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

Arachidonic acid induces phosphorylation of an 18 kDa protein in electrically permeabilised rat islets of Langerhans

Arachidonic acid (AA) was shown to induce concentration-dependent, calcium-independent, in situ phosphorylation of a protein of approximate molecular weight 18 kDa in electrically permeabilised rat islets of Langerhans. This protein did not appear to be a substrate for protein kinase C (PKC) since stimulation of PKC by 4 beta phorbol myristate acetate (4 beta PMA) did not result in 32P incorporation into an 18 kDa protein, and since AA-induced phosphorylation was observed in islets in which PKC had been down-regulated by prolonged exposure of islets to 4 beta PMA. These results suggest that AA stimulates protein phosphorylation by a mechanism other than PKC activation.

A role for the arachidonic acid cascade in fast synaptic modulation: ion channels and transmitter uptake systems as target proteins

Recent evidence indicates that arachidonic acid (AA) and its metabolites play a fast messenger role in synaptic modulation in the CNS. 12-Lipoxygenase derivatives are released by Aplysia sensory neurons in response to inhibitory transmitters and directly target a class of K+ channels, increasing the probability of their opening. In this way, hyperpolarization is achieved and action potentials are shortened, leading to synaptic depression. Other types of K+ channels in vertebrate excitable cells have been found to be sensitive to arachidonic acid, lipoxygenase products, and polyunsaturated fatty acids (PUFA). In the mammalian CNS, arachidonic acid is released upon stimulation of N-methyl-D-aspartate (NMDA)-type glutamate receptors. We found that arachidonic acid inhibits the rate of glutamate uptake in both neuronal synaptic terminals and astrocytes. Neither biotransformation nor membrane incorporation are required for arachidonic acid to exert this effect. The phenomenon, which is rapid and evident at low microM concentrations of AA, may involve a direct interaction with the glutamate transporter or its lipidic microenvironment on the outer side of the cell membrane. Polyunsaturated fatty acids mimic arachidonate with a rank of potency parallel to the degree of unsaturation. Since the effect of glutamate on the synapses is terminated by diffusion and uptake, a slowing of the termination process may potentiate glutamate synaptic efficacy. However, excessive extracellular accumulation of glutamate may lead to neurotoxicity.

NMDA receptor-mediated arachidonic acid release in neurons: role in signal transduction and pathological aspects

The N-methyl-D-aspartate (NMDA)-sensitive subtype of glutamate receptor, which gates Ca(2+)-permeable ion channels, is known for its role in learning and memory formation, in the induction of long-term potentiation, and also in seizure activity and neurotoxicity. In primary cultures of cerebellar neurons, agonists of NMDA receptors induce a dose-dependent release of [3H]arachidonic acid ([3H]AA), which is potentiated by activation of the glycine-positive modulatory site and inhibited by NMDA receptor antagonists. NMDA receptor-induced [3H]AA release is inhibited by quinacrine and partially depends on the presence of extracellular calcium. The [3H]AA release is not sensitive, however, to pretreatment with pertussis or cholera toxin, which suggests a Ca(2+)-dependent activation of phospholipase A2 not employing G proteins. Pretreatment of cultures with the natural and semisynthetic sphingolipids GT1b and PKS 3, respectively, inhibits NMDA receptor-mediated [3H]AA release. We also demonstrated glutamate-evoked [3H]AA release from rat hippocampal slices, which is NMDA receptor mediated, calcium dependent and sensitive to quinacrine. Arachidonic acid and its metabolites have been shown to play a role as second messengers and to modulate neuronal activity. Moreover, they are thought to act as transsynaptic modulators in the mechanism of NMDA receptor-induced long-term potentiation in the hippocampus. Their role in ischemic brain pathology has also been postulated. Our experiments on cultured cerebellar granule cells, incubated in a Mg(2+)-free medium deprived of glucose and oxygen, demonstrated a time-dependent stimulation of [3H]AA release. This release was inhibited by antagonists of NMDA receptors and by quinacrine. Stimulation of NMDA-sensitive glutamate receptors and the subsequent calcium-mediated activation of phospholipase A2 may play a role in the in vivo release of arachidonic acid during brain ischemia. This hypothesis is supported by the observation that the enhanced level of thromboxane B2 in the gerbil brain after 5 min of global ischemia is reduced by the systemic application of either the NMDA antagonist MK-801 or the ganglioside GM1.

Lipoxygenase metabolites of arachidonic and linoleic acids modulate the adhesion of tumor cells to endothelium via regulation of protein kinase C

12(S)-hydroxyeicosatetraenoic acid (12[S]-HETE) and 13(S)-hydroxyoctadecadienoic acid (13[S]-HODE), lipoxygenase metabolites of arachidonic acid and linoleic acid, respectively, previously have been suggested to regulate tumor cell adhesion to endothelium during metastasis. Adhesion of rat Walker carcinosarcoma (W256) cells to a rat endothelial cell monolayer was enhanced after treatment with 12(S)-HETE and this 12(S)-HETE enhanced adhesion was blocked by 13(S)-HODE. Protein kinase inhibitors, staurosporine, calphostin C, and 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine, inhibited the 12(S)-HETE enhanced W256 cell adhesion. Depleting W256 cells of protein kinase C (PKC) with phorbol 12-myristate-13-acetate abolished their ability to respond to 12(S)-HETE. Treatment of W256 cells with 12(S)-HETE induced a 100% increase in membrane-associated PKC activity whereas 13(S)-HODE inhibited the effect of 12(S)-HETE on PKC translocation. High-performance liquid chromatographic analysis revealed that in W256 cells 12-HETE and 13-HODE were two of the major lipoxygenase metabilites of arachidonic acid and linoleic acid, respectively. Therefore, these two metabolites may provide an alternative signaling pathway for the regulation of PKC. Further, these findings suggest that the regulation of tumor cell adhesion to endothelium by 12(S)-HETE and 13(S)-HODE may be a PKC-dependent process.

