Evolving roles of activins and inhibins in ovarian cancer pathophysiology

Activins and inhibins are unique members of the transforming growth factor-β (TGFβ) family of growth factors, with the ability to exert autocrine, endocrine, and paracrine effects in a wide range of complex physiologic and pathologic processes. Although first isolated within the pituitary, emerging evidence suggests broader influence beyond reproductive development and function. Known roles of activin and inhibin in angiogenesis and immunity along with correlations between gene expression and cancer prognosis suggest potential roles in tumorigenesis. Here, we present a review of the current understanding of the biological role of activins and inhibins as it relates to ovarian cancers, summarizing the underlying signaling mechanisms and physiologic influence, followed by detailing their roles in cancer progression, diagnosis, and treatment.

Activin B and Activin C Have Opposing Effects on Prostate Cancer Progression and Cell Growth

Current prognostic and diagnostic tests for prostate cancer are not able to accurately distinguish between aggressive and latent cancer. Members of the transforming growth factor-β (TGFB) family are known to be important in regulating prostate cell growth and some have been shown to be dysregulated in prostate cancer. Therefore, the aims of this study were to examine expression of TGFB family members in primary prostate tumour tissue and the phenotypic effect of activins on prostate cell growth. Tissue cores of prostate adenocarcinoma and normal prostate were immuno-stained and protein expression was compared between samples with different Gleason grades. The effect of exogenous treatment with, or overexpression of, activins on prostate cell line growth and migration was examined. Activin B expression was increased in cores containing higher Gleason patterns and overexpression of activin B inhibited growth of PNT1A cells but increased growth and migration of the metastatic PC3 cells compared to empty vector controls. In contrast, activin C expression decreased in higher Gleason grades and overexpression increased growth of PNT1A cells and decreased growth of PC3 cells. In conclusion, increased activin B and decreased activin C expression is associated with increasing prostate tumor grade and therefore have potential as prognostic markers of aggressive prostate cancer.

Characterizing Endocrine Status, Tumor Hypoxia and Immunogenicity for Therapy Success in Epithelial Ovarian Cancer

Epithelial ovarian cancer is predominantly diagnosed at advanced stages which creates significant therapeutic challenges. As a result, the 5-year survival rate is low. Within ovarian cancer, significant tumor heterogeneity exists, and the tumor microenvironment is diverse. Tumor heterogeneity leads to diversity in therapy response within the tumor, which can lead to resistance or recurrence. Advancements in therapy development and tumor profiling have initiated a shift from a "one-size-fits-all" approach towards precision patient-based therapies. Here, we review aspects of ovarian tumor heterogeneity that facilitate tumorigenesis and contribute to treatment failure. These tumor characteristics should be considered when designing novel therapies or characterizing mechanisms of treatment resistance. Individual patients vary considerably in terms of age, fertility and contraceptive use which innately affects the endocrine milieu in the ovary. Similarly, individual tumors differ significantly in their immune profile, which can impact the efficacy of immunotherapies. Tumor size, presence of malignant ascites and vascular density further alters the tumor microenvironment, creating areas of significant hypoxia that is notorious for increasing tumorigenesis, resistance to standard of care therapies and promoting stemness and metastases. We further expand on strategies aimed at improving oxygenation status in tumors to dampen downstream effects of hypoxia and set the stage for better response to therapy.

Receptor binding competition: A paradigm for regulating TGF-β family action

The transforming growth factor (TGF)-β family is a group of structurally related, multifunctional growth factors, or ligands that are crucially involved in the development, regulation, and maintenance of animal tissues. In humans, the family counts over 33 members. These secreted ligands typically form multimeric complexes with two type I and two type II receptors to activate one of two distinct signal transduction branches. A striking feature of the family is its promiscuity, i.e., many ligands bind the same receptors and compete with each other for binding to these receptors. Although several explanations for this feature have been considered, its functional significance has remained puzzling. However, several recent reports have promoted the idea that ligand-receptor binding promiscuity and competition are critical features of the TGF-β family that provide an essential regulating function. Namely, they allow a cell to read and process multi-ligand inputs. This capability may be necessary for producing subtle, distinctive, or adaptive responses and, possibly, for facilitating developmental plasticity. Here, we review the molecular basis for ligand competition, with emphasis on molecular structures and binding affinities. We give an overview of methods that were used to establish experimentally ligand competition. Finally, we discuss how the concept of ligand competition may be fundamentally tied to human physiology, disease, and therapy.

