Decrease in cytosolic calcium/phospholipid-dependent protein kinase activity following phorbol ester treatment of EL4 thymoma cells

SNARE protein SNAP25 regulates the chloride-transporter KCC2 in neurons

Inhibitory synaptic neurotransmission mediated by GABA requires a low concentration of chloride ions (Cl-) in neurons, which is established and maintained by the potassium-chloride co-transporter 2 (KCC2). While KCC2-interacting proteins are known to regulate KCC2 protein level and function, specific KCC2-interacting partners are still being identified and characterized. We asked whether SNAP25, an integral component of the SNARE-complex and a novel KCC2 interactor, regulates KCC2 protein and function in mice. We demonstrated that SNAP25 interacts with KCC2, and that this interaction is regulated by protein kinase C (PKC)-mediated phosphorylation. We also discovered that SNAP25 knockdown decreases total KCC2 in cortical neurons, and reduces the strength of synaptic inhibition, as demonstrated through a depolarization of the reversal potential for GABA (EGABA), indicating reduced KCC2 function. Our biochemical and electrophysiological data combined demonstrate that SNAP25 regulates KCC2 membrane expression and function, and in doing so, regulates inhibitory synaptic transmission.

Electrophysiological and Imaging Analysis of GFP-Tagged Protein Kinase C γ Translocation in Cerebellar Purkinje Cells

The cerebellum contains the highest density of protein kinase C (PKC) in the central nervous system. PKCγ, the major isotype accounting for over half of the PKCs in the cerebellum, is expressed exclusively in Purkinje cells (PCs). Inactivated PKCγ, which is localized in the cytoplasm of PC dendrites and soma, begins to translocate to the cell membrane upon activation. However, the physiological conditions that induce PKCγ translocation in PC remain largely unknown. Here, we virally expressed PKCγ-GFP in PCs and examined the conditions that induced its translocation to PC dendrites by whole-cell patch clamp analysis combined with confocal GFP fluorescence imaging. A single or repetitive (150 pulses at 5 Hz for 30 s) electrical stimulation to a climbing fiber (CF), which produced a complex spike(s) in PC, failed to induce translocation of PKCγ-GFP to the dendritic shaft of PCs. Direct current injection (+ 2 nA for 3 s) to PC also did not induce the translocation, although PCs generated simple spikes continuously at high rates. In contrast, high-frequency parallel fiber (PF) stimulation (50 pulses at 50 Hz for 1 s), which triggered action potentials followed by sustained depolarization (known as mGluR1-mediated slow depolarization), caused translocation of cytoplasmic PKCγ-GFP to the membrane. Low-frequency PF stimulation (150 pulses at 5 Hz for 30 s) induced continuous simple spike firing but did not induce translocation. These results suggest that CF-triggered depolarization, which causes Ca²⁺ influx through voltage-gated Ca²⁺ channels throughout PC dendrites and somas, is insufficient to induce the translocation of PKCγ, instead requiring high-frequency PF stimulation that activates mGluR1.

M1 and M2 mAChRs activate PDK1 and regulate PKC βI and ε and the exocytotic apparatus at the NMJ

Neuromuscular junctions (NMJ) regulate cholinergic exocytosis through the M₁ and M₂ muscarinic acetylcholine autoreceptors (mAChR), involving the crosstalk between receptors and downstream pathways. Protein kinase C (PKC) regulates neurotransmission but how it associates with the mAChRs remains unknown. Here, we investigate whether mAChRs recruit the classical PKCβI and the novel PKCε isoforms and modulate their priming by PDK1, translocation and activity on neurosecretion targets. We show that each M₁ and M₂ mAChR activates the master kinase PDK1 and promotes a particular priming of the presynaptic PKCβI and ε isoforms. M₁ recruits both primed-PKCs to the membrane and promotes Munc18-1, SNAP-25, and MARCKS phosphorylation. In contrast, M₂ downregulates PKCε through a PKA-dependent pathway, which inhibits Munc18-1 synthesis and PKC phosphorylation. In summary, our results discover a co-dependent balance between muscarinic autoreceptors which orchestrates the presynaptic PKC and their action on ACh release SNARE-SM mechanism. Altogether, this molecular signaling explains previous functional studies at the NMJ and guide toward potential therapeutic targets.

Functional lipidomics of vascular endothelial cells in response to laminar shear stress

Laminar shear stress generated by blood flow stimulates endothelial cells and activates signal transduction, which plays an important role in vascular homeostasis. Several lines of evidence indicate that membrane and intracellular lipids are involved in the signal transduction of biomechanical stresses. In this study, we performed global profiling of cellular lipids from human pulmonary artery endothelial cells (HPAEC) exposed to laminar shear stress. A total of 761 species of lipids were successfully annotated, with 198 of these species significantly changed in response to shear stress for 24 hours. Ether-linked lipids containing an alkyl moiety with a medium chain length (C11-C14) were uniquely upregulated, and the administration of their biosynthetic precursor 1-O-dodecyl-rac-glycerol attenuated phorbol 12-myristate 13-acetate (PMA) induced vascular cell adhesion molecule-1 (VCAM-1) expression. Given the pro-inflammatory and atherogenic roles of VCAM-1, our findings suggest that the induction of a specific group of lipids (ie, ether-linked lipids with medium length alkyl side chain) may confer atheroprotective and anti-inflammatory roles to vascular endothelial cells under flow conditions.

Protein kinase C fusion proteins are paradoxically loss of function in cancer

Within the AGC kinase superfamily, gene fusions resulting from chromosomal rearrangements have been most frequently described for protein kinase C (PKC), with gene fragments encoding either the C-terminal catalytic domain or the N-terminal regulatory moiety fused to other genes. Kinase fusions that eliminate regulatory domains are typically gain of function and often oncogenic. However, several quality control pathways prevent accumulation of aberrant PKC, suggesting that PKC fusions may paradoxically be loss of function. To explore this topic, we used biochemical, cellular, and genome editing approaches to investigate the function of fusions that retain the portion of the gene encoding either the catalytic domain or regulatory domain of PKC. Overexpression studies revealed that PKC catalytic domain fusions were constitutively active but vulnerable to degradation. Genome editing of endogenous genes to generate a cancer-associated PKC fusion resulted in cells with detectable levels of fusion transcript but no detectable protein. Hence, PKC catalytic domain fusions are paradoxically loss of function as a result of their instability, preventing appreciable accumulation of protein in cells. Overexpression of a PKC regulatory domain fusion suppressed both basal and agonist-induced endogenous PKC activity, acting in a dominant-negative manner by competing for diacylglycerol. For both catalytic and regulatory domain fusions, the PKC component of the fusion proteins mediated the effects of the full-length fusions on the parameters examined, suggesting that the partner protein is dispensable in these contexts. Taken together, our findings reveal that PKC gene fusions are distinct from oncogenic fusions and present a mechanism by which loss of PKC function occurs in cancer.

Protein Kinase C: Targets to Regenerate Brain Injuries?

