PlexinD1 Is a Novel Transcriptional Target and Effector of Notch Signaling in Cancer Cells

The secreted semaphorin Sema3E controls cell migration and invasiveness in cancer cells. Sema3E-receptor, PlexinD1, is frequently upregulated in melanoma, breast, colon, ovarian and prostate cancers; however, the mechanisms underlying PlexinD1 upregulation and the downstream events elicited in tumor cells are still unclear. Here we show that the canonical RBPjk-dependent Notch signaling cascade controls PlexinD1 expression in primary endothelial and cancer cells. Transcriptional activation was studied by quantitative PCR and promoter activity reporter assays. We found that Notch ligands and constitutively activated intracellular forms of Notch receptors upregulated PlexinD1 expression; conversely RNAi-based knock-down, or pharmacological inhibition of Notch signaling by gamma-secretase inhibitors, downregulated PlexinD1 levels. Notably, both Notch1 and Notch3 expression positively correlates with PlexinD1 levels in prostate cancer, as well as in other tumor types. In prostate cancer cells, Sema3E-PlexinD1 axis was previously reported to regulate migration; however, implicated mechanisms were not elucidated. Here we show that in these cells PlexinD1 activity induces the expression of the transcription factor Slug, downregulates E-cadherin levels and enhances cell migration. Moreover, our mechanistic data identify PlexinD1 as a pivotal mediator of this signaling axis downstream of Notch in prostate cancer cells. In fact, on one hand, PlexinD1 is required to mediate cell migration and E-cadherin regulation elicited by Notch. On the other hand, PlexinD1 upregulation is sufficient to induce prostate cancer cell migration and metastatic potential in mice, leading to functional rescue in the absence of Notch. In sum, our work identifies PlexinD1 as a novel transcriptional target induced by Notch signaling, and reveals its role promoting prostate cancer cell migration and downregulating E-cadherin levels in Slug-dependent manner. Collectively, these findings suggest that Notch-PlexinD1 signaling axis may be targeted to impair prostate cancer cell invasiveness and metastasis.

Tumour growth inhibition and anti-metastatic activity of a mutated furin-resistant Semaphorin 3E isoform

Secreted Semaphorin 3E (Sema3E) promotes cancer cell invasiveness and metastatic spreading. The pro-metastatic activity of Sema3E is due to its proteolytic fragment p61, capable of transactivating the oncogenic tyrosine kinase ErbB2 that associates with the Sema3E receptor PlexinD1 in cancer cells. Here, we show that a mutated, uncleavable variant of Sema3E (Uncl-Sema3E) binds to PlexinD1 like p61-Sema3E, but does not promote the association of PlexinD1 with ErbB2 nor activates the ensuing signalling cascade leading to metastatic spreading. Furthermore, Uncl-Sema3E competes with endogenous p61-Sema3E produced by tumour cells, thereby hampering their metastatic ability. Uncl-Sema3E also acts independently as a potent anti-angiogenic factor. It activates a PlexinD1-mediated signalling cascade in endothelial cells that leads to the inhibition of adhesion to extracellular matrix, directional migration and cell survival. The putative therapeutic potential of Uncl-Sema3E was validated in multiple orthotopic or spontaneous tumour models in vivo, where either local or systemic delivery of Uncl-Sema3E-reduced angiogenesis, growth and metastasis, even in the case of tumours refractory to treatment with a soluble vascular endothelial growth factor trap. In summary, we conclude that Uncl-Sema3E is a novel inhibitor of tumour angiogenesis and growth that concomitantly hampers metastatic spreading.

The tetraspanin CD151 is required for Met-dependent signaling and tumor cell growth

