Gene expression profiling of in vivo UVB-irradiated human epidermis

The calcium-activated chloride channel-associated protein rCLCA2 is expressed throughout rat epidermis, facilitates apoptosis and is downmodulated by UVB

The rodent chloride channel regulatory proteins mCLCA2 and its porcine and human homologues pCLCA2 and hCLCA2 are expressed in keratinocytes but their localization and significance in the epidermis have remained elusive. hCLCA2 regulates cancer cell migration, invasion and apoptosis, and its loss predicts poor prognosis in many tumors. Here, we studied the influences of epidermal maturation and UV-irradiation (UVR) on rCLCA2 (previous rCLCA5) expression in cultured rat epidermal keratinocytes (REK) and correlated the results with mCLCA2 expression in mouse skin in vivo. Furthermore, we explored the influence of rCLCA2 silencing on UVR-induced apoptosis. rClca2 mRNA was strongly expressed in REK cells, and its level in organotypic cultures remained unchanged during the epidermal maturation process from a single cell layer to fully differentiated, stratified cultures. Immunostaining confirmed its uniform localization throughout the epidermal layers in REK cultures and in rat skin. A single dose of UVR modestly downregulated rClca2 expression in organotypic REK cultures. The immunohistochemical staining showed that CLCA2 localized in basal and spinous layers also in mouse skin, and repeated UVR induced its partial loss. Interestingly, silencing of rCLCA2 reduced the number of apoptotic cells induced by UVR, suggesting that by facilitating apoptosis, CLCA2 may protect keratinocytes against the risk of malignancy posed by UVB-induced corrupt DNA.

Dose and time effects of solar-simulated ultraviolet radiation on the in vivo human skin transcriptome

Background

Terrestrial ultraviolet (UV) radiation causes erythema, oxidative stress, DNA mutations and skin cancer. Skin can adapt to these adverse effects by DNA repair, apoptosis, keratinization and tanning.

Objectives

To investigate the transcriptional response to fluorescent solar‐simulated radiation (FSSR) in sun‐sensitive human skin in vivo.

Methods

Seven healthy male volunteers were exposed to 0, 3 and 6 standard erythemal doses (SED). Skin biopsies were taken at 6 h and 24 h after exposure. Gene and microRNA expression were quantified with next generation sequencing. A set of candidate genes was validated by quantitative polymerase chain reaction (qPCR); and wavelength dependence was examined in other volunteers through microarrays.

Results

The number of differentially expressed genes increased with FSSR dose and decreased between 6 and 24 h. Six hours after 6 SED, 4071 genes were differentially expressed, but only 16 genes were affected at 24 h after 3 SED. Genes for apoptosis and keratinization were prominent at 6 h, whereas inflammation and immunoregulation genes were predominant at 24 h. Validation by qPCR confirmed the altered expression of nine genes detected under all conditions; genes related to DNA repair and apoptosis; immunity and inflammation; pigmentation; and vitamin D synthesis. In general, candidate genes also responded to UVA1 (340–400 nm) and/or UVB (300 nm), but with variations in wavelength dependence and peak expression time. Only four microRNAs were differentially expressed by FSSR.

Conclusions

The UV radiation doses of this acute study are readily achieved daily during holidays in the sun, suggesting that the skin transcriptional profile of ‘typical’ holiday makers is markedly deregulated.

What's already known about this topic?

The skin's transcriptional profile underpins its adverse (i.e. inflammation) and adaptive molecular, cellular and clinical responses (i.e. tanning, hyperkeratosis) to solar ultraviolet radiation.

Few studies have assessed microRNA and gene expression in vivo in humans, and there is a lack of information on dose, time and waveband effects.

What does this study add?

Acute doses of fluorescent solar‐simulated radiation (FSSR), of similar magnitude to those received daily in holiday situations, markedly altered the skin's transcriptional profiles.

The number of differentially expressed genes was FSSR‐dose‐dependent, reached a peak at 6 h and returned to baseline at 24 h.

The initial transcriptional response involved apoptosis and keratinization, followed by inflammation and immune modulation. In these conditions, microRNA expression was less affected than gene expression.

Linked Comment:Hart. Br J Dermatol 2020; 182:1328–1329.

