Differential expression of the lenticular antioxidant system in laboratory animals: a determinant of species predilection to oxidative stress-induced ocular toxicity?

Current Trends in the Pharmacotherapy of Cataracts

Cataracts, one of the leading causes of preventable blindness worldwide, refers to lens degradation that is characterized by clouding, with consequent blurry vision. As life expectancies improve, the number of people affected with cataracts is predicted to increase worldwide, especially in low-income nations with limited access to surgery. Although cataract surgery is considered safe, it is associated with some complications such as retinal detachment, warranting a search for cheap, pharmacological alternatives to the management of this ocular disease. The lens is richly endowed with a complex system of non-enzymatic and enzymatic antioxidants which scavenge reactive oxygen species to preserve lens proteins. Depletion and/or failure in this primary antioxidant defense system contributes to the damage observed in lenticular molecules and their repair mechanisms, ultimately causing cataracts. Several attempts have been made to counteract experimentally induced cataract using in vitro, ex vivo, and in vivo techniques. The majority of the anti-cataract compounds tested, including plant extracts and naturally-occurring compounds, lies in their antioxidant and/or free radical scavenging and/or anti-inflammatory propensity. In addition to providing an overview of the pathophysiology of cataracts, this review focuses on the role of various categories of natural and synthetic compounds on experimentally-induced cataracts.

N-nitrosamines induced infertility and hepatotoxicity in male rabbits

N-nitrosamines are widely spread environmental pollutants of well-known toxicity and carcinogenicity in various animal species. These compounds are metabolically activated by cytochrome P450 system predominantly in the liver and in other tissues into more active metabolites leading to generation of both alkylating agents that alkylate DNA and reactive oxygen species. In the current study, we investigated the influence of four types of N-nitrosamines that are commonly present in the environment [methyethylnitrosamine, (MEN), diethylnitrosamine (DEN), diphenylnitroasamine (DPN) and dimethylnitrosamine (DMN)] on both livers and testes of male rabbits through assessment of 17 β-hydroxysteroid dehydrogenase (17 β-HSD) activity. The protein expression of the three cytochrome P450s (CYP11A1, CYP19A1, and CYP21A2) is involved in the steroidogenesis. The levels of testosterone (T) and estradiol (E2) were also determined in the plasma of N-nitrosamines-treated rabbits after one, four-, eight- and twelve weeks of treatment of male New Zealand rabbits with an oral dose of 0.5 mg/kg B.W/day of each compound. In addition, activities of glutathione reductase (GR), superoxide dismutase (SOD), catalase (CAT) and levels of free radicals measured as thiobarbituric acid reactive substances (TBARS), and reduced glutathione (GSH) level were quantified in both livers and testes. The present study showed that levels of free radicals (TBARS) were markedly increased, whereas GSH levels were depleted in the tissues of both livers and testes after treatment of rabbits with any of N-nitrosamines. In addition, all tested N-nitrosamines inhibited the activities of antioxidant enzyme activities (GR, GST, SOD, and CAT) in hepatic and testicular tissues of rabbits after 12 weeks of treatment. Histopathological examination showed that N-nitrosamines caused lymphocytic infiltration with vascular degeneration and necrosis, congestion of central vein with RBCs hemolysis, dilated sinusoids, as well as fibrosis around portal areas were seen in hepatic tissues. In the testes, histopathological examination displayed disorganized seminiferous tubules with degeneration of germinal epithelium and Sertoli cells. Also, spermatogenic cells had pyknotic nuclei and others were detached from basement membranes of seminiferous tubules, edema was seen between seminiferous tubules. Moreover, the present data showed that MEN and DEN down-regulated the protein expression of both CYP19A1 and 21A2 in both livers and testes of male rabbits. In addition, both MEN and DEN decreased levels of testosterone and estradiol in plasma of treated rabbits. On the one hand, DMN and DPN markedly up-regulated the protein expression of CYP19A1 in both hepatic and testicular tissues of treated rabbits. These compounds potentially increased estradiol and decreased testosterone levels. On the other hand, no correlation was found between the expression of CYP11A1 and levels of both testosterone and estradiol. It is concluded that most of tested N-nitrosamines induce different changes, which could be a new mechanism of infertility due to exposure to N-nitrosamines from different environmental sources.

