Localization of the 230-kilodalton bullous pemphigoid antigen in cultured keratinocytes: formation of a prehemidesmosome

Transient translocation of hemidesmosomal bullous pemphigoid antigen 1 from cytosol to membrane fractions by 12-O-tetradecanoylphorbol-13-acetate treatment and Ca2+-switch in a human carcinoma cell line

We previously showed that 12-O-tetradecanoylphorbol-13-acetate (TPA) and Ca2+-switch from low (0.07 mM) to normal (1.87 mM) concentration in culture medium, which were also linked to activation of protein kinase C (PKC), lead to phosphorylation of 180 kDa-bullous pemphigoid antigen (BPAG) 2, but not of 230 kDa-BPAG1, and possibly to its disassembly from hemidesmosomes in a human squamous cell carcinoma cell line (DJM-1). In this study, we examined the effects of TPA and Ca2+-switch on intracellular localization of BPAG1 by immuno-blotting and immuno-fluorescence microscopy with monoclonal antibodies to the antigen after sub-cellular fractionation. In DJM-1 cells cultured in low Ca2+ medium, BPAG1 was detected as phosphate buffered saline-soluble (cytosolic), Triton X-100 soluble (roughly membrane-associated) and Triton X-100 insoluble (cytoskeleton-bound) forms, whereas in normal Ca2+-grown cells only as cytosolic and cytoskeleton-bound forms. In normal Ca2+-cultured cells, TPA (50 nM) caused a complete translocation of BPAG1 from cytosol to membrane fractions within 10 min, that was inhibited by pretreatment with H7 (a selective PKC inhibitor) at 40 microM. After 30 min and 4 h of TPA-treatment, BPAG1 was exclusively detected in cytoskeleton fractions. Morphologically, immuno-fluorescence microscopy showed that treatment caused a marked reduction of BPAG1 from the cytoplasm and generated a linear pattern at cell-cell contacts, suggesting translocation of BPAG1 from the cytosol to the plasma membrane. In contrast, the Ca2+-switch from low to normal caused a prominent increase of BPAG1, both in cytosolic and membrane-associated forms after 4 h, that was inhibited both with H7 and cycloheximide (an inhibitor of protein synthesis) at 70 microM, suggesting a role for PKC and BPAG1 synthesis in these Ca2+-induced effects. These results suggest that TPA and Ca2+-switch induced BPAG1 translocation to membrane fractions possibly mediated by PKC-activation. Furthermore, whereas TPA affects the redistribution of BPAG1 among their pools without inducing their synthesis, Ca2+-switch induces both membrane translocation and synthesis of BPAG1, suggesting involvement of signaling other than PKC pathways in control of BPAG1 synthesis.

Adhesion receptors in health and disease

Cell adhesion molecules have been recognized to play a major role in a variety of physiological and pathological phenomena. They determine the specificity of cell-cell binding and the interactions between cells and extracellular matrix proteins. Some of them may also function as receptors that trigger intracellular pathways and participate in cellular processes like migration, proliferation, differentiation, and cell death. The receptors that mediate adhesion between epithelial cells that are discussed in this review include integrins, selectins, the immunoglobulin superfamily members, and cadherins. The intent of this review is to inform the reader about recent advances in cellular and molecular functions of certain receptors, specifically those that are considered important in cell adhesion. We have deliberately not provided all-inclusive detailed information on every molecule, but instead, have presented a generalized overview in order to give the reader a global perspective. This information will be useful in enhancing the reader's understanding of the molecular pathology of diseases and recognizing the potential role of these receptors and ligands as therapeutic agents.

Identification and functional separation of retinoic acid receptor neutral antagonists and inverse agonists

Inverse agonists are ligands that are capable of repressing basal receptor activity in the absence of an agonist. We have designed a series of C-1-substituted acetylenic retinoids that exhibit potent antagonism of retinoic acid receptor (RAR)-mediated transactivation. Comparison of these related retinoid antagonists for their ability to repress basal RAR transcriptional activity demonstrates that the identity of the C-1 substituent differentiates these ligands into two groups: RAR inverse agonists and neutral antagonists. We show that treatment of cultured human keratinocytes with a RAR inverse agonist, but not a RAR neutral antagonist, leads to the repression of the serum-induced differentiation marker MRP-8. While RAR-selective agonists also repress expression of MRP-8, cotreatment with a RAR inverse agonist and a RAR agonist results in a mutual repression of their individual inhibitory activities, indicating the distinct modes of action of these two disparate retinoids in modulating MRP-8 expression. Our data indicate that RARs, like beta2-adrenoreceptors, are sensitive to inverse agonists and that this new class of retinoids will provide insight into the molecular mechanisms of RAR function in skin and other responsive tissues.

