Inducible expression of filaggrin increases keratinocyte susceptibility to apoptotic cell death

Rare Filaggrin Variants Are Associated with Pustular Skin Diseases in Asians

Reactive pustular eruptions (RPEs) can manifest in a variety of conditions, including pustular psoriasis (PP) and adult-onset immunodeficiency syndrome due to anti-interferon-γ autoantibody (AOID). These RPEs can be attributed to different causes, one of which is genetic factors. However, the genetic basis for pustular skin diseases remains poorly understood. In our study, we conducted whole-exome sequencing on a cohort of 17 AOID patients with pustular reactions (AOID-PR) and 24 PP patients. We found that 76% and 58% of the AOID-PR and PP patients, respectively, carried rare genetic variations within the filaggrin (FLG) gene family. A total of 12 out of 21 SNPs on FLG had previously received clinical classifications, with only p.Ser2706Ter classified as pathogenic. In contrast, none of the FLG3 SNPs identified in this study had prior clinical classifications. Overall, these variations had not been previously documented in cases of pustular disorders, and two of them were entirely novel discoveries. Immunohistochemical analysis of skin biopsies revealed that FLG variants like p.Ser860Trp, p.Gly3903Ter, p.Gly2440Glu, and p.Glu2133Asp caused reductions in FLG levels similar to the pathogenic FLG p.Ser2706Ter. These results highlight rare FLG variants as potential novel genetic risk factors contributing to pustule formation in both AOID and PP.

In silico analysis of the profilaggrin sequence indicates alterations in the stability, degradation route, and intracellular protein fate in filaggrin null mutation carriers

Background: Loss of function mutation in FLG is the major genetic risk factor for atopic dermatitis (AD) and other allergic manifestations. Presently, little is known about the cellular turnover and stability of profilaggrin, the protein encoded by FLG. Since ubiquitination directly regulates the cellular fate of numerous proteins, their degradation and trafficking, this process could influence the concentration of filaggrin in the skin. Objective: To determine the elements mediating the interaction of profilaggrin with the ubiquitin-proteasome system (i.e., degron motifs and ubiquitination sites), the features responsible for its stability, and the effect of nonsense and frameshift mutations on profilaggrin turnover. Methods: The effect of inhibition of proteasome and deubiquitinases on the level and modifications of profilaggrin and processed products was assessed by immunoblotting. Wild-type profilaggrin sequence and its mutated variants were analysed in silico using the DEGRONOPEDIA and Clustal Omega tool. Results: Inhibition of proteasome and deubiquitinases stabilizes profilaggrin and its high molecular weight of presumably ubiquitinated derivatives. In silico analysis of the sequence determined that profilaggrin contains 18 known degron motifs as well as multiple canonical and non-canonical ubiquitination-prone residues. FLG mutations generate products with increased stability scores, altered usage of the ubiquitination marks, and the frequent appearance of novel degrons, including those promoting C-terminus-mediated degradation routes. Conclusion: The proteasome is involved in the turnover of profilaggrin, which contains multiple degrons and ubiquitination-prone residues. FLG mutations alter those key elements, affecting the degradation routes and the mutated products' stability.

Extracellular epimorphin impairs expression and processing of profilaggrin in HaCaT keratinocytes

The expression and processing of filaggrin, a filament-associated protein in the skin epidermis, is closely associated with keratinocyte cornification. The large precursor profilaggrin (Pro-FLG) is initially detected at the granular layer in keratohyalin granules, subsequently processed into 10 to 12 filaggrin monomers (mFLGs) for keratin assembly, and ultimately degraded into smaller peptides that behave as natural moisturizing factor (NMF) at the outermost epidermis. We previously reported that epimorphin (EPM) extruded upon external stimuli severely perturbs epidermal terminal differentiation. Using HaCaT keratinocytes with inducible expression and recombinant EPM and FLG, we investigated the effect of extracellular EPM on the expression profile of filaggrin. As expression and processing of Pro-FLG in primary keratinocytes are accompanied with apoptotic cell death, we employed HaCaT keratinocytes that grow and express filaggrin mRNA in standard culture medium. In response to ectopic stimulation with extracellular EPM, Pro-FLG expression decreased with elimination of keratohyalin granules in the cells, with filaggrin mRNA remained constant and profilaggrin processing was not accelerated. Additionally, using a recombinant form of mFLG engineered for intracellular localization, we found that extracellular EPM hindered proteolytic cleavage of mFLG for production of NMF. Taken together, extracellularly extruded EPM, an epidermal cornification blocker, not only decreases Pro-FLG expression but also reduces the production of NMF in HaCaT keratinocytes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-022-00566-8.

