Detoxification of H2O2 by cultured rabbit lens epithelial cells: participation of the glutathione redox cycle

Minimizing Oxidative Stress in the Lens: Alternative Measures for Elevating Glutathione in the Lens to Protect against Cataract

Oxidative stress plays a major role in the formation of the cataract that is the result of advancing age, diabetes or which follows vitrectomy surgery. Glutathione (GSH) is the principal antioxidant in the lens, and so supplementation with GSH would seem like an intuitive strategy to counteract oxidative stress there. However, the delivery of glutathione to the lens is fraught with difficulties, including the limited bioavailability of GSH caused by its rapid degradation, anatomical barriers of the anterior eye that result in insufficient delivery of GSH to the lens, and intracellular barriers within the lens that limit delivery of GSH to its different regions. Hence, more attention should be focused on alternative methods by which to enhance GSH levels in the lens. In this review, we focus on the following three strategies, which utilize the natural molecular machinery of the lens to enhance GSH and/or antioxidant potential in its different regions: the NRF2 pathway, which regulates the transcription of genes involved in GSH homeostasis; the use of lipid permeable cysteine-based analogues to increase the availability of cysteine for GSH synthesis; and the upregulation of the lens's internal microcirculation system, which is a circulating current of Na+ ions that drives water transport in the lens and with it the potential delivery of cysteine or GSH. The first two strategies have the potential to restore GSH levels in the epithelium and cortex, while the ability to harness the lens's internal microcirculation system offers the exciting potential to deliver and elevate antioxidant levels in its nucleus. This is an important distinction, as the damage phenotypes for age-related (nuclear) and diabetic (cortical) cataract indicate that antioxidant delivery must be targeted to different regions of the lens in order to alleviate oxidative stress. Given our increasing aging and diabetic populations it has become increasingly important to consider how the natural machinery of the lens can be utilized to restore GSH levels in its different regions and to afford protection from cataract.

Mapping Glucose Uptake, Transport and Metabolism in the Bovine Lens Cortex

Purpose: To spatially correlate the pattern of glucose uptake to glucose transporter distributions in cultured lenses and map glucose metabolism in different lens regions. Methods: Ex vivo bovine lenses were incubated in artificial aqueous humour containing normoglycaemic stable isotopically-labelled (SIL) glucose (5 mM) for 5 min-20 h. Following incubations, lenses were frozen for subsequent matrix-assisted laser desorption/ionisation (MALDI) imaging mass spectrometry (IMS) analysis using high resolution mass spectrometry. Manually dissected, SIL-incubated lenses were subjected to gas chromatography-mass spectrometry (GC-MS) to verify the identity of metabolites detected by MALDI-IMS. Normal, unincubated lenses were manually dissected into epithelium flat mounts and fibre cell fractions and then subjected to either gel-based proteomic analysis (Gel-LC/MS) to detect facilitative glucose transporters (GLUTs) by liquid chromatography tandem mass spectrometry (LC-MS/MS). Indirect immunofluorescence and confocal microscopy of axial lens sections from unincubated fixed lenses labelled with primary antibodies specific for GLUT 1 or GLUT 3 were utilised for protein localisation. Results: SIL glucose uptake at 5 min was concentrated in the equatorial region of the lens. At later timepoints, glucose gradually distributed throughout the epithelium and the cortical lens fibres, and eventually the deeper lens nucleus. SIL glucose metabolites found in glycolysis, the sorbitol pathway, the pentose phosphate pathway, and UDP-glucose formation were mapped to specific lens regions, with distinct regional signal changes up to 20 h of incubation. Spatial proteomic analysis of the lens epithelium detected GLUT1 and GLUT3. GLUT3 was in higher abundance than GLUT1 throughout the epithelium, while GLUT1 was more abundant in lens fibre cells. Immunohistochemical mapping localised GLUT1 to epithelial and cortical fibre cell membranes. Conclusion: The major uptake site of glucose in the bovine lens has been mapped to the lens equator. SIL glucose is rapidly metabolised in epithelial and fibre cells to many metabolites, which are most abundant in the metabolically more active cortical fibre cells in comparison to central fibres, with low levels of metabolic activity observed in the nucleus.

Hallmarks of lens aging and cataractogenesis

Lens homeostasis and transparency are dependent on the function and intercellular communication of its epithelia. While the lens epithelium is uniquely equipped with functional repair systems to withstand reactive oxygen species (ROS)-mediated oxidative insult, ROS are not necessarily detrimental to lens cells. Lens aging, and the onset of pathogenesis leading to cataract share an underlying theme; a progressive breakdown of oxidative stress repair systems driving a pro-oxidant shift in the intracellular environment, with cumulative ROS-induced damage to lens cell biomolecules leading to cellular dysfunction and pathology. Here we provide an overview of our current understanding of the sources and essential functions of lens ROS, antioxidative defenses, and changes in the major regulatory systems that serve to maintain the finely tuned balance of oxidative signaling vs. oxidative stress in lens cells. Age-related breakdown of these redox homeostasis systems in the lens leads to the onset of cataractogenesis. We propose eight candidate hallmarks that represent common denominators of aging and cataractogenesis in the mammalian lens: oxidative stress, altered cell signaling, loss of proteostasis, mitochondrial dysfunction, dysregulated ion homeostasis, cell senescence, genomic instability and intrinsic apoptotic cell death.

Protein posttranslational modification (PTM) by glycation: Role in lens aging and age-related cataractogenesis

Crystallins, the most prevalent lens proteins, have no turnover throughout the entire human lifespan. These long-lived proteins are susceptible to post-synthetic modifications, including oxidation and glycation, which are believed to be some of the primary mechanisms for age-related cataractogenesis. Thanks to high glutathione (GSH) and ascorbic acid (ASA) levels as well as low oxygen content, the human lens is able to maintain its transparency for several decades. Aging accumulates substantial changes in the human lens, including a decreased glutathione concentration, increased reactive oxygen species (ROS) formation, impaired antioxidative defense capacity, and increased redox-active metal ions, which induce glucose and ascorbic acid degradation and protein glycation. The glycated lens crystallins are either prone to UVA mediated free radical production or they attract metal ion binding, which can trigger additional protein oxidation and modification. This vicious cycle is expected to be exacerbated with older age or diabetic conditions. ASA serves as an antioxidant in the human lens under reducing conditions to protect the human lens from damage, but ASA converts to the pro-oxidative role and causes lens protein damage by ascorbylation in high oxidation or enriched redox-active metal ion conditions. This review is dedicated in honor of Dr. Frank Giblin, a great friend and superb scientist, whose pioneering and relentless work over the past 45 years has provided critical insight into lens redox regulation and glutathione homeostasis during aging and cataractogenesis.

Characterisation of Glutathione Export from Human Donor Lenses

Purpose

To investigate whether human donor lenses are capable of exporting reduced glutathione.

