Author: Pooja Sharma
ABSTRACT
Activin-A belongs to the member of the TGF-beta superfamily of proteins produced by many types of cell throughout the development. They are homodimers of the β-subunit. The disulfide-linked subunits give rise to three proteins: βA-βA (activin-A), βA-βB (activin-AB), βB-βB (activin-B). Activin interacts with Type 1 and Type 2 threonine/serine kinases to signal SMAD proteins to regulate the various types of functions. Their function includes wound healing, differentiation, cell proliferation, apoptosis, and metabolism. Human activin-A has 100% identity of amino acid sequence to rat, mouse, bovine, porcine and feline proteins. Activin plays an important role in the embryo implantation and diagnosis of ectopic pregnancy. In it, they regulate the endometrial receptivity and trophoblast activity. Also, activin-A is used in the reduction of luteinization of human luteinized granulosa cells. Also, they maintain the pluripotency in the absence of feeder layers of human embryonic stem cells. Therefore, activins have many roles which are further described in this article.
INTRODUCTION
Activins were first isolated from “Porcine Follicular Fluid” in 1986. Porcine Follicular Fluids are gonadal proteins. They stimulate the synthesis and secretion of the pituitary follicle stimulating hormone (FSH). They are homodimeric proteins that are present in three different isoforms. Each isoform of activin is consists of two beta subunits (14 kDa) that are linked by disulfide bonds. They belong to the superfamily of transforming growth factor – β (TGF-β), a group of molecules that have similar structure and function. They are synthesized in various tissues and their expression levels are elevated in the pituitary gland, hypothalamus, placenta, and adrenal. Activins interact with Type 1 and Type 2 threonine/serine kinases to signal SMAD proteins that regulate the variety of functions. They participate in various physiological processes such as neural survival, angiogenesis, cellular differentiation and remodeling, cell adhesion, apoptosis and tissue repair.
Activin A is a homodimer of two chains of beta A which is biologically inactive until the propeptide of N-terminal is cleaved from each other Human Activin A has 100% similar amino acid sequence to rat, mouse, bovine, porcine and feline proteins.
In the reproductive process, activin A plays a key role in embryogenesis, expression of gonadotropins (Follicle Stimulating Hormone - FSH) and Luteinizing Hormone (LH) and maturation of ovarian follicles. Activin A is distributed in various organs during embryogenesis and also in spermatocytes and eggs. Also, they are important to support the survival and proliferation of granulosa cells.
For example- The best-understood example of these proteins is structure shows how the pro-domains dimerize by disulfide linkage and forms a latent complex with the mature growth factor.
STRUCTURE
Activins are dimeric proteins belongs to the superfamily of transforming growth factor-β (TGF-β), that are similar in structure but functionally diverse growth factors. It includes the Drosophila decapentaplegic gene product that is TGF-βs, bone morphogenetic proteins.
Activins are homodimers of the β-subunit. The disulfide-linked subunits give rise to three proteins: βA-βA (activin-A), βA-βB (activin-AB), βB-βB (activin-B). In recent, the another new three β-subunits have been cloned, that is βC, βD and βE, but there is no information available on the dimeric proteins. Therefore, the data which shows the formation of heterodimers of βC-activin in human liver and prostate. Out of these, when activin B is in its native form, then it is not identified in gonadal fluids. Like the other two activins, recombinant βB- βB is biologically active, in increasing the release of Follicle Stimulating Hormone (FSH) from cultured pituitary cells. That’s why the three forms of activin were considered to be the members of a hypothalamus-pituitary-gonadal axis. Due to this, they named as activins because they showed a stimulatory action in contrary to inhibins, on the pituitary secretion of FSH.
IMPORTANT ROLES OF ACTIVIN PROTEIN
- Role of Activins in Embryo Implantation and in the Diagnosis of Ectopic Pregnancy The implantation process (a complex process) and the invasion of trophoblast are crucial for the successful establishment of the pregnancy. Normally, when the receptivity of the implantation window and uterine is maximal, then an embryo implant in the endometrium. Receptivity of endometrial involves the expression of molecular factors that governs the feto-maternal dialogue and initiate the decidualization of endometrial stroma. Candidate molecules such as cytokines and growth factors, are secreted by epithelial and endometrial cells and they are described as major regulators of blastocyst implantation. Ectopic pregnancy i.e., EP is a form of abnormal pregnancy in which fertilized ovum implants outside the intrauterine cavity and the region of ampullary of the fallopian tube, which is the most common site of implantation. Ectopic pregnancy is worldwide health problem which leads to the maternal morbidity and/or death during the first trimester. Activins and their related proteins play a key role in the regulation of endometrial receptivity, the activity of trophoblast and in the implantation of an embryo. Pathological expression of these proteins have been connected to abnormal implantation and failing in the early pregnancy at the tissue levels and serum levels. Although, the serum activin-A have a diagnostic value of a single measurement in differentiating abnormal pregnancy to normal pregnancy is controversial.
