Activins, inhibins, and follistatins: further thoughts on a growing family of regulators

Follistatin production by skin fibroblasts and its regulation by dexamethasone

Activin and follistatin (FS) appear to play a role in the development of the skin and its appendages, in the inflammatory process, angiogenesis, and in wound healing. Although there is information on the expression of activin subunits and receptors in fibroblasts and keratinocytes, there are no reports on the regulation of FS expression in these cells. In the present study we analyzed the splicing variants of FS mRNAs in fibroblasts from genital and nongenital skin by RT-PCR and northern analysis, and examined the induction of FS mRNA and protein by hormones and growth factors in skin fibroblasts from human and nonhuman primates. FS mRNA was highly expressed in all fibroblast strains with similar expression regardless of donor species (human or monkey), donor age (neonate or adult), or the organ from which the fibroblast strains were established (skin or pituitary, genital or non-genital skin). Moreover, the band density corresponding to FS-288 was <5-10% of the value for FS-315 in skin fibroblasts as in all other tissues examined. Fibroblast FS mRNA and protein production were biphasically regulated by dexamethasone: low concentrations (0.01 and 0.1 nM) increased whereas higher concentrations (>1 nM) suppressed FS expression. On the other hand, androgens, activin and PACAP38 were without effect. These data establish cultured skin fibroblasts as a model to study FS gene expression in humans, and support a role for follistatin in the normal immune response and in the anti-inflammatory actions of glucocorticoids.

Testosterone modulates the dendritic architecture of arcuate neuroendocrine neurons in adult male rats

Recent studies have demonstrated that gonadectomy of adult male rats induces dendritic growth of neuroendocrine neurons in the arcuate nucleus. We have hypothesized that these changes are secondary to the loss of testosterone negative feedback. In the present study, we examined the effects of testosterone replacement on the dendritic morphology of arcuate neuroendocrine neurons in castrated rats. Rats were orchidectomized and implanted with silastic capsules designed to produce physiological levels of plasma testosterone (n=9) or empty silastic capsules (n=9) for 2 months. Retrograde labeling with systemically injected Fluoro-Gold, followed by intracellular injection of labeled neurons in a fixed slice preparation, were used to visualize arcuate neuroendocrine neurons. Quantitative analysis of dendritic morphology was performed using three-dimensional computer reconstruction. Serum levels of LH (luteinizing hormone) and testosterone were measured by radioimmunoassay. Treatment of castrated rats with physiological levels of testosterone significantly reduced dendritic length, volume and terminal branch number relative to the castrated rats receiving empty silastic capsules. Dendritic spine density was also greater in the testosterone-treated animals, although the total numbers of spines per dendrite was not significantly different between the two groups. In addition, testosterone replacement was effective in reducing serum LH to levels found in intact rats. These studies demonstrate that testosterone replacement suppresses the dendritic outgrowth of arcuate neuroendocrine neurons that occurs in response to castration. The parallel changes in dendritic arbor and serum LH after castration and hormone replacement suggests that the suppressive effects of testosterone are related to steroid negative feedback.

Follicle-stimulating hormone regulation of inhibin alpha- and beta(B)-subunit and follistatin messenger ribonucleic acid in cultured avian granulosa cells

FSH regulation of inhibin alpha-, beta(B)-subunit and follistatin mRNA was investigated in cultured chicken granulosa cells, which were isolated and pooled according to size from the F(4) + F(5) follicles, small yellow follicles (SYF), and large white follicles (LWF). In experiment 1 (four replicate experiments), granulosa cells were cultured, and the effect of FSH (50 ng/ml) on the growth of cells from the different follicles was examined at 24 and 48 h of culture. Cell viability was >95% for all of the granulosa cell cultures at 24 and 48 h. At 24 h, the number of granulosa cells in both the FSH-treated and the untreated cultures for all follicle types was numerically greater than the number of cells originally plated. At 48 h, FSH-treated cultures for all follicle types had twice (P: < 0. 05) the number of cells as the untreated cultures. In experiment 2 (three replicate experiments), FSH increased expression of the mRNA for inhibin alpha-subunit in LWF granulosa cells at 4 and 24 h to detectable levels and increased inhibin alpha-subunit protein accumulation to detectable levels by 24 h in granulosa cells from the LWF. FSH also increased (P: < 0.05) mRNA levels for the inhibin alpha-subunit at 4 and 24 h in SYF granulosa cells and at 24 h in F(4) + F(5) granulosa cells. The effects of FSH on follistatin and ss(B)-subunit were variable with respect to follicle development and culture duration. These results suggest that FSH plays an important role in stimulating the production of mRNA and protein for the inhibin alpha-subunit in small prehierarchical follicles.

Roles of the activin regulatory system in fish reproduction

Activin (βAβA, βAβB, and βBβb) is a dimeric growth factor with diverse biological activities in vertebrate reproduction. Activin exerts its actions by binding to its specific type II and type I receptors. The activity of activin is regulated by follistatin, its binding protein, and the antagonists inhibin and antivin. All major components of the activin-inhibin-follistatin system have been identified in fish except the α subunit of inhibin. Using goldfish as a model, we have demonstrated that activin is expressed in the pituitary and the recombinant goldfish activin B has novel inverse effects on the expression of GTH β subunits. Activin increases the mRNA level of GTH-Iβ while significantly suppressing the expression of GTH-IIβ. We have also demonstrated the expression of activin and its receptors in the goldfish and zebrafish ovary. Using an in vitro ovarian follicle incubation as the system, we have investigated the involvement of the activin system in the process of final oocyte maturation. Our evidence clearly indicates that activin has potent effect of promoting final oocyte maturation, and that it may play a role in mediating the stimulatory effect of pituitary gonadotropin in the event of oocyte maturation. Key words: activin, inhibin, follistatin, fish, reproduction.

Hepatocyte growth factor induces branching tubulogenesis in MDCK cells by modulating the activin-follistatin system

BACKGROUND

The activin-follistatin system is expressed in tubular cells of the kidney. The present study was conducted to examine the role of the activin-follistatin system in tubulogenesis using Madin-Darby canine kidney (MDCK) cells as a model system.

