Semaphorin SEMA3F and VEGF have opposing effects on cell attachment and spreading

Asymmetric and parallel subgenome selection co-shape common carp domestication

BACKGROUND

The common carp (Cyprinus carpio) might best represent the domesticated allopolyploid animals. Although subgenome divergence which is well-known to be a key to allopolyploid domestication has been comprehensively characterized in common carps, the link between genetic architecture underlying agronomic traits and subgenome divergence is unknown in the selective breeding of common carps globally.

RESULTS

We utilized a comprehensive SNP dataset in 13 representative common carp strains worldwide to detect genome-wide genetic variations associated with scale reduction, vibrant skin color, and high growth rate in common carp domestication. We identified numerous novel candidate genes underlie the three agronomically most desirable traits in domesticated common carps, providing potential molecular targets for future genetic improvement in the selective breeding of common carps. We found that independently selective breeding of the same agronomic trait (e.g., fast growing) in common carp domestication could result from completely different genetic variations, indicating the potential advantage of allopolyploid in domestication. We observed that candidate genes associated with scale reduction, vibrant skin color, and/or high growth rate are repeatedly enriched in the immune system, suggesting that domestication of common carps was often accompanied by the disease resistance improvement.

CONCLUSIONS

In common carp domestication, asymmetric subgenome selection is prevalent, while parallel subgenome selection occurs in selective breeding of common carps. This observation is not due to asymmetric gene retention/loss between subgenomes but might be better explained by reduced pleiotropy through transposable element-mediated expression divergence between ohnologs. Our results demonstrate that domestication benefits from polyploidy not only in plants but also in animals.

SEMA3B-AS1 suppresses colorectal carcinoma progression by inhibiting Semaphorin 3B-dependent VEGF signaling pathway activation

Mounting evidence has demonstrated the considerable regulatory effects of long noncoding RNAs (lncRNAs) in the tumorigenesis and progression of various carcinomas. LncRNA Semaphorin 3B (SEMA3B) antisense RNA 1 (SEMA3B-AS1) has been found to be dysregulated in a few carcinomas recently. However, its potential function and mechanism in colorectal carcinoma (CRC) have not yet been examined. Here we show that SEMA3B-AS1 acts as a crucial regulator of CRC progression. We found that SEMA3B-AS1 expression was downregulated in CRC cell lines and tissues. Downregulation of SEMA3B-AS1 was significantly associated with poor survival in CRC patients. Overexpression of SEMA3B-AS1 reduced the cell growth and metastasis of CRC in vivo and in vitro. In addition, SEMA3B-AS1 promoted the expression of its sense-cognate gene SEMA3B, a member of the Semaphorin family (SEMAs), by recruiting EP300 to induce H3K9 acetylation at the SEMA3B promoter. Furthermore, we proved that SEMA3B-AS1 suppressed CRC angiogenesis by affecting the vascular endothelial growth factor signaling pathway activation which was regulated by the SEMA3B-NRP1 axis. Our work unravels a novel mechanism of SEMA3B-AS1 in the inhibition of CRC malignant progression and highlights its probability as a new promising diagnostic marker and therapeutic target for CRC interventions.

Advances in SEMA3F regulation of clinically high-incidence cancers

Cancer has become a leading cause of morbidity and mortality in recent years. Its high prevalence has had a severe impact on society. Researchers have achieved fruitful results in the causative factors, pathogenesis, treatment strategies, and cancer prevention. Semaphorin 3F (SEMA3F), a member of the signaling family, was initially reported in the literature to inhibit the growth, invasion, and metastasis of cancer cells in lung cancer. Later studies showed it has cancer-inhibiting effects in malignant tumors such as breast, colorectal, ovarian, oral squamous cell carcinoma, melanoma, and head and neck squamous carcinoma. In contrast, recent studies have reported that SEMA3F is expressed more in hepatocellular carcinoma than in normal tissue and promotes metastasis of hepatocellular carcinoma. We chose lung, breast, colorectal, and hepatocellular carcinomas with high clinical prevalence to review the roles and molecular mechanisms of SEMA3F in these four carcinomas. We concluded with an outlook on clinical interventions for patients targeting SEMA3F.

Endothelial VEGFR Coreceptors Neuropilin-1 and Neuropilin-2 Are Essential for Tumor Angiogenesis

Neuropilin (NRP) expression is highly correlated with poor outcome in multiple cancer subtypes. As known coreceptors for VEGFRs, core drivers of angiogenesis, past investigations have alluded to their functional roles in facilitating tumorigenesis by promoting invasive vessel growth. Despite this, it remains unclear as to whether NRP1 and NRP2 act in a synergistic manner to enhance pathologic angiogenesis. Here we demonstrate, using NRP1 ECKO , NRP2 ECKO , and NRP1/NRP2 ECKO mouse models, that maximum inhibition of primary tumor development and angiogenesis is achieved when both endothelial NRP1 and NRP2 are targeted simultaneously. Metastasis and secondary site angiogenesis were also significantly inhibited in NRP1/NRP2 ECKO animals. Mechanistic studies revealed that codepleting NRP1 and NRP2 in mouse-microvascular endothelial cells stimulates rapid shuttling of VEGFR-2 to Rab7+ endosomes for proteosomal degradation. Our results highlight the importance of targeting both NRP1 and NRP2 to modulate tumor angiogenesis.

Significance

The findings presented in this study demonstrate that tumor angiogenesis and growth can be arrested completely by cotargeting endothelial NRP1 and NRP2. We provide new insight into the mechanisms of action regulating NRP-dependent tumor angiogenesis and signpost a novel approach to halt tumor progression.

Endothelial Semaphorin 3fb regulates Vegf pathway-mediated angiogenic sprouting

Vessel growth integrates diverse extrinsic signals with intrinsic signaling cascades to coordinate cell migration and sprouting morphogenesis. The pro-angiogenic effects of Vascular Endothelial Growth Factor (VEGF) are carefully controlled during sprouting to generate an efficiently patterned vascular network. We identify crosstalk between VEGF signaling and that of the secreted ligand Semaphorin 3fb (Sema3fb), one of two zebrafish paralogs of mammalian Sema3F. The sema3fb gene is expressed by endothelial cells in actively sprouting vessels. Loss of sema3fb results in abnormally wide and stunted intersegmental vessel artery sprouts. Although the sprouts initiate at the correct developmental time, they have a reduced migration speed. These sprouts have persistent filopodia and abnormally spaced nuclei suggesting dysregulated control of actin assembly. sema3fb mutants show simultaneously higher expression of pro-angiogenic (VEGF receptor 2 (vegfr2) and delta-like 4 (dll4)) and anti-angiogenic (soluble VEGF receptor 1 (svegfr1)/ soluble Fms Related Receptor Tyrosine Kinase 1 (sflt1)) pathway components. We show increased phospho-ERK staining in migrating angioblasts, consistent with enhanced Vegf activity. Reducing Vegfr2 kinase activity in sema3fb mutants rescues angiogenic sprouting. Our data suggest that Sema3fb plays a critical role in promoting endothelial sprouting through modulating the VEGF signaling pathway, acting as an autocrine cue that modulates intrinsic growth factor signaling.

