TOSO interacts with SYK and enhances BCR pathway activation in chronic lymphocytic leukemia

Background:TOSO,also named Fas inhibitory molecule 3(FAIM3),has recently been identified as an immunoglobulin M(IgM)Fc receptor(FcμR).Previous studies have shown that TOSO is specifically over-expressed in chronic lymphocytic leukemia(CLL).However,the functions of TOSO in CLL remain unknown.The B-cell receptor(BCR)signaling pathway has been reported to be constitutively activated in CLL.Here,we aimed to investigate the functions of TOSO in the BCR signaling pathway and the pathogenesis of CLL.Methods:We over-expressed TOSO in B-cell lymphoma cell lines(Granta-519 and Z138)by lentiviral transduction and knocked down TOSO by siRNA in primary CLL cells.The over-expression and knockdown of TOSO were confirmed at the RNA level by polymerase chain reaction and protein level by Western blotting.Co-immunoprecipitation with TOSO antibody followed by liquid chromatography coupled with tandem mass spectrometry(IP/LCMS)was used to identify TOSO interacting proteins.Western blotting was performed to detect the activation status of BCR signaling pathways as well as B-cell lymphoma 2(BCL-2).Flow cytometry was used to examine the apoptosis of TOSO-over-expressing B lymphoma cell lines and TOSO-down-regulated CLL cells via the staining of Annexin V and 7-AAD.One-way analyses of variance were used for intergroup comparisons,while independent samples t tests were used for two-sample comparisons.Results:From IP/LCMS,we identified spleen tyrosine kinase(SYK)as a crucial candidate of TOSO-interacting protein and confirmed it by co-immunoprecipitation.After stimulation with anti-IgM,TOSO over-expression increased the phosphorylation of SYK,and subsequently activated the BCR signaling pathway,which could be reversed by a SYK inhibitor.TOSO knockdown in primary CLL cells resulted in reduced SYK phosphorylation as well as attenuated BCR signaling pathway.The apoptosis rates of the Granta-519 and Z138 cells expressing TOSO were(8.46±2.90)%and(4.20±1.21)%,respectively,significantly lower than the rates of the control groups,which were(25.20±4.60)%and(19.72±1.10)%,respectively(P<0.05 for both).The apoptosis rate was reduced after knocking down TOSO in the primary CLL cells.In addition,we also found that TOSO down-regulation in primary cells from CLL patients led to decreased expression of BCL-2 as well as lower apoptosis,and vice versa in the cell line.Conclusions:TOSO might be involved in the pathogenesis of CLL by interacting with SYK,enhancing the BCR signaling pathway,and inducing apoptosis resistance.