Pleiotropic control of glucose and hormone responses by PRL1, a nuclear WD protein, in Arabidopsis.

The prl1 mutation localized by T-DNA tagging on Arabidopsis chromosome 4-44 confers hypersensitivity to glucose and sucrose. The prl1 mutation results in transcriptional derepression of glucose responsive genes defining a novel suppressor function in glucose signaling. The prl1 mutation also augments the sensitivity of plants to growth hormones including cytokinin, ethylene, abscisic acid, and auxin; stimulates the accumulation of sugars and starch in leaves; and inhibits root elongation. PRL1 encodes a regulatory WD protein that interacts with ATHKAP2, an alpha-importin nuclear import receptor, and is imported into the nucleus in Arabidopsis. Potential functional conservation of PRL1 homologs found in other eukaryotes is indicated by nuclear localization of PRL1 in monkey COS-1 cells and selective interaction of PRL1 with a nuclear protein kinase C-betaII isoenzyme involved in human insulin signaling.

Clarification of the binding mode of teleocidin and benzolactams to the Cys2 domain of protein kinase Cdelta by synthesis of hydrophobically modified, teleocidin-mimicking benzolactams and computational docking simulation.

Phorbol esters (12-O-tetradecanoylphorbol 13-acetate; TPA) and teleocidins are known to be potent tumor promoters and to activate protein kinase C (PKC) by binding competitively to the enzyme. The relationship between the chemical structures and the activities of these compounds has attracted much attention because of the marked structural dissimilarities. The benzolactam 5, with an eight-membered lactam ring and benzene ring instead of the nine-membered lactam ring and indole ring of teleocidins, reproduces the active ring conformation and biological activities of teleocidins. Herein we describe the synthesis of benzolactams with hydrophobic substituents at various positions. Structure-activity data indicate that the existence of a hydrophobic region between C-2 and C-9 and the steric factor at C-8 play critical roles in the appearance of biological activities. We also computationally simulated the docking of teleocidin and the modified benzolactam molecules to the Cys2 domain structure observed in the crystalline complex of PKCdelta with phorbol 13-acetate. Teleocidin and benzolactams fitted well into the same cavity as phorbol 13-acetate. Of the three functional groups hydrogen-bonding to the protein, two hydrogen-bonded with protein atoms in common with phorbol 13-acetate, but the third one hydrogen-bonded with a different protein atom from that in the case of phorbol 13-acetate. The model explains well the remarkable difference in activity between 5 and its analogue having a bulky substituent at C-8.

Interaction of protein kinase C zeta with ZIP, a novel protein kinase C-binding protein.

The atypical protein kinase C (PKC) member PKC-zeta has been implicated in several signal transduction pathways regulating differentiation, proliferation or apoptosis of mammalian cells. We report here the identification of a cytoplasmic and membrane-associated protein that we name zeta-interacting protein (ZIP) and that interacts with the regulatory domain of PKC-zeta but not classic PKCs. The structural motifs in ZIP include a recently defined ZZ zinc finger as a potential protein binding module, two PEST sequences and a novel putative protein binding motif with the consensus sequence YXDEDX5SDEE/D. ZIP binds to the pseudosubstrate region in the regulatory domain of PKC-zeta and is phosphorylated by PKC-zeta in vitro. ZIP dimerizes via the same region that promotes binding to PKC-zeta suggesting a competitive situation between ZIP:ZIP and ZIP:PKC-zeta complexes. In the absence of PKC-zeta proper subcellular localization of ZIP is impaired and we show that intracellular targeting of ZIP is dependent on a balanced interaction with PKC-zeta. Taking into account the recent isolation of ZIP by others in different contexts we propose that ZIP may function as a scaffold protein linking PKC-zeta to protein tyrosine kinases and cytokine receptors.

Activation of protein kinase C subtypes alpha, gamma, delta, epsilon, zeta, and eta by tumor-promoting and nontumor-promoting agents.

