Protein kinase C-alpha and -epsilon are enriched in growth cones of differentiating SH-SY5Y human neuroblastoma cells

Synthesis and biological evaluation of selective histone deacetylase 6 inhibitors as multifunctional agents against Alzheimer's disease

Histone deacetylase 6 (HDAC6) is a potential target for Alzheimer's disease (AD). In this study, a series of novel phenothiazine-, memantine-, and 1,2,3,4-tetrahydro-γ-carboline-based HDAC6 inhibitors with a variety of linker moieties were designed and synthesized. As a hydrochloride salt, the phenothiazine-based hydroxamic acid W5 with a pyridyl-containing linker motif was identified as a high potent and selective HDAC6 inhibitor. It inhibited HDAC6 with an IC₅₀ of 2.54 nM and was more than 290- to 3300-fold selective over other HDAC isoforms. In SH-SY5Y cells, W5 dose-dependently increased the acetylated α-tubulin levels and reduced the hyperphosphorylated tau proteins at Ser396. As an effective metal chelator, W5 inhibited Cu²⁺-induced Aβ_1-42 aggregation and disaggregated Cu²⁺-Aβ_1-42 oligomers, and showed protective effects on the SH-SY5Y cells against Aβ_1-42- as well as Cu²⁺-Aβ_1-42 induced cell damages, serving as a potential ligand to target AD metal dyshomeostasis. Moreover, W5 promoted the differentiated neuronal neurite outgrowth, increased the mRNA expression of the recognized neurogenesis markers, GAP43, N-myc, and MAP-2. Therefore, W5 might be a good lead for the development of novel HDAC6 inhibitors targeting multi-facets of AD.

A Unique GSK-3β inhibitor B10 Has a Direct Effect on Aβ, Targets Tau and Metal Dyshomeostasis, and Promotes Neuronal Neurite Outgrowth

Due to the complicated pathogenesis of Alzheimer's disease (AD), the development of multitargeted agents to simultaneously interfere with multiple pathological processes of AD is a potential choice. Glycogen synthase kinase-3β (GSK-3β) plays a vital role in the AD pathological process. In this study, we discovered a novel 1H-pyrrolo[2,3-b]pyridine derivative B10 as a GSK-3β inhibitor that features with a quinolin-8-ol moiety to target the metal dyshomeostasis of AD. B10 potently inhibited GSK-3β with an IC₅₀ of 66 ± 2.5 nM. At the concentration of 20 μM, B10 increased β-catenin abundance (β-catenin/GAPDH: 0.83 ± 0.086 vs. 0.30 ± 0.016), phosphorylated GSK-3β at Ser9 (p-GSK-3β/GAPDH: 0.53 ± 0.045 vs. 0.35 ± 0.012), and decreased the phosphorylated tau level (p-tau/GAPDH: 0.33 ± 0.065 vs. 0.83 ± 0.061) in SH-SY5Y cells. Unlike other GSK-3β inhibitors, B10 had a direct effect on Aβ by inhibiting Aβ_1-42 aggregation and promoting the Aβ_1-42 aggregate disassociation. It selectively chelated with Cu²⁺, Zn²⁺, Fe^3+, and Al³⁺, and targeted AD metal dyshomeostasis. Moreover, B10 effectively increased the mRNA expression of the recognized neurogenesis markers, GAP43, N-myc, and MAP-2, and promoted the differentiated neuronal neurite outgrowth, possibly through the GSK-3β and β-catenin signal pathways. Therefore, B10 is a potent and unique GSK-3β inhibitor that has a direct on Aβ and serves as a multifunctional anti-AD agent for further investigations.

Overexpression of tissue-nonspecific alkaline phosphatase increases the expression of neurogenic differentiation markers in the human SH-SY5Y neuroblastoma cell line

Patients suffering from the rare hereditary disease hypophosphatasia (HPP), which is based on mutations in the ALPL gene, tend to develop central nervous system (CNS) related issues like epileptic seizures and neuropsychiatric illnesses such as anxiety and depression, in addition to well-known problems with the mineralization of bones and teeth. Analyses of the molecular role of tissue-nonspecific alkaline phosphatase (TNAP) in transgenic SH-SY5Y(TNAPhigh) neuroblastoma cells compared to SH-SY5Y(TNAPlow) cells indicate that the enzyme influences the expression levels of neuronal marker genes like RNA-binding protein, fox-1 homolog 3 (NEUN) and enolase 2, gamma neuronal (NSE) as well as microtubule-binding proteins like microtubule-associated protein 2 (MAP2) and microtubule-associated protein tau (TAU) during neurogenic differentiation. Fluorescence staining of SH-SY5Y(TNAPhigh) cells reveals TNAP localization throughout the whole length of the developed projection network and even synapsin Ι co-localization with strong TNAP signals at some spots at least at the early time points of differentiation. Additional immunocytochemical staining shows higher MAP2 expression in SH-SY5Y(TNAPhigh) cells and further a distinct up-regulation of tau and MAP2 in the course of neurogenic differentiation. Interestingly, transgenic SH-SY5Y(TNAPhigh) cells are able to develop longer cellular processes compared to control cells after stimulation with all-trans retinoic acid (RA). Current therapies for HPP prioritize improvement of the bone phenotype. Unraveling the molecular role of TNAP in extraosseous tissues, like in the CNS, will help to improve treatment strategies for HPP patients. Taking this rare disease as a model may also help to dissect TNAP's role in neurodegenerative diseases and even improve future treatment of common pathologies.

