Molecular cloning and expression of the cDNA for alpha 3 subunit of human alpha 3 beta 1 (VLA-3), an integrin receptor for fibronectin, laminin, and collagen

Chondroitin Sulfate Proteoglycan 4 as a Marker for Aggressive Squamous Cell Carcinoma

Chondroitin sulfate (CS) proteoglycan 4 (CSPG4) is a cell surface proteoglycan that is currently under investigation as a marker of cancer malignancy, and as a potential target of anticancer drug treatment. CSPG4 acts as a driver of tumourigenesis by regulating turnover of the extracellular matrix (ECM) to promote tumour cell invasion, migration as well as inflammation and angiogenesis. While CSPG4 has been widely studied in certain malignancies, such as melanoma, evidence is emerging from global gene expression studies, which suggests a role for CSPG4 in squamous cell carcinoma (SCC). While relatively treatable, lack of widely agreed upon diagnostic markers for SCCs is problematic, especially for clinicians managing certain patients, including those who are aged or infirm, as well as those with underlying conditions such as epidermolysis bullosa (EB), for which a delayed diagnosis is likely lethal. In this review, we have discussed the structure of CSPG4, and quantitatively analysed CSPG4 expression in the tissues and pathologies where it has been identified to determine the usefulness of CSPG4 expression as a diagnostic marker and therapeutic target in management of malignant SCC.

Basement Membrane Regulates Fibronectin Organization Using Sliding Focal Adhesions Driven by a Contractile Winch

We have discovered that basement membrane and its major components can induce rapid, strikingly robust fibronectin organization. In this new matrix assembly mechanism, α5β1 integrin-based focal adhesions slide actively on the underlying matrix toward the ventral cell center through the dynamic shortening of myosin IIA-associated actin stress fibers to drive rapid fibronectin fibrillogenesis distal to the adhesion. This mechanism contrasts with classical fibronectin assembly based on stable or fixed-position focal adhesions containing αVβ3 integrins plus α5β1 integrin translocation into proximal fibrillar adhesions. On basement membrane components, these sliding focal adhesions contain standard focal adhesion constituents but completely lack classical αVβ3 integrins. Instead, peripheral α3β1 or α2β1 adhesions mediate initial cell attachment but over time are switched to α5β1 integrin-based sliding focal adhesions to assemble fibronectin matrix. This basement-membrane-triggered mechanism produces rapid fibronectin fibrillogenesis, providing a mechanistic explanation for the well-known widespread accumulation of fibronectin at many organ basement membranes.

Integrin α3 is involved in non-enveloped hepatitis E virus infection

Hepatitis E virus (HEV) causes acute and fulminant hepatitis worldwide. Although enveloped (e) and non-enveloped (ne) forms of HEV have been discovered, host factors involved in infection, including receptors, remain to be elucidated. Here, we identified integrin α3 (encoded by ITGA3), a protein that binds and responds to the extracellular matrix, as an essential host factor for HEV infection. Integrin α3 expression was lower in four HEV-non-permissive cell subclones than in an HEV-permissive subclone. ITGA3 knockout cells lost HEV permissibility, suggesting that integrin α3 is critical for HEV infection. Stable expression of integrin α3 in an HEV-non-permissive subclone provided permissibility only to infection by neHEV; expression of integrin α3 lacking the ectodomain did not. Direct interaction between neHEV and the integrin α3 ectodomain was confirmed by co-precipitation using a soluble integrin α3-Fc. These results strongly suggest that integrin α3 is a key molecule for cellular attachment and entry of neHEV.

Integrin α2β1 Expression Regulates Matrix Metalloproteinase-1-Dependent Bronchial Epithelial Repair in Pulmonary Tuberculosis

Pulmonary tuberculosis (TB) is caused by inhalation of Mycobacterium tuberculosis, which damages the bronchial epithelial barrier to establish local infection. Matrix metalloproteinase-1 plays a crucial role in the immunopathology of TB, causing breakdown of type I collagen and cavitation, but this collagenase is also potentially involved in bronchial epithelial repair. We hypothesized that the extracellular matrix (ECM) modulates M. tuberculosis-driven matrix metalloproteinase-1 expression by human bronchial epithelial cells (HBECs), regulating respiratory epithelial cell migration and repair. Medium from monocytes stimulated with M. tuberculosis induced collagenase activity in bronchial epithelial cells, which was reduced by ~87% when cells were cultured on a type I collagen matrix. Matrix metalloproteinase-1 had a focal localization, which is consistent with cell migration, and overall secretion decreased by 32% on type I collagen. There were no associated changes in the specific tissue inhibitors of metalloproteinases. Decreased matrix metalloproteinase-1 secretion was due to ligand-binding to the α2β1 integrin and was dependent on the actin cytoskeleton. In lung biopsies, samples from patients with pulmonary TB, integrin α2β1 is highly expressed on the bronchial epithelium. Areas of lung with disrupted collagen matrix showed an increase in matrix metalloproteinases-1 expression compared with areas where collagen was comparable to control lung. Type I collagen matrix increased respiratory epithelial cell migration in a wound-healing assay, and this too was matrix metalloproteinase-dependent, since it was blocked by the matrix metalloproteinase inhibitor GM6001. In summary, we report a novel mechanism by which α2β1-mediated signals from the ECM modulate matrix metalloproteinase-1 secretion by HBECs, regulating their migration and epithelial repair in TB.

Peripheral blood AKAP7 expression as an early marker for lymphocyte-mediated post-stroke blood brain barrier disruption

Our group recently identified 16 genes whose peripheral blood expression levels are differentially regulated in acute ischemic stroke. The purpose of this study was to determine whether the early expression levels of any of these 16 genes are predictive for post-stroke blood brain barrier (BBB) disruption. Transcriptional expression levels of candidate genes were measured in peripheral blood sampled from ischemic stroke patients at emergency department admission, and BBB permeability was assessed at 24 hour follow up via perfusion-weighted imaging. Early heightened expression levels of AKAP7, a gene encoding a protein kinase A-binding scaffolding molecule, were significantly associated with BBB disruption 24 hours post-hospital admission. We then determined that AKAP7 is predominantly expressed by lymphocytes in peripheral blood, and strongly co-expressed with ITGA3, a gene encoding the adhesion molecule integrin alpha 3. Subsequent in vitro experiments revealed that heightened expression of AKAP7 and ITGA3 in primary human lymphocytes is associated with a highly adherent phenotype. Collectively, our results suggest that AKAP7 expression levels may have clinical utility as a prognostic biomarker for post-stroke BBB complications, and are likely elevated early in patients who later develop post-stroke BBB disruption due to the presence of an invasive lymphocyte population in the peripheral blood.

Regulation of ionizing radiation-induced adhesion of breast cancer cells to fibronectin by alpha5beta1 integrin

Ionizing radiation (IR) is commonly used for cancer therapy, however, its potential influence on cancer metastatic potential remains controversial. In this study, we elucidated the role of integrins in regulation of IR-altered adhesion between breast cancer cells and extracellular matrix (ECM) proteins, which is a key step in the initial phase of metastasis. Our data suggest that the extent of effect that ionizing radiation had on cell adhesion depended on the genetic background of the breast cancer cells. Ionizing radiation was a better adhesion inducer for p53-mutated cells, such as MDA-MB-231 cells, than for p53 wild-type cells, such as MCF-7 cells. While IR-induced adhesions between MDA-MB-231 cells to fibronectin, laminin, collagen I and collagen IV, only blocking of the adhesion between α5β1 integrin and fibronectin using anti-α5β1 integrin antibody could completely inhibit the radiation-induced adhesion of the cells. A soluble Arg-Gly-Asp peptide, the binding motif for fibronectin binding integrins, could also reduce the adhesion of the cells to fibronectin with or without ionizing radiation exposure. The inhibition of the cell-fibronectin interaction also affected, but did not always correlate with, transwell migration of the cancer cells. In addition, our data showed that the total expression of α5 integrin and surface expression of α5β1 integrin were increased in the cells treated with ionizing radiation. The increased surface expression of α5β1 integrin, along with the adhesion between the cells and fibronectin, could be inhibited by both ataxia telangiectasia mutated (ATM) and Rad3-related (ATR) kinase inhibitors. These results suggested that ATM/ATR-mediated surface expression of α5β1 integrin might play a central role in regulation of ionizing radiation-altered adhesion.

