Different isozymes of protein kinase C mediate feedback inhibition of phospholipase C and stimulatory signals for exocytosis in rat RBL-2H3 cells

Uncoupling of ATP-induced inositol phosphate formation and Ca(2+) mobilization by phorbol ester in canine cultured tracheal epithelial cells

The regulation of the increase in inositol phosphates (IPs) production and intracellular Ca(2+) concentration ([Ca(2+)](i)) by protein kinase C (PKC) was investigated in canine cultured tracheal epithelial cells (TECs). Pretreatment of TECs with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min attenuated the ATP- and UTP-induced IPs formation and Ca(2+) mobilization. The concentrations of PMA that gave half-maximal (EC(50)) inhibition of ATP- and UTP-induced IPs accumulation and an increase in [Ca(2+)](i) were 5-10 and 4-12 nM, respectively. Prior treatment of TECs with staurosporine (1 microM), a PKC inhibitor, partially inhibited the ability of PMA to attenuate ATP- and UTP-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Furthermore, analysis of cell extracts by Western blotting with antibodies against different PKC isozymes revealed that TECs expressed PKC-alpha, -betaI, -betaII, -gamma, -delta, -epsilon, -theta, and -zeta. With PMA treatment of the cells for various times, translocation of PKC-alpha, -betaI, -betaII, -gamma, -delta, -epsilon, and -theta from the cytosol to the membrane was seen after 5- and 30-min and 2- and 4-h treatment. However, 6-h treatment caused a partial down-regulation of these PKC isozymes. PKC-zeta was not significantly translocated and down-regulated at any of the times tested. In conclusion, these results suggest that activation of PKC may inhibit the phosphoinositide (PI) hydrolysis and consequently attenuate the [Ca(2+)](i) increase or inhibit independently both responses to ATP and UTP. The translocation of PKC-alpha, -betaI, -betaII, -delta, -epsilon, -gamma, and -theta induced by PMA caused an attenuation of ATP- and UTP-induced IPs accumulation and Ca(2+) mobilization in TECs.

Ouabain-induced enhancement of rat mast cells response. Modulation by protein phosphorylation and intracellular pH

The digitalic glicoside ouabain induces potentiation of rat mast cell histamine release in response to several stimuli, which is mediated by Na+/Ca2+ exchanger. In this work, we studied the effect of ouabain on cytosolic calcium, intracellular pH and histamine release with Ca2+ ionophore A23187 in conditions designed to maximize ouabain-induced potentiation of rat mast cells response. The effect of protein kinase C (PKC), cAMP and phosphatase inhibition was also tested. Ouabain induced an enhancement in histamine release, cytosolic calcium and intracellular pH. The adenylate cyclase activator forskolin reduced the effect of ouabain on histamine release and intracellular pH, but enhanced the effect on cytosolic calcium. PKC activator PMA enhanced the effect of ouabain on histamine release and cytosolic calcium, without affecting intracellular pH. A PKC inhibitor, GF-109203X, reduced ouabain-induced enhancement of histamine release and intracellular pH, but increased the enhancement on cytosolic calcium. Finally, inhibition of protein phosphatases 1 and 2A with okadaic acid, increased the effect of ouabain on histamine release and intracellular pH, but reduced cytosolic calcium in presence of ouabain. This result suggest that ouabain-induced potentiation of rat mast cell histamine release with A23187 is modulated by kinases, and this modulation may be carried out by changes in intracellular alkalinization. However, the mechanism underlying cellular alkalinization remains to be elucidated.

HCO(3)(-) ions modify the role of PKC isoforms in the modulation of rat mast cell functions

PKC and the intracellular calcium signal are two well-known intracellular signaling pathways implicated in the induction of mast cell exocytosis. Both signals are modified by the presence or absence of HCO(3)(-) ions in the external medium. In this work, we studied the regulation of the exocytotic process by PKC isozymes and its relationship with HCO(3)(-) ions and PKC modulation of the calcium entry. The calcium entry, induced by thapsigargin and further addition of calcium, was inhibited by PMA, a PKC activator, and enhanced by 500 nM GF109203X, which inhibits Ca(2+)-independent PKC isoforms. PMA inhibition of the Ca(2+) entry was reverted by 500 and 50 nM GF109203X, which inhibit Ca(2+)-independent and Ca(2+)-dependent isoforms, respectively, and Gö6976, a specific inhibitor of Ca(2+)-dependent PKCs. Thus, activation of Ca(2+)-dependent and Ca(2+)-independent PKC isoforms inhibit Ca(2+) entry in rat mast cells, either in a HCO(3)(-)-buffered or a HCO(3)(-)-free medium. PMA, GF109203X, Gö6976 and rottlerin, a specific inhibitor of PKC delta, were also used to study the role of PKC isoforms in the regulation of exocytosis induced by thapsigargin, ionophore A23187 and PMA. The results demonstrate that Ca(2+)-dependent PKC isoforms inhibit exocytosis in a HCO(3)(-)-dependent way. Moreover, Ca(2+)-independent PKC delta was the main isoform implicated in promotion of Ca(2+)-dependent mast cell exocytosis in the presence or absence of HCO(3)(-). The role of PKC isoforms in the regulation of mast cell exocytosis depends on the stimulus and on the presence or absence of HCO(3)(-) ions in the medium, but it is independent of PKC modulation of the Ca(2+) entry.

Tyrosine phosphorylation of the linker for activator of T cells in mast cells by stimulation with the high affinity IgE receptor

Aggregation of the high affinity IgE receptors (FcepsilonRI) on basophils and mast cells, members of the immune receptor family, initiates a cascade of events that results in the release of inflammatory mediators. This pathway involves the activation of several protein-tyrosine kinases, including Lyn, Syk, Btk, and Fak that induce the tyrosine phosphorylation of various proteins. The linker for activation of T cells (LAT), was originally found as a ZAP-70 tyrosine kinase substrate that linked T cell receptors to cellular activation, and was expressed in T cells, NK cells and mast cells. Here we show that LAT expressed in the RBL-2H3 rat mast cell line is tyrosine-phosphorylated after aggregation of FcepsilonRI. The tyrosine phosphorylation of the LAT was dramatically enhanced after receptor aggregation. Furthermore, a tyrosine-phosphorylated 80-kDa protein associated with LAT transiently after receptor aggregation. GST fusion proteins containing parts of PLCgamma or PI3 kinase can bind LAT. These results suggest that LAT plays an important role not only in T cell, but also in mast cell activation, and that the association among these signaling molecules is critical for FcepsilonRI-mediated intracellular signal transduction in mast cells.

Intracellular pH on translocation of protein kinase C isozymes in rat pinealocytes

In rat pinealocytes, cytoplasmic alkalization causes protein kinase C (PKC) translocation, but the isozyme involved is not known. In this study, we investigated the effect of cytoplasmic alkalization on membrane-associated PKCalpha, delta, epsilon, and zeta, four isozymes present in the rat pineal gland. Treatment with NH(4)Cl, which had no effect on PKCzeta, caused a sustained increase in membrane-associated PKCalpha, delta, and epsilon that lasted for at least 60 min. The effect of NH(4)Cl on PKCalpha, delta, and epsilon was reduced by sodium propionate, an agent that counteracts the effect of NH(4)Cl on intracellular pH. Both sodium propionate and 5-(N,N-hexamethylene)amiloride (HMA), two treatments that abolished the effect of norepinephrine on cytoplasmic alkalization, also reduced norepinephrine-mediated increases in membrane-associated PKCalpha, delta, and epsilon. In contrast, these two treatments did not have an effect on the increase in membrane-associated PKC isozymes caused by 4beta-phorbol 12-myristate 13-acetate (PMA), an active phorbol ester, even though HMA was effective in abolishing PMA-mediated increases in intracellular pH. These results, apart from demonstrating that cytoplasmic alkalization by itself can cause translocation of PKCalpha, delta, and epsilon in rat pinealocytes, also indicate that the norepinephrine-stimulated cytoplasmic alkalization plays an important role in transducing signals from the adrenergic receptor to selective PKC isozymes. However, PKC translocation stimulated directly by PMA does not appear to be sensitive to changes in intracellular pH.

