Reperfusion after liver transplantation in rats differentially activates the mitogen-activated protein kinases

The injury resulting from cold ischemia and warm reperfusion during liver transplantation is a major clinical problem that limits graft success. Kupffer cell activation plays a pivotal role in reperfusion injury, and Kupffer cell products, including free radicals and tumor necrosis factor α (TNF-α), are implicated as damaging agents. However, the second messengers and signaling pathways that are activated by the stress of hepatic ischemia/reperfusion remain unknown. The purpose of this study is to assess the activation of the three known vertebrate mitogen activated protein kinase (MAPKs) and the activating protein 1 (AP-1) transcription factor in response to ischemia and reperfusion in the transplanted rat liver. There was a potent, sustained induction of c-jun N-terminal kinase (JNK), but not of the related MAPKs extracellular signal-regulated kinases (ERK) or p38, upon reperfusion after transplantation. TNF-α messenger RNA (mRNA) levels and transcription factors AP-1 and nuclear factor-κB (NF-κB) were induced in the liver after 60 minutes of reperfusion. Finally, there was an elevation of ceramide, but not diacylglycerol or sphingosine, in the transplanted liver. Ceramide is a second messenger generated by TNF-α treatment and is an activator of JNK. Because JNK activation preceded the elevations in ceramide and TNF-α mRNA, these results suggest that increased hepatic TNF-α and ceramide may perpetuate JNK induction, but that they are not the initiating signals of JNK activation during reperfusion injury in the transplanted liver.