Introduction

Phytoestrogens are polyphenolic compounds mainly present in legumes. The term phytoestrogen was derived from Greek word phyto means plant and estrogen means a hormone regulates female reproductive functions such as fertility in vertebrates. Phytoestrogens are available in the form of flavonoid and nonflavonoid groups, flavonoids include isoflavones, coumestans, etc. and nonflavonoid includes lignans as shown in Figure 1. Especially soybean is the richest source of isoflavones such as daidzein, genistein and glycitein and also found in flowers, fruits, vegetables, cereals and beverages (Viggiani, Polimeno, Leo, & Barone, 2019). Phytoestrogens are able to produce potential health benefits on different systems of human body by exerting agonist and antagonistic effects on estrogen receptors (ERs) such as ERα (Estrogen Receptor α) and ERβ (Estrogen Receptor β), because of their structural similarity with endogenous female reproductive hormone 17β-estradiol (Kitajewska, Szela, Maslyak, Sklyaroy, & I, 2014; Ko, 2014).

Cardiovascular disease (CVD) is the major cause of mortality in developing countries. Atherosclerosis is the major cause of coronary heart disease (CHD), and it is a common form of cardiovascular disease, and is initiated by inflammatory response of the vascular endothelial cells following injury (Krizova, Dadakova, Kasparovska, & Kasparovsky, 2019; Pabich & Materska, 2019). Similar to endogenous estrogens, phytoestrogens may produce beneficial effects on cardiovascular diseases through the estrogen receptors present in the vascular endothelium, vascular smooth muscle cells, intracellular lipid metabolism, extracellular matrix synthesis and vascular inflammation mediates through esrogen receptors (Rizzo & Baroni, 2018; Sirotkin & Harrath, 2014).

Epidemiological studies reveal that premenopausal women are less prone to development of CVD when compared to men. Female protection against CVD is associated with sex hormone levels as the incidence and severity of CVD increases in postmenopausal women. Worldwide women are suffering with reduced quality of life by menopausal symptoms. Estrogen levels are declined in postmenopausal condition, leads to vasomotor symptoms combine with increased probabilities of bone fractures, cardiovascular problems and changes in blood cholesterol levels (Sathyapalan et al., 2018).

Current treatment for vasomotor symptoms is hormone replacement therapy (HRT) but this therapy increases risk of cancer in uterus, breast and ovaries because of they are rich in ERα (Poluzzi et al., 2013). Estradiol show equal affinity towards both estrogen receptors this results in cancer of tissues rich in ERα because of overexpression of cells, but isoflavones have strong affinity to ERβ and weak affinity to ERα.Through ERβ selectivity isoflavones produce beneficial effects and minimise the risk of cancer. Phytoestrogens have been used in the Population that consumes soy-based food products less affected with cardiovascular disease based on this Asian people less affected with cardiovascular disease compared with western population because of their higher intake of soy-based diet (Abdelrazek et al., 2019; Krizova et al., 2019).

Classification of Phytoestrogens

Phytoestrogens may be classified as flavonoids and non-flavonoids. The flavonoids include Isoflavones (genistein, daidzein, glycitein, biochanin A and formononetin) and Coumestans (coumestrol, wedelolactone, plicadin), and nonflavonoids include lignans as shown in Figure 1. (Krizova et al., 2019; Pabich et al., 2019).

Absorption, Distribution, Metabolism and Excretion of Isoflavones

Mostly isoflavones are available as inactive glycoside conjugates in diet. After ingestion these are hydrolysed partly by the β-glucosidase present in the small intestine and mainly by bacterial β-glucosidase present in large intestine to form aglycones (Ko, 2014). These aglyconesare absorbed in to systemic circulation through the absorptive areas of intestine. Intestinal flora transforms aglycones in to bioactive compounds they are structurally similar to estradiol and therefore have a similar affinity towards cellular estrogen receptors. Colonicbacteria convert aglycones to inactive compounds, these are excreted through faeces. After absorption the primary and secondary aglycones are conjugated in part with sulfates and mainly with glucuronic acid. These conjugated products are freely circulate in the blood stream and produce their therapeutic benefit until they are excreted in the urine and bile as metabolites. When they are excreted in bile in to the intestine again absorbed by the intestinal mucosa and enter enterohepatic circulation or are excreted in faeces as conjugated forms and bacterial end products. After oral administration of isoflavones the peak plasma concentration attained at 7.2 to 7.4 hours. They remain for 24 hr in circulation, with an average 6-8 hours of plasma half-life. (Cederroth & Nef, 2009; Pabich et al., 2019)

Mechanism of action of isoflavones

Isoflavones may exert their biological action interacted through estrogen receptors. There are two types of estrogen receptors such as ERα (Estrogen Receptor α) and ERβ (Estrogen Receptor β) to exert estrogenic action of 17β-estradiol. Both the estrogen receptors may occupied with isoflavones but isoflavones show more affinity towards ERβ compare with ERα. Estradiol shows similar affinity towards ERα and ERβ (Krizova et al., 2019; Viggiani et al., 2019).