Diverse effects of sphingosine on calcium mobilization and influx in differentiated HL-60 cells

Sphingosine induces a biphasic increase in cytosolic-free Ca(2+)([Ca(2+)](i)) with an initial peak followed by a sustained increase in HL-60 cells differentiated into neutrophil-like cells. The initial peak is not affected by the presence of ethylene glycol bis (beta-aminoethyl ether) N, N, N', N-tetraacetic acid (EGTA) in the buffer and appears to be dependent on conversion of sphingosine to sphingosine -1-phosphate (S1P) by sphingosine kinase, since it is blocked by the presence of N, N-dimethylsphingosine (DMS), which, like sphingosine, causes a sustained increase itself. The sustained increase that is elicited by sphingosine or DMS is abolished by the presence of EGTA in the buffer. The sustained sphingosine-induced Ca(2+)influx does not appear due to Ca(2+)influx through store-operated Ca(2+)(SOC) channels, since the influx is not inhibited by SKF 96365, nor is it augmented by loperamide. In addition, sphingosine and DMS attenuate the Ca(2+)influx through SOC channels that occurs after depletion of intracellular stores by ATP or thapsigargin. Both the initial peak and the sustained increase in [Ca(2+)](i)elicited by sphingosine can be blocked by phorbol 12-myristate 13-acetate (PMA)-elicited activation of protein kinase C. Thus, in HL-60 cells sphingosine causes a mobilization of Ca(2+)from intracellular Ca(2+)stores, which requires conversion to S1P, while both sphingosine and DMS elicit a Ca(2+)influx through an undefined Ca(2+)channel and cause a blockade of SOC channels.

Molecular consequences of human mast cell activation following immunoglobulin E-high-affinity immunoglobulin E receptor (IgE-FcepsilonRI) interaction

The cross-linking by immunoglobulin E of its high-affinity receptor, FcepsilonRI, on mast cells initiates a complex series of biochemical events leading to degranulation and the synthesis and secretion of eicosanoids and cytokines through the action of transcription factors, such as nuclear factor-kappaB. The initial activation involves the phosphorylation of FcepsilonRI beta- and gamma-subunits through the actions of the tyrosine kinases lyn and syk. For the purposes of description, the subsequent events may be grouped in three cascades characterized by the key proteins involved. First, the phospholipase C-inositol phosphate cascade activates protein kinase C and is largely responsible for calcium mobilization and influx. Second, activation of Ras and Raf via mitogen-activated protein kinase causes the production of arachidonic acid metabolites. Third, the generation of sphingosine and sphingosine-1-phosphate occurs through activation of sphingomyelinase. While the early signaling events tend to be specific for the cited cascades, there is an increasing overlap of activated proteins with the downstream propagation of the signal. It is the balanced interaction between these proteins that culminates in degranulation, synthesis, and release of eicosanoids and cytokines.

Inhibition of bradykinin-induced phosphoinositide hydrolysis and Ca2+ mobilisation by phorbol ester in rat cultured vascular smooth muscle cells

Regulation of the increase in inositol phosphate (IP) production and intracellular Ca2+ concentration ([Ca2+]i by protein kinase C (PKC) was investigated in cultured rat vascular smooth muscle cells (VSMCs). Pretreatment of VSMCs with phorbol 12-myristate 14-acetate (PMA, 1 microM) for 30 min almost abolished the BK-induced IP formation and Ca2+ mobilisation. This inhibition was reduced after incubating the cells with PMA for 4 h, and within 24 h the BK-induced responses were greater than those of control cells. The concentrations of PMA giving a half-maximal (pEC50) and maximal inhibition of BK induced an increase in [Ca2+]i, were 7.8 +/- 0.3 M and 1 microM, n = 8, respectively. Prior treatment of VSMCs with staurosporine (1 microM), a PKC inhibitor, inhibited the ability of PMA to attenuate BK-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Paralleling the effect of PMA on the BK-induced IP formation and Ca2+ mobilisation, the translocation and downregulation of PKC isozymes were determined by Western blotting with antibodies against different PKC isozymes. The results revealed that treatment of the cells with PMA for various times, translocation of PKC-alpha, betaI, betaII, delta, epsilon, and zeta isozymes from the cytosol to the membrane were seen after 5 min, 30 min, 2 h, and 4 h of treatment. However, 24-h treatment caused a partial downregulation of these PKC isozymes in both fractions. Treatment of VSMCs with 1 microM PMA for either 1 or 24 h did not significantly change the K(D) and Bmax of the BK receptor for binding (control: K(D) = 1.7 +/- 0.2 nM; Bmax = 47.3 +/- 4.4 fmol/mg protein), indicating that BK receptors are not a site for the inhibitory effect of PMA on BK-induced responses. In conclusion, these results demonstrate that translocation of PKC-alpha, betaI, betaII, delta, epsilon, and zeta induced by PMA caused an attenuation of BK-induced IPs accumulation and Ca2+ mobilisation in VSMCs.

Gastrin effects on isolated rat enterochromaffin-like cells following long-term hypergastrinaemia in vivo

The enterochromaffin-like (ECL) cells play an important role in the regulation of gastric acid secretion. They respond to gastrin by a prompt increase in histamine secretion, an effect which is mediated by the CCK-(B)/gastrin receptor acting through the IP(3)/DAG pathway. In the rat, long-term treatment with acid secretion inhibitors induces hypergastrinaemia which, in turn, results in ECL cell hypertrophy and hyperplasia. The aim of the present study was to evaluate various functional parameters in acutely isolated rat ECL cells, following long-term hypergastrinaemia in vivo. Rats were treated with vehicle or a supramaximal daily dose of omeprazole for more than 10 weeks to ensure ECL cell hyperplasia. ECL cells were isolated from vehicle-treated animals and 24, 72 and 120 h after the last dose of omeprazole. The functional activity of the acutely isolated ECL cells was determined by measuring gastrin-and forskolin-induced histamine secretion. Changes in cytosolic free calcium upon gastrin stimulation were monitored by digital video imaging. ECL cells successively regained their ability to respond to gastrin following long-term hypergastrinaemia, reaching close to vehicle-treated levels 120 h after the last dose of omeprazole. In the rat, the response pattern of the ECL cells appears to normalise in parallel with the normalisation of plasma gastrin levels.

