Pharmacological and Physiological

23 Pharmacological and Physiological

Effects of Ginseng

An-Sik Chung and Kyung-Joo Cho Korea Advanced Institute of Science and Technology

Daejeon, South Korea Jong Dae Park

KT & G Central Research Institute Daejeon, South Korea

I. INTRODUCTION Ginseng is a medicinal plant widely used for the treatment of various diseases

such as cancer, diabetes, and cardiovascular diseases, as well as for improving immune function and vitality. In this chapter, we review recent studies on the effects of ginseng on various diseases, particularly on diabetes, hypertension, and cancer. Ginseng improves glucose homeostasis and insulin sensitivity. It exerts cytotoxic and antimetastatic activities against various kinds of cancer cell lines, and induces differentiation or apoptosis of several cancer cells. Furthermore, ginseng has been reported to have an antineoplastic effect by enhancing immune function. An antihypertensive effect of ginseng is shown to occur by the enhanced synthesis and release of nitric oxide (NO). In addition, ginseng exerts antiatherosclerotic and antiplatelet effects. This chapter pro- vides useful information on the pharmacological and clinical usage, and also

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suggests a method to conduct further study on ginseng to identify its potential effects.

The root of Panax ginseng C. A. Meyer, known as Korean ginseng, has been a valuable and important folk medicine in East Asian countries, including China, Korea, and Japan, for about 2000 years. Panax is derived from the word ‘‘panacea,’’ which means a cure-all for diseases and a source of longevity as well as physical strength and resistance. As the use of traditional Chinese herbs for medicinal and dietary purposes becomes increasingly popular in Western countries, sales of P. ginseng are increasing in North America and Europe as well as other parts of the world.

Active constituents found in most ginseng species include ginsenosides, polysaccharides, peptides, polyacetylenic alcohols, and fatty acids (1). The major active components in P. ginseng are the ginsenosides, a group of saponins with triterpenoid dammarane structure (2). More than 30 ginseno- sides have been isolated (3), and novel structures continue to be identified, particularly from Panax quinquefolius and Panax japonica as well as P. ginseng berry (4,5). Pharmacological effects of ginseng have been demon- strated in cancer, diabetes, and the cardiovascular, immune, and central nervous systems including antistress and antioxidant activity (6). This review focuses on the effects of ginseng on diabetes, anticancer activity, and the immune, and cardiovascular systems.

II. STRUCTURAL PROPERTY OF GINSENG Ginsenosides, known as saponins, are the major components of ginseng (Fig.

1). Ginsenosides have a steroidal skeleton with a modified side chain at C-20 (7,8). They differ from one another by the type of sugar moieties, their num- ber, and their site of attachment. Among the saponins, the genuine sapogenins 20(S)-protopanaxa-diol and -triol have been identified as 20(S) 12h-hydroxyl- and 20(S) 6a, 12h-dihydroxy-dammarenediol-II, respectively (9). Some part-

ly deglycosylated saponins such as ginsenoside Rh 1 , Rh 2 , and Rg 3 are obtained from red ginseng as artifacts produced during steaming. Stepwise deglycosylated compounds such as compound K and 20(S)-protopanaxadiol can be generated through metabolic transformation by human intestinal

bacteria. Ginsenoside Rg 1 is converted into 20(S)-protopanaxatriol via ginsenoside Rh1. The binding of the sugar has been shown to influence biological activity. Rh 1 and Rh 2 are structurally similar, but have different activity. Ginsenoside Rh 2 has been shown to decrease growth of B16-BL6 melanoma cells, and stimulates melanogenesis and cell-to-cell adhesiveness. However, Rh 1 has no effect on cell growth and cell-to-cell adhesiveness, but stimulates melanogenesis (10). Although both Rh 2 and Rh 3 induce differen- tiation of promyelocytic leukemia HL-60 cells into morphological and

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F IGURE 1 Structure of ginsenosides.

