The size of the intracellular beta 1-integrin precursor pool regulates maturation of beta 1-integrin subunit and associated alpha-subunits

Ubiquitin-specific peptidase 22 controls integrin-dependent cancer cell stemness and metastasis

Integrins play critical roles in connecting the extracellular matrix and actin. While the upregulation of integrins is thought to promote cancer stemness and metastasis, the mechanisms underlying their upregulation in cancer stem cells (CSCs) remain poorly understood. Herein, we show that USP22 is essential in maintaining breast cancer cell stemness by promoting the transcription of integrin β1 (ITGB1). Both genetic and pharmacological inhibition of USP22 largely impaired breast CSCs self-renewal and prevented their metastasis. Reconstitution of integrin β1 partially rescued USP22-null breast cancer metastasis. USP22 functions as a bona fide deubiquitinase to protect the proteasomal degradation of the forkhead box M1 (FoxM1), a transcription factor for tumoral ITGB1 gene transcription. Immunohistochemistry staining detected a positive correlation among USP22, FoxM1, and integrin β1 in human breast cancers. Collectively, our study identifies the USP22-FoxM1-integrin β1 signaling axis as critical for cancer stemness and offers a potential target for antitumor therapy.

Integrin Crosstalk Contributes to the Complexity of Signalling and Unpredictable Cancer Cell Fates

Integrins are heterodimeric cell surface receptors composed of α and β subunits that control adhesion, proliferation and gene expression. The integrin heterodimer binding to ligand reorganises the cytoskeletal networks and triggers multiple signalling pathways that can cause changes in cell cycle, proliferation, differentiation, survival and motility. In addition, integrins have been identified as targets for many different diseases, including cancer. Integrin crosstalk is a mechanism by which a change in the expression of a certain integrin subunit or the activation of an integrin heterodimer may interfere with the expression and/or activation of other integrin subunit(s) in the very same cell. Here, we review the evidence for integrin crosstalk in a range of cellular systems, with a particular emphasis on cancer. We describe the molecular mechanisms of integrin crosstalk, the effects of cell fate determination, and the contribution of crosstalk to therapeutic outcomes. Our intention is to raise awareness of integrin crosstalk events such that the contribution of the phenomenon can be taken into account when researching the biological or pathophysiological roles of integrins.

Psoriasiform skin disease in transgenic pigs with high-copy ectopic expression of human integrins α2 and β1

Psoriasis is a complex human-specific disease characterized by perturbed keratinocyte proliferation and a pro-inflammatory environment in the skin. Porcine skin architecture and immunity are very similar to that in humans, rendering the pig a suitable animal model for studying the biology and treatment of psoriasis. Expression of integrins, which is normally confined to the basal layer of the epidermis, is maintained in suprabasal keratinocytes in psoriatic skin, modulating proliferation and differentiation as well as leukocyte infiltration. Here, we generated minipigs co-expressing integrins α2 and β1 in suprabasal epidermal layers. Integrin-transgenic minipigs born into the project displayed skin phenotypes that correlated with the number of inserted transgenes. Molecular analyses were in good concordance with histological observations of psoriatic hallmarks, including hypogranulosis and T-lymphocyte infiltration. These findings mark the first creation of minipigs with a psoriasiform phenotype resembling human psoriasis and demonstrate that integrin signaling plays a key role in psoriasis pathology.

Summary: A cloned porcine disease model to advance topical treatment in the debilitating skin disorder psoriasis.

RNF-121 is an endoplasmic reticulum-membrane E3 ubiquitin ligase involved in the regulation of beta-integrin

RNF-121 is an E3 ligase RING finger protein that is localized to the ER in Caenorhabditis elegans and functions in the UPR and ERAD pathways. The β subunit of the heterodimeric integrin receptor was identified as a substrate for RNF-121, suggesting a link between ERAD and cell adhesion through the regulation of β-integrin.