Acute or chronic injury to the central nervous system (CNS), causes neuronal death and irreversible cognitive deficits or sensory-motor alteration. Despite the capacity of the adult CNS to generate new neurons from neural stem cells (NSC), neuronal replacement following an injury is a restricted process, which does not naturally result in functional regeneration. Therefore, potentiating endogenous neurogenesis is one of the strategies that are currently being under study to regenerate damaged brain tissue. The insignificant neurogenesis that occurs in CNS injuries is a consequence of the gliogenic/non-neurogenic environment that inflammatory signaling molecules create within the injured area. The modification of the extracellular signals to generate a neurogenic environment would facilitate neuronal replacement. However, in order to generate this environment, it is necessary to unearth which molecules promote or impair neurogenesis to introduce the first and/or eliminate the latter. Specific isozymes of the protein kinase C (PKC) family differentially contribute to generate a gliogenic or neurogenic environment in injuries by regulating the ADAM17 mediated release of growth factor receptor ligands. Recent reports describe several non-tumorigenic diterpenes isolated from plants of the Euphorbia genus, which specifically modulate the activity of PKC isozymes promoting neurogenesis. Diterpenes with 12-deoxyphorbol or lathyrane skeleton, increase NPC proliferation in neurogenic niches in the adult mouse brain in a PKCβ dependent manner exerting their effects on transit amplifying cells, whereas PKC inhibition in injuries promotes neurogenesis. Thus, compounds that balance PKC activity in injuries might be of use in the development of new drugs and therapeutic strategies to regenerate brain injuries.

Protein kinase Cα gain-of-function variant in Alzheimer's disease displays enhanced catalysis by a mechanism that evades down-regulation

Conventional protein kinase C (PKC) family members are reversibly activated by binding to the second messengers Ca²⁺ and diacylglycerol, events that break autoinhibitory constraints to allow the enzyme to adopt an active, but degradation-sensitive, conformation. Perturbing these autoinhibitory constraints, resulting in protein destabilization, is one of many mechanisms by which PKC function is lost in cancer. Here, we address how a gain-of-function germline mutation in PKCα in Alzheimer's disease (AD) enhances signaling without increasing vulnerability to down-regulation. Biochemical analyses of purified protein demonstrate that this mutation results in an ∼30% increase in the catalytic rate of the activated enzyme, with no changes in the concentrations of Ca²⁺ or lipid required for half-maximal activation. Molecular dynamics simulations reveal that this mutation has both localized and allosteric effects, most notably decreasing the dynamics of the C-helix, a key determinant in the catalytic turnover of kinases. Consistent with this mutation not altering autoinhibitory constraints, live-cell imaging studies reveal that the basal signaling output of PKCα-M489V is unchanged. However, the mutant enzyme in cells displays increased sensitivity to an inhibitor that is ineffective toward scaffolded PKC, suggesting the altered dynamics of the kinase domain may influence protein interactions. Finally, we show that phosphorylation of a key PKC substrate, myristoylated alanine-rich C-kinase substrate, is increased in brains of CRISPR-Cas9 genome-edited mice containing the PKCα-M489V mutation. Our results unveil how an AD-associated mutation in PKCα permits enhanced agonist-dependent signaling via a mechanism that evades the cell's homeostatic down-regulation of constitutively active PKCα.

Synaptic Activity and Muscle Contraction Increases PDK1 and PKCβI Phosphorylation in the Presynaptic Membrane of the Neuromuscular Junction

Conventional protein kinase C βI (cPKCβI) is a conventional protein kinase C (PKC) isoform directly involved in the regulation of neurotransmitter release in the neuromuscular junction (NMJ). It is located exclusively at the nerve terminal and both synaptic activity and muscle contraction modulate its protein levels and phosphorylation. cPKCβI molecular maturation includes a series of phosphorylation steps, the first of which is mediated by phosphoinositide-dependent kinase 1 (PDK1). Here, we sought to localize PDK1 in the NMJ and investigate the hypothesis that synaptic activity and muscle contraction regulate in parallel PDK1 and cPKCβI phosphorylation in the membrane fraction. To differentiate the presynaptic and postsynaptic activities, we abolished muscle contraction with μ-conotoxin GIIIB (μ-CgTx-GIIIB) in some experiments before stimulation of the phrenic nerve (1 Hz, 30 min). Then, we analyzed total and membrane/cytosol fractions of skeletal muscle by Western blotting. Results showed that PDK1 is located exclusively in the nerve terminal of the NMJ. After nerve stimulation with and without coincident muscle contraction, total PDK1 and phosphorylated PDK1 (pPDK1) protein levels remained unaltered. However, synaptic activity specifically enhanced phosphorylation of PDK1 in the membrane, an important subcellular location for PDK1 function. This increase in pPDK1 coincides with a significant increase in the phosphorylation of its substrate cPKCβI also in the membrane fraction. Moreover, muscle contraction maintains PDK1 and pPDK1 but increases cPKCβI protein levels and its phosphorylation. Thus, even though PDK1 activity is maintained, pcPKCβI levels increase in concordance with total cPKCβI. Together, these results indicate that neuromuscular activity could induce the membrane targeting of pPDK1 in the nerve terminal of the NMJ to promote the phosphorylation of the cPKCβI, which is involved in ACh release.

Conventional protein kinase C in the brain: 40 years later

Protein kinase C (PKC) is a family of enzymes whose members transduce a large variety of cellular signals instigated by the receptor-mediated hydrolysis of membrane phospholipids. While PKC has been widely implicated in the pathology of diseases affecting all areas of physiology including cancer, diabetes, and heart disease-it was discovered, and initially characterized, in the brain. PKC plays a key role in controlling the balance between cell survival and cell death. Its loss of function is generally associated with cancer, whereas its enhanced activity is associated with neurodegeneration. This review presents an overview of signaling by diacylglycerol (DG)-dependent PKC isozymes in the brain, and focuses on the role of the Ca2+-sensitive conventional PKC isozymes in neurodegeneration.

Enzastaurin: A lesson in drug development

Enzastaurin is an orally administered drug that was intended for the treatment of solid and haematological cancers. It was initially developed as an isozyme specific inhibitor of protein kinase Cβ (PKCβ), which is involved in both the AKT and MAPK signalling pathways that are active in many cancers. Enzastaurin had shown encouraging preclinical results for the prevention of angiogenesis, inhibition of proliferation and induction of apoptosis as well as showing limited cytotoxicity within phase I clinical trials. However, during its assessment in phase II and III clinical trials the efficacy of enzastaurin was poor both in combination with other drugs and as a single agent. In this review, we will discuss the development of enzastaurin from drug design to clinical testing, exploring target identification, validation and preclinical assessment. Finally, we will consider the clinical evaluation of enzastaurin as an example of the challenges associated with drug development. In particular, we discuss the poor translation of drug efficacy from preclinical animal models, inappropriate end point analysis, limited standards in phase I clinical trials, insufficient use of biomarker analysis and also patient stratification, all of which contributed to the failure to achieve approval of enzastaurin as an anticancer therapeutic.

Smooth muscle-protein translocation and tissue function

Smooth muscle (SM) tissue is a complex organization of multiple cell types and is regulated by numerous signaling molecules (neurotransmitters, hormones, cytokines, etc.). SM contractile function can be regulated via expression and distribution of the contractile and cytoskeletal proteins, and activation of any of the second messenger pathways that regulate them. Spatial-temporal changes in the contractile, cytoskeletal or regulatory components of SM cells (SMCs) have been proposed to alter SM contractile activity. Ca(2+) sensitization/desensitization can occur as a result of changes at any of these levels, and specific pathways have been identified at all of these levels. Understanding when and how proteins can translocate within the cytoplasm, or to-and-from the plasmalemma and the cytoplasm to alter contractile activity is critical. Numerous studies have reported translocation of proteins associated with the adherens junction and G protein-coupled receptor activation pathways in isolated SMC systems. Specific examples of translocation of vinculin to and from the adherens junction and protein kinase C (PKC) and 17 kDa PKC-potentiated inhibitor of myosin light chain phosphatase (CPI-17) to and from the plasmalemma in isolated SMC systems but not in intact SM tissues are discussed. Using both isolated SMC systems and SM tissues in parallel to pursue these studies will advance our understanding of both the role and mechanism of these pathways as well as their possible significance for Ca(2+) sensitization in intact SM tissues and organ systems.

