Involvement of CaM kinase II in gonadotropin-releasing hormone-induced activation of MAP kinase in cultured hypothalamic neurons

Calcium/Calmodulin-Dependent Kinases in the Hypothalamus, Pituitary, and Pineal Gland: An Overview

We review the literature on the little-known roles of specific CaMKs in regulating endocrine functions of the pineal gland, the pituitary gland, and the hypothalamus. Melatonin activates hippocampal CaMKII, which then influences dendritogenesis. In the pituitary gland, the signal pathways activated by the CaMK in lower vertebrates, such as fishes, differ from those of mammals. In the teleost anterior pituitary, the activation of CaMKII induces the expression of somatolactin by glucagon b. In rats and humans, CaMKIVs have been associated with gonadotropes and thyrotropes and CaMKII with several types of human tumor cells and with a specific signaling pathway. Neuropeptides such as vasopressin and endothelin are also involved in the CaMKII signaling chain, as is the CaMKIIδ isoform which participates in generating the circadian rhythms of the suprachiasmatic nucleus. What arises from this review is that most of the hypothalamic CaMKs are involved in activities of the endocrine brain. Furthermore, among the CaMKs, type II occurs with the highest frequency followed by CaMKIV and CaMKI.

Roles of Pyk2 in signal transduction after gonadotropin-releasing hormone receptor stimulation

The receptor for gonadotropin-releasing hormone (GnRH) is highly expressed in hypothalamic neurons. It has been reported that GnRH treatment of cultured GnRH neurons (GT1-7 cells) activated proline-rich tyrosine kinase 2 (Pyk2), and Pyk2 was involved in the activation of extracellular signal-regulated protein kinase 1 (ERK1) and ERK2 (ERK1/2). In the present study, we first examined the possibility that GnRH treatment might activate epidermal growth factor receptor (EGFR). We found that activation of EGFR after GnRH treatment for 5 min was much less than after EGF or heparin-binding EGF treatment. Next, we examined whether or not Pyk2 bound to growth factor receptor-binding protein 2 (Grb2). We overexpressed FLAG-fused Pyk2 in GT1-7 cells, and immunoprecipitated Pyk2 using an anti-FLAG antibody. The binding of Pyk2 to Grb2 was detected only after GnRH treatment. In contrast, a site-directed mutant of Pyk2 wherein tyrosine 881 was mutated to phenylalanine did not bind to Grb2. Studies with small interfering RNA and inhibitors indicated that the activation of Grb2/Ras/Raf/MEK was a major pathway to ERK1/2 activation after the short-term treatment of GT1-7 cells with GnRH.

Up-regulation of DUSP5 and DUSP6 by gonadotropin-releasing hormone in cultured hypothalamic neurons, GT1-7 cells

Gonadotropin-releasing hormone (GnRH) is secreted from hypothalamic neurons (GnRH neurons) and stimulates anterior pituitary gonadotrophs to synthesize and secrete gonadotropins. In addition to gonadotrophs, GnRH neurons also express GnRH receptors, and the autocrine action of GnRH is reportedly involved in the regulation of functions of GnRH neurons. There is accumulating evidence that extracellular signal-regulated kinase (ERK), one of mitogen-activated protein kinases (MAPKs), is activated by GnRH and involved in various effects of GnRH in GnRH neurons. In the present study, we performed microarray analysis to examine the types of genes whose expression was regulated by GnRH in immortalized mouse GnRH neurons (GT1-7 cells). We found that 257 genes among 55,681 genes examined were up-regulated after 30-min treatment of GT1-7 cells with GnRH. These up-regulated genes included four dual-specificity MAPK phosphatases (DUSPs), DUSP1, DUSP2, DUSP5, and DUSP6. Reverse transcription-polymerase chain reaction analysis confirmed that the mRNA levels of DUSP5 and DUSP6 were robustly increased within 30 min. U0126, an inhibitor of ERK activation, completely inhibited the increases in the mRNA levels of DUSP5 and DUSP6. Immunoblotting analysis revealed that ERK activation peaked at 5 min and declined steeply at 60 min, whereas DUSP5 and DUSP6 proteins were increased from 60 min. It was notable that down-regulation of DUSP6 augmented GnRH-induced ERK activation approximately 1.7-fold at 60 min. These results suggested that the up-regulation of DUSP6 regulates the duration of ERK activation at least in part.

