Sigma 1 receptor stimulation protects against oxidative damage through suppression of the ER stress responses in the human lens

Stimulation of sigma-1 receptors is reported to protect against oxidative stress. The present study uses cells and tissue from the human lens to elucidate the relationship between the sigma 1 receptor, ER stress and oxidative stress-induced damage. Exposure of the human lens cell line FHL124 to increasing concentrations of H(2)O(2) led to reduced cell viability and increased apoptosis. In response to 30 μM H(2)O(2), levels of the ER stress proteins BiP, ATF6 and pEIF2α were significantly increased within 4h of exposure. Expression of the sigma 1 receptor was markedly increased in response to H(2)O(2). Application of 10 and 30 μM (+)-pentazocine, a sigma 1 receptor agonist, significantly inhibited the H(2)O(2) induced cell death. (+)-Pentazocine also suppressed the oxidative stress induced reduction of pro-caspase 12 and suppressed the induction of the ER stress proteins BiP and EIF2α. When applied to cultured human lenses, (+)-pentazocine protected against apoptotic cell death, LDH release and against H(2)O(2) induced opacification. These data demonstrate that stimulation of the sigma 1 receptor provides significant protection from oxidative damage and is, therefore, a putative therapeutic approach to delay the onset of diseases that may be triggered by oxidative damage, including cataract formation.

MMP2 activity is critical for TGFβ2-induced matrix contraction--implications for fibrosis

PURPOSE

The fibrotic lens disorder posterior capsule opacification (PCO) develops in millions of patients following cataract surgery. PCO characteristics are extensive extracellular matrix (ECM) production and contraction of the posterior lens capsule, resulting in light-scattering ECM modification (wrinkling). The pro-fibrotic cytokine transforming growth factor beta (TGFβ) is central to PCO development. This study aimed to elucidate the role of the ECM modulators matrix metalloproteinases (MMPs) in TGFβ-mediated PCO formation.

METHODS

The human lens epithelial cell-line FHL-124 and human capsular bag models were employed. Gene expression of MMP family members was determined by oligonucleotide microarray and quantitative real-time RT-PCR. MMP2 and MT1-MMP protein levels were analyzed by ELISA and Western blotting, respectively. Matrix contraction was determined using an FHL-124 patch contraction assay; at end-point, cells were stained with Coomassie brilliant blue and area was determined using image analysis software. Cell coverage and wrinkle formation on the posterior capsule were also assessed using human capsular bag models.

RESULTS

Active TGFβ2 (10 ng/mL) increased gene and protein levels of MMP2 and MT1-MMP and induced matrix contraction in FHL-124 cells. Specific siRNA inhibition of MT1-MMP did not suppress TGFβ2-induced matrix contraction. Active TGFβ2-mediated contraction was prevented by broad-spectrum MMP inhibitor GM6001 (25 μM), MMP2 siRNA, and MMP2 neutralizing antibody (4 μg/mL). TGFβ2-induced wrinkle formation was attenuated in human capsular bags treated with MMP2 neutralizing antibody (20 μg/mL).

CONCLUSIONS

MMP2 plays a critical role in TGFβ2-mediated matrix contraction, which appears to be independent of MT1-MMP. MMP2 inhibition provides a novel strategy for the treatment of PCO and potentially other fibrotic disorders.

Oligonucleotide microarray analysis of human lens epithelial cells: TGFbeta regulated gene expression

PURPOSE

Transforming growth factor beta (TGFbeta), a pro-fibrotic cytokine has been proposed a causative factor in the progression of lens pathologies including posterior capsule opacification (PCO), a condition that occurs after cataract surgery. This study employs oligonucleotide microarrays to provide a global profile of gene expression in FHL 124 cells, to identify changes in gene expression following treatment with TGFbeta1 and TGFbeta2, and to enable putative genes relating to TGFbeta regulation and PCO to be identified.

METHODS

Routinely cultured FHL 124 cells maintained in serum free Eagle's Minimum Essential Medium (EMEM) were treated with either TGFbeta1 or TGFbeta2 at 10 ng/ml for 24 h then total RNA extraction was carried out. Total RNA (16 microg) was used to analyze gene expression by spotted oligonucleotide microarray hybridization. The spotted oligonucleotide microarrays employed contained 13,971 oligonucleotide probes, each designed to be specific for an individual gene. Array images were analyzed using GenePix Pro 3.0, followed by raw data import into GeneSpring 7.0 where a cross gene error model (CGEM) filter was applied. Data was subjected to LoWess normalization prior to comparison of the different treatment groups. Quantitative real-time polymerase chain reaction (QRT-PCR) was used to validate the oligonucleotide microarray data, using a select number of genes exhibiting differential expression.

RESULTS

A total of 301 genes were up-regulated by more than 1.5 fold in FHL 124 cells by both TGFbeta1 and TGFbeta2. Many of these up-regulated genes had biological functions relevant to lens epithelial cells including roles in contraction, transdifferentiation and as extracellular matrix (ECM) components. A total of 164 genes were down-regulated by more that 1.5 fold in FHL 124 cells by both TGFbeta1 and TGFbeta2. Many of these down-regulated genes have biological functions including roles in apoptosis, signaling, and as anti-oxidants. Following treatment with TGFbeta1 and TGFbeta2, QRT-PCR successfully validated the differential changes in gene expression detected by oligonucleotide microarrays.

CONCLUSIONS

TGFbeta1 and TGFbeta2 regulate the gene expression of genes that have important roles in human lens epithelial cell biology. Most importantly, TGFbeta induces the gene expression of a number of fibrotic markers which may have a role in promoting the development of PCO such as transdifferentiation markers, contractile factors, and ECM components.

Relative suppression of the sodium-dependent Vitamin C transport in mouse versus human lens epithelial cells

Vitamin C is a major antioxidant and UV absorbent in the human lens. In the rodent lens, the levels are very low for unknown reasons. Searching for clues to explain this suppression, we investigated the comparative uptake of Vitamin C in cultured human and mouse lens epithelial cells. When compared to human HLE-B3 lens epithelial cells, (14)C-ASA uptake was 4- to 10-fold impaired in confluent mouse lens 17EM15 (p < 0.0001) and 21EM15 (p < 0.001) cells, respectively. High glucose concentrations reduced the uptake by 30-50% in all cells (p < 0.005). Incubation of cells with 6-deoxy-6-fluoro-ascorbic (F-ASA), i.e. a probe specific for the sodium-dependent Vitamin C uptake (SVCT2), revealed a 10-fold uptake suppression into mouse 17EM15 relative to human HLE-B3 and JAR choriocarcinoma cells (a control), that could be overcome by overexpressing hSVCT2 using two different promoter constructs. The relative Vitamin C uptake differences suggest either low expression of SVCT2, molecular differences between the transporters themselves or their biological regulation, since a recent study has shown that exogenous feeding of ascorbic acid to rats increased only modestly lenticular uptake (Mody et al., Acta Ophthalmol Scand 83: 228-223, 2005). Elucidation of the mechanism by which SCVT2 activity is suppressed in mouse lens may help unravel a major question of evolutionary significance for night vision in nocturnal animals.