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The application of transcriptional benchmark dose modeling for deriving thresholds of effects associated with solar-simulated ultraviolet radiation exposure

Considerable data has been generated to elucidate the transcriptional response of cells to ultraviolet radiation (UVR) exposure providing a mechanistic understanding of UVR-induced cellular responses. However, using these data to support standards development has been challenging. In this study, we apply benchmark dose (BMD) modeling of transcriptional data to derive thresholds of gene responsiveness following exposure to solar-simulated UVR. Human epidermal keratinocytes were exposed to three doses (10, 20, 150 kJ/m² ) of solar simulated UVR and assessed for gene expression changes 6 and 24 hr postexposure. The dose-response curves for genes with p-fit values (≥ 0.1) were used to derive BMD values for genes and pathways. Gene BMDs were bi-modally distributed, with a peak at ∼16 kJ/m² and ∼108 kJ/m² UVR exposure. Genes/pathways within Mode 1 were involved in cell signaling and DNA damage response, while genes/pathways in the higher Mode 2 were associated with immune response and cancer development. The median value of each Mode coincides with the current human exposure limits for UVR and for the minimal erythemal dose, respectively. Such concordance implies that the use of transcriptional BMD data may represent a promising new approach for deriving thresholds of actinic effects. Environ. Mol. Mutagen. 59:502-515, 2018. © 2018 The Authors Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Ultraviolet B radiation down-regulates ULK1 and ATG7 expression and impairs the autophagy response in human keratinocytes

Autophagy is a self-digestive pathway that helps to maintain cellular homeostasis, and many autophagy-related gene (ATG)s involved the regulation of the autophagy process. Ultraviolet light is a common stressor of skin, but it is unclear how autophagy is regulated after ultraviolet exposure in epidermal keratinocytes. Here, we found that the mRNAs of some key ATG genes such as ULK1, ATG5 and ATG7 exhibited significantly lower levels in the skin tissues of the face and chest with solar ultraviolet exposure, compared with perineal skin. Interestingly, UVB radiation down-regulated the expression of ULK1, ATG3 and ATG7, and it inhibited the autophagy flux via a mechanistic target of rapamycin (MTOR)-independent pathway in human keratinocytes. The inhibition of autophagy in UVB-treated keratinocytes cannot be restored by treatment with the MTOR-dependent autophagy inducer rapamycin. Importantly, UVB treatment perturbs the conversion of microtubule-associated protein 1 light chain 3 (LC3)-I to LC3-II and LC3-II turnover in response to treatment with MTOR inhibitors (Torin 1 and pp242), as well as endoplasmic reticular stress (A23187 and tunicamycin), inositol pathway (L690,330) and autophagy inducers (resveratrol and STF62247). Our study demonstrates that UVB radiation down-regulates several key autophagy-related proteins and impairs the autophagy response in keratinocytes. This study demonstrates a linkage between autophagy and skin disorders associated with ultraviolet exposure.

Skinomics: past, present and future for diagnostic microarray studies in dermatology

Easily accessible, skin was among the first targets analyzed using 'omics' and dermatology embraced the approaches very early. Microarrays have been used to define disease markers, identify transcriptional changes and even trace the course of treatment. Melanoma and psoriasis have been explored using microarrays. Particularly noteworthy is the multinational mapping of psoriasis susceptibility loci. The transcriptional changes in psoriasis have been identified using hundreds of biopsies. Epidermal keratinocytes have been studied because they respond to UV light, infections, inflammatory and immunomodulating cytokines, toxins and so on. Epidermal differentiation genes are being characterized and are expressed in human epidermal stem cells. Exciting discoveries defining human skin microbiomes have opened a new field of research with great medical potential. Specific to dermatology, the non-invasive skin sampling for microarray studies, using tape stripping, has been developed; it promises to advance dermatology toward 'omics' techniques directly applicable to the personalized medicine of the future.

Upregulation of MMP12 and its activity by UVA1 in human skin: potential implications for photoaging

UVA1 constitutes around 75% of the terrestrial UV radiation, and most of the output of artificial tanning sources. However, the molecular effects of UVA1 in human skin in vivo are surprisingly poorly understood. We have examined time-dependent whole-genome expression, along with mRNA and protein changes in the skin after one minimal erythema dose of spectrally pure UVA1 (50 J cm(-2)) and 300 nm UVB (30 mJ cm(-2)). After 24 hours, the genes induced to the greatest extent were those involved in extracellular matrix remodeling with both UVA1 (P=5.5e-7) and UVB (P=2.9e-22). UVA1 and UVB caused different effects on matrix metalloproteinase (MMP) expression: UVB induced MMP1, MMP3, and MMP10 mRNA at 24 hours to a much greater extent than UVA1. MMP12 induction by UVA1 at 6 hours is marked and much greater than that by UVB. We have found that MMP12 mRNA induction by UVA1 resulted in expression of MMP12 protein, which is functional as an elastase. This induction of elastase activity did not occur with UVB. We hypothesize that the UVA1 induction of MMP12 mediates some of its photoaging effects, particularly by contributing to elastin degeneration in late solar elastosis. MMP12 is a good marker of UVA1 exposure.

