Regulation of activin βA mRNA level by cAMP

Calcium-regulated expression of activin A in RBL-2H3 mast cells

The present study examined the regulatory expression of activin A, a potent growth and differentiation factor, in rat basophilic leukemia (RBL-2H3) mast cells. Treatment of RBL-2H3 cells sensitized with anti-dinitrophenyl IgE with multivalent dinitrophenyl led to a clear increase in RT-PCR products of inhibin/activin beta(A). The steady-state mRNA of inhibin/activin beta(A) was also induced by increasing cytosolic Ca(2+) concentration with ionomycin, which required de novo protein synthesis, and was regulated at the transcriptional level. Pretreatment of RBL-2H3 cells with antagonists or inhibitors for the calmodulin pathway blocked ionomycin-dependent inhibin/activin beta(A) transcription and mRNA induction, suggesting the involvement of calmodulin-dependent kinase (CaMK) and calcineurin. The ionomycin-dependent inhibin/activin beta(A) induction was also partially blocked by preincubation with c-Jun NH(2)-terminal kinase (JNK) and p38 kinase inhibitors, but not with MEK1 inhibitor. These results suggest that inhibin/activin beta(A) gene activation is achieved by the JNK and p38 kinase activation through the calmodulin pathway in mast cells.

Reversible increase of serum activin A levels in women with Graves' disease

The aim of this study was to analyze the serum levels of activin A in hyperthyroid patients with Graves' disease. Serum activin A and FSH levels were measured in a total of 93 females (64 regularly cycling and 29 post-menopausal). Of these, 20 were hyperthyroid patients with Graves disease, 33 were euthyroid goitrous patients (20 had autoimmune thyroiditis AT and 13 only had goiter) representing the internal control group and 40 were healthy subjects representing the external control group. Serum levels of activin A were higher in goitrous patients with AT than in control subjects (p=0.0388). Activin A levels were almost doubled in the cycling and in post-menopausal hyperthyroid women (0.91+/-0.21 vs 0.43+/-0.07 microg/l; p<0.0001 and 0.92+/-0.22 vs 0.48+/-0.24 microg/l; p=0.0001, respectively). In 10 cycling hyperthyroid patients, studied even after methimazole treatment, that increase was substantially reversed, once euthyroidism was attained (p=0.002). These findings indicate that thyroid function and autoimmune processes significantly affect serum levels of activin A in patients with Graves' disease.

Up-regulation of the expression of activins in the pancreatic duct by reduction of the beta-cell mass

Activins expressed in progenitor cells of the pancreas regulate differentiation of endocrine cells during development. Neogenesis of beta-cells takes place in adult animals under some conditions, and beta-cells are thought to arise from precursors locating in the pancreatic duct. In the present study, we investigated whether or not activins are expressed in the duct where beta-cell neogenesis is initiated. mRNA for the beta(A)- and beta(B)-subunits was expressed in isolated mouse pancreatic ducts. Immunohistochemically, the beta(A)-subunit was detected in the pancreatic duct and colocalized with cytokeratin, a marker of ductal cells. The beta(A)-subunit was also expressed in nestin-positive cells in the duct. Likewise, the beta(B)-subunit was detected in the pancreatic duct. In addition, mRNA for the type II and type IIB activin receptors was expressed in the duct. Expression of mRNA for two activin subunits was markedly increased after streptozotocin injection. Similarly, the mRNA expression was up-regulated after partial pancreatectomy. These results indicate that activins are expressed in the pancreatic duct and are up-regulated shortly after the reduction of the beta-cell mass. Induction of activins in the duct may be a critical step in the initiation of beta-cell neogenesis.