Semaphorin 3F Serves as a Tumor Suppressor in Esophageal Squamous Cell Carcinoma and is Associated With Lymph Node Metastasis in Disease Progression

Background:

Esophageal squamous cell carcinoma is one of the leading aggressive malignancies with high mortality. Semaphorin 3F has been reported to be involved in lymphangiogenesis by interacting the vascular endothelial growth factor C/neuropilin 2 axis. This study aimed to assess the clinical and functional role of semaphorin 3F and preliminarily evaluate the relationship between semaphorin 3F and lymph node metastasis in esophageal squamous cell carcinoma.

Methods:

The messenger RNA expression of semaphorin 3F was analyzed using quantitative real-time polymerase chain reaction. The expression differences of semaphorin 3F between patients having esophageal squamous cell carcinoma with and without lymph node metastasis were assessed, and the correlation of semaphorin 3F with vascular endothelial growth factor C and neuropilin 2 was estimated. The prognostic value of semaphorin 3F was evaluated using Kaplan-Meier survival curves and Cox regression analysis. Gain- and loss-functional cell experiments were performed to explore the biological function of semaphorin 3F, vascular endothelial growth factor C, and neuropilin 2.

Results:

The messenger RNA expression of semaphorin 3F was reduced in esophageal squamous cell carcinoma tissues compared with normal tissues, and lower semaphorin 3F expression was observed in patients having esophageal squamous cell carcinoma with positive lymph node metastasis. Semaphorin 3F expression was associated with lymph node metastasis and negatively correlated with vascular endothelial growth factor C and neuropilin 2. Lower semaphorin 3F expression was related to a poor overall survival of esophageal squamous cell carcinoma and served as an independent prognostic indicator. In esophageal squamous cell carcinoma cells, semaphorin 3F messenger RNA expression was also decreased compared with normal cells, and the overexpression of semaphorin 3F could significantly inhibit cell proliferation, migration, and invasion. The downregulation of vascular endothelial growth factor C and neuropilin 2 could inhibit cell proliferation, migration, and invasion of esophageal squamous cell carcinoma cells.

Conclusion:

All data indicate that semaphorin 3F serves as a potential prognostic biomarker and tumor suppressor of esophageal squamous cell carcinoma and may be involved in the lymph node metastasis development through regulating neuropilin 2.

Characterization of Class-3 Semaphorin Receptors, Neuropilins and Plexins, as Therapeutic Targets in a Pan-Cancer Study

Class-3 semaphorins (SEMA3s), initially characterized as axon guidance cues, have been recognized as key regulators for immune responses, angiogenesis, tumorigenesis and drug responses. The functions of SEMA3s are attributed to the activation of downstream signaling cascades mainly mediated by cell surface receptors neuropilins (NRPs) and plexins (PLXNs), yet their roles in human cancers are not completely understood. Here, we provided a detailed pan-cancer analysis of NRPs and PLXNs in their expression, and association with key signal transducers, patient survival, tumor microenvironment (TME), and drug responses. The expression of NRPs and PLXNs were dysregulated in many cancer types, and the majority of them were further dysregulated in metastatic tumors, indicating a role in metastatic progression. Importantly, the expression of these genes was frequently associated with key transducers, patient survival, TME, and drug responses; however, the direction of the association varied for the particular gene queried and the specific cancer type/subtype tested. Specifically, NRP1, NRP2, PLXNA1, PLXNA3, PLXNB3, PLXNC1, and PLXND1 were primarily associated with aggressive phenotypes, whereas the rest were more associated with favorable prognosis. These data highlighted the need to study each as a separate entity in a cancer type- and subtype-dependent manner.

A pan-cancer study of class-3 semaphorins as therapeutic targets in cancer

BACKGROUND

Initially characterized as axon guidance factors, semaphorins also have been implicated to have critical roles in multiple physiological and developmental functions, including the regulation of immune responses, angiogenesis, organ formation, and the etiology of multiple forms of cancer. Moreover, their contribution in immunity and the regulation of tumour microenvironment is becoming increasingly recognized. Here, we provide a comprehensive analysis of class-3 semaphorins, the only secreted family of genes among veterbrate semaphorins, in terms of their expression profiles and their association with patient survival. We also relate their role with immune subtypes, tumour microenvironment, and drug sensitivity using a pan-cancer study.

RESULTS

Expression profiles of class-3 semaphorins (SEMA3s) and their association with patient survival and tumour microenvironment were studied in 31 cancer types using the TCGA pan-cancer data. The expression of SEMA3 family varies in different cancer types with striking inter- and intra- cancer heterogeneity. In general, our results show that SEMA3A, SEMA3C, and SEMA3F are primarily upregulated in cancer cells, while the rest of SEMA3s are mainly down-regulated in the tested tumours. The expression of SEMA3 family members was frequently associated with patient overall survival. However, the direction of the association varied with regards to the particular SEMA3 isoform queried and the specific cancer type tested. More specifically, SEMA3A and SEMA3E primarily associate with a poor prognosis of survival, while SEMA3G typically associates with survival advantage. The rest of SEMA3s show either survival advantage or disadvantage dependent on cancer type. In addition, all SEMA3 genes show significant association with immune infiltrate subtypes, and they also correlate with level of stromal cell infiltration and tumour cell stemness with various degrees. Finally, our study revealed that SEMA3 genes, especially SEMA3C and SEMA3F may contribute to drug induced cancer cell resistance.

CONCLUSIONS

Our systematic analysis of class-3 semaphorin gene expression and their association with immune infiltrates, tumour microenvironment and cancer patient outcomes highlights the need to study each SEMA3 member as a separate entity within each specific cancer type. Also our study validated the identification of class-3 semaphorin signals as promising therapeutic targets in cancer although further laboratory validation still needed.

