Sequestration of phosphoinositides by mutated MARCKS effector domain inhibits stimulated Ca(2+) mobilization and degranulation in mast cells

Protein kinase C β (PKCβ) participates in antigen-stimulated mast cell degranulation mediated by the high-affinity receptor for immunoglobulin E, FcεRI, but the molecular basis is unclear. We investigated the hypothesis that the polybasic effector domain (ED) of the abundant intracellular substrate for protein kinase C known as myristoylated alanine-rich protein kinase C substrate (MARCKS) sequesters phosphoinositides at the inner leaflet of the plasma membrane until MARCKS dissociates after phosphorylation by activated PKC. Real-time fluorescence imaging confirms synchronization between stimulated oscillations of intracellular Ca(2+) concentrations and oscillatory association of PKCβ-enhanced green fluorescent protein with the plasma membrane. Similarly, MARCKS-ED tagged with monomeric red fluorescent protein undergoes antigen-stimulated oscillatory dissociation and rebinding to the plasma membrane with a time course that is synchronized with reversible plasma membrane association of PKCβ. We find that MARCKS-ED dissociation is prevented by mutation of four serine residues that are potential sites of phosphorylation by PKC. Cells expressing this mutated MARCKS-ED SA4 show delayed onset of antigen-stimulated Ca(2+) mobilization and substantial inhibition of granule exocytosis. Stimulation of degranulation by thapsigargin, which bypasses inositol 1,4,5-trisphosphate production, is also substantially reduced in the presence of MARCKS-ED SA4, but store-operated Ca(2+) entry is not inhibited. These results show the capacity of MARCKS-ED to regulate granule exocytosis in a PKC-dependent manner, consistent with regulated sequestration of phosphoinositides that mediate granule fusion at the plasma membrane.     

Multicolor imaging of Ca(2+) and protein kinase C signals using novel epifluorescence microscopy.

Dynamic changes in intracellular free Ca(2+) concentrations ([Ca(2+)](i)s) control many important cellular events, including binding of Ca(2+)-calmodulin (Ca(2+)-CaM) and phosphorylation by protein kinase C (PKC). The two signals compete for the same domains in certain substrates, such as myristoylated alanine-rich PKC-substrate (MARCKS). To observe the convergence and relative time of arrival of CaM and PKC signals at their shared domain of MARCKS, we need to image cells that are loaded with more than two fluorescent dyes at a reasonable speed. We have developed a simple and powerful multicolor imaging system using conventional fluorescence microscopy. The epifluorescence configuration uses a glass reflector and rotating filter wheels for excitation and emission paths. As it is free of dichroic (multichroic) mirrors, multiple fluorescence images can be acquired rapidly regardless of the colors of fluorophores. We visualized Ca(2+)-CaM and PKC together with the dynamics of their common target, MARCKS, in single live cells. Receptor-activation resulted in translocation of MARCKS from the plasma membrane to cytosol through its phosphorylation by PKC. By observing fluorescence resonance energy transfer, we also obtained direct evidence that Ca(2+)-CaM binds MARCKS to drag it away from the membrane in circumstances when Ca(2+)-mobilization predominates over PKC activation.     

A continuous fluorescence assay for protein kinase C

A 6-acryloyl-2-dimethylaminonapthalene (acrylodan)-labeled 25-amino acid peptide (acrylodan-CKK-KKRFSFKKSFKLSGFSFKKNKK-COO-), containing the protein kinase C (PKC) phosphorylation sites of brain myristoylated alanine-rich kinase C substrate protein, undergoes a 20% fluorescence decrease when it is phosphorylated by phospholipid/calcium-dependent protein kinase (PKC). This fluorescence decrease is dependent on the presence of PKC, calcium (half-maximal stimulation at pCa = 6.2), phosphatidylserine, diacylglycerol, or phorbol-12-myristate-13-acetate (half-maximal stimulation at 2 nM) and ATP, and correlates well (r = 0.997) with [32P]phosphate incorporation into the peptide. This fluorescence assay allows detection of 0.02 nM PKC, while similar concentrations of cyclic AMP-dependent or type II calmodulin-dependent protein kinases produced no change in peptide fluorescence. The method can be used to assay purified PKC as well as activity in crude brain homogenates. Incubation of PKC with staurosporine inhibits the fluorescence decrease with an IC50 of 2 nM. Thus the fluorescence decrease that occurs in the acrylodan-peptide provides a continuous fluorescence assay for PKC activity.     

