MITOGEN-ACTIVATED PROTEIN-KINASE TRANSLOCATES TO THE NUCLEUS DURING ISCHEMIA AND IS ACTIVATED DURING REPERFUSION

Growth factors and various cellular stresses are known to activate mit ogen-activated protein (MAP) kinase, which plays a role in conveying s ignals from the cytosol to the nucleus. The phosphorylation of MAP kin ase is thought to be a prerequisite for translocation. Here, we invest igate the translocation and activation of MAP kinase during ischaemia and reperfusion in perfused rat heart. Ischaemia (0-40 min) induces th e translocation of MAP kinase from the cytosol fraction to the nuclear fraction. Immunohistochemical observation shows that MAP kinase stain ing in the nucleus is enhanced after ischaemia for 40 min. Unexpectedl y, tyrosine phosphorylation of MAP kinase is unchanged in the nuclear fraction during ischaemia, indicating that unphosphorylated MAP kinase translocates from the cytosol to the nucleus. During reperfusion (0-3 0 min), after ischaemia for 20 min, tyrosine phosphorylation of MAP ki nase in the nuclear fraction is increased with a peak at 10 min of rep erfusion. The activation is confirmed by MAP kinase activity with simi lar kinetics to the tyrosine phosphorylation. However, the amount of M AP kinase in the fraction is almost constant during reperfusion for 10 min. Although an upstream kinase for MAP kinase, MAP kinase/extracell ular signal-regulated kinase kinase (MEK)-1, remains in the cytosol th roughout ischaemia and reperfusion, MEK-2, another upstream kinase for MAP kinase, is constantly present in the nucleus as well as in the cy toplasm, based on analyses by fractionation and immunohistochemistry. Furthermore, MEK-2 activity in the nuclear fraction is rapidly increas ed during post-ischaemic reperfusion. These findings demonstrate that nuclear MAP kinase is activated by tyrosine phosphorylation during rep erfusion, probably by MEK-2.