CD9 expression in gastric cancer and its significance

Separation and isolation of CD9-positive extracellular vesicles from plasma using flow cytometry

Extracellular vesicles (EVs) are nanosized (∼30-1000 nm) lipid-enclosed particles released by a variety of cell types. EVs are found in biological fluids and are considered a promising material for disease detection and monitoring. Given their nanosized properties, EVs are difficult to isolate and study. In complex biological samples, this difficulty is amplified by other small particles and contaminating proteins making the discovery and validation of EV-based biomarkers challenging. Developing new strategies to isolate EVs from complex biological samples is of significant interest. Here, we evaluate the utility of flow cytometry to isolate particles in the nanoscale size range. Flow cytometry calibration was performed and 100 nm nanoparticles and ∼124 nm virus were used to test sorting capabilities in the nanoscale size range. Next, using blood plasma, we assessed the capabilities of flow cytometry sorting for the isolation of CD9-positive EVs. Using flow cytometry, CD9-positive EVs could be sorted from pre-enriched EV fractions and directly from plasma without the need for any EV pre-enrichment isolation strategies. These results demonstrate that flow cytometry can be employed as a method to isolate subpopulations of EVs from biological samples.

Prognostic value and multifaceted roles of tetraspanin CD9 in cancer

CD9 is a crucial regulator of cell adhesion in the immune system and plays important physiological roles in hematopoiesis, blood coagulation or viral and bacterial infections. It is involved in the transendothelial migration of leukocytes which might also be hijacked by cancer cells during their invasion and metastasis. CD9 is found at the cell surface and the membrane of exosomes affecting cancer progression and therapy resistance. High expression of CD9 is mostly associated with good patients outcome, with a few exceptions. Discordant findings have been reported for breast, ovarian, melanoma, pancreatic and esophageal cancer, which might be related to using different antibodies or inherent cancer heterogeneity. According to in vitro and in vivo studies, tetraspanin CD9 is not clearly associated with either tumor suppression or promotion. Further mechanistic experiments will elucidate the role of CD9 in particular cancer types and specific conditions.

Increased CD9 expression predicts favorable prognosis in human cancers: a systematic review and meta-analysis

Background

CD9 is implicated in cancer progression and metastasis by its role in suppressing cancer cell proliferation and survival. However, the prognostic and clinicopathological significance of CD9 expression is controversial. Therefore, the current meta-analysis was conducted to determine the prognostic and clinicopathological significance of CD9 expression in cancer patients.

Methods

Eligible studies were selected through database search of PubMed, Embase and Cochrane library up to April 5 2020. The necessary data were extracted from the included studies. Pooled hazard ratio (HR) and odds ratio (OR) with 95% confidence interval (CI) were calculated to evaluate the prognostic and clinicopathological significance of CD9 expression in cancer patients.

Results

A total of 17 studies consisting of 3456 cancer patients were included in this meta-analysis. An increased CD9 expression was significantly associated with a more favorable overall survival (OS) (HR 0.47, 95% CI 0.31–0.73, p = 0.001) and disease-free survival (DFS) (HR 0.48, 95% CI 0.30–0.79, p = 0.003). In subgroup analysis of cancer type, an increased CD9 expression was associated with increased OS in breast cancer and digestive system cancer, and with increased DFS in head and neck cancer and leukemia/lymphoma. Additionally, an increased CD9 expression significantly correlated with lower overall stage (OR 0.45, 95% CI 0.29–0.72, p = 0.001).

Conclusion

An increased CD9 expression was associated with favorable survival in cancer patients suggesting that CD9 expression could be a valuable survival factor in cancer patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02152-y.

Multi-layered proteogenomic analysis unravels cancer metastasis directed by MMP-2 and focal adhesion kinase signaling

The role of matrix metalloproteinase-2 (MMP-2) in tumor cell migration has been widely studied, however, the characteristics and effects of MMP-2 in clinical sample of metastatic colorectal cancer (CRC) remain poorly understood. Here, in order to unveil the perturbed proteomic signal during MMP-2 induced cancer progression, we analyzed plasma proteome of CRC patients according to disease progression, HCT116 cancer secretome upon MMP-2 knockdown, and publicly available CRC tissue proteome data. Collectively, the integrative analysis of multi-layered proteomes revealed that a protein cluster containing EMT (Epithelial-to-Mesenchymal Transition)-associated proteins such as CD9-integrin as well as MMP-2. The proteins of the cluster were regulated by MMP-2 perturbation and exhibited significantly increased expressions in tissue and plasma as disease progressed from TNM (Tumor, Node, and Metastasis) stage I to II. Furthermore, we also identified a plausible association between MMP-2 up-regulation and activation of focal adhesion kinase signaling in the proteogenomic analysis of CRC patient tissues. Based on these comparative and integrative analyses, we suggest that the high invasiveness in the metastatic CRC resulted from increased secretion of MMP-2 and CD9-integrin complex mediated by FAK signaling activation.

