Isozymes of protein kinase C in human neutrophils and their modification by two endogenous proteinases

Crystal-induced neutrophil activation: XI. Implication and novel roles of classical protein kinase C

Monosodium urate (MSU) crystals are among the most potent proinflammatory stimuli, and an innate immune inflammatory response to the crystal surface is involved in the pathology of gouty arthritis. Furthermore, MSU crystals have recently been identified as danger signals able to induce the maturation of dendritic cells. Release of the crystals into the joint cavity promotes an acute inflammation characterized by a massive infiltration of neutrophils that leads to tissue damage. Protein kinase C (PKC) represents a family of serine/threonine kinases that play central signaling roles in multiple cellular responses. This family of kinases is divided into three subfamilies based on second messenger requirements: conventional (or classical), novel, and atypical. Despite their role in signal transduction, very little is known about the involvement of the PKC family in the inflammatory reaction induced by MSU crystals. In the present study, we show that MSU crystals activate conventional PKC isoforms, and that this activation is necessary for the MSU crystal-induced degranulation and generation of a chemotactic activity in the supernatants of MSU crystal-stimulated human neutrophils. Evidence is also obtained that the tyrosine kinase Syk is a substrate of PKC and that the PKC-mediated serine phosphorylation of Syk is necessary to its interaction with the regulatory subunit of PI3K kinases (p85) and thus to the subsequent activation of these lipid kinases. These results suggest novel means of modulating neutrophil responses (through the specific regulation of PKC) during the acute phase of MSU crystal-induced inflammation.

The atypical protein kinase C in Aplysia can form a protein kinase M by cleavage

In vertebrates, a brain-specific transcript from the atypical protein kinase C (PKC) zeta gene encodes protein kinase M (PKM) zeta, a constitutively active kinase implicated in the maintenance of synaptic plasticity and memory. We have cloned the atypical PKC from Aplysia, PKC Apl III. We did not find a transcript in Aplysia encoding PKMzeta, and evolutionary analysis of atypical PKCs suggests formation of this transcript is restricted to vertebrates. Instead, over-expression of PKC Apl III in Aplysia sensory neurons leads to production of a PKM fragment of PKC Apl III. This cleavage was induced by calcium and blocked by calpain inhibitors. Moreover, nervous system enriched spliced forms of PKC Apl III show enhanced cleavage. PKC Apl III could also be activated through phosphorylation downstream of phosphoinositide 3-kinase. We suggest that PKM forms of atypical PKCs play a conserved role in memory formation, but the mechanism of formation of these kinases has changed over evolution.

Selective and transient activation of protein kinase C alpha by fumonisin B1, a ceramide synthase inhibitor mycotoxin, in cultured porcine renal cells

Fumonisin B(1) (FB(1)), a potent and naturally occurring mycotoxin produced by the fungus Fusarium verticillioides, has been implicated in fatal and debilitating diseases in animals and humans. FB(1) affects a variety of cell signaling proteins including protein kinase C (PKC); a serine/threonine kinase, involved in a number of signal transduction pathways that include cytokine induction, carcinogenesis and apoptosis. The aim of this study was to investigate the short-term temporal and concentration-dependent effects of FB(1) on PKC isoforms present in LLC-PK(1) cells in relation to the FB(1)-induced accumulation of sphinganine and sphingosine utilizing various inhibitors and activators. Our studies demonstrated that FB(1) (0.1-1 microM) selectively and transiently activated PKCalpha at 5 min, without affecting PKC-delta, -epsilon and -zeta isoforms. At higher FB(1) concentrations and later time points (15-120 min), PKCalpha membrane concentrations declined to untreated levels. The observed increase in cytosol PKCalpha protein expression at 15 min was not associated with an increase in its activity or protein biosynthesis. Calphostin C, a PKC inhibitor, abrogated the FB(1)-induced translocation of PKCalpha. Pre-incubation with the PKC activator, phorbol 12-myristate 13-acetate, resulted in an additive effect on membrane translocation of PKCalpha. Intracellular sphinganine and sphingosine concentrations were unaltered at the time points tested. Myriocin, a specific inhibitor of serine palmitoyltransferase, the first enzyme in de novo sphingolipid biosynthesis, did not prevent the FB(1)-induced PKCalpha cytosol to membrane redistribution. Altering PKCalpha and its signal transduction pathways may be of importance in the ability of FB(1) to exert its toxicity via apoptosis and/or carcinogenesis.

Fumonisin B1-induced apoptosis is associated with delayed inhibition of protein kinase C, nuclear factor-kappaB and tumor necrosis factor alpha in LLC-PK1 cells

Fumonisin B1 (FB1), the most potent of the fumonisin mycotoxins, is a carcinogen and causes a wide range of species-specific toxicoses. FB1 modulates the activity of protein kinase C (PKC), a family of phospholipid-dependent serine/threonine kinases that play important role in modulating a variety of biologic responses ranging from regulation of cell growth to cell death. Although it has been demonstrated that FB1 induces apoptosis in many cell lines, the precise mechanism of apoptosis is not fully understood. In this study, we investigated the membrane localization of various PKC isoforms, PKC enzyme activity, and its downstream targets, namely nuclear factor-kappa B (NF-kappaB), tumor necrosis factor alpha (TNFalpha), and caspase 3, in porcine renal epithelial (LLC-PK1) cells. FB1 repressed cytosol to membrane translocation of PKC-alpha, -delta, -epsilon, and -zeta isoforms over 24-72 h. The FB1-induced membrane PKC repression was corroborated by a concentration-dependent decrease in total PKC activity. Exposure of cells to phorbol 12-myristate 13-acetate (PMA) for this duration also resulted in repressed PKC membrane localization and activity comparable to FB1. Exposure of cells to FB1 (10 microM) was associated with inhibition of cytosol to nuclear translocation of NF-kappaB and NF-kappaB-DNA binding at 72 h. The expression of TNFalpha was significantly inhibited at 24 and 48 h in response to 1 and 10 microM FB1. Increased caspase 3 activity was observed in LLC-PK1 cells exposed to > or =1 microM FB1 at 48 h. PMA also increased the caspase 3 activity at 24 and 48 h. Results suggest that FB1-induced apoptosis involves the activation of caspase 3, which is associated with the repression of PKC and possibly its down-stream effectors, NF-kappaB and TNFalpha.

Inhibition of neutrophil O(2)(-) production by unsymmetrical methylene derivatives of benzopyrans: their use as potential antiinflammatory agents

Some unsymmetrical derivatives of benzopyrans 9 were synthesized and tested to verify their PKC inhibitory activity. For this purpose, the Mannich bases of 7-hydroxycoumarins 6 were treated with 2-(dialkylamino)benzopyran-4-ones or 3-(dialkylamino)naphtho[2,1-b]pyran-1-ones 8 in the presence of acetic or propionic anhydride, yielding compounds 9. Human neutrophils stimulated with either PMA and f-MLF were used as the cellular model. The efficiency of the compounds 9 was established on their capacity to reduce the O(2)(-) production by activated human neutrophils. Compounds 9d and 9f, bearing an acetoxy group in position 7 of the chromone moiety, seem to counteract the neutrophil activation efficiently.

