Requirement of an AP-1 site in the calcium response region of the involucrin promoter

Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

Investigation into the Anti-Acne Effects of Castanea sativa Mill Leaf and Its Pure Ellagitannin Castalagin in HaCaT Cells Infected with Cutibacterium acnes

Acne vulgaris is a prevalent skin disorder affecting many young individuals, marked by keratinization, inflammation, seborrhea, and colonization by Cutibacterium acnes (C. acnes). Ellagitannins, known for their antibacterial and anti-inflammatory properties, have not been widely studied for their anti-acne effects. Chestnut (Castanea sativa Mill., C. sativa), a rich ellagitannin source, including castalagin whose acne-related bioactivity was previously unexplored, was investigated in this study. The research assessed the effect of C. sativa leaf extract and castalagin on human keratinocytes (HaCaT) infected with C. acnes, finding that both inhibited IL-8 and IL-6 release at concentrations below 25 μg/mL. The action mechanism was linked to NF-κB inhibition, without AP-1 involvement. Furthermore, the extract displayed anti-biofilm properties and reduced CK-10 expression, indicating a potential role in mitigating inflammation, bacterial colonization, and keratosis. Castalagin's bioactivity mirrored the extract's effects, notably in IL-8 inhibition, NF-κB inhibition, and biofilm formation at low μM levels. Other polyphenols, such as flavonol glycosides identified via LC-MS, might also contribute to the extract's biological activities. This study is the first to explore ellagitannins' potential in treating acne, offering insights for developing chestnut-based anti-acne treatments pending future in vivo studies.

Activating mutations in JAK2 and CALR differentially affect intracellular calcium flux in store operated calcium entry

BACKGROUND

Calcium (Ca2+) signaling regulates various vital cellular functions, including integrin activation and cell migration. Store-operated calcium entry (SOCE) via calcium release-activated calcium (CRAC) channels represents a major pathway for Ca2+ influx from the extracellular space in multiple cell types. The impact of JAK2-V617F and CALR mutations which are disease initiating in myeloproliferative neoplasms (MPN) on SOCE, calcium flux from the endoplasmic reticulum (ER) to the cytosol, and related key signaling pathways in the presence or absence of erythropoietin (EPO) or thrombopoietin (TPO) is poorly understood. Thus, this study aimed to elucidate the effects of these mutations on the aforementioned calcium dynamics, in cellular models of MPN.

METHODS

Intracellular Ca2+ levels were measured over a time frame of 0-1080 s in Fura-2 AM labeled myeloid progenitor 32D cells expressing various mutations (JAK2-WT/EpoR, JAK2-V617F/EpoR; CALR-WT/MPL, CALR-ins5/MPL, and del52/MPL). Basal Ca2+ concentrations were assessed from 0-108 s. Subsequently, cells were stimulated with EPO/TPO in Ca2+-free Ringer solution, measuring Ca2+ levels from 109-594 s (store depletion). Then, 2 mM of Ca2+ buffer resembling physiological concentrations was added to induce SOCE, and Ca2+ levels were measured from 595-1080 s. Fura-2 AM emission ratios (F340/380) were used to quantify the integrated Ca2+ signal. Statistical significance was assessed by unpaired Student's t-test or Mann-Whitney-U-test, one-way or two-way ANOVA followed by Tukey's multiple comparison test.

RESULTS

Following EPO stimulation, the area under the curve (AUC) representing SOCE significantly increased in 32D-JAK2-V617F cells compared to JAK2-WT cells. In TPO-stimulated CALR cells, we observed elevated Ca2+ levels during store depletion and SOCE in CALR-WT cells compared to CALR-ins5 and del52 cells. Notably, upon stimulation, key components of the Ca2+ signaling pathways, including PLCγ-1 and IP3R, were differentially affected in these cell lines. Hyper-activated PLCγ-1 and IP3R were observed in JAK2-V617F but not in CALR mutated cells. Inhibition of calcium regulatory mechanisms suppressed cellular growth and induced apoptosis in JAK2-V617F cells.

CONCLUSIONS

This report highlights the impact of JAK2 and CALR mutations on Ca2+ flux (store depletion and SOCE) in response to stimulation with EPO and TPO. The study shows that the JAK2-V617F mutation strongly alters the regulatory mechanism of EpoR/JAK2-dependent intracellular calcium balance, affecting baseline calcium levels, EPO-induced calcium entry, and PLCγ-1 signaling pathways. Our results reveal an important role of calcium flux in the homeostasis of JAK2-V617F positive cells.

Dihydromyrcenol Modulates Involucrin Expression through the Akt Signaling Pathway

The epidermis serves as a protective barrier against external threats and is primarily composed of keratinocytes, which ultimately form corneocytes. Involucrin, a protein integral to the cornified envelope, plays a pivotal role in preserving the functional integrity of the skin barrier. Previous studies have shown that Akt plays an important role in keratinocyte differentiation and skin barrier development. This study investigated whether dihydromyrcenol (DHM), a plant-derived terpene, could increase involucrin production in keratinocytes and sought to elucidate the possible underlying mechanisms. To accomplish this objective, we assessed the alterations in involucrin by DHM through quantitative PCR and Western blot on the HaCaT cell line. The changes in the promoter levels were investigated using luciferase assays. Furthermore, upstream mechanisms were explored through the use of siRNA and inhibitors. To strengthen our findings, the results were subsequently validated in primary cells and 3D skin equivalents. DHM significantly increased involucrin mRNA and protein levels in a concentration-dependent manner. In addition, the Fyn-Akt signaling pathway was found to be required for DHM-induced involucrin expression, as inhibition of Fyn or Akt blocked the increase in involucrin mRNA induced by DHM. The transcription factor Sp1, which is recognized as one of the transcription factors for involucrin, was observed to be activated in response to DHM treatment. Moreover, DHM increased epidermal thickness in a 3D human skin model. These findings suggest that the modulation of involucrin expression with DHM could improve skin barrier function and highlight the importance of manipulating the Akt pathway to achieve this improvement.

TRPC1 channel clustering during store-operated Ca2+ entry in keratinocytes

Skin is the largest organ in the human body with ∼95% of its surface made up of keratinocytes. These cells maintain a healthy skin barrier through regulated differentiation driven by Ca²⁺-transcriptional coupling. Many important skin conditions arise from disruption of this process although not all stages are fully understood. We know that elevated extracellular Ca²⁺ at the skin surface is detected by keratinocyte Gα_q-coupled receptors that signal to empty endoplasmic reticulum Ca²⁺ stores. Orai channel store-operated Ca²⁺ entry (SOCE) and Ca²⁺ influx via "canonical" transient receptor potential (TRPC)-composed channels then activates transcription factors that drive differentiation. While STIM-mediated activation of Orai channels following store depletion is well defined, how TRPC channels are activated is less clear. Multiple modes of TRPC channel activation have been proposed, including 1) independent TRPC activation by STIM, 2) formation of Orai-TRPC-STIM complexes, and 3) the insertion of constitutively-active TRPC channels into the membrane during SOCE. To help distinguish between these models, we used high-resolution microscopy of intact keratinocyte (HaCaT) cells and immunogold transmission electron microscopy (TEM) of HaCaT plasma membrane sheets. Our data shows no evidence of significant insertion of Orai1 or TRPC subunits into the membrane during SOCE. Analysis of transmission electron microscopy data shows that during store-depletion and SOCE, Orai1 and TRPC subunits form separate membrane-localized clusters that migrate towards each other. This clustering of TRPC channel subunits in keratinocytes may support the formation of TRPC-STIM interactions at ER-plasma membrane junctions that are distinct from Orai-STIM junctions.