Protein kinase C (PKC) subtypes alpha, gamma, delta, epsilon, zeta, and eta have been expressed using the baculovirus expression system. The partially purified PKC subtypes have been studied for their substrate specificities and phospholipid-independent activation by various chemically different nontumor- and tumor-promoting agents, as well as their inhibition of kinase activity by staurosporine and two related compounds. An endogenous PKC-like kinase activity of Sf9 cells was detected and analyzed for cofactor requirements and inhibition. Protamine sulfate was most efficiently phosphorylated by all of the PKC subtypes tested, although this phosphorylation was independent of phosphatidylserine (PS) and diacylglycerol (DAG) or 12-O-tetradecanoylphorbol 13-acetate (TPA). Except for PKC-zeta, all subtypes tested phosphorylated myelin basic protein (MBP), histone, or a peptide derived from the pseudosubstrate region of PKC-alpha in a PS/DAG-dependent manner but to varying extents. Among the various agents tested, TPA most efficiently stimulated the kinase activities of the PKC subtypes in a phospholipid-dependent manner. Phorbol 12,13-dibutyrate (PDBu) was less effective than TPA but displayed no major difference among the subtypes. Activation of PKC-alpha by bryostatin-1 reached only half of the TPA response whereas the other subtypes were activated more effectively. The weak tumor promoter resiniferonol 9,13,14-orthophenyl acetate (ROPA) mainly stimulated PKC-alpha and PKC-gamma at 1 microM concentration, whereas PKC-epsilon and PKC-eta were much less activated. Sapintoxin D, mezerein, indolactam V, and resiniferatoxin at concentrations of 1-100 nM preferentially activated PKC-alpha in a DAG-like manner, whereas at 1 microM other subtypes were activated as well. Preferential activation of PKC-alpha was also noted for tinyatoxin and thapsigargin, but their mode of activation is unclear because these two compounds did not compete for the phorbol ester binding of the PKC subtypes as the other agents did. Of the three PKC inhibitors tested, staurosporine most efficiently inhibited kinase activity of the PKC subtypes, whereas K252a and CGP 41251 were at least 10 times less effective. However, K252a showed certain specificity for inhibition of PKC-alpha, and CGP 41251 failed to inhibit PKC-epsilon and PKC-zeta. Given the different substrate specificities and modes of activation by various tumor-promoting and nontumor-promoting agents, as well as the different sensitivities towards different inhibitors, our results indicate a divergence of individual PKC subtypes in signal transduction.

Immunodeficiency in protein kinase cbeta-deficient mice.

Cross-linking of the antigen receptor on lymphocytes by antigens or antibodies to the receptor results in activation of enzymes of the protein kinase C (PKC) family. Mice homozygous for a targeted disruption of the gene encoding the PKC-betaI and PKC-betaII isoforms develop an immunodeficiency characterized by impaired humoral immune responses and reduced cellular responses of B cells, which is similar to X-linked immunodeficiency in mice. Thus PKC-betaI and PKC-betaII play an important role in B cell activation and may be functionally linked to Bruton's tyrosine kinase in antigen receptor-mediated signal transduction.

Ceramide-binding and activation defines protein kinase c-Raf as a ceramide-activated protein kinase.

Interleukin 1 is the prototype of an inflammatory cytokine, and evidence suggests that it uses the sphingomyelin pathway and ceramide production to trigger mitogen-activated protein kinase (MAPK) activation and subsequent gene expression required for acute inflammatory processes. To identify downstream signaling targets of ceramide, a radioiodinated photoaffinity labeling analog of ceramide ([125I] 3-trifluoromethyl-3-(m-iodophenyl)diazirine-ceramide) was employed. It is observed that ceramide specifically binds to and activates protein kinase c-Raf, leading to a subsequent activation of the MAPK cascade. Ceramide does not bind to any other member of the MAPK module nor does it bind to protein kinase C-zeta. These data identify protein kinase c-Raf as a specific molecular target for interleukin 1 beta-stimulated ceramide formation and demonstrate that ceramide is a lipid cofactor participating in regulation of c-Raf activity.

Protein kinase C beta II specifically binds to and is activated by F-actin.