We report on the characterization of RNF-121, an evolutionarily conserved E3 ligase RING finger protein that is expressed in the endoplasmic reticulum (ER) of various cells and tissues in Caenorhabditis elegans. Inactivation of RNF-121 induced an elevation in BiP expression and increased the sensitivity of worms to ER stress. Genetic analysis placed RNF-121 downstream of the unfolded protein response (UPR) regulator protein kinase-like endoplasmic reticulum kinase (PERK). We identify PAT-3::GFP, the β subunit of the heterodimeric integrin receptors, as an RNF-121 substrate; whereas induction of RNF-121 expression reduced the level of PAT-3::GFP in the gonad distal tip cells, inhibition of RNF-121 led to the accumulation of stably bound PAT-3::GFP inclusions. Correspondingly, overexpression of RNF-121 during early stages of gonad development led to aberrations in germline development and gonad migration that overlap with those observed after PAT-3 inactivation. The formation of these gonad abnormalities required functional ER-associated degradation (ERAD) machinery. Our findings identify RNF-121 as an ER-anchored ubiquitin ligase that plays a specific role in the ERAD pathway by linking it to the regulation of the cell adhesion integrin receptors.

DNAzymes to mouse beta1 integrin mRNA in vivo: targeting the tumor vasculature and retarding cancer growth

Previously, we designed a DNAzyme (beta1DE) targeting the human beta1 integrin subunit, which efficiently digested the mRNA of the beta1 integrin subunit and downregulated beta1 integrin expression in endothelial cells. This DNAzyme blocked the adhesion of endothelial cells and abolished their ability to form microcapillary tubes in Matrigel. In our present study, we demonstrate that beta1DE effectively inhibited neovascularization in Matrigel plugs (BALB/c mice, n=20) and solid human carcinoma tumors developed in nude mice (BALB/cA nude (nu-/-)-B6.Cg-Foxn1(nu)) (n=30) using prostate carcinoma cells PC-3 (n=15) and colon adenocarcinoma cells CX1.1 (n=15). When injected intratumorally, it significantly reduced the tumor size and number of microvessels developed by both CX1.1 and PC-3 cells within the 3 weeks of experiment duration. Thus, DNAzymes targeting beta1 integrin genes can inhibit multiple key tumorigenic processes in vitro and in vivo and may serve as useful anti-cancer agents.

Identification of N-glycan-binding proteins for E3 ubiquitin ligases

N-glycans serve as a degradation signal by the SCF(Fbx2) ubiquitin ligase complex in the cytosol. Fbx2, an F-box protein, binds specifically to proteins attached with N-linked high-mannose type oligosaccharides, and subsequently contributes to ubiquitination of glycoproteins. Pre-integrin beta1 is identified as one of the Fbx2 targets. These two proteins bind in the cytosol after inhibition of the proteasome. These results indicate that SCF(Fbx2) ubiquitinates N-linked glycoproteins, which are translocated from the endoplasmic reticulum to the cytosol by the quality control mechanism. This chapter describes methods, including a binding protein assay for N-glycans, a ubiquitination assay for N-linked glycoproteins with SCF(Fbx2) ubiquitin ligase complex, an overlay assay for the detection of Fbx2 binding proteins, and a pull-down assay for the interaction between Fbx2 and N-linked glycoproteins, used to identify N-glycan-binding proteins for E3 ubiquitin ligases.

Expression patterns of integrins on pleomorphic adenoma and adenoid cystic carcinoma: study on specimens and in vitro investigation of the effects of extracellular matrix on the expression of these adhesion molecules

BACKGROUND

Pleomorphic adenoma (PA) and adenoid cystic carcinoma (ACC) are neoplasms of distinct behaviour, showing similar origin, cell components and marked presence of extracellular matrix (ECM). Interactions between cells and ECM are important in the biology of tumours, being partially mediated by integrins. This study investigated these interactions on PA and ACC using paraffin-embedded tissue and an in vitro model of these conditions.

METHODS

Expression of integrins in paraffin-embedded samples was assessed by immunohistochemistry. Cells from PA and ACC were characterized using immunofluorescence, and integrin patterns of expression were investigated on cells cultivated on different ECM proteins.

RESULTS

Luminal cells of both PA and ACC were more intensely positive for integrins than myoepithelial cells. In vitro studies revealed that PA cells expressed more integrins than ACC cells regardless the ECM protein present.

CONCLUSIONS

This study revealed particular patterns of integrin expression in both specimens and in vitro models of PA and ACC. This might prove useful for a better understanding of the biology of these lesions.

