Effects of activation of protein kinase C on the agonist-induced stimulation and inhibition of cyclic AMP formation in intact human platelets

Neurochemical and morphological studies on differentiation of NG108-15 cells by phorbol ester and forskolin

12-O-tetradecanoylphorbol 13-acetate (TPA), forskolin or dibutyryl cAMP induced neurite outgrowth and inhibition of cell growth in NG108-15 cells. TPA, forskolin and dibutyryl cAMP significantly increased specific activity of choline acetyltransferase. Forskolin markedly stimulated cAMP accumulation, but not TPA, suggesting that forskolin could induce differentiation by increasing the cAMP content via adenylate cyclase activation, but TPA-induced differentiation seems not to be due to the raise of the cAMP level. Incubation of the cells with TPA, forskolin or dibutyryl cAMP for 24 h resulted in enhancement of 50 mM K(+)-evoked Ca(2+) influx and neurite elongation, although incubation with these agents for 1 h didn't affect these events. From these results, it is suggested that TPA and forskolin induce differentiation of NG108-15 cells to acetylcholine neurons via different mechanisms: protein kinase C activation by TPA and cAMP-dependent protein kinase activation by forskolin. In addition, it is likely that Ca(2+) channels in cells differentiated by TPA, forskolin or dibutyryl cAMP become sensitive to depolarization.

Potentiation of ganodermic acid S on prostaglandin E(1)-induced cyclic AMP elevation in human platelets

Ganodermic acid S (GAS), isolated from the Chinese medicinal fungus Ganoderma lucidum (Fr.) Karst (Polyporaceae), exhibits inhibitory effects on platelet responses to various aggregating agonists. Our study demonstrated that GAS also participated in potentiating the response of human gel-filtered platelets to prostaglandin (PG) E(1). GAS at <20 microM did not show any significant change of basal cyclic AMP level in gel-filtered platelets. However, GAS potentiated the PGE(1)-evoked cyclic AMP level in a bell-shaped, concentration-dependent manner. The agent at 7.5 microM enhanced the level up to 1.8-fold of that evoked by PGE(1) alone. Collagen did not inhibit the PGE(1)-induced cyclic AMP level in platelets pretreated with GAS at 6 to 7.5 microM. In the presence of 7.5 microM GAS, the agent enhanced the inhibition of PGE(1) on platelet response to collagen in: phosphorylation of myosin light chain and pleckstrin; alpha-granule secretion; cell aggregation and protein-tyrosine phosphorylation. In addition, the agent along with PGE(1) almost abolished the dense-granule secretion and thromboxane (TX) B(2) formation. The results suggest that GAS played an additional role in potentiating the PGE(1)-induced cyclic AMP synthesis. GAS and PGE(1) inhibited additively the platelet response to collagen.

Altered postreceptor signal transduction of formyl-Met-Leu-Phe receptors in polymorphonuclear leukocytes of patients with non-insulin-dependent diabetes mellitus

The signal transduction of the formyl-Met-Leu-Phe (FMLP) receptor in polymorphonuclear leukocytes (PMNLs) from patients with non-insulin-dependent diabetes mellitus (NIDDM) was compared to that of PMNLs obtained from healthy volunteers. According to our previous studies in this group of patients neither the decrease in insulin binding capacity nor the enhanced insulin-degrading enzyme activity was involved. In control PMNLs, 10 nM FMLP induced a pertussis toxin-sensitive increase in phosphatidyl inositol (PI) cleavage and a subsequent Ca2+ signaling from the intracellular pools. On the other hand, the FMLP-induced protein kinase C (PKC) activation and translocation into the membrane could not be detected in these cells via the measurement of 32P incorporation into histone. In contrast, in PMNLs of this special group of patients suffering from NIDDM the FMLP stimulus produced a significantly low increase in PI cleavage and Ca2+ signaling from the intracellular pools. Moreover, in resting PMNLs of these patients with NIDDM, not only the [Ca2+]i but also the membrane-bound PKC activity was found to be significantly increased. In addition, PKC translocation into the cell membrane of diabetic PMNLs could be further triggered with FMLP as judged by the measurement of 32P incorporation into histone. Based on these results, it appears that the signaling of FMLP receptors in PMNLs of some NIDDM patients may have an alternative pathway through Ca2+ influx from extracellular medium, arachidonic acid cascade, and PKC activation.