Calcium-Inducible MAPK/AP-1 Signaling Drives Semaphorin 3A Expression in Normal Human Epidermal Keratinocytes

Epidermal keratinocytes are primarily involved in the expression of semaphorin (Sema) 3A, which is involved in the regulation of cutaneous innervation. However, the mechanisms underlying the intracellular signaling of Sema3A expression in keratinocytes remain unknown. We herein investigated the signaling mechanisms for the induction of Sema3A expression in normal human epidermal keratinocytes (NHEKs). Sema3A expression is transiently increased in calcium-stimulated NHEKs, whereas it is markedly decreased in terminally differentiated NHEKs. Sema3A mRNA is mainly localized in the stratum basale and stratum suprabasale of the epidermis. We cloned the 5'-flanking region of the Sema3A gene and identified a critical region for Sema3A promoter activity within -134 base pairs of the start codon. We found transcription factor binding sites, including that for activator protein (AP)-1, in this region. Sema3A expression was increased by the co-overexpression of JunB and Fra-2 in the presence of 0.1 or 1.4 mM calcium. The calcium-mediated transient upregulation of Sema3A expression was significantly suppressed by mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) 1/2 or AP-1 inhibitors. These results demonstrate that the calcium-mediated transient upregulation of Sema3A in NHEKs is involved in the MEK/ERK and AP-1 signaling axis. Therefore, Sema3A mRNA may be expressed in the lower epidermis under controlled conditions by calcium via the MAPK-AP-1 axis.

The Effect of Adiponectin on the Regulation of Filaggrin Expression in Normal Human Epidermal Keratinocytes

Background

Adiponectin, an adipokine secreted from adipocytes, affects energy metabolism and also shows anti-diabetic and anti-inflammatory properties. Recent studies have reported that adiponectin plays a role in regulating skin inflammation.

Objective

This study aimed to investigate the effect of adiponectin on the expression of filaggrin (FLG) in normal human epidermal keratinocytes (NHEKs).

Methods

NHEKs were serum-starved for 6h before being treated with adiponectin. Afterward, cell viability was assessed by MTT assay. We also treated with calcium, interleukin (IL)-4, and IL-13 to provide positive and negative comparative controls, respectively. Gene mRNA expression was quantified using real time reverse transcription polymerase chain reaction, and protein expression was evaluated using Western blot. To evaluate the relationship among mitogen-activated protein kinases (MAPKs), activator protein 1 (AP-1), and FLG, we also treated cells with inhibitors for MAPKs JNK, p38, and ERK1/2.

Results

FLG and FLG-2 mRNA expression in NHEKs significantly increased after treatment with 10 µg/ml adiponectin. Adiponectin also restored FLG and FLG-2 mRNA expression that was otherwise inhibited by treatment with IL-4 and IL-13. Adiponectin induced FLG expression via AP-1 and MAPK signaling.

Conclusion

Adiponectin positively regulated the expression of FLG and could be useful as a therapeutic agent to control diseases related to disrupted skin barrier function.

Skin Barrier and Calcium

Epidermal barrier formation and the maintenance of barrier homeostasis are essential to protect us from the external environments and organisms. Moreover, impaired keratinocytes differentiation and dysfunctional skin barrier can be the primary causes or aggravating factors for many inflammatory skin diseases including atopic dermatitis and psoriasis. Therefore, understanding the regulation mechanisms of keratinocytes differentiation and skin barrier homeostasis is important to understand many skin diseases and establish an effective treatment strategy. Calcium ions (Ca²⁺) and their concentration gradient in the epidermis are essential in regulating many skin functions, including keratinocyte differentiation, skin barrier formation, and permeability barrier homeostasis. Recent studies have suggested that the intracellular Ca²⁺ stores such as the endoplasmic reticulum (ER) are the major components that form the epidermal calcium gradient and the ER calcium homeostasis is crucial for regulating keratinocytes differentiation, intercellular junction formation, antimicrobial barrier, and permeability barrier homeostasis. Thus, both Ca²⁺ release from intracellular stores, such as the ER and Ca²⁺ influx mechanisms are important in skin barrier. In addition, growing evidences identified the functional existence and the role of many types of calcium channels which mediate calcium flux in keratinocytes. In this review, the origin of epidermal calcium gradient and their role in the formation and regulation of skin barrier are focused. We also focus on the role of ER calcium homeostasis in skin barrier. Furthermore, the distribution and role of epidermal calcium channels, including transient receptor potential channels, store-operated calcium entry channel Orai1, and voltage-gated calcium channels in skin barrier are discussed.

Nicotine-induced cellular stresses and autophagy in human cancer colon cells: A supportive effect on cell homeostasis via up-regulation of Cox-2 and PGE(2) production

Nicotine, one of the active components in cigarette smoke, has been described to contribute to the protective effect of smoking in ulcerative colitis (UC) patients. Furthermore, the nicotinic acetylcholine receptor α7 subunit (α7nAChR) expressed on immune cells, is an essential regulator of inflammation. As intestinal epithelial cells also express α7nAChR, we investigated how nicotine could participate in the homeostasis of intestinal epithelial cells. First, using the human adenocarcinoma cell line HT-29, we revealed that nicotine, which triggers an influx of extracellular Ca(2+) following α7nAChR stimulation, induces mitochondrial reactive oxygen species (ROS) production associated with a disruption of the mitochondrial membrane potential and endoplasmic reticulum stress. This results in caspase-3 activation, which in turn induces apoptosis. Additionally, we have shown that nicotine induces a PI3-K dependent up-regulation of cyclooxygenase-2 (Cox-2) expression and prostaglandin E2 (PGE2) production. In this context, we suggest that this key mediator participates in the cytoprotective effects of nicotine against apoptosis by stimulating autophagy in colon cancer cells. Our results provide new insight into one potential mechanism by which nicotine could protect from UC and suggest an anti-inflammatory role for the cholinergic pathway at the epithelial cell level.

Novel mechanisms for the vitamin D receptor (VDR) in the skin and in skin cancer

The VDR acting with or without its principal ligand 1,25(OH)2D regulates two central processes in the skin, interfollicular epidermal (IFE) differentiation and hair follicle cycling (HFC). Calcium is an important co-regulator with 1,25(OH)2D at least of epidermal differentiation. Knockout of the calcium sensing receptor (CaSR) in addition to VDR accelerates the development of skin cancer in mice on a low calcium diet. Coactivators such as mediator 1 (aka DRIP205) and steroid receptor coactivator 3 (SRC3) regulate VDR function at different stages of the differentiation process, with Med 1 essential for hair follicle differentiation and early stages of epidermal differentiation and proliferation and SRC3 essential for the latter stages of differentiation including formation of the permeability barrier and innate immunity. The corepressor of VDR, hairless (HR), is essential for hair follicle cycling, although its effect on epidermal differentiation in vivo is minimal. In its regulation of HFC and IFE VDR controls two pathways-wnt/β-catenin and sonic hedgehog (SHH). In the absence of VDR these pathways are overexpressed leading to tumor formation. Whereas, VDR binding to β-catenin may block its activation of TCF/LEF1 sites, β-catenin binding to VDR may enhance its activation of VDREs. 1,25(OH)2D promotes but may not be required for these interactions. Suppression of SHH expression by VDR, on the other hand, requires 1,25(OH)2D. The major point of emphasis is that the role of VDR in the skin involves a number of novel mechanisms, both 1,25(OH)2D dependent and independent, that when disrupted interfere with IFE differentiation and HFC, predisposing to cancer formation. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Calcium, Orai1, and epidermal proliferation

Ca(2+) influx controls essential epidermal functions, including proliferation, differentiation, cell migration, itch, and barrier homeostasis. The Orai1 ion channel allows capacitive Ca(2+) influx after Ca(2+) release from the endoplasmic reticulum, and it has now been shown to modulate epidermal atrophy. These findings reveal new interactions among various Ca(2+) signaling pathways and uncover novel functions for Ca(2+) signaling via the Orai1 channel.

