Current Topics: Reviews
Effects of Benzophenones from Mango Leaves on Lipid Metabolism
2019 Volume 67 Issue 7 Pages 634-639
Details
2019 Volume 67 Issue 7 Pages 634-639
The mango tree (Mangifera indica L.) is a tropical, perennial, woody evergreen plant belonging to the Anacardiaceae. In traditional medicine, dried mango tree leaves were considered useful in treating diabetes and respiratory infections. In this paper, we review the phytochemical research on mango leaves and the mechanisms of benzophenones in lipid metabolism regulation. Thirty-six benzophenones have been isolated from mango leaves; among them, mangiferin is the major compound. Structure–activity relationships of benzophenones in lipid accumulation and the mechanisms of action of mangiferin in lipid metabolism are summarized. After oral administration, mangiferin is partly converted to its active metabolite, northyariol, which contributes to the activation of sirtuin-1 and liver kinase B1 and increases the intracellular AMP level and AMP/adenosine triphosphate ratio, followed by AMP-activated protein kinase phosphorylation, leading to increased phosphorylation of sterol regulatory element-binding protein-1c. Current evidence supports ethnopharmacological uses of mango leaves in diabetes and points toward potential future applications.
The mango tree (Mangifera indica L.) is a tropical, perennial, woody evergreen plant belonging to the Anacardiaceae. The consumption of mango leaves has a long history, and the use of mango leaves to make tea has become a common trend to help treat diabetes and diabetes-associated blood vessel problems.1) In traditional Chinese medicine,2) Indian medicine,3) and African folk medicine,4) dried mango tree leaves were considered useful in treating diabetes and respiratory infections. Acute toxicity testing of a 70% ethanol extract of mango tree leaves showed no significant abnormality at the maximal dose (18.4 g/kg) in ICR mice, and no significant pathological, hematological, or biochemical toxic effects were detected in Sprague–Dawley rats administered the extract for 3 consecutive months at a dose of 900 mg/kg/d. This evidence indicates that mango leaf intake in the usual forms is safe.5)
The chemical constituents of mango leaves include phenolic acid and terpenes,6) flavonoids,7) and benzophenones.8) Among them, mangiferin, a glucosyl benzophenone, is the major compound.9) Recently, it has been reported that mango leaves and their compounds exert biological activity, such as myocardial ischemia-reperfusion protection,10) promoting urate excretion through the kidney,11) and antihypertensive,12) antidiabetic, and antihyperlipidemic effects.13)
Based on previous research, we summarize the phytochemical information on mango leaves and the mechanisms of action of benzophenones on lipid metabolism regulation in this review, which may be beneficial for the administration of nutritional supplements or new drug development using mango leaves or their benzophenones.
Traditionally, mango leaves were used for the treatment of diabetes. High triglyceride (TG) levels are the key factor linked to insulin resistance and elevated blood glucose levels in diabetes. Elevated TG levels are also a feature of metabolic syndrome, which increases the risk of heart disease and stroke. Recently, the effects of mango leaf extract on serum TG levels have been examined in high cholesterol diet-induced diabetic rats. The oral administration of mango leaf water extract (100, 200, and 400 mg/kg/d) improved the serum glucose and lipid profiles.14) This result confirmed the effects of mango leaf extract on TG metabolism, but the mechanism of action and active substance remained unclear.
