Alternative, nonapoptotic programmed cell death: mediation by arrestin 2, ERK2, and Nur77

The nuclear receptor subfamily 4 group A1 in human disease

Nuclear receptor 4A1 (NR4A1), a member of the NR4A subfamily, acts as a gene regulator in a wide range of signaling pathways and responses to human diseases. Here, we provide a brief overview of the current functions of NR4A1 in human diseases and the factors involved in its function. A deeper understanding of these mechanisms can potentially improve drug development and disease therapy.

Malayoside, a cardenolide glycoside extracted from Antiaris toxicaria Lesch, induces apoptosis in human non-small lung cancer cells via MAPK-Nur77 signaling pathway

Lung cancer is the leading cause of cancer deaths in the world. Non-small cell lung cancer (NSCLC), with poor prognosis and resistance to chemoradiotherapy, is the most common histological type of lung cancer. Therefore, it is necessary to develop new and more effective treatment strategy for NSCLC. Nur77, an orphan member of the nuclear receptor superfamily, induces apoptosis in cancer cells including NSCLC cells, by high expression and translocation to mitochondria. Small molecules trigger expression and mitochondrial localization of Nur77 may be an ideal anti-cancer drug candidate. Here, we report malayoside, a cardiac glycoside in the extract of Antiaris toxicaria Lesch., had different sensitivities to NSCLC cells. Malayoside induced apoptosis in NCI-H460 cells. Meanwhile, malayoside induced Nur77 expression and mitochondrial localization, and its induction of apoptosis was Nur77-dependent. To investigate the molecular mechanism of malayoside inducing Nur77 and apoptosis, we found that malayoside activated MAPK signaling pathway, including both ERK and p38 phosphorylation. The suppression of MAPK signaling activation inhibited the expression of Nur77 and apoptosis induced by malayoside. Our studies in nude mice showed that malayside potently inhibited the growth of tumor cells in vivo. Furthermore, the anti-cancer effect of malayosidwas in vivo was also related to the elevated expression of Nur77, p-ERK, and p-p38 proteins. Our results suggest that malayoside possesses an anti-NSCLC activity in vitro and in vivo mainly via activation of MAPK-Nur77 signaling pathway, indicating that malayoside is a promising chemotherapeutic candidate for NSCLC.

Maduramicin triggers methuosis-like cell death in primary chicken myocardial cells

Maduramicin frequently induces severe cardiotoxicity in broiler chickens as well as in humans who consume maduramicin accidentally. Apoptosis and non-apoptotic cell death occur concurrently in the process of maduramicin-induced cardiotoxicity; however, the underlying mechanism of non-apoptotic cell death is largely unknown. Here, we report the relationship between maduramicin-caused cytoplasmic vacuolization and methuosis-like cell death as well as the underlying mechanism in primary chicken myocardial cells. Maduramicin induced a significant increase of cytoplasmic vacuoles with a degree of cell specificity in primary chicken embryo fibroblasts and chicken hepatoma cells (LMH), along with a decrease of ATP and an increase of LDH. The accumulated vacuoles were partly derived from cellular endocytosis rather than the swelling of endoplasm reticulum, lysosomes, and mitochondria. Moreover, the broad-spectrum caspase inhibitor carbobenzoxy-Val-Ala-Asp-fluoromethylketone (z-VAD-fmk) did not prevent maduramicin-induced cytoplasmic vacuolization. DNA ladder and cleavage of PARP were not observed in chicken myocardial cells during maduramicin exposure. Pretreatment with 3-methyladenine (3-MA) and cholorquine (CQ) of chicken myocardial cells did not attenuate cytoplasmic vacuolization and cytotoxicity, although LC3 and p62 were activated. Bafilomycin A1 almost completely prevented the generation of cytoplasmic vacuoles and significantly attenuated cytotoxicity induced by maduramicin, along with downregulation of K-Ras and upregulation of Rac1. Taken together, "methuosis" due to excessive cytoplasmic vacuolization mediates the cardiotoxicity of maduramicin. This provides new insights for understanding a nonclassical form of cell death in the field of drug-induced cytotoxicity.

The nuclear receptor NR4A1 is regulated by SUMO modification to induce autophagic cell death

NR4A is a nuclear receptor protein family whose members act as sensors of cellular environment and regulate multiple processes such as metabolism, proliferation, migration, apoptosis, and autophagy. Since the ligand binding domains of these receptors have no cavity for ligand interaction, their function is most likely regulated by protein abundance and post-translational modifications. In particular, NR4A1 is regulated by protein abundance, phosphorylation, and subcellular distribution (nuclear-cytoplasmic translocation), and acts both as a transcription factor and as a regulator of other interacting proteins. SUMOylation is a post-translational modification that can affect protein stability, transcriptional activity, alter protein-protein interactions and modify intracellular localization of target proteins. In the present study we evaluated the role of SUMOylation as a posttranslational modification that can regulate the activity of NR4A1 to induce autophagy-dependent cell death. We focused on a model potentially relevant for neuronal cell death and demonstrated that NR4A1 needs to be SUMOylated to induce autophagic cell death. We observed that a triple mutant in SUMOylation sites has reduced SUMOylation, increased transcriptional activity, altered intracellular distribution, and more importantly, its ability to induce autophagic cell death is impaired.

Dysregulation of Macropinocytosis Processes in Glioblastomas May Be Exploited to Increase Intracellular Anti-Cancer Drug Levels: The Example of Temozolomide

Macropinocytosis is a clathrin-independent endocytosis of extracellular fluid that may contribute to cancer aggressiveness through nutrient supply, recycling of plasma membrane and receptors, and exosome internalization. Macropinocytosis may be notably triggered by epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR), two well-known markers for glioblastoma aggressiveness. Therefore, we studied whether the expression of key actors of macropinocytosis is modified in human glioma datasets. Strong deregulation has been evidenced at the mRNA level according to the grade of the tumor, and 38 macropinocytosis-related gene signatures allowed discrimination of the glioblastoma (GBM) samples. Honokiol-induced vacuolization was then compared to vacquinol-1 and MOMIPP, two known macropinocytosis inducers. Despite high phase-contrast morphological similarities, honokiol-induced vacuoles appeared to originate from both endocytosis and ER. Also, acridine orange staining suggested differences in the macropinosomes’ fate: their fusion with lysosomes appeared very limited in 3-(5-methoxy -2-methyl-1H-indol-3-yl)-1-(4-pyridinyl)-2-propen-1-one (MOMIPP)-treated cells. Nevertheless, each of the compounds markedly increased temozolomide uptake by glioma cells, as evidenced by LC-MS. In conclusion, the observed deregulation of macropinocytosis in GBM makes them prone to respond to various compounds affecting their formation and/or intracellular fate. Considering that sustained macropinocytosis may also trigger cell death of both sensitive and resistant GBM cells, we propose to envisage macropinocytosis inducers in combination approaches to obtain dual benefits: increased drug uptake and additive/synergistic effects.

The role of PAK1 in the sensitivity of kidney epithelial cells to ischemia-like conditions

Kidney ischemia, characterized by insufficient supply of oxygen and nutrients to renal epithelial cells, is the main cause of acute kidney injury and an important contributor to mortality world-wide. Earlier research implicated a G-protein coupled receptor (NK1R) in the death of kidney epithelial cells in ischemia-like conditions. P21-associated kinase 1 (PAK1) is involved in signalling by several G-proteins. We explored the consequences of PAK1 inhibition for cell survival under the conditions of reduced glucose and oxygen. Inhibition of PAK1 by RNA interference, expression of a dominant-negative mutant or treatment with small molecule inhibitors greatly reduced the death of cultured kidney epithelial cells. Similar protection was achieved by treating the cells with inhibitors of MEK1, in agreement with the prior reports on PAK1-MEK1 connection. Concomitant inhibition of NK1R and PAK1 offered no better protection than inhibition of NK1R alone, consistent with the two proteins being members of the same pathway. Furthermore, NK1R, PAK and MEK inhibitors reduced the induction of TRAIL in ischemia-like conditions. Considering the emerging role of TRAIL in ischemia-mediated cell death, this phenomenon may contribute to the protective effects of these small molecules. Our findings support further exploration of PAK and MEK inhibitors as possible agents to avert ischemic kidney injury.

