The Exploration of Natural Compounds for Anti-Diabetes from Distinctive Species Garcinia linii with Comprehensive Review of the Garcinia Family

Approximately 400 Garcinia species are distributed around the world. Previous studies have reported the extracts from bark, seed, fruits, peels, leaves, and stems of Garcinia mangostana, G. xanthochymus, and G. cambogia that were used to treat adipogenesis, inflammation, obesity, cancer, cardiovascular diseases, and diabetes. Moreover, the hypoglycemic effects and underlined actions of different species such as G. kola, G. pedunculata, and G. prainiana have been elucidated. However, the anti-hyperglycemia of G. linii remains to be verified in this aspect. In this article, the published literature was collected and reviewed based on the medicinal characteristics of the species Garcinia, particularly in diabetic care to deliberate the known constituents from Garcinia and further focus on and isolate new compounds of G. linii (Taiwan distinctive species) on various hypoglycemic targets including α-amylase, α-glucosidase, 5′-adenosine monophosphate-activated protein kinase (AMPK), insulin receptor kinase, peroxisome proliferator-activated receptor gamma (PPARγ), and dipeptidyl peptidase-4 (DPP-4) via the molecular docking approach with Gold program to explore the potential candidates for anti-diabetic treatments. Accordingly, benzopyrans and triterpenes are postulated to be the active components in G. linii for mediating blood glucose. To further validate the potency of those active components, in vitro enzymatic and cellular function assays with in vivo animal efficacy experiments need to be performed in the near future.


α-Amylase and α-Glucosidase
To regulate the postprandial blood glucose level, diabetic patients took carbohydrate hydrolase inhibitors such as α-glucosidase and α-amylase to avoid hyperglycemia. α-amylase and α-glucosidase are the key enzymes to hydrolyze carbohydrates and help glucose ingestion [30]. Therefore, diabetic patients have to control their blood glucose by using clinical drugs such as Precose ® (Acarbose) and Glyset ® (Miglitol) [4] or other anti-diabetic natural compounds [27] to prolong hydrolysis of carbohydrates against hyperglycemia. In cumulative studies, a few crude extracts from the Garcinia species, e.g., G. cambogia, G. xanthochymus, G. kola, and G. mangostana [10][11][12][13][14], containing biflavonoids, polyphenols, and xanthones, also inhibit the enzyme activity of α-amylase and α-glucosidase. Therefore, the extracts are able to help diabetic patients to control their blood glucose levels by the inhibition of carbohydrate hydrolysis. Our docking results ( Figure 1) showed that benzopyrans and triterpenes had a higher binding affinity with α-amylase and α-glucosidase than with biflavonoid and phenolic compounds. Additionally, α-tocopherolquinone (a kind of benzopyrans) and squalene (a kind of triterpenes) had a high binding affinity with α-amylase and α-glucosidase to prolong carbohydrate hydrolyzation, reduce the absorption of glucose and mediate the blood glucose level.    The binding affinity of benzopyrans, triterpenes, stigmastane, biflavonoid, and phenolic on α-amylase, α-glucosidase, AMPK, insulin receptor kinase, PPARγ, and DPP4. Molecular docking was performed by Gold program. Ranking/ChemPLP score presents the order of score value. The model setup was genetic algorithms (GA) run 10 times, a GA search efficiency 200%, removal of water and hydrogen, and ChemPLP scoring. ChemPLP used hydrogen bonding and multiple linear potentials to model Van der Waals and repulsive terms. α-Tocopherolquinone (a kind of benzopyrans) and squalene (a kind of triterpenes) had a higher binding affinity than the reference drug, Acarbose with α-amylase and α-glucosidase prolonging the carbohydrates hydrolyzed to reduce the absorption of glucose and regulate blood glucose levels. Interestingly, α-tocopherolquinone also had a higher binding affinity than reference drugs (Metformin, Chaetochromin, and GW9662) with AMPK1, AMPK2, PPARγ, and IRK templates, respectively; and binding signals would stimulate insulin secretion in contrast to Squalene, which only had a binding affinity with AMPK1. However, α-tocopherolquinone and Squalene still had a stronger binding affinity than Sitagliptin (reference drug) with DDP4 template that could prevent incretins from being digested by DDP4 and promote skeletal cells' uptake of glucose from the blood.

