PEA-15 Modulates TNFα Intracellular Signaling in Astrocytes

Targetting PED/PEA-15 for diabetes treatment

INTRODUCTION

PED/PEA-15 is an ubiquitously expressed protein, involved in the regulation of proliferation and apoptosis. It is commonly overexpressed in Type 2 Diabetes (T2D) and in different T2D-associated comorbidities, including cancer and certain neurodegenerative disorders. Areas covered: In mice, Ped/Pea-15 overexpression impairs glucose tolerance and, in combination with high fat diets, further promotes insulin resistance and T2D. It also controls β-cell mass, altering caspase-3 activation and the expression of pro- and antiapoptotic genes. These changes are mediated by PED/PEA-15-PLD1 binding. Overexpression of PLD1 D4 domain specifically blocks Ped/Pea-15-PLD1 interaction, reverting the effect of Ped/Pea-15 in vivo. D4α, a D4 N-terminal peptide, is able to displace Ped/Pea-15-PLD1 binding, but features greater stability in vivo compared to the entire D4 peptide. Here, we review early mechanistic studies on PED/PEA-15 relevance in apoptosis before focusing on its role in cancer and T2D. Finally, we describe potential therapeutic opportunities for T2D based on PED/PEA-15 targeting. Expert opinion: T2D is a major problem for public health and economy. Thus, the identification of new molecules with pharmacological activity for T2D represents an urgent need. Further studies with D4α will help to identify smaller pharmacologically active peptides and innovative molecules of potential pharmacological interest for T2D treatment.

Estradiol attenuates down-regulation of PEA-15 and its two phosphorylated forms in ischemic brain injury

Estradiol exerts a neuroprotective effect against focal cerebral ischemic injury through the inhibition of apoptotic signals. Phosphoprotein enriched in astrocytes 15 (PEA-15) is mainly expressed in brain that perform anti-apoptotic functions. This study investigated whether estradiol modulates the expression of PEA-15 and two phosphorylated forms of PEA-15 (Ser 104 and Ser 116) in middle cerebral artery occlusion (MCAO)-induced injury and glutamate exposure-induced neuronal cell death. Adult female rats were ovariectomized to remove endogenous estradiol and treated with vehicle or estradiol prior to MCAO. Focal cerebral ischemia was induced by MCAO and cerebral cortices were collected 24 h after MCAO. Western blot analysis indicated that estradiol prevents the MCAO-induced decrease in PEA-15, phospho-PEA-15 (Ser 104), phospho-PEA-15 (Ser 116). Glutamate exposure induced a reduction in PEA-15, phospho-PEA-15 (Ser 104), phospho-PEA-15 (Ser 116) in cultured neurons, whereas estradiol treatment attenuated the glutamate toxicity-induced decrease in the expression of these proteins. It has been known that phosphorylation of PEA-15 is an important step in carrying out its anti-apoptotic function. Thus, these findings suggest that the regulation of PEA-15 phosphorylation by estradiol contributes to the neuroprotective function of estradiol in ischemic brain injury.

Quantitative proteomics reveals that PEA15 regulates astroglial Aβ phagocytosis in an Alzheimer's disease mouse model

Amyloid-beta (Aβ) deposition plays a crucial role in the progression of Alzheimer's disease (AD). The Aβ deposited extracellularly can be phagocytosed and degraded by surrounding activated astrocytes, but the precise mechanisms underlying Aβ clearance mediated by astrocytes remain unclear. In this study, we performed tandem mass tag-based quantitative proteomic analysis on the cerebral cortices of 5-month-old APP/PS1 double-transgenic mice. Among the 2668 proteins quantified, 35 proteins were upregulated and 12 were downregulated, with most of these proteins being shown here for the first time to be differently expressed in the APP/PS1 mouse. The altered proteins were involved in molecular transport, lipid metabolism, autophagy, inflammation, and oxidative stress. One specific protein, PEA15 (phosphoprotein enriched in astrocytes 15 kDa) upregulated in APP/PS1 mice, was verified to play a critical role in astrocyte-mediated Aβ phagocytosis. Furthermore, PEA15 levels were determined to increase with age in APP/PS1 mice, indicating that Aβ stimulated the upregulation of PEA15 in the APP/PS1 mouse. These results highlight the function of PEA15 in astrocyte-mediated Aβ phagocytosis, and thus provide novel insight into the molecular mechanism underlying Aβ clearance. The protein-expression profile revealed here should offer new clues to understand the pathogenesis of AD and potential therapeutic targets for AD.

