Signal transduction by the α6β4 integrin: charting the path between laminin binding and nuclear events

Regulation of Kinase Signaling Pathways by α6β4-Integrins and Plectin in Prostate Cancer

Hemidesmosomes (HDs) are adhesive structures that ensure stable anchorage of cells to the basement membrane. They are formed by α6β4-integrin heterodimers and linked to intermediate filaments via plectin. It has been reported that one of the most common events during the pathogenesis of prostate cancer (PCa) is the loss of HD organization. While the expression levels of β4-integrins are strongly reduced, the expression levels of α6-integrins and plectin are maintained or even elevated, and seem to promote tumorigenic properties of PCa cells, such as proliferation, invasion, metastasis, apoptosis- and drug-resistance. In this review, we discuss the potential mechanisms of how HD components might contribute to various cellular signaling pathways to promote prostate carcinogenesis. Moreover, we summarize the current knowledge on the involvement of α6β4-integrins and plectin in PCa initiation and progression.

EGFR-dependent tyrosine phosphorylation of integrin β4 is not required for downstream signaling events in cancer cell lines

In epithelial cancers, the epidermal growth factor receptor (EGFR) and integrin α6β4 are frequently overexpressed and found to synergistically activate intracellular signaling pathways that promote cell proliferation and migration. In cancer cells, the β4 subunit is phosphorylated at tyrosine residues not normally recognized as kinase substrates; however, the function of these phosphotyrosine residues in cancer cells is a subject of much debate. In EGFR-overexpressing carcinoma cells, we found that the Src family kinase (SFK) inhibitor PP2 reduces β4 tyrosine phosphorylation following the activation of EGFR. However, siRNA mediated knockdown of the SFKs Src, Fyn, Yes and Lyn, individually or in combination, did not affect the EGF-induced phosphorylation of β4. Using phospho-peptide affinity chromatography and mass spectrometry, we found that PLCγ1 binds β4 at the phosphorylated residues Y1422/Y1440, but were unable to verify this interaction in A431 carcinoma cells that overexpress the EGFR. Furthermore, using A431 cells devoid of β4 or reconstituted with phenylalanine specific mutants of β4, the activation of several downstream signaling pathways, including PLCγ/PKC, MAPK and PI3K/Akt, were not substantially affected. We conclude that tyrosine-phosphorylated β4 does not enhance EGFR-mediated signaling in EGFR-overexpressing cells, despite the fact that this integrin subunit is highly tyrosine phosphorylated in these cells.

The Tumor Microenvironment: Focus on Extracellular Matrix

The extracellular matrix (ECM) regulates the development and maintains tissue homeostasis. The ECM is composed of a complex network of molecules presenting distinct biochemical properties to regulate cell growth, survival, motility, and differentiation. Among their components, proteoglycans (PGs) are considered one of the main components of ECM. Its composition, biomechanics, and anisotropy are exquisitely tuned to reflect the physiological state of the tissue. The loss of ECM's homeostasis is seen as one of the hallmarks of cancer and, typically, defines transitional events in tumor progression and metastasis. In this chapter, we discuss the types of proteoglycans and their roles in cancer. It has been observed that the amount of some ECM components is increased, while others are decreased, depending on the type of tumor. However, both conditions corroborate with tumor progression and malignancy. Therefore, ECM components have an increasingly important role in carcinogenesis and this leads us to believe that their understanding may be a key in the discovery of new anti-tumor therapies. In this book, the main ECM components will be discussed in more detail in each chapter.

Knockdown of LRP/LR induces apoptosis in pancreatic cancer and neuroblastoma cells through activation of caspases

The 37kDa/67kDa laminin receptor (LRP/LR) serves various physiological and pathological roles such as enhancing tumour-related processes including metastasis, angiogenesis, cellular viability and telomerase activation in cancerous cell lines. The present study investigates the effect of siRNA mediated downregulation of LRP/LR on pancreatic cancer (AsPC-1) and neuroblastoma (IMR-32) cells. MTT and BrdU assays revealed that siRNA mediated downregulation of LRP resulted in a significant reduction in cell viability and cell proliferation. In addition, knock-down of LRP resulted in phosphatidylserine externalization, diminished nuclear integrity and significantly enhanced caspase-3 activity, which is indicative of apoptosis. LRP downregulation resulted in a significant increase in caspase-8 activity in IMR-32 cells and enhanced caspase-8 and 9 activity in AsPC-1 cells. These data recommend siRNA mediated knock-down of LRP as a potential therapeutic avenue for the treatment of pancreatic cancer and neuroblastoma.

Up-regulation of integrin α6β4 expression by mitogens involved in dairy cow mammary development

In dairy cows, the extracellular microenvironment varies significantly from the virgin state to lactation. The function of integrin α6β4 is dependent on cell type and extracellular microenvironment, and the precise expression profile of α6β4 and its effects on mammary development remain to be determined. In the present study, real-time PCR and immunohistochemistry were used to analyze the expression and localization of integrin α6β4 in Holstein dairy cow mammary glands. The effects of integrin α6β4 on the proliferation induced by mammogenic mitogens were identified by blocking integrin function in purified dairy cow mammary epithelial cells (DCMECs). The results showed that the localization of β4 subunit and its exclusive partner the α6 subunit were not consistent but were co-localized in basal luminal cells and myoepithelial cells, appearing to prefer the basal surface of the plasma membrane. Moreover, α6 and β4 subunit messenger RNA (mRNA) levels changed throughout the stages of dairy cow mammary development, reflected well by protein levels, and remained higher in the virgin and pregnancy states, with duct/alveolus morphogenesis and active cell proliferation, than during lactation, when growth arrest is essential for mammary epithelial cell differentiation. Finally, the upregulation of integrin expression by both mammogenic growth hormone and insulin-like growth factor-1 and the inhibited growth of DCMECs by function-blocking integrin antibodies confirmed that integrin α6β4 was indeed involved in dairy cow mammary development.

FOXN1 (GFP/w) reporter hESCs enable identification of integrin-β4, HLA-DR, and EpCAM as markers of human PSC-derived FOXN1(+) thymic epithelial progenitors

Thymic epithelial cells (TECs) play a critical role in T cell maturation and tolerance induction. The generation of TECs from in vitro differentiation of human pluripotent stem cells (PSCs) provides a platform on which to study the mechanisms of this interaction and has implications for immune reconstitution. To facilitate analysis of PSC-derived TECs, we generated hESC reporter lines in which sequences encoding GFP were targeted to FOXN1, a gene required for TEC development. Using this FOXN1^GFP/w line as a readout, we developed a reproducible protocol for generating FOXN1-GFP⁺ thymic endoderm cells. Transcriptional profiling and flow cytometry identified integrin-β4 (ITGB4, CD104) and HLA-DR as markers that could be used in combination with EpCAM to selectively purify FOXN1⁺ TEC progenitors from differentiating cultures of unmanipulated PSCs. Human FOXN1⁺ TEC progenitors generated from PSCs facilitate the study of thymus biology and are a valuable resource for future applications in regenerative medicine.

