UV-A induces persistent genomic instability in human keratinocytes through an oxidative stress mechanism

Ultraviolet-A (UV-A, 320 to 400 nm) radiation comprises 95% of the solar ultraviolet radiation (UVR) reaching the earth's surface. It has been associated experimentally and epidemiologically with malignant melanoma. In this study we investigated whether UV-A radiation can induce a persistent, heritable hypermutability in mammalian cells similar to that observed following ionising radiation (IR). Using the immortalized human skin keratinocyte cell line HaCaT we found that UV-A radiation does lead to a continuing reduction in plating efficiency, an increased "spontaneous" mutant fraction, and an increase in micronucleus formation up to 21 d after initial exposure. Reversal of these effects using catalase may indicate a role for hydrogen peroxide in this phenomenon. These results add to the significance of UV-A radiation as a risk factor in skin carcinogenesis.     

In vitro photostability and photoprotection studies of a novel 'multi-active' UV-absorber

This paper reports on the synthesis and properties of a new UV-absorber (OC-NO) based on the most popular UV filter worldwide, ethylhexyl methoxycinnamate (OMC) in which the methoxy group has been replaced with a pyrrolidine nitroxide bearing antioxidant activity. This sunscreen active has therefore both UV-absorbing and antioxidant properties which could ideally address both the UV-B and UV-A skin photo-damage. For broad-spectrum coverage, the combinations of OC-NO with two commonly used UV-A absorbers (BMDBM and DHHB) were also studied. The results obtained reveal that OC-NO: (a) is as photostable as OMC after UV-A exposure; (b) acts as free radical scavenger as demonstrated by EPR and chemical studies; (c) reduces UV-A and UV-A+BMDBM induced lipid peroxidation in liposomes and cells, measured as reduced TBARS levels and increased C11-BODIPY red fluorescence, respectively; (d) has comparable antioxidant activity to that of vitamin E and BHT commonly used in skin care formulations; (e) is non-cytotoxic to human skin fibroblasts as assessed with the MTT assay when exposed to increasing doses of UV-A; and (f) OC-NO+DHHB is a promising, photostable broad spectrum UV-filter combination that concomitantly reduces UV-induced free radical damage. These results suggest that nitroxide/antioxidant-based UV-absorbers may pave the way for the utilization of 'multi-active' ingredients in sunscreens thereby reducing the number of ingredients in these formulations.     

Perception of solar UV radiation by plants: photoreceptors and mechanisms

About 95% of the ultraviolet (UV) photons reaching the Earth’s surface are UV-A (315–400 nm) photons. Plant responses to UV-A radiation have been less frequently studied than those to UV-B (280–315 nm) radiation. Most previous studies on UV-A radiation have used an unrealistic balance between UV-A, UV-B, and photosynthetically active radiation (PAR). Consequently, results from these studies are difficult to interpret from an ecological perspective, leaving an important gap in our understanding of the perception of solar UV radiation by plants. Previously, it was assumed UV-A/blue photoreceptors, cryptochromes and phototropins mediated photomorphogenic responses to UV-A radiation and “UV-B photoreceptor” UV RESISTANCE LOCUS 8 (UVR8) to UV-B radiation. However, our understanding of how UV-A radiation is perceived by plants has recently improved. Experiments using a realistic balance between UV-B, UV-A, and PAR have demonstrated that UVR8 can play a major role in the perception of both UV-B and short-wavelength UV-A (UV-A_sw, 315 to ∼350 nm) radiation. These experiments also showed that UVR8 and cryptochromes jointly regulate gene expression through interactions that alter the relative sensitivity to UV-B, UV-A, and blue wavelengths. Negative feedback loops on the action of these photoreceptors can arise from gene expression, signaling crosstalk, and absorption of UV photons by phenolic metabolites. These interactions explain why exposure to blue light modulates photomorphogenic responses to UV-B and UV-A_sw radiation. Future studies will need to distinguish between short and long wavelengths of UV-A radiation and to consider UVR8’s role as a UV-B/UV-A_sw photoreceptor in sunlight.

In sunlight, UVR8 mediates the perception of both UV-B and short-wavelength UV-A radiation with its sensitivity moderated by blue light perceived through cryptochromes.     

In vivo activation of human immunodeficiency virus type 1 long terminal repeat by UV type A (UV-A) light plus psoralen and UV-B light in the skin of transgenic mice.