BIOLOGICAL SIGNIFICANCE

Activated astrocytes are known to clear the Aβ deposited in the extracellular milieu, which is why they play a key role in regulating the progression of Alzheimer's disease (AD). However, the molecular mechanism underlying astrocyte-mediated Aβ phagocytosis and degradation remains unclear. By performing tandem mass tag-based quantitative proteomic analysis, we identified 47 proteins that were differentially expressed in APP/PS1 double-transgenic. To our knowledge, this is the first time most of these proteins have been reported to exhibit altered expression in the mouse model of AD. Furthermore, our results indicate that one of the proteins upregulated in the APP/PS1 mouse, PEA15 (phosphoprotein enriched in astrocytes 15 kDa), regulates astroglial phagocytosis of Aβ. Our findings provide new insights into the molecular mechanism underlying Aβ clearance in AD. The altered profile of protein expression in APP/PS1 mice described here should offer valuable clues to understand the pathogenesis of AD and facilitate the identification of potential targets for the treatment of AD.

Phosphoprotein enriched in astrocytes (PEA)-15: a potential therapeutic target in multiple disease states

Phosphoprotein enriched in astrocytes-15 (PEA-15) is a cytoplasmic protein that sits at an important junction in intracellular signalling and can regulate diverse cellular processes, such as proliferation and apoptosis, dependent upon stimulation. Regulation of these processes occurs by virtue of the unique interaction of PEA-15 with other signalling proteins. PEA-15 acts as a cytoplasmic tether for the mitogen-activated protein kinases, extracellular signal-regulated kinase 1/2 (ERK1/2) preventing nuclear localisation. In order to release ERK1/2, PEA-15 requires to be phosphorylated via several potential pathways. PEA-15 (and its phosphorylation state) therefore regulates many ERK1/2-dependent processes, including proliferation, via regulating ERK1/2 nuclear translocation. In addition, PEA-15 contains a death effector domain (DED) which allows interaction with other DED-containing proteins. PEA-15 can bind the DED-containing apoptotic adaptor molecule, Fas-associated death domain protein (FADD) which is also dependent on the phosphorylation status of PEA-15. PEA-15 binding of FADD can inhibit apoptosis as bound FADD cannot participate in the assembly of apoptotic signalling complexes. Through these protein–protein interactions, PEA-15-regulated cellular effects have now been investigated in a number of disease-related studies. Changes in PEA-15 expression and regulation have been observed in diabetes mellitus, cancer, neurological disorders and the cardiovascular system. These changes have been suggested to contribute to the pathology related to each of these disease states. As such, new therapeutic targets based around PEA-15 and its associated interactions are now being uncovered and could provide novel avenues for treatment strategies in multiple diseases.

Brief alcohol exposure alters transcription in astrocytes via the heat shock pathway

