Phorbol Ester Enhancement of Neurotransmitter Release from Rat Brain Synaptosomes

Neurotransmitter release from rat brain synaptosomes was measured following pretreatment with various phorbol esters. Ca2+-dependent, evoked neurotransmitter release was increased by phorbol esters that were active in stimulating protein kinase C. Protein kinase C activation was demonstrated by increased incorporation of 32P into 87-kilodalton phosphoprotein, a specific substrate for that kinase. Inactive phorbol esters had no effect on neurotransmitter release or on the phosphorylation of 87-kilodalton phosphoprotein. The increased release was observed in either crude cortical synaptosomal fractions (P2) or purified cortical synaptosomal fractions. The enhancement was found for all neurotransmitters (norepinephrine, acetylcholine, gamma-aminobutyric acid, serotonin, dopamine, and aspartate), all brain regions (cerebral cortex, hippocampus, and corpus striatum), and all secretagogues (elevated extracellular K+ level, veratridine, or A23187) examined. It was also observed at all calcium concentrations present during stimulation of release. The phorbol ester enhancement of Ca2+-dependent release occurred whether or not calcium was present during pretreatment. These results indicate that stimulation of protein kinase C leads to an enhanced sensitivity of the stimulus-secretion coupling processes to calcium within the nerve terminal. The results support the possibility that presynaptic activation of protein kinase C modulates nerve terminal neurotransmitter release in the CNS.     

Subcellular distribution of protein kinase C/phorbol ester receptors in differentiating mouse keratinocytes

The activation of protein kinase C (PKC) by diacylglycerol or tumor promoters plays a pivotal role in signal transduction and subsequent activation of cellular processes. Since the activity of this enzyme is dependent on its immediate lipid domain, its relative distribution within the cell may be an important regulatory mechanism. We report here a relative decrease in PKC/phorbol ester receptor associated with the particulate fraction of mouse keratinocytes induced to differentiate by two separate systems. First, proliferating keratinocytes maintained in low Ca2+ (0.09 mM) serum-free medium were induced to differentiate rapidly by the addition of Ca2+ (1.8 mM). A 1.4-fold decrease in the percent of total phorbol receptor binding activity present in the particulate fraction and concomitant increase in binding in the cytosol fraction was evident 20 min after the Ca2+ addition. Second, in keratinocytes that differentiate over a 6 day cultivation period in serum-containing medium with Ca2+ concentration of 1.8 mM, a significant decrease in the percent of the phorbol receptor binding activity present in the particulate fraction was observed as the culture begins to differentiate on days 3 and 4. Maximal phorbol ester binding in the particulate fraction corresponded to the proliferative phase of the culture (day 2), while lower levels of PKC/phorbol ester binding to particulate fractions were noted during the early differentiative phase (days 3 and 4). Addition of the synthetic diacylglycerols 1-oleoyl-2-acetylglycerol or L-alpha-1,2 dioctanyl glycerol at 30 micrograms/ml to proliferating keratinocyte cultures induced a modest increase in two markers of terminal differentiation: cornified envelope formation and transglutaminase levels. These findings, taken together, support the hypothesis that PKC activation plays a role in the initial signalling events for keratinocyte differentiation.     

Possible involvement of protein kinase C and calcium ion in growth factor-induced expression of c-myc oncogene in Swiss 3T3 fibroblasts

The addition of platelet-derived growth factor and fibroblast growth factor to quiescent cultures of Swiss 3T3 fibroblasts rapidly induced protein kinase C activation and Ca2+ mobilization and afterwards markedly increased c-myc mRNA levels. 1-Oleoyl-2-acetylglycerol, a membrane-permeable synthetic diacylglycerol, and 12-O-tetradecanoylphorbol 13-acetate, a tumor-promoting phorbol ester, stimulated protein kinase C activation without Ca2+ mobilization. Inversely, Ca2+ ionophores, A23187 and ionomycin, elicited Ca2+ mobilization without protein kinase C activation. Both protein kinase C-activating and Ca2+-mobilizing agents were able to increase c-myc mRNA levels in an additive manner. Prolonged treatment of the cells with phorbol 12,13-dibutyrate, another protein kinase C-activating phorbol ester, led to the down-regulation and complete disappearance of protein kinase C. In these cells, 1-oleoyl-2-acetylglycerol and 12-O-tetradecanoylphorbol 13-acetate did not increase c-myc mRNA levels, but platelet-derived growth factor, fibroblast growth factor, and the Ca2+ ionophores, all of which still induced Ca2+ mobilization, stimulated the increase of c-myc mRNA levels. These results strongly suggest that both protein kinase C and Ca2+ may be involved in platelet-derived growth factor- as well as fibroblast growth factor-induced expression of the c-myc oncogene in Swiss 3T3 cells.     

