Engagement of phytoestrogens in breast cancer suppression: Structural classification and mechanistic approach

Highlights

• Incidence of cancer especially hormone dependant breast cancer is a major concern in postmenopausal women.

• Phytoestrogens or natural estrogens can be an effective alternative for synthetic estrogens.

• Phytoestrogens can decrease the breast cancer by acting through various mechanisms.

Abstract

Cancer is the world’s devastating disease, and breast cancer is the most common reason for the death of women worldwide. Many synthetic drugs and medications are provided with their beneficial actions, but all of these have side effects and resistance problems. Natural remedies are coming forward to overcome the disadvantages of synthetic drugs. Among the natural categories, phytoestrogens having a structural similarity of mammalian oestradiol proves its benefit with various mechanisms not only in the treatment of breast cancer but even to prevent the occurrence of postmenopausal symptoms. Phytoestrogens are plant-derived compounds that were utilized in ancient medications and traditional knowledge for its sex hormone properties. Phytoestrogens exert pleiotropic effects on cellular signalling and show effects on estrogen-dependent diseases. However, because of activation/inhibition of steroid hormonal receptor ER-α or ER-β, these compounds induce or inhibit steroid hormonal (estrogen) action and, therefore, have the potential to disrupt hormone (estrogen) signalling pathway. In this review, we have discussed and summarize the effect of certain phytoestrogens and their possible mechanisms that can substantiate advantageous benefits for the treatment of post-menopausal symptoms as well as for breast cancer.

Introduction

Cancer is triggered by collective genetic and non-genetic alterations prompted by environmental aspects that cause inappropriate activation or inactivation of particular genes leading to neoplastic transformations, or abnormal cell growth [1]. Cancer rises subsequently in such a way that a series of genetic mutations remove the typical check on cell growth. Cancerous cells continue to divide and propagate to produce tumors [2]. They can occupy adjacent structures and spread via the lymph or blood to distant organs. Some of the biological mechanisms that change a normal cell into a cancer cell are identified while others are not known [3]. Malignant growth varies from most different illnesses in that it can create at any phase throughout everyday life and in any body organ [4]. Cancer can be categorized based on where primary cancer grows and on the type of tumor [5]. Lung cancer, breast cancer (BC), colorectal cancer, stomach cancer, liver cancer, and cervical cancer are the most common forms of cancer which has been diagnosed worldwide [3].

Among all types of cancers, BC is one of the major causes of death in women worldwide [6]. In BC, there is a continuous reproduction and uncontrolled growth of cells in the breast. Abnormal cells cluster each other and lead to the formation of tumor [7]. It has been reported that BC can originate either from lobules of the breast and/or from ducts, which are responsible for carrying the milk from lobules to mammilla [8]. BC is affecting 2.1 million women each year, and also causes the highest number of cancer-related deaths among women. It is estimated that 627,000 women died from BC in 2018, i.e. about 15% of all women’s death rate worldwide [9]. However, due to lack of awareness, absence of screening programs, irregular check-ups, there is an increased risk of BC in rural areas than urban ones [10].

17β-Estradiol (estrogen/E2) is a major estrogen which is secreted by the ovaries and synthesized from cholesterol in the graafian follicles, corpus luteum, and the placenta. It is involved in the development of BC via stimulation and proliferation of cells. Women typically in their 40s or 50s when there is a complete or permanent menstrual cessation due to loss of function in ovaries that creates both, eggs and female hormone (estrogen), this stage is known as menopause [11]. When the menstrual cycle stops in women, there is a decrease in the concentration of circulating estrogen, which causes several unpleasant symptoms, including urogenital atrophy, hot flushes, mood swings, osteoporosis [12]. For the treatment of postmenopausal symptoms, especially osteoporosis, hormone replacement therapy (HRT)/estrogen replacement therapy is said to be an effective and preventive method. However, prolonged exposure of these hormones leads to an increased risks of BC called hormone-dependent BC and ovarian cancer [11,13,14]. Several synthetic compounds available that can specifically bind and induce conformational changes of ERs hence can combat post-menopausal symptoms. Selective Estrogen Receptor Modulators (SERMs) are among the category of drugs which are used as a first-line treatment for menopause induced osteoporosis. Because of their anti-estrogenic effect, tamoxifen, one of the SERMs, has been commonly used to treat menopausal symptoms as well as BC but has the severe side effect of stimulating increased risk of endometrial cancer [15]. Recent therapies, including radiotherapy, hormonal therapy, and adjuvant therapy are useful, but due to side effects and toxicity issues, there is no cure for patients with an advanced stage of BC. Recent research also established new mechanisms, biomarkers, pathways, and agents that are likely to be effective in the treatment of BC [16]. There is an unmet requirement for agents that can be able to minimize the occurrence of side effects in women treated with BC therapies [17]. The most common side effects of SERMs and aromatase inhibitors (AIs) which are generally prescribed to BC patients are menstrual discharge, vaginal dryness, dyspareunia, and arthralgia [18]. Tamoxifen treatment results in a greater rate of vaginal bleeding and discharge, endometrial cancer, venous thromboembolism, and stroke cases. AI shows a higher rate of musculoskeletal complications (including arthralgia, bone deterioration, and fractures), and dyspareunia (vaginal dryness) [19]. By looking at this issue of side-effects, toxicity, and resistance, research is now converging on natural remedies for the treatment of post-menopausal symptoms as well as BC. In the natural category of plant-derived constituents, phytoestrogens are creating interest because of their structural similarity to mammalian estrogen (E2) [20].

