Ultraviolet radiation-induced suppression of natural killer cell activity is enhanced in xeroderma pigmentosum group A (XPA) model mice

Photobiological information obtained from XPA gene-deficient mice

The XPA gene-deficient mouse, an animal model of xeroderma pigmentosum (XP), develops enhanced photobiologic reactions including acute inflammation, immunosuppression and skin carcinogenesis, because of the defect in the excision repair of ultraviolet-induced DNA lesions. The results strongly suggest that nuclear DNA is an important chromophore to initiate acute and chronic skin damages. The model mouse is a useful experimental animal not only to investigate the mechanisms of photosensitivity in XP, but also to study physiological photobiology in humans, because photobiologic reactions are greatly intensified in this mouse.

Development of a New Mouse Model (Xeroderma Pigmentosum A-Deficient, Stem Cell Factor-Transgenic) of Ultraviolet B-Induced Melanoma

It is well established that exposure to sunlight or ultraviolet radiation (UVR) is the major environmental risk factor for the development of skin neoplasms. To date, however, there have been few appropriate mouse models available for studying the role of UVR in melanoma carcinogenesis, mainly because of the murine lack of the epidermal melanocyte, which is a major source of origin of human melanoma. In this study, we established xeroderma pigmentosum group A gene-deficient, stem cell factor-transgenic mice, which are defective in the repair of damaged DNA and do have epidermal melanocytes. The mice were exposed to UVR three times a week for 10 wk. More than 30% of the irradiated mice developed tumors of melanocyte origin that metastasized to the lymph nodes. Histologically, proliferated cells exhibited lentigo maligna melanoma or nodular melanoma. Immunohistochemistry confirmed that the tumor cells were characteristic of melanoma. Non-irradiated mice did not develop skin tumors spontaneously. The newly generated model mouse might be useful for studying the photobiological aspects of human melanoma, because the mice developed melanoma from epidermal melanocytes only after UVR exposures.

Ultraviolet Radiation-Induced Impairment of Tumor Rejection Is Enhanced in Xeroderma Pigmentosum A Gene-Deficient Mice

Xeroderma pigmentosum (XP)A gene-deficient mice display dermatologic abnormalities similar to human XP, such as enhanced ultraviolet (UV)-induced acute inflammation and high incidence of UVB-induced skin cancer. We have previously reported that UVB-induced immunosuppression of contact hypersensitivity was greatly enhanced in XPA mice. In the present study, we examined the effects of UVB radiation on tumor rejection in XPA mice. Tumor cells established from UVB-induced squamous cell carcinoma in XPA mice were injected subcutaneously. No difference in the development of tumors was observed between the non-irradiated XPA and wild-type mice. Tumors developed, grew in size, and reached the maximum at 7-10 d after the inoculation. Thereafter, all tumors decreased in size and were completely rejected by 4 wk in both strains of mice. When tumor cells were inoculated into the skin that had been irradiated with 50-150 mJ per cm2 of UVB, tumor grew in 60% (12 of 20) of the XPA mice, but only in 4% (one of 23) of wild-type mice. Phenotyping of tumor-infiltrating cells revealed that the migration of natural killer cells and CD8+ T cells was inhibited in UVB-irradiated XPA mice. These data suggest that enhanced UVB-induced impairment of tumor rejection could be partially involved in the cancer development of XP patients.

Photochemotherapy inhibits angiogenesis and induces apoptosis of endothelial cells in vitro

BACKGROUND

Photochemotherapy has long been used in the treatment of psoriasis; however, its mechanism has not been completely elucidated. Psoriasis is now regarded as an angiogenesis-related disease. Recent studies indicated that the inhibition of angiogenesis by photochemotherapy could be an underlying mechanism. It was found that photochemotherapy can downregulate the expression of angiogenic factors in keratinocytes. However, the direct effect of photochemotherapy on endothelial cells has not been studied.

METHODS

In this study, we determined the effect of photochemotherapy on the proliferation of human microvascular endothelial cells through MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and cell cycle analysis. The migration assay and in vitro tube formation assay were used to investigate the migration properties and tube formation ability of human microvascular endothelial cells after psoralen plus UVA (PUVA) treatment. The apoptosis of endothelial cells elicited by photochemotherapy was also analyzed with fluorescence-activated cell sorting analysis (FACS).

RESULTS

UVA (0.8-5.0 J/cm(2)) irradiation with the presence of 8-methoxypsoralen (8-MOP) (300 ng/ml) resulted in a dose-dependent reduction in the cell viabilities of endothelial cells. FACS data showed an accumulation of cells in G0/G1 phase of cell cycle and apoptotic features of cell death after UVA irradiation with psoralen. The migration properties and tube formation ability of endothelial cells were dramatically inhibited by photochemotherapy.

CONCLUSION

Our results showed that photochemotherapy inhibits angiogenesis and induces apoptosis of human microvascular endothelial cells in vitro, which may be a possible mechanism of photochemotherapy in the treatment of psoriasis.

