Expression of beta 1B integrin isoform in CHO cells results in a dominant negative effect on cell adhesion and motility

mTOR pathway inhibition alters proliferation as well as differentiation of neural stem cells

Introduction

Neural stem cells (NSCs) are essential for both embryonic development and adult neurogenesis, and their dysregulation causes a number of neurodevelopmental disorders, such as epilepsy and autism spectrum disorders. NSC proliferation and differentiation in the developing brain is a complex process controlled by various intrinsic and extrinsic stimuli. The mammalian target of rapamycin (mTOR) regulates proliferation and differentiation, among other cellular functions, and disruption in the mTOR pathway can lead to severe nervous system development deficits. In this study, we investigated the effect of inhibition of the mTOR pathway by rapamycin (Rapa) on NSC proliferation and differentiation.

Methods

The NSC cultures were treated with Rapa for 1, 2, 6, 24, and 48 h. The effect on cellular functions was assessed by immunofluorescence staining, western blotting, and proliferation/metabolic assays.

Results

mTOR inhibition suppressed NSC proliferation/metabolic activity as well as S-Phase entry by as early as 1 h of Rapa treatment and this effect persisted up to 48 h of Rapa treatment. In a separate experiment, NSCs were differentiated for 2 weeks after treatment with Rapa for 24 or 48 h. Regarding the effect on neuronal and glial differentiation (2 weeks post-treatment), this was suppressed in NSCs deficient in mTOR signaling, as evidenced by downregulated expression of NeuN, MAP2, and GFAP. We assume that the prolonged effect of mTOR inhibition is realized due to the effect on cytoskeletal proteins.

Discussion

Here, we demonstrate for the first time that the mTOR pathway not only regulates NSC proliferation but also plays an important role in NSC differentiation into both neuronal and glial lineages.

Effects of CD49d-targeted antisense-oligonucleotide on α4 integrin expression and function of acute lymphoblastic leukemia cells: Results of in vitro and in vivo studies

We recently demonstrated the effectiveness of blocking CD49d with anti-functional antibodies or small molecule inhibitors as a rational targeted approach to the treatment of acute leukemia in combination with chemotherapy. Antisense oligonucleotide promises to be no less specific than antibodies and inhibitors, but more interesting for pharmacokinetics and pharmacodynamics. We addressed this using the published CD49d antisense drug ATL1102. In vitro, we incubated/nucleofected the ALL cell line Kasumi-2 with ATL1102. In vivo, immunodeficient hosts were engrafted with primary ALL cells and treated with ATL1102. Changes in expression of CD49d mRNA and CD49d protein, and of cooperating gene products, including ß1 integrin and CXCR4, as well as survival in the mouse experiments were quantified. We observed dose-dependent down-regulation of CD49d mRNA and protein levels and its partner integrin ß1 cell surface protein level and, up-regulation of CXCR4 surface expression. The suppression was more pronounced after nucleofection than after incubation, where down-regulation was significant only at the higher doses. In vivo effects of ATL1102 were not sufficient to translate into “clinical” benefit in the leukemia model. In summary, antisense oligonucleotides are successful tools for specifically modulating gene expression but sufficient delivery to down-regulate CD49d in vivo may be difficult to achieve.

The conformational state of hERG1 channels determines integrin association, downstream signaling, and cancer progression

Ion channels regulate cell proliferation, differentiation, and migration in normal and neoplastic cells through cell-cell and cell-extracellular matrix (ECM) transmembrane receptors called integrins. K⁺ flux through the human ether-à-go-go-related gene 1 (hERG1) channel shapes action potential firing in excitable cells such as cardiomyocytes. Its abundance is often aberrantly high in tumors, where it modulates integrin-mediated signaling. We found that hERG1 interacted with the β₁ integrin subunit at the plasma membrane of human cancer cells. This interaction was not detected in cardiomyocytes because of the presence of the hERG1 auxiliary subunit KCNE1 (potassium voltage-gated channel subfamily E regulatory subunit 1), which blocked the β₁ integrin-hERG1 interaction. Although open hERG1 channels did not interact as strongly with β₁ integrins as did closed channels, current flow through hERG1 channels was necessary to activate the integrin-dependent phosphorylation of Tyr³⁹⁷ in focal adhesion kinase (FAK) in both normal and cancer cells. In immunodeficient mice, proliferation was inhibited in breast cancer cells expressing forms of hERG1 with impaired K⁺ flow, whereas metastasis of breast cancer cells was reduced when the hERG1/β₁ integrin interaction was disrupted. We conclude that the interaction of β₁ integrins with hERG1 channels in cancer cells stimulated distinct signaling pathways that depended on the conformational state of hERG1 and affected different aspects of tumor progression.

The novel α4B murine α4 integrin protein splicing variant inhibits α4 protein-dependent cell adhesion

Integrins affect the motility of multiple cell types to control cell survival, growth, or differentiation, which are mediated by cell-cell and cell-extracellular matrix interactions. We reported previously that the α9 integrin splicing variant, SFα9, promotes WT α9 integrin-dependent adhesion. In this study, we introduced a new murine α4 integrin splicing variant, α4B, which has a novel short cytoplasmic tail. In inflamed tissues, the expression of α4B, as well as WT α4 integrin, was up-regulated. Cells expressing α4B specifically bound to VCAM-1 but not other α4 integrin ligands, such as fibronectin CS1 or osteopontin. The binding of cells expressing WT α4 integrin to α4 integrin ligands is inhibited by coexpression of α4B. Knockdown of α4B in metastatic melanoma cell lines results in a significant increase in lung metastasis. Expression levels of WT α4 integrin are unaltered by α4B, with α4B acting as a regulatory subunit for WT α4 integrin by a dominant-negative effect or inhibiting α4 integrin activation.

C-terminal COOH of integrin β1 is necessary for β1 association with the kindlin-2 adapter protein

Protein-protein interactions are driving forces in cellular processes. As a prime example, transmembrane integrins link extracellular matrix and intracellular proteins, resulting in bidirectional signaling that regulates cell migration, proliferation, differentiation, and survival. Here we provide the first evidence that interaction between the integrin β1 cytoplasmic tail and kindlin-2, a member of a family of adapters implicated in human disease pathogenesis, is mainly governed by the β1 C-terminal carboxylate moiety and is required for laterality organ development in zebrafish. Affinity measurements indicate that this unusual protein-protein interaction mode is coordinated by a putative carboxylate-binding motif in the kindlin-2 FERM subdomain F3. Contrary to the C terminus of proteins that engage PDZ domains, the C-terminal three residues of β1, per se, do not contribute to kindlin-2 binding or to laterality organ development. Thus, by employing zebrafish as an in situ physiological tool to correlate protein structure and function, we have discovered an unexpected association chemistry between an integrin and a key adapter involved in integrin signaling.

A novel mutation in β integrin reveals an integrin-mediated interaction between the extracellular matrix and cki-1/p27KIP1

The cell-extracellular matrix (ECM) interaction plays an essential role in maintaining tissue shapes and regulates cell behaviors such as cell adhesion, differentiation and proliferation. The mechanism by which the ECM influences the cell cycle in vivo is poorly understood. Here we demonstrate that the β integrin PAT-3 regulates the localization and expression of CKI-1, a C. elegans homologue of the cyclin dependent kinase inhibitor p27(KIP1). In nematodes expressing wild type PAT-3, CKI-1::GFP localizes primarily to nucleoli in hypodermal cells, whereas in animals expressing mutant pat-3 with a defective splice junction, CKI-1::GFP appears clumped and disorganized in nucleoplasm. RNAi analysis links cell adhesion genes to the regulation of CKI-1. RNAi of unc-52/perlecan, ina-1/α integrin, pat-4/ILK, and unc-97/PINCH resulted in abnormal CKI-1::GFP localization. Additional RNAi experiments revealed that the SCF E3 ubiquitin-ligase complex genes, skpt-1/SKP2, cul-1/CUL1 and lin-23/F-box, are required for the proper localization and expression of CKI-1, suggesting that integrin signaling and SCF E3 ligase work together to regulate the cellular distribution of CKI-1. These data also suggest that integrin plays a major role in maintaining proper CKI-1/p27(KIP1) levels in the cell. Perturbed integrin signaling may lead to the inhibition of SCF ligase activity, mislocalization and elevation of CKI-1/p27(KIP1). These results suggest that adhesion signaling is crucial for cell cycle regulation in vivo.

