Polarized expression of HD1: relationship with the cytoskeleton in cultured human colonic carcinoma cells

Case reports of immune-related cystitis and the antibody combination hypothesis

Immune-related cystitis is a rare condition, and its diagnostic criteria and pathogenesis are not yet fully understood. Here, we report two cases of immune-related cystitis. Both patients were previously diagnosed with lung squamous cell carcinoma and received combined treatment with immune checkpoint inhibitors and chemotherapy, leading to hemorrhagic cystitis. We reviewed the cystoscopic images and pathological features of previous cases and found that autoantibodies against hemidesmosomes may be the cause of immune-related cystitis, proposing the "antibody combination" hypothesis to explain the tissue specificity of the condition.

The Tongue Squamous Carcinoma Cell Line Cal27 Primarily Employs Integrin α6β4-Containing Type II Hemidesmosomes for Adhesion Which Contribute to Anticancer Drug Sensitivity

Integrins are heterodimeric cell surface glycoproteins used by cells to bind to the extracellular matrix (ECM) and regulate tumor cell proliferation, migration and survival. A causative relationship between integrin expression and resistance to anticancer drugs has been demonstrated in different tumors, including head and neck squamous cell carcinoma. Using a Cal27 tongue squamous cell carcinoma model, we have previously demonstrated that de novo expression of integrin αVβ3 confers resistance to several anticancer drugs (cisplatin, mitomycin C and doxorubicin) through a mechanism involving downregulation of active Src, increased cell migration and invasion. In the integrin αVβ3 expressing Cal27-derived cell clone 2B1, αVβ5 expression was also increased, but unrelated to drug resistance. To identify the integrin adhesion complex (IAC) components that contribute to the changes in Cal27 and 2B1 cell adhesion and anticancer drug resistance, we isolated IACs from both cell lines. Mass spectrometry (MS)-based proteomics analysis indicated that both cell lines preferentially, but not exclusively, use integrin α6β4, which is classically found in hemidesmosomes. The anticancer drug resistant cell clone 2B1 demonstrated an increased level of α6β4 accompanied with increased deposition of a laminin-332-containing ECM. Immunofluorescence and electron microscopy demonstrated the formation of type II hemidesmosomes by both cell types. Furthermore, suppression of α6β4 expression in both lines conferred resistance to anticancer drugs through a mechanism independent of αVβ3, which implies that the cell clone 2B1 would have been even more resistant had the upregulation of α6β4 not occurred. Taken together, our results identify a key role for α6β4-containing type II hemidesmosomes in regulating anticancer drug sensitivity.

Comparative interactomics analysis reveals potential regulators of α6β4 distribution in keratinocytes

The integrin α6β4 and cytoskeletal adaptor plectin are essential components of type I and type II hemidesmosomes (HDs). We recently identified an alternative type II HD adhesion complex that also contains CD151 and the integrin α3β1. Here, we have taken a BioID proximity labeling approach to define the proximity protein environment for α6β4 in keratinocytes. We identified 37 proteins that interacted with both α6 and β4, while 20 and 78 proteins specifically interacted with the α6 and β4 subunits, respectively. Many of the proximity interactors of α6β4 are components of focal adhesions (FAs) and the cortical microtubule stabilizing complex (CMSC). Though the close association of CMSCs with α6β4 in HDs was confirmed by immunofluorescence analysis, CMSCs have no role in the assembly of HDs. Analysis of the β4 interactome in the presence or absence of CD151 revealed that they are strikingly similar; only 11 different interactors were identified. One of these was the integrin α3β1, which interacted with α6β4 more strongly in the presence of CD151 than in its absence. These findings indicate that CD151 does not significantly contribute to the interactome of α6β4, but suggest a role of CD151 in linking α3β1 and α6β4 together in tetraspanin adhesion structures.

Summary: Comparative interactomics analysis reveals close proximity of HDs, FAs and CMSCs, and a role of CD151 in linking α3β1 and α6β4 together in an alternative type II HD-like adhesion complex.

Tetraspanin CD151 and integrin α3β1 contribute to the stabilization of integrin α6β4-containing cell-matrix adhesions

Tetraspanin CD151 has been suggested to regulate cell adhesion through its association with laminin-binding integrins α3β1 and α6β4; however, its precise function in keratinocyte adhesion remains elusive. In this study, we investigated the role of CD151 in the formation and maintenance of laminin-associated adhesions. We show that CD151, through binding to integrin α3β1, plays a critical role in the stabilization of an adhesion structure with a distinct molecular composition of hemidesmosomes with tetraspanin features. These hybrid cell-matrix adhesions, which are formed early during cell adhesion and spreading and at later stages of cell spreading, are present in the central region of the cells. They contain the CD151-α3β1/α6β4 integrin complexes and the cytoskeletal linker protein plectin, but are not anchored to the keratin filaments. In contrast, hemidesmosomes, keratin filament-associated adhesions that contain integrin α6β4, plectin, BP180 (encoded by COL17A1) and BP230 (encoded by DST), do not require CD151 for their formation or maintenance. These findings provide new insights into the dynamic and complex regulation of adhesion structures in keratinocytes and the pathogenic mechanisms underlying skin blistering diseases caused by mutations in the gene for CD151.

Laminin-332-integrin interaction: a target for cancer therapy?

