Reduced and total ascorbate in guinea pig eye tissues in response to dietary intake

Glycative stress as a cause of macular degeneration

People are living longer and rates of age-related diseases such as age-related macular degeneration (AMD) are accelerating, placing enormous burdens on patients and health care systems. The quality of carbohydrate foods consumed by an individual impacts health. The glycemic index (GI) is a kinetic measure of the rate at which glucose arrives in the blood stream after consuming various carbohydrates. Consuming diets that favor slowly digested carbohydrates releases sugar into the bloodstream gradually after consuming a meal (low glycemic index). This is associated with reduced risk for major age-related diseases including AMD, cardiovascular disease, and diabetes. In comparison, consuming the same amounts of different carbohydrates in higher GI diets, releases glucose into the blood rapidly, causing glycative stress as well as accumulation of advanced glycation end products (AGEs). Such AGEs are cytotoxic by virtue of their forming abnormal proteins and protein aggregates, as well as inhibiting proteolytic and other protective pathways that might otherwise selectively recognize and remove toxic species. Using in vitro and animal models of glycative stress, we observed that consuming higher GI diets perturbs metabolism and the microbiome, resulting in a shift to more lipid-rich metabolomic profiles. Interactions between aging, diet, eye phenotypes and physiology were observed. A large body of laboratory animal and human clinical epidemiologic data indicates that consuming lower GI diets, or lower glycemia diets, is protective against features of early AMD (AMDf) in mice and AMD prevalence or AMD progression in humans. Drugs may be optimized to diminish the ravages of higher glycemic diets. Human trials are indicated to determine if AMD progression can be retarded using lower GI diets. Here we summarized the current knowledge regarding the pathological role of glycative stress in retinal dysfunction and how dietary strategies might diminish retinal disease.

Glyoxalase System as a Therapeutic Target against Diabetic Retinopathy

Hyperglycemia, a defining characteristic of diabetes, combined with oxidative stress, results in the formation of advanced glycation end products (AGEs). AGEs are toxic compounds that have adverse effects on many tissues including the retina and lens. AGEs promote the formation of reactive oxygen species (ROS), which, in turn, boost the production of AGEs, resulting in positive feedback loops, a vicious cycle that compromises tissue fitness. Oxidative stress and the accumulation of AGEs are etiologically associated with the pathogenesis of multiple diseases including diabetic retinopathy (DR). DR is a devastating microvascular complication of diabetes mellitus and the leading cause of blindness in working-age adults. The onset and development of DR is multifactorial. Lowering AGEs accumulation may represent a potential therapeutic approach to slow this sight-threatening diabetic complication. To set DR in a physiological context, in this review we first describe relations between oxidative stress, formation of AGEs, and aging in several tissues of the eye, each of which is associated with a major age-related eye pathology. We summarize mechanisms of AGEs generation and anti-AGEs detoxifying systems. We specifically feature the potential of the glyoxalase system in the retina in the prevention of AGEs-associated damage linked to DR. We provide a comparative analysis of glyoxalase activity in different tissues from wild-type mice, supporting a major role for the glyoxalase system in the detoxification of AGEs in the retina, and present the manipulation of this system as a therapeutic strategy to prevent the onset of DR.

Vitreous humor and albumin augment the proliferation of cultured retinal precursor cells

Intravitreal injection is an important delivery route for studies involving the transplantation of various types of precursor cells to the retina; however, the effect on these cells of exposure to the vitreous microenvironment has not been specifically investigated. Here vitreous humor was evaluated for the potential to influence the proliferation of rat retinal precursor cells in vitro. Cells were isolated at embryonic day 19 and plated in standard proliferation medium in the presence or absence of fluid expressed from porcine vitreous humor. Cellular proliferation at different concentrations of vitreous fluid supplementation was quantified by using a (3)H-thymidine incorporation assay. Active components of vitreous fluid were partially characterized by gel filtration chromatography (GFC) and UV spectral analysis. The effect of each vitreous fraction on proliferation was determined as well. Results showed that addition of 20% vitreous fluid to primary rat retinal cultures significantly increased (3)H-thymidine incorporation compared with growth medium without vitreous supplementation. A vitreous fraction showing growth-promoting activity was localized to a molecular mass range <1000 Da, consistent with ascorbic acid. Ascorbic acid was confirmed in vitreous fluid by UV spectral analysis. Growth-augmenting activity was present in higher molecular mass vitreous fractions, consistent with protein components. Albumin, the major protein in vitreous fluid, was found to augment proliferation. Because vitreous-associated augmentation of retinal precursor proliferation remains an epidermal growth factor-dependent phenomenon, the proliferative status of transplanted cells in the vitreous cavity is likely determined by a combination of factors.

