The generation of hydrogen peroxide by the UVA irradiation of human lens proteins

Proposed Role for Internal Lens Pressure as an Initiator of Age-Related Lens Protein Aggregation Diseases

The process that initiates lens stiffness evident in age-related lens protein aggregation diseases is thought to be mainly the result of oxidation. While oxidation is a major contributor, the exposure of lens proteins to physical stress over time increases susceptibility of lens proteins to oxidative damage, and this is believed to play a significant role in initiating these diseases. Accordingly, an overview of key physical stressors and molecular factors known to be implicated in the development of age-related lens protein aggregation diseases is presented, paying particular attention to the consequence of persistent increase in internal lens pressure.

Acceleration of age-induced proteolysis in the guinea pig lens nucleus by in vivo exposure to hyperbaric oxygen: A mass spectrometry analysis

Hyperbaric oxygen (HBO) treatment of animals or ocular lenses in culture recapitulates many molecular changes observed in human age-related nuclear cataract. The guinea pig HBO model has been one of the best examples of such treatment leading to dose-dependent development of lens nuclear opacities. In this study, complimentary mass spectrometry methods were employed to examine protein truncation after HBO treatment of aged guinea pigs. Quantitative liquid chromatography-mass spectrometry (LC-MS) analysis of the membrane fraction of guinea pig lenses showed statistically significant increases in aquaporin-0 (AQP0) C-terminal truncation, consistent with previous reports of accelerated loss of membrane and cytoskeletal proteins. In addition, imaging mass spectrometry (IMS) analysis spatially mapped the acceleration of age-related αA-crystallin truncation in the lens nucleus. The truncation sites in αA-crystallin closely match those observed in human lenses with age. Taken together, our results suggest that HBO accelerates the normal lens aging process and leads to nuclear cataract.

An investigation of early radiation damage in rainbow trout eye-lenses

As part of the wider interest in the effects of ionizing radiation on non-human biota, this investigation was carried out to study early radiation damage to the eye-lenses of rainbow trout. Lenses were cultured and irradiated to doses of 1.1 Gy and 2.2 Gy with low-energy X-rays of 40 kV. Laser focal analysis was used to track changes in focal lengths across the lenses post-irradiation. Changes in focal length variability (FLV) were measured to determine whether this could give an indication of the early effects of radiation on lens health. No statistically significant differences in FLV between the control and irradiated lenses within 10 days post-irradiation were observed. FLV was found to be 0.09 ± 0.02 mm for 2.2 Gy lenses, 0.06 ± 0.01 mm for 1.1 Gy lenses, and 0.11 ± 0.02 mm for control lenses at the end of the observation period.

Cataract Development by Exposure to Ultraviolet and Blue Visible Light in Porcine Lenses

Background and Objectives: Cataract is still the leading cause of blindness. Its development is well researched for UV radiation. Modern light sources like LEDs and displays tend to emit blue light. The effect of blue light on the retina is called blue light hazard and is studied extensively. However, its impact on the lens is not investigated so far. Aim: Investigation of the impact of the blue visible light in porcine lens compared to UVA and UVB radiation. Materials and Methods: In this ex-vivo experiment, porcine lenses are irradiated with a dosage of 6 kJ/cm2 at wavelengths of 311 nm (UVB), 370 nm (UVA), and 460 nm (blue light). Lens transmission measurements before and after irradiation give insight into the impact of the radiation. Furthermore, dark field images are taken from every lens before and after irradiation. Cataract development is illustrated by histogram linearization as well as faults coloring of recorded dark field images. By segmenting the lens in the background's original image, the lens condition before and after irradiation could be compared. Results: All lenses irradiated with a 6 kJ/cm2 reveal cataract development for radiation with 311 nm, 370 nm, and 460 nm. Both evaluations reveal that the 460 nm irradiation causes the most cataract. Conclusion: All investigated irradiation sources cause cataracts in porcine lenses-even blue visible light.

Evidence of Highly Conserved β-Crystallin Disulfidome that Can be Mimicked by In Vitro Oxidation in Age-related Human Cataract and Glutathione Depleted Mouse Lens

Low glutathione levels are associated with crystallin oxidation in age-related nuclear cataract. To understand the role of cysteine residue oxidation, we used the novel approach of comparing human cataracts with glutathione-depleted LEGSKO mouse lenses for intra- versus intermolecular disulfide crosslinks using 2D-PAGE and proteomics, and then systematically identified in vivo and in vitro all disulfide forming sites using ICAT labeling method coupled with proteomics. Crystallins rich in intramolecular disulfides were abundant at young age in human and WT mouse lens but shifted to multimeric intermolecular disulfides at older age. The shift was ∼4x accelerated in LEGSKO lens. Most cysteine disulfides in β-crystallins (except βA4 in human) were highly conserved in mouse and human and could be generated by oxidation with H(2)O(2), whereas γ-crystallin oxidation selectively affected γC23/42/79/80/154, γD42/33, and γS83/115/130 in human cataracts, and γB79/80/110, γD19/109, γF19/79, γE19, γS83/130, and γN26/128 in mouse. Analysis based on available crystal structure suggests that conformational changes are needed to expose Cys42, Cys79/80, Cys154 in γC; Cys42, Cys33 in γD, and Cys83, Cys115, and Cys130 in γS. In conclusion, the β-crystallin disulfidome is highly conserved in age-related nuclear cataract and LEGSKO mouse, and reproducible by in vitro oxidation, whereas some of the disulfide formation sites in γ-crystallins necessitate prior conformational changes. Overall, the LEGSKO mouse model is closely reminiscent of age-related nuclear cataract.

