Activation-dependent degradation of protein kinase C eta

Prolonged activation of protein kinase Cs (PKCs) by long-term treatment of cells with phorbol ester tumor promoters down-regulates the expression of many PKCs. To investigate the molecular mechanisms involved in the down-regulation of PKC eta, we expressed the novel PKCs eta and θ and various mutant forms in baby hamster kidney cells. Upon overexpression, constitutively active PKC eta, but not wild type or kinase-dead PKC eta, underwent rapid degradation to generate several lower molecular weight polypeptides. When co-expressed with active kinases, kinase-dead PKC eta with a pseudosubstrate site mutation designed to give an active conformation was down-regulated while the wild type PKC eta was not. These results suggest requirements for kinase activity and an active conformation for down-regulation of PKC eta. Treatment with the proteasome inhibitors N-Ac-Leu-Leu-norleucinal and lactacystin led to accumulation of PKC eta proteolytic products and potentially ubiquitinated forms. While wild type PKC eta localizes mostly to the detergent-soluble fraction of the cell, a significant portion of full-length constitutively active PKC eta and of kinase-dead, active conformation PKC eta were found in the detergent-insoluble fraction. Several proteolytic fragments of constitutively active PKC eta also were found in the detergent insoluble fraction. These full-length and proteolytic fragments of PKC eta in the detergent-insoluble fraction accumulated further in the presence of proteasome inhibitors. These data suggest that active conformation PKC eta accumulates in the detergent-insoluble compartment, is degraded by proteolysis in the presence of kinase activity, and that the cleavage products undergo further degradation via ubiquitin-mediated degradation in the proteasome. Oncogene (2000) 19, 4263 - 4272

Phorbol 12-myristate 13-acetate induces protein kinase ceta-specific proliferative response in astrocytic tumor cells

Protein kinase C (PKC) activation has been implicated in cellular proliferation in neoplastic astrocytes. The roles for specific PKC isozymes in regulating this glial response, however, are not well understood. The aim of this study was to characterize the expression of PKC isozymes and the role of PKC-eta expression in regulating cellular proliferation in two well characterized astrocytic tumor cell lines (U-1242 MG and U-251 MG) with different properties of growth in cell culture. Both cell lines expressed an array of conventional (alpha, betaI, betaII, and gamma) and novel (theta and epsilon) PKC isozymes that can be activated by phorbol myristate acetate (PMA). Another novel PKC isozyme, PKC-eta, was only expressed by U-251 MG cells. In contrast, PKC-delta was readily detected in U-1242 MG cells but was present only at low levels in U-251 MG cells. PMA (100 nm) treatment for 24 h increased cell proliferation by over 2-fold in the U-251 MG cells, whereas it decreased the mitogenic response in the U-1242 MG cells by over 90%. When PKC-eta was stably transfected into U-1242 MG cells, PMA increased cell proliferation by 2.2-fold, similar to the response of U-251 MG cells. The cell proliferation induced by PMA in both the U-251 MG and U-1242-PKC-eta cells was blocked by the PKC inhibitor bisindolylmaleimide (0.5 micrometer) and the MEK inhibitor, PD 98059 (50 micrometer). Transient transfection of wild type U-251 with PKC-eta antisense oligonucleotide (1 micrometer) also blocked the PMA-induced increase in [(3)H]thymidine incorporation. The data demonstrate that two glioblastoma lines, with functionally distinct proliferative responses to PMA, express different novel PKC isozymes and that the differential expression of PKC-eta plays a determining role in the different proliferative capacity.

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.

Contributions to maxima in protein kinase C activation

In many lipid systems, the activity of protein kinase C (PKC) exhibits a peak followed by a decline as the mol % of one component is increased. In these systems, an increase in one lipid component is always at the expense of another or accompanied by a change in total lipid concentration. Here we report that in saturated phosphatidylserine (PS)/phosphatidylcholine (PC)/diacylglycerol (DAG) mixtures, increasing PS or DAG at the expense of PC revealed an optimal mol % PS, dependent on mol % DAG, with higher mol % PS diminishing activity. The decrease at high mol % PS is probably not attributable simply to more gel-phase lipid due to the higher melting temperature of saturated PS versus PC because a similar peak in activity occurred in unsaturated lipid systems. Increasing the total lipid concentration at suboptimal mol % PS provided the same activity as higher mol % PS at lower total lipid concentration. However, at optimal mol % PS, activity increased and then decreased as a function of total lipid concentration. PKC autophosphorylation also exhibited an optimum as a function of mol % PS, and increasing the PKC concentration increased the mol % PS at which activity decreased, both for autophosphorylation and for heterologous phosphorylation. Formation of two-dimensional crystals of PKC on lipid monolayers also exhibited a peak as a function of mol % PS, and the unit cell size of the crystals formed shifts from 50 x 50 A at low mol % PS to 75 x 75 A at higher PS. Collectively, these data suggest the existence of optimal lipid compositions for PKC activation, with increased quantity of these domains serving to dilute out enzyme-substrate aggregates and/or enzyme-enzyme aggregates on the lipid surface.

Differential downstream functions of protein kinase Ceta and -theta in EL4 mouse thymoma cells

Sensitive EL4 mouse thymoma cells (s-EL4) respond to phorbol esters with growth inhibition, adherence to substrate, and production of cytokines including interleukin 2. Since these cells express several of the phorbol ester-sensitive protein kinase C (PKC) isozymes, the function of each isozyme remains unclear. Previous studies demonstrated that s-EL4 cells expressed substantially more PKCeta and PKCtheta than did EL4 cells resistant to phorbol esters (r-EL4). To examine potential roles for PKCeta and PKCtheta in EL4 cells, wild type and constitutively active versions of the isozymes were transiently expressed using a Sindbis virus system. Expression of constitutively active PKCeta, but not PKCtheta, in s- and r-EL4 cells altered cell morphology and cytoskeletal structure in a manner similar to that of phorbol ester treatment, suggesting a role for PKCeta in cytoskeletal organization. Prolonged treatment of s-EL4 cells with phorbol esters results in inhibition of cell cycling along with a decreased expression of most of the PKC isozymes, including PKCtheta. Introduction of virally expressed PKCtheta, but not PKCeta, overcame the inhibitory effects of the prolonged phorbol ester treatment on cell cycle progression, suggesting a possible involvement of PKCtheta in cell cycle regulation. These results support differential functions for PKCeta and PKCtheta in T cell activation.

Influence of lipid on the structure and phosphorylation of protein kinase C alpha substrate peptides

The structure and phosphorylation of two protein kinase C (PKC) alpha substrate peptides were investigated in varying lipid systems using enzyme activity assays and circular dichroism (CD) spectroscopy. The alpha-peptide, which exhibits the typical PKC alpha substrate motif and is based on the pseudosubstrate region of PKCalpha, was phosphorylated to a similar extent in bovine brain phosphatidylserine vesicles or diheptanoylphosphatidylcholine (PC7) micelles (both with 5 mol % 1,2-dioleoyl-sn-glycerol), whereas neuromodulin (NM)-peptide, which does not exhibit this motif by virtue of its primary structure, was phosphorylated to a much lesser extent in the PC7 micellar system. CD spectra of the peptides indicated that NM-peptide underwent a dramatic structural change in the presence of dimyristoylphosphatidylserine (DMPS) vesicles, whereas spectra acquired in PC7 micelles were similar to those acquired in buffer alone. No significant structural change was observed in the alpha-peptide in the presence of either lipid. PKC activity assays conducted with a series of NM-peptides successively substituted with nitroxide spin labels at each residue position suggested that several residues distal to the phosphorylation site are necessary for substrate recognition. The effect of these substitutions is not consistent with the binding of the NM-peptide to PKC in an extended structure, but is consistent with the binding of this peptide in a helical conformation. Furthermore, the docking of a helical NM-peptide to the substrate binding site of PKC suggests that the interaction is energetically feasible. These results suggest that PKC may recognize some non-linear substrate motifs and that lipid binding may convert a protein into a better PKC substrate.

