Macrophage migration inhibitory factor (MIF) is a pluripotent cytokine that has an essential role in the pathophysiology of experimental allergic inflammation. Recent findings suggest that MIF is involved in several allergic disorders, including atopic dermatitis (AD). In this study, the role of MIF in allergic skin inflammation was examined using a murine model of AD elicited by epicutaneous sensitization with ovalbumin (OVA). We observed the number of skin-infiltrating eosinophils to significantly increase in OVA-sensitized MIF transgenic (Tg) mice compared with their wild-type (WT) littermates. On the other hand, eosinophils were virtually absent from the skin of MIF knockout (KO) mice and failed to infiltrate their skin after repeated epicutaneous sensitization with OVA. The mRNA expression levels of eotaxin and IL-5 were significantly increased in OVA-sensitized skin sites of MIF Tg mice, but were significantly decreased in MIF KO mice in comparison with the levels in WT littermates. Eotaxin expression was induced by IL-4 stimulation in fibroblasts in MIF Tg mice, but not in MIF KO mice. These findings indicate that MIF can induce eosinophil accumulation in the skin. Therefore, the targeted inhibition of MIF might be a promising new therapeutic strategy for allergic skin diseases.

D-Serine is an endogenous coagonist of the N-methyl-D-aspartate (NMDA)–type glutamate receptor in the central nervous system and its synthesis is catalyzed by serine racemase (SR). Recently, the NMDA receptor has been found to be expressed in keratinocytes (KCs) of the skin and involved in the regulation of KC growth and differentiation. However, the localization and role of SR in the skin remain unknown. Here, using SR-knockout (SR-KO) mice as the control, we demonstrated the localization of the SR protein in the granular and cornified layer of the epidermis of wild-type (WT) mice and its appearance in confluent WT KCs. We also demonstrated the existence of a mechanism for conversion of L-serine to D-serine in epidermal KCs. Furthermore, we found increased expression levels of genes involved in the differentiation of epidermal KCs in adult SR-KO mice, and alterations in the barrier function and ultrastructure of the epidermis in postnatal day 5 SR-KO mice. Our findings suggest that SR in the skin epidermis is involved in the differentiation of epidermal KCs and the formation of the skin barrier.

Chronic ultraviolet (UV) exposure can increase the occurrence of p53 mutations, thus leading to a dysregulation of apoptosis and the initiation of skin cancer. Therefore, it is extremely important that apoptosis is induced quickly after UV irradiation, without any dysregulation. Recent studies have suggested a potentially broader role for macrophage migration inhibitory factor (MIF) in growth regulation via its ability to antagonize p53-mediated gene activation and apoptosis. To further elucidate the possible role of MIF in photocarcinogenesis, the acute and chronic UVB effect in the skin was examined using macrophage migration inhibitory factor transgenic (MIF Tg) and wild-type (WT) mice. The MIF Tg mice exposed to chronic UVB irradiation began to develop skin tumors after approximately 14 weeks, whereas the WT mice began to develop tumors after 18 weeks. A higher incidence of tumors was observed in the MIF Tg in comparison with the WT mice after chronic UVB irradiation. Next, we clarified whether the acceleration of photo-induced carcinogenesis in the MIF Tg mice was mediated by the inhibition of apoptosis There were fewer sunburned cells in the epidermis of the MIF Tg mice than the WT mice after acute UVB exposure. The epidermis derived from the MIF Tg mice exhibited substantially decreased levels of p53, bax and p21 after UVB exposure in comparison with the WT mice. Collectively, these findings suggest that chronic UVB exposure enhances MIF production, which may inhibit the p53-dependent apoptotic processes and thereby induce photocarcinogenesis in the skin.