For many years, extracellular matrix (ECM) was considered to function as a tissue support and filler. However, we now know that ECM proteins control many cellular events through their interaction with cell-surface receptors and cytoplasmic signaling pathways. For example, they regulate cell proliferation, cell division, cell adhesion, cell migration, and apoptosis. We focus in this review on a laminin isoform, laminin-332 (formerly termed laminin-5), a major component of the basement membrane (BM) of skin and other epithelial tissues. It is composed of 3 subunits (alpha3beta3 and gamma3 and interacts with at least two integrin receptors expressed by epithelial cells (alpha3beta1 and alpha6beta4 integrin. Mutations in either laminin-332 or integrin alpha6beta4 result in junctional epidermolysis bullosa, a blistering skin disease, while targeting of laminin-332 by autoantibodies in cicatricial pemphigoid leads to dysadhesion of epithelial cells from their underlying connective tissue. Abnormal expression of laminin-332 and its integrin receptors is also a hallmark of certain tumor types and is believed to promote invasion of colon, breast and skin cancer cells. Moreover, there is emerging evidence that laminin-332 and its protease degradation products are not only found at the leading front of several tumors but also likely induce and/or promote tumor cell migration. Thus, in this review, we focus specifically on the role of laminin-332 and its integrin receptors in adhesion, proliferation, and migration/invasion of cancer cells. Finally, we discuss strategies for the development of laminin-332-based antagonists for the treatment of malignant tumors.

Intermediate filaments: a role in epithelial polarity

Intermediate filaments have long been considered mechanical components of the cell that provide resistance to deformation stress. Practical experimental problems, including insolubility, lack of good pharmacological antagonists, and the paucity of powerful genetic models have handicapped the research of other functions. In single-layered epithelial cells, keratin intermediate filaments are cortical, either apically polarized or apico-lateral. This review analyzes phenotypes of genetic manipulations of simple epithelial cell keratins in mice and Caenorhabditis elegans that strongly suggest a role of keratins in apico-basal polarization and membrane traffic. Published evidence that intermediate filaments can act as scaffolds for proteins involved in membrane traffic and signaling is also discussed. Such a scaffolding function would generate a highly polarized compartment within the cytoplasm of simple epithelial cells. While in most cases mechanistic explanations for the keratin-null or overexpression phenotypes are still missing, it is hoped that investigators will be encouraged to study these as yet poorly understood functions of intermediate filaments.

Comparative analysis of the expression of ERBIN and Erb-B2 in normal human skin and cutaneous carcinomas

BACKGROUND

ERBIN is a binding partner of Erb-B2, an orphan receptor within the Erb-B family critically involved in the regulation of cell growth and differentiation. Although its function remains unclear, ERBIN is thought to affect the polarity of epithelial cells and cell growth via the Ras signalling pathway.

OBJECTIVES

To examine and compare the tissue distribution and the expression levels of ERBIN and Erb-B2 in normal skin and in cutaneous carcinomas.

METHODS

Fifteen cases of basal cell carcinoma (BCC), 12 cases of squamous cell carcinoma (SCC) and five cases of keratoacanthoma (KA) were analysed by immunohistochemistry on paraffin-embedded sections using anti-ERBIN and anti-Erb-B2 antibodies.

RESULTS

ERBIN and Erb-B2 had a similar distribution in normal human skin. They were primarily localized at the plasma membrane in differentiated keratinocytes and in duct cells from eccrine glands, whereas they were localized diffusely in the cytoplasma of basal keratinocytes. In both SCC and KA the subcellular distribution of ERBIN and Erb-B2 remained unchanged, whereas both proteins were redistributed from the plasma membrane into cytosolic aggregates in BCC.

CONCLUSIONS

The subcellular localization of ERBIN in normal human skin is similar to that of Erb-B2 and varies with cell differentiation. Based on our findings and on the biological activities of Erb-B2, it is conceivable that disturbed expression or functioning of ERBIN and Erb-B2 is implicated in the development of the malignant phenotype of BCC.

Both type-I hemidesmosomes and adherens-type junctions contribute to the cell–substratum adhesion system in myoepithelial cells

Myoepithelial cells present in exocrine glands cause secretion from the glands by contraction. They have mixed characteristics with regard to cytoskeletal elements, containing both epithelial-type intermediate filaments and smooth muscle-type myofilaments. For further characterization, myoepithelial cells from bovine apocrine sweat glands and tracheal glands were here examined with special attention to the cell-substratum adhesion system. Immunofluorescence microscopy using a panel of antibodies against adherens-type junctional and hemidesmosomal proteins demonstrated two types of cell-substratum junctions in myoepithelial cells from both glands. Type-I hemidesmosomes (HDs) consisting of plectin, BP230, integrin alpha6beta4, and BP180 were thus observed as punctate arrays longitudinally arranged along myoepithelial cell surfaces, while adherens-type junctions were similarly evident as linear rib-like structures. Double-label immunofluoresence revealed the two junctions to be distributed in a mutually exclusive or independent manner. Electron microscopy further demonstrated that apocrine myoepithelial cells surround secretory epithelial cells completely, without any gaps, HDs being abundant along the basement membrane, but with no distinct structures in the inter-hemidesmosomal regions. Immunoelectron microscopy, however, revealed an interhemidesmosomal localization of vinculin, pointing to the existence of adherens-type junctions. Secretory epithelial cells in tracheal glands were found not to be completely covered with myoepithelial cells, so that more than half of them are directly attached to the basement membrane, where they form type II-HDs lacking BP230 and BP180, but no detectable adherens junctions, like epidermal basal cells and sebaceous gland cells. These observations demonstrate that, in addition to their cytoskeleton, myoepithelial cells have both epithelial- and smooth muscle-type cell-substratum adhesion structures, i.e. HDs and dense plaque-like adherens junctions.