Epithelial stem cells as mucosal antigen-delivering cells: A novel AIDS vaccine approach

A key obstacle limiting development of an effective AIDS vaccine is the inability to deliver antigen for a sufficient period of time resulting in weak and transient protection. HIV transmission occurs predominantly across mucosal surfaces; therefore, an ideal vaccine strategy would be to target HIV at mucosal entry sites to prevent infection. Such a novel strategy relies on the activation of mucosal immune response via presentation of viral antigens by the mucosal epithelial cells. The use of a terminally differentiated epithelial cell promoter to drive expression of antigens leading to viral protein production in the upper layers of the epithelium is central to the success of this approach. Our results show that when administered intradermally to mice, a GFP-reporter gene under the transcriptional control of the involucrin promoter is expressed in the upper layers of the epidermis and, although transduced cells were very low in number, high and sustained anti-GFP antibody production is observed in vivo. A subsequent experiment investigates the effectiveness of GFP-tagged replication-competent SIVdeltaNef and GFP-tagged replication-deficient SIVdeltaVifdeltaNef constructs under the transcriptional control of the involucrin promoter. Optimal conditions for production of pseudotyped VSV-G viral particles destined to transduce basal epithelial stem cells at the mucosal sites of entry of SIV in our animal model were determined. Altogether, the data demonstrate the feasibility of an epithelium-based vaccine containing involucrin-driven viral antigen encoding sequences that integrate into epithelial stem cells and show long-term expression in the upper layer of the epithelium even after multiple cycle of epithelia renewal. Such epithelium-based vaccine should elicit a long-term immunity against HIV/SIV infection at the site of entry of the virus.

Role of the calcium-sensing receptor in calcium regulation of epidermal differentiation and function

The epidermis is a stratified squamous epithelium composed of proliferating basal and differentiated suprabasal keratinocytes. It serves as the body's major physical and chemical barrier against infection and harsh environmental insults, as well as preventing excess water loss from the body into the atmosphere. Calcium is a key regulator of the proliferation and differentiation in keratinocytes. Elevated extracellular Ca(2+) concentration ([Ca(2+)]o) raises the levels of intracellular free calcium ([Ca(2+)]i), promotes cell-cell adhesion, and activates differentiation-related genes. Keratinocytes deficient in the calcium-sensing receptor fail to respond to [Ca(2+)]o stimulation and to differentiate, indicating a role for the calcium-sensing receptor in transducing the [Ca(2+)]o signal during differentiation. The concepts derived from in vitro gene knockdown experiments have been evaluated and confirmed in three mouse models in vivo.

Calcium regulation of keratinocyte differentiation

Calcium is the major regulator of keratinocyte differentiation in vivo and in vitro. A calcium gradient within the epidermis promotes the sequential differentiation of keratinocytes as they traverse the different layers of the epidermis to form the permeability barrier of the stratum corneum. Calcium promotes differentiation by both outside-in and inside-out signaling. A number of signaling pathways involved with differentiation are regulated by calcium, including the formation of desmosomes, adherens junctions and tight junctions, which maintain cell-cell adhesion and play an important intracellular signaling role through their activation of various kinases and phospholipases that produce second messengers that regulate intracellular free calcium and PKC activity, critical for the differentiation process. The calcium receptor plays a central role by initiating the intracellular signaling events that drive differentiation in response to extracellular calcium. This review will discuss these mechanisms.

Vitamin D and the skin: Physiology and pathophysiology

The keratinocytes of the skin are unique in being not only the primary source of vitamin D for the body, but in possessing both the enzymatic machinery to metabolize the vitamin D produced to active metabolites (in particular 1,25(OH)(2)D) and the vitamin D receptor (VDR) that enables the keratinocytes to respond to the 1,25(OH)(2)D thus generated. Numerous functions of the skin are regulated by vitamin D and/or its receptor. These include inhibition of proliferation, stimulation of differentiation including formation of the permeability barrier, promotion of innate immunity, regulation of the hair follicle cycle, and suppression of tumor formation. Regulation of these actions is exerted by a number of different coregulator complexes including the coactivators vitamin D receptor interacting protein (DRIP) complex also known as Mediator and the steroid receptor coactivator (SRC) family (of which SRC 2 and 3 are found in keratincytes), the inhibitor hairless (Hr), and β-catenin whose impact on VDR function is complex. Different coregulators appear to be involved in different VDR regulated functions. This review will examine the various functions of vitamin D and its receptor in the skin, and explore the mechanisms by which these functions are regulated.

Suppression of matrix metalloproteinase-9 expression in undifferentiated, non-apoptotic keratinocytes is abrogated by the cleavage of poly(ADP-ribose) polymerase-1

Matrix metalloproteinase (MMP)-9, an enzyme that degrades the extracellular matrix, has been implicated as a key enzyme in the process of tissue remodeling. This study demonstrates the regulation of MMP-9 transcription through a gene regulatory element in its promoter (the KRE-M9 element). The KRE-M9-binding protein was purified and identified as poly(ADP-ribose) polymerase-1 (PARP-1), which inhibits the transcription of MMP-9 similar to involucrin. This regulation occurs in non-apoptotic keratinocytes using the distinctive culture conditions of high and low Ca(2+) levels. PARP cleavage, which occurs during apoptosis, results in de-repression of MMP-9 promoter activity. These data clarify a new role of PARP-1 and suggest a physiologically relevant connection between caspase activation and MMP-9 expression.

Effects of human papillomavirus (HPV) type 16 oncoproteins on the expression of involucrin in human keratinocytes

BACKGROUND

The human papillomavirus (HPV) life cycle is closely linked to keratinocyte differentiation. Oncogenic HPV infection has been shown to hamper the normal differentiation of keratinocytes; however, the underlying mechanisms responsible for this phenomenon are yet to be clarified. Here, we aimed to study the effects of HPV16 E6 and E7 oncogenes on the expression of involucrin (IVL), an established marker of keratinocyte differentiation, in human foreskin keratinocyte (HFK) cells.

RESULTS

The differentiation of HFK cells by serum and high calcium significantly increased both the mRNA and the protein levels of IVL. The E6 and E7 oncoproteins of HPV16 together caused strong down-regulation of IVL mRNA and protein both in proliferating and in differentiating HFK cells. To study the effects of HPV oncogenes on the IVL promoter, we made transient transfection assays and luciferase tests and found that HPV 16 E6 but not E7 repressed IVL promoter activity in proliferating HFK cells. The inhibitory effect of HPV 16 E6 on the human IVL promoter could be localised to the proximal regulatory region (PRR) of the gene.

CONCLUSIONS

These results suggest that the down-regulation of IVL promoter activity by HPV 16 E6 significantly contribute to the inhibition of endogenous IVL expression by the HPV 16 oncoproteins. In contrast, the down-regulation of endogenous IVL expression by HPV16 E7 is probably not caused by a direct and specific effect of E7 on the IVL promoter.

TRAIL-induced keratinocyte differentiation requires caspase activation and p63 expression

Cornification, the terminal differentiation of keratinocytes, is a special form of programmed cell death in the skin. In this article, we report that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce the expression of the keratinocyte differentiation markers involucrin and type 1 transglutaminase in normal human epidermal keratinocytes. The induction of differentiation occurs mainly under the activation of caspases 3 and 8, and apoptosis can also be triggered. Inhibition of these apoptotic caspases attenuates both apoptosis and differentiation of keratinocytes caused by TRAIL but barely affects the induction of differentiation caused by calcium and phorbol 12-myristate 13-acetate. Differential regulation of extracellular signal-regulated kinase and p38 activation by TRAIL is also observed. Moreover, the degradation of p63 is induced by TRAIL-elicited caspase activation. However, the existence of p63 is essential for the initiation of keratinocyte differentiation by TRAIL because knockdown of ΔNp63 decreases TRAIL-induced differentiation. Taken together, our results suggest that TRAIL can be an inducer of both differentiation and apoptosis in human keratinocytes, and that caspases critically mediate these processes. This study identifies a new role of apoptotic caspases for terminal differentiation of keratinocytes and further elucidates the molecular pathways involved in this unique model of cell death.

Protein kinase C delta and eta differently regulate the expression of loricrin and Jun family proteins in human keratinocytes

Barrier function of the epidermis is maintained by precise expression of keratinocyte-specific structural proteins to form the cornified cell envelope (CE). Loricrin, a major component of the CE, is expressed at the late stage of keratinocyte differentiation. In this study, we reveal the isoform-specific function of protein kinase C (PKC) in the regulation of loricrin expression. Both PKCdelta and PKCeta have been recognized as differentiation-promoting isoforms. However, loricrin expression was inversely controlled by PKCdelta and PKCeta in cultured keratinocytes and 3D skin culture; i.e. loricrin expression was decreased by PKCdelta and increased by PKCeta. To clarify the mechanisms that PKCdelta and PKCeta oppositely regulate the loricrin expression, we examined the expression of activator protein-1 (AP-1) family proteins, which modulate the transcription of loricrin and are downstream molecules of PKC. PKCdelta decreased c-Jun expression, whereas PKCeta increased JunD, which are positive regulators of loricrin transcription. These findings suggest that inverse effects of PKCdelta and PKCeta on loricrin expression attributes to the expression of c-Jun and JunD.

