Cardiotoxin III suppresses hepatocyte growth factor–stimulated migration and invasion of MDA‐MB‐231 cells

The hepatocyte growth factor (HGF)/c‐Met signalling pathway is deregulated in most cancers and associated with a poor prognosis in breast cancer. Cardiotoxin III (CTX III), a basic polypeptide isolated from Naja naja atra venom, has been shown to exhibit anticancer activity. In this study, we use HGF as an invasive inducer to investigate the effect of CTX III on MDA‐MB‐231 cells. When cells were treated with non‐toxic doses of CTX III, CTX III inhibited the HGF‐promoted cell migration and invasion. CTX III significantly suppressed the HGF‐induced c‐Met phosphorylation and downstream activation of phosphatidylinositol 3‐kinase (PI3k)/Akt and extracellular signal‐regulated kinase (ERK) 1/2. Additionally, CTX III similar to wortmannin (a PI3K inhibitor) and U0126 (an upstream kinase regulating ERK1/2 inhibitor) attenuated cell migration and invasion induced by HGF. This effect was paralleled by a significant reduction in phosphorylation of IκBα kinase and IκBα and nuclear translocation of nuclear factor κB (NF‐κB) as well as a reduction of matrix metalloproteinase‐9 (MMP‐9) activity. Furthermore, the c‐Met inhibitor PHA665752 inhibited HGF‐induced MMP‐9 expression, cell migration and invasion, as well as the activation of ERK1/2 and PI3K/Akt, suggesting that ERK1/2 and PI3K/Akt activation occurs downstream of c‐Met activation. Taken together, these findings suggest that CTX III inhibits the HGF‐induced invasion and migration of MDA‐MB‐231 cells via HGF/c‐Met‐dependent PI3K/Akt, ERK1/2 and NF‐κB signalling pathways, leading to the downregulation of MMP‐9 expression. Copyright © 2014 John Wiley & Sons, Ltd.     

Geranylgeranyltransferase I mediates BDNF‐induced synaptogenesis

Geranylgeranyltransferase I (GGT) is a prenyltransferase that mediates lipid modification of Rho small GTPases, such as Rho, Rac, and Cdc42, which are important for neuronal synaptogenesis. Although GGT is expressed in brain extensively, the function of GGT in central nerves system is largely unknown so far. We have previously demonstrated that GGT promotes the basal and neuronal activity and brain‐derived neurotrophic factor (BDNF)‐induced dendritic morphogenesis of cultured hippocampal neurons and cerebellar slices. This study is to explore the function and mechanism of GGT in neuronal synaptogenesis. We found that the protein level and activity of GGT gradually increased in rat hippocampus from P7 to P28 and subcellular located at synapse of neurons. The linear density of Synapsin 1 and post‐synaptic density protein 95 increased by over‐expression of GGT β, while reduced by inhibition or down‐regulation of GGT. In addition, GGT and its known substrate Rac was activated by BDNF, which promotes synaptogenesis in cultured hippocampal neurons. Furthermore, BDNF‐induced synaptogenesis was eliminated by GGT inhibition or down‐regulation, as well as by non‐prenylated Rac1 over‐expression. Together, our data suggested that GGT mediates BDNF‐induced neuronal synaptogenesis through Rac1 activation.     

Combined signaling through ERK, PI3K/AKT, and RAC1/p38 is required for met-triggered cortical neuron migration

Cell migration is a complex biological process playing a key role in physiological and pathological conditions. During central nervous system development, positioning and function of cortical neurons is tightly regulated by cell migration. Recently, signaling events involving the urokinase-type plasminogen activator receptor, which is a key regulator for the activation of hepatocyte growth factor (HGF), have been implicated in modulating cortical neuron migration. However, the intracellular pathways controlling neuronal migration triggered by the HGF receptor Met have not been elucidated. By combining pharmacological and genetic approaches, we show here that the Ras/ERK pathway and phosphatidylinositol 3-kinase (PI3K) are both required for cortical neuron migration. By dissecting the downstream signals necessary for this event, we found that Rac1/p38 and Akt are required, whereas the c-Jun N-terminal kinase (JNK) and mTOR/p70(s6k) pathways are dispensable. This study demonstrates that concomitant activation of the Ras/ERK, PI3K/Akt, and Rac1/p38 pathways is required to achieve full capacity of cortical neurons to migrate upon HGF stimulation.     

