Oxidative Inactivation of the Proteasome in Retinal Pigment Epithelial Cells: A POTENTIAL LINK BETWEEN OXIDATIVE STRESS AND UP-REGULATION OF INTERLEUKIN-8

Oxidative stress and inflammation are implicated in the pathogenesis of many age-related diseases. Stress-induced overproduction of inflammatory cytokines, such as interleukin-8 (IL-8), is one of the early events of inflammation. The objective of this study was to elucidate mechanistic links between oxidative stress and overproduction of IL-8 in retinal pigment epithelial (RPE) cells. We found that exposure of RPE cells to H₂O₂, paraquat, or A2E-mediated photooxidation resulted in increased expression and secretion of IL-8. All of these oxidative stressors also inactivated the proteasome in RPE cells. In contrast, tert-butylhydroperoxide (TBH), a lipophilic oxidant that did not stimulate IL-8 production, also did not inactivate the proteasome. Moreover, prolonged treatment of RPE cells with proteasome-specific inhibitors recapitulated the stimulation of IL-8 production. These data suggest that oxidative inactivation of the proteasome is a potential mechanistic link between oxidative stress and up-regulation of the proinflammatory IL-8. The downstream signaling pathways that govern the production of IL-8 include NF-κB and p38 MAPK. Proteasome inhibition both attenuated the activation and delayed the turnoff of NF-κB, resulting in biphasic effects on the production of IL-8. Prolonged proteasome inhibition (>2 h) resulted in activation of p38 MAPK via activation of MKK3/6 and increased the production of IL-8. Chemically inhibiting the p38 MAPK blocked the proteasome inhibition-induced up-regulation of IL-8. Together, these data indicate that oxidative inactivation of the proteasome and the related activation of the p38 MAPK pathway provide a potential link between oxidative stress and overproduction of proinflammatory cytokines, such as IL-8.