Direct Activation Of Protein Kinase C Research Articles

Oxidized-LDL enhances coronary vasoconstriction by increasing the activity of protein kinase C isoforms alpha and epsilon.

Oxidized low-density lipoprotein (ox-LDL) plays a critical role in the development of atherosclerotic coronary vasospasm; however, the cellular mechanisms involved are not fully understood. We tested the hypothesis that ox-LDL enhances coronary vasoconstriction by increasing the activity of specific protein kinase C (PKC) isoforms in coronary smooth muscle. Active stress was measured in de-endothelialized porcine coronary artery strips; cell contraction and [Ca(2+)](i) were monitored in single coronary smooth muscle cells loaded with fura-2; and the cytosolic and particulate fractions were examined for PKC activity and reactivity with isoform-specific anti-PKC antibodies with Western blots. Ox-LDL (100 microgram/mL) caused slow but significant increases in active stress to 1.3+/-0.4x10(3) N/m(2) and cell contraction (10%) that were completely inhibited by GF109203X (10(-6) mol/L), an inhibitor of Ca(2+)-dependent and -independent PKC isoforms, with no significant change in [Ca(2+)](i). 5-Hydroxytryptamine (5-HT; 10(-7) mol/L) and KCl (24 mmol/L) caused increases in cell contraction and [Ca(2+)](i) that were inhibited by the Ca(2+) channel blocker verapamil (10(-6) mol/L). Ox-LDL enhanced coronary contraction to 5-HT and KCl with no additional increases in [Ca(2+)](i). Direct activation of PKC by phorbol 12-myristate13-acetate (PMA; 10(-7) mol/L) caused a contraction similar in magnitude and time course to ox-LDL-induced contraction and enhanced 5-HT- and KCl-induced contraction with no additional increases in [Ca(2+)](i). The ox-LDL-induced enhancement of 5-HT and KCl contraction was inhibited by Gö6976 (10(-6) mol/L), an inhibitor of Ca(2+)-dependent PKC isoforms. Both ox-LDL and PMA caused an increase in PKC activity in the particulate fraction, a decrease in the cytosolic fraction, and an increase in the particulate/cytosolic PKC activity ratio. Western blots revealed the Ca(2+)-dependent PKC-alpha and the Ca(2+)-independent PKC-delta, -epsilon, and -zeta isoforms. In unstimulated tissues, PKC-alpha- and -epsilon were mainly cytosolic, PKC-delta was mainly in the particulate fraction, and PKC-zeta was equally distributed in the cytosolic and particulate fractions. Ox-LDL alone or PMA alone caused translocation of PKC-epsilon from the cytosolic to particulate fraction, whereas the distribution pattern of PKC-alpha, -delta, and -zeta remained unchanged. 5-HT (10(-7) mol/L) alone and KCl alone did not change PKC activity. In tissues pretreated with ox-LDL or PMA, 5-HT and KCl caused additional increases in PKC-alpha activity. Native LDL did not significantly affect coronary contraction, [Ca(2+)](i), or PKC activity. These results suggest that ox-LDL causes coronary contraction via activation of the Ca(2+)-independent PKC-epsilon and enhances the contraction to [Ca(2+)](i)-increasing agonists by activating the Ca(2+)-dependent PKC-alpha. Activation of PKC-alpha and -epsilon may represent a possible cellular mechanism by which ox-LDL could enhance coronary vasospasm.

Characteristics of the free cytosolic calcium timelag following IgE-mediated stimulation of human basophils: significance for the nonreleasing basophil phenotype

These studies examine characteristics of the quiescent period (timelag) of the free cytosolic calcium ([Ca++]i) elevation that follows stimulation of human basophils through the IgE receptor. Previous studies established that the [Ca++]i timelag was sensitive to the rate of ligand binding, but little else is known about this response characteristic. The [Ca++]i timelag could be lengthened using antigenic stimulation that is rapid but only weakly induces secretion: tenfold differences in the "strength" of the stimulus, as assessed by histamine release, are associated with threefold differences in the timelag. Inhibiting p53/56lyn kinase with low concentrations of the specific inhibitor, PP1, lengthened the [Ca++]i timelag dramatically. PP1 was also found to delay the onset of syk phosphorylation and histamine release. Staurosporine and genistein, which are known to inhibit early tyrosine kinases, had, at best, only modest effects on the [Ca++]i timelag. Specific inhibitors of protein kinase C (PKC) had no effect on the [Ca++]i timelag, and direct activation of PKC with PMA had only very modest effects on the timelag. Contrary to expectations, basophils with the so-called nonreleasing phenotype demonstrated an IgE-mediated [Ca++]i response at the single-cell level. However, the length of [Ca++]i timelag in nonreleasing basophils was threefold longer than normally found in releasing basophils. Furthermore, the [Ca++]i response was significantly more asynchronous than in releasing basophils and lacking in a sustained [Ca++]i elevation. These studies indicate that the [Ca++]i timelag following stimulation through the IgE receptor is sensitive to inhibition of lyn kinase but not other agents that have been demonstrated to inhibit early tyrosine kinases previously. However, only one characteristic of the [Ca++]i response phenotype of nonreleasing basophils--the [Ca++]i timelag but not the absence of a sustained [Ca++]i elevation--could be mimicked by inhibition of lyn kinase with PP1.

