Role of substrate in determining the phospholipid specificity of protein kinase C activation

Ex vivo editing of human hematopoietic stem cells for erythroid expression of therapeutic proteins

Targeted genome editing has a great therapeutic potential to treat disorders that require protein replacement therapy. To develop a platform independent of specific patient mutations, therapeutic transgenes can be inserted in a safe and highly transcribed locus to maximize protein expression. Here, we describe an ex vivo editing approach to achieve efficient gene targeting in human hematopoietic stem/progenitor cells (HSPCs) and robust expression of clinically relevant proteins by the erythroid lineage. Using CRISPR-Cas9, we integrate different transgenes under the transcriptional control of the endogenous α-globin promoter, recapitulating its high and erythroid-specific expression. Erythroblasts derived from targeted HSPCs secrete different therapeutic proteins, which retain enzymatic activity and cross-correct patients’ cells. Moreover, modified HSPCs maintain long-term repopulation and multilineage differentiation potential in transplanted mice. Overall, we establish a safe and versatile CRISPR-Cas9-based HSPC platform for different therapeutic applications, including hemophilia and inherited metabolic disorders.

A platform for systemic therapeutic transgene expression independent of patient mutations needs a safe and highly transcribed locus. Here the authors ex vivo edit HPSCs using CRISPR-Cas9 to integrate transgenes under the α-globin promoter to achieve erythroid specific expression.

Phosphatidylinositol 4,5-bisphosphate decreases the concentration of Ca2+, phosphatidylserine and diacylglycerol required for protein kinase C α to reach maximum activity

The C2 domain of PKCα possesses two different binding sites, one for Ca(2+) and phosphatidylserine and a second one that binds PIP2 with very high affinity. The enzymatic activity of PKCα was studied by activating it with large unilamellar lipid vesicles, varying the concentration of Ca(2+) and the contents of dioleylglycerol (DOG), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphadidylserine (POPS) in these model membranes. The results showed that PIP2 increased the Vmax of PKCα and, when the PIP2 concentration was 5 mol% of the total lipid in the membrane, the addition of 2 mol% of DOG did not increase the activity. In addition PIP2 decreases K0.5 of Ca(2+) more than 3-fold, that of DOG almost 5-fold and that of POPS by a half. The K0.5 values of PIP2 amounted to only 0.11 µM in the presence of DOG and 0.39 in its absence, which is within the expected physiological range for the inner monolayer of a mammalian plasma membrane. As a consequence, PKCα may be expected to operate near its maximum capacity even in the absence of a cell signal producing diacylglycerol. Nevertheless, we have shown that the presence of DOG may also help, since the K0.5 for PIP2 notably decreases in its presence. Taken together, these results underline the great importance of PIP2 in the activation of PKCα and demonstrate that in its presence, the most important cell signal for triggering the activity of this enzyme is the increase in the concentration of cytoplasmic Ca(2+).

Inhibition of Chk1 by the G2 DNA damage checkpoint inhibitor isogranulatimide

Inhibitors of the G2 DNA damage checkpoint can selectively sensitize cancer cells with mutated p53 to killing by DNA-damaging agents. Isogranulatimide is a G2 checkpoint inhibitor containing a unique indole/maleimide/imidazole skeleton identified in a phenotypic cell-based screen; however, the mechanism of action of isogranulatimide is unknown. Using natural and synthetic isogranulatimide analogues, we show that the imide nitrogen and a basic nitrogen at position 14 or 15 in the imidazole ring are important for checkpoint inhibition. Isogranulatimide shows structural resemblance to the aglycon of UCN-01, a potent bisindolemaleimide inhibitor of protein kinase Cβ (IC50, 0.001 μmol/L) and of the checkpoint kinase Chk1 (IC50, 0.007 μmol/L). In vitro kinase assays show that isogranulatimide inhibits Chk1 (IC50, 0.1 μmol/L) but not protein kinase Cβ. Of 13 additional protein kinases tested, isogranulatimide significantly inhibits only glycogen synthase kinase-3β (IC50, 0.5 μmol/L). We determined the crystal structure of the Chk1 catalytic domain complexed with isogranulatimide. Like UCN-01, isogranulatimide binds in the ATP-binding pocket of Chk1 and hydrogen bonds with the backbone carbonyl oxygen of Glu85 and the amide nitrogen of Cys87. Unlike UCN-01, the basic N15 of isogranulatimide interacts with Glu17, causing a conformation change in the kinase glycine-rich loop that may contribute importantly to inhibition. The mechanism by which isogranulatimide inhibits Chk1 and its favorable kinase selectivity profile make it a promising candidate for modulating checkpoint responses in tumors for therapeutic benefit.

Regulation of PKC alpha activity by C1-C2 domain interactions

In this study, the role of interdomain interactions involving the C1 and C2 domains in the mechanism of activation of PKC was investigated. Using an in vitro assay containing only purified recombinant proteins and the phorbol ester, 4 beta-12-O-tetradecanoylphorbol-13-acetate (TPA), but lacking lipids, it was found that PKC alpha bound specifically, and with high affinity, to a alpha C1A-C1B fusion protein of the same isozyme. The alpha C1A-C1B domain also potently activated the isozyme in a phorbol ester- and diacylglycerol-dependent manner. The level of this activity was comparable with that resulting from membrane association induced under maximally activating conditions. Furthermore, it was found that alpha C1A-C1B bound to a peptide containing the C2 domain of PKC alpha. The alpha C1A-C1B domain also activated conventional PKC beta I, -beta II, and -gamma isoforms, but not novel PKC delta or -epsilon. PKC delta and -epsilon were each activated by their own C1 domains, whereas PKC alpha, -beta I, -beta II, or -gamma activities were unaffected by the C1 domain of PKC delta and only slightly activated by that of PKC epsilon. PKC zeta activity was unaffected by its own C1 domain and those of the other PKC isozymes. Based on these findings, it is proposed that the activating conformational change in PKC alpha results from the dissociation of intra-molecular interactions between the alpha C1A-C1B domain and the C2 domain. Furthermore, it is shown that PKC alpha forms dimers via inter-molecular interactions between the C1 and C2 domains of two neighboring molecules. These mechanisms may also apply for the activation of the other conventional and novel PKC isozymes.

