Euonymus alatus: A Review on Its Phytochemistry and Antidiabetic Activity

Euonymus alatus (E. alatus) is a medicinal plant used in some Asian countries for treating various conditions including cancer, hyperglycemia, and diabetic complications. This review outlines the phytochemistry and bioactivities of E. alatus related to antidiabetic actions. More than 100 chemical constituents have been isolated and identified from E. alatus, including flavonoids, terpenoids, steroids, lignans, cardenolides, phenolic acids, and alkaloids. Studies in vitro and in vivo have demonstrated the hypoglycemic activity of E. alatus extracts and its certain constituents. The hypoglycemic activity of E. alatus may be related to regulation of insulin signaling and insulin sensitivity, involving PPARγ and aldose reductase pathways. Further studies on E. alatus and its bioactive compounds may help to develop new agents for treating diabetes and diabetic complications.


Antidiabetic Activity
The effects of E. alatus extracts have been tested in vivo. In streptozotocin (STZ) treated diabetic rats, an aqueous extract of E. alatus reduced the body weight, the fasting plasma glucose level, and glucose tolerance. The serum levels of insulin, glucagon, cholesterol, and triglyceride were also reduced [31]. Similar results were obtained in high-fat plus low dose STZ diabetic rats, showing that E. alatus treated rats had lower levels of fasting blood glucose and insulin and decreased levels of blood lipids and inflammatory mediators (TNF-, C-reactive protein), indicating that E. alatus can improve the glucose-lipid metabolism and insulin resistance in diabetic conditions [32]. Park et al. also demonstrated that an ethanol extract of E. alatus reduced the body weight, increased insulin sensitivity, and corrected the associated hyperinsulinemia and hyperlipidemia in high-fat diet-induced hyperglycemic and hyperlipidemic ICR mice [33].
The antihyperglycemic effect of E. alatus may involve a protection of functional islet cells since E. alatus treated animals were shown with more positive staining of islet cells than those in diabetic controls [34]. Other studies in ICR mice indicate that E. alatus may affect glucose and lipid homeostasis via a regulation of hepatic lipogenesis related genes (SREBPla, FAS, and GAPT) and PPAR gene expressions in periepididymal fat. The plausible mechanism of hypoglycemic and hypolipidemic actions of E. alatus extract is illustrated in Figure 5 [33].
In addition, a study showed that E. alatus protected rats from experimental diabetic nephropathy induced by uninephrectomy plus STZ treatment, with 12-week administration of E. alatus extract and irbesartan (positive control) decreased HbA1c and pathological changes (extracellular matrix expansion and glomerulosclerosis) in kidney and improved blood lipids profile and kidney function; the effect was associated with a downregulation of transform growth factor 1 expression [35]. In addition, E. alatus was shown to inhibit polyol pathway, which is known to be associated with chronic diabetic complications such as neuropathy, nephropathy, and retinopathy [36].
Fang et al. studied the antidiabetic effects of different fractions of E. alatus extracts (including petroleum ether, diethyl ether, ethyl acetate, n-butanol, and water fraction) in alloxan-induced diabetic mice and high-fat diet diabetic mice and found that ethyl acetate fraction significantly reduced plasma glucose and glucose tolerance in both normal and diabetic mice [37] and also reduced total cholesterol and triglyceride contents and increased SOD activity in diabetic mice [37]. Further analysis revealed that the main components in the ethyl acetate fraction were flavonoids and phenolic acids, including quercetin and kaempferol, which were known for their antioxidant activities [37]. In another study, different extract fractions of E. alatus, including aqueous, diethyl ether, and ethyl acetate fractions, were tested in alloxan induced diabetic mice at a dose of 10 g/kg and it was found that the aqueous extract was the most active in decreasing blood glucose and lipid levels and improved glucose tolerance [6]. Thus, E. alatus may contain multiple active antidiabetic constituents. Similarly, a study on the hypoglycemic fractions of six fractions of E. alatus extracts (including petroleum ether, ethyl acetate, n-butanol, water, residue, and rectified polysaccharide) in diabetic rats found that the fractions of petroleum ether, water, and ethyl acetate had significant antidiabetic effects. Fractions of n-butanol and rectified polysaccharide reduced blood creatinine, and other fractions reduced urea level. The residue fraction decreased the low-density lipoprotein (LDL) and cholesterol contents. The body weight was increased by the treatment with all fractions except rectified polysaccharide. These results indicate that different active compounds in these fractions may be responsible for the observed effects of E. alatus, including antidiabetic, antihyperlipidemic, kidney function improvement, blood viscosity decrease, and body weight affecting [38], and the active antidiabetic compounds are likely to be from the petroleum ether, water, and ethyl acetate fractions. In another study, an ethyl acetate extract of E. alatus was shown with hypoglycemic effect, and four compounds were isolated from this fraction and identified as p-hydroxybenzoic acid (EA-1), protocatechuic acid (EA-2), 4-hydroxy-3-methoxybenzoic acid (EA-3), and 3, 5-dimethoxy-4-hydroxybenzoic acid (EA-4) [8]. Others reported identification of six compounds with hyperglycemic activity from the 90% ethanol extracts of E. alatus, including aromadendrin, epifriedelanol, protocatechuic acid,sitosterol, quercetin, and rutin [39]. The active components in protecting experimental diabetic nephropathy as mentioned above have also been suggested to be concentrated in ethyl acetate and n-butanol fractions [36,40], though the nature of these compounds is still not identified. Jeong et al. (2015) studied the inhibitory effects of 23 compounds isolated from E. alatus on protein tyrosine phosphatases 1B (PTP1B) and -glucosidase activities and found that lupenone, lupeol, taraxerol, p-propoxybenzoic acid, 1-feruloyl--D-glucoside, and 3-hydroxycoumarinflavanol exhibited inhibitory activity against PTP1B with IC 50 values ranging from 5.6 to 18.4 M. 24R-methyllophenol, arborinone, and p-propoxybenzoic acid were shown with a similar    [20] 111 Grasshopper ketone C OH O HO [49] Evidence-Based Complementary and Alternative Medicine 9   [22].
Studies on kaempferol and quercetin, the active constituents of E. alatus, demonstrated that these compounds improved insulin-stimulated glucose uptake in mature 3T3-L1 adipocytes [41]. kaempferol and quercetin were shown to act as weak partial agonists in the PPAR reporter gene assay, without inducing differentiation of 3T3-L1 preadipocytes as traditional PPAR agonists. When kaempferol and quercetin were added together with the PPAR agonist rosiglitazone, the 3T3-L1 differentiation was inhibited in a dose-dependent manner. Competitive ligand-binding assay confirmed that kaempferol and quercetin competed with rosiglitazone at the same binding pocket site as PPAR . These compounds were also shown with significant inhibitory effects on NO production in response to lipopolysaccharide treatment in macrophage cells in which the PPAR was overexpressed. These findings suggest that kaempferol and quercetin may act on multiple targets to ameliorate hyperglycemia [41].
Ivorra et al. studied the effects of daucosterol ( -sitosterol 3--glucoside) and its aglycone ( -sitosterol) on plasma insulin and glucose levels in normo-and hyperglycemic rats and found that oral administration of daucosterol or -sitosterol increased the fasting plasma insulin levels. In addition, both compounds improved the oral glucose tolerance and increased glucose-induced insulin secretion [42]. In addition, rutin, one of the constituents of E. alatus, has been shown to decrease the plasma levels of glucose and lipids and increase the expression of PPAR mRNA and protein in skeletal muscles of db/db mice [43]. Rutin was also demonstrated with an AR inhibition activity (IC 50 3.01 M) [44]. Quercitrin, a flavonoid glycoside in E. alatus, was demonstrated as a noncompetitive AR inhibitor. It blocked polyol accumulation in intact rat lenses incubated in medium containing high concentration of sugars [45]. Other compounds, such as linarin (acacetin-7-O--D-rutinoside), have also been demonstrated with inhibitory activity against -glucosidase [46].

