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

The urothelial gene regulatory network: understanding biology to improve bladder cancer management

The urothelium is a stratified epithelium composed of basal cells, one or more layers of intermediate cells, and an upper layer of differentiated umbrella cells. Most bladder cancers (BLCA) are urothelial carcinomas. Loss of urothelial lineage fidelity results in altered differentiation, highlighted by the taxonomic classification into basal and luminal tumors. There is a need to better understand the urothelial transcriptional networks. To systematically identify transcription factors (TFs) relevant for urothelial identity, we defined highly expressed TFs in normal human bladder using RNA-Seq data and inferred their genomic binding using ATAC-Seq data. To focus on epithelial TFs, we analyzed RNA-Seq data from patient-derived organoids recapitulating features of basal/luminal tumors. We classified TFs as "luminal-enriched", "basal-enriched" or "common" according to expression in organoids. We validated our classification by differential gene expression analysis in Luminal Papillary vs. Basal/Squamous tumors. Genomic analyses revealed well-known TFs associated with luminal (e.g., PPARG, GATA3, FOXA1) and basal (e.g., TP63, TFAP2) phenotypes and novel candidates to play a role in urothelial differentiation or BLCA (e.g., MECOM, TBX3). We also identified TF families (e.g., KLFs, AP1, circadian clock, sex hormone receptors) for which there is suggestive evidence of their involvement in urothelial differentiation and/or BLCA. Genomic alterations in these TFs are associated with BLCA. We uncover a TF network involved in urothelial cell identity and BLCA. We identify novel candidate TFs involved in differentiation and cancer that provide opportunities for a better understanding of the underlying biology and therapeutic intervention.

The role of enhancers in psoriasis and atopic dermatitis

Regulatory elements, particularly enhancers, play a crucial role in disease susceptibility and progression. Enhancers are DNA sequences that activate gene expression and can be affected by epigenetic modifications, interactions with transcription factors (TFs) or changes to the enhancer DNA sequence itself. Altered enhancer activity impacts gene expression and contributes to disease. In this review, we define enhancers and the experimental techniques used to identify and characterize them. We also discuss recent studies that examine how enhancers contribute to atopic dermatitis (AD) and psoriasis. Articles in the PubMed database were identified (from 1 January 2010 to 28 February 2023) that were relevant to enhancer variants, enhancer-associated TFs and enhancer histone modifications in psoriasis or AD. Most enhancers associated with these conditions regulate genes affecting epidermal homeostasis or immune function. These discoveries present potential therapeutic targets to complement existing treatment options for AD and psoriasis.

A Kaleidoscope of Keratin Gene Expression and the Mosaic of Its Regulatory Mechanisms

Keratins are a family of intermediate filament-forming proteins highly specific to epithelial cells. A combination of expressed keratin genes is a defining property of the epithelium belonging to a certain type, organ/tissue, cell differentiation potential, and at normal or pathological conditions. In a variety of processes such as differentiation and maturation, as well as during acute or chronic injury and malignant transformation, keratin expression undergoes switching: an initial keratin profile changes accordingly to changed cell functions and location within a tissue as well as other parameters of cellular phenotype and physiology. Tight control of keratin expression implies the presence of complex regulatory landscapes within the keratin gene loci. Here, we highlight patterns of keratin expression in different biological conditions and summarize disparate data on mechanisms controlling keratin expression at the level of genomic regulatory elements, transcription factors (TFs), and chromatin spatial structure.

ΔNp63 drives dysplastic alveolar remodeling and restricts epithelial plasticity upon severe lung injury

The lung exhibits a robust, multifaceted regenerative response to severe injuries such as influenza infection, during which quiescent lung-resident epithelial progenitors participate in two distinct reparative pathways: functionally beneficial regeneration via alveolar type 2 (AT2) cell proliferation and differentiation, and dysplastic tissue remodeling via intrapulmonary airway-resident basal p63+ progenitors. Here we show that the basal cell transcription factor ΔNp63 is required for intrapulmonary basal progenitors to participate in dysplastic alveolar remodeling following injury. We find that ΔNp63 restricts the plasticity of intrapulmonary basal progenitors by maintaining either active or repressive histone modifications at key differentiation gene loci. Following loss of ΔNp63, intrapulmonary basal progenitors are capable of either airway or alveolar differentiation depending on their surrounding environment both in vitro and in vivo. Uncovering these regulatory mechanisms of dysplastic repair and lung basal cell fate choice highlight potential therapeutic targets to promote functional alveolar regeneration following severe lung injuries.