Synergism between subthreshold concentrations of thrombin and 12-O-tetradecanoylphorbol 13-acetate in platelet activation

A comparison was made between the time courses and interdependence of platelet aggregation, serotonin release, and cytosolic free Ca2+ concentration in the same sample of platelets loaded with [14C]-serotonin and Ca2+-sensitive photoprotein aequorin. In 100 micrograms/ml aspirin-treated platelets, neither 0.01 U/ml thrombin nor 50nM TPA, an active phorbol ester, induced significant aggregation, serotonin release, or a rise in the intracellular calcium concentration. However, when these two agents were added together, marked aggregation and release were observed without a change in the cytosolic free Ca2+ concentration. No correlation was observed between the extent of the synergistic effects and time of preincubation with TPA. Potentiatory effects of protein kinase C on receptor-mediated agonists need to be considered in platelet activation.     

Protein kinase C and meiotic maturation of surf clam oocytes

We report here that phorbol ester, a potent activator of protein kinase C, induces germinal vesicle breakdown in surf clam oocytes. However, phorbol ester-induced activation is slow and is not accompanied by an increased Ca2+ influx. Simultaneous additions of phorbol ester and various amounts of K+ ions, which induce Ca2+ influx of different amplitudes, result in successful activation within the normal time schedule at K+ concentrations inefficient alone in activating the oocytes. In vivo, increased protein phosphorylation triggered by phorbol ester amounts to about one third that seen after fertilization. These results suggest that increased Ca2+ influx and protein kinase C activation act in synergy to cause resumption of meiotic maturation in these oocytes.     

Monoclonal antibodies against type II rat brain protein kinase C

Monoclonal antibodies (8/1, 10/10, and 25/3) against rat brain type II protein kinase C were used for the immunochemical characterization of this kinase. These antibodies immunoprecipitated the type II protein kinase C in a dose-dependent manner but did neither to the type I nor III isozyme. Immunoblot analysis of the tryptic fragments from protein kinase C revealed that all three antibodies recognized the 27-38-kDa fragments, the phospholipid/phorbol ester-binding domain, but not the 45-48-kDa fragments, the kinase catalytic domain. The immune complexes of the kinase and the antibodies retained 70-80% of the kinase activity which was dependent on Ca2+ and phosphatidylserine and further activated by diacylglycerol or tumor-promoting phorbol ester. With antibody 8/1, the kinetic parameters with respect to Km for ATP and histone and K alpha for phosphatidylserine and phorbol 12,13-dibutyrate were not significantly influenced. However, the antibody causes variable effects on the K alpha for Ca2+ under different assay conditions. When determined in the presence of phosphatidylserine, the K alpha for Ca2+ was reduced by an order of magnitude (37 +/- 8 to 2.0 +/- 1.8 microM); in the presence of phosphatidylserine and phorbol 12,13-dibutyrate, the K alpha for Ca2+ was not significantly altered; and in the presence of phosphatidylserine and dioleoylglycerol, the kinase became an apparently Ca2+-independent enzyme. The effects of antibody 8/1 on the kinetic parameters of the enzyme for phorbol ester binding were different from those for kinase activity. This antibody causes a 20-30% reduction in phorbol ester binding and a 2-fold increase (1.9 +/- 0.2 to 3.9 +/- 0.3 micrograms/ml) in the concentration of phosphatidylserine required for half-maximal binding, but is without significant influence on those parameters for Ca2+ and phorbol 12,13-dibutyrate. The differential effects of antibody 8/1 on kinase activity and phorbol ester binding with respect to the kinetic parameter of phosphatidylserine suggest that the roles of this phospholipid in supporting phorbol ester binding and kinase activation are different. In the presence of the antibody, the autophosphorylations of the phospholipid/phorbol ester-binding domain and the kinase domain were reduced; the reduction was more pronounced for the former than for the latter. These results suggest that the epitope for antibody 8/1 is localized within the phospholipid/phorbol ester-binding domain at the region adjacent to the kinase domain so that the autophosphorylations of both domains are affected.     

Ca2+ mobilization primes protein kinase C in human platelets. Ca2+ and phorbol esters stimulate platelet aggregation and secretion synergistically through protein kinase C.

Low concentrations of Ca2+-mobilizing agonists such as vasopressin, platelet-activating factor, ADP, the endoperoxide analogue U44069 and the Ca2+ ionophore A23187 enhance the binding of [3H]phorbol 12,13-dibutyrate (PdBu) to intact human platelets. This effect is prevented by preincubation of platelets with prostacyclin (except for A23187). Adrenaline, which does not increase Ca2+ in the platelet cytosol, does not enhance the binding of [3H]PdBu to platelets. In addition, all platelet agonists except adrenaline potentiate the phosphorylation of the substrate of protein kinase C (40 kDa protein) induced by PdBu. Potentiation of protein kinase C activation is associated with increased platelet aggregation and secretion. Stimulus-induced myosin light-chain phosphorylation and shape change are not significantly affected, but formation of phosphatidic acid is decreased in the presence of PdBu. The results may indicate that low concentrations of agonists induce in intact platelets the translocation of protein kinase C to the plasma membrane by eliciting mobilization of Ca2+, and thereby place the enzyme in a strategic position for activation by phorbol ester. Such activation enhances platelet aggregation and secretion, but at the same time suppresses activation of phospholipase C. Therefore, at least part of the synergism evoked by Ca2+ and phorbol ester is mediated through a single pathway which involves protein kinase C. It is likely that the priming of protein kinase C by prior Ca2+ mobilization occurs physiologically in activated platelets.     