CD151, a transmembrane protein of the tetraspanin family, is implicated in the regulation of cell-substrate adhesion and cell migration through physical and functional interactions with integrin receptors. In contrast, little is known about the potential role of CD151 in controlling cell proliferation and survival. We have previously shown that β4 integrin, a major CD151 partner, not only acts as an adhesive receptor for laminins but also as an intracellular signaling platform promoting cell proliferation and invasive growth upon interaction with Met, the tyrosine kinase receptor for hepatocyte growth factor (HGF). Here we show that RNAi-mediated silencing of CD151 expression in cancer cells impairs HGF-driven proliferation, anchorage-independent growth, protection from anoikis, and tumor progression in xenograft models in vivo. Mechanistically, we found that CD151 is crucially implicated in the formation of signaling complexes between Met and β4 integrin, a known amplifier of HGF-induced tumor cell growth and survival. CD151 depletion hampered HGF-induced phosphorylation of β4 integrin and the ensuing Grb2-Gab1 association, a signaling pathway leading to MAPK stimulation and cell growth. Accordingly, CD151 knockdown reduced HGF-triggered activation of MAPK but not AKT signaling cascade. These results indicate that CD151 controls Met-dependent neoplastic growth by enhancing receptor signaling through β4 integrin-mediated pathways, independent of cell-substrate adhesion.

Sema3E-Plexin D1 signaling drives human cancer cell invasiveness and metastatic spreading in mice

Semaphorin 3E (Sema3E) is a secreted molecule implicated in axonal path finding and inhibition of developmental and postischemic angiogenesis. Sema3E is also highly expressed in metastatic cancer cells, but its mechanistic role in tumor progression was not understood. Here we show that expression of Sema3E and its receptor Plexin D1 correlates with the metastatic progression of human tumors. Consistent with the clinical data, knocking down endogenous expression of either Sema3E or Plexin D1 in human metastatic carcinoma cells hampered their metastatic potential when injected into mice, while tumor growth was not markedly affected. Conversely, overexpression of exogenous Sema3E in cancer cells increased their invasiveness, transendothelial migration, and metastatic spreading, although it inhibited tumor vessel formation, resulting in reduced tumor growth in mice. The proinvasive and metastatic activity of Sema3E in tumor cells was dependent on transactivation of the Plexin D1-associated ErbB2/Neu oncogenic kinase. In sum, Sema3E-Plexin D1 signaling in cancer cells is crucially implicated in their metastatic behavior and may therefore be a promising target for strategies aimed at blocking tumor metastasis.

Abstract 3398: Double-faced Sema3E-PlexinD1 signaling inhibits tumor growth but promotes metastatic spreading

Semaphorins comprise a wide family of secreted and membrane-bound signals, originally found in neural development, and then implicated in a range of functions: from angiogenesis, to the immune response, to cancer progression. We have previously shown that high-affinity semaphorin receptors are found in the families of the plexins and of the neuropilins. Moreover, additional molecules interact with semaphorin receptors in cell-context dependent manner, featuring a complex scenario of multiple potential signaling pathways. The role of semaphorin signaling in tumor progression is currently under close scrutiny; it is known that cancer cells release semaphorins regulating their own behavior as well as that of cells in the tumor microenvironment, such as endothelial cells and recruited leukocytes.

In this work, we focused on the function of Semaphorin 3E (Sema3E) and its high affinity receptor PlexinD1, and on their potential role to regulate cancer progression. Here we found that Sema3E and PlexinD1 are highly expressed in human metastatic tumors. Moreover, we provided experimental evidences that Sema3E released by tumor cells is a double-faced signal, acting via two distinctive signaling pathways. In paracrine manner, Sema3E-PlexinD1 signaling can elicit endothelial cell repulsion, thereby reducing vessel density and tumor growth. Conversely, Sema3E autocrine signaling in tumor cells thrusts their invasiveness, trans-endothelial migration, and metastatic spreading, through the trans-activation of PlexinD1-associated oncogenic kinases. Notably, by silencing the endogenous expression of either Sema3E or PlexinD1 the metastatic potential of cancer cells is significantly reduced, identifying this pathway as a major driver of the metastatic spreading. In sum, Sema3E autocrine signaling in cancer cells is crucially implicated to induce a metastatic behavior, and it appears as a promising target for strategies aimed at blocking tumor metastasis.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3398.

Co-expression of plexin-B1 and Met in human breast and ovary tumours enhances the risk of progression

Background: Plex-B1, the receptor of Sema4D, has been implicated in tumour growth, angiogenesis and metastasis. The binding of Sema4D to Plex-B1 can trigger the activation of Met tyrosine kinase, thereby promoting cell dissociation and invasive growth. We tested the hypothesis that the expression of Plex-B1, either alone or in association with Met, can be of predictive value for tumour progression.