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FOXN1-GFP reporter hESC lines were generated

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KGF promotes the proliferation of FOXN1-GFP⁺ cells

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FOXN1-GFP⁺ cells express TEC-associated genes

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ITGB4, HLA-DR, and EpCAM can be used to purify FOXN1⁺ TEC progenitors (219)

Anti-LRP/LR specific antibody IgG1-iS18 impedes adhesion and invasion of liver cancer cells

Two key events, namely adhesion and invasion, are pivotal to the occurrence of metastasis. Importantly, the 37 kDa/67 kDa laminin receptor (LRP/LR) has been implicated in enhancing these two events thus facilitating cancer progression. In the current study, the role of LRP/LR in the adhesion and invasion of liver cancer (HUH-7) and leukaemia (K562) cells was investigated. Flow cytometry revealed that the HUH-7 cells displayed significantly higher cell surface LRP/LR levels compared to the poorly-invasive breast cancer (MCF-7) control cells, whilst the K562 cells displayed significantly lower cell surface LRP/LR levels in comparison to the MCF-7 control cells. However, Western blotting and densitometric analysis revealed that all three tumorigenic cell lines did not differ significantly with regards to total LRP/LR levels. Furthermore, treatment of liver cancer cells with anti-LRP/LR specific antibody IgG1-iS18 (0.2 mg/ml) significantly reduced the adhesive potential of cells to laminin-1 and the invasive potential of cells through the ECM-like Matrigel, whilst leukaemia cells showed no significant differences in both instances. Additionally, Pearson's correlation coefficients suggested direct proportionality between cell surface LRP/LR levels and the adhesive and invasive potential of liver cancer and leukaemia cells. These findings suggest the potential use of anti-LRP/LR specific antibody IgG1-iS18 as an alternative therapeutic tool for metastatic liver cancer through impediment of the LRP/LR- laminin-1 interaction.

Bullous pemphigoid: role of complement and mechanisms for blister formation within the lamina lucida

Bullous pemphigoid (BP), an autoimmune subepidermal blistering skin disease, demonstrates tense blisters with or without widespread erythema, blistering along the lamina lucida, immunoglobulin G and/or complement deposits at the basement membrane zone, and the presence of circulating autoantibodies against hemidesmosomal molecules. These autoantibodies usually react against 180-kDa and/or 230-kDa proteins, designated as BP180 and BP230, respectively. The precise blistering mechanisms after autoantibodies bind to antigens are not fully understood. Immune complexes are thought to initially activate the complement cascade, which may induce activation of proteases and/or cytokines and cause dermal-epidermal separation. However, why does separation run specifically within the lamina lucida in a space as narrow as 500 nm wide? This review mainly focuses on the possible mechanisms of BP-specific blistering and how separation occurs along the lamina lucida, based on existing evidence.

Autonomous isolation, long-term culture and differentiation potential of adult salivary gland-derived stem/progenitor cells

Salivary gland stem/progenitor cells belong to the endodermal lineage and may serve as good candidates to replace their dysfunctional counterparts. The objective of this study was to isolate large numbers of salivary gland tissue-derived stem cells (SGSCs) from adult rats in order to develop a clinically applicable method that does not involve sorting or stem cell induction by duct ligation. We analysed SGSCs isolated from normal rat salivary glands to determine whether they retained the major characteristics of stem cells, self-renewal and multipotency, especially with respect to the various endodermal cell types. SGSCs expressed high levels of integrin α6β1 and c-kit, which are surface markers of SGSCs. In particular, the integrin α6β1(+) /c-kit(+) salivary gland cells maintained the morphology, proliferation activity and multipotency of stem cells for up to 92 passages in 12 months. Furthermore, we analysed the capacity of SGSCs to differentiate into endoderm lineage cell types, such as acinar-like and insulin-secreting cells. When cultured on growth factor reduced matrigel, the morphology of progenitor cells changed to acinar-like structures and these cells expressed the acinar cell-specific marker, α-amylase, and tight junction markers. Moreover, reverse transcription-polymerase chain reaction (RT-PCR) data showed increased expression of pancreatic cell markers, including insulin, Pdx1, pan polypeptide and neurogenin-3, when these cells formed pancreatic clusters in the presence of activin A, exendin-4 and retinoic acid. These data demonstrate that adult salivary stem/progenitor cells may serve as a potential source for cell therapy in salivary gland hypofunction and diabetes.

Expression of laminin-5 and integrins in actinic cheilitis and superficially invasive squamous cell carcinomas of the lip

The progression of carcinogenesis entails the detachment of cells, invasion and migration of neoplastic cells. Alterations in epithelial adhesion and basement membrane proteins might mediate the early stages of carcinogenesis. This study investigated the expression of adhesion molecules and the basement membrane protein laminin-5 in actinic cheilitis (AC) and incipient squamous cell carcinoma of the lower lip to understand early photocarcinogenesis. Ln-5γ2 chain as well as α3, β1 subunits of α3β1 heterodimer and β4 subunit of integrin α6β4 were evaluated by immunohistochemistry in 16 cases of AC and 16 cases of superficially invasive squamous cell carcinoma (SISCC). Most AC cases showed reduced expression of β1, β4 and α3 integrins, and SISCCs lacked β1, β4 and α3 integrins in the invasive front. AC cases were negative for the Ln-5γ2 chain. Five cases of SISCC (31%) showed heterogeneous Ln-5γ2 chain expression in the invasive front of the tumor. Integrin β1, β4 and α3 expression is lost during the early stages of lip carcinogenesis. Expression of Ln-5γ2 in the invasive front in cases and its correlation with tumor progression suggest that it mediates the acquisition of the migrating and invading epithelial cell phenotype.