Possible role of cortactin phosphorylation by protein kinase Cα in actin-bundle formation at growth cone

BACKGROUND INFORMATION

Cortactin contributes to growth cone morphogenesis by forming with dynamin, ring-shaped complexes that mechanically bundle and stabilise F-actin. However, the regulatory mechanism of cortactin action is poorly understood.

RESULTS

Immunofluorescence microscopy revealed that protein kinase C (PKC) α colocalises with cortactin at growth cone filopodia in SH-SY5Y neuroblastoma cells. PKC activation by phorbol 12-myristate 13-acetate causes cortactin phosphorylation, filopodial retraction and F-actin-bundle loss. Moreover, PKCα directly phosphorylates cortactin in vitro at S135/T145/S172, mitigating both cortactin's actin-binding and actin-crosslinking activity, whereas cellular expression of a phosphorylation-mimetic cortactin mutant hinders filopodial formation with a significant decrease of actin bundles.

CONCLUSIONS

Our results indicate that PKC-mediated cortactin phosphorylation might be implicated in the maintenance of growth cone.

The C1 domain-targeted isophthalate derivative HMI-1b11 promotes neurite outgrowth and GAP-43 expression through PKCα activation in SH-SY5Y cells

Protein kinase C (PKC) is a family of serine/threonine phosphotransferases ubiquitously expressed and involved in multiple cellular functions, such as proliferation, apoptosis and differentiation. The C1 domain of PKC represents an attractive drug target, especially for developing PKC activators. Dialkyl 5-(hydroxymethyl)isophthalates are a novel group of synthetic C1 domain ligands that exhibit antiproliferative effect in HeLa cervical carcinoma cells. Here we selected two isophthalates, HMI-1a3 and HMI-1b11, and characterized their effects in the human neuroblastoma cell line SH-SY5Y. Both of the active isophthalates exhibited significant antiproliferative and differentiation-inducing effects. Since HMI-1b11 did not impair cell survival even at the highest concentration tested (20μM), and supported neurite growth and differentiation of SH-SY5Y cells, we focused on studying its downstream signaling cascades and effects on gene expression. Consistently, genome-wide gene expression microarray and gene set enrichment analysis indicated that HMI-1b11 (10μM) induced changes in genes mainly related to cell differentiation. In particular, further studies revealed that HMI-1b11 exposure induced up-regulation of GAP-43, a marker for neurite sprouting and neuronal differentiation. These effects were induced by a 7-min HMI-1b11 treatment and specifically depended on PKCα activation, since pretreatment with the selective inhibitor Gö6976 abolished the up-regulation of GAP-43 protein observed at 12h. In parallel, we found that a 7-min exposure to HMI-1b11 induced PKCα accumulation to the cytoskeleton, an effect that was again prevented by pretreatment with Gö6976. Despite similar binding affinities to PKC, the isophthalates had different effects on PKC-dependent ERK1/2 signaling: HMI-1a3-induced ERK1/2 phosphorylation was transient, while HMI-1b11 induced a rapid but prolonged ERK1/2 phosphorylation. Overall our data are in accordance with previous studies showing that activation of the PKCα and ERK1/2 pathways participate in regulating neuronal differentiation. Furthermore, since PKC has been classified as one of the cognitive kinases, and activation of PKC is considered a potential therapeutic strategy for the treatment of cognitive disorders, our findings suggest that HMI-1b11 represents a promising lead compound in research aimed to prevent or counteract memory impairment.

Protein kinase C activity blocks neuropeptide Y-mediated inhibition of glutamate release and contributes to excitability of the hippocampus in status epilepticus

The unbalanced excitatory/inhibitory neurotransmitter function in the neuronal network afflicted by seizures is the main biochemical and biophysical hallmark of epilepsy. The aim of this work was to identify changes in the signaling mechanisms associated with neuropeptide Y (NPY)-mediated inhibition of glutamate release that may contribute to hyperexcitability. Using isolated rat hippocampal nerve terminals, we showed that the KCl-evoked glutamate release is inhibited by NPY Y2 receptor activation and is potentiated by the stimulation of protein kinase C (PKC). Moreover, we observed that immediately after status epilepticus (6 h postinjection with kainate, 10 mg/kg), the functional inhibition of glutamate release by NPY Y2 receptors was transiently blocked concomitantly with PKC hyperactivation. The pharmacological blockade of seizure-activated PKC revealed again the Y2 receptor-mediated inhibition of glutamate release. The functional activity of PKC immediately after status epilepticus was assessed by evaluating phosphorylation of the AMPA receptor subunit GluR1 (Ser-831), a substrate for PKC. Moreover, NPY-stimulated [35S]GTPgammaS autoradiographic binding studies indicated that the common target for Y2 receptor and PKC on the inhibition/potentiation of glutamate release was located downstream of the Y2 receptor, or its interacting G-protein, and involves voltage-gated calcium channels.