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.

Ascorbic acid reversibly inhibits proliferation of retinal pigment epithelial cells

PURPOSE

Proliferation control in adult retinal pigment epithelial (ARPE) cells is an essential factor in the clinical management of proliferative vitreoretinopathy (PVR). Factors which inhibit PVR and which are without toxic potential are therefore of interest in controlling proliferation. The aim of the present study was to gain insight into a possible function of high intraocular ascorbic acid levels as a physiological modulator of proliferation.

METHODS

Adult retinal pigment epithelial cells were incubated in vitro with increasing concentrations of ascorbic acid (0.5-4 mmol, pH 7.4). Cell proliferation was assayed by the bromide-deoxy-uridine (BrdU) assay. The culture medium (CM) containing ascorbic acid was replaced with normal CM and the recovery of proliferation was measured after 24 hours. In order to be able to distinguish between proliferation inhibition, apoptosis, necrosis and recovery of proliferation, we performed TUNEL assays and fluorescence analysis cell-counter (FAC) analysis.

RESULTS

Ascorbic acid significantly inhibits ARPE cell proliferation if it is present in concentrations above 2 mmol. Proliferation resumed in all ARPE cell cultures after pre-incubation with ascorbic acid, indicating that direct toxicity of ascorbic acid is a negligible factor. The time-point and extent of recovery in proliferation was dependent on the initial ascorbic acid concentration. Fluorescence-labelled cell counts on apoptosis markers (FAC) data showed some induction of apoptosis and necrosis after incubation with 4 mmol ascorbic acid.

CONCLUSIONS

Ascorbic acid has a dose-dependent influence on the proliferation of vital ARPE cells. This possibly reflects the role of ascorbic acid at a physiological level within the vitreous cavity in preventing proliferative vitreoretinopathy (PVR). These findings may stimulate the development of new strategies in the clinical treatment of PVR.

2001 assessment of nutritional influences on risk for cataract

It is clear that oxidative stress is associated with compromises to the lens. Recent literature indicates that antioxidants may ameliorate that risk and may actually decrease risk for cataract. This article will briefly review the etiology of cataract. It will also review the epidemiologic information with emphasis on roles for vitamins C and E and carotenoids. More thorough recent reviews are available.

Effects of ascorbic acid on retinal pigment epithelial cells

PURPOSE

Evaluation of effects of ascorbic acid on cell characteristics of dedifferentiated porcine retinal pigment epithelial (pRPE) cells.

METHODS

pRPE cells were incubated in vitro with increasing concentrations of ascorbic acid (0.25-1.5 mMol). Cell proliferation was assayed by measuring the incorporation of 5-bromo-2'-deoxy-uridine (BrdU) into cellular DNA. Migration and contraction properties were studied on a cell permissive porous membrane and collagen gels, respectively. Phenotypic changes in response to ascorbic acid and its derivative ascorbic acid 2-phophate were evaluated by microscopy and indirect immunofluorescence.

RESULTS

Ascorbic acid significantly inhibits cell proliferation, migration, and contraction in concentrations of 1 mMol or more. Under the influence of at least 1 mMol ascorbic acid dedifferentiated pRPE cells exhibited a pigmented status within 24 hours. Addition of 500 U/ml catalase prevented the antiproliferative effect of ascorbic acid and the formation of pigment. Concentrations of 0.5 mMol ascorbic acid as well as 1 mMol ascorbic acid 2-phosphate promoted differentiation of cell phenotype. Furthermore, ascorbic acid 2-phosphate supported the formation of in vivo-like epithelial structures.

CONCLUSIONS

Ascorbic acid has an influence on vital cell characteristics such as proliferation, migration, contraction and differentiation of pRPE cells. As dedifferentiation of these cells is an integral part in the development of proliferative vitreoretinopathy (PVR), ascorbic acid should be taken into consideration as a supplement in the clinical management of this disease.

Oral vitamins C and E as additional treatment in patients with acute anterior uveitis: a randomised double masked study in 145 patients

AIM

To investigate the effect of additional oral vitamins C and E on acute anterior uveitis.

METHODS

A placebo controlled double masked study on the effect of vitamin C 500 mg in combination with vitamin E 100 mg twice daily in 145 patients with acute anterior uveitis. As a primary end point variable, laser cell/flare measurements were performed. Best corrected and stenopeic visual acuity (VA) testing and clinical variable scores were measured.

