Selective translocation of protein kinase C-delta in PC12 cells during nerve growth factor-induced neuritogenesis

Role of protein kinase Cδ in dopaminergic neurotoxic events

The pro-apoptotic role of Protein kinase Cδ (PKCδ), a member of the novel PKC subfamily, has been well-documented in various pathological conditions. In the central nervous system, the possible role of PKCδ has been studied, mainly in the condition of dopaminergic loss. It has been suggested that the phosphorylation of PKCδ at tyrosine 311 residue (Tyr³¹¹) by redox-sensitive Src family kinases (SFKs) is critical for the caspase-3-mediated proteolytic cleavage, which produces the constitutively active cleaved form of PKCδ. Mitochondrial translocation of cleaved PKCδ has been suggested to facilitate mitochondria-derived apoptosis and oxidative burdens. Moreover, it has been suggested that PKCδ contribute to neuroinflammation through the transformation of microglia into the pro-inflammatory M1 phenotype and the assembly of membrane NADPH oxidase in dopaminergic impairments. Interestingly, mitochondrial respiratory chain inhibitors or neuroinflammogens have shown to induce PKCδ activation in dopaminergic systems. Thus, PKCδ activation may be one of the pivotal causes of neuropathologic events, and could amplify these processes further in a positive feedback manner. Furthermore, PKCδ may play an intermediary role in connecting each neuropathologic event. This review affords insight into the role of PKCδ in various dopaminergic neurotoxic models, which could provide a potential target for mitigating dopaminergic neurotoxicity.

Estrogen-induced cell signaling in the sexually dimorphic nucleus of the rat preoptic area: potential involvement of cofilin in actin dynamics for cell migration

Estrogen is a key factor to induce the sexually dimorphic nucleus (SDN) in the preoptic area (POA) of the rat brain. Identification of estrogen-dependent signaling pathways at SDN in POA during the critical period is a prerequisite for elucidating the mechanism. In the present study, we treated female rats with/without 17β-estradiol (E2) at birth, designated as postnatal day 1 (P1), and prepared total RNA from brain slices containing SDN for DNA microarray analysis. Among the estrogen-responsive genes identified, protein kinase C-delta (PKC-δ) was significantly up-regulated by E2 at P5. We examined the downstream effectors of PKC-δ protein by Western blotting and found an E2-induced PKC-δ/Rac1/PAK1/LIMK1/cofilin pathway. In the pathway, E2 suppressed the phosphorylation (inactive form) of cofilin. This result was supported by immunohistochemistry, where the phosphorylation/dephosphorylation of cofilin occurred at SDN, which suggests that cell migration is a cue to create sexual dimorphism in POA.

Bryostatin-1 vs. TPPB: dose-dependent APP processing and PKC-α, -δ, and -ε isoform activation in SH-SY5Y neuronal cells

Activation of the α-secretase processing pathway of amyloid precursor protein (APP) is recognized as an important mechanism which diverts APP processing from production of beta-amyloid (Aβ) to non toxic sAPPα, decreasing Alzheimer’s disease (AD) plaque formation and AD-associated cognitive deficits. Two potent classes of PKC modulators can activate the α-secretase pathway, the benzo/indolactams and bryostatin/bryologues. While both modulate PKC-dependent APP processing, no direct comparisons of their relative pharmacological potencies have been accomplished which could assist in the development of AD therapies. In this study, we measured the activation of α-secretase APP processing and PKC-α, -δ, and -ε induced by the benzolactam-APP modulator TPPB and bryostatin-1 in the neuroblastoma cell line SH-SY5Y which expresses APP and α- and β-secretase processing mechanisms. Bryostatin-1 produced a more rapid, potent, and sustained activation of α-secretase APP processing than TPPB and selectively activated PKC-δ and PKC-ε. Although TPPB also activated α-secretase, its potency was approximately 10- to 100-fold lower, possibly reflecting lower PKC-δ and -ε activation. Because bryostatin-1 is a highly potent PKC-δ and -ε activator which activates α-secretase APP processing, further characterization of bryostatin-1/bryologues may help refine their use as important tools for the clinical management of AD.

p48 Ebp1 acts as a downstream mediator of Trk signaling in neurons, contributing neuronal differentiation

Two Ebp1 isoproteins, p48 and p42, regulate cell survival and differentiation distinctively. Here we show that p48 is the major isoform in hippocampal neurons and is localized throughout the entire neuron. Notably, reduction of p48 Ebp1 expression inhibited BDNF-mediated neurite outgrowth in hippocampal neurons. The p48 protein acts as a downstream effector of the Trk receptor, which mediates the functions of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in hippocampal cells. Trk receptor activation by both NGF and BDNF induced phosphorylation of Ebp1 at the S360 upon the activation of protein kinase Cδ (PKCδ) and triggered dissociation of p48 from retinoblastoma (Rb). Although both NGF and BDNF activate mitogen-activated protein kinase (MAPK; extracellular signal-related kinase (ERK)) as well as phosphatidylinositide 3-kinase (PI3K)/Akt, their activation is regulated in different time-frame upon growth factor specificity, especially, eliciting PKCδ mediated p48 S360 phosphorylation. Thus, p48 Ebp1 contributes to neuronal cell differentiation and growth factor specificity through the activation of PKCδ, acting as a crucial downstream effector of neurotrophin signaling.

Neurotrophins induce neuregulin release through protein kinase Cdelta activation

Proper, graded communication between different cell types is essential for normal development and function. In the nervous system, heart, and for some cancer cells, part of this communication requires signaling by soluble and membrane-bound factors produced by the NRG1 gene. We have previously shown that glial-derived neurotrophic factors activate a rapid, localized release of soluble neuregulin from neuronal axons that can, in turn promote proper axoglial development (Esper, R. M., and Loeb, J. A. (2004) J. Neurosci. 24, 6218-6227). Here we elucidate the mechanism of this localized, regulated release by implicating the delta isoform of protein kinase C (PKC). Blocking the PKC delta isoform with either rottlerin, a selective antagonist, or small interference RNA blocks the regulated release of neuregulin from both transfected cells and primary neuronal cultures. PKC activation also leads to the rapid phosphorylation of the pro-NRG1 cytoplasmic tail on serine residues adjacent to the membrane-spanning segment, that, when mutated markedly reduce the rate of NRG1 activity release. These findings implicate this specific PKC isoform as an important factor for the cleavage and neurotrophin-regulated release of soluble NRG1 forms that have important effects in nervous system development and disease.

