Protective effect of ascorbate against oxidative stress in the mouse lens

Synergetic effects of polyethylene microplastic and abamectin pesticides on the eyes of zebrafish larvae and adults through activation of apoptosis signaling pathways

Although the toxicity of microplastics (MPs) and pesticides has recently been described, the possible effects of combining these pollutants are poorly understood. Thus, we evaluated the potential impact of exposure to polyethylene MP (PE-MP) and abamectin (ABM) (alone and combined) in zebrafish. After five days, the combined exposure to MP and ABM decreased the survival rate compared to exposures to individual pollutants. A significant increase in reactive oxygen species (ROS), lipid peroxidation, apoptosis, and impairment in antioxidant response was observed in zebrafish larvae. Morphological changes in the eyes of zebrafish significantly increased in the combined exposure group than in the individual exposure. Furthermore, the bax and p53 expression (specific apoptotic genes) was significantly upregulated after the combined exposure to PE-MP and ABM. So, the synergetic effect of MP and ABM cannot be ignored, and further research on other higher models is required to confirm its consequences.

Oxidative Stress in the Anterior Ocular Diseases: Diagnostic and Treatment

The eye is a metabolically active structure, constantly exposed to solar radiations making its structure vulnerable to the high burden of reactive oxygen species (ROS), presenting many molecular interactions. The biomolecular cascade modification is caused especially in diseases of the ocular surface, cornea, conjunctiva, uvea, and lens. In fact, the injury in the anterior segment of the eye takes its origin from the perturbation of the pro-oxidant/antioxidant balance and leads to increased oxidative damage, especially when the first line of antioxidant defence weakens with age. Furthermore, oxidative stress is related to mitochondrial dysfunction, DNA damage, lipid peroxidation, protein modification, apoptosis, and inflammation, which are involved in anterior ocular disease progression such as dry eye, keratoconus, uveitis, and cataract. The different pathologies are interconnected through various mechanisms such as inflammation, oxidative stress making the diagnostics more relevant in early stages. The end point of the molecular pathway is the release of different antioxidant biomarkers offering the potential of predictive diagnostics of the pathology. In this review, we have analysed the oxidative stress and inflammatory processes in the front of the eye to provide a better understanding of the pathomechanism, the importance of biomarkers for the diagnosis of eye diseases, and the recent treatment of anterior ocular diseases.

Zinc supplementation modifies brain tissue transcriptome of Apis mellifera honeybees

BACKGROUND

Bees are the most important group of pollinators worldwide and their populations are declining. In natural conditions, Apis mellifera depends exclusively on food from the field to meet its physiological demands. In the period of scarcity, available resources are insufficient and artificial supplementation becomes essential for maintaining the levels of vitamins, proteins, carbohydrates, and minerals of colonies. Among these minerals, zinc is essential in all living systems, particularly for the regulation of cell division and protein synthesis, and is a component of more than 200 metalloenzymes.

RESULTS

The total RNA extracted from the brain tissue of nurse bees exposed to different sources and concentrations of zinc was sequenced. A total of 1,172 genes in the treatment that received an inorganic source of zinc and 502 genes that received an organic source of zinc were found to be differentially expressed among the control group. Gene ontology enrichment showed that zinc can modulate important biological processes such as nutrient metabolism and the molting process.

CONCLUSIONS

Our results indicate that zinc supplementation modulates the expression of many differentially expressed genes and plays an important role in the development of Apis mellifera bees. All the information obtained in this study can contribute to future research in the field of bee nutrigenomics.

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

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

Vitamin C and the Lens: New Insights into Delaying the Onset of Cataract

Cataracts or clouding of the lens is the leading cause of blindness in the world. Age and diabetes are major risk factors, and with an increasing aging and diabetic population, the burden of cataracts will grow. Cataract surgery is an effective way to restore vision; however, alternatives to cataract surgery are required to reduce the looming cataract epidemic. Since it is well established that oxidative damage plays a major role in the etiology of cataracts, antioxidants have been promoted as therapies to delay and/or prevent cataracts. However, many antioxidant interventions including vitamin C have produced mixed results as anti-cataract therapies. Progress has been made towards our understanding of lens physiology and the mechanisms involved in the delivery and uptake of antioxidants to the lens which may guide future studies aimed at addressing some of the inconsistencies seen in previous animal and human studies. Of interest is the potential for vitamin C based supplements in delaying the onset of cataracts post vitrectomy which occurs in up to 80% of patients within two years. These targeted approaches are required to reduce the burden of cataract on hospitals and improve the quality of life of our aging and diabetic population.