Transmembrane-4 superfamily proteins associate with activated protein kinase C (PKC) and link PKC to specific beta(1) integrins

Translocation of conventional protein kinases C (PKCs) to the plasma membrane leads to their specific association with transmembrane-4 superfamily (TM4SF; tetraspanin) proteins (CD9, CD53, CD81, CD82, and CD151), as demonstrated by reciprocal co-immunoprecipitation and covalent cross-linking experiments. Although formation and maintenance of TM4SF-PKC complexes are not dependent on integrins, TM4SF proteins can act as linker molecules, recruiting PKC into proximity with specific integrins. Previous studies showed that the extracellular large loop of TM4SF proteins determines integrin associations. In contrast, specificity for PKC association probably resides within cytoplasmic tails or the first two transmembrane domains of TM4SF proteins, as seen from studies with chimeric CD9 molecules. Consistent with a TM4SF linker function, only those integrins (alpha(3)beta(1), alpha(6)beta(1), and a chimeric "X3TC5" alpha(3) mutant) that associated strongly with tetraspanins were found in association with PKC. We propose that PKC-TM4SF-integrin structures represent a novel type of signaling complex. The simultaneous binding of TM4SF proteins to the extracellular domains of the integrin alpha(3) subunit and to intracellular PKC helps to explain why the integrin alpha3 extracellular domain is needed for both intracellular PKC recruitment and PKC-dependent phosphorylation of the alpha(3) integrin cytoplasmic tail.

Amyloid precursor protein and amyloid beta peptide in human platelets. Role of cyclooxygenase and protein kinase C

The main component of Alzheimer's disease (AD) senile plaques is amyloid-beta peptide (Abeta), a proteolytic fragment of the amyloid precursor protein (APP). Platelets contain both APP and Abeta and may contribute to the perivascular amyloid deposition seen in AD. However, no data are available concerning the biochemical mechanism(s) involved in their formation and release by these cells. We found that human platelets released APP and Abeta following activation with collagen or arachidonic acid. Inhibition of platelet cyclooxygenase (COX) reduced APP but not Abeta release following those stimuli. In contrast, activation of platelets by thrombin and calcium ionophore caused release of both APP and Abeta in a COX-independent fashion. Ex vivo studies showed that, despite suppression of COX activity, administration of aspirin did not modify Abeta or APP levels in serum or plasma, suggesting that this enzyme plays only a minor role in vivo. We examined the regulation of APP cleavage and release from activated platelets and found that cleavage requires protein kinase C (PKC) activity and is regulated by the intracellular second messengers phosphatidylinositol 2-phosphate and Ca(2+). Our data provide the first evidence that in human platelets COX is a minor component of APP secretion whereas PKC plays a major role in the secretory cleavage of APP. By contrast, Abeta release may represent secretion of preformed peptide and is totally independent of both COX and PKC activity.

Protein kinase C zeta phosphorylates a subset of selective sites of the NADPH oxidase component p47phox and participates in formyl peptide-mediated neutrophil respiratory burst

Generation of superoxide anion by the multiprotein complex NADPH phagocyte oxidase is accompanied by extensive phosphorylation of its 47-kDa protein component, p47(phox), a major cytosolic component of this oxidase. Protein kinase C zeta (PKC zeta), an atypical PKC isoform expressed abundantly in human polymorphonuclear leukocytes (PMN), translocates to the PMN plasma membrane upon stimulation by the chemoattractant fMLP. We investigated the role of PKC zeta in p47(phox) phosphorylation and in superoxide anion production by human PMN. In vitro incubation of recombinant p47(phox) with recombinant PKC zeta induced a time- and concentration-dependent phosphorylation of p47(phox) with an apparent K(m) value of 2 microM. Phosphopeptide mapping analysis of p47(phox) showed that PKC zeta phosphorylated fewer selective sites in comparison to "conventional" PKCs. Serine 303/304 and serine 315 were identified as targets of PKC zeta by site-directed mutagenesis. Stimulation of PMN by fMLP induced a rapid and sustained plasma membrane translocation of PKC zeta that correlated to that of p47(phox). A cell-permeant-specific peptide antagonist of PKC zeta inhibited both fMLP-induced phosphorylation of p47(phox) and its membrane translocation. The antagonist also inhibited the fMLP-induced production of oxidant (IC(50) of 10 microM), but not that induced by PMA. The inhibition of PKC zeta expression in HL-60 neutrophil-like cells using antisense oligonucleotides (5 and 10 microM) inhibited fMLP-promoted oxidant production (27 and 50%, respectively), but not that induced by PMA. In conclusion, p47(phox) is a substrate for PKC zeta and participates in the signaling cascade between fMLP receptors and NADPH oxidase activation.

Activation of PKC is sufficient to induce an apoptotic program in salivary gland acinar cells

Accumulating evidence suggests that specific isoforms of PKC may function to promote apoptosis. We show here that activation of the conventional and novel isoforms of PKC with 12-O-tetradecanoyl phorbol-13- ester (TPA) induces apoptosis in salivary acinar cells as indicated by DNA fragmentation and activation of caspase-3. TPA-induced DNA fragmentation, caspase-3 activation, and morphologic indicators of apoptosis, can be enhanced by pretreatment of cells with the calpain inhibitor, calpeptin, prior to the addition of TPA. Analysis of PKC isoform expression by immunoblot shows that TPA-induced downregulation of PKC alpha and PKC delta is delayed in cells pre-treated with calpeptin, and that this correlates with an increase of these isoforms in the membrane fraction of cells. TPA-induced apoptosis is accompanied by biphasic activation of the c-jun-N-terminal kinase (JNK) pathway and inactivation of the extracellular regulated kinase (ERK) pathway. Expression of constitutively activated PKC alpha or PKC delta, but not kinase negative mutants of these isoforms, or constitutively activated PKC epsilon, induces apoptosis in salivary acinar cells, suggesting a role for these isoforms in TPA-induced apoptosis. These studies demonstrate that activation of PKC is sufficient for initiation of an apoptotic program in salivary acinar cells. Cell Death and Differentiation (2000) 7, 1200 - 1209.

Mechanisms involved in the augmentation of arachidonic acid-induced free-radical generation from rat neutrophils following hypoxia-reoxygenation

Polymorphonuclear leukocytes are known to play an important role in hypoxia/ischemia and reoxygenation injury. The present study was undertaken to investigate the involvement of protein kinase C, calmodulin, and cyclic adenosine monophosphate in the augmentation of the free-radical generation observed after hypoxia-reoxygenation (H-R). Free-radical generation from the rat polymorphonuclear leukocytes was measured as the arachidonic acid (1-5x10(-5) M)-induced luminol-dependent chemiluminescence response, which was augmented following H-R. The increase in free-radical generation after H-R was completely blocked by the pretreatment of cells with PKC inhibitor H(7), whereas indomethacin (a cyclo-oxygenase inhibitor) or forskolin (an adenylate cyclase activator) failed to modulate the H-R-dependent response. However, W(7)-a calcium/calmodulin (Ca(2+)/CaM) antagonist-partially reduced the augmented free-radical generation observed in the H-R cells. Results obtained thus suggest the possible involvement of protein kinase C and calcium in the augmentation of the free-radical generation response following H-R.

Ethanol increases superoxide anion production stimulated with 4beta-phorbol 12-myristate 13-acetate in human polymorphonuclear leukocytes. Involvement of protein kinase C

Stimulation of human polymorphonuclear leukocytes (PMNs) with PMA initiates a cascade of events leading to the production and release of superoxide anion (O-2), a major component in anti-bacterial defense. Generation of O-2 by PMA-stimulated PMNs occurs through the translocation and activation of protein kinase C (PKC). In this study, using freshly isolated PMNs, we examined the effect of ethanol on this response to PMA. Our results show that the basal production of O-2 was not affected by ethanol. In contrast, the response induced by PMA was potentiated by ethanol. This potentiation was observed even at high doses of PMA (200 nM) which alone had stimulated the O-2 response maximally. This enhanced response was not due to an increase of PMA uptake by PMNs. The maximal effect was obtained when the cells were preincubated with 80 mM of ethanol before PMA stimulation. Measurement of PKC activity in the cytosolic and membrane fractions showed that pretreatment of PMNs with ethanol increased twofold the PMA-stimulated PKC activity in the membrane fraction. Furthermore, Western blot analysis verified that this increase in PKC activity in the membrane fraction was linked to an increase in the translocation of PKC-alpha and -beta isoforms to the membrane. These results suggest that ethanol potentiates PMA-induced O-2 production through increasing PKC translocation and activity in PMNs.

