Slit protein-mediated inhibition of CXCR4-induced chemotactic and chemoinvasive signaling pathways in breast cancer cells

A roundabout way to cancer

The Slit family of secreted proteins and their transmembrane receptor, Robo, were originally identified in the nervous system where they function as axon guidance cues and branching factors during development. Since their discovery, a great number of additional roles have been attributed to Slit/Robo signaling, including regulating the critical processes of cell proliferation and cell motility in a variety of cell and tissue types. These processes are often deregulated during cancer progression, allowing tumor cells to bypass safeguarding mechanisms in the cell and the environment in order to grow and escape to new tissues. In the past decade, it has been shown that the expression of Slit and Robo is altered in a wide variety of cancer types, identifying them as potential therapeutic targets. Further, studies have demonstrated dual roles for Slits and Robos in cancer, acting as both oncogenes and tumor suppressors. This bifunctionality is also observed in their roles as axon guidance cues in the developing nervous system, where they both attract and repel neuronal migration. The fact that this signaling axis can have opposite functions depending on the cellular circumstance make its actions challenging to define. Here, we summarize our current understanding of the dual roles that Slit/Robo signaling play in development, epithelial tumor progression, and tumor angiogenesis.

A systematic analysis on DNA methylation and the expression of both mRNA and microRNA in bladder cancer

BACKGROUND

DNA methylation aberration and microRNA (miRNA) deregulation have been observed in many types of cancers. A systematic study of methylome and transcriptome in bladder urothelial carcinoma has never been reported.

METHODOLOGY/PRINCIPAL FINDINGS

The DNA methylation was profiled by modified methylation-specific digital karyotyping (MMSDK) and the expression of mRNAs and miRNAs was analyzed by digital gene expression (DGE) sequencing in tumors and matched normal adjacent tissues obtained from 9 bladder urothelial carcinoma patients. We found that a set of significantly enriched pathways disrupted in bladder urothelial carcinoma primarily related to "neurogenesis" and "cell differentiation" by integrated analysis of -omics data. Furthermore, we identified an intriguing collection of cancer-related genes that were deregulated at the levels of DNA methylation and mRNA expression, and we validated several of these genes (HIC1, SLIT2, RASAL1, and KRT17) by Bisulfite Sequencing PCR and Reverse Transcription qPCR in a panel of 33 bladder cancer samples.

CONCLUSIONS/SIGNIFICANCE

We characterized the profiles between methylome and transcriptome in bladder urothelial carcinoma, identified a set of significantly enriched key pathways, and screened four aberrantly methylated and expressed genes. Conclusively, our findings shed light on a new avenue for basic bladder cancer research.

Glucocorticoid regulation of SLIT/ROBO tumour suppressor genes in the ovarian surface epithelium and ovarian cancer cells

The three SLIT ligands and their four ROBO receptors have fundamental roles in mammalian development by promoting apoptosis and repulsing aberrant cell migration. SLITs and ROBOs have emerged as candidate tumour suppressor genes whose expression is inhibited in a variety of epithelial tumours. We demonstrated that their expression could be negatively regulated by cortisol in normal ovarian luteal cells. We hypothesised that after ovulation the locally produced cortisol would inhibit SLIT/ROBO expression in the ovarian surface epithelium (OSE) to facilitate its repair and that this regulatory pathway was still present, and could be manipulated, in ovarian epithelial cancer cells. Here we examined the expression and regulation of the SLIT/ROBO pathway in OSE, ovarian cancer epithelial cells and ovarian tumour cell lines. Basal SLIT2, SLIT3, ROBO1, ROBO2 and ROBO4 expression was lower in primary cultures of ovarian cancer epithelial cells when compared to normal OSE (P<0.05) and in poorly differentiated SKOV-3 cells compared to the more differentiated PEO-14 cells (P<0.05). Cortisol reduced the expression of certain SLITs and ROBOs in normal OSE and PEO-14 cells (P<0.05). Furthermore blocking SLIT/ROBO activity reduced apoptosis in both PEO-14 and SKOV-3 tumour cells (P<0.05). Interestingly SLIT/ROBO expression could be increased by reducing the expression of the glucocorticoid receptor using siRNA (P<0.05). Overall our findings indicate that in the post-ovulatory phase one role of cortisol may be to temporarily inhibit SLIT/ROBO expression to facilitate regeneration of the OSE. Therefore this pathway may be a target to develop strategies to manipulate the SLIT/ROBO system in ovarian cancer.

A novel role for Slit2/Robo1 axis in modulating HIV-1 replication in T cells

OBJECTIVE

Slit2 is a secreted glycoprotein that has been shown to possess anti-inflammatory properties. In addition, Slit2 has been shown to modulate CXCR4-mediated functional effects in T cells. However, its role in HIV-1 pathogenesis is not known. The objective of the current study is to analyze the role of Slit2 in modulating HIV-1 replication in T cells.

METHODS

The effect of endogenous Slit2 expression of HIV-1 replication in T cells was studied by transient overexpression or downregulation of Slit2. The role of exogenous Slit2 was studied by analyzing the effect of soluble Slit2 protein on HIV-1 replication in T-cell lines and peripheral blood mononuclear cells (PBMCs).

RESULTS

Studies on T-cell lines revealed a higher expression of Slit2 in Jurkat T cells compared with MT4 cells. We observed that downregulation of Slit2 in Jurkat T cells using Slit2-specific small inhibitor RNA enhanced HIV-1 replication. However, overexpression of Slit2 in MT4 cells and PBMCs reduced HIV-1 replication. As Slit2 is a secretory protein, we further analyzed the role of soluble Slit2 on HIV-1 virus replication using various cell lines and PBMCs. Our data indicated that exogenous Slit2 inhibited replication of both X4-tropic and R5-tropic HIV-1 viruses. Further studies revealed that Slit2 mediated its functional effects by binding to Robo1 receptor.

CONCLUSION

Taken together, our results describe Slit2/Robo1 axis as a novel modulator of HIV-1 replication in T cells. These studies may contribute to the understanding of the immunopathogenesis of HIV-1 infection.

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.

Slit-2 facilitates interaction of P-cadherin with Robo-3 and inhibits cell migration in an oral squamous cell carcinoma cell line

Slits are a group of secreted glycoproteins that act as molecular guidance cues in cellular migration. Recently, several studies demonstrated that Slit-2 can operate as candidate tumour suppressor protein in various tissues. In this study, we show Slit-2 expression in basal cell layers of normal oral mucosa colocalized with P-cadherin expression. In contrast, there is a loss of Slit-2 and P-cadherin expression in mucosa of oral squamous cell carcinoma (OSCC). Our in vitro investigations reveal a correlation of P-cadherin and Slit-2 expression: OSCC cells with induced P-cadherin expression (PCI52_PC) display an increased Slit-2 expression. However, abrogating P-cadherin function with a function-blocking antibody decreases Slit-2 secretion confirming a direct link between P-cadherin and Slit-2. Moreover, experiments with OSCC cells show that Slit-2 interferes with a Wnt related signalling pathway, which in turn affects Slit-2 expression in a feedback loop. Functionally, transwell migration assays demonstrate a Slit-2 dose-dependent decrease of PCI52_PC cell migration. However, there is no influence on migration in mock control cells. Responsible for this migration block might be an interaction of P-cadherin with Roundabout (Robo)-3, a high affinity receptor of Slit-2. Indeed, proximity ligation assays exhibit P-cadherin/Robo-3 interactions on PCI52_PC cells. Additionally, we detect a modulation of this interaction by addition of recombinant Slit-2. Down-regulation of Robo-3 expression via small interfering RNA neutralizes Slit-2 induced migration block in PCI52_PC cells. In summary, our experiments show antitumorigenic effects of Slit-2 on P-cadherin expressing OSCC cells supposedly via modulation of Robo-3 interaction.