Protein kinase C-alpha phosphorylation of specific serines in the connecting segment of the beta 4 integrin regulates the dynamics of type II hemidesmosomes

Although the regulation of hemidesmosome dynamics during processes such as epithelial migration, wound healing, and carcinoma invasion is important, the mechanisms involved are poorly understood. The integrin alpha 6 beta 4 is an essential component of the hemidesmosome and a target of such regulation. Epidermal growth factor (EGF) can induce hemidesmosome disassembly by a mechanism that involves serine phosphorylation of the beta 4 integrin subunit. Using a combination of biochemical and mutational analyses, we demonstrate that EGF induces the phosphorylation of three specific serine residues (S(1356), S(1360), and S(1364)) located within the connecting segment of the beta 4 subunit and that phosphorylation on these residues accounts for the bulk of beta 4 phosphorylation stimulated by EGF. Importantly, phosphorylation of these serines is critical for the ability of EGF to disrupt hemidesmosomes. Using COS-7 cells, which assemble hemidesmosomes type II upon exogenous expression of the alpha 6 beta 4 integrin, we observed that expression of a beta 4 construct containing Ser-->Ala mutations of S(1356), S(1360), and S(1364) reduced the ability of EGF to disrupt hemidesmosomes and that this effect appears to involve cooperation among these phosphorylation sites. Moreover, expression of Ser-->Asp mutants that mimic constitutive phosphorylation reduced hemidesmosome formation. Protein kinase C-alpha (PKC-alpha) is the kinase responsible for phosphorylating at least two of these serines, based on in vitro kinase assays, peptide mapping, and mutational analysis. Together, these results highlight the importance of serine phosphorylation in regulating type II hemidesmosome disassembly, implicate a cluster of serine residues within the connecting segment of beta 4, and argue for a key role for PKC-alpha in regulating these structures.

Hemidesmosomes: molecular organization and their importance for cell adhesion and disease

In the skin, basal epithelial cells constantly divide to renew the epidermis. The newly formed epithelial cells then differentiate in a process called keratinization, ultimately leading to the death of these cells and a pile-up of cell material containing vast amounts of keratins. The basal keratinocytes in skin are attached to their underlying basement membrane via specialized adhesion complexes termed hemidesmosomes (HDs). These complexes ascertain stable adhesion of the epidermis to the dermis, and mutations in components of these complexes often result in tissue fragility and blistering of the skin. In this review, we will describe the various hemidesmosomal proteins in detail as well as, briefly, the protein families to which they belong. Specifically, we will report the protein-protein interactions involved in the assembly of hemidesmosomes and their molecular organization. Some signaling pathways involving primarily the alpha6beta4 integrin will be discussed, since they appear to profoundly modulate the assembly and function of hemidesmosomes. Furthermore, the importance of these hemidesmosomal components for the maintenance of tissue homeostasis and their involvement in various clinical disorders will be emphasized. Finally, we will present a model for the assembly of HDs, based on our present knowledge.

The association of the tetraspanin D6.1A with the alpha6beta4 integrin supports cell motility and liver metastasis formation

The metastatic subline of a rat pancreatic adenocarcinoma differs from the non-metastasizing subline by overexpression of 5 membrane molecules: CD44 variant isoforms, EpCAM, the tetraspanin D6.1A, an uPAR-related molecule and, as described here, the alpha6beta4 integrin. An antibody-defined molecule was identified by mass spectrometry and cloning as alpha6beta4 integrin. Transfection-induced expression of alpha6beta4 in the non-metastasizing subline did not support migration on laminin 5 or tumor progression. However, when the non-metastasizing subline was doubly transfected to express alpha6beta4 and the D6.1A tetraspanin, intraperitoneally injected tumor cells frequently formed liver metastasis. For the following reasons we assume that metastasis formation is supported by an interaction between alpha6beta4 and D6.1A. (i) The 2 molecules can associate and co-localize. (ii) Co-localization is strengthened by PKC stimulation. (iii) PKC stimulation, which induces a migratory phenotype, leads to a redistribution of alpha6beta4/D6.1A complexes. In resting cells, the molecules co-localize at the trail of the cell; during PKC stimulation they become transiently internalized and are (re-)expressed in the leading lamella. Thus, in the appropriate milieu, i.e. intraperitoneally, alpha6beta4 changes from an adhesion-supporting towards a migration-supporting molecule by its association with a tetraspanin. The findings provide a convincing experimental explanation for the repeatedly described involvement of alpha6beta4 in tumor progression.

