Requirement of pyk2 for the activation of the MAP kinase cascade induced by Ca(2+) (but not by PKC or G protein) in PC12 cells

Ganglioside GQ1b induces dopamine release through the activation of Pyk2

Growing evidence indicates that GQ1b, one of the gangliosides members, contributes to synaptic transmission and synapse formation. Previous studies have shown that GQ1b could enhance depolarization induced neurotransmitter release, while the role of GQ1b in asynchronous release is still largely unknown. Here in our result, we found low concentration of GQ1b, but not GT1b or GD1b (which were generated from GQ1b by plasma membrane-associated sialidases), evoked asynchronous dopamine (DA) release from both clonal rat pheochromocytoma PC12 cells and rat striatal slices significantly. The release peaked at 2 min after GQ1b exposure, and lasted for more than 6 min. This effect was caused by the enhancement of intracellular Ca(2+) and the activation of Pyk2. Inhibition of Pyk2 by PF-431396 (a dual inhibitor of Pyk2 and FAK) or Pyk2 siRNA abolished DA release induced by GQ1b. Moreover, Pyk2 Y402, but not other tyrosine site, was phosphorylated at the peaking time. The mutant of Pyk2 Y402 (Pyk2-Y402F) was built to confirm the essential role of Y402 activation. Further studies revealed that activated Pyk2 stimulated ERK1/2 and p-38, while only the ERK1/2 activation was indispensable for GQ1b induced DA release, which interacted with Synapsin I directly and led to its phosphorylation, then depolymerization of F-actin, thus contributed to DA release. In conclusion, low concentration of GQ1b is able to enhance asynchronous DA release through Pyk2/ERK/Synapsin I/actin pathway. Our findings provide new insights into the role of GQ1b in neuronal communication, and implicate the potential application of GQ1b in neurological disorders.

Postsynaptic clustering and activation of Pyk2 by PSD-95

The tyrosine kinase Pyk2 plays a unique role in intracellular signal transduction by linking Ca(2+) influx to tyrosine phosphorylation, but the molecular mechanism of Pyk2 activation is unknown. We report that Pyk2 oligomerization by antibodies in vitro or overexpression of PSD-95 in PC6-3 cells induces trans-autophosphorylation of Tyr402, the first step in Pyk2 activation. In neurons, Ca(2+) influx through NMDA-type glutamate receptors causes postsynaptic clustering and autophosphorylation of endogenous Pyk2 via Ca(2+)- and calmodulin-stimulated binding to PSD-95. Accordingly, Ca(2+) influx promotes oligomerization and thereby autoactivation of Pyk2 by stimulating its interaction with PSD-95. We show that this mechanism of Pyk2 activation is critical for long-term potentiation in the hippocampus CA1 region, which is thought to underlie learning and memory.

UTP and ATP increase extracellular signal-regulated kinase 1/2 phosphorylation in bovine chromaffin cells through epidermal growth factor receptor transactivation

Adenosine triphosphate (ATP) is coreleased with catecholamines from adrenal medullary chromaffin cells in response to sympathetic nervous system stimulation and may regulate these cells in an autocrine or paracrine manner. Increases in extracellular signal-regulated kinase (ERK) 1/2 phosphorylation were observed in response to ATP stimulation of bovine chromaffin cells. The signaling pathway involved in ATP-mediated ERK1/2 phosphorylation was investigated via Western blot analysis. ATP and uridine 5′-triphosphate (UTP) increased ERK1/2 phosphorylation potently, peaking between 5 and 15 min. The mitogen-activated protein kinase (MAPK/ERK)-activating kinase (MEK) inhibitor PD98059 blocked this response. UTP, which is selective for G-protein-coupled P2Y receptors, was the most potent agonist among several nucleotides tested. Adenosine 5′-O-(3-thio) triphosphate (ATPγS) and ATP were also potent agonists, characteristic of the P2Y₂ or P2Y₄ receptor subtypes, whereas agonists selective for P2X receptors or other P2Y receptor subtypes were weakly effective. The receptor involved was further characterized by the nonspecific P2 antagonists suramin and reactive blue 2, which each partially inhibited ATP-mediated ERK1/2 phosphorylation. Inhibitors of protein kinase C (PKC), protein kinase A (PKA), Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and phosphoinositide-3 kinase (PI3K) had no effect on ATP-mediated ERK1/2 phosphorylation. The Src inhibitor PP2, epidermal growth factor receptor (EGFR) inhibitor AG1478, and metalloproteinase inhibitor GM6001 decreased ATP-mediated ERK1/2 phosphorylation. These results suggest nucleotide-mediated ERK1/2 phosphorylation is mediated by a P2Y₂ or P2Y₄ receptor, which stimulates metalloproteinase-dependent transactivation of the EGFR.

