Abstract 376: Ischemic Preconditioning Regulates Myocardial Viability Through The Proteasomal Regulation Of Two Opposing PKC Isozymes
The response of the myocardium to an ischemic insult is regulated by two opposing PKC isozymes, δ and ϵPKC. We therefore wanted to determine how two highly homologous kinases could play such distinct roles in cardiac ischemia/reperfusion (I/R) injury. Using an ex vivo rat model of myocardial infarction, we determined that short bouts of ischemia and reperfusion prior to the prolonged ischemic event (IPC) diminished δPKC and enhanced ϵPKC translocation to the mitochondria. In addition, cellular levels of δPKC decreased by 60% in response to IPC, whereas the levels of ϵPKC remained unchanged. Prolonged ischemia induced a 50% decline in the ATP-dependent proteasome activity and increased the accumulation of unfolded proteins during reperfusion; both of which were completely prevented by IPC. Pharmacological inhibition of the proteasome and selective inhibition of ϵPKC during IPC restored δPKC translocation to the mitochondria and elevated the levels of unfolded proteins while increasing myocardial injury associated with I/R. (Note that none of these treatments had any effect on viability under normoxic conditions.) Interestingly, administration of a specific peptide activator of ϵPKC during I/R completely prevented the loss in proteasome activity seen during ischemia. Importantly, increased myocardial injury was the result of restoring the δPKC-mediated I/R pro-apoptotic phenotype by decreasing pro-survival signaling and increasing cytochrome c release into the cytosol. Taken together, this study shows that IPC prevents I/R injury by protecting ATP-dependent 26S proteasome function, which in turn decreases the accumulation of the pro-apoptotic kinase, δPKC, at cardiac mitochondria resulting in the accumulation of the pro-survival kinase, ϵPKC. Additionally, ϵPKC seems to be a novel regulator of proteasome function as it relates to I/R injury. The proteasome can therefore be viewed as a sensor of cellular viability, determining the ratio of pro-apoptotic δPKC and pro-survival ϵPKC at the mitochondria and thus the ultimate fate of the cell.