Revisiting the Roles of Filaggrin in Atopic Dermatitis

The discovery in 2006 that loss-of-function mutations in the filaggrin gene (FLG) cause ichthyosis vulgaris and can predispose to atopic dermatitis (AD) galvanized the dermatology research community and shed new light on a skin protein that was first identified in 1981. However, although outstanding work has uncovered several key functions of filaggrin in epidermal homeostasis, a comprehensive understanding of how filaggrin deficiency contributes to AD is still incomplete, including details of the upstream factors that lead to the reduced amounts of filaggrin, regardless of genotype. In this review, we re-evaluate data focusing on the roles of filaggrin in the epidermis, as well as in AD. Filaggrin is important for alignment of keratin intermediate filaments, control of keratinocyte shape, and maintenance of epidermal texture via production of water-retaining molecules. Moreover, filaggrin deficiency leads to cellular abnormalities in keratinocytes and induces subtle epidermal barrier impairment that is sufficient enough to facilitate the ingress of certain exogenous molecules into the epidermis. However, although FLG null mutations regulate skin moisture in non-lesional AD skin, filaggrin deficiency per se does not lead to the neutralization of skin surface pH or to excessive transepidermal water loss in atopic skin. Separating facts from chaff regarding the functions of filaggrin in the epidermis is necessary for the design efficacious therapies to treat dry and atopic skin.

Chondroitin 6-sulfate represses keratinocyte proliferation in mouse skin, which is associated with psoriasis

Chondroitin sulfates are implicated in epidermal biology, but functional significance of chondroitin sulfates remains unclear. Here, we report that chondroitin 6-sulfate is important for the maintenance of epidermal homeostasis. Mice deficient in chondroitin 6-O-sulfotransferase-1 (C6st-1), which is involved in biosynthesis of chondroitin 6-sulfate, exhibited keratinocyte hyperproliferation and impaired skin permeability barrier function. Chondroitin 6-sulfate directly interacted with the EGF receptor and negatively controlled ligand-induced EGF receptor signaling. Normal function of hyperproliferative C6st-1-knockout mouse-derived keratinocytes was rescued by treatment with exogenous chondroitin 6-sulfate. Epidermal hyperplasia, induced using imiquimod, was more severe in C6st-1-knockout mice than in C6st-1 wild-type mice. Taken together, these findings indicate that chondroitin 6-sulfate represses keratinocyte proliferation in normal skin, and that the expression level of C6st-1 may be associated with susceptibility to psoriasis.

Development of the Vestibular Lamina in Human Embryos: Morphogenesis and Vestibule Formation

The vestibular lamina (VL) is a transient developmental structure that forms the lip furrow, creating a gap between the lips/cheeks and teeth (oral vestibule). Surprisingly, little is known about the development of the VL and its relationship to the adjacent dental lamina (DL), which forms the teeth. In some congenital disorders, such as Ellis-van Creveld (EVC) syndrome, development of the VL is disrupted and multiple supernumerary frenula form, physically linking the lips and teeth. Here, we assess the normal development of the VL in human embryos from 6.5 (CS19) to 13 weeks of development, showing the close relationship between the VL and DL, from initiation to differentiation. In the anterior lower region, the two structures arise from the same epithelial thickening. The VL then undergoes complex morphogenetic changes during development, forming a branched structure that separates to create the vestibule. Changing expression of keratins highlight the differentiation patterns in the VL, with fissure formation linked to the onset of filaggrin. Apoptosis is involved in removal of the central portion of the VL to create a broad furrow between the future cheek and gum. This research forms an essential base to further explore developmental defects in this part of the oral cavity.