Lipid-dependent activation of protein kinase C-alpha by normal alcohols

Significant stimulation of protein kinase C-alpha (PKCalpha) by n-alcohols was observed in characterized lipid systems composed of phosphatidylcholine/phosphatidylserine/dioleoylglycerol (PC/PS/DO). The logarithm of the alcohol concentrations to achieve half-maximal PKC stimulation (ED(50)) and of the maximal PKC stimulation by alcohols were both linear functions of alcohol chain length, consistent with the Meyer-Overton effect. Binding of phorbol esters to PKC was not significantly affected by octanol. Octanol increased, up to 4-fold, the affinity of PKC binding to the lipid bilayers in both the absence and presence of DO. However, octanol increased PKC activity much more significantly than it enhanced binding of the enzyme to the lipid bilayers, suggesting that the stimulation of PKC is not merely a reflection of the increase in PKC bilayer binding affinity. (31)P NMR experiments did not reveal formation of non-lamellar phases with octanol. Differential scanning calorimetry suggested that alcohols, like diacylglycerol, induce formation of compositionally distinct domains and the maximal enzyme activity with alcohol resided roughly in the putative domain-coexistence region. These results suggest that alcohols are mimicking diacylglycerol in activating PKC, not by binding to the high affinity phorbol ester binding site, but by altering lipid structure and by enhancing PKC-bilayer binding.

Inhibition of membrane lipid-independent protein kinase Calpha activity by phorbol esters, diacylglycerols, and bryostatin-1

The activity of membrane-associated protein kinase C (PKC) has previously been shown to be regulated by two discrete high and low affinity binding regions for diacylglycerols and phorbol esters (Slater, S. J., Ho, C., Kelly, M. B., Larkin, J. D., Taddeo, F. J., Yeager, M. D., and Stubbs, C. D. (1996) J. Biol. Chem. 271, 4627-4631). PKC is also known to interact with both cytoskeletal and nuclear proteins; however, less is known concerning the mode of activation of this non-membrane form of PKC. By using the fluorescent phorbol ester, sapintoxin D (SAPD), PKCalpha, alone, was found to possess both low and high affinity phorbol ester-binding sites, showing that interaction with these sites does not require association with the membrane. Importantly, a fusion protein containing the isolated C1A/C1B (C1) domain of PKCalpha also bound SAPD with low and high affinity, indicating that the sites may be confined to this domain rather than residing elsewhere on the enzyme molecule. Both high and low affinity interactions with native PKCalpha were enhanced by protamine sulfate, which activates the enzyme without requiring Ca2+ or membrane lipids. However, this "non-membrane" PKC activity was inhibited by the phorbol ester 4beta-12-O-tetradecanoylphorbol-13-acetate (TPA) and also by the fluorescent analog, SAPD, opposite to its effect on membrane-associated PKCalpha. Bryostatin-1 and the soluble diacylglycerol, 1-oleoyl-2-acetylglycerol, both potent activators of membrane-associated PKC, also competed for both low and high affinity SAPD binding and inhibited protamine sulfate-induced activity. Furthermore, the inactive phorbol ester analog 4alpha-TPA (4alpha-12-O-tetradecanoylphorbol-13-acetate) also inhibited non-membrane-associated PKC. In keeping with these observations, although TPA could displace high affinity SAPD binding from both forms of the enzyme, 4alpha-TPA was only effective at displacing high affinity SAPD binding from non-membrane-associated PKC. 4alpha-TPA also displaced SAPD from the isolated C1 domain. These results show that although high and low affinity phorbol ester-binding sites are found on non-membrane-associated PKC, the phorbol ester binding properties change significantly upon association with membranes.

Evidence for a regulatory binding site for arginine-rich peptides on protein kinase C

The peptides N-biotinyl-RRRCLRRL and N-biotinyl-RKRCLRRL covalently modify protein kinase C (PKC) through reaction of the Cys sulfhydryl group with the active site of the enzyme. The labeling of PKC occurs only in the presence of the cofactors phosphatidylserine, diacylglycerol, and Ca2+ but not in their absence. Low concentrations of the Arg-rich substrate, R4YGSR6Y greatly increase the extent of the reaction of these biotinylated peptides with PKC in the presence of lipid cofactors but in the absence of calcium. This effect can be observed at 50 nM R4YGSR6Y and suggests the presence of a high-affinity binding site for Arg-rich peptides which is separate from the active site but which enhances accessibility of the active site. The study also demonstrates the utility of the biotinylated peptides as active site labels which can detect the conformational change accompanying the activation of PKC.

Does Ca2+ channel blockade modulate the antidepressant-induced changes in mechanisms of adrenergic transduction?