Estrogen binds to membrane and cytoplasmic estrogen receptors produce rapid nongenomic and genomic based cellular effects respectively. ERα and ERβ both are shown important role in modulation of cardiovascular diseases (Desmawati & Sulastri, 2019).

In vascular endothelial cells rapid activation of endothelial nitric oxide synthase (eNOS) to release nitric oxide to produce vasodilation as shown in Figure 2, endothelial cell growth and migration, platelet inhibition and prevention of thrombus formation this mechanism happened through activation of membrane bound ERα receptor initiate rapid signal transduction through kinases activation such as phosphatidylinositol-3-OH kinase (PI3K), mitogen -activated protein kinase (MAPK) and eNOS . ERα regulates activation of superoxide dismutase 2 results in decrease in formation of reactive oxygen species (ROS) results in antioxidant activity in mitochondria and prevents mitochondrial dysfunction. ERβ maintains normal function of superoxide dismutase 2 .In vascular smooth muscle cells proliferation leads to vascular related diseases, estrogen controls proliferation by activating ERα and ERβ associated with caveolin-1 present at plasma membrane of the vascular smooth muscle cell upon activation of ERα stimulates the various phosphatases reverses the phosphorylation results in a decrease of cell growth and migration .In cardiomyocytes through activation of ERβ decreases the cardiac hypertrophy. Mitochondria are the main source of energy i.e ATP and play an important role in regulation of myocardial contraction, calcium homeostasis and regulates cell death and apoptosis by forming reactive oxygen species (ROS) (Mahmoodzadeh & Dworatzek, 2019).

Blood pressure lowering effects of isoflavones

Prevention of cardiovascular diseases is mainly depending on maintenance of normal arterial function. Low level of NO production in blood vessel leads to vasoconstriction results in hypertension. Hypertension increases the risk of cardiovascular disease by increasing the endothelial injury through activation of proinflammatory mediators such as angiotensin II, causes vasoconstriction results in hypertension and activates lipoxygenase, the key enzyme in smooth muscles generates free radicals and formation of oxidized LDL cholesterol. Increased availability of oxidized form of LDL cholesterol in blood vessels leads to atheroma plaque formation. Isoflavones reduce the blood pressure by activating genes produces endothelial nitric oxide, interacts with estrogen receptors (Garg, Lule, Malik, & Tomar, 2016; Liu et al., 2012). Isoflavones have blood pressure lowering effect and the blood vessel endothelium rich in ERβ receptors are target for isoflavones, activation of receptors results in expression of endothelial nitric oxide synthase (eNOS) produces nitric oxide (NO) is a potent vasorelaxant decreases elevated blood pressure (Ramdath, Padhi, Sarfaraz, Renwick, & Duncan, 2017). Six months of isoflavone administration in postmenopausal women has been shown to increase endothelial vasodilation and reduce the number of cell adhesion molecules such as intracellular adhesion molecule 1, vascular cell adhesion protein 1, and E-selectin. Increased renal blood flow and inhibition of angiotensin converting enzyme (ACE) also occurred with isoflavones (Krizova et al., 2019; Pabich et al., 2019).

One of the research results reveal that attenuation of raise in blood pressure by hastening of nitric oxide (NO) production and prevention of inflammation occurred with isoflavones.A meta-analysis of 11studies showed that soy isoflavones having blood pressure lowering activity in hypertensive patients not in normal individuals (Krizova et al., 2019).

Liu et al. meta-analysis results concluded that ingestion of 65-153 mg/day soy isoflavones along with the soy protein 1-12 months decreased blood pressure in hypertensives but this effect was not observed in normotensive subject (Liu et al., 2012).

Tao sun et al., studied vasorelaxant and antihypertensive effects of formononetin through endothelium-dependent and endothelium-independent mechanisms. Endothelium dependent pathway produce vasorelaxant effect by releasing NO and endothelium independent pathway produces vasorelaxant effect probably due to inhibition of voltage dependent Ca+2channels and intracellular Ca+2 release. Vasorelaxant effect of formononetin studied by using segments of rat mesenteric arteries, renal arteries, cerebral arteries, coronary arteries and abdominal aortas precontraction induced with KCl or U46619 or phenylephrine. Formononetin produced concentration-dependent relaxation the rat artery segments (Tao, Rui, & Cao, 2011).

Antioxidant activity of isoflavones

Isoflavones produce a considerable antioxidant property along with estrogenic activity due to the availability of two hydroxyl groups in the chemical structure like daidzein. Aglycones are more active than glycosides. The antioxidant effect of isoflavones was evaluated by both in vitro and in vivo experimental models.Administrationof genistein in to mice and rats enhanced activation of antioxidative enzymes in epidermis and intestine was observed. (Krizova et al., 2019; Pandey & Rizvi, 2009).