Growth factor receptor signalling in human lens cells: role of the calcium store

In the human lens, stimulation of tyrosine-kinase coupled growth factor receptors such as epidermal growth factor receptor (EGFR) can induce calcium release from endoplasmic reticulum (ER) stores. The present study investigated the impact of calcium store inactivation on EGFR signalling, cell growth and death in a well-characterised human lens cell line (FHL124). FHL124 cells were routinely cultured in Eagle's minimum essential medium (EMEM) supplemented with 10% foetal calf serum (FCS) and seeded on 24-well plates (DNA and protein synthesis), tissue culture dishes (growth assay, western immunoblot), and glass coverslips (immunocytochemistry). DNA and protein synthesis rates were quantified by measuring the incorporation of (3)H-thymidine and (35)S-methionine into FHL124 cells in serum-free EMEM or EMEM supplemented with thapsigargin (Tg) (100 nM and 1 microM). Longer-term growth was assessed by quantifying the increase in area over time of a circular patch of seeded cells. EGFR was identified using anti-EGFR mouse monoclonal antibody and visualised by fluorescence microscopy with ALEXA 488 conjugated secondary antibody. Programmed cell death was determined by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) assay method. Activation of the mitogen-activated protein kinase (MAPK) signalling protein extracellular signal-regulated kinase (ERK) and the cell cycle proteins CDK2 and P27(kip1) were detected by western immunoblot techniques. Inactivation by > or =100 nM Tg inhibited both protein and DNA synthesis although the effect on the latter was greatest. The cell cycle activator CDK2 was reduced by Tg, while the inhibitor P27(kip1) was increased along with the percentage of apoptotic cells. A single, maximal epidermal growth factor (EGF) (10 ng ml(-1)) exposure induced receptor internalization and increased ERK phosphorylation. Both internalisation and ERK activation were unaffected by the presence of Tg. However, reduced internalisation and ERK activation followed repeated EGF applications in the presence of Tg. Additionally, ERK activation by submaximal EGF concentrations was reduced by store depletion. An intact endoplasmic reticulum calcium store therefore plays a significant role in human lens cell survival and growth.

TGFbeta2 influences alpha5-beta1 integrin distribution in human lens cells

TGFbeta plays a central role in posterior capsule opacification, in which cell proliferation and matrix deposition, accompanied by capsular wrinkling, are largely responsible for the increased light scatter involved. Human FHL124 cells were plated onto uncoated glass coverslips to form circular patches so that the central cells reached confluency while the peripheral cells grew outwards. Cell patches were exposed to serum free (SF) EMEM (control) or TGFbeta supplemented (10 ng ml(-1)) EMEM. Fibronectin (Fn), alpha5beta1 integrin and F-actin were localized by immunofluorescence techniques and analysed by confocal microscopy. In the confluent, central cells in SF medium alpha5beta1 showed a punctate distribution while Fn was present in strongly staining fibres. TGFbeta had no effect on integrin or Fn distribution in confluent cells. In the peripheral, motile cells of the patches in SF conditions alpha5beta1 was localized in well-defined focal adhesion plaques at the ends of actin stress fibres, while Fn was distributed in a punctate perinuclear pattern. TGFbeta had a profound dispersing effect on the integrin causing a widespread distribution of alpha5beta1 in the membrane with no apparent association with the actin filaments. The cells had a more fibroblastic morphology with increased deposition of Fn near the nucleus. All the TGFbeta-induced changes were inhibited by the TGFbeta antibody CAT152 (Cambridge Antibody Technology). Culture with a function-blocking alpha5 antibody or Fn antibody resulted in detachment of the peripheral cells from the patches, but the central cells remained intact. The patch culture method therefore provides a convenient means of investigating the differences between confluent and growing lens cells both in terms of the patterns of alpha5beta1 integrin and Fn and also in the response of the molecular arrangements of both to TGFbeta2.

Stress-induced ATP release from and growth modulation of human lens and retinal pigment epithelial cells

ATP release has been shown to occur following stimulation in several cellular systems. This study was undertaken to determine if lens and retinal epithelial cells release ATP in response to physiological stresses and to elucidate a possible role for ATP. Analysis of human aqueous humour samples showed a mean ATP level of 37.8+/-7.7 nM. Hyper-osmotic stress induced a dose- and time-dependent release of ATP. Both cell types were found to proliferate in serum-free medium, and the addition of ATP and adenosine at concentrations as low as 0.1 nM inhibited growth. Gene profiling also demonstrated the presence of the ectonucleotidases CD39 and CD73 and the A1 adenosine receptor on both cell types.

Characterisation of TGF-β2 signalling and function in a human lens cell line

There is increasing evidence implicating Transforming growth factor beta (TGF-beta) in pathological states of the lens. However, the underlying signalling mechanisms in human cells have not been fully examined. We have therefore investigated in a human lens cell line, FHL 124, the signalling characteristics of TGF-beta and Smad proteins. Moreover, we have tested the effectiveness of a fully human monoclonal anti-TGF-beta2 antibody, CAT-152, in suppressing TGF-beta2 induced changes in a number of conditions. FHL 124 cells were routinely cultured in Eagle's minimum essential medium (EMEM) supplemented with 10% FCS. Characterisation of the cell line was determined using Affymetrix gene microarrays and compared to native human lens epithelium. Cells were serum starved for 24 hr prior to exposure to TGF-beta2 in the presence and absence of CAT-152. Non-stimulated cells served as controls. Smad 4 localisation was observed by immunocytochemistry. To study Smad-dependent transcriptional activity, cells were transfected with SBE4-luc, an artificial smad-specific reporter, using Fugene-6. Transcriptional activity was determined by luciferase activity. Gene expression was assessed using reverse transcriptase-polymerase chain reaction (RT-PCR). Proliferation was determined by 3H-thymidine DNA incorporation. Growth and contraction were assessed using a scratch and patch assay. Affymettrix gene microarrays identified 99.5% homology between FHL 124 cells and the native lens epithelium with respect to expression pattern of the 22,270 genes on the chip. Moreover, FHL 124 cells expressed phenotypic markers, alphaA-crystallin and pax6 along with lens epithelial cell specific marker FoxE3. Immunocytochemical studies revealed the presence of Smad 4 which following TGF-beta2 exposure accumulated in the cell nucleus. Furthermore, Smad-dependent transcriptional activity was also stimulated. TGF-beta2 enhanced the expression of mRNA levels of alpha smooth muscle actin (alphaSMA) and connective tissue growth factor (CTGF). Exposure to TGF-beta2 resulted in a relatively small inhibition of 3H-thymidine incorporation of FHL 124 cells. However, a more marked contractile effect was also observed. In serum-supplemented medium, growth rates and TGF-beta induced contraction were enhanced. Treatment with 0.1-10 microg ml(-1) CAT-152 dose-dependently inhibited 10 ng ml(-1) TGF-beta2 induced effects in the presence and absence of serum. Exposure of FHL 124 cells to TGF-beta therefore induces Smad translocation, transcription, expression of transdifferentiation markers and induces marked contraction. Treatment with CAT-152 can effectively inhibit these responses. TGF-beta2 induced changes can also persist long after the period of exposure and when in the presence of serum TGF-beta induced contraction is enhanced. The work presented therefore demonstrates a platform technology to study TGF-beta2 signalling in human lens epithelial cells and provides evidence to show TGF-beta2 can be a potent factor in the development of posterior capsule opacification following cataract surgery.

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.

