Journal of Cell Science 113, 1101-1109 (2000) 1101 Printed in Great Britain © The Company of Biologists Limited 2000 JCS1056 COMMENTARY Positive and negative regulation of TGF-β signaling Kohei Miyazono Department of Biochemistry, The Cancer Institute of the Japanese Foundation for Cancer Research (JFCR), and Research for the Future Program, the Japan Society for the Promotion of Science, 1-37-1 Kami-ikebukuro, Toshima-ku, Tokyo 170-8455, Japan *Author for correspondence (e-mail: [email protected]) Published on WWW 7 March 2000 SUMMARY Cytokines of the transforming growth factor β (TGF-β) which limit the magnitude of signals and terminate superfamily, including TGF-βs, activins and bone signaling. Negative regulation is also important for morphogenetic proteins (BMPs), bind to specific formation of gradients of morphogens, which is crucial in serine/threonine kinase receptors and transmit developmental processes. In addition, other signaling intracellular signals through Smad proteins. Upon ligand pathways regulate TGF-β and BMP signaling through stimulation, Smads move into the nucleus and function as cross-talk. Nearly 20 BMP isoforms have been identified, components of transcription complexes. TGF-β and BMP and their activities are regulated by various extracellular signaling is regulated positively and negatively through antagonists. Regulation of TGF-β signaling might be tightly various mechanisms. Positive regulation amplifies signals linked to tumor progression, since TGF-β is a potent to a level sufficient for biological activity. Negative growth inhibitor in most cell types. regulation occurs at the extracellular, membrane, cytoplasmic and nuclear levels. TGF-β and BMP signaling is often regulated through negative feedback mechanisms, Key words: TGF-β, BMP, activin, antagonist, Smad INTRODUCTION to two different types of serine/threonine kinase receptor: type I and type II (Fig. 1; Heldin et al., 1997; Massague, Transforming growth factor (TGF)-β and related factors are 1998; Zhang and Derynck, 1999). The type II receptor multifunctional cytokines that regulate growth, kinases are constitutively active; upon ligand binding, the differentiation, adhesion and apoptosis of various cell types type II receptors activate the type I receptor kinases through (Roberts and Sporn, 1990). More than 30 proteins have been phosphorylation of the juxtamembrane domain (mainly the identified as members of the TGF-β superfamily, which glycine-serine-rich domain or GS domain) of type I receptors. includes TGF-βs, activins and bone morphogenetic proteins The type I receptor kinases then activate intracellular (BMPs). Activins and BMPs play important roles in early substrates, the central signal messengers being Smad proteins embryogenesis (Kingsley, 1994; Hogan, 1996; Harland and (Miyazono et al., 2000). Smads include three subclasses: Gerhart, 1997); activins induce dorsal mesoderm in Xenopus receptor-regulated Smads (R-Smads), common-partner embryos, whereas BMPs induce ventral mesoderm. BMPs Smads (Co-Smads) and inhibitory Smads (I-Smads). R- also play critical roles in morphogenesis of various tissues. Smads are anchored to the cell membrane through TGF-β plays an important role in early embryonic membrane-bound proteins, including Smad-anchor for development (Goumans et al., 1999), but might play more receptor activation (SARA; Tsukazaki et al., 1998). R-Smads crucial roles at relatively late stages of development and in directly interact with and become phosphorylated by type I adult tissues. TGF-β acts as a potent growth inhibitor for most receptors. R-Smads then form complexes with Co-Smads and types of cells, including epithelial cells, endothelial cells, migrate into the nucleus, where they regulate transcription of hematopoietic cells and lymphocytes (Roberts and Sporn, target genes (Fig. 1). In mammals, Smad2 and Smad3 are 1990; Miyazono et al., 1994). In addition, TGF-β functions TGF-β/activin-specific R-Smads, whereas Smad1, Smad5 as a fibrogenic factor and is responsible for tissue sclerosis and, presumably, Smad8 are BMP-specific R-Smads. Smad4 of liver, kidney, lung, skin and other tissues. is the only Co-Smad in mammals, but two Co-Smads, TGF-β and related factors are produced as dimeric Smad4α and Smad4β, have been identified in Xenopus precursors, in which the C-terminal portions form active (Howell et al., 1999; Masuyama et al., 1999). Smad6 and ligands following proteolytic processing (Miyazono et al., Smad7 act as I-Smads. 