Astrocytes are critical for maintaining homeostasis in the central nervous system (CNS), and also participate in the genomic response of the brain to drugs of abuse, including alcohol. In this study, we investigated ethanol regulation of gene expression in astrocytes. A microarray screen revealed that a brief exposure of cortical astrocytes to ethanol increased the expression of a large number of genes. Among the alcohol-responsive genes (ARGs) are glial-specific immune response genes, as well as genes involved in the regulation of transcription, cell proliferation, and differentiation, and genes of the cytoskeleton and extracellular matrix. Genes involved in metabolism were also upregulated by alcohol exposure, including genes associated with oxidoreductase activity, insulin-like growth factor signaling, acetyl-CoA, and lipid metabolism. Previous microarray studies performed on ethanol-treated hepatocyte cultures and mouse liver tissue revealed the induction of almost identical classes of genes to those identified in our microarray experiments, suggesting that alcohol induces similar signaling mechanisms in the brain and liver. We found that acute ethanol exposure activated heat shock factor 1 (HSF1) in astrocytes, as demonstrated by the translocation of this transcription factor to the nucleus and the induction of a family of known HSF1-dependent genes, the heat shock proteins (Hsps). Transfection of a constitutively transcriptionally active Hsf1 construct into astrocytes induced many of the ARGs identified in our microarray study supporting the hypothesis that HSF1 transcriptional activity, as part of the heat shock cascade, may mediate the ethanol induction of these genes. These data indicate that acute ethanol exposure alters gene expression in astrocytes, in part via the activation of HSF1 and the heat shock cascade.

An astrocyte-specific proteomic approach to inflammatory responses in experimental rat glaucoma

PURPOSE

To delineate astrocyte-mediated inflammatory processes in glaucoma, we analyzed proteomic responses of retinal astrocytes in an experimental rat model using a cell-specific approach.

METHODS

IOP elevation was induced in rats by hypertonic saline injections into episcleral veins. Enriched samples of astrocytes were isolated through the immunomagnetic cell selection process established originally for retinal ganglion cell (RGC) sampling. Ocular hypertensive and control samples were collected by pooling from rat eyes matched for the cumulative IOP exposure. Protein expression was analyzed complementarily by quantitative two-dimensional capillary liquid chromatography and linear ion trap mass spectrometry (LC-MS/MS) followed by quantitative Western blot analysis and retinal tissue immunolabeling using specific antibodies to selected proteins.

RESULTS

Following validation of enriched astrocyte samples, LC-MS/MS analysis resulted in the identification of over 2000 proteins with high confidence. Bioinformatic comparison analysis of the high-throughput MS/MS data along with the findings of immunoblotting and immunohistochemistry supported distinct responses of ocular hypertensive astrocytes during the experimental paradigm, which exhibited predominantly cellular activation and immune/inflammatory responses as opposed to activation of cell death signaling in ocular hypertensive RGCs. Inflammatory responses of astrocytes in experimental glaucoma included up-regulation of a number of immune mediators/regulators linked to TNF-α/TNFR signaling, nuclear factor kappa-B (NF-κB) activation, autophagy regulation, and inflammasome assembly.

CONCLUSIONS

These findings validate an astrocyte-specific approach to quantitatively identify proteomic alterations in experimental glaucoma, and highlight many immune mediators/regulators characteristic of the inflammatory responses of ocular hypertensive astrocytes. By dissecting the complexity of prior data obtained from whole tissue, this pioneering approach should enable astrocyte responses to be defined and new treatments targeting astrocytes to be developed.

Gingko biloba Extract (EGb 761) Attenuates the Focal Cerebral Ischemic Injury-Induced Decrease in Astrocytic Phosphoprotein PEA-15 Levels

EGb 761 is an extract of Gingko biloba that is neuroprotective against focal cerebral ischemic injury. PEA-15 (phosphoprotein enriched in astrocytes 15) modulates cell proliferation and apoptosis. In this study, we investigated whether EGb 761 regulates the expression of PEA-15 and two phosphorylated forms of PEA-15 (Ser 104 and Ser 116) in middle cerebral artery occlusion (MCAO)-induced injury. Adult male rats were treated with vehicle or EGb 761 (100 mg/kg) prior to MCAO and cerebral cortices were collected 24 h after MCAO. A reduction in expression of PEA-15 and its phosphorylated forms induced by MCAO injury was detected using a proteomic approach. EGb 761 pretreatment prevented the ischemic injury-induced decrease in PEA-15 expression. Western blot analysis demonstrated that EGb 761 attenuates the injury-induced reduction in PEA-15, phospho-PEA-15 (Ser 104), phospho-PEA-15 (Ser 116). Phosphorylation of PEA-15 influences its anti-apoptotic function; a decrease in PEA-15 phosphorylation induces apoptotic cell death. The maintenance of PEA-15 phosphorylation by EGb 761 pretreatment during cerebral ischemic injury indicates that EGb 761 is a neuroprotective against cerebral ischemic injury.

