In vivo relevance for photoprotection by the vitamin D rapid response pathway

Inactivation of the vitamin D receptor enhances susceptibility of murine skin to UV-induced tumorigenesis

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is the biologically active ligand for the vitamin D receptor (VDR). VDR(-/-) mice have a hair follicle-cycling defect resulting in alopecia. However, mice lacking 25-hydroxyvitamin D(3) 1alpha-hydroxylase (CYP27B1(-/-)), and having no circulating 1,25(OH)(2)D(3), have normal follicular function. These mouse models indicate that VDR functions independently of 1,25(OH)(2)D(3) in regulating hair-follicle cycling. Here, we show that VDR(-/-) mice rapidly develop chemically induced skin tumors, whereas CYP27B1(-/-) and wild-type mice do not, indicating that VDR, and not the 1,25(OH)(2)D(3) ligand, is essential for protection against skin tumorigenesis. Because the majority of human skin cancer results from exposure to UV, the susceptibility of VDR(-/-) mice to this carcinogen was also evaluated. VDR(-/-) mice developed UV-induced tumors more rapidly and with greater penetrance than did VDR(+/+) mice. p53 protein levels were upregulated at similar rates in UV-treated keratinocytes of VDR(-/-) and VDR(+/+) mice. However, rates of thymine-dimer repair and UV-induced apoptosis were significantly lower in VDR(-/-) epidermis compared with the wild type epidermis. UV-induced epidermal thickening was also attenuated in VDR(-/-) skin, indicating that VDR plays a critical role in the repair and removal of severely damaged keratinocytes and adaptation of the skin to chronic UV exposure.

Vitamin D analogs: therapeutic applications and mechanisms for selectivity

The vitamin D endocrine system plays a central role in mineral ion homeostasis through the actions of the vitamin D hormone, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], on the intestine, bone, parathyroid gland, and kidney. The main function of 1,25(OH)(2)D(3) is to promote the dietary absorption of calcium and phosphate, but effects on bone, kidney and the parathyroids fine-tune the mineral levels. In addition to these classical actions, 1,25(OH)(2)D(3) exerts pleiotropic effects in a wide variety of target tissues and cell types, often in an autocrine/paracrine fashion. These biological activities of 1,25(OH)(2)D(3) have suggested a multitude of potential therapeutic applications of the vitamin D hormone for the treatment of hyperproliferative disorders (e.g. cancer and psoriasis), immune dysfunction (autoimmune diseases), and endocrine disorders (e.g. hyperparathyroidism). Unfortunately, the effective therapeutic doses required to treat these disorders can produce substantial hypercalcemia. This limitation of 1,25(OH)(2)D(3) therapy has spurred the development of vitamin D analogs that retain the therapeutically important properties of 1,25(OH)(2)D(3), but with reduced calcemic activity. Analogs with improved therapeutic indices are now available for treatment of psoriasis and secondary hyperparathyroidism in chronic kidney disease, and research on newer analogs for these indications continues. Other analogs are under development and in clinical trials for treatment of various types of cancer, autoimmune disorders, and many other diseases. Although many new analogs show tremendous promise in cell-based models, this article will limit it focus on the development of analogs currently in use and those that have demonstrated efficacy in animal models or in clinical trials.

The controversial role of vitamin D in the skin: immunosuppression vs. photoprotection

Vitamin D is produced in the skin by ultraviolet (UV) B radiation (290-320 nm). The active metabolite 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] is made systemically by hydroxylation of vitamin D in the liver and the kidney, but also locally in the epidermis, which suggests that 1,25(OH)(2)D(3) may have important functions in the skin. 1,25(OH)(2)D(3) has opposing effects: it can mimic immunosuppressive effects caused by UV irradiation in some models, or reverse UV-induced DNA damage and immunosuppression in other models. 1,25(OH)(2)D(3) exerts effects on Langerhans cells that are characteristic of those associated with UV radiation (UVR)-induced suppression of contact hypersensitivity, and topical application of the vitamin D analogue calcipotriene suppresses contact hypersensitivity in human subjects to a similar extent as UVR. However, 1,25(OH)(2)D(3) decreases DNA damage both in vitro when added to human skin cells in culture before and after UVR, and in vivo when applied to mouse skin after UVR. Furthermore, topical 1,25(OH)(2)D(3) applied to mouse skin after UVR reversed the immunosuppressive effect of UVR in a contact hypersensitivity model. This review will discuss the role of 1,25(OH)(2)D(3) as either a mediator of UVR-induced immune suppression or as a photoprotective molecule against UVR-induced DNA damage and immune suppression.

Topically applied 1,25-dihydroxyvitamin D3 enhances the suppressive activity of CD4+CD25+ cells in the draining lymph nodes

The immunomodulatory effects of vitamin D have been described following chronic oral administration to mice or supplementation of cell cultures with 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the active form of vitamin D. In this study, topically applied 1,25(OH)(2)D(3), enhanced the suppressive capacity of CD4(+)CD25(+) cells from the draining lymph nodes. The effects of topical 1,25(OH)(2)D(3) were compared with those of UVB irradiation, which is the environmental factor required for 1,25(OH)(2)D(3) production in skin. CD4(+) cells from the skin-draining lymph nodes (SDLN) of either 1,25(OH)(2)D(3)-treated or UVB-irradiated mice had reduced capacity to proliferate to Ags presented in vitro, and could suppress Ag-specific immune responses upon adoptive transfer into naive mice. This regulation was lost upon removal of CD4(+)CD25(+) cells. Furthermore, purified CD4(+)CD25(+) cells from the SDLN of 1,25(OH)(2)D(3)-treated or UVB-irradiated mice compared with equal numbers of CD4(+)CD25(+) cells from control mice had increased capacity to suppress immune responses in both in vitro and in vivo assay systems. Following the sensitization of recipient mice with OVA, the proportion of CD4(+)Foxp3(+) cells of donor origin significantly increased in recipients of CD4(+)CD25(+) cells from the SDLN of 1,25(OH)(2)D(3)-treated mice, indicating that these regulatory T cells can expand in vivo with antigenic stimulation. These studies suggest that 1,25(OH)(2)D(3) may be an important mediator by which UVB-irradiation exerts some of its immunomodulatory effects.