'Crosstalk': a pivotal role for protein kinase C in modulating relationships between signal transduction pathways

Effects of staurosporine on ACTH-mediated stimulation of aldosterone production

Incubation of rat adrenal glomerulosa cells with low concentrations (up to 50 nM) of the protein kinase (PKC) inhibitor staurosporine (ST) inhibited aldosterone (ALDO) and cyclic AMP (cAMP) production stimulated by adrenocorticotropic hormone (ACTH) and cholera toxin. Only higher concentrations (1.6 microM) of staurosporine inhibited dibutyryl-cAMP- and forskolin-induced stimulation of aldosterone production. cAMP levels were increased only with low concentrations of the PKC inhibitor. This latter increase was avoided by treatment with a maximal concentration of isobutylmethylxanthine (MIX). Our results suggest that: (1) second messengers other than cAMP are involved in ACTH action; (2) staurosporine inhibits different kinases involved in ACTH action in a dose-dependent manner; (3) the protein kinase inhibited by high concentrations of staurosporine appears to be the cAMP-dependent kinase, PKA; and (4) the protein kinase inhibited by low concentrations of staurosporine remains to be identified. This latter species is suggested as being involved in mediating ACTH-induced activation of Gs.

Regulation of type V adenylyl cyclase by PMA-sensitive and -insensitive protein kinase C isoenzymes in intact cells

Abstract Type V adenylyl cyclase (AC) was stably over-expressed in HEK293 cells (293AC-V). Forskolin-stimulated cAMP accumulation in 293AC-V was 5 times as great as that in control cells. PMA, a protein kinase C (PKC) activator, enhanced cAMP accumulation in 293AC-V cells dose-and time-dependently and this enhancement was abolished by staurosporine. Insulin also enhanced cAMP accumulation in 293AC-V cells. Co-transfection of PKC-zeta, but not PKC-alpha, potentiated the effects of insulin. These data suggest that type V AC activity is regulated in cells by PKC isoenzymes through different extracellular stimuli.

Streptozotocin-induced diabetes elicits the phosphorylation of hepatocyte Gi2 alpha at the protein kinase C site but not at the protein kinase A-controlled site

Streptozotocin-induced diabetes caused a profound increase in the steady-state level of phosphorylation of the alpha-subunit of the adenylate cyclase inhibitory protein Gi2 in hepatocytes. Unlike hepatocytes from control animals, those from streptozotocin-diabetic animals showed no increase in the phosphorylation of Gi2 alpha in response to a challenge with the protein kinase C activator phorbol myristate acetate. However, a stimulatory effect of 8-bromo-cAMP on Gi2 alpha phosphorylation was evident in hepatocytes from diabetic animals but this was severely reduced compared with that observed in hepatocytes from normal animals. Two-dimensional tryptic phosphopeptide mapping showed that Gi2 alpha in resting hepatocytes from diabetic animals was phosphorylated exclusively at the protein kinase C site (C-site) but no labelling was evident at the protein kinase A-regulated site (AN-site). Treatment of hepatocytes from diabetic animals with phorbol myristate acetate did not change this pattern of labelling. In contrast, challenge of hepatocytes from diabetic animals with 8-bromo-cAMP led to the appearance of a new labelled phosphopeptide that was consistent with labelling at the AN-site. Analysis of the C-site and AN-site phosphopeptides from hepatocytes of diabetic animals treated with 8-bromo-cAMP showed that the increase in labelling of Gi2 alpha caused by this ligand could be attributed almost entirely to labelling at the AN-site. Thus streptozotocin diabetes appears to cause enhanced labelling of hepatocyte Gi2 alpha by exclusively increasing phosphorylation at the C-site. It is suggested that the increased labelling at the C-site reflects an augmentation of the protein kinase C signalling system in hepatocytes from streptozotocin-induced diabetic animals. This may have wide-spread functional consequences for these cells and may result either from an increased protein kinase C activity and/or a reduction in protein phosphatase 1 and/or 2A activity.