XPA gene-deficient, SCF-transgenic mice with epidermal melanin are resistant to UV-induced carcinogenesis

Photobiologic investigations have been performed using animals without epidermal melanocytes. We developed xeroderma pigmentosum group A gene-deficient (XPA (-/-)), stem cell factor transgenic (SCF-Tg) mice, which one defective in nucleotide excision repair and have epidermal melanocytes, and investigated protective effects of epidermal melanin against UV-induced injuries. When irradiated to UVB, XPA (-/-) mice developed greatly enhanced responses including acute inflammation, cyclobutane pyrimidine dimer (CPD) formation, keratinocyte apoptosis, depletion of Langerhans cells and immunosuppression of contact hypersensitivity, but XPA (-/-), SCF-Tg mice showed much less responses to the same dose of UVB. XPA (-/-), SCF-Tg mice did not develop skin cancers after repeated exposures to UVB for 30 wk at a total dose of 72 J per cm(2), which induced a significant number of tumors even in wild-type, XPA (+/+) mice, and was lethal dose for XPA (-/-) mice. Dimethylbenz (alpha) anthracence (DMBA) induces DNA damages, which require XPA protein to be repaired. Topical application of DMBA produced a significant inflammation, CPD formation, apoptosis, immunosuppression, and skin cancers in XPA (-/-), SCF-Tg mice as well as XPA (-/-) mice. These findings indicate that epidermal melanin has a high ability to protect DNA damage by UVB radiation, and thereby, prevent UV-induced inflammation, immunosuppression, and carcinogenesis.

UV-induced skin damage

Solar radiation induces acute and chronic reactions in human and animal skin. Chronic repeated exposures are the primary cause of benign and malignant skin tumors, including malignant melanoma. Among types of solar radiation, ultraviolet B (290-320 nm) radiation is highly mutagenic and carcinogenic in animal experiments compared to ultraviolet A (320-400 nm) radiation. Epidemiological studies suggest that solar UV radiation is responsible for skin tumor development via gene mutations and immunosuppression, and possibly for photoaging. In this review, recent understanding of DNA damage caused by direct UV radiation and by indirect stress via reactive oxygen species (ROS) and DNA repair mechanisms, particularly nucleotide excision repair of human cells, are discussed. In addition, mutations induced by solar UV radiation in p53, ras and patched genes of non-melanoma skin cancer cells, and the role of ROS as both a promoter in UV-carcinogenesis and an inducer of UV-apoptosis, are described based primarily on the findings reported during the last decade. Furthermore, the effect of UV on immunological reaction in the skin is discussed. Finally, possible prevention of UV-induced skin cancer by feeding or topical use of antioxidants, such as polyphenols, vitamin C, and vitamin E, is discussed.

Photobiologic and photoimmunologic characteristics of XPA gene-deficient mice

Xeroderma pigmentosum group A (XPA) gene-deficient mice cannot repair UV-induced DNA damage and easily develop skin cancers by UV irradiation. Just like human XP patients, homozygous (-/-) mice developed stronger longer-lasting acute inflammation than did wild-type mice after a single irradiation with UVB. Moreover, the model mice showed more severe UV-induced damage of keratinocytes and Langerhans cells than did the control mice. UVB-induced local and systemic immunosuppression was greatly enhanced in the (-/-) mice. Treatment with indomethacin, an inhibitor of prostaglandin (PG) synthesis, inhibited UV-induced inflammation and abrogated immunosuppression. In XPA-deficient mice, the amount of PGE2 and the expression level of COX-2 mRNA greatly increased after UVB irradiation compared with wild-type mice. These results suggest that the excess DNA photoproducts remaining in XPA-deficient cells after UV radiation induce COX-2 expression and subsequently produce a high amount of PGE2, which causes the enhancement of inflammation and immunosuppression. In XPA-deficient mice, the natural killer cell activity significantly decreased after repeated exposures to UVB. Our experimental data indicate that cancer development in XP patients involves not only mutagenesis due to the defect in DNA repair, but also the enhanced UV-immunosuppression and intensified impairment of natural killer function.

UV-induced skin carcinogenesis in xeroderma pigmentosum group A (XPA) gene-knockout mice with nucleotide excision repair-deficiency

Nucleotide excision repair (NER) removes a wide variety of lesions from the genome and is deficient in the genetic disorder, xeroderma pigmentosum (XP). In this paper, an in vitro analysis of the XP group A gene product (XPA protein) is reported. Results of an analysis on the pathogenesis of ultraviolet (UV)-B-induced skin cancer in the XPA gene-knockout mouse are also described: (1) contrary to wild type mice, significant bias of p53 mutations to the transcribed strand and no evident p53 mutational hot spots were detected in the skin tumors of XPA-knockout mice. (2) Skin cancer cell lines from UVB-irradiated XPA-knockout mice had a decreased mismatch repair activity and an abnormal cell cycle checkpoint, suggesting that the downregulation of mismatch repair helps cells escape killing by UVB and that mismatch repair-deficient clones are selected for during the tumorigenic transformation of XPA (-/-) cells. (3) The XPA-knockout mice showed a higher frequency of UVB-induced mutation in the rpsL transgene at a low dose of UVB-irradiation than the wild type mice. CC-->TT tandem transition, a hallmark of UV-induced mutation, was detected at higher frequency in the rpsL transgene in the XPA-knockout mice than the wild type mice. This rpsL/XPA mouse system will be useful for further analysing the role of NER in the mutagenesis induced by various carcinogens. (4) The UVB-induced immunosuppression was greatly enhanced in the XPA-knockout mice. It is possible that an enhanced impairment of the immune system by UVB irradiation is involved in the high incidence of skin cancer in XP.

