Klf4 is a transcription factor required for establishing the barrier function of the skin

Nuclear localization of KLF4 is associated with an aggressive phenotype in early-stage breast cancer

PURPOSE

The Krüppel-like transcription factor KLF4/GKLF induces both malignant transformation and a slow-growth phenotype in vitro. Although KLF4 expression is increased in most cases of breast cancer, it was unknown whether these cases represent a distinct subtype with a different clinical outcome.

EXPERIMENTAL DESIGN

We examined expression of KLF4 by immunostaining 146 cases of human primary infiltrating ductal carcinoma of the breast. Staining patterns were correlated with clinical outcome and with established prognostic factors.

RESULTS

Subcellular localization exhibited case-to-case variation. Tumors with high nuclear staining and low cytoplasmic staining were termed type 1. For patients with early-stage disease (i.e., stage I or IIA), type 1 staining was associated with eventual death because of breast cancer (hazard ratio, 2.8; 95% confidence interval, 1.23-6.58; P = 0.011). The association was stronger in patients with early-stage cancer and small primary tumors (i.e., < or =2.0 cm in diameter; hazard ratio, 4.3; 95% confidence interval, 1.75-10.62; P < 0.001). For patients with early-stage disease, multivariate analysis indicated that type 1 staining was independently associated with outcome (adjusted hazard ratio 2.6; 95% confidence interval, 1.10-6.05; P = 0.029). Type 1 staining was also associated with high histological grade (P = 0.032), increased expression of Ki67 (P = 0.016), and reduced expression of BCL2 (P = 0.032). In vitro, KLF4 was localized within the nucleus of transformed RK3E epithelial cells, consistent with a nuclear function of this transcription factor during induction of malignant transformation.

CONCLUSIONS

The results suggest that localization of KLF4 in the nucleus of breast cancer cells is a prognostic factor and identify KLF4 as a marker of an aggressive phenotype in early-stage infiltrating ductal carcinoma.

Developmental diethylstilbestrol exposure alters genetic pathways of uterine cytodifferentiation

The formation of a simple columnar epithelium in the uterus is essential for implantation. Perturbation of this developmental process by exogenous estrogen, such as diethylstilbestrol (DES), results in uterine metaplasia that contributes to infertility. The cellular and molecular mechanism underlying this transformation event is not well understood. Here we use a combination of global gene expression analysis and a knockout mouse model to delineate genetic pathways affected by DES. Global gene expression profiling experiment revealed that neonatal DES treatment alters uterine cell fate, particularly in the luminal epithelium by inducing abnormal differentiation, characterized by the induction of stratified epithelial markers including members of the small proline-rich protein family and epidermal keratins. We show that Msx2, a homeodomain transcription factor, functions downstream of DES and is required for the proper expression of several genes in the uterine epithelium including Wnt7a, PLAP, and K2.16. Finally, Msx2-/- uteri were found to exhibit abnormal water trafficking upon DES exposure, demonstrating the importance of Msx2 in tissue responsiveness to estrogen exposure. Together, these results indicate that developmental exposure to DES can perturb normal uterine development by affecting genetic pathways governing uterine differentiation.

Lethal, neonatal ichthyosis with increased proteolytic processing of filaggrin in a mouse model of Netherton syndrome

Netherton syndrome is an autosomal recessive multisystemic disorder characterized by congenital ichthyosiform erythroderma, hair shaft defects and atopy, caused by mutations within the human SPINK5 gene. To investigate the development of this disease, we have cloned mouse spink5 and created mice with a mutated premature stop codon at amino acid R820X, to produce an allele that closely mimics a point mutation (E827X) in human SPINK5. Newborn spink5(R820X/R820X) mice develop a lethal, severe ichthyosis with a loss of skin barrier function and dehydration, resulting in death within a few hours of birth, similar to that observed in patients with severe Netherton syndrome. Epidermal barrier function is compromised because of the stratum corneum becoming spontaneously detached in the newborn mice, and this is probably compounded by the reduced mechanical strength detected in the cornified envelopes. Biochemical analysis of skin from newborn wild-type and spink5(R820X/R820X) mice revealed a substantial increase in the proteolytic processing of profilaggrin into its constituent filaggrin monomers. Filaggrin functions to organize keratin filaments into highly ordered macrofibrils that crisscross the cornified cells of the stratum corneum imparting structural integrity, and defects in filaggrin processing occur in a number of forms of congenital ichthyosis. These data suggest that in the absence of the serine protease inhibitor spink5, there is an abnormal increase in the processing of profilaggrin, resulting in an overabundance of filaggrin monomers, and that this may play a direct role in the observed deficit in the adhesion of the stratum corneum and the severely compromised epidermal barrier function.

Pax9 is required for filiform papilla development and suppresses skin-specific differentiation of the mammalian tongue epithelium

The epidermis is a derivative of the surface ectoderm. It forms a protective barrier and specific appendages including hair, nails, and different eccrine glands. The surface ectoderm also forms the epithelium of the oral cavity and tongue, which develop a slightly different barrier and form different appendages such as teeth, filiform papillae, taste papillae, and salivary glands. How this region-specific differentiation is genetically controlled is largely unknown. We show here that Pax9, which is expressed in the epithelium of the tongue but not in skin, regulates several aspects of tongue-specific epithelial differentiation. In Pax9-deficient mice filiform papillae lack the anterior-posterior polarity, a defect that is associated with temporal-spatial changes in Hoxc13 expression. Barrier formation is disturbed in the mutant tongue and genome-wide expression profiling revealed that the expression of specific keratins (Krt), keratin-associated proteins, and members of the epidermal differentiation complex is significantly down-regulated. In situ hybridization demonstrated that several 'hard' keratins, Krt1-5, Krt1-24, and Krt2-16, are not expressed in the absence of Pax9. Notably, specific 'soft' keratins, Krt2-1 and Krt2-17, normally weakly expressed in the tongue but present at high levels in skin and in orthokeratinized oral dysplasia are up-regulated in the mutant tongue epithelium. This result indicates a partial trans-differentiation to an epithelium with skin-specific characteristics. Together, our findings show that Pax9 regulates appendage formation in the mammalian tongue and identify Pax9 as an important factor for the region-specific differentiation of the surface ectoderm.

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Identification of Aberrantly Methylated Genes in Association with Adult T-Cell Leukemia

In this study, we identified 53 aberrantly hypermethylated DNA sequences in adult T-cell leukemia (ATL) cells using methylated CpG island amplification/representational difference analysis method. We also observed a proportionate increase in the methylation density of these regions with disease progression. Seven genes, which were expressed in normal T cells, but suppressed in ATL cells, were identified near the hypermethylated regions. Among these silenced genes, Kruppel-like factor 4 (KLF4) gene is a cell cycle regulator and early growth response 3 (EGR3) gene is a critical transcriptional factor for induction of Fas ligand (FasL) expression. Treatment with 5-aza-2'-deoxycytidine resulted in the recovery of their transcription, indicating that their silencing might be associated with DNA hypermethylation. To study their functions in ATL cells, we transfected recombinant adenovirus vectors expressing KLF4 and EGR3 genes. Expression of KLF4 induced apoptosis of ATL cells whereas enforced expression of EGR3 induced the expression of FasL gene, resulting in apoptosis. Thus, suppressed expression of EGR3 enabled ATL cells to escape from activation-induced cell death mediated by FasL. Our results showed that the methylated CpG island amplification/representational difference analysis method allowed the isolation of hypermethylated DNA regions specific to leukemic cells and thus shed light on the roles of DNA methylation in leukemogenesis.

Complex redundancy to build a simple epidermal permeability barrier

To survive the transition from an aqueous in utero to a terrestrial ex utero environment, mice and humans must construct an epidermal permeability barrier in utero. Terminally differentiated epidermal cells, lipids and tight junctions are all essential to achieve this barrier. Recent analyses of mouse mutants with defects in structural components of the terminally differentiated epidermal cell, catalyzing enzymes, lipid processing, transcriptional regulators and the intercellular junctions have highlighted their essential function in establishing the epidermal permeability barrier. Particularly interesting examples include modulation of the expression of transglutaminase 1 enzyme, the transcription factor Klf4 and the claudin tight junction proteins. However, careful analysis of the various mutant phenotypes during embryonic development, as neonates and either as adults or transplanted skin, has revealed much more about the redundancy and compensatory mechanisms of the system. Molecular analysis of the various mouse mutants has demonstrated common pathways to compensate for loss of the epidermal barrier.