Laminin chains in the basement membranes of human thymus

In recent studies, the alpha 2 chain of laminin (Ln) has been suggested to be the only laminin alpha chain expressed in mouse and human thymus. We have now used chain-specific monoclonal antibodies and indirect immunofluorescence microscopy to study the expression of laminin chains in samples of foetal and 6-year-old human thymus. The subepithelial basement membrane of the capsule of foetal 16-to 18-week thymus presented a bright immunoreactivity for Ln alpha 1, alpha 3, beta 1, beta 3 and gamma 1 chains but not for alpha 2 chain, suggesting the expression of laminins-1 and-5. Most cortical and medullary epithelial cells, including Hassall's corpuscles, however, lacked laminin immunoreactivity. Immunoreactivity for Ln beta 2 chain was only seen in basal laminae of larger blood vessels. In thymic specimens from 6-year-old children, immunoreactivity for the laminin alpha 1, alpha 3, beta 1, beta 3 and gamma 1 chains was invariably found in subepithelial basement membrane of the capsule and that for laminin alpha 2 chain was now also distinct but more heterogeneous. Furthermore, the thymic subepithelial basement membrane of the capsule at all stages showed immunoreactivity for collagen type VII, forming the anchoring fibres in epithelial basement membranes. The subcapsular thymic epithelium also showed immunoreactivity for the BP 230 antigen and beta 4 integrin subunit, both components of hemidesmosomes. The present results show that the thymic subepithelial basement membrane of the capsule presents properties which are commonly seen in stratified and combined epithelia, and are compatible with suggestions of the antigenic similarity of thymic epithelial cells and keratinocytes.

Structural analysis of the predicted coiled-coil rod domain of the cytoplasmic bullous pemphigoid antigen (BPAG1). Empirical localization of the N-terminal globular domain-rod boundary

The bullous pemphigoid antigen BPAG1 is required for keratin filament linkage to the hemidesmosome, an adhesion complex in epithelial basal cells. BPAG1 structural organization is similar to the intermediate filament-associated proteins desmoplakin I (DPI) and plectin. All three proteins have predicted dumbbell-like structure with central alpha-helical coiled-coil rod and regions of N- and C-terminal homology. To characterize the size of the N-terminal globular domain in BPAG1, two polypeptides spanning possible boundaries with the coiled-coil rod domain of BPAG1 were expressed in Escherichia coli. BP-1 (Mr = 111,000), containing amino acids 663-1581 of BPAG1 (Sawamura, D., Li, K., Chu, M.-L., and Uitto, J. (1991) J. Biol. Chem. 266, 17784-17790), and BP-1A, with a 186 amino acid N-terminal deletion, were purified. BP-1 and BP-1A behave as highly asymmetric dimers in aqueous solution according to velocity sedimentation and gel filtration. Both have globular heads with rod-like tails of roughly equal length, 55-60 nm, upon rotary shadowing. BP-1A content of alpha-helix, determined by circular dichroism, is approximately 90%, consistent with alpha-helical coiled-coil formation in the rod-like tails. The estimated rod length, 383 +/- 57 amino acids (0.15 nm/amino acid), implies that globular folding in the BPAG1 N-terminal extends to the end of N-terminal homology with DPI and plectin. These findings support the existence of a common domain structure in the N-terminal regions of the BPAG1/DPI/plectin family.

