Equol inhibits growth, induces atresia, and inhibits steroidogenesis of mouse antral follicles in vitro

Abstract

Equol is a non-steroidal estrogen metabolite produced by microbial conversion of daidzein, a major soy isoflavone, in the gut of some humans and many animal species. Isoflavones and their metabolites can affect endogenous estradiol production, action, and metabolism, potentially influencing ovarian follicle function. However, no studies have examined the effects of equol on intact ovarian antral follicles, which are responsible for sex steroid synthesis and further development into ovulatory follicles. Thus, the present study tested the hypothesis that equol inhibits antral follicle growth, increases follicle atresia, and inhibits steroidogenesis in the adult mouse ovary. To test this hypothesis, antral follicles isolated from adult CD-1 mice were cultured with vehicle control (dimethyl sulfoxide; DMSO) or equol (600 nM, 6 μM, 36 μM, 100 μM) for 48 and 96 h. Every 24 h, follicle diameters were measured to monitor growth. At 48 and 96 h, the culture medium was subjected to measurement of hormone levels, and the cultured follicles were subjected to gene expression analysis. Additionally, follicles were histologically evaluated for signs of atresia after 96 h of culture. The results indicate that equol (100 μM) inhibited follicle growth, altered the mRNA levels of bcl2-associated X protein and B cell leukemia/lymphoma 2, and induced follicle atresia. Further, equol decreased the levels of estradiol, testosterone, androstenedione, and progesterone, and it decreased mRNA levels of cholesterol side-chain cleavage, steroid 17-α-hydroxalase, and aromatase. Collectively, these data indicate that equol inhibits growth, increases atresia, and inhibits steroidogenesis of cultured mouse antral follicles.

Effects of isoliquiritigenin on ovarian antral follicle growth and steroidogenesis

“…Previous studies with genistein, a commonly used phytoestrogen found in soy, showed that it could induce atresia of rat follicles [34] and inhibit growth of mouse antral follicles via Cdkn1a upregulation [35]. Equol, a metabolite of the soy phytoestrogen daidzein, was found to inhibit mouse antral follicle growth and increase follicle atresia [36]. The isoliquiritigenin-induced inhibition of follicle growth in the present study may thus potentially be due to disruption of cell cycle regulators causing cell cycle arrest, increased apoptosis causing follicle atresia, or a combination of both.…”

“…It was also found to decrease estradiol levels in porcine granulosa cells [44]. Equol was found to inhibit the production of estradiol, testosterone, and progesterone by mouse antral follicles in vitro [36]. Daily consumption of soy products containing genistein and daidzein for one month reduced circulating plasma levels of estradiol and progesterone in healthy, premenopausal women [42].…”

Urinary equol is associated with bioavailable testosterone but not total testosterone in women

“…One in vitro study showed that equol decreased the mRNA expression of CYP11A1, a key enzyme in the conversion of cholesterol to pregnenolone. As pregnenolone is the steroid precursor of testosterone, the downregulation of CYP11A1 reduced the production of testosterone in an in vitro ovarian follicle culture system [16]. Additionally, equol directly diminished the ability of antral follicles to produce testosterone by increasing the ratio of Bax to Bcl2 mRNA in these follicles and thus dysregulating the apoptotic pathway [16].…”

“…As pregnenolone is the steroid precursor of testosterone, the downregulation of CYP11A1 reduced the production of testosterone in an in vitro ovarian follicle culture system [16]. Additionally, equol directly diminished the ability of antral follicles to produce testosterone by increasing the ratio of Bax to Bcl2 mRNA in these follicles and thus dysregulating the apoptotic pathway [16]. Equol also inhibited the secretion of testosterone from antral follicles by reducing the levels of estrogens, thus inhibiting the production of cyclic adenosine monophosphate [46,47].…”

“…Equol, a major gut floradependent metabolite of daidzein [14], has a longer half-life [10] and stronger estrogenic/anti-estrogenic properties, compared with daidzein and genistein [11,15]. A few in vitro and animal studies revealed that equol exerted modulating hormone-like effects (e.g., testosterone [16][17][18][19][20], and SHBG [21,22]) and could alter the blood levels of androgens [23]. Accordingly, equol might affect blood androgens in humans.…”