Phorbol ester induces desensitization of adenylate cyclase and phosphorylation of the beta-adrenergic receptor in turkey erythrocytes

Évolution des idées sur le mécanisme d'action des antidépresseurs : le concept d'hétéro-régulation des récepteurs

Des lès premières années d'utilisation clinique des antidépresseurs, les neurones à sérotonine (5-HT) et à noradrénaline (NA) ont été considérés comme les sites d'action privilégiés de ces médicaments.

En vingt ans, les idées sur leurs mécanismes d'action ont évolué parallèlement à la progression des connaissances et à l'obtention de nouvelles molécules actives :

C'est la stimulation - directe ou indirecte - des récepteurs sérotoninergiques (5-HT2) qui serait nécessaire à l'obtention d'un découplage β lors d'une stimulation des récepteurs β-adrénergiques.

Les molécules antidepressives n'auraient d'action thérapeutique qu'à condition d'agir de façon synergique sur les transmissions sérotoninergique et noradrénergique.

Receptor sequestration in response to β-arrestin-2 phosphorylation by ERK1/2 governs steady-state levels of GPCR cell-surface expression

MAPKs are activated in response to G protein-coupled receptor (GPCR) stimulation and play essential roles in regulating cellular processes downstream of these receptors. However, very little is known about the reciprocal effect of MAPK activation on GPCRs. To investigate possible crosstalk between the MAPK and GPCRs, we assessed the effect of ERK1/2 on the activity of several GPCR family members. We found that ERK1/2 activation leads to a reduction in the steady-state cell-surface expression of many GPCRs because of their intracellular sequestration. This subcellular redistribution resulted in a global dampening of cell responsiveness, as illustrated by reduced ligand-mediated G-protein activation and second-messenger generation as well as blunted GPCR kinases and β-arrestin recruitment. This ERK1/2-mediated regulatory process was observed for GPCRs that can interact with β-arrestins, such as type-2 vasopressin, type-1 angiotensin, and CXC type-4 chemokine receptors, but not for the prostaglandin F receptor that cannot interact with β-arrestin, implicating this scaffolding protein in the receptor's subcellular redistribution. Complementation experiments in mouse embryonic fibroblasts lacking β-arrestins combined with in vitro kinase assays revealed that β-arrestin-2 phosphorylation on Ser14 and Thr276 is essential for the ERK1/2-promoted GPCR sequestration. This previously unidentified regulatory mechanism was observed after constitutive activation as well as after receptor tyrosine kinase- or GPCR-mediated activation of ERK1/2, suggesting that it is a central node in the tonic regulation of cell responsiveness to GPCR stimulation, acting both as an effector and a negative regulator.

Autonomic nervous system and immune system interactions

The present review assesses the current state of literature defining integrative autonomic-immune physiological processing, focusing on studies that have employed electrophysiological, pharmacological, molecular biological, and central nervous system experimental approaches. Central autonomic neural networks are informed of peripheral immune status via numerous communicating pathways, including neural and non-neural. Cytokines and other immune factors affect the level of activity and responsivity of discharges in sympathetic and parasympathetic nerves innervating diverse targets. Multiple levels of the neuraxis contribute to cytokine-induced changes in efferent parasympathetic and sympathetic nerve outflows, leading to modulation of peripheral immune responses. The functionality of local sympathoimmune interactions depends on the microenvironment created by diverse signaling mechanisms involving integration between sympathetic nervous system neurotransmitters and neuromodulators; specific adrenergic receptors; and the presence or absence of immune cells, cytokines, and bacteria. Functional mechanisms contributing to the cholinergic anti-inflammatory pathway likely involve novel cholinergic-adrenergic interactions at peripheral sites, including autonomic ganglion and lymphoid targets. Immune cells express adrenergic and nicotinic receptors. Neurotransmitters released by sympathetic and parasympathetic nerve endings bind to their respective receptors located on the surface of immune cells and initiate immune-modulatory responses. Both sympathetic and parasympathetic arms of the autonomic nervous system are instrumental in orchestrating neuroimmune processes, although additional studies are required to understand dynamic and complex adrenergic-cholinergic interactions. Further understanding of regulatory mechanisms linking the sympathetic nervous, parasympathetic nervous, and immune systems is critical for understanding relationships between chronic disease development and immune-associated changes in autonomic nervous system function.

Natriuretic peptides as regulatory mediators of secretory activity in the digestive system

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are members of the natriuretic peptide family best known for their role in blood pressure regulation. However, in recent years all the natriuretic peptides and their receptors have been described in the gastrointestinal tract, digestive glands and central nervous system, as well as implicated in the regulation of digestive gland functions. The current review highlights the regulatory role of ANP and CNP in pancreatic and other digestive secretions. ANP and CNP stimulate basal as well as induced pancreatic secretion and modify bicarbonate and chloride secretions. Whereas ANP and CNP exert effects directly on pancreatic cells, CNP also acts through a vago-vagal reflex. At high doses both peptides attenuate pancreatic secretion induced by high doses of secretin through the PLC/PKC pathway. With regards to other digestive secretions, ANP and CNP decrease bile secretion in the rat. ANP does not induce salivation by itself but enhances stimulated salivary secretion and modifies salivary composition in rat parotid as well as submandibular glands. In rat pancreatic, hepatic, parotid and submandibular tissues, the NPR-C receptor mediates mostly peripheral responses whereas NPR-A and NPR-B receptors, which are coupled to guanylate cyclase, likely mediate the central response. In addition, ANP modulates gastric acid secretion via a vagal-dependent mechanism. In the intestine, ANP and CNP decrease water and sodium chloride absorption through an increase in cGMP levels. Overall, these findings indicate that ANP and CNP are members of the large group of regulatory peptides affecting digestive secretions.

Involvement of protein kinase C and cyclic adenosine 3',5'-monophosphate-dependent kinase in steroidogenic acute regulatory protein expression and steroid biosynthesis in Leydig cells

This study investigated the roles of the protein kinase C (PKC) and protein kinase A (PKA) pathways in regulating constitutive steroidogenesis and steroidogenic acute regulatory (STAR; herein designated by its common name, StAR) protein in R2C Leydig tumor cells. Inhibition of PKC and phospholipase C resulted in significant decreases in steroid production, phosphorylation of cAMP-responsive element binding (CREB) protein, and Star gene transcription under basal conditions in R2C cells. These observations were corroborated in MA-10 and mLTC-1 Leydig tumor cell lines, in which activation of PKC by phorbol-12-myristate-13-acetate (PMA, 10 nM) increased CREB phosphorylation and total StAR (tot-StAR) protein expression. However, induction of StAR protein by PMA did not result in the expected concomitant increase in steroids because PKC failed to phosphorylate StAR, the biologically active form of the protein. However, in conjunction with PMA, minor increases in PKA activity using submaximal doses of (Bu)2cAMP (0.05-0.1 mM; a concentration range insufficient for induction of StAR), were able to stimulate dramatic increases in both phospho-StAR (P-StAR) and steroid production. Human chorionic gonadotropin stimulation also resulted in a further enhancement in P-StAR and progesterone production when added to PMA-treated MA-10 cells. Similar results for tot-StAR and P-StAR expression were observed in primary cultures of immature rat Leydig cells treated with PMA and submaximal doses of (Bu)2cAMP. In summary, the present study demonstrates that basal activities of both PKC and PKA play important roles in the constitutive steroidogenic characteristics of R2C cells. This study also demonstrates for the first time a role for PMA-induced PKC in StAR protein regulation and the requirement for submaximal doses of cAMP to produce steroids in Leydig cells.