Loss of keratins 8 and 18 leads to alterations in α6β4-integrin-mediated signalling and decreased neoplastic progression in an oral-tumour-derived cell line

Keratins 8 and 18 (K8 and K18) are predominantly expressed in simple epithelial tissues and perform both mechanical and regulatory functions. Aberrant expression of K8 and K18 is associated with neoplastic progression and invasion in squamous cell carcinomas (SCCs). To understand the molecular basis by which K8 promotes neoplastic progression in oral SCC (OSCC), K8 expression was inhibited in AW13516 cells. The K8-knockdown clones showed a significant reduction in tumorigenic potential, which was accompanied by a reduction in cell motility, cell invasion, decreased fascin levels, alterations in the organization of the actin cytoskeleton and changes in cell shape. Furthermore, K8 knockdown led to a decrease in α6β4 integrin levels and α6β4-integrin-dependent signalling events, which have been reported to play an important role in neoplastic progression in epithelial tissues. Therefore, modulation of α6β4 integrin signalling might be one of the mechanisms by which K8 and K18 promote malignant transformation and/or progression in OSCCs.

Loss of alpha-dystroglycan laminin binding in epithelium-derived cancers is caused by silencing of LARGE

The interaction between epithelial cells and the extracellular matrix is crucial for tissue architecture and function and is compromised during cancer progression. Dystroglycan is a membrane receptor that mediates interactions between cells and basement membranes in various epithelia. In many epithelium-derived cancers, β-dystroglycan is expressed, but α-dystroglycan is not detected. Here we report that α-dystroglycan is correctly expressed and trafficked to the cell membrane but lacks laminin binding as a result of the silencing of the like-acetylglucosaminyltransferase (LARGE) gene in a cohort of highly metastatic epithelial cell lines derived from breast, cervical, and lung cancers. Exogenous expression of LARGE in these cancer cells restores the normal glycosylation and laminin binding of α-dystroglycan, leading to enhanced cell adhesion and reduced cell migration in vitro. Our findings demonstrate that LARGE repression is responsible for the defects in dystroglycan-mediated cell adhesion that are observed in epithelium-derived cancer cells and point to a defect of dystroglycan glycosylation as a factor in cancer progression.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

Sealing the skin barrier around transcutaneous implants

Conventional amputation prostheses rely on the attachment of the socket to the stump, which may lead to soft-tissue complications. Intraosseous transcutaneous amputation prostheses (ITAPs) allow direct loading of the skeleton, but their success is limited by infection resulting from breaching of the skin at the interface with the implant. Keratinocytes provide the skin’s primary barrier function, while hemidesmosomes mediate their attachment to natural ITAP analogues. Keratinocytes must attach directly to the surface of the implant. We have assessed the proliferation, morphology and attachment of keratinocytes to four titaniumalloy surfaces in order to determine the optimal topography in vitro. We used immunolocalisation of adhesion complex components, scanning electron microscopy and transmission electron microscopy to assess cell parameters.

We have shown that the proliferation, morphology and attachment of keratinocytes are affected by the surface topography of the biomaterials used to support their growth. Smoother surfaces improved adhesion. We postulate that a smooth topography at the point of epithelium-ITAP contact could increase attachment in vivo, producing an effective barrier of infection.

Loss of alpha6 integrins in keratinocytes leads to an increase in TGFbeta and AP1 signaling and in expression of differentiation genes

Mice lacking the alpha6 integrin chain die at birth with severe skin blistering. To further study the function of alpha6 integrin in skin, we generated conditionally immortalized cell lines from the epidermis of wild-type and alpha6 deficient mouse embryos. Mutant cells presented a decreased adhesion on laminin 5, the major component of the basement membrane in the skin, and on laminins 10/11 and 2. A DNA array analysis revealed alterations in the expression of extracellular matrix (ECM) components including laminin 5, cytoskeletal elements, but also membrane receptors like the hemidesmosomal components integrin beta4 and collagen XVII, or growth factors and signaling molecules of the TGFbeta, EGF, and Wnt pathways. Finally, an increase of several epidermal differentiation markers was observed in cells and tissue at the protein level. Further examination of the mutant tissue revealed alterations in the filaggrin signal. These differences may be linked to an upregulation of the TGFbeta and the Jun/Fos pathways in mutant keratinocytes. These results are in favor of a role for integrin alpha6beta4 in the maintenance of basal keratinocyte properties and epidermal homeostasis.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

A lipid raft-associated 67kDa laminin receptor mediates suppressive effect of epigallocatechin-3-O-gallate on FcεRI expression

(-)-Epigallocatechin-3-O-gallate (EGCG), a major green tea polyphenol, has previously exhibited a suppressive effect on the expression of the high-affinity IgE receptor (FcepsilonRI). This effect has been shown to be elicited by interaction with the plasma membrane microdomain lipid rafts. Recently, we have identified the 67 kDa laminin receptor (67LR) as a cell surface EGCG receptor that mediates an anti-cancer action. Here we show that the 67LR is highly associated with lipid rafts on human basophilic KU812 cells. Experiments using 67LR-enhanced and -reduced cells revealed that the EGCG's ability to downregulate FcepsilonRI expression correlated with the amount of 67LR. Thus, these results suggest that the lipid raft-associated 67LR plays an important role in mediating the FcepsilonRI-suppressive action of EGCG.

Laminin-induced signaling in tumor cells

Laminin is the main non-collagenous glycoprotein found in the basement membrane. The various laminin isoforms are involved in many physiological and pathological processes, including cancer dissemination. The interaction of cancer cells with laminin was identified as a key event in tumor invasion and metastasis. Laminin effects are mediated by laminin receptors that are divided into two groups: integrin and non-integrin receptors. Activation of a specific signal transduction pathway in the cell depends on various factors and may be altered when normal tissue becomes neoplastic. Laminin signals via multiple signal transduction pathways involving various components such as G-proteins, intracellular calcium, phospholipase D, mitogen activated protein kinases, phosphatases, focal adhesion kinase, small GTPases of the Rho family, and cytoskeleton components. This review focuses on the role of laminin in tumor progression, its signaling via the non-integrin 67kDa laminin receptor and via integrins and the reciprocal relations between these receptors in certain tumors.

Ly6 family member C4.4A binds laminins 1 and 5, associates with galectin-3 and supports cell migration

C4.4A is a member of the Ly6 family, with low homology to uPAR. It has been detected mainly on metastasizing carcinoma cells and proposed to be involved in wound healing. So far, C4.4A has been observed as an orphan receptor, and its functional activity has not been explored. Using recombinant rat C4.4A (rrC4.4A) made in a eukaryotic expression system, we demonstrate by immunohistology that C4.4A ligands are strongly expressed in tissues adjacent to squamous epithelia of, e.g., tongue and esophagus, the expression pattern partly overlapping with laminin (LN) and complementing the C4.4A expression that is found predominantly on the basal layers of squamous epithelium. ELISA screening of several components of the extracellular matrix revealed selective binding of rrC4.4A to LN1 and LN5 and that transfection of the BSp73AS tumor line with C4.4A cDNA (BSp73AS-1B1) promoted LN1 and LN5 binding. Binding of BSp73AS-1B1 to LN5 and, less markedly, LN1 induced spreading, lamellipodia formation and migration. C4.4A also associates with galectin-3 in nontransformed tissues and tumor lines. There is evidence that the association of C4.4A with galectin-3 influences LN adhesion. C4.4A was described originally as a metastasis-associated molecule. Our findings that LN1 and LN5 are C4.4A ligands, that galectin-3 associates with C4.4A and that C4.4A ligand binding confers a migratory phenotype are well in line with the supposed metastasis association.