The two most closely related isoenzymes of protein kinase C (PKC), PKC betaI and betaII, are distinct but highly homologous isoenzymes derived via alternative splicing of the same gene product. In this study, PKC betaII, but not PKC betaI, translocated to the actin cytoskeleton upon stimulation of cells with phorbol esters. In cells, antibodies to PKC betaII, but not to PKC betaI, co-immunoprecipitated actin. Using an actin-binding co-sedimentation assay, we show in vitro that PKC betaII, but not PKC betaI, binds to actin specifically. This binding was inhibited by peptides based on sequences unique to PKC betaII; thus defining an actin-binding site in PKC betaII that is not present in PKC betaI. The binding of PKC betaII to actin was not inhibited by kinase inhibitors of PKC (sphingosine and staurosporine), suggesting that prior activation and/or substrate phosphorylation are not required for the interaction of PKC betaII with actin. On the other hand, the interaction of PKC betaII with actin resulted in marked enhancement of autophosphorylation of PKC betaII and in an alteration in substrate specificity. These studies serve to define a novel functional domain in the carboxyl-terminal region of PKC beta, which is involved in directing isoenzyme-specific protein-protein interactions, and consequently, isoenzyme-specific functions in vivo.

Soluble human interleukin-6 receptor. Expression in insect cells, purification and characterization.

The extracellular domain of the human interleukin-6 (IL-6) receptor, comprising 339 amino acids following the signal peptide, has been expressed in baculovirus-infected insect cells (Sf158). When the soluble receptor secreted into the culture medium was purified by affinity chromatography, using IL-6 immobilized on Sepharose, 6 mg soluble receptor was isolated from 1 l conditioned medium of Sf158 suspension cultures. A molar absorption coefficient of 9.3 x 10(4) l.mol-1.cm-1 was calculated from the ultraviolet spectrum of the soluble IL-6 receptor. After SDS/PAGE and silver staining, an apparent molecular mass of 48 kDa was estimated for the purified protein. Deglycosylation with peptide N-glycosidase F resulted in an increase in electrophoretic mobility and a decrease in the apparent molecular mass from 48 kDa to about 41-44 kDa. As expected, the soluble human IL-6 receptor bound human 125I-labeled IL-6 with low affinity (Kd = 500 pM). Furthermore, the binding of soluble human IL-6 receptor to immobilized IL-6 was studied using real-time interaction analysis. The recombinant soluble receptor showed biological activity on HepG2 cells stably transfected with a cDNA coding for IL-6 (HepG2-IL-6 cells). Haptoglobin mRNA synthesis was induced by the soluble IL-6 receptor at concentrations as low as 10 ng/ml. Five monoclonal antibodies were generated. Two groups of antibodies were identified mapping to amino acids 1-67 and 68-143 of the soluble IL-6 receptor, respectively. The plasma clearance of soluble 125I-labeled IL-6 receptor in the absence and presence of IL-6 was studied in rats as a model system. The kinetics was biphasic. Soluble IL-6 receptor/IL-6 complexes were cleared more rapidly than the soluble receptor alone. Intravenously injected soluble 125I-labeled IL-6 receptor, as well as complexes with IL-6, rapidly accumulated in liver and to a lesser extent in skeletal muscle, skin and kidneys. Subsequently, the radioactivity appeared in the gut content.

Evidence for different signalling pathways of PKC zeta and ras-p21 in Xenopus oocytes.

Considerable effort has been devoted to identifying critical steps in mitogenic signal transduction pathways. Recently, the atypical PKC zeta isoform has attracted great interest since it has been reported to induce GVBD in Xenopus oocytes and transformation of NIH3T3 fibroblasts, two processes closely linked with the regulation of cell division. Furthermore, PKC zeta has been proposed as an essential effector for ras-p21 function and therefore may be an essential component of the signalling pathway(s) activated by mitogens. In this study we have analysed the responses induced in Xenopus oocytes after microinjection of purified recombinant PKC zeta protein. Microinjection of PKC zeta induced the early activation of MPF which precedes GVBD and also induced the activation of MAP kinase and S6 kinase II. The activation of MPF, MAP kinase and S6 kinase II by PKC zeta was sensitive to cycloheximide, while induction of GVBD was independent of protein synthesis. These results indicate that PKC zeta induces the activation of at least two pathways, only one of them leading to the activation of MAP kinase. By contrast, neither the induction of GVBD nor the activation of MPF, MAPK and S6 kinase II induced by the ras-p21 protein were dependent on protein synthesis. Thus, the comparison of these responses suggests that PKC zeta most likely does not mediate the ras-induced signal transduction pathway in Xenopus laevis oocytes.