Salivary gland tumours: immunoexpression of integrins beta 1, beta 3 and beta 4

BACKGROUND

Integrins, heterodimeric transmembranic adhesion molecules composed of alpha and beta subunits, have been implicated in normal and neoplastic biological processes. We investigated the patterns of expression of integrins subunits beta 1, beta 3 and beta 4 in neoplasms derived from the terminal segment of salivary glands in order to understand their expression patterns in the different structures of these tumours.

METHODS

Immunohistochemistry using 'catalized signal amplification' (CSA) detection system was applied to paraffin-embedded specimens of polymorphous low-grade adenocarcinoma, adenoid cystic carcinoma and acinic cell carcinoma. Normal salivary glands were used as controls.

RESULTS

Immunoexpression of integrins showed that neoplastic structures of the tumours studied tend to mimic the patterns seen in the normal structures of salivary glands from which they are derived. Anaplastic cells were negative for all integrins.

CONCLUSIONS

This study revealed detailed topography of integrins in malignant tumours derived from intercalated acinar segment of salivary gland and this might be useful in the diagnosis of these lesions, especially of fine-needle aspiration (FNA) products or small specimens derived form incisional biopsies.

E3 ubiquitin ligase that recognizes sugar chains

N-glycosylation of proteins in the endoplasmic reticulum (ER) has a central role in protein quality control. Here we report that N-glycan serves as a signal for degradation by the Skp1-Cullin1-Fbx2-Roc1 (SCF(Fbx2)) ubiquitin ligase complex. The F-box protein Fbx2 (ref. 4) binds specifically to proteins attached to N-linked high-mannose oligosaccharides and subsequently contributes to ubiquitination of N-glycosylated proteins. Pre-integrin beta 1 is a target of Fbx2; these two proteins interact in the cytosol after inhibition of the proteasome. In addition, expression of the mutant Fbx2 Delta F, which lacks the F-box domain that is essential for forming the SCF complex, appreciably blocks degradation of typical substrates of the ER-associated degradation pathway. Our results indicate that SCF(Fbx2) ubiquitinates N-glycosylated proteins that are translocated from the ER to the cytosol by the quality control mechanism.

Antisense oligonucleotides to the integrin receptor subunit alpha(5) decrease fibronectin fragment mediated cartilage chondrolysis

OBJECTIVE

To investigate involvement of the integrin alpha(5) subunit of the classical fibronectin receptor in cartilage chondrolytic activities of fibronectin fragments (Fn-f).

DESIGN

Bovine chondrocytes and cartilage explants were cultured in the presence of antisense oligonucleotide (ASO), or sense (SO) or scrambled sequence oligonucleotide (SCO) corresponding to the bovine alpha(5) subunit. The effects of the oligonucleotides on mRNA and protein expression of the alpha(5) subunit were analysed by rtPCR and Western blotting, respectively. To test effects on Fn-f activities, three different Fn-f were first added to serum or serum-free cultures, followed by addition of oligonucleotides and the effects on Fn-f mediated proteoglycan (PG) degradation, cartilage PG depletion and PG and general protein synthesis suppression were tested.

RESULTS

The ASO decreased alpha(5) mRNA and protein expression to 69% and 55%, respectively, in monolayer cultures and decreased protein expression 67% in cartilage explants, while SO and SCO were ineffective. The ASO partially reversed the ability of the Fn-fs to suppress PG and general protein synthesis in cartilage explant and high density chondrocyte cultures. Concentrations of ASO from 1 nM to 5 microM effectively suppressed Fn-f activities in particular assays and the effects were reversible, while SO and SCO were not significantly effective. ASO also suppressed, in a dose-dependent and reversible fashion, the ability of the Fn-fs to enhance degradation and release of PG from cartilage explants. The ASO were also effective in suppressing the ability of an antibody to the alpha(5) subunit to enhance PG degradation, but were ineffective in blocking endotoxin or IL-1beta enhanced degradation.

CONCLUSIONS

These data implicate the alpha(5) integrin subunit in Fn-f mediated activities, consistent with a role for the alpha(5)beta(1) integrin in this pathway.

