Diverse mechanisms of myocardial p38 mitogen-activated protein kinase activation - Evidence for MKK-independent activation by a TAB1-associated mechanism contributing to injury during myocardial ischemia

The ischemic activation of p38alpha mitogen-activated protein kinase ( p38alpha-MAPK) is thought to contribute to myocardial injury. Under other circumstances, activation is through dual phosphorylation by MAPK kinase 3 (MKK3). Therefore, the mkk3(-/-) murine heart should be protected during ischemia. In retrogradely perfused mkk3(-/-) and mkk3(-/-) mouse hearts subjected to 30 minutes of global ischemia and 120 minutes of reperfusion, infarction/risk volume was similar ( 50 +/- 5 versus 51 +/- 4, P = 0.93, respectively), as was intraischemic p38-MAPK phosphorylation ( 10 minutes ischemia as percent basal, 608 +/- 224 versus 384 +/- 104, P = 0.43, respectively). This occurred despite undetectable activation of MKK3/6 in mkk3(-/-) hearts. However, tumor necrosis factor (TNF)-induced p38-MAPK phosphorylation was markedly diminished in mkk3(-/-) vs mkk3(-/-) hearts ( percent basal, 127 +/- 23 versus 540 +/- 267, respectively, P = 0.04), suggesting an MKK-independent activation mechanism by ischemia. Hence, we examined p38-MAPK activation by TAB1-associated autophosphorylation. In wild-type mice and mkk3(-/-) mice, the p38-MAPK catalytic site inhibitor SB203580 ( 1 mumol/L) diminished phosphorylation during ischemia versus control ( 10 minutes ischemia as percent basal, 143 +/- 2 versus 436 +/- 96, P = 0.003, and 122 +/- 25 versus 623 +/- 176, P = 0.05, respectively) and reduced infarction volume (infarction/risk volume, 57 +/- 5 versus 36 +/- 3, P < 0.001, and 50 +/- 5 versus 29 +/- 3, P = 0.003, respectively) but did not alter TNF-induced activation, although in homogenates of ischemic hearts but not TNF-exposed hearts, p38-MAPK was associated with TAB1. Furthermore, adenovirally expressed wild-type and drug-resistant p38α-MAPK, lacking the SB203580 binding site, was phosphorylated when H9c2 myoblasts were subjected to simulated ischemia. However, SB203580 (1 μmol/L) did not prevent the phosphorylation of resistant p38α-MAPK. These findings suggest the ischemic activation of p38-MAPK contributing to myocardial injury is by TAB1-associated autophosphorylation.