Lens epithelial cell apoptosis appears to be a common cellular basis for non-congenital cataract development in humans and animals

Cellular lipid peroxidation end-products induce apoptosis in human lens epithelial cells

Hydrogen peroxide (H(2)O(2)), an oxidant present in high concentrations in the aqueous humor of the elderly eyes, is known to impart toxicity to the lens---apoptosis being one of the toxic events. Since H(2)O(2) causes lipid peroxidation leading to the formation of reactive end-products, it is important to investigate whether the end-products of lipid peroxidation are involved in the oxidation-induced apoptosis in the lens. 4-Hydroxynonenal (HNE), a major cytotoxic end product of lipid peroxidation, has been shown to mediate oxidative stress-induced cell death in many cell types. It has been shown that HNE is cataractogenic in micromolar concentrations in vitro, however, the underlying mechanism is not yet clearly understood. In the present study we have demonstrated that H(2)O(2) and the lipid derived aldehydes, HNE and 4-hydroxyhexenal (HHE), can induce dose- and time-dependent loss of cell viability and a simultaneous increase in apoptosis involving activation of caspases such as caspase-1, -2, -3, and -8 in the cultured human lens epithelial cells. Interestingly, we observed that Z-VAD, a broad range inhibitor of caspases, conferred protection against H(2)O(2)- and HNE-induced apoptosis, suggesting the involvement of caspases in this apoptotic system. Using the cationic dye JC-1, early apoptotic changes were assessed following 5 h of HNE and H(2)O(2) insult. Though HNE exposure resulted in approximately 50% cells to undergo early apoptotic changes, no such changes were observed in H(2)O(2) treated cells during this period. Furthermore, apoptosis, as determined by quantifying the DNA fragmentation, was apparent at a much earlier time period by HNE as opposed to H(2)O(2). Taken together, the results demonstrate the apoptotic potential of the lipid peroxidation end-products and suggest that H(2)O(2)-induced apoptosis may be mediated by these end-products in the lens epithelium.

Natural evolution of Elschnig pearl posterior capsule opacification after posterior capsulotomy

PURPOSE

To study the natural evolution of Elschnig pearl posterior capsule opacification (PCO) after neodymium:YAG posterior capsulotomy.

SETTING

Instituto Oftalmológico Murciano, Murcia, Spain.

METHODS

This prospective study comprised 173 eyes that had a posterior capsulotomy to treat Elschnig pearl PCO. Patients were included in the study at different stages after posterior capsulotomy and followed using serial anterior segment photographs for 18 to 48 months.

RESULTS

During the first year after the capsulotomy, pearls proliferated and migrated toward the capsulotomy edge in 83% of the cases, leading to the formation of a collar-like ring of pearls around the capsulotomy. During the second year, an increase in cells was observed in 45% of cases; in 45% of eyes, there were fewer pearls. The amount of pearls decreased in 48%, 52%, and 26% of cases at 3, 4, and 5 years, respectively, and were completely gone in 6%, 17%, and 45%. After 5 years, 17% of eyes had a reduction in pearls and 67% had no pearls.

CONCLUSIONS

Elschnig pearl PCO is a self-limiting process. After posterior capsulotomy, cells proliferated and attempted to occlude the capsular opening. Later, a reduction in the amount of cells was observed, leading to a completely clear posterior capsule in most eyes.

Human bcl-2 gene attenuates the ability of rabbit lens epithelial cells against H2O2-induced apoptosis through down-regulation of the alpha B-crystallin gene

It is well established that the proto-oncogene, bcl-2, can prevent apoptosis induced by a variety of factors. Regarding the mechanism by which BCL-2 prevents cell death, one theory suggests that it acts by protecting cells from oxidative stress. In the lens system, oxidative stress-induced apoptosis is implicated in cataractogenesis. To explore the possibility of anti-apoptotic gene therapy development for cataract prevention and also to further test the anti-oxidative stress theory of BCL-2 action, we have introduced the human bcl-2 gene into an immortalized rabbit lens epithelial cell line, N/N1003A. The stable expression clones of both vector- and bcl-2-transfected cells have been established. Treatment of the two cell lines with H(2)O(2) revealed that bcl-2-transfected cells were less capable of detoxifying H(2)O(2) than the control cells. Moreover, bcl-2-transfected cells are more susceptible to H(2)O(2)-induced apoptosis. To explore why bcl-2-transfected cells have reduced resistance to H(2)O(2)-induced apoptosis, we examined the expression patterns of several relevant genes and found that expression of the alphaB-crystallin gene was distinctly down-regulated in bcl-2-transfected cells compared with that in vector-transfected cells. This down-regulation was specific because a substantial inhibition of BCL-2 expression through antisense bcl-2 RNA significantly restored the level of alphaB-crystallin and, moreover, enhanced the ability of the bcl-2-transfected cells against H(2)O(2)-induced apoptosis. Introduction of a mouse alphaB-crystallin gene into bcl-2-transfected cells also counteracted the BCL-2 effects. Down-regulation of alphaB-crystallin gene was largely derived from changed lens epithelial cell-derived growth factor activity. Besides, alphaB-crystallin prevents apoptosis through interaction with procaspase-3 and partially processed procaspase-3 to prevent caspase-3 activation. Together, our results reveal that BCL-2 can regulate gene expression in rabbit lens epithelial cells. Through down-regulation of the alphaB-crystallin gene, BCL-2 attenuates the ability of rabbit lens epithelial cells against H(2)O(2)-induced apoptosis.

