Comparative study of estrogenic activities of phytoestrogens using OECD in vitro and in vivo testing methods

Receptor mediated biological activities of phytoestrogens

Phytoestrogens are plant-derived compounds that have chemical structures and functions similar to estrogen. Phytoestrogens act as ligand-inducible transcription factors involved in cellular growth by binding to estrogen receptors (ERs), specifically ER alpha (ERα) and beta (ERβ). Through this mechanism, phytoestrogens have a physiological function similar to that of the female hormone 17β-estradiol (E2), which can be useful in treating osteoporosis, cardiovascular disease, and cancer. Furthermore, phytoestrogens have been found to elicit various cellular responses depending on their affinity for ERs; in particular, they show a greater affinity with for ERβ. This study aimed to comprehensively analyze the mode of action of eight phytoestrogens, namely kaempferol, coumestrol, glycitein, apigenin, daidzein, genistein, equol, and resveratrol, by evaluating their estrogenic activity as ER ligands. Based on the bioluminescence resonance energy transfer (BRET)-based ER dimerization and transactivation assay results, all the phytoestrogens tested were identified as estrogen agonists by mediating ERα and ERβ dimerization. The specific binding and functions of ERα and ERβ were distinguished by differentiating between their dimerization activity. In addition, this study contributes to advancing our understanding of the overall mechanism of action involving both ERs.

The anti-aromatase and anti-estrogenic activity of plant products in the treatment of estrogen receptor-positive breast cancer

Despite being the focal point of decades of research, female breast cancer (BC) continues to be one of the most lethal cancers in the world. Given that 80 % of all diagnosed BC cases are estrogen receptor-positive (ER+) with carcinogenesis driven by estrogen-ERα signalling, current standard of care (SOC) hormone therapies are geared towards modulating the function and expression levels of estrogen and its receptors, ERα and ERβ. Currently, aromatase inhibitors (AIs), selective ER modulators (SERMs) and selective ER degraders (SERDs) are clinically prescribed for the management and treatment of ER+ BC, with the anti-aromatase activity of AIs abrogating estrogen biosynthesis, while the anti-estrogenic SERMs and SERDs antagonise and degrade the ER, respectively. The use of SOC hormone therapies is, however, significantly hampered by the onset of severe side-effects and the development of resistance. Given that numerous studies have reported on the beneficial effects of plant compounds and/or extracts and the multiple pathways through which they target ER+ breast carcinogenesis, recent research has focused on the use of dietary chemopreventive agents for BC management. When combined with SOC treatments, several of these plant components and/or extracts have demonstrated improved efficacy and/or synergistic impact. Moreover, despite a lack of in vivo investigations, plant products are generally reported to have a lower side-effect profile than SOC therapies and are therefore thought to be a safer therapeutic choice. Thus, the current review summarizes the findings from the last five years regarding the anti-aromatase and anti-estrogenic activity of plant products, as well as their synergistic anti-ER+ BC effects in combination with SOC therapies.

Phytoestrogens: Chemistry, potential health benefits, and their medicinal importance

Phytoestrogens, also known as xenoestrogens, are secondary metabolites derived from plants that have similar structures and biological effects as human estrogens. These compounds do not directly affect biological functions but can act as agonists or antagonists depending on the level of endogenous estrogen in the body. Phytoestrogens may have an epigenetic mechanism of action independent of estrogen receptors. These compounds are found in more than 300 plant species and are synthesized through the phenylpropanoid pathway, with specific enzymes leading to various chemical structures. Phytoestrogens, primarily phenolic compounds, include isoflavonoids, flavonoids, stilbenes, and lignans. Extensive research in animals and humans has demonstrated the protective effects of phytoestrogens on estrogen-dependent diseases. Clinical trials have also shown their potential benefits in conditions such as osteoporosis, Parkinson's disease, and certain types of cancer. This review provides a concise overview of phytoestrogen classification, chemical diversity, and biosynthesis and discusses the potential therapeutic effects of phytoestrogens, as well as their preclinical and clinical development.