A direct redox regulation of protein kinase C isoenzymes mediates oxidant-induced neuritogenesis in PC12 cells

In this study, we have used the PC12 cell model to elucidate the mechanisms by which sublethal doses of oxidants induce neuritogenesis. The xanthine/xanthine oxidase (X/XO) system was used for the steady state generation of superoxide, and CoCl(2) was used as a representative transition metal redox catalyst. Upon treatment of purified protein kinase C (PKC) with these oxidants, there was an increase in its cofactor-independent activation. Redox-active cobalt competed with the redoxinert zinc present in the zinc-thiolates of the PKC regulatory domain and induced the oxidation of these cysteine-rich regions. Both CoCl(2) and X/XO induced neurite outgrowth in PC12 cells, as determined by an overexpression of neuronal marker genes. Furthermore, these oxidants induced a translocation of PKC from cytosol to membrane and subsequent conversion of PKC to a cofactor-independent form. Isoenzyme-specific PKC inhibitors demonstrated that PKCepsilon plays a crucial role in neuritogenesis. Moreover, oxidant-induced neurite outgrowth was increased with a conditional overexpression of PKCepsilon and decreased with its knock-out by small interfering RNA. Parallel with PKC activation, an increase in phosphorylation of the growth-associated neuronal protein GAP-43 at Ser(41) was observed. Additionally, there was a sustained activation of extracellular signal-regulated kinases 1 and 2, which was correlated with activating phosphorylation (Ser(133)) of cAMP-responsive element-binding protein. All of these signaling events that are causally linked to neuritogenesis were blocked by antioxidant N-acetylcysteine (both L and D-forms) and by a variety of PKC-specific inhibitors. Taken together, these results strongly suggest that sublethal doses of oxidants induce neuritogenesis via a direct redox activation of PKCepsilon.

Comparison of the PKCalpha and the PKCepsilon C1b domains: identification of residues critical for PKCepsilon-mediated neurite induction

We showed earlier that over-expression of protein kinase C (PKC) epsilon induces neurite outgrowth. The effect is mediated by a region (PKCepsilonPSC1V3) encompassing the pseudosubstrate, the two C1 domains and part of the V3 region, and is independent of the catalytic activity of the enzyme. In this region, residues immediately N-terminal of the C1b domain are crucial for neurite outgrowth. However, in this study we show that the PKCepsilon C1b domain itself is necessary for neurite induction, since a mutant in which the PKCepsilon C1b domain has been replaced with the C1b domain from PKCalpha, PKCepsilonPSC1a(alphaC1b)V3 lacks neurite-inducing capacity. The molecular basis for the importance of the PKCepsilon C1b domain was investigated by mutation studies of the PKCalpha C1b domain. Point mutations were done in the PKCalpha C1b domain of the PKCepsilonPSC1a(alphaC1b)V3 construct, in which the PKCalpha residues were mutated into the corresponding residues in PKCepsilon. This highlighted residues in the C-terminal part of the primary sequence of the C1b domain, located in the base of the C1b domain, as important for neurite outgrowth. The mutations S48P, D32K and L49N all influenced neurite induction positively. Furthermore, the mutation of L49N alone was sufficient to make PKCepsilonPSC1a(alphaC1b)V3 neuritogenic in phorbol ester-stimulated cells, and mutation of this residue in full-length PKCepsilon into the corresponding residue in PKCalpha, N291L reduced the neurite-inducing effect of PKCepsilon. In conclusion, we have identified residues in the PKCepsilon C1b domain, in particular Asn49, that are essential for neurite outgrowth.

Cdc42 is involved in PKCepsilon- and delta-induced neurite outgrowth and stress fibre dismantling

We have shown that protein kinase C (PKC)epsilon, independently of the catalytic domain, induces outgrowth of cellular processes via its regulatory domain in both neural cells and fibroblasts. This was accompanied by stress fibre loss. Here, we have examined the role of the small GTPases, Rac1, and Cdc42, in these PKC-mediated morphological and cytoskeletal changes. Both constitutively active and dominant negative Rac1 and Cdc42 attenuated the PKC-mediated outgrowth of processes. The suppression was larger for Cdc42 than for Rac1. The PKC-mediated dismantling of the stress fibres in both HiB5 and fibroblasts was inhibited by the expression of the Cdc42 mutants whereas they had smaller effects on the stress fibre dismantling induced by the ROCK inhibitor, Y-27632, indicating a more crucial role for Cdc42 in the PKC-mediated pathway. We conclude that Cdc42 is an important downstream factor in the pathway through which PKC mediates morphological and cytoskeletal effects.

Staurosporine-induced growth inhibition of glioma cells is accompanied by altered expression of cyclins, CDKs and CDK inhibitors

Staurosporine was found to bring about complete growth inhibition of human glioma cell lines. U87 MG cells were arrested in S phase while U373 MG cells in G2/M phase on staurosporine treatment. Consistent with this observation, no change in G1 phase regulators viz., Cyclin D1, D3 and CDK4 was seen on staurosporine treatment. The levels of CDK2, CDC2, Cyclin A and Cyclin B proteins decreased, while the levels of CDK inhibitors viz., p21 and p27 were found to increase on staurosporine treatment. The mRNA levels of CDK2 and CDC2 genes were also found to decrease on staurosporine treatment. Thus apart from staurosporine's known direct inhibitory effect on CDK2 and CDC2 activities, staurosporine was found to down-regulate activities of these two kinases by modulating the expression of the kinases themselves as well that of their activating partners (Cyclins) and their inhibitors.

Protein kinase C and the regulation of the actin cytoskeleton

Protein kinase C (PKC) isoforms are central components in intracellular networks that regulate a vast number of cellular processes. It has long been known that in most cell types, one or more PKC isoforms influences the morphology of the F-actin cytoskeleton and thereby regulates processes that are affected by remodelling of the microfilaments. These include cellular migration and neurite outgrowth. This review focuses on the role of classical and novel PKC isoforms in migration and neurite outgrowth, and highlights some regulatory steps that may be of importance in the regulation by PKC of migration and neurite outgrowth. Many studies indicate that integrins are crucial mediators both upstream and downstream of PKC in inducing morphological changes. Furthermore, a number of PKC substrates, directly associated with the microfilaments, such as MARCKS, GAP43, adducin, fascin, ERM proteins and others have been identified. Their potential role in PKC effects on the cytoskeleton is discussed.

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.

Aberrant δPKC activation in the spinal cord of Wobbler mouse: a model of motor neuron disease

Protein kinase C (PKC) was suggested to play a role in the pathology of amyotrophic lateral sclerosis (ALS) patients. Activation of PKC delta (deltaPKC) modulates mitochondrially induced apoptosis. The goal of the present study was to define whether deltaPKC activation occurs in Wobbler mouse spinal cord (a model of motor neuron disease). The level of deltaPKC in the soluble fraction was significantly decreased in the spinal cord of Wobbler mice, which was associated with a significant increase in deltaPKC cleavage. Since caspase-3 is known to cleave deltaPKC, we determined caspase-3 activation in the Wobbler mice spinal cord, immunohistochemically. The results demonstrated intense immunoreactivity for activated caspase-3 in corticospinal tract motor neurons of Wobbler mice spinal cord. We hypothesize from these results that caspase-3 activation cleaves deltaPKC, which in turn promotes an aberrant signal transduction pathway in the Wobbler spinal cord.

Ceramide influences neurite outgrowth and neuroblastoma cell apoptosis regulated by novel protein kinase C isoforms

We have previously seen that protein kinase C (PKC) epsilon induces neurite outgrowth and that PKCdelta and PKCtheta elicit apoptosis in neuroblastoma cells. In this study we investigate the effects of cell-permeable C(2)-ceramide on these events in SK-N-BE(2) neuroblastoma cells. C(2)-ceramide abolishes neurite formation induced by overexpression of PKCepsilon and, in cells overexpressing PKCdelta or PKCtheta, ceramide treatment leads to apoptosis. Exposure to C(2)-ceramide also suppressed neurite outgrowth induced by retinoic acid, but ceramide did not abrogate neurite induction by treatment with the ROCK inhibitor Y-27632, demonstrating that C(2)-ceramide is not a general inhibitor of neurite outgrowth. The neurite-suppressing effect occurs independently of cell-death. Furthermore, C(2)-ceramide relocated PKCepsilon and the isolated regulatory domain of PKCepsilon from the cytosol to the perinuclear region. In contrast, neither the localization of PKCdelta nor of PKCtheta was affected by C(2)-ceramide. Taken together, the data indicate that the neurite-inhibiting effect of C(2)-ceramide treatment may be caused by a re-localization of PKCepsilon and thus identify a functional consequence of ceramide effects on PKCepsilon localization.

