Hydrochlorothiazide Enhances UVA‐Induced DNA Damage

The UVA is currently thought to be carcinogenic because, similar to UVB, it induces the formation of cyclobutane pyrimidine dimers (CPDs). Various drugs have been reported to cause photosensitive drug eruptions as an adverse effect. Although the precise mechanism of photosensitive drug eruption remains to be elucidated, it is generally accepted that free radicals and other reactive molecules generated via UV‐irradiated drugs play important roles in the pathogenesis of photosensitive drug eruptions. The waveband of concern for photo‐reactive drugs is UVA‐visible light, but some extend into the UVB region. We tested whether photosensitive drugs could enhance CPD formation after UVA exposure by using isolated DNA in the presence of several reported photosensitive drugs using high‐performance liquid chromatography. We found that the diuretic agent hydrochlorothiazide (HCT) significantly enhanced the production of TT dimers over a wide range of UVA. Furthermore, we investigated whether UVA plus HCT could enhance CPD production in xeroderma pigmentosum model mice defective in nucleotide excision repair. Immunofluorescence studies showed that CPD formation in the skin significantly increased after 365 nm narrow‐band UVA irradiation in the presence of HCT, compared with that in wild‐type mice. HCT could be used with caution because of its enhancement of UVA‐induced DNA damage.     

Protective Effect of Tropical Highland Blackberry Juice (Rubus adenotrichos Schltdl.) Against UVB‐Mediated Damage in Human Epidermal Keratinocytes and in a Reconstituted Skin Equivalent Model

Solar ultraviolet (UV) radiation, particularly its UVB (280–320 nm) spectrum, is the primary environmental stimulus leading to skin carcinogenesis. Several botanical species with antioxidant properties have shown photochemopreventive effects against UVB damage. Costa Rica's tropical highland blackberry (Rubus adenotrichos) contains important levels of phenolic compounds, mainly ellagitannins and anthocyanins, with strong antioxidant properties. In this study, we examined the photochemopreventive effect of R. adenotrichos blackberry juice (BBJ) on UVB‐mediated responses in human epidermal keratinocytes and in a three‐dimensional (3D) reconstituted normal human skin equivalent (SE). Pretreatment (2 h) and posttreatment (24 h) of normal human epidermal keratinocytes (NHEKs) with BBJ reduced UVB (25 mJ cm^?2)‐mediated (1) cyclobutane pyrimidine dimers (CPDs) and (2) 8‐oxo‐7,8‐dihydro‐2′‐deoxyguanosine (8‐oxodG) formation. Furthermore, treatment of NHEKs with BBJ increased UVB‐mediated (1) poly(ADP‐ribose) polymerase cleavage and (2) activation of caspases 3, 8 and 9. Thus, BBJ seems to alleviate UVB‐induced effects by reducing DNA damage and increasing apoptosis of damaged cells. To establish the in vivo significance of these findings to human skin, immunohistochemistry studies were performed in a 3D SE model, where BBJ was also found to decrease CPDs formation. These data suggest that BBJ may be developed as an agent to ameliorate UV‐induced skin damage.     

Baicalin Protects Keratinocytes from Toll‐like Receptor‐4 Mediated DNA Damage and Inflammation Following Ultraviolet Irradiation

UVB radiation contributes to both direct and indirect damage to the skin including the generation of free radicals and reactive oxygen species (ROS), inflammatory responses, immunosuppression and gene mutations, which can ultimately lead to photocarcinogenesis. A plant‐derived flavonoid, baicalin, has been shown to have antioxidant, anti‐inflammatory and free radical scavenging activities. Previous studies from our laboratory have shown that in murine skin, Toll‐like receptor‐4 (TLR4) enhanced both UVB‐induced DNA damage and inflammation. The aim of this study was to investigate the efficacy of baicalin against TLR4‐mediated processes in the murine keratinocyte PAM 212 cell line. Our results demonstrate that treating keratinocytes with baicalin both before and after UV radiation (100 mJ cm^?2) significantly inhibited the level of intracellular ROS and decreased cyclobutane pyrimidine dimers and 8‐Oxo‐2′‐deoxyguanosine (8‐oxo‐dG)—markers of DNA damage. Furthermore, cells treated with baicalin demonstrated an inhibition of TLR4 and its downstream signaling molecules, MyD88, TRIF, TRAF6 and IRAK4. TLR4 pathway inhibition resulted in NF‐κB inactivation and down‐regulation of iNOS and COX‐2 protein expression. Taken together, baicalin treatment effectively protected keratinocytes from UVB‐induced inflammatory damage through TLR pathway modulation.     

