Regulation of cell migration by the integrin beta subunit ectodomain

Twenty years of the G protein-coupled estrogen receptor GPER: Historical and personal perspectives

Estrogens play a critical role in many aspects of physiology, particularly female reproductive function, but also in pathophysiology, and are associated with protection from numerous diseases in premenopausal women. Steroids and the effects of estrogen have been known for ∼90 years, with the first evidence for a receptor for estrogen presented ∼50 years ago. The original ancestral steroid receptor, extending back into evolution more than 500 million years, was likely an estrogen receptor, whereas G protein-coupled receptors (GPCRs) trace their origins back into history more than one billion years. The classical estrogen receptors (ERα and ERβ) are ligand-activated transcription factors that confer estrogen sensitivity upon many genes. It was soon apparent that these, or novel receptors may also be responsible for the "rapid"/"non-genomic" membrane-associated effects of estrogen. The identification of an orphan GPCR (GPR30, published in 1996) opened a new field of research with the description in 2000 that GPR30 expression is required for rapid estrogen signaling. In 2005-2006, the field was greatly stimulated by two studies that described the binding of estrogen to GPR30-expressing cell membranes, followed by the identification of a GPR30-selective agonist (that lacked binding and activity towards ERα and ERβ). Renamed GPER (G protein-coupled estrogen receptor) by IUPHAR in 2007, the total number of articles in PubMed related to this receptor recently surpassed 1000. In this article, the authors present personal perspectives on how they became involved in the discovery and/or advancement of GPER research. These areas include non-genomic effects on vascular tone, receptor cloning, molecular and cellular biology, signal transduction mechanisms and pharmacology of GPER, highlighting the roles of GPER and GPER-selective compounds in diseases such as obesity, diabetes, and cancer and the obligatory role of GPER in propagating cardiovascular aging, arterial hypertension and heart failure through the stimulation of Nox expression.

Virus-induced Ca2+ influx extends survival of west nile virus-infected cells

West Nile virus (WNV) infection leads to rapid and sustained Ca(2+) influx. This influx was observed with different strains of WNV and in different types of cells. Entry during virion endocytosis as well as through calcium channels contributed to the Ca(2+) influx observed in WNV-infected cells. Ca(2+) influx was not detected after infection with vesicular stomatitis virus (VSV) and occurred only through endocytosis in Sindbis virus-infected cells. Caspase 3 cleavage and activation of several kinases, including focal adhesion kinase (FAK), mitogen-activated extracellular signal-regulated protein kinase (ERK1/2), and protein-serine kinase B alpha (Akt), at early times after WNV infection were shown to be dependent on Ca(2+) influx. Although the activation of these kinases was sustained in virus-infected cells throughout infection, UV-inactivated WNV induced only a transient activation of FAK and ERK1/2 at early times after infection. The Ca(2+)-dependent FAK activation observed in WNV-infected cells was not mediated by alphavbeta3 integrins. Reduction of Ca(2+) influx at early times of infection by various treatments decreased the viral yield and delayed both the early transient caspase 3 cleavage and the activation of FAK, Akt, and ERK signaling. The results indicate that Ca(2+) influx is required for early infection events needed for efficient viral replication, possibly for virus-induced rearrangement of the endoplasmic reticulum (ER) membrane. Increased caspase 3 cleavage at both early (transient) and late times of infection correlated with decreased activation of the FAK and ERK1/2 pathways, indicating a role for these kinases in extending the survival of flavivirus-infected cells.

Adhesion behaviors of human trophoblast cells by contact with endothelial cells

Although it is still not clear whether migratory trophoblasts reach the spiral arteries by migration within blood vessels against blood flow or by a mechanism of directional cell division/proliferation, this process involves the attachment and adhesion of trophoblasts to endothelial cells lining the blood vessel walls. This raises the possibility that the cell-cell contact with endothelial cells may regulate trophoblast cell adhesion behaviors according to the surrounding flow condition. To test this, the adhesion forces of early gestation human trophoblast cells (TCs) cultured on glass slides coated with type I rat collagen or cultured with human umbilical vein endothelial cells (HUVECs) were measured quantitatively using a micropipette aspiration technique. Then, the resistance of TCs co-cultured with HUVECs to flow-induced shear stress was assessed with a flow chamber technique. The results showed that the adhesion force of TCs to glass slides coated with collagen was positively correlated with the concentration of collagen. By contact with endothelial cells, the adhesion force and the resistance to shear stress for the TCs were significantly enhanced. The interdiction of integrin beta1 interaction remarkably reduced the adhesion forces of TCs to endothelial cells, hence their resistance to shear stress. The results therefore suggest that the contacts of TCs with endothelial cells enhance the adhesion forces of human TCs, partially by regulating with the integrin beta1 according to the flow condition (i.e., the shear stress) in such a way to prevent the TCs from being carried downstream by flowing blood.