Oral intake of α‑glucosyl‑hesperidin ameliorates selenite‑induced cataract formation

Hesperetin is a natural flavonoid with robust antioxidant properties. Our previous study reported that hesperetin can prevent cataract formation. However, an important consideration regarding hesperetin consumption is the limited bioavailability due to its poor solubility. The present study investigated the anti‑cataract effects of α‑glucosyl hesperidin in vivo and in vitro using a selenite‑induced cataract model. SD rats (age, 13 days) were orally administered PBS (0.2 ml) or α‑glucosyl hesperidin (200 mg/kg) on days 0, 1 and 2. Sodium selenite was subcutaneously administered to the rats 4 h after the first oral administration on day 0. Antioxidant levels in the lens and blood were measured on day 6. In vitro, human lens epithelial cells were treated with sodium selenite (10 µM) and/or hesperetin (50 or 100 mM) for 24 h and analyzed for apoptosis markers using sub‑G1 population and Annexin V‑FITC/propidium iodide staining and DNA ladder formation. α‑glucosyl hesperidin treatment significantly reduced the severity of selenite‑induced cataract. The level of antioxidants was significantly reduced in the selenite‑treated rats compared with in the controls; however, they were normalized with α‑glucosyl hesperidin treatment. In vitro, hesperetin could significantly reduce the number of cells undergoing apoptosis induced by sodium selenite in human lens epithelial cell lines. Overall, oral consumption of α‑glucosyl hesperidin could delay the onset of selenite‑induced cataract, at least in part by modulating the selenite‑induced cell death in lens epithelial cells.

Ascorbic acid and protein glycation in vitro

The aim of the study was to compare the effects of ascorbic acid (AA) in vitro in the absence and in the presence of cell-dependent recycling. In a cell-free system, AA enhanced glycoxidation of bovine serum albumin (BSA) by glucose and induced BSA glycation in the absence of sugars. On the other hand, AA did not affect erythrocyte hemolysis, glycation of hemoglobin and erythrocyte membranes, and inactivation of catalase, protected against inactivation of acetylcholinesterase of erythrocytes incubated with high glucose concentrations and enhanced the loss of glutathione. These results can be explained by assumption that AA acts as a proglycating agent in the absence of recycling while is an antiglycating agent when metabolic recycling occurs.

Glycation of bovine serum albumin by ascorbate in vitro: Possible contribution of the ascorbyl radical?

Ascorbic acid (AA) has been reported to be both pro-and antiglycating agent. In vitro, mainly proglycating effects of AA have been observed. We studied the glycation of bovine serum albumin (BSA) induced by AA in vitro. BSA glycation was accompanied by oxidative modifications, in agreement with the idea of glycoxidation. Glycation was inhibited by antioxidants including polyphenols and accelerated by 2,​2′-​azobis-​2-​methyl-​propanimidamide and superoxide dismutase. Nitroxides, known to oxidize AA, did not inhibit BSA glycation. A good correlation was observed between the steady-state level of the ascorbyl radical in BSA samples incubated with AA and additives and the extent of glycation. On this basis we propose that ascorbyl radical, in addition to further products of AA oxidation, may initiate protein glycation.

•

Ascorbic acid (AA) induced glycation of bovine serum albumin (BSA) in vitro.

•

Antioxidants, including polyphenols, inhibited glycation.

•

Nitroxides, known to oxidize AA, did not protect from glycation.

•

BSA glycation was accelerated by AAPH and superoxide dismutase.

•

Good correlation was found between the level of ascorbyl radical and extent of glycation.

•

We postulate that ascorbyl radical is able to induce protein glycation.

Effects of long-acting somatostatin analogues on redox systems in rat lens in experimental diabetes