functional granulocytes, the potency of Rh 2 is higher (11). Another factor that contributes to structural difference between ginsenosides is stereochemistry at C-20. Although both 20(S)- and 20(R)-ginsenoside Rg 2 inhibit acetycholine- evoked secretion of catecholamines from cultured bovine adrenal chromaffin cells, the 20(S) isomer has a greater inhibitory effect (12). Structural changes after oral administration also contribute to diversity. Certain ginsenosides

such as Rb 1 and Rg 1 are poorly absorbed after injection (13). Rb 1 was

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hydrolyzed to compound K by intestinal flora (14); compound K was shown to increase the cytotoxicity of antineoplastic drugs (15) and to induce apoptosis in B16-BL6 melanoma cells (16).

The other major bioactive ingredient of ginseng is polysaccharide. Acidic polysaccharides found in ginseng such as ginsan (17,18), ginsenan S- IA, and ginsenan S-IIA (19) have been reported to have potential immuno- logical activities. Ginseng’s immunomodulatory effects may be one of its antitumor mechanisms. Ginseng polysaccharide GH1 reduces blood glucose and liver glycogen of mice (20). A high-output inducible nitric oxide synthase (iNOS) was shown in female BALB/c mice administered intraperitoneally (i.p.) with acidic polysaccharide from ginseng (21). Rhamnogalacturonan II from the leaves of P. ginseng C. A. Meyer acts as a macrophage Fc receptor expression-enhancing polysaccharide (22).

These structure-activity relationships are useful for understanding previously identified, bioactive components of ginseng, and to isolate and develop new therapeutic ingredients of ginseng.

III. REDUCTION OF BLOOD GLUCOSE LEVEL AND IMPROVEMENT OF DIABETES TREATMENT

Diabetes, affecting almost 3% of the world’s population, is one of the major global health problems. In particular, there is a high incidence among the elderly population. It has been shown that the root of P. ginseng and other ginseng species has antihyperglycemic activity in vitro (23,24) and in vivo (25– 28). More than 90% of patients with diabetes have type 2 diabetes, which is related to aging and diet. Although type 2 diabetes is more common and has serious complications, even reducing life expectancy by 8–10 years (29), most in vivo animal studies using ginseng have been conducted using type 1 rather than type 2 diabetes models. In this study, we focus on the effects of ginseng on type 2 diabetes.

The root of P. ginseng has been used to improve glucose homeostasis and insulin sensitivity (30) and clinically to treat type 2 diabetes (2,31). It has been observed that blood glucose level falls significantly in genetically obese diabetic mice after treatment with a single 90 mg/kg ginseng root extract at an i.p. dose (28). It has also been demonstrated that 3 g of American ginseng root given 40 min before a test meal significantly lowered blood glucose level in nondiabetic subjects and type 2 diabetic patients (32). Oral administration of P. ginseng root to diabetic KKAy mice for 4 weeks reduced blood glucose levels similar to that of an insulin sensitizer (rosiglitazone)-treated group (33). Moreover, ginseng therapy for type 2 diabetes elevates mood, improves psychophysical performance, and reduces fasting blood glucose and body weight. A 200-mg dose of ginseng improves glycated hemoglobin, serum lipid,

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amino-terminal propeptide concentration, and physical activity. These obser- vations suggest that ginseng is beneficial for patients with type 2 diabetes and to prevent development of diabetes in nondiabetic subjects.

The main component of P. ginseng is ginsenosides (ginseng saponins). Ginsenoside Rb 2 was found to be the most effective component of ginseno- sides for streptozotocin-diabetic rats (27). Rats treated with ginsenoside Rb 2 had a significant decrease in blood glucose levels with increased activity of glucokinase and decreased activity of glucose-6-phosphatase. Recently, anti- hyperglycemic and antiobese effects of P. ginseng berry extract have been demonstrated, and its major constituent, ginsenoside Re, has been observed (5). Treatment with the berry extract by daily i.p. injection for 12 days in obese diabetic C56BL/6J mice reduced glucose to levels similar to normal control value; after treatment the mice also had significantly improved glucose tolerance. The improvement of blood glucose level in the extract-treated ob/ ob mice is associated with a significant reduction in serum insulin level in fed and fasting mice. A hyperinsulinemic-euglycemic clamp study revealed more than a twofold increase in the rate of insulin-stimulated glucose disposal in treated ob/ob mice. In addition, the extract-treated ob/ob mice lost a signif- icant amount of weight, which was associated with a significant reduction in food intake and very significant increase in energy expenditure and body temperature.

