The human sunburn reaction: histologic and biochemical studies

p53-regulated apoptosis is differentiation dependent in ultraviolet B-irradiated mouse keratinocytes

Previous studies from our laboratory, using p53 transgenic mice, have suggested that ultraviolet (UV) light-induced keratinocyte apoptosis in the skin is not affected by overexpression of mutant p53 protein. To further elucidate a possible role for p53 in UV-induced keratinocyte cell death, we now examine apoptosis in skin and isolated keratinocytes from p53 null (-/-) mice and assess the influence of cell differentiation on this process. In vivo, using this knockout model, epidermal keratinocytes in p53-/- mice exhibited only a 5.2-fold increase in apoptosis after 2000 J/m2 UVB irradiation compared with a 26.3-fold increase in normal control animals. If this p53-dependent apoptosis is important in elimination of precancerous, UV-damaged keratinocytes, then it should be active in the undifferentiated cells of the epidermal basal layer. To test this hypothesis, we examined the effect of differentiation on UV-induced apoptosis in primary cultures of murine and human keratinocytes. Apoptosis was p53-independent in undifferentiated murine keratinocytes, which exhibited relative resistance to UVB-induced killing with only a 1.5-fold increase in apoptosis in p53+/+ cells and a 1.4-fold increase in p53-/- cells. Differentiated keratinocytes, in contrast, showed a 9.4-fold UVB induction of apoptosis in p53+/+ cells, almost three times the induction observed in p53-/- cells. This UV-induced difference in apoptosis was observed when keratinocytes were cultured on type IV collagen substrate, but not on plastic alone. Western blotting of UV-irradiated, differentiated keratinocytes did not support a role for either Bax or Bcl-2 in this process. In support of these findings in mice, cell death in human cultured keratinocytes also occurred in a differentiation-associated fashion. We conclude that p53-induced apoptosis eliminates damaged keratinocytes in the differentiated cell compartment, but this mechanism is not active in the basal, undifferentiated cells and is therefore of questionable significance in protection against skin cancer induction.

Efficacy of micronized titanium dioxide-containing compounds in protection against UVB-induced immunosuppression in humans in vivo

Micronized pigment-containing sunscreens may provide a good alternative to chemical sunscreens in protection against ultraviolet (UV) B-induced immunosuppression. The metal particles in these products are likely to remain on the skin surface where they can offer broadband protection for both the UVA and UVB regions. We have tested the protective capacity of three titanium dioxide (TiO2)-containing compounds in humans in vivo. The effect on sunburn cell formation has been investigated using transmission electron microscopy, while the mixed epidermal cell lymphocyte reaction (MECLR) has been used as a model for immunosuppression. Furthermore, the influence of titanium on the integrity of the stratum corneum barrier (intercellular lipids and desmosomes) has been examined using freeze fracture electron microscopy. We find that all three compounds protect against sunburn cell formation. The immunoprotection studies show that one of the three compounds does not prevent UVB-induced changes of the MECLR responses. Application of this compound without subsequent UVB irradiation also induces a significant decrease of the MECLR responses. Moreover, the same compound affects the intercellular lipid layers, and desmosomes cannot be detected. The deleterious effect of this compound is probably caused by an incomplete hydrolysis during the TiO2 synthesis. Our findings indicate that micronized pigment-containing compounds can offer good protection against short-term UVB-induced immunomodulation in humans in vivo. However, accurate screening of the synthesis of these compounds is a prerequisite for their safe use as sunscreening agents in human subjects.

Aberrant timing in epidermal expression of inducible nitric oxide synthase after UV irradiation in cutaneous lupus erythematosus

Photosensitivity is a main criterion for the diagnosis of systemic lupus erythematosus (LE), and ultraviolet (UV) irradiation plays a key role in the pathogenesis of cutaneous LE. Patients with a tentative diagnosis of LE are routinely tested for skin lesion development after experimental UV irradiation, providing an ideal opportunity to evaluate early, preclinical events involved in the pathogenesis of LE. Several reports have shown expression of the cytokine-inducible nitric oxide synthase (iNOS) in autoimmune diseases. Therefore, we investigated the role of iNOS expression at mRNA and protein level in the pathogenesis of LE lesions. Skin biopsies from patients with different subtypes of LE were examined, and iNOS expression was found in six of 18 biopsies from cutaneous LE patients and two of three biopsies from systemic LE patients. In biopsies taken 4-20 d after UV irradiation, epidermal iNOS expression was seen in all patients (n = 10) after UVB and in four of 10 patients provoked by UVA. In healthy controls (n = 8) epidermal iNOS expression was detected 24 h after UV irradiation, persisting for another day before subsiding on day 3. In LE patients (n = 8) the exact reverse situation was seen: an iNOS-specific signal was undetectable in keratinocytes for 2 d after UV irradiation, but became positive on day 3 and persisted for up to 25 d in the evolving skin lesions. Our findings demonstrate a time-restricted, UV-induced iNOS expression in human skin; moreover, the results indicate that both the kinetics of iNOS induction as well as the time span of local iNOS expression may be critical to the development of cutaneous LE lesions.

Dermal mast cells determine susceptibility to ultraviolet B-induced systemic suppression of contact hypersensitivity responses in mice

Different strains of mice have varying susceptibilities to ultraviolet radiation (UV) of wavelength 280-320 nm (UVB) for 50% suppression of systemic contact hypersensitivity (CHS) responses. Prevalence of histamine-staining dermal mast cells in different strains of mice (C57BL/ 6J, DBA/2, BALB/c) correlated directly with their susceptibility to UVB-induced systemic immunosuppression. BALB/c mice carrying Uvs1, a major locus for susceptibility to UV-induced immunosuppression, contained greater numbers of dermal mast cells than BALB/c mice of the same parental origin. Strains of mice that were differentiated on their susceptibility to UVB-induced downregulation of systemic CHS responses were similar in their susceptibility to histamine-induced immunomodulation. Histamine, but not UVB irradiation, decreased systemic CHS responses in mast cell-depleted mice (W f/W f). Reconstitution of the dorsal skin of W f/W f mice with bone marrow-derived mast cell precursors from nonmutant mice rendered the mice susceptible to UVB irradiation for systemic suppression of CHS responses. UVB irradiation did not suppress delayed type hypersensitivity responses to allogeneic spleen cells in W f/W f mice. In contrast, UV irradiation suppressed CHS responses in W f/W f mice when hapten was applied to the irradiated site. This study demonstrates that dermal mast cells are necessary for the induction of systemic suppression of CHS responses by UVB radiation, and suggests that mast cell- derived histamine is one component of this UVB-induced systemic immunosuppression.

Do inflammatory processes contribute to radiation induced erythema observed in the skin of humans?

BACKGROUND

Two prospective trials were designed to determine whether there may be a role for inflammatory mediators in human skin erythema at both high and low doses per fraction and for 'out of field' effects.

METHODS

Trial 1. Effects of topical indomethacin (1%) and hydrocortisone (1%) applied before and during radiotherapy were compared for erythema induced by 20 Gy in four fractions (n = 26, 6 MV). Trial 2. Effects of topical hydrocortisone (1 %) applied before and during radiotherapy and no medication were compared for erythema induced by 1, 3, 5 and 7 Gy in five fractions (n = 21, 120 kV). Erythema was measured using reflectance spectrophotometry (RFS) and laser Doppler (LD) on a weekly basis.