Regulation of HPV18 Genome Replication, Establishment and Persistence by Sequences in the Viral Upstream Regulatory Region

During persistent human papillomavirus infection, the viral genome replicates as an extrachromosomal plasmid that is efficiently partitioned to daughter cells during cell division. We have previously shown that an element which overlaps the HPV18 transcriptional enhancer promotes stable DNA replication of replicons containing the viral replication origin. Here we perform comprehensive analyses to elucidate the function of this maintenance element. We conclude that no unique element or binding site in this region is absolutely required for persistent replication and partitioning, and instead propose that the overall chromatin architecture of this region is important to promote efficient use of the replication origin. These results have important implications on the genome partitioning mechanism of papillomaviruses. Importance Persistent infection with oncogenic HPVs is responsible for ∼5% human cancers. The viral DNA replicates as an extrachromosomal plasmid and is partitioned to daughter cells in dividing keratinocytes. Using a complementation assay that allows us to separate viral transcription and replication, we provide insight into viral sequences that are required for long term replication and persistence in keratinocytes. Understanding how viral genomes replicate persistently for such long periods of time will guide the development of anti-viral therapies.

Minimal essential region for krüppel-like factor 5 expression and the regulation by specificity protein 3-GC box binding

Krüppel-like factor 5 (KLF5) transcriptionally controls the proliferation-differentiation balance of epithelium and is overexpressed in carcinomas. Although genomic region modifying KLF5 expression is widespread in different types of cells, the region that commonly regulates basal expression of the genes across cell-types is uncertain. In this study we determined the minimal essential region for the expression and its regulatory transcription factors using oral carcinoma cells. A reporter assay defined a 186bp region downstream of the transcription start site and a cluster of six GC boxes (GC1-GC6) as the minimal essential region. Mutation in the GC1 or GC6 regions but not other GC boxes significantly decreased the reporter expression. The decrease by the GC1 mutation was reproduced in the 2kbp full-length promoter, but not by the GC6 mutation. Additionally, specificity proteins (Sp) that can be expressed in epithelial cells and bind GC box, Sp3 co-localized with KLF5 in oral epithelium and carcinomas and chromatin immunoprecipitation analyses showed Sp3 as the prime GC1-binding protein. Inhibition of Sp-GC box binding by mithramycin A and knockdown of Sp3 by the short interfering RNA decreased expression of the reporter gene and endogenous KLF5. These data demonstrate that a 186bp region is the minimal essential region and that Sp3-GC1 binding is essential to the basal expression of KLF5.

Induction of differentiation in psoriatic keratinocytes by propylthiouracil and fructose

Psoriasis is characterized by uncontrolled proliferation and poor differentiation. Sirtuin1 (SIRT1) a class III deacetylase, crucial for differentiation in normal keratinocytes, is reduced in psoriasis. Down regulated SIRT1 levels may contribute to poor differentiation in psoriasis. In addition, the levels of early differentiation factors Keratin1 (K1) and Keratin10 (K10) are depleted in psoriasis. We attempted to study a possible effect of fructose, a SIRT1 upregulator and Propylthiouracil (PTU) to augment differentiation in psoriatic keratinocytes. Keratinocytes were cultured from lesional biopsies obtained from psoriatic patients and control cells were obtained from patients undergoing abdominoplasty. Cells were treated with fructose and PTU individually. K1 and K10 transcript levels were measured to evaluate early differentiation; SIRT1 protein expression was also studied to decipher its role in the mechanism of differentiation. The K1, K10 transcript levels, SIRT1 protein and transcript levels in fructose treated psoriatic keratinocytes were improved. This suggests keratinocyte differentiation was induced by fructose through SIRT1 upregulation. Whereas PTU induced differentiation, as confirmed by improved K1, K10 transcript levels followed a non-SIRT1 mechanism. We conclude that the use of fructose and PTU may be an adjunct to the existing therapies for psoriasis.

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Fructose induces differentiation of psoriatic keratinocytes through SIRT1 upregulation.

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Propylthiouracil (PTU) improves K1 and K10 to induce differentiation in psoriasis.

