Selective activation of the gamma-subspecies of protein kinase C from bovine cerebellum by arachidonic acid and its lipoxygenase metabolites

Pre-incubation of synaptosomes with arachidonic acid potentiates inhibition of [3H]D-aspartate transport

The ability of low micromolar concentrations of the polyunsaturated fatty acid, arachidonic acid (cis-5,8,11,14-eicosatetraenoic acid) to inhibit the high-affinity, sodium-dependent transport of [3H]D-aspartate into purified synaptosomes of rat brain has been examined. Pre-incubation of the synaptosomes with arachidonic acid for 10-60 min produced a marked potentiation of the response to 10 microM arachidonic acid compared to co-incubation, and the threshold for inhibition of [3H]D-aspartate transport occurred at a concentration of 1 microM. Minimal inhibition of transport was seen with the unsaturated fatty acids, cis-oleic (cis-9-octadecenoic acid) and cis-linolenic (cis-9,12,15-octadecatrienoic acid), nor with the 20-carbon saturated fatty acid, arachidic acid (n-eicosanoic acid). Inclusion of the cyclo-oxygenase inhibitor, nor-dihydroguaretic acid (NDGA), in the presence of 5 microM arachidonic acid did not alter the inhibition of [3H]D-aspartate transport between 0-10 min, but did enhance the response at longer pre-incubation times. Inhibition of [3H]D-aspartate transport by arachidonic acid persisted during addition of the calcium ionophore, A23187, whereas removal of calcium ions from the incubation medium potentiated the response to arachidonic acid. The results are discussed in terms of the physiological relevance of the inhibition of glutamate transport by arachidonic acid, and suggest that regulation of inhibition of the glutamate transporter by arachidonic acid may be achieved by changes in the extracellular, as well as the intracellular, concentration of calcium ions.

12(S)-hydroxyeicosatetraenoic acid and 13(S)-hydroxyoctadecadienoic acid regulation of protein kinase C-alpha in melanoma cells: role of receptor-mediated hydrolysis of inositol phospholipids

Protein kinase C (PKC) isoenzymes are essential components of cell signaling. In this study, we investigated the regulation of PKC-alpha in murine B16 amelanotic melanoma (B16a) cells by the monohydroxy fatty acids 12(S)-hydroxyeicosatetraenoic acid [12(S)-HETE] and 13(S)-hydroxyoctadecadienoic acid [13(S)-HODE]. 12(S)-HETE induced a translocation of PKC-alpha to the plasma membrane and focal adhesion plaques, leading to enhanced adhesion of B16a cells to the matrix protein fibronectin. However, 13(S)-HODE inhibited these 12(S)-HETE effects on PKC-alpha. A receptor-mediated mechanism of action for 12(S)-HETE and 13(S)-HODE is supported by the following findings. First, 12(S)-HETE triggered a rapid increase in cellular levels of diacylglycerol and inositol trisphosphate in B16a cells. 13(S)-HODE blocked the 12(S)-HETE-induced bursts of both second messengers. Second, the 12(S)-HETE-increased adhesion of B16a cells to fibronectin was sensitive to inhibition by a phospholipase C inhibitor and pertussis toxin. Finally, a high-affinity binding site (Kd = 1 nM) for 12(S)-HETE was detected in B16a cells, and binding of 12(S)-HETE to B16a cells was effectively inhibited by 13(S)-HODE (IC50 = 4 nM). In summary, our data provide evidence that regulation of PKC-alpha by 12(S)-HETE and 13(S)-HODE may be through a guanine nucleotide-binding protein-linked receptor-mediated hydrolysis of inositol phospholipids.

Purification and characterization of a fatty acid-activated protein kinase (PKN) from rat testis

PKN, a novel protein kinase with a catalytic domain homologous to that of the protein kinase C (PKC) family and unique N-terminal leucine-zipper-like sequences, was identified by molecular cloning from a human hippocampus cDNA library [Mukai and Ono (1994) Biochem. Biophys. Res. Commun. 199, 897-904]. Recently we partially purified recombinant PKN from COS7 cells transfected with the cDNA construct encoding human PKN, and demonstrated that the recombinant PKN was activated by unsaturated fatty acids and limited proteolysis [Mukai, Kitagawa, Shibata et al. (1994) Biochem. Biophys. Res. Commun. 204, 348-356]. The present work has focused on the further purification and characterization of PKN from native rat tissue. Immunochemical measurement revealed that PKN was found in every tissue, and was especially abundant in testis, spleen and brain; subcellular fractionation of rat brain showed that half of the PKN was localized in the soluble cytosolic fraction. PKN was purified approx. 8000-fold to apparent homogeneity from the cytosolic fraction of rat testis by DEAE-cellulose chromatography, ammonium sulphate fractionation and chromatography on butyl-Sepharose, heparin-Sepharose, Mono Q and protamine-CH-Sepharose. The enzyme migrates as a band of apparent molecular mass 120 kDa. Using serine-containing peptides based on the pseudosubstrate sequence of PKC-delta as phosphate acceptors, the kinase activity was stimulated several-fold by 40 microM unsaturated fatty acids or by detergents such as 0.04% sodium deoxycholate and 0.004% SDS. In the absence of modifiers, protamine sulphate, myelin basic protein and synthetic peptides based on the pseudosubstrate site of PKCs or ribosomal S6 protein were good substrates for phosphorylation by the kinase. In the presence of 40 microM arachidonic acid the kinase activity of PKN for these phosphate acceptors was increased 2-18-fold. The autophosphorylation activity of purified PKN was partially inhibited by pretreatment with alkaline phosphatase. These properties appear to distinguish PKN from many protein kinases isolated previously.

Requirement for protein kinase C activation in basic fibroblast growth factor-induced human endothelial cell proliferation

The intracellular signaling mechanisms that mediate basic fibroblast growth factor (bFGF)-induced angiogenesis have not been fully identified. In particular, whether activation of the intracellular enzyme protein kinase C (PKC) is necessary or sufficient for bFGF-induced mitogenesis of human endothelial cells is not clear. Accordingly, the effect of bFGF stimulation on the Ca2+ increase and PKC activity of normal human endothelial cells (HEC) was studied, as was the effect of inhibition of PKC and the distribution of PKC isoenzymes in these cells. The addition of bFGF to cultured HEC increased overall PKC activity in the absence of an increase in intracellular Ca2+ and markedly stimulated their proliferation, as did the addition of PKC-activating phorbol esters. bFGF-induced proliferation was prevented by the PKC inhibitors chelerythrine and H-7 and by downregulation of PKC after prolonged incubation with phorbol esters. In contrast, these inhibitors did not prevent HEC proliferation induced by epidermal growth factor. Because of the failure of bFGF to increase Ca2+, we determined whether bFGF-induced proliferation could be mediated by novel or atypical PKC isoenzymes (which are not regulated by Ca2+). Investigation of the isoenzyme distribution of confluent and subconfluent HEC by immunoblotting, Northern transfer analysis, and polymerase chain reaction of reverse-transcribed RNA revealed the presence of several novel and atypical isoenzymes (PKC-delta, -eta, -theta, and -zeta) as well as small amounts of the conventional (Ca(2+)-regulated) isoenzymes PKC-alpha and -beta. Activation of PKC by bFGF, in the absence of an increase in intracellular Ca2+, suggests that one or more of these Ca(2+)-independent PKC isoenzymes are both necessary and sufficient for HEC proliferation after bFGF.