Methods

Human lenses of varying ages were cultured in artificial aqueous humor for 1 hour under hypoxic conditions to mimic the physiologic environment and reduced glutathione (GSH) and oxidized glutathione (GSSG) levels measured in the media and in the lens.

Results

Human donor lenses released both GSH and GSSG into the media. Donor lenses cultured in the presence of acivicin, a γ-glutamyltranspeptidase inhibitor, exhibited a significant increase in GSSG levels (P < 0.05), indicating that GSSG undergoes degradation into its constituent amino acids. Screening of GSH/GSSG efflux transporters revealed Mrp1, Mrp4, and Mrp5 to be present at the transcript level, but only Mrp5 was expressed at the protein level. Blocking Mrp5 function with the Mrp inhibitor MK571 led to a significant decrease in GSSG efflux (P < 0.05), indicating that Mrp5 is likely to be involved in mediating GSSG efflux. Measurements of efflux from the anterior and posterior surface of the lens revealed that GSH and GSSG efflux occurs at both surfaces but predominantly at the anterior surface.

Conclusions

Human lenses export GSH and GSSG into the surrounding ocular humors, which can be recycled by the lens to maintain intracellular GSH homeostasis or used by neighboring tissues to maintain GSH levels.

Translational Relevance

Early removal of a clear lens, as occurs to treat myopia and presbyopia, would eliminate this GSH reservoir and reduce the supply of GSH to other tissues, which, over time, may have clinical implications for the progression of other ocular diseases associated with oxidative stress.

Lens Biology and Biochemistry

The primary function of the lens resides in its transparency and ability to focus light on the retina. These require both that the lens cells contain high concentrations of densely packed lens crystallins to maintain a refractive index constant over distances approximating the wavelength of the light to be transmitted, and a specific arrangement of anterior epithelial cells and arcuate fiber cells lacking organelles in the nucleus to avoid blocking transmission of light. Because cells in the lens nucleus have shed their organelles, lens crystallins have to last for the lifetime of the organism, and are specifically adapted to this function. The lens crystallins comprise two major families: the βγ-crystallins are among the most stable proteins known and the α-crystallins, which have a chaperone-like function. Other proteins and metabolic activities of the lens are primarily organized to protect the crystallins from damage over time and to maintain homeostasis of the lens cells. Membrane protein channels maintain osmotic and ionic balance across the lens, while the lens cytoskeleton provides for the specific shape of the lens cells, especially the fiber cells of the nucleus. Perhaps most importantly, a large part of the metabolic activity in the lens is directed toward maintaining a reduced state, which shelters the lens crystallins and other cellular components from damage from UV light and oxidative stress. Finally, the energy requirements of the lens are met largely by glycolysis and the pentose phosphate pathway, perhaps in response to the avascular nature of the lens. Together, all these systems cooperate to maintain lens transparency over time.

Mitochondria induce oxidative stress, generation of reactive oxygen species and redox state unbalance of the eye lens leading to human cataract formation: disruption of redox lens organization by phospholipid hydroperoxides as a common basis for cataract disease

The aging eye appears to be at considerable risk from oxidative stress. Lipid peroxidation (LPO) is one of the mechanisms of cataractogenesis, initiated by enhanced promotion of oxygen free radicals in the eye fluids and tissues and impaired enzymatic and non-enzymatic antioxidant defenses of the crystalline lens. The present study proposes that mitochondria are one of the major sources of reactive oxygen species (ROS) in mammalian and human lens epithelial cells and that therapies that protect mitochondria in lens epithelial cells from damage and reduce damaging ROS generation may potentially ameliorate the effects of free radical-induced oxidation that occur in aging ocular tissues and in human cataract diseases. It has been found that rather than complete removal of oxidants by the high levels of protective enzyme activities such as superoxide dismutase (SOD), catalase, lipid peroxidases in transparent lenses, the lens conversely, possess a balance between peroxidants and antioxidants in a way that normal lens tends to generate oxidants diffusing from lenticular tissues, shifting the redox status of the lens to become more oxidizing during both morphogenesis and aging. Release of the oxidants (O(2)(-)·, H(2)O(2) , OH·, and lipid hydroperoxides) by the intact lenses in the absence of respiratory inhibitors indicates that these metabolites are normal physiological products inversely related to the lens life-span potential (maturity of cataract) generated through the metal-ion catalyzed redox-coupled pro-oxidant activation of the lens reductants (ascorbic acid, glutathione). The membrane-bound phospholipid (PL) hydroperoxides escape detoxification by the lens enzymatic reduction. The lens cells containing these species would be vulnerable to peroxidative attack which trigger the PL hydroperoxide-dependent chain propagation of LPO and other damages in membrane (lipid and protein alterations). The increased concentrations of primary LPO products (diene conjugates, lipid hydroperoxides) and end fluorescent LPO products were detected in the lipid moiety of the aqueous humor samples obtained from patients with cataract as compared to normal donors. Since LPO is clinically important in many of the pathological effects and aging, new therapeutic modalities, such as patented N-acetylcarnosine prodrug lubricant eye drops, should treat the incessant infliction of damage to the lens cells and biomolecules by reactive lipid peroxides and oxygen species and "refashion" the affected lens membranes in the lack of important metabolic detoxification of PL peroxides. Combined in ophthalmic formulations with N-acetylcarnosine, mitochondria-targeted antioxidants are promising to become investigated as a potential tool for treating a number of ROS-related ocular diseases, including human cataracts.

The Etiology of Steroid Cataract

Steroid-induced posterior subcapsular cataracts (PSCs) exhibit three main distinctive characteristics: (i) association only with steroids possessing glucocorticoid activity, (ii) involvement of aberrant migrating lens epithelial cells, and (iii) a central posterior location. The first characteristic suggests a key role for glucocorticoid receptor activation and subsequent changes to the transcription of specific genes. Glucocorticoid receptor activation is associated in many cell types with proliferation, suppressed differentiation, a reduced susceptibility to apoptosis, altered transmembrane transport, and enhancement of reactive oxygen species activity. Glucocorticoids may be capable of inducing changes to the transcription of genes in lens epithelial cells that are related to many of these cellular processes. This review examines the various mechanisms that have been proposed to account for the development of PSC in the context of recent DNA array studies. Additionally, given that the glucocorticoid receptor can also engender wide-ranging indirect activities, glucocorticoids could also indirectly affect the lens through the responses of other cells within the ocular compartment and/or through effects on cells at more remote locations. These indirect mechanisms, which, for example, could be mediated through alterations to the intraocular levels of growth factors that normally orchestrate lens development and maintain lens homeostasis, are also discussed. Although the mechanism of steroid cataract induction remains unknown, glucocorticoid-induced gene transcription events in lens epithelial cells, and also other intraocular or systemic cells, likely interact to generate steroid cataracts. Finally, although evidence for glucocorticoid-protein adduct formation in the lens is inconclusive, the generation of such adducts cannot yet be discounted as a contributing factor and must necessarily be retained in discussions of the etiology of steroid cataract.