Ferulic acid prevents the cerebral ischemic injury-induced decreases of astrocytic phosphoprotein PEA-15 and its two phosphorylated forms

Ferulic acid protects neuronal cells against focal cerebral ischemic injury through its anti-oxidative and anti-inflammatory effects. Phosphoprotein enriched in astrocytes 15 (PEA-15) is known to modulate various cellular processes including cell proliferation, apoptosis, and survival. This study was investigated whether ferulic acid can regulate the levels of PEA-15 and its two phosphorylated forms (Ser 104 and Ser 116) in a cerebral ischemic injury model and in neuronal cells exposed to glutamate. A middle cerebral artery occlusion (MCAO) was performed to induce focal cerebral ischemic injury. Adult male rats were immediately treated with vehicle or ferulic acid (100 mg/kg) at the beginning of the MCAO, and then cerebral cortices were collected 24h after MCAO. The decrease in PEA-15 level after ischemic injury was detected using a proteomic approach. Ferulic acid administration prevented the ischemic injury-induced decrease of PEA-15 level. Moreover, Western blot analysis clearly confirmed that ferulic acid attenuates the ischemic injury-induced decreases in PEA-15, phospho-PEA-15 (Ser 104), and phospho-PEA-15 (Ser 116) levels. Glutamate exposure induced significant reductions in the levels of PEA-15 and the two phospho-PEA-15 (Ser 104 and Ser 116) in cultured hippocampal neuron, while pretreatment with ferulic acid prevented the glutamate toxicity-induced decreases in these proteins levels. The decrease of phospho-PEA-15 protein level indicates that the anti-apoptotic function of PEA-15 was being inhibited. Thus, these results suggest that ferulic acid protects neuronal cells against ischemic injury by maintenance of phospho-PEA-15 protein levels.

Melatonin prevents down-regulation of astrocytic phosphoprotein PEA-15 in ischemic brain injury

Melatonin functions as a free-radical scavenger and has a neuroprotective effect against ischemic brain damage. PEA-15 (phosphoprotein enriched in astrocytes 15) regulates various cellular processes including cell proliferation and apoptosis. In this study, we investigated whether melatonin regulates the levels of PEA-15 and the two phosphorylated forms of PEA-15 (Ser 104 and Ser 116) in a middle cerebral artery occlusion (MCAO)-induced injury model and neuronal cells exposed to glutamate. Adult male rats were treated with vehicle or melatonin (5 mg/kg) prior to MCAO, and cerebral cortex tissues were collected 24 h after MCAO. PEA-15 levels after ischemic brain injury were monitored using a proteomic approach. Melatonin pretreatment prevented the ischemic injury-induced reduction in PEA-15 levels. Moreover, Western blot analysis demonstrated that melatonin attenuated the ischemic injury-induced reduction in PEA-15, phospho-PEA-15 (Ser 104), and phospho-PEA-15 (Ser 116) levels. Neuronal cells exposed to glutamate showed decreased expression of PEA-15, phospho-PEA-15 (Ser 104), and phospho-PEA-15 (Ser 116), while melatonin pretreatment prevented the glutamate toxicity-induced decreases in the levels of these proteins. The reduction in the levels of phospho-PEA-15 proteins indicates the inhibition of anti-apoptotic function of PEA-15. Together, in vivo and in vitro results suggest that melatonin protects neurons against ischemic injury by maintaining levels of phospho-PEA-15 proteins.

