Photocarcinogenesis and inhibition of intercellular adhesion molecule 1 expression in cells of DNA-repair-defective individuals

DNA Damage: From Chronic Inflammation to Age-Related Deterioration

To lessen the "wear and tear" of existence, cells have evolved mechanisms that continuously sense DNA lesions, repair DNA damage and restore the compromised genome back to its native form. Besides genome maintenance pathways, multicellular organisms may also employ adaptive and innate immune mechanisms to guard themselves against bacteria or viruses. Recent evidence points to reciprocal interactions between DNA repair, DNA damage responses and aspects of immunity; both self-maintenance and defense responses share a battery of common players and signaling pathways aimed at safeguarding our bodily functions over time. In the short-term, this functional interplay would allow injured cells to restore damaged DNA templates or communicate their compromised state to the microenvironment. In the long-term, however, it may result in the (premature) onset of age-related degeneration, including cancer. Here, we discuss the beneficial and unrewarding outcomes of DNA damage-driven inflammation in the context of tissue-specific pathology and disease progression.

Persistence of repair proteins at unrepaired DNA damage distinguishes diseases with ERCC2 (XPD) mutations: cancer-prone xeroderma pigmentosum vs. non-cancer-prone trichothiodystrophy

Patients with xeroderma pigmentosum (XP) have a 1,000-fold increase in ultraviolet (UV)-induced skin cancers while trichothiodystrophy (TTD) patients, despite mutations in the same genes, ERCC2 (XPD) or ERCC3 (XPB), are cancer-free. Unlike XP cells, TTD cells have a nearly normal rate of removal of UV-induced 6-4 photoproducts (6-4PP) in their DNA and low levels of the basal transcription factor, TFIIH. We examined seven XP, TTD, and XP/TTD complex patients and identified mutations in the XPD gene. We discovered large differences in nucleotide excision repair (NER) protein recruitment to sites of localized UV damage in TTD cells compared to XP or normal cells. XPC protein was rapidly localized in all cells. XPC was redistributed in TTD, and normal cells by 3 hr postirradiation, but remained localized in XP cells at 24-hr postirradiation. In XP cells recruitment of other NER proteins (XPB, XPD, XPG, XPA, and XPF) was also delayed and persisted at 24 hr (p<0.001). In TTD cells with defects in the XPD, XPB, or GTF2H5 (TTDA) genes, in contrast, recruitment of these NER proteins was reduced compared to normals at early time points (p<0.001) and remained low at 24 hr postirradiation. These data indicate that in XP persistence of NER proteins at sites of unrepaired DNA damage is associated with greatly increased skin cancer risk possibly by blockage of translesion DNA synthesis. In contrast, in TTD, low levels of unstable TFIIH proteins do not accumulate at sites of unrepaired photoproducts and may permit normal translesion DNA synthesis without increased skin cancer.

Six genes associated with the clinical phenotypes of individuals with deficient and proficient DNA repair

Xeroderma pigmentosum (XP) is a genetic disorder characterised by hypo-/hyperpigmentation, increased sensitivity to ultraviolet (UV)-radiation and an up to 2000-fold increased skin cancer risk. Cells from XP-patients are defective in nucleotide excision repair (NER) responsible for repair of UV-induced DNA damage. This defect accounts for their mutator phenotype but does not predict their increased skin cancer risk. Therefore, we carried out array analysis to measure the expression of more than 1000 genes after UVB-irradiation in XP cells from three complementation groups with different clinical severity (XP-A, XP-D, XP-F) as well as from patients with normal DNA repair but increased skin cancer risk (≥2 basal or squamous cell carcinoma at age <40yrs). Of 144 genes investigated, 20 showed differential expression with p < 0.05 after irradiation of cells with 100 mJ/cm(2) of UVB. A subset of six genes showed a direct association of expression levels with clinical severity of XP in genes affecting carcinogenesis relevant pathways. Genes identified in XP cells could be confirmed in cells from patients with no known DNA repair defects but increased skin cancer risk. Thus, it is possible to identify a small gene subset associated with clinical severity of XP patients also applicable to individuals with no known DNA repair defects.

The DNA repair gene XPD/ERCC2 polymorphisms Arg156Arg (exon 6) and Lys751Gln (exon 23) are closely associated

In the context of a molecular epidemiology study dealing with the effects of individual genetic susceptibility on childhood respiratory morbidity, DNA repair genotypes for the XPD/ERCC2 gene in exon 6 (Arg156Arg) and exon 23 (Lys751Gln) have been analyzed by PCR/RFLP assays in DNA samples isolated from the fetal parts of placentas. The study was performed using a cohort of 729 children born in 1994-1998 in two districts of the Czech Republic. On the basis of these data, we tested the association between the two genotypes. The principal finding of this study is that the exon 6 and exon 23 polymorphisms in the XPD/ERCC2 gene are tightly associated, with persons who are homozygous CC in exon 23 being mostly (81%) homozygous CC in exon 6, and persons homozygous AA in exon 6 mostly (88%) homozygous AA in exon 23. This strong association may have serious consequences for the interpretation of cancer susceptibility and other molecular epidemiology studies dealing with the XPD6 and XPD23 genotypes, since the observed effects of the silent XPD6 polymorphism might be, in fact, the result of XPD23 polymorphism, which is connected with an amino acid substitution in the resulting XPD protein.

Decreased human epidermal antigen-presenting cell activity after ultraviolet A exposure: dose–response effects and protection by sunscreens

BACKGROUND

Ultraviolet (UV) exposure of human skin causes immunosuppression that contributes to the growth of skin cancer. The contribution of UVA in these processes is still a matter of debate.