METHODS

Tubulogenesis was assessed using MDCK cells cultured in collagen gel. The effect of recombinant human activin A on tubulogenesis was examined. Blockade of the action of endogenous activin was achieved by either adding follistatin or transfection of dominant-negative mutant of the type II activin receptor. The production of activin A was examined by Northern blotting, in situ hybridization, and Western blotting.

RESULTS

MDCK cells expressed mRNA for the betaA subunit of activin. These cells formed spherical cysts when cultured in collagen gel. Hepatocyte growth factor (HGF) added to the spherical cysts induced branching tubulogenesis. When activin A was added together with HGF, activin A blocked the branching tubulogenesis induced by HGF, and the activin-treated cells were scattered. Conversely, follistatin, an antagonist of activin A, induced branching tubulogenesis qualitatively similar to that induced by HGF. Adenovirus vector-mediated transfer of the gene encoding truncated type II activin receptor, which acts as a dominant negative mutant, also induced branching tubulogenesis. Finally, HGF markedly inhibited the production of activin A in MDCK cells cultured in collagen gel.

CONCLUSION

Activin A produced in MDCK cells tonically inhibits branching tubulogenesis, and HGF induced branching tubulogenesis mainly by blocking the production of activin A.

A model of retinal cell differentiation in the chick embryo

This article presents an overview of retinal cell differentiation in the chick embryo, in the context of a hypothetical model based on information generated during the last several years. The model proposes that: (1) most (if not all) proliferating neuroepithelial cells have the potential to give rise to a progeny comprising two or more different cell types; (2) the time at which cells undergo their terminal mitosis does not determine their differentiated fate; (3) many postmitotic precursor cells remain plastic (i.e., uncommitted) for some time after terminal mitosis, during which they encounter position-dependent signals as they migrate toward their definitive laminar position within the retina; (4) as a consequence of these inductive stimuli, precursor cells that migrate to different retinal layers express different transcriptional regulators; (5) morphologically undifferentiated precursor cells are committed to cell type-specific, complex patterns of differentiation, which they can express even when isolated from their normal microenvironment, and (6) even after precursor cells become committed to a specific identity, additional inductive signals are necessary for the cells to complete the development of a fully mature phenotype. The article presents a summary of the supportive evidence, as well as a critical evaluation of the model, and concludes with an overview of unanswered questions regarding retinal cell differentiation and a brief evaluation of the prospects for further progress in this field.

Estradiol regulation of follistatin and inhibin alpha- and beta(B)-subunit mRNA in avian granulosa cells

Estradiol modulation of granulosa cell growth and regulation of follistatin and inhibin alpha- and beta(B)-subunit mRNA were investigated in cultured chicken granulosa cells. Granulosa cells were isolated and pooled according to size from the F(4) + F(5), small yellow (SYF), and large white (LWF) follicles. Isolated and dispersed granulosa cells were then cultured in the absence or presence of 1 x 10(-5) M 17 beta-estradiol. In Experiment 1 (n = 4 replications) the effect of estradiol on the growth of granulosa cells from the different-sized follicles was examined at 24 and 48 h of culture. Untreated and treated granulosa cells from all three follicle sizes proliferated during culture, and cell viability for all cultures was over 95% throughout the experiment. After 48 h the untreated cultures for all follicle types had 1.6 to 2.2 times (P < 0.05) more cells than the estradiol-treated cultures. In Experiment 2 (n = 3 replications), the cultures were terminated at 4 and 24 h after plating. Follistatin mRNA levels were higher in estradiol-treated cells at 24 h in F(4) + F(5) follicles, at 4 and 24 h in the SYF, and at 4 h in the LWF. beta(B)-subunit mRNA levels were also increased by estradiol at 4 h in F(4) + F(5) cells and at 4 and 24 h in the LWF. Steady state mRNA levels for the alpha-subunit were higher (P < 0.05) in estradiol-treated cultures at 4 and 24 h in F(4) + F(5) follicles and at 24 h in the SYF. Immunoreactive alpha-subunit protein, however, was not increased by estradiol treatment. Thus, whereas estradiol inhibited granulosa cell growth, it exerted a generally stimulatory effect on the expression of FS and the inhibin alpha- and beta(B)-subunit mRNA.

Insertion of Inhbb into the Inhba locus rescues the Inhba-null phenotype and reveals new activin functions

The activins (dimers of betaA or betaB subunits, encoded by the genes Inhba and Inhbb, respectively) are TGF-beta superfamily members that have roles in reproduction and development. Whereas mice homozygous for the Inhba-null allele demonstrate disruption of whisker, palate and tooth development, leading to neonatal lethality, homozygous Inhbb-null mice are viable, fertile and have eye defects. To determine if these phenotypes were due to spatiotemporal expression differences of the ligands or disruption of specific ligand-receptor interactions, we replaced the region of Inhba encoding the mature protein with Inhbb, creating the allele Inhbatm2Zuk (hereafter designated InhbaBK). Although the craniofacial phenotypes of the Inhba-null mutation were rescued by the InhbaBK allele, somatic, testicular, genital and hair growth were grossly affected and influenced by the dosage and bioactivity of the allele. Thus, functional compensation within the TGF-beta superfamily can occur if the replacement gene is expressed appropriately. The novel phenotypes in these mice further illustrate the usefulness of insertion strategies for defining protein function.

