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Physiological roles of activins in the human ovary

Chang, Hsun-Ming; Leung, Peter C.K.

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doi: 10.1097/JBR.0000000000000016
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Abstract

Introduction

Activins, members of the pleiotropic transforming growth factor-β (TGF-β) superfamily, are homo- or heterodimers of inhibin β subunits that have a broad range of biological effects in various tissues throughout the developmental stages of mammals.[1] Initially isolated and purified from porcine ovarian follicular fluid in 1986, activins were shown to induce the release of follicle stimulating hormone (FSH) from cultured anterior pituitary cells in rats.[2] It has been demonstrated that these multi-functional growth factors play essential roles not only in endocrine feedback control in the gonadotropin system but also in autocrine and paracrine modulation of both reproductive and nonreproductive tissues, including the brain, liver, kidneys, and ovaries.[1] In the human ovary, growing follicles express both inhibin βA and inhibin βB, resulting in the production of the following three main activins: activin A (βA and βA), activin B (βB and βB), and activin AB (βA and βB).[3–5] Additionally, follicular fluid obtained from antral follicles contains all mature forms of activin A, activin B, and activin AB in many species, including humans.[3,5] The expression and localization of these activins, activin functional receptors, and activin-binding proteins (inhibins, follistatins, and follistatin-like 3) in developing follicles suggest that activins may regulate follicular development in a paracrine/autocrine manner.[6,7]

Over the past few decades, the effects that activins exert on reproductive biology have been intensively investigated with regard to germ cell development, Sertoli cell proliferation, ovarian follicular development, endometrial repair following menstruation, decidualization, and pregnancy maintenance.[8] Moreover, studies have revealed that activins are involved in the modulation of the final stage of ovarian folliculogenesis, which is characterized by gonadotrophin responsiveness, steroidogenesis, oocyte maturation, cumulus cell expansion, extracellular matrix (ECM) formation, and luteal function.[8,9] However, most information regarding the functional roles of the activin system in the regulation of ovarian function was obtained from studies performed in animal models. Although the fundamental processes and regulatory mechanisms that govern ovarian biology are highly conserved across a diversity of mammalian species, the physiological roles of activins in the human ovary remain poorly characterized. Thus, in this article, we systemically review the extensive literature on the connections between activins and ovarian biology and disorders with the aim of promoting the development of potential novel preventive, diagnostic, and therapeutic approaches for patients suffering from activin-related reproductive abnormalities.

Activins and functional receptors

Activins and inhibins are functionally antagonistic extracellular signaling molecules that belong to the TGF-β superfamily.[10] These growth factors were first identified in the follicular fluid of bovine and porcine ovarian follicles, in which they were able to enhance (activins) or attenuate (inhibins) the production and secretion of FSH by anterior pituitary cells.[2,11] Similar to other members of the TGF-β superfamily, activins are disulfate-linked homo- or heterodimers composed of two inhibin β subunits. At present, four inhibin β subunits (βA, βB, βC, and βE) have been identified in mammals, including humans.[12–14] Combinations of these β subunits can potentially give rise to several homodimeric activin proteins (activin A, B, C, D, and E) and heterodimeric activin proteins (activin AB, AC, AD, and AE).[12–14] Inhibins are heterodimeric glycoproteins composed of an inhibin α subunit and one of the two inhibin subunits: βA in inhibin A and βB in inhibin B.[15]

Despite the fact that they share a relatively high sequence identity, inhibin βA and inhibin βB have been shown to play distinct functional roles in many target tissues and cells.[16] Their differential expression patterns and receptor- or antagonist-binding affinities were proposed as an explanation for the specific effects of their isoforms (activin A vs activin B). Indeed, inhibin βA and inhibin βB null mice exhibit subtly different phenotypes.[17,18] Transgenic mouse models constructed to exhibit both targeted depletion of the inhibin βA gene (Inha) and transgenic overexpression of the inhibin βB gene (Inhb) showed that the insertion of inhibin βB only partially rescued inhibin βA deletion-induced loss of function.[19]