OBJECTIVES

The purpose of our study was first to find a dose-response effect of UVA exposure on human epidermal antigen-presenting cell (APC) activity and to evaluate the protective capacity of two sunscreen formulations against a high level of acute UVA exposure. We also tried to evaluate the protective capacity afforded by the same sunscreens against UVA-induced clinical changes such as redness and pigmentation.

METHODS

The functional assessment of the alloantigen-presenting capacity of epidermal cells prepared from skin keratotome samples 3 days after UVA exposure was measured with a mixed epidermal cell-lymphocyte reaction (MECLR) in each healthy volunteer (n = 16). Redness and pigmentation were assessed by chromametry 24 h after exposure to a single UVA dose.

RESULTS

In vivo UVA exposure to 15, 30 and 60 J cm(-2) resulted in a dose-dependent decrease in purified allogeneic T cell (CD4+ T cells) proliferation induced by UVA-irradiated epidermal cells. The epidermal APC function was significantly decreased with a suberythemal exposure corresponding to 15 J cm(-2). The decrease, partial and not statistically different between 30 and 60 J cm(-2), exhibits a plateau-response effect. There was no correlation between the decrease of the epidermal APC function and the intensity of erythema and persistent pigment darkening. Both sunscreen formulations strongly inhibited the UVA-induced reduction of MECLR at 90 J cm(-2).

CONCLUSION

Our results clearly demonstrate that UVA impairs the APC activity of the epidermal cells and thus may contribute to UV-induced immunosuppression in humans. They also indicate that erythema and immunosuppression have different dose-response curves in the UVA range. The two sunscreen formulations afforded a significant protection against the decrease in epidermal APC activity induced by exposure to a high UVA dose (90 J cm(-2)).

Ultraviolet B radiation-induced immunosuppression: molecular mechanisms and cellular alterations

About 30 years ago, the discovery of the connection between UV radiation and the immune system triggered the field of photoimmunology. In that time, many aspects were studied, and a complex picture emerged. UV absorption results in multi-tiered molecular and cellular UV radiation-induced events, eventually affecting the immune system. The shorter wavelengths of the UV spectrum, i.e. UVB appear to be the most critical players for impairing immune reactions. This review summarizes and discusses UVB radiation-induced effects on the skin, considering the primary efferent molecular events following energy absorption of UVB radiation, ending with the various afferent cellular changes, such as induction of regulatory T cells.

Xeroderma pigmentosum: from symptoms and genetics to gene-based skin therapy

Xeroderma pigmentosum (XP) is a rare, recessively inherited genodermatosis prone to ultraviolet (UV)-induced skin neoplasms from keratinocyte origin, i.e. basal and squamous cell carcinoma. Cells from classic XP patients fail to properly eliminate UV-induced DNA lesions by the nucleotide excision repair (NER) mechanism. A variant form of XP, called XP-V suffers from faulty translesion synthesis. We review here recent data on XP gene products whose alterations affect NER and result in one of the 7 complementation groups of XP. Encouraging results of retrovirus-based genetic correction of XP keratinocytes are summarized and support realistic prospects of gene therapy for the XP-C complementation group.

Ectoin: an effective natural substance to prevent UVA-induced premature photoaging

With the help of a new 'UVA stress model', it was shown that Ectoin protects the skin from the effects of UVA-induced cell damage in a number of different ways. Using cell cultures, high-performance thin-layer chromatography, gel electrophoresis mobility shift assays, reverse transcriptase polymerase chain reaction, ion exchange chromatography and UV spectroscopy, it was demonstrated that the UVA-induced second messenger release, transcription factor AP-2 activation, intercellular adhesion molecule-1 expression and mitochondrial DNA mutation could be prevented. The results obtained clearly demonstrate that Ectoin counteracts the effects of UVA-induced and accelerated skin aging at different cell levels.

Xeroderma pigmentosum: from genetics to hopes and realities of cutaneous gene therapy

Xeroderma pigmentosum (XP) is a rare genodermatosis transmitted as an autosomal and recessive trait. XP patients are highly photosensitive and prone to develop skin tumours in sun-exposed areas. Biochemical and genetic studies have demonstrated that nucleotide excision repair, the most versatile DNA repair mechanism, is deficient in XP cells, leading to ultraviolet-induced hypermutagenesis and a predisposition of XP patients to cancer. Cloning of XP genes responsible for the disease, together with the poor efficacy of classical pharmacological treatments, have motivated approaches towards cutaneous gene therapy of the XP. The author's group have successfully reconstructed XP skin in vitro from XP keratinocytes and fibroblasts. More recently, the possibility to fully revert the phenotype of XP keratinocytes after retrovirus-mediated transfer of the adequate wild-type XP gene in XP keratinocytes was demonstrated. Reconstruction of genetically corrected XP skin in vitro constitutes a new hope toward cutaneous gene therapy of the XP.

Mitochondrial cytochrome c release mediates ceramide-induced activator protein 2 activation and gene expression in keratinocytes

The intracellular signaling pathway(s) through which second messenger ceramides induce gene expression in human cells has not yet been characterized. In the present study, ceramide-induced expression of intercellular adhesion molecule-1 (ICAM-1), which requires activation of transcription factor activator protein 2 (AP-2), was found to be mediated through a mitochondrial pathway. Inhibitors of mitochondrial electron transport chain (e.g. rotenone, thenoyltrifluoroacetone, and antimycin A) reduced ceramide-induced ICAM-1 expression. Stimulation of human keratinocytes with cell-permeant ceramides at concentrations that did not induce apoptosis (no activation of caspases 3, 8, and 9 and no nucleosomal fragmentation) but caused AP-2 activation and ICAM-1 induction released cytochrome c (cyt c) from mitochondria into the cytoplasm of cells. This cyt c release was an indispensable prerequisite for effective ceramide signaling, because its inhibition by modulating the mitochondrial megachannel with bonkrekic acid or carboxyatractyloside prevented ceramide-induced AP-2 activation and ICAM-1 expression. Analysis of the interaction between cyt c and AP-2 revealed that cyt c oxidized AP-2 and that this redox regulation greatly enhanced the DNA binding capacity of AP-2. Mitochondria thus have a previously unrecognized function in signaling ceramide-induced transcription factor activation and gene regulation.

Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients

Mutations in the XPD gene result in xeroderma pigmentosum (XP) and trichothiodystrophy (TTD), the phenotypes of which are often intricate. To understand the genotype/phenotype relationship, we engineered recombinant TFIIHs in which XPD subunits carry amino acid changes found in XPD patients. We demonstrate that all the XPD mutations are detrimental for XPD helicase activity, thus explaining the NER defect. We also show that TFIIH from TTD patients, but not from XP patients, exhibits a significant in vitro basal transcription defect in addition to a reduced intracellular concentration. Moreover, when XPD mutations prevent interaction with the p44 subunit of TFIIH, transactivation directed by certain nuclear receptors is inhibited, regardless of TTD versus XP phenotype, thus explaining the overlapping symptoms. The implications of these mutations are discussed using a structural model of the XPD protein. Our study provides explanations for the nature and the severity of the various clinical features.

Xeroderma Pigmentosum: Mondscheinkinder. Xeroderma Pigmentosum: Children of the Moon

Xeroderma pigmentosum is based on a genetic defect in the DNA repair system, which is diagnosed in early childhood. Xeroderma pigmentosum is a rare disorder, which is transmitted in an autosomal recessive manner. Children with xeroderma pigmentosum display hypersensitivity to ultraviolet (UV) radiation. These patients experience serious sunburns with minimal exposure and then develop poikiloderma in the sun-exposed areas. Squamous cell carcinomas, basal cell carcinomas and malignant melanomas all appear during childhood. The majority of patients do not reach adult, but die from metastatic cutaneous malignancies. Genetically, xeroderma pigmentosum is differentiated into 7 complementation groups (XP-A to XP-G) and the xeroderma pigmentosum variants (XP-V). The assignment to the specific complementation group is made by fusing of xeroderma pigmentosum fibroblasts. Xeroderma pigmentosum must be distinguished from other so-called DNA repair deficiency syndromes, including Cockayne syndrome and trichothiodystrophy. A topical DNA repair enzyme appears to be helpful. A recombinant liposomal encapsulated T4 endonuclease V repairs UV-induced cyclobutane-pyrimidine dimers. Direct curative treatment of xeroderma pigmentosum could be achieved with gene therapy in future. Transfection of an intact repair gene which specifically codes for the missing repair protein could open new possibilities in the therapy of xeroderma pigmentosum.

Genetic reversion of inherited skin disorders

Human epidermis is a squamous stratified epithelium whose integrity relies on balanced processes of cell attachment, proliferation, and differentiation. In monogenic skin dermatoses, such as mecano-bullous diseases, or DNA repair deficiencies such as the xeroderma pigmentosum (XP), alterations of skin integrity may have devastating consequences as illustrated by the extremely high epidermal cancer proneness of XP patients. The lack of efficient pharmacological treatments, the easy accessibility of skin, and the possibility of long term culture and genetic manipulations ex vivo of epidermal keratinocytes, have encouraged approaches toward gene transfer and skin therapy prospects. We review here some of the human genetic disorders that exhibit major traits in skin, as well as requirements and difficulties inherent to approaches aimed at stable phenotypic correction.

Photoimmunology and nucleotide excision repair: impact of transcription coupled and global genome excision repair

Ultraviolet (UV) light generates damage to DNA which is removed by a versatile mechanism called nucleotide excision repair (NER). There are two subpathways for NER: the transcription coupled repair (TCR) pathway which removes DNA damage from actively transcribed genes and the global genome repair pathway which removes damage throughout the genome. Most types of DNA lesions are processed more rapidly by TCR than by GGR. It is widely accepted that immunological processes play a pivotal role in the generation of skin tumours induced by exposure to ultraviolet light and first evidence is emerging that GGR and TCR play different roles in skin reactions such as erythema and delayed type hypersensitivity. The relationship between UV-induced responses of the skin and the two NER subpathways is discussed.

Defective dendritic cell maturation in a child with nucleotide excision repair deficiency and CD4 lymphopenia

We report a case of a combined immunodeficiency (CID) in a child affected by trichothiodystrophy (TTD) characterized by an altered response to ultraviolet (UV) light due to a defect in the XPD gene. The XPD gene encodes a subunit of the transcription factor II H (TFIIH), a complex involved in nucleotide-excision repair (NER) and basal transcription. Our patient showed neurological and immune system abnormalities, including CD4 + lymphopenia never previously reported in TTD patients. In vitro immunological studies revealed a marked reduction in T-cell proliferation in response to mitogens and CD3 cross-linking which was partially recovered by the addition of anti-CD28 antibody or exogenous interleukin-2. The patient's T cells displayed alterations in T-cell receptor (TCR/CD3) proximal signalling characterized by marked reduction in Lck kinase activity coupled with a constitutive hyperactivation of Fyn kinase. Despite these alterations, normal levels of Lck and Fyn proteins were detected. The role of antigen-presenting cells (APCs) in the pathogenesis of the T-cell defect was investigated by analysing dendritic cells (DCs) generated from the patient's blood monocytes. In these cells, flow cytometry revealed significantly reduced expression of the CD86 co-stimulatory molecules and HLA glycoproteins. In addition, the patient's DCs showed a decreased ability to stimulate naive T lymphocytes. Overall, the results of our study suggest that a defective TFIIH complex might result in alterations in T cells and DC functions leading to a severe immunodeficiency.

