Scholarly article on topic 'AMP-activated protein kinase: An emerging target for ginseng'

AMP-activated protein kinase: An emerging target for ginseng Academic research paper on "Biological sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Journal of Ginseng Research
OECD Field of science
Keywords
{AMPK / cancer / ginsenosides / "metabolic disease" / " Panax ginseng "}

Abstract of research paper on Biological sciences, author of scientific article — Kyong Ju Jeong, Go Woon Kim, Sung Hyun Chung

Abstract The adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a key sensor of cellular energy. Once activated, it switches on catabolic pathways generating adenosine triphosphate (ATP), while switching off biosynthetic pathways consuming ATP. Pharmacological activation of AMPK by metformin holds a therapeutic potential to reverse metabolic abnormalities such as type 2 diabetes and nonalcoholic fatty liver disease. In addition, altered metabolism of tumor cells is widely recognized and AMPK is a potential target for cancer prevention and/or treatment. Panax ginseng is known to be useful for treatment and/or prevention of cancer and metabolic diseases including diabetes, hyperlipidemia, and obesity. In this review, we discuss the ginseng extracts and ginsenosides that activate AMPK, we clarify the various mechanisms by which they achieve this, and we discuss the evidence that shows that ginseng or ginsenosides might be useful in the treatment and/or prevention of metabolic diseases and cancer.

Academic research paper on topic "AMP-activated protein kinase: An emerging target for ginseng"

J Ginseng Res 38 (2014) 83-88

Review article

AMP-activated protein kinase: An emerging target for ginseng

Kyong Ju Jeong, Go Woon Kim, Sung Hyun Chung*

Department of Pharmacology and Clinical Pharmacy, College of Pharmacy, KyungHee University, Seoul, Korea

ARTICLE INFO

Article history:

Received 14 August 2013

Received in Revised form

22 October 2013

Accepted 19 November 2013

Available online 18 December 2013

Keywords:

cancer

ginsenosides

metabolic disease

Panax ginseng

ABSTRACT

The adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a key sensor of cellular energy. Once activated, it switches on catabolic pathways generating adenosine triphosphate (ATP), while switching off biosynthetic pathways consuming ATP. Pharmacological activation of AMPK by metformin holds a therapeutic potential to reverse metabolic abnormalities such as type 2 diabetes and nonalcoholic fatty liver disease. In addition, altered metabolism of tumor cells is widely recognized and AMPK is a potential target for cancer prevention and/or treatment. Panax ginseng is known to be useful for treatment and/or prevention of cancer and metabolic diseases including diabetes, hyperlipidemia, and obesity. In this review, we discuss the ginseng extracts and ginsenosides that activate AMPK, we clarify the various mechanisms by which they achieve this, and we discuss the evidence that shows that ginseng or ginsenosides might be useful in the treatment and/or prevention of metabolic diseases and cancer.

Copyright © 2013, The Korean Society of Ginseng, Published by Elsevier. All rights reserved.

1. Introduction

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a somewhat old kinase because its activity was first documented in 1973 as a negative regulator of acetyl-coenzyme A (CoA) carboxylase (ACC) and 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGR) in the biosynthesis of fatty acids and cholesterol, respectively [1,2]. AMPK is a highly preserved sensor of cellular energy status, and appears to exist in essentially all eukaryotes as heterotrimeric complexes composed of a catalytic a subunit and regulatory b and g subunits. The a subunit contains the kinase domain, with the conserved threonine residue that is the target for upstream kinases [liver kinase B1 (LKB1) and Ca2+-activated calmodulin-dependent kinase kinases (CaMKKs)] located within the activation loop. Phosphorylation at Thr172 is required for ki-nase activity and function in all species from yeast to man, and with the human kinase, causes >100-fold activation [3]. In mammals, all three subunits have multiple isoforms encoded by distinct genes (a1, a2; g1, g2, g3), which assemble to form up to 12 het-

erotrimeric combinations [4]. The functions of the different subunit isoforms remain unclear, although there is tissue-specific

expression of some isoforms, and there is evidence that different isoforms may target complexes to specific subcellular locations. Because the energy status of the cell is a crucial factor in all aspects of cell function, it is not surprising that AMPK has umpteen downstream targets whose phosphorylation mediates dramatic changes in cell metabolism, cell growth, and other functions.

Obesity and the metabolic syndrome represent a major health problem in both Western and developing countries. Considering the role of AMPK in regulating energy balance at both the cellular and whole-body levels, this kinase occupies a pivotal position in studies regarding obesity, diabetes, and the metabolic syndrome [5]. By direct phosphorylation of metabolic enzymes and transcription factors, AMPK switches on catabolic pathways, such as the uptake of glucose and fatty acids, and their metabolism by mito-chondrial oxidation and glycolysis. In addition, AMPK switches off anabolic pathways, such as the synthesis of glucose, glycogen, and lipids in the liver. By promoting muscle glucose uptake and metabolism and by inhibiting hepatic gluconeogenesis, AMPK activation can explain the antidiabetic action of metformin. Type 2 diabetes is primarily caused by insulin resistance, which is strongly associated with excess triglyceride storage in liver and muscle. By switching

* Corresponding author. Department of Pharmacology and Clinical Pharmacy, College of Pharmacy, Kyung Hee University, 1 Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Korea.