Calcium signalling in growth cone migration

Growth cones, the motile structures at the tips of advancing axons and dendrites, respond to a wide range of cues by either turning towards or away from the cue. Cytosolic calcium signals appear to mediate a large fraction of both types of response. Calcium signals can be generated by influx through plasma membrane channels or by release from intracellular stores. While neurotransmitters can elicit calcium influx through ionotropic receptors, other chemical cues open plasma membrane voltage gated calcium channels by a mechanism other than a change of membrane voltage. In general attractive cues generate spatially and temporally restricted calcium increases that are difficult to detect using conventional indicators. One target for these calcium signals is calmodulin dependent protein kinase II. Repulsive cues generate spatially and temporally more diffuse calcium increases that can be more readily detected using fluorescent indicators. One target for these is the phosphatase calcineurin, which may act by dephosphorylating GAP43 and allowing the latter to cap actin filaments.

Neuronal ELAV proteins enhance mRNA stability by a PKCalpha-dependent pathway

More than 1 in 20 human genes bear in the mRNA 3' UTR a specific motif called the adenine- and uridine-rich element (ARE), which posttranscriptionally determines its expression in response to cell environmental signals. ELAV (embryonic lethal abnormal vision) proteins are the only known ARE-binding factors that are able to stabilize the bound mRNAs, thereby positively controlling gene expression. Here, we show that in human neuroblastoma SH-SY5Y cells, neuron-specific ELAV (nELAV) proteins (HuB, HuC, and HuD) are up-regulated and redistributed by 15 min of treatment with the activators of PKC phorbol esters and bryostatin-1. PKC stimulation also induces nELAV proteins to colocalize with the translocated PKCalpha isozyme preferentially on the cytoskeleton, with a concomitant increase of nELAV threonine phosphorylation. The same treatment promotes stabilization of growth-associated protein 43 (GAP-43) mRNA, a well known nELAV target, and induces an early increase in GAP-43 protein concentration, again only in the cytoskeletal cell fraction. Genetic or pharmacological inactivation of PKCalpha abolishes nELAV protein cytoskeletal up-regulation, GAP-43 mRNA stabilization, and GAP-43 protein increase, demonstrating the primary role of this specific PKC isozyme in the cascade of nELAV recruitment. Finally, in vivo PKC activation is associated with an up-regulation of nELAV proteins in the hippocampal rat brain. These findings suggest a model for gene expression regulation by nELAV proteins through a PKCalpha-dependent pathway that is relevant for the cellular programs in which ARE-mediated control plays a pivotal role.

Effects of Thimerosal on NGF Signal Transduction and Cell Death in Neuroblastoma Cells

Signaling through neurotrophic receptors is necessary for differentiation and survival of the developing nervous system. The present study examined the effects of the organic mercury compound thimerosal on nerve growth factor signal transduction and cell death in a human neuroblastoma cell line (SH-SY5Y cells). Following exposure to 100 ng/ml NGF and increasing concentrations of thimerosal (1 nM-10 microM), we measured the activation of TrkA, MAPK, and PKC-delta. In controls, the activation of TrkA MAPK and PKC-delta peaked after 5 min of exposure to NGF and then decreased but was still detectable at 60 min. Concurrent exposure to increasing concentrations of thimerosal and NGF for 5 min resulted in a concentration-dependent decrease in TrkA and MAPK phosphorylation, which was evident at 50 nM for TrkA and 100 nM for MAPK. Cell viability was assessed by the LDH assay. Following 24-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence or absence of NGF was 596 nM and 38.7 nM, respectively. Following 48-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence and absence of NGF was 105 nM and 4.35 nM, respectively. This suggests that NGF provides protection against thimerosal cytotoxicity. To determine if apoptotic versus necrotic cell death was occurring, oligonucleosomal fragmented DNA was quantified by ELISA. Control levels of fragmented DNA were similar in both the presence and absence of NGF. With and without NGF, thimerosal caused elevated levels of fragmented DNA appearing at 0.01 microM (apoptosis) to decrease at concentrations >1 microM (necrosis). These data demonstrate that thimerosal could alter NGF-induced signaling in neurotrophin-treated cells at concentrations lower than those responsible for cell death.