DNAzymes to beta 1 and beta 3 mRNA down-regulate expression of the targeted integrins and inhibit endothelial cell capillary tube formation in fibrin and matrigel

A novel approach based on DNA-cleaving deoxyribozymes (DNAzymes) was developed to control expression of beta(1) and beta(3) integrins in endothelial cells. To engineer a specific cleavage site in mRNA, the flanking domains of DNAzymes were derived from oligodeoxynucleotides complementary to sequences corresponding to 1053-1070 and 1243-1267 in beta(1) and beta(3) mRNA, respectively. Phosphorothioate analogues of these antisense oligodeoxynucleotides, designated beta1-1053 and beta3-1243, significantly inhibited expression of beta(1) and beta(3) integrin subunits in endothelial and K562 cells at the level of mRNA and protein synthesis. They also specifically decreased the cell surface expression of corresponding subunits in endothelial cells and K562 cells, as measured by flow cytometry. In functional tests, beta1-1053 and beta3-1243 markedly reduced adhesion of cells to fibronectin and vitronectin, respectively. We designed DNAzymes to beta(1) and beta(3) mRNAs containing a 15-deoxynucleotide catalytic domain that was flanked by two substrate recognition segments of 8 and 10 deoxynucleotides for beta(1) and beta(3) DNAzymes, respectively. Both DNAzymes in the presence of Mg(2+) specifically cleaved their substrates, synthetic beta(1) and beta(3) mRNA fragments. Although DNAzymes were partially modified with phosphorothioate and with 2'-O-methyl groups at both the 5' and 3' ends indicated similar kinetic parameters, they were significantly more potent than the unmodified DNAzymes because of their much higher resistance to nuclease degradation. Similar to the antisense oligonucleotides, DNAzymes abolished microvascular endothelial cell capillary tube formation in fibrin and Matrigel. In conclusion, DNAzymes to beta(1) and beta(3) mRNAs with 2'-O-methyl modifications are potentially useful as gene-inactivating agents and may ultimately provide a therapeutic means to inhibit angiogenesis in vivo.

Suppression of integrin expression and tumorigenicity by sulfation of lactosylceramide in 3LL Lewis lung carcinoma cells

To investigate the cellular functions of sulfated glycosphingolipids, we introduced the cerebroside sulfotransferase (CST) gene into J5 cells, a subclone of 3LL Lewis lung carcinoma cells. The J5 cells lack acidic glycosphingolipids but accumulate their common biosynthetic precursor, lactosylceramide. We established the stable CST transfectants, J5/CST-1 and J5/CST-2 clones, highly expressing sulfated lactosylceramide (SM3). Both clones exhibited more spherical morphology in comparison to mock transfectant, and their adhesiveness to fibronectin and laminin was significantly lower. The loss of cell-substratum interactions in these SM3-expressing cells could be attributed to decreased expression of integrins (alpha(5), alpha(6), and beta(1)) on the cell surface and their whole cellular levels. However, the levels of H-2K(b) and H-2D(b) antigens remained unchanged. Reverse transcriptase-polymerase chain reaction and Northern blot analyses for these integrins exhibited significant decrease of beta(1) gene expression in J5/CST-1 and 2, but there was no change in the levels of alpha(5) and alpha(6) transcripts. Deglycosylation by endoglycosidase H treatment clearly demonstrated that the precursor form of beta(1) integrin, possessing high mannose oligosaccharide chains, was preferentially decreased in the CST transfectants. These results demonstrate that endogenous SM3 negatively regulates beta(1) integrin expression at the transcriptional level, and the decrease of alpha integrin proteins in the CST transfectants was due to the post-transcriptional modification. We suggest the putative importance of the intracellular pre-beta(1) integrin pool for normal integrin maturation and subsequent function. Although the rates of cell proliferation in vitro for mock and CST transfectants were similar, tumorigenicity of J5/CST-1 and -2 cells inoculated into syngeneic C57/BL6 mice was greatly decreased or even absent. This was probably due to global loss of the efficient cell-matrix interactions, which are essential for the development of malignant tumors in vivo. Thus, we showed the evidence that cellular SM3 negatively regulates the cell-substratum interaction, resulting in the loss of tumorigenicity.