Stimulation of MAP kinase by v-raf transformation of fibroblasts fails to induce hyperphosphorylation of transfected tau.

A proportion of the microtubule-associated protein, tau, is in an elevated state of phosphorylation in foetal and adult brain whereas all of the tau in paired helical filaments, which are characteristic of Alzheimer's disease is hyperphosphorylated; it is important therefore to elucidate the mechanisms that regulate tau phosphorylation. Here we describe results that show that although MAP kinase can hyperphosphorylate tau in vitro, activation of MAP kinase in transformed fibroblasts does not result in hyperphosphorylation of transfected tau, whereas glycogen synthase kinase-3 beta (GSK-3 beta) when co-transfected with tau does result in tau hyperphosphorylation. The findings imply that GSK-3 beta may be a stronger candidate than MAP kinase for inducing tau hyperphosphorylation in vivo.

Kinase activities of c-Mos and v-Mos proteins: a single amino acid exchange is responsible for constitutive activation of the 124 v-Mos kinase.

The Mos protein kinase is a serine-/threonine-specific protein kinase with a crucial role in meiotic cell divisions in vertebrates. Several oncogenic derivatives of the c-Mos protein have been discovered in murine retroviruses. These proteins have acquired mutations and exhibit different degrees of protein kinase activity in vitro. In an attempt to understand the factors governing Mos protein kinase activity we have compared the kinase activities of the wild-type c-Mos protein and two v-Mos proteins (strain HT1 and MSV124) after expression in insect cells. Only the 124 v-Mos protein showed kinase activity in vitro as measured by autophosphorylation, vimentin phosphorylation or by phosphorylation and activation of MAP kinase kinase. By domain swapping and site-directed mutagenesis we identified a single point mutation in the 124 v-Mos protein (Arg145-->Gly) which is responsible for its constitutive activity. This residue is located in the alpha-helix C of the kinase domain close to the ATP binding fold and is conserved in all known c-Mos proteins. Introduction of the corresponding mutation into HT1 v-Mos and into murine c-Mos activated both proteins for autophosphorylation, vimentin phosphorylation and for signalling via MAP kinase kinase in vitro. We hypothesize that the Arg145-->Gly mutation found in 124 v-Mos mimicks a conformational change which might be an obligatory step in the activation of c-Mos in vivo.

Platelet-derived growth factor and angiotensin II stimulate the mitogen-activated protein kinase cascade in renal mesangial cells: comparison of hypertrophic and hyperplastic agonists.

Exposure of mesangial cells to platelet-derived growth factor (PDGF) BB caused a significant stimulation of cell proliferation and protein synthesis, as measured by [3H]thymidine incorporation and [3H]leucine incorporation respectively. In contrast, cells treated with angiotensin II had no significant increase in [3H]thymidine incorporation, but demonstrated a marked increase in [3H]leucine incorporation. Furthermore, angiotensin II significantly increased total protein content per cell. These data show that, whereas PDGF-BB is a mitogen and stimulates mesangial-cell hyperplasia, angiotensin II causes hypertrophy of the cells without hyperplasia. Treatment of mesangial cells with PDGF and angiotensin II rapidly and dose-dependently stimulated mitogen-activated protein (MAP) kinase activity, as shown by an assay for activity in vitro using myelin basic protein as a substrate, and by immunoprecipitation of 32P-labelled cells with specific antibodies against the 42 kDa and 44 kDa mitogen-activated protein kinases p42mapk and p44mapk, respectively. Whereas stimulation with PDGF-BB caused a potent and sustained (for more than 30 min) phosphorylation and activation of p42mapk and p44mapk, as well as of the upstream activators MAP kinase kinase and c-Raf, the effect of angiotensin II was less potent, reaching a peak at 5-10 min and thereafter declining rapidly. In summary, these results suggest that PDGF-BB and angiotensin II differ in their potency and duration of activation of the MAP kinase cascade, which may explain why PDGF-BB is a potent mitogen for mesangial cells, whereas angiotensin II only triggers mesangial-cell hypertrophy.

