Structure and regulation of the envoplakin gene

Envoplakin Inhibits Macrophage Polarization by Altering the Inflammatory Tumor Microenvironment of Melanoma Through the RAS / ERK Signaling Pathway

Purpose

Tumor growth induces the tumor margin to become a transition zone rich in immune cells. EVPL is a potential prognostic biomarker for melanoma. Melanoma is difficult to cure because of its high metastasis, so it is urgent to find effective genes to inhibit tumor progression and regulate tumor microenvironment.

Methods

Firstly, differentially expressed genes (DEGs) among normal skin, nevus and melanoma samples in GSE3189 were screened. Bioinformatics was used to further explore the hub genes and enriched pathways closely related to the inflammatory response of DEGs in melanoma. We selected EVPL, which is associated with the Ras/Raf signaling pathway, for in vitro study. CCK-8, colony formation, wound healing, Transwell and flow cytometry assays were respectively used to evaluate the proliferation, migration, invasion, and apoptosis of cancer cells. Enzyme-linked immunosorbent assay was conducted for the monitoring of changes in the tumor microenvironment. To evaluate the effect of EVPL on macrophage recruitment, we established a co-culture system in a Transwell chamber. The polarization of macrophages was examined after treatment of cells with RAS/ERK signaling inhibitors SCH772984 and sh-EVPL. Additionally, changes in the expression of pathway proteins were measured by Western blot.

Results

Among the screened hub genes, EVPL was associated with the Ras/Raf pathway, a key signaling pathway in melanoma, and may be involved in regulating the inflammatory microenvironment of melanoma. Oe-EVPL was proved to suppress melanoma cell malignant progression. By inhibiting EVPL expression, the inhibitory effects on melanoma progression induced by the addition of SCH772984 were reversed. Furthermore, EVPL was found to inhibit the expression of chemokines, the recruitment of macrophages, and the polarization of macrophages through the Ras/Raf/ERK signaling pathway.

Conclusion

EVPL can inhibit the progression of melanoma through the RAS/ERK signaling pathway, change the inflammatory tumor microenvironment of melanoma, and inhibit the recruitment of macrophages.

Mechanism of intermediate filament recognition by plakin repeat domains revealed by envoplakin targeting of vimentin

Plakin proteins form critical connections between cell junctions and the cytoskeleton; their disruption within epithelial and cardiac muscle cells cause skin-blistering diseases and cardiomyopathies. Envoplakin has a single plakin repeat domain (PRD) which recognizes intermediate filaments through an unresolved mechanism. Herein we report the crystal structure of envoplakin's complete PRD fold, revealing binding determinants within its electropositive binding groove. Four of its five internal repeats recognize negatively charged patches within vimentin via five basic determinants that are identified by nuclear magnetic resonance spectroscopy. Mutations of the Lys1901 or Arg1914 binding determinants delocalize heterodimeric envoplakin from intracellular vimentin and keratin filaments in cultured cells. Recognition of vimentin is abolished when its residues Asp112 or Asp119 are mutated. The latter slot intermediate filament rods into basic PRD domain grooves through electrosteric complementarity in a widely applicable mechanism. Together this reveals how plakin family members form dynamic linkages with cytoskeletal frameworks.

Plakin proteins link cell junctions to cytoskeletal frameworks, and their disruption within epithelial and cardiac muscle cells cause skin blistering diseases and cardiomyopathies. Here the authors use structural biology approaches to reveal the mechanism that allows plakins to recognize intermediate filaments within the cytoskeleton.

Desmosomes: regulators of cellular signaling and adhesion in epidermal health and disease

Desmosomes are intercellular junctions that mediate cell-cell adhesion and anchor the intermediate filament network to the plasma membrane, providing mechanical resilience to tissues such as the epidermis and heart. In addition to their critical roles in adhesion, desmosomal proteins are emerging as mediators of cell signaling important for proper cell and tissue functions. In this review we highlight what is known about desmosomal proteins regulating adhesion and signaling in healthy skin-in morphogenesis, differentiation and homeostasis, wound healing, and protection against environmental damage. We also discuss how human diseases that target desmosome molecules directly or interfere indirectly with these mechanical and signaling functions to contribute to pathogenesis.

