Estradiol Signaling at the Heart of Folliculogenesis: Its Potential Deregulation in Human Ovarian Pathologies

Abstract

Estradiol (E2) is a major hormone controlling women fertility, in particular folliculogenesis. This steroid, which is locally produced by granulosa cells (GC) within ovarian follicles, controls the development and selection of dominant preovulatory follicles. E2 effects rely on a complex set of nuclear and extra-nuclear signal transduction pathways principally triggered by its nuclear receptors, ERα and ERβ. These transcription factors are differentially expressed within follicles, with ERβ being the predominant ER in GC. Several ERβ splice isoforms have been identified and display specific structural features, which greatly complicates the nature of ERβ-mediated E2 signaling. This review aims at providing a concise overview of the main actions of E2 during follicular growth, maturation, and selection in human. It also describes the current understanding of the various roles of ERβ splice isoforms, especially their influence on cell fate. We finally discuss how E2 signaling deregulation could participate in two ovarian pathogeneses characterized by either a follicular arrest, as in polycystic ovary syndrome, or an excess of GC survival and proliferation, leading to granulosa cell tumors. This review emphasizes the need for further research to better understand the molecular basis of E2 signaling throughout folliculogenesis and to improve the efficiency of ovarian-related disease therapies.

Keywords: estradiol; estrogen receptors isoforms; folliculogenesis; granulosa cell tumors; granulosa cells; polycystic ovary syndrome.

Figure 1

Schematic representation of ESR2 gene and human ERβ protein isoforms (ERβ1–5). Within the 5′ untranslated region of ESR2, three distinct promoters are found (OK, ON, and E1). Exons 1–8 are represented by boxes, and the introns are represented by lines. Exons are represented in different colors, and their encoding protein domains are represented by the corresponding colors. The use of alternative acceptor sites leads to the production of five different proteins (ERβ1, ERβ2, ERβ3, ERβ4, and ERβ5). All these isoforms share common exons 1–7 but differ in their alternative exon 8 (b, d, e, c, and a respectively). For protein isoforms, from the N-terminus to C-terminus, the N-terminal domain (NTD)/activation function 1 (AF-1) is colored in blue, the DNA binding domain (DBD) is colored in dark gray, the hinge domain is colored in clear brown, and the ligand-binding domain (LBD)/activation function 2 (AF-2) is colored in dark blue. At the end of the protein, the alternative splicing produces proteins identical in their first 468 amino acids but differing in the sequence corresponding to the end of the ligand binding of ERβ1, which modifies their capacity to bind ligands. Numbers indicate the amino acids of the protein.

Figure 2

Working model for the role of E2 signaling in PCOS follicles. In granulosa cells (GC) of small antral follicles, FSH stimulates the expression of the aromatase that converts thecal-derived androgens (i.e., testosterone (T) produced upon LH stimulation) to estrogens (E2). E2 contained in the follicular fluid (FF) synergizes with FSH to promote follicular growth and further participates in the selection of the dominant preovulatory follicle [1,3]. The FF of normal antral follicle contains a large set of proteins and hormones such as high levels of inhibins and E2 together with low concentrations of anti-Müllerian hormone (AMH), interleukin-6 (IL-6), epidermal growth factor (EGF), or T. Contrary to thecal cells that principally express ERα, GC mainly express ERβ isoforms [29,30]. Among them, only ERβ1 binds to E2 [20]. ERβ1 as well as ERβ4 possess pro-apoptotic activities that may influence follicles’ fate [30]. In PCOS follicles, abnormal high levels of LH stimulate the T production of thecal cells. In addition to deficit in FSH, endogenous inhibitors (principally steroids) present in the FF [101,102] down-regulate aromatase activity, resulting in lower E2 production. Lower levels of E2 might participate in a higher production of AMH, IL-6, and/or EGF present in PCOS FF [83,105,115,118,119]. Low intra-follicular E2 concentrations may compromise the maturation of developing follicles and favor the accumulation of small antral follicles. The GC of PCOS follicles prematurely express LH receptors (LH-R) [80], which participate in the early maturation of these follicles. The expression levels of each ERβ isoform are still unknown. Specific ERβ isoforms expression might influence follicular fate and contribute to the high number of follicles that characterizes this pathology [78]. LH: Luteinizing hormone; FSH: Follicle-stimulating hormone; LH-R: LH receptor.

Figure 3

Working hypothesis for a role of E2 signaling in GCT. ERβ is expressed in all GCT [31,136,137]. There is a heterogeneity of ER subtypes expression among GCT [31,136,137,139,140,141,142]. In GCT, all ERβ isoforms are localized in the nucleus [31,33], except for ERβ2, which is also present in the cytosol, possibly interacting with mitochondrial Bcl-2 [33] to promote GCT survival. GCT produce E2 that could bind to GPER1 and promote the inhibition of GCT migration and invasion, in the absence of ERα [135] (A). In addition, E2 could bind to ERβ1, which would either homodimerize or heterodimerize with ERβ5 or ERβ2 [20], in the absence of ERα, to regulate pro-survival or pro-apoptotic gene expression, through DNA interactions with ERE, AP-1, or SP-1 sites [56]. ERβ1, ERβ2, and ERβ5 might also trigger non-genomic pathways that still need to be identified in GCT. (B) In the presence of ERα, E2 binding would induce its homo- or heterodimerization with other ER subtypes that would finally regulate the expression of other sets of pro-survival genes. ERβ isoforms inhibit the transcriptional activity of ERα [22]. In addition, E2 could also bind to ERβ1 or ERα, which are expressed in endothelial cells, to respectively reduce or stimulate angiogenesis [166,167]. The presence of specific ER subtypes probably orientates the fate of GCT.