E-mail address: suchung@khu.ac.kr (S.H. Chung).

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

1226-8453/$ — see front matter Copyright © 2013, The Korean Society of Ginseng, Published by Elsevier. All rights reserved. http://dx.doi.org/10.1016/j.jgr.2013.11.014

off the synthesis of fatty acids and triglycerides and enhancing fat oxidation, AMPK activation might also explain the insulin-sensitizing action of metformin.

The uncontrolled proliferation of cancer cells is supported by a corresponding adjustment of energy metabolism. Nowadays, altered metabolism of tumor cells is widely recognized as an emerging hallmark and a potential drug target in cancer cells. Protein synthesis is the best-characterized process regulated by the mammalian target of rapamycin complex 1 (mTORC1). mTORC1 plays a key role in translational control by phosphorylating lots of translation regulators, including S6 kinase 1 (S6K1) [6]. The synthesis of fatty acids, triglycerides, cholesterol, RNA, and proteins are all upregulated in tumor cells. Notably, because protein synthesis requires a myriad of cellular energy, AMPK activation induced by metabolic stress significantly inhibits protein synthesis, resulting in AMPK—mTORC1 crosstalk: AMPK attenuates mTORC1 signaling through phosphorylation and activation of tuberous sclerosis 2 [7], a negative regulator of mTORC1. AMPK also directly phosphorylates Raptor, which induces 14-3-3 binding to raptor and repression of mTORC1 activity [8]. Other findings that AMPK caused the inhibition of progress through the cell cycle [9], and that the mechanism of AMPK activation required the presence of the tumor suppressor LKB1 [10—12] also gave us the idea that AMPK activators might be beneficial in the prevention and/or treatment of cancer. AMPK activation switches off all of these pathways and would therefore be expected to exert an antitumor effect, reinforced by its ability to cause cell-cycle arrest. These effects of AMPK might explain the tumor suppressor effects of the upstream kinase LKB1 [13], as well as findings that metformin usage reduces the risk of cancer in diabetics [14] and that metformin and other AMPK activators (phenformin, A-769662) delay the onset of tumorigenesis in a mouse model [15].

Over recent years, a plethora of naturally occurring compounds including ginseng and ginsenosides have been reported to activate AMPK in intact cells. These natural products include resveratrol from grapes [16], epigallocatechin-3-gallate (EGCG) from green tea and capsaicin from chili peppers [17], curcumin from turmeric [18], as well as four compounds derived from traditional Chinese medicine, berberine from Chinese Goldthread [19], hispidulin from Snow Lotus [20], licochalcone A from Glycyrrhiza and Brassica rapa [21], and betulinic acid from Betula [22]. Ginseng is one of the most popular and bestselling herbal medicines worldwide. Ginseng has been used as a medicine and/or as a neutraceutical by healthy and ill individuals all around the world. Many clinical and animal studies on ginseng have been performed to characterize its therapeutic properties, which include improving physical performance [23,24] and sexual function [25,26], treating cancer [27,28], diabetes [29—31], and hypertension [32,33]. In this article, we review the mechanisms by which AMPK is activated by ginseng extracts or ginsenosides, well-known active components found in ginseng. Ginseng was used for preventing and/or treating metabolic disorders and cancer prior to when it was realized that ginseng and ginsenosides seem to be AMPK activators.

2. Pharmacological activities of ginseng as an AMPK activator

AMPK activators derived from medicinal plants have disparate chemical structures and it was difficult to see how they activate AMPK. However, it has now been shown that some inhibit mito-chondrial function, either inhibiting the respiratory chain (berberine and licochalcone A) or the adenosine triphosphate (ATP) synthase (EGCG and resveratrol), or acting as an uncoupler (curcumin). Consistent with the idea that this is how they activate AMPK, berberine and resveratrol increased the AMP:ATP ratio in cultured cells and failed to activate AMPK in cells expressing the

AMP/ADP-insensitive R531G [34]. Why do so many plants produce compounds that are mitochondrial inhibitors and hence AMPK activators? Respiratory chain and ATP synthase might have potential binding sites for xenobiotic compounds, and the production of mitochondrial poisons might be a suitable mechanism for plants to deter infection by pathogens. To date, 31 English language articles were published according to a search of the PubMed database using keywords "ginseng", "ginsenoside", and "AMPK". Among them, 19 articles are related to metabolic diseases, six articles are related to cancer, and six articles are related to other pharmacological activities, including two review articles.

