Identification of PKC-isoform-specific biological actions using pharmacological approaches

Move over protein kinase C, you've got company: alternative cellular effectors of diacylglycerol and phorbol esters

Diacylglycerol is an essential second messenger in mammalian cells. The most prominent intracellular targets of diacylglycerol and of the functionally analogous phorbol esters belong to the protein kinase C (PKC) family. However, at least five alternative types of high-affinity diacylglycerol/phorbol-ester receptor are known: chimaerins, protein kinase D, RasGRPs, Munc13s and DAG kinase gamma. Recent evidence indicates that these have functional roles in diacylglycerol second messenger signalling in vivo and that several cellular processes depend on these targets rather than protein kinase C isozymes. These findings contradict the still prevalent view according to which all diacylglycerol/phorbol-ester effects are caused by the activation of protein kinase C isozymes. RasGRP1 (in Ras/Raf/MEK/ERK signalling) and Munc13-1 (in neurotransmitter secretion) are examples of non-PKC diacylglycerol/phorbol-ester receptors that mediate diacylglycerol and phorbol-ester effects originally thought to be caused by PKC isozymes. In the future, pharmacological studies on PKC must be complemented with alternative experimental approaches to allow the separation of PKC-mediated effects from those caused by alternative targets of the diacylglycerol second messenger pathway. The examples of RasGRP1 and Munc13-1 show that detailed genetic analyses of C(1)-domain-containing non-PKC diacylglycerol/phorbol-ester receptors in mammals are ideally suited to achieve this goal.

Group-I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to extracellular signal-regulated kinase (ERK) activation via distinct, but overlapping, signalling pathways

The coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the extracellular signal-regulated protein kinase (ERK) pathway has been studied in Chinese hamster ovary cell-lines where receptor expression is under inducible control. Both mGlu receptors stimulated comparable, robust and agonist concentration-dependent ERK activations in the CHO cell-lines. The mGlu1a receptor-mediated ERK response was almost completely attenuated by pertussis toxin (PTx) pretreatment, whereas the mGlu5a-ERK response, and the phosphoinositide response to activation of either receptor, was PTx-insensitive. mGlu1a and mGlu5a receptor coupling to ERK occurred via mechanisms independent of phosphoinositide 3-kinase activity and intracellular and/or extracellular Ca2+ concentration. While acute treatment with a protein kinase C (PKC) inhibitor did not attenuate agonist-stimulated ERK activation, down-regulation of PKCs by phorbol ester treatment for 24 h did attenuate both mGlu1a and mGlu5a receptor-mediated responses. Further, inhibition of Src non-receptor tyrosine kinase activity by PP1 attenuated the ERK response generated by both receptor subtypes, but only mGlu1a receptor-ERK activation was attenuated by PDGF receptor tyrosine kinase inhibitor AG1296. These findings demonstrate that, although expressed in a common cell background, these closely related mGlu receptors utilize different G proteins to cause ERK activation and may recruit different tyrosine kinases to facilitate this response.

Differential expression of protein kinase C isoenzymes related to high nitric oxide synthase activity in a T lymphoma cell line

Protein kinase C (PKC) is critical for T lymphocyte activation and proliferation, while nitric oxide synthase (NOS) may function both as an activator or inhibitor of T cell apoptosis. Both enzymatic activities were studied in T lymphoma cells in comparison to normal and activated T lymphocytes. Here we show a higher translocation of PKC in BW5147 lymphoma cells than in mitogen-stimulated T lymphocytes. Tumor cells overexpressed PKC zeta isoform, while high levels of the PKC beta isotype were found in mitogen-stimulated T lymphocytes. Moreover, tumoral T cells showed high NOS activity, almost undetectable in normal or stimulated T lymphocytes. PKC and NOS inhibitors or the intracellular delivery of an anti-PKC zeta antibody diminished both NO production and proliferation in tumor cells. These results suggest that atypical PKC zeta isoform expression and its association with NOS activity regulation would participate in the multistep process leading to BW5147 cell malignant transformation.

Protein kinase C inhibitors counteract the ethanol effects on myelin basic protein expression in differentiating CG-4 oligodendrocytes

Abnormal formation of myelin appears to be one defect contributing to the development of the neuropathology associated with the fetal alcohol syndrome. Using the CG-4 cell line we previously showed that 25-75 mM EtOH downregulates the expression of myelin basic protein (MBP) in differentiating oligodendrocytes (OLGs) without affecting morphological development (Dev. Brain Res. 128 (2001) 9). Here we showed that a relatively low concentration of 12-phorbol-13-myristate acetate (PMA) mimicked the EtOH-caused inhibition of MBP expression without affecting morphology. The inhibition of MBP expression by 100 mM EtOH or 1 nM PMA was completely counteracted by three inhibitors of protein kinase C (PKC), bisindolylmaleimide I, chelerythrine chloride, and calphostin C, indicating that EtOH downregulated MBP expression by activating PKC. We investigated whether the EtOH activation resulted, in part, from upregulation of the expression of PKC isozymes. Of 11 PKC isozymes examined, CG-4 OLGs expressed nine; PKCs alpha, beta1, beta2; delta, epsilon, eta; lambda, zeta; mu; while PKC isozymes gamma and theta were not detected. Only five PKC isozymes, alpha, beta1, beta2, eta, and mu, displayed developmental changes in expression. However, EtOH did not upregulate the early expression of any PKC isozyme during the first 2 days of differentiation, the developmental stage when it downregulates MBP expression in CG-4 cells. The similar effects of PMA and EtOH indicate that EtOH delays MBP expression by activating at least one phorbol ester-sensitive PKC isozyme in oligodendrocytes without upregulating its expression.

Phorbol ester-mediated neurotensin secretion is dependent on the PKC-alpha and -delta isoforms

Neurotensin (NT) plays an important role in gastrointestinal secretion, motility, and growth. The mechanisms regulating NT secretion are not entirely known. Our purpose was to define the role of the PKC signaling pathway in secretion of NT from BON cells, a human pancreatic carcinoid cell line that produces and secretes NT peptide. We demonstrated expression of all 11 PKC isoforms at varying levels in untreated BON cells. Expression of PKC-alpha, -beta2, -delta, and -mu isoforms was most pronounced. Immunofluorescent staining showed PKC-alpha and -mu expression throughout the cytoplasm and in the membrane. Also, significant fluorescence of PKC-delta was noted in the nucleus and cytoplasm. Treatment with PMA induced translocation of PKC-alpha, -delta, and -mu from cytosol to membrane. Activation of PKC-alpha, -delta, and -mu was further confirmed by kinase assays. Addition of PKC-alpha inhibitor Gö-6976 at a nanomolar concentration, other PKC inhibitors Gö-6983 and GF-109203X, or PKC-delta-specific inhibitor rottlerin significantly inhibited PMA-mediated NT release. Overexpression of either PKC-alpha or -delta increased PMA-mediated NT secretion compared with control cells. We demonstrated that PMA-mediated NT secretion in BON cells is associated with translocation and activation of PKC-alpha, -delta, and -mu. Furthermore, inhibition of PKC-alpha and -delta blocked PMA-stimulated NT secretion, suggesting a critical role for these isoforms in NT release.

The different responses of rat glutamate transporter type 2 and its mutant (tyrosine 403 to histidine) activity to volatile anesthetics and activation of protein kinase C

Glutamate transporters play an important role in homeostasis of extracellular glutamate, a major excitatory neurotransmitter and a potential neurotoxin. In mammalian brain, glutamate transporter type 2 (EAAT2) is the most abundant form. Studies of molecular structures demonstrated that tyrosine 403 is critical in regulating the ion selectivity and transport mode of EAAT2. We hypothesized that wild type EAAT2 and its mutant at tyrosine 403 have different responses to volatile anesthetics, commonly used anesthetics that have been shown to affect glutamate transporter activity and decrease extracellular glutamate concentrations. We used site-directed mutagenesis and oocyte expression systems to test the hypothesis. Volatile anesthetics did not affect the activity of wild type EAAT2, isolated from rat hippocampus. When tyrosine 403 was replaced by histidine (Y403H), volatile anesthetics (isoflurane or halothane) at clinically relevant concentrations significantly decreased the transporter activity. Okadaic acid, a phosphatase inhibitor, significantly prolonged the isoflurane-induced inhibition. This inhibition was reversed by staurosporine and calphostin C, two protein kinase C (PKC) inhibitors, but not by the third PKC inhibitor, chelerythrine. Phorbol 12-myristate 13-acetate, a PKC activator, inhibited the activity of both wild type and Y403H EAAT2. This inhibition was also reversed by the same two PKC inhibitors but not by the third one. These results suggest that the switch of tyrosine 403 to histidine rendered EAAT2 sensitive to volatile anesthetics, a phenomenon that may require protein phosphorylation. PKC may be involved in the regulation of the activity of both wild type and Y403H EAAT2.