Activin and inhibin signaling: From regulation of physiology to involvement in the pathology of the female reproductive system

Activins and inhibins - comprising activin A, B, AB, C and E, and inhibin A and B isoforms - belong to the transforming growth factor beta (TGFβ) superfamily. They regulate several biological processes, including cellular proliferation, differentiation and invasiveness, to enhance the formation and functioning of many human tissues and organs. In this review, we have discussed the role of activin and inhibin signaling in the physiological and female-specific pathological events that occur in the female reproductive system. The up-to-date evidence indicates that these cytokines regulate germ cell development, follicular development, ovulation, uterine receptivity, decidualization and placentation through the activation of several signaling pathways; and that their dysregulated expression is involved in the pathogenesis and pathophysiology of the numerous diseases, including pregnancy complications, that disturb reproduction. Hence, some of the isoforms have been suggested as potential biomarkers and therapeutic targets for the management of some of these diseases. Tackling the research directions highlighted in this review will enhance a detailed comprehension and the clinical utility of these cytokines.

Role of activin C in normal ovaries and granulosa cell tumours of mice and humans

Activins and inhibins play important roles in the development, growth and function of the ovary. Mice lacking inhibin develop granulosa cell tumours in their ovaries that secrete activin A, and these tumours are modulated by increased activin C expression. The aim of the present study was to identify where activin C is expressed in mouse and human ovaries and whether overexpression of activin C modulates normal follicular development in mice. Immunohistochemical staining for the activin βC subunit was performed on sections from mouse and human ovaries and human adult granulosa cell tumours. Stereology techniques were used to quantify oocyte and follicular diameters, and the percentage of different follicular types in ovaries from wild-type mice and those underexpressing inhibin α and/or overexpressing activin C. Staining for activin βC was observed in the oocytes, granulosa cells, thecal cells and surface epithelium of mouse and human ovaries, and in the granulosa-like cells of adult granulosa cell tumours. Overexpression of activin C in mice did not alter follicular development compared with wild-type mice, but it did modulate the development of abnormal early stage follicles in inhibin α-null mice. These results provide further evidence of a role for activin C in the ovary.

ALK4-SMAD2/3-SMAD4 signaling mediates the activin A-induced suppression of PTX3 in human granulosa-lutein cells

As one of the members of the transforming growth factor-β (TGF-β) superfamily, activin A plays an important role in regulating follicular development and oocyte maturation. Pentraxin 3 (PTX3) is the key component that promotes the process of cumulus expansion during mammalian ovulation. At present, the regulation of PTX3 expression in human granulosa cells remains largely unknown. This study aimed to examine the effects of activin A on the expression of PTX3 in human granulosa-lutein (hGL) cells and to investigate the underlying molecular mechanisms. Using an established immortalized hGL cell line (SVOG) and primary hGL cells as study models, we demonstrated that activin A significantly increased the phosphorylation of SMAD2 and SMAD3, which suppressed the expression of PTX3 at both the mRNA and protein levels. Additionally, these effects induced by activin A were completely reversed by pretreatment with the TGF-β type I receptor inhibitor SB431542 and knockdown of ALK4. Furthermore, knockdown of SMAD2, SMAD3, or SMAD4 completely reversed the activin A-induced suppressive effects on PTX3 expression. Notably, the ChIP analyses demonstrated that phosphorylated SMADs could bind to human PTX3 promoter. Collectively, our results showed that the ALK4-SMAD2/3-SMAD4 signaling pathway most likely mediates the suppressive effect of activin A on PTX3 expression in hGL cells.