Arsenite suppression of involucrin transcription through AP1 promoter sites in cultured human keratinocytes

While preserving keratinocyte proliferative ability, arsenite suppresses cellular differentiation markers by preventing utilization of AP1 transcriptional response elements. In present experiments, arsenite had a dramatic effect in electrophoretic mobility supershift analysis of proteins binding to an involucrin promoter AP1 response element. Without arsenite treatment, binding of JunB and Fra1 was readily detected in nuclear extracts from preconfluent cultures and was not detected a week after confluence, while c-Fos was detected only after confluence. By contrast, band shift of nuclear extracts from arsenite treated cultures showed only JunB and Fra1 binding in postconfluent as well as preconfluent cultures. Immunoblotting of cell extracts showed that arsenite treatment prevented the loss of Fra1 and the increase in c-Fos proteins that occurred after confluence in untreated cultures. Chromatin immunoprecipitation assays demonstrated substantial reduction of c-Fos and acetylated histone H3 at the proximal and distal AP1 response elements in the involucrin promoter and of coactivator p300 at the proximal element. Alteration of AP1 transcription factors was also examined in response to treatment with four metal containing compounds (chromate, vanadate, hemin, divalent cadmium) that also suppress involucrin transcription. These agents all influenced transcription at AP1 elements in a transcriptional reporter assay, but exhibited less effect than arsenite on binding activity assessed by mobility shift and chromatin immunoprecipitation and displayed variable effects on AP1 protein levels. These findings help trace a mechanism by which transcriptional effects of arsenite become manifest and help rationalize the unique action of arsenite, compared to the other agents, to preserve proliferative ability.

Defective TPA signalling compromises HaCat cells as a human in vitro skin carcinogenesis model

HaCat cells, a human keratinocyte line, are commonly utilised as an in vitro cell model for toxicity testing and the discernment of processes of chemically induced skin carcinogenesis. Here, as part of an ongoing program of carcinogenesis research, we tested the genomic transcriptional response of two keratinocyte cell lines HaCat (human) and Pam212 (mouse) to 12-O-tetradecanoylphorbol 13-acetate (TPA), one of the most studied skin carcinoma promoting agents, and compared this with the response in primary keratinocytes. Differences in the genomic response profile indicated an insufficiency in the MEK/ERK pathway signalling in HaCat but not Pam212 cells compared to primary keratinocytes. TPA can also activate NFkappaB and so we tested whether this was also deficient in the HaCat cells using TNFalpha which signals directly to NFkappaB. By this method NFkappaB was found to be equally active in both HaCat and Pam212 cells. Analysis of ERK phosphorylation showed that while TPA mediated ERK phosphorylation occurred in both cell lines it was more robust and difficult to inhibit in Pam212 cells suggesting that there may be an insufficiency in this step in HaCat cells leading to a reduced response. Overall these data indicate that caution should be employed when using HaCat cells as an in vitro skin model for biochemical research or toxicological evaluation.

Overexpression of protease-activated receptor-1 contributes to melanoma metastasis via regulation of connexin 43

Protease-activated receptor-1 (PAR-1) is a key player in melanoma metastasis with higher expression seen in metastatic melanoma cell lines and tissue specimens. cDNA microarray and Western blot analyses reveal that the gap junctional intracellular communication molecule connexin 43 (Cx-43), known to be involved in tumor cell diapedesis and attachment to endothelial cells, is significantly decreased after PAR-1 silencing in metastatic melanoma cell lines. Furthermore, Cx-43 promoter activity was significantly inhibited in PAR-1-silenced cells, suggesting that PAR-1 regulates Cx-43 at the transcriptional level. Chromatin immunoprecipitation studies showed a reduction in the binding of SP-1 and AP-1 transcription factors to the promoter of Cx-43. Both transcription factors have been shown previously to be required for maximal Cx-43 promoter activity. These results were corroborated by mutating the AP-1 and SP-1 binding sites resulting in decreased Cx-43 promoter activity in PAR-1-positive cells. Moreover, as Cx-43 has been shown to facilitate arrest of circulating tumor cells at the vascular endothelium, melanoma cell attachment to endothelial cells was significantly decreased in PAR-1-silenced cells, with this effect being abrogated after PAR-1 rescue. Herein, we report that up-regulation of PAR-1 expression, seen in melanoma progression, mediates high levels of Cx-43 expression. As both SP-1 and AP-1 transcription factors act as positive regulators of Cx-43, our data provide a novel mechanism for the regulation of Cx-43 expression by PAR-1. Indeed, Cx-43 expression was restored following PAR-1 rescue in PAR-1-silenced cells. Taken together, our data support the tumor promoting function of Cx-43 in melanoma.

Vitamin D receptor and coactivators SRC2 and 3 regulate epidermis-specific sphingolipid production and permeability barrier formation

The vitamin D receptor (VDR) is a nuclear hormone receptor that controls transcription of target genes. It exerts its biological effects through transcriptional coactivators. Previously, we identified two distinct classes of VDR coactivators, VDR-interacting protein (DRIP) and steroid receptor coactivator (SRC) at different stages of keratinocyte differentiation. Here, we determined the functions of VDR and coactivators in lipid production and permeability barrier formation. Silencing of either VDR, SRC2, or SRC3 resulted in decreases in specific glucosylceramide (GlcCer) species but not other lipids such as cholesterol and free fatty acids. Their silencing also caused decreased transcription of fatty acid elongase and ceramide glucosyltransferase, which are critical for the synthesis of epidermis-unique GlcCer species, and defects in lamellar body formation associated with decreased expression of the lipid transporter ATP-binding cassette transporter protein 12. VDR null mice exhibit abnormal barrier function with altered lipid composition in vivo. These results demonstrate that VDR and coactivators SRC2 and SRC3, which are also involved in other nuclear receptors as well, are critical for epidermis-specific sphingolipid production and barrier formation. In contrast, DRIP silencing had no apparent effect on these processes indicating that the two classes of coactivators are differentially utilized.

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

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

Involucrin Expression Is Decreased in Hailey–Hailey Keratinocytes Owing to Increased Involucrin mRNA Degradation

Hailey-Hailey disease (HHD) (MIM 16960) is an autosomal-dominant blistering skin disease caused by a mutation in the Ca2+-ATPase ATP2C1 (protein SPCA1), responsible for controlling Ca2+ concentrations in the cytoplasm and Golgi in human keratinocytes. Cytosolic Ca2+ concentrations, in turn, play a major role in the regulation of keratinocyte differentiation. To study how ATP2C1 function impacts keratinocyte differentiation, we assessed involucrin expression in HHD keratinocytes. Involucrin is a protein that makes up the cornified envelope of keratinocytes and is expressed in response to increased intracellular Ca2+ concentrations. Even though HHD keratinocytes suffer from abnormally high cytosolic Ca2+, we found that these cells expressed lower involucrin protein levels at both low and high extracellular Ca2+ concentrations when compared with normal control keratinocytes. Decreased involucrin protein levels were caused by lower involucrin mRNA levels in HHD keratinocytes. Decreased involucrin mRNA, in turn, was caused by increased rates of involucrin mRNA degradation. Ca2+-sensitive involucrin AP-1 promotor activity was increased, both in HHD keratinocytes and in an small interfering RNA (siRNA) experimental model, suggesting compensatory promoter upregulation in the face of increased mRNA degradation. This report provides new insights into differentiation defects in HHD and its relationship to Ca2+ signaling.