Tea Flavanols Block Advanced Glycation of Lens Crystallins Induced by Dehydroascorbic Acid

Growing evidence has shown that ascorbic acid (ASA) can contribute to protein glycation and the formation of advanced glycation end products (AGEs), especially in the lens. The mechanism by which ascorbic acid can cause protein glycation probably originates from its oxidized form, dehydroascorbic acid (DASA), which is a reactive dicarbonyl species. In the present study, we demonstrated for the first time that four tea flavanols, (-)-epigallocatechin 3-O-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin 3-O-gallate (ECG), and (-)-epicatechin (EC), could significantly trap DASA and consequently form 6C- or 8C-ascorbyl conjugates. Among these four flavanols, EGCG exerted the strongest trapping efficacy by capturing approximate 80% of DASA within 60 min. We successfully purified and identified seven 6C- or 8C-ascorbyl conjugates of flavanols from the chemical reaction between tea flavanols and DASA under slightly basic conditions. Of which, five ascorbyl conjugates, EGCGDASA-2, EGCDASA-2, ECGDASA-1, ECGDASA-2 and ECDASA-1, were recognized as novel compounds. The NMR data showed that positions 6 and 8 of the ring A of flavanols were the major active sites for trapping DASA. We further demonstrated that tea flavanols could effectively inhibit the formation of DASA-induced AGEs via trapping DASA in the bovine lens crystallin-DASA assay. In this assay, 8C-ascorbyl conjugates of flavanols were detected as the major adducts using LC-MS. This study suggests that daily consumption of beverages containing tea flavanols may prevent protein glycation in the lens induced by ascorbic acid and its oxidized products.

Influence of the dark/light rhythm on the effects of UV radiation in the eyestalk of the crab Neohelice granulata

Crustaceans are interesting models to study the effects of ultraviolet (UV) radiation, and many species may be used as biomarkers for aquatic contamination of UV radiation reaching the surface of the Earth. Here, we investigated cell damage in the visual system of crabs Neohelice granulata that were acclimated to either 12L:12D, constant light, or constant dark, and were exposed to UVA or UVB at 12:00h (noon). The production of reactive oxygen species (ROS), antioxidant capacity against peroxyl radicals (ACAP), lipid peroxidation (LPO) damage, catalase activity, and pigment dispersion in the eye were evaluated. No significant differences from the three groups of controls (animals acclimated to 12L:12D, or in constant light, or not exposed to UV radiation) were observed in animals acclimated to 12L:12D, however, crabs acclimated to constant light and exposed to UV radiation for 30min showed a significant increase in ROS concentration, catalase activity, and LPO damage, but a decrease in ACAP compared with the controls. Crabs acclimated to constant darkness and exposed to UV for 30min showed a significantly increased ROS concentration and LPO damage, but the ACAP and catalase activity did not differ from the controls (animals kept in the dark while the experimental group was being exposed to UV radiation). Pigment dispersion in the pigment cells of eyes of animals acclimated to constant light was also observed. The results indicate that UVA and UVB alter specific oxidative parameters; however, the cell damage is more evident in animals deviated from the normal dark/light rhythm.

Lens aging: effects of crystallins

The primary function of the eye lens is to focus light on the retina. The major proteins in the lens--alpha, beta, and gamma-crystallins--are constantly subjected to age-related changes such as oxidation, deamidation, truncation, glycation, and methylation. Such age-related modifications are cumulative and affect crystallin structure and function. With time, the modified crystallins aggregate, causing the lens to increasingly scatter light on the retina instead of focusing light on it and causing the lens to lose its transparency gradually and become opaque. Age-related lens opacity, or cataract, is the major cause of blindness worldwide. We review deamidation, and glycation that occur in the lenses during aging keeping in mind the structural and functional changes that these modifications bring about in the proteins. In addition, we review proteolysis and discuss recent observations on how crystallin fragments generated in vivo, through their anti-chaperone activity may cause crystallin aggregation in aging lenses. We also review hyperbaric oxygen treatment induced guinea pig and 'humanized' ascorbate transporting mouse models as suitable options for studies on age-related changes in lens proteins.

Measurement of lens protein aggregation in vivo using dynamic light scattering in a guinea pig/UVA model for nuclear cataract

The role of UVA radiation in the formation of human nuclear cataract is not well understood. We have previously shown that exposing guinea pigs for 5 months to a chronic low level of UVA light produces increased lens nuclear light scattering and elevated levels of protein disulfide. Here we have used the technique of dynamic light scattering (DLS) to investigate lens protein aggregation in vivo in the guinea pig/UVA model. DLS size distribution analysis conducted at the same location in the lens nucleus of control and UVA-irradiated animals showed a 28% reduction in intensity of small diameter proteins in experimental lenses compared with controls (P < 0.05). In addition, large diameter proteins in UVA-exposed lens nuclei increased five-fold in intensity compared to controls (P < 0.05). The UVA-induced increase in apparent size of lens nuclear small diameter proteins was three-fold (P < 0.01), and the size of large diameter aggregates was more than four-fold in experimental lenses compared with controls. The diameter of crystallin aggregates in the UVA-irradiated lens nucleus was estimated to be 350 nm, a size able to scatter light. No significant changes in protein size were detected in the anterior cortex of UVA-irradiated lenses. It is presumed that the presence of a UVA chromophore in the guinea pig lens (NADPH bound to zeta crystallin), as well as traces of oxygen, contributed to UVA-induced crystallin aggregation. The results indicate a potentially harmful role for UVA light in the lens nucleus. A similar process of UVA-irradiated protein aggregation may take place in the older human lens nucleus, accelerating the formation of human nuclear cataract.

Induction of ultraviolet cataracts in vitro: prevention by pyruvate

Ultraviolet (UV) radiation is one of the important cataract risk factors. The present studies examined the hypothesis that this effect is due to the UV penetration through the cornea and subsequent induction of a photochemical generation of reactive species of oxygen (ROS) in the aqueous and lens. The hypothesis was ascertained by rat lens organ culture studies conducted under UV (365 nm), with media containing micromolar levels of riboflavin, with and without pyruvate, the latter acting as an ROS scavenger. The implication of ROS in the UV-induced damage was confirmed by measurements of peroxide generation. Damage to the lens was assessed physiologically by measuring the decrease in its active transport of rubidium ions. Biochemically, it was assessed by measuring the lowering of adenosine triphosphate and glutathione. The incorporation of pyruvate in the medium protected the lens against these deleterious effects. That the beneficial effect of pyruvate is attributable to its ROS-scavenging property was proven by the peroxide depletion in its presence, commensurate with its own utilization in parallel. A protective effect of this keto acid against UV-induced tissue damage has been shown for the first time, suggesting its clinical usefulness against UV irradiation-induced pathologies. Hence, further studies on the possible protective effects of such alpha-keto acids against UV damage are in progress.