Inhibition of migration and invasion of carcinoma cells by urokinase-derived antagonists of alphavbeta5 integrin activation

We previously showed that, while binding to urokinase receptor (uPAR) through its growth factor domain (GFD, residues 1-49), urokinase (uPA) can engage alphavbeta5 integrin through an internal domain (CP, residues 132-158). This novel uPA/alphavbeta5 interaction promotes cytoskeletal rearrangements and directional cell migration (Franco et al., J Cell Sci 2006;119:3424-34). We now show that treatment of cells with phosphomimic uPA (uPA138E/303E, serine 138 and 303 substituted with glutamic acid) strongly inhibits matrix-induced cell migration. Unlike uPA, binding of uPA138E/303E to cell surface did not induce F-actin enriched protruding structures and caused a 5-fold reduction in cell translocation speed, as determined by video tracking of living cells. Inhibition of migration was found to be independent of uPAR, since uPA variants lacking the GFD domain, but carrying the relevant Ser to Glu substitutions were as effective inhibitor as uPA138E/303E. Through several independent approaches, we established that the phosphomimics specifically bind to alphavbeta5 integrin through the CP region carrying the S138E mutation. This interaction blocks integrin activation, as determined by a decreased affinity of alphavbeta5 to vitronectin and a reduced association of the beta5 cytoplasmic tail with talin. Finally, stable expression of uPA138E/303E in human squamous carcinoma cells prevented tumor cell invasion in vivo. Thus, when expressed in cancer cells, the inhibitory phosphomimic effect was dominant over the effect of endogenously produced uPA. These results shed light on the regulation of cell migration by uPA phosphorylation and provide a realistic opportunity for a novel antiinvasive/metastatic therapeutic intervention.

Syndecan-1 regulates alphavbeta5 integrin activity in B82L fibroblasts

B82L mouse fibroblasts respond to fibronectin or vitronectin via a syndecan-1-mediated activation of the alphavbeta5 integrin. Cells attached to syndecan-1-specific antibody display only filopodial extension. However, the syndecan-anchored cells extend lamellipodia when the antibody-substratum is supplemented with serum, or low concentrations of adsorbed vitronectin or fibronectin, that are not sufficient to activate the integrin when plated alone. Integrin activation is blocked by treatment with (Arg-Gly-Asp)-containing peptides and function-blocking antibodies that target alphav integrins, as well as by siRNA-mediated silencing of beta5 integrin expression. In addition, alphavbeta5-mediated cell attachment and spreading on high concentrations of vitronectin is blocked by competition with recombinant syndecan-1 ectodomain core protein and by downregulation of mouse syndecan-1 expression by mouse-specific siRNA. Taking advantage of the species-specificity of the siRNA, rescue experiments in which human syndecan-1 constructs are expressed trace the activation site to the syndecan-1 ectodomain. Moreover, both full-length mouse and human syndecan-1 co-immunoprecipitate with the beta5 integrin subunit, but fail to do so if the syndecan is displaced by competition with soluble, recombinant syndecan-1 ectodomain. These results suggest that the ectodomain of the syndecan-1 core protein contains an active site that assembles into a complex with the alphavbeta5 integrin and regulates alphavbeta5 integrin activity.

Adhesion, migration and communication in melanocytes and melanoma

Under normal conditions, homeostasis determines whether a cell remains quiescent, proliferates, differentiates, or undergoes apoptosis. In this state of homeostasis, keratinocytes control melanocyte growth and behaviour through a complex system of paracrine growth factors and cell-cell adhesion molecules. Alteration of this delicate homeostatic balance and can lead to altered expression of cell-cell adhesion and cell communication molecules and to the development of melanoma. Melanoma cells escape from this control by keratinocytes through three major mechanisms: (1) down-regulation of receptors important for communication with keratinocytes such as E-cadherin, P-cadherin, desmoglein and connexins, which is achieved through growth factors produced by fibroblasts or keratinocytes; (2) up-regulation of receptors and signalling molecules not found on melanocytes but important for melanoma-melanoma and melanoma-fibroblast interactions such as N-cadherin, Mel-CAM, and zonula occludens protein-1 (ZO-1); (3) loss of anchorage to the basement membrane because of an altered expression of the extracellular-matrix binding integrin family. In the current review, we describe the alterations in cell-cell adhesion and communication associated with melanoma development and progression, and discuss how a greater understanding of these processes may aid the future therapy of this disease.

A role for Mer tyrosine kinase in αvβ5 integrin-mediated phagocytosis of apoptotic cells