Arachidonic acid and free fatty acids as second messengers and the role of protein kinase C

In addition to serving as the precursor to a plethora of eicosanoids and other bioactive molecules, arachidonate may function as a bona fide second messenger. A number of studies have documented the ability of arachidonate to regulate the function of multiple targets in vitro systems. This has been particularly well established and studied with the activation of protein kinase C by arachidonate in a mechanism distinct from activation by diacylglycerol. In cells, arachidonate induces a number of activities, many of which may be independent of further metabolism to eicosanoids; suggesting possible direct action of arachidonate. This review summarizes the current state of knowledge on the possible second messenger function of arachidonate with specific emphasis on the regulation of protein kinase C.

Evidence that protein kinase C and mitogen activated protein kinase are not involved in the mechanism by which insulin stimulates translation in L6 myoblasts

Insulin stimulated a concentration-dependent increase in protein synthesis in L6 myoblasts which was significant at 1 nM. This response was not prevented by the transcription inhibitor, actinomycin D. The protein kinase C (PKC) inhibitor, Ro-31-8220, and downregulation of PKC by prolonged incubation of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA), had no effect on the ability of insulin to stimulate protein synthesis whilst completely blocking the response to TPA. In contrast, insulin failed to enhance protein synthesis significantly in the presence of either ibuprofen, a selective cyclooxygenase inhibitor or rapamycin, an inhibitor of the 70 kDa S6 kinase. When cell extracts were prepared and assayed for total myelin basic protein kinase activity, a stimulatory effect of insulin was not observed until the concentration approached 100-fold (i.e. 100 nM) that required to elicit increases in protein synthesis. Upon fractionation on a Mono-Q column, 100 nM insulin increased the activity of 3 peaks which phosphorylated myelin basic protein. Two of these peaks were identified as the 42 and 44 kDa forms of Mitogen Activated Protein (MAP) kinase by immunoblotting. In contrast, 1 nM insulin had no effect on the activity of these peaks. The data suggest that physiologically relevant concentrations of insulin do not stimulate translation in L6 cells through either PKC or the 42/44 kDa isoforms of MAP kinase and that this response is, at least in part, mediated through the activation of the 70 kDa S6 kinase by cyclooxygenase metabolites.

Arachidonic acid: toxic and trophic effects on cultured hippocampal neurons

Arachidonic acid (20:4) is a component of membrane lipids that has been implicated as a messenger both in physiological and pathophysiological processes, including ischemic injury and synaptic plasticity. In order to clarify direct trophic or toxic effects of arachidonic acid on central neurons, primary cultures of rat hippocampal neurons were exposed to arachidonic acid under chemically-defined conditions. Arachidonic acid present in the culture medium at concentrations over 5 x 10(-6) M showed profound toxicity, whereas at lower concentrations (10(-6) M) it significantly supported the survival of hippocampal neurons. These effects were not mimicked by oleic acid (18:1) or palmitic acid (16:0). The toxic action of 10(-5) M arachidonic acid was markedly and significantly prevented by a lipoxygenase inhibitor nordihydroguaiaretic acid (10(-6) M). AA861 and baicalein (each at 10(-6) M), a selective inhibitor for 5- and 12-lipoxygenase, respectively, also showed a significant protective effect, whereas cyclooxygenase inhibitor indomethacin (10(-5) M) had no effect. The toxic action was also prevented by an antioxidant alpha-tocopherol (10(-6) M), but not by superoxide dismutase (100 U/ml) or catalase (200 U/ml). The trophic effect of 10(-6) M arachidonic acid was not suppressed by the treatments listed above. At lower concentrations (10(-7)-10(-6) M), arachidonic acid promoted neurite elongation, which was not inhibited by nordihydroguaiaretic acid or indomethacin. Overall, arachidonic acid has both trophic and toxic actions on cultured hippocampal neurons, part of which involves its metabolism by lipoxygenases. The mechanisms and the physiological significance of these effects are discussed.

Kinase requirement for retinal growth cone motility

Since cytoplasmic Ca2+ levels are reported to regulate neurite elongation, we tested whether calcium-activated kinases might be necessary for growth cone motility and neurite elongation in explant cultures of goldfish retina. Kinase inhibitors and activators were locally applied by micropipette to retinal growth cones and the responses were observed via phase-contrast videomicroscopy. In some cases, growth rates were also quantified over several hours after general application in the medium. The selective inhibitors of protein kinase C, calphostin C (0.1-1 microM) and chelerythrin (up to 50 microM), caused no obvious changes in growth cones or neurite elongation, and activators of PKC (phorbols, arachidonic acid, and diacylglycerol) also were generally without effects, although phorbols slowed the growth rate. Inhibitors of protein kinase A and tyrosine kinases also produced no obvious effects. The calmodulin antagonists, calmidazolium (0.1 microM), trifluoperazine (100 microM), and CGS9343B (50 microM), however, caused a reversible growth cone arrest with loss of filopodia and lamellipodia. The growth cone became a club-shaped swelling which sometimes moved a short distance back the shaft, leaving evacuated filaments at points of strong filopodial attachments. A similar reversible growth cone arrest occurred with the general kinase inhibitors: H7 at 200 but not at 100 microM, and staurosporine at 100 but not 10 nM, suggesting possible involvement of a calmodulin-dependent kinase (camK) rather than PKC. The selective inhibitor of camKII, KN-62 (tested up to 50 microM), produced no effects, but the specific myosin light-chain kinase (MLCK) inhibitors ML-7 (3-5 microM) and ML-9 (5-10 microM) reversibly reproduced the effect, suggesting that MLCK rather than camKII is necessary for growth cone motility. The MLCK inhibitors' effects both on growth cone morphology and on F-actin filaments (rhodamine-phalloidin staining) were similar to those caused by cytochalasin D (5 microM), and are discussed in light of findings that inhibiting MLCK disrupts actin filaments in astrocytes and fibroblasts.

Localization of protein kinase C subspecies in the rabbit retina

The localization of PKC subspecies alpha, beta, gamma, epsilon and zeta was studied immunocytochemically in the rabbit retina. Conventional, Ca(2+)-sensitive PKC subtypes alpha, beta, gamma were all localized in different neuronal populations. The zeta-subspecies, which does not require Ca2+ for activation, was colocalized with PKC-alpha. PKC-epsilon, which is independent of Ca2+ and DAG, was colocalized with PKC-beta. Some populations of neurons, including cone bipolar cells, contained none of the PKC-subspecies studied. These results imply a cellular segregation of different signaling pathways in mammalian retina.

Alterations of protein kinase C isozyme and substrate proteins in mouse brain after electroconvulsive seizures

Protein kinase C (PKC) activity, Western blot analysis of PKC alpha, beta, gamma, epsilon and zeta with isozyme-specific antibodies, endogenous substrate protein phosphorylation, and Western blot analysis of neuromodulin, were studied in mouse brain after repeated electroconvulsive shock. The PKC isozymes and endogenous substrates in the crude cytosolic and membrane fractions were partially purified on DE-52 columns eluted with buffer containing 100 or 200 mM KCl. The kinase activity assayed by phosphorylation of exogenous histone was increased in the 200 mM KCl eluates of both the cytosol and membrane fractions from electroshocked mice. Further analysis by immunoblotting demonstrated that this increased activity was due to an increase in the PKC gamma isozyme. The level of the novel type isozymes, epsilon and zeta, was not altered in electroshocked mice. An in vitro phosphorylation study showed that the endogenous substrate, 17 kDa neurogranin, was mostly eluted by 100 mM KCl. In contrast, the 43 kDa neuromodulin only appeared in the 200 mM KCl eluate, according to autoradiography, SDS-PAGE and Western blot analysis; its level was found to be increased in the membrane fraction of electroshocked mice, as demonstrated by in vitro phosphorylation studies. Therefore, an increase in both PKC gamma and neuromodulin contributed to the increased phosphorylation of neuromodulin during electroshock seizure.