Tetraspanins: Novel Molecular Regulators of Gastric Cancer

Gastric cancer is the fourth and fifth most common cancer worldwide in men and women, respectively. However, patients with an advanced stage of gastric cancer still have a poor prognosis and low overall survival rate. The tetraspanins belong to a protein superfamily with four hydrophobic transmembrane domains and 33 mammalian tetraspanins are ubiquitously distributed in various cells and tissues. They interact with other membrane proteins to form tetraspanin-enriched microdomains and serve a variety of functions including cell adhesion, invasion, motility, cell fusion, virus infection, and signal transduction. In this review, we summarize multiple utilities of tetraspanins in the progression of gastric cancer and the underlying molecular mechanisms. In general, the expression of TSPAN8, CD151, TSPAN1, and TSPAN4 is increased in gastric cancer tissues and enhance the proliferation and invasion of gastric cancer cells, while CD81, CD82, TSPAN5, TSPAN9, and TSPAN21 are downregulated and suppress gastric cancer cell growth. In terms of cell motility regulation, CD9, CD63 and CD82 are metastasis suppressors and the expression level is inversely associated with lymph node metastasis. We also review the clinicopathological significance of tetraspanins in gastric cancer including therapeutic targets, the development of drug resistance and prognosis prediction. Finally, we discuss the potential clinical value and current limitations of tetraspanins in gastric cancer treatments, and provide some guidance for future research.

Assessment of TSPAN Expression Profile and Their Role in the VSCC Prognosis

The role and prognostic value of tetraspanins (TSPANs) in vulvar squamous cell carcinoma (VSCC) remain poorly understood. We sought to primarily determine, at both the molecular and tissue level, the expression profile of the TSPANs CD9, CD63, CD81, and CD82 in archived VSCC samples (n = 117) and further investigate their clinical relevance as prognostic markers. Our studies led us to identify CD63 as the most highly expressed TSPAN, at the gene and protein levels. Multicomparison studies also revealed that the expression of CD9 was associated with tumor size, whereas CD63 upregulation was associated with histological diagnosis and vascular invasion. Moreover, low expression of CD81 and CD82 was associated with worse prognosis. To determine the role of TSPANs in VSCC at the cellular level, we assessed the mRNA levels of CD63 and CD82 in established metastatic (SW962) and non-metastatic (SW954) VSCC human cell lines. CD82 was found to be downregulated in SW962 cells, thus supporting its metastasis suppressor role. However, CD63 was significantly upregulated in both cell lines. Silencing of CD63 by siRNA led to a significant decrease in proliferation of both SW954 and SW962. Furthermore, in SW962 particularly, CD63-siRNA also remarkably inhibited cell migration. Altogether, our data suggest that the differential expression of TSPANs represents an important feature for prognosis of VSCC patients and indicates that CD63 and CD82 are likely potential therapeutic targets in VSCC.

CD9, a tetraspanin target for cancer therapy?

In the present minireview, we intend to provide a brief history of the field of CD9 involvement in oncogenesis and in the metastatic process of cancer, considering its potential value as a tumor-associated antigenic target. Over the years, CD9 has been identified as a favorable prognostic marker or predictor of metastatic potential depending on the cancer type. To understand its implications in cancer beside its use as an antigenic biomarker, it is essential to know its physiological functions, including its molecular partners in a given cell system. Moreover, the discovery that CD9 is one of the most specific and broadly expressed markers of extracellular membrane vesicles, nanometer-sized entities that are released into extracellular space and various physiological body fluids and play a role in intercellular communication under physiological and pathological conditions, notably the establishment of cancer metastases, has added a new dimension to our knowledge of CD9 function in cancer. Here, we will discuss these issues as well as the possible cancer therapeutic implications of CD9, their limitations, and pitfalls.

The tetraspanin CD9 controls migration and proliferation of parietal epithelial cells and glomerular disease progression

The mechanisms driving the development of extracapillary lesions in focal segmental glomerulosclerosis (FSGS) and crescentic glomerulonephritis (CGN) remain poorly understood. A key question is how parietal epithelial cells (PECs) invade glomerular capillaries, thereby promoting injury and kidney failure. Here we show that expression of the tetraspanin CD9 increases markedly in PECs in mouse models of CGN and FSGS, and in kidneys from individuals diagnosed with these diseases. Cd9 gene targeting in PECs prevents glomerular damage in CGN and FSGS mouse models. Mechanistically, CD9 deficiency prevents the oriented migration of PECs into the glomerular tuft and their acquisition of CD44 and β1 integrin expression. These findings highlight a critical role for de novo expression of CD9 as a common pathogenic switch driving the PEC phenotype in CGN and FSGS, while offering a potential therapeutic avenue to treat these conditions.

In both focal segmental glomerulosclerosis (FSGS) and crescentic glomerulonephritis (CGN), kidney injury is characterised by the invasion of glomerular tufts by parietal epithelial cells (PECs). Here Lazareth et al. identify the tetraspanin CD9 as a key regulator of PEC migration, and find its upregulation in FSGS and CGN contributes to kidney injury in both diseases.

CD9 Expression in Tumor Cells Is Associated with Poor Prognosis in Patients with Invasive Lobular Carcinoma

Purpose

We investigated the prognostic significance of CD9 expression in tumor cells of patients with invasive lobular carcinoma (ILC).

Methods

CD9 expression was evaluated by immunohistochemistry in 113 ILC tissue samples. Correlation of CD9 expression with the patients' clinicopathological parameters and overall survival was assessed.

