Phytoestrogens and reproductive biology

Abstract

Phytoestrogens are naturally occurring plant substances that can either mimic or antagonize the action of endogenous estrogens. This is because of the similarity of the functional structure of phytoestrogens and endogenous estrogens. In premenopausal women, phytoestrogen intake might induce a decrease in luteinizing hormone, follicle‐stimulating hormone and estradiol (E₂), which are associated with a longer follicular phase. The circulating transport protein, sex hormone‐binding globulin, is increased, resulting in less cellular availability of E₂. Phytoestrogens inhibit the activities of E₂ synthetic enzymes through adenylate cyclase and tyrosine kinase cascades. This might decrease of risk of hormone dependent cancers. A phytoestrogen‐rich diet might reproduce normal body composition, affecting the course of polycystic ovary syndrome (PCOS). Some herbs used in traditional Japanese medicine contain phytoestrogens that influence endogenous hormone levels to directly regulate the pituitary‐ovarian system, in particular, the chemotactic effects on ovaries. (Reprod Med Biol 2005; 4: 225 –229)

INTRODUCTION

PHYTOESTROGENS ARE PLANT compounds with estrogen‐like properties. Certain plants are used in traditional Japanese medicine and plant therapy might be ascribed, such as the pomegranate being associated with fertility; ¹ the Thai vine, Pueraria mirifica, being used as a rejuvenant and aphrodisiac; ² and hops are believed to lower libido. ³ The Allen–Doisy bioassay for estrogens ⁴ was developed and showed that plant extracts exhibited estrogenic activity. ⁵ Since then, some plants have been found to show estrogenic activity on bioassay or to contain estrogenic compounds. ⁶ In addition, phytoestrogens are of economic importance, being linked to the infertility condition in sheep known as Clover Disease. ⁷

Phytoestrogens are naturally occurring plant substances that mimic estrogen, yet they are usually non‐steroidal structures and can either mimic or antagonize the actions of endogenous estrogens, ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² whereas phytoestrogens can competitively bind to estrogen receptors in the reproductive organs with a much lower affinity than naturally occurring estrogens. ¹³ , ¹⁴

These phytoestrogens can be divided into three categories: (i) the isoflavones found in legumes; (ii) the resorcylic acid lactones (estrogenic mycotoxins) produced by the fungus Fusarium roseum; and (iii) the coumestans that are also found in legumes. In reproductive medicine, genistein and daidzein, which are isoflavones found in soybeans and the leaves of the Indian breadroot (Psoralea corylifolia), ¹⁵ might be considered to be phytoestrogens which have some effect on both sexual development and normal reproductive function. In the present study, the relationship between phytoestrogens and reproductive biology has been reviewed.

EFFECTS OF PHYTOESTROGENS

IN TERMS OF the possibility that phytoestrogen intake could affect levels of endogenous hormones, an inverse relationship between urinary excretion of enterolactone and plasma luteinizing hormone (LH) level has been reported, ¹⁶ yet the causal link between phytoestrogen intake and circulating levels of endogenous estrogens remains to be established. High soy supplementation among postmenopausal women showed neither estrogenic effects observed in vaginal cytology nor significant changes in serum follicle‐stimulating hormone (FSH), LH and sex hormone‐binding globulin (SHBG) levels. ¹⁷ , ¹⁸ No significant effect of soy consumption on cycle length was reported. ¹⁹ On the contrary, although premenopausal women were fed diets consisting of different soybean products with isoflavones, follicular phase length was significantly increased and peak progesterone, FSH and LH levels were suppressed. ²⁰ Daily flax seed supplementation increased luteal phase duration but with no difference in follicular phase length in normally cycling women. ²¹ In premenopausal women who were fed daidzein and genistein, there was a significant decrease in estradiol (E₂) and dehydroepiandrostenedione sulfate, and a non‐significant increase in menstrual cycle length, ²² yet there was an endogeneous influence on the absorption and metabolism of the compounds. ²³

Menstrual cycle length was found to be shorter for breast cancer patients than for control subjects. ²² It might be consistent with the evidence that menstrual cycle length is 2–3 days longer in Asian women than in Western women, which could be partly because of the consumption of substantial amounts of phytoestrogens present in soy. ¹⁶ , ²⁴ Some of the reported biological effects of diets consisting of soy products containing isoflavones are similar to those induced by the potent synthetic antiestrogen, tamoxifen. Tamoxifen, when used therapeutically in breast cancer patients, led to a decrease in circulating concentrations of LH and FSH. ²⁵ The decrease in LH and FSH levels were consistently associated with a longer follicular phase. At the present time, it might be possible for phytoestrogens to be attributed to hormonal changes in women with high soy consumption.