- Activin A Reduces Luteinisation Of Human Luteinized Granulosa Cells In reproductive biology, one of the fundamental processes is the dominant follicle’s transition into the corpus luteum. It Involves the follicular cell’s transformation in the most active part of the body that is, steroidogenic endocrine gland. Luteinisation is the process where follicular granulosa cells are differentiated into the granulosa-lutein cells of the corpus luteum which involves marked and disparate the morphological, biochemical and cellular changes. Then, the granulosa cells become terminally differentiated, in such a way that they do not divide again and develop the enzyme machinery that is necessary for massive progesterone synthesis. This is the highly organized process, that is tightly orchestrated by steroid hormones, growth factors, and gonadotropins, and is associated with structural and functional changes that characterize the transition of follicular-luteal. In a conception cycle, a viable corpus luteum is necessary to the success of early gestation, while, in a non-conception cycle, its demise is essential for the next wave of folliculogenesis. In the luteinization of granulosa cells, the molecular pathways are not entirely clear. While the luteinizing hormone (LH) surge or addition of human chorionic gonadotrophin (hCG) or LH initiate luteinisation, experimental work clearly shows that in the absence of LH, the removal of granulosa cells from the follicle causes spontaneous luteinisation. Actually, this hampers the study of follicular granulosa cells in vitro. Therefore, it is believed that luteinisation is a pathway of differentiation that is programmed before antral formation, and there is only way by which the follicles can escape this fate is by inhibitory factors (Wehrenberg & Rune 2000). It is suggested that in order to discourage luteinisation the follicle itself can provide a milieu (Channing et al. 1980, Eppig et al. 1997), and inhibitor(s) of such magnitude may actually be present in follicular fluid (Ledwitz-Rigby et al. 1977, Channing et al. 1978) or it come directly from the oocyte itself (Vanderhyden & Macdonald 1998, Brankin et al. 2003). Therefore, it appears Luteinizing Hormone (LH) surge is able to remove such disrupt connections between granulosa cells and cells, inhibitory factors and/or induce genes that facilitate luteinisation. One molecule that may have a role in the luteinisation’s prevention is activin A. Activin A is a dimeric glycoprotein and member of the transforming growth factor (TGF)-β superfamily that is found in the follicular fluid and can delay the luteinisation and/or atresia of granulosa cell by decreasing basal and hCG-induced progesterone in human (Rabinovici et al. 1990, Di Simone et al. 1994), sheep (Shidaifat et al. 2001), monkey (Brannian et al. 1992) and goat (Shidaifat 2001) granulosa cells. In addition, at the end of the luteal phase, action of activin A may be involved in promoting luteolysis (Myers et al. 2007a). It is believed that one of the roles of hCG may be to inhibit activin action during maternal recognition of pregnancy. Therefore, it is hypothesized that activin A was anti-luteal and, LH surge is used to remove activin A and facilitate luteinisation, at the start of the luteal phase. At the end of the luteal phase, action of activin A is increased to facilitate luteolysis and hCG at early pregnancy, acting through the LH receptor, which continues to inhibit the action of activin A and facilitate luteal maintenance.