Ubiquitin-activating enzyme (E1) isoforms in lens epithelial cells: origin of translation, E2 specificity and cellular localization determined with novel site-specific antibodies

Lens development and response to peroxide stress are associated with dramatic changes in protein ubiquitination, reflecting dynamic changes in activity of the ubiquitin-activating enzyme (E1). Two isoforms of E1 (E1A and E1B) have been identified in lens cells although only one E1 mRNA, containing three potential translational start sites, has been detected. Novel, site-specific antibodies to E1 were generated and the hypothesis that the two isoforms of E1 are translated from alternative initiation codons of a single mRNA was tested. Antibodies raised against E1A-N peptide (Met(1)to Cys(23)of E1A) reacted only with E1A by immunoblot and immunoprecipitation. Antibodies raised against E1B-N peptide (Met(1)to Glu(25)of E1B or Met(41)to Glu(65)of E1A) and E1AB-C peptide (His(1030)to Arg(1058)of E1A or His(990)to Arg(1018)of E1B) reacted with both E1A and E1B. These results indicate that (1) E1A and E1B contain the same C-terminal residues; (2) E1A contains the N terminal sequence of E1B; and (3) E1B does not contain the N terminal sequence of E1A. The two isoforms of lens E1 are therefore translated from a single mRNA. Specifically, E1A is translated from the first initiation codon, and E1B translated from the second initiation codon. E1A and E1B were affinity-purified, and their ability to 'charge' ubiquitin carrier proteins (E2s) with activated ubiquitin was compared in a cell-free system. E1A and E1B were indistinguishable with respect to charging different E2s. However, E1 immunolocalization studies with human lens epithelial cells indicate that E1A and E1B are preferentially localized to the nucleus and cytosol, respectively. This observation suggests that E1A and E1B ubiquitinate different proteins and serve different functions in intact cells.

Hepatocyte growth factor function and c-Met expression in human lens epithelial cells

PURPOSE

Hepatocyte growth factor (HGF) and its receptor c-met perform a multitude of functions. However, despite the significant degree of study of HGF and c-met in numerous tissues and cell types, relatively few investigations have been performed on the lens. In the current study, therefore, the role of HGF and the receptor c-met in human lens epithelial cells was investigated.

METHODS

Anterior epithelium and capsular bags were prepared from human donor eyes and maintained in Eagle's minimum essential medium (EMEM) in a 5% CO(2) atmosphere at 35 degrees C. In addition, the human lens cell line FHL124, was routinely cultured and seeded onto glass coverslips (c-met immunodetection), 12-well plates (DNA and protein synthesis), and tissue culture dishes (migration). c-Met was detected by immunocytochemistry and fluorescence-activated cell scanning (FACS). HGF was measured using enzyme-linked immunosorbent assay (ELISA) techniques. Proliferation and protein synthesis were determined by [(3)H]thymidine and (35)S-methionine incorporation into DNA and proteins, respectively. Migration was assessed using a scratch-wound assay and time-lapse video microscopy.

RESULTS

HGF was detected at all stages of culture of capsular bags in protein-free medium. Moreover, c-met was present on the native epithelium and after mechanical trauma was seen to be upregulated. Immunolocalization and FACS analysis demonstrated c-met expression on FHL124 cells throughout the whole population. Furthermore, FACS analysis showed that serum-maintained cells sustained a higher level of receptor expression relative to serum-deprived cells. Additionally, HGF was found to stimulate proliferation, protein synthesis, and migratory responses.

CONCLUSIONS

c-Met receptors are expressed in native epithelium, capsular bag cultures, and FHL124 cells. Receptor is distributed across the entire cell population; however, this expression is environmentally and mechanically sensitive. HGF is also present in capsular bags at all stages of culture. In addition, HGF can stimulate migration, proliferation, and protein synthesis. It therefore appears that a multifunctional autocrine loop involving HGF and c-met is in place and could be important in the development of posterior capsule opacification.

Induction of heme oxygenase-1 modulates cis-aconitase activity in lens epithelial cells

Heme oxygenase-1 is the heme catabolic enzyme induced in human dermal fibroblasts by environmental stress. We report an increase of heme oxygenase-1 message in lens epithelial cells after exposure to UVA radiation, followed by a 10-fold increase of protein expression. The size of message was larger than previously demonstrated for fibroblasts. The relationship between heme oxygenase-1 activation and iron metabolism was investigated by measurement of activities of both cytosolic and mitochondrial cis-aconitase enzymes. A 2-fold increase in mitochondrial cis-aconitase activity in UVA-exposed cells coincided with the time of maximal heme oxygenase-1 expression. We propose that modulation of cis-aconitase activity at the translational level by an increase of cellular iron is an important consequence of heme oxygenase-1 activation. This might be a novel aspect of the protective role of heme oxygenase-1 in modulating the response of cells challenged with oxidative stress.

Protection from oxidative insult in glutathione depleted lens epithelial cells

It has previously been shown that TEMPOL, n-propyl gallate and deferoxamine, compounds that limit the availability of Fe+2 and prevent the generation of hydroxyl radicals, protect cultured rabbit lens epithelial cells from H2O2-induced damage. In view of the importance of glutathione as an antioxidant and the decrease in GSH that is known to accompany most forms of cataract, we investigated whether these compounds protected cultured lens epithelial cells from H2O2 when the cells were artificially depleted of glutathione. Treatment of lens epithelial cells with 1-chloro-2,4-dinitrobenzene (CDNB), a compound that irreversibly binds to glutathione, or buthionine sulfoximine (BSO), an inhibitor of glutathione biosynthesis, reduced the glutathione content to an average of 15-20% of the control values without a concomitant increase in oxidized glutathione. Morphological changes were assessed by phase contrast and electron microscopy. In order to assess growth, cells in 5 ml serum-free MEM were exposed to an initial concentration of 0. 05 mm H2O2 (for 50,000 cells) or 2 doses of 0.5 mm H2O2 (for 800,000 cells). After exposure to H2O2, medium was replaced with MEM plus 8% rabbit serum; cells were fed on days 3 and 6 and counted on day 7. When 50,000 or 800,000 cells with decreased glutathione were exposed to 0.05 or 0.5 mm H2O2 the H2O2 was cytotoxic, whereas cells treated with H2O2 alone remained viable but showed inhibited proliferation. An unexpected finding was that cells continued to remove H2O2 from the medium at normal rates even when the GSH level was reduced. Cells treated with CDNB or BSO alone exhibited morphological and growth properties comparable to untreated cells. Cells treated with CDNB or BSO and then with H2O2 exhibited decreased cell-to-cell contact, nuclear shrinkage, and arborization when viewed with phase-contrast microscopy and showed extensive nuclear and cytoplasmic degeneration at the EM level. Cell death was determined by dye exclusion and confirmed by video microscopy. When cells were treated with CDNB or BSO and subsequently treated with TEMPOL, n-propyl gallate or deferoxamine and then challenged with H2O2 cytotoxicity was prevented and the cells were capable of growth. The data show that H2O2 was not lethal to glutathione-depleted lens epithelial cells when they were treated with compounds that prevented the generation of reactive oxygen species. In addition, the results indicate that GSH has an important protective role independent of its ability to decompose H2O2 via glutathione peroxidase.

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.

