Exposure of the skin to UV radiation initiates a variety of harmful effects on human health, including squamous and basal cell carcinoma and melanoma, as well as premature aging of the skin and susceptibility to infection (7
). UVB radiation has multiple effects on the immune system (6
). There is ample clinical and experimental evidence to suggest that immune factors contribute to the pathogenesis of sunlight-induced skin cancer in mice and probably in humans as well (10
). Chronically immunosuppressed patients living in regions of intense sun exposure experience an exceptionally high rate of skin cancer (Reviewed in 24
). This observation is consistent with the hypothesis that immune surveillance is an important mechanism designed to prevent the generation and maintenance of neoplastic cells. As UVB-induced immunosuppression has been implicated in the development of photocarcinogenesis, we examined the efficacy of GTPs in drinking water on UVB-induced immunosuppression using local and systemic models of CHS in C3H/HeN mice, and investigated the possible mechanism of prevention of UVB-induced immunosuppression by GTPs. The results presented here show that administration of GTPs in drinking water inhibits UVB-induced suppression of CHS response to DNFB in both local and systemic models of CHS. These data provide a first line of evidence that prevention of photocarcinogenesis by drinking GTPs may be, at least in part, due to the prevention of UVB-induced immunosuppression in mice.
In terms of the mechanisms by which GTPs mediate the inhibition of UVB-induced immunosuppression, our data demonstrate that treatment of mice with GTPs rapidly remove or repair UVB-induced DNA damage in the form of CPDs in UVB-exposed skin site, and reduces the emigration of CPD+
antigen presenting cells from the epidermis to draining lymph nodes. There is evidence that UV-induced DNA damage is the molecular trigger for the migration of Langerhans cells (antigen presenting cells in the epidermis) from the skin to the draining lymph nodes (13
). The UV-induced DNA damage also impairs the antigen presenting capacity of Langerhans cells which results in a lack of sensitization and the induction of tolerance to contact sensitizers (21
). We observed that administration of GTPs in drinking water of mice inhibited the migration of epidermal antigen presenting cells to DLN in mice, indicating that treatment of GTPs might be able to repair UV-induced CPD in the mice. We speculate that, as the migrating antigen presenting cells in the epidermis were either not damaged or were repaired in mice they were able to present Ag to T-cells in the DLN resulting in induction of sensitization to DNFB after challenge. Further, the numbers of CPD+
cells were significantly higher in the non-GTPs-fed mice in the subcapsular sinus to the paracortical region of the lymph nodes, including the interfollicular areas, which are the sites of T-cell localization. Thus, the damaged DNA in the lymph nodes of non-GTPs-fed mice may adversely affect the ability of the antigen presenting cells to present Ag to T-cells thus abrogating sensitization after DNFB challenge. In contrast, the reverse was observed in GTPs-fed mice, and that may be one of the reasons that GTPs prevent UVB-induced immunosuppression in mice.
NER is the main mechanism of repair in mammalian cells for the removal of UV radiation-induced DNA damage. Since the treatment of GTPs enhances the removal or repair of UVB-induced DNA damage, we further examined whether the removal or repair of UV-induced CPDs by GTPs is mediated via induction of NER genes. Our real-time PCR data indicate that treatment of mice with GTPs increases the levels of some NER genes (e.g.
) in UVB-exposed skin sites compared to non-GTPs-fed mice and that may have contributed in the rapid repair of damaged DNA in mouse skin. However, GTPs have no effect on some other NER genes (e.g.
, DDB1 and DDB2). It suggests that the function of GTPs is NER gene-specific. The role of NER was further confirmed by assessing the effect of GTPs on UVB-induced immunosuppression in XPA-/-
mice and data were compared with the XPA+/+
(proficient) mice. Treatment of mice with GTPs in drinking water prevents UVB-induced suppression of CHS response in XPA+/+
mice but do not prevent in XPA-/-
mice further support our observations that inhibition of UVB-induced immunosuppression by GTPs require functional NER genes. This observation was important as the treatment of GTPs do not remove or repair UVB-induced DNA damage in XPA-/-
mice but repair in XPA+/+
mice which were exposed to UVB. Importantly, exposure of mice to UV radiation suppresses CHS response in both XPA-/-
mice. It suggests that UV-induced immunosuppression is mediated through other mechanisms in addition to DNA damage. These may include: (i) UV-induced suppression of IL-12 (19
). IL-12 stimulates immune system through the development of Th1 cell types; (ii) stimulation of IL-10 in UV-irradiated skin which is considered as an immunosuppressive cytokine (24
). Further to confirm our hypothesis and verify our present data, we used NER-deficient cells from XPA
-patients and NER-proficient cells from healthy persons. Cells derived from patients suffering from xeroderma pigmentosum either lack or have reduced DNA repair capacity due to genetic mutations in several components of the NER. The XPA
complementation type represents the most severe phenotype, because the XPA
gene is the most crucial component in the repair process and, thus, cells lacking the XPA
gene are completely deficient in NER (25
). Our dot-blot analysis indicated that GTPs were able to remove UV-induced CPDs in NER-proficient cells (XPA+/+
) but was not able to remove or repair in NER-deficient (XPA-/-
) human fibroblast cells. These observations indicate that repair of UV-induced DNA damage by GTPs is mediated through the NER mechanism or GTPs-induced DNA repair requires functional NER. These findings have important implications for the chemopreventive mechanism of skin cancer protection by GTPs, and identify a new mechanism by which GTPs prevent UV-induced immunosuppression. Based on the information obtained in this study, it can be suggested that the consumption of 5-6 cups (one cup=150 ml) of green tea (1 g green tea leaves/150 ml) per day by humans may provide the same level of photoprotective effect in human system. However, the magnitude of photoprotective effect or UVB-induced immunosuppression by green tea may differ person to person based on the differences in race, and intensity and exposure time of UV radiation.
Taken together, our data indicate that the prevention of UV radiation-induced immunosuppression by drinking GTPs is mediated through rapid repair of UVB-induced DNA damage and that requires NER. As UV-induced DNA damage and immunosuppression play an important role in melanoma and nonmelanoma skin cancers, it is tempting to suggest that drinking green tea should be further investigated as a chemopreventive agent for the prevention of skin cancers in humans, and its possible use in future practice of medicine.