Cyclosporine A inhibits MRTF-SRF signalling through Na+/K+ ATPase inhibition & Actin remodelling

Calcineurin Inhibitors (CNI) are the pillars of immunosuppression in transplantation. However, they display a potent nephrotoxicity whose mechanisms remained widely unsolved. We used an untargeted quantitative proteomic approach (iTRAQ technology) to highlight new targets of CNI in renal proximal tubular cells (RPTCs). CNI-treated RPTCs proteome displayed an over-representation of Actin-binding proteins with a CNI-specific expression profile. Cyclosporine A (CsA) induced F-Actin remodelling and depolymerisation, decreased F-Actin-stabilizing, polymerization-promoting Cofilin (CFL) oligomers and inhibited the G-Actin-regulated serum responsive factor (SRF) pathway. Inhibition of CFL canonical phosphorylation pathway reproduced CsA effects; however, Ser3, an analogue of the phosphorylation site of CFL prevented the effects of CsA which suggests that CsA acted independently from the canonical CFL regulation. CFL is known to be regulated by the Na+/K+-ATPase. Molecular docking calculations evidenced 2 inhibiting sites of CsA on Na+/K+-ATPase and a 23% decrease in Na+/K+-ATPase activity of RPTCs was observed with CsA. Ouabain, a specific inhibitor of Na+/K+-ATPase also reproduced CsA effects on Actin organization and SRF activity. Altogether, these results described a new original pathway explaining CsA nephrotoxicity.