Transgenic overexpression of cdx1b induces metaplastic changes of gene expression in zebrafish esophageal squamous epithelium

Cdx2 has been suggested to play an important role in Barrett's esophagus or intestinal metaplasia (IM) in the esophagus. To investigate whether transgenic overexpression of cdx1b, the functional equivalent of mammalian Cdx2 in zebrafish, may lead to IM of zebrafish esophageal squamous epithelium, a transgenic zebrafish system was developed by expressing cdx1b gene under the control of zebrafish keratin 5 promoter (krt5p). Gene expression in the esophageal squamous epithelium of wild-type and transgenic zebrafish was analyzed by Affymetrix microarray and confirmed by in situ hybridization. Morphology, mucin expression, cell proliferation, and apoptosis were analyzed by hematoxylin & eosin (HE) staining, Periodic acid Schiff (PAS) Alcian blue staining, proliferating cell nuclear antigen (PCNA) immunohistochemical staining, and TUNEL assay as well. cdx1b was found to be overexpressed in the nuclei of esophageal squamous epithelial cells of the transgenic zebrafish. Ectopic expression of cdx1b disturbed the development of this epithelium in larval zebrafish and induced metaplastic changes in gene expression in the esophageal squamous epithelial cells of adult zebrafish, that is, up-regulation of intestinal differentiation markers and down-regulation of squamous differentiation markers. However, cdx1b failed to induce histological IM, or to modulate cell proliferation and apoptosis in the squamous epithelium of adult transgenic zebrafish.

Oxidation and Nitrosylation of Cysteines Proximal to the Intermediate Filament (IF)-binding Site of Plectin

As an intermediate filament (IF)-based cytolinker protein, plectin plays a key role in the maintenance of cellular cytoarchitecture and serves at the same time as a scaffolding platform for signaling cascades. Consisting of six structural repeats (R1-6) and harboring binding sites for different IF proteins and proteins involved in signaling, the plectin C-terminal domain is of strategic functional importance. Depending on the species, it contains at least 13 cysteines, 4 of which reside in the R5 domain. To investigate the structural and biological functions of R5 cysteines, we used cysteine-to-serine mutagenesis and spectroscopic, biochemical, and functional analyses. Urea-induced unfolding experiments indicated that wild-type R5 in the oxidized, disulfide bond-mediated conformation was more stable than its cysteine-free mutant derivative. The binding affinity of R5 for vimentin was significantly higher, however, when the protein was in the reduced, more relaxed conformation. Of the four R5 cysteines, one (Cys4) was particularly reactive as reflected by its ability to form disulfide bridges with R5 Cys1 and to serve as a target for nitrosylation in vitro. Using immortalized endothelial cell cultures from mice, we show that endogenous plectin is nitrosylated in vivo, and we found that NO donor-induced IF collapse proceeds dramatically faster in plectin-deficient compared with wild-type cells. Our data suggest an antagonistic role of plectin in nitrosylation (oxidative stress)-mediated alterations of IF cytoarchitecture and a possible role of R5 Cys4 as a regulatory switch.

Differential recognition of response elements determines target gene specificity for p53 and p63

p63 is a member of the p53 tumor suppressor gene family, which regulates downstream target gene expression by binding to sequence-specific response elements similar to those of p53. By using oligonucleotide expression microarray analysis and analyzing the promoters of p63-induced genes, we have identified novel p63-specific response elements (p63-REs) in the promoter regions of EVPL and SMARCD3. These p63-REs exhibit characteristic differences from the canonical p53-RE (RRRCWWGYYY) in both the core-binding element (CWWG) as well as the RRR and/or YYY stretches. Luciferase assays on mutagenized promoter constructs followed by electromobility shift analysis showed that p53 preferentially activates and binds to the RRRCATGYYY sequence, whereas p63 preferentially activates RRRCGTGYYY. Whereas EVPL protein is highly expressed in epithelial cells of the skin and pharynx in the p63+/+ mouse, it is undetectable in these tissues in the p63-/- mouse. Our results indicate that p63 can regulate expression of specific target genes such as those involved in skin, limb, and craniofacial development by preferentially activating distinct p63-specific response elements.

Co-assembly of Envoplakin and Periplakin into Oligomers and Ca2+-dependent Vesicle Binding

Plakin family members envoplakin and periplakin have been shown to be part of the cornified cell envelope in terminally differentiating stratified squamous epithelia. In the present study, purified recombinant human envoplakin and periplakin were used to investigate their properties and interactions. We found that envoplakin was insoluble at physiological conditions in vitro, and co-assembly with periplakin was required for its solubility. Envoplakin and periplakin formed soluble complexes with equimolar stoichiometry. Chemical cross-linking revealed that the major soluble form of all periplakin constructs and of envoplakin/periplakin rod domains was a dimer, although co-assembly of the full-length proteins resulted in formation of higher order oligomers. Electron microscopy of rotary-shadowed periplakin demonstrated thin flexible molecules with an average contour length of 88 nm for the rod-plus-tail fragment, and immunolabeling EM confirmed the molecule as a parallel, in-register, dimer. Both periplakin and envoplakin/periplakin oligomers were able to bind synthetic lipid vesicles whose composition mimicked the cytoplasmic side of the plasma membrane of eukaryotic cells. This binding was dependent on anionic phospholipids and Ca(2+). These findings raise the possibility that envoplakin and periplakin bind to the plasma membrane upon elevation of intracellular [Ca(2+)] in differentiating keratinocytes, where they serve as a scaffold for cornified cell envelope assembly.