Results

CD9 expression was detected in 48 (42.5%) ILC patients. However, no significant relation could be determined between CD9 expression and the clinicopathological parameters of the patient including tumor size, lymph node metastasis, lymphovascular invasion, histologic grade, expression of hormone receptors, human epidermal growth factor receptor 2 status, and Ki-67 labeling index. Patients with CD9 expression had worse overall survival (p = 0.051) and disease-free survival (DFS, p = 0.014) compared to patients without CD9 expression. Multivariate analysis revealed that CD9 expression was an independent prognostic factor for DFS (p = 0.049).

Conclusion

CD9 expression in tumor cells could be a significant prognostic marker in patients with ILC.

Therapeutic antibodies against cancer stem cells: a promising approach

Monoclonal antibodies have been extensively used to treat malignancy along with routine chemotherapeutic drugs. Chemotherapy for metastatic cancer has not been successful in securing long-term remission of disease. This is in part due to the resistance of cancer cells to drugs. One aspect of the drug resistance is the inability of conventional drugs to eliminate cancer stem cells (CSCs) which often constitute less than 1-2% of the whole tumor. In some tumor types, it is possible to identify these cells using surface markers. Monoclonal antibodies targeting these CSCs are an attractive option for a new therapeutic approach. Although administering antibodies has not been effective, when combined with chemotherapy they have proved synergistic. This review highlights the potential of improving treatment efficacy using functional antibodies against CSCs, which could be combined with chemotherapy in the future.

Tspan2: a tetraspanin protein involved in oligodendrogenesis and cancer metastasis

Tetraspanin 2 (Tspan2) is one of the less well-characterised members of the tetraspanin superfamily, and its precise function in different human tissue types remains to be explored. Initial studies have highlighted its possible association in neuroinflammation and carcinogenesis. In the central nervous system, Tspan2 may contribute to the early stages of the oligodendrocyte differentiation into myelin-forming glia. Furthermore, in human lung cancer, Tspan2 could be involved in the progression of the tumour metastasis by modulating cancer cell motility and invasion functions. In this review, we discuss the available evidence for the potential role of Tspan2 and introduce possible strategies for disease targeting.

Prognostic significance of CD9 expression differs between tumour cells and stromal immune cells, and depends on the molecular subtype of the invasive breast carcinoma

AIMS

CD9, a tetraspanin transmembrane protein, modulates cell motility, migration, and proliferation. The aim of this study was to investigate the prognostic significance of CD9 expression in patients with invasive breast carcinoma (IBC).

METHODS AND RESULTS

CD9 expression was evaluated in tissue microarrays of 1349 IBC samples via immunohistochemistry. CD9 expression in tumour cells (T-CD9 expression) and CD9 expression in stromal immune cells (S-CD9 expression) were analysed separately. T-CD9 expression was observed in 732 (54.3%) cases, and was associated with lymph node metastasis, histological type, lymphovascular invasion, high histological grade, HER2 positivity, a high Ki67 labelling index, and distant metastasis. S-CD9 expression was observed in 833 (61.7%) cases, and was associated with large tumour size, histological type, high histological grade, negative hormone receptors, HER2 positivity, a high Ki67 labelling index, and tumour-infiltrating lymphocytes. Patients with T-CD9 expression had shorter disease-free survival (DFS) than those without T-CD9 expression in the univariate and multivariate analyses. However, S-CD9 expression correlated significantly with a favourable DFS in the univariate and multivariate analyses. In the subgroup analysis, T-CD9 expression and S-CD9 expression were independent markers for DFS in luminal A and luminal B (HER2-negative) subgroups, respectively.

CONCLUSIONS

T-CD9 expression could be a biomarker for poor prognosis in luminal A IBC, whereas S-CD9 expression could be a marker of good prognosis in luminal B (HER2-negative) IBC. Therefore, tumour compartment-specific analyses considering molecular subtypes are necessary to study the prognostic significance of CD9 expression in IBC.

CD9 Expression in Colorectal Carcinomas and Its Prognostic Significance

Background

CD9, a member of the tetraspanin superfamily, is a tumor suppressor in many malignancies. The aim of this study was to evaluate the immunohistochemical expression of CD9 in colorectal carcinomas (CRCs) and determine clinicopathological and prognostic significance of its expression.

Methods

The CD9 expression status of 305 CRCs was evaluated using a semi-quantitative scoring system in tumor cells (T-CD9) and immune cells (I-CD9) by classifying the results as high and low expression.

Results

High T-CD9 (T-CD9 [+]) expression was detected in 175 samples (57.6%) and high I-CD9 (I-CD9 [+]) expression was detected in 265 samples (86.9%). Using Kaplan-Meier survival analysis, the T-CD9 (+) group showed a tendency for better disease-free survival (DFS) (p = .057). In left-sided tumors, DFS was significantly longer in the T-CD9 (+) group (p = .021) but no statistical significance was observed with right-sided tumors (p = .453). I-CD9 (+) CRCs significantly correlated with well/moderately differentiation (p = .014). In Kaplan-Meier analysis, the I-CD9 (+) group had a tendency towards worse DFS compared to the I-CD9 (–) group (p = .156). In combined survival analysis of T-CD9 and I-CD9, we found that the longest DFS was among patients in the T-CD9 (+)/I-CD9 (–) group, whereas the T-CD9 (–)/I-CD9 (+) group showed the shortest DFS (p = .054).

Conclusions

High expression of T-CD9 was associated with a favorable DFS, especially in left-sided CRCs. Combined evaluation of T-CD9 and I-CD9 is required to determine the comprehensive prognostic effect of CD9 in CRCs.