Identification of a candidate integrin-fraction associated with the activated form of the PDGF-receptor

The activated PDGF-receptor has been shown to coimmunoprecipitate together with alpha v beta 3 integrin out of the 15,000g soluble supernatant of non-ionic detergent cell lysates. I have now further characterized this complex by ultracentrifugation analysis. The ultracentrifugation-conditions were chosen so that the phosphorylated form of the PDGF-receptor was pelleted out of the 15,000g soluble supernatant. Together with the tyrosine-phosphorylated PDGF-receptor small amounts of integrins, cytoskeletal- and extracellular matrix proteins were recovered in the pelleted material. The results show that (i) the candidate-fraction of integrins interacting with the activated PDGF-receptor is small compared to the overall integrin population in the cell lysate, (ii) several proteins known to be present in focal adhesions such as FAK, talin, and vinculin are absent from the integrin-growth factor receptor complexes, while on the other hand (iii) a tyrosine-phosphorylated protein migrating at 120 kDa was highly enriched in the ultracentrifugation-pellet, and finally (iv) non-ionic detergent cell lysates appear to contain quantitatively small fractions of complexed proteins that are qualitatively distinct from their total cellular population. Thus, the separation of protein-complexes from the total cellular proteom may be instrumental for the investigation of cellular protein complexes in general.

Expression of antisense to integrin subunit beta 3 inhibits microvascular endothelial cell capillary tube formation in fibrin

alpha(v)beta(3) antagonists are potent angiogenesis inhibitors, and several different classes of inhibitors have been developed, including monoclonal antibodies, synthetic peptides, and small organic molecules. However, each class of inhibitor works by the same principal, by blocking the binding of ligands to alpha(v)beta(3). In an effort to develop an alpha(v)beta(3) inhibitor that down-regulates the actual level of alpha(v)beta(3), we developed an antisense strategy to inhibit alpha(v)beta(3) expression in vitro. beta(3) antisense expressed in endothelial cells specifically down-regulated alpha(v)beta(3) and inhibited capillary tube formation, with the extent of down-regulation correlating with the extent of tube formation inhibition. This inhibition was matrix-specific, since tube formation was not inhibited in Matrigel. These findings support the notion that alpha(v)beta(3) is required for an essential step of angiogenesis in fibrin, namely capillary tube formation. These results suggest that pseudogenetic inhibition of beta(3) integrins using antisense techniques may ultimately provide a therapeutic means to inhibit angiogenesis in vivo.

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.

Embryonic neurons adapt to the inhibitory proteoglycan aggrecan by increasing integrin expression

The primary mediators of cell migration during development, wound healing and metastasis, are receptors of the integrin family. In the developing and regenerating nervous system, chondroitin sulfate proteoglycans (CSPGs) inhibit the integrin-dependent migration of neuronal growth cones. Here we report that embryonic sensory neurons cultured on the growth-promoting molecule laminin in combination with the inhibitory CSPG aggrecan rapidly adapt to inhibition. Adaptation is associated with a two- to threefold increase in the levels of RNA and surface protein for two laminin receptors, integrin alpha6beta1 and alpha3beta1, indicating that integrin expression is regulated by aggrecan. Increased integrin expression is associated both with increases in neuronal cell adhesion/outgrowth and with decreases in the ability of aggrecan to inhibit cell adhesion. Directly increasing integrin expression by adenoviral infection is sufficient to eliminate the inhibitory effects of aggrecan, indicating that upregulation of integrin receptors may promote neuronal regeneration in the presence of inhibitory matrix components.

Embryonic neurons adapt to the inhibitory proteoglycan aggrecan by increasing integrin expression

The primary mediators of cell migration during development, wound healing and metastasis, are receptors of the integrin family. In the developing and regenerating nervous system, chondroitin sulfate proteoglycans (CSPGs) inhibit the integrin-dependent migration of neuronal growth cones. Here we report that embryonic sensory neurons cultured on the growth-promoting molecule laminin in combination with the inhibitory CSPG aggrecan rapidly adapt to inhibition. Adaptation is associated with a two- to threefold increase in the levels of RNA and surface protein for two laminin receptors, integrin alpha6beta1 and alpha3beta1, indicating that integrin expression is regulated by aggrecan. Increased integrin expression is associated both with increases in neuronal cell adhesion/outgrowth and with decreases in the ability of aggrecan to inhibit cell adhesion. Directly increasing integrin expression by adenoviral infection is sufficient to eliminate the inhibitory effects of aggrecan, indicating that upregulation of integrin receptors may promote neuronal regeneration in the presence of inhibitory matrix components.