Interaction of extracellular matrix and activin-A in the initiation of follicle growth in the mouse ovary

The precise mechanism for the initiation of follicle growth and progression through the earliest stages of follicle development remains largely unknown. Activins play a role during early follicle development, and evidence suggests that the extracellular matrix plays a role during later stages of follicular growth. We investigated the role of activin-A and extracellular matrix in follicle growth initiation and early follicular development in the mouse ovary. Ovaries were collected from 5-day-old mice and cultured for 10 days on polylysine, collagen, or laminin in the presence or absence of recombinant human activin-A. Follicle density, indices of follicle growth initiation (primary:primordial follicle [PY:PD] and primary:total follicle [PY:TF] ratios), ratios of multilayer follicle:total follicle (ML:TF), and follicle growth rates were compared between groups. Follicle densities were significantly higher in the extracellular matrix treatment group compared with the polylysine group (P < 0.01). Also, compared with polylysine, both collagen and laminin significantly increased indices of follicle growth initiation (PY:PD ratio: P < 0.001, odds ratio of 3.3; PY:TF ratio: P < 0.001, odds ratio of 2.5), and these were not altered by activin treatment. In the absence of activin-A, exposure to neither collagen nor laminin had an effect on multilayer follicle development. When activin-A was added, collagen and laminin had opposing effects on multilayer follicle development. Activin-A stimulated multilayer follicle development in the presence of laminin (ML:TF ratio: P = 0.01, odds ratio of 10.8), whereas it suppressed follicle growth in collagen (P = 0.01). Activin-A did not affect the ML:TF ratio in the polylysine-treated groups. These results strongly suggest that extracellular matrix components and activin-A interact with each other, and that they regulate follicle growth initiation and multilayer follicle development.

Measurement of dimeric inhibins and effects of active immunization against inhibin alpha-subunit on plasma hormones and testis morphology in the developing cockerel

Inhibins and activins are implicated as endocrine regulators of follicle-stimulating hormone production and of testicular steroidogenesis and spermatogenesis in mammals. The potential involvement of these proteins in cockerels was investigated by measurement of circulating inhibin A, inhibin B, total inhibin alpha-subunit immunoreactivity (ir-alpha), activin A, LH, FSH, and testosterone from the juvenile state through to sexual maturity. Plasma inhibin A remained low between 6 to 12 wk of age and increased approximately threefold (P < 0.05) to a prepubertal peak between Weeks 14 to 18, followed by a gradual decline to the end of the study (Week 24). Although plasma FSH levels were not correlated to inhibin A before Week 16 (r = -0.17), they were negatively correlated from Week 18 (r = -0.49; P < 0.005). Inhibin B levels were below the assay detection limit until 16 wk of age but thereafter rose steadily in parallel with FSH (r = 0.27; P < 0.02) and testosterone (r = 0.35; P < 0.005). Thus, inhibins A and B showed divergent profiles during sexual maturation. Plasma ir-alpha levels were much higher than dimeric inhibin levels throughout, although the relative difference varied with age. Plasma activin A levels were below the assay detection at all times. Juvenile cockerels were actively immunized against a synthetic chicken inhibin alpha-subunit peptide conjugate to determine effects on plasma hormones and on testicular weight, morphology, and activin A content. Immunization generated circulating antibodies that bound (125)I-bovine 32-kDa inhibin but did not affect plasma FSH or testosterone levels at any stage of development. However, immunization reduced postpubertal plasma LH levels (P < 0.05) and promoted increased testicular weight (24%; P < 0.01) and total testicular activin A content (42%; P < 0.001) at 24 wk. Testis weight of immunized birds was positively correlated with inhibin antibody titer (r = 0.61; P < 0.05). Live weight gain was not affected by immunization. Morphometric analysis of testis sections showed that inhibin immunization had no effect on the fractional volume of the seminiferous tubule wall, seminiferous tubule lumen, or interstitial tissue area. Likewise, seminiferous tubule surface area and surface area:volume ratios were not different from controls. These findings support differential roles for inhibins A and B in regulating the pituitary-testicular axis during sexual maturation in the cockerel but highlight the need for more detailed studies to distinguish between potential endocrine and local intragonadal roles of inhibin-related peptides and to elucidate the mechanism by which immunization against inhibin alpha-subunit promotes testis enlargement without raising plasma FSH.

Activin, inhibin, and follistatin in zebrafish ovary: expression and role in oocyte maturation

Activins, inhibins, and follistatins are important regulators of mammalian reproduction. However, their roles in lower vertebrates are poorly understood. In this study, we examined the expression of activin A, inhibin A, and follistatins in the zebrafish ovary and determined their role in final oocyte maturation. Using reverse transcription-polymerase chain reaction with primers specific for activin/inhibin beta(A) subunit and for follistatins, we detected DNA fragments of the expected size, which, upon sequencing, conformed to activin/inhibin beta(A) and follistatin. Western blot analysis using an antibody against activin/inhibin beta(A) subunit revealed two bands with sizes similar to those of activin A and inhibin A. The expression of follistatins was also confirmed by Western blot analysis. These results suggest that activin A, an inhibin A-like molecule, and follistatins are expressed in the zebrafish ovary. In cultured zebrafish follicles, activin A and inhibin A both induced final oocyte maturation in a dose-dependent manner. The effects of activin A and inhibin A were blocked by their binding protein, follistatin-288. Interestingly, follistatin-288 also inhibited final oocyte maturation induced by gonadotropin and by maturation-inducing hormone (MIH), suggesting that activin A and/or inhibin A may be local regulators mediating gonadotropin- and MIH-induced final oocyte maturation. Taken together, these findings suggest that activin A and inhibin A are paracrine regulators of ovarian functions in fish.

The roles of activins, inhibins and estrogen in early committed follicles

The hypothesis that activin and inhibin are autocrine/paracrine mediators of ovarian folliculogenesis has a solid basis. In mouse and rat models, granulosa cells (GC) of committed follicles express mRNA and protein for the activin/inhibin subunits and mRNA for the activin receptors (type I and II). Dimeric inhibin-A and -B are produced by postnatal ovarian cell dispersates and (GC) in culture. Similar levels of inhibin-A and -B are produced by postnatal ovarian cells, but thereafter as the ovary develops, inhibin-A becomes the predominant form. Activin was more effective than transforming growth factor-beta (TGF-beta) in enhancing follicle stimulating hormone (FSH)-stimulated inhibin production by ovarian cells. Evidence for a local regulatory role of estrogen in the ovary is also accumulating. Murine models of estrogen receptor (ERalpha or ERbeta) disruption produce mice with abnormal ovarian phenotypes. Female mice, which lack the capacity to produce estrogen (ArKO mice), have arrested folliculogenesis, no corpora lutea, elevated levels of luteinising hormone (LH), FSH and testosterone and are infertile. These data are consistent with autocrine/paracrine actions of activin in the early growth of committed follicles and estrogen in follicular maturation.