The effects of long-acting somatostatin analogues, angiopeptin (AGP) and Sandostatin (SMS), on the early decline in the lens content of glutathione (GSH), ATP and NADPH and increase in sorbitol were studied in STZ diabetic rats, and comparison was made with the effect of insulin. Three factors prompted this study: (i) the known increase in IGF-1 in ocular tissue in diabetes and antagonistic effect of somatostatins, (ii) the known effect of IGF-1 in increasing lens aldose reductase and (iii) the lack of effect of somatostatins on diabetic hyperglycaemia, the latter enabling a differentiation to be made between effects of hyperglycaemia per se and site(s) of IGF-1/somatostatins. All four metabolites studied showed a significant restoration towards the normal control level after 7 days of treatment with AGP and SMS, and AGP was more effective on levels of GSH and ATP. A significant correlation was found between GSH and ATP across all groups at 7 days treatment. The redox state changes in diabetes include both NADP+/NADPH and NAD+/NADH in the conversion of glucose to sorbitol and via sorbitol dehydrogenase to fructose with a linked decrease in ATP formation via NAD+/NADH regulation of the glycolytic pathway. The interlinked network of change includes the requirement for ATP in the synthesis of GSH. The present study points to possible loci of action of somatostatins in improving metabolic parameters in the diabetic rat lens via effects on aldose reductase and/or glucose transport at GLUT 3.

The effects of L-arginine, alone and combined with vitamin C, on mineral status in relation to its antidiabetic, anti-inflammatory, and antioxidant properties in male rats on a high-fat diet

The aim of this study was to evaluate the influence of the intake of L-arginine alone and of L-arginine with vitamin C on mineral concentration in rats fed with a high-fat diet, and to assess the lipid glucose, insulin, and total antioxidant status (TAS) and tumor necrosis factor (TNF) alpha serum levels that result. Wistar rats were assigned to groups fed with either a standard control diet (C), a diet high in fat (FD), a diet high in fat with L-arginine, or a diet high in fat with L-arginine and vitamin C. After 6 weeks, the length and weight of the rats were measured, and the animals were euthanized. The liver, spleen, kidneys, pancreas, heart, and gonads were collected, as were blood samples. The total serum cholesterol, triglyceride, fasting glucose, insulin, TAS, and TNF alpha levels were measured. The tissue calcium, magnesium, iron, zinc, and copper concentrations were determined. It was found that L-arginine supplementation diminished the effect of the modified diet on the concentration of iron in the liver and spleen and of copper in heart. At the same time, it was observed that L-arginine supplementation reduced the effect of the high-fat diet on insulin, TNF alpha, and TAS. The combination of L-arginine and vitamin C produced a similar effect on the mineral levels in the tissues as did L-arginine used alone. Moreover, positive correlations between serum insulin and iron in the liver, between TNF alpha and iron in the liver, and between TNF alpha and copper in the heart were observed. The level of TAS in serum was inversely correlated with the copper level in the heart and the iron level in the liver. We concluded that the beneficial influence of L-arginine on insulin, TAS, and TNF alpha serum level is associated with changes in the iron and copper status in rats fed with a high-fat diet. No synergistic effect of L-arginine and vitamin C in the biochemical parameters or in the mineral status in rats fed with the modified diet was observed.

Establishment of a lens epithelial cell line from a canine mature cataract

The aim of this study was to establish a lens epithelial cell (LEC) line originated from a cataract of a dog. An anterior capsulorhexis specimen from a dog naturally developing mature cataracts was obtained prior to routine phacoemulsification cataract extraction. The primary lens epithelial cells were transfected with expression plasmid DNA encoding the large T antigen of replication origin-defective simian virus 40 (SV40), and then a colony was cloned using a glass cylinder. The primary cells stopped proliferation in three passages, while the transfected cells remained proliferative. Functional analysis of Na-dependent vitamin C transporter (SVCT) indicated that the Km value toward ascorbic acid (vitamin C) was 19.9 ± 2.8 µM, and RT-PCR analysis showed that SVCT2 was observed in this cell line while SVCT1 was not, which is one of the characteristics of LECs. Western blot analysis and cytoimmunochemistry indicated immortalized cells produced a protein with a molecular mass of 25 kDa, which reacted with an antibody to αB-crystallin within the whole cytosol. The cloned cell line, termed cdLEC, grew well and could be propagated over 250 times by basically splitting at 1:20. These results indicate that cdLEC may also provide a useful in vitro system for the study of the pathophysiology of cataract.

Modification of intracellular level of free radicals and apoptosis in cultured human endotheliocytes and carcinoma cells

The intracellular levels of superoxide O(2)(-)and nitric oxide NO were directly measured under intravital conditions in cultured human endotheliocytes ECV 304 and carcinoma cells HeLa G-63. Comparative analysis of changes in the intracellular levels of superoxide and NO induced by ascorbic acid revealed a negative correlation between NO and O(2)(-)levels, whose strength depended on concentration of the acid. In pharmacological concentrations, ascorbic acid induced apoptotic death of carcinoma cells, but did not trigger apoptosis of endotheliocytes. The study demonstrated a possible way of cytotoxic action of ascorbic acid in pharmacological concentrations.