Celastrol Attenuates Cadmium-Induced Neuronal Apoptosis via Inhibiting Ca2+ -CaMKII-Dependent Akt/mTOR Pathway

Cadmium (Cd), an environmental and industrial pollutant, affects the nervous system and consequential neurodegenerative disorders. Recently, we have shown that celastrol prevents Cd-induced neuronal cell death partially by suppressing Akt/mTOR pathway. However, the underlying mechanism remains to be elucidated. Here, we show that celastrol attenuated Cd-elevated intracellular-free calcium ([Ca²⁺ ]_i ) level and apoptosis in neuronal cells. Celastrol prevented Cd-induced neuronal apoptosis by inhibiting Akt-mediated mTOR pathway, as inhibition of Akt with Akt inhibitor X or ectopic expression of dominant negative Akt reinforced celastrol's prevention of Cd-induced phosphorylation of S6K1/4E-BP1 and cell apoptosis. Furthermore, chelating intracellular Ca²⁺ with BAPTA/AM or preventing [Ca²⁺ ]_i elevation using EGTA potentiated celastrol's repression of Cd-induced [Ca²⁺ ]_i elevation and consequential activation of Akt/mTOR pathway and cell apoptosis. Moreover, celastrol blocked Cd-elicited phosphorylation of CaMKII, and pretreatment with BAPTA/AM or EGTA enhanced celastrol's suppression of Cd-increased phosphorylation of CaMKII in neuronal cells, implying that celastrol hinders [Ca²⁺ ]_i -mediated CaMKII phosphorylation. Inhibiting CaMKII with KN93 or silencing CaMKII attenuated Cd activation of Akt/mTOR pathway and cell apoptosis, and this was strengthened by celastrol. Taken together, these data demonstrate that celastrol attenuates Cd-induced neuronal apoptosis via inhibiting Ca²⁺ -CaMKII-dependent Akt/mTOR pathway. Our findings underscore that celastrol may act as a neuroprotective agent for the prevention of Cd-induced neurodegenerative disorders. J. Cell. Physiol. 232: 2145-2157, 2017. © 2016 Wiley Periodicals, Inc.

Crosstalk between Ca2+ signaling and mitochondrial H2O2 is required for rotenone inhibition of mTOR signaling pathway leading to neuronal apoptosis

Rotenone, a neurotoxic pesticide, induces loss of dopaminergic neurons related to Parkinson's disease. Previous studies have shown that rotenone induces neuronal apoptosis partly by triggering hydrogen peroxide (H₂O₂)-dependent suppression of mTOR pathway. However, the underlying mechanism is not fully understood. Here, we show that rotenone elevates intracellular free calcium ion ([Ca²⁺]_i) level, and activates CaMKII, resulting in inhibition of mTOR signaling and induction of neuronal apoptosis. Chelating [Ca²⁺]_i with BAPTA/AM, preventing extracellular Ca²⁺ influx using EGTA, inhibiting CaMKII with KN93, or silencing CaMKII significantly attenuated rotenone-induced H₂O₂ production, mTOR inhibition, and cell death. Interestingly, using TTFA, antimycin A, catalase or Mito-TEMPO, we found that rotenone-induced mitochondrial H₂O₂ also in turn elevated [Ca²⁺]_i level, thereby stimulating CaMKII, leading to inhibition of mTOR pathway and induction of neuronal apoptosis. Expression of wild type mTOR or constitutively active S6K1, or silencing 4E-BP1 strengthened the inhibitory effects of catalase, Mito-TEMPO, BAPTA/AM or EGTA on rotenone-induced [Ca²⁺]_i elevation, CaMKII phosphorylation and neuronal apoptosis. Together, the results indicate that the crosstalk between Ca²⁺ signaling and mitochondrial H₂O₂ is required for rotenone inhibition of mTOR-mediated S6K1 and 4E-BP1 pathways. Our findings suggest that how to control over-elevation of intracellular Ca²⁺ and overproduction of mitochondrial H₂O₂ may be a new approach to deal with the neurotoxicity of rotenone.