H(2)O(2)-mediated oxidative stress activates NF-kappa B in lens epithelial cells

In the mammalian lens, intracellular oxidants produced by photo-oxidative processes and exposure to toxic chemicals constitute stresses that produce cellular oxidative damage, result in changes in gene expression, and are causally related to cataract formation. Currently, it is believed that H(2)O(2) is the major oxidant to which the lens is exposed. In this report, we examine the activation and regulation of the oxidant-sensitive transcription factor, NF-kappa B, by H(2)O(2)-mediated oxidative stress in lens epithelial cells. Lens epithelial cells treated with H(2)O(2) demonstrated at 1 h a strong activation of NF-kappa B which returned to basal levels by 2 h. Under proteasome inhibition using both MG132 and lactacystin, H(2)O(2)-mediated activation of NF-kappa B was prevented, implicating the involvement of proteasome degradation of I kappa B proteins as being necessary for this activation. However, Western blot analysis demonstrated no degradation of I kappa B-alpha, -beta, or -epsilon associated with H(2)O(2)-mediated NF-kappa B activation. In comparison, when cells were treated with the cytokine TNF-alpha, NF-kappa B was strongly activated and degradation of both I kappa B-alpha and -beta was observed. These results clearly demonstrate that H(2)O(2)-mediated oxidative stress activates NF-kappa B in lens epithelial cells, which may subsequently lead to changes in gene expression. The results also reveal that different signaling pathways in the activation of NF-kappa B in lens epithelial cells are utilized by H(2)O(2) and TNF-alpha. These different pathways of NF-kappa B activation may be required to effect specific NF-kappa B-dependent gene expression in response to these different stimuli.

Removal of oxidatively damaged proteins from lens cells by the ubiquitin-proteasome pathway

Understanding how oxidized proteins are removed is important since accumulation of such damaged proteins is causally related to cellular and organismic dysfunction, disease and aging. Previous work showed that activity of the ubiquitin-proteasome pathway (UPP) in lens cells increased during recovery from oxidative stress ( Shang et al., 1997b : J. Biol. Chem. 272, 23086-93). In this study we sought to determine if the up-regulation of the UPP during recovery from oxidative stress has a role in selective removal of oxidized proteins from the cells. In cells which were not exposed to peroxide, inhibition of the proteasome with MG132 or clasto-lactacystin beta-lactone had little effect on protein carbonyl levels. However, inhibition of the proteasome in the 20 microM peroxide-treated cells caused an approximate 60% increase in levels of protein carbonyl and an approximate 100% increase in levels of ubiquitin conjugates. The carbonyl-containing proteins that accumulated in the presence of the proteasome inhibitor co-localized with high molecular mass ubiquitin-protein conjugates. Furthermore, isolated carbonyl-containing proteins from H2O2-treated cells were ubiquitinated, and ubiquitin-conjugates were enriched with carbonyl-containing proteins. The diminished effect of proteasome inhibitors on protein carbonyl levels, together with the robust increase in ubiquitin-protein conjugates and accompanied increases in oxidized proteins, upon exposure to 60 microM H2O2 indicate that the proteasomal step of the UPP is more susceptible to oxidative inactivation than the ubiquitination step. In fact, oxidative stress is associated with a hyperactivation of the ubiquitin-activating enzyme. These data indicate that the UPP plays a role in removal of oxidatively damaged proteins from cells and that attenuation of the UPP activity may result in cytotoxic accumulation of damaged proteins, possibly including the ubiquitinated forms.