Measurement of protection afforded by ultraviolet-absorbing window film using an in vitro model of photodamage

BACKGROUND AND OBJECTIVES

The effects of chronic sun damage including telangiectasias, solar lentigos, rhytides, enlarged pores, sagging skin, and pre-cancerous and cancerous growths are among the most common presenting complaints in a dermatologist's office. These changes are often worse on the driver's side of the face, emphasizing the role of UVA exposure received while driving in producing these changes. This study was undertaken to measure the ability of car window glass alone and in combination with ultraviolet (UV)-absorbing film to reduce UV-damage as measured using an established in vitro model of photoprotection. STUDY DESIGN MATERIALS AND METHODS: Using the 3T3 neutral red uptake photoprotection assay with solar simulating radiation (SSR) administered by a xenon arc solar simulator, we measured the photoprotection ability of auto glass, window film that filters UV radiation, and the combination of window film and auto glass.

RESULTS

As measured by the 3T3 neutral red uptake photoprotection assay, auto glass reduced cell death from SSR by 29%, while window film reduced it 90%, and the combination of auto glass and film reduced cell death by 93%, when compared to unfiltered SSR.

CONCLUSIONS

Window film that filters UV radiation results in dramatic reductions in cytotoxicity when measured by the neutral red uptake photoprotection assay. Widespread use of window film provides an ever-present barrier to ultraviolet A (UVA) exposure and could potentially reduce the detrimental effects of UVA, including photoaging, skin cancer, and ocular damage, such as cataracts. In addition, such film is essential for patients suffering from conditions sensitive to UV radiation, such as lupus erythematosis.

UVA-induced oxidative damage and cytotoxicity depend on the mode of exposure

The reciprocity rule (Bunsen-Roscoe law) states that a photochemical reaction is directly proportional to the total energy dose, irrespective of the dose distribution. In photomedicine the validity of this law is usually taken for granted, although the influence of radiation intensity and dose distribution are largely unknown. We have examined in a tissue culture model the effects of fractionated versus single dose exposure to UV from a metal halide source on survival, DNA synthesis, glutathione, and oxidative membrane damage. Exposure to fractionated UVA was followed by an increased rate of cell death compared to single dose exposure, when intervals between fractions where short (10-120 min). Longer intervals had the opposite effect. Corresponding results were obtained for DNA synthesis (BrdU incorporation). The increased cytotoxicity of dose fractionation with short intervals could not be abrogated by non-enzymatic antioxidants (astaxanthin, ascorbic acid, alpha-tocopherol). Fractionated irradiation with short intervals led to higher degree of depletion of glutathione (GSH) and to enhanced formation of thiobarbituric acid reactive substances (TBARS) in comparison to an identical single dose. Long intervals between fractions induced opposite effects. Taken together, these data indicate that immediately after UVA exposure cells are more sensitive to a further oxidative attack making repeated exposure with short intervals more cytotoxic than continuous single dose UVA. This might have implications also for responses to UVA in vivo and further studies will have to extend these findings to the situation in healthy and diseased human skin.

Age-related nuclear cataract—oxidation is the key

Age is by far the biggest risk factor for cataract, and it is sometimes assumed that cataract is simply an amplification of this aging process. This appears not to be the case, since the lens changes associated with aging and cataract are distinct. Oxidation is the hallmark of age-related nuclear (ARN) cataract. Loss of protein sulfhydryl groups, and the oxidation of methionine residues, are progressive and increase as the cataract worsens until >90% of cysteine and half the methionine residues are oxidised in the most advanced form. By contrast, there may be no significant oxidation of proteins in the centre of the lens with advancing age, even past age 80. The key factor in preventing oxidation seems to be the concentration of nuclear glutathione (GSH). Provided that nuclear GSH levels can be maintained above 2 mm, it appears that significant protein oxidation and posttranslational modification by reactive small molecules, such as ascorbate or UV filter degradation products, is not observed. Adequate coupling of the metabolically-active cortex, the source of antioxidants such as GSH, to the quiescent nucleus, is crucial especially since it would appear that the cortex remains viable in old lenses, and even possibly in ARN cataract lenses. Therefore it is vital to understand the reason for the onset of the lens barrier. This barrier, which becomes apparent in middle age, acts to impede the flow of small molecules between the cortex and the nucleus. The barrier, rather than nuclear compaction (which is not observed in human lenses), may contribute to the lowered concentration of GSH in the lens nucleus after middle age. By extending the residence time within the lens centre, the barrier also facilitates the decomposition of intrinsically unstable metabolites and may exacerbate the formation of H(2)O(2) in the nucleus. This hypothesis, which is based on the generation of reactive oxygen species and reactive molecules within the nucleus itself, shifts the focus away from theories for cataract that postulated a primary role for oxidants generated outside of the lens. Unfortunately, due to marked variability in the lenses of different species, there appears at present to be no ideal animal model system for studying human ARN cataract.

Long-term lens organ culture system to determine age-related effects of UV irradiation on the eye lens

Aging of the eye lens represents the life-long accumulation of damage. Factors responsible for age-related cataract are unknown because medical evaluations of aged populations demonstrate a wide range of systemic diseases and medical disorders. There are some main suspected factors, which may contribute to accumulated age-related damage in the eye lens. (1) Diseases, such as diabetes, substantially increase the probability of cataract formation in the age group from 40 to 49, and double or triple this probability for ages 50 to 69. (2) Drugs, including systemic medications such as steroids. (3) Environmental factors, such as UV radiation, heat and electromagnetic radiation. Our study represents an effort to determine the effects of suspected cataractogenic factors on the eye lens. The experiments are performed using a unique long-term lens organ culture system of bovine lenses. In our system it is possible to give controlled amounts of insult and monitor changes in lens optical quality throughout the culture period of 8-15 days. The optical properties, monitored in association with biochemical analysis of lens epithelium, cortex and nuclear samples, help in determining the mechanisms of cataract formation. The present study investigates mechanisms by which UV-A radiation at 365 nm causes damage to the lens. It is believed that solar radiation is one of the major environmental factors involved in lens cataractogenesis. Bovine lenses were placed in our special culture cells for pre-incubation of 24 hr followed by irradiation of 29 or 33 J cm(-2). The lenses were maintained in the cells during irradiation. After irradiation, lens optical quality was monitored throughout the culture period and lens epithelium was taken for enzyme analysis. Using the culture system we learned that: (a) young lenses (less than one-year-old) are less sensitive to UV radiation than 3-year-old lenses; (b) the lenses have the ability to recover in organ culture conditions; (c) applying the insult in one step results in less damage than dividing the same insult in 4 steps with 24 hr interval between each one; and (d) the damage from UV is greater if the intervals between each irradiation stage are insufficient to permit full recovery.