GENE PROFILING: IMPLICATIONS IN DERMATOLOGY

DNA microarrays are capable of following the level of expression of, virtually, all genes in a human tissue. This has been employed to determine the aberrant gene expression profiles in many skin diseases, including ultraviolet light damage, inflammatory processes and cancers. Because of its accessibility, skin also served as one of the initial targets of basic research using DNA microarrays. Both the epidermis and dermis have been extensively investigated. Development of bed-side uses of DNA arrays, and the concomitant price reduction of the materials and methods of microarray analyses, holds great promise for improved diagnosis, treatment and prevention of dermatologic disorders.

Gene profiling of narrowband UVB-induced skin injury defines cellular and molecular innate immune responses

The acute response of human skin to UVB radiation has not been fully characterized. We sought to define the cutaneous response at 24 hours following narrowband UVB (NB-UVB, 312-nm peak), a therapeutically relevant source of UVB, using transcriptional profiling, immunohistochemistry, and immunofluorescence. There were 1,522 unique differentially regulated genes, including upregulated genes encoding antimicrobial peptides (AMPs) (S100A7, S100A12, human beta-defensin 2, and elafin), as well as neutrophil and monocyte/dendritic cell (DC) chemoattractants (IL-8, CXCL1, CCL20, CCL2). Ingenuity pathway analysis demonstrated activation of innate defense and early adaptive immune pathways. Immunohistochemistry confirmed increased epidermal staining for AMPs (S100A7, S100A12, human beta-defensin 2, and elafin). Inflammatory myeloid CD11c(+)BDCA1(-) DCs were increased in irradiated skin, which were immature as shown by minimal colocalization with DC-LAMP, and coexpressed inflammatory markers tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand in irradiated skin. There were increased BDCA3(+) DCs, a cross-presenting DC subtype with immunosuppressive functions, and these cells have not been previously characterized as part of the response to UVB. These results show that the acute response of human skin to erythemogenic doses of NB-UVB includes activation of innate defense mechanisms, as well as early infiltration of multiple subtypes of inflammatory DCs, which could serve as a link between innate and adaptive immunity.

SKINOMICS: Transcriptional Profiling in Dermatology and Skin Biology

Recent years witnessed the birth of bioinformatics technologies, which greatly advanced biological research. These 'omics' technologies address comprehensively the entire genome, transcriptome, proteome, microbiome etc. A large impetus in development of bioinformatics was the introduction of DNA microarrays for transcriptional profiling. Because of its accessibility, skin was among the first organs analyzed using DNA microarrays, and dermatology among the first medical disciplines to embrace the approach. Here, DNA microarray methodologies and their application in dermatology and skin biology are reviewed. The most studied disease has been, unsurprisingly, melanoma; markers of melanoma progression, metastatic potential and even melanoma markers in blood have been detected. The basal and squamous cell carcinomas have also been intensely studied. Psoriasis has been comprehensively explored using DNA microarrays, transcriptional changes correlated with genomic markers and several signaling pathways important in psoriasis have been identified. Atopic dermatitis, wound healing, keloids etc. have been analyzed using microarrays. Noninvasive skin sampling for microarray studies has been developed. Simultaneously, epidermal keratinocytes have been the subject of many skin biology studies because they respond to a rich variety of inflammatory and immunomodulating cytokines, hormones, vitamins, UV light, toxins and physical injury. The transcriptional changes occurring during epidermal differentiation and cornification have been identified and characterized. Recent studies identified the genes specifically expressed in human epidermal stem cells. As dermatology advances toward personalized medicine, microarrays and related 'omics' techniques will be directly applicable to the personalized dermatology practice of the future.