Heme oxygenase synthesis is induced in cultured lens epithelium by hyperbaric oxygen or puromycin

We showed previously that treatment of cultured rabbit lens epithelial cells (LECs) with hyperbaric oxygen (HBO) produced DNA strand-breaks, caused reversible inhibition of protein synthesis and induced the synthesis of a 32 kD protein. In the present work, we employed immunostaining procedures to identify the 32 kD protein as heme oxygenase-1 (HO-1). Increased synthesis of the enzyme was observed as early as 12 hr after HBO-treatment, reached a maximum at 18 hr and was not detectable at 36 hr. Exposure of the cells to hemin also increased the synthesis of HO-1. An HBO-induced inhibition of protein synthesis and the subsequent induction of HO-1 was also observed in the capsule-epithelium of cultured rabbit lenses. For both LECs and the cultured lens, only HO-1 and not heme oxygenase-2 was HBO-inducible. Use of the antioxidant dimethylthiourea with HBO-treated lenses or LECs did not alter the observed effects on protein synthesis or the induction of HO-1. In contrast to results obtained with 50 atm O2, a pressure of 25 atm O2 inhibited protein synthesis only slightly and failed to induce synthesis of the 32 kD protein (although, as shown previously, identical exposure of LECs to 25 atm O2 significantly damaged DNA). Inhibition of protein synthesis in LECs and cultured lenses with the use of puromycin also induced synthesis of HO-1. Both hemin (10 micron), a source of iron, and 50 atm O2 produced a three-fold increase in the concentration of ferritin, a natural iron chelator, in LECs two days after exposure; no effects on ferritin levels were observed after 1 or 3 days. The finding that the increase in ferritin concentration occurred in the cells significantly after hemin- or HBO-induced synthesis of heme oxygenase indicates that chelatable iron rather than the heme molecule itself may have been the primary agent responsible for inducing ferritin synthesis. The data suggest that HBO-induced synthesis of HO-1 in the lens epithelium may be the result of an inhibition of protein synthesis, possibly leading to an accumulation of heme, rather than a direct protective response against oxidative stress.

Expression of Crystallins, Pax6, Filensin, CP49, MIP, and MP20 in lens-derived cell lines

PURPOSE

Cell lines are the systems of choice to analyze cellular functions related to the particular organ system. For lens research, three cell lines are widely used: N/N1003A (derived from rabbit lenses), alpha TN4, and NKR-11 (both of murine origin). The aim of the current study was to characterize these particular cell lines with respect to their expression of genes that are considered to be lens specific or expressed preferentially in the lens, such as crystallins, Pax6, Filensin, CP49, MIP, and MP20.

METHODS

alpha A- and alpha B-crystallin cDNA from rabbit lenses were sequenced. The expression of various genes was analyzed by reverse transcription-polymerase chain reaction using specific primers and mRNA from three lens-derived cell lines. For control, the expression of the selected genes was compared in nonlenticular tissues of mouse as well as in non-lens-derived murine cell lines (EF43, NIH-3T3, and L929).

RESULTS

None of the transcripts for beta B2-crystallin, gamma-crystallins, MIP, MP20, filensin, and CP49 could be detected in the lens-derived cell lines. Transcripts for alpha A-crystallin were amplified in alpha TN4, but not in N/N1003A and NKR-11 cells. Pax6, a master control gene of eye development, is expressed in all three lens-derived cell lines and, additionally, in cell lines of neuronal origin, but not in corneal endothelial cells and in the currently used control cell lines.

CONCLUSIONS

Three cell lines of lenticular origin were tested for expression of genes that were found abundantly in the lens. The observed expression of Pax6 in all lens-derived cell lines allows their use in the analysis of corresponding signal chains.

Regional differences in the distribution of catalase in the epithelium of the ocular lens

Oxidative stress is thought to play a major role in cataract formation. The present experiments are aimed at gaining a better understanding of the systems that protect the lens from damage by reactive oxygen species. The aqueous humor normally contains hydrogen peroxide (H2O2), a compound capable of generating reactive oxygen species. The systems protecting the ocular lens from oxidative damage are primarily confined to the epithelium, a single layer of cells on the anterior side of the organ directly beneath the lens capsule. When cultured rabbit lenses were challenged with a single dose of 0.2 mM H2O2, cells in the peripheral region of the epithelium survived; those in the central region died. Here we investigate the histochemical and immunoperoxidase distributions of catalase, an enzyme which detoxifies H2O2, in cells from the peripheral and central regions of the epithelium on flat mount preparations of the epithelium. In a flat mount, the entire population of lens epithelial cells can be viewed on one preparation. The reaction product for catalase activity and its immunoperoxidase localization were more intense in peripheral epithelial cells than in cells throughout the central epithelium. Treatment of cultured lens epithelial cells or rabbit lenses with 3-aminotriazole or potassium cyanide, inhibitors of catalase, reduced or abolished the histochemical reaction product. Ultrastructural cytochemistry confirmed the presence of catalase in microperoxisomes of the epithelial cells from whole lenses. The decreased level of catalase throughout the central epithelium may account for the increased susceptibility of these cells to H2O2-induced cell death.

Calcium mobilisation modulates growth of lens cells

The modulating effect of calcium cell signalling agonists on tissue growth was studied in a rabbit lens cell line (NN1003A). Calcium mobilisation was measured after Fura-2 incorporation and growth assayed either by direct Coulter counting or [3H]-thymidine incorporation. Transient increases in cytoplasmic calcium were elicited by rabbit serum, histamine, ATP and PDGF. Thapsigargin induced a prolonged increase and all of the above agonists failed to elicit a response after thapsigargin. Rabbit serum and PDGF both increased cell growth in a concentration-dependent manner. While histamine and ATP had little effect in serum-free medium, they reduced serum-stimulated growth. Acetylcholine and FGF did not produce a marked rise in cytoplasmic calcium and neither did they modulate growth. Both thapsigargin and caffeine greatly inhibited growth. These findings indicate that, in lens cells, agonists that mobilise calcium, whether by acting through G-protein or tyrosine kinase receptors, also modulate lens cell growth. Agents such as thapsigargin and caffeine that inactivate the same calcium store also inhibit growth.

Enhanced prostaglandin synthesis after ultraviolet-B exposure modulates DNA synthesis of lens epithelial cells and lowers intraocular pressure in vivo

PURPOSE

To study the functional significance of prostaglandin synthesis after ultraviolet-B (UVB) exposure of cultured human lens epithelial cells and rabbit eyes in vivo.

METHODS

Prostaglandin E2 (PGE2) was assayed using a radioimmunoassay (RIA) and mass spectroscopy. An immortalized human lens epithelial cell line (HLE-B3) was exposed to UV irradiation, and the synthesis of PGE2 was compared with the rabbit lens epithelial cell line N/N1003A. Intact human lenses were exposed to UVB in organ culture. [3H]Thymidine incorporation was measured in cultured lens epithelial cells by incubation with the radiolabel. The effects of isobutyl methyl xanthine (IBMX), an inhibitor of phosphodiesterase and of dibutyryl cyclic adenosine monophosphate (cAMP), an analog of cAMP, on PGE2 synthesis and DNA synthesis, were determined. Rabbit eyes were exposed to UVB radiation in vivo. Intraocular pressure was measured at specific times after exposure. Aqueous humor was remove from rabbit eyes, and its PGE2 content was measured by RIA.