Protein kinase C-mediated down-regulation of beta(2)-adrenergic receptor and gene expression in rat C6 glioma cells

We investigated the regulation of beta(2)-adrenergic receptors (beta(2)AR) by protein kinase C (PKC) in rat C6 glioma cells at the levels of receptor activity, protein expression and gene expression. Cells exposed to 4beta-phorbol-12-myristate-13-acetate (PMA), a potent activator of PKC, exhibited a time- and concentration-dependent decrease in beta(2)AR binding activity. Maximum down-regulation was approximately 50% by 24 h and western blot analysis revealed a parallel decrease in beta(2)AR protein. In addition, PMA treatment resulted in an acute desensitization of beta(2)AR-stimulated cyclic AMP response prior to any reduction in receptor levels. PMA exposure also affected steady-state beta(2)AR mRNA levels in a time-dependent, biphasic manner. During the first 4 h, levels decreased by approximately 60% and then slowly recovered to approximately 75% of control by 24 h. As the reduction in receptor mRNA was not due to a decrease in its stability, we examined beta(2)AR gene transcription by nuclear run-on assays. Transcriptional activity in nuclei from C6 cells treated with PMA for 2 h was reduced by 70% compared to controls. Thus PKC can regulate beta(2)AR at least two levels: the first being an acute desensitization of receptor function, and the second being a more prolonged repression of receptor gene transcription that in turn results in decreased receptor expression.

Growth hormone secretagogue actions on the pituitary gland: multiple receptors for multiple ligands?

1. Growth hormone (GH) secretion is thought to occur under the reciprocal regulation of two hypothalamic hormones, namely GH-releasing hormone (GHRH) and somatostatin (SRIF), through their engagement with specific cell-surface receptors on the anterior pituitary somatotropes. 2. In addition to GHRH and SRIF, synthetic GH-releasing peptides (GHRP) or GH secretagogue(s) (GHS) regulate GH release through the activation of a novel receptor, the GHS receptor (GHS-R). 3. The cloning of the GHS-R from human, swine and rat identifies a novel G-protein-coupled receptor involved in the control of GH secretion and supports the existence of an undiscovered hormone that may activate this receptor. 4. Varieties of intracellular signalling systems are suggested to mediate the action of GHS, which include changes in intracellular free Ca2+ ([Ca2+]i), cAMP, protein kinases A and C, phospholipase C etc. 5. With regard to the use of signalling systems by GHS, especially a new form of GHRP or GHRP-2, a clear species difference has been demonstrated, supporting the possibility of more than one type of GHS-R.

G protein-coupled receptors as targets for prolactin actions

Prolactin (PRL) is known to be involved in a wide range of biological functions including osmoregulation, lactation, reproduction, and immunomodulation. The first step in PRL action involves its interaction with a specific membrane receptor that belongs to the cytokine receptor superfamily. In spite of the lack of a kinase domain, receptors of the cytokine superfamily induce tyrosine phosphorylation of cellular substrates including the receptors. The role of PRL in female reproductive functions is well known and a direct effect on ovarian and testicular steroidogenesis has been established. In the ovary, PRL binds to a specific membrane receptor and exerts an inhibitory effect on follicular steroidogenesis. This effect is the result of an impairment involving FSH stimulation of G protein-coupled receptors (GPCR) and cyclic AMP-mediated activation of aromatase cytochrome P450 gene expression. This observation may indicate a direct connection between tyrosine phosphorylation and follicle-stimulating hormone (FSH) receptor (FSHR) transduction pathways, as is the case for growth factor receptors with intrinsic tyrosine kinase activity, which share several downstream signaling elements with GPCRs. Some studies leading to our understanding of these pathways are reviewed.

Role of protein kinase C and cAMP in fluoxetine effects on human T-cell proliferation

In this work, we studied the effect of fluoxetine on human T-lymphocyte proliferation using optimal and suboptimal concanavalin A concentrations. In particular, we analyzed the influence of fluoxetine on the kinases that are involved in intracellular signalling after stimulation with mitogens. We found that fluoxetine promoted the Ca2+ -mediated proteolysis of protein kinase C (PKC) and increased cyclic-AMP (cAMP) levels, thereby impairing lymphocyte proliferation, when optimal concanavalin A concentrations were used. In contrast, when suboptimal concanavalin A concentrations were used, fluoxetine only increased PKC translocation, without modifying cAMP levels, leading to T-cell proliferation. According to our results, fluoxetine has a dual effect on T-cell proliferation by modulating the PKC and protein kinase A pathways. This mechanism is thought to be mediated through Ca2+ mobilization.

Targeting of tissue plasminogen activator into the regulated secretory pathway of neuroendocrine cells

Plasma levels of tissue plasminogen activator (tPA) increase rapidly in response to specific vasoactive agents, trauma, and neural stimulation. This response has been attributed to acute release of tPA from stored pools within the vascular endothelium and from catecholamine storage vesicles of chromaffin cells. We have tested directly whether tPA can be sorted into the regulated secretory pathway using the murine pituitary-derived neuroendocrine cell line AtT-20 transfected with tPA cDNA. Clones of AtT-20 cells expressing tPA were isolated, and targeting of tPA into the regulated secretory pathway was demonstrated by (1) stimulation of tPA secretion with 8-bromo-cAMP, the secretagogue which promotes the release of dense granule contents; (2) colocalization with ACTH, an endogenous protein that is stored in dense core granules; and (3) retention of newly synthesized tPA in the cell for prolonged periods of time. Laser scanning confocal microscopy analysis of cells immunostained with antibodies to tPA and ACTH showed colocalization at the tips of the neuritic processes under the cytoplasmic membrane, a region where dense granules are known to migrate after maturation. Treatment of the cells with 5 mM 8-bromo-cAMP for 30 min resulted in a 2.41+/-0.36-fold increase in tPA secretion. Both the magnitude of the stimulatory effect and the fraction of the intracellular tPA released were the same regardless of the tPA expression level in the various clones. Pulse-chase experiments showed that a portion of newly synthesized tPA is retained in the cell for at least 4 h and is released into the culture medium in response to 8-bromo-cAMP. These studies indicate that tPA, under the appropriate conditions, can be targeted into the regulated secretory pathway and can be stored for later release by cellular stimuli.

Supramaximal CCK and CCh concentrations abolish VIP potentiation by inhibiting adenylyl cyclase activity

Exocrine pancreatic secretion stimulated by vasoactive intestinal polypeptide (VIP), which acts through the adenylyl cyclase-cAMP pathway, is potentiated by stimulation with other secretagogues such as CCK and carbachol (CCh). However, the potentiating effect is abolished by the same secretagogues at supramaximal concentrations. In the present study, we examined the mechanisms by which supramaximal concentrations of CCK octapeptide (CCK-8) or CCh reduce the VIP-induced potentiation of amylase secretion from isolated rat pancreatic acini. VIP-stimulated amylase secretion was potentiated by submaximal stimulatory concentrations of CCK-8 and CCh but was reduced by the same reagents at higher concentrations. Supramaximal concentrations of CCK-8 or CCh also reduced forskolin-induced potentiation of amylase release but did not reduce that induced by 8-bromo-cAMP. Moreover, supramaximal concentrations of CCK-8 or CCh inhibited VIP-stimulated intracellular cAMP production as well as adenylyl cyclase activity. 12-O-tetradecanoylphorbol 13-acetate (TPA) also reduced the magnitude of the potentiation of amylase release caused by VIP plus CCK-8 or CCh, although TPA itself decreased neither VIP-stimulated adenylyl cyclase activity nor intracellular cAMP accumulation. These results indicate that supramaximal concentrations of CCK-8 and CCh reduce the potentiating effect of VIP and forskolin on amylase secretion by inhibiting the adenylyl cyclase activity. In addition, protein kinase C is suggested to be partly implicated in this inhibitory mechanism. The mechanisms that lead to such inhibition may be interlinked but distinct from each other.