Impact of semaphorin expression on prognostic characteristics in breast cancer

Breast cancer is one of the major causes of cancer-related deaths among women worldwide. Aberrant regulation of various growth factors, cytokines, and other proteins and their receptors in cancer cells drives the activation of various oncogenic signaling pathways that lead to cancer progression. Semaphorins are a class of proteins which are differentially expressed in various types of cancer including breast cancer. Earlier, these proteins were known to have a major function in the nerve cell adhesion, migration, and development of the central nervous system. However, their role in the regulation of several aspects of tumor progression has eventually emerged. There are over 30 genes encoding the semaphorins, which are divided into eight subclasses. It has been reported that some members of semaphorin classes are antiangiogenic and antimetastatic in nature, whereas others act as proangiogenic and prometastatic genes. Because of their differential expression and role in angiogenesis and metastasis, semaphorins emerged as one of the important prognostic factors for appraising breast cancer progression.

Sema3F (Semaphorin 3F) Selectively Drives an Extraembryonic Proangiogenic Program

OBJECTIVE

Molecular pathways governing blood vessel patterning are vital to vertebrate development. Because of their ability to counteract proangiogenic factors, antiangiogenic secreted Sema3 (class 3 semaphorins) control embryonic vascular morphogenesis. However, if and how Sema3 may play a role in the control of extraembryonic vascular development is presently unknown.

APPROACH AND RESULTS

By characterizing genetically modified mice, here, we show that surprisingly Sema3F acts instead as a selective extraembryonic, but not intraembryonic proangiogenic cue. Both in vivo and in vitro, in visceral yolk sac epithelial cells, Sema3F signals to inhibit the phosphorylation-dependent degradation of Myc, a transcription factor that drives the expression of proangiogenic genes, such as the microRNA cluster 17/92. In Sema3f-null yolk sacs, the transcription of Myc-regulated microRNA 17/92 cluster members is impaired, and the synthesis of Myc and microRNA 17/92 foremost antiangiogenic target Thbs1 (thrombospondin 1) is increased, whereas Vegf (vascular endothelial growth factor) signaling is inhibited in yolk sac endothelial cells. Consistently, exogenous recombinant Sema3F inhibits the phosphorylation-dependent degradation of Myc and the synthesis of Thbs1 in mouse F9 teratocarcinoma stem cells that were in vitro differentiated in visceral yolk sac epithelial cells. Sema3f-/- mice placentas are also highly anemic and abnormally vascularized.

CONCLUSIONS

Sema3F functions as an unconventional Sema3 that promotes extraembryonic angiogenesis by inhibiting the Myc-regulated synthesis of Thbs1 in visceral yolk sac epithelial cells.

Altered gene expression in early postnatal monoamine oxidase A knockout mice

We reported previously that monoamine oxidase (MAO) A knockout (KO) mice show increased serotonin (5-hydroxytryptamine, 5-HT) levels and autistic-like behaviors characterized by repetitive behaviors, and anti-social behaviors. We showed that administration of the serotonin synthesis inhibitor para-chlorophenylalanine (pCPA) from post-natal day 1 (P1) through 7 (P7) in MAO A KO mice reduced the serotonin level to normal and reverses the repetitive behavior. These results suggested that the altered gene expression at P1 and P7 may be important for the autistic-like behaviors seen in MAO A KO mice and was studied here. In this study, Affymetrix mRNA array data for P1 and P7 MAO A KO mice were analyzed using Partek Genomics Suite and Ingenuity Pathways Analysis to identify genes differentially expressed versus wild-type and assess their functions and relationships. The number of significant differentially expressed genes (DEGs) varied with age: P1 (664) and P7 (3307) [false discovery rate (FDR) <0.05, fold-change (FC) >1.5 for autism-linked genes and >2.0 for functionally categorized genes]. Eight autism-linked genes were differentially expressed in P1 (upregulated: NLGN3, SLC6A2; down-regulated: HTR2C, MET, ADSL, MECP2, ALDH5A1, GRIN3B) while four autism-linked genes were differentially expressed at P7 (upregulated: HTR2B; downregulated: GRIN2D, GRIN2B, CHRNA4). Many other genes involved in neurodevelopment, apoptosis, neurotransmission, and cognitive function were differentially expressed at P7 in MAO A KO mice. This result suggests that modulation of these genes by the increased serotonin may lead to neurodevelopmental alteration in MAO A KO mice and results in autistic-like behaviors.

The role of the semaphorins in cancer

The semaphorins were initially characterized as axon guidance factors, but have subsequently been implicated also in the regulation of immune responses, angiogenesis, organ formation, and a variety of additional physiological and developmental functions. The semaphorin family contains more then 20 genes divided into 7 subfamilies, all of which contain the signature sema domain. The semaphorins transduce signals by binding to receptors belonging to the neuropilin or plexin families. Additional receptors which form complexes with these primary semaphorin receptors are also frequently involved in semaphorin signaling. Recent evidence suggests that semaphorins also fulfill important roles in the etiology of multiple forms of cancer. Some semaphorins have been found to function as bona-fide tumor suppressors and to inhibit tumor progression by various mechanisms while other semaphorins function as inducers and promoters of tumor progression.

Understanding lymphangiogenesis in knockout models, the cornea, and ocular diseases for the development of therapeutic interventions

A major focus of cancer research for several decades has been understand the ability of tumors to induce new blood vessel formation, a process known as angiogenesis. Unfortunately, only limited success has been achieved in the clinical application of angiogenesis inhibitors. We now know that lymphangiogenesis, the growth of lymphatic vessels, likely also plays a major role in tumor progression. Thus, therapeutic strategies targeting lymphangiogenesis or both lymphangiogenesis and angiogenesis may represent promising approaches for treating cancer and other diseases. Importantly, research progress toward understanding lymphangiogenesis is significantly behind that related to angiogenesis. A PubMed search of "angiogenesis" returns nearly 80,000 articles, whereas a search of "lymphangiogenesis" returns 2,635 articles. This stark contrast can be explained by the lack of molecular markers for identifying the invisible lymphatic vasculature that persisted until less than 2 decades ago, combined with the intensity of research interest in angiogenesis during the past half century. Still, significant strides have been made in developing strategies to modulate lymphangiogenesis, largely using ocular disease models. Here we review the current knowledge of lymphangiogenesis in the context of knockout models, ocular diseases, the biology of activators and inhibitors, and the potential for therapeutic interventions targeting this process.