We investigated the effects of mango leaf 70% ethanol extract (ME) on serum lipid metabolism in KK-Ay mice. After 8-week administration, mesentery fat was significantly reduced, and the ratio of mesentery fat to body weight showed a tendency to decrease in the 500-mg/kg/d group. From week 2 until the end of ME administration, serum TG and glucose levels were significantly decreased in a dose-dependent manner (from 200 to 500 mg/kg). From 4 weeks of ME administration, the serum free fatty acid (FFA) level was reduced in the treatment groups. After the final ME administration, all animals were fasted for 20 h, and then the serum and liver biochemical indexes were measured. Serum glucose, TG, and total cholesterol (TC), but not FFA, levels were reduced in the ME-treated groups. In the liver, TG, TC, FFA, and low-density lipoprotein cholesterol (LDL-C) were decreased in the ME-treated groups.15)
Previous research results suggested that the mechanism of action of mango leaf extract on lipid metabolism might be related to energy homeostasis. AMP-activated protein kinase (AMPK) is a key sensor of fuel and energy status regulating glucose and lipid metabolism. Under energy-stress conditions, AMP combines with the AMPKγ subunit-cystathionine synthase of alanine and threonine, causing changes in AMPKα subunit structure, after which the Thr172 subunit of AMPKα is phosphorylated by upstream kinase, activating AMPK.16) The activation of AMPK can improve insulin resistance by increasing fatty acid oxidation and reducing TG synthesis, which contribute to the inhibition of malonyl CoA and acetyl CoA carboxylase (ACC) activity.17) A low AMPK level or AMPK activation failure is considered to be a key factor causing metabolic disease.18) AMPK is involved in whole-body energy consumption and production and it is the only protein kinase targeted in the treatment of metabolic syndrome to date.19)
Focusing on the activation of the AMPK-signaling pathway, we performed protein and gene expression analyses in KK-Ay mice after 8-week administration of ME. Although there was no change in the AMPK protein level in the ME-treated group, ME administration can substantially increase AMPK phosphorylation. In addition, the ME-treated group showed significantly decreased expression levels of ACC (2.8-fold), hormone-sensitive lipase (HSL) (1.6-fold), fatty acid synthase (FAS) (1.8-fold), and peroxisome proliferator activated receptor (PPAR)-γ (4.0-fold). These results suggest that ME-regulated lipid homeostasis may occur through upregulation of the AMPK-signaling pathway15) (Fig. 1).
Benzophenones are the representative components in the Mangifera genus, such as mangiferin, present in the bark, stems, flowers, and leaves of mango trees. The mangiferin content was reported to be more than 2% (w/w) in dried mango leaves collected in Hainan province, P. R. China, in different seasons.20) Thirty-six benzophenones have been isolated and identified from mango leaves8,21–25) (Table 1), and their chemical structures are shown in Fig. 2.
| No. | Compound | Reference |
|---|---|---|
| 1 | Iriflophenone-3-C-β-glucoside | 8) |
| 2 | Mangiferoside A | 8) |
| 3 | Foliamangiferoside B | 21) |
| 4 | Mangiferoside A1 | 8) |
| 5 | Foliamangiferoside A4 | 21) |
| 6 | Mangiferoside A2 | 8) |
| 7 | Foliamangiferoside A3 | 21) |
| 8 | Maclurin-3-C-β-D-glucoside | 8) |
| 9 | 2,4,4′,6-Tetrahydroxy-3′-methoxy-benzophenone-3-C-β-D-glucopyranoside | 22) |
| 10 | Maclurin 3-C-(2-O-galloyl)-β-D-glucoside | 23) |
| 11 | Maclurin 3-C-(6″-O-p-hydroxybenzoyl)-β-D-glucoside | 24) |
| 12 | Maclurin 3-C-(2,3-di-O-galloyl)-β-D-glucoside | 23) |
| 13 | Maclurin 3-C-(2″-O-p-hydroxybenzoyl-6″-O-galloyl)-β-D-glucoside | 24) |
| 14 | Maclurin 3-C-(2″-O-galloyl-6″-O-p-hydroxybenzoyl)-β-D-glucoside | 24) |
| 15 | Maclurin 3-C-(2″,3″,6″-tri-O-galloyl)-β-D-glucoside | 24) |
| 16 | Iriflophenone-3-C-(2-O-p-hydroxybenzoyl)-β-D-glucopyranoside | 8) |
