Mitogen-Activated Protein Kinases
Mitogen-activated protein (MAP) kinases are a family of serine/threonine kinases that play a key role in transducing chemical and physical extracellular signals into a variety of intracellular responses. These protein kinases are among the most highly studied signaling molecules, as reflected by the more than 97,000 papers on “MAP kinase” listed in PubMed as of August 2016. This chapter provides a broad overview of the different subfamilies of MAP kinases. Each subfamily will then be treated in more detail in individual chapters of this encyclopedia.
The discovery of MAP kinases goes back to 1987 when Ray and Sturgill reported the identification of a novel insulin-stimulated serine/threonine kinase activity from extracts of 3T3-L1 adipocytes that was capable of phosphorylating microtubule-associated protein-2 in vitro (Ray and Sturgill 1987). As a result, the enzyme was called microtubule-associated protein (MAP) kinase (Ray and Sturgill 1988). Partial purification of the novel kinase revealed that its activation was accompanied by phosphorylation on threonine and tyrosine residues, suggesting that it may be a direct substrate of the insulin receptor or another insulin-regulated tyrosine kinase (Ray and Sturgill 1988). Subsequent work indicated that the insulin-activated MAP kinase was identical to phosphoproteins of 41–43 kDa, previously shown to be phosphorylated on tyrosine in response to a variety of mitogens and growth factors (Rossomando et al. 1989). This led to the redesignation of the MAP acronym from microtubule-associated protein to mitogen-activated protein.
The first sequence of a mammalian MAP kinase, the insulin-stimulated MAP kinase named extracellular signal-regulated kinase 1 (Erk1), was reported in 1990 by the group of Melanie Cobb (Boulton et al. 1990). Interestingly, this protein kinase was found to be homologous to two recently identified budding yeast protein kinases, Kss1p and Fus3p, involved in cell cycle arrest in response to mating pheromones. This finding provided the first evidence for the conservation of MAP kinase functions across eukaryotic evolution. Later on, the discovery of additional MAP kinase pathways, first in yeast and then in mammalian cells, led to the concept of multiple MAP kinase subfamilies (Avruch 2007; Pearson et al. 2001; Widmann et al. 1999).
The initial observation that MAP kinase contains both phosphothreonine and phosphotyrosine, followed by the demonstration that dephosphorylation of either residue leads to inactivation of the kinase (Anderson et al. 1990) suggested that MAP kinase could serve to integrate information from two different signaling pathways. Intense efforts were devoted to identify these upstream MAP kinase kinase(s). Surprisingly, purification of a MAP kinase activator revealed that the threonine and tyrosine phosphorylation activities reside in the same polypeptide (Ahn et al. 1992). Shortly thereafter, a series of studies established that the MAP kinase kinases are themselves activated by phosphorylation by upstream MAP kinase kinase kinases, defining the basic architecture of MAP kinase pathways.
Subfamilies of MAP Kinases
Structure and Regulation of MAP Kinases
At the structural level, MAP kinases are composed of a catalytic kinase domain flanked by amino- and carboxy-terminal extensions of varying lengths (Fig. 1). All the family members display more than 40% identity to the founding member Erk1 in the kinase domain. One feature that distinguishes the different MAP kinase subfamilies is the sequence of the activation loop, which is the site of activating phosphorylation by upstream protein kinases. Several MAP kinases possess the motif Thr-Xxx-Tyr in the activation loop and are phosphorylated by a family of dual-specificity protein kinases known as MAP kinase kinases (MKKs) or MAP kinase or ERK kinases (MEKs). However, the Thr-Xxx-Tyr motif is absent in the MAP kinases Erk3, Erk4, and Nlk. Erk3 and Erk4 bear the motif Ser-Glu-Gly instead, whereas Nlk has a glutamic acid residue at the position of the tyrosine. The MAP kinase Erk7 has the Thr-Xxx-Tyr motif in its loop but is activated by a different mechanism. Based on these structural and regulatory features, it has been proposed to divide MAP kinases into classical and atypical enzymes (Coulombe and Meloche 2007). Classical MAP kinases, which are phosphorylated and activated by MKK family members, include the four subfamilies Erk1/Erk2, Jnk1/Jnk2/Jnk3, p38α/p38β/p38γ/p38δ, and Erk5. Atypical MAP kinases, which are not substrates of MKKs, include the subfamilies Erk3/Erk4, Erk7, and Nlk (Fig. 1).