Mechanisms involved in enhancement of osteoclast formation by activin-A

Several growth factors in bone tissues are reported to be associated with osteoclastogenesis. Activin-A, a member of the transforming growth factor-β (TGF-β) family is known to be present in bone tissues and an important regulator in osteoclastogenesis with SMAD-mediated signaling being crucial for inducing osteoclast differentiation. In the present study, we examined the effect and underlying mechanisms of activin-A on osteoclast formation in vitro culture systems. Activin-A enhanced osteoclast formation in both mouse bone marrow cells and monocyte/macrophage cell line RAW 264.7 cells induced by receptor activator of nuclear factor kappa B (NF-κB) ligand (RANKL) and/or macrophage stimulating factor (M-CSF). We also found that activin-A stimulated bone resorption and actin ring formation induced by RANKL and/or M-CSF. Furthermore, activin-A enhanced RANKL-induced expression of nuclear factor of activated T cell cytoplasmic 1 (NFATc1), a key regulator of osteoclastogenesis, thereby increasing osteoclastogenesis-related marker gene expression, including tartrate-resistant acid phosphatase, osteoclast stimulatory transmembrane protein, and cathepsin K. Blockage of receptor binding by follistatin, an activing-binding protein suppressed the activin-A-mediated stimulation of NFATc1. In addition, activin-A increased RANKL-induced c-fos expression without significantly affecting the NF-κB and mitogen-activated protein kinase (MAPK) signaling pathway. Pre-treatment of the cells with a specific inhibitor of SMAD2/3 attenuated the activin-A-induced expression of NFATc1 and co-immunoprecipitation assay revealed that treatment with activin-A increased physical interaction of phosphorylated-c-fos and phosphorylated-SMAD2 protein induced by RANKL. These results suggest that activin-A enhances RANKL-induced osteoclast formation mediated by interaction of c-fos and smad2/3.

Soluble activin type IIB receptor improves fracture healing in a closed tibial fracture mouse model

Fractures still present a significant burden to patients due to pain and periods of unproductivity. Numerous growth factors have been identified to regulate bone remodeling. However, to date, only the bone morphogenetic proteins (BMPs) are used to enhance fracture healing in clinical settings. Activins are pleiotropic growth factors belonging to the TGF-β superfamily. We and others have recently shown that treatment with recombinant fusion proteins of activin receptors greatly increases bone mass in different animal models by trapping activins and other ligands thus inhibiting their signaling pathways. However, their effects on fracture healing are less known. Twelve-week old male C57Bl mice were subjected to a standardized, closed tibial fracture model. Animals were divided into control and treatment groups and were administered either PBS control or a soluble activin type IIB receptor (ActRIIB-Fc) intraperitoneally once a week for a duration of two or four weeks. There were no significant differences between the groups at two weeks but we observed a significant increase in callus mineralization in ActRIIB-Fc-treated animals by microcomputed tomography imaging at four weeks. Bone volume per tissue volume was 60%, trabecular number 55% and bone mineral density 60% higher in the 4-week calluses of the ActRIIB-Fc-treated mice (p<0.05 in all). Biomechanical strength of 4-week calluses was also significantly improved by ActRIIB-Fc treatment as stiffness increased by 64% and maximum force by 45% (p<0.05) compared to the PBS-injected controls. These results demonstrate that ActRIIB-Fc treatment significantly improves healing of closed long bone fractures. Our findings support the previous reports of activin receptors increasing bone mass but also demonstrate a novel approach for using ActRIIB-Fc to enhance fracture healing.

Over-Expression of Activin-βC Is Associated with Murine and Human Prostate Disease

Activins are members of the TGF-β superfamily and have been linked to prostate cancer. There are four mammalian activin subunits (β_A, β_B, β_C, and β_E) that dimerize to form functional proteins. The role of activin-A (β_A-β_A) has been relatively well characterized and has been shown to generally inhibit growth in the prostate. In contrast, little is known about the biological function of the β_C and β_E subunits. Previous work indicated activin-C (β_C-β_C) to be an antagonist of activin-A. This is important because resistance to activin-A growth inhibition occurs during prostate cancer progression. This paradox is not currently well understood. Hence, we hypothesize that local expression of the activin-β_C subunit antagonizes activin-A-dependent growth inhibition and represents a key factor contributing to acquired insensitivity to activin-A observed in prostate cancer progression. To test our hypothesis, we characterized the ventral prostate lobes of 9-month-old transgenic mice over-expressing activin-β_C and examined the expression of activin-β_A, activin-β_C, and the activin intracellular signaling factor, Smad-2, in human prostate diseases. Prostate epithelial cell hyperplasia, low-grade prostatic intraepithelial neoplasia (PIN) lesions, alterations in cell proliferation, and reduced Smad-2 nuclear localization were evident in mice over-expressing activin-β_C. Increased activin-β_A and -β_C subunit immunoreactive scores and decreased Smad-2 nuclear localization were also evident in human prostate cancer. This study suggests that over-expression of activin-β_C is associated with murine and human prostate pathologies. We conclude that the activin-β_C subunit may have therapeutic and/or diagnostic implications in human prostate disease.