Conventional protein kinase C mediates phorbol-dibutyrate-induced cytoskeletal remodeling in a7r5 smooth muscle cells

Phorbol dibutyrate (PDBu) induced the formation of podosome-like structures together with partial disassembly of actin stress fibers in A7r5 smooth muscle cells. These podosomes contained alpha-actinin, F-actin, and vinculin and exhibit a tubular, column-like structure arising perpendicularly from the bottom of PDBu-treated cells. The conventional protein kinase C (PKC) antagonist, GO6976, inhibited PDBu-induced cytoskeletal remodeling at 0.1 microM, whereas the novel PKC antagonist, rottlerin, was ineffective at 10 microM. PDBu induced the translocation of the conventional PKC-alpha but not the novel PKC-delta to the sites of podosome formation in A7r5 cells. Although partial disassembly of actin stress fibers was observed in both Y-27632- and PDBu-treated cells, focal adhesions were much reduced in number and size only in Y-27632-treated cells. Furthermore, PDBu restored focal adhesions in Y-27632-treated cells. Live video fluorescence microscopy of alpha-actinin GFP revealed a lag phase of about 20 min prior to the rapid formation and dynamic reorganization of podosomes during PDBu treatment. These findings suggest that conventional PKCs mediate PDBu-induced formation of dynamic podosome-like structures in A7r5 cells, and Rho-kinase is unlikely to be the underlying mechanism. The podosome columns could represent molecular scaffolds where PKC-alpha phosphorylates regulatory proteins necessary for Ca(2+) sensitization in smooth muscle cells.

Regulation of the neuronal glutamate transporter excitatory amino acid carrier-1 (EAAC1) by different protein kinase C subtypes

In previous studies, we have shown that activation of protein kinase C (PKC) rapidly (within minutes) increases the activity and cell surface expression of the glutamate transporter EAAC1 in two systems that endogenously express this transporter (C6 glioma cells and cocultures of neurons and astrocytes). However, the magnitude of the increase in activity is greater than the increase in cell surface expression. In addition, certain compounds completely block the increase in cell surface expression but only partially attenuate the increase in activity. We hypothesized that PKC increases EAAC1 activity by increasing cell surface expression and catalytic efficiency and that two different subtypes of PKC mediate these effects. To address these hypotheses, the PKC subtypes expressed by C6 glioma cells were identified. Of the PKC subtypes that are activated by phorbol esters, only PKCalpha, PKCdelta, and PKCepsilon were observed. Gö6976, a compound that blocks PKCalpha at concentrations that do not inhibit PKCdelta or PKCepsilon, partially inhibited the increase in uptake but completely abolished the increase in EAAC1 cell surface expression. The 'Gö6976-insensitive' increase in activity was not associated with a change in total transporter expression but was associated with an increase in the V(max). Na(+)-dependent glycine transport was not increased, providing indirect evidence that the Gö6976-insensitive increase in activity was not caused by a change in the Na(+) electrochemical gradient required for activity. Finally, by down-regulating different subtypes of PKC, we found evidence that PKCepsilon mediates the increase in EAAC1 activity that is independent of changes in cell surface expression and found further evidence that PKCalpha mediates the increase in cell surface expression. The potential relationship of the present work with a previously identified role for PKCalpha in certain forms of synaptic plasticity is discussed.

Activation of protein kinase C beta II by the stereo-specific phosphatidylserine receptor is required for phagocytosis of apoptotic thymocytes by resident murine tissue macrophages

We showed previously that protein kinase C (PKC) is required for phagocytosis of apoptotic leukocytes by murine alveolar (AMø) and peritoneal macrophages (PMø) and that such phagocytosis is markedly lower in AMø compared with PMø. In this study, we examined the roles of individual PKC isoforms in phagocytosis of apoptotic thymocytes by these two Mø populations. By immunoblotting, AMø expressed equivalent PKC eta but lower amounts of other isoforms (alpha, betaI, betaII, delta, epsilon, mu, and zeta), with the greatest difference in betaII expression. A requirement for PKC betaII for phagocytosis was demonstrated collectively by phorbol 12-myristate 13-acetate-induced depletion of PKC betaII, by dose-response to PKC inhibitor Ro-32-0432, and by use of PKC betaII myristoylated peptide as a blocker. Exposure of PMø to phosphatidylserine (PS) liposomes specifically induced translocation of PKC betaII and other isoforms to membranes and cytoskeleton. Both AMø and PMø expressed functional PS receptor, blockade of which inhibited PKC betaII translocation. Our results indicate that murine tissue Mø require PKC betaII for phagocytosis of apoptotic cells, which differs from the PKC isoform requirement previously described in Mø phagocytosis of other particles, and imply that a crucial action of the PS receptor in this process is PKC betaII activation.

Inhibition by Wnt-1 or Wnt-3a of nerve growth factor-induced differentiation of PC12 cells is reversed by bisindolylmaleimide-I but not by several other PKC inhibitors

Wnt-1 and Wnt-3a exhibit redundancy in neural crest development. We have found that they do not produce the same effects on PC12 cells, which were obtained from the adrenal medulla, a neural crest derivative. However, both Wnt-1 or Wnt-3a inhibit nerve growth factor (NGF)-induced neurite outgrowth. The inhibition is reversed by the protein kinase C (PKC) inhibitor, bisindolylmaleimide-I, but it did not reverse Wnt-1-induced activation of the canonical Wnt pathway. The Wnt-1 inhibitory effect was not reversed by several other PKC inhibitors, by phorbol ester-induced down-regulation of PKC, or by pertussis toxin, which is known to inhibit another Wnt signaling pathway, the Wnt/Ca(2+) pathway. We suggest that bisindolylmaleimide-I acts by affecting either a pathway downstream from Lef-1/Tcf in the canonical pathway or a Wnt signaling pathway other than the canonical pathway. In either case, the bisindolylmaleimide-I sensitivity of this pathway should aid in its identification.

Protein kinase mediated upregulation of endothelin A, endothelin B and 5-hydroxytryptamine 1B/1D receptors during organ culture in rat basilar artery

1. Organ culture has been shown to upregulate both endothelin (ET) and 5-hydroxytryptamine 1B/1D (5-HT(1B/1D)) receptors in rat cerebral arteries. The purpose of the present study was to investigate the involvement of protein kinases, especially protein kinases C (PKC) and A (PKA) in this process. 2. The effect of inhibiting protein kinases during organ culture with staurosporine (unspecific protein kinase inhibitor), RO 31-7549 (specific inhibitor of classical PKC's) and H 89 (specific inhibitor of PKA) was examined using in vitro pharmacological examination of cultured vessel segments with ET-1 (unspecific ET(A) and ET(B) agonist), S6c (specific ET(B) agonist) and 5-CT (5-HT(1) agonist). Levels of mRNA coding for the ET(A), ET(B), 5-HT(1B) and 5-HT(1D) receptors were analysed using real-time RT-PCR. 3. Classical PKC's are critically involved in the appearance of the ET(B) receptor; co-culture with RO 31-7549 abolished the contractile response (6.9 +/- 1.8%) and reduced the ET(B) receptor mRNA by 44 +/- 4% as compared to the cultured control. Correlation between decreased ET(B) receptor mRNA and abolished contractile function indicates upstream involvement of PKC. 4. Inhibition of PKA generally had an enhancing effect on the induced changes giving rise to a 7-25% increase in E(max) in response to ET-1, S6c and 5-CT as compared to the cultured control. 5. Staurosporine inhibited the culture induced upregulation of the response of both the ET(A) and the 5-HT(1B/1D) receptors, but had no significant effect on the mRNA levels of these receptors. This lack of correlation indicates an additional downstream involvement of protein kinases.

Sympathoexcitation by bradykinin involves Ca2+-independent protein kinase C

Bradykinin has long been known to excite sympathetic neurons via B(2) receptors, and this action is believed to be mediated by an inhibition of M-currents via phospholipase C and inositol trisphosphate-dependent increases in intracellular Ca(2+). In primary cultures of rat superior cervical ganglion neurons, bradykinin caused an accumulation of inositol trisphosphate, an inhibition of M-currents, and a stimulation of action potential-mediated transmitter release. Blockade of inositol trisphosphate-dependent signaling cascades failed to affect the bradykinin-induced release of noradrenaline, but prevented the peptide-induced inhibition of M-currents. In contrast, inhibition or downregulation of protein kinase C reduced the stimulation of transmitter release, but not the inhibition of M-currents, by bradykinin. In cultures of superior cervical ganglia, classical (alpha, betaI, betaII), novel (delta, epsilon), and atypical (zeta) protein kinase C isozymes were detected by immunoblotting. Bradykinin induced a translocation of Ca(2+)-independent protein kinase C isoforms (delta and epsilon) from the cytosol to the membrane of the neurons, but left the cellular distribution of other isoforms unchanged. This activation of Ca(2+)-independent protein kinase C enzymes was prevented by a phospholipase C inhibitor. The bradykinin-dependent stimulation of noradrenaline release was reduced by inhibitors of classical and novel protein kinase C isozymes, but not by an inhibitor selective for Ca(2+)-dependent isoforms. These results demonstrate that bradykinin B(2) receptors are linked to phospholipase C to simultaneously activate two signaling pathways: one mediates an inositol trisphosphate- and Ca(2+)-dependent inhibition of M-currents, the other one leads to an excitation of sympathetic neurons independently of changes in M-currents through an activation of Ca(2+)-insensitive protein kinase C.