Methods: The expression and distribution of Plex-B1 and Met were investigated by immunohistochemistry and immunofluorescence in 50 human neoplasias originating in the breast and ovary, and correlated with clinical–pathological data at diagnosis.

Results: Plex-B1 and Met were individually expressed in 14% and in 24% of the tumours, respectively. Plex-B1 and Met were co-expressed in 24/50 cases (48%), and in the majority of these (83%) Met was tyrosine phosphorylated. The expression of Plex-B1 or Met alone showed no significant correlation with tumour aggressiveness, whereas advanced stage tumours (III–IV) frequently showed Plex-B1–Met double-positive (9/13). Tumours co-expressing Plex-B1 and Met were characterised by worse grading and higher incidence of lymph node metastases. Out of 22 tumours with lymph node metastases, as many as 19 were Plex-B1 and Met double-positive (p=0.0008), and 17 expressed phosphorylated Met (p=0.002).

Conclusions: Plex-B1 assumes a predictive value for unfavourable outcome when co-expressed with Met.

Expression of basic fibroblast growth factor, its receptors and syndecans in bladder cancer

Basic fibroblast growth factor (bFGF) is a heparin-binding cationic protein involved in a variety of pathological conditions including angiogenesis and solid tumour growth. The basic fibroblast growth factor receptor (FGFR) family comprises at least 4 high affinity tyrosine kinase receptors that require syndecans for their function. Mounting evidence indicates that syndecans, that bind both bFGF and their FGFRs, will act as stimulators, whereas syndecans that only bind bFGF will act as inhibitors of signaling by sequestering the growth factor. Recent findings have highlighted the importance of syndecans in urological cancers. The aim of this study is to investigate the expression of bFGF, its receptors (R1 and R2) and syndecans (1-4) in invasive urothelial carcinoma and normal-looking urothelium by Western blotting, RT-PCR, and immunohistochemistry analyses. Interestingly, bFGF, FGFR1 and FGFR2 protein levels statistically increased in bladder cancer tissues. mRNA of FGFR1 and syndecans (1-4), showed a statistically significant increase while an mRNA increase in the other molecules analysed was not significant. bFGF, its receptors and syndecan immunostaining were mainly present in the urothelium both in normal-looking tissues and urothelial neoplastic cells. In conclusion, our data report that the bFGF, FGFR and syndecan expressions are altered in bladder tumours.

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.

MicroRNAs impair MET-mediated invasive growth

MicroRNAs (miRNA) are a recently identified class of noncoding, endogenous, small RNAs that regulate gene expression, mainly at the translational level. These molecules play critical roles in several biological processes, such as cell proliferation and differentiation, development, and aging. It is also known that miRNAs play a role in human cancers where they can act either as oncogenes, down-regulating tumor suppressor genes, or as onco-suppressors, targeting molecules critically involved in promotion of tumor growth. One of such molecules is the tyrosine kinase receptor for hepatocyte growth factor, encoded by the MET oncogene. The MET receptor promotes a complex biological program named "invasive growth" that results from stimulation of cell motility, invasion, and protection from apoptosis. This oncogene is deregulated in many human tumors, where its most frequent alteration is overexpression. In this work, we have identified three miRNAs (miR-34b, miR-34c, and miR-199a*) that negatively regulate MET expression. Inhibition of these endogenous miRNAs, by use of antagomiRs, resulted in increased expression of MET protein, whereas their exogenous expression in cancer cells blocked MET-induced signal transduction and the execution of the invasive growth program, both in cells expressing normal levels of MET and in cancer cells overexpressing a constitutively active MET. Moreover, we show that these same miRNAs play a role in regulating the MET-induced migratory ability of melanoma-derived primary cells. In conclusion, we have identified miRNAs that behave as oncosuppressors by negatively targeting MET and might thus provide an additional option to inhibit this oncogene in tumors displaying its deregulation.