Two-dimensional crystals of protein kinase C

Three two-dimensional (2D) crystal forms of protein kinase C (PKC) alpha and three of PKC delta have been grown on lipid monolayers composed of dioleoylphosphatidylcholine: dioleoylphosphatidylserine: (45:50:5 molar ratio). In the absence of DO, two additional 2D crystals of PKC delta are seen, suggesting that the presence of diolein (DO) alters the conformation of intact PKC at the lipid surface. Reconstructions of electron micrographs of these eight lattices show good reproducibility and indicate that several are appropriate for three-dimensional reconstruction to 20 A resolution.

Activation of protein kinase C alpha and delta by bile acids: correlation with bile acid structure and diacylglycerol formation

The feedback repression of cholesterol 7alpha-hydroxylase transcriptional activity and mRNA levels by taurocholate (TCA) occurs via a protein kinase C (PKC)-dependent signal. To determine whether bile acids could activate PKC indirectly via generation of diacylglycerol (DG), their effects on DG levels in primary cultures of rat hepatocytes were determined using a DG kinase assay. To determine whether bile acids might activate PKC isozymes more directly, their effects on PKC alpha and delta purified from baculovirus expression systems were examined in phosphatidylserine/phosphatidylcholine/Triton X-100 (PS/PC/TX) mixed micelles. Addition of tauroursodeoxycholate (TUDCA), taurocholate (TCA), or taurodeoxycholate (TDCA) (50 microM) to the cells rapidly (15 min) increased DG content in cultured rat hepatocytes to 105%, 155%, and 130%, respectively, as compared to untreated control cultures. Addition of TCA increased PKC alpha specific activity with EC50 of approximately 400 nM; maximal activity was observed with 5 microM. Other taurine-conjugated bile acids (5 microM) increased PKC alpha specific activity (pmol/min/microg protein) in proportion to their relative hydrophobicity: PS/PC/TX 17 +/- 2; + TUDCA 29 +/- 18; + TCA 68 +/-13; + TDCA 166 +/- 21; and, taurochenodeoxycholate 178 +/- 20 (P vs. PS/PC/TX = 0.54, 0.019, 0.002, and 0.001, respectively); unconjugated bile acids gave similar results (r2 for activity vs. hydrophobicity index 0.59). Taurine-conjugated bile acid interaction enthalpies, as determined by dimyristoyl-phosphatidylcholine chromatography, were more highly correlated (r2 = 0.96) with PKC alpha activation than with the hydrophobicity index. TCA also stimulated the activity of purified PKCdelta with EC50 of approximately 150 nM and maximally (2.7-fold) at 1 microM. Free and taurine-conjugated bile acids (1 microM) increased PKCdelta activity according to their hydrophobicity index (r2 = 0.89) and interaction enthalpies (r2 = 0.96).

A role for protein kinase CbetaI in the regulation of Ca2+ entry in Jurkat T cells

T cell activation leading to cytokine production and cellular proliferation involves a regulated increase and subsequent decrease in the intracellular concentration of Ca2+ ([Ca2+]i). While much is understood about agonist-induced increases in [Ca2+]i, less is known about down-regulation of this pathway. Understanding the mechanism of this down-regulation is critical to the prevention of cell death that can be the consequence of a sustained elevation in [Ca2+]i. Protein kinase C (PKC), activated by the diacylglycerol produced as a consequence of T cell receptor engagement, has long been presumed to be involved in this down-regulation, although the precise mechanism is not wholly clear. In this report we demonstrate that activation of PKC by phorbol esters slightly decreases the rate of Ca2+ efflux from the cytosol of Jurkat T cells following stimulation through the T cell receptor or stimulation in a receptor-independent manner by thapsigargin. On the other hand, phorbol ester treatment dramatically reduces the rate of Ca2+ influx following stimulation. Phorbol ester treatment is without an effect on Ca2+ influx in a different T cell line, HSB. Down-regulation of PKCbetaI expression by 18-h phorbol ester treatment is associated with a loss of the response to acute phorbol ester treatment in Jurkat cells, suggesting that PKCbetaI may be the isozyme responsible for the effects on Ca2+ influx. Electroporation of an anti-PKCbetaI antibody, but not antibodies against PKCalpha or PKCgamma, led to an increase in the rate of Ca2+ influx following stimulation. Taken together, these data suggest that PKCbetaI may be a component of the down-regulation of increases in [Ca2+]i associated with Jurkat T cell activation.

Selective up-regulation of protein kinase C eta in phorbol ester-sensitive versus -resistant EL4 mouse thymoma cells

Stimulation of sensitive EL4 mouse thymoma cells (s-EL4) with phorbol esters results in production of interleukin 2 (IL-2), adherence to a plastic substrate, and growth inhibition, whereas a phorbol ester-resistant variant (r-EL4) fails to respond. Previous studies revealed substantially decreased expression of protein kinase C (PKC) epsilon in the r-EL4 versus s-EL4 cells. This work has been extended to examine the more recently described PKC isozymes. Western and Northern analyses revealed a marked decrease in PKC eta and theta in r-EL4 as compared to s-EL4 cells. Treatment of these lines with phorbol ester for 24 h resulted in down-regulation of all PKC isozymes examined except PKC eta, which was up-regulated in the s-EL4 cells at the time of maximal IL-2 production. Two newly isolated EL4 clones, resistant to phorbol ester-induced growth inhibition but still exhibiting the phorbol ester-induced adherence and IL-2 production, both expressed PKC eta and theta. Collectively, these observations suggest a dissociation of growth inhibition from adherence and IL-2 production pathways and a potential role for PKC eta in the latter.

Activation of protein kinase C by coexisting diacylglycerol-enriched and diacylglycerol-poor lipid domains

To test the hypothesis that activation of protein kinase C (PKC) is related to the interface between coexisting diacylglycerol- (DAG-) enriched and DAG-poor phases, the thermotropic phase behavior of the ternary mixtures dimyristoylphosphatidylcholine (DMPC)/dimyristoylphosphatidylserine (DMPS)/dioleoylglycerol (DO), DMPC/DMPS/1-palmitoyl-2-oleoylglycerol (PO), and DMPC/DMPS/dimyristoylglycerol (DM) was analyzed and compared with the ability of the lipid mixtures to support PKC activity. Differential scanning calorimetry (DSC) was used to monitor the gel-to-liquid crystalline phase transition as a function of the mole fraction of DO (chiDO), PO (chiPO), or DM (chiDM) in DMPC/DMPS (1:1) multilamellar vesicles (MLVs) and of chiDO in large unilamellar vesicles (LUVs). The addition of DAG at low mole fractions gave rise to the appearance of two or more overlapping transitions. The phase boundaries of the ternary mixtures deduced from the partial phase diagrams were chiDO = approximately 0.10 and approximately 0.3 for DMPC/DMPS/DO, chiPO = approximately 0.05 and approximately 0.4 for DMPC/DMPS/PO, and chiDM = approximately 0.025 and approximately 0.5-0.6 for DMPC/ DMPS/DM. Above these mole fractions of DAG, the transitions again became very sharp. The ability of the lipid mixtures to support activity of PKC alpha and PKC eta was examined below and above the gel-to-liquid crystalline phase transition. In the gel phase, PKC activity went through a maximum as a function of increasing mole fraction of each DAG and was restricted to lipid compositions in which coexisting phases were observed. Maximal activity decreased with increasing saturation of the DAG. In the fluid state, maximal PKC activity was shifted to higher DO mole fractions and the peak was much broader. Collectively, these data support a role for both the presence and nature of interface between compositionally distinct domains in activation of PKC.