Regulation of platelet glycoprotein IIb/IIIa (integrin alpha IIB beta 3) function via the thrombin receptor

Binding sites on glycoprotein (GP) IIb/IIIa exposed by 0.5 unit/ml alpha-thrombin are insensitive to prostaglandin I2 (PGI2), in contrast with sites exposed by ADP or platelet-activating factor. Here we show that the thrombin receptor agonist peptide (TRAP) (SFLLRN; 15 microM) opens almost the same number of GPIIb/IIIa molecules as 0.5 unit/ml alpha-thrombin (64840 +/- 8920 compared with 81050 +/- 6030 molecules of fibronectin bound/platelet), but these sites rapidly close on addition of PGI2. To investigate whether alpha-thrombin and TRAP initiate different signalling pathways, we measured phospholipase C (PLC)-mediated control of GPIIb/IIIa and its sensitivity to cyclic AMP. Optimal concentrations of alpha-thrombin and TRAP activated PLC maximally, but TRAP induced only about 50% protein kinase C PKC) activation after 10 min stimulation compared with alpha-thrombin. These concentrations also suppressed PGI2-induced cyclic AMP accumulation, with alpha-thrombin inducing complete inhibition and TRAP about 10% less. Direct activation of PKC by phorbol 12-myristate 13-acetate confirmed earlier observations that PGI2-induced cyclic AMP accumulation is partly inhibited via PKC. Applying different concentration of alpha-thrombin, TRAP or a combination of alpha-thrombin and the thrombin receptor inhibitory peptide (TRIP) (Mpr-F-Cha-Cha-RKPNDK-NH2; 800 microM) (Mpr, 3-mercaptopropionic acid; Cha, cyclohexylalanine), we show that the different means of stimulating the thrombin receptor all suppressed PGI2-induced cyclic AMP accumulation via (i) activation of PKC and (ii) activation of the heterotrimeric G-protein, Gi. We conclude that complete inhibition of cyclic AMP accumulation requires activation of both PKC and Gi, as observed with 0.5 unit/ml alpha-thrombin. Although TRAP almost fully exposes GPIIb/IIIa, its activation of PKC is incomplete, enabling PGI2 to raise cyclic AMP concentration from 1.4 +/- 0.7 to 4.1 +/- 1.3 nmol/10(11) platelets (P < 0.005) which is sufficient to close exposed GPIIb/IIIa molecules.

Potentiation of adenylyl cyclase in human peripheral blood mononuclear cells by cell-activating stimuli

The isoprenaline-induced production of cAMP in human peripheral blood mononuclear cells (PBMC) was potentiated significantly by incubating PBMC with isoprenaline in the presence of phytohaemagglutinin (PHA), Concanavalin A (Con A) or A23187. This potentiation, that proved to be dependent on the concentration of PHA, Con A or A23187, increased the maximal response but did not cause a change in the potency of isoprenaline. Potentiation could not be induced by the phorbol ester phorbol-myristate acetate, suggesting that protein kinase C-dependent pathways are not likely to be involved in potentiation of adenylyl cyclase. Potentiation could be inhibited by chelating extracellular Ca2+ with EGTA and also by N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamine, an inhibitor of calmodulin. Potentiation could not be induced by preincubation of PBMC with PHA, suggesting that transient biochemical changes are involved. It was concluded from these results that potentiation in PBMC probably involves the activation of Ca2+/calmodulin-dependent adenylyl cyclase subtypes. Potentiation of the adenylyl cyclase activity could be an important physiological mechanism in vivo preventing cells from becoming "over stimulated".

Heterologous sensitization of adenylate cyclase activity by serotonin in the rat cerebral cortex

In vitro exposure of rat cerebrocortical slices to microM concentrations of serotonin (5HT) results in an increased response of adenylate cyclase to isoproterenol (ISO). No change in the affinity of the beta-adrenoceptor toward the agonist was found after 5HT exposure when measuring ISO displacement of [3H]CGP 12177 binding. A similar increase of adenylate cyclase response was also found when using VIP as a stimulatory agent. The dose-response curve of adenylate cyclase to the GTP analogue, GppNHp, was modified by 5HT, which promotes a significantly higher maximal response without altering the potency of GppNHp. Forskolin-stimulated adenylate cyclase activity was not affected by 5HT. Serotonergic 5HT2 receptors are involved in the sensitization of adenylate cyclase to GppNHp, since the selective 5HT2 antagonist ketanserin inhibits the effect of 5HT, whereas the 5HT2 agonist DOI mimics 5HT. The involvement of 5HT2 receptor-coupled activation of protein kinase C is also demonstrated: direct protein kinase C activators such as phorbol esters and s,n-dioctanoylglycerol behave in the same manner as 5HT, while the protein kinase C inhibitor CGP 41251 prevents 5HT from increasing adenylate cyclase responsiveness to GppNHp. Moreover, in vitro exposure of cortical slices to 5HT results in reduced inhibition of adenylate cyclase by somatostatin. Since no change was observed at the receptor level and in the direct stimulation of the catalytic subunit of the enzyme, we propose that 5HT might accomplish the sensitization of adenylate cyclase through protein kinase C by inactivating the inhibitory coupling protein Gi and facilitating the interaction of the exogenous GppNHp with the stimulatory coupling protein Gs.

