Genomic organization of the human integrin beta4 gene

Molecular architecture and function of the hemidesmosome

Hemidesmosomes are multiprotein complexes that facilitate the stable adhesion of basal epithelial cells to the underlying basement membrane. The mechanical stability of hemidesmosomes relies on multiple interactions of a few protein components that form a membrane-embedded tightly-ordered complex. The core of this complex is provided by integrin α6β4 and P1a, an isoform of the cytoskeletal linker protein plectin that is specifically associated with hemidesmosomes. Integrin α6β4 binds to the extracellular matrix protein laminin-332, whereas P1a forms a bridge to the cytoplasmic keratin intermediate filament network. Other important components are BPAG1e, the epithelial isoform of bullous pemphigoid antigen 1, BPAG2, a collagen-type transmembrane protein and CD151. Inherited or acquired diseases in which essential components of the hemidesmosome are missing or structurally altered result in tissue fragility and blistering. Modulation of hemidesmosome function is of crucial importance for a variety of biological processes, such as terminal differentiation of basal keratinocytes and keratinocyte migration during wound healing and carcinoma invasion. Here, we review the molecular characteristics of the proteins that make up the hemidesmosome core structure and summarize the current knowledge about how their assembly and turnover are regulated by transcriptional and post-translational mechanisms.

Molecular architecture and function of the hemidesmosome

Hemidesmosomes are multiprotein complexes that facilitate the stable adhesion of basal epithelial cells to the underlying basement membrane. The mechanical stability of hemidesmosomes relies on multiple interactions of a few protein components that form a membrane-embedded tightly-ordered complex. The core of this complex is provided by integrin α6β4 and P1a, an isoform of the cytoskeletal linker protein plectin that is specifically associated with hemidesmosomes. Integrin α6β4 binds to the extracellular matrix protein laminin-332, whereas P1a forms a bridge to the cytoplasmic keratin intermediate filament network. Other important components are BPAG1e, the epithelial isoform of bullous pemphigoid antigen 1, BPAG2, a collagen-type transmembrane protein and CD151. Inherited or acquired diseases in which essential components of the hemidesmosome are missing or structurally altered result in tissue fragility and blistering. Modulation of hemidesmosome function is of crucial importance for a variety of biological processes, such as terminal differentiation of basal keratinocytes and keratinocyte migration during wound healing and carcinoma invasion. Here, we review the molecular characteristics of the proteins that make up the hemidesmosome core structure and summarize the current knowledge about how their assembly and turnover are regulated by transcriptional and post-translational mechanisms.

Analysis on the relevance of asthma susceptibility with the alteration of integrin β 4 expression

Accumulated research has suggested the importance of the adhesion molecules modulation as therapeutic approach for bronchial asthma. Adhesion molecules expression alteration contributes to the pathogenesis of asthma. In order to probe the roles of expression imbalance of adhesion molecules in asthma pathogenesis, expression profiling of adhesion molecules was performed using cDNA microarray assay. The results showed that the expression pattern of adhesion molecules was altered in peripheral blood leucocytes of asthma patients. In this study, we focused on one of the abnormally expressed molecule, integrin β4, which was down-regulated in all asthma patients, to analyze the relevance of asthma susceptibility with the alteration of integrin β4 expressions. Real time PCR was used to verify the down-regulation of integrin β4 in additional 38 asthma patients. Next, the 5'flanking region of integrin β4 DNA were amplified, sequenced and site-directed mutagenesis technology in correspondent variation sites were carried out. Among 4 variation sites found in 5' flanking region of integrin β4, 3 were related to asthma susceptibility: -nt1029 G/A, -nt 1051 G/A, and -nt 1164 G/C. A reduction of human integrin β4 promoter activity was observed at mutants of these sites. This study demonstrates that various adhesion molecules in asthma patients are abnormally expressed. Mutations in 5' flanking region result in reduced integrin β4 expression, which is related to increased risk of asthma.

Comparative studies of vertebrate Beta integrin genes and proteins: ancient genes in vertebrate evolution

Intregins are heterodimeric α- and β-subunit containing membrane receptor proteins which serve various cell adhesion roles in tissue repair, hemostasis, immune response, embryogenesis and metastasis. At least 18 α- (ITA or ITGA) and 8 β-integrin subunits (ITB or ITGB) are encoded on mammalian genomes. Comparative ITB amino acid sequences and protein structures and ITB gene locations were examined using data from several vertebrate genome projects. Vertebrate ITB genes usually contained 13-16 coding exons and encoded protein subunits with ~800 amino acids, whereas vertebrate ITB4 genes contained 36-39 coding exons and encoded larger proteins with ~1800 amino acids. The ITB sequences exhibited several conserved domains including signal peptide, extracellular β-integrin, β-tail domain and integrin β-cytoplasmic domains. Sequence alignments of the integrin β-cytoplasmic domains revealed highly conserved regions possibly for performing essential functions and its maintenance during vertebrate evolution. With the exception of the human ITB8 sequence, the other ITB sequences shared a predicted 19 residue α-helix for this region. Potential sites for regulating human ITB gene expression were identified which included CpG islands, transcription factor binding sites and microRNA binding sites within the 3'-UTR of human ITB genes. Phylogenetic analyses examined the relationships of vertebrate beta-integrin genes which were consistent with four major groups: 1: ITB1, ITB2, ITB7; 2: ITB3, ITB5, ITB6; 3: ITB4; and 4: ITB8 and a common evolutionary origin from an ancestral gene, prior to the appearance of fish during vertebrate evolution. The phylogenetic analyses revealed that ITB4 is the most likely primordial form of the vertebrate β integrin subunit encoding genes, that is the only β subunit expressed as a constituent of the sole integrin receptor 'α6β4' in the hemidesmosomes of unicellular organisms.