Efficient phagocytosis of apoptotic cells is crucial for many cellular processes. One of earliest signals to the phagocyte is the expression of phosphatidylserine (PS) on the outer surface of the apoptotic cell that provides a potent `eat-me' signal. Recognition of PS occurs either directly, via PS receptor (PS-R), or indirectly via αvβ5(3) integrin or Mer-family tyrosine kinases through the opsonizing proteins milk fat globule-EGF factor 8 protein (MFG-E8), or growth arrest specific factor-6 (Gas6), respectively. Because Mer and αvβ5 integrin share PS-dependent recognition signals, we investigated their post-receptor signaling cascades following receptor activation. Using a constitutively active form for Mer (CDMer) or Gas6 as a ligand to stimulate Mer, we found that Mer activation induced a post-receptor signaling cascade involving Src-mediated tyrosine phosphorylation of FAK on Tyr861, the recruitment of FAKTyr861 to the αvβ5 integrin, and increased formation of p130CAS/CrkII/Dock180 complex to activate Rac1. Coexpression of Mer with αvβ5 integrin had a synergistic effect on Rac1 activation, lamellipodial formation and the phagocytosis of apoptotic cells. Interestingly, Gas6 or CDMer failed to stimulate p130CAS tyrosine phosphorylation or phagocytosis in β5-deficient CS-1 cells or in mutant β5ΔC-expressing cells, suggesting that Mer is directionally and functionally linked to the integrin pathway. The present data indicate that receptors that recognize apoptotic cells in the context of PS functionally crosstalk to amplify intracellular signals to internalize apoptotic cells. Moreover, our data link another PS-dependent signal to the CrkII/Dock180/Rac1 module.

Interaction of West Nile virus with alpha v beta 3 integrin mediates virus entry into cells

The functional receptor for the flavivirus West Nile (WNV) infection has been characterized in this study with a combination of biochemical and molecular approaches. A 105-kDa protease-sensitive glycoprotein that binds WNV was isolated from the plasma membrane of cells permissive to WNV infection. The protein was subjected to peptide sequencing, and this glycoprotein was identified as a member of the integrin superfamily. Infection of WNV was shown to be markedly inhibited in Vero cells pretreated with blocking antibodies against alpha(v)beta(3) integrin and its subunits by receptor competition assay. It was also noted that cells pretreated with antibodies against alpha(v)beta(3) integrin can effectively inhibit flavivirus Japanese encephalitis but to a lesser extent flavivirus dengue infections. West Nile virus entry is independent of divalent cations and is not highly blocked by arginine-glycine-aspartic acid (RGD) peptides, suggesting that the interaction between the virus and alpha(v)beta(3) integrin is not highly dependent on the classical RGD binding motif. In addition, gene silencing of the beta(3) integrin subunit in cells has resulted in cells largely resistant to WNV infection. In contrast, expression of recombinant human beta(3) integrin substantially increased the permissiveness of CS-1 melanoma cells for WNV infection. Soluble alpha(v)beta(3) integrin can also effectively block WNV infection in a dose-dependent manner. Furthermore, WNV infection also triggered the outside-in signaling pathway via the activation of integrin-associated focal adhesion kinase. The identification of alpha(v)beta(3) integrin as a receptor for WNV provides insight into virus-receptor interaction, hence creating opportunities in the development of anti-viral strategies against WNV infection.

Peptide Inhibition of Catalytic and Noncatalytic Activities of Matrix Metalloproteinase-9 Blocks Tumor Cell Migration and Invasion

Migration of invasive cells appears to be dependent on matrix metalloproteinases (MMPs) anchored on the cell surface through integrins. We have previously demonstrated an interaction between the integrin alpha-subunit I domain and the catalytic domain of MMP-9. We now show that there is also an interaction between the integrin beta subunit and MMP-9. Using phage display, we have developed MMP-9 inhibitors that bind either to the MMP-9 catalytic domain, the collagen binding domain, or the C-terminal hemopexin-like domain. The C-terminal domain-binding peptide mimics an activation epitope in the stalk of the integrin beta chain and inhibits the association of MMP-9 C-terminal domain with alpha(V)beta(5) integrin. Unlike other MMP-9 binding peptides, it does not directly inhibit catalytic activity of MMP-9, but still prevents proenzyme activation and cell migration in vitro and tumor xenograft growth in vivo. We also find an association between MMP-9 and urokinase-plasminogen activator receptor and find that urokinase-plasminogen activator receptor is cleaved by MMP-9. Collectively, we have defined molecular details for several interactions mediated by the different MMP-9 domains.

Soluble urokinase-type plasminogen activator receptor inhibits cancer cell growth and invasion by direct urokinase-independent effects on cell signaling

The urokinase-type plasminogen activator receptor (uPAR) is released from human cancers and is readily detected in blood. In animal models, soluble uPAR (SuPAR) antagonizes cancer progression; however, the mechanism by which SuPAR functions in vivo remains unclear. It is generally thought that SuPAR scavenges uPA and prevents its interaction with membrane-anchored uPAR. In this study, we demonstrate a novel molecular mechanism by which SuPAR may inhibit cancer progression. We show that SuPAR has the potential to directly and in a uPA-independent manner block the signaling activity of membrane-anchored uPAR. Whether SuPAR inhibits signaling is cell type-specific, depending on the state of the endogenous uPA-uPAR signaling system. In uPAR-deficient cells that lack endogenous uPAR signaling, including uPAR-/-murine embryonic fibroblasts and human embryonal kidney 293 cells, SuPAR functions as a partial signaling agonist that activates ERK/mitogen-activated protein kinase. By contrast, in cells with potent autocrine uPA-uPAR signaling systems, including MDA-MB 231 breast cancer cells and low density lipoprotein receptor-related protein-1-deficient murine embryonic fibroblasts, SuPAR substantially decreases ERK activation. The mechanism probably involves competitive displacement of membrane-anchored uPAR-uPA complex from signaling adaptor proteins. As a result of its effects on cell signaling, SuPAR blocks cell growth and inhibits cellular invasion of Matrigel. Cleavage of SuPAR by proteinases increases its signaling agonist activity and reverses its inhibitory effects on growth and invasion. Thus, proteolytic cleavage represents a molecular switch that neutralizes the anticancer activity of SuPAR.

