TGFbeta-stimulated Smad1/5 phosphorylation requires the ALK5 L45 loop and mediates the pro-migratory TGFbeta switch

EMBO J. 2009 Jan 21;28(2):88-98. doi: 10.1038/emboj.2008.266. Epub 2008 Dec 18.


During the course of breast cancer progression, normally dormant tumour-promoting effects of transforming growth factor beta (TGFbeta), including migration, invasion, and metastasis are unmasked. In an effort to identify mechanisms that regulate the pro-migratory TGFbeta 'switch' in mammary epithelial cells in vitro, we found that TGFbeta stimulates the phosphorylation of Smad1 and Smad5, which are typically associated with bone morphogenetic protein signalling. Mechanistically, this phosphorylation event requires the kinase activity and, unexpectedly, the L45 loop motif of the type I TGFbeta receptor, ALK5, as evidenced by studies using short hairpin RNA-resistant ALK5 mutants in ALK5-depleted cells and in vitro kinase assays. Functionally, Smad1/5 co-depletion studies demonstrate that this phosphorylation event is essential to the initiation and promotion of TGFbeta-stimulated migration. Moreover, this phosphorylation event is preferentially detected in permissive environments such as those created by tumorigenic cells or oncogene activation. Taken together, our data provide evidence that TGFbeta-stimulated Smad1/5 phosphorylation, which occurs through a non-canonical mechanism that challenges the notion of selective Smad phosphorylation by ALK5, mediates the pro-migratory TGFbeta switch in mammary epithelial cells.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Activins / pharmacology
  • Animals
  • Benzamides / pharmacology
  • Bone Morphogenetic Proteins / physiology
  • Breast Neoplasms
  • Cell Line
  • Cell Line, Tumor
  • Cell Movement*
  • Cell Proliferation
  • Cell Transformation, Neoplastic / metabolism
  • Dioxoles / pharmacology
  • Humans
  • Mice
  • Phosphorylation
  • Protein Binding
  • Protein Isoforms / antagonists & inhibitors
  • Protein Isoforms / metabolism
  • Protein-Serine-Threonine Kinases / antagonists & inhibitors
  • Protein-Serine-Threonine Kinases / genetics
  • Protein-Serine-Threonine Kinases / metabolism*
  • Receptor, Transforming Growth Factor-beta Type I
  • Receptors, Transforming Growth Factor beta / antagonists & inhibitors
  • Receptors, Transforming Growth Factor beta / genetics
  • Receptors, Transforming Growth Factor beta / metabolism*
  • Signal Transduction
  • Smad1 Protein / metabolism*
  • Smad5 Protein / metabolism*
  • Transforming Growth Factor beta / pharmacology
  • Transforming Growth Factor beta / physiology*


  • 4-(5-benzo(1,3)dioxol-5-yl-4-pyridin-2-yl-1H-imidazol-2-yl)benzamide
  • Benzamides
  • Bone Morphogenetic Proteins
  • Dioxoles
  • Protein Isoforms
  • Receptors, Transforming Growth Factor beta
  • Smad1 Protein
  • Smad1 protein, mouse
  • Smad5 Protein
  • Smad5 protein, mouse
  • Transforming Growth Factor beta
  • activin A
  • Activins
  • Protein-Serine-Threonine Kinases
  • Receptor, Transforming Growth Factor-beta Type I
  • TGFBR1 protein, human
  • Tgfbr1 protein, mouse