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

Keratinocyte junctions and the epidermal barrier: how to make a skin-tight dress

Although intercellular junctions are known to be the major regulators of permeability of simple epithelia, they had not been thought to be important in regulating the permeability of stratified mammalian epithelia. Furuse et al. (2002)(this issue) demonstrate that functional tight junctions may indeed be a necessary part of the permeability barrier of the skin.

Getting under the skin of epidermal morphogenesis

At the surface of the skin, the epidermis serves as the armour for the body. Scientists are now closer than ever to understanding how the epidermis accomplishes this extraordinary feat, and is able to survive and replenish itself under the harshest conditions that face any tissue. By combining genetic engineering with cell-biological studies and with human genome data analyses, skin biologists are discovering the mechanisms that underlie the development and differentiation of the epidermis and hair follicles of the skin. This explosion of knowledge paves the way for new discoveries into the genetic bases of human skin disorders and for developing new therapeutics.

A subtractive gene expression screen suggests a role of transcription factor AP-2 alpha in control of proliferation and differentiation

The transcription factor AP-2 alpha has been implicated as a cell type-specific regulator of gene expression during vertebrate embryogenesis based on its expression pattern in neural crest cells, ectoderm, and the nervous system in mouse and frog embryos. AP-2 alpha is prominently expressed in cranial neural crest cells, a population of cells that migrate from the lateral margins of the brain plate during closure of the neural tube at day 8-9 of embryonic development. Homozygous AP-2 alpha mutant mice die perinatally with cranio-abdominoschisis, full facial clefting, and defects in cranial ganglia and sensory organs, indicating the importance of this gene for proper development. By using a subtractive cloning approach, we identified a set of genes repressed by AP-2 alpha that are described to retard cellular proliferation and induce differentiation and apoptosis. We show that these target genes are prematurely expressed in AP-2 alpha mutant mice. One of the genes isolated, the Krüppel-box transcription factor KLF-4 implicated in induction of terminal differentiation and growth regulation, is found expressed in mutant embryonic fibroblasts. We show that fibroblasts lacking AP-2 alpha display retarded growth but no enhanced apoptosis. Based on these data we suggest that AP-2 alpha might be required for cell proliferation by suppression of genes inducing terminal differentiation, apoptosis, and growth retardation.

The zebrafish klf gene family

The Krüppel-like factor (KLF) family of genes encodes transcriptional regulatory proteins that play roles in differentiation of a diverse set of cells in mammals. For instance, the founding member KLF1 (also known as EKLF) is required for normal globin production in mammals. Five new KLF genes have been isolated from the zebrafish, Danio rerio, and the structure of their products, their genetic map positions, and their expression during development of the zebrafish have been characterized. Three genes closely related to mammalian KLF2 and KLF4 were found, as was an ortholog of mammalian KLF12. A fifth gene, apparently missing from the genome of mammals and closely related to KLF1 and KLF2, was also identified. Analysis demonstrated the existence of novel conserved domains in the N-termini of these proteins. Developmental expression patterns suggest potential roles for these zebrafish genes in diverse processes, including hematopoiesis, blood vessel function, and fin and epidermal development. The studies imply a high degree of functional conservation of the zebrafish genes with their mammalian homologs. These findings further the understanding of the KLF genes in vertebrate development and indicate an ancient role in hematopoiesis for the Krüppel-like factor gene family.

Krüppel-like factor 4 (gut-enriched Krüppel-like factor) inhibits cell proliferation by blocking G1/S progression of the cell cycle

Krüppel-like factor 4 (KLF4) is an epithelial cell-enriched, zinc finger-containing transcription factor, the expression of which is associated with growth arrest. Previous studies show that constitutive expression of KLF4 inhibits DNA synthesis but the manner by which KLF4 exerts this effect is unclear. In the present study, we developed a system in which expression of KLF4 is controlled by a promoter that is induced upon treatment of cells containing the receptors for the insect hormone, ecdysone, with ponasterone A, an ecdysone analogue. The rate of proliferation of a stably transfected colon cancer cell line, RKO, was significantly decreased following addition of ponasterone A when compared with untreated cells. Flow cytometric analyses indicated that the inducible expression of KLF4 caused a block in the G(1)/S phase of the cell cycle. A similar block was observed when ecdysone receptor-containing RKO cells were infected with a replication-defective recombinant adenovirus containing an inducible KLF4 and treated with ponasterone A. Results of these studies provide evidence that the inhibitory effect of KLF4 on cell proliferation is mainly exerted at the G(1)/S boundary of the cell cycle.