Bartonella henselae engages inside-out and outside-in signaling by integrin β1 and talin1 during invasome-mediated bacterial uptake

The VirB/D4 type IV secretion system (T4SS) of the bacterial pathogen Bartonella henselae (Bhe) translocates seven effector proteins (BepA-BepG) into human cells that subvert host cellular functions. Two redundant pathways dependent on BepG or the combination of BepC and BepF trigger the formation of a bacterial uptake structure termed the invasome. Invasome formation is a multi-step process consisting of bacterial adherence, effector translocation, aggregation of bacteria on the cell surface and engulfment, and eventually, complete internalization of the bacterial aggregate occurs in an F-actin-dependent manner. In the present study, we show that Bhe-triggered invasome formation depends on integrin-β1-mediated signaling cascades that enable assembly of the F-actin invasome structure. We demonstrate that Bhe interacts with integrin β1 in a fibronectin- and VirB/D4 T4SS-independent manner and that activated integrin β1 is essential for both effector translocation and the actin rearrangements leading to invasome formation. Furthermore, we show that talin1, but not talin2, is required for inside-out activation of integrin β1 during invasome formation. Finally, integrin-β1-mediated outside-in signaling by FAK, Src, paxillin and vinculin is necessary for invasome formation. This is the first example of a bacterial entry process that fully exploits the bi-directional signaling capacity of integrin receptors in a talin1-specific manner.

Extended interaction of beta1 integrin subunit-deficient cells (GD25) with surfaces modified with fibronectin-derived peptides: Culture optimization, adhesion and cytokine panel studies

The modification of biomaterials with extracellular matrix-mimicking factors is a common technique used to influence the cellular response through integrin-mediated signaling. The inherent limitations of antibody-inhibition studies necessitate the use of complementary methods to block integrin function to confirm cell-surface interaction. In this study, we employed a beta1 integrin-deficient cell line, GD25, to investigate the role of beta1 subunit in cell adhesion and subsequent cytokine (granulocyte macrophage colony stimulating factor; interleukin (IL)-1alpha; IL-1beta; IL-6; monocyte chemoattractant protein-1; regulated upon activation, normal T-cell expressed, and secreted; tumor necrosis factor-alpha) release kinetics in the presence of tissue culture polystyrene (TCPS) and semi-interpenetrating polymer networks (sIPN) modified with fibronectin (FN)-mimic peptides (RGD, PHSRN). Culture conditions (i.e. seeding density, medium, serum supplementation) were optimized for long-term observation. Differences in cell adhesion, cell viability and cytokine release behavior were dependent on the presence of the beta1 integrin subunit, FN, sIPN cast method and peptide identity. By comparing two complementary techniques for assaying integrin function, we observed both similarities (i.e. decreased adhesion to FN-absorbed TCPS and increased IL-1beta release at 96h) and differences (i.e. no difference in adhesion or IL-1beta release in the presence of different sIPN surfaces) when the function of the beta1 subunit was blocked in cell adhesion and signaling in the presence of biomaterials.

CD24 expression causes the acquisition of multiple cellular properties associated with tumor growth and metastasis

The glycosylphosphatidylinositol-anchored membrane protein CD24 functions as an adhesion molecule for P-selectin and L1 and plays a role in B-cell development and neurogenesis. Over the last few years, a large body of literature has also implicated CD24 expression in tumorigenesis and progression. Here, we show that ectopic CD24 expression can be sufficient to promote tumor metastasis in experimental animals. By developing a doxycycline-inducible system for the expression of CD24 in breast cancer cells, we have also analyzed the cellular properties that CD24 expression influences. We found that CD24 expression increased tumor cell proliferation. Furthermore, in addition to promoting binding to P-selectin, CD24 expression also indirectly stimulated cell adhesion to fibronectin, collagens I and IV, and laminin through the activation of alpha3beta1 and alpha4beta1 integrin activity. Moreover, CD24 expression supported rapid cell spreading and strongly induced cell motility and invasion. CD24-induced proliferation and motility were integrin independent. Together, these observations implicate CD24 in the regulation of multiple cell properties of direct relevance to tumor growth and metastasis.

Altered expression of integrins in RSV-transformed chick epiphyseal chondrocytes

Chondrocytes have been shown to express both in vivo and in vitro a number of integrins of the beta1-, beta3- and beta5-subfamilies (Biorheology 37 (2000) 109). Normal and v-Src-transformed chick epiphyseal chondrocytes (CEC) display different adhesion properties. While normal CEC with time in culture tends to increase their adhesion to the substrate by organizing focal adhesions and actin stress fibers, v-Src-transformed chondrocytes display a refractile morphology and disorganization of actin cytoskeleton. We wondered whether the reduced adhesion and spreading of v-Src-transformed chondrocytes could be ascribed to changes in integrin expression and/or function. Integrin expression by normal CEC is studied and compared to v-Src-transformed chick chondrocytes, using monoclonal and polyclonal antibodies to integrins alpha- and beta-chains. We show the presence of alpha1-, alpha3-, alphav-, alpha6-, beta1- and beta3-chains on CEC, with very low levels of alpha2- and alpha5-chains. Alphav chain associates with multiple beta subunits in normal and transformed chondrocytes. With the exception of alpha1- and alpha2-chains, the levels of the integrin chains analyzed are higher in transformed chondrocytes as compared with normal chondrocytes. In spite of the increased levels of integrin expression, transformed chondrocytes exhibit loss of focal adhesion and actin stress fibers and low adhesion activity on several extracellular matrix constituents. These observations raise the possibility that, in addition to its effects on global pattern of integrin expression, v-Src can influence integrin function in chondrocytes.

Role of the β1-integrin cytoplasmic tail in mediating invasin-promoted internalization of Yersinia

Invasin of Yersinia pseudotuberculosis binds to β1-integrins on host cells and triggers internalization of the bacterium. To elucidate the mechanism behind the β1-integrin-mediated internalization of Yersinia, a β1-integrin-deficient cell line, GD25, transfected with wild-type β1A, β1B or different mutants of the β1A subunit was used. Both β1A and β1B bound to invasin-expressing bacteria, but only β1A was able to mediate internalization of the bacteria. The cytoplasmic region of β1A, differing from β1B, contains two NPXY motifs surrounding a double threonine site. Exchanging the tyrosines of the two NPXYs to phenylalanines did not inhibit the uptake, whereas a marked reduction was seen when the first tyrosine (Y783) was exchanged to alanine. A similar reduction was seen when the two nearby threonines (TT788-9) were exchanged with alanines. It was also noted that cells affected in bacterial internalization exhibited reduced spreading capability when seeded onto invasin, suggesting a correlation between the internalization of invasin-expressing bacteria and invasin-induced spreading. Likewise, integrins defective in forming peripheral focal complex structures was unable to mediate uptake of invasin-expressing bacteria.

Disruption of C-terminal cytoplasmic domain of betaPS integrin subunit has dominant negative properties in developing Drosophila

We have analyzed a set of new and existing strong mutations in the myospheroid gene, which encodes the betaPS integrin subunit of Drosophila. In addition to missense and other null mutations, three mutants behave as antimorphic alleles, indicative of dominant negative properties. Unlike null alleles, the three antimorphic mutants are synthetically lethal in double heterozygotes with an inflated (alphaPS2) null allele, and they fail to complement very weak, otherwise viable alleles of myospheroid. Two of the antimorphs result from identical splice site lesions, which create a frameshift in the C-terminal half of the cytoplasmic domain of betaPS. The third antimorphic mutation is caused by a stop codon just before the cytoplasmic splice site. These mutant betaPS proteins can support cell spreading in culture, especially under conditions that appear to promote integrin activation. Analyses of developing animals indicate that the dominant negative properties are not a result of inefficient surface expression, or simple competition between functional and nonfunctional proteins. These data indicate that mutations disrupting the C-terminal cytoplasmic domain of integrin beta subunits can have dominant negative effects in situ, at normal levels of expression, and that this property does not necessarily depend on a specific new protein sequence or structure. The results are discussed with respect to similar vertebrate beta subunit cytoplasmic mutations.

The integrin cytoplasmic domain-associated protein ICAP-1 binds and regulates Rho family GTPases during cell spreading

Using two-hybrid screening, we isolated the integrin cytoplasmic domain-associated protein (ICAP-1), an interactor for the COOH terminal region of the beta1A integrin cytoplasmic domain. To investigate the role of ICAP-1 in integrin-mediated adhesive function, we expressed the full-length molecule in NIH3T3 cells. ICAP-1 expression strongly prevents NIH3T3 cell spreading on extracellular matrix. This inhibition is transient and can be counteracted by coexpression of a constitutively activated mutant of Cdc42, suggesting that ICAP-1 acts upstream of this GTPase. In addition, we found that ICAP-1 binds both to Cdc42 and Rac1 in vitro, and its expression markedly inhibits activation of these GTPases during integrin-mediated cell adhesion to fibronectin as detected by PAK binding assay. In the attempt to define the molecular mechanism of this inhibition, we show that ICAP-1 reduces both the intrinsic and the exchange factor-induced dissociation of GDP from Cdc42; moreover, purified ICAP-1 displaces this GTPase from cellular membranes. Together, these data show for the first time that ICAP-1 regulates Rho family GTPases during integrin-mediated cell matrix adhesion, acting as guanine dissociation inhibitor.