Reverse proteomic antibody screening identifies anti adhesive VHH targeting VLA-3

Therapeutic approaches aimed at targeting tumor surface markers using monoclonal antibodies provide a powerful strategy in cancer treatment. Here we report selection of single variable domains (VHH) of llama heavy chain antibodies, using a VHH-phage-display library. A reverse proteomic approach was used to identify the cognate proteins recognized by enriched VHH on HeLa cells. One of these VHH bound the integrin alpha 3 beta 1 (VLA-3) and was further characterized. Most interestingly, this VHH could inhibit VLA-3 mediated cell-matrix adhesion. Our approach provides a fast and efficient method to screen for novel cell surface markers on normal and tumor cells that may find diagnostic or therapeutic application in disease management or treatment.

Identification of integrin alpha3 as a new substrate of the adenovirus E4orf6/E1B 55-kilodalton E3 ubiquitin ligase complex

The human adenovirus E4orf6 and E1B55K proteins promote viral replication by targeting several cellular proteins for degradation. The E4orf6 product has been shown by our group and others to form an E3 ubiquitin ligase complex that contains elongins B and C and cullin family member Cul5. E1B55K associates with this complex, where it is believed to function primarily to introduce bound substrates for degradation via proteasomes. In addition to p53, its first known substrate, the E4orf6/E1B 55-kDa complex (E4orf6/E1B55K) was shown to promote the degradation of Mre11 and DNA ligase IV; however, additional substrates are believed to exist. This notion is strengthened by the fact that none of these substrates seems likely to be associated with additional functions shown to be mediated by the E4orf6-associated E3 ubiquitin ligase complex, including export of late viral mRNAs and blockage of export of the bulk cellular mRNAs from the nucleus. In an attempt to identify new E4orf6/E1B55K substrates, we undertook a proteomic screen using human p53-null, non-small-cell lung carcinoma H1299 cells expressing either E4orf6 protein alone or in combination with E1B55K through infection by appropriate adenovirus vectors. One cellular protein that appeared to be degraded by E1B55K in combination with the E4orf6 protein was a species of molecular mass approximately 130 kDa that was identified as the integrin alpha3 subunit (i.e., very late activation antigen 3 alpha subunit). Preliminary analyses suggested that degradation of alpha3 may play a role in promoting release and spread of progeny virions.

Responses of endothelial cell and astrocyte matrix-integrin receptors to ischemia mimic those observed in the neurovascular unit

BACKGROUND AND PURPOSE

Apposition of endothelial cells and astrocyte foot processes to the basal lamina matrix is postulated to underlie the cerebral microvessel permeability barrier. Focal cerebral ischemia induces rapid loss of select matrix-binding integrins from both cell compartments in the nonhuman primate. This study is the first to examine the conditions underlying integrin loss from these cell-types during ischemia in vitro and their relation to the changes in vivo.

METHODS

The impact of normoxia or standardized oxygen-glucose deprivation on integrin expression by murine primary cerebral endothelial cells and astrocytes grown on matrix substrates (collagen IV, laminin, and perlecan) of the basal lamina were quantitatively assessed by flow cytometry.

RESULTS

Endothelial cell expression of the beta1 and alpha 5 subunits significantly increased on all matrix ligands, whereas astrocytes displayed modest significant decreases in alpha 5 and alpha 6 subunits. Oxygen-glucose deprivation produced a further significant increase in subunit beta1 expression by both cell types, but a clear decrease in both alpha1 and alpha 6 subunits by murine astrocytes.

CONCLUSIONS

Ischemia in vitro significantly increased endothelial cell beta1 expression, which is consistent with the increase in beta1 transcription by microvessels peripheral to the ischemic core. The loss of alpha1 and alpha 6 integrins from murine astrocytes is identical to that seen in the nonhuman primate in vivo. These findings establish both isolated murine cerebral endothelial cells and astrocytes as potential integrin response cognates of microvascular cells of the neurovascular unit in primates, and allow determination of the mechanisms of their changes to ischemia.

Novel surface patterning approaches for tissue engineering and their effect on cell behavior

Methods for the creation of specially designed surfaces for use in the preparation of tailor-made tissue constructs with the ultimate aim of tissue engineering are reviewed here. Fundamental aspects of cell adhesion, proliferation and differentiation and the parameters involved in these processes are discussed. A survey of recent micro- and nano-technological methods for creating physical and chemical cues on tissue engineering carriers is presented. This overview is supported with data from the literature on various applications of different cells on materials with widely differing chemistries and physical properties. Interactions between different cell types and micro- and nano-fabricated substrates are summarized.

Changes in adhesive and migratory characteristics of hepatocellular carcinoma (HCC) cells induced by expression of alpha3beta1 integrin

The invasive and metastatic potentials of hepatocellular carcinoma are positively correlated with the expression level of alpha3beta1 integrin, a high-affinity adhesion receptor for laminin isoforms including laminin-5. In this study, we investigated changes in the adhesive and invasive behaviors of human HCC HepG2 cells after transfection with cDNA for alpha3 integrin in order to elucidate the direct involvement of this integrin in these cellular processes. We introduced cDNA for splice variants of alpha3 integrin (alpha3A and alpha3B) into the cells, and selected two transfectant clones (HepG2-3A and HepG2-3B), which express the alpha3A and alpha3B integrins, respectively. Both transfectant cells adhered almost equally to laminin-5-coated plates in an alpha3 integrin-dependent manner, indicating that transfected alpha3Abeta1 and alpha3Bbeta1 integrins were functionally active in these cells. The migratory and invasive potentials of the transfectant cells were assessed by scratch wound assay and in vitro chemoinvasion assay. The results demonstrated that the migration of HepG2-3A and HepG2-3B cells but not of mock transfectant (HepG2-M) cells was stimulated on the plates coated with laminin-5. Furthermore, HepG2-3A and HepG2-3B cells were found to be more invasive into laminin-5-containing matrices than were HepG2-M cells. These results strongly suggest that enhanced expression of alpha3beta1 integrin on HCC cells is directly involved in their malignant phenotypes such as invasion and metastasis.

Characterization of the promoter for the alpha3 integrin gene in various tumor cell lines: roles of the Ets- and Sp-family of transcription factors

The alpha3beta1 integrin is an adhesion receptor for extracellular matrix proteins, including laminin isoforms, and plays crucial roles in the organization of epithelial and endothelial tissues. The aberrant expression of this adhesion molecule on tumor cells is associated with their invasive and metastatic potentials. In the present study, we analyzed the elements essential for alpha3 integrin gene expression in various tumor cell lines with different tissue origins by luciferase assay. An approximately 0.3 kb fragment of the 5'-flanking region of the mouse alpha3 integrin gene (-260/+84, relative to the major transcription start site) showed strong promoter activity in all six examined tumor cell lines. However, we found that these cell lines could be divided into two groups according to the level of dependency on the putative Ets-transcription factor binding motif located at -133. This motif was previously shown to be crucial for alpha3 integrin expression in MKN1 gastric carcinoma cells. The gene expression in one group of cell lines was upregulated mainly by the Ets motif, whereas that in the other group was less dependent on the Ets motif. We then postulated that additional regulatory elements were responsible for the expression of alpha3 integrin, and found that a GC-rich motif at -69 was another important element. An electrophoretic mobility shift assay using specific antibodies and a Western blot analysis of nuclear proteins revealed that the Sp3-transcription factor bound to this GC-rich motif. These results suggest that the Sp3 and Ets transcription factors cooperatively regulate alpha3 integrin gene expression and that the contribution of each element depends on the type of tumor cells.