Transgenic mouse models support HCR as an effector gene in the PSORS1 locus

Genetic susceptibility for psoriasis is regulated to the greatest extent by the PSORS1 locus. Three psoriasis-associated susceptibility alleles have been identified within it, namely, HLACw6, HCR*WWCC and CDSN*5, but strong linkage disequilibrium between them has made it difficult to distinguish their individual genetic effects, and animal models to study their effects are not known. To study the function of HCR, we engineered transgenic mice with either a non-risk allele of HCR or the HCR*WWCC risk allele under the control of the cytokeratin-14 promoter. These choices were motivated by the apparently dominant effect of PSORS1 on psoriasis susceptibility and the physiological expression of HCR in basal keratinocytes. Transgenic mice appeared phenotypically normal and histologically their skin was indistinguishable from wild-type mice. Expression studies using Affymetrix arrays suggested that the HCR risk allele has specific functional consequences relevant to the pathogenesis of psoriasis. Comparison of gene expression changes between non-risk and risk allele mice revealed similarities to previous observations in human psoriatic skin, including upregulation of cytokeratins 6, 16 and 17 in risk allele mice. We also observed changes in the expression of genes associated with terminal differentiation and formation of the cornified cell envelope. Our results support the concept that HCR may constitute an essential gene in the PSORS1 locus. These observations are also compatible with a model that a susceptibility gene for psoriasis induces changes that are contributory but not sufficient by itself to produce the clinical phenotype.

AP-2 transcription factor family member expression, activity, and regulation in human epidermal keratinocytes in vitro

The AP-2 transcription factor family is presumed to play an important role in the regulation of the keratinocyte squamous differentiation program; however, limited functional data are available to support this. In the present study, the activity and regulation of AP-2 were examined in differentiating human epidermal keratinocytes. We report that (1) AP-2 transcriptional activity decreases in differentiated keratinocytes but remains unchanged in differentiation-insensitive squamous cell carcinoma cell lines, (2) diminished AP-2 transcriptional activity is associated with a loss of specific DNA-bound AP-2 complexes, and (3) there is an increase in the ability of cytoplasmic extracts, derived from differentiated keratinocytes, to phosphorylate AP-2 alpha and AP-2 beta when cells differentiate. In contrast, extracts from differentiation-insensitive squamous cell carcinoma cells are unable to phosphorylate AP-2 proteins. Finally, the phosphorylation of recombinant AP-2 alpha by cytosolic extracts from differentiated keratinocytes is associated with decreased AP-2 DNA-binding activity. Combined, these data indicate that AP-2 trans-activation and DNA-binding activity decrease as keratinocytes differentiate, and that this decreased activity is associated with an enhanced ability to phosphorylate AP-2 alpha and beta.

KLF2 Is a novel transcriptional regulator of endothelial proinflammatory activation

The vascular endothelium is a critical regulator of vascular function. Diverse stimuli such as proinflammatory cytokines and hemodynamic forces modulate endothelial phenotype and thereby impact on the development of vascular disease states. Therefore, identification of the regulatory factors that mediate the effects of these stimuli on endothelial function is of considerable interest. Transcriptional profiling studies identified the Kruppel-like factor (KLF)2 as being inhibited by the inflammatory cytokine interleukin-1β and induced by laminar shear stress in cultured human umbilical vein endothelial cells. Overexpression of KLF2 in umbilical vein endothelial cells robustly induced endothelial nitric oxide synthase expression and total enzymatic activity. In addition, KLF2 overexpression potently inhibited the induction of vascular cell adhesion molecule-1 and endothelial adhesion molecule E-selectin in response to various proinflammatory cytokines. Consistent with these observations, in vitro flow assays demonstrate that T cell attachment and rolling are markedly attenuated in endothelial monolayers transduced with KLF2. Finally, our studies implicate recruitment by KLF2 of the transcriptional coactivator cyclic AMP response element–binding protein (CBP/p300) as a unifying mechanism for these various effects. These data implicate KLF2 as a novel regulator of endothelial activation in response to proinflammatory stimuli.

The Kruppel-like KLF4 Transcription Factor, a Novel Regulator of Urokinase Receptor Expression, Drives Synthesis of This Binding Site in Colonic Crypt Luminal Surface Epithelial Cells

The urokinase-type plasminogen activator receptor (u-PAR) plays a central role in cell migration, growth, and invasion and is regulated, in part, transcriptionally. In mice, u-PAR expression is restricted to a few tissues, one of which is the colon. We therefore screened a colon expression library for regulators of u-PAR promoter activity and identified a zinc finger protein bearing consensus sequences to the Kruppel-like family of transcription factors and showing partial homology with one of the members, KLF4. Like u-PAR, KLF4 expression is predominant in the luminal surface epithelial cells of the colonic crypt, and we hypothesized that u-PAR synthesis in these cells is directed by this transcription factor. Colon cells from KLF4 null mice showed a dramatic reduction in u-PAR protein compared with wild-type mice. Conversely, KLF4 expression in HCT116 colon cancer cells increased the amount of u-PAR protein/mRNA. Transient transfection of KLF4 with a reporter driven by 5'-deleted u-PAR promoter fragments indicated the requirement of the proximal 200 base pairs for optimal expression. Mobility-shifting experiments demonstrated binding of KLF4 to multiple regions of the u-PAR promoter (-154/-128, -105/-71, and -51/-24), and chromatin immunoprecipitation assays confirmed the binding of KLF4 to the endogenous promoter. Deletion of the -144/-123 promoter region diminished but did not eliminate the ability of KLF4 to transactivate the u-PAR promoter, suggesting cooperativity of these binding sites with respect to activation of gene expression. In conclusion, we have identified KLF4 as a novel regulator of u-PAR expression that drives the synthesis of u-PAR in the luminal surface epithelial cells of the colon.

Discovery of Ca2+-relevant and differentiation-associated genes downregulated in esophageal squamous cell carcinoma using cDNA microarray

To identify genes that are differentially expressed in human esophageal squamous cell carcinoma (ESCC), we have developed a cDNA microarray representing 34 176 clones to analyse gene expression profiles in ESCC. A total of 77 genes (including 31 novel genes) were downregulated, and 15 genes (including one novel gene) were upregulated in cancer tissues compared with their normal counterparts. Immunohistochemistry and Northern blot analysis were carried out to verify the cDNA microarray results. It was revealed that genes involved in squamous cell differentiation were coordinately downregulated, including annexin I, small proline-rich proteins (SPRRs), calcium-binding S100 proteins (S100A8, S100A9), transglutaminase (TGM3), cytokeratins (KRT4, KRT13), gut-enriched Krupple-like factor (GKLF) and cystatin A. Interestingly, most of the downregulated genes encoded Ca(2+)-binding or -modulating proteins that constitute the cell envelope (CE). Moreover, genes associated with invasion or proliferation were upregulated, including genes such as fibronectin, secreted protein acidic and rich in cystein (SPARC), cathepsin B and KRT17. Functional analysis of the alteration in the expression of GKLF suggested that GKLF might be able to regulate the expression of SPRR1A, SPRR2A and KRT4 in ESCC. This study provides new insights into the role of squamous cell differentiation-associated genes in ESCC initiation and progression.

Identification of MoKA, a novel F-box protein that modulates Krüppel-like transcription factor 7 activity

KLF7, a member of the Krüppel-like transcription factor family, is believed to regulate neurogenesis and cell cycle progression. Here, a yeast two-hybrid screen for KLF7 cofactors in the developing nervous system identified a novel 140-kDa protein named MoKA, for modulator of KLF7 activity. Interaction between MoKA and KLF7 was confirmed by the in vitro glutathione S-transferase pull-down assay and by coimmunoprecipitation of the proteins overexpressed in mammalian cells. Functional assays documented that MoKA is a KLF7 coactivator, and in situ hybridizations identified the developing nervous system and the adult testes as two sites of MoKA and Klf7 coexpression. Chromatin immunoprecipitation experiments demonstrated KLF7 binding to the p21(WAF1/Cip1) gene while transient transfection assays documented KLF7 stimulation of the p21(WAF1/Cip1) proximal promoter. Additional tests revealed that distinct structural motifs of MoKA direct interaction with KLF7 and shuttling between the nucleus and cytoplasm of asynchronously cycling cells. Altogether, our results strongly suggest that MoKA and KLF7 interact functionally to regulate gene expression during cell differentiation and identify the cell cycle regulator p21(WAF1/Cip1) as one of the targeted genes.

