Mitogen-activated protein kinases (MAPKs) are important signal transducing enzymes, unique to eukaryotes, that are involved in many facets of cellular regulation. Initial research concentrated on defining the components and organization of MAPK signalling cascades, but recent studies have begun to shed light on the physiological functions of these cascades in the control of gene expression, cell proliferation and programmed cell death.

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Mammalian MAP kinase signalling cascades

Specificity of receptor tyrosine kinase signaling: Transient versus sustained extracellular signal-regulated kinase activation

Signal transduction by the JNK group of MAP kinases.

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

/pdf/mitogen-activated-protein-map-kinase-pathways-regulation-and-3nsauvepr2.pdf

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions.

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.

/pdf/signaling-to-nf-kappab-1zyb7vgt7s.pdf

Signaling to NF-kappaB.

A 5-week-old male infant presented with severe bacterial infections and poor wound healing, suggesting a neutrophil defect. Neutrophils from this patient exhibited decreased chemotaxis, polarization, azurophilic granule secretion, and superoxide anion (O2−) production but had normal expression and up-regulation of CD11b. Rac2, which constitutes >96% of the Rac in neutrophils, is a member of the Rho family of GTPases that regulates the actin cytoskeleton and O2− production. Western blot analysis of lysates from patient neutrophils demonstrated decreased levels of Rac2 protein. Addition of recombinant Rac to extracts of the patient neutrophils reconstituted O2− production in an in vitro assay system. Molecular analysis identified a point mutation in one allele of the Rac2 gene resulting in the substitution of Asp57 by an Asn (Rac2D57N). Asp57 is invariant in all defined GTP-binding proteins. Rac2D57N binds GDP but not GTP and inhibits oxidase activation and O2− production in vitro. These data represent the description of an inhibitory mutation in a member of the Rho family of GTPases associated with a human immunodeficiency syndrome.

https://www.pnas.org/content/pnas/97/9/4654.full.pdf

Human neutrophil immunodeficiency syndrome is associated with an inhibitory Rac2 mutation

MEKKs, GCKs, MLKs, PAKs, TAKs, and tpls: upstream regulators of the c-Jun amino-terminal kinases?

Sequential protein kinase reactions controlling cell growth and differentiation

Targeted disruption of the gene encoding MEK kinase 1 (MEKK1), a mitogen-activated protein kinase (MAPK) kinase kinase, defined its function in the regulation of MAPK pathways and cell survival. MEKK1−/− embryonic stem cells from mice had lost or altered responses of the c-Jun amino-terminal kinase (JNK) to microtubule disruption and cold stress but activated JNK normally in response to heat shock, anisomycin, and ultraviolet irradiation. Activation of JNK was lost and that of extracellular signal-regulated protein kinase (ERK) was diminished in response to hyperosmolarity and serum factors in MEKK1−/− cells. Loss of MEKK1 expression resulted in a greater apoptotic response of cells to hyperosmolarity and microtubule disruption. When activated by specific stresses that alter cell shape and the cytoskeleton, MEKK1 signals to protect cells from apoptosis.

https://science.sciencemag.org/content/sci/282/5395/1911.full.pdf

Role of MEKK1 in cell survival and activation of JNK and ERK pathways defined by targeted gene disruption.

Cyclic AMP (cAMP)-dependent protein kinase A (PKA) stimulates the transcription of many eucaryotic genes by catalyzing the phosphorylation of the cAMP-regulatory element binding protein (CREB). Conversely, the attenuation or inhibition of cAMP-stimulated gene transcription would require the dephosphorylation of CREB by a nuclear protein phosphatase. In HepG2 cells treated with the protein serine/threonine (Ser/Thr) phosphatase inhibitor okadaic acid, dibutyryl-cAMP-stimulated transcription from the phosphoenolpyruvate carboxykinase (PEPCK) promoter was enhanced over the level of PEPCK gene transcription observed in cells treated with dibutyryl-cAMP alone. This process was mediated, at least in part, by a region of the PEPCK promoter that binds CREB. Likewise, okadaic acid prevents the dephosphorylation of PKA-phosphorylated CREB in rat liver nuclear extracts and enhances the ability of PKA to stimulate transcription from the PEPCK promoter in cell-free reactions. The ability of okadaic acid to enhance PKA-stimulated transcription in vitro was entirely dependent on the presence of CREB in the reactions. The phospho-CREB (P-CREB) phosphatase activity present in nuclear extracts coelutes with protein Ser/Thr phosphatase type 2A (PP2A) on Mono Q, amino-hexyl Sepharose, and heparin agarose columns and was chromatographically resolved from nuclear protein Ser/Thr-phosphatase type 1 (PP1). Furthermore, P-CREB phosphatase activity in nuclear extracts was unaffected by the heat-stable protein inhibitor-2, which is a potent and selective inhibitor of PP1. Nuclear PP2A dephosphorylated P-CREB 30-fold more efficiently than did nuclear PP1. Finally, when PKA-phosphorylated CREB was treated with immunopurified PP2A and PP1, the PP2A-treated CREB did not stimulate transcription from the PEPCK promoter in vitro, whereas the PP1-treated CREB retained the ability to stimulate transcription. Nuclear PP2A appears to be the primary phosphatase that dephosphorylates PKA-phosphorylated CREB.

Gary L. Johnson

Papers

Human neutrophil immunodeficiency syndrome is associated with an inhibitory Rac2 mutation

MEKKs, GCKs, MLKs, PAKs, TAKs, and tpls: upstream regulators of the c-Jun amino-terminal kinases?

Sequential protein kinase reactions controlling cell growth and differentiation

Role of MEKK1 in cell survival and activation of JNK and ERK pathways defined by targeted gene disruption.

Nuclear protein phosphatase 2A dephosphorylates protein kinase A-phosphorylated CREB and regulates CREB transcriptional stimulation.