"Unscheduled" DNA synthesis in lens epithelium following ultraviolet irradiation

The Role of the Reactive Oxygen Species and Oxidative Stress in the Pathomechanism of the Age-Related Ocular Diseases and Other Pathologies of the Anterior and Posterior Eye Segments in Adults

The reactive oxygen species (ROS) form under normal physiological conditions and may have both beneficial and harmful role. We search the literature and current knowledge in the aspect of ROS participation in the pathogenesis of anterior and posterior eye segment diseases in adults. ROS take part in the pathogenesis of keratoconus, Fuchs endothelial corneal dystrophy, and granular corneal dystrophy type 2, stimulating apoptosis of corneal cells. ROS play a role in the pathogenesis of glaucoma stimulating apoptotic and inflammatory pathways on the level of the trabecular meshwork and promoting retinal ganglion cells apoptosis and glial dysfunction in the posterior eye segment. ROS play a role in the pathogenesis of Leber's hereditary optic neuropathy and traumatic optic neuropathy. ROS induce apoptosis of human lens epithelial cells. ROS promote apoptosis of vascular and neuronal cells and stimulate inflammation and pathological angiogenesis in the course of diabetic retinopathy. ROS are associated with the pathophysiological parainflammation and autophagy process in the course of the age-related macular degeneration.

Polymorphisms of DNA repair genes XPD and XRCC1 and risk of cataract development

The association between oxidative or ultraviolet (UV) light induced DNA damage in the lens epithelium and the development of lens opacities, and the existence of DNA repair in lens epithelial cells have been reported. Polymorphisms of DNA repair enzymes may affect repair efficiency. In this study, we aimed to determine the frequency of polymorphisms in two DNA repair enzyme genes, xeroderma pigmentosum complementation group D (XPD) codon 751 and X-ray cross-complementing group 1 (XRCC1) codon 399, in a sample of Turkish patients with maturity onset cataract. By using polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP), we analysed XRCC1-Arg399Gln and XPD-Lys751Gln polymorphisms in 195 patients with cataract (75 patients with cortical, 53 with nuclear, 37 with posterior subcapsular, and 30 with mixed type) and in 194 otherwise healthy control group of similar age. There was a significant difference between frequencies for XPD-751 Gln/Gln genotype in cataract patients (12%) and healthy controls (20%) (P=0.008, OR=0.40, 95% CI=0.20-0.81). After stratification by the cataract subtypes, XPD-751 Gln/Gln genotype was found to be significantly different in patients with cortical (4%) type cataract in respect to control subjects (20%) (P=0.038, OR=0.16, 95% CI=0.04-0.64). In addition, the allele frequency of the C (Gln)-allele of XPD-Lys751Gln was found to be significantly different in mixed type cataract group (P=0.008, OR=0.48, 95% CI: 0.26-0.90). No statistically significant difference was found for the genotypic and allelic distributions of the polymorphisms in XRCC1 gene between the groups. These findings suggest that polymorphism in XPD codon 751 may be associated with the development of maturity onset cataract.

Photorepair of ultraviolet radiation (UVR)-induced pyrimidine dimers in lens epithelial DNA of Monodelphis domestica

The repair of UV radiation-induced pyrimidine dimers has been measured in lens epithelial DNA of the marsupial Monodelphis domestica using a pyrimidine dimer-specific endonuclease from Micrococcus luteus. Approximately 40% of the initially induced dimers were repaired during 90 min exposures to photoreactivating light. This capacity of the lens epithelium to photorepair pyrimidine dimers may provide a means with which to determine whether pyrimidine dimers in lens epithelial DNA are involved in UV radiation-induced pathologic changes of the lens.