Activation of adenosine A2A receptor up-regulates BDNF expression in rat primary cortical neurons

As a member of neurotrophin family, brain derived neurotrophic factor (BDNF) plays critical roles in neuronal development, differentiation, synaptogenesis, and neural protection from the harmful stimuli. There have been reported that adenosine A2(A) receptor subtype is widely distributed in the brain regions, such as hippocampus, striatum, and cortex. Adenosine A2(A) receptor is colocalized with BDNF in brain regions and the functional interaction between A2(A) receptor stimulation and BDNF action has been suggested. In this study, we investigated the possibility that the activation of A2(A) receptor modulates BDNF production in rat primary cortical neuron. CGS21680, an adenosine A2(A) receptor agonist, induced BDNF expression and release. An antagonist against A2(A) receptor, ZM241385, prevented CGS21680-induced increase in BDNF production. A2(A) receptor stimulation induced the activation of Akt-GSK-3β signaling pathway and the blockade of the signaling pathway with specific inhibitors abolished the increase in BDNF production, possibly via modulation of ERK1/2-CREB pathway. The physiological roles of A2(A) receptor-induced BDNF production was demonstrated by the protection of neurons from the excitotoxicity and increased neurite extension as well as synapse formation from immature and mature neurons. Taken together, activation of A2(A) receptor regulates BDNF production in rat cortical neuron, which provides neuro-protective action.     

Hepatocyte growth factor and c-Met promote dendritic maturation during hippocampal neuron differentiation via the Akt pathway

During central nervous system development, growth factors and their associated receptor protein tyrosine kinases regulate many neuronal functions such as neurite extension and dendrite maturation. Hepatocyte growth factor (HGF) and its receptor, c-Met, can promote formation of neurites and enhance elaboration of dendrites in mature neurons, but their effects on the early stages of dendrite maturation in hippocampal neurons and the signaling pathways by which they promote dendrite formation have not been studied. Exogenous HGF treatment effectively enhanced the phosphorylation and activation of c-Met in cultured hippocampal neurons at 4 days in vitro. HGF treatment increased the number of dendrites and promoted dendrite elongation in these neurons. Consistent with these results, HGF activated Akt, which phosphorylates glycogen synthase kinase-3beta (GSK-3beta) to inactivate it, and reduced phosphorylation of microtubule-associated protein 2 (MAP2), which can promote microtubule polymerization and dendrite elongation when dephosphorylated. Conversely, pharmacological inhibition of c-Met with its specific inhibitor, PHA-665752, or genetic knock-down of c-Met with short hairpin RNAs (shRNAs) suppressed HGF-induced phosphorylation of Akt and GSK-3beta, increased phosphorylation of MAP2, and reduced dendrite number and length in cultured hippocampal neurons. Moreover, suppressing c-Met with PHA-665752 or by shRNA decreased MAP2 expression. Inhibiting Akt activity with the phosphoinositide-3-kinase inhibitor LY294002 or Akt inhibitor X suppressed HGF-induced phosphorylation of GSK-3beta, increased MAP2 phosphorylation, and blocked the ability of HGF to enhance dendritic length. These observations indicate that HGF and c-Met can regulate the early stages of dendrite maturation via activation of the Akt/GSK-3beta pathway.     

HGF regulates the development of cortical pyramidal dendrites

Although hepatocyte growth factor (HGF) and its receptor tyrosine kinase MET are widely expressed in the developing and mature central nervous system, little is known about the role of MET signaling in the brain. We have used particle-mediated gene transfer in cortical organotypic slice cultures established from early postnatal mice to study the effects of HGF on the development of dendritic arbors of pyramidal neurons. Compared with untreated control cultures, exogenous HGF promoted a highly significant increase in dendritic growth and branching of layer 2 pyramidal neurons, whereas inactivation of endogenous HGF with function-blocking, anti-HGF antibody caused a marked reduction in size and complexity of the dendritic arbors of these neurons. Furthermore, pyramidal neurons transfected with an MET dominant-negative mutant receptor likewise had much smaller and less complex dendritic arbors than did control transfected neurons. Our results indicate that HGF plays a role in regulating dendritic morphology in the developing cerebral cortex.     

MET inhibitor PHA-665752 suppresses the hepatocyte growth factor-induced cell proliferation and radioresistance in nasopharyngeal carcinoma cells

Although ionizing radiation (IR) has provided considerable improvements in nasopharyngeal carcinoma (NPC), in subsets of patients, radioresistance is still a major problem in the treatment. In this study, we demonstrated that irradiation induced MET overexpression and activation, and the aberrant MET signal mediated by hepatocyte growth factor (HGF) induced radioresistance. We also found that MET inhibitor PHA-665752 effectively suppressed HGF induced cell proliferation and radioresistance in NPC cells. Further investigation indicated that PHA-665752 suppressed the phosphorylation of the Akt, ERK1/2, and STAT3 proteins in a dose-dependent manner. Our data indicated that the combination of IR with a MET inhibitor, such as PHA-665752, might be a promising therapeutic strategy for NPC.     

