ERK1/2 Regulates Intracellular ATP Levels through α-Enolase Expression in Cardiomyocytes Exposed to Ischemic Hypoxia and Reoxygenation

Extracellular signal-regulated kinase 1/2 (ERK1/2) is known to function in cell survival in response to various stresses; however, the mechanism of cell survival by ERK1/2 remains poorly elucidated in ischemic heart. Here we applied functional proteomics by two-dimensional electrophoresis to identify a cellular target of ERK1/2 in response to ischemic hypoxia. Approximately 1500 spots were detected by Coomassie Brilliant Blue staining of a sample from unstimulated cells. The staining intensities of at least 50 spots increased at 6-h reoxygenation after 2-h ischemic hypoxia. Of the 50 spots that increased, at least 4 spots were inhibited in the presence of PD98059, a MEK inhibitor. A protein with a molecular mass of 52 kDa that is strongly induced by ERK1/2 activation in response to ischemic hypoxia and reoxygenation was identified as α-enolase, a rate-limiting enzyme in the glycolytic pathway, by liquid chromatography-mass spectrometry and amino acid sequencing. The expressions of the α-enolase mRNA and protein are inhibited during reoxygenation after ischemic hypoxia in the cells containing a dominant negative mutant of MEK1 and treated with a MEK inhibitor, PD98059, leading to a decrease in ATP levels. α-Enolase expression is also observed in rat heart subjected to ischemia-reperfusion. The induction of α-enolase by ERK1/2 appears to be mediated by c-Myc. The introduction of the α-enolase protein into the cells restores ATP levels and prevents cell death during ischemic hypoxia and reoxygenation in these cells. These results show that α-enolase expression by ERK1/2 participates in the production of ATP during reoxygenation after ischemic hypoxia, and a decrease in ATP induces apoptotic cell death. Furthermore, α-enolase improves the contractility of cardiomyocytes impaired by ischemic hypoxia. Our results reveal that ERK1/2 plays a role in the contractility of cardiomyocytes and cell survival through α-enolase expression during ischemic hypoxia and reoxygenation.