Phytoestrogens (PEs) are naturally occurring phytochemicals that mimic the effect of estrogen and therefore, can treat post-menopausal symptoms without side effects [14]. Inside the body, phytoestrogen resembles the structure of the mammalian endoestrogen, i.e. 17-β estradiol. The distance between the two hydroxyl groups of estrogen and phytoestrogen (genistein) is almost the same, i.e. 14.5 Å (Fig. 1) hence can bind to the estrogen receptor (ER). It has been observed that along with the similar distance of hydroxyl group of estradiol and genistein, both have a similar binding pattern when docked with ER-α receptor (PDB ID: 4XI3) suggesting that both compounds bind in the same binding pocket. Moreover, the amino acids with which they bind in the binding pocket of ER-α receptor was also same i.e. histidine (HIS 524), phenylalanine (PHE 404) and Glutamine (GLU 353). Moreover, type of interaction with these amino acids were similar such as hydroxyl group of both estradiol (on 3rd and 17th C) and genistein (on 7th and 4’ C atom) form hydrogen bonds with histidine and glutamine while aromatic rings of both have pi-pi stacking interaction with phenyl alanine. These results were also supported by a similar study by Manas et al. where studied the interaction of these compounds in ER-α and ER-β [21]. They have both agonistic and antagonistic action hence are beneficial when there is a deficiency of estrogens in the body as well as when there is overexpression of ER because of the increased amount of estrogens in the body [22]. It has also been reported that genistein, like estrogen, can also promote cancer in a dose dependant manner in HeLa cell line. Genistein has shown to increase the expression of ER-α, Akt and NF-κB p65 proteins in estrogen dependant PI3K/Akt–NF–κB pathway and thus can stimulate cell proliferation through this pathway [23]. The phytoestrogens act as an antagonist in the breast and have antagonistic actions in other parts of the body. Resveratrol is an example of phytoestrogen that prevents the development of different cells in the breast (MCF-7 and MBA-MB-231) by modulating the expression of specific transcription factors that are correlated with cell cycle regulation, angiogenesis, metastasis, and apoptosis.

Moreover, the stereochemistry of different phytoestrogens plays an important role in their activity. One such study was conducted by Muthaya et al. where they observed that the two equol (metabolite of daidzein) enantiomers R-(−)-equol and S–(−)-equol might act through two estrogen receptors, ERα and ERβ in breast cancer and therefore effect differently. It was found that after conversion by gut microflora, the estrogenic activity of S–(−)-equol exceeds than that of daidzein (Fig. 2). Interestingly, the two enantiomers of equol show differential activity on the two estrogen receptor i.e., ERα and ERβ, with R-(−)-equol having binding affinity ERα, while S–(−)-equol having a binding affinity with ERβ [24].

