Low Dose Ultraviolet B Irradiation Increases Hyaluronan Synthesis in Epidermal Keratinocytes via Sequential Induction of Hyaluronan Synthases Has1–3 Mediated by p38 and Ca2+/Calmodulin-dependent Protein Kinase II (CaMKII) Signaling

Hyaluronan, a major epidermal extracellular matrix component, responds strongly to different kinds of injuries. This also occurs by UV radiation, but the mechanisms involved are poorly understood. The effects of a single ultraviolet B (UVB) exposure on hyaluronan content and molecular mass, and expression of genes involved in hyaluronan metabolism were defined in monolayer and differentiated, organotypic three-dimensional cultures of rat epidermal keratinocytes. The signals regulating the response were characterized using specific inhibitors and Western blotting. In monolayer cultures, UVB increased hyaluronan synthase Has1 mRNA already 4 h postexposure, with a return to control level by 24 h. In contrast, Has2 and Has3 were persistently elevated from 8 h onward. Silencing of Has2 and especially Has3 decreased the UVB-induced accumulation of hyaluronan. p38 and Ca²⁺/calmodulin-dependent protein kinase II pathways were found to be involved in the UVB-induced up-regulation of Has2 and Has3 expression, respectively, and their inhibition reduced hyaluronan deposition. However, the expressions of the hyaluronan-degrading enzymes Hyal1 and Hyal2 and the hyaluronan receptor Cd44 were also up-regulated by UVB. In organotypic cultures, UVB treatment also resulted in increased expression of both Has and Hyal genes and shifted hyaluronan toward a smaller size range. Histochemical stainings indicated localized losses of hyaluronan in the epidermis. The data show that exposure of keratinocytes to acute, low dose UVB increases hyaluronan synthesis via up-regulation of Has2 and Has3. The simultaneously enhanced catabolism of hyaluronan demonstrates the complexity of the UVB-induced changes. Nevertheless, enhanced hyaluronan metabolism is an important part of the adaptation of keratinocytes to radiation injury.     

Advances in resolving the heterogeneity and dynamics of keratinocyte differentiation

The mammalian skin is equipped with a highly dynamic stratified epithelium. The maintenance and regeneration of this epithelium is supported by basally located keratinocytes, which display stem cell properties, including lifelong proliferative potential and the ability to undergo diverse differentiation trajectories. Keratinocytes support not just the surface of the skin, called the epidermis, but also a range of ectodermal structures including hair follicles, sebaceous glands, and sweat glands. Recent studies have shed light on the hitherto underappreciated heterogeneity of keratinocytes by employing state-of-the-art imaging technologies and single-cell genomic approaches. In this mini review, we highlight major recent discoveries that illuminate the dynamics and cellular mechanisms that govern keratinocyte differentiation in the live mammalian skin and discuss the broader implications of these findings for our understanding of epithelial and stem cell biology in general.     

Human primary keratinocytes show restricted ability to up-regulate suppressor of cytokine signaling (SOCS)3 protein compared with autologous macrophages

Suppressor of cytokine signaling (SOCS)3 belongs to a family of proteins that are known to exert important functions as inducible feedback inhibitors and are crucial for the balance of immune responses. There is evidence for a deregulated immune response in chronic inflammatory skin diseases. Thus, it was the aim of this study to investigate the regulation of SOCS proteins involved in intracellular signaling pathways occurring during inflammatory skin diseases and analyze their impact on the course of inflammatory responses. Because we and others have previously described that the cytokine IL-27 has an important impact on the chronic manifestation of inflammatory skin diseases, we focused here on the signaling induced by IL-27 in human primary keratinocytes compared with autologous blood-derived macrophages. Here, we demonstrate that SOCS3 is critically involved in regulating the cell-specific response to IL-27. SOCS3 was found to be significantly up-regulated by IL-27 in macrophages but not in keratinocytes. Other STAT3-activating cytokines investigated, including IL-6, IL-22, and oncostatin M, also failed to up-regulate SOCS3 in keratinocytes. Lack of SOCS3 up-regulation in skin epithelial cells was accompanied by prolonged STAT1 and STAT3 phosphorylation and enhanced CXCL10 production upon IL-27 stimulation compared with macrophages. Overexpression of SOCS3 in keratinocytes significantly diminished this enhanced CXCL10 production in response to IL-27. We conclude from our data that keratinocytes have a cell type-specific impaired capacity to up-regulate SOCS3 which may crucially determine the course of chronic inflammatory skin diseases.     