Keratinocytes display normal proliferation, survival and differentiation in conditional β4-integrin knockout mice

The α6β4 integrin is located at the basal surface of keratinocytes, in hemidesmosomal structures that mediate stable adhesion of epidermal cells to the underlying basement membrane component laminin-5. The absence of α6β4 integrin causes junctional epidermolysis bullosa, a severe blistering disease of the skin leading to perinatal death, confirming its essential role in mediating strong keratinocyte adhesion. Several studies have suggested that α6β4 integrin can also regulate signaling cascades that control cell proliferation, survival and migration through a mechanism independent of its adhesive function. We have generated a conditional knockout mouse strain, in which the gene encoding the β4 integrin subunit (Itgb4) was inactivated only in small stretches of the skin. These mice were viable and permitted an accurate analysis of the consequences of the loss of β4 on various biological processes by comparing β4-positive and -negative parts of the skin in the same animal. Despite the complete loss of hemidesmosomes in regions lacking α6β4 integrin, the distribution of a range of adhesion receptors and basement membrane proteins was unaltered. Moreover, loss of α6β4 did not affect squamous differentiation, proliferation or survival, except for areas in which keratinocytes had detached from the basement membrane. These in vivo observations were confirmed in vitro by using immortalized keratinocytes – derived from β4-subunit conditional knockout mice – from which the gene encoding β4 had been deleted by Cre-mediated recombination. Consistent with the established role of α6β4 in adhesion strengthening, its loss from cells was found to increase their motility. Our findings clearly demonstrate that, after birth, epidermal differentiation, proliferation and survival all proceed normally in the absence of α6β4, provided that cell adhesion is not compromised.

Laminin-5 induces osteogenic gene expression in human mesenchymal stem cells through an ERK-dependent pathway

The laminin family of proteins is critical for managing a variety of cellular activities including migration, adhesion, and differentiation. In bone, the roles of laminins in controlling osteogenic differentiation of human mesenchymal stem cells (hMSC) are unknown. We report here that laminin-5 is found in bone and expressed by hMSC. hMSC isolated from bone synthesize laminin-5 and adhere to exogenous laminin-5 through alpha3beta1 integrin. Adhesion to laminin-5 activates extracellular signal-related kinase (ERK) within 30 min and leads to phosphorylation of the osteogenic transcription factor Runx2/CBFA-1 within 8 d. Cells plated on laminin-5 for 16 d express increased levels of osteogenic marker genes, and those plated for 21 d deposit a mineralized matrix, indicative of osteogenic differentiation. Addition of the ERK inhibitor PD98059 mitigates these effects. We conclude that contact with laminin-5 is sufficient to activate ERK and to stimulate osteogenic differentiation in hMSC.

Laminin-induced signaling in tumor cells: the role of the M(r) 67,000 laminin receptor

The expression of the M(r) 67,000 laminin receptor, a nonintegrin laminin receptor, was found to be up-regulated in neoplastic cells and to directly correlate with invasion and metastatic potential. In the present study, we investigated the role of laminin receptor in mediating laminin effects and the involvement of the mitogen-activated protein kinases (MAPK) cascades and dual-specificity phosphatases in laminin signaling in human melanoma cells. Using stable transfection of A375SM melanoma cells, we established lines expressing reduced or elevated laminin receptor. The antisense-transfected cells demonstrated reduced attachment to laminin and reduced invasion through Matrigel-coated filters. In addition, both matrix metalloproteinase-2 (MMP-2) mRNA expression and activity were significantly reduced in the antisense-transfected cells. Antisense-transfected cells showed a reduction in mRNA level of the alpha6B integrin subunit isoform, whereas no change in the mRNA level of the alpha6A isoform was observed. We found that exogenous laminin reduced the phosphorylated (active) form of extracellular signal-regulated kinase, c-Jun NH(2)-terminal protein kinase, and p38 in all of the cells, irrespective of the expression of the laminin receptor. Furthermore, the phosphorylation of extracellular signal-regulated kinase, c-Jun NH(2)-terminal protein kinase, and p38 was significantly higher in the cell lines expressing reduced laminin receptor, regardless of the exposure to exogenous laminin. This increase of MAPK phosphorylation was accompanied by a significant reduction in MKP-1 phosphatase mRNA level and a significant increase in PAC-1 phosphatase mRNA level. In conclusion, our results confirm the involvement of the laminin receptor in different mechanisms related to tumor dissemination and provide first evidence of the involvement of MAPK and dual-specificity phosphatases in its signal transduction pathway.

Epithelial cell motility on laminin-5: regulation by matrix assembly, proteolysis, integrins and erbB receptors

Cell migration plays a central role in a wide variety of biological events, including embryogenesis, inflammatory immune response, wound healing, or cancer invasion. Tight regulation of cell motility is a prerequisite for normal development and maintenance of an organism, and to avoid metastatic spread of tumor cells. An important determinant of migratory efficiency is the substrate over which a cell migrates. Laminin-5 (Ln-5) is an extracellular matrix component prominent in basement membranes and as such it is a substrate in direct contact with epithelial cells. Interestingly, Ln-5 has been shown to both stimulate and downregulate epithelial cell migration. In this article, we plan to give an overview on the different mechanisms cells employ to regulate their migratory behavior on Ln-5. We will discuss how proteolytic processing of Ln-5 acts as posttranslational modification that plays a major role in the regulation of cell migration. The different proteolytic Ln-5 species may bind to distinct cell surface receptors called integrins, which translate substrate binding into a specific cellular response that triggers cell motility. Furthermore, interaction between Ln-5-binding integrins and other transmembrane and cytoplasmic proteins increases complexity and may allow fine-tuning of cell migration in response to the cellular environment.