Another example is of a natural phenylpropanoids isolated from the heartwood of Chamaecyparis obtuse, hinokiresinol. The compound was found to possess estrogen receptor binding affinity, but both geometrical isomers and enantiomers of hinokiresinol have shown a difference in their anticancer activity. Its geometrical isomers are trans-hinokiresinol (1) which is called as hinokiresinol, and the cis-hinokiresinol (2) is called as nyasol (Fig. 2). It was found that the anticancer activity of cis isomer of hinokiresinol was greater around one order of magnitude than that of the trans isomer. Even there was an activity and binding difference among cis isomers both compounds proven to exhibit binding affinity for the estrogen receptor but (3R)-cis-Hinokiresinol (2a), and (3S)-cis-Hinokiresinol (2b) have an IC₅₀ value of 400 nM and 60 nM. The (3S)-cis-Hinokiresinol (2b) exhibited highest binding affinity then the (3R)-cis-Hinokiresinol (2a) [25].

The incidence of BC is much lower in Asian countries compared to the western population. The average daily intake of phytoestrogens in the diet of the Asian population is 20–50 mg, whereas in Western countries it is < 1 mg. This could be related to dietary habits, i.e. vegetation from Asian countries and meat from Western people [26]. The beneficial effects of reduced risk of breast and prostate cancers and lower postmenopausal symptoms, osteoporosis, and cardiovascular diseases have been associated with Asian human populations consuming high soy concentrations, especially isoflavones (daidzein and genistein) [27].

Phytoestrogens (PEs), also known as plant-derived xenoestrogens or dietary estrogens, are polyphenolic or non-steroidal compounds that are considered as secondary metabolites, derived from plants [28]. Among the polyphenolic compounds isoflavonoids, stilbenes, lignans, and coumestans conveyed to show estrogen-mediated response called endogenic effects [[29], [30], [31]]. It has been found that consumption of a PEs-rich diet may have protective effects on estrogen-related conditions, such as prostate and breast cancer, osteoporosis, and cardiovascular diseases. Phytoestrogens have antioxidation, anticancer, phytoestrogenic, antiatherosclerosis, and antiosteoporosis activity.

The major source of Isoflavones producing estrogenic activity are beans, legumes, soy products such as tofu, yoghurt, and soy noodle, soy sauce [32]. The lignans are majorly found in cereals, fruit, and vegetables [33]. The lignans are present in higher concentrations in flaxseed known as linseed and are present in lower concentrations in whole grain, cereals, vegetables, fruit, and seeds (Hwang & Choi, 2015). Different categories of plant-derived phytoestrogens with their examples are classified in Table 1.

Chemically, polyphenols are the substances that consist of a phenyl ring and substituted by one or more hydroxyl groups [56]. In plants, they are synthesized by the shikimic acid pathway. Structurally they possess C6 (ring A) - C3 (ring C) – C6 (ring B), i.e., two phenyl rings with a chromate ring that is the backbone of phenolic compounds [30,57]. Based on the substitution of hydroxyl group at different positions on chromate ring, they can be classified as flavones, flavanones, chalcones, aurones, anthocyanidins, and isoflavonoids [58]. While the non-flavonoids can be differentiated into phenols as phenolic acids (phenylacetic acids or benzoic acids, and hydroxycinnamic acids), benzaldehyde derivatives, acetophenones, benzophenones, and polyphenols like curcuminoids, tannins (e.g., hydrolyzable tannins), coumarin, xanthones, stilbenoids, lignans, and secoiridoids (Estrela et al., 2017). The use of PEs or plant-derived estrogen has been increased in the diet because of the health-promoting action of the dietary plants in BC. It delays the onset of many diseases, such as cardiovascular, postmenopausal symptoms, and post-menopausal osteoporosis [59].

There are two main classes of polyphenols: flavonoids and non-flavonoids (Fig. 2). The flavonoid group consists of two benzene rings bound by a C-ring of the heterocyclic pyrone. The group of non-flavonoids contains more complex structures including stilbenes, tannins of phenolic acids, and lignans [60]. PEs are generally present in the form of inactive glucoside. There is a structural similarity to estrogen that resembles the structure of heterocyclic phenols when there is an enzymatic metabolism of the lignans in the gastrointestinal tract. Also, secoisolariciresinol and matairesinol are converted into estradiol and enterolactone respectively, in a plant that is also known as estrogenically active lignans [61]. Isoflavones, coumestans, and lignans, occur in either plants or their seeds. Resorcylic acid lactones are produced by molds that commonly contaminate cereal crops and hence are termed as mycoestrogens [62]. There is often more than one class of PEs in a single plant. Soybean is abundant in isoflavones, for instance, while soy sprout is a potent source of coumestrol, the primary coumestan. Isoflavones in soy products have antioxidant properties and act as phytoestrogens [63].