Ascorbic acid promotes detoxification and elimination of 4-hydroxy-2(E)-nonenal in human monocytic THP-1 cells

4-Hydroxy-2(E)-nonenal (HNE), a reactive aldehyde derived from oxidized lipids, has been implicated in the pathogenesis of cardiovascular and neurological diseases, in part by its ability to induce oxidative stress and by protein carbonylation in target cells. The effects of intracellular ascorbic acid (vitamin C) on HNE-induced cytotoxicity and protein carbonylation were investigated in human THP-1 monocytic leukemia cells. HNE treatment of these cells resulted in apoptosis, necrosis, and protein carbonylation. Ascorbic acid accumulated in the cells at concentrations of 6.4 or 8.9 mM after treatment with 0.1 or 1 mM ascorbate in the medium for 18 h. Pretreatment of cells with 1.0 mM ascorbate decreased HNE-induced formation of reactive oxygen species and formation of protein carbonyls. The protective effects of ascorbate were associated with an increase in the formation of GSH-HNE conjugate and its phase 1 metabolites, measured by LC-MS/MS, and with increased transport of GSH conjugates from the cells into the medium. Ascorbate pretreatment enhanced the efflux of the multidrug resistant protein (MRP) substrate, carboxy-2',7'-dichlorofluorescein (CDF), and it prevented the HNE-induced inhibition of CDF export from THP-1 cells, suggesting that the protective effect of ascorbate against HNE cytotoxicity is through modulation of MRP-mediated transport of GSH-HNE conjugate metabolites. The formation of ascorbate adducts of HNE was observed in the cell exposure experiments, but it represented a minor pathway contributing to the elimination of HNE and to the protective effects of ascorbate.

Effects of Mobile Phones on Oxidant/Antioxidant Balance in Cornea and Lens of Rats

PURPOSE

To investigate the effects of mobile-phone-emitted radiation on the oxidant/antioxidant balance in corneal and lens tissues and to observe any protective effects of vitamin C in this setting.

METHODS

Forty female albino Wistar rats were assigned to one of four groups containing 10 rats each. One group received a standardized daily dose of mobile phone radiation for 4 weeks. The second group received this same treatment along with a daily oral dose of vitamin C (250 mg/kg). The third group received this dose of vitamin C alone, while the fourth group received standard laboratory care and served as a control. In corneal and lens tissues, malondialdehyde (MDA) levels and activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) were measured with spectrophotometric methods.

RESULTS

In corneal tissue, MDA level and CAT activity significantly increased in the mobile phone group compared with the mobile phone plus vitamin C group and the control group (p < 0.05), whereas SOD activity was significantly decreased (p < 0.05). In the lens tissues, only the MDA level significantly increased in the mobile phone group relative to mobile phone plus vitamin C group and the control groups (p < 0.05). In lens tissue, significant differences were not found between the groups in terms of SOD, GSH-Px, or CAT (p > 0.05).

CONCLUSIONS

The results of this study suggest that mobile telephone radiation leads to oxidative stress in corneal and lens tissues and that antioxidants such as vitamin C can help to prevent these effects.

Drinking water supplementation with ascorbate is not protective against UVR-B-induced cataract in the guinea pig

PURPOSE

To study if ascorbate supplementation decreases ultraviolet radiation (UVR)-induced cataract development in the guinea pig.

METHODS

Sixty 6-9-week-old pigmented guinea pigs received drinking water supplemented with or without 5.5 mm l-ascorbate for 4 weeks. After supplementation, 40 animals were exposed unilaterally in vivo under anaesthesia to 80 kJ/m(2) UVR-B. One day later, the animals were killed and lenses were extracted. Degree of cataract was quantified by measurement of intensity of forward lens light scattering. Lens ascorbate concentration was determined with high-performance liquid chromatography (HPLC) with UVR detection at 254 nm. Twenty animals were used as non-exposed control.