Depolarization activates ERK and proline-rich tyrosine kinase 2 (PYK2) independently in different cellular compartments in hippocampal slices

In the hippocampus, extracellular signal-regulated kinase (ERK) and the non-receptor protein proline-rich tyrosine kinase 2 (PYK2) are activated by depolarization and involved in synaptic plasticity. Both are also activated under pathological conditions following ischemia, convulsions, or electroconvulsive shock. Although in non-neuronal cells PYK2 activates ERK through the recruitment of Src-family kinases (SFKs), the link between these pathways in the hippocampus is not known. We addressed this question using K(+)-depolarized rat hippocampal slices. Depolarization increased the phosphorylation of PYK2, SFKs, and ERK. These effects resulted from Ca(2+) influx through voltage-gated Ca(2+) channels and were diminished by GF109203X, a protein kinase C inhibitor. Inhibition of SFKs with PP2 decreased PYK2 tyrosine phosphorylation dramatically, but not its autophosphorylation on Tyr-402. Moreover, PYK2 autophosphorylation and total tyrosine phosphorylation were profoundly altered in fyn-/- mice, revealing an important functional relationship between Fyn and PYK2 in the hippocampus. In contrast, ERK activation was unaltered by PP2, Fyn knock-out, or LY294002, a phosphatidyl-inositol-3-kinase inhibitor. ERK activation was prevented by MEK inhibitors that had no effect on PYK2. Immunofluorescence of hippocampal slices showed that PYK2 and ERK were activated in distinct cellular compartments in somatodendritic regions and nerve terminals, respectively, with virtually no overlap. Activation of ERK was critical for the rephosphorylation of a synaptic vesicle protein, synapsin I, following depolarization, underlining its functional importance in nerve terminals. Thus, in hippocampal slices, in contrast to cell lines, depolarization-induced activation of non-receptor tyrosine kinases and ERK occurs independently in distinct cellular compartments in which they appear to have different functional roles.

Calcium induces cell survival and proliferation through the activation of the MAPK pathway in a human hormone-dependent leukemia cell line, TF-1

Survival and proliferation of cells of a human myelo-erythroid CD34+ leukemia cell line (TF-1) depend on the presence of granulocyte-macrophage colony-stimulating factor or interleukin-3. Upon hormone withdrawal these cells stop proliferating and undergo apoptotic process. In this report we demonstrate that a controlled increase in [Ca2+]i induces hormone-independent survival and proliferation of TF-1 cells. We found that moderate elevation of [Ca2+]i by the addition of cyclopiasonic-acid protected TF1 cells from apoptosis. Furthermore, a higher, but transient elevation of [Ca2+]i by ionomycin treatment induced cell proliferation. In both cases caspase-3 activity was reduced, and Bcl-2 was up-regulated. Higher elevation of [Ca2+]i by ionomycin induced MEK-dependent biphasic ERK1/2 activation, sufficient to move the cells from G0/G1 to S/M phases. Meanwhile, activation of ERK1/2, phosphorylation of the Elk-1 transcription factor, and, consequently, a substantial elevation of Egr-1 and c-Fos levels and AP-1 DNA binding were observed. Moderate elevation of [Ca2+]i, on the other hand, caused a delayed monophasic activation of ERK1/2 and Elk-1 that was accompanied with only a small increase of Egr-1 and c-Fos levels and AP-1 DNA binding. The specific MEK-1 kinase inhibitor, PD98059, inhibited all the effects of increasing [Ca2+]i, indicating that the MAPK/ERK pathway activation is essential for TF-1 cell survival and proliferation. Based on these results we suggest that the elevation of the [Ca2+]i may influence the cytokine dependence of hemopoietic progenitors and may contribute to pathological hematopoiesis.