Protein kinase C and meiotic regulation in isolated mouse oocytes.

In this study, the possible role of protein kinase C (PKC) in mediating both positive and negative actions on meiotic maturation in isolated mouse oocytes has been examined. When cumulus cell-enclosed oocytes (CEO) were cultured for 17-18 hr in a medium containing 4 mM hypoxanthine (HX) to maintain meiotic arrest, each of the five different activators and five different antagonists of PKC stimulated germinal vesicle breakdown (GVB) in a dose-dependent fashion. One of the activators, phorbol-12-myristate 13-acetate (PMA), also triggered GVB in CEO arrested with isobutylmethylxanthine or guanosine, but not in those arrested with dibutyryl cyclic AMP. When denuded oocytes (DO) were cultured for 3hr in inhibitor-free medium, all PKC activators suppressed maturation (<10% GVB compared to 94% in controls), while the effect of PKC antagonists was negligible. Four of the five antagonists reversed the meiosis-arresting action of HX in DO. PMA transiently arrested the spontaneous maturation of both CEO and DO, with greater potency in DO. The stimulatory action of PMA in HX-arrested oocytes was dependent on cumulus cells, because meiotic induction occurred in CEO but not DO. PKC activators also preferentially stimulated cumulus expansion when compared to antagonists. A cell-cell coupling assay determined that the action of PMA on oocyte maturation was not due to a loss of metabolic coupling between the oocyte and cumulus oophorus. Finally, Western analysis demonstrated the presence of PKCs alpha, beta1, delta, and eta in both cumulus cells and oocytes, but only PKC epsilon was detected in the cumulus cells. It is concluded that direct activation of PKC in the oocyte suppresses maturation, while stimulation within cumulus cells generates a positive trigger that leads to meiotic resumption.

The adhesion signaling molecule p190 RhoGAP is required for morphogenetic processes in neural development.

Rho GTPases direct actin rearrangements in response to a variety of extracellular signals. P190 RhoGAP (GTPase activating protein) is a potent Rho regulator that mediates integrin-dependent adhesion signaling in cultured cells. We have determined that p190 RhoGAP is specifically expressed at high levels throughout the developing nervous system. Mice lacking functional p190 RhoGAP exhibit several defects in neural development that are reminiscent of those described in mice lacking certain mediators of neural cell adhesion. The defects reflect aberrant tissue morphogenesis and include abnormalities in forebrain hemisphere fusion, ventricle shape, optic cup formation, neural tube closure, and layering of the cerebral cortex. In cells of the neural tube floor plate of p190 RhoGAP mutant mice, polymerized actin accumulates excessively, suggesting a role for p190 RhoGAP in the regulation of +Rho-mediated actin assembly within the neuroepithelium. Significantly, several of the observed tissue fusion defects seen in the mutant mice are also found in mice lacking MARCKS, the major substrate of protein kinase C (PKC), and we have found that p190 RhoGAP is also a PKC substrate in vivo. Upon either direct activation of PKC or in response to integrin engagement, p190 RhoGAP is rapidly translocated to regions of membrane ruffling, where it colocalizes with polymerized actin. Together, these results suggest that upon activation of neural adhesion molecules, the action of PKC and p190 RhoGAP leads to a modulation of Rho GTPase activity to direct several actin-dependent morphogenetic processes required for normal neural development.

Protein kinase C activates non-capacitative calcium entry in human platelets.

1. In many non-excitable cells Ca2+ influx is mainly controlled by the filling state of the intracellular Ca2+ stores. It has been suggested that this store-mediated or capacitative Ca2+ entry is brought about by a physical and reversible coupling of the endoplasmic reticulum with the plasma membrane. Here we provide evidence for an additional, non-capacitative Ca2+ entry mechanism in human platelets. 2. Changes in cytosolic Ca2+ and Sr2+ were measured in human platelets loaded with the fluorescent indicator fura-2. 3. Depletion of the internal Ca2+ stores with thapsigargin plus a low concentration of ionomycin stimulated store-mediated cation entry, as demonstrated upon Ca2+ or Sr2+ addition. Subsequent treatment with thrombin stimulated further divalent cation entry in a concentration-dependent manner. 4. Direct activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol also stimulated divalent cation entry, without evoking the release of Ca2+ from intracellular stores. Cation entry evoked by thrombin or activators of PKC was abolished by the PKC inhibitor Ro-31-8220. 5. Unlike store-mediated Ca2+ entry, jasplakinolide, which reorganises actin filaments into a tight cortical layer adjacent to the plasma membrane, did not inhibit divalent cation influx evoked by thrombin when applied after Ca2+ store depletion, or by activators of PKC. Thrombin also activated Ca2+ entry in platelets in which the release from intracellular stores and store-mediated Ca2+ entry were blocked by xestospongin C. 6. These results indicate that the non-capacitative divalent cation entry pathway is regulated independently of store-mediated entry and does not require coupling of the endoplasmic reticulum and the plasma membrane. These results support the existence of a mechanism for receptor-evoked Ca2+ entry in human platelets that is independent of Ca2+ store depletion. This Ca2+ entry mechanism may be activated by occupation of G-protein-coupled receptors, which activate PKC, or by direct activation of PKC, thus generating non-capacitative Ca2+ entry alongside that evoked following the release of Ca2+ from the intracellular stores.