Acyl-CoA-binding protein is a potent m-calpain activator

Acyl-CoA-binding protein, a 20-kDa homodimer that exerts many physiological functions, promotes activation of the classic calpain forms, most markedly that of the m-isozyme. This protein factor was purified from rat skeletal muscle and was also expressed in Escherichia coli. Both native and recombinant acyl-CoA-binding proteins show the same molecular properties and an identical capacity to decrease the [Ca(2+)] required for m-calpain activity. The binding of long-chain acyl-CoAs to acyl-CoA-binding protein does not modify the activating effect on calpains. Acyl-CoA-binding protein seems to be involved in the m-calpain regulation process, whereas the previously identified UK114 activator is a specific modulator of micro-calpain. Acyl-CoA-binding protein is proposed as a new component of the Ca(2+)-dependent proteolytic system. A comparative analysis among levels of classic calpains and their activator proteins is also reported.

Inhibition of calpain blocks platelet secretion, aggregation, and spreading

Previous studies have indicated that the Ca(2+)-dependent protease, calpain, is activated in platelets within 30-60 s of thrombin stimulation, but specific roles of calpain in platelets remain to be identified. To directly test the functions of calpain during platelet activation, a novel strategy was developed for introducing calpain's specific biological inhibitor, calpastatin, into platelets prior to activation. This method involves treatment of platelets with a fusion peptide, calpastat, consisting of the cell-penetrating signal sequence from Kaposi's fibroblast growth factor connected to a calpain-inhibiting consensus sequence derived from calpastatin. Calpastat specifically inhibits thrombin peptide (SFLLR)-induced alpha-granule secretion (IC(50) = 20 microM) during the first 30 s of activation, thrombin-induced platelet aggregation (IC(50) = 50 microM), and platelet spreading on glass surfaces (IC(50) = 34 microM). Calpastat-Ala, a mutant peptide in which alanine is substituted at conserved calpastatin residues, lacks calpain inhibitory activity and fails to inhibit secretion, aggregation, or spreading. The peptidyl calpain inhibitors calpeptin, MDL 28,170 (MDL) and E64d also inhibit secretion, aggregation and spreading, but require 3-10-fold higher concentrations than calpastat for biological activity. Together, these findings demonstrate that calpain regulates platelet secretion, aggregation, and spreading and indicate that calpain plays an earlier role in platelet activation following thrombin receptor stimulation than had been previously detected.

Degradation of protein kinase Malpha by mu-calpain in a mu-calpain-protein kinase Calpha complex

In previous studies, we isolated and identified a mu-calpain-PKCalpha complex from rabbit skeletal muscle. At the same time we pointed out that an association between mu-calpain and PKCalpha could occur at the level of the plasma membrane of muscle cells, and that PKCalpha could thus be considered as a potential mu-calpain substrate. In the present study, using the mu-calpain-PKCalpha complex as a model, we report that mu-calpain is activated in the combined presence of physiological calcium concentrations (less than 1 microM) and phosphatidylserine. Furthermore our data also show that: (1) there exists a correlation between the appearance of autolyzed mu-calpain forms and PKCalpha hydrolysis which leads to the formation of PKMalpha; (2) in certain experimental conditions, autolyzed mu-calpain forms are able to hydrolyze PKMalpha independently of the presence of diacylglycerol.

Protein kinase calpha regulates human monocyte O-2 production and low density lipoprotein lipid oxidation

Our previous studies have shown that human native low density lipoprotein (LDL) can be oxidized by activated human monocytes. In this process, both activation of protein kinase C (PKC) and induction of superoxide anion (O-2) production are required. PKC is a family of isoenzymes, and the functional roles of individual PKC isoenzymes are believed to differ based on subcellular location and distinct responses to regulatory signals. We have shown that the PKC isoenzyme that is required for both monocyte O-2 production and oxidation of LDL is a member of the conventional PKC group of PKC isoenzymes (Li, Q., and Cathcart, M. K. (1994) J. Biol. Chem. 269, 17508-17515). The conventional PKC group includes PKCalpha, PKCbetaI, PKCbetaII, and PKCgamma. With the exception of PKCgamma, each of these isoenzymes was detected in human monocytes. In these studies, we investigated the requirement for select PKC isoenzymes in the process of monocyte-mediated LDL lipid oxidation. Our data indicate that PKC activity was rapidly induced upon monocyte activation with the majority of the activity residing in the membrane/particulate fraction. This enhanced PKC activity was sustained for up to 24 h after activation. PKCalpha, PKCbetaI, and PKCbetaII protein levels were induced upon monocyte activation, and PKCalpha and PKCbetaII substantially shifted their location from the cytosol to the particulate/membrane fraction. To distinguish between these isoenzymes for regulating monocyte O-2 production and LDL oxidation, PKCalpha or PKCbeta isoenzyme-specific antisense oligonucleotides were used to selectively suppress isoenzyme expression. We found that suppression of PKCalpha expression inhibited both monocyte-mediated O-2 production and LDL lipid oxidation by activated human monocytes. In contrast, inhibition of PKCbeta expression (including both PKCbetaI and PKCbetaII) did not affect O-2 production or LDL lipid oxidation. Further studies demonstrated that the respiratory burst oxidase responsible for O-2 production remained functionally intact in monocytes with depressed levels of PKCalpha because O-2 production could be restored by treating the monocytes with arachidonic acid. Taken together, our data reveal that PKCalpha, and not PKCbetaI or PKCbetaII, is the predominant isoenzyme required for O-2 production and maximal oxidation of LDL by activated human monocytes.

Protein kinase C isoforms in normal and chronic myeloid leukemic neutrophils. Distinct signal for PKC alpha by immunodetection on PVDF membrane, decreased expression of PKC alpha and increased expression of PKC delta in leukemic neutrophils

Neutrophils from patients with Chronic Myeloid Leukemia (CML) exhibit defects in several functions. They also show altered phosphorylation-dephosphorylation patterns of several proteins on stimulation with phorbol myristate acetate--a direct activator of protein kinase C (PKC). Since PKC mediates several functions of the neutrophil, in this study we investigate the PKC isoform profile and subcellular distribution in normal and CML neutrophils in an attempt to elucidate their role in CML signalling. Our results show the presence of PKC alpha, betaI, betaII and delta in both the cell types. A distinct and clear signal was obtained for PKC alpha, the isoform reported to be absent or present in very low amounts in normal neutrophils. In addition, PKC alpha was present in significantly lower levels in CML neutrophils while the PKC delta isoform was found in significantly higher amounts in the CML cytosol as compared to that in normal cells. PKC alpha, betaI, betaII and delta isoforms could not be detected in the nucleus of unstimulated normal and CML neutrophils. The altered levels of PKC alpha and delta may be one of the causes for the defects in function exhibited by the leukemic cells.