Impaired ultraviolet-B-induced cytokine induction in xeroderma pigmentosum fibroblasts

Xeroderma pigmentosum is a rare, autosomal recessive disease in which patients develop excessive solar damage at an early age and have a 1000-fold increased risk of developing cutaneous neoplasms. Xeroderma pigmentosum can be classified into seven complementation groups (A-G) with defects in different DNA nucleotide excision repair genes. Xeroderma pigmentosum patients also have impaired immune function including reduced natural killer cell activity and impaired induction of interferon-gamma. We hypothesized that altered cytokine induction may contribute to the immune defect in xeroderma pigmentosum patients. We examined cytokine mRNA expression after ultraviolet B irradiation using reverse transcriptase polymerase chain reaction in fibroblasts derived from five xeroderma pigmentosum patients in complementation groups A, C, and D and in complemented XP-A and XP-D cells. Cytokines interleukin-1beta and interleukin-6 displayed impaired ultraviolet B induction whereas interleukin-8 had normal induction in the xeroderma pigmentosum fibroblasts. Stable complementation of XP-A and XP-D cell lines increased ultraviolet-B-induced interleukin-1beta and interleukin-6 expression. These results demonstrate a deficient response of xeroderma pigmentosum fibroblasts to ultraviolet B in terms of cytokine interleukin-1beta and interleukin-6 induction but normal interleukin-8 induction and exhibit a role for DNA repair in cytokine induction.

New developments in photoprotection of human skin

Because of increased leasure time, the growing popularity of staying outdoors and of holidays in the sun, it has become more and more important to study the molecular and photobiological effects that ultraviolet (UV) radiation exerts on human skin. Information obtained from these studies is being used to constantly improve the quality of sunscreen preparations containing organic and inorganic filters. It is also fostering the development of antioxidants and active agents that can be used in combination with or in addition to UV filters to provide better photoprotection for human skin. In addition, these studies provide novel test models which allow to prove or disprove the efficacy of a given sunscreen preparation for biological endpoints that differ from the sunburn reaction and are of direct relevance for photocarcinogenesis and photoaging. In this review, two examples are given to illustrate this development: (1) the use of topically applied DNA repair enzymes to prevent UVB-radiation-induced damage and (2) the development of a novel assay that is based on the detection of UVA-radiation-induced mutations in mitochondrial DNA and can be used to study the efficacy of protective measures against photoaging of human skin.

Effects of solar radiation on the human immune system

On UV irradiation of the skin, a complex cascade of immunological changes results, initiated by cutaneous chromophores and ending in suppression of some local and systemic immune responses. In this review, the stages in this process are outlined first, concentrating on the roles of DNA and urocanic acid as photoreceptors. Evidence indicating UV-induced immunomodulation of delayed hypersensitivity and resistance to infectious diseases in human subjects follows. Aspects of genetic susceptibility to the immunosuppressive effects of UV exposure and extrapolation of the data obtained in animal models to the human situation are included. Finally uncertain and unknown factors relating to the impact of UV on the human immune system are discussed.

Contrasting effects of an ultraviolet B and an ultraviolet A tanning lamp on interleukin-6, tumour necrosis factor-alpha and intercellular adhesion molecule-1 expression

BACKGROUND

Recent studies have demonstrated that a tanning lamp emitting predominantly ultraviolet (UV) A induces significant yields of the type of potentially mutagenic DNA damage that are associated with the onset of skin cancer (i.e. cyclobutane pyrimidine dimers). UV-induced immunosuppression is also an important event leading to skin cancer.

OBJECTIVES

To the modulation of key immunological molecules following exposure to a broad-spectrum UVB lamp and a predominantly UVA-emitting tanning lamp using model in vitro systems.

METHODS

We compared secretion and mRNA expression of interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha in normal human epidermal keratinocytes, and interferon (IFN)-gamma-induced intracellular adhesion molecule (ICAM)-1 in normal human fibroblasts irradiated in vitro with a broad-spectrum UVB lamp or with a Philips 'Performance' tanning lamp.

RESULTS

With broad-spectrum UVB irradiation, upregulation of IL-6 and TNF-alpha mRNA was detected 6 h after irradiation, and a dose-dependent increase of cytokines in the supernatants of irradiated cells was found 24 h after irradiation. In contrast, there was no cytokine secretion and little evidence for mRNA upregulation following exposure to a tanning lamp. When cells were exposed first to broad-spectrum UVB, then the tanning lamp, UVB-induced cytokine secretion was inhibited, although mRNA levels were upregulated to a level close to that observed with UVB alone. By using a Schott WG 320 nm filter to attenuate the level of UVB relative to UVA emitted by the tanning lamp, the inhibition of cytokine secretion was shown to be associated with UVA exposure. Both UV sources inhibited IFN-gamma-induced ICAM-1 mRNA expression in a dose-dependent fashion. By using a Schott WG 335 nm filter, inhibition of ICAM-1 mRNA expression by the tanning lamp was shown to be associated with UVB exposure.

CONCLUSIONS

These results suggest that UV sources emitting different levels of UVA and UVB have differential effects on the modulation of different immunoregulatory molecules, and indicate that there are potential interactions between these wavelengths.