The novel protein kinase C epsilon isoform at the adult neuromuscular synapse: location, regulation by synaptic activity-dependent muscle contraction through TrkB signaling and coupling to ACh release

Background

Protein kinase C (PKC) regulates a variety of neural functions, including neurotransmitter release. Although various PKC isoforms can be expressed at the synaptic sites and specific cell distribution may contribute to their functional diversity, little is known about the isoform-specific functions of PKCs in neuromuscular synapse. The present study is designed to examine the location of the novel isoform nPKCε at the neuromuscular junction (NMJ), their synaptic activity-related expression changes, its regulation by muscle contraction, and their possible involvement in acetylcholine release.

Results

We use immunohistochemistry and confocal microscopy to demonstrate that the novel isoform nPKCε is exclusively located in the motor nerve terminals of the adult rat NMJ. We also report that electrical stimulation of synaptic inputs to the skeletal muscle significantly increased the amount of nPKCε isoform as well as its phosphorylated form in the synaptic membrane, and muscle contraction is necessary for these nPKCε expression changes. The results also demonstrate that synaptic activity-induced muscle contraction promotes changes in presynaptic nPKCε through the brain-derived neurotrophic factor (BDNF)-mediated tyrosine kinase receptor B (TrkB) signaling. Moreover, nPKCε activity results in phosphorylation of the substrate MARCKS involved in actin cytoskeleton remodeling and related with neurotransmission. Finally, blocking nPKCε with a nPKCε-specific translocation inhibitor peptide (εV1-2) strongly reduces phorbol ester-induced ACh release potentiation, which further indicates that nPKCε is involved in neurotransmission.

Conclusions

Together, these results provide a mechanistic insight into how synaptic activity-induced muscle contraction could regulate the presynaptic action of the nPKCε isoform and suggest that muscle contraction is an important regulatory step in TrkB signaling at the NMJ.

Tonic and phasic smooth muscle contraction is not regulated by the PKCα - CPI-17 pathway in swine stomach antrum and fundus

Regulation of myosin light chain phosphatase (MLCP) via protein kinase C (PKC) and the 17 kDa PKC-potentiated inhibitor of myosin light chain phosphatase (CPI-17) has been reported as a Ca²⁺ sensitization signaling pathway in smooth muscle (SM), and thus may be involved in tonic vs. phasic contractions. This study examined the protein expression and spatial-temporal distribution of PKCα and CPI-17 in intact SM tissues. KCl or carbachol (CCh) stimulation of tonic stomach fundus SM generates a sustained contraction while the phasic stomach antrum generates a transient contraction. In addition, the tonic fundus generates greater relative force than phasic antrum with 1 µM phorbol 12, 13-dibutyrate (PDBu) stimulation which is reported to activate the PKCα – CPI-17 pathway. Western blot analyses demonstrated that this contractile difference was not caused by a difference in the protein expression of PKCα or CPI-17 between these two tissues. Immunohistochemical results show that the distribution of PKCα in the longitudinal and circular layers of the fundus and antrum do not differ, being predominantly localized near the SM cell plasma membrane. Stimulation of either tissue with 1 µM PDBu or 1 µM CCh does not alter this peripheral PKCα distribution. There are no differences between these two tissues for the CPI-17 distribution, but unlike the PKCα distribution, CPI-17 appears to be diffusely distributed throughout the cytoplasm under relaxed tissue conditions but shifts to a primarily peripheral distribution at the plasma membrane with stimulation of the tissues with 1 µM PDBu or 1 µM CCh. Results from double labeling show that neither PKCα nor CPI-17 co-localize at the adherens junction (vinculin/talin) at the membrane but they do co-localize with each other and with caveoli (caveolin) at the membrane. This lack of difference suggests that the PKCα - CPI-17 pathway is not responsible for the tonic vs. phasic contractions observed in stomach fundus and antrum.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

Protein kinase C, an elusive therapeutic target?

Protein kinase C (PKC) has been a tantalizing target for drug discovery ever since it was first identified as the receptor for the tumour promoter phorbol ester in 1982. Although initial therapeutic efforts focused on cancer, additional indications--including diabetic complications, heart failure, myocardial infarction, pain and bipolar disorder--were targeted as researchers developed a better understanding of the roles of eight conventional and novel PKC isozymes in health and disease. Unfortunately, both academic and pharmaceutical efforts have yet to result in the approval of a single new drug that specifically targets PKC. Why does PKC remain an elusive drug target? This Review provides a short account of some of the efforts, challenges and opportunities in developing PKC modulators to address unmet clinical needs.

Novel PKCα-mediated phosphorylation site(s) on cofilin and their potential role in terminating histamine release

PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation, and the novel phosphorylation contributes to F-actin remodeling by regulating the ability of cofilin to bind to 14-3-3ζ and to depolymerize and/or sever F-actin. This highly regulated mechanism is necessary for the proper termination of degranulation.

Using specific inhibitors, kinase-negative mutants, and small interfering RNA against protein kinase Cα (PKCα) or PKCβI, we find that PKCβI positively regulates degranulation in rat basophilic leukemia–2H3 cells, whereas PKCα negatively regulates degranulation. Mass spectrometric and mutagenic analyses reveal that PKCα phosphorylates cofilin at Ser-23 and/or Ser-24 during degranulation. Overexpression of a nonphosphorylatable form (S23,24A), but not that of a mutant-mimicking phosphorylated form (S23,24E), increases degranulation. Furthermore, the S23,24A mutant binds to F-actin and retains its depolymerizing and/or cleavage activity; conversely, the S23,24E mutant is unable to sever actin filaments, resulting in F-actin polymerization. In addition, the S23,24E mutant preferentially binds to the 14-3-3ζ protein. Fluorescence-activated cell sorting analysis with fluorescein isothiocyanate–phalloidin and simultaneous observation of degranulation, PKC translocation, and actin polymerization reveals that during degranulation, actin polymerization is dependent on PKCα activity. These results indicate that a novel PKCα-mediated phosphorylation event regulates cofilin by inhibiting its ability to depolymerize F-actin and bind to 14-3-3ζ, thereby promoting F-actin polymerization, which is necessary for cessation of degranulation.

PKCδ activation mediates angiogenesis via NADPH oxidase activity in PC-3 prostate cancer cells

BACKGROUND

PKCδ is generally known as a pro-apoptotic and anti-proliferative enzyme in human prostate cancer cells.

METHODS

Here, we investigated the role of PKCδ on the growth of PC-3 human prostate cancer cells in vivo and in vitro.

RESULTS

We found that sustained treatment with a specific PKCδ activator (ψδ receptor for active C kinase, ψδRACK) increased growth of PC-3 xenografts. There was increased levels of HIF-1α, vascular endothelial growth factor and CD31-positive cells in PC-3 xenografts, representative of increased tumor angiogenesis. Mechanistically, PKCδ activation increased the levels of reactive oxygen species (ROS) by binding to and phosphorylating NADPH oxidase, which induced its activity. Also, PKCδ-induced activation of NADPH oxidase increased the level of HIF-1α.