We investigated how the L-type calcium channel blockade (CCB) with nifedipine affects the cyclic AMP responses to noradrenaline or isoproterenol in cerebral cortical slices from rats receiving antidepressant treatments that induce (electroconvulsive shock, imipramine) or do not induce (amitriptyline) beta-downregulation. To assess the role of protein kinase C (PKC) in receptor crosstalk under CCB conditions, the cyclic AMP responses were tested also in the presence of a PKC activator, TPA. CCB alone induced no changes, but modulated the action of those antidepressants that down regulate the beta-adrenergic system. Chronic ECS and imipramine treatments were differently affected. ECS, under conditions of CCB, down regulated the response to isoproterenol in the presence of TPA, while imipramine ceased to block the TPA-potentiation of cyclic AMP responses. Thus, CCB affects the processes related to the antidepressant-induced changes on the crosstalk between alpha1- and beta-adrenergic receptors, depending on the specific properties of the antidepressant.

Conformation of a protein kinase C substrate NG(28-43), and its analog in aqueous and sodium dodecyl sulfate micelle solutions

A peptide corresponding to the neuronal protein neurogranin (NG) residues 28-43, NG(28-43), and its analog, [A35]NG(28-43), have been investigated by NMR, electron paramagnetic resonance (EPR), and circular dichroism (CD) spectroscopies. The peptides existed in aqueous solution predominantly in random form. However, a nascent helical structure was detected in the central region of the parent peptide from NMR data. Furthermore, a helical structure can be detected for both peptides with greater induced secondary structure for the parent peptide in the presence of sodium dodecyl sulfate (SDS) micelle. The formation of micelles for SDS was confirmed by results from EPR as well as 13C NMR. As shown by CD experiments, helical conformer was induced for NG(28-43) in vesicular solution containing phosphatidyl serine (PS), whereas no helix can be discerned for the peptide in phosphatidyl choline (PC)-containing vesicular solution. Together with the induction of the peptide into helix in SDS micellar solution as suggested by both NMR and CD data, these results underscored the electrostatic contribution to the interaction of the PKC substrate peptides and proteins with membrane. According to NMR and CD data, a dynamic equilibrium existed between free and micelle-bound states for the peptide. Moreover, proton-deuterium exchange results and SDS-induced linewidth broadening of proton resonances allowed delineation of the orientation of the amphipathic helix on the surface of SDS micelle. The result was supported by spin label experiments that indicated F35 of NG(28-43) interacted strongly with the hydrocarbon interior of micelle. Based on the experimental findings, a working model was proposed that attempted to partly explain the roles played by the nonpolar amino acid near the phosphorylation site, by the negatively charged phospholipids, and by the basic amino acids of the substrate.

Protein kinase C in IL-2 signal transduction

Interleukin-2 (IL-2), secreted principally by activated helper T-cells, plays a pivotal role in the generation and regulation of the immune response. The various biologic functions of IL-2 have been the focus of intensive study over the years and have been well worked out. By contrast, an understanding of the intracellular signals coupled to the IL-2 receptor and responsible for mediating IL-2 effects in T-cells is far less developed, and the role that protein kinase C (PKC) may play in the various cellular responses to IL-2 receptor activation is unclear. In this article we will discuss IL-2, its receptors, and IL-2 signal transduction in relation to the physiological roles PKC activation may play in IL-2-mediated activation of T-cells and other hematopoietic cells.

Selective phosphorylation of cationic polypeptide aggregated with phosphatidylserine/diacylglycerol/Ca2+/detergent mixed micelles by Ca(2+)-independent but not Ca(2+)-dependent protein kinase C isozymes

Mixed micelles containing Nonidet P40 (NP-40) (829 microM or 4.8 mM), phosphatidylserine (PS) (14.5 or 8 mol%), and 1,2-diacylglycerol (DG) (0.5 or 1 mol%) when preincubated with protein kinase C (PKC) assay mixture containing cationic substrate and CaCl2 (400 microM) formed aggregates in a time-, temperature-, and substrate concentration-dependent manner with a t1/2 approximately 3-12 min (22 degrees C). Concomitant with the formation of these aggregates there was a substantial loss of substrate phosphorylation catalyzed by the Ca(2+)-dependent PKC alpha, beta, and gamma but not the Ca(2+)-independent PKC, delta and epsilon. All cationic PKC substrates tested, neurogranin peptide analog, neurogranin, and histone III-S, formed aggregates with PS/DG/NP-40/Ca2+ mixed micelles in a time-dependent fashion. The poly(cationic-anionic) PKC substrate protamine sulfate also forms aggregates with the mixed micelles in the presence of Ca2+, but without affecting the substrate phosphorylation by the kinase. Under similar conditions, but at 4 degrees C, neither aggregation nor loss of cationic substrate phosphorylation was observed. Another nonionic detergent, octyl glucoside, behaved similarly to NP-40. Phosphatidylinositol (PI) and phosphatidylglycerol like PS, were effective in forming aggregates with NP-40/cationic polypeptide/DG/Ca2+ as monitored by light scattering, yet without affecting substrate phosphorylation. Phosphorylation of cationic substrates by M-kinase, derived from trypsinized PKC beta, was also greatly diminished by the aggregation. In contrast, [3H]phorbol 12,13-dibutyrate binding to PKC beta was unaffected. Formation of the aggregates that were selectively utilized by the Ca(2+)-independent PKCs was dependent on the ratio of cationic substrate to the number of mixed micelles.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane components can modulate the substrate specificity of protein kinase C