The membrane binding kinetics of full-length PKCα is determined by membrane lipid composition

Protein kinase Cα (PKCα) is activated by its translocation to the membrane. Activity assays show the importance of PIP(2) in determining the specific activity of this enzyme. A FRET stopped flow fluorescence study was carried out to monitor the rapid kinetics of protein binding to model membranes containing POPC/POPS/DOG and eventually PIP(2). The results best fitted a binding mechanism in which protein bound to the membrane following a two-phase mechanism with a first bimolecular reaction followed by a slow unimolecular reaction. In the absence of PIP(2), the rapid protein binding rate was especially dependent on POPS concentration. Formation of the slow high affinity complex during the second phase seems to involve specific interactions with POPS and DOG since it is only sensitive to changes within relatively low concentration ranges of these lipids. Both the association and dissociation rate constants fell in the presence of PIP(2). We propose a model in which PKCα binds to the membranes via a two-step mechanism consisting of the rapid membrane initial recruitment of PKCα driven by interactions with POPS and/or PIP(2) although interactions with DOG are involved too. PKCα searches on the lipid bilayer in two dimensions to establish interactions with its specific ligands.     

Fluorimetric studies of protein kinase C interactions with phospholipids

Dansyl-phosphatidylserine (D-PS) was used as a fluorescence probe to study interactions between protein kinase C (PKC) with phospholipid vesicles. It was found that D-PS fluorescence (520 nm) was enhanced by PKC (excited at 285 nm). The fluorescence energy transfer, indicative of a close association of PKC with D-PS vesicles, was differentially modulated by various phospholipids, depending upon their effects on PKC activation state and the manners in which they were present. PKC inhibitors (e.g. polymyxin B and ether lipids) potently inhibited the PKC-enhanced D-PS fluorescence. It is suggested that certain spatial arrangements between PKC and its phospholipid cofactor are essential for the enzyme activation and that D-PS would be a useful probe to study fluorimetrically such interactions.     

Distribution of active protein kinase C in smooth muscle.

To localize activated protein kinase C (PKC) in smooth muscle cells, an antibody directed to the catalytic site of the enzyme was used to assess PKC distribution by immunofluorescence techniques in gastric smooth muscle cells isolated from Bufo marinus. An antibody to vinculin was used to delineate the cell membrane. High-resolution three-dimensional images of immunofluorescence were obtained from a series of images collected through focus with a digital imaging microscope. Cells were untreated or treated with agents that increase PKC activity (10 microM carbachol for 1 min, 1 microM phorbol 12-myristate 13-acetate (PMA) for 10 min), or have no effect on PKC activity (1 micrometer 4-alpha phorbol, 12,13-didecanoate (4-alpha PMA)). In unstimulated cells, activated PKC and vinculin were located and organized at the cell surface. Cell cytosol labeling for activated PKC was sparse and diffuse and was absent for vinculin. After treatment with carbachol, which stimulates contraction and PKC activity, in addition to the membrane localization, the activated PKC exhibited a pronounced cytosolic fibrillar distribution and an increased total fluorescence intensity relative to vinculin. The distributions of activated PKC observed after PMA but not 4-alpha PMA were similar to those observed with carbachol. Our results indicate that in resting cells there is a pool of activated PKC near the cell membrane, and that after stimulation activated PKC is no longer membrane-confined, but is present throughout the cytosol. Active PKC appears to associate with contractile filaments, supporting a possible role in modulation of contraction.     

In vitro linoleic acid activation of protein kinase C

The importance of membrane fluidity in the activation of protein kinase C (PKC) was examined using the membrane fluidizer, linoleic acid, in a well-defined model membrane system. Biochemical and biophysical properties of the system were monitored. Linoleic acid activated PKC to a level of 50% of that observed for diacylglycerol. In contrast, linoleic acid did not directly interact with the phorbol ester binding site as did diacylglycerol. This was determined by the lack of involvement of the ionizable group of the fatty acid with activity and the enhancement of phorbol ester binding by linoleic acid and its ester analogs. The membrane fluidity of this model membrane system in the presence of linoleic acid was increased as determined by fluorescence polarization. This increased the availability of phospholipids, thus, explaining the linoleic acid-induced enhancement of phorbol ester binding. The PKC conformation as determined from intrinsic tryptophan fluorescence spectra was different for lipid mixtures containing linoleic acid or diacylglycerol correlating with the difference in biochemical activation properties. This study provides evidence that membrane fluidization is not the predominant function of the lipid activator in PKC activation, but may play a role in obtaining the preferred membrane state for maximal activation.     