Identification of novel genes that regulate androgen receptor signaling and growth of androgen-deprived prostate cancer cells

Prostate cancer progression to castration refractory disease is associated with anomalous transcriptional activity of the androgen receptor (AR) in an androgen-depleted milieu. To identify novel gene products whose downregulation transactivates AR in prostate cancer cells, we performed a screen of enzymatically-generated shRNA lenti-libraries selecting for transduced LNCaP cells with elevated expression of a fluorescent reporter gene under the control of an AR-responsive promoter. The shRNAs present in selected populations were analyzed using high-throughput sequencing to identify target genes. Highly enriched gene targets were then validated with siRNAs against selected genes, testing first for increased expression of luciferase from an AR-responsive promoter and then for altered expression of endogenous androgen-regulated genes in LNCaP cells. We identified 20 human genes whose silencing affected the expression of exogenous and endogenous androgen-responsive genes in prostate cancer cells grown in androgen-depleted medium. Knockdown of four of these genes upregulated the expression of endogenous AR targets and siRNAs targeting two of these genes (IGSF8 and RTN1) enabled androgen-independent proliferation of androgen-dependent cells. The effects of IGSF8 appear to be mediated through its interaction with a tetraspanin protein, CD9, previously implicated in prostate cancer progression. Remarkably, homozygous deletions of IGSF8 are found almost exclusively in prostate cancers but not in other cancer types. Our study shows that androgen independence can be achieved through the inhibition of specific genes and reveals a novel set of genes that regulate AR signaling in prostate cancers.

Targeted therapies in gastric cancer and future perspectives

Advanced gastric cancer (AGC) is associated with a high mortality rate and, despite multiple new chemotherapy options, the survival rates of patients with AGC remains poor. After the discovery of targeted therapies, research has focused on the new treatment options for AGC. In the last two decades, many targeted molecules were developed against AGC. Currently, two targeted therapy molecules have been approved for patients with AGC. In 2010, trastuzumab was the first molecule shown to improve survival in patients with HER2-positive AGC as part of a first-line combination regimen. In 2014, ramucirumab was the second targeted molecule to improve survival rates and was suggested as treatment for patients with AGC who had progressed after first-line platinum plus fluoropyrimidine with or without anthracycline chemotherapy. Ramucirumab was the first targeted therapy acting as a single agent in patients with advanced gastroesophageal cancers. Although these two molecules were introduced into clinical use, many other promising molecules have been tested in phase I-II trials. It is obvious that in the near future many different targeted therapies will be in use for treatment of AGC. In this review, the current status of targeted therapies in the treatment of AGC and gastroesophageal junction tumors, including HER (2-3) inhibitors, epidermal growth factor receptor inhibitors, tyrosine kinase inhibitors, antiangiogenic agents, c-MET inhibitors, mammalian target of rapamycin inhibitors, agents against other molecular pathways fibroblast growth factor, Claudins, insulin-like growth factor, heat shock proteins, and immunotherapy, will be discussed.

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.

Novel CD9-targeted therapies in gastric cancer

There are 33 human tetraspanin proteins, emerging as key players in malignancy, the immune system, fertilization, cellular signaling, adhesion, morphology, motility, proliferation, and tumor invasion. CD9, a member of the tetraspanin family, associates with and influences a variety of cell-surface molecules. Through these interactions, CD9 modifies multiple cellular events, including adhesion, migration, proliferation, and survival. CD9 is therefore considered to play a role in several stages during cancer development. Reduced CD9 expression is generally related to venous vessel invasion and metastasis as well as poor prognosis. We found that treatment of mice bearing human gastric cancer cells with anti-CD9 antibody successfully inhibited tumor progression via antiproliferative, proapoptotic, and antiangiogenic effects, strongly indicating that CD9 is a possible therapeutic target in patients with gastric cancer. Here, we describe the possibility of CD9 manipulation as a novel therapeutic strategy in gastric cancer, which still shows poor prognosis.

Role of motility-related protein-1 in promoting the development of several types of cancer (Review)

Motility-related protein-1 (CD9), a type of cell surface glycoprotein comprising a four-pass transmembrane domain that forms multimeric complexes with other cell surface proteins, belongs to the tetraspanins family. From previous studies, we know that CD9 is considered to function primarily as a progression and metastasis suppressor in a variety of cancers, including breast, non-small cell lung colon and myeloma. However, an expanding body of literature has shown the contradictory outcome that tetraspanin CD9 is also vital in promoting cancer progression in several types of cancer. This review summarizes the recent studies on CD9 and concludes that it does not always act as a progression and metastasis suppressor. Conversely, in specific cases, CD9 may promote tumor progression through the following three aspects: Facilitating tumor cell transmigration, increasing tumor cell motility and hastening the growth of some cancers. In addition, CD9 appears to be an important marker of cancer stem cells in certain types of tumor.

