beta-1 Integrins mediate tumour cell adhesion to quiescent endothelial cells in vitro

In vitro Studies of Transendothelial Migration for Biological and Drug Discovery

Inflammatory diseases and cancer metastases lack concrete pharmaceuticals for their effective treatment despite great strides in advancing our understanding of disease progression. One feature of these disease pathogeneses that remains to be fully explored, both biologically and pharmaceutically, is the passage of cancer and immune cells from the blood to the underlying tissue in the process of extravasation. Regardless of migratory cell type, all steps in extravasation involve molecular interactions that serve as a rich landscape of targets for pharmaceutical inhibition or promotion. Transendothelial migration (TEM), or the migration of the cell through the vascular endothelium, is a particularly promising area of interest as it constitutes the final and most involved step in the extravasation cascade. While in vivo models of cancer metastasis and inflammatory diseases have contributed to our current understanding of TEM, the knowledge surrounding this phenomenon would be significantly lacking without the use of in vitro platforms. In addition to the ease of use, low cost, and high controllability, in vitro platforms permit the use of human cell lines to represent certain features of disease pathology better, as seen in the clinic. These benefits over traditional pre-clinical models for efficacy and toxicity testing are especially important in the modern pursuit of novel drug candidates. Here, we review the cellular and molecular events involved in leukocyte and cancer cell extravasation, with a keen focus on TEM, as discovered by seminal and progressive in vitro platforms. In vitro studies of TEM, specifically, showcase the great experimental progress at the lab bench and highlight the historical success of in vitro platforms for biological discovery. This success shows the potential for applying these platforms for pharmaceutical compound screening. In addition to immune and cancer cell TEM, we discuss the promise of hepatocyte transplantation, a process in which systemically delivered hepatocytes must transmigrate across the liver sinusoidal endothelium to successfully engraft and restore liver function. Lastly, we concisely summarize the evolving field of porous membranes for the study of TEM.

The vacuolar-type ATPase inhibitor archazolid increases tumor cell adhesion to endothelial cells by accumulating extracellular collagen

The vacuolar-type H⁺-ATPase (v-ATPase) is the major proton pump that acidifies intracellular compartments of eukaryotic cells. Since the inhibition of v-ATPase resulted in anti-tumor and anti-metastatic effects in different tumor models, this enzyme has emerged as promising strategy against cancer. Here, we used the well-established v-ATPase inhibitor archazolid, a natural product first isolated from the myxobacterium Archangium gephyra, to study the consequences of v-ATPase inhibition in endothelial cells (ECs), in particular on the interaction between ECs and cancer cells, which has been neglected so far. Human endothelial cells treated with archazolid showed an increased adhesion of tumor cells, whereas the transendothelial migration of tumor cells was reduced. The adhesion process was independent from the EC adhesion molecules ICAM-1, VCAM-1, E-selectin and N-cadherin. Instead, the adhesion was mediated by β1-integrins expressed on tumor cells, as blocking of the integrin β1 subunit reversed this process. Tumor cells preferentially adhered to the β1-integrin ligand collagen and archazolid led to an increase in the amount of collagen on the surface of ECs. The accumulation of collagen was accompanied by a strong decrease of the expression and activity of the protease cathepsin B. Overexpression of cathepsin B in ECs prevented the capability of archazolid to increase the adhesion of tumor cells onto ECs. Our study demonstrates that the inhibition of v-ATPase by archazolid induces a pro-adhesive phenotype in endothelial cells that promotes their interaction with cancer cells, whereas the transmigration of tumor cells was reduced. These findings further support archazolid as a promising anti-metastatic compound.

miR-720 is a downstream target of an ADAM8-induced ERK signaling cascade that promotes the migratory and invasive phenotype of triple-negative breast cancer cells

BACKGROUND

ADAM8 (a disintegrin and metalloproteinase 8) protein promotes the invasive and metastatic phenotype of triple-negative breast cancer (TNBC) cells. High ADAM8 expression in breast cancer patients is an independent predictor of poor prognosis. Here, we investigated whether ADAM8 regulates specific miRNAs, their roles in aggressive phenotype, and potential use as biomarkers of disease.