A mechanism underlying stimulation and inhibition of protein kinase C by lyso-PC: A role of membrane physical state

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Identification of an integral plasma membrane pool of protein kinase C in mammalian tissues and cells

Protein kinase C (PKC) is a family of serine/threonine protein kinases that are pivotal in cellular regulation. Since its discovery in 1977, PKCs have been known as cytosolic and peripheral membrane proteins. However, there are reports that PKC can insert into phospholipids vesicles in vitro. Given the intimate relationship between the plasma membrane and the activation of PKC, it is important to determine whether such "membrane-inserted" form of PKC exists in mammalian cells or tissues. Here, we report the identification of an integral plasma membrane pool for all the 10 PKC isozymes in vivo by their ability to partition into the detergent-rich phase in Triton X-114 phase partitioning, and by their resistance to extractions with 0.2 M sodium carbonate (pH 11.5), 2 M urea and 2 M sodium chloride. The endogenous integral membrane pool of PKC in mouse fibroblasts is found to be acutely regulated by phorbol ester or diacylglycerol, suggesting that this pool of PKC may participate in cellular processes known to be regulated by PKC. At least for PKC(alpha), the C2-V3 region at the regulatory domain of the kinase is responsible for membrane integration. Further exploration of the function of this novel integral plasma membrane pool of PKC will not only shed new light on molecular mechanisms underlying its cellular functions but also provide new strategies for pharmaceutical modulation of this important group of kinases.     

Interaction of a pseudosubstrate peptide of protein kinase C and its myristoylated form with lipid vesicles: Only the myristoylated form translocates into the lipid bilayer

Lipopeptides derived from protein kinase C (PKC) pseudosubstrates have the ability to cross the plasma membrane in cells and modulate the activity of PKC in the cytoplasm. Myristoylation or palmitoylation appears to promote translocation across membranes, as the non-acylated peptides are membrane impermeant. We have investigated, by fluorescence spectroscopy, how myristoylation modulates the interaction of the PKC pseudosubstrate peptide KSIYRRGARRWRKL with lipid vesicles and translocation across the lipid bilayer. Our results indicate that myristoylated peptides are intimately associated with lipid vesicles and are not peripherally bound. When visualized under a microscope, myristoylation does appear to facilitate translocation across the lipid bilayer in multilamellar lipid vesicles. Translocation does not involve large-scale destabilization of the bilayer structure. Myristoylation promotes translocation into the hydrophobic interior of the lipid bilayer even when the non-acylated peptide has only weak affinity for membranes and is also only peripherally associated with lipid vesicles.     

Interaction of a pseudosubstrate peptide of protein kinase C and its myristoylated form with lipid vesicles: only the myristoylated form translocates into the lipid bilayer

Lipopeptides derived from protein kinase C (PKC) pseudosubstrates have the ability to cross the plasma membrane in cells and modulate the activity of PKC in the cytoplasm. Myristoylation or palmitoylation appears to promote translocation across membranes, as the non-acylated peptides are membrane impermeant. We have investigated, by fluorescence spectroscopy, how myristoylation modulates the interaction of the PKC pseudosubstrate peptide KSIYRRGARRWRKL with lipid vesicles and translocation across the lipid bilayer. Our results indicate that myristoylated peptides are intimately associated with lipid vesicles and are not peripherally bound. When visualized under a microscope, myristoylation does appear to facilitate translocation across the lipid bilayer in multilamellar lipid vesicles. Translocation does not involve large-scale destabilization of the bilayer structure. Myristoylation promotes translocation into the hydrophobic interior of the lipid bilayer even when the non-acylated peptide has only weak affinity for membranes and is also only peripherally associated with lipid vesicles.     