Cross talk between beta(1) and alpha(V) integrins: beta(1) affects beta(3) mRNA stability

There is increasing evidence that a fine-tuned integrin cross talk can generate a high degree of specificity in cell adhesion, suggesting that spatially and temporally coordinated expression and activation of integrins are more important for regulated cell adhesive functions than the intrinsic specificity of individual receptors. However, little is known concerning the molecular mechanisms of integrin cross talk. With the use of beta(1)-null GD25 cells ectopically expressing the beta(1)A integrin subunit, we provide evidence for the existence of a cross talk between beta(1) and alpha(V) integrins that affects the ratio of alpha(V)beta(3) and alpha(V)beta(5) integrin cell surface levels. In particular, we demonstrate that a down-regulation of alpha(V)beta(3) and an up-regulation of alpha(V)beta(5) occur as a consequence of beta(1)A expression. Moreover, with the use of GD25 cells expressing the integrin isoforms beta(1)B and beta(1)D, as well as two beta(1) cytoplasmic domain deletion mutants lacking either the entire cytoplasmic domain (beta(1)TR) or only its "variable" region (beta(1)COM), we show that the effects of beta(1) over alpha(V) integrins take place irrespective of the type of beta(1) isoform, but require the presence of the "common" region of the beta(1) cytoplasmic domain. In an attempt to establish the regulatory mechanism(s) whereby beta(1) integrins exert their trans-acting functions, we have found that the down-regulation of alpha(V)beta(3) is due to a decreased beta(3) subunit mRNA stability, whereas the up-regulation of alpha(V)beta(5) is mainly due to translational or posttranslational events. These findings provide the first evidence for an integrin cross talk based on the regulation of mRNA stability.

A novel syndrome of variant leukocyte adhesion deficiency involving defects in adhesion mediated by beta1 and beta2 integrins

Leukocyte adhesion deficiency type I (LAD-1) is a disorder associated with severe and recurrent bacterial infections, impaired extravascular targeting and accumulation of myeloid leukocytes, altered wound healing, and significant morbidity that is caused by absent or greatly diminished surface expression of integrins of the beta2 class. We report clinical features and analysis of functions of cells from a patient with a myelodysplastic syndrome and infectious complications similar to those in the severe form of LAD-1, but whose circulating neutrophils displayed normal levels of beta2 integrins. Analysis of adhesion of these cells to immobilized ligands and to endothelial cells and assays of cell-cell aggregation and chemotaxis demonstrated a profound defect in adhesion mediated by beta2 integrins indicative of a variant form of LAD-1. A novel cell line established from Epstein-Barr virus-transformed lymphoblasts from the subject demonstrated deficient beta2 integrin-dependent adhesive function similar to that of the primary leukocytes. In addition, these cells had markedly impaired beta1 integrin-dependent adhesion. Sequence analysis and electrophoretic mobility of beta1 and beta2 proteins from the cell line demonstrated that the defects were not a result of structural abnormalities in the integrin subunit chains themselves and suggest that the adhesive phenotype of these cells is due to one or more abnormalities of inside-out signaling mechanisms that regulate the activity of integrins of these classes. These features define a unique LAD-1 variant syndrome that may reveal important insights that are generally relevant to inside-out signaling of integrins, a molecular process that is as yet incompletely understood.

Contact with fibrillar collagen inhibits melanoma cell proliferation by up-regulating p27KIP1

It is known that the extracellular matrix regulates normal cell proliferation, and it is assumed that anchorage-independent malignant cells escape this regulatory function. Here we demonstrate that human M24met melanoma cells remain responsive to growth regulatory signals that result from contact with type I collagen and that the effect on proliferation depends on the physical structure of the collagen. On polymerized fibrillar collagen, M24met cells are growth arrested at the G(1)/S checkpoint and maintain high levels of p27(KIP1) mRNA and protein. In contrast, on nonfibrillar (denatured) collagen, the cells enter the cell cycle, and p27(KIP1) is down-regulated. These growth regulatory effects involve contact between type I collagen and the collagen-binding integrin alpha(2)beta(1), which appears restricted in the presence of fibrillar collagen. Thus melanoma cells remain sensitive to negative growth regulatory signals originating from fibrillar collagen, and the proteolytic degradation of fibrils is a mechanism allowing tumor cells to escape these restrictive signals.

The association of CRKII with C3G can be regulated by integrins and defines a novel means to regulate the mitogen-activated protein kinases

In studies to define mechanisms of ERK activation in Chinese hamster ovary cells, we have observed an inverse correlation between CRKII-C3G complex formation and ERK activity. That is, we were able to coprecipitate the guanine nucleotide exchange factor C3G with the adaptor protein CRKII in lysates from suspended cells that had low ERK activity, but we could not do so or could do so less efficiently in lysates of adherent cells with increased ERK activity. Consistent with the presence of a functional CRKII-C3G complex, we detected more GTP-loaded RAP1 in suspension than adherent lysates. Overexpression of cDNAs encoding B-RAF, CRKII W109L, and PTP1B C215S activated ERK in suspension cells, the latter two constructs also disrupting CRKII-C3G complex formation. Finally, we have also observed that certain integrin alpha subunit cytoplasmic splice variants differentially regulate ERK1/2 but also in a manner that correlated with levels of a CRKII-C3G complex. Thus, these data suggest the involvement of integrins in an ERK suppression pathway mediated by CRKII-C3G complex formation and downstream signaling from activated RAP1.

beta1B integrin subunit contains a double lysine motif that can cause accumulation within the endoplasmic reticulum

Human epidermal keratinocytes are one of the few cell types that express the beta1B splice variant of the beta1 integrin subunit. Although in transfection experiments beta1B acts as a dominant negative inhibitor of cell adhesion, we found that beta1B was expressed at very low levels in keratinocytes, both in vivo and in culture, and had a predominantly cytoplasmic distribution, concentrated within the endoplasmic reticulum. To examine why beta1B accumulated in the cytoplasm, we prepared chimeras between CD8alpha and the beta1A and beta1B integrin cytoplasmic domains. In transfected HeLa cells, both constructs reached the cell surface but the rate of maturation of the beta1B chimera was considerably retarded relative to beta1A. The beta1B cytoplasmic domain contains two lysine residues that resemble the double lysine motif characteristic of many proteins that are resident within the endoplasmic reticulum. Mutation of each lysine individually to serine had no effect on CD8beta1B maturation, but when both residues were mutated the rate of CD8beta1B maturation increased to that of CD8beta1A. Further analysis of beta1B function in keratinocytes must, therefore, take into account the low abundance of the isoform relative to beta1A and the potential for beta1B to accumulate in the endoplasmic reticulum.

Expression of integrin subunit beta1B in integrin beta1-deficient GD25 cells does not interfere with alphaVbeta3 functions

We have expressed the beta1B integrin subunit in beta1-deficient GD25 cells to examine beta1B functions without the interference of endogenous beta1A expression. As previously reported [Retta et al., 1998, Mol. Biol. Cell 9, 715-731], the beta1B integrins did not mediate cell adhesion under normal culture conditions, while the presence of 0.3 mM Mn(2+) allowed beta1B integrins to support adhesion. Mn(2+), as well as the small soluble peptide GRGDS, induced a beta1B conformation, which was recognized by the mAb 9EG7, a marker for active or ligand-bound integrins. beta1B integrins were found to localize to a subset of focal contacts in a ligand-independent manner on fibronectin, but not on vitronectin. However, clustering of beta1B did not induce tyrosine phosphorylation of FAK, p130(Cas), or paxillin, as studied by beta1B-mediated adhesion, to fibronectin in the presence of Mn(2+) or to anti-beta1 antibody in DMEM. Induction of ligand-occupied conformation by the GRGDS peptide during the adhesion to anti-beta1 antibody also failed to trigger FAK phosphorylation. Stimulation of tyrosine phosphorylation on FAK, p130(Cas), and paxillin by adhesion via integrin alphaVbeta3 to fibronectin or vitronectin was not disturbed in GD25-beta1B cells compared to the untransfected GD25 cells, nor were any negative effects of beta1B observed on alphaVbeta3-mediated cell attachment, spreading, and actin organization, or on the cell proliferation rate. These results show that the reported negative effects of beta1B on adhesive events do not apply to alphaVbeta3-dependent interactions and suggest that they may specifically act on beta1 integrins.

Integrin subunits (beta)1C-1 and (beta)1C-2 expressed in GD25T cells are retained and degraded intracellularly rather than localised to the cell surface

We have previously identified the integrin (beta)1C-2 and characterised the distribution of (beta)1C-1 and (beta)1C-2 transcripts in various cell lines and normal cells. In this study we have investigated the expression of the two (beta)1C-variants in integrin (beta)1 deficient mouse GD25T cells. After stable transfection of the GD25T cells with cDNAs coding for (beta)1A, (beta)1C-1 and (beta)1C-2, the cell surface expression of the (beta)1C-1 and (beta)1C-2 variants was found to be very low while the (beta)1A variant was expressed at high levels. Northern blot analysis showed that the level of (beta)1-transcript in the (beta)1C-1 and (beta)1C-2 clones was equal or higher than in the (beta)1A clones. Metabolic labelling and deglycosylation by endoglycosidase H treatment clearly demonstrated that the majority of the (beta)1C-1 and (beta)1C-2 chains did not become maturely glycosylated, nor did they dimerize with (alpha) subunits. After 20 hours of chase, the labelled (beta)1C-1 and (beta)1C-2 chains had been gradually degraded, whereas immature (beta)1A was converted into the maturely glycosylated form during the same period of time. Immunostaining showed intracellular (beta)1 localisation in the (beta)1C-1 and (beta)1C-2 expressing clones, while in the (beta)1A expressing clones the (beta)1 chains were mainly localised to focal adhesion sites and along fibronectin fibres. Taken together, we have shown that expression of both integrin (beta)1C-1 and (beta)1C-2 in GD25T cells result in very low cell surface expression compared with the normal (beta)1A isoform. Instead, both (beta)1C-1 and (beta)1C-2 chains remain in the endoplasmic reticulum until they are intracellularly degraded.