Transcription of alpha2 integrin gene in osteosarcoma cells is enhanced by tumor promoters

Integrin alpha2beta1 is a heterodimeric transmembrane receptor for collagens. In osteogenic cells the expression of alpha2beta1 integrin is induced by both Kirsten sarcoma virus and chemical transformation. The association of alpha2 integrin with transformed cell phenotype was studied further by testing the effects of two tumor promoters, 12-O-tetradecanoylphorbol 13-acetate (TPA) and okadaic acid (OA), on human MG-63 osteosarcoma cells. TPA, an activator of protein kinase C, increased the cell surface expression of alpha2 integrin and the corresponding mRNA levels. Nuclear run-on assays indicated that TPA activated the transcription of alpha2 integrin gene. TPA also slightly increased the expression of alpha3 integrin but had no effect on the transcription of alpha5, alphav, or beta1 integrin subunits. OA, an inhibitor of serine/threonine phosphatases, increased alpha2 integrin gene transcription and mRNA levels, but in contrast to TPA, OA decreased alpha3 integrin expression. The increased expression of alpha2 integrin on TPA-treated MG-63 cells led to faster cell spreading on type I collagen. Our results link the enhanced transcription of alpha2 integrin gene to tumor progression and show the independent regulation of alpha2 integrin compared to other integrin genes.

Oncogenic Ki-ras but not oncogenic Ha-ras blocks integrin beta1-chain maturation in colon epithelial cells

Human colorectal tumors commonly contain mutations in Ki-ras but rarely, if ever, in Ha-ras. The selectivity for Ki-ras mutations in this tumor was explored using the HD6-4 colon epithelial cell line which contains no ras mutations. After adhesion to an extracellular matrix, HD6-4 cells polarize into columnar goblet cells with distinct apical and basal regions. Stable HD6-4 transfectants were made with mini-gene constructs of the oncogenic cellular Ki-ras4BG12V gene, the oncogenic Ha-rasG12V gene, or mini-gene constructs of wild-type Ki-ras4B as a control. Ki-ras mutations, but not Ha-ras mutations, disrupted colon epithelial cell apicobasal polarity and adhesion to collagen I and laminin. Three Ha-ras transfectants and three Ki-ras transfectants exhibited Ras proteins expressing the Val-12 mutation by Western blotting with pan-rasG12V antibody. Only wild-type Ki-ras transfectant cells and oncogenic Ha-ras transfectant cells synthesized the mature, fully glycosylated forms of beta1 integrin. Instead of the mature integrin beta1-chain, a faster migrating beta1-chain intermediate was detected on the cell surface and in the cytoplasm of the oncogenic Ki-ras transfectants. Expression of the oncogenic Ki-ras gene caused the altered beta1 integrin maturation because phosphorothiolated antisense oligonucleotides to Ki-ras reduced expression of both the mutant Ki-Ras protein and the aberrant integrin beta1-chain and increased expression of the mature integrin beta1-chain. Altered glycosylation generated the new beta1 integrin form since integrin core beta1-chain proteins of the same molecular weight were yielded in Ki-ras, Ha-ras, and control transfectants after removal of sugar residues with endoglycosidase F or following tunicamycin treatment to inhibit glycosylation. The selective effect of oncogenic Ki-ras on beta1 integrin glycosylation was not due to selective activation of mitogen-activated protein kinases because both mutated Ki- and Ha-ras genes activated this pathway and increased cell proliferation. Since blocking the glycosylation of integrin beta1-chain inhibited the adherence, polarization, and subsequent differentiation of colon epithelial cells, the selective effects of the oncogenic cellular Ki-ras gene on integrin beta1-chain glycosylation may account, at least in part, for the selection of Ki-ras mutations in human colon tumors.