Novel roles for Slits and netrins: axon guidance cues as anticancer targets?

Over the past few years, several genes, proteins and signalling pathways that are required for embryogenesis have been shown to regulate tumour development and progression by playing a major part in overriding antitumour safeguard mechanisms. These include axon guidance cues, such as Netrins and Slits. Netrin 1 and members of the Slit family are secreted extracellular matrix proteins that bind to deleted in colorectal cancer (DCC) and UNC5 receptors, and roundabout receptors (Robos), respectively. Their expression is deregulated in a large proportion of human cancers, suggesting that they could be tumour suppressor genes or oncogenes. Moreover, recent data suggest that these ligand-receptor pairs could be promising targets for personalized anticancer therapies.

Angiocrine factors modulate tumor proliferation and motility through EphA2 repression of Slit2 tumor suppressor function in endothelium

It is well known that tumor-derived proangiogenic factors induce neovascularization to facilitate tumor growth and malignant progression. However, the concept of "angiocrine" signaling, in which signals produced by endothelial cells elicit tumor cell responses distinct from vessel function, has been proposed, yet remains underinvestigated. Here, we report that angiocrine factors secreted from endothelium regulate tumor growth and motility. We found that Slit2, which is negatively regulated by endothelial EphA2 receptor, is one such tumor suppressive angiocrine factor. Slit2 activity is elevated in EphA2-deficient endothelium. Blocking Slit activity restored angiocrine-induced tumor growth/motility, whereas elevated Slit2 impaired growth/motility. To translate our findings to human cancer, we analyzed EphA2 and Slit2 expression in human cancer. EphA2 expression inversely correlated with Slit2 in the vasculature of invasive human ductal carcinoma samples. Moreover, analysis of large breast tumor data sets revealed that Slit2 correlated positively with overall and recurrence-free survival, providing clinical validation for the tumor suppressor function for Slit2 in human breast cancer. Together, these data support a novel, clinically relevant mechanism through which EphA2 represses Slit2 expression in endothelium to facilitate angiocrine-mediated tumor growth and motility by blocking a tumor suppressive signal.

The neuronal repellent SLIT2 is a target for repression by EZH2 in prostate cancer

The neuronal repellent SLIT2 is repressed in a number of cancer types primarily through promoter hypermethylation. SLIT2, however, has not been studied in prostate cancer. Through genome-wide location analysis we identified SLIT2 as a target of Polycomb group (PcG) protein EZH2. The EZH2-containing Polycomb repressive complexes bound to the SLIT2 promoter inhibiting its expression. SLIT2 was down-regulated in a majority of metastatic prostate tumors exhibiting a negative correlation with EZH2. This repressed expression could be restored by methylation inhibitors or EZH2-suppressing compounds. In addition, a low level of SLIT2 expression was associated with aggressive prostate, breast and lung cancers. Functional assays showed that SLIT2 inhibited prostate cancer cell proliferation and invasion. Thus, this study demonstrated for the first time epigenetic silencing of SLIT2 in prostate tumors, and supported SLIT2 as a potential biomarker for aggressive solid tumors. Importantly, PcG-mediated repression may serve as a precursor for the silencing of SLIT2 by DNA methylation in cancer.

The SLIT-ROBO pathway: a regulator of cell function with implications for the reproductive system

The secreted SLIT glycoproteins and their Roundabout (ROBO) receptors were originally identified as important axon guidance molecules. They function as a repulsive cue with an evolutionarily conserved role in preventing axons from migrating to inappropriate locations during the assembly of the nervous system. In addition the SLIT-ROBO interaction is involved in the regulation of cell migration, cell death and angiogenesis and, as such, has a pivotal role during the development of other tissues such as the lung, kidney, liver and breast. The cellular functions that the SLIT/ROBO pathway controls during tissue morphogenesis are processes that are dysregulated during cancer development. Therefore inactivation of certain SLITs and ROBOs is associated with advanced tumour formation and progression in disparate tissues. Recent research has indicated that the SLIT/ROBO pathway could also have important functions in the reproductive system. The fetal ovary expresses most members of the SLIT and ROBO families. The SLITs and ROBOs also appear to be regulated by steroid hormones and regulate physiological cell functions in adult reproductive tissues such as the ovary and endometrium. Furthermore several SLITs and ROBOs are aberrantly expressed during the development of ovarian, endometrial, cervical and prostate cancer. This review will examine the roles this pathway could have in the development, physiology and pathology of the reproductive system and highlight areas for future research that could further dissect the influence of the SLIT/ROBO pathway in reproduction.

Slit2 inhibits glioma cell invasion in the brain by suppression of Cdc42 activity

Acquisition of insidious invasiveness by malignant glioma cells involves multiple genetic alterations in signaling pathways. Slit2, a chemorepulsive factor, controls cell migration of neuronal and glial cells during development and inhibits chemotaxic migration of various types of cells in vitro. However, the role of Slit2 in vitro remains controversial, and the biological significance of Slit2 expression in cancer cell invasion in vivo has not yet been determined. In the present study, we characterized the effects of Slit2 expression on the migration and invasion of invasive glioma cells in vitro and in vivo. By reverse transcriptase polymerase chain reaction (PCR) analyses, Slit2 was found to be expressed at lower levels in primary glioma specimens and invasive glioma cells compared with normal human brain cells and astrocytes. Ectopic expression of Slit2 or treatment with recombinant Slit2 on glioma cells attenuates cell migration and invasion through inhibition of Cdc42 activity in vitro. Cellular depletion of Robo1, a cognate receptor for Slit2, prevented Slit2 inhibition of Cdc42 activity and glioma cell migration. In vivo, expression of Slit2 by invasive SNB19 glioma cells markedly inhibited glioma cell infiltration into the brain of mice. Moreover, impediment of glioma cell invasion by Slit2 did not affect the expression of N-cadherin and beta-catenin in glioma cells. These results provide the first evidence demonstrating that Slit2-Robo1 inhibits glioma invasion through attenuating Cdc42 activity in vitro and in the brain. Understanding the mechanisms of Slit2-Robo1 inhibition of glioma cell invasion will foster new treatments for malignant gliomas.