RESULTS

Cultured human lens epithelial cells (HLE), like rabbit lens epithelial cells (RLE), showed a dose-dependent increase in basal PGE2 synthesis 24 hours after UVB exposure. However, the amount of PGE2 synthesis was 2000-fold higher in the rabbit cells. Ultraviolet-B radiation enhanced the incorporation of [3H]thymidine in lens epithelial cells. Pretreatment of cells with indomethacin reduce PGE2 synthesis and [3H]thymidine incorporation. The human and rabbit cells responded in a similar manner to changes in DNA synthesis after UVB exposure. The addition of IBMX or dbcAMP to indomethacin-treated, UVB-exposed cells restored DNA synthesis toward the levels observed in the UVB-exposed cells. An increase in the concentration of cAMP was observed in lens epithelial cells exposed to exogenous PGE2. PGE2 synthesis in intact human lenses also increased twofold 24 hours after UVB exposure. Exposure of the rabbit eye in vivo to an optimal dose of UVB produced an increase in the PGE2 levels of the lens and the aqueous humor. Measurements of the intraocular pressure (IOP) of the animals showed a decrease in IOP by 2.21 +/- 0.66 and 6.45 +/- 0.79 mm Hg (mean +/- SEM, P = 0.004, t-test) at 6 and 24 hours after UVB exposure, respectively. The decrease in IOP was prevented by pretreatment with indomethacin. Exposure of the rabbit lens to UVB radiation in vivo enhanced [3H]thymidine incorporation twofold into the lens. Pretreatment of rabbits with indomethacin before exposure reduced this response.

CONCLUSIONS

Results indicate that UVB exposure enhances PGE2 synthesis in HLE cultures as well as in rabbit lenses irradiated in vivo. This increased PGE2 synthesis is related to the increase in DNA synthesis observed after UVB treatment. The modulation of DNA synthesis in cultured lens epithelial cells after UVB exposure may be mediated by a cAMP-dependent mechanism.

Establishment of epithelial lines from cryopreserved lenses and capsule-epithelial preparations

PURPOSE

To determine if lens epithelial lines can be established from cryopreserved whole rabbit lenses and from cryopreserved capsule-epithelial preparations (CEPs).

METHODS

Lenses or freshly isolated CEPs were cryopreserved and subsequently thawed. Thawed whole lenses were cultured for 48 hours in growth medium and fixed, and whole mounts were examined for mitosis. In addition, CEPs were peeled from cryopreserved lenses and placed in tissue culture. Viability of cryopreserved cells was assessed measuring attachment efficiency and growth.

RESULTS

Whole mounts from cryopreserved lenses that were thawed and placed in organ culture in a serum-containing medium exhibited numerous mitotic figures. Freshly isolated CEPs that were cryopreserved and CEPs from cryopreserved lenses generated cell lines. Attachment efficiency was 90% within 3 hours of plating. When 50,000 cells from cryopreserved CEPs were cultured in growth medium, 10(6) cells were noted after 7 days of culture. The cells completed 27 population doublings and showed no sign of senescence.

CONCLUSIONS

Rabbit lens epithelial cell lines can be initiated from cryopreserved lenses or CEPs.

Hydrogen peroxide affects specific epithelial subpopulations in cultured rabbit lenses

PURPOSE

To investigate the effect of hydrogen peroxide on the epithelial cells of cultured rabbit lenses.

METHODS

Lenses were cultured in minimum essential medium containing a single dose of 0.03, 0.1, or 0.2 mM H2O2. Three hours later the medium was replaced with peroxide-free minimum essential medium. Lenses were also treated with 0.5 mM 1,3-bis(2-chloroethyl)-1 nitrosourea (BCNU) to lower the activity of glutathione reductase and then exposed to 0.03 mM H2O2 maintained nearly constant by glucose oxidase. After H2O2 treatment, lenses were fixed and whole mounts of the epithelium were prepared or lenses were processed for electron microscopy.

RESULTS

Cells exposed to a single dose of 0.03 mM H2O2 appeared normal; 0.1 mM H2O2 was not cytotoxic. Exposure to 0.2 mM H2O2 elicited swelling in cells in the pre-equatorial region (30 minutes) followed by the formation of islands of cells in the pre-equatorial region at 1 hour. Central epithelial cells appeared normal at 1 hour, were swollen at 3 hours and dead at 24 hours. By 48 hours, dead cells were found in the pre-equatorial and central regions. Cells in the peripheral region of the epithelium did not exhibit cytotoxicity. If lenses were pretreated with BCNU and then challenged with a maintained level of 0.03 mM H2O2, cytotoxicity was induced in the central and pre-equatorial regions. Cells in the peripheral region survived BCNU-H2O2 treatment.

CONCLUSIONS

Cells in the peripheral region of cultured lenses were more resistant to H2O2 cytotoxicity than cells in the central and pre-equatorial regions. The antioxidant defense or repair systems for H2O2-induced damage do not appear to be uniformly distributed in subpopulations of the lens epithelium.

Lens epithelium: a primary target of UVB irradiation

Cultured rabbit lenses and cultured rabbit lens epithelial cells were irradiated with UV to correlate morphological changes in the epithelium with physiological changes in the whole lens during the development of UV-induced cataract. Two UV spectral ranges were utilized; one spanned 290 to 340 nm and was designated near-UV, the other was a narrower, pure UVB region: 303 to 313 nm, designated UVB. Irradiation with either spectrum of the anterior surface of whole lenses caused opacification and a dose-dependent loss of ion homeostasis as measured by Na+ and Ca2+ concentrations in whole lenses. It was determined that cation pump activity, assessed by 86Rb uptake, continued to decline steadily during culture after UV irradiation. Whole mount preparations of the epithelial cell layer of UVB-irradiated lenses revealed morphological changes within 2 hr of irradiation and cell death after 20 hr. Following posterior irradiation of whole lenses, the epithelial cells remained viable and lenses remained transparent during 3 days of culture, presumably because UV photons did not reach the epithelium. Absorption of UV photons by posterior fiber cell membranes and proteins did not cause opacification. To learn more about the epithelial damage, cultured rabbit lens epithelial cells were irradiated, UVB treatment retarded growth over a 7-day period in cultured cells. The surviving cells at day 7 were abnormal in appearance and the potassium concentration was approximately 50% less than controls, a finding which may explain the previously reported reduction in protein synthesis by UVB irradiation. Collectively, the data suggest that UV cataract is initiated by damage to the epithelium, including a change in membrane permeability leading to loss of ion homeostasis in the lens.

Differentiation and angiogenic growth factor message in two mammalian lens epithelial cell lines

Lens epithelial cells in culture can sometimes be induced to form spheroid aggregates termed lentoid bodies, composed of cells exhibiting various characteristics of the more highly differentiated lens fiber cells. However, lentoid bodies are often slow to form, and the ability to produce them declines with serial subculture. It was therefore of interest to establish and/or characterize lens epithelial cell lines capable of forming lentoid bodies. The differentiation state was assessed in lentoid bodies formed by each of two lens epithelial cell lines, the transformed alpha TN4 cell line from mouse and the nontransformed N/N1135A cell line from rabbit. Lentoid and monolayer cultures of each cell line were examined for transcripts of the lens-specific alpha A-crystallin ("alpha A"), gamma D-crystallin ("gamma D"; formerly gamma 1-crystallin) and MP26 genes. alpha TN4 lentoid bodies contained 2.5 times the alpha A RNA found in monolayer cells, but lacked detectable gamma D and MP26 RNA. None of the three markers were detected in either lentoid or monolayer N/N1135A cultures grown under the conditions described. Lentoid body formation alone, therefore, does not indicate the extent of differentiation occurring. At least some of the changes in cell adhesion occurring during lentoid body formation involve laminin-like and fibronectin-like interactions, and are reminiscent of those observed during embryonic lens formation. Finally, vascular endothelial growth factor mRNA was absent from the lens but present in alpha TN4 cells, suggesting a mechanism whereby the lens tumors of the founder mouse became vascularized.