RESULTS

Trial 1. A bi-phasic reaction time course was suggested in two-thirds of the cases. The first phase did not appear to be influenced by hydrocortisone cream but the second was significantly attenuated. Indomethacin had no effect on either reaction phase. Erythema measured several centimetres outside of the field was reduced by hydrocortisone but not by indomethacin. Trial 2. Trial 2 confirmed the presence of measurable erythema, invisible to the eye, that coincided in its time course to the first phase of erythema noted in trial 1. This reaction was more intense than predicted by the LQ formula and was non-significantly attenuated by topical hydrocortisone. RFS readings proved to be less subject to inter- and intra-patient variations than the LD unit used.

CONCLUSION

Inflammatory responses may play a role in the mediation of the erythematous response to radiation in human skin. Further studies are warranted.

Skin changes in "screen dermatitis" versus classical UV- and ionizing irradiation-related damage--similarities and differences

An increasing number of persons say that they get cutaneous problems as well as symptoms from certain internal organs, such as the central nervous system (CNS) and the heart, when being close to electric equipment. A major group of these patients are the users of video display terminals (VDTs), who claim to have subjective and objective skin- and mucosa-related symptoms, such as pain, itch, heat sensation, erythema, papules, and pustules. The CNS symptoms are, e.g. dizziness, tiredness, and headache. Erythema, itch, heat sensation, edema and pain are also common symptoms of sunburn (UV dermatitis). Alterations have been observed in cell populations of the skin of patients suffering from so-called "screen dermatitis" similar to those observed in the skin damaged due to ultraviolet (UV) light or ionizing radiation. In "screen dermatitis" patients a much higher number of mast cells have been observed. It is known that UVB irradiation induces mast cell degranulation and release of TNF-alpha. The high number of mast cells present in the "screen dermatitis" patients and the possible release of specific substances, such as histamine, may explain their clinical symptoms of itch, pain, edema and erythema. The most remarkable change among cutaneous cells, after exposure with the above-mentioned irradiation sources, is the disappearance of the Langerhans' cells. This change has also been observed in "screen dermatitis" patients, again pointing to a common cellular and molecular basis. The results of this literature study demonstrate that highly similar changes exist in the skin of "screen dermatitis" patients, as regards the clinical manifestations as well as alterations in the cell populations, and in skin damaged by UV light or ionizing radiation.

Modulation of ultraviolet light-induced epidermal damage: beneficial effects of tocopherol

Oxygen free radicals have been shown to result from and mediate deleterious effects of ultraviolet radiation on the skin. The purpose of this study was to determine if topical DL-alpha-tocopherol (vitamin E) could reduce ultraviolet-induced damage to the epidermis. Twenty mice were treated with either ethanol or a 1:1 mixture of tocopherol and ethanol. Treatments consisted of once-daily 0.1-ml topical applications for 1 week, followed by irradiation with 0.30 mW/cm2 of ultraviolet B irradiation. A statistically significant decrease in Schiff base formation was noted between tocopherol-treated animals and their controls. Histologic study revealed a statistically significant increase in epidermal thickness in tocopherol-treated skin versus controls or vehicle alone. The thicker epidermis was accompanied by the presence of parakeratosis, implicating increased proliferation as the cause of the increasing thickness. The number of sunburn cells was decreased by tocopherol treatment. Tocopherol protection from ultraviolet irradiation may have been due to both direct protection from free radicals and indirect protection by means of increased epidermal thickness. The demonstration of beneficial effects of tocopherol administration suggests that further studies in clinically relevant models to define optimal dosage, frequency of administration, vehicle, and quantitation of the possible protective effects afforded to Langerhans cells may be useful.

Photochemistry and photobiology of actinic erythema: defensive and reparative cutaneous mechanisms

Sunlight is part of our everyday life and most people accept it as beneficial to our health. With the advance of our knowledge in cutaneous photochemistry, photobiology and photomedicine over the past four decades, the terrestrial solar radiation has become a concern of dermatologists and is considered to be a major damaging environmental factor for our skin. Most photobiological effects (e.g., sunburn, suntanning, local and systemic immunosuppression, photoaging or dermatoheliosis, skin cancer and precancer, etc.) are attributed to ultraviolet radiation (UVR) and more particularly to UVB radiation (290-320 nm). UVA radiation (320-400 nm) also plays an important role in the induction of erythema by the photosensitized generation of reactive oxygen species (singlet oxygen (1O2), superoxide (O2.-) and hydroxyl radicals (.OH)) that damage DNA and cellular membranes, and promote carcinogenesis and the changes associated with photoaging. Therefore, research efforts have been directed at a better photochemical and photobiological understanding of the so-called sunburn reaction, actinic or solar erythema. To survive the insults of actinic damage, the skin appears to have different intrinsic defensive mechanisms, among which antioxidants (enzymatic and non-enzymatic systems) play a pivotal role. In this paper, we will review the basic aspects of the action of UVR on the skin: a) photochemical reactions resulting from photon absorption by endogenous chromophores; b) the lipid peroxidation phenomenon, and c) intrinsic defensive cutaneous mechanisms (antioxidant systems). The last section will cover the inflammatory response including mediator release after cutaneous UVR exposure and adhesion molecule expression.

The spectral dependence for UVA-induced cumulative damage in human skin

The wavelength dependence for UVA-induced cumulative damage was investigated in human skin. Epidermal changes (stratum corneum thickening, viable epidermal thickening sunburn cell production), as well as dermal alterations (lysozyme deposition, inflammation), were used as indices of cumulative photoperturbation. UVA wavelengths between 320 nm and 345 nm were more effective than longer wavelengths (360-400 nm) in inducing viable epidermal thickening. Similarly, the shorter wavelengths (320-345 nm) elicited more sunburn cells, although these differences did not reach statistical significance. All UVA bands were equally effective in inducing the dermal markers. At equal fluences, wavelengths > 400 nm produced no measurable cutaneous alterations. These findings suggest that (i) chronic epidermal and dermal damages have different spectral dependence and (ii) the action spectrum for dermal damage in the UVA is broad, extending up to 400 nm, and is different from the acute erythema spectrum in humans.

Solar-simulated ultraviolet irradiation induces selective influx of CD4+ T lymphocytes in normal human skin

The proportion and composition of the human cutaneous CD3+ T lymphocyte population was determined in situ following a single exposure to physiological, erythema-inducing doses of simulated solar radiation, mainly consisting of UV radiation. Biopsies were taken 1, 2 and 7 days after local irradiation of normal volunteers with 1, 2 and 4 MED by a xenonarc lamp and immunohistochemistry was performed on cryostat sections. Ultraviolet radiation caused an initial decrease of intraepidermal CD3+ T-cell numbers or even could lead to T-cell depletion 24 and 48 h postirradiation, and this was followed by an infiltration of T cells in the epidermis as determined 1 week after UV exposure. The number of dermal CD3+ T cells was increased 24 h after irradiation, reached a maximum at 48 h and subsequently declined at day 7, though remained significantly higher than the unirradiated control. Double staining demonstrated that the CD3+ T cells, which immigrated into the (epi)dermis upon UV exposure, coexpressed CD4 but not CD8. Therefore the CD4/CD8 ratio in skin was markedly increased during the first week upon UV exposure. Our time course study shows that UV radiation affects the T-cell population within human skin by depleting the majority of epidermal T cells and initiating a selective influx of CD4+ T cells.