The axonal repellent, Slit2, inhibits directional migration of circulating neutrophils

In inflammatory diseases, circulating neutrophils are recruited to sites of injury. Attractant signals are provided by many different chemotactic molecules, such that blockade of one may not prevent neutrophil recruitment effectively. The Slit family of secreted proteins and their transmembrane receptor, Robo, repel axonal migration during CNS development. Emerging evidence shows that by inhibiting the activation of Rho-family GTPases, Slit2/Robo also inhibit migration of other cell types toward a variety of chemotactic factors in vitro and in vivo. The role of Slit2 in inflammation, however, has been largely unexplored. We isolated primary neutrophils from human peripheral blood and mouse bone marrow and detected Robo-1 expression. Using video-microscopic live cell tracking, we found that Slit2 selectively impaired directional migration but not random movement of neutrophils toward fMLP. Slit2 also inhibited neutrophil migration toward other chemoattractants, namely C5a and IL-8. Slit2 inhibited neutrophil chemotaxis by preventing chemoattractant-induced actin barbed end formation and cell polarization. Slit2 mediated these effects by suppressing inducible activation of Cdc42 and Rac2 but did not impair activation of other major kinase pathways involved in neutrophil migration. We further tested the effects of Slit2 in vivo using mouse models of peritoneal inflammation induced by sodium periodate, C5a, and MIP-2. In all instances, Slit2 reduced neutrophil recruitment effectively (P<0.01). Collectively, these data demonstrate that Slit2 potently inhibits chemotaxis but not random motion of circulating neutrophils and point to Slit2 as a potential new therapeutic for preventing localized inflammation.

Site-specific metastasis formation: chemokines as regulators of tumor cell adhesion, motility and invasion

The metastatic spread of tumors is a well-coordinated process in which different types of cancers tend to form metastases in defined organs. The formation of site-specific metastases requires full compatibility between the intrinsic properties of the tumor cells and the tumor microenvironment. It was recently found that chemokines which are expressed in specific loci promote the adhesion, migration and invasion of tumor cells that express the corresponding receptor(s). Of the different members of the family, the CXCL12 chemokine and its cognate CXCR4 receptor are the prototypes of this process, although other members of the family (e.g. CCR7 and CCR10) also play a role in determination of the metastatic spread. This commentary addresses the fundamental roles of chemokines and their receptors in site-specific metastasis, with emphasis on CXCL12-CXCR4. The article also describes some of the efforts that were performed thus far in order to identify the intracellular components involved in this process. The focus is put on the roles played by proteins that regulate adhesion and migration of tumor cells in response to CXCL12, including mainly focal adhesion kinase (FAK), Pyk2/RAFTK and members of the Rho family of GTPases (RhoA, Rac, Cdc42). This is followed by discussion of open questions that need to be addressed in future research, and of the potential therapeutic implications of the findings that are available to date in this field.

Deubiquitinating enzyme USP33/VDU1 is required for Slit signaling in inhibiting breast cancer cell migration

Slit regulates migration of not only neurons, but also nonneuronal cells, such as leukocytes and cancer cells. Slit effect on cancer cell migration has not been well-characterized. In this study, we used several different assays to examine Slit effect on breast cancer cell migration in vitro. We show that ubiquitin-specific protease 33 (USP33)/VDU1, originally identified as a von Hippel-Lindau tumor suppressor (VHL) protein-interacting deubiquitinating enzyme, binds to the Robo1 receptor, and that USP33 is required for Slit responsiveness in breast cancer cells. Slit induces redistribution of Robo1 from intracellular compartments to the plasma membrane in a USP33-dependent manner. Slit impairs directional migration of breast cancer cells without affecting their migration speed. This inhibitory effect is Robo-mediated and USP33-dependent. These data uncover a previously unknown function of USP33 and reveal a new player in Slit-Robo signaling in cancer cell migration.

Slit2 inhibits growth and metastasis of fibrosarcoma and squamous cell carcinoma

Slits are a group of secreted glycoproteins that play a role in the regulation of cell migration. Previous studies suggested that Slit2 might be a tumor-suppressor gene. However, it remained to be determined whether Slit2 suppressed tumor growth and metastasis in animal models. We showed that Slit2 expression was decreased or abolished in human esophageal squamous cell carcinomas (SCCs) compared to normal tissues by in situ hybridization. Stable transfection of human SCC A431 and fibrosarcoma HT1080 cells with Slit2 gene suppressed tumor growth in athymic nude mice. Apoptosis in Slit2-transfected tumors was increased, whereas proliferating cells were decreased, suggesting a mechanism for Slit2-mediated tumor suppression. This was supported by further analysis indicating that antiapoptotic molecules Bcl-2 and Bcl-xl and cell cycle molecules Cdk6 and Cyclin D1 were down-regulated in Slit2-transfected tumors. Furthermore, wound healing and Matrigel invasion assays showed that the transfection with Slit2 inhibited tumor cell migration and invasion. Slit2-transfected tumors showed a high level of keratin 8/18 and a low level of N-cadherin expression compared to empty vector-transfected tumors. More importantly, Slit2 transfection suppressed the metastasis of HT1080 tumor cells in lungs after intravenous inoculation. Collectively, our study has demonstrated that Slit2 inhibits tumor growth and metastasis of fibrosarcoma and SCC and that its effect on cell cycle and apoptosis signal pathways is an important mechanism for Slit2-mediated tumor suppression.

The Slit/Robo system suppresses hepatocyte growth factor-dependent invasion and morphogenesis

The Slit protein acts through the Roundabout receptor as a paracrine chemorepellent in axon guidance and as an inhibitor in leukocyte chemotaxis, but its role in epithelial cell motility and morphogenesis remains largely unexplored. We report that nontransformed epithelial cells and cancerous cells empower the Slit-2/Robo1 signaling system to limit outward migration in response to motogenic attractants and to remain positionally confined within their primitive location. Short hairpin RNA-mediated depletion of SLIT-2 or ectopic expression of a soluble decoy Robo enhance hepatocyte growth factor (HGF)-induced migration, matrix invasion, and tubulogenesis, concomitantly with the up-regulation of Cdc-42 and the down-modulation of Rac-1 activities. Accordingly, autocrine overexpression or exogenous administration of Slit-2 prevent HGF-triggered motile responses, reduce Cdc-42 activation, and stimulate Rac-1. This antimigratory activity of Slit-2 derives from the inhibition of actin-based protrusive forces and from an increased adhesive strength of cadherin-mediated intercellular contacts. These results disclose a novel function for Slit and Robo in the inhibition of growth factor-mediated epithelial cell motility and morphogenesis, invoking a critical role for both molecules as natural antagonists of neoplastic invasive growth.

Quantification of SLIT-ROBO transcripts in hepatocellular carcinoma reveals two groups of genes with coordinate expression

BACKGROUND

SLIT-ROBO families of proteins mediate axon pathfinding and their expression is not solely confined to nervous system. Aberrant expression of SLIT-ROBO genes was repeatedly shown in a wide variety of cancers, yet data about their collective behavior in hepatocellular carcinoma (HCC) is missing. Hence, we quantified SLIT-ROBO transcripts in HCC cell lines, and in normal and tumor tissues from liver.

METHODS

Expression of SLIT-ROBO family members was quantified by real-time qRT-PCR in 14 HCC cell lines, 8 normal and 35 tumor tissues from the liver. ANOVA and Pearson's correlation analyses were performed in R environment, and different clinicopathological subgroups were pairwise compared in Minitab. Gene expression matrices of cell lines and tissues were analyzed by Mantel's association test.