UV irradiation has been shown to activate the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) in cell culture; however, only limited studies have been described in vivo. UV light has been categorized as UV-A (400 to 315 nm), -B (315 to 280 nm), or -C (less than 280 nm); the longer wavelengths are less harmful but more penetrative. Highly penetrative UV-A radiation constitutes the vast majority of UV sunlight reaching the earth's surface but is normally harmless. UV-B irradiation is more harmful but less prevalent than UV-A. In this report, the HIV-1 LTR-luciferase gene in the skin of transgenic mice was markedly activated when exposed to UV-B irradiation. The LTR in the skin of transgenic mice pretreated topically with a photosensitizing agent (psoralen) was also activated to similar levels when exposed to UV-A light. A 2-h exposure to sunlight activated the LTR in skin treated with psoralen, whereas the LTR in skin not treated with psoralen was activated after 7 h of sunlight exposure. The HIV-1 LTR-beta-galactosidase reporter gene was preferentially activated by UV-B irradiation in a small population of epidermal cells. The transgenic mouse models carrying HIV-1 LTR-luciferase and LTR-beta-galactosidase reporter genes have been used to demonstrate the in vivo UV-induced activation of the LTR and might be used to evaluate other environmental factors or pharmacologic substances that might potentially activate the HIV-1 LTR in vivo.     

Metabolism of a physiological amount of all-trans-retinol in the vitamin A-deficient rat

Because only retinol and not all-trans-retinoic acid (atRA) can satisfy all of the functions of vitamin A, we have investigated the retinol metabolites in tissues of vitamin A-deficient (VAD) rats responding to a radioactive dose of [20-(3)H]all-trans-retinol. As expected, atRA is the major vitamin A metabolite present in the target tissues of VAD rats given a physiological dose (1 microg) of [20-(3)H]all-trans-retinol (atROL). Both atROL and atRA were detected by high-performance liquid chromatographic (HPLC) analysis of the radioactivity extracted from the liver, kidney, small intestine, lung, spleen, bone, skin, or testis of these animals. Novel retinol metabolites were observed in the aqueous extracts from the testis, lung, and skin. However, these metabolites were detected in very small amounts and were not characterized further. Importantly, neither 9-cis-retinoic acid (9cRA), 9-cis-retinol (9cROL), nor 13-cis-retinoic acid (13cRA) was present in detectable amounts. The amounts of atRA varied in each tissue, ranging from 0.29 +/- 0.05 fmol of RA/g of tissue in the femurs to 12.9 +/- 4.3 fmol of RA/g of tissue in the kidneys. The absence of 9cRA in vivo was not due to degradation of this retinoid during the extraction procedure or HPLC analysis of the extracted radioactivity. As atROL completely fulfills all of the physiological roles of vitamin A, and 9cRA is not detected in any of the tissues analyzed, these results suggest that 9cRA may have no physiological relevance in the rat.     

The evolution of human skin coloration

Skin color is one of the most conspicuous ways in which humans vary and has been widely used to define human races. Here we present new evidence indicating that variations in skin color are adaptive, and are related to the regulation of ultraviolet (UV) radiation penetration in the integument and its direct and indirect effects on fitness. Using remotely sensed data on UV radiation levels, hypotheses concerning the distribution of the skin colors of indigenous peoples relative to UV levels were tested quantitatively in this study for the first time. The major results of this study are: (1) skin reflectance is strongly correlated with absolute latitude and UV radiation levels. The highest correlation between skin reflectance and UV levels was observed at 545 nm, near the absorption maximum for oxyhemoglobin, suggesting that the main role of melanin pigmentation in humans is regulation of the effects of UV radiation on the contents of cutaneous blood vessels located in the dermis. (2) Predicted skin reflectances deviated little from observed values. (3) In all populations for which skin reflectance data were available for males and females, females were found to be lighter skinned than males. (4) The clinal gradation of skin coloration observed among indigenous peoples is correlated with UV radiation levels and represents a compromise solution to the conflicting physiological requirements of photoprotection and vitamin D synthesis. The earliest members of the hominid lineage probably had a mostly unpigmented or lightly pigmented integument covered with dark black hair, similar to that of the modern chimpanzee. The evolution of a naked, darkly pigmented integument occurred early in the evolution of the genus Homo. A dark epidermis protected sweat glands from UV-induced injury, thus insuring the integrity of somatic thermoregulation. Of greater significance to individual reproductive success was that highly melanized skin protected against UV-induced photolysis of folate (Branda & Eaton, 1978, Science201, 625-626; Jablonski, 1992, Proc. Australas. Soc. Hum. Biol.5, 455-462, 1999, Med. Hypotheses52, 581-582), a metabolite essential for normal development of the embryonic neural tube (Bower & Stanley, 1989, The Medical Journal of Australia150, 613-619; Medical Research Council Vitamin Research Group, 1991, The Lancet338, 31-37) and spermatogenesis (Cosentino et al., 1990, Proc. Natn. Acad. Sci. U.S.A.87, 1431-1435; Mathur et al., 1977, Fertility Sterility28, 1356-1360).As hominids migrated outside of the tropics, varying degrees of depigmentation evolved in order to permit UVB-induced synthesis of previtamin D(3). The lighter color of female skin may be required to permit synthesis of the relatively higher amounts of vitamin D(3)necessary during pregnancy and lactation. Skin coloration in humans is adaptive and labile. Skin pigmentation levels have changed more than once in human evolution. Because of this, skin coloration is of no value in determining phylogenetic relationships among modern human groups.     