Expression and distribution of distinct variants of E-MAP-115 during proliferation and differentiation of human intestinal epithelial cells

Epithelial cell proliferation and differentiation occur concomitant with striking remodeling of the cytoskeleton. Microtubules (MTs) play important roles in these processes, during which the MTs themselves are reorganized and stabilized by microtubule-associated proteins (MAPs). Among the proteins classified as structural MAPs, E-MAP-115 (also named ensconsin) is preferentially expressed in cells of epithelial origin. The aims of this study were, first, to determine if E-MAP-115, like other MAPs, is expressed as different isoforms during differentiation and, second, to perform a detailed analysis of the expression and distribution of any E-MAP-115 variants detected in intestinal epithelial cells during their polarization/differentiation. It was our expectation that these data would help us to develop hypotheses concerning the role of this MAP in epithelial development. We report the expression of three E-MAP-115 transcripts encoding isoforms of 115, 105, and 95 kDa; two display an expression gradient inverse to the third one as Caco-2 cells progress from proliferation through the stages of differentiation. To monitor the proteins produced from each transcript, we used purified polyclonal antibodies against synthetic peptides contained within the 115, 105, and 95 kDa isoforms to assay proliferating and differentiating CaCo-2 cells. Our results indicate that the expression and MT-binding capacity of the 115, 105, and 95 kDa isoforms vary upon proliferation/differentiation of the cells. E-MAP-115 proteins colocalize with MTs in proliferative and differentiated Caco-2 cells; in vivo, they are expressed in both crypt and villus epithelial cells where they are mainly concentrated at the apical pole of the cells.

Proteomic analysis of nuclear proteins from proliferative and differentiated human colonic intestinal epithelial cells

Self-renewing tissues such as the intestine contain progenitor proliferating cells which subsequently differentiate. Cell proliferation and differentiation involve gene regulation processes which take place in the nucleus. A human intestinal epithelial cell line model (Caco2/TC7) which reproduces these dynamic processes has been used to perform proteomic studies on nuclear proteins. Nuclei from Caco2/TC7 cells at proliferative and differentiated stages were purified by subcellular fractionation. After two-dimensional gel electrophoresis separation and ruthenium staining, 400 protein spots were detected by image analysis. Eighty-five spots corresponding to 60 different proteins were identified by matrix-assisted laser desorption/ionization mass spectrometry in nuclei from proliferative cells. Comparison of nuclear proteomes from proliferative or differentiated cells by differential display resulted in the identification of differentially expressed proteins such as nucleolin, hnRNP A2/B1 and hnRNP A1. By using Western blot analysis, we found that the expression and number of specific isoforms of these nuclear proteins decreased in differentiated cells. Immunocytochemistry experiments also showed that in proliferative cells nucleolin was distributed in nucleoli-like bodies. In contrast, hnRNPs A2/B1 and A1 were dispersed throughout the nucleus. This study of the nuclear proteome from intestinal epithelial cells represents the first step towards the establishment of a protein database which will be a valuable resource in future studies on the differential expression of nuclear proteins in response to physiological, pharmacological and pathological modulations.

Dynamics of the alpha6beta4 integrin in keratinocytes

The integrin alpha6beta4 has been implicated in two apparently contrasting processes, i.e., the formation of stable adhesions, and cell migration and invasion. To study the dynamic properties of alpha6beta4 in live cells two different beta4-chimeras were stably expressed in beta4-deficient PA-JEB keratinocytes. One chimera consisted of full-length beta4 fused to EGFP at its carboxy terminus (beta4-EGFP). In a second chimera the extracellular part of beta4 was replaced by EGFP (EGFP-beta4), thereby rendering it incapable of associating with alpha6 and thus of binding to laminin-5. Both chimeras induce the formation of hemidesmosome-like structures, which contain plectin and often also BP180 and BP230. During cell migration and division, the beta4-EGFP and EGFP-beta4 hemidesmosomes disappear, and a proportion of the beta4-EGFP, but not of the EGFP-beta4 molecules, become part of retraction fibers, which are occasionally ripped from the cell membrane, thereby leaving "footprints" of the migrating cell. PA-JEB cells expressing beta4-EGFP migrate considerably more slowly than those that express EGFP-beta4. Studies with a beta4-EGFP mutant that is unable to interact with plectin and thus with the cytoskeleton (beta4(R1281W)-EGFP) suggest that the stabilization of the interaction between alpha6beta4 and LN-5, rather than the increased adhesion to LN-5, is responsible for the inhibition of migration. Consistent with this, photobleaching and recovery experiments revealed that the interaction of beta4 with plectin renders the bond between alpha6beta4 and laminin-5 more stable, i.e., beta4-EGFP is less dynamic than beta4(R1281W)-EGFP. On the other hand, when alpha6beta4 is bound to laminin-5, the binding dynamics of beta4 to plectin are increased, i.e., beta4-EGFP is more dynamic than EGFP-beta4. We suggest that the stability of the interaction between alpha6beta4 and laminin-5 is influenced by the clustering of alpha6beta4 through the deposition of laminin-5 underneath the cells. This clustering ultimately determines whether alpha6beta4 will inhibit cell migration or not.

Overexpression of laminin alpha1 chain in colonic cancer cells induces an increase in tumor growth

Laminins represent a growing family of glycoproteins constituting the basement membrane. They are known to direct many biological processes. With respect to carcinogenesis, laminins play an important role in cell adhesion, mitogenesis, differentiation and even metastasis. To further study the biological significance of laminin-1 (composed of alpha1, beta1 and gamma1 chains) in intestinal cell differentiation or tumorigenesis, an alpha1-laminin expression vector was introduced into the HT29 colonic cancer cells, in which laminin alpha1 chain is not expressed. Upon transfection of the alpha1 chain, the alpha1beta1gamma1 trimer was found secreted in the media along with free alpha1 chain as assessed by immunoprecipitation. The presence of the laminin alpha1 chain did not significantly modify the levels of the other laminin chains nor the integrins expressed by the HT29 cells. In spite of similar growth properties with the control cells in vitro (plastic dish, soft agar), the laminin alpha1 transfectants showed a significantly increased tumor growth when injected in nude mice. Histologic and immunohistochemic examination of the laminin alpha1-expressing tumors points to an increased recruitment of the host stromal and vascular cells, without modification in the differentiation profile and invasion potential. In parallel, a clear accumulation of laminin-10 (alpha5beta1gamma1) at the carcinoma/stromal interface and a segregation of the integrin beta4 subunit at the basal pole of the cancer cells occurred, compared to control tumors. Overall, our observations emphasize the importance of laminin-1 as a chemoattractant of both stromal and vascular cells and in epithelial/stromal cell interactions for the organization of the basement membrane and segregation of integrins leading to an epithelial cell growth signal. Such a sequence of events is reminiscent of what occurs during development.

