Certain stimuli, such as inflammatory signals and oxidative stress, can trigger the phosphorylation of IkB and lead to the ubiquitination and degradation of IkB by the proteasome. After dissociating from IkB, the active form of NF-kB translocates into the nucleus and promotes the transcription of related genes. Proteasome inhibitors have been demonstrated to be beneficial in several pathologic conditions, including autoimmune disorders in animal models and cancers in human. Proteasome inhibitors are also shown to have organ-protective effects in experimental IR injury of the brain, heart and kidney. Blockage of NF-kB activation is thought to account for the majority of protective effects by proteasome inhibition. The effect of proteasome inhibitors on retinal IR injury has never been studied. Bortezomib, previously known as PS-341, LDP- 341 and MLM341, is a 26S proteasome inhibitor. It is approved by the FDA for use in the Crizotinib c-Met inhibitor treatment of multiple myeloma. We have demonstrated that bortezomib had anti-inflammatory effects in endotoxin-induced uveitis of rats in a previous study. Concerning retinal IR injury, we hypothesized that bortezomib could inhibit the activation of NF-kB and associated inflammatory mediators, alleviate oxidative stress in the retina and reduce retinal neuron death and ganglion cell apoptosis. To provide evidence for these predictions, we designed an animal study to investigate the effect of bortezomib on pressure-induced IR injury in the rat retina. Our study demonstrated that pretreatment with bortezomib could dose-dependently reduce the b-wave decrement in ERG, and PCR and western blot analysis showed that pretreatment with bortezomib could decrease the expression of inflammatory mediators and production of p53 and bax. IF staining of retinal sections revealed that bortezomib decreased tissue oxidative stress, activation of NF-kB and recruitment of CD 68 cells resulting from IR insult. We observed that cell apoptosis, revealed by in situ TUNEL staining, was attenuated by bortezomib. IF staining with Neu-N indicated that retinal ganglion cells were largely preserved in the high-dose bortezomib group after 7 days of initiating IR injury. Currently, the clinical application of bortezomib is primarily in the treatment of human hematologic diseases, such as multiple myeloma, mantle cell lymphoma, and acute graft-versus-host disease after allograft bone marrow transplantation. Other potential applications of bortezomib, such as the anti-inflammatory effects in autoimmune diseases and the protective effects in IR injury of brain, heart, and kidney, are under investigation and currently limited to use in animal models. Although not fully understood, the postulated mechanism of the drug��s effect is primarily achieved through the inhibition of NF-kB activation, which in turn reduces the transcription of inflammation- related genes and avoids the induction of inflammatory cascades. Therefore, a proteasome inhibitor could protect target tissues from inflammation-induced oxidative stress and cell damage. Whether proteasome function is altered in the process of IR injury is still controversial. Increased proteasome activity has been observed in an experimental study of IR injury in liver tissue, as well as in the retina demonstrated by our study. Moreover, proteasome inhibitors theoretically normalize the proteasome function and exert anti-inflammatory effects through the inactivation of NF-kB. However, impaired proteasome function has been found in some animal studies of IR injury in brain and heart tissue. A plausible explanation for this impairment is the depletion of ATP during ischemia, which inhibits the conversion of 20S to 26S proteasome. Another possibility is that the oxidative protein and lipid products interfere with the enzyme activity of proteasomes. Furthermore, the functional impairment of proteasomes will lead to the intracellular accumulation of oxidative and ubiquitinated proteins, resulting in a vicious cycle. Paradoxically, the use of proteasome inhibitors in these animal models has been demonstrated to lead to a tissue-protective effect, which may be related to the time of drug administration. It has been postulated that short-term use of proteasome inhibitors in the acute stage of IR injury, during which the proteasome function was only minimally affected, may be more beneficial than persistent use of the drug in the SB431542 chronic stage. Our study demonstrated that the activation of NF-kB was involved in the pathogenesis of IR injury in the retina, which was compatible with the results of other animal studies. In these studies, the activation of NF-kB was found to peak at 12 to 24 hours after the injury.