17beta-estradiol stimulates MAPK signaling pathway in human lens epithelial cell cultures preventing collapse of mitochondrial membrane potential during acute oxidative stress

17beta-estradiol (17beta-E2) protects against H2O2-mediated depletion of intracellular ATP and lessens the degree of depolarization of mitochondrial membrane potential (DeltaPsi(m)) in cultured lens epithelial cells consequential to oxidative insult. We now report that 17beta-E2 acts as a positive regulator of the survival signal transduction pathway, MAPK which, in turn, acts to stabilize DeltaPsi(m) in effect, attenuating the extent of depolarization of mitochondrial membrane potential in the face of acute oxidative stress. The SV-40 viral transformed human cell line, HLE-B3 was treated with 17beta-E2 over a time course of 60 min and phosphorylation of ERK1/2 was analyzed by Western blot. ERK1/2 was phosphorylated within 5-15 min in the presence of 17beta-E2. Cell cultures were exposed to the MEK1/2 inhibitor, UO126, subsequent to H2O2+/-17beta-E2 treatment and the DeltaPsi(m) examined using JC-1, a potentiometric dye which serves as an indicator for the state of mitochondrial membrane potential. UO126 treatment attenuated ERK1/2 phosphorylation irrespective of whether estradiol was administered. Mitochondrial membrane depolarization resulting from H2O2 stress was substantially greater in the presence of UO126. The greater the extent of depolarization, the less effective 17beta-E2 treatment was in checking mitochondrial membrane depolarization, indicating that the relative degree of ERK phosphorylation influences mitochondrial stability with oxidative insult. The data support a positive correlation between 17beta-E2 stimulation of ERK1/2 phosphorylation and mitochondrial stabilization that would otherwise cause a complete collapse of DeltaPsi(m).

Fatty acid cytotoxicity to human lens epithelial cells

Data obtained with the neutral red cytotoxicity assay reveal that human lens epithelial cells in culture are highly sensitive to low micromolar concentrations of unsaturated, cis-configured fatty acids in the following order: arachidonic acid>linolenic acid=linoleic acid=oleic acid, whereas the saturated fatty acids are much less effective. Though the cytotoxic effects of the unsaturated fatty acids could not be discerned from effects of their oxidation products, the fact that oleic acid is equally cytotoxic as linoleic acid or linolenic acid as well as previously reported findings with bovine lens epithelial cells support the idea that the unsaturated fatty acid molecules directly account for the cytotoxicity and not their products of lipid peroxidation. Bleb formation and cell retraction are early morphological signs of fatty acid-induced lens cell damage. These cellular alterations are accompanied by an aggregation of intermediate filaments in a first step, whereas the disorganization of microfilaments occurs at a later time and only at higher fatty acid concentrations. Measurements of protein-, RNA- and DNA-synthesis turned out to be much less sensitive parameters for the fatty acid-induced damage of lens cells. The uptake rate of linoleic acid by human lens cells is relatively high (4.35 fmol sec(-1) per 1000 cells), 30 and 50% higher as compared with diploid human embryonal lung fibroblasts and chemically transformed mouse fibroblasts, respectively. Saturation kinetics in combination with competition between linoleic acid, oleic acid and palmitic acid on one hand and ineffectiveness of trypsin and DIDS treatment on the other hand hint at cytoplasmic fatty acid binding proteins as receptors with high binding affinity (5.55 micromol l(-1), calculated for the linoleic acid-albumin complex) to be involved in the fatty acid uptake in human lens cells. Cellular fatty acid uptake is mainly influenced by the albumin concentrations present in physiological solutions. Albumin determinations in aqueous humor from 177 cataract patients reveal an age-dependent, statistically significant albumin rise with average values below 2 micromol l(-1) up to the age of 40 years to about 4 micromol l(-1) at the age between 80 and 90 years with single values up to 10 micromol l(-1). Using physiological fatty acid mixtures it is demonstrated that fatty acid-induced lens cell damage is strongly increased by elevated albumin concentrations found in aqueous humor of the elderly, who already have cataracts. Free fatty acid induced lens cell damage as a possible cause for age-dependent cataracts as well as a molecular link between systemic diseases such as diabetes and cataract formation is discussed.

Effects of variation in superoxide dismutases (SOD) on oxidative stress and apoptosis in lens epithelium

Among the critical antioxidant enzymes that protect the cells against oxidative stress are superoxide dismutases: CuZnSOD (Sod1) and MnSOD (Sod2). The latter is also implicated in apoptosis. To determine the importance of these enzymes in protection against reactive oxygen species (ROS) in the lens, we analysed DNA strand breaks in lens epithelium from transgenic and knockout (Sod1) mice following exposure to H2O2 in organ culture. Since Sod2 knockouts do not survive, comparison was made of lenses of partially-deficient (heterozygote) for Sod2 and the wild-type controls which have twice the enzyme level. Antioxidant potential of Sod2 was also studied in human lens epithelial cells (SRA01/04) in which the enzyme was up- and down-regulated by transfection with plasmids containing sense and antisense human cDNA for MnSOD. DNA strand breaks in the epithelium of Sod1 knockouts and Sod2 heterozygotes were much greater than in the corresponding wild-type or in transgenic mice over-expressing the enzymes when the lenses were exposed to H2O2. The functional role of Sod2 in apoptosis was examined in cultured human lens epithelial cells. Cells with higher enzyme levels were more resistant to the cytotoxic effects of H2O2, O2- and UV-B radiation. Furthermore, Sod2-deficient cells showed dramatic mitochondrial damage, cytochrome C leakage, caspase 3 activation and increased apoptotic cell death when they were challenged with O2-. Thus, mitochondrial enzyme (Sod2) deficiency plays an important role in the initiation of apoptosis in the lens epithelium.

Sequential effects of ultraviolet radiation on the histomorphology, cell density and antioxidative status of the lens epithelium--an in vivo study

In vivo progressive effects of UV irradiation on the lens epithelium were studied using various histomorphological and biochemical parameters. Fifteen day old rat pups were exposed to 600 mW/m2 of radiation, including UV-A and UV-B, 12 h daily for 90, 120, 150 and 180 days. Biochemical parameters such as protein-bound and non-protein-bound sulfhydryl groups in both soluble and insoluble fractions and enzymes, which play an important role in combating the oxidative stress, were studied. Decreased cell density of lens epithelial cells (LEC) was observed in all three zones along with the decrease in the levels of soluble sulfhydryls (S-SH), glutathione reductase (GR), superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT). On the other hand, an increase in insoluble sulfhydryls was observed. Because of the decrease in S-SH and GR activities, the LEC became vulnerable to oxidative stress. Decreased activities of SOD, GPx and CAT suggest elevated oxidative stress. This effect of UV radiation may lead to cell death that may be responsible for the observed decrease in the cell density in all three zones of the lens epithelium.