RESULTS

Genewise hierarchical clustering revealed two subgroups with coordinate expression pattern in both the HCC cell lines and tissues: ROBO1, ROBO2, SLIT1 in one cluster, and ROBO4, SLIT2, SLIT3 in the other, respectively. Moreover, SLIT-ROBO expression predicted AFP-dependent subgrouping of HCC cell lines, but not that of liver tissues. ROBO1 and ROBO2 were significantly up-regulated, whereas SLIT3 was significantly down-regulated in cell lines with high-AFP background. When compared to normal liver tissue, ROBO1 was found to be significantly overexpressed, while ROBO4 was down-regulated in HCC. We also observed that ROBO1 and SLIT2 differentiated histopathological subgroups of liver tissues depending on both tumor staging and differentiation status. However, ROBO4 could discriminate poorly differentiated HCC from other subgroups.

CONCLUSION

The present study is the first in comprehensive and quantitative evaluation of SLIT-ROBO family gene expression in HCC, and suggests that the expression of SLIT-ROBO genes is regulated in hepatocarcinogenesis. Our results implicate that SLIT-ROBO transcription profile is bi-modular in nature, and that each module shows intrinsic variability. We also provide quantitative evidence for potential use of ROBO1, ROBO4 and SLIT2 for prediction of tumor stage and differentiation status.

The malignant phenotype of breast cancer cells is reduced by COX-2 silencing

The cyclooxygenase (COX) pathway is currently targeted for therapeutic intervention in different cancers. We have previously shown that silencing of COX-2 in the poorly differentiated metastatic breast cell line MDA-MB-231 by RNA interference markedly delayed tumor onset and inhibited metastasis. To understand the functional effects of COX-2 silencing underlying the inhibition of tumor growth and metastasis previously reported, we investigated changes in these cells for a number of cancer-associated phenotypes. Cyclooxygenase-2-silenced cells were less able to acidify tissue culture medium, a response that could partly be attributed to decreased lactate production or export detected by reduced lactate in the medium. Consistent with the significantly reduced transcript levels of hyaluronan synthase 2, an enzyme responsible for the total level of hyaluronan secreted by these cells, COX-2 silencing resulted in lower hyaluronan levels secreted in culture medium. Inhibition of human umbilical vein endothelial cell network association in a coculture assay was also observed in COX-2-silenced cells. These data highlight the functional role of COX-2 in pathways that mediate increased malignancy.

SLITs suppress tumor growth in vivo by silencing Sdf1/Cxcr4 within breast epithelium

The genes encoding Slits and their Robo receptors are silenced in many types of cancer, including breast, suggesting a role for this signaling pathway in suppressing tumorigenesis. The molecular mechanism underlying these tumor-suppressive effects has not been delineated. Here, we show that loss of Slits, or their Robo1 receptor, in murine mammary gland or human breast carcinoma cells results in coordinate up-regulation of the Sdf1 and Cxcr4 signaling axis, specifically within mammary epithelium. This is accompanied by hyperplastic changes in cells and desmoplastic alterations in the surrounding stroma. A similar inverse correlation between Slit and Cxcr4 expression is identified in human breast tumor tissues. Furthermore, we show in a xenograft model that Slit overexpression down-regulates CXCR4 and dominantly suppresses tumor growth. These studies classify Slits as negative regulators of Sdf1 and Cxcr4 and identify a molecular signature in hyperplastic breast lesions that signifies inappropriate up-regulation of key prometastatic genes.

Novel regulated expression of the SLIT/ROBO pathway in the ovary: possible role during luteolysis in women

The human corpus luteum (CL) undergoes luteolysis, associated with marked tissue and vascular remodeling, unless conception occurs and the gland is rescued by human chorionic gonadotropin (hCG). In Drosophila the Slit gene product, a secreted glycoprotein, acts as a ligand for the roundabout (robo) transmembrane receptor. Together they influence the guidance and migration of neuronal and nonneuronal cells. In vertebrates three Slit (Slit1, Slit2, Slit3) and four Robo (Robo1, Robo2, Robo3/Rig-1, Robo4/Magic Robo) genes have been identified. ROBO1, SLIT2, and SLIT3 are also inactivated in human cancers and may regulate apoptosis and metastasis. Because processes such as apoptosis and tissue remodeling occur during the regression of the CL, the aim of this study was to investigate the expression, regulation, and effects of the SLIT and ROBO genes in human luteal cells. Immunohistochemistry and RT-PCR revealed that SLIT2, SLIT3, ROBO1, and ROBO2 are expressed in luteal steroidogenic cells and fibroblast-like cells of the human CL. Furthermore, using real-time quantitative PCR, expression of SLIT2, SLIT3, and ROBO2 was maximal in the late-luteal phase and significantly reduced after luteal rescue in vivo with exogenous hCG (P<0.05). Additionally, hCG significantly inhibited SLIT2, SLIT3, and ROBO2 expression in cultured luteinized granulosa cells (P<0.05). Blocking SLIT-ROBO activity increased migration and significantly decreased levels of apoptosis in primary cultures of luteal cells (P<0.05). Overall, these results suggest the SLIT/ROBO pathway could play an important role in luteolysis in women.

Epidermal growth factor promotes breast cancer cell chemotaxis in CXCL12 gradients

The chemokine receptor CXCR4 and its ligand CXCL12 play an important role in breast cancer invasion and metastasis, and induce the chemotaxis of various types of cancer cells. Previous studies of CXCL12-induced chemotaxis have, for the most part, relied on endpoint assays (e.g., transwell assays) that provide poor control over the cell microenvironment. Specifically, these assays lacked the ability to dissect the role that autocrine and paracrine growth factors play in chemokine-induced cancer cell chemotaxis. Here, we employ a microfluidic chemotaxis chamber that allows the effects of specific exogenous factors on cell migration to be directly characterized, without the interference of autocrine/paracrine signaling. Using this approach, we investigated the migration of MDA-MB-231 breast cancer cells in well-defined CXCL12 gradients. We found that CXCL12 alone failed to stimulate chemotaxis of these cells; however, when the CXCL12 gradient was supplemented with a uniform stimulus of either EGF or conditioned media, a directional response was induced. This dependence on growth factor signaling points to the importance of autocrine and paracrine factors in determining the migratory response of the cells, and may play an important role in cancer metastasis.

Slit-2 induces a tumor-suppressive effect by regulating beta-catenin in breast cancer cells

SLIT-2 is considered as a candidate tumor suppressor gene, because it is frequently inactivated in various cancers due to hypermethylation of its promoter region and allelic loss. However, the exact mechanism of its tumor-suppressive effect has not been elucidated. Here, we observed that Slit-2-overexpressing breast cancer cells exhibited decreased proliferation and migration capabilities compared with control cells under in vitro conditions. These results were confirmed in vivo in mouse model systems. Mice injected with MCF-7/Slit-2 cells showed a 60-70% reduction in tumor size compared with mice injected with MCF-7/VC cells both in the absence and presence of estrogen. Upon further elucidation, we observed that Slit-2 mediates the tumor-suppressive effect via a coordinated regulation of the beta-catenin and PI3K signaling pathways and by enhancing beta-catenin/E-cadherin-mediated cell-cell adhesion. Our study for the first time reveals that Slit-2-overexpressing breast cancer cells exhibit tumor suppressor capabilities through the novel mechanism of beta-catenin modulation.