An ultraviolet (UV-A) absorbing biopterin glucoside from the marine planktonic cyanobacterium Oscillatoria sp.

We have investigated the physiological response of marine planktonic cyanobacteria to UV-A (320–390 nm) irradiation. Here, we report the isolation of a UV-A absorbing pigment from a UV-A resistant strain of Oscillatoria. This pigment has been purified, and its structure determined to be biopterin glucoside (BG), a compound chemically related to the pteridine pigments found in butterfly wings. A UV-A sensitive isolate did not produce significant levels of this chromophore. UV-A radiation was very effective in eliciting synthesis of BG. In addition, increased UV-A radiation, increased intracellular levels of BG. These data suggest that BG may protect the cyanobacterium from adverse effects of UV-A radiation. Correspondence to: T. Matsunaga     

Elevated UV-B radiation incident on Quercus robur leaf canopies enhances decomposition of resulting leaf litter in soil

We examined whether the exposure of Quercus robur L. to elevated UV-B radiation (280–315 nm) during growth would influence leaf decomposition rate through effects on litter quality. Saplings were exposed for eight months at an outdoor facility in the UK to a 30% elevation above the ambient level of erythemally weighted UV-B radiation under UV-B treatment arrays of fluorescent lamps filtered with cellulose diacetate, which transmitted both UV-B and UV-A (315–400 nm) radiation. Saplings were exposed to elevated UV-A alone under control arrays of lamps filtered with polyester and to ambient radiation under unenergised arrays of lamps. Abscised leaves from saplings were enclosed in 1 mm² mesh nylon bags, placed in a Quercus–Fraxinus woodland and were sampled at 0.11, 0.53, 1.10 and 1.33 years for dry weight loss, chemical composition and saprotrophic fungal colonization. At abscission, litters from UV-A control arrays had ≈ 7.5% higher lignin/nitrogen ratios than those from UV-B treatment and ambient arrays (P < 0.06). Dry weight loss of leaves treated with elevated UV-B radiation during growth was 2.5% and 5% greater than that of leaves from UV-A control arrays at 0.53 and 1.33 years, respectively. Litter samples from UV-B treatment arrays lost more nitrogen and phosphorus than samples from ambient arrays and more carbon than samples from UV-A control arrays. The annual fractional weight loss of litter from UV-B treatment arrays was 8% and 6% greater than that of litter from UV-A control and ambient arrays, respectively. Regression analyses indicated that the increased decomposition rate of UV-B treated litters was associated with enhanced colonization of leaves by basidiomycete fungi, the most active members of the soil fungal community, and that the frequency of these fungi was negatively associated with the initial lignin/nitrogen ratio of leaves.     

The induction and repair of DNA damage and its influence on cell death in primary human fibroblasts exposed to UV-A or UV-C irradiation

Irradiation with UV-A of normal human fibroblasts in phosphate-buffered saline induced cell death, measured as lack of colony-forming ability. A specially filtered sunlamp, emitting wavelengths greater than 330 nm, was used as UV-A source. After UV-A irradiation, single-strand breaks (alkali-labile bonds) could be detected in DNA; these lesions were rapidly repaired. The induction of these single-strand breaks was almost eliminated when irradiation was performed in the presence of catalase. However, catalase, when present during UV-A irradiation, did not reduce cell death of the fibroblasts. Excision repair, monitored as unscheduled DNA synthesis, was induced strongly by irradiation with UV-C (predominantly 254 nm), but could not be detected after UV-A irradiation. Moreover, very little accumulation of incision breaks during post-irradiation incubation with hydroxyurea and 1-beta-D-arabinofuranosylcytosine (araC) was detected after UV-A. This is consistent with the low amount of pyrimidine dimers (measured as UV-endonuclease susceptible sites) induced by UV-A. Xeroderma pigmentosum fibroblasts of complementation group A, which are extremely sensitive to UV-C irradiation, showed the same sensitivity to UV-A as normal fibroblasts. The results indicate that lethality by UV-A wavelengths greater than 330 nm is caused by lesions other than single-strand breaks (alkali-labile bonds) and pyrimidine dimers.     