Antihepatotoxic effect of tadehaginoside, extracted from Tadehagi triquetrum (L.), against CCl4-lesioned rats through activating the Nrf2 signaling pathway and attenuating the inflammatory response

Recently, an increasing number of studies suggest that oxidative stress and inflammation are associated with hepatocellular injuries. Thus, we aimed to evaluate the potential hepatoprotective role of tadehaginoside (TA) on liver lesions induced by carbon tetrachloride (CCl4). The results in vitro suggested that TA dose-dependently suppressed the cell proliferation of HepG2 cells, whereas the phosphorylated level of IκBα in cells was effectively inactivated. The study in vivo showed that TA significantly lowered the serum concentrations of alanine aminotransferase (ALT), aspartate aminotransferase (AST), immunoglobulin E (IgE), and leukotriene (LT) in CCl4-lesioned rats. Pathological examination indicated that CCl4-induced hepatocellular damage was effectively mitigated by TA treatment. Meanwhile, the contents of γ-glutamylcysteine synthetase (γ-GCS), glutathione (GSH), and catalase (CAT) in liver tissue were gradually elevated. In addition, cytochrome c oxidase (COX) mRNA expression in hepatocytes was markedly upregulated, and nuclear factor E2-related factor 2 (Nrf2) and Kelch-like ECH-associated protein 1 (Keapl) levels were progressively increased. Furthermore, the tumor necrosis factor alpha (TNF-α) and nuclear factor-kappa B (NF-κB)-expressed protein were downregulated. These findings demonstrate that tadehaginoside effectively protects against CCl4-induced oxidative injury and inflammatory reaction in hepatocytes, in which the underlying mechanisms are involved in activating the Nrf2 signaling pathway and inhibiting the NF-κB pathway, thereby attenuating oxidative stress and reducing the inflammation in liver cells.

Puerarin attenuates neuronal degeneration in the substantia nigra of 6-OHDA-lesioned rats through regulating BDNF expression and activating the Nrf2/ARE signaling pathway

An increasing number of studies suggest that oxidative stress is associated with the Parkinsonian process. This study evaluated the potential neuroprotective role of puerarin (PR) on lesioned substantia nigra (SN) induced by 6-hydroxydopamine (6-OHDA). Data from a rotational test showed that PR treatment significantly decreased apomorphine-induced rotations. Both the dopamine (DA) content in the SN and the endogenous expression of brain-derived neurotrophic factor (BDNF) were also elevated by the treatment. Pathological examination showed that dopaminergic neuronal degeneration in the SN was attenuated by PR treatment. Meanwhile, the contents of γ-glutamylcysteine synthetase (γ-GCS), glutathione (GSH) and catalase (CAT) in SN tissue were gradually elevated. Additionally, cytochrome c oxidase (COX) mRNA expression in the SN was markedly up-regulated. At the same time, nuclear factor E2-related factor 2 (Nrf2) and Kelch-like ECH-associated protein 1 (Keapl) levels were progressively increased by the PR treatment. Our findings indicated that puerarin effectively protects against 6-OHDA-mediated oxidative stress injury in SN neurons, in which the underlying mechanisms are involved in modulating BDNF expression and activating the Nrf2/ARE signaling pathway.

Effect of vanadium on renal Na+,K+-ATPase activity in diabetic rats: a possible role of leptin

Several researches attempt to protect diabetic patients from the development of nephropathy. Involvement of leptin and renal Na+,K+-ATPase enzyme in diabetic nephropathy (DN) development is a recent field for researches. Vanadium, as a trace element with insulin mimetic effect, may act synergistically with insulin to protect against the development of DN. Sixty male Sprague Dawley rats were divided into six groups: control group (C), vanadium control group (CV), streptozotocin-induced diabetic group (D), insulin-treated diabetic group (DI), vanadium-treated diabetic group (DV), and combined insulin and vanadium-treated diabetic group. Six weeks later, systolic blood pressure (SBP) was measured and retro-orbital blood samples were collected to estimate glycosylated hemoglobin (HbA(₁c)), serum sodium (Na+) and creatinine, blood urea nitrogen (BUN) and plasma leptin levels. Preparation of microsomal fraction of renal tissue homogenate for estimation of Na+,K+-ATPase activity was done. The D group showed a significant increase in SBP, HbA(₁c), serum Na+, creatinine, and BUN levels and Na+,K+-ATPase activity in microsomal fraction of renal tissue homogenate while plasma leptin level decreased significantly compared with C and CV groups. Both DI and DV groups showed a significant improvement in all the above measured parameters compared with D group while there were no significant changes between the DI and DV groups. Concomitant treatment with insulin and vanadium resulted in a significant improvement in all the measured parameters compared to each alone. Vanadium in combination with insulin ameliorates DN markers and reduces renal Na+,K+-ATPase overactivity in diabetic rats. An effect that may be partially mediated through correction of hypoleptinemia observed in these animals.