Artocarpin‐enriched (Artocarpus altilis) Heartwood Extract Provides Protection Against UVB‐induced Mechanical Damage in Dermal Fibroblasts

This study aimed to evaluate the protective effect of artocarpin‐enriched (Artocarpus altilis) heartwood extract on the mechanical properties of UVB‐irradiated fibroblasts. Human skin fibroblasts were pretreated with 50 μg/mL^?1 extract and later irradiated with UVB (200 mJ/cm^?2). They were then cultured within three‐dimensional of free‐floating and tense collagen lattices. The pretreatment of fibroblasts with the extract prior to UVB radiation showed cells protection against UVB‐induced suppression of α‐SMA expression, fibroblast migration and contraction. These results reveal that the extract prevents mechanical damages induced by UVB irradiation in fibroblast‐embedded collagen lattices, and therefore, has a potential as a natural photo‐protectant.     

An Experimental Population Study of Nucleotide Excision Repair as a Risk Factor for UVB‐induced Melanoma

Nucleotide excision repair (NER) is the primary defense against the DNA damage implicit in skin cancer formation and is negatively affected by chronic exposure to UVB radiation. However, in situ and in vitro studies consistently yield equivocal results when addressing individual DNA repair capacity and melanoma susceptibility. The primary objective of this study was to determine if individual global NER capacity is a risk factor for melanoma formation in a prominent UVB‐inducible melanoma model, hybrid Xiphophorus fishes. After neonatal UVB irradiation, adult tumor‐bearing and tumor‐free fish were given a challenge UVB dose and (6–4) photoproduct repair was quantified in individual fish at 24 h using radioimmunoassay. Despite considerable inter‐individual variation in repair capacity, ranging from 13% to 91%, we found no difference in mean NER capacity between fish with and without melanomas, thus detaching global NER from melanomagenesis. Furthermore, despite epidemiological data indicating that sex and age are important risk factors underlying melanoma susceptibility, we found no difference in mean NER rates among the sexes or as a function of age. We conclude with a discussion of the apparent paradox of how inter‐individual variation in NER is not a risk factor given the clear evidence that DNA damage underlies melanoma susceptibility.     

Tolerance to Solar Ultraviolet‐B Radiation in the Citrus Red Mite,An Upper Surface User of Host Plant Leaves

Plant‐dwelling mites are potentially exposed to solar ultraviolet‐B (UVB) radiation that causes deleterious and often lethal effects, leading most mites to inhabit the lower (underside) leaf surfaces. However, in species of spider mite belonging to the Genus Panonychus, a substantial portion of individuals occur on upper leaf surfaces. We investigated whether the upper leaf surfaces of citrus trees are favorable for P. citri, and to what extent they are tolerant to UVB radiation. If eggs are not adequately protected from UVB damage, females may avoid ovipositing on the upper surfaces of sunny leaves. To test this, we conducted laboratory experiments using a UVB lamp, and semioutdoor manipulative experiments. As a result, P. citri eggs are tolerant to UVB. Field studies revealed that the ratio of eggs and adult females on upper leaf surfaces were larger for shaded than for sunny leaves. However, 64–89% of eggs hatched successfully even on sunny upper leaf surfaces. Nutritional evaluation revealed that whether on sunny or shaded leaves, in fecundity and juvenile development P. citri reaped the fitness benefits of upper leaf surfaces. Consequently, P. citri is tolerant to UVB damage, and inhabiting the upper surfaces of shaded leaves is advantageous to this mite.     