Loss of proteolytically processed filaggrin caused by epidermal deletion of Matriptase/MT-SP1

Profilaggrin is a large epidermal polyprotein that is proteolytically processed during keratinocyte differentiation to release multiple filaggrin monomer units as well as a calcium-binding regulatory NH₂-terminal filaggrin S-100 protein. We show that epidermal deficiency of the transmembrane serine protease Matriptase/MT-SP1 perturbs lipid matrix formation, cornified envelope morphogenesis, and stratum corneum desquamation. Surprisingly, proteomic analysis of Matriptase/MT-SP1–deficient epidermis revealed the selective loss of both proteolytically processed filaggrin monomer units and the NH₂-terminal filaggrin S-100 regulatory protein. This was associated with a profound accumulation of profilaggrin and aberrant profilaggrin-processing products in the stratum corneum. The data identify keratinocyte Matriptase/MT-SP1 as an essential component of the profilaggrin-processing pathway and a key regulator of terminal epidermal differentiation.

Conditional targeting of E-cadherin in skin: insights into hyperproliferative and degenerative responses

Loss of E-cadherin has been associated with human cancers, and yet in the early mouse embryo and the lactating mammary gland, the E-cadherin null state results in tissue dysfunction and cell death. Here we targeted loss of E-cadherin in skin epithelium. The epidermal basal layer responded by elevating P-cadherin, enabling these cells to maintain adherens junctions. Suprabasal layers upregulated desmosomal cadherins, but without classical cadherins, terminal differentiation was impaired. Progressive hyperplasia developed with age, a possible consequence of proliferative maintenance in basal cells coupled with defects in terminal differentiation. In contrast, hair follicles lost integrity of the inner root sheath and hair cuticle without apparent elevation of cadherins. These findings suggest that, if no compensatory mechanisms exist, E-cadherin loss may be incompatible with epithelial tissue survival, whereas partial compensation can result in alterations in differentiation and proliferation.

Enterocyte differentiation marker intestinal alkaline phosphatase is a target gene of the gut-enriched Kruppel-like factor

We have examined the role that the transcription factor gut-enriched Krüppel-like factor (KLF4 or GKLF) plays in activating the enterocyte differentiation marker gene intestinal alkaline phosphatase (IAP). A yeast one-hybrid screen was used to identify proteins interacting with a previously identified cis-element (IF-III) located within the human IAP gene promoter. DNA-protein interactions were determined by using EMSA. Northern blot analysis was used to study RNA expression in human colon cancer RKO cells engineered to overexpress KLF4. Transient transfections with IAP-luciferase reporter constructs were used to characterize the mechanisms by which KLF4 activates IAP transcription. The yeast one-hybrid screen and EMSA identified KLF4 as binding to IF-III. RKO cells induced to overexpress KLF4 demonstrated a corresponding dose-dependent increase in IAP expression, and EMSA with nuclear extract from these cells confirmed that KLF4 binds to the IF-III element. Transient transfections revealed that KLF4 transactivated the IAP gene largely via a critical segment in the IAP promoter that includes the IF-III cis-element. Mutant KLF4 constructs failed to fully activate IAP. We have identified the enterocyte differentiation marker IAP as a KLF4 target gene. IAP transactivation by KLF4 is likely mediated through a critical region located within the proximal IAP promoter region.

A transforming growth factor-beta control element required for SM alpha-actin expression in vivo also partially mediates GKLF-dependent transcriptional repression

We previously demonstrated that a conserved transforming growth factor-beta control element (TCE) within the 5'-region of the smooth muscle cell (SMC) differentiation marker gene SM alpha-actin could mediate both transcriptional activation and repression in cultured SMCs through interaction with members of the zinc finger Kruppel-like transcription factor (KLF) family. The aims of the present studies were to: 1) determine the role of the SM alpha-actin TCE in vivo through mutagenesis studies in transgenic mice and 2) further characterize the possible role and mechanisms by which the TCE-binding factor GKLF/KLF4 induces repression of SMC marker genes in various SMC model systems in vitro. Our results showed that the TCE was required for SM alpha-actin promoter activity in transgenic mice in vivo. Results of transient transfection studies showed that GKLF-induced repression of a SM alpha-actin promoter/luciferase reporter gene partially depended on the TCE. Furthermore, a GKLF overexpressing adenovirus inhibited whereas GKLF morpholino antisense oligos increased expression of endogenous SMC marker genes. Results of chromatin immunoprecipitation assays showed GKLF binding to TCE containing regions of various SMC marker gene promoters within intact chromatin. Finally, results of co-transfection studies showed that overexpression of IKLF/KLF5 reversed GKLF-dependent repression thus supporting a model of reciprocal activation-repression of SMC gene expression by different members of the KLF gene family.

Identification of the Drosophila progenitor of mammalian Krüppel-like factors 6 and 7 and a determinant of fly development

The Krüppel-like transcription factors (KLFs) represent a family of 15 different zinc finger proteins of the C(2)H(2) type that are involved in vertebrate development and which control cell proliferation, growth and differentiation. Structural-functional considerations have segregated KLF6 and KLF7 into a phylogenetically distinct group. Here we report the identification of Luna, the Drosophila progenitor of the mammalian KLF6/KLF7 group. This conclusion is based on the near sequence identity, as well as the comparable location of the DNA-binding domains and nuclear localization signals of the insect and mammalian proteins. The homology extends to the composition and function of the amino-terminal segment of Luna which, similarly to the mammalian counterparts, stimulates transcription in a reporter gene assay. We also present preliminary in vivo evidence of Luna involvement in embryonic development and cell differentiation. First, luna RNA interference and luna overexpression during early Drosophila embryogenesis leads to developmental arrest at different embryonic stages. Second, targeted perturbation of luna expression in the forming compound eye interferes with terminal cell differentiation, but not cell specification. We therefore propose that Luna is a novel transcriptional determinant of Drosophila development.

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.

The Krüppel-like factor epiprofin is expressed by epithelium of developing teeth, hair follicles, and limb buds and promotes cell proliferation

We identified a cDNA clone for epiprofin, which is preferentially expressed in teeth, by differential hybridization using DNA microarrays from an embryonic day 19.5 mouse molar cDNA library. Sequence analysis revealed that this cDNA encodes a member of the Krüppel-like factor family containing three characteristic C2H2-type zinc finger motifs. The full-length cDNA was obtained by the 5' Cap capture method. Except for its 5'-terminal sequence, the epiprofin mRNA sequence is almost identical to the predicted sequence of Krüppel-like factor 14/Sp6 (specificity protein 6), which was previously identified in expressed sequence tag data bases and GenBank by an Sp1 zinc finger DNA-binding domain search (Scohy, S., Gabant, P., Van Reeth, T., Hertveldt, V., Dreze, P. L., Van Vooren, P., Riviere, M., Szpirer, J., and Szpirer, C. (2000) Genomics 70, 93-101). This sequence difference is due to differences in the assignment of the location of exon 1. In situ hybridization revealed that epiprofin mRNA is expressed by proliferating dental epithelium, differentiated odontoblast, and also hair follicle matrix epithelium. In addition, whole mount in situ hybridization showed transient expression of epiprofin mRNA in cells of the apical ectodermal ridge in developing limbs and the posterior neuropore. Transfection of an epiprofin expression vector revealed that this molecule is localized in the nucleus and promotes cell proliferation. Thus, epiprofin is a highly cell- and tissue-specific nuclear protein expressed primarily by proliferating epithelial cells of teeth, hair follicles, and limbs that may function in the development of these tissues by regulating cell growth.

