SCF^FBXL19 Mediates Rac3Ubiquitination And Degradation

BackgroundRac3is a small GTPase multifunctional protein that regulates cell adhesion, migration,and differentiation. It has been considered as an oncogene in breast cancer; however, its rolein esophageal cancer and the regulation of its stability have not been studied. F-box proteinsare major subunits within the Skp1-Cullin-1-F-box (SCF) E3ubiquitin ligases that recognizeparticular substrates for ubiquitination and proteasomal degradation. Recently, we have shownthat SCFFBXL19targets Rac1and RhoA, thus regulating Rac1and RhoA ubiquitination anddegradation. Here, we demonstrate the role of FBXL19in the regulation of Rac3site-specificubiquitination and stability. Expression of TGFβ1is associated with poor prognosis ofesophageal cancer. TGFβ1reduces tumor suppressor, E-cadherin, expression in variousepithelial-derived cancers. Here we investigate the role of FBXL19-mediated Rac3degradation in TGFβ1-induced E-cadherin down-regulation in esophageal cancer cells.MethodsFBXL19-regulated endogenous and over-expressed Rac3stability were determined byimmunoblotting and co-immunoprecipitation. Esophageal cancer cells (OE19and OE33) wereused to investigate TGFβ1-induced E-cadherin down-regulation by Immunoblotting andImmunostaining.ResultsOverexpression of FBXL19decreased endogenous and over-expressed Rac3expressionby interacting and polyubiquitinating Rac3, while down-regulation of FBXL19suppressedRac3degradation. Lysine166within Rac3was identified as an ubiquitination acceptor site.The FBXL19variant with truncation at the N-terminus resulted in an increase in Rac3degradation; however, the FBXL19variant with truncation at the C-terminus lost its ability tointeract with Rac3and ubiquitinate Rac3protein. Further, we found that Rac3plays a criticalrole in TGFβ1-induced E-cadherin down-regulation in esophageal cancer cells. Over- expression of FBXL19attenuated TGFβ1-induced E-cadherin down-regulation andesophageal cancer cells elongation phenotype.ConclusionsCollectively these data unveil that FBXL19functions as an antagonist of Rac3byregulating its stability and regulates the TGFβ1-induced E-cadherin down-regulation. Thisstudy will provide a new potential therapeutic strategy to regulate TGFβ1signaling, thussuppressing esophageal tumorgenesis.