UV-induced cutaneous photobiology

Ultraviolet radiation (UVR) present in sunlight is a major environmental factor capable of affecting human health and well being. The organ primarily affected by UVR is the skin, which is composed of a variety of different cell types. Here, UVR is needed for production of active vitamin D as well as producing undesirable effects such as sunburn, premature cutaneous photoaging, and promoting skin cancer development. Depending on the radiation dose, UVR influences virtually every cutaneous cell type investigated differently. Since the end of the nineteenth century, sun exposure has been known to induce skin cancer, which is now the human malignancy with the most rapidly increasing incidence. In several experimental models, mid-range UVR has been demonstrated to be the major cause of UV-induced cutaneous tumors. The stratospheric ozone layer protecting the terrestrial surface from higher quantum energy solar radiation is being damaged by industrial activities resulting in the possibility of increased UVR exposure in the future. Investigations in the field of experimental dermatology have shown that within the skin an immunosurveillance system exists that may be able to detect incipient neoplasms and to elicit a host responses against it. This article reviews the literature on studies designed to investigate the effects of UVR on cutaneous cellular components, with special focus on the immune system within the skin and the development of UV-induced cancer.

Enhanced inflammation and immunosuppression by ultraviolet radiation in xeroderma pigmentosum group A (XPA) model mice

Xeroderma pigmentosum group A (XPA) gene-deficient mice were developed by gene targeting in mouse embryonic stem cells. To examine whether these XPA-model mice display photodermatologic abnormalities similar to those in human xeroderma pigmentosum, we investigated the effects of acute ultraviolet radiation on the homozygous (-/-) mice compared to the wild type (+/+) and heterozygous (+/-) mice. A single irradiation with ultraviolet B or topical psoralen plus ultraviolet A treatment induced stronger and longer lasting ear swelling in the (-/-) mice than in the (+/+) and (+/-) mice. Histologic changes including epidermal necrosis, cell infiltration, and sunburn cell formation after ultraviolet B radiation were more prominent in the (-/-) model mice than in the control mice. The (-/-) model mice showed damage of ADPase(+)Langerhans cells at a lower ultraviolet B dose than did the control mice. Moreover, the reappearance of ADPase(+)Langerhans cells after ultraviolet B radiation was delayed in the (-/-) mice compared to the control mice. Although contact hypersensitivity was induced equally in all mice, ultraviolet B-induced local and systemic immunosuppression were greatly enhanced in the (-/-) model mice. The data suggest that the XPA gene-deficient mice may be a useful model of human XPA, because the responses to UV radiation in the mice were very similar to those in the patients with XPA. Moreover, it is possible that enhanced ultraviolet immunosuppression is involved in the development of skin cancers in xeroderma pigmentosum.

The immunohistochemical effects of a single challenge with an intermediate dose of ultraviolet B on normal human skin

Ultraviolet B (UVB) irradiation has extensively been advocated for use in the investigation of cutaneous inflammation in vivo. Mostly doses above the threshold of skin damage have been used. Therefore it is not clear whether the changes observed are specific effects of UVB or to a certain extent represent wound healing. In this study the dose-dependent effects of UVB on normal human skin were assessed using histology and immunohistochemistry. The dose of 1 MED was chosen as a dose unducing tissue changes with adequate morphology: no toxicity but evident immunohistochemical changes. The sequential effects of this 1 MED of UVB were studied for up to 14 days after irradiation, using immunohistochemistry with a panel of monoclonal antibodies. Substantial effects were observed, mainly on proliferation and differentiation; the markers for inflammation did not reveal major changes. This model might be a promising approach to evaluate the effect of drugs on epidermal proliferation and differentiation in vivo.

UVB radiation affects the mobility of epidermal growth factor receptors in human keratinocytes and fibroblasts

Growth factor receptors transmit biological signals for the stimulation of cell growth in vitro and in vivo and their autocrine stimulation may be involved in tumorigenesis. It is therefore, of great value to understand receptor reactions in response to ultraviolet (UV) light which certain normal human cells are invaribly exposed to during their growth cycle. UV irradiation has recently been shown to deplete antioxidant enzymes in human skin. The aims of the present study were a) to compare the lateral mobility of epidermal growth factor receptors (EGF-R) in cultured human keratinocytes and human foreskin fibroblasts, b) to investigate effects of ultraviolet B radiation on the mobility of EGF-R in these cells, and c) study the response of EGF-R on addition of antioxidant enzymes. The epidermal growth factor receptors were labeled with rhodaminated EGF, the lateral diffusion was determined and the fraction of mobile EGF-R assessed with the fluorescence recovery after photobleaching (FRAP). We found that human keratinocytes display a higher basal level of EGF-R mobility than human skin fibroblasts, viz. with diffusion coefficients (D +/- standard error of the mean, SEM) of 4.2 +/- 0.2 x 10(-10) cm2/s, and 1.8 +/- 0.2 x 10(-10) cm2/s, respectively. UVB-irradiated fibroblasts showed an almost four-fold increase in the diffusion coefficient; D was 6.3 +/- 0.3 x 10(-10) cm2/s. The keratinocytes, however, displayed no significant increase in receptor diffusion after irradiation; D was 5.1 +/- 0.8 x 10(-10) cm2/s. In both cell types the percentage of EGF-R fluorescence recovery after photobleaching, i.e. the fraction of mobile receptors, was significantly increased after irradiation. In keratinocytes it increased from 69% before irradiation to 78% after irradiation. Analogous figures for fibroblasts were 61% and 73%. The effect of UVB on fibroblast receptors was abolished by prior addition of superoxide dismutase (SOD) and catalase (CAT). It is concluded that UVB radiation of fibroblasts and keratinocytes can affect their biophysical properties of EGF-R. The finding that addition of antioxidant enzymes prevented the UVB effect in fibroblasts may indicate the involvement of reactive oxygen metabolites.

Comparison of changes in endothelial adhesion molecule expression following UVB irradiation of skin and a human dermal microvascular cell line (HMEC-1)

We have assessed the pattern of dermal endothelial adhesion molecule expression following broadband UVB irradiation in vivo and in vitro. Skin biopsies were taken from 4 human volunteers at baseline and at 4, 8 and 24 h post-irradiation with 2.5 minimal erythema doses of UVB. Sections were stained immunohistochemically for E-selectin, intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1). CD31 and neutrophil elastase. The effect of direct UVB irradiation on E-selectin, ICAM-1 and VCAM-1 was examined in a human dermal microvascular endothelial cell line, HMEC-1. Cultured HMEC-1 were irradiated with 2.5-40 mJ/cm2 of UVB, and assessed for adhesion molecule expression by immunofluorescence microscopy and fluorescence-activated cell sorter analysis. In vivo, E-selectin was minimally expressed on EC at baseline and was induced by 4 h following irradiation, P < 0.01. ICAM-1 was moderately expressed at baseline and appeared mildly induced at 24 h, although this did not reach statistical significance. VCAM-1 was weakly expressed in unirradiated skin while CD31 was moderately expressed, but neither was induced by UVB irradiation. A significant neutrophilic infiltrate appeared by 8 h and was maximal at 24 h, P < 0.05. Neutrophil infiltration correlated with E-selectin expression, r = 0.96. In HMEC-1, ICAM-1 was upregulated at 24 h post-irradiation, with an increase in mean channel fluorescence from 100% at baseline to 145 (SD12)% at 24 h, P < 0.05. No change was seen in expression of E-selectin, VCAM-1 or CD31. These studies support the involvement of endothelial adhesion molecules E-selectin and ICAM-1 in UVB-induced inflammation. Whereas ICAM-1 is upregulated by direct irradiation of endothelial cells, E-selectin stimulation appears to be an indirect effect.