Identification of tissue-specific genes in nasopharyngeal epithelial tissue and differentially expressed genes in nasopharyngeal carcinoma by suppression subtractive hybridization and cDNA microarray

Suppression subtractive hybridization (SSH) was performed for isolation of tissue-specific genes in nasopharyngeal epithelial tissue, by use of cDNAs from human adult nasopharyngeal epithelial tissue as tester and mixed cDNAs from esophagus, lung, liver, heart, stomach, spleen, skeletal muscle, kidney, and skin as drivers. Fourteen differentially expressed genes in nasopharyngeal epithelial tissue were obtained. Among these genes, LPLUNC1 and SPLUNC1 were confirmed to be specifically expressed in nasopharyngeal epithelial tissue and the trachea. A novel transcript of SPLUNC1, which we designate NASG, was found. We also combined SSH and cDNA microarray hybridization to identify genes whose expressions were altered in nasopharyngeal carcinoma (NPC). We used NPC cell line HNE1 and primary human embryo nasopharyngeal epithelial cells in one SSH experiment, and NPC biopsies and normal adult nasopharyngeal epithelial tissue in another. Some 1,200 SSH inserts from four subtractive cDNA libraries were arrayed onto nylon membranes by use of robotic printing. Differential gene expression was verified by hybridizing of the membranes with radioactively labeled first-strand cDNA from NPC cell line HNE1, primary human embryo nasopharyngeal epithelial cells, NPC biopsies, and normal adult nasopharyngeal epithelial tissue. Seventeen differentially expressed genes in NPC were obtained. Among these genes, we identified SPLUNC1 and LPLUNC1 to be down-expressed in NPC biopsies (34/48, 33/48).

GATA-3: an unexpected regulator of cell lineage determination in skin

Multipotent skin stem cells give rise to epidermis and its appendages, including the hair follicle. The Lef-1/Tcf family of Wnt-regulated transcription factors plays a major role in specification of the hair shaft, but little is known about how the equally important hair channel, the inner root sheath (IRS), develops in concert to shape and guide the hair. In a microarray screen to search for transcriptional regulators of hair follicle morphogenesis, we identified GATA-3, a key regulator of T-cell lineage determination. Surprisingly, this transcription factor is essential for stem cell lineage determination in skin, where it is expressed at the onset of epidermal stratification and IRS specification in follicles. GATA-3-null/lacZ knock-in embryos can survive up to embryonic day 18.5 (E18.5), when they fail to form the IRS. Skin grafting unveiled additional defects in GATA-3-null hairs and follicles. IRS progenitors failed to differentiate, whereas cortical progenitors differentiated, but produced an aberrant hair structure. Curiously, some GATA-3-null progenitor cells expressed mixed IRS and hair shaft markers. Taken together, these findings place GATA-3 with Lef-1/Wnts at the crossroads of the IRS versus hair shaft cell fate decision in hair follicle morphogenesis. This newfound function for GATA-3 in skin development strengthens the parallels between the differentiation programs governing hair follicle and lymphocyte differentiation.

Downregulation and growth inhibitory effect of epithelial-type Krüppel-like transcription factor KLF4, but not KLF5, in bladder cancer

Krüppel-like factors (KLFs) are key transcriptional regulators of cell differentiation and proliferation. Among the KLF family, the expression of KLF4 (GKLF) and KLF5 (IKLF) is highly restricted in the epithelial cells of several organs such as the gut and skin, and it has been reported that these epithelial-type KLF genes may be involved in colon carcinogenesis. Recently we found that Klf4 and Klf5 genes were significantly expressed in the developmental bladder epithelium of mice as well. Therefore, in this report we studied the involvement of the KLF4 and KLF5 genes in bladder carcinogenesis. First, we analyzed the expression of KLF4 and KLF5 in a variety of human bladder cancer cell lines and surgical specimens by RNA blot and in situ hybridization analyses. Both genes were highly expressed in the normal bladder epithelium, whereas KLF4, but not KLF5, was frequently downregulated in bladder cancer cell lines and cancer tissues. We then transduced the KLF4 and KLF5 genes into the bladder cancer cell lines using adenoviral vectors to examine the biological activities of the genes on those cells. The transduction of KLF4, but not KLF5, suppressed cell growth and induced apoptosis. Our study suggests that inactivation of KLF4 is one of the frequent steps towards bladder carcinogenesis.

Unique Keratinocyte-Specific Effects of Interferon-γ that Protect Skin from Viruses, Identified Using Transcriptional Profiling

Interferon (IFN)-γ, is a multifunctional, immunomodulatory cytokine with cell type-specific antiviral activities, particularly important in skin, where it is implicated in many diseases ranging from warts to psoriasis and cancer. Since epidermis is our first line of defence against many viruses, we investigated the molecular processes regulated by IFN-γ in keratinocytes using DNA microarrays. We identified the IFN-γ-regulated keratinocyte-specific genes and antiviral processes. Exclusively in keratinocytes, IFN-γ-induced tight junction proteins, presumably to deny viruses paracellular routes of infection. Furthermore, differing from published data, we find that IFN-γ suppressed the expression of keratinocytes differentiation markers including desmosomal proteins, cornified envelope components and suprabasal cytokeratins. Inhibition of differentiation may interfere with the epidermal tropism of viruses that require differentiating cells for growth, for example, papillomaviruses. As in other cell types, IFN-γ induced HLA, cell adhesion and proteasome proteins, facilitating leukocyte attraction and antigen-presentation by keratinocytes. IFN-γ also induced chemokine/cytokines specific for mononuclear cells. IFN-γ suppressed the expression of over 100 genes responsible for cell cycle, DNA replication and RNA metabolism, thereby shutting down many nuclear processes and denying viruses a healthy cell in which to replicate. Thus, uniquely in keratinocytes, IFN-γ initiates a well-organized molecular programme boosting host antiviral defences, obstructing viral entry, suppressing cell proliferation and impeding differentiation.

Dynamic expression of Krüppel-like factor 4 (Klf4), a target of transcription factor AP-2alpha during murine mid-embryogenesis

Krüppel-like factor 4 (Klf4) belongs to the family of transcription factors that are thought to be involved in the regulation of epithelial and germ cell differentiation, based on their expression in postproliferative cells of the skin, gut, and testes. Gene ablation experiments suggest that Klf4 plays a role in keratinocyte differentiation, since mice lacking Klf4 fail to establish proper barrier function and, as a consequence, die postnatally due to dehydration. Recent studies have shown that Klf4 is also expressed in postnatal male mice, in postmeiotic sperm cells undergoing terminal differentiation into sperm cells. However, prior to the current study, the expression pattern of Klf4 during early and mid-embryogenesis had not been examined. Here we demonstrate that Klf4 transcripts can be detected from embryonic day 4.5 (E4.5) on in the developing conceptus, and that Klf4 expression before E10 is restricted to extraembryonic tissues. The embryo proper displays a highly dynamic and changing Klf4 signal from E10 of murine development on. In addition to being expressed in a stripe of mesenchymal cells extending from the forelimb bud rostrally over the branchial arches to the developing eye, Klf4 is also expressed in the mesenchyme surrounding the nasal pit at day E11.5. In addition, Klf4 has been detected in the apical ectodermal ridge and adjacent mesenchymal cells in the limb buds, and in mesenchymal cells of the developing body wall in trunk areas. These findings suggest that Klf4 plays an important role in regulating cellular proliferation, which underlies the morphogenetic changes that shape the developing embryo.

