Glutamate induces focal adhesion kinase tyrosine phosphorylation and actin rearrangement in heterologous mGluR1-expressing CHO cells via calcium/calmodulin signaling

The histidine-rich calcium binding protein (HRC) promotes tumor metastasis in hepatocellular carcinoma and is upregulated by SATB1

The histidine-rich calcium binding protein (HRC) is a regulator of Ca²⁺-homeostasis. Herein, we found that HRC was frequently upregulated in human hepatocellular carcinoma (HCC) tissues, and its expression was correlated with tumor size and metastasis. Moreover, HRC expression was positively related to the metastatic potential of HCC cell lines. Knockdown of HRC suppressed cell invasion and migration in vitro, whereas ectopic expression of HRC resulted in increased cell invasion and migration in vitro and intrahepatic and lung metastasis in vivo. Interestingly, the pro-invasion and pro-migration effects of HRC were associated with focal adhesion turnover, which was a consequence of FAK phosphorylation. Further experiments showed that HRC induced phospho-FAK, focal adhesion turnover and cell migration through Ca²⁺/CaM singaling. We found that HRC increased [Ca²⁺]_i by inhibiting the expression of SERCA2. In addition, upregulation of HRC in HCC was attributed to SATB1, which is known to promote HCC metastasis. Ectopic expression of SATB1 enhanced HRC gene transcription by activating AP-1 in mainly a JNK-dependent manner. Our findings highlight HRC as a potential therapeutic target for HCC treatment.

Researching glutamate - induced cytotoxicity in different cell lines: a comparative/collective analysis/study

Although glutamate is one of the most important excitatory neurotransmitters of the central nervous system, its excessive extracellular concentration leads to uncontrolled continuous depolarization of neurons, a toxic process called, excitotoxicity. In excitotoxicity glutamate triggers the rise of intracellular Ca²⁺ levels, followed by up regulation of nNOS, dysfunction of mitochondria, ROS production, ER stress, and release of lysosomal enzymes. Excessive calcium concentration is the key mediator of glutamate toxicity through over activation of ionotropic and metabotropic receptors. In addition, glutamate accumulation can also inhibit cystine (CySS) uptake by reversing the action of the CySS/glutamate antiporter. Reversal of the antiporter action reinforces the aforementioned events by depleting neurons of cysteine and eventually glutathione’s reducing potential. Various cell lines have been employed in the pursuit to understand the mechanism(s) by which excitotoxicity affects the cells leading them ultimately to their demise. In some cell lines glutamate toxicity is exerted mainly through over activation of NMDA, AMPA, or kainate receptors whereas in other cell lines lacking such receptors, the toxicity is due to glutamate induced oxidative stress. However, in the greatest majority of the cell lines ionotropic glutamate receptors are present, co-existing to CySS/glutamate antiporters and metabotropic glutamate receptors, supporting the assumption that excitotoxicity effect in these cells is accumulative. Different cell lines differ in their responses when exposed to glutamate. In this review article the responses of PC12, SH-SY5Y, HT-22, NT-2, OLCs, C6, primary rat cortical neurons, RGC-5, and SCN2.2 cell systems are systematically collected and analyzed.

Glutamate involvement in calcium-dependent migration of astrocytoma cells

BACKGROUND

Astrocytoma are known to have altered glutamate machinery that results in the release of large amounts of glutamate into the extracellular space but the precise role of glutamate in favoring cancer processes has not yet been fully established. Several studies suggested that glutamate might provoke active killing of neurons thereby producing space for cancer cells to proliferate and migrate. Previously, we observed that calcium promotes disassembly of integrin-containing focal adhesions in astrocytoma, thus providing a link between calcium signaling and cell migration. The aim of this study was to determine how calcium signaling and glutamate transmission cooperate to promote enhanced astrocytoma migration.

METHODS

The wound-healing model was used to assay migration of human U87MG astrocytoma cells and allowed to monitor calcium signaling during the migration process. The effect of glutamate on calcium signaling was evaluated together with the amount of glutamate released by astrocytoma during cell migration.

