Ginger extract and selenium supplementation: A promising approach to improve diabetic retinopathy

• Introduction
• Methods
• Results
• Discussion

Abstract

Purpose: Diabetic retinopathy results from damage to small blood vessels and retinal neurons due to the overproduction of reactive oxygen species and overexpression of TRPM2 and TRPV1. Hence, inhibition of these events by ginger and selenium may reduce diabetes-induced ocular damage. Therefore, the aim of this study was to investigate the therapeutic effects of ginger, selenium, and their combinations on apoptosis, inflammation, insulin resistance, oxidative damage, and the expression of TRPM2 and TRPV1.

Methods: Seventy-two adult male Wistar rats were divided into nine groups as follows: control, diabetes, diabetes-ginger (100 mg/kg), diabetes-selenium (50, 100, and 150 µg/kg), and diabetes-ginger (100 mg/kg)–selenium (50, 100, and 150 µg/kg).

Results: Diabetes increased the expression of protein and genes of TRPM2 and TRPV1, and it induced oxidative damage by increasing malondialdehyde levels and decreasing superoxide dismutase, glutathione peroxidase, and catalase enzyme activities. Diabetes induced apoptosis by increasing BAX and caspase-3 gene expression and decreasing Bcl2 in eye tissue when compared to the control group. However, treatment with ginger (100 mg/kg), selenium (50, 100, and 150 µg/kg), and their combinations improved these situations in the diabetic groups compared to the diabetic group.

Conclusions: Diabetes induced retinopathy by inducing oxidative damage, inflammation, apoptosis, and upregulation of TRPM2 and TRPV1. However, treatments with selenium, ginger, and their combinations improved diabetic retinopathy by inhibiting oxidative damage, inflammation, and apoptosis and downregulating protein and gene expression of TRPM2 and TRPV1. The results of this study suggest that ginger and selenium can be a good treatment to inhibit the progression of diabetic retinopathy.

Diabetic retinopathy is a common complication of diabetes that affects the eyes, leading to vision impairment or even blindness if left untreated. Diabetic retinopathy develops due to prolonged exposure to high blood glucose levels, causing damage to the blood vessels supplying the retina [1,2]. This damage triggers a cascade of events leading to oxidative stress, inflammation, and apoptosis in retinal cells. These processes contribute to the progression of diabetic retinopathy [2]. Transient receptor potential melastatin 2 (TRPM2) and transient receptor potential vanilloid 1 (TRPV1) are two ion channels that have been implicated in the pathogenesis of diabetic retinopathy [3,4]. TRPM2 is a calcium-permeable ion channel that plays a role in oxidative stress–induced cell death [4]. In diabetic retinopathy, increased oxidative stress is observed due to high glucose levels and impaired antioxidant defense mechanisms. Studies have shown that TRPM2 expression and activity are upregulated in the retina of diabetic animals and patients [5,6]. Overactivation of TRPM2 contributes to retinal cell death and inflammation through calcium influx and activation of downstream signaling pathways [2,6]. TRPV1 is a nonselective cation channel primarily known for its role in pain sensation. However, recent studies have suggested its involvement in diabetic retinopathy as well [7,8]. Activation of TRPV1 has been shown to promote inflammation, oxidative stress, and vascular dysfunction, all of which are key features of diabetic retinopathy [8,9]. In animal models of diabetes, increased TRPV1 expression and activity have been observed in the retina, leading to neurovascular damage [10]. Overall, TRPM2 and TRPV1 overactivation due to diabetes contributes to oxidative stress, inflammation, and cell death in the retina [11]. Targeting these ion channels may hold therapeutic potential for managing this sight-threatening complication of diabetes. However, further research is needed to fully understand their precise roles and develop effective treatments targeting these channels.

Recent studies suggest that herbal and nutrient supplementations, such as ginger and selenium supplementation, may hold potential in mitigating these detrimental effects by targeting specific molecular pathways [12,13]. Ginger (Zingiber officinale) has been widely recognized for its anti-inflammatory, antioxidant, and antiapoptotic properties [13,14]. Its active components, such as gingerols and shogaols, have demonstrated protective effects against various diseases [15]. Recent studies have shown that ginger reduced inflammation markers and inhibited oxidative stress by scavenging free radicals [16-18]. Selenium is an essential trace element known for its antioxidant properties. Some studies suggest that selenium supplementation may have a protective effect against diabetic retinopathy by inhibiting oxidative stress and inflammation [18-20]. Oxidative stress is known to contribute to the development and progression of diabetic retinopathy. Selenium acts as a cofactor for some antioxidant enzymes, such as glutathione peroxidase, which helps neutralize harmful free radicals [9,20]. By enhancing the antioxidant defense system, selenium may potentially inhibit the oxidative stress associated with diabetic retinopathy. Additionally, selenium has been shown to have anti-inflammatory properties. Chronic inflammation plays a crucial role in the pathogenesis of diabetic retinopathy [21]. By modulating inflammatory mediators, selenium potentially inhibits inflammation in the retina and the progression of diabetic retinopathy. However, it is important to note that more research is needed to establish a definitive relationship between selenium and diabetic retinopathy [22]. The optimal dose of selenium supplementation for the prevention or management of diabetic retinopathy is unclear. It is always recommended to consult with a health care professional before starting any supplementation regimen. Recent research suggests that combining ginger with some trace elements, such as zinc and selenium, enhances their individual therapeutic effects on diabetic retinopathy. These supplementations may target TRPM2 and TRPV1 ion channels, which play crucial roles in oxidative stress, autophagy, and apoptosis in retinal cells in patients with diabetes. Therefore, the aim of this study was to investigate the anti-inflammatory, antioxidative, and antiapoptotic effects of different doses of selenium in combination with ginger extract in the eye tissue of diabetic rats.