Expression of the gut-enriched Krüppel-like factor (Krüppel-like factor 4) gene in the human colon cancer cell line RKO is dependent on CDX2

Gut-enriched Krüppel-like factor (GKLF or KLF4) is a zinc finger-containing, epithelial-specific transcription factor, that functions as a suppressor of cell proliferation. We previously showed that GKLF expression is decreased in intestinal and colonic adenomas, respectively, from multiple intestinal neoplasia (Min) mice and familial adenomatous polyposis (FAP) patients. This study shows that GKLF is induced upon activation of the adenomatous polyposis coli (APC) gene. However, among several human colon cancer cell lines surveyed, expression of GKLF is lowest in RKO, a line with wild-type APC and beta-catenin. RKO contains a mutated allele that encodes the putative tumor suppressor homeodomain protein, CDX2. We show that wild-type CDX2 activates the GKLF promoter and that the mutated CDX2 has a dominant negative effect on wild-type function. Our results may help explain the exceedingly low levels of GKLF expression detected in this cell line, which may in turn contribute to the tumor phenotype.

Sp1 and krüppel-like factor family of transcription factors in cell growth regulation and cancer

The Sp/KLF family contains at least twenty identified members which include Sp1-4 and numerous krüppel-like factors. Members of the family bind with varying affinities to sequences designated as 'Sp1 sites' (e.g., GC-boxes, CACCC-boxes, and basic transcription elements). Family members have different transcriptional properties and can modulate each other's activity by a variety of mechanisms. Since cells can express multiple family members, Sp/KLF factors are likely to make up a transcriptional network through which gene expression can be fine-tuned. 'Sp1 site'-dependent transcription can be growth-regulated, and the activity, expression, and/or post-translational modification of multiple family members is altered with cell growth. Furthermore, Sp/KLF factors are involved in many growth-related signal transduction pathways and their overexpression can have positive or negative effects on proliferation. In addition to growth control, Sp/KLF factors have been implicated in apoptosis and angiogenesis; thus, the family is involved in several aspects of tumorigenesis. Consistent with a role in cancer, Sp/KLF factors interact with oncogenes and tumor suppressors, they can be oncogenic themselves, and altered expression of family members has been detected in tumors. Effects of changes in Sp/KLF factors are context-dependent and can appear contradictory. Since these factors act within a network, this diversity of effects may arise from differences in the expression profile of family members in various cells. Thus, it is likely that the properties of the overall network of Sp/KLF factors play a determining role in regulation of cell growth and tumor progression.

Transcription factor LKLF is sufficient to program T cell quiescence via a c-Myc-dependent pathway

T lymphocytes circulate in a quiescent state until they encounter cognate antigen bound to the surface of an antigen-presenting cell. The molecular pathways that regulate T cell quiescence remain largely unknown. Here we show that forced expression of the lung Krüppel-like transcription factor (LKLF) in Jurkat T cells is sufficient to program a quiescent phenotype characterized by decreased proliferation, reduced cell size and protein synthesis and decreased surface expression of activation markers. Conversely, LKLF-deficient peripheral T cells produced by gene targeting showed increased proliferation, increased cell size and enhanced expression of surface activation markers in vivo. LKLF appeared to function, at least in part, by decreasing expression of the proto-oncogene encoding c-Myc. Forced expression of LKLF was associated with markedly decreased c-Myc expression. In addition, many effects of LKLF expression were mimicked by expression of the dominant-negative MadMyc protein and rescued by overexpression of c-Myc. Thus, LKLF is both necessary and sufficient to program quiescence in T cells and functions, in part, by negatively regulating a c-Myc--dependent pathway.