First identified from a human liver cDNA library, the inhibin βC subunit shares high sequence identity with the inhibin βA and βB subunits (approximately 51% and 53%, respectively).[12] In the human reproductive system, the inhibin βC mRNA was localized in the epididymis, testis, ovary, and placenta.[20] Despite the formation of activin C, activin AC, and activin BC, the inhibin βC subunit does not heterodimerize with inhibin α to form inhibin C.[21] Compared with the major roles of activin A and activin B in the regulation of ovarian follicular function, the physiological role of activin C in the ovary is largely unknown. Inhibin βC subunits can heterodimerize with inhibin βA and βB subunits to form activin AC and activin BC, respectively.[21,22] However, the results of our previous studies showed that unlike activin A, B, and AB, activin AC lacks biological activity in the activin-responsive system.[23,24] The endogenous inhibin βC subunit has been proposed to antagonize other activins by heterodimerizing with inhibin βA or inhibin βB. Interestingly, overexpressing inhibin βC reduced the level of activin A both in vivo and in vitro.[25,26]

As members of the TGF-β superfamily, activins exert their cellular effects by binding to two transmembrane serine/threonine kinase receptors: type I and type II receptors.[27] Dimeric activins initially bind to two type II receptors, and the resulting ligand-receptor complex then recruits and activates two type I receptors, eventually leading to the phosphorylation of downstream intracellular signaling molecules.[28] To date, five distinct type II receptors (activin receptor type 2A [ACVR2A], activin receptor type 2B [ACVR2B], bone morphogenetic protein receptor type 2 [BMPR2], TGF-β receptor 2 [TβR2], and anti-Mullerian hormone receptor type 2 [AMHR2]) and seven type I receptors (activin receptor like-kinase 1–7, also known as ALK1–7) have been identified and shown to mediate the cellular activities induced by the members of the TGF-β superfamily.[28] Among these five type II receptors, ACVR2A and ACVR2B bind to activins with high affinity, and ALK4 (also known as activin receptor type IB, ACVR1B) is the primary type I receptor for activins.[29,30] In many cells, the binding activities of these receptors, when bound by activins, are regulated by extracellular activin-binding proteins, including follistatins and follistatin-related gene proteins.[31] Furthermore, activin-induced cellular signaling is controlled by specific intracellular receptor-interacting proteins that contain PZD domains, such as adaptors and scaffolding proteins.[31]

The activin signal transduction pathway

In the canonical TGF-β signaling pathways, ligand-receptor binding leads to the activation of type I receptors via the phosphorylation of their intracellular kinase domains, which in turn results in the activation and phosphorylation of downstream signal transducers, including receptor-regulated SMADs (R-SMADs).[27] Among the eight R-SMADs, SMAD2 and SMAD3 respond to activins and mediate the activin-induced signaling pathway.[27] Once activated, SMAD2/3 associates with a common SMAD (co-SMAD or SMAD4) to form a complex that can then translocate to the nucleus to interact with various transcription factors to regulate target gene expression.[28,32] In addition to the SMAD-mediated canonical pathway, other intracellular signaling cascades have been reported to be triggered by activins. For instance, mitogen-activated protein kinase family members, including the p38 mitogen-activated protein kinase, phosphoinositide 3-kinase/protein kinase B, and c-Jun N-terminal kinase, have been shown to mediate the downstream effects of activin-receptor complexes.[33–35] Taken together, these data indicate that the differential cell responses induced by activins may trigger either SMAD-dependent or SMAD-independent signaling pathways in a cell type-dependent manner.[36]

Expression of inhibin subunits and localization of activins in the human ovary

Most studies aiming to explore the expression of activins and activin receptors have been performed in mammalian ovaries, especially adult rat ovaries.[37] Various isoforms of activin and their cognate receptors are expressed during different stages in the developing follicles of rat ovaries. In addition, data collected from clinical samples have revealed that several activins are detected in follicular fluid. In this review, we focused on the expression profiles and potential physiological roles of activins in human ovaries. The expression, cellular localization, and follicular fluid concentration of activin/inhibin ligands are listed in Table 1.

Table 1
Table 1:
Localization of activin/inhibin ligands in the human ovary.