- In the absence of Feeder Layers, Activin A Maintains Pluripotency of Human Embryonic Stem Cells When the cells are grown on mouse embryonic feeder layers (mEFs), pluripotency of human embryonic stem cells (hECs) is maintained, on laminin or matrigel supplemented with conditioned medium (CM) from mEFs or on human feeder layers. Additionally, as in the case with mouse embryonic stem cells (mESCs), signals received from the feeder layers that do not operate through the leukemia inhibitory factor (LIF)/gp130 pathway. Consequently, alternate signaling pathways activates when it comes in contact with hESCs to feeder layers and/or soluble factors(s) which are present in the conditioned medium, that mediates the maintenance of pluripotency. It is showed that the morphology and growth of hESC line human skin fibroblast (HSF6) were similar when it is grown on either mEFs or on laminin or matrigel supplemented with conditioned media from mEFs. In the absence of a mEF-conditioned medium, HSF6 cells grown rapidly lose the pluripotency markers Nanog, Oct-4, and TRA-1-60, that indicates the loss of “stemness”. Thus, the conditioned medium contains soluble factors which are secreted by the feeder layers that helps in maintaining the pluripotency. In a study, it is reported that the hESCs has grown on laminin in the presence of activin A, keratinocyte growth factor (KGF), and nicotinamide (NIC) remain undifferentiated during continuous growth over 20 passages.
- Activin Signaling – An Emerging Target For Therapeutic Interventions After the purification and identification of activins as the regulators of follicle-stimulating hormone (FSH) from the anterior pituitary, activins play important roles in the hypothalamus-pituitary-gonadal axis have been described. However, the activity of activin is not limited to reproductive tissues. Activins and the related factors have pleiotropic actions in extra-gonadal tissues.
- Activin and Their Regulators in Metabolic Disorders Activin signaling is required for the development of the exocrine and endocrine pancreas, and the dysregulation of the activin signaling pathway that contributes to the genesis of metabolic diseases. In human embryonic stem cells (hESCs), activin B mediates the induction of homeoprotein Pdx1 (a key regulator of an endocrine pancreas that regulates the formation of pancreas islet). Type 1 receptor-like ALK7 for activin B, activin AB and nodal, is expressed abundantly in pancreatic beta (β) cells and adipose tissues, and help in regulating insulin biosynthesis and secretion.
- Activin and Myostatin in Muscular Diseases Myostatin is the skeletal muscle-specific member of the TGF- β family, that restricts muscle growth and determines skeletal muscle mass. Myostatin signals through activin type 1 receptors like ALK 4 & 5 & type 2 receptors. Mice with targeted deletion of the myostatin gene have increased muscle mass of 25-30% resulting from hypertrophy and hyperplasia. Myostatin is considered to be a good drug target for therapeutics that stimulates the growth of skeletal muscle may be useful for muscle wasting conditions such as sarcopenia, muscular dystrophy, cachexia. Whereas activins & TGF- β function in almost every type of cell. Myostatin specifically affects the growth of skeletal muscle. Thus, targeting myostatin is a rational therapeutic strategy to increase the mass of skeletal muscle.
- Activin and BMP Signaling in Osteoporosis and Bone Formation Although both activin and BMP regulate the formation of bone, their modes of action are distinct. BMPs are inducers of osteoblast differentiation. Activins are expressed mainly in bone tissues, and regulate the bone formation by controlling the functions of both osteoblast and osteoclast. Different from the BMP’s activity, activins enhance the receptor activator of NF-κB ligand that mediates the osteoclast differentiation, and it acts as a commitment factors for osteoclastogenesis. Both anabolic and antiresorptive drugs are useful for the treatment of osteoporosis. Bisphosphonates, modulators of selective estrogen receptor and estrogen are currently available drugs for antiresorptive, whereas parathyroid which is a human recombinant hormone is an anabolic drug. The extracellular domain of ActRIIA is stabilized by fusion to Immunoglobulin G-Fc (IgG-Fc) increases bone mass and strength by inhibition of activin, and it is a novel promising agent for the osteoporosis in early human trials. Thus, activin and BMP pathway are the therapeutic targets for the treatment of low bone mass.
- Roles of Activins and Their Related Growth Factors In Cancer One of the major activities of activins is the inhibition of cancer cell growth in the early stage of cancer development. Activin facilitates the signaling either by Crypto silencing or FLRG silencing which inhibits human breast cancer cell growth. Mutations in several genes are involved in the activin signaling pathway, characterized in cancers. Two polyadenine tracts of 8-bp of the ACVR2 gene were targets for the frameshift mutations in gastrointestinal cancers with the instability of microsatellite. Gene mutations in somatic ACVR1B have been found in pancreatic carcinoma and Smad 2 & 4 are mutated in pancreatic carcinomas and colorectal. Thus, activin receptors dysregulation and activin/Smads of TGF-β is directly involved in carcinogens. Cancer cells produce TGF- β has immunosuppressive effects, which results in the evasion of cancers by the immune system from destruction. But, TGF- β kinase (inhibitor) reverses this effect, which inhibits the growth of cell and enhances the immunogenicity of cancer cells.