Propyl gallate is a superoxide dismutase mimic and protects cultured lens epithelial cells from H2O2 insult

n-Propyl gallate (nPG) is a food preservative that is generally regarded as safe by the US FDA. It suppresses oxidation in biological systems. The mechanism by which nPG acts in biological systems is uncertain. We investigated whether nPG protected cultured lens epithelial cells from H(2)O(2)-induced damage. Cells were treated with H(2)O(2) or with nPG and then H(2)O(2). H(2)O(2) inhibited growth, caused membrane blebbing, decreased lactate production, increased the level of GSSG, decreased the levels of GSH, ATP and NAD(+), and G3PDH activity, stimulated the hexose monophosphate shunt and induced single-strand breaks in DNA. nPG prevented the H(2)O(2)-induced growth inhibition, membrane blebbing, drop in NAD(+) and single-strand breaks in DNA. The mechanism by which nPG acts at the chemical level was investigated using electron paramagnetic resonance (EPR), direct spectrophotometric kinetic measurements, and cyclic voltammetry. When nPG at low concentrations (nM to microM) was mixed with a large excess of O(2)(-)*, the superoxide signal was destroyed as indicated by UV visible spectroscopy and EPR. Kinetic analysis indicated that nPG dismutated O(2)(-)* in repetitive additions of superoxide with little loss of activity. The rate constant for the overall reaction of nPG with O(2)(-)* was ca. 10(6)M(-1)s(-1). nPG had a very low specific binding constant for Fe(2+) as determined by cyclic voltammetry. The evidence indicates that nPG dismutates the superoxide ion in a catalytic manner.

The presence of a human UV filter within the lens represents an oxidative stress

It has recently been demonstrated that, with age, UV filters such as 3-hydroxykynurenine glucoside, bind to proteins in the human lens. This covalent interaction leads to colouration of the normal lens, and results from the instability of the kynurenine side chain. Other primate UV filters, in addition to containing the same side chain, can also be readily oxidized. One such compound is 3-hydroxykynurenine (3OHKyn). It has been proposed that oxidation of bound and/or free UV filters, such as 3OHKyn may give rise to the lens colouration associated with age-related nuclear cataract. Therefore it has become important to understand the oxidation of 3OHKyn within the lens. In this study, intact bovine lenses (which lack UV filters) were incubated with 3OHKyn and various lens parameters monitored. The effect of exposure to hyperbaric oxygen (HBO) was also assessed, both alone, and in combination with prior 3OHKyn incubation. Glutathione (GSH), protein sulfhydryl and protein-bound sulfhydryl levels, as well as soluble protein content and gel filtration profiles, were obtained for cortical and nuclear regions after defined periods of incubation. The presence of the primate UV filter, 3OHKyn, at concentrations similar to those present in the human lens, was shown to produce considerable oxidative stress within the lens, as judged by its effect on GSH. This effect was noted under normobaric conditions, but was exacerbated by increased oxygen. Exposure of lenses to HBO caused a marked fall in GSH in cortical and nuclear regions. This effect was exaggerated in the presence of 3OHKyn. HBO treatment also lead to a fall in protein sulfhydryl content, however, this was only partial (approximately 1 mol SH per mol protein) and changed only slowly, even with extended periods of exposure to HBO, suggesting that most crystallin sulfhydryl groups may be buried. 3OHKyn did not appreciably affect this oxidation although it did cause an increase in the level of protein-bound sulfhydryl. HBO treatment produced a more than two-fold increase in protein-bound sulfhydryl content in the cortex. There was little influence of 3OHKyn alone on protein solubility, even with extended periods of incubation, however, incubation for 72 hr in the presence of HBO caused a significant increase in insoluble protein particularly in the nucleus. This insolubilization was further increased in the presence of 3OHKyn. FPLC profiles showed that the proportion of gamma and beta crystallins in the soluble fraction decreased following HBO, suggesting that these may be involved in disulfide bond formation. This study demonstrates that a readily oxidized compound, such as the primate UV filter 3OHKyn, represents an oxidative stress within the lens and that such oxidative processes can be exacerbated if the concentration of oxygen within the lens is increased. We speculate that this factor may account for the evolution of unusually high levels of glutathione reductase in human lenses.

Reduced levels of rat lens antioxidant vitamins upon in vitro UVB irradiation

Ultraviolet (UV) radiation is one of the major risk factors of cataractogenesis. UV radiation induced damage to the eye lens is believed to be mediated through reactive oxygen species. Antioxidant defense systems, enzymatic and non-enzymatic, resist this damage. In the present study, the levels of rat lens endogenous antioxidants, L-ascorbic acid, alpha-tocopherol and beta-carotene, have been determined by HPLC upon in vitro UVB irradiation. UVB irradiation for 24 h (300 nm; 100 µW/cm(2)) of three months old rat lens suspended in RPMI medium, leads to 69-89% decrease in endogenous levels of these antioxidants. The addition of ascorbic acid (2 mM), alpha-tocopherol (2.5 µM) or beta-carotene (10 µM), separately to the medium during irradiation significantly prevented the decrease in their endogenous levels, thereby suggesting a protective role for these antioxidant micronutrients against photodamage to the eye lens.

Glutathione: a vital lens antioxidant

The reducing compound glutathione (GSH) exists in an unusually high concentration in the lens where it functions as an essential antioxidant vital for maintenance of the tissue's transparency. In conjunction with an active glutathione redox cycle located in the lens epithelium and superficial cortex, GSH detoxifies potentially damaging oxidants such as H2O2 and dehydroascorbic acid. Recent studies have indicated an important hydroxyl radical-scavenging function for GSH in lens epithelial cells, independent of the cells' ability to detoxify H2O2. Depletion of GSH or inhibition of the redox cycle allows low levels of oxidant to damage lens epithelial targets such as Na/K-ATPase, certain cytoskeletal proteins and proteins associated with normal membrane permeability. The level of GSH in the nucleus of the lens is relatively low, particularly in the aging lens, and exactly how the compound travels from the epithelium to the central region of the organ is not known. Recently, a cortical/nuclear barrier to GSH migration in older human lenses was demonstrated by Sweeney et al. The relatively low ratio of GSH to protein -SH in the nucleus of the lens, combined with low activity of the glutathione redox cycle in this region, makes the nucleus especially vulnerable to oxidative stress, as has been demonstrated with use of in vivo experimental animal models such as hyperbaric oxygen, UVA light and the glutathione peroxidase knockout mouse. Effects observed in these models, which are currently being utilized to investigate the mechanism of formation of human senile nuclear cataract, include an increase in lens nuclear disulfide, damage to nuclear membranes and an increase in nuclear light scattering. A need exists for development of therapeutic agents to slow age-related loss of antioxidant activity in the nucleus of the human lens to delay the onset of cataract.