Trichothiodystrophy: update on the sulfur-deficient brittle hair syndromes

Trichothiodystrophy (TTD) refers to a heterogeneous group of autosomal recessive disorders that share the distinctive features of short, brittle hair and an abnormally low sulfur content. Within the spectrum of the TTD syndromes are numerous interrelated neuroectodermal disorders. The TTD syndromes show defective synthesis of high-sulfur matrix proteins. Abnormalities in excision repair of ultraviolet (UV)-damaged DNA are recognized in about half of the patients. Three distinct autosomal recessive syndromes are associated with nucleotide excision repair (NER) defects: the photosensitive form of TTD, xeroderma pigmentosum, and Cockayne syndrome. The unifying feature of these conditions is exaggerated sensitivity to sunlight and UV radiation. In contrast to patients with xeroderma pigmentosum, no increase of skin cancers in patients with TTD has been observed. Genetically, 3 complementation groups have been characterized among photosensitive patients with TTD. Most patients exhibit mutations on the two alleles of the XPD gene. Rarely, mutated XPB gene or an unidentified TTD-A gene may result in TTD. In UV-sensitive TTD, the TFIIH transcription factor containing XPB and XPD helicase activities necessary for both transcription initiation and DNA repair is damaged. Beyond deficiency in the NER pathway, it is hypothesized that basal transcription may be altered leading to decreased transcription of specific genes. Depressed RNA synthesis may account for some clinical features, such as growth retardation, neurologic abnormalities, and brittle hair and nails. Therefore the attenuated expression of some proteins in differentiated cells is most likely explained by a mechanism distinct from DNA repair deficiency. The first transgenic mouse models for NER deficiencies have been generated. The TTD mouse as well as related cell models will provide important tools to understand the complex relationships between defects in DNA repair, low-sulfur hair shaft disorders, and the genotype-phenotype correlates for this constellation of inherited disorders, including the lack of predisposition to cancer in patients with TTD.

Ultraviolet AI exposure of human skin results in Langerhans cell depletion and reduction of epidermal antigen-presenting cell function: partial protection by a broad-spectrum sunscreen

BACKGROUND

Ultraviolet (UV) B-induced effects on the skin immune system have been extensively investigated, but little is known regarding the immunological changes induced by UVA exposure of human skin. Recent data assessing the protection afforded by sunscreens against photoimmunosuppression stress the need for broad-spectrum sunscreens with an adequate UVA protection.

OBJECTIVES

The purpose of this study was first to determine the changes observed in epidermal Langerhans cells (ELC) density and epidermal antigen-presenting cell (APC) activity after exposure of human skin to UVAI (340-400 nm) radiation, and secondly to assess the immune protection afforded in vivo by a sunscreen formulation containing a long wavelength UVA filter with a low UVA protection factor (UVA-PF = 3).

METHODS

Epidermal cell (EC) suspensions were prepared from skin biopsies 3 days after exposure to a single dose of UVAI (either 30 or 60 J cm(-2)).

RESULTS

Flow-cytometric analysis of EC suspensions revealed that exposure to 60 J cm(-2) UVAI resulted in a decreased number of ELC without infiltration of CD36+ DR+ CD1a- antigen-presenting macrophages into the epidermis, and a significant reduction of HLA-DR expression on viable ELC. In vivo exposure to both 30 and 60 J cm(-2) resulted in a decreased allogeneic CD4+ T-cell proliferation induced by UVAI-irradiated ECs. The sunscreen application partially prevented (57 +/- 9%) the decrease in epidermal allogeneic APC activity induced by 60 J cm(-2) UVAI.

CONCLUSIONS

In vivo UVAI exposure of human skin results in a decreased number of ELC and in a downregulation of epidermal APC activity. This last effect is partially prevented by prior application of a sunscreen with a low UVAI-PF value. These results indicate that increasing the absorption of UV filters for long UVA wavelengths may lead to an improved immune protection.

Trichothiodystrophy, a transcription syndrome

Trichothiodystrophy (TTD) is a rare genetic disorder characterized by a hair dysplasia and associated with numerous symptoms affecting mainly organs derived from the neuroectoderm. About half of TTD patients exhibit photosensitivity because their nucleotide-excision repair pathway (NER) does not remove UV-induced DNA lesions efficiently. However, they do not present the skin cancer susceptibility expected from such an NER disorder. Their deficiencies result from phenotype-specific mutations in either XPB or XPD. These genes encode the helicase subunits of TFIIH, a DNA repair factor that is also required for transcription of class II genes. Thus, time- and tissue-specific impairments of transcription might explain the developmental and neurological symptoms of TTD. In a third group of photosensitive patients, TTD-A, no mutation has been identified, although TFIIH amount is reduced.

Cutaneous gene transfer and therapy: the present and the future

The easy accessibility of the skin as a therapeutic target provides an exciting potential for this organ for the development of gene therapy protocols for cutaneous diseases and a variety of metabolic disorders. Thus far, full phenotypic reversion of a diseased phenotype has been achieved in vivo for junctional epidermolysis bullosa and X-linked or lamellar ichthyosis and in vitro for xeroderma pigmentosum. These recessive skin diseases are characterized by skin blistering, abnormalities in epidermal differentiation and increased development of skin cancers, respectively. Corrective gene delivery at both molecular and functional levels was achieved by transduction of cultured skin cells using retroviral vectors carrying the specific curative cDNA. These positive results should prompt clinical trials based on transplantation of artificial epithelia reconstructed ex vivo using genetically modified keratinocytes. Promising results have also been obtained in phenotypic reversion of cells isolated from patients suffering from a number of metabolic diseases such as gyrate atrophy, familial hypercholesterolemia or phenylketonuria. In these diseases transplantation of autologous artificial epithelia expressing the transgenes of interest or direct transfer of the DNA to the skin represents a potential therapeutic approach for the systemic delivery of active molecules. Successful cutaneous gene therapy trials, however, require development of protocols for efficient gene transfer to epidermal stem cells, and information about the host immune response to the recombinant polypeptides produced by the implanted keratinocytes. The availability of spontaneous animal models for genodermatoses will validate the gene therapy approach in preclinical trials.