METHODS

Microarray analysis was performed on RNA from MDA-MB-231 cells after transient ADAM8 knockdown using TaqMan miRNA cards. Changes in miRNA levels were confirmed using two ADAM8 siRNAs in TNBC cell lines. Kinase inhibitors, β1-integrin antagonist antibody, and different forms of ADAM8 were employed to elucidate the signaling pathway required for miR-720 expression. miR-720 levels were modulated using a specific antagomiR or a mimic, and effects on aggressive phenotype of TNBC cells were determined using Boyden chamber and 3D-Matrigel outgrowth assays. Plasma was isolated from mice before and after implantation of MDA-MB-231 cells and analyzed for miR-720 levels. Serum samples of TNBC patients were evaluated for their ADAM8 and miR-720 levels.

RESULTS

We identified 68 miRNAs differentially regulated upon ADAM8 knockdown, including decreased levels of secreted miR-720. Ectopic overexpression of wild-type ADAM8 or forms that lack metalloproteinase activity similarly induced miR-720 levels. The disintegrin and cysteine-rich domains of ADAM8 were shown to induce miR-720 via activation of a β1-integrin to ERK signaling cascade. Knockdown of miR-720 led to a significant decrease in migratory and invasive abilities of TNBC cells. Conversely, miR-720 overexpression rescued these properties. A profound increase in plasma levels of miR-720 was detected 7 days after TNBC cell inoculation into mouse mammary fat pads when tumors were barely palpable. Concordantly, miR-720 levels were found to be significantly higher in serum samples of TNBC patients with high ADAM8 expression.

CONCLUSIONS

We have shown for the first time that miR-720 is induced by ADAM8 signaling via ERK and plays an essential role in promoting the aggressive phenotype of TNBCs. miR-720 is elevated in serum of patients with ADAM8-high TNBC and, in a group with other miRNAs downstream of ADAM8, holds promise as a biomarker for early detection of or treatment response of ADAM8-positive TNBCs.

When the endothelium scores an own goal: endothelial cells actively augment metastatic extravasation through endothelial-mesenchymal transition

Endothelial-mesenchymal transition (EndMT) is an important mechanism during organ development and in certain pathological conditions. For example, EndMT contributes to myofibroblast formation during organ fibrosis, and it has been identified as an important source of cancer-associated fibroblasts, facilitating tumor progression. Recently, EndMT was proposed to modulate endothelial function during intravasation and extravasation of metastatic tumor cells. Evidence suggests that endothelial cells are not passive actors during transendothelial migration (TEM) of cancer cells, as there are profound changes in endothelial junctional protein expression, signaling, permeability, and contractility. This review describes these alterations in endothelial characteristics during TEM of metastatic tumor cells and discusses them in the context of EndMT. EndMT could play an important role during metastatic intravasation and extravasation, a novel hypothesis that may lead to new therapeutic approaches to tackle metastatic disease.

Endothelial-mesenchymal transition of brain endothelial cells: possible role during metastatic extravasation

Cancer progression towards metastasis follows a defined sequence of events described as the metastatic cascade. For extravasation and transendothelial migration metastatic cells interact first with endothelial cells. Yet the role of endothelial cells during the process of metastasis formation and extravasation is still unclear, and the interaction between metastatic and endothelial cells during transendothelial migration is poorly understood. Since tumor cells are well known to express TGF-β, and the compact endothelial layer undergoes a series of changes during metastatic extravasation (cell contact disruption, cytoskeletal reorganization, enhanced contractility), we hypothesized that an EndMT may be necessary for metastatic extravasation. We demonstrate that primary cultured rat brain endothelial cells (BEC) undergo EndMT upon TGF-β1 treatment, characterized by the loss of tight and adherens junction proteins, expression of fibronectin, β1-integrin, calponin and α-smooth muscle actin (SMA). B16/F10 cell line conditioned and activated medium (ACM) had similar effects: claudin-5 down-regulation, fibronectin and SMA expression. Inhibition of TGF-β signaling during B16/F10 ACM stimulation using SB-431542 maintained claudin-5 levels and mitigated fibronectin and SMA expression. B16/F10 ACM stimulation of BECs led to phosphorylation of Smad2 and Smad3. SB-431542 prevented SMA up-regulation upon stimulation of BECs with A2058, MCF-7 and MDA-MB231 ACM as well. Moreover, B16/F10 ACM caused a reduction in transendothelial electrical resistance, enhanced the number of melanoma cells adhering to and transmigrating through the endothelial layer, in a TGF-β-dependent manner. These effects were not confined to BECs: HUVECs showed TGF-β-dependent SMA expression when stimulated with breast cancer cell line ACM. Our results indicate that an EndMT may be necessary for metastatic transendothelial migration, and this transition may be one of the potential mechanisms occurring during the complex phenomenon known as metastatic extravasation.