Bone morphogenetic protein-2 is a regulator of cell adhesion

Bone morphogenetic proteins (BMPs) are a group of peptide growth factors closely related to transforming growth factors-beta. The BMPs are suggested to play an essential role in bone development and they are strong candidate molecules to be used clinically to improve fracture healing. BMPs are also involved in the differentiation of several other tissues during embryogenesis. Here, we show that human recombinant BMP-2 regulates cell-matrix interactions by modifying the expression of integrin-type receptors. The synthesis of alpha3 integrin was down-regulated by BMP-2 in two osteogenic sarcoma-derived cell lines, Saos-2 and HOS, and also in human fetal chondrocytes. BMP-2 had no effect on the expression of alpha1, alpha2, alpha5, alpha6, and alphaV integrins. BMP-2 reduced the expression of alpha3 integrin subunit at mRNA level. Laminin-5 was shown to be the ligand for alpha3beta1 integrin on Saos cells and BMP-2 decreased the ability of Saos cells to attach to it. These results suggest that BMP-2 has a specific effect on the alpha3beta1 integrin-mediated cell adhesion to laminin-5. Given the fact that BMP-2 is expressed in osteosarcomas, in addition to in bone, this mechanism is putatively important especially in bone development and tumors. We also studied the effect of BMP-2 on a human keratinocyte cell line, HaCaT. In HaCaT cells, the expression of alpha2 integrin was strongly down-regulated by BMP-2, whereas its effect on the expression of alpha3 integrin was smaller. We suggest that the effects of BMP-2 may be partially mediated by specifically altered cell adhesion.

Intracellular single-chain antibody inhibits integrin VLA-4 maturation and function

A single-chain antibody construct was prepared containing the VH and VL regions of anti-(integrin alpha 4) antibody HP1/2, an interchain linker and a KDEL endoplasmic reticulum retention sequence. Intracellular expression of this single-chain antibody caused cell-surface expression of alpha 4 beta 1 integrin to be decreased by 80% on selected RD cells and by 65-100% on selected Jurkat cells, relative to mock transfectants. Immunoprecipitation from single-chain-antibody-transfected cells showed that the single-chain antibody was complexed with the integrin alpha 4 and beta 1 subunits, and the diminished sizes of alpha 4 and beta 1 were consistent with impaired maturation. Furthermore, cell adhesion to alpha 4 beta 1 ligands [VCAM-1 (vascular cell adhesion molecule-1), FN40 (40 kDa chymotryptic fragment of fibronectin) and CS1] was greatly impaired in both RD and Jurkat cells, and cell spreading on immobilized FN40 protein was almost completely eliminated. Thus we conclude that intracellular single-chain antibodies may be used to reduce or eliminate cell-surface expression of a specific integrin, with specific functional consequences. This approach should be generally applicable to other integrin subunits.

Restricted expression and function of laminin 1-binding integrins in normal and malignant oral mucosal keratinocytes

Squamous cell carcinoma of the oral cavity spreads by initial invasion of the laminin-rich basement membrane. We examined the adhesion and motility of human oral SCC cells and normal mucosal keratinocytes and found that the SCC cells readily attached and migrated on laminin 1 substrates but migrated poorly on collagen type I and fibronectin. The normal keratinocytes, however, adhered poorly to and were non-motile on laminin 1 yet readily and preferentially attached and migrated on fibronectin and collagen type I. Analysis with blocking anti-integrin antibodies showed that the SCC cells used the alpha 6 beta 1 complex to attach and migrate on laminin 1 and that this activity was confined to the E8 long arm fragment of laminin. Affinity chromatography on laminin-Sepharose columns revealed that the SCC cells, but not normal keratinocytes, expressed high levels of the alpha 6 beta 1 laminin 1 receptor. Metabolic pulse-chase analysis indicated that in contrast to the SCC cells, keratinocytes did not have a stable pool of beta 1 subunit precursor. Preferential pairing of alpha 6 with beta 4 and the deficiency in pre-beta 1 levels appear to account for the failure of keratinocytes to form significant alpha 6 beta 1 complex. Additionally, the presence of laminin 1 in co-coating experiments blocked keratinocyte adhesion to other immobilized ligands, such as collagen type I or fibronectin. This anti-adhesive effect seemed to reflect a general paralysis of cell adhesive function, since laminin 1 also diminished the adhesion of keratinocytes to substrates coated with immobilized anti-integrin subunit antibody. The inhibitory activity of laminin 1 resided in the E1' and E8 fragments, and not in the E3, E4 or G domains. Collectively, our results indicate that laminin 1 is a restrictive ligand for normal keratinocytes, apparently because of their failure to assemble and express the alpha 6 beta 1 complex or other functional laminin receptors and their sensitivity to the anti-adhesive activity of laminin itself. The elevated expression of alpha 6 beta 1 following malignant conversion of muscosal keratinocytes promotes their migration on laminin, a process important during invasion and metastasis.