Structurally, the main isoforms of activins and inhibins are disulfide-linked homo- or heterodimers that form inhibin α, inhibin βA or inhibin βB. In human, these inhibin subunits have been identified in both normal and polycystic ovaries.[38–40] Specifically, Northern blot analysis showed that the inhibin α gene (INHA) was expressed in granulosa cells (GCs) and theca cells (TCs) in human preovulatory follicles.[40] Using immunohistochemical staining, one study demonstrated that the inhibin α, inhibin βA, and inhibin βB proteins were localized to GCs and TCs in antral follicles but not early preantral follicles in adult human ovaries.[41] Additionally, these inhibin subunits are highly expressed in the GCs and TCs of aromatase-positive antral follicles, indicating that activins and inhibins are involved in the modulation of ovarian steroidogenesis.[41] Furthermore, the fact that both inhibin α and inhibin βA mRNAs are localized in TCs indicates that the inhibin A in these cells may exert an autocrine effect.[39] Interestingly, inhibin α and inhibin βA are localized to GCs in atretic follicles, suggesting potential roles of activin A and inhibin A in regulating follicular atresia.[41] Our previous study also showed that the mRNAs and proteins of all 3 inhibin subunits (inhibin α, βA, and βB) were expressed in primary and immortalized luteinized GCs in periovulatory follicles obtained from patients who underwent in vitro fertilization.[42] A recent study performed using immunohistochemistry showed that inhibin βC was localized to oocytes, GCs, TCs, and the surface epithelium in normal human ovaries and GC tumors.[43] The same study also demonstrated that overexpressing inhibin βC in inhibin α-null mice modulated the development of abnormal early stage follicles, indicating that activin C plays a functional role in the rat ovary.[43] However, further studies are required to confirm this finding.

Clinical data obtained from patients undergoing in vitro fertilization have suggested that activins and inhibins are detectable in follicular fluid and that some of them are correlated with the quality of retrieved oocytes (Table 1). For instance, the follicular levels of both inhibin A and inhibin B are highly correlated with the number of recoverable oocytes.[44] However, the follicular fluid levels of both inhibins are not associated with subsequent fertilization rates of oocytes.[44] During the development of antral follicles, the follicular fluid levels of activin A declined as the follicles enlarged.[44]

Expression of activin receptors and signal transducers in the human ovary

In the human ovary, the cognate receptors and canonical signal transducers of activin ligands have been detected in different follicular compartments at various stages in growing follicles (Table 2). ACVR2A immunoreactivity was detected in oocytes obtained from secondary follicles and antral follicles.[41] Additionally, low levels of ACVR2A were identified in GCs that showed no reactivity in the TCs of secondary and antral follicles.[41] Immunostaining for ACVR2B was detected only in GCs and not in TCs in antral follicles.[41] Both SMAD2 and SMAD4 proteins were detected in oocytes and GCs in all sizes of preantral follicles.[41] The SMAD2 protein was also found in the GCs of antral follicles, while the SMAD4 protein was found in both GCs and TCs in antral follicles.[41] Furthermore, the results of our studies have demonstrated that the mRNAs and proteins of all SMADs (SMAD2, SMAD3, and SMAD4) are expressed in luteinized GCs in human periovulatory follicles (Table 2).[23]

Table 2
Table 2:
Localization of activin receptors and signal transducers in the human ovary.

Activins and primordial germ cell development

During the gastrulation stage of embryo development, a number of germline stem cells called primordial germ cells (PGCs) migrate to the primitive gonadal fold and later proliferate and increase in cell number.[45] These PGCs subsequently differentiate into primordial follicles, which are characterized as oocytes surrounded by somatic cells.[46] The molecular mechanisms by which growth factors modulate the developmental processes underlying the formation of primordial follicles are different between rodents and humans.[47,48] In mouse embryos, PGCs develop from pluripotent epiblast cells and are triggered by a synergistic effect between extraembryonic cell-derived bone morphogenetic protein (BMP)4 and BMP8B.[49] Additionally, the number of mouse PGCs can be increased by induction with visceral endoderm-derived BMP2.[49] In human fetal ovaries, BMP4-induced up-regulation promoted cell apoptosis and modulated the number of post-migratory PGCs.[48] Prior to the formation of human primordial follicles, the combined effect of oocyte-derived growth differentiation factor 9 and somatic cell-derived activin βA signaling determined selective germ cell survival.[50] Indeed, in vitro studies demonstrated that treatment with activin A increased the number of oogonia and enhanced the proliferation rate of oogonia.[50] Collectively, these findings suggest that activin plays a crucial role in regulating germ cell proliferation and primordial follicle formation in humans (Fig. 1).

Figure 1
Figure 1:
Schematic diagram summarizing physiological roles of activins in the human ovary. The putative physiological roles of activins in regulating human ovarian functions, including primordial germ cell development, follicular growth and development, ovarian steroidogenesis, ECM remodeling, oocyte maturation, ovulation, and luteal function are shown. COX-2 = cyclooxygenase-2, ECM = extracellular matrix, FSH = follicle stimulating hormone, GC = granulosa cell, LH = luteinizing hormone, LOX = lysyl oxidase, PGE2 = prostaglandin E2.

