Activation of purified human protein kinase C alpha and beta I isoenzymes in vitro by Ca2+, phosphatidylinositol and phosphatidylinositol 4,5-bisphosphate

Aging bone marrow mesenchymal stromal cells have altered membrane glycerophospholipid composition and functionality

Human mesenchymal stem/stromal cells (hMSC) are increasingly used in advanced cellular therapies. The clinical use of hMSCs demands sequential cell expansions. As it is well established that membrane glycerophospholipids (GPL) provide precursors for signaling lipids that modulate cellular functions, we studied the effect of the donor's age and cell doublings on the GPL profile of human bone marrow MSC (hBMSC). The hBMSCs, which were harvested from five young and five old adults, showed clear compositional changes during expansion seen at the level of lipid classes, lipid species, and acyl chains. The ratio of phosphatidylinositol to phosphatidylserine increased toward the late-passage samples. Furthermore, 20:4n-6-containing species of phosphatidylcholine and phosphatidylethanolamine accumulated while the species containing monounsaturated fatty acids (FA) decreased during passaging. Additionally, in the total FA pool of the cells, 20:4n-6 increased, which happened at the expense of n-3 polyunsaturated FAs, especially 22:6n-3. The GPL and FA correlated with the decreased immunosuppressive capacity of hBMSCs during expansion. Our observations were further supported by alterations in the gene expression levels of several enzymes involved in lipid metabolism and immunomodulation. The results show that extensive expansion of hBMSCs harmfully modulates membrane GPLs, affecting lipid signaling and eventually impairing functionality.

Serine 34 phosphorylation of rho guanine dissociation inhibitor (RhoGDIalpha) links signaling from conventional protein kinase C to RhoGTPase in cell adhesion

Conventional protein kinase C (PKC) isoforms are essential serine/threonine kinases regulating many signaling networks. At cell adhesion sites, PKCα can impact the actin cytoskeleton through its influence on RhoGTPases, but the intermediate steps are not well known. One important regulator of RhoGTPase function is the multifunctional guanine nucleotide dissociation inhibitor RhoGDIα that sequesters several related RhoGTPases in an inactive form, but it may also target them through interactions with actin-associated proteins. Here, it is demonstrated that conventional PKC phosphorylates RhoGDIα on serine 34, resulting in a specific decrease in affinity for RhoA but not Rac1 or Cdc42. The mechanism of RhoGDIα phosphorylation is distinct, requiring the kinase and phosphatidylinositol 4,5-bisphosphate, consistent with recent evidence that the inositide can activate, localize, and orient PKCα in membranes. Phosphospecific antibodies reveal endogenous phosphorylation in several cell types that is sensitive to adhesion events triggered, for example, by hepatocyte growth factor. Phosphorylation is also sensitive to PKC inhibition. Together with fluorescence resonance energy transfer microscopy sensing GTP-RhoA levels, the data reveal a common pathway in cell adhesion linking two essential mediators, conventional PKC and RhoA.

Redundancy of biological regulation as the basis of emergence of multidrug resistance

Active efflux of xenobiotics is a major mechanism of cell adaptation to environmental stress. The ATP-dependent transmembrane transporter P-glycoprotein (Pgp) confers long-term cell survival in the presence of different toxins, including anticancer drugs (this concept is referred to as multidrug resistance, or MDR). The vital importance of this mechanism for cell survival dictates the reliability and promptness of its acquisition. To fulfill this requirement, the MDR1 gene that encodes Pgp in humans must be readily upregulated in cells that express low to null levels of MDR1 mRNA prior to stress. The MDR1 gene and a stable MDR phenotype can be induced after short-term exposure of cells to a variety of cues. This effect is implemented by activation of MDR1 transcription and mRNA stabilization. The MDR1 message abundance is regulated by mechanisms generally involved in stress response, namely activation of phospholipase C, protein kinase C and mitogen-activated protein kinase cascades, mobilization of intracellular Ca2+, and nuclear factor kappa B activation. Furthermore, the proximal MDR1 promoter sites critical for induction are not unique for the MDR1 gene; they are common regulatory elements in eukaryotic promoters. Moreover, MDR1 induction can result from activation of (an) intermediate gene(s) whose product(s), in turn, directly activate(s) the MDR1 promoter and/or cause(s) mRNA stabilization. Redundancy of signal transduction and transcriptional mechanisms is the basis for the virtually ubiquitous inducibility of the MDR1 gene. Thus, the complex network of MDR1 regulation ensures rapid emergence of pleiotropic resistance in cells.