2.1. Effects on metabolic diseases

Beneficial effects of ginseng and its active ingredients on metabolic disorders have been known from many clinical and animal studies. Table 1 summarizes the effects of ginseng associated with AMPK activation in animal and cell studies. AMPK phosphorylates serine residues surrounded by a well-defined recognition motif [8,35]. Fig. 1 shows targets involved in the acute and chronic regulation of metabolism. Ginseng or ginsenosides can work on one specific target and pathway or more than one target, or even other targets not shown in Fig. 1, including glycolysis, lipolysis, glycogen synthesis, protein synthesis, forkhead box transcription factor class O1/3a (FOXO1/3a) target genes, genes involved in oxidative stress resistance, cytochrome P450 drug metabolism genes, and amplitude and period of expression of circadian genes.

(1) AMPK activates glucose transporter 4 (GLUT4)-mediated glucose uptake in muscle via phosphorylation of TBC1 domain family member 1 (TBC1D1) [36]. Leeetal [37] demonstrated that higher expression levels of GLUT4 and its transcription factor (myocyte enhancer factor 2, MEF-2) were observed in the gastrocnemius muscle of Korean red ginseng (KRG)-treated Otsuka Long-Evans Tokushima Fatty (OLETF) rats compared with untreated rats.

(2) AMPK activates fatty acid uptake via translocation of the transporter CD36 to the plasma membrane [38]. Kim et al [39] showed that compound K (CK) increased gene expressions of peroxisome proliferator activated receptor a (PPARa) and CD36, a transcriptional regulator for lipid catabolism and uptake in human hepatoma cells.

(3) AMPK activates fatty acid oxidation by phosphorylating and inactivating the mitochondria-associated isoform of ACC2, thus lowering malonyl-CoA, an inhibitor of fatty acid uptake into mitochondria via the carnitine palmitoyltransferase system [40]. Shen et al [41] demonstrated that Rb1 reduced fatty liver in obese rats, and this effect was primarily due to increased fatty acid oxidation via activation of the AMPK signaling pathway.

(4) AMPK inhibits fatty acid synthesis by directly phosphorylating and inactivating the cytosolic isoform of ACC1 [42].

(5) AMPK inhibits triglyceride and phospholipid synthesis by causing inactivation of the first enzyme involved in their synthesis, glycerol-3-phosphate acyl transferase (GPAT) [43]. Yuan et al [44] demonstrated that CK has a beneficial effect on lipid metabolism via activation of AMPK in the liver of C57BL/ksJ db/ db mice. CK (also known as IH-901) significantly reduced the expressions of sterol response element binding protein 1 (SREBP1) and its target genes such as fatty acid synthase (FAS), stearoyl-CoA desaturase 1 (SCD1), and GPAT in the liver of mice.

(6) AMPK inhibits cholesterol synthesis by direct phosphorylation and inactivation of HMGR [45]. Lee et al [46] showed that ginsenoside Rg3 reduces lipid accumulation in HepG2 cells. Rg3 decreased mRNA expression of SREBP2, a transcriptional regulator of genes involved in cholesterol metabolism, and expression of HMGR, which catalyzes a rate-limiting step in

Table 1

Effects of Ginseng on Metabolic Diseases in Relation to AMPK Activation

Material

Cell line/animal

Dose/duration

Effects and molecular mechanism

CK (1H-901)

Ginsam

HFD-fed Long-Evans rats Rat primary hepatocytes

H411E cells

C2C12 myotubes HepG2 cells 3T3-L1 adipocytes C2C12 myotubes 3T3-L1 adipocytes

C57BL/KsJ db mice C2C12 myotubes

HepG2 cells

HepG2 cells HFD-fed C57BL/6J mice 3T3-L1 adipocytes

C2C12 myotubes

HepG2 cells

HepG2 cells

Otsuka Long-Evans Tokushima Fatty rats

Otsuka Long-Evans Tokushima Fatty rats

10 mg/kg, i.p. for 4 weeks 0.01-1 mM 50-200 mM

0.1-10 mM

20-80 mM

10-100 mM 0.001-0.1 mM

10-25 mg/kg, p.o. for 6 weeks 5-20 mM

5-20 mM

5-20 mM

5-20 mg/kg, p.o. for 3 weeks 0.001-0.1 mM

10-40 mM

10-40 mM

5-20 mM

200 mg/kg, p.o. for 40 weeks 300-500 mg/kg, p.o. for 8 weeks

Decreased hepatic fat accumulation [41]

Enhanced fatty acid oxidation via increase in CPT1 activity Increased AMP/ATP ratio

Inhibited gluconeogenesis via induction of SHP gene [56]

expression and suppression of ROS—JNK pathway in palmitate-induced insulin resistance Induced glucose uptake and p38 MAPK phosphorylation [57]

AMPK and p38 activation was mediated by ROS production Decreased hepatic triglyceride and cholesterol levels Inhibited expression of SREBP-2 and HMGR