Modulation of lens epithelial cell proliferation by enhanced prostaglandin synthesis after UVB exposure

PURPOSE

The goals of this investigation were to examine the synthesis of prostaglandins after UVB exposure of lens epithelial cells and to investigate their role in cell proliferation.

METHODS

Cultured rabbit lens epithelial cells (cell line N/N1003A) were exposed to low levels of UV radiation. Prostaglandins were assayed by radioimmunoassay; products of arachidonic acid metabolism were analyzed by thin-layer chromatography and mass spectroscopy. Cell proliferation was measured by [3H]thymidine incorporation and proliferative autoradiography.

RESULTS

Cultured lens epithelial cells exposed to UVB radiation showed a dose-dependent increase in basal prostaglandin synthesis measured 24 hours after UV exposure. At an optimal dose (250 J/m2) of UVB, prostaglandin E2 (PGE2) synthesis was enhanced tenfold. Product identity was confirmed using thin-layer chromatography and mass spectroscopy with authentic standards. Incubation of irradiated cells with exogenous arachidonic acid followed by extraction and thin-layer chromatography revealed that the cultures produced PGE2, prostaglandin I2 (measured as 6-keto-prostaglandin F1 alpha), prostaglandin F2 alpha, and hydroxyeicosatetraenoic acid. The synthesis of all of these products was enhanced threefold in cells exposed to 250-J/m2 UVB. Indomethacin pretreatment eliminated the synthesis of prostaglandins, further confirming their identity. To discover the relationship between PGE2 synthesis and irradiation-induced cell proliferation, [3H]thymidine incorporation into DNA was determined 24 or 48 hours after exposure. These experiments revealed a fivefold increase in incorporation induced by UVB exposure. UVB-enhanced incorporation of thymidine was eliminated by pretreatment of cultures with indomethacin to eliminate PG synthesis. However, when 100 nM PGE2 was added to the indomethacin-treated irradiated cultures, incorporation of the label was restored toward the level detected in the UVB-stimulated cells. Addition of other prostaglandins to the cultures was ineffective.

CONCLUSIONS

The results indicate that the synthesis of PGE2 is enhanced by exposure of lens epithelial cells to UVB radiation. PGE2 seems to play a specific role in cell proliferation after UV exposure. This increase in PGE2 synthesis may be important in posterior subcapsular cataract formation in humans and in animals exposed to UVB radiation in vivo.

Action spectrum for cytotoxicity in the UVA- and UVB-wavelength region in cultured lens epithelial cells

PURPOSE

This study was done to quantitate the biologic effects of different wavelengths of radiation in the UVA- and UVB-wavelength region on cultured rabbit lens epithelial cells.

METHODS

An action spectrum for UV-induced cytotoxicity as measured by colony-forming ability was determined using six different monochromatic wavelengths from 297 to 405 nm in rabbit lens epithelial cell line N/N1003A. Cell survival was determined by clonogenic assay. Fluence rates were monitored with a calibrated radiometer.

RESULTS

Survival curves show that cell killing was most efficient at 297 nm. After quantum correction, the efficiency of 297-nm radiation in cell killing was 7 times greater than was 302-nm radiation. Radiation at 297 nm was more than 170, 340, 560, and 2000 times as effective in cell killing as 313-, 325-, 334-, and 365-nm radiation, respectively. The action spectrum had a shape similar to the DNA absorption spectrum in the UVB region, suggesting that DNA may be one of the critical targets for damage to the cells. At wavelengths longer than 313 nm, the shape of the action spectrum deviated from the DNA absorption spectrum.

CONCLUSIONS

Cytotoxicity of UV radiation in cultured lens epithelial cells varies greatly with wavelength within the UVA and UVB regions. Different mechanisms may predominate in the two wavelength regions. Cultured cells may provide a suitable system for investigating the mechanisms by which UV radiation damages lens epithelial cells and leads to cataract formation.

The superoxide dismutase mimic TEMPOL protects cultured rabbit lens epithelial cells from hydrogen peroxide insult

The superoxide dismutase mimic, 4-hydroxy TEMPO (TEMPOL), was used to investigate the mechanism by which H2O2 damages cultured rabbit lens epithelial cells and to identify some of the targets of H2O2 insult. Most studies aimed at determining the mechanism by which H2O2 exerts its cytotoxic effect have used iron chelators to prevent the generation of the damaging hydroxyl radical. Since TEMPOL does not chelate transition metals, we were afforded an additional means of investigating the mechanism by which H2O2 exerts its cytotoxicity. Cells at low or high density were cultured in MEM containing 5 mM TEMPOL and exposed to a single sub-lethal dose of 0.05 or 0.5 mM H2O2, respectively. Analysis of EPR spectra indicated that TEMPOL was stable in MEM, did not destroy H2O2 and penetrated the intracellular fluid. TEMPOL prevented or curtailed the H2O2-induced inhibition of cell growth, blebbing of the cell membrane, the decrease in NAD+, the activation of poly ADP-ribose polymerase, an enzyme involved in DNA repair, and limited the induction of single strand breaks in DNA normally brought about by H2O2. TEMPOL did not prevent the H2O2-induced decrease in reduced glutathione, lactate production, and the activity of glyceraldehyde 3-phosphate dehydrogenase, or the H2O2-induced increases in oxidized glutathione and hexose monophosphate shunt activity. Addition of TEMPOL 1-15 min after exposure of cells to H2O2 offered partial protection from the inhibition of cell division. TEMPOL at 5 mM did not inhibit cell growth. These results, coupled with our other findings suggest that some of the H2O2-induced damage in cultured rabbit LECs is mediated by intracellular redox-active metals involved in the Haber-Weiss cycle. Cellular changes not protected by TEMPOL, including attack of H2O2 on the thiol groups of GSH (mediated through glutathione peroxidase) and G3PDH, are likely brought about by H2O2 itself and not by reactions of oxygen free-radicals generated from H2O2.