Integrin family of cell adhesion molecules in the injured brain: regulation and cellular localization in the normal and regenerating mouse facial motor nucleus

Integrins are a large family of heterodimeric glycoproteins that play a crucial role in cell adhesion during development, inflammation, and tissue repair. In the current study, we investigated the localization of different integrin subunits in the mouse facial motor nucleus and their regulation after transection of the facial nerve. In the normal mouse brain, there was clear immunoreactivity for alpha5-, alpha6-, and beta1-integrin subunits on blood vessel endothelia and for alphaM- and beta2-subunits on resting parenchymal microglia. Facial nerve transection led to an up-regulation of the beta1-subunit on the axotomized neurons and an increase in the alpha4-, alpha5-, alpha6-, beta1-, alphaM-, alphaX-, and beta2-subunits on the adjacent, activated microglia. Quantification of the microglial integrins revealed two different expression patterns. The subunits alpha5 and alpha6 showed a monophasic increase with a maximum at day 4, the alphaM-subunit a biphasic regulation, with an early peak at day 1 and an elevated plateau between day 14 and 42. At day 14, there was also an influx of lymphocytes immunoreactive for the alpha4beta1- and alphaLbeta2-integrins, which aggregated at sites of neural debris and phagocytotic microglia. This finding was accompanied by a significant increase of the alpha5beta1-integrin on blood vessel endothelia. In summary, facial axotomy is followed by a strong and cell-type-specific expression of integrins on the affected neurons and on surrounding microglia, lymphocytes, and vascular endothelia. The presence of several, strikingly different temporal patterns suggests a selective involvement of these molecules in the different adhesive events during regeneration in the central nervous system.

Focal adhesion targeting: the critical determinant of FAK regulation and substrate phosphorylation

The focal adhesion kinase (FAK) is discretely localized to focal adhesions via its C-terminal focal adhesion-targeting (FAT) sequence. FAK is regulated by integrin-dependent cell adhesion and can regulate tyrosine phosphorylation of downstream substrates, like paxillin. By the use of a mutational strategy, the regions of FAK that are required for cell adhesion-dependent regulation and for inducing tyrosine phosphorylation of paxillin were determined. The results show that the FAT sequence was the single region of FAK that was required for each function. Furthermore, the FAT sequence of FAK was replaced with a focal adhesion-targeting sequence from vinculin, and the resulting chimera exhibited cell adhesion-dependent tyrosine phosphorylation and could induce paxillin phosphorylation like wild-type FAK. These results suggest that subcellular localization is the major determinant of FAK function.

Integrins: alternative splicing as a mechanism to regulate ligand binding and integrin signaling events

Integrins are a family of transmembrane proteins composed of heterodimers of alpha and beta subunits. With their extracellular domain they bind extracellular matrix proteins or other cell surface molecules, and their cytoplasmic domain binds to cytoskeletal and signaling proteins. Thus, they are in an ideal position to transfer information from the extracellular environment to the interior of the cell and vice versa. For several integrin subunits, alternative splicing of mRNA leads to variations in the sequence of both extracellular and cytoplasmic domains. Many integrin splice variants have specific expression patterns, but for some time, functional differences between these variants were not evident. Recent experiments using transfected cell lines and gene targeting of specific splice variants have contributed significantly to our understanding of the function of these splice variants. The results indicate that alternative splicing is a mechanism to subtly regulate the ligand binding and signaling activity of integrins.

R-Ras signals through specific integrin alpha cytoplasmic domains to promote migration and invasion of breast epithelial cells

Specificity and modulation of integrin function have important consequences for cellular responses to the extracellular matrix, including differentiation and transformation. The Ras-related GTPase, R-Ras, modulates integrin affinity, but little is known of the signaling pathways and biological functions downstream of R-Ras. Here we show that stable expression of activated R-Ras or the closely related TC21 (R-Ras 2) induced integrin-mediated migration and invasion of breast epithelial cells through collagen and disrupted differentiation into tubule structures, whereas dominant negative R-Ras had opposite effects. These results imply novel roles for R-Ras and TC21 in promoting a transformed phenotype and in the basal migration and polarization of these cells. Importantly, R-Ras induced an increase in cellular adhesion and migration on collagen but not fibronectin, suggesting that R-Ras signals to specific integrins. This was further supported by experiments in which R-Ras enhanced the migration of cells expressing integrin chimeras containing the alpha2, but not the alpha5, cytoplasmic domain. In addition, a transdominant inhibition previously noted only between integrin beta cytoplasmic domains was observed for the alpha2 cytoplasmic domain; alpha2beta1-mediated migration was inhibited by the expression of excess alpha2 but not alpha5 cytoplasmic domain-containing chimeras, suggesting the existence of limiting factors that bind the integrin alpha subunit. Using pharmacological inhibitors, we found that R-Ras induced migration on collagen through a combination of phosphatidylinositol 3-kinase and protein kinase C, but not MAPK, which is distinct from the other Ras family members, Rac, Cdc42, and N- and K-Ras. Thus, R-Ras communicates with specific integrin alpha cytoplasmic domains through a unique combination of signaling pathways to promote cell migration and invasion.

Mutational analysis of cell cycle inhibition by integrin beta1C

Integrin beta1C is an alternatively spliced cytoplasmic variant of the beta1 subunit that potently inhibits cell cycle progression. In this study, we analyzed the requirements for growth suppression by beta1C. A chimera containing the extracellular/transmembrane domain of the Tac subunit of the human interleukin 2 receptor (gp55) fused to the cytoplasmic domain of beta1C (residues 732-805) strongly inhibited growth in mouse 10T1/2 cells even at low expression levels, whereas chimeras containing the beta1A, beta1B, beta1D, beta3, and beta5 cytoplasmic domains had weak and variable effects. The beta1C cytoplasmic domain is composed of a membrane proximal region (732-757) common to all beta1 variants and a COOH-terminal 48-amino acid domain (758-805) unique to beta1C. The beta1C-specific domain (758-805) was sufficient to block cell growth even when expressed as a soluble cytoplasmic green fluorescent protein fusion protein. These results indicate that growth inhibition by beta1C does not require the intact receptor and can function in the absence of membrane targeting. Analysis of deletions within the beta1C-specific domain showed that the 18-amino acid sequence 775-792 is both necessary and sufficient for maximal growth inhibition, although the 13 COOH-terminal residues (793-805) also had weak activity. Finally, beta1C is known to be induced in endothelial cells in response to tumor necrosis factor and is down-regulated in prostate epithelial cells after transformation. The green fluorescent protein/beta1C (758-805) chimera blocked growth in the human endothelial cell line EV304 and in the transformed prostate epithelial cell line DU145, consistent with a role for beta1C as a growth inhibitor in vivo.

RhoA activity is required for fibronectin assembly and counteracts beta1B integrin inhibitory effect in FRT epithelial cells

FRT thyroid epithelial cells synthesize fibronectin and organize a network of fibronectin fibrils at the basal surface of the cells. Fibronectin fibril formation is enhanced by the overexpression of the ubiquitous beta1A integrin and is inhibited by the expression of the dominant-negative beta1B subunit. We tested the hypotheses that RhoA activity might mediate the integrin-dependent fibronectin fibrillogenesis and might counteract beta1B integrin inhibitory effect. FRT-beta1A cells were transfected with a vector carrying a dominant negative form of RhoA (RhoAN19) or treated with the C3 transferase exoenzyme. Both treatments inhibited fibronectin assembly and caused loss of actin microfilaments and adhesion plaques. On the other hand, FRT-beta1B cells were transfected with the constitutively activated form of RhoA (RhoAV14) or treated with the E. coli cytotoxic necrotizing factor 1, which directly activates RhoA. Either treatment restored microfilament and adhesion plaque assembly and promoted fibronectin fibril organization. A great increase in fibronectin fibril assembly was also obtained by treatment of FRT-beta1B cells with TGF-beta. Our data indicate that RhoA is required to promote fibronectin matrix assembly in FRT cells and that the activation of the signal transduction pathway downstream of RhoA can overcome the inhibitory effect of beta1B integrin.