Puerarin was studied for antioxidant property in pretreated neonatal murine cardiomyocytes with puerarin (50, 100 µM) decreased NADPH oxidase activity, inhibited ROS production and activated oxidative stress protective signaling pathways as shown in Figure 2. (Lim, Kim, & Kim, 2013; Yoon & Park, 2014).

Lipid lowering effect of isoflavones

Epidemiological studies recommended that the ingestion of soy and soy products for longer period decreases the cardiovascular risks. In 38 research studies meta-analysis results showed that the average intake of soy protein 47g per day a reduces bad cholesterol levels and increases good cholesterol levels and protect from cardiovascular risks. (Krizova et al., 2019; Rizzo et al., 2018).

Low density lipoproteins (LDL) are oxidized with free radicals present in walls of the blood vessels and accumulate then causes thrombosis, leads to cardiovascular disorders. The LDL cholesterol lowering effect of soy protein was already established. Zhan and Ho carried out meta-analysis results indicating that consumption of soy protein containing isoflavones notably reduced serum total cholesterol, LDL-cholesterol, and triglyceride, and remarkably increased high-density lipoprotein (HDL) cholesterol but it depends on duration of intake, sex, and initial plasma concentration of the individual as shown in Figure 2. (Pabich et al., 2019).

Puerariae radix is an edible crude drug used for different health problem. The important chemical constituents of this herb are flavonoids, coumarins and isoflavones, such as daidzein, genistein and puerarin and they are responsible to produce different pharmacological actions. P.lobata total isoflavones (PTIF), which contain the unique isoflavone, puerarin, studied on menopausal symptoms. Puerarin has shown significant protection against menopausal symptoms such as dyslipidemia and osteoporosis by producing strong estrogenic activity compared with other isoflavones (Lim et al., 2013).

Isoflavones effect on myocardial and vascular cell proliferation

Chunmei Liang et al., evaluated the formononetin action on cell proliferation and migration by using of human umblical vein endothelial cells (HUVECs). Formononetin is the isoflavone one of the main active constituent of Chinese herbal medicine clover and it showed protective effect on the human umblical vascular endothelial cells and the molecular mechanisms may correlate with insulin like growth factor 1 receptor (IGF-1R) and intercellular adhesion molecule 1 (ICAM-1). Insulin like growth factor 1(IGF-1) binds with insulin like growth factor receptor (IGF-1R) to inhibit endothelial cell apoptosis and promote endothelial cell proliferation, migration and angiogenesis. ICAM-1produce adhesion between leukocytes and endothelial cells and serve as the initiating event of angiogenesis (Liang, Zhou, Sui, & Huang, 2019)

Barbara Deodato et al., evaluated phytoestrogen genistein cardioprotective action in experimental myocardial ischaemia reperfusion injury in rats. Genistein, the principal isoflavone present in soy is structurally similar to estradiol, binds to estrogen receptors and produce estrogenic activity in some tissues. Genistein administration lowered myocardial necrosis and myeloperoxidase (MPO) activity both in the area at risk and in the necrotic area, decreased serum creatinine phosphokinase activity(CPK), increased myocardial contractility, decreased occurrence of ventricular arrhythmias, reduced serum and macrophages levels of tumour necrosis factor-α(TNF-α) and blunted intracellular adhesion molecule-1(ICAM-1) expression in the injured myocardium (22) (Deodato et al., 1999).

Isoflavone formononetin offered protection to rat cardiomyocyte H9c2 cells against oxygen glucose deprivation and reoxygenation by inhibiting ROS formation and promoting glycogen synthase kinase -3β (GSK -3β) phosphorylation. In ischaemic reperfusion injury the major cause of cell death is opening of mitochondrial permeability transition pore (mPTP). GSK-3β stimulates the mPTP opening and there by induces mitochondrial dysfunction. Phosphorylation regulates the enzymatic activity of GSK-3β. Phosphorylation occurs at tyrosine 216 increases the activity and at serine 9 significantly decreases the enzymatic activity of GSK-3β (Cheng, Xia, Han, & Rong, 2016).

Conclusions

Isoflavones are the group of chemical substances and sub class of phytoestrogens, these are present in legumes such as soybean, clover, kudzu and beverages, especially soybean rich in isoflavone content. Commonly available isoflavones are genistein, daidzein, formononetin and glycitein have nutritional and therapeutic value because of their estrogenic and antiestrogenic action. Epidemiological studies showing that increased intake of isoflavones reduce the hazard of cardiac problems, menopausal symptoms, osteoporosis and cancer. Although, the information available on protective role of isoflavones in cardiovascular diseases, still there is need to evaluate the clear mechanism responsible for protective action.

Acknowledgements

Authors wish to thank DST-FIST, New Delhi for their infrastructure support to Department of Pharmacology, JSS College of Pharmacy, JSS Academy of Higher Education and Research, P.B. 20,Rocklands,Ooty-643001, Tamilnadu,India and funding support from JSS AHER.

Conflict of Interest

The authors declare no conflict of interest.

Funding Support

None.