Tcf3 and Lef1 regulate lineage differentiation of multipotent stem cells in skin

In skin, multipotent stem cells generate the keratinocytes of the epidermis, sebaceous gland, and hair follicles. In this paper, we show that Tcf3 and Lef1 control these differentiation lineages. In contrast to Lef1, which requires Wnt signaling and stabilized beta-catenin to express the hair-specific keratin genes and control hair differentiation, Tcf3 can act independently of its beta-catenin interacting domain to suppress features of epidermal terminal differentiation, in which Tcf3 is normally shut off, and promote features of the follicle outer root sheath (ORS) and multipotent stem cells (bulge), the compartments which naturally express Tcf3. These aspects of Tcf3's action are dependent on its DNA binding and Groucho repressor-binding domains. In the absence of its beta-catenin interacting domain, Lef1's behavior (Delta NLef1) seems to be markedly distinct from that of Delta NTcf3. Delta NLef1 does not suppress epidermal differentiation and promote ORS/bulge differentiation, but rather suppresses hair differentiation and gives rise to sebocyte differentiation. Taken together, these findings provide powerful evidence that the status of Tcf3/Lef complexes has a key role in controlling cell fate lineages in multipotent skin stem cells.

Structural organization and regulation of the small proline-rich family of cornified envelope precursors suggest a role in adaptive barrier function

The protective barrier provided by stratified squamous epithelia relies on the cornified cell envelope (CE), a structure synthesized at late stages of keratinocyte differentiation. It is composed of structural proteins, including involucrin, loricrin, and the small proline-rich (SPRR) proteins, all encoded by genes localized at human chromosome 1q21. The genetic characterization of the SPRR locus reveals that the various members of this multigene family can be classified into two distinct groups with separate evolutionary histories. Whereas group 1 genes have diverged in protein structure and are composed of three different classes (SPRR1 (2x), SPRR3, and SPRR4), an active process of gene conversion has counteracted diversification of the protein sequences of group 2 genes (SPRR2 class, seven genes). Contrasting with this homogenization process, all individual members of the SPRR gene family show specific in vivo and in vitro expression patterns and react selectively to UV irradiation. Apparently, creation of regulatory rather than structural diversity has been the driving force behind the evolution of the SPRR gene family. Differential regulation of highly homologous genes underlines the importance of SPRR protein dosage in providing optimal barrier function to different epithelia, while allowing adaptation to diverse external insults.

Developmental expression of mouse Krüppel-like transcription factor KLF7 suggests a potential role in neurogenesis

To identify potential functions for the Krüppel-like transcription factor KLF7, we have determined the spatiotemporal pattern of gene expression during embryogenesis and in the adult organism. We show that the profile of Klf7 expression predominantly involves the central and peripheral nervous systems and is broadly identified by three separate phases. The first phase occurs early in embryogenesis with increasingly strong expression in the spinal cord, notably in motor neurons of the ventral horn, in dorsal root ganglia, and in sympathetic ganglia. The second robust phase of Klf7 expression is confined to the early postnatal cerebral cortex and is downregulated thereafter. The third phase is characterized by high and sustained expression in the adult cerebellum and dorsal root ganglia. Functionally, these three phases coincide with establishment of neuronal phenotype in embryonic spinal cord, with synaptogenesis and development of mature synaptic circuitry in the postnatal cerebral cortex, and with survival and/or maintenance of function of adult sensory neurons and cerebellar granule cells. Consistent with Klf7 expression in newly formed neuroblasts, overexpression of the gene in cultured fibroblasts and neuroblastoma cells repressed cyclin D1, activated p21, and led to G1 growth arrest. Based on these data, we argue for multiple potential functions for KLF7 in the developing and adult nervous system; they include participating in differentiation and maturation of several neuronal subtypes and in phenotypic maintenance of mature cerebellar granule cells and dorsal root ganglia.

