Mechanistic analysis of S-(1,2-dichlorovinyl)-L-cysteine-induced cataractogenesis in vitro

Placenta as a target of trichloroethylene toxicity

Trichloroethylene (TCE) is an industrial solvent and a common environmental contaminant detected in thousands of hazardous waste sites. Risk of exposure is a concern for workers in occupations that use TCE as well as for residents who live near industries that use TCE or who live near TCE-contaminated sites. Although renal, hepatic and carcinogenic effects of TCE have been documented, less is known about TCE impacts on reproductive functions despite epidemiology reports associating maternal TCE exposure with adverse pregnancy outcomes. Toxicological evidence suggests that the placenta mediates at least some of the adverse pregnancy outcomes associated with TCE exposure. Toxicology studies show that the TCE metabolite, S-(1,2-dichlorovinyl)-l-cysteine (DCVC) generates toxic effects such as mitochondrial dysfunction, apoptosis, oxidative stress, and release of prostaglandins and pro-inflammatory cytokines in placental cell lines. Each of these mechanisms of toxicity have significant implications for placental functions and, thus, ultimately the health of mother and developing child. Despite these findings there remain significant gaps in our knowledge about effects of TCE on the placenta, including effects on specific placental cell types and functions as well as sex differences in response to TCE exposure. Due to the critical role that the placenta plays in pregnancy, future research addressing some of these knowledge gaps could lead to significant gains in public health.

Reactive Oxygen Stimulation of Interleukin-6 Release in the Human Trophoblast Cell Line HTR-8/SVneo by the Trichlorethylene Metabolite S-(1,2-Dichloro)-l-Cysteine

Trichloroethylene (TCE) is a common environmental pollutant associated with adverse reproductive outcomes in humans. TCE intoxication occurs primarily through its biotransformation to bioactive metabolites, including S-(1,2-dichlorovinyl)-l-cysteine (DCVC). TCE induces oxidative stress and inflammation in the liver and kidney. Although the placenta is capable of xenobiotic metabolism and oxidative stress and inflammation in placenta have been associated with adverse pregnancy outcomes, TCE toxicity in the placenta remains poorly understood. We determined the effects of DCVC by using the human extravillous trophoblast cell line HTR-8/SVneo. Exposure to 10 and 20 μM DCVC for 10 h increased reactive oxygen species (ROS) as measured by carboxydichlorofluorescein fluorescence. Moreover, 10 and 20 μM DCVC increased mRNA expression and release of interleukin-6 (IL-6) after 24-h exposure, and these responses were inhibited by the cysteine conjugate beta-lyase inhibitor aminooxyacetic acid and by treatments with antioxidants (alpha-tocopherol and deferoxamine), suggesting that DCVC-stimulated IL-6 release in HTR-8/SVneo cells is dependent on beta-lyase metabolic activation and increased generation of ROS. HTR-8/SVneo cells exhibited decreased mitochondrial membrane potential at 5, 10, and 20 μM DCVC at 5, 10, and 24 h, showing that DCVC induces mitochondrial dysfunction in HTR-8/Svneo cells. The present study demonstrates that DCVC stimulated ROS generation in the human placental cell line HTR-8/SVneo and provides new evidence of mechanistic linkage between DCVC-stimulated ROS and increase in proinflammatory cytokine IL-6. Because abnormal activation of cytokines can disrupt trophoblast functions necessary for placental development and successful pregnancy, follow-up investigations relating these findings to physiologic outcomes are warranted.

