Current knowledge and future direction of research on soy isoflavones as a therapeutic agents

Abstract

Isoflavones, the most abundant phytoestrogens in Soy beans, are structurally similar to 17beta-estradiol. The antioxidant property of the soy isoflavones, namely, genistein and daidzein is well established in different experimental models and also in clinical studies. The compounds have been found effective in the management of diabetes by acting on peroxisome proliferator-activated receptors. It reduces the risk of coronary heart disease by reducing the level of low-density lipoprotein and triglycerides. Soy isoflavones have the potential in the treatment of osteoporosis to act on osteoclasts further to inhibit tyrosine kinase. Among the soy isoflavones, genistein is the potential compound found effective in the treatment of cancer by acting on androgen receptor further to inhibit tyrosine kinases. In this article, various aspects of the diverse biological activities of soy isoflavones and their potential clinical implications with mechanism of action, especially in the treatment and prevention of diabetes, cardiovascular diseases, cancer, osteoporosis, neuroprotection, and also future area of research on soy isoflavones are reviewed and discussed.

INTRODUCTION

Legumes play an important role in the traditional diets of many regions of the world. Glycine max (Leguminosae), commonly known as soy bean or soya bean (SB) is unique among the legumes. It is an annual plant that may vary in growth, habit, and height. It may grow prostrate, not growing to higher than 20 cm (7.8 inches), or even stiffly erect up to 2 m (6.5 feet) in height. The mature seed is rich in proteins and is a concentrated source of isoflavones.[1–3] For centuries, SBs and soy protein have been consumed as a staple food and are an important source of high-quality proteins.[4,5] The rich history of soy began 5000 years ago on the windy plains of Eastern Asia. According to the Chinese tradition, SB was one of the five sacred crops named by the Chinese emperor Sheng-Nung, who reigned 5000 years ago.[6] Historians maintain that Sheng-Nung mentioned SB in his “Ben Tsao Gang Mu,” written in the year 2838 B.C. Soy was once cultivated by Chinese farmers, but it spread gradually across China, Korea, Japan, India, and European countries, and finally to America.[7]

As SBs have many important nutritious components, such as proteins, fats, carbohydrates, and α-tocopherol, long-term continuous intake of this legume might effectively prevent senility.[8] Over the last few years, research has focused on SB constituents, such as isoflavones, phytosterols, saponins, water- and fat-soluble vitamins, and minerals.[3] The isoflavones, namely, genistin and daidzin, are metabolized to genistein and daidzein.[9] They are considered to be essential for the healthy functioning of bowels, heart, kidney, liver, and stomach.[10,11] Soy proteins have many beneficial effects in humans: reduction of serum cholesterol levels and reduction in the risk for coronary heart disease (CHD), reduction in the risk for breast cancer, reduction in the risk for osteoporosis in women, and alleviation of the disturbances caused by menopause.[12] Thisth review article highlights the health benefits and lacunae of soy isoflavones as major nutraceutical compounds.

COMPOSITION

SB contains about 39.6% good-quality proteins, 20% cholesterol-free oil, 33% carbohydrates, and reasonable amounts of minerals, including calcium and iron, as compared with conventional source of proteins. SBs are largely replacing milk and other sources of animal proteins, which are expensive and not readily available, as suitable substitutes for high-quality proteins.[13] Soy does not have lactose. Hence, soy milk is suited for lactose-intolerant people.[14] SBs contain a variety of bioactive components, including saponins, protease inhibitors, phytic acid, all essential amino acids, and isoflavones [Figure 1]. Isoflavones belong to a class of compounds generally known as phytoestrogens, plant compounds that have estrogen-like structures.[15,16]

Figure 1.

Different chemical constituents in soya bean

SOY ISOFLAVONES

Isoflavones are found in varying amounts in legumes, but their significant source is SBs.[17] Isoflavones are phytoestrogens and have a chemical structure similar to that of estrogen [Figure 2]. The two primary isoflavones in SB are genistein and daidzein, along with other glycosides. A method for the quantification of daidzein, genistein, glycitein, and other glucosides and acetylglucosides in soy extracts has been developed and validated.[18] Isoflavones are metabolized by gut microflora; their sugar portion is removed from the glycosides, forming the active compounds genistein and daidzein, which are then absorbed in the intestine. Daidzein is more bioavailable than genistein, because it has a longer half-life in the intestine. The genistein is degraded about twice as fast and is therefore less absorbed. However, the isoflavones are also degraded by microflora to other metabolites, including the isoflavonoid equol.[15] Together with isoflavones, there are two other main classes of phytoestrogens: lignans and coumestans.[19] Isoflavones are associated with the protein fraction; hence, they are present only in the whole SB and other high-protein secondary products.[20,21] Soy isoflavones are being studied intensively to clarify the physiologic effects they exert. In some cases, the isoflavones may be one of the key factors in SB that have a disease-fighting potential.