Purification and characterization of three distinct types of protein phosphatase catalytic subunits in bovine platelets

The catalytic subunits of bovine platelet protein phosphatases were separated into three distinct forms by chromatography on heparin-Sepharose. Each phosphatase was further purified to apparent homogeneity as judged in sodium dodecyl sulfate-polyacrylamide gel yielding single protein bands of 37, 41, and 36 kDa. The 37-kDa phosphatase was excluded from heparin-Sepharose and preferentially dephosphorylated the alpha-subunit of phosphorylase kinase. It was stimulated by polycations (polybrene or histone H1) and was inhibited by okadaic acid (IC50 = 0.3 nM), but its activity was not influenced by inhibitor-2 or heparin. The 41-kDa phosphatase was eluted from heparin-Sepharose by 0.20-0.25 M NaCl and preferentially dephosphorylated the beta-subunit of phosphorylase kinase. It was stimulated by polycations and inhibited by okadaic acid (IC50 = 2 nM), but its activity was not affected by inhibitor-2 or heparin. The 36-kDa phosphatase was eluted from heparin-Sepharose by 0.45-0.50 M NaCl and preferentially dephosphorylated the beta-subunit of phosphorylase kinase. It was inhibited by inhibitor-2, heparin, histone H1, and okadaic acid (IC50 = 70 nM). The 37- and 36-kDa phosphatases can be classified as type-2A and type-1 enzymes, respectively. The 41-kDa phosphatase does not precisely fit the criteria of either type, showing only partial similarities to both type-1 and type-2A enzymes and it may represent a novel type of protein phosphatase in bovine platelets.

ADP receptor-induced activation of guanine-nucleotide-binding proteins in human platelet membranes

ADP receptor-regulated binding of the labeled GTP analog, guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTP[gamma S]), to guanine-nucleotide-binding proteins (G proteins) was studied in human platelet membranes. The potent ADP receptor agonist, 2-methyl-thio-adenosine 5'-diphosphate (2MeSADP), a non-hydrolyzable analog of ADP, increased the binding of [35S]GTP[gamma S] without apparent lag phase. Under optimal conditions, i.e. in the presence of GDP (1-10 microM), 2MeSADP increased the binding up to about threefold, with half-maximal and maximal increase observed at 10 nM and 1 microM 2MeSADP, respectively. ADP itself increased the binding of [35S]GTP[gamma S] by maximally about twofold, with half-maximal increase occurring at 0.1 microM ADP. The agonist-induced stimulation was competitively antagonized by the ADP receptor(s) antagonist, (1S)-adenosine 5'-O-(1-thiotriphosphate) [(Sp)-ATP[alpha S]]. Other platelet receptor agonists known to act through receptors coupled to G proteins also increased binding of [35S]GTP[gamma S] in human platelet membranes, but without being inhibited by (Sp)-ATP[alpha S]. The data presented indicate that the platelet ADP receptor(s) can interact with and efficiently activate G proteins, the nature of which remains to be identified.

Phosphorylation of the inhibitory guanine-nucleotide-binding protein as a possible mechanism of inhibition by protein kinase C of agonist-induced Ca2+ mobilization in human platelet

Increases in the intracellular Ca2+ concentration of human platelets caused by receptor agonists, such as thrombin, 9,11-epithio-11,12-methanothromboxane A2 (STA2), platelet-activating factor (PAF) and arginine-vasopressin, were inhibited by prior addition of 12-O-tetradecanoylphorbol 13-acetate (TPA) in time-dependent and concentration-dependent manners. The inhibitions were mostly reversed by staurosporine, and inhibitor of protein kinase C, added 1 min before TPA. Prior treatment of platelets with thrombin or STA2, the efficacious Ca2+ mobilizer, suppressed the increase in the intracellular Ca2+ concentration of the cells to other agonists, but treatment with less efficacious PAF or vasopressin did not. The heterologous receptor desensitizations were also reversed by staurosporine. The antibody, directed against the carboxy-terminal region of the alpha subunits 1 and 2 of the inhibitory guanine-nucleotide-binding proteins (Gi1 alpha and Gi2 alpha), was raised in rabbit and was used to immunoprecipitate Gi alpha in 32P-labeled platelets. The radioactivity was detected in Gi alpha after incubation of 32P-labeled platelets with TPA, thrombin or STA2, but not in the cells incubated with PAF or vasopressin. The time-dependency or concentration-dependency of TPA-induced phosphorylation of Gi alpha was similar to the dependency of its inhibitory action on agonist-induced Ca2+ mobilization. Thus, strong activation of Ca2+/phospholipid-dependent protein kinase C by phorbol ester or agonists of certain Ca(2+)-mobilizing receptors leads to phosphorylation of the alpha subunit of guanine-nucleotide-binding protein, thereby impairing the coupling of the G protein to receptors as a feedback regulatory component of the receptor-triggered intracellular Ca(2+)-mobilizing system.