Morphological and physiological responses of bean plants to supplemental UV radiation in a Mediterranean climate

During the last few decades many experiments have been performed to evaluate the responses of plants to enhanced solar UV-B radiation (280–320 nm) that may occur because of stratospheric ozone depletion; most of them were performed in controlled environment conditions where plants were exposed to low photosynthetically active radiation (PAR) levels and high UV-B irradiance. Since environmental radiative regimes can play a role in the response of plants to UV-B enhancement, it appears doubtful whether it is valid to extrapolate the results from these experiments to plants grown in natural conditions. The objective of this work was to evaluate the effects on physiology and morphology of a bean (Phaseolus vulgaris L.) cultivar Nano Bobis, exposed to supplemental UV radiation in the open-air. UV-B radiation was supplied by fluorescent lamps to simulate a 20% stratospheric ozone reduction. Three groups of plants were grown: control (no supplemental UV), UV-A treatment (supplementation in the UV-A band) and UV-B treatment (supplemental UV-B and UV-A radiation). Each group was replicated three times. After 33 days of treatment plants grown under UV-B treatment had lower biomass, leaf area and reduced leaf elongation compared to UV-A treatment. No significant differences were detected in photosynthetic parameters, photosynthetic pigments and UV-B absorbing compounds among the three groups of plants. However, plants exposed to UV-A treatment showed a sort of 'stimulation' of their growth when compared to the control. The results of this experiment showed that plants may be sensitive to UV-A radiation, thus it is difficult to evaluate the specific effects of UV-B (280–320 nm) radiation from fluorescent lamps and it is important to choose the appropriate control. Environmental conditions strongly affect plant response to UV radiation so further field studies are necessary to assess the interaction between UV-B exposure and meteorological variability.     

BLUE LIGHT AND UV-A RADIATION CONTROL THE SYNTHESIS OF MYCOSPORINE-LIKE AMINO ACIDS IN CHONDRUS CRISPUS (FLORIDEOPHYCEAE)

The induction of UV-absorbing compounds known as mycosporine-like amino acids (MAAs) by red, green, blue, and white light (43% ambient radiation greater than 390 nm) was examined in sublittoral Chondrus crispus Stackh. Fresh collections or long-term cultures of sublittoral thalli, collected from Helgoland, North Sea, Germany, and containing no measurable amounts of MAAs, were exposed to filtered natural radiation for up to 40 days. The MAA palythine (λ_max 320 nm) was synthesized in thalli in blue light to the same extent observed in control samples in white light. In contrast, thalli in green or red light contained only trace amounts of MAAs. After the growth and synthesis period, the photosynthetic performance of thalli in each treatment, measured as pulse amplitude modulated chlorophyll fluorescence, was assessed after a defined UV dose in the laboratory. Thalli with MAAs were more resistant to UV than those without, and exposure to UV-A+B was more damaging than UV-A in that optimal (F_v/F_m) and effective (φ_II) quantum yields were lower and a greater proportion of the primary electron acceptor of PSII, Q, became reduced at saturating irradiance. However, blue light-grown thalli were generally more sensitive than white light control samples to UV-A despite having similar amounts of MAAs. The most sensitive thalli were those grown in red light, which had significantly greater reductions in F_v/F_m and φ_II and greater Q reduction. Growth under UV radiation alone had been shown previously to lead to the synthesis of the MAA shinorine (λ_max 334 nm) rather than palythine. In further experiments, we found that preexposure to blue light followed by growth in natural UV-A led to a 7-fold increase in the synthesis of shinorine, compared with growth in UV-A or UV-A+B without blue light pretreatment. We hypothesize that there are two photoreceptors for MAA synthesis in C. crispus, one for blue light and one for UV-A, which can act synergistically. This system would predispose C. crispus to efficiently synthesize UV protective compounds when radiation levels are rising, for example, on a seasonal basis. However, because the UV-B increase associated with artificial ozone reduction will not be accompanied by an increase in blue light, this triggering mechanism will have little additional adaptive value in the face of global change unless a global UV-B increase positively affects water column clarity.     