The cationic amphiphile, cholesteryl-3 beta-carboxyamidoethylene-trimethylammonium iodide, can alter the substrate specificity of protein kinase C (PKC). The phosphorylation of histone catalyzed by PKC requires the binding of the enzyme to phospholipid vesicles. This cationic amphiphile reduces both the binding of PKC to lipid and as a consequence its rate of phosphorylation of histone. In contrast, PKC bound to large unilamellar vesicles (LUVs) composed of 50 mol% POPS, 20 mol% POPC, and 30 mol% of this amphiphile catalyzes protamine sulfate phosphorylation by an almost 4 fold greater rate. This activation requires phosphatidylserine (PS) and is inhibited by Ca2+. The extent of activation is affected by the time of incubation of PKC with LUVs. This data suggests a novel mechanism by which PKC-dependent signal transduction pathways may be altered by altering the protein targets of this enzyme.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Differential effects of fatty acid and phospholipid activators on the catalytic activities of a structurally novel protein kinase from rat liver

The lipid responsiveness of the structurally unique protein kinase, referred to as PAK-1, recently isolated from rat liver [(1994) J. Biol. Chem. 269, in press], is characterised by the high sensitivity (low micromolar) of its ribosomal S6(229-239) peptide kinase activity to both cardiolipin and the cis-unsaturated fatty acids and insensitivity to phosphatidylserine. Autophosphorylation of PAK-1 exhibited even greater sensitivity (submicromolar) to cardiolipin, but was relatively less affected by phosphatidylserine. Oleate, the most potent activator of PAK-1's peptide kinase activity was relatively ineffectual with autophosphorylation. These and other unusual characteristics, including high levels of basal catalytic activities, suggest a novel mechanism of regulation distinct from that of the protein kinase Cs.

Inhibition of protein kinase C of human neutrophils by phosphatidylcholines

Pulmonary surfactant phospholipids, particularly phosphatidylcholines (PCs), inhibit superoxide generation in neutrophils. The activation of neutrophils is in part dependent on protein kinase C (PKC). To investigate the mechanism of action of PCs in inhibition of neutrophil activation, we studied the effect of PCs, commonly found in pulmonary surfactant, on the distribution of PKC in intact resting and stimulated neutrophils as well as on their kinase activity in vitro. In general, in contrast to PCs with saturated fatty acyl moieties, PCs with unsaturated fatty acyl moieties inhibited PKC activity in vitro. To determine the effect of PCs on the activation of PKC in intact neutrophils, neutrophils preincubated with PCs for 2 hours were stimulated by 1,2-sn-dioctanoylglycerol (diC8) or 4-beta-phorbol 12-beta-myristate 13-alpha-acetate (PMA). The cytosolic PKC activity did not change in cells preincubated with PCs after stimulation with diC8 (3.76 +/- 0.83 units vs 3.90 +/- 1.48 units), as would be expected in control cells (3.06 +/- 0.41 units to 1.01 +/- 0.29 units). In contrast to diC8, PMA-induced translocation of PKC to the membrane was unaffected by PCs. These data suggest that specific PCs not only inhibit neutrophil PKC in vitro but can also affect its translocation in response to stimulation by diacylglycerol in intact neutrophils.

Protein kinase C and annexins: unusual calcium response elements in the cell

Protein kinase C and the annexins appear to share some unusual and potentially important membrane- and calcium-binding properties. While these proteins are calcium response elements, they are not calcium-binding proteins in the formal sense; at intracellular calcium concentrations, they only bind significant amounts of calcium when membranes or other suitable surfaces are present. The number of calcium ions bound per protein is large (> 8) and this stoichiometry, at the protein-membrane interface, may provide the large number of contact points needed for the very high-affinity interaction that is observed. The further ability of annexins and PKC to form structures with properties of integral membrane proteins may be important to provide a type of long-term cell signalling that produces a constitutively active kinase or ion channel activity. Selectivity for phospholipids in bilayer form is modest with respect to the acidic phospholipids but there is a surprising preference for phosphatidylethanolamine as the neutral phospholipid matrix. Along with other unusual properties, these proteins offer the potential for unique types of cell regulation events.

Sphingolipid metabolism and signal transduction: inhibition of in vitro phospholipase activity by sphingosine

Sphingosine inhibits protein kinase C activity in vitro and has been used to implicate this enzyme in signal transduction and cell function. We report that sphingosine directly inhibits phospholipases A2 and D. Sphingosine inhibits Ca(2+)-dependent phospholipases A2 from Naja naja, porcine pancreas, Crotalus adamanteus, human disc and neutrophil in a dose-dependent manner with IC50 values ranging from 5-40 microM using [1-14C]oleate-labelled autoclaved E. coli (20 microM) as substrate. Inhibition is comparable using the same concentrations (20 microM) of [1-14C]oleate-labelled C. albicans or E. coli, or aqueous dispersions of 1-acyl-2-[1-14C]linoleoylglycerophosphoethanolamine or -choline. Sphinganine and stearylamine are as inhibitory as sphingosine; monoolein is less inhibitory (IC50 = 70 microM), while octylamine, N-acetylsphingosine, sphingomyelin and ceramide have no effect. Inhibition is relieved by increasing concentrations of substrate phospholipid. The molar ratio of sphingosine to phospholipid required for 50% inhibition ranges from 0.5 to 1.0 with 2-100 microM E. coli phospholipid. In contrast, sphingosine has a biphasic effect on the hydrolysis of E. coli by S. chromofuscus phospholipase D; concentrations less than or equal to 25 microM stimulate activity while concentrations greater than 25 microM are inhibitory. Addition of Triton X-100 eliminates both the stimulatory and inhibitory effects of sphingosine on phospholipase D activity.