Effects of UVB irradiation on keratinocyte growth factor (KGF) and receptor (KGFR) expression in cultured human keratinocytes

Keratinocyte growth factor (KGF) and its receptor (KGFR) are thought to play important roles in normal keratinocyte growth and differentiation. Since UVB radiation is known to influence keratinocyte growth, we sought to determine whether UVB would alter the expression of KGF and KGFR. Using a reverse-transcription coupled polymerase chain reaction (RT-PCR), the present study examined the expression of KGF and KGFR mRNA in cultured normal human keratinocytes exposed to UVB irradiation. Total cellular RNA was extracted from cultured keratinocytes at various time points after irradiation, reverse transcribed and used for PCR amplification using primers specific for KGF and KGFR. Constitutive expression of KGFR mRNA, but not KGF mRNA, was detected in normal cultured human keratinocytes. After UVB irradiation at 300 J/m2, the KGF mRNA remained undetectable while the KGFR mRNA level was significantly decreased. The down-regulation of KGFR mRNA expression was also confirmed by Northern blot analysis. Immunohistochemical studies demonstrated a decreased positive signal of KGFR in human keratinocytes after UVB irradiation. Our results suggest a possible role for the KGF-KGFR signalling pathway in the skin after exposure to UVB, and that UVB-induced growth inhibition of keratinocytes in hyperproliferative skin disorders may be related to downregulation of KGFR.

Photoimmunology of lupus erythematosus and dermatomyositis: a speculative review

David Norris has proposed a four step model for the pathogenesis of LESSD (1): (1) exposure to UV light induces the release of proinflammaotry epidermal and dermal mediators such as IL-1 and TNF-alpha; (2) these mediators induce changes in epidermal and dermal cells including the induction of adhesion molecules and promotion of the translocation of normally intracellular autoantigen such as Ro/SS-A to the surface of epidermal cells; (3) autoantibody from the circulation binds to autoantigens such as Ro/SS-A that have been translocated to the surface of epidermal keratinocytes and (4) keratinocyte cytotoxicity ensues as the results of lymphoid cells that have been recruited from the circulation recognizing and responding to the Fc domains of autoantibody molecules bound to autoantigen expressed on the surface of keratinocytes (i.e. antibody-dependent cell-mediated cytotoxicity). Although this remains among the most attractive of hypotheses for the explanation of Ro/SS-A antibody-associated forms of LESSD such as SCLE and neonatal LE, it does not address the pathogenesis of other forms of LESSD such as DLE, which are not associated with high-level Ro/SS-A antibody production or other known autoantibody specificities (low-level Ro/SS-A autoantibody production has been noted recently in DLE patients (155)). In addition, this hypothesis implies that the fundamental abnormality in SCLE and neonatal LE is the production of high levels of Ro/SS-A autoantibody; however, equally high levels of Ro/SS-A antibodies having similar molecular specificities are frequently encountered in other conditions such as Sjögren's syndrome in which LESSD is seen only infrequently. Also, a nude mouse model of anti-Ro/SS-A autoantibody in deposition grafted human skin has been developed; however, no inflammation or epidermal injury occurs in these animals (83). Most work has indicated that the action spectrum for Ro/SS-A autoantigen modulation in human epidermal keratinocytes is limited to the UVB spectrum; however, recent studies have suggested that UVA is involved in the elicitation of certain forms of photosensitive cutaneous LE such as SCLE. The hypothesis that CD4+ T cells that are specific for autoantigens in the skin whose expression is altered by UVR exposure might play a role in the expression of LESSD needs to be explored further. Because LE is thought to be a polygenic autoimmune disease, it is possible that polymorphism of genes that govern the skin's response to UVR might be involved in the pathogenesis of photosensitive LESSD. Candidate genes would include: The Ro/SS-A autoantigenic polypeptides and h-YRNA; cytokines, cytokine receptors and adhesion molecules induced on epidermal keratinocytes and dermal endothelial cells by UVR; molecules involved in DNA repair; components of pathways leading to the generation and quenching of oxygen free radicals and components of the UVR-induced apoptosis cascade. Unfortunately, so little is known about DM photosensitivity that it is difficult to even speculate about pathogenetic mechanisms that might be involved other than to extrapolate from the observations and currents of thought relating to photosensitive cutaneous LE. A truly limiting aspect of our understanding in this area has been the absence of working models of the patterns of inflammation seen in LESSD and cutaneous DM. Until such models become available, considering the limitations of human studies, alone it is highly likely that reviews of this subject will continue to be based on much in speculation as observation.

Cell-mediated immunosuppressive mechanisms induced by UV radiation

Soon after UV exposure, mast cells degranulate, possibly because of the release of the mediators and cytokines from the epidermis, and there are subsequent vascular changes and cellular infiltration. Within a few hours, the soluble mediator milieu of UV-exposed skin becomes exceedingly complex and replete with interactions. Leukocytes newly entering the skin, as well as those already in the skin, must respond to these inflammatory signals. Altered antigen presentation and immune suppression likely derive from alterations induced in the APC that comprise the post-UV leukocyte population of the skin. Many of these mechanisms may explain the effectiveness of phototherapy in atopic dermatitis and psoriasis.

Upregulation of nitric oxide synthase in cultured human keratinocytes after ultraviolet B and bradykinin

Ultraviolet B (UVB) irradiation of the skin has been reported to upregulate nitric oxide synthase (NOS) activity with enhancement of nitric oxide (NO) formation. Bradykinin, a known stimulator of NO production, is produced in the skin within minutes of UVB irradiation. The combined effect of UVB and bradykinin on NOS was therefore examined in a cultured human keratinocyte (KC) line. Activity was determined in KC homogenates by the recovery of [3H]L-citrulline using labeled L-arginine as the substrate in the presence of mM NADPH. Monoclonal antibodies to specific isoforms of NOS that cross-react with their human counterparts were used to determine the isoform(s) in control, UVB, bradykinin treated and UVB and bradykinin treated KC. Human KC express NOS activity which is lowest at confluence and highest during proliferation. UVB increased NOS activity when a set dose of irradiation was administered from 32.2-48.3 mJ/cm2 but was inhibitory after 64.4 and 80.5 mJ/cm2. Thirty min after 10(-6) M bradykinin, NOS activity nearly doubled followed by return of activity to control levels at 60 min. Activity after UVB and bradykinin was only slightly higher than that observed with bradykinin alone. Immunochemically, an isoform of M(r) 155 kDa was detected in control cells with the antibody for the constitutive brain enzyme, bNOS. Recovery of this isoform increased after UVB treatment as well as after bradykinin which was time dependent. When both stimulants were used, the recovery of the 155 kDa enzyme was markedly enhanced, unlike the enzyme activity findings. These data indicate that the expression of NOS activity under unstimulated conditions in human KC in culture is due to the constitutive NOS found in neuronal tissue, bNOS. The recovery of bNOS increased after UVB and after bradykinin while the combination of both resulted in the synergistic increase in bNOS protein with only a marginal further increase in NOS activity.