Cytoplasmic vacuolization in cell death and survival

Cytoplasmic vacuolization (also called cytoplasmic vacuolation) is a well-known morphological phenomenon observed in mammalian cells after exposure to bacterial or viral pathogens as well as to various natural and artificial low-molecular-weight compounds. Vacuolization often accompanies cell death; however, its role in cell death processes remains unclear. This can be attributed to studying vacuolization at the level of morphology for many years. At the same time, new data on the molecular mechanisms of the vacuole formation and structure have become available. In addition, numerous examples of the association between vacuolization and previously unknown cell death types have been reported. Here, we review these data to make a deeper insight into the role of cytoplasmic vacuolization in cell death and survival.

Substance P Receptor Signaling Mediates Doxorubicin-Induced Cardiomyocyte Apoptosis and Triple-Negative Breast Cancer Chemoresistance

Doxorubicin (DOX), an anthracycline, is broadly considered the most active single agent available for treating breast cancer but has been known to induce cardiotoxicity. Although DOX is highly effective in treating triple-negative breast cancer (TNBC), DOX can have poor outcomes owing to induction of chemoresistance. There is an urgent need to develop new therapies for TNBC aimed at improving DOX outcome and DOX-induced cardiotoxicity. Substance P (SP), a neuropeptide involved in pain transmission is known to stimulate production of reactive oxygen species (ROS). Elevated cardiac ROS is linked with heart injury and failure. We investigated the role of SP in chemotherapy-associated death of cardiomyocytes and chemoresistance. We showed that pretreating a cardiomyocyte cell line (H9C2) and a TNBC cell line (MDA-MB 231) with aprepitant, a SP receptor antagonist that is routinely used to treat chemotherapy-associated associated nausea, decreased DOX-induced reduction of cell viability, apoptotic cell death, and ROS production in cardiomyocytes and increased DOX-induced reduction of cell viability, apoptotic cell death, and ROS production in TNBC cells compared with cells treated with DOX alone. Our findings demonstrate the ability of aprepitant to decrease DOX-induced killing of cardiomyocytes and to increase cancer cell sensitivity to DOX, which has tremendous clinical significance.

Antagonist of the neurokinin-1 receptor curbs neuroinflammation in ex vivo and in vitro models of Lyme neuroborreliosis

Background

Lyme neuroborreliosis (LNB) can affect both the peripheral (PNS) and the central nervous systems (CNS); it is caused by the spirochete Borrelia burgdorferi. The neuropeptide substance P (SP) is an important mediator of both neuroinflammation and blood-brain barrier dysfunction, through its NK₁ receptor. Increased levels of SP have been shown to correlate with cell death. The present study used both ex vivo and in vitro models of experimentation to determine if the inflammatory mediator production and concomitant cell death caused by exposure of neural tissues and cells to B. burgdorferi could be attenuated by treatment with a NK₁ receptor antagonist.

Methods

We incubated normal rhesus frontal cortex tissue explants (CNS) and primary cultures of rhesus dorsal root ganglia cells (PNS) with live B. burgdorferi and tested the effectiveness of the NK₁ receptor antagonist L703,606 in attenuating inflammatory immune responses and neuronal and glial damage. Culture supernatants and tissue lysates were subjected to multiplex ELISA to quantify immune mediators, while the cells were evaluated for apoptosis by the in situ TUNEL assay. In addition, we identified immune mediators and producer cells in tissue sections by immunofluorescence staining and confocal microscopy.

Results

Co-incubation of both CNS tissues and PNS cells with the NK₁ receptor antagonist attenuated bacterially induced increases in inflammatory cytokine and chemokine production, particularly, IL-6, CXCL8, and CCL2, and reduced apoptosis levels. Confocal microscopy confirmed that neurons and glial cells are sources of these immune mediators. These results suggest that NK₁R antagonist treatment is able to reduce downstream pro-inflammatory signaling, thereby indicating that its systemic administration may slow disease progression.

Conclusions

We propose that SP contributes to neurogenic inflammation in LNB, and provide data to suggest that an NK₁ receptor antagonist may represent a novel neuroprotective therapy.

Non-apoptotic cell death associated with perturbations of macropinocytosis

Although macropinocytosis is widely recognized as a distinct form of fluid-phase endocytosis in antigen-presenting dendritic cells, it also occurs constitutively in many other normal and transformed cell types. Recent studies have established that various genetic or pharmacological manipulations can hyperstimulate macropinocytosis or disrupt normal macropinosome trafficking pathways, leading to accumulation of greatly enlarged cytoplasmic vacuoles. In some cases, this extreme vacuolization is associated with a unique form of non-apoptotic cell death termed “methuosis,” from the Greek methuo (to drink to intoxication). It remains unclear whether cell death related to dysfunctional macropinocytosis occurs in normal physiological contexts. However, the finding that some types of cancer cells are particularly vulnerable to this unusual form of cell death has raised the possibility that small molecules capable of altering macropinosome trafficking or function might be useful as therapeutic agents against cancers that are resistant to drugs that work by inducing apoptosis. Herein we review examples of cell death associated with dysfunctional macropinocytosis and summarize what is known about the underlying mechanisms.

Differential roles for Bim and Nur77 in thymocyte clonal deletion induced by ubiquitous self-antigen

Negative selection, primarily mediated through clonal deletion of self-reactive thymocytes, is critical for establishing self-tolerance and preventing autoimmunity. Recent studies suggest that the molecular mechanisms of negative selection differ depending on the thymic compartment and developmental stage at which thymocytes are deleted. Using the physiological HY(cd4) TCR transgenic model of negative selection against ubiquitous self-antigen, we previously found that one of the principal mediators implicated in clonal deletion, Bim, is required for caspase-3 activation but is ultimately dispensable for negative selection. On the basis of these data, we hypothesized that Nur77, another molecule thought to be a key mediator of clonal deletion, could be responsible for Bim-independent deletion. Despite comparable Nur77 induction in thymocytes during negative selection, Bim deficiency resulted in an accumulation of high-affinity-signaled thymocytes as well as impairment in caspase-mediated and caspase-independent cell death. Although these data suggested that Bim may be required for Nur77-mediated cell death, we found that transgenic Nur77 expression was sufficient to induce apoptosis independently of Bim. However, transgenic Nur77-induced apoptosis was significantly inhibited in the context of TCR signaling, suggesting that endogenous Nur77 could be similarly regulated during negative selection. Although Nur77 deficiency alone did not alter positive or negative selection, combined deficiency in Bim and Nur77 impaired clonal deletion efficiency and significantly increased positive selection efficiency. Collectively, these data shed light on the different roles for Bim and Nur77 during ubiquitous Ag-mediated clonal deletion and highlight potential differences from their reported roles in tissue-restricted Ag-mediated clonal deletion.