Molecular cloning and expression analysis of the complementary deoxyribonucleic acid for chicken inhibin/activin beta(B) subunit

Inhibins and activins are dimeric peptide hormones that play an integral role in the intraovarian regulation of folliculogenesis. The domestic hen, with its well-defined follicular hierarchy, provides a unique model in which to study the role of these hormones in follicular development. In the present study, the complete coding sequence and deduced amino acid sequence for the chicken inhibin/activin beta(B) subunit has been determined from cDNA clones isolated from a chicken ovarian granulosa cell library. This beta(B)-subunit cDNA predicts a precursor protein of 392 amino acids containing the mature C-terminal 115 amino acid beta(B) subunit. When compared to the beta(B) subunit isolated from a variety of species, the chicken cDNA clone showed high nucleotide identity in the full-length coding region (>70%) and in the mature coding region (>80%). In addition, the deduced amino acid sequence of chicken beta(B) subunit showed greater than 95% identity compared to other species in the mature peptide region. Expression of the beta(B)-subunit mRNA was detected by reverse transcription-polymerase chain reaction in both gonadal and extragonadal tissues. Northern blot analysis detected expression in the gonadal tissues only, specifically in the granulosa tissue from the F3-F5 follicles, small yellow follicles (SYF), large white follicles, and immature and mature rooster testes. A major transcript of approximately 4.1 kilobases (kb) and three minor transcripts of approximately 8.4 kb, 6.5 kb, and 1.7 kb were detected in the SYF granulosa samples. To examine the expression pattern of the beta(B) subunit around the stage of follicle selection, the SYF granulosa was subdivided into two groups: 6-8 mm and 9-12 mm. Quantification of RNA expression (n = 3) showed that expression of the beta(B) subunit was maximal in the 6-8 mm SYF. Activin B, as well as other intraovarian signals, may regulate early follicle selection and/or development in the chicken.

Regulation of germ cell and Sertoli cell development by activin, follistatin, and FSH

We have demonstrated a role for activin A, follistatin, and FSH in male germ cell differentiation at the time when spermatogonial stem cells and committed spermatogonia first appear in the developing testis. Testis fragments from 3-day-old rats were cultured for 1 or 3 days with various combinations of these factors, incubated with bromodeoxyuridine (BrdU) to label proliferating cells, and then processed for stereological analysis and detection of BrdU incorporation. Gonocyte numbers were significantly elevated in cultures treated with activin, while the combination of FSH and the activin antagonist, follistatin, increased the proportion of spermatogonia in the germ cell population after 3 days. All fragment groups treated with FSH contained a significantly higher proportion of proliferating Sertoli cells, while activin and follistatin each reduced Sertoli cell division. In situ hybridization and immunohistochemistry on normal rat testes demonstrated that gonocytes, but not spermatogonia, contain the activin beta(A) subunit mRNA and protein. In contrast, gonocytes first expressed follistatin mRNA and protein at 3 days after birth, concordant with the transition of gonocytes to spermatogonia. Collectively, these data demonstrate that germ cells have the potential to regulate their own maturation through production of endogenous activin A and follistatin. Sertoli cells were observed to produce the activin/inhibin beta(A) subunit, the inhibin alpha subunit, and follistatin, demonstrating that these cells have the potential to regulate germ cell maturation as well as their own development. These findings indicate that local regulation of activin bioactivity may underpin the coordinated development of germ cells and somatic cells at the onset of spermatogenesis.

The role of activin and transforming growth factor-beta in the regulation of organ mass in the rat liver

The present study was conducted to assess the role of activin(s) in the regulatory mechanism to maintain constant liver mass. To this end, we infused follistatin, an activin antagonist, into the portal vein of the rat. Follistatin induced DNA synthesis, as assessed by bromodeoxy uridine labeling, in intact livers. Small peaks of bromodeoxy uridine labeling were observed after 3 and 18 hours of infusion, and a large peak was observed after 48 hours. In follistatin-treated rats, the DNA content of the liver was significantly elevated after 72 hours and returned to the basal value within 120 hours. Likewise, liver weight increased significantly after 60 and 72 hours, but returned to the control value within 120 hours. Apoptosis of hepatocytes, assessed by the Tdt-mediated, dUTP-biotin nick end labeling method was observed after 72 hours or later. Messenger RNA (mRNA) expression of hepatocyte growth factor, transforming growth factor-alpha, tumor necrosis factor-alpha, and interleukin-6 did not increase after the addition of follistatin. The mRNA expression and immunoreativity of transforming growth factor-beta increased after the administration of follistatin. These results suggest that the blockade of activin action leads to the initiation of DNA synthesis in the intact liver. Activins may tonically inhibit hepatocyte growth in the intact liver. Transforming growth factor-beta may also act to maintain constant liver mass when activin action is blocked.

Activin A-induced HepG2 liver cell apoptosis: involvement of activin receptors and smad proteins

A balance between cell proliferation and apoptosis is important for regulating normal liver function. Proteins of the transforming growth factor-beta superfamily are known to be important mediators of apoptosis in the liver. In this study we demonstrate that activin A potently induces apoptotic cell death in a hepatoma cell line, HepG2 cells. To determine the roles of activin receptors and downstream signaling proteins in activin A-induced apoptosis in these cells, the activin signaling pathway was analyzed using the transcription of an activin-responsive reporter gene, p3TP-Lux, as an assay. Although individual activin receptors had little effect on transcriptional activity, coexpression of an activin type I receptor and a type II receptor significantly increased both basal and activin-induced transcriptional activation, with the combination ofreceptors IB and IIB being the most potent. Similarly, expression of individual Smad proteins had only a modest effect on reporter gene activity, but the combination of Smad2 and Smad4 strongly stimulated transcription. Activin signaling induced a rapid relocation of Smad2 to the nucleus, as determined using a green fluorescence protein-Smad2 fusion protein. In contrast, green fluorescence protein-Smad4 remained localized to the cytoplasm unless it was coexpressed with Smad2. In agreement with the transcriptional response assays, overexpression or suppression of activin signaling components in HepG2 cells altered apoptosis. Overexpression of receptors IB and IIB or Smad proteins 2 and 4 stimulated apoptosis, whereas dominant negative mutant forms of the activin type IIB receptor or Smad2 blocked activin-stimulated apoptosis. These studies suggest that signaling from the cell surface to the nucleus through Smad proteins is a required component of the activin A-induced cell death process in liver cells.