Lens epithelial cell apoptosis is an early event in the development of UVB-induced cataract

Epidemiological and experimental studies have revealed that exposure to UV can induce cataractogenesis. To investigate the mechanism of this induction, viability of the lens epithelial cells from UVB-treated rat lenses were examined. Irradiation of the cultured rat lenses with 8 J/s/m2 UVB for 60 min triggers lens epithelial cell apoptosis as determined by terminal deoxyribonucleotide transferase (TdT) labeling and DNA fragmentation assays. The apoptotic lens epithelial cells were initially found in the equatorial region and then quickly appeared in both equatorial and central regions. The percentage of apoptotic cells continuously increased during the postirradiation incubation. After a 5-h post-UVB incubation, more than 50% of the lens epithelial cells were apoptotic. By 24 h, all of the lens epithelial cells in the irradiated lenses were dead through apoptosis. Associated with this apoptotic process is a large upregulation of the proto-oncogene, c-fos. Opacification appears to follow the death of lens epithelial cells occurring first in the equatorial region and then in the central area. This is also true of classical cataract parameters such as non-protein thiol and wet weight, which are significantly modified only after appreciable epithelial cell apoptosis. Together, these results suggest that the rapid apoptotic death of the lens epithelial cells induced by UVB initiates cataract development.

In vivo studies on the effect of UV-radiation on the eye lens in animals

Ultraviolet light is a non-ionizing radiation that induces photochemical reactions in the tissue. Its spectral A and B ranges are partially absorbed by the cornea and/or lens thus causing damage on the cellular, cell physiological and molecular level. UV-A does not seem to damage the cornea permanently and its effects in the lens have a very prolonged latency period. Typical reactions of the cornea are oedema, punctuate keratitis (photoelectric keratitis) and neovascularization. In the lens all reactions that could be evidenced, were located in the epithelium and in the outer cortical fiber cells. In vivo UV-A induces swelling and slight vacuolation of the anterior suture system, but apart from these transient effects, only very limited permanent damage could be demonstrated. UV-B induces the formation of an anterior subcapsular cataract, starting also with vacuolation of the suture system. These morphological characteristics can be visualized at the slitlamp microscope. Histologically, sutural irregularities (UV-A) and epithelial hyperplasia with capsular multiplication (UV-B) as well as disintegration of the anterior suture system could be observed. Patho-physiologically, a reduction of lens fresh weight (UV-B) as well as changes of the equilibrium of reduced and oxidized glutathione (GSH/GSSG) could be demonstrated. On the protein-biochemical level, changes in the ratio of water-soluble versus water-insoluble protein could be evidenced, as well as effects on specific crystallin fractions, namely alpha-crystallin. In addition, the appearance of a newly synthetized 31 kDa protein could be demonstrated in UV-B irradiated mice.

The role of the lens epithelium in development of UV cataract

In view of renewed interest in the lens epithelium as the initiation site for cataract development, it seemed timely to review recent studies which appear to establish UV damage in the lens epithelium as the cause of UV cataract. While UV photons can and do interact with lens proteins in the cortex and nucleus, experimental results from cultured lenses and tissue cultured epithelial cells also demonstrate both mutagenic and cytotoxic effects in the epithelium. This minireview examines UV-induced changes in lens physiology that appear to follow epithelial cell damage, including inactivation of critical enzymes of transport and metabolic processes. Changes in membrane function include altered cation transport, increased permeability, and altered biosynthesis. One potential scenario for the propagation of damage from the epithelium to the underlying fiber cells includes calcium elevation, an early event in cataract development and critical to many physiological processes.