Alzheimer's disease-like phosphorylation of the microtubule-associated protein tau by glycogen synthase kinase-3 in transfected mammalian cells.

BACKGROUND: Paired helical filaments (PHFs) are a characteristic pathological feature of Alzheimer's disease; their principal component is the microtubule-associated protein tau. The tau in PHFs (PHF-tau) is hyperphosphorylated, but the cellular mechanisms responsible for this hyperphosphorylation have yet to be elucidated. A number of kinases, including mitogen-activated protein (MAP) kinase, glycogen synthase kinase (GSK)-3 alpha, GSK-3 beta and cyclin-dependent kinase-5, phosphorylate recombinant tau in vitro so that it resembles PHF-tau as judged by its reactivity with a panel of antibodies capable of discriminating between normal tau and PHF-tau, and by a reduced electrophoretic mobility that is characteristic of PHF-tau. To determine whether MAP kinase, GSK-3 alpha and GSK-3 beta can also induce Alzheimer's disease-like phosphorylation of tau in mammalian cells, we studied the phosphorylation status of tau in primary neuronal cultures and transfected COS cells following changes in the activities of MAP kinase and GSK-3. RESULTS: Activating MAP kinase in cultures of primary neurons or transfected COS cells expressing tau isoforms did not increase the level of phosphorylation for any PHF-tau epitope investigated. But elevating GSK-3 activity in the COS cells by co-transfection with GSK-3 alpha or GSK-3 beta decreased the electrophoretic mobility of tau so that it resembled that of PHF-tau, and induced reactivity with eight PHF-tau-selective monoclonal antibodies. CONCLUSIONS: Our data indicate that GSK-3 alpha and/or GSK-3 beta, but not MAP kinase, are good candidates for generating PHF-type phosphorylation of tau in Alzheimer's disease. The involvement of other kinases in the generation of PHFs cannot, however, be eliminated. Our results suggest that aberrant regulation of GSK-3 may be a pathogenic mechanism in Alzheimer's disease.

Signalling from TPA to MAP kinase requires protein kinase C, raf and MEK: reconstitution of the signalling pathway in vitro.

The phorbol ester PMA/TPA (phorbol 12-myristate 13-acetate) is a potent tumor promoter which mimics distinct intracellular signalling events triggered by activated growth factor receptors, e.g. the activation of MAP kinases. The largest known family of TPA-binding proteins comprise members of the protein kinase C (PKC) family although other TPA-binding proteins outside the PKC family have recently been identified. In this report we addressed the mechanism and the pathway by which TPA induces the activation of MAPkinases. Using recombinant proteins and in vitro phosphorylation reactions we identified the components in the signal transduction pathway from TPA to MAPkinase and we show that the activation of MAPkinase by TPA requires the presence of protein kinase C, c-raf and the MAPkinase activator MEK. We also find that the activation of raf autophosphorylation in vitro correlates with the ability of Raf to signal to MAPkinase. Thus the activation of Raf by PKC apparently can trigger the same signalling pathway as oncogenic Raf or Raf activation by ras in combination with tyrosine phosphorylation.

Protein kinase C and mammary cell differentiation: involvement of protein kinase C alpha in the induction of beta-casein expression.

Treatment of HC11 mouse mammary epithelial cells with the lactogenic hormones dexamethasone, insulin, and prolactin (DIP) leads to cellular differentiation and production of the milk protein beta-casein. The following experimental evidence suggests the involvement of protein kinase C (PKC) in DIP induced signal transduction. Down-regulation of PKC by 12-O-tetradecanoylphorbol-13-acetate or addition of CGP 41251, a selective inhibitor of PKC, inhibited beta-casein protein expression induced by DIP in HC11 cells. This inhibition occurs at the level of transcription, since the DIP mediated activation of a beta-casein promoter-luciferase reporter construct or of mammary gland specific factor (MGF), an essential transcription factor for beta-casein promoter activity, was also inhibited by CGP 41251. Inhibition or down-regulation of PKC reduced the activation of MGF by prolactin as well. PKC-alpha, the only conventional PKC isoform expressed in HC11 cells, is most likely involved in the DIP induced beta-casein expression. (a) Only PKC-alpha and PKC-epsilon are down-regulated by 12-O-tetradecanoylphorbol-13-acetate whereas PKC-delta and PKC-zeta are not. (b) Of the PKC isoforms expressed in HC11 cells, CGP 41251 inhibits PKC-alpha more potently than PKC-delta, PKC-epsilon, and PKC-zeta. The IC50 for the inhibition of beta-casein synthesis, MGF activation, and beta-casein promoter activity by CGP 41251 correlated well with the IC50 of PKC-alpha inhibition. (c) Finally, only PKC-alpha translocated to membrane fractions after DIP or prolactin treatment. Taken together, these data indicate that PKC-alpha plays an important role in the signaling pathway activated by prolactin during beta-casein induction.