RESULTS

Supplementation increased lens ascorbate concentration significantly. In UVR-exposed animals, mean 95% confidence intervals (CIs) for animal-averaged lens ascorbate concentration (micromol/g wet weight lens) were 0.54 +/- 0.07 (no ascorbate) and 0.83 +/- 0.05 (5.5 mm ascorbate). In non-exposed control animals, mean 95% CIs for animal-averaged lens ascorbate concentration (micromol/g wet weight lens) were 0.72 +/- 0.12 (0 mm ascorbate) and 0.90 +/- 0.15 (5.5 mm ascorbate). All non-exposed lenses were devoid of cataract. Superficial anterior cataract developed in all UVR-exposed lenses. The lens light scattering was 39.2 +/- 14.1 milli transformed equivalent diazepam concentration (m(tEDC)) without and 35.9 +/- 14.0 m(tEDC) with ascorbate supplementation.

CONCLUSION

Superficial anterior cataract develops in lenses exposed to UVR-B. Ascorbate supplementation is non-toxic to both UVR-B-exposed lenses and non-exposed control lenses. Ascorbate supplementation does not reduce in vivo lens forward light scattering secondary to UVR-B exposure in the guinea pig.

Changes in redox ratio and protein glycation in precataractous lens from fructose-fed rats: effects of exogenous L-carnitine

1. Rats fed high dietary fructose are documented to form an acquired model of insulin resistance. The present study measured the effects of administration of L-carnitine (CA) on lens protein glycation, oxidative stress and redox homeostasis in rats fed a high-fructose diet. 2. Animals were divided into four groups: (i) an untreated control group (fed starch diet); (ii) an untreated fructose-group (fed a high-fructose diet); (iii) a CA-treated (300 mg/kg per day), fructose-fed group; and (iv) a CA-treated, starch-fed group. After 60 days treatment, lenses were dissected and multiple oxidative stress markers, glycation of proteins and the ratio of oxidized to reduced glutathione (GSSG/GSH) were determined. 3. A significant decline in enzyme and non-enzyme anti-oxidants and an increase in lipid peroxidation products, protein oxidation, protein glycation, GSSG/GSH ratio and aldehyde formation were observed in lens samples obtained from fructose-fed rats. Administration of CA to fructose-fed rats significantly attenuated oxidative damage and protein glycation and returned levels of anti-oxidants to near those seen in the control group. 4. The results of the present study indicate that dietary fructose disturbs lens integrity and exogenous CA may safeguard the lens by preventing glycation and oxidative stress.

Vitamin C mediates chemical aging of lens crystallins by the Maillard reaction in a humanized mouse model

Senile cataracts are associated with progressive oxidation, fragmentation, cross-linking, insolubilization, and yellow pigmentation of lens crystallins. We hypothesized that the Maillard reaction, which leads browning and aroma development during the baking of foods, would occur between the lens proteins and the highly reactive oxidation products of vitamin C. To test this hypothesis, we engineered a mouse that selectively overexpresses the human vitamin C transporter SVCT2 in the lens. Consequently, lenticular levels of vitamin C and its oxidation products were 5- to 15-fold elevated, resulting in a highly compressed aging process and accelerated formation of several protein-bound advanced Maillard reaction products identical with those of aging human lens proteins. These data strongly implicate vitamin C in lens crystallin aging and may serve as a model for protein aging in other tissues particularly rich in vitamin C, such as the hippocampal neurons and the adrenal gland. The hSVCT2 mouse is expected to facilitate the search for drugs that inhibit damage by vitamin C oxidation products.

Oxidation, antioxidants and cataract formation: a literature review

PURPOSE

This review aims to provide a literature survey of the association between photo-oxidation of lens proteins and lipid peroxidation with the genesis of age-related cataract in laboratory studies using rodent models, in epidemiological and interventional studies in humans.

MATERIALS AND METHODS

A Medline search using initial search terms lens, oxidation, antioxidant, and diet was employed to search for research papers covering the areas noted above from 1995 to 2005. Literature cited in those papers was also reviewed to provide as comprehensive a coverage of research work as possible.

RESULTS

Lens protein photo-oxidation and lipid peroxidation are widely acknowledged as important steps in age-related cataractogenesis. Dietary antioxidants are central in retarding cataractogenesis, although most evidence for this is gained from laboratory-based work on relatively unphysiologic rodent cataract models, using antioxidant regimes that could not be sustained in clinical practice. Most research in humans is retrospective epidemiology although some interventional research has been undertaken, with mixed results.

CONCLUSIONS

Dietary antioxidants are likely to be important in retarding cataractogenesis in older animals and in humans. Work on companion animals could provide a valuable stepping stone between rodent-based laboratory work and human interventional studies.