Inhibited adipocyte differentiation by activation of AMPK [58]

and inhibition of PPAR-g 20(S)-Rg3 showed higher pharmacological effects in [59]

insulin secretion and AMPK activation than 20(R)-Rg3 Enhanced glucose uptake and stimulated GLUT4 [60]

translocation by activation of AMPK and PI3K pathway Inhibited TG accumulation Plasma glucose decreased by 20.7% at 25 mg/kg Plasma insulin increased by 3.4 times in 25 mg/kg-treated mice Stimulated glucose uptake and overexpression of GLUT4

via activation of AMPK and PI3K—Akt pathway Inhibited TG accumulation

Inhibited lipogenesis by modulating LKB1—AMPK—SREBP1 signaling pathway, and stimulated lipolysis via upregulations of PPAR-a and CD36 Inhibited hepatic glucose production and lipogenesis via [50]

activation of AMPK signaling pathway Lowered blood glucose and TG levels by 18.9% and 29.5%

in 20 mg/kg of Re-treated mice Enhanced glucose uptake by inducing mRNA and protein [60]

expression of GLUT4 Enhanced TG accumulation

Improved insulin resistance [61]

Enhanced glucose uptake by overexpression of GLUT4 via

activation of AMPK Inhibited hepatic glucose production by phosphorylation [55]

of LKB1, AMPK, and FoxO1 PEPCK and G6Pase activities were decreased

Inhibited hepatic glucose production by phosphorylation [48]

of GSKb and induction in SHP gene expression was mediated by AMPK activation

Improved insulin sensitivity [37]

Promoted fatty acid oxidation and enhanced mitochondrial biogenesis and glucose utilization by activation of AMPK Plasma insulin levels were lowered, and this effect was [62]

related to overexpression of GLUT4 by activation of AMPK and PPAR-g

CK, compound K; CPT1, carnitine palmitoyltransferase-1; FoxOl, forkhead box class O1; G6Pase; glucose-6-phosphatase; GLUT4, glucose transporter 4; GSK3b, glycogen synthase kinase 3b; HFD, high fat diet; HMGR, 3-hydroxy-3-methyl glutaryl coenzyme A reductase; i.p., intraperitoneal administration; JNK, c-Jun NH2-terminal kinase; KRG, Korean red ginseng; LKB1, liver kinase Bl; MAPK, mitogen-activated protein kinase; PEPCK, phosphoenolpyruvate carboxykinase; P13K, phosphatidylinositol 3-kinase; p.o., oral administration; PPAR, peroxisome proliferator-activated receptor; ROS, reactive oxygen species; SHP, orphan nuclear receptor small heterodimer partner; SREBP, sterol regulatory element binding protein; TG, triglyceride.

cholesterol synthesis, was also suppressed in a time-dependent manner.

(7) AMPK phosphorylates cyclic AMP response element binding protein (CREB)-regulated transcription coactivator- 2 (CRTC2), causing it to bind 14-3-3 proteins, thus retaining it in the cytoplasm and inhibiting activation of genes encoding gluco-neogenic enzymes such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) in the liver [47]. Yuan et al [48] revealed that ginsenoside Rg2 inhibits hepatic glucose production through activation of the AMPK signaling pathway. Although phosphorylation of CRTC2 by Rg2 was not shown in this article, Rg2 reduced phosphorylation of CREB causing interruption of the formation of the CREB—CRTC2 complex, which resulted in suppression of gene expression of gluconeogenic enzymes such as PEPCK and G6Pase.

(8) AMPK phosphorylates SREBP1, preventing its proteolytic processing and translocation into the nucleus and thus inhibiting transcription of lipogenic genes, including those encoding ACC1 and FAS [49]. Our group reported that CK and Re attenuate hepatic lipid accumulation in HepG2 cells [39,50]. CK and Re attenuated the expression of SREBP1, central to the intracellular surveillance of lipid catabolism and de novo biogenesis, in time-and dose-dependent manners. Genes for SCD1 and FAS, well-known target molecules of SREBP1, were also suppressed.

2.2. Effects on cancer

Beneficial effects of ginseng or ginsenosides on cancer associated with the AMPK signaling pathway were reported since 2009,

GLUT4 gene expression " glucose uptake

Fatty acid uptake & oxidation

Fatty acid & TG synthesis

Cholesterol synthesis

Glucose synthesis

Fig. 1. Acute and chronic metabolic effects of adenosine monophosphate (AMP)-activated protein kinase (AMPK) activation. See text for numbering and key to acronyms. Blue arrows indicate activation, red lines with a bar at the end indicate inhibition. Suppression of hepatic glucose production and lipid accumulation by ginsenosides were mainly mediated by liver kinase B1 (LKB1)—AMPK signaling pathways. ACC, Acetyl coenzyme A carboxylase; ATP, adenosine triphosphate; CaMKK, calmodulin-dependent kinase kinase; CRTC, regulated transcription coactivator; GLUT4, glucose transporter 4; G6Pase, glucose-6-phosphatase; GPAT, glutamine phosphoribosylpyrophosphate amidotransferase; HMGR, 3-hydroxy-3-methylglutaryl-coenzyme A reductase; PEPCK, phosphoenolpyruvate carboxykinase; SREBP, sterol response element binding protein; TG, triglyceride.

