The lens has a transparent, elastic avascular refractive organization and plays an important role in visual formation. Cataracts are characterized by gradual accumulation of cloudy deposits on the ocular lens and have been a leading cause of visual impairment and blindness worldwide for centuries. Although modern cataract surgery is safe and effective, there are still many problems, such as high costs, loss of normal functions of postoperative eyes, and a high incidence of after-cataract. As the aging population increases, cataracts have become an increasingly serious issue [
1,
2]. Thus, there is a great demand for safe, effective, and inexpensive agents to prevent or delay the onset of cataracts.
Cataracts are multifactorial eye diseases associated with several risk factors such as aging, diabetes, exposure to sunlight, and hypertension. However, oxidative stress caused by reactive oxygen species (ROS) has long been regarded as the major mechanism by which cells are damaged and cataracts are formed [
3‐
6]. Under physiological conditions, lenses can compensate for a mild degree of oxidant stress and remove oxidative damaged molecules by elevating endogenous antioxidants such as reduced glutathione (GSH) and activating anti-oxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), which play important roles in protecting the lens against oxidative stress. However, in some cases such as aging, ROS production is excessive or the ability of the lens to scavenge ROS decreases, and oxidative stress injuries may occur and then cataracts are formed [
3,
4].
Because of the major role of oxidative stress in cataract formation, natural antioxidants with high activity and few side effects have attracted increasing attention to delay the onset or progression of cataracts [
7‐
10].
Moringa oleifera, also known as horseradish tree or drumstick tree, belongs to the
Moringaceae family and has been used in nutritious foods and traditional medicines for the treatment of various diseases such as rheumatism, inflammation, and diabetes in many Asian countries [
11]. Particularly,
Moringa oleifera is one of the best known and most widely distributed species that is rich in natural antioxidants [
12,
13].
Moringa oleifera leaf extracts was reported to inhibit the ROS formation induced by H
2O
2 and enhanced the activities and mRNA expressions of SOD and CAT in KB cells [
14] and in HEK-293 Cells [
15]. Recently the
Moringa oleifera leaf extract was reported to protect yeast cells against oxidative stress induced by cadmium and H
2O
2 through the reduction of intracellular ROS levels [
16]. Regular intake of
Moringa oleifera leaves through diet decreased the lipid per oxidation and increase the SOD and CAT activities in a diabetes-induced oxidative stress model in rats [
17].
Moringa oleifera seed extract can inhibit the ROS formation induced by high fat diet in mice [
18].
Moringa oleifera root extract attenuated beryllium-induced oxidative stress in rats [
19]. All these significant antioxidant activities of
Moringa oleifera from both in vitro as well as in vivo studies suggest that
Moringa oliefera may inhibit the cataract formation induced by oxidative stress. Although some studies have reported that the
Moringa oleifera leaf extract has potential inhibitory effects on high sugar-induced cataract in goat lens in vitro [
20] and selenite-induced cataract in rat pups [
21], no study have actually been conducted on the protective effects of
Moringa oleifera on oxidative stress induced cataract. In addition, compared with the traditional uses of the leaves, flowers, and seeds of
Moringa oleifera, its stem is not often consumed, and the stem may even be considered as an agricultural by-product. However,
Moringa oleifera stem is very abundant and inexpensive. Thus, any health benefit from it may reach a large part of the population. Therefore, it is worthwhile to investigate the potential effects of
Moringa oleifera stem on delaying the onset or progression of cataracts induced by oxidative stress. In addition, PPARs (including three isoforms: α, γ, and β/δ) are ligand-activated transcription factors of the nuclear hormone receptor and play key roles in maintaining glucose and lipid homeostasis by modulating gene expression. Recent studies indicate that PPARs have potential benefits to improve or prevent various vision-threatening eye diseases such as diabetic retinopathy, glaucoma, and diabetic macular edema [
22‐
24]. Therefore, in this research, we also evaluate the effect of
Moringa oleifera stem on the expression of PPARs.
Lens organ cultures provide a simple and effective platform to screen for candidate compounds that protect against cataract formation [
25,
26]. Hydrogen peroxide (H
2O
2) is the main intracellular ROS in the aqueous humor, which causes cataract development [
27,
28], and is often used to induce cataract formation in vitro [
8]. Therefore, in the present study, we focused on the protective effect of
Moringa oleifera stem extract (MOSE) against cataract formation and explored its underlying mechanism using a cataract formation model induced by H
2O
2 in lens organ culture. Luteolin is a flavonoid present in the leaves and stems of many plants and some reports indicate that luteolin exerts protective effects on selenite [
29,
30] and STZ [
31]-induced cataracts. So luteolin was used as a reference for its established antioxidant property in our research.