Activins and follicular development

The spatial and temporal expression pattern of activin subunits, functional receptors, and signal transduction molecules in human developing follicles indicate that activins may participate in the developmental processes observed in germ cells and their surrounding somatic cells. An in vitro study showed that treatment with exogenous activin A (100 ng/mL) increased the number of oogonia and enhanced the germ cell proliferation rate in cultured human ovarian fragments.[51] Targeted depletion of ACVR2A in mice increased the number of atretic follicles and decreased the number of corpora lutea.[17] Additionally, exogenous activin A has been shown to promote cell proliferation in GCs in both mice and rats.[52,53] These results provide evidence supporting the notion that activins promote the differentiation and development of primordial follicles into antral follicles during post-natal follicular development (Fig. 1).

Activins and ovarian steroidogenesis

In human, the products of ovarian steroidogenesis are indispensable not only for the maintenance of a normal female phenotype but also to support normal follicular functions. Any imbalance in the production of these steroids may lead to one of several hormonal and ovulatory dysfunctions, including anovulation, estrogen deficiency, abortion, infertility, premature ovarian insufficiency, and even endometrial cancers.[54] Therefore, the process of ovarian steroid synthesis must be highly modulated to sustain female reproductive health. Recent studies have shown that normal ovarian steroidogenesis is highly dependent on both endocrine (regulated by pituitary-derived gonadotropins, FSH and luteinizing hormone [LH]) and autocrine/paracrine (regulated by intra-ovarian factors) systems.[54] A previous study performed using various mammalian models, including humans, has indicated that activins are critical regulators of ovarian steroidogenesis.[8] Indeed, the targeted depletion of Smad proteins (Smad2, Smad3, or Smad4) in the GCs of mice resulted in an ovary-specific phenotype that manifested as cumulus cell defects and premature luteinization, indicating that an activin-mediated functional signaling event is required for normal ovarian steroid production (Fig. 1).[55,56]

Our previous studies showed that all three activin isoforms (activin A, B, and AB) regulated ovarian steroid production by inducing estradiol production while suppressing progesterone production in luteinized human GCs.[23,24] Additionally, we showed that gonadotropins, FSH and LH increased the production of estradiol and progesterone, while pretreatment with activins enhanced FSH-induced estradiol but suppressed LH-induced progesterone production in human GCs.[23] Specifically, activins increased estradiol production by up-regulating the expression of aromatase (a key enzyme for estradiol production) and enhanced the cellular effects of FSH by up-regulating the expression of FSH receptors.[23] In contrast, activins decreased progesterone production by down-regulating the expression of the steroidogenic acute regulatory protein (step-limited regulatory protein for progesterone production) and attenuated cell responsiveness to LH by down-regulating the expression of LH receptor.[23] All these findings suggest that activins may be involved in the maturation processes of growing follicles, most likely by acting as both a maturation stimulator and a luteinization inhibitor (Fig. 1). Moreover, there is evidence of crosstalk between intrafollicular activins and pituitary gonadotropins, which collaborates to regulate human ovarian functions.

Activins and oocyte maturation

Studies performed in animal models have demonstrated that activins play functional roles in the acquisition of oocyte competence during the final stages of mammalian oogenesis. The addition of recombinant activin A enhanced the in vitro maturation of bovine oocytes and their subsequent post-cleavage development.[57] Similarly, activin increased the proportion of cleaved bovine oocytes (cumulus-enclosed or cumulus-free oocytes) that reached the blastocyst stage.[58] In contrast, the addition of an activin antagonist, follistatin, to cultured bovine oocytes decreased the maturation rates of oocytes.[58] The effects of activins on oocyte maturation in humans remain unclear because of the limited availability of experimental samples. A prospective randomized controlled study performed using ovarian tissues obtained from reproductive women revealed that immature oocytes cultured in the presence of activin A exhibited higher incidences of germinal vesicle breakdown (an indicator of oocyte nuclear maturation) and development into fully mature (metaphase II, MII) oocytes.[59] These findings provide invaluable insights into the roles of activins in the regulation of oocyte maturation and suggest that these findings could be clinically applied to determine the optimal milieu to promote the developmental potential of immature oocytes obtained from infertile patients undergoing in vitro maturation (Fig. 1).