Differential regulation of activin A for hepatocyte growth and fibronectin synthesis in rat liver injury

BACKGROUND/AIMS

Both hepatocyte growth and production of extracellular matrix such as fibronectin are essential for liver regeneration. Although activin A is reported to inhibit DNA replication in rat hepatocytes, the role of activin A for liver regeneration after acute injury has not been fully assessed. This study investigated the mechanism by which hepatocyte growth is regulated by activin A during liver regeneration and the effects of activin A on extracellular matrix production.

METHODS

The mRNA for betaA subunit of activin A and activin receptors in hepatocytes and hepatic stellate cells after CCl4 administration were studied by Northern blotting. Binding of 125I-activin A was tested in these cells. Effects of activin A were examined by DNA, collagen and fibronectin synthesis.

RESULTS

betaA mRNA was expressed in quiescent hepatocytes, and this expression peaked 12 h after CCl4 administration. Activin receptor mRNAs and cross-linked ligand/receptor complexes were expressed in hepatocytes and hepatic stellate cells However, these levels decreased specifically in hepatocytes at 24 h and had normalized by 72 h. The down-regulation of activin receptor was also observed after partial hepatectomy. Antiproliferative response to activin A decreased in hepatocytes at 24 h. Activin A stimulated production of fibronectin by hepatic stellate cells, but the synthesis of collagen was only slightly elevated in hepatic stellate cells following activin stimulation.

CONCLUSIONS

The down-regulation of activin receptors in hepatocytes may be partly responsible for these cells becoming responsive to mitogenic stimuli. The increase of activin A at the early stage of liver injury has the potential to contribute to the regulation of fibronectin production in hepatic stellate cells.

A role of insulin-like growth factor I for follicle-stimulating hormone receptor expression in rat granulosa cells

The present study was undertaken to identify the mechanisms underlying the effect of insulin-like growth factor I (IGF-I) on FSH receptor (FSHR) in rat granulosa cells. Treatment with FSH produced a substantial increase in FSHR mRNA level, as was expected, while concurrent treatment with increasing concentrations of IGF-I brought about dose-dependent increases in FSH-induced FSHR mRNA, with a maximal response 2.8-fold greater than that induced by FSH alone. IGF-I, either alone or in combination with FSH, did not affect intracellular cAMP levels, whereas it enhanced the effect of 8-bromo (Br)-cAMP on FSHR mRNA production. Taken together, these findings suggest that the ability of IGF-I to enhance FSH action concerning the induction of FSHR is exerted at sites distal to cAMP generation. We then investigated whether the effect of IGF-I and FSH on FSHR mRNA levels was the result of increased transcription and/or altered mRNA stability. The rates of FSHR mRNA gene transcription, assessed by nuclear run-on transcription assay, were not increased by the addition of IGF-I. On the other hand, the decay curves for the 2. 4-kilobase (kb) FSHR mRNA transcript in primary granulosa cells significantly altered the slope of the FSHR mRNA decay curve in the presence of IGF-I and increased the half-life of the FSHR mRNA transcript. These data suggest a possible role for changes in FSHR mRNA stability in the IGF-I-induced regulation of FSHR in rat granulosa cells. Treatment with activin produced a substantial increase in FSHR mRNA level, as was expected, and concurrent treatment with IGF-I did not affect activin-induced FSHR mRNA. Our data suggest that the IGF-I effect on FSHR expression is related to cAMP production induced by FSH and may maintain FSHR mRNA level because of prolonged FSHR mRNA stability.

Expression of activin A and follistatin core proteins by human prostate tumor cell lines

Activin and follistatin (FS) messenger RNA and protein are expressed and localized to human prostate tissue from men with high grade cancer and to human prostate tumor cell lines LNCaP, DU145, and PC3. Although activin A induces apoptosis and inhibits cell proliferation in LNCaP cells, PC3 cells are insensitive to the effect of exogenous addition of activin A. The results of this study show that activin A and FS are produced and can be measured by specific enzyme-linked immunosorbent assays in PC3 cells and media but are not detectable in LNCaP cells. Over 10 days in culture, the production of activin A by PC3 cells declines and is inversely correlated (r = -0.779) to FS288 production, which steadily increases and is significantly elevated compared with Day 1 of culture. The presence of FS288 and FS315 proteins was confirmed by immunocytochemistry and showed that only PC3 cells produced the FS288 isoform. Western blotting of PC3 cell media confirmed the presence of the FS288 isoform. Blockade of FS288 activity with a neutralizing antibody rendered PC3 cells responsive to activin A, as measured by inhibition of proliferation. Collectively, these results suggest that PC3 tumor cells are insensitive to activin A because they produce measurable amounts of activin ligand and FS288 protein, which is capable of blocking the autocrine response of these cells to activin A.