The use of rat lens explant cultures to study the mechanism of drug-induced cataractogenesis

Lens explant cultures were used to assess the mechanism of drug-induced cataractogenic potential of NVS001, a peroxisome proliferator-activated receptor delta (PPARδ) agonist, which resulted in cataract in all treated animals during a 13-week rat study. Ciglitazone, a PPARγ agonist and cataractogenic compound, was used as a positive control to validate this model. Rat lenses were extracted and cultured in medium supplemented with antibiotics for 24-h preincubation pretreatment. Lenses showing no signs of damage at the end of the preincubation pretreatment period were randomized into five experimental groups, (1) untreated control, (2) 0.1% dimethyl sulphoxide control, (3) 10μM NVS001, (4) 10μM ciglitazone, and (5) 10μM acetaminophen (negative control). Lenses were treated every 24 h after preincubation pretreatment for up to 48 h. Samples for viability, histology, and gene expression profiling were collected at 4, 24, and 48 h. There was a time-dependent increase in opacity, which correlated to a decrease in viability measured by adenosine triphosphate levels in NVS001 and ciglitazone-treated lenses compared with controls. NVS001 and ciglitazone had comparable cataractogenic effects after 48 h with histology showing rupture of the lens capsule, lens fiber degeneration, cortical lens vacuolation, and lens epithelial degeneration. Furthermore, no changes were seen when lenses were treated with acetaminophen. Gene expression analysis supported oxidative and osmotic stress, along with decreases in membrane and epithelial cell integrity as key factors in NVS001-induced cataracts. This study suggests that in vitro lens cultures can be used to assess cataractogenic potential of PPAR agonists and to study/understand the underlying molecular mechanism of cataractogenesis in rat.

A current practice for predicting ocular toxicity of systemically delivered drugs

The ability to predict ocular side effects of systemically delivered drugs is an important issue for pharmaceutical companies. Although animal models involving standard clinical ophthalmic examinations and postmortem microscopic examinations of eyes are still used to identify ocular issues, these methods are being supplemented with additional in silico, in vitro, and in vivo techniques to identify potential safety issues and assess risk. The addition of these tests to a development plan for a potential new drug provides the opportunity to save time and money by detecting ocular issues earlier in the program. This review summarizes a current practice for minimizing the potential for systemically administered, new medicines to cause adverse effects in the eye.

Ciglitazone-induced lenticular opacities in rats: in vivo and whole lens explant culture evaluation

The cataractogenic potential of the thiazolidinedione ciglitazone (CIG) was investigated in vivo and in vitro. In the rat, CIG caused a dose-dependent (30-300 mg/kg/day) increase in incidence and severity of nuclear cataract formation during a 3-month nonclinical safety assessment study. Potential mechanisms of toxicity were surveyed using whole rat lens explants exposed to CIG with or without various inhibitors of cataract formation. In vitro, CIG caused a concentration-(0.375-30 muM) and time-dependent (3-24 h) change in biochemical [ATP content or mitochondrial reduction of the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium (MTT) and reduced glutathione (GSH) content] and morphometric (lens wet weight and clarity) markers of damage. Within 3 h of exposure, 7.5 muM CIG decreased lens ATP content 37 +/- 7% (percentage of difference from control, p < 0.05). After 24 h of exposure, lens ATP content, MTT reduction, and GSH content declined 57 +/- 5, 30 +/- 28, and 42 +/- 8%, respectively. Lens wet weight increased 17 +/- 4% with a concomitant decrement in lens clarity. Pretreating lenses with the mitochondrial calcium uniport inhibitor ruthenium red (RR) partially or fully protected lenses from toxicity. In contrast, the antioxidant dithiothreitol, aldose reductase inhibitor sorbinil, and selective cell-permeable calpain inhibitors [calpain II inhibitor and (2S,3S)-trans-epoxysuccinyl-l-leucylamido-3-methylbutane ethyl ester (E64d)] were ineffective in providing protection under the present testing conditions. Early and selective changes in lenticular ATP content and the partial or full protective effect of RR suggest that alterations in lens bioenergetics may play an important role in CIG-induced cataract formation. Lens explant cultures were successfully used to select two thiazolidinediones that lacked cataractogenic activity when evaluated in 3-month rat safety assessment studies.