Predicting prognosis using molecular profiling in estrogen receptor-positive breast cancer treated with tamoxifen

Background

Estrogen receptor positive (ER+) breast cancers (BC) are heterogeneous with regard to their clinical behavior and response to therapies. The ER is currently the best predictor of response to the anti-estrogen agent tamoxifen, yet up to 30–40% of ER+BC will relapse despite tamoxifen treatment. New prognostic biomarkers and further biological understanding of tamoxifen resistance are required. We used gene expression profiling to develop an outcome-based predictor using a training set of 255 ER+ BC samples from women treated with adjuvant tamoxifen monotherapy. We used clusters of highly correlated genes to develop our predictor to facilitate both signature stability and biological interpretation. Independent validation was performed using 362 tamoxifen-treated ER+ BC samples obtained from multiple institutions and treated with tamoxifen only in the adjuvant and metastatic settings.

Results

We developed a gene classifier consisting of 181 genes belonging to 13 biological clusters. In the independent set of adjuvantly-treated samples, it was able to define two distinct prognostic groups (HR 2.01 95%CI: 1.29–3.13; p = 0.002). Six of the 13 gene clusters represented pathways involved in cell cycle and proliferation. In 112 metastatic breast cancer patients treated with tamoxifen, one of the classifier components suggesting a cellular inflammatory mechanism was significantly predictive of response.

Conclusion

We have developed a gene classifier that can predict clinical outcome in tamoxifen-treated ER+ BC patients. Whilst our study emphasizes the important role of proliferation genes in prognosis, our approach proposes other genes and pathways that may elucidate further mechanisms that influence clinical outcome and prediction of response to tamoxifen.

Slit2 involvement in glioma cell migration is mediated by Robo1 receptor

Slit and Robo proteins are evolutionarily conserved molecules whose interaction underlies axon guidance and neuronal precursor cell migration. During development secreted Slit proteins mediate chemorepulsive signals on cells expressing Robo receptors. Because similar molecular mechanisms may be utilized in glioma cell invasion and neuroblast migration, we studied the expression of Slit2 and its transmembrane receptor Robo1 as well as their functional role in migration in glioma cells. qRT-PCR and immunohistochemistry of human specimens revealed that Slit2 was distinctly expressed by non-neoplastic neurons, but at only very low levels in fibrillary astrocytoma and glioblastoma. Robo1 also was mainly restricted to neurons in the normal brain, whereas astrocytic tumor cells in situ as well as glioblastoma cell lines overexpressed Robo1 at mRNA and protein levels. Recombinant human Slit2 in a concentration of 0.45 nM was repulsive for glioma cell lines in a modified Boyden chamber assay. RNAi-mediated knockdown of Robo1 in glioma cell lines neutralized the repulsive effect of Slit2, demonstrating that Robo1 served as the major Slit2 receptor. Our findings suggest that a chemorepulsive effect mediated by interaction of Slit2 and Robo1 participates in glioma cell guidance in the brain.

Slits and Roundabouts in cancer, tumour angiogenesis and endothelial cell migration

Angiogenesis describes the development of new blood vessels from pre-existing vessels. The hijacking of this physiological process by tumours allows them to develop their own supplies of nutrients and oxygen, enabling their growth and metastasis. A large body of literature has accumulated over the last 20 years relating to angiogenesis, including signalling pathways involved in this process. One such pathway uses Slit-Roundabout proteins that are implicated in the development of cancers and tumour angiogenesis. The Roundabout family of receptors are large, single-pass transmembrane cell surface receptors involved in directing cell migration in response to their cognate Slit ligands. Although best known for their role in neuronal development, Slits and Roundabouts have now been implicated in myogenesis, leukocyte chemotaxis and tumour angiogenesis, confirming that the Robo signalling pathway functions across multiple cell types. We review here the evidence for a role for Slits and Roundabouts in cancer. In particular, we focus on the role of Robo1 and Robo4 in tumour angiogenesis and discuss the signalling pathways downstream of these proteins mediating endothelial cell migration.

The neuronal guidance cue Slit2 induces targeted migration and may play a role in brain metastasis of breast cancer cells

Cell migration is essential in many diverse processes ranging from embryonic development to wound healing and immune response. Cancer cells have recently been shown to utilize chemoattraction mechanisms mediated by chemokines and their respective receptors, e.g., the CXCL12/CXCR4 pathway normally found in leukocytes. Here we show that Slit2, a secreted protein signaling through the Roundabout (Robo) receptor as a chemorepellent in axon guidance and neuronal migration, acts as a potent chemoattractant for breast cancer cells. Comparing cell lines specifically metastasizing to either brain or bone, we found significant differences in their responses to CXCL12 and Slit2 treatments, suggesting a role for Slit/Robo signaling in brain metastasis.

Stromal-cell derived factor-1 regulates epithelial ovarian cancer cell invasion by activating matrix metalloproteinase-9 and matrix metalloproteinase-2

The aim of this study was to test the hypothesis that stromal-cell derived factor-1 (SDF-1)/chemokine receptor type 4 (CXCR4)-mediated metastasis of ovarian cancer cells to peritoneal cavity would be related with matrix metalloproteinase-9 (MMP-9) and matrix metalloproteinase-2 (MMP-2) activation. Invasion assay was used to evaluate the functional interaction between SDF-1 and CXCR4. Real-time polymerase chain reaction was performed to analyze the changes of CXCR4, MMP-2 and MMP-9 at the mRNA level upon SDF-1 stimulation. Western blot was used for observing the changes of CXCR4 protein expression and zymograph assay for comparing MMP-2 and MMP-9 activities after SDF-1 induction. In ovarian cancer cells, SDF-1 stimulation resulted in increased CXCR4 expression at both the transcription and protein levels. SDF-1 increased cell invasion in a CXCR4-dependent manner, and this increase was related with the activation of MMPs. Moreover, SDF-1-CXCR4 interaction activated MMP-9 at the both transcription and protein levels and MMP-2 only at the post-translation level. SDF-1-CXCR4 interaction confers on ovarian cancer cells a remarkable potential to activate MMPs for subsequently invading the peritoneal cavity.

Organ selectivity in metastasis: regulation by chemokines and their receptors

Cancer metastasis results from a non-random process, in which organ selectivity by the tumor cells is largely determined by factors that are expressed at the remote organs that eventually turn into preferred sites of metastasis formation. These factors support the consecutive steps required for metastasis formation, including tumor cell adhesion to microvessel walls, extravasation into target tissue and migration. Of the different components that regulate organ selectivity, instrumental roles were recently attributed to chemokines and their receptors. The present review presents the rationale standing behind the first studies looking at the potential involvement of chemokine-related components in organ selectivity. Based on these studies and many others that followed, the current paradigm is that chemokines that are expressed at specific organs determine to large extent organ specificity by promoting tumor cell adhesion to microvessel walls, by facilitating processes of extravasation into the target tissue and by inducing tumor cell migration. Moreover, chemokines can possibly support additional steps that are required for "successful" establishment of metastases, such as tumor cell proliferation and survival. The review focuses on the CXCL12-CXCR4 pair as the role model in our current understanding of chemokine involvement in organ selectivity. This review also describes the prominent roles played by CCR7 and its corresponding chemokine ligands (CCL21, CCL19) in lymph node metastasis, and of the CCR10-CCL27 axis in melanoma skin survival and metastasis. Overall, the present discussion describes chemokines as important constituents of the tumor microenvironment at metastatic sites, dictating directionality of chemokine receptor-expressing tumor cells, facilitating their adhesion and extravasation, and eventually contributing to organ selectivity.