Peroxisome Proliferator-Activated Receptor Gamma (PPARγ)
The peroxisome proliferator-activated receptor (PPAR) is a nuclear receptor superfamily and has three isotypes α, δ, and γ that can regulate lipid metabolism, inflammation, and insulin sensitivity as well as insulin production and secretion for treating diabetes [40][41][42]. PPARγ could mediate lipid mobilization, glucose metabolism, inflammatory response, and adipokines production and secretion [41,43]. Henceforth, cumulative studies emerged and showed PPARγ ligands that could promote triglyceride storage in fat that was implicated in insulin resistance and control adipocyte-secreted hormones [41]. In clinical treatments, Rosiglitazone is an agonist of PPARγ that could ameliorate the memory of Alzheimer patients and even increase insulin sensitivity for diabetes [44]. In traditional therapy, thiazolidinedione (TZD) was usually used to treat diabetes patients but TZD promotes triglyceride storage that causes adverse effects such as headache, muscle soreness, obesity, edema, etc. [45]. Previously, the extract of G. cambogia contained (−)-hydroxycitric acid (HCA), which was found to be an active ingredient used to treat obesity and obesity-related diseases, e.g., diabetes, atherosclerosis, etc. [46]. The results (Figure 1) showed that α-tocopherolquinone, 6β-Hydroxystigmast-4-en-3-one, 1,6-Dihydroxy-3,5-dimethoxyxanthen-9-one, and 1,6-Dihydroxy-5-methoxyxanthone stimulated insulin sensitivity, and in virtual screening via the binding affinity of GW9662 (reference drug), which is lower than those of compounds isolated from G. linii.

Dipeptidyl-Peptidase 4 (DPP-4) and Glucagon-Like Peptide 1 (GLP-1)
The dipeptidyl peptidase-4 (DPP-4) could hydrolyze glucagon-like peptide 1 (GLP-1) or gastric inhibitory polypeptide (GIP) and lead to negative effects on the concentration of incretins (GLP-1 and GIP), insulin secretion, and glucose tolerance due to DPP4 gene expression [47]. Consequently, some diabetes patients may take a DPP4 inhibitor such as Sitagliptin to increase insulin secretion for diabetes therapy and ameliorate the therapeutic effect of GLP-1 [48,49]. The GLP-1 was treated with DPP4 inhibitors against diabetes from 2005 to 2007 and still had adverse effects such as rhinopharyngitis and upper respiratory tract infections [50]. Therefore, some cumulative studies indicated that natural compounds, e.g., rutin, curcumin, antroquinonol, quercetin, and 16-hydroxy-cleroda-3, 13-dien-15, 16-olide (HCD), could inhibit DPP4 activity, such as the inhibitory efficacy of curcumin and quercetin, better than Sitagliptin [36,37,51]. A previous report showed that the extract of the G. cambogia fruit, which contains hydroxycitric acid (HCA), could decrease the serum insulin levels and prolong intestinal tracts to absorb glucose as well as to potentially change incretins (GLP-1, GIP) secretions [10,52]. Taken altogether, the extract of G. cambogia could regulate blood glucose levels, treat metabolic syndromes, and lead to weight loss. To increase insulin sensitivity, our docking results showed that benzopyrans, triterpenes, stigmastane, and biflavonoids were found to act as insulin receptor agonists and promoted glucose uptake in skeletal cells from blood. Hereafter, incretins are degraded by DPP4 and lead to pancreatic β cells to decrease secretions of insulin (Figure 1). Of note, the reference drug, Sitagliptin, plays a major role in inhibiting the activation of DPP4. Obviously, our data indicated that α-tocopherolquinone and squalene had stronger binding affinity with DPP4 as an inhibitor than with Sitagliptin to prevent incretin (GLP-1) degraded by DPP4.