Antiangiogenic and antitumor effects of a protein kinase Cbeta inhibitor in human breast cancer and ovarian cancer xenografts

In cell culture, the compound 317615 2HCl, a potent inhibitor of VEGF-stimulated HUVEC proliferation, was not very effective against MX-1 breast cancer cells (IC50= 8.1 microM) or SKOV-3 ovarian carcinoma cells (IC50 = 9.5 microM). Exposure to combinations of paclitaxel or carboplatin and 317615 x 2HCl with MX-1 cells in culture resulted in cell survival that reflected primarily additivity of the two agents. Exposure of SKOV-3 cells to paclitaxel or carboplatin along with 317615 2HCl resulted in cell survivals that reflected additivity of 317615 x 2HCl with paclitaxel and greater-than-additive cytotoxicity with carboplatin. Administration of 317615 x 2HCI orally twice daily to nude mice bearing subcutaneous MX-1 tumors or SKOV-3 tumors resulted in a decreased number of intratumoral vessels as determined by CD31 and CD105 staining with decreases of 35% and 43% in MX-1 tumors and 60% and 75% in SKOV-3 tumors, respectively. 317615 x 2HCl was an active antitumor agent against the MX-1 xenograft and increased the tumor growth delay produced by paclitaxel by 1.7-fold and the tumor growth delay produced by carboplatin by 3.8-fold. Administration of 317615 x 2HCl also increased the tumor growth delay produced by fractionated radiation therapy in the MX-1 tumor. Treatment with 317615 x 2HCl alone increased the lifespan of animals bearing intraperitoneal SKOV-3 xenografts by 1.9 fold compared with untreated control animals. The combination of paclitaxel and 317615 x 2HCl resulted in 100% 120-day survival of SKOV-3 bearing animals. Administration of 317615 x 2HCl along with carboplatin to animals bearing the SKOV-3 tumor produced a 1.8-fold increase in lifespan compared with carboplatin alone. 317615 x 2HCl is a promising new antiangiogenic agent that is in early phase clinical testing.

Possible involvement of M5 muscarinic receptor in the enhancing actions of the novel gastroprokinetic agent Z-338 on nifedipine-sensitive voltage-dependent Ca2+ currents in guinea pig stomach

We investigated the effects of the novel gastroprokinetic agent Z-338 (N-(N-N'-diisopropylaminoethyl)-[2-(2-hydroxy-4,5-dimethoxybenzoylamino)-1,3-thiazole-4-yl] carboxyamide monohydrochloride trihydrate) on L-type voltage-dependent Ca2+ currents (ICa) in guinea pig gastric myocytes by using the whole-cell patch clamp technique. Bath-applied acetylcholine (ACh) produced biphasic effects on ICa, i.e., enhancement (1-100 nM) and inhibition (1-100 microM), both of which were abolished by pretreatment with atropine (10 microM) or intracellular perfusion of GDPbetaS (500 microM). Z-338 (> or = 1 nM, ED50: 120 nM) mimicked the enhancing effects of ACh, but did not inhibit ICa. The effects of Z-338 and ACh were non-additive and blocked by atropine and GDPbetaS, but not by pertussis toxin (PTX) pretreatment (500 ng/ml). ACh (> or = 1 microM) induced slow inward currents via activation of the muscarinic receptor/PTX-sensitive G-protein pathway, but Z-338 was devoid of these effects. Neither pirenzepine (1 microM), AF-DX116 (1 microM), nor oxybutynin (100 nM) could prevent Z-338 (1 microM) and ACh (10 nM) from enhancing ICa, whilst 4-DAMP (100 nM) blocked the effects of Z-338 and ACh. Bath-application of protein kinase C (PKC) activator PDBu (phorbol-12,13-dibutyrate) (250 nM) enhanced ICa, and conversely, pipette inclusion of PKC inhibitor peptide (150 microM) abolished the effects of ACh and Z-338 on ICa. These results collectively suggest that although contribution of the M3 receptor is not excluded, the major actions of Z-338 on gastric myocytes are potentiation of ICa through activation of M5-like receptor.

Up-regulation of the G(q/11alpha) protein and protein kinase C during the development of sensitization to morphine-induced hyperlocomotion

It has been recognized that protein kinase C (PKC) pathway is involved in the synaptic plasticity. The present study was then designed to examine the changes in G(q/11alpha) and G(betagamma) subunits and PKC activity on sensitization to the morphine-induced hyperlocomotion. Repeated subcutaneous administration of morphine every 72 h produced sensitization to the morphine-induced hyperlocomotion. In morphine-sensitized mice, the protein level of G(q/11alpha) subunit in the limbic forebrain including the nucleus accumbens, but not in the lower midbrain containing the ventral tegmental area, was markedly increased, whereas the levels of G(betagamma) subunit were not altered in either areas. Under these conditions, the levels of membrane-bound phosphorylated-PKC in the limbic forebrain was clearly up-regulated by intermittent morphine treatment. We also found the lack of changes in the level of the regulator of G protein signaling 4, which is a specific G(q/11alpha)-dependent GTPase activating protein, in the limbic forebrain obtained from morphine-sensitized mice. These results indicate that the up-regulation of membrane-bound PKC following intermittent morphine treatment results from the increase in levels of G(q/11alpha) protein. In order to investigate the direct involvement of PKC in the morphine-induced hyperlocomotion, the locomotion induced by acute morphine treatment in the presence or absence of a PKC inhibitor was measured. A specific PKC inhibitor Ro-32-0432 given intracerebroventricularly caused a dose-dependent inhibition of morphine-induced hyperlocomotion. These findings suggest that the up-regulation of G(q/11alpha)-dependent PKC activity in membranes of the limbic forebrain is implicated in the development of sensitization to morphine-induced hyperlocomotion in mice.

Differentiated HL-60 cells are a valid model system for the analysis of human neutrophil migration and chemotaxis

We have carried out a detailed comparison of the motile properties of differentiated HL-60 cells and human peripheral blood neutrophils. We compared the effects of chemotactic stimuli and of inhibitors of signalling proteins on morphology, chemokinesis and chemotaxis of neutrophils and differentiated HL-60 cells using videomicroscopy and a filter assay for chemotaxis. We also assessed expression of signalling and cytoskeletal proteins using Western blotting. Chemotactic peptide induced a front-tail polarity in HL-60 cells comparable to that of neutrophils. Chemokinetic and chemotactic responses to chemotactic peptide were also very similar for both cell types, concerning mean speed of migration, the fraction of migrated cells and the concentration of stimulus optimal for activation. The cytokine interleukin-8 was in contrast clearly less effective in activating motile responses of differentiated HL-60 cells as compared to neutrophils. An important functional role of Rho-activated kinases and phosphatidylinositol 3-kinase in motile responses of HL-60 cells, consistent with their upregulation during differentiation, could be confirmed using inhibitors with specificity for the corresponding enzymes. The only difference observed here between HL-60 cells and neutrophils concerned the differential effects of a protein kinase C inhibitor.In summary, the results presented here show that differentiated HL-60 cells, stimulated with chemotactic peptide, are a valid model system to study molecular mechanisms of neutrophil emigration.

Mechanism of 17-beta-estradiol-induced Erk1/2 activation in breast cancer cells. A role for HER2 AND PKC-delta

Activation of mitogen-activated protein kinase (Erk/MAPK) is a critical signal transduction event for estrogen (E(2))-mediated cell proliferation. Recent studies from our group and others have shown that persistent activation of Erk plays a major role in cell migration and tumor progression. The signaling mechanism(s) responsible for persistent Erk activation are not fully characterized, however. In this study, we have shown that E(2) induces a slow but persistent activation of Erk in MCF-7 breast carcinoma cells. The E(2)-induced Erk activation is dependent on new protein synthesis, suggesting that E(2)-induced growth factors play a major role in Erk activation. When MCF-7 cells were treated with E(2) in the presence of an anti-HER-2 monoclonal antibody (herceptin), 60-70% of E(2)-induced Erk activation is blocked. In addition, when untreated MCF-7 cells were exposed to conditioned medium from E(2)-treated cells, Erk activity was significantly enhanced. Furthermore Erk activity was blocked by an antibody against HER-2 or by heregulin (HRG) depletion from the conditioned medium through immunoprecipitation. In contrast, epidermal growth factor receptor (Ab528) antibody only blocked 10-20% of E(2)-induced Erk activation, suggesting that E(2)-induced Erk activation is predominantly mediated through the secretion of HRG and activation of HER-2 by an autoctine/paracrine mechanism. Inhibition of PKC-delta-mediated signaling by a dominant negative mutant or the relatively specific PKC-delta inhibitor rottlerin blocked most of the E(2)-induced Erk activation but had no effect on TGF alpha-induced Erk activation. By contrast inhibition of Ras, by inhibition of farnesyl transferase (Ftase-1) or dominant negative (N17)-Ras, significantly inhibited both E(2)- and TGF alpha-induced Erk activation. This evaluation of downstream signaling revealed that E(2)-induced Erk activation is mediated by a HRG/HER-2/PKC-delta/Ras pathway that could be crucial for E(2)-dependent growth-promoting effects in early stages of tumor progression.

