-related deaths in men in most Western countries, with the majority of deaths attributed to late-stage metastatic disease. CD151 may prove to be a valuable prognostic marker for treatment stratification and is a possible antimetastatic target.

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.