Mezerein and 12-deoxyphorbol 13-isobutyrate, protein kinase C ligands with differential biological activities, do not distinguish PKC-isotypes alpha, beta 1, beta 2, and gamma.

Previous in vitro experiments have shown that the phorbol-like diterpenes 12-deoxyphorbol 13-isobutyrate (dPB), and possibly mezerein, have multiple biological target sites which differ from one another in apparent affinity for dPB by 12.5-780 fold and for mezerein by 24-fold. These two compounds are thus very important ligands because of their potential PKC isotype-selectivity. In the present study they were found to have binding affinities differing by a maximum of only 1.6-fold among recombinant protein kinase C (PKC) isotypes alpha, beta 1, beta 2, and gamma (the "A-group") in a [3H]phorbol dibutyrate binding assay. The apparent Ki's were 92-140 nM for dPB and 68-92 nM for mezerein. Our results are consistent with short-term 12-deoxyphorbol ester-induced mouse skin inflammation being mediated at least in part by one or more A-group PKC isotypes. The data also indicate that the pharmacologically distinguishable target sites previously established for mezerein and dPB must include one or more binding sites not found in the A-group of PKC isotypes and that mezerein has a high-affinity, non-A-group target site in brain.

Increased Kit/SCF receptor induced mitogenicity but abolished cell motility after inhibition of protein kinase C.

The product of the c-kit proto-oncogene, denoted Kit/SCF-R, encodes a tyrosine kinase receptor for stem cell factor (SCF). Kit/SCF-R induces proliferation, differentiation or migration of cells within the hematopoietic, gametogenic and melanogenic lineages at different developmental stages. We report here that protein kinase C (PKC) mediates phosphorylation of Kit/SCF-R on serine residues in response to SCF or PMA in intact cells. The phosphorylation inhibits SCF-induced tyrosine autophosphorylation of Kit/SCF-R. In vitro studies showed that PKC phosphorylated the Kit/SCF-R directly on serine residues and inhibited autophosphorylation of Kit/SCF-R, as well as its kinase activity towards an exogenous substrate. The PKC-induced phosphorylation did not affect Kit/SCF-R ligand binding affinity. Inhibition of PKC led to increased SCF-induced tyrosine autophosphorylation, as well as increased SCF-induced mitogenicity. In contrast, PKC was necessary for SCF-induced motility responses, including actin reorganization and chemotaxis. Our data suggest that PKC is involved in a negative feedback loop which regulates the Kit/SCF-R and that the activity of PKC determines whether the effect of SCF will be preferentially mitogenic or motogenic.

Selective regulation of expression of protein kinase C (PKC) isoenzymes in multidrug-resistant MCF-7 cells. Functional significance of enhanced expression of PKC alpha.

The multidrug resistance (MDR) phenotype induces cross-resistance to many chemotherapeutic agents in cancer cells. Protein kinase C (PKC) has been implicated in the regulation of the MDR phenotype. In order to determine the role of specific PKC isoenzymes in regulating the MDR phenotype, the expression and activity of PKC isoenzymes in the human breast cancer cell line, MCF-7-WT, and an MDR subline, MCF-7-MDR, were examined. The MDR phenotype was associated with a 10-fold increase in calcium-dependent PKC activity as well as a 10-fold decrease in calcium-independent activity was due to a selective increase in the activity was due to a selective increase in the expression of PKC alpha as determined by Western blot analysis and hydroxylapatite chromatography. This increase in expression of PKC alpha was regulated at the message level as demonstrated by Northern blot analysis. The decrease in calcium-independent activity was caused by a decrease in the expression of PCK delta and epsilon. The significance of the increase in PKC alpha expression was then demonstrated by a commensurate 11-fold increase in the basal and stimulated phosphorylation of the myristolated alanine-rich C kinase substrate. Phosphorylation of P-glycoprotein, the cellular mediator of the MDR phenotype, was increased > 20-fold in the unstimulated MCF-7-MDR cell line and its phosphorylation was further increased 2-fold in response to phorbol 12-myristate 13-acetate. These changes paralleled the increases in P-glycoprotein pump function and the MDR phenotype underscoring the role for PKC alpha in regulating P-glycoprotein phosphorylation and function.