Protein-bound kynurenine is a photosensitizer of oxidative damage

Human lens proteins become progressively modified by tryptophan-derived UV filter compounds in an age-dependent manner. One of these compounds, kynurenine, undergoes deamination at physiological pH, and the product binds covalently to nucleophilic residues in proteins via a Michael addition. Here we demonstrate that after covalent attachment of kynurenine, lens proteins become susceptible to photo-oxidation by wavelengths of light that penetrate the cornea. H2O2 and protein-bound peroxides were found to accumulate in a time-dependent manner after exposure to UV light (lambda > 305-385 nm), with shorter-wavelength light giving more peroxides. Peroxide formation was accompanied by increases in the levels of the protein-bound tyrosine oxidation products dityrosine and 3,4-dihydroxyphenylalanine, species known to be elevated in human cataract lens proteins. Experiments using D2O, which enhances the lifetime of singlet oxygen, and azide, a potent scavenger of this species, are consistent with oxidation being mediated by singlet oxygen. These findings provide a mechanistic explanation for UV light-mediated protein oxidation in cataract lenses, and also rationalize the occurrence of age-related cataract in the nuclear region of the lens, as modification of lens proteins by UV filters occurs primarily in this region.

Peripheral light focusing as a potential mechanism for phakic dysphotopsia and lens phototoxicity

Our aim was to examine secondary image formation in the anterior segment caused by peripheral light focusing (PLF) in the human cornea, and in particular the crystalline lens. Non-sequential ray-tracing (OptiCAD) was applied to an anatomically based human eye model, which incorporates a gradient index crystalline lens. For analysis of the limbal effect, we varied the incident angle from 100 to 122 degrees, while for the crystalline lens effect, the incident angle was varied from 60 to 90 degrees. The corneal shapes studied included central radii from 7.4 to 8.2 mm with a range of shape factors. In each case, we computed the peak and average intensities, and the area of exposure at the limbus or lens periphery. The computation was repeated with a previous model eye for comparison. For the limbal effect, a peak intensity gain of x22.5 was found at an incident angle of 104 degrees which compares well with previous results. The average intensity gain at this angle was x7.5 over an area of 0.23 mm2. Steeper corneal curvature produced a greater PLF effect. For the crystalline lens effect, maximum UVA (365 nm) intensity gain peaked at x8.6 at 84 degrees with average intensity gain of x2.3. The area of UVA exposure peaked at 4.7 mm2 at 70 degrees. A relatively wide range (30 degrees ) of incident angles produced peak PLF gains of x3 or more in the lens. Significant focusing of light is directed to the nasal limbus, and to a lesser extent to the crystalline lens over a broad range of incident angles. PLF in the nasal cornea is reduced by an order of magnitude when a UV-blocking soft contact lens is used. The concentration levels and intraocular sites of PLF action on UV and visible light suggest a new mechanism of phakic dysphotopsia and lens phototoxicity.

The effects of sub-solar levels of UV-A and UV-B on rabbit corneal and lens epithelial cells

The purpose of this work was to establish whether exposing cultured rabbit corneal and lens epithelial cells to ultraviolet radiation equivalent to several hours under the sun would damage the cells. Confluent rabbit corneal epithelial cells were irradiated with broadband UV-A or UV-B, and confluent lens epithelial cells were irradiated with broadband UV-A. The maximum dose of UV-A was 6.3 J cm(-2) and that of UV-B was 0.60 J cm(-2). Damage to corneal epithelial cell was studied using the terminal deoxynucleotidyl transferase mediated dUTP-X nick end labeling (TUNEL) assay and damage to lens epithelial cell was studied using the single cell gel electrophoresis (comet) assay and trypan blue exclusion assay. Lipid peroxidation was assayed using the thiobarbituric acid reaction. Both UV-B and UV-A induced cell death in corneal epithelial cells with different latent periods. UV-A damage included cell death, decreased viability and increased lipid peroxidation of lens epithelial cell. In addition, UV irradiation of the corneal and lens epithelial cells decreased the activity of catalase to thirty to fifty percent of its original value, while the activities of glutathione peroxidase and superoxide dismutase did not decrease within experimental error. Thus, even sub-solar UV radiation can cause irreversible damage to corneal and lens epithelial cells.

Ultraviolet radiation modulates nuclear factor kappa B activation in human lens epithelial cells