Pentylenetetrazole-induced chemoshock affects protein kinase C and substrate proteins in mouse brain

Protein kinase C (PKC) activity, western blot analysis of PKC alpha, beta, gamma, epsilon, and zeta by isozyme-specific antibodies, and in vitro phosphorylation of endogenous substrate proteins were studied in the mice brain after pentylenetetrazole-induced chemoshock. The PKC isozymes and endogenous substrates in the crude cytosolic and membrane fractions were partially purified by DE-52 columns eluted with buffer A containing 100 or 200 mM KCl. This method consistently separates cytosolic and membrane proteins and various PKC isoforms. The 100 mM KCl eluates from DE-52 columns contain more PKC alpha and beta in both cytosol and membrane than the 200 mM KCl eluates, whereas PKC gamma, epsilon, and zeta appear in equal amounts in these two eluates. The kinase activity assayed by phosphorylation of exogenous histone was increased in the chemoshocked mice in both the cytosol and membrane of 200 mM KCl eluates. In further analysis by immunoblotting, this increased activity was found to be due to the increase in content of PKC gamma isozyme. As for novel-type epsilon and zeta isozymes, they were not altered in the chemoshocked mice. From autoradiography, the endogenous substrate 17-kDa neurogranin, which was shown below 21 kDa, was mostly eluted by 100 mM KCl from the DE-52 column, whereas 43-kDa neuromodulin, which was also demonstrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, only appeared in the 200 mM KCl eluates. The in vitro phosphorylation of neuromodulin was found to be increased in the chemoshocked mice. Therefore, the increased phosphorylation of neuromodulin and increased content of the PKC gamma isoform were involved in the pentylenetetrazole-induced chemoshock.

C-kinase manipulations disrupt activity-driven retinotopic sharpening in regenerating goldfish retinotectal projection

Regenerating optic axons initially branch over a wide area in tectum to form a crude retinotopic map. The map is sharpened, and retinotopically appropriate synapses are stabilized via NMDA receptors that detect, via summation of EPSPs, the coincident activity of neighboring ganglion cells that make synapses onto common tectal cells. Sharpening shares a number of properties with long-term potentiation (LTP) in hippocampus. This study tested whether protein kinase C (PKC) activation is necessary for sharpening as it is for LTP. Intraocular (IO) or intracranial (IC) injections of kinase inhibitors or activators were made every other day from 19 to 37 days postcrush (sensitive period), and the projections formed were later recorded. Retinotopic sharpening was prevented by IC injection of the following agents: (1) general kinase inhibitors sphingosine and H7 (100-200 microM in fluid above brain), (2) active but not inactive phorbols (TPA, 1 microM), and (3) calphostin C (1 microM), a specific and irreversible PKC inhibitor. The mature projection on the opposite tectum, however, when examined was not unsharpened. Lack of sharpening was reflected in multiunit fields at each tectal point that averaged 27 degrees-30 degrees versus 11 degrees in Ringers and inactive phorbol control regenerates. Intraocular injections of either TPA (1 microM), or calphostin C (1 microM) also prevented sharpening (26 degrees and 32 degrees multiunit fields), suggesting action on PKC axonally transported to the presynaptic terminals. Calphostin C had no noticeable effect on the firing patterns of retinal ganglion cells. The endogenous activator of PKC, arachidonic acid (AA), disrupted sharpening at 20 microM or higher (IC injection, 32 degrees multiunit fields), while a control fatty acid, elaidic acid, had no effect. Although AA at 5 microM showed no effect, and diacylglycerol at 5 microM exhibited only small effects, together they produced a large synergistic effect (32 degrees multiunit fields). Such synergy mirrors the synergy in the activation of several isoforms of PKC. Actual concentrations in the extradural fluid around the brain were assayed via injections of 3H-AA. Levels fell about sixfold after a day and by an additional fivefold the second day before the next injection. The results confirm that activity-driven retinotopic sharpening is very sensitive to manipulations of kinases, especially PKC.

Role of specific isoforms of protein kinase C in angiotensin II and lipoxygenase action in rat adrenal glomerulosa cells

Evidence indicates that the lipoxygenase (LO) pathway of arachidonic acid is a key mediator of angiotensin II (AII)-induced aldosterone synthesis in adrenal glomerulosa cells. Although protein kinase C (PKC) may play a role in AII action, the precise PKC isoforms involved and whether LO products can activate PKC is not clear. We therefore evaluated the effect of AII and LO products such as 12- and 15-hydroxyeicosatetraenoic acids (HETEs) on PKC activation in isolated rat adrenal glomerulosa cells. PKC activity was measured by the phosphorylation of a PKC specific peptide while the PKC isoforms were identified by Western immunoblotting using antibodies that recognize the alpha, beta, gamma or epsilon isoforms of PKC. Treatment of the cells for 15 min with AII (10[-8]M) or the LO products 12- or 15-HETE caused a marked increase in PKC activity in membrane fractions with reciprocal decreases in the cytosolic PKC activity. Rat glomerulosa cells expressed only the alpha, and epsilon isoforms of PKC. AII increased membrane bound levels of both PKC-alpha and -epsilon (1.9- and 1.5-fold, respectively), whereas the LO products predominantly activated PKC-epsilon. Reciprocal decreases in immunoreactive cytosolic PKC levels were seen. AII-induced aldosterone synthesis was blocked by H-7 and retinal as well as by a PKC-specific pseudosubstrate inhibitor, PKC(19-36). These results suggest that AII and LO pathway-induced actions in the adrenal glomerulosa may be mediated by specific PKC isoforms.

Polyunsaturated fatty acids do not activate protein kinase C in the testis of the goldfish (Carassius auratus)

Earlier studies have established that polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid and docosahexaenoic acid inhibit steroid production in the goldfish testis. As PUFA inhibit testicular steroidogenesis in the rat through activation of protein kinase C (PKC), the present studies were undertaken to characterize the properties of PKC in the goldfish testis and to test the effects of selected PUFA on PKC activity. PKC activity was quantified in goldfish testis homogenate following partial purification by DEAE-cellulose chromatography by determining the transfer of radiolabelled phosphate from [γ - (32)P]ATP to histone III-S. Testicular PKC activity was defined by the amount of protein phosphorylation in the presence of phosphatidylserine, phasphatidylcholine, Ca(2+) ions and diolein (a 1,2-diacylglycerol analog) above that obtained in response to Ca(2+) ions alone. Western blot analysis of a crude testis homogenate using an antibody specific to the α and β isoforms of mammalian PKC led to the identification a single band of protein (80 kD) that co-migrated with PKC from rabbit brain cytosol. Addition of arachidonic, eicosapentaenoic or docosahexaenoic acids failed to activate PKC. However, PKC activity stimulated by phospholipid, Ca(2+) ions and diolein was inhibited in a dose related fashion by all of these fatty acids. These studies suggest that the inhibitory effects of EPA and DHA on testicular steroidogenesis are not mediated by activation of PKC. The lack of effect of PUFA on PKC activity in the goldfish testis suggests that either the distribution of PKC isoforms differs between the testis of mammals and fish or that PKC is not activated by PUFA in the goldfish.

