The TAB1-p38α complex aggravates myocardial injury and can be targeted by small molecules.

Inhibiting MAPK14 (p38α) diminishes cardiac damage in myocardial ischemia. During myocardial ischemia, p38α interacts with TAB1, a scaffold protein, which promotes p38α autoactivation; active p38α (pp38α) then transphosphorylates TAB1. Previously, we solved the X-ray structure of the p38α-TAB1 (residues 384-412) complex. Here, we further characterize the interaction by solving the structure of the pp38α-TAB1 (residues 1-438) complex in the active state. Based on this information, we created a global knock-in (KI) mouse with substitution of 4 residues on TAB1 that we show are required for docking onto p38α. Whereas ablating p38α or TAB1 resulted in early embryonal lethality, the TAB1-KI mice were viable and had no appreciable alteration in their lymphocyte repertoire or myocardial transcriptional profile; nonetheless, following in vivo regional myocardial ischemia, infarction volume was significantly reduced and the transphosphorylation of TAB1 was disabled. Unexpectedly, the activation of myocardial p38α during ischemia was only mildly attenuated in TAB1-KI hearts. We also identified a group of fragments able to disrupt the interaction between p38α and TAB1. We conclude that the interaction between the 2 proteins can be targeted with small molecules. The data reveal that it is possible to selectively inhibit signaling downstream of p38α to attenuate ischemic injury.

TAB1-Induced Autoactivation of p38α Mitogen-Activated Protein Kinase Is Crucially Dependent on Threonine 185.

p38α mitogen-activated protein kinase is essential to cellular homeostasis. Two principal mechanisms to activate p38α exist. The first relies on dedicated dual-specificity kinases such as mitogen-activated protein kinase kinase (MAP2K) 3 (MKK3) or 6 (MKK6), which activate p38α by phosphorylating Thr180 and Tyr182 within the activation segment. The second is by autophosphorylation of Thr180 and Tyr182 in cis, mediated by p38α binding the scaffold protein TAB1. The second mechanism occurs during myocardial ischemia, where it aggravates myocardial infarction. Based on the crystal structure of the p38α-TAB1 complex we replaced threonine 185 of p38α with glycine (T185G) to prevent an intramolecular hydrogen bond with Asp150 from being formed. This mutation did not interfere with TAB1 binding to p38α. However, it disrupted the consequent long-range effect of this binding event on the distal activation segment, releasing the constraint on Thr180 that oriented its hydroxyl for phosphotransfer. Based on assays performed in vitro and in vivo, the autoactivation of p38α(T185G) was disabled, while its ability to be activated by upstream MAP2Ks and to phosphorylate downstream substrates remained intact. Furthermore, myocardial cells expressing p38α(T185G) were resistant to injury. These findings reveal a mechanism to selectively disable p38α autoactivation and its consequences, which may ultimately circumvent the toxicity associated with strategies that inhibit p38α kinase activity under all circumstances, such as with ATP-competitive inhibitors.

Redox-dependent dimerization of p38α mitogen-activated protein kinase with mitogen-activated protein kinase kinase 3.