Activation of Protein Kinase C Stimulates the Dephosphorylation of Natriuretic Peptide Receptor-B at a Single Serine Residue: A POSSIBLE MECHANISM OF HETEROLOGOUS DESENSITIZATION

The binding of atrial natriuretic peptide and C-type natriuretic peptide (CNP) to the guanylyl cyclase-linked natriuretic peptide receptors A and B (NPR-A and -B), respectively, stimulates increases in intracellular cGMP concentrations. The vasoactive peptides vasopressin, angiotensin II, and endothelin inhibit natriuretic peptide-dependent cGMP elevations by activating protein kinase C (PKC). Recently, we identified six in vivo phosphorylation sites for NPR-A and five sites for NPR-B and demonstrated that the phosphorylation of these sites is required for ligand-dependent receptor activation. Here, we show that phorbol 12-myristate 13-acetate, a direct activator of PKC, causes the dephosphorylation and desensitization of NPR-B. In contrast to the CNP-dependent desensitization process, which results in coordinate dephosphorylation of all five sites in the receptor, phorbol 12-myristate 13-acetate treatment causes the dephosphorylation of only one site, which we have identified as Ser(523). The conversion of this residue to alanine or glutamate did not reduce the amount of mature receptor protein as indicated by detergent-dependent guanylyl cyclase activities or Western blot analysis but completely blocked the ability of PKC to induce the dephosphorylation and desensitization of NPR-B. Thus, in contrast to previous reports suggesting that PKC directly phosphorylates and inhibits guanylyl cyclase-linked natriuretic peptide receptors, we show that PKC-dependent dephosphorylation of NPR-B at Ser(523) provides a possible molecular explanation for how pressor hormones inhibit CNP signaling.

Oxidized-Ldl Enhances Coronary Vasoconstriction by Increasing the Activity of α and ε Protein Kinase C Isoforms

31 Oxidized low density lipoprotein (ox-LDL) plays a critical role in the development of atherosclerotic coronary vasospasm; however, the cellular mechanisms involved are not fully understood. We tested the hypothesis that ox-LDL enhances coronary vasoconstriction by increasing the activity of specific protein kinase C (PKC) isoforms in coronary smooth muscle. Active stress was measured in de-endothelialized porcine coronary artery strips, [Ca 2+ ] i was monitored in single coronary smooth muscle cells loaded with fura-2, and the whole tissue, cytosolic and particulate fractions were examined for PKC activity and reactivity with isoform-specific anti-PKC antibodies using Western blot analysis. Ox-LDL (100 μg/ml) caused a slow but significant increase in active stress to 1.2±0.2 x10 3 N/m 2 , that was completely inhibited by the PKC inhibitors staurosporine and calphostin C, with no significant increase in [Ca 2+ ] i . Ox-LDL enhanced coronary contraction to increasing concentrations of 5-hydroxytryptamine (5-HT) and extracellular KCl with no additional increases in [Ca 2+ ] i . Direct activation of PKC by phorbol 12,13-dibutyrate (PDBu, 0.1 μM) caused a contraction similar in magnitude and time course to ox-LDL induced contraction. PDBu also enhanced 5-HT and KCl-induced contraction with no additional increases in [Ca 2+ ] i . Both ox-LDL and PDBu caused an increase in PKC activity in the particulate fraction and a decrease in the cytosolic fraction and increased the particulate/cytosolic PKC activity ratio. Western blot analysis revealed α-, δ-, ε- and ζ-PKC isoforms. In unstimulated tissue, α- and ε-PKC were mainly cytosolic, δ-PKC was mainly in the particulate fraction, while ζ-PKC was equally distributed in the cytosolic and particulate fractions. Ox-LDL and PDBu caused translocation of α- and ε-PKC from the cytosolic to particulate fraction, with minimal effect on the distribution of δ-PKC and ζ-PKC. Native LDL did not significantly affect coronary contraction, [Ca 2+ ] i or PKC activity. These results suggest that ox-LDL enhances coronary vasoconstriction via activation and translocation of α- and ε-PKC isoforms in coronary smooth muscle.

