Downregulation of Smurf2 ubiquitin ligase in pancreatic cancer cells reversed TGF-β-induced tumor formation

Smad ubiquitin regulatory factor 2 (Smurf2) is an E3 ubiquitin ligase that regulates transforming growth factor beta (TGF-beta)/Smad signaling and is implicated in a wide range of cellular responses. However, the exact mechanism whereby Smurf2 controls TGF-beta-induced signaling pathways remains unknown. Here, we identified the relationship between the alternate TGF-beta signaling pathways: TGF-beta/PI3K/Akt/beta-catenin and TGF-beta/Smad2/3/FoxO1/PUMA and Smurf2. The results showed that TGF-beta promoted proliferation, invasion, and migration of human pancreatic carcinoma (PANC-1) cells through the PI3K/Akt/beta-catenin pathway. Inhibiting the PI3K/Akt signal transformed the TGF-beta-induced cell response from promoting proliferation to Smad2/3/FoxO1/PUMA-mediated apoptosis. The activation of Akt inhibited the phosphorylation/activation of Smad3 and promoted the phosphorylation/inactivation of FoxO1, inhibiting the nuclear translocation of both Smad3 and FoxO1 and inhibiting the expression of PUMA, a key apoptotic mediator. However, downregulation of Smurf2 in PANC-1 cells removed Akt-mediated suppression of Smad3 and FoxO1, allowing TGF-beta-induced phosphorylation/activation of Smad2/3, dephosphorylation/activation of FoxO1, nuclear translocation of both factors, and activation of PUMA-mediated apoptosis. Downregulation of Smurf2 also decreased invasion and migration in TGF-beta-induced PANC-1 cells. The in vivo experiments also revealed that downregulation of Smurf2 delayed the growth of xenograft tumors originating from PANC-1 cells especially when treated with TGF-beta. Taken together, these results indicate that expression of Smurf2 plays a central role in the determination and activation/inhibition of particular cellular pathways and the ultimate fate of cells induced by TGF-beta. An increased understanding of the intricacies of the TGF-beta signaling pathway may provide a new anti-cancer therapeutic target.