1993; Kingsley, 1994). The secreted TGF-β-like factors bind Signaling by TGF-β-like factors is regulated in both positive 1102 K. Miyazono and others and negative fashions, and is tightly controlled temporally and EXTRACELLULAR ANTAGONISTS spatially through multiple mechanisms at the extracellular, membrane, cytoplasmic and nuclear levels. Positive regulation TGF-β is secreted as a latent complex, which must be activated could be critical for amplification of signaling by TGF-β-like to exhibit its biological effects (Saharinen et al., 1999). TGF- factors. Negative regulation plays an important role in βs are synthesized as precursor forms; the N-terminal portions restriction and termination of signaling, and often occurs of the TGF-β precursors are cleaved off, but remain bound to through a negative feedback loop (Fig. 2). Negative regulation the C-terminal active dimers and maintain them in inactive is also crucial in early embryonic development and forms (Miyazono et al., 1993). In contrast, activins and BMPs morphogenetic processes, limiting the range of signaling by do not form such TGF-β-like latent complexes and therefore TGF-β-like factors and forming a gradient of ligand activity. do not require prior activation to exert biological effects; Signaling by TGF-β-like factors is also regulated through however, their activities are tightly regulated by specific cross-talk with other signal transduction pathways, including antagonists. Two different types of antagonist have been MAP kinase pathways and JAK/STAT pathways. Perturbation identified: those that directly bind ligands, and those that of the negative regulation of TGF-β signaling might be linked belong to the TGF-β superfamily and interfere with binding of to the pathogenesis of various clinical disorders, especially ligands to specific receptors. progression of tumors. Ligand-binding antagonists Various antagonists that directly bind BMPs have been POSITIVE REGULATION OF TGF-β AND BMP identified. These include noggin, chordin, cerberus and its SIGNALING related proteins, and follistatin. Cerberus, gremlin, caronte, DAN and other structurally related proteins are collectively Positive regulation of TGF-β and BMP signaling, especially termed the DAN family (Hsu et al., 1998). Proteins of the DAN the induction of ligands and their signaling components, often family have a conserved cystine-knot motif, which is also occurs through the action of TGF-β-like factors themselves. found in other growth factors, including TGF-β-like factors For example, three mammalian isoforms of TGF-β (TGF-β1, (Pearce et al., 1999; Rodriguez Esteban et al., 1999). However, TGF-β2, and TGF-β3) are auto- and cross-induced by different other BMP antagonists lack sequence similarity with each TGF-β isoforms (Bascom et al., 1989; Kim et al., 1990; other. O’Reilly et al., 1992). Nodal and its related proteins, which Why are there so many antagonists of BMPs? One important play an important role in early embryogenesis and act through reason may be that these antagonists have distinct expression activin receptors and Smad2 (Nomura and Li, 1998), are also profiles and regulate different biological responses in vivo. induced by nodal signaling (Meno et al., 1999). In certain types Noggin and chordin are secreted by Spemann’s organizer, and of cell, TGF-β receptors might be induced by ligand induce neural tissue from ectoderm and dorsalize ventral stimulation (Bloom et al., 1996). mesoderm (Piccolo et al., 1996; Zimmerman et al., 1996). Transcription factors that function as targets of TGF-β- Cerberus plays an essential role in formation of head-like like factors are also induced by ligand stimulation. TGF-β structure. A cerberus-like protein, caronte, plays a critical role induces production of a transcription factor, Runx3 (formerly in the establishment of left-right asymmetry (Rodriguez termed PEBP2αC/Cbfa3/AML2), in B lymphocytes (Shi and Esteban et al., 1999; Yokouchi et al., 1999). Limb development Stavnezer, 1998). Newly synthesized Runx3 in turn forms a is controlled by various BMP antagonists, including noggin, complex with Smad3 activated by TGF-β, and cooperatively chordin, follistatin and gremlin, which have distinct roles in induces IgA class switching in B lymphocytes (Hanai et al., limb morphogenesis (McMahon et al., 1998; Capdevila et al., 1999). c-Jun is induced by TGF-β (Wong et al., 1999) and 1999; Merino et al., 1999). Noggin is also involved in hair- regulates the transcription of target genes in concert with follicle induction (Botchkarev et al., 1999). Smads (Zhang et al., 1998; Liberati et al., 1999). Another important reason may be that these antagonists have Smad signaling is also positively modulated through cross- different affinities for various BMP isoforms (some of them are talk with other signaling pathways. Smads might be activated termed growth/differentiation factors or GDFs) as well as other by tyrosine kinase receptor signals under certain conditions (de factors. Both noggin and chordin directly and specifically bind Caestecker et al.,