Complement System and MAPK Signaling in Calcineurin-Inhibitor Induced Nephrotoxicity.

Abstract

Background: The gap between organ demand and supply to be used for transplantation is wide and getting wider. One way to improve the situation is to prolong allograft survival. One entity that significantly contributes to renal allograft loss is calcineurin inhibitor (CNI) nephrotoxicity (CIN). Various mechanisms are discussed to play a role in CIN pathogenesis; one of which is complement mediated injury. Purpose: To investigate the impact of CNIs on intracellular signaling, complement regulators and complement activation. Methods / Results: We have performed experiments utilizing two proximal tubule cell lines of human origin, HK2 and hTERT-RPTCs. CyclosporinA (CsA) and tacrolimus (FK506) treatment induced activation/phosphorylation of MAPK1/-2 in both cell lines. This was associated with a significant decrease in protein levels of suppressor of cytokine signaling (SOCS)-3, which we have shown recently to act as a negative regulator of MAPK1/-2 signaling. Hence, one might hypothesize that SOCS-3 downregulation occurs prior to MAPK signaling and enhances MAPK activation. In order to investigate the regulation of complement regulatory proteins DAF (CD55) and MCP (CD46), which inhibit or accelerate disassembly of C3 convertase and inhibit C3b and C4b, we assessed protein expression levels by Western Blotting and found that CNI treatment caused a down regulation of DAF and MCP. Thus one might speculate that a part of CIN may be attributed to dysregulated complement system activation by MAPK1/-2-induced down regulation of complement regulators DAF/MCP. Finally, we measured cellular proliferation by 3H-thymidine incorporation in cells treated with CsA and FK506 in heat-inactivated normal human serum (HIS) and compared it with the effects in normal human serum (NHS). Interestingly, cellular growth with HIS was approx. 20% better than with NHS. Thus, reduced cell growth could be attributed to enhanced complement system induced cell death and therefore may be of relevance for development of CIN. Conclusions: Our results suggest that CsA and FK506 induce MAPK1/-2 phosphorylation and the down regulation of SOCS-3. This is associated with down regulation of DAF and MCP, an activation of the complement system, which might play a role in pathogenesis of CIN.