RESULTS

Laser flare measurements (ph/s) before treatment were 207.1 (SD 258) in the vitamin group and 143.6 (156) in the placebo group. After 3 days corresponding values were 80.2 (129) and 54.7 (82), after 7 days 89.2 (187) (12.5) and 85.8 (208), after 14 days 47.1 (109.5) and 40.5 (116) after 28 days 23.1 (53.6) and 23.1 (48), and after 56 days 15.6 (26) and 15.3 (17). There was no significant difference in time trend between the two treatment groups (RMANOVA; p = 0.53). Baseline VA (logMAR) was 0.106 (0.241) in the vitamin group and 0.128 (0.456) in the placebo group. VA after 3 days was 0. 236 (0.293) and 0.344 ( 0.489), after 7 days 0.204 (0.292) and 0.292 (0.479), after 14 days 0.162 (0.274) and 0.193 (0.454), after 28 days 0.096 (0.232) and 0.158 (0.436), and 0.026 (0.213) and 0.106 (0. 437) after 56 days. Although no significant difference in time trend was detected, evaluation of the VA data of the last time point (56 days) by means of the Mann-Whitney test showed a significantly better VA in the vitamin group (p = 0.01).

CONCLUSIONS

There was no significant effect of vitamins C and E on laser flare measurements. The significant effect of the oral vitamins on visual acuity at 8 weeks after start of the oral vitamins C and E may indicate a protective effect in patients with acute anterior uveitis.

Vitamin C in human and guinea pig aqueous, lens and plasma in relation to intake

PURPOSE

Given the recent correlation between nutrition and risk for eye disease, there is keen interest in a possible correlation between nutrient intake and eye-tissue nutrient levels. In this work, the objective was (1) to determine, for the first time, the relation between dietary intake of vitamin C and eye tissue levels of the vitamin in free-living humans, (2) to determine the relation between levels of the vitamin in plasma, lens and aqueous, and (3) to compare this information to data gathered for a carefully reared group of guinea pigs that were fed different levels of vitamin C.

METHODS

Two hundred sixty-five cataract patients (mean age = 72 years) from a clinical practice were recruited for this study. One hundred thirty-two patients provided the dietary intake data via a food frequency questionnaire, which we used for this work. Plasma, aqueous humor, and lens samples were obtained at the time of lentectomy and preserved for vitamin C analysis. Comparable samples were obtained from male Hartley white guinea pigs that were fed known amounts of vitamin C. Linear and log10-linear statistical models were also used to characterize the relation between vitamin C intake and human ocular tissue levels of the vitamin and to examine potential confounding and the effect of modification by age and sex.

RESULTS

In humans, plasma and aqueous vitamin C concentrations were related to intake in a log-linear fashion, with slopes of 0.03 mM plasma vitamin C/log10-mg daily vitamin C intake and 0.41 mM aqueous vitamin C/log10-mg daily vitamin C intake. The best fit of vitamin C levels in lens and diet predicts a linear relationship with a sex-adjusted slope of 0.00094 mM lens vitamin C/mg daily vitamin C intake, although a log-linear relation can also be modeled. In guinea pigs, diet was related to eye tissue and plasma levels of the vitamin by a log10 linear relationship in all cases. Vitamin C in human lens was linearly related to plasma and aqueous vitamin C with slopes of 8.8 and 0.23, respectively. Vitamin C in aqueous was related to plasma in a log10-linear fashion with a slope of 1.6 mM aqueous vitamin C/log10 mM plasma vitamin C. In guinea pigs, vitamin C in plasma was related to aqueous and lens vitamin C by log10-linear relationships, whereas lens and aqueous vitamin C were clearly linearly related.

CONCLUSIONS

Plasma and aqueous appear to be saturated in humans with intakes of < 250 mg vitamin C/day. However, a saturating relationship between lens vitamin C and dietary intake in humans was not indicated in this study, although such a relationship is seen in guinea pigs. Intertissue relations between vitamin C levels in humans and guinea pigs are similar for some but not all relations.

Determination of ascorbic acid in human vitreous humor by high-performance liquid chromatography with UV detection

PURPOSE

Ascorbic acid (AA) accumulates in vitreous at a concentration several times higher than in plasma. It has been suggested that AA may serve as an antioxidant that protects ocular tissues from free radical attack. There are many reports about the concentration of AA in ocular tissues. However, AA in adult human vitreous humor has not been determined. We measured concentrations of AA from pathologic human vitreous samples and compared the results.

METHODS

AA was measured by high-performance liquid chromatography (HPLC) with UV detection. Human vitreous humor was collected from patients undergoing pars plana vitrectomy.

