Molecular imaging of TGFβ-induced Smad2/3 phosphorylation reveals a role for receptor tyrosine kinases in modulating TGFβ signaling.

PURPOSE The dual modality of TGFβ, both as a potent tumor suppressor and a stimulator of tumor progression, invasion, and metastasis, make it a critical target for therapeutic intervention in human cancers. The ability to carry out real-time, noninvasive imaging of TGFβ-activated Smad signaling in live cells and animal models would significantly improve our understanding of the regulation of this unique signaling cascade. To advance these efforts, we developed a highly sensitive molecular imaging tool that repetitively, noninvasively, and dynamically reports on TGFBR1 kinase activity. EXPERIMENTAL DESIGN The bioluminescent TGFβR1 reporter construct was developed using a split firefly luciferase gene containing a functional sensor of Smad2 phosphorylation, wherein inhibition of TGFβ receptor1 kinase activity leads to an increase in reporter signaling. The reporter was stably transfected into mammalian cells and used to image in vivo and in vitro bioluminescent activity as a surrogate for monitoring TGFBR1 kinase activity. RESULTS The reporter was successfully used to monitor direct and indirect inhibition of TGFβ-induced Smad2 and SMAD3 phosphorylation in live cells and tumor xenografts and adapted for high-throughput screening, to identify a role for receptor tyrosine kinase inhibitors as modulators of TGFβ signaling. CONCLUSION The reporter is a dynamic, noninvasive imaging modality for monitoring TGFβ-induced Smad2 signaling in live cells and tumor xenografts. It has immense potential for identifying novel effectors of R-Smad phosphorylation, for validating drug-target interaction, and for studying TGFβ signaling in different metastasis models.