[Radiation-induced cataract: physiopathologic, radiobiologic and clinical aspects]

Cataractogenesis is a widely reported late effect of irradiated crystalline lens. In this review the authors discussed the different aspects of radiation cataract pathogenesis, and the different mechanisms involved in the lens opacification, particularly the epithelium modifications such as epithelial cell death. The authors also reported the influence of radiation exposure on cataract formation following total body irradiation (TBI) and autologous or allogeneic bone marrow transplantation for hematologic malignancies. Moreover, the radiobiological parameters are not studied for the crystalline lens of human. We applied for the first time the linear-quadratic (LQ) and biological effective dose (BED) concept to TBI data. The calculated value of alpha/beta of 1 Gy is in the range of the values reported for the other late responding tissues. The other risk factors for cataract development after TBI such as age, gender, central nervous system boost, long-term steroid therapy and heparin administration are discussed. In terms of cataract or sicca syndrome prevention, numerous compounds have been successfully tested in experimental models or used for the prevention of radiation-induced xerostomia in patients treated for head and neck cancer. However, none of them has been clinically evaluated for ocular radiation late effects prevention. In this report the authors discussed some of the radioprotectors potentially interesting for radiation-induced cataract or sicca syndrome prevention.

Caspase-3 is actively involved in okadaic acid-induced lens epithelial cell apoptosis

Phosphorylation and dephosphorylation are important cellular events regulating major metabolic activities such as signal transduction, gene expression, cell cycle progression, and apoptosis. It is well documented that okadaic acid, a potent inhibitor of protein phosphatase-1 (PP-1) and -2A (PP-2A), can induce apoptosis in a variety of cell lines. Our recent studies have revealed that in the immortal rabbit lens epithelial cell line, N/N1003A, inhibition of PP-1, but not PP-2A, leads to rapid apoptosis of the lens epithelial cells. This induction of cell death is associated with up-regulated expression of a set of genes, including the tumor-suppressor gene, p53, and the proapoptotic gene, bax. In the present study, we demonstrate that inhibition of PP-1 by okadaic acid in the primary cultures of rat lens epithelial cells also leads to apoptotic death. Moreover, we show that the cysteine protease, caspase-3, is important in the execution of okadaic acid-induced apoptosis. Treatment of the primary cultures of rat lens epithelial cells with 100 nM okadaic acid up-regulates expression of caspase-3 at the mRNA, protein, and enzyme activity levels. Inhibition of the caspase-3 activity with a chemically synthesized inhibitor prevents okadaic acid-induced apoptosis in rat lens epithelial cells. Similar results are also observed in the immortal cell line N/N1003A. Furthermore, stable expression of the mouse gene encoding lens alphaB crystallin inhibits okadaic acid-induced apoptosis, and this inhibition is associated with repression of the okadaic acid-induced up-regulation of caspase-3 activity. Taken together, these results demonstrate that caspase-3 is actively involved in okadaic acid-induced lens epithelial cell apoptosis.

Transcriptional up-regulation of the protooncogenes c-fos and c-jun following vitreous removal and short-term in vitro culture of bovine lenses

Chemical (mainly oxidative) and mechanical (anterior capsule injury) stresses have been reported to up-regulate the expression of the protooncogenes c-fos and c-jun in the lens. Another potentially stressful, yet largely unexplored condition, inherent to all experiments requiring the in vitro culturing of isolated lenses, is vitreous removal. Based on the results of an extensive RNA gel blot analysis conducted on epithelial/capsule preparations isolated from calf lenses dissected and cultured under different conditions, we show, here, that lens isolation and short-term culture (1-2.5 hr, without any significant GSH depletion) result in a strong and time-dependent up-regulation of the c-jun and c-fos mRNAs. This response, which relies on transcriptional protooncogene activation and is more intense for c-fos than for c-jun, is in part prevented by the preservation of the lens-vitreous contact, but not by the culture of vitreous-stripped lenses on a vitreous bed. Supplementation of the culture medium with the antioxidant N -acetyl-cysteine slightly reduced the c-jun, but not the c-fos response. Protooncogene up-regulation thus appears to be mainly determined by the disruption of critical lens-vitreous interactions. Since this response takes place in the epithelial cells, these data also point to the existence of a communication mechanism whereby a posteriorly applied mechanical stress is transmitted to, and perceived by, the anterior lens surface.