Continuous infusion of substance P into rat striatum alleviates nociceptive behavior via phosphorylation of extracellular signal-regulated kinase 1/2

Intraplantar injection of 0.4% formalin into the rat hind paw leads to a biphasic nociceptive response; an 'acute' phase (0-15 min) and 'tonic' phase (16-120 min), which is accompanied by significant phosphorylation of extracellular signal-regulated kinase (ERK)1/2 in the contralateral striatum at 120 min post-formalin injection. To uncover a possible relationship between the slow-onset substance P (SP) release and increased ERK1/2 phosphorylation in the striatum, continuous infusion of SP into the striatum by reverse microdialysis (0.4 μg/mL in microdialysis fiber, 1 μL/min) was performed to mimic volume neurotransmission of SP. Continuous infusion for 3 h of SP reduced the duration of 'tonic' phase nociception, and this SP effect was mediated by neurokinin 1 (NK1) receptors since pre-treatment with NK1 receptor antagonist CP96345 (10 μM) blocked the effect of SP infusion. However, formalin-induced 'tonic' phase nociception was significantly prolonged following acute injection of the MAP/ERK kinase 1/2 inhibitor PD0325901 (100 pmol) by microinjection. The coinfusion of SP and PD0325901 significantly increased the 'tonic' phase of nociception. These data demonstrate that volume transmission of striatal SP triggered by peripheral nociceptive stimulation does not lead to pain facilitation but a significant decrease of tonic nociception by the activation of the SP-NK1 receptor-ERK1/2 system. Noxious stimulation induces a slow-onset substance P (SP) release as a volume transmitter, activating extra-synaptic NK1 receptors, and evokes phosphorylation of extracellular signal-regulated kinase (ERK) 1/2. The SP-NK1-ERK1/2 system in the striatum decreases tonic nociception.

Ozone-induced airway epithelial cell death, the neurokinin-1 receptor pathway, and the postnatal developing lung

Children are uniquely susceptible to ozone because airway and lung growth continue for an extensive period after birth. Early-life exposure of the rhesus monkey to repeated ozone cycles results in region-specific disrupted airway/lung growth, but the mediators and mechanisms are poorly understood. Substance P (SP), neurokinin-1 receptor (NK-1R); and nuclear receptor Nur77 (NR4A1) are signaling pathway components involved in ozone-induced cell death. We hypothesize that acute ozone (AO) exposure during postnatal airway development disrupts SP/NK-1R/Nur77 pathway expression and that these changes correlate with increased ozone-induced cell death. Our objectives were to 1) spatially define the normal development of the SP/NK-1R/Nur77 pathway in conducting airways; 2) compare how postnatal age modulates responses to AO exposure; and 3) determine how concomitant, episodic ozone exposure modifies age-specific acute responses. Male infant rhesus monkeys were assigned at age 1 mo to two age groups, 2 or 6 mo, and then to one of three exposure subgroups: filtered air (FA), FA+AO (AO: 8 h/day × 2 days), or episodic biweekly ozone exposure cycles (EAO: 8 h/day × 5 days/14-day cycle+AO). O3 = 0.5 ppm. We found that 1) ozone increases SP/NK-1R/Nur77 pathway expression in conducting airways, 2) an ozone exposure cycle (5 days/cycle) delivered early at age 2 mo resulted in an airway that was hypersensitive to AO exposure at the end of 2 mo, and 3) continued episodic exposure (11 cycles) resulted in an airway that was hyposensitive to AO exposure at 6 mo. These observations collectively associate with greater overall inflammation and epithelial cell death, particularly in early postnatal (2 mo), distal airways.

NR4A nuclear receptors are orphans but not lonesome

The NR4A subfamily of nuclear receptors consists of three mammalian members: Nur77, Nurr1, and NOR-1. The NR4A receptors are involved in essential physiological processes such as adaptive and innate immune cell differentiation, metabolism and brain function. They act as transcription factors that directly modulate gene expression, but can also form trans-repressive complexes with other transcription factors. In contrast to steroid hormone nuclear receptors such as the estrogen receptor or the glucocorticoid receptor, no ligands have been described for the NR4A receptors. This lack of known ligands might be explained by the structure of the ligand-binding domain of NR4A receptors, which shows an active conformation and a ligand-binding pocket that is filled with bulky amino acid side-chains. Other mechanisms, such as transcriptional control, post-translational modifications and protein-protein interactions therefore seem to be more important in regulating the activity of the NR4A receptors. For Nur77, over 80 interacting proteins (the interactome) have been identified so far, and roughly half of these interactions has been studied in more detail. Although the NR4As show some overlap in interacting proteins, less information is available on the interactome of Nurr1 and NOR-1. Therefore, the present review will describe the current knowledge on the interactomes of all three NR4A nuclear receptors with emphasis on Nur77.

Roles of proteolysis in regulation of GPCR function

The enzymatic activity of peptidases must be tightly regulated to prevent uncontrolled hydrolysis of peptide bonds, which could have devastating effects on biological systems. Peptidases are often generated as inactive propeptidases, secreted with endogenous inhibitors, or they are compartmentalized. Propeptidases become active after proteolytic removal of N-terminal activation peptides by other peptidases. Some peptidases only become active towards substrates only at certain pHs, thus confining activity to specific compartments or conditions. This review discusses the different roles proteolysis plays in regulating GPCRs. At the cell-surface, certain GPCRs are regulated by the hydrolytic inactivation of bioactive peptides by membrane-anchored peptidases, which prevent signalling. Conversely, cell-surface peptidases can also generate bioactive peptides, which directly activate GPCRs. Alternatively, cell-surface peptidases activated by GPCRs, can generate bioactive peptides to cause transactivation of receptor tyrosine kinases, thereby promoting signalling. Certain peptidases can signal directly to cells, by cleaving GPCR to initiate intracellular signalling cascades. Intracellular peptidases also regulate GPCRs; lysosomal peptidases destroy GPCRs in lysosomes to permanently terminate signalling and mediate down-regulation; endosomal peptidases cleave internalized peptide agonists to regulate GPCR recycling, resensitization and signalling; and soluble intracellular peptidases also participate in GPCR function by regulating the ubiquitination state of GPCRs, thereby altering GPCR signalling and fate. Although the use of peptidase inhibitors has already brought success in the treatment of diseases such as hypertension, the discovery of new regulatory mechanisms involving proteolysis that control GPCRs may provide additional targets to modulate dysregulated GPCR signalling in disease.

The nuclear receptor NR4A1 induces a form of cell death dependent on autophagy in mammalian cells

The control of cell death is a biological process essential for proper development, and for preventing devastating pathologies like cancer and neurodegeneration. On the other hand, autophagy regulation is essential for protein and organelle degradation, and its dysfunction is associated with overlapping pathologies like cancer and neurodegeneration, but also for microbial infection and aging. In the present report we show that two evolutionarily unrelated receptors--Neurokinin 1 Receptor (NK(1)R,) a G-protein coupled receptor, and Insulin-like Growth Factor 1 Receptor (IGF1R), a tyrosine kinase receptor--both induce non-apoptotic cell death with autophagic features and requiring the activity of the autophagic core machinery proteins PI3K-III, Beclin-1 and Atg7. Remarkably, this form of cell death occurs in apoptosis-competent cells. The signal transduction pathways engaged by these receptors both converged on the activation of the nuclear receptor NR4A1, which has previously been shown to play a critical role in some paradigms of apoptosis and in NK(1)R-induced cell death. The activity of NR4A1 was necessary for IGF1R-induced cell death, as well as for a canonical model of cell death by autophagy induced by the presence of a pan-caspase inhibitor, suggesting that NR4A1 is a general modulator of this kind of cell death. During cell death by autophagy, NR4A1 was transcriptionally competent, even though a fraction of it was present in the cytoplasm. Interestingly, NR4A1 interacts with the tumor suppressor p53 but not with Beclin-1 complex. Therefore the mechanism to promote cell death by autophagy might involve regulation of gene expression, as well as protein interactions. Understanding the molecular basis of autophagy and cell death mediation by NR4A1, should provide novel insights and targets for therapeutic intervention.