The sustained second phase of hormone-stimulated diacylglycerol accumulation does not activate protein kinase C in GH3 cells

Numerous hormones activate cells through receptor-regulated hydrolysis of phosphoinositides resulting in elevated cellular diacylglycerol (DAG), an activator of protein kinase C (PKC). Our previous studies showed that thyrotropin-releasing hormone (TRH) treatment of GH3 cells stimulated a rapid (less than 10 s) but transient (less than 60 s) association of cytosolic PKC with the membrane. In this study, we investigated the roles of hormone-stimulated Ca2+ and DAG levels in initiating and terminating the membrane association of PKC. The initial effects of TRH were not mimicked by elevating CA2+ levels, however, inhibiting TRH-stimulated Ca2+ increases blocked hormone-stimulated PKC translocation. Hence, the TRH stimulation of both Ca2+ and DAG levels were essential for the initial PKC translocation. The termination of PKC membrane association could not be attributed to proteolysis of PKC nor to limiting Ca2+ levels. Treatment of cells with phorbol diesters potentiated and prolonged the effects of TRH on PKC translocation, suggesting that DAG levels limited the membrane association of PKC. Since TRH stimulated a sustained increase in DAG levels, DAG composition was analyzed. There was a marked shift in DAG from tetraenoic (at 15 s) to more saturated DAGs at longer times. In addition, increases in plasma membrane DAG in response to TRH were transient rather than sustained. We propose that the TRH stimulation of PKC translocation is short-lived due to the metabolism of plasma membrane DAGs which are effective in promoting PKC activation. In contrast, DAGs which accumulate in intracellular membranes during the sustained phase of TRH treatment appear to be ineffective as activators of PKC.     

Chemical modifications of resveratrol for improved protein kinase C alpha activity

Resveratrol (1) is a naturally occurring phytoalexin that affects a variety of human disease models, including cardio- and neuroprotection, immune regulation, and cancer chemoprevention. One of the possible mechanisms by which resveratrol affects these disease states is by affecting the cellular signaling network involving protein kinase C alpha (PKCα). PKCα is a member of the family of serine/threonine kinases, whose activity is inhibited by resveratrol. To study the structure-activity relationship, several monoalkoxy, dialkoxy and hydroxy analogs of resveratrol have been synthesized, tested for their cytotoxic effects on HEK293 cells, measured their effects on the membrane translocation properties of PKCα in the presence and absence of the PKC activator TPA, and studied their binding with the activator binding domain of PKCα. The analogs showed less cytotoxic effects on HEK293 cells and caused higher membrane translocation (activation) than that of resveratrol. Among all the analogs, 3, 16 and 25 showed significantly higher activation than resveratrol. Resveratrol analogs, however, inhibited phorbol ester-induced membrane translocation, and the inhibition was less than that of resveratrol. Binding studies using steady state fluorescence spectroscopy indicated that resveratrol and the analogs bind to the second cysteine-rich domain of PKCα. The molecular docking studies indicated that resveratrol and the analogs interact with the protein by forming hydrogen bonds through its hydroxyl groups. These results signify that molecules developed on a resveratrol scaffold can attenuate PKCα activity and this strategy can be used to regulate various disease states involving PKCα.     

In situ observation of mobility and anchoring of PKCbetaI in plasma membrane

We employed fluorescence correlation spectroscopy (FCS) to analyze the characteristics of biomolecules in living cells. Protein kinase C (PKC) changes its subcellular localization from cytosol to the plasma membrane by its ligand. Using FCS, we found PKCbetaI labeled with enhanced green fluorescent protein freely diffusing in cytosol. Upon 12-O-tetradecanoylphorbol-13-acetate activation, a large part of PKCbetaI is anchored in the plasma membrane but some PKCbetaI still moves freely near the plasma membrane. These results indicate that a diffusion-driven transport mechanism is appropriate for the molecular mechanism of the PKCbetaI localization change.     