Signaling pathways involved in dephosphorylation and localization of the actin-binding protein cofilin in stimulated human neutrophils

We have studied activation-induced dephosphorylation of proteins in human neutrophils loaded with [32P]orthophosphate using two-dimensional gel electrophoresis and autoradiography. A major phosphoprotein of 20 kDa in resting neutrophils was markedly dephosphorylated upon activation of cells with chemotactic peptide or phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC). Using a monoclonal anti-cofilin antibody, this phosphoprotein could be shown to be identical with cofilin, a protein implicated in actin filament remodeling. Signaling pathways leading to this dephosphorylation were further characterized. To define the role of PKC isoforms in cofilin dephosphorylation, we used different PKC inhibitors. Gö 6976 (10 microM), which inhibits preferentially PKC alpha and beta, did not prevent PMA-induced dephosphorylation of cofilin, whereas Ro 31-8220 and CGP 41,251 (10 microM), which act also on Ca(2+)-independent PKC isoforms, almost completely suppressed this event. The lack of effect of Gö 6976 was not due to insufficient entry into the cells, as this drug suppressed PMA-induced increases in protein phosphorylation. Ca(2+)-independent PKC isoforms, rather than PKC alpha or beta, may thus be involved in PMA-induced cofilin dephosphorylation. In contrast, Ro 31-8220 did not inhibit chemotactic peptide-induced cofilin dephosphorylation, suggesting here a PKC-independent pathway. The phosphatase inhibitor okadaic acid (1-2 microM) attenuated phosphorylation of cofilin in resting cells. This reduced level was not further attenuated by PMA. Phosphatases 1 and/or 2A may thus control cofilin phosphorylation in resting cells and contribute to PMA-induced cofilin dephosphorylation. Dephosphorylation of cofilin induced by PMA, chemotactic peptide, or okadaic acid was always accompanied by a shift of cofilin to the cell periphery into F-actin-rich areas. These findings suggest a role of cofilin in stimulus-dependent actin remodeling in motile neutrophils.

The N-formyl peptide receptor: a model for the study of chemoattractant receptor structure and function

N-formyl peptides, such as fMet-Leu-Phe, are one of the most potent chemoattractants for phagocytic leukocytes. The interaction of N-formyl peptides with their specific cell surface receptors has been studied extensively and used as a model system for the characterization of G-protein-coupled signal transduction in phagocytes. The cloning of the N-formyl peptide receptor cDNA from several species and the identification of homologous genes have allowed detailed studies of structural and functional aspects of the receptor. Recent findings that the receptor is expressed in nonhematopoietic cells and that nonformylated peptides can activate the receptor suggest potentially novel functions and the existence of additional ligands for this receptor.

Evidence for the involvement of protein kinase C inhibition by norathyriol in the reduction of phorbol ester-induced neutrophil superoxide anion generation and aggregation

Norathyriol, a xanthone aglycone, inhibited superoxide anion (O2-) generation and O2 consumption in phorbol 12-myristate 13-acetate (PMA)-activated rat neutrophils in a concentration-dependent manner. In addition, norathyriol inhibited PMA- but enhanced formylmethionyl-leucyl-phenylalanine (fMLP)-induced neutrophil aggregation. Norathyriol suppressed neutrophil cytosolic protein kinase C as well as rat brain protein kinase C over the same range of concentrations at which it inhibited the respiratory burst. Norathyriol did not affect [3H]phorbol 12,13-dibutyrate ([3H]PDB) binding to neutrophil cytosolic protein kinase C, but effectively attenuated trypsin-treated rat brain protein kinase C activity. Moreover, norathyriol was found to be a noncompetitive inhibitor with respect to ATP and peptide substrate (N-terminal acetylated, amino acid sequence 4-14 of the myelin basic protein, Ac-MBP-(4-14)). Unlike staurosporine, norathyriol did not affect porcine heart protein kinase A activity. On the immunoblot analysis of protein kinase C subcellular distribution, the PMA-induced translocation of protein kinase C-beta from the cytosol to the membrane was not affected by norathyriol. These results show that the inhibition by a plant product, norathyriol, of PMA-induced respiratory burst and aggregation is, at least partly, attributed to the direct suppression of protein kinase C activity through blockade of the catalytic region, but is not due to interference with the membrane translocation of protein kinase C during PMA-induced cell activation.

Phosphatidic acid-mediated phosphorylation of the NADPH oxidase component p47-phox. Evidence that phosphatidic acid may activate a novel protein kinase

Phosphatidic acid (PA), generated by phospholipase D activation, has been linked to the activation of the neutrophil respiratory burst enzyme, NADPH oxidase; however, the intracellular enzyme targets for PA remain unclear. We have recently shown (McPhail, L. C., Qualliotine-Mann, D., and Waite, K. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7931-7935) that a PA-activated protein kinase is involved in the activation of NADPH oxidase in a cell-free system. This protein kinase phosphorylates numerous endogenous proteins, including p47-phox, a component of the NADPH oxidase complex. Phospholipids other than PA were less effective at inducing endogenous protein phosphorylation. Several of these endogenous substrates were also phosphorylated during stimulation of intact cells by opsonized zymosan, an agonist that induces phospholipase D activation. We sought to identify the PA-activated protein kinase that phosphorylates p47-phox. The PA-dependent protein kinase was shown to be cytosolic. cis-Unsaturated fatty acids were poor inducers of protein kinase activity, suggesting that the PA-activated protein kinase is not a fatty acid-regulated protein kinase (e.g. protein kinase N). Chromatographic techniques separated the PA-activated protein kinase from a number of other protein kinases known to be activated by PA or to phosphorylate p47-phox. These included isoforms of protein kinase C, p21 (Cdc42/Rac)-activated protein kinase, and mitogen-activated protein kinase. Gel filtration chromatography indicated that the protein kinase has an apparent molecular size of 125 kDa. Screening of cytosolic fractions from several cell types and rat brain suggested the enzyme has widespread cell and tissue distribution. The partially purified protein kinase was sensitive to the same protein kinase inhibitors that diminished NADPH oxidase activation and was independent of guanosine 5'-3-O-(thio)triphosphate and Ca2+. Phosphoamino acid analysis showed that serine and tyrosine residues were phosphorylated on p47-phox by this kinase(s). These data indicate that one or more potentially novel protein kinases are targets for PA in neutrophils and other cell types. Furthermore, a PA-activated protein kinase is likely to be an important regulator of the neutrophil respiratory burst by phosphorylation of the NADPH oxidase component p47-phox.

Translocation of protein kinase C isoforms in rat neutrophils

In this study, we examined the protein kinase C (PKC) isoforms present in cytosol and membrane fractions of rat neutrophils by Western blotting analysis with monoclonal antibodies against PKC isoforms and demonstrated that rat neutrophils express at least three conventional PKCs (cPKC), alpha, beta and gamma, four novel PKCs (nPKC), delta, epsilon, theta and mu, and three atypical PKCs (aPKC), iota, lambda and zeta, although PKC lambda and zeta were barely detected. Cells stimulated with phorbol 12-myristate 13-acetate (PMA) induce a sustained and marked translocation of cPKC and nPKC from the cytosol to particulate fraction. A concentration-dependence of PMA on the membrane translocation of PKC isoforms was observed. Treatment with formyl-Met-Leu-Phe (fMLP), in contrast with PMA, caused a transient and less prominent association of cPKC and nPKC with particulate fraction. However, the distribution of PKC iota isoform was affected neither by fMLP nor by PMA. These data indicate that the rat neutrophils contain PKCs of three isoform families and the membrane translocation of cPKC and nPKC was observed in cells in response to PMA as well as to fMLP.