The role of aldose reductase in sugar cataract formation: aldose reductase plays a key role in lens epithelial cell death (apoptosis)

Since aldose reductase is localized primarily in lens epithelial cells, osmotic insults induced by the accumulation of sugar alcohols occur first in these cells. To determine whether the accumulation of sugar alcohols can induce lens epithelial cell death, galactose-induced apoptosis has been investigated in dog lens epithelial cells. Dog lens epithelial cells were cultured in Dulbecco's modified Eagle's mimimum essential medium (DMEM) supplemented with 20% fetal calf serum (FCS). After reaching confluence at fifth passage, the medium was replaced with the same DMEM medium containing 50 mM D-galactose and the cells were cultured for an additional 2 weeks. Almost all of the cells cultured in galactose medium were stained positively for apoptosis with the terminal deoxynucleotidyl transferance-mediated biotin-dUTP nick end labeling (TUNEL) technique. Agarose gel electrophoresis of these cells displayed obvious DNA fragmentation, known as a ladder formation. All of these apoptotic changes were absent in similar cells cultured in galactose medium containing 1 microM of the aldose reductase inhibitor AL 1576. Addition of AL 1576 also reduced the cellular galactitol levels from 123+/-10 microgram/10(6) cells (n=5) to 3.9+/-1.9 microgram/10(6) cells (n=5). These observations confirm that galactose induced apoptosis occurs in dog lens epithelial cells. Furthermore, the prevention of apoptosis by an aldose reductase inhibitor suggests that this apoptosis is linked to the accumulation of sugar alcohols.

hTERT can function with rabbit telomerase RNA: regulation of gene expression and attenuation of apoptosis

Telomerase is a specialized DNA polymerase that adds telomeric sequences onto chromosome ends. The functional telomerase complex contains a telomerase reverse transcriptase (TERT) and also a telomerase RNA (TR). Although it is well established that the human telomerase reverse transcriptase (hTERT) can function well in different human cell lines, it has not been shown whether it is compatible with telomerase template RNA from other species. Here we report that the expressed hTERT is functionally compatible with rabbit telomerase template RNA (rTR) as demonstrated by TRAP assay. The direct interaction between hTERT and rTR is further confirmed by immunoprecipitation-linked RT-PCR in which rTR is detected from the complex immunoprecipitated by an anti-hTERT antibody. The hTERT expressed in rabbit lens epithelial cells demonstrates two major functions: modulation of expression of other genes and attenuation of apoptosis. Thus, telomerase has a variety of functions besides telomere synthesis, and the template RNA is functionally conserved between human and rabbit.

Evidence for apoptosis in the selenite rat model of cataract

The purposes of this experiment were (1) to determine if apoptosis was accelerated during formation of selenite cataract, and (2) to determine the role of calpains and caspases in lens apoptosis. Evidence for apoptosis in selenite-injected rats included: approximately 7-8% of epithelial cells in germinative zone were positive, disappearance of the nuclear membrane, condensation of the chromatin, and breakdown of PARP. Activation of calpains was indicated by characteristic limited proteolysis of crystallins, breakdown of alpha-spectrin to 150/145 kDa fragments, hydrolysis of vimentin, and autolytic breakdown of m-calpain. Selenite cataract did not have an appreciable effect on the mRNA levels for caspase-3, calpains, and calpastatin. This indicated the increased enzyme activity of m-calpain and caspase-3 in selenite cataract occurred at the enzyme level rather than by upregulation of mRNAs. Increased calpain and caspase activity may be linked to the selenite-induced apoptosis. Such data are important because they indicate that apoptosis may be a fairly early event in selenite cataract.

Cataractogenesis in neonatal Sprague-Dawley rats by N-methyl-N-nitrosourea

Cataract was induced by a single intraperitoneal injection of 100 mg/kg N-methyl-N-nitrosourea (MNU) to 0-, 5-, 10-, 15-, or 20-day-old male and female Sprague-Dawley rats. In day 0, 5, 10, and 15 MNU-treated rats, mature cataracts were constantly seen 7, 14, 14, and 30 days after dosing, respectively. In the day 20 MNU-treated rats, only subcapsular cataract was seen 30 days after dosing. Therefore, the rats exposed to MNU at an earlier age caused cataract more rapidly and severely. In the day 0 MNU-treated rats, 7-methyldeoxyguanosine DNA adduct was detected in the lens epithelial nuclei 12 hours after MNU dosing, followed by apoptosis, which was confirmed by morphology, by TUNEL signals, and by DNA ladder and peaked 3 days after MNU dosing. In the apoptosis cascade, upregulation of Bax, downregulation of Bcl-2, and increased CPP32 protease (caspase-3) activity were seen 12 hours after MNU dosing. Therefore, the pathogenesis of MNU-induced cataract was associated with DNA adduct formation in the lens epithelial cell nuclei leading to apoptosis by upregulation of Bax protein, downmodulation of Bcl-2 protein, and activation of caspase-3.