Neural differentiation of mesenchymal stem cells influences chemotactic responses to HGF

Recently, mesenchymal stem cells (MSCs) have been extensively used for cell‐based therapies in neuronal degenerative disease. Although much effort has been devoted to the delineation of factors involved in the migration of MSCs, the relationship between the chemotactic responses and the differentiation status of these cells remains elusive. Here, we report that MSCs in varying neural differentiation states display different chemotactic responses to hepatocyte growth factor (HGF): first, the number of chemotaxing MSCs and the optimal concentrations of HGF that induced the peak migration varied greatly; second, time‐lapse video analysis showed that MSCs in certain differentiation state migrated more efficiently toward HGF; third, the phosphorylation levels of Akt, ERK1/2, SAPK/JNK, and p38MAPK were closely related to the differentiation levels of MSCs subjected to HGF; and finally, although inhibition of ERK1/2 signaling significantly attenuated HGF‐stimulated transfilter migration of both undifferentiated and differentiating MSCs, abolishment of PI3K/Akt, p38MAPK, or SAPK/JNK signaling only decreased the number of migrated cells in certain differentiation state(s). Blocking of PI3K/Akt or MAPK signaling impaired the migration efficiency and/or speed, the extent of which depends on the cell differentiation states. Meanwhile, F‐actin rearrangement, which is essential for MSCs chemotaxis, was induced by HGF, and the time points of cytoskeletal reorganization were different among these cells. Collectively, these results demonstrate that neural differentiation of MSCs influences their chemotactic responses to HGF: MSCs in varying differentiation states possess different migratory capacities, thereby shedding light on optimization of the therapeutic potential of MSCs to be employed for neural regeneration after injury. J. Cell. Physiol. 228: 149–162, 2013. © 2012 Wiley Periodicals, Inc.     

The role of agrin in synaptic development,plasticity and signaling in the central nervous system

Development of the neuromuscular junction (NMJ) requires secretion of specific isoforms of the proteoglycan agrin by motor neurons. Secreted agrin is widely expressed in the basal lamina of various tissues, whereas a transmembrane form is highly expressed in the brain. Expression in the brain is greatest during the period of synaptogenesis, but remains high in regions of the adult brain that show extensive synaptic plasticity. The well-established role of agrin in NMJ development and its presence in the brain elicited investigations of its possible role in synaptogenesis in the brain. Initial studies on the embryonic brain and neuronal cultures of agrin-null mice did not reveal any defects in synaptogenesis. However, subsequent studies in culture demonstrated inhibition of synaptogenesis by agrin antisense oligonucleotides or agrin siRNA. More recently, a substantial loss of excitatory synapses was found in the brains of transgenic adult mice that lacked agrin expression everywhere but in motor neurons. The mechanisms by which agrin influences synapse formation, maintenance and plasticity may include enhancement of excitatory synaptic signaling, activation of the “muscle-specific” receptor tyrosine kinase (MuSK) and positive regulation of dendritic filopodia. In this article I will review the evidence that agrin regulates synapse development, plasticity and signaling in the brain and discuss the evidence for the proposed mechanisms.     

BMP7‐induced dendritic growth in sympathetic neurons requires p75NTR signaling

Dendritic morphology is a critical determinant of neuronal connectivity, and in postganglionic sympathetic neurons, tonic activity correlates directly with the size of the dendritic arbor. Thus, identifying signaling mechanisms that regulate dendritic arborization of sympathetic neurons is important to understanding how functional neural circuitry is established and maintained in the sympathetic nervous system. Bone morphogenetic proteins (BMPs) promote dendritic growth in sympathetic neurons; however, downstream signaling events that link BMP receptor activation to dendritic growth are poorly characterized. We previously reported that BMP7 upregulates p75^NTR mRNA in cultured sympathetic neurons. This receptor is implicated in controlling dendritic growth in central neurons but whether p75^NTR regulates dendritic growth in peripheral neurons is not known. Here, we demonstrate that BMP7 increases p75^NTR protein in cultured sympathetic neurons, and this effect is blocked by pharmacologic inhibition of signaling via BMP type I receptor. BMP7 does not trigger dendritic growth in sympathetic neurons dissociated from superior cervical ganglia (SCG) of p75^NTR nullizygous mice, and overexpression of p75^NTR in p75^NTR?/? neurons is sufficient to cause dendritic growth even in the absence of BMP7. Morphometric analyses of SCG from wild‐type versus p75^NTR nullizygous mice at 3, 6, and 12 to 16 weeks of age indicated that genetic deletion of p75^NTR does not prevent dendritic growth but does stunt dendritic maturation in sympathetic neurons. These data support the hypotheses that p75^NTR is involved in downstream signaling events that mediate BMP7‐induced dendritic growth in sympathetic neurons, and suggest that p75^NTR signaling positively modulates dendritic complexity in sympathetic neurons in vivo. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1003–1013, 2016     