Preferential release of catecholamine from permeabilized PC12 cells by alpha- and beta-type protein kinase C subspecies

Protein kinase C (PKC) is now recognized as comprising two groups of closely related subspecies. The PKC gamma enzyme is apparently present only in central nervous tissues, and hence was expected to participate in neurotransmitter release. We have utilized a 'depletion-insertion' method to identify the PKCs participating in the exocytotic response. PC12 cells were 'down-regulated' by prior treatment (24 h) with phorbol 12-myristate 13-acetate (PMA; 1 microM), which nearly abolished endogenous PKC activity. Down-regulated PC12 cells were loaded with [3H]dopamine, permeabilized with digitonin, and recombinant or purified PKCs were inserted and activated with a low dose of PMA (20 nM). Among group A PKCs, PKC alpha was the most effective activator of [3H]dopamine release (215%), followed by beta II (185%) and beta I (150%). PKC gamma had no consistent effect on neurotransmitter release. PC12 cells express PKC alpha and PKC beta, but not PKC gamma, as revealed by Northern-blot analysis. We therefore postulate that PKC alpha and PKC beta participate in neurotransmitter release, whereas PKC gamma might be involved in other neuronal functions.

Rapid purification and characterization of protein kinase C from bovine retinal rod outer segments

A rapid FPLC procedure for the purification of protein kinase C from bovine rod outer segments is described. The enzyme is essentially homogeneous after purification and exhibits a molecular mass of approximately 85 kDa, as determined by SDS/PAGE. From its chromatographic behaviour on hydroxyapatite, and from Western-blotting experiments using isoenzyme-specific antibodies, we were able to identify the bovine rod outer segment protein kinase C as being of the alpha or type-III form. The purified protein kinase C has a specific activity of 1066 nmol 32P.min-1.mg protein-1, and shows a 30-fold activation upon the addition of the effectors Ca2+, PtdSer and 1,2-diacylglycerol. Arachidonic acid and linoleic acid were also found to enhance significantly the activity of the purified enzyme.

Differential expression of the beta I- and beta II-PKC subspecies in the postnatal developing rat brain; an immunocytochemical study

Differential expression of protein kinase C subspecies, beta I- and beta II-PKC, derived from a single gene by alternative splicing was evidenced in the postnatal developing rat brain. Immunoblot analysis of the PKC subspecies in the whole developing brain showed that beta I-PKC was present at birth and then gradually increased, while beta II-PKC was not present at birth or on postnatal day 3, then increased rapidly from day 7 to the maximum value seen in the adult brain. Under light microscopy, beta I-PKC immunoreactivities seen at birth were the most intense in the brainstem and intense in the diagonal bundle and globus pallidus. beta I-PKC immunoreactivities in these neurons weakened from day 7 and disappeared in the adult brain, while in the cerebral cortex, triangular septal nucleus and pontine nucleus beta I-PKC immunoreactivities were week at birth and then gradually increased. beta II-PKC immunoreactivities were first visible in neurons on day 7 and increased progressively. beta I- and beta II-PKCs were not co-localized in a neuron, as far as examined. The immunoreactivities of beta I-PKC at birth were localized in growth cone-like structures as well as in the dendrites and perikarya. Similarly, alpha-PKC was also present at birth in the growth cone-like structure. Immunoblot analysis revealed that beta I-PKC was present at birth in the growth cone-rich fraction from the hindbrain but not in that from the forebrain, while alpha-PKC was found in the growth cone-rich fraction from both the forebrain and the hindbrain. beta II- and gamma-PKC were not detected in the growth cone-rich fraction from either forebrain or hindbrain. These findings suggest that beta I- and beta II-PKC play a role in different stages of development and in different neurons; both beta-subspecies may be involved in postnatal developing neuronal functions while only beta I-PKC plays functional roles in the growth cone, in the prenatal developmental stage.

Calcium influx through 'L'-type channels into rat anterior pituitary cells can be modulated in two ways by protein kinase C (PKC-isoform selectivity of 1,2-dioctanoyl sn-glycerol?)

The depolarisation-induced influx of 45Ca2+ into anterior pituitary tissue and GH3 cells through 'L'-type, nimodipine-sensitive channels was investigated. In anterior pituitary prisms, phorbol esters, activators of protein kinase C, caused an enhancement of K(+)-induced 45Ca2+ influx. However, in the GH3 anterior pituitary cell line, phorbol esters inhibited K(+)-induced 45Ca2+ influx. The modulation by phorbol esters in both tissues was stereo-specific and time- and concentration-dependent. The diacylglycerol analogue, 1,2-dioctanoyl sn-glycerol was able to mimic the phorbol ester-induced enhancement of calcium influx into anterior pituitary pieces, but was ineffective in GH3 cells. 1,2-Dioctanoyl sn-glycerol may selectively activate an isoform of protein kinase C which is responsible for enhanced 'L'-type Ca(2+)-channel activity.

Protein kinase c-beta-like immunoreactivity in the developing and adult rat adrenal gland

Protein kinase c-beta-like immunoreactivity was studied in the adrenal gland of adult rats and at different pre- and postnatal stages of development (E17-P21) with an antibody specific to both the beta 1 and beta 2 subtypes of the kinase. In the adult rat adrenal gland, the immunoreactivity was seen in numerous nerve fibres in the adrenal medulla both in bundles and individually forming occasionally dense networks around chromaffin cell groups. Several protein kinase c-beta-immunoreactive fibres were also observed transversing the adrenal cortex towards the medulla. No remaining immunoreactive fibres two weeks after transection of the splanchnic nerve could be seen; nor was any immunoreactivity observed in the chromaffin cells of the adrenal medulla or in the cortical cells, but some faintly immunoreactive ganglion cells were detected in the adrenal medulla. The amount and distribution of protein kinase c-beta-like immunoreactivity in the fetal and developing adrenals was very similar to that seen in the adrenal glands of adult rats. On the basis of its localization, the beta-subtype of protein kinase c does not appear to be directly involved in the release of catecholamines from the adrenal medulla, but it might have a role in the regulation of neurotransmitter release from preganglionic cholinergic neurons.