Deficient tyrosine phosphorylation of c-Cbl and associated proteins in phorbol ester-resistant EL4 mouse thymoma cells

Two tyrosine phosphoproteins in phorbol ester-sensitive EL4 (S-EL4) mouse thymoma cells have been identified as the p120 c-Cbl protooncogene product and the p85 subunit of phosphatidylinositol 3-kinase. Tyrosine phosphorylation of p120 and p85 increased rapidly after phorbol ester stimulation. Phorbol ester-resistant EL4 (R-EL4) cells expressed comparable amounts of c-Cbl and phosphatidylinositol 3-kinase protein but greatly diminished tyrosine phosphorylation. Co-immunoprecipitation experiments revealed complexes of c-Cbl with p85, and of p85 with the tyrosine kinase Lck in phorbol ester-stimulated S-EL4 but not in unstimulated S-EL4 or in R-EL4 cells. In vitro binding of c-Cbl with Lck SH2 or SH3 domains was detected in both S-EL4 and R-EL4 cells, suggesting that c-Cbl, p85, and Lck may form a ternary complex. In vitro kinase assays revealed phosphorylation of p85 by Lck only in phorbol ester-stimulated S-EL4 cells. Collectively, these results suggest that Cbl-p85 and Lck-p85 complexes may form in unstimulated S-EL4 and R-EL4 cells but were not detected due to absence of tyrosine phosphorylation of p85. Greatly decreased tyrosine phosphorylation of c-Cbl and p85 in the complexes may contribute to the failure of R-EL4 cells to respond to phorbol ester.

Lipid lateral heterogeneity in phosphatidylcholine/phosphatidylserine/diacylglycerol vesicles and its influence on protein kinase C activation

To test the hypothesis that the activation of protein kinase C (PKC) is influenced by lateral heterogeneities of the components of the lipid bilayer, the thermotropic phase behavior of dimyristoylphosphatidylcholine (DMPC)/dimyristoylphosphatidylserine (DMPS)/dioleoylglycerol (DO) vesicles was compared with the activation of PKC by this system. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to monitor the main transition (i.e., the gel-to-fluid phase transition) as a function of mole fraction DO (chi(DO)) in DMPC/DO, DMPS/DO, and [DMPC/DMPS (1:1, mol/mol)]/DO multilamellar vesicles (MLVs). In each case, when chi(DO) < or approximately 0.3, DO significantly broadened the main transition and shifted it to lower temperatures; but when chi(DO) > approximately 0.3, the main transition became highly cooperative, i.e., narrow, again. The coexistence of overlapping narrow and broad transitions was clearly evident in DSC thermograms from chi(DO) approximately 0.1 to chi(DO) approximately 0.3, with the more cooperative transition growing at the expense of the broader one as chi(DO) increased. FTIR spectroscopy, using analogs of DMPC and DMPS with perdeuterated acyl chains, showed that the melting profiles of all three lipid components in [DMPC/DMPS (1:1, mol/mol)]/DO MLVs virtually overlay when chi(DO) = 0.33, suggesting that a new type of phase, with a phospholipid/DO mole ratio near 2:1, is formed in this system. Collectively, the results are consistent with the coexistence of DO-poor and DO-rich domains throughout the compositions chi(DO) approximately 0.1 to chi(DO) approximately 0.3, even at temperatures above the main transition. Comparison of the phase behavior of the binary mixtures with that of the ternary mixtures suggests that DMPS/DO interactions may be more favorable than DMPC/DO interactions in the ternary system, especially in the gel state. PKC activity was measured using [DMPC/DMPS (1:1, mol/mol)]/DO MLVs as the lipid activator. At 35 degrees C (a temperature above the main transition of the lipids), PKC activity increased gradually with increasing chi(DO) from chi(DO) approximately 0.1 to chi(DO) approximately 0.4, and activity remained high at higher DO contents. In contrast, at 2 degrees C (a temperature below the main transition), PKC activity exhibited a maximum between chi(DO) approximately 0.1 and chi(DO) approximately 0.3, and at higher DO contents activity was essentially constant at 20-25% of the activity at the maximum. We infer from these results that the formation of DO-rich domains is related to PKC activation, and when the lipid is in the gel state, the coexistence of DO-poor and DO-rich phases also contributes to PKC activation.

Activation of protein kinase C by lysophosphatidic acid: dependence on composition of phospholipid vesicles

Lysophosphatidic acid (LPA) has attracted recent attention as a major serum-derived regulator implicated in responses to vascular injury and inflammation, in tumour invasiveness and in neuronal signalling and remodelling. Although the possibility of a specific G-protein-coupled LPA receptor protein has been suggested, characterization of such a receptor is lacking. Since LPA can activate protein kinase C (PKC) pathways in many cells and PKC activators mimic many LPA effects, the possibility of more direct LPA effects on PKC was investigated. Phosphatidylcholine (PC)/phosphatidylserine (PS)/diacylglycerol (DAG) lipid vesicles of defined acyl chain composition were used to activate the enzyme. At total concentrations of saturated PC/PS + DAG vesicles (2-3 mM) that provided maximal PKC activation, 1-10 mol % [18:1]-LPA led to a further approx. 2-fold activation of PKC alpha. At lower lipid concentrations, a greater increase was observed with LPA concentrations up to 16-20 mol %. Higher concentrations of LPA were inhibitory. The LPA activation of PKC was dependent on the presence of DAG, PS and Ca2+. [18:1]-Lysophosphatidylcholine produced similar PKC activation in PC/PS/DAG vesicles. [14:0]-LPA was less effective, and longer-chain saturated lysolipids were ineffective. In unsaturated PC/PS vesicles, very little to no effect of LPA was discernable. These results suggest that physiologically or pathologically relevant concentrations of LPA can contribute to PKC activation depending on the composition of the lipid membrane. We hypothesize that LPA may affect the formation of lipid domains that are recognized by the enzyme.

Protein kinase C in cyclic stretch-induced nerve growth factor production by urinary tract smooth muscle cells

Cyclic stretch of cultured urinary tract smooth muscle cells has been used to mimic some of the events that occur with bladder obstruction. The stretch stimulus induces production of nerve growth factor (NGF), which has been implicated in changes in bladder innervation. Stretch-induced NGF production was blocked by actinomycin. Involvement of protein kinase C (PKC) in the stretch-induced NGF production is strongly suggested by the following observations. Phorbol ester activators of PKC mimicked the stretch response as did platelet-derived growth factor (PDGF), which acts, in part, through generation of endogenous diacylglycerols. Both stretch- and PDGF-induced NGF production were blocked by prolonged incubation with phorbol ester to downregulate PKC. Western blot analysis confirmed partial downregulation of the Ca(2+)-dependent PKC-alpha and PKC-beta 1 and near complete downregulation of the Ca(2+)-independent PKC isozymes delta, epsilon, and zeta. The involvement of PKC in transducing a physical stimulus (stretch) into a biochemical response (NGF production) has implications for novel types of therapeutic intervention in ailments such as bladder obstruction.