A forty-two year voyage through vision research

This presentation is an overview of my involvement in vision research and the factors and individuals that influenced my career in this field over the last 42 years. It also summarizes my research interests and contributions in the areas of aqueous humor dynamics, transport of various substances across blood-aqueous barrier and in the lens. The metabolism and function of glutathione in the lens and the development of tissue culture of human lens epithelium as a model system to study its role in lens and cataract formation are reviewed.

The hydroxyl radical in lens nuclear cataractogenesis

Cataract is the major cause of blindness; the most common form is age-related, or senile, cataract. The reasons for the development of cataract are unknown. Here we demonstrate that nuclear cataract is associated with the extensive hydroxylation of protein-bound amino acid residues, which increases with the development of cataract by up to 15-fold in the case of DOPA. The relative abundance of the oxidized amino acids in lens protein (assessed per parent amino acid) is DOPA > o- and m-tyrosine > 3-hydroxyvaline, 5-hydroxyleucine > dityrosine. Nigrescent cataracts, in which the normally transparent lens becomes black and opaque, contain the highest level of hydroxylated amino acids yet observed in a biological tissue: for example, per 1000 parent amino acid residues, DOPA, 15; 3-hydroxyvaline, 0.3; compared with dityrosine, 0.05. The products include representatives of the hydroperoxide and DOPA pathways of protein oxidation, which can give rise to secondary reactive species, radical and otherwise. The observed relative abundance corresponds closely with that of products of hydroxyl radical or metal-dependent oxidation of isolated proteins, and not with the patterns resulting from hypochlorite or tyrosyl-radical oxidation. Although very little light in the 300-400-nm range passes the cornea and the filter compounds of the eye, we nevertheless also demonstrate that photoxidation of lens proteins with light of 310 nm, the part of the spectrum in which protein aromatic residues have residual absorbance, does not give rise to the hydroxylated aliphatic amino acids. Thus the post-translational modification of crystallins by hydroxyl radicals/Fenton systems seems to dominate their in vivo oxidation, and it could explain the known features of such nuclear cataractogenesis.

Thioltransferase is present in the lens epithelial cells as a highly oxidative stress-resistant enzyme

The redox homeostasis is controlled by several enzyme systems. Sulfhydryl groups in lens proteins are very sensitive to oxidative stress and can easily conjugate with nonprotein thiols (S-thiolation) to form protein-thiol mixed disulfides. We have observed an elevation of protein S-S-glutathione (PSSG) and protein-S-S-cysteine (PSSC) in cataractous lenses from humans and from animal models subjected to oxidative stress. We also observed that these protein-thiol mixed disulfides could be spontaneously dissociated and lowered to basal levels if the lens which was pre-exposed to H2O2 was subsequently cultured in H2O2-free medium. This suggests that the lens has a system to repair oxidative damage through dethiolation thereby restoring its redox homeostasis. In other tissues, an enzyme, thioltransferase (TTase), has been shown to be responsible for thiol/disulfide regulation. We recently demonstrated the presence of this enzyme in the lens and in cultured lens epithelial cells. Here, we investigated the response of TTase to H2O2 stress and its possible repair function in cultured lens epithelial cells. Rabbit lens epithelial cell line N/N 1003A was raised to confluence, trypsinized and plated at 0.8 million cells per 60 mm culture dish. The cells were incubated overnight in Eagle's minimum essential medium (MEM) with 1% rabbit serum and then in serum-free MEM for 30 min before a bolus of 0.5 mm H2O2 was added. At intervals of 5, 15, 30 min and up to 3 hr, the cells were harvested and used for enzyme assays for TTase, glutathione reductase (GR), glutathione peroxidase (GPx) and glyceraldehyde-3-phosphate dehydrogenase (G-3PD). Free GSH, total SH and PSSG and PSSC were also determined. Hydrogen peroxide in the medium was measured at each time point. Cells incubated without H2O2 were used as controls. The results showed that the H2O2 concentration was reduced to 50% within 30 min and was undetectable at 2 hr. Cellular GSH dropped to 40% within 5 min and stayed at this level before it began to increase at 90 min and completely recovered by 2 hr. The total SH groups were similar to free GSH. PSSG and PSSC increased 6.5 and 2 times respectively before 30 min and then decreased when GSH started to recover. G-3PD was most sensitive to H2O2 and lost 95% activity within 5 min. The activity was regained quickly when H2O2 diminished in the medium. A similar but less severe pattern was observed in both GPx (60% loss at 60 min) and GR (30% loss at 90 min). In contrast, TTase activity remained constant during the entire 3 hr. Only when a higher dose of H2O2 (0.8-1.0 mM) was used, did TTase activity show a brief loss (<30% at 60 min) and a swift recovery. Cells exposed to H2O2 exhibited a normal morphology with no evidence of DNA fragmentation. The lens epithelial cells showed a remarkable ability to repair the early damages induced by H2O2. The unusual oxidative stress-resistant property displayed by TTase, coupled with its known function suggest that it plays an important role in the repair of oxidative damage.

Oxidative defense enzyme activity and mRNA levels in lenses of diabetic rats

This study examines the mRNA expression and enzyme activity of oxidative defense enzymes during the course of streptozotocin-induced hyperglycemic cataract development. Diabetes was produced in 5-wk-old male Sprague-Dawley rats by administering streptozotocin ip and mRNA expression and enzyme activity were monitored on d 4, 8, 12, 16, 20, 40, 60, and 80; concomitantly, the onset and progress of cataract was followed by digital image analysis. Peak enzyme activity and mRNA expression were attained between d 20 and 40. Although catalase and glutathione peroxidase maintained high levels of mRNA expression through d 60, induction of CuZu-superoxide dismutase was transient, with the activity and mRNA levels returning to baseline values by d 40. There was a pronounced increase in aldose reductase activity, which gradually declined to basal levels by d 60; however, the mRNA levels remained unaltered. Other changes included a progressive loss of lenticular transparency, which declined to 40% of control by d 80. The role of antioxidant defense enzymes and, more interestingly, aldose reductase in combating oxidative stress in diabetic cataractogenesis is discussed.

A comparison of antioxidant enzyme activities in organ-cultured rhesus monkey lenses following peroxide challenge

PURPOSE

To analyze the activities of catalase, glutathione peroxidase and superoxide dismutase, three enzymes involved in the detoxification of reactive oxygen species in organ-cultured Rhesus monkey lenses.

METHODS

Lenses freshly obtained from Rhesus monkeys were incubated at 37 degrees C for 2 h and assessed for lens integrity. Lenses were then oxidatively stressed by exposure to a bolus of hydrogen peroxide. The three enzyme activities were assayed 2, 4 and 24 h after exposure to the peroxide challenge.