Gonadotropin regulation of inhibin alpha-subunit mRNA and immunoreactive protein in cultured chicken granulosa cells

Gonadotropin regulation of the inhibin alpha-subunit was investigated in chicken granulosa cell cultures. Granulosa layers were isolated from the F(1) and F(3) + F(4) follicles from three to four hens, pooled according to size, dispersed, and cultured (n = 3 replications for each experiment). In Experiments 1 and 2 either ovine LH or FSH was added to the cultures at doses of 0, 5, and 25 ng/ml. The cultures were terminated at 4, 24, and 48 h after plating. For both follicle sizes the expression of mRNA for the inhibin alpha-subunit was less (P < 0.05) at 24 and 48 h in untreated cells than in those treated with both doses of LH. Expression of the mRNA for the inhibin alpha-subunit was significantly increased only by the 25 ng/ml dose of FSH and only in the F(1) follicle at 24 and 48 h compared to the untreated cells. After 48 h of culture, immunoreactive alpha-subunit protein accumulation was greater for both follicle types in the media of cells treated with the highest dose of LH and FSH than in the media from untreated cells. In Experiment 3, doses of 0, 5, 25, or 50 ng/ml of either LH or FSH were added to F(1) and F(3) + F(4) granulosa cells. All cultures were terminated at 48 h. LH and FSH increased the expression of the mRNA and immunoreactive protein for the inhibin alpha-subunit equally in a time-dependent manner. These experiments indicate that gonadotropins enhance the expression of both the mRNA and the protein for the inhibin alpha-subunit in chicken granulosa cells.

'Paracrine' control of spermatogenesis

Spermatogenesis is a remarkably complex but precise process yielding highly differentiated haploid germ cells from diploid stem cells. Although many factors have been implicated in the paracrine control of spermatogenesis, functional proof is only available for a few regulators. Among those are androgens, growth factors and stem cell factor. Cell- and organ-specific genetargeting will provide important insights into the relevance of local factors controlling male gametogenesis. As testicular communication frequently occurs between rather remote cells and compartments, it is proposed that the term 'local' rather than 'paracrine' mediators/factors should be used, since the latter term refers to communication amongst neighbouring cells (and mainly via diffusion).

TGF-beta Family Members and Gonadal Development

Several members of the transforming growth factor beta (TGF-beta) family are involved in gonadal development; namely, TGF-beta itself, inhibins, activins, anti-Müllerian hormone (AMH) and GDF-9. These proteins do not affect initial gonadal organogenesis but play either a stimulatory or inhibitory role in the division and differentiation of gonadal cells and in meiotic maturation in the female. Furthermore, as shown by transgenic mouse technology, both AMH and inhibin act as tumor suppressors.

Thirty-seven candidate genes for polycystic ovary syndrome: strongest evidence for linkage is with follistatin

Polycystic ovary syndrome (PCOS) is a common endocrine disorder of women, characterized by hyperandrogenism and chronic anovulation. It is a leading cause of female infertility and is associated with polycystic ovaries, hirsutism, obesity, and insulin resistance. We tested a carefully chosen collection of 37 candidate genes for linkage and association with PCOS or hyperandrogenemia in data from 150 families. The strongest evidence for linkage was with the follistatin gene, for which affected sisters showed increased identity by descent (72%; chi(2) = 12.97; nominal P = 3.2 x 10(-4)). After correction for multiple testing (33 tests), the follistatin findings were still highly significant (P(c) = 0.01). Although the linkage results for CYP11A were also nominally significant (P = 0.02), they were no longer significant after correction. In 11 candidate gene regions, at least one allele showed nominally significant evidence for population association with PCOS in the transmission/disequilibrium test (chi(2) >/= 3.84; nominal P < 0.05). The strongest effect in the transmission/disequilibrium test was observed in the INSR region (D19S884; allele 5; chi(2) = 8.53) but was not significant after correction. Our study shows how a systematic screen of candidate genes can provide strong evidence for genetic linkage in complex diseases and can identify those genes that should have high (or low) priority for further study.

Control of the expression of luteinizing hormone receptor by local factors in rat granulosa cells

To identify the mechanisms underlying the hormone-dependent induction and maintenance of luteinizing hormone receptor (LH-R) in rat granulosa cells, the effect of follicle-stimulating hormone (FSH) and local factors on the LH-R mRNA levels were studied. LH-R mRNA levels of the cells incubated with FSH decreased rapidly after medium removal, and readdition of FSH with the fresh medium did not restore these levels. On the other hand, 8-bromoadenosine 3,5-cyclic monophosphate significantly enhanced the expression of LH-R mRNA after medium removal, while the level of LH-R mRNA was lower than that of the cells replaced by original medium including FSH. In addition, the incubation with 8-Br-cAMP produced dose-dependent responses for LH-R mRNAs and enhanced the activity of 1379 bp of the LH-R 5'-flanking region, while the level of LH-R mRNA decreased 3 days after medium removal. Further studies were undertaken to assess the role of factors in maintaining the LH receptor once induced by FSH. Since FSH and cAMP increase follistatin production in granulosa cells, we examined the effect of follistatin on LH-R induction in the presence of activin and FSH. Activin induced LH-R in the presence of FSH significantly, and follistatin antagonized this effect in a dose-dependent manner. However, insulinlike growth factor-I (IGF-I) induced LH-R mRNA in the presence of FSH even after medium change. IGF-I might be one of the important factors that act in the medium to maintain LH-R levels in granulosa cells.

Molecular cloning of testican-2: defining a novel calcium-binding proteoglycan family expressed in brain

We have screened a human cDNA library using an expressed sequence tag related to the BM-40/secreted protein, acidic and rich in cysteine (SPARC)/osteonectin family of proteins and isolated a novel cDNA. It encodes a protein precursor of 424 amino acids that consists of a signal peptide, a follistatin-like domain, a Ca2+-binding domain, a thyroglobulin-like domain, and a C-terminal region with two putative glycosaminoglycan attachment sites. The protein is homologous to testican-1 and was termed testican-2. Testican-1 is a proteoglycan originally isolated from human seminal plasma that is also expressed in brain. Northern blot hybridization of testican-2 showed a 6.1-kb mRNA expressed mainly in CNS but also found in lung and testis. A widespread expression in multiple neuronal cell types in olfactory bulb, cerebral cortex, thalamus, hippocampus, cerebellum, and medulla was detected by in situ hybridization. A recombinant fragment consisting of the Ca2+-binding EF-hand domain and the thyroglobulin-like domain of testican-2 showed a reversible Ca2+-dependent conformational change in circular dichroism studies. Testican-1 and -2 form a novel Ca2+-binding proteoglycan family built of modular domains with the potential to participate in diverse steps of neurogenesis.