Signature of B-CLL with different prognosis by Shrunken centroids of surface antigen expression profiling

With the aim of identifying the immunophenotypic profile of B-cell chronic lymphocytic leukemia (B-CLL) subsets with different prognosis, we investigated by flow cytometry the expression of 36 surface antigens in 123 cases, all with survivals. By analyzing results with unsupervised (hierarchical and K-means clustering) algorithms, three distinct immunophenotypic groups (I, II, and III) were identified, group I (51/123) with longer survivals, as compared to the group II (36/123) and III (36/123). The immunophenotypic signatures of these groups, as determined by applying the nearest Shrunken centroids method as class predictor, were characterized by the coordinated and differential expression of 12 surface markers, that is, group I: above-average expression of CD62L, CD54, CD49c, and CD25, below-average expression of CD38; group II: above-average expression of CD38, CD49d, CD29, and CD49e; and group III: below-average expression of the above markers, overexpression of CD23, CD20, SmIg, and CD79b. As opposed to groups II-III, group I B-CLLs lacked expression of ZAP-70 and activation-induced cytidine deaminase in the majority of cases, while more frequently had mutated IgV(H) genes and IgV(H) mutations consistent with antigen-driven selection. Our findings contribute to improve the immunophenotypical identification of disease subsets with different prognosis and suggest a set of surface antigens to be employed as prognosticators in routine diagnostic/prognostic procedures.

Physiological and Pathological Roles of α3β1 Integrin

Alpha3beta1 integrin has been considered to be a mysterious adhesion molecule due to the pleiotropy in its ligand-binding specificity. However, recent studies have identified laminin isoforms as high-affinity ligands for this integrin, and demonstrated that alpha3beta1 integrin plays a number of essential roles in development and differentiation, mainly by mediating the establishment and maintenance of epithelial tissues. Furthermore, alpha3beta1 integrin is also implicated in many other biological phenomena, including cell growth and apoptosis, angiogenesis and neural functions. This integrin receptor forms complexes with various other membrane proteins, such as the transmembrane-4 superfamily proteins (tetraspanins), cytoskeletal proteins and signaling molecules. Recently, lines of evidence have been reported showing that complex formation regulates integrin functions in cell adhesion and migration, signal transduction across cell membranes, and cytoskeletal organization. In addition to these roles in physiological processes, alpha3beta1 integrin performs crucial functions in various pathological processes, especially in wound healing, tumor invasion and metastasis, and infection by pathogenic microorganisms.

Mass Spectrometric Based Mapping of the Disulfide Bonding Patterns of Integrin α Chains

Integrins are one of the major mediators of cellular adherence. Structurally the component alpha and beta chains are characterized by extensive intrachain disulfide bonding. The assignment of these bonds is currently based on homology with the chains of the integrin alphaIIbbeta3. However, recent crystallographic analysis of the soluble alphaVbeta3 construct indicates that the alphaV chain displays bonding patterns different from those predicted for alphaIIb. In an effort to define the disulfide bonding patterns in integrins, we have used mass spectrometric based approaches to map the human alpha3, alpha5, alphaV, and alphaIIb. The results indicate that there are differences in the disulfide patterns of the alpha chains. These do not correlate with the integrin capacity to bind ligands as all integrins used in the present study displayed functional activity. The differences were observed in the bonding patterns linking the heavy (H) and light (L) components of the of the alpha chains. It was also possible to assign the location in alpha5 of an additional disulfide bond involving a pair of cysteines not present in alphaV or alphaIIb. This second bond between the H and L chains of alpha5 has not been previously described. These results indicate that not all integrin species display the same disulfide bonding patterns. They also highlight the need for caution in the use of assignments based on sequence homology.

Two novel mutations in the alpha IIb calcium-binding domains identify hydrophobic regions essential for alpha IIbbeta 3 biogenesis

The recently published crystal structure of the external domains of alphaVbeta3 confirms the prediction that the aminoterminal portion of alphaV, which shares 40% homology with alphaIIb, folds into a beta-propeller structure and that the 4 calcium-binding domains are positioned on the bottom of the propeller. To gain insight into the role of the calcium-binding domains in alphaIIb biogenesis, we characterized mutations in the second and third calcium-binding domains of alphaIIb in 2 patients with Glanzmann thrombasthenia. One patient inherited a Val298Phe mutation in the second domain, and the other patient inherited an Ile374Thr mutation in the third domain. Mammalian cell expression studies were performed with normal and mutant alphaIIb and beta3 cDNA constructs. By flow cytometry, expression of alphaIIb Val298Phe/beta3 in transfected cells was 28% of control, and expression of alphaIIbIle374Thr/beta3 was 11% of control. Pulse-chase analyses showed that both mutant pro-alphaIIb subunits are retained in the endoplasmic reticulum and degraded. Mutagenesis studies of the Val298 and Ile374 residues showed that these highly conserved, branch-chained hydrophobic residues are essential at these positions and that biogenesis and expression of alphaIIbbeta3 is dramatically affected by structural variations in these regions of the calcium-binding domains. Energy calculations derived from a new model of the alphaIIb beta-propeller indicate that these mutations interfere with calcium binding. These data suggest that the alphaIIb calcium-binding domains play a key structural role in the beta-propeller, and that the structural integrity of the calcium-binding domains is critical for integrin biogenesis.

Characterization of the promoter for the mouse alpha 3 integrin gene

The alpha 3 beta 1 integrin is an adhesion receptor for extracellular matrix proteins including isoforms of laminin, and the changes of its expression level in various cancer cells are thought to cause their malignant phenotypes. We have cloned an approximately 4 kb DNA fragment of the 5'-flanking region of the murine alpha 3 integrin gene and analyzed its promoter activity. Transfection of MKN1 gastric carcinoma cells with serially truncated segments of the 5'-flanking region linked to a luciferase gene indicated that a 537-bp SalI/SacI fragment upstream of exon 1 was sufficient to promote high level gene expression. By 5'-rapid amplification of cDNA ends (5'-RACE) using a cap site-labeled cDNA library, we determined one major and one minor transcription start sites in this region. The murine alpha 3 integrin gene was found to contain a CCAAT box, but to lack a TATA box. Luciferase assay following transfection with a series of deletion constructs of the SalI/SacI fragment revealed that the sequence between positions -260 and -119 bp (relative to the major transcription start site) is required for efficient transcription in gastric carcinoma cells. The sequence analysis of this segment showed the presence of several consensus sequences for transcription factors including Ets, GATA and MyoD/E-box binding factors. The introduction of mutation in one of the Ets-binding sequences greatly decreased its promoter activity, suggesting that the transcription of the alpha 3 integrin gene in these cells is regulated by the Ets-family of transcription factors.

Intergrin gene expression profiles of humanhepatocellular carcinoma

AIM: To investigate gene expression profiles of intergrin genes in hepatocellular carcinoma (HCC) through the usage of Atlas Human Cancer Array membranes, semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and Northern blot.

METHODS: Hybridization of cDNA array membrane was performed with α³²P-labeled cDNA probes synthesized from RNA isolated from hepatocellular carcinoma and adjacent non-cirrhotic liver. AtlasImage, which is a software specific to array, was used to analyze the result. RT-PCR of 24 pairs specimen and Northern blot of 4 pairs specimen were used to confirm the expression pattern of some intergrin genes identified by Atlas arrays hybridization.

RESULTS: Among 588 genes spotted in membrane, 17 genes were related to intergrin. Four genes were up-regulated, such as intergrin alpha8, beta1, beta7 and beta8 in HCC. Whereas there were no genes down-regulated in HCC. RT-PCR and Northern blot analysis of intergrin beta1 gene gave results consistent with cDNA array findings.