Subcellular localisation of BAG-1 and its regulation of vitamin D receptor-mediated transactivation and involucrin expression in oral keratinocytes: implications for oral carcinogenesis

In oral cancers, cytoplasmic BAG-1 overexpression is a marker of poor prognosis. BAG-1 regulates cellular growth, differentiation and survival through interactions with diverse proteins, including the vitamin D receptor (VDR), a key regulator of keratinocyte growth and differentiation. BAG-1 is expressed ubiquitously in human cells as three major isoforms of 50 kDa (BAG-1L), 46 kDa (BAG-1M) and 36 kDa (BAG-1S) from a single mRNA. In oral keratinocytes BAG-1L, but not BAG-1M and BAG-1S, enhanced VDR transactivation in response to 1alpha,25-dihydroxyvitamin D3. BAG-1L was nucleoplasmic and nucleolar, whereas BAG-1S and BAG-1M were cytoplasmic and nucleoplasmic in localisation. Having identified the nucleolar localisation sequence in BAG-1L, we showed that mutation of this sequence did not prevent BAG-1L from potentiating VDR activity. BAG-1L also potentiated transactivation of known vitamin-D-responsive gene promoters, osteocalcin and 24-hydroxylase, and enhanced VDR-dependent transcription and protein expression of the keratinocyte differentiation marker, involucrin. These results demonstrate endogenous gene regulation by BAG-1L by potentiating nuclear hormone receptor function and suggest a role for BAG-1L in 24-hydroxylase regulation of vitamin D metabolism and the cellular response of oral keratinocytes to 1alpha,25-dihydroxyvitamin D3. By contrast to the cytoplasmic BAG-1 isoforms, BAG-1L may act to suppress tumorigenesis.

TRPV6 is a Ca2+ entry channel essential for Ca2+-induced differentiation of human keratinocytes

Ca(2+) is an essential factor inducing keratinocyte differentiation due to the natural Ca(2+) gradient in the skin. However, the membrane mechanisms that mediate calcium entry and trigger keratinocyte differentiation had not previously been elucidated. In this study we demonstrate that Ca(2+)-induced differentiation up-regulates both mRNA and protein expression of a transient receptor potential highly Ca(2+)-selective channel, TRPV6. The latter mediates Ca(2+) uptake and accounts for the basal [Ca(2+)](i) in human keratinocytes. Our results show that TRPV6 is a prerequisite for keratinocyte entry into differentiation, because the silencing of TRPV6 in human primary keratinocytes led to the development of impaired differentiated phenotype triggered by Ca(2+). The expression of such differentiation markers as involucrin, transglutaminase-1, and cytokeratin-10 was significantly inhibited by small interfering RNA-TRPV6 as compared with differentiated control cells. TRPV6 silencing affected cell morphology and the development of intercellular contacts, as well as the ability of cells to stratify. 1,25-Dihydroxyvitamin D3, a cofactor of differentiation, dose-dependently increased TRPV6 mRNA and protein expression in human keratinocytes. This TRPV6 up-regulation led to a significant increase in Ca(2+) uptake in both undifferentiated and differentiated keratinocytes. We conclude that TRPV6 mediates, at least in part, the pro-differentiating effects of 1,25-dihydroxyvitamin D3 by increasing Ca(2+) entry, thereby promoting differentiation. Taken together, these data suggest that the TRPV6 channel is a key element in Ca(2+)/1,25-dihydroxyvitamin D3-induced differentiation of human keratinocytes.

Identification and characterization of small-molecule inducers of epidermal keratinocyte differentiation

An essential function of the human epidermis is the maintenance of a protective barrier against the environment. As a consequence, keratinocytes, which make up this layer of the skin, undergo an elaborate process of self-renewal, terminal differentiation, and cell death. Misregulation of these processes can lead to several human diseases, including psoriasis and basal cell and squamous cell carcinomas. To identify novel regulators of keratinocyte differentiation, a cell-based screen of small-molecule libraries was carried out for molecules that induce terminal differentiation of normal human epidermal keratinocytes. One class of molecules was identified, the 2-(3,4,5-trimethoxyphenylamino)-pyrrolo[2,3-d]pyrimidines, which were shown to induce differentiation of epidermal progenitor cells to terminally differentiated keratinocytes. These molecules serve as useful mechanistic probes of the cellular differentiation programs that regulate the formation and homeostasis of the epidermis and may lead to novel therapeutic approaches for the treatment of skin hyperproliferative disorders.

The role of proline-rich protein tyrosine kinase 2 in differentiation-dependent signaling in human epidermal keratinocytes

Non-receptor tyrosine kinase proline-rich protein tyrosine kinase 2 (Pyk2) functions as an integrator of multiple signaling pathways involved in the regulation of fundamental cellular processes. Pyk2 expression, regulation, and functions in skin have not been examined. Here we investigated the expression and subcellular localization of Pyk2 in human epidermis and in primary human keratinocytes, and studied the mechanisms of Pyk2 activation by differentiation-inducing stimuli, and the role of Pyk2 as a regulator of keratinocyte differentiation. We demonstrate that Pyk2 is abundantly expressed in skin keratinocytes. Notably, the endogenous Pyk2 protein is predominantly localized in keratinocyte nuclei throughout all layers of healthy human epidermis, and in cultured human keratinocytes. Pyk2 is activated by treatment with keratinocyte-differentiating agents, 12-O-tetradecanoylphorbol-13-acetate and calcium via a mechanism that requires intracellular calcium release and functional protein kinase C (PKC) and Src activities. Particularly, differentiation-promoting PKC delta and PKC eta elicit Pyk2 activation. Our data show that Pyk2 increases promoter activity and endogenous protein levels of involucrin, a marker of keratinocyte terminal differentiation. This regulation is associated with increased expression of Fra-1 and JunD, activator protein-1 transcription factors known to be required for involucrin expression. Altogether, these results provide insights into Pyk2 signaling in epidermis and reveal a novel role for Pyk2 in regulation of keratinocyte differentiation.

Opposing action of curcumin and green tea polyphenol in human keratinocytes

Persistent environmental insult can convert a normal cell into a cancer cell. However, various natural chemopreventive agents called antioxidants can retard this progression. We have recently explored the effects of several chemopreventive agents, including green tea polyphenol and curcumin, on normal human keratinocyte function. Our findings suggest that a bioactive polyphenol from green tea, (-)-epigallocatechin-3-gallate (EGCG), acts to increase involucrin gene expression, suggesting that EGCG treatment enhances normal human keratinocyte differentiation. Mechanistic studies indicate that EGCG alters mitogen-activated protein kinase cascade function to activate involucrin gene transcription via a Ras, MEKK1, MEK3, ERK1/2-p38delta cascade that targets AP1 and CAATT enhancer binding protein transcription factors. These findings suggest that EGCG may inhibit disease progression by promoting keratinocyte differentiation. Parallel studies indicate that not all antioxidants produce a similar response. Curcumin, an antioxidant derived from the turmeric, antagonizes the EGCG-dependent response by interfering in this signaling pathway. These studies suggest that different antioxidant may produce antagonistic effects in tissues.

Heptachlor epoxide induces a non-capacitative type of Ca2+ entry and immediate early gene expression in mouse hepatoma cells

The effects of the organochlorine (OC) liver tumor promoter heptachlor epoxide (HE) and a related non-tumor promoting OC, delta-hexachlorocyclohexane (delta-HCH), on the dynamics of intracellular calcium (Ca2+) were investigated in mouse 1c1c7 hepatoma cells. HE induced a non-capacitative, Ca2+ entry-like phenomenon, which was transient and concentration-dependent with 10 and 50 microM HE. The plasma membrane Ca2+ channel blocker SKF-96365 antagonized this HE-induced Ca2+ entry. delta-HCH failed to induce Ca2+ entry, rather it antagonized the HE-induced Ca2+ entry. Both HE and delta-HCH induced Ca2+ release from endoplasmic reticulum (ER) at treatment concentrations as low as 10 microM; at 50 microM, the former induced 5x as much Ca2+ release as the latter. The HE-induced Ca2+ release from the ER was antagonized using the IP3 receptor/channel blocker xestospongin C, suggesting that HE induces ER Ca2+ release through the IP3 receptor/channel pore. These results show that the effect of HE on cellular Ca2+ mimics that of mitogens such as epidermal and hepatocyte growth factors. They also provide insight into the similarities and differences between tumorigenic and non-tumorigenic OCs, in terms of the mechanisms and the extent of the [Ca2+]i increased by these agents.