Atopic dermatitis (AD) is a common chronic inflammatory skin disease associated with various factors, including immunological abnormalities and exposure to allergens. Astaxanthin (AST) is a xanthophyll carotenoid that has recently been demonstrated to have anti-inflammatory effects and to regulate the expression of inflammatory cytokines. Thus, we investigated whether AST could improve the dermatitis and pruritus in a murine model of AD using NC/Nga mice. In addition to a behavioral evaluation, the effects of AST on the AD were determined by the clinical skin severity score, serum IgE level, histological analyses of skin, and by reverse transcription-PCR and Western blotting analyses for the expression of inflammation-related factors. AST (100 mg/kg) or vehicle (olive oil) was orally administered once day and three times a week for 26 days. When compared with vehicle-treated group, the administration of AST significantly reduced the clinical skin severity score. In addition, the spontaneous scratching in AD model mice was reduced by AST administration. Moreover, the serum IgE level was markedly decreased by the oral administration of AST compared to that in vehicle-treated mice. The number of eosinophils, total and degranulated mast cells all significantly decreased in the skin of AST-treated mice compared with vehicle-treated mice. The mRNA and protein levels of eotaxin, MIF, IL-4, IL-5 and L-histidine decarboxylase were significantly decreased in the skin of AST-treated mice compared with vehicle-treated mice. These results suggest that AST improves the dermatitis and pruritus in AD via the regulation of the inflammatory effects and the expression of inflammatory cytokines.

UV radiation indirectly regulates melanogenesis in melanocytes through a paracrine regulatory mechanism involving keratinocytes. Protease-activated receptor (PAR)-2 activation induces melanosome transfer by increasing phagocytosis of melanosomes by keratinocytes. This study demonstrated that macrophage migration inhibitory factor (MIF) stimulated PAR-2 expression in human keratinocytes. In addition, we showed that MIF stimulated stem cell factor (SCF) release in keratinocytes; however, MIF had no effect on the release of endothelin-1 or prostaglandin E2 in keratinocytes. In addition, MIF had no direct effect on melanin and tyrosinase synthesis in cultured human melanocytes. The effect of MIF on melanogenesis was also examined using a three-dimensional reconstituted human epidermal culture model, which is a novel, commercially available, cultured human epidermis containing functional melanocytes. Migration inhibitory factor induced an increase in melanin content in the epidermis after a 9-day culture period. Moreover, melanin synthesis induced by UV-B stimulation was significantly down-regulated by anti-MIF antibody treatment. An in vivo study showed that the back skin of MIF transgenic mice had a higher melanin content than that of wild-type mice after 12 weeks of UV-B exposure. Therefore, MIFmediated melanogenesis occurs mainly through the activation of PAR-2 and SCF expression in keratino- Exposure to UV radiation leads to various short-term deleterious cutaneous effects, including sunburn and immunosuppression, and long-term consequences that lead to premature aging, including hyperpigmentation.1 UV radiation indirectly regulates melanogenesis in melanocytes through a paracrine regulatory mechanism involving keratinocytes. UV-B-induced pigmentation occurs when human keratinocytes exposed to UV-B are stimulated to produce and secrete several mediators that trigger the activation of melanocytes and act as potent mitogens and melanogens for human melanocytes.2-4 The two main paracrine melanogenic cytokines, stem cell factor (SCF) and endothelin (ET)-1, have been demonstrated to play pivotal roles in skin pigmentation, including UV-Binduced pigmentation. 5 In addition, prostaglandins (PGs) are key mediators of diverse functions in the skin; and several reports 6,7 have suggested that PGs mediate postinflammatory pigmentary changes by modulating melanin synthesis and melanocyte dendricity.Protease-activated receptor (PAR)-2 is a member of a novel G-protein-coupled seven-transmembrane receptor family.8 These receptors are irreversibly activated through proteolytic cleavage of their amino termini. Subsequent to proteolytic cleavage, the newly exposed NH2 terminus acts as a tethered peptide ligand, which binds and activates the receptor. Protease-activated receptor-2 is involved in skin pigmentation because it increases the phagocytosis of melanosomes by keratinocytes.9 UV ir-