Eicosanoid activation of protein kinase C epsilon: involvement in growth cone repellent signaling

Exposure of growing neurons to thrombin or semaphorin 3A stimulates a receptor-mediated signaling cascade that results in collapse of their growth cones. This collapse response necessitates eicosanoid production, as we have shown earlier. The present report investigates whether and which protein kinase C (PKC) isoforms may be activated by such eicosanoids. To examine these questions, we isolated growth cones from fetal rat brain and tested whether thrombin or the eicosanoid, 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE), could activate endogenous growth cone PKC. We show that both thrombin and 12(S)-HETE stimulate the phosphorylation of the myristoylated alanine-rich protein kinase C substrate, an 87-kDa adhesion site protein. Furthermore, we show both with immunoprecipitated and with recombinant PKC that 12(S)-HETE activation is selective for the epsilon isoform and does not require accessory proteins. Last, we demonstrate that PKC activation is necessary for thrombin-induced growth cone collapse. These data indicate that eicosanoid-mediated repellent effects result from the direct and selective activation of PKCepsilon and suggest the involvement of myristoylated alanine-rich protein kinase C substrate phosphorylation in growth cone collapse.

Molecular cloning and identification of a putative PKC epsilon cDNA from Limulus polyphemus brain

The protein kinase C (PKC) family of enzymes is broadly distributed and has been implicated in a diverse array of cellular functions. Recent evidence supporting PKC involvement in the regulation of the Limulus choline cotransporter prompted us to clone PKC from a Limulus central nervous system (CNS) cDNA library. An Aplysia californica calcium independent PKC (Apl II) cDNA probe was used to screen the library and 5' RACE SMART PCR was used to obtain the full-length sequence. The resulting cDNA, which included 5' and 3' nontranslation regions, was 4675 bp. Analysis of the encoded peptide sequence using the Swiss-prot database revealed at least 58% identity to PKC epsilon. A commercial polyclonal antibody against PKC epsilon was used in Western blots to positively label a 30 kDa protein from Limulus CNS and the expressed fusion protein of the encoded sequence. These data support the presence of a newly identified PKC-like homolog in Limulus which likely represents a PKC epsilon equivalent.

Selective association of protein kinase C with 14-3-3 zeta in neuronally differentiated PC12 Cells. Stimulatory and inhibitory effect of 14-3-3 zeta in vivo

The 14-3-3 protein is a family of highly conserved acidic proteins found in a wide range of eukaryotes from yeast to mammals. 14-3-3 acts as an adapter protein and interacts with signaling molecules including protein kinase C (PKC). Although 14-3-3 zeta was originally characterized as an endogenous PKC inhibitor, it was reported to activate PKC in vitro, but the in vivo regulation of PKC by 14-3-3 is still not well understood. To examine the regulation of PKC by 14-3-3 in the cell, we have generated a sub-cell line, PC12-B3, that stably expresses FLAG epitope-tagged 14-3-3 zeta isoform in PC12 cells. Here we show that PKC-alpha and PKC-epsilon become associated with 14-3-3 zeta when the cells are neuronally differentiated by nerve growth factor. We found that the immunoprecipitate by anti-FLAG antibody contains constitutive and autonomous Ca(2+)-independent non-classical PKC activity. In contrast, the FLAG immunoprecipitate has no Ca(2+)-dependent classical PKC activity despite the fact that PKC-alpha is present in the FLAG immunoprecipitate from differentiated PC12-B3 cells. Our results show that the association with 14-3-3 zeta has distinct effects on classical PKC and non-classical PKC activity.

A role for protein kinase C and its substrates in the action of valproic acid in the brain: implications for neural plasticity

Valproic acid (VPA) is a broad-spectrum anticonvulsant with well-documented teratogenic effects, but whose mechanism of action is largely unknown. In the present study we have examined the effects of VPA on the expression of two prominent substrates for protein kinase C (PKC) in the brain, MARCKS and GAP-43, which have been implicated in actin-membrane plasticity and neurite outgrowth during neuronal differentiation, respectively, and are essential to normal brain development. Immortalized hippocampal HN33 cells exposed to VPA exhibited reduced MARCKS protein expression and demonstrated increased GAP-43 protein expression, with concomitant alterations in cellular morphology, including an increase in the number and length of neurites and accompanied by a reduction in cell growth rate. The effects of VPA were observed at clinically relevant concentrations following chronic (>1 day) VPA exposure. We also present evidence for a VPA-induced alteration in PKC activity, as well as temporal changes in individual PKC isozyme expression. Inhibition of PKC with the PKC-selective inhibitor, LY333531, prevented the VPA-induced down-regulation of membrane-associated MARCKS, but had no effect on the cytosolic MARCKS reduction or the GAP-43 up-regulation. Inhibition of PKC by LY333531 enhanced the differentiating effects of VPA; additionally, LY333531 alone induced greater neurite outgrowth in this cell line. Collectively, these data indicate that VPA induces neuronal differentiation, associated with a reduction in MARCKS expression and an increase in GAP-43 expression, consistent with the hypothesis that a reduction in MARCKS at the membrane may be permissive for cytoskeletal plasticity during neurite outgrowth.

Activation of Ras, Raf-1 and protein kinase C in differentiating human neuroblastoma cells after treatment with phorbolester and NGF

The human neuroblastoma cell line SH-SY5Y/TrkA differentiates in vitro and acquires a sympathetic phenotype in response to phorbolester (activator of protein kinase C, PKC) in the presence of serum or growth factors, or nerve growth factor (NGF). We have now investigated to what extent phorbolester and NGF cause activation of Ras and Raf-1 and the involvement of PKC in this response in differentiating SH-SY5Y/TrkA cells. NGF stimulated increased accumulation of Ras-GTP and a threefold activation of Raf-1. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA) had no effect on the amount of Ras-GTP but led to a smaller activation of Raf-1. NGF caused a limited increase in phosphorylation of Raf-1 compared with TPA, and NGF-induced Raf activity was independent of PKC. Analysis of phosphorylation of the endogenous PKC substrate myristoylated alanine-rich C-kinase substrate (MARCKS), and of subcellular distribution of PKC-alpha, -delta, and -epsilon revealed that NGF only caused a very small activation of PKC in SH-SY5Y/TrkA cells. The results identify Raf-1 as a target for both TPA- and NGF-induced signals in differentiating SH-SY5Y/TrkA cells and demonstrate that signalling to Raf-1 was mediated via distinct mechanisms.

Differential regulation of primary protein kinase C substrate (MARCKS, MLP, GAP-43, RC3) mRNAs in the hippocampus during kainic acid-induced seizures and synaptic reorganization

In the mature hippocampus, kainic acid seizures lead to excitotoxic cell death and synaptic reorganization in which granule cell axons (mossy fibers) form ectopic synapses on granule cell dendrites. In the present study, we examined the expression of four major, developmentally regulated protein kinase C (PKC) substrates (MARCKS, MLP, GAP-43, RC3), which have different subcellular and regional localizations in the hippocampus at several time points (6 hr, 12 hr, 18 hr, 24 hr, 48 hr, 5 days, or 15 days) following kainic acid seizures using in situ hybridization. Consistent with previous reports, following kainate seizures, GAP-43 mRNA expression exhibited a delayed and protracted elevation in the granule cell layer, which peaked at 24 hr, whereas expression in fields CA1 and CA3 remained relatively unchanged. Conversely, RC3 mRNA expression exhibited a delayed reduction in the granule cell layer that was maximal at 18 hr, as well as a reduction CA1 at 48 hr, whereas CA3 levels did not change. MARCKS mRNA expression in the granule cell layer and CA1 remained stable following kainate, although an elevation was observed in subfield CA3c at 12 hr. Similarly, MLP mRNA expression did not change in the granule cell layer or CA1 following kainate but exhibited a protracted elevation in subfields CA3b,c beginning at 6 hr post-kainate. Collectively these data demonstrate that different PKC substrate mRNAs exhibit unique expression profiles and regulation in the different cell fields of the mature hippocampus following kainic acid seizures and during subsequent synaptic reorganization. The expression profiles following kainate seizures bear resemblance to those observed during postnatal hippocampal development, which may indicate the recruitment of common regulatory mechanisms.

Facial motor neuron regeneration induces a unique spatial and temporal pattern of myristoylated alanine-rich C kinase substrate expression

We have previously shown that the myristoylated alanine-rich C kinase substrate, a primary protein kinase C substrate in brain that binds and cross-links filamentous actin, is enriched in neuronal growth cones and is developmentally regulated in brain. Here we examined myristoylated alanine-rich C kinase substrate expression in the facial motor nucleus during axonal regeneration following facial nerve axotomy or facial nerve resection lesions, which impede regeneration, or following motor neuron degeneration induced by the retrograde neurotoxin ricin. For comparative purposes, the protein kinase C substrates myristoylated alanine-rich C kinase substrate-like protein and growth-associated protein-43 were examined in parallel. Myristoylated alanine-rich C kinase substrate messenger RNA exhibited a robust increase in both neurons and non-neuronal cells in the facial motor nucleus beginning four days after axotomy, peaked at seven days (2.5-fold), and declined back to baseline levels by 40 days. Myristoylated alanine-rich C kinase substrate protein similarly exhibited a twofold elevation in the facial motor nucleus determined four and 14 days post-axotomy. Following nerve resection, myristoylated alanine-rich C kinase substrate messenger RNA levels increased at seven days and returned to baseline levels by 40 days. Unlike myristoylated alanine-rich C kinase substrate messenger RNA, myristoylated alanine-rich C kinase substrate-like messenger RNA levels did not increase in the facial motor nucleus at any time point following nerve axotomy or resection, whereas growth-associated protein-43 messenger RNA exhibited a rapid (one day) and prolonged (40 days) elevation in facial motor nucleus neurons following either nerve axotomy or resection. Ricin-induced degeneration of facial motor neurons elevated myristoylated alanine-rich C kinase substrate and myristoylated alanine-rich C kinase substrate-like messenger RNAs in both microglia (lectin-positive) and astrocytes (glial fibrillary acidic protein-positive).Collectively, these data demonstrate that myristoylated alanine-rich C kinase substrate exhibits a unique expression profile in the facial motor nucleus following facial nerve lesions, and it is proposed that myristoylated alanine-rich C kinase substrate may serve to mediate actin-membrane cytoskeletal plasticity in both neurons and glial cells in response to protein kinaseC-mediated signaling during nerve regeneration and degeneration.