Differentiating human keratinocytes are deficient in p53 but retain global nucleotide excision repair following ultraviolet radiation

Terminally differentiating keratinocytes constitute the predominant cell type within the skin epidermis and play an important role in the overall photobiology of human skin following ultraviolet radiation. However, the DNA repair capacity of differentiating keratinocytes is unclear, and little is known regarding how such repair activity is regulated in these cells. We systematically compared the global genomic nucleotide excision repair response of cultured undifferentiated human keratinocytes to those that were allowed to differentiate in 1.2 mM Ca(2+), in some cases supplemented with phorbol ester or Vitamin C. Differentiated cells ceased replication and expressed typical markers of differentiation. Following ultraviolet radiation, keratinocytes that were differentiated up to 12 days removed cyclobutane pyrimidine dimers and pyrimidine(6,4)pyrimidone photoproducts from the global genome as efficiently as undifferentiated cells. However, following the onset of calcium-induced differentiation, basal levels of p53 were nearly undetectable by 12 days of differentiation when global repair activity was unaffected. Following ultraviolet radiation, induction of p53 following ultraviolet radiation was abrogated by 6 days of calcium-induced differentiation. Basal levels of mRNA encoding the DNA damage recognition proteins, XPC and DDB2, were relatively insensitive to differentiation and p53 levels. However, following ultraviolet radiation, inductions of mRNA encoding the DNA damage recognition proteins, DDB2 and XPC, were differentially affected by differentiation. Rapid loss of DDB2 mRNA induction was associated with differentiation, while XPC mRNA induction diminished more slowly with differentiation. These results indicate that human keratinocytes preserve global nucleotide excision repair as well as expression of genes encoding key DNA damage recognition proteins well into the terminal differentiation process, perhaps using mechanisms other than p53.

The Fos family of transcription factors and their role in tumourigenesis

Members of the Fos family (c-Fos, FosB and its smaller splice variants, Fra-1 and Fra-2) dimerise with Jun proteins to form the AP-1 transcription factor complex. Based on the rapidly growing amount of data from experimental studies, animal models and investigations on clinical tumour samples, this review summarises the current knowledge about the role of these proteins in carcinogenesis. In addition to c-Fos, which has oncogenic activity and is frequently overexpressed in tumour cells, Fra-1 seems to play a role in the progression of many carcinomas. The results obtained from various studies show different implications for these transcription factors according to tumour type, i.e., Fra-1 overexpression enhances the motility and invasion of breast and colorectal cancer cells, but inhibits the tumourigenicity of cervical carcinoma cell lines. Knowledge about regulation of invasion and metastasis in different malignant tumours in vivo might open promising perspectives to targeted therapeutic approaches.

Epidermal growth factor (EGF)-mediated DNA-binding activity of AP-1 is attenuated in senescent human epidermal keratinocytes

The proliferative responses of cells to mitogens decrease during aging, and this may result from age-related defects in signal transduction in response to mitogens. In this study, we have investigated the age-related alteration of responses to epidermal growth factor (EGF) in cultured human keratinocytes that were senesced in vitro by repeated passage. The stimulation with EGF increased the DNA-binding activity of activator protein 1 (AP-1), an important transcription factor for cell proliferation, in young keratinocytes, whereas the binding activity showed little or slight change in the senescent cells. The induced DNA-binding activity of AP-1 in young cells was inhibited by PD 98059, an inhibitor of MEK, and partially inhibited by GF 109203X, an inhibitor of protein kinase C. Western blot analysis demonstrated that EGF induced dramatic increase in the phosphorylation of EGF receptor (EGFR) and extracellular signal-regulated kinases (ERK) in young cells, while this phosphorylation was much less profound in senescent cells. Finally, the application of EGF to young cells resulted in increased phosphorylation of Fra-2, a Fos protein component of the Jun/Fos heterodimer AP-1 complex. This EGF-induced Fra-2 phosphorylation was attenuated in senescent cells. Taken together, our study suggests that the signal transduction mediated by EGF/ERK pathway is altered in senescent human keratinocytes, and this change may be attributed, in part, to the decreased AP-1 transcription activity observed in senescent keratinocytes.

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.

Corticotropin-releasing hormone induces keratinocyte differentiation in the adult human epidermis

Previously we documented that human epidermis exclusively expresses corticotropin releasing hormone receptor 1 (CRH-R1). To define the role of CRH in the epidermis, we investigated its effects on differentiation of normal human adult epidermal keratinocytes. Thus, CRH inhibited proliferation in a dose dependent fashion and significantly decreased Ki-67 antigen expression. This effect was independent of either the presence or the absence of growth factors in the medium. Flow cytometry analysis demonstrated that CRH inhibited the transition from G0/1 to S phase of the cell cycle, which was accompanied by an increased expression of cdk inhibitor p16 (Ink4a) protein. The antiproliferative effect was attenuated by protein kinase C inhibitor (GF109203X) but not by H89 (protein kinase A inhibitor), PD98059, or SB203580 (MAP kinase inhibitors). The cell cycle withdrawal was associated with the induction of keratinocyte differentiation. Thus, CRH stimulated the expression of cytokeratin 1 and involucrin, and inhibited cytokeratin 14 on both mRNA and protein levels. It also increased cell granularity and cell size. Furthermore, CRH induced signal transduction cascade that included stimulation of inositol 1,4,5-triphosphate, which was time and dose dependent. CRH also increased activator protein-1 DNA binding activity with JunD identified as the most important element. Thus, activation of CRH-R1 induces a non-random and sequential signal transduction cascade governing both keratinocyte differentiation and the inhibition of cell proliferation through G0/1 arrest. We propose that this program, triggered by CRH interaction with CRH-R1, includes induction of a transduction pathway involving the sequential activation of phospholipase C, protein kinase C, activator protein-1 (including Jun D), and p16.

Activator protein 1 DNA binding activity is decreased in lesional psoriatic skin compared with nonlesional psoriatic skin

BACKGROUND

Psoriasis is a common benign skin disease characterized by hyperproliferation and abnormal differentiation of keratinocytes. The transcription factor activator protein 1 (AP-1) is known to play an important role in cell proliferation and differentiation.

OBJECTIVES

To investigate AP-1 DNA binding activity in psoriatic skin.

METHODS

Keratome biopsies were taken from patients with plaque-type psoriasis. Electrophoretic mobility shift assays were used to determine the AP-1 DNA binding activity, whereas Western and Northern blotting was used to determine Jun and Fos protein and mRNA expression.

RESULTS

We found that AP-1 DNA binding activity was almost completely abolished in lesional psoriatic skin compared with nonlesional psoriatic skin. Furthermore, experiments revealed that the protein and mRNA expression of the AP-1 subunits c-Fos, Fra-1 and c-Jun was reduced in lesional psoriatic skin compared with nonlesional psoriatic skin, whereas the protein and mRNA expression of the subunit JunB was increased. Topical application of the vitamin D analogue calcipotriol under occlusion to involved psoriatic skin for 4 days resulted in an increase in AP-1 DNA binding activity, and an increase in the protein and mRNA expression of c-Fos, Fra-1 and c-Jun, together with a decrease in JunB protein and mRNA expression.

CONCLUSIONS

Together, these results suggest that the activity of the transcription factor AP-1 is impaired in lesional psoriatic skin and that this impairment may be important for the disturbed epidermal growth observed in psoriasis.