Dietary fish oil reduces basal and ultraviolet B-generated PGE2 levels in skin and increases the threshold to provocation of polymorphic light eruption

The sunburn response is markedly reduced by dietary fish oil rich in omega-3 polyunsaturated fatty acids. Because prostaglandins mediate the vasodilatation, we examined the effect of fish oil on ultraviolet (UV) B-induced prostaglandin metabolism. In addition we assessed the potential photoprotective effect of fish oil in light-sensitive patients. Thirteen patients with polymorphic light eruption received dietary supplements of fish oil rich in omega-3 polyunsaturated fatty acids for 3 months. At baseline and 3 months, the minimal erythema dose of UVB irradiation was determined, and a graded UVA challenge given to a forearm to assess the threshold dose for papule provocation. Suction blisters were raised on the other forearm, on control skin, and on skin irradiated with four times the minimal erythema dose of UVB 24 h previously, and blister fluid prostaglandin E2 was measured by radioimmunoassay. Following 3 months of fish oil, the mean minimal erythema dose of UVB irradiation increased from 19.8 +/- 2.6 to 33.8 +/- 3.7 mJ/cm2 (mean +/- SEM), p < 0.01. The UVA provocation test was positive in 10 patients at baseline, and after 3 months nine of these showed reduced sensitivity to papule provocation, p < 0.001. Before fish oil, PGE2 increased from 8.6 (SEM 2.1) ng/ml in control skin to 27.2 (11) ng/ml after UVB, p < 0.01. Following 3 months of fish oil, PGE2 decreased to 4.1 (1) and 9.6 (2.4) ng/ml in control and irradiated skin, respectively, p < 0.05. Reduction of UV-induced inflammation by fish oil may be due, at least partially, to lowered prostaglandin E2 levels. The photoprotection against UVA-provocation of a papular response suggests a clinical application for fish oil in polymorphic light eruption.

Dorsal skin reactions to sunlight and artificial ultraviolet light in hairless descendants of Mexican hairless dogs

Dorsal skin reactions to irradiation with two different types of ultraviolet (UV) light sources (sunlight and artificial UV light) were investigated in hairless descendants of Mexican hairless dogs. The total energy dose of solar UV exposure and artificial UV irradiation was 90 kJ/m2 and 40 kJ/m2, respectively. Histological examinations were done up to 7 days after UV irradiation (7 DAI). At 1 DAI, the solar UV-exposed skin did not show marked changes, while artificial UV irradiated skin exhibited a visible erythematous reaction and prominent histological alterations such as epidermal thickening, appearance of sunburn cells and deformation elastic fibers. At 4 DAI of solar UV-exposure, the skin color became moderately dark and noticeable pigmentation developed in the epidermis. In contrast, at 4 DAI of artificial UV irradiation, there still remained moderate degeneration in the epidermis and dermis, and delayed tanning was weak. At 7 DAI of solar exposure, delayed suntan reactions became more prominent. Histologically, there were heavy pigmentation of melanin granules mainly in the stratum basale. On the other hand, artificial UV irradiated skin showed less pigmentation. Thus, solar exposure provoked remarkable pigmentation while artificial UV irradiation brought about severe sunburn reactions in the dorsal skin of hairless dogs.

Quantitative assessment of cumulative damage from repetitive exposures to suberythemogenic doses of UVA in human skin

Daily exposures to relatively small suberythemogenic fluences of UVA (50-200 kJ/m2) for 8 days resulted in cumulative morphological skin alterations indicative of early tissue injury. Histologically, irradiated skin revealed epidermal hyperplasia, inflammation and deposition of lysozyme along the dermal elastic fiber network. Sunburn cells were also present within the epidermis. These changes were quantified by image analysis and were found to be related to the cumulative UVA fluence. A long UVA waveband (UVAI, 340-400 nm) was as effective as a broad UVA band (320-400 nm), suggesting that these changes are induced by longer UVA wavelengths.

Involvement of substance P in ultraviolet irradiation-induced inflammation in rat skin

The possible involvement of substance P released from primary afferents in rat skin was investigated in cutaneous inflammation following ultraviolet (UV) irradiation. Recordings from c-fibres innervating the UV-exposed hindpaw skin showed long-lasting low-frequency (0.8-1.25 Hz) spontaneous activity. Spontaneously active c-fibres increased to constitute 35.3% of the total population 72 h after UV exposure. Immunohistochemical analysis of substance P-containing nerve fibres in hindpaw skin revealed a significant increase in substance P immunoreactivity 24 h after UV irradiation. Average length of substance P-immunolabelled nerve fibres was about two times higher in UV-exposed compared to control skin. UV-induced oedema was investigated in rat ears using an ear-swelling test. Intradermal injection of either peptide (Spantide) or nonpeptide (CP-96,345) substance P antagonists and epicutaneous application of CP-96,345 reduced UV-induced oedema significantly in the late phase of sunburn (> 12 h after UV exposure). The UV-induced increase in skin blood flow was investigated in hindpaw skin up to 72 h by the laser Doppler technique. Epicutaneous application of CP-96,345 reduced erythema significantly between 12 and 72 h after UV exposure. Thus, our findings suggest the involvement of neurogenic substance P as a proinflammatory mediator in the late phase of UV-induced cutaneous inflammation in rats.

Ultraviolet-B-induced apoptosis of keratinocytes: evidence for partial involvement of tumor necrosis factor-alpha in the formation of sunburn cells

Irradiation with ultraviolet (UV) B radiation results in the formation of apoptotic keratinocytes called sunburn cells. Recently, it was demonstrated that keratinocytes can release tumor necrosis factor-alpha (TNF-alpha), which is known to cause apoptosis in particular cells. In addition, it has been shown that UVB light induces the release of TNF-alpha by keratinocytes and that keratinocytes express the 55-kD receptor for TNF-alpha. Therefore, we investigated whether TNF-alpha is involved in UV-induced apoptosis of keratinocytes. Normal human keratinocytes and HaCaT cells were exposed to UVB light, and apoptosis was examined by nick translation evaluated by fluorescence-activated cell sorter analysis. UVB induced apoptosis in a dose-dependent manner, which was confirmed by electron microscopy. Addition of a polyclonal antibody directed against human TNF-alpha immediately after UVB exposure was able to reduce DNA fragmentation. However, it was not possible to rescue all cells from apoptosis. To prove whether TNF-alpha is also involved in vivo in UVB-induced apoptosis of keratinocytes, Balb/c mice were exposed to UVB on their abdomens, skin biopsies were performed 24 h later, and sunburn cells were counted. A single dose of 2000 J/m2 caused a significant induction of sunburn cells. Subcutaneous injection of a polyclonal antibody directed against murine TNF-alpha immediately after UVB treatment resulted in a significant but incomplete reduction of sunburn cells, whereas injection of a rabbit IgG as a control had no effect. In both the in vitro and in vivo systems, application of recombinant TNF-alpha alone either to untreated keratinocytes or into normal murine skin did not induce sunburn cells. Thus, these data demonstrate that TNF-alpha is involved in UVB-induced apoptosis, but by itself is not able to induce sunburn cells. This further supports the notion that UVB-induced apoptosis of keratinocytes is a multifactorial event.