Insulin Receptor Kinase (IRK)
α-subunits of insulin receptors receive signal insulin, which triggers tyrosine kinase of β-subunits (Insulin receptor kinase, IRK) to form intracellular auto-phosphorylation at Tyr1158, Tyr1162, and Tyr1163 [53]. Once the insulin receptors are activated, they promote PI3K to phosphorylate PIP2; and, further, PIP3 leads the PDK1/2 activation. When AKT was phosphorylated by receiving the signal, the downstream AS160 would prompt glucose transporter 4 (GLUT4) translocation and uptake glucose into the cells [54]. Previously, some natural compounds have been demonstrated such as (+)-antroquinonol isolated from Antrodia cinnamomea [55], rutin (a kind of flavonoid) isolated from Toona sinensis Roem [53], and the phenolics isolated from coffee silverskins and husks [56] that result in lowered glucose levels. All of these compounds could enhance the activation of IRK to promote the skeletal tissues to absorb glucose and, consequently, ameliorate insulin resistance by reducing blood glucose levels in the diabetic patients. Therefore, in this study, we collated research from the literature by the application of the Garcinia species for various anti-diabetes treatments. Previous literature revealed that G. xanthochymus, G. kola, G. mangostana, G. pedunculata, and G. prainiana contained natural compounds, e.g., biflavonoids, xanthone, HCA, and depsidone, which could augment IRK activity and regulate the blood glucose levels for diabetic patients [10,15,16,25,57]. Accordingly, our docking data revealed that only α-tocopherolquinone had a higher binding affinity with IRK than a reference drug (Chaetochromin), suggesting that α-tocopherolquinone acts as an anti-hyperglycemic compound to heighten IRK activity (Figure 1). Table 1. Summary of the Garcinia species on specific targets of anti-diabetes with basic findings.

Species. Molecular Targets Basic Findings
G. cambogia α-Glucosidase, PPARγ, DPP4 Small intestinal exposure to HCA resulted in a modest reduction in glycemia of healthy individuals [20]. Mixture (GE containing HCA as an active ingredient, PE, anti-adipogenic activity) reduced the expression of adipogenesis-related factors C/EBP-α, PPARγ, and FAS [46]. Insulin resistance did not develop in HCA-SX-supplemented rats via lowered fasting plasma insulin and glucose [58,59].

Conclusions and Future Remarks
In ancient societies, the Garcinia species were used as a daily supply, e.g., building material, food additives, fruit juice, jam, and dye. However, natural compounds that are isolated from the bark, seeds, fruits, peels, leaves, and stems of some Garcinia species such as G. kola, G. pedunculata, G. prainiana, G. mangostana, G. xanthochymus, and G. cambogia have been reported to have a variety of medicinal values. These compounds are applied to treat adipogenesis, inflammation, obesity, cancer, cardiovascular diseases, and diabetes. Predominantly, the isolated natural compounds of G. linii in this study are employed to do molecule docking with α-amylase, α-glucosidase, AMPK, IRK, PPARγ, and DPP4, respectively. Of note, our docking data revealed that the ChemPLP scores for Benzopyrans, Flavonols, Polyphenol, Stigmastane, and Triterpenes isolated from G. linii had a higher AMPK affinity when compared with Metformin; and, alternatively, Garcinia linii alone or in combination with Metformin can have a greater potential to alleviate side-effects or elevate applicable time (e.g., cumulative effect) by reducing Metformin dosage. These results demonstrated that benzopyrans and triterpenes had a stronger binding affinity with anti-diabetic target molecules as a template than reference drugs, e.g., Acarbose with α-Amylase and α-Glucosidase, Metformin with AMPK, Sitagliptin with DPP4, Chaetochromin with IRK, and GW9662 with PPARγ. According to this evidence, benzopyrans and triterpenes are suggested to be the active components in G. linii for mediating blood glucose. To further validate the potency of these active components, compounds purified and subsequently the enzyme activity test, an in vitro cellular function assay and an in vivo animal efficacy experiment need to be conducted to investigate their potential role in anti-diabetes and anti-hyperglycemia in the future. Acknowledgments: We sincerely thanks Max K Leong (Department of Chemistry, NDHU, Hualien, Taiwan) for his help for this work accomplishment.

Conflicts of Interest:
The authors declare no conflict of interest.