Adhesion complexes implicated in intestinal epithelial cell-matrix interactions

This article review summarizes data on cell-substratum adhesion complexes involved in the regulation of cellular functions in the intestine. We first focus on the molecular composition of the two main adhesion structures-the beta1 integrin-adhesion complex and the hemidesmosome-found in vivo and in two human intestinal cell lines. We also report the key findings on the cellular behavior and response to the extracellular matrix that involve integrins, the main transmembrane anchors of these complexes. How the dynamics of cell/extracellular matrix interactions contribute to cell migration, proliferation, differentiation, and tumorigenicity is discussed in the light of the data provided by the human intestinal cells.

The tetraspan molecule CD151, a novel constituent of hemidesmosomes, associates with the integrin alpha6beta4 and may regulate the spatial organization of hemidesmosomes

CD151 is a cell surface protein that belongs to the tetraspan superfamily. It associates with other tetraspan molecules and certain integrins to form large complexes at the cell surface. CD151 is expressed by a variety of epithelia and mesenchymal cells. We demonstrate here that in human skin CD151 is codistributed with alpha3beta1 and alpha6beta4 at the basolateral surface of basal keratinocytes. Immunoelectron microscopy showed that CD151 is concentrated in hemidesmosomes. By immunoprecipitation from transfected K562 cells, we established that CD151 associates with alpha3beta1 and alpha6beta4. In beta4-deficient pyloric atresia associated with junctional epidermolysis bullosa (PA-JEB) keratinocytes, CD151 and alpha3beta1 are clustered together at the basal cell surface in association with patches of laminin-5. Focal adhesions are present at the periphery of these clusters, connected with actin filaments, and they contain both CD151 and alpha3beta1. Transient transfection studies of PA-JEB cells with beta4 revealed that the integrin alpha6beta4 becomes incorporated into the alpha3beta1-CD151 clusters where it induces the formation of hemidesmosomes. As a result, the amount of alpha3beta1 in the clusters diminishes and the protein becomes restricted to the peripheral focal adhesions. Furthermore, CD151 becomes predominantly associated with alpha6beta4 in hemidesmosomes, whereas its codistribution with alpha3beta1 in focal adhesions becomes partial. The localization of alpha6beta4 in the pre-hemidesmosomal clusters is accompanied by a strong upregulation of CD151, which is at least partly due to increased cell surface expression. Using beta4 chimeras containing the extracellular and transmembrane domain of the IL-2 receptor and the cytoplasmic domain of beta4, we found that for recruitment of CD151 into hemidesmosomes, the beta4 subunit must be associated with alpha6, confirming that integrins associate with tetraspans via their alpha subunits. CD151 is the only tetraspan identified in hemidesmosomal structures. Others, such as CD9 and CD81, remain diffusely distributed at the cell surface. In conclusion, we show that CD151 is a major component of (pre)-hemidesmosomal structures and that its recruitment into hemidesmosomes is regulated by the integrin alpha6beta4. We suggest that CD151 plays a role in the formation and stability of hemidesmosomes by providing a framework for the spatial organization of the different hemidesmosomal components.

Formation of hemidesmosome-like structures in the absence of ligand binding by the (alpha)6(beta)4 integrin requires binding of HD1/plectin to the cytoplasmic domain of the (beta)4 integrin subunit

Hemidesmosomes are adhesion structures that mediate anchorage of epithelial cells to the underlying basement membrane. We have previously shown that the (alpha)6(beta)4 integrin can induce the assembly of these multi-protein structures independent of binding to its ligand laminin-5 (ligand-independent formation of hemidesmosomes). Our results suggested a role for HD1/plectin, which binds to the cytoplasmic domain of the (beta)4 integrin subunit, in controlling the clustering of hemidesmosomal components at the basal side of the cell. Using keratinocytes derived from patients lacking HD1/plectin, we now show that ligand-independent formation of hemidesmosomal clusters indeed requires HD1/plectin, in contrast to the ligand-dependent assembly of hemidesmosomes. No clustering of the (alpha)6(beta)4 integrin, or of the bullous pemphigoid antigens BP180 and BP230, was seen when HD1/plectin-deficient keratinocytes were plated on fibronectin or type IV collagen. In (β)4-deficient keratinocytes, expression of an interleukin 2 receptor (IL2R) transmembrane chimera containing the (beta)4 cytoplasmic tail with the mutation R1281W, which abrogates HD1/plectin binding, resulted in a diffuse distribution of the chimeric receptor. In contrast, a (beta)4(R1281W) mutant that can associate with (alpha)6 and bind ligand, was found to be directed to the basal surface of the cells, at sites where laminin-5 was deposited. In addition, this mutant induced clustering of BP180 and BP230 at these sites. Together, these results show that the formation of hemidesmosomes requires binding of either ligand or HD1/plectin to the (beta)4 integrin subunit. Intriguingly, we found that IL2R/(beta)4 chimeras become localized in pre-existing hemidesmosomes of HD1/plectin-deficient keratinocytes, and that this localization requires a domain in the (beta)4 cytoplasmic tail that is also required for HD1/plectin binding (residues 1115–1356). Because this part of (beta)4 lacks the BP180 binding site, and since we show in this study that it is unable to interact with the same part on another (beta)4 molecule, we suggest that the chimera becomes incorporated into hemidesmosomes of HD1/plectin-deficient keratinocytes by interacting with an as yet unidentified hemidesmosomal component.