Importance of phosphatidylethanolamine for association of protein kinase C and other cytoplasmic proteins with membranes

Biological membranes exhibit an asymmetric distribution of phospholipids. Phosphatidylserine (PS) is an acidic phospholipid that is found almost entirely on the interior of the cell where it is important for interaction with many cellular components. A less well understood phenomenon is the asymmetry of the neutral phospholipids, where phosphatidylcholine (PC) is located primarily on exterior membranes while phosphatidylethanolamine (PE) is located primarily on interior membranes. The effect of these neutral phospholipids on protein-phospholipid associations was examined using four cytoplasmic proteins that bind to membranes in a calcium-dependent manner. With membranes containing PS at a charge density characteristic of cytosolic membranes, protein kinase C and three other proteins with molecular masses of 64, 32, and 22 kDa all showed great selectively for membranes containing PE rather than PC as the neutral phospholipid; the calcium requirements for membrane-protein association of the 64- and 32-kDa proteins were about 10-fold lower with membranes containing PE; binding of the 22-kDa protein to membranes required the presence of PE and could not even be detected with membranes containing PC. Variation of the PS/PE ratio showed that membranes containing about 20% PS/60% PE provided optimum conditions for binding and were as effective as membranes composed of 100% PS. Thus, PE, as a phospholipid matrix, eliminated the need for membranes with high charge density and/or reduced the calcium concentrations needed for protein-membrane association. A surprising result was that PKC and the 64- and 32-kDa proteins were capable of binding to neutral membranes composed entirely of PE/PC or PC only. The different phospholipid headgroups altered only the calcium required for membrane-protein association. For example, calcium concentrations at the midpoint for association of the 64-kDa protein with membranes containing PS, PE/PC, or PC occurred at 6, 100, and 20,000 microM, respectively. Thus, biological probes detected major differences in the surface properties of membranes containing PE versus PC, despite the fact that both of these neutral phospholipids are often thought to provide "inert" matrices for the acidic phospholipids. The selectivity for membranes containing PE could be a general phenomenon that is applicable to many cytoplasmic proteins. The present study suggested that the strategic location of PE on the interior of the membranes may be necessary to allow some membrane-protein associations to occur at physiological levels of calcium and PS.

Lipid-dependent membrane enzymes. Kinetic modelling of the activation of protein kinase C by phosphatidylserine

A previously developed kinetic theory for lipid-dependent membrane enzymes (Sandermann, H. (1982) Eur. J. Biochem. 127, 123-128) is used to examine the activation of protein kinase C by phosphatidylserine. Hill-coefficients ranging up to 11 have been reported for activation in mixed micelles with Triton X-100. On the basis of this uniquely high degree of cooperativity, protein kinase C has been postulated to represent a new class of lipid-dependent membrane enzymes (Newton, A. and Koshland, D.E., Jr. (1989) J. Biol. Chem. 264, 14909-14915). In contrast, activation in the absence of Triton X-100 has led to Hill-coefficients of only less than or equal to 2.6. In order to resolve the apparent discrepancy, activation is now considered to involve binding of PS monomers to interacting sites on the enzyme, a non-activating PS trapping process also occurring in the presence of Triton X-100. Estimates for trapping are made for several sets of published data for micellar activation. The kinetic model developed here successfully fits each data set using a Hill-coefficient of only 3.0. An influence of Ca2+/ions or of a two-step mechanism of lipid-protein interaction are considered as possible molecular explanations. It is concluded (i) that lipid activation of protein kinase C may proceed without unique cooperativity and (ii) that ligand trapping could provide another means for 'threshold-type' kinetic regulation of membrane enzyme and receptor systems.

Different mechanisms of beta-adrenoceptor down-regulation by chronic imipramine and electroconvulsive treatment: possible role for protein kinase C

The aim of this study was to find out how protein kinase C (PKC) is involved in down-regulation of the beta-adrenoceptor in cortical slices of rats subjected to antidepressant treatments. The responses of the cyclic AMP generating system to forskolin, isoproterenol, and noradrenaline were tested in the absence and presence of a PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA). The antidepressive treatments applied were chronic administration of imipramine and electroconvulsive shock. The potentiating effect of the phorbol ester on cyclic AMP response to isoproterenol was retained in imipramine-treated animals and even accentuated in rats subjected to electroconvulsive treatment; the TPA effect on noradrenaline-induced cyclic AMP response was blunted in rats receiving imipramine, but augmented in those receiving electroconvulsive treatment. In imipramine-treated rats the beta-down-regulation was still evident in the presence of TPA; after electroconvulsive treatment the phorbol ester-induced potentiation was so high that no significant beta-down-regulation could be observed. No procedure affected the response to forskolin. The beta-down-regulation that develops during chronic imipramine treatment differs from that caused by chronic electroconvulsive treatment; in both cases it is not related to the direct effect on adenylate cyclase.

Polyphosphoinositides as activators of PKC-dependent synapsin I phosphorylation

The effect of PIP2 and diacylglycerol (products of polyphosphoinositide turnover) on the activation level of phosphorylation of the human brain neurospecific protein SI by PKC from the same source was studied. The apparent activation constant of the phosphorylation process was shown to decrease in the presence of PIP2 from 1.1 micrograms/ml for PI and from 0.8 micrograms/ml to 0.6 microgram/ml for PS; the value of 0.4 microgram/ml in the latter case was detected merely after the addition of DOG into the reaction mixture. Polyphosphoinositides are suggested to play a role in activating PKC-mediated phosphorylation of SI in nerve terminals.

