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

<i>Background:</i> 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-ĸB (NF-ĸB) is altered upon I/R injury. However, few studies have focused on the compositional change of NF-ĸB complexes. In the current study, we observed different NF-ĸB dimers in I/R and IPC, and postulated that p50 homodimers might represent the predominant NF-ĸB activation in renal IPC. <i>Methods: </i>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-ĸB activities, composition of NF-ĸB, and expression of IL-18. <i>Results:</i> Compared to the I/R group, NF-ĸB activity decreased significantly in the IPC group. Supershift assays were then performed to examine the NF-ĸB subunits in the binding complexes. In both the I/R and IPC groups, the composition of NF-ĸB contained p65 and p50. The densities of these two supershift bands were almost equivalent in the I/R group, suggesting the NF-ĸB 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-ĸB 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. <i>Conclusion:</i> In conclusion, our investigation suggests that: (1) the general activity of NF-ĸB is attenuated in IPC kidneys, and (2) the remaining activation of NF-ĸB 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.