Adenosine mediates sustained adrenergic desensitization in the rat heart via activation of protein kinase C

Adenosine attenuates the myocardial metabolic and contractile responses induced by ss-adrenergic stimulation. Our study was conducted to investigate the longevity of this antiadrenergic action after adenosine exposure. Adenosine (33 micromol/L) was infused into isolated perfused rat hearts for 1, 5, 30, or 60 minutes, and the adrenergic responsiveness (AR) to isoproterenol (10(-8) mol/L) was determined at the end of each infusion period and during a 45-minute adenosine washout period. Interstitial levels of adenosine, as determined from epicardial surface transudates, returned to preinfusion levels within 10 minutes of washout. The duration of adenosine infusion had no effect on the extent of attenuation of AR at the end of the infusion. Whereas AR returned to preadenosine levels with washout of shorter adenosine infusions (1 and 5 minutes), there was a slow and incomplete recovery of AR after the longer exposures (30 and 60 minutes) to adenosine. The magnitude of this persistent antiadrenergic effect (PAE) of adenosine at 15 minutes of washout was proportional to the epicardial concentration of adenosine during infusion of the nucleoside. Infusion of adenosine either with the nonselective adenosine receptor antagonist 8-p-sulfophenyl theophylline or with the selective A1-receptor antagonist 1,3-dipropyl, 8-cyclopentylxanthine, abolished the PAE during the washout period. In addition, the PAE could be demonstrated only with the selective A1-receptor agonist 2-chloro-N6-cyclopentyladenosine and not with the selective A3-receptor agonist 4-aminobenzyl-5'-N methylcarboxamido-adenosine. When the protein kinase C (PKC) inhibitor chelerythrine was coadministered with adenosine, the PAE of adenosine was not apparent during adenosine washout. A 30-minute infusion of phenylephrine, an alpha-adrenergic agonist that enhances PKC activity, produced a PAE that lasted for up to 30 minutes of washout. This effect was prevented by the coinfusion of chelerythrine. Thus, it is concluded that the PAE of adenosine is determined by the myocardial concentration of this nucleoside and is manifested when myocardial concentrations of adenosine returned to baseline levels. Moreover, a 5-minute duration of adenosine exposure is required for the expression of the PAE. This latter effect seems to be dependent on adenosine-induced PKC activation via A1-receptors.

Protein kinase C-mediated down-regulation of beta1-adrenergic receptor gene expression in rat C6 glioma cells

In the current study, we investigated the mechanism by which protein kinase C (PKC) regulates the expression of beta1-adrenergic receptor (beta1AR) mRNA in rat C6 glioma cells. Exposure of the cells to 4beta-phorbol-12-myristate-13-acetate (PMA), an activator PKC, resulted in a down-regulation of both beta1AR binding sites and mRNA levels in a time- and concentration-dependent manner. This effect was not observed with phorbol esters that do not activate PKC and was blocked by bisindolylmaleimide, a specific PKC inhibitor. Activation of PKC did not reduce the half-life of beta1AR mRNA but significantly decreased the activity of the beta1AR promoter, as determined by reporter analysis. A putative response element, with partial homology to a consensus cAMP response element, was identified by mutation analysis of the promoter at positions -343 to -336, relative to the translational start site. Mutation of this putative regulatory element, referred to as a beta1AR-PKC response element, completely blocked the PKC-mediated down-regulation of beta1AR promoter activity. Gel mobility shift analysis detected two specific bands when C6 cell extracts were incubated with a labeled DNA probe containing the beta1AR-PKC response element sequence. Formation of one of these bands was inhibited by an oligonucleotide probe containing a consensus CRE and disrupted by an antibody for cAMP response element binding protein. Based on these studies, we propose that the PKC-induced down-regulation of beta1AR gene transcription in C6 cells is mediated in part by a cAMP response element binding protein-dependent mechanism acting on a novel response element.

Identification of tumor-specific paclitaxel (Taxol)-responsive regulatory elements in the interleukin-8 promoter

Paclitaxel (Taxol) is a novel chemotherapeutic drug that is effective against breast and ovarian cancers. Although the primary target of paclitaxel is microtubules, its efficacy exceeds that of conventional microtubule-disrupting agents, suggesting that it may have additional cellular effects. Previously, we demonstrated that paclitaxel can induce interleukin-8 (IL-8) gene expression at the transcriptional level in subsets of human ovarian cancer lines. In this as well as the previous report, we present evidence that this ability is not linked to the lipopolysaccharide pathway of IL-8 gene induction. The present study identifies the cis-acting elements and trans-acting factors involved in this induction by transfecting DNA constructs containing the 5'-flanking region of the IL-8 gene linked to the chloramphenicol acetyltransferase reporter gene into paclitaxel-responsive and nonresponsive ovarian cancer cells (responsiveness refers to the IL-8 response). Paclitaxel only activated the IL-8 promoter in responsive cells. The AP-1 and NF-kappaB binding sites in the IL-8 promoter are required for activation by paclitaxel; in contrast, a C/EBP site required for IL-8 promoter activation in other cell types is not involved. Gel shift assays demonstrate that paclitaxel causes a marked increase in protein binding to the NF-kappaB and AP-1 consensus binding sequences in the paclitaxel-responsive ovarian cells, but not the nonresponsive cells. The induction of NF-kappaB and AP-1 binding is reduced by the addition of protein kinase C inhibitors and cyclic AMP effector, respectively. These results demonstrate a molecular mechanism for cell-specific paclitaxel-induced IL-8 gene expression which may have clinical relevance.

Role of chondroitin 4-sulphate as a receptor for polycation induced human platelet aggregation