Protein kinase C isoforms in iris sphincter smooth muscle: differential effects of phorbol ester on contraction and cAMP accumulation are species specific

Objectives were to identify PKC isoforms in iris sphincter isolated from rabbit, cat, dog and bovine irides, to determine their subcellular distribution, and to investigate the effects of the phorbol ester, PDBu, on contraction and cAMP accumulation in this tissue. Using six isoform (alpha, beta, gamma, epsilon, delta, zeta)-specific polyclonal antibodies, PKC alpha, beta, epsilon, delta, and zeta were detected in the four species, whereas PKC gamma was detected only in dog and bovine. PKC alpha and epsilon are the most abundant isoforms in this tissue. PKC alpha is mainly cytosolic in rabbit and bovine and membrane associated in cat and dog. PKC gamma is equally distributed in cytosol and membrane fractions of bovine, but mostly cytosolic in dog. PKC beta, delta and epsilon are mainly membraneous and PKC zeta is mainly cytosolic in all species. PDBu (100 nM) induced a contractile response in rabbit- and cat-, but not in dog and bovine, sphincters, and increased cAMP accumulation in rabbit, cat, dog and bovine by 111, 130, 458 and 294%, respectively. Therefore, the lack of effect of PDBu on contraction in dog and bovine, as compared to rabbit and cat, may be due: (a) to the presence of PKC gamma isoform, and (b) to the stronger stimulatory effects of the phorbol ester on cAMP production in the non-contracting species. In addition to demonstrating the presence of various PKC isoforms in the iris sphincter and the activation of adenylyl cyclase by this protein kinase, we have shown that the distribution of the PKC isoforms in this tissue is species specific. Furthermore, our data suggest that there may be specific physiological functions associated with each of the PKC isoforms and that PKC is involved in the contractile response of some but not all smooth muscles.

Differential desensitization response of the neonatal and adult rat somatotroph to growth hormone-releasing hormone and phorbol ester

Elevated levels of circulating growth hormone (GH) in the perinatal animal may be caused in part by relative resistance to the desensitizing effects of GH secretagogues. We compared the effects of 4-day exposure of primary pituitary cell cultures from adult male and 2-day-old rats to GH-releasing hormone (GHRH; 10 nM) or 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 1 microM) on subsequent acute GH response to these secretagogues. Prolonged exposure to GHRH reduced subsequent GHRH-induced GH release from pituitary cells of both age groups, but the reduction in GH response was significantly less in neonates than adults. In addition, GH secretion from neonatal pituitaries rose progressively during each day of GHRH exposure, to reach levels almost 7 times basal; by contrast, GH secretion from adult pituitaries increased only transiently and then declined. Prolonged exposure to TPA reduced the subsequent GH response to TPA equally in neonates and adults, but differentially affected the GH response to GHRH; TPA exposure reduced the GH response to GHRH in neonates, but not in adults. These data suggest a fundamental difference between the GH regulatory processes of neonatal and adult pituitaries. The ability of the somatotroph to exhibit attenuated GH response on exposure to secretagogue is developmentally regulated, and relative resistance of the immature somatotroph to homologous desensitization by GHRH may contribute to elevated serum GH levels during the perinatal period.

Signalling mechanisms of endothelin-induced mitogenesis and melanogenesis in human melanocytes

To understand the signalling mechanisms involved in the dual stimulatory effects of endothelin-1 (ET-1) on DNA synthesis and melanization in cultured human melanocytes, we analysed the biological profile of ET-1 receptor and determined the effects of ET-1 on the protein kinase C, cyclic AMP system and mitogen-activated protein kinase (MAP kinase) in comparison with their relevant stimulants. The photoaffinity labelling of ET-1 receptors with Denny-Jaff reagents revealed an ET-1 receptor with a molecular mass of 51 kDa in human melanocytes. The ET(A) receptor subtype-sensitive antagonist BQ123(50 nM) or pertussis toxin (100 ng/ml) significantly suppressed the ET-1-induced intracellular calcium mobilization, indicating the presence of pertussis toxin-sensitive G-protein-coupled ET(A) receptors. An assay of protein kinase C activity revealed that 10nM ET-1 translocated cytosolic protein kinase C to membrane-bound protein kinase C within 5 min of the start of incubation. In contrast, receptor-mediated melanocyte activation by ET-1 was accompanied by an elevated level of cyclic AMP (4-fold over control) after 10-60 min of incubation, whereas 60 min of incubation of human melanocytes with c-Kit or c-Met ligands such as stem cell factor (10 nM) or basic fibroblast growth factor (10 nM) did not elevate the cyclic AMP level. We have also demonstrated that a specific tyrosine kinase inhibitor, tyrphostin B-42 (10 microM), inhibited the ET-1-induced growth stimulation, suggesting the involvement of the tyrosine kinase pathway in growth stimulation. Consistently, an assay of MAP kinase revealed that ET-1 caused a 10-fold activation of MAP kinase after 5 min of incubation with human melanocytes in a similar way to tyrosine kinase ligands such as stem cell factor and hepatocyte growth factor. Further, the DNA synthesis stimulated by the c-Kit ligand stem cell factor at a concentration of 1 nM was synergistically enhanced by 5 nM ET-1. These results suggest that ET-induced dual cellular events in human melanocytes are closely associated with cross-talk between the protein kinase C and A and tyrosine kinase pathways.