Barrier function of the skin: "la raison d'être" of the epidermis

The primary function of the epidermis is to produce the protective, semi-permeable stratum corneum that permits terrestrial life. The barrier function of the stratum corneum is provided by patterned lipid lamellae localized to the extracellular spaces between corneocytes. Anucleate corneocytes contain keratin filaments bound to a peripheral cornified envelope composed of cross-linked proteins. The many layers of these specialized cells in the stratum corneum provide a tough and resilient framework for the intercellular lipid lamellae. The lamellae are derived from disk-like lipid membranes extruded from lamellar granules into the intercellular spaces of the upper granular layer. Lysosomal and other enzymes present in the extracellular compartment are responsible for the lipid remodeling required to generate the barrier lamellae as well as for the reactions that result in desquamation. Lamellar granules likely originate from the Golgi apparatus and are currently thought to be elements of the tubulo-vesicular trans-Golgi network. The regulation of barrier lipid synthesis has been studied in a variety of models, with induction of several enzymes demonstrated during fetal development and keratinocyte differentiation, but an understanding of this process at the molecular genetic level awaits further study. Certain genetic defects in lipid metabolism or in the protein components of the stratum corneum produce scaly or ichthyotic skin with abnormal barrier lipid structure and function. The inflammatory skin diseases psoriasis and atopic dermatitis also show decreased barrier function, but the underlying mechanisms remain under investigation. Topically applied "moisturizers" work by acting as humectants or by providing an artificial barrier to trans-epidermal water loss; current work has focused on developing a more physiologic mix of lipids for topical application to skin. Recent studies in genetically engineered mice have suggested an unexpected role for tight junctions in epidermal barrier function and further developments in this area are expected. Ultimately, more sophisticated understanding of epidermal barrier function will lead to more rational therapy of a host of skin conditions in which the barrier is impaired.

Comparative proteomic profiling of murine skin

Mammalian skin is regularly exposed to different environmental stresses, each of which results in specific compensatory changes in protein expression that can be assessed by proteomic analysis. We have established a reference proteome map of BALB/c murine skin allowing the resolution of greater than 500 protein spots in a single two-dimensional polyacrylamide gel. Forty-four protein spots, corresponding to 28 different cutaneous proteins, were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and the Mascot online database searching algorithm. Twenty-five proteins were expressed at higher levels in the epidermis, whereas only nine were found predominantly in the subepidermal tissues. A subset of protein spots exhibited strain-specific expression. Proteins of diverse function were identified, including those involved in stress response, apoptosis, growth inhibition, the maintenance of structural integrity, translational control, energy metabolism, calcium binding, cholesterol transport, and the scavenging of free radicals. Prohibitin expression was detected cutaneously, with more abundant protein and mRNA levels in the epidermis. Five molecular chaperones including protein di-sulfide isomerase, 78 kDa glucose-regulated protein precursor, heat shock protein 60 (HSP60), HSP70, and HSP27 were also identified. Of these, HSP27 expression was confined mainly to the epidermis, and expression of protein disulfide isomerase was found primarily in the subepidermal tissues. Proteomic analysis of skin following heat or cold shock resulted in increased levels of HSP27, HSP60, and HSP70 suggesting involvement of these chaperones in the cutaneous response mechanism to temperature stress. These data establish numerous reference markers within the proteome map of murine skin and provide an important framework for future efforts aimed at characterization of the epidermal and subepidermal responses to environmental changes.

Mice with targeted disruption of the fatty acid transport protein 4 (Fatp 4, Slc27a4) gene show features of lethal restrictive dermopathy

The fatty acid transport protein family is a group of evolutionarily conserved proteins that are involved in the cellular uptake and metabolism of long and very long chain fatty acids. However, little is known about their respective physiological roles. To analyze the functional significance of fatty acid transport protein 4 (Fatp4, Slc27a4), we generated mice with a targeted disruption of the Fatp4 gene. Fatp4-null mice displayed features of a neonatally lethal restrictive dermopathy. Their skin was characterized by hyperproliferative hyperkeratosis with a disturbed epidermal barrier, a flat dermal-epidermal junction, a reduced number of pilo-sebaceous structures, and a compact dermis. The rigid skin consistency resulted in an altered body shape with facial dysmorphia, generalized joint flexion contractures, and impaired movement including suckling and breathing deficiencies. Lipid analysis demonstrated a disturbed fatty acid composition of epidermal ceramides, in particular a decrease in the C26:0 and C26:0-OH fatty acid substitutes. These findings reveal a previously unknown, essential function of Fatp4 in the formation of the epidermal barrier.

Laser capture microdissection-based in vivo genomic profiling of wound keratinocytes identifies similarities and differences to squamous cell carcinoma

Keratinocytes undergo a dramatic phenotypic conversion during reepithelialization of skin wounds to become hyperproliferative, migratory, and invasive. This transient healing response phenotypically resembles malignant transformation of keratinocytes during squamous cell carcinoma progression. Here we present the first analysis of global changes in keratinocyte gene expression during skin wound healing in vivo, and compare these changes to changes in gene expression during malignant conversion of keratinized epithelium. Laser capture microdissection was used to isolate RNA from wound keratinocytes from incisional mouse skin wounds and adjacent normal skin keratinocytes. Changes in gene expression were determined by comparative cDNA array analyses, and the approach was validated by in situ hybridization. The analyses identified 48 candidate genes not previously associated with wound reepithelialization. Furthermore, the analyses revealed that the phenotypic resemblance of wound keratinocytes to squamous cell carcinoma is mimicked at the level of gene expression, but notable differences between the two tissue-remodeling processes were also observed. The combination of laser capture microdissection and cDNA array analysis provides a powerful new tool to unravel the complex changes in gene expression that underlie physiological and pathological remodeling of keratinized epithelium.

Ectopic expression of kruppel like factor 4 (Klf4) accelerates formation of the epidermal permeability barrier

Dysfunction of the epidermal permeability barrier can result in dehydration, electrolyte imbalance and poor thermoregulation. Immature skin is a portal of entry for infectious agents and potential toxins in topically applied substances. As the skin is one of the last organs to mature in utero, premature infants born before 34 weeks gestation are at great risk for complications. The transcription factor kruppel-like factor 4 (Klf4), has been shown by a targeted ablation, to have an essential function in barrier acquisition. We investigated whether Klf4 expression in utero is sufficient to establish the epidermal barrier. Specifically, we generated lines of mice that express Klf4 from a tetracycline inducible promoter when crossed with transgenic mice expressing the tetracycline transactivator tTA from the epidermal keratin 5 promoter. These mice exhibit acceleration in barrier acquisition as manifest by the exclusion of a dye solution one day earlier in development than controls. Underlying this dye impermeability are morphological changes, including an increased number of stratified layers, expression of terminal differentiation markers and assembly of cornified envelopes. By all criteria, Klf4 ectopic expression accelerates the normal process of terminal differentiation. Premature barrier acquisition in these mice follows the normal pattern rather than the pattern of the transgene promoter, indicating that there are fields of competence in which KLF4 acts. Although other transgenic mice have perturbed barrier acquisition, these mice are the first to accelerate the process of barrier establishment. These studies show that KLF4 regulates barrier acquisition and provides an animal model for studying how to accelerate the process of barrier acquisition for the premature infant.

Overexpression of the calcium sensing receptor accelerates epidermal differentiation and permeability barrier formation in vivo

The calcium sensing receptor (CaSR) has emerged as an important mediator of a wide range of Ca(2+)-dependent physiological responses (Ca(2+) signaling) in various tissues. To explore the role of CaSR in the epidermis, we utilised the keratin 14 promoter to express CaSR cDNA constitutively in the basal cells of the stratified squamous epithelium of transgenic mice. Analysis of the transgenic mice revealed that a sensitized response to CaSR signaling accelerates the epidermal differentiation program with the precocious formation of the epidermal permeability barrier (EPB) during development and an accelerated hair growth at birth. Our observations indicate that overexpression of CaSR in the undifferentiated basal cells leads to changes in the differentiation program of the transgenic epidermis, including the stimulation of keratins 1 and 6 as well as the overexpression of several markers of terminal differentiation such as filaggrin, loricrin and involucrin. Our data suggest that the observed modifications in the differentiation pathway are a consequence of a CaSR-induced enhancement of Ca(2+) signaling involving cross-talk with other signaling pathways (e.g. EGF and Wnt/Ca(2+)). These studies provide new insights into the role of CaSR in epidermal differentiation including EPB development and hair follicle morphogenesis.