Recognition of the Neural Chemoattractant Netrin-1 by Integrins α6β4 and α3β1 Regulates Epithelial Cell Adhesion and Migration

Netrins, axon guidance cues in the CNS, have also been detected in epithelial tissues. In this study, using the embryonic pancreas as a model system, we show that Netrin-1 is expressed in a discrete population of epithelial cells, localizes to basal membranes, and specifically associates with elements of the extracellular matrix. We demonstrate that alpha6beta4 integrin mediates pancreatic epithelial cell adhesion to Netrin-1, whereas recruitment of alpha6beta4 and alpha3beta1 regulate the migration of CK19+/PDX1+ putative pancreatic progenitors on Netrin-1. These results provide evidence for the activation of epithelial cell adhesion and migration by a neural chemoattractant, and identify Netrin-1/integrin interactions as adhesive/guidance cues for epithelial cells.

Integrins in epithelial cell polarity: using antibodies to analyze adhesive function and morphogenesis

Epithelial cells polarize in response to cell-substratum and cell-cell adhesive interactions. Contacts between cells and proteins of the extracellular matrix are mediated by integrin receptors. Of the 24 recognized integrin heterodimers, epithelial cells typically express four or more distinct integrins, with the exact complement dependent on the tissue of origin. Investigation of the roles of integrins in epithelial cell polarization has depended on the use of function-blocking antibodies both to determine ligand specificity of individual integrins and to disrupt and redirect normal morphogenesis. In this article we describe techniques for employing function-blocking anti-integrin antibodies in adhesion assays of the polarized Madin-Darby canine kidney (MDCK) cell line and to demonstrate the involvement of beta1 integrins in collagen-induced tubulocyst formation. These techniques can be easily expanded to other antibodies and epithelial cell lines to characterize specific functions of individual integrins in epithelial morphogenesis.

Cleavage of corneal basement membrane components by ethanol exposure in laser-assisted subepithelial keratectomy

PURPOSE

To determine the anatomic cleavage plane after exposure to 20% ethanol for approximately 20 to 25 seconds to create an epithelial flap in laser-assisted subepithelial keratectomy (LASEK).

SETTING

Ocular Surface Research & Education Foundation, Miami, Florida, and Hermann Eye Center Refractive Surgery Center, Houston, Texas, USA.

METHODS

Immunofluorescence staining using monoclonal antibodies against laminin 5, collagen VII, and integrins beta(1) and beta(4) was performed to determine the anatomic location of the cleavage plane in an epithelial flap created by 20-second exposure to 20% ethanol in cadaver eyes and in epithelial flaps obtained from LASEK patients.

RESULTS

Immunofluorescence staining to laminin 5 and integrin beta(4) was patchy in the lifted flap and the remaining corneal basement membrane. Immunostaining to collagen VII, the main component of anchoring fibrils, remained exclusively in the corneal bed. Immunostaining to integrin beta(1), present in the pericellular location of all epithelial cell layers, remained exclusively in the epithelial flap. This finding was consistent in cadaver corneas and LASEK epithelial flaps.

CONCLUSIONS

The cleavage plane of the ethanol-induced corneal epithelial flap is located between the lamina lucida and the lamina densa of the basement membrane, where integrin beta(4) interacts with laminin 5 to form hemidesmosomes.

Dysmyelinating sensory-motor neuropathy in merosin-deficient congenital muscular dystrophy

A 20-year-old man with mild myopathy, external ophthalmoparesis, epilepsy, and diffuse white matter hyperintensity in the brain on magnetic resonance imaging had partial merosin deficiency in muscle and absent merosin in the endoneurium. Motor and sensory nerve conduction velocities were slow. Nerve biopsy showed reduction of large myelinated fibers, short internodes, enlarged nodes, excessive variability of myelin thickness, tomacula, and uncompacted myelin, but no evidence of segmental demyelination, naked axons, or onion bulbs. Thus, in congenital muscular dystrophy, merosin expression may be dissociated in different tissues, and the neuropathy is sensory-motor and due to abnormal myelinogenesis.

Increased expression of integrin beta-4 in papillary thyroid carcinoma with gross lymph node metastasis

Although the prognosis is generally good for patients with papillary thyroid carcinoma, gross nodal metastasis of carcinoma has a poor prognosis. It is necessary to clarify how carcinoma progresses to gross nodal metastasis in order to establish a cure. The adhesion molecule integrin beta-4 is considered to be related to cell migration and metastasis in many carcinomas. In the present study, we examined integrin beta-4 expression in 65 cases of human papillary thyroid carcinoma using immunohistochemical methods. Expression of integrin beta-4 was found in all papillary carcinomas, but in few normal thyrocytes. Interestingly, integrin beta-4 expression in the carcinomas with gross (> or =3 cm) lymph node metastasis was significantly higher than that in carcinomas with small (<3 cm) or no lymph node metastasis. These results suggest that integrin beta-4 expression in thyroid carcinoma may play a role in the development of gross lymph node metastasis of papillary carcinomas.

EGF-R signaling through Fyn kinase disrupts the function of integrin alpha6beta4 at hemidesmosomes: role in epithelial cell migration and carcinoma invasion

We have examined the mechanism and functional significance of hemidesmosome disassembly during normal epithelial cell migration and squamous carcinoma invasion. Our findings indicate that a fraction of EGF receptor (EGF-R) combines with the hemidesmosomal integrin alpha6beta4 in both normal and neoplastic keratinocytes. Activation of the EGF-R causes tyrosine phosphorylation of the beta4 cytoplasmic domain and disruption of hemidesmosomes. The Src family kinase inhibitors PP1 and PP2 prevent tyrosine phosphorylation of beta4 and disassembly of hemidesmosomes without interfering with the activation of EGF-R. Coimmunoprecipitation experiments indicate that Fyn and, to a lesser extent, Yes combine with alpha6beta4. By contrast, Src and Lck do not associate with alpha6beta4 to a significant extent. A dominant negative form of Fyn, but not Src, prevents tyrosine phosphorylation of beta4 and disassembly of hemidesmosomes. These observations suggest that the EGF-R causes disassembly of hemidesmosomes by activating Fyn, which in turn phosphorylates the beta4 cytoplasmic domain. Neoplastic cells expressing dominant negative Fyn display increased hemidesmosomes and migrate poorly in vitro in response to EGF. Furthermore, dominant negative Fyn decreases the ability of squamous carcinoma cells to invade through Matrigel in vitro and to form lung metastases following intravenous injection in nude mice. These results suggest that disruption of hemidesmosomes mediated by Fyn is a prerequisite for normal keratinocyte migration and squamous carcinoma invasion.