Beta4 integrin subunit gene expression correlates with tumor size and nuclear grade in early breast cancer

In vitro data support a role for the alpha6beta4 integrin in tumor cell migration and invasion, particularly in breast carcinoma cells, but clinical data on this potentially important integrin are limited. The beta4 integrin subunit has been shown to cluster with genes characteristic of basal/myoepithelial cells in cDNA microarray analyses of breast cancer, and the subset of breast cancers with increased expression of genes characteristic of basal/myoepithelial cells appears to be particularly aggressive. The purpose of this study was to determine whether alpha6beta4 integrin expression correlates with aggressive clinicopathologic features of breast cancer and whether expression of this integrin has prognostic significance in early breast cancer. We evaluated tumor expression of the beta4 integrin subunit gene in a cohort of patients with early invasive breast carcinoma by in situ hybridization and correlated expression levels with multiple clinicopathologic characteristics. We also evaluated expression of laminin-5 protein, the principal ligand of alpha6beta4, in this patient cohort. Although we observed a slight trend towards decreased disease-free survival for patients whose tumors had high beta4 gene expression and coexpression of laminin-5, this did not reach statistical significance (P=0.11). However, we did observe a correlation between beta4 mRNA expression and both tumor size (P=0.01) and tumor nuclear grade (P<0.01). These results do not demonstrate prognostic significance for beta4 gene expression and/or laminin-5 protein expression in early breast cancer, but increased beta4 gene expression in larger tumors and in higher grade tumors does support a potential role for the alpha6beta4 integrin in tumor progression.

Novel and recurrent mutations in the integrin beta 4 subunit gene causing lethal junctional epidermolysis bullosa with pyloric atresia

In this study we examined two unrelated patients affected with the lethal variant of junctional epidermolysis bullosa with pyloric atresia (PA-JEB) who were found to carry mutations in the integrin beta4 subunit gene (ITGB4). Although in both patients Northern blot analysis showed only a 50% reduction in the level of ITGB4 transcript, a complete lack (patient 1) or a strong reduction (patient 2) of beta4 immunoreactivity was observed in the skin. Using immunoprecipitation analysis, integrin beta4 could not be visualized in patient 1 cells while a markedly reduced amount (approximately 20%) of normal sized beta4 chains was detected in patient 2. These data suggested the presence of ITGB4 mutations that interfere with both mRNA and protein stability. Using molecular analysis, patient 1 was shown to be a compound heterozygous for a single amino acid deletion (deltaN318) and a not yet identified mutation that induces a very rapid decay of the encoded mRNA transcript. Patient 2 was, instead, a compound heterozygous for a novel 4-bp tandem duplication (4298-4299ins4) and a previously described missense mutation (R252C). Our data support the notion that PA-JEB lethal phenotypes associated with a markedly decreased/absent alpha6beta4 expression can be due not only to the presence of null alleles, but also to specific mutations leading to protein instability and/or altered function.

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

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

Cloning and characterization of the human beta4-integrin gene promoter and enhancers

The cell-surface adhesion molecule alpha6beta4-integrin is a receptor for laminins and a component of hemidesmosomes. beta4-Integrin expression is restricted to proliferating basal keratinocytes in the epidermis and is suppressed when differentiation commences. Altered beta4-integrin expression levels correlate significantly with the aggressive behavior of cancers. In order to clarify the mechanisms that regulate transcription of the beta4-integrin gene, we cloned its 5'-flanking region. This 5'-flanking region was found to have a high G + C content and not to contain either TATA or CAAT boxes. Nested delimitation and reporter analyses mapped a basal promoter to nucleotides -106 to +105, surrounding the most proximal transcription initiation site. Gel retardation and mutational analyses revealed that cooperation between AP1 and Ets, interacting with other factors, mediated the promoter activity. In addition to the promoter element, enhancer activity was found in the first intron (+1905/+3933) and in a sequence upstream of the promoter region (-414/-107). These findings should facilitate our understanding of the regulation of beta4-integrin gene expression in processes such as cell growth and differentiation, apoptosis, and cancer development and metastasis.