The tumor suppressor semaphorin 3B triggers a prometastatic program mediated by interleukin 8 and the tumor microenvironment

Semaphorins are a large family of evolutionarily conserved morphogenetic molecules originally identified for their repelling role in axonal guidance. Intriguingly, semaphorins have recently been implicated in cancer progression (Neufeld, G., T. Lange, A. Varshavsky, and O. Kessler. 2007. Adv. Exp. Med. Biol. 600:118–131). In particular, semaphorin 3B (SEMA3B) is considered a putative tumor suppressor, and yet we found that it is expressed at high levels in many invasive and metastatic human cancers. By investigating experimental tumor models, we confirmed that SEMA3B expression inhibited tumor growth, whereas metastatic dissemination was surprisingly increased. We found that SEMA3B induced the production of interleukin (IL) 8 by tumor cells by activating the p38–mitogen-activated protein kinase pathway in a neuropilin 1–dependent manner. Silencing the expression of endogenous SEMA3B in tumor cells impaired IL-8 transcription. The release of IL-8, in turn, induced the recruitment of tumor-associated macrophages and metastatic dissemination to the lung, which could be rescued by blocking IL-8 with neutralizing antibodies. In conclusion, we report that SEMA3B exerts unexpected functions in cancer progression by fostering a prometastatic environment through elevated IL-8 secretion and recruitment of macrophages coupled to the suppression of tumor growth.

Evidence for a role of TGF-beta1 in the expression and regulation of alpha-SMA in fetal growth restricted placentae

There is evidence that alpha-smooth muscle actin (alpha-SMA) is a protein that plays a pivotal role in the production of contractile forces and it is induced by transforming growth factor-beta1 (TGF-beta1). We have analysed the expression of alpha-SMA, TGF-beta1, its receptor RI and the activator phospho-Smad2 in (a) fetal growth restriction pre-eclamptic placentae characterised by early onset and absence of end diastolic velocities in the umbilical arteries (FGR-AED) and (b) control placentae accurately matched for gestational age. The study was performed by immunohistochemical, quantitative Western blotting, ELISA, RT-PCR and in vitro analyses. We found that TGF-beta1 stimulates alpha-SMA production in chorionic villi cultured in vitro. In addition, we observed that in vivo TGF-beta1 concentration is significantly higher in FGR-AED placental samples than in control placentae and that this growth factor could have a paracrine action on villous stroma myofibroblasts expressing TGF-beta1 receptors and phospho-Smad2. Indeed, we report that alpha-SMA undergoes a redistribution in FGR-AED placental villous tree, i.e. we show that alpha-SMA is enhanced in medium and small stem villi and significantly decreased in the peripheral villi. Our data allow us to consider TGF-beta1 and alpha-SMA as key molecules related to FGR-AED placental villous tree phenotypic changes responsible for increased impedance to blood flow observable in this pathology.

Restricted innervation of uterus and placenta during pregnancy: evidence for a role of the repelling signal Semaphorin 3A

Because data from the literature suggest a lack of innervation of the placenta, we have investigated placenta, umbilical cord, and uterus to identify the molecules that play a role in regulating innervation in these organs. Neuropilin-1 and Plexin-A1 are cell surface proteins that form a receptor complex for Semaphorin 3A (Sema 3A), a secreted molecule mediating repelling signals for axonal growth cones. We have analyzed the expression of Neuropilin-1, Plexin-A1, and Semaphorin 3A in the above-mentioned tissues on the hypothesis that these molecules could regulate innervation in these organs during gestation. We found that nervous fibers are only present in the proximal part of the umbilical cord, close to the newborn, and in nongestational uterine tissues. In contrast, nervous fibers are not present in the distal segment of the umbilical cord, in the placenta and in the uterine tissues during gestation. We also found that Sema 3A receptors, Neuropilin-1 and Plexin-A1, are expressed by the nervous fibers of the proximal part of the umbilical cord, whereas Sema 3A is secreted in the umbilical cord, in the placenta, and in gestational uterine tissues. We report that a factor secreted in the umbilical cord induces the collapse of neurite growth cones in vitro and provide evidence that this factor is Sema 3A. In summary, our results suggest that the chemorepulsive signals mediated by Sema 3A play an important role in preventing nerve fibers growth in the umbilical cord and in gestational uterine tissues. The inhibition of nerve growth into the myometrium as well as into the placenta could be considered fundamental processes to preserve the fetus from external stressful events.

