TGF-β signaling by Smad proteins

https://doi.org/10.1016/S1359-6101(99)00025-8Get rights and content

Abstract

Smads are signal transducers for the members of the transforming growth factor-β (TGF-β) superfamily. Bone morphogenetic proteins (BMPs) and their receptors induce differentiation of C2C12 cells into osteoblast-like cells. Using an adenoviral expression vector system, we showed that receptor-regulated Smads (R-Smads) activated by BMPs can induce the differentiation of C2C12 cells. Inhibitory Smads (I-Smads) interfere with the osteoblast differentiation of C2C12 cells by preventing the nuclear translocation of R-Smads. After translocation into the nucleus, Smad oligomers regulate the transcription of target genes through binding to DNA directly, interaction with other DNA binding proteins, and recruitment of transcriptional co-activators or co-repressors. Through interaction with different transcription factors and transcriptional co-activators or co-repressors, Smads may exhibit specific effects in various cell types.

Introduction

Members of the transforming growth factor-β (TGF-β) superfamily are multifunctional cytokines, which include TGF-βs, activins, and bone morphogenetic proteins (BMPs). These cytokines bind to two different types of serine/threonine kinase receptors (type I and type II), and activate intracellular substrates, e.g. Smad proteins [1], [2], [3]. The type II receptor kinases are constitutively active, and transactivate the type I receptor kinases, which then transmit signals inside cells. Thus, the type I receptors act downstream of the type II receptors, and determine the specificity of intracellular signals.

Smads are subdivided into three subclasses based on their structure and function. Receptor-regulated Smads (R-Smads) have a C-terminal Ser-Ser-X-Ser motif, and are directly phosphorylated by the type I receptor kinases. R-Smads then form complexes with common-partner Smads (Co-Smads) and translocate into the nucleus, where they regulate the transcription of target genes. Inhibitory Smads (I-Smads) are induced by ligand stimulation and interfere with the receptor activation or complex formation of R-Smads. In this review, I will focus on the biological activities of Smads determined using adenoviral expression vectors, and the function of Smads in the nucleus.

Section snippets

Bioactivity of Smads: adenoviral expression vector-based approach

C2C12 cells are undifferentiated mouse mesenchymal cells which differentiate into mature myocytes after serum deprivation in culture. If they are treated with BMPs, e.g. BMP-2, BMP-6 or BMP-7/OP-1 (osteogenic protein-1), they differentiate into osteoblast-like cells, and synthesize alkaline phosphatase [4], [5], [6]. Using adenoviral expression vectors, we examined the biological activities of Smads and compared them with ligands or constitutively active forms of type I receptors.

Function of Smads in the nucleus

After translocation into the nucleus, Smads regulate the transcription of target genes by directly binding to consensus DNA sequences, interacting with other DNA binding proteins, and recruiting transcriptional co-activators or co-repressors (Fig. 4).

Conclusion

Here I have described the biological activity of Smads determined using the adenoviral expression vector system. The adenoviral vector system will be useful for study of the biological activity of Smads in vivo, as shown for the lung fibrosis induced by bleomycin in mice [15]. The adenovirus system may also be helpful in studying the in vitro bioactivity of these proteins in cells which are difficult to transfect genes. In fact, more than 80% of C2C12 cells could be transfected using the

Acknowledgements

I would like to thank the colleagues in my laboratory and collaborators in other laboratories. This work was supported by grants-in-aid from the Ministry of Education, Science, Sports, and Culture of Japan, Research for the Future Program of the Japan Society for the Promotion of Science, and the Creative BioMolecules Fund.

References (42)

  • Y. Sun et al.

    Interaction of the Ski oncoprotein with Smad3 regulates TGF-β signaling

    Mol. Cell

    (1999)
  • C.-H. Heldin et al.

    TGF-β signalling from cell membrane to nucleus through SMAD proteins

    Nature

    (1997)
  • J. Massagué

    TGF-β signal transduction

    Annu. Rev. Biochem.

    (1998)
  • Miyazono K, ten Dijke P, Heldin C-H. TGF-β signaling by Smad proteins. Adv Immunol (in press),...
  • T. Katagiri et al.

    Bone morphogenetic protein-2 converts the differentiation pathway of C2C12 myoblasts into the osteoblast lineage

    J. Cell. Biol.

    (1994)
  • K. Takeda et al.

    Identification of a novel bone morphogenetic protein-responsive gene that may function as a noncoding RNA

    J. Biol. Chem.

    (1998)
  • T. Ebisawa et al.

    Characterization of bone morphogenetic protein-6 signaling pathways in osteoblast differentiation

    J. Cell. Sci.

    (1999)
  • Fujii M, Takeda K, Imamura T, Sampath TK, Enomoto S, Kawabata M, Kato M, Ichijo H, Miyazono K. Roles of bone...
  • P. ten Dijke et al.

    Identification of type I receptors for osteogenic protein-1 and bone morphogenetic protein-4

    J. Biol. Chem.

    (1994)
  • M. Macı́as-Silva et al.

    Specific activation of Smad1 signaling pathways by the BMP7 type I receptor, ALK2

    J. Biol. Chem.

    (1998)
  • S. Itoh et al.

    Transforming growth factor β1 induces nuclear export of inhibitory Smad7

    J. Biol. Chem.

    (1998)
  • Cited by (0)

    View full text