F-box protein complex FBXL19 regulates TGFβ1-induced E-cadherin down-regulation by mediating Rac3 ubiquitination and degradation

Background: Rac3 is a small GTPase multifunctional protein that regulates cell adhesion, migration, and differentiation. It has been considered as an oncogene in breast cancer; however, its role in esophageal cancer and the regulation of its stability have not been studied. F-box proteins are major subunits within the Skp1-Cullin-1-F-box (SCF) E3 ubiquitin ligases that recognize particular substrates for ubiquitination and proteasomal degradation. Recently, we have shown that SCFFBXL19 targets Rac1 and RhoA, thus regulating Rac1 and RhoA ubiquitination and degradation. Here, we demonstrate the role of FBXL19 in the regulation of Rac3 site-specific ubiquitination and stability. Expression of TGF beta 1 is associated with poor prognosis of esophageal cancer. TGF beta 1 reduces tumor suppressor, E-cadherin, expression in various epithelial-derived cancers. Here we investigate the role of Gamma BXL19-mediated Rac3 degradation in TGF beta 1-induced E-cadherin down-regulation in esophageal cancer cells. Methods: FBXL19-regulated endogenous and over-expressed Rac3 stability were determined by immunoblotting and co-immunoprecipitation. Esophageal cancer cells (OE19 and OE33) were used to investigate TGF beta 1-induced E-cadherin down-regulation by Immunoblotting and Immunostaining. Results: Overexpression of FBXL19 decreased endogenous and over-expressed Rac3 expression by interacting and polyubiquitinating Rac3, while down-regulation of FBXL19 suppressed Rac3 degradation. Lysine166 within Rac3 was identified as an ubiquitination acceptor site. The FBXL19 variant with truncation at the N-terminus resulted in an increase in Rac3 degradation; however, the FBXL19 variant with truncation at the C-terminus lost its ability to interact with Rac3 and ubiquitinate Rac3 protein. Further, we found that Rac3 plays a critical role in TGF beta 1-induced E-cadherin down-regulation in esophageal cancer cells. Over-expression of FBXL19 attenuated TGF beta 1-induced E-cadherin down-regulation and esophageal cancer cells elongation phenotype. Conclusions: Collectively these data unveil that FBXL19 functions as an antagonist of Rac3 by regulating its stability and regulates the TGF beta 1-induced E-cadherin down-regulation. This study will provide a new potential therapeutic strategy to regulate TGF beta 1 signaling, thus suppressing esophageal tumorigenesis.