Renal Protection by Ischemic Preconditioning Is Associated with p50/p50 Homodimers

Background: Although recent studies indicate that renal ischemia preconditioning (IPC) protects the kidney from ischemia-reperfusion (I/R) injury, the precise protection mechanism is still unknown. It has been widely recognized that the activity of nuclear factor-kappa B (NF-kappa B) is altered upon I/R injury. However, few studies have focused on the compositional change of NF-kappa B complexes. In the current study, we observed different NF-kappa B dimers in I/R and IPC, and postulated that p50 homodimers might represent the predominant NF-kappa B activation in renal IPC. Methods: 24 male SD rats were treated with a sham operation, I/R or IPC. While the right kidney was removed in all animals, I/R was induced by left renal artery clamping for 45 min, and IPC was induced by left renal artery clamping for 2 min, reperfused for 5 min, and repeated for 3 cycles before 45 min occlusion. After 24 h of reperfusion, we assessed NF-kappa B activities, composition of NF-kappa B, and expression of IL-18. Results: Compared to the I/R group, NF-kappa B activity decreased significantly in the IPC group. Supershift assays were then performed to examine the NF-kappa B subunits in the binding complexes. In both the I/R and IPC groups, the composition of NF-kappa B contained p65 and p50. The densities of these two supershift bands were almost equivalent in the I/R group, suggesting the NF-kappa B composition was p50/p65 dimers. However, the densities of p50 bands were more than twice that of p65 in the IPC group and the levels of Bcl-3 increased in parallel in these rats, suggesting that p50/p50 homodimers might dominate the NF-kappa B activities in renal IPC. Finally, the expression of IL-18, a marker of acute kidney injury, was significantly increased in the area of tubulointerstitium in the I/R group and attenuated in the IPC group. Conclusion: In conclusion, our investigation suggests that: (1) the general activity of NF-kappa B is attenuated in IPC kidneys, and (2) the remaining activation of NF-kappa B is dominated with p50/p50 homodimer. Both of these effects might subsequently downregulate IL-18 expression, as well as provide renal protective effects of IPC. Though we focused on the phenomenological observations in this study, the detailed mechanism is still to be determined. Copyright (c) 2009 S. Karger AG, Basel