Dietary curcumin prevents ocular toxicity of naphthalene in rats

Administration of naphthalene is known to cause cataract formation in rats and rabbits and naphthalene-initiated cataract is frequently used as a model for studies on senile cataract in humans. Oxidative stress has been implicated in the mechanism of naphthalene-induced cataract. Curcumin, a constituent of turmeric, a spice used in Indian curry dishes, is an effective antioxidant and is known to induce the enzymes of glutathione-linked detoxification pathways in rats. During the present studies, we have examined whether low levels of dietary curcumin could prevent naphthalene-induced opacification of rat lens. The presence of apoptotic cells in lens epithelial cells was also examined by catalytically incorporating labeled nucleotide to DNA with either Klenow fragment of DNA polymerase or by terminal deoxynucleotidyl transferase (TdT), which forms polymeric tail using the principle of TUNEL assay. The results of these studies demonstrated that the rats treated with naphthalene and kept on a diet supplemented with only 0.005% (w/w) curcumin had significantly less opacification of lenses as compared to that observed in rats treated only with naphthalene. Our studies also demonstrate, for the first time, that naphthalene-initiated cataract in lens is accompanied and perhaps preceded by apoptosis of lens epithelial cells and that curcumin attenuates this apoptotic effect of naphthalene.

Stimulus-specific and cell type-specific cascades: emerging principles relating to control of apoptosis in the eye

Apoptosis has a critical role in development, homeostasis, wound healing, and the pathophysiology of disease in the organs of multicellular organisms. It has been implicated in these processes in retina, lens, cornea, trabecular meshwork, optic nerve, and the central nervous system pathways that contribute to vision. Considerable interest has been focused on inhibiting apoptosis to control disease and wound healing processes in which programmed cell death is thought to have a critical role. A simplified view led to the search for effective inhibitors of 'the final common pathway for apoptosis'. Recent studies have provided important insights into the modulators that participate in and regulate the apoptosis cascades which are activated in response to cytokines, ionizing radiation, chemotherapeutic agents, growth factor deprivation, and other stimulators of cell death. These studies lead to the inescapable conclusion that the apoptosis pathways are not only stimulus-specific, but also cell-type specific. These observations have important implications related to development of pharmacological strategies for controlling apoptosis-associated disease and apoptosis-initiated wound healing.

Gap junctions containing alpha8-connexin (MP70) in the adult mammalian lens epithelium suggests a re-evaluation of its role in the lens

A missense mutation in one of the three lens connexins, alpha8-connexin, has been recently shown to be the genetic basis of the zonular pulverant lens cataract. This connexin had been considered to be expressed only in lens fibre cells. The present studies show that alpha8-connexin is also expressed in the lens epithelial cell layer. For this study, the distribution of gap junctions in the adult bovine lens has been investigated by confocal immunofluorescence microscopy using antibodies against alpha8-connexin (MP70) and alpha1-connexin (Cx43). In addition to the anticipated localisation of alpha8-connexin to the broad faces of lens fibre cells as reported in other species, alpha8-connexin was also found colocalized with alpha1-connexin at plaques in the lateral epithelial-epithelial plasma membranes of the bovine lens. These data suggest that mixed alpha8-connexin/alpha1-connexin plaques are between epithelial cells at their apico-lateral plasma membranes, rather than between epithelial and fibre cells. Indeed, freeze fracture analyses of the epithelial-fibre cell interface failed to reveal gap junctions connecting the epithelium and the underlying fibre cells. Importantly, microdissection and subsequent immunoblotting of lens epithelium samples confirmed the immunolocalisation results. The data suggest mature mammalian lens epithelial cells could form either heteromeric, heterotypic and/or mixed homomeric-homotypic gap junctional complexes with unique physiological properties, an important point when considering the role of epithelial cell connexins in cataractogenesis.

Posterior capsule opacification. Part 1: Experimental investigations

Posterior capsule opacification (PCO) is the most frequent complication associated with decreased vision after cataract surgery. Previous methods of preventing PCO have not proven to be practical, effective, and safe for routine clinical procedure, but some novel concepts and methods have recently been developed. This 2-part review looks at clinical and experimental investigations of PCO, focusing on developments since 1992. Clinical aspects will be presented in a later issue. This paper addresses (1) in vitro models for PCO research; (2) pathophysiology and molecular biology of lens epithelial cells (LECs); (3) prevention of PCO. Of special interest are methods of culturing human LECs obtained by capsulotomy during cataract surgery, including those obtained with an intact capsular bag, to provide an in vitro model for investigating the pathophysiology of LECs; the effect of a sharp bend in the lens capsule that induces contact inhibition of migrating LECs; more specific inhibition of migrating LECs using an immunotoxin, b-FGF-saporin, or EDTA and RGD-peptides.