A long-term depression of AMPA currents in cultured cerebellar Purkinje neurons

Cerebellar long-term depression (LTD) is a model of synaptic plasticity in which conjunctive stimulation of parallel fiber and climbing fiber inputs to a Purkinje neuron induces a persistent depression of the parallel fiber-Purkinje neuron synapse. We report that an analogous phenomenon may be elicited in the cultured mouse Purkinje neuron when iontophoretic glutamate application and depolarization of the Purkinje neurons are substituted for parallel fiber and climbing fiber stimulation, respectively. The induction of LTD in these cerebellar cultures requires activation of both ionotropic (AMPA) and metabotropic quisqualate receptors, together with depolarization in the presence of external Ca2+. This postsynaptic alteration is manifest as a depression of glutamate or AMPA currents, but not aspartate or NMDA currents. These results strengthen the contention that the expression of cerebellar LTD is at least in part postsynaptic and provide evidence that activation of both ionotropic and metabotropic quisqualate receptors are necessary for LTD induction.

Synergistic action of diacylglycerol and unsaturated fatty acid for protein kinase C activation: its possible implications

Kinetic properties of the purified alpha, beta, and gamma subspecies of protein kinase C (PKC) to respond to diacylglycerol, phosphatidylserine (PtdSer), and Ca2+ were reinvestigated in the presence of several fatty acids. Although responses of these enzyme subspecies to the lipids slightly differed from one another, the reaction velocity of these subspecies was significantly enhanced by synergistic action of diacylglycerol and a cis-unsaturated fatty acid. Arachidonic, oleic, linoleic, linolenic, and docosahexaenoic acids were active in this role, whereas saturated fatty acids such as palmitic and stearic acids were inactive. Elaidic acid was also inactive. In the presence of both PtdSer and diacylglycerol, the cis-unsaturated fatty acids increased further an apparent affinity of PKC to Ca2+ and allowed the enzyme to exhibit almost full activation at nearly basal levels of Ca2+ concentration. The concentration of fatty acid giving rise to the maximum activation of enzyme was approximately 20-50 microM. The result presented herein implies that the receptor-mediated release of unsaturated fatty acids from phospholipids may take part, in synergy with diacylglycerol, in the activation of PKC even when the Ca2+ concentration is low. A possibility arises, then, that the activation of PKC is an integral part of the signal-induced degradation cascade of various membrane phospholipids, which is initiated by the actions of phospholipase C and phospholipase A2.

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

GTP-binding proteins and potassium channels involved in synaptic plasticity and learning

Inhibition of potassium channels is possibly the first step in the sequence of biochemical events leading to memory formation. These channels appear to be regulated directly or indirectly by GTP-binding proteins (G proteins), which may themselves be affected by phosphorylation and dephosphorylation in response to elevated calcium levels or other phenomena resulting from the blockage of the potassium channels. A wide variety of cellular phenomena, from transcriptional changes to axonal transport, are thus capable of being initiated by these events.

Long-lasting change in the membrane-associated protein kinase C activity in the hippocampal kindled rat

The effect of hippocampal kindling on protein kinase C (PKC) activity and protein concentration was investigated in rat amygdala/pyriform cortex (AM/PC) and right (contralateral) and left (ipsilateral) hippocampus (HIPP). There was no difference in cytosolic PKC activity between control and kindled groups in any part of the brain. The membrane-associated PKC activity was altered as follows. One week after the last seizure, it was significantly increased in both right (by 26%, P less than 0.05) and left HIPP (by 30%, P less than 0.02). Four weeks after the last seizure, it was significantly increased in the AM/PC (by 14%, P less than 0.02), right HIPP (by 37%, P less than 0.01) and left HIPP (by 24%, P less than 0.05). The protein concentrations in the crude cytosolic extracts prior to elution of PKC through DE-52 columns were significantly increased in the AM/PC (by 11%, P less than 0.05) and right HIPP (by 18%, P less than 0.02) 4 weeks after the last seizure. In the membrane extracts, there was a significant increase by 23% (P less than 0.02) in the left HIPP 1 week after the last seizure. In the fraction co-eluted with PKC, a significant increase in protein concentration of the cytosolic preparation was confirmed in the AM/PC (by 12%, P less than 0.05) as well as in the left HIPP (by 15%, P less than 0.05) 4 and 1 weeks respectively after the last seizure.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the activity of protein kinase C and the differential subcellular redistribution of its isozymes in the rat striatum during and following transient forebrain ischemia

The changes in the levels of protein kinase C [PKC(alpha, beta II, gamma)] were studied in cytosolic and particulate fractions of striatal homogenates from rats subjected to 15 min of cerebral ischemia induced by bilateral occlusion of the common carotid arteries and following 1 h, 6 h, and 48 h of reperfusion. During ischemia the levels of PKC(beta II) and -(gamma) increased in the particulate fraction to 390% and 590% of control levels, respectively, concomitant with a decrease in the cytosolic fraction to 36% and 20% of control, respectively, suggesting that PKC is redistributed from the cytosol to cell membranes. During reperfusion the PKC(beta II) levels in the particulate fraction remained elevated at 1 h postischemia and decreased to below control levels after 48 h reperfusion, whereas PKC(gamma) rapidly decreased to subnormal levels. In the cytosol PKC(beta II) and -(gamma) decreased to 25% and 15% of control levels at 48 h, respectively. The distribution of PKC(alpha) did not change significantly during ischemia and early reperfusion. The PKC activity in the particulate fraction measured in vitro by histone IIIS phosphorylation in the presence of calcium, 4 beta-phorbol 13-myristate 12-acetate, and phosphatidylserine (PS) significantly decreased by 52% during ischemia, and remained depressed over the 48-h reperfusion period. In the cytosolic fraction PKC activity was unchanged at the end of ischemia, and decreased by 47% after 6 h of reperfusion. The appearance of a stable cytosolic 50-kDa PKC-immunoreactive peptide or an increase in the calcium- and PS-independent histone IIIS phosphorylation was not observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoplasmic lipid bodies of neutrophils: formation induced by cis-unsaturated fatty acids and mediated by protein kinase C