Figure 2.

Chemical structure of soy isoflavones, such as daidzein, genistein, and glycitein, Daidzein is 4’, 7- dihydroxyisoflayone, genistein is 4’5, 7-trihydroxy isoflavones and glycitein is 7,4’ - dihydroxy-6-methoxyisoflayone.

ANTIOXIDANT ACTIVITY OF SOY ISOFLAVONES

A considerable amount of research is being done to investigate the potential role of antioxidants in reducing cell damage that can contribute to the development of cancer. Genistein has been shown to have antioxidant properties. It was found that genistein has a potential antioxidant effect on hydrogen peroxide production by 12-O-tetrade-canoylphorbol-13-acetate- activated HL-60 cells. Feeding genistein to mice for 30 days also significantly enhanced the activity of antioxidant enzymes in the skin and small intestine.[22,23] In vivo soy isoflavones (150 and 250 ppm) showed antioxidant property by elevating the antioxidant enzyme activity, such as catalase and superoxide dismutase, in various organs of rat where as tofu containing approximately 50 ppm isoflavones had better effects than the soy isoflavones. This indicated that molecules other than isoflavones may have a synergistic effect on in vivo antioxidant enzyme inductions of tofu.[24] Another study reveals that daidzein enhances catalase promoter activity at 100 μmol/L in a reporter gene assay and at 200 μmol/L in Northern blot experiments but shows only little antioxidant capacity.[25]

In another study, stroke patients were given a soy creme product (10.6% protein) that reduced lipid peroxidation in molecules of low-density lipoprotein (LDL) cholesterol, very-low-density lipoprotein cholesterol, and high-density lipoproteins (HDL).[26] Another study found that genistein inhibited the oxidation of LDL cholesterol in the presence of copper ions or superoxide/nitric acid radicals in vitro. In addition, this research suggested that genistein effectively protected human endothelial cells from damage by oxidized lipoproteins.[27] Another study was conducted on six healthy volunteers who consumed three soy bars per day for 2 weeks; each bar contained 12 mg of genistein and 7 mg of daidzein. Serum LDL cholesterol, which was isolated at baseline, after 2 weeks of soy bar consumption, and after soy was discontinued, was subjected to copper-mediated oxidation in vitro. Compared with off-soy values, during soy intake, the lag phases of LDL cholesterol oxidation curves were prolonged by a mean of 20 min (P < 0.02), which indicated a reduced susceptibility to oxidation.[28] Studies also found that the oxidative metabolites of genistein and daidzein showed higher antioxidant activities than the positive controls of quercetin and ascorbic acid in different in vitro assays.[29–32]

EFFECT IN THE MANAGEMENT OF DIABETES

Consuming foods with a low glycemic index (GI) has been shown to improve blood sugar control and blood lipid values in patients with non-insulin-dependent diabetes. Legumes, especially SBs, have a very low GI and are valuable foods to be included in a diabetic diet.[33–36] Studies also found that isoflavones in SBs significantly reduced the risk of type II diabetes by decreasing GI.[37] This has been attributed to many factors, including their fiber, tannin, and phytic acid contents.[38–40] In support of this statement, there are findings from a prospective study showing that women who consumed diets with a high GI were 40% more likely to develop diabetes than those consuming low-GI diets, even after controlling for several diabetes risk factors.[41] Thus, beans may be a particularly important food for individuals with diabetes and those with an elevated risk of developing diabetes.

Blood sugar control may also be improved by choosing carbohydrates that are high in soluble fiber. Some researchers believe that fiber has no measurable benefit unless it is added to the diet in very large amounts.[42] Soy fiber is extremely fermentable in humans, and therefore, may have more physiologic benefits than some other types.[43] A study using 25 g of supplemental soy fiber per day found that it significantly reduced insulin response to an oral glucose challenge—and reduced total plasma cholesterol in hypercholesterolemic patients.[44] Soy isoflavones improve lipid and glucose metabolism by acting as antidiabetic peroxisome proliferator-activated receptors, which are promiscuous nuclear receptors that regulate the transcription of genes involved in lipid and glucose homeostasis and lipid metabolism within the cell.[45] Soy isoflavones increase insulin secretion without any change in the glucose disposal as well as decrease the plasma adiponectin concentrations.[46] Studies also found that genistein increases glucose-stimulated insulin secretion in cell lines and mouse pancreatic islets at micromolar concentrations via a cAMP-dependent protein kinase mechanism.[47]