The muscle-specific laminin receptor alpha7 beta1 integrin negatively regulates alpha5 beta1 fibronectin receptor function

alpha7 beta1 is the major integrin complex expressed in differentiated muscle cells where it functions as a laminin receptor. In this work we have expressed the alpha7 integrin subunit in CHO cells to investigate the functional properties of this receptor. After transfection with alpha7 CHO cells acquired the ability to adhere and spread on laminin 1 consistent with the laminin receptor activity of the alpha7 beta1. alpha7 transfectants, however, showed a 70% reduction in the ability to adhere to fibronectin and were unable to assemble a fibronectin matrix. The degree of reduction was inversely related to the level of alpha7 expression. To define the mechanisms underlying this adhesive defect we analyzed surface expression and functional properties of the alpha5 beta1 fibronectin receptor. Although cell surface expression of alpha5 beta1 was reduced by a factor of 20-25% in alpha7 transfectants compared to control untransfected cells, this slight reduction was not sufficient to explain the dramatic reduction in cell adhesion (70%) and matrix assembly (close to 100%). Binding studies showed that the affinity of 125I-fibronectin for its surface receptor was decreased by 50% in alpha7 transfectants, indicating that the alpha5 beta1 integrin is partially inactivated in these cells. Inactivation can be reversed by Mn2+, a cation known to increase integrin affinity for their ligands. In fact, incubation of cells with Mn2+ restored fibronectin binding affinity, adhesion to fibronectin, and assembly of fibronectin matrix in alpha7 transfectants. These data indicate that alpha7 expression leads to the functional down regulation of alpha5beta1 integrin by decreasing ligand binding affinity and surface expression. In conclusion, the data reported establish the existence of a negative cooperativity between alpha7 and alpha5 integrins that may be important in determining functional regulation of integrins during myogenic differentiation.

Interaction of the integrin beta1 cytoplasmic domain with ICAP-1 protein

In a yeast two-hybrid screen, a protein named ICAP-1 (beta1 integrin cytoplasmic domain associated protein) associated with the integrin beta1 cytoplasmic tail but not with tails from three other integrin beta subunits (beta2, beta3, and beta5) or from seven different alpha subunits. Likewise in human cells, ICAP-1 associated specifically with the beta1 but not beta2, beta3, or beta5 tails. The carboxyl-terminal 14 amino acids of beta1 were critical for ICAP-1 interaction. ICAP-1 is a ubiquitously expressed protein of 27 and 31 kDa, with the smaller form being preferentially solubilized by Triton X-100. Phosphorylation of both 27- and 31-kDa forms was constitutive but was increased by 1.5-2-fold upon cell spreading on fibronectin, compared with poly-L-lysine. Also, ICAP-1 contributes to beta1 integrin-dependent migration because (i) ICAP-1 transfection markedly increased chemotactic migration of COS7 cells through fibronectin-coated but not vitronectin-coated porous filters, and (ii) support of beta1-dependent cell migration (in Chinese hamster ovary cells transfected with various wild type and mutant beta1 forms) correlated with ICAP-1 association. In summary, ICAP-1 (i) associates specifically with beta1 integrins, (ii) is phosphorylated upon beta1 integrin-mediated adhesion, and (iii) may regulate beta1-dependent cell migration.

Differential expression of laminin receptors in human hepatocellular carcinoma

BACKGROUND

Laminin receptors are involved in cell-extracellular matrix interactions in malignant cells that show invasion and metastasis. Hepatocellular carcinoma frequently shows early invasion into blood vessels, and intrahepatic and extrahepatic metastases. However, the role of laminin receptors in hepatocellular carcinoma is unknown.

AIMS

To examine the expression of mRNA for laminin receptors and their isoforms in hepatocellular carcinoma.

METHODS

The expression of several laminin receptors, including alpha1 integrin, alpha6 integrin and its isoforms alpha6A and alpha6B, beta1 integrin and its isoforms beta1A and beta1B, and 32kD/67kDa laminin binding protein was examined in human hepatocellular carcinomas and non-cancerous liver tissues using the reverse transcription polymerase chain reaction.

RESULTS

Alpha6 Integrin, beta1 integrin, and laminin binding protein showed notably increased expression in hepatocellular carcinoma, compared with non-cancerous liver tissue, although the alpha1 integrin did not show a significant change. Furthermore, beta1B integrin, a splicing variant of beta1 integrin, was overexpressed in hepatocellular carcinoma while the beta1A integrin isoform did not show significant changes between hepatocellular carcinoma and surrounding non-cancerous liver tissue.

CONCLUSIONS

The differential upregulation of laminin receptors and their splicing isoforms was shown in hepatocellular carcinoma, suggesting that certain laminin receptors and their isoforms may be involved in the development and progression of hepatocellular carcinoma.

Beta1C integrin in epithelial cells correlates with a nonproliferative phenotype: forced expression of beta1C inhibits prostate epithelial cell proliferation

The expression of the beta1C integrin, an alternatively spliced variant of the beta1 subunit, was investigated in human adult and fetal tissues. In the adult, beta1C immunoreactivity was found in nonproliferative, differentiated simple, and/or pseudostratified epithelia in prostate glands and liver bile ducts. In contrast, beta1C was undetectable in stratified squamous epithelium of the epidermis and/or in hepatocytes. Luminal prostate epithelial cells expressed beta1C in vivo and in vitro, but no beta1C was seen in basal cells, which are proliferating cells. Fetal prostate expressed beta1C in differentiated glands that had a defined lumen, but not in budding glands, indicating that beta1C is a marker of prostate epithelium differentiation. The beta1C and the common beta1A variants are differentially distributed: beta1A was found in luminal and basal epithelial as well as in stromal cells in the prostate. In the liver, beta1C and beta1A were coexpressed in biliary epithelium, whereas vascular cells expressed only beta1A. Because we found beta1C in nonproliferative and differentiated epithelium, we investigated whether beta1C could have a causal role in inhibiting epithelial cell proliferation. The results showed that exogenous expression of a beta1C, but not of a beta1A, cytoplasmic domain chimeric construct, completely inhibited thymidine incorporation in response to serum by prostate cancer epithelial cells. Consistent with these in vitro results, beta1C appeared to be downregulated in prostate glands that exhibit regenerative features in benign hyperplastic epithelium. These data show that the presence of beta1C integrins in epithelial cells correlates with a nonproliferative, differentiated phenotype and is growth inhibitory to prostate epithelial cells in vitro. These findings indicate a novel pathophysiological role for this integrin variant in epithelial cell proliferation.

Interactions between growth factors and integrins: latent forms of transforming growth factor-beta are ligands for the integrin alphavbeta1

The multipotential cytokine transforming growth factor-beta (TGF-beta) is secreted in a latent form. Latency results from the noncovalent association of TGF-beta with its processed propeptide dimer, called the latency-associated peptide (LAP); the complex of the two proteins is termed the small latent complex. Disulfide bonding between LAP and latent TGF-beta-binding protein (LTBP) produces the most common form of latent TGF-beta, the large latent complex. The extracellular matrix (ECM) modulates the activity of TGF-beta. LTBP and the LAP propeptides of TGF-beta (isoforms 1 and 3), like many ECM proteins, contain the common integrin-binding sequence RGD. To increase our understanding of latent TGF-beta function in the ECM, we determined whether latent TGF-beta1 interacts with integrins. A549 cells adhered and spread on plastic coated with LAP, small latent complex, and large latent complex but not on LTBP-coated plastic. Adhesion was blocked by an RGD peptide, and cells were unable to attach to a mutant form of recombinant LAP lacking the RGD sequence. Adhesion was also blocked by mAbs to integrin subunits alphav and beta1. We purified LAP-binding integrins from extracts of A549 cells using LAP bound to Sepharose. alphavbeta1 eluted with EDTA. After purification in the presence of Mn2+, a small amount of alphavbeta5 was also detected. A549 cells migrated equally on fibronectin- and LAP-coated surfaces; migration on LAP was alphavbeta1 dependent. These results establish alphavbeta1 as a LAP-beta1 receptor. Interactions between latent TGF-beta and alphavbeta1 may localize latent TGF-beta to the surface of specific cells and may allow the TGF-beta1 gene product to initiate signals by both TGF-beta receptor and integrin pathways.