SMAD3 Activation: A Converging Point of Dysregulated TGF-Beta Superfamily Signaling and Genetic Aberrations in Granulosa Cell Tumor Development?

Ovarian granulosa cell tumors (GCTs) are rare gynecologic tumors in women. Due to the rarity and limited research efforts invested, the etiology of GCTs remains poorly defined. A landmark study has discovered the mutation of forkhead box L2 (FOXL2) as a genetic hallmark of adult GCTs in the human. However, our understanding of the role of cell signaling in GCT development is far from complete. Increasing lines of evidence highlight the importance of TGF-beta (TGFB) superfamily signaling in the pathogenesis of GCTs. This review draws on findings using genetically modified mouse models and human patient specimens and cell lines to reveal SMAD3 activation as a potentially key converging point of dysregulated TGFB superfamily signaling and genetic aberrations in GCT development. It is anticipated that deciphering the role of TGFB superfamily signaling cascades in ovarian tumorigenesis will help develop new therapeutic approaches for GCTs by targeting core signaling elements essential for tumor initiation, growth, and progression.

Activins in reproductive biology and beyond

BACKGROUND

Activins are members of the pleiotrophic family of the transforming growth factor-beta (TGF-β) superfamily of cytokines, initially isolated for their capacity to induce the release of FSH from pituitary extracts. Subsequent research has demonstrated that activins are involved in multiple biological functions including the control of inflammation, fibrosis, developmental biology and tumourigenesis. This review summarizes the current knowledge on the roles of activin in reproductive and developmental biology. It also discusses interesting advances in the field of modulating the bioactivity of activins as a therapeutic target, which would undoubtedly be beneficial for patients with reproductive pathology.

METHODS

A comprehensive literature search was carried out using PUBMED and Google Scholar databases to identify studies in the English language which have contributed to the advancement of the field of activin biology, since its initial isolation in 1987 until July 2015. 'Activin', 'testis', 'ovary', 'embryonic development' and 'therapeutic targets' were used as the keywords in combination with other search phrases relevant to the topic of activin biology.

RESULTS

Activins, which are dimers of inhibin β subunits, act via a classical TGF-β signalling pathway. The bioactivity of activin is regulated by two endogenous inhibitors, inhibin and follistatin. Activin is a major regulator of testicular and ovarian development. In the ovary, activin A promotes oocyte maturation and regulates granulosa cell steroidogenesis. It is also essential in endometrial repair following menstruation, decidualization and maintaining pregnancy. Dysregulation of the activin-follistatin-inhibin system leads to disorders of female reproduction and pregnancy, including polycystic ovary syndrome, ectopic pregnancy, miscarriage, fetal growth restriction, gestational diabetes, pre-eclampsia and pre-term birth. Moreover, a rise in serum activin A, accompanied by elevated FSH, is characteristic of female reproductive aging. In the male, activin A is an autocrine and paracrine modulator of germ cell development and Sertoli cell proliferation. Disruption of normal activin signalling is characteristic of many tumours affecting reproductive organs, including endometrial carcinoma, cervical cancer, testicular and ovarian cancer as well as prostate cancer. While activin A and B aid the progression of many tumours of the reproductive organs, activin C acts as a tumour suppressor. Activins are important in embryonic induction, morphogenesis of branched glandular organs, development of limbs and nervous system, craniofacial and dental development and morphogenesis of the Wolffian duct.

CONCLUSIONS

The field of activin biology has advanced considerably since its initial discovery as an FSH stimulating agent. Now, activin is well known as a growth factor and cytokine that regulates many aspects of reproductive biology, developmental biology and also inflammation and immunological mechanisms. Current research provides evidence for novel roles of activins in maintaining the structure and function of reproductive and other organ systems. The fact that activin A is elevated both locally as well as systemically in major disorders of the reproductive system makes it an important biomarker. Given the established role of activin A as a pro-inflammatory and pro-fibrotic agent, studies of its involvement in disorders of reproduction resulting from these processes should be examined. Follistatin, as a key regulator of the biological actions of activin, should be evaluated as a therapeutic agent in conditions where activin A overexpression is established as a contributing factor.