Species-specific microRNA roles elucidated following astrocyte activation

MicroRNAs (miRNAs) are short non-coding RNAs that play a central role in regulation of gene expression by binding to target genes. Many miRNAs were associated with the function of the central nervous system (CNS) in health and disease. Astrocytes are the CNS most abundant glia cells, providing support by maintaining homeostasis and by regulating neuronal signaling, survival and synaptic plasticity. Astrocytes play a key role in repair of brain insults, as part of local immune reactivity triggered by inflammatory or pathological conditions. Thus, astrocyte activation, or astrogliosis, is an important outcome of the innate immune response, which can be elicited by endotoxins such as lipopolysaccharide (LPS) and cytokines such as interferon-gamma (IFN-γ). The involvement of miRNAs in inflammation and stress led us to hypothesize that astrogliosis is mediated by miRNA function. In this study, we compared the miRNA regulatory layer expressed in primary cultured astrocyte derived from rodents (mice) and primates (marmosets) brains upon exposure to LPS and IFN-γ. We identified subsets of differentially expressed miRNAs some of which are shared with other immunological related systems while others, surprisingly, are mouse and rat specific. Of interest, these specific miRNAs regulate genes involved in the tumor necrosis factor-alpha (TNF-α) signaling pathway, indicating a miRNA-based species-specific regulation. Our data suggests that miRNA function is more significant in the mechanisms governing astrocyte activation in rodents compared to primates.

The PEA-15 protein regulates autophagy via activation of JNK

PEA-15/PED (phosphoprotein enriched in astrocytes 15 kDa/phosphoprotein enriched in diabetes) is a death effector domain-containing protein which is known to modulate apoptotic cell death. The mechanism by which PEA-15 inhibits caspase activation and increases ERK (extracellular-regulated kinase) activity is well characterized. Here, we demonstrate that PEA-15 is not only pivotal in the activation of the ERK pathway but also modulates JNK (c-Jun N-terminal kinase) signaling. Upon overexpression of PEA-15 in malignant glioma cells, JNK is potently activated. The PEA-15-induced JNK activation depends on the phosphorylation of PEA-15 at both phosphorylation sites (serine 104 and serine 116). The activation of JNK is substantially inhibited by siRNA-mediated down-regulation of endogenous PEA-15. Moreover, we demonstrate that glioma cells overexpressing PEA-15 show increased signs of autophagy in response to classical autophagic stimuli such as ionizing irradiation, serum deprivation, or rapamycin treatment. In contrast, the non-phosphorylatable mutants of PEA-15 are not capable of promoting autophagy. The inhibition of JNK abrogates the PEA-15-mediated increase in autophagy. In conclusion, our data show that PEA-15 promotes autophagy in glioma cells in a JNK-dependent manner. This might render glioma cells more resistant to adverse stimuli such as starvation or ionizing irradiation.

Identification of proteins regulated by estradiol in focal cerebral ischemic injury--a proteomics approach

Estradiol protects neuronal cells against permanent and focal ischemic brain damage. We identified the proteins that are expressed following estradiol administration during cerebral ischemia in an animal model. Adult female rats were ovariectomized and treated with oil or estradiol prior to middle cerebral artery occlusion (MCAO) to induce cerebral ischemia, and brains were collected 24h after MCAO. Protein analysis was performed on the cerebral cortex using two-dimensional gel electrophoresis. Protein spots with difference in intensity between oil- and estradiol-treated groups were identified by mass spectrometry. Among these proteins, levels of protein phosphatase 2A (PP2A) and astrocytic phosphoprotein PEA-15 were significantly decreased in the oil-treated group in comparison to the estradiol-treated group. Moreover, Western blot analysis demonstrated that estradiol treatment prevents injury-induced decrease of PP2A and PEA-15 levels during both MCAO-induced injury and glutamate exposure in HT22 cells. In contrast, levels of the 60kDa heat shock protein (Hsp 60) were significantly increased in oil-treated animals, while estradiol prevented the injury-induced increase of Hsp 60. The results of this study provide an evidence that estradiol protects neuronal cells against ischemic brain injury through the up- and down-modulation of specific proteins.