Rapid tyrosine phosphorylation of an 85,000 M(r) protein after phorbol ester stimulation of EL4 thymoma cells

Early signalling events between protein kinase C (PKC) activation and lymphokine transcription were compared between phorbol ester-sensitive and -resistant EL4 cell lines which do or do not respond with interleukin 2 (IL2) production, respectively. The earliest event detected in the sensitive cell line was a dramatic increase in the tyrosine phosphorylation of an 85,000 M(r) protein (p85; 30 s), followed by mobility shifts of raf-1, mitogen-activated protein kinase kinase (MEK), mitogen-activated protein (MAP) kinase, lck and ZAP-70 (within 5 min). In contrast, p85 was not detected in the resistant cell line and lck and raf-1 mobility shifts exhibited delayed kinetics. Both vanadate and okadaic acid blocked the phorbol ester-stimulated p85 tyrosine phosphorylation in the sensitive cell line, suggesting that a phosphatase activity downstream of PKC activation may be required for p85 tyrosine phosphorylation. Characterization of p85 and its regulation should help elucidate some of the earliest events in this PKC pathway.

Regulation of intestinal guanylate cyclase by the heat-stable enterotoxin of Escherichia coli (STa) and protein kinase C

The heat-stable enterotoxin of Escherichia coli (STa) stimulates membrane-bound guanylate cyclase in intestinal epithelium and induces fluid and ion secretion. Using the T84 human colon carcinoma cell line as a model, we observed that phorbol esters markedly enhanced STa-stimulated cyclic GMP accumulation in T84 cells (C. S. Weikel, C. L. Spann, C. P. Chambers, J. K. Crane, J. Linden, and E. L. Hewlett, Infect. Immun. 58:1402-1407, 1990). In this study we document that the phorbol ester treatment increases 125I-STa-binding sites as well as membrane-bound guanylate cyclase activity in T84 cells and provide evidence that both effects are mediated by phosphorylation. Guanylate cyclase activity was increased approximately 50% in membranes prepared from intact T84 cells treated with phorbol-12,13-dibutyrate (beta-PDB) and after treatment of homogenates with beta-PDB in a manner dependent on ATP, MgCl2, and cytosol. Similarly, treatment of membranes with purified bovine brain protein kinase C in the presence of appropriate cofactors and beta-PDB resulted in an increase in STa-stimulated guanylate cyclase activity of about 70%. Likewise, the number of 125I-STa-binding sites was increased by about 25 to 40% in membranes prepared from intact cells or homogenates treated with beta-PDB; no effect on binding affinity (Kd = 0.15 nM) was noted. These experiments suggest that protein kinase C may phosphorylate the STa receptor-guanylate cyclase or a closely related protein and increase guanylate cyclase activity. The stimulatory effects of protein kinase C on STa-sensitive guanylate cyclase are opposite in direction to the profound inhibitory effects of the kinase on atrial natriuretic peptide-stimulated guanylate cyclase, demonstrating differential regulation by protein kinases within the guanylate cyclase-receptor family.

Activation and inhibition of protein kinase C isozymes alpha and beta by Gd3+

Gd3+ was evaluated as a probe for Ca2+ sites on protein kinase C (PKC) by studying its ability to replace Ca2+ in activation of PKC isozymes II (beta) and III (alpha) in the lipid systems phosphatidylserine/1,2-dioleoyl-sn-glycerol (PS/DO) and diheptanoylphosphatidylcholine (PC7)/DO. PKC beta was stimulated by Ca2+ or Gd3+ in PS/DO whereas activity in PC7/DO was independent of these metals. Thus, it is suggested that Gd3+ replaces Ca2+ at a site involving metal-lipid interactions. High concentrations of Ca2+ or Gd3+ inhibited activity in both lipid systems. Analysis of the Gd3+ inhibition in the PC7/DO system suggests that it is due to formation of GdATP, which competes at the MgATP site. Activity of PKC alpha was dependent on low concentrations of Ca2+ in both lipid systems. The ability of Gd3+ to substitute for Ca2+ could not be evaluated in the PS system due to the inability to completely remove contaminating Ca2+ without chelating buffers. Successful reduction of contaminating Ca2+ was achieved in the PC7 system but Gd3+ failed to substitute for Ca2+ in activating PKC alpha and only caused inhibition. This is consistent with binding of Gd3+ to a Ca2+ site at or near the active site of the enzyme rather than to a site on the lipid. These results indicate that interactions between PKC and Gd3+ are complex, involving occupation of more than one class of sites. Conditions for separately evaluating the individual sites can be manipulated by selection of isozyme and lipid system.

High-affinity Ca(2+)- and substrate-binding sites on protein kinase C alpha as determined by nuclear magnetic resonance spectroscopy

Water proton nuclear magnetic resonance (NMR) relaxation rates were used to identify metal sites on protein kinase C (PKC) isozymes alpha and beta using paramagnetic Gd3+ as a probe. The paramagnetic effect of Gd3+ on water proton relaxation was enhanced with PKC isozymes alpha and beta in the presence of diheptanoylphosphatidylcholine/1,2-dioleoyl-sn-glycerol (PC7/DO). The data are consistent with a single class of metal-binding sites on PKC beta and two classes of sites on PKC alpha: a single high-affinity site with a KD for Gd3+ of 0.2 microM and a larger class of sites with a lower affinity for Gd3+. Titration with Ca2+ abolished the observed enhancement of water proton relaxation by the PKC alpha.Gd3+ complex, consistent with displacement of Gd3+ by Ca2+. Titrations of the PKC alpha.Gd3+ complex with Co(NH3)4ATP, a substitution-inert analogue of ATP, caused a substantial decrease in the observed water proton relaxation enhancement, consistent with formation of a ternary enzyme.metal.substrate complex with a KPKC alpha.Gd.[CoATP] of 30-100 nM. Titration of the metal enzyme complex with a model peptide substrate derived from the pseudosubstrate sequence of PKC alpha caused a similar decrease in enhancement at stoichiometric concentrations consistent with the formation of a PKC alpha.Gd3+.peptide complex with a KPKC alpha.Gd.[peptide] of less than or equal to 13 nM. Titrations of the fully formed PKC alpha.Gd3+.peptide complex with Co(NH3)4ATP caused a further decrease in enhancement consistent with formation of a quaternary complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of phospholipid unsaturation on protein kinase C activation

To examine the hypothesis that physical features of the membrane contribute to protein kinase C activation, phosphatidylcholine/phosphatidylserine/diolein (70:25:5) vesicles of defined acyl chain composition were tested for their ability to activate the enzyme. Maximal activation was found to correlate with the mole percent unsaturation in the system. Unsaturation could be provided by either the phosphatidylserine or the phosphatidylcholine component. Vesicles containing 5 mol% diolein but lacking any unsaturation in the phospholipid did not support activity, indicating that acidic head groups alone are not sufficient for activity. The saturated lipid vesicles could be rendered effective but only at very high (25 mol%) concentrations of diolein. The degree of acyl chain unsaturation and the positioning of the double bond had little effect on the activity, suggesting that the effect of the unsaturation is due to some physical property of the lipid rather than to a specific lipid-protein interaction. Addition of cholesterol to both saturated and unsaturated systems indicated that fluidity, as assessed by fluorescence anisotropy, did not correlate with activity. These results suggest that a physical property of the membrane other than fluidity is important for the activation of protein kinase C. A model for protein kinase C activation involving phase separation and/or head group spacing is discussed.

Defective induction of Jun and Fos-related proteins in phorbol ester-resistant EL4 mouse thymoma cells

Treatment of sensitive EL4 mouse thymoma cells with phorbol esters causes growth inhibition, adherence to substrate and production of several lymphokines including Interleukin 2. Resistant cells lack all of these responses. Since production of Interleukin 2 mRNA is dependent on protein synthesis, and the Interleukin 2 gene has a phorbol ester responsive element, we examined both cell lines for expression of the various Jun and Fos species which bind to this element. Phorbol ester induced c-fos, jun-B, and jun-D RNAs within 20 min in both cell lines. Fos-B was similarly induced in sensitive cells but induction was delayed and greatly enhanced in resistant cells. C-jun RNA induction was detected only in sensitive cells. Western analysis confirmed the induction of c-Jun and a Fos-related protein in sensitive cells only. Southern analysis indicated that both cell lines contain c-jun and fra-1 genes. These results suggest that defective induction of c-Jun and/or Fos-related proteins may contribute to the absence of phorbol ester-induced lymphokine production in resistant EL4 cells.

