TGF-β receptor signaling

https://doi.org/10.1016/S0304-419X(97)00017-6Get rights and content

Introduction

The last fifteen years have seen an explosive progress in our knowledge of how cell proliferation and differentiation are regulated by extracellular factors. A few growth factors were initially characterized following extensive purification and are now often considered as prototypes for families of structurally related factors. Concomitant with their characterization came the recognition that these related factors mediate their activities through a large number of cell surface receptors, which based on structural features, can also be divided in families. Most receptors for growth stimulatory factors are cell surface transmembrane proteins that either contain a cytoplasmic tyrosine kinase domain or associate with a cytoplasmic tyrosine kinase. Ligand-induced autophosphorylation of these kinases on tyrosines then initiates the signaling cascades that lead to altered cell proliferation. Usually, structurally related growth factors interact with structurally related receptors and distinct receptor families generally mediate activities of families of growth factors. Thus, the combinatorial interactions between multiple growth factors and receptors are likely to result in the transfer of a complex body of information through multiple signaling cascades into the nucleus, where consequent changes in cell proliferation and differentiation and in gene transcription are initiated (for reviews: 1, 2, 3, 4, 5, 6, 7).

The rapid progress in our knowledge of growth factors and their receptors is well illustrated by the characterization of a superfamily of growth factors related to transforming growth factor-β (the TGF-β superfamily; Fig. 1), and, more recently, of a receptor family for these factors. TGF-β was initially identified in the medium of several transformed and non-transformed cell lines 8, 9. The high levels of TGF-β1 in blood platelets allowed the purification 10, 11and subsequent cDNA cloning [12]of TGF-β1. TGF-β1 is a disulfide-linked homodimer of two 112 amino acid polypeptides with a characteristic pattern of nine cysteines 11, 12. Subsequently, two additional TGF-β species, TGF-β2 and -β3, each encoded by separate genes, were identified 13, 14. All three TGF-βs are synthesized as precursors with large prosegments 13, 14, which are cleaved from the C-terminal mature monomer presumably through the activity of a KEX-like protease [15]. Two molecules of the prosegment interact noncovalently with the mature TGF-β dimer and maintain it in a biologically inactive form which is unable to bind to the receptors 16, 17, 18. A subsequent activation event, often resulting from proteolytic cleavage of the prosegments by plasmin, is required to release the biologically active TGF-β dimer that binds to the TGF-β receptors [19].

Following the molecular characterization of TGF-β1, many structurally related factors have been identified (Fig. 1). The TGF-β isoforms have nine characteristic cysteines and seven of these conserved in a defined spacing pattern in all members of the TGF-β superfamily. In addition, all TGF-β-related factors are disulfide-linked dimers, usually homodimers although several of them are able to form heterodimers with properties that are different from homodimers (for reviews: 20, 21, 22). The conserved cysteine pattern of all TGF-β superfamily members strongly suggests similar disulfide configurations and three-dimensional structures which are likely to resemble the extended butterfly-like knot structure of TGF-β2, in which the two polypeptides are linked by a single disulfide bridge 23, 24, 25. In contrast with the mature sequences, the prosegments of the TGF-β-related polypeptides are only minimally conserved. Whereas the TGF-β prosegment keeps TGF-β in a latent complex and may play a role as chaperone during protein export 26, 27, the roles of the prosegments of other TGF-β related proteins have been poorly studied.

A large number of proteins from Drosophila to mammals belong to the TGF-β superfamily and additional members are likely to be identified (for reviews: 20, 21, 22). The biological activities of most factors have been poorly defined, but developmental studies suggest important roles in tissue differentiation and pattern formation. The latter is illustrated by the effect of Dpp in Drosophila, which plays a key role in dorsal-ventral patterning and in the morphogenesis of various structures. The high sequence conservation of these factors throughout evolution is apparent with Drosophila Dpp and its vertebrate homologs BMP-2 and -4 which can functionally replace each other, and with Drosophila 60A and its homologs BMP-5, -6, -7, -8, and -9 (for review see [22]). However, no TGF-β homologs have been identified in Drosophila or C. elegans. Besides their roles in development and cell differentiation, most TGF-β superfamily members also modulate cell proliferation and often inhibit cell growth. The antiproliferative activity of TGF-β has received considerable attention, since not only it allows insight into the processes that lead to growth arrest, but also this activity stands in sharp contrast with the activities of the well-studied mitogenic growth factors. Finally, TGF-β-related factors also enhance or decrease the expression of various genes and are major inducers of extracellular matrix protein and integrin synthesis (for review: [28]).