Ultraviolet B irradiation of skin induces mast cell degranulation and release of tumour necrosis factor-alpha

In the 'sunburn' response in skin, dermal blood vessels are activated and traffic of dendritic Langerhans' cells altered. While these changes have been attributed to the cytokine TNF-alpha, the source of this acutely released TNF has not been identified. This report demonstrates that the 'sunburn' response, both in vivo and in vitro, is accompanied by rapid degranulation of cutaneous mast cells, with consequential release of intracellular stores of TNF. Epidermal keratinocytes were only minor contributors to local TNF production. Expression of the TNF-inducible CD62E (E-selectin/ELAM-1) and CD54 adhesion molecules on cutaneous endothelium occurred 2 h following mast cell degranulation, and this event was sensitive to blockade of mast cells with disodium cromoglycate. These results indicate that TNF release in skin in the acute sunburn response can largely be attributed to mast cells.

Possible role of 1,25-dihydroxyvitamin D3-induced metallothionein in photoprotection against UVB injury in mouse skin and cultured rat keratinocytes

Previously cadmium chloride was successfully used to prevent sunburn cell (SBC) induction in mouse skin in vivo and to promote human cell survival in vitro after UVB exposure, indicating a protective effect of cadmium-induced metallothionein (MT) with the property of anti-oxidant action. Although cadmium is a potent inducer of MT, the cytotoxic metal is not available for clinical use. The aim of this study is to affirm MT gene expression by the active for of vitamin D3, 1,25-dihydroxyvitamin D3, [1,25(OH)2D3] in cultured keratinocytes and examine in vivo and in vitro the photo-protective effects of 1,25(OH)2D3. Northern hybridization with human MT-IIa cDNA showed a significant increase in the gene expression of MT in the cells treated with 1,25(OH)2D3. Intraperitoneal injection and topical application of 1,25(OH)2D3 caused a significant reduction in SBC formation in mouse skin after UVB administration. The experiment showed the existence of an optimum level of 1,25(OH)2D3 for reducing photodamage. The cells exposed to 1,25(OH)2D3 showed increased tolerance (cell survival) to UVB injury. 1,25(OH)2D3-induced MT may act as a radical scavenger in oxygen-mediated UV injury including SBC formation in the skin. These results indicate that 1,25(OH)2D3 may be practically applied to humans for the purpose of photoprotection.

Reduction of erythema in hairless guinea pigs after cutaneous sulfur mustard vapor exposure by pretreatment with niacinamide, promethazine and indomethacin

Erythema is the initial symptom that occurs after sulfur mustard (HD) cutaneous exposure. The time course of HD-induced erythema is similar to that observed after UV irradiation, which can be reduced by indomethacin. Sulfur mustard lethality is decreased by using promethazine, which is an antihistamine. Niacinamide can reduce microvesication after HD vapor exposure in hairless guinea pig (HGP) skin. The present study examines the effect of the combined administration of niacinamide, indomethacin and promethazine used alone or in all possible combinations on the degree of erythema and histopathologic skin damage after HD exposure in HGP. Niacinamide (750 mg kg-1, i.p.), promethazine (12.5 mg kg-1, i.m.) or indomethacin (4 mg kg-1, p.o.) used singly or in combination was given as a 30-min pretreatment before an 8-min HD vapor cup skin exposure. Using a combination pretreatment of niacinamide, promethazine and indomethacin, erythema was reduced at 4 (91%) and 6 (55%) h, but not 24 h after HD. The incidence of histopathological skin changes (microvesicles, follicular involvement, epidermal necrosis, intracellular edema and pustular epidermatitis) 24 h after HD was not reduced. This study indicates that HD-induced erythema may result from several different mechanisms, including inflammation, histamine release and DNA damage. It is suggested that two phases of inflammation may occur: an early phase sensitive to antihistamines and non-steroidal antiinflammatory drugs and a late phase of extensive cell damage that was not sensitive to these drug pretreatments.

Cumulative effects from repeated exposures to suberythemal doses of UVB and UVA in human skin

BACKGROUND

The skin is repeatedly exposed to solar UV radiation. Long-term photodamage is a consequence of cumulative UV radiation injury. Hence an examination of the repetitive effects of UV exposure is more likely to yield clues to the early alterations that lead to photoaged skin than a single exposure.

OBJECTIVE

We examined the effects of repetitive low-dose UV irradiation on human skin with the aim of identifying UVA-induced effects that may have a different wavelength dependence than acute erythema.

METHODS

Areas on the lower part of the back were each exposed to a suberythemal dose (0.5 minimal erythema dose [MED]) of solar simulated radiation (290 to 400 nm) and of UVA (320 to 400 nm) once daily, 5 days a week, for 28 doses. One site was also treated daily with a sunscreen having a sun protection factor of 22 and then exposed to 11 MEDs of solar simulated radiation for the same duration. Epidermal and dermal changes were analyzed and quantified by histochemical stains in combination with computer-assisted image analysis of tissue sections.

RESULTS

At equal 0.5 MED doses, UVA induced greater cumulative changes than solar simulated radiation, as assessed by development of a greater cumulative erythema response in the first week of treatment, the presence of epidermal hyperplasia and stratum corneum thickening, depletion of Langerhans cells, dermal inflammatory infiltrates, and deposition of lysozyme on elastin fibers. These changes were not prevented by the sunscreen. A single short-term dose of UVA did not elicit these changes.

CONCLUSION

These findings suggest that UVA may contribute significantly to long-term actinic damage and that the spectral dependence for cumulative damage does not parallel the action spectrum for acute injury (erythema) in human beings.

Both UVA and UVB induce cytoskeleton-dependent surface blebbing in epidermoid cells

Data on the morphological changes induced by UVA or UVB irradiation of A431 epidermoid cells in culture are presented. After irradiation with different doses of UVB (120-2400 J m-2) or UVA (10(4)-10(5) J m-2), the membrane and cytoskeleton of these cells were analysed by immunofluorescence and scanning electron microscopy at different times after exposure (0-48 h). Both UVA and UVB alter microtubules and microfilaments and surface blebs are formed after UV irradiation. In particular, UVB induces multiple small blebs on the cells, while UVA induces one single large bleb on each cell. Since cytoskeletal damage and surface blebbing of this type are also induced by oxidative stress, these results add to the body of evidence indicating that UV radiation is capable of pro-oxidant behaviour. Specifically, the morphological changes described in this paper are reminiscent of the modifications which accompany epidermal keratinocytes during their transformation to sunburn cells after UV irradiation. The physiological implications of these findings are discussed.