Alpha v beta 5 integrin-dependent programmed cell death triggered by a peptide mimic of annexin V

The diverse cytoplasmic domain sequences within the various integrin subunits are critical for integrin-mediated signaling into the cell (outside-in signaling) and for activation of ligand binding affinity (inside-out signaling). Here we introduce an approach based on phage display technology to identify molecules that specifically interact with the cytoplasmic domain of the beta 5 integrin subunit. We show that a peptide selected for binding specifically to the beta 5 cytoplasmic domain (VVISYSMPD) induces apoptosis upon internalization. The cell death process induced by VVISYSMPD is sensitive to modulation by growth factors and by protein kinase C (PKC), and it cannot be triggered in beta 5 null cells. Finally, we show that the VVISYSMPD peptide is a mimic of annexin V. Our results suggest a functional link between the alpha v beta 5 integrin, annexin V, and programmed cell death. We propose the term "endothanatos" to designate this phenomenon.

ERK and RhoA differentially regulate pseudopodia growth and retraction during chemotaxis

Nonmotile cells extend and retract pseudopodia-like structures in a random manner, whereas motile cells establish a single dominant pseudopodium in the direction of movement. This is a critical step necessary for cell migration and occurs prior to cell body translocation, yet little is known about how this process is regulated. Here we show that myosin II light chain (MLC) phosphorylation at its regulatory serine 19 is elevated in growing and retracting pseudopodia. MLC phosphorylation in the extending pseudopodium was associated with strong and persistent amplification of extracellular-regulated signal kinase (ERK) and MLC kinase activity, which specifically localized to the leading pseudopodium. Interestingly, inhibition of ERK or MLC kinase activity prevented MLC phosphorylation and pseudopodia extension but not retraction. In contrast, inhibition of RhoA activity specifically decreased pseudopodia retraction but not extension. Importantly, inhibition of RhoA activity specifically blocked MLC phosphorylation associated with retracting pseudopodia. Inhibition of either ERK or RhoA signals prevents chemotaxis, indicating that both pathways contribute to the establishment of cell polarity and migration. Together, these findings demonstrate that ERK and RhoA are distinct pathways that control pseudopodia extension and retraction, respectively, through differential modulation of MLC phosphorylation and contractile processes.

Adhesion and migration of extracellular matrix-stimulated breast cancer

BACKGROUND

Extracellular matrix (ECM) components, such as vitronectin and fibronectin, have been shown to enhance the metastatic potential of breast cancer cells. We hypothesized that ECM binding to integrin receptors on breast cancer cells influenced cellular adhesion and migration.

MATERIALS AND METHODS

Adhesion assays were performed using breast cancer cell lines MDA-MB-435 and MDA-MB-231 and various concentrations of vitronectin or fibronectin. Migration assays were performed using the same cell lines and invasion chambers with 8 microm pore polycarbonate membranes. Blocking antibodies and specific peptidomimetic inhibitors to integrin receptors were used to identify the integrin subunits reacting with vitronectin and fibronectin.

RESULTS

While both breast cancer cell lines adhered to and migrated toward vitronectin and fibronectin, MDA-MB-435 had a higher maximum binding to vitronectin and MDA-MB-231 had a higher maximum binding to fibronectin. Anti-beta1 antibody inhibited the adhesion and migration of MDA-MB-231 to fibronectin and the adhesion of MDA-MB-231 to vitronectin but had no effect on vitronectin-induced adhesion or migration of MDA-MB-435. The alpha(v)beta3/alpha(v)beta5 antagonist, SB 265123, inhibited MDA-MB-231 and MDA-MB-435 adhesion and migration to vitronectin but had no effect on migration to fibronectin in either cell line.

CONCLUSIONS

We conclude that the integrin subunits beta1, alpha(v)beta3, and alpha(v)beta5 can be involved in breast cancer cell adhesion and migration to vitronectin and fibronectin. Because more than one integrin inhibitor was required to block adhesion or migration in the cell lines studied, breast cancer therapy based on integrin antagonists would most likely require concomitant use of multiple agents.