HFD-STZ–induced diabetes increased protein and gene expression of TRPM2 and TRPV1, and it induced inflammation and oxidative damage by increasing the amount of TNF-α, IL-1β, and MDA and decreasing the amount of SOD, GPx, and CAT enzymes. It also induced apoptosis by increasing the expression of BAX and caspase 3 genes and decreasing Bcl2 in eye tissue. However, treatment with ginger extract, different doses of selenium, and their combinations improved these situations.

TRPM2 and TRPV1 channels are upregulated in diabetic retinopathy and contribute to the progression of retinopathy by inducing oxidative damage, apoptosis, and inflammation in retinal cells [30-32]. It has been reported that TRPV1, TRPV4, and TRPM2 channels are activated by oxidative stress [33,34]. DNA damage caused by free radicals and PARP-1 activation also causes the activation of the TRPM2 channel [35]. Our results in the diabetic group showed that protein and gene expression of TRPM2 and TRPV1 are parallel with changes in factors involved in causing oxidative damage, inflammation, and apoptosis. Previous studies have shown that TRP channels are sensitive to reduced intracellular thiol levels and can be inhibited by antioxidant treatment. Increasing intracellular thiol enhances cell survival by improving mitochondrial function and reducing related apoptotic cascades. In fact, these findings clearly indicate that improving the function of TRPM2 and TRPV1 channels and maintaining the appropriate concentrations of antioxidant precursors, such as thiol, can prevent the occurrence of events after their dysfunction (i.e., oxidative damage, inflammation, and apoptosis). In support of this evidence, the results of western blot analysis showed that the activity of TRPM2 and TRPV1 channels improved in a dose-dependent manner by treating with different doses of selenium. However, treatment with selenium in doses of 50 and 100 µg/kg did not have much effect on the expression of the genes of these channels, and the 150-µg/kg dose was able to significantly change their expression. It seems that this effect is due to indirect gene changes resulting from selenium consumption. Probably, in this case, selenium changed the expression of TRPM2 and TRPV1 genes through its intracellular effects. Selenium inhibits activation of TRP channels and intracellular calcium signaling. In addition, the reduction of intracellular calcium decreases mitochondrial depolarization and prevents excessive production of free radicals. Supplementation with selenium, in line with our results, prevents apoptotic cell death by reducing caspase 3 and 9 activities. Selenium is also a cofactor for the antioxidant GPx in reducing excessive reactive oxygen species production in cytosolic compartments. Therefore, the treatments with selenium at different doses improved diabetic retinopathy by inhibiting oxidative damage, inflammation, and apoptosis via the downregulation of TRPM2 and TRPV1 activity.

Under these conditions, ginger appears to have inhibited the protein and gene expression of TRPM2 and TRPV1 channels, as well as factors contributing to oxidative damage and inflammation [14,17,36]. Ginger’s bioactive compounds, such as gingerols and shaogols, are directly and indirectly responsible for inhibiting inflammation and oxidative damage [12,15]. These compounds also exert genomic and nongenomic control over the activity of enzymes involved in sugar and lipid metabolism, insulin action, and the modulation of pathways like the pentose phosphate pathway [12,15,37]. Reducing glucose levels and improving serum insulin levels are likely key factors in mitigating diabetes-related complications, particularly retinopathy. Consistent with its effects on glucose metabolism and insulin function, ginger’s free radical scavenging ability likely contributed to reduced reactive oxygen species levels. This, in turn, appears to have further inhibited inflammation and oxidative damage, in addition to reducing TRPM2 and TRPV1 protein and gene expression.