CONCLUSIONS

Our results using tumors from the PC-3 xenograft model suggest that PKCδ activation increases angiogenic activity in androgen-independent PC-3 prostate cancer cells by increasing NADPH oxidase activity and HIF-1α levels and thus may partly be responsible for increased angiogenesis in advanced prostate cancer.

Are zinc-finger domains of protein kinase C dynamic structures that unfold by lipid or redox activation?

Protein kinase C (PKC) is activated by lipid second messengers or redox action, raising the question whether these activation modes involve the same or alternate mechanisms. Here we show that both lipid activators and oxidation target the zinc-finger domains of PKC, suggesting a unifying activation mechanism. We found that lipid agonist-binding or redox action leads to zinc release and disassembly of zinc fingers, thus triggering large-scale unfolding that underlies conversion to the active enzyme. These results suggest that PKC zinc fingers, originally considered purely structural devices, are in fact redox-sensitive flexible hinges, whose conformation is controlled both by redox conditions and lipid agonists.

Sustained inhibition of PKCα reduces intravasation and lung seeding during mammary tumor metastasis in an in vivo mouse model

Metastasis is the major reason for breast cancer-related deaths. Although there is a host of indirect evidence for a role of PKCα in primary breast cancer growth, its role in the molecular pathways leading to metastasis have not been comprehensively studied. By treating mice with αV5-3, a novel peptide inhibitor selective for PKCα, we were able to determine how PKCα regulates metastasis of mammary cancer cells using a syngeneic and orthotopic model. The primary tumor growth was not affected by αV5-3 treatment. However, the mortality rate was reduced and metastasis in the lung decreased by more than 90% in the αV5-3-treated mice relative to the control-treated mice. αV5-3 treatment reduced intravasation by reducing MMP-9 activities. αV5-3 treatment also reduced lung seeding of tumor cells and decreased cell migration, effects that were accompanied by a reduction in NFκB-activity and cell surface levels of the CXCL12 receptor, CXCR4. αV5-3 treatment caused no apparent toxicity in non-tumor bearing naïve mice. Rather, inhibiting PKCα protected against liver damage and increased the number of immune cells in tumor-bearing mice. Importantly, αV5-3 showed superior efficacy relative to anti-CXCR4 antibody in reducing metastasis, in vivo. Together, these data show that pharmacological inhibition of PKCα effectively reduces mammary cancer metastasis by targeting intravasation and lung seeding steps in the metastatic process and suggest that PKCα-specific inhibitors, such as αV5-3, can be used to study the mechanistic roles of PKCα specifically and may provide a safe and effective treatment for the prevention of lung metastasis of breast cancer patients.

Highly Specific Modulators of Protein Kinase C Localization: Applications to Heart Failure

Heart failure (HF) in which the blood supply does not match the body's needs, affects 10% of the population over 65 years old. The protein kinase C (PKC) family of kinases has a key role in normal and disease states. Here we discuss the role of PKC in HF and focus on the use of specific PKC regulators to identify the mechanism leading to this Pathology and potential leads for therapeutics.

Synaptic activity-related classical protein kinase C isoform localization in the adult rat neuromuscular synapse

Protein kinase C (PKC) is essential for signal transduction in a variety of cells, including neurons and myocytes, and is involved in both acetylcholine release and muscle fiber contraction. Here, we demonstrate that the increases in synaptic activity by nerve stimulation couple PKC to transmitter release in the rat neuromuscular junction and increase the level of alpha, betaI, and betaII isoforms in the membrane when muscle contraction follows the stimulation. The phosphorylation activity of these classical PKCs also increases. It seems that the muscle has to contract in order to maintain or increase classical PKCs in the membrane. We use immunohistochemistry to show that PKCalpha and PKCbetaI were located in the nerve terminals, whereas PKCalpha and PKCbetaII were located in the postsynaptic and the Schwann cells. Stimulation and contraction do not change these cellular distributions, but our results show that the localization of classical PKC isoforms in the membrane is affected by synaptic activity.

A simple analogue of tumor-promoting aplysiatoxin is an antineoplastic agent rather than a tumor promoter: development of a synthetically accessible protein kinase C activator with bryostatin-like activity

Protein kinase C (PKC) is widely recognized as a therapeutic target in intractable diseases such as cancer, Alzheimer's disease (AD), and acquired immune deficiency syndrome (AIDS). While inhibition of PKC is a general therapeutic strategy for the treatment of cancer, PKC activators are potential therapeutic agents for AD and AIDS. However, concerns have been raised about their therapeutic use since PKC activators such as phorbol esters exhibit potent tumor-promoting activities. Naturally occurring bryostatin 1 (bryo-1), prostratin, and 12-deoxyphorbol 13-phenylacetate (DPP) are fascinating PKC activators without tumor-promoting activities. Bryo-1 is currently in clinical trials for the treatment of cancer and is also effective against AD. Prostratin and DPP are attractive candidates for the adjunctive treatment of human immunodeficiency virus (HIV) infection. However, their limited availability from natural sources and synthetic complexity have hampered further development as therapeutic agents. We report here easy access (22 steps) to a simple analogue (1) of the tumor-promoting aplysiatoxin (ATX) as a novel PKC activator with anticancer and anti-tumor-promoting activities. Anticancer activities of 1 against several human cancer cell lines were comparable to those of bryo-1. Moreover, 1 as well as bryo-1 significantly inhibited the Epstein-Barr virus early antigen (EBV-EA) induction by the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA), whereas ATX strongly induced EBV-EA. This inhibitory effect is characteristic of antitumor promoters. Compound 1 as well as bryo-1 displayed significant binding and activation of PKCdelta and induced its translocation to the nuclear membrane in CHO-K1 cells. This study provides a synthetically accessible PKC activator with bryo-1-like activities, which could be another therapeutic lead for cancer, AD, and AIDS.

Centrosomal PKCbetaII and pericentrin are critical for human prostate cancer growth and angiogenesis

Angiogenesis is critical in the progression of prostate cancer. However, the interplay between the proliferation kinetics of tumor endothelial cells (angiogenesis) and tumor cells has not been investigated. Also, protein kinase C (PKC) regulates various aspects of tumor cell growth, but its role in prostate cancer has not been investigated in detail. Here, we found that the proliferation rates of endothelial and tumor cells oscillate asynchronously during the growth of human prostate cancer xenografts. Furthermore, our analyses suggest that PKCbetaII was activated during increased angiogenesis and that PKCbetaII plays a key role in the proliferation of endothelial cells and tumor cells in human prostate cancer; treatment with a PKCbetaII-selective inhibitor, betaIIV5-3, reduced angiogenesis and tumor cell proliferation. We also find a unique effect of PKCbetaII inhibition on normalizing pericentrin (a protein regulating cytokinesis), especially in endothelial cells as well as in tumor cells. PKCbetaII inhibition reduced the level and mislocalization of pericentrin and normalized microtubule organization in the tumor endothelial cells. Although pericentrin has been known to be up-regulated in epithelial cells of prostate cancers, its level in tumor endothelium has not been studied in detail. We found that pericentrin is up-regulated in human tumor endothelium compared with endothelium adjacent to normal glands in tissues from prostate cancer patients. Our results suggest that a PKCbetaII inhibitor such as betaIIV5-3 may be used to reduce prostate cancer growth by targeting both angiogenesis and tumor cell growth.