Activins and ECM remodeling

Recently, research regarding oocyte-somatic cell interactions has been of considerable interest.[54,60] Within ovarian follicles, oocytes act in a coordinated manner with their surrounding somatic cells to promote the maturation of the oocyte and the differentiation of follicles. The developmental processes underlying folliculogenesis and oogenesis depend on continuous signaling interactions between the oocyte and the somatic cells of the follicle. Additionally, intercellular communication between the germ cell and supporting somatic cells are heavily reliant on the well-developed architecture that supports the physical rigidity of the ovarian follicle.[61] In the ovarian follicle, the ECM performs several essential cell functions; for example, the ECM provides structural support, produces steroid hormones, allows various growth factors and cytokines access to the follicle, promotes oocyte maturation, and modulates important cellular activities (ie, proliferation, differentiation, aggregation, survival, and apoptosis).[61] Lysyl oxidase (LOX) initiates covalent cross-linking between ECM backbone components (elastin and collagen) and thereby acts as the key enzyme that triggers the formation and maintenance of ECM.[62] LOX is highly expressed in the GCs of many mammals, including humans.[63–65] Notably, the expression of LOX in GCs is positively correlated with the competence of the corresponding oocytes, indicating that ECM and its principal enzyme, LOX, play crucial roles in regulating follicular development and oocyte maturation.[66] Recent studies have indicated the involvement of intrafollicular factors in the regulation of LOX expression and activities. Among these factors, members of the TGF-β superfamily have been demonstrated to increase the expression and activity of LOX.[65,67,68] In particular, two structure-related members of the TGF-β superfamily, activin A and growth differentiation factor 8, increased LOX expression and activity by up-regulating connective tissue growth factor (also known as CCN2) in human GCs (Fig. 1).[65,68]

Activins and ovulation

In mammals, ovulation leads to rupture of an ovarian follicle and is a prerequisite event for fertilization and subsequent embryonic development. This specific reproductive event is triggered by the LH surge. In response to a surge in gonadotropin-releasing hormone, the LH surge up-regulates the expression of the following epidermal growth factor (EGF)-like growth factors: amphiregulin, epiregulin, and betacellulin.[69,70] These EGF-like growth factors, in turn, induce a series of morphological and functional changes in periovulatory follicles, including oocyte maturation, ECM modulation, and cumulus-oocyte complex expansion.[71] Because it lies downstream of the LH surge, prostaglandin E2 (PGE2) is a critical mediator that triggers the ovulation event.[72] Prostaglandins are synthesized by several enzymes, including the cyclooxygenase (COX) enzymes COX-1 and COX-2, which catalyze the limiting step of the production process.[73] We previously showed that all three EGF-like growth factors increased the production of PGE2 by up-regulating the expression of COX-2 in human luteinized GCs.[70] In the rat ovary, activin A induced premature superovulation and enhanced the formation of corpus luteum.[74] Although there is no direct evidence showing that members of the TGF-β superfamily are involved in the regulatory processes underlying ovulation in humans, our results indicate that activin A can increase the expression of COX-2 and the production of PGE2 in human GCs (Fig. 1).[75]

Activins and luteal function

Originally derived from GCs, TCs, capillaries, and fibroblasts, the corpus luteum is a glandular structure that secretes mainly progesterone, which plays a vital role in regulating the menstrual cycle and maintaining pregnancy.[76] After ovulation, interactions among many luteotropic and luteolytic mediators lead to the development of the corpus luteum, a process that is characterized by luteal formation, maintenance, and regression (if no conception occurs).[76] Therefore, luteinization is a highly orchestrated process that is controlled by various inducers (such as gonadotropins, FSH and LH) and inhibitors (such as growth factors derived from oocytes and TCs).[76] Before ovulation, these luteinization inhibitors persistently exert suppressive effects against the production of progesterone by growing follicles. The dysregulation of related luteinization inhibitors may result in premature luteinisation, affecting embryo implantation and placentation.[76] Activins are principal luteinization inhibitors that suppress basal and human chorionic gonadotropin-, FSH-, or LH-induced progesterone production in human GCs (Fig. 1).[23,77] In human GCs, the production of bioactive mature activin A can be induced by 2 theca-derived growth factors, BMP4 and BMP7.[42] All 3 intrafollicular factors (activin A, BMP4, and BMP7) regulate luteal function in a coordinated manner by suppressing the production of progesterone in human GCs.[23,78] Intriguingly, the addition of human chorionic gonadotropin in cultured human luteinized GCs up-regulated the expression of follistatin (activin-binding protein) to completely reverse the effects of endogenous activin A.[79] However, both inhibin α and inhibin βA are highly expressed in the corpus luteum, and the bioactive form of inhibin A reaches its peak level at the mid-luteal phase of the menstrual cycle, indicating a luteolytic role for this factor.[80]