Several studies have investigated the mechanism responsible for the antidiabetic effect of P. ginseng. Postulated mechanisms of lowering blood glucose by ginseng are as follows.

A. Modulation of Insulin Secretion Some ginseng fractions have been observed to increase the blood insulin level

and glucose-stimulated insulin secretion in alloxan-induced diabetic mice (24). This effect may also be mediated by increased NO synthesis of ginseng. Recently, it has been shown that NO stimulates glucose-dependent secretion of insulin in rat pancreatic islet cells (34).

B. Modulation of Digestion and Absorption of Carbohydrates An inhibition of neuronal discharge frequency from the gastric compartment

of the brain stem in rats has been observed (35), and inhibition of gastric secretion by Asian ginseng has also been observed in rats (36). The anti- diabetic effect of ginseng root has also been shown to block intestinal glucose absorption and inhibit hepatic glucose-6-phosphatase activity (33). These results suggest that ginseng triggers digestion of food and decreases the rate of carbohydrate absorption into the portal hepatic circulation.

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C. Control of Blood Glucose Level Two recent epidemiological cohort studies found that a reduction in dietary

glycemic index (GI, an indicator of carbohydrate’s ability to raise blood glucose level) between the highest and lowest quintiles decreases the risk of developing diabetes (37,38). The difference in GI between these quintiles for nondiabetic subjects and for subjects with type 2 diabetes has been observed after oral administration of ginseng before oral glucose administration (32). It has been suggested that ginseng is useful for healthy people to prevent diabetes, and for type 2 diabetes patients to improve glycemic control (39). More studies are required to confirm that ginseng administration decreases the GI. If this is the case, ginseng may be useful in the conventional treatment of diabetes. Before ginseng’s therapeutic benefit in these areas is claimed, studies of the efficacy of long-term administration using hemoglobin A1c (HbA1c) as a surrogate end-point marker and dose response are required.

D. Effect on Glucose Transport P. ginseng has been shown to increase both glucose transporter-2 protein in

the liver of normal and hyperglycemic mice (40) and glucose uptake into sheep erythrocytes in a dose-dependent manner (41). Recently, it has been shown that insulin-stimulated glucose uptake in rat skeletal muscles and adipose tissue is NO-dependent (42). Evidence suggests that ginseng can increase NO. Increased NO synthesis by ginseng in the endothelium of the lungs, heart, and kidney and in the corpus cavernosum has been detected (3).

E. Regulation of Adipogenic Transcription Factor Peroxisome Proliferator Activated Receptor- ;

Treatment with white ginseng rootlet in KKAy mice has been shown to increase peroxisome proliferator activated receptor-g (PPARg) (33), a pivotal regulator of adipocyte differentiation. Pharmaceutical ligands for PPARg, including the thiazolidinedione (TZD) class of drugs, are potent insulin sensitizers that impact whole-body glucose utilization (43,44).

IV. ANTICARCINOGENIC ACTIVITY The main weapons in the war against cancer have been early detection and

surgical removal, radiotherapy, chemotherapy, and attempts to develop gene therapy. However, the result have been less than ideal, and the dominant strategy is now changing from therapeutic approaches to prevention of cancer by identifying effective natural products as chemopreventive agents. One promising candidate with cancer-preventive effects is ginseng. Its usefulness

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as an anticarcinogen has been shown by extensive preclinical and epidemio- logical studies. The following details anticarcinogenic effects of ginseng based on its diverse mechanisms.