The kinase p38α MAPK (p38α) plays a pivotal role in many biological processes. p38α is activated by canonical upstream kinases that phosphorylate the activation region. The purpose of our study was to determine whether such activation may depend on redox-sensing cysteines within p38α. p38α was activated and formed a disulfide-bound heterodimer with MAP2K3 (MKK3) in rat cardiomyocytes and isolated hearts exposed to H2O2 This disulfide heterodimer was sensitive to reduction by mercaptoethanol and was enhanced by the thioredoxin-reductase inhibitor auranofin. We predicted that Cys-119 or Cys-162 of p38α, close to the known MKK3 docking domain, were relevant for these redox characteristics. The C119S mutation decreased whereas the C162S mutation increased the dimer formation, suggesting that these two Cys residues act as vicinal thiols, consistent with C119S/C162S being incapable of sensing H2O2 Similarly, disulfide heterodimer formation was abolished in H9C2 cells expressing both MKK3 and p38α C119S/C162S and subjected to simulated ischemia and reperfusion. However, the p38α C119S/C162S mutants did not exhibit appreciable alteration in activating dual phosphorylation. In contrast, the anti-inflammatory agent 10-nitro-oleic acid (NO2-OA), a component of the Mediterranean diet, reduced p38α activation and covalently modified Cys-119/Cys-162, probably obstructing MKK3 access. Moreover, NO2-OA reduced the dephosphorylation of p38α by hematopoietic tyrosine phosphatase (HePTP). Furthermore, steric obstruction of Cys-119/Cys-162 by NO2-OA pretreatment in Langendorff-perfused murine hearts prevented the p38-MKK3 disulfide dimer formation and attenuated H2O2-induced contractile dysfunction. Our findings suggest that cysteine residues within p38α act as redox sensors that can dynamically regulate the association between p38 and MKK3.

Mechanism and consequence of the autoactivation of p38α mitogen-activated protein kinase promoted by TAB1.

p38α mitogen-activated protein kinase (p38α) is activated by a variety of mechanisms, including autophosphorylation initiated by TGFß-activated kinase 1 binding protein 1 (TAB1) during myocardial ischemia and other stresses. Chemical-genetic approaches and coexpression in mammalian, bacterial and cell-free systems revealed that mouse p38α autophosphorylation occurs in cis by direct interaction with TAB1(371-416). In isolated rat cardiac myocytes and perfused mouse hearts, TAT-TAB1(371-416) rapidly activates p38 and profoundly perturbs function. Crystal structures and characterization in solution revealed a bipartite docking site for TAB1 in the p38α C-terminal kinase lobe. TAB1 binding stabilizes active p38α and induces rearrangements within the activation segment by helical extension of the Thr-Gly-Tyr motif, allowing autophosphorylation in cis. Interference with p38α recognition by TAB1 abolishes its cardiac toxicity. Such intervention could potentially circumvent the drawbacks of clinical pharmacological inhibitors of p38 catalytic activity.

A chemical genetic approach reveals that p38alpha MAPK activation by diphosphorylation aggravates myocardial infarction and is prevented by the direct binding of SB203580.

The use of nonselective pharmacological inhibitors has resulted in controversy regarding the mechanism and consequences of p38 activation during myocardial infarction. Classic p38 inhibitors such as SB203580 rely on a critical "gatekeeper" threonine residue for binding. We addressed these controversies by using mice in which the p38alpha alleles were targeted to cause substitution of the gatekeeper residue and resistance to inhibition. In homozygous drug-resistant compared with wild-type hearts, SB203580 failed to inhibit the activating phosphorylation of p38 or to reduce the infarction caused by myocardial ischemia. However, BIRB796, a p38 inhibitor not reliant on the gatekeeper for binding, similarly reduced p38-activating phosphorylation and infarction in both wild-type and knock-in mice, thereby excluding a nonspecific inhibitor-dependent phenotype resulting from the targeting strategy. Furthermore, the activation during myocardial ischemia involved phosphorylation of both the threonine and tyrosine residues in the activation loop of p38 despite the phosphorylation of the threonine alone being sufficient to create the epitope for dual phosphospecific antibody binding. Finally, SB203580 failed to reduce infarction in heterozygous drug-resistant hearts, suggesting that near complete inhibition of p38alpha kinase activity is necessary to elicit protection. These results indicate that, during myocardial ischemia, p38alpha (i) is the dominant-active p38 isoform, (ii) contributes to infarction, (iii) is responsible for the cardioprotective effect of SB203580, and (iv) is activated by a mechanism consistent with autodiphosphorylation despite this necessitating the phosphorylation of a tyrosine residue by an archetypal serine/threonine kinase.

Targeting p38-MAPK in the ischaemic heart: kill or cure?