Inactivation of tyrosinase photoinduced by pterin

Tyrosinase catalyzes in mammals the first and rate-limiting step in the biosynthesis of the melanin, the main pigment of the skin. Pterins, heterocyclic compounds able to photoinduce oxidation of DNA and its components, accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder in which the protection against UV radiation fails due to the lack of melanin. Aqueous solutions of tyrosinase were exposed to UV-A irradiation (350nm) in the presence of pterin, the parent compound of oxidized pterins, under different experimental conditions. The enzyme activity in the irradiated solutions was determined by spectrophotometry and HPLC. In this work, we present data that demonstrate unequivocally that the enzyme is photoinactivated by pterin. The mechanism of the photosensitized process involves an electron transfer from tyrosinase to the triplet excited state of pterin, formed after UV-A excitation of pterin. The biological implications of the results are discussed.     

Who, what, where and when-influences on cutaneous vitamin D synthesis

The synthesis of vitamin D in skin is a two-stage process that begins with the production of previtamin D after irradiation of 7-dehydrocholesterol by ultraviolet (UV) radiation. A number of personal and environmental factors control the probability of a suitable UV photon reaching a molecule of 7-dehydrocholesterol in the skin. These are astronomical factors that govern the solar zenith angle (SZA), and the local state of the atmosphere, determining the available solar UV radiation; skin pigmentation and age, determining competing absorbers of UV radiation and available 7-dehydrocholesterol; individual behaviour in the local surroundings, determining exposure of unprotected skin to available UV radiation. The only one of these influences that can be determined unequivocally for any situation is the SZA. The other influences must be considered either as individual case studies, or be represented by "typical" and "idealised" situations for the weather, skin and behaviour. At large SZAs there is insufficient solar UV radiation to initiate significant vitamin D synthesis. At smaller SZAs assessment of solar exposure necessary for vitamin D synthesis can only be indicative and application of any such assessment necessarily requires awareness of both self- and the local environment.     

Vitamin A--a pregnancy hazard alert.

Vitamin A is essential to human health, but concerns have arisen recently regarding its potential teratogenicity. Human and animal birth defects have been associated with the use of the vitamin A analogue, isotretinoin, or Accutane, for acne treatment, although the association of such defects with vitamin A itself is unclear. The federal Food and Drug Administration is evaluating the health issues surrounding vitamin A and, together with the manufacturer, has developed restrictions and label warnings to ensure the appropriate use of Accutane. We also have evaluated these issues, with concerns about the possible teratogenicity of high vitamin A intake during pregnancy. Practitioners should be familiar with the possible hazard of excessive dosages of vitamin A and its analogues. Vitamin A daily doses of higher than 8,000 IU for pregnant woman are not necessary for good health and are not recommended. Foods high in beta-carotene can provide the necessary amounts of vitamin A and, in contrast to the synthetic analogues, their use has not been associated with vitamin A toxicity or teratogenicity in humans or animals.     

Spectral balance and UV-B sensitivity of soybean: a field experiment

Soybean [Glycine max (L.) Merr.] cultivar Essex was grown and tested for sensitivity to UV-B radiation (280–320 nm) under different combinations of UV-A (320–400 nm) and PFD (400–700 nm) radiation in four simultaneous field experiments. The radiation conditions were effected with combinations of filtered solar radiation and UV-B and UV-A lamps electronically modulated to track ambient radiation. Significant UV-B-caused decreases in total aboveground production and growth were seen only when PFD and UV-A were reduced to less than half their flux in sunlight. When PFD was low, UV-A appeared to be particularly effective in mitigating UV-B damage. However, when PFD was high, substantial UV-A did not appear to be required for UV-B damage mitigation. Leaf chlorophyll fluorescence did not indicate photosynthetic damage under any radiation combination. With UV-B, leaves in all experiments exhibited increased UV-absorbing pigments and decreased whole-leaf UV transmittance. Results of these field experiments indicate difficulties in extrapolating from UV-B experiments conducted in glasshouse or growth cabinet conditions to plant UV-B sensitivity in the field. Implications for UV radiation weighting functions in evaluating atmospheric ozone reduction are also raised.     