ERK1/2 antagonizes glycogen synthase kinase-3beta-induced apoptosis in cortical neurons

Inhibition of glycogen synthase kinase-3beta (GSK3beta) is one of the mechanisms by which phosphatidylinositol 3-kinase (PI3K) activation protects neurons from apoptosis. Here, we report that inhibition of ERK1/2 increased the basal activity of GSK3beta in cortical neurons and that both ERK1/2 and PI3K were required for brain-derived neurotrophic factor (BDNF) suppression of GSK3beta activity. Moreover, cortical neuron apoptosis induced by expression of recombinant GSK3beta was inhibited by coexpression of constitutively active MKK1 or PI3K. Activation of both endogenous ERK1/2 and PI3K signaling pathways was required for BDNF to block apoptosis induced by expression of recombinant GSK3beta. Furthermore, cortical neuron apoptosis induced by LY294002-mediated activation of endogenous GSK3beta was blocked by expression of constitutively active MKK1 or by BDNF via stimulation of the endogenous ERK1/2 pathway. Although both PI3K and ERK1/2 inhibited GSK3beta activity, neither had an effect on GSK3beta phosphorylation at Tyr-216. Interestingly, PI3K (but not ERK1/2) induced the inhibitory phosphorylation of GSK3beta at Ser-9. Significantly, coexpression of constitutively active MKK1 (but not PI3K) still suppressed neuronal apoptosis induced by expression of the GSK3beta(S9A) mutant. These data suggest that activation of the ERK1/2 signaling pathway protects neurons from GSK3beta-induced apoptosis and that inhibition of GSK3beta may be a common target by which ERK1/2 and PI3K protect neurons from apoptosis. Furthermore, ERK1/2 inhibits GSK3beta activity via a novel mechanism that is independent of Ser-9 phosphorylation and likely does not involve Tyr-216 phosphorylation.     

Cannabinoid receptor 1 induces a biphasic ERK activation via multiprotein signaling complex formation of proximal kinases PKCε, Src, and Fyn in primary neurons

Cannabinoid receptors 1 (CB1Rs) play important roles in the regulation of dendritic branching, synapse density, and synaptic transmission through multiple G-protein-coupled signaling systems, including the activation of the extracellular signal-regulated kinases ERK1/2. The proximal signaling interactions leading to ERK1/2 activation by CB1R in CNS remain, however, unclear. Here, we present evidence that the CB1R agonist methanandamide induced a biphasic and sustained activation of ERK1/2 in primary neurons derived from E7 telencephalon. We show that E7 neurons natively express high levels of CB1R message and protein, the majority of which associates with PKC? at basal conditions. We now demonstrate that the first peak of ERK activation by CB1R was mediated by the sequential activation of G(q), PLC, and PKC?, selectively, and that the CB1R-activated PKC? acutely formed transient signaling modules containing activated Src and Fyn. A second pool of CB1Rs, coupled to PTX-sensitive activation of G(i/o), utilized as effectors additional Src and Fyn molecules to generate a second, additional wave of ERK activation at 15 min. Concurrently to these intermolecular signaling interactions, cytoskeleton-associated proteins MARCKS and p120catenin were drastically modified by phosphorylation of PKC and Src, respectively. These receptor-proximal signaling events correlated well with induction of neuritic outgrowth in the long term. Our data provide evidence for multiprotein signaling complex formation in the coupling of CB1R to activation of ERK in CNS neurons, and may elucidate several of the less understood acute effects of cannabinoid drugs.     

