Differential activation of extracellular signal-regulated kinase 1 and a related complex in neuronal nuclei
National Institute of Environmental Health Sciences, National Institutes of Health, MD F2-04, PO Box 12233, Research Triangle Park, NC 27709, USA. Brain Cell Biology
(Impact Factor: 3.25).
01/2007; 35(4-6):267-81. DOI: 10.1007/s11068-008-9018-7
The extracellular signal-regulated kinases 1 and 2 (ERKs 1/2) are known to participate in regulating transcription in response to moderate depolarization, such as synaptic stimulation, but how the same active enzyme can differentially regulate distinct transcriptional programs induced with abnormal depolarization (high potassium) is unknown. We hypothesized that ERK1 or 2 accomplishes this differential nuclear response through close association with other proteins in stable complexes. In support of this hypothesis, we have found that immunoreactivity for an apparent high molecular weight complex containing phospho-ERK1 increased in response to synaptic stimulation, but decreased in response to high potassium; p-ERK immunoreactivity at 44/42 kDa increased in both cases. Evidence supporting the conclusion that the band of interest contained ERK1 in a complex, as opposed to it being an unrelated protein crossreacting with antibodies against p-ERK, is that ERK1 (p44 MAPK) and 14-3-3 protein were electroeluted from the 160-kDa band cut from a gel. We also found the nuclear complexes to be exceptionally durable, suggesting a role for the crosslinking enzyme, transglutaminase, in its stabilization. In addition, we found other components of the ERK pathway, including MEK, ERK2, p90RSK, and Elk-1, migrating at higher-than-expected weights in brain nuclei. These results describe a novel stable complex of ERK1 in neuronal nuclei that responds differentially to synaptic and depolarizing stimulation, and thus may be capable of mediating gene transcription in a way distinct from the monomeric protein.
Available from: jbc.org
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ABSTRACT: The serine/threonine kinase B-Raf is the second most frequently occurring human oncogene after Ras. Mutations of B-Raf occur
with the highest incidences in melanoma, and the most common mutant, V600E, renders B-Raf constitutively active. The sodium
proton exchanger isoform 1 (NHE1) is a ubiquitously expressed plasma membrane protein responsible for regulating intracellular
pH, cell volume, cell migration, and proliferation. A screen of protein kinases that bind to NHE1 revealed that B-Raf bound
to the cytosolic regulatory tail of NHE1. Immunoprecipitation of NHE1 from HeLa and HEK cells confirmed the association of
B-Raf with NHE1 in vivo. The expressed and purified C-terminal 182 amino acids of the NHE1 protein were also shown to associate with B-Raf protein
in vitro. Because treatment with the kinase inhibitor sorafenib decreased NHE1 activity in HeLa and HEK cells, we examined the role
of B-Raf in regulating NHE1 in malignant melanoma cells. Melanoma cells with the B-RafV600E mutation demonstrated increased resting intracellular pH that was dependent on elevated NHE1 activity. NHE1 activity after
an acute acid load was also elevated in these cell lines. Moreover, inhibition of B-Raf activity by either sorafenib, PLX4720,
or siRNA reduction of B-Raf levels abolished ERK phosphorylation and decreased NHE1 activity. These results demonstrate that
B-Raf associates with and stimulates NHE1 activity and that B-RafV600E also increases NHE1 activity that raises intracellular pH.
Available from: Siiri E Iismaa
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ABSTRACT: Molecular deletion of transglutaminase 2 (TG2) has been shown to improve function and survival in a host of neurological conditions including stroke, Huntington's disease, and Parkinson's disease. However, unifying schemes by which these cross-linking or polyaminating enzymes participate broadly in neuronal death have yet to be presented. Unexpectedly, we found that in addition to TG2, TG1 gene expression level is significantly induced following stroke in vivo or due to oxidative stress in vitro. Forced expression of TG1 or TG2 proteins is sufficient to induce neuronal death in Rattus norvegicus cortical neurons in vitro. Accordingly, molecular deletion of TG2 alone is insufficient to protect Mus musculus neurons from oxidative death. By contrast, structurally diverse inhibitors used at concentrations that inhibit TG1 and TG2 simultaneously are neuroprotective. These small molecules inhibit increases in neuronal transamidating activity induced by oxidative stress; they also protect neurons downstream of pathological ERK activation when added well after the onset of the death stimulus. Together, these studies suggest that multiple TG isoforms, not only TG2, participate in oxidative stress-induced cell death signaling; and that isoform nonselective inhibitors of TG will be most efficacious in combating oxidative death in neurological disorders.
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ABSTRACT: Transglutaminases (TGs) are multifunctional, calcium-dependent enzymes that have been recently implicated in stroke pathophysiology. Classically, these enzymes are thought to participate in cell injury and death in chronic neurodegenerative conditions via their ability to catalyze covalent, nondegradable crosslinks between proteins or to incorporate polyamines into protein substrates. Accumulating lines of inquiry indicate that specific TG isoforms can shuttle into the nucleus when they sense pathologic changes in calcium or oxidative stress, bind to chromatin and thereby transduce these changes into transcriptional repression of genes involved in metabolic or oxidant adaptation. Here, we review the evidence that supports principally a role for one isoform of this family, TG2, in cell injury and death associated with hemorrhagic or ischemic stroke. We also outline an evolving model in which TG2 is a critical mediator between pathologic signaling and epigenetic modifications that lead to gene repression. Accordingly, the salutary effects of TG inhibitors in stroke may derive from their ability to restore homeostasis by removing inappropriate deactivation of adaptive genetic programs by oxidative stress or extrasynaptic glutamate receptor signaling.Journal of Cerebral Blood Flow & Metabolism advance online publication, 10 April 2013; doi:10.1038/jcbfm.2013.53.
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