Involvement of protein kinase C in the modulation of 1alpha,25-dihydroxy-vitamin D3-induced 45Ca2+ uptake in rat and chick cultured myoblasts

The calciotropic hormone 1alpha,25-dihydroxy-vitamin D3 (1,25(OH)2D3) has been shown to stimulate both rat and chick myoblast 45Ca2+ uptake via modulation of dihydropyridine-sensitive L-type calcium channels through phosphorylation by the cAMP/protein kinase A pathway. We further investigated the involvement of protein kinases in 1,25(OH)2D3-signal transduction on cultured myoblasts. The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) was found to rapidly stimulate myoblast 45Ca2+ uptake, mimicking 1,25(OH)2D3. The effects of PMA were time- (1-5 min) and dose (50-100 nM)-dependent, were mimicked by 1,2-dioctanoylglycerol (DOG) and were specific, since the inactive analogue 4alpha-phorbol was without effect. Analogously to the hormone, PMA-enhanced 45Ca2+ uptake was suppressed by the Ca2+-channel blocker nifedipine (5 microM). 1-(5-isoquinolynsulfonyl)-2-methylpiperazine (H-7), a PKC inhibitor, and down-regulation of PKC by prolonged exposure to PMA (1 microM, 24 h), abolished both PMA and hormone effects on rat and chick cells. As in chick myoblasts, 1,25(OH)2D3 activated PKC in rat myoblasts, with translocation of activity from the cytosol to the cell membrane. Treatment of myoblasts with PMA (100 nM) plus 1,25(OH)2D3 (1 nM) greatly potentiated 45Ca2+ uptake than either agent alone. PMA also increased myoblast cAMP content. These results suggest the involvement of PKC in the mechanism by which 1,25(OH)2D3 rapidly stimulates calcium uptake in both mammalian and avian myoblasts.

Effect of ischemic preconditioning of the myocardium on cAMP

Reduction of cAMP has been implicated in the protection of ischemic preconditioning (IP), but until now, this possibility has not been directly addressed. In this study, we found that in the in vivo rabbit heart 10 to 30 minutes of sustained regional ischemia was accompanied by a nearly twofold rise in cAMP levels. This increase in cAMP was attenuated when sustained ischemia was preceded by IP induced with a single cycle of transient ischemia and reperfusion (TI/R) and prevented when ischemia was preceded by three cycles of TI/R. The mechanism of cAMP reduction by IP does not involve activation of protein kinase C (PKC), since the PKC inhibitor polymyxin B (24 mg/kg) did not raise cAMP levels during sustained ischemia in IP hearts. Furthermore, this effect is also not mediated by reduced responsiveness of the beta-adrenergic effector pathway, since both nonischemic hearts and hearts subjected to three cycles of TI/R exhibited similar increases in cAMP in response to 5 micrograms/kg isoproterenol. However, propranolol (0.75 mg/kg) abolished the rise in cAMP levels observed during sustained ischemia in control hearts but did not reduce cAMP levels further in IP hearts. These data indicate that the ischemia-induced rise in cAMP levels in control hearts was mediated by activation of the beta-adrenergic receptor. Taken together with data demonstrating that beta-adrenergic responsiveness was not affected by IP, these data support the conclusion that the lack of elevation in cAMP levels observed during sustained ischemia in IP hearts is mediated by an attenuation of norepinephrine release. To examine whether the protection of IP against necrosis was mediated by the lack of elevation in cAmp levels, we determined whether the infarct size-limiting effect of IP could be blocked by NKH477, an activator of adenylyl cyclase. Four groups or rabbits were subjected to 30 minutes of in vivo regional ischemia and 90 minutes of reperfusion. Control hearts (n = 10) had 53.6 +/- 5.5% infarction of the area at risk. IP with three cycles of transient ischemia limited infarct size to 3.2 +/- 1.3% (N = 13, p < .0001). NKH477 (45 micrograms/kg) increased average cAMP levels in IP hearts during sustained ischemia to levels similar to those in untreated control hearts. However, NKH477 did not block IP (50.2 +/- 7.7% of the area at risk was infarcted in the control +NKH477 group [n = 10] versus 10.0 +/- 5.9% in the IP + NKH477 group [n = 7], P < .05). Therefore, we conclude that although IP lowers cAMP levels during sustained ischemia, this effect is not necessary for its protection against necrosis, since raising cAMP does not block this protection of IP.