Synthesis, conformational analysis, and biological evaluation of 1-hexylindolactam-V10 as a selective activator for novel protein kinase C isozymes

Conventional and novel protein kinase C (PKC) isozymes are the main targets of tumor promoters. We developed 1-hexylindolactam-V10 ( 5) as a selective activator for novel PKC isozymes that play important roles in various cellular processes related to tumor promotion, ischemia--reperfusion injury in the heart, and Alzheimer's disease. The compound existed as a mixture of three conformers. The trans-amide restricted analogues of 5 ( 14 and 15) hardly bound to PKC isozymes, suggesting that the active conformation of 5 could be that with a cis-amide. Compound 5 selectively translocated novel PKC isozymes over conventional PKC isozymes in HeLa cells at 0.1-1 microM. These results suggest that 5 could be useful for the functional analysis of novel PKC isozymes.

Immunogold electron microscopic demonstration of distinct submembranous localization of the activated gammaPKC depending on the stimulation

We examined the precise intracellular translocation of gamma subtype of protein kinase C (gammaPKC) after various extracellular stimuli using confocal laser-scanning fluorescent microscopy (CLSM) and immunogold electron microscopy. By CLSM, treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a slow and irreversible accumulation of green fluorescent protein (GFP)-tagged gammaPKC (gammaPKC-GFP) on the plasma membrane. In contrast, treatment with Ca(2+) ionophore and activation of purinergic or NMDA receptors induced a rapid and transient membrane translocation of gammaPKC-GFP. Although each stimulus resulted in PKC localization at the plasma membrane, electron microscopy revealed that gammaPKC showed a subtle but significantly different localization depending on stimulation. Whereas TPA and UTP induced a sustained localization of gammaPKC-GFP on the plasma membrane, Ca(2+) ionophore and NMDA rapidly translocated gammaPKC-GFP to the plasma membrane and then restricted gammaPKC-GFP in submembranous area (<500 nm from the plasma membrane). These results suggest that Ca(2+) influx alone induced the association of gammaPKC with the plasma membrane for only a moment and then located this enzyme at a proper distance in a touch-and-go manner, whereas diacylglycerol or TPA tightly anchored this enzyme on the plasma membrane. The distinct subcellular targeting of gammaPKC in response to various stimuli suggests a novel mechanism for PKC activation.

Effects of long-term elevated glucose on collagen formation by mesangial cells

Glomerulosclerosis is one of the complications of diabetes that occurs after many years of uncontrolled hyperglycemia. Mesangial cells (MCs) exposed to high glucose (HG) for short periods have shown that transforming growth factor-beta (TGF-beta) and activated diacylglycerol-dependent protein kinase C (PKC) mediate increased collagen formation. Our study examined collagen formation by MCs exposed to HG for 8 weeks. Exposure to HG in overnight culture resulted in the activation of all PKC isoforms. In contrast, 8-week exposure to HG resulted in the persistent activation of PKC-delta, did not change PKC-alpha or -beta activity, and decreased PKC-epsilon activity while increasing collagen I and IV gene and protein expression. Collagen IV accumulation was reversed by specific PKC-delta inhibition. Collagen IV gene expression was completely normalized by TGF-beta neutralization; however, this was associated with plasminogen activator inhibitor-1 (PAI-1) overexpression and a modest reduction in collagen protein. Our studies suggest that prolonged exposure to HG results in PKC-delta-driven collagen accumulation by MCs mediated by PAI-1 but independent of TGF-beta.

Pituitary adenylate cyclase-activating polypeptide-induced differentiation of embryonic neural stem cells into astrocytes is mediated via the beta isoform of protein kinase C

We have found previously that pituitary adenylate cyclase-activating polypeptide (PACAP) increases the number of astrocytes generated from cultured mouse neural stem cells (NSCs) via a mechanism that is independent of the cyclic AMP/protein kinase A pathway (Ohno et al., 2005). In the present study, the signaling pathway involved in the differentiation process was further investigated. PACAP-induced differentiation was inhibited by the phospholipase C inhibitor, U73122, the protein kinase C (PKC) inhibitor, chelerythrine, and the intracellular calcium chelator, BAPTA-AM, and was mimicked by phorbol 12-myristate 13-acetate (PMA), but not by 4alpha-PMA. These results suggest that the PACAP-generated signal was mediated via the PACAP receptor, PAC1 stimulated heterotrimeric G-protein, resulting in activation of phospholipase C, followed by calcium- and phospholipid-dependent protein kinase C (cPKC). To elucidate the involvement of the different isoforms of cPKC, their gene and protein expression were examined. Embryonic NSCs expressed alpha and betaII PKC, but lacked PKCgamma. When NSCs were exposed to 2 nM PACAP, protein expression levels of the betaII isoform transiently increased two-fold before differentiation, returning to basal levels by Day 4, whereas the level of PKCalpha increased linearly up to Day 6. Overexpression of PKCbetaII with adenovirus vector synergistically enhanced differentiation in the presence of 1 nM PACAP, whereas expression of the dominant-negative mutant of PKCbetaII proved inhibitory. These results indicate that the beta isoform of PKC plays a crucial role in the PACAP-induced differentiation of mouse embryonic NSCs into astrocytes.

The protein kinase C pathway mediates cardioprotection induced by cardiac-specific overexpression of fibroblast growth factor-2

Elucidation of protective mechanisms against ischemia-reperfusion injury is vital to the advancement of therapeutics for ischemic heart disease. Our laboratory has previously shown that cardiac-specific overexpression of fibroblast growth factor-2 (FGF2) results in increased recovery of contractile function and decreased infarct size following ischemia-reperfusion injury and has established a role for the mitogen-activated protein kinase (MAPK) signaling cascade in the cardioprotective effect of FGF2. We now show an additional role for the protein kinase C (PKC) signaling cascade in the mediation of FGF2-induced cardioprotection. Overexpression of FGF2 (FGF2 Tg) in the heart resulted in decreased translocation of PKC-delta but had no effect on PKC-alpha, -epsilon, or -zeta. In addition, multiple alterations in PKC isoform translocation occur during ischemia-reperfusion injury in FGF2 Tg hearts as assessed by Western blot analysis and confocal immunofluorescent microscopy. Treatment of FGF2 Tg and nontransgenic (NTg) hearts with the PKC inhibitor bisindolylmaleimide (1 micromol/l) revealed the necessity of PKC signaling for FGF2-induced reduction of contractile dysfunction and myocardial infarct size following ischemia-reperfusion injury. Western blot analysis of FGF2 Tg and NTg hearts subjected to ischemia-reperfusion injury in the presence of a PKC pathway inhibitor (bisindolylmaleimide, 1 micromol/l), an mitogen/extracellular signal-regulated kinase/extracellular signal-regulated kinase (MEK/ERK) pathway inhibitor (U-0126, 2.5 micromol/l), or a p38 pathway inhibitor (SB-203580, 2 micromol/l) revealed a complicated signaling network between the PKC and MAPK signaling cascades that may participate in FGF2-induced cardioprotection. Together, these data suggest that FGF2-induced cardioprotection is mediated via a PKC-dependent pathway and that the PKC and MAPK signaling cascades are integrally connected downstream of FGF2.