Lipid bodies, nonmembrane-bound cytoplasmic inclusions, serve as repositories of esterified arachidonate and are increased in cells associated with inflammatory reactions. We have evaluated stimuli and mechanisms responsible for lipid body formation within human polymorphonuclear leukocytes (PMNs). Arachidonic acid and oleic acid stimulated dose-dependent formation of lipid bodies over 0.5-1 h. Other C20 and C18 fatty acids were less active and demonstrated rank orders as follows: cis-unsaturated fatty acids were much more active than trans-fatty acids, and activity diminished with decreasing numbers of double bonds. Lipid bodies elicited in vitro with cis-fatty acids were ultrastructurally identical to lipid bodies present in PMNs in vivo. Lipid body induction was not because of fatty acid-elicited oxidants or fatty acid-induced ATP depletion. Cis-fatty acid-induced activation of protein kinase C (PKC) was involved in lipid body formation as evidenced by the capacity of other PKC activators, 1-oleoyl-2-acetyl-glycerol and two active phorbol esters, phorbol myristate acetate, and phorbol 12,13 dibutyrate, but not an inactive phorbol, to induce lipid body formation. The PKC inhibitor, 1-O-hexadecyl-2-O-methyl-glycerol, inhibited PMN lipid body formation induced by oleic and arachidonic acids and by 1-oleoyl-2-acetyl-glycerol and phorbol myristate acetate. Other PKC inhibitors (staurosporine, H-7) also inhibited lipid body formation. Formation of lipid bodies in PMNs is a specific cellular response, stimulated by cis-fatty acids and diglycerides and apparently mediated by PKC, which results in the mobilization and deposition of lipids within discrete, ultrastructurally defined cytoplasmic domains.

Synaptosomal protein kinase C subspecies: A. Dynamic changes in the hippocampus and cerebellar cortex concomitant with synaptogenesis

The expression of protein kinase C (PKC) subspecies in synaptosomes prepared from a number of adult brain regions was compared. Cerebral cortical and thalamic/striatal synaptosomes were found to express three peaks of enzyme activity upon hydroxyapatite chromatography, corresponding to the type I(gamma), type II(beta), and type III(alpha) subspecies. Synaptosomes prepared from either the hippocampus or the cerebellar cortex, however, contained only two major peaks, corresponding to the alpha- and beta-subspecies, with barely detectable levels of the gamma-subspecies, even though these tissue areas were enriched in the latter enzyme. When the ontogenic pattern of hippocampal synaptosomal PKC subspecies was examined, it was found that at postnatal day 7, significant quantities of the gamma-subspecies were present and that this subspecies reached its peak levels at around postnatal day 14, before steadily declining to its adult level. Similar changes were observed also for the gamma-subspecies in cerebellar cortex synaptosomes. The dynamic changes in the synaptosomal PKC subspecies take place at a critical period in the development of the rat brain, concomitant with an active period of synaptogenesis, suggesting that it may play a role in synaptogenesis.

Protein kinase C subspecies in adult rat hippocampal synaptosomes. Activation by diacylglycerol and arachidonic acid

Synaptosomes isolated from the adult rat hippocampus contain the alpha- and beta-subspecies of protein kinase C (PKC), but not the gamma-subspecies which is abundantly expressed in the pyramidal cells in this brain region. Although the gamma-subspecies is known to respond significantly to free arachidonic acid, it is found that both the alpha- and beta-subspecies are also activated dramatically by arachidonic acid in synergistic action with diacylglycerol. Oleic, linoleic, and linolenic acids are all active. It is possible that unsaturated fatty acids may take part in the activation of alpha- and beta-subspecies of PKC which are present in the presynaptic nerve endings terminating at the hippocampal pyramidal cells.

Characterization of lipoxins by combined gas chromatography and electron-capture negative ion chemical ionization mass spectrometry: formation of lipoxin A4 by stimulated human whole blood

The lipoxins are a recent addition to the family of bioactive products derived from arachidonic acid. Here, we have prepared pentafluorobenzyl ester, trimethylsilyl ether derivatives of lipoxin A4, lipoxin B4 and pentadeuterolipoxin A4 and have characterized these products by electron-capture negative ion chemical ionization gas chromatography/mass spectrometry (NICI GC/MS). Lipoxin A4 (5S,6R,15S-trihydroxy-7,9,13-trans-11-cis-eicosa-tetraenoic acid; LXA4) was quantified following extraction from whole blood by stable isotopic dilution utilizing deuterium-labeled LXA4 as internal standard and selected ion monitoring of the [M--pentafluorobenzyl] anions. Studies with a second tritiated internal standard (e.g. [11,12-3H]LXA4) also showed that the recovery of LXA4 was greater than 80% following solid-phase extraction from whole blood, and greater than 90% from isolated cells. In addition, neither isolated neutrophils nor platelets oxidatively metabolized [11,12-3H]LXA4 when incubated in the presence or absence of stimuli. Whole blood incubated with either the ionophore of divalent cations (A23187), thrombin, or thrombin plus the chemotactic peptide formylmethionyl-leucine-phenylalanine generated both LXA4 and thromboxane, which were quantified by stable isotope dilution. The ratio of thromboxane to LXA4 formed by stimulated whole blood ranged from approximately 2:1 to 20:1. These results indicate that the lipoxins display suitable characteristics as their respective pentafluorobenzyl ester, trimethylsilyl ether derivatives for quantification by electron-capture NICI GC/MS. Moreover, they provide evidence that LXA4 can be generated from endogenous sources in whole blood following exposure to physiologically relevant stimuli.

Lipoxins: eicosanoids carrying intra- and intercellular messages

The lipoxins are a recent addition to the family of biologically active products derived from arachidonic acid. Compounds of this series contain a conjugated tetraene structure and can be generated by the actions of the major lipoxygenases of human tissues (5-, 12-, and 15-LO's). Biosynthesis of the lipoxins from cellular sources of unesterified arachidonic acid is triggered by the initial actions of either the 15-LO or 5-LO followed by additional reactions. Recent results indicate that lipoxins are also generated by receptor-mediated events during cell-cell interactions with the transcellular metabolism of key intermediates. Lipoxin A4 and lipoxin B4 each possess a unique spectrum of biological activities unlike those of other eicosanoids in both in vivo and in vitro systems. Lipoxin A4 stimulates changes in the microvasculature and can block some of the proinflammatory effects of leukotrienes (in vivo). Lipoxin A4 and lipoxin B4 both inhibit natural killer cells (in vitro), and lipoxin B4 displays selective actions on hematopoietic cells. The finding that lipoxin A4 activates isolated protein kinase C suggests that it may also serve an intracellular role in its cell of origin before it is released to the extracellular milieu. Thus, cell-cell interactions, along with multiple oxygenations by lipoxygenases, generate compounds that can regulate cellular responses by serving as both intra- and intercellular messages.