Expression of Klf9 and Klf13 in mouse development

Klf9 and Klf13 are members of the C(2)H(2) zinc finger family of transcription factors that are thought to be involved in regulating basal transcription. The mRNA localization of Klf9 and Klf13 during development was determined by in situ hybridization of mouse E8, E11, E13 and E16 embryo sections. The data showed that Klf9 and Klf13 are widely expressed at all the mouse embryo stages examined. Whilst the expression patterns of the two genes largely overlap there are differences in the localization or level of expression in some tissues. At E11, both genes are expressed in high levels in the cephalic mesenchyme whilst Klf13 and not Klf9 is expressed at high levels in the developing heart at E8 and E11. In the gut and bladder at E16, Klf13 is expressed in the epithelial cell layer whereas Klf9 is expressed in both the muscle and epithelial layers. Both Klf9 and Klf13 are expressed at high levels in the epidermis at E11, E13 and E16.

Site-specific acetylation by p300 or CREB binding protein regulates erythroid Krüppel-like factor transcriptional activity via its interaction with the SWI-SNF complex

Recruitment of modifiers and remodelers to specific DNA sites within chromatin plays a critical role in controlling gene expression. The study of globin gene regulation provides a convergence point within which to address these issues in the context of tissue-specific and developmentally regulated expression. In this regard, erythroid Krüppel-like factor (EKLF) is critical. EKLF is a red cell-specific activator whose presence is crucial for establishment of the correct chromatin structure and high-level transcriptional induction of adult beta-globin. We now find, by metabolic labeling-immunoprecipitation experiments, that EKLF is acetylated in the erythroid cell. EKLF residues acetylated by CREB binding protein (CBP) in vitro map to Lys-288 in its transactivation domain and Lys-302 in its zinc finger domain. Although site-specific DNA binding by EKLF is unaffected by the acetylation status of either of these lysines, directed mutagenesis of Lys-288 (but not Lys-302) decreases the ability of EKLF to transactivate the beta-globin promoter in vivo and renders it unable to be superactivated by coexpressed p300 or CBP. In addition, the acetyltransferase function of CBP or p300 is required for superactivation of wild-type EKLF. Finally, acetylated EKLF has a higher affinity for the SWI-SNF chromatin remodeling complex and is a more potent transcriptional activator of chromatin-assembled templates in vitro. These results demonstrate that the acetylation status of EKLF is critical for its optimal activity and suggest a mechanism by which EKLF acts as an integrator of remodeling and transcriptional components to alter chromatin structure and induce adult beta-globin expression within the beta-like globin cluster.

Keratinocyte growth factor induces hyperproliferation and delays differentiation in a skin equivalent model system

Keratinocyte growth factor (KGF) is a paracrine mediator of epithelial cell growth. To examine the direct effects of KGF on the morphogenesis of the epidermis, we generated skin equivalents in vitro by seeding human keratinocytes on the papillary surface of acellular dermis and raising them up to the air-liquid interface. KGF was either added exogenously or expressed by keratinocytes via a recombinant retrovirus encoding KGF. KGF induced dramatic changes to the 3-dimensional organization of the epidermis including pronounced hyperthickening, crowding, and elongation of the basal cells, flattening of the rete ridges, and a ripple-like pattern in the junction of stratum corneum and granular layers. Quantitative immunostaining for the proliferation antigen, Ki67, revealed that in addition to increasing basal proliferation, KGF extended the proliferative compartment by inducing suprabasal cell proliferation. KGF also induced expression of the integrin alpha 5 beta 1 and delayed expression of keratin 10 and transglutaminase. However, barrier formation of the epidermis was not disrupted. These results demonstrate for the first time that a single growth factor can alter the 3-dimensional organization and proliferative function of an in vitro epidermis. In addition to new strategies for tissue engineering, such a well-defined system will be useful for analyzing growth factor effects on the complex links between cell proliferation, cell movement and differentiation within a stratified tissue.