Roles of activins in ovarian pathology

Accumulating evidence indicates that inhibin subunits and their cognate receptors and signaling transducers are expressed in the oocytes and their surrounding somatic cells. Indeed, activins and inhibins act as crucial regulators of follicular and oocyte development in the primordial follicle, preantral follicular, antral follicle, and corpus luteum in addition to exerting effects on ovulation. The dysregulation of the activin/inhibin system may affect PGC formation, follicular growth, GC proliferation, ovarian steroidogenesis, oocyte maturation, ovulation, and luteal function, leading to reproductive pathology, female infertility, or failure in childbirth. Relevant animal studies performed using transgenic mice (conditional knock-in or knock-out) have increased our understanding of the individual roles of activins and inhibins in regulating ovarian function. For example, the targeted depletion of inhibin α in mouse embryos led to ovarian tumors and infertility, indicating that inhibin α is a tumor-suppressor gene,[52] and Acvr2a deficient female mice exhibited infertility, indicating a functional role for activin signaling in reproduction.[17]

Activin A and reproductive aging

Reproductive aging is the gradual decline and eventual cessation of ovarian activity; this process can occur in women who experience a decline in fertility that precedes menopause by several years. This process is characterized by accelerated ovarian follicular development associated with a monotropic rise of FSH.[81] Studies have shown that this elevation in FSH serum levels is caused by a decrease in inhibin levels and an increase in activin levels.[82–84] Compared with cycling, young women, women of mid-reproductive and peri-menopausal ages have higher serum activin A levels.[82–84] Specifically, the serum levels of activin A were 2-fold higher throughout the menstrual cycle in females aged 43–47 years than in younger women (aged 19–38 years).[83] In contrast, older women have lower serum inhibin B levels during the follicular phase and lower serum inhibin A levels during the luteal phase.[83] Similarly, data from another clinical study showed that the follicular fluid concentrations of activin A and follistatin (but not inhibins) in the dominant follicle were significantly higher in women aged 40–45 years than in women aged 20–25 years.[85] Animal studies have shown that activin A promotes premature superovulation in the rat ovary.[74] These studies suggest that elevated activin A levels may induce premature superovulation in older women, leading to reproductive aging. We hypothesized that this result is most likely derived from the local effects of activin A in the ovary as the levels of FSH were not affected at this stage. Furthermore, an in vitro study performed using a microarray analysis showed that the transcripts of inhibin α and ACVR2B in human cumulus cells were lower in older women (older than 37 years).[86] Collectively, these findings suggest that serum levels of activin A level may be a reliable marker to represent female reproductive age.

Activins and polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common cause of anovulatory infertility (approximately 75%) and affects 5–10% of women of reproductive age.[87] This unique female endocrinopathy is characterized by two of the following three criteria: menstrual irregularity, hyperandrogenism, and polycystic ovary morphology.[87] Studies have shown that PCOS is associated with imbalanced ratios among activins and their binding proteins (antagonists), follistatins and inhibins. Two clinical cohort studies performed at the same time demonstrated that serum levels of activin were lower while serum levels of follistatin were higher in women with PCOS than in normal controls.[88,89] A follow-up study revealed that circulating activin A levels were lower while circulating follistatin and inhibin B levels were higher in women with PCOS.[90] However, a community-based population study did not confirm these previous findings and instead found that serum activin A and activin B levels were not different between women with PCOS and normal controls, even though serum follistatin levels were higher in women with PCOS.[91] Therefore, the dysregulation of the activin/inhibin system might be a causative factor of PCOS. Future studies aimed at addressing the pathophysiological roles of activins and inhibins in the pathogenesis of PCOS will be of great interest.