1. Proteoglycans provide negatively charged sites on the surface of platelets, leukocytes and endothelial cells. Since chondroitin 4-sulphate is the main proteoglycan present on the platelet surface, the role of this molecule in mediating the activation of human platelets by polylysine was studied. 2. Platelets were desensitized with phorbol 12-myristate 13-acetate (PMA, 10 nM) 5 min before the addition of polylysine to platelet-rich plasma (PRP). Changes in the intracellular Ca2+ concentration were measured in fura2-am (2 microM) loaded platelets and protein phosphorylation was assessed by autoradiography of the electrophoretic profile obtained from [32P]-phosphate labelled platelets. The release of dense granule contents was measured in [14C]-5-hydroxytryptamine loaded platelets and the synthesis of thromboxane (TXA2) was assessed by radioimmunoassay. Surface chondroitin 4-sulphate proteoglycan was degraded by incubating platelets with different concentrations of chondroitinase AC (3 min, 37 degrees C). The amount of chondroitin 4-sulphate remaining in the platelets was then quantified after proteolysis and agarose gel electrophoresis. 3. The addition of PMA to PRP before polylysine inhibited the aggregation by 88 +/- 18% (n = 3). Staurosporine (1 microM, 5 min) prevented the PMA-induced inhibition. Chondroitinase AC (4 pu ml-1 to 400 muu ml-1, 3 min) abolished the polylysine-induced aggregation in PRP but caused only a discrete inhibition of ADP-induced aggregation. The concentration of chrondroitin 4-sulphate in PRP (0.96 +/- 0.2 microgram/10(8) platelets, n = 3) and in washed platelets (WP; 0.35 +/- 0.1 microgram/10(8) platelets, n = 3) was significantly reduced following incubation with chondroitinase AC (PRP = 0.63 +/- 0.1 microgram/10(8) platelets and WP = 0.08 +/- 0.06 microgram/10(8) platelets). 4. Washed platelets had a significantly lower concentration of chondroitin 4-sulphate than platelets in PRP. The addition of polylysine to WP induced a rapid increase in light transmission which was not accompanied by TXA2 synthesis or the release of dense granule contents. This effect was not inhibited by sodium nitroprusside (SNP), iloprost, EDTA or the peptide RGDS. This event was accompanied by the discrete phosphorylation of plekstrin and myosin light chain, which were inhibited by staurosporine (10 microM, 10 min). The hydrolysis of platelet surface chondroitin 4-sulphate strongly reduced the polylysine-induced phosphorylation. 5. Our results indicate that polylysine activates platelets through a specific receptor which could be the proteoglycan chondroitin 4-sulphate present on the platelet membrane.

Protein kinase C deficiency blocks recovery from agonist-induced desensitization

Protein phosphorylation is central to agonist-induced attenuation of the function of G-protein-linked receptors. Stable expression of RNA antisense to specific protein kinase mRNAs permitted analysis of loss-of-function mutants of A431 human epidermoid carcinoma cells, lacking protein kinase A, protein kinase C, or beta-adrenergic receptor kinase. Deficiency of protein kinase C, but not the others, amplified rather than attenuated agonist-induced desensitization. In wild-type cells, the t1/2 for recovery from desensitization was approximately 25 min following removal of agonist. In the protein kinase C-deficient cells, no resensitization was observed even 60 min after agonist removal. Like protein kinase C-deficiency, inhibition of protein kinase C with bisindolylmaleimide or calphostin C blocked resensitization. Resensitization was suppressed by FK506, an inhibitor of protein phosphatase 2B, mimicking protein kinase C-deficiency, but in a non-additive manner. The data reveal protein kinase C and protein phosphatase 2B to be critical elements of resensitization.

Attenuation of stimulated Ca2+ influx in colonic epithelial (HT29) cells by cAMP

In HT29 colonic epithelial cells agonists such as carbachol (CCH) or ATP increase cytosolic Ca2+ activity ([Ca2+]i) in a biphasic manner. The first phase is caused by inositol 1,4,5-trisphophate-(Ins P3-) mediated Ca2+ release from their respective stores and the second plateau phase is mainly due to stimulated transmembraneous Ca2+ influx. The present study was undertaken to examine the effect of increased adenosine 3',5'-cyclic monophosphate (cAMP) (forskolin 10 micromol/l = FOR) on the Ca2+ transient in the presence of CCH (100 micromol/l). In unpaired experiments it was found that FOR induced a depolarization and reduced cytosolic Ca2+ ([Ca2+]i, measured as the fura-2 fluorescence ratio 340/380 nm) significantly. Dideoxyforskolin had no such effect. The effect of FOR was abolished when the cells were depolarized by a high-K+ solution. In further paired experiments utilizing video imaging in conjunction with whole-cell patch-clamp, [Ca2+]i was monitored separately for the patch-clamped cell and three to seven neighbouring cells. In the presence of CCH, FOR reduced [Ca2+]i uniformly from a fluorescence ratio (345/380) of 2.9 +/- 0.12 to 1.8 +/- 0.07 in the patch-clamped cell and its neighbours (n = 48) and depolarized the membrane voltage (Vm) of the patch-clamped cells significantly and reversibly from -54 +/- 7.4 to -27 +/- 5.9 mV (n = 6). In additional experiments Vm was depolarized by 15-54 mV by various increments in the bath K+ concentration. This led to corresponding reductions in [Ca2+]i. Irrespective of the cause of depolarization (high K+ or FOR) there was a significant correlation between the change in Vm and change in [Ca2+]i. These data indicate that the cAMP-mediated attenuation of Ca2+ influx is caused by the depolarization produced by this second messenger.

Protein kinase C-dependent modulation of stimulatory guanine nucleotide binding protein of fetal rat skin keratinocytes

Although the protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA) has been known to induce heterologous desensitization of the epidermal adenylate cyclase, the precise mechanism of PMA action remains unknown. Effects of PMA on the receptor-G-protein-adenylate cyclase system of fetal rat skin keratinocytes (FRSK) were investigated. Choleratoxin catalysed the ADP ribosylation of 45 kDa and 52 kDa membrane proteins and islet activating protein (IAP) catalysed the ADP ribosylation of a 40 kDa membrane protein. Incubation of FRSK with PMA decreased the cholera toxin-catalysed ADP ribosylation of the membrane protein, but not the IAP-catalysed ADP ribosylation. The effect of PMA on the cholera toxin-catalysed ADP ribosylation was inhibited by the PKC inhibitor, H-7 (1-(5-isoquinolinesulfonyl)-2-methyl piperazine dihydrochloride). 1-Oleoyl-2-acetylglycerol (OAG), a membrane-permeable diacylglycerol analogue, also decreased the cholera toxin-catalysed ADP ribosylation, but 4-0-methyl PMA, a very weak PKC activator, had no effect. Keratinocytes are known to express the guanine nucleotide binding proteins, Gsalpha, Gi2alpha, and Gi3alpha. Immunoblot analysis of the PMA-treated FRSK showed no detectable difference in the amount of Gsalpha, Gi2alpha, Gi3alpha, or the beta subunit of the G-protein. PMA significantly decreased the beta-adrenergic adenylate cyclase response and cholera toxin-induced cyclic AMP accumulation, while it markedly increased forskolin-induced cyclic AMP accumulation. These results indicate that phorbol esters affect the stimulatory guanine nucleotide binding protein (Gs) of FRSK via a PKC-dependent pathway.

Protein kinase C regulates pleckstrin by phosphorylation of sites adjacent to the N-terminal pleckstrin homology domain

Pleckstrin is a substrate for protein kinase C in activated platelets that contains at its N and C termini two of the pleckstrin homology (PH) domains that have been proposed to mediate protein-protein and protein-lipid interactions. We have recently shown that pleckstrin can inhibit agonist-induced phosphoinositide hydrolysis and that this inhibition requires an intact N-terminal PH domain (residues 6 to 99). In the present studies, we have identified the sites of phosphorylation in pleckstrin and examined their contribution to pleckstrin function. In human platelets activated with thrombin or phorbol esters, and in COS-1 cells expressing pleckstrin, a combination of phosphopeptide analysis and site-directed mutagenesis shows that three residues in the intervening sequence between the two pleckstrin PH domains become phosphorylated: Ser113, Thr114, and Ser117. Replacing all three of these sites with glycine decreased phosphorylation by > 90% and reduced pleckstrin's ability to inhibit phosphoinositide hydrolysis by as much as 80%. Replacing the phosphorylation sites with alanine residues had a similar effect, while substitution with aspartate, glutamate, or lysine residues produced pleckstrin variants that were fully active even in the absence of phosphorylation. These results suggest that phosphorylation enhances pleckstrin's activity by introducing a cluster of charges into a region adjacent to, but not within, the N-terminal PH domain. This may have an allosteric effect on the N-terminal PH domain, regulating its interaction with other molecules necessary for the inhibition of phosphoinositide hydrolysis.