Klf6 is a zinc finger protein expressed in a cell-specific manner during kidney development

Molecular mechanisms that are responsible for the development of the renal collecting duct system during embryogenesis are still poorly understood. A mouse cDNA encoding a zinc finger protein, called Klf6, which is a member of the Krüppel-like family of transcription factors, has been cloned. Northern blot analyses showed that Klf6 was already expressed in 11.5-d postconception mouse embryos and that its expression persisted after birth. They also disclosed that Klf6 had a restricted pattern of expression. In situ hybridization experiments using mouse embryos showed that during kidney development, Klf6 was expressed selectively in the Wolffian duct and in its derivatives. During mesonephros development, it was expressed in the Wolffian duct but not in the mesonephric mesenchyme. Thereafter, Klf6 was expressed in the ureteric bud and its branches and in the collecting ducts, whereas it was not expressed in tubular structures that derive from the metanephric mesenchyme. Glomeruli were not labeled during early stages of differentiation, and it is only at the capillary stage that a staining of the mesangial area was observed, which persisted after birth. This pattern of expression is strikingly similar to the one of GATA-3, which is another zinc finger protein. It suggests that Klf6 may play a role during kidney development and in particular during the development of the renal collecting duct system, possibly in association with GATA-3.

mKlf7, a potential transcriptional regulator of TrkA nerve growth factor receptor expression in sensory and sympathetic neurons

Development of the nervous system relies on stringent regulation of genes that are crucial to this process. TrkA, the receptor for nerve growth factor (NGF), is tightly regulated during embryonic development and is essential for the survival and differentiation of neural crest-derived sensory and sympathetic neurons. We have previously identified a mouse TrkA enhancer and have characterized several cis regulatory elements that are important for appropriate TrkA expression in vivo. We now report the cloning of a novel gene encoding a Kruppel-like factor from a mouse dorsal root ganglion expression library. This Kruppel-like factor, named mKlf7, binds specifically to an Ikaros core binding element that is crucial for in vivo TrkA enhancer function. Using in situ hybridization, we demonstrate that mKlf7 is coexpressed with TrkA in sensory and sympathetic neurons during embryogenesis and in adulthood. These data are consistent with the idea that mKlf7 may directly regulate TrkA gene expression in the peripheral nervous system.

Mouse Trefoil factor genes: genomic organization, sequences and methylation analyses

The mammalian Trefoil Factors (TFFs), TFF1/pS2, TFF2/SP and TFF3/ITF, are expressed and secreted throughout the gastrointestinal tract with a specific and complementary pattern. These proteins exhibit common functions in the protection and repair process of the gastrointestinal epithelial barrier. Here, we report the clustered organization of the three mouse TFF genes in a 40 kb DNA segment, in a head to tail orientation in the following order: TFF1, TFF2, and TFF3. Computer comparison of the mouse TFF promoter sequences to their human counterparts revealed conserved boxes in both mouse and human genes. Promoter methylation analyses showed that, in tissues where these genes are normally expressed, the proximal promoters of TFF1 and TFF2 are specifically not methylated and that of TFF3 is partially demethylated. In contrast, in organs that do not express TFFs, the promoters of the three genes are methylated. These findings strongly argue for the involvement of epigenetic mechanisms in the regulation of TFF expression in normal and pathological conditions.

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.

Caspase-14: analysis of gene structure and mRNA expression during keratinocyte differentiation

Caspase-14 is expressed in a tissue-specific manner in mouse skin. Here we determined the complete caspase-14 cDNA sequence of human caspase-14 by rapid amplification of cDNA ends. Sequence comparison with a cosmid clone containing genomic DNA revealed that the human caspase-14 gene comprises seven exons. Facultative utilization of a cryptic splice acceptor site within intron 5 leads to the formation of two mRNA species. In situ hybridization of human skin showed that caspase-14 is expressed in the uppermost layer of living epidermal cells, i.e., the granular layer, in hair follicles and sebaceous glands. In vitro caspase-14 transcription was low in subconfluent cultures but strongly increased when keratinocyte differentiation was simulated by maintaining cells at confluence for several days. This transcriptional upregulation was suppressed in the presence of a high extracellular calcium concentration. Our findings show that caspase-14 is regulated at the level of transcription during keratinocyte differentiation in vitro and in vivo.