| 17 | Iriflophenone 3-C-(2-O-galloyl)-β-D-glucoside | 23) |
| 18 | Mangiferoside C1 | 8) |
| 19 | Mangiferoside C3 | 8) |
| 20 | Foliamangiferoside C7 | 21) |
| 21 | Foliamangiferoside C6 | 21) |
| 22 | Foliamangiferoside C5 | 21) |
| 23 | Foliamangiferoside C2 | 8) |
| 24 | Foliamangiferoside C4 | 21) |
| 25 | Iriflophenone 3-C-(2″,6″-di-O-galloyl)-β-D-glucoside | 24) |
| 26 | Iriflophenone 3-C-(2″,3″,6″-tri-O-galloyl)-β-D-glucoside | 24) |
| 27 | Iriflophene | 25) |
| 28 | 2,4′,6-Trihydroxy-4-methoxybenzophenone | 22) |
| 29 | Iriflophenone-2-O-β-D-glucopyranoside | 22) |
| 30 | 2,4′,6-Trihydroxy-4-methoxybenzophenone-2-O-β-D-glucopyranoside | 8) |
| 31 | 4,4′,6-Trihydroxybenzophenone-2-O-α-L-arabinofuranoside | 22) |
| 32 | 4,4′,6-Trihydroxybenzophenone-2-O-(2″),3-C-(1″)-1″-desoxy-β-fructopyranoside | 25) |
| 33 | 4′,6-Dihydroxy-4-methoxybenzophenone-2-O-(2″),3-C-(1″)-1″-desoxy-β-fructopyranoside | 25) |
| 34 | 4,4′,6-Trihydroxybenzophenone-2-O-(2″),3-C-(1″)-1″-desoxy-β-fructo-furanoside | 25) |
| 35 | Aquilarinoside A | 22) |
| 36 | 4′,6-Dihydroxy-4-methoxybenzophenone-2-O-(2″),3-C-(1″)-1″-desoxy-α-L-fructofuranoside | 22) |
Benzophenones exhibit a range of biological activities including antifungal, anti-human immunodeficiency virus (HIV), antimicrobial, antioxidant, antidiabetic, antiviral, and cytotoxic activity.26,27) In 2011, our group studied the SAR of benzophenones isolated from mango leaves from the viewpoint of TG accumulation in 3T3-L1 cells. We found that there was no direct link between the bioactivity and cyclization or uncyclization in the C-ring. In the A-ring, methylation of the 4-hydroxyl group weakened the inhibitory effect on TG accumulation. In the B-ring, 3′,4′-dihydroxyl-substituted benzophenones showed the same activity as the 3′-hydroxyl-substituted type, and the activity of the 3′-methoxy-4′-hydroxy type became more moderate than that of the mono- or dihydroxy type8) (Fig. 3).
Niu et al.28) reported that mangiferin, the major benzophenone in mango leaves, improved dyslipidemia by increasing AMPK phosphorylation and its downstream proteins including CD36 and carnitine palmitoyl transferase 1 (CPT-1), as well as decreasing diacylglycerol acyltransferase (DGAT)-2 and ACC both in vivo and in vitro. The results verified the possible mechanism of action of mango leaf extract on lipid metabolism through the AMPK-signaling pathway.
When the effects of benzophenones isolated from mango leaves on AMPK were examined in 3T3-L1 cells, the results indicated that most benzophenones activate AMPK and downregulate sterol regulatory element binding protein (SREBP)-1c and FAS expression, resulting in decreased intracellular TG accumulation.8,21) These results suggested that benzophenones are likely the active substances in mango leaves affecting lipid metabolism.
Mice exposed to a high-fat diet (HFD) combined with mangiferin exhibited a substantial shift in the respiratory quotient from fatty acid toward carbohydrate utilization. Mangiferin administration significantly increased glucose oxidation in the muscle of HFD-fed mice without affecting fatty acid oxidation. These results indicate that mangiferin redirects fuel utilization toward carbohydrates.29) Oral administration of mangiferin (50 and 150 mg/kg body weight) to HFD-fed hamsters significantly decreased final body weight, liver weight, and visceral fat-pad weight; serum TG and FFA concentrations and hepatic TG levels; hepatic and muscle total FFA content; and PPAR-α, CD36, and CPT-1 mRNA expression, while downregulating SREBP-1c, ACC, DGAT-2, and microsomal triglyceride transfer protein gene expression in the liver.30)
Mangiferin exhibits various bioactivities on lipid metabolism and in metabolic disorders. The most consistent phenotype is that mangiferin stimulates lipolysis and suppresses lipogenesis, thereby reducing lipid accumulation and consequently preventing hyperlipidemia.31) However, the exact mechanisms of action remain unclear.