Flavonoids are the polyphenolic compounds that are biosynthetically formed through the phenylpropanoid-acetate pathway via chalcone synthase and condensation reactions with malonyl CoA [64,65]. Flavonoids can be categorized into several subfamilies (Fig. 3) by the degree of oxidation of the oxygenated heterocyclic such as flavanols, flavanones, flavonols (mainly flavan-3-ols), isoflavones and anthocyanidins [57]. Due to the structural similarity of non-steroidal PEs with E2, several PEs may bind to both ER-α and ER-β. The isoflavonoids are a subtype of flavonoids that are characterized by the migration of the phenolic group from the C-3 to the C-2 position of the chromate ring [66]. Isoflavones are present mainly in the plants belonging to family Fabaceae, which comprise legumes such as soy, peanut, and clover. Isoflavones were also present in the family of Iridaceae, and the Euphorbiaceae derived mostly from soy and red clover. On a dry basis, the soy seeds show high levels of formononetin and biochanin A, i.e., 729 μg/g. Raw soybeans produce 1.2–4.2 mg/g isoflavones while high protein soy ingredients i.e. soy flour produce 1.1–1.4 mg/g on the dry weight [67], while neutraceuticals products (soy foods) like tofu, yogurt, and tempeh burger contain 6–20% of isoflavones [29,67,68]. Daidzein and genistein are the two major isoflavones present in soy. Daidzein is a kind of isoflavone rich in soybean with a molecular weight of 254.24 g/mol. In Asia, soy consumption can be as large as 30–50 g per day, and genistein plasma concentrations have been calculated from 0.1 to 10 mM [69]. Although with an affinity about 5000 times lower than that of the estrogens, isoflavones can function as partial agonists or ER antagonists, likely by aromatase enzyme inhibition (CYP19), the enzyme that catalyzes the conversion of androstenedione and testosterone to oestrone and estradiol [70]. While soy fermentation may reduce the number of isoflavonoids present by a factor of 2–3, isoflavonoids bioavailability in fermented products is higher, so urinary excretion rates are similar for people who consume fermented and unfermented products [29,71]. Compounds such as apigenin, found in parsley or chamomile, reflect flavones. Apigenin has beneficial effects on human well-being. The average flavone consumption is very minimal and is measured at between 0.3 and 1.6 mg/day [72].

There are several naturally occurring isoflavones as shown in Fig. 4. After consumption, there is a metabolism of isoflavones (daidzein, genistein) in the gastrointestinal tract. The genistein and daidzein will be formed after the metabolism of biochanin A and formononetin respectively. Further, daidzein can also be metabolized into dihydrodaidzein, and then into O-desmethylangolensin (O-DMA) and equol while genistein is metabolized into dihydrogenistein, and then into 6′-hydroxy-O-DMA (Fig. 4) [29,68]. Isoflavones have drawn special interest because of its role in improving postmenopausal symptoms such as osteoporosis and hot flushes. Various epidemiological and clinical studies assessed the effects of isoflavones on menopausal symptoms such as decreased hot flushes, increase in bone mineral density, reduction in LDL and total cholesterol, and the excretion of bone resorption biomarkers [67,68]. An isoflavone isosideroxylin (Fig. 4) derived from the ethanolic extract of leaves of Leiophyllum buxifolium showed an antiproliferative effect against MDA-MB-231 (ER -) and MCF-7 (ER +) BC cell lines [73]. Liquiritigenin was isolated from the roots of Glycyrrhizae uralensis Fisch and described to activate the ERE-tk-luciferase, as well as three native ER regulatory elements via NKG2E (killer cell lectin-like receptor), CECR6 (cat eye syndrome chromosome region candidate), and NKD (naked cuticle homolog). It also activates native target genes transfected with ER-β but not ER-α because of its 20-fold higher affinity for ER-β than ER-α [74]. Hesperetin (3′,5,7-trihydroxy-4-methoxyflavanone), is a flavone that is mainly present in citrus fruits, and also present in its glycosidic form as hesperidin. Hesperidin is metabolized to hesperetin by the intestinal bacteria before absorption and used as a prodrug. In a study, it is found that hesperetin inhibits cell proliferation in a dose-dependent manner and causing cell cycle arrest at the G1 phase. Treatment of MCF-7 cells with hesperetin for 48–72 h displayed down regulation of cyclin-dependent kinases (CDKs) and cyclins in G1 phase [75]. Biochanin A exhibits biphasic regulation on MCF-7 cells, since at a concentration <10 μg/mL, it increases cell growth and de novo synthesis while at a concentration >30 μg/mL, it inhibits cell growth and DNA synthesis (IC₅₀ = 40 μg/ml) [76]. Isoliquiritigenin (ISL) is a flavonoid, isolated from the liquorice root and found to exhibit ER-α dependent growth-promoting effects on BC cells. It induces apoptosis and inhibits cellular growth by deactivating PI3K/Akt and by downregulating arachidonic acid (AA) metabolic pathway [77]. In Table 2, flavonoids are discussed along with their respective mechanism of action that has been identified.