Regulation of 5-hydroxytryptamine-induced signal transduction in canine cultured aorta smooth muscle cells by phorbol ester

Regulation of the increase in inositol phosphates (IPs) production and intracellular Ca2+ concentration ([Ca2+]i) by protein kinase C (PKC) was investigated in cultured canine aorta smooth muscle cells (ASMCs). Stimulation of ASMCs by 5-hydroxytryptamine (5-HT) led to IPs formation and caused an initial transient [Ca2+]i peak followed by a sustained elevation of [Ca2+]i in a concentration-dependent manner. Pretreatment of ASMCs with phorbol 12-myristate 13-acetate (PMA) for 30 min almost abolished the 5-HT-induced IPs formation and Ca2+ mobilization. This inhibition was reduced after long-term incubating the cells with PMA. Prior treatment of ASMCs with staurosporine or GF109203X, PKC inhibitors, inhibited the ability of PMA to attenuate 5-HT-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. In parallel with the effect of PMA on the 5-HT-induced IP formation and Ca2+ mobilization, the translocation and down-regulation of PKC isozymes were determined by Western blotting with antibodies against different PKC isozymes. The results revealed that treatment of ASMCs with PMA for various times, translocation of PKC-alpha, betaI, betaII, delta, epsilon, theta, and zeta isozymes from the cytosol to the membrane was seen after 5-min, 30-min, 2-h, and 4-h treatment. However, 24-h treatment caused a partial down-regulation of these PKC isozymes. In conclusion, these results demonstrate that translocation of PKC-alpha, betaI, betaII, delta, epsilon, theta, and zeta induced by PMA caused an attenuation of 5-HT-induced IPs accumulation and Ca2+ mobilization in ASMCs.

Overactivation of phospholipase C-gamma1 renders platelet-derived growth factor beta-receptor-expressing cells independent of the phosphatidylinositol 3-kinase pathway for chemotaxis

We have previously shown that porcine aortic endothelial cells expressing the Y934F platelet-derived growth factor (PDGF) beta-receptor mutant respond to PDGF-BB in a chemotaxis assay at about 100-fold lower concentration than do wild-type PDGF beta-receptor-expressing cells (Hansen, K., Johnell, M., Siegbahn, A. , Rorsman, C., Engström, U., Wernstedt, C., Heldin, C.-H., and Rönnstrand, L. (1996) EMBO J. 15, 5299-5313). Here we show that the increased chemotaxis correlates with increased activation of phospholipase C-gamma1 (PLC-gamma1), measured as inositol-1,4, 5-trisphosphate release. By two-dimensional phosphopeptide mapping, the increase in phosphorylation of PLC-gamma1 was shown not to be selective for any site, rather a general increase in phosphorylation of PLC-gamma1 was seen. Specific inhibitors of protein kinase C, bisindolylmaleimide (GF109203X), and phosphatidylinositol 3-kinase (PI3-kinase), LY294002, did not affect the activation of PLC-gamma1. To assess whether increased activation of PLC-gamma1 is the cause of the hyperchemotactic behavior of the Y934F mutant cell line, we constructed cell lines expressing either wild-type or a catalytically compromised version of PLC-gamma1 under a tetracycline-inducible promoter. Overexpression and concomitant increased activation of wild-type PLC-gamma1 in response to PDGF-BB led to a hyperchemotactic behavior of the cells, while the catalytically compromised PLC-gamma1 mutant had no effect on PDGF-BB-induced chemotaxis. Furthermore, in cells expressing normal levels of PLC-gamma1, chemotaxis was inhibited by LY294002. In contrast, the increase in chemotactic response seen upon overexpression of PLC-gamma1 was not inhibited by the PI3-kinase inhibitor LY294002. These observations suggest the existence of two different pathways which mediate PDGF-induced chemotaxis; depending on the cellular context, the PI3-kinase pathway or the PLC-gamma1 pathway may dominate.

Ca2+ and phorbol ester effect on the mast cell phosphoprotein induced by cromolyn

Several phosphoproteins are involved in stimulus-secretion coupling. The beta and gamma subunits of immunoglobulin E binding protein (FC epsilonRI) and three other protein bands get phosphorylated during stimulation of mast cell secretion. These additional proteins of 42, 59 and 68 kDa are also phosphorylated when secretion is stimulated by compound 48/80 (C48/80). A 78 kDa band, however, is phosphorylated as secretion wanes after stimulation with C48/80 and by the anti-allergic drug disodium cromoglycate (cromolyn). Phosphorylation was blocked by protein kinase C inhibitors. We investigated the isozyme involved by first showing that a cation ionophore prevented the phosphorylation of the 78 kDa protein, while a Ca2+ chelator did not affect phosphorylation even though it enhanced the inhibitory effect of cromolyn. This protein was identified as moesin by immunoprecipitation. Protein kinase C activators had no effect on 78 kDa protein phosphorylation either in the presence or absence of Ca2+ ions, but prevented its phosphorylation by cromolyn. Protein phosphatase inhibitors prolonged the duration, but not the amount of phosphate incorporated in the 78 kDa protein band while cromolyn had no effect on protein phosphatase action in vitro. The insensitivity of the 78 kDa protein phosphorylation to calcium and protein kinase C activators suggests that an atypical protein kinase C isozyme may be involved. Western blot analysis identified the presence of isozymes alpha, beta, delta and zeta, of which only the latter fits the profile suggested by the present findings.

Adrenergic regulation of translocation of protein kinase C isozymes in rat pinealocytes

Protein kinase C (PKC) is involved in the alpha1-adrenergic-potentiation of beta-adrenergic stimulated cyclic nucleotide responses in rat pinealocytes. In the present study, the PKC isozymes expressed in rat pinealocytes and their regulation by norepinephrine (NE) were investigated. Western blot analysis identified PKC alpha (a classical PKC isozyme), PKC delta and epsilon (novel PKC isozymes), and PKC zeta: (atypical PKC isozymes). NE caused an increase in PKC alpha, delta, and epsilon, but not PKC zeta, in the particulate fraction. BAPTA-AM, which clamps intracellular Ca2+, reduced NE mediated translocation of PKC alpha, delta, and epsilon. Subjecting the animals to stimulus deprivation, which altered adrenergic-stimulated cyclic nucleotide responses, had no effect on the expression of PKC alpha, delta, epsilon, and zeta. Overnight treatment with 4beta-phorbol 12-myristate 13-acetate, an activator of PKC, down-regulated PKC alpha, delta, and epsilon, but not PKC zeta. Our results indicate that all three classes of PKC isozymes (PKC alpha, delta, epsilon, and zeta are expressed in the rat pineal gland. However, selective activation of these PKC isozymes does not appear to account for the differences in the pineal cAMP and cGMP responses to stimulation.

Ca2+ and protein kinase C signaling for histamine and sulfidoleukotrienes released from human cultured mast cells

Human cultured mast cells (HCMC) release histamine and sulfidoleukotrienes (LTs) upon IgE-FcepsilonRI-mediated mast cell activation. We analyzed the Ca2+ and PKC signaling in HCMC and compared it to that in rodent mast cells. In HCMC, after IgE-mediated stimulation, an elevation of [Ca2+]i and PKC translocation to the membrane fraction was observed. As concerns Ca2+ signaling, 1) IgE-mediated histamine and LTs release was abolished after Ca2+ depletion, and the reconstitution of Ca2+ recovered the release of histamine and LTs. As regards PKC signaling, 1) staurosporine inhibited IgE-mediated mediator release. 2) PKC-downregulated mast cells did not release histamine and LTs. A23187 and PMA synergistically potentiated the activation of extracellular-regulated kinase and synergistically induced histamine and LTs release. These results demonstrated that HCMC might be useful for analysis of the signal transduction pathway for mediator release, such as histamine and LTs.

Protein kinase C delta

The protein kinase C (PKC) family consists of 11 isoenzymes that, due to structural and enzymatic differences, can be subdivided into three groups: The Ca(2+)-dependent, diacylglycerol (DAG)-activated cPKCs (conventional PKCs: alpha, beta 1, beta 2, gamma); the Ca(2+)-independent, DAG-activated nPKCs (novel PKCs: delta, epsilon, eta, theta, mu), and the Ca(2+)-dependent, DAG non-responsive aPKCs (atypical PKCs: zeta, lambda/iota). PKC mu is a novel PKC, but with some special structural and enzymatic properties.