From a drug metabolism perspective, as an xanthone carbon glycoside, the poor solubility of mangiferin in most aqueous and organic solvents makes it difficult to cross the intestinal tract barrier and maintain its original structure after undergoing liver metabolism. Previous studies by another group showed that the bioavailability of oral mangiferin was 1.2% in rats.32)
After oral administration of mangiferin to healthy rats at a dose of 200 mg/kg, after gut flora degradation and liver phase I and II metabolism more than 30 types of mangiferin metabolites could be detected in vivo.33) The metabolic pathways of mangiferin were assumed to be deglycosylated, dehydroxylated, methylated, glycosylated, glucuronidated, and sulfated transformations. Tissue distribution analysis revealed that the amount of mangiferin was less than the amounts of its metabolites in the liver.34) That finding was consistent with our results, implying that mangiferin might not be the final active compound for regulating lipid metabolism. Accordingly, without in vivo results, in vitro experiments are neither appropriate nor sufficient to reveal the actual mechanism of oral mangiferin.
Using phytochemical methods, we isolated the metabolites of mangiferin from mouse urine after long-term oral administration and identified the main ones using spectrographic methods. A ultra performance liquid chromatography-quartet time-of-flight (UPLC-QTOF)-MS method was developed and optimized to identify the mangiferin metabolites in mouse liver, allowing the detection of the compounds mangiferin, norathyriol, 1,3,7-trihydroxy-6-methoxyxanthone, and 1,7-dihydroxyxanthone. Consistent with the previous report,33) liver distribution analysis results indicated that less mangiferin was accumulated compared with the amounts of its metabolites (Fig. 4). These results suggested that not only the original form of mangiferin but also its metabolites may be active constituents in regulating liver lipid metabolism.
Based on liver metabolite analysis, we tested the inhibitory effects of mangiferin metabolites on TG accumulation in oleic acid-induced HepG2 cells. Interestingly, the mangiferin metabolic product norathyriol decreased the intracellular TG content more markedly than mangiferin. In addition, when the mechanism of action of norathyriol on lipid metabolism was investigated, we found that it was mediated by the activation of sirtuin-1 (SIRT-1), liver kinase B1 (LKB1), and elevation of the intracellular AMP level and AMP/ATP ratio, followed by AMPK phosphorylation, resulting in SREBP-1c phosphorylation and inactivation.34)
It was reported that norathyriol is a competitive inhibitor of protein tyrosine phosphatase 1B35) and uridine 5′-diphosphate (UDP)-glucuronosyltransferase isoforms36) to improve glucose homeostasis. Combined with research on the effects of norathyriol on lipid metabolism, the role of mangiferin in metabolic syndrome was partly clarified (Fig. 5).
ACC: acetyl-CoA carboxylase; ACL: ATP-citrate lyase; AMPKα: AMP-activated protein kinase α; ATGL: adipose triglyceride lipase; CaMKK: calmodulin-dependent protein kinase kinase; CPT-1: carnitine palmitoyl transferase 1; DAG: diacylglycerols; DGAT: diacylglycerol acyltransferase; FAS: fatty acid synthase; LDLR: low-density lipoprotein receptor; HSL: hormone-sensitive lipase; LKB1: liver kinase B1; MAG: monoacylglycerol; MGL: monoacylglycerol lipase; PTP1B: protein tyrosine phosphatase 1B; SCAP: SREBP cleavage-activating protein; SCD: stearoyl-CoA desaturase; Sirt1: sirtuin1; SREBP-1c: sterol regulatory element-binding protein 1c; TAG (TG): triglyceride; VLDL: very low-density lipoprotein.
Active constituents from traditional medicine are an important resource for new drug development. This paper reviewed recent research on the phytochemistry, pharmacology, and molecular biology of mango leaves and summarized how mango leaves and their constituent benzophenones regulate lipid metabolism. Mangiferin, a benzophenone glucoside, is present not only in Mangifera sp. but also in other medicinal herbs and food, such as the root of Anemarrhena asphodeloides, the stems of Salacia sp. and Iris unguicularis, and leaves of Bombax ceiba. Mangiferin has great potential for the development of new drugs or functional food based on its important role in regulating lipid metabolism.
This research was supported by Grants from the National Natural Science Foundation of China (81173524; 81673688) and the Important Drug Development Fund, Ministry of Science and Technology of China (2018ZX09735-002).
The authors declare no conflict of interest.