The Barrier Molecules Junction Plakoglobin, Filaggrin, and Dystonin Play Roles in Melanoma Growth and Angiogenesis

OBJECTIVE

To understand role of barrier molecules in melanomas.

BACKGROUND

We have reported poor patient survival and low immune infiltration of melanomas that overexpress a set of genes that include filaggrin (FLG), dystonin (DST), junction plakoglobin (JUP), and plakophilin-3 (PKP3), and are involved in cell-cell adhesions. We hypothesized that these associations are causal, either by interfering with immune cell infiltration or by enhancing melanoma cell growth.

METHODS

FLG and DST were knocked out by CRISPR/Cas9 in human DM93 and murine B16-F1 melanoma cells. PKP3 and JUP were overexpressed in murine B16-AAD and human VMM39 melanoma cells by lentiviral transduction. These cell lines were evaluated in vitro for cell proliferation and in vivo for tumor burden, immune composition, cytokine expression, and vascularity.

RESULTS

Immune infiltrates were not altered by these genes. FLG/DST knockout reduced proliferation of human DM93 melanoma in vitro, and decreased B16-F1 tumor burden in vivo. Overexpression of JUP, but not PKP3, in B16-AAD significantly increased tumor burden, increased VEGF-A, reduced IL-33, and enhanced vascularity.

CONCLUSIONS

FLG and DST support melanoma cell growth in vitro and in vivo. Growth effects of JUP were only evident in vivo, and may be mediated, in part, by enhancing angiogenesis. In addition, growth-promoting effects of FLG and DST in vitro suggest that these genes may also support melanoma cell proliferation through angiogenesis-independent pathways. These findings identify FLG, DST, and JUP as novel therapeutic targets whose down-regulation may provide clinical benefit to patients with melanoma.

Dysregulated function of normal human epidermal keratinocytes in the absence of filaggrin

The aim of the present study was to investigate the impact of filaggrin knockdown on the function of normal human epidermal keratinocytes (NHEKs). Filaggrin expression levels in NHEKs were knocked down by lentivirus (LV) encoding small hairpin RNA (shRNA), with control cells infected with nonsense shRNA or not infected. Cell migration and invasion were assayed using Transwell inserts, cell adhesion and proliferation by the Cell Counting kit-8 assay, and apoptosis and cell cycle progression by flow cytometry. shRNA efficiently suppressed expression of filaggrin protein. The LV group had significantly decreased cell migration, adhesion and proliferation, and increased apoptosis compared with the control groups (P=0.027). In addition, the proportion of cells in G₁ and G₂ phases were significantly increased in the LV group compared with control groups (P=0.018). The results of the present study demonstrate that filaggrin knockdown inhibits NHEK migration, adhesion and proliferation, promotes apoptosis and disturbs cell cycle progression.

AKT1-mediated Lamin A/C degradation is required for nuclear degradation and normal epidermal terminal differentiation

Nuclear degradation is a key stage in keratinocyte terminal differentiation and the formation of the cornified envelope that comprises the majority of epidermal barrier function. Parakeratosis, the retention of nuclear material in the cornified layer of the epidermis, is a common histological observation in many skin diseases, notably in atopic dermatitis and psoriasis. Keratinocyte nuclear degradation is not well characterised, and it is unclear whether the retained nuclei contribute to the altered epidermal differentiation seen in eczema and psoriasis. Loss of AKT1 function strongly correlated with parakeratosis both in eczema samples and in organotypic culture models. Although levels of DNAses, including DNase1L2, were unchanged, proteomic analysis revealed an increase in Lamin A/C. AKT phosphorylates Lamin A/C, targeting it for degradation. Consistent with this, Lamin A/C degradation was inhibited and Lamin A/C was observed in the cornified layer of AKT1 knockdown organotypic cultures, surrounding retained nuclear material. Using AKT-phosphorylation-dead Lamin A constructs we show that the retention of nuclear material is sufficient to cause profound changes in epidermal terminal differentiation, specifically a reduction in Loricrin, Keratin 1, Keratin 10, and filaggrin expression. We show that preventing nuclear degradation upregulates BMP2 expression and SMAD1 signalling. Consistent with these data, we observe both parakeratosis and evidence of increased SMAD1 signalling in atopic dermatitis. We therefore present a model that, in the absence of AKT1-mediated Lamin A/C degradation, DNA degradation processes, such as those mediated by DNAse 1L2, are prevented, leading to parakeratosis and changes in epidermal differentiation.