RESULTS

Freshly dissected lenses placed in organ culture exhibited a 20% decrease in catalase activity within 2 h. During the course of a 24 h incubation, catalase activity continued to decrease to a level 58% below that of freshly dissected monkey lenses. In contrast, the activity levels of both glutathione peroxidase and superoxide dismutase increased dramatically within the first 2 h of organ culture, with superoxide dismutase being most affected. Although glutathione peroxidase activity declined with incubation time, its level at the end of 24 h was still 36% greater than that of the fresh lenses. Superoxide dismutase activity remained elevated throughout the 24 h incubation period. The addition of a bolus of 0.25mM H2O2 to monkey lenses in culture had no effect on catalase activity. Two h after the peroxide insult, glutathione peroxidase activity decreased in comparison to control levels while the activity of superoxide dismutase increased by 43%. After 24 h, superoxide dismutase activity returned to values equivalent to the controls. In lenses challenged with 0.50mM H2O2, catalase and glutathione peroxidase activities decreased at 2 h, while superoxide dismutase activity increased 67% above control levels. At subsequent timepoints, catalase activity increased and reached control levels. In contrast, glutathione peroxidase activity continued to decrease with time eventually reaching fresh lens levels. Superoxide dismutase activity levels remained elevated and were equivalent to control values at 24 h.

CONCLUSIONS

The data indicate that placement of monkey lenses into an organ culture system represents an environmental change sufficient to cause a response in antioxidant enzyme levels. The addition of H2O2 to this environment caused only superoxide dismutase to be stimulated above control lens levels.

Lens epithelial cell mRNA. I. Cloning and sequencing of a messenger RNA with a basic motif/leucine-rich domain specifically expressed in rat lens epithelial cells

By methods of subtraction-hybridization of lambda ZAP cDNA libraries, prepared from 4-week-old rat lens epithelial cells (capsule) and lens fiber cells (decapsulated lens), we have isolated a specific cDNA clone whose target mRNA is about 600 b long. Northern blot hybridization analysis data showed that the target mRNA was preferentially expressed in the lens epithelial cells; it was not found in the retina or in non-ocular tissues. The complete sequence of the mRNA was obtained both by the 5' and 3' rapid amplification of cDNA ends (5'-RACE, 3'-RACE), and by sequencing of a clone containing the full-length cDNA insert. It contains 599 b. with an open reading frame at nucleotide (nt) 30 (ORF30). The base sequence appears to represent a complete sequence of the target mRNA, with a poly(A) signal at nt 563-569, and a poly(A) tail at nt 582 (GenBank accession #U15149). In situ hybridization showed that the target mRNA was localized in the anterior epithelial cells, and in the elongating cells in the bow region. The encoded protein of 57 amino acids (aa) contains a basic and an acidic domain. It is rich in leucine, which is located at the basic region of the ORF at every fourth aa, and at the acidic region at eight aa intervals. Therefore, there is a periodicity in the appearance of leucine residues in the deduced primary sequence, as has been seen in DNA binding proteins. Search of GenBank, EMBL and Swiss-Prot did not yield significant homology to any of the entered sequences in those databanks. Our data demonstrate that we have isolated a cDNA clone containing novel sequences specifically expressed in lens epithelial cells.

Peroxide-metabolizing systems of the crystalline lens

The ability of transparent and cataractous human, rabbit and mice lenses to metabolize hydrogen peroxide in the surrounding medium was evaluated. Using a chemiluminescence method in a system of luminol-horseradish peroxidase and a photometric technique, the temperature-dependent kinetics of H2O2 decomposition by lenses were measured. The ability of opaque human lenses to catalyze the decomposition of 10(-4) M H2O2 was significantly decreased. However, this was reversed by the addition of GSH to the incubation medium. Incubation of the mice lenses with the initial concentration H2O2 10(-4) M led to partial depletion of GSH in normal and cataractous lenses. Human cataractous lenses showed decreased activities of glutathione reductase, glutathione peroxidase (catalyzing reduction of organic hydroperoxides including hydroperoxides of lipids), superoxide dismutase, but no signs of depletion in activities of catalase or glutathione peroxidase (utilizing H2O2). The findings indicated an impairment in peroxide metabolism of the mature cataractous lenses compared to normal lenses to be resulted from a deficiency of GSH. An oxidative stress induced by accumulation of lipid peroxidation products in the lens membranes during cataract progression could be considered as a primary cause of GSH deficiency and disturbance of the redox balance in the lens.

Effect of pigmentation on glutathione redox cycle antioxidant defense in whole as well as different regions of human cataractous lens

'Reactive oxygen species', generated within the lens, are implicated in the deepening of nuclear pigmentation leading to browning of the human cataractous lens. The present study, which was carried out in senile brunescent cataracts, deals with changes in the antioxidant glutathione redox cycle defense system during the progression of browning (yellow to brown) in different regions of the human lens. A significant reduction in the levels of reduced glutathione was noted in the cortical and nuclear regions of the brown lens as compared to yellow lens, despite normal glutathione reductase activity. The rate limiting enzyme of the hexosemonophosphate shunt pathway, glucose 6-phosphate dehydrogenase, and the important oxidant scavenger enzyme, glutathione peroxidase, showed significant changes in opposite directions (glucose 6-phosphate dehydrogenase increased and glutathione peroxidase decreased), only in the cortical region of the brown lens as compared to yellow lens. The significance of the present findings in relation to the overall biochemical mechanisms underlying browning of the lens tissue in human cataracts is discussed.

Effects of aging in vitro on intracellular proteolysis in cultured rabbit lens epithelial cells in the presence and absence of serum

Alterations in proteolytic capabilities have been associated with abnormalities in the aged eye lens, but in vivo tests of this hypothesis have been difficult to pursue. To simulate aging, we cultured cells from an 8-yr-old rabbit to early (population-doubling level 20 to 30) and late (population-doubling level greater than 125) passage. Long-lived (t1/2 greater than 10 h) and short-lived (t1/2 less than 10 h) intracellular proteins were labeled with [3H]leucine, and the ability of the cells to mount a proteolytic response to the stress of serum withdrawal was determined. For early passage cells, the average t1/2 of long-lived proteins in the presence and absence of serum was 62 and 39 h, respectively. For late-passage cells, the average t1/2 of long-lived proteins in the presence and absence of serum was 58 and 43 h, respectively. The net increase in intracellular proteolysis in the absence of serum was 59 and 35% for early and late-passage cells, respectively. Thus, in vitro-aged rabbit lens epithelial cells amount only 60% the proteolytic response to serum removal shown in "younger" cells. The enhanced ability of early passage cells to respond to serum removal seems to involve lower homeostatic levels of proteolysis in the presence of serum and greater enhancement of proteolysis in the absence of serum. Less than 2% of the protein is in the pool of short-lived proteins. Rates of proteolysis of short-lived proteins in the presence and absence of serum were indistinguishable. With respect to basal proteolytic rates in the presence of serum and ability to mount a proteolytic response upon serum withdrawal, these rabbit lens epithelial cells are similar to bovine lens epithelial cells and fibroblasts.