The degrees of UVB-induced erythema and pigmentation correlate linearly and are reduced in a parallel manner by topical anti-inflammatory agents

To examine whether it is possible to evaluate the degree of ultraviolet B (UVB)-induced inflammation by measuring the degree of hyperpigmentation, we investigated the relationship between UVB-induced erythema and the subsequent pigmentation quantitatively. At 24 h and 7 d after irradiation with erythemogenic doses of UVB to the backs of 16 Japanese subjects, the degree of induced erythema (delta erythema index) and that of pigmentation (delta melanin index) were examined by an image analytic method using a videomicroscope interfaced with a computer. The relationship between two indices was linear in each subject, and the correlation coefficient was 0.83 when evaluated using whole data. The slope of the regression line for the delta melanin index against delta erythema index tended to become steeper as non-irradiated skin color became darker (r = 0.63), suggesting that more efficient melanogenesis takes place after the same level of inflammation in the subject with darker skin. Both erythema and hyperpigmentation were suppressed significantly and in a parallel manner by corticosteroids and indomethacin applied topically immediately after UVB irradiation. These results imply that the post-inflammatory hyperpigmentation correlates closely with the severity of the prior inflammation and that chemical mediators released in the inflammatory process have considerable influence on the melanogenesis. We conclude that the measurement of UVB-induced hyperpigmentation can be utilized for the assessment of topical anti-inflammatory agents, unless these have direct actions on the tyrosinase activity of melanocytes.

Ultraviolet irradiation induces c-fos but not c-Ha-ras proto-oncogene expression in human epidermis

The link between sun exposure and skin cancer is well established, but the mechanism of photocarcinogenesis is still incompletely understood. In vitro experimentation has shown that induction of the c-fos proto-oncogene occurs in cultured human keratinocytes after ultraviolet exposure, and c-Ha-ras mutations are commonly present in human skin neoplasms removed from chronically sun-exposed sites. In the present study, the effect of UV irradiation on the expression of these two proto-oncogenes was examined. The sun-protected volar forearm of six subjects was exposed to a standardized erythemogenic dose of solar-simulated light, and punch biopsies were obtained after 1 h and 24 h from the irradiated area and a nearby shielded area. Expression of c-fos, determined by in situ hybridization of histologic cross-sections, was detected in the basal and lower epidermal layers in all biopsies. However, at 1 h there was a marked increase that returned to baseline by 24 h. c-Ha-ras mRNA could not be detected by riboprobe hybridization in any of the biopsy specimens. Our data demonstrate transient induction of c-fos but not c-Ha-ras expression, at least at the timepoints studied, following a modest UV exposure in normal skin. This phenomenon may lead to the subsequent constitutive over-expression and super-inducibility of c-fos observed in cultured keratinocytes derived from photodamaged skin and may facilitate the development of skin cancer.

Active oxygen mechanisms of UV inflammation

Active oxygen radicals are important in the pathogenesis of UV irradiation injury. The initiating mechanisms involve the generation of hydroxyl radicals, superoxide, and organic hydroperoxides due to photochemical reactions. These active oxygen species lead to DNA strand breakage, mutation and the generation of inflammatory mediators such as cytokines and arachidonic acid metabolites which amplify the irradiation-induced inflammation. Several compounds have recently been utilized to successfully decrease these effects. Improved understanding of the mechanisms by which active oxygen species induce injury in skin now promises improved treatment.

Activation of neutrophil membrane-associated oxidative metabolism by ultraviolet radiation

Exposure of human neutrophils to ultraviolet radiation (UVR) in vitro was accompanied by activation of superoxide generation and preferential release of secondary granules. These pro-oxidative interactions of UVR with neutrophils were dependent on intact cellular membrane-associated oxidative metabolism and were mediated almost exclusively by the UVB component of UVR. Irradiation of neutrophils was also associated with release of arachidonic acid from membrane phospholipids, implicating involvement of phospholipase A2 (PLA2) in the pro-oxidative activity of UVR. The pro-oxidative interactions of UVR with neutrophils were mimicked by coincubation of the cells with reagent arachidonate or lysophosphatidylcholine (LPC), whereas the PLA2 inhibitor 4-p-bromophenacyl bromide, as well as the LPC- and arachidonate complex-forming agent alpha-tocopherol, inhibited these pro-oxidative interactions of UVR with phagocytes. Because phagocyte-derived reactive oxidants are cytotoxic, immunosuppressive, and carcinogenic, these agents are potential mediators of UVR-mediated tissue damage and tumorigenesis.

Neutrophils, differentiated macrophages, and monocyte/macrophage antigen presenting cells infiltrate murine epidermis after UV injury

We asked whether, as in humans, a population of antigen-presenting macrophages infiltrates the epidermis of ultraviolet (UV)-exposed BALB/c mice. Using three-color flow cytometry on cell suspensions plus in situ immunofluorescence microscopy, the phenotype of normal Langerhans cells was class II major histocompatibility complex (MHC+), CD11b+, NLDC-145+, BM8+ CD45+ and homogeneous. By contrast, in epidermal cells harvested 3 d following UV (UV-EC), there were two subsets of class II MHC+ cells: 1) class II MHChi CD11b+, and 2) class II MHClo CD11b-. Neither expressed the Langerhans cell markers BM8 and NLDC-145. In addition, there were two major populations of class II MHC- CD11b+ cells; half of these expressed the GR-1 neutrophil marker. Langerhans and dendritic epidermal T cells were markedly reduced after UV injury. By electron microscopy, immunomagnetic bead-purified CD11b+ cells in UV-EC were comprised of neutrophils, differentiated macrophages, and mononuclear cells with prominent lysosomes, but no Birbeck granules; the class II MHC+ subset resembled a monocytic cell in between differentiated macrophages and indeterminate dendritic cells. Functionally, immediately following in vivo UV exposure, the allogeneic antigen-presenting cell capacity of UV-EC was reduced to 21 +/- 6% of control epidermal cells (C-EC); by 3 d, antigen-presenting cell activity of UV-EC had recovered to 59 +/- 11% of C-EC, although at this time NLDC-145+ Langerhans cells had reached their lowest number. The recovered antigen-presenting cell activity was critically dependent upon the class II MHChiCD11b+ cells. Sensitization of BALB/c mice through skin that contained these antigen-presenting cells (3 d after UV) resulted in tolerance to dinitrofluorobenzene. By contrast, sensitization through UV-exposed skin immediately after the exposure resulted in unresponsiveness without tolerance, demonstrating temporal association of tolerance with leukocytic infiltration. In summary, murine epidermis responds to an acute UV injury in vivo with an initial abrogation of antigen-presenting activity followed by epidermal infiltration with neutrophils, differentiated macrophages, and monocytic antigen-presenting cells that are distinct from Langerhans cells with regard to expression of Langerhans cell markers and ultrastructure.

Clinical relevance of antibodies to Ro/SS-A and La/SS-B in subacute cutaneous lupus erythematosus and related conditions

Ro/SS-A autoantibodies are frequently associated with subacute cutaneous lupus erythematosus, neonatal lupus erythematosus and Sjögren's syndrome. The Ro/SS-A autoantigen is a ribonucleoprotein complex consisting of at least four protein components and four small cytoplasmic RNA components designated hY RNA 1, 3, 4 and 5. Three of the Ro/SS-A peptides have been isolated and cloned. The function of this ribonucleoprotein complex is as yet unknown.