Estrogens and androgens are bound to transport proteins SHBG and albumin in the circulation. Only a small portion of these steroids (e.g. <2%) are transported in the free form, which is thought to be biologically active and taken up by the tissues. Therefore, steroidal activity is dependent on serum concentration changes in SHBG, which is synthesized in the liver. Lignans and isoflavones stimulate the synthesis of SHBG by liver cancer cells in culture. ²⁶ , ²⁷ Urinary lignan concentrations were positively or inversely correlated with SHBG or free E₂ concentration, respectively. ¹⁸ However, the observational associations between phytoestrogens and SHBG found in clinical trials involving phytoestrogen intake were not found in several short‐term studies. ¹⁸ , ¹⁹ , ²⁸ SHBG, which plausibly influenced endogeneous hormonal levels, did not change in levels with a diet of soy protein. ²⁰ , ²¹ A causal association between intake levels of phytoestrogens and an increase in SHBG concentrations remains to be minimal or plausible.

Polycystic ovary syndrome (PCOS) is associated with anovulation, hypersecretion of androgen, LH and insulin, and abnormal insulin‐like growth factor metabolism. These are unlikely to be the result of a single autosomal genetic defect. ²⁹ Phytoestrogens are known to cause a polycystic ovary‐like anovulatory condition in animals, ⁷ and in some reports,menstrual cycle length might be changed in women on a soy diet. ¹⁷ , ²⁰ The cellular activities of insulin and insulin‐like growth factor, related to PCOS pathogenesis, are mediated by tyrosine kinases, which might be inhibited by isoflavones as described later. Nutrition appears to modulate the progress of PCOS, often with resumption of ovulation after weight loss. A soy bean diet might normalize nutrition and reproduction, whereas obese women become ovulatory when slim. ³⁰ The possibility of dietary phytoestrogens affecting the progress of PCOS in humans might be plausible.

EFFECTS OF HERBS

TRADITIONAL HERBAL MEDICINE, for example Unkei‐to, uses herbs such as Panax ginseng, Opiopogonis tuber and Glycyrrhizale radix, which contain steroids and isoflavones. It might be effective in the treatment of pituitary–ovarian dysfunction in young women. ³¹ In particular, Unkei‐to has been found to stimulate the normal pituitary reponse of gonadotropin‐releasing hormone (GnRH) and improve normal pulsatile secretion of FSH and LH, resulting in an induction of ovulation. ³¹ In anovulatory patients with high plasma LH concentrations, including patients with PCOS, Unkei‐to has been found to induce significant improvement of the plasma LH level ³¹ and directly stimulate the synthesis and release of LH and FSH in cultured rat pituitary cells, ³² as well as the secretion of steroid hormones in rat ovarian cell culture. ³³ These findings suggest that Unkei‐to has direct regulatory effects on the rat pituitary–ovarian system, although the effect of Unkei‐to on the human ovary remains to be clarified.

In addition to known endocrine factors such as gonadotropic and ovarian steroid hormones, certain cytokines have been proposed to play endocrine and/or paracrine roles in mammalian ovarian follicular development and ovulation. ³⁴ , ³⁵ , ³⁶ More recent reports support the existence of an intraovarian cytokine system that includes receptors and binding proteins. ³³ , ³⁶ , ³⁷ Consideration of these findings leads to the possibility that Unkei‐to might directly regulate ovarian function, at least in part by inducing the secretion of cytokine. Unkei‐to was found to stimulate the secretions of E₂ and progesterone from highly luteinized granulosa cells obtained from in vitro fertilization patients; the effect on E₂ secretion occurred with 0.3 µg/mL, whereas a significant effect on progesterone secretion was obtained at 10 µg/mL. The Unkei‐to stimulation of E₂ secretion could be accounted for by the effects of its ingredients; Panax ginseng, Paeonia lactiflora Pallas and Cinnamomun cassia Blume. Exposure of the cells to Unkei‐to caused dose‐dependent increases in the concentrations of interleukin (IL)‐1β, IL‐6 and IL‐8 in the culture medium. ³⁸ These results indicate that Unkei‐to has direct stimulatory effects on human granulosa cells to stimulate the steroidogenesis and the secretion of cytokines. The various beneficial actions of Unkei‐to on the ovary might result from a combination of different herbal ingredients with stimulatory effects on both steroidogenesis and the ovulatory process within the ovary, as well as with stimulatory effects on the hypothalamus–pitutary axis.