Conventional PKCs regulate the temporal pattern of Ca2+ oscillations at fertilization in mouse eggs

In mammalian eggs, sperm-induced Ca²⁺ oscillations at fertilization are the primary trigger for egg activation and initiation of embryonic development. Identifying the downstream effectors that decode this unique Ca²⁺ signal is essential to understand how the transition from egg to embryo is coordinated. Here, we investigated whether conventional PKCs (cPKCs) can decode Ca²⁺ oscillations at fertilization. By monitoring the dynamics of GFP-labeled PKCα and PKCγ in living mouse eggs, we demonstrate that cPKCs translocate to the egg membrane at fertilization following a pattern that is shaped by the amplitude, duration, and frequency of the Ca²⁺ transients. In addition, we show that cPKC translocation is driven by the C2 domain when Ca²⁺ concentration reaches 1–3 μM. Finally, we present evidence that one physiological function of activated cPKCs in fertilized eggs is to sustain long-lasting Ca²⁺ oscillations, presumably via the regulation of store-operated Ca²⁺ entry.

A new phosphatidylinositol 4,5-bisphosphate-binding site located in the C2 domain of protein kinase Calpha

In view of the interest shown in phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) as a second messenger, we studied the activation of protein kinase Calpha by this phosphoinositide. By using two double mutants from two different sites located in the C2 domain of protein kinase Calpha, we have determined and characterized the PtdIns(4,5)P(2)-binding site in the protein, which was found to be important for its activation. Thus, there are two distinct sites in the C2 domain: the first, the lysine-rich cluster located in the beta3- and beta4-sheets and which activates the enzyme through direct binding of PtdIns(4,5)P(2); and the second, the already well described site formed by the Ca(2+)-binding region, which also binds phosphatidylserine and a result of which the enzyme is activated. The results obtained in this work point to a sequential activation model, in which protein kinase Calpha needs Ca(2+) before the PtdIns(4,5)P(2)-dependent activation of the enzyme can occur.

Syndecan-4-mediated signalling

The paradigm of cell surface proteoglycan function has been centered on the role of the ectoplasmic heparan sulfate (HS) chains as acceptors of a wide array of ligands, including extracellular matrix (ECM) proteins and soluble growth factors. Within this picture, the core proteins were assigned only a passive role of carrying the glycosaminoglycan (GAG) chains without direct participation in mediating outside-in signals generated by the binding of the above ligands. It appears now, however, that, side by side with the integrins and the tyrosine kinase receptors, the core proteins of the syndecan family of transmembrane proteoglycans are involved in signaling. The highly conserved tails of all the four members of the syndecan family contain a carboxy-terminal PDZ (Postsynaptic density 95, Disk large, Zona occludens-1)-binding motif, capable of forming multimolecular complexes through the binding of PDZ adaptor proteins. The cytoplasmic tail of the ubiquitously expressed syndecan-4 is distinct from the other syndecans in its capacity to bind phosphatidylinositol 4,5-bisphosphate (PIP2) and to activate protein kinase C (PKC) alpha. These properties may confer on syndecan-4 specific and unique signaling functions.

The role of protein kinase C in the development of the complications of diabetes

Diabetes mellitus produces a state of chronic hyperglycemia which in turn leads to the development of severe complications including retinopathy, nephropathy, neuropathy, and atherosclerosis. Many different mechanisms have been put forward to attempt to explain how glucose elevations can damage these various organ systems. Protein kinase C activation is one of the sequelae of hyperglycemia and is thought to play a role in the development of diabetic complications. There are multiple mechanisms for its activation in the diabetic state and multiple downstream effects attributable to that activation. The role of protein kinase C activation in the development of the above-mentioned complications of diabetes is discussed in this chapter. In addition, the potential use of isoform-specific inhibitors of protein kinase C for the treatment of diabetic complications is proposed.

