EphA2 cleavage by MT1-MMP triggers single cancer cell invasion via homotypic cell repulsion

Immunohistochemical demonstration of EphA2 processing by MT1-MMP in invasive cutaneous squamous cell carcinoma

Erythropoietin-producing hepatocellular receptor-2 (EphA2) overexpression is prevalent in many types of human cancers, and it has been reported that high EphA2 expression is correlated with malignancy. Recent studies revealed that processing of EphA2 by cleaving off the N-terminal portion by membrane-type 1 matrix metalloproteinase (MT1-MMP) promotes invasion via stimulation of Ras in cancer cells in vitro. The objectives of this study were to investigate the presence and role of EphA2 processing in cutaneous squamous cell carcinoma (SCC) tissues. EphA2 (C-terminal and N-terminal) and MT1-MMP expression patterns and levels were analyzed immunohistochemically in SCC (n = 70) and Bowen disease (BD; n = 20). Levels of MT1-MMP and EphA2 expression were evaluated using digital image analysis. Proximity between MT1-MMP and EphA2 in cancer cells and its effect on EphA2 processing were investigated using a combination of in situ proximity ligation assay (PLA) and Western blotting. Immunohistochemical analyses showed that levels of EphA2 N-terminal expression were significantly lower than those of EphA2 C-terminal expression in SCC, whereas levels of EphA2 C- and N-terminal expression were similar in BD. Western blotting showed processed EphA2 fragments in human SCC tissues. Expression levels of MT1-MMP, EphA2, and processed EphA2 fragments were higher in SCC than BD. Proximity between MT1-MMP and EphA2 in SCC was demonstrated by in situ PLA. Our results suggest possible involvement of MT1-MMP processing of EphA2 in invasiveness of cutaneous SCC.

Downregulation of human intercellular adhesion molecule-1 attenuates the metastatic ability in human breast cancer cell lines

Intercellular adhesion molecule-1 (ICAM-1) is a cell surface glycoprotein that belongs to immunoglobulin superfamily and plays an important role in tumor cell expansion or metastasis. However, the detailed mechanisms of ICAM-1 in breast cancer remain unclear. In this study, we evaluated the expression level of ICAM-1 in breast cancer using tissue microarray and clinical tissue specimens by immunohistochemical method, and the results revealed that ICAM-1 is highly expressed in the breast cancer tissues. To investigate whether ICAM-1 can affect the metastasis ability in breast cancer, we knocked down ICAM-1 expression in breast cancer cell line MCF-7 by using lentivirus-mediated RNA interference (RNAi). As a result, we stably silenced ICAM-1 expression in MCF-7 cells by infection with lentivirus expressing green fluorescent protein (GFP), the change of metastatic ability of MCF-7 cells was assessed by wound-healing assay, Transwell assay or clone formation assay. Our results showed that silencing of ICAM-1 can inhibit the metastatic ability of MCF-7 cell lines in vitro significantly, and the decreased migration and invasion was accompanied by a reduction of MMP-14. These results implying that ICAM-1 might be involved in the progression of breast cancer metastasis and lentivirus-mediated silencing of ICAM-1 might be a potential therapeutic approach for the treatment of breast cancer.

Screening for tumor suppressors: Loss of ephrin receptor A2 cooperates with oncogenic KRas in promoting lung adenocarcinoma

Lung adenocarcinoma, a major form of non-small cell lung cancer, is the leading cause of cancer deaths. The Cancer Genome Atlas analysis of lung adenocarcinoma has identified a large number of previously unknown copy number alterations and mutations, requiring experimental validation before use in therapeutics. Here, we describe an shRNA-mediated high-throughput approach to test a set of genes for their ability to function as tumor suppressors in the background of mutant KRas and WT Tp53. We identified several candidate genes from tumors originated from lentiviral delivery of shRNAs along with Cre recombinase into lungs of Loxp-stop-Loxp-KRas mice. Ephrin receptorA2 (EphA2) is among the top candidate genes and was reconfirmed by two distinct shRNAs. By generating knockdown, inducible knockdown and knockout cell lines for loss of EphA2, we showed that negating its expression activates a transcriptional program for cell proliferation. Loss of EPHA2 releases feedback inhibition of KRAS, resulting in activation of ERK1/2 MAP kinase signaling, leading to enhanced cell proliferation. Intriguingly, loss of EPHA2 induces activation of GLI1 transcription factor and hedgehog signaling that further contributes to cell proliferation. Small molecules targeting MEK1/2 and Smoothened hamper proliferation in EphA2-deficient cells. Additionally, in EphA2 WT cells, activation of EPHA2 by its ligand, EFNA1, affects KRAS-RAF interaction, leading to inhibition of the RAS-RAF-MEK-ERK pathway and cell proliferation. Together, our studies have identified that (i) EphA2 acts as a KRas cooperative tumor suppressor by in vivo screen and (ii) reactivation of the EphA2 signal may serve as a potential therapeutic for KRas-induced human lung cancers.