A. Effects on Tumor Cell Cytotoxicity and Differentiation Saponin and nonsaponin compounds have been reported to show cytotoxic

activities against various kinds of cancer cell lines in culture. The major active components are ginsenoside Rh 2 , a peculiar component of red ginseng, and also polyacetylenes, panaxydol, panaxynol, and panaxytriol. In addition, ace- tylpanaxydol and panaxydolchlorohydrin, showing cytotoxicity against lymph- oid leukemia L1210, have been isolated from Korean ginseng root (45). The critical early even in the cytotoxicity of panaxytriol was identified as ATP depletion resulting from a direct inhibition of mitochondrial respiration (46). Further experimental evidence suggested that polyacetylenes inhibited the synthesis of macromolecules such as DNA, RNA, and protein in L1210 cells, resulting in cytotoxicity (17).

Ginseng was found to have the ability to induce the transformation of neoplastic cells into normal cells. Ginsenoside Rh 2 inhibited the growth and colony-forming ability of Morris hepatoma cells in soft agar suspension culture, and stimulated serum protein synthesis of these cells, thus converting the cell characteristics both functionally and morphologically to those resembling original normal liver cells, a process known as ‘‘redifferentiation or reverse transformation’’ (47). Similarly, ginsenosides Rh1 and Rh2 have been shown to cause differentiation of F9 teratocarcinoma stem cells via binding to a steroid receptor (48). Accordingly, recent studies on therapy for various types of cancers have focused on drugs that induce differentiation of maturation-resistant cells causing the related disease. Furthermore, ginseno-

side Rh 2 has been found to significantly induce B16 cell differentiation and to increase melanin synthesis in B16 cells (49).

B. Antimetastatic Effect and Inhibitory Activity on Multidrug Resistance (MDR)

Generally, primary tumors are not fatal. Instead, most cancer patients succumb to metastases—multiple, widespread tumor colonies established by malignant cells that detach themselves from the original tumor and travel through the body, often to distant sites. Some invading cells penetrate body cavities, the blood, lymph, or spinal fluids and are then released (50). Ginseng saponin has recently received a great deal of attention for its effects in inhibiting invasion and metastasis of cancer cells. 20(R)- and 20(S)-ginseno-

side Rg 3 was found to inhibit the lung metastasis of tumor cells such as B16-

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BL-6 melanoma and colon 26M3.1, when they were orally administered at a dose of 100–1000 Ag per mouse. The mechanism of their antimetastatic effect is considered to be related to inhibition of the adhesion and invasion of tumor cells, and also to antiangiogenesis activity (51). Glucocorticoid receptor- induced downregulation of matrix metalloproteinase (MMP)-9 by panaxa- diol and panaxatriol appear to be associated with the reduced invasive capacity of HT1080, a highly metastatic human fibrosarcoma cell line (52).

Multiple administration of ginsenoside Rb 2 after the intravenous inoculation of B16-BL6 melanoma cells resulted in significant inhibition of tumor- associated angiogenesis responsible for the inhibition of lung tumor metas- tasis (53). Ginsenoside Rb2 has been reported to inhibit invasion to the basement membrane via MMP-2 suppression in some endometrial cancers such as HHUA and HEC-1-A cells, and is projected for use as a medicine for inhibition of secondary spreading of uterine endometrial cancers (54). It was

shown that ginsenoside Rg 3 inhibited experimental pulmonary metastasis by highly metastatic mouse melanoma B16FE7 cells, which was mediated by inhibiting the 1-oleoyl-lysophosphatidic acid (LPA)-triggered rise of intra- cellular Ca 2+ (55). While these results suggest the usefulness of ginsenoside