Application of Gene Expression Trajectories Initiated from ErbB Receptor Activation Highlights the Dynamics of Divergent Promoter Usage

Understanding how cells use complex transcriptional programs to alter their fate in response to specific stimuli is an important question in biology. For the MCF-7 human breast cancer cell line, we applied gene expression trajectory models to identify the genes involved in driving cell fate transitions. We modified trajectory models to account for the scenario where cells were exposed to different stimuli, in this case epidermal growth factor and heregulin, to arrive at different cell fates, i.e. proliferation and differentiation respectively. Using genome-wide CAGE time series data collected from the FANTOM5 consortium, we identified the sets of promoters that were involved in the transition of MCF-7 cells to their specific fates versus those with expression changes that were generic to both stimuli. Of the 1,552 promoters identified, 1,091 had stimulus-specific expression while 461 promoters had generic expression profiles over the time course surveyed. Many of these stimulus-specific promoters mapped to key regulators of the ERK (extracellular signal-regulated kinases) signaling pathway such as FHL2 (four and a half LIM domains 2). We observed that in general, generic promoters peaked in their expression early on in the time course, while stimulus-specific promoters tended to show activation of their expression at a later stage. The genes that mapped to stimulus-specific promoters were enriched for pathways that control focal adhesion, p53 signaling and MAPK signaling while generic promoters were enriched for cell death, transcription and the cell cycle. We identified 162 genes that were controlled by an alternative promoter during the time course where a subset of 37 genes had separate promoters that were classified as stimulus-specific and generic. The results of our study highlighted the degree of complexity involved in regulating a cell fate transition where multiple promoters mapping to the same gene can demonstrate quite divergent expression profiles.

Control of cellular motility by neuropilin-mediated physical interactions

The neuropilin (Nrp) family consists of multifunctional cell surface receptors with critical roles in a number of different cell and tissue types. A core aspect of Nrp function is in ligand-dependent cellular migration, where it controls the multistep process of cellular motility through integration of ligand binding and receptor signaling. At a molecular level, the role of Nrp in migration is intimately connected to the control of adhesive interactions and cytoskeletal reorganization. Here, we review the physiological role of Nrp in cellular adhesion and motility in the cardiovascular and nervous systems. We also discuss the emerging pathological role of Nrp in tumor cell migration and metastasis, providing motivation for continued efforts toward developing Nrp inhibitors.

Semaphorin-3F functions as a tumor suppressor in colorectal cancer due to regulation by DNA methylation

Semaphorin-3F (SEMA3F) is a member of the class III semaphorin family, and is seen as a candidate tumor suppressor gene. The aims of this study were to evaluate the effect of SEMA3F in colorectal cancer (CRC) patients, and to explore the mechanism for that SEMA3F suppresses tumor progression and metastasis. The expression levels of SEMA3F in the colorectal cancer tissues and corresponding non-tumor colorectal tissues were determined by Western blotting and real-time quantitative PCR (qRT-PCR). In addition, we evaluate the effects of SEMA3F on CRC cell migration and colony formation in vitro. Subsequently, quantitative methylation-specific PCR (qMSP) was used to detect the DNA methylation status in the CpG islands of SEMA3F gene promoter in normal colon and colorectal cancer cell lines, colorectal cancer tissues and corresponding non-tumor colorectal tissues. We found that SEMA3F was downregulated in the protein (P < 0.01) and mRNA (P < 0.001) levels in CRC tissues as compared to matched adjacent non-tumor tissues. Moreover, MSP assay showed high levels of SEMA3F gene promoter methylation in the CpG islands in some CRC cell lines and tissue samples. Furthermore, SEMA3F expression was reactivated in CRC cell lines after treatment with 5-Aza-CdR, demethylation of SW620 cells resulted in cell colony formation and invasion inhibition. These findings suggest DNA methylation of promoter CpG island-mediated silencing of the tumor suppressor SEMA3F gene plays an important role in the carcinogenesis of CRC.

Neuropilin-2 mediates lymphangiogenesis of colorectal carcinoma via a VEGFC/VEGFR3 independent signaling

Lymphangiogenesis critically contributes to the lymphatic metastasis of colorectal carcinomas (CRCs), but the underlying mechanism of CRC lymphangiogenesis remains largely elusive. We have previously demonstrated that Semaphorin-3F (SEMA3F) is critically involved in CRC metastasis, and the receptor of SEMA3F, neuropilin-2 (NRP2), originally described as an axon guiding chemorepulsant implicated in nerve development, has been suggested in promoting lymphangiogenesis via acting as an obligate co-receptor of VEGFR3 cooperatively enhancing the activity of VEGF-C. Our present study revealed that in colorectal carcinomas, NRP2 expression levels of tumor-associated lymphatic endothelial cells (LECs) are significantly correlated with the density of tumor lymphatic vessels. In vitro, activation of NRP2 in LECs substantially facilitates their migration, sprouting, and tubulogenesis capacity via regulating the rearrangement of cytoskeleton polarity. In vivo model further showed that in the xenografts generated from SEMA3F knockdown CRC cells, NRP2 is substantially activated in tumor-associated LECs, resulting in a significantly increased tumor lymphangiogenesis. Further evidence demonstrated that CRC cell induces the activation of NRP2 in LECs to promote tumor lymphangiogenesis via integrinα9β1/FAK/Erk pathway independent VEGF-C/VEGFR3 signaling. Our study for the first time revealed the novel molecular mechanism of NRP2-mediated-lymphangiogenesis in CRCs, suggesting NRP2 as a potential therapeutic target in preventing lymphatic metastasis of CRCs.

Semaphorin-3F suppresses the stemness of colorectal cancer cells by inactivating Rac1

Tumor cell stemness has been recognized as a key contributor to tumor initiation, progression and recurrence. Our previous studies have found that semaphorin-3F (SEMA3F), an axon guidance molecule in the development of central nervous system, inhibited the growth and metastasis of colorectal cancer (CRC). However, a possible role for SEMA3F in regulating cancer cell stemness remains unknown. Here, we report a novel mechanism of the acquirement of stemness of CRC cells regulated by SEMA3F. Knockdown of SEMA3F significantly promoted the self-renewal and tumorigenicity of CRC cells, and increased the expression of stemness-associated genes, while overexpressing SEMA3F reduced the stemness of CRC cells. Mechanistically, GTP-Rac1 was involved in SEMA3F mediated regulation of CRC cell stemness by targeting the Wnt/β-catenin pathway. Clinically, GTP-Rac1 expression was inversely correlated with SEMA3F levels in CRC samples and patients with SEMA3F(low)/GTP-Rac1(high) CRC showed poorer prognosis. Our findings demonstrate the ability of SEMA3F to inhibit the stemness of human CRC cells by suppressing Rac1 activation, which suggests a novel therapeutic approach for CRC.