Mechanisms of DNA damage response to targeted irradiation in organotypic 3D skin cultures

DNA damage (caused by direct cellular exposure and bystander signaling) and the complex pathways involved in its repair are critical events underpinning cellular and tissue response following radiation exposures. There are limited data addressing the dynamics of DNA damage induction and repair in the skin particularly in areas not directly exposed. Here we investigate the mechanisms regulating DNA damage, repair, intracellular signalling and their impact on premature differentiation and development of inflammatory-like response in the irradiated and surrounding areas of a 3D organotypic skin model. Following localized low-LET irradiation (225 kVp X-rays), low levels of 53BP1 foci were observed in the 3D model (3.8±0.28 foci/Gy/cell) with foci persisting and increasing in size up to 48 h post irradiation. In contrast, in cell monolayers 14.2±0.6 foci/Gy/cell and biphasic repair kinetics with repair completed before 24 h was observed. These differences are linked to differences in cellular status with variable level of p21 driving apoptotic signalling in 2D and accelerated differentiation in both the directly irradiated and bystander areas of the 3D model. The signalling pathways utilized by irradiated keratinocytes to induce DNA damage in non-exposed areas of the skin involved the NF-κB transcription factor and its downstream target COX-2.

Understanding engineered nanomaterial skin interactions and the modulatory effects of ultraviolet radiation skin exposure

The study of engineered nanomaterials for the development of technological applications, nanomedicine, and nano-enabled consumer products is an ever-expanding discipline as is the concern over the impact of nanotechnology on human environmental health and safety. In this review, we discuss the current state of understanding of nanomaterial skin interactions with a specific emphasis on the effects of ultraviolet radiation (UVR) skin exposure. Skin is the largest organ of the body and is typically exposed to UVR on a daily basis. This necessitates the need to understand how UVR skin exposure can influence nanomaterial skin penetration, alter nanomaterial systemic trafficking, toxicity, and skin immune function. We explore the unique dichotomy that UVR has on inducing both deleterious and therapeutic effects in skin. The subject matter covered in this review is broadly informative and will raise awareness of potential increased risks from nanomaterial skin exposure associated with specific occupational and life style choices. The UVR-induced immunosuppressive response in skin raises intriguing questions that motivate future research directions in the nanotoxicology and nanomedicine fields.

Keratinocytes express cytokines and nerve growth factor in response to neuropeptide activation of the ERK1/2 and JNK MAPK transcription pathways

Sensory neurons innervating the skin can release neuropeptides that are believed to modulate cellular proliferation, wound healing, pigmentation, and keratinocyte innate immune responses. While the ability of neuropeptides to stimulate keratinocyte production of inflammatory mediators has been demonstrated, there is no information concerning the mechanisms by which neuropeptide activation of keratinocyte cell surface receptors ultimately leads to the up-regulation of mediator production. In this study we used a keratinocyte cell line to identify the presence of substance P (SP) and calcitonin gene-related peptide (CGRP) receptors on keratinocytes and examined the effects of SP and CGRP stimulation on keratinocyte neuropeptide signaling, cell proliferation, and interleukin-1β (IL-1β), interleukin-6 (IL-6), tumor necrosis factor α (TNF-α), and nerve growth factor (NGF) expression. Neuropeptide stimulation caused an up-regulation of neuropeptide receptor expression in keratinocytes and a dramatic increase in keratinocyte secretion of SP and CGRP, suggesting possible autocrine or paracrine stimulatory effects and amplification of neuropeptide signaling. Both SP and CGRP concentration-dependently stimulated cellular proliferation and the expression and secretion of inflammatory cytokines and NGF in keratinocytes. SP also activated all 3 families of mitogen activated protein kinase (MAPK) and nuclear factor κB (NFκB) in keratinocytes, while CGRP only activated p38 and extracellular signal related kinase1/2 (ERK1/2) MAPKs. Neuropeptide stimulated inflammatory mediatory production in keratinocytes was reversed by ERK1/2 and JNK inhibitors. The current study is the first to observe; 1) that CGRP stimulates keratinocyte expression of CGRP and its receptor complex, 2) that SP and CGRP stimulate IL-6 and TNF-α secretion in keratinocytes, 3) that SP activated all three MAPK families and the NFκB transcriptional signaling pathway in keratinocytes, and 4) that SP and CGRP stimulated inflammatory mediator production in keratinocytes is dependent on ERK1/2 and JNK activation. These studies provide evidence suggesting that disruption of ERK1/2 and JNK signaling may potentially be an effective therapy for inflammatory skin diseases and pain syndromes mediated by exaggerated sensory neuron-keratinocyte signaling.