Postnatal exposure history and airways: oxidant stress responses in airway explants

Postnatally, the lung continues to grow and differentiate while interacting with the environment. Exposure to ozone (O(3)) and allergens during postnatal lung development alters structural elements of conducting airways, including innervation and neurokinin abundance. These changes have been linked with development of asthma in a rhesus monkey model. We hypothesized that O(3) exposure resets the ability of the airways to respond to oxidant stress and that this is mediated by changes in the neurokinin-1 receptor (NK-1R). Infant rhesus monkeys received episodic exposure to O(3) biweekly with or without house dust mite antigen (HDMA) from 6 to 12 months of age. Age-matched monkeys were exposed to filtered air (FA). Microdissected airway explants from midlevel airways (intrapulmonary generations 5-8) for four to six animals in each of four groups (FA, O(3), HDMA, and HDMA+O(3)) were tested for NK-1R gene responses to acute oxidant stress using exposure to hydrogen peroxide (1.2 mM), a lipid ozonide (10 μM), or sham treatment for 4 hours in vitro. Airway responses were measured using real-time quantitative RT-PCR of NK-1R and IL-8 gene expression. Basal NK-1R gene expression levels were not different between the exposure groups. Treatment with ozonide or hydrogen peroxide did not change NK-1R gene expression in animals exposed to FA, HDMA, or HDMA+O(3). However, treatment in vitro with lipid ozonide significantly increased NK-1R gene expression in explants from O(3)-exposed animals. We conclude that a history of prior O(3) exposure resets the steady state of the airways to increase the NK-1R response to subsequent acute oxidant stresses.

Caspase-3 activation as a bifurcation point between plasticity and cell death

Death-mediating proteases such as caspases and caspase-3 in particular, have been implicated in neurodegenerative processes, aging and Alzheimer's disease. However, emerging evidence suggests that in addition to their classical role in cell death, caspases play a key role in modulating synaptic function. It is remarkable that active caspases-3, which can trigger widespread damage and degeneration, aggregates in structures as delicate as synapses and persists in neurons without causing acute cell death. Here, we evaluate this dichotomy, and discuss the hypothesis that caspase-3 may be a bifurcation point in cellular signaling, able to orient the neuronal response to stress down either pathological/apoptotic pathways or towards physiological cellular remodeling. We propose that temporal, spatial and other regulators of caspase activity are key determinants of the ultimate effect of caspase-3 activation in neurons. This concept has implications for differential roles of caspase-3 activation across the lifespan. Specifically, we propose that limited caspase-3 activation is critical for synaptic function in the healthy adult brain while chronic activation is involved in degenerative processes in the aging brain.

Singapore grouper iridovirus, a large DNA virus, induces nonapoptotic cell death by a cell type dependent fashion and evokes ERK signaling

Virus induced cell death, including apoptosis and nonapoptotic cell death, plays a critical role in the pathogenesis of viral diseases. Singapore grouper iridovirus (SGIV), a novel iridovirus of genus Ranavirus, causes high mortality and heavy economic losses in grouper aquaculture. Here, using fluorescence microscopy, electron microscopy and biochemical assays, we found that SGIV infection in host (grouper spleen, EAGS) cells evoked nonapoptotic programmed cell death (PCD), characterized by appearance of cytoplasmic vacuoles and distended endoplasmic reticulum, in the absence of DNA fragmentation, apoptotic bodies and caspase activation. In contrast, SGIV induced typical apoptosis in non-host (fathead minnow, FHM) cells, as evidenced by caspase activation and DNA fragmentation, suggesting that SGIV infection induced nonapoptotic cell death by a cell type dependent fashion. Furthermore, viral replication was essential for SGIV induced nonapoptotic cell death, but not for apoptosis. Notably, the disruption of mitochondrial transmembrane potential (ΔΨm) and externalization of phosphatidylserine (PS) were not detected in EAGS cells but in FHM cells after SGIV infection. Moreover, the extracellular signal-regulated kinase (ERK) signaling was involved in SGIV infection induced nonapoptotic cell death and viral replication. This is a first demonstration of ERK-mediated nonapoptotic cell death induced by a DNA virus. These findings contribute to understanding the mechanisms of iridovirus pathogenesis.

Endothelin-converting enzyme-1 regulates trafficking and signalling of the neurokinin 1 receptor in endosomes of myenteric neurones

Neuropeptide signalling at the plasma membrane is terminated by neuropeptide degradation by cell-surface peptidases, and by β-arrestin-dependent receptor desensitization and endocytosis. However, receptors continue to signal from endosomes by β-arrestin-dependent processes, and endosomal sorting mediates recycling and resensitization of plasma membrane signalling. The mechanisms that control signalling and trafficking of receptors in endosomes are poorly defined. We report a major role for endothelin-converting enzyme-1 (ECE-1) in controlling substance P (SP) and the neurokinin 1 receptor (NK(1)R) in endosomes of myenteric neurones. ECE-1 mRNA and protein were expressed by myenteric neurones of rat and mouse intestine. SP (10 nM, 10 min) induced interaction of NK(1)R and β-arrestin at the plasma membrane, and the SP-NK(1)R-β-arrestin signalosome complex trafficked by a dynamin-mediated mechanism to ECE-1-containing early endosomes, where ECE-1 can degrade SP. After 120 min, NK(1)R recycled from endosomes to the plasma membrane. ECE-1 inhibitors (SM-19712, PD-069185) and the vacuolar H(+)ATPase inhibitor bafilomycin A(1), which prevent endosomal SP degradation, suppressed NK(1)R recycling by >50%. Preincubation of neurones with SP (10 nM, 5 min) desensitized Ca(2+) transients to a second SP challenge after 10 min, and SP signals resensitized after 60 min. SM-19712 inhibited NK(1)R resensitization by >90%. ECE-1 inhibitors also caused sustained SP-induced activation of extracellular signal-regulated kinases, consistent with stabilization of the SP-NK(1)R-β-arrestin signalosome. By degrading SP and destabilizing endosomal signalosomes, ECE-1 has a dual role in controlling endocytic signalling and trafficking of the NK(1)R: promoting resensitization of G protein-mediated plasma membrane signalling, and terminating β-arrestin-mediated endosomal signalling.

ERK phosphorylation predicts synergism between gemcitabine and the epidermal growth factor receptor inhibitor AG1478

BACKGROUND

Clinical trials combining epidermal growth factor receptor (EGFR) inhibitors with gemcitabine-based chemotherapy in non-small cell lung cancer (NSCLC) have not produced a survival advantage. This may be caused by antagonism between the two drugs or mutations that promote such, possibly RAS mutation. Furthermore, ERK, a critical growth regulator downstream of RAS, may play a role. This study aimed to explore the relationship between ERK, synergy/antagonism and cell cycle arrest in combination treatment.

METHODS

A549 (mutant KRAS), H322 (wildtype KRAS) and siRNA-mediated KRAS knockdown A549 were treated with gemcitabine and/or the EGFR inhibitor AG1478 and analyzed with median effect analysis. Cell cycle distribution and ERK phosphorylation were assessed using flow cytometry and ELISA, respectively. Effect on cytotoxicity after ERK inhibition by U0126 was also assessed.

RESULTS

Cytotoxic interaction was dose dependent with antagonism at high dose AG1478. G1 arrest was observed with both high dose AG1478 and high dose gemcitabine and therefore was inconsistently associated with antagonism. Furthermore, ERK phosphorylation was increased by gemcitabine and its suppression by AG1478 was related to antagonism particularly in H322. ERK's effect in antagonism was further confirmed by using U0126. Greater antagonism was observed in the KRAS mutant cell line and KRAS knockdown by siRNA resulted in increased sensitivity to AG1478 as well as combination treatment.

CONCLUSION

Our findings are consistent with a model in which ERK phosphorylation favors synergy and the outcome depends on the balance between gemcitabine-induced and AG1478-inhibited ERK phosphorylation. KRAS mutation confers resistance to AG1478 as well as combination treatment.