Identification of intracellular receptor proteins for activated protein kinase C

Protein kinase C (PKC) translocates from the cytosol to the particulate fraction on activation. This activation-induced translocation of PKC is thought to reflect PKC binding to the membrane lipids. However, immunological and biochemical data suggest that PKC may bind to proteins in the cytoskeletal elements in the particulate fraction and in the nuclei. Here we describe evidence for the presence of intracellular receptor proteins that bind activated PKC. Several proteins from the detergent-insoluble material of the particulate fraction bound PKC in the presence of phosphatidylserine and calcium; binding was further increased with the addition of diacylglycerol. Binding of PKC to two of these proteins was concentration-dependent, saturable, and specific, suggesting that these binding proteins are receptors for activated C-kinase, termed here "RACKs." PKC binds to RACKs via a site on PKC distinct from the substrate binding site. We suggest that binding to RACKs may play a role in activation-induced translocation of PKC.

Activation and regulation of protein kinase C enzymes

Protein Kinase C (PKC) has been a principal regulatory enzyme whose function has been intensely investigated in the past decade. The primary features of this family of enzymes includes phosphorylation of serine and threonine residues located on basic proteins and peptide in a manner that is stimulated by calcium, phospholipid, and either diacylglycerol or phorbol esters. An additional intriguing feature of the enzymes is its ability to form two membrane-associated states, one of which is calcium dependent and reversible and the second is an irreversible complex which has the characteristics of an intrinsic membrane protein. Formation of the irreversible membrane-bound form is greatly facilitated by calcium and the tumor-promoting phorbol esters but does not appear to include covalent changes in the PKC structure. The intrinsic membrane-bound form is a very different enzyme in that its activity is no longer dependent on the other cofactors. It is proposed that formation of the irreversible membrane-bound form may be a mechanism for generating long-term cell regulation events where transient cell signals and second messengers induce long-term changes in the distribution of an enzyme in the cell. This property may be common to a number of regulatory proteins that are known to be distributed between the cytosol and membrane-fractions in the cell. Unfortunately, many problems have confronted study of PKC mechanism using the in vitro assay. This assay involves aggregation of the substrate, phospholipid, and enzyme to form a discontinuous mixture. Such a complex system prevents straightforward interpretation of enzyme kinetic data. Although many compounds affect the in vitro activity of PKC, most appear to accomplish this by relatively uninteresting mechanisms such as interference with the aggregation process. While some highly potent inhibitors undoubtedly interact directly with PKC, they also inhibit other enzymes and there are no entirely specific inhibitors of PKC known. Speculation on the possible roles of PKC in cell regulation are abundant and exciting. However, delineation of the regulatory roles of PKC may require another decade of intense effort.

Protein kinase C and regulation of the local competence of Xenopus ectoderm

The limited competence of embryonic tissue to respond to an inductive signal has an essential, regulatory function in embryonic induction. The molecular basis for the competence of Xenopus ectoderm to differentiate into neural tissue was investigated. Dorsal mesoderm or 12-O-tetradecanoyl phorbol-13-acetate (TPA) caused in vivo activation of protein kinase C (PKC) and neural differentiation mainly in dorsal ectoderm and to a lesser extent in ventral ectoderm. These data correlate with the observations that PKC preparations from dorsal and ventral ectoderm differ, the dorsal PKC preparation being more susceptible to activation by TPA and diolein than is the ventral PKC preparation. Monoclonal antibodies against the bovine PKC alpha plus beta or gamma isozymes immunostained dorsal and ventral ectoderm, respectively, which suggests different localizations of PKC isozymes. These results suggest that PKC participates in the establishment of embryonic competence.

Ether lipid (AMG) exhibits both synergistic and inhibitory interactions with the ionophore A23187 in mast cell histamine release

Antineoplastic ether lipids with the structure 1-O-long-chain-alkyl-2-O-methylglycero-3-phosphocholine (AMG-PC) have direct tumour cytotoxic as well as immunomodulatory effects. Their tumouricidal action has been related to protein kinase C inhibition by the dialkylglycerol metabolite (AMG). The present investigation explores the influence of AMG (1-O-hexadecyl-2-O-methyl-sn-glycerol) on histamine release from isolated rat mast cells, which have a well-characterized response to protein kinase C activators. AMG could both enhance and antagonize responses to the ionophore A23187 and to A23187 in combination with the phorbol ester TPA. The synergistic effect was maximum at 2-5 microM AMG and could increase the response to A23187 more than 10-fold. Maximal inhibitory effect was found after preincubation with 20 microM AMG, irrespective of the ionophore concentration and the presence of TPA. The synergistic effect of AMG was dependent on energy and calcium, indicating non-cytotoxic mechanisms. The interaction between AMG and A23187 resembles previous findings with TPA and suggests an activation of protein kinase C.

Protein phosphorylation associated with the stimulation of neutrophils. Modulation of superoxide production by protein kinase C and calcium

Neutrophils and other phagocytic cells of the immune system possess a superoxide-generating oxidase system which is essential for the efficient killing of microbes. The system is activated by a wide variety of stimuli, some of which operate through pathways involving protein kinase C (PKC), while others appear not to. The PKC-dependent pathway is probably the major signal transduction route for most of the stimuli. Alterations in cellular Ca2+ and diglyceride levels can have a pronounced stimulatory effect on this pathway by their ability to synergistically activate PKC. This review discusses PKC, the different interactions of this kinase with the plasmalemma that are important in superoxide production, the synergy between Ca2+ and diglyceride, and the nature of the phosphoproteins involved. Evidence supporting the existence of the PKC-independent pathway is also reviewed.