CONCLUSION: Investigation of these intergrin genes should help to disclose the molecular mechanism of the cell adhesion, invasive and metastasis of HCC. A few genes are reported to have changed in HCC for the first time. The quick and high-throughout method of profiling gene expression by cDNA array provides us overview of key factors that may involved in HCC, and may find the clue of the study of HCC metastasis and molecular targets of anti-metastasis therapy. The precise relationship between the altered genes and HCC is a matter of further investigation.

Expression of beta1 integrins during periosteal chondrogenesis

OBJECTIVE

The interactions between integrins and extracellular matrix proteins are known to modulate cell behavior, and may be involved in regulating cartilage formation and repair. The purpose of this study was to determine the patterns and localization of expression of the beta1 integrins during cartilage formation by periosteum, which is used to repair articular cartilage.

DESIGN

Periosteal explants from 2-month-old rabbit medial proximal tibiae were cultured in agarose suspension for 0 to 6 weeks, with 10 ng/ml transforming growth factor-beta1 added for the first 2 days of culture. Integrin expressions were measured by reverse transcriptase-polymerase chain reaction (RT-PCR) and localized by immunohistochemistry.

RESULTS

Normal periosteum expressed the alpha1, alpha3, alpha5, beta1 subunits at low levels, and the proteins for all but the alpha3 subunits were identified by immunohistochemistry in the periosteum. Significant two- to five-fold up-regulation of the mRNA expression of the alpha1, alpha3, alpha5 and beta1 integrin subunits during the early proliferative stage of chondrogenesis was observed. The initial change was a five-fold increase in alpha5 expression on day 2 and a two-fold increase in alpha3 expression. On day 5, alpha1 expression was up-regulated (four-fold). beta1 expression was broadly up-regulated (three to four-fold) from day 5 to 14. In the early stage of chondrocyte differentiation, after day 14, alpha1 expression was down-regulated, while there was upregulation of alpha3 (three-fold), alpha5 (three-fold) and beta1 (four-fold) expressions. Thereafter, alpha1 expression was down-regulated, while alpha3, alpha5 and beta1 expressions were up-regulated again during matrix synthesis. Immunohistochemistry confirmed this late decrease in alpha1 levels and increase in alpha3, alpha5 and beta1 levels in chondrocytes.

CONCLUSIONS

These observations indicate that the beta1 integrins play an important role in the process of chondrogenesis in periosteum.

Functional comparison of the alpha3A and alpha3B cytoplasmic domain variants of the chicken alpha3 integrin subunit

Integrin alpha3beta1 can be alternatively spliced to generate alpha3A and alpha3B cytoplasmic domain variants that are conserved among vertebrates. To identify distinct functions of these variants, we transfected cells with intact alpha3 integrins or chimeric receptors. alpha3Abeta1 and alpha3Bbeta1 each localized to focal contacts in keratinocytes on an extracellular matrix rich in laminin-5, to which both are known to bind with high affinity. However, alpha3B accumulated intracellularly in keratinocytes on collagen, suggesting that laminin binding may stabilize alpha3Bbeta1 surface expression. Neither alpha3 cytoplasmic domain affected recruitment of chimeric alpha5 integrins to fibronectin-induced focal contacts, and either substituted for the alpha5 cytoplasmic domain in alpha5beta1-mediated cell migration. However, the alpha5/alpha3B chimera localized to cell-cell borders in MDCK or CHO cells to a lesser extent than did the alpha5/alpha3A chimera. To determine whether the alpha3 cytoplasmic domains conferred distinct localization to a nonintegrin protein, we transfected cells with interleukin-2 receptor (IL-2R) chimeras containing the alpha3 cytoplasmic domains. The IL-2R/alpha3A chimera was expressed efficiently on the cell surface, while the IL-2R/alpha3B chimera accumulated intracellularly. Our findings suggest that the alpha3B cytoplasmic domain harbors a retention signal that is regulated in an intact integrin and can alter cell surface expression and distribution of alpha3beta1.

Gingival, dermal, and periodontal ligament fibroblasts express different extracellular matrix receptors

BACKGROUND

Fibroblasts are the predominant cells of the periodontal ligament and the gingiva and have important roles in the function and regeneration of the tooth support apparatus. The goal of this study was to investigate the possible differences in the adhesion properties and expression of extracellular matrix (ECM) receptors among different fibroblast populations.

METHODS

The adhesion of gingival (GF), dermal (DF), and periodontal ligament fibroblast (PDLF) cultures to ECM proteins (fibronectin, laminin, vitronectin, RGD peptide, collagen type I, and collagen type IV) adsorbed to tissue culture plastic was evaluated fluorometrically. Quantitative reverse transcription-polymerase chain reactions (RT-PCR) were performed using primers specific for 19 integrin subunits to quantify ECM receptor transcript expression.

RESULTS

Our data demonstrated that GF and PDLF adhere to vitronectin and collagen types I and IV more avidly than do DF. PDLF adhered well to laminin, whereas GF and DF did not. Quantitation of integrin expression demonstrated that the different fibroblast types expressed different integrin transcripts, further demonstrating their innate differences.

CONCLUSIONS

The 3 fibroblast types studied behave differently and expressed different ECM receptors. However, gingival fibroblasts and periodontal ligament fibroblasts are more similar in their attachment and integrin expression than either is to dermal fibroblasts. Therefore, experiments using DF will not necessarily be valid for oral tissues.

Role of alpha(3)beta(1) integrin in tubulogenesis of Madin-Darby canine kidney cells

BACKGROUND

We isolated several Madin-Darby canine kidney (MDCK) subclones that exhibit different degrees of branching tubulogenesis in lower concentrations of collagen gel. The M634 clone formed cell aggregates in 0.3% collagen gel, but developed branching tubules vigorously in 0.1% collagen gel. In contrast, the Y224 clone formed cysts in 0.3% collagen gel and displayed fewer branching structures in 0.1% collagen gel. Morphologically, M634 cells exhibited higher levels of cell scattering as well as collagen-induced cell migration than Y224. We conducted this study to delineate the underlying mechanism of branching tubulogenesis in M634 cells.

METHODS

Components of the focal contact machinery were analyzed in both cell lines, including the extracellular matrix glycoproteins fibronectin, laminin, and vitronectin; cytoskeleton-associated elements alpha-actinin, talin, and vinculin; and receptors for extracellular matrix and alpha(2), alpha(3), alpha(5), alpha(v), beta(1), and beta(3) integrins. Furthermore, we established several stable transfectants of alpha(3) integrin antisense RNA in M634 cells to examine the role of alpha(3)beta(1) integrin in branching morphogenesis directly.

RESULTS

There were no obvious differences in levels of the focal adhesion complex proteins between M634 and Y224 cells, except that the content of the alpha(3) and beta1 integrins were 1.2- and 0.6-fold higher in M634 cells, respectively. The expression of alpha(3) integrin antisense RNA significantly lowered the levels of alpha(3) integrin mRNA and protein. The potential of cell scattering, migration, and branching tubulogenesis in M634 cells was inhibited according to the decrease in alpha(3) integrin expression.

CONCLUSION

Our data indicate that expression of alpha(3)beta(1) integrin regulates cell scattering, migration, and branching tubulogenesis of MDCK cells, possibly via adhesion to or serving as a signaling molecule for type I collagen.

Puromycin aminonucleoside suppresses integrin expression in cultured glomerular epithelial cells

Puromycin aminonucleoside (PAN)-induced nephrosis is a well-described model of human idiopathic nephrotic syndrome, but the mechanism of PAN's effect is not completely understood. Because PAN injection into rats results in retraction of glomerular epithelial cell foot processes and glomerular epithelial cell detachment, it was hypothesized that PAN might alter the contacts between these cells and the glomerular basement membrane. The major integrin expressed by glomerular epithelial cells is alpha3beta1, which mediates attachment of these cells to extracellular matrix proteins including type IV collagen. T-SV 40 immortalized human glomerular epithelial cells were used to study PAN's effects on alpha3beta1 expression, as well as that of podocalyxin and the slit diaphragm component ZO-1. Glomerular epithelial cells were seeded into plastic flasks and allowed to attach and proliferate for 48 h. The cells were then incubated for another 48 h in media containing 0, 0.5, or 5.0 microg/ml PAN. PAN exposure resulted in dose-dependent decreases in alpha3 and beta1 expression, both at the protein level and at the mRNA level. This was accompanied by a significant decrease in the adhesion of glomerular epithelial cells to type IV collagen. PAN did not affect ZO-1 protein expression. Treatment with PAN increased the expression of podocalyxin at the protein and mRNA levels. Reduced glomerular epithelial cell expression of alpha3beta1 integrins and impaired adhesion to type IV collagen may contribute to the glomerular epithelial cell detachment from glomerular basement membrane seen in the PAN nephrosis model.