Protein kinase C and phospholipase C mediate alpha 1- and beta-adrenoceptor intercommunication in rat hypothalamic slices

These experiments examined the mechanism by which phenylephrine enhances beta-adrenoceptor-stimulated cyclic AMP formation in rat hypothalamic and preoptic area slices. To this end we manipulated phospholipase C. phospholipase A2, and protein kinase C activity in slices and assessed the effects of these manipulations on phenylephrine augmentation of isoproterenol-stimulated cyclic AMP generation. Since previous work indicated that estrogen enhances the alpha 1-component of cyclic AMP formation, we examined slices from both gonadectomized and estrogen-treated animals. The alpha 1-antagonist prazosin eliminated phenylephrine augmentation of the beta-response, suggesting that alpha 1-adrenergic receptors mediate the potentiation of cyclic AMP formation. Inhibition of protein kinase C by H7 attenuated the alpha 1-augmentation of beta-stimulated cyclic AMP formation. Staurosporine, a more potent protein kinase C inhibitor, completely abolished the alpha 1-augmenting response. In addition, phenylephrine potentiation of the isoproterenol response was not observed if protein kinase C was first stimulated directly with a synthetic diacylglycerol (1-oleoyl-2-acetyl-sn-glycerol) or phorbol ester (phorbol 12,13-dibutyrate). Neomycin, an inhibitor of phospholipase C, decreased alpha 1-receptor enhancement of beta-stimulated cyclic AMP formation, whereas quinacrine, an inhibitor of phospholipase A2, did not. The data suggest that the postreceptor mechanism involved in alpha 1-adrenergic receptor potentiation of cyclic AMP generation in hypothalamic and preoptic area slices includes activation of phospholipase C and protein kinase C.

Dual effect of arachidonic acid on protein kinase C isoenzymes isolated from rabbit thymus cells

Type II and type III isoenzymes of protein kinase C isolated from rabbit thymus cells were activated at relatively low concentrations but were inhibited at higher concentrations of arachidonic acid. Activation by cis-unsaturated fatty acids required Ca2+; the maximal activity was approached at about 10(-6) M Ca2+ concentration. The kinetics of activation and inhibition by arachidonic acid depended strongly on the nature of the substrate (synthetic oligopeptide or H1 histone), on the concentration of the protein substrate and on the stage of purification of the isoenzyme preparation investigated. Activation seemed to be favoured at high protein concentrations.

N-methyl-D-aspartate-sensitive glutamate receptors induce calcium-mediated arachidonic acid release in primary cultures of cerebellar granule cells

In primary cultures of cerebellar granule cells, glutamate, aspartate, and N-methyl-D-aspartate (NMDA) induced a dose-dependent release of [3H]arachidonic acid ([3H]AA) which was selective for these agonists and was inhibited by NMDA receptor antagonists. The agonist-induced [3H]AA release was reduced by quinacrine at concentrations that inhibited phospholipase A2 (PLA2) but affected neither the activity of phospholipase C (PLC) nor the hydrolysis of phosphoinositides induced by glutamate or quisqualate. Thus, the increased formation of AA was due to the receptor-mediated activation of PLA2 rather than to the action of PLC followed by diacylglycerol lipase. The receptor-mediated [3H]AA release was dependent on the presence of extracellular Ca2+ and was mimicked by the Ca2+ ionophore ionomycin. Pretreatment of granule cells with either pertussis or cholera toxin failed to inhibit the receptor-mediated [3H]AA release. Hence, in cerebellar granule cells, the stimulation of NMDA-sensitive glutamate receptors leads to the activation of PLA2 that is mediated by Ca2+ ions entering through the cationic channels functioning as effectors of NMDA receptors. A coupling through a toxin-sensitive GTP-binding protein can be excluded.

Protein kinase C is translocated to cell membranes during cerebral ischemia

The subcellular distribution of PKC(alpha) and PKC(gamma) was studied in homogenates of cerebral cortex from rats subjected to 10 and 15 min of ischemia and 15 min of ischemia followed by 1 h, 6 h, 24 h, 48 h, and 7 days of reperfusion. During ischemia no significant changes in the levels of PKC (alpha) were seen. During the first hour of reperfusion, a transient 2.5-fold (P less than 0.05) increase in PKC (alpha) levels was observed in the particulate fraction. In contrast, a three-fold increase of PKC(gamma) in the particulate fraction concomitant with a 40% decrease in the cytosol was noted during ischemia. In the postischemic phase the levels in the cytosol decreased to 35% of control values at 2 days following ischemia, with a concomitant decrease in the particulate fraction to control levels. The redistribution of PKC to the cell membranes during and following ischemia could be due to ischemia induced receptor activation, increased levels of diacylglycerols, arachidonate and intracellular calcium, and may be of importance for the development of ischemic neuronal damage.

Nordihydroguaiaretic acid inhibits urokinase synthesis by phorbol myristate acetate-stimulated LLC-PK1 cells

Protein kinase C (PKC) activation is regulated by Ca2+, phospholipids, diacylglycerol (DAG) and fatty acids. Phorbol myristate acetate (PMA) which mimics the effect of DAG on PKC induces transcriptional activation of the urokinase-type plasminogen activator (u-PA) gene in LLC-PK1 cells. We examined in the present work the relationships between PKC activity, fatty acids, and u-PA synthesis in this cell line. We showed that H7, an inhibitor of PKC, inhibited the PMA-induced u-PA synthesis by LLC-PK1 cells. PMA-induced u-PA synthesis was enhanced by eicosatetraynoic acid (ETYA), a competitive inhibitor of both the lipoxygenase and cyclooxygenase pathways and inhibited by nordihydroguaiaretic acid (NDGA), an inhibitor of the lipoxygenase pathway. Three other unrelated lipoxygenase inhibitors (phenidone 100 microM, BW755 50 microM and diethylcarbamazine 50 microM) had no effect on u-PA biosynthesis. Two polyunsaturated fatty acids other than ETYA, arachidonic acid and linoleic acid, also potentiated the PMA effect and a lipoxygenase derivative, 12 hydroxyeicosatetraenoic acid (12 HETE), did not modify the basal and PMA-stimulated u-PA syntheses. PKC activity purified from cytosol of LLC-PK1 cells was stimulated by addition of 16 nM PMA in vitro and this effect was blunted by simultaneous addition of 5 microM NDGA. By Northern blot analysis using a pig u-PA cDNA probe we found that PMA increased the steady state level of u-PA mRNA after 2 h of incubation and that NDGA inhibited this effect. These data suggest that NDGA inhibits PMA-stimulated PKC activity in intact cells leading to a decrease of u-PA mRNA level and u-PA biosynthesis in PMA-stimulated LLC-PK1 cells. Polyunsaturated fatty acids have opposite effects.