Magnetic stimulation modulates structural synaptic plasticity and regulates BDNF–TrkB signal pathway in cultured hippocampal neurons

Repetitive transcranial magnetic stimulation (rTMS) is a neuropsychiatric tool that can be used to investigate the neurobiology of learning and cognitive function. Few studies have examined the effects of low frequency (?1 Hz) magnetic stimulation (MS) on structural synaptic plasticity of neurons in vitro, thus, the current study examined its effects on hippocampal neuron and synapse morphology, as well as synaptic protein markers and signaling pathways. Similarly, both intensities of low frequency magnetic stimulation (1 Hz) activated brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase B (TrkB) pathways, including the pathways for mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and for phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt). Specifically, low intensity magnetic stimulation (LIMS, 1.14 Tesla, 1 Hz) promoted more extensive dendritic and axonal arborization, as well as increasing synapses density, thickening PSD (post synaptic density) and upregulation of synaptophysin (SYN), growth associated protein 43 (GAP43) and post synaptic density 95 (PSD95). Conversely, high intensity magnetic stimulation (HIMS, 1.55 Tesla, 1 Hz) appeared to be detrimental, reducing dendritic and axonal arborization and causing apparent structural damage, including thinning of PSD, less synapses and disordered synaptic structure, as well as upregulation of GAP43 and PSD95, possibly for their ability to mitigate dysfunction. In conclusion, we infers that low frequency magnetic stimulation participates in regulating structural synaptic plasticity of hippocampal neurons via the activation of BDNF–TrkB signaling pathways.     

PI3-K/Akt and ERK pathways activated by VEGF play opposite roles in MPP+-induced neuronal apoptosis

Vascular endothelial growth factor (VEGF), a specific pro-angiogenic peptide, has shown neuroprotective effects in the Parkinson’s disease (PD) models, but the underlying mechanisms remain elusive. In this study, the neuroprotective properties of VEGF on 1-methyl-4-phenylpyridinium ion (MPP⁺)-induced neurotoxicity in primary cerebellar granule neurons were investigated. Pretreatment of VEGF prevented MPP⁺-induced neuronal apoptosis in a concentration- and time-dependent manner. And this prevention was blocked by PTK787/ZK222584, a VEGF receptor-2 specific inhibitor. Both inhibition of the Akt pathway and activation of the extracellular signal-regulated kinase (ERK) pathway contribute to MPP⁺-induced neuronal apoptosis. VEGF reversed the inhibition of phosphoinositide 3-kinase (PI3-K)/Akt pathway caused by MPP⁺, but further enhanced the activation of ERK induced by MPP⁺. Interestingly, VEGF and PD98059 (an ERK kinase inhibitor) play a synergistic role in protecting neurons from MPP⁺-induced toxicity. Collectively, these findings suggest that the PI3-K/Akt and ERK pathways activated by VEGF play opposite roles in MPP⁺-induced neuronal apoptosis. This finding offers not only a new and clinically significant modality as to how VEGF exerts its neuroprotective effects but also a novel therapeutic strategy for PD by differentially regulating PD-associated signaling pathways.     

High intensity ERK signal mediates hepatocyte growth factor-induced proliferation inhibition of the human hepatocellular carcinoma cell line HepG2

Hepatocyte growth factor (HGF) induces growth stimulation of a variety of cell types, but it also induces growth inhibition of several types of tumor cell lines. The molecular mechanism of the HGF-induced growth inhibition of tumor cells remains obscure. We have investigated the intracellular signaling pathway involved in the antiproliferative effect of HGF on the human hepatocellular carcinoma cell line HepG2. HGF induced strong activation of ERK in HepG2 cells. Although the serum-dependent proliferation of HepG2 cells was inhibited by the MEK inhibitor PD98059 in a dose-dependent manner, 10 microM PD98059 reduced the HGF-induced strong activation of ERK to a weak activation; and as a result, the proliferation inhibited by HGF was completely restored. Above or below this specific concentration, the restoration was incomplete. Expression of constitutively activated Ha-Ras, which induces strong activation of ERK, led to the proliferation inhibition of HepG2 cells, as was observed in HGF-treated HepG2 cells. This inhibition was suppressed by the MEK inhibitor. Furthermore, HGF treatment and expression of constitutively activated Ha-Ras changed the hyperphosphorylated form of the retinoblastoma tumor suppressor gene product pRb to the hypophosphorylated form. This change was inhibited by the same concentration of MEK inhibitor needed to suppress the proliferation inhibition. These results suggest that ERK activity is required for both the stimulation and inhibition of proliferation of HepG2 cells; that the level of ERK activity determines the opposing proliferation responses; and that HGF-induced proliferation inhibition is caused by cell cycle arrest, which results from pRb being maintained in its active hypophosphorylated form via a high-intensity ERK signal in HepG2 cells.