Effect of tumour-promoting phorbol ester, thrombin and vasopressin on translocation of three distinct protein kinase C isoforms in human platelets and regulation by calcium.

Protein kinase C (PKC) acts in synergy with Ca2+ mobilization for the activation of platelets. Three different PKC subtypes that specifically react with antibodies to alpha- beta- and zeta-PKC have been detected in human platelets. We have compared the subcellular redistribution of these isoforms in platelets after exposure to the tumour-promoting phorbol ester phorbol 12-myristate 13-acetate (PMA) and to two physiological agonists, thrombin and vasopressin. In the presence of PMA, beta-PKC is most rapidly translocated to membranes, followed by zeta-PKC and alpha-PKC [membrane contents of 39 +/- 6, 31 +/- 4 and 24 +/- 4% (means +/- S.E.M.) respectively after 2 min incubation]. In contrast, both thrombin and vasopressin induced a biphasic translocation of PKC isoforms. For both agonists, the first phase of translocation occurred within 1 min and was identical for the three isoforms. However, during the second phase, the translocation of zeta-PKC by thrombin and vasopressin differed [membrane contents (mean +/- S.E.M.) of 24 +/- 3 and 46 +/- 4% respectively after 10 min]. These results suggest a differential activation of zeta-PKC by vasopressin and thrombin. PMA-induced translocation of alpha-PKC was decreased from 278 +/- 27 to 198 +/- 24 (mean +/- S.E.M., P = 0.02; percentage increase over control value) in the presence of 1 mM-EDTA, whereas chelation of intracellular Ca2+ by Quin2-AM does not influence this response. These results suggest that the PMA-induced translocation of alpha-PKC depends on the presence of 1 mM concentration of extracellular Ca2+. In addition, the chelation of either extracellular or intracellular Ca2+ inhibited both vasopressin- and thrombin-induced translocation of all three isoforms, suggesting that Ca2+ is an important requirement for the translocation of alpha-, beta- and zeta-PKC by physiological agonists. In conclusion, the translocation of PKC varies between different isoforms and between different agonists.

Activation of the c-Raf protein kinase by protein kinase C phosphorylation.

The product of the c-raf-1 proto-oncogene is a cytoplasmic serine/threonine protein kinase that appears to be activated in signal transduction from a variety of cell-surface receptors. The mechanism of c-Raf activation upon stimulation of cell-surface receptors is not clear, but there seem to exist multiple pathways of activation which involve tyrosine and/or serine phosphorylation of the c-Raf protein in vivo. The activated state of Raf is reflected in an increased apparent molecular weight of the Raf protein in sodium dodecyl sulfate-polyacrylamide gels owing to hyperphosphorylation. The tumor promoter 12-O-tetradecanoyl phorbol 13-acetate (TPA) is one of the agents able to induce this hyperphosphorylation of Raf in vivo, suggesting that protein kinase C (PKC) may be involved in the activation of c-Raf in particular situations. Using recombinant baculoviruses expressing PKC and Raf polypeptides, we show here that conventional PKC types (alpha, beta, gamma) but not novel types (delta, zeta, eta) or the unrelated Mos kinase are able to activate c-Raf in a TPA-dependent manner upon coexpression in insect cells. Direct phosphorylation of the Raf protein with PKC in vitro also enhanced the kinase activity of c-Raf, suggesting that c-Raf acts immediately downstream of PKC in a protein kinase cascade which is triggered by TPA and may lead to transcriptional activation of TPA-inducible genes and tumor promotion.