and there are six articles published up to the present time. Recently, our group reported that CK and Rg3 induce apoptosis via the CaMKK—AMPK signaling pathway in HT-29 colon cancer cells, and these activities were confirmed using either compound C (a chemical inhibitor of AMPK) or small interfering RNA (siRNA) for AMPK or STO-609 (a chemical inhibitor of CaMKK) [51,52]. Kim et al [53] also reported that CK inhibits cell growth, induces apoptosis via generation of reactive oxygen species, as well as decreasing cyclooxygenase-2 expression and prostaglandin E2 levels. These effects were induced via an AMPK-dependent pathway and were abrogated by a specific AMPK inhibitor, compound C [53]. More recently, Hwang et al [54] reported that 20-O-b-D-glucopyranosyl-20(S)-protopanaxadiol (20-GPPD), a metabolite of ginseng saponin, causes apoptosis of colon cancer cells through the induction of cytoplasmic Ca2+. 20-GPPD decreased cell viability, increased annexin V-positive early apoptosis, and induced sub-G1 accumulation and nuclear condensation of CT-26 murine colon cancer cells. Although 20-GPPD-induced activation of AMPK played a key role in the apoptotic death of CT-26 cells, LKB1, a well-known upstream kinase of AMPK, was not involved in this activation [54].

Although many studies support the tumor-suppressive role of AMPK, some evidence suggests that the metabolic function of AMPK might be overridden by oncogenic signals so that tumor cells use AMPK activation as a survival strategy to gain growth. During certain stages of tumor development, AMPK might act as protective

machinery against metabolic stress such as nutrient deprivation and hypoxia. Thus, investigation to define at which stage of cancer progression might represent a more relevant strategy to employ AMPK activation for cancer treatment is clearly warranted.

3. Perspectives

AMPK is a critical metabolic sensor that finely regulates the energy homeostasis of cells. Therefore, it has been suggested as a potential target for metabolic disorders and cancer. A plethora of chemical agents reported to activate AMPK exist, most notably metformin and 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR). Most of these chemicals, except A-769662, known to be a direct AMPK activator developed in 2005 by Abbott Laboratories, Abbott Park, Illinois, USA, activate AMPK indirectly with some other effects. At this time, we do not know exactly how ginseng or ginsenosides activate AMPK although LKB1 [39,48,50,55] or the calcium-dependent pathway involving phosphorylation of AMPK by CAMKK would be suggested. As alternative or additional explanations, mechanisms involving either an increase in the AMP:ATP ratio [41], inhibition of mitochondrial ATP synthesis, or the SIRT1-dependent pathway via increase in nicotinamide adenine dinucleotide (NAD+) levels should be tested to elucidate further how ginseng or ginse-nosides activate AMPK. Despite recent advances in the mechanistic understanding of AMPK activation by ginseng or ginsenosides,

several key questions still remain. Is there a positive correlation between antimetabolic or anticancer activities of ginseng (and ginse-nosides) and the AMPK signaling pathway as a primary target? If yes, how do ginseng or ginsenosides activate AMPK? Do they activate AMPK directly or indirectly? What are the therapeutic and toxico-logical consequences of AMPK activation? The AMPK field of research is now well developed and should provide new and exciting novelties regarding the application of AMPK in preventive and clinical medicine. With the concerted research efforts of many laboratories, these challenges may be addressed soon.

Conflicts of interest

All authors declare no conflicts of interest.

Acknowledgments

This work was supported by a grant from the Kyung Hee University in 2013 (KHU-20130535).

References

[1] Beg ZH, Allmann DW, Gibson DM. Modulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity with cAMP and with protein fractions of rat liver cytosol. Biochem Biophys Res Commun 1973;54:1362-9.

[2] Carlson CA, Kim KH. Regulation of hepatic acetyl coenzyme A carboxylase by phosphorylation and dephosphorylation. J Biol Chem 1973;248:378-80.

[3] Stein SC, Woods A, Jones NA, Davison MD, Carling D. The regulation of AMP-activated protein kinase by phosphorylation. Biochem J 2000;345:437-43.

[4] Hardie dG. AMP-activated/SNF1 protein kinases: conserved guardians of cellular energy. Nat Rev Mol Cell Biol 2007;8:774-85.

[5] Zhang BB, Zhou G, Li CAMPK. an emerging drug target for diabetes and the metabolic syndrome. Cell Metab 2009;9:407-16.

[6] Ma XM, Blenis J. Molecular mechanisms of mTOR-mediated translational control. Nat Rev Mol Cell Biol 2009;10:307-18.

[7] Inoki K, Zhu T, Guan KL. TSC2 mediates cellular energy response to control cell growth and survival. Cell 2003;115:577-90.

[8] Gwinn DM, Shackelford DB, Egan DF, Mihaylova MM, Mery A, Vasquez DS, Turk BE, Shaw RJ. AMPK phosphorylation of raptor mediates a metabolic checkpoint. Mol Cell 2008;30:214-26.