Rg 3 in preventing cancer spread, further in vivo studies are necessary to clarify te antimetastatic effects of ginseng. One of the major side effects to the effective treatment of human malignancies is the acquisition of broad anticancer drug resistance by tumor cells. This phenomenon has been termed ‘‘multidrug resistance (MDR).’’ Therefore, development of new modulators for the inhibition of drug re- sistance is required. MDR inhibitory activity was determined by measuring cytotoxicity to MDR cells using MDR human fibrocarcinoma KB V20C, which is resistant to 20 nM of vincristine and expresses a high level of mdr1

gene. 20(S)-Ginsenoside Rg 3 , saponin of red ginseng, was found to have the most potent inhibitory activity on MDR, and its ID 50 (dose for 50%

3 did not affect the growth of normal cells. Moreover, ginsenoside Rg 3 (40 mg/kg, i.p.) potentiated adriamycin chemotherapy in P388/ADR BDF1 mice expressing MDR. Ginsenoside Rg 3 appears to compete with vincristine for binding to p-glycoprotein (Pgp) and blocks the efflux of anticancer drugs (56). It is also reported that quasipanaxatriol, 20(S)-protopanaxatriol, ginsenoside

5 M. In cytotoxicity assays, ginsenoside Rg

Rh 2 , and compound K, the metabolic final product of protopanaxadiol saponin, greatly enhanced the cytotoxicity of the anticancer drugs in P388/ ADM cells (adriamycin-resistant P388 leukemia cells) (15). The reversal of daunomycin resistance in P388/ADM by quasipanaxatriol with a double bond introduced at C-20 of protopanxatriol was found to show effective ac- cumulation of drugs mediated by daunomycin-efflux blockage. These results

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suggest that further clinical trials of ginseng components in reversal of Pgp- associated MDR are highly feasible.

C. Anticarcinogenic Activities and Synergistic Effects in Combination with Chemical Therapeutic Agents

Several investigations were carried out to evaluate the inhibitory or preventive effects of ginseng on carcinogenesis induced by various chemical carcinogens. The prolonged administration of Korean red ginseng extract inhibited the incidence and the proliferation of tumors induced by 7,12-dimethylbenz (a) anthracene (DMBA), urethane, and aflatoxin B1 (57). The chemopreventive potential of ginseng was evaluated using DMBA-induced skin tumorigenesis (papillomagenesis) in male Swiss albino mice. There was a marked reduction not only in tumor incidence but also in cumulative tumor frequency at the initiation phase of tumorigenesis, with a small reduction at the promotional stage, suggesting the anticarcinogenic activities of ginseng (58). Ginsenosides

Rg 3 and Rg 5 were found to show statistically significant reduction of lung tumor incidence in a newly established 9-week, medium-term anticarcinoge- nicity test model of lung tumors in mice, and ginsenoside Rh 2 showed a tendency to decrease the incidence, indicating that these ginsenosides are active anticarcinogenic compounds (59). The inhibitory effects of ginseng on the development of 1,2-dimethylhydrazine (DMH)-induced aberrant crypt foci (ACF) in the colon were investigated in rats. Dietary administration of red ginseng in combination with DMH suppressed colon carcinogenesis in rats associated partly with inhibition of cell proliferation, acting on ACF in the colonic mucosa (60). In addition, an anticarcinogenic effect of red ginseng on the development of liver cancer induced by diethylnitrosamine (DEN) in rats was identified in preventive and curative groups (61).

More recently, it has been reported that less glycosylated protopanax- adiol derivatives are effective in cancer prevention and some oleanane-type pentacyclic triterpenoid compounds show anticarcinogenic activities in two- stage anticancer promotion experiments in vitro and in vivo (9). Ginsenoside

Rh 2 has been described to have diverse effects on the expression of the trans- formed phenotype in BALB/c 3T3 cells, and augments the metastatic po- tential in an experimental metastasis assay (62). The estrogenic potential of American ginseng extract to induce the expression of pS2, an estrogen- regulated gene, was evaluated in breast cancer cell lines MCF-7, T-47D, and BT-20. It was found that American ginseng exhibits estrogen-like effects on estrogen-receptor-positive breast cancer cells by inducing pS2 expression, suggesting that it may play a protective role against breast cancer (63). This is further supported by the observation that American ginseng inhibits breast

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cancer cell growth by transcriptional activation of the p21 gene, a universal cell cycle inhibitor, independent of p53 (64). It is also reported that concurrent use of American ginseng extract and breast cancer therapeutic agents results in a significant suppression of cancer cell growth, suggesting its synergistic effects on breast cancer therapeutics (65).