The Sod2 mutant mouse as a model for oxidative stress: a functional proteomics perspective

Oxidative stress has been implicated in the pathogenesis of numerous human diseases and disorders, but the mechanistic basis often remains enigmatic. The Sod2 mutant mouse, which is sensitized to mitochondrial stress, is an ideal mutant model for studying the role of oxidative stress in a diverse range of complications arising from mitochondrial dysfunction and diminished antioxidant defense. To fully appreciate the widespread molecular consequences under increased oxidative stress, a systems approach utilizing proteomics is able to provide a global overview of the complex biological changes, which a targeted single biomolecular approach cannot address fully. This review focuses on the applications of mass spectrometry and functional proteomics in the Sod2 mouse. The combinatorial approach provides novel insights into the interplay of chemistry and biology, free radicals and proteins, thereby augmenting our understanding of how redox perturbations influence protein dynamics. Ultimately, this knowledge can lead to the development of free radical-targeted therapies.

Age-related cataract progression in five mouse models for anti-oxidant protection or hormonal influence

Five mouse models with known alterations of resistance to oxidative damage were compared by slit lamp examination for the presence and degree of advancement of age-related cataract in young adult and old animals along with wild type controls. A group of young and old normal C57BL/6Jax mice were examined first to constitute a standard, and they were found to exhibit age-related cataract development. Following this, four models on the C57BL/6 background with imposed genetic alterations affecting anti-oxidant enzyme presence or activity, and one outbred model in which a deletion blocked the growth hormone/IGF-1 axis, were similarly examined. There was no evidence of foetal or juvenile cataract development in any of these models, and an age-related severity for lens opacities was shown between young adult and old mice in all groups. Model 1, mice null for the anti-oxidant gene glutathione peroxidase-1 (GPX1) had significantly advanced cataracts in older mice vs. same age controls. In mouse model 2 hemizygous knockout of SOD2 (MnSOD) did not affect age-related cataract development. In model 3 combining the GPX1 and SOD2 deficiencies in the same animal did not advance cataract development beyond that of the GPX1 null alone. In model 4 the addition of anti-oxidant protection in the lens by transfection of human catalase targeted only to the mitochondria resulted in a significant delay in cataract development. The 5th model, growth hormone receptor knockout (GHR-/-) mice, also demonstrated a significant reduction in age-related cataract development, as well as dwarfism. These findings, in general, support the oxidative theory of age-related cataract development. The exception, the partial deletion of SOD2 in the hemizygous KO model, probably did not represent a sufficiently severe deprivation of anti-oxidant protection to produce pathologic changes in the lens.

Applications of cytotoxicity assays and pre-lethal mechanistic assays for assessment of human hepatotoxicity potential

While drug toxicity (especially hepatotoxicity) is the most frequent reason cited for withdrawal of an approved drug, no simple solution exists to adequately predict such adverse events. Simple cytotoxicity assays in HepG2 cells are relatively insensitive to human hepatotoxic drugs in a retrospective analysis of marketed pharmaceuticals. In comparison, a panel of pre-lethal mechanistic cellular assays hold the promise to deliver a more sensitive approach to detect endpoint-specific drug toxicities. The panel of assays covered by this review includes steatosis, cholestasis, phospholipidosis, reactive intermediates, mitochondria membrane function, oxidative stress, and drug interactions. In addition, the use of metabolically competent cells or the introduction of major human hepatocytes in these in vitro studies allow a more complete picture of potential drug side effect. Since inter-individual therapeutic index (TI) may differ from patient to patient, the rational use of one or more of these cellular assay and targeted in vivo exposure data may allow pharmaceutical scientists to select drug candidates with a higher TI potential in the drug discovery phase.