Articular chondrocytes from aging rats respond poorly to insulin-like growth factor-1: an altered signaling pathway

This study investigates the effect of insulin-like growth factor-1 (IGF-1) and phorbol 12-myrystate 13-acetate (PMA) on 3H-thymidine, 35SO 4 and 3H -glycine incorporations, adenosine 3′:5′-cyclic monophosphate (cAMP) production and protein kinase C (PKC) activation in cultured rat articular chondrocyte monolayers (RACM) derived from animals of different ages. It was found that IGF-1 stimulates all these cellular functions in cultures derived from all age groups in a concentration dependent manner, although the cells from 14-month old animals responded poorly. IGF-1 also induces in cells from 1-month old rats an increase in the expression of mRNAs specific for aggrecan and type II collagen molecules as shown with RT-PCR. These effects are mediated via IGF-1 interaction with specific receptors because the monoclonal antibody against the receptor protein suppresses more than 60% of the ligand-induced DNA synthesis. PMA, a direct PKC activator, potentiated IGF-1-induced effects in all cells but much more strongly in cells from young than in cells from 14-month old animals. The age-related failure of RACM to respond adequately to IGF-1 was correlated with a decrease in IGF-1-induced cAMP production, and IGF-1-induced and PMA-induced PKC activations. These results show that IGF-1 regulates the synthesis of DNA, proteoglycans (PG) and collagen II at the level of transcription and suggest that the reduced response of cell monolayers derived from 14-month old rats to IGF-1 is probably due to a failure of old cells to adequately transduce IGF-1 receptor-generated downstream signaling.

Endothelin-1 and photoreleased diacylglycerol increase L-type Ca2+ current by activation of protein kinase C in rat ventricular myocytes.

The amphotericin B-perforated whole-cell patch clamp technique was used to determine the modulation of L-type Ca2+ channels by protein kinase C (PKC)-mediated pathways in adult rat ventricular myocytes. Application of 10 nM endothelin-1 (ET-1) increased peak Ca2+ current (ICa) by 28.2 +/- 2.5 % (n = 13) and slowed current decay. These effects were prevented by the endothelin receptor antagonist PD145065 (10 microM) and by the PKC inhibitor chelerythrine (8 microM). To establish if direct activation of PKC mimicked the ET-1 effect, the active and inactive phorbol esters (phorbol-12-myristate-13-acetate and 4alpha-phorbol-12, 13-didecanoate) were tested. Both phorbol esters (100 nM) resulted in a small (approximately 10%) increase in ICa, suggesting PKC-independent effects. Bath application of dioctanoylglycerol (diC8), a diacylglycerol (DAG) analogue which is capable of directly activating PKC, caused a gradual decline in peak ICa (50.4 +/- 6.2 %, n = 5) and increased the rate of current decay. These effects were unaffected by the PKC inhibitor chelerythrine (8 microM). Intracellular photorelease of caged diC8 with 3 or 10 s exposure to UV light produced a concentration-dependent increase in peak ICa (20. 7 +/- 8.5 % (n = 8) for 3 s UV and 60.8 +/- 11.4 % (n = 13) for 10 s UV), which could be inhibited by chelerythrine. Our results demonstrate that both ET-1 and intracellularly photoreleased diC8 increase ICa by a PKC-mediated pathway, which is in direct contrast to the PKC-independent inhibition of ICa produced by bath-applied diC8. We conclude that specific cellular pools of DAG are crucially important in the regulation of ICa by PKC.

Involvement of proline-rich tyrosine kinase 2 in platelet activation: tyrosine phosphorylation mostly dependent on alphaIIbbeta3 integrin and protein kinase C, translocation to the cytoskeleton and association with Shc through Grb2.

Proline-rich tyrosine kinase 2 (Pyk2) (also known as RAFTK, CAKbeta or CADTK) has been identified as a member of the focal adhesion kinase (FAK) family of protein-tyrosine kinases and it has been suggested that the mode of Pyk2 activation is distinct from that of FAK. In the present study we investigated the mode of Pyk2 activation in human platelets. When platelets were stimulated with thrombin, Pyk2, as well as FAK, was markedly tyrosine-phosphorylated, in a manner mostly dependent on alphaIIbbeta3 integrin-mediated aggregation. The residual Pyk2 tyrosine phosphorylation observed in the absence of platelet aggregation was completely abolished by pretreatment with BAPTA/AM [bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester]. The Pyk2 phosphorylation was inhibited by protein kinase C (PKC) inhibitors at concentrations that inhibited platelet aggregation. In contrast, direct activation of PKC with the active phorbol ester PMA induced the tyrosine phosphorylation of Pyk2 and FAK but only when platelets were fully aggregated with the exogenous addition of fibrinogen (the ligand for alphaIIbbeta3 integrin). Furthermore, PMA-induced Pyk2 (and FAK) tyrosine phosphorylation was also observed when platelets adhered to immobilized fibrinogen. The activation of the von Willebrand factor (vWF)--glycoprotein Ib pathway with botrocetin together with vWF failed to induce Pyk2 (and FAK) tyrosine phosphorylation. Most Pyk2 and FAK was present in the cytosol and membrane skeleton fractions in unstimulated platelets. When platelets were stimulated with thrombin, both Pyk2 and FAK were translocated to the cytoskeleton in an aggregation-dependent manner. In immunoprecipitation studies, Pyk2, as well as FAK, seemed to associate with Shc through Grb2. With the use of glutathione S-transferase fusion proteins containing Shc-SH2, Grb2-SH2, and Grb2 N-terminal and C-terminal SH3 domains, it was implied that the proline-rich region of Pyk2 (and FAK) binds to the N-terminal SH3 domain of Grb2 and that the phosphotyrosine residue of Shc binds to the SH2 domain of Grb2. Although Pyk2 and FAK have been reported to be differentially regulated in many cell types, our results suggest that, in human platelets, the mode of Pyk2 activation is mostly similar to that of FAK, in terms of alphaIIbbeta3 integrin-dependent and PKC-dependent tyrosine phosphorylation. Furthermore, Pyk2, as well as FAK, might have one or more important roles in post-aggregation tyrosine phosphorylation events, in association with the cytoskeleton and through interaction with adapter proteins including Grb2 and Shc.