Uncoupling of bradykinin-induced phosphoinositide hydrolysis and Ca2+ mobilization by phorbol ester in canine cultured tracheal epithelial cells

1. Regulation of the increase in inositol phosphates (IPs) production and intracellular Ca2+ concentration ([Ca2+]i by protein kinase C (PKC) was investigated in canine cultured tracheal epithelial cells (TECs). Stimulation of TECs by bradykinin (BK) led to IPs formation and caused an initial transient [Ca2+]i peak in a concentration-dependent manner. 2. Pretreatment of TECs with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min attenuated the BK-induced IPs formation and Ca2+ mobilization. The maximal inhibition occurred after incubating the cells with PMA for 2 h. 3. The concentrations of PMA that gave half-maximal (pEC50) inhibition of BK-induced IPs accumulation and an increase in [Ca2+]i were 7.07 M and 7.11 M, respectively. Inactive phorbol ester, 4alpha-phorbol 12,13-didecanoate at 1 microM, did not inhibit these responses. Prior treatment of TECs with staurosporine (1 microM), a PKC inhibitor, inhibited the ability of PMA to attenuate BK-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. 4. In parallel with the effect of PMA on the BK-induced IPs formation and Ca2+ mobilization, the translocation and down-regulation of PKC isozymes were determined. Analysis of cell extracts by Western blotting with antibodies against different PKC isozymes revealed that TECs expressed PKC-alpha, betaI, betaII, gamma, delta, epsilon, theta and zeta. With PMA treatment of the cells for various times, translocation of PKC-alpha, betaI, betaII, gamma, delta, epsilon and theta from cytosol to the membrane was seen after 5 min, 30 min, 2 h, and 4 h treatment. However, 6 h treatment caused a partial down-regulation of these PKC isozymes. PKC-zeta was not significantly translocated and down-regulated at any of the times tested. 5. Treatment of TECs with 1 microM PMA for either 30 min or 6 h did not significantly change the KD, and Bmax receptor for BK binding (control: KD=1.7+/-0.3 nM; Bmax=50.5+/-4.9 fmol/mg protein), indicating that BK receptors are not a site for the inhibitory effect of PMA on BK-induced responses. 6. In conclusion, these results suggest that activation of PKC may inhibit the phosphoinositide hydrolysis and consequently attenuate the [Ca2+]i increase or inhibit independently both responses to BK. The translocation of pKC-alpha, betaI, betaII, delta, epsilon, gamma, and theta induced by PMA caused an attenuation of BK-induced IPs accumulation and Ca2+ mobilization in TECs.

Expression of Protein Kinase C Isozymes in Human Basophils: Regulation by Physiological and Nonphysiological Stimuli

The expression of protein kinase C (PKC) isozymes in human basophils and the regulation of PKC isozymes during basophil activation by phorbol 12-myristate 13-acetate (PMA) ± ionomycin, f-met-leu-phe (FMLP), and anti-IgE antibody were examined. In human basophils (> 98% purity), PKCβΙ, βΙΙ, δ, and  were expressed, PKC was difficult to detect, and PKCγ and η were undetectable. In unstimulated basophils, PKCβI and βII were found primarily in the cytosol fraction (95% ± 3% of total and 98% ± 1%, respectively). Within 5 minutes of stimulation with PMA (100 ng/mL), both PKCβI and βII were translocated to the membrane fraction (85% ± 4% and 83% ± 6%, respectively). In resting cells, 48% ± 3% and 61% ± 10% of PKCδ and , respectively, existed in the membrane fraction. Within 1 minute of stimulation with PMA, 90% ± 6% of PKC was found in the membrane fraction, however, no translocation of PKCδ was apparent. Stimulation with FMLP caused modest translocation (≈20%) of all PKC isozymes by 1 minute, whereas stimulation with anti-IgE antibody led to no detectable changes in PKC location throughout a 15-minute period of measurement. However, concentrations of PMA and ionomycin that alone caused no PKC translocation and little histamine release, together caused significant histamine release but no apparent PKC translocation. Studies with bis-indolylmaleimide analogs showed inhibition of PMA-induced, but not anti–IgE-induced, histamine release. These pharmacological studies suggest that PKC does not play a prodegranulatory role in human basophil IgE-mediated secretion.

© 1998 by The American Society of Hematology.

Expression of Protein Kinase C Isozymes in Human Basophils: Regulation by Physiological and Nonphysiological Stimuli

The expression of protein kinase C (PKC) isozymes in human basophils and the regulation of PKC isozymes during basophil activation by phorbol 12-myristate 13-acetate (PMA) ± ionomycin, f-met-leu-phe (FMLP), and anti-IgE antibody were examined. In human basophils (> 98% purity), PKCβΙ, βΙΙ, δ, and  were expressed, PKC was difficult to detect, and PKCγ and η were undetectable. In unstimulated basophils, PKCβI and βII were found primarily in the cytosol fraction (95% ± 3% of total and 98% ± 1%, respectively). Within 5 minutes of stimulation with PMA (100 ng/mL), both PKCβI and βII were translocated to the membrane fraction (85% ± 4% and 83% ± 6%, respectively). In resting cells, 48% ± 3% and 61% ± 10% of PKCδ and , respectively, existed in the membrane fraction. Within 1 minute of stimulation with PMA, 90% ± 6% of PKC was found in the membrane fraction, however, no translocation of PKCδ was apparent. Stimulation with FMLP caused modest translocation (≈20%) of all PKC isozymes by 1 minute, whereas stimulation with anti-IgE antibody led to no detectable changes in PKC location throughout a 15-minute period of measurement. However, concentrations of PMA and ionomycin that alone caused no PKC translocation and little histamine release, together caused significant histamine release but no apparent PKC translocation. Studies with bis-indolylmaleimide analogs showed inhibition of PMA-induced, but not anti–IgE-induced, histamine release. These pharmacological studies suggest that PKC does not play a prodegranulatory role in human basophil IgE-mediated secretion.

© 1998 by The American Society of Hematology.

GnRH signalling pathways and GnRH-induced homologous desensitization in a gonadotrope cell line (alphaT3-1)

Exposure of the gonadotrope cells to gonadotropin-releasing hormone (GnRH) reduces their responsiveness to a new GnRH stimulation (homologous desensitization). The time frame as well as the mechanisms underlying this phenomenon are yet unclear. We studied in a gonadotrope cell line (alphaT3-1) the effects of short as well as long term GnRH pretreatments on the GnRH-induced phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities, by measuring the production of IP3, total inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that although rapid desensitization of GnRH-induced IP3 formation did not occur in these cells, persistent stimulation of cells with GnRH or its analogue resulted in a time-dependent attenuation of GnRH-elicited IPs formation. GnRH-induced IPs desensitization was potentiated after direct activation of PKC by the phorbol ester TPA, suggesting the involvement of distinct mechanisms in the uncoupling exerted by either GnRH or TPA on GnRH-stimulated PI hydrolysis. The levels of individual phosphoinositides remained unchanged under any desensitization condition applied. Interestingly, while the GnRH-induced PLA2 activity was rapidly desensitized (2.5 min) after GnRH pretreatments, the neuropeptide-evoked PLD activation was affected at later times, indicating an important time-dependent contribution of these enzymatic activities in the sequential events underlying the GnRH-induced homologous desensitization processes in the gonadotropes. Under GnRH desensitization conditions, TPA was still able to induce PLD activation and to further potentiate the GnRH-evoked PLD activity. AlphaT3-1 cells possess several PKC isoforms which, except PKCzeta, were differentially down-regulated by TPA (PKCalpha, betaII, delta, epsilon, eta) or GnRH (PKCbetaII, delta, epsilon, eta). In spite of the presence of PKC inhibitors or down-regulation of PKC isoforms by TPA, the desensitizing effect of the neuropeptide on GnRH-induced IPs, AA and PEt formation remained unchanged. In conclusion, in alphaT3-1 cells the GnRH-induced homologous desensitization affects the GnRH coupling with PLC, PLA2 and PLD by mechanism(s) which do not implicate TPA-sensitive PKC isoforms, but likely reflect time-dependent modification(s) on the activation processes of the enzymes.