Protein kinase C (PKC) delta regulates PKCalpha activity in a Syndecan-4-dependent manner

The phosphorylation state of Ser(183) in the cytoplasmic tail of syndecan-4 determines the binding affinity of the cytoplasmic tail to phosphatidylinositol 4,5-bisphosphate (PIP(2)), the capacity of the tail to multimerize, and its ability to activate protein kinase C (PKC) alpha. We sought to identify the kinase responsible for this phosphorylation and to determine its downstream effects on PKCalpha activity and on endothelial cell function. Among several PKC isoenzymes tested, only PKCalpha and -delta were able to specifically phosphorylate Ser(183) in vitro. However, studies in cultured endothelial cells showed that the phosphorylation level of syndecan-4 was significantly reduced in endothelial cells expressing a dominant negative (DN) PKCdelta but not a DN PKCalpha mutant. Syndecan-4/PIP(2)-dependent PKCalpha activity was significantly increased in PKCdelta DN cells, while PKCdelta overexpression was accompanied by decreased PKCalpha activity. PKCdelta-overexpressing cells exhibited a significantly lower proliferation rate and an impaired tube formation in response to FGF2, which were mirrored by similar observations in PKCalpha DN endothelial cells. These findings suggest that PKCdelta is the kinase responsible for syndecan-4 phosphorylation, which, in turn, attenuates the cellular response to FGF2 by reducing PKCalpha activity. The reduced PKCalpha activity then leads to impaired endothelial cell function. We conclude that PKCdelta regulates PKCalpha activity in a syndecan-4-dependent manner.

Mesangial cell protein kinase C isozyme activation in the diabetic milieu

High-glucose-induced activation of mesangial cell protein kinase C (PKC) contributes significantly to the pathogenesis of diabetic nephropathy. Excess glucose metabolism through the polyol pathway leads to de novo synthesis of both diacylglyerol (DAG) and phosphatidic acid, which may account for increased mesangial cell PKC-alpha, -beta, -delta, -epsilon, and -zeta activation/translocation observed within 48-h exposure to high glucose. Raised intracellular glucose causes generation of reactive oxygen species that may directly activate PKC isozymes and enhance their reactivity to vasoactive peptide signaling. In both diabetic rodent models of diabetes and cultured mesangial cells, PKC-beta appears to be the key isozyme required for the enhanced expression of transforming growth factor-beta(1), initiation of early accumulation of mesangial matrix protein, and increased microalbuminuria. Enhanced collagen IV expression by mesangial cells in response to vasoactive peptide hormone stimulation, e.g., endothelin-1, requires PKC-beta, -delta, -epsilon and -zeta. Loss of mesangial cell contractility to potent vasoactive peptides and coincident F-actin disassembly are due to high-glucose-activation of PKC-zeta. Inhibition of mesangial cell PKC isozyme activation in high glucose may prove to be the next important treatment for diabetic nephropathy.

Protein kinase C and calcium/calmodulin-activated protein kinase II (CaMK II) suppress nicotinic acetylcholine receptor gene expression in mammalian muscle. A specific role for CaMK II in activity-dependent gene expression

Nicotinic acetylcholine receptor (nAChR) gene expression is regulated by both muscle activity and increased intracellular calcium. This regulation is an important developmental event that rids receptors from the extrajunctional region of the developing muscle fiber. In avian muscle, it has been proposed that muscle activity suppresses nAChR gene expression via calcium-activated protein kinase C (PKC)-dependent phosphorylation of the myogenic transcription factor, myogenin. Here, we examined the role that PKC and other kinases play in mediating calcium- and activity-dependent suppression of nAChR genes in rat primary myotubes. We found that although activated PKC could regulate nAChR promoter activity and transiently suppressed both nAChR and myogenin gene expression, it did not appear to be required for calcium- or activity-dependent control of nAChR gene expression in mammalian muscle. Neither depletion of PKC from myotubes nor specific pharmacological inhibition of PKC blocked the suppression of nAChR gene expression produced by calcium or muscle depolarization. In contrast, we provide evidence that calcium/calmodulin-activated protein kinase II participates in mediating the effects of muscle depolarization on nAChR and myogenin gene expression.

Conventional protein kinase C isoforms regulate human dopamine transporter activity in Xenopus oocytes

The hypothesis that specific protein kinase C (PKC) isoforms regulate dopamine transporter (DAT) function was tested in Xenopus laevis oocytes expressing human (h)DAT. Activation of conventional PKCs (cPKCs) and novel PKCs (nPKCs) using 10 nM phorbol 12-myristate 13-acetate (PMA) significantly inhibited DAT-associated transport currents. This effect was reversed by isoform-non-selective PKC inhibitors, selective inhibitors of cPKCs and deltaPKC, and by Ca2+ chelation. By contrast, the epsilonPKC translocation inhibitor peptide had no effect on PMA-induced inhibition of hDAT transport-associated currents. Thus, the primary mechanism by which PMA regulates hDAT expressed in oocytes appears to be by activating cPKC(s).

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.

Desensitization of group I metabotropic glutamate receptors in rat sympathetic neurons

Desensitization of heterologously expressed metabotropic glutamate receptor 5a (mGluR5a) was examined in rat sympathetic neurons. Calcium currents in cells expressing mGluR5a exhibited substantial inhibition in response to glutamate exposure. In the continued presence of glutamate, inhibition attenuated rapidly over the course of about a minute. Desensitization was eliminated when a nonhydrolyzable ATP analogue was substituted for ATP in the pipette solution, suggesting that desensitization was mediated by a phosphorylation event. Next, pharmacological agents were used to investigate the nature of the kinase involved in desensitization. Desensitization was sensitive to the nonspecific kinase inhibitor, staurosporine, but not H-7, another nonspecific kinase inhibitor. Inhibitors of myosin light chain kinase and calmodulin-dependent kinase were without effect on desensitization. However, desensitization was sensitive to the protein kinase C inhibitor bisindolymaleimide. In contrast, Gö6976, a selective inhibitor of conventional protein kinase C isoforms, was without effect. In addition, desensitization persisted in the presence of 10 mM intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, a fast Ca(2+) chelator. Finally, overexpression of wild-type calmodulin, which can bind mGluR5 and inhibit phosphorylation, did not alter mGluR desensitization. Two Ca(2+)-binding-deficient calmodulin mutants were also without effect. These data indicate a role for nonconventional protein kinase C isoforms as a mediator of mGluR5 desensitization and that the phosphorylation of mGluR5a that competes with calmodulin binding does not mediate desensitization.

Sigma(2)-receptor regulation of dopamine transporter via activation of protein kinase C

The elucidation of the mechanisms underlying sigma(2)-receptor activation and signal transduction is crucial to the understanding of sigma(2)-receptor function. Previous studies in our laboratory have demonstrated sigma(2)-receptor-mediated regulation of the dopamine transporter (DAT) as measured by amphetamine-stimulated release of [(3)H]dopamine (DA) from both rat striatal slices and PC12 cells. The regulation of the DAT in the PC12 cell model was dependent upon activation of Ca(2+)/calmodulin-dependent kinase II. We have now studied the second messenger systems involved in sigma(2)-receptor-mediated regulation of amphetamine-stimulated [(3)H]DA release in rat striatal slices, including Ca(2+)/calmodulin-dependent kinase II, protein kinase C, and sources of calcium required for the enhancement of release produced by sigma(2)-receptor activation. The Ca(2+)/calmodulin-dependent kinase II inhibitors 1-[N,O-bis-(5-isoquionolinesulfonyl)]-N-methyl-L-tyrosyl-4-phenylpiperazine and N-[2-[[[3-(4'-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4'-methoxy-benzenesulfonamide phosphate did not significantly affect the (+)-pentazocine-mediated enhancement of amphetamine-stimulated [(3)H]DA release. However, we found that an inhibitor of protein kinase C, 3-[1-[3-(dimethylamino)propyl]-1H-indol-3-yl)-1H-pyrrole-2,5-dione, blocks the (+)-pentazocine-mediated enhancement in rat striatal slices. The protein kinase C activator phorbol 12-myristate 13-acetate, but not the inactive isophorbol 4 alpha,9 alpha,12 alpha,13 alpha,20-pentahydroxytiglia-1,6-dien-3-one, enhanced the amphetamine-stimulated [(3)H]DA release comparable to the enhancement seen by (+)-pentazocine alone. Additionally, the L-type voltage-dependent calcium channel inhibitor nitrendipine or prior treatment with thapsigargin, but not the N-type voltage-dependent calcium channel omega-conotoxin MVIIA, attenuated the (+)-pentazocine-mediated enhancement. Together, these data suggest that activation of sigma(2)-receptors results in the regulation of DAT activity via a calcium- and protein kinase C-dependent signaling mechanism.