Structural insights into the Slit-Robo complex

Slits are large multidomain leucine-rich repeat (LRR)-containing proteins that provide crucial guidance cues in neuronal and vascular development. More recently, Slits have been implicated in heart morphogenesis, angiogenesis, and tumor metastasis. Slits are ligands for the Robo (Roundabout) receptors, which belong to the Ig superfamily of transmembrane signaling molecules. The Slit-Robo interaction is mediated by the second LRR domain of Slit and the two N-terminal Ig domains of Robo, but the molecular details of this interaction and how it induces signaling remain unclear. Here we describe the crystal structures of the second LRR domain of human Slit2 (Slit2 D2), the first two Ig domains of its receptor Robo1 (Ig1-2), and the minimal complex between these proteins (Slit2 D2-Robo1 Ig1). Slit2 D2 binds with its concave surface to the side of Ig1 with electrostatic and hydrophobic contact regions mediated by residues that are conserved in other family members. Surface plasmon resonance experiments and a mutational analysis of the interface confirm that Ig1 is the primary domain for binding Slit2. These structures provide molecular insight into Slit-Robo complex formation and will be important for the development of novel cancer therapeutics.

Slit-2/Robo-1 modulates the CXCL12/CXCR4-induced chemotaxis of T cells

Slit, which mediates its function by binding to the Roundabout (Robo) receptor, has been shown to regulate neuronal, dendritic, and leukocyte migration. However, the molecular mechanism by which the Slit/Robo complex inhibits the migration of cells is not well defined. Here, we showed that Slit-2 can inhibit the CXCL12-induced chemotaxis and transendothelial migration of T cells and monocytes. We observed that CXCR4 associates with Robo-1 and that Slit-2 treatment enhances this association with the Robo-1 receptor. Robo-1 is a single-pass transmembrane receptor whose intracellular region contains four conserved motifs designated as CC0, CC1, CC2, and CC3. Structural and functional analyses of Robo receptors revealed that interaction of the CC3 motif with the CXCR4 receptor may regulate the CXCL12-induced chemotaxis of T cells. We further characterized Slit-2-mediated inhibition of the CXCL12/CXCR4 chemotactic pathway and found that Slit-2 can block the CXCL12-induced activation of the Src and Lck kinases but not Lyn kinase. Although Slit-2 did not inhibit the CXCL12-induced activation of MAPKs, it did inhibit the Akt phosphorylation and Rac activation induced by this chemokine. Altogether, our studies indicate a novel mechanism by which the Slit/Robo complex may inhibit the CXCR4/CXCL12-mediated chemotaxis of T cells.

Dynamic stromal-epithelial interactions during progression of MCF10DCIS.com xenografts

MCF10DCIS.com cells form comedo type ductal carcinoma in situ in immune-deficient mice before forming invasive ductal carcinoma. As the lesions mature, both stromal and epithelial cells undergo phenotypic changes detected by immunohistochemistry. Myofibroblasts are present before the formation of carcinoma in situ and after development of invasive carcinoma. MCF10DCIS. com lesions develop a myoepithelial layer prior to exhibiting a basement membrane surrounding the ductal mass. TGFbeta1 is initially expressed by the epithelial cells but is expressed by stroma in invasive carcinoma. Stromal derived factor-1 is detected in epithelial cells in early carcinoma in situ but is produced in stromal cells in invasive carcinoma. The receptor CXCR4 is expressed by epithelial cells in the xenografts at all times, as is the hepatocyte growth factor receptor c-met. MCF10DCIS.com xenografts illustrate the dynamic interplay of epithelium and stroma in the development of carcinoma in situ and subsequent invasive carcinoma. Although the phenotype of the epithelial cells may be dependent upon the stroma, the malignant epithelium induces the development of the stroma necessary for progression to the invasive stage. (c) 2007 Wiley-Liss, Inc.

Promoter hypermethylation of p16INK4A, p14ARF, CyclinD2 and Slit2 in serum and tumor DNA from breast cancer patients

Epigenetic mechanisms such as DNA methylation play important role in cancer. Epigenetic alterations involved in the onset and progression of breast cancer may serve as biomarkers for early detection and prediction of disease prognosis. Furthermore, using body fluids such as serum offers a non-invasive method to procure multiple samples for biomarker analyses. The aim of this study is to determine the correlation between methylation status of multiple cancer genes, p16(INK4A), p14(ARF), Cyclin D2 and Slit2 in invasive ductal carcinoma of the breast and paired serum DNA and clinicopathological parameters. Of the 36 breast cancer patients investigated, 31 (86%) tumors and 30 (83%) paired sera showed methylation of at least one of these 4 genes. Methylation frequencies varied from 27% for CyclinD2, 44% for p16(INK4A), 47% for p14(ARF) to 58% for Slit2. There was concordance between DNA methylation in tumor and paired serum DNA of each gene. This study underscores the potential utility of DNA methylation based screening of serum as a surrogate marker for tumor DNA methylation status of these genes in breast cancer. Further, expression profile of p16(INK4A) could be linked to epigenetic events, thus suggesting this pathway as a potential target for therapeutic strategies based on reversal of epigenetic silencing.

Intravascular cell-to-cell adhesive interactions and bone metastasis

Metastatic cancer spread to bones, causing intractable pain, pathological fractures, spinal cord compression, and ultimately death, represents massive clinical problem. Intravascular cell-to-cell heterotypic (between cancer and other types of cells) and homotypic (between cancer cells) adhesive interactions, leading to the establishment of metastatic deposits in bone marrow vasculature, represent important rate-limiting steps in bone metastasis. In this review, we discuss molecular and cellular mechanisms underpinning metastasis-associated intravascular cell-to-cell adhesive interactions, their role in a multi-step metastatic cascade, and a potential for therapeutic targeting of early metastasis-associated adhesive events.

Inhibition of medulloblastoma cell invasion by Slit

Invasion of brain tumor cells has made primary malignant brain neoplasms among the most recalcitrant to therapeutic strategies. We tested whether the secreted protein Slit2, which guides the projection of axons and developing neurons, could modulate brain tumor cell invasion. Slit2 inhibited the invasion of medulloblastoma cells in a variety of in vitro models. The effect of Slit2 was inhibited by the Robo ectodomain. Time-lapse videomicroscopy indicated that Slit2 reduced medulloblastoma invasion rate without affecting cell direction or proliferation. Both medulloblastoma and glioma tumors express Robo1 and Slit2, but only medulloblastoma invasion is inhibited by recombinant Slit2 protein. Downregulation of activated Cdc42 may contribute to this differential response. Our findings reinforce the concept that neurodevelopmental cues such as Slit2 may provide insights into brain tumor invasion.