Toxicogenomic analysis of chlorine vapor-induced porcine skin injury

Chlorine is an industrial chemical that can cause cutaneous burns. Understanding the molecular mechanisms of tissue damage and wound healing is important for the selection and development of an effective post-exposure treatment. This study investigated the effect of cutaneous chlorine vapor exposure using a weanling swine burn model and microarray analysis. Ventral abdominal sites were exposed to a mean calculated chlorine vapor concentration of 2.9 g/L for 30 min. Skin samples were harvested at 1.5 h, 3 h, 6 h, and 24 h post-exposure and stored in RNAlater(®) until processing. Total RNA was isolated, processed, and hybridized to Affymetrix GeneChip(®) Porcine Genome Arrays. Differences in gene expression were observed with respect to sampling time. Ingenuity Pathways Analysis revealed seven common biological functions among the top ten functions of each time point, while canonical pathway analysis revealed 3 genes (IL-6, IL1A, and IL1B) were commonly shared among three significantly altered signaling pathways. The transcripts encoding all three genes were identified as common potential therapeutic targets for Phase II/III clinical trial, or FDA-approved drugs. The present study shows transcriptional profiling of cutaneous wounds induced by chlorine exposure identified potential targets for developing therapeutics against chlorine-induced skin injury.

Transcriptional profiling shows altered expression of wnt pathway- and lipid metabolism-related genes as well as melanogenesis-related genes in melasma

Melasma is a commonly acquired hyperpigmentary disorder of the face, but its pathogenesis is poorly understood and its treatment remains challenging. We conducted a comparative histological study on lesional and perilesional normal skin to clarify the histological nature of melasma. Significantly, higher amounts of melanin and of melanogenesis-associated proteins were observed in the epidermis of lesional skin, and the mRNA level of tyrosinase-related protein 1 was higher in lesional skin, indicating regulation at the mRNA level. However, melanocyte numbers were comparable between lesional and perilesional skin. A transcriptomic study was undertaken to identify genes involved in the pathology of melasma. A total of 279 genes were found to be differentially expressed in lesional and perilesional skin. As was expected, the mRNA levels of a number of known melanogenesis-associated genes, such as tyrosinase, were found to be elevated in lesional skin. Bioinformatics analysis revealed that the most lipid metabolism-associated genes were downregulated in lesional skin, and this finding was supported by an impaired barrier function in melasma. Interestingly, a subset of Wnt signaling modulators, including Wnt inhibitory factor 1, secreted frizzled-related protein 2, and Wnt5a, were also found to be upregulated in lesional skin. Immunohistochemistry confirmed the higher expression of these factors in melasma lesions.

Image-guided genomic analysis of tissue response to laser-induced thermal stress

The cytoprotective response to thermal injury is characterized by transcriptional activation of "heat shock proteins" (hsp) and proinflammatory proteins. Expression of these proteins may predict cellular survival. Microarray analyses were performed to identify spatially distinct gene expression patterns responding to thermal injury. Laser injury zones were identified by expression of a transgene reporter comprised of the 70 kD hsp gene and the firefly luciferase coding sequence. Zones included the laser spot, the surrounding region where hsp70-luc expression was increased, and a region adjacent to the surrounding region. A total of 145 genes were up-regulated in the laser irradiated region, while 69 were up-regulated in the adjacent region. At 7 hours the chemokine Cxcl3 was the highest expressed gene in the laser spot (24 fold) and adjacent region (32 fold). Chemokines were the most common up-regulated genes identified. Microarray gene expression was successfully validated using qRT- polymerase chain reaction for selected genes of interest. The early response genes are likely involved in cytoprotection and initiation of the healing response. Their regulatory elements will benefit creating the next generation reporter mice and controlling expression of therapeutic proteins. The identified genes serve as drug development targets that may prevent acute tissue damage and accelerate healing.