Is the lens canned?

The ocular lens somehow remains pellucid despite bombardment by ultraviolet radiation and endogenous hydrogen peroxide (present in the humoral fluids which bathe this tissue). The lens and adjacent aqueous and vitreous humors contain exceptionally high concentrations of reducing substances, particularly ascorbic acid, thought to be important in lenticular oxidant defense. However, in the presence of traces of transition metals, or when exposed to ultraviolet radiation, ascorbic acid readily reacts with oxygen, yielding hydrogen peroxide, and damaging lens crystallins. We propose the alternative hypothesis that the real antioxidant function of ascorbic acid, particularly that in the aqueous and vitreous humors, may be effecting the conversion of oxygen to water. Because the lens lacks a blood supply, coupled reactions of ascorbic acid with oxygen in the humoral fluid spaces should produce a metabolically sustained anaerobiosis. If so, nature may have preinvented the process of canning, wherein food (or in this case, the lens) is preserved by a combination of sterility and anoxia.

The relative roles of the glutathione redox cycle and catalase in the detoxification of H2O2 by cultured rabbit lens epithelial cells

The relative roles of the glutathione redox cycle and catalase in the detoxification of H2O2 were investigated in cultured rabbit lens epithelial cells. Exposure of cells to H2O2 was carried out following inhibition of either of the two antioxidant systems. Two different procedures were used to expose the cells to extracellular H2O2, one in which a low, steady state level of 0.025 mM H2O2 was maintained in the culture medium with the use of glucose oxidase and the other in which H2O2 was added to the medium as a single pulse at levels ranging from 0.03 to 0.5 mM. When lens cells were treated with a low, steady state level of H2O2, the glutathione redox cycle was the primary means of defense against oxidative damage. Cells with fully active catalase but with inhibited glutathione reductase were not able to resist the cytotoxic effects of a 0.025 mM level of extracellular H2O2. Under these conditions the cells were nearly completely depleted of reduced glutathione within 15 min. The cellular damage observed after 1.5 hr of culture included loss of cell-to-cell contact, rounding up of the cells and formation of numerous blebs. In contrast, cells with completely inhibited catalase but with an unimpaired glutathione redox cycle suffered few damaging effects from a 3-hr exposure to 0.025 mM H2O2. When lens cells were pulsed with a single challenge of 0.5 mM H2O2, both the glutathione redox cycle and catalase were found to be essential for survival of the cells. While control cells were able to withstand the pulse of H2O2, cells with impaired activities of either the glutathione redox cycle or catalase were killed. Control cells treated with 0.5 mM H2O2 may have been protected from damage by the fact that the cellular level of GSH never dropped below 35% of normal. The cause of cell death following inhibition of catalase appeared to be related to an inability of the cells to remove peroxide from the culture medium, at a rapid rate, following the H2O2-pulse. Although cells with impaired glutathione reductase activity removed H2O2 from the medium at a rate comparable to that of control cells (due to uninhibited catalase activity), they did not survive the challenge.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha B-crystallin is expressed in kidney epithelial cell lines and not in fibroblasts

We have recently shown the presence of alpha B-crystallin in non-ocular tissues of diverse embryological origins such as the heart, brain, spinal cord, kidney, retina, etc. Using an alpha B-crystallin-specific antiserum and immunofluorescence, immunoblotting, immunoprecipitation and peptide mapping with Staphylococcus aureus protease, we demonstrate differential expression of alpha B-crystallin in epithelial and fibroblast cell lines. alpha B-Crystallin was detectable only in epithelial cell lines such as MDBK, MDCK, LLCPK1 and JTC-12, and was not observed in two kidney fibroblast cell lines, one skin fibroblast cell line, and one corneal fibroblast cell line. Differential expression of the alpha B-crystallin gene was also confirmed by Northern blot analysis of the RNAs isolated from these cell lines. These data suggest a cell-type-specific role for alpha B.

Effect of inhibition of the glutathione redox cycle on the ultrastructure of peroxide-treated rabbit epithelial cells

Our previous studies have shown that exposure of cultured rabbit lenses to physiological levels of hydrogen peroxide, following inhibition of the glutathione redox cycle, leads to the formation of distinct vacuoles in the anterior region of the lens at the germinative zone between the epithelium and lens fibers. In the present study the ultrastructure of H2O2-induced membrane damage in the intact lens and in cultured lens epithelial cells was examined by scanning and transmission electron microscopy (SEM and TEM), following the inhibition of glutathione reductase with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Lenses treated with BCNU/H2O2 exhibited swollen epithelial cells which were observed only above the peroxide-induced vacuoles. The apical surface of the swollen cells had membrane blebs which protruded into the underlying vacuolar space. The appearance of the blebs coincided with a change in the organization of the layer of microfilaments which is normally associated with the apical surface of the cell. Cultured lens epithelial cells treated with BCNU/H2O2 showed membrane blebs which increased in size and number with the duration of exposure. Initially, the blebs were seen only on certain regions of the cell surface with other regions appearing normal. TEM revealed a disorganization of microfilaments in the BCNU/H2O2 treated cells. Neither BCNU nor H2O2 alone affected the morphology of intact lenses or of cultured lens epithelial cells. In culture, isolated lens epithelial cells exposed to BCNU/H2O2 were more susceptible to damage than contiguous cells. While the exact mechanism by which H2O2-induced damage leads to bleb formation on the cell surface is not known, the inability of the cells to detoxify H2O2 due to the inhibition of glutathione reductase results in the disturbance of membrane cytoskeleton and a focal weakening of the cell surface. These results indicate a correlation between the active glutathione redox cycle in lens epithelium and maintenance of normal cytoskeletal protein organization.

Effects of age and dietary vitamin C on the contents of ascorbic acid and acid-soluble thiol in lens and aqueous humour of guinea-pigs

Variations in ascorbate and thiol concentration in lens and aqueous humour, with age and vitamin C nutrition, are of potential biological importance. To study these relationships, Dunkin-Hartley guinea-pigs were maintained for periods of up to 1.3 yr on diets containing either high or low (marginal) vitamin C. Ascorbate contents of liver, spleen, adrenals, lens and aqueous humour, and acid-soluble thiol of lens and aqueous humour were measured. High vitamin C intake maintained ascorbate levels in the internal organs between five and 30 times the level attained by the low vitamin C intake and aqueous humour vitamin C was 10-20 times higher at high vitamin C intake. Lens ascorbate, however, was only about twice as high at high vitamin C intake than at low intake, and at both intake levels it declined steadily to about half its initial value, after 1.3 yr. Thus an animal aged 1.3 yr on the low intake had about one-quarter to one-fifth of the lens ascorbate level of a young animal receiving a generous intake. Acid-soluble thiol in the lens, in contrast to ascorbic acid, increased significantly with age but was not significantly affected by dietary vitamin C intake. Acid-soluble thiol in the aqueous humour was only 0.5-2% of the concentration found in lens, and unlike the lens thiol level, it declined with age. No sex differences were observed for ascorbate or thiol levels either in lens or in aqueous humour.