Activins and ovarian cancers

In human, activin A is regarded as a cancer modulator that is involved in carcinogenesis in various tissues and organs.[82,92,93] Depending on its interactions with other signaling pathways and the cell types involved, activin A may either enhance tumor progression or inhibit tumorigenesis in certain cancers.[92,94] Interestingly, while activin B acts as a cancer modulator in a manner similar to that of activin A,[95,96] activin C is regarded as a tumor suppressor.[97] One study found that in human ovarian cancers, the expression of inhibin α was decreased, while activin signaling activity was increased.[98] The same study group also found that activin C acted as an antagonist for activin A by competitively binding to ACVR2A and ACVR2B receptors, which further modulated downstream SMAD pathways.[98] Our in vitro study showed that activin A can increase cell proliferation in human GC tumors.[99] In contrast, an activin A-induced increase in cancer cell proliferation was attenuated by the addition of follistatin or the activin type I receptor inhibitor SB431542.[99] In addition, positive immunostaining for both inhibin βA and inhibin βB was detected in specimens obtained from ovarian malignant germ cell tumors.[100] Similarly, serum levels of activin B were higher in patients with GC tumors.[101] Indeed, animal studies have confirmed that in mice, the targeted depletion of inhibin α induced the overexpression of activin A and the subsequent development of ovarian tumors during the early fetal stage.[52] Taken together, the findings in the literature indicate that the overexpression of inhibin βA and inhibin βB or the dysregulation of the activin signaling pathway may be involved in the development of various human ovarian and germ cell tumors.

Clinical applications and therapeutic potential

Since activin βA signaling affected the formation of human primordial follicles and treatment with activin A increased the number and enhanced the proliferation rate of oogonia,[50] we hypothesize that enhancing agents that target ovary-specific activin signaling or activin functions could have therapeutic potential in patients with primary ovarian insufficiency who would like to preserve their fertility and ovarian function. Animal studies have provided insights into the role of activin A in the positive regulation of oocyte maturation.[57,58] A clinical trial performed in human also demonstrated that adding activin A enhanced nuclear maturation and developmental competence in human immature oocytes.[59] Thus, supplementation with activin isoforms in the culture medium could exert a beneficial effect on immature oocytes during the process of in vitro oocyte maturation.

In many tissues, activins act as pleiotropic regulators of development, metabolism, homeostasis, wound repair, immune response, and carcinogenesis.[102] The activin system has been shown to be dysregulated in inflammation, fibrosis, cachexia, and several cancers.[93] Over the past decade, studies have assessed the therapeutic potential and effects of activin antagonists, including follistatins, inhibins, synthetic activin antagonists, synthetic activin inhibitors, and soluble activin type II receptors.[103–105] Intriguingly, some of these activin antagonists have been successfully applied to reverse activin-induced cachexia in rodents.[103] However, further assessment is required to investigate the safety, efficacy, and administration route of these activin antagonist-based reagents before they can be translated into pharmaceutical applications in humans. At present, therapeutic blocking of activin signaling has not been applied in reproductive pathology or diseases. As our understanding of the expression, dysregulation, and molecular mechanisms involved in the activin system increases, the development of novel therapeutic strategies for patients with related ovarian pathologies, infertility, and germ cell cancers will become more plausible.

Conclusion

Initially identified as potent FSH stimulators, activins have been demonstrated to play multifaceted roles in ovarian biology, including primordial germ cell development, follicular growth and development, ovarian steroidogenesis, ECM remodeling, oocyte maturation, ovulation, and luteal function. Within the functional unit of the female reproductive system, locally produced intrafollicular activins act in synergistic and complementary ways to affect the growing follicle and promote the development of a mature, competent oocyte. The abnormal expression of activins or the dysregulation of activin signaling may cause infertility or several ovarian pathologies, such as reproductive aging, PCOS, and ovarian cancers. A detailed illustration of the expression profile, activities, molecular mechanisms of the members of the activin system that function in the ovary is essential to obtain a more comprehensive understanding of normal follicular functions and the mechanisms by which activins act as communicators between the pituitary and the ovaries. This information will promote the development of new therapeutic approaches for treating related ovarian diseases and female infertility.

Acknowledgments

We would like to thank all the subjects involved in the study for their participation.

Author contributions

HMC and PCKL participated in the design and writing of manuscript, performance of the literature review. Both authors approved the final version of the manuscript.

Financial support

This work was supported by the Foundation Scheme Grant FDN-143317 to PCKL.

Conflicts of interest

The authors declare that they have no conflicts of interest.

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Keywords:

activin; follicular development; ovarian aging; ovarian cancer; ovary; polycystic ovary

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