Electrostatics of phosphoinositide bilayer membranes. Theoretical and experimental results

We made fluorescence, electron paramagnetic resonance (EPR), electrophoretic mobility, and ionizing electrode measurements to study the effect of the monovalent lipid phosphatidylinositol (PI) and the trivalent lipid phosphatidylinositol 4,5-bisphosphate (PIP2) on the electrostatic potential adjacent to bilayer membranes. When the membranes were formed from mixtures of PI and the zwitterionic lipid phosphatidylcholine (PC), the Gouy-Chapman-Stern (GCS) theory described adequately the dependence of potential on distance (0, 1, 2 nm) from the membrane, mole % negative lipid, and [KCI]. Furthermore, all EPR and fluorescence probes reported identical surface potentials with a PC/PI membrane. With PC/PIP2 membranes, however, the anionic (coion) probes reported less negative potentials than the cationic (counterion) probes; the deviations from the GCS theory were greater for the coions than the counterions. Discreteness-of-charge theories based on the Poisson-Boltzmann equation incorrectly predict that deviations from the GCS theory should be greater for counterions than for coions. We discuss a consistent statistical mechanical theory that takes into account three effects ignored in the GCS theory: the finite size of the ions in the double layer, the electrical interaction between pairs of ions (correlation effects), and the mobile discrete nature of the surface charges. This theory correctly predicts that deviations from GCS theory should be negligible for monovalent lipids, significant for trivalent lipids, and greater for coions than for counterions.

Distribution of tumor promoters in lipid membranes and changes in membrane structure

The interaction of tumor promoters differing in molecular structure, namely, 12-O-tetradecanoylphorbol 13-acetate (TPA) and teleocidin, with dipalmitoylphosphatidylcholine (DPPC) vesicles was studied. Investigation by Fourier transform infrared spectroscopy clarified the differences between the tumor promoters in the mode of interaction with lipid bilayer membranes. The temperature dependence of the bandwidth of the C-H or C = O stretching absorption of lipid molecules in the presence of tumor promoters relative to that in pure DPPC vesicles indicated that TPA is incorporated into the hydrophobic core of the lipid bilayer membrane whilst teleocidin binds predominantly to the membrane surface. However, both tumor promoters tend to restrict the motion of lipid molecules in membranes. The same conclusion was derived from measurements of steady-state fluorescence polarization, which showed that tumor promoters decreased the membrane fluidity. On the other hand, carboxyfluorescein (CF) leakage from vesicles was enhanced by the addition of TPA below the phase-transition temperature, whereas the effect of teleocidin on steady-state CF leakage was not as significant. It is considered that the difference in the profile of the TPA-induced increase in CF leakage compared to that of teleocidin might be ascribable to a different binding site for each tumor promoter in the membranes.

Structural requirements for long-chain (sphingoid) base inhibition of protein kinase C in vitro and for the cellular effects of these compounds

Sphingosine, sphinganine, and other long-chain (sphingoid) bases inhibit protein kinase C in vitro and block cellular responses to agonists that are thought to act via this enzyme. To gain further insight into the mechanism of this inhibition, a series of long-chain analogues differing in alkyl chain length (11-20 carbon atoms), stereochemistry, and headgroup were examined for (a) inhibition of protein kinase C activity in vitro, (b) the neutrophil respiratory burst in response to phorbol myristate acetate (PMA), (c) the PMA-induced differentiation of HL-60 cells, and (d) the growth of Chinese hamster ovary cells. In every instance, the effects were maximal with the 18-carbon homologues, which are the same length as the predominant naturally occurring long-chain base (sphingosine). The lower potency of the shorter chain homologues was partially due to decreased uptake by cells. Small differences were obtained with the four stereoisomers of sphingosine (i.e., D and L forms of erythro- and threo-sphingosine), with N-methyl derivatives of the different sphingosine homologues, and with simpler alkylamines (e.g., stearylamine). The potency of the different headgroup analogues may be affected by the degree of protonation at the assay pH. The pKa of sphingosine was measured to be 6.7; the pKa varied among the analogues. These findings establish that the major structural features required for inhibition of protein kinase C and cellular processes dependent on this enzyme are the presence of a free amino group and an aliphatic side chain and that other groups have more subtle effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cis-unsaturated fatty acids on aortic protein kinase C activity

Long-chain cis-unsaturated fatty acids could substitute for phosphatidylserine and activate bovine aortic protein kinase C in assays with histone as substrate. The optimal concentration was 24-40 microM for oleic, linoleic and arachidonic acids. With arachidonic acid, the Ka for Ca2+ was 130 microM and kinase activity was maximal at 0.5 mM-Ca2+. Diolein only slightly activated the oleic acid-stimulated enzyme at low physiological Ca2+ concentrations (0.1 and 10 microM). Oleic acid also stimulated kinase C activity, determined with a Triton X-100 mixed-micellar assay. Under these conditions, the fatty acid activation was absolutely dependent on the presence of diolein, but a Ca2+ concentration of 0.5 mM was still required for maximum kinase C activity. The effect of fatty acids on protein kinase C activity was also investigated with the platelet protein P47 as a substrate, since the properties of kinase C can be influenced by the choice of substrate. In contrast with the results with histone, fatty acids did not stimulate the phosphorylation of P47 by the aortic protein kinase C. Activation of protein kinase C by fatty acids may allow the selective phosphorylation of substrates, but the physiological significance of fatty acid activation is questionable because of the requirement for high concentrations of Ca2+.