Hemidesmosome ontogeny in digit skin of the human fetus

Hemidesmosomes are junctional complexes involved in the attachment of epidermal basal keratinocytes to the basement membrane. To try to understand better the sequence of events in the morphogenesis of hemidesmosomes, we undertook an ultrastructural analysis of hemidesmosome formation in fetal and neonatal digit skin. Hemidesmosomes, defined as membrane-associated densities or plaques, were counted and scored for three morphological characteristics: (1) the presence of a sub-basal dense plate, (2) association with anchoring filaments within the lamina lucida and (3) contacts with intermediate filaments. No hemidesmosomes were evident at 7 weeks' gestational age. Between 9 and 15 weeks the number of hemidesmosomes increased by about fourfold (from 20.6 +/- 3.8 (SD) to 95.5 +/- 8.4 per 40 micro m of basal cell plasma membrane; P < 0.01). The association of hemidesmosomes with intermediate filaments and anchoring filaments also increased after 15 weeks (P < 0.05). Early attachment plaques first appeared as triangular focal densities on the basal plasma membrane with the appearance of sub-basal dense plates, which later became both larger and more electron dense. By 15 weeks, an inner plaque could be distinguished from the outer plaque, which coincided with a closer association with intermediate filaments. Hemidesmosomes appeared fully developed by 15 weeks' gestation. This study illustrates the structural relationship of hemidesmosomes to both intra- and extracellular filaments, suggesting close functional interactions. The complexity of the hemidesmosome plaque is also revealed early during development.

Continual assembly of half-desmosomal structures in the absence of cell contacts and their frustrated endocytosis: a coordinated Sisyphus cycle

It is widely assumed that the coordinate assembly of desmosomal cadherins and plaque proteins into desmosome-typical plaque-coated membrane domains, capable of anchoring intermediate-sized filaments (IF), requires cell-to-cell contacts and a critical extracellular Ca2+ concentration. To test this hypothesis we studied several cell lines grown for years in media with less than 0.1 mM Ca2+ to steady-state low Ca2+ medium (LCM) conditions, particularly the human keratinocyte line HaCaT devoid of any junctional cell contact (HaCaT-L cells). Using immunolocalization and vesicle fractionation techniques, we found that the transmembrane glycoprotein, desmoglein (Dsg), colocalized with the plaque proteins, desmoplakin and plakoglobin. The sites of coassembly of desmosomal molecules in HaCaT-L cells as well as in HaCaT cells directly brought into LCM were identified as asymmetric plaque-coated plasma membrane domains (half-desmosomes) or as special plaque-associated cytoplasmic vesicles, most of which had formed endocytotically. The surface exposure of Dsg in these half-desmosomes was demonstrated by the binding, in vivo, of antibodies specific for an extracellular Dsg segment which also could cross-bridge them into symmetric quasi-desmosomes. Otherwise, these half-desmosomes were shown in LCM to be taken up endocytotically. Half-desmosomal assemblies were also seen in uncoupled cells in normal Ca2+ medium. We conclude that, in the absence of intercellular contacts, assembly of desmosomal proteins at the cell surface takes place, resulting in transient half-desmosomes which then, in LCM and without a stable partner connection to the adjacent cell, can be endocytotically resumed. This frustrated cycle of synthesis and assembly maintains an ensemble of molecules characteristic of epithelial differentiation and the potential to form desmosomes, even when the final junctional structure cannot be formed. We propose that these half-desmosomal structures are general cell structures of epithelial and other desmosome-forming cells.

Expression pattern of the bullous pemphigoid-180 antigen in normal and neoplastic epithelia

BP180 is a 180kDa hemidesmosomal protein recognized by bullous pemphigoid (BP) and pemphigoid gestationis (PG) autoantibodies. Recent cloning and sequence analysis performed by our laboratory have revealed that BP180 is a transmembrane protein with a long extracellular collagen-like region. A rabbit polyclonal antibody has been generated against a recombinant protein, designated GST-N delta 1, containing a segment of the BP180 ectodomain. The resulting antiserum, RN delta 1A, was shown to specifically react with BP180 on immunoblot, and labelled the extracellular region of the epidermal hemidesmosome on immunoelectron microscopy. A panel of normal and neoplastic human tissues were analysed by indirect immunofluorescence (IF) and RN delta 1A, to determine the distribution of BP180. A total of nine basal cell carcinomas (BCCs) and four squamous cell carcinomas (SCCs) of the skin were also studied. Intense IF staining was seen along the basement membrane zone (BMZ) of the epidermis, hair follicles, and the periphery of sebaceous gland lobules. The sebaceous lobules showed more intense staining in areas close to the duct. The epithelial BMZ of the following tissues also reacted with RN delta 1A: cornea, ocular conjunctiva, buccal mucosa, upper oesophagus, placenta (amnion placentum), umbilical cord and transitional epithelium of the bladder. The epithelium of the jejunum and ovary failed to react with RN delta 1A. Staining of the BCCs and SCCs was variable. Five of six nodular BCCs showed some anti-BP180 staining at the tumour-stromal interface, although the level of staining was less intense than that observed in the overlying normal epidermis. All three morphoeic BCCs analysed in this investigation did not show any staining with RN delta 1A. Three of four SCCs showed weak staining at the tumour-stromal interface. Thus, the tissue distribution of BP180 paralleled that of hemidesmosomes, and expression of this protein was found to be decreased or absent in cutaneous neoplasms.

