Background: Transforming growth factor beta1 (TGFbeta1) is a potent inhibitor of epithelial cell growth, thus playing an important role in tissue homeostasis. Most carcinoma cells exhibit a reduced sensitivity for TGFbeta1 mediated growth inhibition, suggesting TGFbeta1 participation in the development of these cancers. The tumor suppressor gene DPC4/SMAD4, which is frequently inactivated in carcinoma cells, has been described as a key player in TGFbeta1 mediated growth inhibition. However, some carcinoma cells lacking functional SMAD4 are sensitive to TGFbeta1 induced growth inhibition, thus requiring a SMAD4 independent TGFbeta1 pathway.
Results: Here we report that mature TGFbeta1 is a ligand for the integrin alphaVbeta6, independent of the common integrin binding sequence motif RGD. After TGFbeta1 binds to alphaVbeta6 integrin, different signaling proteins are activated in TGFbeta1-sensitive carcinoma cells, but not in cells that are insensitive to TGFbeta1. Among others, interaction of TGFbeta1 with the alphaVbeta6 integrin resulted in an upregulation of the cell cycle inhibitors p21/WAF1 and p27 leading to growth inhibition in SMAD4 deleted as well as in SMAD4 wildtype carcinoma cells.
Conclusions: Our data provide support for the existence of an alternate TGFbeta1 signaling pathway that is independent of the known SMAD pathway. This alternate pathway involves alphaVbeta6 integrin and the Ras/MAP kinase pathway and does not employ an RGD motif in TGFbeta1-sensitive tumor cells. The combined action of these two pathways seems to be necessary to elicit a complete TGFbeta1 signal.