Activation and protein kinase C-dependent nuclear accumulation of ERK in differentiating human neuroblastoma cells

The human neuroblastoma cell line SH-SY5Y is a well characterized model for sympathetic neuronal differentiation in vitro. Several differentiation protocols exist, one of which, the addition of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) in the presence of serum, has been thoroughly studied. Wild-type SH-SY5Y cells are unresponsive to nerve growth factor (NGF), but cells transfected with the high-affinity NGF receptor TrkA (SH-SY5Y/TrkA) differentiate in response to NGF. In the present study, we have addressed the existence of a differentiation-specific mode of activation and subcellular distribution of the extracellular signal-regulated kinases ERK1 and ERK2 in SH-SY5Y/wt and SH-SY5Y/TrkA. Both TPA and NGF induced a sustained activation and nuclear accumulation of ERK that was accompanied by transactivation of a serum response element (SRE)-driven reporter and of the c-fos gene. However, activation and nuclear accumulation of ERK were not sufficient to induce neuronal differentiation in SH-SY5Y, as demonstrated by the response to TPA in serum-free cultures. Nuclear accumulation but not activation of ERK was demonstrated to require active protein kinase C (PKC). The effect of specific PKC inhibitors on subcellular distribution of ERK and ERK-dependent transcription suggests a functional role for PKC in the regulation of nuclear ERK activity in SH-SY5Y neuroblastoma cells.

Anti-proliferative effect of two novel palmitoyl-carnitine analogs, selective inhibitors of protein kinase C conventional isoenzymes

Previous studies have shown that palmitoyl-carnitine is an anti-proliferative agent and a protein kinase C inhibitor. Two new palmitoyl-carnitine analogs were synthesized by replacing the ester bond with a metabolically more stable ether bond. An LD50 value in the nM range was found in anti-proliferative assays using HL-60 cells and was dependent on the alkyl-chain length. The inhibitory action of these water-soluble compounds on protein kinase C in vitro was greatly increased with respect to palmitoyl-carnitine and was dependent on the length of the alkyl chain. Its effect was mediated by an increase in the enzyme's requirement for phosphatidylserine. Inhibition of the in situ phosphorylation of a physiological platelet protein kinase C substrate and of phorbol ester-induced differentiation of HL-60 cells was also observed. Finally, to test for isoenzyme selectivity, several human recombinant protein kinase C isoforms were used. Only the Ca2+-dependent classic protein kinase Cs (alpha, betaIota, betaIotaIota and gamma) were inhibited by these compounds, yet the activities of casein kinase I, Ca2+/calmodulin-dependent kinase and cAMP-dependent protein kinase were unaffected. Thus, these novel inhibitors appear to be both protein kinase C and isozyme selective. They may be useful in assessing the individual roles of protein kinase C isoforms in cell proliferation and tumor development and may be rational candidates for anti-neoplasic drug design.

PKCepsilon, via its regulatory domain and independently of its catalytic domain, induces neurite-like processes in neuroblastoma cells

To investigate the role of protein kinase C (PKC) isoforms in regulation of neurite outgrowth, PKCalpha, betaII, delta, and epsilon fused to enhanced green fluorescent protein (EGFP) were transiently overexpressed in neuroblastoma cells. Overexpression of PKCepsilon-EGFP induced cell processes whereas the other isoforms did not. The effect of PKCepsilon-EGFP was not suppressed by the PKC inhibitor GF109203X. Instead, process formation was more pronounced when the regulatory domain was introduced. Overexpression of various fragments from PKCepsilon regulatory domain revealed that a region encompassing the pseudosubstrate, the two C1 domains, and parts of the V3 region were necessary and sufficient for induction of processes. By deleting the second C1 domain from this construct, a dominant-negative protein was generated which suppressed processes induced by full-length PKCepsilon and neurites induced during retinoic acid- and growth factor-induced differentiation. As with neurites in differentiated neuroblastoma cells, processes induced by the PKCepsilon- PSC1V3 protein contained alpha-tubulin, neurofilament-160, and F-actin, but the PKCepsilon-PSC1V3-induced processes lacked the synaptic markers synaptophysin and neuropeptide Y. These data suggest that PKCepsilon, through its regulatory domain, can induce immature neurite-like processes via a mechanism that appears to be of importance for neurite outgrowth during neuronal differentiation.

