Early signaling with iron and copper in ischemic preconditioning of the heart

Iron and copper play major roles in biological systems, catalyzing free radical production and consequently causing damage. The relatively high levels of these metals, which are mobilized into the coronary flow following prolonged ischemia, have been incriminated as key players in reperfusion injury to the heart (M. Chevion et al., Proc. Nat. Acad Sci. USA 90:1102-6 (1993) & E. Berenshtein et al., J Mol. Cell. Cardiol. 29:3025-34 (1997)). In the present communication we investigated other roles of iron - providing protection to the ischemic heart via preconditioning (PC). PC was accomplished by subjecting isolated rat hearts to three episodes of 2 min ischemia separated by 3 min of reperfusion. Prolonged ischemia followed the PC phase. PC hearts (group I) were compared to hearts subjected to normal perfusion (group II, no ischemia) and to ischemia without PC (group III). Group I showed a marked improvement in the recovery of hemodynamic function versus group III. Biochemical parameters further substantiated the PC protection provided to group I against prolonged ischemia. Correspondingly, group I presented markedly lower redistribution and mobilization of iron and copper into the coronary flow, following prolonged ischemia, as evinced from the decrease in total levels, and in the "free" fraction (redox active levels) of either iron or copper. During the PC phase no loss of cardiac function was observed. A small wave of redistribution and mobilization of iron and copper (typically less than 4-8% of the value of 35 min ischemia) was recorded. The cellular content of ferritin measured in the heart was significantly higher in group I than in group III (0.90 and 0.54, respectively). Also, iron-saturation of ferritin was significantly lower for PC hearts, compared to both group II & III (0.22 versus 0.32 and 0.31 mug/mug, for 35 min ischemia, respectively). These findings are in accord with the proposal that intracellular re-distribution and mobilization of small levels of iron, during PC, cause a switch between cellular IRP-1 and aconitase, reversing the inhibitory control of translation of the ferritin message, and allowing a subsequent rapid accumulation of this iron-storage protein. It is proposed that iron plays a dual role: (i) It serves as a signaling pathway for the accumulation of ferritin following the PC phase. This iron is not involved in cardiac injury, but rather prepares the heart against future high levels of "free" iron, and thus reduces the degree of myocardial damage after prolonged ischemia. (ii) High levels of iron (and copper) are mobilized following prolonged ischemia and cause tissue damage. It is tempting to speculate that by analogy to iron and ferritin, copper and its binding protein - metallothionein, are involved not only in reperfusion injury to the heart, but also in cardiac protection by PC.