Structure of the rat inhibin and activin betaA-subunit gene and regulation in an ovarian granulosa cell line

We have isolated the rat inhibin and activin betaA-subunit gene, which is composed of three exons, and have characterized a 571-bp region upstream from the transcriptional start site that functions as a promoter in transient transfection studies in an ovarian granulosa cell line, GRMO2. Deletion analysis of the 571-bp promoter region has identified DNA sequences between -362 bp and -110 bp to be essential in mediating basal promoter activity and activation by forskolin (FSK) and/or 12-O-tetradecanoylphorbol-13-acetate (TPA). Within this region, a variant CRE (cAMP response element) has been identified at -120 bp. Point mutations in the variant CRE substantially reduce the ability of FSK and/or TPA to induce promoter activity in GRMO2 cells. A single nucleotide change in the variant CRE, which converts it to a consensus CRE, does not enhance promoter activity in response to FSK and/or TPA, but rather reduces promoter activity to the same extent as the other inactivating mutation in the variant CRE, suggesting that this element does not act as a classical CRE. Consistent with this, electrophoretic mobility shift assays performed using antibodies to a variety of cAMP and phorbol ester-responsive transcription factors indicate that the AP-1 family proteins jun-B and fos-B are present in the protein complex binding to the variant CRE. Overexpression of jun-B and fos-B in GRMO2 cells resulted in a robust activation of the betaA-subunit promoter. Our results suggest that this novel variant CRE sequence mediates both cAMP and phorbol ester regulation through its interactions with AP-1family proteins.

Regulation of activin-A production by human amnion, decidua and placenta in vitro by pro-inflammatory cytokines

Activin-betaA subunits are expressed by the human placenta and extraplacental membranes at term and preterm. The regulation of activin-A production by these tissues has not been characterized to date, however. To determine the effects on activin-A production of pro-inflammatory cytokines, amnion, decidual and placental cells were isolated by enzyme dispersion and treated in primary culture with interleukin-1beta (IL-1beta) and tumour necrosis factor-alpha (TNF-alpha). Activin-A production (determined by ELISA) by amnion, decidual and placental cultures was 1.2 +/-0.27, 31.1+/-9.9, and 50.7+/-28.5 pg/microg protein/16 h, respectively (mean+/-SEM; n=5-7 experiments). Both IL-1beta and TNF-alpha stimulated activin-A production in a concentration-dependent fashion in all cultures; maximal stimulation was achieved at 0.25-1.0 ng/ml IL-1beta and 25-50 ng/ml TNF-alpha, respectively. In amnion, decidual and placental cultures IL-1beta stimulated activin-A production to 747+/-274, 190+/-11, and 254+/-60.2 per cent of controls, while TNF-alpha stimulated production to 312+/-81.5, 194+/-22.5, and 193+/-12.5 per cent, respectively (mean+/-SEM; n=5; P<0.05 by ANOVA). These studies show for the first time that pro-inflammatory cytokines are potent stimulators of activin-A production by intrauterine tissues. This may provide an explanation for the elevated concentrations of activin-A measured in the sera of some women in preterm labour.

Equine inhibin/activin beta A-subunit mRNA is expressed in the endometrial gland, but not in the trophoblast, during pregnancy

The expression of both inhibin alpha- and inhibin/activin beta A-subunit mRNA was examined in equine uteroplacental tissues collected during pregnancy (days 90 to 300). Northern blot analysis revealed that 5 transcripts (7.0, 4.1, 3.4, 2.6, 1.5 kb) of beta A-subunit were present, and the most abundantly expressed transcript was the 1.5 kb one. Relatively high levels of the 1.5 kb transcript were seen in the second trimester of pregnancy compared to what was found in the third trimester. To identify the tissue localization of beta A-subunit mRNA, in situ hybridization was performed, and the positive signal was observed exclusively in the endometrial glands, but not in the fetal placental tissue (trophoblast) at days 150, 210, and 300 of pregnancy. On the other hand, inhibin alpha-subunit transcript could not be detected at any stage of pregnancy examined either by Northern blot analysis or in situ hybridization. Although the factor(s) regulating the gene expression of beta A-subunit in this equine tissue is currently unknown, these results suggest that activin, but not inhibin, is predominantly produced in the endometrial glands of the pregnant mare, and thus produced activin may play a paracrine or endocrine role during pregnancy in this species.

Alternative poly(A) site selection in complex transcription units: means to an end?