Pleckstrin inhibits phosphoinositide hydrolysis initiated by G-protein-coupled and growth factor receptors. A role for pleckstrin's PH domains

Pleckstrin is a 40-kDa protein present in platelets and leukocytes that contains two PH domains separated by a 150-residue intervening sequence. Pleckstrin is a major substrate for protein kinase C, but its function is unknown. The present studies examine the effects of pleckstrin on second messenger generation. When expressed in cos-1 or HEK-293 cells, pleckstrin inhibited 1) the G alpha-mediated activation of phospholipase C beta initiated by thrombin, M1-muscarinic acetylcholine, and angiotensin II receptors, 2) the stimulation of phospholipase C beta by constitutively active Gq alpha, 3) the G beta gamma-mediated activation of phospholipase C beta caused by alpha 2A-adrenergic receptors, and 4) the tyrosine phosphorylation-mediated activation of phospholipase C gamma caused by Trk A. However, pleckstrin had no effect on either the stimulation or inhibition of adenylyl cyclase. The inhibition of phosphoinositide hydrolysis caused by pleckstrin was similar in magnitude to that caused by activating protein kinase C with phorbol 12-myristate 13-acetate (PMA). When combined, pleckstrin and PMA had an additive effect, inhibiting phosphoinositide hydrolysis by as much as 90%. Structure-function analysis highlighted the role of pleckstrin's N-terminal PH domain in these events. Although deleting the C-terminal PH domain had no effect, deleting the N-terminal PH domain abolished activity (but not expression) and mutating a highly conserved tryptophan residue within the N-terminal PH domain decreased activity by one-third. Notably, however, a pleckstrin variant in which the N-terminal PH domain was replaced with a second copy of the C-terminal PH domain was nearly as active as native pleckstrin. These results show that: 1) pleckstrin can inhibit pathways leading to both phospholipase C beta- and phospholipase C gamma-mediated phosphoinositide hydrolysis, 2) this inhibition affects activation of phospholipase C beta mediated by either G alpha or G beta gamma, but does not affect the regulation of adenylyl cyclase activity by G alpha or G beta gamma, 3) although pleckstrin is a substrate for protein kinase C, the effects of pleckstrin and PMA are at least partially independent, 4) the inhibition caused by pleckstrin appears to be mediated by the PH domain at the N terminus, rather than the C terminus of the molecule, and 5) location of the two PH domains within the molecule clearly contributes to their individual activity.2+1

Activation of protein kinase C enhances the phosphorylation of the type B interleukin-8 receptor and stimulates its degradation in non-hematopoietic cells

We have previously characterized the stably transfected, clonally selected human placental cell line, 3ASubE P-3, which overexpresses the type B interleukin-8 receptor (IL-8RB) and responds to the chemokine melanoma growth stimulatory activity (MGSA) with enhanced phosphorylation of this receptor. In work described here, we demonstrate that the MGSA-enhanced phosphorylation of this receptor is mediated via a process involving pertussis toxin-sensitive G proteins. Furthermore, treatment of the 3ASubE P-3 cells with either 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1,2-dioctanoyl-sn-glycerol (diC8), two different activators of protein kinase C (PKC), results in a concentration-dependent increase in the phosphorylation of the IL-8RB. Inhibition of PKC, by treatment with staurosporin (50 nM for 2 h), or down-regulation of PKC, by prolonged treatment with TPA (400 nM for 40 h) suppresses the TPA-enhanced receptor phosphorylation, but has no effect on the MGSA-enhanced receptor phosphorylation. These data suggest that the isoforms of PKC that are sensitive to these manipulations may not play a role in mediating the MGSA-enhanced phosphorylation of the IL-8RB. TPA treatment also results in a time-dependent decrease in 125I-MGSA binding to the 3ASubE P-3 cells. A 30-min treatment with 400 nM TPA results in approximately a 50% decrease in binding, whereas a 2-h treatment essentially eliminates specific binding of 125I-MGSA to these cells. The TPA-induced decrease in 125I-MGSA binding is accompanied by enhanced degradation of the IL-8RB, as indicated by Western blot analysis and pulse-chase experiments, suggesting a potential role for PKC as a negative regulator of the IL-8RB. MGSA treatment (50 nM for 2 h) also stimulates receptor degradation in the 3ASubE P-3 cells, indicating that this receptor is down-regulated in response to prolonged exposure to its ligand. In similar studies conducted on the promonocytic cell line, U937, MGSA treatment of the U937 cells resulted in receptor phosphorylation, whereas PKC activation failed to significantly modulate the phosphorylation state of the IL-8RB. Treatment of the U937 cells with MGSA, TPA, or diC8 resulted in a loss of receptor protein present in these cell types. These data imply that MGSA signaling through the IL-8RB is similar in both the non-hematopoietic and hematopoietic cell types, whereas activation of PKC by TPA or diC8 elicits different responses in these two distinct cell types.

Long term phorbol ester treatment down-regulates the beta 3-adrenergic receptor in 3T3-F442A adipocytes

The role of protein kinase C (PKC) in the regulation of the beta 3-adrenergic receptor (beta 3-AR) gene was examined in murine 3T3-F442A adipocytes, which express this receptor subtype at a high level. We also investigated the involvement of this kinase in the modulation of beta 3-AR gene expression by insulin. Long term exposure of 3T3-F442A adipocytes to phorbol 12-myristate 13-acetate (PMA) decreased beta 3-AR mRNA content in a time- and concentration-dependent manner, with maximal changes observed at 6 h (6.5-fold decrease) and at 100 nM PMA. This inhibition was selective for beta 3-AR transcripts, since beta 1- and beta 2-AR mRNA content remained unchanged. Also, (-)-[125I]cyanopindolol saturation and competition binding experiments on adipocyte membranes indicated that PMA induced an approximately 2-fold decrease in beta 3-AR expression, while that of the two other subtypes was not affected. This correlated with a lower efficacy of beta 3-AR agonists to stimulate adenylyl cyclase. Conversely, long term exposure to PMA did not alter adenylyl cyclase activity in response to guanosine 5'-O-(3-thiotriphosphate) or forskolin. The inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate did not repress beta 3-AR mRNA levels. Inhibition of beta 3-AR mRNA by PMA was suppressed by the PKC-selective inhibitor bisindolylmaleimide, and was not observed in PKC-depleted cells, indicating that PKC was involved in this response. mRNA turnover experiments showed that the half-life of beta 3-AR transcripts was not affected by long term PMA exposure. When 3T3-F442A adipocytes were pretreated with PMA for 24 h to down-regulate PKC, or with bisindolylmaleimide, the insulin-induced inhibition of beta 3-AR mRNA levels was reduced by 44-67%. These findings demonstrate that sustained PKC activation exerts a specific control of beta 3-AR gene expression and is involved, at least in part, in the modulation by insulin of this adrenergic receptor subtype.