Inhibition of cromolyn-induced phosphorylation of a 78-kDa protein by phorbol esters in rat peritoneal mast cells

Disodium cromoglycate (cromolyn) is a well documented inhibitor of immunologically-induced histamine release from rat peritoneal mast cells and has been shown to stimulate the phosphorylation of a mast cell protein of apparent molecular mass 78,000 Da (78 kDa), an event which may be involved in terminating secretion. Here we aimed to determine the role of the ubiquitous enzyme, protein kinase C, in the phosphorylating activity of cromolyn by examining the effects of phorbol esters (activators of protein kinase C) on protein phosphorylation in [32P]orthophosphate loaded rat peritoneal mast cells. Protein kinase C-activating phorbol esters such as 12-O-tetradecanoyl phorbol-13-acetate (TPA) and 4beta-phorbol 12,13-dibutyrate (PdBu) were found to potently inhibit cromolyn-induced phosphorylation when added to mast cells simultaneously with cromolyn (IC50 22 and 79 nM respectively). 4Alpha-phorbol 12,13-didecanoate (PdD), a phorbol ester which does not activate protein kinase C, had no effect on cromolyn-induced phosphorylation. Addition of TPA to mast cells previously exposed to cromolyn for 60 sec (i.e. when 78-kDa protein phosphorylation is maximal) also caused a very rapid dephosphorylation of the 78-kDa protein. Phosphorylation of the 78-kDa protein can also be induced by dibutyryl cyclic GMP and this action was similarly inhibited by TPA and PdBu. Cromolyn inhibited secretion induced by anti-IgE, but not by TPA, and thus inhibition of secretion by cromolyn is further correlated to its phosphorylation of the 78-kDa protein. The data suggest that the inhibitory action of cromolyn on mast cell secretion and phosphorylation of the 78-kDa protein are not mediated through a phorbol ester-sensitive protein kinase C, but more likely that such an enzyme could be involved in regulating dephosphorylation of the 78-kDa protein. Further explanations for this novel dephosphorylating activity of phorbol esters are discussed.

Suppression of thyrotropin receptor-G protein-phospholipase C coupling by activation of protein kinase C in thyroid carcinoma cells

In human thyroid follicular cells TSH exerts its action on growth and function at least via two distinct pathways, the adenylate cyclase cascade and the phospholipase Cbeta (PLCbeta)-mediated inositol phosphate generation. We investigated the effect of TSH on activation of phosphoinositide hydrolysis and inositol phosphate generation by PLCbeta in HTh74 thyroid carcinoma cells that express functional TSH receptors and in HTC-TSHr thyroid carcinoma cells that are devoid of endogenous TSH receptors but express recombinant human TSH receptors. In both cell lines, TSH up to concentrations of 300 mU/ml failed to stimulate myo-inositol 1,4,5-trisphosphate and myo-inositol-tetrakisphosphate generation, but led to a decrease in these compounds within 1 min of stimulation. However, ATP and bradykinin increased concentrations of inositol phosphates in both thyroid carcinoma cell lines. In contrast, in differentiated FRTL5 thyroid cell line and CHO-TSHr cell line expressing recombinant human TSH receptors, TSH elicited a significant increase in myo-inositol 1,4,5-trisphosphate and its metabolic derivatives. However, when HTC-TSHr cells were pretreated with calphostin C or staurosporine, inhibitors of protein kinase C, a TSH concentration of 20 mU/ml enhanced generation of inositol phosphates in these cells. From our data we conclude that in HTC-TSHr and HTh74 thyroid carcinoma cells, the coupling within the TSH receptor-Gq protein-PLCbeta signaling pathway is impaired compared to that in nontransformed cells. It is conceivable that this is at least in part dependent on the level of protein kinase C activation in these cells.

Feedback regulation of extracellular ATP-stimulated phosphoinositide hydrolysis by protein kinase C-alpha in bovine glomerular endothelial cells

In glomerular endothelial cells, extracellular ATP stimulates a phospholipase C with subsequent hydrolysis of polyphosphoinositides and an increase in cytosolic free Ca2+ concentration ([Ca2+]i). Short-term (30 min) pretreatment of endothelial cells with 12-O-tetradecanoylphorbol 13-acetate (TPA), a potent activator of protein kinase C (PKC), decreases the ATP-stimulated phosphoinositide degradation and Ca2+ mobilization. However, this inhibition was lost after incubating the cells for four hours with TPA. Longer-term pretreatment (10 to 48 hr) even potentiated ATP-induced phosphoinositide breakdown and Ca2+ mobilization. In addition, pretreating the cells for 30 minutes with the specific PKC inhibitor Ro 31-8220 dose-dependently increased ATP-stimulated phosphoinositide hydrolysis, thus clearly indicating a regulatory role for PKC in the inositol lipid signaling pathway in glomerular endothelial cells. By using specific antibodies recognizing the different PKC isoenzymes, it is observed that glomerular endothelial cells express five isoenzymes: PKC-alpha, -delta, -epsilon, -zeta and -theta. No PKC-beta, -gamma, -eta and -mu isoenzymes were detected. On exposure to TPA, a complete depletion of PKC-alpha is observed within four hours. In contrast, PKC-epsilon was more resistant to phorbol ester, and even after 48 hours of TPA treatment, only 60% of PKC-epsilon was down-regulated. PKC-theta decreased very slowly from the cytosol (47% left after 24 hr of phorbol ester treatment) and translocated to the Triton X100-insoluble fraction. Moreover, PKC-delta and PKC-zeta were not significantly affected by 48 hours of phorbol ester incubation. Thus, only PKC-alpha is depleted with a kinetic that corresponds to the loss of feedback inhibition of ATP-stimulated phosphoinositide turnover. In the next step, [Ca2+]i changes were measured in single cells loaded with Fura-2 after microinjection of neutralizing PKC isoenzyme-specific antibodies. Injection of antibodies specific for PKC-alpha potently increased Ca2+ mobilization in response to ATP stimulation when compared to cells injected with buffer only or antibodies specific for PKC-epsilon. These results provide evidence that PKC-alpha mediates feedback inhibition of ATP-stimulated phosphoinositide hydrolysis in glomerular endothelial cells.

