The dual nature of TGFβ signalling in the suppression and the paradoxical promotion of breast tumours remains relatively mysterious. Signal transduction through the Smad-TGFβ pathway is known to be an inducer of cell cycle arrest and has been shown to have a role in early epithelial tumour suppression. Paradoxically, in breast carcinomas, prolonged signaling through the TGFβ receptor promotes latent tumour progression, invasiveness and the epithelial-to-mesenchymal transition of tumour cells. Known mutations in the major TGFβ signalling proteins do not offer a sufficient explanation for this dual role. Previously, we had found that the RING finger protein, RNF11, was over expressed in high-grade breast tumors, and was capable of antagonizing Smurf2 mediated inhibition of TGFbeta signalling. Here we show that RNF11 is capable of enhancing the Smad-TGFβ signalling pathway directly. We show that RNF11 binds directly to Smad4, the common Smad for TGFβ, Activin, and BMP signalling. Through this interaction, RNF11 enhances the transactivation activity of Smad4 and of Smad4-Samd2 and Smad4-Smad3 transcription complexes. In addition, we show that RNF11 enhancement of Smad4 activity occurs through an ubiquitin-mediated mechanism, which leads to the accumulation of the Smad4 protein. Cellular assays show that RNF11 can dramatically affect the survival and growth of breast cancer cells in vitro and in vivo. Taken together, we elucidate a novel paradigm for Smad4 and R-Smad interactions that proceeds through an ubiquitin-dependant protein interaction mechanism. These observations indicate a role for RNF11 in prolonged TGFβ signalling, enhanced Smad4 function and possibly a dual role in early tumour suppression and late tumour progression.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA