Current Molecular Pharmacology, 2012, 5, 195-204 195 Activin Receptor Signaling: A Potential Therapeutic Target for Osteoporosis Sutada Lotinun*,1, R. Scott Pearsall2, William C. Horne1 and Roland Baron1,3 1Department of Oral Medicine, Infection and Immunity, Harvard School of Dental Medicine, Boston, MA 02115, USA; 2Acceleron Pharma, Inc., 128 Sidney Street, Cambridge, MA 02139, USA; 3Endocrine Unit, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA Abstract: Current antiresorptive therapies not only prevent bone loss by decreasing osteoclastic bone resorption but also inhibit bone formation. Dual anabolic antiresorptive agents may be required to cure severe osteoporosis by preventing further bone loss and increasing bone mass to normal levels. Recent studies have demonstrated that activin signaling plays a crucial role in the skeleton. Activins, like other TGF-β superfamily members, transduce their signals through type I and II receptor serine/threonine kinases. The binding of activins to activin type IIA (ActRIIA) or type IIB (ActRIIB) receptors induces the recruitment and phosphorylation of an activin type I receptor (ALK4 and/or ALK7), which then phosphorylates the Smad2 and Smad3 intracellular signaling proteins. Activin signaling is down-regulated by inhibins, follistatin and other proteins, which antagonize activin signaling by a variety of mechanisms. A soluble chimeric protein composed of the extracellular domain of ActRIIA fused to IgG-Fc binds to circulating ligands such as activin A and prevents signaling through the endogenous receptor. In cynomolgus monkeys, the ActRIIA soluble receptor increases bone volume by decreasing bone resorption and increasing bone formation, leading to enhanced mechanical strength and bone quality. In addition, a single dose of the soluble ActRIIA-Fc fusion protein increased serum BSALP and PINP and decreased serum CTX and TRACP 5b in postmenopausal women. These data provide evidence of a dual anabolic antiresorptive effect of the soluble ActRIIA-Fc fusion protein in the skeleton. Therefore, targeting activin receptor signaling may be useful for therapeutic intervention in osteoporosis. Keywords: Activin, follistatin, inhibin, osteoblast, osteoclast, osteoporosis. INTRODUCTION available in the United State is the 1-34 fragment of recombinant human parathyroid hormone (PTH) or Osteoporosis is one of the most severe problems teriparatide. PTH dramatically increases bone mass and affecting the quality of life in the elderly. There are more reduces fracture risk. However, the use of teriparatide as a than 1 million osteoporotic fractures per year in the United treatment for osteoporosis is limited to 24 months in the States, the most devastating of which are hip fractures. United State and 18 months in Europe [5]. Several novel Approximately 20% of individuals over the age of 50 years anabolic agents targeting various signaling pathways are die within a year of a hip fracture, and many others require currently in development. Increasing canonical Wnt long-term care in nursing homes. Current treatments for signaling using antibodies that prevent the binding of osteoporosis are inadequate due to limited efficacy; negative regulators of Wnt signaling, such as sclerostin and approximately 50% of patients taking the presently approved dickkopf-1 (DKK-1), to the co-receptor lipoprotein-receptor- drugs suffer a subsequent fracture [1]. The antiresorptive related protein (LRP) 5/6 has emerged as a promising agents used in the treatment of osteoporosis prevent bone strategy to increase bone mass [6-8]. However, concerns resorption but also inhibit bone formation. Long-term about the long-term safety of Wnt inhibitors have not been treatment with bisphosphonates is associated with an addressed. impairment of bone formation and osteonecrosis of the jaw The activin/inhibin/follistatin system has also been and may also lead to reduced mechanical strength due to implicated in the regulation of bone homeostasis and age- accumulated strain-induced microfractures in the absence of bone remodeling [2-4]. In spite of the widely appreciated related bone loss [9], and it has been suggested that decreased gonadal inhibin secretion and the consequent magnitude of this problem, there is still a critical gap in our increase in activin activity is an important contributory factor understanding of how to effectively reverse bone loss in the in the perimenopausal increase in bone resorption [10]. elderly. Consistent with this, it has recently been shown in animal Due to the limitations of antiresorptive agents and the models [11-13] and a phase I clinical trial [14] that a soluble therapeutic value of inducing bone formation, particularly in ActRIIA-Fc fusion protein represents a potential anabolic severe osteoporosis, a major effort has focused on agent. identifying and developing agents that exert an anabolic In this review, we focus on recent advances in the effect on bone. The only approved bone anabolic agent elucidation of activin signaling, with a particular emphasis on activin interactions with their receptors, activin *Address correspondence to this author at the Department of Oral Medicine, antagonists and the effects of activins on bone turnover. We Infection and Immunity, Harvard School of Dental Medicine, Boston, MA also discuss the evidence that the soluble ActRIIA-Fc fusion 02115, USA; Tel: (617) 432-7328; Fax: (617) 432-1897; protein is a promising potential therapeutic agent for treating E-mail: [email protected] osteoporosis and related diseases. 1874-4702/12 $58.00+.00 © 2012 Bentham Science Publishers 196 Current Molecular Pharmacology, 2012, Vol. 5, No. 2 Lotinun et al. ACTIVIN STRUCTURE AND SIGNALING are able to bind ligands in the absence of type I receptor. However, they are unable to initiate signal transduction Activin Isoforms without forming a complex with an activin type I receptor. Activins are members of the transforming growth factor- Canonical activin signaling involves activation of Smad β (TGF-β) superfamily that elicit diverse biological family proteins and translocation to the nucleus (Fig. 1). The responses in reproductive, hematopoietic, immune, central high affinity binding of activins to ActRIIA or ActRIIB nervous and musculoskeletal system [15-16]. Other members receptors induces the recruitment ALK4 and/or ALK7 and in the TGF-β superfamily include TGF-βs, bone phosphorylation of the regulatory GS domain, followed by morphogenetic proteins (BMPs), myostatin, nodal, growth activation of Smad2 and Smad3. The receptor-activated and differentiation factors (GDFs), anti-Muellerian hormone Smads subsequently form a complex with the common co- (AMH) and others [17-18]. Activins were first purified as Smad (Smad4) allowing for translocation into the nucleus dimers consisting of disulfide-linked β subunits and where they bind to the promoter sequences of target genes identified as local mediators capable of stimulating pituitary and regulate gene transcription and cellular function. In follicle-stimulating hormone (FSH) release [19]. Four addition to the canonical signaling pathway, other non- different β -subunits of activin (βA, βB, βC and βE) have canonical or Smad-independent effectors, including ERK1/2, been described in mammals and a βD subunit has been JNK and p38 MAPK, PKC, Akt, IκB-α and Wnt/β-catenin identified in Xenopus [20-22]. The βA and βB subunits are have been reported in several tissues [27-33]. However, the detectable in most tissues with high expression in link between the activated receptor complexes and non- reproductive organs whereas the βC and βE subunits are canonical signaling remains to be defined. These signaling predominantly expressed in liver. Homo- and cascades may modulate the Smad-dependent responses, or heterodimerization of βA and βB subunits gives rise to three they may act on distinct downstream effectors in different biologically active glycoproteins, including activin A (βA- cell types. βA), activin B (βB- βB) and activin AB (βA- βB). ACTIVIN ANTAGONISTS Activin Receptors and Signal Transduction Activin signaling is regulated by multiple factors that act Activins, like other TGF-β family members, initiate their at the intracellular, membrane or extracellular levels. signal transduction cascades through two types of single Intracellularly, the inhibitory Smad7 prevents activin- transmembrane serine-threonine kinase receptors, called type mediated phosphorylation of Smad2 and Smad3, thereby I and type II receptors, which are approximately 55 and 70 blocking the signaling pathway [34]. The binding of activin kDa, respectively. Upon the ligand binding to receptors, two to its receptor is antagonized by inhibins, which block the type I and two type II receptors form a tetrameric complex. activin binding site and by BAMBI (BMP and activin Seven type I receptors, activin receptor-like kinase 1-7 receptor membrane bound inhibitor), a pseudo-receptor that (ALK1-7) and 5 type II receptors (TβRII, BMPRII, sequesters activin. Cripto blocks the recruitment of the type I AMHRII, ActRIIA and ActRIIB) have been identified in the receptor by the ligand-occupied type II receptor. Finally, TGF-β receptor superfamily [23]. Various combinations of extracellular antagonists, including follistatin and follistatin- the type I and type II receptors mediate signaling by different like 3, bind to ligands with high affinity and interfere with ligands in a cell-specific manner. These ligand/receptor receptor engagement.