Influence of the activity of glutathione reductase on the response of cultured lens epithelial cells from young and old rabbits to hydrogen peroxide

Our previous studies on cultured rabbit lens epithelial cells from 4-day-old rabbits showed that the glutathione redox cycle plays an important role in detoxifying H2O2, a potentially damaging oxidant present in the aqueous humor. Here we report the effect of donor age and cell density on the ability of cultured rabbit lens epithelial cells to detoxify H2O2. Lens epithelial cells (8 x 10(5] from a 4-day-old and an 8-year-old rabbit were cultured for 3 hr in minimal essential medium (MEM) or in MEM containing 0.01-0.1 mM H2O2 maintained with glucose oxidase. We determined the effect of H2O2 on the level of reduced glutathione (GSH), hexose monophosphate shunt activity, cell growth, and morphology. For growth studies, cells were exposed to the desired concentration of H2O2 for 3 hr and then cultured in MEM plus 10% rabbit serum for 7 days and counted. Young and old untreated cells contained high levels (30-40 nmol/8 x 10(5) cells) of GSH. Cells from 4-day-old rabbits tolerated 0.03 mM H2O2 with no effect on GSH and a minimal decrease in subsequent cell growth. However, in the older cells, GSH and growth were substantially diminished following treatment with 0.03 mM H2O2. Cells plated out at high density (8 x 10(5] were more tolerant of 0.03 mM H2O2 than cells plated out at low density (5 x 10(4]. Maximum shunt activity in the younger cells exposed to H2O2 was twice that of the older cells and occurred at a higher level of H2O2 (0.04 compared with 0.03 mM). Enzyme activities in untreated young and old cells were comparable for hexokinase, glucose-6-phosphate dehydrogenase, and glutathione peroxidase. However, glutathione reductase activity was 50% lower in the cells from the 8-year-old rabbit. The toxicity of H2O2 to cultured lens epithelial cells was directly related to donor age and inversely related to cell density. The damage in the older lens epithelial cells at 0.03 mM H2O2 was apparently due, in part, to a diminished response of the glutathione redox cycle to oxidative challenge.

Peroxide-induced effects on lens cation transport following inhibition of glutathione reductase activity in vitro

Previous studies from this laboratory have shown that the normal lens can tolerate exposure to 0.05 mM H2O2 without apparent damage and that this is due in part to an active glutathione redox cycle. The present studies were designed to investigate the role of glutathione reductase in protecting cation transport systems in the lens against potentially damaging effects of peroxide. Pre-treatment of rabbit lenses with 0.5 mM 1.3-bis(2-chloroethyl)-1-nitrosourea (BCNU), a relatively specific inhibitor of glutathione reductase, brought about a 71% inhibition of the enzyme in the capsule-epithelia of the lenses. Subsequent exposure of the lenses for 3 hr to a constant level of 0.05 mM H2O2 in culture medium produced significant accumulation of oxidized glutathione (GSSG) in the lens epithelium and severe effects on the electrolyte balance in the lens, on the activity of Na, K-ATPase and on the accumulation and efflux of 86Rb. The effects included a 35% decrease in activity of Na, K-ATPase, a 10 mM increase in the concentration of Na+ and an 8 mM decrease in K+. BCNU-H2O2 treatment also resulted in loss of transparency of the lenses in the form of vacuoles present in the anterior, subcapsular region, encircling the entire periphery of the organ near the germinative zone of the epithelium. Treatment with either BCNU or 0.05 mM H2O2 alone had only minimal effects on accumulation of GSSG in the epithelium, on lens transparency and on the parameters of cation transport which were investigated. When lenses were treated with 0.05 mM H2O2 alone and then placed in normal medium to measure the accumulation of 86Rb it was found that the cation pump was stimulated 20% above the normal level of activity. Levels of H2O2 higher than 0.05 mM without BCNU pre-treatment produced significant inhibition of Na, K-ATPase and the effects of 0.3 mM H2O2 on cation transport and GSSG accumulation were comparable to those of BCNU-0.05 mM H2O2. While inhibition of the activities of glutathione reductase and Na, K-ATPase in the lenses was found to be irreversible, a partial recovery of the Na+ level and nearly complete recovery of the K+ level were observed when treated lenses were cultured in normal medium for an additional 6 hr. In addition, the rate of efflux of 86Rb which was significantly faster from the BCNU-H2O2-treated lenses compared with the controls, was found to return to the control value during the recovery period.(ABSTRACT TRUNCATED AT 400 WORDS)

Proton NMR spectroscopy of glucose consumption by cultured lens epithelial cells

Proton NMR spectroscopy was performed on media collected from cultured lens epithelial cells of the rabbit eye incubated with Krebs-Ringer's solution containing 5.5mM 13C-glucose (labeled at the C-1 position). Comparing proton resonance intensities of the lactate-methyl group of the C-3 carbon (both 12C and 13C) enabled us to quantify the hexose monophosphate shunt (HMPS) activity. Our results showed that the epithelial cells remained stable in both lactate production and HMPS activity for at least 8 hours. In addition, although tBHP (1mM) resulted in an increase of glucose flow through the HMPS, the rate of lactate production was not affected. In contrast, KCN (2mM) caused a 72% increase of lactate production and a slight (6%) decrease of glucose consumption through the HMPS.

The effect of H2O2 on lens epithelial cell glutathione

Investigation of lens epithelial cells indicates that under normal conditions, essentially all of the detectable cellular glutathione is in a reduced state. However, exposure to levels of H2O2 in the range found in the aqueous fluid of cataract patients causes rapid, very large changes in the glutathione redox ratios. Immediately following short-term exposure to 0.15-0.2 mM H2O2, reduced glutathione drops to 19% of its normal level and the remainder of the total glutathione is found in the oxidized form. Within the next few minutes, the redox ratio returns to normal. However, total glutathione levels remain approximately 20% below normal even one hour after exposure to H2O2. With exposure to a higher concentration of H2O2, a greater loss of glutathione is observed. The results suggest that the glutathione redox ratios change dramatically as a result of oxidative insult but quickly return to normal when the oxidative insult is removed. The formation of mixed glutathione-protein disulfide was also observed but only after long-term (1 hour) exposure to a high level (0.6 mM) of H2O2.