N-methyl-D-aspartate receptor-induced translocation of protein kinase C to the nucleus in rat cerebellar slices

Rat cerebellar slices were incubated in absence and presence of N-methyl-D-aspartate and then used to prepare a purified nuclear fraction. The purity of the nuclear fraction was assessed by electron microscopy and measurements of Na+, K(+)-ATPase activity. The presence of protein kinase C in nuclear fractions was measured by [3H]phorbol dibutyrate binding. Treatment of cerebellar slices with N-methyl-D-aspartate caused a significant, two-fold increase in the density of nuclear [3H]phorbol dibutyrate binding sites, indicating the translocation of protein kinase C to the nuclear fraction. The effect of N-methyl-D-aspartate was prevented by the presence of the N-methyl-D-aspartate receptor antagonist (+)5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801). These results suggest a possible role for protein kinase C in mediating N-methyl-D-aspartate-induced nuclear events.

Arachidonate activation of protein kinase C may be involved in the stimulation of protein synthesis by insulin in L6 myoblasts

Insulin stimulated protein synthesis in L6 myoblasts but did not increase the labelling of DAG or the release of phosphocholine from phosphatidylcholine. The DAG lipase inhibitor, RHC 80267, more than doubled the amount of label appearing in DAG but did not stimulate protein synthesis. Even in the presence of the DAG lipase inhibitor insulin failed to have any effect on DAG labelling, and conversely RHC 80267 did not modify the insulin-induced increase in protein synthesis. These results suggest that endogenous DAG production is not involved in the stimulation of protein synthesis by insulin. However, exogenous diacylglycerols (1-oleoyl-2-acetyl glycerol and 1-stearoyl-2-arachidonoyl glycerol) both stimulated protein synthesis in L6 myoblasts. The efficacy of the former (arachidonate-free) DAG suggested that their action was by activation of protein kinase C rather than by arachidonate release and prostaglandin formation. Ibuprofen, an inhibitor of cyclo-oxygenase failed to block the effects of insulin whereas a second cyclo-oxygenase inhibitor, indomethacin had only a partial inhibitory effect. The protein kinase C (PKC) inhibitor, RO-31-8220, totally blocked the effect of insulin. Since indomethacin is also recognised to inhibit phospholipase A2, the data suggests that insulin acts on protein synthesis in myoblasts by arachidonate activation of PKC.

Heterologous sensitization of adenylate cyclase activity by serotonin in the rat cerebral cortex

In vitro exposure of rat cerebrocortical slices to microM concentrations of serotonin (5HT) results in an increased response of adenylate cyclase to isoproterenol (ISO). No change in the affinity of the beta-adrenoceptor toward the agonist was found after 5HT exposure when measuring ISO displacement of [3H]CGP 12177 binding. A similar increase of adenylate cyclase response was also found when using VIP as a stimulatory agent. The dose-response curve of adenylate cyclase to the GTP analogue, GppNHp, was modified by 5HT, which promotes a significantly higher maximal response without altering the potency of GppNHp. Forskolin-stimulated adenylate cyclase activity was not affected by 5HT. Serotonergic 5HT2 receptors are involved in the sensitization of adenylate cyclase to GppNHp, since the selective 5HT2 antagonist ketanserin inhibits the effect of 5HT, whereas the 5HT2 agonist DOI mimics 5HT. The involvement of 5HT2 receptor-coupled activation of protein kinase C is also demonstrated: direct protein kinase C activators such as phorbol esters and s,n-dioctanoylglycerol behave in the same manner as 5HT, while the protein kinase C inhibitor CGP 41251 prevents 5HT from increasing adenylate cyclase responsiveness to GppNHp. Moreover, in vitro exposure of cortical slices to 5HT results in reduced inhibition of adenylate cyclase by somatostatin. Since no change was observed at the receptor level and in the direct stimulation of the catalytic subunit of the enzyme, we propose that 5HT might accomplish the sensitization of adenylate cyclase through protein kinase C by inactivating the inhibitory coupling protein Gi and facilitating the interaction of the exogenous GppNHp with the stimulatory coupling protein Gs.

Conformational analysis of arachidonic and related fatty acids using molecular dynamics simulations

Arachidonic acid has recently gained attention as a result of current evidence indicating that it may play the role of a 'second messenger' in signal transduction processes. In order to gain insight into the mechanism behind its action, quenched molecular dynamics simulations were performed on arachidonic (20:4) and related fatty acids: linoleic (18:2), oleic (18:1), arachidic (20:0), and stearic (18:0). The angle-iron structure, representative of arachidonic acid in the crystal or very-low-temperature state, readily gave way at higher temperature to a dominant hairpin structure whereby the COOH end of arachidonic acid comes into close proximity with the C14-15 pi-bond resulting in a packed pi-bond-rich loop. The lowest energy conformer for arachidonic acid was found to be 10.65 kcal/mol below that of the energy-minimized crystal structure. In the case of saturated fatty acids, the crystal all-trans conformation remained the lowest energy form. Analysis of conformational energy contours for carbon-carbon torsion angles representative of fatty acids suggest that the flexibility of arachidonic acid is, in part, a result of the relative torsional freedom of C-C (single) bonds located between or adjacent to C = C (double) bonds. It is hypothesized that the ability of arachidonic acid to form packed structures with curved regions containing pi-bonds may allow for hydrophobic interactions with proteins, and/or hydrogen bonding between the pi-bonds of arachidonic acid and polar groups of the protein structures.

Elevated glucose and angiotensin II increase 12-lipoxygenase activity and expression in porcine aortic smooth muscle cells

The lipoxygenase (LO) pathway of arachidonate metabolism has been suggested to play a key role in atherosclerosis and in mediating several actions of angiotensin II (AII). However, the relationship between LO activation and factors linked to accelerated diabetic vascular disease such as hyperglycemia and AII is not known. We have investigated the effect of high glucose (HG; 25 mM) and AII on LO activity as well as LO protein and mRNA expression in porcine aortic vascular smooth muscle cells (PVSMCs). We observed that cells cultured in HG had significantly higher levels of the cell-associated LO products 12- and 15-hydroxyeicosatetraenoic acids (HETEs). AII added to cells grown in HG specifically further increased only cell-associated 12-HETE levels. Using immunoblot analysis and reverse transcriptase PCRs, we demonstrated the presence in PVSMCs of porcine leukocyte-type 12-LO protein and mRNA, respectively. Furthermore, the levels of both were markedly upregulated by AII as well as by HG. These studies suggest that enhanced 12-LO activity and expression are mechanisms for accelerated vascular disease produced by HG and AII in diabetes mellitus.

Immunocytochemical localization of alpha-, beta I-, beta II- and gamma-subspecies of protein kinase C in the motor and premotor cortices of the rhesus monkey

We obtained evidence for the localization of alpha-, beta I-, beta II- and gamma-subspecies of protein kinase C (PKC) in the monkey motor and premotor cortices (Brodmann's areas 4 and 6). In Brodmann's area 4, the immunoreactivity for the alpha-PKC was present in horizontal and round cells in the layers I and II, and small pyramidal cells in layer III and also in the glial cells in subcortical white matter. The alpha-PKC immunopositive glial cells contained GFAP-immunoreactive product. The beta I-PKC immunoreactivity was present in the round cells in layer I and in the pyramidal cells in the layer V, including Betz cells. The beta II-PKC immunoreactivity was observed as small dots in perikarya of the small and medium-sized pyramidal cells in layers II, III, V and VI, but not in layer I. The gamma-PKC immunoreactive cell bodies were observed in layers II, III and VI, and most of the immunoreactive cells were pyramidal. Intense gamma-PKC immunoreactivity was found in the neuropils of layers I and II. Similar distributions of four PKC subspecies were seen in Brodmann's area 6, except that beta I-PKC immunoreactive Betz cells were not present. The unique localization of PKC subspecies suggested that each PKC subspecies was involved in the specific function in motor and premotor cortices of the rhesus monkey.