Interferon-mediated block in cell cycle and altered integrin expression in a differentiated salivary gland cell line (HSG) cultured on Matrigel

Sjögren's syndrome (SS), an idiopathic, autoimmune exocrinopathy, is partly characterized by diminished salivary flow, acinar cell atrophy, and increased expression of several cytokines. Several in vivo characteristics of the sialoadenitis are also evident in a human salivary gland ductal epithelial cell line (HSG) treated with cytokines. HSG cells differentiate to the acinar phenotype when cultured on Matrigel (Becton Dickinson, Bedford, MA), a basement membrane extract. To elucidate mechanisms of salivary gland pathology, the effects of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on cell cycle progression and integrin expression were evaluated in HSG acinarlike cells. Flow cytometry experiments showed that cytokine treatment for 2 days arrested cells in G(1) phase of the cell cycle, and this preceded significant morphologic changes and decreased viability. Whereas only modest cytokine-mediated increases in protein expression for the alpha 3 and beta 1 integrin subunits were seen by immunoprecipitation, a form of alpha 3 integrin displaying enhanced electrophoretic mobility was evident after 6 days of cytokine treatment. To our knowledge, this is the first report demonstrating an IFN-mediated alteration in the electrophoretic mobility of integrin subunits. From this study, it was evident that the combination of IFN-gamma and TNF-alpha resulted in a block in G(1) phase for acinar cells before accumulation of the alpha 3 integrin variant or significant degenerative cellular changes. Information from the present and previous studies suggests that cytokines may alter the pattern of integrin expression and block cell cycle progression in salivary gland cells grown in three-dimensional acinarlike clusters. These experiments may provide a new cell culture model to study the effects of cytokines in normal and diseased salivary glands, including SS.

Branching morphogenesis during kidney development

Epithelial tissues such as kidney, lung, and breast arise through branching morphogenesis of a pre-existing epithelial structure. They share common morphological stages and a need for regulation of a similar set of developmental decisions--where to start; when, where, and in which direction to branch; and how many times to branch--decisions requiring regulation of cell proliferation, apoptosis, invasiveness, and cell motility. It is likely that similar molecular mechanisms exist for the epithelial branching program. Here we focus on the development of the collecting system of the kidney, where, from recent data using embryonic organ culture, cell culture models of branching morphogenesis, and targeted gene deletion experiments, the outlines of a working model for branching morphogenesis begin to emerge. Key branching morphogenetic molecules in this model include growth factors, transcription factors, distal effector molecules (such as extracellular matrix proteins, integrins, proteinases and their inhibitors), and genes regulating apoptosis and cell proliferation.

Mouse keratinocytes immortalized with large T antigen acquire alpha3beta1 integrin-dependent secretion of MMP-9/gelatinase B

Remodeling of the extracellular matrix during tissue development, wound repair and tumor cell invasion depends on the coordinated regulation of cell adhesion receptors, matrix proteins and enzymes that proteolyse the extracellular matrix. Integrin alpha3beta1 is a major receptor on epidermal keratinocytes for laminin-5 in the cutaneous basement membrane and is required for normal basement membrane organization during skin development. alpha3beta1 is also expressed at high levels in the majority of adherent transformed cells and in most tumors, and it could have similar roles in extracellular matrix remodeling during tumorigenesis and cell invasion. In the present study, we show that alpha3beta1 expression is required in immortalized mouse keratinocytes (MK) for the production of the matrix metalloproteinase MMP-9/gelatinase B, an MMP that is coexpressed with alpha3beta1 in epithelial cell carcinomas and during wound healing, and contributes to the invasive potential of some tumor cells. MMP-9 was expressed in MK cells derived from wild-type mice, but not in MK cells derived from alpha3-null mice. Reconstitution of alpha3beta1 expression in alpha3-null MK cells through transfection with the alpha3 subunit restored MMP-9 secretion, indicating an alpha3beta1-dependent pathway for MMP-9 production. alpha3beta1-dependent expression of MMP-9 was associated with the immortalized phenotype, since nonimmortalized, primary keratinocytes required soluble growth factors, but not alpha3beta1, for efficient expression of MMP-9. Our results suggest that an alpha3beta1-independent pathway(s) for MMP-9 production is suppressed in keratinocytes immortalized with large T antigen, and that an alpha3beta1-dependent pathway is required for sustained production of MMP-9 in the absence of other pathways.

Comparative analysis of integrin expression on monocyte-derived macrophages and monocyte-derived dendritic cells

Both macrophages (MAC) and dendritic cells (DC) are members of the mononuclear phagocyte system (MPS) with monocytes (MO) as common precursor cells. Cells of the MPS are able to take up, process and present antigens to T lymphocytes, thereby inducing a primary or secondary immune response. Adhesion molecules are of crucial importance for the interaction of antigen-presenting cells with immune cells, especially T lymphocytes. By representational difference analysis, we identified CD49c (VLA-3), a member of the beta1-integrin family of adhesion receptors, as differentiation-associated antigen in MO-derived MAC. In contrast, MO-derived DC did not express CD49c mRNA. These data prompted us to compare the integrin expression pattern of MAC and DC. Both cell types showed a low expression of the alpha-chains of the beta1-integrins CD49a, CD49b, CD49d and CD49e, whereas a marked difference was observed for CD49c and CD49f. Expression of both integrins increased during MO to MAC differentiation, but was not detectable on DC. In parallel the beta1-chain (CD29) was clearly up-regulated during MO to MAC differentiation but was only weakly expressed on DC. On the other hand, the beta2-integrins CD11a, CD11b, CD11c and CD18 were all expressed on MAC and DC. Beside their role in cell-cell interaction and adhesion, beta2-integrins are also known as possible binding molecules for bacteria and lipopolysaccharide (LPS), especially for high LPS concentrations. Therefore we investigated the LPS response of MAC versus DC in terms of tumour necrosis factor-alpha (TNF-alpha) release. DC were less responsive to low doses of LPS, which can easily be explained by the very low CD14 expression on DC compared for MAC. In contrast, the TNF-alpha response was comparable to MAC when DC were stimulated with high LPS concentrations. Our results show a specific, differentiation-dependent pattern of beta1- and beta2-integrin expression on in vitro-generated MAC and DC. We suggest that the high expression of CD11/CD18 on DC could be involved in the LPS binding of DC. As LPS is not only an activation but also a differentiation stimulus for DC, the expression of CD11/CD18 on DC may be important for the successful maturation of DC and thereby the initiation of a primary immune response.

