"Stress-responsive" mitogen-activated protein kinases (c-Jun N-terminal kinases and p38 mitogen-activated protein kinases) in the myocardium.

Abstract

The best-characterized subfamilies of the mitogen-activated protein kinase (MAPK) superfamily are the extracellularly responsive kinases (ERKs) and the two “stress-responsive” MAPK subfamilies, namely, the c-Jun N-terminal kinases (JNKs) and the p38-MAPKs.1 2 3 4 5 As yet, no single nomenclature has been determined, and the synonyms currently in use are summarized in Table 1⇓. The ERK cascade is the most thoroughly studied of the MAPK cascades, and it is activated principally by G protein–coupled receptor (GPCR) agonists in cardiac myocytes. We have reviewed this topic recently,6 and we will not discuss it in any depth here. The regulation of the JNK and p38-MAPK cascades in the myocardium (Figure 1⇓) forms the principal subject of this review. Figure 1. A scheme for the activation of stress-responsive MAPK cascades. Exposure of cultured cardiac myocytes and whole hearts to stresses or GPCR agonists leads to activation of the JNKs and p38-MAPKs. Cellular stresses induce reorganization of the cytoskeleton and activation of the small G proteins, Rac and Cdc42. GPCR agonists activate the small G protein, Ras, through a PKC-dependent mechanism. Ras is recognized to participate in the activation of the ERK cascade and, either directly or indirectly, may activate the stress-responsive MAPKs (JNKs and p38-MAPKs). The JNKs are activated by MKK4 and MKK7, whereas p38-MAPKs are activated by MKK3 and MKK6 (see Table 2⇓). The upstream activators (the MKKKs) of the stress-responsive MKKs have not been clearly defined but possibly include MEKKs (MAPK or ERK kinase kinases), mixed lineage kinases (MLKs), and/or p21-activated kinases (PAKs). Substrates for the stress-responsive MAPKs include transcription factors, which regulate the transcriptional changes, and MAPKAPK2, which phosphorylates Hsp25/27 and may thereby confer cytoprotection. CHOP indicates C/EBP homologous protein; MEF2C myocyte enhancer factor 2C. View this table: Table 1. Mitogen-Activated Protein Kinase (MAPK) Subfamilies An ever-increasing number of isoforms of …