The emerging role of class-3 semaphorins and their neuropilin receptors in oncology

The semaphorins, discovered over 20 years ago, are a large family of secreted or transmembrane and glycophosphatidylinositol -anchored proteins initially identified as axon guidance molecules crucial for the development of the nervous system. It has now been established that they also play important roles in organ development and function, especially involving the immune, respiratory, and cardiovascular systems, and in pathological disorders, including cancer. During tumor progression, semaphorins can have both pro- and anti-tumor functions, and this has created complexities in our understanding of these systems. Semaphorins may affect tumor growth and metastases by directly targeting tumor cells, as well as indirectly by interacting with and influencing cells from the micro-environment and vasculature. Mechanistically, semaphorins, through binding to their receptors, neuropilins and plexins, affect pathways involved in cell adhesion, migration, invasion, proliferation, and survival. Importantly, neuropilins also act as co-receptors for several growth factors and enhance their signaling activities, while class 3 semaphorins may interfere with this. In this review, we focus on the secreted class 3 semaphorins and their neuropilin co-receptors in cancer, including aspects of their signaling that may be clinically relevant.

The role of axon guidance factor semaphorin 6B in the invasion and metastasis of gastric cancer

OBJECTIVE

To investigate the role of semaphorin 6B in gastric cancer invasion and metastasis.

METHODS

Immunohistochemistry for semaphorin 6B was performed on gastric cancer tumour tissue samples in this retrospective study. Levels of semaphorin 6B protein and mRNA were determined in gastric cancer cell lines by Western blotting and quantitative reverse transcription-polymerase chain reaction, respectively. The human gastric cancer cell line SGC-7901 was transfected with small interfering RNA targeting semaphorin 6B; effects on cell adhesion, migration and invasion were determined by cell adhesion assay, transwell chamber migration assay and wound healing assay, respectively.

RESULTS

Tumour tissue samples from 220 patients were analysed. In vivo, semaphorin 6B immunopositivity correlated with tumour differentiation, lymph node metastasis and distant metastasis but not patient age, sex or tumour stage. Semaphorin 6B gene silencing significantly suppressed adhesion, migration and invasion of gastric cancer cells in vitro.

CONCLUSIONS

Semaphorin 6B is related to tumour differentiation and metastasis in vivo, and tumour cell migration, adhesion and invasion in vitro. Semaphorin 6B may represent a reliable biomarker for diagnosis, evaluation and gene-targeted therapy of gastric cancer.

The antitumor activity of a doxorubicin loaded, iRGD-modified sterically-stabilized liposome on B16-F10 melanoma cells: in vitro and in vivo evaluation

Considering the fact that iRGD (tumor-homing peptide) demonstrates tumor-targeting and tumor-penetrating activity, and that B16-F10 (murine melanoma) cells overexpress both αv integrin receptor and neuropilin-1 (NRP-1), the purpose of this study was to prepare a novel doxorubicin (DOX)-loaded, iRGD-modified, sterically-stabilized liposome (SSL) (iRGD-SSL-DOX) in order to evaluate its antitumor activity on B16-F10 melanoma cells in vitro and in vivo. The iRGD-SSL-DOX was prepared using a thin-film hydration method. The characteristics of iRGD-SSL-DOX were evaluated. The in vitro leakage of DOX from iRGD-SSL-DOX was tested. The in vitro tumor-targeting and tumor-penetrating characteristics of iRGD-modified liposomes on B16-F10 cells were investigated. The in vivo tumor-targeting and tumor-penetrating activities of iRGD-modified liposomes were performed in B16-F10 tumor-bearing nude mice. The antitumor effect of iRGD-SSL-DOX was evaluated in B16-F10 tumor-bearing C57BL/6 mice in vivo. The average particle size of the iRGD-SSL-DOX was found to be 91 nm with a polydispersity index (PDI) of 0.16. The entrapment efficiency of iRGD-SSL-DOX was 98.36%. The leakage of DOX from iRGD-SSL-DOX at the 24-hour time point was only 7.5%. The results obtained from the in vitro flow cytometry and confocal microscopy, as well as in vivo biodistribution and confocal immunofluorescence microscopy experiments, indicate that the tumor-targeting and tumor-penetrating activity of the iRGD-modified SSL was higher than that of unmodified SSL. In vivo antitumor activity results showed that the antitumor effect of iRGD-SSL-DOX against melanoma tumors was higher than that of SSL-DOX in B16-F10 tumor-bearing mice. In conclusion, the iRGD-SSL-DOX is a tumor-targeting and tumor-penetrating peptide modified liposome which has significant antitumor activity against melanoma tumors.

Caveolin-1 regulates Rac1 activation and rat pulmonary microvascular endothelial hyperpermeability induced by TNF-α

A multiplicity of vital cellular and tissue level functions are controlled by caveolin-1 and it is considered to be an important candidate for targeted therapeutics. Rac1-cortactin signaling plays an important role in maintaining the functions of the endothelial barrier in microvascular endothelial cells. The activity of Rac1 has been shown to be regulated by caveolin-1. Therefore, the present study investigated the consequences of down-regulating caveolin-1 and the subsequent changes in activity of Rac1 and the endothelial barrier functions in primary rat pulmonary microvascular endothelial cells (RPMVECs). RPMVECs were transfected with a small hairpin RNA duplex to down-regulate caveolin-1 expression. This procedure significantly increased the activity of Rac1. Moreover, down-regulation of caveolin-1 attenuated TNF-α-induced decrease in TER, increase in the flux of FITC-BSA and the disappearance of cortactin from the cell periphery in RPMVEC. Rac1 inhibitors significantly abolished this barrier-protective effect induced by down-regulation of caveolin-1 in response to TNF-α in RPMVECs. In conclusion, our data suggest a mechanism for the regulation of Rac1 activity by caveolin-1, with consequences for activation of endothelial cells in response to TNF-α.

Semaphorin signals tweaking the tumor microenvironment

Solid tumors not only comprise malignant cells but also other nonmalignant cell types, forming a unique microenvironment that can strongly influence the behavior of tumor cells. Recent advances in the understanding of cancer biology have highlighted the functional role of semaphorins. In fact, semaphorins form a family of molecular signals known to guide and control cell migration during embryo development and in adults. Tumor cells express semaphorins as well as their receptors, plexins and neuropilins. It has been shown that semaphorin signaling can regulate tumor cell behavior. Moreover, semaphorins are important regulators of tumor angiogenesis. Conversely, very little is known about the functional relevance of semaphorin signals for tumor-infiltrating stromal cells, such as leukocytes. In this chapter, we review the current knowledge on the functional role of semaphorins in cancer progression, and we focus on the emerging role of semaphorins in mediating the cross talk between tumor cells and different tumor stromal cells.