Inhibition of 5-hydroxytryptamine-induced phosphoinositide hydrolysis and Ca2+ mobilization in canine cultured tracheal smooth muscle cells by phorbol ester

1. Regulation of the increase in inositol-1,4,5-trisphosphate (IP3) production and intracellular Ca2+ concentration ([Ca2+]i by protein kinase C (PKC) was investigated in canine cultured tracheal smooth muscle cells (TSMCs). Stimulation of TSMCs by 5-hydroxytryptamine (5-HT) caused an initial transient [Ca2+]i peak followed by a sustained elevation of [Ca2+]i in a concentration-dependent manner. 2. Pretreatment of TSMCs with phorbol 12-myristate 13-acetate (PMA, 1 microM) for 30 min blocked the 5-HT-induced IP3 formation and Ca2+ mobilization. This inhibition was reduced after the cells had been incubated with PMA for 8 h, and within 48 h the 5-HT-induced Ca2+ mobilization reached the same extent as control cells. 3. The concentration of PMA that gave half-maximal inhibition of 5-HT-induced increase in [Ca2+]i was 4 nM. Pretreatment of TSMCs with staurosporine (1 microM) of GF109203X (0.1 microM), PKC inhibitors, inhibited the ability of PMA to attenuate 5-HT-induced responses, suggesting that the inhibitory effect of PMA was mediated through the activation of PKC. 4. In parallel with the effect of PMA on 5-HT-induced IP3 formation and Ca2+ mobilization, the translocation and down-regulation of PKC isozymes were determined by Western blot analysis in TSMCs. Analysis of cell extracts by Western blotting with antibodies against different PKC isozymes revealed that TSMCs expressed PKC-alpha, beta I, beta II, delta, epsilon, theta and zeta. With PMA treatment of the cells for various times, translocation of PKC-alpha, beta I, beta II, delta, epsilon, and theta from the cytosol to the membrane was seen after 5 min, 30 min, 2 h, and 4 h treatment. However, 24 h treatment caused a partial down-regulation of these PKC isozymes PKC-zeta was not significantly translocated and down-regulated at any of the times tested. 5. In conclusion, these results suggest that activation of PKC may inhibit the receptor-mediated phosphoinositide hydrolysis and consequently attenuate the [Ca2+]i increase or inhibit both responses independently. The translocation of PKC-alpha, beta I, beta II, delta, epsilon, and theta induced by PMA caused an attenuation of 5-HT-stimulated IP3 accumulation and Ca2+ mobilization in TSMCs.

Differential activation of protein kinase C delta and epsilon gene expression by gonadotropin-releasing hormone in alphaT3-1 cells. Autoregulation by protein kinase C

The effect of gonadotropin-releasing hormone (GnRH) upon protein kinase C (PKC) delta and PKCepsilon gene expression was investigated in the gonadotroph-derived alphaT3-1 cell line. Stimulation of the cells with a stable analog [D-Trp6]GnRH (GnRH-A) resulted in a rapid elevation of PKCepsilon mRNA levels (1 h), while PKCdelta mRNA levels were elevated only after 24 h of incubation. The rapid elevation of PKCepsilon mRNA by GnRH-A was blocked by pretreatment with a GnRH antagonist or actinomycin D. The PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA), but not the Ca2+ ionophore ionomycin, mimicked the rapid effect of GnRH-A upon PKCepsilon mRNA elevation. Additionally, the rapid stimulatory effect of GnRH-A was blocked by the selective PKC inhibitor GF109203X, by TPA-mediated down-regulation of endogenous PKC, or by Ca2+ removal. Interestingly, serum-starvation (24 h) advanced the stimulation of PKCdelta mRNA levels by GnRH-A and the effect could be detected at 1 h of incubation. The rapid effect of GnRH-A upon PKCdelta mRNA levels in serum-starved cells was mimicked by TPA, but not by ionomycin, and was abolished by down-regulation of PKC or by Ca2+ removal. Preactivation of alphaT3-1 cells with GnRH-A for 1 h followed by removal of ligand and serum resulted in elevation of PKCdelta mRNA levels after 24 h of incubation. Western blot analysis revealed that GnRH-A and TPA stimulated (within 5 min) the activation and some degradation of PKCdelta and PKCepsilon. We conclude that Ca2+ and PKC are involved in GnRH-A elevation of PKCdelta and PKCepsilon mRNA levels, with Ca2+ being necessary but not sufficient, while PKC is both necessary and sufficient to mediate the GnRH-A response. A serum factor masks PKCdelta but not PKCepsilon mRNA elevation by GnRH-A, and its removal exposes preactivation of PKCdelta mRNA by GnRH-A which can be memorized for 24 h. PKCdelta and PKCepsilon gene expression evoked by GnRH-A is autoregulated by PKC, and both isotypes might participate in the neurohormone action.

Role of phospholipase Cbeta3 phosphorylation in the desensitization of cellular responses to platelet-activating factor

Platelet-activating factor (PAF) stimulates a diverse array of cellular responses through receptors coupled to G proteins that activate phospholipase C (PLC). Truncation of the cytoplasmic tail of the receptor to remove phosphorylation sites (mutant PAF receptor, mPAFR) results in enhancement of PAF-stimulated responses. Here we demonstrate that PAF or phorbol 12-myristate 13-acetate (PMA) pretreatment inhibited wild type PAFR-induced PLC-mediated responses by approximately 90%, whereas these responses to the phosphorylation-deficient mPAFR were inhibited by approximately 50%, despite normal G protein coupling, suggesting a distal inhibitory locus. PAF and PMA, as well as a membrane permeable cyclic AMP analog, stimulated phosphorylation of PLCbeta3. A protein kinase C (PKC) inhibitor blocked phosphorylation of PLCbeta3 stimulated by PAF and PMA but not by cAMP. Activation of protein kinase A (PKA) by cAMP did not result in inhibition of Ca2+ mobilization stimulated by PAF. In contrast, cAMP did inhibit the response to formylpeptide chemoattractant receptor. These data suggest that homologous desensitization of PAF-mediated responses is regulated via phosphorylation at two levels in the signaling pathway, one at the receptor and the other at PLCbeta3 mediated by PKC but not by PKA. Phosphorylation of PLCbeta3 by PKA could explain the inhibition of formylpeptide chemoattractant receptor signaling by cAMP. As PAF and formylpeptide chemoattractant receptors activate PLC via different G proteins, phosphorylation of PLCbeta3 by PKC and PKA could provide distinct regulatory control for classes of G protein-coupled receptors.

Negative regulation of MAP kinase by diacylglycerol-dependent mechanisms via G protein-coupled receptors in rat basophilic RBL-2H3 (ml) cells

Carbachol and 5'-(N-ethylcarboxamido)-adenosine (NECA), stimulants of G protein-coupled receptors, induce MAP kinase activation in the muscarinic ml receptor-transfected mast cell line, RBL-2H3 (ml) cells. The phospholipase C inhibitor neomycin and the phosphatidate phosphohydrolase inhibitor propranolol augmented MAP kinase activation induced by carbachol and NECA without affecting the antigen-induced MAP kinase activation. Furthermore, the duration of MAP kinase activation induced by carbachol or NECA was also prolonged by neomycin and propranolol. The specific protein kinase C inhibitor Ro 31-8425 enhanced the carbachol- or NECA-induced MAP kinase activation. These findings suggest that the MAP kinase activation mediated by the G protein-coupled receptors is negatively regulated by diacylglycerol and activated protein kinase C(s).

Structure and expression of the Caenorhabditis elegans protein kinase C2 gene. Origins and regulated expression of a family of Ca2+-activated protein kinase C isoforms

The molecular and cellular basis for concerted Ca2+/lipid signaling in Caenorhabditis elegans was investigated. A unique gene (pkc-2) and cognate cDNAs that encode six Ca2+/diacylglycerol-stimulated PKC2 isoenzymes were characterized. PKC2 polypeptides (680-717 amino acid residues) share identical catalytic, Ca2+-binding, diacylglycerol-activation and pseudosubstrate domains. However, sequences of the N- and C-terminal regions of the kinases diverge. PKC2 diversity is partly due to differential activation of transcription by distinct promoters. Each promoter precedes an adjacent exon that encodes 5'-untranslated RNA, an initiator AUG codon and a unique open reading frame. PKC2 mRNAs also incorporate one of two 3'-terminal exons via alternative splicing. Cells that are capable of receiving and propagating signals carried by Ca2+/diacylglycerol were identified by assessing activities of pkc-2 gene promoters in transgenic C. elegans and visualizing the distribution of PKC2 polypeptides via immunofluorescence. Highly-selective expression of certain PKC2 isoforms was observed in distinct subsets of neurons, intestinal and muscle cells. A low level of PKC2 isoforms is observed in embryos. When L1 larvae hatch and interact with the external environment PKC2 content increases 10-fold. Although 77- and 78-kDa PKC2 isoforms are evident throughout post-embryonic development, an 81-kDa isoform appears to be adapted for function in L1 and L2 larvae.