- Activins Activities in The Brain In the central nervous, activins and activin receptors are highly expressed and have crucial roles in the neural development. The expression of inhibin βA mRNA, that encodes activin A and is induced by an excitatory synaptic input. It is induced in granule cell neurons of the hippocampus by high-frequency synaptic stimuli that produce LTP (Long Term Potentiation). The number of synaptic contacts is increased by activin by modulating dynamics of an actin in the spine of the neurons, which may be responsible for LTP establishment. This modulation is mediated by the MAP kinase cascades. Evidence indicates that activin also has neuroprotective and neurotrophic effects on selective neurons. Treatment by recombinant activin following ischemic injury rescues neurons from damage. Neurons which are overexcited protected by the neurotrophic effect of basic FGF (Fibroblast growth Factor), which depends on the induction of an activin-A.
- Activin-A is Active in Inflammation in COPD COPD (Chronic Obstructive Pulmonary Disease) is characterized by limitation of irreversible airflow in which chronic inflammation of the airways plays a major role. The persistent inflammation is triggered by inhaled toxic substances, for example- cigarette smoke. Signaling pathways such as WNT, transforming growth factor (TGF)- β and Sonic hedgehog have all been linked to COPD, just because of associations of genetic or because of differential gene and expression of protein in lung tissue. Although these pathways were originally known for their role in development, it is now clear that these pathways also play crucial roles in tissue inflammation and tissue repair. Activin-A is a member of the TGF- β superfamily which is an important regulator of embryonic development, hematopoiesis and a wide range of tightly regulated biological processes, including tissue repair and immunity. Dysregulation of activin-A may contribute to the disease development. In recent, increased in the expression of activin-A has been demonstrated in pulmonary hypertension, asthma, and acute lung injury. Inflammatory cytokines, oxidative stress, toll-like receptor ligands stimulate the production and expression of activin-A, which in turn it regulates the release of an inflammatory cytokine, explaining its role in inflammatory responses. Follistatin, the endogenous activin-A inhibitor has the ability to counteract activin-A activity. Due to its involvement in the inflammatory responses, it is rational to connect the activin-A to the pathogenesis of inflammatory diseases. An imbalance between follistatin and activin-A potentially leads to excess in activin-A mediated inflammatory responses. However, the roles of follistatin and activin-A have not yet been established in Chronic Obstructive Pulmonary Disease (COPD). In the tissue (Verhamme et al.), it is demonstrated for the first time that the activin-A is an important regulator of the smoke of cigarette which induces inflammation in COPD. It is found that activin-A is activated and elevated in the airway epithelium, alveolar macrophages and airway smooth muscle of COPD patients. These findings were further validated in vivo to demonstrate that the cigarette smoke induces marked expression of activin-A in the lungs and bronchoalveolar lavage (BAL) fluid of mice. In in-vitro, the expression of activin-A is increased after cigarette smoke exposure of human bronchial epithelial cells, whereas the expression of follistatin was reduced. These data suggests that the smoke of cigarette is a major factor involved in the upregulated of activin-A. The limitation of airflow, lung tissues of smokers had significantly higher level of activin-A mRNA expression is compared with non-smokers subjects. Whereas the increase in the expression of activin-A in airway epithelium was specific for COPD. This study reveals that inhibition of activin-A signaling by administrating of its endogenous inhibitor follistatin, which attenuates cigarette smoke induce production of monocyte chemoattractant protein-1, interleukin (IL)-6, keratinocyte-derived chemokine and TDF-β1, tumor necrosis factor (TNF)-α, and induces the release of anti-inflammatory cytokine IL-10. Administration of follistatin reduces the number of inflammatory cells, such as macrophages, monocytes, CD4+ and CD8+ cells and neutrophils, in the BAL fluid of cigarette smoke-exposed mice. It is concluded that activin-A is not an innocent bystander but it plays an important and active role in cigarette smoke-induced in pulmonary inflammation.
References:
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About Author / Additional Info:
I am pursuing M.Sc in Biotechnology from Chaudhary Charan Singh University, Meerut