Investigation into the mechanism of the loss of laminin 5 (alpha3beta3gamma2) expression in prostate cancer

Laminin 5 is a pivotal hemidesmosomal protein involved in cell stability, migration, and anchoring filament formation. Protein and gene expression of the alpha3, beta3, and gamma2 chains of laminin 5 were investigated in normal and invasive prostate carcinoma using immunohistochemistry, Northern analysis, and in situ hybridization. Laser capture microdissection of normal and carcinomatous glands, in conjunction with RNA amplification and reverse Northern analysis, were used to confirm the gene expression data. Protein and mRNA expression of all three laminin 5 chains were detected in the basal cells of normal glands. In contrast, invasive prostate carcinoma showed a loss of beta3 and gamma2 protein expression with variable expression of alpha3 chains. Despite the loss of protein expression, there was retention of beta3 and gamma2 mRNA expression as detected by in situ hybridization, Northern and reverse Northern analysis. Our findings imply that an altered mechanism of translation of beta3 or gamma2 mRNAs into functional proteins contributes to failure of anchoring filaments and hemidesmosomal formation. The resultant hemidesmosome instability or loss would suggest a less stable epithelial-stromal junction, increased invasion and migration of malignant cells, and disruption of normal integrin signaling pathways.

Tyrosine phosphorylation of the beta 4 integrin cytoplasmic domain mediates Shc signaling to extracellular signal-regulated kinase and antagonizes formation of hemidesmosomes

Ligation of the alpha(6)beta(4) integrin induces tyrosine phosphorylation of the beta(4) cytoplasmic domain, followed by recruitment of the adaptor protein Shc and activation of mitogen-activated protein kinase cascades. We have used Far Western analysis and phosphopeptide competition assays to map the sites in the cytoplasmic domain of beta(4) that are required for interaction with Shc. Our results indicate that, upon phosphorylation, Tyr(1440), or secondarily Tyr(1422), interacts with the SH2 domain of Shc, whereas Tyr(1526), or secondarily Tyr(1642), interacts with its phosphotyrosine binding (PTB) domain. An inactivating mutation in the PTB domain of Shc, but not one in its SH2 domain, suppresses the activation of Shc by alpha(6)beta(4). In addition, mutation of beta(4) Tyr(1526), which binds to the PTB domain of Shc, but not of Tyr(1422) and Tyr(1440), which interact with its SH2 domain, abolishes the activation of ERK by alpha(6)beta(4). Phenylalanine substitution of the beta(4) tyrosines able to interact with the SH2 or PTB domain of Shc does not affect incorporation of alpha(6)beta(4) in the hemidesmosomes of 804G cells. Exposure to the tyrosine phosphatase inhibitor orthovanadate increases tyrosine phosphorylation of beta4 and disrupts the hemidesmosomes of 804G cells expressing recombinant wild type beta(4). This treatment, however, exerts a decreasing degree of inhibition on the hemidesmosomes of cells expressing versions of beta(4) containing phenylalanine substitutions at Tyr(1422) and Tyr(1440), at Tyr(1526) and Tyr(1642), or at all four tyrosine phosphorylation sites. These results suggest that beta(4) Tyr(1526) interacts in a phosphorylation-dependent manner with the PTB domain of Shc. This event is required for subsequent tyrosine phosphorylation of Shc and signaling to ERK but not formation of hemidesmosomes.

True epithelial hyperplasia in the thymus of early-onset myasthenia gravis patients: implications for immunopathogenesis

The early-onset myasthenia gravis (EOMG) thymus shows characteristic medullary epithelial bands (MEB), greatly expanded perivascular infiltrates and fenestrations of the intervening basement membranes. We now compare epithelial expression of epidermal growth factor receptor (EGFR) and many integrins in EOMG and control samples. The main differences are striking/consistent thickening (in MEB) of what is normally a monolayer of perivascular epithelium, with focal protrusion into the infiltrates. This evidently hyperplastic epithelial subpopulation also strongly expresses EGFR and certain integrins. We suggest that its enhanced interactions with the locally increased extracellular matrix protein deposits may play an important role in autosensitization.

Differential expression of laminin alpha chains during proliferative and differentiation stages in a model for skin morphogenesis

The purpose of this study was to determine the mRNA and protein expression of laminin alpha chains at various stages of in vitro skin morphogenesis. Fibroblasts in mono-cultures express low levels of the mRNA of laminin alpha1,alpha2, alpha3 and alpha4 chains. When co-cultured with keratinocytes for 28 days, they expressed the mRNA for all these chains. Keratinocytes in monolayer expressed the laminin alpha3 chain mRNA and very low levels of the mRNA of the alpha1 and alpha2 chains, although, when recombined with fibroblasts they also expressed laminin alpha1and alpha2 mRNA, but not the laminin alpha4 mRNA. Immunocytochemistry of cells in co-culture showed that laminin alpha1, alpha3 and alpha5 chains were expressed in the epidermis, while the laminin alpha2, beta1, and gamma1 chains were noted in the dermis and at the epidermo-dermal interface. The laminin alpha1chain was first expressed during the proliferative stage (14-21 days) and the laminin alpha2 and alpha5 chains appeared later, during the differentiation stage (28-42 days). The above results suggest that epithelial-mesenchymal interactions are involved in the expression of laminin alpha chain mRNA during in vitro skin morphogenesis. In addition, there is distinct temporal and spatial expression of these chains during proliferative and differentiation stages, possibly reflecting different functions.

Cell migration and the anatomic control of thymocyte precursor differentiation

The thymus performs several essential functions during the steady-state production of T lymphocytes in adults, including expansion of the precursor pool, differentiation into multiple lineages and screening for TCRs with restricted specificities. Other than those functions attributed to the TCR, most of the factors that control these processes remain undefined. One potential mechanism for such control may be related to the movement of precursor cells between distinct anatomical compartments in the thymus. Histological studies show that the majority of CD4- CD8- cells are found in the subcapsular region. However; vascular tissues that support the migration of precursor cells into the thymus (postcapillary venules) are located deep in the tissue, near the cortico-medullary junction. This implies that blood-borne cells entering the thymus must transit outward across the cortex in order to accumulate in the SCR. Differentiation of DN cells into the CD4+ 8+ stage correlates with a reversal in polarity and migration inward, while mature cells ultimately transit the CMJ in the opposite direction of cells first entering the organ. Here we review evidence for a model in which differentiation is induced and proliferation is controlled by this progressive translocation of immature precursors through discrete stromal compartments. In addition, we attempt to summarize what is known about the molecular mechanisms that may support polarized migration of early CD4- 8- thymocytes in the adult, as well as how and where the relevant differentiative and/or proliferative signals may be compartmentalized.