Oxidative stress, apoptosis, and inflammation are believed to play a significant role in the progression of diabetic retinopathy [12,38-40]. Oxidative stress is a major contributor to retinal cell damage in diabetic retinopathy [38]. Our results showed that treatment with selenium at different doses inhibited MDA levels and improved SOD, CAT, and GPx levels in eye tissue in diabetic rats. Selenium acts as a cofactor in the thiol structure as a main precursor of various antioxidants such as GSH (glutathione) and Se-GPx (selenium-dependent glutathione peroxidase). A decrease in thiol level causes a disturbance in the activity of glutathione peroxidase and is one of the reasons for the disturbance in the antioxidant defense mechanisms. The findings of the diabetic groups treated with ginger extract, different doses of selenium, and their combination showed that ginger with selenium works synergistically to scavenge free radicals, reduce lipid peroxidation, and restore antioxidant enzyme activity. By inhibiting oxidative damage, these supplements may help preserve retinal cell function. Our results, in line with previous studies [14,36,41-43], showed that 100 mg/kg of ginger extract indicates a potential antioxidative activity in diabetic animal models and patients with diabetes. Moreover, ginger extract exhibited anti-inflammatory and antiapoptotic activities, as previously reported, due to the presence of bioactive compounds such as gingerols and shaogls [14,15,36].

Treatment with the combination of ginger (100 mg/kg) and different doses of selenium (50, 100, 150 µg/kg) downregulated protein and gene expression of TRPM2 and TRPV1, as well as inhibited oxidative damage, inflammation, and apoptosis in eye tissue of diabetic rats. There is limited research specifically investigating the synergistic effects of ginger and selenium on inhibiting apoptosis, oxidative damage, and inflammation or even TRPM2 and TRPV1 activity. However, both ginger and selenium have been individually studied for their potential antiapoptotic and inflammatory properties and inhibition of oxidative damage [30,32,37]. As discussed, ginger and selenium have both been studied for their potential anti-inflammatory and antioxidant effects [15,17,36,44]. When used together, they may exhibit synergistic effects in inhibiting inflammation and oxidative damage. Ginger contains various bioactive compounds such as gingerols, shogaols, and paradols, which have been shown to possess anti-inflammatory properties by inhibiting the production of proinflammatory molecules such as cytokines and prostaglandins [14,37]. These compounds have been shown to inhibit apoptosis by modulating various signaling pathways involved in cell death regulation [12,15,37]. For example, ginger extract has been found to suppress apoptosis by reducing the expression of proapoptotic proteins (e.g., Bax) and increasing the expression of antiapoptotic proteins (e.g., Bcl-2) [12]. Ginger extract can also improve the levels of some trace elements and total homocysteine, as well as prevent oxidative damage induced by ethanol in rat eye and testes [17,36]. Ginger also exerts its antiapoptotic effects by reducing oxidative stress and inflammation, which are known triggers for apoptosis [45]. Moreover, ginger contains compounds called piperine [37,44], which may enhance the absorption and bioavailability of certain nutrients, including selenium. By consuming ginger along with selenium-rich foods or supplements, the body can better absorb and utilize selenium for its anti-inflammatory and antioxidant effects [46]. Additionally, individual responses for ginger may vary based on factors such as dosage, form of supplementation (e.g., fresh ginger, ginger extract, selenium supplements), and overall health status [47].

While there is no specific research on the combined effects of ginger and selenium on inhibiting apoptosis, therefore, it is plausible that their individual antiapoptotic properties may synergistically contribute to enhanced protection against cell death. However, further studies are needed to explore this potential synergy and elucidate the underlying mechanisms involved. Overall, the combination of ginger and selenium works synergistically to inhibit inflammation by reducing proinflammatory molecules, enhancing antioxidant activity, modulating immune responses, and improving nutrient absorption. This combination may provide greater protection against inflammatory conditions and oxidative damage compared to using ginger or selenium alone [36]. Selenium has been shown to inhibit apoptosis by regulating multiple signaling pathways involved in cell death regulation [9,30]. For instance, selenium supplementation has been found to increase the expression of antiapoptotic proteins (e.g., Bcl-2) and decrease the expression of pro-apoptotic proteins (e.g., Bax) [9,30]. Additionally, selenium can enhance the activity of antioxidant enzymes, thereby reducing oxidative stress–induced apoptosis. However, it is important to note that more research is needed to establish a clear relationship between selenium supplementation and its effects on diabetic retinopathy in humans. Additionally, individual responses to selenium supplementation may vary, and it should be used under medical supervision [48]. Overall, there is some evidence suggesting the potential benefits of selenium for individuals with diabetic retinopathy. Therefore, the combination of ginger together with selenium in different doses has the potential to inhibit oxidative damage, inflammation, and apoptosis, thereby downregulating TRPM2 and TRPV1 channels. An important point regarding high doses of selenium is the toxic effects. Although we conducted a dose-response study to determine the healing effects of selenium, we did not observe toxic effects in high doses of selenium (100 and 150 µg/kg) in this study. There were also no significant differences between them in some parameters. This lack of significant difference in doses of 100 and 150 µg/kg may indicate the occurrence of cell tolerance or the preparation of cell defense mechanisms to initiate damaging events. In other words, taking these doses for more than the duration of this study or a higher dose may be toxic.