Cooperativity between the Ras-ERK and Rho-Rho kinase pathways in urokinase-type plasminogen activator-stimulated cell migration

Binding of the urokinase-type plasminogen activator (uPA) to its receptor activates diverse cell signaling pathways. How these signals are integrated so that cell physiology is altered remains unclear. In this study, we demonstrated that migration of MCF-7 breast cancer cells and HT-1080 fibrosarcoma cells on serum-coated surfaces is stimulated by agents that activate ERK, including uPA, epidermal growth factor, and constitutively active MEK1. The promigratory activity of these agents was entirely blocked not only by the MEK-specific antagonist PD098059, but also by antagonists of the Rho-Rho kinase pathway, including Y-27632 and dominant-negative RhoA (RhoA-N19). uPA did not significantly increase the level of GTP-bound RhoA, suggesting that the constitutive activity of the Rho-Rho kinase pathway may be sufficient to support ERK-stimulated cell migration. Paradoxically, Y-27632 and RhoA-N19 increased ERK phosphorylation in MCF-7 cells, providing further evidence that ERK activation alone does not promote cell migration when Rho kinase is antagonized. When MCF-7 cell migration was stimulated by ERK-independent processes such as expression of the beta(3) integrin subunit or changing the substratum to type I collagen, Y-27632 and RhoA-N19 failed to inhibit the response. This study supports a model in which the Ras-ERK and Rho-Rho kinase pathways cooperate to promote cell migration. Neutralizing either pathway is sufficient to block the response to agents that stimulate cell migration by activating ERK.

Angiogenesis inhibitors in clinical development for lung cancer

The use of tumor angiogenesis as a therapeutic target is based on extensive literature showing the dependence of tumors on the process of angiogenesis for growth, invasion, and metastasis. Seminal work performed by Folkman three decades ago determined that tumors beyond the size of approximately 2 mm require angiogenesis for subsequent growth and development. This basic hypothesis stimulated research in the field of angiogenesis and has resulted in the identification of factors that both enhance and inhibit this "angiogenic switch." The intent of this article is to present data on several angiogenesis inhibitors that are currently undergoing clinical evaluation in cancer patients. These agents may be particularly useful in the treatment of lung cancer, both as adjunctive therapy in early-stage or locally advanced disease, as well as in combination strategies with platinum-based therapy in metastatic disease. Although angiogenesis inhibitors have been in clinical trials for the past decade, there has been a shift in recent years towards the development of more mechanism-based and receptor-targeted agents. Interestingly, no antiangiogenic agent has been approved as such for use in cancer, perhaps because of the challenges involved in the clinical development of these novel agents. These include the potential requirement for long-term administration, difficulties in deriving biologically efficacious doses in early clinical trials, and the inability to use tumor regression as a primary endpoint in phase II trials.

Alpha(v) integrin, p38 mitogen-activated protein kinase, and urokinase plasminogen activator are functionally linked in invasive breast cancer cells

We reported previously that endogenous p38 MAPK activity is elevated in invasive breast cancer cells and that constitutive p38 MAPK activity is important for overproduction of uPA in these cells (Huang, S., New, L., Pan, Z., Han, J., and Nemerow, G. R. (2000) J. Biol. Chem. 275, 12266-12272). However, it is unclear how elevated endogenous p38 MAPK activity is maintained in invasive breast cancer cells. In the present study, we found that blocking alpha(v) integrin functionality with a function-blocking monoclonal antibody or down-regulating alpha(v) integrin expression with alpha(v)-specific antisense oligonucleotides significantly decreased the levels of active p38 MAPK and inhibited cell-associated uPA expression in invasive breast cancer MDA-MB-231 cells. These results suggest a function link between alpha(v) integrin, p38 MAPK activity, and uPA expression in invasive tumor cells. We also found that vitronectin/alpha(v) integrin ligation specifically induced p38 MAPK activation and uPA up-regulation in invasive MDA-MB-231 cells but not in non-invasive MCF7 cells. Finally, using a panel of melanoma cells, we demonstrated that the cytoplasmic tail of alpha(v) integrin subunit is required for alpha(v) integrin ligation-induced p38 MAPK activation.

Ectodomain shedding of L1 adhesion molecule promotes cell migration by autocrine binding to integrins

The L1 adhesion molecule plays an important role in axon guidance and cell migration in the nervous system. L1 is also expressed by many human carcinomas. In addition to cell surface expression, the L1 ectodomain can be released by a metalloproteinase, but the biological function of this process is unknown. Here we demonstrate that membrane-proximal cleavage of L1 can be detected in tumors and in the developing mouse brain. The shedding of L1 involved a disintegrin and metalloproteinase (ADAM)10, as transfection with dominant-negative ADAM10 completely abolishes L1 release. L1-transfected CHO cells (L1-CHO) showed enhanced haptotactic migration on fibronectin and laminin, which was blocked by antibodies to alpha v beta 5 and L1. Migration of L1-CHO cells, but not the basal migration of CHO cells, was blocked by a metalloproteinase inhibitor, indicating a role for L1 shedding in the migration process. CHO and metalloproteinase-inhibited L1-CHO cells were stimulated to migrate by soluble L1-Fc protein. The induction of migration was blocked by alpha v beta 5-specific antibodies and required Arg-Gly-Asp sites in L1. A 150-kD L1 fragment released by plasmin could also stimulate CHO cell migration. We propose that ectodomain-released L1 promotes migration by autocrine/paracrine stimulation via alpha v beta 5. This regulatory loop could be relevant for migratory processes under physiological and pathophysiological conditions.