Optical effects of UV-A and UV-B radiation on the cultured bovine lens

The effects of repeated exposures to UV-A (335 nm) and UV-B (305 nm) radiation on the crystalline lens were studied by treating cultured bovine lenses daily or weekly. The effects of irradiation on lens optical quality were monitored using an automated scanning laser system that records both relative transmittance and focal length across the lens. Relatively low radiant exposures of UV-B were used (0.06, 0.03, 0.01 J/cm2) compared to UV-A (1.44 J/cm2). In total, 38 treated lenses and 32 controls were cultured for times ranging from 400-1000 hours. Results indicate that this range of UV-B exposure may represent the threshold for in vitro UV-B induced opacification. Lenses treated weekly with 0.06 J/cm2 UV-B showed a significant decrease in transmittance compared to controls 69 hours after the first treatment and an increase in focal length variability. The ability of the lens to repair itself, as found in a previous single dose study, was absent after repeated doses. Lenses exposed daily to 0.03 and 0.01 J/cm2 UV-B showed no significant change in transmittance or focal length variability compared to controls. Daily exposure to 1.44 J/cm2 UV-A resulted in no significant change in transmittance or focal length variability compared to controls.

DNA single strand breaks in human lens epithelial cells from patients with cataract

In recent years, there has been renewed interest in the association between DNA damage to the lens epithelium and the development of lens opacities. Although a number of in vitro studies have indicated that lens epithelial cells are susceptible to a variety of DNA damaging insults and that these cells possess the capacity to repair such damage, no previous studies have directly addressed whether DNA damage is associated with human cataract in vivo. Utilizing samples of lens epithelial cells obtained from patients undergoing cataract surgery, the percentage of cells containing DNA single strand breaks was directly determined by the single-cell gel assay (SGA) method. Non-cataractous human Eye Bank lenses of similar ages to the cataractous samples and calf lenses were used as controls. In approximately 50% of the cataractous samples analyzed, the proportion of cells containing DNA single strand breaks was significantly higher than in control lenses. No relationship between age and DNA damage was noted. These findings are consistent with the hypothesis that in some human patients with cataract, DNA damage in the lens epithelial cell population may be related to the development of lens fiber cell opacity.

Near-UV radiation disrupts filamentous actin in lens epithelial cells

Ultraviolet radiation in the near range (UVA) causes lens opacification and disrupts the actin cytoskeleton in rabbit and gray squirrel lenses. Changes were noted using transmission electron microscopy of tangential sections and rhodaminephalloidin fluorescence microscopy of epithelial whole mounts of irradiated and unirradiated lenses, and corresponded with gross cataract formation. Irradiated lenses lacked microfilament polygonal arrays at the inner surface of the apical plasma membrane (i.e., in the cell pole next to the lens fibers) in lens epithelia of both species; a condensed actin bundle was present instead. This bundle, and scattered small actin clumps in the cytoplasm, were identified by immunogold TEM, using a specific antibody and a secondary antibody conjugated with colloidal gold. Similar techniques showed breakdown of tubulin and vimentin, but after longer intervals than for the breakdown of actin. Generalized cytologic damage was also present in epithelial cells, but not in the underlying cortical lens fibers. Damage began to occur after 4 hr of irradiation and became more severe with increased exposure. Shielded controls remained clear, had normal cytology and polygonal arrays, and no clumping of actin filaments.

UV-B radiation and the optical properties of cultured bovine lenses

The effect of UV-B radiation on the crystalline lens was examined by subjecting bovine lenses in culture to varying low exposures at 300 nm. Lens optical quality was monitored on a long-term basis (to 1000 hrs.) with an automated scanning laser system that recorded both change in relative scatter and focal length across each lens. Data were collected for 20 lens positions at each scan. Radiant exposure levels consisted of 0.5, 0.25, 0.125, 0.06 and 0.03 Jcm-2. Twenty irradiated lenses were compared to twelve untreated controls. All of the irradiated lenses showed changes in scatter and focal length relative to the controls. Most (about 75%) of the treated lenses showed significant increases in scatter (200-400%) and focal length (10-20%) at 40 to 60 hours after exposure. A similar time frame for lens damage was noted by visual inspection. Exposure to UV-B at the above doses did not affect culture longevity.

