Ischemia associated injury of the myocardium is caused by oxidative damage during reperfusion. Myocardial protection by ischemic preconditioning (IPC) was shown to be mediated by a transient '
iron-signal' that leads to the accumulation of
apoferritin and sequestration of reactive
iron released during the
ischemia. Here we identified the source of this '
iron signal' and evaluated its role in the mechanisms of cardiac protection by hypoxic preconditioning. Rat hearts were retrogradely perfused and the effect of proteasomal and lysosomal
protease inhibitors on
ferritin levels were measured. The
iron-signal was abolished,
ferritin levels were not increased and cardiac protection was diminished by inhibition of the
proteasome prior to IPC. Similarly, double amounts of
ferritin and better recovery after ex vivo
ischemia-and-reperfusion (I/R) were found in hearts from in vivo
hypoxia pre-conditioned animals. IPC followed by normoxic perfusion for 30 min ('delay') prior to I/R caused a reduced
ferritin accumulation at the end of the
ischemia phase and reduced protection. Full restoration of the IPC-mediated cardiac protection was achieved by employing lysosomal inhibitors during the 'delay'. In conclusion, proteasomal protein degradation of
iron-
proteins causes the generation of the '
iron-signal' by IPC, ensuing de-novo
apoferritin synthesis and thus, sequestering reactive
iron. Lysosomal
proteases are involved in subsequent
ferritin breakdown as revealed by the use of specific pathway inhibitors during the 'delay'. We suggest that proteasomal
iron-protein degradation is a stress response causing an expeditious cytosolic
iron release thus, altering
iron homeostasis to protect the myocardium during I/R, while lysosomal
ferritin degradation is part of housekeeping
iron homeostasis.