Protective Effects of Coumestrol on Metabolic Dysfunction and Its Estrogen Receptor-Mediated Action in Ovariectomized Mice

Coumestrol, a phytoestrogen compound found in various plants, has been shown to act as a potent estrogen receptor (ER) agonist, with a higher binding affinity for ERβ than for ERα. However, there is currently limited information regarding its beneficial effects in postmenopausal disorders and its ER-mediated mechanisms. Herein, we investigated the effects of coumestrol (subcutaneous or oral treatment) on metabolic dysfunction in ovariectomized (OVX) mice fed a high-fat diet, in comparison with the effects of 17β-estradiol (E2) replacement. Coumestrol was administered daily at a dose of 5 mg/kg for 10 weeks. Coumestrol treatment through the subcutaneous route stimulated uterine growth in OVX mice at a level lower than that of E2. E2 and coumestrol prevented body fat accumulation, adipocyte hypertrophy, and hepatic steatosis, and enhanced voluntary physical activity. Coumestrol showed estrogen-mimetic effects in the regulation of the protein expressions involved in browning of white fat and insulin signaling, including increased hepatic expression of fibroblast growth factor 21. Importantly, the metabolic effects of coumestrol (oral administration at 10 mg/kg for 7 weeks) were mostly abolished following co-treatment with an ERβ-selective antagonist but not with an ERα-selective antagonist, indicating that the metabolic actions of coumestrol in OVX mice are primarily mediated by ERβ. These findings provide important insights into the beneficial effects of coumestrol as a phytoestrogen supplement for the prevention and treatment of postmenopausal symptoms.

Bisphenol AF Promoted the Growth of Uterus and Activated Estrogen Signaling Related Targets in Various Tissues of Nude Mice with SK-BR-3 Xenograft Tumor

Environmental estrogens can promote the growth, migration, and invasion of breast cancer. However, few studies evaluate adverse health impacts of environmental estrogens on other organs of breast cancer patients. Therefore, the present study investigated the effects of environmental estrogen bisphenol AF (BPAF) on the main organs of female Balb/cA nude mice with SK-BR-3 xenograft tumor by detecting the organ development and gene expression of targets associated with G protein-coupled estrogen receptor 1 (GPER1)-mediated phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) and mitogen-activated protein kinase (MAPK) signaling pathways in hypothalamus, ovary, uterus, liver, and kidney. The results showed that BPAF at 20 mg/kg bw/day markedly increased the uterine weight and the uterine coefficient of nude mice compared to SK-BR-3 bearing tumor control, indicating that BPAF promoted the growth of uterus due to its estrogenic activity. Additionally, BPAF significantly up-regulated the mRNA relative expression of most targets related to nuclear estrogen receptor alpha (ERα) and GPER1-mediated signaling pathways in the hypothalamus, followed by the ovary and uterus, and the least in the liver and kidney, indicating that BPAF activated different estrogen activity related targets in different tissues. In addition, BPAF markedly up-regulated the mRNA expression of GPER1 in all tested tissues, and the molecular docking showed that BPAF could dock into GPER1. Because gene change is an early event of toxicity response, these findings suggested that BPAF might aggravate the condition of breast cancer patients through exerting its estrogenic activity via the GPER1 pathway in various organs.

Naringenin and Phytoestrogen 8-Prenylnaringenin Protect against Islet Dysfunction and Inhibit Apoptotic Signaling in Insulin-Deficient Diabetic Mice

It has been shown that citrus flavanone naringenin and its prenyl derivative 8-prenylnaringenin (8-PN) possess various pharmacological activities in in vitro and in vivo models. Interestingly, it has been proposed that prenylation can enhance biological potentials, including the estrogen-like activities of flavonoids. The objective of this study was to investigate the anti-diabetic potential and molecular mechanism of 8-PN in streptozotocin (STZ)-induced insulin-deficient diabetic mice in comparison with naringenin reported to exhibit hypoglycemic effects. The oral administration of naringenin and 8-PN ameliorated impaired glucose homeostasis and islet dysfunction induced by STZ treatment. These protective effects were associated with the suppression of pancreatic β-cell apoptosis and inflammatory responses in mice. Moreover, both naringenin and 8-PN normalized STZ-induced insulin-signaling defects in skeletal muscles and apoptotic protein expression in the liver. Importantly, 8-PN increased the protein expression levels of estrogen receptor-α (ERα) in the pancreas and liver and of fibroblast growth factor 21 in the liver, suggesting that 8-PN could act as an ERα agonist in the regulation of glucose homeostasis. This study provides novel insights into the mechanisms underlying preventive effects of naringenin and 8-PN on the impairment of glucose homeostasis in insulin-deficient diabetic mice.