Molecular characterization of the follicle defects in the growth differentiation factor 9-deficient ovary

Growth differentiation factor-9 (GDF-9), a secreted member of the transforming growth factor-beta superfamily, is expressed at high levels in the mammalian oocyte beginning at the type 3a primary follicle stage. We have previously demonstrated that GDF-9-deficient female mice are infertile because of an early block in folliculogenesis at the type 3b primary follicle stage. To address the molecular defects that result from the absence of GDF-9, we have analyzed the expression of several important ovarian marker genes. The major findings of our studies are as follows: 1) There are no detectable signals around GDF-9-deficient follicles for several theca cell layer markers [i.e. 17alpha-hydroxylase, LH receptor (LHR), and c-kit, the receptor for kit ligand]. This demonstrates that in the absence of GDF-9, the follicles are incompetent to emit a signal that recruits theca cell precursors to surround the follicle; 2) The primary follicles of GDF-9-deficient mice demonstrate an up-regulation of kit ligand and inhibin-alpha. This suggests that these two important secreted growth factors, expressed in the granulosa cells, may be directly regulated in a paracrine fashion by GDF-9. Up-regulation of kit ligand, via signaling through c-kit on the oocyte, may be directly involved in the increased size of GDF-9-deficient oocytes and the eventual demise of the oocyte; 3) After loss of the oocyte, the cells of the GDF-9-deficient follicles remain in a steroidogenic cluster that histologically resembles small corpora lutea. However, at the molecular level, these cells are positive for both luteal markers (e.g. LHR and P-450 side chain cleavage) and nonluteal markers (e.g. inhibin alpha and P-450 aromatase). This demonstrates that initially the presence of the oocyte prevents the expression of luteinized markers, but that the absence of GDF-9 at an early timepoint alters the differentiation program of the granulosa cells; and 4) As demonstrated by staining with either proliferating cell nuclear antigen (PCNA) or Ki-67 and TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) labeling, the granulosa cells of GDF-9-deficient type 3b primary follicles fail to proliferate but also fail to undergo cell death. This suggests that granulosa cells of type 3b follicles require GDF-9 for continued growth and also to become competent to undergo apoptosis, possibly through a differentiation event Thus, these studies have enlightened us as to the paracrine roles of GDF-9 as well as the normal steps of granulosa cell and theca cell growth and differentiation within ovarian follicles.

Dimeric inhibin A and B production are differentially regulated by hormones and local factors in rat granulosa cells

In the present study, we have examined the role of hormones and growth factors in regulating dimeric inhibin production in immature rat granulosa cells. Purified granulosa cells from estrogen-primed immature rats were cultured under defined conditions. Inhibins A and B in the culture media were measured using a two-site enzyme-linked immunosorbent assay specific for each dimer. Under basal conditions, granulosa cells produced 14-fold more inhibin A than inhibin B (inhibin A, 2.0; inhibin B, 0.14 ng/ml, measured against human standards; average A/B apparent ratio, 14). Addition of increasing doses of FSH elicited dose-dependent increases in both inhibins, the effects being more pronounced on inhibin A than on inhibin B (9.4- and 4.1-fold increases, respectively; average A/B ratio, 34). Estradiol, when added alone, stimulated inhibin A production 3- to 6-fold, whereas minor changes were observed in inhibin B production. Insulin-like growth factor-I produced a similar stimulation of both inhibins (3-fold stimulation over control). This growth factor, however, induced a marked dissociation in the sensitivity of inhibins A and B to FSH stimulation, with maximal stimulation of inhibin B observed at comparatively lower concentrations of the gonadotropin. Transforming growth factor-beta (TGF-beta, 5 ng/ml) had a more marked stimulatory effect on inhibin B than on inhibin A production (7- to 14-fold vs. 2- to 5-fold for inhibin B and A, respectively). A more pronounced differential stimulation of inhibin B was also exerted by another member of the TGF-beta superfamily, activin A (A/B ratio, 0.66). This preferential stimulation of inhibin B by TGF-beta and activin A was amplified in the presence of FSH. Coculture of rat granulosa cells with freshly isolated bovine oocytes was also associated with a marked stimulation of inhibin B production (100-fold increase) and a comparatively lower stimulation of inhibin A (10-fold increase; A/B ratio, 1). The discrepancy between the proportion of inhibin dimers in serum (A/B ratio, 0.13) and those produced by untreated granulosa cells may suggest that intraovarian factors, such as TGF-beta, activin A, or oocyte-derived factor(s), are responsible for the shift of the ratio toward the predominance of inhibin B.

Identification of high affinity binding sites for inhibin on ovine pituitary cells in culture

The aim of this study was to identify and characterize binding sites for inhibin in primary cultures of ovine anterior pituitary cells. Recombinant human 31-kDa inhibin A was iodinated by an optimized lactoperoxidase procedure. Fractionation of the labeled protein by gel filtration chromatography on Sephadex G-100 in 0.1 M HCl yielded two immunoactive peak regions, the second of which was bioactive as assessed by in vitro bioassay, with a ratio of bioactivity/immunoactivity of 0.62-0.77 and an iodine incorporation ratio of 1.7-2.0 mol 125I/mol inhibin. The specific binding of purified [125I]inhibin to cultured ovine pituitary cells varied with time, temperature, and cell number. Displacement of the tracer by unlabeled inhibin, as assessed by Scatchard analysis, revealed two binding sites with average Kd values of 0.28 and 3.9 nM and with approximately 250 and 3100 binding sites/anterior pituitary cell, respectively. There was little cross-reaction between inhibin and activin A (<2%), transforming growth factor-beta (<0.2%), or follistatin (<<0.1%). Examination of cell lines that were not expected to have inhibin receptors showed that there was no specific binding of inhibin to human leukemia (Jurkat) cells, whereas the binding to human embryonic kidney (293) cells was displaced by both inhibin and activin with a similar degree of cross-reaction, which suggests binding to an activin receptor. It is concluded that inhibin-binding sites with high affinity and specificity have been identified on ovine pituitary cells, consistent with both inhibin action on the pituitary and the presence of the putative inhibin receptor.