Natural stilbenes are an important group of polyphenolic non-flavonoid phytochemicals with a nucleus of 1,2-diphenylethylene [85]. There are over 400 natural stilbenes, but they are contained in a restricted and heterogeneous community of plant families as the main enzyme engaged in stilbene biosynthesis, stilbene synthase, is not ever-present [86]. Many distant botanical families, such as Gnetaceae, Pinaceae, Cyperaceae, Fabaceae, Dipterocarpaceae, and Vitaceae, are known for their high stilbene content [87].

Stilbene shares similar substrates with flavonoids in plants as well as biosynthetic pathways [88]. Although most plants produce flavonoids, stilbenes are synthesized by only a few species of plants. Beginning with phenylalanine, the biosynthetic pathway of stilbenes undergoes several enzymatic reactions to generate p-coumaroyl-CoA [89,90]. Resveratrol is then produced by aldol reaction in the presence of malonyl-CoA and stilbene synthase. Resveratrol is converted to produce pterostilbene by a reaction facilitated by O-methyltransferase, pterostilbene is found to be more bioavailable (80%) than resveratrol (20%) even with all of the structural similarity [89].

There are several structural analogs of stilbenes as shown in Fig. 5, such as Pinosylvin and hydroxylated derivatives such as piceatannol, also methylated, and glycosylated derivatives such as desoxyrhapotigenin, pterostilbene, pinositilbene and piceid, resveratroloside respectively. All these types of analogs exhibit antiproliferative activity again different types of cancer cells, as well they show higher oral bioavailability and biological properties than resveratrol [42,91,92]. Among them, the most active compounds tended to be trans-resveratrol trimethyl ether and pinostilbene, which have already been reported to be 100 times more cytotoxic in cancer cell lines than resveratrol [93,94]. Studies of structure-activity relationships with resveratrol analogs showed that phenolic hydroxyl or methyl groups are the main structural determinants of the activity of a molecule. Therefore, resveratrol’s molecular scaffold was used as a starting point for synthesizing a new derivative of resveratrol for enhancing and improving its chemo preventive activities [41,91]. Resveratrol, a bioactive component of red wine, gets converted into piceatannol (PIC) and plays a crucial role in a procarcinogenic process. About 25 μM PIC marginally elevated p21 protein levels within 30 min in MCF-7 cells while 150 μM PIC triggered MCF-7 cell death. It is considered that MCF-7 cells are caspase-3-deficient and are, therefore, less sensitive to drug- and radiation-triggered apoptosis [95].

The beneficial health effects of resveratrol and other stilbenes have been identified [96]. Because of their antioxidant, cell death stimulation, anti-inflammatory and other effects (Table 3) associated with low toxicity under in vivo conditions, stilbenes have an exceptional capacity for the prevention and treatment of various diseases including cancer [97]. These positive health effects include lifetime expansion, weight loss, and prevention of cardiovascular, neurodegenerative, and stroke-inducing brain damage, cancer, and metastasis of cancer [98]. All stilbenoids contain two aromatic rings bound by a methylene bridge backbone where either methyl, prenyl, hydroxyl, methoxy, or geranyl can be substituted, and sugars can be mixed to create glycosides, creating a collection of compounds of various chemical structures and properties [90]. Stilbenes are present mainly in the trans-isomer due to increased stability; although, cis-isomer is sometimes found in some plants. Resveratrol from red wine is the main dietary source of phytoestrogenic stilbenes. Although there are two isomers of resveratrol, cis and trans, only the trans form has been reported to be estrogenic [99].