Phosphatidylinositol-4,5-bisphosphate mediates the interaction of syndecan-4 with protein kinase C

Recent studies have demonstrated that the cytoplasmic tail of syndecan-4, a widely expressed transmembrane proteoglycan, can activate protein kinase Calpha in vitro, in combination with phosphatidylinositol-4,5-bisphosphate (PI-4,5-P(2)). Syndecan-4 is involved in growth factor binding as well as in adhesion to extracellular matrix proteins, while PI-4,5-P(2) synthesis is modulated by growth factor and adhesion-generated signaling. The cooperative activation of PKCalpha by the proteoglycan and the phosphatidylinositol may constitute, therefore, an essential part of the cell's response to these extracellular signals. To characterize the activation mechanism of PKCalpha, we addressed here the nature of the interplay between syndecan-4, PI-4,5-P(2), and PKCalpha by measuring their mutual binding affinities and the specificity of their interactions. We found that the cytoplasmic tail of syndecan-4 is unlikely to bind directly to PKCalpha, and that this interaction critically depends on PI-4,5-P(2). The PI-4,5-P(2) specificity of the activation of PKCalpha is conferred by the cytoplasmic tail of syndecan-4, which has higher binding affinity for this phosphatidylinositol over phosphatidylinositol-3,4-bisphosphate and the -3,4,5-trisphospate. The activation is specific to PKCalpha and does not encompass the novel protein kinase C delta isoenzyme.

Protein tyrosine phosphorylation activates rat splenic type II phosphatidylinositol 4-kinase in vitro

Regulation of phosphatidylinositol 4-kinase (PtdIns 4-kinase) by protein tyrosine phosphorylation has been indirect and the effects of phosphorylation are debatable. Rat splenic type II PtdIns 4-kinase was phosphorylated in vitro with protein tyrosine kinases from Con A stimulated splenic lymphocytes. Stoichiometric analysis showed one mole of phosphate was incorporated per mole of PtdIns 4-kinase. Tyrosine phosphorylation increased the enzyme activity by 3-fold. Kinetic analysis showed a reduction in Km for PtdIns and an increase in Vmax. Dephosphorylation with protein phosphotyrosine phosphatase abolished the activation of PtdIns 4-kinase while protein phosphatase 2A had no effect. Protein tyrosine phosphorylation and activation of PtdIns 4-kinase appear to be tissue specific.

The sevenfold way of PKC regulation

Protein kinase C (PKC) is a family of enzymes that are physiologically activated by 1,2-diacylglycerol (DAG) and other lipids. To date, 11 different isozymes, alpha, betaI, betaII, gamma, delta, epsilon, nu, lambda(iota), mu, theta and zeta, have been identified. On the basis of their structure and activators, they can be divided into three groups, two of which are activated by DAG or its surrogate, phorbol 12-myristate 13-acetate (PMA). PKC isozymes are remarkably different in number and prevalence in different cell lines and tissues. When activated, the isozymes bind to membrane phospholipids or to receptors that are located in and anchor the enzymes in a subcellular compartment. Some PKCs may also be activated in their soluble form. These enzymes phosphorylate serine and threonine residues on protein substrates, perhaps the best known of which are the myristoylated, alanine-rich C kinase substrate and nuclear lamins A, B and C. The enzymes clearly play a role in signal transduction, and, because of the importance of PMA as a tumor promoter, they are thought to affect some aspect of cell cycling. How PKC takes part in the regulation of cell transformation, growth, differentiation, ruffling, vesicle trafficking and gene expression, however, is largely unknown.

Lipid products of phosphoinositide 3-kinase and phosphatidylinositol 4',5'-bisphosphate are both required for ADP-dependent platelet spreading