Stimulation of the ERK pathway by GTP-loaded Rap1 requires the concomitant activation of Ras, protein kinase C, and protein kinase A in neuronal cells

The small GTPases Ras or Rap1 were suggested to mediate the stimulatory effect of some G protein-coupled receptors on ERK activity in neuronal cells. Accordingly, we reported here that pituitary adenylate cyclase-activating polypeptide (PACAP), whose G protein-coupled receptor triggers neuronal differentiation of the PC12 cell line via ERK1/2 activation, transiently activated Ras and induced the sustained GTP loading of Rap1. Ras mediated peak stimulation of ERK by PACAP, whereas Rap1 was necessary for the sustained activation phase. However, PACAP-induced GTP-loading of Rap1 was not sufficient to account for ERK activation by PACAP because 1) PACAP-elicited Rap1 GTP-loading depended only on phospholipase C, whereas maximal stimulation of ERK by PACAP also required the activity of protein kinase A (PKA), protein kinase C (PKC), and calcium-dependent signaling; and 2) constitutively active mutants of Rap1, Rap1A-V12, and Rap1B-V12 only minimally stimulated the ERK pathway compared with Ras-V12. The effect of Rap1A-V12 was dramatically potentiated by the concurrent activation of PKC, the cAMP pathway, and Ras, and this potentiation was blocked by dominant-negative mutants of Ras and Raf. Thus, this set of data indicated that GPCR-elicited GTP loading of Rap1 was not sufficient to stimulate efficiently ERK in PC12 cells and required the permissive co-stimulation of PKA, PKC, or Ras.

Co-requirement of cyclic AMP- and calcium-dependent protein kinases for transcriptional activation of cholecystokinin gene by protein hydrolysates

Little is known about the mechanisms by which protein-derived nutrients regulate hormone gene expression in the intestine. We have previously reported that protein hydrolysates (i.e. peptones), which are representative of the protein fraction in the lumen, increased cholecystokinin (CCK) gene transcription in the STC-1 enteroendocrine cell line. In the present work, we examined the intracellular events evoked by peptones to stimulate CCK gene transcription. In STC-1 cells, peptones stimulated cyclic AMP production and protein kinase A (PKA) activity. This was associated with a nuclear translocation of the PKA catalytic subunit and with a PKA-dependent phosphorylation of the CRE-binding protein (CREB) at Ser(133). Using transient transfection experiments and reporter luciferase assays, we show that peptone-stimulated transcriptional activity of the CCK gene promoter was significantly decreased when the PKA pathway was inhibited. Furthermore, the intracellular calcium chelator 1,2-bis-(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-tetra(acetoxymethyl)ester completely inhibited peptone-induced stimulation of the CCK gene promoter activity, phosphorylation of CREB, and PKA activity. Peptones increased, in a calcium-dependent manner, the phosphorylation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) and the MEK inhibitor PD98059 decreased the peptone-induced stimulation of CCK gene promoter activity. This stimulation was also reduced by 30% in the presence of the calcium/calmodulin-dependent protein kinase (CaMK) inhibitor KN-93. Total inhibition was obtained when the PKA, ERK, and CaMK pathways were simultaneously blocked with appropriate inhibitors to these pathways. These results demonstrate the simultaneous involvement of cAMP- and calcium-dependent protein kinases in the stimulation of intestinal CCK gene transcription by protein-derived nutrients.