Non-enzymatic triggering of the ceramide signalling cascade by solar UVA radiation

Ceramide is a key component of intracellular stress responses. Evidence is provided for a novel mechanism of ceramide formation that mediates solar ultraviolet (UV) A radiation-induced expression of the intercellular adhesion molecule (ICAM)-1. Similarly to UVA radiation, ceramide stimulation of human keratinocytes induced ICAM-1 mRNA expression and activated the ICAM-1 promoter through transcription factor AP-2. Ceramide-activated AP-2 and ceramide-induced ICAM-1 reporter gene activation were abrogated through deletion of the AP-2 binding site. UVA radiation increased the level of ceramide in keratinocytes and inhibition of sphingomyelin synthesis prevented UVA radiation-induced ICAM-1 expression. Hitherto, two pathways have been identified for ceramide accumulation: hydrolysis from sphingomyelin through neutral and acid sphingomyelinases, and de novo synthesis by ceramide synthase. UVA radiation did not activate any of these enzymes. Ceramide generation in UVA-irradiated cells, however, was inhibited by singlet oxygen quenchers and mimicked in unirradiated cells by a singlet oxygen-generating system. In addition, UVA radiation and singlet oxygen both generated ceramide in protein-free, sphingomyelin-containing liposomes. This study indicates that singlet oxygen triggers a third, non-enzymatic mechanism of ceramide formation.

Ultraviolet-B-induced apoptosis and cytokine release in xeroderma pigmentosum keratinocytes

We have assessed the ability of xeroderma pigmentosum and normal keratinocytes grown out from skin biopsies to undergo apoptosis after irradiation with ultraviolet B. Keratinocytes have been studied from xeroderma pigmentosum complementation groups A (three biopsies), C (three biopsies), D (one biopsy), xeroderma pigmentosum variant (two biopsies), and Cockayne syndrome (one biopsy). The three xeroderma pigmentosum group A and the xeroderma pigmentosum group D samples were at least six times more sensitive than normal cells to ultraviolet B-induced apoptosis. The xeroderma pigmentosum variant samples showed intermediate susceptibility. Xeroderma pigmentosum group C samples proved heterogeneous: one showed high sensitivity to apoptosis, whereas two showed near normal susceptibility. The Cockayne syndrome sample showed the high susceptibility of xeroderma pigmentosum groups A and D only at a higher fluence. These results suggest that the relationships between repair deficiency, apoptosis, and susceptibility to skin cancer are not straightforward. Ultraviolet B-induced skin cancer is also thought to be due in part to ultraviolet B-induced impairment of immune responses. The release of the inflammatory cytokines interleukin-6 and tumor necrosis factor-alpha from cultured xeroderma pigmentosum keratinocytes tended to occur at lower fluences than in normals, but was less extensive, and was more readily inhibited at higher fluences of ultraviolet B.

Neurotrophin-4 production by human epidermal keratinocytes: increased expression in atopic dermatitis

Chronic inflammatory conditions of human skin, such as prurigo lesions of atopic dermatitis, are characterized clinically by intense pruritus and histologically by increased innervation. Regulation of skin innervation is thought to depend on neurotrophic factors. In this study, human skin cells were identified as a source of neurotrophins. Cultured keratinocytes expressed neurotrophin-4, whereas dermal fibroblasts expressed neurotrophin-3. In vitro stimulation with interferon-gamma, a marker cytokine for atopic eczema, induced keratinocyte neurotrophin-4 production, which was able to support growth of a neuroglioblastoma-derived cell line. In vivo, immunohistochemistry of human skin for neurotrophins showed neurotrophin-4 staining in the epidermal layer and neurotrophin-3 staining in the dermal compartment. Neurotrophin-4 but not neurotrophin-3 expression was markedly increased in interferon-gamma-injected skin. Prurigo lesions of atopic dermatitis skin were characterized by intense epidermal staining for neurotrophin-4, suggesting a pathophysiologic role for this neurotrophin in the increased innervation characteristic for these skin lesions. This study demonstrates differential expression and regulation of neurotrophins in human skin. It also identifies keratinocyte-derived neurotrophin-4 as a possible link between the immune and the nerve system of human skin.

Effects of deoxyribonucleosides and cell-stimulation on frequencies of ultraviolet-B-induced micronuclei in human peripheral blood lymphocytes

In the present study the involvement of deoxyribonucleotides (dNTPs) in the clastogenicity of ultraviolet-B (UVB) in unstimulated peripheral blood lymphocytes (G(0)-PBLs) was investigated. This was studied by analyzing the frequency of UVB-induced micronuclei (MN), either after adding a cocktail of the four deoxyribonucleosides to the PBLs immediately after exposure to UVB, or by stimulating the cells before exposure. In total, PBLs obtained from two different donors were investigated. For both donors, it could be demonstrated that addition of deoxyribonucleosides to UVB-irradiated G(0)-PBLs resulted in a significant reduction of the clastogenic effect of UVB. A gradual reduction of the clastogenic effect of UVB could also be realized by irradiating PBLs that were progressively more stimulated with the lectin PHA before exposure. The latter finding is explained by upregulation of intracellular pool sizes of dNTPs in stimulated PBLs.

UV damage causes uncontrolled DNA breakage in cells from patients with combined features of XP-D and Cockayne syndrome

Nucleotide excision repair (NER) removes damage from DNA in a tightly regulated multiprotein process. Defects in NER result in three different human disorders, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS). Two cases with the combined features of XP and CS have been assigned to the XP-D complementation group. Despite their extreme UV sensitivity, these cells appeared to incise their DNA as efficiently as normal cells in response to UV damage. These incisions were, however, uncoupled from the rest of the repair process. Using cell-free extracts, we were unable to detect any incision activity in the neighbourhood of the damage. When irradiated plasmids were introduced into unirradiated XP-D/CS cells, the ectopically introduced damage triggered the induction of breaks in the undamaged genomic DNA. XP-D/CS cells thus have a unique response to sensing UV damage, which results in the introduction of breaks into the DNA at sites distant from the damage. We propose that it is these spurious breaks that are responsible for the extreme UV sensitivity of these cells.