Protein kinase C and linoleic acid-induced inhibition of melanogenesis

Linoleic acid has been shown to inhibit melanogenesis in cultured B16 mouse melanoma cells. We report here the possible involvement of protein kinase C (PKC) in linoleic acid-induced inhibition of melanogenesis in B16 cells. A single PKC subspecies (alpha-PKC) was detected in B16 cells. The enzyme was activated by linoleic acid in vitro. The effective concentrations at which PKC was activated (25 microM; maximum response) were consistent with those for the inhibition of melanogenesis in cell culture system. In addition, the permeable diacylglycerol 1-oleoyl-2-acetyl glycerol that activates PKC also inhibits melanogenesis at 100 microM. These results suggest that activation of PKC plays a pivotal role in the linoleic acid-induced inhibition of melanogenesis in B16 cells.

Arachidonic acid modulates hippocampal calcium current via protein kinase C and oxygen radicals

Arachidonic acid (AA) is a second messenger liberated via receptor activation of phospholipase A2 or diacylglycerol-lipase. We used whole-cell voltage clamp of acutely isolated hippocampal CA1 pyramidal cells to investigate the hypothesis that AA modulates Ca2+ channel current (ICa) via activation of protein kinase C (PKC) and generation of free radicals. AA depressed ICa in a dose- and time-dependent manner similar to that previously reported for the action of phorbol esters on ICa. A similar depression was seen with a xanthine-based free radical generating system. The specific PKC inhibitor PKCI (19-36), the protein kinase inhibitor H-7, and the superoxide free radical scavenger SOD each blocked ICa depression by 70%-80%. Complete block of the AA response occurred when SOD was used simultaneously with a PKC inhibitor. These data suggest that PKC and free radicals play a role in AA-induced suppression of ICa.

Neuron-specific protein F1/GAP-43 shows substrate specificity for the beta subtype of protein kinase C

We determined whether the beta or gamma protein kinase C (PKC) subtypes implicated in long-term potentiation (LTP) selectively regulates protein F1 phosphorylation. Purified bovine PKC subtypes and recombinant PKC subtypes activated by phosphatidylserine (PS) and calcium were tested for their relative ability to phosphorylate purified rat protein F1 (a.k.a. GAP-43). After equalizing enzyme activity against histone, the recombinant beta II PKC phosphorylated protein F1 to a 6 fold greater extent than the recombinant gamma PKC. Bovine beta I PKC phosphorylated protein F1 to a 3 fold greater extent than bovine gamma PKC. Even when PS was replaced by lipoxin B4, which can selectively increase gamma PKC activity, beta I PKC was still superior to gamma PKC in phosphorylating protein F1. Taken together with previous cellular studies of brain showing parallel levels of expression of beta PKC mRNA and protein F1 mRNA, the present results make it attractive to propose that beta PKC regulates protein F1 phosphorylation during the development of synaptic plasticity.

Key role of diacylglycerol-mediated 12-lipoxygenase product formation in angiotensin II-induced aldosterone synthesis

We have shown earlier that the 12-lipoxygenase product of arachidonic acid (AA), 12-hydroxyeicosatetraenoic acid (12-HETE), plays an important role in mediating angiotensin II (AII)-induced aldosterone secretion (J. Clin. Invest. (1987) 80, 1763). In the present study, we have evaluated whether diacylglycerol (DG) is the source of arachidonic acid giving rise to this 12-HETE. Treatment of rat adrenal glomerulosa cells with a DG lipase inhibitor, RHC 80267, which prevents conversion of DG to AA and HETEs, blocked AII-induced aldosterone and 12-HETE formation. In contrast, a DG kinase inhibitor, R59022, which prevents conversion of DG to phosphatidic acid, potentiated AII-induced aldosterone and 12-HETE formation. These two inhibitors block DG metabolism which would be expected to lead to increased DG levels and protein kinase C activity and AII-induced steroidogenesis. However, only R59022 potentiated AII action while RHC 80267 was inhibitory. This suggests that conversion of DG to AA and 12-HETE is important for AII action. Further proof for this was obtained by measuring [3H]AA-labeled DG levels. The combination of the inhibitors significantly potentiated AII-induced DG formation even though this same combination was inhibitory on AII-induced aldosterone and 12-HETE. Thus, the inhibitory effect of RHC 80267 is due to blockade of AA release and not of DG formation. These results suggest that DG plays a dual role in AII action, both as an activator of protein kinase C and as a source of AA for 12-HETE formation.

Differential inhibition of protein kinase C subtypes

Catalytic properties of protein kinase C isoforms purified from rat brain and bovine adrenocortical tissues were examined. The results showed that known inhibitors of PKC activity such as gossypol and H-7 were active on all the three isolated enzyme isoforms with similar IC50 values. However, whereas the type III brain isozyme activity was not affected by a preincubation with phosphatidylserine (PS), the same treatment resulted in a virtually complete loss of the type I and II isoform activities within 4 min at 30 degrees. This kinase inactivation caused by PS preincubation was prevented in the presence of ATP-Mg2+ or its competitive inhibitor H-7. These findings indicate that the type III isoform can clearly be distinguished from the other members of the PKC family by this specific property. This approach was used to confirm the characterization of the single form of PKC detected in bovine adrenocortical tissue as a type III isotype. This specific behavior toward phosphatidylserine suggests that the molecular organization of the phospholipid sensitive, regulatory domain of the PKC isoform III with regard to its catalytic site and thus its mechanism of activation may differ from that of other PKC isotypes.