The effect of photochemical stress upon the lenses of normal and glutathione peroxidase-1 knockout mice

This communication investigates the effect of oxidative stress upon the lenses of young normal and glutathione peroxidase-1 (GSHPx-1) Knockout mice. Both normal and knockout lenses have similar biochemical and morpholigical characteristics and the elimination of GSHPx-1 only decreases slightly the ability of the lens to degrade H2O2. Examination of the effect of a 4 hr photochemical stress on morphological characteristics indicates that there is comparable damage in the normal and knockout lenses in the epithelial and bow regions while the posterior region remains normal. However, at 24 hrs post-insult, the normal lenses appear to recover somewhat in the bow region while the knockout bow and posterior regions have extensive damage. In contrast to the morphological data, the biochemical parameters (14C)choline transport and (3H)thymidine incorporation are affected to a somewhat greater extent in the knockout lenses than in normal lenses. While both of these parameters are further affected in the 24 hr post-insult period, there is no further change in the relative effects upon normal and knockout lenses. Non-protein thiol is affected in a similar manner in both lens types. The effect upon biochemical parameters of tertiary butyl hydroperoxide (TBHP) insult was similar to H2O2 and photochemical stress. The overall conclusion is that young GSHPx-1 knockout lenses handle oxidative stress somewhat less effectively than comparable normal lenses but non-stressed knockout lenses appear normal. These results differ from observations reported by Reddy et al. (1997) under somewhat different conditions.

Changes in the nucleolar and coiled body compartments precede lamina and chromatin reorganization during fibre cell denucleation in the bovine lens

Nuclear elimination accompanies differentiation in such specialized cell types such as erthyrocytes and lens fibre cells. It also accompanies apoptosis which has suggested that similar processes could operate in both. Denucleation occurs in the lens in order to reduce light scatter and this process is often disrupted in cataract. Using the adult bovine lens as a model system, nuclear changes accompanying denucleation are described with particular emphasis on the lamina, nucleolar and coiled body compartments in lens nuclei. Nuclear shape, chromatin reorganization and chromatin breakdown were also monitored to correlate the timing of events. Rearrangement of both A- and B-type nuclear lamins occurred in parallel with chromatin condensation and preceded changes in nuclear shape. The earliest changes detected in this study occurred in the coiled body and nucleolar compartments using coilin and fibrillarin antibodies respectively, suggesting that a shutdown in transcription is an early event in denucleation. Fibrillarin redistributed from an open floret pattern to several condensed spots which gradually decreased in intensity and eventually disappeared. Coilin, however, was localized in several microfoci prior to being reorganized into fewer larger foci. Prior to chromatin condensation, coilin redistributed to the nucleolar compartment and was absent from nuclei where chromatin had begun to condense. Such nuclei were positive by TUNEL staining. In contrast to the nucleus, mitochondrial degradation in lens fibre cells was a rapid process and involved a relatively sharp transition between positive and negative fibre cells for two mitochondrial specific markers, BAP 37 and prohibitin. A link between the changes in the nuclear lamina and chromatin with the initiation of mitochondrial fragmentation was also observed. Therefore, it is possible that the signal for the initiation of denucleation could originate from the mitochondria as proposed for apoptosis. Differences between apoptosis and lens fibre cell denucleation were noted and included the timescale of nuclear changes as well as the persistence of a nuclear remnant. These studies suggest that transcriptional shutdown precedes lamina reorganization and chromatin breakdown during lens fibre cell denucleation.

Disruption of alpha3 connexin gene leads to proteolysis and cataractogenesis in mice

Gap junction channels formed by alpha3 (Cx46) and alpha8 (Cx50) connexin provide pathways for communication between the fiber cells in the normal transparent lens. To determine the specific role of alpha3 connexin in vivo, the alpha3 connexin gene was disrupted in mice. Although the absence of alpha3 connexin had no obvious influence on the early stages of lens formation and the differentiation of lens fibers, mice homozygous for the disrupted alpha3 gene developed nuclear cataracts that were associated with the proteolysis of crystallins. This study establishes the importance of gap junctions in maintaining normal lens transparency by providing a cell-cell signaling pathway or structural component for the proper organization of lens membrane and cytoplasmic proteins.