Extrathyroidal actions of antithyroid thionamides

Some compounds having thionamide structure inhibit thyroid functions. Such antithyroid thionamides include mercaptomethylimidazole (methimazole), thiourea and propylthiouracil, of which mercaptomethylimidazole is widely used to treat hyperthyroidism. Undesirable side effects develop from these drugs due to extrathyroidal actions. Antithyroid thionamides inhibit lactoperoxidase which contributes to the antibacterial activities of a number of mammalian exocrine gland secretions that protect a variety of mucosal surfaces. These drugs stimulate both gastric acid and pepsinogen secretions, thereby augmenting the severity of gastric ulcers and preventing wound healing. Increased gastric acid secretion is partially due to the H2 receptor activation, and also through the stimulation of the parietal cell by intracellular generation of H2O2 following inactivation of the gastric peroxidase-catalase system. Severe abnormalities may develop in blood cells and the immune system after thionamide therapy. It causes agranulocytosis, aplastic anemia, and purpura along with immune suppression. Olfactory and auditory systems are also affected by these drugs. Thionamide affects the sense of smell and taste and also causes loss of hearing. It binds to the Bowman's glands in the olfactory mucosa and causes extensive lesion in the olfactory mucosa. Thionamides also affect gene expression and modulate the functions of some cell types. A brief account of the chemistry and metabolism of antithyroid thionamides, along with their biological actions are presented.

Effects of quercetin on antioxidant defense in streptozotocin-induced diabetic rats

In light of evidence that some complications of diabetes mellitus may be caused or exacerbated by oxidative damage, we investigated the effects of subacute treatment with the antioxidant quercetin on tissue antioxidant defense systems in streptozotocin-induced diabetic Sprague-Dawley rats (30 days after streptozotocin induction). Quercetin, 2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one, was administered at a dose of 10mg/kg/day, ip for 14 days, after which liver, kidney, brain, and heart were assayed for degree of lipid peroxidation, reduced and oxidized glutathione content, and activities of the free-radical detoxifying enzymes catalase, superoxide dismutase, glutathione peroxidase, and glutathione reductase. Treatment of normal rats with quercetin increased serum AST and increased hepatic concentration of oxidized glutathione. All tissues from diabetic animals exhibited disturbances in antioxidant defense when compared with normal controls. Quercetin treatment of diabetic rats reversed only the diabetic effects on brain oxidized glutathione concentration and on hepatic glutathione peroxidase activity. By contrast, a 20% increase in hepatic lipid peroxidation, a 40% decline in hepatic glutathione concentration, an increase in renal (23%) and cardiac (40%) glutathione peroxidase activities, and a 65% increase in cardiac catalase activity reflect intensified diabetic effects after treatment with quercetin. These results call into question the ability of therapy with the antioxidant quercetin to reverse diabetic oxidative stress in an overall sense.

The use of explant lens culture to assess cataractogenic potential

Explanted cultures of crystalline lenses have been used to investigate mechanisms of xenobiotic-induced cataract formation. However, very few studies have utilized mechanistic information to predict the cataractogenic potential of structurally diverse xenobiotics. The present investigation outlines how visual assessment of lens clarity, biochemical endpoints of toxicity, and mechanisms of lenticular opacity formation can be used to select compounds with a lower probability of causing cataract formation in vivo. The rat lens explant culture system has been used to screen thiazolidinediones against ciglitazone for their direct cataractogenic potential in vitro. The two compounds that were selected as development candidates (englitazone and darglitazone) did not produce cataracts in rats exposed daily for 3 months. The culture system has also been used to illustrate that the lens is capable of metabolizing compounds to reactive intermediates. In this example, the toxicity of S-(1,2-dichlorovinyl)-L-cysteine (DCVC), a model cataractogen, was attenuated by inhibiting lenticular cysteine conjugate beta-lyase metabolism using aminooxyacetic acid. Finally, this model was used retrospectively to investigate the cataractogenic potential of CJ-12,918 and CJ-13,454 in rats. These compounds showed differences in the incidence of cataract formation in vivo based on differences in hepatic metabolism and penetration of parent drug and metabolites into the lens. The rank order of cataractogenic potential in vitro correlated better with in vivo results when an induced S9 microsomal fraction was added to the culture media. However, the model did not correctly predict the cataractogenic potential of ZD2138, a structurally similar compound. These studies illustrate the use of explant culture to assess mechanisms of cataract formation and outline its use and limitations for predicting cataractogenic potential in vivo.