[9] Imamura K, Ogura T, Kishimoto A, Kaminishi M, Esumi H. Cell cycle regulation via p53 phosphorylation by a 5'-AMP activated protein kinase activator, 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside, in a human hepa-tocellular carcinoma cell line. Biochem Biophys Res Commun 2001;287:562-7.

[10] Hawley SA, Boudeau J, Reid JL, Mustard KJ, Udd L, Mäkelä TP, Alessi DR, Hardie DG. Complexes between the LKB1 tumor suppressor, STRAD alpha/ beta and MO25 alpha/beta are upstream kinases in the AMP-activated protein kinase cascade. J Biol 2003;2:28.

[11] Woods A, Johnstone SR, Dickerson K, Leiper FC, Fryer LG, Neumann D, Schlattner U, Wallimann T, Carlson M, Carling D. LKB1 is the upstream kinase in the AMP-activated protein kinase cascade. Curr Biol 2003;13:2004-8.

[12] Shaw RJ, Kosmatka M, Bardeesy N, Hurley RL, Witters LA, DePinho RA, Cantley LC. The tumor suppressor LKB1 kinase directly activates AMP-activated kinase and regulates apoptosis in response to energy stress. Proc Natl Acad Sci U S A 2004;101:3329-35.

[13] Alessi DR, Sakamoto K, Bayascas JR. LKB1-dependent signaling pathways. Annu Rev Biochem 2006;75:137-63.

[14] Evans JM, Donnelly LA, Emslie-Smith AM, Alessi DR, Morris AD. Metformin and reduced risk of cancer in diabetic patients. BMJ 2005;330:1304-5.

[15] Huang X, Wullschleger S, Shpiro N, McGuire VA, Sakamoto K, Woods YL, McBurnie W, Fleming S, Alessi DR. Important role of the LKB1-AMPK pathway in suppressing tumorigenesis in PTEN-deficient mice. Biochem J 2008;412: 211-21.

[16] Baur JA, Pearson KJ, Price NL, Jamieson HA, Lerin C, Kalra A, Prabhu VV, Allard JS, Lopez-Lluch G, Lewis K, et al. Resveratrol improves health and survival of mice on a high-calorie diet. Nature 2006;444:337-42.

[17] Hwang JT, Park IJ, Shin JI, Lee YK, Lee SK, Baik HW, Ha J, Park OJ. Genistein, EGCG, and capsaicin inhibit adipocyte differentiation process via activating AMP-activated protein kinase. Biochem Biophys Res Commun 2005;338:694-9.

[18] Lim HW, Lim HY, Wong KP. Uncoupling of oxidative phosphorylation by curcumin: implication of its cellular mechanism of action. Biochem Biophys Res Commun 2009;389:187-92.

[19] Turner N, Li JY, Gosby A, To SW, Cheng Z, Miyoshi H, Taketo MM, Cooney GJ, Kraegen EW, James DE, et al. Berberine and its more biologically available derivative, dihydroberberine, inhibit mitochondrial respiratory complex I: a mechanism for the action of berberine to activate AMP-activated protein ki-nase and improve insulin action. Diabetes 2008;57:1414-8.

[20] Lin YC, Hung CM, Tsai JC, Lee JC, Chen YL, Wei CW, Kao JY, Way TD. Hispidulin potently inhibits human glioblastoma multiforme cells through activation of AMP-activated protein kinase (AMPK). J Agric Food Chem 2010;58:9511-7.

[21] Quan HY, Kim SJ, Kim DY, Jo HK, Kim GW, Chung SH. Licochalcone A regulates hepatic lipid metabolism through activation of AMP-activated protein kinase. Fitoterapia 2013;86:208-16.

[22] Quan HY. Kim do Y, Kim SJ, Jo HK, Kim GW, Chung SH. Betulinic acid alleviates non-alcoholic fatty liver by inhibiting SREBP1 activity via the AMPK-mTOR-SREBP signaling pathway. Biochem Pharmacol 2013;85:1330-40.

[23] Kulaputana O, Thanakomsirichot S, Anomasiri W. Ginseng supplementation does not change lactate threshold and physical performances in physically active Thai men. J Med Assoc Thai 2007;90:1172-9.

[24] Bahrke M, Morgan W, Stagner A. Is ginseng an ergogenic aid? Int J Sport Nutr Metab 2009;19:298-322.

[25] Jang DJ, Lee MS, Shin BC, Lee YC, Ernst E. Red ginseng for treating erectile dysfunction: a systematic review. Br J Clin Pharmacol 2008;66:444-50.

[26] Choi YD, Park CW, Jang J, Kim SH, Jeon HY, Kim WG, Lee SJ, Chung WS. Effects of Korean ginseng berry extract on sexual function in men with erectile dysfunction: a multicenter, placebo-controlled, double-blind clinical study. Int J Impot Res 2013;25:45-50.