Hyperbaric oxygen inhibits the growth of cultured rabbit lens epithelial cells without affecting glutathione level

Studies on human patients and experimental animals indicate that hyperbaric O2 can opacify the lens nucleus and damage the lens epithelium in vivo. Here we investigate the effects of hyperbaric O2 on cultured rabbit lens epithelial cells (LECs). When the cells were exposed to 50 atm O2 (99% O2 + 1% CO2) for 3 hr there were no immediate effects on morphology, viability and transport processes (uptake of 86Rb and 14C-alpha AIB). In addition, the O2 treatment did not lower the high level of reduced glutathione or increase the low level of oxidized glutathione. However, 50 atm O2 did produce a near doubling in the glycolytic rate which maintained ATP at levels only slightly lower than normal. Although the 3-hr O2 treatment was not lethal, it completely inhibited cell division for 2 days. After 2 days, growth was initiated and, at day 7 the rate of growth was faster than the controls (control cells were treated with ambient air or 50 atm N2 for 3 hr). Cells treated with 8 atm O2 for 3 hr exhibited a slowed rate of growth, relative to controls, while exposure to 2 atm O2, did not inhibit mitosis. Changes in morphology (multilayering and elongation) of cells exposed to 50 atm O2, but not the controls, were evident 7 days after the 3-hr exposure. The incorporation of [35S]methionine into individual polypeptides and [3H]thymidine into DNA was significantly inhibited immediately following a 3-hr treatment with 50 atm O2, but both parameters recovered within 2 days. DNA strand breaks were observed in LECs following hyperbaric O2 treatment as low as 4 atm O2 for 3 hr and increased with higher pressures of O2, but not N2. Treatment with 50 atm O2 nearly doubled the activity of the DNA repair enzyme, poly-ADP-ribose polymerase, and decreased the level of its substrate NAD+; the latter effect was reduced by 3-aminobenzamide, an inhibitor of the enzyme. Thus, although LECs tolerated brief exposures to high pressures of O2 without cell death, DNA damage occurred at relatively low pressures of O2. All of the effects of hyperbaric O2 on LECs occurred without any alteration of the normal levels of reduced and oxidized glutathione. It appears that GSH is important in maintaining cell viability during exposure to an elevated level of O2, but that it is incapable of preventing O2-induced effects on growth and DNA.

Effect of ultraviolet-B radiation on protein synthesis in cultured lens epithelial cells

Exposure of cultured rabbit lens epithelial cells to repetitive doses of UV-B radiation delays their growth and alters the synthesis of specific proteins. Irradiated cells on the shoulder of the survival curve exhibited a dose-dependent decrease in growth when subcultured in serum-supplemented medium. UV-B irradiation did not affect the subsequent attachment efficiency of the cells. Control and UV-B irradiated cells were incubated with [35S]methionine and the pattern of protein synthesis in the cells was analyzed by SDS-PAGE and autoradiography. Analysis of the labeled proteins from cells exposed to UV-B radiation showed the induction of a 32 kD polypeptide and the loss of a 26 kD polypeptide compared with controls. Analysis of the proteins released by the UV-B irradiated cells into the culture medium revealed the 50% loss of a 37 kD radiolabeled protein compared with controls. The alteration of protein synthesis in lens epithelial cells by UV-B radiation may contribute to cataract formation.

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)

Susceptibility of lens epithelial membrane SH groups to hydrogen peroxide

Although membrane SH groups are thought to be targets of oxidative insults, no measurement of lens epithelial membrane SH groups following exposure to potentially damaging oxidants has been reported. Here we investigate the effect of hydrogen peroxide, an oxidant found in the aqueous humor, and of p-chloromercuriphenylsulfonic acid (p-chloromecuribenzene-sulfonic acid) (PCMBS), a relatively impermeant sulfhydryl probe, on membrane SH groups and ion homeostasis in cultured lens epithelial cells. Exposure to PCMBS caused a 10% loss of membrane SH groups, an increase in sodium and calcium levels, and a decrease in potassium, but did not affect the intracellular level of glutathione (GSH). After 5 min of exposure to an initial concentration of 1.0 mM hydrogen peroxide, GSH declined from 14.1 mM to 3 mM, there was a 20% loss of membrane SH groups and within 1 hr, potassium declined from 132 to 116 mM. Cells that were exposed to 0.1 or 0.5 mM peroxide did not exhibit significant loss of membrane SH groups and did not show a decrease in GSH comparable to that found in cells treated with 1 mM peroxide. The peroxide induced loss of membrane SH groups and subsequent change in ion homeostasis occurred only when there was a rapid and sustained loss of intracellular glutathione. Thus lens epithelial cell membrane SH groups are not only important in ion regulation but are targets of hydrogen peroxide when the intracellular level of GSH is significantly diminished.

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.

Histochemical localization of catalase in cultured lens epithelial cells

The diaminobenzidine (DAB) technique was used to investigate the localization of the peroxidatic activity of catalase in cultured lens epithelial cells at the ultrastructural level. Cultured rabbit, bovine and mouse lens epithelial cells incubated in an alkaline DAB reaction mixture contained catalase-positive microperoxisomes which were randomly distributed throughout the cytoplasm. The reaction product of catalase was abolished when cells were treated with 3-amino-1-H-1,2,4-triazole, a specific inhibitor of catalase, and was not detected when DAB or H2O2 was omitted from the incubation medium, or when the pH of the reaction mixture was lowered to 7.0. The fine structure of lens epithelial cells that were constantly exposed to 0.025 or 0.05 mM hydrogen peroxide was also determined. Lens epithelial cells that were constantly exposed to 0.05 mM H2O2 for 1-3 h exhibited damaged mitochondria, a phenomenon that did not occur when the cells were exposed to 0.025 mM peroxide.

A rabbit lens epithelial cell line supports expression of an exogenous crystallin gene characteristic of lens fiber cell differentiation

Cell lines derived from the lens generally fail to maintain synthesis of crystallins in long-term culture. Here we demonstrate that the N/N1003A line of undifferentiated lens epithelial cells, derived from a newborn rabbit, does not produce detectable levels of alpha-, beta- or gamma-crystallin transcripts, yet is capable of supporting the transient expression of the mouse gamma 2-crystallin promoter, a promoter which is active only in terminally differentiated lens fiber cells in vivo. Analysis of a set of deletion constructs suggested that sequences required for activity of the mouse promoter in N/N1003A cells are similar, but not identical, to those previously shown to be essential in primary chick embryo lens explants. Therefore, these results suggest that different transcriptional factors may be capable of supporting lens-specific activity of the mouse gamma 2 promoter. In addition, this cell line, N/N1003A, should be useful for investigations on the elements regulating gamma-crystallin gene expression.

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.

Retention of lens specificity in long-term cultures of diploid rabbit lens epithelial cells