Further identification of protein kinase C isozymes in mouse epidermis

In the current study, the protein kinase C (PKC) isozymes present in mouse epidermis have been identified using immunological and chromatographic methods. Six PKC isozymes, PKC alpha, PKC beta, PKC gamma, PKC delta, PKC epsilon, and PKC zeta, were identified in unfractionated epidermal preparations by protein immunoblotting. The subcellular distribution and presence of these isozymes was further verified by hydroxyapatite (HA) chromatography with the exception of PKE epsilon, which could not be detected following HA chromatography. The five PKC isozymes recovered following HA chromatography were detected in both epidermal cytosol and particulate fractions, although PKC delta was found in a much higher proportion relative to the other PKC isozymes in the particulate fraction using histone H1 as the substrate. The biochemical properties of the epidermal PKC isozymes partially purified by HA chromatography agreed with those reported for other tissues and further supported their immunological identification in epidermal preparations. The activities of HA chromatography peaks corresponding to PKC alpha, PKC beta, and PKC gamma were found to be dependent on both Ca2+ and phosphatidylserine (PtdSer), whereas, the activities of HA peaks corresponding to PKC delta and PKC zeta were Ca(2+)-independent but PtdSer-dependent. The HA peak corresponding to PKC gamma also displayed a characteristic biphasic modulation by arachidonic acid (activation at low, inactivation at high concentrations) and inactivation by preincubation with PtdSer. PKC zeta activity was also characteristic, in that it was dependent on PtdSer and was not increased by the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate. Some differences in substrate specificity were also observed between the epidermal PKC isozymes. The presence of multiple isozymes of PKC in mouse epidermis suggests that the different isozymes may play distinct roles in signal transduction and tumor promotion in this tissue.

How is protein kinase C activated in CNS

Protein kinase C (PKC) enzyme family consists of the Ca(2+)-dependent and -independent subgroups of phospholipid/diacylglycerol (DAG)-stimulated serine/threonine protein kinases. These enzymes exhibit distinct cellular and subcellular localizations in CNS and subtle differences in their biochemical characteristics and substrate specificities. It is believed that each of these isoenzymes respond differently to different input signals. However, detailed mechanism for the functioning of these enzymes in vivo is largely unknown; this is in part due to the absence of specific activator, inhibitor, or substrate for each of these enzymes. Recent advances in biochemical, biophysical, and molecular characterizations have defined certain structural features important to confer the stimulatory responses of these enzymes to Ca2+, DAG or phorbol ester, and Zn2+; other features important for the binding of anionic phospholipids, Ca2+/phospholipid complexes, and cis-unsaturated fatty acids have not yet been characterized. Activation of PKC requires the increase in [Ca2+]i and DAG and/or cis-unsaturated fatty acids. Ca2+ promotes the interactions of the Ca(2+)-dependent subgroup of PKCs with membrane phosphatidylserine (PS) and the enzymes become partially active when simultaneously associated with phosphatidylinositol 4,5-bisphosphate or fully active when DAG is available. Free fatty acids such as arachidonic acid, generated by the activation of phospholipase A2, could synergize with DAG to activate the enzyme maximally. The Ca(2+)-independent subgroup of PKCs also become active when associated with PS at elevated level of DAG. Sustained activation of PKCs leads to the conversion of these enzymes into membrane-inserted and membrane protein-associated forms, which may be responsible for certain long-term neural responses. Activation of PKC results in the phosphorylation of cellular proteins; among them, several calmodulin (CaM)-binding proteins are the prominent substrates of these kinases. Phosphorylation of these proteins by PKC favors the release of CaM, which is required for the Ca2+/CaM-dependent enzymes. Thus, activation of PKCs can lead to diverse cellular responses through such amplification steps. Future studies should be directed at the elucidation of the activation of each PKC isoform in vivo to correlate with the physiological responses.

Protein kinase C isoenzymes: divergence in signal transduction?

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Effect of inhibitors of eicosanoid metabolism on release of [3H]noradrenaline from the human neuroblastoma, SH-SY5Y

Nordihydroguaiaretic acid (NDGA; a lipoxygenase inhibitor), LY-270766 (an inhibitor of 5-lipoxygenase), and the diacylglycerol lipase inhibitor RG 80267 completely eliminated potassium-evoked release of [3H]-noradrenaline ([3H]NA) from the human neuroblastoma clone SH-SY5Y with IC50 values of 10, 15, and 30 microM, respectively. In contrast, these inhibitors only partially inhibited carbachol-evoked release and had little effect on the calcium ionophore A23187-evoked release of NA in this cell line. Arachidonic acid partially inhibited potassium- and A23187-evoked release but did not reverse the inhibition of potassium-evoked release observed in the presence of RG 80267. These studies suggest that arachidonic acid (or its lipoxygenase products) are not important intermediates in the regulation of exocytosis in SH-SY5Y. This conclusion is strengthened by our studies in which SH-SY5Y cells were grown in medium supplemented with bovine serum albumin-linoleic acid (50 microM). Under these conditions there was a selective increase in content of membrane polyunsaturated fatty acids of the omega 6 series, including arachidonic acid; however, these changes did not effect potassium-, veratridine-, carbachol-, or calcium ionophore-evoked release of [3H]NA.

Characteristics of arachidonic-acid-mediated brain protein kinase C activation: evidence for concentration-dependent heterogeneity

Arachidonic acid (AA) activates brain protein kinase C (PKC) in a specific manner, and which differs from that of diacylglycerol (DG)-mediated PKC activation in cofactor Ca2+ and phosphatidylserine (PtdSer) requirements. We presently report that characteristics of AA-mediated activation are heterogenous, and are dependent upon the concentrations of AA. Highly sensitive PKC activation (HS) occurring at concentrations of 20 microM AA can be distinguished from less sensitive PKC activation (LS) requiring concentrations of at least 160 microM AA, on the basis of the effects of phorbol ester TPA or DG, phosphatidylcholine (PtdCho) and sodium deoxycholate (DOC). TPA, like DG suppressed the HS reaction whereas it enhanced the LS reaction. PtdCho, a phospholipid which does not affect DG-mediated activation, also prevented the HS reaction without affecting the LS reaction. This latter was inhibited at 100 microM DOC, a concentration which slightly stimulated the HS reaction. The substrate specificity was also different in the two reactions: the preferential substrate for PKC in HS was histone type VII-S, while it was histone type V-S in LS. Both reactions were similarly affected by PtdSer. In 0.1 mM CaCl2, PtdSer stimulated AA-mediated activation without evoking additive responses while this phospholipid prevented this activation in 0.5 mM EGTA, suggesting that AA and PtdSer bind PKC on the same or related sites. Together these results provide evidence for the existence of different modes of AA-mediated PKC activation with unique characteristics which presumably involve two different binding sites for AA on the same molecule and/or different PKC isoforms.