Methylmercury inhibits TrkA signaling through the ERK1/2 cascade after NGF stimulation of PC12 cells

Using PC12 cells as a model of neuronal differentiation, we have shown that acute exposure to methylmercury (CH3Hg) inhibits nerve growth factor (NGF)-induced activation of TrkA. In the present study, we examined the effects of CH3Hg on pathways activated by NGF. NGF-induced phosphorylation of ERK1/2 in PC12 cells was time-dependent. Concurrent exposure to CH3Hg and NGF for 2.5 min resulted in a concentration-dependent inhibition of ERK1/2 phosphorylation (EC50 = 0.018 microM). However, NGF-stimulated ERK1/2 phosphorylation was not altered after 5 min of exposure to CH3Hg. In vitro studies revealed that CH3Hg did not directly inhibit the ERK kinase MEK. As reported in other neuronal tissue, CH3Hg can inhibit PKC activity in vitro. Incubation of PC12 cell lysates with CH3Hg produced a concentration-dependent inhibition of PKC activity that was significant at 0.3-10 microM. Further studies using recombinant enzymes examined the effect of CH3Hg on PKC isoforms expressed in PC12 cells. CH3Hg inhibited PKCdelta, and zeta activity in a concentration-dependent manner at higher concentrations (3-10 microM), while a significant increase in PKCalpha activity was observed at lower concentrations (0.1 microM). However, CH3Hg had no affect on NGF-induced PKC activity in intact cells. These results show that CH3Hg inhibition of NGF-stimulated TrkA activation in PC12 cells decreases downstream signaling through the Raf/MEK/ERK cascade. In intact cells PKC does not appear to be a primary target for CH3Hg.

Induction of neurites by the regulatory domains of PKCdelta and epsilon is counteracted by PKC catalytic activity and by the RhoA pathway

We have shown that protein kinase C (PKC) epsilon, independently of its kinase activity, via its regulatory domain (RD), induces neurites in neuroblastoma cells. This study was designed to evaluate whether the same effect is obtained in nonmalignant neural cells and to dissect mechanisms mediating the effect. Overexpression of PKCepsilon resulted in neurite induction in two immortalised neural cell lines (HiB5 and RN33B). Phorbol ester potentiated neurite outgrowth from PKCepsilon-overexpressing cells and led to neurite induction in cells overexpressing PKCdelta. The effects were potentiated by blocking the PKC catalytic activity with GF109203X. Furthermore, kinase-inactive PKCdelta induced more neurites than the wild-type isoform. The isolated regulatory domains of novel PKC isoforms also induced neurites. Experiments with PKCdelta-overexpressing HiB5 cells demonstrated that phorbol ester, even in the presence of a PKC inhibitor, led to a decrease in stress fibres, indicating an inactivation of RhoA. Active RhoA blocked PKC-induced neurite outgrowth, and inhibition of the RhoA effector ROCK led to neurite outgrowth. This demonstrates that neurite induction by the regulatory domain of PKCdelta can be counteracted by PKCdelta kinase activity, that PKC-induced neurite outgrowth is accompanied by stress fibre dismantling indicating an inactivation of RhoA, and that the RhoA pathway suppresses PKC-mediated neurite outgrowth.

Cholecystokinin-stimulated protein kinase C-delta kinase activation, tyrosine phosphorylation, and translocation are mediated by Src tyrosine kinases in pancreatic acinar cells

Protein kinase C-delta (PKC-delta) is involved in growth, differentiation, tumor suppression, and regulation of other cellular processes. PKC-delta activation causes translocation, tyrosine phosphorylation, and serine-threonine kinase activity. However, little is known about the ability of G protein-coupled receptors to activate these processes or the mediators involved. In the present study, we explored the ability of the neurotransmitter/hormone, CCK, to stimulate these changes in PKC-delta and explored the mechanisms. In rat pancreatic acini under basal conditions, PKC-delta is almost exclusively located in cytosol. CCK and TPA stimulated a rapid PKC-delta translocation to membrane and nuclear fractions, which was transient with CCK. CCK stimulated rapid tyrosine phosphorylation of PKC-delta and increased kinase activity. Using tyrosine kinase (B44) and a tyrosine phosphatase inhibitor (orthovanadate), changes in both CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation were shown to correlate with changes in its kinase activity but not translocation. Both PKC-delta tyrosine phosphorylation and activation occur exclusively in particulate fractions. The Src kinase inhibitors, SU6656 and PP2, but not the inactive related compound, PP3, inhibited CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation and activation. In contrast, PP2 also had a lesser effect on CCK- but not TPA-stimulated PKC-delta translocation. CCK stimulated the association of Src kinases with PKC-delta, demonstrated by co-immunoprecipitation. These results demonstrate that CCKA receptor activation results in rapid translocation, tyrosine phosphorylation, and activation of PKC-delta. Stimulation of PKC-delta translocation precedes tyrosine phosphorylation, which is essential for activation to occur. Activation of Src kinases is essential for the tyrosine phosphorylation and kinase activation to occur and plays a partial role in translocation.

Hippocampal gene expression profiling in spatial discrimination learning

Learning and long-term memory are thought to involve temporally defined changes in gene expression that lead to the strengthening of synaptic connections in selected brain regions. We used cDNA microarrays to study hippocampal gene expression in animals trained in a spatial discrimination-learning paradigm. Our analysis identified 19 genes that showed statistically significant changes in expression when comparing Nai;ve versus Trained animals. We confirmed the changes in expression for the genes encoding the nuclear protein prothymosin(alpha) and the delta-1 opioid receptor (DOR1) by Northern blotting or in situ hybridization. In additional studies, laser-capture microdissection (LCM) allowed us to obtain enriched neuronal populations from the dentate gyrus, CA1, and CA3 subregions of the hippocampus from Nai;ve, Pseudotrained, and spatially Trained animals. Real-time PCR examined the spatial learning specificity of hippocampal modulation of the genes encoding protein kinase B (PKB, also known as Akt), protein kinase C(delta) (PKC(delta)), cell adhesion kinase(beta) (CAK(beta), also known as Pyk2), and receptor protein tyrosine phosphatase(zeta/beta) (RPTP(zeta/beta)). These studies showed subregion specificity of spatial learning-induced changes in gene expression within the hippocampus, a feature that was particular to each gene studied. We suggest that statistically valid gene expression profiles generated with cDNA microarrays may provide important insights as to the cellular and molecular events subserving learning and memory processes in the brain.