Expression of CD9 and CD82 in clear cell renal cell carcinoma and its clinical significance

CD9 and CD82, members of the tetraspanin family, act as metastasis suppressors in many human malignant tumors, but the role of these molecules is not well known in clear cell renal cell carcinoma (CCRCC). This study was designed to evaluate the immunohistochemical expression of CD9 and CD82 in 644 cases of CCRCC and to determine the clinicopathologic and prognostic significance of their expression. The percentage of positive tumor cells was evaluated, and the expression was classified into 2 categories: low expression (less than 10% positive cells) or high expression (more than 10% positive cells) for CD9 expression and negative (no positive cells) or positive for CD82 expression. CD9 high expression was found in 303 (47.0%) patients, and CD82 positivity was found in 98 (15.2%) patients. High expression of CD9 was statistically associated with older patients (p=0.003). The cases showing positive immunoreactivity for CD82 exhibited a high stage (p<0.001) and high nuclear grade (p<0.001). The overall, cancer-specific and progression-free survival rates were significantly higher in patients with a CD82-negative profile compared to patients with a CD82-positive profile (p<0.001). Although the biological function of CD82 in CCRCC remains unclear, the CCRCC patients with CD82 positive expression show poor prognosis.

CD9 negatively regulates CD26 expression and inhibits CD26-mediated enhancement of invasive potential of malignant mesothelioma cells

CD26/dipeptidyl peptidase IV is a cell surface glycoprotein which consists of multiple functional domains beside its ectopeptidase site. A growing body of evidence indicates that elevated expression of CD26 correlates with disease aggressiveness and invasive potential of selected malignancies. To further explore the molecular mechanisms involved in this clinical behavior, our current work focused on the interaction between CD26 and CD9, which were recently identified as novel markers for cancer stem cells in malignant mesothelioma. We found that CD26 and CD9 co-modulated and co-precipitated with each other in the malignant mesothelioma cell lines ACC-MESO1 and MSTO-211H. SiRNA study revealed that depletion of CD26 led to increased CD9 expression, while depletion of CD9 resulted in increased CD26 expression. Consistent with these findings was the fact that gene transfer of CD26 into CD26-negative MSTO-211H cells reduced CD9 expression. Cell invasion assay showed that overexpression of CD26 or gene depletion of CD9 led to enhanced invasiveness, while CD26 gene depletion resulted in reduced invasive potential. Furthermore, our work suggested that this enhanced invasiveness may be partly mediated by α5β1 integrin, since co-precipitation studies demonstrated an association between CD26 and α5β1 integrin. Finally, gene depletion of CD9 resulted in elevated protein levels and tyrosine phosphorylation of FAK and Cas-L, which are downstream of β1 integrin, while depletion of CD26 led to a reduction in the levels of these molecules. Collectively, our findings suggest that CD26 potentiates tumor cell invasion through its interaction with α5β1 integrin, and CD9 negatively regulates tumor cell invasion by reducing the level of CD26-α5β1 integrin complex through an inverse correlation between CD9 and CD26 expression. Our results also suggest that CD26 and CD9 serve as potential biomarkers as well as promising molecular targets for novel therapeutic approaches in malignant mesothelioma and other malignancies.

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.

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.

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.

A functional genetic screen reveals new regulators of β1-integrin activity

β1 integrins constitute a large group of widely distributed adhesion receptors, which regulate the ability of cells to interact with their surroundings. This regulation of the expression and activity of integrins is crucial for tissue homeostasis and development and contributes to inflammation and cancer. We report an RNA interference screen to uncover genes involved in the regulation of β1-integrin activity using cell spot microarray technology in cancer cell lines. Altogether, ten cancer and two normal cell lines were used to identify regulators of β1 integrin activity. Cell biological analysis of the identified β1-integrin regulatory genes revealed that modulation of integrin activity can influence cell invasion in a three-dimensional matrix. We demonstrate with loss-of-function and rescue experiments that CD9 activates and MMP8 inactivates β1 integrins and that both proteins associate with β1 integrins in cells. Furthermore, CD9 and MMP8 regulate cancer cell extravasation in vivo. Our discovery of new regulators of β1-integrin activity highlight the complexity of integrin activity regulation and provide a set of new genes involved in regulation of integrin function.

Targeting CD9 produces stimulus-independent antiangiogenic effects predominantly in activated endothelial cells during angiogenesis: a novel antiangiogenic therapy

The precise roles of tetraspanin CD9 are unclear. Here we show that CD9 plays a stimulus-independent role in angiogenesis and that inhibiting CD9 expression or function is a potential antiangiogenic therapy. Knocking down CD9 expression significantly inhibited in vitro endothelial cell migration and invasion induced by vascular endothelial growth factor (VEGF) or hepatocyte growth factor (HGF). Injecting CD9-specific small interfering RNA (siRNA-CD9) markedly inhibited HGF- or VEGF-induced subconjunctival angiogenesis in vivo. Both results revealed potent and stimulus-independent antiangiogenic effects of targeting CD9. Furthermore, intravitreous injections of siRNA-CD9 or anti-CD9 antibodies were therapeutically effective for laser-induced retinal and choroidal neovascularization in mice, a representative ocular angiogenic disease model. In terms of the mechanism, growth factor receptor and downstream signaling activation were not affected, whereas abnormal localization of integrins and membrane type-1 matrix metalloproteinase was observed during angiogenesis, by knocking down CD9 expression. Notably, knocking down CD9 expression did not induce death and mildly inhibited proliferation of quiescent endothelial cells under conditions without an angiogenic stimulus. Thus, CD9 does not directly affect growth factor-induced signal transduction, which is required in angiogenesis and normal vasculature, but is part of the angiogenesis machinery in endothelial cells during angiogenesis. In conclusion, targeting CD9 produced stimulus-independent antiangiogenic effects predominantly in activated endothelial cells during angiogenesis, and appears to be an effective and safe antiangiogenic approach. These results shed light on the biological roles of CD9 and may lead to novel antiangiogenic therapies.