Persistent activation of protein kinase Calpha is not necessary for expression of cerebellar long-term depression

Protein kinase Calpha (PKCalpha) plays a major role in the induction of long-term depression (LTD) in a cerebellar Purkinje cell (PC). The sequential activation model for classical PKC states that PKCalpha translocates to the plasma membrane by binding Ca(++) and then becomes fully activated by binding diacylglycerol (DAG), which enables estimation of the activity by monitoring its localization. Here, we performed simultaneous electrophysiological recording and fluorescence imaging in a cultured PC expressing GFP-tagged PKCalpha. When a PC was depolarized, PKCalpha transiently translocated to the plasma membrane in a Ca(++)-dependent manner. Application of membrane permeable DAG or the blocker of DAG lipase prolonged the translocation. These results suggest that the sequential activation model is applicable to PCs. Conjunctive applications of glutamate and depolarization pulse induced LTD, but did not prolong the translocation. Thus, our results imply that persistent activation of PKCalpha is not necessary for the expression of LTD.

Peptides Derived from the C2 Domain of Protein Kinase Cϵ (ϵPKC) Modulate ϵPKC Activity and Identify Potential Protein-Protein Interaction Surfaces

Peptides derived from protein kinase C (PKC) modulate its activity by interfering with critical protein-protein interactions within PKC and between PKC and PKC-binding proteins (Souroujon, M. C., and Mochly-Rosen, D. (1998) Nat. Biotechnol. 16, 919-924). We previously demonstrated that the C2 domain of PKC plays a critical role in these interactions. By focusing on epsilonPKC and using a rational approach, we then identified one C2-derived peptide that acts as an isozyme-selective activator and another that acts as a selective inhibitor of epsilonPKC. These peptides were used to identify the role of epsilonPKC in protection from cardiac and brain ischemic damage, in prevention of complications from diabetes, in reducing pain, and in protecting transplanted hearts. The efficacy of these two peptides led us to search for additional C2-derived peptides with PKC-modulating activities. Here we report on the activity of a series of 5-9-residue peptides that are derived from regions that span the length of the C2 domain of epsilonPKC. These peptides were tested for their effect on PKC activity in cells in vivo and in an ex vivo model of acute ischemic heart disease. Most of the peptides acted as activators of PKC, and a few peptides acted as inhibitors. PKC-dependent myristoylated alanine-rich C kinase substrate phosphorylation in epsilonPKC knock-out cells revealed that only a subset of the peptides were selective for epsilonPKC over other PKC isozymes. These epsilonPKC-selective peptides were also protective of the myocardium from ischemic injury, an epsilonPKC-dependent function (Liu, G. S., Cohen, M. V., Mochly-Rosen, D., and Downey, J. M. (1999) J. Mol. Cell. Cardiol. 31, 1937-1948), and caused selective translocation of epsilonPKC over other isozymes when injected systemically into mice. Examination of the structure of the C2 domain from epsilonPKC revealed that peptides with similar activities clustered into discrete regions within the domain. We propose that these regions represent surfaces of protein-protein interactions within epsilonPKC and/or between epsilonPKC and other partner proteins; some of these interactions are unique to epsilonPKC, and others are common to other PKC isozymes.

RBCK1, a protein kinase CbetaI (PKCbetaI)-interacting protein, regulates PKCbeta-dependent function

RBCK1 (RBCC protein interacting with PKC 1) has originally been identified as a protein kinase CbetaI (PKCbetaI)-binding partner by a two-hybrid screen and as one of the gene transcripts that increases during adult cardiac hypertrophy. To address whether RBCK1 and PKCbetaI functions are interconnected, we used cultured neonatal myocytes where we previously found that the activity of PKCbetaI is required for an increase in cell size, also called hypertrophy. In this study, we showed that acute treatment of cardiac myocytes with phenylephrine, a prohypertrophic stimulant, transiently increased the association of RBCK1 with PKCbetaI within 1 min. A prolonged phenylephrine treatment also resulted in an increase of the interaction of the two proteins. Endogenous RBCK1 protein levels increased upon phenylephrine-induced hypertrophy. Further, adenovirus-based RBCK1 overexpression in the absence of phenylephrine increased cardiac cell size. This RBCK1-mediated hypertrophy required PKCbeta activity, since the increase in cell size was inhibited when the RBCK1-expressing cells were treated with PKCbeta-selective antagonists, supporting our previous observation that both PKCbetaI and PKCbetaII are required for hypertrophy. Unexpectedly, RBCK1-induced increased cell size was inhibited by phenylephrine. This effect correlated with a decrease in the level of both PKCbeta isoforms. Most importantly, RNA interference for RBCK1 significantly inhibited the increase in cell size of cardiac myocytes following phenylephrine treatment. Our results suggest that RBCK1 binds PKCbetaI and is a key regulator of PKCbetaI function in cells and that, together with PKCbetaII, the three proteins are essential for developmental hypertrophy of cardiac myocytes.

WISP-2/CCN5 Is Involved As a Novel Signaling Intermediate in Phorbol Ester-Protein Kinase Cα-Mediated Breast Tumor Cell Proliferation

PMA and active phorbol esters stimulate the proliferation of various tumor cells, including ER-positive human breast tumor cell lines. However, the specific signaling pathways involved in the PMA-induced mitogenic effect on breast tumor cells have not been fully elucidated. In the present study, we explored the mechanisms associated with the mitogenic influence of PMA on breast tumor cells. Following an acute exposure (i.e., within 2 to 6 h) to PMA (50 nM), a mitogenic effect was observed on WISP-2/CCN5-positive breast tumor cell lines, including MCF-7, ZR-75-1 and SKBR-3 cells, and induction of WISP-2/CCN5 mRNA expression paralleled the observed mitogenic proliferation. This effect was undetected in WISP-2/CCN5 negative MDA-MB-231 breast tumor cells or human mammary epithelial cells with or without ER-alpha transfection. The mitogenic effect of PMA was perturbed by short hairpin RNA (shRNA)-mediated inhibition of WISP-2/CCN5 signaling in MCF-7 cells. Moreover, the upregulation of WISP-2/CCN5 by PMA is not ER dependent but is instead mediated through a complex PKCalpha-MAPK/ERK and SAPK/JNK signaling pathway, which leads to growth stimulation of MCF-7 breast tumor cells. These series of experiments provide the first evidence that WISP-2/CCN5 is a novel signaling molecule that critically participates in the mitogenic action of PMA on noninvasive, WISP-2/CCN5-positive breast tumor cells through PKCalpha-dependent, multiple molecular signal transduction pathways.

Design and synthesis of 8-octyl-benzolactam-V9, a selective activator for protein kinase C epsilon and eta

Conventional (alpha, betaI, betaII, gamma) and novel (delta, epsilon, eta, theta) protein kinase C (PKC) isozymes are main targets of tumor promoters, such as phorbol esters and indolactam-V (ILV). We have recently found that 1-hexyl derivatives of indolinelactam-V (2, 3), in which the indole ring of ILV was replaced with the indoline ring, showed a binding preference for novel PKCs over conventional PKCs. To develop a new ILV analogue displaying increased synthetic accessibility and improved binding selectivity for novel PKCs, we have designed 8-octyl-benzolactam-V9 (4), a simple analogue without the pyrrolidine moiety of 2 and 3. Compound 4 showed significant binding selectivity for isolated C1B domains of novel PKCs. Moreover, 4 translocated PKC epsilon and eta from the cytoplasm to the plasma membrane of HeLa cells at 1 microM, whereas other PKC isozymes did not respond even at 10 microM. These results indicate that 4 could be a selective activator for PKC epsilon and eta.

