Trends in Endocrinology & Metabolism
Dose-dependent effects of phytoestrogens on bone
Introduction
Phytoestrogens are plant-derived non-steroidal compounds that bind to estrogen receptors (ERs) and have estrogen-like activity [1]. They have attracted much attention among public and medical communities because of their potential beneficial role in prevention and treatment of cardiovascular diseases, osteoporosis, diabetes and obesity, menopausal symptoms, renal diseases and various cancers 2, 3. They can provide health benefits only when consumed at sufficient levels [1]; conversely, they have been categorized as endocrine disruptors that cause environmental problems and deleterious effects on reproductive systems [4]. Some phytoestrogens are well known enzyme inhibitors [5] but their biological effects differ from those of estrogen, antiestrogen and tyrosine kinase inhibitors [6]. Therefore, identifying their molecular mechanisms of action is essential for understanding both the beneficial and adverse effects of phytoestrogens.
Proposed molecular mechanisms are based on their ER-mediated and/or enzyme-inhibiting effects. ER-mediated action has focused on the preferential binding of phytoestrogens to ERβ, which acts as a dominant-negative regulator of estrogen signalling 7, 8. Some phytoestrogens inhibit tyrosine kinases, and mitogen-activated protein kinases (MAPKs), which are crucial for cellular functions [5]. However, the biological effects of phytoestrogens cannot be fully explained by these mechanisms [6].
We recently identified peroxisome proliferator-activated receptors (PPARs) as additional molecular targets of phytoestrogens 9, 10. PPARs are crucial targets in many Western diseases [11] in which phytoestrogens appear to have beneficial effects. Regardless of their enzyme inhibitory actions, phytoestrogens can dose-dependently activate PPARs and induce divergent effects on osteogenesis and adipogenesis. As a result, the balance between concurrently activated ERs and PPARs determines the dose-dependent biological effects of phytoestrogens. As shown in our studies, dominant ER-mediated effects (an increase in osteogenesis and a decrease in adipogenesis) can only be seen at low concentrations of phytoestrogens, whereas dominant PPARγ-mediated effects (a decrease in osteogenesis and an increase in adipogenesis) are only evident at high concentrations. Therefore, the dose-dependent effects of phytoestrogens on bone should be emphasized. Furthermore, even when phytoestrogens exert dominant ER- or PPARγ-mediated effects, their pleiotropic actions exist in a wide range of concentrations. For example, the major phytoestrogen genistein at a physiologically obtainable concentration of 5 μM exerted typical estrogenic effects in osteoprogenitor KS483 cells – upregulation of osteogenesis and downregulation of adipogenesis 9, 12. However, when ERs were blocked by the antiestrogen compound ICI182780 (AstraZeneca, www.astrazeneca.com), genistein downregulated osteogenesis and upregulated adipogenesis (Figure 1). These results suggest that genistein at 5 μM concurrently activated both ERs and PPARγ [9]. This review highlights these pleiotropic actions and the implications for finding precise beneficial doses in vivo and in clinical trials.
Section snippets
Phytoestrogens
Phytoestrogens are divided into three classes: isoflavones, coumestans and lignans. In addition, some flavonoids, such as apigenin, quercetin, kaempferol and naringenin, are also classed as phytoestrogens [1]. The most extensively studied phytoestrogens are isoflavones, which are found in high concentrations in soybeans (∼0.2–1.6 mg g−1 dry weight) [1]. Depending on diet, human plasma concentrations of isoflavones can vary by more than 100-fold, and can reach 6 μM [1]. Rodent diets used in
Dose-dependent effects of isoflavones on bone in vitro
Isoflavones stimulate osteogenesis at low concentrations and inhibit osteogenesis at high concentrations in osteoblasts and osteoprogenitor cells 9, 10. Isoflavones also influence the production of cytokines derived from osteoblasts and osteoprogenitor cells in a similar biphasic manner [15]. By contrast, bone resorption data show that, instead of having biphasic effects, isoflavones only inhibit osteoclast formation and activity [16].
Effects of isoflavones on bone in vivo
Biphasic dose-dependent responses have also been seen in vivo, suggesting that isoflavones might influence bone formation rather than bone resorption [1]. There are also data indicating that isoflavones affect both bone formation and bone resorption [16]. Owing to the limited dose selection, time and length of exposure, and different responses in various models, it is not surprising that some experiments showed bone-sparing effects, whereas others did not, or even showed contradictory results 1
ER-mediated actions
Isoflavones bind to both ERα and ERβ with different affinities. They bind to ERα with an affinity that is ∼100 times lower than that of E2. By contrast, there is a great difference in their binding affinities for ERβ. With the exception of genistein, which has a similar binding affinity for ERβ to that of E2, isoflavones have a lower affinity for ERβ 45, 46. However, the binding affinity is unlikely to account entirely for distinct transcriptional actions [8].