Thromboxane receptor stimulation inhibits adenylate cyclase and reduces cyclic AMP-mediated inhibition of ADP-evoked responses in fura-2-loaded human platelets

Stimulation of human platelets with the thromboxane A2 analogue, U46619, after treatment with prostaglandin E1 or forskolin, reduced the inhibition of ADP-evoked Mn2+ influx and the release of Ca2+ from intracellular stores. U46619 decreased the elevated concentration of 3',5'-cyclic AMP in platelets that were pretreated with prostaglandin E1. These results suggest that occupation of prostaglandin H2/thromboxane A2 receptors, like those for other agonists, inhibits adenylate cyclase activity, which can contribute to the promotion of platelet activation.

Prostaglandin-concentration-dependent desensitization of adenylate cyclase in human erythroleukaemia (HEL) cells is abolished by pertussis toxin and enhanced by induction by dimethyl sulphoxide

Prostaglandin-regulated cyclic AMP metabolism in human erythroleukaemia (HEL) cells was similar to that previously described in platelets [Ashby (1989) Mol. Pharmacol. 36, 866-873], displaying prostaglandin-concentration-dependent desensitization that could be explained by the presence of separate stimulatory and inhibitory prostaglandin receptors. Pertussis toxin abolished prostaglandin-concentration-dependent desensitization, indicating that the process is mediated through a pertussis toxin-sensitive GTP-binding protein. Treatment of HEL cells for 4 days with the inducer dimethyl sulphoxide enhanced prostaglandin-concentration-dependent desensitization, but did not alter the initial rate of cyclic AMP synthesis or the amount of Gi2 alpha measured by immunoblotting, suggesting that the inhibitory receptor was selectively induced by changing the cells to a more platelet-like form.

Elevation of platelet cyclic AMP level by thromboxane A2/prostaglandin H2 receptor agonists

Arachidonic acid (AA)- or thromboxane A2/prostaglandin H2 (TXA2/PGH2) analog (STA2 and U-46619)-induced aggregations yielded a bell-shaped dose-response curve. The inhibitory mechanism by high concentrations of the agonists was examined. STA2 elevated cAMP level of platelet in a dose-dependent manner. And the aggregation was affected by metabolic inhibitors of cAMP. AA also rised cAMP level, and the rise was suppressed by indomethacin. These results indicate that the reduction of aggregation by high dose of the agonists is through cAMP elevation. The cAMP elevation was not suppressed by ruling out phospholipase C effects by chelation of cytoplasmic Ca2+ and inhibition of protein kinase C (PKC). These results suggest that the cAMP elevation is not due to activation of phospholipase C-linked TXA2/PGH2 receptor. 13-APA, an antagonist of TXA2/PGH2 receptor, suppressed the cAMP elevation, although ONO-3708, another antagonist, had no effect. As to be expected from this result, inhibitory effect of 13-APA on high STA2 level-induced aggregation was weaker than that of ONO-3708. The antagonists did not inhibit PGE1- or PGD2-induced cAMP elevation. These findings suggest that platelet has adenylate cyclase-linked TXA2/PGH2 receptor.

Effect of triggering epidermal Fc gamma receptors on the interleukin-2- and interleukin-6-induced upregulation of Ia antigen expression by murine epidermal Langerhans cells: the role of prostaglandins and cAMP