Protein kinase C isozyme expression in phorbol ester-sensitive and -resistant EL4 thymoma cells

To investigate whether differential protein kinase C isozyme expression in phorbol ester-sensitive and -resistant EL4 thymoma cells could account for the difference in phorbol ester responsiveness, we purified and characterized isozymes from the two cell lines. In both cell types, two peaks of protein kinase C activity were resolved on hydroxylapatite following DEAE-cellulose and phenyl-Superose chromatography. Western blot analysis showed that the first peak corresponded to protein kinase C-beta and the second to protein kinase C-alpha. Two-dimensional phosphotryptic mapping of the purified alpha and beta isozymes did not reveal any reproducible differences between sensitive and resistant EL4 cells. Nor were any differences between the cell types observed in the cytosolic versus membrane localization of alpha and beta protein kinase C. Northern blot analysis showed the expression of mRNA for protein kinase C-alpha, -beta, -delta and -epsilon in both cell lines, and the absence of mRNA for gamma or zeta. Although no major differences in expression of alpha, beta, or delta mRNA between sensitive and resistant EL4 cells were detectable, expression of protein kinase C-epsilon mRNA in resistant cells was only 20-25% of that in sensitive. Western blot analysis with anti-protein kinase C-epsilon antibodies showed the presence of the epsilon-isozyme in sensitive cells and the absence of detectable amounts in resistant cells. Although protein kinase C-epsilon constitutes only a small portion of the total protein kinase C in sensitive cells, the possibility is raised that decreased protein kinase C-epsilon expression may contribute to the failure of resistant EL4 cells to respond to phorbol esters.

Differential activation of protein kinase C isozymes by short chain phosphatidylserines and phosphatidylcholines

To investigate the importance of the physical state of phospholipids for activation of protein kinase C, we have used short chain phospholipids, which, depending on their concentration, can exist as either monomers or micelles. We previously reported that short chain phosphatidylcholines (PC) can activate protein kinase C at concentrations that correlate with the critical micelle concentration of the activating lipid (Walker, J. M., and Sando, J. J. (1988) J. Biol. Chem. 263, 4537-4540). We have now expanded this work to short chain phosphatidylserine (PS) systems in order to examine the role of Ca2(+)-phospholipid interactions in the activation process. Short chain PS were synthesized from corresponding PC and purified by reverse-phase high pressure liquid chromatography. Use of the short chain system has revealed significant differences in the activation of type II and type III protein kinase C isozymes. The type II isozyme required Ca2+ in the presence of long chain PS vesicles; in the presence of the short chain phospholipid micelles (PC or PS), most of the activity was Ca2+ independent. Addition of diacylglycerol caused a small increase in type II activity in all phospholipid systems. In contrast, type III protein kinase C was Ca(+)-dependent in all of the lipid systems. The concentration of Ca2+ required to activate type III protein kinase C was independent of the phospholipid type despite large differences in the ability of these lipids to bind Ca2+. This isozyme required diacylglycerol only in the PC micelle system or with vesicles composed of long chain saturated PS. The presence of short chain PS micelles or long chain PS with unsaturated fatty acyl chains rendered this Ca2(+)-dependent protein kinase C virtually diacylglycerol independent. These results are consistent with a model in which type II protein kinase C requires Ca2+ primarily for membrane association, a requirement which is bypassed with the micelle system, whereas type III protein kinase C has an additional Ca2+ requirement for activity that does not involve Ca2(+)-phospholipid interactions.

Phosphorylation of protein kinase C by casein kinase-1

Because phosphorylation of protein kinase C (PKC) may provide a mechanism for regulation of this enzyme, we have examined the ability of two other kinases to phosphorylate PKC. Our results show that casein kinase 1 (CK-1), but not casein kinase 2 (CK-2), can phosphorylate PKC in the absence of Ca2+ and phospholipids. The 32P incorporation into PKC in the presence of Ca2+ and phospholipids is also enhanced by CK-1.

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.

Activation of protein kinase C by short chain phosphatidylcholines

The acidic phospholipid requirement for protein kinase C (Ca2+/phospholipid-dependent enzyme) activation has been well established, although the molecular nature of this lipid-protein interaction is unclear. The additional requirement for Ca2+ has provided the basis for several models involving charge interactions. We now report that short chain neutral phosphatidylcholines also activate the kinase. Examination of a large series of phosphatidylcholines of varying acyl chain length revealed a close correlation between the ability to form micelles and the ability to support kinase activity. Peak activation occurred in the concentration range just before the critical micelle concentration of each phospholipid. Activation was absolutely dependent on the presence of Ca2+ and diacylglycerol. The possible roles of Ca2+ and phospholipid in the activation process are reexamined in light of these unexpected results.

Phorbol esters increase adenylate cyclase activity and stability in pituitary membranes

In this report, we demonstrate that calcium and phorbol esters enhance cAMP production in GH4C1 cell homogenates. The mechanism for this is a reduction in the rate of decay of adenylate cyclase activity over the course of the assay. Purified protein kinase C can reconstitute calcium- and phorbol ester-dependent adenylate cyclase. Phorbol ester-activated protein kinase C increases both the initial rate of cAMP synthesis and reduces the time-dependent decay of adenylate cyclase activity in membrane preparations. The rate of cAMP production is fit to an equation derived from a model which assumes that adenylate cyclase initially exists in a high activity state which decays exponentially into a low activity state. We suggest that protein kinase C can both prevent the decay of the high activity state and convert the low activity state into the high activity state.

Differences between human and goose erythrocytes in response to phorbol esters and expression of phorbol ester receptors

Phorbol esters inhibited the uptake of a fluorescent glucose analogue in goose but not in human erythrocytes. Specific phorbol-12,13-dibutyrate (PDB) binding sites were identified in both goose and human erythrocytes. In the absence of Ca2+ and phospholipid, PDB binding in whole cell lysates was similar to that in intact cells, but addition of Ca2+ (0.5 mM) and phosphatidyl serine (96 micrograms/ml) caused a 4-fold increase in the binding detected in lysates. Nonlinear least-squares analysis of the PDB binding isotherm revealed that the data for lysates from both goose and human cells were best fit by a two-site model, with goose erythrocytes having approximately 3 times as many sites per class of receptors. Subcellular fractionation of human lysates indicated that the high (Kd = 3.6 +/- 2.2 nM) and low (Kd = 20 +/- 5 nM) affinity sites could be accounted for by the contributions from cytosol and crude membrane, respectively. Separation of the high and low affinity sites was not achieved in goose lysates. PDB binding to intact goose erythrocytes exhibited the lower affinity (Kd approximately 30 nM) and was enhanced approximately 2-fold by incubation at 37 degrees C relative to incubation at 4 degrees C. This was due to an increased Bmax, with no change in Kd of the whole cell binding. Human erythrocytes did not demonstrate this temperature-enhanced binding of PDB to intact cells. These data are consistent with a temperature-induced translocation of PDB receptors from cytosol to membrane in goose erythrocytes. The failure of human erythrocytes to respond to PDB is not due to an absence of PDB receptors but may be related to the diminished number of receptors or to the lack of a temperature-induced increase in whole cell receptor number.