ERα mediates most actions of
Conclusions
Biphasic dose-dependent effects of phytoestrogens are the result of the concurrent activation of ERs and PPARs, which produce divergent actions in the same cell–tissue system (Figure 4). This molecular mechanism can explain the dose-dependent effects of phytoestrogens with or without enzyme-inhibiting action. Although this mechanism was proposed on the basis of observations in bone, it could also apply to other cellular systems. Indeed, using ER-positive or negative MCF7, T47D and MDA-231
Acknowledgements
We thank S.E. Papapoulos and J.A. Romijn for their critical reading of this manuscript.
References (66)
- et al.
Beneficial role of dietary phytoestrogens in obesity and diabetes
Am. J. Clin. Nutr.
(2002) Phyto-oestrogens
Best Pract. Res. Clin. Endocrinol. Metab.
(2003)Cell signaling and regulators of cell cycle as molecular targets for prostate cancer prevention by dietary agents
Biochem. Pharmacol.
(2000)Soy isoflavones-phytoestrogens and what else?
J. Nutr.
(2004)Estrogen receptor β-selective transcriptional activity and recruitment of coregulators by phytoestrogens
J. Biol. Chem.
(2001)Peroxisome proliferator-activated receptor γ (PPARγ) as a molecular target for the soy phytoestrogen genistein
J. Biol. Chem.
(2003)PPARs and the complex journey to obesity
Nat. Med.
(2004)- et al.
Animal models impacted by phytoestrogens in commercial chow: implications for pathways influenced by hormones
Lab. Invest.
(2001) - et al.
Dietary phytoestrogens and their effect on bone: evidence from in vitro and in vivo, human observational, and dietary intervention studies
Am. J. Clin. Nutr.
(2003) Physiological concentrations of genistein stimulate the proliferation and protect against free radical-induced oxidative damage of MC3T3-E1 osteoblast-like cells
Nutr. Rev.
(2001)
Daidzein enhances osteoblast growth that may be mediated by increased bone morphogenetic protein (BMP) production
Biochem. Pharmacol.
Stimulatory effect of daidzein in osteoblastic MC3T3-E1 cells
Biochem. Pharmacol.
Effects of phytoestrogens and environmental estrogens on osteoblastic differentiation in MC3T3-E1 cells
Toxicology
The effects of estrogen on osteoprotegerin, RANKL, and estrogen receptor expression in human osteoblasts
Bone
Effects of genistein on expression of bone markers during MC3T3-E1 osteoblastic cell differentiation
J. Nutr. Biochem.
Inhibitory effect of genistein on bone resorption in tissue culture
Biochem. Pharmacol.
Difference in effective dosage of genistein on bone and uterus in ovariectomized mice
Biochem. Biophys. Res. Commun.
The clinical importance of the metabolite equol – a clue to the effectiveness of soy and its isoflavones
J. Nutr.
Soy protein and isoflavones: their effects on blood lipids and bone density in postmenopausal women
Am. J. Clin. Nutr.
Beyond ERα and ERβ: estrogen receptor binding is only part of the isoflavone story
J. Nutr.
Cloning and function of rabbit peroxisome proliferator-activated receptor δ/β in mature osteoclasts
J. Biol. Chem.
Peroxisome proliferator-activated receptor activators modulate the osteoblastic maturation of MC3T3-E1 preosteoblasts
FEBS Lett.
Peroxisome proliferator-activated receptors α and γ are activated by indomethacin and other non-steroidal anti-inflammatory drugs
J. Biol. Chem.
Binding of prostaglandins to human PPARγ: tool assessment and new natural ligands
Eur. J. Pharmacol.
Soy isoflavones exert antidiabetic and hypolipidemic effects through the PPAR pathways in obese Zucker rats and murine RAW 264.7 cells
J. Nutr.
Activation of peroxisome proliferator-activated receptor-γ inhibits the Runx2-mediated transcription of osteocalcin in osteoblasts
J. Biol. Chem.
Activation of peroxisome proliferator-activated receptor-γ pathway inhibits osteoclast differentiation
J. Biol. Chem.
Frequency of stromal lineage colony forming units in bone marrow of peroxisome proliferator-activated receptor-α-null mice
Bone
Expression of peroxisome proliferator-activated receptor PPARδ promotes induction of PPARγ and adipocyte differentiation in 3T3C2 fibroblasts
J. Biol. Chem.
Peroxisome proliferator-activated receptor δ (PPARδ)-mediated regulation of preadipocyte proliferation and gene expression is dependent on cAMP signaling
J. Biol. Chem.
Dietary phyto-oestrogens and bone health
Proc. Nutr. Soc.
Phytoestrogen-low diet for endocrine disruptor studies
J. Agric. Food Chem.
Estrogen receptor β acts as a dominant regulator of estrogen signaling
Oncogene
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