An assessment of transcriptional changes in porcine skin exposed to bromine vapor

Bromine is an industrial chemical that can cause severe cutaneous burns. This study was a preliminary investigation into the effect of cutaneous exposure to bromine vapor using a weanling swine burn model and microarray analysis. Ventral abdominal sites were exposed to a mean calculated bromine vapor concentration of 0.69 g L(-1) for 10 or 20 min. At 48 h postexposure, total RNA from skin samples was isolated, processed, and hybridized to Affymetrix GeneChip Porcine Genome Arrays. Expression analysis revealed that bromine vapor exposure for 10 or 20 min promoted similar transcriptional changes in the number of significantly modulated probe sets. A minimum of 83% of the probe sets was similar for both exposure times. Ingenuity pathways analysis revealed eight common biological functions among the top 10 functions of each experimental group, in which 30 genes were commonly shared among 19 significantly altered signaling pathways. Transcripts encoding heme oxygenase 1, interleukin-1β, interleukin 2 receptor gamma chain, and plasminogen activator inhibitor-1 were identified as common potential therapeutic targets for Phase II/III clinical trial or FDA-approved drugs. The present study is an initial assessment of the transcriptional responses to cutaneous bromine vapor exposure identifying molecular networks and genes that could serve as targets for developing therapeutics for bromine-induced skin injury.

In vivo identification of solar radiation-responsive gene network: role of the p38 stress-dependent kinase

Solar radiation is one of the most common threats to the skin, with exposure eliciting a specific protective cellular response. To decrypt the underlying mechanism, we used whole genome microarrays (Agilent 44K) to study epidermis gene expression in vivo in skin exposed to simulated solar radiation (SSR). We procured epidermis samples from healthy Caucasian patients, with phototypes II or III, and used two different SSR doses (2 and 4 J/cm(2)), the lower of which corresponded to the minimal erythemal dose. Analyses were carried out five hours after irradiation to identify early gene expression events in the photoprotective response. About 1.5% of genes from the human genome showed significant changes in gene expression. The annotations of these affected genes were assessed. They indicated a strengthening of the inflammation process and up-regulation of the JAK-STAT pathway and other pathways. Parallel to the p53 pathway, the p38 stress-responsive pathway was affected, supporting and mediating p53 function. We used an ex vivo assay with a specific inhibitor of p38 (SB203580) to investigate genes the expression of which was associated with active p38 kinase. We identified new direct p38 target genes and further characterized the role of p38. Our findings provide further insight into the physiological response to UV, including cell-cell interactions and cross-talk effects.

Ultraviolet radiation-induced transcription is associated with gene-specific histone acetylation

UVR is an important environmental carcinogen and a powerful modulator of the cutaneous immune system. Exposure to UVR activates many signaling pathways leading to changes in the expression of several hundred genes. While the covalent modification of histones has been shown to play a central role in regulating gene expression, the impact of UVR on histone modifications and the contribution of histone acetyltransferases (HATs) and histone deacetylases (HDACs) to the UVR-induced transcriptional response have not been completely characterized. In this report, we have examined the impact of UVR on histone H3 K9/14 acetylation. The potential role of UVR-induced histone acetylation in the UVR transcriptional response was also explored using the HAT inhibitor curcumin and HDAC inhibitor trichostatin A (TSA). We found that UVR caused an increase in histone H3 acetylation within the promoter regions of ATF3, COX2, IL-8, MKP1 and MnSOD. In most of the regions examined, histone H3 acetylation peaked 24 h after UVR and then returned to baseline levels by 72 h. The induction of ATF3, COX2 and MKP1 was blocked in the presence of curcumin at doses that decrease in vivo histone H3 acetylation but not at lower doses that do not affect acetylation levels. We also provide evidence that for ATF3, a transcriptional superinduction occurs after repeat exposures to UVR that can be recapitulated when the second UVR exposure is replaced with TSA treatment. Thus, UVR can alter histone acetylation within human keratinocytes and these changes may contribute to the UVR-transcriptional response.

Glomerular targets for autoantibodies in lupus nephritis--an apoptotic origin

Systemic lupus erythematosus (SLE) is an autoimmune syndrome where different organs may individually or simultaneously be affected. Whether SLE is one disease entity or represents a variety of intrinsically unrelated organ manifestations is unknown. Variability of clinical presentations of SLE argues against the former. This does not, however, exclude that certain organ manifestations may be pathogenetically linked. It is believed that in situ binding of anti-dsDNA antibodies by nucleosomes is involved in organ manifestations in SLE. This review will focus on nature and origin of target structures for anti-dsDNA and antinucleosome antibodies in glomerular capillary and mesangial matrix membranes. We will particularly discuss the potential role of apoptosis and release of apoptotic chromatin in terms of their putative impact in SLE.

Gene expression profiling analysis of solar lentigo in relation to immunohistochemical characteristics

BACKGROUND

Solar lentigo appears as dark brown spots that occur on sun-exposed areas and is considered to be a hallmark of aged skin. Although considerable knowledge about acute pigmentation has recently been accumulated, little is yet known about the mechanisms underlying chronic- and delayed-type hyperpigmentation, such as solar lentigo.