Exposure to ultraviolet radiation (UVR) is a known risk factor for cataract, but the molecular mechanisms involved have not been elucidated. We hypothesized that exposure to UVR would modulate the activation of nuclear factor kappa-B (NF-kappa B) within the human lens epithelium, since NF-kappa B is a key regulator of cellular responses to UVR stress in other cell types. Human lens epithelial (HLE) cells were exposed to acute physiological doses of ultraviolet A (UVAR), B (UVBR), C (UVCR) radiation, or interleukin-1 beta (IL-1 beta) and NF-kappa B activation was measured by electrophoretic shift assay (EMSA). Phosphorylation of I kappa B in response to UVAR was measured by Western blotting. Irradiation of HLE cells with UVAR (0-1100 J/m(2)) did not reduce cell survival, while UVBR (400-1600 J/m(2)) and UVCR (300-900 J/m(2)) significantly reduced HLE cell survival. EMSA analysis of HLE nuclear proteins indicated activation of NF-kappa B, but not activator protein-1 (AP-1), by UVAR. The effects of UVBR and UVCR were less pronounced. Exposure of HLE cells to UVAR (0-900 J/m(2)) followed by a 30-min incubation resulted in a dose-dependent activation of NF-kappa B. UVAR-induced NF-kappa B activation in HLE cells was evident 10 min postirradiation, maximal at 60 min and returned to control levels by 120 min. Western blot analysis of phosphorylation of the NF-kappa B inhibitory protein, I kappa B, revealed that UVAR activates NF-kappa B via a mechanism involving the phosphorylation of I kappa B-alpha; this effect was dose-dependent. Supershift analysis demonstrated that UVAR and IL-1 beta activate the transcriptionally active p65/p50 NF-kappa B dimer. These studies demonstrate that UVAR activates NF-kappa B in HLE cells in a time- and dose-dependent manner via signaling through I kappa B-alpha. The activation of NF-kappa B in HLE cells by UVAR may have implications for the development and progression of cataract and other related ocular disorders.

A structural study of the retinal photoreceptor, plexiform and ganglion cell layers following exposure to UV-B and UV-C radiation in the albino rat

Over the last two decades, ultraviolet radiation levels (UV), reaching the Earth's surface, have been increasing at a rate of 1.5% per each 1% loss of the ozone layer. Moreover, artificial UV-sources have also proliferated and contributed to the rising UV-stress that many organisms have to face. To assess how the vertebrate retina responds to an exposure of short wavelength UV, we focused our attention on the rat retina, observing photoreceptor (containing outer and inner segments of rods and cones), inner plexiform, and ganglion cell layers by light and transmission electron microscopy using conventional and cytochemical techniques. We analyzed how cells of the layers in question responded to a 30 min exposure to UV-C and UV-B radiation with doses of 7200 and 590 J/cm(2), respectively. The results show that there are significant changes in the nuclei and cytoplasmic organelles of the exposed retinae when compared with those of the unexposed controls. The changes include an increase in heterochromatin, distension of rough endoplasmic reticulum, mitochondrial disruptions, and increases in the number of myelin bodies. The recorded morphological changes, especially those of the ganglion cells, are suggestive of apoptotic processes and show that the exposure of vertebrate retina to wavelengths ranging from 254 to 312 nm can produce alterations that are likely to impact negatively on the retina's proper functioning.

Separation of the yellow chromophores in individual brunescent cataracts

Quantitative changes in the 330 nm absorbing chromophores and 350/450 nm fluorophores of water-soluble (WS) and water-insoluble (WI) proteins of individual human cataract lenses were characterized and compared with aged normal human lens. Twenty-five brunescent cataract lenses from India were selected from five different stages (types I-V) based upon the color of the lens. The WS and WI proteins from each lens were collected and subjected to an extensive enzymatic digestion procedure under argon. The lens protein digests were separated by Bio-Gel P-2 size-exclusion chromatography and individual peaks were analyzed further by reversed-phase HPLC. The total WI proteins increased and the total WS protein decreased with the development of cataract, especially in the late stages of cataract (III-V). The total 330 nm absorbance and 350/450 nm fluorescence of the WI fraction also increased, however, the A(330) and fluorescence per mg lens protein were constant except for type V (black) lenses. Bio-Gel P-2 chromatography separated the chromophores and fluorophores into four fractions. The main fraction (designated as peak 2+3) from the cataract WI proteins was several times higher than that present in aged normal human lens WI proteins. A significant increase of this fraction was observed in WI proteins, but not in WS proteins with cataract development. Similarly, fractions 1 and 4 in the WI proteins also increased gradually but fraction 5 did not. Reversed-phase HPLC resolved fraction (2+3) of the water-insoluble sonicate supernatant proteins into four 330 nm absorbing peaks and eight fluorescent peaks. Among these peaks, a late-eluting peak (peak 8) increased 10 to 15-fold with the progress of cataract, and accounted for 80% of the total chromophores in type V lenses. This peak may represent limit digests of advanced glycation end-products (AGEs) derived protein cross-links. HPLC profiles of fraction 5 from both WS and WI proteins showed numerous new peaks which were not observed in either WS protein from cataract or WI proteins from aged normal human. The severe coloration and the higher levels of numerous novel chromophores and fluorophores in brunescent cataractous lenses reveal the possibility that a different chemistry occurs during cataract development.

Advanced glycation endproducts as UVA photosensitizers of tryptophan and ascorbic acid: consequences for the lens

Upon aging, the lens accumulates brown fluorophores, mainly derived from the Maillard reaction between vitamin C oxidation products and crystallins lysine residues. At the same time, the concentration of UVA filters decreases, allowing some radiation to be absorbed by lenticular advanced glycation endproducts (AGEs). This paper quantifies the photosensitizing activity of AGEs at various oxygen pressures, and compares it to that of lenticular riboflavin (RF). Solutions containing the sensitizer and the substrates tryptophan (Trp) and ascorbate (AH(-)) were irradiated at 365 nm. We show that the AGEs-photosensitized Trp oxidation rate increases with AGEs concentration and is optimal at 5% oxygen, the pressure in the lens. By contrast, for AH(-), the photooxidation rate increases with oxygen concentration. Despite the higher quantum yield of RF-depending reactions, its low concentration as compared to that of AGEs in aging lenses induces significantly higher Trp and AH(-) photodegradation rates with AGEs than with RF. As ascorbate is more rapidly photodegraded than Trp, the antioxidant competitively protects Trp from oxidation up to 1 mM, although not absolutely. We conclude that in the aging lens, AH(-) exerts a strong UVA protecting activity, but does not impede some Trp residue to be photodegraded proportionally to the AGEs concentration.