Chromium-picolinate induced ocular changes: Protective role of ascorbic acid

Chromium-picolinate (Cr-picolinate) is a popular nutritional supplement; however its safety has been questioned with regard to its ability to act as a clastogen. The aim of the present work was to evaluate the biochemical, histological and morphological changes in the cornea and lens following oral administration of Cr-picolinate and the possible protective effect of Vitamin C. Ninety male Sprague-Dawley rats were divided into five groups included the control group, the groups treated with Cr-picolinate (0.8 and 1.5 mg/100 g b.w.) alone or in combination with Vitamin C (0.5 mg/100 g b.w.) for 8 weeks. The results indicated that the high dose of Cr-picolinate induced a significant decrease in SOD, GSH, Na(+)-, K(+)-ATPase levels, and a significant increase in MDA level. Severe morphological and histological changes in the cornea and lens accompanied with a decrease in the total soluble protein of the lens homogenate and changes in the crystalline fractions in lens. Vitamin C supplementation succeeded to restore these changes to great extent. It could be concluded that consumption of Cr-picolinate for a long time induced several hazards to cornea and lens. Supplementation with extra amounts of Vitamin C may be useful to restrain the Cr-picolinate induced ocular changes.

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.

Ultraviolet radiation-B-induced cataract in albino rats: maximum tolerable dose and ascorbate consumption

PURPOSE

To investigate the maximum tolerable dose (MTD) for cataract induced by ultraviolet radiation-B (UVB) in 7-week-old albino rats and to study the effect of UVB eye exposure on lens ascorbate content.

METHODS

Fifty 7-week-old albino Sprague Dawley rats were unilaterally exposed in vivo to 300-nm UVB under anaesthesia, receiving 0, 0.25, 3.5, 4.3 and 4.9 kJ/m(2). The MTD was estimated based on lens forward light scattering measurements. Lens ascorbate content was determined in the processed lens using high performance liquid chromatography with UVR detection.

RESULTS

Animals exposed to UVB doses >or=3.5 kJ/m(2) developed cortical cataracts. The MTD for avoidance of UVB-induced cataract was estimated to 3.01 kJ/m(2). UVB exposure decreased lens ascorbate concentration in the exposed lens in line with UVB dose, H(e), according to the models: C = C(NonCo) + C(Co)e(-kH(e) ) for exposed lenses; C = C(NonCo) + C(Co) for non-exposed lenses, and C(d) = C(Co)(e(-kH(e) ) - 1). Parameters for consumable and non-consumable ascorbate were estimated to C(NonCo) = 0.04 and C(Co) = 0.11 micromol/g wet weight of lens. For lens ascorbate difference, tau = 1/k = 0.86 kJ/m(2). A total of 63% of UVB consumable ascorbate has been consumed after only tau = 0.86 kJ/m(2), while MTD(2.3 : 16) = 3.01 kJ/m(2), indicating that ascorbate decrease is in the order of 3.5 times more sensitive to detecting UVR damage in the lens than forward light scattering.

CONCLUSIONS

The MTD for avoidance of UVB-induced cataract in the 7-week-old albino Sprague Dawley rat was estimated to be 3.01 kJ/m(2). In vivo UVB exposure of the rat eye decreases lens ascorbate content following an exponential decline, and suprathreshold doses cause greater effect than subthreshold doses.

The effect of N-acetyl-l-cysteine and ascorbic acid on visible-light-irradiated camphorquinone/N,N-dimethyl-p-toluidine-induced oxidative stress in two immortalized cell lines

Recent studies have revealed that visible-light (VL)-irradiated camphorquinone (CQ), in the presence of a tertiary amine (e.g., N,N-dimethyl-p-toluidine, DMT), generates initiating radicals that may indiscriminately react with molecular oxygen forming reactive oxygen species (ROS). In this study, the ability of the antioxidants N-acetyl-l-cysteine (NAC) and ascorbic acid (AA) to reduce intracellular oxidative stress induced by VL-irradiated CQ/DMT or VL-irradiated hydrogen peroxide (H(2)O(2)) was assessed in an immortalized Murine cementoblast cell line (OCCM.30) and an immortalized Murine fibroblast cell line, 3T3-Swiss albino (3T3). Intracellular oxidative stress was measured with the membrane permeable dye, 2',7'-dichlorodihydrofluorescein diacetate (H(2)DCF-DA). VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) each produced significantly (p<0.001) elevated intracellular oxidative levels in both cell types compared to intracellular ROS levels in VL-irradiated untreated cells. OCCM.30 cementoblasts were found to be almost twice as sensitive to VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) treatment compared to 3T3 fibroblasts. Furthermore, 10mm NAC and 10mm AA each eliminated oxidative stress induced by VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) in both cell types. Our results suggest that NAC and AA may effectively reduce or eliminate oxidative stress in cells exposed to VL-irradiated CQ/DMT following polymerization.