Cooperative regulation of the invasive and metastatic phenotypes by different domains of the type I insulin-like growth factor receptor beta subunit

The receptor for the type 1 insulin-like growth factor (IGF-I) regulates multiple cellular functions impacting on the metastatic phenotype of tumor cells, including cellular proliferation, anchorage-independent growth, survival, migration, synthesis of the 72-kDa type IV collagenase and invasion. We have used site-directed mutagenesis to generate domain-specific mutants of the receptor beta subunit to analyze the role of specific tyrosines in the regulation of the invasive/metastatic phenotype. Poorly invasive M-27 carcinoma cells expressing low receptor numbers were transfected with a plasmid vector expressing IGF-I receptor cDNA in which single or multiple tyrosine codons in the kinase domain, namely Tyr-1131, Tyr-1135, and Tyr-1136 or the C-terminal tyrosines 1250 and 1251 were substituted with phenylalanine. Changes in the invasive and metastatic properties were analyzed relative to M-27 cells expressing the wild type receptor. We found that cells expressing the Y1131F,Y1135F,Y1136F or Y1135F receptor mutants lost all IGF-IR-dependent functions and their phenotypes were indistinguishable from, or suppressed relative to, the parent line. The Y1250F,Y1251F substitution abolished anchorage-independent growth, cell spreading, and the anti-apoptotic effect of IGF-I whereas all other IGF-IR-dependent phenotypes were either unperturbed (i.e. mitogenicity) or only partially reduced (migration and invasion). The results identify three types of receptor-dependent functions in this model: those dependent only on an intact kinase domain (DNA synthesis), those dependent equally on kinase domain and Tyr-1250/1251 signaling (e.g. apoptosis, soft agar cloning) and those dependent on kinase domain and enhanced through Tyr-1250/1251 signaling (migration, invasion). They suggest that signals derived from both regions of the receptor cooperate to enhance tumor metastasis.

High PKC alpha and low E-cadherin expression contribute to high migratory activity of colon carcinoma cells

The protein kinase C (PKC) is a family of serine/threonine kinases that are key regulatory enzymes involved in growth, differentiation, cytoskeletal reorganization, tumor promotion, and migration. We investigated the functional involvement of PKC isotypes and of E-cadherin in the regulation of the locomotion of six human colon-adenocarcinoma cell lines. The different levels of the PKC alpha and the E-cadherin expression have predictable implications in the spontaneous locomotory activity. With the use of PKC alpha--specific inhibitors (safingol, Go6976) as well as the PKC delta--specific inhibitor rottlerin, we showed that only PKC alpha plays a major role in the regulation of tumor cell migration. The results were verified by knocking out the translation of PKC isozymes with the use of an antisense oligonucleotide strategy. After stimulation with phorbol ester we observed a translocation and a colocalization of the activated PKC alpha at the plasma membrane to the surrounding extracellular matrix. Furthermore, we investigated the functional involvement of E-cadherin in the locomotion with the use of a blocking antibody. A high level of PKC alpha expression together with a low E-cadherin expression was strongly related to a high migratory activity of the colon carcinoma cells. This correlation was independent of the differentiation grade of the tumor cell lines.

Tumor cell invasion is promoted by activation of protease activated receptor-1 in cooperation with the alpha vbeta 5 integrin

The first prototype of the protease activated receptor (PAR) family, the thrombin receptor PAR1, plays a central role both in the malignant invasion process of breast carcinoma metastasis and in the physiological process of placental implantation. The molecular mechanism underlying PAR1 involvement in tumor invasion and metastasis, however, is poorly defined. Here we show that PAR1 increases the invasive properties of tumor cells primarily by increased adhesion to extracellular matrix components. This preferential adhesion is accompanied by the cytoskeletal reorganization of F-actin toward migration-favoring morphology as detected by phalloidin staining. Activation of PAR1 increased the phosphorylation of focal adhesion kinase and paxillin, and the induced formation of focal contact complexes. PAR1 activation affected integrin cell-surface distribution without altering their level of expression. The specific recruitment of alpha(v)beta(5) to focal contact sites, but not of alpha(v)beta(3) or alpha(5)beta(1), was observed by immunofluorescent microscopy. PAR1 overexpressing cells showed selective reciprocal co-precipitation with alpha(v)beta(5) and paxillin but not with alpha(v)beta(3) that remained evenly distributed under these conditions. This co-immunoprecipitation failed to occur in cells containing the truncated form of PAR1 that lacked the entire cytoplasmic portion of the receptor. Thus, the PAR1 cytoplasmic tail is essential for conveying the cross-talk and recruiting the alpha(v)beta(5) integrin. While PAR1 overexpressing cells were invasive in vitro, as reflected by their migration through a Matrigel barrier, invasion was further enhanced by ligand activation of PAR1. Moreover, the application of anti-alpha(v)beta(5) antibodies specifically attenuated this PAR1 induced invasion. We propose that the activation of PAR1 may lead to a novel cooperation with the alpha(v)beta(5) integrin that supports tumor cell invasion.