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Fructose and PTU can be used as adjunct to existing therapies for psoriasis.

Genome-wide screening of aberrant DNA methylation which associated with gene expression in mouse skin cancers

Epigenetic alteration of genomic DNA is a common and key process in carcinogenesis. There is considerable evidence indicating that some of the somatic alterations occurring during carcinogenesis in humans also involve the same processes as those observed in mice. Therefore, we analyzed mouse skin cancer tissues induced by the 2-stage carcinogenesis model to identify skin tumor-specific differentially methylated regions (ST-DMRs) during the multistep carcinogenesis process. We have previously identified ST-DMRs using the restriction landmark genomic scanning (RLGS) technique and reported that some of the mouse ST-DMRs were also epigenetically modified in human cancers, such as melanoma, neuroblastoma, and brain tumor. These results encouraged us to pursue global methylation screening in mouse skin carcinogenesis. Using the methylated DNA immunoprecipitation (MeDIP) method combined with the NimbleGen promoter plus CpG island (CpGi) array, we identified 615 ST-DMRs. In combination with global gene expression analysis, 91 of these ST-DMRs were shown to be located on or around the genes differentially expressed between normal skin and tumor tissues, including a candidate human tumor suppressor gene Tfap2e. As observed in human colorectal cancers, Tfap2e was methylated at a CpGi located in intron 3 and downregulated in skin tumors. Our results identified aberrant methylated regions that were associated with gene expression regulation during carcinogenesis, which may indicate critical genetic regions also involved in human carcinogenesis. © 2013 Wiley Periodicals, Inc.

Early intra-amniotic gene transfer using lentiviral vector improves skin blistering phenotype in a murine model of Herlitz junctional epidermolysis bullosa

Mutations of the LAMB3 gene cause a lethal form of junctional epidermolysis bullosa (JEB). We hypothesized that early intra-amniotic gene transfer in a severe murine model of JEB would improve or correct the skin phenotype. Time-dated fetuses from heterozygous LAMB3(IAP) breeding pairs underwent ultrasound guided intra-amniotic injection of lentiviral vector encoding the murine LAMB3 gene at embryonic day 8 (E8). Gene expression was monitored by immunohistochemistry. The transgenic laminin-β3 chain was shown to assemble with its endogenous partner chains, resulting in detectable amounts of laminin-332 in the basement membrane zone of skin and mucosa. Ultrastructually, the restoration of ∼60% of hemidesmosomal structures was also noted. Although we could correct the skin phenotype in 11.9% of homozygous LAMB3(IAP) mice, none survived beyond 48 h. However, skin transplants from treated E18 homozygous LAMB3(IAP) fetuses maintained normal appearance for 6 months with persistence of normal assembly of laminin-332. These results demonstrate for the first time long-term phenotypic correction of the skin pathology in a severe model of JEB by in vivo prenatal gene transfer. Although survival remained limited due to the limitations of this mouse model, this study supports the potential for treatment of JEB by prenatal gene transfer.

A genome-wide analysis of open chromatin in human tracheal epithelial cells reveals novel candidate regulatory elements for lung function

BACKGROUND

Distal cell-type-specific regulatory elements may be located at very large distances from the genes that they control and are often hidden within intergenic regions or in introns of other genes. The development of methods that enable mapping of regions of open chromatin genome wide has greatly advanced the identification and characterisation of these elements.

METHODS

Here we use DNase I hypersensitivity mapping followed by deep sequencing (DNase-seq) to generate a map of open chromatin in primary human tracheal epithelial (HTE) cells and use bioinformatic approaches to characterise the distribution of these sites within the genome and with respect to gene promoters, intronic and intergenic regions.

RESULTS

Genes with HTE-selective open chromatin at their promoters were associated with multiple pathways of epithelial function and differentiation. The data predict novel cell-type-specific regulatory elements for genes involved in HTE cell function, such as structural proteins and ion channels, and the transcription factors that may interact with them to control gene expression. Moreover, the map of open chromatin can identify the location of potentially critical regulatory elements in genome-wide association studies (GWAS) in which the strongest association is with single nucleotide polymorphisms in non-coding regions of the genome. We demonstrate its relevance to a recent GWAS that identifies modifiers of cystic fibrosis lung disease severity.