Intestinal restitution: progression of actin cytoskeleton rearrangements and integrin function in a model of epithelial wound healing

Superficial injury involving the mucosa of the gastrointestinal tract heals by a process termed restitution that involves epithelial sheet movement into the damaged area. The forces that drive epithelial sheet movement are only partially understood, although it is known to involve changes in the morphology of cells bordering the damage, such as the formation of large, flat, cytoplasmic extensions termed lamellae. We investigated the mechanism of epithelial sheet movement by following the response of the actin cytoskeleton and specific integrins (alpha6beta4, alpha6beta1, and alpha3beta1) to wounding. To model this event in vitro, monolayers of T84 cells, well-differentiated colon carcinoma cells, were damaged by aspiration and the ensuing response was analyzed by a combination of time-lapse video microscopy, fluorescence confocal microscopy and antibody inhibition assays. We show that wound healing begins with retraction of the monolayer. alpha6beta4 integrin is localized on the basal surface in structures referred to as type II hemidesmosomes that persist throughout this early stage. We hypothesize that these structures adhere to the substrate and function to retard retraction. Once retraction ceases, the wound is contracted initially by actin purse strings and then lamellae. Purse strings and lamellae produce a pulling force on surrounding cells, inducing them to flatten into the wound. In the case of lamellae, we detected actin suspension cables that appear to transduce this pulling force. As marginal cells produce lamellae, their basal type II hemidesmosomes disappear and the alpha6 integrins appear evenly distributed over lamellae surfaces. Antibodies directed against the alpha6 subunit inhibit lamellae formation, indicating that redistribution of the alpha6 integrins may contribute to the protrusion of these structures. Antibodies directed against the alpha3beta1 integrin also reduce the size and number of lamellae. This integrin's contribution to lamellae extension is most likely related to its localization at the leading edge of emerging protrusions. In summary, wounds in epithelial sheets initially retract, and then are contracted by first an actin purse string and then lamellae, both of which serve to pull the surrounding cells into the denuded area. The alpha6 integrins, particularly alpha6beta4, help contain retraction and both the alpha6 integrins and alpha3beta1 integrin contribute to lamellae formation.

Regulation of the type II hemidesmosomal plaque assembly in intestinal epithelial cells

Hemidesmosomes (HDs) are cellular junctions that anchor epithelial cells to the extracellular matrix (ECM) and are associated morphologically with the cytoskeleton. Hemidesmosomal molecular components include two proteins involved in linking intermediate filaments, HD1/plectin and BP230, and two transmembrane proteins, BP180 and the alpha6beta4 integrin, a laminin receptor. In cells lacking BP230 and BP180, HD1/plectin still associates with alpha6beta4 integrin, forming HD-like structures, called type II HDs. In the present study, we used an intestinal epithelial cell line that expresses HD1/plectin and the alpha6beta4 integrin to investigate the regulation of assembly of these proteins in type II HDs. These compounds were found to be clustered at sites of cell-ECM contact and their polarized localization was influenced by either cell confluency or extracellular matrix deposition. Conventional and immunoelectron microscopy showed that HD1/plectin and the beta4 integrin subunit are colocalized in an adhesion structure. Using cytoskeleton-disrupting drugs and confocal microscopy, we demonstrated that type II HDs are made up of numerous individual plaques whose assembly into a cluster requires actin filaments, but not microtubules.

Biology and function of hemidesmosomes

Hemidesmosomes are cell-substratum adhesion sites that connect the extracellular matrix to the keratin cytoskeleton. Our knowledge of the function of these structures has greatly increased as a result of studies on patients with aberrant expression of hemidesmosome components and studies using targeted inactivation of mouse genes encoding these components. Insight into the formation of hemidesmosomes, as well as into protein-protein interactions that occur in these junctional complexes, has recently been gained by in vitro cell transfections, blot overlay and yeast two-hybrid assays. In addition, recent results indicate that the alpha6 beta4 integrin is involved in the transduction of signals that are induced by the extracellular matrix and which modulate processes as diverse as cell proliferation, differentiation, apoptosis, migration and tissue morphogenesis. Thus it seems that hemidesmosomes do not merely maintain dermo-epidermal adhesion and tissue integrity, but that they are also implicated in intracellular signaling. Here we discuss recently published data on the biology and function of hemidesmosomes.

Intestinal epithelial-mesenchymal cell interactions

Intestinal morphogenesis, as well as maintenance of the stem cell population and of the steady state between cell proliferation and differentiation, results from controlled cell interactions. There is growing evidence that the mesenchymal cells control epithelial cell behavior via their own expression and induction in the epithelial cells of key regulatory genes. This heterologous cross talk involves basement membrane molecules and paracrine factors. New in vitro/in vivo cellular models allowed us to analyze various mesenchymal cell phenotypes and to show that they exhibit different inductive properties on epithelial cells and that their proliferation and metabolic properties are differentially modulated by cytokines. Finally the epithelial-mesenchymal unit is controlled by hormonal and exogenous factors.