Many genes have been described and characterized which result in alternative polyadenylation site use at the 3'-end of their mRNAs based on the cellular environment. In this survey and summary article 95 genes are discussed in which alternative polyadenylation is a consequence of tandem arrays of poly(A) signals within a single 3'-untranslated region. An additional 31 genes are described in which polyadenylation at a promoter-proximal site competes with a splicing reaction to influence expression of multiple mRNAs. Some have a composite internal/terminal exon which can be differentially processed. Others contain alternative 3'-terminal exons, the first of which can be skipped in some cells. In some cases the mRNAs formed from these three classes of genes are differentially processed from the primary transcript during the cell cycle or in a tissue-specific or developmentally specific pattern. Immunoglobulin heavy chain genes have composite exons; regulated production of two different Ig mRNAs has been shown to involve B cell stage-specific changes in trans -acting factors involved in formation of the active polyadenylation complex. Changes in the activity of some of these same factors occur during viral infection and take-over of the cellular machinery, suggesting the potential applicability of at least some aspects of the Ig model. The differential expression of a number of genes that undergo alternative poly(A) site choice or polyadenylation/splicing competition could be regulated at the level of amounts and activities of either generic or tissue-specific polyadenylation factors and/or splicing factors.

Inhibin, activin and follistatin in the human placenta--a new family of regulatory proteins

The family of inhibin-related proteins has been investigated extensively in the last decade. It is composed of three members: inhibin, activin and follistatin. Inhibin and activin are chemically related, while follistatin acts as an activin-binding protein. Initially identified as regulators of pituitary follice stimulating hormone (FSH) secretion, inhibin, activin and follistatin have more recently been characterized as growth factors, embryo modulators and immune factors. Human placenta, amnion, chorion and maternal decidua express mRNAs for inhibin, activin and follistatin, and the presence of both immunoreactive and bioactive proteins has been demonstrated. The proteins are present in maternal and fetal circulation, and are measurable in amniotic fluid with changes related to gestational age and to the occurrence of gestational diseases. Various biological actions have been described in embryo and intrauterine tissues, which suggest a role for these proteins in the development of the gestational unit. However, several questions remain to be elucidated. The chemical forms of inhibin, activin and follistatin produced by human placenta and the mechanisms involved in the regulation of their secretion are largely unknown. The nature of the receptors for these proteins and the physiological implications of receptor activation have not yet been elucidated and this will require further investigation.

Human activin betaA gene. Identification of novel 5' exon, functional promoter, and enhancers

On the basis of cDNA cloning, primer extension, and transfection experiments, we identified a novel 5' exon of the human activin betaA subunit gene, and found its enhancer and promoter regions as well as multiple transcription start sites. A series of deletion and mutation analyses of the enhancer sequences defined the 45-base pair core region (DR-1 core) containing two short elements with similarity to AP-1 (12-O-tetradecanoylphorbol-13-acetate response element; TRE) and CREB/ATF (cyclic AMP response element; CRE) binding sites, both of which were necessary for full enhancer activity. Gel shift and antibody supershift assays using DR-1 core region revealed the formation of two specific DNA-protein complexes, one of which could be partially dissociated by a competing oligonucleotide containing a single copy of the consensus TRE, but the other of which contained neither CREB/ATF nor AP-1 as major components. Although 12-O-tetradecanoylphorbol-13-acetate and cAMP induced the activin enhancer/promoter-driven CAT activity, such drug induction was obscured when either the TRE- or CRE-like elements were mutated in the native promoter context. Our results demonstrate that the promoter and enhancer regions identified here are essential for maintaining the efficient promoter activity of the human activin betaA subunit gene.

Differential regulation of inhibin/activin alpha- and beta A-subunit and follistin mRNAs by cyclic AMP and phorbol ester in cultured human granulosa-luteal cells