Formation of a novel 20-hydroxylated metabolite of lipoxin A4 by human neutrophil microsomes

Lipoxin A4 (LXA4) is a biologically active compound produced from arachidonic acid via interactions of lipoxygenases. Incubation of LXA4 either with human neutrophils or with the neutrophil microsomes leads to formation of a polar compound on a reverse-phase high-performance liquid chromatography. We have identified the metabolite as 20-hydroxy-LXA4, a novel metabolite of arachidonic acid, on the basis of ultraviolet spectrometry and gas chromatography-mass spectrometry. The LXA4 omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, by antibodies raised against NADPH-cytochrome P-450 reductase, or competitively by leukotriene B4 (LTB4) and LTB5, substrates of LTB4 omega-hydroxylase. These findings indicate that the formation of 20-hydroxy-LXA4 is catalyzed by a neutrophil cytochrome P-450, the LTB4 omega-hydroxylase.

Role of polyunsaturated fatty acids as signal transducers: amplification of signals from growth factor receptors by fatty acids in mammary epithelial cells

The growth, morphogenesis and differentiation of milk producing epithelial tissues in the developing mammary glands require interaction with extracellular matrices and stimulation by hormones, growth factors and essential fatty acids. In primary culture, the proliferation of mammary epithelial cells (MEC), induced by epidermal growth factor (EGF), is enhanced and sustained by linoleate and its eicosanoid metabolites. Since a combination of linoleic acid (18:2 omega 6) and prostaglandin E2 or cAMP has synergistic effect on EGF-stimulated growth, it is suggested that additional cAMP-dependent protein kinase A (PK-A) independent pathways may also contribute to the linoleate effect on EGF action. Possible involvement of Ca2+/phospholipid-dependent protein kinase C (PK-C) is explored. Both linoleate and arachidonate can activate Type-II and Type-III protein kinase-C in MEC and a PK-C inhibitor can block growth stimulation by EGF and fatty acids. Like 12-O-Tetradecanoly phorbol-13-acetate (TPA), a PK-C activator which also enhances EGF-stimulated growth of MEC, linoleate can phosphorylate a 40-42 KD protein. EGF itself can stimulate transient phosphorylation of the same protein in MEC cultures but when supplemented with linoleate, which does not influence the ligand binding affinity of EGF-receptors, the transient phosphorylation signal in 40-42 KD protein is sustained.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular signaling by hydrolysis of phospholipids and activation of protein kinase C

Hydrolysis of inositol phospholipids by phospholipase C is initiated by either receptor stimulation or opening of Ca2+ channels. This was once thought to be the sole mechanism to produce the diacylglycerol that links extracellular signals to intracellular events through activation of protein kinase C. It is becoming clear that agonist-induced hydrolysis of other membrane phospholipids, particularly choline phospholipids, by phospholipase D and phospholipase A2 may also take part in cell signaling. The products of hydrolysis of these phospholipids may enhance and prolong the activation of protein kinase C. Such prolonged activation of protein kinase C is essential for long-term cellular responses such as cell proliferation and differentiation.

Potencies of protein kinase C inhibitors are dependent on the activators used to stimulate the enzyme

The aim was to examine systematically the potencies of protein kinase C inhibitors as a function of the kinase activator. Protein kinase C is activated by at least four stimulators: calcium plus phosphatidylserine (Ca/PS), phorbol 12-myristate 13-acetate plus PS (PS/PMA), arachidonic acid plus calcium (Ca/AA) and the synthetic peptide activator PCK530-558. With histone or GS1-12 as substrates, protein kinase C was maximally activated by Ca/PS, or to maxima of 62%, 89% or 82% with PS/PMA, Ca/AA or PKC530-558, respectively. One group of inhibitors, including H-7 and staurosporine, were equipotent, regardless of the activator. All other inhibitors showed variable selectivity, dependent upon the activator. A second group of inhibitors, including sphingosine and lipophosphoglycan, were eight or 200 times more potent for inhibition of PS/PMA-stimulated activity (relative to Ca/PS) and a third group, including retinal and palmitoylcarnitine, were 14 or 262 times more potent towards Ca/PS-stimulated activity. A final group (rhodamine 6G) was nine times more potent when Ca/AA was the activator. Similar results were obtained using the endogenous substrates dephosphin or MARCKS in synaptosol. Phosphorylation of MARCKS was stimulated by PS/PMA or Ca/PS, while phosphorylation of dephosphin was stimulated only by Ca/PS. The phosphorylation of either by Ca/PS-activated kinase was nine times more potently inhibited by palmitoylcarnitine, while phosphorylation of MARCKS by PS/PMA-activated kinase was 10 times more potently inhibited by sphingosine. H-7 inhibited both at similar concentrations. A model encompasses these differences in potency if the inhibitors are divided into four groups (A-D) according to their competitive inhibition with the appropriate activator or at the active site. The non-selective inhibitors interact at the active sites of protein kinase C (group A). The compounds which preferentially inhibit PS/PMA-activated kinase (sphingosine and lipophosphoglycan) are competitive inhibitors of PMA and 1,2-diacylglycerol (group B), those selective for Ca/PS-activated kinase (palmitoylcarnitine and retinal) are competitive with PS (group C) and those selective for Ca-AA activation (rhodamine 6G) are likely to be competitive with fatty acid (group D). Therefore, the effectiveness of protein kinase C inhibitors is dependent upon the activator employed.

Protein kinase C subspecies in rabbit corneal epithelium: increased activity of alpha subspecies during wound healing

Protein kinase C (PKC) has been implicated in cell proliferation and differentiation. Multiple forms of PKC have been isolated, principally from the brain where PKC is most abundant. In rabbit corneal epithelium, two distinct major peaks of PKC activity were resolved by hydroxyapatite column chromatography. Peak 2, with 65% of the total PKC activity, corresponds to alpha-PKC, based on its mobility in the column and Western blot analysis using specific monoclonal antibodies. Peak 1 did not react with either polyclonal or monoclonal antibodies to PKC alpha-, beta-, and gamma-isoforms suggesting the presence of isoforms specific to the corneal epithelium, or of another member of the PKC family. To investigate possible changes in the amounts of the various PKC subspecies during wound healing, the enzyme activities of the isolated subspecies were assayed 2, 5, and 7 days after corneal de-epithelialization. Two days after wounding, by which time the migratory limbal epithelium had covered the denuded area, total PKC activity was unchanged but alpha-PKC activity had increased to 77% of the total activity, compared with 65% in non-wounded epithelium. An increased proportion of alpha-PKC activity was also observed 5 and 7 days after wounding, during which time proliferation of epithelium continued. We hypothesize that alpha-PKC plays a role in long-term responses after injury such as gene expression and corneal epithelial proliferation. Moreover, these studies indicate that the cornea provides a good model of in vivo wound healing for PKC studies.

Protein-kinase-C inhibitor calphostin C reduces B16 amelanotic melanoma cell adhesion to endothelium and lung colonization

We recently reported that the Ca(2+)- and phospholipid-dependent protein kinase, protein kinase C (PKC), was involved in rat Walker carcinosarcoma cell adhesion to large-vessel endothelium. We extended our studies to explore the role of this kinase in the adhesion to small-vessel endothelium and lung colonization of murine B16 amelanotic melanoma (B16a). Subpopulations of B16a cells, which differ in lung-colonization potentials, were isolated by centrifugal elutriation from solid tumors. In this study, we demonstrate that cells from a high metastatic sub-population (HM340), when compared with cells from a low metastatic sub-population (LM180), exhibit elevated levels of total cellular as well as membrane-bound PKC. The increase in PKC in cells from the HM340 correlates positively to their increased ability to adhere to murine pulmonary-microvessel endothelial-cell monolayer, and to form pulmonary colonies in syngeneic mice. Calphostin C, a potent and selective PKC inhibitor, decreases in a dose-dependent manner the adhesion to endothelium and the lung colonization of cells from both the low and the high metastatic sub-populations with IC50 at sub-micromolar concentrations. In conclusion, our results suggest that PKC may be a key element in regulating tumor-cell metastasis and that PKC inhibitors may be anti-metastatic agents.