Receptor crosstalk: effects of prolonged carbachol exposure on beta 1-adrenoceptors and adenylyl cyclase activity in neonatal rat ventricular myocytes

Supersensitivity of adenylyl cyclase after exposure to inhibitory agonists is a general means of cellular adaptation. We hypothesized that such "crosstalk" between muscarinic cholinergic agonists, beta 1-adrenoceptors, and adenylyl cyclase may be an important mechanism of cardiac adaptation to interventions that enhance vagal activity. We used primary cultures of neonatal rat ventricular myocytes and measured beta-adrenoceptors by radioligand binding and adenylyl cyclase activity by a single column method. Carbachol induced a time- and dose-dependent reversible decrease in cell surface beta 1-adrenoceptors. The peak effect occurred after 20 h of exposure to 100 microM carbachol which caused a decrease in the maximum number of binding sites for the beta-adrenoceptor antagonist 3H-CGP-12177 from 42.3 +/- 3.4 to 33.0 +/- 2.6 fmol/mg protein (n = 12, P < 0.03) without a change in antagonist affinity. Loss of cell surface receptors was prevented by atropine and by the protein kinase C inhibitor H7. The decrease in cell surface receptors was not accompanied by receptor internalization as assessed by equilibrium binding experiments in a cytosolic fraction using 125I-iodocyanopindolol. In contrast to the well-known acute inhibitory effects of carbachol on adenylyl cyclase activation, prolonged carbachol exposure preserved (-)-isoprenaline-stimulated adenylyl cyclase activity and enhanced postreceptor stimulated adenylyl cyclase activity. Carbachol did not further enhance adenylyl cyclase activity after pretreatment with pertussis toxin. The protein kinase C inhibitor chelerythrine prevented the carbachol induced enhancement of forskolin-stimulated adenylyl cyclase activity. We conclude that prolonged incubation with carbachol in rat neonatal ventricular myocytes causes a reduction in cell surface beta 1-Adrenoceptor density. beta 1-Adrenoceptor-mediated adenylyl cyclase activity is preserved and postreceptor-mediated adenylyl cyclase activity is augmented. Our data suggest that carbachol-stimulated protein kinase C activity may play a key role in the prolonged muscarinic regulation of adenylyl cyclase activity.

Cross-talk of parathyroid hormone-responsive dual signal transduction systems in osteoblastic osteosarcoma cells: its role in PTH-induced homologous desensitization of intracellular calcium response

The present study was designed to characterize the cross-talk of parathyroid hormone (PTH)-responsive dual signal transduction systems (cAMP-dependent protein kinase (PKA) and calcium/protein kinase C [PKC]) and its participation in PTH-induced homologous desensitization of intracellular calcium ([Ca2+]i) in osteoblastic UMR-106 cells. Although our recent study revealed that prolonged (more than 2 h) pretreatment with PKC-activating phorbol ester, phorbol 12-myristate 13-acetate (PMA) significantly decreased the PTH-stimulated cAMP production, pretreatment with PMA (10(-7) and 10(-6) M) but not 10(-6) M 4 alpha-phorbol 12,13-didecanoate (PDD), incapable of activating PKC for 30 min significantly augmented 10(-7) M hPTH-(1-34)-stimulated cAMP production. H-7 (50 microM), a PKC inhibitor, significantly antagonized this PMA-induced effect. Pretreatment with 10(-6) M PMA for 30 min did not affect PTH receptor binding but significantly augmented a cAMP responsiveness to 10(-5) M forskolin and 1 microgram/ml cholera toxin. Pertussis toxin (0.5 microgram/ml) did not affect the PMA-induced augmentation of the PTH-stimulated cAMP production. PTH caused a complete homologous desensitization of [Ca2+]i response within 30 min. Pretreatment with 10(-4) M dibutyryl cAMP for 30 min and 6 h significantly reduced and completely blocked the PTH-induced increase in [Ca2+]i, respectively. Pretreatment with 10(-4) M Sp-cAMPs, a direct PKA activator, for 30 min completely blocked the PTH-induced increase in [Ca2+]i. Rp-cAMPS (10(-4) M), an antagonist of PKA, slightly but significantly antagonized the PTH-induced homologous desensitization of [Ca2+]i response. The present study indicates that the time of exposure to PKC activation is a critical determinant in modulating the cAMP system, while PKA activation counterregulatorily acts on the [Ca2+]i system, and that PKA activation is linked to the PTH-induced homologous desensitization of [Ca2+]i response.

Phorbol ester has different effects on forskolin and beta-adrenergic-stimulated cAMP accumulation in mouse parotid acini

Phorbol 12-myristate 13-acetate (TPA) augmented the effects of forskolin, and inhibited the effects of isoproterenol on cAMP accumulation in mouse parotid acini. Treatment of intact cells with the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (MIX), blocked TPA inhibition of isoproterenol but not forskolin-stimulated cAMP accumulation. TPA also caused the translocation of protein kinase C (PKC) from cytosol to membrane. Pre-treatment of parotid acini with TPA for 30 min enhanced the forskolin and isoproterenol-stimulated adenylate cyclase activity in isolated parotid membranes. Addition of purified PKC (pPKC) to parotid membranes mimicked the effects of TPA in increasing cAMP synthesis; the effects were blocked in the absence of calcium and phospholipid, and in the presence of the synthetic peptide (PKC 19-36). Purified PKC also mimicked the effects of TPA in augmenting forskolin and isoproterenol-stimulated adenylate cyclase activities in the cell-free system. Data suggest that the differential regulation of forskolin and isoproterenol-stimulated cAMP accumulation by TPA results from modification of enzymes that synthesize and degrade cAMP.

Regulation of adenylyl cyclase in rabbit iris ciliary body

The enzyme adenylyl cyclase has been shown to be important in the regulation of intraocular pressure. We therefore studied the activity of adenylyl cyclase (AC) activity in the rabbit iris/ciliary body (I/CB) after pre-treatment with the beta-adrenergic agonist isoproterenol (ISO) which activates cAMP dependent protein kinase A, and phorbol 12,13 dibutyrate (PDB) which activates protein kinase C. When I/CB was pre-treated with ISO (10 microM) or PDB (1 microM), attenuated AC activity (approximately 35%) resulted when the activity of the enzyme was assessed by rechallenge with isoproterenol. However, when AC activity was assessed by rechallenge with forskolin or prostaglandin, enhanced activity resulted. In an effort to identify the mechanism of this apparent heterologous regulation of AC, studies were performed that showed no significant changes in the density of beta-adrenergic receptors or the affinity of the receptors for the ligand (125I)-Iodopindolol occurred in ISO or PDB treated tissue. Similarly, in membranes prepared from ISO or PDB treated tissue, no significant changes in the functional activity of the guanine nucleotide binding proteins Gi or Gs could be ascertained as assessed by somatostatin inhibition of forskolin-stimulated AC (to assess Gi function), or in an adenylyl cyclase complementation assay (to assess Gs function). However, AC activity stimulated by Mn2+ and purified Gs was enhanced (approximately 2X) following isoproterenol or phorbol ester pre-treatment, suggesting that an alteration at the level of the catalytic subunit of AC resulted from ISO or PDB pretreatment. Therefore, the assessment of net changes in receptor coupled AC activity induced by phorbol esters or isoproterenol appears to be dependent on the drug used to rechallenge the AC system and cAMP production is dependent on the sum of diverse effects on multiple components of the AC pathway.