Immunohistochemical distribution of the tetraspanin CD9 in normal porcine tissues

The tetra-membrane-spanning protein, CD9 is a 24-27 kDa cell surface glycoprotein expressed in a wide variety of human cells being involved in a variety of cell processes, including signaling, adhesion, motility, fertilization and tumor cells metastasis. By means of a polyclonal antibody (N1) raised against recombinant swine CD9 protein, we studied the immunohistochemical expression of CD9 on different normal swine tissues. Immunochemistry shows that swine CD9 was distribute in a similar form than in human tissues, being present on epithelial cells of lung, liver, kidney, skin, tonsil, testis (epididymo), gut mucosa, uterus and mama. Furthermore, polyclonal antibody against swine CD9 reacts with white matter from cerebrum and cerebellum, peripheral nerves fibers and Hassal corpuscle from thymus and ovum. Platelets react strongly with our antibody, but monocytes and neutrophils react lightly. These results suggest that CD9 antigen should play a similar functional role in swine and human and therefore studies on CD9 on swine as an animal model would allow new knowledge about its role in adhesion, fertilization and tumor metastasis among other important biomedical processes.

Prognostic significance of CD9 expression in locally advanced gastric cancer treated with surgery and adjuvant chemoradiotherapy

The tetraspanin transmembrane protein CD9 plays an important role in inhibiting cell motility in numerous neoplastic cell lines, including lung, gastric, pancreatic, and bladder carcinomas. The prognostic importance of CD9 in the survival of gastric carcinoma patients has not been examined to date, and in the present study, we attempted to define its prognostic value. The study included 49 (35 men and 14 women) patients with locally advanced (stages II-IV) gastric cancer. The median age was 55 years (range, 22-73 years). Surgery was the initial treatment for all patients, followed by adjuvant chemoradiotherapy. Tissue sections were evaluated immunohistochemically with a monoclonal anti-CD9 antibody. Of the 49 patients with gastric adenocarcinoma, 11 (22.4%) were CD9-positive, and 38 (77.6%) were CD9-negative. A significant prognostic value in disease-free survival and overall survival was observed in T classification and CD9 positivity. In conclusion, CD9 expression in gastric cancer appears to be associated with poor prognosis.

Expression and function of CD9 in melanoma cells

CD9, a member of the tetraspanin family, functions as an organizer in "tetraspanin webs," through interacting with other cell adhesion molecules. It plays a role in differentiation, fertilization, and cell migration. We investigated the expression and function of CD9 in melanoma. CD9 protein expression in B16 mouse melanoma and six human melanoma cell lines was decreased compared to normal melanocytes. B16F1 clones stably overexpressing CD9 had reduced ability to form colonies in soft agar; however, paradoxically these overexpressing clones had increased ability to invade Matrigel. Similarly, transient overexpression of CD9 in the human metastatic melanoma cell line WM9 dramatically decreased anchorage-independent growth, while transient overexpression of CD9 in the radial growth phase cell line SbCl2 resulted in the gain of Matrigel invasion activity. DNA sequencing of CD9 cDNA from all six human melanoma cell lines did not show deletions, insertions, or mutations. Treatment of all six human melanoma cell lines with the histone deacetylase inhibitor trichostatin A increased CD9 levels. The DNA methylation inhibitor 5-aza-cytidine also increased CD9 protein levels with greater increases seen in cell lines derived from more malignant melanomas.

A novel therapeutic strategy with anti-CD9 antibody in gastric cancers

BACKGROUND

CD9 is a member of the tetraspanins, and has been shown to be involved in a variety of cellular activities such as motility, cell signaling, proliferation, adhesion, and metastasis. However, very little is known about the involvement of CD9 in the process of development of primary tumors. In the present study, we investigated whether anti-CD9 monoclonal antibody (ALB6) has antitumor effects in human gastric cancer cell xenografts.

METHODS

Human gastric cancer cell lines (MKN-28) (5 x 10(6) cells/animal) were inoculated subcutaneously into the dorsal region of SCID mice (five mice in each group). After a tumor was visualized, animals were assigned to either the ALB6 treatment group or the control IgG treatment group (100 microg/body/time, intravenous, three times per week. Day 1, 4, and 7 of first week). Then tumor volumes were monitored every day. Proliferation of tumor was analyzed by 5-bromo-2'-deoxyuridine (BrdU) immunostaining, apoptosis was determined by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling (TUNEL) methods, and angiogenesis was assessed by counting the number of CD34-positive endothelial cells.

RESULTS

Tumor volume was significantly suppressed (1,682 +/- 683 mm(3) versus 4,507 +/- 1,012 mm(3); P = 0.049), the BrdU labeling indexes were significantly decreased (10.9 +/- 1.1% versus 17.2 +/- 1.4%; P = 0.009), the apoptotic indexes were significantly increased (1.98 +/- 0.48% versus 0.72 +/- 0.09%; P = 0.034), and tumor microvessel densities were significantly suppressed (671,922 +/- 34,505 pixels/mm(2) versus 1,135,043 +/- 36,086 pixels/mm(2); P = 0.037) in the ALB6 treatment group compared with the control IgG treatment group.