Involvement of Protein Kinase D1 in Signal Transduction from the Protein Kinase C Pathway to the Tyrosine Kinase Pathway in Response to Gonadotropin-releasing Hormone

The receptor for gonadotropin-releasing hormone (GnRH) belongs to the G protein-coupled receptors (GPCRs), and its stimulation activates extracellular signal-regulated protein kinase (ERK). We found that the transactivation of ErbB4 was involved in GnRH-induced ERK activation in immortalized GnRH neurons (GT1-7 cells). We found also that GnRH induced the cleavage of ErbB4. In the present study, we examined signal transduction for the activation of ERK and the cleavage of ErbB4 after GnRH treatment. Both ERK activation and ErbB4 cleavage were completely inhibited by YM-254890, an inhibitor of Gq/11 proteins. Down-regulation of protein kinase C (PKC) markedly decreased both ERK activation and ErbB4 cleavage. Experiments with two types of PKC inhibitors, Gö 6976 and bisindolylmaleimide I, indicated that novel PKC isoforms but not conventional PKC isoforms were involved in ERK activation and ErbB4 cleavage. Our experiments indicated that the novel PKC isoforms activated protein kinase D (PKD) after GnRH treatment. Knockdown and inhibitor experiments suggested that PKD1 stimulated the phosphorylation of Pyk2 by constitutively activated Src and Fyn for ERK activation. Taken together, it is highly possible that PKD1 plays a critical role in signal transduction from the PKC pathway to the tyrosine kinase pathway. Activation of the tyrosine kinase pathway may be involved in the progression of cancer.

Desensitization by different strategies of epidermal growth factor receptor and ErbB4

Four transmembrane tyrosine kinases constitute the ErbB protein family: epidermal growth factor receptor (EGFR) or ErbB1, ErbB2, ErbB3, and ErbB4. In general, the structure and mechanism of the activation of these members are similar. However, significant differences in homologous desensitization are known between EGFR and ErbB4. Desensitization of ligand-occupied EGFR occurs by endocytosis, while that of ErbB4 occurs by selective cleavage at the cell surface. Because ErbB4 is abundantly expressed in neurons from fetal to adult brains, elucidation of the desensitization mechanism is important to understand neuronal development and synaptic functions. Recently, it has become clear that heterologous desensitization of EGFR and ErbB4 are induced by endocytosis and cleavage, respectively, similar to homologous desensitization. It has been reported that heterologous desensitization of EGFR is induced by serine phosphorylation of EGFR via the p38 mitogen-activated protein kinase (p38 MAP kinase) pathway in various cell lines, including alveolar epithelial cells. In contrast, the protein kinase C pathway is involved in ErbB4 cleavage. In this review, we will describe recent advances in the desensitization mechanisms of EGFR and ErbB4, mainly in alveolar epithelial cells and hypothalamic neurons, respectively.

hsBAFF promotes proliferation and survival in cultured B lymphocytes via calcium signaling activation of mTOR pathway

B-cell activating factor of the TNF family (BAFF, also called BLyS, TALL-1, THANK, or zTNF4) has revealed its critical function in B lymphocyte proliferation and survival, as well as the pathogenesis of autoimmune disease. However, the molecular mechanisms of excess BAFF-extended aggressive B lymphocytes have not been completely defined. Here we show that excessive hsBAFF-elevated [Ca(2+)]i activated mammalian target of rapamycin (mTOR) signaling pathway, leading to proliferation and survival in B lymphocytes. This is supported by the findings that intracellular Ca(2+) chelator (BAPTA/AM) or mTOR inhibitor (rapamycin) abolished the events. Sequentially, we observed that preventing [Ca(2+)]i elevation using EGTA or 2-APB dramatically inhibited hsBAFF activation of mTOR signaling, as well as cell growth and survival, suggesting that hsBAFF-induced extracellular Ca(2+) influx and ER Ca(2+) release elevates [Ca(2+)]i contributing to B lymphocyte proliferation and survival via activation of mTOR signaling. Further, we noticed that pretreatment with BAPTA/AM, EGTA or 2-APB blocked hsBAFF-increased phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII), and inhibiting CaMKII with KN93 attenuated hsBAFF-activated mTOR signaling, as well as cell growth and survival, revealing that the effects of hsBAFF-elevated [Ca(2+)]i on mTOR signaling as well as proliferation and survival in B lymphocytes is through stimulating phosphorylation of CaMKII. The results indicate that hsBAFF activates mTOR pathway triggering B lymphocyte proliferation and survival by calcium signaling. Our findings suggest that manipulation of intracellular Ca(2+) level or CaMKII and mTOR activity may be exploited for the prevention of excessive BAFF-induced aggressive B lymphocyte disorders and autoimmune diseases.