Nucleotide excision repair and human syndromes

DNA damage is implicated in cancer and aging, and several DNA repair mechanisms exist that safeguard the genome from these deleterious consequences. Nucleotide excision repair (NER) removes a wide diversity of lesions, the main of which include UV-induced lesions, bulky chemical adducts and some forms of oxidative damage. The NER process involves the action of at least 30 proteins in a 'cut-and-paste'-like mechanism. The consequences of a defect in one of the NER proteins are apparent from three rare recessive syndromes: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and the photosensitive form of the brittle hair disorder trichothiodystrophy (TTD). Sun-sensitive skin is associated with skin cancer predisposition in the case of XP, but remarkably not in CS and TTD. Moreover, the spectrum of clinical symptoms differs considerably between the three syndromes. CS and TTD patients exhibit a spectrum of neurodevelopmental abnormalities and, in addition, TTD is associated with ichthyosis and brittle hair. These typical CS and TTD abnormalities are difficult to comprehend as a consequence of defective NER. This review briefly describes the biochemistry of the NER process, summarizes the clinical features of the NER disorders and speculates on the molecular basis underlying these pleitropic syndromes.

Enzyme plus light therapy to repair DNA damage in ultraviolet-B-irradiated human skin

Ultraviolet-B (UVB) (290-320 nm) radiation-induced cyclobutane pyrimidine dimers within the DNA of epidermal cells are detrimental to human health by causing mutations and immunosuppressive effects that presumably contribute to photocarcinogenesis. Conventional photoprotection by sunscreens is exclusively prophylactic in nature and of no value once DNA damage has occurred. In this paper, we have therefore assessed whether it is possible to repair UVB radiation-induced DNA damage through topical application of the DNA-repair enzyme photolyase, derived from Anacystis nidulans, that specifically converts cyclobutane dimers into their original DNA structure after exposure to photoreactivating light. When a dose of UVB radiation sufficient to induce erythema was administered to the skin of healthy subjects, significant numbers of dimers were formed within epidermal cells. Topical application of photolyase-containing liposomes to UVB-irradiated skin and subsequent exposure to photoreactivating light decreased the number of UVB radiation-induced dimers by 40-45%. No reduction was observed if the liposomes were not filled with photolyase or if photoreactivating exposure preceded the application of filled liposomes. The UVB dose administered resulted in suppression of intercellular adhesion molecule-1 (ICAM-1), a molecule required for immunity and inflammatory events in the epidermis. In addition, in subjects hypersensitive to nickel sulfate, elicitation of the hypersensitivity reaction in irradiated skin areas was prevented. Photolyase-induced dimer repair completely prevented these UVB radiation-induced immunosuppressive effects as well as erythema and sunburn-cell formation. These studies demonstrate that topical application of photolyase is effective in dimer reversal and thereby leads to immunoprotection.

Activation of p70 ribosomal protein S6 kinase is an essential step in the DNA damage-dependent signaling pathway responsible for the ultraviolet B-mediated increase in interstitial collagenase (MMP-1) and stromelysin-1 (MMP-3) protein levels in human dermal fibroblasts

Ultraviolet B (UVB) irradiation has been shown to stimulate the expression of matrix-degrading metalloproteinases via generation of DNA damage and/or reactive oxygen species. Matrix-degrading metalloproteinases promote UVB-triggered detrimental long term effects like cancer formation and premature skin aging. Here, we were interested in identifying components of the signal transduction pathway that causally link UVB-mediated DNA damage and induction of matrix-degrading metalloproteinase (MMP)-1/interstitial collagenase and MMP-3/stromelysin-1 in human dermal fibroblasts in vitro. The activity of p70 ribosomal S6 kinase, a downstream target of the FK506-binding protein-12/rapamycin-associated protein kinase (FRAP) kinase (RAFT1, mTOR), was identified to be 4.8 +/- 0.8-fold, and MMP-1 and MMP-3 protein levels 2.4- and 11.5-fold increased upon UVB irradiation compared with mock-irradiated controls. The FRAP kinase inhibitor rapamycin and the DNA repair inhibitor aphidicolin significantly suppressed the UVB-mediated increase in p70 ribosomal S6 kinase activity by 50-65% and MMP-1 and MMP-3 protein levels by 34-68% and 42-88% compared with UVB-irradiated fibroblasts. By contrast, the interleukin-1beta-mediated increase in MMP-1 and MMP-3 protein levels could not be suppressed by rapamycin. Collectively, our data suggest that the FRAP-controlled p70 ribosomal S6 kinase is an essential component of a DNA damage-dependent, but not of the interleukin-1/cell membrane receptor-dependent signaling.

Photoimmunology, DNA repair and photocarcinogenesis

In recent years major progress has been made in identifying the molecular mechanisms by which UV radiation modulates the immune system of the skin. From these studies it appears that the generation of DNA damage and the subsequent activation of DNA repair enzymes play a critical role in the generation of UV-B-induced immunosuppression. These studies have made use of cells from both nucleotide excision repair (NER)-deficient individuals and mice. Results obtained from these studies have important clinical implications for DNA-repair-deficient patients in particular and for effective photoprotection of human skin in general.