Quantitative analysis of exocytosis visualized by a video-enhanced light/fluorescence microscope reveals two distinct components of exocytosis in RBL-2H3 cells

Rat basophilic leukemia (RBL-2H3) cells, which exhibit Ca2+-dependent secretion of granules when stimulated with antigen or the Ca2+-ionophore A23187, were observed under a video-enhanced light/fluorescence microscope. Exocytotic events of individual granules were visualized in individual cells stimulated with antigen or A23187. Exocytosis of granules stimulated with A23187 showed two peaks in the time course. The earlier one was inhibited by selective inhibitors of protein kinase C (Ro31-8425, Ro31-8220, and chelerythrine) and the other was inhibited by an inhibitor of phosphatidate hydrolase, propranolol. Exocytosis by antigen stimulation, however, showed only one peak, which was inhibited by the selective inhibitors of protein kinase C, but not by propranolol. These results indicate that at least two distinct components of exocytosis exist in RBL-2H3 cells.

Characterization of the 78 kDa mast cell protein phosphorylated by the antiallergic drug cromolyn and homology to moesin

Mast cells (MC) can be stimulated to secrete by cross-linking immunoglobulin E bound to specific surface receptors, as well as in response to polycationic molecules such as substance P and compound 48/80. The antiallergic drug disodium cromoglycate (cromolyn) inhibited MC secretion and rapidly incorporated phosphate into a 78 kDa protein, speculated to be its mode of action. This protein was purified by single and two-dimensional gel electrophoresis, and was shown to be phosphorylated primarily on serine residues by protein kinase C. Partial amino acid sequencing of two generated fragments was identical to that of portions of mouse moesin, a member of the band 4.1 superfamily of proteins, with no definitive function known to date. Polyclonal antibodies raised against the rat basophil leukemia cell moesin cDNA expressed in Escherichia coli immunoprecipitated the 78 kDa phosphoprotein quantitatively, and immunocytochemistry localized it to the plasma membrane. Reversible phosphorylation of this 78 kDa phosphoprotein could affect its possible cytoskeletal binding through which it may regulate stimulus-secretion coupling in MC.

Phenylarsine oxide (PAO)-mediated activation of phospholipase D in rat basophilic leukemia (RBL-2H3) cells: possible involvement of calcium and protein kinase C

Addition of phenylarsine oxide (PAO) to [3H]oleic acid-labeled rat basophilic leukemia (RBL-2H3) cells gave rise to the remarkable formation of [3H]phosphatidylbutanol (PBut), a specific product of phospholipase D (PLD) activation. Preincubation of cells with 2,3-dimercaptopropanol (DMP) or dithiothreitol (DTT), compounds containing sulfhydryls, prevented PAO-stimulated [3H]PBut formation, indicating that PAO-stimulated PLD through interacting with vicinal thiol groups. Treatment of cells with PAO resulted in increase in intracellular Ca2+ concentration without significant production of inositol phosphates. Removal of extracellular free Ca2+ by chelating with EGTA was found to inhibit [3H]PBut formation by PAO. Incubation of cells with 20 nM phorbol 12-myristate 13-acetate (PMA) for 6 h caused down-regulation of protein kinase C (PKC) alpha and beta isozymes, whereas it had no effect on PKC delta, epsilon and zeta isozymes. Under this condition, decrease in PAO-stimulated [3H]PBut formation was observed to occur with a concomitant decrease in the level of PKC alpha and beta isozymes. These results suggest that a covalent bridge between vicinal thiol groups of cell surface proteins induced by PAO potentiates PLD activation and that PAO-induced PLD activation is regulated by Ca2+ and PKC alpha and/or beta isozymes.

Differential effects of the protein kinase C activator phorbol 12-myristate 13-acetate on calcium responses and secretion in adherent and suspended RBL-2H3 mucosal mast cells

Adhesion of RBL-2H3 mucosal mast cells to fibronectin-coated surfaces has been linked to changes in secretion and tyrosine kinase activity. We now show that adhesion affects the sensitivity of RBL cells to the protein kinase C activator phorbol 12-myristate 13-acetate (PMA). In suspended cells, PMA inhibited antigen-induced calcium influx (as measured by manganese influx) and changes in intracellular free calcium and had complex effects on antigen-stimulated secretion. However, in adherent cells PMA had little effect on these responses. Suspended cells only secreted in response to thapsigargin if they were co-treated with PMA, while adherent cells secreted in response to thapsigargin alone. The thapsigargin-induced secretion in adherent cells was inhibited by protein kinase C down-regulation and by the protein kinase C inhibitor GF 109203X, but not by calphostin C. We suggest that protein kinase C is constitutively activated in adherent cells, possibly due to modification of the regulatory domain of the enzyme.

Aggregation of the Fc epsilon RI in mast cells induces the synthesis of Fos-interacting protein and increases its DNA binding-activity: the dependence on protein kinase C-beta

The ability of c-Fos to dimerize with various proteins creates transcription complexes which can exert their regulatory function on a variety of genes. One of the transcription factors that binds to c-Fos is the newly discovered Fos-interacting protein (FIP). In this report we present evidence for the regulation of the synthesis of FIP by a physiological stimulus. We found that the aggregation of the mast cell high affinity receptor for IgE (Fc epsilon RI) induced the synthesis of FIP and increased its DNA binding activity. Moreover, down-regulation of the isoenzyme protein kinase C-beta (PKC-beta) by a specific antisense phosphorothioate oligonucleotide resulted in profound inhibition of FIP-Fos DNA binding activity. Thus, aggregation of the Fc epsilon RI on mast cells elicits a PKC-beta dependent signaling pathway which regulates FIP-Fos DNA binding activity.

Phorbol ester stimulation of phosphatidylcholine synthesis in four cultured neural cell lines: correlations with expression of protein kinase C isoforms

Phosphatidylcholine (PtdCho) can provide lipid second messengers involved in signal transduction pathways. As a measure of phospholipid turnover in response to extracellular stimulation, we investigated differential enhancement of [3H]choline incorporation into PtdCho by phorbol esters. In C6 rat glioma and SK-N-SH human neuroblastoma cells, [3H]PtdCho synthesis was 2-4 fold stimulated by beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA) when [3H]choline was incubated simultaneously with, or 15 min prior to, beta-TPA treatment. By contrast, in N1E-115 mouse and SK-N-MC human neuroblastoma cells, phorbol esters had no appreciable effect on [3H]choline incorporation; however, in all cells, 200 microM oleic acid enhanced PtdCho synthesis, indicating a stimulable process. Alterations by thymeleatoxin (TMT), an activator of conventional PKC isoforms (alpha, beta and gamma), were similar to beta-TPA. We investigated whether expression of specific PKC isoforms might correlate with these effects of phorbol esters on PtdCho synthesis. All cell lines bound phorbol esters, had PKC activity that was translocated by phorbol esters and differentially expressed isoforms of PKC. Northern and western blot analyses, using specific cDNA and antibodies for PKC-alpha, -beta, -gamma, -delta, -epsilon, and -zeta, revealed that expression of alpha-isoform predominated in C6 and SK-N-SH cells. In contrast, TPA-responsive beta-isoform predominated in SK-N-MC cells. gamma-PKC was not detected in any cells and only in C6 cells was PKC-delta present and translocated by beta-TPA treatment. PKC-epsilon was not detected in SK-N-MC cell lines but translocated with TPA treatment in the other three cell lines. PKC-zeta was present in all cells but was unaltered by TPA treatment. Accordingly, stimulation of PtdCho turnover by phorbol esters correlated only with expression of PKC-alpha; presence of PKC-beta alone was insufficient for a TPA response.