RESULTS

We observed that glutamate stimulates motility in serum-starved cells, whereas in the presence of serum, inhibitors of glutamate receptors reduce migration. Migration speed was also reduced in presence of an intracellular calcium chelator. During migration, cells displayed spontaneous Ca(2+) transients. L-THA, an inhibitor of glutamate re-uptake increased the frequency of Ca(2+) oscillations in oscillating cells and induced Ca(2+) oscillations in quiescent cells. The frequency of migration-associated Ca(2+) oscillations was reduced by prior incubation with glutamate receptor antagonists or with an anti-β1 integrin antibody. Application of glutamate induced increases in internal free Ca(2+) concentration ([Ca(2+)]i). Finally we found that compounds known to increase [Ca(2+)]i in astrocytomas such as thapsigagin, ionomycin or the metabotropic glutamate receptor agonist t-ACPD, are able to induce glutamate release.

CONCLUSION

Our data demonstrate that glutamate increases migration speed in astrocytoma cells via enhancement of migration-associated Ca(2+) oscillations that in turn induce glutamate secretion via an autocrine mechanism. Thus, glutamate receptors are further validated as potential targets for astrocytoma cancer therapy.

GABAB receptor activation protects neurons from apoptosis via IGF-1 receptor transactivation

The G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) play key roles in cell-cell communication. Several studies revealed important synergisms between these two types of receptors, with some of the actions of either receptor being mediated through transactivation of the other. Among the large GPCR family, GABA(B) receptor is activated by the neurotransmitter GABA, and is expressed in most neurons where it mediates slow and prolonged inhibition of synaptic transmission. Here we show that this receptor is involved in the regulation of life and death decisions of cerebellar granule neurons (CGNs). We show that specific activation of GABA(B) receptor can protect neurons from apoptosis through a mechanism that involves transactivation of the IGF-1 receptor (IGF-1R). Further work demonstrated that this cross talk was dependent on G(i/o)-protein, PLC, cytosolic Ca(2+), and FAK1 but independent of PKC, while IGF-1R-induced signaling involved Src kinase, PI3 kinase, and Akt activation. These results reveal a new function for this important GPCR and further highlight the importance of functional cross-talk networks between GPCRs and RTKs. Our results reveal GABA(B) receptor as a potential drug target for the treatment of neurodegenerative disorders.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

The activation of MEK-ERK1/2 by glutamate receptor-stimulation is involved in the regulation of RPE proliferation and morphologic transformation

Retinal pigment epithelial (RPE) cells are the main cell type involved in the pathogenesis of proliferative vitreoretinopathy (PVR). As a result from retinal detachment or surgical procedures, RPE comes in contact with glutamate from serum, glial release and the injured retina. The purpose of this study was to explore a possible role for glutamate in the development of PVR, mediated by the receptor-stimulated activation of the ERK1/2 MAPK pathway, the alteration of cell proliferation and the transdifferentiation of RPE cells, using rat RPE cells in culture as a model system. We demonstrated the expression in these cells of Group I metabotropic-and ionotropic AMPA/KA and NMDA glutamate receptors (GluRs), predominantly of the NMDA subtype, which are targeted to the membrane, and exhibit pharmacological and biochemical characteristics equivalent to those previously established in brain tissue. Proliferation was measured by MTS-reduction colorimetric assay, and actin cytoskeleton dynamics was visualized by immunoflurescence using alpha-sma specific antibodies. Activation of metabotropic, AMPA and NMDA receptors by glutamate induced the time-and dose-dependent phosphorylation of ERK1/2, assessed by Western blot analysis, in parallel to a significant increase in cell proliferation and a decrease in alpha-sma expression and its recruitment into stress fibers. These effects were all prevented by the inhibition of MEK. Hence, results suggest that glutamate could be involved in the generation of PVR, through a GluR-mediated increase in proliferation and phenotypic transformation, cause-effect related to the activation of ERK1/2.