The clinical development of the bryostatins

The bryostatins are a group of novel macrocyclic lactones derived from the marine bryozoan, Bugula neritina. In vitro evidence indicates that their main mechanism of action is modulation of protein kinase C (PKC) activity. Phase I studies suggested significant antineoplastic activity against several tumor types and defined the main dose-limiting toxicity as myalgia. Bryostatin-1 has subsequently been investigated extensively in phase II clinical trials as a single agent, although trial design has been hampered by lack of human pharmacokinetic data. Results have been generally disappointing but in vitro and animal data suggests an important role for bryostatin-1 in combination with cytotoxic agents. Preliminary results of phase I studies support these observations but further work needs to be done to define the future role of the bryostatins in the clinic.

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.

Targeting protein kinase C: new therapeutic opportunities against high-grade malignant gliomas?

A large body of evidence suggests that the abnormal phenotype of neoplastic astrocytes, including their excessive proliferation rate and high propensity to invade surrounding tissues, results from mutations in critical genes involved in key cellular events. These genetic alterations can affect cell-surface-associated receptors, elements of signaling pathways, or components of the cell cycle clock, conferring a gain or a loss of relevant metabolic functions of the cells. The understanding of such phenomena may allow the development of more efficacious forms of cancer treatment. Examples are therapies specifically directed against overexpressed epidermal growth factor receptor, hyperactive Ras, excessively stimulated Raf-1, overproduced ornithine decarboxylase, or aberrantly activated cyclin-dependent kinases. The applicability of some of these approaches is now being assessed in patients suffering from primary malignant central nervous system tumors that are not amenable to current therapeutic modalities. Another potentially useful therapeutic strategy against such tumors involves the inhibition of hyperactive or overexpressed protein kinase C (PKC). This strategy is justified by the decrease in cell proliferation and invasion following inhibition of the activity of this enzyme observed in preclinical glioma models. Thus, interference with PKC activity may represent a novel form of experimental cancer treatment that may simultaneously restrain the hyperproliferative state and the invasive capacity of high-grade malignant gliomas without inducing the expected toxicity of classical cytotoxic agents. Of note, the experimental use of PKC-inhibiting agents in patients with refractory high-grade malignant gliomas has indeed led to some clinical responses. The present paper reviews the current status of the biochemistry and molecular biology of PKC, as well as the possibilities for developing novel anti-PKC-based therapies for central nervous system malignancies.

Phosphorylation-dependent regulation of phospholipase D2 by protein kinase C delta in rat Pheochromocytoma PC12 cells

Many studies have shown that protein kinase C (PKC) is an important physiological regulator of phospholipase D (PLD). However, the role of PKC in agonist-induced PLD activation has been mainly investigated with a focus on the PLD1, which is one of the two PLD isoenzymes (PLD1 and PLD2) cloned to date. Since the expression of PLD2 significantly enhanced phorbol 12-myristate 13-acetate (PMA)- or bradykinin-induced PLD activity in rat pheochromocytoma PC12 cells, we investigated the regulatory mechanism of PLD2 in PC12 cells. Two different PKC inhibitors, GF109203X and Ro-31-8220, completely blocked PMA-induced PLD2 activation. In addition, specific inhibition of PKC delta by rottlerin prevented PLD2 activation in PMA-stimulated PC12 cells. Concomitant with PLD2 activation, PLD2 became phosphorylated upon PMA or bradykinin treatment of PC12 cells. Moreover, rottlerin blocked PMA- or bradykinin-induced PLD2 phosphorylation in PC12 cells. Expression of a kinase-deficient mutant of PKC delta using adenovirus-mediated gene transfer inhibited the phosphorylation and activation of PLD2 induced by PMA in PC12 cells, suggesting the phosphorylation-dependent regulation of PLD2 mediated by PKC delta kinase activity in PC12 cells. PKC delta co-immunoprecipitated with PLD2 from PC12 cell extracts, and associated with PLD2 in vitro in a PMA-dependent manner. Phospho-PLD2 immunoprecipitated from PMA-treated PC12 cells and PLD2 phosphorylated in vitro by PKC delta were resolved by two-dimensional phosphopeptide mapping and compared. At least seven phosphopeptides co-migrated, indicating the direct phosphorylation of PLD2 by PKC delta inside the cells. Immunocytochemical studies of PC12 cells revealed that after treatment with PMA, PKC delta was translocated from the cytosol to the plasma membrane where PLD2 is mainly localized. These results suggest that PKC delta-dependent direct phosphorylation plays an important role in the regulation of PLD2 activity in PC12 cells.

Protein kinase C delta (PKCdelta) is required for protein tyrosine phosphatase mu (PTPmu)-dependent neurite outgrowth

Protein tyrosine phosphatase mu (PTPmu) is an adhesion molecule in the immunoglobulin superfamily and is expressed in the developing nervous system. We have shown that PTPmu can promote neurite outgrowth of retinal ganglion cells and it regulates neurite outgrowth mediated by N-cadherin (S. M. Burden-Gulley and S. M. Brady-Kalnay, 1999, J. Cell Biol. 144, 1323-1336). We previously demonstrated that PTPmu binds to the scaffolding protein RACK1 in yeast and mammalian cells (T. Mourton et al., 2001, J. Biol. Chem. 276, 14896-14901). RACK1 is a receptor for activated protein kinase C (PKC). In this article, we demonstrate that PKC is involved in PTPmu-dependent signaling. PTPmu, RACK1, and PKCdelta exist in a complex in cultured retinal cells and retinal tissue. Using pharmacologic inhibition of PKC, we demonstrate that PKCdelta is required for neurite outgrowth of retinal ganglion cells on a PTPmu substrate. These results suggest that PTPmu signaling via RACK1 requires PKCdelta activity to promote neurite outgrowth.

Protein kinase Cepsilon actin-binding site is important for neurite outgrowth during neuronal differentiation

We have previously shown that protein kinase Cepsilon (PKCepsilon) induces neurite outgrowth via its regulatory domain and independently of its kinase activity. This study aimed at identifying mechanisms regulating PKCepsilon-mediated neurite induction. We show an increased association of PKCepsilon to the cytoskeleton during neuronal differentiation. Furthermore, neurite induction by overexpression of full-length PKCepsilon is suppressed if serum is removed from the cultures or if an actin-binding site is deleted from the protein. A peptide corresponding to the PKCepsilon actin-binding site suppresses neurite outgrowth during neuronal differentiation and outgrowth elicited by PKCepsilon overexpression. Neither serum removal, deletion of the actin-binding site, nor introduction of the peptide affects neurite induction by the isolated regulatory domain. Membrane targeting by myristoylation renders full-length PKCepsilon independent of both serum and the actin-binding site, and PKCepsilon colocalized with F-actin at the cortical cytoskeleton during neurite outgrowth. These results demonstrate that the actin-binding site is of importance for signals acting on PKCepsilon in a pathway leading to neurite outgrowth. Localization of PKCepsilon to the plasma membrane and/or the cortical cytoskeleton is conceivably important for its effect on neurite outgrowth.