We have shown previously that ADP released upon platelet adhesion mediated by alphaIIb beta3 integrin triggers accumulation of phosphatidylinositol 3',4'-bisphosphate (PtdIns-3,4-P2) (Gironcel, D. , Racaud-Sultan, C., Payrastre, B., Haricot, M., Borchert, G., Kieffer, N., Breton, M., and Chap, H. (1996) FEBS Lett. 389, 253-256). ADP has also been involved in platelet spreading. Therefore, in order to study a possible role of phosphoinositide 3-kinase in platelet morphological changes following adhesion, human platelets were pretreated with specific phosphoinositide 3-kinase inhibitors LY294002 and wortmannin. Under conditions where PtdIns-3, 4-P2 synthesis was totally inhibited (25 microM LY294002 or 100 nM wortmannin), platelets adhered to the fibrinogen matrix, extended pseudopodia, but did not spread. Moreover, addition of ADP to the medium did not reverse the inhibitory effects of phosphoinositide 3-kinase inhibitors on platelet spreading. Although synthetic dipalmitoyl PtdIns-3,4-P2 and dipalmitoyl phosphatidylinositol 3',4', 5'-trisphosphate restored only partially platelet spreading, phosphatidylinositol 4',5'-bisphosphate (PtdIns-4,5-P2) was able to trigger full spreading of wortmannin-treated adherent platelets. Following 32P labeling of intact platelets, the recovery of [32P]PtdIns-4,5-P2 in anti-talin immunoprecipitates from adherent platelets was found to be decreased upon treatment by wortmannin. These results suggest that the lipid products of phosphoinositide 3-kinase are required but not sufficient for ADP-induced spreading of adherent platelets and that PtdIns-4,5-P2 could be a downstream messenger of this signaling pathway.

Synthesis and protein kinase C inhibitory activities of balanol analogues with modification of 4-hydroxybenzamido moiety

A series of racemic balanol analogues with modification of the benzamido moiety of balanol have been synthesized and evaluated for their inhibitory activities against human protein kinase C isozymes (PKC-alpha, -beta I, -beta II, -gamma, -delta, -epsilon, and -eta). The structural modification includes replacement of the 4-hydroxyphenyl group with variously substituted phenyl rings, substitution of the amide linkage with a sulfonamide or an ester, and replacement of the 4-hydroxyphenyl substructure with a hydroxyl substituted indole or a hydroxybenzyl group. in general, these analogues were found to be less potent than balanol, but a number of analogues were identified with improved isozyme selectivity. The structure-activity relationship studies of these analogues also indicated that (1) the optimal general PKC inhibition requires a free 4-hydroxyl group in the benzamido portion of the molecule, (2) the amide linkage of the benzamido moiety is important for PKC inhibition, and (3) the conformation associated with the benzamido moiety seems to have a profound effect on PKC inhibition. The requirement of a free 4-hydroxyl group in conjunction with an appropriate conformation of the benzamido moiety for optimal PKC inhibition suggests that the 4-hydroxyphenyl group may be involved in a specific inhibitor-enzyme interaction important for PKC inhibition.

Protein kinase C from bat brain: the enzyme from a hibernating mammal

Protein kinase C (PKC) from brain of euthermic and hibernating bats (Myotis lucifugus) showed only one form as determined by hydroxylapatite chromatography, compared with three forms found in rat brain. Cross-reaction with antibodies to rabbit alpha, beta, and gamma isozymes showed that bat brain contained only PKC(gamma). During hibernation the activity of PKC in bat brain decreased to 63% of the euthermic value but the percentage that was membrane-associated did not change. Bat and rat brain PKC(gamma) were purified to homogeneity. Both enzymes phosphorylated all three of the substrates tested (FKKSFKL-NH2 peptide substrate, histone H1, protamine), the bat enzyme having significantly higher K(m) values than rat PKC for both peptide and histone. Both enzymes required phospholipids and Ca2+ for activation with rat brain PKC depending almost exclusively on phosphatidylserine. Bat PKC, however, made use of other phospholipids and showed relative activities of 100:81:33:42 for euthermic PKC and 100:91:45:35 for hibernator PKC with phosphatidylserine, phosphatidylinositol, phosphatidylcholine, and phosphatidylethanolamine (each at 50 microM), respectively. Activation of bat PKC by phosphatidylserine was temperature sensitive, being 3.5-fold at 4 degrees C (hibernating body temperature) compared with 14-18-fold at 33 degrees C (near euthermic body temperature). Arrhenius plots for bat brain PKC showed a sharp break below 10 degrees C; activation energies below this temperature were 11.5- and 5.2-fold greater than at higher temperatures for the enzyme from hibernating versus euthermic animals. By contrast, plots for the rat enzyme were linear over the range 0-42 degrees C. The data suggest that a sharp suppression of PKC activity by several mechanisms (reduced total activity, low temperature effects on activity and sensitivity to phospholipids) may be important to overall metabolic rate suppression during hibernation.