ADAM8 expression in invasive breast cancer promotes tumor dissemination and metastasis

The transmembrane metalloprotease-disintegrin ADAM8 mediates cell adhesion and shedding of ligands, receptors and extracellular matrix components. Here, we report that ADAM8 is abundantly expressed in breast tumors and derived metastases compared to normal tissue, especially in triple-negative breast cancers (TNBCs). Furthermore, high ADAM8 levels predicted poor patient outcome. Consistently, ADAM8 promoted an aggressive phenotype of TNBC cells in culture. In a mouse orthotopic model, tumors derived from TNBC cells with ADAM8 knockdown failed to grow beyond a palpable size and displayed poor vascularization. Circulating tumor cells and brain metastases were also significantly reduced. Mechanistically, ADAM8 stimulated both angiogenesis through release of VEGF-A and transendothelial cell migration via β1-integrin activation. In vivo, treatment with an anti-ADAM8 antibody from the time of cell inoculation reduced primary tumor burden and metastases. Furthermore, antibody treatment of established tumors profoundly decreased metastases in a resection model. As a non-essential protein under physiological conditions, ADAM8 represents a promising novel target for treatment of TNBCs, which currently lack targeted therapies and frequently progress with fatal dissemination.

αB-crystallin: a novel regulator of breast cancer metastasis to the brain

PURPOSE

Basal-like breast tumors are typically (ER/PR/HER2) triple-negative and are associated with a high incidence of brain metastases and poor clinical outcomes. The molecular chaperone αB-crystallin is predominantly expressed in triple-negative breast cancer (TNBC) and contributes to an aggressive tumor phenotype in preclinical models. We investigated the potential role of αB-crystallin in brain metastasis in TNBCs.

EXPERIMENTAL DESIGN

αB-crystallin expression in primary breast carcinomas and brain metastases was analyzed by immunohistochemistry among patients with breast cancer with brain metastases. αB-crystallin was overexpressed or silenced in two different TNBC cell lines. The effects on cell adhesion to human brain microvascular endothelial cells (HBMEC) or extracellular matrix proteins, transendothelial migration, and transmigration across a HBMEC/astrocyte coculture blood-brain barrier (BBB) model were examined. In addition, the effects of overexpressing or silencing αB-crystallin on brain metastasis in vivo were investigated using orthotopic TNBC models.

RESULTS

In a cohort of women with breast cancer brain metastasis, αB-crystallin expression in primary breast carcinomas was associated with poor overall survival and poor survival after brain metastasis, even among patients with TNBC. Stable overexpression of αB-crystallin in TNBC cells enhanced adhesion to HBMECs, transendothelial migration, and BBB transmigration in vitro, whereas silencing αB-crystallin inhibited these events. αB-crystallin promoted adhesion of TNBC cells to HBMECs, at least in part, through an α3β1 integrin-dependent mechanism. αB-crystallin overexpression promoted brain metastasis, whereas silencing αB-crystallin inhibited brain metastasis in orthotopic TNBC models.

CONCLUSION

αB-crystallin is a novel regulator of brain metastasis in TNBC and represents a potential biomarker and drug target for this aggressive disease.

Role of the protein tyrosine kinase Syk in regulating cell-cell adhesion and motility in breast cancer cells

The expression of the Syk protein tyrosine kinase in breast cancer cells is inversely correlated with invasive growth and metastasis. The expression of Syk inhibits cell motility while supporting the formation of cell clusters by enhancing cell-cell contacts and promoting the redistribution of the adhesion proteins cortactin and vinculin to these contacts. Syk associates physically with cortactin and catalyzes its phosphorylation on tyrosine. The clustering of integrins leads to the phosphorylation of Syk and of numerous cellular proteins in a manner dependent on the activity of the kinase and on the presence of tyrosine 342 located in the linker B region. The ability of Syk to participate in integrin-mediated protein tyrosine phosphorylation correlates well with its ability to inhibit cell motility.