Nerve growth factor stimulates multisite tyrosine phosphorylation and activation of the atypical protein kinase C's via a src kinase pathway

Atypical protein kinase C (PKC) isoforms are required for nerve growth factor (NGF)-initiated differentiation of PC12 cells. In the present study, we report that PKC-iota becomes tyrosine phosphorylated in the membrane coincident with activation posttreatment with nerve growth factor. Tyrosine phosphorylation and activation of PKC-iota were inhibited in a dose-dependent manner by both PP2 and K252a, src and TrkA kinase inhibitors. Purified src was observed to phosphorylate and activate PKC-iota in vitro. In PC12 cells deficient in src kinase activity, both NGF-induced tyrosine phosphorylation and activation of PKC-iota were also diminished. Furthermore, we demonstrate activation of src by NGF along with formation of a signal complex including the TrkA receptor, src, and PKC-iota. Recruitment of PKC-iota into the complex was dependent on the tyrosine phosphorylation state of PKC-iota. The association of src and PKC-iota was constitutive but was enhanced by NGF treatment, with the src homology 3 domain interacting with a PXXP sequence within the regulatory domain of PKC-iota (amino acids 98 to 114). Altogether, these findings support a role for src in regulation of PKC-iota. Tyrosine 256, 271, and 325 were identified as major sites phosphorylated by src in the catalytic domain. Y256F and Y271F mutations did not alter src-induced activation of PKC-iota, whereas the Y325F mutation significantly reduced src-induced activation of PKC-iota. The functional relevance of these mutations was tested by determining the ability of each mutant to support TRAF6 activation of NF-kappaB, with significant impairment by the Y325F PKC-iota mutant. Moreover, when the Y352F mutant was expressed in PC12 cells, NGF's ability to promote survival in serum-free media was reduced. In summary, we have identified a novel mechanism for NGF-induced activation of atypical PKC involving tyrosine phosphorylation by c-Src.

A PKCbeta isoform mediates phorbol ester-induced activation of Erk1/2 and expression of neuronal differentiation genes in neuroblastoma cells

Protein kinase C (PKC) activation induces neuronal differentiation of SH-SY5Y neuroblastoma cells. This study examines the role of PKCbeta isoforms in this process. The PKCbeta-specific inhibitor LY379196 had no effect on 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced neurite outgrowth from SH-SY5Y neuroblastoma cells. On the other hand, PKCbeta inhibition suppressed the TPA-stimulated increase in neuropeptide Y mRNA, activation of neuropeptide Y gene promoter elements, and phosphorylation of Erk1/2. The TPA-induced increase in neuropeptide Y expression was also inhibited by the MEK inhibitor PD98059. These data indicate that activation of a PKCbeta isoform, through a pathway involving Erk1/2, leads to increased expression of neuronal differentiation genes in neuroblastoma cells.

Expression and regulation of phospholipase D during neuronal differentiation of PC12 cells

To assess a possible role for phospholipase D (PLD) in PC12 cell signal transduction and differentiation, we have investigated the expression of PLD in PC12 cells and found that the differentiation factor, nerve growth factor (NGF) increased PLD1 protein expression and phorbol 12-myristate 13 acetate (PMA)-induced PLD activity. During neuronal differentiation, this effect showed correlation to the protein expression levels of classical protein kinase C (PKC) isozymes, PKC-alpha and -beta II, but there was no significant increase in the protein level of RhoA, another regulatory factor for PLD activation. Interestingly, PLD1 was associated with PKC-alpha or beta II, and its association gradually increased as NGF-induced neuronal differentiation progressed. PKC inhibitor, Ro-31-8220, caused a significant inhibition of neurite outgrowth and PLD activity. Furthermore, PLD1 was constitutively associated with the Shc adaptor molecule, the overexpression of which is known to induce PLD activity and to induce neurite outgrowth. Taken together, the data in this study suggests that PLD1 is closely implicated in neuronal differentiation of PC12 cells.

Modulation of PKCδ tyrosine phosphorylation and activity in salivary and PC-12 cells by Src kinases

Protein kinase C (PKC) δ becomes tyrosine phosphorylated in rat parotid acinar cells exposed to muscarinic and substance P receptor agonists, which initiate fluid secretion in this salivary cell. Here we examine the signaling components of PKCδ tyrosine phosphorylation and effects of phosphorylation on PKCδ activity. Carbachol- and substance P-promoted increases in PKCδ tyrosine phosphorylation were blocked by inhibiting phospholipase C (PLC) but not by blocking intracellular Ca2+ concentration elevation, suggesting that diacylglycerol, rather than d- myo-inositol 1,4,5-trisphosphate production, positively modulated this phosphorylation. Stimuli-dependent increases in PKCδ activity in parotid and PC-12 cells were blocked in vivo by inhibitors of Src tyrosine kinases. Dephosphorylation of tyrosine residues by PTP1B, a protein tyrosine phosphatase, reduced the enhanced PKCδ activity. Lipid cofactors modified the tyrosine phosphorylation-dependent PKCδ activation. Two PKCδ regulatory sites (Thr-505 and Ser-662) were constitutively phosphorylated in unstimulated parotid cells, and these phosphorylations were not altered by stimuli that increased PKCδ tyrosine phosphorylation. These results demonstrate that PKCδ activity is positively modulated by tyrosine phosphorylation in parotid and PC-12 cells and suggest that PLC-dependent effects of secretagogues on salivary cells involve Src-related kinases.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.

Regulation of protein kinase C-delta and -epsilon isoforms by phorbol ester treatment of LLC-PK1 renal epithelia

BACKGROUND

LLC-PK1 renal epithelia are a widely used model for proximal tubular physiology and differentiation. Protein kinase C (PKC) has been observed to play a role in both processes. This study examines the subcellular distribution and down-regulation of PKC-delta and PKC-epsilon isoforms in phorbol ester-treated LLC-PK1 epithelia.

METHODS

Cells were treated with 10-7 mol/L 12-O-tetradecanoyl phorbol 13-acetate (TPA) for up to seven days and were extracted as total cell lysates as well as cytosolic, membrane-associated (Triton-X soluble) and a third (Triton-X insoluble) fraction. The expression and cellular localization of PKC-delta and PKC-epsilon isoforms were then detected using Western immunoblot and immunofluorescence.

RESULTS

Based on the use of an anti-PKC-delta monoclonal antibody, TPA was observed to cause a rapid decrease in total PKC-delta content, which then returned to near control levels by seven days of treatment. Immunofluorescence indicated that PKC-delta had a cytoskeletal localization within the cells, and a subtle cytoskeletal rearrangement occurred upon exposure to TPA. Western immunoblots showed that PKC-delta did not undergo the expected membrane translocation upon activation by TPA, but simply disappeared immediately from the cytosolic compartment. Conventional cell fractionation procedures such as homogenization and Triton extraction prior to Western immunoblot will, however, fail to evaluate completely PKC-delta in LLC-PK1 epithelia because of the highly stringent measures necessary to extract PKC-delta from the cytoskeletal compartment of these cells. Furthermore, we observed that a second (polyclonal) PKC-delta antibody may recognize phosphorylated forms of PKC-delta, which went unrecognized by the other antibody. PKC-epsilon was present in the cytosol, membrane, and Triton-X-insoluble fractions of the cells. TPA treatment resulted in a partial translocation of PKC-epsilon to both the membrane and Triton-X-insoluble fractions of the cell, but total PKC-epsilon remained essentially unchanged.

CONCLUSIONS

The present data indicate that the localization of PKC-delta and subsequent redistribution within the LLC-PK1 cells in response to TPA treatment is highly unique and distinct from that of PKC-epsilon and PKC-alpha. An important methodological finding is that one given antibody may not recognize all phosphoproteins of a given PKC isoform.