Isoenzymes of protein kinase C: differential involvement in apoptosis and pathogenesis

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Synthesis and protein kinase C inhibitory activities of balanol analogs with replacement of the perhydroazepine moiety

Balanol is a potent protein kinase C (PKC) inhibitor that is structurally composed of a benzophenone diacid, a 4-hydroxybenzamide, and a perhydroazepine ring. A number of balanol analogs in which the perhydroazepine moiety is replaced have been synthesized and their biological activities evaluated against both PKC and cAMP-dependent kinase (PKA). The results suggested that the activity and the isozyme/kinase selectivity of these compounds are largely related to the conformation about this nonaromatic structural element of the molecules.

Profilin and gelsolin stimulate phosphatidylinositol 3-kinase activity

Actin-binding proteins such as profilin and gelsolin bind to phosphatidylinositol (PI) 4,5-bisphosphate (PI 4,5-P2) and regulate the concentration of monomeric actin. We report here that profilin and gelsolin stimulate PI 3-kinase-mediated phosphorylation of PI 4,5-P2 (lipid kinase activity) in a concentration-dependent manner. This effect is specific to profilin and gelsolin because other cytoskeletal proteins such as tau or actin do not affect PI 3-kinase activity. In addition to lipid kinase activity, PI 3-kinase also has protein kinase activity: it phosphorylates proteins (p85 subunit of PI 3-kinase). However, the protein kinase activity of PI 3-kinase was not affected in the presence of profilin. Kinetic analysis, as a function of varying concentrations of ATP and PI 4,5-P2, showed that profilin affects the Vmax of PI 3-kinase without affecting k(m). Profilin may also affect PI 3-kinase activity by its direct association to the enzyme because dot-blot analysis using antibody to glutathione S-transferase (GST) suggested that GST-85 kDa, a fusion protein of PI 3-kinase, binds to profilin. However, PI 3-kinase did not affect the actin-sequestering ability of profilin (determined by pyrene-labeled actin), which indicates that actin and p85 do not share a common binding site on profilin. These studies suggest that profilin and gelsolin may control the generation of 3-OH phosphorylated phosphoinositides, which in turn may regulate the actin polymerization.

Synthesis and protein kinase C inhibitory activities of acyclic balanol analogs that are highly selective for protein kinase C over protein kinase A

A series of balanol analogs in which the perhydroazepine ring and the p-hydroxybenzamide moiety were combined into an acyclic linked unit have been prepared and evaluated for their inhibitory properties against the serine/threonine kinase PKC. Several low-micromolar to low-nanomolar inhibitors of the alpha, beta I, beta II, gamma, delta, epsilon and eta PKC isozymes were prepared. In general, these acyclic balanol analogs were found to be highly selective for PKC over the serine/threonine kinase PKA. The type and number of atoms linking the benzophenone ester to the p-hydroxyphenyl group necessary for optimal PKC inhibition were investigated. The most potent compounds contained a three-carbon linker in which the carboxamide moiety of balanol had been replaced by a methylene group. The effect of placing substituents on the three-carbon chain was also investigated. The preferred compounds contained either a 2-benzenesulfonamido (6b) or a 1-methyl (21b) substituent. The preferred compounds 6b and 21b were tested against a panel of serine/threonine kinases and found to be highly selective for PKC. The more active enantiomer of 6b, (S)-12b, was 3-10-fold more active than the R-enantiomer against the PKC isozymes. The effect of making the analogs more rigid by making the three-carbon chain part of a five-membered ring, but with retention of the methylene replacement for the carboxamide moiety, led to potent PKC inhibitors including anti-substituted pyrrolidine analog 35b and the most potent PKC inhibitor in the series, anti-substituted cyclopentane analog 29b. The anti cyclopentane analog 29b, was a low-micromolar inhibitor of the PMA-induced superoxide burst in neutrophils, and its carboxylic ester was a high-nanomolar inhibitor of neutrophils. Finally esterification of 21b, (S)-12b, and 35b turned these potent PKC inhibitors into low-micromolar inhibitors of neutrophils.