Ultraviolet light induced DNA damage and repair in bovine lens epithelial cells

DNA damage caused by UV-B and UV-A irradiation and the rate of repair of such damage was quantitated in bovine lens epithelial cell cultures using a modified alkaline elution methodology. Two enzymes, bacteriophage T4 endonuclease V, which cleaves at the site of pyrimidine dimers, and E. coli endonuclease III, which cleaves at the site of thymine glycols, were utilized. Pyrimidine dimers were not detected after UV-A irradiation of lens cultures with up to 400 J/m2. In contrast, after exposure to as little as 2 J/m2 of UV-B irradiation, large numbers of pyrimidine dimers were observed. At higher fluences, thymine glycols were also found. Significant levels of DNA-DNA crosslinking were suggested by reduced rates of elution of DNA from cells treated with both UV-B irradiation and H2O2 in comparison to treatment with H2O2 alone. Protein-DNA crosslinks, in contrast, were not observed. The rate of repair of UV-B induced DNA damage was quantitated by harvesting cells at various times after the UV-B exposure. Single-strand breaks were never observed immediately after UV-B exposure but appeared later during the repair phase. In contrast to the repair of H2O2 induced DNA damage, which is largely completed within 30 min of exposure, more than 50% of the UV-B light induced DNA damage remained unrepaired five hours after exposure. This difference between the rate of repair of H2O2 and UV-B induced DNA damage could provide valuable insights into the nature of DNA damaging agents in the lens environment and may reflect underlying differences in the potential for epithelial cell DNA mutation in response to various DNA damaging insults.

Liquid scintillation counting of 3H-thymidine incorporated into rat lens DNA

DNA synthesis in the lens has previously been localized by autoradiography following incorporation of 3H-thymidine. For the quantification of DNA synthesis in the lens, pooling of lenses and extraction of the DNA for liquid scintillation counting, has formerly been adapted. In the present investigation a method has been developed for the extraction of the unincorporated tracer from whole lenses after short time incubation in a medium containing 3H-thymidine. The 3H-thymidine incorporated into individual lenses was then detected by liquid scintillation counting after dissolution of the lenses. The sources of the variation in the method are evaluated.

Ocular fluorometry: principles, fluorophores, instrumentation, and clinical applications

Ocular fluorometry is rapidly evolving as a versatile technique for research and diagnosis in ophthalmology. The main reasons for this increasing success are 1) the ideal characteristics of the eye as an optical device for excitation of tissue fluorescence and for the detection of the fluorescent emission; 2) the development of novel fluorometric techniques, including differential and time-resolved fluorescence spectroscopy; and 3) the increasing use of coupling geometries with high-resolution and high spatial selectivity. Both endogenous and exogenous fluorophores are of interest to ocular fluorometry. The most significant among endogenous fluorophores are the fluorescing pigments of the lens and of the retinal pigment epithelium (RPE). The nature, topography, and fluorescence properties of such pigments depend on age and pathology and on the level of light exposure. Exogenous fluorophores of interest are both intentionally induced and unintentionally accumulated drugs (some of which are phototoxic). Laser-based fluorometric techniques play a leading role in ocular fluorometry. The peculiar properties of the laser for the excitation of fluorescence make this source a favorite candidate for ocular fluorometry both in vitro and in vivo.

Effect of an aldose reductase inhibitor, AL-1576, on the development of UV-B and X-ray cataract

The effect of an aldose reductase inhibitor, AL-1576, on the development of UV-B and X-ray-induced cataract was studied with 100 female Brown-Norway rats. Histological studies were made with 32 eyes. A new histological procedure enabled detailed information on UV or X-ray-induced impairment on the lens, as well as on the potential efficiency of anticataract drugs. No definite effect of the aldose reductase inhibitor, AL-1576, on the development of UV or X-ray-induced cataract could be found. It may be concluded that AL-1576 cannot prevent damage caused by irradiation, like that of other oxidative influences. Disadvantageous effects of AL-1576 on the lens could be excluded.