RESULTS

AA was quantified in vitreous humor of proliferative diabetic retinopathy (PDR), proliferative vitreoretinopathy (PVR), macular hole (MH), idiopathic premacular fibrosis (PMF), and Terson syndrome (Terson). The concentrations of AA were 120.9 +/- 36.3 micrograms/ml (mean +/- SD), 129.8 +/- 36.6, 311.5 +/- 126.7, 446.9 +/- 154.2 and 406.0 +/- 22.0, respectively. There was no significant difference between the PDR and the PVR groups (unpaired t-test). Patients with PDR and PVR showed significantly lower concentrations of AA than those with MH, PMF, and Terson (p < 0.01).

CONCLUSIONS

These findings suggest that increased oxidative stress may be produced in the ocular tissues of eyes with PDR and PVR, and AA appears to be consumed (oxidized) in performing its protective role.

Dietary calorie restriction in the Emory mouse: effects on lifespan, eye lens cataract prevalence and progression, levels of ascorbate, glutathione, glucose, and glycohemoglobin, tail collagen breaktime, DNA and RNA oxidation, skin integrity, fecundity, and cancer

The Emory mouse is the best model for age-related cataract. In this work we compare the effects of feeding a control diet (C) with a diet restricted (R) by 40% relative to C animals. In the R animals, median lifespan was extended by 40%. The proportion of R mice with advanced cataract was lower than C mice as early as 5 months of age. The mean grade of cataract was lower in R animals, beginning at 11 months and continuing until the end of the study. Ascorbate levels in R plasma and liver were 41-56% of C animals. There was no difference between diet groups with respect to lens ascorbate. Aging was associated with a decrease in ascorbate in lenses and kidneys in C and R mice. By 22 months, R animals had 48% higher liver glutathione levels than C mice. Liver glutathione levels were maximal at 12 months. Plasma glucose levels were > 27% lower in R animals at 6.5 and 22 months, and there was a 14% increase in glucose levels upon aging for both diet groups. In R mice, glycohemoglobin levels were 51% lower and tail collagen breaktime was decreased by 40%, even in younger animals. Collagen breaktime increased > 360% upon aging for both diet groups. Rates of production of urinary oxo8dG and oxo8G were higher in R animals compared with C animals, and increased upon aging. C animals exhibited more cancer and dermatological lesions, but less tail tip necrosis and inflamed genitals than R mice. These data allow evaluation of several theories of aging.

Cataract: relationship between nutrition and oxidation

Opacification of the lens, or cataract, is causally related to the precipitation of proteins or other constituents upon aging. Proteins in the lens are unusually long lived and are subject to extensive damage, including (photo)oxidation. Accumulation of damaged proteins also appears to be due in part to attenuated activity of some proteolytic pathways, which in younger tissue may serve to identify and remove such moieties. The damaged proteins accumulate, aggregate, and precipitate. Compared with other health problems, surgery to remove cataract and related visits to physicians consume the largest proportion of the Medicare budget, i.e., $3.2 billion annually in the United States. The situation is exacerbated in many parts of the world where there is a dearth of ophthalmologists to perform the required number of procedures. Historically efforts to delay cataract assumed a low profile in ophthalmologic research. Recent data, however, indicate that consuming elevated levels of antioxidants such as ascorbate, carotenoids, and tocopherol is associated with delayed development of various forms of cataract. The same beneficial relationship to vision pertains to plasma antioxidant status and to fruit and vegetable intake. Thus, it seems that assuring optimal antioxidant intake can extend lens function. It has been estimated that in the United States over half of the cataract extractions and associated costs would be obviated if cataract could be delayed by 10 years. The data reviewed indicate that optimizing nutrition will help achieve that objective.

Effect of photooxidation on the eye lens and role of nutrients in delaying cataract

The function of the eye lens is to collect and focus light on the retina. To do so, it must remain clear during the decades of life. Upon aging, lens constituents are damaged and precipitate in opacities called senile cataracts. Laboratory and epidemiologic data indicate that the damage is due in part to light and active forms of oxygen. Antioxidant nutrients - ascorbate, carotenoids, and tocopherol - appear to offer protection against cataract. Fifty million persons worldwide are blind due to cataract, and, in the U.S., there are 1.2 million cataract surgeries performed at an annual cost (including physician visits) of over $3.2 billion. It has been estimated that a 10-year delay in the development of cataract would eliminate the need for half the surgeries. Since it will not be possible to replace most of the damaged lenses, it is essential to determine the efficacy of supplying adequate levels of antioxidant nutrients early in life to preserve lens function.