Quantification of Cyclobutane Pyrimidine Dimers Induced by UVB Radiation in Conidia of the Fungi Aspergillus fumigatus,Aspergillus nidulans,Metarhizium acridum and Metarhizium robertsii

Conidia are responsible for reproduction, dispersal, environmental persistence and host infection of many fungal species. One of the main environmental factors that can kill and/or damage conidia is solar UV radiation. Cyclobutane pyrimidine dimers (CPD) are the major DNA photoproducts induced by UVB. We examined the conidial germination kinetics and the occurrence of CPD in DNA of conidia exposed to different doses of UVB radiation. Conidia of Aspergillus fumigatus, Aspergillus nidulans and Metarhizium acridum were exposed to UVB doses of 0.9, 1.8, 3.6 and 5.4 kJ m⁻². CPD were quantified using T4 endonuclease V and alkaline agarose gel electrophoresis. Most of the doses were sublethal for all three species. Exposures to UVB delayed conidial germination and the delays were directly related both to UVB doses and CPD frequencies. The frequencies of dimers also were linear and directly proportional to the UVB doses, but the CPD yields differed among species. We also evaluated the impact of conidial pigmentation on germination and CPD induction on Metarhizium robertsii. The frequency of dimers in an albino mutant was approximately 10 times higher than of its green wild-type parent strain after exposure to a sublethal dose (1.8 kJ m⁻²) of UVB radiation.     

Protective Effect of Curcumin Against Acute Ultraviolet B Irradiation‐induced Photo‐damage

Ultraviolet B (UVB) irradiation is one of the most dangerous insults for skin and causes sunburn, erythema, photoaging and photocarcinogenesis. Curcumin (diferuloylmethane), a yellow spice derived from dried rhizomes of Curcuma longa, has been shown to possess significant anti‐inflammatory, antioxidant, anticarcinogenic, antimutagenic, anticoagulant and anti‐infective effects. However, the protective effects of curcumin against acute photo‐damage are poorly understood. In this study, we investigated the photoprotective effects of curcumin against UVB‐induced acute photo‐damage in hairless mice and immortalized human keratinocytes (HaCaT). Topical application of curcumin significantly inhibited acute UVB (540 mJ cm^?2, for 3 successive days)‐induced inflammatory cells, collagen accrementition derangement and lipid peroxidation, and effectively induced NF‐E2‐related factor 2 (Nrf2) nuclear accumulation in uncovered (Uncv) hairless mice skin. Treatment of HaCaT cells with curcumin significantly attenuated acute UVB (300 mJ cm^?2)‐induced lactate dehydrogenase release, intracellular reactive oxygen species production and DNA damage, activated the expression of the phase II detoxifying enzymes and promoted DNA repair activity. The photoprotective effect provided by curcumin was potential associated with modulation of Nrf2‐dependent antioxidant response. Our study suggested that curcumin is a potential agent for preventing and/or treating UV radiation‐induced acute inflammation and photoaging.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

The Reversibility of UV‐B Induced Alterations in Optical Properties of the Rabbit Cornea Depends on Dose of UV Irradiation

Solar UVB radiation evokes photokeratitis, accompanied by increased corneal hydration and changes in corneal transparency, resulting in increased light absorption. Corneal optical properties are disturbed and visual acuity decreased. The aim of this study was to investigate the reversibility of these UVB‐induced changes. Rabbit corneas were irradiated with UVB doses of 0.5 J cm^?2 or 1.01 J cm^?2 during 4 days. Some rabbits were sacrificed after the last irradiation and some 2 months later. Corneas were investigated spectrophotometrically for light absorption, and corneal hydration was evaluated by central corneal thickness with an ultrasonic pachymeter. Corneal impression cytologies were examined immunohistochemically for proinflammatory cytokines and malondialdehyde. The increased corneal light absorption, hydration and the staining of immunohistochemical markers found in corneas after irradiation returned to normal values during 2 months in corneas irradiated with the lower UVB dose. In contrast, in corneas irradiated with the higher UVB dose, a moderate but statistically significant increase in corneal light absorption, hydration and positive immunohistochemical stainings remained as residual changes. This was in contrast to normal corneas, where the staining of proinflammatory cytokines as well as malondialdehyde was negative. In conclusion, the reversibility of UVB‐induced disturbances was dependent on UVB dose.     

Single-cell gel/comet assay applied to the analysis of UV radiation-induced DNA damage in Rhodomonas sp. (Cryptophyta).