Following incubation of murine epidermis in medium containing either interleukin-2 or interleukin-6, there is significant upregulation in the density of Ia+ epidermal Langerhans cells (to 159% and 175% of control, respectively). This cytokine-induced upregulation is abrogated by either rabbit or human IgG due to triggering of Fc gamma receptors. In contrast, human IgA does not inhibit the effect of interleukin-2 or interleukin-6. Using different isotypes of murine IgG, we have demonstrated that all subclasses are capable of inhibiting the cytokine-induced enhancement of Ia antigen, although IgG1 and IgG2b must be heat aggregated to be effective. The IgG-mediated events are dependent on prostaglandin synthesis because they can be blocked by the cyclooxygenase inhibitor indomethacin, 10 micrograms/ml. The responsible PG appears to be PGD2; in contrast to its known inhibitory effect on macrophages, PGE2 does not inhibit the upregulation of Ia antigen on Langerhans cells. In addition, these IgG-mediated events are dependent upon the generation of cAMP because they can be blocked by the adenylate cyclase inhibitor 2',5'-dideoxyadenosine, 1 mM. Despite the apparently central role of PGD2 and cAMP in this process, triggering of the Fc gamma R by different isotypes of IgG blocks upregulation of Ia via at least two different pathways. The inhibition caused by aggregated IgG1 or IgG2b, which bind to Fc gamma RII on Langerhans cells, is abrogated by para-bromophenacylbromide, an inhibitor of phospholipase A2. In contrast, the inhibition caused by monomeric IgG2a, which binds to Fc gamma RI most likely on keratinocytes, or monomeric IgG3, which probably binds to this same Fc gamma RI, is abrogated by staurosporine, an inhibitor of protein kinase C, as well as by W7, a calmodulin antagonist. Finally, 1,2 dioctanoyl-rac-glycerol, an activator of protein kinase C, mimics the Ig-mediated events. Based on these findings, as well as studies using monoclonal antibodies to the murine Fc gamma receptors I and II, we conclude that, as is the case in murine macrophages, triggering of an epidermal Fc gamma RI, most likely on keratinocytes, results in the generation of cAMP via a Ca(++)-dependent protein kinase C pathway, whereas triggering of an epidermal Fc gamma RII, most likely on Langerhans cells, results in the elevation of cAMP via a phospholipase A2-mediated pathway. In contrast to the situation for macrophages, PGD2 is a vital intermediate in both pathways, perhaps because Langerhans cells have receptors for only this prostaglandin.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Chemical and functional analysis of components of adenylyl cyclase from human platelets treated with phorbolesters

Human platelets, prelabeled with [32P]phosphate were treated with tetradecanoylphorbol acetate (TPA) for 5 min at 37 degrees C. Phosphorylation of the components of adenylyl cyclase was determined in membranes using specific antibodies against G-proteins and the catalytic moiety. Less than 0.01 mol of [32P]phosphate/mol could be detected in immunoprecipitates using antibodies against sequences within the alpha-subunit of the GTP binding protein Gi. TPA, however, caused the incorporation of 0.67-1.1 mol of [32P]phosphate per mol of catalyst while 0.13-0.2 mol were found in the absence of TPA. Lack of modification of the alpha-subunit of Gi was also indicated by the results of reconstitution experiments with purified Gi alpha from bovine brain: adenylyl cyclase in membranes from untreated platelets was significantly more inhibited by added G1 alpha, than that from TPA treated cells. While beta, gamma-subunits were like-wise inhibitory no difference dependent on platelet-pretreatment could be observed.

Enhancement of cyclic AMP metabolism in a B cell line by protein kinase C

In a human B cell line in which we previously demonstrated an inverse relationship between cyclic adenosine monophosphate (cAMP) content and immunoglobulin secretion, the phorbol ester, phorbol myristate acetate, (PMA), was shown to augment the cAMP elevating ability of cholera toxin (CT), suggesting a regulatory linkage between the two transmembrane signaling pathways, cAMP and phospholipid (J. Immunol. 141, 1678-1686, 1988). We now extend these studies and provide additional evidence that activated protein kinase C, a principal product of the activation of the hydrolytic phospholipid pathway, plays a direct role in the augmentation of cAMP levels in cells stimulated by diverse cAMP-elevating ligands. Prostaglandin E1 (PGE1), forskolin (FSK) and CT, all of which demonstrated a concentration and time-dependent elevation of intracellular cAMP, produced even greater (up to twofold) elevations of cAMP in the presence of PMA or the diacylglycerol analogs, 1,2-dioctanoylglycerol (DiC8), and 1-oleoyl-2-acetylglycerol (OAG). In the absence of CT, PGE1, or FSK, these protein kinase C activators produced only small increases in cAMP content of the cells. Several tests of protein kinase C specificity in these PMA-, DiC8-, and OAG-induced augmentations were made: (i) only phorbol esters known to activate protein kinase C worked, (ii) PMA augmentation was abolished by down-regulation of protein kinase C, (iii) Staurosporine (a known inhibitor of protein kinase C) selectively inhibited the effects of PMA on cAMP generation and on immunoglobulin secretion in the LA350 cell line.

