During carcinogenesis, Transforming Growth Factor β (TGFβ) signaling is upregulated and promotes epithelial-to-mesenchymal transition (EMT) and fibrosis, two processes that are known to advance cancer progression. Cellular response to TGFβ signaling, however, is contextual, and under homeostatic conditions, suppresses tumorigenesis. Intracellular TGFβ signaling is similarly nuanced. Two intracellular TGFβ signaling molecules, Smad2 and Smad3, are both activated by TGFβ signaling. Knockout of these molecules, individually, revealed that the Smad2 knockout is embryonically lethal, but Smad3 knockout embryos survive to term. This finding supports that cellular response from Smad2 signaling differs from that of Smad3 signaling. In the context of carcinogenesis, Smad3 signaling was shown to promote EMT and fibrosis, whereas Smad2 signaling was shown to suppress EMT and fibrosis despite both molecules being activated by TGFβ signaling. In light of these findings, precise inhibition of Smad3 signaling without disruption of Smad2 signaling is highly desirable.
Specific targeting of Smad3 presents two challenges. First, the therapeutic should be able to differentiate Smad3 from its homologue, Smad2, and specifically inhibit the former. Second, the therapeutic should be able to penetrate the cell membrane and preferably be orally bioavailable. An antibody-based therapeutic would presumably be able to differentiate the Smad3 from Smad2, but would not be cell permeable or orally bioavailable. A small molecule inhibitor, specific inhibitor of Smad3 (SIS3), has been reported in the literature, where the authors describe reduction of phosphorylated Smad3 and extracellular matrix production while not inhibiting phosphorylation of Smad2. In our hands, SIS3 did not show specific inhibition of Smad3 phosphorylation, which prompted us to pursue the discovery of cell-penetrating macrocyclic peptides that function as a Smad3-specific inhibitor. Here, we describe our progress toward identification of Smad3-specific macrocyclic peptides by using in vitro selection.