NK4, an HGF antagonist, prevents hematogenous pulmonary metastasis by inhibiting adhesion of CT26 cells to endothelial cells

Hepatocyte growth factor (HGF) plays a definitive role in invasive, angiogenic, and metastatic activities of tumor cells by binding to the c-Met receptor. NK4, a competitive antagonist for HGF and the c-Met receptor, prevents tumor cell growth and metastasis via its bifunctional properties to act as an HGF antagonist and angiogenesis inhibitor. In the present study, we investigated the inhibitory effectiveness of NK4 on hematogenous pulmonary metastasis of the CT26 murine colon cancer cell line, focusing on tumor cell adhesion to endothelial cells. In an in vitro adhesion assay, HGF facilitated adhesion of CT26 cells to a murine endothelial cell line (F-2) in a dose-dependent manner. Furthermore, the enhancing effect of HGF on CT26-F-2 cell interaction was blocked by NK4 as well as by anti-HGF antibody. Similarly, HGF-induced phosphorylation of focal adhesion kinase (FAK), downstream of integrin signaling, was reduced by NK4 and by anti-HGF antibody. However, distinct integrin expression on the surface of CT26 cells was not altered by HGF. In an in vivo experimental pulmonary metastasis assay, stable NK4 expression potently decreased the number of pulmonary metastatic foci. The NK4-induced suppression of pulmonary metastasis was partially reversed when HGF was intraperitoneally administered in an adhesive phase. These results suggest that NK4 could act on tumor cells to inhibit CT26 adhesion to endothelial cells by reducing FAK phosphorylation, which is regulated by inside-out HGF/c-Met signaling, and thereby suppress hematogenous pulmonary metastasis.

Expression of ecto-5'-nucleotidase (eN, CD73) in cell lines from various stages of human melanoma

Ecto-5'-nucleotidase is a GPI-anchored enzyme localized in cell membrane lipid rafts. Although it is highly expressed in many tumour cells, its specific function during tumorigenesis is unclear. We have found that, among different melanoma cells, upregulated expression of ecto-5'-nucleotidase is associated with a highly invasive phenotype. Analysis of other cell membrane proteins involved in melanoma adhesion and metastasis demonstrated that expression of alpha5, beta1, beta3-integrin subunits and CD44 was elevated gradually in accordance with increasing metastatic potential. Expression of alphav-integrin and caveolin-1 was seen mostly in cells derived from metastatic melanomas. Furthermore, in contrast to N-cadherin, which was unaltered in all lines, we could not detect E-cadherin in any cell type. Functional assays demonstrated that highly expressed ecto-5'-nucleotidase is a catalytically competent protein that is very sensitive to inhibition by concanavalin A. The interaction with concanavalin A also caused increased association of ecto-5'-nucleotidase-rich lipid rafts with much heavier cytoskeletal complexes as determined by density gradient centrifugation. A similar shift towards heavier cytoskeletal fractions also took place with other proteins coexpressed with ecto-5'-nucleotidase, such as alphav, alpha5, beta1 and beta3-integrins, caveolin-1 and CD44. As ConA-induced clustering may reflect the interactions of membrane proteins with extracellular matrix, we also analysed the effect of several extracellular matrix proteins on the in-situ activity of ecto-5'-nucleotidase in WM9 cells and found that tenascin C strongly inhibited ecto-5'-nucleotidase activity and adenosine generation from AMP. We also developed WM9 cells with reduced ecto-5'-nucleotidase expression and tested differences in cell adhesion on various extracellular matrix proteins. WM9 cells attached significantly weaker to tenascin C layer. These observations indicate that expression of ecto-5'-nucleotidase correlates with a number of metastasis-related markers and thus may have a function in this process. Furthermore, our data suggest that, in addition to generating adenosine, ecto-5'-nucleotidase may have independent roles in adhesion and interaction with extracellular matrix components in melanoma.

Interactions of human prostatic epithelial cells with bone marrow endothelium: binding and invasion

Prostate cancer shows a propensity to form secondary tumours within the bone marrow. Such tumours are the major cause of mortality in this disease. We have developed an in vitro system to study the binding of prostate epithelial cells to bone marrow endothelium (BME) and stroma (BMS). The metastatic prostate cancer cell line, PC3 (derived from a bone metastasis), was seeded onto confluent layers of BME and its binding characteristics compared to human umbilical vein endothelial cells (HUVEC), lung endothelium (Hs888Lu) and BMS. The PC3 cell line showed significantly increased binding to BME (P< 0.05) compared to endothelium derived from HUVEC and lung or BMS with maximal binding occurring at 1 h. Following pre-incubation with a β1 integrin antibody PC3 binding to BME was inhibited by 64% (P< 0.001). Antibodies directed against the integrins β4, α2, α4, α5 and the cellular adhesion molecules P-selectin, CD31, VCAM-1 and sialy Lewis X showed no effect on blocking PC3 binding. Primary prostatic epithelial cells from both malignant (n = 11) and non-malignant tissue (n = 11) also demonstrated equivalent levels of increased adhesion to BME and BMS compared to HUVEC, peaking at 24 h. Further studies examined the invasive ability of prostate epithelial cells in response to bone marrow endothelium using Matrigel invasion chamber assays. In contrast to the previous results, malignant cells showed an increase (1000 fold) in invasive ability, whilst non-malignant prostate epithelia did not respond. We have shown that both malignant and non-malignant prostate epithelial cells can bind at equivalent levels and preferentially to primary human bone marrow endothelium in comparison to controls. However, only malignant prostate epithelia show increased invasive ability in response to BME. © 2001 Cancer Research Campaign www.bjcancer.com

Tumor cell adhesion to endothelial cells is increased by endotoxin via an upregulation of beta-1 integrin expression

BACKGROUND

Recent studies have demonstrated that metastatic disease develops from tumor cells that adhere to endothelial cells and proliferate intravascularly. The beta-1 integrin family and its ligand laminin have been shown to be important in tumor-to-endothelial cell adhesion. Lipopolysaccharide (LPS) has been implicated in the increased metastatic tumor growth that is seen postoperatively. We postulated that LPS increases tumor cell expression of beta-1 integrins and that this leads to increased adhesion.

METHODS

The human metastatic colon cancer cell line LS174T was labeled with an enhanced green fluorescent protein (eGFP) using retroviral transfection. Cell cultures were treated with LPS for 1, 2, and 4 h (n = 6 each) and were subsequently cocultured for 30 or 120 min with confluent human umbilical vein endothelial cells (HUVECs), to allow adherence. Adherent tumor cells were counted using fluorescence microscopy. These experiments were carried out in the presence or absence of a functional blocking beta-1 integrin monoclonal antibody (4B4). Expression of beta-1 integrin and laminin on tumor and HUVECs was assessed using flow cytometric analysis. Tumor cell NF-kappaB activation after incubation with LPS was measured.

RESULTS

Tumor cell and HUVEC beta-1 integrin expression and HUVEC expression of laminin were significantly (P < 0.05) enhanced after incubation with LPS. Tumor cell adhesion to HUVECs was significantly increased. Addition of the beta-1 integrin blocking antibody reduced tumor cell adhesion to control levels. LPS increased tumor cell NF-kappaB activation.

CONCLUSIONS

Exposure to LPS increases tumor cell adhesion to the endothelium through a beta-1 integrin-mediated pathway that is NF-kappaB dependent. This may provide a target for immunotherapy directed at reducing postoperative metastatic tumor growth.

Progesterone induces focal adhesion in breast cancer cells MDA-MB-231 transfected with progesterone receptor complementary DNA

