The results of the current study showed that apocynin prevents cataracts, using morphological grading and laboratory analysis. The selenite-induced cataract model has been widely used as a model for evaluating anti-cataract agents due to its rapidity, ease and reproducibility [10, 17–19]. The effects of many pharmacological agents including piceatannol [19], alpha-glucosyl hesperidin [18], hydrogen saline [20], sildenafil [3], and rosmaniric asit [12] in the treatment of cataracts have been frequently investigated in the literature. However, some of these agents may cause secondary metabolic reactions that need to be eliminated in vivo and may affect the metabolic oxidation-reduction reactions in the lens since their reduction is very strong. Therefore, in this experimental study, apocynin belonging to the methoxy-substituted catechol group was used in a selenite-induced cataract model in rats and it was determined that both low and high doses of apocynin administration decreased the cataract grade in lenses. Low-dose apocynin was seen to increase CAT and GR levels in lens tissue but did not change MDA levels in the lens tissue, and high-dose apocynin was observed to increase MDA levels.
Apocynin has been found to lower the levels of reactive oxygen species (ROS) and reduce oxidative damage [21]. Kılıç et al. reported that apocynin decreased neutrophil oxidative burst and neutrophil chemotaxis and thus neutrophil-mediated cell damage. It was emphasized that apocynin has therapeutic and protective effects on pulmonary fibrosis [22]. Al-Shabrawey et al. showed that apocynin administration reduced oxidative stress and protected the eyes from retinal neovascularization in ischemic retinopathy in rats [23]. Another study found that superoxide radical production was significantly reduced and rod cells were protected when apocynin was administered to rats with photoreceptor degeneration in retinal dystrophies [24]. It has also been reported that especially neuronal cells and those related to visual physiology are adversely affected by NADPH oxidase activation, triggering choroidal endothelial cell migration, which is critical in age-related macular degeneration [23, 25]. These positive effects have been demonstrated in conditions such as choroidal neovascularization and diabetic retinopathy [26]. In an in vitro model, NADPH oxidase activation was suppressed by apocynin, resulting in a significant reduction in choroidal neovascularization due to its potent antioxidant effects [27]. Ozer et al. reported improvements in histopathological and ophthalmoscopic findings in experimental proliferative vitreoretinopathy with apocynin treatment for 21 days [28]. Consistent with the literature, the positive effects of apocynin were evaluated on a selenite-induced cataract model in rats in the current study.
According to the oxidative stress hypothesis of cataract formation, ROS cause increased harmful biochemical reactions, including oxidation, cross-linking and aggregation of lens proteins, peroxidation of membrane lipids and apoptosis of lens epithelial cells [29]. MDA is known to be a toxic component in the eye due to its high level of cross-linking capacity with the lipid membrane [30]. In recent studies evaluating the anti-cataractogenic effects of various substances on selenite-induced cataract models, MDA has been widely used as an oxidative stress marker [31, 32]. A previous study showed that an increased MDA level in the retina of rats with diabetic nephropathy regressed and decreased SOD and GSH levels increased with apocynin treatment [33]. In another study, 5-week treatment with 2.4 g/L apocynin (in drinking water) reduced systemic and hepatic oxidative stress in mice fed a high-fat diet [34]. In contrast to the literature, the current study results showed that high-dose apocynin significantly increased the MDA level in lens tissue while low-dose did not alter this level, suggesting that apocynin affects the oxidative stress in a dose-dependent manner in lens tissue, although this effect was not based on lipid peroxidation but another mechanism which may play a role in oxidative stress in cataract formation.
There is a wide range of enzymatic antioxidant defenses which remove free radicals and other ROS, including SOD, CAT, GPx, GR and GST [35]. CAT reactions are the main mechanism for removal of high concentrations of H2O2. CAT protects the tissues from oxidative damage by reacting with oxidant substances [36]. There have been shown to be different effects of apocynin used in animal models on antioxidant enzymes. measured in different tissues [37, 38]. Low-dose (10 mg/kg) and high-dose (20 mg/kg) apocynin have been reported to show similar antioxidant activity in acute lung injury[38] and hyperoxic lung injury [37], but there are no previous studies in which both doses have been studied in the cataract simultaneously. The current study included both doses of apocynin in a selenite-induced cataract model in rats and thus comparisons were made between doses. CAT levels in the lens tissue of both dose groups were significantly higher than those of the non-treated cataract group. In addition, low-dose apocynin was seen to successfully increase CAT levels in the lens tissue to normal levels, suggesting that both doses of apocynin prevented cataract formation by enhancing antioxidant activity.
As an antioxidant marker, GR is the rate-controlling enzyme of the glutathione redox cycle, and it maintains the intracellular GSH level by supporting the integrity of cell membranes and stabilizing the sulfhydryl groups of proteins [39]. Therefore, GFR levels were examined to determine the effects of apocynin in the cataract model. Both doses of apocynin increased GR levels in the lens tissue, while a high dose increased MDA level, suggesting that it may be partially toxic in the cataract model.
Additionally, the solvent we employed for apocynin, DMSO, is commonly employed in dermatological research[40] and with its anti-inflammatory[41] and intracranial edema-reducing[42] properties substantiated by previous studies. Reports suggest that DMSO acts as an antioxidant by scavenging free radicals and stabilizing damaged membranes, thus averting leakage from injured cells [40]. Our study's biochemical and morphological analyses support these assertions, indicating that, in accordance with existing literature, DMSO predominantly impedes cataract formation via antioxidant enzymes [40].
This experimental study has several limitations. As previously mentioned, experimental selenite cataracts are dissimilar from senile human cataracts in some aspects. Therefore, the results obtained in this experimental study cannot be directly related to the development of human cataracts. In addition, there was no positive control group to enable comparisons of therapeutic efficacy between apocynin and some known drugs. However, a strong aspect of this study was that both doses of apocynin prevented cataract formation by regulating the antioxidant systems, which was supported by the laboratory and morphological data. Additional studies are needed to determine the optimal dose of apocynin and its efficacy on other tissues.