Impairment of eye lens cell physiology and optics by broadband ultraviolet A-ultraviolet B radiation

The phototoxicity of ultraviolet A (UVA) alone and UVA plus ultraviolet B (UVB) combined on cultured porcine lenses was investigated by analyzing cellular function as measured with a fluorescence bioassay approach and optical integrity, in terms of sharpness of the lens focus as measured with a scanning laser system. The bioassay consisted of carboxyfluorescein diacetate-acetoxymethyl ester and alamarBlue fluorescent dyes. Aseptically dissected porcine lenses were maintained in modified medium 199 without phenol red supplemented with 1% penicillin-streptomycin and 4% porcine serum. At 1 week of preincubation, baseline measurements were obtained. Then the lenses were treated with single exposures of different UVA and UVB energy levels. The lenses treated with 86 J/cm2 UVA alone showed a significant (P < 0.05) decrease in cellular and optical integrity at 48 h after exposure, whereas those treated with 43 J/cm2 UVA alone did not show significant phototoxic effect. Lenses treated with 15.63 J/cm2 UVA plus 0.019 J/cm2 UVB combined showed significant adverse effects beginning from 48 h after exposure. Also, there was no recovery. These findings show that a high UVA dose alone and relatively low UVA in combination with low UVB radiant exposure can impair lens cellular and optical functions, respectively.

Rate of formation of AGEs during ascorbate glycation and during aging in human lens tissue

The similarity of the yellow chromophores isolated from human cataracts with those from ascorbic acid modified calf lens proteins was recently published [Biochim. Biophys. Acta 1537 (2001) 14]. The data presented here additionally quantify age-dependent increases in individual yellow chromophores and fluorophores in the water-insoluble fraction of normal human lens. The water-insoluble fraction of individual normal human lens was isolated, solubilized by sonication and digested with a battery of proteolytic enzymes under argon to prevent oxidation. The level of A(330)-absorbing yellow chromophores, 350/450 nm fluorophores and total water-insoluble (WI) protein were quantified in each lens. The total yellow chromophores and fluorophores accumulated in parallel with the increase in the water-insoluble protein fraction during aging. The digest from each single human lens was then subjected to Bio-Gel P-2 size-exclusion chromatography. The fractions obtained were further separated by a semi-preparative prodigy C-18 high-performance liquid chromatography (RP-HPLC). Bio-Gel P-2 chromatography showed four major fractions, each of which increased with age. RP-HPLC of the amino acid peak resolved five major A(330)-absorbing peaks and eight fluorescent peaks, and each peak increased coordinately with age. A late-eluting peak, which contained hydrophobic amino acids increased significantly after age 60. Aliquots from an in vitro glycation of calf lens proteins by ascorbic acid were removed and subjected to the same enzymatic digestion. Ascorbic acid-modified calf lens protein digests showed an almost identical profile of chromophores, which also increased in a time-dependent manner. The late-eluting peak, however, did not increase with the time of glycation and may not be an advanced glycation endproduct (AGE) product. The data indicate that the total water-insoluble proteins, individual yellow chromophores and fluorophores increased equally both with aging in normal human lens and during ascorbate glycation in vitro. The major protein modifications, which accumulate during aging, therefore, appear to be AGEs. Whereas the late-eluting peak, which showed poor correlation to ascorbylation, may represent UV filter compounds bound to lens proteins.

Studies on singlet oxygen formation and UVA light-mediated photobleaching of the yellow chromophores in human lenses

The protein-bound chromophores, which increase with aging in the human lens, act as UVA sensitizers, producing almost exclusively singlet oxygen in vitro. Direct irradiation of whole, aged human lenses with high intensity UVA light (200 mW cm(-2) for 24 hr), however, failed to produce singlet oxygen damage, as evidenced by the lack of either His or Trp photodestruction. Total homogenates of human lenses prepared in a cuvette under air did show destruction of His and Trp residues by UVA light, but no destruction was seen when equivalent homogenates were prepared under argon. These data are consistent with the idea that the low oxygen levels in the lens prevent singlet oxygen damage in vivo.UVA irradiation of aged human lenses in culture caused an extensive photobleaching of the yellow chromophores. A time course indicated that the photobleaching increased with time, with significant color loss apparent after 6 hr. Homogenization of the irradiated and dark control lenses in 6 M guanidine-HCl, followed by determination of the difference spectrum, showed approximately 50% bleaching of compounds with a lambda(max) at 355 nm. Similarly, fluorophores with a lambda(max) for excitation of 355 nm and for emission of 420 nm were 50% destroyed by the UVA light. Similar results were obtained in vitro by the anaerobic irradiation of a sonication-solubilized WI fraction from type II brunescent cataracts and from aged human lenses. In this system, there was an initial bleaching of 15% after 30 min of irradiation, followed by a slow increase over the next 6 hr to a final bleaching of 30%. The addition of 1.0 m M ascorbic acid, but not 1.0 m M glutathione (GSH), increased the photobleaching to 60% under argon, and the loss of ascorbate could be detected under these anaerobic conditions. In the presence of air, UVA light produced no photobleaching, but rather caused a three-fold increase in absorbance at 345 nm, which was prevented by the inclusion of 1.0 m M ascorbic acid and almost 50% inhibited by 1.0 m M GSH. The data are consistent with the conversion of the triplet state of the sensitizers to anion and cation radicals in the absence of oxygen. Photobleaching may occur either by dismutation of the anion radical or by reduction of the anion radical by ascorbate via type I chemistry. UVA irradiation of an enriched fraction of sensitizers from a proteolytic digest from type II cataract lenses produced a 63% bleaching at 330 nm in the absence of oxygen, and the almost complete loss of the A(330) absorbing and 350/450 nm fluorescent peaks upon HPLC separation. This loss correlated with the loss of the ability of the irradiated fraction to produce singlet oxygen in vitro upon subsequent UVA irradiation.