Identification of the C-terminal tail domain of AHF/trichohyalin as the critical site for modulation of the keratin filamentous meshwork in the keratinocyte

BACKGROUND

AHF/trichohyalin is a large structural protein abundant in the inner root sheath (IRS) of anagenic hair follicles, which has been thought to mediate the keratin filamentous assembly. However, its functional mechanism is largely unknown.

OBJECTIVE

This study aimed at the identification of the key domain in AHF for keratin association and the establishment of a plausible mechanism for the modulation of the keratin meshwork.

METHODS

Several keratinocyte cell lines were introduced with the full length or several mutants of AHF, together with IRS-specific keratin krt31, and the profile of the AHF granules and the cellular behaviors were carefully analyzed.

RESULTS

Full length of AHF formed small round granules that clearly bound to and aligned on the exogenous keratin filaments in the keratinocytes, severely affected cellular growth, mobility and shape. Intriguingly, the removal of only 6 amino acids around the C-terminal tail of AHF resulted not only in the complete loss of its keratin adherent ability but also in a dramatic enlargement of the granules.

CONCLUSION

We propose a model for cytoskeletal modulation in the IRS of anagenic hair follicles: AHF latches onto the keratin bundles by its C-terminus and rearranges the keratin meshwork by intrinsic cohesive activity for the granule formation.

Expression of differential genes involved in the maintenance of water balance in human skin by Piptadenia colubrina extract

BACKGROUND

Hydration and integrity of the stratum corneum (SC) is an important determinant of skin appearance, metabolism, mechanical properties, and barrier function. The presence of aquaglyceroporins and envelope proteins are crucial to provide greater corneocyte cohesion to keep water and other moisturizers in the skin.

AIMS

In this study, we evaluated the ability of Piptadenia colubrina, a plant native of South American rain forests, in the expression of genes involved in skin capacitance and SC integrity.

METHODS

The expression of genes for aquaporin-3 (AQP3), loricrin, involucrin (INV), and filaggrin (FLG) was measured by real-time PCR, using an in vitro model of human keratinocytes incubated with concentrations of 2.5, 5, 10, and 20 mg/mL of a hydroglycolic extract of P. colubrina (HEPC). The amount of AQP3 protein was also tested by immunohistochemistry in human skin explants. Clinical trials were conducted to evaluate the effects of a gel-cream containing HEPC on the glycerol index and skin capacitance.

RESULTS

Hydroglycolic extract of P. colubrina increased both the expression and immunoreactivity of AQP3 in cultured keratinocytes and human skin explants. The gene induction to envelope proteins FLG and INV was also observed after cell incubation with HEPC. Skin capacitance was significantly improved in human volunteers under treatment with HEPC-containing cream.

CONCLUSIONS

The extract of P. colubrina promotes cellular hydration and induces gene expression of envelope proteins providing greater corneocyte cohesion to keep water and other moisturizers in the skin and an appropriate epidermal adhesion. The in vitro findings were clinically confirmed and encourage the clinical use of this compound in skin care products.