Role of protein kinase C-epsilon in the development of kappa-opioid receptor tolerance to U50,488H in rat ventricular myocytes

The role of protein kinase C-epsilon (PKC-epsilon) in the development of kappa-opioid receptor (kappa-OR) tolerance to the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) (U50,488H), the selective agonist of kappa-OR, was determined in rat ventricular myocytes. Incubation of ventricular myocytes with 1 microM U50,488H for 24 h significantly attenuated the inhibitory effects of 30 microM U50,488H on the electrically-induced [Ca(2+)](i) transient and forskolin-stimulated cyclic AMP accumulation, indicating the development of tolerance to the kappa-OR agonist. Chronic treatment of ventricular myocytes with U50,488H also induced translocation of PKC-epsilon to the particulate fraction. On the other hand, administration of 30 microM U50,488H for 10 min induced translocation of PKC-alpha to the particulate fraction in naïve ventricular myocytes, but not in cells pretreated with 1 microM U50,488H for 24 h. In ventricular myocytes incubated for 24 h with 1 microM U50,488H together with 1 microM chelerythrine or 1 microM GF109203X, PKC inhibitors, or 0.1 microM epsilonV1-2 peptide, a selective inhibitor of PKC-epsilon, 30 microM U50,488H still produced the inhibitory effect on the electrically-induced [Ca(2+)](i) transient as it did in naïve ventricular myocytes. Chronic treatment of ventricular myocytes with U50,488H and chelerythrine also attenuated the development of tolerance to acute U50,488H on cyclic AMP accumulation. Cells exposed to chelerythrine, GF109203X, or epsilonV1-2 peptide alone did not show an altered [Ca(2+)](i) response to U50,488H. These results indicate that activation of PKC-epsilon is a critical step in the development of tolerance in the kappa-OR.

Real time visualization of protein kinase activity in living cells

A library of fluorescently labeled protein kinase C (PKC) peptide substrates was prepared to identify a phosphorylation-induced reporter of protein kinase activity. The lead PKC substrate displays a 2.5-fold change in fluorescence intensity upon phosphorylation. PKC activity is readily sampled in cell lysates containing the activated PKCs. Immunodepletion of conventional PKCs from the cell lysate eliminates the fluorescence response, suggesting that this peptide substrate is selectively phosphorylated by PKCalpha, beta, and gamma. Finally, living cells microinjected with the peptide substrate exhibit a 2-fold increase in fluorescence intensity upon exposure to a PKC activator. These results suggest that peptide-based protein kinase biosensors may be useful in monitoring the temporal and spatial dynamics of PKC activity in living cells.

Inhibition of protein kinase Calpha prevents endothelial cell migration and vascular tube formation in vitro and myocardial neovascularization in vivo

Although protein kinase C (PKC) activation is required for endothelial cell (EC) growth, migration, adhesion, and vessel formation, the role of individual PKC isoenzymes in these events is not defined. Because PKCalpha has been previously linked with enhanced EC migration and response to angiogenic growth factors, we characterized a specific phosphorothioate-modified 21-mer antisense PKCalpha (AS-PKCalpha). AS-PKCalpha (500 nmol/L) prevented the expression of PKCalpha protein by 90% in human ECs and did not reduce the expression of any other PKC isoenzyme. AS-PKCalpha reduced human EC migration by 64% compared with its control oligonucleotide in a "scratch" wounding assay, and AS-PKCalpha reduced human EC adhesion to the extracellular matrix protein vitronectin by 18%. Phosphorylation of mitogen-activated protein kinase (extracellular signal-regulated kinase 1/2) induced by vascular endothelial growth factor was inhibited by 30% in human ECs transfected with AS-PKCalpha. Compared with control, AS-PKCalpha also reduced the number of EC tubes formed in a 3D type I collagen gel assay by 37.5%. Finally, using an osmotic minipump, we infused AS-PKCalpha into mice in which myocardial infarction was induced by coronary ligation and found that the oligonucleotide was primarily taken up by intramyocardial blood vessels. Compared with the results with control oligonucleotide, AS-PKCalpha oligonucleotide inhibited the number of anti-PKCalpha-stained blood vessels by 48% and reduced the total vessel number by 72% as well. In conclusion, the expression of PKCalpha is required for full EC migration, adhesion to vitronectin, vascular endothelial growth factor-induced extracellular signal-regulated kinase activation, and tube formation and is likely to be of importance in myocardial angiogenesis in vivo after ischemia.

Caspase-3-dependent proteolytic cleavage of protein kinase Cdelta is essential for oxidative stress-mediated dopaminergic cell death after exposure to methylcyclopentadienyl manganese tricarbonyl

In the present study, we characterized oxidative stress-dependent cellular events in dopaminergic cells after exposure to an organic form of manganese compound, methylcyclopentadienyl manganese tricarbonyl (MMT). In pheochromocytoma cells, MMT exposure resulted in rapid increase in generation of reactive oxygen species (ROS) within 5--15 min, followed by release of mitochondrial cytochrome C into cytoplasm and subsequent activation of cysteine proteases, caspase-9 (twofold to threefold) and caspase-3 (15- to 25-fold), but not caspase-8, in a time- and dose-dependent manner. Interestingly, we also found that MMT exposure induces a time- and dose-dependent proteolytic cleavage of native protein kinase Cdelta (PKCdelta, 72-74 kDa) to yield 41 kDa catalytically active and 38 kDa regulatory fragments. Pretreatment with caspase inhibitors (Z-DEVD-FMK or Z-VAD-FMK) blocked MMT-induced proteolytic cleavage of PKCdelta, indicating that cleavage is mediated by caspase-3. Furthermore, inhibition of PKCdelta activity with a specific inhibitor, rottlerin, significantly inhibited caspase-3 activation in a dose-dependent manner along with a reduction in PKCdelta cleavage products, indicating a possible positive feedback activation of caspase-3 activity by PKCdelta. The presence of such a positive feedback loop was also confirmed by delivering the catalytically active PKCdelta fragment. Attenuation of ROS generation, caspase-3 activation, and PKCdelta activity before MMT treatment almost completely suppressed DNA fragmentation. Additionally, overexpression of catalytically inactive PKCdelta(K376R) (dominant-negative mutant) prevented MMT-induced apoptosis in immortalized mesencephalic dopaminergic cells. For the first time, these data demonstrate that caspase-3-dependent proteolytic activation of PKCdelta plays a key role in oxidative stress-mediated apoptosis in dopaminergic cells after exposure to an environmental neurotoxic agent.

The role of protein kinase C-delta in PTH stimulation of IGF-binding protein-5 mRNA in UMR-106-01 cells

We have investigated the role of protein kinase C (PKC) signal transduction pathways in parathyroid hormone (PTH) regulation of insulin-like growth factor-binding protein-5 (IGFBP-5) gene expression in the rat osteoblast-like cell line UMR-106-01. Involvement of the PKC pathway was determined by the findings that bisindolylmaleimide I inhibited 40% of the PTH effect, and 1 microM bovine PTH-(3-34) stimulated a 10-fold induction of IGFBP-5 mRNA. PTH-(1-34) and PTH-(3-34) (100 nM) both stimulated PKC-delta translocation from the membrane to the nuclear fraction. Rottlerin, a PKC-delta-specific inhibitor, and a dominant negative mutant of PKC-delta were both able to significantly inhibit PTH-(1-34) and PTH-(3-34) induction of IGFBP-5 mRNA, suggesting a stimulatory role for PKC-delta in the effects of PTH. Phorbol 12-myristate 13-acetate (PMA) stimulated PKC-alpha translocation from the cytosol to the membrane and inhibited approximately 50% of the PTH-(1-34), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate-stimulated IGFBP-5 mRNA levels, suggesting that PKC-alpha negatively regulates protein kinase A (PKA)-mediated induction of IGFBP-5 mRNA. These results suggest that the induction of IGFBP-5 by PTH is both PKA and PKC dependent and PKC-delta is the primary mediator of the effects of PTH via the PKC pathway.

Effect of protein kinase C on endoplasmic reticulum cholesterol

Plasma membrane cholesterol both regulates and is regulated by effector proteins in the endoplasmic reticulum (ER) through a feedback system that is poorly understood. We now show that ER cholesterol varies over a fivefold range in response to experimental agents that act upon protein kinase C (PKC). Agents that activate Ca(2+)-dependent PKC [phorbol-12-myristate-13-acetate (PMA) and bryostatin 1] increased the level of ER cholesterol; inhibitors such as staurosporine and calphostin C decreased it. Rottlerin, a selective inhibitor of the PKC-delta isoform, also increased ER cholesterol. The esterification of plasma membrane cholesterol was altered by protein kinase C-directed agents in a corresponding fashion. Furthermore, the regulatory effect of plasma membrane cholesterol on the esterification of ER cholesterol was blocked by PKC-directed agents. These findings suggest that multiple protein kinase C isoforms participate in the regulation of ER cholesterol and therefore in cholesterol homeostasis.