OBJECTIVES

To clarify further the mechanisms underlying the development of solar lentigo, we carried out gene expression analysis in skin biopsy specimens obtained from human solar lentigines using DNA microarray analysis.

METHODS

Two pairs of skin specimens were obtained from solar lentigo and adjacent sun-exposed normal skin, as well as normal skin on the buttocks of 16 volunteers aged 40-55 years. One set of specimens was frozen and RNA was extracted for microarray and the other set was prepared for histological sections and analysed by antibodies and probes.

RESULTS

Sixty-five genes were upregulated more than 1.8-fold in solar lentigo compared with adjacent control skin and seven melanocyte-related genes were included. Compared with sun-protected skin, many inflammation-related genes were upregulated in solar lentigo, and compared with sun-exposed control skin, upregulation of genes related to fatty-acid metabolism was apparent in solar lentigo. Moreover, we found downregulation of cornified envelope-related genes, which suggests suppression of cornification in the epidermis in solar lentigo. Immunohistochemically, larger numbers of TRP1-positive cells were found in the basal layer of solar lentigo than in normal skin. Fatty acid-related genes were highly expressed in the epidermis as detected by in situ hybridization, and they were much more prominent in the lesional skin of solar lentigo. However, cycling epidermal cells detectable with Ki67 antibody were fewer in the lesional skin of solar lentigo. Expression of filaggrin and involucrin was decreased in the lesional skin, where the number of cell layers of the stratum corneum was significantly higher than in normal skin.

CONCLUSIONS

The results of the present microarray analysis of solar lentigo, demonstrating upregulation of genes related to inflammation, fatty-acid metabolism and melanocytes and downregulation of cornified envelope-related genes, suggest that solar lentigo is induced by the mutagenic effect of repeated ultraviolet light exposures in the past, leading to the characteristic enhancement of melanin production, together with decreased proliferation and differentiation of lesional keratinocytes on the background of chronic inflammation.

DNA microarrays in dermatology and skin biology

Because of its accessibility, skin has been among the first organs analyzed using DNA microarrays. Skin cancers, melanomas, and basal and squamous cell carcinomas have been intensely investigated because they are very frequent and can be fatal. Psoriasis, one of the most common human inflammatory diseases, has been studied comprehensively using DNA microarrays. In addition, epidermal keratinocytes have been the target of many studies because they respond to a rich variety of inflammatory and immunomodulating cytokines, hormones, vitamins, ultraviolet (UV) light, toxins, and physical injury. Because of the ethical considerations, the effects of harmful or dangerous agents on skin have been studied using artificial skin substitutes. Transcriptional mechanisms that regulate epidermal differentiation and cornification have begun to yield their mysteries, and very exciting recent studies identified the genes specifically expressed in epidermal stem cells. Thus, skin has everything: stem cells, differentiation, signaling, inflammation, diseases, and cancer. All these exciting facets of skin have been explored using DNA microarrays. Researchers in skin biology and dermatology were among the first to implement this technology and we expect that they will continue to generate exciting and useful new knowledge.

Distinct pigmentary and melanocortin 1 receptor-dependent components of cutaneous defense against ultraviolet radiation

Genetic variation at the melanocortin 1 receptor (MC1R) is an important risk factor for developing ultraviolet (UV) radiation–induced skin cancer, the most common form of cancer in humans. The underlying mechanisms by which the MC1R defends against UV-induced skin cancer are not known. We used neonatal mouse skin (which, like human skin, contains a mixture of melanocytes and keratinocytes) to study how pigment cells and Mc1r genotype affect the genome-level response to UV radiation. Animals without viable melanocytes (Kit^W-v/Kit^W-v) or animals lacking a functional Mc1r (Mc1r^e/Mc1r^e) were exposed to sunburn-level doses of UVB radiation, and the patterns of large-scale gene expression in the basal epidermis were compared to each other and to nonmutant animals. Our analysis revealed discrete Kit- and Mc1r-dependent UVB transcriptional responses in the basal epidermis. The Kit-dependent UVB response was characterized largely by an enrichment of oxidative and endoplasmic reticulum stress genes, highlighting a distinctive role for pigmented melanocytes in mediating antioxidant defenses against genotoxic stresses within the basal epidermal environment. By contrast, the Mc1r-dependent UVB response contained an abundance of genes associated with regulating the cell cycle and oncogenesis. To test the clinical relevance of these observations, we analyzed publicly available data sets for primary melanoma and melanoma metastases and found that the set of genes specific for the Mc1r-dependent UVB response was able to differentiate between different clinical subtypes. Our analysis also revealed that the classes of genes induced by UVB differ from those repressed by UVB with regard to their biological functions, their overall number, and their size. The findings described here offer new insights into the transcriptional nature of the UV response in the skin and provide a molecular framework for the underlying mechanisms by which melanocytes and the Mc1r independently mediate and afford protection against UV radiation.