In-depth exploration of cerebrospinal fluid by combining peptide ligand library treatment and label-free protein quantification

Cerebrospinal fluid (CSF) is the biological fluid in closest contact with the brain and thus contains proteins of neural cell origin. Hence, CSF is a biochemical window into the brain and is particularly attractive for the search for biomarkers of neurological diseases. However, as in the case of other biological fluids, one of the main analytical challenges in proteomic characterization of the CSF is the very wide concentration range of proteins, largely exceeding the dynamic range of current analytical approaches. Here, we used the combinatorial peptide ligand library technology (ProteoMiner) to reduce the dynamic range of protein concentration in CSF and unmask previously undetected proteins by nano-LC-MS/MS analysis on an LTQ-Orbitrap mass spectrometer. This method was first applied on a large pool of CSF from different sources with the aim to better characterize the protein content of this fluid, especially for the low abundance components. We were able to identify 1212 proteins in CSF, and among these, 745 were only detected after peptide library treatment. However, additional difficulties for clinical studies of CSF are the low protein concentration of this fluid and the low volumes typically obtained after lumbar puncture, precluding the conventional use of ProteoMiner with large volume columns for treatment of patient samples. The method has thus been optimized to be compatible with low volume samples. We could show that the treatment is still efficient with this miniaturized protocol and that the dynamic range of protein concentration is actually reduced even with small amounts of beads, leading to an increase of more than 100% of the number of identified proteins in one LC-MS/MS run. Moreover, using a dedicated bioinformatics analytical work flow, we found that the method is reproducible and applicable for label-free quantification of series of samples processed in parallel.

The genomic response of the ipsilateral and contralateral cortex to stroke in aged rats

Aged rats recover poorly after unilateral stroke, whereas young rats recover readily possibly with the help from the contralateral, healthy hemisphere. In this study we asked whether anomalous, age-related changes in the transcriptional activity in the brains of aged rats could be one underlying factor contributing to reduced functional recovery. We analysed gene expression in the periinfarct and contralateral areas of 3-month- and 18-month-old Sprague Dawley rats. Our experimental end-points were cDNA arrays containing genes related to hypoxia signalling, DNA damage and apoptosis, cellular response to injury, axonal damage and re-growth, cell lineage differentiation, dendritogenesis and neurogenesis. The major transcriptional events observed were: (i) Early up-regulation of DNA damage and down-regulation of anti-apoptosis-related genes in the periinfarct region of aged rats after stroke; (ii) Impaired neurogenesis in the periinfarct area, especially in aged rats; (iii) Impaired neurogenesis in the contralateral (unlesioned) hemisphere of both young and aged rats at all times after stroke and (iv) Marked up-regulation, in aged rats, of genes associated with inflammation and scar formation. These results were confirmed with quantitative real-time PCR. We conclude that reduced transcriptional activity in the healthy, contralateral hemisphere of aged rats in conjunction with an early up-regulation of DNA damage-related genes and pro-apoptotic genes and down-regulation of axono- and neurogenesis in the periinfarct area are likely to account for poor neurorehabilitation after stroke in old rats.

Altered gene expression profiles in the frontal cortex of cirrhotic alcoholics

BACKGROUND

Cirrhosis is the result of chronic liver disease that causes scarring and dysfunction of the liver. The disease is a common concomitant condition resulting from sustained exposure to alcohol. Heavy alcohol use results in brain damage that is generally more severe in cirrhotic compared with noncirrhotic alcoholics. We examined, at the cellular level, gene expression in the frontal cortex of cirrhotic alcoholics.

METHODS

Gene expression profiles were compared between cirrhotic and noncirrhotic alcoholics using approximately 47,000 element cDNA microarrays.