D. Antitumor Activities of Ginsenosides Oral administration of ginsenoside Rh 2 has been reported to have an

inhibitory effect on tumor growth in nude mice bearing human ovarian cancer cells (HRA), resulting in remarkable retardation of the tumor growth. In particular, tumor growth in mice treated with 15, 30, and 120 AM of Rh 2 was significantly inhibited, compared to that in CDDP (cis-diaminedichloro- platinum) (II)-treated mice as well as in untreated mice (66). Further investigation showed that p.o. but not i.p. treatment with ginsenoside Rh 2 resulted in induction of apoptosis in the tumors in addition to augmentation of the natural killer activity in spleen cells from tumor-bearing nude mice, suggesting that an evaluation of the treatment of recurrent or refractory ovarian tumors is warranted (67).

E. Immunomodulatory Antitumor Activities of Ginseng Polysaccharide

It has long been reported that ginseng strengthens the resistance of an organism to harmful physical, chemical, and biological stresses. This capa- bility is related to its regulation of the immune system, which plays an important role in the protective mechanism of the body. Recently, an acidic polysaccharide named ginsan has been isolated from white ginseng, peeled dried ginseng, and identified as an ideal nontoxic antineoplastic immunostim- ulator. It acts by activating multiple effector arms of the immune system and killer cells (Thyl + , AsGM1 + , CD8 + ) (68). Ginsan has been found to generate LAK cells from both NK and T cells by endogenously producing multiple cytokines, contributing to its effectiveness in the immunoprevention and immunotherapy of cancer (17). More recently, red ginseng acidic polysac- charide (RGAP) has also been isolated from Korean red ginseng. Having different chemical molar composition from ginsan, it is reported to have immunomodulatory antitumor activity. RGAP was found to induce induc- ible nitric oxide synthase (iNOS) in mice resulting in NO secretion (21). Peritoneal macrophages from RGAP-treated mice exhibited potent tumor- icidal activities toward P815 and WEHI 164 tumor cells. In addition, treat- ment of RGAP in vivo stimulated tumoricidal activities of natural killer (NK) cells and showed increased life span of sarcoma 180–bearing mice together with decreased tumor weight of B16 tumor–bearing mice. These results

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suggest that activation of macrophages and NK cells serve to enhance in vivo antitumor activities of RGAP (69). GFP, a more active fraction of RGAP, increased the survival rates of male ICR mice transplanted with sarcoma 180

10 times higher than RGAP, and showed more potent tumoricidal activities of NK cells than RGAP. Recently, it has also been demonstrated that polysac- charide fraction of American ginseng, P. quinquefolius, exerts TNF-a-stim- ulating activity on macrophages, and is found to contain glucose, galactose, arabinose, rhamnose, and mannose (70). These results suggest that clinical trials of these ginseng polysaccharides in immunotherapy against cancer are highly feasible.

F. Induction of Apoptosis by Ginsenosides Apoptosis is responsible for a pathological mechanism related to human

diseases such as cancer, autoimmune disease, viral infection, and neurode- generative disorder (71). Ginsenoside Rh 2 has been shown to arrest cell cycle at the G1 phase and to prolong the S phase (72,73). It was reported that ginsenoside Rh 2 induced apoptosis through protein kinase C in human neu- roblastoma SK-N-BE (2) and rat glioma C6Bu-1 cells (17). In addition, ginsenoside Rh 2 has been shown to induce apoptosis independently of Bcl-2, Bcl- X L , or Bax in C6Bu-1 cells (74). In a parallel study, it was found that ginsenoside Rh 2 –induced cell death was mediated by the generated reactive oxygen species and activation of the caspase pathway in a Bcl- X L -independent manner (75). These reports demonstrate that the induction of apoptosis by ginseng may be one of its anticarcinogenic mechanisms.