SrGAP2-Dependent Integration of Membrane Geometry and Slit-Robo-Repulsive Cues Regulates Fibroblast Contact Inhibition of Locomotion

Migrating fibroblasts undergo contact inhibition of locomotion (CIL), a process that was discovered five decades ago and still is not fully understood at the molecular level. We identify the Slit2-Robo4-srGAP2 signaling network as a key regulator of CIL in fibroblasts. CIL involves highly dynamic contact protrusions with a specialized actin cytoskeleton that stochastically explore cell-cell overlaps between colliding fibroblasts. A membrane curvature-sensing F-BAR domain pre-localizes srGAP2 to protruding edges and terminates their extension phase in response to cell collision. A FRET-based biosensor reveals that Rac1 activity is focused in a band at the tip of contact protrusions, in contrast to the broad activation gradient in contact-free protrusions. SrGAP2 specifically controls the duration of Rac1 activity in contact protrusions, but not in contact-free protrusions. We propose that srGAP2 integrates cell edge curvature and Slit-Robo-mediated repulsive cues to fine-tune Rac1 activation dynamics in contact protrusions to spatiotemporally coordinate CIL.

Eph receptor and ephrin function in breast, gut, and skin epithelia

Epithelial cells are tightly coupled together through specialized intercellular junctions, including adherens junctions, desmosomes, tight junctions, and gap junctions. A growing body of evidence suggests epithelial cells also directly exchange information at cell-cell contacts via the Eph family of receptor tyrosine kinases and their membrane-associated ephrin ligands. Ligand-dependent and -independent signaling via Eph receptors as well as reverse signaling through ephrins impact epithelial tissue homeostasis by organizing stem cell compartments and regulating cell proliferation, migration, adhesion, differentiation, and survival. This review focuses on breast, gut, and skin epithelia as representative examples for how Eph receptors and ephrins modulate diverse epithelial cell responses in a context-dependent manner. Abnormal Eph receptor and ephrin signaling is implicated in a variety of epithelial diseases raising the intriguing possibility that this cell-cell communication pathway can be therapeutically harnessed to normalize epithelial function in pathological settings like cancer or chronic inflammation.

The role of proteases in regulating Eph/ephrin signaling

Proteases regulate a myriad of cell functions, both in normal and disease states. In addition to protein turnover, they regulate a range of signaling processes, including those mediated by Eph receptors and their ephrin ligands. A variety of proteases is reported to directly cleave Ephs and/or ephrins under different conditions, to promote receptor and/or ligand shedding, and regulate receptor/ligand internalisation and signaling. They also cleave other adhesion proteins in response to Eph-ephrin interactions, to indirectly facilitate Eph-mediated functions. Proteases thus contribute to Eph/ephrin mediated changes in cell-cell and cell-matrix interactions, in cell morphology and in cell migration and invasion, in a manner which appears to be tightly regulated by, and co-ordinated with, Eph signaling. This review summarizes the current literature describing the function and regulation of protease activities during Eph/ephrin-mediated cell signaling.

MMP14 Cleavage of VEGFR1 in the Cornea Leads to a VEGF-Trap Antiangiogenic Effect

PURPOSE

To determine the possible antiangiogenic effect of metalloproteinase (MMP) 14 cleavage of vascular endothelial growth factor receptor 1 (VEGFR1) in the cornea.

METHODS

Recombinant mouse (rm) VEGFR1 was incubated with various concentrations of recombinant MMP14 to examine proteolysis in vitro. The reaction mixture was analyzed by SDS-PAGE and stained with Coomassie blue. The fragments resulting from rmVEGFR1 cleavage by MMP14 were subjected to Edman degradation, and the amino acid sequences were aligned with rmVEGFR1 sequences. Surface plasmon resonance was used to determine the equilibrium dissociation constant (KD) between MMP14 and rmVEGFR1. The KD value of rmVEGFR1 and the 59.8-kDa cleavage product binding to VEGF-A₁₆₅ was also determined. Cell proliferation assays were performed in the presence of VEGF-A₁₆₅ plus the 59.8-kDa VEGFR1 fragment or VEGF-A₁₆₅ alone.