TGF-β1 was the first factor to be characterized at the molecular level and is widely available for cell biological studies, and is therefore considered as the prototype factor of the large TGF-β superfamily. Together with the extensive knowledge of the activities of TGF-β, the characterization of the TGF-β receptors and associated signaling mechanisms provides a model for receptor signaling by all members of the TGF-β superfamily. cDNA characterization and structural analyses have defined the receptors for TGF-β superfamily members as a distinct group of cell surface receptors different from other growth factor receptors (for review; 21, 29, 30). This review summarizes our current knowledge (based on the literature up to the end of 1996) on the structure and mechanism of signaling of the TGF-β receptors and related receptors for other members of the TGF-β superfamily. Furthermore, we will outline recent advances in our understanding of the potential role of TGF-β receptor signaling as a mechanism of tumor suppression. We will not discuss the role of signaling by TGF-β-related factors and their receptors during development.

Section snippets

Three types of TGF-β receptors

The TGF-β receptors were initially detected using chemical crosslinking of 125I-TGF-β to cell surface binding proteins. Electrophoretic separation of the crosslinked ligand-receptor complexes on gel revealed three types of cell surface proteins in most cells, which, based on their electrophoretic mobility, were named types I, II and III 31, 32. The largest binding protein, i.e. the type III receptor or betaglycan, is often most abundant and usually displays extensive electrophoretic

Structure of betaglycan and endoglin

The type III receptor, also called betaglycan, is in many cell lines and cell types the most abundant TGF-β binding protein at the cell surface at levels up to 200 000 molecules per cell. This protein is widely expressed in fetal and adult tissues, including mesenchymal, epithelial, neuronal and other cell types, but not in endothelial cells and certain types of myoblasts, epithelial and hematopoietic cells 31, 34, 39, 40, 41, 42. The extensive modification of the type III receptor with

Structure of the type II TGF-β receptor

The type II and type I receptors mediate TGF-β signal transduction. Both receptors are transmembrane serine/threonine kinases with a single transmembrane domain. The deduced 567 amino acid polypeptide sequence for the type II TGF-β receptor includes a signal peptide, a 136 amino acid long N-glycosylated extracellular domain and a cytoplasmic domain which largely consists of the kinase domain flanked by short juxtamembrane and C-terminal tails 71, 72(Fig. 3). The large number of cysteines in the

Structure and ligand binding of the type I TGF-β receptors

The type I receptors are, similarly to the type II receptors, transmembrane serine/threonine kinase receptors. They resemble the type II receptors in structure and have considerable sequence homology, although they are generally smaller with a length of 503 to 532 amino acids (Fig. 3). So far, seven different mammalian type I receptors have been identified 102, 103, 104, 105, 106, 107, 108, 109. Their ligand binding properties and abilities to signal in response to ligand have been extensively

Interactions between receptors: the heteromeric receptor complex

The three receptor types discussed above have the ability to associate in homomeric and heteromeric complexes (Fig. 4). Thus, the type II receptors form ligand-independent homodimers 62, 92which are constitutively autophophorylated 95, 96. Similarly, type I receptors also form homodimers independent of ligand-binding (Miettinen and Derynck, unpublished; Wells, Gilboa, Lodish and Henis, unpublished). Whereas type I receptors bind ligand independent of type II receptor expression in the case of

Reporter assays for receptor function

Among the many effects of TGF-β on cell physiology, the two best studied TGF-β responses are the antiproliferative effect and the induction of expression of extracellular matrix proteins. The latter response has allowed the development of a convenient reporter assay to study TGF-β responsiveness and receptor function. In this assay, a modified segment of the promoter for plasminogen activator inhibitor type 1 (PAI-1) drives the expression of luciferase, and the luciferase activity, which

Modulation of receptor signaling

An intriguing question is whether the growth inhibitory and gene induction responses to TGF-β result from a single or two different signalling pathways following receptor activation. Expression of some nuclear oncogenes, such as the adenovirus E1A protein, the human papilloma virus E6-E7 proteins and the SV40 small T and polyoma middle T antigens, inactivates the antiproliferative activity of TGF-β while the ability of TGF-β to induce gene expression is maintained 148, 149. These viral proteins

Effects of TGF-β on cell growth

TGF-β induces a strong growth inhibitory effect on many cell types, including epithelial cells. In contrast, several other cell types, most notably mesenchymal cells, undergo growth stimulation when treated with TGF-β 155, 156, 157, 158. The signaling mechanisms that are at the basis for this growth stimulation are likely to be indirect. Indeed, the stimulation of proliferation may result from a TGF-β-induced expression of endogenous growth factors, such as platelet-derived growth factor 155,