Arachidonic acid and related methyl ester mediate protein kinase C activation in intact platelets through the arachidonate metabolism pathways

Unlike unsaturated fatty acids, which almost fully activated purified brain protein kinase C in a phosphatidylserine- and Ca2(+)-free reaction, related methyl esters were poorly active in vitro. In contrast, methyl arachidonate was revealed to be as potent as arachidonic acid in activating protein kinase C in intact platelets. Arachidonic acid-mediated activation peaked at 20 s while methyl arachidonate-mediated activation plateaued at 2 min when both lipids were added at 50 microM. At concentrations higher than 0.3 mM, all tested unsaturated fatty acids and related methyl esters were weak activators of the enzyme, with the exception of linolenic acid and methyl linolenate which evoked strong enzyme activation. However, inhibitors of arachidonate metabolism blocked both arachidonic-acid and methyl-arachidonate-induced responses. At 5 microM arachidonic acid and methyl arachidonate, protein kinase C activation was due to a cyclooxygenase product(s) whereas at 50 microM the lipoxygenase pathway was mostly involved in the reaction. Therefore, arachidonic acid and its methyl ester activate protein kinase C in platelets mainly through action of their metabolites and eicosanoid synthesis. It is suggested that such indirect protein kinase C activation may account for the tumor-promoting activity of unsaturated fatty acids and related methyl esters.

Lipoxin A4 and lipoxin B4 stimulate the release but not the oxygenation of arachidonic acid in human neutrophils: dissociation between lipid remodeling and adhesion

The profiles of actions of lipoxin A4 (LXA4) and lipoxin B4 (LXB4), two lipoxygenase-derived eicosanoids, were examined with human neutrophils. At nanomolar concentrations, LXA4 and LXB4 each stimulated the release of [1-14C]arachidonic acid from esterified sources in neutrophils. Lipoxin-induced release of [1-14C]arachidonic acid was both dose- and time-dependent and was comparable to that induced by the chemotactic peptide f-met-leu-phe. Time-course studies revealed that lipoxin A4 and lipoxin B4 each induced a biphasic release of [1-14C]arachidonic acid, which was evident within seconds (5-15 sec) in its initial phase and minutes (greater than 30 sec) in the second phase. In contrast, the all-trans isomers of LXA4 and LXB4 did not provoke [1-14C]AA release. Lipoxin-induced release of arachidonic acid was inhibited by prior treatment of the cells with pertussis toxin but not by its beta-oligomers, suggesting the involvement of guaninine nucleotide-binding regulatory proteins in this event. Dual radiolabeling of neutrophil phospholipid classes with [1-14C]arachidonic acid and [3H]palmitic acid showed that phosphatidylcholine was a major source of lipoxin-induced release of [1-14C]arachidonic acid. They also demonstrated that lipoxins rapidly stimulate both formation of phosphatidic acid as well as phospholipid remodeling. Although both LXA4 and LXB4 (10(-8)-10(-6) M) stimulated the release of [1-14C]arachidonic acid, neither compound evoked its oxygenation by either the 5- or 15-lipoxygenase pathways (including the formation of LTB4, 20-COOH-LTB4, 5-HETE, or 15-HETE). LXA4 and LXB4 (10(-7) M) each stimulated the elevation of cytosolic Ca2+ as monitored with Fura 2-loaded cells, albeit to a lesser extent than equimolar concentrations of FMLP. Neither lipoxin altered the binding of [3H]LTB4 to its receptor on neutrophils. In addition, they did not stimulate aggregation or induce adhesion of neutrophils to human endothelial cells. Results indicate that both LXA4 and LXB4 stimulate the rapid remodeling of neutrophil phospholipids to release arachidonic acid without provoking either aggregation or the formation of lipoxygenase-derived products within a similar temporal and dose range. Together they indicate that LXA4 and LXB4 display selective actions with human neutrophils and suggest that these eicosanoids possess unique profiles of action which may regulate neutrophil function during inflammation.

Immunocytochemical localization of the alpha subspecies of protein kinase C in rat brain

The distribution of the alpha subspecies of protein kinase C (PKC) in rat brain was demonstrated immunocytochemically by using polyclonal antibodies raised against a synthetic oligopeptide corresponding to the carboxyl-terminal sequence of alpha-PKC. The alpha-PKC-specific immunoreactivity was widely but discretely distributed in both gray and white matter. The immunoreactivity was associated predominantly with neurons, particularly with perikaryon, dendrite, or axon, but little was seen in the nucleus. Glial cells expressed this PKC subspecies poorly, if at all. The highest density of immunoreactivity was seen in the olfactory bulb, septohippocampal nucleus, indusium griseum, islands of Calleja, intermediate part of the lateral septal nucleus, and Ammon's horn. A moderately high density of the immunoreactivity was seen in the anterior olfactory nucleus, anterior commissure, cingulate cortex, dentate gyrus, compact part of the substantia nigra, interpeduncular nucleus, inferior olive, and olivocerebellar tract. This distribution pattern was consistent with that obtained by in situ hybridization histochemistry. The distribution of alpha-PKC immunoreactivity was different from that of beta I-, beta II-, and gamma-PKC immunoreactivity. These findings suggest that alpha-PKC is involved heavily in the control of specific functions of some restricted neurons.

Lipoxin formation during human neutrophil-platelet interactions. Evidence for the transformation of leukotriene A4 by platelet 12-lipoxygenase in vitro

Human neutrophils from peripheral blood may physically interact with platelets in several settings including hemostasis, inflammation, and a variety of vascular disorders. A role for lipoxygenase (LO)-derived products has been implicated in each of these events; therefore, we investigated the formation of lipoxins during coincubation of human neutrophils and platelets. Simultaneous addition of FMLP and thrombin to coincubations of these cells led to formation of both lipoxin A4 and lipoxin B4, which were monitored by reversed-phase high pressure liquid chromatography. Neither stimulus nor cell type alone induced the formation of these products. When leukotriene A4 (LTA4), a candidate for the transmitting signal, was added to platelets, lipoxins were formed. In cell-free 100,000 g supernatants of platelet lysates, which displayed 12-LO activity, LTA4 was also transformed to lipoxins. Platelet formation of lipoxins was inhibited by the LO inhibitor esculetin and partially sensitive to chelation of Ca2+, while neither acetylsalicylic acid nor indomethacin significantly inhibited their generation. In contrast, neutrophils did not transform LTA4 to lipoxins. Cell-free 100,000 g supernatants of neutrophil lysates converted LTA4 to LTB4. These results indicate that neutrophil-platelet interactions can lead to the formation of lipoxins from endogenous sources and provide a role for platelet 12-LO in the formation of lipoxins from LTA4.