Apoptosis in proliferative vitreoretinal disorders: possible involvement of TGF-beta-induced RPE cell apoptosis

The targeted induction of apoptosis is a novel therapeutic approach to control the unlimited growth of proliferating cells. Since massive proliferation of cells at the vitreoretinal interface is a key feature of proliferative vitreoretinal disorders, we sought to identify apoptosis in epiretinal membranes from patients with proliferative vitreoretinopathy (PVR), proliferative diabetic retinopathy (PDR) and macular pucker. Further, we evaluated the possible induction of apoptosis of retinal pigment epithelial (RPE) cells by transforming growth factor-beta(TGF-beta). Apoptotic cells were identified by in situ DNA end labeling and acridine orange staining on paraffin-embedded tissue sections from epiretinal membranes of patients with all vitreoretinal disorders examined. Labeled nuclei or condensed chromatin were scattered throughout the membranes or occurred in clusters. Most apoptotic cells were RPE-derived, as assessed by cytokeratin immunochemistry. No apoptotic glial cells were detected. In PVR, proliferative activity, as confirmed by Ki-67 immunochemistry, was associated with short history and rapid disease progression. Apoptotic nuclei were observed more frequently in long-standing PVR or slow progression towards traction retinal detachment. TGF-beta was detected in all control vitreous samples by bioassay at concentrations below 20 ng ml-1. TGF-beta levels increased up to 20-fold in pathological vitreous. Marked heterogeneity was observed in all patient groups. The degree of TGF-beta activation was significantly higher in PVR than in PDR. Proapoptotic effects of TGF-beta were demonstrated in cultured human RPE cells by electron microscopy, in situ DNA end labeling, comet assay and a photometric enzyme immunoassay for histone-associated DNA fragments. Apoptosis appears to be a key regulatory mechanism of growth control of specific cell populations in proliferative vitreoretinal disorders. Administration of proapoptotic growth factors such as TGF-beta may provide a novel approach to inhibit cellular proliferation at the vitreoretinal interface.

Hydrogen peroxide-induced expression of the proto-oncogenes, c-jun, c-fos and c-myc in rabbit lens epithelial cells

The involvement of H2O2 in cataract development has been established in both human patients and animal models. At the molecular level H2O2 has been observed to cause damage to DNA, protein and lipid. To explore the oxidative stress response of the lens system at the gene expression level, we have examined the effects of H2O2 on the mRNA change of the proto-oncogenes, c-jun, c-fos and c-myc in a rabbit lens cell line, N/N1003A. H2O2 treatment of the rabbit lens epithelial cells for 60 min induces quick up-regulation of both c-jun and c-fos mRNAs. The maximal induction is 38 fold for c-jun at 150 microM and 72 fold for c-fos at 250 microM H2O2. Treatment of N/N1003A cells with 50-250 microM H2O2 for 60 min leads to a 2-5 fold increase of the c-myc mRNA level. H2O2 also induces an up-regulation in transactivity of the activating protein-1 (AP-1) as shown with a reporter gene driven by a prolactin gene promoter with 4 copies of AP-1 binding sites inserted in the upstream of the promoter. Maximal induction occurs with 150 microM H2O2. In the same system, the antioxidants, N-acetyl-cysteine (NAC) and pyrrolidine dithiocarbamate (PDTC) at concentrations shown to up-regulate the mRNAs of both c-jun and c-fos, also enhance the transactivity of AP-1. NAC and PDTC have different effects in modulating the induction of AP-1 activity by H2O2 and TPA. These results reveal that oxidative stress regulates expression of various regulatory genes in lens systems, which likely affects cell proliferation, differentiation and viability and thus affect normal lens functions.

The contribution of GSH peroxidase-1, catalase and GSH to the degradation of H2O2 by the mouse lens

Utilizing cultured lenses from normal and homozygous glutathione peroxidase (GSHPx-1) knockout mice and inhibitors for GSSG Reductase (GSSG Red), 1,3-bis(2-chlorethyl)-1-nitrosourea (BCNU) and catalase (Cat), 3-aminotriazole (3-AT), the ability to degrade H2O2 was examined at two H2O2 concentrations, 300 microM and 80 microM. It was found that GSHPx-1 contributed about 15% to the H2O2 degradation. The Cat contribution was concentration dependent being about 30% at 300 microM H2O2 and approximately 8% to 15% at 80 microM H2O2. GSH loss measured as nonprotein thiol (NP-SH) was shown to be linked to most of the remaining H2O2 degradation accounting for about 54% to 72% of the H2O2 degradation at 300 microM and 80 microM, respectively. However, based on evaluation of the ability of GSH to nonenzymatically degrade H2O2, it can only account for about 36% at 300 microM and 19% at 80 microM H2O2 of the observed lens H2O2 degradation. It is, therefore, concluded that lens GSH must be involved in other reactions either directly or indirectly related to H2O2 degradation.