TGF-β receptor-associated proteins and their role in signaling

To identify signaling pathways that mediate the activities of TGF-β and related factors, considerable emphasis has been placed on the identification and characterization of proteins that associate with and are phosphorylated by serine/threonine kinase receptors. Coimmunoprecipitation and biochemical analyses have been largely unsuccessful as the primary approach to identify receptor-associated proteins. In contrast, several proteins have been identified in yeast two-hybrid analyses using the

Association of TGF-β signaling with kinase and phosphatase pathways

Kinase cascades downstream from the receptors have also been implicated in TGF-β signaling. TGF-β receptor activation in cells that are growth inhibited by TGF-β may induce enzymatic activities that are also involved in mitogenic signaling. Thus, TGF-β induces a rapid, albeit modest increase in GTP-bound p21ras and activates ras signaling in epithelial cells that are growth inhibited by TGF-β 189, 190. In addition, TGF-β induces a rapid, but modest, activation of p44mapk [191]and, in another

Mad-related proteins as signaling mediators

Recently, genetic approaches in Drosophila and C. elegans and complementary biochemical studies have resulted in the characterization of Mad in Drosophila and its homologs as important signaling mediators of the responses to TGF-β-related factors. The extremely rapid progress in their structural and functional characterization makes any review of this field dated and limited.

TGF-β receptor signaling and tumor suppression

The potent ability of TGF-β to inhibit proliferation of many different normal cell types stands in contrast with the resistance of numerous tumors and tumor cell lines to TGF-β (for review: 231, 232) Escape from growth inhibition by TGF-β has been demonstrated in many tumor types from epithelial or neuro-ectodermal origin [232]. Perhaps most striking is the resistance of various carcinomas, such as gastric [233], colon [234]and small-cell lung carcinomas [235]to the growth inhibitory effect of

Acknowledgements

Research on TGF-β receptor signaling was sponsored by grant CA63101 from NIH to R.D. and a postdoctoral fellowship from American Cancer Society to X.-H.F. We thank members of the laboratory for their contributions to the studies on TGF-β receptor signaling.

First page preview

First page preview
Click to open first page preview

References (286)

  • J. Massagué

    TGF-β signaling: receptors, transducers, and Mad proteins

    Cell

    (1996)
  • S. Cheifetz et al.

    Cellular distribution of type I and type II receptors for transforming growth factor-β

    J. Biol. Chem.

    (1986)
  • S. Cheifetz et al.

    The transforming growth factor-β system, a complex pattern of cross-reactive ligands and receptors

    Cell

    (1987)
  • J. Massagué

    Receptors for the TGF-β family

    Cell

    (1992)
  • F.T. Boyd et al.

    Transforming growth factor-β inhibition of epithelial cell proliferation linked to the expression of a 53 kDa membrane receptor

    J. Biol. Chem.

    (1989)
  • M. Laiho et al.

    Concomitant loss of transforming growth factor (TGF)-β receptors types I and II in TGF-β-resistant cell mutants implicates both receptor types in signal transduction

    J. Biol. Chem.

    (1990)
  • M. Laiho et al.

    Responsiveness to transforming growth factor-β (TGF-β) restored by genetic complementation between cells defective in TGF-β receptors I and II

    J. Biol. Chem.

    (1991)
  • A. Geiser et al.

    Inhibition of growth by transforming growth factor-β following fusion of two non-responsive human carcinoma cell lines

    J. Biol. Chem.

    (1992)
  • J. Massagué

    Subunit structure of a high affinity receptor for type β transforming growth factor. Evidence for a disulfide-linked glycosylated receptor complex

    J. Biol. Chem.

    (1985)
  • J. Massagué et al.

    Cellular receptors for type β transforming growth factor. Ligand binding and affinity labeling in human and rodent cell lines

    J. Biol. Chem.

    (1985)
  • X.-F. Wang et al.

    Expression cloning and characterization of the TGF-β type III receptor

    Cell

    (1991)
  • S. Cheifetz et al.

    The transforming growth factor-β receptor type III is a membrane proteoglycan. Domain structure of the receptor

    J. Biol. Chem.

    (1988)
  • P.R. Segarini et al.

    The high molecular weight receptor to transforming growth factor-β contains glycosaminoglycan chains

    J. Biol. Chem.

    (1988)
  • F. López-Casillas et al.