The bone morphogenetic protein 15 gene is X-linked and expressed in oocytes

We have taken advantage of the sequence relationships among the bone morphogenetic proteins (BMPs) to identify the mouse Bmp15 and human BMP15 genes. The 392-amino acid prepropeptides encoded by these BMP genes exhibit significant homology to each other, although the 70% identity observed between the 125-amino acid mature peptides is considerably lower than that seen in comparisons of other mouse and human orthologs. Both genes share a common structural organization and encode mature peptides that lack the cysteine residue normally involved in the formation of a covalent dimer. In addition, mouse Bmp15 and human BMP15 map to conserved syntenic regions on the X chromosome. We demonstrate, through a combination of Northern blot and in situ hybridization analyses, that mouse Bmp15 is expressed specifically in the oocyte beginning at the one-layer primary follicle stage and continuing through ovulation. Interestingly, BMP-15 is most closely related to and shares a coincident expression pattern with the mouse growth/differentiation factor 9 (GDF-9) gene that is essential for female fertility. Our findings will be important for defining the role of BMP-15 in follicular development.

Positive association between expression of follicle-stimulating hormone beta and activin betaB-subunit genes in boars

This study tested our hypothesis that inhibin/activin (I/A) betaB subunit and not follistatin (FS) gene expression relates positively to plasma FSH concentrations in the anterior pituitary gland of boars. Mature crossbred boars (n = 12) were selected for divergence in plasma FSH concentrations, and their anterior pituitary glands were evaluated for expression of the FSHbeta, I/A ssB, FS, calmodulin, and GnRH receptor (GnRH-R) genes by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and/or RNase protection assays (RPAs). Expression of I/A ssB was greater (p < 0. 01) in the six boars with high FSH than in the six with low FSH; expression of the I/A betaB-subunit gene was positively correlated to that of the FSHbeta gene (RT-PCR: r = 0.96; p < 0.01; RPA: r = 0.68; p < 0.05). In contrast, expression of the FS (p > 0.10), GnRH-R (p > 0. 08), and calmodulin (p > 0.10) genes was similar in the two groups of boars. Additionally, expression of the FSHbeta gene was correlated positively with pituitary and plasma FSH concentrations (r = 0.69 and 0.88, respectively; p < 0.05). These results support the hypothesis that activin B is partially responsible for elevated FSH concentrations in boars. Furthermore, the expression difference of the calmodulin gene observed previously between Meishan and White Composite boars represents a breed difference unrelated to FSH.

Expression pattern of messenger ribonucleic acid for follistatin and the inhibin/activin subunits during follicular and testicular development in gallus domesticus

The expression of mRNA for follistatin and the inhibin/activin subunits was investigated in the follicles of laying hens and the testes of immature and mature roosters. Total RNA was isolated from immature and mature testes and from individual granulosa layers of the F1 through F5 follicles, from a pool of the F6-F8 follicles, from the small yellow follicles, and from the combined granulosa and theca layers of the large white follicles. Northern blot analysis was performed, and a follistatin mRNA transcript of approximately 2. 4 kilobases (kb) was detected in immature testes and in the following follicles: F5, the pool of F6-F8, and small yellow and large white follicles. The greatest expression of follistatin occurred in the small yellow follicles, and in this tissue two minor transcripts of approximately 1.7 and 3.7 kb were also detected. The inhibin alpha subunit was expressed in both testes samples and in all of the follicles except the large white follicles. Expression of the alpha subunit was greatest in the F5 follicle, and expression decreased with follicle maturity. As previously reported, the inhibin/activin ssA subunit was found in the greatest abundance in the F1 follicle, with lesser amounts detected in the other hierarchical follicles and immature testes. In contrast to the ssA subunit, the inhibin/activin ssB subunit was not detected in the four largest hierarchical follicles but was expressed in greatest abundance in the pool of F6-F8 and small yellow follicles. This represents the first report, to our knowledge, of the detection of follistatin mRNA in the hen ovary and rooster testes. The restriction of follistatin mRNA expression to the small follicles suggests that follistatin, by regulating activin and/or inhibin availability, may play a critical role in early follicular development.

Expression of growth differentiation factor-9 messenger ribonucleic acid in ovarian and nonovarian rodent and human tissues

Growth differentiation factor-9 (GDF-9) is a member of the transforming growth factor-beta family that is reported to be expressed exclusively in the ovary, specifically in the oocyte. Female mice deficient in GDF-9 are infertile, suggesting that GDF-9 receptor agonists and antagonists may specifically modulate fertility. We now report that GDF-9 messenger RNA (mRNA) is expressed in nonovarian tissues in mice, rats, and humans. GDF-9 mRNA was detected in mouse and rat ovary, testis, and hypothalamus by Northern blot and RT-PCR analyses. The localization of GDF-9 mRNA specifically in oocytes of the mouse ovary was confirmed by in situ hybridization histochemistry. In mouse testis, although localization in Sertoli cell cytoplasm could not be ruled out, mRNA expression was observed in large pachytene spermatocytes and round spermatids. The expression of GDF-9 mRNA in human tissues was assessed by Northern blot and RT-PCR analyses. GDF-9 mRNA was observed in ovary and testis and, surprisingly, in diverse nongonadal tissues, including pituitary, uterus, and bone marrow. Therefore, GDF-9 mRNA expression in rodents is not exclusive to the ovary, but includes the testis and hypothalamus. Furthermore, human GDF-9 mRNA is expressed not only in the gonads, but also in several extragonadal tissues. The function and relevance of nongonadal GDF-9 mRNA are not known, but may affect strategies for contraception and fertility that are based on GDF-9 activity.