Lessons from loricrin-deficient mice: compensatory mechanisms maintaining skin barrier function in the absence of a major cornified envelope protein

The epidermal cornified cell envelope (CE) is a complex protein-lipid composite that replaces the plasma membrane of terminally differentiated keratinocytes. This lamellar structure is essential for the barrier function of the skin and has the ability to prevent the loss of water and ions and to protect from environmental hazards. The major protein of the epidermal CE is loricrin, contributing approximately 70% by mass. We have generated mice that are deficient for this protein. These mice showed a delay in the formation of the skin barrier in embryonic development. At birth, homozygous mutant mice weighed less than control littermates and showed skin abnormalities, such as congenital erythroderma with a shiny, translucent skin. Tape stripping experiments suggested that the stratum corneum stability was reduced in newborn Lor(-/-) mice compared with wild-type controls. Isolated mutant CEs were more easily fragmented by sonication in vitro, indicating a greater susceptibility to mechanical stress. Nevertheless, we did not detect impaired epidermal barrier function in these mice. Surprisingly, the skin phenotype disappeared 4-5 d after birth. At least one of the compensatory mechanisms preventing a more severe skin phenotype in newborn Lor(-/-) mice is an increase in the expression of other CE components, such as SPRRP2D and SPRRP2H, members of the family of "small proline rich proteins", and repetin, a member of the "fused gene" subgroup of the S100 gene family.

Conditional ablation of beta1 integrin in skin. Severe defects in epidermal proliferation, basement membrane formation, and hair follicle invagination

The major epidermal integrins are alpha3beta1 and hemidesmosome-specific alpha6beta4; both share laminin 5 as ligand. Keratinocyte culture studies implicate both integrins in adhesion, proliferation, and stem cell maintenance and suggest unique roles for alphabeta1 integrins in migration and terminal differentiation. In mice, however, whereas ablation of alpha6 or beta4 results in loss of hemidesmosomes, epidermal polarity, and basement membrane (BM) attachment, ablation of alpha3 only generates microblistering due to localized internal shearing of BM. Using conditional knockout technology to ablate beta1 in skin epithelium, we have uncovered biological roles for alphabeta1 integrins not predicted from either the alpha3 knockout or from in vitro studies. In contrast to alpha3 null mice, beta1 mutant mice exhibit severe skin blistering and hair defects, accompanied by massive failure of BM assembly/organization, hemidesmosome instability, and a failure of hair follicle keratinocytes to remodel BM and invaginate into the dermis. Although epidermal proliferation is impaired, a spatial and temporal program of terminal differentiation is executed. These results indicate that beta1's minor partners in skin are important, and together, alphabeta1 integrins are required not only for extracellular matrix assembly but also for BM formation. This, in turn, is required for hemidesmosome stability, epidermal proliferation, and hair follicle morphogenesis. However, beta1 downregulation does not provide the trigger to terminally differentiate.

Decreased expression of the gut-enriched Krüppel-like factor gene in intestinal adenomas of multiple intestinal neoplasia mice and in colonic adenomas of familial adenomatous polyposis patients

Gut-enriched Krüppel-like factor (GKLF) is a zinc finger-containing transcription factor, the expression of which is associated with growth arrest. We compared Gklf expression in intestinal and colonic adenomas to normal mucosa in multiple intestinal neoplasia (Min) mice and familial adenomatous polyposis (FAP) patients, respectively, using semi-quantitative RT-PCR. In Min mice, the level of Gklf transcript is highest in normal-appearing intestinal tissues and decreases as the size of the adenoma increases. In FAP patients, the level of GKLF transcript is lower in adenomas compared to paired normal-appearing mucosa from the same patient or normal colonic mucosa from control individuals without FAP. The possibility of DNA methylation as a cause for the decreased expression of Gklf in adenomas of Min mice was investigated by methylation-specific PCR. Results indicate that the Gklf gene is not methylated in either normal or tumorous tissues. The findings of our study are therefore consistent with the potential role of GKLF as a negative growth regulator of gut epithelial cells.