Functional characterization of equine neutrophils in response to calcium ionophore A23187 and phorbol myristate acetate ex vivo

Equine neutrophils (PMN) play a critical role in inflammatory processes in horses. The objective of this study was to characterize equine PMN function ex vivo following stimulation with calcium ionophore A23187 (A23187) and phorbol myristate acetate (PMA). These stimulants trigger different branches of the PMN activation process that occurs in vivo. Equine PMN were isolated from the whole blood of six clinically normal geldings using a one-step discontinuous Percoll gradient technique. Neutrophil aggregation, degranulation, and superoxide anion production were evaluated in assay systems which had previously been established to quantitate PMN function. Dose-response curves for A23187 and PMA were derived for the three functions. Results indicate that equine PMN aggregation and superoxide anion production are more responsive to activation by PMA as the maximum change in percent transmittance and maximum nanomoles of superoxide anion produced following PMA stimulation (60.8% and 10.4 nmol per 10(6) cells, respectively) were greater than those values stimulated by A23187 (41.5% and 5.2 nmol per 10(6) cells, respectively). However, degranulation was found to be more responsive to A23187 stimulation (maximum percent degranulation: 56.1%) than to PMA stimulation (maximum percent degranulation: 30.7%). Dose-response curves following A23187 and PMA stimulation revealed that superoxide anion production had the lowest threshold concentration among the three functions. Degranulation had the highest threshold concentration among the three functions for both stimulants. Results indicate that equine PMN functions differ in their dependence on second messengers in the activation pathway. These functions also occur in a dose-dependent manner and differ in the threshold concentrations required for their stimulation.

Investigation of the inhibitory effect of broussochalcone A on respiratory burst in neutrophils

Broussochalcone A, a prenylated chalcone isolated from Broussonetia papyrifera (L.) VENT. (Moraceae), inhibited O2 consumption in formylmethionyl-leucyl-phenylalanine (fMLP)- and phorbol 12-myristate 13-acetate (PMA)-stimulated rat neutrophils in a concentration-dependent manner with IC50 values of 70.3 +/- 4.9 and 63.9 +/- 7.1 microM, respectively. Broussochalcone A did not affect the fMLP-induced increase of cellular inositol trisphosphate (IP3) and [Ca2+]i. However, the enzyme activity of neutrophil cytosolic protein kinase C was effectively suppressed by broussochalcone A. Broussochalcone A had no effect on either [3H]phorbol 12,13-dibutyrate ([3H]PDB) binding to neutrophil cytosolic protein kinase C or on PMA-induced membrane translocation of protein kinase C-beta in neutrophils. Broussochalcone A suppressed the enzyme activity of trypsin-treated rat brain protein kinase C in a concentration-dependent manner. In PMA-activated neutrophil particulate NADPH oxidase, broussochalcone A attenuated superoxide anion radical (O2.-) generation with an IC50 value of 61.8 +/- 5.4 microM. These results show that the inhibitory effect of broussochalcone A on respiratory burst in neutrophils is not mediated by the reduction of phospholipase C activity, but is mediated partly by the suppression of protein kinase C activity through interference with the catalytic region and by the attenuation of O2.- generation from the NADPH oxidase complex.

Chemotactic peptide N-formyl-met-leu-phe activation of p38 mitogen-activated protein kinase (MAPK) and MAPK-activated protein kinase-2 in human neutrophils

Activation of polymorphonuclear leukocytes (PMN) by chemotactic peptides initiates a series of functional responses that serve to eliminate pathogens. The intermediate steps that link engagement of the chemoattractant receptor to the microbicidal responses involve protein kinases that have yet to be identified. In this study we detected in human PMN the presence of p38 mitogen-activated protein kinase (MAPK), which became rapidly tyrosine phosphorylated and activated in response to the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP). Pretreatment of PMN with wortmannin, a phosphatidylinositol 3-kinase inhibitor, or bis-indolylmaleimide, a protein kinase C antagonist, resulted in partial inhibition of p38 phosphorylation upon fMLP stimulation. Similarly, phosphorylation of p38 was only partially inhibited when the fMLP-induced cytosolic calcium transient was prevented. Stimulation of PMN by the chemoattractant also resulted in the rapid phosphorylation and activation of MAPK-activated protein kinase-2 (MAPKAPK-2), which was completely inhibited by the specific p38 inhibitor, SB203580. The physical interaction of p38 with MAPKAPK-2 was studied by coimmunoprecipitation. These two kinases were found to be associated in unstimulated PMN but dissociated upon activation of the cells by fMLP. Together these findings demonstrate the activation of p38 by chemotactic peptides in human PMN by a process involving phosphatidylinositol 3-kinase, protein kinase C, and calcium. p38, in turn, is an upstream activator of MAPKAPK-2.

PKC--a pivotal regulator of early development

Oocytes, eggs and blastomeres of the embryo are special cells that undergo rapid changes in structure and function at developmental transitions. These changes are frequently regulated by cytoplasmic signaling events, particularly at the developmental transition of fertilization, because the genome is largely inactivated at this time. Protein kinase C (PKC) is a signaling agent that acts after the sperm-induced rise in calcium and has a central role in the remodeling of the structure of the egg into the zygote in many species. PKC also acts during other developmental transitions. This kinase serves as a chronometer, which can choreograph the cell's remodeling events in both space and time. Several technical advancements discussed in this review have permitted a better understanding of the actions of PKC.

Inhibition by HAJ11 of respiratory burst in neutrophils and the involvement of protein tyrosine phosphorylation and phospholipase D activation

1. The possible mechanisms of the inhibitory effect of ethyl 2-(3-hydroxyanilino)-4-oxo-4,5-dihydrofuran-3-carboxylate (HAJ11) on the respiratory burst of rat neutrophils in vitro was investigated. 2. HAJ11 caused a reversible and a concentration-dependent inhibition of formyl-Met-Leu-Phe (fMLP)-induced superoxide anion (O2.-) generation (IC50 4.9 +/- 0.7 microM) and O2 consumption (IC50 4.9 +/- 1.5 microM). Concanavalin A (Con A)- and NaF-induced O2.- generation were also suppressed by HAJ11. However, HAL11 was a weak inhibitor of the phorbol 12-myristate 13-acetate (PMA)-induced responses. 3. HAJ11 did not scavenge the /2.- generation in the xanthine-xanthine oxidase system and dihydroxyfumaric acid (DHF) autoxidation. 4. HAJ11 showed no activity on fMLP-induced inositol phosphates formation and [Ca2+]i elevation in intact neutrophils. In addition, HAJ11 had no effect on neutrophil cytosolic phospholipase C (PLC) activity. 5. HAJ11 reduced fMLP-induced phosphatidic acid (PA) (IC50 29.1 +/- 6.5 microM) and phosphatidylethanol (PE+) (IC50 22.6 +/- 1.9 microM) formation in a concentration-dependent manner. HAJ11 also reduced protein tyrosine phosphorylation in neutrophils stimulated by fMLP. 6. HAJ11 was a weak inhibitor of neutrophil cytosolic protein kinase C (PKC) activity, and had a negligible effect on brain PKC. Cellular cyclic nucleotides levels were not altered by HAJ11. In addition, HAJ11 did not affect protein kinase A (PKA) activity. 7. HAJ11 had not effect on the O2.- generation of PMA-activated and arachidonic acid (AA)-activated NADPH oxidase preparations. 8. Taken together these results indicate that the inhibition of respiratory burst by HAJ11 probably mainly occurs through inhibition of protein tyrosine phosphorylation and phospholipase D (PLD) activity.