RESULTS

Widespread differences in transcriptome patterns were observed in cirrhotic compared with noncirrhotic alcoholics and these differences in gene expression accurately distinguished cirrhotic from noncirrhotic alcoholics. Functionally related groups of genes were identified that are involved in cell adhesion, mitochondrial function, synaptic transmission, apoptosis, and cell proliferation. Both astrocytes and neuronal cells were affected at the transcriptional level. The regulated genes are involved in neurite growth, neuronal cell adhesion, synaptic vesicle release, and postsynaptic neurotransmission.

CONCLUSIONS

These changes in the transcriptome likely contribute to the more severe brain dysfunction in cirrhotic alcoholics.

PEA-15 inhibits tumor cell invasion by binding to extracellular signal-regulated kinase 1/2

Phosphoprotein enriched in astrocytes of 15 kDa (PEA-15) binds to extracellular signal-regulated kinase 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinases to alter ERK1/2 cellular localization and target preferences and binds to adaptors in the extrinsic cell death pathway to block apoptosis. Here, we report that PEA-15 protein expression is inversely correlated with the invasive behavior of breast cancer in an immunohistochemical analysis of a breast cancer progression tissue microarray. Short hairpin RNA-mediated inhibition of PEA-15 expression increased the invasion of PEA-15-expressing tumor cells in vitro, suggesting a causative role for PEA-15 in the inhibition of invasion. This causative role was confirmed by the finding that the enforced expression of PEA-15 in invasive tumor cells reduced invasion. The effect of PEA-15 on tumor invasion is mediated by its interaction with ERK1/2 as shown by the following: (a) PEA-15 mutants that fail to bind ERK1/2 did not inhibit invasion; (b) overexpression of ERK1 or activated MAP/ERK kinase (MEK) reversed the inhibitory effect of PEA-15; (c) when an inhibitor of ERK1/2 activation reduced invasion, PEA-15 expression did not significantly reduce invasion further. Furthermore, we find that the effect of PEA-15 on invasion seems to relate to the nuclear localization of activated ERK1/2. PEA-15 inhibits invasion by keeping ERK out of the nucleus, as a PEA-15 mutant that cannot prevent ERK nuclear localization was not able to inhibit invasion. In addition, membrane-localized ERK1, which sequesters endogenous ERK1 to prevent its nuclear localization, also inhibited invasion. These results reveal that PEA-15 regulates cancer cell invasion via its ability to bind ERK1/2 and indicate that nuclear entry of ERK1/2 is important in tumor behavior.

Phorbol esters induce intracellular accumulation of the anti-apoptotic protein PED/PEA-15 by preventing ubiquitinylation and proteasomal degradation

Phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes (PED/PEA)-15 is an anti-apoptotic protein whose expression is increased in several cancer cells and following experimental skin carcinogenesis. Exposure of untransfected C5N keratinocytes and transfected HEK293 cells to phorbol esters (12-O-tetradecanoylphorbol-13-acetate (TPA)) increased PED/PEA-15 cellular content and enhanced its phosphorylation at serine 116 in a time-dependent fashion. Ser-116 --> Gly (PED(S116G)) but not Ser-104 --> Gly (PED(S104G)) substitution almost completely abolished TPA regulation of PED/PEA-15 expression. TPA effect was also prevented by antisense inhibition of protein kinase C (PKC)-zeta and by the expression of a dominant-negative PKC-zeta mutant cDNA in HEK293 cells. Similar to long term TPA treatment, overexpression of wild-type PKC-zeta increased cellular content and phosphorylation of WT-PED/PEA-15 and PED(S104G) but not of PED(S116G). These events were accompanied by the activation of Ca2+-calmodulin kinase (CaMK) II and prevented by the CaMK blocker, KN-93. At variance, the proteasome inhibitor lactacystin mimicked TPA action on PED/PEA-15 intracellular accumulation and reverted the effects of PKC-zeta and CaMK inhibition. Moreover, we show that PED/PEA-15 bound ubiquitin in intact cells. PED/PEA-15 ubiquitinylation was reduced by TPA and PKC-zeta overexpression and increased by KN-93 and PKC-zeta block. Furthermore, in HEK293 cells expressing PED(S116G), TPA failed to prevent ubiquitin-dependent degradation of the protein. Accordingly, in the same cells, TPA-mediated protection from apoptosis was blunted. Taken together, our results indicate that TPA increases PED/PEA-15 expression at the post-translational level by inducing phosphorylation at serine 116 and preventing ubiquitinylation and proteosomal degradation.