Cloning and characterization of the murine beta(3) integrin gene promoter: identification of an interleukin-4 responsive element and regulation by STAT-6

Expression of the alpha(v)beta(3) integrin by murine bone marrow macrophages is regulated by cytokines such as IL-4 and GM-CSF through transcriptional activation of the beta(3) subunit gene. To characterize the molecular mechanisms by which such regulation occurs, we isolated the murine beta(3) integrin promoter. To this end, we first cloned a full length beta(3) cDNA and used the 5'UTR and leader peptide coding sequence to identify genomic clones containing the beta(3) promoter region. The transcriptional start site, identified by primer extension and S1 nuclease assay, is 34 nt upstream of the translation initiation codon. A 1.1 kb fragment of the promoter region drives IL-4 responsive transcription in transiently transfected murine bone marrow macrophages. Deletion analysis of the beta(3) promoter indicates the IL-4 responsive element lies between -465 to -678 nt relative to the transcriptional start site. This promoter fragment contains two overlapping STAT consensus recognition sites and nuclear extracts from BMMs contain an IL-4-inducible DNA binding factor, identified by super shift analysis, as STAT-6. Furthermore, an oligonucleotide which includes the two STAT recognition sites residing in the IL-4 responsive region of the beta(3) promoter, competes for STAT-6 binding. Confirming IL-4 induction of the integrin subunit is specifically mediated by STAT-6, beta(3) mRNA is not enhanced in BMMs derived from STAT-6 deleted mice, which however, retain their capacity to respond to GM-CSF.

Convergence of alpha(v)beta(3) integrin- and macrophage colony stimulating factor-mediated signals on phospholipase Cgamma in prefusion osteoclasts

The macrophage colony stimulating factor (M-CSF) and alpha(v)beta(3) integrins play critical roles in osteoclast function. This study examines M-CSF- and adhesion-induced signaling in prefusion osteoclasts (pOCs) derived from Src-deficient and wild-type mice. Src-deficient cells attach to but do not spread on vitronectin (Vn)-coated surfaces and, contrary to wild-type cells, their adhesion does not lead to tyrosine phosphorylation of molecules activated by adhesion, including PYK2, p130(Cas), paxillin, and PLC-gamma. However, in response to M-CSF, Src(-/-) pOCs spread and migrate on Vn in an alpha(v)beta(3)-dependent manner. Involvement of PLC-gamma activation is suggested by using a PLC inhibitor, U73122, which blocks both adhesion- and M-CSF-mediated cell spreading. Furthermore, in Src(-/-) pOCs M-CSF, together with filamentous actin, causes recruitment of beta(3) integrin and PLC-gamma to adhesion contacts and induces stable association of beta(3) integrin with PLC-gamma, phosphatidylinositol 3-kinase, and PYK2. Moreover, direct interaction of PYK2 and PLC-gamma can be induced by either adhesion or M-CSF, suggesting that this interaction may enable the formation of integrin-associated complexes. Furthermore, this study suggests that in pOCs PLC-gamma is a common downstream mediator for adhesion and growth factor signals. M-CSF-initiated signaling modulates the alpha(v)beta(3) integrin-mediated cytoskeletal reorganization in prefusion osteoclasts in the absence of c-Src, possibly via PLC-gamma.

Regulation of adenovirus membrane penetration by the cytoplasmic tail of integrin beta5

Adenovirus (Ad) cell entry involves sequential interactions with host cell receptors that mediate attachment (CAR), internalization (alphavbeta3 and alphavbeta5), and penetration (alphavbeta5) of the endosomal membrane. These events allow the virus to deliver its genome to the nucleus. While integrins alphavbeta3 and alphavbeta5 both promote Ad internalization into cells, integrin alphavbeta5 selectively facilitates Ad-mediated membrane permeabilization and endosome rupture. In the experiments reported herein, we demonstrate that the intracellular domain of the integrin beta5 subunit specifically regulates Ad-mediated membrane permeabilization and gene delivery. CS-1 melanoma cells expressing a truncated integrin beta5 or a chimeric (beta5-beta3) cytoplasmic tail (CT) supported normal levels of Ad endocytosis but had reduced Ad-mediated gene delivery and membrane permeabilization relative to cells expressing a wild-type integrin beta5. Thin-section electron microscopy revealed that virion particles were capable of being endocytosed into cells expressing a truncated beta5CT, but they failed to escape cytoplasmic vesicles and translocate to the nucleus. Site-specific mutagenesis studies suggest that a C-terminal TVD motif in the beta5CT plays a major role in Ad membrane penetration.

Myosin light chain kinase functions downstream of Ras/ERK to promote migration of urokinase-type plasminogen activator-stimulated cells in an integrin-selective manner