Activin-βC modulates cachexia by repressing the ubiquitin-proteasome and autophagic degradation pathways

BACKGROUND

Cancer-associated cachexia and muscle wasting are considered key determinants of cancer-related death and reduction in the quality of life of cancer patients. A crucial link has been established between activin signaling and skeletal muscle atrophy-hypertrophy. We previously showed that activin-βC, a novel activin-A antagonist, is a tumor modulator that abolishes the cancer-associated cachexia in a mouse genetic model of gonadal tumorigenesis, in which the normal balance of inhibin/activin signalling is disrupted by a targeted mutation in the Inha gene (inhibin α-KO mouse). This study aimed to identify the molecular mechanism by which activin-βC increases survival and abolishes cancer-associated cachexia in α-KO mice. We hypothesized that overexpression of activin-βC modulates the cachexia phenotype by antagonizing the activin signaling pathway and repressing muscle wasting via the ubiquitin-proteasome and the autophagic-lysosomal degradation pathways.

METHODS

Male and female ActC++, α-KO, and α-KO/ActC++ mice and WT littermate controls were studied. Western blot analysis for the specific E3 ubiquitin ligases, atrogin-1 and MuRF1, markers of the autophagic-lysosomal pathway, Beclin-1, p62, and LC3A/B, effectors Smad-2, Smad-3 and myostatin was performed in the gastrocnemius of age-matched mice. Histopathology of the gastrocnemius and survival analysis were also conducted in animals from the same breeding cohort. Serum levels of activin-A, inflammatory cytokines, hormonal profile, and bone density were also assessed.

RESULTS

Increased levels of atrogin-1, MuRF-1, Beclin-1, p62, LC3A/B-I, Smad-2 and serum levels of activin-A were noted in the α-KO mice. These mice developed gonadal cancers followed by severe weight loss, and reduced survival. Overexpression of activin- βC antagonized the activin signaling cascade, attenuating the ubiquitin-proteasome and the autophagic-lysosomal degradation pathways, and reduced serum levels of activin-A. α-KO/ActC++ mice displayed a less aggressive cachectic phenotype, reduced tumor weight, and prolonged survival.

CONCLUSION

Our findings show for the first time a specific effect of activin-βC on muscle wasting and transcription factors involved in muscle protein degradation. The study indicates that activin-βC may be a novel therapy to abrogate cancer-associated weight loss and prolong survival.

Activins and activin antagonists in the human ovary and ovarian cancer

Activins are members of the transforming growth factor β superfamily that play an important role in controlling cell proliferation and differentiation in many organs including the ovary. It is essential that activin signalling be tightly regulated as imbalances can lead to uncontrolled cell proliferation and cancer. This review describes the expression and function of the activins and their known antagonists in both normal and cancerous human ovaries.

Activin-βC modulates gonadal, but not adrenal tumorigenesis in the inhibin deficient mice

Activins and inhibins are involved in the regulation of several biological processes, including reproduction, development and fertility. Deregulation of the inhibin/activin signaling pathway has been implicated in the progression of reproductive and adrenal cancers. Deletion of the inhibin α-subunit results in up-regulation of the circulating levels of activins and this leads to the development of sex-cord stromal tumors followed by a cancer associated-cachexia in mice. When gonadectomy is performed, development of adrenocortical carcinomas is observed. We previously showed that overexpression of activin-βC modulates the development of sex-cord stromal tumors and reduces cancer-cachexia in the inhibin-deficient mice by antagonizing the activin signaling pathway. The adrenal cortex and gonads share in common a large subset of genes, consistent with their common embryonic lineage. Additionally, it has been shown that adrenocortical carcinomas adopt an altered cellular identity resembling the ovary. Therefore, a study to assess the impact of overexpression of activin-βC on the onset of adrenocortical carcinoma in gonadectomized inhibin-deficient mice was warranted. Within the current study we evaluated markers of apoptosis, proliferation, tumor burden, survival analysis and serum levels of activin-A in gonadectomized mice versus sham operated controls. Results showed that overexpression of activin-βC modulated the development of reproductive tumors but had no effect on adrenal tumorigenesis. Our data reinforces the importance of activin-βC in reproductive biology and suggest that activin-βC is a tumor modulator with gonadal specificity.