Liver protein kinase C isozymes: properties and enzyme role in a vertebrate facultative anaerobe

Protein kinase C was purified to homogeneity from liver of the anoxia-tolerant turtle (Trachemys scripta elegans). Two isozymes were present and were identified as PKC alpha and PKC beta by hydroxylapatite chromatography and cross-reaction with specific antibodies to the mammalian isozymes. Kinetic characterization of the isozymes showed that both required phospholipids and Ca2+ for activation and both were inhibited by low concentrations of PKC inhibitors. The PKC alpha was activated more strongly by phosphatidylinositol and lysophosphatidylinositol compared with PKC beta. Treatment with trypsin did not activate turtle PKC isozymes, but generated inactive PKC beta, whereas PKC alpha was resistant to inactivation. Anoxia exposure of turtles in vivo, via submergence in N2-gassed water at 7 degrees C, altered the activity and subcellular distribution of PKC in liver. After 1 hr of anoxic exposure at 7 degrees C, the activity of membrane-bound PKC had increased by 2.4-fold and represented a translocation of 40% of PKC beta and more than 80% of PKC alpha from the cytosol to the membrane-associated fraction. With longer submergence, however, membrane-bound PKC activity was suppressed again. This two-phase response to anoxia by PKC suggests that an activation of PKC, through its translocation to the membrane, is important in mediating the initial metabolic responses to submergence, which include an activation of glycogenolysis during the hypoxia transition period. With sustained anoxia exposure, the subsequent reduction of PKC activity may be part of the overall mechanism of metabolic rate depression that allows endurance of prolonged anoxia.

A phosphatidylinositol (PI) kinase gene family in Dictyostelium discoideum: biological roles of putative mammalian p110 and yeast Vps34p PI 3-kinase homologs during growth and development

Three groups of phosphatidylinositol (PI) kinases convert PI into PI(3)phosphate, PI(4)phosphate, PI(4,5) bisphosphate, and PI(3,4,5)trisphosphate. These phosphoinositides have been shown to function in vesicle-mediated protein sorting, and they serve as second-messenger signaling molecules for regulating cell growth. To further elucidate the mechanism of regulation and function of phosphoinositides, we cloned genes encoding five putative PI kinases from Dictyostelium discoideum. Database analysis indicates that D. discoideum PIK1 (DdPIK1), -2, and -3 are most closely related to the mammalian p110 PI 3-kinase, DdPIK5 is closest to the yeast Vps34p PI 3-kinase, and DdPIK4 is most homologous to PI 4-kinases. Together with other known PI kinases, a superfamily of PI kinase genes has been defined, with all of the encoded proteins sharing a common highly conserved catalytic core domain. DdPIK1, -2, and -3 may have redundant functions because disruption of any single gene had no effect on D. discoideum growth or development. However, strains in which both of the two most highly related genes, DdPIK1 and DdPIK2, were disrupted showed both growth and developmental defects, while double knockouts of DdPIK1 and DdPIK3 and DdPIK2 and DdPIK3 appear to be lethal. The delta Ddpik1 delta Ddpik2 null cells were smaller than wild-type cells and grew slowly both in association with bacteria and in axenic medium when attached to petri plates but were unable to grow in suspension in axenic medium. When delta Ddpik1 delta Ddpik2 null cells were plated for multicellular development, they formed aggregates having multiple tips and produced abnormal fruiting bodies. Antisense expression of DdPIK5 (a putative homolog of the Saccharomyces cerevisiae VPS34) led to a defect in the growth of D. discoideum cells on bacterial lawns and abnormal development. DdPIK5 complemented the temperature-sensitive growth defect of a Schizosaccharomyces pombe delta Svps34 mutant strain, suggesting DdPIK5 encodes a functional homolog of yeast Vps34p. These observations indicate that in D. discoideum, different PI kinases regulate distinct cellular processes, including cell growth, development, and protein trafficking.

Differential effects of spermine on phosphatidylinositol 3-kinase and phosphatidylinositol phosphate 5-kinase

The metabolism of phosphoinositides plays an important role in the signal transduction pathways. We report here that naturally occurring polyamines affect the activities of phosphatidylinositol (PI) 3-kinase and PI 4-phosphate (PIP) 5-kinase differently. While polyamines inhibited the PI 3-kinase activity, they stimulated the activity of PIP 5-kinase in the order of spermine > spermidine > putrescine. Spermine inhibited the PI 3-kinase activity in a concentration-dependent manner with an IC50 of 100 microM. On the other hand, spermine (5 mM) stimulated the activity of PIP 5-kinase 2-3 fold. Kinetic studies of spermine-mediated inhibition of PI 3-kinase revealed that it was noncompetitive with respect to ATP. The effect of Mg2+ and PIP2 concentration on kinase activity was sigmoidal, with spermine inhibiting PI 3-kinase activity at all PIP2 concentrations. While 1 mM calcium stimulated PI 3-kinase activity at submaximal concentrations of Mg2+ (1.25 mM), inhibition was observed at optimal concentration of Mg2+ (2 mM). We propose that spermine may modulate the cellular signal by virtue of its differential effects on phosphoinositide kinases.