Overexpression of Krüppel-like factor 4 in the human colon cancer cell line RKO leads to reduced tumorigenecity

Krüppel-like factor 4 (KLF4) is a zinc-finger-containing transcription factor, the expression of which is enriched in the postmitotic cells of the intestinal epithelium. KLF4 is a target gene of the tumor suppressor adenomatous polyposis coli (APC). We sought to determine the role of KLF4 in suppressing the tumorigenecity of RKO colon cancer cells, which do not express KLF4. We utilized an established system in RKO cells, in which an inducible promoter controls expression of KLF4. Four independent assays were used to assess the effects of KLF4 induction on tumor cells. We find that KLF4 overexpression reduces colony formation, cell migration and invasion, and in vivo tumorigenecity. The mechanism of action of KLF4 does not involve apoptosis. These findings, along with our previous findings that KLF4 induces G1/S arrest, suggest that KLF4 is a cell cycle checkpoint protein that can reduce tumorigenecity of colon cancer cells.

Tight junctions in the skin

Tight junctions (=zonulae occludentes, TJs) function as an effective barrier in simple epithelia. Recent developments in the molecular biology of TJs revealed that TJs also exist in the stratum granulosum and contribute to barrier function in epidermis. Furthermore, several TJ-related junctions were identified in epidermis. In this review article, the history of the investigation into TJs in epidermis and the perspectives of investigation into TJs in dermatology are described.

Transcriptional profiling of Krüppel-like factor 4 reveals a function in cell cycle regulation and epithelial differentiation

Krüppel-like factor 4 (KLF4) is an epithelially enriched, zinc finger-containing transcription factor, the expression of which is associated with growth arrest. Constitutive expression of KLF4 inhibits G1/S transition of the cell cycle but the manner by which it accomplishes this effect is unclear. To better understand the biochemical function of KLF4, we identified its target genes using cDNA microarray analysis in an established human cell line containing inducible KLF4. RNA extracted from induced and control cells were hybridized differentially to microarray chips containing 9600 human cDNAs. In all, 84 genes with significantly increased expression and 107 genes with significantly reduced expression due to KLF4 induction were identified. The affected genes are sorted to several clusters on the basis of functional relatedness. A major cluster belongs to genes involved in cell-cycle control. Within this cluster, many up-regulated genes are inhibitors of the cell cycle and down-regulated genes are promoters of the cell cycle. Another up-regulated gene cluster includes nine keratin genes, of which seven are located in a specific region on chromosome 12. The results indicate that KLF4 is involved in the control of cell proliferation and does so by eliciting changes in expression of numerous cell-cycle regulatory genes in a concerted manner. Furthermore, KLF4 regulates expression of a group of epithelial-specific keratin genes in a manner consistent with a potential locus control region function.

The Krüppel-like factor KLF2 inhibits peroxisome proliferator-activated receptor-gamma expression and adipogenesis

Obesity is an important public health problem associated with a number of disease states such as diabetes and arteriosclerosis. As such, an understanding of the mechanisms governing adipose tissue differentiation and function is of considerable importance. We recently reported that the Krüppel-like zinc finger transcription factor KLF15 can induce adipocyte maturation and GLUT4 expression. In this study, we identify that a second family member, KLF2/Lung Krüppel-like factor (LKLF), as a negative regulator of adipocyte differentiation. KLF2 is highly expressed in adipose tissue, and studies in cell lines and primary cells demonstrate that KLF2 is expressed in preadipocytes but not mature adipocytes. Constitutive overexpression of KLF2 but not KLF15 potently inhibits peroxisome proliferator-activated receptor-gamma (PPARgamma) expression with no effect on the upstream regulators C/EBPbeta and C/EBPdelta. However, the expression of C/EBPalpha and SREBP1c/ADD1 (adipocyte determination and differentiation factor-1/sterol regulatory element-binding protein-1), two factors that feedback in a positive manner to enhance PPARgamma function, was also markedly reduced. In addition, transient transfection studies show that KLF2 directly inhibits PPARgamma2 promoter activity (70% inhibition; p < 0.001). Using a combination of promoter mutational analysis and gel mobility shift assays, we have identified a binding site within the PPARgamma2 promoter, which mediates this inhibitory effect. These data identify a novel role for KLF2 as a negative regulator of adipogenesis.

Covering the limb--formation of the integument

An organism's outermost covering, the integument, has evolved to fulfil a diverse range of functions. Skin provides a physical barrier, an environment for immunological surveillance, and also performs a range of sensory, thermoregulatory and biosynthetic functions. Examination of the skin of limb digits reveals a range of skin types including the thickened hairless epidermis of the toe pads (palmar or plantar epidermis) and thinner epidermis between the hair follicles (interfollicular epidermis) of hairy skin. An important developmental function of skin is to give rise to a diverse group of appendages including hair follicles, with associated sebaceous glands (or feathers and scales in chick), eccrine sweat glands and the nail. A key question is how does this morphological variety arise from the single-layered epithelium covering embryonic limb buds? This review will attempt to address this question by linking the extensive morphological/anatomical data on maturation of epidermis and its appendages with (1) current research into the range, plasticity and location of the putative epidermal stems cells; (2) molecular/microenvironmental regulation of epidermal stem cell lineages and lineage choice; and (3) regulation of the differentiation pathways, focusing on differentiation of the interfollicular epidermis.

Down-regulation of gut-enriched Krüppel-like factor expression in esophageal cancer

AIM: Esophageal carcinoma is one of the most common malignant tumors in China. But the molecular mechanisms of esophageal carcinoma remains unclear. Gut-enriched Krüppel-like factor (GKLF) is a newly identified transcription factor which is expressed abandantly in the epithelial cells of the gastrointestinal tract and deregulation of GKLF was linked to several types of cancer. It is of interest to study the expression and role of GKLF in esophageal carcinoma.

METHODS: Semi-quantitative RT-PCR was used to compare GKLF expression in esophageal squamous cell carcinoma to normal mucosa of the same patients. The serum deprivation inducibility of GKLF was observed in an esophageal squamous cancer cell line by comparison to the primary culture of human fibroblast. The effect of antisense GKLF transfection on the proliferation and adhesion of esophageal squamous cancer cell line was also observed.

RESULTS: The level of GKLF transcript is lower in esophageal squamous cell carcinoma compared to paired normal-appearing mucosa in 14 of 17 of the tumors analyzed. The serum deprivation inducibility of GKLF was greatly decreased in an esophageal squamous cancer cell line compared to the primary culture of human fibroblast. Decreased expression of GKLF in the esophageal cancer cell by antisense GKLF transfection increased its proliferation rate compared with that of vector transfected cell control (P < 0.05). Transfection of antisense GKLF decreased its adhesion ability (P < 0.05).

CONCLUSION: The findings of this study demonstrate the down-regulation of GKLF in esophageal squamous cancer, and suggest that deregulation of GKLF may play a role in initiation and/or progression as well as the metastasis of esophageal squamous cancer.

Role of peroxisome proliferator-activated receptor alpha in epidermal development in utero

The protective function of the skin is mediated by the stratum corneum, the outermost layer of the skin, which is the end-product of epidermal differentiation. Previously, we showed that fetal rat skin explants complete the late-stage milestones of epidermal development when grown in a serum- and growth-factor-free medium, suggesting that endogenous metabolites could regulate the late program that leads to barrier formation. Because a variety of endogenous free fatty acids are known activators, peroxisome proliferator-activated receptor alpha (PPAR-alpha) is a potential candidate for this key regulatory role. Indeed, whereas PPAR-alpha expression is first noted at gestational day 13.5 and peaks between days 14.5 and 15.5, fatty acid synthesis is very active in fetal rodent epidermis peaking at gestational day 17. Furthermore, we have reported that both epidermal differentiation and stratum corneum formation in utero are stimulated by pharmacologic activation of PPAR-alpha. This study was designed to test whether PPAR-alpha plays a physiologic role in epidermal differentiation and stratum corneum formation in utero. In PPAR-alpha-/- mice we observed delayed stratum corneum formation between day 18.5 of gestation and birth. Concurrently, there was diminished beta-glucocerebrosidase activity at the stratum granulosum-stratum corneum junction and a modest decrease in both involucrin and loricrin protein expression, markers of keratinocyte differentiation. Both the number of stratum corneum cell layers was reduced and the processing of the lamellar bilayers was delayed in animals lacking PPAR-alpha, indicating a transient functional defect. In contrast, the lamellar body secretory system as well as rates of epidermal proliferation and cell death appeared normal in PPAR-alpha-/- mice. These results indicate that PPAR-alpha plays a physiologic role during fetal stratum corneum development. The transient and incomplete nature of the developmental delay, however, is consistent with regulation of the late stages of epidermal development by multiple factors.