PMA- and ANG II-induced PKC regulation of the renal Na+-HCO3−cotransporter (hkNBCe1)

The renal electrogenic Na+-HCO3−cotransporter (hkNBCe1) plays a major role in the bicarbonate reabsorption by the kidney. We examined how PMA- and ANG II-activated PKCs regulate hkNBCe1 expressed with or without the ANG II receptors AT1Bin Xenopus laevis oocytes. We found that 10 nM PMA halved the hkNBCe1 current detected in voltage-clamped oocytes. A PKC-specific inhibitor GF-109203X, and a specific inhibitor of Ca-dependent conventional PKCαβγ, GÖ-6976, significantly reduced PMA inhibition. PMA did not alter surface expression of the cotransporters, but it significantly increased hkNBCe1-PKCαβγ membrane association. We found that at 10−6M, ANG II halved the hkNBCe1 current detected in oocytes coexpressing cotransporters with AT1B. A PKC-specific inhibitor GF-109203X, and a PKCε translocation inhibitor εV1–2 peptide as well as BAPTA-AM (but not GÖ-6976), significantly reduced ANG II inhibition. At 10−6M, ANG II significantly decreased surface expression of the cotransporters and increased hkNBCe1-PKCε membrane association. Additionally, we found that at 10−11and 10−10M ANG II stimulated hkNBCe1 current. This effect was blocked by BAPTA-AM and partially reduced by GF-109203X. We also found that ANG II increased intracellular Ca2+in fluo 4-loaded oocytes. Our results suggest that 1) PMA inhibition of hkNBCe1 is mediated by Ca-dependent PKCαβγ and 10 nM PMA does not induce downregulation of cotransporter surface expression. 2) ANG II (10−6M) inhibition of hkNBCe1 is mediated by both Ca-independent PKCε and downregulation of cotransporter surface expression, possibly triggered by intracellular Ca2+mobilization. 3) Similar to proximal tubule, acute ANG II has a biphasic effect on hkNBCe1 coexpressed with AT1Bin X. laevis oocytes.

Impact of PKCdelta on estrogen receptor localization and activity in breast cancer cells

Regulation of estrogen receptor (ER) function in breast cancer cells is a complex process involving different signalling mechanisms. One signal transduction component that appears to influence ER signalling is protein kinase C (PKC). PKCdelta is a particular isoenzyme of the novel PKC subfamily that plays a role in growth control, differentiation and apoptosis. The aim of the present study was to investigate the impact of PKCdelta on the regulation of the transcriptional activity of the human ERalpha. By using 12-O-tetradecanoylphorbol-13-acetate (TPA), Bryostatin1 and Rottlerin, we show that active PKCdelta is a proproliferative factor in estrogen-dependent breast cancer cells. Furthermore, activation of PKCdelta by TPA resulted in activation and nuclear translocation of ERalpha and in an increase of ER-dependent reporter gene expression. Transfection and expression of the regulatory domain RDdelta of PKCdelta, which is inhibitory to PKCdelta, inhibited the TPA-induced ERalpha activation and translocation. ERalpha was not phosphorylated by PKCdelta; however, glycogen synthase kinase-3 (GSK3) was identified as a substrate of PKCdelta. The expression of RDdelta resulted in a decrease of TPA-induced GSK3 phosphorylation and translocation into the nucleus. We suggest that GSK3 plays a role in the PKCdelta-related nuclear translocation of ERalpha.

Protein kinase C-alpha mediates TNF release process in RBL-2H3 mast cells

1 To clarify the mechanism of mast cell TNF secretion, especially its release process after being produced, we utilized an antiallergic drug, azelastine (4-(p-chlorobenzyl)-2-(hexahydro-1-methyl-1H-azepin-4-yl)-1-(2H)- phthalazinone), which has been reported to inhibit TNF release without affecting its production in ionomycin-stimulated RBL-2H3 cells. 2 Such inhibition was associated with the suppression of an ionomycin-induced increase in membrane-associated PKC activity rather than the suppression of Ca2+ influx, suggesting that PKC might be involved in TNF release process. 3 To see whether conventional PKC family (cPKCs) are involved, we investigated the effects of a selective cPKC inhibitor (Gö6976) and an activator (thymeleatoxin) on TNF release by adding them 1 h after cell stimulation. By this time, TNF mRNA expression had reached its maximum. Gö6976 markedly inhibited TNF release, whereas thymeleatoxin enhanced it, showing a key role of cPKC in TNF post-transcriptional process, possibly its releasing step. 4 To determine which subtype of cPKCs could be affected by azelastine, Western blotting and live imaging by confocal microscopy were conducted to detect the translocation of endogenous cPKC (alpha, betaI and betaII) and transfected GFP-tagged cPKC, respectively. Both methods clearly demonstrated that 1 microM azelastine selectively inhibits ionomycin-triggered translocation of (alpha)PKC without acting on betaI or betaIIPKC. 5 In antigen-stimulated cells, such a low concentration of azelastine did not affect either (alpha)PKC translocation or TNF release, suggesting a functional link between (alpha)PKC and the TNF-releasing step. 6 These results suggest that (alpha)PKC mediates the TNF release process and azelastine inhibits TNF release by selectively interfering with the recruitment of (alpha)PKC in the pathway activated by ionomycin in RBL-2H3 cells.

Protein Kinase Cδ Activation Induces Apoptosis in Response to Cardiac Ischemia and Reperfusion Damage

Heart attacks caused by occlusion of coronary arteries are often treated by mechanical or enzymatic removal of the occlusion and reperfusion of the ischemic heart. It is now recognized that reperfusion per se contributes to myocardial damage, and there is a great interest in identifying the molecular basis of this damage. We recently showed that inhibiting protein kinase Cdelta (PKCdelta) protects the heart from ischemia and reperfusion-induced damage. Here, we demonstrate that PKCdelta activity and mitochondrial translocation at the onset of reperfusion mediates apoptosis by facilitating the accumulation and dephosphorylation of the pro-apoptotic BAD (Bcl-2-associated death promoter), dephosphorylation of Akt, cytochrome c release, PARP (poly(ADP-ribose) polymerase) cleavage, and DNA laddering. Our data suggest that PKCdelta activation has a critical proapoptotic role in cardiac responses following ischemia and reperfusion.

Propagation of γPKC translocation along the dendrites of Purkinje cell in γPKC-GFP transgenic mice

To elucidate spatial and temporal profiles of the protein kinase C (PKC) activation in relation to neuronal functions including synaptic plasticity, we tried to detect PKC translocation in living brain slices. We first developed brain region-specific and inducible gammaPKC-GFP transgenic mice using a tetracycline (tet)-regulated system. In the transgenic mice, the expression of gammaPKC-GFP was region-specifically regulated by the promoter and abolished by the administration of doxycycline. Cerebellar slices from the mice were utilized for intracellular recording and fluorescence imaging of gammaPKC-GFP in Purkinje cells. GFP fluorescence was uniformly distributed from soma to dendritic arbor. When mGluR agonists were applied, the intensity was transiently increased at the edge of the dendrite and concomitantly decreased in the cytoplasm, indicating that gammaPKC translocated to the plasma membrane. This transient change in the pattern of GFP fluorescence simultaneously occurred throughout the Purkinje cell dendrites by agonist stimulation. Translocation of gammaPKC-GFP was also induced by electrical stimulation of parallel fibres. However, the event was not restricted at the distal dendrites, propagated forwardly along the dendritic tree and reached to the proximal trunk close to the soma. Time course of the propagation was slower than the electrical signal and Ca(2+) waves and faster than conveying molecules through microtubules. The present results indicate that PKC signals activated locally by parallel fibre input could propagate to the soma through dendrites in living Purkinje neurones. The findings may provide us with a new insight for understanding molecular mechanisms of the synaptic plasticity including cerebellar long-term depression.