Characterization of activators and inhibitors of protein kinase C mu

In order to investigate regulatory mechanisms and to identify potential substrates of a novel member of the protein kinase C (PKC) family, PKC mu, specific antibodies have been raised against unique amino- and carboxy-terminal regions. PKC mu kinase activity was studied upon immunoprecipitation from stably transfected cell lines as well as from the A549 carcinoma cell line expressing the endogenous PKC mu gene. Cell fractionation revealed that PKC mu is predominantly found in the particulate fraction, suggesting an association with the membrane or membrane-bound structures. In vitro kinase assays with immunoprecipitated PKC mu demonstrated a Ca2+ independent enhancement of constitutive autophosphorylation activity by phosphatidylserine. Despite a limited in vitro phorbol ester response, an apparent phorbol ester activation of PKC mu was observed when cell cultures, instead of immunoprecipitated enzyme, were treated with either phorbol 12-myristate 13-acetate or 1,2 dioleoyl-sn-glycerol. Both in vitro autophosphorylation and substrate phosphorylation of myelin basic protein and histone III were enhanced under these conditions. However, long-term treatment with the phorbol ester did not result in downregulation of PKC mu protein levels and kinase activity. Studies with several protein kinase inhibitors revealed a novel sensitivity profile of PKC mu, with no inhibition by calphostin C, reduced sensitivity to staurosporine but, compared to other PKCs, an approximately 60-fold higher sensitivity to the selective PKA inhibitor H89. Together, the data presented here show that localization of PKC mu and regulation of its kinase activity differ from that of other PKCs suggesting a novel function of PKC mu in intracellular signal pathways.

Modulation of murine macrophage nitric oxide synthesis by liposomal phospholipids: correlation with liposome immune adjuvant activity

The influence of alum and liposomal phospholipids on interferon-gamma-(IFN-gamma), IFN-gamma/N-acetylmuramyl-L-alanyl-D-isoglutamine- (MDP) or IFN-gamma/tumor necrosis factor-alpha- (IFN-gamma/TNF-alpha) induced macrophage nitric oxide (NO) synthesis has been investigated. IFN-gamma induced NO synthesis in a dose-dependent manner. TNF-alpha and MDP did not induce NO synthesis, but interacted synergistically with sub-optimal doses of IFN-gamma. Alum strongly inhibited IFN-gamma-induced NO synthesis (ID50 25 microgram/ml). Liposomes composed of dipalmitoylphosphatidylcholine (DPPC) had no effect on IFN-gamma-induced NO synthesis. IFN-gamma-induced NO synthesis was stimulated by DPPC/dimyristoylphosphatidylglycerol (DMPG) liposomes (9:1 mol ratio, ED50 45 nmol phospholipid/ml), and inhibited by DPPC/dipalmitoylphosphatidylethanolamine (DPPE) liposomes (9:1 mol ratio, ID50 > 500 nmol phospholipid/ml), and DPPC/phosphatidylserine (PS) liposomes (7:3 mol ratio, ID50 150 nmol phospholipid/ml). Alum, DPPC/PE and DPPC/PS liposomes also inhibited IFN-gamma/MDP- and IFN-gamma/TNF-alpha-induced NO synthesis. Neither alum or the liposome preparations had significant toxicity towards macrophages in vitro at concentrations that induced maximal inhibition or stimulation of IFN-gamma-induced NO synthesis. Immunization of mice with alum-adsorbed and liposome-incorporated bovine serum albumin (BSA) demonstrated that enhancement or reduction of both IgG antibody and the proportion of IgG2a/IgG2b was correlated with stimulation or inhibition of IFN-gamma-induced NO synthesis.