Effects of systemic indomethacin, meclizine, and BW755C on chronic ultraviolet B-induced effects in hairless mouse skin

Chronic exposure of hairless mice to ultraviolet B (UVB) radiation is associated with inflammation as well as an altered macromolecular composition of the dermis. This study was designed to determine whether or not various systemic anti-inflammatory agents inhibit chronic UVB-induced changes in the macromolecular content of the dermis and, if so, whether each agent had the same or different effects. The agents and doses were chosen for their ability to inhibit the changes induced by a single exposure to UVB radiation (increased vasopermeability, neutrophil accumulation, and skin-fold thickness). Indomethacin, a cyclooxygenase inhibitor, and meclizine, an H1 histamine receptor antagonist, were administered from slow-release pellets. BW755C, a combined cyclooxygenase and lipoxygenase inhibitor, was administered intraperitoneally 30 min prior to UVB exposure. Animals were exposed to UVB three times per week for 20-26 weeks or were unirradiated. The elastin, glycosaminoglycan and collagen content of the skin were determined by measuring the desmosine, uronic acid, and hydroxyproline levels, respectively. The amount of each macromolecule per area of skin increased after chronic UVB exposure. The increase in desmosine was inhibited by indomethacin; the increase in hydroxyproline was inhibited by meclizine and BW755C. None of the agents inhibited the uronic acid increase. These results suggest that chronic inflammation contributes to the dermal changes seen in chronically UVB-exposed skin and that different inflammatory mediators are involved in the increases observed in elastin, glycosaminoglycans, and collagen.

Enhanced keratinocyte prostaglandin synthesis after UV injury is due to increased phospholipase activity

The possibility that increased eicosanoid synthesis in skin after ultraviolet light irradiation is due to enhanced phospholipase activity was examined. [3H]arachidonic acid-labeled human keratinocyte cultures exposed to 30 mJ/cm2 ultraviolet (UV) B were studied 6 h after injury. Bradykinin-stimulated release of [3H]arachidonic acid was increased 1.8-fold over release from control cultures by prior irradiation. In unlabeled cultures, prior irradiation produced a threefold increase in bradykinin-stimulated prostaglandin (PG) E2 synthesis as measured by immunoassay. The relative contribution of increased phospholipase vs. cyclooxygenase activity was therefore examined using stable isotope mass measurements of PGE2. By this method, prior irradiation increased bradykinin-stimulated phospholipase activity 3.5-fold, while no change in total cellular cyclooxygenase activity was observed. The effects of irradiation on phospholipase activity were then assessed in more detail. The activities of phospholipase A2, arachidonoyl-CoA synthetase, and arachidonoyl-CoA lysophosphatide acyltransferase in cell homogenates were determined. No effect of UV exposure on the activity of these enzymes was observed. These results suggest that the increase in prostaglandin synthesis produced after UV irradiation is due to increased phospholipase activity, thus enhancing arachidonate release.

Ultraviolet radiation-induced phospholipase A2 activation occurs in mammalian cell membrane preparations

Ultraviolet erythema in human skin is mediated in part by membrane derivatives of arachidonic acid (AA). UVA (320-400 nm) and UVB (290-320 nm) have been shown to induce release of AA from intact mammalian cells in culture. In order to investigate the mechanism of this release we examined the effect of UVA and UVB on release of [3H] AA from membrane preparations of murine fibroblasts. C3H 10T1/2 cells were prelabelled for 24 h with [3H] AA. The membrane fractions of the cells were separated after lysis by differential centrifugation. The membranes were irradiated in suspension and the [3H] AA released from the membranes was determined by scintillation spectroscopy of supernatants 3-4 h after irradiation. Both UVA and UVB induced release of AA from the membrane preparations. The response to UVB was small but significant, reaching levels approximately 150% of control release at doses of 1,200-4,000 J/m2. The response to UVA was larger; doses of 2.5-5.0 J/cm2 induced release equal to twice control (200%) levels, while doses of 10-20 J/cm2 induced maximal release at levels approximately 400% of control. Time course studies with UVB and UVA showed maximal release at 4 h after irradiation. When the membrane preparations were incubated with a polyclonal anti-phospholipase A2 antibody the UV induced release of [3H] AA was completely inhibited in both UVB (1200 J/m2) and UVA (10 J/cm2) treated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of UV-induced inflammation

The inflammation produced by exposure to ultraviolet (UV) light has been well documented clinically and histologically. However, the mechanisms by which mediators induce this clinical response remain poorly defined. It is clear that photochemistry occurring after UV absorption must be responsible for initiating these events. Some of these underlying mechanisms have been defined. After exposure to UV light, the formation of prostaglandins and the release of histamine are increased. In addition to an increase in the quantity of these mediators, an increase in sensitivity of irradiated tissue to agonist stimulation also occurs. This increased sensitivity may cause tissue to respond to agonist levels previously present. Phospholipase activity also increases, making more substrate available for prostaglandin formation. Oxygen radical-induced peroxidation of membrane lipids caused by irradiation may contribute to increased phospholipase activity. Oxygen-free radicals also participate in sunburn cell formation and in UV-induced decreases in Langerhans cell numbers. Several enzymatic and non-enzymatic mechanisms are present in skin for reducing these highly reactive oxygen species.

The effect of ultraviolet (UVB and PUVA) radiation on the expression of epidermal keratins

Using a panel of monoclonal antibodies directed against keratins (PKK2, CK8.12 and KL1), the effects of ultraviolet B (UVB) and psoralen plus ultraviolet A (PUVA) irradiation on keratin expression in guinea-pig skin were examined immunohistochemically. Following irradiation, whether by UVB or PUVA, rapid alterations in the distribution pattern of keratins were observed in the epidermis. The alterations included the induction of basal cell-type keratins (PKK2 and CK8.12 staining) in the suprabasal layers, with concomitant reduction of the suprabasal-type keratins (KL1 staining). These alterations in keratin expression were observed during the period when DNA synthesis appears to be accelerated by ultraviolet light exposure (5 h-5 days after UVB, and 2-10 days after PUVA irradiation). Therefore, these changes are probably reflections of a proliferative or regenerative state of keratinocytes. This explanation was supported by the result of an experiment involving tape stripping of the epidermal horny layers, which also accelerates DNA synthesis by keratinocytes. Immunohistochemistry appears to be a useful and sensitive method of detecting the effect of ultraviolet light on keratinization.

Melanocyte-stimulating properties of arachidonic acid metabolites: possible role in postinflammatory pigmentation

Normal human epidermal melanocytes became swollen and more dendritic with an increase in the amount of tyrosinase and immunoreactive b-locus protein when they were cultured for 2 days with the following arachidonic acid metabolites: prostaglandin (PG) D2, leukotriene (LT) B4, LTC4, LTD4, LTE4, thromboxane (TX) B2 and 12-hydroxy eicosatetraenoic acid (12-HETE). The effect of LTC4 was particularly strong compared to that of PGE2, about which we have previously reported. On the other hand, PGE1, PGF2 alpha and 6-ketoPGF1 alpha did not show any significant stimulatory effect. These data suggest that arachidonate-derived chemical mediators, especially LTC4, may be responsible for the induction of post-inflammatory hyperpigmentation of the skin.