CONCLUSION

Since HTE cells have many functional similarities with bronchial epithelial cells and other differentiated cells in the respiratory epithelium, these data are of direct relevance to elucidating the molecular basis of normal lung function and lung disease.

Characterization of a novel type I keratin gene and generation of transgenic lines with fluorescent reporter genes driven by its promoter/enhancer in Xenopus laevis

We investigated the characteristics of a novel type I keratin gene in Xenopus laevis during ontogenesis. The transcript was first detected in the posterior region at the late neurula stage, and then restricted to the fin and external gill during embryogenesis. To examine the transcriptional regulation of the keratin gene in vivo, we generated transgenic lines with fluorescent reporter genes driven by its 4.2-kb upstream sequence. The promoter/enhancer activity recapitulated the endogenous gene expression during embryogenesis. Sequential deletion analyses revealed that the regions proximal to the promoter were essential for fin-specific expression. Reporter expression was detected in various organs, including the fin and gill. In particular, robust expression was observed in the developing limbs and gill. The reporter fluorescence rapidly decreased with internal gill resorption during metamorphosis. The transgenic lines carrying the promoter/enhancer should represent valuable tools for elucidating the formation, development and resorption of various organs, especially the gill.

A study of the expression of functional human coagulation factor IX in keratinocytes using a nonviral vector regulated by K14 promoter

Ex vivo gene therapy requires a suitable bioreactor for production and delivery of the gene products into a target tissue, and keratinocyte is suitable model in this regard because of its potential for systemic release of proteins. To establish a keratinocyte-specific expression system, a mammalian-based expression plasmid equipped with a 2,240-bp fragment from the human keratin 14 (k14) gene enhancer/promoter region was constructed and used for the insertion of the human coagulation factor IX (hFIX)-cDNA downstream the K14-derived regulatory elements. The human epidermal keratinocytes isolated from neonatal foreskin were cultivated in keratinocyte serum-free media and transfected with the recombinant plasmid. The K14-promoter-driven expression of recombinant hFIX (rhFIX) was evaluated by performing coagulation test as well as enzyme-linked immunosorbent assay on the cultured media collected from the transfected cells at various stages. The rhFIX corresponding transcript and protein were confirmed by performing reverse transcription PCR as well as immunoblotting experiments, respectively. Based on the coagulation activities obtained from the conditioned media of nine isolated clones, the hFIX expression levels vary from 5% to 39% of normal human plasma. Expression levels of the hFIX obtained in this study are comparable to those reported for viral systems. The obtained data supported the potential of keratinocyte for the expression and secretion of biologically active rhFIX and underscore the importance of the examined cis sequences for enhancing gene expression in a mammalian expression system. Besides, it has provided means for further bioengineering strategies to improve the expression efficiency of the hFIX in keratinocytes and other mammalian host cells.

Activation of PPARbeta/delta causes a psoriasis-like skin disease in vivo

Background

Psoriasis is one of the most frequent skin diseases world-wide. The disease impacts enormously on affected patients and poses a huge financial burden on health care providers. Several lines of evidence suggest that the nuclear hormone receptor peroxisome proliferator activator (PPAR) β/δ, known to regulate epithelial differentiation and wound healing, contributes to psoriasis pathogenesis. It is unclear, however, whether activation of PPARβ/δ is sufficient to trigger psoriasis-like changes in vivo.

Methodology/Principal Findings

Using immunohistochemistry, we define the distribution of PPARβ/δ in the skin lesions of psoriasis. By expression profiling, we confirm that PPARβ/δ is overexpressed in the vast majority of psoriasis patients. We further establish a transgenic model allowing inducible activation of PPARβ/δ in murine epidermis mimicking its distribution in psoriasis lesions. Upon activation of PPARβ/δ, transgenic mice sustain an inflammatory skin disease strikingly similar to psoriasis, featuring hyperproliferation of keratinocytes, dendritic cell accumulation, and endothelial activation. Development of this phenotype requires the activation of the Th17 subset of T cells, shown previously to be central to psoriasis. Moreover, gene dysregulation in the transgenic mice is highly similar to that in psoriasis. Key transcriptional programs activated in psoriasis, including IL1-related signalling and cholesterol biosynthesis, are replicated in the mouse model, suggesting that PPARβ/δ regulates these transcriptional changes in psoriasis. Finally, we identify phosphorylation of STAT3 as a novel pathway activated by PPARβ/δ and show that inhibition of STAT3 phosphorylation blocks disease development.