The Krüppel-like factor KLF15 regulates the insulin-sensitive glucose transporter GLUT4

Resistance to the stimulatory effects of insulin on glucose utilization is a key feature of type 2 diabetes, obesity, and the metabolic syndrome. Recent studies suggest that insulin resistance is primarily caused by a defect in glucose transport. GLUT4 is the main insulin-responsive glucose transporter and is expressed predominantly in muscle and adipose tissues. Whereas GLUT4 has been shown to play a critical role in maintaining systemic glucose homeostasis, the mechanisms regulating its expression are incompletely understood. We have cloned the murine homologue of KLF15, a member of the Krüppel-like family of transcription factors. KLF15 is highly expressed in adipocytes and myocytes in vivo and is induced when 3T3-L1 preadipocytes are differentiated into adipocytes. Overexpression of KLF15 in adipose and muscle cell lines potently induces GLUT4 expression. This effect is specific to KLF15 as overexpression of two other Krüppel-like factors, KLF2/LKLF and KLF4/GKLF, did not induce GLUT4 expression. Both basal (3.3-fold, p < 0.001) and insulin-stimulated (2.4-fold, p < 0.00001) glucose uptake are increased in KLF15-overexpressing adipocytes. In co-transfection assays, KLF15 and MEF2A, a known activator of GLUT4, synergistically activates the GLUT4 promoter. Promoter deletion and mutational analyses provide evidence that this activity requires an intact KLF15-binding site proximal to the MEF2A site. Finally, co-immunoprecipitation assays show that KLF15 specifically interacts with MEF2A. These studies indicate that KLF15 is an important regulator of GLUT4 in both adipose and muscle tissues.

Redox-sensitive vascular smooth muscle cell proliferation is mediated by GKLF and Id3 in vitro and in vivo

Reactive oxygen species such as superoxide and hydroxyl radicals have been implicated in the pathogenic growth of various cell types. The molecular mechanisms involved in redox-sensitive cell growth control are poorly understood. Stimulation of cultured vascular smooth muscle cells (VSMC) with xanthin/xanthin oxidase (X/XO) increases proliferation, whereas stimulation with hydrogen peroxide and Fe3+NTA (H-Fe) causes growth arrest of VSMC. Differential Display led to the identification of two novel, differentially regulated redox-sensitive genes. The dominant negative helix-loop-helix protein Id3 is induced by X/XO and down-regulated by H-Fe. The transcription factor gut-enriched Kruppel-like factor (GKLF) is induced by H-Fe but not by X/XO. Induction of GKLF and inhibition of Id3 via transfection experiments leads to growth arrest, whereas overexpression of Id3 and inhibition of GKLF cause cell growth. Id3 down-regulation is induced via binding of GKLF to the Id3 promotor and concomitantly reduced Id3 gene transcription rate. GKLF induction by H-Fe is mediated through hydroxyl radicals, p38MAP kinase-, calcium-, and protein synthesis-dependent pathways. Id3 is induced by X/XO via superoxide, calcium, p38, and p42/44 MAP kinase. GKLF induces and Id3 depresses expression of p21WAF1/Cip1, p27KIP1, p53. Induction of Id3 is accomplished by angiotensin II via superoxide release. A vascular injury mouse model revealed that Id3 is overexpressed in proliferating vascular tissue in vivo. These findings reveal novel mechanisms of redox-controlled cellular proliferation involving GKLF and Id3 that may have general implications for our understanding of vascular and nonvascular growth control.

Developmental and cell type-specific expression of the zinc finger transcription factor Krüppel-like factor 4 (Klf4) in postnatal mouse testis

The transcription factor Krüppel-like factor 4 (Klf4, formerly gut-enriched Krüppel-like factor, GKLF) is specifically expressed in postproliferative epithelial cells such as those of the gut and the epidermis. The importance of Klf4 in terminal differentiation of keratinocytes has been demonstrated by targeted gene ablation in mice. Klf4-deficient mice fail to establish the barrier function of the skin and die shortly after birth due to dehydration. Based on these findings as well as in vitro studies, Klf4 was characterized as a transcription factor essential for postproliferative differentiation of epithelial cells. However, the testicular Klf4 expression pattern is completely unknown. Here, we demonstrate that Klf4 is developmentally regulated during postnatal testicular development of the mouse. Furthermore, we show strong expression in the postmeiotic germ cells undergoing final differentiation into sperm cells and provide evidence that Klf4 is also expressed in the somatic Sertoli cells. These findings suggest that Klf4 might play an important role in testicular differentiation in mammals.

Opposing effects of Krüppel-like factor 4 (gut-enriched Krüppel-like factor) and Krüppel-like factor 5 (intestinal-enriched Krüppel-like factor) on the promoter of the Krüppel-like factor 4 gene

KLF4 (Krüppel-like factor 4 or gut-enriched Krüppel-like factor, GKLF) and KLF5 (Krüppel-like factor 5 or intestinal-enriched Krüppel-like factor, IKLF) are two closely related members of the zinc finger-containing Krüppel-like factor family of transcription factors. Although both genes are expressed in the intestinal epithelium, their distributions are different: Klf4 is primarily expressed in the terminally differentiated villus cells while Klf5 is primarily in the proliferating crypt cells. Previous studies show that Klf4 is a negative regulator of cell proliferation and Klf5 is a positive regulator of cell proliferation. In this study, we demonstrate that Klf5 binds to a number of cis-DNA elements that have previously been shown to bind to Klf4. However, while Klf4 activates the promoter of its own gene, Klf5 suppresses the Klf4 promoter. Moreover, Klf5 abrogates the activating effect of Klf4 on the Klf4 promoter and Klf4 abrogates the inhibitory effect of Klf5 on the same promoter. An explanation of this competing effect is due to physical competition of the two proteins for binding to cognate DNA sequence. The complementary tissue localization of expression of Klf4 and Klf5 and the opposing effect of the two Klfs on the Klf4 promoter activity may provide a basis for the coordinated regulation of expression of the Klf4 gene in the intestinal epithelium.

Synergistic activation of the rat laminin gamma1 chain promoter by the gut-enriched Kruppel-like factor (GKLF/KLF4) and Sp1

Laminin is a multifunctional heterotrimeric protein present in extracellular matrix where it regulates processes that compose tissue architecture including cell differentiation. Laminin gamma1 is the most widely expressed laminin chain and its absence causes early lethality in mouse embryos. Laminin gamma1 chain gene (LAMC1) promoter contains several GC/GT-rich motifs including the bcn-1 element. Using the bcn-1 element as a bait in the yeast one-hybrid screen, we cloned the gut-enriched Kruppel-like factor (GKLF or KLF4) from a rat mesangial cell library. We show that GKLF binds bcn-1, but this binding is not required for the GKLF-mediated activation of the LAMC1 promoter. The activity of GKLF is dependent on a synergism with another Kruppel-like factor, Sp1. The LAMC1 promoter appears to have multiple GKLF- and Sp1-responsive elements which may account for the synergistic activation. We provide evidence that the synergistic action of GKLF and Sp1 is dependent on the promoter context and the integrity of GKLF activation and DNA-binding domain. GKLF is thought to participate in the switch from cell proliferation to differentiation. Thus, the Sp1-GKLF synergistic activation of the LAMC1 promoter may be one of the avenues for expression of laminin gamma1 chain when laminin is needed to regulate cell differentiation.