RESULTS

Matrix metalloproteinase 14 binds and cleaves rmVEGFR1 to produce 59.8-kDa (N-terminal fragment, Ig domains 1-5), 35-kDa (C-terminal fragment containing IgG and His-tag), and 21-kDa (Ig domains 6-7) fragments. The 59.8-kDa fragment showed binding to VEGF-A₁₆₅ and inhibited VEGF-induced endothelial cell mitogenesis.

CONCLUSIONS

Our findings suggest that VEGFR1 cleavage by MMP14 in the cornea leads to a VEGF-trap effect, reducing the proangiogenic effect of VEGF-A₁₆₅, thereby reducing corneal angiogenesis.

Proteolysis of EphA2 Converts It from a Tumor Suppressor to an Oncoprotein

Eph receptor tyrosine kinases are considered candidate therapeutic targets in cancer, but they can exert opposing effects on cell growth. In the presence of its ligands, Eph receptor EphA2 suppresses signaling by other growth factor receptors, including ErbB, whereas ligand-independent activation of EphA2 augments ErbB signaling. To deploy EphA2-targeting drugs effectively in tumors, the anti-oncogenic ligand-dependent activation state of EphA2 must be discriminated from its oncogenic ligand-independent state. Because the molecular basis for the latter is little understood, we investigated how the activation state of EphA2 can be switched in tumor tissue. We found that ligand-binding domain of EphA2 is cleaved frequently by the membrane metalloproteinase MT1-MMP, a powerful modulator of the pericellular environment in tumor cells. EphA2 immunostaining revealed a significant loss of the N-terminal portion of EphA2 in areas of tumor tissue that expressed MT1-MMP. Moreover, EphA2 phosphorylation patterns that signify ligand-independent activation were observed specifically in these areas of tumor tissue. Mechanistic experiments revealed that processing of EphA2 by MT1-MMP promoted ErbB signaling, anchorage-independent growth, and cell migration. Conversely, expression of a proteolysis-resistant mutant of EphA2 prevented tumorigenesis and metastasis of human tumor xenografts in mice. Overall, our results showed how the proteolytic state of EphA2 in tumors determines its effector function and influences its status as a candidate biomarker for targeted therapy.

MMP16 Mediates a Proteolytic Switch to Promote Cell-Cell Adhesion, Collagen Alignment, and Lymphatic Invasion in Melanoma

Lymphatic invasion and accumulation of continuous collagen bundles around tumor cells are associated with poor melanoma prognosis, but the underlying mechanisms and molecular determinants have remained unclear. We show here that a copy-number gain or overexpression of the membrane-type matrix metalloproteinase MMP16 (MT3-MMP) is associated with poor clinical outcome, collagen bundle assembly around tumor cell nests, and lymphatic invasion. In cultured WM852 melanoma cells derived from human melanoma metastasis, silencing of MMP16 resulted in cell-surface accumulation of the MMP16 substrate MMP14 (MT1-MMP) as well as L1CAM cell adhesion molecule, identified here as a novel MMP16 substrate. When limiting the activities of these trans-membrane protein substrates toward pericellular collagen degradation, cell junction disassembly, and blood endothelial transmigration, MMP16 supported nodular-type growth of adhesive collagen-surrounded melanoma cell nests, coincidentally steering cell collectives into lymphatic vessels. These results uncover a novel mechanism in melanoma pathogenesis, whereby restricted collagen infiltration and limited mesenchymal invasion are unexpectedly associated with the properties of the most aggressive tumors, revealing MMP16 as a putative indicator of adverse melanoma prognosis.

Eph receptors and ephrins: therapeutic opportunities

The erythropoietin-producing hepatocellular carcinoma (Eph) receptor tyrosine kinase family plays important roles in developmental processes, adult tissue homeostasis, and various diseases. Interaction with Eph receptor-interacting protein (ephrin) ligands on the surface of neighboring cells triggers Eph receptor kinase-dependent signaling. The ephrins can also transmit signals, leading to bidirectional cell contact-dependent communication. Moreover, Eph receptors and ephrins can function independently of each other through interplay with other signaling systems. Given their involvement in many pathological conditions ranging from neurological disorders to cancer and viral infections, Eph receptors and ephrins are increasingly recognized as attractive therapeutic targets, and various strategies are being explored to modulate their expression and function. Eph receptor/ephrin upregulation in cancer cells, the angiogenic vasculature, and injured or diseased tissues also offer opportunities for Eph/ephrin-based targeted drug delivery and imaging. Thus, despite the challenges presented by the complex biology of the Eph receptor/ephrin system, exciting possibilities exist for therapies exploiting these molecules.