Involvement of proline-rich tyrosine kinase 2 in platelet activation: tyrosine phosphorylation mostly dependent on αIIbβ3 integrin and protein kinase C, translocation to the cytoskeleton and association with Shc through Grb2

Proline-rich tyrosine kinase 2 (Pyk2) (also known as RAFTK, CAKβ or CADTK) has been identified as a member of the focal adhesion kinase (FAK) family of protein-tyrosine kinases and it has been suggested that the mode of Pyk2 activation is distinct from that of FAK. In the present study we investigated the mode of Pyk2 activation in human platelets. When platelets were stimulated with thrombin, Pyk2, as well as FAK, was markedly tyrosine-phosphorylated, in a manner mostly dependent on αIIbβ3 integrin-mediated aggregation. The residual Pyk2 tyrosine phosphorylation observed in the absence of platelet aggregation was completely abolished by pretreatment with BAPTA/AM [bis-(o-aminophenoxy)ethane-N,N,Nʹ,Nʹ-tetra-acetic acid acetoxymethyl ester]. The Pyk2 phosphorylation was inhibited by protein kinase C (PKC) inhibitors at concentrations that inhibited platelet aggregation. In contrast, direct activation of PKC with the active phorbol ester PMA induced the tyrosine phosphorylation of Pyk2 and FAK but only when platelets were fully aggregated with the exogenous addition of fibrinogen (the ligand for αIIbβ3 integrin). Furthermore, PMA-induced Pyk2 (and FAK) tyrosine phosphorylation was also observed when platelets adhered to immobilized fibrinogen. The activation of the von Willebrand factor (vWF)--glycoprotein Ib pathway with botrocetin together with vWF failed to induce Pyk2 (and FAK) tyrosine phosphorylation. Most Pyk2 and FAK was present in the cytosol and membrane skeleton fractions in unstimulated platelets. When platelets were stimulated with thrombin, both Pyk2 and FAK were translocated to the cytoskeleton in an aggregation-dependent manner. In immunoprecipitation studies, Pyk2, as well as FAK, seemed to associate with Shc through Grb2. With the use of glutathione S-transferase fusion proteins containing Shc-SH2, Grb2-SH2, and Grb2 N-terminal and C-terminal SH3 domains, it was implied that the proline-rich region of Pyk2 (and FAK) binds to the N-terminal SH3 domain of Grb2 and that the phosphotyrosine residue of Shc binds to the SH2 domain of Grb2. Although Pyk2 and FAK have been reported to be differentially regulated in many cell types, our results suggest that, in human platelets, the mode of Pyk2 activation is mostly similar to that of FAK, in terms of αIIbβ3 integrin-dependent and PKC-dependent tyrosine phosphorylation. Furthermore, Pyk2, as well as FAK, might have one or more important roles in post-aggregation tyrosine phosphorylation events, in association with the cytoskeleton and through interaction with adapter proteins including Grb2 and Shc.

Induction of relaxin messenger RNA expression in response to prolactin receptor activation requires protein kinase C delta signaling.