    Structure and expression of the membrane proteoglycan betaglycan, a component of the TGF-β receptor system

    Cell

    (1991)
  • P.R. Segarini et al.

    Membrane binding characteristics of two forms of transforming growth factor-β

    J. Biol. Chem.

    (1987)
  • S. Cheifetz et al.

    Transforming growth factor-β (TGF-β) receptor proteoglycan. Cell surface expression and ligand binding in the absence of glycosaminoglycan chains

    J. Biol. Chem.

    (1989)
  • J.L. Andres et al.

    Purification of the transforming growth factor-β (TGF-β) binding proteoglycan betaglycan

    J. Biol. Chem.

    (1991)
  • J.L. Andres et al.

    Binding of two growth factor families to separate domains of the proteoglycan betaglycan

    J. Biol. Chem.

    (1992)
  • M.-C. Pepin et al.

    Mutagenesis analysis of the membrane-proximal ligand binding site of the TGF-β receptor type III extracellular domain

    FEBS Letters

    (1995)
  • D. Fukushima et al.

    Localization of transforming growth factor β binding site in betaglycan

    J. Biol. Chem.

    (1993)
  • A. Gougos et al.

    Primary structure of endoglin, an RGD-containing glycoprotein of human endothelial cells

    J. Biol. Chem.

    (1990)
  • S. Cheifetz et al.

    Endoglin is a component of the transforming growth factor-β receptor system in human endothelial cells

    J. Biol. Chem.

    (1992)
  • H. Yamashita et al.

    Endoglin forms a heteromeric complex with the signaling receptors for transforming growth factor-β

    J. Biol. Chem.

    (1994)
  • S.A. McCarthy et al.

    Activin-A binds to a heterotrimeric receptor complex on the vascular endothelial cell surface. Evidence for a type III receptor

    J. Biol. Chem.

    (1994)
  • F. López-Casillas et al.

    Betaglycan presents ligand to the TGF-β signaling receptor

    Cell

    (1993)
  • A. Moustakas et al.

    The transforming growth factor receptors types I, II and III form hetero-oligomeric complexes in the presence of ligand

    J. Biol. Chem.

    (1993)
  • H.Y. Lin et al.

    Expression cloning of the TGF-β type II receptor, a functional transmembrane serine/threonine kinase receptor

    Cell

    (1992)
  • A. Suzuki et al.

    Cloning of an isoform of mouse TGF-β type II receptor gene

    FEBS Letters

    (1994)
  • L.S. Mathews et al.

    Expression cloning of an activin receptor, a predicted transmembrane serine kinase

    Cell

    (1991)
  • L. Attisano et al.

    Novel activin receptors: distinct genes and alternative mRNA splicing generate a repertoire of serine/threonine kinase receptors

    Cell

    (1992)
  • T. Nohno et al.

    Identification of a human type II receptor for bone morphogenetic protein-4 that differential heteromeric complexes with bone morphogenetic protein type I receptors

    J. Biol. Chem.

    (1995)
  • A. Letsou et al.

    Drosophila dpp signaling is mediated by the punt gene product: a dual ligand-binding type II receptor of the TGF-β receptor family

    Cell

    (1995)
  • E. Ruberte et al.

    An absolute requirement for both the type II and type I receptors, punt and thick veins, for dpp signaling in vivo

    Cell

    (1995)
  • H.Y. Lin et al.

    The soluble exoplasmic domain of the type II transforming growth factor (TGF)-β receptor. A heterogeneous glycosylated protein with high affinity and selectivity for TGF-β ligands

    J. Biol. Chem.

    (1995)
  • C. Rodriguez et al.

    Cooperative binding of transforming growth factor (TGF)-β2 to the types I and II TGF-β receptors

    J. Biol. Chem.

    (1995)
  • P.A. Hoodless et al.

    MADR1, a MAD-related protein that functions in BMP2 signalling pathways

    Cell

    (1996)
  • H. Nishitoh et al.

    Identification of type I and type II receptors for Growth/Differentiation Factor-5

    J. Biol. Chem.

    (1996)
  • M. Kawabata et al.

    Cloning of a novel type II serine/threonine kinase receptor through interaction with the type I transforming growth factor-β receptor

    J. Biol. Chem.

    (1995)
  • R.-H. Chen et al.

    Homomeric interactions between the type II TGF-β receptors

    J. Biol. Chem.

    (1994)
  • L.S. Mathews et al.

    Characterization of type II activin receptors. Binding, processing and phosphorylation

    J. Biol. Chem.

    (1993)
  • Cited by (0)

    View full text