Phospholipase A2 products retain a neuron specific gamma isoform of PKC on the plasma membrane through the C1 domain--a molecular mechanism for sustained enzyme activity

To clarify molecular mechanism for sustained activation of gamma protein kinase C (gammaPKC), a neuron-specific subtype, we investigated the involvement of phospholipase A2 (PLA2) products in the membrane association of gammaPKC upon activation of G protein coupled purinoceptors in CHO-K1 and NG 108-15 cells. In addition, the functional domain responsible for PLA2-product mediated retention of gammaPKC on the plasma membrane was determined by simultaneously monitoring two different fluorescence-tagged gammaPKCs and mutants in the same living CHO-K1 cells. Purinoceptor activation by UTP induced a transient translocation of gammaPKC from the cytoplasm to the plasma membrane. Interestingly, PLA2 inhibitors, bromoenol lactone (BEL) and arachidonyl-trifluoromethyl ketone (AACOF3), shortened the retention time of gammaPKC on the plasma membrane in cells treated with UTP, while a DAG kinase inhibitor did not affect it. The C1 domain deficient mutant (DeltaC1-gammaPKC) also showed short membrane association compared with wild type gammaPKC, when cells are treated with UTP or arachidonic acid (AA) plus a Ca(2+) ionophore. However, deletion of C1A or C1B subdomains (DeltaC1A-gammaPKC or DeltaC1B-gammaPKC) did not alter the retention time on the plasma membrane, whereas PLA2 inhibitor shortened the retention times of both mutants. These results indicate that PLA2 products prolong the retention of gammaPKC on the plasma membrane through the C1A and/or C1B subdomain in purinoceptor-stimulated CHO-K1 cells. The importance of PLA2 product and C1 domain for the retention of gammaPKC on the membrane was also confirmed using neuronal cell line, suggesting that these are part of molecular machinery for sustaining enzyme activity in neurons.

Amyloid beta peptide directly inhibits PKC activation

A putative protein kinase C (PKC) pseudosubstrate domain in beta amyloid (Abeta) suggests a potential interaction between Abeta and PKC. In this study, we investigated whether and how Abeta interacts with PKC. Abeta peptides inhibited PKC phosphorylation in a dose-dependent manner in cell-free in vitro condition, suggesting a direct interaction between Abeta and PKC. Experiments involving deletion of the Abeta sequence indicated that the putative PKC pseudosubstrate domain (Abeta 28-30) is critical for Abeta-PKC interaction. Addition of Abeta peptides to cultured B103 cells reduced the activated forms of PKCalpha and PKCepsilon. It also inhibited phorbol-12,13-dibutyrate (PDBu)-induced membrane translocation of PKCalpha and PKCepsilon without altering their expression levels, indicating that activation of intracellular PKC is inhibited by treatment of Abeta peptides. These results suggest that Abeta peptides inhibit PKC activation via direct interactions, which may play a role in pathogenesis of AD.

Diacylglycerol's affair with protein kinase C turns 25

This year marks the 25-year anniversary of the discovery by Nishizuka and co-workers that diacylglycerol activates the ubiquitous signal transducer protein kinase C. This discovery placed the lipid second messenger-protein kinase C signaling pathway center stage alongside the cAMP-protein kinase A pathway, which was already established as a fundamental mechanism for transducing extracellular signals.

Autoregulation of bone sialoprotein gene in pre-osteoblastic and non-osteoblastic cells

Regulation of the bone sialoprotein (BSP) gene is important in the differentiation of osteoblasts, in bone matrix mineralization, and in tumor metastasis. We investigated BSP gene transcription by performing functional analysis of the 9256bp of the 5' flanking region of the murine BSP gene containing its promoter. We found that the forced expression of BSP stimulated mouse BSP promoter activity in a dose-dependent manner in both MC3T3-E1 preosteoblast and HEK-293 cell lines, which was transcriptional factor Cbfa1 independent. Co-culture of cells separately expressing BSP promoter reporter and BSP failed to mediate the BSP autoregulation, suggesting that the event might happen intracellularly. Deletion analysis of the BSP promoter indicated that the proximal promoter (110bp) was sufficient to confer this autoregulation. We conclude that the BSP gene is autoregulated in part by a positive feedback on its own promoter.

A Critical Intramolecular Interaction for Protein Kinase Cϵ Translocation

Disruption of intramolecular interactions, translocation from one intracellular compartment to another, and binding to isozyme-specific anchoring proteins termed RACKs, accompany protein kinase C (PKC) activation. We hypothesized that in inactive epsilonPKC, the RACK-binding site is engaged in an intramolecular interaction with a sequence resembling its RACK, termed psiepsilonRACK. An amino acid difference between the psiepsilonRACK sequence in epsilonPKC and its homologous sequence in epsilonRACK constitutes a change from a polar non-charged amino acid (asparagine) in epsilonRACK to a polar charged amino acid (aspartate) in epsilonPKC. Here we show that mutating the aspartate to asparagine in epsilonPKC increased intramolecular interaction as indicated by increased resistance to proteolysis, and slower hormone- or PMA-induced translocation in cells. Substituting aspartate for a non-polar amino acid (alanine) resulted in binding to epsilonRACK without activators, in vitro, and increased translocation rate upon activation in cells. Mathematical modeling suggests that translocation is at least a two-step process. Together our data suggest that intramolecular interaction between the psiepsilonRACK site and RACK-binding site within epsilonPKC is critical and rate limiting in the process of PKC translocation.

[Analysis of molecular mechanism regulating spatio-temporal localization and activity of protein kinase C and diacylglycerol kinase using live imaging]

Protein kinase C (PKC) changes its subcellular localization depending on extracellular signals including hormones and neurotransmitters. Such translocation is referred to as "targeting" in this review. The live imaging technique using GFP has allowed the dynamic movement of PKC to be visualized in living cells and revealed a remarkable diversity in PKC targeting. These studies indicate an importance of targeting in regulating the physiological and isotype-specific function of PKC. Like PKC, diacylglycerol kinase (DGK), which phosphorylates diacylglycerol resulting in attenuation of PKC, subtype-specifically translocates to particular subcellular compartments including the plasma membrane and Golgi complex. In addition, it has been shown that the localization and activation of the two functionally-related kinases are well organized by direct interaction and phosphorylation. This review summarizes diversity in targeting of PKC and DGK and the molecular mechanisms regulating their targeting.

Regulatory proteins with a sense of direction: cell cycle signalling network in Caulobacter

Localization of kinases and other signalling molecules at discrete cellular locations is often an essential component of signal transduction in eukaryotes. Caulobacter crescentus is a small, single-celled bacterium that presumably lacks intracellular organelles. Yet in Caulobacter, the subcellular distribution of several two-component signal transduction proteins involved in the control of polar morphogenesis and cell cycle progression changes from a fairly dispersed distribution to a tight accumulation at one or both poles in a spatial and temporal pattern that is reproduced during each cell cycle. This cell cycle-dependent choreography suggests that similarly to what happens in eukaryotes, protein localization provides a means of modulating signal transduction in bacteria. Recent studies have provided important insights into the biological role and the mechanisms for the differential localization of these bacterial signalling proteins during the Caulobacter cell cycle.