Using conscious rabbits, we examined the effect of ischemic preconditioning (PC) on p44 and p42
mitogen-activated protein kinases (MAPKs). We found that both
isoforms contribute significantly to total MAPK activity in the heart (in-gel
kinase assay: p44, 59 +/- 1%; p42, 41 +/- 1%). Ischemic PC (6 cycles of 4-min occlusion/4-min reperfusion) elicited a pronounced increase in total cellular MAPK activity (+89%). This increase, which occurred exclusively in the nuclear fraction, was contributed by both
isoforms (in-gel
kinase assay: p44, +97%; p42, +210%) and was accompanied by migration of the two
proteins from the cytosolic to the nuclear compartment. In control rabbits,
MAPK kinase (
MEK)1 and MEK2, direct activators of p44 and p42 MAPKs, were located almost exclusively in the cytosolic fraction. Ischemic PC induced a marked increase in cytosolic
MEK activity (+164%), whereas nuclear
MEK activity did not change, indicating that
MEK-induced activation of MAPKs occurred in the cytosolic compartment. Activation of MAPKs after ischemic PC was completely blocked by the
protein kinase C (PKC) inhibitor
chelerythrine. Selective overexpression of
PKC-epsilon in adult rabbit cardiomyocytes induced activation of both p44 and p42 MAPKs and reduced
lactate dehydrogenase release during simulated
ischemia-reperfusion, which was abolished by the
MEK inhibitor
PD-98059. The results demonstrate that 1) ischemic PC induces a rapid activation of p44 and p42 MAPKs in hearts of conscious rabbits; 2) the mechanism of this phenomenon involves activation of p44 and p42 MAPKs in the cytosol and their subsequent translocation to the nucleus; and 3) it occurs via a PKC-mediated signaling pathway. The in vitro data implicate
PKC-epsilon as the specific
isoform responsible for PKC-induced MAPK activation and suggest that p44/p42 MAPKs contribute to
PKC-epsilon-mediated protection against simulated
ischemia. The results are compatible with the hypothesis that p44 and p42 MAPKs may play a role in myocardial adaptations to ischemic stress.