Transforming growth factor-beta1 stimulates contraction of human glioblastoma cell-mediated collagen lattice through enhanced alpha2 integrin expression

Rapid invasiveness is a feature of the highly malignant glioblastoma tumor and is closely related to patient prognosis. The interaction between extracellular matrix (ECM) and cell surface receptors such as integrin heterodimers play a key role in the process of tumor invasion. We investigated the effects of transforming growth factor-beta1 (TGF-beta1), which is a mitogenic factor for glial cells, on integrin expression in T98G human glioblastoma cells using an in vitro model 3-dimensional collagen lattice. Exogenously applied TGF-beta1 dose-dependently enhanced collagen lattice contraction. Among the inhibitory antibodies tested against alpha integrin subunits, the anti-alpha2 antibody, P1-E6, alone prevented the enhanced contractile response by TGF-beta1, whereas any alpha integrin antibody (including P1-E6) had little effect on lattice contraction when cultured without TGF-beta1. RT-PCR analysis revealed that TGF-beta1 strongly increased alpha2 integrin transcript level. Furthermore, pretreatment with antisense phosphorothioate oligodeoxynucleotides against human alpha2 integrin using hemagglutinating virus of Japan (HVJ) liposome-mediated transfer prevented the effects of TGF-beta1 and also reduced the lattice contraction even in the absence of TGF-beta1. This data indicates that increased expression of alpha2 integrin is responsive to enhanced collagen lattice contraction by TGF-beta1. We suggest that TGF-beta1 exerts its effects on the invasive property of glioblastoma cells via upregulation of the alpha2 integrin subunit expression.

Alpha 3 beta 1 adhesion to laminin-5 and invasin: critical and differential role of integrin residues clustered at the boundary between alpha 3 N-terminal repeats 2 and 3

Integrin/ligand interaction is a therapeutic target for many diseases. We previously reported that residues critical for ligand binding are clustered in N-terminal repeat 3 (in the predicted 2-3 loop) of alpha 4, alpha 5 and alpha IIb. Here we have localized residues critical for ligand binding in the alpha 3 subunit of integrin alpha 3 beta 1 with distinct ligand specificity (laminin-5). We identified an alpha 3 epitope common to several function-blocking anti-alpha 3 antibodies at the boundary between repeats 1 and 2 (residues 75-80). We found that swapping the predicted 4-1 loop (residues 153-165) at the boundary between repeats 2 and 3 with the corresponding alpha 4 sequence and mutating Thr-162 and Gly-163 residues in this predicted loop block laminin-5 binding. Thr-162 and Gly-163 and the antibody epitope are separated in the primary structure; however, they are close to each other in the proposed beta-propeller model. Mutating residues recently reported to block (Tyr-186 and Trp-188) or enhance (Asp-122) laminin-5 binding to alpha 3 beta 1 [Krukonis, E. S., Dersch, P., Eble, J. A., and Isberg, R. R.(1998) J. Biol. Chem. 273, 31837-31843] did not affect laminin-5 binding under the assay conditions used. Thr-162 and Gly-163 are not critical for adhesion to invasin, indicating that laminin-5 and invasin may use different recognition mechanisms, and that mutation of Thr-162 and Gly-163 does not drastically affect the integrity of alpha 3 beta 1. These results suggest that residues critical for ligand binding may be similarly (but not identically) located in repeat 3 of the alpha subunit regardless of ligand specificity.

A novel protease-docking function of integrin at invadopodia

Invadopodia are membrane extensions of aggressive tumor cells that function in the activation of membrane-bound proteases occurring during tumor cell invasion. We explore a novel and provocative activity of integrins in docking proteases to sites of invasion, termed invadopodia. In the absence of collagen, alpha(3)beta(1) integrin and the gelatinolytic enzyme, seprase, exist as nonassociating membrane proteins. Type I collagen substratum induces the association of alpha(3)beta(1) integrin with seprase as a complex on invadopodia. The results show that alpha(3)beta(1) integrin is a docking protein for seprase to form functional invadopodia. In addition, alpha(5)beta(1) integrin may participate in the adhesion process necessary for invadopodial formation. Thus, alpha(3)beta(1) and alpha(5)beta(1) integrins play major organizational roles in the adhesion and formation of invadopodia, promoting invasive cell behavior.

Integrins: a role as cell signalling molecules

Integrins form the major family of proteins that mediates cell-matrix interactions. As well as an adhesive function, it is increasingly apparent that integrins can transduce messages via classic signalling pathways and impact upon such fundamental cellular processes as proliferation, apoptosis, differentiation, and motility. Dysregulation of these processes are a feature of many malignancies. Altered integrin expression has been observed in many human tumours, and perturbation of integrin function or expression in experimental systems has demonstrated that altered integrin signalling may directly contribute to the development of the malignant phenotype.

Structure, genetic localization, and identification of the cardiac and skeletal muscle transcripts of the human integrin alpha7 gene (ITGA7)

We have determined the structure and the exon size pattern of the human integrin alpha7 subunit gene (ITGA7), which has been shown to be affected in a form of congenital myopathy. The gene is composed of at least 27 exons spanning a region of about 22.5 kb. The sequence of all exon/intron boundaries was determined and conforms to the GT/AG splicing consensus. We investigated the different splicing forms previously described in human and rodents. The major cytoplasmic variants alpha7A and alpha7B, which are developmentally regulated and tissue specific, were identified in human tissues, as well as the extracellular isoforms X1 and X2. The recently described D variant was detected in adult tissues by RT-PCR but not the C variant. We localized ITGA7 on chromosome 12q13 by high-resolution radiation hybrid mapping between D12S312 and D12S90 and identified a new CA-repeat microsatellite in intron 1.

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.

Selective tetraspan-integrin complexes (CD81/alpha4beta1, CD151/alpha3beta1, CD151/alpha6beta1) under conditions disrupting tetraspan interactions

The tetraspans are molecules with four transmembrane domains which are engaged in multimolecular complexes (the tetraspan web) containing a subset of beta1 integrins (in particular alpha3beta1, alpha4beta1 and alpha6beta1), MHC antigens and several unidentified molecules. The molecules associated with tetraspans are readily detected after immunoprecipitation performed in mild detergents such as Brij 97 or CHAPS. In this study we show that another classical mild detergent, digitonin, dissociated most of these associated molecules, including integrins, from the tetraspans CD9, CD37, CD53, CD63, CD82, Co-029, Talla-1 and NAG-2. In contrast, reciprocal immunoprecipitations from various cell lines demonstrated that two other tetraspans, CD81 and CD151, formed complexes with integrins not disrupted by digitonin. These complexes were CD81/alpha4beta1, CD151/alpha3beta1 and CD151/alpha6beta1. Furthermore, a new anti-CD151 monoclonal antibody (mAb), TS151r, was shown to have a restricted pattern of expression, inversely related to the sum of the levels of expression of alpha6beta1 and alpha3beta1. This mAb was unable to co-precipitate integrins in digitonin, suggesting that its epitope is blocked by the association with integrins. Indeed, the binding of TS151r to the cell surface was quantitatively diminished following alpha3beta1 overexpression. Altogether, these data suggest that, among tetraspans, CD81 interacts directly with the integrin alpha4beta1, and CD151 interacts directly with integrins alpha3beta1 and alpha6beta1. Because all tetraspan-tetraspan associations are disrupted by digitonin, it is likely that the other tetraspans interact indirectly with integrins, through interactions with CD81 or CD151.

Analysis of cultured keratinocytes from a transgenic mouse model of psoriasis: effects of suprabasal integrin expression on keratinocyte adhesion, proliferation and terminal differentiation

Many important transgenic mouse models of benign and neoplastic skin diseases have been generated through the use of promoters that target transgene expression to the different epidermal layers. However, more mechanistic studies of the specific effects of the transgenes on keratinocytes have been hampered by difficulties in culturing keratinocytes from adult mouse epidermis and by the low differentiation potential of many established mouse keratinocyte lines. We have used the Rheinwald & Green technique to cultivate primary adult keratinocytes and to generate keratinocyte lines from transgenic mice which have a sporadic psoriatic phenotype due to expression of human integrin subunits under the control of the involucrin promoter. We show that the transgenes are induced when keratinocytes are placed in suspension and that the transgenic integrins are capable of clustering in focal adhesions and mediating cell adhesion and spreading. We also show that suprabasal integrin expression has no direct effect on proliferation of cells in the underlying basal layer, ruling this out as a possible explanation for the epidermal hyperproliferation observed in the transgenic mice.