Khat Alters the Phenotype of in vitro-reconstructed Human Oral Mucosa

Khat-chewing has been associated with oral lesions including oral cancer, but the mechanisms leading to their development are not known. We hypothesized that khat interferes with the physiological processes of the oral mucosa, such as cell proliferation and differentiation, and aimed at investigating the effects of khat exposure on in vitro-reconstructed human normal buccal mucosa. Khat decreased cell proliferation, epithelial thickness, and cytokeratin 13 expression, while inducing premature expression of p21Waf1/Cip1, transglutaminases, involucrin, and filaggrin. This suggests that khat is able to induce abnormal differentiation of the buccal epithelium. Khat-induced alterations were accompanied by increased levels of p38 and were reversed by p38 inhibition, pointing to p38 as the key player in this process. The morphological changes described herein mirror the in vivo changes previously described in khat users, and demonstrate for the first time that khat induces pathological alterations in human buccal mucosa, providing evidence that raises concerns about the effects of khat use on oral health.

Apoptosis coordinates with proliferation and differentiation during human hair follicle morphogenesis

BACKGROUND

Apoptosis sculptures the most complicated skin appendage, feathers, out of epidermal layers by playing a variety of roles (1). Human hair follicle formation is different from feathers in growth direction and pattern formation of proliferative zone.

OBJECTIVE

To delineate the apoptotic events together with proliferation and differentiation in developing human hair follicle and emphasis on the mechanism and biological meaning of epidermal hair canal.

METHODS

We used TUNEL to examine apoptosis and Ki-67, involucrin, filaggrin immuno-localization to examine proliferation and differentiation.

RESULTS

In hair germs, apoptosis was diffuse in periderm, basal keratinocytes, and mesenchymal cells with high Ki-67 expression, but spared follicular germinative cells with low Ki-67 and high bcl-2 expression. In hair pegs, apoptosis was active in high Ki-67 expression area, like outer root sheath, hair follicle sheath, but spared dermal papilla with low Ki-67 and high bcl-2 expression. In bulbous pegs, apoptosis appeared in companion layer, precortical area, inner root sheath and outer root sheath, but spared bulge area with high bcl-2 expression. Apoptosis resulted in epidermal and subepidermal hair canal formation. Filaggrin and involucrin were expressed in the lining cells of hair canal. CD1a+ cells were densely distributed alone the hair canal before its opening.

CONCLUSION

During human hair follicle morphogenesis, apoptosis coordinates with proliferation to shape the growth zone, creates space to free the hair shaft from follicular wall, and directs a driving force on hair shaft extension. Apoptosis accompanies the terminal differentiation of epidermal hair canal. The bottom becomes interfollicular epidermis after roof shedding and hair exposure. Langerhans cells also populate in the hair canal before its opening. The biological meaning of epidermal hair canal is supposed to prepare the barrier when hair perturbing the intact of epidermis.

Sculpting skin appendages out of epidermal layers via temporally and spatially regulated apoptotic events

Complex skin appendages are built from the epidermal cells through various cell events. Here we used TUNEL and caspase-3 immuno-localization to examine apoptosis in feather morphogenesis. We deduced three modes. In Mode 1A, apoptosis occurs within the localized growth zone (LoGZ) to regulate growth (feather buds). In Mode 1B, morphogen secreting cells are present adjacent to LoGZ and apoptosis may work to remove such signaling centers (barb ridges). In Mode 2, keratinocytes apoptosed before terminal differentiation and left spaces between branches (marginal plate). In Mode 3A, keratinocytes cornified and flaked off to free skin appendages (feather sheath, pulp epithelium). In Mode 3B, keratinized apoptosed epithelial cells became permanent structures (rachis, ramus, barbules). Thus, different apoptotic modes can have different impacts on morphogenesis. We further tested effects of imbalanced Shh on apoptosis. Shh suppression reduced marginal plate apoptosis and caused abnormal differentiation of barbule plates. Shh over-expression enhanced proliferation in barb ridges. Expression of Patched in the barbule plate epithelia implies a paracrine mechanism. The current work complements our recent work on LoGZ to show how adding and removing cell masses in temporally and spatially specific ways are coordinated to sculpt skin appendages from epidermal layers.