A Proteasome Inhibitor-stimulated Nrf1 Protein-dependent Compensatory Increase in Proteasome Subunit Gene Expression Reduces Polycomb Group Protein Level

The polycomb group (PcG) proteins, Bmi-1 and Ezh2, are important epigenetic regulators that enhance skin cancer cell survival. We recently showed that Bmi-1 and Ezh2 protein level is reduced by treatment with the dietary chemopreventive agents, sulforaphane and green tea polyphenol, and that this reduction involves ubiquitination of Bmi-1 and Ezh2, suggesting a key role of the proteasome. In the present study, we observe a surprising outcome that Bmi-1 and Ezh2 levels are reduced by treatment with the proteasome inhibitor, MG132. We show that this is associated with a compensatory increase in the level of mRNA encoding proteasome protein subunits in response to MG132 treatment and an increase in proteasome activity. The increase in proteasome subunit level is associated with increased Nrf1 and Nrf2 level. Moreover, knockdown of Nrf1 attenuates the MG132-dependent increase in proteasome subunit expression and restores Bmi-1 and Ezh2 expression. The MG132-dependent loss of Bmi-1 and Ezh2 is associated with reduced cell proliferation, accumulation of cells in G₂, and increased apoptosis. These effects are attenuated by forced expression of Bmi-1, suggesting that PcG proteins, consistent with a prosurvival action, may antagonize the action of MG132. These studies describe a compensatory Nrf1-dependent, and to a lesser extent Nrf2-dependent, increase in proteasome subunit level in proteasome inhibitor-treated cells and confirm that PcG protein levels are regulated by proteasome activity.     

Population of Rh123dim human keratinocytes form holoclones

The aim of our studies was to develop an efficient strategy to isolate human early epidermal progenitors for experimental and potential clinical purposes. We employed fluorescence-activated cell sorting (FACS) to isolate cells that poorly accumulate metabolic Rhodamin123 (Rh123) dye. We noticed that similarly to a population of β1-integrin bright (β1^bright) cells, a population of Rh123 dull (Rh123^dim) cells is highly enriched for cells growing holoclones, colonies composed of the most primitive keratinocytes. Furthermore, Rh123^dim cells express several morphological features of primitive undifferentiated cells and are also highly motile. We postulate that these cells could become an important source of epidermal progenitors to expand keratinocytes for clinical purposes.     

One step ahead: Role of filopodia in adhesion formation during cell migration of keratinocytes

Cell adhesion is an essential prerequisite for cell function and movement. It depends strongly on focal adhesion complexes connecting the extracellular matrix to the actin cytoskeleton. Especially in moving cells focal adhesions are highly dynamic and believed to be formed closely behind the leading edge. Filopodia were thought to act mainly as guiding cues using their tip complexes for elongation. Here we show for keratinocytes a strong dependence of lamellipodial adhesion sites on filopodia. Upon stable contact of the VASP-containing tip spot to the substrate, a filopodial focal complex (filopodial FX) is formed right behind along the filopodia axis. These filopodial FXs are fully assembled, yet small adhesions containing all adhesion markers tested. Filopodial FXs when reached by the lamellipodium are just increased in size resulting in classical focal adhesions. At the same time most filopodia regain their elongation ability. Blocking filopodia inhibits development of new focal adhesions in the lamellipodium, while focal adhesion maturation in terms of vinculin exchange dynamics remains active. Our data therefore argue for a strong spatial and temporal dependence of focal adhesions on filopodial focal complexes in keratinocytes with filopodia not permanently initiated via new clustering of actin filaments to induce elongation.     

Upregulation of genes orchestrating keratinocyte differentiation,including the novel marker gene ID2, by contact sensitizers in human bulge‐derived keratinocytes

In the epidermis, keratinocytes are involved in physical and first‐line immune protection of the host. In this study, we analyzed the molecular responses to certain contact sensitizers (2,4‐dinitrochlorobenzene and NiSO₄) and irritants (sodium dodecyl sulfate and benzalkonium chloride) in cultured human keratinocytes from the bulge region of a plucked hair follicle (bulge‐derived keratinocytes [BDKs]) and compared these molecular responses to those with the human monocytic leukemia cell line, THP‐1. The BDKs, individually established without invasive biopsies, showed high reactivity to these stimulants. As a primary response to the contact sensitizers, the NRF2‐mediated signaling pathway was upregulated in BDKs and THP‐1. The expression of IL1B and IL8 genes was not induced by the irritants but by the sensitizers in THP‐1. However, the expression of the IL1B and IL8 genes was induced at higher levels by the irritants in BDKs than by the sensitizers. Many genes orchestrating keratinocyte differentiation, including ID2, were significantly upregulated in response to the sensitizers in BDKs but not those in THP‐1. The use of the ID2 gene to discriminate between sensitizers and irritants might be effective as a novel marker for application during in vitro sensitization with BDKs. © 2010 Wiley Periodicals, Inc. J Biochem Mol Toxicol 24:10–20, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jbt.20307