Cyclic adenosine monophosphate and diacylglycerol. Mutually inhibitory second messengers in cultured rat inner medullary collecting duct cells

Studies were performed to examine interactions between the adenylyl cyclase (AC) and phospholipase C (PLC) signaling systems in cultured rat inner medullary collecting duct cells. Stimulation of AC by either arginine vasopressin (AVP) or forskolin or addition of exogenous cAMP inhibits epidermal growth factor (EGF)-stimulated PLC. This inhibition is mediated by activation of cAMP-dependent kinase as it is prevented by pretreatment with the A-kinase inhibitor, N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide (H8) but not by the C-kinase inhibitor, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7). Exposure to EGF eliminates AVP-stimulated cAMP generation. This is not mediated by a cyclooxygenase product as inhibition by EGF is observed even in the presence of the cyclooxygenase inhibitor, flurbiprofen. Inhibition by EGF is not due to an increase in inositol trisphosphate (IP3) as exposure of saponin-permeabilized cells to exogenous IP3 is without effect. Inhibition by EGF is prevented by pretreatment with the C-kinase inhibitor, H7, but not by the A-kinase inhibitor, H8. Exposure to the synthetic diacylglycerol (DAG), dioctanoylglycerol, also inhibits AVP-stimulated AC activity; therefore, inhibition by EGF is due to activation of protein kinase C. Thus, in cultured rat inner medullary collecting duct cells, cAMP and DAG function as mutually inhibitory second messengers with each impairing formation of the other.

12-O-tetradecanoylphorbol 13-acetate and forskolin modify muscarinic receptor-linked Ca2+ mobilization in SH-SY5Y neuroblastoma cells through different mechanisms

The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), which causes differentiation of SH-SY5Y neuroblastoma cells, reduces carbachol binding and carbachol-stimulated Ca2+ mobilization in these cells. The decrease in responsiveness to carbachol is due partially to a reduction in the amount of Ca2+ released by the cells and partially to a decrease in the sensitivity of the cells to carbachol. These effects probably can be attributed to a reduction in muscarinic receptor number and a decrease in receptor affinity, respectively. Forskolin, an alkaloid known to cause an increase in cellular cyclic AMP, enhances Ca2+ influx into the cells without affecting the cytosolic free Ca2+ concentration. The alkaloid causes an apparent restoration of the reduced Ca2+ release, caused by TPA, but does not affect the sensitivity of the cells to carbachol. Forskolin increases the decay of carbachol-induced increase in cytosolic Ca2+. The effects of TPA appear to be linked directly to receptor function, whereas those of forskolin are due to the effect of cyclic AMP on cellular Ca2+ metabolism.

Platelet aggregation induced by alpha 2-adrenoceptor and protein kinase C activation. A novel synergism

Adrenaline or UK 14304 (a specific alpha 2-adrenoceptor agonist) and phorbol ester (phorbol 12,13-dibutyrate; PdBu) or bioactive diacylglycerols (sn-1,2-dioctanoylglycerol; DiC8) synergistically induced platelet aggregation and ATP secretion. The effect on aggregation was more pronounced than the effect on secretion, and it was observed in aspirinized, platelet-rich plasma or suspensions of washed aspirinized platelets containing ADP scavengers. No prior shape change was found. In the presence of adrenaline, DiC8 induced reversible aggregation and PdBu evoked irreversible aggregation that correlated with the different kinetics of DiC8- and PdBu-induced protein kinase C activation. Adrenaline and UK 14304 did not induce or enhance phosphorylation induced by DiC8 or PdBu of myosin light chain (20 kDa), the substrate of protein kinase C (47 kDa), or a 38 kDa protein. Immunoprecipitation studies using a Gcommon alpha antiserum or a Gi alpha antiserum showed that Gi alpha is not phosphorylated after exposure of platelets to PdBu or PdBu plus adrenaline. Adrenaline, PdBu or adrenaline plus PdBu did not cause stimulation of phospholipase C as reflected in production of [32P]phosphatidic acid. Adrenaline caused a small increase of Ca2+ in the platelet cytosol of platelets loaded with Indo-1; this effect was also observed in the absence of extracellular Ca2+. However, under conditions of maximal aggregation induced by adrenaline plus PdBu, no increase of cytosolic Ca2+ was observed. Platelet aggregation induced by PdBu plus adrenaline was not inhibited by a high intracellular concentration of the calcium chelator Quin-2. These experiments indicate that alpha 2-adrenoceptor agonists, known to interact with Gi, and protein kinase C activators synergistically induced platelet aggregation through a novel mechanism. The synergism occurs distally to Gi protein activation and protein kinase C-dependent protein phosphorylation and does not involve phospholipase C activation or Ca2+ mobilization.