Acute effects of UVB radiation on the proliferation and differentiation of keratinocytes

PURPOSE

The effects of UVB radiation on the proliferation and differentiation of epidermal keratinocytes were investigated with respect to timing, dosage, and repeated exposures.

METHODS

Nine healthy volunteers were placed into three subgroups and exposed to UVB radiation on buttock skin using a Waldmann UV 800 unit fitted with Philips TL-20W/12 fluorescent lamps. Three volunteers were given 2 MED of UVB and biopsied at: pre-exposure, 24, 48 and 72 h after UVB exposure. For three volunteers, 1 MED, 2 MED, 3 MED of UVB were applied. After 48 h, biopsies were taken from non-irradiated and irradiated sites. Finally, three volunteers received 1 MED of UVB daily for 5 days, and the non-irradiated and irradiated sites were biopsied 48 h after the final exposure. The expression of proliferation and differentiation markers by keratinocytes were detected by immunohistochemical staining, and the results were analysed quantitatively by image analysis.

RESULTS

The expression of proliferation and differentiation markers was observed prominently 48 h after irradiation. Higher doses of UVB caused an increase in proliferation and differentiation marker expression. Repeated exposures potentiated the effect of UVB radiation.

CONCLUSION

UVB irradiation concomitantly promotes epidermal proliferation and differentiation. Responses were maximal 48 h after irradiation. This effect of UVB increases linearly according to dose and repetition.

Hepatocyte growth factor inhibits anoikis in head and neck squamous cell carcinoma cells by activation of ERK and Akt signaling independent of NFkappa B

Hepatocyte growth factor (HGF), also known as a scatter factor, regulates a variety of biological activities including cell proliferation, survival, migration, and angiogenesis. Importantly, HGF and its receptor c-Met have been found to be associated with metastasis of human head and neck squamous cell carcinoma (HNSCC). Because anoikis resistance plays an important role in tumor progression and metastasis, here we examined whether HGF suppressed suspension-induced apoptosis (anoikis) in HNSCC cells, and if so, we assessed downstream signaling pathways mediated by HGF. We found that HNSCC cells underwent anoikis upon loss of matrix contact, whereas HGF provided protection against it. HGF-induced anoikis resistance was found to be dependent on both ERK and Akt signaling pathways. The inhibition of either ERK or Akt activation abolished HGF-mediated survival. Furthermore, we found that HGF did not activate NFkappaB transcription in HNSCC cells and that HGF-mediated anoikis resistance was independent of NFkappaB. Taken together, our results suggest that anoikis resistance induced by HGF may also play an important role in the progression and metastasis of HNSCC.

Hornerin, a novel profilaggrin-like protein and differentiation-specific marker isolated from mouse skin

A novel mouse cDNA named hornerin was isolated by RNA differential display applied to developing mouse skin. Hornerin, which has 2,496 amino acids, comprises EF-hand domains at the N terminus followed by a spacer sequence and a large repetitive domain, indicating that hornerin is a novel member of the "fused gene"-type cornified envelope precursor protein family. The repetitive domain of hornerin was found to be rich in glycine, serine, and glutamine. Hornerin was expressed in the tongue, esophagus, forestomach, and skin among the adult mouse tissues examined, all of them cornifying stratified epithelium. In the embryonic mouse skin, hornerin mRNA was first detected on gestational day 15.5 in the epidermis coincidentally with the formation of a granular layer. In accordance with this, hornerin was detected in the granular and cornified layers of the mature epidermis. In the granular cells of the epidermis, the hornerin protein was detected in keratohyalin granules together with profilaggrin. Furthermore, Western blot analysis of the mouse skin showed that the hornerin protein was cleaved during the process of epidermal differentiation, indicating possible posttranslational proteolytic processing as is observed in profilaggrin. Differentiation of primary mouse epidermal keratinocytes with 0.12 mm Ca(2+) resulted in the induction of hornerin. These results indicate that hornerin is structurally as well as functionally most similar to profilaggrin among the family members and possibly plays pleiotropic roles, including a role in cornification.