Efficacy of lower doses of vanadium in restoring altered glucose metabolism and antioxidant status in diabetic rat lenses

Vanadium compounds are potent in controlling elevated blood glucose levels in experimentally induced diabetes. However the toxicity associated with vanadium limits its role as therapeutic agent for diabetic treatment. A vanadium compound sodium orthovanadate (SOV) was given to alloxan-induced diabetic Wistar rats in lower doses in combination with Trigonella foenum graecum, a well-known hypoglycemic agent used in traditional Indian medicines. The effect of this combination was studied on lens morphology and glucose metabolism in diabetic rats. Lens, an insulin-independent tissue, was found severely affected in diabetes showing visual signs of cataract. Alterations in the activities of glucose metabolizing enzymes (hexokinase, aldose reductase, sorbitol dehydrogenase, glucose-6-phosphate dehydrogenase) and antioxidant enzymes (glutathione peroxidase, glutathione reductase) besides the levels of related metabolites, [sorbitol, fructose, glucose, thiobarbituric acid reactive species (TBARS) and reduced glutathione (GSH)] were observed in the lenses from diabetic rats and diabetic rats treated with insulin (2 IU/day), SOV (0.6 mg/ml), T. f. graecum seed powder (TSP, 5%) and TSP (5%) in combination with lowered dose of vanadium SOV (0.2 mg/ml), for a period of 3 weeks. The activity of the enzymes, hexokinase, aldose reductase and sorbitol dehydrogenase was significantly increased whereas the activity of glucose-6-phosphate dehydrogenase, glutathione peroxidase and glutathione reductase decreased significantly in lenses from 3 week diabetic rats. Significant increase in accumulation of metabolites, sorbitol, fructose, glucose was found in diabetic lenses. TBARS measure of peroxidation increased whereas the levels of antioxidant GSH decreased significantly in diabetic condition. Insulin restored the levels of altered enzyme activities and metabolites almost to control levels. Sodium orthovanadate (0.6 mg/ml) and Trigonella administered separately to diabetic animals could partially reverse the diabetic changes, metabolic and morphological, while vanadate in lowered dose in combination with Trigonella was found to be the most effective in restoring the altered lens metabolism and morphological appearance in diabetes. It may be concluded that vanadate at lowered doses administered in combination with Trigonella was the most effective in controlling the altered glucose metabolism and antioxidant status in diabetic lenses, these being significant factors involved in the development of diabetic complications, that reflects in the reduced lens opacity.

Lens lipids and maximum lifespan

Unlike in most organs, the lipid composition of lenses varies dramatically among species and with age. The focus of this study is to assess how these changes relate to lifespan. Studies on cataract suggest that the lens may serve as a window into the processes leading to accelerated mortality. As a first step toward elucidating cellular processes in the lens that may serve as markers for accelerated mortality, we examined the correlation between species-dependent and age-related lens lipid compositional differences and maximum life span. We included data from camels, which, even in old age, rarely develop cataracts although they live under adverse conditions. Camel lens lipids were mainly composed of sphingolipids (77%) and phosphatidylcholines (23%). Bovine lens lipid composition was comparable to a previous study, and both bovine lens sphingolipids, phosphatidylcholines and camel lens phosphatidylcholines content fit well (within the 95% confidence limits) in the curve obtained by plotting maximum life spans of other species with sphingolipids and phosphatidylcholines. Lifespan was directly related to lens sphingolipid content and indirectly related to lens phosphatidylcholine content. The camel lens sphingolipid value was significantly above the curve for other species. Except for the camel lens nucleus, lipid order and sphingolipid content were linearly related, p < 0.005 with a slope of 0.85+/-0.07, and intercept of 6.9+/-3.8. Lipid phase transition temperature and sphingolipid content were also linearly related, p = 0.01 with a slope of 0.20+/-0.07, and intercept of 21.7+/-5.3. Our data support the hypothesis that humans have adapted so that their lens membranes have a high sphingolipid content that confers resistance to oxidation, allowing these membranes to stay clear for a relatively longer time than is the case in many other species. Age-related changes in human lens lipid composition may serve as a marker for oxidative stress and may reflect systemic oxidative insult, providing a window into the health of an individual.