[27] Helms S. Cancer prevention and therapeutics: Panax ginseng. Altern Med Rev 2004;9:259-74.

[28] Choi J, Kim TH, Choi TY, Lee MS. Ginseng for health care: a systematic review of randomized controlled trials in Korean literature. PLoS One 2013;8: e59978.

[29] Kim S, Shin BC, Lee MS, Lee H, Ernst E. Red ginseng for type 2 diabetes mel-litus: a systematic review of randomized controlled trials. Chin J Integr Med 2011;17:937-44.

[30] Mucalo I, Rahelic D, Jovanovski E, Bozikov V, Romic Z, Vuksan V. Effect of American ginseng (Panax quinquefolius L.) on glycemic control in type 2 diabetes. Coll Antropol 2012;36:1435-40.

[31] Yuan HD, Kim JT, Kim SH, Chung SH. Ginseng and diabetes: the evidences from in vitro, animal and human studies. J Ginseng Res 2012;36:27-39.

[32] Rhee MY, Kim YS, Bae JH, Nah DY, Kim YK, Lee MM, Kim HY. Effect of Korean red ginseng on arterial stiffness in subjects with hypertension. J Altern Complement Med 2011;17:45-9.

[33] Stavro PM, Woo M, Heim TF, Leiter LA, Vuksan V. North American ginseng exerts a neutral effect on blood pressure in individuals with hypertension. Hypertension 2005;46:406-11.

[34] Hawley SA, Ross FA, Chevtzoff C, Green KA, Evans A, Fogarty S, Towler MC, Brown LJ, Ogunbayo OA, Evans AM, Hardie DG. Use of cells expressing gamma subunit variants to identify diverse mechanisms of AMPK activation. Cell Metab 2010;11:554-65.

[35] Scott JW, Norman DG, Hawley SA, Kontogiannis L, Hardie DG. Protein kinase substrate recognition studied using the recombinant catalytic domain of AMP-activated protein kinase and a model substrate. J Mol Biol 2002;317: 309-23.

[36] Fr0sig C, Pehm0ller C, Birk JB, Richter EA, Wojtaszewski JF. Exercise-induced TBC1D1 Ser237 phosphorylation and 14-3-3 protein binding capacity in human skeletal muscle. J Physiol 2010;588:4539-48.

[37] Lee HJ, Lee YH, Park SK, Kang ES, Kim HJ, Lee YC, Choi CS, Park SE, Ahn CW, Cha BS, et al. Korean red ginseng (Panax ginseng) improves insulin sensitivity and attenuates the development of diabetes in Otsuka Long-Evans Tokushima fatty rats. Metabolism 2009;58:1170-7.

[38] Habets DD, Coumans WA, El Hasnaoui M, Zarrinpashneh E, Bertrand L, Viollet B, Kiens B, Jensen TE, Richter EA, Bonen A, et al. Crucial role for LKB1 to AMPKalpha2 axis in the regulation of CD36-mediated long-chain fatty acid uptake into cardiomyocytes. Biochim Biophys Acta 2009;1791:212-9.

[39] Kim DY, Yuan HD, Chung IK, Chung SH. Compound K, intestinal metabolite of ginsenoside, attenuates hepatic lipid accumulation via AMPK activation in human hepatoma cells. J Agric Food Chem 2009;57:1532-7.

[40] Merrill GF, Kurth EJ, Hardie DG, Winder WW. AICA riboside increases AMP-activated protein kinase, fatty acid oxidation, and glucose uptake in rat muscle. Am J Physiol 1997;273:1107-12.

[41] Shen L, Xiong Y, Wang DQ, Howles P, Basford JE, Wang J, Xiong YQ, Hui DY, Woods SC, Liu M. Ginsenoside Rb1 reduces fatty liver by activating AMP-activated protein kinase in obese rats. J Lipid Res 2013;54:1430-8.

[42] Davies SP, Carling D, Munday MR, Hardie DG. Diurnal rhythm of phosphor-ylation of rat liver acetyl-CoA carboxylase by the AMP-activated protein ki-nase, demonstrated using freeze-clamping. Effects of high fat diets. Eur J Biochem 1992;203:615-23.

[43] Muoio DM, Seefeld K, Witters LA, Coleman RA. AMP-activated kinase reciprocally regulates triacylglycerol synthesis and fatty acid oxidation in liver and muscle: evidence that sn-glycerol-3-phosphate acyltransferase is a novel target. Biochem J 1999;338:783-91.

[44] Yuan HD, Kim SJ, Chung SH. Beneficial effects of IH-901 on glucose and lipid metabolisms via activating adenosine monophosphate-activated protein kinase and phosphatidylinositol-3 kinase pathways. Metabolism 2011;60: 43-51.

[45] Clarke PR, Hardie DG. Regulation of HMG-CoA reductase: identification of the site phosphorylated by the AMP-activated protein kinase in vitro and in intact rat liver. EMBO J 1990;9:2439-46.