Biochemistry and cell biology of phospholipase D in human neutrophils

Neutrophils play a major role host defense against invading microbes. Recent studies have emphasized the importance of the phospholipase D (PLD) in the signalling cascade leading to neutrophil activation. Phospholipase D catalyzes the hydrolysis of phospholipids to generate phosphatidic acid with secondarily generation of diradylglycerol; both of these products have been implicated as second messengers. Herein, we discuss the regulation and the biochemistry of the receptor-regulated PLD in human neutrophils. In vivo and in vitro studies suggest an activation mode in which initial receptor-linked activation of phospholipase C generates diacylglycerol and inositol trisphosphate. The resulting calcium flux along with the diacylglycerol activate a conventional isoform of protein kinase C (PKC), probably PKC beta 1. This PKC, in turn phosphorylates a plasma membrane component resulting in PLD activation and a second outpouring of diradylglycerol. The small GTP-binding proteins, RhoA and ARF, also participate in this process, and synergize with a 50 kDa cytosolic regulatory factor.

Stimulation of protein kinase C redistribution and inhibition of leukotriene B4-induced inositol 1,4,5-trisphosphate generation in human neutrophils by lipoxin A4

1. To test the hypothesis that protein kinase C (PKC) is involved in the inhibitory actions of lipoxin A4 (LXA4) on second messenger generation, we studied the effects of LXA4 on PKC in human neutrophils and on leukotriene B4 (LTB4)-stimulated inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) generation. 2. LXA4, 1 microM, caused a fall in cytosolic PKC-dependent histone phosphorylating activity to 23.5% of basal levels. 3. LXA4, caused an increase in particulate PKC-dependent histone phosphorylating activity with a bell-shaped dose-response fashion; maximal stimulation was observed at 10 nM LXA4. 4. Western blot analysis with affinity-purified antibodies to alpha- and beta-PKC showed that only the beta-PKC isotype was translocated by LXA4. 5. LXA4 inhibited LTB4-stimulated Ins(1,4,5)P3 generation in a bell-shaped fashion with maximal inhibition at 1 nM LXA4. The observed inhibition was dose-dependently removed by pre-incubation with a PKC inhibitor (Ro-31-8220). 6. These results show that LXA4 activates PKC in whole cells and supports a role for PKC activation in the inhibitory action of LXA4 on LTB4-induced Ins(1,4,5)P3 generation. 7. LXA4 (1-1000 nM) pre-incubation did not affect specific binding of [3H]-LTB4 to neutrophils. Thus, the inhibitory effect of LXA4 on LTB4-stimulated Ins(1,4,5)P3 generation could not be attributed to an effect on LTB4 receptors.

Chemoattractant-induced activation of vacuolar H+ pumps and of an H(+)-selective conductance in neutrophils

Upon binding to their receptors on the surface of neutrophils, chemotactic peptides elicit a burst of metabolic activity. The excess acid generated by this burst must be rapidly extruded in order to maintain intracellular pH and preserve normal microbicidal responses. Recently, H(+)-pumping vacuolar-type ATPases (V-pumps) and a H(+)-selective conductance were described in the membrane of neutrophils. However, these systems are virtually quiescent in resting cells. In this report, we analyzed whether the V-pumps and the conductance become active and contribute to pH regulation following cell activation by chemoattractants. Formyl-Met-Leu-Phe (fMLP) was found to stimulate V-pumps, as assessed by the appearance of bafilomycin-sensitive H+ extrusion. Concomitantly, the chemoattractant also activated the H+ conductance, detected as a voltage-dependent and Zn(2+)-sensitive net H+ efflux. In both cases, activation was prevented by treatment with competing antagonistic peptides or with pertussis toxin, implying mediation by a receptor coupled to a heterotrimeric G protein. The signalling pathways downstream of the G proteins were also investigated. Stimulation of neither the V-pump nor the conductance required activation of protein kinase C. An elevation of cytosolic calcium ([Ca2+]i) comparable to that induced by fMLP did not suffice to trigger either transporter. Moreover activation of the conductance remained unaffected when the chemoattractant-induced increase in [Ca2+]i was precluded. In contrast, stimulation of the V-pump was substantially (approximately 50%) depressed when [Ca2+]i was prevented from rising. Tyrosine phosphorylation of several polypeptides accompanies stimulation by fMLP. Prevention of phosphotyrosine accumulation resulted in a pronounced inhibition of H(+)-pumping and of the H+ conductance. Together, these data indicate that engagement of surface receptors by chemotactic peptides can lead to stimulation of two voltage-sensitive pH regulatory pathways, a pump and a conductance, by a pathway that requires tyrosine phosphorylation. Both pathways are capable of sizable H+ extrusion, thereby contributing to pH regulation during the metabolic burst.

Inhibition of neutrophil chemotaxis by protease inhibitors. Differential effect of inhibitors of serine and thiol proteases

Previous studies have demonstrated that neutrophils possess an active serine protease(s) which may be involved in the process of chemotaxis but the precise identity of this enzyme(s) remains to be determined. In this study fourteen different protease inhibitors were tested over a wide concentration range for their ability to inhibit unstimulated neutrophil movement and chemotaxis to C5a, fMLP and IL-8. Pretreatment of neutrophils with aspartyl or metallo-protease inhibitors had no effect on either chemotaxis or random cell movement. The thiol protease inhibitors E-64 and cystatin, as well as the thiol/serine inhibitors antipain and leupeptin, diminished only C5a-induced chemotaxis. Pretreatment of neutrophils with the serine protease inhibitors PMSF or 3,4-DCI significantly reduced chemotaxis to C5a, fMLP and IL-8. The inhibitor of trypsin-like serine proteases, TLCK, and the neutrophil elastase inhibitor MeO-Suc-AAPV-CMK had no inhibitory effect on cell movement. However, two different inhibitors of chymotrypsin-like serine proteases, TPCK and chymostatin, significantly inhibited movement to any chemoattractant. These results suggest that an active chymotrypsin-like serine protease is essential for neutrophils to respond to chemotactic stimuli.

Proteolysis of protein kinase C: mM and microM calcium-requiring calpains have different abilities to generate, and degrade the free catalytic subunit, protein kinase M

Limited proteolysis of protein kinase C (PKC) by calpain under cell free conditions cleaves the regulatory and catalytic PKC subunits, generating a free, co-factor independent catalytic subunit, termed PKM. In the present study, we demonstrate distinct differences in the rate, nature, and lipid-sensitivity of PKC and PKM proteolysis by microM and mM calcium-requiring calpain isozymes (mu calpain or m calpain, respectively). PKC is a preferred substrate for m calpain; not even a 100-fold increase in mu calpain was capable of degrading PKC as fast as in calpain. PKM was generated by both m and mu calpains, but was itself rapidly degraded by m calpain and therefore was only transiently detectable. By contrast, PKM was formed but not degraded by mu calpain, and persisted in the presence of mu calpain long after all PKC had been degraded. Phosphatidyl serine (PS) inhibited PKC hydrolysis by m calpain yet enhanced PKC hydrolysis by mu calpain. The ability of either calpain isoenzyme to degrade [14C]azocasein was unaffected by PS, suggesting that the influence of PS was on PKC conformation. These findings point towards distinct roles for mu and m calpain in PKC regulation.

Identification of a third ubiquitous calpain species--chicken muscle expresses four distinct calpains

In the mammalian calpain system, two isozymes, mu- and m-types, have been well-characterized, and are considered to be conserved in the avian system as well. Thus, chicken calpain, whose large subunit was cloned in 1984, has long been regarded as 'm-type', since chicken also possesses 'mu-type' activity, although its structure has not yet been elucidated. In this study, we identified three kinds of cDNAs encoding distinct chicken calpain large subunits. Two of the three were highly similar to the mammalian mu-type and p94, respectively. The third shows a much higher similarity to mammalian m-type than the first identified chicken calpain, indicating that this molecule, which has been considered as 'm-type', should be renamed. We, therefore, designated it 'mu/m-calpain', because its sequence and Ca(2+)-sensitivity lie between mu- and m-types. Northern blot analyses revealed that chicken mCL and muCL, as well as mu/mCL, show ubiquitous expression, while p94 was detected predominantly in skeletal muscle, as previously reported. Chicken skeletal muscle, therefore, expresses at least four types of calpain, three ubiquitous and one tissue-specific.