A distal region of the human TGM1 promoter is required for expression in transgenic mice and cultured keratinocytes

Background

TGM1(transglutaminase 1) is an enzyme that crosslinks the cornified envelope of mature keratinocytes. Appropriate expression of the TGM1 gene is crucial for proper keratinocyte function as inactivating mutations lead to the debilitating skin disease, lamellar ichthyosis. TGM1 is also expressed in squamous metaplasia, a consequence in some epithelia of vitamin A deficiency or toxic insult that can lead to neoplasia. An understanding of the regulation of this gene in normal and abnormal differentiation states may contribute to better disease diagnosis and treatment.

Methods

In vivo requirements for expression of the TGM1 gene were studied by fusing various lengths of promoter DNA to a reporter and injecting the DNA into mouse embryos to generate transgenic animals. Expression of the reporter was ascertained by Western blotting and immunohistochemistry. Further delineation of a transcriptionally important distal region was determined by transfections of progressively shortened or mutated promoter DNA into cultured keratinocytes.

Results

In vivo analysis of a reporter transgene driven by the TGM1 promoter revealed that 1.6 kilobases, but not 1.1 kilobases, of DNA was sufficient to confer tissue-specific and cell layer-specific expression. This same region was responsible for reporter expression in tissues undergoing squamous metaplasia as a response to vitamin A deprivation. Mutation of a distal promoter AP1 site or proximal promoter CRE site, both identified as important transcriptional elements in transfection assays, did not prevent appropriate expression. Further searching for transcriptional elements using electrophoretic mobility shift (EMSA) and transfection assays in cultured keratinocytes identified two Sp1 elements in a transcriptionally active region between -1.6 and -1.4 kilobases. While mutation of either Sp1 site or the AP1 site singly had only a small effect, mutation of all three sites eliminated nearly all the transcriptional activity.

Conclusions

A distal region of the TGM1 gene promoter, containing AP1 and Sp1 binding sites, is evolutionarily conserved and responsible for high level expression in transgenic mice and in transfected keratinocyte cultures.

Kruppel-like factors regulate the Lama1 gene encoding the laminin alpha1 chain

Laminin-1 (alpha1beta1gamma1), a basement membrane (BM) constituent, has been associated with differentiation processes and also with malignant progression. In the intestinal tissue, the alpha1 chain is expressed and secreted in the subepithelial BM during the developmental period; in the adult rodent tissue, it is restricted to the BM of the dividing cells. To understand how laminin alpha1 chain expression is regulated, we cloned and characterized a 2-kb promoter region of the Lama1 mouse gene. Analysis of the promoter was conducted in the Caco2-TC7 intestinal epithelial cells by transient transfection of serially deleted and site-directed mutated promoter constructs, by electrophoretic mobility shift assays, and expression of selected transcription factors. We determined that a proximal region, which includes an Sp1-binding GC box and a Krüppel-like element, was important for the promoter activity. This region is conserved between the human and mouse genes. Interestingly, two Krüppel-like factors KLF4 and KLF5 exhibit opposing effects on the Lama1 promoter activity that are decreased and increased, respectively, in the intestinal epithelial cells. These data corroborate the complementary expression of KLF4 and KLF5 along the intestinal crypt-villus axis and the parallel expression of KLF5 and laminin alpha1 chain in the crypt region. Finally, we showed that glucocorticoids stimulate the promoter activity. This study is the first characterization of the Lama1 promoter; we identified regulatory elements that may account for the expression pattern of the endogenous protein in the mouse intestine.

Interaction of periplakin and envoplakin with intermediate filaments

Periplakin is a component of desmosomes and the epidermal cornified envelope. Its N-terminal domain interacts with the plasma membrane; it heterodimerises with envoplakin via its rod domain; and its C-terminus interacts with intermediate filaments. Periplakin has the shortest C-terminus of the plakin family, comprising only the linker domain found in all conventional plakins. By transient transfection of COS7 cells and primary human epidermal keratinocytes with deletion mutants of the periplakin C-terminus we mapped sequences required for intermediate filament interaction to two regions of the linker motif that are most highly conserved amongst the plakins. The results were confirmed by overlay assays of the binding of in vitro translated periplakin constructs to keratins and vimentin. We found that envoplakin and periplakin could still associate with each other when parts of their rod domains were deleted and, surprisingly, that removal of the entire rod domain did not completely inhibit their interaction. Co-transfection of constructs containing the C-termini of envoplakin and periplakin suggested that the periplakin C-terminus may stabilise the interaction of the envoplakin C-terminus with intermediate filaments. We conclude that the periplakin C-terminus plays an important role in linking periplakin and envoplakin to intermediate filaments.