Protein kinase C-alpha mediates cigarette smoke extract- and complement factor 5a-stimulated interleukin-8 release in human bronchial epithelial cells

BACKGROUND

Cigarette smoke extract (CSE) activates protein kinase C (PKC) and augments complement factor 5a (C5a)-stimulated release of the proinflammatory cytokine IL-8 in human bronchial epithelial cells (HBEC). We hypothesized that PKC activation by alternative PKC activators will also mediate C5a-stimulated IL-8 release in HBEC.

METHODS

HBEC were treated with phorbol myristate acetate (100 ng/mL), calcium ionophore A23187 (2 nM), or 10 nM cholesterol-3-sulfate in the presence or absence of C5a. Interleukin-8 (IL-8) release was measured by enzyme-linked immunoadsorbent assay.

RESULTS

IL-8 release by PKC activators alone was significantly higher than in unstimulated cells and was further augmented in the presence of C5a. Preincubation with the PKC inhibitor calphostin C (1 microM) significantly suppressed IL-8 release in HBEC treated with CSE and C5a. Preincubation with 10 microM TMB-8 (an intracellular calcium sequester) also significantly suppressed IL-8 release in CSE- and C5a-treated HBEC, suggesting that intracellular calcium is required for CSE- and C5a-mediated IL-8 release. When HBEC were preincubated with 30 nM of the PKCbeta-specific inhibitor LY363196, CSE- and C5a-mediated IL-8 release was not inhibited. However, with higher concentrations of LY363196 (>600 nM), which exceeds the IC50 for PKCbeta greater than 100 fold, CSE- and C5a-mediated IL-8 release was significantly suppressed. Preincubation of HBEC with 100 nM of Gö 6976, a specific PKCalpha inhibitor, significantly inhibited CSE- and C5a-mediated stimulation of IL-8 release.

CONCLUSIONS

Collectively, these data suggest that PKC activators in addition to CSE augment C5a-stimulated IL-8 release from HBEC and that CSE and C5a stimulate IL-8 release in HBEC by activating the calcium-dependent PKCalpha isoform.

In vivo evidence for a role of protein kinase C in peripheral nociceptive processing

1. The present study was designed to characterize the nociceptive response induced by protein kinase C (PKC) peripheral activation and to investigate if this biochemical event is important for the nociceptive response induced by formaldehyde, and bradykinin (BK). 2. Intraplantar injection of phorbol-12,13-didecanoate (PDD; 0.01, 0.1 or 1 microg), a PKC activator, but not of 4 alpha-PDD (inactive analogue), dose-dependently induced thermal hyperalgesia in rats. This response was not observed at the contralateral hindpaw. Intraplantar injection of PDD (0.01, 0.1 or 1 microg) also induced mechanical allodynia. In mice, injection of PDD (0.1 or 1 microg) into the dorsum of the hindpaw induced a spontaneous licking behaviour. 3. Intraplantar co-injection of chelerythrine (10 or 50 microg), a PKC inhibitor, attenuated the thermal hyperalgesia induced by PDD (0.1 microg) in rats. 4. The second phase of the nociceptive response induced by the injection of formaldehyde (0.92%, 20 microl) into the dorsum of mice hindpaws was inhibited by ipsi-, but not contralateral, pre-treatment with chelerythrine (1 microg). 5. Intraplantar injection of BK (10 microg) induced mechanical allodynia in rats. Ipsi- but not contralateral injection of bisindolylmaleimide I (10 microg), a PKC inhibitor, inhibited BK-induced mechanical allodynia. 6. In conclusion, this study demonstrates that PKC activation at peripheral tissues leads to the development of spontaneous nociceptive response, thermal hyperalgesia and mechanical allodynia. Most importantly, it also gives in vivo evidence that peripheral PKC activation is essential for the full establishment of the nociceptive response induced by two different inflammatory stimuli.

Protein kinase C and the development of diabetic vascular complications

Hyperglycemic control in diabetes is key to preventing the development and progression of vascular complications such as retinopathy, nephropathy and neuropathy. Increased activation of the diacylglycerol (DAG)-protein kinase C (PKC) signal transduction pathway has been identified in vascular tissues from diabetic animals, and in vascular cells exposed to elevated glucose. Vascular abnormalities associated with glucose-induced PKC activation leading to increased synthesis of DAG include altered vascular blood flow, extracellular matrix deposition, basement membrane thickening, increased permeability and neovascularization. Preferential activation of the PKCbeta isoform by elevated glucose is reported to occur in a variety of vascular tissues. This has lead to the development of LY333531, a PKCbeta isoform specific inhibitor, which has shown potential in animal models to be an orally effective and nontoxic therapy able to produce significant improvements in diabetic retinopathy, nephropathy, neuropathy and cardiac dysfunction. Additionally, the antioxidant vitamin E has been identified as an inhibitor of the DAG-PKC pathway, and shows promise in reducing vascular complications in animal models of diabetes. Given the overwhelming evidence indicating a role for PKC activation in contributing to the development of diabetic vascular complications, pharmacological therapies that can modulate this pathway, particularly with PKC isoform selectivity, show great promise for treatment of vascular complications, even in the presence of hyperglycemia.

Regulation of SP-B and SP-C secretion in rat type II cells in primary culture

Secretion of lung surfactant phospholipids is a highly regulated process. A variety of physiological and pharmacological agents stimulate surfactant phospholipid secretion in isolated type II cells. Although the lipid and hydrophobic protein components of surfactant are believed to be secreted together by exocytosis of lamellar body contents, regulation of surfactant protein (SP) B and SP-C secretion has not previously been examined. To address the question of whether secretion of SP-B and SP-C is stimulated by the same agonists that stimulate phospholipid secretion, we measured secretion of all four SPs under the same conditions used to measure phosphatidylcholine secretion. Freshly isolated rat type II cells were cultured overnight and exposed to known surfactant phospholipid secretagogues for 2.5 h, after which the amounts of SP-A, SP-B, SP-C, and SP-D in the medium were measured with immunoblotting. Secretion of SP-B and SP-C was stimulated three- to fivefold by terbutaline, 5'-(N-ethylcarboxyamido)adenosine, ATP, 12-O-tetradecanoylphorbol 13-acetate, and ionomycin. Similar to their effects on phospholipid secretion, the stimulatory effects of the agonists were abolished by Ro 31-8220. Secretion of SP-A and SP-D was not stimulated by the secretagogues tested. We conclude that secretion of the phospholipid and hydrophobic protein components of surfactant is similarly regulated, whereas secretion of the hydrophilic proteins is regulated differently.

Protein kinase C inhibitors as novel anticancer drugs

The role of PKC isoforms in signal transduction pathways involved in regulation of the cell cycle, apoptosis, angiogenesis, differentiation, invasiveness, senescence and drug efflux are reviewed, along with the clinical results on the current crop of PKC inhibitors, including midostaurin (PKC-412, CGP 41251, N -benzoylstaurosporine), UCN-01 (7-hydroxystaurosporine), bryostatin 1, perifosine, ilmofosine, Ro 31-8220, Ro 32-0432, GO 6976, ISIS-3521 (CGP 64128A) and the macrocyclic bis (indolyl) maleimides (LY-333531, LY-379196, LY-317615). An appreciation of the complex, often contradictory roles of PKC isoforms in signal transduction pathways involved in cancer is important for interpreting the clinical results observed with PKC inhibitors of varying selectivity. An antisense oligonucleotide, ISIS-3521 and two orally available small molecule inhibitors, LY 333531 and midostaurin, have now advanced to latter stage development for cancer and/or other indications. These compounds have varying levels of selectivity for the PKC isoforms and for the kinase and initial safety and early clinical efficacy have been encouraging. At this stage, the potential of PKC inhibition for the treatment of cancer has not been fully realised. The concurrent inhibition of multiple PKC isoforms may yet provide an improved clinical outcome in treating cancers in view of the complex interrelated roles of the PKC isoforms.

Characterization of functional VIP/PACAP receptors in the human erythroleukemic HEL cell line

The presence of VIP/PACAP receptors was investigated on the human erythroleukemic cell line HEL. Specific binding of [125I]-PACAP or [125I]-VIP on HEL cells or membranes was very low and did not allow to perform competition curves. At 37 degrees C PACAP transiently increased cAMP levels in the presence of the non-specific phosphodiesterase inhibitor IBMX, suggesting rapid desensitization. Kinetic studies revealed that optimal conditions to measure the EC(50) of PACAP(1-27) were 10 min at 20 degrees C. Under those conditions, PACAP-related peptides increased cAMP levels with EC(50) in agreement with the pharmacological profile of the VPAC(1) receptor subtype: PACAP = VIP > [K(15), R(16,) L(27)]VIP(1-7)/GRF(8-27) = [R(16)]ChSn (two VPAC(1) agonists) >> helodermin = secretin. RO 25-1553, a selective activator of VPAC(2) receptor was inactive at 1 microM. Dose-response curves of VPAC(1) agonist molecules (PACAP, VIP, [K(15), R(16), L(27)]VIP(1-7)/GRF(8-27), [R(16)]ChSn) were shifted to the right by the VPAC(1) receptor antagonist [AcHis(1), D-Phe(2), Lys(15), Leu(17)]VIP(3-7)/GRF(8-27), with a K(i) of 3 +/- 1 nM (n = 3). The presence of VPAC(1) receptor mRNA was confirmed by RT-PCR. Preincubation with PACAP or PMA showed that VPAC(1) receptors underwent homologous and heterologous desensitization. This study provides the first evidence for the expression of functional VPAC(1) receptors undergoing rapid desensitization in HEL cells.