Receptor complexes for each of the Class 3 Semaphorins

The Class 3 Semaphorins (Sema3s) are a sub-family of proteins whose known biological roles are varied and growing. The mechanism of action of the Sema3s requires binding to transmembrane receptors that comprise heteromeric complexes of Neuropilins, Plexins and cell adhesion molecules (CAMs). However, knowledge of the receptor components of the Sema3s remains incomplete, and there may be receptor components which are as yet undiscovered. The receptor complexes of the Sema3s share receptor components with each other, and it is the specific combination of these components within a heteromeric complex that is thought to give rise to selective binding and signalling for individual Sema3s. This crosstalk makes it experimentally difficult to define a single holoreceptor for each Sema3. Furthermore, the receptor composition for a given Sema3 may differ between cell types, and change as a function of developmental state or pathological situation. Nevertheless, there are at least some known differences in the constitutive structure of the receptors for the Sema3s. For example in neural cells, Sema3a and Sema3f signal through different Neuropilins (Nrp1 and Nrp2 respectively) and L1cam only appears important for Sema3a signaling, while Nrcam forms a complex with Nrp2. Further complexity arises from crosstalk of other families of ligands (e.g., VEGF) with Sema3 receptor components. Thus the Sema3s, which have been shown as antagonists for each other, can also act as antagonists for other families of molecules. This review compiles experimental evidence describing the receptor components for the Sema3s, detailing the current state of knowledge of which components are important for signaling of each Sema3 before going on to consider possible future directions for the field.

Semaphorins in angiogenesis and tumor progression

The semaphorins were initially described as axon guidance factors, but have recently been implicated in a variety of physiological and developmental functions, including regulation of immune response, angiogenesis, and migration of neural crest cells. The semaphorin family contains more than 30 genes divided into seven subfamilies, all of which are characterized by the presence of a sema domain. The semaphorins transduce their signals by binding to one of the nine receptors belonging to the plexin family, or, in the case of the class 3 semaphorins, by binding to one of the two neuropilin receptors. Additional receptors, which form complexes with these primary semaphorin receptors, are also frequently involved in semaphorin signaling. Recent evidence suggests that some semaphorins can act as antiangiogenic and/or antitumorigenic agents whereas other semaphorins promote tumor progression and/or angiogenesis. Furthermore, loss of endogenous inhibitory semaphorin expression or function on one hand, and overexpression of protumorigenic semaphorins on the other hand, is associated with the progression of some tumor types.

Navigating breast cancer: axon guidance molecules as breast cancer tumor suppressors and oncogenes

Slit, Netrin, Ephrin, and Semaphorin's roles in development have expanded greatly in the past decade from their original characterization as axon guidance molecules (AGMs) to include roles as regulators of tissue morphogenesis and development in diverse organs. In the mammary gland, AGMs are important for maintaining normal cell proliferation and adhesion during development. The frequent dysregulation of AGM expression during tumorigenesis and tumor progression suggests that AGMs also play a crucial role as tumor suppressors and oncogenes in breast cancer. Moreover, these findings suggest that AGMs may be excellent targets for new breast cancer prognostic tests and more effective therapeutic strategies.

Neuropilins: a new target for cancer therapy

Recent investigations highlighted strong similarities between neural crest migration during embryogenesis and metastatic processes. Indeed, some families of axon guidance molecules were also reported to participate in cancer invasion: plexins/semaphorins/neuropilins, ephrins/Eph receptors, netrin/DCC/UNC5. Neuropilins (NRPs) are transmembrane non tyrosine-kinase glycoproteins first identified as receptors for class-3 semaphorins. They are particularly involved in neural crest migration and axonal growth during development of the nervous system. Since many types of tumor and endothelial cells express NRP receptors, various soluble molecules were also found to interact with these receptors to modulate cancer progression. Among them, angiogenic factors belonging to the Vascular Endothelial Growth Factor (VEGF) family seem to be responsible for NRP-related angiogenesis. Because NRPs expression is often upregulated in cancer tissues and correlated with poor prognosis, NRPs expression might be considered as a prognostic factor. While NRP1 was intensively studied for many years and identified as an attractive angiogenesis target for cancer therapy, the NRP2 signaling pathway has just recently been studied. Although NRP genes share 44% homology, differences in their expression patterns, ligands specificities and signaling pathways were observed. Indeed, NRP2 may regulate tumor progression by several concurrent mechanisms, not only angiogenesis but lymphangiogenesis, epithelial-mesenchymal transition and metastasis. In view of their multiples functions in cancer promotion, NRPs fulfill all the criteria of a therapeutic target for innovative anti-tumor therapies. This review focuses on NRP-specific roles in tumor progression.

Endogenous axon guiding chemorepulsant semaphorin-3F inhibits the growth and metastasis of colorectal carcinoma

PURPOSE

To elucidate the role of Semaphorin-3F (SEMA3F), originally described as an axon guiding chemorepulsant implicated in nerve development, in the progression of colorectal carcinoma.

EXPERIMENTAL DESIGN

SEMA3F and its receptor NRP2 were examined in 72 cases of human colorectal carcinoma specimens and cell lines LoVo, SW480, and SW620 with immunohistochemistry and Western blotting. SEMA3F mRNA expression in the frozen tissue specimens and cell lines was examined with quantitative reverse transcriptase-PCR. Confocal laser scanning microscopy was used for detection of cellular localization of the proteins by immunofluorescent staining. MTT assay, flow cytometry, cell adhesion and migration, and xenografts were used to evaluate biological significance of SEMA3F.

RESULTS

SEMA3F was significantly reduced in colorectal carcinoma tissues and cell lines. Overexpression of SEMA3F resulted in reduced proliferation, adhesion to fibronectin, and migratory capability as well as reduced S-phase population and integrin αvβ3 expression of SW480 colon cancer cells. In addition, SEMA3F-overexpressing cells exhibited diminished tumorigenesis when transplanted orthotopically in nude mice and reduced liver metastases. Moreover, transfection of siRNA targeting SEMA3F in colon cancer cells increased their tumorigenicity in vivo.

CONCLUSIONS

Endogenous SEMA3F acts as a suppressor of the growth and metastasis of human colorectal cancer cells.

Integration of genotypic and phenotypic screening reveals molecular mediators of melanoma-stromal interaction

Tumor-endothelium interactions are critical for tumor survival and metastasis. Melanomas can rapidly metastasize early in tumor progression, but the dependence of this aggressive behavior on tumor-stromal interaction is poorly understood. To probe the mechanisms involved, we developed a heterotypic coculture methodology, allowing simultaneous tracking of genomic and phenotypic changes in interacting tumor and endothelial cells in vitro. We found a dramatic rearrangement of endothelial cell networks into patterns reminiscent of vascular beds, even on plastic and glass. Multiple genes were upregulated in the process, many coding for cell surface and secreted proteins, including Neuropilin-2 (NRP2). A critical role of NRP2 in coordinated cell patterning and growth was confirmed using the coculture system. We conclude that NRP2 represents an important mediator of melanoma-endothelial interactions. Furthermore, the described methodology represents a powerful yet simple system to elucidate heterotypic intercellular interactions mediating diverse physiological and pathological processes.