Exocytosis from permeabilized lactating mouse mammary epithelial cells. Stimulation by Ca2+ and phorbol ester, but inhibition of regulated exocytosis by guanosine 5'-[gamma-thio]triphosphate

Lactating mouse mammary epithelial cells secrete large amounts of milk protein via constitutive or regulated exocytotic pathways. Secretion through both pathways was quantified by assaying the release of [35S]methionine-labelled trichloroacetic acid-precipitable proteins from digitonin-permeabilized secretory acini isolated from mammary glands of 10-day-post-partum lactating mice. Protein secretion from the isolated permeabilized cells was either Ca(2+)-dependent (regulated) or Ca(2+)-independent (constitutive). In both cases there was a requirement for ATP. Addition of the phorbol ester phorbol 12-myristate 13-acetate (PMA) caused a marked increase in the percentage protein secretion from the cells in a Ca(2+)-independent manner. However, the non-hydrolysable GTP analogue guanosine 5'-[gamma-thio]triphosphate (GTP[S]) caused a partial inhibition of Ca(2+)-dependent exocytosis, while having no significant effect on Ca(2+)-independent exocytosis. Thus the GTP[S] is exerting its effect on the regulated pathway at a site subsequent to protein sorting and packaging into secretory vesicles at the trans-Golgi network.

Activation of protein kinase C in permeabilized human neuroblastoma SH-SY5Y cells

The activation of protein kinase C was investigated in digitonin-permeabilized human neuroblastoma SH-SY5Y cells by measuring the phosphorylation of the specific protein kinase C substrate myelin basic protein4-14. The phosphorylation was inhibited by the protein kinase C inhibitory peptide PKC19-36 and was associated to a translocation of the enzyme to the membrane fractions of the SH-SY5Y cells. 1,2-Dioctanoyl-sn-glycerol had no effect on protein kinase C activity unless the calcium concentration was raised to concentrations found in stimulated cells (above 100 nM). Calcium in the absence of other activators did not stimulate protein kinase C. Phorbol 12-myristate 13-acetate was not dependent on calcium for the activation or the translocation of protein kinase C. The induced activation was sustained for 10 min, and thereafter only a small net phosphorylation of the substrate could be detected. Calcium or dioctanoylglycerol, when applied alone, only caused a minor translocation, whereas in combination a marked translocation was observed. Arachidonic acid (10 microM) enhanced protein kinase C activity in the presence of submaximal concentrations of calcium and dioctanoylglycerol. Quinacrine and p-bromophenacyl bromide did not inhibit calcium- and dioctanoylglycerol-induced protein kinase C activity at concentrations which are considered to be sufficient for phospholipase A2 inhibition.

Thrombin and histamine rapidly stimulate the phosphorylation of the myristoylated alanine-rich C-kinase substrate in human umbilical vein endothelial cells: evidence for distinct patterns of protein kinase activation

Human alpha-thrombin and histamine each stimulates protein phosphorylation in human umbilical vein endothelial cells (HUVEC). We have identified the most prominent of these phosphoproteins by immunoprecipitation as the human homolog of the widely distributed myristoylated alanine-rich C-kinase substrate (MARCKS). Stimulation by 0.1-10 U/ml of alpha-thrombin produces a time-dependent, sustained (plateau 3-5 min) level of MARCKS phosphorylation. MARCKS phosphorylation requires thrombin catalytic activity but not receptor binding and is also seen in response to stimulation by a peptide, TR (42-55), that duplicates a portion of the thrombin receptor tethered ligand created by thrombin proteolytic activity. One micromolar histamine, like alpha-thrombin, produces sustained phosphorylation of MARCKS (plateau 3-5 min). In contrast, 100 microM histamine results in rapid but transient MARCKS phosphorylation (peak 1-3 min). HUVEC treated with 100 microM histamine for 5 min can be restimulated by alpha-thrombin but not fresh histamine, suggesting that the histamine receptor was desensitized. MARCKS phosphorylation can also be induced by several exogenous protein kinase C (PKC) activators and both alpha-thrombin- and histamine-induced MARCKS phosphorylation are inhibited by the PKC antagonist staurosporine. However, while prolonged PMA pretreatment ablates histamine-induced MARCKS phosphorylation, the ability of thrombin to induce MARCKS phosphorylation is retained. These findings provide evidence for agonist-specific pathways of protein kinase activation in response to thrombin and histamine in HUVEC.

Gonadotropin releasing hormone activates the lipoxygenase pathway in cultured pituitary cells: role in gonadotropin secretion and evidence for a novel autocrine/paracrine loop

The formation and role of arachidonic acid (AA) and its metabolites during gonadotropin releasing hormone- (GnRH-) induced gonadotropin secretion were investigated in primary cultures of rat pituitary cells. Prelabeled cells ([3H]AA) responded to GnRH challenge with increased formation (about 2-fold) of the leukotrienes LTC4, LTD4, and LTE4 as well as 5- and 15-eicosatetraenoic acids (5- and 15-HETE) as identified by HPLC. Formation of leukotrienes and 15-HETE was further verified by specific radioimmunoassays. No significant increase in the formation of 12-HETE or of the cyclooxygenase products prostaglandin E (PGE) and thromboxane A2 by GnRH was noticed. Addition of physiological concentrations of LTC4 enhanced basal LH release, while subphysiological concentrations of LTC4 (10(-15)-10(-12) M) inhibited GnRH-induced LH release by about 35% (p less than 0.02). Using specific lipoxygenase inhibitors L-656,224 and MK 886, we found inhibition of GnRH-induced LH release by about 40% at concentrations known to specifically inhibit the 5-lipoxygenase pathway. The peptidoleukotriene receptor antagonist ICI 198,615 inhibited LTC4- and LTE4-induced LH release and surprisingly also the effect of GnRH on LH release by 40%. The data strongly suggest a role for AA and its lipoxygenase metabolites in the on/off reactions of GnRH upon LH release. The data also present a novel amplification cycle in which newly formed leukotrienes become first messengers and establish an autocrine/paracrine loop.

Phosphatidylcholine-dependent protein kinase C activation. Effects of cis-fatty acid and diacylglycerol on synergism, autophosphorylation and Ca(2+)-dependency

A long-chain neutral phospholipid, dioleoylphosphatidylcholine, was found to support protein kinase C activation by cis-fatty acid and diacylglycerol (DAG). This effect of phosphatidylcholine (PC) is totally dependent on the presence of cis-fatty acid; PC greatly stimulates the cis-fatty acid-induced protein kinase C activity, but it does not activate protein kinase C at all, even in the presence of DAG, if cis-fatty acid is absent. DAG, however, plays a modulatory role in the presence of Ca2+; it further enhances the PC-potentiated cis-fatty acid activation of protein kinase C. Although the activities of all three protein kinase C subtypes tested (types I, II and III) are supported by this PC mechanism, type III is most sensitive to the DAG effect, and it is activated synergistically by cis-fatty acid and DAG. The potency of PC to support the synergistic activation of this subtype is equivalent to that of phosphatidylserine (PS). There are several differences, however, between PC- and PS-supported synergism observed in type III protein kinase C: (1) Ca(2+)-sensitivity is different; PC requires higher concentrations of Ca2+ (10-20 microM-Ca2+) than those required for PS (micromolar Ca2+); (2) PC/cis-fatty acid/DAG-induced autophosphorylation of protein kinase C subtypes (types I, II and III) is very weak, whereas PS/cis-fatty acid/DAG strongly stimulate autophosphorylation of these subtypes under the conditions at which both PC and PS systems fully activate the protein kinase C in terms of histone phosphorylation. These observations suggest that a neutral phospholipid such as PC may also participate in the activation and differential regulation of protein kinase C.