Association of type 3 protein kinase C with focal contacts in rat embryo fibroblasts

We have used immunocytofluorescence techniques to determine the subcellular distribution of the Ca2+, phospholipid-dependent protein kinase, protein kinase C (PKC). Using monoclonal antibodies that are specific for Type 3 (alpha) PKC, we have determined that there are least two pools of PKC in normal rat embryo fibroblasts (REF52 cells): diffuse cytoplasmic and fiber-associated. Extraction with chelators and detergent before fixing and staining removes the cytoplasmic PKC. The fiber-associated staining remains in these cytoskeleton preparations. The cytoskeleton Type 3 PKC staining closely resembles that of the focal contact protein vinculin and colocalizes with another focal contact protein, talin. Cytochalasin, but not colchicine, coordinately disrupts the staining pattern of vinculin and PKC. Activation of PKC by treatment with phorbol esters causes depolymerization of microfilaments and reorganization of vinculin staining. We propose that Type 3 PKC is a modulatory component of the focal contact and has a primary role in regulation of the association of microfilament bundles with the plasma membrane.     

Phorbol ester-induced actin assembly in neutrophils: role of protein kinase C

The shape changes and membrane ruffling that accompany neutrophil activation are dependent on the assembly and reorganization of the actin cytoskeleton, the molecular basis of which remains to be clarified. A role of protein kinase C (PKC) has been postulated because neutrophil activation, with the attendant shape and membrane ruffling changes, can be initiated by phorbol esters, known activators of PKC. It has become apparent, however, that multiple isoforms of PKC with differing substrate specificities exist. To reassess the role of PKC in cytoskeletal reorganization, we compared the effects of diacylglycerol analogs and of PKC antagonists on kinase activity and on actin assembly in human neutrophils. Ruffling of the plasma membrane was assessed by scanning EM, and spatial redistribution of filamentous (F)-actin was assessed by scanning confocal microscopy. Staining with NBD-phallacidin and incorporation of actin into the Triton X-100-insoluble ("cytoskeletal") fraction were used to quantify the formation of (F)-actin. [32P]ATP was used to detect protein phosphorylation in electroporated cells. Exposure of neutrophils to 4 beta-PMA (an activator of PKC) induced protein phosphorylation, membrane ruffling, and assembly and reorganization of the actin cytoskeleton, whereas the 4a-isomer, which is inactive towards PKC, failed to produce any of these changes. Moreover, 1,2-dioctanoylglycerol, mezerein, and 3-(N-acetylamino)-5-(N-decyl-N-methylamino)-benzyl alcohol, which are nonphorbol activators of PKC, also promoted actin assembly. Although these effects were consistent with a role of PKC, the following observations suggested that stimulation of conventional isoforms of the kinase were not directly responsible for actin assembly: (a) Okadaic acid, an inhibitor of phosphatases 1 and 2A, potentiated PMA-induced protein phosphorylation, but not actin assembly; and (b) PMA-induced actin assembly and membrane ruffling were not prevented by the conventional PKC inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine, staurosporine, calphostin C, or sphingosine at concentrations that precluded PMA-induced protein phosphorylation and superoxide production. On the other hand, PMA-induced actin assembly was inhibited by long-chain fatty acid coenzyme A esters, known inhibitors of nuclear PKC (nPKC). We conclude that PMA-induced actin assembly is unlikely to be mediated by the conventional isoforms of PKC, but may be mediated by novel isoforms of the kinase such as nPKC.     

Thrombin-induced translocation of protein kinase C in human megakaryoblastic leukemia cells (MEG-01)

Our previous immunocytochemical study showed that Ca2+ ionophore-induced translocation of protein kinase C (PKC) in human megakaryoblastic leukemia cells (MEG-01) was potentiated by a synthetic diacylglycerol (T. Ito, T. Tanaka, T. Yoshida, K. Onoda, H. Ohta, M. Hagiwara, Y. Itoh, M. Ogura, H. Saito, and H. Hidaka, 1988, J. Cell Biol. 107, 929). In the present study, we analyzed the roles of the intracellular Ca2+ levels ([Ca2+]i) and diacylglycerol (DG) levels in thrombin-induced translocation of PKC using MEG-01 cells. When the cells were treated with thrombin (0.5 U/ml), PKC was translocated from the cytosol to the plasma membrane after 15 s, and the maximal membrane association was observed after 90 s. The [Ca2+]i of the cells rapidly increased (15 s) and reached a maximum level at 60 s which was sustained for a total of 600 s after thrombin addition. The increase in DG was biphasic with the first phase occurring in the first 15 s and the increase during the second phase lasting less than 600 s. The experiments without extracellular Ca2+ indicated that Ca2+ efflux accompanied by DG in the first phase was sufficient to initiate the membrane association of the PKC and that the large Ca2+ influx enhanced the binding. PKC returned to the cytosol within 600 s despite high levels of both [Ca2+]i and DG. We found that a relatively selective PKC inhibitor, H-7, enhanced thrombin-induced translocation of PKC without modulating [Ca2+]i or DG levels. These results indicate that certain protein phosphorylation events, potentially those mediated by PKC, may be responsible for, at least in part, inhibiting membrane association and further activation of the enzyme.     