Ascorbic acid promotes osteoclastogenesis from embryonic stem cells

Ascorbic acid (AA) is known to regulate cell differentiation; however, the effects of AA on osteoclastogenesis, especially on its early stages, remain unclear. To examine the effects of AA throughout the process of osteoclast development, we established a culture system in which tartrate-resistant acid phosphate (TRAP)-positive osteoclasts were induced from embryonic stem cells without stromal cell lines. In this culture system, the number of TRAP-positive cells was strongly increased by the addition of AA during the development of osteoclast precursors, and reducing agents, 2-mercaptoethanol, monothioglycerol, and dithiothreitol, failed to substitute for AA. The effect of AA was stronger when it was added during the initial 4 days during the development of mesodermal cells than when it was added during the last 4 days. On day 4 of the culture period, AA increased the total cell recovery and frequency of osteoclast precursors. Magnetic cell sorting using anti-Flk-1 antibody enriched osteoclast precursors on day 4, and the proportion of Flk-1-positive cells but not that of platelet-derived growth factor receptor alpha-positive cells was increased by the addition of AA. These results suggest that AA might promote osteoclastogenesis of ES cells through increasing Flk-1-positive cells, which then give rise to osteoclast precursors.

Combination of glycemic and oxidative stress in lens: implications in augmentation of cataract formation in diabetes

It is well known that the incidence of cataract is higher in diabetics as compared to non-diabetics. Its rate of maturation is also faster in the diabetics. The precise mechanism of this acceleration is not clearly understood. It is hypothesized that this could be a result of the combination of the metabolic and oxidative stress induced by glycemia itself with the age-associated increase in ambient generation of oxyradical species. In the current studies, we have investigated this possibility using the galactose cataract model. Galactosemia was induced by feeding rats a 50% galactose diet. The increased susceptibility of the glycemic lenses to physiological damage by reactive oxygen species (ROS) was studied by incubating them in Tyrode in the absence and presence of menadione. The resulting physiological damage to the lens was assessed initially in terms of its ability to maintain Na+-K+ ATPase dependent active transport of potassium ions, as represented by the uptake of rubidium ions. Subsequently, the level of ATP, indexing the general metabolic status, and the level of glutathione (GSH), indexing the status of antioxidant reserve, were also determined. The uptake of rubidium in the normal lenses incubated in the presence of the quinone was depressed to more than 50% of the controls run in the basal medium. A similar depression existed in the galactosemic lenses in comparison to the normal lenses. However, in the presence of menadione, the inhibition of the uptake was accentuated further in the case of galactosemic lenses, the uptake here being only 20% of the normal controls. Similarly, the galactosemic lenses were also more susceptible to menadione dependent decrease in ATP and GSH.

In vitro glucose-induced cataract in copper-zinc superoxide dismutase null mice

The purpose of this study was to evaluate the involvement of the superoxide radical in glucose-induced cataract using lenses from mice lacking the cytosolic copper-zinc superoxide dismutase (SOD1). Lenses from wild-type mice and SOD1 null mice were kept in organ culture with either 5.6 or 55.6 mM glucose for 6 days. The cataract formation was followed with digital image analysis and ocular staging. The lens damage was further quantified by analysis of the leakage of lactate dehydrogenase into the medium by the uptake of 86Rb and by determining the water content of the lenses. The formation of superoxide radicals in the lenses was assessed with lucigenin-derived chemiluminescence. Immunohistochemical staining for SOD1 was also performed on murine lenses. The SOD1 null lenses exposed to high glucose developed more cataract showed an increased leakage of lactate dehydrogenase and developed more oedema compared to the control lenses. At 5.6 mM glucose there was no difference between the SOD1 null and wild-type lenses. Staining for SOD1 was seen primarily in the cortex of the wild-type lens. This in vitro model suggests an involvement of the superoxide radical and a protective effect of SOD1 in glucose-induced cataract formation.