Involvement of Ca2+- and cyclic adenosine monophosphate-mediated signaling pathways in photodynamic injury of isolated crayfish neuron and satellite glial cells

To investigate the mechanisms of oxidative injury of neurons and glia, we studied the photodynamic effect on isolated stretch receptor that consists of only two sensory neurons enwrapped by satellite glial cells. Photodynamic therapy (PDT), a potent inducer of oxidative stress, is a prospective method for destruction of brain tumors. PDT induced functional inactivation and necrosis of neurons, necrosis, apoptosis, and proliferation of glial cells. The roles of calmodulin, calmodulin-dependent kinase II, phospholipase C, protein kinases A and C, and phosphodiesterase in these processes were studied by using their inhibitors: fluphenazine, KN-93, D-609, H89, staurosporine, and papaverine, respectively. PDT-induced firing abolishment was enhanced by H89 or papaverine, whereas staurosporine acted oppositely. Fluphenazine or KN-93 reduced necrosis of neurons and glial cells. H89 enhanced necrosis of neurons, whereas staurosporine enhanced necrosis of glial cells. Inhibition of protein kinases A and C enhanced PDT-induced glial apoptosis. Photodynamic gliosis was prevented by KN-93 or staurosporine. These data indicate possible involvement of calmodulin and calmodulin-dependent kinase II in photoinduced necrosis of neurons and glia. Protein kinase C could protect glial cells from necrosis and apoptosis and participate in photoinduced gliosis and loss of neuronal activity. Protein kinase A maintained neuronal firing and protected neurons from photoinduced necrosis and glial cells from apoptosis. Phosphodiesterase reduced necrosis of photosensitized neurons and glia. Thus, Ca(2+)- and cAMP-mediated signaling pathways were involved in photooxidative injury of neurons and glia. Their pharmacological modulation may differently change the efficacy of photodynamic injury of neurons and glial cells.

Activity-Dependent Initiation of a Prolonged Depolarization in Aplysia Bag Cell Neurons: Role for a Cation Channel

The translation of prior activity into changes in excitability is essential for memory and the initiation of behavior. After brief synaptic input, the bag cell neurons of Aplysia californica undergo a nearly 30-min afterdischarge to release egg-laying hormone. The present study examines a prolonged depolarization in cultured bag cell neurons. A 5-Hz, 10-s action potential train elicited a depolarization of about 10 mV, which lasted ≤30 min and was reduced by calmodulin kinase inhibition. Very broad action potentials (resulting from TEA application) decreased prolonged depolarization amplitude, indicating that strong Ca2+ influx did not necessarily promote the response. The prolonged depolarization current ( IPD) was recorded after 5-Hz, 10-s trains of square voltage pulses of varying duration (10–150 ms). Despite Ca2+ influx increasing steadily with pulse duration, IPD was most reliably initiated at 100 ms, suggesting a Ca2+ window or limit exists for triggering IPD. Consistent with this, modestly broader action potentials, evoked by lengthening the train current-pulse duration, resulted in smaller prolonged depolarizations. With respect to the properties of IPD, it displayed a linear current–voltage relationship with a reversal potential of about −45 mV that was shifted to approximately −25 mV by lowering internal K+ or about −56 mV by lowering external Na+ and Ca2+. IPD was blocked by Gd3+, but was not antagonized by MDL-123302A, SKF-96365, 2-APB, tetrodotoxin, or flufenamic acid. Optimal Ca2+ influx may activate calmodulin kinase and a voltage-independent, nonselective cation channel to initiate the prolonged depolarization, thereby contributing to the afterdischarge and reproduction.

Group-I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to extracellular signal-regulated kinase (ERK) activation via distinct, but overlapping, signalling pathways

The coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the extracellular signal-regulated protein kinase (ERK) pathway has been studied in Chinese hamster ovary cell-lines where receptor expression is under inducible control. Both mGlu receptors stimulated comparable, robust and agonist concentration-dependent ERK activations in the CHO cell-lines. The mGlu1a receptor-mediated ERK response was almost completely attenuated by pertussis toxin (PTx) pretreatment, whereas the mGlu5a-ERK response, and the phosphoinositide response to activation of either receptor, was PTx-insensitive. mGlu1a and mGlu5a receptor coupling to ERK occurred via mechanisms independent of phosphoinositide 3-kinase activity and intracellular and/or extracellular Ca2+ concentration. While acute treatment with a protein kinase C (PKC) inhibitor did not attenuate agonist-stimulated ERK activation, down-regulation of PKCs by phorbol ester treatment for 24 h did attenuate both mGlu1a and mGlu5a receptor-mediated responses. Further, inhibition of Src non-receptor tyrosine kinase activity by PP1 attenuated the ERK response generated by both receptor subtypes, but only mGlu1a receptor-ERK activation was attenuated by PDGF receptor tyrosine kinase inhibitor AG1296. These findings demonstrate that, although expressed in a common cell background, these closely related mGlu receptors utilize different G proteins to cause ERK activation and may recruit different tyrosine kinases to facilitate this response.

Metabotropic glutamate receptor 1-induced upregulation of NMDA receptor current: mediation through the Pyk2/Src-family kinase pathway in cortical neurons

The mechanism underlying the upregulation of NMDA receptor function by group I metabotropic glutamate receptors (mGluRs), including mGluR1 and 5, is not known. Here we show that in cortical neurons, brief selective activation of group I mGluRs with (S)-3,5-dihydroxy-phenylglycine (DHPG) induced a Ca(2+)-calmodulin-dependent activation of Pyk2/CAKbeta and the Src-family kinases Src and Fyn that was independent of protein kinase C (PKC). Activation of Pyk2 and Src/Fyn kinases led to increased tyrosine phosphorylation of NMDA receptor subunits 2A and B (NR2A/B) and was blocked by a selective mGluR1 antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester, but not an mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine. Functional linkage between mGluR1 activation and NR2A tyrosine phosphorylation through Pyk2 and Src was also demonstrated after expression of these elements in human embryonic kidney 293 cells. Supporting functional consequences, selective activation of mGluR1 by DHPG induced a potentiation of NMDA receptor-mediated currents that was blocked by inhibiting mGluR1 or Src-family kinases. Furthermore, antagonizing calmodulin or mGluR1, but not PKC, reduced the basal tyrosine phosphorylation levels of Pyk2 and Src, suggesting that mGluR1 may control the basal activity of these kinases and thus the tyrosine phosphorylation levels of NMDA receptors.

Interaction between metabotropic glutamate receptor 7 and alpha tubulin

Metabotropic glutamate receptors (mGluRs) mediate a variety of responses to glutamate in the central nervous system. A primary role for group-III mGluRs is to inhibit neurotransmitter release from presynaptic terminals, but the molecular mechanisms that regulate presynaptic trafficking and activity of group-III mGluRs are not well understood. Here, we describe the interaction of mGluR7, a group-III mGluR and presynaptic autoreceptor, with the cytoskeletal protein, alpha tubulin. The mGluR7 carboxy terminal (CT) region was expressed as a GST fusion protein and incubated with rat brain extract to purify potential mGluR7-interacting proteins. These studies yielded a single prominent mGluR7 CT-associated protein of 55 kDa, which subsequent microsequencing analysis revealed to be alpha tubulin. Coimmunoprecipitation assays confirmed that full-length mGluR7 and alpha tubulin interact in rat brain as well as in BHK cells stably expressing mGluR7a, a splice variant of mGluR7. In addition, protein overlay experiments showed that the CT domain of mGluR7a binds specifically to purified tubulin and calmodulin, but not to bovine serum albumin. Further pull-down studies revealed that another splice variant mGluR7b also interacts with alpha tubulin, indicating that the binding region is not localized to the splice-variant regions of either mGluR7a (900-915) or mGluR7b (900-923). Indeed, deletion mutagenesis experiments revealed that the alpha tubulin-binding site is located within amino acids 873-892 of the mGluR7 CT domain, a region known to be important for regulation of mGluR7 trafficking. Interestingly, activation of mGluR7a in cells results in an immediate and significant decrease in alpha tubulin binding. These data suggest that the mGluR7/alpha tubulin interaction may provide a mechanism to control access of the CT domain to regulatory molecules, or alternatively, that this interaction may lead to morphological changes in the presynaptic membrane in response to receptor activation.