CONCLUSIONS

These results suggest that administration of anti-CD9 antibody to mice bearing human gastric cancer cells successfully inhibits tumor progression via antiproliferative, proapoptotic, and antiangiogenetic effects.

DDX1 is required for testicular tumorigenesis, partially through the transcriptional activation of 12p stem cell genes

Cytogenetic analysis has identified 12p gain as the most frequent abnormality in human testicular germ cell tumors (TGCTs). It has been suggested that amplification and overexpression of stem cell-associated genes, including cyclin-D2, on the human chromosome 12p region are involved in germ cell tumorigenesis. By subtractive cDNA analysis, we identified Ddx1, a member of the DEAD-box protein family, as a gene predominantly expressed in the primordial germ cells of mouse embryos. Knockdown of Ddx1 in a mouse spermatogonia-derived cell line, GC-1spg, by short interference RNA repressed the expression of cyclin-D2, CD9 and GDF3 genes. In the mouse cyclin-D2 gene, a genomic DNA region between -348 and -329 was responsible for transcriptional activation by DDX1 based on reporter and gel shift assays. Similarly, DDX1 knockdown in the human TGCT cell line NEC8 repressed the expression of stem cell-associated genes localized on chromosome 12p13.3, including cyclin-D2, CD9 and NANOG. DDX1-knocked-down TGCT cells could not form solid tumors in nude mice. Furthermore, in situ hybridization revealed that DDX1 mRNA was produced in both seminoma and nonseminoma types of human TGCT samples. We conclude that DDX1 is a critical factor for testicular tumorigenesis.

Tetraspanin CD9 regulates beta 1 integrin activation and enhances cell motility to fibronectin via a PI-3 kinase-dependent pathway

Tetraspanin CD9 regulates cell motility and other adhesive processes in a variety of tissue types. Using transfected Chinese Hamster Ovary cells as our model system, we examined the cellular pathways critical for CD9 promoted cell migration. alpha 5 beta 1 integrin was directly involved as CD9 enhanced migration was abolished by the alpha 5 beta 1 blocking antibody PB1. Furthermore, the ligand mimetic peptide RGDS, significantly upregulated the expression of a beta1 ligand induced binding site (LIBS) demonstrating for the first time that CD9 expression potentiates beta1 integrin high affinity conformation states. CD9 promoted cell motility was significantly blocked by phosphatidylinositol-3 kinase (PI-3K) inhibitors, wortmannin and LY294002, whereas inhibitors targeting protein kinase C or mitogen-activated protein kinase had no effect. PI-3K dominant/negative cDNA transfections confirmed that PI-3K was an essential component. CD9 enhanced the phosphorylation of the PI-3K substrate, Akt, in response to cell adhesion on FN. CD9 expression also upregulated p130Cas phosphorylation and total protein levels; however, p130Cas siRNA knockdown did not alter the motile phenotype. CD9 enhanced migration was also unaffected by serum deprivation suggesting that growth factors were not critical. Our studies demonstrate that CD9 upregulates beta1 LIBS, and in concert with alpha 5 beta 1, enhances cell motility to FN via a PI-3K dependent mechanism.

Functional implications of tetraspanin proteins in cancer biology

Human tetraspanin proteins are a group of 33 highly hydrophobic membrane proteins that can form complexes in cholesterol-rich microdomains, distinct from lipid rafts, on the cell surface in a dynamic and reversible way. These complexes are composed of a core of several tetraspanin proteins that organize other membrane proteins such as integrins, human leukocyte antigen (HLA) antigens and some growth factor receptors. Although most tetraspanin proteins have been studied individually, tetraspanin proteins and their complexes can have effects on cellular adhesion and motility, interactions with stroma or affect signaling by growth factors, and for most of them no ligand has been identified. Functionally these proteins have been mostly studied in cells of lymphoid lineage, but they are present in all cell types. Data is also available for some tumors, where some tetraspanins have been identified as metastasis suppressors, but their significance is still not clear. Some of their implications in tumor biology and the areas that deserve further study are outlined.

Colocalisation of CD9 and mortalin in CD9-induced mitotic catastrophe in human prostate cancer cells

CD9, a member of the tetraspanin family of proteins, is involved in a variety of cellular interactions with many other proteins and molecules. Although CD9 has been implicated in cell fusion, migration and cancer progression, the detailed function of this protein is not completely understood and likely depends on interactions with different protein partners, which are not yet all known. Using co-immunoprecipitation and mass-spectrometric protein sequencing, we have identified in prostate cancer cells, a novel CD9 partner, the 75-kDa protein HSPA9B, also known as mortalin. We further show that introduction and overexpression of wild-type CD9 into human PC-3 prostate cancer cells induces mitotic catastrophe. We also demonstrate, by immunocolocalisation studies, the interaction of CD9 and mortalin in PC-3 cells undergoing mitotic catastrophe. Our results not only identified mortalin as a new CD9 partner, but also clarify the mechanisms by which CD9 may control prostate cancer progression.