Skin cancer is the most common type of cancer in humans and annually accounts for approximately 60,000 deaths worldwide. The most important factors causally linked to skin cancer susceptibility are inadequate protection against ultraviolet (UV) B radiation, fair skin color, and variation of the melanocortin 1 receptor (MC1R) gene. We used cDNA microarrays to measure the genome-wide transcriptional responses to UVB irradiation in the epidermis of neonatal mice (which approximates the human basal epidermis in its cellular composition and general physiology). To investigate how pigment cells (melanocytes) and MC1R afford protection against UVB radiation, we compared results from normal mice to those from mutant mice that lacked either melanocytes (Kit^W-v/Kit^W-v) or a functional Mc1r (Mc1r^e/Mc1r^e). We identified melanocyte- and Mc1r-dependent UVB gene expression profiles in the basal epidermis. Surprisingly, the melanocyte- and Mc1r-dependent UVB responses highlighted distinct functions, with the former largely mediating antioxidant defenses and the latter regulating the cell cycle and susceptibility to oncogenesis. We also demonstrated that a subset of Mc1r-dependent UVB-responsive genes could discriminate among human melanoma subtypes, thereby suggesting a mechanism by which MC1R gene variants may predispose toward skin cancer.

Erbb2 regulates inflammation and proliferation in the skin after ultraviolet irradiation

Exposure to ultraviolet (UV) irradiation is the major cause of nonmelanoma skin cancer, the most common form of cancer in the United States. UV irradiation has a variety of effects on the skin associated with carcinogenesis, including DNA damage and effects on signal transduction. The alterations in signaling caused by UV regulate inflammation, cell proliferation, and apoptosis. UV also activates the orphan receptor tyrosine kinase and proto-oncogene Erbb2 (HER2/neu). In this study, we demonstrate that the UV-induced activation of Erbb2 regulates the response of the skin to UV. Inhibition or knockdown of Erbb2 before UV irradiation suppressed cell proliferation, cell survival, and inflammation after UV. In addition, Erbb2 was necessary for the UV-induced expression of numerous proinflammatory genes that are regulated by the transcription factors nuclear factor-kappaB and Comp1, including interleukin-1beta, prostaglandin-endoperoxidase synthase 2 (Cyclooxygenase-2), and multiple chemokines. These results reveal the influence of Erbb2 on the UV response and suggest a role for Erbb2 in UV-induced pathologies such as skin cancer.

The UVB-induced gene expression profile of human epidermis in vivo is different from that of cultured keratinocytes

In order to obtain a comprehensive picture of the molecular events regulating cutaneous photodamage of intact human epidermis, suction blister roofs obtained after a single dose of in vivo ultraviolet (UV)B exposure were used for microarray profiling. We found a changed expression of 619 genes. Half of the UVB-regulated genes had returned to pre-exposure baseline levels at 72 h, underscoring the transient character of the molecular cutaneous UVB response. Of special interest was our finding that several of the central p53 target genes remained unaffected following UVB exposure in spite of p53 protein accumulation. We next compared the in vivo expression profiles of epidermal sheets to that of cultured human epidermal keratinocytes exposed to UVB in vitro. We found 1931 genes that differed in their expression profiles between the two groups. The expression profile in intact epidemis was geared mainly towards DNA repair, whereas cultured keratinocytes responded predominantly by activating genes associated with cell-cycle arrest and apoptosis. These differences in expression profiles might reflect differences between mature differentiating keratinocytes in the suprabasal epidermal layers versus exponentially proliferating keratinocytes in cell culture. Our findings show that extreme care should be taken when extrapolating from findings based on keratinocyte cultures to changes in intact epidermis.