Evaluation of synthetic sphingosine, lysosphingolipids and glycosphingolipids as inhibitors of functional responses of human neutrophils

Human neutrophils, when exposed to soluble stimuli, aggregate, release oxygenated products of arachidonic acid and generate active oxygen species. Sphingolipid-derived products such as sphingosine and lysosphingolipids have been shown to exert selective actions on a variety of cell types, including neutrophils. Therefore, to determine the structural basis for selective inhibition of neutrophil responses by naturally occurring sphingolipids, seven compounds were prepared by total organic synthesis, and their impact on neutrophils in suspension has been studied. The compounds synthesized included sphingosine, psychosine, lactosyl lysosphingolipid, globotriaosyl (Gb3) lysosphingolipid, galactosyl cerebroside, lactosyl ceramide and Gb3 ceramide. The neutrophil responses studied were aggregation, leukotriene generation and superoxide anion production. When exposed to non-cytotoxic levels of the synthetic compounds, as monitored by exclusion of Trypan Blue, none of the synthetic sphingolipids inhibited A23187-induced aggregation of neutrophils. Only lactosyl lysosphingolipid, at a concentration of 1 microM, significantly inhibited aggregation induced by fMetLeuPhe; the other compounds in this series including sphingosine were without effect at equal molar concentrations (1 microM). Aggregation induced by phorbol 12-myristate 13-acetate (PMA) (0.1 microM) was significantly blocked by only two of the synthetic sphingolipids (1 microM). At concentrations below 1 microM, these inhibitory actions were not evident, nor was it possible to assign a structure-activity relationship for this series of compounds. None of the synthetic sphingolipids effectively inhibited the generation of superoxide anions induced by PMA. In addition, neither synthetic sphingosine nor psychosine affected either the formation or metabolism of leukotriene B4. Taken together, the results provide further evidence that sphingolipids, when added to intact cells, are not potent selective inhibitors of functional responses of human neutrophils.

Lipid activators of protein kinase C

Among the many reported lipid activators of protein kinase C only those of high affinity can be considered true physiological effectors, at present the tumor promoters, e.g., phorbol esters; 1,2-diacyl-sn-glycerols; and phosphatidylinositol 4,5-bisphosphate. Many other compounds (including arachidonic acid) are activators at high, unphysiological concentrations only, and they seem to be sterically unsuited for bonding to the enzyme. Such pseudo-activators possibly act by scrambling the structure of the regulatory moiety of the kinase.

Biology of the protein kinase C family

Protein kinase C (PKC) is composed of a family of isozymes that transduce signals of certain hormones, growth factors, lectins, and neurotransmitters. This review addresses the role of PKC in the regulation of cellular proliferation and its disorders. PKC is directly activated in vivo by the second messenger diacylglycerol, a lipid produced by phospholipase C-catalyzed hydrolysis of phosphatidylinositol and polyphosphoinositides. Diacylglycerol activates PKC by reducing the enzyme's requirement for Ca2+. Phorbol ester tumor promoters and related agents potently activate PKC by a mechanism analogous to that of diacylglycerol, providing evidence that PKC activation is a critical event in tumor promotion. However, the role of PKC activation in tumor promotion is not entirely clear. For example, bryostatin is a potent PKC activator that antagonizes phorbol ester-mediated tumor promotion, and mezerein is a second-stage tumor promoter that potently activates PKC. In addition to studies concerned with tumor promotion, studies of oncogene action also indicate a role for PKC in carcinogenesis. A number of plasma membrane-associated oncogene products and related proteins are PKC substrates, and PKC activation leads to induction of the expression of oncogenes that code for nuclear proteins. PKC is implicated in human breast and colon carcinogenesis. Tumor-promoting bile acids activate PKC, and PKC expression studies in rat colonic epithelial cells and human breast cancer cells indicate a positive role for PKC in the proliferation of the cells. Altered expression of PKC in human colon and breast tumors indicates that PKC isozymes may be useful markers for these diseases.

Transformation of leukotriene A4 to lipoxins by rat kidney mesangial cell

Incubation of rat mesangial cells with leukotriene A4 in the presence of calcium ionophore A23187 led to a substrate dependent formation of lipoxin and its isomers. The major metabolite coeluted with authentic lipoxin A4 (LXA4) and lipoxin B4 (LXB4) in RP-HPLC system, and possessed a characteristic U.V. spectrum and C-value which were identical to authentic standards. GC/MS analysis on LXA4 further demonstrates that the mesangial cell derived LXA4 was identical to that reported by Serhan et al. (1) as LXA4 [5(S), 6,(R), 15(S)-trihydroxy7,9,13-trans-11-cis-eicosatetraenoic acid]. The formation of LXA4 was linear with substrate (LTA4) concentration. No similar products occurred in boiled controls. Incubation of mesangial cell with 15-HPETE failed to produce any lipoxin-like material. The absence of LX-like substance following incubation of 15-HPETE with mesangial cells suggested that 5-lipoxygenase activity is not expressed in mesangial cells under these conditions. The generation of LXA4 from LTA4 in mesangial cells suggested that there is an active 15- or 12- lipoxygenase activity in the kidney. The production of LX may play an important role in the regulation of renal function and the response to inflammatory stimuli.

Two isozymes of PKC found in HL-60 cells show a difference in activation by the phorbol ester TPA

Cytosol from untreated cells and a detergent extract of the particulate fraction from TPA-treated HL-60 cells were analyzed for protein kinase C activity by consecutive column chromatography on Mono Q and hydroxyapatite. From both preparations two separate peaks of enzyme activity were obtained. The first peak, eluting at lower salt concentrations, is activated at lower TPA concentrations (3 X 10(-9) M) than the other (10(-7) M), which was eluted at higher salt concentrations.