Protein kinase C regulates FADD recruitment and death-inducing signaling complex formation in Fas/CD95-induced apoptosis

Activation of protein kinase C (PKC) triggers cellular signals that inhibit Fas/CD95-induced cell death in Jurkat T-cells by poorly defined mechanisms. Previously, we have shown that one effect of PKC on Fas/CD95-dependent cell death occurs through inhibition of cell shrinkage and K(+) efflux (Gómez-Angelats, M., Bortner, C. D., and Cidlowski, J. A. (2000) J. Biol. Chem. 275, 19609-19619). Here we report that PKC alters Fas/CD95 signaling from the plasma membrane to the activation of caspases by exerting a profound action on survival/cell death decisions. Specific activation of PKC with 12-O-tetradecanoylphorbol-13-acetate or bryostatin-1 induced translocation of PKC from the cytosol to the membrane and effectively inhibited cell shrinkage and cell death triggered by anti-Fas antibody in Jurkat cells. In contrast, inhibition of classical PKC isotypes with Gö6976 exacerbated the effect of Fas activation on both apoptotic volume decrease and cell death. PKC activation/inhibition did not affect anti-Fas antibody binding to the cell surface, intracellular levels of FADD (Fas-associated protein with death domain), or c-FLIP (cellular FLICE-like inhibitory protein) expression. However, processing/activation of both caspase-8 and caspase-3 and BID cleavage were markedly blocked upon PKC activation and, conversely, were augmented during PKC inhibition, suggesting a role for PKC upstream of caspase-8 processing and activation. Analysis of death-inducing signaling complex (DISC) formation was carried out to examine the influence of PKC on recruitment of both FADD and procaspase-8 to the Fas receptor. PKC activation blocked FADD recruitment and caspase-8 activation and thus DISC formation in both type I and II cells. In contrast, inhibition of classical PKCs promoted the opposite effect on the Fas pathway by rapidly increasing FADD recruitment, caspase-8 activation, and DISC formation. Together, these data show that PKC finely modulates Fas/CD95 signaling by altering the efficiency of DISC formation.

Ceramide inhibition of NF-kappaB activation involves reverse translocation of classical protein kinase C (PKC) isoenzymes: requirement for kinase activity and carboxyl-terminal phosphorylation of PKC for the ceramide response

Protein kinase C (PKC) is known to activate NF-kappaB whereas the lipid mediator ceramide was recently shown to inhibit activation of this transcription factor (1, 2). In this study, the mechanisms by which ceramide interferes with this pathway were examined in Jurkat leukemia and MCF-7 breast cancer cells. Both exogenous and endogenous ceramide inhibited selectively PKC-mediated activation of NF-kappaB by reverting PKC translocation to the membrane. Next, confocal and immunofluorescence studies were performed to evaluate the direct effects of ceramide on PKC. These studies showed that ceramide inhibited translocation of a green fluorescent protein (GFP)-PKCbeta2 fusion protein in response to PMA. A mutant PKC in which autophosphorylation sites were mutated to alanine (PKC-DA) was resistant to ceramide. A kinase-inactive mutant (PKC-KR) was also resistant to ceramide action, and the results were supported using kinase inhibitors of the enzyme. Finally, overexpression of PKC-DA prevented, at least partly, the ability of ceramide to inhibit activation of NF-kappaB. Taken together, these studies show that ceramide has acute effects on translocation of PKC by inducing reverse translocation, and this reversal requires both the kinase activity of PKC and phosphorylation of the autophosphorylation sites.

Histamine H1 receptor activation stimulates mitogenesis in human astrocytoma U373 MG cells

In human astrocytoma U373 MG cells that express histamine H1 receptors (180 +/- 6 fmol/mg protein) but not H2 or H3 receptors, histamine stimulated mitogenesis as assessed by [3H]-thymidine incorporation (173 +/- 2% of basal; EC50, 2.5 +/- 0.4 microM). The effect of 100 microM histamine was fully blocked by the selective H1 antagonist mepyramine (1 microM) and was markedly reduced (93 +/- 4% inhibition) by the phospholipase C inhibitor U73122 (10 microM). The activator of protein kinase C (PKC) phorbol 12-tetradecanoyl-13-acetate (TPA, 100nM) stimulated [3H]-thymidine incorporation (270 +/- 8% of basal), and this response was not additive with that to 100 microM histamine. The incorporation of [3H]-thymidine induced by 100 microM histamine was partially reduced by the PKC inhibitor Ro 31-8220 (57 +/- 7% inhibition at 300 nM) and by the compound PD 098,059 (30 microM, 62 +/- 14% inhibition), an inhibitor of the mitogen-activated kinase (MAPK) kinases MEK1/MEK2. These results show that histamine H1 receptor activation stimulates the proliferation of human astrocytoma U373 MG cells. The action of histamine appears to be partially mediated by PKC stimulation and MAPK activation.

Transcriptional regulation of cyclooxygenase-2 gene in ovine large luteal cells

There is positive feedback pathway in the ovine large luteal cell, such that prostaglandin (PG) F(2 alpha) stimulation induces intraluteal PGF(2 alpha) production as the result of induction of one of the rate-limiting enzymes in PG production, cyclooxygenase-2 (Cox-2). The objective of the present study was to evaluate the intracellular effector systems and important DNA transcriptional element(s) involved in regulating the Cox-2 gene in ovine large luteal cells. In transient transfection assays, Cox-2 promoter was rapidly induced (4 h) by phorbol didecanoate (a protein kinase [PK] C activator), ionomycin, and cloprostenol (PGF(2 alpha) analogue), with a peak induction at 12 h. Cloprostenol-mediated promoter activation was not blocked by inhibition of various second messenger systems, including PKA, calcium calmodulin kinase II, or mitogen-activated protein kinases. However, myristoylated PKC pseudosubstrate peptide inhibited cloprostenol stimulation of Cox-2 promoter, indicating the critical role of PKC in this stimulation. The Cox-2 promoter could be reduced to 282 base pairs (bp) of the 5' flanking sequence with retention of full inducibility by cloprostenol. Mutation of three critical cis-responsive elements within this 282-bp region (C/EBP, cAMP responsive element [CRE], and E-box) indicated that E-box was critical in both basal and cloprostenol-induced promoter activity. However, there was also significant but less dramatic inhibition of cloprostenol stimulation by mutation of C/EBP and CRE in the Cox-2 promoter, and mutation of all three elements eliminated cloprostenol induction of this promoter. Electrophoretic mobility shift assays of nuclear extracts from large luteal cells revealed that upstream stimulatory factor (USF)-1 and USF-2 bound to the E-box in Cox-2. Thus, PKC directly regulates transcription of the Cox-2 gene in large luteal cells by acting through DNA elements close to the putative transcriptional start point, particularly an E-box region at -50 bp.

Protein kinase C isoforms as therapeutic targets in nervous system disease states

Neuronal tissues display high levels of protein kinase C (PKC) activity and isoform expression. The activation of this enzymatic system is important in the control of short and long term brain functions (ion channel regulation, receptor modulation, neurotransmitter release, synaptic potentiation/depression, neuronal survival) that are related to diverse brain pathologies. This review will describe recent developments in PKC regulation and changes in levels, isoforms and activation in acute and chronic neurodegenerative pathologies as well as in affective and psychic disorders. The recent availability of isoform selective inhibitors and activators may help to understand better the relevance of PKC in central nervous system (CNS) physiology and pathology and to identify new and safer pharmacologic strategies to be tested in different disease states.

