Sunlight, Vitamin D, and Xeroderma Pigmentosum

Sunlight, in particular UV-B radiation, is an important factor for endogenous vitamin D production as 80-90% of the required vitamin D needs to be photosynthesized in the skin. The active form of vitamin D, vitamin D₃ or calcitriol, binds to the ligand-activated transcription factor vitamin D receptor (VDR) for genomic and non-genomic effects. Recently, calcitriol and analogs have been shown to have antiproliferative effects in mouse and human BCC and SCC cell lines in vitro. As UV radiation plays a critical role in the photosynthesis of vitamin D, stringent sun protection, as recommended for xeroderma pigmentosum (XP) patients, may impact their vitamin D levels.XP is a rare autosomal recessive disorder with a worldwide prevalence of 1 in 1,000,000. XP can be divided into seven different complementation groups: XP-A to XP-G. The complementation groups correspond with the underlying gene defect. Defects in these genes lead to a defective nucleotide excision repair (NER), which is necessary to remove UV-induced DNA damage such as the UV photoproducts cyclobutane pyrimidine dimers (CPD) and 6-4 pyrimidine-pyrimidone (6-4 PP) dimer. Additionally, a variant form with a mutation in the translational polymerase η gene (PolH), also called XP variant (XPV), exists. Patients with XPV show a defect in translesion synthesis. Due to their inability to repair UV-induced lesions, XP patients exhibit an increased risk for UV-induced nonmelanoma skin cancer (NMSC) such as basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) as well as melanoma. Although no curative therapy for XP exists today, numerous options for the treatment and prophylaxis of skin cancer have become available.

UV-B-Induced CPD Photolyase Gene Expression is Regulated by UVR8-Dependent and -Independent Pathways in Arabidopsis

Plants have evolved various mechanisms that protect against the harmful effects of UV-B radiation (280-315 nm) on growth and development. Cyclobutane pyrimidine dimer (CPD) photolyase, the repair enzyme for UV-B-induced CPDs, is essential for protecting cells from UV-B radiation. Expression of the CPD photolyase gene (PHR) is controlled by light with various wavelengths including UV-B, but the mechanisms of this regulation remain poorly understood. In this study, we investigated the regulation of PHR expression by light with various wavelengths, in particular low-fluence UV-B radiation (280 nm, 0.2 µmol m(-2) s(-1)), in Arabidopsis thaliana seedlings grown under light-dark cycles for 7 d and then adapted to the dark for 3 d. Low-fluence UV-B radiation induced CPDs but not reactive oxygen species. AtPHR expression was effectively induced by UV-B, UV-A (375 nm) and blue light. Expression induced by UV-A and blue light was predominantly regulated by the cryptochrome-dependent pathway, whereas phytochromes A and B played a minor but noticeable role. Expression induced by UV-B was predominantly regulated by the UVR8-dependent pathway. AtPHR expression was also mediated by a UVR8-independent pathway, which is correlated with CPD accumulation induced by UV-B radiation. These results indicate that Arabidopsis has evolved diverse mechanisms to regulate CPD photolyase expression by multiple photoreceptor signaling pathways, including UVR8-dependent and -independent pathways, as protection against harmful effects of UV-B radiation.