Cellular and molecular features of lens differentiation: a review of recent advances

In this paper, the more recent literature pertaining to differentiation in the developing vertebrate lens is reviewed in relation to previous work. The literature reviewed reveals that the developing lens has been, and will continue to be, a useful model system for the examination of many fundamental processes occurring during embryonic development. Areas of lens development reviewed here include: the induction and early embryology of the lens; lens cell culture techniques; the role of growth factors and cytokines; the involvement of gap junctions in lens cell-cell communication; the role of cell adhesion molecules, integrins, and the extracellular matrix; the role of the cytoskeleton; the processes of programmed cell death (apoptosis) and lens fibre cell denucleation; the involvement of Pax and Homeobox genes; and crystallin gene regulation. Finally, some speculation is provided as to possible directions for further research in lens development.

Apoptosis in ocular disease: a molecular overview

Apoptosis is a form of genetically programmed cell death that can be induced by a variety of different stimuli. It is often referred to as a form of cellular suicide. Typically, apoptosis is characterized by the condensation and shrinkage of the cellular nucleus and cytoplasm, followed by the complete fragmentation of the cell and subsequent phagocytosis of the debris by surrounding cells. Although important during development, and also for maintaining homeostasis in some adult tissues, apoptosis can also be associated with disease processes. Recent laboratory studies indicate that apoptosis is a mechanism of cell death in several important ocular diseases including glaucoma, retinitis pigmentosa, cataract formation, retinoblastoma, retinal ischemia, and diabetic retinopathy. This review summarizes the results of these studies and provides a brief description of some of the key molecules that are involved in the genetic regulation of apoptosis. It is possible that a complete understanding of how these molecules function may someday lead to new treatment options aimed at blocking the death of cells in a variety of ocular diseases.

Effect of age on alteration of glutathione metabolism following chronic cigarette smoke inhalation in mice

Cigarette smoke is the most common oxidant stress in daily life and may affect the antioxidant capacity in humans and animals. The antioxidant functions may play an important role in preventing age-related disorders. However, influences of chronic cigarette smoke on the antioxidant capacity of visceral organs have not been investigated in the age. Senescene-accelerated mice (SAM) are good models for studying physiologic and/or pathologic aging. A senescene-prone strain, SAMP2, shows characteristics of premature aging. The senescence-resistant strain, SAMR1, exhibits relatively normal aging. In this study we examined the effects of chronic cigarette smoke exposure on the glutathione (GSH) metabolism of visceral organs in the two strains of mice that were 6 and 18 months old. After a 4-week cigarette or air exposure, total GSH and oxidized GSH (GSSG) in the organs were examined. In the young (6-month-old) mice, exposure to cigarette smoke caused a significant decrease of GSH in liver, blood, and lung of SAMP2 but not in those of SAMR1. In the aged (18-month-old) mice reduced GSH with a marked increase of GSSG were found in liver of both strains of SAM following cigarette smoke exposure. The baseline values of GSH and the GSSG/GSH ratio after air exposure were slightly changed with age, and the values after exposure to cigarette smoke were changed markedly with advancing age. These results indicate that GSH metabolism may be impaired by chronic cigarette smoke exposure in mice and that aged mice are more susceptible to cigarette smoke than young mice.

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

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

The concentration of light in the human lens

PURPOSE

This thesis explores the idea that light energy, especially ultraviolet light, contributes to the unequal distribution of cataract around the world and to the development of cortical opacities.

METHODS

In the first section, the thesis reviews historical concepts of the function of the lens and the nature of cataract, epidemiologic data on the global distribution of cataract, and clinical observations of the predominant location of cortical opacification. Second, computer ray tracings and geometric optics demonstrate the passage of light of varying angle of incidence within the lens. Third, two models of the human eye are used to study the refraction of light by the cornea and lens and illustrate the concentration of energy at the equatorial plane of the lens.

RESULTS

Cataract prevalence increases with proximity to the earth's equator, and cortical cataract is most common in the inferior and inferonasal lens. Theoretical studies and the eye models both demonstrate that the concentration of light within the lens increases with angle of incidence, and the eye models suggest that the inferior and inferonasal lens receives significantly more energy than other sections of the lens.

CONCLUSION

The prevalence of cataract and exposure to ultraviolet energy both increase with decreasing latitude. The most common location of cortical cataract in the inferonasal lens is consistent with the greater dose of light energy received by this portion of the lens. These studies suggest that the global distribution of cataract and the development of cortical cataract are at least in part dependent on the dose of ultraviolet light received by the lens.