Immunolocalization of integrins in the normal and neoplastic colonic epithelium

Cryosections of normal colon (NC), tubular and villous adenomas (TA, VA), and variably differentiated colon adenocarcinomas (CA) were immunostained with monoclonal antibodies to alpha 1-6 and alpha v, and beta 1-4 integrin subunits; select samples were stained for cytokeratin (Ck) 20 and villin. In NC, alpha 2 staining was strongest in crypt cells; alpha 1,3 and alpha v, and beta 1,3 and beta 4, and Ck 20 and villin predominated in superficial enterocytes. In TA and VA, monolayered glands showed integrin, Ck 20 and villin patterns that differed slightly from both crypt and superficial enterocytes. Complex glands in VA showed decreased integrin staining and basal polarization; Ck 20 and villin were strong only in luminal cells. CA showed overall weaker integrin staining than adenomas. Regardless of invasion depth, well formed malignant glands mimicked TA; pleomorphic glands mimicked VA with focal basal integrin polarization and solid clusters displayed scanty integrins, uneven Ck 20, and villin in occasional cells. Diverse integrins in crypt compared with superficial enterocytes reflect changing adhesive requirements as cells migrate and terminally differentiate. Decreasing expression and altered distribution of integrins, Ck 20 and villin noted in TA, VA, and in CA of increasing grade indicate that certain adhesive and cytoskeletal features more closely relate to glandular architecture than to depth of invasion.

Production of basement membrane components by a reconstructed epidermis cultured in the absence of serum and dermal factors

A fully differentiated epithelium displaying features of human epidermis was obtained in vitro by culturing second-passage normal human keratinocytes for 14 days in defined medium and on an inert polycarbonate filter substratum at the air-liquid interface. Vertical sections stained for histology and indirect immunofluorescence studies showed that the 'basal' cells synthesize and secrete all major markers of hemidesmosomes and the lamina lucida. Components of the lamina densa are also expressed. Collagen VII is synthesized, but not secreted. Ultrastructural studies showed the presence of hemidesmosomes with major dense plaques and anchoring filaments, and a basement membrane-like structure was clearly identified. These results show that epidermal cells are able to produce hemidesmosomes and to secrete the major components of the dermo-epidermal junction in the absence of serum and dermal factors, suggesting that basement membrane synthesis and hemidesmosome assembly are not dependent on the presence of dermis.

The matrix secreted by 804G cells contains laminin-related components that participate in hemidesmosome assembly in vitro

Hemidesmosomes are important adhesion devices found in epithelial cells. They connect the intermediate filament cytoskeleton network with components of the basement membrane zone. 804G cells are an unusual epithelial cell line, since they form bona fide hemidesmosomes when plated on glass or plastic. In this study we tested an hypothesis: that this ability is a consequence of an extracellular component produced by the 804G cells. As probes for our study we generated a rabbit antiserum (J18) and monoclonal antibodies against components of urea-solubilized 804G matrix. Antibodies in the J18 serum recognize major lectin-binding polypeptides of 150, 140 and 135 kDa in the 804G matrix. A monoclonal antibody (5C5) that shows reactivity with the 150 and 135 kDa polypeptides in western immunoblots immunoprecipitates all three molecular mass species, indicating that these polypeptides are part of a matrix complex. Moreover, one, at least, of these matrix elements is immunologically related to laminin, since J18 antibodies selected on fusion protein fragments of a newly characterized laminin variant, laminin B2t (Kallunki et al., J. Cell Biol., 119, 679–694, 1992), react with the 140 kDa polypeptide component of the 804G cell matrix. To undertake functional analyses of 804G matrix, cells of the human epidermal carcinoma line SCC12, which do not assemble bona fide hemidesmosomes in vitro, were cultured on 804G matrix for 24 h and then analysed by confocal immunofluorescence and electron microscopy. In SCC12 cells maintained on 804G cell matrix, hemidesmosomal antigens localize in a distinctive leopard spot pattern that mirrors the distribution of 804G matrix elements. Furthermore, ultrastructural analysis reveals that the 804G cell matrix supports the formation of ‘mature’ hemidesmosomes by SCC12 cells. Thus 804G cell matrix is a remarkable tool for hemidesmosome studies and it will now be of great importance to determine the exact composition of the 804G matrix, especially its structural and antigenic relationship to laminins.