Regulation of protrusion shape and adhesion to the substratum during chemotactic responses of mammalian carcinoma cells

We report here the first direct observation of chemotaxis to EGF by rat mammary carcinoma cells. When exposed to a gradient of EGF diffusing from a micropipette, MTLn3 cells displayed typical ameboid chemotaxis, extending a lamellipod-like protrusion and moving toward the pipette. Using a homogeneous upshift in EGF to model stimulated lamellipod extension (J. E. Segall et al., 1996, Clin. Exp. Metastasis 14, 61-72), we analyzed the relationship between adhesion and chemoattractant-stimulated protrusion. Exposure to EGF led to a rapid remodeling of the adhesive contacts on adherent cells, in synchrony with extension of a flat lamellipod over the substratum. EGF-stimulated lamellipods still extended in the presence of adhesion-blocking peptides or over nonadhesive surfaces. They were, however, slightly shorter and retracted rapidly under those conditions. The major protrusive structure observed on well-spread, adherent cells, after EGF stimulation was a flat broad lamellipod, whether or not in contact with the substratum, while cells in suspension showed transient protrusive activity over the entire cell surface. We conclude that the initial adhesive status of the cell conditions the shape of the outcoming protrusion. Altogether our results suggest that, although adhesive contacts are not necessary for lamellipod extension, they play a role in stabilizing the protrusion as well as in the control of its final shape and amplitude.

beta1D integrin inhibits cell cycle progression in normal myoblasts and fibroblasts

Integrins are alphabeta heterodimeric transmembrane receptors involved in the regulation of cell growth and differentiation. The beta1 integrin subunit is widely expressed in vivo and is represented by four alternatively spliced cytoplasmic domain isoforms. beta1D is a muscle-specific variant of beta1 integrin and a predominant beta1 isoform in striated muscles. In the present study we showed that expression of the exogenous beta1D integrin in C2C12 myoblasts and NIH 3T3 or REF 52 fibroblasts inhibited cell proliferation. Unlike the case of the common beta1A isoform, adhesion of beta1D-transfected C2C12 myoblasts specifically via the expressed integrin did not activate mitogen-activated protein kinases. The beta1D-induced growth inhibitory signal was shown to occur late in the G1 phase of the cell cycle, before the G1-S transition. Ha-(12R)Ras, but not (Delta22W)Raf-1 oncogene, was able to overcome completely the beta1D-triggered cell growth arrest in NIH 3T3 fibroblasts. Since perturbation of the beta1D amino acid sequence in beta1A/beta1D chimeric integrins decreased the growth inhibitory signal, the entire cytoplasmic domain of beta1D appeared to be important for this function. However, an interleukin-2 receptor-beta1D chimera containing the cytoplasmic domain of beta1D still efficiently inhibited cell growth, showing that the ectodomain and the ligand-binding site in beta1D were not required for the growth inhibitory signal. Together, our data showed a new specific function for the alternatively spliced beta1D integrin isoform. Since the onset of beta1D expression during myodifferentiation coincides with the timing of myoblast withdrawal from the cell cycle, the growth inhibitory properties of beta1D demonstrated in this study might reflect the major function for this integrin in commitment of differentiating skeletal muscle cells in vivo.

Distinct involvement of beta3 integrin cytoplasmic domain tyrosine residues 747 and 759 in integrin-mediated cytoskeletal assembly and phosphotyrosine signaling

We have investigated the structural requirements of the beta3 integrin subunit cytoplasmic domain necessary for tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin during alphav beta3-mediated cell spreading. Using CHO cells transfected with various beta3 mutants, we demonstrate a close correlation between alphav beta3-mediated cell spreading and tyrosine phosphorylation of FAK and paxillin, and highlight a distinct involvement of the NPLY747 and NITY759 motifs in these signaling processes. Deletion of the NITY759 motif alone was sufficient to completely prevent alphav beta3-dependent focal contact formation, cell spreading, and FAK/paxillin phosphorylation. The single Y759A substitution induced a strong inhibitory phenotype, while the more conservative, but still phosphorylation-defective, Y759F mutation restored wild type receptor function. Alanine substitution of the highly conserved Tyr747 completely abolished alphav beta3-dependent formation of focal adhesion plaques, cell spreading, and FAK/paxillin phosphorylation, whereas a Y747F substitution only partially restored these events. As none of these mutations affected receptor-ligand interaction, our results suggest that the structural integrity of the NITY759 motif, rather than the phosphorylation status of Tyr759 is important for beta3-mediated cytoskeleton reorganization and tyrosine phosphorylation of FAK and paxillin, while the presence of Tyr at residue 747 within the NPLY747 motif is required for optimal beta3 post-ligand binding events.

beta1-integrin cytoplasmic subdomains involved in dominant negative function

The beta1-integrin cytoplasmic domain consists of a membrane proximal subdomain common to the four known isoforms ("common" region) and a distal subdomain specific for each isoform ("variable" region). To investigate in detail the role of these subdomains in integrin-dependent cellular functions, we used beta1A and beta1B isoforms as well as four mutants lacking the entire cytoplasmic domain (beta1TR), the variable region (beta1COM), or the common region (beta1 deltaCOM-B and beta1 deltaCOM-A). By expressing these constructs in Chinese hamster ovary and beta1 integrin-deficient GD25 cells (Wennerberg et al., J Cell Biol 132, 227-238, 1996), we show that beta1B, beta1COM, beta1 deltaCOM-B, and beta1 deltaCOM-A molecules are unable to support efficient cell adhesion to matrix proteins. On exposure to Mn++ ions, however, beta1B, but none of the mutants, can mediate cell adhesion, indicating specific functional properties of this isoform. Analysis of adhesive functions of transfected cells shows that beta1B interferes in a dominant negative manner with beta1A and beta3/beta5 integrins in cell spreading, focal adhesion formation, focal adhesion kinase tyrosine phosphorylation, and fibronectin matrix assembly. None of the beta1 mutants tested shows this property, indicating that the dominant negative effect depends on the specific combination of common and B subdomains, rather than from the absence of the A subdomain in the beta1B isoform.

Autocrine motility factor and the extracellular matrix. II. Degradation or remodeling of substratum components directs the motile response of tumor cells

Autocrine motility factor is a tumor-secreted cytokine which regulates cellular growth and motility by a receptor-mediated pathway. In the accompanying report (Part I of II), it was demonstrated that high (K1735-M1) and low (K1735-C1.11) metastatic murine melanoma cells display distinct adhesion and spreading characteristics which correlate with their differential spontaneous and stimulated migrations on the extracellular matrix components fibronectin, laminin and collagen IV. These parameters were further related to discrete profiles of focal adhesion plaque integrity and reorganization. Here we describe unique migration patterns observed in these murine melanoma cells which reflect differences in degradation and/or remodeling of the cellular substratum. These profiles of matrix interaction were influenced distinctly by autocrine motility factor and dictated by both substrate composition and cellular phenotype. Since activation of the autocrine motility factor receptor stimulates invasion of a reconstituted basement membrane and enhances experimental metastasis by high- but not low-metastatic K1735 cells, differences in the invasive phenotypes of these cells may be due in part to their differential responses to external stimuli coupled with internal propensities toward either matrix degradation and migration (high-metastatic cells) or matrix remodeling and stasis (low-metastatic cells).

Identification of beta1C-2, a novel variant of the integrin beta1 subunit generated by utilization of an alternative splice acceptor site in exon C

A new splice variant of the human integrin subunit beta1 has been identified and designated beta1C-2. It differs from the previously reported beta1C (in this report designated beta1C-1) by 18 nucleotides, and is generated by splicing from exon 6 to an alternative splice acceptor site within exon C, causing an in-frame deletion of six amino acids of the cytoplasmic region of beta1C-1. The beta1C-2 mRNA is present in several human cell lines and tissues at low levels, similarly to beta1C-1. In peripheral T-lymphocytes, beta1C-2 is the selectively expressed isoform. Neither beta1C-1 nor beta1C-2 mRNA could be detected in mouse tissues, and Southern hybridization of a mouse genomic beta1 clone with a human exon-C-specific probe failed to identify a corresponding mouse exon. The antisense orientation of exon C is highly homologous to an Alu element. Since Alu elements are restricted to primates, the beta1C-1 and beta1C-2 variants of the integrin subunit beta1 are specific for these species. The protein coded for by the beta1C-2 cDNA can be expressed and localized to the surface of beta1 deficient mouse cells. However, while stable transformed clones expressing high levels of the beta1A were commonly found, the beta1C-1 and beta1C-2 expressing clones expressed barely detectable amounts of the beta1 protein. Hence, high levels of beta1C-2 may be incompatible with cell proliferation, as previously suggested for beta1C-1.

Beta1B integrin interferes with matrix assembly but not with confluent monolayer polarity, and alters some morphogenetic properties of FRT epithelial cells

Beta1B is a beta1 integrin splice variant that differs from the ubiquitous beta1A in the terminal portion of the cytosolic tail. The expression of this variant in CHO cells results in reduced fibroblast adhesion and motility (Balzac, E et al., J. Cell Biol. 127, 557-565 (1994)). We have evaluated the phenotypic changes induced by the expression of beta1B in the FRT epithelial cell line. Stable transfectants of FRT cells expressing beta1B or beta1A human integrins were obtained. The transfected integrins associated with the endogenous alpha subunits and were delivered to the plasma membrane. Beta1B expressing cells attached less efficiently and spread less on fibronectin, laminin or type IV collagen coated dishes. A great reduction of fibronectin fibrils associated to the basal membrane of non-confluent beta1B transfected cells was observed. This was paralleled by the disappearance of microfilament bundles and loss of basally located focal adhesions. On the contrary, upon beta1A transfection, a higher amount of fibronectin fibrils, together with microfilament bundles and focal adhesions, was observed. Expression of beta1B did not significantly modify the ability to manifest the polarized phenotype when cells were grown to confluence on filters in two-chamber-systems. Beta1B-transfected cells showed reduced motile properties when embedded as aggregates in type I collagen gels. Moreover, formation of polarized cysts in suspension culture was impaired. The results show that beta1B, by interfering with focal adhesion organization, microfilament and fibronectin assembly, cell spreading and migration, affects some morphogenetic properties of FRT epithelial cells.