Interaction of parathyroid hormone-related peptide-responsive dual signal transduction systems in osteoblastic osteosarcoma cells: role in PTHrP-induced homologous desensitization

In osteoblastic UMR-106 cells, 10(-7) M human (h) PTH-related peptide (PTHrP)-(1-34) significantly induced the formation of total inositol phosphates to the same degree as 10(-7) M hPTH-(1-34), confirming that in addition to cAMP-dependent protein kinase (PKA), PTHrP possesses another signal transduction system, calcium/protein kinase C (Ca/PKC). Experiments were therefore performed to characterize the cross talk of these dual-signal transduction systems and its participation in the PTHrP-induced homologous desensitization of cAMP and cytosolic calcium (Cai) response in osteoblasts. Preincubation with 10(-7) M hPTHrP-(1-34) caused homologous desensitization, resulting in a remarkable decrease in cAMP accumulation in response to further exposure to PTHrP. This effect was significant after 2 h pretreament and reached a maximum at 6 h. Pretreatment with the PKC-activating phorbol ester phorbol 12-myristate-13-acetate (PMA, 10(-6) M) for 30 minutes and 6 h caused a significant increase and decrease in cAMP responsiveness to PTHrP, respectively. Pretreatment with calcium ionophores (A23187 or ionomycin, 10(-6) M), not for 30 minutes but for 6 h, caused a significant decrease in cAMP responsiveness to PTHrP. H-7 (an inhibitor of PKC, 50 microM) significantly blocked not only PMA- but also PTHrP-induced desensitization of the cAMP response. PTHrP caused the complete homologous desensitization of an increase in Cai within 30 minutes. Pretreatment with dibutyryl-cAMP (10(-4) M) for 30 minutes caused significant inhibition of the PTHrP-induced increase in Cai, and pretreatment with Sp-cAMPS (10(-4) M), a direct activator of PKA, for 30 minutes completely blocked the PTHrP-induced increase in Cai.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross talk of dual-signal transduction systems in the regulation of DNA synthesis by parathyroid hormone in osteoblastic osteosarcoma cells

There has been recent evidence that calcium/protein kinase C (Ca/PKC) messenger system as well as adenylate cyclase are involved in the signal transduction stimulated by PTH. We therefore examined the role of these dual-signal transduction systems and the interaction of these systems in the regulation of DNA synthesis by PTH in the osteoblastic osteosarcoma cells, UMR-106. As recently reported, 10(-4) M Sp-cAMPS, a direct activator of cAMP-dependent protein kinase (PKA), and 10(-4) M dibutyryl-cAMP, as well as hPTH-(1-34), caused the significant inhibition of [3H]thymidine incorporation (TdR). Both A23187 and ionomycin (10(-8)-10(-6) M) inhibited TdR in a dose-dependent manner, with a minimal effective dose at 10(-7) M. Although 10(-6) M phorbol 12-myristate 13-acetate (PMA) caused slight but significant stimulation of TdR by itself, it augmented not only dibutyryl-cAMP- but also Sp-cAMPS-induced inhibition of TdR. On the other hand, 4 alpha-phorbol 12,13-didecanoate, incapable of activating PKC, failed to augment these cAMP analogs-induced effects. Pretreatment with 50 microM H-7, an inhibitor of PKC, not only abolished the PMA-induced augmentation of effect by cAMP analogs but also significantly blocked the PTH-induced inhibitory effect on TdR. Pretreatment with 10(-6) M PMA, which downregulates PKC, significantly inhibited the PTH-induced suppression of TdR. Combined treatment with cAMP analog (dibutyryl-cAMP or Sp-cAMPS) and calcium ionophore (A23187 or ionomycin) caused additive effects on TdR, and PMA used in combination with both cAMP analog and calcium ionophore induced the further inhibition of TdR.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of high-threshold Ca current and spontaneous postsynaptic transient currents by phorbol 12,13-diacetate, 1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7), and monosialoganglioside (GM1) in CA1 pyramidal neurons of rat hippocampus in vitro

Phorbol esters, which activate protein kinase C (PKC), enhance synaptic transmission in the CA1 subfield of hippocampus, both in situ and in vitro. The increase in synaptic transmission could be the consequence of enhanced Ca influx into nerve terminals, and perhaps a more general increase in voltage-dependent Ca currents. The effects of phorbol 12,13-diacetate (PDAc) on the high-voltage activated (HVA) Ca currents, as well as spontaneous transient currents were therefore investigated by intracellular recording in hippocampal slices. PDAc selectively augmented, by 45% +/- 10%, the early peak of the HVA Ca current (but not its sustained component), and also spontaneous inhibitory postsynaptic currents. The inactive phorbol ester, 4 alpha-PDAc, had no comparable effects. The actions of PDAc were reversible on prolonged washing, and they were antagonized by the PKC inhibitors (1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7) and monosialoganglioside (GM1). In addition, GM1, which also activates the Ca/calmodulin-dependent kinase, enhanced spontaneous excitatory postsynaptic currents, while inhibiting the IPSCs. It is concluded that activation of PKC increases HVA (probably N-type) Ca current and facilitates ongoing GABAergic IPSCs.

Agonist receptors and G proteins as mediators of platelet activation

Recent studies have helped to define the earliest events of signal transduction in platelets, particularly those involved in the generation of second messengers. The best-understood of these events are those which involve guanine nucleotide binding regulatory proteins. G proteins are heterotrimers comprised of alpha, beta and gamma subunits, each of which can exist in multiple forms. Some, but not all, of the known variants of G alpha are substrates for ADP-ribosylation by pertussis toxin, a modification which disrupts the flow of information from receptor to effector. The G proteins that have been identified in platelets to date are Gs, Gi1, Gi2, Gi3, Gz and Gq. Gs and one or more of the Gi family members regulate cAMP formation by adenylylcyclase. Gi may also be responsible for the pertussis toxin-sensitive activation of phospholipase C which occurs when platelets are activated by thrombin. Gq is thought to be responsible for the pertussis toxin-resistant activation of phospholipase C by TxA2. Gz does not have an established role, but has the unique property of being phosphorylated by protein kinase C during platelet activation. Recent efforts to clone the receptors that interact with G proteins in platelets have been successful for epinephrine, thrombin, TxA2 and platelet activating factor. Each of these resembles other G protein-coupled receptors, being comprised of a single polypeptide with 7 transmembrane domains. In the case of thrombin, receptor activation is thought to involve a unique mechanism in which thrombin cleaves its receptor, creating a new N-terminus that can serve as a tethered ligand. Peptides corresponding to the tethered ligand can mimic the effects of thrombin, while antibodies to the same domain inhibit platelet activation. Shortly after activation, thrombin receptors become resistant to re-activation by thrombin. This desensitization, which appears to be due to a combination of proteolysis, phosphorylation and internalization, provides a potential mechanism for limiting the duration of thrombin-initiated signals in platelets.