Nerve growth factor-induced phosphorylation of SNAP-25 in PC12 cells: a possible involvement in the regulation of SNAP-25 localization

Synaptosomal-associated protein of 25 kDa (SNAP-25), a t-SNARE protein essential for neurotransmitter release, is phosphorylated at Ser187 following activation of cellular protein kinase C by treatment with phorbol 12-myristate 13-acetate. However, it remains unclear whether neuronal activity or an endogenous ligand induces the phosphorylation of SNAP-25. Here we studied the phosphorylation of SNAP-25 in PC12 cells using a specific antibody for SNAP-25 phosphorylated at Ser187. A small fraction of SNAP-25 was phosphorylated when cells were grown in the absence of nerve growth factor (NGF). A brief treatment with NGF that was enough to activate the mitogen-activated protein kinase signal transduction pathway did not increase the phosphorylation of SNAP-25; however, phosphorylation was up-regulated after a prolonged incubation with NGF. Up-regulation was transitory, and maximum phosphorylation (a fourfold increase over basal phosphorylation) was achieved between 36 and 48 h after the addition of NGF. Immunofluorescent microscopy showed that SNAP-25 was localized primarily in the plasma membrane, although a significant population was also present in the cytoplasm. Quantitative microfluorometry revealed that prolonged treatment with NGF resulted in a preferential localization of SNAP-25 in the plasma membrane. A mutational study using a fusion protein with green fluorescent protein as a tag indicated that the point mutation of Ser187 to Ala abolished the NGF-dependent relocalization. A population of SNAP-25 in the plasma membrane was not increased by a point mutation at Ser187 to Glu; however, it was increased by prolonged treatment with NGF, indicating that the SNAP-25 phosphorylation is essential, but not sufficient, for the NGF-induced relocation to the plasma membrane. Our results suggest a close temporal relationship between the up-regulation of SNAP-25 phosphorylation and its relocation, and NGF-induced differentiation of PC12 cells.

Differential action of nerve growth factor and phorbol ester TPA on rat synaptosomal PKC isoenzymes

The subcellular redistribution of protein kinase C family members (alpha, beta, gamma, delta, epsilon and zeta isoforms) was examined in response to treatment with 12-O-tetradecanoyl-phorbol-13 acetate (TPA) or nerve growth factor (NGF) in a synaptosomal-enriched P2 fraction from rat brain. Treatment with TPA affected members of the classical-PKC family (alpha, beta and gamma), resulting in a final loss of total protein of each isoenzyme. The kinetics of changes of members of the novel-PKC family are different, the delta isoform being translocated, but not down-regulated, while the epsilon isoform showing only a slight diminishing of immunoreactivity in the soluble and particulate fractions. The atypical-PKC zeta isoform was not translocated in response to TPA. Incubation with NGF induced a loss of immunoreactivity of the cytosolic alpha, beta and epsilon isoforms, but the membrane fractions of these isoforms were not appreciably affected. In contrast, a marked translocation from cytosol to membrane was observed in the case of the gamma and delta isoforms. The zeta isoform presented a slight translocation from the particulate fraction to the soluble fraction. Thus, the results show that the effects of TPA and NGF on PKC isoforms are not coincident in synaptosomes, the 6 isoform being activated and not down-regulated by both treatments, whereas the gamma isoform is only down-regulated in the case of TPA, but presents sustained translocation with NGF, indicating that PKC isoform-specific degradation pathways exist in synaptic terminals. The effects of NGF on PKC isoforms coexist with an increase in NGF-induced polyphosphoinositide hydrolysis, suggesting the participation of phospholipases.

Protein kinase Cdelta mediates neurogenic but not mitogenic activation of mitogen-activated protein kinase in neuronal cells

In several neuronal cell systems, fibroblast-derived growth factor (FGF) and nerve growth factor (NGF) act as neurogenic agents, whereas epidermal growth factor (EGF) acts as a mitogen. The mechanisms responsible for these different cellular fates are unclear. We report here that although FGF, NGF, and EGF all activate mitogen-activated protein (MAP) kinase (extracellular signal-related kinase [ERK]) in rat hippocampal (H19-7) and pheochromocytoma (PC12) cells, the activation of ERK by the neurogenic agents FGF and NGF is dependent upon protein kinase Cdelta (PKCdelta), whereas ERK activation in response to the mitogenic EGF is independent of PKCdelta. Antisense PKCdelta oligonucleotides or the PKCdelta-specific inhibitor rottlerin inhibited FGF- and NGF-induced, but not EGF-induced, ERK activation. In contrast, EGF-induced ERK activation was inhibited by the phosphatidylinositol-3-kinase inhibitor wortmannin, which had no effect upon FGF-induced ERK activation. Rottlerin also inhibited the activation of MAP kinase kinase (MEK) in response to activated Raf, but had no effect upon c-Raf activity or ERK activation by activated MEK. These results indicate that PKCdelta functions either downstream from or in parallel with c-Raf, but upstream of MEK. Inhibition of PKCdelta also blocked neurite outgrowth induced by FGF and NGF in PC12 cells and by activated Raf in H19-7 cells, indicating a role for PKCdelta in the neurogenic effects of FGF, NGF, and Raf. Interestingly, the PKCdelta requirement is apparently cell type specific, since FGF-induced ERK activation was independent of PKCdelta in NIH 3T3 murine fibroblasts, in which FGF is a mitogen. These data demonstrate that PKCdelta contributes to growth factor specificity and response in neuronal cells and may also promote cell-type-specific differences in growth factor signaling.

Increased expression of phospholipase D1 mRNA during cAMP- or NGF-induced differentiation in PC12 cells

The rat pheochromocytoma PC12 cells have been employed as a useful model to study neuronal differentiation. To gain insight into the molecular events involved in extension of neurites, the differential expression of phospholipase D (PLD) isozymes were examined in cyclic AMP- or nerve growth factor (NGF)-treated PC12 cells. When the cells were incubated with dibutyryl cyclic AMP (dbcAMP), almost all the cells displayed prominent neurite outgrowth at 24 h. The neurites developed in NGF-treated cells at 24 h were shorter than those in the cells treated with dbcAMP. However, most of the NGF-treated cells extended long neurites at day 5. The rPLD1b mRNA increased within 6 h following dbcAMP treatment and maintained a high level up to 24 h. In contrast, the levels of rPLD1a and rPLD2 mRNAs were rather consistent throughout the time course examined. However, when the cells were treated with NGF, rPLD1a and rPLD1b mRNAs, but not rPLD2 mRNA, increased within 2 days and remained elevated up to 5 days. These results suggest the possible implication of PLD1 in PC12 cell differentiation.

Protein kinase C delta

The protein kinase C (PKC) family consists of 11 isoenzymes that, due to structural and enzymatic differences, can be subdivided into three groups: The Ca(2+)-dependent, diacylglycerol (DAG)-activated cPKCs (conventional PKCs: alpha, beta 1, beta 2, gamma); the Ca(2+)-independent, DAG-activated nPKCs (novel PKCs: delta, epsilon, eta, theta, mu), and the Ca(2+)-dependent, DAG non-responsive aPKCs (atypical PKCs: zeta, lambda/iota). PKC mu is a novel PKC, but with some special structural and enzymatic properties.