Several studies have found that exposure to blueberry inhibited breast cancer through in vitro and in vivo. High-bush, low-bush, and silk blueberry, blueberry juice had an anti-proliferative effect on the MDA-MB-231 group of breast cancer cells [105]. In BC, pterostilbene has been shown to have anticancer effects by altering multiple cancer pathways in vitro and in vivo. The anti-cancer effects of pterostilbene have been related to its antioxidant-inducing ability. Alosi et al. discovered that pterostilbene induced apoptosis and inhibited concentration and time-dependent proliferation of MDA-MB-231 and MCF-7, BC cells [42]. Resveratrol, a stilbene, is described as a possible chemopreventive agent found in the human diet and is considered a PE because of its potent estrogenic and even superestrogenic properties (in combination with E2) in mammalian cancer [93]. Resveratrol has a lower binding affinity for ERs as compared to estradiol but binds to ER-α and ER-β and activating them to show a full agonist action [106,107]. Resveratrol functions as a selective estrogen receptor modulator (SERM) and resveratrol’s effects depend on the type of cell and target organ, as well as the presence of endogenous estrogen [106].

Lignans is one of the class of PEs which has protective effects on the health of humans. These are abundant in various parts of the plants, including root, hardwood, bark, rhizomes, stem, fruits, leaves, and seeds [108]. In western diets, they are an important source of PEs [109]. The medicinal value of lignans was noticed in ancient times when different plant extracts containing lignans were used as folk remedies [110]. Several new lignans have been discovered by the isolation from traditional medicines. Because of their structural similarity to mammalian estrogen, they may act as a weak estrogen hence possesses estrogenic potency [111]. Several cytotoxic lignans have therapeutic value, such as podophyllotoxin, used in cancer therapy [112]. Most non-toxic lignans are the components of human nutrition, found at high concentrations in oilseeds, in grains such as wheat, rye, oats, and berries [113]. The phytolignans are present in abundance in flaxseed, whole-grain pieces of bread, vegetables, tea while fruits contain lower content of lignans except for strawberries and cranberries [114]. Flaxseed is one of the substantial dietary sources of the lignan that contains secoisolariciresinol and matairesinol [115,116].

Lignans can be classified into different types (Fig. 6) such as dibenzyl butyrolactone, furofuran, furan, and dibenzylbutane, containing scaffolds. The matairesinol, arctigenin and nortrachelogenin are dibenzyl butyrolactone type; pinoresinol and phillygenin are a furofuran type. The furan types of lignans are categorized into lariciresinol and anhydro-secoisolariciresinol and last, the dibenzylbutane type of lignans contains secoisolariciresinol and isolariciresinol [117].

Lignans are bearing two units of phenylpropanoid (C6–C3) chemical assemblies [110]. There are two dimers secoisolariciresinol and matairesinol that are not estrogenic by themselves but are readily transformed into mammalian lignans, enterodiol, and enterolactone respectively, which can have estrogenic effects [116]. Enterolactone (ENL) shows the antiestrogenic effect by inhibiting the growth of MCF-7, which was induced by the estradiol. Likewise, in the absence of the estradiol, it stimulates the proliferation of these cells [117]. The conversion occurs by the gut microflora, and the mammalian lignans are readily absorbed [118].

Almost all lignans can be converted to enterolactone (ENL), except nortrachelogenin and tracheloside (Fig. 7). In rodents, oral administration of plant lignans such as lariciresinol, secoisolariciresinol diglucoside, 7-hydroxymatairesinol, arctiin, sesamine, and enterodiol, stimulated their metabolism to enterolactone, by intestinal micro flora. This could enhance the urinary excretion of enterolactone (ENL) and displays the inhibition of estrogen-responsive mammary tumors in intact rats [118,119]. The significance of ENL formation is further confirmed by the absence of anticarcinogenic effects of structurally related plant lignans, i.e. nortrachelogenin, and tracheloside. Both of them have a C-8 hydroxy group, rendering them non-convertible to ENL (Fig. 7), and they neither inhibited the growth nor the multiplicity of carcinogen-induced mammalian tumors in rats [118].