Hemidesmosomes and their unique transmembrane protein BP180

Hemidesmosomes are adhesion complexes responsible for linking keratin intermediate filaments of stratified and complex epithelia to components of the extracellular matrix such as collagen fibrils. Over the past several years, it has become clear that there are at least five hemidesmosomal proteins, including HD1/plectin and BP230 as cytoplasmic plaque proteins and integrin alpha6beta4 and BP180 as transmembrane proteins. Among them, BP180 is unique as a transmembrane protein because of its collagenous extracellular domain. Recent biochemical and ultrastructural analyses have revealed its molecular configuration and nature as a major component of anchoring filaments connecting hemidesmosomes to the basement membrane. These results indicate that BP180 is a new type of adhesion receptor. In addition to biochemical analyses of these hemidesmosomal proteins, recent studies on patients with inherited skin blistering diseases and on knockout mice have demonstrated roles in hemidesmosome formation and stabilization, as well as unexpected, novel functions.

Human colonic cancer cells synthesize and adhere to laminin-5. Their adhesion to laminin-5 involves multiple receptors among which is integrin alpha2beta1

In the mature gut, laminin-5 is expressed at the basal aspect of the differentiating epithelial cells. In vitro, we show that three more or less differentiated human colonic cancer HT29 cell lines produce and deposit laminin-5; they predominantly synthesize and secrete the 440 kDa form of laminin-5 that comprises the unprocessed 155 kDa gamma2 chain, as determined by immunoprecipitation analysis. In contrast, the highly differentiated colon carcinoma Caco-2 cells produce almost no laminin-5. Using anti-integrin antibodies, we show that adhesion of the two colonic cancer cell lines to laminin-5 is mediated by multiple integrin receptors including those for alpha3beta1, alpha6beta1 and alpha6beta4 integrins like in other cell types. In addition, the implication of integrin alpha2beta1 in this adhesion process is demonstrated for the first time. This has been shown by cell adhesion inhibition experiments, solid phase assays and confocal analysis. Together with previous in situ observations, these data provide a baseline knowledge for the understanding of the regulation of laminin-5 in normal and pathological intestine.

Cellular and molecular partners involved in gut morphogenesis and differentiation

The intestinal mucosa represents an interesting model to study the cellular and molecular basis of epithelial-mesenchymal cross-talk participating in the development and maintenance of the digestive function. This cross-talk involves extracellular matrix molecules, cell-cell and cell-matrix adhesion molecules as well as paracrine factors and their receptors. The cellular and molecular unit is additionally regulated by hormonal, immune and neural inputs. Such integrated cell interactions are involved in pattern formation, in proximodistal regionalization, in maintenance of a gradient of epithelial proliferation and differentiation, and in epithelial cell migration. We focus predominantly on two aspects of these integrated interactions in this paper: (i) the role of basement membrane molecules, namely laminins, in the developmental and spatial epithelial behaviour; and (ii) the importance of the mesenchymal cell compartment in these processes.

Ligand-independent role of the beta 4 integrin subunit in the formation of hemidesmosomes

Recently, we have shown that a region within the beta4 cytoplasmic domain, encompassing the second fibronectin type III (FNIII) repeat and the first 27 amino acids of the connecting segment, is critical for the localization of alpha6 beta4 in hemidesmosomes. In addition, this region was shown to regulate the distribution of HD1/plectin in transfected cells. In order to investigate the function of the beta4 extracellular and cytoplasmic domains in the assembly and integrity of hemidesmosomes, we have constructed chimeric receptors consisting of the extracellular and transmembrane domains of the interleukin 2 receptor (IL2R), fused to different parts of the beta4 cytoplasmic domain. These chimeras are expressed as single subunits at the plasma membrane. The results show that the first and the second FNIII repeat, together with the first part of the connecting segment (in total a stretch of 241 amino acids spanning amino acids 1,115 to 1,356) are both essential and sufficient for the localization of beta4 in pre-existing hemidesmosomes. Moreover, expression of the IL2R/beta4 chimeric constructs in COS-7 and CHO cells, which do not express alpha6 beta4 or the bullous pemphigoid (BP) antigens but do express HD1/plectin, revealed that the stretch of 241 amino acids is sufficient for inducing the formation of type II hemidesmosomes. Expression of the IL2R/beta4 chimeras in a keratinocyte cell line derived from a patient lacking beta4 expression, showed that amino acids 1,115 to 1,356 can also induce the formation of type I hemidesmosomes. We further demonstrate that type I and II hemidesmosomes can also be formed upon adhesion of alpha6 beta4-expressing cells to fibronectin. These findings establish that the beta4 extracellular domain is not essential for the induction of hemidesmosome assembly. Moreover, they demonstrate that binding of alpha6 beta4 to ligand, and heterodimerization of alpha6 with beta4, are not required for hemidesmosome formation. This indicates that the assembly of hemidesmosomes can be regulated from within the cell.