Research resource: Comparative nuclear receptor atlas: basal and activated peritoneal B-1 and B-2 cells

Naïve murine B cells are typically divided into three subsets based on functional and phenotypic characteristics: innate-like B-1 and marginal zone B cells vs. adaptive B-2 cells, also known as follicular or conventional B cells. B-1 cells, the innate-immune-like component of the B cell lineage are the primary source of natural antibodies and have been shown to modulate autoimmune diseases, human B-cell leukemias, and inflammatory disorders such as atherosclerosis. On the other hand, B-2 cells are the principal mediators of the adaptive humoral immune response and represent an important pharmacological target for various conditions including rheumatoid arthritis, lupus erythematosus, and lymphomas. Using the resources of the Nuclear Receptor Signaling Atlas program, we used quantitative real-time PCR to assess the complement of the 49 murine nuclear receptor superfamily expressed in quiescent and toll-like receptor (TLR)-stimulated peritoneal B-1 and B-2 cells. We report the expression of 24 nuclear receptors in basal B-1 cells and 25 nuclear receptors in basal B-2 cells, with, in some cases, dramatic changes in response to TLR 4 or TLR 2/1 stimulation. Comparative nuclear receptor profiling between B-1 and peritoneal B-2 cells reveals a highly concordant expression pattern, albeit at quantitatively dissimilar levels. We also found that splenic B cells express 23 nuclear receptors. This catalog of nuclear receptor expression in B-1 and B-2 cells provides data to be used to better understand the specific roles of nuclear receptors in B cell function, chronic inflammation, and autoimmune disease.

Pathological and ultrastructural changes in cultured cells induced by venom from the ectoparasitic wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae)

The ectoparasitic wasp Nasonia vitripennis produces a proteinaceous venom that induces death in fly hosts by non-paralytic mechanisms. Previous in vitro assays have suggested that the primary cause of cell and tissue death is oncosis, a non-programmed cell death (PCD) pathway characterized by cellular swelling and lysis. However, ultrastructural analyses of BTI-TN-5B1 cells exposed to LC(99) doses of wasp venom revealed cellular changes more consistent with apoptosis and/or non-apoptotic PCD than oncosis or necrosis: By 3h after incubation with venom, susceptible cells displayed indentations in the nuclear membranes, large nucleoli, and extensive vacuolization throughout the cytoplasm. In the vast majority of venom treated cells, annexin V bound to the plasma membrane surface within 15 min after treatment, a characteristic consistent with translocation of phosphatidylserine to the cell surface during the early stages of apoptosis. Likewise, mitochondrial transmembrane potential was depressed in cells within 15 min in venom-treated cells, an event that occurred in the absence of mitochondrial swelling or rupturing of cristae. Active caspase 3 was detected by fluorescent labeling in nearly all venom treated cells 3h after exposure to venom, and in turn, the potent caspase 3 inhibitor Z-VAD-FMK attenuated the morphological changes elicited by wasp venom and afforded protection to BTI-TN-5B1-4 cells.

Growth arrest and induction of apoptotic and non-apoptotic programmed cell death by, Physalis minima L. chloroform extract in human ovarian carcinoma Caov-3 cells

ETHNOPHARMACOLOGICAL RELEVANCE

The decoction of the whole plant of Physalis minima L. is traditionally consumed to treat cancer. Its anticancer property has been previously verified (using in vitro cytotoxicity assays) against NCI-H23 lung, CORL23 lung and MCF7 breast cancer cell lines but the mechanism underlying the anticancer potency towards ovarian carcinoma cells remain unclear.

AIM OF THE STUDY

The present study is aimed to systematically determine the cytotoxicity and possible cell death mechanism elicited by the chloroform extract of Physalis minima in human ovarian Caov-3 carcinoma.

MATERIALS AND METHODS

Cytotoxicity of the extract was measured using the methylene blue assay. The mechanism of cell death was determined using four independent methods, namely DeadEnd assay to label the DNA fragmentation nuclei cells, RT-PCR analysis to determine the mRNA expression level of three apoptotic genes (c-myc, p53 and caspase-3 genes), Transmission Electron Microscope (TEM) analysis to describe the ultra structural characteristics and annexin V and propidium iodide staining to confirm the types and stages of cell deaths.

RESULTS

Cytotoxicity screening of the extract on Caov-3 cells exhibited concentration- and time-dependent inhibitory effects. A combination of apoptotic and autophagic programmed cell death was detected. The apoptotic characteristic was initially determined by DNA fragmentation followed by the expression of c-myc and p53 genes that was much earlier than caspase-3. Apoptotic ultra structural changes (including clumping and magination of chromatin, blebbing and convolution of nucleus membrane and formation of apoptotic bodies) and autophagy (Type II non-apoptotic programmed cell death) with distinct vacuolated morphology were detected in TEM analysis. The existence of these programmed cell deaths was then corroborated using annexin V and propidium iodide staining.

CONCLUSIONS

The chloroform extract of Physalis minima exerted anticancer effect due to a combination of apoptotic and autophagic cell death mechanisms on Caov-3 cells. The induction of these programmed cell deaths was mediated via c-myc, p53 and caspase-3 dependent pathway. The results could provide a valuable insight in cancer therapy.

Taxol induces paraptosis independent of both protein synthesis and MAPK pathway

Our recent studies have shown that high concentration of taxol induced a caspase-independent paraptosis-like cell death and cytoplasmic vacuolization derived predominantly from endoplasmic reticulum (ER) swelling in human lung carcinoma cell lines (ASTC-a-1). In this report, we further explored the relationship between taxol-induced cell death and vacuolization, and the roles of protein synthesis, mitogen-activated protein kinase kinases (MEK), c-jun N-terminal kinase (JNK) and P38 in taxol-induced paraptosis. Enhanced green fluorescent protein (EGFP) was used to probe the cell morphological change, while ER-targeted red fluorescent protein (er-RFP) was used to probe ER spatial distribution. Real-time monitoring of the ER swelling dynamics during the formation of vacuolization inside single living cells co-expressing EGFP and er-RFP further demonstrated that taxol-induced cytoplasmic vacuolization was from the ER restructuring due to fusion and swelling. PI staining showed that taxol-induced vacuolization was not necrosis. These results further demonstrated that the taxol-induced cell death was neither apoptosis nor necrosis, and fitted the criteria of paraptosis characterized by cytoplasmic vacuolization, caspase-independence, lack of apoptotic morphology and insensitivity to broad caspase inhibitor. Our data further indicated that taxol-induced paraptosis required neither protein synthesis nor the participation of MEK, JNK, and P38, which was different from the insulin-like growth factor I receptor (IGFIR)-induced paraptosis. These results suggest that high concentration of taxol activates an alternative paraptotic cell death pathway.

Substance P antagonists as a therapeutic approach to improving outcome following traumatic brain injury

Although a number of secondary injury factors are known to contribute to the development of morphological injury and functional deficits following traumatic brain injury, accumulating evidence has suggested that neuropeptides, and in particular substance P, may play a critical role. Substance P is released early following acute injury to the CNS as part of a neurogenic inflammatory response. In so doing, it facilitates an increase in the permeability of the blood-brain barrier and the development of vasogenic edema. At the cellular level, substance P has been shown to directly result in neuronal cell death; functionally, substance P has been implicated in learning and memory, mood and anxiety, stress mechanisms, emotion-processing, migraine, emesis, pain, and seizures, all of which may be adversely affected after brain injury. Inhibition of post-traumatic substance P activity, either by preventing release or by antagonism of the neurokinin-1 receptor, has consistently resulted in a profound decrease in development of edema and marked improvements in functional outcome. This review summarizes the current evidence supporting a role for substance P in acute brain injury.