EFFECT ON CARDIOVASCULAR DISEASE

Diabetes raises the risk for heart disease and atherosclerosis. Cardiovascular disease is 2–4 times as common in diabetic patients as in the general population. CHD is the major cause of death in most developed countries and is rapidly increasing in prevalence in the developing countries. More than one in four Americans have some form of cardiovascular disease.[48] In the United States, death rates from cardiovascular disease exceed 1 million annually and the total cost is estimated to exceed $120 billion, the largest disease-related cost to health. Although many risk factors, such as cigarette smoking and hypertension, contribute to the risk for CHD, lipid abnormalities are the major factors. LDL plays a central role in the atherosclerotic process. LDL penetrates the walls of blood vessels where it is oxidized by free radicals and accumulates as a gruel-like material that blocks the blood vessel to cause thrombosis. HDL cholesterol has a protective effect and acts to prevent LDL oxidation and remove the cholesterol that accumulates in the blood vessel wall.[49] Soy protein exerts several antiatherogenic effects. First, it decreases LDL cholesterol levels significantly. Second, it tends to increase HDL cholesterol levels. This is rather unique since most dietary interventions, such as oat bran intake or decreased saturated fat intake, significantly decrease HDL cholesterol levels.[50] Third, soy isoflavones, plant chemicals unique to SB, have antioxidant properties that prevent LDL from oxidation.[32]

There is a great deal of evidence that soy protein helps lower blood cholesterol levels.[51–54] Replacing animal protein with soy protein in the diet lowers the total and LDL cholesterol levels in people with high cholesterol levels. A meta-analysis of 38 research studies concluded that soy protein lowers total and LDL cholesterol and triglycerides, without lowering HDL cholesterol in hypercholesterolemic humans. In these studies, the average consumption of soy protein was 47 g per day. The greatest decreases in cholesterol were seen in those with the highest starting levels. Even adding soy protein to an omnivorous diet has been shown to produce this effect, which also emphasized that 25 g of soy protein per day may be enough to lower cholesterol levels.[55–57] The research study with rhesus monkeys found that isoflavones associated with soy protein enhance the cholesterol-lowering effects.[58] Another study suggests that soy protein inhibits LDL oxidation.[26]

A meta-analysis of 27 studies concluded that replacing dietary saturated fat with unsaturated fat lowers total cholesterol levels and also improves the ratio of HDL to LDL cholesterol.[59] Various protein fractions and isoflavone components were actively studied for antiatherogenic effects and the demonstration expressed that estrogen receptor-α is required for atheroprotection by soy protein; this provides an important new mechanistic insight.[60] Soy protein isoflavones stimulate the transcription factor sterol regulatory element-binding protein. The reduction of this factor decreases the expression of several lipogenic enzymes, thus decreasing the serum and hepatic triglycerides, as well as LDL cholesterol.[61]

Cardioprotective effects of soy isoflavones may be due in part to a variety of actions, including a favorable effect on the blood lipid profile and inhibition of LDL cholesterol oxidation. The precise mechanism by which soy isoflavones the blood lipid profile is unknown. One possible mechanism is altered hepatic metabolism, with enhanced removal of LDL cholesterol and very-low-density-lipoprotein cholesterol by hepatocytes.

EFFECT ON CANCER

Cancer is the second leading cause of death in the United States. Scientific evidence indicates that one third of cancer deaths in the United States are due to dietary factors.[62] SB compounds, namely, isoflavones, saponins, phytates, protease inhibitors, and phytosterols, are identified as anticancer drugs.[63] Isoflavones are weak estrogens. They can function as antiestrogens by binding with estrogen receptors in place of the potent physiologic estrogens, thus blocking them from exerting their effects.[64] Theoretically, this would reduce the risk for cancers, which are stimulated by estrogen, such as certain breast cancers. Genistein arrests cell cycle progression at the G2–M transition, includes apoptosis, has antioxidant properties, modifies eicosanoid metabolism, and inhibits angiogenesis.[65]