Structure and regulation of the envoplakin gene

Envoplakin, a member of the plakin family of proteins, is a component of desmosomes and the epidermal cornified envelope. To understand how envoplakin expression is regulated, we have analyzed the structure of the mouse envoplakin gene and characterized the promoters of both the human and mouse genes. The mouse gene consists of 22 exons and maps to chromosome 11E1, syntenic to the location of the human gene on 17q25. The exon-intron structure of the mouse envoplakin gene is common to all members of the plakin family: the N-terminal protein domain is encoded by 21 small exons, and the central rod domain and the C-terminal globular domain are coded by a single large exon. The C terminus shows the highest sequence conservation between mouse and human envoplakins and between envoplakin and the other family members. The N terminus is also conserved, with sequence homology extending to Drosophila Kakapo. A region between nucleotides -101 and 288 was necessary for promoter activity in transiently transfected primary keratinocytes. This region is highly conserved between the human and mouse genes and contains at least two different positively acting elements identified by site-directed mutagenesis and electrophoretic mobility shift assays. Mutation of a GC box binding Sp1 and Sp3 proteins or a combined E box and Krüppel-like element interacting with unidentified nuclear proteins virtually abolished promoter activity. 600 base pairs of the mouse upstream sequence was sufficient to drive expression of a beta-galactosidase reporter gene in the suprabasal layers of epidermis, esophagus, and forestomach of transgenic mice. Thus, we have identified a regulatory region in the envoplakin gene that can account for the expression pattern of the endogenous protein in stratified squamous epithelia.

The gut-enriched Kruppel-like factor (Kruppel-like factor 4) mediates the transactivating effect of p53 on the p21WAF1/Cip1 promoter

An important mechanism by which the tumor suppressor p53 maintains genomic stability is to induce cell cycle arrest through activation of the cyclin-dependent kinase inhibitor p21(WAF1/Cip1) gene. We show that the gene encoding the gut-enriched Krüppel-like factor (GKLF, KLF4) is concurrently induced with p21(WAF1/Cip1) during serum deprivation and DNA damage elicited by methyl methanesulfonate. The increases in expression of both Gklf and p21(WAF1/Cip1) due to DNA damage are dependent on p53. Moreover, during the first 30 min of methyl methanesulfonate treatment, the rise in Gklf mRNA level precedes that in p21(WAF1/Cip1), suggesting that GKLF may be involved in the induction of p21(WAF1/Cip1). Indeed, GKLF activates p21(WAF1/Cip1) through a specific Sp1-like cis-element in the p21(WAF1/Cip1) proximal promoter. The same element is also required by p53 to activate the p21(WAF1/Cip1) promoter, although p53 does not bind to it. Potential mechanisms by which p53 activates the p21(WAF1/Cip1) promoter include a physical interaction between p53 and GKLF and the transcriptional induction of Gklf by p53. Consequently, the two transactivators cause a synergistic induction of the p21(WAF1/Cip1) promoter activity. The physiological relevance of GKLF in mediating p53-dependent induction of p21(WAF1/Cip1) is demonstrated by the ability of antisense Gklf oligonucleotides to block the production of p21(WAF1/Cip1) in response to p53 activation. These findings suggest that GKLF is an essential mediator of p53 in the transcriptional induction of p21(WAF1/Cip1) and may be part of a novel pathway by which cellular responses to stress are modulated.