Unscheduled DNA synthesis in lens epithelium after in vivo exposure to UV radiation in the 300 nm wavelength region

A quantitative autoradiographic method was developed to study the pattern of DNA synthesis in the rat lens epithelium after in vivo exposure to UV radiation in the 300 nm wavelength region. It was found that UV radiation (peak transmission = 298 nm, half-width = +/- 10 nm) induces unscheduled DNA synthesis and that the proportion of nuclei in S-phase concurrently is reduced indicating an inhibition of the scheduled DNA synthesis. The registered unscheduled DNA synthesis is believed to be excision repair of DNA damage induced by the UV radiation. Excision repair in lens epithelial cells could be one mechanism involved in the correlation between exposure to sunlight and cataracts.

Endogenous nuclease activity in chick embryo lens cells

The temporal and spatial sequence of nuclear disappearance during the terminal differentiation of lens fiber cells could be due to an impairment of the DNA repair pathways or to the appearance of an active DNA degradation process. The results presented here favor the second hypothesis. A single-stranded DNA nuclease activity and a double-stranded DNA nuclease activity have been found in chick embryo fiber cells. Moreover, there is a good correspondence between the variations of the nuclease activity and the stages of differentiation of the different samples analyzed.

Evolution of the distribution, proliferation and ultraviolet repair capacity of rat lens epithelial cells as a function of maturation and aging

The symmetrical organization of lens epithelium was a determining factor in this study whose purpose was to investigate the growth of this tissue during development, maturation and aging. By direct observation it was possible to score all the nuclei, to determine four different zones of cellular density and to monitor them during the whole lifespan. The central zone is mainly quiescent and the proliferation at the periphery accounts for the low steady growth rate of the tissue. With aging there is a continuous decrease in the mitotic index and a cell size enlargement. These cells were able to perform unscheduled DNA synthesis in vitro after ultraviolet irradiation. There is an age-related decrease in DNA repair, but most of it occurs during development (until 59 weeks of age) and then remains constant (59-173 weeks). These results mean that in a differentiated pure cell population aging is not directly related to decline in unscheduled DNA synthesis.

Quantitation of DNA repair in brain cell cultures: implications for autoradiographic analysis of mixed cell populations

Quantitation of DNA repair in the mixed cell population of mouse embryo brain cultures has been assessed by autoradiographic analysis of unscheduled DNA synthesis following UV-irradiation. The proportion of labelled neurons and the grain density over neuronal nuclei are both less than the corresponding values for glial cells. The nuclear geometries of these two classes of cell are very different. Partial correction for the different geometries by relating grain density to nuclear area brings estimates of neuronal and glial DNA repair synthesis more closely in line. These findings have general implications for autoradiographic measurement of DNA repair in mixed cell populations and in differentiated versus dividing cells.

Dermatological and ocular examinations in rabbits chronically photosensitized with methoxsalen

Four groups of female Dutch-belted rabbits (Oryctulagus cuniculus) were given methoxsalen (12 mg/kg) or placebo by oral intubation and 1 hr later were exposed to UVA for either 2 or 8 hr. This procedure was repeated 5 days each week for 18 mo. A fifth group received no drug and no UVA exposure. The skin of the animals given methoxsalen and UVA showed signs of acute and chronic phototoxicity. Multiple peripheral blood parameters of hepatic, renal and hematologic function were normal and were not different between groups. Complete ophthalmoscopic examinations were performed periodically. No cataracts were seen in any of the animals. This data provides the perspective that in one species the daily dose of methoxsalen and UVA required to induce chronic cutaneous photosensitization is lower than the daily dose required to induce cataracts. It is inadvisable to interpret this data as suggesting that no risk exists for patients being treated with oral methoxsalen photochemotherapy. The experimental evidence supporting photosensitization as a cause of cataracts and implicating a role of lens DNA in this cataractogenesis is reviewed. Because methoxsalen-UVA alterations of lens DNA or protein could lead to delayed onset of cataracts, and because of the serious nature and potential preventability of phototoxic lens opacification, appropriate protective eye wear is recommended for all patients receiving oral psoralen photochemotherapy.