Interaction of the guidance molecule Slit with cellular receptors

Slits are large secreted glycoproteins characterized by an unusual tandem of four LRR (leucine-rich repeat) domains in their N-terminal half. Slit proteins were initially described as repulsive guidance cues in neural development, but it has become clear that they have additional important functions, for instance in the vasculature and immune system. Genetic studies have identified two types of cellular receptors for Slits: Robos (Roundabout) and the HS (heparan sulphate) proteoglycan syndecan. The intracellular signalling cascade downstream of Robo activation is slowly being elucidated, but the mechanism of transmembrane signalling by Robo has remained obscure. No active signalling role for syndecan has yet been demonstrated. Slit-HS interactions may be important for shaping the presumed Slit gradient or presenting Slit at its target cell surface. Recent studies have mapped the binding sites for Robos and HS/heparin to discrete Slit domains. Robos bind to the second LRR domain of Slit, whereas HS/heparin binds with very high affinity to the C-terminal portion of Slit. Slit activity is likely to be modulated by physiological proteolytic cleavage in the region separating the Robo and HS/heparin-binding sites.

Differential expression of novel potential regulators in hematopoietic stem cells

The hematopoietic system is an invaluable model both for understanding basic developmental biology and for developing clinically relevant cell therapies. Using highly purified cells and rigorous microarray analysis we have compared the expression pattern of three of the most primitive hematopoietic subpopulations in adult mouse bone marrow: long-term hematopoietic stem cells (HSC), short-term HSC, and multipotent progenitors. All three populations are capable of differentiating into a spectrum of mature blood cells, but differ in their self-renewal and proliferative capacity. We identified numerous novel potential regulators of HSC self-renewal and proliferation that were differentially expressed between these closely related cell populations. Many of the differentially expressed transcripts fit into pathways and protein complexes not previously identified in HSC, providing evidence for new HSC regulatory units. Extending these observations to the protein level, we demonstrate expression of several of the corresponding proteins, which provide novel surface markers for HSC. We discuss the implications of our findings for HSC biology. In particular, our data suggest that cell–cell and cell–matrix interactions are major regulators of long-term HSC, and that HSC themselves play important roles in regulating their immediate microenvironment.

Hematopoietic, or blood-forming, stem cells (HSC) are responsible for the continual replenishment of all blood cells throughout life. This ability to both renew themselves and give rise to expanded populations of differentiating and mature cells is a hallmark of stem cells and is therefore an area of intense research. The rarity of HSC as well as their location in the bone marrow environment has made it difficult to identify the genes that regulate these properties. The earliest stages of blood development begins with the long-term (LT) repopulating HSC that then differentiate into short-term (ST) repopulating HSC and non-self renewing multipotent progenitors (MPP). The authors investigated the gene expression differences in these highly purified populations that differ mainly in their capacity to self renew, and identified a number of genes specific to each of these populations. Intriguingly, many of these genes code for proteins that are involved in cell–cell and cell–matrix interactions that were not previously identified on these populations. These novel discoveries will, together with future experiments, enhance our understanding of the basic biology of stem cells and their clinical uses.

Promoter hypermethylation-mediated inactivation of multiple Slit-Robo pathway genes in cervical cancer progression

BACKGROUND

Cervical Cancer (CC) exhibits highly complex genomic alterations. These include hemizygous deletions at 4p15.3, 10q24, 5q35, 3p12.3, and 11q24, the chromosomal sites of Slit-Robo pathway genes. However, no candidate tumor suppressor genes at these regions have been identified so far. Slit family of secreted proteins modulates chemokine-induced cell migration of distinct somatic cell types. Slit genes mediate their effect by binding to its receptor Roundabout (Robo). These genes have shown to be inactivated by promoter hypermethylation in a number of human cancers.

RESULTS

To test whether Slit-Robo pathway genes are targets of inactivation at these sites of deletion, we examined promoter hypermethylation of SLIT1, SLIT2, SLIT3, ROBO1, and ROBO3 genes in invasive CC and its precursor lesions. We identified a high frequency of promoter hypermethylation in all the Slit-Robo genes resulting in down regulated gene expression in invasive CC, but the inhibitors of DNA methylation and histone deacetylases (HDACs) in CC cell lines failed to effectively reactivate the down-regulated expression. These results suggest a complex mechanism of inactivation in the Slit-Robo pathway in CC. By analysis of cervical precancerous lesions, we further show that promoter hypermethylation of Slit-Robo pathway occurs early in tumor progression.

CONCLUSION

Taken together, these findings suggest that epigenetic alterations of Slit-Robo pathway genes (i) play a role in CC development, (ii) further delineation of molecular basis of promoter methylation-mediated gene regulation provides a potential basis for epigenetic-based therapy in advanced stage CC, and (iii) form epigenetic signatures to identify precancerous lesions at risk to progression.

CXCL12/CXCR4 signaling activates Akt-1 and MMP-9 expression in prostate cancer cells: the role of bone microenvironment-associated CXCL12

BACKGROUND

Hematopoietic cells home to bone by means of chemo-attraction to marrow chemokines, and interaction of chemokines with their receptors leads to the expression/activation of adhesion molecules and proteases. Recent evidence suggests that similar mechanisms may be active in cancer metastasis. Previously, we showed that metalloproteases (MMPs), and in particular MMP-9, play a role in prostate cancer (PC) expansion in bone.

METHODS

We used a variety of methods including RT-PCR, immunohistochemistry, ELISA, gelatin zymography, cellular motility and invasion, and subcellular fractionation of PC cells applied to in vivo and in vitro models.

RESULTS

Here we showed that (a) CXCL12/CXCR4 axis is expressed in PC bone metastasis; (b) exogenous CXCL12 induced MMP-9 expression by PC cells; (c) bone stromal cells and bone tissue conditioned media induced the migration of PC cells in a CXCR4-dependent manner; (d) pharmacological inhibition of PI3 kinase and MAP kinase pathways abrogated CXCL12-induced MMP-9 expression and invasion of PC cells; (e) exogenous CXCL12 induced Akt1 phosphorylation is indispensable for proMMP-9 secretion, migration, and invasion of PC cells; (f) CXCR4 was localized to lipid rafts in PC cells and initiated Akt phosphorylation.

CONCLUSIONS

These data suggest that chemoattractive mechanisms involve migration of cancer cells towards bone tissue, and that cell signaling induced by binding of the chemokine to its receptor leads to the activation of multiple signaling pathways and subsequent secretion of MMP-9 into the local environment. These findings provide a link between chemoattractive mechanisms, growth of tumor cells in bone, and tumor-enhanced bone matrix turnover.