Daphnoretin-induced respiratory burst in rat neutrophils is, probably, mainly through protein kinase C activation

Daphnoretin, a dicoumarin isolated from Wikstroemia indica C.A. Mey. (Thymelaceae), induced superoxide anion (O2-) formation in rat neutrophils in a concentration-dependent manner. Addition of staurosporine reduced daphnoretin-induced respiratory burst. Removal of extracellular free Ca2+ by EGTA did not affect the respiratory burst of neutrophils in response to daphnoretin. Prior exposure of neutrophils to phorbol 12-myristate 13-acetate (PMA) or daphnoretin reduced the O2- formation caused by a subsequent challenge with PMA and daphnoretin, but potentiated the response caused by a subsequent addition of formyl-Met-Leu-Phe (fMLP). Like PMA, daphnoretin did not increase the [Ca2+]i during cell activation. In neutrophil suspension, daphnoretin increased the membrane associated protein kinase C activity. In the presence of Ca2+ and phosphatidyl-serine, daphnoretin also activated protein kinase C isolated from cytosolic fraction of resting neutrophils. Staurosporine inhibited the direct activation of protein kinase C caused by daphnoretin as well as by PMA. Daphnoretin reduced the [3H]Phorbol-12,13-dibutyrate ([3H]PDB) binding to the neutrophil cytosolic protein kinase C in a concentration-dependent manner with an IC50 value of 1.77 +/- 0.37 microM. These results indicate that daphnoretin, like PMA, may direct activation of protein kinase C which in turn activated NADPH oxidase and elicited respiratory burst.

Degradation of protein kinase C alpha and its free catalytic subunit, protein kinase M, in intact human neuroblastoma cells and under cell-free conditions. Evidence that PKM is degraded by mM calpain-mediated proteolysis at a faster rate than PKC

Proteolytic cleavage of protein kinase C (PKC) under cell-free conditions generates a co-factor independent, free catalytic subunit (PKM). However, the difficulty in visualizing PKM in intact cells has generated controversy regarding its physiological relevance. In the present study, treatment of SH-SY-5Y cells with 2-O-tetradecanoylphorbol 13-acetate resulted in complete down-regulation of PKC within 24 h without detection of PKM. By contrast, low levels of PKM were transiently detected following ionophore-mediated calcium influx under conditions which induced no detectable PKC loss. PKM was not detected during rapid cell-free degradation of partially purified SH-SY-5Y PKC alpha by purified human brain mM calpain. However, when the kinetics of PKC degradation were slowed by lowering levels of calpain, PKM was transiently detected. PKM was also only transiently observed following calpain-mediated degradation of purified rat brain PKC alpha. Densitometric analyses indicated that, once formed, PKM was degraded approximately 10 times faster than PKC. These data provide an explanation as to why PKM is difficult to observe in situ, and indicate that PKM should not be considered as an 'unregulated' kinase, since its persistence is apparently strictly regulated by proteolysis.

Immunochemical identification and translocation of protein kinase C zeta in human neutrophils

Western blots of human polymorphonuclear leukocyte (PMN) extracts were immunostained with antibodies specific for various protein kinase C (PKC) isoforms. Two bands corresponding to PKC type zeta with apparent molecular masses of 81 kDa and 76 kDa were identified in the cytosolic fraction of resting cells, in addition to PKC types alpha and beta. PKC zeta was apparently abundant, like PKC beta, whereas PKC delta, -epsilon, and -gamma were not detectable. Following short stimulation (5 min) of PMN with phorbol-12-myristate-13-acetate (1 microgram/ml), physical translocation of PKC zeta from the cytosol to the plasma membrane fraction occurred, although this isoform does not bind phorbol esters. These data show that, in addition to the two calcium-dependent isoenzymes alpha and beta, human PMN express a calcium-independent isoenzyme zeta which translocates in stimulated cells, suggesting a role in the regulation of antibacterial activities.

Differential phosphorylation in normal and leukemic granulocytes in response to phorbol 12-myristate 13-acetate

Granulocytes from the peripheral blood of patients with chronic myeloid leukemia (CML) exhibit a number of functional defects. To explore the relationship of these aberrations to signal transduction, granulocytes from normal subjects and CML patients were labelled with 32Pi, stimulated with phorbol myristate acetate (PMA) and the phosphoproteins (Pps) in the unstimulated and stimulated cells analyzed by 2D-SDS-PAGE followed by autoradiography. Results show that there are six distinct reproducibly phosphorylated proteins referred to as Pp1-Pp6 identifiable in the basal patterns of the resting granulocytes. Amongst these, Pp1 and Pp5 are more intensely phosphorylated and Pp3 is very faint or absent in unstimulated CML cells, relative to the normal granulocytes. On stimulation of normal cells with PMA, Pp1, Pp3, Pp4 and Pp6 exhibit distinct patterns of phosphorylation-dephosphorylation. In the CML cells, however, Pp1 and Pp4 are unresponsive to PMA. We conclude that PKC-mediated functions involving Pp1, Pp3 and Pp4 are most probably defective in CML cells.

New era of calpain research. Discovery of tissue-specific calpains

The recent discovery of several new calpain species other than the two species thus far studied reveals that calpain, especially the calpain large subunit, constitutes a family comprising at least six members that can be classified into ubiquitous (mu, m- and mu/m-types) and tissue-specific (p94 or nCL-1 specific for skeletal muscle, and nCL-2 and -2' specific for stomach) calpains. The newly identified tissue-specific calpains have various characteristics distinct from conventional calpains in structure, manner of expression, and enzyme activity. Unique features of tissue specific calpains are discussed together with the evolutionary view of the calpain large subunit.

Effect of protein kinase C inhibitors on invasiveness of human melanoma clones expressing different levels of protein kinase C isoenzymes

The involvement of protein kinase C (PKC) in the mechanism of chemotaxis and invasiveness of human melanoma has been studied in 6 clones of 665/2 cell line characterized by a different integrin profile, differentiation grade and in vitro invasive ability. The levels of total protein kinase C activity revealed a direct correlation with the chemotactic and invasive ability of these clones. Protein kinase C inhibitors, sphingosine and staurosporine, reduced chemotaxis and invasiveness of the highly invasive clone 2/60, while 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) was ineffective. Immunofluorescence analysis revealed high levels of protein kinase C alpha in clone 2/60, while the less invasive clone 2/21 expressed low levels of protein kinase C alpha and beta, but surprisingly appreciable levels of protein kinase C gamma. Downregulation with phorbol 12-myristate 13-acetate (TPA) did not affect invasiveness of clone 2/60 unless the compound was present during the assay. H7 strongly increased invasiveness of clone 2/21 and was able to reverse the inhibitory effect of TPA on clone 2/60. Preliminary experiments showed higher levels of diacylglycerol in clones with lower protein kinase C, suggesting a constitutive downregulation of the enzyme in low invasive clones. Our results support a role for protein kinase C in the invasion process, but point out the complexity of the mechanism which might involve the proteolytic fragment of the enzyme, protein kinase M.