Pinoresinol is an important polyphenolic compound and structurally simplest lignan which occur in large quantities in woody or fibrous plants. Being a dimer of coniferyl alcohol formed via phenylpropanoid pathway, pinoresinol occur both as (+)-pinoresinol and (−)-pinoresinol and act as a plant defence system against fungus in their reproductive organs and seeds [120,121]. Among various bioactivities of pinoresinol anticancer is of main concern, as there is handsome amount of literature in support of it. In a study, pinoresinol rich virgin olive oil extracts was tested for their anticancer properties in HER2-positive and HER2-negative breast carcinomas using in vitro MTT assay, HER2-specific ELISAs and immunoblotting. It was found that (+)-acetoxypinoresinol and (+)-pinoresinol rich fractions at have strong tumoricidal (IC₅₀ = 51 μM and 73 μM, respectively) and apoptosis inducing activity as detected by cell dealth ELISA detecting apoptosis induced DNA-histone fragmentation [122]. Also, in another study (+)-pinoresinol have shown antibreast cancer activity independent of estrogen receptor status i.e. in both ER+ (MCF-7) and ER- (MDA-MB-231) breast cancer cell lines [123]. Besides breast cancer pinoresinol has found to be active against leukaemias, colon cancers and hepatocellular carcinoma through different mechanisms [121,124,125].

Arctiin is an essential lignan of Arctium lappa, recently reported to induce growth inhibition of different forms of human tumor cells, including osteosarcoma, lung, colorectal, cervical, BC, melanoma, transformed renal cells, and prostate cancer with the downregulation of cyclin D1 protein expression (Table 4) [126]. Arctigenin and arctiin, both have been presented to have anti-cancer effects and are involved in signalling pathways such as the ROS/MAPK/Bcl-2, AMPK [127]. FAK has been re-counted to be overexpressed in cells of BC. It has been reported that enterolactone reduced the growth of new and established tumors and exerts an antitumor effect by blocking the FAK/paxillin signalling pathway by regulating the expression of cell proliferation and cell cycle-associated genes (Table 4) [128]. One of the possible mechanisms of action of 7-hydroxymatairesinol is that it decreases local estrogen production by inhibiting 17-hydroxysteroid dehydrogenase type I and aromatase enzyme (Table 4) [115]. It has been found that Lariciresinol causes G0-G1 mitotic phase arrest in breast cancer cells while increasing tumor suppressor protein maspin levels that decrease cell invasion, metastasis, and angiogenesis [129]. Sesamine helps in reducing the growth of BC cells in athymic mice at high serum concentrations of estrogen in part by down-regulating the expressions of EGFR and MAPK [130]. Nortrachelogenin inhibits BC cell growth through the Akt pathway [131]. Tracheloside is a lignan glycoside that is majorly found in the Trachelospermum species, and it is the major active constituent of Crathamus tinctorius belonging to family Compositae. It is similar to arctiin and act as phytoestrogen by activation of the Erk1/2 and PI3K/Akt pathways [132].

Coumestans are another class of polyphenols found in plants that exhibits estrogenic activity. In 1957, coumestrol was isolated by Bickoff et al. from strawberry clover, alfalfa/lucerne and ladino clover then were classified under phytoestrogens (Trifolium fragiferum L., Fabaceae; Medicago sativa L., Fabaceae; Trifolium repens L., Fabaceae; [133]. The main dietary source of coumestrol is legumes, however low levels have been reported in brussel, sprouts, and spinach. Clover and soybean sprouts are reported to have the highest concentration [134]. In bean sprouts and fodder plants, coumestans have earned very little scientific consideration [133]. Their involvement in fodder crops was related to confounding livestock reproductive efficiency. Studies reported that feeding cattle with 37 ppm (mg/kg) or more coumestrol resulted in negative estrogenic effects such as udder development, steering bulls and prolapsed vagina, cervix, and rectum [102,135].

Coumestrol and 4′-methoxycoumestrol (Fig. 8) are the two major examples of coumestans with phytoestrogenic activity [136]. Insect and fungal attacks can cause plants to produce coumestrol or 4′-methoxycoumestrol as phytoalexins [102]. Phytoalexins are often secondary metabolites formed as a result of injury or disturbance which help plants to defend against pests, bacteria, and viruses [137]. Coumestrol and genistein are more closely related to ER-β than the other PEs [138]. The in vitro results conveyed that coumestrol prevents bone resorption and enhances bone mineralization [139].

