Ischaemic preconditioning improves proteasomal activity and increases the degradation of deltaPKC during reperfusion

The response of the myocardium to an ischaemic insult is regulated by two highly homologous protein kinase C (PKC) isozymes, delta and epsilonPKC. Here, we determined the spatial and temporal relationships between these two isozymes in the context of ischaemia/reperfusion (I/R) and ischaemic preconditioning (IPC) to better understand their roles in cardioprotection.

Using an ex vivo rat model of myocardial infarction, we found that short bouts of ischaemia and reperfusion prior to the prolonged ischaemic event (IPC) diminished deltaPKC translocation by 3.8-fold and increased epsilonPKC accumulation at mitochondria by 16-fold during reperfusion. In addition, total cellular levels of deltaPKC decreased by 60 +/- 2.7% in response to IPC, whereas the levels of epsilonPKC did not significantly change. Prolonged ischaemia induced a 48 +/- 11% decline in the ATP-dependent proteasomal activity and increased the accumulation of misfolded proteins during reperfusion by 192 +/- 32%; both of these events were completely prevented by IPC. Pharmacological inhibition of the proteasome or selective inhibition of epsilonPKC during IPC restored deltaPKC levels at the mitochondria while decreasing epsilonPKC levels, resulting in a loss of IPC-induced protection from I/R. Importantly, increased myocardial injury was the result, in part, of restoring a deltaPKC-mediated I/R pro-apoptotic phenotype by decreasing pro-survival signalling and increasing cytochrome c release into the cytosol.

Taken together, our findings indicate that IPC prevents I/R injury at reperfusion by protecting ATP-dependent 26S proteasomal function. This decreases the accumulation of the pro-apoptotic kinase, deltaPKC, at cardiac mitochondria, resulting in the accumulation of the pro-survival kinase, epsilonPKC.