Stress-induced mitogen-activated protein kinase (MAP) p38 is activated in various forms of heart failure, yet its effects on the intact heart remain to be established. Targeted activation of p38 MAP kinase in ventricular myocytes was achieved in vivo by using a gene-switch transgenic strategy with activated mutants of upstream kinases MKK3bE and MKK6bE. Transgene expression resulted in significant induction of p38 kinase activity and premature death at 7-9 weeks. Both groups of transgenic hearts exhibited marked interstitial fibrosis and expression of fetal marker genes characteristic of cardiac failure, but no significant hypertrophy at the organ level. Echocardiographic and pressure-volume analyses revealed a similar extent of systolic contractile depression and restrictive diastolic abnormalities related to markedly increased passive chamber stiffness. However, MKK3bE-expressing hearts had increased end-systolic chamber volumes and a thinned ventricular wall, associated with heterogeneous myocyte atrophy, whereas MKK6bE hearts had reduced end-diastolic ventricular cavity size, a modest increase in myocyte size, and no significant myocyte atrophy. These data provide in vivo evidence for a negative inotropic and restrictive diastolic effect from p38 MAP kinase activation in ventricular myocytes and reveal specific roles of p38 pathway in the development of ventricular end-systolic remodeling.heart failure ͉ conditional transgenesis

Abstract-p38 Mitogen-activated protein kinase (MAPK) is one of the most ancient signaling molecules and is involved in multiple cellular processes, including cell proliferation, cell growth, and cell death. In the heart, enhanced activation of p38 MAPK is associated with ischemia/reperfusion injury and the onset of heart failure. In the present study, we investigated the function of p38 MAPK in regulating cardiac contractility and its underlying mechanisms. In cultured adult rat cardiomyocytes, activation of p38 MAPK by adenoviral gene transfer of an activated mutant of its upstream kinase, MKK3bE, led to a significant reduction in baseline contractility, compared with uninfected cells or those infected with a control adenoviral vector (Adv-␤-galactosidase). The inhibitory effect of MKK3bE on contractility was largely prevented by coexpressing a dominant-negative mutant of p38 MAPK or treating cells with a p38 MAPK inhibitor, SB203580. Conversely, inhibition of endogenous p38 MAPK activity by SB203580 rapidly and reversibly enhanced cell contractility in a dose-dependent manner, without altering L-type Ca 2ϩ currents or Ca 2ϩ i transients. MKK3bE-induced p38 activation had no significant effect on pH i , whereas SB203580 had a minor effect to elevate pH i . Furthermore, activation of p38 MAPK was unable to increase troponin I phosphorylation. Thus, we conclude that the negative inotropic effect of p38 MAPK is mediated by decreasing myofilament response to Ca 2ϩ , rather than by altering Ca Key Words: p38 mitogen-activated protein kinase Ⅲ cardiac contractility Ⅲ excitation-contraction coupling Ⅲ troponin I Ⅲ intracellular pH M itogen-activated protein kinase (MAPK) superfamily is one of the most important signal transduction systems conserved in all eukaryotes. 1-4 There are three major subgroups identified, including the extracellular signal-regulated kinase (ERK1/2), p38 MAPK, and c-jun-NH 2 -terminal kinase (JNK). p38 MAPK is a subfamily of stress-activated MAPKs (SAPKs) known to be involved in a multitude of cellular events, such as inflammation, cell injury, cell proliferation or differentiation, and cell growth or death. [5][6][7] In the heart, activation of p38 MAPK has been observed in pressure overload or ischemia/infarction-induced cardiac hypertrophy and heart failure in humans 8,12,13 and animal models. 14 -19 In cultured cardiac myocytes, activation of p38 MAPK induces myocyte hypertrophy and apoptosis 18,20 and is also implicated in the preconditioning process and ischemia/reperfusion injury. [21][22][23][24][25] Our recent studies have shown that ␤-adrenergic stimulation is able to activate p38 MAPK via a protein kinase A (PKA)-dependent mechanism and that activation of p38 MAPK provides a negative feedback to PKA-mediated positive contractile response in intact cardiac myocytes. 26 Furthermore, evidence from our recent in vivo studies in transgenic mice has demonstrated that cardiacspecific activation of p38 MAPK markedly attenuates cardiac contractility. 27 However, the mechanism underlying the...