Effect of phorbol ester treatment on receptor-mediated versus G-protein-activator-mediated responses in platelets. Evidence for a two-site action of phorbol ester at the level of G-protein function

In platelets, and in several other cell systems, pre-treatment with protein kinase C activators such as phorbol 12-myristate 13-acetate (PMA) results in the inhibition of receptor-mediated responses, suggesting that protein kinase C may play an important role in the termination of signal transduction. In the present study, we have attempted to locate the site of action of phorbol ester by comparing thrombin-induced (i.e. receptor-mediated) platelet activation with that induced by guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and NaF, two agents which by-pass the receptor and initiate platelet responses by directly modulating G-protein function. After a 10 s pre-treatment with PMA (16 nM), dense-granule secretion induced by thrombin (0.2 unit/ml), GTP[S] (40 microM) and NaF (30 mM) was potentiated, resulting in a greater than additive response to agent plus PMA. However, after a 5 min pre-treatment, thrombin-induced secretion alone was inhibited, whereas PMA plus GTP[S]/NaF-induced release remained greater than additive. [32P]Phosphatidate formation in response to all three agents, in contrast, was inhibited by 50-70% in PMA (5 min)-treated platelets. That secretion induced by these agents is a protein kinase C-dependent event was demonstrable by using staurosporine, a protein kinase C inhibitor which at concentrations of 1-10 nM inhibited (70-90%) PMA-induced as well as thrombin- and NaF-induced secretion and protein phosphorylation. In membranes from PMA-treated platelets, thrombin-stimulated GTPase activity was significantly enhanced compared with that in untreated membranes (59% versus 82% increase over basal activity). The results suggest that inhibition of receptor-mediated responses by PMA may be directed towards two sites relating to G-protein activation: (i) receptor-stimulated GTPase activity and (ii) G-protein-phospholipase C coupling. Furthermore, the lack of inhibition of NaF- and GTP[S]-induced secretion by PMA suggests that different mechanisms may be involved in thrombin-induced and G-protein-activator-induced secretion.

Prostacyclin inhibits platelet aggregation induced by phorbol ester or Ca2+ ionophore at steps distal to activation of protein kinase C and Ca2+-dependent protein kinases

Suspensions of aspirin-treated, 32P-prelabelled, washed platelets containing ADP scavengers in the buffer were activated with either phorbol 12,13-dibutyrate (PdBu) or the Ca2+ ionophore A23187. High concentrations of PdBu (greater than or equal to 50 nM) induced platelet aggregation and the protein kinase C (PKC)-dependent phosphorylation of proteins with molecular masses of 20 (myosin light chain), 38 and 47 kDa. No increase in cytosolic Ca2+ was observed. Preincubation of platelets with prostacyclin (PGI2) stimulated the phosphorylation of a 50 kDa protein [EC50 (concn. giving half-maximal effect) 0.6 ng of PGI2/ml] and completely abolished platelet aggregation [ID50 (concn. giving 50% inhibition) 0.5 ng of PGI2/ml] induced by PdBu, but had no effect on phosphorylation of the 20, 38 and 47 kDa proteins elicited by PdBu. The Ca2+ ionophore A23187 induced shape change, aggregation, mobilization of Ca2+, rapid phosphorylation of the 20 and 47 kDa proteins and the formation of phosphatidic acid. Preincubation of platelets with PGI2 (500 ng/ml) inhibited platelet aggregation, but not shape change, Ca2+ mobilization or the phosphorylation of the 20 and 47 kDa proteins induced by Ca2+ ionophore A23187. The results indicate that PGI2, through activation of cyclic AMP-dependent kinases, inhibits platelet aggregation at steps distal to protein phosphorylation evoked by protein kinase C and Ca2+-dependent protein kinases.

Thrombin exerts a dual effect on stimulated adenylate cyclase in hamster fibroblasts, an inhibition via a GTP-binding protein and a potentiation via activation of protein kinase C

Previous studies in Chinese-hamster fibroblasts (CCL39 line) indicate that an important signalling pathway involved in thrombin's mitogenicity is the activation of a phosphoinositide-specific phospholipase C, mediated by a pertussis-toxin-sensitive GTP-binding protein (Gp). The present studies examine the effects of thrombin on the adenylate cyclase system and the interactions between the two signal transduction pathways. We report that thrombin exerts two opposite effects on cyclic AMP accumulation stimulated by cholera toxin, forskolin or prostaglandin E1. (1) Low thrombin concentrations (below 0.1 nM) decrease cyclic AMP formation. A similar inhibition is induced by A1F4-, and both thrombin- and A1F4- -induced inhibitions are abolished by pertussis toxin. (2) Increasing thrombin concentration from 0.1 to 10 nM results in a progressive suppression of adenylate cyclase inhibition and in a marked enhancement of cyclic AMP formation in pertussis-toxin-treated cells. A similar stimulation is induced by an active phorbol ester, and thrombin-induced potentiation of adenylate cyclase is suppressed by down-regulation of protein kinase C. Therefore, we conclude that (1) the inhibitory effect of thrombin on adenylate cyclase is the direct consequence of the activation of a pertussis-toxin-sensitive inhibitory GTP-binding protein (Gi) possibly identical with Gp, and (2) the potentiating effect of thrombin on cyclic AMP formation is due to stimulation of protein kinase C, as an indirect consequence of Gp activation. Our results suggest that the target of protein kinase C is an element of the adenylate cyclase-stimulatory GTP-binding protein (Gs) complex. At low thrombin concentrations, activation of phospholipase C is greatly attenuated by increased cyclic AMP, leading to predominance of the Gi-mediated inhibition.