The ability of PRL or rat placental lactogen (rPL)-1 to induce relaxin mRNA expression was analyzed in a luteinized rat granulosa cell culture model. PRL receptor activation induced relaxin mRNA expression in a concentration- and time-dependent manner. High concentrations of PRL receptor agonist, equivalent to those of the second half of pregnancy in rats, were required to elicit relaxin mRNA expression. A 40-fold induction of relaxin mRNA was observed in cells treated 24 h with 1 microg/ml of rPL-1. Estrogen enhanced relaxin expression induced by PRL but did not affect relaxin expression on its own. PRL/rPL-1 induction of relaxin expression was independent of the extracellular regulated kinase (ERK) members of the mitogen-activated protein kinase (MAPK) pathway, based on the inability of the ERK kinase inhibitor PD98059 to block induction of relaxin expression. PRL/rPL-1 induction of relaxin expression required protein kinase C (PKC) delta, based on the ability of the preferential PKC delta inhibitor rottlerin to abolish induction of relaxin expression. Direct activation of PKC by phorbol myristate acetate, however, was not sufficient to promote induction of relaxin mRNA expression. Stats (signal transducers and activators of transcription) 3 and 5 DNA binding activities were induced by PRL/rPL-1 treatment of luteinized granulosa cells but only Stat 3 DNA binding was reduced by rottlerin. PRL/rPL-1 treatment of luteinized granulosa cells resulted in increased phosphorylation on tyrosine-705 and serine-727 of Stat 3, and these responses were reduced and blocked, respectively, by rottlerin. Tyrosine and serine phosphorylations of Stat 3 in the corpus luteum were also increased in the second half of pregnancy when PL levels are highest. Stat 3, but not Stat 1 or 5, coimmunoprecipitated with luteal PKC delta during pregnancy; Stat 3 transiently coimmunoprecipitated with PKC delta from luteinized granulosa cells in response to PRL receptor activation; and Stat 3/PKC delta complex formation required PKC delta kinase activity. Taken together, these results show that PKC delta is obligatory for PRL/rPL-1-dependent relaxin expression, that PKC delta complexes with Stat 3 in response to PRL receptor activation, and that PKC delta is involved in the regulation of Stat 3 phosphorylation downstream of the PRL receptor. These results demonstrate that PRL/rPL-1 promotes relaxin expression in luteal cells and that this event is mediated, at least in part, via PKC delta.

Protein kinase C enhances the rapidly activating delayed rectifier potassium current, IKr, through a reduction in C-type inactivation in guinea-pig ventricular myocytes.

1. The rapidly activating delayed rectifier potassium current, IKr, was studied in guinea-pig ventricular myocytes in the presence of thiopentone, which blocks the more slowly activating component of the delayed rectifier potassium current, IKs, and using whole cell perforated patch clamp or switched voltage clamp with sharp electrodes to minimise intracellular dialysis. 2. Activation of protein kinase A (PKA) by isoprenaline or forskolin caused an increase in IKr tail currents. Following a 300 ms depolarising step to +20 mV, mean tail current amplitude was increased 47 +/- 12% by isoprenaline, and 73 +/- 13% by forskolin. No increase in IKr was observed when IKr was studied using whole cell ruptured patch clamp and there was no change in the reversal potential of IKr in the presence of isoprenaline. 3. The rectification of the current sensitive to E4031, a selective IKr blocker, was markedly reduced in the presence of isoprenaline and the region of negative slope was absent. This is consistent with a reduction in the inactivation of IKr and was supported by the finding that IKr, in the presence of isoprenaline, was somewhat less sensitive to block. E4031 (5 microM) blocked only 81 +/- 5% of IKr in the presence of isoprenaline compared to 100 +/- 0% in control. 4. The forskolin- and isoprenaline-induced increases in IKr were inhibited by staurosporine and by the selective protein kinase C (PKC) inhibitor bisindolymaleimide I. Direct activation of PKC by phorbol dibutyrate increased IKr tail currents by 24 +/- 5%. Both the isoprenaline- and forskolin-induced increases in IKr were inhibited when calcium entry was reduced by block of ICa with nifedipine or when myocytes were pre-incubated in BAPTA-AM. 5. The selective PKA inhibitor KT5720 prevented the isoprenaline-induced increase in IKr only when the increase in ICa was also suppressed. 6. These data show a novel mechanism of regulation of IKr by PKC and this kinase was activated by beta-adrenoceptor stimulation. IKr seems to be enhanced through a reduction in the C-type inactivation which underlies the rectification of the channel and such a mechanism may occur in other channels with this type of inactivation.

CCK stimulates mob-1 expression and NF-kappaB activation via protein kinase C and intracellular Ca(2+).

Supraphysiological concentrations of cholecystokinin (CCK) induce chemokine expression in rat pancreatic acini through the activation of the transcription factor NF-kappaB. In the current study, the intracellular signals involved in these pathophysiological effects of CCK were investigated. CCK induction of mob-1 expression in isolated rat pancreatic acini was blocked by the protein kinase C (PKC) inhibitors GF-109203X and Ro-32-0432 and by the intracellular Ca(2+) chelator BAPTA. CCK induced NF-kappaB nuclear translocation, and DNA binding was also blocked by GF-109203X and BAPTA. Direct activation of PKC with TPA induced mob-1 chemokine expression and activated NF-kappaB DNA binding to a similar extent as did CCK. Increasing intracellular Ca(2+) using ionomycin had no effect on mob-1 mRNA levels or NF-kappaB activity. Both CCK and TPA treatments decreased inhibitory kappaB-alpha (IkappaB-alpha) levels, whereas ionomycin had no effect. However, the effects of TPA on IkappaB-alpha degradation were less complete than for CCK. In combination, TPA and ionomycin degraded IkappaB-alpha to a similar extent as CCK. Therefore, activation of NF-kappaB and mob-1 expression by supraphysiological CCK is likely mediated by both PKC activation and elevated intracellular Ca(2+).

Short-chain fatty acids reduce expression of specific protein kinase C isoforms in human colonic epithelial cells.