Clinical Evidence.
There have been limited clinical studies, mostly case reports, on the antidiabetic actions of E. alatus containing formulae (Table 8). In addition, a controlled trial, involving two groups of patients (40 patients in each group) with impaired glucose tolerance, showed that the group treated with diet and exercise intervention plus E. alatus formula for 1 month had significantly reduced blood glucose levels, compared to that in the diet and exercise control group. The effective rate was 80% in the E. alatus treatment group, compared to that of 55% in the control group [47]. It should be pointed out in most of these studies that E. alatus was not used lone, but in combination with other herbs; thus it is not clear if the observed effects are due to E. alatus or through interactions with other herbs. Thus, there may be potential bias in these findings. The current evidence for the clinical efficacy for treating diabetes is still weak. Nevertheless, these findings warrant further studies.

Conclusion
There is an increasing interest in E. alatus as a potential antidiabetic agent. More than 100 chemical constituents have been isolated and identified from E. alatus. The main chemical classes include flavonoids, terpenoids, steroids, phenylpropanoids, cardenolides, phenolic acids, and alkaloids. E. alatus has been demonstrated with hyperglycemic activity in vivo. The hypoglycemic activity E. alatus may be related to its effects on insulin signaling and glucose metabolism, including stimulating insulin secretion, improving affinity Fasting blood glucose, 2 h postprandial blood glucose values Treatment group: 32 cases effective (80%) and 8 cases no effect (20%) Control group: 22 cases effective (55%) and 18 cases no effect (45%) [47] of insulin and receptor, increasing insulin sensitivity and tolerance, and reducing insulin resistance. It may also act as PPAR agonist and aldose reductase inhibitor. Further study on the bioactive compounds of E. alatus and its pharmacology may help to develop new agents for treating diabetes and diabetic complications.