As described above, the protective influence of ginseng against cancer has been shown by extensive preclinical and epidemiological studies; how- ever, these effects need to be carefully investigated by scientific clinical trials focusing on the major cancer killers such as stomach, lung, liver, and colorectal cancers.

V. ANTIHYPERTENSIVE EFFECTS High blood pressure is associated with decreased life expectancy and in-

creased risk of stroke, coronary heart disease, and other end-organ diseases such as renal failure. Ginseng contains active compounds that normalize blood pressure. The effect of a certain drug on blood pressure can be analyzed by investigating the effect of the drug on the smooth muscle of blood vessels. It is well established that blood vessel smooth muscle tone is regulated by the available intracellular Ca 2+ concentration, which in turn is profoundly influenced by interaction of the cellular membrane and sarcoplasmic reticu- lum in the smooth muscle. It was found that both protopanxatriol and

528 Chung et al. protopanaxadiol saponins inhibit Ca 2+ binding to the cellular membrane;

protopanaxatriol is approximately 180% more potent than protopanaxadiol ginsenosides (76). It was reported that ginseng induced no significant change in blood pressure in subjects with normal blood pressure, but had a normal- izing effect on subjects with abnormal blood pressure (77). It has recently been reported that vasodilation and protective effects of ginsenoside Rg1 against free radical injury might be related to enhanced synthesis and release of NO, demonstrating the usefulness of ginseng in treatment of pulmonary and systemic hypertension (78). The detailed mechanism and evidence of antihy- pertensive effects of ginseng are as follows.

A. Endothelium-Dependent Vasorelaxation of Ginsenosides Endothelium plays an important role in regulating vascular tone by releasing

several vasoactive autacoids, including prostacyclin, endothelium-derived re- laxing factor (EDRF), and endothelium-derived hyperpolarizing factor (EDHF) (79). EDRF has been identified as NO, which is produced from L -arginine by NOS (80). NO relaxes blood vessels mostly by stimulating soluble guanylyl cyclase, which leads to an increased production of cGMP in vascular smooth muscle (81). Ginsenosides have been found to lower blood pressure in a dose- dependent manner in rats at doses of 10–100 mg/kg. This effect is mediated by release of endothelium-derived NO, enhancing the accumulation of cGMP (82). Recently, it has also been reported that ginseng can improve vascular endothelial dysfunction in patients with hypertension, possibly by increasing

synthesis of NO. Protopanaxatriol and its purified ginsenosides Rg 1 and Re caused endothelium-dependent relaxation, which is associated with the formation of cGMP. In contrast, protopanaxadiol or ginsenosides Rg 1 and Re did not affect vascular tone or production of cGMP in rat aorta. Moreover, ginsenosides Rg 1 and Re were less effective endothelium-depen- dent vasodilators than total ginsenosides and protopanaxatriol (83). Ginse- noside Rg 3 belongs to the protopanaxadiol group of red ginseng, steamed and dried ginseng. Present findings indicate that ginsenoside Rg 3 effectively stimulates NO formation in endothelial cells, which accounts for the endo- thelim-dependent relaxation and production of cGMP in the rat aorta.

Ginsenoside Rg 3 –induced endothelium-dependent relaxation was markedly inhibited by a nonselective K + channel blocker, but not by an ATP-sensitive K + channel blocker (84). These findings show that ginsenoside Rg 3 activates tetraethylammonium-sensitive K + channels in endothelial cells, which pre- sumably leads to an influx of Ca 2+ and the subsequent activation of the en- dothelial NOS (eNOS) (85).