Conclusions

Activation of PPARβ/δ in the epidermis is sufficient to trigger inflammatory changes, immune activation, and signalling, and gene dysregulation characteristic of psoriasis.

An active role of the DeltaN isoform of p63 in regulating basal keratin genes K5 and K14 and directing epidermal cell fate

Background

One major defining characteristic of the basal keratinocytes of the stratified epithelium is the expression of the keratin genes K5 and K14. The temporal and spatial expression of these two genes is usually tightly and coordinately regulated at the transcriptional level. This ensures the obligate pairing of K5 and K14 proteins to generate an intermediate filament (IF) network that is essential for the structure and function of the proliferative keratinocytes. Our previous studies have shown that the basal-keratinocyte restricted transcription factor p63 is a direct regulator of K14 gene.

Methodology/Principal Findings

Here we provide evidence that p63, specifically the ΔN isoform also regulates the expression of the K5 gene by binding to a conserved enhancer within the 5′ upstream region. By using specific antibodies against ΔNp63, we show a concordance in the expression between basal keratins and ΔNp63 proteins but not the TAp63 isoforms during early embryonic skin development. We demonstrate, that contrary to a previous report, transgenic mice expressing ΔNp63 in lung epithelium exhibit squamous metaplasia with de novo induction of K5 and K14 as well as transdifferentiation to the epidermal cell lineage. Interestingly, the in vivo epidermal inductive properties of ΔNp63 do not require the C-terminal SAM domain. Finally, we show that ΔNp63 alone can restore the expression of the basal keratins and reinitiate the failed epidermal differentiation program in the skin of p63 null animals.

Significance

ΔNp63 is a critical mediator of keratinocyte stratification program and directly regulates the basal keratin genes.

Keratin 7 promoter selectively targets transgene expression to normal and neoplastic pancreatic ductal cells in vitro and in vivo

Keratin 7 is expressed in simple epithelia but is expressed at low or undetectable levels in gastrointestinal epithelial cells. In the pancreas, it is present in ductal but not in acinar cells. K7 mRNA is overexpressed in pancreatic cancers. Here we use luciferase reporter assays to analyze the tissue-specific regulatory elements of murine keratin 7 (Krt7) promoter in vitro and in vivo. All elements required for appropriate cell and tissue specificity in reporter assays are present within the Krt7 -234 bp sequence. This fragment appears more selective to pancreatic ductal cells than the Krt19 promoter. GC-rich sequences corresponding to putative Sp1, AP-2 binding sites are essential for in vitro activity. Krt7-LacZ transgenic mice were generated to analyze in vivo activity. Sequences located 1.5 or 0.25 kb upstream of the transcription initiation site drive reporter expression to ductal, but not acinar, cells in transgenic mice. LacZ mRNA was detected in the pancreas as well as in additional epithelial tissues--such as the intestine and the lung--using both promoter constructs. An AdK7Luc adenovirus was generated to assess targeting selectivity in vivo by intravenous injection to immunocompetent mice and in a xenograft model of pancreatic cancer. The -0.25 kb region showed pancreatic selectivity, high activity in pancreatic cancers, and sustained transgene expression in xenografts. In conclusion, the krt7 promoter is useful to target pancreatic ductal adenocarcinoma cells in vitro and in vivo.

NFATc1 balances quiescence and proliferation of skin stem cells

Quiescent adult stem cells reside in specialized niches where they become activated to proliferate and differentiate during tissue homeostasis and injury. How stem cell quiescence is governed is poorly understood. We report here that NFATc1 is preferentially expressed by hair follicle stem cells in their niche, where its expression is activated by BMP signaling upstream and it acts downstream to transcriptionally repress CDK4 and maintain stem cell quiescence. As stem cells become activated during hair growth, NFATc1 is downregulated, relieving CDK4 repression and activating proliferation. When calcineurin/NFATc1 signaling is suppressed, pharmacologically or via complete or conditional NFATc1 gene ablation, stem cells are activated prematurely, resulting in precocious follicular growth. Our findings may explain why patients receiving cyclosporine A for immunosuppressive therapy display excessive hair growth, and unveil a functional role for calcium-NFATc1-CDK4 circuitry in governing stem cell quiescence.