Relationship in humans between ascorbic acid consumption and levels of total and reduced ascorbic acid in lens, aqueous humor, and plasma

The relationships between plasma, aqueous humor and lens ascorbic acid levels are examined in 131 samples from 127 patients. Mean ascorbate intake for nonsupplemented individuals was 148 mg/day or over two times the recommended daily allowance. A subset of 44 patients participated in a trial to assess the impact of vitamin C supplementation of 2 grams per day on aqueous and lens ascorbic acid levels. Such supplementation significantly increased both total and reduced ascorbic acid levels in plasma and aqueous and total ascorbic acid in the lens. Correlation coefficients relating total and reduced ascorbic acid levels in the three tissues ranged from 0.42 to 0.19 (p less than 0.05 for all correlation coefficients). Over 60% of the ascorbate was present in the reduced form in plasma and aqueous, and about 50% of the lens ascorbate was in the reduced form.

Effect of species differences and dietary vitamin C on the concentration of ascorbate- and acid-soluble thiol in fish eye

Data presented confirm the essentiality of modification of the dinitrophenylhydrazine (DNPH) method to analyze the total ascorbic acid and dehydroascorbic acid in ocular tissues and stress the need of corrections for the interfering substances. Variations in ascorbate and thiol concentrations in the lens, retina and aqueous humour of freshwater fish belonging to the Cyprinidae family were examined. The interspecific variability of ascorbate concentration was highest in the aqueous humour and lowest in the retina. The high ascorbate concentration in the retina seems to reflect the importance of the sense of vision in fish life-style as compared to chemo- and acoustico lateralis senses. The regional distribution of the total ascorbate is in the order of decreasing concentrations: retina, lens and aqueous humour. However, the retinal ascorbate is almost exclusively in the oxidized form, and the lenticular ascorbate is almost exclusively in the reduced form. Thiol concentration in the lens is five- to tenfold that in the retina and aqueous humour. This explains the oxidation status of ascorbate in different eye compartments of the eye. After 30 days on diets containing various levels of ascorbic acid or ascorbic acid sulphate, the ascorbate concentration in the eye compartments of common carp (Cyprinus carpio L.) was determined. Ocular tissue can be used to monitor the development of the ascorbate status in fish, and the retina is the most responsive tissue to the enhanced or depleted ascorbate levels.

Protein oxidation and proteolytic degradation. General aspects and relationship to cataract formation

1) Intracellular proteins are subject to oxidative and photooxidative denaturation. 2) Proteolytic systems recognize and selectively degrade oxidatively denatured, and photooxidatively denatured proteins. By degrading mildly denatured proteins these proteolytic systems prevent further oxidative/photooxidative damage which could otherwise result in the formation of cross-linked (undigestible) proteins, or protein fragments with toxic biological activities. Proteolytic systems also provide amino acids for the synthesis of new (replacement) proteins. 3) A 700,000 dalton neutral endoproteinase, which we have called macroxyproteinase or M.O.P., appears to be mostly responsible for the degradation of oxidatively denatured proteins. M.O.P. has been shown to function in red blood cells and in the eye lens, and appears to also exist in many other mammalian cell types. 4) Cataract is a disease associated with aging, and with photooxidative denaturation (and cross-linking) of lens crystallins and other proteins. 5) Both cataract and aging of lens cells are associated with declining proteolytic capacity and diminished antioxidant protection. 6) Lens aging and in vivo photooxidative stress can cause opacity ("cataract"), cross-linking of crystallins, and diminished proteolytic capacity. 7) High levels of dietary ascorbate increase ascorbate concentrations in lens tissue, and are associated with greater resistance of lens proteins and lens proteolytic enzymes to oxidative/photooxidative stress in vitro.

Associations between nutrition and cataract

Blindness due to opacification of the lens, or cataract, afflicts 50 million persons worldwide. In the United States over 541,000 cataract extractions are done annually at a cost of over $3.8 billion. Conservative estimates indicate that the prevalences of cataracts in Americans aged 65-75 and 75-85 years are 18% and 46%, respectively. Cataracts are even more prevalent in some other populations. It is estimated that the need for cataract extractions would be diminished by half if onset of cataract could be delayed by only ten years. Hypotheses regarding the etiology of cataract include oxidative perturbations of protein metabolism, diverse pathologic conditions, and perhaps glycation of lens proteins. Epidemiologic data indicate that elevated plasma levels of specific nutrients (i.e., carotenoids, ascorbate, tocopherol, and taurine) are associated with diminished incidence of certain types of cataract. Biochemical evidence suggests that each of these compounds can delay photooxidative damage to lens proteins. Roles in lens metabolism for selenium and tryptophan have been suggested. Elucidation of mechanisms by which caloric restriction delays cataract development is a promising area of current research.