Carcinogen-induced inflammation and immunosuppression are enhanced in xeroderma pigmentosum group A model mice associated with hyperproduction of prostaglandin E2

Xeroderma pigmentosum group A (XPA) gene-deficient mice easily developed skin cancers by the application of topical chemical carcinogens as well as by UV irradiation. As certain chemical carcinogens have been shown to be immunosuppressive, we examined the inflammatory and immunosuppressive effects of dimethylbenz(a)anthracene (DMBA) on XPA mice. Compared with wild-type mice, XPA mice showed greater ear swelling and reduction of epidermal Langerhans cells after DMBA application. Topical application of DMBA impaired the induction of contact hypersensitivity, initiated either locally or at distant sites. These DMBA-induced local and systemic immunosuppressions were more greatly enhanced in XPA mice than in wild-type mice. DMBA application induced pronounced production of PGE(2), IL-10, and TNF-alpha in the skin of XPA mice. Treatment with indomethacin, a potent inhibitor of PG biosynthesis, inhibited DMBA-induced inflammation and local immunosuppression. In XPA mice, increased serum IL-10 was detected after DMBA treatment. Excess production of PGE(2), TNF-alpha, and IL-10 after DMBA application may be involved in the enhanced local and systemic immunosuppression in DMBA-treated XPA mice. Susceptibility to DMBA-induced skin tumors in XPA mice may be due to easy impairment of the immune system by DMBA in addition to a defect in the repair of DMBA-DNA adduct. Enhanced immunosuppression by chemical carcinogens as well as the mutagenicity of these mutagens might be associated with the high incidence of internal malignancies seen in XP patients. Moreover, these results supported the hypothesis that persistent DNA damage is a trigger for the production of immunoregulatory cytokines.

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.

Studies of in vivo mutations in rpsL transgene in UVB-irradiated epidermis of XPA-deficient mice

We have established xeroderma pigmentosum group A (XPA) gene-knockout mice with nucleotide excision repair (NER) deficiency, which rapidly developed skin tumors when exposed to a low dose of chronic UV like XP-A patients, confirming that the NER process plays an important role in preventing UVB-induced skin cancer. To examine the in vivo mutation in the UVB-irradiated epidermis, we established XPA (-/-), (+/-) and (+/+) mice carrying the Escherichia coli rpsL transgene with which the mutation frequencies and spectra in the UVB-irradiated epidermal tissue can be examined conveniently. The XPA (-/-) mice showed a higher frequency of UVB-induced mutation in the rpsL transgene with a low dose (150 J/m(2)) of UVB-irradiation than the XPA (+/-) and (+/+) mice, while, at a high dose (900 J/m(2)) they showed almost the same frequency of mutation as the XPA (+/-) and (+/+) mice, probably because of cell death in the epidermis of the XPA (-/-) mice. However, CC-->TT tandem transition, a hallmark of UV-induced mutation, was detected at higher frequency in the XPA (-/-) mice than the XPA (+/-) and (+/+) mice at both doses of UVB. This rpsL/XPA mouse system will be useful for further analyzing the role of NER in the mutagenesis and carcinogenesis induced by various carcinogens.

Indications and action mechanisms of phototherapy

Recently phototherapy has become one of the most commonly used modalities for the treatment of a variety of skin diseases, although the action mechanisms have not been fully understood. The inhibitory effect of UVR on DNA synthesis may be one of the actions for proliferating skin diseases. However, phototherapy is also used for the treatment of allergic or autoimmune diseases. It has been confirmed that the skin is an important immunologic organ whose constitutive cells are all involved in immunologic reactions. We have investigated the effects of PUVA and UVB radiation on the immunocompetent cells, including Langerhans cells, T lymphocytes, mast cells, endothelial cells and natural killer cells. Exposure to UVR inhibits contact sensitization to haptens applied not only to the irradiated skin area but also to the non-irradiated distant skin when the exposure dose is relatively high and/or the application skin area is large. In addition, hapten-specific tolerance develops by the generation of suppressor T cells. Phototherapy is also useful for immediate type hypersensitivity such as urticaria. Action mode in the case may be the inhibitory effects of UVR on histamine release from mast cells. The results obtained from these experiments suggest that phototherapy exerts its anti-allergic and anti-inflammatory effects through immunosuppression.