Similarity of the yellow chromophores isolated from human cataracts with those from ascorbic acid-modified calf lens proteins: evidence for ascorbic acid glycation during cataract formation

Chromatographic evidence supporting the similarity of the yellow chromophores isolated from aged human and brunescent cataract lenses and calf lens proteins ascorbylated in vitro is presented. The water-insoluble fraction from early stage brunescent cataract lenses was solubilized by sonication (WISS) and digested with a battery of proteolytic enzymes under argon to prevent oxidation. Also, calf lens proteins were incubated with ascorbic acid for 4 weeks in air and submitted to the same digestion. The percent hydrolysis of the proteins to amino acids was approximately 90% in every case. The content of yellow chromophores was 90, 130 and 250 A(330) units/g protein for normal human WISS, cataract WISS and ascorbate-modified bovine lens proteins respectively. Aliquots equivalent to 2.0 g of digested protein were subjected to size-exclusion chromatography on a Bio-Gel P-2 column. Six peaks were obtained for both preparations and pooled. Side by side thin-layer chromatography (TLC) of each peak showed very similar R(f) values for the long wavelength-absorbing fluorophores. Glycation with [U-(14)C]ascorbic acid, followed by digestion and Bio-Gel P-2 chromatography, showed that the incorporated radioactivity co-eluted with the A(330)-absorbing peaks, and that most of the fluorescent bands were labeled after TLC. Peaks 2 and 3 from the P-2 were further fractionated by preparative Prodigy C-18 reversed-phase high-performance liquid chromatography. Two major A(330)-absorbing peaks were seen in peak 2 isolated from human cataract lenses and 5 peaks in fraction 3, all of which eluted at the same retention times as those from ascorbic acid glycated calf lens proteins. HPLC fractionation of P-2 peaks 4, 5 and 6 showed many A(330)-absorbing peaks from the cataract WISS, only some of which were identical to the asorbylated proteins. The major fluorophores, however, were present in both preparations. These data provide new evidence to support the hypothesis that the yellow chromophores in brunescent lenses represent advanced glycation endproducts (AGEs) probably due to ascorbic acid glycation in vivo.

The optical properties of the anterior segment of the eye: implications for cortical cataract

Epidemiological studies have correlated cortical cataract with exposure to light and have suggested that this is due primarily to relatively short wavelengths of ultraviolet radiation (UV-B). In addition, some cellular and animal models also implicate UV-B. In order to evaluate the likely role of different wavelengths of light in the etiology of cortical cataracts, the optical characteristics of several animal models were ascertained and compared to the primate. This study shows that the mouse model absorbs UV-B almost exclusively whereas other animal models such as the rabbit and the guinea pig also contain chromophores that absorb UV-A. The absorptive characteristics of the human lens varies drastically with age. The young lens absorbs primarily UV-A, whereas with age, there are increases in absorptions at 320 nm and out to wavelengths as long as 550 nm. By sectioning human lenses it was found that these changes in absorption properties increased toward the central and the nuclear regions. These absorptive characteristics were then compared to the amount of light reaching the surface of the lens. It was found that UV-B is a minor component of total energy reaching the surface of the human lens and old human lens proteins absorb 2 orders of magnitude more UV-A and visible light than UV-B. It is concluded that it is premature to exclude UV-A or even visible light in the etiology of human cortical cataracts.

The effect of vitamin E acetate on ultraviolet-induced mouse skin carcinogenesis

Despite the benefits of sunscreens, ultraviolet (UV) exposure can still lead to skin cancer. In this study we investigated the effect of topical application of the antioxidant vitamin E acetate (VEA) on the inhibition of UV-induced carcinogenesis. Hairless SKH-1 mice received 5.2 mg of VEA 30 min before (VEA/UV) or after (UV/ VEA) a single minimal erythemic dose of UV light. Vehicle-control animals received acetone 30 min before UV exposure (Ace/UV). After 24 h, cyclobutane dimer repair was twofold and 1.5-fold greater in the UVNEA and VEA/UV groups, respectively. Expression of p53 protein in the UV/VEA group was maximum at 12 h after UV exposure, whereas in the Ace/UV- and VEA/UV-treated mice, maximum p53 immunostaining was statistically higher at 15 h (P = 0.03). DNA synthesis as determined by 5-bromo-2'-deoxyuridine incorporation was twofold higher after 15 h in all groups but was not statistically different among treatment groups. Protein levels of cyclin D1 and p21 were increased in both VEA groups by 6 h. In addition, VEA treatments delayed tumor formation and yield for the first 20 wk, although this difference was lost by 30 wk. The telomerase activity of carcinomas from the UV/VEA-treated mice was statistically lower than that of the Ace/UV-treated mice (P = 0.05). This study showed that although VEA may mitigate some of the initial events associated with UV irradiation such as DNA damage and p53 expression, it has limited potential in preventing UV-induced proliferation and tumor formation.

UVA irradiation of human lens proteins produces residual oxidation of ascorbic acid even in the presence of high levels of glutathione

The oxidation products of ascorbic acid (AscH-) can rapidly glycate and crosslink lens proteins in vitro, producing fluorophores and browning products similar to those present in cataractous lenses. The accumulation of AscH- oxidation products, however, would largely be prevented by the millimolar levels of glutathione (GSH) present in human lens. Here we investigate whether protein aggregation could allow the oxidation of AscH- by UVA-induced reactive oxygen species in the presence of physiological levels of GSH. The metal-catalyzed oxidation of 1.0 mM AscH- by 50 microM Cu(II) was almost complete after 1 h, but no oxidation was seen in the presence of GSH concentrations as low as 0.5 mM. UVA irradiation of protein aggregates from human lens, which accumulated more than 2.0 mM singlet oxygen after 1 h, caused a 50-60% oxidation of 1.0 mM AscH-. The addition of 204 mM GSH, however, decreased AscH- oxidation by less than half, and 30% of the AscH- was oxidized even in the presence of 15 mM GSH. This diminished protection may be due, in part, to the ability of AscH-, but not GSH, to penetrate to the sites of singlet oxygen generation located within the protein. Consistent with this hypothesis, greater GSH protection was seen when a proteolytic digest of the human proteins was subjected to the same irradiation or when singlet oxygen was chemically generated from 3-(4-methyl-1-naphthyl)propionic acid endoperoxide (MNPAE) at 37 degrees C in the medium. The addition of 50 microM Cu(II) had no effect on the rate of degradation of dehydroascorbic acid (DHA). Singlet oxygen, either UVA- or MNPAE-generated, increased the rate of DHA loss. This secondary oxidation of DHA by singlet oxygen would allow the accumulation of AscH- oxidation products was not reducible by GSH. Therefore, the data presented here argue that the protein aggregation seen in older human lenses may permit oxidized AscH--induced crosslinking to occur even at physiological GSH levels.