The p38-MAPK pathway plays an important role in myocardial ischaemia/reperfusion injury and has been implicated in the regulation of cardiac gene expression, myocyte hypertrophy, inflammation, energetic metabolism, contractility, proliferation and apoptosis. The activation of p38-MAPK by dual phosphorylation during myocardial ischaemia aggravates lethal injury. However, under other circumstances activation can protect the heart, and recent evidence suggests that the mechanism of p38-MAPK activation may differ by circumstance. Determining the precise mechanism of activation during myocardial ischaemia is of considerable importance, since it may allow prevention of the detrimental, but not the beneficial, activation of p38-MAPK and lead to the identification of the relevant signalling molecules to be targeted for pharmaceutical intervention.

Inhibition of p38 MAPK activity fails to attenuate contractile dysfunction in a mouse model of low-flow ischemia.

OBJECTIVE: The basal activity of p38 MAPK has recently been shown to impair myocardial contractility. This kinase is activated by ischemia and short-term hibernation. We hypothesized that p38 MAPK activation may contribute to the contractile deficit that characterizes low-flow ischemia. METHODS: In Langendorff-perfused isolated C57BL/6 mouse hearts, perfusion pressure was reduced from 85 to 15 or 30 mm Hg for 120 min to induce ischemic left ventricular dysfunction. The effect of the p38 MAPK inhibitor SB203580 (1 microM/l) on contractile function and p38 MAPK activation was assessed. RESULTS: Reduction in perfusion pressure to 15 or 30 mm Hg was accompanied by stable reductions in coronary flow (83+/-2% and 66+/-2%, respectively) and developed pressure (84+/-2% and 61+/-3%), with minimal infarction (15.6+/-0.69% and 10.6+/-0.98% of LV myocardium, respectively), but marked activation of p38 MAPK (reflected in pHSP27 1092+/-326% basal and 996+/-301% basal, respectively). The p38 MAPK inhibitor SB203580, present during the last 60 min of reduced pressure perfusion, prevented p38 MAPK activation (pHSP27 281+/-92% basal, p=0.01 and 186+/-72% basal, p=0.01) but, despite the presence of a contractile reserve, had no effect on developed pressure. Similarly, early treatment with SB203580 started 5 min after the onset of reduced flow also failed to attenuate contractile dysfunction. CONCLUSION: The p38 MAPK activation that accompanies short-term hibernation does not appear to contribute to the contractile deficit.

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 micromol/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 p38alpha-MAPK, lacking the SB203580 binding site, was phosphorylated when H9c2 myoblasts were subjected to simulated ischemia. However, SB203580 (1 micromol/L) did not prevent the phosphorylation of resistant p38alpha-MAPK. These findings suggest the ischemic activation of p38-MAPK contributing to myocardial injury is by TAB1-associated autophosphorylation.

Varying susceptibility to myocardial infarction among C57BL/6 mice of different genetic background.

Genetically manipulated mouse lines are invaluable to investigate the effects of a single gene on sensitivity to ischemia. When choosing appropriate controls, we were concerned that intrinsic, strain-independent but colony-dependent differences may influence the susceptibility to ischemia. We, therefore, compared the infarct:risk volume ratio (I:R%) after 30-min global ischemia in Langendorff-perfused hearts from outbred C57BL/6 mice with that in wild-type mice derived from heterozygote x heterozygote crosses of two different in-house C57BL/6 mouse lines with targeted disruption of an MKK3 or MAPKAPK2 allele. Despite similar hemodynamic characteristics, I:R% in outbred C57BL/6 hearts was significantlysmaller (40.8 +/- 2.8%) than in C57BL/6 MAPKAPK2 wild types (65.8 +/- 4.5%, P = 0.0003) and significantly larger than in C57BL/6 MKK3 wild types (23.7 +/- 2.9%, P = 0.002). Therefore, inherent colony substrain-dependent differences appear to influence the susceptibility to infarction in response to global ischemia, underscoring the importance of using colony-matched wild-type controls in murine studies of myocardial ischemia.