Regulation of calcium-dependent [3H]noradrenaline release from rat cerebrocortical synaptosomes by protein kinase C and modulation of the actin cytoskeleton.

The effects that active phorbol esters, staurosporine, and changes in actin dynamics, might have on Ca2+ -dependent exocytosis of [3H]-labelled noradrenaline, induced by either membrane-depolarizing agents or a Ca2+ ionophore, have been examined in isolated nerve terminals in vitro. Depolarization-induced openings of voltage-dependent Ca2+ channels with 30 mM KCl or 1 mM 4-aminopyridine induced limited exocytosis of [3H]noradrenaline, presumably from a readily releasable vesicle pool. Application of the Ca2+ ionophore calcimycin (10 microM) induced more extensive [3H]noradrenaline release, presumably from intracellular reserve vesicles. Stimulation of protein kinase C with phorbol 12-myristate,13-acetate increased release evoked by all secretagogues. Staurosporine (1 microM) had no effect on depolarization-induced release, but decreased ionophore-induced release and reversed all effects of the phorbol ester. When release was induced by depolarization, internalization of the actin-destabilizing agent DNAase I into the synaptosomes gave a slight increase in [3H]NA release and strongly increased the potentiating effect of the phorbol ester. In contrast, when release was induced by the Ca2+ ionophore, DNAase I had no effect, either in the absence or presence of phorbol ester. The results indicate that depolarization of noradrenergic rat synaptosomes induces Ca2+ -dependent release from a releasable pool of staurosporine-insensitive vesicles. Activation of protein kinase C increases this release by staurosporine-sensitive mechanisms, and destabilization of the actin cytoskeleton further increases this effect of protein kinase C. In contrast, ionophore-induced noradrenaline release originates from a pool of staurosporine-sensitive vesicles, and although activation of protein kinase C increases release from this pool, DNAase I has no effect and also does not change the effect of protein kinase C. The results support the existence of two functionally distinct pools of secretory vesicles in noradrenergic CNS nerve terminals, which are regulated in distinct ways by protein kinase C and the actin cytoskeleton.     

Phorbol ester receptors and protein kinase C in primary neuronal cultures: development and stimulation of endogenous phosphorylation

Embryonic rat neurons cultured in defined medium, essentially in the absence of glia, were highly enriched in phorbol ester receptors. The neurons displayed a single class of phorbol 12,13-dibutyrate binding sites with a maximum binding capacity, after 10 d in culture, of 18.6 pmol/mg protein and an apparent dissociation constant of 7.1 nM. Phorbol ester binding sites were associated with protein kinase C, which represented a major protein kinase activity in primary neuronal cultures. Ca2+-phosphatidylserine-sensitive phosphorylation of endogenous substrates was more marked than that observed in the presence of cyclic AMP or Ca2+ and calmodulin. Phorbol ester receptors and protein kinase C levels were critically dependent on the culture age. Thus, about a 20-fold increase in binding sites occurred during the first week in culture and was accompanied by a corresponding increase in Ca2+-phosphatidylserine-sensitive protein phosphorylation in soluble neuronal extracts. These changes largely paralleled a similar rise in phorbol ester binding during fetal development in vivo. The apparent induction of phorbol ester receptors was specific relative to other cellular proteins and could be inhibited by cycloheximide or Actinomycin D. Phosphorylation of endogenous substrates in intact cultured neurons paralleled the age-dependent increase in protein kinase C. Furthermore, 32P incorporation into several major phosphoproteins was markedly augmented by treating the neuronal cultures with phorbol esters. Such phosphorylation events may provide a clue to the significance of protein kinase C in developing neurons.     

Synergistic activation of protein kinase Calpha, -betaI, and -gamma isoforms induced by diacylglycerol and phorbol ester: roles of membrane association and activating conformational changes

Protein kinase Calpha (PKCalpha) has been shown to contain two discrete activator sites with differing binding affinities for phorbol esters and diacylglycerols. The interaction of diacylglycerol with a low-affinity phorbol ester binding site leads to enhanced high-affinity phorbol ester binding and to a potentiated level of activity [Slater, S. J., Ho, C., Kelly, M. B., Larkin, J. D. , Taddeo, F. J., Yeager, M. D., and Stubbs, C. D. (1996) J. Biol. Chem. 271, 4627-4631]. In this study, the mechanism of this enhancement of activity was examined with respect to the Ca2+ dependences of membrane association and accompanying conformational changes that lead to activation. The association of PKCalpha with membranes containing 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1, 2-dioleoylglycerol (DAG), determined from tryptophan to dansyl-PE resonance energy transfer (RET) measurements, was found to occur at relatively low Ca2+ levels (相似文献