Inhibition of hemopoietic growth factor-induced proliferation by adenosine diphosphate-ribosylation inhibitors

The effects of adenosine diphosphate (ADP) ribosylation inhibitors on hematopoietic growth factor-induced proliferation were examined. Significant inhibition of interleukin-3 (IL-3), colony-stimulating factor 1, and lung conditioned media-induced clonal agar growth of normal murine hematopoietic cells by 10 mmol/L nicotinamide (NAM), 10 mmol/L 3-aminobenzamide (3AB), and 5 mmol/L N1-methylnicotinamide (1MN) was noted. Nicotinic acid, a related compound that does not inhibit ADP ribosylation, failed to inhibit the growth factor-mediated proliferation. NAM (10 mmol/L), 3AB (10 mmol/L), and 1MN (5 mmol/L) also prevented IL-3 and phorbol ester-stimulated 3H-thymidine incorporation into the IL-3-responsive FDC-P1 cell line. Exposure of FDC-P1 cells to 10 mmol/L NAM led to a significant decrease in nuclear poly-(ADP-ribose) levels. Exposure of FDC-P1 cells to 5 mmol/L 1MN did not affect the interaction of the phorbol ester receptor, protein kinase-C (PK-C), with the cell membrane as determined by assay of phorbol ester binding in cytosol and membrane preparations. Nor did it affect the catalytic activity of PK-C as determined by assaying the in vitro phosphorylation of histone H1 by cytosolic kinase preparations from FDC-P1 as well as EL4 thymoma cells. 1MN markedly enhanced the inhibitory effects of phorbol esters on DNA synthesis of EL4 cells even at concentrations (1.25 mmol/L) that had no effects on DNA synthesis in the absence of phorbol esters. Our findings demonstrate that (a) active ADP ribosylation inhibitors interfere with growth factor-induced proliferation of murine hematopoietic cells and (b) the inhibition occurs at a step that follows the activation and translocation of PK-C and is more closely linked to DNA synthesis.

Absence of protein kinase C in nuclei of EL4 mouse thymoma cells

Since evidence indicates that phorbol ester-induced production of interleukin 2 requires transcription, we investigated the possibility that the phorbol ester receptor acts directly in the nuclei of EL4 thymoma cells. Using a procedure that minimized plasma membrane contamination (as measured by 5'-nucleotidase activity) and maintained the integrity of the double nuclear membrane, we were unable to detect specific binding of [3H]phorbol 12,13-dibutyrate in nuclei of unstimulated cells. Treatment of cells with phorbol 12,13-dibutyrate (100 nM, 37 degrees C) for up to 6 h did not cause appearance of phorbol ester binding capacity in nuclei (4 +/- 8% of homogenate value; 5'-nucleotidase activity = 10 +/- 3%) despite translocation of 40% of the cytosolic binding capacity to the plasma membrane fraction. The failure to detect nuclear binding capacity in treated cells was not due to occupation of nuclear sites with unlabeled ligand; effective exchange binding was demonstrated by recovery of total homogenate binding capacity in treated cells of 82 +/- 13% of that in untreated cells. Treatment of isolated nuclei with DNase to liberate DNA binding proteins also failed to reveal any nuclear phorbol ester binding capacity. Assay of nuclei for protein kinase C enzymatic activity gave similar negative results. These data argue strongly against a direct action of the intact phorbol ester receptor (or the phorbol ester binding fragment) in the transcriptional activation of interleukin 2 in EL4 cells but cannot rule out the possibility of a role for the catalytic fragment.

Phorbol esters induce interleukin 2 mRNA in sensitive but not in resistant EL4 cells

Phorbol ester sensitive EL4 cells become growth-inhibited and produce interleukin 2 when treated with phorbol-12, 13-dibutyrate. Resistant cells lack both responses. To determine whether the defect in phorbol ester-resistant EL4 cells occurs pre- or post-transcriptionally, a hybridization assay for interleukin 2 mRNA was developed using two synthetic oligonucleotides complementary to mouse interleukin 2 mRNA as probes. Both probes hybridized to a 1-kilobase band in RNA from phorbol ester-treated sensitive cells. This RNA was detectable within 3 h of phorbol ester administration, and accumulation peaked by 12 h. The 1-kilobase band was induced in a concentration-dependent manner by 4-beta-phorbol-12, 13-dibutyrate but not by the inactive analog, 4-alpha-phorbol-12, 13-dibutyrate. No bands hybridizing with the interleukin 2 probe were detected in RNA isolated from unstimulated cells or from phorbol ester-resistant EL4 cells at any time up to 24 h following phorbol ester stimulation. The accumulation of the RNA in sensitive cells was blocked when the protein synthesis inhibitors, cycloheximide (75 microM) or puromycin (90 microM) were added within 1 h of the addition of phorbol ester. If cycloheximide was added 2 or more h after phorbol ester treatment, superinduction of the 1-kilobase band was observed. These results indicate that the failure of phorbol ester-resistant EL4 cells to produce interleukin 2 is due to a defect proximal to interleukin 2 transcription and that the accumulation of interleukin 2 mRNA in phorbol ester-sensitive EL4 cells requires protein synthesis.

Inhibition of phorbol ester stimulated interleukin 2 production by copper(II) complexes

Superoxide dismutase mimetic copper(II) complexes, such as copper(II)(3,5-diisopropylsalicylate)2 (CuDIPS), inhibit phorbol ester stimulated tumor promotion in mouse skin. Therefore, CuDIPS was tested as a potential inhibitor of another effect of phorbol esters, induction of interleukin 2 (IL2) synthesis, in the mouse thymoma cell line EL4. CuDIPS inhibited phorbol ester induced IL2 production in a concentration dependent manner with a 50% inhibitory concentration of about 10 microM. However, the ligand 3,5-diisopropylsalicylic acid also inhibited the induction of IL2 by phorbol esters (50% inhibitory concentration, 15 microM). Since the superoxide dismutase mimetic activity of CuDIPS is not stable in the presence of ethylenediaminetetraacetic acid, the effects of CuDIPS could be due to the free ligand and not to the intact metallocomplex. Consequently, a series of extremely stable copper(II) macrocyclic compounds was synthesized, and the reduction potential, superoxide dismutase mimetic activity, and ability to inhibit phorbol ester induced IL2 production were determined for each. Of the copper(II) macrocyclic complexes studied, only the most potent superoxide dismutase mimetic compound was found to inhibit phorbol ester induced IL2 production. Copper(II) complexes had to be added no later than 4 following phorbol ester administration to be effective inhibitors of the IL response, suggesting that these compounds act subsequent to the binding of phorbol esters but prior to the transcription of IL2 messenger RNA. Adherence of EL4 cells to substrate in response to phorbol esters was unaffected by copper(II) compounds. In summary, copper(II) compounds with appropriate reduction potentials can act within a defined time period to inhibit some, but not all, of the effects of phorbol esters on EL4 cells.

Lack of direct effect of 2,3,7,8-tetrachlorodibenzo-P-dioxin (TCDD) on protein kinase C activity in EL4 cells

In vivo administration of TCDD produces an increase in the level of Protein Kinase C in the hepatic plasma membrane. We have studied the direct effects of TCDD on cultured EL4 thymoma cells, which contain a large amount of Protein Kinase C and respond to phorbol esters with rapid translocation of the kinase to the membrane, followed by growth inhibition, adherence to substrate and production of interleukin 2. TCDD (10-1000 nM) did not compete with 3H-phorbol dibutyrate for binding to cytosolic Protein Kinase C, and had no effect on Protein Kinase C activity in vitro. TCDD did not stimulate translocation of Protein Kinase C to the membrane, and did not affect phorbol ester-stimulated translocation. TCDD did not inhibit EL4 cell growth or affect phorbol ester induced growth inhibition, and failed to stimulate production of interleukin 2. Thus, TCDD does not appear to activate Protein Kinase C in EL4 cells.