To investigate the effects of red ginseng on blood pressure, change of blood pressure and heart rate after intravenous injection of red ginseng was

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studied in conscious normotensive and one-kidney, one-clip Goldblatt hy- pertensive rats. Experimental evidence indicates that the NO-releasing effect of red ginseng, like other NO donors, partly contributes to hypotensive effects (86). Clinical study has been performed to estimate the effect of Korean red ginseng on vascular endothelial cell dysfunction in patients with hyperten- sion. To assess the function of the vascular endothelial cell, changes of forearm blood flow to infusion of acetylcholine, sodium nitroprusside, and bradykinin in incremental doses were measured by venous occlusion plethys- mography. In the ginseng-treated hypertensive group, forearm blood flows at the highest dose of acetylcholine and bradykinin were significantly higher than those of the nontreated hypertensive group. It was found that Korean red ginseng could improve the vascular endothelial dysfunction in patients with hypertension possibly by increasing NO synthesis (87). These results support the claim that ginseng has pharmacological activities on circulatory diseases including hypertension.

B. Antiatherosclerosis and Antiplatelet Effects of Ginsenosides

Endothelial cell damage is considered to be the initial step in the genesis of thrombosis and arteriosclerosis, the common precursors of cardiovascular disoders. Platelet hyperfunction, such as enhanced platelet aggregation

associated with overproduction of thromboxane A 2 (TXA 2 ), a potent platelet aggregative and vasoconstrictive substance, has been frequently encountered in patients with cardiovascular thrombotic diseases. Prostaglandin I 2 (PGI 2 ),

a potent antiplatelet aggregative and vasodilatatory substance produced in vascular walls, is also reported to synthesize less in patients with atheroscle- rotic changes than in normal subjects. Panaxynol was found to markedly inhibit the aggregation of platelets induced by collagen, arachidonic acid, and platelet-activating factor (PAF), while ginsenosides had no significant effect on the aggregation. It was suggested that panaxynol is the most potent antiplatelet agent in ginseng and its mechanism of action is chiefly due to the inhibition of thromboxane formation (88). In addition, it was reported that panaxynol inhibited the aggregation, release, and thromboxane forma-

tion in rabbit platelets, while ginsenosides Ro, Rg 1 , and Rg 2 suppressed only the release (89). It has also been demonstrated that ginsenoside Rg 1 inhibited platelet activation induced by TXA 2 through inhibition of TXA 2 -induced

Ca 2+ mobilization, and ginsenoside Rg 3 induced TXA 2 -induced platelet aggregation. Ginsenoside Rc was shown to stimulate in vitro PGI 2 formation by cultured rat vascular smooth muscle cells through enhanced gene expres- sion of cyclooxygenase (90). These results suggest that ginsenoside Rg 1 and Rg 3 , which have antiplatelet and antiatherosclerotic effects, may have clinical

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potential for the prevention and the treatment of certain thrombotic and atherosclerotic disorders. Furthermore, American ginseng extract was proved to be associated with the inhibition of thrombin-induced endothelin release due to NO release (91). This result suggests that American ginseng may play a therapeutic role in facilitating the hemodynamic balance of vascular endo- thelial cells.

VI. SUMMARY AND FUTURE WORK For centuries, ginseng has been a highly valued herb in the East. Recently,

many clinical trials using ginseng have been undertaken in Western countries as well as in East Asia. In this chapter, we have summarized the diverse pharmacological and physiological effects of ginseng on various diseases such as diabetes, cancer, and cardiovascular diseases. However, although there is a wealth of evidence suggesting that ginseng can be useful for the treatment of various diseases, a great deal of vital research remains to be solved. First, numerous experiments reported have been performed in animal models instead of humans. Therefore, a large-scale, controlled clinical study is needed to validate these results in terms of their applicability to humans. Second, there is still little evidence on ginseng’s effectiveness at molecular levels. Understanding of molecular regulation of ginseng is necessary to apply it clinically, and also to discover new therapeutic effects. Third, the standard- ization of ginseng extract to yield constant results in the treatment of ginseng to cells, animals, and human is also urgently needed.

ACKNOWLEDGMENT This work was supported by the Aging and Apoptosis Research Center Pro-

gram Grant (R11-079) from the Korea Science and Engineering Foundation.

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