Regulation of the human involucrin gene promoter by co-activator proteins

Stratified squamous epithelial cells undergo an orderly process of terminal differentiation that is characterized by specific molecular and morphological changes, including expression of the cornified envelope protein involucrin. Significant progress has been made in characterizing the upstream regulatory region of the involucrin gene. Binding sites for AP-1 (activator protein 1) and Sp1 transcription factors were shown to be important for involucrin promoter activity and tissue-specific expression. Defective terminal differentiation is often characterized by decreased or lack of involucrin expression. Recently, a dominant-negative construct of the transcriptional co-activator P/CAF [p300/CBP-associated factor, where CBP stands for CREB (cAMP-response-element-binding protein)-binding protein] was shown to inhibit involucrin expression in immortalized keratinocytes [Kawabata, Kawahara, Kanekura, Araya, Daitoku, Hata, Miura, Fukamizu, Kanzaki, Maruyama and Nakajima (2002) J. Biol. Chem. 277, 8099-8105]. Loss of expression or inactivation of other co-activators has also been demonstrated [Suganuma, Kawabata, Ohshima, and Ikeda (2002) Proc. Natl. Acad. Sci. U.S.A. 99, 13073-13078]. In the present study, we re-expressed CBP and P/CAF in immortalized keratinocyte lines that had lost expression of these co-activator proteins. Re-expression of these proteins restored calcium- and RA (retinoic acid)-responsive involucrin expression in these cells. RA and calcium signalling induced exchange of CBP and P/CAF occupancy at the AP-1 sites of the involucrin promoter. CBP and P/CAF inductions of the involucrin expression were not dependent on MEK (mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase), p38, protein kinase C or CaM kinase (calcium/calmodulin-dependent kinase) signalling. Kinase-induced changes in involucrin promoter activity directly resulted from changes in AP-1 protein expression. We concluded that CBP and P/CAF are important regulators of involucrin expression in stratified squamous epithelial cells.

Extracellular calcium induces COX-2 in osteoblasts via a PKA pathway

We have shown that extracellular calcium [Ca(+2)](e) induces cyclooxygenase-2 (COX-2) expression and prostaglandin E(2) (PGE(2)) production via an ERK signaling pathway in osteoblasts. In this study, we examined the roles of protein kinase C (PKC) and A (PKA) signaling pathways in the [Ca(+2)](e) induction of COX-2 in primary calvarial osteoblasts from mice transgenic for -371 bp of the COX-2 promoter fused to a luciferase reporter. Neither PKC specific inhibitors nor downregulation of the PKC pathway by phorbol myristate acetate (PMA) affected the [Ca(+2)](e) stimulation of COX-2 mRNA or promoter activity. In contrast, PKA inhibitors, used at doses that inhibited forskolin-stimulated luciferase activity by 90%, reduced [Ca(+2)](e)-stimulated COX-2 mRNA expression and promoter activity by 80-90%. [Ca(+2)](e) also stimulated a 2- to 3-fold increase in cAMP production. Hence, the [Ca(+2)](e) induction of COX-2 mRNA expression and promoter activity was independent of the PKC pathway and dependent on the PKA signaling pathway.

Antioxidants regulate normal human keratinocyte differentiation

Cancer begins with a normal cell that, due to persistent environmental insult, is transformed, via a series of progressively more insidious steps, into a cancer cell. A major goal of chemopreventive therapy is to alter the normal cell response to the environmental agent with the goal of inhibiting disease progression. (-)-Epigallocatechin-3-gallate (EGCG) is an important bioactive green tea antioxidant that possesses remarkable cancer chemopreventive properties. We have recently explored the hypothesis that EGCG prevents cancer by promoting keratinocyte differentiation. Based on our findings, we argue that EGCG acts to enhance the differentiation of normal keratinocytes. This is a potentially important finding, as it represents a novel mechanism of disease inhibition by EGCG--cancer preventive "differentiation therapy". However, not all antioxidant chemopreventive agents work by this mechanism. Curcumin, for example, inhibits the differentiation-promoting activity of EGCG. This report discusses the mechanism of EGCG and curcumin action in regulating expression of involucrin, a marker of keratinocyte differentiation.

Differential gene induction of human beta-defensins (hBD-1, -2, -3, and -4) in keratinocytes is inhibited by retinoic acid

Human skin is able to mount a fast response against invading harmful bacteria through the rapid production of inducible peptide antibiotics such as the human beta-defensins (hBD). To gain more insight into the role and regulation of inducible beta-defensins in the innate immunity of human skin, we investigated whether gene induction of the human beta-defensins hBD-1, -2, -3, and -4 in keratinocytes is regulated in a similar manner. Therefore, we performed a comparative study of gene expression of these four hBD in primary cultured keratinocytes using real-time PCR. A basal mRNA expression was observed for all four hBD in primary keratinocytes, which strongly increased for hBD-2, -3, and -4 during Ca(2+)-induced differentiation of the keratinocytes. This effect was completely abolished when the keratinocytes were pre-treated with all-trans-retinoic acid (RA). Furthermore, the differential induction of hBD-2, -3, and -4 gene expression in keratinocytes by proinflammatory cytokines, phorbol-myristate-acetate (PMA), and bacteria was inhibited by more than 90% when the keratinocytes were pre-incubated with RA. Inhibition of IL-1beta-mediated hBD-2 induction through RA was further confirmed by gene reporter assays and western-blot analysis. We conclude that RA is a potent inhibitor of beta-defensin induction in keratinocytes and might downregulate the inducible innate chemical defense system of human skin.

Bone marrow stromal cells, preadipocytes, and dermal fibroblasts promote epidermal regeneration in their distinctive fashions

Mesenchymal cell types, under mesenchymal-epithelial interaction, are involved in tissue regeneration. Here we show that bone marrow stromal cells (BMSCs), subcutaneous preadipocytes, and dermal fibroblasts distinctively caused keratinocytes to promote epidermal regeneration, using a skin reconstruction model by their coculture with keratinocytes. Three mesenchymal cell types promoted the survival, growth, and differentiation of keratinocytes, whereas BMSCs and preadipocytes inhibited their apoptosis. BMSCs and preadipocytes induced keratinocytes to reorganize rete ridge- and epidermal ridge-like structures, respectively. Keratinocytes with fibroblasts or BMSCs expressed the greatest amount of interleukin (IL)-1alpha protein, which is critical for mesenchymal-epithelial cross-talk in skin. Keratinocytes with or without three mesenchymal supports displayed another cross-talk molecule, c-Jun protein. Without direct mesenchymal-epithelial contact, the rete ridge- and epidermal ridge-like structures were not replicated, whereas the other phenomena noted above were. DNA microarray analysis showed that the mesenchymal-epithelial interaction affected various gene expressions of keratinocytes and mesenchymal cell types. Our results suggest that not only skin-localized fibroblasts and preadipocytes but also BMSCs accelerate epidermal regeneration in complexes and that direct contact between keratinocytes and BMSCs or preadipocytes is required for the skin-specific morphogenesis above, through mechanisms that differ from the IL-1alpha/c-Jun pathway.

The Effect of LXR Activators on AP-1 Proteins in Keratinocytes

Oxysterols, via activation of liver X receptor (LXR), regulate keratinocyte differentiation by stimulating transglutaminase cross-linking of several constituent proteins leading to the formation of the cornified envelope. We previously reported that oxysterols increase the expression of one of these cross-linked proteins, involucrin, and that this effect can be abolished by mutations of the distal activator protein (AP)-1 response element in the involucrin promoter. Furthermore, oxysterols increase AP-1 binding in an electrophoretic gel mobility shift assay and increase the expression of an AP-1 reporter. In this study, we describe the individual components of the AP-1 complex that are involved in the oxysterol-mediated AP-1 activation and stimulation of keratinocyte differentiation. We identified Fra-1 within the AP-1 DNA binding complex by supershift analysis of nuclear extracts from oxysterol-treated, cultured keratinocytes and confirmed that oxysterol treatment increased the levels of Fra-1 by western blot analysis. Additionally, on Western and Northern analysis, oxysterol treatment increased two other AP-1 proteins, Jun-D and c-Fos, whereas Fra-2, Jun-B, and c-Jun were not changed. Similar alterations in AP-1 proteins occurred when 25-OH-cholesterol or non-steroidal LXR agonists (GW3965, TO-901317) were used. These results indicate that oxysterols induce specific AP-1 proteins, thereby activating involucrin, one of the genes required for epidermal differentiation.