Malondialdehyde-acetaldehyde-adducted bovine serum albumin activates protein kinase C and stimulates interleukin-8 release in bovine bronchial epithelial cells

Previous study results have demonstrated that cigarette smoke or acetaldehyde rapidly stimulates protein kinase C (PKC)-mediated release of interleukin-8 (IL-8) in bovine bronchial epithelial cells (BECs). Low concentrations of acetaldehyde combine synergistically with malondialdehyde to increase significantly maximal BEC PKC activity at 48 to 96 h stimulation. Because more than 95% of alcoholics are cigarette smokers, we hypothesized that malondialdehyde, an inflammation product of lipid peroxidation, and acetaldehyde, both a product of ethanol metabolism and a component of cigarette smoke, might stimulate PKC-mediated IL-8 release in BECs by malondialdehyde-acetaldehyde (MAA) adduct formation, rather than as free aldehydes. Protein kinase C activity is maximally elevated in BECs treated with 50 microg/ml of BSA-MAA from approximately 1 to 3 h. This activity subsequently begins to decrease by 4 to 6 h, with a return to baseline unstimulated kinase activity levels by 24 h. No activation of cyclic AMP-dependent protein kinase (PKA) or cyclic GMP-dependent protein kinase (PKG) was observed in BSA-MAA-treated BECs. The MAA adduct activation of PKC was followed by a fourfold to tenfold greater release of IL-8 over that observed for both BECs exposed to media only and BSA control-treated BECs. Protein kinase C activation and IL-8 release were blocked by pretreating BECs with 1 microM calphostin C or 100 nM of the PKC alpha-specific inhibitor, Go 6976. Isoform-specific inhibitors to PKC beta, PKC delta, and PKC zeta failed to inhibit completely MAA adduct-stimulated PKC or IL-8 release. Results of these studies indicate that metabolites derived from ethanol and cigarette smoke, such as acetaldehyde and malondialdehyde, form adducts that stimulate airway epithelial cell PKC alpha-mediated release of promigratory cytokines.

Relationship between thromboxane/prostacyclin ratio and diabetic vascular complications

To elucidate the relationship between the thromboxane A2/prostacyclin (TXA2/PGI2) ratio and diabetic complications, the levels of 11-dehydro-thromboxane B2 and 2,3-dinor-6-keto-prostaglandin F1alpha, the urinary metabolites of thromboxane A2 and prostacyclin, were measured in diabetics by gas chromatography/selected ion monitoring. We compared the TXA2/PGI2 ratio in healthy volunteers and diabetics. The TXA2/PGI2 ratio of diabetics was significantly higher than that of healthy volunteers and we could reconfirm the hypercoagulable condition in diabetics. We also investigated the difference of TXA2/PGI2 levels in diabetics with retinopathy and neuropathy. The TXA2/PGI2 ratio of diabetics with retinopathy showed significantly higher level than without retinopathy. However, the TXA2/PGI2 ratio of diabetics with neuropathy was the same as without neuropathy. These results suggest that the TXA2/PGI2 ratio reflects the pathological conditions of diabetes, especially the change of vasculature. The monitoring and improvement of TXA2/PGI2 ratio could be useful for the prevention of diabetic vascular complications.

Chronic and acute regulation of Na+/Cl- -dependent neurotransmitter transporters: drugs, substrates, presynaptic receptors, and signaling systems

Na+/Cl- -dependent neurotransmitter transporters, which constitute a gene superfamily, are crucial for limiting neurotransmitter activity. Thus, it is critical to understand their regulation. This review focuses primarily on the norepinephrine transporter, the dopamine transporter, the serotonin transporter, and the gamma-aminobutyric acid transporter GAT1. Chronic administration of drugs that alter neurotransmitter release or inhibit transporter activity can produce persistent compensatory changes in brain transporter number and activity. However, regulation has not been universally observed. Transient alterations in norepinephrine transporter, dopamine transporter, serotonin transporter, and GAT1 function and/or number occur in response to more acute manipulations, including membrane potential changes, substrate exposure, ethanol exposure, and presynaptic receptor activation/inhibition. In many cases, acute regulation has been shown to result from a rapid redistribution of the transporter between the cell surface and intracellular sites. Second messenger systems involved in this rapid regulation include protein kinases and phosphatases, of which protein kinase C has been the best characterized. These signaling systems share the common characteristic of altering maximal transport velocity and/or cell surface expression, consistent with regulation of transporter trafficking. Although less well characterized, arachidonic acid, reactive oxygen species, and nitric oxide also alter transporter function. In addition to post-translational modifications, cytoskeleton interactions and transporter oligomerization regulate transporter activity and trafficking. Furthermore, promoter regions involved in transporter transcriptional regulation have begun to be identified. Together, these findings suggest that Na+/Cl- -dependent neurotransmitter transporters are regulated both long-term and in a more dynamic manner, thereby providing several distinct mechanisms for altering synaptic neurotransmitter concentrations and neurotransmission.

Effect of the postprandial state on nontraditional risk factors

Hyperglycemia is the cause of chronic microvascular complications in diabetes. Increased macrovascular disease occurring in diabetes is multifactorial, with hyperglycemia being only 1 of the factors responsible. A major deficiency in our understanding of the role of hyperglycemia in the pathogenesis of chronic diabetic complications is the contribution made by exaggerated postprandial glycemic excursions, in contrast to that made by chronic fasting hyperglycemia. Hyperglycemia is thought to cause chronic diabetic complications through > or = 1 of the following biochemical mechanisms: (1) an exaggerated polyol pathway, (2) protein glycation and the formation of advanced glycation end products, (3) excessive activation of protein kinase C (PKC) isoenzymes in vascular and mesangial cells, and (4) greater oxidative stress. The focus of this article is how postprandial hyperglycemia and glucose excursions might specifically activate > or = 1 of these mechanisms and how they might contribute to the development of the chronic complications of diabetes by causing endothelial dysfunction, a procoagulant state, carbonyl stress, and/or vascular and oxidative effects secondary to PKC activation.

Therapeutic potential of PKC inhibitors in painful diabetic neuropathy

Diabetic neuropathy accompanied by anomalies in pain perception is one of the most frequent complications in insulin-dependent diabetes in humans. Many clinical and experimental studies have suggested that diabetes or hyperglycaemia alters pain sensitivity. In humans, diabetic neuropathy can be associated with burning, tactile hypersensitivity. Behavioural reactions of hyperalgesia in animal models of diabetes have been described. However, the aetiology of these disturbances is still unknown, although metabolic factors such as hyperglycaemia or neurotransmitter alteration may be involved. Activation of protein kinase C (PKC) has been implicated in changes in pain perception. Phorbol esters, which activate PKC, enhance the thermal hyperalgesia in diabetic mice and enhance nociceptive responses after tissue injury induced by formalin. Electrophysiological experiments have shown that activation of PKC leads to long-lasting enhancement of excitatory amino acid-mediated currents in dorsal horn neurones and trigeminal neurones. Thus, activation of PKC may underlie the neuronal sensitisation that produces hyperalgesia in diabetic neuropathy.

Proteinase-activated receptor-2-mediated activation of stress-activated protein kinases and inhibitory kappa B kinases in NCTC 2544 keratinocytes

In this study we examined the regulation of the stress-activated protein (SAP) kinases and inhibitory kappa B kinases (IKKs) through stimulation of the novel G-protein-coupled receptor proteinase-activated receptor-2 in the human keratinocyte cell line NCTC2544. Trypsin and the peptide SLIGKV stimulated a time-dependent increase in both c-Jun N-terminal kinase and p38 mitogen-activated protein kinase activity. Trypsin also stimulated NF kappa B-DNA binding and the activation of the upstream kinases IKK alpha and -beta. Phorbol 12-myristate 13-acetate also strongly activated both SAP kinases and IKK isoforms, suggesting the potential for a protein kinase C-mediated regulatory mechanism underlying the effects of trypsin. Pre-incubation with selective protein kinase C (PKC) inhibitors GF109203X and Gö6983, or transfection of dominant negative (DN)-PKC alpha, abolished phorbol 12-myristate 13-acetate-mediated c-Jun N-terminal kinase activity, although it only partially inhibited the response to trypsin. In contrast, Gö6983 reduced trypsin-stimulated p38 mitogen-activated protein kinase activity to a greater extent than GF109203X, although DN-PKC alpha or PKC zeta had no substantial effect. Additionally, inhibitors of PKC partially reduced trypsin-stimulated IKK alpha activity but abolished that of IKK beta, whereas DN-PKC alpha but not DN-PKC zeta substantially reduced trypsin-stimulated Flag-IKK beta activity. This study shows for the first time proteinase-activated receptor-2-mediated stimulation of both SAP kinase and IKK signaling and differing roles for PKC isoforms in the regulation of each pathway.

Regulation of surfactant secretion

Lung surfactant is synthesized in the alveolar type II cell. Its lipids and hydrophobic proteins (SP-B and SP-C) are stored in lamellar bodies and secreted by regulated exocytosis. In contrast, the hydrophilic proteins (SP-A and SP-D) appear to be secreted independently of lamellar bodies. Regulation of surfactant secretion is mediated by at least three distinct signaling mechanisms: activation of adenylate cyclase with formation of cAMP and activation of cAMP-dependent protein kinase; activation of protein kinase C; and a Ca(2+)-regulated mechanism that likely results in the activation of Ca(2+)-calmodulin-dependent protein kinase. These signaling mechanisms are activated by a variety of agonists, some of which may have a physiological role. ATP is one such agent and it activates all three signaling mechanisms. There is increasing information on the identity of several of the signaling proteins involved in surfactant secretion although others remain to be established. In particular the identity of the phospholipase C, protein kinase C and phospholipase D isomers expressed in the type II cell and/or involved in surfactant secretion has been established. Distal steps in the secretory pathway beyond protein kinase activation as well as the physiological regulation of surfactant secretion, are major issues that need to be addressed.