Linking integrin alpha6beta4-based cell adhesion to the intermediate filament cytoskeleton: direct interaction between the beta4 subunit and plectin at multiple molecular sites

Recent studies with patients suffering from epidermolysis bullosa simplex associated with muscular dystrophy and the targeted gene disruption in mice suggested that plectin, a versatile cytoskeletal linker and intermediate filament-binding protein, may play an essential role in hemidesmosome integrity and stabilization. To define plectin's interactions with hemidesmosomal proteins on the molecular level, we studied its interaction with the uniquely long cytoplasmic tail domain of the beta4 subunit of the basement membrane laminin receptor integrin alpha6beta4 that has been implicated in connecting the transmembrane integrin complex with hemidesmosome-anchored cytokeratin filaments. In vitro binding and in vivo cotransfection assays, using recombinant mutant forms of both proteins, revealed their direct interaction via multiple molecular domains. Furthermore, we show in vitro self-interaction of integrin beta4 cytoplasmic domains, as well as disruption of intermediate filament network arrays and dislocation of hemidesmosome-associated endogenous plectin upon ectopic overexpression of this domain in PtK2 and/or 804G cells. The close association of plectin molecules with hemidesmosomal structures and their apparent random orientation was indicated by gold immunoelectron microscopy using domain-specific antibodies. Our data support a model in which plectin stabilizes hemidesmosomes, via directly interlinking integrin beta4 subunits and cytokeratin filaments.

The integrin alpha6beta4 functions in carcinoma cell migration on laminin-1 by mediating the formation and stabilization of actin-containing motility structures

Functional studies on the alpha6beta4 integrin have focused primarily on its role in the organization of hemidesmosomes, stable adhesive structures that associate with the intermediate filament cytoskeleton. In this study, we examined the function of the alpha6beta4 integrin in clone A cells, a colon carcinoma cell line that expresses alpha6beta4 but no alpha6beta1 integrin and exhibits dynamic adhesion and motility on laminin-1. Time-lapse videomicroscopy of clone A cells on laminin-1 revealed that their migration is characterized by filopodial extension and stabilization followed by lamellae that extend in the direction of stabilized filopodia. A function-blocking mAb specific for the alpha6beta4 integrin inhibited clone A migration on laminin-1. This mAb also inhibited filopodial formation and stabilization and lamella formation. Indirect immunofluorescence microscopy revealed that the alpha6beta4 integrin is localized as discrete clusters in filopodia, lamellae, and retraction fibers. Although beta1 integrins were also localized in the same structures, a spatial separation of these two integrin populations was evident. In filopodia and lamellae, a striking colocalization of the alpha6beta4 integrin and F-actin was seen. An association between alpha6beta4 and F-actin is supported by the fact that alpha6beta4 integrin and actin were released from clone A cells by treatment with the F-actin- severing protein gelsolin and that alpha6beta4 immunostaining at the marginal edges of clone A cells on laminin-1 was resistant to solubilization with Triton X-100. Cytokeratins were not observed in filopodia and lamellipodia. Moreover, alpha6beta4 was extracted from these marginal edges with a Tween-40/deoxycholate buffer that solubilizes the actin cytoskeleton but not cytokeratins. Three other carcinoma cell lines (MIP-101, CCL-228, and MDA-MB-231) exhibited alpha6beta4 colocalized with actin in filopodia and lamellae. Formation of lamellae in these cells was inhibited with an alpha6-specific antibody. Together, these results indicate that the alpha6beta4 integrin functions in carcinoma migration on laminin-1 through its ability to promote the formation and stabilization of actin-containing motility structures.

Key role of the Cdx2 homeobox gene in extracellular matrix-mediated intestinal cell differentiation

To explore the role of homeobox genes in the intestine, the human colon adenocarcinoma cell line Caco2-TC7 has been stably transfected with plasmids synthesizing Cdx1 and Cdx2 sense and antisense RNAs. Cdx1 overexpression or inhibition by antisense RNA does not markedly modify the cell differentiation markers analyzed in this study. In contrast, Cdx2 overexpression stimulates two typical markers of enterocytic differentiation: sucrase-isomaltase and lactase. Cells in which the endogenous expression of Cdx2 is reduced by antisense RNA attach poorly to the substratum. Conversely, Cdx2 overexpression modifies the expression of molecules involved in cell-cell and cell-substratum interactions and in transduction process: indeed, E-cadherin, integrin-beta4 subunit, laminin-gamma2 chain, hemidesmosomal protein, APC, and alpha-actinin are upregulated. Interestingly, most of these molecules are preferentially expressed in vivo in the differentiated villi enterocytes rather than in crypt cells. Cdx2 overexpression also results in the stimulation of HoxA-9 mRNA expression, an homeobox gene selectively expressed in the colon. In contrast, Cdx2-overexpressing cells display a decline of Cdx1 mRNA, which is mostly found in vivo in crypt cells. When implanted in nude mice, Cdx2-overexpressing cells produce larger tumors than control cells, and form glandular and villus-like structures. Laminin-1 is known to stimulate intestinal cell differentiation in vitro. In the present study, we demonstrate that the differentiating effect of laminin-1 coatings on Caco2-TC7 cells is accompanied by an upregulation of Cdx2. To further document this observation, we analyzed a series of Caco2 clones in which the production of laminin-alpha1 chain is differentially inhibited by antisense RNA. We found a positive correlation between the level of Cdx2 expression, that of endogenous laminin-alpha1 chain mRNA and that of sucrase-isomaltase expression in these cell lines. Taken together, these results suggest (a) that Cdx1 and Cdx2 homeobox genes play distinct roles in the intestinal epithelium, (b) that Cdx2 provokes pleiotropic effects triggering cells towards the phenotype of differentiated villus enterocytes, and (c) that Cdx2 expression is modulated by basement membrane components. Hence, we conclude that Cdx2 plays a key role in the extracellular matrix-mediated intestinal cell differentiation.