Pubertal exposure to genistein that is administered via either injections or fed has consistently been found to reduce the incidence and/or multiplicity of subsequent carcinogen-induced, estrogen-dependent mammary tumors.[66] Genistein inhibits the proliferation of MDA-MB-231 human breast cancer cells in cell culture and the probable mechanism is inhibition of the cell cycle at G2–M.[67] In vitro studies have demonstrated that genistein and daidzein were shown to inhibit the proliferation of a number of murine and human neuroblastoma cell lines.[68–70] Soy isoflavones has been shown to act on androgen receptor further to inhibit tyrosine kinases, and thereby block the growth and proliferation of cancer cells[71] [Figure 3]. Genistein selectively inhibits cancer cells, and leaves normal cells alone, and also inhibits angiogenesis; it is also a potent antioxidant that blocks the formation of oxygen free radicals, which are involved in cancer promotion.[72,73]

Figure 3.

Inhibition of growth and proliferation of cancer cells by soy isoflavones

EFFECT ON OSTEOPOROSIS

Osteoporosis is characterized by reduced bone mass and structural deterioration of bone tissue; it occurs in women and is primarily related to aging and hormone deficiency.[74] Drugs that act on resorptive pathway (responsible for bone loss) and to build new and improved skeletons are specifically recommended to treat bone loss.[75] Experimental studies on genistein showed a positive effect on osteoporotic bone by decreasing osteoclastic resorption factor, such as collagen C-telopeptide, and increasing osteoblastic formation markers, such as bone-alkaline phosphatase.[76] Genistein has also shown to selectively antagonize the bone catabolic effects of parathyroid hormone in osteoblasts by reducing parathyroid hormone-induced increases in soluble receptor activator of nuclear factor-xB ligand and reversing decreases in osteoproteger in expression in vitro.[77,78]

The mechanism of action of isoflavones seems to be to act independently on osteoclasts via nonestrogenic mechanisms because there are no estrogen receptors in the nuclei of osteoclasts. Isoflavones act on osteoclasts by inhibition of tyrosine kinase.[79] Although the mechanisms of isoflavones on osteoporosis are still not completely known, evidence from in vitro studies suggests that they act in multiple ways, via genomic and nongenomic pathways and via both osteoclasts and osteoblasts. Soy isoflavones stimulate the activity and proliferation of bone-building cells, namely, osteoblasts, to maintain bone mass against the action of osteoclast cells, which release acid and enzyme to dissolve bone. The osteoblast cells produce a collagen core and coat it with an adhesive substance, which is present in the bone and finally calcium adheres to the collagen to form a new bone tissue[80] [Figure 4].

Figure 4.

Mechanism of action of soy isoflavones in the treatment of osteoporosis, (a) Osteoclast cells carried via bloodstream to bone, (b) Cells fimly attach themselves to the bone, (c) Cells releases acid and enzymes that dissolve the bone, (d) Osteoclast cells disappears once their function is completed, (e) Soy isoflavones stimulates the activity and proliferation of osteoblasts (bond-building), (f) The cells produce a collagen core and coat it with an adhesive substance, (g) Calcium adheres to the collagen, forming new bone tissue

ADVERSE EFFECT OF SOY ISOFLAVONES

Soy isoflavones in moderation are probably not dangerous, as few studies have indicated adverse effects. However, large doses have been shown to increase apoptosis and cell degeneration, and in some cancer regimes, once the cancer has progressed beyond the hormone-dependent stage, high doses of isoflavones may be contraindicated.[81]

SUMMARY AND CONCLUSION

Most of the studies evaluated the potential of soy isoflavones on various biomarkers or diseases, although, several of the endpoints, such as LDL and bone mineral density, do have known meaningful correlations with clinical outcomes. Cardiovascular surrogate endpoints were assessed by the largest number of studies. Overall, soy isoflavone was found to have some effect on lipids. However, the duration of these studies were generally short, and it is uncertain whether the results would be sustained. Furthermore, it is not clear whether the renal protective effects of soy protein are caused by the isoflavones (daidzein and genistein) and lignans or some other component that need to be discussed in future. Till date, even though some epidemiologic data and clinical studies suggest potential protective effects on specific target organs, further controlled, randomized studies are necessary to determine the real benefits of soy isoflavone for the treatment and management of chronic diseases at different dose levels.

Nevertheless, more studies are needed to better understand the exact mechanisms, in vivo target sites, stability, and safety, prior to using as a potential therapeutic agent. This review article suggests to healthy individuals as well as patients to include caution in using soy isoflavone supplements until clinical trials are completed. Clinical study with soy isoflavones in patients with chronic diseases should be a high priority.