[46] Lee S, Lee MS, Kim CT, Kim IH, Kim Y. Ginsenoside Rg3 Reduces lipid accumulation with AMP-activated protein kinase (AMPK) activation in HepG2 cells. Int J Mol Sci 2012;13:5729-39.

88 J Ginseng Res

[47] Koo SH, Flechner L, Qi L, Zhang X, Screaton RA, Jeffries S, Hedrick S, Xu W, Boussouar F, Brindle P, et al. The CREB coactivator TORC2 is a key regulator of fasting glucose metabolism. Nature 2005;437:1109-11.

[48] Yuan HD, Kim DY, Quan HY, Kim SJ, Jung MS, Chung SH. Ginsenoside Rg2 induces orphan nuclear receptor SHP gene expression and inactivates GSK3ß via AMP-activated protein kinase to inhibit hepatic glucose production in HepG2 cells. Chem Biol Interact 2012;195:35-42.

[49] Li Y, Xu S, Mihaylova MM, Zheng B, Hou X, Jiang B, Park O, Luo Z, Lefai E, Shyy JY, et al. AMPK phosphorylates and inhibits SREBP activity to attenuate hepatic steatosis and atherosclerosis in diet-induced insulin-resistant mice. Cell Metab 2011;13:376-88.

[50] Quan HY, Yuan HD, Jung MS, Ko SK, Park YG, Chung SH. Ginsenoside Re lowers blood glucose and lipid levels via activation of AMP-activated protein kinase in HepG2 cells and high-fat diet fed mice. Int J Mol Med 2012;29:73-80.

[51] Kim DY, Park MW, Yuan HD, Lee HJ, Kim SH, Chung SH. Compound K induces apoptosis via CAMK-IV/AMPK pathways in HT-29 colon cancer cells. J Agric Food Chem 2009;57:10573-8.

[52] Yuan HD, Quan HY, Zhang Y, Kim SH, Chung SH. 20(S)-Ginsenoside Rg3-induced apoptosis in HT-29 colon cancer cells is associated with AMPK signaling pathway. Mol Med Rep 2010;3:825-31.

[53] Kim AD, Kang KA, Zhang R, Lim CM, Kim HS, Kim DH, Jeon YJ, Lee CH, Park J, Chang WY, et al. Ginseng saponin metabolite induces apoptosis in MCF-7 breast cancer cells through the modulation of AMP-activated protein kinase. Environ Toxicol Pharmacol 2010;30:134-40.

[54] Hwang JA, Hwang MK, Jang Y, Lee EJ, Kim JE, Oh MH, Shin DJ, Lim S, Go Ji, Oh U, et al. 20-0-ß-d-glucopyranosyl-20(S)-protopanaxadiol, a metabolite of

4;38:83—88

ginseng, inhibits colon cancer growth by targeting TRPC channel-mediated calcium influx. J Nutr Biochem 2013;24:1096—104.

[55] Kim SJ, Yuan HD, Chung SH. Ginsenoside Rg1 suppresses hepatic glucose production via AMP-activated protein kinase in HepG2 cells. Biol Pharm Bull 2010;33:325—8.

[56] Lee KT, Jung TW, Lee HJ, Kim SG, Shin YS, Whang WK. The antidiabetic effect of ginsenoside Rb2 via activation of AMPK. Arch Pharm Res 2011;34: 1201—8.

[57] Lee MS, Hwang JT, Kim SH, Yoon S, Kim MS, Yang HJ, Kwon DY. Ginsenoside Rc, an active component of Panax ginseng, stimulates glucose uptake in C2C12 myotubes through an AMPK-dependent mechanism. J Ethnopharmacol 2010;127:771—6.

[58] Hwang JT, Lee MS, Kim HJ, Sung MJ, Kim HY, Kim MS, Kwon DY. Antiobesity effect of ginsenoside Rg3 involves the AMPK and PPAR-g signal pathways. Phytother Res 2009;23:262—6.

[59] Park MW, Ha J, Chung SH. 20(S)-ginsenoside Rg3 enhances glucose-stimulated insulin secretion and activates AMPK. Biol Pharm Bull 2008;31:748—51.

[60] Huang YC, Lin CY, Huang SF, Lin HC, Chang WL, Chang TC. Effect and mechanism of ginsenosides CK and Rg1 on stimulation of glucose uptake in 3T3-L1 adipocytes. J Agric Food Chem 2010;58:6039—47.

[61] Lee HM, Lee OH, Kim KJ, Lee BY. Ginsenoside Rg1 promotes glucose uptake through activated AMPK pathway in insulin-resistant muscle cells. Phytother Res 2012;26:1017—22.

[62] Lim S, Yoon JW, Choi SH, Cho BJ, Kim JT, Chang HS, Park HS, Park KS, Lee HK, Kim YB, et al. Effect of ginsam, a vinegar extract from Panax ginseng, on body weight and glucose homeostasis in an obese insulin-resistant rat model. Metabolism 2009;58:8—15.