ERK and cell death: ERK1/2 in neuronal death

Extracellular signal-regulated kinase (ERK) is a versatile protein kinase that regulates many cellular functions. Growing evidence suggests that ERK1/2 plays a crucial role in promoting cell death in a variety of neuronal systems, including neurodegenerative diseases. It is believed that the magnitude and the duration of ERK1/2 activity determine its cellular function. In this review, we summarize recent evidence for a role of ERK1/2 in neuronal death. Furthermore, we discuss the mechanisms involved in ERK1/2 mediating neuronal death.

Activation of calcitonin gene-related peptide receptor during ozone inhalation contributes to airway epithelial injury and repair

The authors investigated the importance of the neuropeptide, calcitonin gene-related peptide (CGRP), in epithelial injury, repair, and neutrophil emigration after ozone exposure. Wistar rats were administered either a CGRP-receptor antagonist (CGRP(8-37)) or saline and exposed to 8 hours of 1-ppm ozone or filtered air with an 8-hour postexposure period. Immediately after exposure, ethidium homodimer was instilled into lungs as a marker of necrotic airway epithelial cells. After fixation, airway dissected lung lobes were stained for 5'-bromo-2'-deoxyuridine, a marker of epithelial proliferation. Positive epithelial cells were quantified in specific airway generations. Rats treated with CGRP(8-37) had significantly reduced epithelial injury in terminal bronchioles and reduced epithelial proliferation in proximal airways and terminal bronchioles. Bronchoalveolar lavage and sections of terminal bronchioles showed no significant difference in the number of neutrophils emigrating into airways in CGRP(8-37)-treated rats. The airway epithelial cell line, HBE-1, showed no difference in the number of oxidant stress positive cells during exposure to hydrogen peroxide and a range of CGRP(8-37) doses, demonstrating no antioxidant effect of CGRP(8-37). We conclude that activation of CGRP receptors during ozone inhalation contributes to airway epithelial injury and subsequent epithelial proliferation, a critical component of repair, but does not influence neutrophil emigration into airways.

Common pathways for activation of proinflammatory gene expression by G protein-coupled receptors in primary lung epithelial and endothelial cells

Acute lung injury is associated with an inflammatory response resulting from the action of multiple mediators. Many proinflammatory mediators released during lung injury exert effects by binding to G protein-coupled receptors (GPCRs). The authors' earlier studies showed that substance P (SP), a ligand for the tachykinin 1 receptor, induced nuclear factor (NF)-kappa B activation and interleukin (IL)-8 up-regulation through a G(q)-dependent pathway. Here the authors extend these findings by examining effects of multiple ligands for G(q)-coupled GPCRs in primary human small airway epithelial cells (SAECs) and rat lung microvessel endothelial cells (RLMVECs). SP, bradykinin, protease activated receptor 2 agonist, and platelet-activating factor (PAF) stimulated IL-8 production in SAECs, whereas only SP and PAF up-regulated CINC-1 (a rat IL-8 homolog) in RLMVECs. Using signaling inhibitors, the authors investigated PAF-induced IL-8 expression and SP-induced CINC-1 expression in primary cells. Signaling cascades were similar in SAECs and RLMVECs and involved phospholipase C/calcium/protein kinase C (PKC) and Ras/Raf/Erk pathways. In addition, the tyrosine kinase inhibitor AG 17 and the proteasome inhibitor MG132 significantly reduced IL-8 and CINC-1 expression induced by GPCR ligands. The results demonstrate a common signaling pathway in primary lung epithelial and endothelial cells, suggesting a generalized mechanism for the induction of proinflammatory gene expression by G(q)-coupled GPCRs following lung injury.

Endosomal endothelin-converting enzyme-1: a regulator of beta-arrestin-dependent ERK signaling

Neuropeptide signaling at the cell surface is regulated by metalloendopeptidases, which degrade peptides in the extracellular fluid, and beta-arrestins, which interact with G protein-coupled receptors (GPCRs) to mediate desensitization. beta-Arrestins also recruit GPCRs and mitogen-activated protein kinases to endosomes to allow internalized receptors to continue signaling, but the mechanisms regulating endosomal signaling are unknown. We report that endothelin-converting enzyme-1 (ECE-1) degrades substance P (SP) in early endosomes of epithelial cells and neurons to destabilize the endosomal mitogen-activated protein kinase signalosome and terminate signaling. ECE-1 inhibition caused endosomal retention of the SP neurokinin 1 receptor, beta-arrestins, and Src, resulting in markedly sustained ERK2 activation in the cytosol and nucleus, whereas ECE-1 overexpression attenuated ERK2 activation. ECE-1 inhibition also enhanced SP-induced expression and phosphorylation of the nuclear death receptor Nur77, resulting in cell death. Thus, endosomal ECE-1 attenuates ERK2-mediated SP signaling in the nucleus to prevent cell death. We propose that agonist availability in endosomes, here regulated by ECE-1, controls beta-arrestin-dependent signaling of endocytosed GPCRs.

Plasticity of hippocampus following perinatal asphyxia: effects on postnatal apoptosis and neurogenesis

Asphyxia during delivery produces long-term deficits in brain development, including hippocampus. We investigated hippocampal plasticity after perinatal asphyxia, measuring postnatal apoptosis and neurogenesis. Asphyxia was performed by immersing rat fetuses with uterine horns removed from ready-to-deliver rats into a water bath for 20 min. Caesarean-delivered pups were used as controls. The animals were euthanized 1 week or 1 month after birth. Apoptotic nuclear morphology and DNA breaks were assessed by Hoechst and TUNEL assays. Neurogenesis was estimated by bromodeoxyuridine/MAP-2 immunocytochemistry, and the levels and expression of proteins related to apoptosis and cell proliferation were measured by Western blots and in situ hybridization, respectively. There was an increase of apoptosis in CA1, CA3, and dentate gyrus (DG) and cell proliferation and neurogenesis in CA1, DG, and hilus regions of hippocampus 1 week after asphyxia. The increase of apoptosis in CA3 and cell proliferation in the suprapyramidal band of DG was still observed 1 month following asphyxia. There was an increase of BAD, BCL-2, ERK2, and bFGF levels in whole hippocampus and bFGF expression in CA1 and CA2 and hilus at P7 and P30. There was a concomitant decrease of phosphorylated-BAD (Ser112) levels. The increase of BAD levels supports the idea of delayed cell death after perinatal asphyxia, whereas the increases of BCL-2, ERK2, and bFGF levels suggest the activation of neuroprotective and repair pathways. In conclusion, perinatal asphyxia induces short- and long-term regionally specific plastic changes, including delayed cell death and neurogenesis, involving pro- and antiapoptotic as well as mitogenic proteins, favoring hippocampal functional recovery.

Accumulation of rhodopsin in late endosomes triggers photoreceptor cell degeneration

Progressive retinal degeneration is the underlying feature of many human retinal dystrophies. Previous work using Drosophila as a model system and analysis of specific mutations in human rhodopsin have uncovered a connection between rhodopsin endocytosis and retinal degeneration. In these mutants, rhodopsin and its regulatory protein arrestin form stable complexes, and endocytosis of these complexes causes photoreceptor cell death. In this study we show that the internalized rhodopsin is not degraded in the lysosome but instead accumulates in the late endosomes. Using mutants that are defective in late endosome to lysosome trafficking, we were able to show that rhodopsin accumulates in endosomal compartments in these mutants and leads to light-dependent retinal degeneration. Moreover, we also show that in dying photoreceptors the internalized rhodopsin is not degraded but instead shows characteristics of insoluble proteins. Together these data implicate buildup of rhodopsin in the late endosomal system as a novel trigger of death of photoreceptor neurons.