Suprabasin, a novel epidermal differentiation marker and potential cornified envelope precursor

The suprabasin gene is a novel gene expressed in mouse and human differentiating keratinocytes. We identified a partial cDNA encoding suprabasin using a suppression subtractive hybridization method between the proliferative basal and differentiating suprabasal populations of the mouse epidermis. A 3' gene-specific probe hybridized to transcripts of 0.7- and 2.2-kb pairs on Northern blots with specific detection in differentiated keratinocytes of stratified epithelia. The mouse gene was mapped to chromosome 7 by fluorescence in situ hybridization. This region is syntenic to human chromosome band 19q13.1, which contained the only region in the data bases with homology to the mouse suprabasin sequence. During embryonic mouse development, suprabasin mRNA was detected at day 15.5, coinciding with epidermal stratification. Suprabasin was detected in the suprabasal layers of the epithelia in the tongue, stomach, and epidermis. Differentiation of cultured primary epidermal keratinocytes with 0.12 mm Ca(2+) or 12-O-tetradecanoylphorbol-13-acetate treatment resulted in the induction of suprabasin. The 2.2-kb cDNA transcript encodes a protein of 72 kDa with a predicted isoelectric point of 6.85. The translated sequence has an amino-terminal domain, a central domain composed of repeats rich in glycine and alanine, and a carboxyl-terminal domain. The alternatively spliced 0.7-kb transcript encodes a smaller protein that shares the NH(2)- and COOH-terminal regions but lacks the repeat domain region. Cross-linking experiments indicate that suprabasin is a substrate for transglutaminase 2 and 3 activity. Altogether, these results indicate that the suprabasin protein potentially plays a role in the process of epidermal differentiation.

Novel alternative splicings of BPAG1 (bullous pemphigoid antigen 1) including the domain structure closely related to MACF (microtubule actin cross-linking factor)

BPAG1 (bullous pemphigoid antigen 1) was originally identified as a 230-kDa hemidesmosomal protein and belongs to the plakin family, because it consists of a plakin domain, a coiled-coil rod domain and a COOH-terminal intermediate filament binding domain. To date, alternatively spliced products of BPAG1, BPAG1e, and BPAG1n are known. BPAG1e is expressed in epithelial tissues and localized to hemidesmosomes, on the other hand, BPAG1n is expressed in neural tissues and muscles and has an actin binding domain at the NH(2)-terminal of BPAG1e. BPAG1 is also known as a gene responsible for Dystonia musculorum (dt) neurodegeneration syndrome of the mouse. Another plakin family protein MACF (microtubule actin cross-linking factor) has also an actin binding domain and the plakin domain at the NH(2)-terminal. However, in contrast to its high homology with BPAG1 at the NH(2)-terminal, the COOH-terminal structure of MACF, including a microtubule binding domain, resembles dystrophin rather than plakins. Here, we investigated RNAs and proteins expressed from the BPAG1 locus and suggest novel alternative splicing variants, which include one consisting of the COOH-terminal domain structure homologous to MACF. The results indicate that BPAG1 has three kinds of cytoskeletal binding domains and seems to play an important role in linking the different types of cytoskeletons.

Gene targeting of envoplakin, a cytoskeletal linker protein and precursor of the epidermal cornified envelope

Envoplakin, a member of the plakin family of cytoskeletal linker proteins, is localized in desmosomes of stratified epithelial cells and is a component of the epidermal cornified envelope. Gene targeting in mouse embryonic stem cells was used to generate a null allele of envoplakin. No envoplakin transcripts from the targeted allele could be detected in the skin of newborn mice. Mice homozygous for the targeted allele were born in the normal Mendelian ratio and were fertile. They did not develop any discernible pathological phenotype up to the age of 1 year. The ultrastructural appearance of cornified envelopes from adult epidermis was indistinguishable between wild-type and knockout mice, and there was no evidence that the absence of envoplakin affected the subcellular distribution of periplakin or desmoplakin, two other plakins found in desmosomes. The proportion of immature cornified envelopes in the epidermis of newborn mice was greater in envoplakin-null animals than in heterozygous littermates or wild-type mice, and the envelopes had a larger surface area. This correlated with a slight delay in barrier acquisition during embryonic development. We conclude that although envoplakin is part of the scaffolding on which the cornified envelope is assembled, it is not essential for envelope formation or epidermal barrier function.