Rabbit lens epithelial cells from newborn animals exhibited limited growth when cultured under standard conditions. Cell lines were generated when explants from individual lenses were cultured in medium supplemented with conditioned medium or untreated rabbit serum. All lines exhibited a stable epithelial morphology. One line, N/N1003A, was examined extensively with respect to its growth, ploidy, and maintenance of lens-specific functions. Cells at population-doubling level (pdl) 120 exhibited a normal chromosomal banding pattern, were diploid, were non-tumorigenic in vivo, did not grow in suspension culture, and did not exhibit sustained growth in medium supplemented with low concentrations of serum. The shape of the growth curves and the final density for cells at pdl 24 and 181 exposed to various concentrations of serum were identical. The cells showed no diminution in growth as a function of in vitro age. The cells retained lens-specific functions. Proteins were isolated from cells at pdl 40 and 170, and were separated on polyacrylamide gels. Western immunoblot analysis using antiserum to alpha-crystallin, a tissue-specific protein found in lens epithelial cells in vivo, indicated the presence of alpha-A- and alpha-B-crystallin polypeptides. The cells also contained the transcription factors required for activating the murine alpha-A-crystallin gene promoter, which is known to function with precise tissue specificity. When an expression vector including the bacterial chloramphenicol acetyltransferase (CAT) gene controlled by the alpha-A-crystallin gene promoter was introduced into the lens epithelial cells, the CAT gene was expressed.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Although it has been shown that cultured rabbit lenses can adequately defend against the 0.03-0.05 mM level of H2O2 normally found in aqueous humor, the contribution of the epithelium in this process has not been well defined. In the present study, the peroxide-detoxifying ability of the epithelium is evaluated in cultured rabbit lens cells established from 4-6-day-old rabbits and compared to that of skin fibroblasts from rabbits of the same age. When cells were cultured in medium containing H2O2, the concentration of peroxide rapidly decreased; however, various concentrations could be maintained for 3-hr periods by using glucose oxidase to enzymically generate H2O2. At an extracellular level of 0.03 mM H2O2, the rate of detoxification of peroxide by epithelial cells was 2 mumol H2O2 (8 x 10(5) cells)-1 3 hr-1, twice as fast as that for fibroblasts. Epithelial cells contained a high level of reduced glutathione (GSH) equal to 36 nmol (8 x 10(5) cells)-1, twice that present in the fibroblasts. The concentration of GSH in 8 x 10(5) epithelial cells, a number of cells normally present in one intact rabbit lens epithelium, remained constant during 3 hr of exposure to H2O2 levels as high as 0.03 mM, even though the amount of H2O2 taken up under these conditions was sufficient to oxidize completely the cellular GSH every 2 min. In contrast, the GSH content of fibroblasts declined at levels of peroxide above 0.01 mM. Participation of the glutathione redox cycle in the H2O2-detoxification process was demonstrated from studies of hexose monophosphate shunt (HMPS) activity as measured by oxidation of [1-14C]-labeled glucose. The oxidation of [1-14C]-glucose in epithelial cells was stimulated 13 times that of controls during exposure to 0.04-0.05 mM H2O2, while the corresponding increase in oxidation of [6-14C]-labeled glucose was only 1.6 times. In contrast, maximum shunt activity in fibroblasts occurred at 0.03-0.04 mM H2O2 and was six times the control value. The growth potential of the cells following a 3-hr exposure to H2O2 was also used as a measure of oxidant toxicity in both cell types. Concentrations of H2O2 up to 0.03 mM had no effect on the growth of 8 x 10(5) epithelial cells but did diminish the growth of the same number of fibroblasts. Cell density was found to be an important parameter in the ability of the cells to tolerate H2O2.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin growth factor and epidermal growth factor trigger mitosis in lenses cultured in a serum-free medium

The mitogenicity of insulin, insulin growth factor (IGF), and epidermal growth factor (EGF) was evaluated on rabbit lenses cultured in medium KEI-4. IGF, the most highly purified of the insulin-like growth factors was a potent mitogen for mammalian lens epithelia cells. IGF and EGF triggered cell proliferation throughout the normally amitotic central and pre-equatorial region of the epithelium. The mitotic response elicited by IGF and EGF was dose dependent, was preceded by DNA synthesis, exceeded that engendered by equimolar insulin, and exhibited a chronology identical to that brought about by crystalline insulin. Lenses cultured in KEI-4 alone or in KEI-4 supplemented with growth hormone, proinsulin, the A and/or B chain of insulin, or MSA, another of the insulin-like growth factors belonging to the somatomedin family, did not show a mitotic response. Simultaneous exposure of the lens to IGF and EGF resulted in an increase in the total number of mitotic figures over that obtained with equimolar concentrations of IGF and EGF. Our results suggest that IGF or other insulin-like growth factors may be capable of regulating cell division in the mammalian lens in vivo. That the lens epithelium responded to IGF and EGF may indicate that lens epithelial cells are subject to multiple hormonal interaction. Since growth factors appear to be cell type specific, information obtained from the rabbit lens epithelium should be useful in delineating the factors and conditions required for the growth of cultured human lens cells.

Thrombin induces cell division in rabbit lenses cultured in a completely defined serum-free medium

Experiments were initiated to gain an understanding of the environmental factors that may regulate injury-induced mitosis and wound healing in the mammalian lens. The addition of thrombin or trypsin to a completely defined serum-free medium stimulated cell proliferation and migration in the cultured mammalian lens. A 30 min exposure of the rabbit lens to highly purified thrombin induced DNA synthesis and mitosis throughout the normally amitotic central region of the lens epithelium. Lenses exposed to thrombin for 24 or 52 hr exhibited cell migration and mitosis. The mitotic response brought about by thrombin was totally curtailed by hirudin and antithrombin III. Prothrombin, papain, or pepsin were not mitogenic toward the cultured lens. A 30 min exposure of the lens to trypsin induced cell division and migration, a response that did not occur in the presence of trypsin inhibitors. Lenses cultured in a trypsin-containing medium for 24 hr showed extensive cell death throughout the entire central region of the epithelium. In addition, an endogenous serine protease, plasminogen activator, was detected in cultured rabbit lens epithelial cells. Wound healing in the lens in vivo is accompanied by cellular migration and mitosis. The present experiments demonstrate that a highly purified serine protease, thrombin, which is present at the site of lenticular injury in vivo, is capable of inducing mitosis and migration in lens epithelia. The results suggest that thrombin or other exogenous and endogenous serine proteases might contribute to the process of wound healing in the ocular lens.

Establishment and characterization of a lens epithelial cell line from an eight year old rabbit

Although information is available on the in vitro properties of lens epithelia of young adult animals from several species, few, if any reports document the conditions required for the initiation and long-term culture of lens epithelium from animals beyond their medium life-span. We report here on the conditions required for the culture of lens cells from an 8 year old rabbit. New Zealand White rabbits have a median life-span of approximately 7 years. Primary culture was initiated in MEM supplemented with 10% rabbit serum. Cells reached confluency within 25 days, responded to serum in a dose dependent manner and had an average doubling time of 23 h during the logarithmic growth phase. Cells increased in number in a dose dependent manner when insulin, insulin growth factor, epidermal growth factor (EGF), or fibroblast growth factor (FGF) was added to the culture medium. Thus, lens epithelia from this very old rabbit retained the ability to respond to highly purified growth factors. Cells exposed to a medium supplemented with insulin, EGF and FGF showed a five-fold increase in number at day 7 of culture, a value exceeding that brought about by the individual growth factors. An examination of chromosomal preparations indicated that the cells were aneuploid. Whether the aneuploidy was acquired in vitro or is a normal adjunct of aging in the lens in vivo is unknown. Proteins extracted from this line contained polypeptides that migrated to the position of and had apparent molecular weights of lens proteins.

Both human and newborn rabbit lens epithelial cells exhibit similar limited growth properties in tissue culture

Human lens cells from 5-91-year old individuals were cultured in 8 different basal media containing fetal bovine, adult bovine, rabbit or human serum or human plasma or in a serum-containing medium supplemented with insulin, epidermal growth factor, fibroblast growth factor plus other hormones or trace elements. Cultures were initiated from explants of the capsule and epithelium or following enzymatic dissociation of cells from the capsule. Under all conditions the epithelial cells had a limited doubling potential. As a function of time in culture, cells enlarged, displayed numerous filaments and exhibited apparent in vitro senescence. Lens epithelia from 4-6 day old rabbits cultured under identical conditions mimicked the behavior of human lens cells. Lens epithelia from newborn rabbits may be a suitable model for investigating the basis of apparent in vitro senescence in this cell type and could help in defining the conditions required for the long-term growth of human lens cells. The limited growth of human lens epithelia suggests that these cells require tissue-specific nutrients or hormonal supplements not present in standard tissue culture media.