Recombinant bacterial expression and purification of human fragile X mental retardation protein isoform 1

The loss of expression of the fragile X mental retardation protein (FMRP) leads to fragile X syndrome. FMRP has two types of RNA binding domains, two K-homology domains and an arginine-glycine-glycine box domain, and it is proposed to act as a translation regulator of specific messenger RNA. The interest to produce sufficient quantities of pure recombinant FMRP for biochemical and biophysical studies is high. However, the recombinant bacterial expression of FMRP has had limited success, and subsequent recombinant eukaryotic and in vitro expression has also resulted in limited success. In addition, the in vitro and eukaryotic expression systems may produce FMRP which is posttranslationally modified, as phosphorylation and arginine methylation have been shown to occur on FMRP. In this study, we have successfully isolated the conditions for recombinant expression, purification and long-term storage of FMRP using Escherichia coli, with a high yield. The expression of FMRP using E. coli renders the protein devoid of the posttranslational modifications of phosphorylation and arginine methylation, allowing the study of the direct effects of these modifications individually and simultaneously. In order to assure that FMRP retained activity throughout the process, we used fluorescence spectroscopy to assay the binding activity of the FMRP arginine-glycine-glycine box for the semaphorin 3F mRNA and confirmed that FMRP remained active.

Molecular and clinical aspects of targeting the VEGF pathway in tumors

Tumor angiogenesis is a complex process resulting from many signals from the tumor microenvironment. From preclinical animal models to clinical trials and practice, targeting tumors with antiangiogenic therapy remains an exciting area of study. Although many scientific advances have been achieved, leading to the development and clinical use of antiangiogenic drugs such as bevacizumab, sorafenib, and sunitinib, these therapies fall short of their anticipated benefits and leave many questions unanswered. Continued research into the complex signaling cascades that promote tumor angiogenesis may yield new targets or improve upon current therapies. In addition, the development of reliable tools to track tumor responses to antiangiogenic therapy will enable a better understanding of current therapeutic efficacy and may elucidate mechanisms to predict patient response to therapy.

Expression by midbrain dopamine neurons of Sema3A and 3F receptors is associated with chemorepulsion in vitro but a mild in vivo phenotype

Here we explore the role of semaphorin 3A and 3F (Sema3A, Sema3F) in the formation of the mesotelencephalic pathway. We show that Sema3A and 3F are expressed in the ventral mesencephalon (VM) of E13.5 rat embryos; the receptors Neuropilin 1 and Neuropilin 2, and co-receptors L1CAM, NrCAM, and Plexins A1 and A3 but not A4 are expressed by VM dopaminergic neurons; these neurons bind Sema3A and 3F in vitro which induces collapse of their growth cones and elicits, with different potencies, a repulsive response; and this response is absent in axons from Nrp1 and Nrp2 null embryos. Despite these in vitro effects, only very mild anatomical defects were detected in the organization of the mesotelencephalic pathway in embryonic and adult Nrp1 or Nrp2 null mice. However, the dopaminergic meso-habenular pathway and catecholaminergic neurons in the parafascicular and paraventricular nuclei of the thalamus were significantly affected in Nrp2 null mice. These data are consistent with a model whereby Sema3A and 3F, in combination with other guidance molecules, contributes to the navigation of DA axons to their final synaptic targets.

Hormonal regulation and distinct functions of semaphorin-3B and semaphorin-3F in ovarian cancer

Semaphorins comprise a family of molecules that influence neuronal growth and guidance. Class-3 semaphorins, semaphorin-3B (SEMA3B) and semaphorin-3F (SEMA3F), illustrate their effects by forming a complex with neuropilins (NP-1 or NP-2) and plexins. We examined the status and regulation of semaphorins and their receptors in human ovarian cancer cells. A significantly reduced expression of SEMA3B (83 kDa), SEMA3F (90 kDa), and plexin-A3 was observed in ovarian cancer cell lines when compared with normal human ovarian surface epithelial cells. The expression of NP-1, NP-2, and plexin-A1 was not altered in human ovarian surface epithelial and ovarian cancer cells. The decreased expression of SEMA3B, SEMA3F, and plexin-A3 was confirmed in stage 3 ovarian tumors. The treatment of ovarian cancer cells with luteinizing hormone, follicle-stimulating hormone, and estrogen induced a significant upregulation of SEMA3B, whereas SEMA3F was upregulated only by estrogen. Cotreatment of cell lines with a hormone and its specific antagonist blocked the effect of the hormone. Ectopic expression of SEMA3B or SEMA3F reduced soft-agar colony formation, adhesion, and cell invasion of ovarian cancer cell cultures. Forced expression of SEMA3B, but not SEMA3F, inhibited viability of ovarian cancer cells. Overexpression of SEMA3B and SEMA3F reduced focal adhesion kinase phosphorylation and matrix metalloproteinase-2 and matrix metalloproteinase-9 expression in ovarian cancer cells. Forced expression of SEMA3F, but not SEMA3B in ovarian cancer cells, significantly inhibited endothelial cell tube formation. Collectively, our results suggest that the loss of SEMA3 expression could be a hallmark of cancer progression. Furthermore, gonadotropin- and/or estrogen-mediated maintenance of SEMA3 expression could control ovarian cancer angiogenesis and metastasis.

Role of class 3 semaphorins and their receptors in tumor growth and angiogenesis

Class 3 semaphorins (SEMA3) were first identified as glycoproteins that negatively mediate neuronal guidance by binding to neuropilin and repelling neurons away from the source of SEMA3. However, studies have shown that SEMA3s are also secreted by other cell types, including tumor cells, where they play an inhibitory role in tumor growth and angiogenesis (specifically SEMA3B and SEMA3F). SEMA3s primarily inhibit the cell motility and migration of tumor and endothelial cells by inducing collapse of the actin cytoskeleton via neuropilins and plexins. Besides binding to SEMA3s, neuropilin also binds the protumorigenic and proangiogenic ligand vascular endothelial growth factor (VEGF). Although some studies attribute the antitumorigenic and antiangiogenic properties of SEMA3s to competition between SEMA3s and VEGF for binding to neuropilin receptors, several others have shown that SEMA3s display growth-inhibitory activity independent of competition with VEGF. A better understanding of these molecular interactions and the role and signaling of SEMA3s in tumor biology will help determine whether SEMA3s represent potential therapeutic agents. Herein, we briefly review (a) the role of SEMA3s in mediating tumor growth, (b) the SEMA3 receptors neuropilins and plexins, and (c) the potential competition between SEMA3s and VEGF family members for neuropilin binding.