Muscle beta1D integrin reinforces the cytoskeleton-matrix link: modulation of integrin adhesive function by alternative splicing

Expression of muscle-specific beta1D integrin with an alternatively spliced cytoplasmic domain in CHO and GD25, beta1 integrin-minus cells leads to their phenotypic conversion. beta1D-transfected nonmuscle cells display rounded morphology, lack of pseudopodial activity, retarded spreading, reduced migration, and significantly enhanced contractility compared with their beta1A-expressing counterparts. The transfected beta1D is targeted to focal adhesions and efficiently displaces the endogenous beta1A and alphavbeta3 integrins from the sites of cell-matrix contact. This displacement is observed on several types of extracellular matrix substrata and leads to elevated stability of focal adhesions in beta1D transfectants. Whereas a significant part of cellular beta1A integrin is extractable in digitonin, the majority of the transfected beta1D is digitonin-insoluble and is strongly associated with the detergent-insoluble cytoskeleton. Increased interaction of beta1D integrin with the actin cytoskeleton is consistent with and might be mediated by its enhanced binding to talin. In contrast, beta1A interacts more strongly with alpha-actinin, than beta1D. Inside-out driven activation of the beta1D ectodomain increases ligand binding and fibronectin matrix assembly by beta1D transfectants. Phenotypic effects of beta1D integrin expression in nonmuscle cells are due to its enhanced interactions with both cytoskeletal and extracellular ligands. They parallel the transitions that muscle cells undergo during differentiation. Modulation of beta1 integrin adhesive function by alternative splicing serves as a physiological mechanism reinforcing the cytoskeleton- matrix link in muscle cells. This reflects the major role for beta1D integrin in muscle, where extremely stable association is required for contraction.

Spatial and temporal expression of the beta1D integrin during mouse development

The beta1D protein is a recently characterized isoform of the integrin beta1 subunit that is present in cardiac and skeletal muscles. In this study, we have examined the expression of beta1D in different types of skeletal muscle and in cardiac muscle and studied its distribution during mouse development, using new monoclonal antibodies specific for beta1D. Immunoprecipitation studies revealed that, while beta1A is strongly expressed in proliferating C2C12 myoblasts, beta1D is only expressed after their differentiation to myotubes. In these myotubes, beta1D is associated with different alpha subunits, namely alpha3A, alpha5, alpha7A, or alpha7B. Initially, during embryogenesis, the alpha1A subunit is the only beta1 variant expressed in skeletal and cardiac muscle. The beta1D subunit is first detected in skeletal muscle at E17.5, whereas in cardiac muscle its expression begins around the time of birth. Later the expression of beta1A in skeletal and cardiac muscle becomes restricted to capillary cells, whereas beta1D eventually becomes the only variant expressed in adult cardiac and skeletal muscle cells. The switch from the beta1A to the beta1D subunit in cardiac muscle cells coincides with the expression of alpha7. In adults there is a distinct concentration of beta1D at the myotendinous junctions of muscle fibers and at costameres in both cardiac and skeletal muscle. In addition, beta1D is present at intercalated discs in cardiac muscle and at neuromuscular junctions in skeletal muscle cells. The amount of beta1D in different types of skeletal muscle (fast, slow, and mixed-type) was similar, but cardiac muscle expressed almost five times as much of this protein. We suggest that beta1D plays a role in the maintenance of the cytoarchitecture of mature muscle and in the functional integrity of the muscle cells.

CD44 isoform expression in periodontal tissues: cell-type specific regulation of alternative splicing

CD44 functions as a receptor for various extracellular matrices and plays crucial roles in homotypic and heterotypic cell-cell interactions. Recently, the molecular structure of CD44 has been extensively analyzed and multiple isoforms produced by alternative splicing of messenger RNA have been identified. In this study, we examined the expression of CD44 isoforms on different cell types isolated from periodontal tissue. In order to examine tissue differences in CD44 isoform expression, we established in vitro cell culture of human gingival fibroblasts (HGF), human periodontal ligament cells (HPDL) and human gingival epithelial cells (HGEC). These cells all expressed CD44 protein and messenger RNA. However, immunoprecipitation and Northern blot analysis revealed that HGEC expressed larger CD44 isoforms than HGF and HPDL. Reverse transcription-polymerase chain reaction with primers flanking the insertion site of alternatively spliced exons was used to study details of the heterogeneity. All cells examined expressed a major band in the absence of alternatively spliced exons and additional larger bands. In particular, HGEC contained more abundant high molecular mass species. In vitro stimulation by IL-1 beta, TNF alpha or phorbol 12-myristate 13-acetate induced an increase in total CD44 messenger RNA in HGF but not change in overall patterns of CD44 isoform expression. However, the isoform expression of HGEC was sensitive to cell density. The amount of larger isoform was decreased by culturing cells beyond confluence. These findings suggest that CD44 isoform expression is cell type-specifically regulated in periodontium and altered according to growth phase of HGEC.

Alternatively spliced variants: a new view of the integrin cytoplasmic domain

A large number of studies have underscored a major role for the integrin alpha beta cytoplasmic domains in the modulation of cell functions. Cytoplasmic domain variants of the beta 1, beta 3, beta 4, alpha 3, alpha 6 and alpha 7 subunits have been described. These molecules are generated by alternative splicing events and are expressed in a cell- or tissue-type specific manner. Some of these variants (beta 1C, beta 1D, alpha 6A and alpha 7A) are predominantly expressed upon differentiation and have been shown to be regulated during development. The studies on the structure-function relationship of the integrin variant subunits, published between 1989 and now, will be reviewed here for the first time. The results demonstrate that differences in the cytoplasmic domain do not affect either the alpha beta heterodimer formation or the ligand specificity. Instead, alternatively spliced integrin cytoplasmic domains appear to be essential modulators of receptor localization, cell proliferation and migration, as well as phosphorylation of signaling molecules. These observations lead to the current hypothesis that cell-type specific regulation of alternatively spliced integrin cytoplasmic domains may provide a highly specialized mechanism to control cell growth and intracellular signaling pathways.

NPXY motifs control the recruitment of the alpha5beta1 integrin in focal adhesions independently of the association of talin with the beta1 chain

With the exception of the divergent beta4 and beta8 chains, the integrin beta subunit cytoplasmic domains are short and highly conserved sequences. Consensus motifs are found among the different cytoplasmic beta chains. Experiments using chimeric receptors demonstrated that the 47 amino acids of the beta1 subunit cytoplasmic domain contain sufficient information to target integrins to adhesion plaques. Three clusters of amino acids, named cyto-1, cyto-2 and cyto-3, seem to contribute to this localization. Cyto-2 and cyto-3 exhibit NPXY motifs. At present, the exact function of these motifs remains unknown but it is likely that these sequences are involved in protein-protein interactions. Although NPXY motifs often act as internalization signals at the cytoplasmic tail of membrane receptors, our previous results showed that the two NPXY motifs are not responsible for the alpha5beta1 integrin endocytosis. Herein, we address the question of the role of the two highly conserved NPXY motifs found in the beta1 cytoplasmic domain, and which correspond to the conserved domains cyto-2 and cyto-3. We demonstrate that, within the integrin beta1 cytoplasmic tail, the two NPXY motifs are required for the recruitment of the integrin in focal adhesions. In addition, our results indicate that these two motifs control but do not belong to the talin-binding sites. Finally, the analysis of the phenotypes of NPXY mutants reveals that the interaction of talin with the beta1 cytosolic domain is not sufficient to target the integrins to focal adhesions.

Differential signaling after beta1 integrin ligation is mediated through binding of CRKL to p120(CBL) and p110(HEF1)

CRKL is an SH2-SH3-SH3 adapter protein that is a major substrate of the BCR/ABL oncogene. The function of CRKL in normal cells is unknown. In cells transformed by BCR/ABL we have previously shown that CRKL is associated with two focal adhesion proteins, tensin and paxillin, suggesting that CRKL could be involved in integrin signaling. In two hematopoietic cell lines, MO7e and H9, we found that CRKL rapidly associates with tyrosine-phosphorylated proteins after cross-linking of beta1 integrins with fibronectin or anti-beta1 integrin monoclonal antibodies. The major tyrosine-phosphorylated CRKL-binding protein in the megakaryocytic MO7e cells was identified as p120(CBL), the cellular homolog of the v-Cbl oncoprotein. However, in the lymphoid H9 cell line, the major tyrosine-phosphorylated CRKL-binding protein was p110(HEF1). In both cases, this binding was mediated by the CRKL SH2 domain. Interestingly, although both MO7e and H9 cells express p120(CBL) and p110(HEF1), beta1 integrin cross-linking induces tyrosine phosphorylation of p120(CBL) (but not p110(HEF1)) in MO7e cells and of p110(HEF1) (but not p120(CBL)) in H9 cells. In both cell types, CRKL is constitutively complexed to C3G, SOS, and c-ABL through its SH3 domains, and the stoichiometry of these complexes does not change upon integrin ligation. Thus, in different cell types CRKL and its SH3-associated proteins may form different multimeric complexes depending on whether p120(CBL) or p110(HEF1) is tyrosine-phosphorylated after integrin ligation. The shift in association of CRKL and its SH3-associated proteins from p120(CBL) to p110(HEF1) could contribute to different functional outcomes of "outside-in" integrin signaling in different cells.