Granulosa cell-derived inhibin A (a dimer of alpha- and beta A-subunits), activin A (a homodimer of beta A-subunits) and the activin-binding protein follistatin are important regulators of human ovarian steroidogenesis. We here studied how 8-bromo-cAMP (8br-cAMP), a protein kinase A activator, and 12-O-tetradecanoylphorbol 13-acetate (TPA), a protein kinase C activator, affect the steady-state levels of alpha- and beta A-subunit and follistatin mRNAs in cultured human granulosa-luteal cells. 8br-cAMP induced alpha- and beta A-subunit and follistatin steady-state mRNA levels in a time- and concentration-dependent manner. The levels of alpha-subunit mRNAs were stimulated by 8br-cAMP in a sustained manner with a maximal induction seen at the time points 24 and 48 h. By contrast, beta A-subunit and follistatin mRNA levels were rapidly and transiently induced by 8br-cAMP with maximal effects observed at 3 h and 8 h, respectively. TPA did not affect basal alpha-subunit mRNA levels but it rapidly induced beta A-subunit mRNAs at 3 h and the stimulation was still evident at 48 h. TPA induced follistatin mRNA levels with kinetics similar to 8br-cAMP but to a lesser extent. Moreover, 8br-cAMP and TPA stimulated beta A-subunit and follistatin mRNA levels synergistically at 3 h. By contrast, TPA had a potent inhibitory effect on 8br-cAMP- and hCG-induced alpha-subunit levels. Neither 8br-cAMP nor TPA regulated inhibin/activin beta B-subunit mRNA levels. Taken together the activation of protein kinase-A and -C by 8br-cAMP and TPA, respectively, lead to clearly differential responses in the steady-state levels of inhibin activin alpha- and beta A-subunit and follistatin mRNAs. These results suggest that the inhibin A vs. activin A ratio as well as follistatin levels are regulated by multiple second-messenger pathways in the human ovary.

Tissue-specific variation in the length of the 5' untranslated region of the beta A-inhibin mRNA in sheep

The 5' untranslated region (UTR) of beta A inhibin mRNA was compared in a variety of sheep tissues, using primer extension. Considerable variation in the length and number of 5' extended products were noted between tissues. Specific bands were noted in ovarian follicular RNA, which were also present in samples from corpora lutea, stroma, and placental cotyledon RNA. Other extended products were observed in RNA from corpora lutea, stroma, cotyledon, pituitary, bone marrow, frontal cortex, medial basal hypothalamus, adrenal, liver, and kidney, which were not present or weakly represented in follicular RNA. Additional tissue-specific bands were noted in testis and bone marrow RNA. No specific differences in the lengths of the 5' UTR of the beta A inhibin mRNA were observed in sheep homozygous for the Booroola fecundity gene FecB, in any tissue studied. The coding region of ovine beta A inhibin cDNA was sequenced and a genetic polymorphism confirmed within or close to the ovine beta A inhibin gene. We conclude that the beta A inhibin gene is expressed widely in the sheep. Furthermore there is variation in the length of the 5' UTR of beta A inhibin mRNA between male and female gonads and other tissues, implying that expression of this gene is differentially controlled. However, the FecB mutation does not affect mRNA splicing events or the initiation site used in ovarian transcription. The mechanism by which the FecB mutation influences the amounts of beta A inhibin mRNA, follicle-stimulating hormone (FSH) secretion and ovulation rate has still to be elucidated.

Genomic cloning and sequence analyses of the bovine alpha-, beta A- and beta B-inhibin/activin genes. Identification of transcription factor AP-2-binding sites in the 5'-flanking regions by DNase I footprinting

Inhibins and activins are dimeric peptide hormones that regulate the circulating levels of follicle-stimulating hormone (FSH). In turn, FSH stimulates inhibin gene expression in the ovarian follicle; studies to date suggest that this effect is mediated by cAMP and that a cAMP-responsive element, identified in the 5'-flanking region of the alpha-inhibin gene, at least partially effects this response. To explore further the transcriptional regulation of the inhibin/activin genes, we have isolated and sequenced the 5'-flanking regions of the bovine alpha-, beta A- and beta B-inhibin/activin subunit genes and have analysed these regions by primer-extension analysis and DNase I footprinting with the transcription factor AP-2. Analyses indicated that all three gene promoter regions have a number of AP-2-binding sites that are resistant to competition by poly(dI-dC), suggesting that cAMP may control the inhibin/activin ratio by operating through alternative signal-transduction pathways or that inhibin/activin gene expression may be controlled by signals operating through the protein kinase C pathway. A comparison of the DNA sequences protected by AP-2 against DNase I digestion revealed a consensus AP-2-binding site of 5'-GSCCCDSS-3', where S represents a base pairing involving three (C or G) hydrogen bonds and D represents any base other than C. The nucleotide sequences of the bovine beta-subunit structural genes also are reported.