Hydrogen peroxide-and fetal bovine serum-induced DNA synthesis in vascular smooth muscle cells: positive and negative regulation by protein kinase C isoforms

Hydrogen peroxide and fetal bovine serum stimulate DNA synthesis in growth-arrested smooth muscle cells with remarkably similar kinetics and cell density dependence. However, while stimulation with fetal bovine serum results in cell proliferation, that by H2O2 is followed by cell death. Depletion of conventional and novel protein kinase C isoforms, resulting from a long treatment with phorbol-12-myristate-13-acetate, further increases H2O2-induced DNA synthesis. On the other hand, the specific protein kinase C inhibitor calphostin C abolished the increased DNA synthesis promoted by fetal bovine serum or H2O2. H2O2 increases protein kinase C activity in smooth muscle cells. This effect is markedly reduced, but not abolished, by down-regulation of the alpha, delta and epsilon protein kinase C isoforms. Thus, the zeta isoform of protein kinase C, which is not down-regulated, may be responsible for the residual H2O2 stimulation of protein kinase C. In conclusion, the results obtained show that H2O2 stimulates protein kinase C activity and DNA synthesis in growth-arrested smooth muscle cells: these events are not followed by cell proliferation but rather by cell death. This H2O2 stimulated DNA synthesis appears to be negatively controlled by alpha, delta and epsilon isoforms and positively controlled by the zeta isoform of protein kinase C.

Differential regulation of phospholipase D and phospholipase C by protein kinase C-beta and -delta in liver macrophages

We have studied activation of phospholipase (PL) C and PLD in liver macrophages labelled with [3H]arachidonic acid. Zymosan, phorbol 12-myristate 13-acetate (PMA), A23187 and fluoride but not arachidonic acid or lipopolysaccharide (LPS) induce an activation of PLD ([3H]phosphatidylethanol (PEt) accumulation). An activation of PLC ([3H]diacylglycerol (DAG) accumulation) is measured with zymosan, PMA and fluoride but not with A23187, LPS or arachidonic acid whereas inositol phosphates are formed with zymosan, only. Removal of extracellular calcium reduces the formation of [3H]PEt and [3H]DAG while pretreatment of the cells with dexamethasone reduces [3H]PEt formation, only. PMA- and zymosan-induced activation of PLD and PMA-induced activation of PLC both seem to be mediated by protein kinase (PK) C-beta whereas zymosan-induced activation of PLC is negatively controlled by PKC-delta. We could furthermore present evidence that the release of [3H]arachidonic acid in these cells occurs independent of an activation of PLD.

Staurosporine inhibits inositol phosphate formation in bovine adrenal medullary cells

The effect of protein kinase C activators and inhibitors on histamine-stimulated phospholipase C in bovine adrenal medullary cells has been investigated. The protein kinase C activators, phorbol 12,13-dibutyrate (PDB) or sn-1,2-dioctanoylglycerol (DOG), inhibited histamine-stimulation of phospholipase C. This inhibition was prevented by the protein kinase C-selective inhibitor Ro 31-8220 (3-[1-[3-(2-isothioureido) propyl]indol-3-yl]-4-(1-methylindol-3-yl)-3-pyrrolin-2,5-dio ne) but not the broad spectrum protein kinase inhibitor staurosporine. Indeed staurosporine on its own inhibited both the histamine-stimulated response and, in permeabilized cells, phospholipase C activated by Ca2+. Staurosporine inhibition of phospholipase C is unlikely to be mediated via protein kinase A or Ca2+/calmodulin-dependent protein kinase because it was not reproduced by selective inhibition of these kinases. Staurosporine treatment, however, reduced inositol phospholipid levels in stimulated cells. Thus staurosporine and Ro 31-8220, two widely used protein kinase C inhibitors, have quite different effects on phospholipase C activation. Furthermore, staurosporine may cause this inhibition through a reduction in the level of phospholipase C substrate.

Regulation of adenylyl cyclase by protein kinase A

The changing relationship between stimuli and responses after prolonged receptor stimulation is a general feature of hormonal signaling systems, termed desensitization. This phenomenon has been best exemplified in the covalent modification of the G protein-linked catecholamine receptors. However, other components within this signaling pathway can be involved in desensitization. Here we present evidence that desensitization occurs at the level of the effector enzyme itself through phosphorylation. Type V adenylyl cyclase (AC) is the major isoform expressed in the heart. Using purified enzymes, we demonstrate that protein kinase A (PKA) directly phosphorylates and thereby inhibits type V AC catalytic activity. This inhibition was negated in the presence of PKA inhibitor. Analysis of enzyme kinetics revealed that this inhibition was due to a decrease in the catalytic rate, not to a decrease in the affinity for the substrate ATP. Our results indicate that AC catalytic activity can be regulated through PKA-mediated phosphorylation, suggesting another mechanism of desensitization for receptor pathways which signal via increases in intracellular cAMP.

Expression of protein kinase C isozymes in hippocampal neurones in culture

Several protein kinase C (PKC) isozymes were analyzed by immunoblot and immunocytochemistry in cultures of hippocampal neurones at several stages of differentiation. Our findings reveal the existence of two distinct patterns of expression. Firstly, conventional PKC isozymes alpha, beta and gamma, that are expressed at very low levels during the initial stages and then increase continuously with time of culture. Secondly, novel PKC isozymes delta, epsilon and zeta, whose contents increase very early to reach a maximum after three days of culture and then progressively decline. Specific proteolysis for PKC isozymes beta and gamma was observed throughout the period studied. The developmental profile obtained for the different PKC isozymes is discussed in relation to the differentiation of hippocampal neurones in culture.

Protein kinase C isoenzymes in mouse harderian gland. Differential expression of the alpha- and epsilon-isoforms during pregnancy. Protein kinase C-OC

Protein kinase C (PKC) is known to be involved in the regulation of exocytosis in different cell lines and tissues. Experiments were designed to determine whether the Harderian gland of CD-1 mouse produces PKC isoenzymes and whether the expression of the isoforms changes during pregnancy. The presence of the isoenzymes was assessed by immunoblotting experiments using extract of total Harderian gland and polyclonal antisera specific for nine different PKC isoforms. Antisera giving a positive staining on Western blots were subsequently used for immunohistochemical investigation using a secondary antibody conjugated to alkaline phosphatase. Immunoblotting experiments revealed that the Harderian gland from female mouse expresses PKC isoforms-alpha, -epsilon, -zeta and -eta. These isoforms were also detected in the Harderian gland from 13-day pregnant mouse; however, striking quantitative changes were seen concerning the alpha- and epsilon-isoforms. The 80-kDa native from of PKC-alpha almost doubled in the pregnant mouse in comparison with normal female mouse whereas the amount of 50-kDa catalytic domain did not change. Protein kinase C-epsilon appeared as a 92- to 93-kDa form and a 67-kDa form. While the 92- to 93-kDa protein was expressed to a similar extent in both types of mouse, the 67-kDa form was more abundant in the Harderian gland from normal female mouse. These data were corroborated by immunohistochemical experiments and showing a diffuse and granular staining of the adenomeres. These observations demonstrate for the first time (to our knowledge) that the mouse Harderian gland produces several PKC isoenzymes that could be involved in the regulation of exocytosis and/or other functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.