Localization of arachidonate 12-lipoxygenase in canine brain tissues

The cytosol fraction from a thoroughly irrigated canine cerebrum was subjected to immunoaffinity chromatography using a monoclonal antibody against porcine leukocyte 12-lipoxygenase. Arachidonate 12-lipoxygenase eluted from the column with some retardation. The enzyme, with a specific activity of 9 nmol/min/mg of protein, converted arachidonic acid to 12(S)-hydroperoxy-5,8,10,14-eicosatetraenoic acid. The enzyme was active not only with arachidonic acid, but also with linoleic and alpha-linolenic acids. In contrast, 12-lipoxygenase of canine platelets was almost inactive with linoleic and alpha-linolenic acids, and the platelet enzyme was also distinguished from the cerebral enzyme in terms of reactivity with the anti-12-lipoxygenase antibody. 12-Lipoxygenase activity was also detected in the cytosol fractions of other parts of canine brain: basal ganglia, hippocampus, cerebellum, olfactory bulb, and medulla oblongata.

Parallel effects of arachidonic acid on insulin secretion, calmodulin-dependent protein kinase activity and protein kinase C activity in pancreatic islets

A potential role of arachidonic acid in the modulation of insulin secretion was investigated by measuring its effects on calmodulin-dependent protein kinase and protein kinase C in islet subcellular fractions. The results were interpreted in the light of arachidonic acid effects on insulin secretion from intact islets. Arachidonic acid could replace phosphatidylserine in activation of cytosolic protein kinase C (K0.5 of 10 microM) and maximum activation was observed at 50 microM arachidonate. Arachidonic acid did not affect the Ca2+ requirement of the phosphatidylserine-stimulated activity. Arachidonic acid (200 microM) inhibited (greater than 90%) calmodulin-dependent protein kinase activity (K0.5 = 50-100 microM) but modestly increased basal phosphorylation activity (no added calcium or calmodulin). Arachidonic acid inhibited glucose-sensitive insulin secretion from islets (K0.5 = 24 microM) measured in static secretion assays. Maximum inhibition (approximately 70%) was achieved at 50-100 microM arachidonic acid. Basal insulin secretion (3 mM glucose) was modestly stimulated by 100 microM arachidonic acid but in a non-saturable manner. In perifusion secretion studies, arachidonic acid (20 microM) had no effect on the first phase of glucose-induced secretion but nearly completely suppressed second phase secretion. At basal glucose (4 mM), arachidonic acid induced a modest but reproducible biphasic insulin secretion response which mimicked glucose-sensitive secretion. However, phosphorylation of an 80 kD protein substrate of protein kinase C was not increased when intact islets were incubated with arachidonic acid, suggesting that the small increases in insulin secretion seen with arachidonic acid were not mediated by protein kinase C. These data suggest that arachidonic acid generated by exposure of islets to glucose may influence insulin secretion by inhibiting the activity of calmodulin-dependent protein kinase but probably has little effect on protein kinase C activity.

Synergistic activation of type III protein kinase C by cis-fatty acid and diacylglycerol

Micromolar concentrations of cis-fatty acid synergistically activate type III protein kinase C with diacylglycerol. This synergistic effect occurs at low concentrations of cis-fatty acid and diacylglycerol, and it is capable of inducing almost full activation of this protein kinase C subtype at a physiologically relevant Ca2+ concentration (2 microM). The synergistic activation mode can be observed even in the absence of Ca2+, but micromolar Ca2+ significantly enhances the type III protein kinase C activation. cis-Fatty acid also augments the diacylglycerol-induced activation of other subtypes (type I and II), although the effect is smaller than that observed in type III. Neither the diacylglycerol- nor the cis-fatty acid-dependent mode of activation can fully activate any of these subtypes at a physiological concentration of Ca2+ (2 microM). Our results suggest that the generation of three second messengers, i.e. the increase in intracellular Ca2+ concentration and the generation of both cis-fatty acid and diacylglycerol in the cell, may be necessary signals for protein kinase C activation, particularly for type III protein kinase C.

The occurrence of three isoenzymes of protein kinase C (alpha, beta and gamma) in retinas of different species

The localisation and immunochemical identification of 3 different forms of protein kinase C (PKC-alpha, PKC-beta and PKC-gamma) in retinas of different species were analysed by immunohistochemistry and SDS-PAGE-Western blotting, respectively. Only in some cases was there a correlation between the findings from each procedure. One reason for the lack of correlation could be the small amounts of PKC present in some retinas, which made detection possible only by first concentrating the antigen by SDS-PAGE and then carrying out Western blotting. Another possible reason is that an antibody recognises unknown antigens immunohistochemically, but, because of their specific characteristics, they are denatured when subjected to SDS-PAGE and Western blotting and therefore remain undetected. PKC-beta immunoreactivity is present in rabbit, frog and goldfish retinas but absent from the rat retina. However, SDS-PAGE and Western blotting experiments showed that the PKC-beta isoenzyme is absent from the fish retina but present in the rat retina. PKC-beta immunoreactivity in rabbit retina is present in ganglion and/or amacrine cells; in the frog retina the enzyme is associated with some bipolar cells. In the goldfish retina, PKC-beta is associated with a large population of cells in the ganglion cell layer as well as with some amacrine cell bodies. PKC-alpha is present primarily in bipolar cells of rat, fish and rabbit retinas and was not detected by immunohistochemistry or blotting experiments in the frog retina. SDS-PAGE and Western blotting of retinal extracts from different species showed that PKC-gamma occurs in the rabbit where it was associated with ganglion and/or amacrine cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoxin generation by human megakaryocyte-induced 12-lipoxygenase

Eicosanoid biosynthesis was examined with a human megakaryocytic cell line (Dami). Megakaryocytes incubated with [1-14C]arachidonic acid and either ionophore A23187 or thrombin generated both thromboxane and 12-hydroxyheptadecatrienoic acid (HHTrE). Exposure to phorbol myristate acetate (PMA) for 1 through 9 days induced differentiation and revealed an increase in the conversion of [1-14C]arachidonate to cyclooxygenase- and lipoxygenase (LO)-derived products. The LO-derived product was identified as 12S-HETE by its physical characteristics including GC/MS and chiral column SP-HPLC. PMA-treated Dami cells did not generate 5-HETE, leukotrienes or lipoxins from exogenous arachidonic acid while they did convert leukotriene A4 (LTA4) to lipoxin A4, lipoxin B4 and their respective all-trans isomers. In addition, COS-M6 cells transfected with a human 12-lipoxygenase cDNA and incubated with either arachidonic acid or LTA4 generated 12-HETE and lipoxins, respectively. The lipoxin profile generated by transfected COS-M6 cells incubated with LTA4 was similar to that generated by the PMA-treated Dami cells. Results indicate that human megakaryocytes can transform arachidonate and LTA4 to bioactive eicosanoids and that the 12-lipoxygenase appears upon further differentiation of these cells. In addition, they indicate that the 12-LO of human megakaryocytes and the 12-LO expressed by transfected COS cells can generate both lipoxins A4 and B4. Together they suggest that the human 12-LO can serve as a model of LX-synthetase activity with LTA4.