Monoclonal antibody ONS-M21 recognizes integrin alpha3 in gliomas and medulloblastomas

The monoclonal antibody ONS-M21 recognizes an antigen found on the surface of glioma and medulloblastoma cells but does not react with the antigens of normal brain tissue. We purified and identified the 140-kDa protein by means of an antibody-binding affinity column. This 140-kDa antigen has sequences homologous to the amino-terminal region and five parts of the internal domain of integrin alpha3. When the integrin alpha3-related sequences was amplified and used to analyse the mRNA of glioma and medulloblastoma surgical specimens, the transcription level of integrin alpha3 mRNA appeared to be quantitatively correlated with the grade of malignancy. These findings suggest that the ONS-M21 antibody, which reacts with integrin alpha3, might be useful in the diagnosis of gliomas and medulloblastomas.

Molecular cloning and developmental expression of the Xenopus homolog of integrin alpha 4

Integrin receptors containing an alpha 4 subunit mediate cell-cell adhesion by binding to VCAM and MadCAM-1 in addition to supporting cell-extracellular matrix (ECM) adhesion by binding to the alternatively spliced V-region of fibronectin (FN). Studies in chick and mouse embryos have implicated these integrins in neural crest migration, myotube formation, heart development, and placentation. Because integrin-FN adhesive interactions have been shown to play essential roles in mammalian development, studies were initiated of integrin alpha 4 in amphibian embryos, which are better suited to experimental analyses of the earliest stages of embryogenesis. Here, the cDNA cloning and pattern of expression of the Xenopus laevis homolog of integrin alpha 4 is reported. Xenopus alpha 4 is 55% identical at the amino-acid level to both its human and mouse counterparts, including conservation of an alpha 4-specific protease cleavage site, 11 potential N-linked glycosylation sites, and 24 cysteine residues. In situ hybridization analysis reveals that transcripts encoding alpha 4 are expressed in epidermis and the branchial arches. Although alpha 4 transcripts can be detected as early as gastrulation, the protein is observed only after tailbud stages of development and is spatially restricted to the epidermis and gills of tadpole stage embryos. From these data it is concluded that Xenopus integrin alpha 4 has structural features in common with other vertebrate alpha 4 homologs, but is detected in a more restricted tissue distribution during development than alpha 4 in other species.

Cre-loxP-mediated inactivation of the alpha6A integrin splice variant in vivo: evidence for a specific functional role of alpha6A in lymphocyte migration but not in heart development

Two splice variants of the alpha6 integrin subunit, alpha6A and alpha6B, with different cytoplasmic domains, have previously been described. While alpha6B is expressed throughout the development of the mouse, the expression of alpha6A begins at 8.5 days post coitum and is initially restricted to the myocardium. Later in ontogeny, alpha6A is found in various epithelia and in certain cells of the immune system. In this study, we have investigated the function of alpha6A in vivo by generating knockout mice deficient for this splice variant. The Cre- loxP system of the bacteriophage P1 was used to specifically remove the exon encoding the cytoplasmic domain of alpha6A in embryonic stem cells, and the deletion resulted in the expression of alpha6B in all tissues that normally express alpha6A. We show that alpha6A-/- mice develop normally and are fertile. The substitution of alpha6A by alpha6B does not impair the development and function of the heart, hemidesmosome formation in the epidermis, or keratinocyte migration. Furthermore, T cells differentiated normally in alpha6A-/- mice. However, the substitution of alpha6A by alpha6B leads to a decrease in the migration of lymphocytes through laminin-coated Transwell filters and to a reduction of the number of T cells isolated from the peripheral and mesenteric lymph nodes. Lymphocyte homing to the lymph nodes, which involves various types of integrin-ligand interactions, was not affected in the alpha6A knockout mice, indicating that the reduced number of lymph node cells could not be directly attributed to defects in lymphocyte trafficking. Nevertheless, the expression of alpha6A might be necessary for optimal lymphocyte migration on laminin in certain pathological conditions.

Migration of breast epithelial cells on Laminin-5: differential role of integrins in normal and transformed cell types

We examined the role of Laminin-5 (Ln-5) an extracellular matrix component of breast gland basement membrane, in supporting migration of normal (HUMEC), immortalized (MCF-10A), and malignant breast epithelial cells that exhibit different degrees of metastatic potential (MDA-MB-435>MDA-MB-231>MCF-7). HUMEC, MCF-10A, and MCF-7 cells all adhered to purified Ln-5 through the alpha3beta1 integrin receptor in adhesion assays. However, HUMEC and MCF-10A cells remained statically adherent, while MCF-7 cells migrated on Ln-5 in Transwell and colloidal gold displacement assays. Anti-alpha3 integrin antibodies blocked migration of MCF-7 cells on Ln-5. MDA-MB-231 and MDA-MB-435 cells bound and migrated on Ln-5 through a beta1 integrin receptor that is insensitive to antibodies that block the function of alpha1, alpha2, alpha3, alpha4, alpha5, alpha6, and alphaV integrin subunits. Migration of all cell types tested was blocked by CM6, a monoclonal antibody directed to a cell adhesion site on the alpha3 chain of Ln-5. Thus, Ln-5 may play an important role in regulating adhesion and migration in normal and transformed breast epithelium. Our results indicate that the type of integrin utilized by breast cells to interact with Ln-5, as well as its functional state, may determine whether cells will be statically adherent or migratory on Ln-5.

Isolation, cloning, and sequence analysis of the integrin subunit alpha10, a beta1-associated collagen binding integrin expressed on chondrocytes

We have found that chondrocytes express a novel collagen type II-binding integrin, a new member of the beta1-integrin family. The integrin alpha subunit, which has a Mr of 160 kDa reduced, was isolated from bovine chondrocytes by collagen type II affinity purification. The human homologue was obtained by screening a human chondrocyte library with a bovine cDNA probe. Cloning and cDNA sequence analysis of the human integrin alpha subunit designated alpha10 show that it shares the general structure of other integrin alpha subunits. The predicted amino acid sequence consists of a 1167-amino acid mature protein, including a signal peptide (22 amino acids), a long extracellular domain (1098 amino acids), a transmembrane domain (25 amino acids), and a short cytoplasmic domain (22 amino acids). The extracellular part contains a 7-fold repeated sequence, an I-domain (199 amino acids) and three putative divalent cation-binding sites. The deduced amino acid sequence of alpha10 is 35% identical to the integrin subunit alpha2 and 37% identical to the integrin subunit alpha1. Northern blot analysis shows a single mRNA of 5.4 kilobases in chondrocytes. A peptide antibody against the predicted sequence of the cytoplasmic domain of alpha10 immunoprecipitated two proteins with masses of 125 and 160 kDa from chondrocyte lysates under reducing conditions. The peptide antibody specifically stained chondrocytes in tissue sections of human articular cartilage, showing that alpha10 beta1 is expressed in cartilage tissue.

Recombinant soluble human alpha 3 beta 1 integrin: purification, processing, regulation, and specific binding to laminin-5 and invasin in a mutually exclusive manner

Using insect cells, we expressed large quantities of soluble human integrin alpha 3 beta 1 ectodomain heterodimers, in which cytoplasmic and transmembrane domains were replaced by Fos and Jun dimerization motifs. In direct ligand binding assays, soluble alpha 3 beta 1 specifically bound to laminin-5 and laminin-10, but not to laminin-1, laminin-2, fibronectin, various collagens, nidogen, thrombospondin, or complement factors C3 and C3b. Soluble alpha 3 beta1 integrin also bound to invasin, a bacterial surface protein, that mediates entry of Yersinia species into the eukaryotic host cell. Invasin completely displaced laminin-5 from the alpha 3 beta 1 integrin, suggesting sterically overlapping or identical binding sites. In the presence of 2 mM Mg2+, alpha 3 beta 1's binding affinity for invasin (Kd = 3.1 nM) was substantially greater than its affinity for laminin-5 (Kd > 600 nM). Upon addition of 1 mM Mn2+, or activating antibody 9EG7, binding affinity for both laminin-5 and invasin increased by about 10-fold, whereas the affinity decreased upon addition of 2 mM Ca2+. Thus, functional regulation of the purified soluble integrin alpha 3 beta 1 ectodomain heterodimer resembles that of wild-type membrane-anchored beta 1 integrins. The integrin alpha 3 subunit was entirely cleaved into disulfide-linked heavy and light chains, at a newly defined cleavage site located C-terminal of a tetrabasic RRRR motif. Within the alpha 3 light chain, all potential N-glycosylation sites bear N-linked mannose-rich carbohydrate chains, suggesting an important structural role of these sugar residues in the stalk-like region of the integrin heterodimer. In conclusion, studies of our recombinant alpha 3 beta 1 integrin have provided new insights into alpha 3 beta1 structure, ligand binding function, specificity, and regulation.