{beta}-Arrestin-2 Mediates Anti-apoptotic Signaling through Regulation of BAD Phosphorylation

β-Arrestins, originally discovered as terminators of G protein-coupled receptor signaling, have more recently been appreciated to also function as signal transducers in their own right, although the consequences for cellular physiology have not been well understood. Here we demonstrate that β-arrestin-2 mediates anti-apoptotic cytoprotective signaling stimulated by a typical 7-transmembrane receptor the angiotensin ATII 1A receptor, expressed endogenously in rat vascular smooth muscle cells or by transfection in HEK-293 cells. Receptor stimulation leads to concerted activation of two pathways, ERK/p90RSK and PI3K/AKT, which converge to phosphorylate and inactivate the pro-apoptotic protein BAD. Anti-apoptotic effects as well as pathway activities can be stimulated by an angiotensin analog (SII), which has been previously shown to activate β-arrestin but not G protein-dependent signaling, and are abrogated by β-arrestin-2 small interfering RNA. These findings establish a key role for β-arrestin-2 in mediating cellular cytoprotective functions by a 7-transmembrane receptor and define the biochemical pathways involved.

Exogenous application of persephin influences phosphatidylinositol-3 kinase and MAPK/ERK signalling and enhances proliferation during early development in retinospheres

Persephin (PSPN), a member of the glial cell line-derived neurotrophic factor family, and its implication in the retina is not well understood but might be an interesting therapeutic target for degenerative diseases. Although, PSPN is lost in the chicken during evolution, its target, the GDNF family receptor alpha 4 (GFRalpha4), is still expressed in a temporal and spatial pattern in the developing retina. We used this "knockout-precondition" to study the bioactivity and the effect of exogenous PSPN application and subsequent GFRalpha activation during retinal development in vitro without impairments of endogenous PSPN. Retinospheres, derived from dissociated chicken retina of embryonic day 6, were treated with PSPN and intracellular signalling was monitored. Additionally, PSPN was added during cultivation of the retinospheres and immunhistochemical stainings and Western blotting were performed to evaluate changes in proliferation, apoptosis and differentiation. Exogenous applied PSPN enhanced phosphatidylinositol-3-kinase (PI-3K) signalling and decreased signalling of mitogen-activated protein kinases (MAPK). Most importantly early retinal proliferation was enhanced and glutamine synthetase expression was decreased whereas differentiation of major retinal cell types was not changed. In contrast to GDNF, PSPN is exclusively influencing early progenitors whereas differentiation is not effected and seems to be regulated through PSPN-independent mechanisms. Since the binding site of PSPN and therefore the target of potential therapeuticals, is well conserved among species and is with high probability not able to bind other members of the GDNF-family, these results might be assigned to other species including mammals and humans.

Extracellular signal-regulated kinases (ERK) and protein kinase C (PKC) activities are involved in the modulation of Nur77 and Nor-1 expression by dopaminergic drugs

The dopamine system is the main target of antipsychotic and psychostimulant drugs. These drugs induce intracellular events that culminate in the transcription of immediate early genes, such as c-fos. Another class of transcription factors, namely, the nuclear receptor subgroup called Nurs (Nur77, Nurr1 and Nor-1), has recently been associated with behavioral and biochemical effects mediated by dopamine. However, the signaling cascade leading to modulation of Nur mRNA levels in the brain has never been investigated. In the present study, we explore in vivo using specific kinase inhibitors the role of mitogen-associated and extracellular signal-regulated kinases (MEK) and protein kinase C (PKC) in the modulation of Nur expression induced by dopamine receptor drugs. Modulation of Nur77 expression by a dopamine D(2) receptor antagonist is associated with MEK and PKC activities, whereas only the PKC activity participates in the modulation of Nor-1 expression. Both MEK and PKC activities also participate in the modulation of Nur77 mRNA levels induced by dopamine receptor agonists, whereas a selective MEK activity is associated with the modulation of Nor-1 mRNA levels. Interestingly, modulation of dopamine drug-induced locomotor activities by kinase inhibitors is in accordance with the effects on Nur77, but not Nor-1, expression. Taken together, the results indicate that signaling events leading to modulation of Nur77 and Nor-1 expression following dopamine receptor interacting drugs are distinct. Considering that orphan nuclear receptors of the Nur subgroup display an important ligand-independent constitutive activity, characterization of the signaling cascades involved in the regulation of their expression represents an important step for understanding their role in dopamine system physiology and pathophysiology.

Activation of neurokinin-1 receptors during ozone inhalation contributes to epithelial injury and repair

We investigated the importance of neurokinin (NK)-1 receptors in epithelial injury and repair and neutrophil function. Conscious Wistar rats were exposed to 1 ppm ozone or filtered air for 8 hours, followed by an 8-hour postexposure period. Before exposure, we administered either the NK-1 receptor antagonist, SR140333, or saline as a control. Ethidium homodimer was instilled into lungs as a marker of necrotic airway epithelial cells. After fixation, whole mounts of airway dissected lung lobes were immunostained for 5-bromo-2'-deoxyuridine, a marker of epithelial proliferation. Both ethidium homodimer and 5-bromo-2'-deoxyuridine-positive epithelial cells were quantified in specific airway generations. Rats treated with the NK-1 receptor antagonist had significantly reduced epithelial injury and epithelial proliferation compared with control rats. Sections of terminal bronchioles showed no significant difference in the number of neutrophils in airways between groups. In addition, staining ozone-exposed lung sections for active caspase 3 showed no apoptotic cells, but ethidium-positive cells colocalized with the orphan nuclear receptor, Nur77, a marker of nonapoptotic, programmed cell death mediated by the NK-1 receptor. An immortalized human airway epithelial cell line, human bronchial epithelial-1, showed no significant difference in the number of oxidant stress-positive cells during exposure to hydrogen peroxide and a range of SR140333 doses, demonstrating no antioxidant effect of the receptor antagonist. We conclude that activation of the NK-1 receptor during acute ozone inhalation contributes to epithelial injury and subsequent epithelial proliferation, a critical component of repair, but does not influence neutrophil emigration into airways.

Alpha 1-antichymotrypsin/SerpinA3 is a novel target of orphan nuclear receptor Nur77

Nur77 is one member of the nuclear receptor superfamily. As a transcription factor, Nur77 participates in a variety of biological processes, including T cell development, inflammatory responses, steroid hormone synthesis, and hepatic glucose metabolism. It typically acts via binding to the Nur77 responsive element (NBRE) in the promoter regions of its target genes. In the present study, we identified a novel Nur77-regulated gene, alpha1-antichymotrypsin/SerpinA3, via an approach combining computational prediction and wet-laboratory validations. First, we identified 483 candidate genes via a human genome-wide scan for NBREs in their proximal promoters. Three out of 14 function-associated genes were further identified to be transactivated by Nur77 in luciferase reporter gene assays in HEK 293T cells. The transactivation assay proved that the NBRE (-182 to -175) in the SerpinA3 promoter region is a novel Nur77-dependent functional motif in HEK 293T and HepG2 cells. Electrophoretic mobility shift and chromatin immunoprecipitation assays demonstrated that Nur77 physically associates with the SerpinA3 promoter region both in vitro and in vivo. Nur77 overexpression and RNA interference-mediated Nur77 gene knockdown analysis confirmed that SerpinA3 is indeed a novel Nur77-targeted gene. These data may throw light on the function of Nur77 in inflammatory responses and acute-phase reactions as well as the development of Alzheimer's disease.