Dissection of a complex enhancer element: maintenance of keratinocyte specificity but loss of differentiation specificity

In this report, we explored the mechanisms underlying keratinocyte-specific and differentiation-specific gene expression in the skin. We have identified five keratinocyte-specific, open chromatin regions that exist within the 6 kb of 5' upstream regulatory sequence known to faithfully recapitulate the strong endogenous keratin 5 (K5) promoter and/or enhancer activity. One of these, DNase I-hypersensitive site (HSs) 4, was unique in that it acted independently to drive abundant and keratinocyte-specific reporter gene activity in culture and in transgenic mice, despite the fact that it was not essential for K5 enhancer activity. We have identified evolutionarily conserved regulatory elements and a number of their associated proteins that bind to this compact and complex enhancer element. The 125-bp 3' half of this element (referred to as 4.2) is by far the smallest known strong enhancer element possessing keratinocyte-specific activity in vivo. Interestingly, its activity is restricted to a subset of progeny of K5-expressing cells located within the sebaceous gland. The other half of HSs 4 (termed 4.1) possesses activity to suppress sebocyte-specific expression and induce expression in the channel (inner root sheath) cells surrounding the hair shaft. Our findings lead us to a view of keratinocyte gene expression which is determined by multiple regulatory modules, many of which contain AP-2 and/or Sp1/Sp3 binding sites for enhancing expression in skin epithelium, but which also harbor one or more unique sites for the binding of factors which determine specificity. Through mixing and matching of these modules, additional levels of specificity are obtained, indicating that both transcriptional repressors and activators govern the specificity.

The zinc-finger transcription factor Klf4 is required for terminal differentiation of goblet cells in the colon

Klf4 (formerly GKLF) is a zinc-finger transcription factor expressed in the epithelia of the skin, lungs, gastrointestinal tract and several other organs. In vitro studies have suggested that Klf4 plays an important role in cell proliferation and/or differentiation. Mice homozygous for a null mutation in Klf4 die within 15 hours of birth and show selective perturbation of late-stage differentiation structures in the epidermis, but the function of Klf4 in the gastrointestinal tract has not been investigated. To address this issue, we have generated Klf4(-/-) mice by homologous recombination in embryonic stem cells. In this study, we provide the first in vivo evidence that Klf4 is a goblet cell-specific differentiation factor in the colon. Klf4(-/-) mice exhibit normal cell proliferation and cell death rates in the colon on postnatal day 1. However, Klf4(-/-) mice demonstrate a 90% decrease in the number of goblet cells in the colon, show abnormal expression of the goblet cell-specific marker Muc2 by in situ hybridization, have abnormal staining of the colonic epithelium with Alcian Blue for acidic mucins, and lack normal goblet cell morphology by ultrastructural analysis. All other epithelial cell types are present in the colon of Klf4(-/-) mice. In summary, Klf4 plays a crucial role in colonic epithelial cell differentiation in vivo.

Human Krüppel-like factor5/KLF5: synergy with NF-kappaB/Rel factors and expression in human skin and hair follicles

In this report we describe the identification of Krüppel-like factor 5 (KLF5/BTEB2) in a yeast one-hybrid screen using a keratinocyte-specific, NF-kappaB binding site as bait. The KLF5 cDNA encodes a larger protein of 457 aa rather than the earlier reported protein of 209 aa. The full-length KLF5 functions as a transactivator in HepG2 cells, and the stimulation of cells with 12-0-tetradecanoylphorbol-13-acetate (TPA) can modulate its transcriptional activity. Overexpression of KLF5 leads to an increase in the TPA response from VLTRE, a TPA-inducible enhancer element that shows keratinocyte specificity with respect to Rel/NF-kappaB binding. The KLF5-mediated transcriptional increase is not observed in the presence of overexpressed NF-kappaB inhibitor, IkappaBalpha. Cotransfection of KLF5 and the p65 subunit of NF-kappaB, results in a synergistic transactivation of the VLTRE-luciferase reporter. The KLF5 mRNA and the protein is expressed in keratinocytes and throughout the adult human epidermis. Its expression is especially strong in the matrix and the inner root sheath cuticle layer of the hair follicle, sebaceous glands and sweat glands. Considering the TPA-responsiveness and expression pattern, we propose that KLF5 like another member of its family KLF4/GKLF may play an important role in skin morphogenesis and carcinogenesis potentially via its interaction with NF-kappaB factors.

Expression of the C-C chemokine MIP-3 alpha/CCL20 in human epidermis with impaired permeability barrier function

External assault to the skin is followed by an epidermal response including synthesis of DNA, lipids, cytokines and migration of antigen presenting cells. MIP-3 alpha (CCL20, LARC, Exodus-1, Scya20) is a recently described C-C chemokine, predominantly expressed in extralymphoid tissue, which is known to direct migration of dendritic cell precursors and memory lymphocytes to sites of antigen invasion. We assessed the expression of MIP-3 alpha in human skin using semi-quantitative polymerase chain reaction. In vivo, MIP-3 alpha mRNA was constitutively expressed at low levels in untreated human epidermis. After acute disruption of the epidermal permeability barrier MIP-3 alpha mRNA was upregulated in the epidermal fraction, whereas dermal MIP-3 alpha mRNA levels remained unchanged. In vitro, MIP-3 alpha was increased in cultured keratinocytes treated with IL-1 alpha and TNF-alpha and was present in immature and mature dendritic cells, THP-1 monocytic cells and activated T cells. Finally, skin biopsies from patients with psoriasis, contact dermatitis and mycosis fungoides showed abundant expression. In biopsies from atopic dermatitis and graft vs. host disease a weak signal was present, whereas no expression was found in scleroderma and toxic epidermal necrolysis. We conclude that regulation of MIP-3 alpha mRNA is part of the epidermal response to external assault. Its upregulation may represent a danger signal for increased immunosurveillance in barrier disrupted skin and inflammatory skin conditions with impaired barrier function to counteract potential antigen invasion.

Permeability barrier dysfunction in transgenic mice overexpressing claudin 6

A defective epidermal permeability barrier (EPB) in premature birth remains a leading cause of neonatal death as a result of its associated complications, which include poor temperature stability, infection by micro-organisms through the skin, and the outflow of water. Despite its importance in survival, the mechanisms involved in the formation and maintenance of the EPB are not well understood. To address the possibility that claudins, a new superfamily of tight junctional molecules, are involved, we engineered transgenic mice with claudin 6 (Cldn6) overexpressed via the involucrin (Inv) promoter. Interestingly, the Inv-Cldn6 transgenic animals die within 2 days of birth, apparently due to the lack of an intact EPB as evidenced by increased water loss and the penetration of X-gal through the skin. Barrier dysfunction was manifested biochemically by the aberrant expression of late epidermal differentiation markers, including K1, filaggrin, loricrin, transglutaminase 3, involucrin, repetin, members of the SPRR family and the transcriptional regulator Klf4. The overall claudin profile of the epidermis was also modified. Our data suggest that repetin and SPRR1A and 2A are downregulated in response to the downregulation of Klf4 in the transgenic animals, which would contribute to decreased protein crossbridging leading to fragile, defective cornified envelopes. These results provide new insights into the role of claudin 6 in epithelial differentiation and EPB formation. In addition, the epidermal phenotype of these transgenic mice, which is very reminiscent of that in pre-term infant skin, suggest that they will be an important and novel model for studies on human premature EPB-related morbidity.

Claudin-based tight junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice

The tight junction (TJ) and its adhesion molecules, claudins, are responsible for the barrier function of simple epithelia, but TJs have not been thought to play an important role in the barrier function of mammalian stratified epithelia, including the epidermis. Here we generated claudin-1-deficient mice and found that the animals died within 1 d of birth with wrinkled skin. Dehydration assay and transepidermal water loss measurements revealed that in these mice the epidermal barrier was severely affected, although the layered organization of keratinocytes appeared to be normal. These unexpected findings prompted us to reexamine TJs in the epidermis of wild-type mice. Close inspection by immunofluorescence microscopy with an antioccludin monoclonal antibody, a TJ-specific marker, identified continuous TJs in the stratum granulosum, where claudin-1 and -4 were concentrated. The occurrence of TJs was also confirmed by ultrathin section EM. In claudin-1-deficient mice, claudin-1 appeared to have simply been removed from these TJs, leaving occludin-positive (and also claudin-4-positive) TJs. Interestingly, in the wild-type epidermis these occludin-positive TJs efficiently prevented the diffusion of subcutaneously injected tracer (approximately 600 D) toward the skin surface, whereas in the claudin-1-deficient epidermis the tracer appeared to pass through these TJs. These findings provide the first evidence that continuous claudin-based TJs occur in the epidermis and that these TJs are crucial for the barrier function of the mammalian skin.