The Eph tyrosine kinase receptors EphB2 and EphA2 are novel proteolytic substrates of tissue factor/coagulation factor VIIa

Tissue factor (TF) binds the serine protease factor VIIa (FVIIa) to form a proteolytically active complex that can trigger coagulation or activate cell signaling. Here we addressed the involvement of tyrosine kinase receptors (RTKs) in TF/FVIIa signaling by antibody array analysis and subsequently found that EphB2 and EphA2 of the Eph RTK family were cleaved in their ectodomains by TF/FVIIa. We used N-terminal Edman sequencing and LC-MS/MS analysis to characterize the cleaved Eph isoforms and identified a key arginine residue at the cleavage site, in agreement with the tryptic serine protease activity of FVIIa. Protease-activated receptor 2 (PAR2) signaling and downstream coagulation activity was non-essential in this context, in further support of a direct cleavage by TF/FVIIa. EphB2 was cleaved by FVIIa concentrations in the subnanomolar range in a number of TF expressing cell types, indicating that the active cellular pool of TF was involved. FVIIa caused potentiation of cell repulsion by the EphB2 ligand ephrin-B1, demonstrating a novel proteolytical event to control Eph-mediated cell segregation. These results define Eph RTKs as novel proteolytical targets of TF/FVIIa and provide new insights into how TF/FVIIa regulates cellular functions independently of PAR2.

Eph- and ephrin-dependent mechanisms in tumor and stem cell dynamics

The erythropoietin-producing hepatocellular (Eph) receptors comprise the largest family of receptor tyrosine kinases (RTKs). Initially regarded as axon-guidance and tissue-patterning molecules, Eph receptors have now been attributed with various functions during development, tissue homeostasis, and disease pathogenesis. Their ligands, ephrins, are synthesized as membrane-associated molecules. At least two properties make this signaling system unique: (1) the signal can be simultaneously transduced in the receptor- and the ligand-expressing cell, (2) the signaling outcome through the same molecules can be opposite depending on cellular context. Moreover, shedding of Eph and ephrin ectodomains as well as ligand-dependent and -independent receptor crosstalk with other RTKs, proteases, and adhesion molecules broadens the repertoire of Eph/ephrin functions. These integrated pathways provide plasticity to cell-microenvironment communication in varying tissue contexts. The complex molecular networks and dynamic cellular outcomes connected to the Eph/ephrin signaling in tumor-host communication and stem cell niche are the main focus of this review.

EphA4 receptor shedding regulates spinal motor axon guidance

BACKGROUND

Proteolytic processing of axon guidance receptors modulates their expression and functions. Contact repulsion by membrane-associated ephrins and Eph receptors was proposed to be facilitated by ectodomain cleavage, but whether this phenomenon is required for axon guidance in vivo is unknown.

RESULTS

In support of established models, we find that cleavage of EphA4 promotes cell-cell and growth cone-cell detachment in vitro. Unexpectedly, however, a cleavage resistant isoform of EphA4 is as effective as a wild-type EphA4 in redirecting motor axons in limbs. Mice in which EphA4 cleavage is genetically abolished have motor axon guidance defects, suggesting an important role of EphA4 cleavage in nonneuronal tissues such as the limb mesenchyme target of spinal motor neurons. Indeed, we find that blocking EphA4 cleavage increases expression of full-length EphA4 in limb mesenchyme, which-via cis-attenuation-apparently reduces the effective concentration of ephrinAs capable of triggering EphA4 forward signaling in the motor axons.

CONCLUSIONS

We propose that EphA4 cleavage is required to establish the concentration differential of active ephrins in the target tissue that is required for proper axon guidance. Our study reveals a novel mechanism to regulate guidance decision at an intermediate target based on the modulation of ligand availability by the proteolytic processing of the receptor.

Targeting tumor invasion: the roles of MDA-9/Syntenin

INTRODUCTION

Melanoma differentiation-associated gene - 9 (MDA-9)/Syntenin has become an increasingly popular focus for investigation in numerous cancertypes. Originally implicated in melanoma metastasis, it has diverse cellular roles and is consistently identified as a regulator of tumor invasion and angiogenesis. As a potential target for inhibiting some of the most lethal aspects of cancer progression, further insight into the function of MDA-9/Syntenin is mandatory.

AREAS COVERED

Recent literature and seminal articles were reviewed to summarize the latest collective understanding of MDA-9/Syntenin's role in normal and cancerous settings. Insights into its participation in developmental processes are included, as is the functional significance of the N- and C-terminals and PDZ domains of MDA-9/Syntenin. Current reports highlight the clinical significance of MDA-9/Syntenin expression level in a variety of cancers, often correlating directly with reduced patient survival. Also presented are assessments of roles of MDA-9/Syntenin in cancer progression as well as its functions as an intracellular adapter molecule.