Regulation of Involucrin Gene Expression

The epidermis is a dynamic renewing structure that provides life-sustaining protection from the environment. The major cell type of the epidermis, the epidermal keratinocyte, undergoes a carefully choreographed program of differentiation. Alteration of these events results in a variety of debilitating and life-threatening diseases. Understanding how this process is regulated is an important current goal in biology. In this review, we summarize the literature regarding regulation of involucrin, an important marker gene that serves as a model for understanding the mechanisms that regulate the differentiation process. Current knowledge describing the role of transcription factors and signaling cascades in regulating involucrin gene expression are presented. These studies describe a signaling cascade that includes the novel protein kinase C isoforms, Ras, MEKK1, MEK3, and a p38delta-extracellular signal regulated kinase 1/2 complex. This cascade regulates activator protein one, Sp1, and CCATT/enhancer-binding protein transcription factor DNA binding to two discrete involucrin promoter regions, the distal- and proximal-regulatory regions, to regulate involucrin gene expression.

The human involucrin gene is transcriptionally repressed through a tissue-specific silencer element recognized by Oct-2

Involucrin is an important marker of epithelial differentiation which expression is upregulated just after basal cells are pushed into the suprabasal layer in stratified epithelia. Several transcription factors and regulatory elements had been described as responsible for turning on the gene. However, it is evident that in basal cell layer, additional mechanisms are involved in keeping the gene silent before the differentiation process starts. In this work, we located a potential transcriptional silencer in a 52bp sequence whose integrity is necessary for silencing the proximal enhancer promoter element (PEP) in multiplying keratinocytes. Octamer-binding sites were noticed in this fragment and the specific binding of Oct-2 transcription factor was detected. Oct-2 appears to be implicated in an epithelial-specific repression activity recorded only in keratinocytes and C33-A cell line. Overexpression of Oct-2 repressed the involucrin promoter activity in epithelial cells and in the presence of the silencer element.

Calcium and 1,25(OH)2D: interacting drivers of epidermal differentiation

Both calcium and 1,25(OH)(2)D promote the differentiation of keratinocytes in vitro. The autocrine or paracrine production of 1,25(OH)(2)D by keratinocytes combined with the critical role of the epidermal calcium gradient in regulating keratinocyte differentiation in vivo suggest the physiologic importance of this interaction. The interactions occur at a number of levels. Calcium and 1,25(OH)(2)D synergistically induce involucrin, a protein critical for cornified envelope formation. The involucrin promoter contains an AP-1 site essential for calcium and 1,25(OH)(2)D induction and an adjacent VDRE essential for 1,25(OH)(2)D but not calcium induction. Calcium regulates coactivator complexes that bind to the Vitamin D receptor (VDR). Nuclear extracts from cells grown in low calcium contain an abundance of DRIP(205), whereas calcium induced differentiation leads to reduced DRIP(205) and increased SRC 3 which replaces DRIP in its binding to the VDR. In vivo models support the importance of 1,25(OH)(2)D-calcium interactions in epidermal differentiation. The epidermis of 1alphaOHase null mice fails to form a normal calcium gradient, has reduced expression of proteins critical for barrier function, and shows little recovery of the permeability barrier when disrupted. Thus in vivo and in vitro, calcium and 1,25(OH)(2)D interact at multiple levels to regulate epidermal differentiation.

Two distinct coactivators, DRIP/mediator and SRC/p160, are differentially involved in VDR transactivation during keratinocyte differentiation

Cell programs such as proliferation and differentiation involve the sequential activation and repression of gene expression. Vitamin D, via its active metabolite 1,25-dihydroxyvitamin D (1,25(OH)(2)D(3)), controls the proliferation and differentiation of a number of cell types, including keratinocytes, by directly regulating transcription. Two classes of coactivators, the Vitamin D receptor (VDR) interacting proteins (DRIP/mediator) and the p160 steroid receptor coactivator family (SRC/p160), control the actions of nuclear hormone receptors, including the Vitamin D receptor. However, the relationship between these two classes of coactivators is not clear. Using GST-VDR affinity beads, we have identified the DRIP/mediator complex as the major VDR binding complex in proliferating keratinocytes. After the cells differentiated, members of the SRC/p160 family were identified in the complex but not major DRIP subunits. Both DRIP205 and SRC-3 potentiated Vitamin D-induced transcription in proliferating cells, but during differentiation, DRIP205 was no longer effective. These results indicate that these two distinct coactivators are differentially involved in Vitamin D regulation of gene transcription during keratinocyte differentiation, suggesting that these coactivators are part of the means by which the temporal sequence of gene expression is regulated during the differentiation process.

Matrix Metalloproteinase 9 Expression is Coordinately Modulated by the KRE-M9 and 12-O-Tetradecanoyl-Phorbol-13-Acetate Responsive Elements

To investigate the pathophysiologic role of matrix metalloproteinase 9 (MMP-9), we analyzed the mechanism of its transcriptional regulation in keratinocytes and in HT1080 fibrosarcoma cells in culture. The KRE-M9 element, which is located between the 12-O-tetradecanoyl-phorbol-13-acetate responsive element (TRE) and the transcription initiation site in the MMP-9 promoter, is essential for MMP-9 transcription in the absence of the TRE. The KRE-M9 binding protein, however, is shown to be a repressor of transcription rather than an activator; we found several times higher transcriptional activity when the KRE-M9 element was mutated. In contrast, activator protein 1 proteins (AP-1) are shown to activate transcription of MMP-9 by binding to the TRE, which is located adjacent to the KRE-M9 element. Moreover, we found that the KRE-M9 binding protein could serve as a differentiation repressing factor 1 (DRF-1) as shown by the decrease in levels of this protein with differentiation. In addition, the TRE binding protein is able to bind to the KRE-M9 to some extent. These results indicate that the coordinated modulation of MMP-9 transcription via the TRE and the KRE-M9 elements is important in epidermal and mesenchymal tissues. Our findings could facilitate consideration of the molecular mechanism in a variety of pathophysiologic conditions with which MMP-9 is involved.

Squamous cell carcinomas fail to respond to the prodifferentiating actions of 1,25(OH)2D: why?

1,25(OH)2D regulates a number of cellular events which contribute to its ability to stimulate differentiation of the keratinocyte. 1,25(OH)2D raises the intracellular calcium (Cai) level in part by increasing the expression of the calcium receptor (CaR). This sensitizes the cell to extracellular calcium, triggering the signaling pathway coupled to the CaR, which results in a rise in Cai. 1,25(OH)2D induces the family of phospholipases C (PLC). These enzymes mediate the hydrolysis of phosphatidyl inositol bisphosphate (PIP2) to form inositol tris phosphate (IP3) and diacylglycerol (DG), which stimulate calcium release from intracellular stores and activate protein kinases C (PKC), respectively. The CaR and other G protein coupled receptors signal through PLC-beta, whereas tyrosine kinase growth factor receptors such as the EGF receptor signal through PLC-gamma. Calcium and PKC regulate the expression of genes in part by controlling the levels and activity of AP-1 transcription factors. 1,25(OH)2D also directly induces structural genes such as involucrin, a substrate for transglutaminase, which crosslinks it to other substrates to form the cornified envelope. 1,25(OH)2D regulates gene expression by activating the vitamin D receptor (VDR), a transcription factor, which, in combination with the retinoid X receptor (RXR) or retinoid A receptor (RAR), binds to its vitamin D response elements (VDRE) in the promoters of genes whose expression it regulates. The VDR also binds to one of two coactivator complexes, Mediator/DRIP (VDR interacting proteins) or p160/SRC (steroid hormone receptor complex), complexes which link the VDR to the RNA polymerase complex. We have recently discovered that the binding of VDR to these complexes is sequential. Binding to Mediator/DRIP occurs in the undifferentiated keratinocyte, but as the cell differentiates, DRIP(205) (the key protein of the DRIP complex binding to the VDR) levels fall, and p160/SRC binding takes over. We hypothesize that this sequential replacement of Mediator/DRIP by p160/SRC is critical for differentiation. Squamous cell carcinomas (SCC) fail to respond to the prodifferentiating actions of 1,25(OH)2D. These cells have normal levels of VDR and normal binding of VDR to VDREs. However, they fail to down-regulate DRIP(205) such that the p160/SRC complex fails to bind to VDR. This lack of sequential binding of these coactivator complexes to the VDR, we believe, maintains the cell in a state of continued proliferation and blocks the ability of 1,25(OH)2D to induce the expression of genes required for the differentiation process.