Upregulation of ID protein by growth and differentiation factor 5 (GDF5) through a smad-dependent and MAPK-independent pathway in HUVSMC.

GDF5 (growth and differentiation factor five), a member of the TGF-beta superfamily, binds specifically to BMPR1b, BMPR2 and ACTR2a receptors forming a heterodimeric complex, thereby inducing phosphorylation of smad1, 5, 8 and translocation to the nucleus. ID1 (inhibitor of differentiation or DNA binding) is essential for G1 to S phase transition inhibiting DNA binding thereby playing an important role in the control of differentiation, proliferation and angiogenesis. The objective of this study was, therefore, to characterize the signal transduction pathway of GDF5, especially the involvement of ID1, in human umbilical vein smooth muscle cells (HUVSMC). We observed the expression of BMPR1a, BMPR1b, BMPR2, ACTR2a, smad1, smad 5, ID1, ID2 and ID3 in HUVSMC. Application of GDF5 upregulated ID1 and ID3 expression by involvement of the smad signaling pathway. GDF5 caused phorsphorylation of smad1 followed by upregulation of ID1 and ID3. Co-incubation with anti-GDF5 prevented these effects. GDF5 significantly inhibited phosphorylation of p38 MAPK and induced phosphorylation of ERK. The specific inhibitor of p38 MAPK or ERK, SB203580 or U0126 did not induce ID protein expression. Smad1 siRNA transfection inhibited the upregulation of ID protein. GDF5 had chemotactic activity in HUVSMC; this effect was partly blocked by transfection of smad1 or ID1 siRNA. Our results indicate that GDF5 induces ID1 and ID3 in HUVSMC by a smad-dependent, MAPK-independent pathway. GDF5 binds to specific receptors, thereby inducing phosphorylation and translocation of smad1 to the nucleus where it is involved in the regulation of transcription. Since ID1 has been shown to be crucial for cell cycle control, we propose that GDF5 could be involved in the process of angiogenesis.