EFFECTS ON METABOLIC TRANSDUCTION

THE ACTION OF genistein is targeted on steroidogenic enzymes, whereas genistein inhibits the conversion of estrone to E₂ by inhibiting 17β‐hydroxysteroid dehydrogenase (17β‐HSD) type 1, ³⁹ therefore exerting antiestrogenic effects by inhibiting the production on active estrogen, E₂. Genistein inhibits the reductive/oxidative activity of 17β‐HSD type 5, inhibiting the conversions of androstenedione to testosterone and androstenediol to androsterone, respectively. ⁴⁰ In addition, although testosterone and androstenedione are substrates for the action of aromatase, which converts testosterone to E₂ and androstenedione to estrone, genistein might inhibit the conversion of these androgenic precursors to E₂ or estrone in the estrogen‐dependent tissues, in terms of the evidence that genistein exerts a weak inhibitory activity on aromatase. ⁴¹

Genistein is a potent inhibitor of tyrosine kinase activity. ⁴² The stimulatory pathway for steroidogenesis is through a cyclic adenosine monophosphate (AMP)‐stimulated signaling system. Cross‐talk between adenylate cyclase‐and tyrosine kinase (TK)‐dependent signaling pathways controls steroidogenesis. ⁴³ Genistein directly inhibits 3 and 17β‐hydroxysteroid dehydrogenase activity. ³⁹ Genistein is a non‐steroidal phytoestrogen with the molecular structure of a polycyclic phenol, ⁴⁴ which inhibits epidermal growth factor (EGF) receptor and various tyrosine kinases. ⁴³ , ⁴⁵ Genistein blocks EGF‐, insulin‐ and growth hormone (GH)‐induced, and tyrosine mitogen‐activated protein (MAP)‐kinase mediated proliferations of several fibroblastic cell lines. ⁴⁶ , ⁴⁷ In terms of the effects of genistein on reproductive tissues, genistein prevented EGF‐induced and tyrosine kinase‐mediated suppressions of apoptosis in rat follicles and granulose cells, ⁴⁸ and inhibited ovarian follicular development in the hamster, ⁴⁹ nuclear maturation of mouse ⁵⁰ , ⁵¹ , ⁵² and pig ⁵⁰ , ⁵¹ , ⁵² , ⁵³ oocytes in vitro, and preimplantation mouse embryo development. ⁵¹ The effects of lower doses of genistein on oogenesis and embryogenesis and on the secretory activity of mammalian ovarian cells are generally stimulatory. ⁵⁴ Genistein is a specific and potent inhibitor of the protein tyrosine kinase (PTK) activity of the EGF receptor, ⁴² suggesting that the anticancer effects of genistein might be mediated by its inhibitory effects on crucial PTK. ⁵⁶ The inhibitory action of genistein on PTK activity in relation to reproductive function remains to be studied, with most studies focusing on its estrogenic and antiestrogenic actions. Numerous growth factors ⁵⁶ and cytokines ⁵⁷ transmit modulatory effects of phytoestrogens on steroidogenesis and on their signal transduction mechanisms involving PTK in the reproductive organs. Thus, it is possible that phytoestrogens might influence some of these paracrine and autocrine actions and alter the normal processes of steroidogenesis in reproduction.

EFFECTS ON DEVELOPMENT

ALTHOUGH LEGUMES HAVE provided humans and animals with a major source of protein, ⁵⁸ there have been no reports on known effects of phytoestrogens in fetuses and infants. Some studies on coumestrol, one of the coumestan phytoestrogens, suggest that neonatal exposure to this compound in markedly higher than normal dietary doses might affect sexual differentiation in rats. ⁵⁹ Coumestrol is less commonly found in human diets than isoflavones. Neonatal exposure to genistein (an isoflavone) prevented the formation of mammary cancer in rats. ⁶⁰ The interperpretation is complex. Species differences ⁶¹ and age differences ⁶² affect the type of estrogen agonist or antagonist activity of different compounds. Extrapolation directly to humans is not plausible.

CONCLUSION

THE RELATIONSHIP BETWEEN phytoestrogen and reproductive biology has been reviewed. Phytoestrogens occur in plant diets and traditional medicines, plausibly exerting the beneficial effects on female reproduction. They show direct and indirect effects on the pituitary–ovarian axis. Those effects might be shown in the improvement of ovulatory disorders and in the decrease of the risk of hormone‐dependent cancers. These benefits, therefore, might be obtained by the intake of a phytoestrogen‐rich diet.