Growth factors and their receptors: fundamental molecules for human placental development

In this review we present data concerning the localization of some important growth factors and their receptors in the human placenta. We focus our attention on molecules playing a fundamental role in angiogenesis and morphologic processes as beta-FGF, EGF and TGF-beta. The distribution of these growth factors and their receptors in the placental villi during gestation suggests that these molecules play a pivotal role in growth and differentiation of the villous tree.

Sema3A and neuropilin-1 expression and distribution in rat white adipose tissue

Semaphorins are cell surface and/or soluble signals that exert an inhibitory control on axon guidance. Sema3A, the vertebrate-secreted semaphorin, binds to neuropilin-1, which together with plexins constitutes the functional receptor. To verify whether Sema3A is produced by white adipocytes and, in that case, to detect its targets in white adipose tissue, we studied the cell production and tissue distribution of Sema3A and neuropilin-1 in rat retroperitoneal and epididymal adipose depots. Sema3A and neuropilin-1 were detected in these depots by Western blotting. The immunohistochemical results showed that Sema3A is produced in, and possibly secreted by, smooth muscle cells of arteries and white adipocytes. Accordingly, neuropilin-1 was found on perivascular and parenchymal nerves. Such a pattern of distribution is in line with a role for secreted Sema3A in the growth and plasticity of white adipose tissue nerves. Indeed, after fasting, when white adipocytes are believed to be overstimulated by noradrenaline and rearrangement of the parenchymal nerve supply may occur, adipocytic expression of Sema3A is reduced. Finally, the presence of neuropilin-1 in some white adipocytes raises the interesting possibility that Sema3A also exerts an autocrine-paracrine role on these cells.

PPARgamma expression in normal human placenta, hydatidiform mole and choriocarcinoma

Peroxisome proliferator-activated receptor (PPAR) gamma belongs to a subclass of nuclear hormone receptors that execute their transcriptional functions as heterodimers with the retinoid X receptors (RXR). PPARgamma plays a pivotal role in cellular differentiation. This study investigated PPARgamma protein expression in normal human placentas, hydatidiform moles and choriocarcinoma, using immunohistochemical and Western blot analyses. In first trimester normal placenta, PPARgamma was mainly localized in the nuclei of the villous cytotrophoblastic cells, whereas at term it was mainly localized in the nuclei of the syncytiotrophoblast. Extravillous cytotrophoblast of cell islands and cell columns also showed nuclear PPARgamma immunostaining. A striking result was the altered expression patterns of PPARgamma in pathological tissues; PPARgamma showed a reduced immunostaining in the trophoblastic diseases. In hydatidiform moles, PPARgamma was mainly localized in the nuclei of the trophoblastic collections of the pathological villi and in the extravillous trophoblastic cells, whereas in the choriocarcinoma, only a few trophoblastic cells showed weak PPARgamma nuclear immunostaining. These findings suggest an involvement of PPARgamma in trophoblast differentiation during normal placental development. The down-regulation of PPARgamma expression in the gestational trophoblastic diseases analysed in this study provides a new insight into the progression of these pathologies.

DNA vaccination with full-length or truncated neu induces protective immunity against the development of spontaneous mammary tumors in HER-2/neu transgenic mice

Genetic immunization against tumor antigens is an effective way to induce an immune response able to oppose cancer progression. Overexpression of HER-2/neu can lead to neoplastic transformation and has been found in many human primary breast cancers. We constructed DNA expression vectors encoding the full-length neu oncogene of rat cDNA (pCMV-NeuNT), the neu extracellular domain (pCMV-ECD), or the neu extracellular and transmembrane domains (pCMV-ECD-TM). We evaluated whether i.m. injection of these plasmids induces protection against the development of mammary tumors occurring spontaneously in FVB/N neu-transgenic mice. We found that pCMV-ECD-TM induced the best protection, whereas both pCMV-ECD and pCMV-NeuNT were less effective. The coinjection with a bicistronic vector for murine IL-12 increased the efficacy of pCMV-ECD and pCMV-NeuNT plasmids, and led to the same protection obtained with pCMV-ECD-TM alone. Anti-neuECD antibodies were detected in pCMV-ECD-TM vaccinated mice and, after coinjection with pCMV-IL12 plasmids, they appeared also in animals immunized with pCMV-ECD. Our data demonstrate the effectiveness of DNA vaccination using truncated Neu plasmids in inducing antitumor protection in a spontaneous mammary tumor model.