Photoirradiation products of flavin derivatives,and the effects of photooxidation on guanine

Photoirradiation in the presence of riboflavin led to guanine oxidation and the formation of imidazolone. Meanwhile, riboflavin itself was degraded by ultraviolet light A (UV-A) and visible light (VIS) radiation, and the end product was lumichrome. VIS radiation in the presence of riboflavin oxidized guanine similarly to UV-A radiation. Although UV-A radiation with lumichrome oxidized guanine, VIS radiation with lumichrome did not. Thus, UV-A radiation with riboflavin can oxidize guanine even if riboflavin is degraded to lumichrome. In contrast, following VIS radiation degradation of riboflavin to lumichrome, VIS radiation with riboflavin is hardly capable of oxidizing guanine. The consequences of riboflavin degradation and guanine photooxidation can be extended to flavin mononucleotide and flavin adenine dinucleotide. In addition, we report advanced synthesis; carboxymethylflavin was obtained by oxidation of formylmethylflavin with chlorite and hydrogen peroxide; lumichrome was obtained by heating of formylmethylflavin in 50% AcOH; lumiflavin was obtained by incubation of formylmethylflavin in 2 M NaOH, followed by isolation by step-by-step concentration.     

Pathophysiology of photoaging of human skin: focus on neutrophils.

UV-induced skin damage is the result of a complex cascade of events. Many studies have focused on the skin effects induced by UV-B or UV-A separately. Recently a UV-source that emits UV-B and UV-A together in a ratio comparable to daily sunlight has been introduced: i.e. solar simulated radiation (SSR). By exposing human skin type I-III to erythematogenic doses of UV (> or =1 MED) emitted by a SSR source we have noticed that: (a) neutrophils are initially the main infiltrating cell type in the dermis and (b) these infiltrating cells are the a key source of in vivo enzymatically [corrected] active enzymes such as elastase, [corrected] matrix metallo proteinases-1 and -9 (MMPs-1 and -9). These enzymes are relevant to the process of photoaging, as they break down the extracellular matrix. Keratinocytes and fibroblasts also produce matrix degrading enzymes, but to a lesser extent. Our results indicate a primary role for infiltrating neutrophils in the initial steps of photoaging. This is further supported by the observation that after exposure of skin type VI to physical doses of SSR, equivalent to those used for skin types I-III, no neutrophils and neutrophil-derived enzymatic activity were observed, explaining why skin type VI is [corrected] less susceptible to photoaging than skin types [corrected] I-III. Statement: Although most of the data, referred to, have been published, the current perspective in which they are put together is completely novel and has not been published elsewhere.     

Do panther chameleons bask to regulate endogenous vitamin D3 production?

Basking by ectothermic vertebrates is thought to have evolved for thermoregulation. However, another beneficial effect of sunlight exposure, specifically the ultraviolet B (UV-B) component, includes endogenous production of vitamin D(3). In the laboratory, panther chameleons exhibited a positive phototaxis to greater visible, ultraviolet A (UV-A) and UV-B light. However, with equivalent high irradiances of UV-A or UV-B, their response to UV-B was significantly greater than it was to UV-A. Exposure of in vitro skin patches of panther chameleons to high UV-B (90 microW/cm(2)) for 1 h significantly enhanced vitamin D(3) concentration. Voluntary exposure to higher UV-B irradiance (70 vs. 1 microW/cm(2)) resulted in greater circulating 25-hydroxyvitamin D(3) in female panther chameleons (604 vs. 92 ng/mL). Depending on dietary intake of vitamin D(3), chameleons adjusted their exposure time to UV-B irradiation as if regulating their endogenous production of this vital hormone. When dietary intake was low (1-3 IU/g), they exposed themselves to significantly more UV-producing light; when intake was high (9-129 IU/g), they exposed themselves to less. Vitamin D(3) photoregulation seems to be an important additional component of the function of basking.     

Modulation of the immune system by UV radiation: more than just the effects of vitamin D?

Humans obtain most of their vitamin D through the exposure of skin to sunlight. The immunoregulatory properties of vitamin D have been demonstrated in studies showing that vitamin D deficiency is associated with poor immune function and increased disease susceptibility. The benefits of moderate ultraviolet (UV) radiation exposure and the positive latitude gradients observed for some immune-mediated diseases may therefore reflect the activities of UV-induced vitamin D. Alternatively, other mediators that are induced by UV radiation may be more important for UV-mediated immunomodulation. Here, we compare and contrast the effects of UV radiation and vitamin D on immune function in immunopathological diseases, such as psoriasis, multiple sclerosis and asthma, and during infection.