Genomic organization of the human alpha 3 integrin subunit gene

The alpha 3 beta 1 integrin is a receptor for various laminin isoforms and plays an important role in the maintenance of tissue integrity. We have characterized the genomic structure of the complete gene for the human alpha 3 integrin subunit. The gene contains 26 exons spanning a region of 36.3 kb of genomic DNA. Its structure closely resembles that of alpha 6, another of the three integrin alpha-subunits that are part of laminin receptors, except that it lacks the corresponding exon 5A, which encodes an X1 region in the extracellular domain of alpha 6. However, the alpha 3 gene contains the equivalent of an exon 5B for an X2-like region, which in alpha 6 and alpha 7 is present only in certain tissue-specific alternative transcripts. The two A and B cytoplasmic variants of alpha 3, which are common to the three laminin receptor integrin alpha-subunits, are encoded by separate exons.

The role of laminins in basement membrane function

Laminins are a family of multifunctional macromolecules, ubiquitous in basement membranes, and represent the most abundant structural noncollagenous glycoproteins of these highly specialised extracellular matrices. Their discovery started with the difficult task of isolating molecules produced by cultivated cells or extracted from tissues. The development of molecular biology techniques has facilitated and accelerated the identification and the characterisation of new laminin variants making it feasible to identify full-length polypeptides which have not been purified. Further, genetically engineered laminin fragments can be generated for studies of their structure-function relationship, permitting the demonstration that laminins are involved in multiple interactions with themselves, with other components of the basal lamina, and with cells. It endows laminins with a central role in the formation, the architecture, and the stability of basement membranes. In addition, laminins may both separate and connect different tissues, i.e. the parenchymal and the interstitial connective tissues. Laminins also provide adjacent cells with a mechanical scaffold and biological information either directly by interacting with cell surface components, or indirectly by trapping growth factors. In doing so they trigger and control cellular functions. Recently, the structural and biological diversity of the laminins has started to be elucidated by gene targeting and by the identification of laminin defects in acquired or inherited human diseases. The consequent phenotypes highlight the pivotal role of laminins in determining heterogeneity in basement membrane functions.

Bacterial lipopolysaccharide contamination of commercial collagen preparations may mediate dendritic cell maturation in culture

Dendritic cells (DC) are potent antigen presenting cells, which are responsible for the initiation of naive T and T-dependent immune responses. The present studies were based upon recent reports that commercial collagen I preparations induce the maturation of human DC in vitro. We show that human blood monocyte-derived (GM-CSF and IL-4 cultured) DC pulsed on collagen I-coated plates undergo a dose-dependent increase in stimulatory capacity in oxidative mitogenesis assays. This is accompanied by the upregulation of costimulatory molecules (CD40, CD80, CD86), CD25, ICAM-1 and the DC-specific marker CD83. The maturation effect is more potent than TNF-alpha, which is a known mediator of DC function. However, bacterial lipopolysaccharide (LPS), a powerful inducer of DC maturation, was found to be present at very high levels in one commercial collagen solution that was tested. The effect of LPS upon DC maturation was similar to culture with collagen. Furthermore, a different collagen I preparation with low levels of LPS contamination was less effective at inducing DC maturation, while spiking the collagen solution with LPS prior to plastic coating equalised these effects. Finally, human monocyte-derived DC were found not to express typical collagen receptors VLA-1, 2 and 3. We therefore propose that LPS contamination may at least partially explain reported collagen I induced DC maturation.

Negative cooperativity between alpha 3 beta 1 and alpha 2 beta 1 integrins in human mammary carcinoma MDA MB 231 cells

The alpha 3 beta 1 integrin has been implicated as a receptor for several matrix components, including collagen, fibronectin, and laminins. The function of alpha 3 beta 1 seems to be very versatile involving cell adhesion to or migration on ECM, establishment of cell-cell contacts in aggregates, as well as linkage to intracellular tyrosine phosphorylation cascades. Here we report a strong induction of attachment of alpha 3 beta 1 integrin expressing human breast carcinoma cell line MDA MB 231 to matrix proteins by two alpha 3 integrin subunit function-blocking monoclonal antibodies (P1B5 and ASC-1). In contrast, stimulation of adhesion to ECM by inhibitory alpha 3 integrin-specific antibodies was not observed in the alpha 3 beta 1 integrin-expressing nonmalignant human mammary epithelial cell line MCF-10A or the human breast carcinoma cell line MDA MB 468 that expressed relatively low amounts of alpha 3 beta 1 integrin at the cell surface. This increase was specific for collagens and not observed on fibronectin or laminin. Physiological concentrations of bivalent cations were not required. MAb P1B5 did not induce homotypic aggregation of MDA MB 231 cells. The P1B5-induced increase in cell attachment to collagens could be prevented but not reduced below control levels by blocking mAb to the alpha 2 integrin subunit. Function blocking anti-alpha 5 integrin subunit mAb was without effect while anti-beta 1-mAb completely abolished adhesion. Our data indicate that negative cooperativity between integrins results in transdominant inhibition of alpha 2 beta 1 function by alpha 3 beta 1 in human MDA MB 231 but not MDA MB 468 tumor cells or nonmalignant MCF-10A cells.

Divalent cations and ligands induce conformational changes that are highly divergent among beta1 integrins

Here we show striking differences in conformational regulation among beta1 integrins. Upon manganese stimulation, a beta1 epitope defined by monoclonal antibody (mAb) 9EG7 was induced strongly (on alpha4beta1), moderately (on alpha5beta1), weakly (on alpha2beta1), or was scarcely detectable (on alpha6beta1 and alpha3beta1). Comparable results were seen for the beta1 epitope defined by mAb 15/7. Likewise, soluble ligands caused strong (alpha4beta1), moderate (alpha5beta1), weak (alpha2beta1, alpha6beta1), or minimal (alpha3beta1) induction of the 9EG7 epitope. Exchange or deletion of alpha chain cytoplasmic tails did not alter Mn2+-induced 9EG7 epitope levels. Upon removal of calcium by EGTA or EDTA, the hierarchy of 9EG7 epitope induction was similar (alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), except that EGTA reduced rather than induced 9EG7 expression on alpha4beta1. Thus in contrast to other beta1 integrins, calcium uniquely supports constitutive expression of the 9EG7 epitope on alpha4beta1. Likewise, calcium supported vascular cell adhesion molecule-stimulated 9EG7 appearance on alpha4beta1, whereas calcium inhibited ligand-induced 9EG7 epitope on other integrins. Constitutive expression of 9EG7 on alpha4beta1 was eliminated by a D698E mutation in alpha4, suggesting that Asp-698 may play a key role in maintaining atypical alpha4beta1 response to calcium. In conclusion, our results (i) demonstrate that mAb such as 9EG7 and 15/7 have limited diagnostic utility as reporters of ligand or Mn2+ occupancy for beta1 integrins, (ii) indicate pronounced differences in conformational flexibilities (alpha4beta1 > alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), (iii) allow us to hypothesize that beta1 integrins may differ markedly in conformation-dependent inside-out signaling, and (iv) have uncovered an atypical alpha4beta1 response to calcium that requires alpha4 Asp-698.