EXPERT OPINION

Multiple studies demonstrate the importance of MDA-9/Syntenin in tumor invasion and progression. Through the use of novel drug design approaches, this protein may provide a worthwhile therapeutic target. As many conventional therapies do not address, or even enhance, tumor invasion, an anti-invasive approach would be a worthwhile addition in cancer therapy.

Actin-associated protein palladin promotes tumor cell invasion by linking extracellular matrix degradation to cell cytoskeleton

Basal-like breast carcinomas, characterized by unfavorable prognosis and frequent metastases, are associated with epithelial-to-mesenchymal transition. During this process, cancer cells undergo cytoskeletal reorganization and up-regulate membrane-type 1 matrix metalloproteinase (MT1-MMP; MMP14), which functions in actin-based pseudopods to drive invasion by extracellular matrix degradation. However, the mechanisms that couple matrix proteolysis to the actin cytoskeleton in cell invasion have remained unclear. On the basis of a yeast two-hybrid screen for the MT1-MMP cytoplasmic tail-binding proteins, we identify here a novel Src-regulated protein interaction between the dynamic cytoskeletal scaffold protein palladin and MT1-MMP. These proteins were coexpressed in invasive human basal-like breast carcinomas and corresponding cell lines, where they were associated in the same matrix contacting and degrading membrane complexes. The silencing and overexpression of the 90-kDa palladin isoform revealed the functional importance of the interaction with MT1-MMP in pericellular matrix degradation and mesenchymal tumor cell invasion, whereas in MT1-MMP-negative cells, palladin overexpression was insufficient for invasion. Moreover, this invasion was inhibited in a dominant-negative manner by an immunoglobulin domain-containing palladin fragment lacking the dynamic scaffold and Src-binding domains. These results identify a novel protein interaction that links matrix degradation to cytoskeletal dynamics and migration signaling in mesenchymal cell invasion.

EphA receptors regulate prostate cancer cell dissemination through Vav2-RhoA mediated cell-cell repulsion

Metastatic prostate cancer cells display EphB receptor-mediated attraction when they contact stromal fibroblasts but EphA-driven repulsion when they contact one another. The impact of these ‘social’ interactions between cells during cancer cell invasion and the signalling mechanisms downstream of Eph receptors are unclear. Here we show that EphA receptors regulate prostate cancer cell dissemination in a 2D dispersal assay and in a 3D cancer cell spheroid assay. We show that EphA receptors signal via the exchange factor Vav2 to activate RhoA and that both Vav2 and RhoA are required for prostate cancer cell–cell repulsion. Furthermore, we find that in EphA2/EphA4, Vav2 or RhoA siRNA-treated cells, contact repulsion can be restored by partial microtubule destabilisation. We propose that EphA–Vav2–RhoA-mediated repulsion between contacting cancer cells at the tumour edge could enhance their local invasion away from the primary tumour.

Involvement of plasminogen cascade proteins in the invasion of pancreatic cancer cells

AIM: To discuss the potential role of plasminogen cascade proteins in the dissociation and subsequent invasion of pancreatic cancer cells.

METHODS: The expression of plasminogen, urokinase type plasminogen activator (uPA) and uPA receptor (uPAR) was detected by Western blot and immunocytochemistry in cell lines and by immunohistochemistry in tissue samples of pancreatic cancer. The correlation between expression of plasminogen cascade proteins and cell dissociation and invasion was analyzed.

RESULTS: Plasminogen, uPA and uPAR were strongly expressed in conditioned medium of dissociated pancreatic cancer cells (PC-1.0), but weakly expressed in conditioned medium of non-dissociated pancreatic cancer cells (PC-1). uPA treatment significantly induced the expression of plasminogen and uPAR in conditioned medium of non-dissociated pancreatic cancer cells (PC-1). Stronger expression of plasminogen and uPAR was observed at the invasive front end than at the center of human pancreatic cancer tissue. Plasmin treatment induced matrix metalloproteinase-2 (MMP-2), MMP-7 and MMP-9 expression in PC-1 cells. Treatment with plasmin or uPA obviously induced invasiveness and dissociation of cell colonies in PC-1 cells.

CONCLUSION: The plasminogen cascade is involved in cell dissociation in the early stage of invasion of pancreatic cancer cells. The plasminogen cascade may be a potential molecular target for anti-invasion and anti-metastasis therapy for pancreatic cancer.