Activation of protein kinase C by short chain phospholipid micelles

PKC (80 kDa) can be cleaved by limited proteolysis into distinct catalytic (50 kDa) and regulatory (32-35 kDa) fragments. After cleavage, the catalytic fragment is active in the absence of Ca2+, phospholipid, or DAG while the regulatory fragment is found associated with phospholipid and continues to bind phorbol esters in a Ca2(+)- and PS-dependent manner (28, 29). In the holoenzyme, the association of the regulatory domain with the membrane may be important to release the catalytic domain from inhibition by the regulatory domain. We have presented evidence indicating that effective membrane binding occurs through interaction with the hydrophobic and/or interfacial regions of the bilayer, and does not result from binding to individual phospholipids. In vivo and in vitro studies suggest that the binding event is carefully regulated. An important function of Ca2+ may be to modify the local structure of the membrane, and thus affect the ability of PKC to associate with it. For at least one of the isozymes, however, Ca2+ may also play an additional role at a site distant from the membrane, suggesting the possibility that the isozymes may be differentially regulated.

Synthetic peptides derived from the nonmuscle myosin light chains are highly specific substrates for protein kinase C

The phosphorylation of synthetic peptides derived from the NH2-terminal sequence of smooth-muscle myosin was studied with purified protein kinase C. The protein kinase C phosphorylation domain included both serine residues and threonine residues in the sequence SSKRAKAKTTKKR(G), denoted myosin light chain (1-13) (MLC(1-13)). Kinetic analysis of MLC(1-13) and truncated peptides derived from the parent peptide established that removal of the serine residues had little effect on protein kinase C reactivity. MLC(1-13) had a V/K of 2.4 min-1.mg-1, whereas the V/K of MLC(3-13) was 3.0 min-1.mg-1. Removal of Lys-3 resulted in a 50% decrease in V/K which was attributable to a 50% decrease in apparent Vmax.Arg-4 was established as a significant protein kinase C specificity determinant, since the apparent Km increased 7-fold and the Vmax decreased 3-fold when the parent peptide was truncated at that residue. All peptides studied required calcium and lipid effectors for full activity with protein kinase C, indicating that they are Class C substrates as defined by Bazzi and Nelsestuen (Biochemistry 26 (1987) 5002) for protein kinase C. Other protein kinases, including cyclic AMP- and cyclic GMP-dependent protein kinase, S6/H4 kinase, myosin light-chain kinase and calcium/calmodulin-dependent kinase II, had little or no activity with these peptides. In studies on the purification of lymphosarcoma protein kinase C by several chromatographic procedures, the results showed that the myosin light-chain peptides can provide convenient and well-characterized substrates for purification and mechanistic studies of protein kinase C biochemistry.

Synthetic branched-chain analogues of distearoylphosphatidylcholine: structure-activity relationship in inhibiting and activating protein kinase C

A series of distearoylphosphatidylcholine (DSPC) analogues having various branched alkyl chains were synthesized and tested for their abilities to regulate protein kinase C (PKC). The greatest improvement (about 3-fold) in the PKC inhibitory activity over that seen for the parental lipid (i.e., DSPC) was accomplished by substitution of 8-methylstearate at sn-2 and 16-methylstearate at both sn-1 and sn-2 positions of glycerol; substitutions at both sn-1 and sn-2 with 8-methylstearate, on the other hand, caused a decrease (about 4-fold) in its inhibitory activity. Introduction of butyl, phenyl, or keto functions to various positions in the fatty alkyl chain substituted at both sn-1- and sn-2 positions imparted upon the DSPC analogues an ability to potently stimulate PKC to an extent comparable to those attainable by diacylglycerol or phorbol ester; the analogues having substitution only at the sn-2 position, in comparison, had no or reduced stimulatory activity. The butyl, phenyl, and keto analogues of DSPC, as with DSPC itself and its methyl analogues, inhibited PKC at high concentrations. Kinetic analysis indicated that the methyl DSPC analogues inhibited the enzyme competitively with respect to phosphatidylserine (PS; a phospholipid cofactor) and Ca2+. The butyl analogues activated the enzyme without affecting its affinity for PS or Ca2+, indicating a mechanism different from that seen for diacylglycerol or phorbol ester. The inhibitory activity of the methyl DSPC analogues and the stimulatory activity of the butyl DSPC analogues were reduced when PKC was activated by phorbol ester. Both classes of the analogues were unable to compete for the binding of [3H]phorbol dibutyrate to PKC.(ABSTRACT TRUNCATED AT 250 WORDS)

Constitutive activity of membrane-inserted protein kinase C

Incubation of purified protein kinase C (PKC) with phospholipid vesicles produced two populations of membrane-bound PKC: one population was dissociated by calcium chelation and the other was not. The second population appeared to be inserted into the membrane. The activity of membrane-inserted PKC was Ca2+-independent and was only modestly sensitive to phorbol esters. Insertion was caused by high calcium concentrations or by phorbol esters plus low calcium. These conditions correlated with those needed to activate PKC; insertion into the membrane may be a primary mechanism of PKC activation. PKC may be a long-term cell regulator which becomes inserted into the membrane upon appearance of the second messengers, calcium and diacylglycerol, and remains in an active membrane-bound state when the second messengers have been removed.