Induction of epithelial-mesenchymal transition by flagellin in cultured lung epithelial cells

Toll-like receptor 5 (TLR5) recognizes bacterial flagellin and activates host inflammatory responses, mainly through activation of the NF-κB pathway. Although pulmonary fibrosis occurs in some cases of lung infection by flagellated bacteria, the pathological roles of TLR5 stimulation in pulmonary fibrosis have yet to be elucidated. In the present study, we first confirmed that flagellin activated the NF-κB pathway in cultured A549 alveolar epithelial cells. Next, we examined the types of genes whose expression was modulated by flagellin in the cells. Microarray analysis of gene expression indicated that flagellin induced a change in gene expression that had a similar trend to transforming growth factor-β1 (TGF-β1), a key factor in the induction of epithelial-mesenchymal transition (EMT). Biochemical analysis revealed that TGF-β1 and flagellin increased the level of fibronectin protein, while they reduced the level of E-cadherin protein after 30 h of treatment. Interestingly, simultaneous treatment with TGF-β1 and flagellin significantly augmented these EMT-related changes. Flagellin strongly activated p38 MAP kinase, and the activation was sustained for longer than 30 h. SB203580, an inhibitor of p38 MAP kinase, inhibited the upregulation of fibronectin by both flagellin and TGF-β1. Simultaneous treatment with TGF-β1 and flagellin augmented the activation of p38 MAP kinase by TGF-β1 or flagellin alone. These results strongly suggest that flagellin cooperates with TGF-β1 in the induction of EMT in alveolar epithelial cells.

CaMKII is involved in cadmium activation of MAPK and mTOR pathways leading to neuronal cell death

Cadmium (Cd), a toxic environmental contaminant, induces neurodegenerative diseases. Recently, we have shown that Cd elevates intracellular free calcium ion ([Ca(2+) ](i) ) level, leading to neuronal apoptosis partly by activating mitogen-activated protein kinases (MAPK) and mammalian target of rapamycin (mTOR) pathways. However, the underlying mechanism remains to be elucidated. In this study, we show that the effects of Cd-elevated [Ca(2+) ](i) on MAPK and mTOR network as well as neuronal cell death are through stimulating phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII). This is supported by the findings that chelating intracellular Ca(2+) with 1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester or preventing Cd-induced [Ca(2+) ](i) elevation using 2-aminoethoxydiphenyl borate blocked Cd activation of CaMKII. Inhibiting CaMKII with KN93 or silencing CaMKII attenuated Cd activation of MAPK/mTOR pathways and cell death. Furthermore, inhibitors of mTOR (rapamycin), c-Jun N-terminal kinase (SP600125) and extracellular signal-regulated kinase 1/2 (U0126), but not of p38 (PD169316), prevented Cd-induced neuronal cell death in part through inhibition of [Ca(2+) ](i) elevation and CaMKII phosphorylation. The results indicate that Cd activates MAPK/mTOR network triggering neuronal cell death, by stimulating CaMKII. Our findings underscore a central role of CaMKII in the neurotoxicology of Cd, and suggest that manipulation of intracellular Ca(2+) level or CaMKII activity may be exploited for prevention of Cd-induced neurodegenerative disorders.