Ascorbate in the rat lens: dependence on dietary intake

PURPOSE

To establish a method for sample preparation to measure ascorbate in whole lenses and to investigate whether lens ascorbate concentration is dependent on dietary ascorbate intake.

METHODS

Four groups of 3 young Sprague-Dawley rats each were fed chow containing L-ascorbate, either 0.0, 5.7, 57.0 or 114.0 mmol/kg for a duration of 4 weeks. Thereafter, each rat was sacrificed. The lens was extracted, photographed, and lens wet weight was measured. The lens was homogenized in 1.0 ml of 0.25% metaphosphoric acid, the homogenate was centrifuged and the supernatant ultrafiltered. The filtrate was injected into an ion exchange, reversed-phase Polypore H HPLC column equipped with a 254-nm ultraviolet detector. Samples were calibrated against an L-ascorbate standard. Polynomial regression analysis was performed on the data.

RESULTS

All lenses were devoid of cataract. A 95% confidence interval for baseline content of ascorbate without any dietary intake was estimated to be 0.16+/-0.01 micromol/g wet weight of lens. The lens ascorbate concentration increased linearly with dietary ascorbate intake with an increased rate, estimated as a 95% confidence interval of 0.33+/-0.18 (micromol ascorbate) (g lens)-1)(mol ascorbate)-1 (kg chow) with r2=0.62.

CONCLUSION

Lens ascorbate concentration linearly increases with dietary ascorbate intake without cataract development in the rat. The currently presented method for sample preparation to measure the whole-lens content of ascorbate is applicable.

Ascorbate in the guinea pig lens: dependence on drinking water supplementation

PURPOSE

To investigate whether lens ascorbate concentration can be elevated with drinking water supplementation.

METHODS

Pigmented guinea pigs received drinking water supplemented with L-ascorbate, concentration 0.00, 2.84, 5.68 or 8.52 mm for a duration of 4 weeks. In addition, the chow fed to all animals contained 125 mmol L-ascorbate per kg of chow. At the end of the supplementation period, the guinea pigs were killed. Each lens was extracted. The lens was processed and ascorbate concentration was measured using high performance liquid chromatography (HPLC) with 254 nm ultraviolet radiation detection. The data were analysed with regression.

RESULTS

At the end of the test period, all lenses were devoid of cataract as observed by slit-lamp examination. All lenses contained a detectable concentration of ascorbate. Estimated 95% confidence intervals for mean animal-averaged lens ascorbate concentrations (micromol/g wet weight of whole lens) per group were 0.51 +/- 0.04 (0.00 mm; n = 6), 0.70 +/- 0.18 (2.84 mm; n = 6), 0.71 +/- 0.11 (5.68 mm; n = 5), and 0.71 +/- 0.06 (8.52 mm; n = 6). Animal-averaged lens ascorbate concentration [Asc(lens)] (micromol/g wet weight lens) increased with ascorbate supplementation in drinking water [Asc(water)] (M), in agreement with the model: [Asc(lens)] = A - Be(-kAsc(water)].

CONCLUSION

Lens ascorbate concentration increases with drinking water supplementation in the guinea pig without cataract development. The currently presented method for measurement of whole lens ascorbate content is suitable.

Effect of alpha-ketoglutarate against selenite cataract formation

We have previously shown that pyruvate protects against reactive oxygen species (ROS) induced damage to lens in vitro. It has also a significant effect against cataract development. Its effectiveness has been ascribed to the presence of alpha-keto-carboxylate group in the molecule, acting as a scavenger of ROS. Hence, it was felt desirable to determine if other alpha-keto-acids could have similar effects. These studies have hence been conducted with alpha-ketoglutarate (alpha-KG), a compound with greater stability and without any known significant effect on the glycolysis. Its effectiveness has been assessed by monitoring cataract development in rat pups given sodium selenite. A large percentage of such animals (about 80%) developed nuclear opacity 7-8 days after its administration. In animals treated with alpha-ketoglutarate, the incidence of cataracts was only 23%. The agent therefore has a very substantial anticataractogenic effect, as apparent by direct slit lamp examination followed by photography, as well as by examination of the isolated lenses through transillumination. The significance of the ophthalmologic findings was apparent also by better physiological maintenance of the tissue, reflected by higher levels of ATP and GSH. In view of these in vivo beneficial effects, studies are in progress to identify the biochemical and metabolic sites of its action. Whether the effectiveness is related only to its action as a ROS scavenger or it could be contributed also by some metabolic effects independent of ROS remains to be determined.