Modulation of cell proliferation by the integrin cytoplasmic domain

Integrin adhesion receptors modulate cell functions, including cell proliferation and survival. The beta 1C integrin, an alternatively spliced form of beta 1A, containing a unique cytoplasmic domain sequence, inhibits cell growth in vitro. In vivo, the expression of beta 1C correlates with a benign, nonproliferative phenotype in epithelial cells. The studies discussed in this article indicate that modulation of cell proliferation, in normal or pathological conditions, might be achieved by the regulated expression of variant integrin subunits.

The role of conserved amino acid motifs within the integrin beta3 cytoplasmic domain in triggering focal adhesion kinase phosphorylation

Integrin-mediated adhesion of cells to extracellular matrix proteins triggers a variety of intracellular signaling pathways including a cascade of tyrosine phosphorylations. In many cell types, the cytoplasmic focal adhesion tyrosine kinase, FAK, appears to be the initial protein that becomes tyrosine-phosphorylated in response to adhesion; however, the molecular mechanisms regulating integrin-triggered FAK phosphorylation are not understood. Previous studies have shown that the integrin beta1, beta3, and beta5 subunit cytoplasmic domains all contain sufficient information to trigger FAK phosphorylation when expressed in single-subunit chimeric receptors connected to an extracellular reporter. In the present study, beta3 cytoplasmic domain deletion and substitution mutants were constructed to identify amino acids within the integrin beta3 cytoplasmic domain that regulate its ability to trigger FAK phosphorylation. Cells transiently expressing chimeric receptors containing these mutant cytoplasmic domains were magnetically sorted and assayed for the tyrosine phosphorylation of FAK. Analysis of these mutants indicated that structural information in both the membrane-proximal and C-terminal segments of the beta3 cytoplasmic domain is important for triggering FAK phosphorylation. In the C-terminal segment of the beta3 cytoplasmic domain, the highly conserved NPXY motif was found to be required for the beta3 cytoplasmic domain to trigger FAK phosphorylation. However, the putative FAK binding domain within the N-terminal segment of the beta3 cytoplasmic domain was found to be neither required nor sufficient for this signaling event. We also demonstrate that the serine 752 to proline mutation, known to cause a variant of Glanzmann's thrombasthenia, inhibits the ability of the beta3 cytoplasmic domain to signal FAK phosphorylation, suggesting that a single mutation in the beta3 cytoplasmic domain can inhibit both "inside-out" and "outside-in" integrin signaling.

Localization of cranin (dystroglycan) at sites of cell-matrix and cell-cell contact: recruitment to focal adhesions is dependent upon extracellular ligands

We report that cranin (dystroglycan) can become recruited to focal adhesions of cultured rat REF 52 fibroblasts and human aortic smooth muscle cells. Within mature focal adhesions, cranin was present within the plaque region defined by beta 1 integrin, vinculin and phosphotyrosine staining, but occupied a larger domain corresponding to the terminal segments of stress fibers that was more precisely co-extensive with the cytoskeletal proteins alpha-actinin, utrophin and aciculin. When REF 52 fibroblasts were plated on different substrata in the absence of protein synthesis and secretion in serum-free medium, focal clusters of cranin readily formed within 2 hours on matrix proteins that bind cranin directly (laminin or agrin) which were maintained as the focal adhesions became mature. In contrast, cranin failed to become targeted to cell-substratum attachment sites, either at early or later times, when cells were plated on a variety of other substrata that elicit formation of focal adhesions but do not bind cranin directly (fibronectin, vitronectin, collagen type IV, or anti-beta 1 integrin antibody TS2/16). These data strongly suggest that targeting of cranin to focal adhesions was dependent upon the presence of an extracellular ligand capable of binding cranin directly. However, some cultured nonmuscle cell lines (e.g., human umbilical vein endothelial cells, NIH 3T3 and CHO cells) failed to localize cranin to focal adhesions, even when plated on laminin. Cranin was also enriched at cell-cell adherens-type junctions of human normal breast MCF-10 epithelial cells, and at growth cones of E17 rat hippocampal axons. That cranin can become targeted to sites of cell-cell and cell-substratum contact in diverse cell types supports the hypothesis that cranin may be involved in mediating or regulating cell adhesion. The absence of muscle-specific and synapse-specific proteins within fibroblasts and epithelial cells provides a different context for thinking about cranin (dystroglycan) that may aid in discerning general principles of its structure and function.

Focal adhesions, contractility, and signaling

Focal adhesions are sites of tight adhesion to the underlying extracellular matrix developed by cells in culture. They provided a structural link between the actin cytoskeleton and the extracellular matrix and are regions of signal transduction that relate to growth control. The assembly of focal adhesions is regulated by the GTP-binding protein Rho. Rho stimulates contractility which, in cells that are tightly adherent to the substrate, generates isometric tension. In turn, this leads to the bundling of actin filaments and the aggregation of integrins (extracellular matrix receptors) in the plane of the membrane. The aggregation of integrins activates the focal adhesion kinase and leads to the assembly of a multicomponent signaling complex.

Dominant-negative effect on adhesion by myelin Po protein truncated in its cytoplasmic domain

The myelin Po protein is believed to hold myelin together via interactions of both its extracellular and cytoplasmic domains. We have already shown that the extracellular domains of Po can interact in a homophilic manner (Filbin, M.T., F.S. Walsh, B.D. Trapp, J.A. Pizzey, and G.I. Tennekoon. 1990. Nature (Lond.). 344:871-872). In addition, we have shown that for this homophilic adhesion to take place, the cytoplasmic domain of Po must be intact and most likely interacting with the cytoskeleton; Po proteins truncated in their cytoplasmic domains are not adhesive (Wong, M.H., and M.T. Filbin, 1994. J. Cell Biol. 126:1089-1097). To determine if the presence of these truncated forms of Po could have an effect on the functioning of the full-length Po, we coexpressed both molecules in CHO cells. The adhesiveness of CHO cells expressing both full-length Po and truncated Po was then compared to cells expressing only full-length Po. In these coexpressors, both the full-length and the truncated Po proteins were glycosylated. They reached the surface of the cell in approximately equal amounts as shown by an ELISA and surface labeling, followed by immunoprecipitation. Furthermore, the amount of full-length Po at the cell surface was equivalent to other cell lines expressing only full-length Po that we had already shown to be adhesive. Therefore, there should be sufficient levels of full-length Po at the surface of these coexpressors to measure adhesion of Po. However, as assessed by an aggregation assay, the coexpressors were not adhesive. By 60 min they had not formed large aggregates and were indistinguishable from the control transfected cells not expressing Po. In contrast, in the same time, the cells expressing only the full-length Po had formed large aggregates. This indicates that the truncated forms of Po have a dominant-negative effect on the adhesiveness of the full-length Po. Furthermore, from cross-linking studies, full-length Po, when expressed alone but not when coexpressed with truncated Po, appears to cluster in the membrane. We suggest that truncated Po exerts its dominant-negative effect by preventing clustering of full-length Po. We also show that colchicine, which disrupts microtubules, prevents adhesion of cells expressing only the full-length Po. This strengthens our suggestion that an interaction of Po with the cytoskeleton, either directly or indirectly, is required for adhesion to take place.

Down-regulation of beta 1C integrin, an inhibitor of cell proliferation, in prostate carcinoma

The beta 1C integrin, a member of the cell adhesion receptor superfamily, is an alternatively spliced variant of the beta 1A subunit and, in contrast to its wild-type counterpart, inhibits cell proliferation in vitro. The expression of beta 1C integrin in tumor cell growth was investigated. In benign and neoplastic human prostate tissues, immunohistochemical analysis performed using affinity-purified antibodies specific for beta 1C demonstrated a predominant epithelial expression of beta 1C in benign prostate glands with marked staining of the apical, basal, and lateral surfaces. In the adjacent prostate adenocarcinoma glands, the beta 1C variant was dramatically down-regulated in 27 of 34 (79%) analyzed cases, whereas the expression and distribution of its wild-type counterpart, beta 1A, remained unchanged. Tumors exhibiting different Gleason's patterns showed that beta 1C was down-regulated in comparison with the benign tissue regardless of the histological grade. Immunoblotting analysis, using affinity-purified antibodies specific for beta 1C, was performed, in a quantitative manner, to compare beta 1C expression in benign and tumor prostate tissue. The results showed that beta 1C was expressed in benign prostate tissue whereas it was undetectable in prostate adenocarcinoma. Taken together, these data show that beta 1C integrin down-regulation in prostate tissues correlates with a neoplastic phenotype consistent with its in vitro growth-inhibitory properties. These findings indicate a novel pathophysiological role for this integrin variant in tumorigenesis.