Coumestrol was isolated from the leaves of Glycine max., exposed to have an anti-cancer effect by inhibiting the phosphotransferase activity of CKII towards β-casein (Table 5). It is found that coumestrol with an IC₅₀ of about 50 μM resulted in 50% inhibition of MCF-7 human breast cancer cells [140]. The over-expression of CK2 is linked with several human cancers and thus may be a potential cancer therapy target. Coumestrol has been reported as an effective inhibitor of CK-2.4^’-Methoxycoumestrol partly prevented the development of cancer cells by down-regulating CK2-specific Akt phosphorylation [141].

Zearalenone (ZEA), a lactone with a 14-member macrocyclic β-resorcyclic acid (Fig. 9), is a non-steroidal estrogenic mycotoxin produced by numerous species of Fusarium as a secondary metabolite, including F. culmorum, F. roseum, F. graminearum, and others [142]. As a result of its strong estrogenic activity, natural exposure to ZEA in contaminated food has been implicated as a cause of female reproductive changes [143]. ZEA is rapidly absorbed and initially metabolized by the intestinal tissue and hepatocytes after oral exposure; this initiates the biotransformation of the compound into its main biologically active metabolites, α- and β-Zearalenol (α- and β-ZOL). ZEA and its metabolites show estrogenic activity which is regulated by their binding affinity to estrogen receptors (ER), and they’re as active as coumestrol and genistein [144]. Unlike the phytoestrogen that shows their affinity towards ER-β, the ZEA, and its reductive metabolites show equal binding affinity towards both ER-α and ER-β receptors [145]. Aflatoxin B1 (Fig. 9), another potential estrogenic mycotoxin, is converted to aflatoxicol and poses negative effects on dairy cows with a direct activity on oocyte or embryo thereby altering maternal homeostasis [146,147]. BPA (Fig. 9) is known to be a derivative of xenoestrogen and is an endocrine-disrupting chemical that may potentially influence human health. Some of BPA action is due to its ability to bind the classical nuclear estrogen receptors (ER), ERα, and ERβ as a weak estrogen agonist. Recent studies have found that BPA can bind strongly to the estrogen receptor-γ (ERR-γ) that has previously been known as an orphan receptor [148].

Researchers in environmental sciences have recently raised an alarm that many of industrial wastes are weakly estrogenic and could act as estrogen mimics (xenoestrogens), thereby disrupting normal endocrine function, leading to reproductive failure and cancer of estrogen-sensitive tissues [72]. Many people suggest that xenoestrogen levels to which humans are exposed daily are insignificant relative to the daily exposure to natural phytoestrogens found abundantly in common foods consumed by humans [149]. However, the molecular structure of exogenous natural and synthetic estrogens may be very similar to, or strikingly different from, the parent hormone estradiol [143]. Despite their structural diversity, all of the exogenous estrogens, when consumed either as natural components (PEs) or contaminants (xenoestrogens, mycoestrogens), can bind to estrogen receptors (ERs) in target cells of the tissues. These mycoestrogens might act as agonistic or antagonistic estrogen-like molecules and hence can have anticancer activity by a number of mechanisms (Table 6) [72].

One compound which does not fall in the categories given above but is polyphenolic compound, i.e. gossypol. Gossypol extracted from cotton plant (genus Gossypium) and Thespesia populnea, both members of the family Malvaceae. It is a known male contraceptive due to its spermicidal activity. Chemically gossypol consists of two naphthalene rings joined by an inter-naphthyl bond between 2nd and 20th carbon atom and exist in two enantiomeric forms (+)-gossypol and (−)-gossypol (Fig. 10) [152,153]. In a study on anti-breast cancer activity of gossypol, the compound was tested against various breast cancer cell lines such as MCF-7, ZR-75-1, MDA-MB-231, MDA-MB-468 and T47D and it was observed that gossypol significantly inhibited the expression of Mouse double minute 2 (MDM2) and vascular endothelial growth factor (VEGF) proteins. It also inhibits the crosstalk between the MDM2 and VEGF mRNA which otherwise help in its translation which in turn results in apoptosis and anti-angiogenic activity [154]. Based on the literature, it is a general consensus that (−)-gossypol has better activity in various forms of cancer including breast cancer. It hasalso been undergone in phase II clinical trial for the treatment of prostate cancer [[154], [155], [156]].