Protein kinase C-delta activation and tyrosine phosphorylation in platelets

Several protein kinase C (PKC) isoforms are expressed in human platelets. We report that PKC-delta is tyrosine phosphorylated within 30 s of platelet activation by thrombin. This correlated with a 2-3-fold increase in the kinase activity of PKC-delta relative to unstimulated platelets. The tyrosine phosphorylated PKC-delta isoform was associated with the platelet particulate (100,000 x g insoluble) fraction. Alpha(IIb)beta3 integrin mediated platelet adhesion to fibrinogen did not significantly affect PKC-delta activity. Tyrosine phosphorylation of PKC-delta was similarly not detected in fibrinogen adherent platelet lysates. Treatment of the platelets with mAb 7E3 prior to the addition of thrombin blocked aggregation having no effect on the thrombin induced PKC-delta activation. We conclude that PKC-delta is activated in platelets by an alpha(IIb)beta3 independent pathway.

Protein kinase C-alpha levels are inversely associated with growth rate in cultured human dermal fibroblasts

Human dermal fibroblasts are known to express the alpha, delta, epsilon, and zeta isoforms of protein kinase C (PKC). We asked whether the growth of human dermal fibroblasts correlates with expression of a particular PKC isoform. Of total PKC activity measured in the presence of calcium, a condition permissive for activation of all PKC isoforms, 75%) was contributed by PKC-alpha, suggesting that PKC-alpha is the dominant isoform in human dermal fibroblasts. We then further studied PKC-alpha under different culture conditions and in cultures derived from different aged donors. In both subconfluent and confluent cultures, total PKC activity and the level of PKC-alpha protein were consistently higher in slowly proliferating adult cells than in more rapidly proliferating newborn cells. Moreover, in newborn fibroblasts density strongly influenced these parameters. At subconfluent density, when cells were dividing exponentially, total PKC activity was 345+/-63 cpm/,ug protein; whereas at confluent density, when cells were growth arrested, it was 6-7 fold higher, 2334+/-50 cpm/ug protein. Immunoblot analysis using a specific monoclonal antibody against PKC-alpha exhibited a similar 6-7 fold increase in the level of PKC-alpha protein at confluent density. However, in adult cells, density had no influence on the already high total activity or level of PKC-alpha. To further determine whether the increases in the levels of total PKC activity and the alpha isoform correlate with the decreased growth rate, a characteristic of both adult donor-derived and confluent cells, total PKC activity and the level of PKC-alpha in subconfluent quiescent cells was compared to that in paired exponentially growing cells at the same density. Total PKC activity was 8836+/-71 cpm/microg protein in subconfluent quiescent cells versus 4415+/-175 cpm/microg protein in dividing cells. The level of PKC-alpha protein was also 2-3 fold higher in quiescent than in growing cultures. However, the amount of PKC-alpha mRNA in these two conditions was identical as determined by northern blot analysis. Taken together, these results suggest an inverse relationship between the levels of total PKC activity and PKC-alpha protein and fibroblast growth rate that is regulated at the post-transcriptional level.

Gene expression in the electrically stimulated differentiation of PC12 cells

Cell differentiation of PC12 cells was electrically induced to grow neurites in the absence of nerve growth factor (NGF) on the electrode surface, of which potential was modulated by a rectangular wave of potential. The electric stimulation induced the c-fos expression which is essential for cell differentiation. Non-specific calcium channel blocker, lanthanum ion, inhibited the electrically induced differentiation, while NGF-induced differentiation was not suppressed. An L-type calcium channel blocker, nifedipine, also inhibited the electrically induced calcium influx and c-fos expression. Moreover, a stretch-activated (SA) channel blocker, gadolinium ion, inhibited the electrically stimulated differentiation by blocking the calcium influx, but gave no prominent effects on the potassium ion-induced differentiation. Chelerythrine, a specific protein kinase C (PKC) inhibitor, almost inhibited the cell differentiation by the electric stimulation but not by the NGF treatment. These results indicate that the alternative potential may stimulate cell differentiation through a PKC cascade.

Activation of P2Y2 receptors by UTP and ATP stimulates mitogen-activated kinase activity through a pathway that involves related adhesion focal tyrosine kinase and protein kinase C

We examined downstream signaling events that followed the exposure of PC12 cells to extracellular ATP and UTP, and we compared the effects of these P2 receptor agonists with those of growth factors and other stimuli. Based on early findings, we focused particular attention on the mitogen-activated protein (MAP) kinase pathway. ATP and/or UTP produced increases in tyrosine phosphorylation of multiple proteins, including p42 MAP (ERK2) kinase, related adhesion focal tyrosine kinase (RAFTK) (PYK2, CAKbeta), focal adhesion kinase (FAK), Shc, and protein kinase Cdelta (PKCdelta). MAP (ERK2) kinase activity (quantified by substrate phosphorylation) was increased by UTP, ATP, phorbol 12-myristate 13-acetate, ionomycin, and growth factors. UTP and ATP were equipotent (EC50 approximately 25 microM) in stimulating MAP kinase activity, suggesting that these effects were mediated via the Gi-linked P2Y2 (P2U) receptor. Consistent with this, the UTP- and ATP-promoted activation of MAP kinase was diminished in pertussis toxin-treated cells. Treatment of cells with pertussis toxin also reduced both the UTP-dependent increases in intracellular calcium ion concentration ([Ca2+]i) and the tyrosine phosphorylation of RAFTK. Similarly, when [Ca2+]i elevation was prevented using BAPTA and EGTA, the activation of MAP kinase by UTP and ionomycin was blocked, and the tyrosine phosphorylation of RAFTK was reduced. The UTP-promoted increase in MAP kinase activity was partially reduced in cells in which PKC was down-regulated, suggesting that both PKC-dependent and PKC-independent pathways were involved. PKCdelta, which increases MAP kinase activity in some systems, became tyrosine-phosphorylated within 15 s of exposure of cells to ATP or UTP; but epidermal growth factor, nerve growth factor, and insulin had little effect. UTP also promoted the association of Shc with Grb2. These results suggest that the P2Y2 receptor-initiated activation of MAP kinase was dependent on the elevation of [Ca2+]i, involved the recruitment of Shc and Grb2, and was mediated by RAFTK and PKC.

Physiological levels of beta-amyloid peptide stimulate protein kinase C in PC12 cells

Alzheimer's beta-amyloid peptide (A beta) is normally present at nanomolar concentrations in body fluids and in the medium of cultured cells. In vitro experiments have shown that A beta has neurotrophic effects and can promote neuronal adhesion and elongation of axon-like processes. In an attempt to understand the molecular mechanisms underlying such effects, we have recently reported that nanomolar doses of A beta can stimulate protein tyrosine phosphorylation and activate phosphatidylinositol-3-kinase in neuronal cells. Here we show evidence that A beta can also activate protein kinase C, a serine/threonine kinase, in PC12 cells. First, using a serine-containing S6 peptide as an exogenous substrate, we found that nanomolar levels of A beta peptides 1-40 or 1-42 significantly stimulated an S6 phosphorylating kinase activity, whereas the A beta40-1 reverse sequence peptide had no effect. Down-regulation of PKC by prolonged (18 h) treatment with 1 microM PMA prevented the A beta-induced S6 phosphorylation. Using a more specific PKC substrate, N-terminal acetylated peptide (4-14) from myelin basic protein, we then demonstrated that A beta indeed increased PKC activity and that this activity could be blocked by the PKC inhibitor, staurosporine. Finally, immunoblotting experiments showed that A beta induced translocation of PKCgamma from cytosol to membrane and also significantly reduced cytosolic PKCalpha levels. Taken together, these data suggest that physiological levels of A beta can regulate PKC activity.