Protein kinase C isotypes and signal-transduction in human neutrophils: selective substrate specificity of calcium-dependent beta-PKC and novel calcium-independent nPKC

Neutrophils possess at least two phospholipid-dependent forms of protein kinase C, a classical Ca/PS/DG-dependent beta-isotype of protein kinase C and a Ca-independent but PS/DG-dependent novel protein kinase C (nPKC) which we now demonstrate to have different substrate specificities. Activation of human neutrophils triggers assembly of an NADPH oxidase in the membrane and generation of O2-. A role for the major Ca-dependent isotype beta-PKC in neutrophils is proposed in stimulus-induced phosphorylation and association of a cytosolic 47 kDa protein (p47-phox) with the membrane NADPH oxidase. In this study we demonstrate that purified beta-PKC and nPKC have very different substrate specificities; beta-PKC but not nPKC phosphorylated both endogenous and recombinant p47-phox. In addition, beta-PKC but not nPKC phosphorylated [ser25]PKC(19-31), the substrate peptide based on a sequence in the Ca-dependent alpha, beta and gamma-isotypes. Pseudosubstrate(19-36), derived from the C-terminus of Ca-dependent PKC isotypes, inhibited beta-PKC but not nPKC activity using either Histone IIIS or peptide(19-31) as substrate. Pseudosubstrate(19-36) also inhibited beta-PKC catalyzed phosphorylation of endogenous and recombinant p47-phox. Pseudosubstrate(19-36) also inhibited the O2- generation triggered by GTP gamma S in electroporated neutrophils by 50%. 32P-Labelled neutrophils electroporated in the presence of GTP gamma S showed phosphorylation of multiple cytosolic proteins including a 47 kDa band, and phosphorylation of membrane-associated 34 kDa, 47 kDa and 54 kDa proteins. Pseudosubstrate(19-36) inhibited phosphorylation of p47-phox in the membrane but not in the cytosol. These findings suggest translocatable, Ca-dependent isotypes of PKC such as beta-PKC may play a role in the phosphorylation of membrane associated p47-phox and the assembly or maintenance of an active NADPH oxidase.

Ethanol exposure increases total protein kinase C activity in human lymphocytes

Human circulating lymphocytes were isolated and incubated with ethanol. Cytosolic, membrane-bound and total detergent extractable protein kinase C (PKC) activities were measured. Exposure to ethanol (100 mm) resulted in an increase in PKC activity, with membrane-associated PKC activity increasing with respect to cytosolic activity at 5 min of exposure. Higher concentrations of ethanol up to 200 mm were associated with increases in total detergent extractable PKC activity. Ethanol was the most potent of a series of straight chain alcohols studied for their effects on detergent-extractable PKC activity.

Novel antibodies specific for proteolyzed forms of protein kinase C: production of anti-peptide antibodies available for in situ analysis of intracellular limited proteolysis

We show here a novel method for the in situ analysis of proteolyzed proteins in a cell. As a model, we focused on protein kinase C (PKC) beta, which is cleaved at a specific site between the catalytic and regulatory domains by calpain, the intracellular calcium-activated neutral proteinase. To detect proteolyzed PKC beta 'cleavage-site-directed antibodies', which specifically recognize the amino-terminal region of the catalytic fragment but do not cross-react with the unproteolyzed enzymes, were raised using synthetic peptide. The synthetic peptide used in this study was QGTKVPEEKTT, corresponding to the amino-terminal region of the catalytic fragment from human PKC beta generated by calpain. Rabbits were immunized with the synthetic peptide after conjugation with a carrier protein. Antibodies obtained reacted with the 46-kDa catalytic fragment of PKC beta, whereas they did not cross-react with unproteolyzed enzyme nor other fragments with different amino-termini. Thus, our antibody is specific to the amino-terminal sequence QGTKVPEEKTT, but does not recognize the same sequence located internally in native PKC beta. When human monoblast U937 cells were treated with calcium ionophore, the catalytic fragment of PKC beta was detected in the cytosol by immunoblotting with the antibody. However, this antibody did not bind unproteolyzed 80-kDa PKC beta, although this form was dominant in the cytosol of the calcium ionophore-treated cells. We could also detect comparable amounts of catalytic fragment in the calcium ionophore-treated cells by immunocytochemical staining with the same antibody. Our method was applied to examine the proteolysis of PKC beta in neutrophils stimulated with various reagents.

Proteinases and proteinase inhibitors as modulators of animal cell growth

1. Three distinct lines of evidence indicate that proteinases are involved in the growth of cultured animal cells. 2. Endogenous growth-related proteinases have been identified, and exogenous proteinases can also stimulate cell proliferation, probably by different mechanisms. In some cases, higher concentrations of proteinases are cytotoxic. 3. Proteinase inhibitors, not surprisingly, inhibit cell growth, but can also be mitogenic at sub-inhibitory concentrations. 4. There must, therefore, be at least three major cellular processes in which proteinases or proteinase inhibitors can operate to exert a direct effect on cell proliferation. 5. Details of one action of an exogenous proteinase, typified by thrombin and the thrombin receptor, are becoming clear at the molecular level, but thrombin probably activates at least two intracellular signalling systems, as well as acting as a growth inhibitor in some situations. 6. Much remains to be investigated in other examples.

Sequence comparison among muscle-specific calpain, p94, and calpain subunits

While conventional calpains, m- and mu-calpains named according to their calcium-dependence, are expressed in almost every tissues, mRNA of newly identified p94, which has a significant sequence similarity to the conventional calpain large subunits, is abundantly expressed only in skeletal muscle. In addition to this specific expression, p94 is distinct from conventional calpains in that it contains three unique regions showing no similarity to conventional calpain subunits. When rat and human p94 are compared, overall sequence similarity is 94.0%, which is close to those for m- and mu-calpain large subunits; 93.1% and 95.4% between human and rabbit, respectively, suggesting the evolutionary importance of p94. These calpain large subunit proteins, p94, m- and mu-types, can be considered to constitute a super family, whose p94, m- and mu-types represent the three major types. Sequences of the calpain large-subunit family members, including the recently reported Schistosoma calpain, are compared. Their evolutionary correlation and function are discussed on the basis of the results thus far obtained.

Protein kinase C of smooth muscle

The primary mechanism of regulation of smooth muscle contraction involves the phosphorylation of myosin catalyzed by Ca2+/calmodulin-dependent myosin light chain kinase. However, additional mechanisms, both Ca(2+)-dependent and Ca(2+)-independent, can modulate the contractile state of smooth muscle. Protein kinase C was first implicated in the regulation of smooth muscle contraction with the observation that phorbol esters induce slowly developing, sustained contractions. Protein kinase C occurs in at least four Ca(2+)-dependent (alpha, beta I, beta II, and gamma) and four Ca(2+)-independent (delta, epsilon, zeta, and eta) isoenzymes. Only the alpha, beta, epsilon, and zeta isoenzymes have been identified in smooth muscle. Both classes of isoenzymes have been implicated in the regulation of smooth muscle contraction. However, the physiologically important protein substrates of protein kinase C have not yet been identified. Specific isoenzymes may be activated by different contractile agonists, and individual isoenzymes exhibit some degree of substrate specificity. Prolonged activation of protein kinase C can result in its proteolysis to the constitutively active catalytic fragment protein kinase M, which would dissociate from the sarcolemma and phosphorylate proteins such as myosin that are inaccessible to membrane-bound protein kinase C. Protein kinase M induces relaxation of demembranated smooth muscle fibers contracted at submaximal Ca2+ concentrations. We suggest that protein kinase C plays two distinct roles in regulating smooth muscle contractility. Stimuli triggering phosphoinositide turnover or phosphatidylcholine hydrolysis induce translocation of protein kinase C (probably specific isoenzymes) to the sarcolemma, phosphorylation of protein, and a slow contraction. Prolonged association of the kinase with the membrane may lead to proteolysis and release into the cytosol of protein kinase M, resulting in myosin phosphorylation and relaxation.