Frequencies of hprt(-) mutations and bcl-2 translocations in circulating human lymphocytes are correlated with United Kingdom sunlight records

Between 1983 and 1995 we have monitored human populations for evidence of exposure to environmental mutagens, taking blood samples to measure hprt(-) mutant frequency in T cells and more recently bcl-2 t(14:18) translocation frequency in B cells. We have now analysed data from 785 assays on 448 blood samples from 308 normal subjects and find that there is a highly significant statistical correlation between hprt(-) mutant frequency and the sunlight record for the 3 weeks prior to taking the blood sample. We discuss the weaknesses in retrospective studies of this nature and the possibility of spurious epidemiological correlations that may result. More controlled experiments can be envisaged that would give a firmer basis to the statistical associations observed. hprt(-) mutations in T cells show little evidence of a UV fingerprint, so that the correlation may be due to immunomodulation rather than mutation. We also find a correlation between the sunlight record and bcl-2 translocation. This translocation is found at a low frequency in the B cells of many normal subjects and is the commonest translocation observed in non-Hodgkin's lymphoma. Our results strengthen the case for a link between sunlight and this increasingly common cancer.

Cancer from the outside, aging from the inside: mouse models to study the consequences of defective nucleotide excision repair

In recent years, mouse models have been generated to study the syndromes associated with a defect in nucleotide excision repair (NER). Thus, via conventional knockout gene targeting or by mimicking patient-specific alleles, mouse models for xeroderma pigmentosum (XP), Cockayne syndrome (CS) and photosensitive trichothiodystrophy (TTD) have been obtained. The generation of this series of mouse mutants allows in vivo investigation of some intriguing questions that have puzzled the field, such as the paradoxical absence of cancer development in TTD and CS despite their NER deficiencies, and the role of the ERCC1 gene in mitotic recombination and cross-link repair. Other interesting issues include the pathophysiology of the non-NER related clinical symptoms in TTD and CS patients and the proposed involvement of NER and transcription in the process of aging. This review will focus on data obtained thus far and discuss further utilization of the mouse mutants for unraveling some of the fascinating and medically relevant aspects associated with defects in NER and related processes.

Induction of interleukin-6 production by ultraviolet radiation in normal human epidermal keratinocytes and in a human keratinocyte cell line is mediated by DNA damage

The sunburn reaction is the most common consequence of human exposure to ultraviolet radiation (UVR), and is mediated at least in part by interleukin-6 (IL-6). The aim of this study was to determine if DNA is a major chromophore involved in the induction of IL-6 following UV irradiation of a human epidermoid carcinoma cell line (KB), and of normal human epidermal keratinocytes. We first confirmed that IL-6 release was associated with enhanced levels of IL-6 mRNA transcripts. The wavelength dependence for IL-6 release was then investigated by irradiating the cells at defined wavelengths (254, 302, 313, 334, and 365 nm) with a monochromator. The maximum effect on IL-6 release was observed at 254 nm with only low levels of induction observed at wavelengths above 313 nm. The wavelength dependence for UV-induced IL-6 release was similar to that for DNA absorption or for the induction of cyclobutane pyrimidine dimers (CPD). To determine whether UV-induced DNA damage mediated IL-6 secretion, the role of CPD was investigated by treating keratinocytes with photosomes (photolyase encapsulated in liposomes) followed by photoreactivating light. This photoreversal procedure led to a reduction in the levels of the UVC-induced secretion of IL-6, which in normal human keratinocytes was unambiguously associated with repair of CPD. We conclude that the release of IL-6 from human keratinocytes following short-wave UVC and UVB irradiation is mediated by DNA damage and that CPD play an important role in this process.

Xeroderma pigmentosum: spinal cord astrocytoma with 9-year survival after radiation and isotretinoin therapy

BACKGROUND

Patients with xeroderma pigmentosum (XP) frequently develop sunlight-induced skin cancer. Infrequently, internal neoplasms may also occur. A 21-year-old patient with XP, who had many skin cancers, developed a rare internal tumour - a grade II diffuse fibrillary spinal cord astrocytoma - during a break in a therapeutic trial of isotretinoin for skin cancer prevention. Treatment of neoplasms in XP patients presents special difficulties because of their defect in DNA repair.

OBJECTIVE

The study objective was to raise awareness of the cancer surveillance process in XP patients and the concerns involved in choice of therapy.

METHODS

Since the spinal cord tumour was inoperable, the patient was treated with x-radiation, continued on isotretinoin treatment and was followed closely for tumour response.

RESULTS

Despite sensitivity to sunlight, the patient had a normal acute response to the x-ray treatment without excessive skin reaction. Serial examinations by magnetic resonance imaging (MRI) starting 8 months after x-ray treatment was initiated, showed a marked gadolinium enhancement followed by regression. This clearing was first seen at 2 years after biopsy and persisted to at least 9 years after treatment.

CONCLUSION

In contrast to the exaggerated sensitivity to UV radiation, XP patients may tolerate therapeutic doses of x-radiation. Isotretinoin treatment may have contributed to the good response of this spinal cord astrocytoma.

Nuclear and non-nuclear targets of genotoxic agents in the induction of gene expression. Shared principles in yeast, rodents, man and plants

The interplay between environmental cues and the genetic response is decisive for the development, health and well-being of an organism. For some environmental factors a narrow margin separates beneficial and toxic impacts. With the increasing exposure to UV-B this dichotomy has reached public attention. This review will be concerned with the mechanisms that mediate a cellular genetic response to noxious agents. The toxic stimuli find access to the regulatory network inside cells by interacting at several points with cellular molecules - a process that converts the 'outside information' into 'cellular language'. As a consequence of such interactions, many adverse agents cause massive signal transduction and changes of gene expression. There is an interesting conservation of the mechanisms from yeast to man. An understanding of the genetic programs and of their phenotypic consequences is lagging behind.