Modulation of angiotensin II type 2 receptor mRNA in rat hypothalamus and brainstem neuronal cultures by growth factors

This study investigates the regulatory effects of growth factors upon angiotensin II type 2 (AT2) mRNA levels in neurons co-cultured from newborn rat hypothalamus and brainstem. Incubation of cultured neurons with nerve growth factor (NGF; 5-50 ng/ml) caused time-dependent changes in the steady-state levels of AT2 receptor mRNA. Short-term (0.5-1.0 h) incubations with NGF resulted in significant increases in AT2 receptor mRNA, whereas longer-term incubations (4-24 h) caused significant decreases. Activation of NGF receptors is known to stimulate phospholipase C-gamma and subsequently activate protein kinase C (PKC). Incubation of cultures with the PKC activator, phorbol-12-myristate-13-acetate (PMA; 100 nM), caused temporal changes in AT2 receptor mRNA levels similar to those observed with NGF. By contrast, insulin (0.1-10 microg/ml) elicited only significant decreases in AT2 receptor mRNA levels. The observed abilities of NGF and insulin to regulate the expression of AT2 receptor mRNA are consistent with the fact that the AT2 receptor gene promoter region contains several cis DNA regulatory elements that respond to growth factor-stimulated transcription factors. These novel observations which show that NGF and insulin can regulate AT2 receptor mRNA in neurons derived from neonatal rat CNS lend support to the idea that AT2 receptors have a role in development and differentiation.

Differential expression of Rho family GTP-binding proteins and protein kinase C isozymes during C6 glial cell differentiation

The differential expression of Rho family of low molecular weight GTP-binding proteins and protein kinase C (PKC) isozymes were examined during differentiation of rat C6 glial cells to astrocytic phenotypes induced by dibutyryl cAMP (dbcAMP)/theophylline. The cells showed rapid and distinct morphological changes, resembling stellate astrocytes at 12 h after the treatment. The treated cells had a round cell body that extended several long processes each with a beaded appearance. In addition to morphological changes, Western blot analysis revealed that S-100 protein, known as a glial cell differentiation marker, increased and reached the maximal level (approximately 6-fold increase) at 24 h following the addition of dbcAMP. In the control experiments with cells cultured in the absence of serum but also without dbcAMP/theophylline, morphological changes were marginal and apparent increases of S-100 protein were not observed by Western blotting. In response to dbcAMP/theophylline treatment, RhoA showed increases in the mRNA level followed by the protein level, as inferred by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting, respectively. Rac1 and Cdc42 proteins were undetectable by Western blot analyses. In PKC isozymes, increases were observed in PKC beta 1, epsilon, and zeta by RT-PCR, and in beta 1 and epsilon by Western blotting. Among them, PKC epsilon showed the most distinct changes. Its mRNA level transiently increased from 3 to 6 h and then decreased even below the basal level at 18 h after the treatment. In contrast, Western blot analysis revealed that PKC epsilon gradually increased time-dependently to 24 h (approximately 6-fold increase), and remained elevated until 48 h. These results suggested that RhoA and PKC epsilon, and probably also PKC beta 1 and PKC zeta, were closely implicated in C6 cell differentiation.

Involvement of protein kinase C in nerve growth factor- and K-252a-stimulated calcium uptake into PC12 cells

Both nerve growth factor (NGF) and K-252a stimulate the uptake of calcium into PC12 cells. Stimulation by either is prevented by pretreatment of the cells with the tumor promoter phorbol 12-myristate 13-acetate (PMA), suggesting an involvement of protein kinase C in the stimulation. The effect of PMA is specific in that the calcium uptake stimulated by either the L-type channel agonist BAY K 8644 or by ATP is not altered in PMA-pretreated cells. An involvement of kinase C is also suggested by the inhibition of NGF- or K-252a-stimulated calcium uptake by the kinase C inhibitors staurosporine and calphostin C. Inhibition by the isoform-specific agents GO 6976 and thymeleatoxin implicates one of the classic calcium-sensitive isoforms of kinase C. The close similarity in the profiles of inhibition of NGF-stimulated and K-252a-stimulated calcium uptake by the various effectors suggests that NGF and K-252a act on calcium uptake through some of the same signaling elements.

Protein kinase C-delta mRNA is down-regulated transcriptionally and post-transcriptionally by 12-O-tetradecanoylphorbol-13-acetate

Activation of protein kinase C-delta (PKC-delta) by 12-O-tetradecanoylphorbol-13-acetate (TPA) is followed by a gradual decrease in detectable protein 12-24 h later in the mouse B lymphoma cell line A20. Down-regulation is associated with TPA-induced proteolysis and a 50-86% decrease in PKC-delta mRNA 0.5-24 h post-treatment which is due to both a 50% decrease in transcription and accelerated degradation of PKC-delta mRNA as determined using the pulse-chase method. Destabilization of PKC-delta mRNA is also observed when actinomycin D is added to cells pretreated with TPA for 2 h; however, addition of actinomycin D or cycloheximide prior to TPA treatment blocks destabilization. Addition of PKC inhibitors to TPA-treated cells also blocks destabilization of PKC-delta mRNA. Cells treated with TPA for 4 h contain an activity not found in control cells which destabilizes PKC-delta mRNA but not glyceraldehyde-3-phosphate dehydrogenase mRNA in vitro. Addition of TPA to control extracts fails to increase degradation of PKC-delta mRNA in vitro, suggesting that treatment of intact cells is required to induce the synthesis of a factor(s) that destabilizes PKC-delta mRNA. This factor(s) then acts along with transcriptional and post-translational regulatory mechanisms to down-regulate PKC-delta.

Overexpression of epsilon-protein kinase C enhances nerve growth factor-induced phosphorylation of mitogen-activated protein kinases and neurite outgrowth

Protein kinase C (PKC) activation enhances neurite outgrowth in several cell lines and primary neurons. The PKC isozymes that mediate this response are unknown. One clue to their identity has come from studies using PC12 cells treated with ethanol. In these cells, ethanol increases levels of delta-PKC and epsilon-PKC and markedly enhances nerve growth factor (NGF)-induced neurite outgrowth and activation of mitogen-activated protein (MAP) kinases by a PKC-dependent mechanism. Since these findings suggest that delta-PKC or epsilon-PKC can promote neural differentiation, we studied neurite outgrowth in stably transfected PC12 cell lines that overexpress these isozymes. Overexpression of epsilon-PKC markedly increased NGF-induced neurite outgrowth. This effect was blocked by down-regulating PKC or by treating cells with the PKC inhibitor GF 109203X. In addition, overexpression of epsilon-PKC enhanced NGF-induced phosphorylation of MAP kinases. In contrast, overexpression of delta-PKC did not alter responses to NGF. These results demonstrate that epsilon-PKC promotes NGF-induced neurite outgrowth by enhancing NGF signal transduction. These findings suggest a role for epsilon-PKC in neural differentiation and plasticity.

Protein kinase C and growth regulation of malignant gliomas

This article reviews the role of the signal transduction enzyme protein kinase C in the regulation of growth of malignant gliomas, and describes how targetting this enzyme clinically can provide a novel approach to glioma therapy.