Substrates for protein kinase C in cytosol of EL4 mouse thymoma cells

Phorbol esters trigger production of interleukin 2 by EL4 thymoma cells via an interaction with specific receptors, now considered to be identical with protein kinase C. Several in vitro substrates for protein kinase C were characterized by incubating cytosol from phorbol ester-responsive and -nonresponsive cells with [32P]adenosine triphosphate and CaCl2 with or without phosphatidylserine and diolein and separating proteins by gel electrophoresis. Phosphorylation of these proteins was calcium dependent in the range of 1-100 microM and stimulated by 10-150 micrograms of phosphatidylserine per ml. Calcium concentrations above 500 microM inhibited 32P incorporation and decreased phospholipid stimulation. Phorbol-12-myristate-13-acetate stimulated phosphorylation of these proteins, with a maximal concentration of 10 nM, providing strong evidence that these are protein kinase C substrates. The substrates for protein kinase C coeluted with the enzyme after binding to a phosphatidylserine affinity column in a calcium-dependent manner. Molecular weights of the protein kinase C substrates in sensitive cell cytosol were approximately 92,000, 84,000, 70,000, 67,000, 53,000, 45,000, 40,000, 36,000, and 20,000. A similar EL4 line which has phorbol ester receptors and protein kinase C, but does not produce interleukin 2 in response to phorbol esters, lacked the Mr 45,000 substrate and often also lacked the Mr 40,000 and 36,000 substrates. These proteins were also analyzed by two-dimensional electrophoresis. These results provide evidence of differences in the two cell lines in the ability of some proteins to serve as substrates for protein kinase C. Four proteins in a highly purified preparation of protein kinase C, at molecular weights of 66,000, 74,000, and 78,000 (all with pI 6.5-7.1) and of 62,000 (pI 6.2-6.4), were protein kinase C substrates, one of which is probably protein kinase C.

Stimulation of porcine jejunal ion secretion in vivo by protein kinase-C activators

Microbial toxins act through cyclic nucleotide dependent (cAMP or cGMP) or cyclic nucleotide independent pathways to cause intestinal ion secretion. To explore the calcium dependent, cyclic nucleotide independent pathway that is postulated to involve protein kinase C activation, we measured protein kinase C activity and phorbol ester binding in isolated intestinal epithelial cells and examined the effects of the C-kinase activators, phorbol myristate acetate, phorbol dibutyrate, and 4-beta-phorbol-12,13-didecanoate, in weaned pig jejunum in vivo. We demonstrated both protein kinase C activity and specific phorbol ester binding in porcine jejunal epithelial cells. Phorbol myristate acetate, phorbol dibutyrate, and 4-beta-phorbol-12,13-didecanoate (10(-5) M) each caused striking secretory responses at 5 h with accumulation of Na+, K+, Cl-, and HCO3- intraluminally. In contrast, 4-alpha-phorbol and 4-alpha-phorbol-12,13-didecanoate, which do not affect protein kinase C, allowed normal net absorption of all electrolytes from the intestinal lumen equivalent to controls with only Ringer's lactate. Time course studies revealed significant secretion within 30 min after exposure to the C-kinase activators. These data suggest an important role for protein kinase C activation in intestinal ion secretion.

Characterization of the chromobindins. Soluble proteins that bind to the chromaffin granule membrane in the presence of Ca2+

A group of proteins that bind to the chromaffin granule membrane in the presence of Ca2+ has been isolated by affinity chromatography of bovine adrenal medullary cytosol on granule membranes coupled to Sepharose 4B. Twenty-two of these proteins were resolved into classes depending upon the Ca2+ concentration at which they were eluted from the affinity column (40 or 0.1 microM), upon their affinities for native granule membranes or for liposomes prepared from extracted granule lipids, and upon the requirement of seven of the proteins for ATP in the cytosol fraction and column buffers to promote binding. The molecular weights and isoelectric points of these proteins were determined by two-dimensional electrophoresis. Two of the granule-binding proteins were identified: synexin and calmodulin. Calmodulin was found to bind to seven specific granule membrane proteins after diffusion of 125I-labeled calmodulin into an acrylamide gel of membrane proteins separated by electrophoresis in the presence of sodium dodecyl sulfate. A phospholipid-activated protein kinase activity, possibly due to protein kinase C, was present in the granule-binding fraction. Two major granule-binding proteins were found to present a pattern in two-dimensional electrophoresis that was very similar to but shifted slightly toward the basic end of the gel from the pattern generated by light chains associated with clathrin in adrenal medullary coated vesicles. In the chromaffin cell, these proteins, by associating with the granule membrane in the presence of an increased cytosolic Ca2+ concentration, might play a variety of roles in the process of exocytosis.

Identification of high-affinity phorbol ester receptor in cytosol of EL4 thymoma cells: requirement for calcium, magnesium, and phospholipids

A specific high-affinity phorbol ester binding component has been identified in the cytosol of an EL4 mouse thymoma line by using conditions similar to those for demonstrating activity of a calcium/phospholipid-dependent protein kinase. Specific binding is absolutely dependent on acidic phospholipids (maximal binding at 96 micrograms of phosphatidylserine per ml or 200 micrograms of phosphatidylinositol per ml) and is greatly enhanced by addition of calcium (0.5 mM) and magnesium (75 mM). Scatchard analysis of the cytosolic binding component indicated a Kd of 4.2 +/- 2.5 nM with 1.8 +/- 0.6 x 10(4) sites per cell compared to a Kd of 11.9 +/- 4.4 nM and 4.1 +/- 1.0 x 10(4) sites per cell for the membrane receptor. Consistent with the existence of at least two phorbol ester binding sites in EL4 cells was the observation of curvilinear Scatchard plots for binding to whole homogenates. The cytosolic binding showed the same order of potency for binding phorbol ester analogs as has been observed for intact cells. These results further support the similarity between phorbol ester receptors and the calcium/phospholipid-dependent protein kinase and suggest that the cytosolic receptor may be involved in subsequent phorbol ester effects in EL4 cells including loss of the kinase activity from the cytosol and production of T-cell growth factor.

Identification of phorbol ester receptors in T-cell growth factor-producing and -nonproducing EL4 mouse thymoma cells

Two lines of EL4 mouse thymoma cells, one which responds to phorbol ester tumor promoters with production of T-cell growth factor and inhibition of proliferation and one which does not respond, have been examined for the presence of specific phorbol ester-binding components. Both lines displayed saturable specific binding as determined by using [20-3H]phorbol 12,13-dibutyrate in a whole-cell-binding assay. Specific binding in each line was maximal within 10 min at 37 degrees but rapidly decreased to about 30% within 30 min. Maximal binding at 4 degrees was reached after 50 min and was stable for at least 8 hr; however, this level was only about 30% of the maximum obtained at 37 degrees. Saturation of the specific binding after 3 hr at 4 degrees obtained at a concentration (30 to 50 nM) which is consistent with that yielding maximal T-cell growth factor production in the responding line but greater than that at which inhibition of proliferation was detected in those cells. Scatchard analysis of these data was consistent with the existence of a single class of binding sites with a Kd of 11 +/- 0.6 (S.D.) nM for the T-cell growth factor-producing cells and 18 +/- 1.9 nM for the nonproducing line. The numbers of sites per cell measured in the responding and nonresponding lines were 5.6 +/- 1.3 x 10(4) and 7.1 +/- 0.2 x 10(4), respectively. Competition by a series of phorbol esters for [20-3H]phorbol 12,13-dibutyrate binding in the responding line showed the same order of potency as production of T-cell growth factor and inhibition of proliferation by the analogs. These data support identification of the phorbol ester-binding component in the responding cells as the receptor mediating T-cell growth factor production. The presence of binding components in unresponsive cells in numbers and with characteristics indistinguishable from those of responsive cells suggests that the failure to respond is due to modification of a step which succeeds the initial binding event.