Phosphatidic acid activation of protein kinase C-zeta overexpressed in COS cells: comparison with other protein kinase C isotypes and other acidic lipids

Phosphatidic acid (PA) is produced rapidly in agonist-stimulated cells, but the physiological function of this PA is unknown. We have examined the effects of PA on distinct isoforms of protein kinase C (PKC) using a new cell-free assay system. Addition of PA to cytosol from COS cells overexpressing PKC-alpha, -epsilon or -zeta differentially-activated all three isotypes, as shown by PKC autophosphorylation, and prominent phosphorylation of multiple endogenous substrates. In the absence of Ca2+, the diacylglycerol-insensitive zeta-isotype of PKC was most strongly activated by both PA and bisPA, a newly identified product of activated phospholipase D, with each lipid inducing its own profile of protein phosphorylation. BisPA was also a strong activator of PKC-epsilon, but a weak activator of PKC-alpha. Ca2+, at > or = 0.1 microM, inhibited PA and bisPA activation of PKC-zeta, but did not affect PKC-epsilon activation. In contrast, PKC-alpha was strongly activated by PA only in the presence of Ca2+. BisPA-induced phosphorylations mediated by PKC-zeta could be mimicked in part by other acidic phospholipids and unsaturated fatty acids. PA activation of PKC-zeta was unique in that PA not only stimulated PKC-zeta-mediated phosphorylation of distinctive substrates, but also caused an upward shift in electrophoretic mobility of PKC-zeta, which was not observed with other acidic lipids or with PKC-alpha or -epsilon. We have presented evidence that this mobility shift is not caused by PKC-zeta autophosphorylation, but it coincides with physical binding of PA to PKC-zeta. These results suggest that in cells stimulated under conditions where intracellular Ca2+ is at (or has returned to) basal level, PA may be a physiological activator of PKC-zeta.

Activation and substrate specificity of the human protein kinase C alpha and zeta isoenzymes

Protein kinase C (PKC), a class of serine/threonine kinases activated by Ca2+ and/or phospholipids, is involved in a variety of cellular processes such as proliferation, differentiation and secretion. Nine members of the PKC gene family are known; these are differentially expressed in eukaryotic cells and can be divided into two sub-groups: the Ca(2+)-dependent (classical) PKC isoenzymes alpha, beta I, beta II and gamma, and the Ca(2+)-independent neoPKC isoenzymes delta, epsilon, zeta, eta and theta. A detailed biochemical characterisation of these PKC isoenzymes is one prerequisite for the elucidation of their distinct roles within cellular signal transduction. In this study, we report the cloning of a human PKC-zeta cDNA, its expression in recombinant baculovirus-infected insect cells and the partial purification of the PKC-zeta isoenzyme. In comparison to highly purified human PKC alpha, a representative of the classical PKC subgroup, purified PKC zeta was characterised with respect to activator requirement, substrate specificity, proteolytic activation and sensitivity towards PKC inhibitors. In contrast to PKC alpha, PKC zeta exhibits a constitutive kinase activity which is independent of Ca2+, phosphatidylserine and diacylglycerol. Arachidonic acid alone or a combination of gamma-linolenic acid and phosphatidylserine slightly enhance PKC zeta activity. In the presence of the classical PKC activators phosphatidylserine/diacylglycerol, PKC alpha phosphorylates a PKC-alpha pseudosubstrate-derived peptide, an epidermal-growth-factor-receptor-derived peptide, histone III-S and myelin basic protein to an equal extent, whilst PKC zeta phosphorylates only the PKC-alpha-derived peptide. However, arachidonic acid greatly diminishes PKC-alpha activity towards the epidermal-growth-factor-receptor-derived peptide, histone III-S and myelin basic protein, but enhances PKC-zeta activity towards the PKC-alpha-derived peptide. These results indicate a possible modulation of substrate specificity of these two PKC isoenzymes by (the binding of) different activators (to their regulatory domains). In the case of PKC zeta, this finding is strengthened by the fact that the epidermal growth factor receptor-derived peptide, which is not a substrate for the holoenzyme, is significantly phosphorylated by a protein fragment generated by limited proteolysis and comprising only the kinase domain. Furthermore, PKC zeta, in contrast to PKC alpha, is insensitive to PKC inhibitors known to interfere either with the regulatory or the catalytic domain and cannot be activated by phorbol ester treatment of NIH 3T3 cells or insect cells, overexpressing the respective PKC isoenzyme. The potential implications of these findings on the mechanism(s) of activation and the substrate specificity of PKC zeta are discussed.

Protein kinase C isoenzymes: divergence in signal transduction?

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