The single-cell gel/comet assay is an electrophoretic technique used to detect single-strand breaks in DNA. Damage is assessed examining individual cells under an epifluorescent microscope. UV-induced DNA damage consists mostly of the formation of pyrimidine dimers; therefore, most of the damage cannot be detected using a standard comet assay. The enzyme T4 endonuclease V breaks DNA strands at sites of pyrimidine dimers. The main objective of this work is to evaluate the comet assay to detect UV-induced damage in DNA after an initial treatment of cells with T4 endonuclease V. This work was conducted on Rhodomonas sp. (Cryptophyta), a marine unicellular flagellate. Cells of Rhodomonas sp. were exposed to 12 h visible + ultraviolet-A + ultraviolet-B (VIS + UVA + UVB) and VIS (control), with and without T4 endonuclease V. Cells exposed to VIS + UVA + UVB showed approximately 200% more damage than control if these were treated with T4 endonuclease V. Rhodomonas sp. were exposed to 3, 6, 9 and 12 h of VIS, VIS + UVA and VIS + UVA + UVB. Damage induced by VIS + UVA + UVB as detected by the comet assay increased along with exposure time. However, damage caused by VIS and VIS + UVA remained relatively constant at all times. Results of this study indicate that the comet assay is more sensitive to UV radiation damage when used in conjunction with T4 endonuclease V. This modification of the comet assay can be used as an alternative technique to detect DNA damage in single cells caused by UV radiation.     

Effects and Mechanism of Nicotinamide Against UVA‐ and/or UVB‐mediated DNA Damages in Normal Melanocytes

Melanoma incidences are increasing rapidly, and ultraviolet (UV) radiation from the sun is believed to be its major contributing factor. UV exposure causes DNA damage in skin which may initiate cutaneous skin cancers including melanoma. Melanoma arises from melanocytes, the melanin‐producing skin cells, following genetic dysregulations resulting into hyperproliferative phenotype and neoplastic transformation. Both UVA and UVB exposures to the skin are believed to trigger melanocytic hyperplasia and melanomagenesis. Melanocytes by themselves are deficient in repair of oxidative DNA damage and UV‐induced photoproducts. Nicotinamide, an active form of vitamin B3 and a critical component of the human body's defense system has been shown to prevent certain cancers including nonmelanoma skin cancers. However, the mechanism of nicotinamide's protective effects is not well understood. Here, we investigated potential protective effects and mechanism of nicotinamide against UVA‐ and/or UVB‐ induced damage in normal human epidermal melanocytes. Our data demonstrated an appreciable protective effect of nicotinamide against UVA‐ and/or UVB‐ induced DNA damage in melanocytes by decreasing both cyclobutane pyrimidine dimers and 8‐hydroxy‐2′‐deoxyguanosine levels. We found that the photoprotective response of nicotinamide was associated with the activation of nucleotide excision repair genes and NRF2 signaling. Further studies are needed to validate our findings in in vivo models.     

DETECTION OF UVR-INDUCED DNA DAMAGE IN MOUSE EPIDERMIS in vivo USING ALKALINE ELUTION

Abstract— Alkaline elution has been used to detect ultraviolet radiation (UVR)-induced DNA damage in the epidermis of C3H/Tif hr/hr mice. This technique detects DNA damage in the form of single-strand breaks and alkali-labile sites (SSB) formed directly by UVA (320–400 nm) or indirectly by UVB (280–320 nm). The latter induces DNA damage such as cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6–4)-photoproducts, which are then converted into transient SSB by cellular endonucleases, during nucleotide excision repair (NER). The irradiation system used had a spectral output similar in effect to solar UVR, with the UVB component inducing 94% of the edema response observed in mice. Consequently, the majority of SSB detected were those formed via NER of UVB-induced photoadducts. The number of SSB detected immediately after 8 kj/m² (2.7 minimum erythema doses determined at 48 h post-UVR [MED]) was low, indicating the formation of only small numbers of transient SSB. When DNA repair inhibitors hydroxyurea and 1 -β-D-arabinofuranosylcytosine were administered (intraperi-toneally) to mice 30 min before UVR, they prevented sealing of the DNA SSB formed during NER. A four-fold increase in the number of SSB detected resulted, which was found to be linearly related to the UVR dose. The SSB induced by 2 kj/m² (less than an MED) were readily detected, with the ear showing lower numbers of SSB than the dorsum. When repair inhibitors were added post-UVR, the rate of formation of SSB declined rapidly with time of administration, reflecting repair of DNA lesions. After a UVR dose of 6 kj/m² (2 MED), 50% of the initial repair-dependent SSB had been removed after approximately 2 h in the ear and 4 h in the dorsum; no more SSB appeared to be incised by 24 h post-UVR. The technique described is an efficient and highly sensitive one for the quantification of SSB induced in UV-irradiated skin samples in vivo.     