Promoter characterization of the human and mouse epilysin (MMP-28) genes

Epilysin (MMP-28) is a recently cloned member of the matrix metalloproteinase family (Lohi et al., J. Biol. Chem. 276 (2001) 10134). It is expressed at highest levels in the skin by basal and suprabasal keratinocytes, and in testis by developing germ cells. To characterize the epilysin promoter, we isolated a 3.0 kb fragment of human genomic DNA containing 5'-flanking sequence of the epilysin gene, and a corresponding 660 bp fragment from the mouse. The 5'-flanking sequences contain no typical TATA-boxes or CCAAT sequences close to the translation initiation sites. RNase protection assay revealed that two transcription start sites are utilized in the human epilysin gene, situated 210 and 230 bp upstream from the translation start site. The promoter contains a GT-box, situated 300 bp upstream from the translation start site, with homology to the consensus binding site for transcription factors of the Sp family. This site is perfectly conserved between the human and mouse promoters. For reporter gene assays a series of constructs with fragments of increasing length of the epilysin promoter were coupled to the firefly luciferase gene. Reporter gene assays indicated that deletion or mutation of the GT-box dramatically reduces the transcriptional activity both in keratinocytes and in spermatogonia. Gel mobility shift assays showed that several nuclear proteins bind specifically to this sequence. Supershift assays with antibodies specific for members of the Sp family identified Sp1 and Sp3 as components of these protein/DNA complexes and hence as possible regulators of the epilysin gene. Our results indicate that the epilysin promoter has distinctive structural and functional features, which may control the unique expression and regulation patterns of the epilysin gene.

Identification and dissection of an enhancer controlling epithelial gene expression in skin

Keratins 14 and 5 are the structural hallmarks of the basal keratinocytes of the epidermis and outer root sheath (ORS) of the hair follicle. Their genes are controlled in a tissue-specific manner and thus serve as useful tools to elucidate the regulatory mechanisms involved in keratinocyte-specific transcription. Previously we identified several keratinocyte-specific DNase I hypersensitive sites (HSs) in the 5' regulatory sequences of the K14 gene and showed that a 700-bp regulatory domain encompassing HSs II and III can confer epidermal and ORS-specific gene expression in transgenic mice in vivo. Although HS II harbored much of the transactivation activity in vitro, it was not sufficient to restrict expression to keratinocytes in vivo. We now explore the HS III regulatory element. Surprisingly, this element on its own confers gene expression to the keratinocytes of the inner root sheath (IRS) of the hair follicle, whereas a 275-bp DNA fragment containing both HSs II and III shifts the expression from the IRS to the basal keratinocytes and ORS in vivo. Electrophoretic mobility-shift assays and mutational studies of HSs III reveal a role for CACCC-box binding proteins, Sp1 family members, and other factors adding to the list of previously described factors that are involved in keratinocyte-specific gene expression. These studies highlight a cooperative interaction of the two HSs domains and strengthen the importance of combinatorial play of transcription factors that govern keratinocyte-specific gene regulation.

Subcellular distribution of envoplakin and periplakin: insights into their role as precursors of the epidermal cornified envelope

Envoplakin and periplakin are two plakins that are precursors of the epidermal cornified envelope. We studied their distribution and interactions by transfection of primary human keratinocytes and other cells. Full-length periplakin localized to desmosomes, the interdesmosomal plasma membrane and intermediate filaments. Full length envoplakin also localized to desmosomes, but mainly accumulated in nuclear and cytoplasmic aggregates with associated intermediate filaments. The envoplakin rod domain was required for aggregation and the periplakin rod domain was necessary and sufficient to redistribute envoplakin to desmosomes and the cytoskeleton, confirming earlier predictions that the proteins can heterodimerize. The linker domain of each protein was required for intermediate filament association. Like the NH(2) terminus of desmoplakin, that of periplakin localized to desmosomes; however, in addition, the periplakin NH(2) terminus accumulated at cell surface microvilli in association with cortical actin. Endogenous periplakin was redistributed from microvilli when keratinocytes were treated with the actin disrupting drug Latrunculin B. We propose that whereas envoplakin and periplakin can localize independently to desmosomes, the distribution of envoplakin at the interdesmosomal plasma membrane depends on heterodimerization with periplakin and that the NH(2) terminus of periplakin therefore plays a key role in forming the scaffold on which the cornified envelope is assembled.