Genetic analysis of integrin function in man: LAD-1 and other syndromes

The integrins are cell membrane receptors composed of alpha and beta subunits which orchestrate adhesive events in all tissues of the body (Hynes, R.O., 1992. Integrins: versatility, modulation, and signalling in cell adhesion. Cell 69, 11-25; and Hynes, R.O., 1999. Cell adhesion: old and new questions. Trends Cell Biol. 9, M33-37). At present 18 alpha subunits and 8 beta subunits have been identified which are loosely organised into families. There are three inherited autosomal recessive diseases in man which involve germline mutations in genes coding for integrins. Leukocyte adhesion deficiency-1 (LAD-1) is the result of mutations in the beta2 subunit of the CD11/CD18 integrins, LFA-1, Mac-1, p150,95 and alphadbeta2. The bleeding disorder Glanzmann thrombasthenia is caused by mutations in either the alpha or beta subunit of the platelet integrin, alphaIIbbeta3. Thirdly, it is now recognised than one of the variants of the usually lethal skin blistering disorder, epidermolysis bullosa (JEB-PA), is caused by mutation in either the alpha or beta subunit of the epithelial hemidesmosome integrin, alpha6beta4. Many of the mutations cause defective alphabeta heterodimer formation. The majority of the beta subunit mutations are in the conserved N-terminal region known as the betaI domain. It is suggested that this region participates in alphabeta heterodimer formation.

The N terminus of the transmembrane protein BP180 interacts with the N-terminal domain of BP230, thereby mediating keratin cytoskeleton anchorage to the cell surface at the site of the hemidesmosome

In epidermal cells, the keratin cytoskeleton interacts with the elements in the basement membrane via a multimolecular junction called the hemidesmosome. A major component of the hemidesmosome plaque is the 230-kDa bullous pemphigoid autoantigen (BP230/BPAG1), which connects directly to the keratin-containing intermediate filaments of the cytoskeleton via its C terminus. A second bullous pemphigoid antigen of 180 kDa (BP180/BPAG2) is a type II transmembrane component of the hemidesmosome. Using yeast two-hybrid technology and recombinant proteins, we show that an N-terminal fragment of BP230 can bind directly to an N-terminal fragment of BP180. We have also explored the consequences of expression of the BP230 N terminus in 804G cells that assemble hemidesmosomes in vitro. Unexpectedly, this fragment disrupts the distribution of BP180 in transfected cells but has no apparent impact on the organization of endogenous BP230 and alpha6beta4 integrin. We propose that the BP230 N terminus competes with endogenous BP230 protein for BP180 binding and inhibits incorporation of BP180 into the cell surface at the site of the hemidesmosome. These data provide new insight into those interactions of the molecules of the hemidesmosome that are necessary for its function in integrating epithelial and connective tissue types.

Cell-matrix adhesions differentially regulate fascin phosphorylation

Cell adhesion to individual macromolecules of the extracellular matrix has dramatic effects on the subcellular localization of the actin-bundling protein fascin and on the ability of cells to form stable fascin microspikes. The actin-binding activity of fascin is down-regulated by phosphorylation, and we used two differentiated cell types, C2C12 skeletal myoblasts and LLC-PK1 kidney epithelial cells, to examine the hypothesis that cell adhesion to the matrix components fibronectin, laminin-1, and thrombospondin-1 differentially regulates fascin phosphorylation. In both cell types, treatment with the PKC activator 12-tetradecanoyl phorbol 13-acetate (TPA) or adhesion to fibronectin led to a diffuse distribution of fascin after 1 h. C2C12 cells contain the PKC family members alpha, gamma, and lambda, and PKCalpha localization was altered upon cell adhesion to fibronectin. Two-dimensional isoelectric focusing/SDS-polyacrylamide gels were used to determine that fascin became phosphorylated in cells adherent to fibronectin and was inhibited by the PKC inhibitors calphostin C and chelerythrine chloride. Phosphorylation of fascin was not detected in cells adherent to thrombospondin-1 or to laminin-1. LLC-PK1 cells expressing green fluorescent protein (GFP)-fascin also displayed similar regulation of fascin phosphorylation. LLC-PK1 cells expressing GFP-fascin S39A, a nonphosphorylatable mutant, did not undergo spreading and focal contact organization on fibronectin, whereas cells expressing a GFP-fascin S39D mutant with constitutive negative charge spread more extensively than wild-type cells. In contrast, C2C12 cells coexpressing S39A fascin with endogenous fascin remained competent to form microspikes on thrombospondin-1, and cells that expressed fascin S39D attached to thrombospondin-1 but did not form microspikes. Blockade of PKCalpha activity by TPA-induced down-regulation led to actin association of wild-type fascin in fibronectin-adherent C2C12 and LLC-PK1 cells but did not alter the distribution of S39A or S39D fascins. The association of fascin with actin in fibronectin-adherent cells was also evident in the presence of an inhibitory antibody to integrin alpha5 subunit. These novel results establish matrix-initiated PKC-dependent regulation of fascin phosphorylation at serine 39 as a mechanism whereby matrix adhesion is coupled to the organization of cytoskeletal structure.

Binding of integrin alpha6beta4 to plectin prevents plectin association with F-actin but does not interfere with intermediate filament binding

Hemidesmosomes are stable adhesion complexes in basal epithelial cells that provide a link between the intermediate filament network and the extracellular matrix. We have investigated the recruitment of plectin into hemidesmosomes by the alpha6beta4 integrin and have shown that the cytoplasmic domain of the beta4 subunit associates with an NH(2)-terminal fragment of plectin that contains the actin-binding domain (ABD). When expressed in immortalized plectin-deficient keratinocytes from human patients with epidermol- ysis bullosa (EB) simplex with muscular dystrophy (MD-EBS), this fragment is colocalized with alpha6beta4 in basal hemidesmosome-like clusters or associated with F-actin in stress fibers or focal contacts. We used a yeast two-hybrid binding assay in combination with an in vitro dot blot overlay assay to demonstrate that beta4 interacts directly with plectin, and identified a major plectin-binding site on the second fibronectin type III repeat of the beta4 cytoplasmic domain. Mapping of the beta4 and actin-binding sites on plectin showed that the binding sites overlap and are both located in the plectin ABD. Using an in vitro competition assay, we could show that beta4 can compete out the plectin ABD fragment from its association with F-actin. The ability of beta4 to prevent binding of F-actin to plectin explains why F-actin has never been found in association with hemidesmosomes, and provides a molecular mechanism for a switch in plectin localization from actin filaments to basal intermediate filament-anchoring hemidesmosomes when beta4 is expressed. Finally, by mapping of the COOH-terminally located binding site for several different intermediate filament proteins on plectin using yeast two-hybrid assays and cell transfection experiments with MD-EBS keratinocytes, we confirm that plectin interacts with different cytoskeletal networks.