Role of Sp1 in Transcription of Human ATP2A2 Gene in Keratinocytes

The ATP2A2 gene encodes Ca2+-dependent ATPase, the dysfunction of which causes Darier disease. In this study, we analyzed the promoter structure of the human ATP2A2 gene using primary normal human keratinocytes (NHK). Reporter assays showed that deletion of -550/-529, -488/-472, -390/-362, or -42/-21 resulted in a significant decrease in human ATP2A2 promoter activity. Electrophoretic mobility shift assay (EMSA) showed that Sp1 is a transcription factor that binds to the -550/-529 and -488/-472 regions of the promoter. Chromatin immunoprecipitation (ChIP) assay demonstrated that Sp1, but not Sp3, binds to the promoter region of the ATP2A2 gene in NHK cells in vivo. Knockdown of Sp1 expression by small interfering RNA resulted in a marked reduction in ATP2A2 promoter activity and ATP2A2 mRNA levels in NHK, suggesting that Sp1 positively transactivates the ATP2A2 promoter in NHK. This is early evidence demonstrating that Sp1 plays an important and positive role in ATP2A2 gene expression in NHK in vivo and in vitro.

Areca nut extract treatment down-regulates involucrin in normal human oral keratinocyte through P13K/AKT activation

Areca (betel) is an important etiological factor linked to the high prevalence of oral carcinoma and other oral diseases in South Asians. Involucrin is a key component of the cornified envelop and a differentiation marker of keratinocyte. In this study, we found that 5 microg/ml non-toxic areca nut extract (ANE) treatment resulted in the 0.5-fold down-regulation of involucrin and disruption in involucrin distribution in normal human oral keratinocyte (NHOK). Progressive down-regulation of involucrin during oral carcinogenesis was noted. Activation of AKT by 1.7-fold and up-regulation of COX-2 by 2-fold were elicited following ANE treatment in NHOK. Treatment with PI3K/AKT blockers reverted the down-regulation of involucrin. ANE also down-regulated involucrin by 0.6-fold and disturbed both cornified envelope and cell aggregation in calcium-induced differentiated NHOK. However, such phenomena seemed to be independent from the ANE-associated COX-2 activation. The ANE-associated down-regulation of involucrin through AKT pathway could underlie the areca-associated epithelial pathogenesis.

Mechanisms regulating epithelial stratification

The epidermis is a stratified epithelium that functions as a barrier protecting the organism from dehydration, mechanical trauma, and microbial insults. This barrier function is established during embryogenesis through a complex and tightly controlled stratification program. Whereas the morphological changes that occur during epidermal development have been extensively studied, the molecular mechanisms that govern this process remain poorly understood. In this review we summarize the current advances that have been made in understanding the molecular mechanisms that regulate epidermal morphogenesis.

A functional enhancer of keratin14 is a direct transcriptional target of deltaNp63

Keratin14 (K14) is a prototypic marker of dividing basal keratinocytes where its gene is transcribed at high levels. Transcriptional regulation of K14 is governed by an evolutionarily conserved functional enhancer marked by DNase 1 hypersensitive sites present upstream of the gene. This enhancer is sufficient to confer epidermal-specific gene expression, which is mediated in part by binding of members of activator protein-2 (AP)-2, AP-1, Ets, and Sp1 families of transcription factors. Here we provide evidence that a keratinocyte-specific nuclear protein identified as deltaNp63 binds to a conserved motif within this enhancer. Interestingly, the selective expression profile of deltaNp63 in various cell lines correlates with both the nuclear complex and the expression of K14. Biochemical studies reveal that deltaNp63 can bind to a specific DNA sequence present in the K14 enhancer and this binding leads to transactivation. In addition, chromatin immunoprecipitation experiments with deltaNp63-specific antibodies demonstrate that the enhancer is occupied by deltaNp63 in cultured keratinocytes and in mouse skin epidermal cells in vivo. Finally, we show that ectopic expression of either p63 isoform (deltaN or TA) can induce de novo expression of K14. These studies provide a potential mechanism by which deltaNp63 directly governs the expression of K14 in a keratinocyte-specific manner.