Phorbol ester- and calcium-induced reorganization of 180-kDa bullous pemphigoid antigen on the ventral surface of cultured human keratinocytes as studied by immunofluorescence and immunoelectron microscopy

The hemidesmosome is an adhesion structure of the epidermal-dermal junction in keratinocytes. When keratinocytes migrate laterally or upward to differentiate, they must control the formation and disintegration of the hemidesmosomes. When keratinocytes are cultured in low-calcium (below 0.1 mM) medium, all cells behave like basal cells, adhere to the culture dish, and proliferate without differentiation. The calcium addition induces the differentiation. A bullous pemphigoid antigen, 180-kDa BPA, has been shown to be a component of the hemidesmosome. Using a monoclonal antibody to the 180-kDa BPA and a human squamous cell carcinoma cell line (DJM-1 cells), the fate of hemidesmosomes was studied after the addition of calcium to low-calcium-grown cells and 12-tetradecanoylphorbol-13-acetate (TPA) to high-calcium (1.87 mM) grown cells by immunofluorescence and immunoelectron microscopy. The antigen was distributed evenly as fine dots on the entire ventral surface of low-calcium cells, whereas they formed a peculiar, concentric ring or arch arrangement on the ventral surface of high-calcium cells. Immunoelectron microscopy revealed the deposits of gold particles at sites on the membrane surface, where some filamentous or electron-dense materials were associated, although the complete structure of hemidesmosomes was not formed. They deposited directly onto the membrane surface in low-calcium cells and with a distance of 20-50 nm from the membrane surface in high-calcium cells. The calcium addition caused a profound reduction of the 180-kDa BPA-positive area for 30 to 120 min and then formed the high-calcium-ring pattern after 4 to 6 h. A similar calcium response was seen in normal human keratinocytes. TPA (16 nM) treatment caused disintegration of the ring pattern in high-calcium DJM-1 cells. This was inhibited with a protein kinase C (PKC) inhibitor. H7 (20 microM). These results suggest that the hemidesmosome is a dynamic structure and PKC can be one of the major factors in controlling the hemidesmosome, since it is known that the low-high calcium shift induces a calcium influx and a PKC activation, and TPA activates PKC in keratinocytes.

Identification of a new hemidesmosomal protein, HD1: a major, high molecular mass component of isolated hemidesmosomes

Hemidesmosomes (HDs) mediate cell adhesion to the extracellular matrix and have morphological association with intermediate-sized filaments (IFs) through cytoplasmic plaques. Though several proteins have been located in HDs, most of them have not been well characterized, with the exception of the 230-kD antigen of bullous pemphigoid (BP), an autoimmune skin blistering disease. Only recently we have succeeded in isolating HDs from bovine corneal epithelial cells and in identifying five major components on SDS-PAGE (Owaribe K., Y. Nishizawa, and W. W. Franke. 1991. Exp. Cell Res. 192:622-630). In this study we report on immunological characterization of one of the major components, termed HD1, with an apparent molecular mass of 500 kD. Immunofluorescence microscopy showed colocalization of HD1 with BP antigen at the basement membrane zone of those tissues that have typical HDs, including skin epidermis, corneal and tracheal epithelia, and myoepithelium. In cultured keratinocytes, HD1 demonstrated colocalization with BP antigen in the precise way, while being absent from focal adhesions. Immunoelectron microscopy revealed that an epitope of HD1 was located on the cytoplasmic side of HDs. Taking all these results together, we conclude that HD1 is a new hemidesmosomal component. Interestingly, HD1 also exists in endothelial and glial cells, which lack typical HDs.