Modulation of octopamine-mediated production of cyclic AMP by phorbol-ester-sensitive protein kinase C in an insect cell line

The presence of protein kinase C (EC 2.7.1.37) in an insect cell line has been demonstrated. Phorbol 12-myristate 13-acetate (PMA), in micromolar concentrations, activated protein kinase C with a translocation of the enzyme from the cytosol to the particulate fraction. Cyclic AMP production in the presence of PMA, octopamine and a combination of both increased in a dose-dependent and time-dependent fashion. The biologically inactive 4 alpha-phorbol 12,13-didecanoate had no effect on protein kinase C activity or on octopamine-mediated cyclic AMP production. Pretreatment of the cells with pertussis toxin had no effect on the response of cells to octopamine or PMA. However, pretreatment with cholera toxin resulted in increased cyclic AMP production which was further enhanced when both cholera toxin and PMA were used in combination. Our data indicate that the octopamine-mediated cyclic AMP production is modulated by protein kinase C.

Effect of phorbol 12-myristate 13-acetate on collagen-induced signal transduction in rabbit platelet

Investigations were made on the inhibitory effect of phorbol 12-myristate 13-acetate (PMA), a powerful activator on protein kinase C, on collagen-induced signal transduction in washed rabbit platelets. Upon activation of the platelets with a low-dose of collagen (5 micrograms/ml), which was suppressed by 10 microM indomethacin, pretreatment of the platelets with 2 nM PMA caused prolongation of lag phase (2 min) before the onsets of the aggregation and ATP secretion as compared with PMA-untreated platelets (30 sec). Under this condition, appearance of the cell responses including the phosphatidic acid formation, thromboxane (Tx) generation and Ca2+-influx was similarly retarded for 2-3 min, whereas arachidonic acid liberation from the membrane phospholipids was not significantly affected by the PMA pretreatment. After such lag phase, every response appeared rapidly and reached almost the control value (without PMA). Upon activation of the same platelets with a high-dose of collagen (50 micrograms/ml), which was only half suppressible by indomethacin, PMA in the presence of indomethacin almost completely suppressed the phosphatidic acid formation as well as the aggregation and ATP secretion. Thus, our results suggest that collagen-platelet interaction may elicit direct activation of phospholipase A2 and C, and that the latter enzyme activation may be regulated by a negative effect of protein kinase C. However, the phospholipase A2 activation may be regulated by a mechanism independent of such effect. In PMA-pretreated platelets in response to a low-dose of collagen, the prolonged lag phase for aggregation appears to be due to impaired conversion of liberated arachidonic acid to TxA2.

Prostaglandin E1: a review

Work on the structure of prostaglandin E1 (PGE1), isolated from natural sources, was completed 25 years ago (1). Shortly after, methods for the chemical synthesis of PG with their natural configuration were developed in the laboratories of the UpJohn Company (2) and of E. J. Corey (3) and, by the late sixties, PGE1 became widely available. The information since accumulated about its biological and clinical effects is more substantial than for any other PG. This review will draw together some of this information, focusing on recent studies of its mechanisms of action.

Protein kinase C potentiates isoproterenol-mediated cyclic AMP production without modifying the homologous desensitization process in J774 cells

The J774 murine macrophage cells possess a beta 2-adrenergic receptor coupled to adenylate cyclase, which can be regulated by homologous desensitization. Stimulation of protein kinase C by phorbol esters or oleoyl acetyl glycerol potentiates two-to-threefold the isoproterenol-induced cyclic AMP accumulation. These promoters act at a post-receptor level, since the number and affinity of the beta-adrenergic receptors, measured by use of the hydrophilic ligand [3H]CGP-12177, are not modified. In addition, the effect of cholera toxin is similarly increased and pretreatment of the cells with pertussis toxin prevents the action of phorbol esters. On the other hand, these promoters are ineffective on isoproterenol-induced desensitization and the rates of receptor segregation and recovery remain unchanged. Therefore, protein kinase C modulates the isoproterenol-stimulated adenylate cyclase, whereas it is inactive on the homologous desensitization process.