A Novel pH/Light‐Triggered Surface for DNA Adsorption and Release

A simple strategy for the immobilization of Cy3‐labeled single strand DNA (Cy3‐ssDNA) on a Si(001) surface and its release under control of both light and pH stimuli is presented. In order to prepare a dual pH/light‐triggered surface, positively chargeable azobenzene molecules are self‐assembled on the Si(001) surface. The surface wettability of this substrate can be changed under influence of both light and pH conditions. The substrates can be positively charged under mildly acidic conditions. The pH‐sensitive behavior of the film allows binding of Cy3‐ssDNA on the functionalized Si(001) surface through e?ective electrostatic interactions with the negatively charged polynucleotide backbone. Moreover, irradiation of the film with UVA light induces trans–cis isomerization of the azobenzene units on the surface. As a result, the binding a?nity for DNA decreases due to the changing surface hydrophilicity. In order to understand and control the reversible photoswitchable mechanism of this surface, water contact angles are measured after UVA and visible light irradiation. The release of DNA from a dual pH/light‐sensitive sample is performed using fluorescence microscopy. The results show that irradiation of the film with UVA light induces trans–cis isomerization of the photoresponsive azobenzene units; this leads to significant changes in the surface hydrophilicity and reduces the binding affinity for DNA.     

Fisetin Inhibits UVB‐induced Cutaneous Inflammation and Activation of PI3K/AKT/NFκB Signaling Pathways in SKH‐1 Hairless Mice

Solar ultraviolet B (UVB) radiation has been shown to induce inflammation, DNA damage, p53 mutations and alterations in signaling pathways eventually leading to skin cancer. In this study, we investigated whether fisetin reduces inflammatory responses and modulates PI3K/AKT/NFκB cell survival signaling pathways in UVB‐exposed SKH‐1 hairless mouse skin. Mice were exposed to 180 mJ cm^?2 of UVB radiation on alternate days for a total of seven exposures, and fisetin (250 and 500 nmol) was applied topically after 15 min of each UVB exposure. Fisetin treatment to UVB‐exposed mice resulted in decreased hyperplasia and reduced infiltration of inflammatory cells. Fisetin treatment also reduced inflammatory mediators such as COX‐2, PGE₂ as well as its receptors (EP1–EP4) and MPO activity. Furthermore, fisetin reduced the level of inflammatory cytokines TNFα, IL‐1β and IL‐6 in UVB‐exposed skin. Fisetin treatment also reduced cell proliferation markers as well as DNA damage as evidenced by increased expression of p53 and p21 proteins. Further studies revealed that fisetin inhibited UVB‐induced expression of PI3K, phosphorylation of AKT and activation of the NFκB signaling pathway in mouse skin. Overall, these data suggest that fisetin may be useful against UVB‐induced cutaneous inflammation and DNA damage.     

Distinct Role of Sesn2 in Response to UVB‐Induced DNA Damage and UVA‐Induced Oxidative Stress in Melanocytes

Ultraviolet (UV) radiation, including both UVB and UVA irradiation, is the major risk factor for causing skin cancer including melanoma. Recently, we have shown that Sesn2, a member of the evolutionarily conserved stress‐inducible protein family Sestrins (Sesn), is upregulated in human melanomas as compared to melanocytes in normal human skin, suggesting an oncogenic role of Sesn2. However, the role of Sesn2 in UVB and UVA response is unknown. Here, we demonstrated that both UVB and UVA induce Sesn2 upregulation in melanocytes and melanoma cells. UVB induces Sesn2 expression through the p53 and AKT3 pathways. Sesn2 negatively regulates UVB‐induced DNA damage repair. In comparison, UVA induces Sesn2 upregulation through mitochondria but not Nrf2. Sesn2 ablation increased UVA‐induced Nrf2 induction and inhibits UVA‐induced ROS production, indicating that Sesn2 acts as an upstream regulator of Nrf2. These findings suggest previously unrecognized mechanisms in melanocyte response to UVB and UVA irradiation and potentially in melanoma formation.     