Axonal transport is inhibited by a protein kinase C inhibitor in cultured isolated mouse dorsal root ganglion cells

We investigated roles of protein kinase C (PKC) and Ca2+/calmodulin-dependent protein II (CAM II) kinase activities in the maintenance of axonal transport in cultured isolated mouse dorsal root ganglion (DRG) cells. Video-enhanced microscopic recordings revealed that the PKC inhibitor chelerythrine (1 microM) reduced anterograde and retrograde axonal transport, while the CAM II kinase inhibitor KN-62 (10 microM) had no effect. Morphological observation showed that neurite growth was prevented by the presence of chelerythrine (1 microM). From these results, we conclude that PKC activity is required to maintain axonal transport and thereby neurite growth.

Mammalian homologue of the Caenorhabditis elegans UNC-76 protein involved in axonal outgrowth is a protein kinase C zeta-interacting protein

By the yeast two-hybrid screening of a rat brain cDNA library with the regulatory domain of protein kinase C zeta (PKCzeta) as a bait, we have cloned a gene coding for a novel PKCzeta-interacting protein homologous to the Caenorhabditis elegans UNC-76 protein involved in axonal outgrowth and fasciculation. The protein designated FEZ1 (fasciculation and elongation protein zeta-1) consisting of 393 amino acid residues shows a high Asp/Glu content and contains several regions predicted to form amphipathic helices. Northern blot analysis has revealed that FEZ1 mRNA is abundantly expressed in adult rat brain and throughout the developmental stages of mouse embryo. By the yeast two-hybrid assay with various deletion mutants of PKC, FEZ1 was shown to interact with the NH2-terminal variable region (V1) of PKCzeta and weakly with that of PKCepsilon. In the COS-7 cells coexpressing FEZ1 and PKCzeta, FEZ1 was present mainly in the plasma membrane, associating with PKCzeta and being phosphorylated. These results indicate that FEZ1 is a novel substrate of PKCzeta. When the constitutively active mutant of PKCzeta was used, FEZ1 was found in the cytoplasm of COS-7 cells. Upon treatment of the cells with a PKC inhibitor, staurosporin, FEZ1 was translocated from the cytoplasm to the plasma membrane, suggesting that the cytoplasmic translocation of FEZ1 is directly regulated by the PKCzeta activity. Although expression of FEZ1 alone had no effect on PC12 cells, coexpression of FEZ1 and constitutively active PKCzeta stimulated the neuronal differentiation of PC12 cells. Combined with the recent finding that a human FEZ1 protein is able to complement the function of UNC-76 necessary for normal axonal bundling and elongation within axon bundles in the nematode, these results suggest that FEZ1 plays a crucial role in the axon guidance machinery in mammals by interacting with PKCzeta.

The regulation of neurotransmitter secretion by protein kinase C

The effect of protein kinase C (PKC) on the release of neurotransmitters from a number preparations, including sympathetic nerve endings, brain slices, synaptosomes, and neuronally derived cell lines, is considered. A comparison is drawn between effects of activation of PKC on neurotransmitter release from small synaptic vesicles and large dense-cored vesicles. The enhancement of neurotransmitter release is discussed in relation to the effect of PKC on: 1. Rearrangement of the F-actin-based cytoskeleton, including the possible role of MARCKS in this process, to allow access of large dense-cored vesicles to release sites on the plasma membrane. 2. Phosphorylation of key components in the SNAP/SNARE complex associated with the docking and fusion of vesicles at site of secretion. 3. Ion channel activity, particularly Ca2+ channels.

Protein kinase C-dependent tyrosine phosphorylation of p130cas in differentiating neuroblastoma cells

The cell signaling docking protein p130cas became tyrosine-phosphorylated in SH-SY5Y human neuroblastoma cells during induced differentiation with 12-O-tetradecanoylphorbol-13-acetate (TPA) and serum or a combination of basic fibroblast growth factor (bFGF) and insulin-like growth factor-I (IGF-I). The differentiating cells develop a neuronal phenotype with neurites and growth cones and sustained activation of protein kinase C (PKC) and pp60c-src. The TPA-induced p130cas phosphorylation increased within 5 min of stimulation and persisted for at least 4 days, whereas bFGF/IGF-I-induced p130cas phosphorylation was biphasic. However, the increase in tyrosine phosphorylation of p130cas was not restricted to differentiation inducing stimuli. The phosphorylation was blocked by the specific PKC inhibitor GF 109203X, and transient transfection with active PKC-epsilon induced p130cas tyrosine phosphorylation. pp60c-src, known to directly phosphorylate p130cas in other cell systems, was not activated after stimulation with TPA or bFGF/IGF-I for up to 30 min, and the initial p130cas phosphorylation was resistant to the Src family kinase inhibitor herbimycin A. However, in long term stimulated cells, herbimycin A blocked the induced phosphorylation of p130cas. Also, overexpression of src induced phosphorylation of p130cas. p130cas protein and phosphorylated p130cas were present in growth cones isolated from differentiated SH-SY5Y cells. Inhibition of PKC activity in differentiating cells with GF 109203X leads to a rapid retraction of growth cone filopodia, and p130cas phosphorylation decreased transiently (within minutes). Growth cones isolated from these cells were virtually devoid of phosphorylated p130cas. These data suggest a function for p130cas as a PKC downstream target in SH-SY5Y cells and possibly also in their growth cones.