Regulated expression of human filaggrin in keratinocytes results in cytoskeletal disruption, loss of cell-cell adhesion, and cell cycle arrest

Filaggrin is an intermediate filament (IF)-associated protein that aggregates keratin IFs in vitro and is thought to perform a similar function during the terminal differentiation of epidermal keratinocytes. To further explore the role of filaggrin in the cytoskeletal rearrangement that accompanies epidermal differentiation, we generated keratinocyte cell lines that express human filaggrin using a tetracycline-inducible promoter system. Filaggrin expression resulted in reduced keratinocyte proliferation and caused an alteration in cell cycle distribution consistent with a post-G1 phase arrest. Keratin filament distribution was disrupted in filaggrin-expressing lines, while the organization of actin microfilaments and microtubules was more mildly affected. Evidence for direct interaction of filaggrin and keratin IFs was seen by overlay assays of GFP-filaggrin with keratin proteins in vitro and by filamentous filaggrin distribution in cells with low levels of expression. Cells expressing moderate to high levels of filaggrin showed a rounded cell morphology, loss of cell-cell adhesion, and compacted cytoplasm. There was also partial or complete loss of the desmosomal proteins desmoplakin, plakoglobin, and desmogleins from cell-cell borders, while the distribution of the adherens junction protein E-cadherin was not affected. No alterations in keratin cytoskeleton, desmosomal protein distribution, or cell shape were observed in control cell lines expressing beta-galactosidase. Filaggrin altered the cell shape and disrupted the actin filament distribution in IF-deficient SW13 cells, demonstrating that filaggrin can affect cell morphology independent of the presence of a cytoplasmic IF network. These studies demonstrate that filaggrin, in addition to its known effects on IF organization, can affect the distribution of other cytoskeletal elements including actin microfilaments, which can occur in the absence of a cytoplasmic IF network. Further, filaggrin can disrupt the distribution of desmosome proteins, suggesting an additional role(s) for this protein in the cytoskeletal and desmosomal reorganization that occurs at the granular to cornified cell transition during terminal differentiation of epidermal keratinocytes.

Epithelial structural proteins of the skin and oral cavity: function in health and disease

Epithelial tissues function to protect the organism from physical, chemical, and microbial damage and are essential for survival. To perform this role, epithelial keratinocytes undergo a well-defined differentiation program that results in the expression of structural proteins which maintain the integrity of epithelial tissues and function as a protective barrier. This review focuses on structural proteins of the epidermis and oral mucosa. Keratin proteins comprise the predominant cytoskeletal component of these epithelia. Keratin filaments are attached to the plasma membrane via desmosomes, and together these structural components form a three-dimensional array within the cytoplasm of epithelial cells and tissues. Desmosomes contain two types of transmembrane proteins, the desmogleins and desmocollins, that are members of the cadherin family. The desmosomal cadherins are linked to the keratin cytoskeleton via several cytoplasmic plaque proteins, including desmoplakin and plakoglobin (gamma-catenin). Epidermal and oral keratinocytes express additional differentiation markers, including filaggrin and trichohyalin, that associate with the keratin cytoskeleton during terminal differentiation, and proteins such as loricrin, small proline-rich proteins, and involucrin, that are cross-linked into the cornified envelope by transglutaminase enzymes. The importance of these cellular structures is highlighted by the large numbers of genetic and acquired (autoimmune) human disorders that involve mutations in, or autoantibodies to, keratins and desmosomal and cornified envelope proteins. While much progress has been made in the identification of the structural proteins and enzymes involved in epithelial differentiation, regulation of this process is less clear. Both calcium and retinoids influence epithelial differentiation by altering the transcription of target genes and by regulating activity of enzymes critical in epithelial differentiation, such as transglutaminases, proteinases, and protein kinases. These studies have furthered our understanding of how epithelial tissue and cell integrity is maintained and provide a basis for the future treatment of skin and oral disorders by gene therapy and other novel therapeutics.