Activation of protein kinase C zeta induces serine phosphorylation of VAMP2 in the GLUT4 compartment and increases glucose transport in skeletal muscle

Insulin stimulates glucose uptake into skeletal muscle tissue mainly through the translocation of glucose transporter 4 (GLUT4) to the plasma membrane. The precise mechanism involved in this process is presently unknown. In the cascade of events leading to insulin-induced glucose transport, insulin activates specific protein kinase C (PKC) isoforms. In this study we investigated the roles of PKC zeta in insulin-stimulated glucose uptake and GLUT4 translocation in primary cultures of rat skeletal muscle. We found that insulin initially caused PKC zeta to associate specifically with the GLUT4 compartments and that PKC zeta together with the GLUT4 compartments were then translocated to the plasma membrane as a complex. PKC zeta and GLUT4 recycled independently of one another. To further establish the importance of PKC zeta in glucose transport, we used adenovirus constructs containing wild-type or kinase-inactive, dominant-negative PKC zeta (DNPKC zeta) cDNA to overexpress this isoform in skeletal muscle myotube cultures. We found that overexpression of PKC zeta was associated with a marked increase in the activity of this isoform. The overexpressed, active PKC zeta coprecipitated with the GLUT4 compartments. Moreover, overexpression of PKC zeta caused GLUT4 translocation to the plasma membrane and increased glucose uptake in the absence of insulin. Finally, either insulin or overexpression of PKC zeta induced serine phosphorylation of the GLUT4-compartment-associated vesicle-associated membrane protein 2. Furthermore, DNPKC zeta disrupted the GLUT4 compartment integrity and abrogated insulin-induced GLUT4 translocation and glucose uptake. These results demonstrate that PKC zeta regulates insulin-stimulated GLUT4 translocation and glucose transport through the unique colocalization of this isoform with the GLUT4 compartments.     

Basal protein kinase Cδ activity is required for membrane localization and activity of TRPM4 channels in cerebral artery smooth muscle cells

The melastatin (M) transient receptor potential channel (TRP) channel TRPM4 is a critical regulator of vascular smooth muscle cell membrane potential and contractility. We recently reported that PKCδ activity influences smooth muscle cell excitability by promoting translocation of TRPM4 channel protein to the plasma membrane. Here we further investigate the relationship between membrane localization of TRPM4 protein and channel activity in native cerebral arterial myocytes. We find that TRPM4 immunolabeling is primarily located at or near the plasma membrane of freshly isolated cerebral artery smooth muscle cells. However, siRNA mediated downregulation of PKCδ or brief (15 min) inhibition of PKCδ activity with rottlerin causes TRPM4 protein to move away from the plasma membrane and into the cytosol. In addition, we find that PKCδ inhibition diminishes TRPM4-dependent currents in smooth muscle cells patch clamped in the amphotericin B perforated patch configuration. We conclude that TRPM4 channels are mobile in native cerebral myocytes and that basal PKCδ activity supports excitability of these cells by maintaining localization TRPM4 protein at the plasma membrane.     

Quenching of fluorescence in membrane protein by hypocrellin B

The hypocrellin B (HB) was used as a fluorescence quencher to study the basic physical characteris-tics of HB in membrane systems, including the diffusion speed of quencher from aqueous phase into membrane phase, the partition coefficient (P) of quencher between membrane and water, and the fluorescence quenching constant of protein (Ksv; Kq). The experimental results show that the quenching of fluorescence in membrane protein by HB can be determined by the principle of dynamic quenching. The experimental process of fluorescence quenching was ob-served in detail by using the ESR technique. The signal of HB" was found to arise from an electron transfer from ex-cited trytophan to HB.