Immunocytochemical distribution of Ca(2+)-independent protein kinase C subtypes (delta, epsilon, and zeta) in regenerating axonal growth cones of rat peripheral nerve

In the peripheral nerve, regenerating axonal sprouts usually emanate at nodes of Ranvier, and extend as growth cones along the inner surface of Schwann cells and/or through Schwann cell columns in the distal nerve segment. In order to elucidate the significance of Ca(2+)-independent protein kinase C in nerve regeneration, localizations of delta, epsilon and zeta subtypes were examined immunocytochemically in sprouts and growth cones of regenerating axons, as well as in normal intact nerves in the rat sciatic nerve. In normal nerves, intense immunoreactivities of delta, epsilon and zeta subtypes were present in axons of both myelinated and unmyelinated fibres. Subcellularly, the distribution of these subtypes in the axoplasm was patchy, and discontinuous in the axolemma and subaxolemmal peripheral zones of myelinated nerves. Some thin myelinated axons showed no immunoreactivity for epsilon subtype. Schwann cells of both myelinated and unmyelinated fibres had moderate immunoreactivities for each subtype. In areas of nerve regeneration, axonal sprouts at nodes of Ranvier, and growth cones extending along Schwann cell basal laminae, had intense immunoreactivities for delta, epsilon and zeta subtypes which are distributed diffusely throughout the axoplasm, and on the entire axolemma. In the sprouts, immunoreactivity for epsilon subtype was strong on the axolemma, but weak or almost absent in the axoplasm. These data, together with those of our previous study, indicate that Ca(2+)-independent protein kinase C subtypes (delta, epsilon and zeta) have basically the same distribution patterns as those of Ca(2+)-dependent subtypes in sprouts and growth cones of regenerating axons, as well as in normal intact axons; albeit epsilon subtype is somewhat different in distribution and intensity from delta and zeta subtypes. It is suggested that Ca(2+)-independent subtypes are involved in maintaining growth cone activities along with the Ca(2+)-dependent subtypes.

Selective activation by bryostatin-1 demonstrates unique roles for PKC epsilon in neurite extension and tau phosphorylation

Phorbol esters such as 12-O-tetradeonyl phorbol-13 acetate (TPA) induce a time-dependent biphasic effect on protein kinase C (PKC)-mediated events by fostering translocation of cytosolic (latent) PKC to the plasma membrane (where it is activated). Continued treatment, however, depletes the cell's entire PKC complement and induces a functional stake of PKC inhibition. Previous studies from several laboratories have demonstrated that long-term TPA treatment, like treatment with PKC inhibitors, induces neuronal differentiation. Bryostatin-1 also induces translocation and overall downregulation of PKC following long-term treatment, yet, unlike TPA or PKC inhibitors, does not induce neuronal differentiation, promoting controversy regarding the role of PKC inhibition in neuronal differentiation. We demonstrate herein that, despite overall downregulation in human neuroblastoma cells, membrane-associated levels of one PKC isoform (PKC epsilon) are actually increased following long-term bryostatin-1 treatment. Since previous studies have implicated this PKC isoform in phosphorylation of the microtubule-associated protein tau and in neuritogenesis, we examined the consequences of long-term bryostatin treatment on these phenomena. Treatment with 25 n-100 M bryostatin-1 for 72 h increased tau phosphorylation and inhibited neuritogenesis. By contrast, treatment with either TPA or the PKC inhibitor staurosporine did not induce tau phosphorylation and induced neurite elaboration. Bryostatin-1 antagonized neurite induction by staurosporine. These findings provide additional evidence for a unique role of PKC epsilon in the regulation of tau phosphorylation and neuronal differentiation, and demonstrate that bryostatin-1 can function under certain conditions as a selective PKC epsilon activator even following long-term treatment.

Differentiation and survival influences of growth factors in human neuroblastoma

Human neuroblastoma cell lines are established from high-stage, highly malignant tumours. Despite this and the fact that these tumours are arrested at an early, immature stage, many cell lines have the capacity to undergo neuronal differentiation under proper growth conditions. One such cell line is the noradrenergic SH-SY5Y cell line. These cells can be induced to mature by a variety of modalities, resulting in different mature phenotypes. The use of this cell system as a model to study the stem cell character of neuroblastoma is reviewed and discussed. In particular, we focus on growth factor dependencies in the SH-SY5Y system, and compare that to the normal situation, i.e. growth factor control of sympathetic neuronal and neuroendocrine differentiation during human and rat embryogenesis.