Since the effects of progesterone are mediated mainly via estrogen-dependent progesterone receptor (PR), the expression of the effects of progesterone may be masked or overridden by the influence of estrogen under conditions in which priming with estrogens is required. We have established a PR-positive but estrogen receptor-alpha (ER-alpha) negative breast cancer cell model by transfecting PR cDNA into ER-alpha- and PR-negative MDA-MB-231 cells in order that the functions of progesterone can be studied independently of estrogens. We have demonstrated using this model that progesterone markedly inhibited cell growth. We have also discovered that progesterone induced remarkable changes in cell morphology and specific adhesion structures. Progesterone-treated cells became considerably more flattened and well spread than vehicle-treated control cells. This was associated with a striking increase of stress fibers, both in number and diameter, and increased focal contacts as shown by the staining of focal adhesion proteins paxillin and talin. There were also distinct increases in tyrosine phosphorylation of focal adhesion protein paxillin and focal adhesion kinase in association with increased focal adhesion. The staining of tyrosine-phosphorylated proteins was concentrated at focal adhesions in progesterone-treated cells. More interestingly, monoclonal antibody (Ab) to beta1 integrin was able to inhibit progesterone-induced cell spreading and formation of actin cytoskeleton. To our knowledge, this is the first report describing a direct effect of progesterone in inducing spreading and adhesion of breast cancer cells, and beta1-integrin appeared to play an essential role in the effect. It is known that the initial step of tumor metastasis is the breakaway of tumor cells from primary tumor mass when they lose the ability to attach. Hence, progesterone-induced cell spreading and adhesion may have significant implications in tumor metastasis.

[Adhesion molecules and cancer]

INTRODUCTION

This review was aimed at summarizing recent advances in the understanding of cell adhesion in order to discuss the possible relevance of new knowledge to the exploration of cancer patients and elaboration of therapeutic strategies.

CURRENT KNOWLEDGE AND KEY POINTS

During the last 10 years, many adhesion molecules were identified, thus allowing to determine their tissue distribution and functional regulation. The concept of adhesiveness was refined. It is now well known that adhesive rate (i.e., the minimal contact time required for bond formation) and binding strength (i.e., the minimal force required to detach bound cells) are distinct parameters. They may be regulated independently, and influence the cell behavior in different ways. It is now possible to achieve accurate control of tumor cell adhesiveness, either by inhibiting an adhesive mechanism (through monoclonal antibodies, competitive ligands, or inhibition of receptor expression with antisense strategy or gene knock-out) or by promoting a binding mechanism (with receptor transfection or pro-inflammatory stimulation).

FUTURE PROSPECTS AND PROJECTS

Recent progress opens new possibilities for diagnosis and treatment. First, the interpretation of experimental data may be improved. Cell adhesive behavior is not entirely accounted for by the density of membrane adhesion receptors. Indeed, adhesion is influenced by receptor connection to the cytoskeleton and structure of the cell coat. An adhesion receptor may be anti-metastatic through an increase in tumor cohesion and cell differentiation, or pro-metastatic, through facilitation of cell migration towards a target tissue. New therapeutic strategies may include anti-adhesive procedure aimed at preventing metastasis formation. The potential importance of a better control of inflammatory processes is also emphasized in view of the influence of these processes on the expression of adhesion molecules.

Endothelial cell intracellular Ca2+ concentration is increased upon breast tumor cell contact and mediates tumor cell transendothelial migration

Tumor cell extravasation is a determinant step in the process of hematogenous metastasis. The signal transduction pathways involved in the interactions between tumor cells and the vascular endothelium during transendothelial migration are still undefined. In the present study, we have investigated the influence of human breast adenocarcinoma cells (MCF7) on human umbilical vein endothelial cell (HUVEC) intracellular Ca2+ concentration ([Ca2+]i). We show that the contact between MCF7 cells and a confluent HUVEC monolayer induces an immediate and transient increase in HUVEC [Ca2+]i. This [Ca2+]i rise could not be elicited by tumor cell-conditioned medium, isolated tumor cell membranes, inert beads or normal breast epithelial cells, demonstrating the involvement of specific recognition mechanisms between MCF7 cells and HUVEC. Depletion of HUVEC intracellular Ca2+ stores by the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin as well as the selective depletion of inositol 1,4,5-triphosphate (IP3)-sensitive Ca2+ stores by prior activation of HUVEC using histamine resulted in a complete inhibition of tumor cell-induced [Ca2+]i elevation. Similar results were obtained when HUVEC monolayers were treated with the tyrosine kinase inhibitor herbimycin A, suggesting a role for tyrosine kinase-associated cell surface receptors in tumor cell-endothelial cell interactions. The depletion of HUVEC intracellular Ca2+ stores by thapsigargin was also shown to delay MCF7-induced endothelial cell disjunction, to prevent their spreading on the subendothelial extracellular matrix and transendothelial migration in vitro. These results suggest that transient changes in endothelial [Ca2+]i may govern multiple steps of tumor cell extravasation.