An autoregulatory loop directs the tissue-specific expression of p63 through a long-range evolutionarily conserved enhancer

p63, a p53 family member, is essential for the development of various stratified epithelia and is one of the earliest markers of many ectodermal structures, including the epidermis, oral mucosa, apical ectodermal ridge, and mammary gland. Genetic regulatory mechanisms controlling p63 spatial expression during development have not yet been defined. Using a genomic approach, we identified an evolutionarily conserved cis-regulatory element, located 160 kb downstream of the first p63 exon, which functions as a keratinocyte-specific enhancer and is sufficient to recapitulate expression of the endogenous gene during mouse embryogenesis. Dissection of the p63 enhancer activity revealed a positive autoregulatory loop in which the p63 proteins directly bind to and are essential regulators of the enhancer. Accordingly, transactivating p63 isoforms induce endogenous p63 expression in cells that do not normally express this gene, whereas dominant negative isoforms suppress p63 expression in keratinocytes. In addition the transcription factor AP-2 also binds to the enhancer and cooperates with p63 to induce its activity. These results demonstrate that a long-range autoregulatory loop is involved in the regulation of p63 expression during embryonic development and in adult cells.

AP-2alpha: a regulator of EGF receptor signaling and proliferation in skin epidermis

AP-2 transcription factors have been implicated in epidermal biology, but their functional significance has remained elusive. Using conditional knockout technology, we show that AP-2α is essential for governing the balance between growth and differentiation in epidermis. In vivo, epidermis lacking AP-2α exhibits elevated expression of the epidermal growth factor receptor (EGFR) in the differentiating layers, resulting in hyperproliferation when the receptors are activated. Chromatin immunoprecipitation and promoter activity assays identify EGFR as a direct target gene for AP-2α repression, and, in the absence of AP-2α, this is manifested primarily in excessive EGF-dependent phosphoinositol-3 kinase/Akt activity. Together, our findings unveil a hitherto unrecognized repressive role for AP-2α in governing EGFR gene transcription as cells exit the basal layer and withdraw from the cell cycle. These results provide insights into why elevated AP-2α levels are often associated with terminal differentiation and why tumor cells often display reduced AP-2α and elevated EGFR proteins.

Regulation of histone deacetylase 4 expression by the SP family of transcription factors

Histone deacetylases mediate critical cellular functions but relatively little is known about mechanisms controlling their expression, including expression of HDAC4, a class II HDAC implicated in the modulation of cellular differentiation and viability. Endogenous HDAC4 mRNA, protein levels and promoter activity were all readily repressed by mithramycin, suggesting regulation by GC-rich DNA sequences. We validated consensus binding sites for Sp1/Sp3 transcription factors in the HDAC4 promoter through truncation studies and targeted mutagenesis. Specific and functional binding by Sp1/Sp3 at these sites was confirmed with chromatin immunoprecipitation (ChIP) and electromobility shift assays (EMSA). Cotransfection of either Sp1 or Sp3 with a reporter driven by the HDAC4 promoter led to high activities in SL2 insect cells (which lack endogenous Sp1/Sp3). In human cells, restored expression of Sp1 and Sp3 up-regulated HDAC4 protein levels, whereas levels were decreased by RNA-interference-mediated knockdown of either protein. Finally, variable levels of Sp1 were in concordance with that of HDAC4 in a number of human tissues and cancer cell lines. These studies together characterize for the first time the activity of the HDAC4 promoter, through which Sp1 and Sp3 modulates expression of HDAC4 and which may contribute to tissue or cell-line-specific expression of HDAC4.

TAp63alpha induces AP-2gamma as an early event in epidermal morphogenesis

Epidermal morphogenesis begins with the commitment of the single-layered surface ectoderm to initiate a stratification program, a process that requires the expression of the transcription factor TAp63alpha. To determine the molecular mechanism by which TAp63alpha induces genes associated with the commitment to stratification, such as K14, we have used a combination of in vitro and in vivo approaches. Our initial gene expression profiling studies suggested that TAp63alpha could regulate one or more AP-2 genes, which have been implicated in development and maintenance of the epidermis. We now demonstrate that TAp63alpha directly induces AP-2gamma expression in embryonic epidermis, when commitment to stratification occurs. Furthermore, we show that, in the absence of AP-2gamma, TAp63alpha fails to induce K14 expression in vitro. Our data identify AP-2gamma as the first in vivo target gene of TAp63alpha, and provide novel insights into the molecular mechanisms associated with early events in epidermal morphogenesis.