Expression of functionally distinct variants of the beta(4)A integrin subunit in relation to the differentiation state in human intestinal cells

Integrins are important mediators of cell-laminin interactions. In the small intestinal epithelium, which consists of spatially separated proliferative and differentiated cell populations located, respectively, in the crypt and on the villus, laminins and laminin-binding integrins are differentially expressed along the crypt-villus axis. One exception to this is the integrin alpha(6)beta(4), which is thought to be ubiquitously expressed by intestinal cells. However, in this study, a re-evaluation of the beta(4) subunit expression with different antibodies revealed that two forms of beta(4) exist in the human intestinal epithelium. Furthermore, we show that differentiated enterocytes express a full-length 205-kDa beta(4)A subunit, whereas undifferentiated crypt cells express a novel beta(4)A subunit that does not contain the COOH-terminal segment of the cytoplasmic domain (beta(4)A(ctd-)). This new form was not found to arise from alternative beta(4) mRNA splicing. Moreover, we found that these two beta(4)A forms can associate into alpha(6)beta(4)A complexes; however, the beta(4)A(ctd-) integrin expressed by the undifferentiated crypt cells is not functional for adhesion to laminin-5. Hence, these studies identify a novel alpha(6)beta(4)A(ctd-) integrin expressed in undifferentiated intestinal crypt cells that is functionally distinct.

The beta4 integrin subunit rescues A431 cells from apoptosis through a PI3K/Akt kinase signaling pathway

To study whether alpha6beta4 integrin regulates apoptosis, human A431 cells were plated on bacteria plates in the presence or absence of mAb beta4. In the absence of mAb beta4, A431 cells demonstrated morphological characteristics of apoptosis by 24 h and most cells died by 48 h. In contrast, in the presence of mAb beta4, cells remained viable, and at the end of 48 h, 70-80% of cells survived. Treatment of A431 cells with mAb beta4 resulted in tyrosine phosphorylation of the p85 subunit of PI3 kinase; PI3 kinase activity increased within 15 min and peaked at 60 min. Stimulation of beta4 in A431 cells resulted in a time-dependent phosphorylation of Akt with a concomitant and parallel phosphorylation of Bad. Inactivation of PI3 kinase with inhibitors blocked the anti-apoptotic effect induced by mAb beta4. These are the first results to suggest that ligation of alpha6beta4 integrin protects cells from apoptosis through a PI3K/Akt kinase signaling pathway.

Division of labor among the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor to signal morphogenesis and beta-casein expression in mammary epithelial cells

Contact of cultured mammary epithelial cells with the basement membrane protein laminin induces multiple responses, including cell shape changes, growth arrest, and, in the presence of prolactin, transcription of the milk protein beta-casein. We sought to identify the specific laminin receptor(s) mediating the multiple cell responses to laminin. Using assays with clonal mammary epithelial cells, we reveal distinct functions for the alpha6beta4 integrin, beta1 integrins, and an E3 laminin receptor. Signals from laminin for beta-casein expression were inhibited in the presence of function-blocking antibodies against both the alpha6 and beta1 integrin subunits and by the laminin E3 fragment. The alpha6-blocking antibody perturbed signals mediated by the alpha6beta4 integrin, and the beta1-blocking antibody perturbed signals mediated by another integrin, the alpha subunit(s) of which remains to be determined. Neither alpha6- nor beta1-blocking antibodies perturbed the cell shape changes resulting from cell exposure to laminin. However, the E3 laminin fragment and heparin both inhibited cell shape changes induced by laminin, thereby implicating an E3 laminin receptor in this function. These results elucidate the multiplicity of cell-extracellular matrix interactions required to integrate cell structure and signaling and ultimately permit normal cell function.

Significant role of laminin-1 in branching morphogenesis of mouse salivary epithelium cultured in basement membrane matrix

Mouse submandibular epithelium shows branching morphogenesis in mesenchyme-free conditions when covered with a basement membrane matrix (Matrigel) in medium supplemented with epidermal growth factor. In the present study, the role of laminin-1 (LN1), a major glycoprotein of Matrigel, in this culture system was defined. When the epithelium was cultured in a LN1-nidogen gel, the epithelium showed much branching, comparable to that observed with Matrigel. By electron microscopy, only a felt-like matrix was formed on the epithelial surface in the LN1-nidogen gel cultures, while an organized basal lamina structure was formed on the epithelial surface in direct or transfilter recombination cultures with mesenchyme. Next, the epithelium covered with Matrigel was cultured in medium containing either biologically active peptides from LN1, IKVAV-including peptide (2097-2108), AG10 (2183-2194), AG32 (2370-2381) or AG73 (2719-2730) from the alpha1 chain, or YIGSR-including peptide (926-933) from the beta1 chain. Only AG73 (RKRLQVQLSIRT from the alpha1 chain carboxyl-terminal globular domain) inhibited the epithelial branching in Matrigel. These results suggest that LN1-nidogen can support the branching morphogenesis of submandibular epithelium even if LN1-nidogen is not assembled into an intact basal lamina, and that the AG73 sequence is an important site on LN1, which interacts with submandibular epithelial cells.

Structure and function of hemidesmosomes: more than simple adhesion complexes

The attachment of cells to the extracellular matrix is of crucial importance in the maintenance of tissue structure and integrity. In stratified epithelia such as in skin as well as in other complex epithelia multiprotein complexes called hemidesmosomes are involved in promoting the adhesion of epithelial cells to the underlying basement membrane. In the past few years our understanding of the role of hemidesmosomes has improved considerably. Their importance has become apparent in clinical conditions, in which absence or defects of hemidesmosomal proteins result in devastating blistering diseases of the skin. Molecular genetic studies have increased our knowledge of the function of the various components of hemidesmosomes and enabled the characterization of protein-protein interactions involved in their assembly. It has become clear that the alpha6beta4 integrin, a major component of hemidesmosomes, is able to transduce signals from the extracellular matrix to the interior of the cell, that critically modulate the organization of the cytoskeleton, proliferation, apoptosis, and differentiation. Nevertheless, our knowledge of the mechanisms regulating the functional state of hemidesmosomes and, hence, the dynamics of cell adhesion, a process of crucial importance in development, wound healing or tumor invasion, remains limited. The aims of this review are to highlight the recent progresses of our knowledge on the organization and assembly of hemidesmosomes, their involvement in signaling pathways as well as their participation in clinical pathologic conditions.