Specific receptors for phorbol esters in lymphoid cell populations: role in enhanced production of T-cell growth factor

Phorbol ester tumor promoters act synergistically with concanavalin A to cause production of T-cell growth factor by normal human peripheral blood lymphocytes. A specific, saturable, binding component which may mediate the phorbol ester effect has been identified by using [20-3H]phorbol 12,13-dibutyrate in a whole-cell binding assay. Specific binding is maximal with 5 min at 37 or 23 degrees C but the level of bound ligand rapidly decreases to about 50% within 1 hr. At 4 degrees C, 2 hr are required to reach maximal binding, and the binding is stable for at least 20 hr. Binding is reversible at 37 and 4 degrees C with time courses similar to those for initial binding at the respective temperatures. Saturation of the specific binding occurs at a concentration (approximately 30 nM) consistent with that producing maximal T-cell growth factor activity. Scatchard analysis of the binding after 30 min at 37 degrees C demonstrates a lower Kd (9 nM) than that determined after 2 hr at 4 degrees C (22 nM). The median number of sites per cell for six donors was 2 X 10(5) (range, 1.3-4 X 10(5). Other tumor-promoting phorbol esters compete for [20-3H]phorbol 12,13-dibutyrate binding in approximate proportion to their activity in stimulating T-cell growth factor production. Phorbol, 4-alpha-phorbol didecanoate, dexamethasone, retinoic acid, butyric acid, and dimethyl sulfoxide do not compete for specific binding.

Molybdate inhibition of glucocorticoid receptor inactivation and transformation

The inactivation of glucocorticoid receptors that occurs when cytosol is heated at 25 degrees C is blocked reversibly by molybdate and slowed by some other phosphatase inhibitors such as fluoride and glucose 1-phosphate. Molybdate is also capable of preventing nonenzymatic inactivation of unbound receptors caused by exposure to salt or precipitation with ammonium sulfate at 0 degrees C. Inactivation of unbound receptors caused by Sephadex G-50 gel filtration is prevented by all three inhibitors. Both molybdate and tungstate block temperature-dependent transformation of glucocorticoid.receptor complexes to the DNA-binding state, where fluoride and glucose 1-phosphate have no effect. Transformation brought about at 0 degrees C by salt, ammonium sulfate precipitation, or gel filtration is also blocked by both molybdate and tungstate. Tungstate differs from molybdate in that it has little or no effect on receptor inactivation. Fluoride and glucose 1-phosphate do not inhibit transformation. These observations support the proposal that molybdate and tungstate are interacting through a reversible association with the glucocorticoid receptor itself. We propose that they may act by forming a complex with a phosphate moiety on the receptor.

Transport of C5 dicarboxylate compounds by Pseudomonas putida

Induced glutarate and 2-oxoglutarate uptake and transport by Pseudomonas putida were investigated in whole cells and membrane vesicles, respectively. Uptake of 2-oxoglutarate, but not glutarate, was against a concentration gradient to 1.7-fold greater than the initial extracellular concentration. Membrane vesicles transported 2-oxoglutarate and glutarate against gradients to intramembrane concentrations fivefold greater than the initial extravesicle concentrations. The rates of transport of both compounds were greatest in the presence of the artificial electron donor system phenazine methosulfate-ascorbate. Malate and D-lactate were the only naturally occurring compounds that served as electron donors. Uptake and transport were inhibited by KCN, NaN3, and 2,2-dinitrophenol. Kinetic parameters of transport were: glutarate, apparent Km--1.22 mM, Vmax--400 nmol/min per mg of membrane protein; 2-oxoglutarate, apparent Km--131 microM, Vmax--255 nmol/min per mg of membrane protein. Studies of competitive inhibition indicated a common system for transport of five C5 dicarboxylate compounds. The apparent Km and Ki values with 2-oxoglutarate as a substrate placed the substrate affinity for transport in the order 2-oxoglutarate greater than glutarate greater than D-2-hydroxyglutarate and L-2-hydroxyglutarate greater than glutaconate.

Activation of thymocyte glucocorticoid receptors to the steroid binding form. The roles of reduction agents, ATP, and heat-stable factors

The specific glucocorticoid binding capacity in cytosol preparations of rat thymocytes decays with a half-life of 4 h at 0 degrees C or 20 min at 25 degrees C. Phosphatase inhibitors (molybdate, fluoride, glucose 1-phosphate) added alone do not prevent this inactivation. Dithiothreitol (2 mM) has a large stabilizing effect on the binding capacity at 0 degrees C but only a small effect at 25 degrees C. Addition of 10 mM molybdate plus 2 mM dithiothreitol totally prevents inactivation for at least 8 h at 25 degrees C as well as at 0 degrees C. Fluoride (100 mM) also retards the inactivation if added with dithiothreitol. Addition of dithiothreitol at 25 degrees C to inactivated cytosol receptors results in partial activation of the binding capacity. Addition of dithiothreitol to receptors inactivated at 25 degrees C in the presence of molybdate allows total reactivation of the binding capacity to the maximum zero time value. If binding capacity is inactivated by preincubation of the cytosol at 25 degrees C, addition of ATP with dithiothreitol enhances the activation observed with only dithiothreitol. This ATP stimulated activation is optimal at 1 to 3 mM. ATP (10 mM) is required when molybdate is added to prevent simultaneous inactivation. ADP, GTP, CTP, and UTP have some activating capacity but the effects of all nucleotides are inhibited by the ATP analog, adenyl-5'-yl (beta, gamma-methylene)diphosphonate. ATP-dependent activation can also be prevented with 50 mM EDTA, and addition of magnesium partially overcomes the EDTA inhibition. Dithiothreitol activation of thymocyte glucocorticoid binding capacity can also be enhanced by addition of a heat-stable preparation from thymocytes, L cells, or liver. Sephadex G-25 chromatography, assay of ATP, and inhibition of the activation with adenyl-5'-yl (beta, gamma-methylene)diphosphonate suggest that these preparations contain varying amounts of endogenous reducing equivalents and ATP as well as a larger heat stable factor. Maximum activation is obtained by adding dithiothreitol, ATP, molybdate, and the larger heat-stable factor. These results suggest that stabilization and activation of glucocorticoid binding capacity in thymocytes requires phosphorylation as well as reduction of the receptor itself or of some other component required for the steroid binding reaction.

ATP-dependent activation of L cell glucocorticoid receptors to the steroid binding form

The specific glucocorticoid binding capacity in cytosols prepared from L929 mouse fibroblasts (L cells) is inactivated with a half-life of approximately 2 h at 25 degrees C. As previously published, this inactivation can be prevented with 10 mM molybdate and markedly slowed by addition of other phosphatase inhibitors such as glucose 1-phosphate and fluoride. We have now found that ATP (5 to 10 mM) also slows the rate of this inactivation. After extensively inactivating the receptor by preincubating cytosol at 25 degrees C for 4 and preventing further inactivation by addition of molybdate, addition of ATP results in reactivation of the steroid binding capacity. Maximal reactivation of 40 to 70% is achieved with 5 to 10 mM ATP. The activation is temperature-dependent and specific for ATP. ADP, GTP, CTP, and UTP do not cause activation and preliminary results indicate no effect of cyclic nucleotides in this system. If activation is prevented by addition of 10 mM EDTA to the cytosol, addition of 3 to 10 mM magnesium permits ATP-dependent activation of the binding capacity. The level of reactivation can be enhanced by addition of a heat-stable factor prepared from the same L cell supernatant. These results support the proposal that L cell glucocorticoid receptors can be activated to the glucocorticoid binding state by an ATP-dependent phosphorylation mechanism.