The relative UV sensitizer activity of purified advanced glycation endproducts

The oxidation products of ascorbic acid react with lens proteins to form advanced glycation endproducts (AGE) that are capable of generating reactive oxygen species when irradiated with UVA light. L-Threose, the most active of these oxidation products, was reacted with N-acetyl lysine and six AGE peaks were isolated by RP-HPLC. Each peak exhibited fluorescence and generated superoxide anion and singlet oxygen in response to UV light. Solutions of these AGE peaks (50 micrograms/mL) generated 5-10 nmol/mL of superoxide anion during a 30 min irradiation. This activity was 100-fold less than the superoxide anion generated by kynurenic acid and 400-fold less than riboflavin. Ultraviolet irradiation generated from 1.2 to 2.7 mumol/mL of singlet oxygen with the purified threose AGE compounds. This activity was similar to that seen with other purified AGE compounds (pentosidine, LM-1 and Ac-FTP) and with kynurenine and 3-OH kynurenine. This considerable singlet oxygen formation, however, was still 40-fold less than that obtained with kynurenic acid and 100-fold less than riboflavin under the same irradiation conditions. In spite of this lower sensitizer efficiency, the purified AGE generated 20-60-fold more singlet oxygen on a weight basis than either crude ascorbic acid glycated proteins or a preparation of water-insoluble proteins from aged normal human lenses. On a molar basis, therefore, AGE could account for the sensitizer activity in these protein preparations if they represented less than 1% of the total amino acids.

Quantitation of the singlet oxygen produced by UVA irradiation of human lens proteins

Ultraviolet irradiation of aged human lens proteins in vitro causes extensive photolytic damage of His and Trp residues. Protection by sodium azide argues for a process mediated by singlet oxygen (1O2). In the work described here, the synthesis of 1O2 was measured by the bleaching of N,N-dimethyl-4-nitrosoaniline (RNO), the oxidation of added histidine and the oxidation of furfuryl alcohol. To obtain a more accurate value for 1O2 generation, a known quantity of 1O2 was generated by the thermal dissociation of 3-(4-methyl-naphthyl)propionic acid endoperoxide, and the efficiency of each assay method to report on the 1O2 generated was determined. The values obtained were 0.003 mol of RNO bleached/mol of 1O2 generated, 0.55 mol of furfuryl alcohol oxidized/mol 1O2 and 0.5 mol of His oxidized/mol 1O2 generated. Irradiation of the human lens proteins with UVA light produced from 2.1 to 2.4 mM of 1O2 by RNO bleaching, 2.6-2.8 mM 1O2 by furfuryl alcohol oxidation and up to 1.9 mM of 1O2 by histidine oxidation during a 1 h irradiation period. The average value (2.2 mM of 1O2) corresponds to the theoretical production of 30 nmol of singlet oxygen at UVA light intensities equivalent to a 1 h exposure to sunlight at noon in the northern hemisphere.

8-Oxodeoxyguanosine formation in the DNA of cultured cells after exposure to H2O2 alone or with UVB or UVA irradiation

The objective of the present study was to establish whether H2O2 alone or in the presence of UVA or UVB would give rise to formation of the oxidatively damaged DNA base 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) in cultured adult rat liver (ARL-18) epithelial cells. Hydrogen peroxide alone at 5 mM increased 8-oxo-dG levels by 42% of that of culture control. Compared to culture control, UVB exposure at a dose of 0.63 J/cm2 elevated 8-oxo-dG levels only 8.4%. In the presence of 5 mM H2O2 + UVB (0.63 J/cm2), 8-oxo-dG levels were elevated 155% above culture control suggesting a synergistic effect. A UVA dose of 10 J/cm2 did not elevate 8-oxo-dG levels above culture control. In the presence of 5 mM H2O2 plus UVA (12 J/cm2), 8-oxo-dG levels were elevated 310% above controls compared with an increase of 75.8% above control levels at the same dose in the absence of H2O2. These results reveal that both UVA or UVB can promote H2O2 generation of reactive oxygen species (ROS) in whole cells resulting in an increase in the formation of 8-oxo-dG, although the photodynamic generation of ROS from H2O2 occurs with a much higher efficiency in the presence of UVB. Our study also demonstrates that 8-oxo-dG can be generated in cellular DNA of whole cells exposed to H2O2 and UVA or UVB, indicating that the ROS generated in whole cell systems are long enough lived to migrate to the nucleus and cause DNA damage.

Quantitation of the reactive oxygen species generated by the UVA irradiation of ascorbic acid-glycated lens proteins

The oxidation products of ascorbic acid rapidly glycate proteins and produce protein-bound, advanced glycation endproducts. These endproducts can absorb UVA light and cause the photolytic oxidation of proteins (Ortwerth, Linetsky and Olesen, Photochem. Photobiol. 62, 454-463, 1995), which is mediated by the formation of reactive oxygen species. A dialyzed preparation of calf lens proteins, which had been incubated for 4 weeks with 20 mM ascorbic acid in air, was irradiated for 1 h with 200 mW/cm2 of absorbed UVA light (gamma > 338 nm), and the concentration of individual oxygen free radicals was measured. Superoxide anion attained a level of 76 microM as determined by the superoxide dismutase (SOD)-dependent increase in hydrogen peroxide formation and of 52 microM by the SOD-inhibitable reduction of cytochrome c. Hydrogen peroxide formation increased linearly to 81 microM after 1 h. Neither superoxide anion nor hydrogen peroxide, however, could account for the UVA photolysis of Trp and His seen in this system. Singlet oxygen levels approached 1.0 mM as measured by the oxidation of histidine, which was consistent with singlet oxygen measurements by the bleaching of N,N-dimethyl-4-nitrosoaniline. High concentrations of sodium azide, a known singlet oxygen quencher, inhibited the photolytic destruction of both His and Trp. Little or no protein damage could be ascribed to hydroxyl radical based upon quenching experiments with added mannitol. Therefore, superoxide anion and H2O2 were generated by the UVA irradiation of ascorbate advanced glycation endproducts, however, the major reactive oxygen species formed was singlet oxygen.