BMP4-dependent expression of Xenopus Grainyhead-like 1 is essential for epidermal differentiation

Morphogen-dependent epidermal-specific transacting factors have not been defined in vertebrates. We demonstrate that a member of the grainyhead transcription factor family, Grainyhead-like 1 (XGrhl1) is essential for ectodermal ontogeny in Xenopus laevis. Expression of this factor is restricted to epidermal cells. Moreover, XGrhl1 is regulated by the BMP4 signaling cascade. Disruption of XGrhl1 activity in vivo results in a severe defect in terminal epidermal differentiation, with inhibition of XK81A1 epidermal keratin gene expression, a key target of BMP4 signaling. Furthermore, transcription of the XK81A1 gene is modulated directly by binding of XGRHL1 to a promoter-localized binding motif that is essential for high-level expression. These results establish a novel developmental role for XGrhl1 as a crucial tissue-specific regulator of vertebrate epidermal differentiation.

p63 and epithelial appendage development

Abstract Epithelial appendages share a common developmental program that relies on extensive interactions between epithelia and adjacent mesenchyme. The transcription factor p63 has a critical role in epithelial appendage development in both vertebrates and non-vertebrates. Both mice and zebrafish lacking p63 expression fail to develop epithelial appendages and other structures that develop as a result of epithelial-mesenchymal interactions. Furthermore, dominantly inherited mutations in p63 are the cause of a subset of human ectodermal dysplasias, which are characterized by developmental abnormalities in epithelia and epithelial appendages. While the importance of p63 for epithelial appendage development is evident, the molecular mechanisms by which p63 functions are largely unknown. In this review, we will discuss the current knowledge of the developmental role of p63 and the implications for epithelial appendage development.

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.

Abnormal epidermal differentiation and impaired epithelial-mesenchymal tissue interactions in mice lacking the retinoblastoma relatives p107 and p130

The functions of p107 and p130, members of the retinoblastoma family, include the control of cell cycle progression and differentiation in several tissues. Our previous studies suggested a role for p107 and p130 in keratinocyte differentiation in vitro. We now extend these data using knockout animal models. We found impaired terminal differentiation in the interfollicular keratinocytes of p107/p130-double-null mice epidermis. In addition, we observed a decreased number of hair follicles and a clear developmental delay in hair, whiskers and tooth germs. Skin grafts of p107/p130-deficient epidermis onto NOD/scid mice showed altered differentiation and hyperproliferation of the interfollicular keratinocytes, thus demonstrating that the absence of p107 and p130 results in the deficient control of differentiation in keratinocytes in a cell-autonomous manner. Besides normal hair formation, follicular cysts, misoriented and dysplastic follicles, together with aberrant hair cycling, were also observed in the p107/p130 skin transplants. Finally, the hair abnormalities in p107/p130-null skin were associated with altered Bmp4-dependent signaling including decreased DeltaNp63 expression. These results indicate an essential role for p107 and p130 in the epithelial-mesenchimal interactions.

Metamorphosis-dependent transcriptional regulation of xak-c, a novel Xenopus type I keratin gene

Anuran larvae transform their epidermis to the adult counterpart during metamorphosis. The major event of this process is the proliferation of larval epidermal basal cells and their differentiation into adult ones. The present study isolated novel type I keratin cDNA dubbed xak-c (Xenopus adult keratin-c) that was exclusively expressed in adult epidermal basal cells. The gene started its expression in the larval epidermis at the onset of metamorphosis. Thyroid hormone (TH) induced the precocious expression of the gene in the epidermis of premetamorphic tadpoles. To study the transcriptional regulation of this gene in relation to epidermal metamorphosis, a 2.8 kb 5'-flanking region of xak-c was cloned and its promoter activity was investigated. Gene constructs were made so as to contain the xak-c promoter region and gene of EGFP or luciferase as a reporter gene and were transfected into various types of cells, which revealed that the 5'-flanking region had an epidermal cell-specific transcriptional activity in both anurans and mammals. Larval skin tissues of Xenopus were transfected with the constructs and cultured in the presence and absence of TH, which showed that the promoter region is responsive to TH, although the region did not contain the consensus TH response element-like sequence. In sharp contrast, the promoter region did not respond to TH in the adult skin, clearly indicating that the cloned region contains specific sequences that respond to metamorphosis-dependent transcription factor(s).