UVA and UVB‐Induced 8‐Methoxypsoralen Photoadducts and a Novel Method for their Detection by Surface‐Enhanced Laser Desorption Ionization Time‐of‐Flight Mass Spectrometry (SELDI‐TOF MS)

UVA‐activated psoralens are used to treat hyperproliferative skin conditions due to their ability to form DNA photoadducts, which impair cellular processes and may lead to cell death. Although UVA (320–400 nm) is more commonly used clinically, studies have shown that UVB (280–320 nm) activation of psoralen can also be effective. However, there has been no characterization of UVB‐induced adduct formation in DNA alone. As psoralen derivatives have a greater extinction coefficient in the UVB region (11 800 cm^?1 M^?1 at 300 nm) compared with the UVA region (2016 cm^?1 M^?1 at 365 nm), a greater extent of adduct formation is expected. SELDI‐TOF, a proteomic technique that combines chromatography with mass spectrometry, was used to detect photoadduct formation in an alternating A–T oligonucleotide. 8‐Methoxypsoralen (8‐MOP) and DNA solutions were irradiated with either UVA or UVB. An adduct peak was obtained with SELDI‐TOF. For UVB‐activated 8‐MOP, the extent of adducts was three times greater than for UVA. HPLC ESI‐MS analysis showed that UVB irradiation yielded high levels of 3,4‐monoadducts (78% of total adducts). UVA was more effective than UVB at conversion of 4′,5′‐monoadducts to crosslinks (17% vs 4%, respectively). This report presents a method for comparing DNA binding efficiencies of interstrand crosslink inducing agents.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

Effects of Ultraviolet Radiation (UVA+UVB) on Young Gametophytes of Gelidium floridanum: Growth Rate,Photosynthetic Pigments,Carotenoids, Photosynthetic Performance,and Ultrastructure

This study investigated the effects of radiation (PAR+UVA+UVB) on the development and growth rates (GRs) of young gametophytes of Gelidium floridanum. In addition, photosynthetic pigments were quantified, carotenoids identified, and photosynthetic performance assessed. Over a period of 3 days, young gametophytes were cultivated under laboratory conditions and exposed to photosynthetically active radiation (PAR) at 80 μmol photons m^?2 s^?1 and PAR+UVA (0.70 W m^?2)+UVB (0.35 W m^?2) for 3 h per day. The samples were processed for light and electron microscopy to analyze the ultrastructure features, as well as carry out metabolic studies of GRs, quantify the content of photosynthetic pigments, identify carotenoids and assess photosynthetic performance. PAR+UVA+UVB promoted increase in cell wall thickness, accumulation of floridean starch grains in the cytoplasm and disruption of chloroplast internal organization. Algae exposed to PAR+UVA+UVB also showed a reduction in GR of 97%. Photosynthetic pigments, in particular, phycoerythrin and allophycocyanin contents, decreased significantly from UV radiation exposure. This result agrees with the decrease in photosynthetic performance observed after exposure to ultraviolet radiation, as measured by a decrease in the electron transport rate (ETR), where values of ETRmax declined approximately 44.71%. It can be concluded that radiation is a factor that affects the young gametophytes of G. floridanum at this stage of development.     

PYRIMIDINE DIMER FORMATION IN HUMAN SKIN

Cyclobutyl pyrimidine dimers are major photoproducts formed upon irradiation of DNA with ultraviolet light. We have developed a method for detecting as few as one pyrimidine dimer per million bases in about 50 ng of non-radioactive DNA, and have used this method to quantitate dimer yields in human skin DNA exposed in situ to UV. We found that UVA radiation (320–400 nm) produces detectable levels of dimers in the DNA of human skin. We also measured UVB-induced dimer yields in skin of individuals of differing sun sensitivity and found higher yields in individuals with higher UVB minimal erythema doses and greater sun sensitivity. These approaches should provide important information on damage induced in human skin upon exposure to natural or artificial sources of ultraviolet radiation.