Urokinase-type plasminogen activator (uPA) activates the mitogen activated protein (MAP) kinases, extracellular signal-regulated kinase (ERK) 1 and 2, in diverse cell types. In this study, we demonstrate that uPA stimulates migration of MCF-7 breast cancer cells, HT 1080 fibrosarcoma cells, and uPAR-overexpressing MCF-7 cells by a mechanism that depends on uPA receptor (uPAR)-ligation and ERK activation. Ras and MAP kinase kinase (MEK) were necessary and sufficient for uPA-induced ERK activation and stimulation of cellular migration, as demonstrated in experiments with dominant-negative and constitutively active mutants of these signaling proteins. Myosin light chain kinase (MLCK) was also required for uPA-stimulated cellular migration, as determined in experiments with three separate MLCK inhibitors. When MCF-7 cells were treated with uPA, MLCK was phosphorylated by a MEK-dependent pathway and apparently activated, since serine-phosphorylation of myosin II regulatory light chain (RLC) was also increased. Despite the transient nature of ERK phosphorylation, MLCK remained phosphorylated for at least 6 h. The uPA-induced increase in MCF-7 cell migration was observed selectively on vitronectin-coated surfaces and was mediated by a beta1-integrin (probably alphaVbeta1) and alphaVbeta5. When MCF-7 cells were transfected to express alphaVbeta3 and treated with uPA, ERK was still phosphorylated; however, the cells did not demonstrate increased migration. Neutralizing the function of alphaVbeta3, with blocking antibody, restored the ability of uPA to promote cellular migration. Thus, we have demonstrated that uPA promotes cellular migration, in an integrin-selective manner, by initiating a uPAR-dependent signaling cascade in which Ras, MEK, ERK, and MLCK serve as essential downstream effectors.

Identification of a region in the integrin beta3 subunit that confers ligand binding specificity

Many integrin adhesion receptors bind ligands containing the Arg-Gly-Asp (RGD) peptide motif. Most integrins exhibit considerable specificity for particular ligands and can distinguish among the many conformations of RGD. In this study we identify the domain of the integrin beta subunit involved in determining ligand binding specificity. Chimeras of beta3 and beta5, the most homologous integrin beta subunits, were expressed with alphav on the surface of human 293 cells. The ligand binding phenotype of each chimera was assessed using the ligands Fab-9 and fibrinogen, both of which have a binding preference for alphavbeta3. The results of the study show that when exons C and D of the beta3 subunit (residues 95-233) are substituted into beta5, the chimera gained the ability to bind Fab-9 with an affinity close to that of wild-type alphavbeta3. This chimera was able to mediate cell adhesion to fibrinogen. Furthermore, the swap of only a 39-residue segment of this larger domain, beta3 residues 164-202, into the backbone of beta5 enabled the chimeric integrin to bind soluble Fab-9. This small domain is highly divergent among the integrin beta subunits, suggesting that it may play a role in determining ligand selection by all integrins.

Influence of Interleukin-3 and Other Growth Factors on α4β1 Integrin-Mediated Adhesion and Migration of Human Hematopoietic Progenitor Cells

The mechanisms by which hematopoietic progenitor cells are normally anchored in stromal niches and yet can be mobilized by specific growth factors are poorly understood. It is likely, however, that integrins and their extracellular matrix (ECM) ligands play a key role in this process, and recent evidence suggests that integrin function is modulated by signals originating from activated growth factor receptors. We have now examined this further by studying the role of growth factors on α4β1 integrin-mediated adhesion of human CD34+ hematopoietic progenitor cells to specific recombinant fibronectin fragments coated onto tissue culture dishes. Cells were prepared from cord blood and peripheral blood harvests. During a 30-minute adhesion assay a mean of 74% of CD34 cells attached to the so-called H120 fragment of fibronectin, which contains the strongest α4β1 integrin-binding sequence. The level of cell adhesion was significantly reduced by low concentrations of interleukin-3 (IL-3) (2.5 to 10 ng/mL), whereas stem cell factor (SCF ) and granulocyte colony-stimulating factor (G-CSF ) at these concentrations did not affect adherence of the cells. Migratory behavior of CD34 cells was examined using fibronectin fragments adsorbed onto a Transwell filter. The H120 fragment supported much higher levels of cell migration than the H0 fragment of fibronectin, which contains a weak α4β1 integrin binding sequence. Over a 16-hour assay, migration of peripheral blood progenitor cells was increased slightly by SCF and by G-CSF. However, a marked stimulation was observed with IL-3, which significantly increased migration. Similar effects were noted with cord blood cells, although a small proportion of cells were able to migrate in the absence of growth factors. These results indicate that there is a highly selective and functional link between the α4β1 integrin and IL-3/IL-3–receptor that could affect the position of stem and progenitor cells in the marrow stroma and influence their growth and development.

Localization of matrix metalloproteinase MMP-2 to the surface of invasive cells by interaction with integrin alpha v beta 3

SUMMARY

Cellular invasion depends on cooperation between adhesive and proteolytic mechanisms. Evidence is provided that the matrix metalloproteinase MMP-2 can be localized in a proteolytically active form on the surface of invasive cells, based on its ability to bind directly integrin alpha v beta 3. MMP-2 and alpha v beta 3 were specifically colocalized on angiogenic blood vessels and melanoma cells in vivo. Expression of alpha v beta 3 on cultured melanoma cells enabled their binding to MMP-2 in a proteolytically active form, facilitating cell-mediated collagen degradation. In vitro, these proteins formed an SDS-stable complex that depended on the noncatalytic C-terminus of MMP-2, since a truncation mutant lost the ability to bind alpha v beta 3. These findings define a single cell-surface receptor that regulates both matrix degradation and motility, thereby facilitating directed cellular invasion.