Coordinating ERK/MAPK signalling through scaffolds and inhibitors

The pathway from Ras through Raf and MEK (MAPK and ERK kinase) to ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase) regulates many fundamental cellular processes. Recently, a number of scaffolding proteins and endogenous inhibitors have been identified, and their important roles in regulating signalling through this pathway are now emerging. Some scaffolds augment the signal flux, but also mediate crosstalk with other pathways; certain adaptors target MEK-ERK/MAPK complexes to subcellular localizations; others provide regulated inhibition. Computational modelling indicates that, together, these modulators can determine the dynamic biological behaviour of the pathway.

Ischemia-induced programmed cell death in astrocytes

Astrocytes are essential for neuronal survival and function, neurogenesis, and neural repair. Although astrocytes are more resistant than neurons to most stress conditions in vitro, certain astrocyte subtypes, such as the glial fibrillary acidic protein (GFAP)-negative protoplasmic astrocytes that predominate in gray matter structures, may be equally or more sensitive than neurons to ischemia in vivo. Programmed cell death differs from passive, necrotic death in that cell constituents actively participate in cell demise. Like neurons, astrocytes undergo programmed cell death during normal development. Cell culture studies have shown that astrocytes can be induced to undergo apoptosis and other forms of programmed cell death by many factors relevant to ischemia, including acidosis, oxidative stress, substrate deprivation, and cytokines. Animal models of cerebral ischemia have confirmed nuclear condensation and upregulation of Bax and caspases in a subset of astrocytes exposed to ischemia, especially in immature brain. A causal role for these events in astrocyte death is supported by improved astrocyte survival after inhibition of caspase-dependent cell death pathways. Astrocyte survival is also improved by blocking the poly(ADP-ribose)-1 cell death pathway. Markers of programmed cell death are generally less evident and less widespread in astrocytes than in neighboring neurons. However, most studies to date have relied only on markers of classical apoptosis. In addition, these studies have relied almost exclusively on GFAP to identify astrocytes. Since most protoplasmic astrocytes are poorly immunoreactive for GFAP, the extent of ischemia-induced programmed cell death in this cell type remains uncertain.

Phosphorylation of phosphoprotein enriched in astrocytes (PEA-15) regulates extracellular signal-regulated kinase-dependent transcription and cell proliferation

Cell cycle progression is dependent on the nuclear localization and transcriptional effects of activated extracellular signal-regulated kinase (ERK)1 and ERK2 mitogen-activated protein (MAP) kinases (ERK1/2). Phosphoprotein enriched in astrocytes (PEA-15) binds ERK1/2 and inhibits their nuclear localization, thus blocking cell proliferation. Here, we report that phosphorylation of PEA-15 blocks its interaction with ERK1/2 in vitro and in vivo and that phosphorylation of both Ser104 and Ser116 is required for this effect. Using phosphomimetic and nonphosphorylatable mutants of PEA-15, we found that PEA-15 phosphorylation abrogates its capacity to block the nuclear localization and transcriptional activities of ERK1/2; this phosphorylation therefore enables the proliferation of cells that express high levels of PEA-15. Additionally, we report that PEA-15 phosphorylation can modulate nontranscriptional activities of ERK1/2, such as the modulation of the affinity of integrin adhesion receptors. Finally, we used a novel anti-phospho-specific PEA-15 antibody to establish that PEA-15 is phosphorylated in situ in normal mammary epithelium. These results define a novel posttranslational mechanism for controlling the subcellular localization of ERK1/2 and for specifying the output of MAP kinase signaling.