LIM1215 colon cancer cells were used as a model of human colonic epithelium to examine the effects of butyrate on protein kinase C (PKC) activity and isoform expression. On Western blot analysis, LIM1215 cells express the PKC isoforms alpha, beta, epsilon, zeta, and lambda, but not gamma, straight theta, or micro. Treatment with 2 mM butyrate for 48 h reduced cellular PKC activity up to 50% and specifically reduced the expression of PKC alpha and PKC epsilon. Similar results were obtained using Caco-2 colon cancer cells. These effects were neither a consequence of the induction of differentiation itself nor the result of direct or indirect activation of PKC. Although dependent on gene transcription and protein synthesis, the effect was not due to a reduction in the synthesis of PKC protein. Butyrate's effect was independent of its beta-oxidation but was mimicked, at least in part, by trichostatin A, an inhibitor of histone deacetylase.

Congenital disorders of platelet signal transduction.

After injury to the blood vessel, platelets adhere to the exposed subendothelium by a process (adhesion) that involves the interaction of a plasma protein, von Willebrand factor (vWF), and a specific protein on the platelet surface, glycoprotein Ib (GPIb; the Figure⇓). Adhesion is followed by recruitment of additional platelets that form clumps, a process called aggregation (cohesion). This involves binding of fibrinogen to specific platelet surface receptors—a complex comprising glycoproteins IIb-IIIa (GPIIb-IIIa). Activated platelets release the contents of their granules (secretion or release reaction), such as ADP and serotonin from dense granules, which subsequently cause recruitment of additional platelets. In addition, platelets play a major role in coagulation mechanisms; several key enzymatic reactions occur on the platelet membrane–lipoprotein surface. A number of physiological agonists interact with specific receptors on the platelet surface to induce responses, including a change in platelet shape from discoid to spherical, aggregation, secretion, and thromboxane A2 (TxA2) production. Other agonists such as prostacyclin inhibit these responses. Ligation of the platelet receptors initiates the production or release of several intracellular messenger molecules, including Ca2+ ions, products of phosphoinositide (PI) hydrolysis by phospholipase C (PLC; diacylglycerol [DG] and inositol 1,4,5-triphosphate [InsP3]), TxA2, and cyclic nucleotides (cAMP; the Figure⇓). These subsequently induce or modulate the various platelet responses of Ca2+ mobilization, protein phosphorylation, aggregation, secretion, and liberation of arachidonic acid. The interaction between the agonist receptors and the key intracellular effector enzymes (eg, PLA2, PLC, adenylyl cyclase) is mediated by a group of GTP-binding proteins that are modulated by GTP. As in most secretory cells, platelet activation results in …

Dynamics of munc18-1 phosphorylation/dephosphorylation in rat brain nerve terminals.

Munc18-1 is a mammalian member of the SEC1 protein family implicated in neuronal secretion. Its sequence contains several consensus sites for phosphorylation by protein kinase C (PKC), a kinase known to enhance secretion. We have characterized the phosphorylation of the synaptic munc18-1 pool by endogenous, presynaptic PKC-isoforms. In isolated rat brain nerve terminals, munc18-1 was almost completely nonphosphorylated. Its phosphorylation state increased by 250% on inhibition of endogenous phosphatases and by 1500% on additional, direct PKC activation using phorbol esters. K+-evoked depolarization also increased munc18-1 phosphorylation, by 50% within 5 s in a Ca2+-dependent manner. Munc18-1 phosphorylation in nerve terminals was blocked by PKC inhibitors. Activation of endogenous PKC in nerve terminals inhibited the interaction of synaptic munc18-1 with its binding partner syntaxin-1A by 50%. Munc18-1 antisera precipitated 80% of native, brain-derived munc18-1 from salt solutions, but only 12% from synaptosomal lysates, together with 6% synaptic syntaxin-1A/B; these amounts were not changed by PKC activation. In this 12%, the phosphate incorporation per mole of munc18 was four-fold lower than the total pool. We conclude that the synaptic munc18-1 pool can be readily and rapidly phosphorylated by endogenous presynaptic PKC isoforms. A high constitutive phosphatase activity keeps its basal phosphorylation state low so that PKC activation can increase the phosphorylation state dramatically. These phosphorylation dynamics and the effects on the interaction with syntaxin-1A make munc18-1 a prominent candidate to account for PKC-dependent enhancement of secretion.

Role of protein kinase C in angiotensin II-induced constriction of renal microvessels