Comparative regulation of inhibin, activin and human chorionic gonadotropin production by placental trophoblast cells in culture

In the present study, we investigated the roles of cyclic adenosine monophosphate (cAMP), intracellular calcium, glucocorticoids, protein kinase-C and gonadotrophin-releasing hormone (GnRH) in regulating human chorionic gonadotrophin (hCG), inhibin and activin production in cultured human term placental trophoblast cells. Inhibin and hCG were measured in conditioned media by radioimmunoassay, while putative forms of inhibin and activin were characterized by western blotting using affinity-purified antisera directed against the inhibin alpha- and beta A-subunits. Inhibin and hCG secretion were stimulated by dexamethasone (0.2 microM), GnRH (5-25 microM), calcium ionophore A23187 (0.2-1 microM), phorbol-12-myristate-13-acetate (22 nM) and epinephrine (1 microM), with increasing response over successive 24-h treatment periods. Two molecules Mr approximately 30 and 32 kDa appeared to be the predominant dimeric forms of inhibin secreted by the cells, while 26 kDa activin was present in excess over inhibin. Large amounts of 40-44 kDa protein were detected by the alpha-directed antisera only, which may be a form of the inhibin alpha-subunit precursor protein. Secretion of activin was responsive to phorbol ester-mediated stimulation but not to the presence of GnRH or elevated cAMP concentrations. The divergence in maternal serum inhibin and hCG concentrations during late pregnancy remains unexplained by these findings.

Gonadotropin-releasing hormone (GnRH) and cyclic AMP positively regulate inhibin subunit messenger RNA levels in human placental cells

Bioactive and immunodetectable levels of both inhibin and activin are present in the placenta, raising questions as to the regulatory control of their synthesis. This study was designed to determine the effect of cyclic AMP (cAMP) and gonadotropin-releasing hormone (GnRH) on inhibin subunit gene expression in short-term incubations of placental cells. A semi-quantitative polymerase chain reaction (PCR) technique was used after isolation of total RNA and first strand cDNA synthesis from mechanically dispersed trophoblast-enriched cells obtained from human placentae at term. The level of gene expression of inhibin subunits was higher for beta A and alpha-subunits mRNA compared to the beta B-subunit mRNA as determined by PCR in combination with Southern blotting or Northern hybridization. Steady-state levels of beta-actin mRNA did not change throughout the 6-h incubation period and was used as a control of PCR amplification of the respective inhibin subunit gene transcripts following treatments with 8-bromo cAMP or GnRH. 8-bromo cAMP dose-dependently increased all three inhibin subunit gene transcripts with maximal responses seen at 150 microM (alpha-subunit mRNA 2.3-fold, beta A-subunit mRNA 1.8-fold and beta B-subunit mRNA 2.8-fold over control). GnRH (100 nM) significantly increased inhibin alpha and beta B-subunit mRNA levels 2.9-fold and 2.0-fold, respectively (P < 0.01), but not beta A-subunit mRNA. Collectively, the present findings demonstrate that in human term placental cells, gene expression of all inhibin subunits is under the direct influence of cAMP and further support a modulatory role of local GnRH in placental trophoblasts during late pregnancy.

Tumor necrosis factor and interleukin-1 induce activin A gene expression in a human bone marrow stromal cell line

Activin A, a homodimer of the beta A chain, regulates hematopoiesis. In a human bone marrow-derived stromal cell line, KM-102, phorbol myristate acetate, tumor necrosis factor-alpha and interleukin-1 beta induced great increases in beta A chain mRNA levels and production of activin A activities. The phorbol ester-induced beta A chain gene expression was inhibited by cycloheximide and down regulation of protein kinase C, whereas the cytokine-induced expression was little affected by these treatments. These results indicate that the inflammatory cytokines directly stimulate beta A chain gene expression via protein kinase C-independent pathways.