Chromatin compaction and cell death by high molecular weight FGF-2 depend on its nuclear localization, intracrine ERK activation, and engagement of mitochondria

Fibroblast growth factor 2 (FGF-2) is produced as CUG-initiated, 22-34 kDa or AUG-initiated 18 kDa isoforms (hi- and lo-FGF-2, respectively), with potentially distinct functions. We report that expression of hi-FGF-2 in HEK293 cells elicited chromatin compaction preceding cell death with apoptotic features. Nuclear localization of the intact protein was required as expression of a non-nuclear hi-FGF-2 mutant failed to elicit chromatin compaction. Equally ineffective, despite nuclear localization, was the over-expression of the 18 kDa core sequence (lo-FGF-2). Chromatin compaction by hi-FGF-2 was accompanied by increased cytosolic cytochrome C, and was attenuated either by over-expression of Bcl-2 or by a peptide inhibitor of the pro-apoptotic protein Bax. In addition hi-FGF-2 elicited sustained activation of total and nuclear extracellular signal regulated kinase (ERK1/2) by an intracrine route, as it was not prevented by neutralizing anti-FGF-2 antibodies. Inhibition of the ERK1/2 activating pathway by dominant negative upstream activating kinase, or by PD 98059, prevented chromatin compaction by hi-FGF-2. ERK1/2 activation was not affected by the Bax-inhibiting peptide suggesting that it occurred upstream of mitochondrial involvement. We conclude that the hi-FGF-2-induced chromatin compaction and cell death requires its nuclear localization, intracrine ERK1/2 activation and mitochondrial engagement.

‘Sensing’ autoimmunity in type 1 diabetes

Type 1 diabetes (T1D) results from autoimmune-mediated loss of insulin-producing beta-cells. Recent findings suggest that the events controlling T1D development are not only immunological, but also neuronal in nature. In the non-obese diabetic (NOD) mouse model of T1D, a mutant sensory neuron channel, TRPV1, initiates chronic, progressive beta-cell stress, inducing islet cell inflammation. This novel mechanism of organ-specific damage requires a permissive, autoimmune-prone host, but ascribes tissue specificity to the local secretory dysfunction of sensory afferent neurons. In NOD mice, normalizing this neuronal function by administration of the neurotransmitter substance P clears islet cell inflammation, reduces insulin resistance and restores normoglycemia. Here, we discuss this neuro-immuno-endocrine model, its implications and the involvement of sensory neurons in other autoimmune disorders. These developments might provide novel neuronal-based therapeutic interventions, particularly in diabetes.

Distinct mechanisms involving diverse histone deacetylases repress expression of the two gonadotropin beta-subunit genes in immature gonadotropes, and their actions are overcome by gonadotropin-releasing hormone

The gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are produced in the embryonic pituitary in response to delivery of the hypothalamic gonadotropin releasing hormone (GnRH). GnRH has a pivotal role in reestablishing gonadotropin levels at puberty in primates, and for many species with extended reproductive cycles, these are reinitiated in response to central nervous system-induced GnRH release. Thus, a clear role is evident for GnRH in overcoming repression of these genes. Although the mechanisms through which GnRH actively stimulates LH and FSH beta-subunit (FSHbeta) gene transcription have been described in some detail, there is currently no information on how GnRH overcomes repression in order to terminate reproductively inactive stages. We show here that GnRH overcomes histone deacetylase (HDAC)-mediated repression of the gonadotropin beta-subunit genes in immature gonadotropes. The repressive factors associated with each of these genes comprise distinct sets of HDACs and corepressors which allow for differentially regulated derepression of these two genes, produced in the same cell by the same regulatory hormone. We find that GnRH activation of calcium/calmodulin-dependent protein kinase I (CaMKI) plays a crucial role in the derepression of the FSHbeta gene involving phosphorylation of several class IIa HDACs associated with both the FSHbeta and Nur77 genes, and we propose a model for the mechanisms involved. In contrast, derepression of the LH beta-subunit gene is not CaMK dependent. This demonstration of HDAC-mediated repression of these genes could explain the temporal shut-down of reproductive function at certain periods of the life cycle, which can easily be reversed by the actions of the hypothalamic regulatory hormone.

Impairments of heat shock protein expression and MAPK translocation in the central nervous system of follitropin receptor knockout mice

The central nervous system is exposed to the chronic oxidative stress during aging when the endogenous defence weakens and the load of reactive oxygen species enhances. Sex hormones and heat shock proteins (Hsps) participate in these responses to stress. Their regulation is disturbed in aging. We assessed the expression of Hsps in hippocampus and cortex of follitropin receptor knockout (FORKO) mice, known to exhibit gender and age-dependent imbalance in sex steroids and gonadotropins. These imbalances could contribute to an impaired regulation of Hsps thereby increasing the risk of developing neurodegenerative disorders. Our study shows that, in the hippocampus the expression of Hsp70 and Hsp25 was reduced in 20-month-old FORKO mice. However, in the cortex both Hsps were significantly down regulated only in elderly females. There is a well-established co-regulation between Hsps and mitogen-activated protein kinases (MAPKs). Significant, gender-specific impairments in the translocation of phosphorylated ERK and JNK were found in the CNS structures in aged FORKO mice. Our results suggest that hormonal imbalances lead to a disturbed subcellular distribution of activated MAPKs which contribute to the impairments of signal transduction networks maintaining normal physiological functions in the cortex and hippocampus that are associated with neurodegenerative changes in aging.

Arrestins: ubiquitous regulators of cellular signaling pathways

The arrestins are a small family of proteins that regulate the signaling and trafficking of G-protein-coupled receptors and also serve as ubiquitous signaling regulators in the cytoplasm and nucleus.

In vertebrates, the arrestins are a family of four proteins that regulate the signaling and trafficking of hundreds of different G-protein-coupled receptors (GPCRs). Arrestin homologs are also found in insects, protochordates and nematodes. Fungi and protists have related proteins but do not have true arrestins. Structural information is available only for free (unbound) vertebrate arrestins, and shows that the conserved overall fold is elongated and composed of two domains, with the core of each domain consisting of a seven-stranded β-sandwich. Two main intramolecular interactions keep the two domains in the correct relative orientation, but both of these interactions are destabilized in the process of receptor binding, suggesting that the conformation of bound arrestin is quite different. As well as binding to hundreds of GPCR subtypes, arrestins interact with other classes of membrane receptors and more than 20 surprisingly diverse types of soluble signaling protein. Arrestins thus serve as ubiquitous signaling regulators in the cytoplasm and nucleus.

Nur77 and retinoid X receptors: crucial factors in dopamine-related neuroadaptation

Dopaminergic systems in the brain adapt in response to various stimuli from the internal and external world, but the mechanisms underlying this process are incompletely understood. Here, we review recent evidence that certain types of transcription factor of the nuclear receptor family, specifically Nur77 and retinoid X receptors, have important roles in adaptation and homeostatic regulation of dopaminergic systems. These findings call for a reassessment of our fundamental understanding of the molecular and cellular basis of dopamine-mediated transmission. Given that diseases such as Parkinson's disease and schizophrenia are thought to involve adaptation of dopamine signalling, these findings might provide new insight into these pathologies and offer new avenues for drug development.

Multiple signaling pathways regulate the transcriptional activity of the orphan nuclear receptor NURR1

The orphan nuclear receptor nurr1 (NR4A2) is an essential transcription factor for the acquisition and maintenance of the phenotype of dopamine (DA)-synthesizing neurons in the mesencephalon. Although structurally related to ligand-regulated nuclear receptors, nurr1 is functionally atypical due to its inability to bind a cognate ligand and to activate transcription following canonical nuclear receptor (NR) rules. Importantly, the physiological stimuli that activate this NR and the signaling proteins that regulate its transcriptional activity in mesencephalic neurons are unknown. We used an affinity chromatography approach and CSM14.1 cells of mesencephalic origin to isolate and identify several proteins that interact directly with nurr1 and regulate its transcriptional activity. Notably, we demonstrate that the mitogen-activated protein kinases, ERK2 and ERK5, elevate, whereas LIM Kinase 1 inhibits nurr1 transcriptional activity. Furthermore, nurr1 recruits ERK5 to a NBRE-containing promoter and is a potential substrate for this kinase. We have identified amino acids in the A/B domain of nurr1 important for mediating the ERK5 activating effects on nurr1 transcriptional activity. Our results suggest that nurr1 acts as a point of convergence for multiple signaling pathways that likely play a critical role in differentiation and phenotypic expression of dopaminergic (DAergic) neurons.