Disintegration of human spermatozoal membranes in seminal plasma decreases the binding capacity of integrins

The in vitro binding capacity of spermatozoal integrins to matrix components after disintegration of sperm membranes was evaluated. The percentage of spermatozoa with functionally-relevant integrins was determined before and after devitalization of spermatozoa, which were resuspended in seminal plasma or in culture medium. The devitalization was performed by cryoshock or by incubation of spermatozoa with triton X-100 in a concentration ranging from 0.01 to 1.0%. The spermatozoal integrins were detected by the binding of anti-integrin antibodies and flow cytometry and the functional activity was monitored by the binding of the spermatozoa to the matrix components in a cell attachment assay. The seminal plasma decreased the binding of anti-integrin antibodies to the spermatozoal surface and the binding of spermatozoa to ligands and matrix components. respectively. In contrast, the expression of fibronectin and laminin on spermatozoa was increased. Not all spermatozoa, which expressed integrins on their surface bound to the ligands in the cell attachment assay. These results suggest that the detectable integrins only partially exert functional relevance. It can be concluded that the spermatozoa with fragile plasma membranes are more prone to functional inactivation of their integrins by the seminal plasma.

Alteration of expression in integrin beta 1-subunit correlates with invasion and metastasis in colorectal cancer

We have investigated the expression of the integrin beta 1 subunit in 51 colorectal cancer tissues using immunoblotting and have determined the relationship between expression and clinical stage. In comparison with normal mucosa the pre-beta-subunit (the precursor of beta 1-subunit) was increased in 15 cases and decreased in two cases, and expression of the beta 1-subunit was decreased in two cases. These alterations of expression of the beta 1-subunit were significantly correlated with lymph node metastasis and depth of invasion (P < 0.01). These results suggest that integrin plays an important role in the invasion and metastasis of colorectal cancer.

Transforming growth factor-beta 1 modulates beta 1 and beta 5 integrin receptors and induces the de novo expression of the alpha v beta 6 heterodimer in normal human keratinocytes: implications for wound healing

The molecular mechanism underlying the promotion of wound healing by TGF-beta 1 is incompletely understood. We report that TGF-beta 1 regulates the regenerative/migratory phenotype of normal human keratinocytes by modulating their integrin receptor repertoire. In growing keratinocyte colonies but not in fully stratified cultured epidermis, TGF-beta 1: (a) strongly upregulates the expression of the fibronectin receptor alpha 5 beta 1, the vitronectin receptor alpha v beta 5, and the collagen receptor alpha 2 beta 1 by differentially modulating the synthesis of their alpha and beta subunits; (b) downregulates the multifunctional alpha 3 beta 1 heterodimer; (c) induces the de novo expression and surface exposure of the alpha v beta 6 fibronectin receptor; (d) stimulates keratinocyte migration toward fibronectin and vitronectin; (e) induces a marked perturbation of the general mechanism of polarized domain sorting of both beta 1 and beta 4 dimers; and (f) causes a pericellular redistribution of alpha v beta 5. These data suggest that alpha 5 beta 1, alpha v beta 6, and alpha v beta 5, not routinely used by keratinocytes resting on an intact basement membrane, act as "emergency" receptors, and uncover at least one of the molecular mechanisms responsible for the peculiar integrin expression in healing human wounds. Indeed, TGF-beta 1 reproduces the integrin expression pattern of keratinocytes located at the injury site, particularly of cells in the migrating epithelial tongue at the leading edge of the wound. Since these keratinocytes are inhibited in their proliferative capacity, these data might account for the apparent paradox of a TGF-beta 1-dependent stimulation of epidermal wound healing associated with a growth inhibitory effect on epithelial cells.