Classical MAP kinases are organized into modules of three sequentially acting protein kinases (Pearson et al. 2001; Widmann et al. 1999). Typically, exposure to an extracellular stimulus induces the oligomerization and/or activation of a cell-surface receptor, often leading to the activation of a small GTPase of the Ras/Rho family, to a MAP kinase kinase kinase kinase, or to the binding of specialized adaptor proteins. These events ultimately lead to the activation of the upstream MAP kinase kinase kinase by a complex mechanism involving phosphorylation events and protein-protein interactions. The MAP kinase kinase kinase then phosphorylates and activates the MAP kinase kinase, which in turn activates the effector MAP kinase by dual phosphorylation of Thr and Tyr residues. Each MAP kinase kinase kinase confers responsiveness to a distinct set of stimuli. Once activated, classical MAP kinases can phosphorylate a vast array of substrates present in all cellular compartments. The regulation of atypical MAP kinases is much less well understood.
MAP Kinases in Physiology and Disease
MAP kinases are activated in response to a variety of extracellular stimuli and cellular perturbations to regulate numerous cellular responses such as gene expression, cell proliferation and differentiation, cell survival, metabolism, motility, cytokine secretion, and adaptation (Cargnello and Roux 2011; Kyriakis and Avruch 2012). Pharmacological and genetic studies have revealed that each of the MAP kinase subfamilies have distinct but sometimes overlapping cellular functions. The best characterized MAP kinase subfamilies are the Erk1/Erk2, Jnk1/Jnk2/Jnk3, and p38α/p38β/p38γ/p38δ. The Erk1/Erk2 MAP kinase pathway plays a major role in the regulation of cell proliferation and differentiation. The Jnk pathway is involved in cell death signaling, metabolism, and immune responses, while the p38 pathway regulates cell differentiation and immune and inflammatory responses.
Deregulated activity of MAP kinase pathways has been linked to many diseases, most notably cancer, inflammatory disorders, diabetes and metabolic syndrome, and neurodegenerative pathologies (Lawrence et al. 2008; Kim and Choi 2010; Kyriakis and Avruch 2012). For example, hyperactivation of the Ras-dependent Raf/Mek/Erk1/Erk2 pathway is frequently observed in human cancer as a result of aberrant activation of receptor tyrosine kinases or gain-of-function mutations in RAS or RAF genes (Schubbert et al. 2007). These findings have prompted the development and clinical evaluation of small-molecule inhibitors of Raf, Mek1/Mek2, and Erk1/Erk2 for the targeted therapy of cancer (Samatar and Poulikakos 2014). Recently, two inhibitors of Raf, vemurafenib and dabrafenib, and one inhibitor of Mek1/Mek2, trametinib, have been approved for the treatment of metastatic melanoma. Several inhibitors of p38 MAP kinase have also entered clinical trials to evaluate their efficacy in rheumatoid arthritis, asthma, and other diseases.
MAP kinases also play a causative role in congenital disorders. Indeed, germline mutations within components of the Erk1/Erk2 MAP kinase signaling pathway are associated with a clinically defined group of developmental syndromes characterized by facial dysmorphism, cardiac malformations, cutaneous and musculoskeletal abnormalities, and cognitive impairment that are collectively termed RASopathies (Rauen 2013). Also, individuals with microdeletions encompassing the Erk2 gene (MAPK1) in distal chromosome 22q11 exhibit a spectrum of craniofacial abnormalities, cardiac defects, and neurodevelopmental deficits (Ben-Shachar et al. 2008).
MAP kinases are components of evolutionarily conserved signaling modules that evolved to control a multitude of physiological responses required to maintain normal cellular and tissue homeostasis. Aberrant activity of MAP kinases has been observed in many human diseases, fostering the development of small-molecule inhibitors targeting these pathways. Considerable progress has been made over the last 25 years in identifying the core components of classical MAP kinase pathways and clarifying their mechanisms of regulation by extracellular stimuli. The physiological functions of classical MAP kinases have been extensively studied by the use of pharmacological inhibitors and genetic approaches. On the other hand, much less is known about the regulation and functions of atypical MAP kinases. Future challenges in the field include defining the individual contributions of MAP kinase isoforms using gene targeting and chemical genetic approaches, systematically characterizing the substrate profile of these enzymes by global proteomic strategies, further our understanding of the spatiotemporal regulation of MAP kinase pathways using mathematical modeling and synthetic biology, and exploiting the therapeutic potential of these enzymes.
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