Protein kinase C impairs the coupling of the GTP-binding protein to LTB4 receptor in neutrophil

In the present study, the mechanism of LTB4 receptor down regulation by protein kinase C (PKC) has been investigated using porcine neutrophil membranes. Pretreatment of intact porcine neutrophils with 12-O-tetradecanoylphorbol-13-acetate (TPA) for 2 min prior to the preparation of plasma membrane, demonstrated a reduced binding sites (Bmax) for LTB4 without altering the receptor affinity (Kd). This effect of TPA on LTB4 receptor binding was found to be due to the activation of PKC as membrane treated with purified PKC (type III) produced the same effect. When membranes from neutrophils pretreated with TPA were exposed to non-hydrolyzable GTP analog, GTP-gamma S, or GMP-PNP, no further decrease in receptor Kd was observed, while the Bmax was reduced to the level observed in TPA treated samples. Treatment of isolated neutrophil membranes with purified PKC reduced the Bmax and blocked the effect of GTP analogs on the receptor affinity. These results suggest that, PKC interrupts the receptor binding to G-protein.

Substance P Is Diminished and Vasoactive Intestinal Peptide Is Augmented in Psoriatic Lesions and These Peptides Exert Disparate Effects on the Proliferation of Cultured Human Keratinocytes

An involvement of neurogenic components in the pathogenesis of psoriatic lesions has been suggested and neuropeptides are thought to play a modulatory role in cutaneous inflammation. In this study, we evaluated the immunoreactivity of the neuropeptides vasoactive intestinal polypeptide (VIP) and substance P (SP) in the skin of patients with chronic plaque psoriasis, by immunohistochemistry and radioimmunoassay. No differences were observed, by immunohistochemistry, in the expression and localization of VIP and SP between psoriatic and normal skin. Using the radioimmunologic technique on whole skin homogenates, VIP levels were significantly increased in psoriatic lesions as compared to normal skin. By contrast, SP levels were significantly lower in lesional and non-lesional psoriatic skin than in normal skin. In addition, we examined the effect of VIP and SP on the proliferation of cultured normal human keratinocytes. VIP (1-28) (1 nM-1 microM) as well as VIP fragments (10-28) (1 nM-1 microM) and (22-28) (1 nM-1 microM) stimulated the proliferation of keratinocytes in a dose-dependent manner, whereas the VIP fragment (1-12) (1 nM-1 microM) was ineffective. The VIP antagonist (N-Ac-Tyr1, D-Phe2)-GRF (1-29)-NH2 (0.1 microM) significantly inhibited the VIP effect on keratinocytes. On the other hand, SP (0.1 microM) not only failed to stimulate keratinocyte growth, but also blocked the VIP-induced stimulation of these cells. The imbalance of cutaneous VIP and SP and their disparate effects on the proliferation of normal human keratinocytes in culture would suggest that these peptides are involved in the pathogenesis of psoriasis and may exert different modulatory activities in the mechanisms underlying the psoriatic lesion.

Phorbol ester and atrial natriuretic peptide receptor response on vascular smooth muscle

At least two types of receptors for natriuretic peptides have been reported: biologically active receptors coupled with guanylate cyclase (atrial natriuretic peptide [ANP]-B receptors) and clearance receptors (ANP-C receptors). To elucidate the role of protein kinase C (PKC) in the regulation of ANP-B receptors, vascular smooth muscle cells in culture were treated with phorbol ester. Incubation with receptor agonists and phorbol ester led to the desensitization of receptor-mediated cyclic guanosine monophosphate (ANP-B receptor response) in rat vascular smooth muscle cells. Although a PKC inhibitor and downregulation of PKC by long-term incubation of cells with phorbol esters blocked the phorbol ester-induced desensitization of the ANP-B receptor response, they did not block the ANP-induced desensitization of the ANP-B receptor response. In addition, when desensitization by phorbol esters was observed, ANP was still capable of desensitization. These observations suggest that the mechanism for regulating ANP-B receptor sensitivity may be both PKC-dependent and PKC-independent and mediated by phorbol esters and ANP, respectively.

Protein kinase C differentially modulates PTH- and PGE2-sensitive adenylate cyclase in osteoblast-like cells

The effects of phorbol 12-myristate 13-acetate (PMA), a known activator of protein kinase C, on receptor-mediated stimulation of adenylate cyclase were evaluated in a rat osteosarcoma cell line (UMR-106) with the osteoblast phenotype. Pretreatment of UMR-106 cells with PMA increased parathyroid hormone (PTH)-stimulated adenylate cyclase activity and inhibited prostaglandin E2 (PGE2)-responsive enzyme activity. In addition, PMA enhanced enzyme activation by forskolin, which is thought to exert a direct stimulatory action on the catalytic subunit of adenylate cyclase. The regulatory effects of PMA were concentration dependent and of rapid onset (less than or equal to 1 min). Treatment with PMA also resulted in translocation of protein kinase C activity from the cytosol to the particulate cell fraction. Pertussis toxin, which attenuates inhibition of adenylate cyclase mediated by the inhibitory guanine nucleotide-binding regulatory protein (Gi), augmented PTH-sensitive adenylate cyclase activity and reduced the incremental increase in PTH response produced by PMA. The results suggest that activation of protein kinase C increases PTH-stimulated adenylate cyclase activity by actions on Gi and/or the catalytic subunit and decreases PGE2 responsiveness by a mechanism involving the PGE2 receptor.

Effect of concanavalin A on intracellular calcium concentration in single blood platelets

Mobilization of Ca++ was estimated in single rabbit blood platelets with digital imaging microscopy. Concanavalin A (Con A) caused a rapid initial increase in intracellular concentration of Ca++ ([Ca++]i) with a latent time of about 20 sec, followed by a sustained increase in [Ca++]i. This effect of Con A was antagonized by alpha-methyl-D-mannose, which already was shown to antagonize the inhibitory effect of Con A on 5-HT transport, indicating that this effect of Con A was also derived from its binding to cell surface glycoproteins. The presence of EGTA in the medium did not affect the initial rise, but inhibited the latter phase of sustained rise. Thus, Con A induced elevation of [Ca++]i was suggested to consist of two different processes: mobilization of Ca++ from the intracellular storage sites and successive Ca++ influx through Ca++ channels. The effect of Con A on the 5-HT transport was tested in the presence of EGTA, a condition where no Ca++ influx occurs. The results indicate that Con A induced inhibition of 5-HT transport was not influenced by EGTA in the medium. It is suggested that the effect of Con A on 5-HT transport might be exerted through the Ca++ mobilization from its intracellular storage sites.

Effect of protein kinase C on amylase secretion and cyclic AMP production in rat pancreatic acinar cells

The authors examined the effects of protein kinase C on secretin-induced amylase release and cyclic AMP production in rat pancreatic acinar cells. Secretin (10(-6) M) and 12-O-tetradecanoyl-phorbol 13-acetate (TPA) (10(-6) M) induced 53% and 60% increase of amylase release from the basal level, respectively during 10 min. Simultaneous addition of TPA and secretin resulted in 42% amylase release from the basal level for 10 min. Suppression of secretin-induced amylase release was evident within 5 min of pretreatment with TPA. TPA showed the same effect on cyclic AMP production; secretin-induced increase of cyclic AMP was suppressed by pretreatment of TPA for 5 min. To explore the mechanism by which TPA inhibits secretin-induced cyclic AMP production, we also examined the effects of protein kinase C purified from rat brain on adenylate cyclase activity in pancreatic acinar membranes. Basal, forskolin- and secretin plus guanosine 5'-[gamma-thio]trisphosphate-stimulated adenylate cyclase activity were inhibited by protein kinase C in the presence of Ca++. These results suggest that protein kinase C might have a role in the inhibitory effect on adenylate cyclase in exocrine pancreas.