Inhibition of nephrin activation by c-mip through Csk-Cbp-Fyn axis plays a critical role in Angiotensin II-induced podocyte damage.

It has been demonstrated that nephrin inactivation plays a critical role in Angiotensin II (AngII)-induced podocyte damage both in in vitro and in vivo, but the underlying molecular mechanisms are still unclear. Recently, c-maf inducing protein (c-mip) has been identified as a key component in the molecular pathogenesis of acquired podocyte diseases. In this study, the role of c-mip on AngII-induced nephrin inactivation and podocyte damage was explored in a mouse podocyte cell line. AngII stimulation caused podocyte damage, presenting with a time and dose dependent cell apoptosis increment, and obvious reorganization of actin cytoskeleton, both of which was remarkably prevented by knockdown of c-mip (siCmip). In AngII stimulated podocyte, c-mip and Csk expressions increased obviously at protein level, and nephrin phosphorylation decreased while Cbp phosphorylation increased. AngII-induced Csk increment and nephrin inactivation was remarkably inhibited by siCmip treatment. AngII stimulation increased the interaction of c-mip and Csk, as well as Csk and Cbp. Notably, the binding of Csk to active form pY418 decreased while the binding of Csk to inactive form pY530 of Src kinase Fyn increased in AngII-stimulated podocyte. Nevertheless, c-mip knockdown prevented AngII-induced reduction of pY418 and increase of pY530. In addition, AngII stimulation significantly decreased the expression of phosphor-Akt (Ser473) and antiapoptotic protein Bcl-2, whereas increased the expression of apoptotic proteins caspase-3 and BAD, all of which were prevented by siCmip treatment. Taken together, our results demonstrated that AngII induced nephrin inactivation and podocyte damage by the novel podocyte protein c-mip through Csk-Cbp-Fyn signaling pathway.