The brain within the tumor: new roles for axon guidance molecules in cancers

Slits, semaphorins and netrins are three families of proteins that can attract or repel growing axons and migrating neurons in the developing nervous system of vertebrates and invertebrates. Recent studies have shown that they are widely expressed outside the nervous system and that they may play important roles in cancers. Several of the genes encoding these proteins are localized on chromosomal region associated with frequent loss-of-heterozygosity in tumors and cancer cell lines and there is also significant hypermethylation of their promoter suggesting that they may act as tumor suppressors. In addition, proteins in all these families and their receptors appear to control the vascularization of the tumors. Last, many axon guidance molecules also regulate cell migration and apoptosis in normal and tumorigenic tissues. Overall, this suggests that molecules that could mimick or block the activity of axon guidance molecules may be used as therapeutic agents for the treatment of malignancy.

Netrin-1 inhibits leukocyte migration in vitro and in vivo

Cell migration plays important roles in embryonic development and inflammation, and this process is highly regulated to ensure tissue homeostasis. A number of barriers exist to prevent the inappropriate migration of leukocytes into healthy peripheral tissues, including retention of these cells in the inactive state and maintenance of the integrity and charge of the vascular endothelium. However, active signals also are likely to exist that can repulse cells or abolish existing cell migration. One such paradigm exists in the developing nervous system, where neuronal migration is mediated by a balance between chemoattractive and chemorepulsive signals. The ability of the guidance molecule netrin-1 to repulse or abolish attraction of neuronal cells expressing the UNC5b receptor makes it an attractive candidate for the regulation of inflammatory cell migration. Here, we show that netrin-1 is expressed on vascular endothelium, where it is regulated by infection and inflammatory cytokines. The netrin-1 receptor UNC5b is strongly expressed by leukocytes, upon which netrin-1 acts as a potent inhibitor of migration to different chemotactic stimuli both in vivo and in vitro. These data suggest that endothelial expression of netrin-1 may inhibit basal cell migration into tissues and that its down-regulation with the onset of sepsis/inflammation may facilitate leukocyte recruitment.

Magic roundabout, a tumor endothelial marker: expression and signaling

Molecular signals that guide blood vessels to specific paths are not fully deciphered, but are thought to be similar to signals that mediate neuronal guidance. These cues are not only critical for normal blood vessel development, but may also play a major role in tumor angiogenesis. In this study, we have demonstrated the tumor endothelial specific expression of a Robo family member, magic roundabout (MRB), functionally characterized its role in endothelial cell migration and defined a signaling pathway that might mediate this function. We show that MRB is differentially over-expressed in tumor endothelial cells versus normal adult endothelial cells in numerous solid tumors. Moreover, over-expression of MRB in endothelial cells activates MRB in a ligand-independent fashion, and activation of MRB via Slit2, a putative ligand, results in inhibition of VEGF and FGF induced migration. We also demonstrate that MRB induced inhibition of endothelial migration is partially mediated by the Ras-Raf-Mek-Erk signaling pathway. We therefore hypothesize that expression of MRB is involved in regulating the migration of endothelial cells during tumor angiogenesis.

Functions of CXCL12 and CXCR4 in breast cancer

The chemokine CXCL12 (SDF-1) and its cognate receptor CXCR4 were first identified in the context of trafficking and homeostasis of immune cells, such as T lymphocytes. Subsequently, it has been determined that CXCR4 regulates several key processes in a wide variety of cancers. Functions of CXCL12 and CXCR4 in cancer first were described in metastatic breast cancer, and more recent studies also have identified roles for this signaling pathway in primary breast tumors. This review focuses on functions of CXCR4 and CXCL12 in primary and metastatic breast cancer, including molecular mechanisms of action and relationships of this pathway to other key regulators of breast cancer progression. We also describe pre-clinical studies indicating the potential to exploit CXCR4 as a new molecular target for diagnosis and treatment of breast cancer in patients.

Sémaphorines et cancers : État des lieux

Semaphorins, first described as axon guidance molecules, play an essential role in neural development, angiogenesis and immunological response. In 1996, two semaphorin genes, SEMA3B and SEMA3F, were isolated from chromosomal region 3p21.3 believed to contain a tumor suppressor gene based on frequent loss of heterozygosity in lung and breast cancer. Since these first studies, several semaphorins have been involved in tumor progression. Some semaphorins have been proposed to have pro-tumoral properties, whereas others have been shown to have tumor suppressive activity. This review summarizes the most recent data implicating semaphorins in cancers.

The p160 family coactivators regulate breast cancer cell proliferation and invasion through autocrine/paracrine activity of SDF-1alpha/CXCL12

Estrogen receptors (ERs) regulate the transcription of genes involved in breast cancer cell proliferation, invasion and metastasis. In addition to ligand concentration, phosphorylation and coactivator/corepressor levels control ER-dependent transcription. In this study, we used MCF-7 breast cancer sublines with variable levels of the steroid receptor coactivator 1 (SRC-1) to investigate the importance of coactivator levels in basal and estrogen-inducible expression of SDF-1alpha/CXCL12, cathepsin D and cMyc. Basal expression of SDF-1alpha and cMyc but not of cathepsin D was substantially lower in a MCF-7 subline lacking SRC-1 ((MCF-7/p2) compared with MCF-7 sublines expressing SRC-1 (MCF-7/p1 and LCC2). Although estrogen efficiently induced SDF-1alpha in MCF-7/p1 cells, very little induction of this gene was observed in MCF-7/p2 cells. The absence of SRC-1 had no effect on estrogen-inducible expression cMyc and cathepsin D suggesting that coactivator levels determine the expression of only a subset of estrogen-regulated genes. Introduction of SRC-1, SRC-2/TIF-2 or SRC-3/AIB1 increased basal expression of SDF-1alpha in MCF-7/p2 cells. Consistent with the role of SDF-1alpha in mediating estrogen-induced proliferation, estrogen failed to increase proliferation of MCF-7/p2 cells. In matrigel invasion assays, conditioned media from MCF-7/p1 but not MCF-7/p2 cells increased invasion of cancer cells expressing metastasis-associated genes and CXCR4, the receptor for SDF-1alpha. These results suggest that coactivators control SDF-1alpha expression, which mediates estrogen-induced proliferation and invasion through autocrine and paracrine mechanisms, respectively. These results also provide a molecular explanation for recent observations linking co-overexpression of coactivators and her2/neu with poor prognosis: coactivators increase SDF-1alpha expression whereas her2/neu stabilize CXCR4 protein.

The axonal attractant Netrin-1 is an angiogenic factor

Blood vessels and nerves often follow parallel trajectories, suggesting that distal targets use common cues that induce vascularization and innervation. Netrins are secreted by the floor plate and attract commissural axons toward the midline of the neural tube. Here, we show that Netrin-1 is also a potent vascular mitogen. Netrin-1 stimulates proliferation, induces migration, and promotes adhesion of endothelial cells and vascular smooth muscle cells with a specific activity comparable to vascular endothelial growth factor and platelet-derived growth factor. Our evidence indicates that the netrin receptor, Neogenin, mediates netrin signaling in vascular smooth muscle cells, but suggests that an unidentified receptor mediates the proangiogenic effects of Netrin-1 on endothelial cells. Netrin-1 also stimulates angiogenesis in vivo and augments the response to vascular endothelial growth factor. Thus, we demonstrate that Netrin-1 is a secreted neural guidance cue with the unique ability to attract both blood vessels and axons, and suggest that other cues may also function as vascular endothelial growth factors.