Role of protein kinase C in angiotensin II-induced constriction of renal microvessels. Although angiotensin II (Ang II) exerts its action through multiple vasomotor mechanisms, the contribution of phosphoinositol hydrolysis products to Ang II-induced renal vasoconstriction remains undetermined. The role of protein kinase C (PKC) in Ang II-induced afferent (AFF) and efferent (EFF) arteriolar constriction was examined using the isolated perfused hydronephrotic rat kidney. Ang II (0.3 nmol/L)-induced EFF constriction was refractory to inhibition of voltage-dependent calcium channels by pranidipine (1 micromol/L, 19 +/- 2% reversal) but was completely reversed by a PKC inhibitor, chelerythrine (1 micromol/L, 96 +/- 2% reversal). Furthermore, direct PKC activation by phorbol myristate acetate (PMA; 1 micromol/L) caused prominent EFF constriction, and this constriction was inhibited by manganese and free calcium medium. In contrast, Ang II-induced AFF constriction was completely abolished by pranidipine (98 +/- 4% reversal) and was partially inhibited by chelerythrine (55 +/- 3% reversal). Although PMA elicited marked AFF constriction, this constriction was insensitive to the calcium antagonist, but was totally inhibited by manganese or free calcium medium. PKC plays an obligatory role in Ang II-induced EFF constriction that requires extracellular calcium entry through nonselective cation channels. In contrast, in concert with our recent findings demonstrating a complete dilation by thapsigargin, Ang II-induced AFF constriction is mainly mediated by inositol trisphosphate (IP3) and voltage-dependent calcium channel pathways, but could not be attributed to the PKC-activated calcium entry pathway (for example, nonselective cation channels). Rather, Ang II-stimulated PKC may cross-talk to the IP3/voltage-dependent calcium channel pathway and could modulate the vasoconstrictor mechanism of the AFF. Thus, the role of PKC during Ang II stimulation differs in AFF and EFF, which may constitute segmental heterogeneity in the renal microvasculature.

Differential Changes in the Phosphorylation of the Protein Kinase C Substrates Myristoylated Alanine‐Rich C Kinase Substrate and Growth‐Associated Protein‐43/B‐50 Following Schaffer Collateral Long‐Term Potentiation and Long‐Term Depression

Activation of protein kinase C (PKC) is one of the biochemical pathways thought to be activated during activity-dependent synaptic plasticity in the brain, and long-term potentiation (LTP) and long-term depression (LTD) are two of the most extensively studied models of synaptic plasticity. Here we have examined changes in the in situ phosphorylation level of two major PKC substrates, myristoylated alanine-rich C kinase substrate (MARCKS) and growth-associated protein (GAP)-43/B-50, after pharmacological stimulation or induction of LTP or LTD in the CA1 field of the hippocampus. We find that direct PKC activation with phorbol esters, K+-induced depolarization, and activation of metabotropic glutamate receptors increase the in situ phosphorylation of both MARCKS and GAP-43/B-50. The induction of LTP increased the in situ phosphorylation of both MARCKS and GAP-43/B-50 at 10 min following high-frequency stimulation, but only GAP-43/B-50 phosphorylation remained elevated 60 min after LTP induction. Furthermore, blockade of LTP induction with the NMDA receptor antagonist D-2-amino-5-phosphonopentanoic acid prevented elevations in GAP-43/B-50 phosphorylation but did not prevent the elevation in MARCKS phosphorylation 10 min following LTP induction. The induction of LTD resulted in a reduction in GAP-43/B-50 phosphorylation but did not affect MARCKS phosphorylation. Together these findings show that activity-dependent synaptic plasticity elicits PKC-mediated phosphorylation of substrate proteins in a highly selective and coordinated manner and demonstrate the compartmentalization of PKC-substrate interactions. Key Words: Protein kinase C-Myristoylated alanine-rich C kinase substrate-Growth-associated protein-43-Long-term potentiation-Long-term depression-(RS)-alpha-Methyl-4-carboxyphenylglycine-D-2-Amino-5-ph osphonopentanoic acid-Glutamate.

Multiple Protein Kinase Pathways Are Involved in Gastrin-releasing Peptide Receptor-regulated Secretion

Gastrin-releasing peptide (GRP) and its amphibian homolog, bombesin, are potent secretogogues in mammals. We determined the roles of intracellular free Ca(2+) ([Ca(2+)](i)), protein kinase C (PKC), and mitogen-activated protein kinases (MAPK) in GRP receptor (GRP-R)-regulated secretion. Bombesin induced either [Ca(2+)](i) oscillations or a biphasic elevation in [Ca(2+)](i). The biphasic response was associated with peptide secretion. Receptor-activated secretion was blocked by removal of extracellular Ca(2+), by chelation of [Ca(2+)](i), and by treatment with inhibitors of phospholipase C, conventional PKC isozymes, and MAPK kinase (MEK). Agonist-induced increases in [Ca(2+)](i) were also inhibited by dominant negative MEK-1 and the MEK inhibitor, PD89059, but not by an inhibitor of PKC. Direct activation of PKC by a phorbol ester activated MAPK and stimulated peptide secretion without a concomitant increase in [Ca(2+)](i). Inhibition of MEK blocked both bombesin- and phorbol 12-myristate 13-acetate-induced secretion. GRP-R-regulated secretion is initiated by an increase in [Ca(2+)](i); however, elevated [Ca(2+)](i) is insufficient to stimulate secretion in the absence of activation of PKC and the downstream MEK/MAPK pathways. We demonstrated that the activity of MEK is important for maintaining elevated [Ca(2+)](i) levels induced by GRP-R activation, suggesting that MEK may affect receptor-regulated secretion by modulating the activity of Ca(2+)-sensitive PKC.