Over-expression of CD9 does not affect in vivo tumorigenic or metastatic properties of human prostate cancer cells

Expression of tetraspanin CD9 protein is downregulated in many tumors. CD9 over-expression also reduces the tumorigenicity of some human cancer cells. Here, we determined if exogenous expression of CD9 affects the properties of human prostate cancer cells. Highly metastatic prostate cancer cells PC-3M-LN4 over-expressing exogenous CD9 were orthotopically injected into the prostate of nude mice. CD9 expression was determined in tumors using PCR and Western immunoblotting techniques. Over-expression of CD9 increased invasiveness of prostate cancer cells in vitro. Animals injected with either parental PC-3M-LN4 or CD9-transfected cells developed tumor and harbored lymph node metastasis. There was no statistical difference in tumor growth between the two cell lines. CD9 did not suppress tumorigenic or metastatic properties of PC-3M-LN4 cells. Our data contrast with published results in other tumor types and likely indicate that other proteins (CD9 partners) are needed for CD9 full anti-tumorigenic action.

Comparative gene expression analysis of ovarian carcinoma and normal ovarian epithelium by serial analysis of gene expression

Despite the poor prognosis of ovarian cancer and the importance of early diagnosis, there are no reliable noninvasive biomarkers for detection in the early stages of disease. Therefore, to identify novel ovarian cancer markers with potential utility in early-stage screening protocols, we have undertaken an unbiased and comprehensive analysis of gene expression in primary ovarian tumors and normal human ovarian surface epithelium (HOSE) using Serial Analysis of Gene Expression (SAGE). Specifically, we have generated SAGE libraries from three serous adenocarcinomas of the ovary and, using novel statistical tools, have compared these to SAGE data derived from two pools of normal HOSE. Significantly, in contrast to previous SAGE-based studies, our normal SAGE libraries are not derived from cultured cell lines. We have also compared our data with publicly available SAGE data obtained from primary tumors and "normal" HOSE-derived cell lines. We have thus identified several known and novel genes whose expressions are elevated in ovarian cancer. These include but are not limited to CLDN3, WFDC2, FOLR1, COL18A1, CCND1, and FLJ12988. Furthermore, we found marked differences in gene expression patterns in primary HOSE tissue compared with cultured HOSE. The use of HOSE tissue as a control for these experiments, along with hierarchical clustering analysis, identified several potentially novel biomarkers of ovarian cancer, including TACC3, CD9, GNAI2, AHCY, CCT3, and HMGA1. In summary, these data identify several genes whose elevated expressions have not been observed previously in ovarian cancer, confirm the validity of several existing markers, and provide a foundation for future studies in the understanding and management of this disease.

Down-regulation of CD9 in human ovarian carcinoma cell might contribute to peritoneal dissemination: morphologic alteration and reduced expression of beta1 integrin subsets

Peritoneal dissemination is one of the main causes of death in cancer patients. Pathophysiology of metastasis has been well investigated, but the mechanism of diffuse spread of tumor colonies in the peritoneal cavity is not fully understood. CD9 is a member of tetraspanin and its down-regulation is known to be involved in poor prognosis. To investigate the significance of the down-regulation of CD9, HTOA, an ovarian carcinoma cell line that highly expressed CD9, was transiently transfected with small interfering RNA (siRNA) against CD9, and CD9-negative cells (HTOA(CD9-)) were purified. HTOA(CD9-) showed altered adhesion patterns on Matrigel, collagen, fibronectin, and laminin compared with those of control siRNA-transfected HTOA (control-HTOA). Flow cytometry and fluorescence cytostainings revealed that the expression levels of integrins beta1, alpha2, alpha3beta1, alpha5, and alpha6 were lower in HTOA(CD9-) than those of control-HTOA. HTOA(CD9-) showed altered expression of junctional and cytoskeletal molecules. By time-lapse video microscopy, control-HTOA showed solid adhesion to extracellular matrix and formed cobblestone pattern, whereas HTOA(CD9-) showed weaker adhesion and were distributed as diffuse spots. To examine whether the expression level of CD9 change during tumor dissemination, HTOA-P, a highly disseminative subclone of HTOA, was established. HTOA-P showed distinctive down-regulation of CD9 at mRNA and protein levels, and showed similar morphologic alteration as HTOA(CD9-) did. These findings indicate that the down-regulation of CD9 may be an acquired event in the process of tumor dissemination. Down-regulated CD9 may attenuate the expression of several integrins and rearrange junctional and cytoskeletal molecules that might contribute to dissemination of ovarian carcinomas.

Tetraspanin microdomains in immune cell signalling and malignant disease

A contemporary goal of researchers in leucocyte signalling has been to uncover how cells physically organize and compartmentalize signalling molecules into efficient, regulated signalling networks. This work has revealed important roles of membrane microdomains that are characterized by their distinctive protein and lipid compositions. Recent studies have demonstrated that besides typical cholesterol- and glycosphingolipid-enriched 'rafts', leucocyte membranes are equipped with a different type of microdomain, made up of tetraspanin proteins. Tetraspanin proteins are involved in the organization of tetraspanin-enriched microdomains by virtue of their capacity to specifically associate with key molecules, including integrins, leucocyte receptors and signalling proteins. The aspects of leucocyte function influenced by tetraspanin microdomains include adhesion, proliferation and antigen presentation. However, the mechanisms by which tetraspanin complexes link to intracellular signalling pathways, are still largely unknown. This review discusses how tetraspanin microdomains might function to regulate signalling in lymphoid and myeloid cells, and how they relate to lipid rafts. In addition, we discuss new insights into the role of tetraspanins in malignant disease.