SPRR1 gene induction and barrier formation occur as coordinated moving fronts in terminally differentiating epithelia

Stratified, terminally differentiated epithelia, such as epidermis and oral epithelia, provide protective barriers against the environment. We recently developed wholemount assays that demonstrate epidermal barrier function during late gestation and showed that epidermal barrier forms at specific sites (epidermal initiation sites), and then spreads around the body as apparent moving fronts. We now ask if this is a fundamental and widespread mode of epithelial developmental change. If so, then the pattern should be apparent when assaying for developmental change other than barrier institution (e.g., gene induction) and similar types of patterned change should be apparent in other types of epithelia. In this study we demonstrate patterned barrier function in a range of additional stratified epithelia from the oral cavity and show that the gene induction pattern of a stratum corneum precursor small proline-rich region protein 1 (SPRR1) precedes barrier function and occurs in the barrier pattern, i.e., gene induction occurs first at initiation sites and propagates across epithelia as apparent moving fronts. These results demonstrate that late gestational developmental change in multiple terminally differentiating epithelia occurs via initiation sites and moving fronts. The pattern precedes barrier formation and results in a developmental gradient that influences gene induction.

Developmental expression of the mouse gene coding for the Krüppel-like transcription factor KLF5

The mammalian Krüppel-like transcription factors are key regulators of multiple morphogenetic programs. Here, we examined the developmental expression of the mouse Klf5 gene and compared it to the established pattern of the Klf4 gene. The results revealed that the two genes are expressed in both overlapping and mutually exclusive patterns. Unlike Klf4, Klf5 mRNA is detected in the E15.5 meninges and in the E.16.5 epithelium of trachea and bronchi. Both genes are co-expressed in the outer layer of the tongue, as well as in the developing epidermis and gut with interesting temporal differences. Klf4 expression in the skin gradually decreases from E15.5 on, whereas Klf5 transcripts continue to accumulate at a fairly high rate in the basal layer of the epidermis. The same sustained activity of Klf5 is already seen in the gastrointestinal tract of the 10.5 day embryo, and is later confined to the base of the intestinal crypts. Maximal Klf4 expression in the gastrointestinal tract is limited to a narrower window of time during the late phase of development.

Transactivation and growth suppression by the gut-enriched Krüppel-like factor (Krüppel-like factor 4) are dependent on acidic amino acid residues and protein-protein interaction

Gut-enriched Krüppel-like factor (GKLF or KLF4) is a pleiotropic (activating and repressive) transcription factor. This study characterizes the mechanisms of transactivation by GKLF. Using a GAL4 fusion assay, the activating domain of murine GKLF was localized to the 109 amino acid residues in the N-terminus. Site-directed mutagenesis showed that two adjacent clusters of acidic residues within this region are responsible for the activating effect. Transactivation by GKLF involves intermolecular interactions as demonstrated by the ability of wild-type, but not mutated, GKLF to compete with the N-terminal activation domain. In addition, wild-type adenovirus E1A, but not a mutated E1A that failed to bind p300/CBP, inhibited transactivation by the N-terminal 109 amino acids of GKLF, suggesting that p300/CBP are GKLF's interacting partners. A physical interaction between GKLF and CBP was demonstrated by glutathione- S -transferase pull-down and by in vivo co-immuno-precipitation experiments. We also showed that the two acidic amino acid clusters are essential for this interaction, since GKLF with mutations in these residues failed to co-immunoprecipitate with CBP. Importantly, the same mutations abrogated the ability of GKLF to suppress cell growth as determined by a colony suppression assay. These studies therefore provide plausible evidence for a structural and functional correlation between the transactivating and growth-suppressing effects of GKLF.

The biology of the mammalian Krüppel-like family of transcription factors

Recent advances in molecular cloning have led to the identification of a large number of mammalian zinc finger-containing transcription factors that exhibit homology to the Drosophila melanogaster protein, Krüppel. Although the amino acid sequences in the zinc finger domains of these Krüppel-like factors (KLFs) are closely related to one another, the regions outside the zinc fingers of the proteins are usually unique. KLFs display seemingly different and broad biological properties with each functioning as an activator of transcription, a repressor or both. This review article provides a current phylogenetic classification of the identified KLFs to date. More importantly, the currently known biological activities of the KLFs in regulating transcription, cell proliferation, differentiation and development are summarized and compared. Further characterization of this interesting protein family should provide additional insights into the their respective regulatory role in various important biological processes.