Proteoglycan signaling co-receptors: roles in cell adhesion, migration and invasion

Signaling co-receptors are diverse, multifunctional components of most major signaling pathways, with roles in mediating and regulating signaling in both physiological and pathophysiological circumstances. Many of these signaling co-receptors, including CD44, glypicans, neuropilins, syndecans and TssRIII/betaglycan are also proteoglycans. Like other co-receptors, these proteoglycan signaling co-receptors can bind multiple ligands, promoting the formation of receptor signaling complexes and regulating signaling at the cell surface. The proteoglycan signaling co-receptors can also function as structural molecules to regulate adhesion, cell migration, morphogenesis and differentiation. Through a balance of these signaling and structural roles, proteoglycan signaling co-receptors can have either tumor promoting or tumor suppressing functions. Defining the role and mechanism of action of these proteoglycan signaling co-receptors should enable more effective targeting of these co-receptors and their respective pathways for the treatment of human disease.

Expression of Semaphorin 3F and Its Receptors in Epithelial Ovarian Cancer, Fallopian Tubes, and Secondary Müllerian Tissues

While semaphorins and their receptors appear to play a role in tumor carcinogenesis, little is known about the role of semaphorin 3F (S3F) in epithelial ovarian cancer (EOC) development. Therefore, we sought to determine the clinical relationship between S3F and its receptors, neuropilin-2 (NP-2) and neuropilin-1 (NP-1) with EOC progression. We analyzed the immunohistological expression of S3F, NP-2, and NP-1 in clinical specimens of normal ovaries (N), benign cystadenomas (Cy), well-differentiated adenocarcinomas (WD), poorly-differentiated adenocarcinomas (PD), inclusion cysts (IC), paraovarian cysts (PC), and fallopian tubes (FT). Tissue sections were evaluated for staining intensity and percentage of immunoreactive epithelia. We found that expression of S3F and NP-2 decreased while NP-1 expression increased with EOC progression. Interestingly, we also found elevated expression of S3F, NP-2, and NP-1 in epithelia of ICs, PCs, and FT. Our findings indicate that loss or deregulation of semaphorin signaling may play an important role in EOC development.

Neuropilin-2 acts as a modulator of Sema3A-dependent glioma cell migration

Semaphorin 3A (Sema3A) is a secreted guidance molecule initially described in the nervous system. This protein is able to control axon growth but also effects on endothelial cells migration. Here, we report that Sema3A acts as a chemorepellent factor for the rat C6 glioma cells and three different human glioma cell lines. Interestingly, Sema3A triggered a chemoattractive response in a fourth human glioma cell line. The nature of the receptor complex ensuring the appropriate signaling was dissected in C6 cells by using function blocking antibodies and gain- or loss-of function experiments using recombinant receptors. Our results demonstrate that neuropilin-1, neuropilin-2 and PlexinA1 are necessary to trigger cell repulsion. The selective blockade of neuropilin-1 or Plexin-A1 switched the chemorepulsive effect of Sema3A into a chemoattractive one. Strikingly, blocking Neuropilin-2 suppressed Sema3A-induced cell migration while overexpression of neuropilin-2 was able to convert the chemorepulsive effect of Sema3A into a chemoattractive one. Our results not only provide additional evidence for a biological function of Sema3A in glioma migration but also reveal part of the receptor complex involved. Hence, our study describes a receptor-based plasticity in cancer cells leading to opposite migration behavior in response to the same extracellular signal.

Semaphorin signaling in cancer cells and in cells of the tumor microenvironment--two sides of a coin

Semaphorins are a large family of secreted and membrane-bound molecules that were initially implicated in the development of the nervous system and in axon guidance. More recently, they have been found to regulate cell adhesion and motility, angiogenesis, immune responses, and tumor progression. Semaphorin receptors, the neuropilins and the plexins, are expressed by a wide variety of cell types, including endothelial cells, bone-marrow-derived cells and cancer cells. Interestingly, a growing body of evidence indicates that semaphorins also have an important role in cancer. It is now known that cancer progression, invasion and metastasis involve not only genetic changes in the tumor cells but also crosstalk between tumor cells and their surrounding non-tumor cells. Through the recruitment of endothelial cells, leukocytes, pericytes and fibroblasts, and the local release of growth factors and cytokines, the tumor microenvironment can mediate tumor-cell survival, tumor proliferation and regulation of the immune response. Moreover, by conferring cancer cells with an enhanced ability to migrate and invade adjacent tissues, extracellular regulatory signals can play a major role in the metastatic process. In this Commentary, we focus on the emerging role of semaphorins in mediating the crosstalk between tumor cells and multiple stromal cell types in the surrounding microenvironment.

Neuropilins in tumor biology

The neuropilin receptors were first discovered as regulators of nervous system development, acting as semaphorin coreceptors with plexins. Subsequently, the neuropilins were identified as receptors for vascular endothelial growth factor. Since those seminal discoveries, additional ligands that bind neuropilins have been described, and many studies have implicated neuropilins in playing key roles in tumor biology. Recent evidence has shown that manipulating neuropilin function can regulate tumor growth and metastasis through effects on vascular biology in the case of neuropilin-1 and lymphatic biology in the case of neuropilin-2. A direct role for neuropilins within in tumor cells has also been postulated. As data continue to accumulate pointing to a role for neuropilins in cancer, the promise for targeting this pathway is beginning to unfold.

Semaphorin 3E expression correlates inversely with Plexin D1 during tumor progression

Plexin D1 (PLXND1) is broadly expressed on tumor vessels and tumor cells in a number of different human tumor types. Little is known, however, about the potential functional contribution of PLXND1 expression to tumor development. Expression of semaphorin 3E (Sema3E), one of the ligands for PLXND1, has previously been correlated with invasive behavior and metastasis, suggesting that the PLXND1-Sema3E interaction may play a role in tumor progression. Here we investigated PLXND1 and Sema3E expression during tumor progression in cases of melanoma. PLXND1 was not expressed by melanocytic cells in either naevi or melanomas in situ, whereas expression increased with invasion level, according to Clark's criteria. Furthermore, 89% of the metastatic melanomas examined showed membranous PLXND1-staining of tumor cells. Surprisingly, expression of Sema3E was inversely correlated with tumor progression, with no detectable staining in melanoma metastasis. To functionally assess the effects of Sema3E expression on tumor development, we overexpressed Sema3E in a xenograft model of metastatic melanoma. Sema3E expression dramatically decreased metastatic potential. These results show that PLXND1 expression during tumor development is strongly correlated with both invasive behavior and metastasis, but exclude Sema3E as an activating ligand.