Estrogen disorders: Interpreting the abnormal regulation of aromatase in granulosa cells (Review).

Aromatase is a cytochrome P450 monooxygenase that encodes CYP19A1 . Its expression is regulated via differential promoter activation in a tissue-specific manner. Aromatase mainly occurs in the ovaries ( 1 ), brain cells ( 2 ) and testes of rodents ( 3 ), as well as in human fat cells ( 4 ) and placental cells ( 5 ). Under the catalytic action of this enzyme, testosterone and androstenedione become demethylated, causing the A ring to be aromatized, to finally produce estrone and estradiol. Moreover, the transforming aromatase is localized in granulosa cells (GCs) of the ovarian follicles ( 6 ). Aromatase plays an important role in GCs. GCs and theca cells synthesize estrogen under the synergy of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In this process, first, the theca cells synthesize androgens and transfer them to ovarian GCs through the basement membrane, after which the androgens are converted into estrogen through the catalytic action of aromatase. The theory that the two types of cells together with LH and FSH function together in estrogen synthesis is collectively termed as '2-cell, 2-gonadotropin hypothesis' ( 7 ). Subsequently, most of the resultant hormone enters the blood-stream and acts on the target organs, such as the breast, while only a small amount of the hormone participates in the ovarian development. GCs are thus extremely important in the process of reproduction; their proliferation and growth determine the maturation of follicles and the production of estrogen. In healthy females prior to menopause, human estrogen is mainly derived from ovarian GCs and the placenta, and the expression of P450 aromatase is significantly higher in the GCs than in other tissues ( 8 ). Hence, GCs are considered as powerful models for studying aromatase and their mechanisms of action. Estrogen plays an important role in the female health and fertility status. It is mainly derived from 3 sources: Estrone is mainly converted from androstenedione of the adrenal gland through the skin and adipose tissue ( 9 , 10 ); estradiol (E2), the most widely effective estrogen, is mainly produced by GCs in the ovaries, and is the main estrogen product synthesized before menopause; estriol (E3), the weakest estrogen, is mainly synthesized in the placenta. Since the ovaries are the main organs which secrete estrogen, the normal expression of aromatase is of utmost importance. Presently, the association between aromatase and the ovarian GCs, as well as the regulatory mechanisms of this enzyme in GCs remain undefined. The present review thus focused on aromatase expression and the molecular regulatory mechanisms in ovarian GCs in order to help interpret estrogen disorders. Potential aromatase inhibitors (AIs) are also discussed an effort to open new research avenues for hormone-dependent diseases and fertility treatment influenced by estrogen-secretion disorders ( Table I ).

Environmental pollution and modification in diets are considered to be important elements affecting female fertility and health issues. Several diseases have been confirmed in relation to estrogen-secretion disorders in the epidemiology and experimental studies. In some patients with estrogen-dependent diseases, high levels of estrogen are accompanied by the overexpression of aromatase ( 15 ). For example, in breast cancer treatment, multiple AIs have been developed, such as exemestane, anastrozole, letrozole and vorozole. The current first-line treatment for breast cancer mainly uses third-generation AIs ( 110 , 111 ). In addition, AIs have begun to be used in the treatment of estrogen-regulated diseases, such as ovarian cancer and endometrial cancer, as well as inducing ovulation. Presently, AIs combined with progesterone and GnRH agonists are mainly used to treat women with endometriosis, along with reducing the risk of ovarian cysts induced by AIs alone. However, symptoms, such as pelvic pain may recur following AI treatment. For women with polycystic ovary syndrome and obesity, letrozole has a higher live birth rate compared with clomiphene citrate, and it has been regarded as the first-line therapy of inducing ovulation. Moreover, AIs are also a good first choice for women with infertility due to the presence of uterine fibroids, the wish to preserve the uterus, or being unsuitable for surgery ( 112 ). Although AIs are effective in the treatment of estrogen-dependent diseases and ovulation induction, osteoarthropathy, menopausal symptoms, intestinal discomfort and drug resistance, which are easily induced following treatment with AIs, remain concerns which require resolutions. A recent study designed and synthesized a novel aromatase inhibitor based on triazole and imidazole ( 113 ). However, the current inhibitors are mainly used in the treatment of breast cancer, albeit for other steroid-dependent diseases and female infertility, and there are a relatively few effective and specific drugs available ( 114 , 115 ), which indicates that the search for novel AIs and other effective drug targets is crucial. GCs, which are the main site of ovarian estrogen production, also generate a large amount of aromatase. Aromatase can promote the biosynthesis of estrogen in GCs, and the production of estrogen can promote the follicular development of GCs and inhibit the apoptosis of GCs. The transcription of genes is mainly controlled by the distal promoter I.1 of the placenta (at 40 kb upstream of the translation start site) and the proximal promoter II of the ovary ( 8 ). FSH can regulate the expression of aromatase by activating the PKA signaling pathway, while simultaneously activating the PI3K ( 85 ), ERK ( 87 ) and other signaling pathways, thereby upregulating the expression of CYP19A1 . The disadvantage of this approach is the lack of substantial research supporting whether the transient activation of P38MAPK caused by FSH can also regulate the expression of aromatase. In addition, owing to biodiversity, individual differences exist in the regulation of aromatase due to different factors, which may present new areas of interest in future research. Understanding the molecular modifications and the mechanisms of action in GCs that can be targeted in the disorders of aromatase secretion for the treatment of diseases caused by ovarian hormones is expected to develop high-efficiency, low-toxic, and side effects of specific drug targets.

The present article summarized and discussed the regulatory characteristics of the CYP19A1 promoter in ovarian GCs, as well as the regulation of aromatase as a downstream effector through multiple signaling pathways. The expression of aromatase in GCs is affected by various factors, and it is one of the main causes of estrogen-dependent diseases and PCOS. Endocrine-disrupting substances in the environment can cause alteration in the expression of aromatase and affect the normal reproduction and sexual differentiation in the human body. Furthermore, some insulin sensitizers have been developed as AIs for the clinical treatment. Other substances, such as FSH, IGF-1 and TGFβ have been proven to promote the expression of aromatase mRNA and protein, although miRNAs, HGF and leptin can partially inhibit or specifically identify the aromatase promoter and downregulate the aromatase transcription, leading to the disturbance in estrogen secretion. Thus, the discovery of potential aromatase inhibitor targets is expected to provide new directions for the treatment of estrogen-dependent diseases and PCOS.