Modulation of cyclic AMP accumulation in glial cells by exogenous phospholipase C

Pretreatment of 1321N1 human astrocytoma cells with exogenously added bacterial phospholipase C (PLC) induced an increase in subsequent stimulation of cyclic AMP accumulation by the beta-adrenergic receptor agonist isoproterenol and by the direct adenylyl cyclase activator forskolin, a phenomenon referred to as sensitization. The direct protein kinase C activator phorbol 12-myristate 13-acetate (PMA) induced a similar sensitization. In contrast, in C62B rat glioma cells both PLC and PMA induced a decrease in subsequent cyclic AMP accumulation stimulated by isoproterenol and little or no change in stimulation by forskolin. Although the effects of PMA were completely abolished by pretreating cells overnight with PMA to down-regulate protein kinase C activity, the effects of PLC were inhibited only partially or not inhibited. Pertussis toxin pretreatment did not inhibit the sensitization induced by PLC, whereas sensitization induced by lysophosphatidic acid (previously shown to involve pertussis toxin-sensitive GTP binding proteins) was completely inhibited. Further studies of these phenomena may reveal novel pathways for regulation of the cyclic AMP signalling pathway.

Insulin and vasopressin elicit inhibition of cholera-toxin-stimulated adenylate cyclase activity in both hepatocytes and the P9 immortalized hepatocyte cell line through an action involving protein kinase C

Incubation of hepatocytes or the SV40-DNA-immortalized hepatocyte P9 cell line with cholera toxin led to a time-dependent activation of adenylate cyclase activity, which occurred after a defined lag period. When added together with cholera toxin, each of the hormones insulin and vasopressin was capable of attenuating the maximum stimulatory effect achieved by cholera toxin over a period of 60 min through a process which could be blocked by the compounds staurosporine and chelerythrine. Attenuating effects on cholera-toxin-stimulated adenylate cyclase activity could also be elicited by using either the protein kinase C (PKC)-stimulating phorbol ester PMA (phorbol 12-myristate 13-acetate) or the protein phosphatase inhibitor okadaic acid. Alkaline phosphatase treatment of membranes reversed the inhibitory effect of PMA. Cholera toxin also stimulated the adenylate cyclase activity of intact CHO (Chinese-hamster ovary) and NIH-3T3 cells, but this activity was insensitive to the addition of PMA. Overexpression of various PKC isoforms in CHO cell lines did not confer sensitivity to inhibition by PMA upon cholera-toxin-stimulated adenylate cyclase activity. Rather, overexpression of the gamma isoform of PKC allowed PMA to stimulate adenylate cyclase activity in CHO cells. It is suggested that the PKC-mediated phosphorylation of a membrane protein attenuates cholera-toxin-stimulated adenylate cyclase activity in hepatocytes and P9 cells. The cellular selectivity of such an action may be due to the target for this inhibitory action of PKC being a particular isoform of adenylate cyclase which provides the major activity in hepatocytes and P9 cells, but not in either CHO or NIH-3T3 cells.

Inhibition of cyclic AMP accumulation by endothelin is pertussis toxin sensitive and calcium independent in isolated adult feline cardiac myocytes

The aims of this study were to determine whether endothelin-1 (ET-1), a positive inotropic agent, altered the production of cyclic AMP (cAMP) in adult feline cardiac myocytes and to characterize the effect with respect to G-protein-coupling and calcium regulation of adenylyl cyclase. ET-1 inhibited basal and/or stimulated cAMP accumulation in the intact cardiac myocyte and in membrane preparations in a dose-dependent manner. In intact cells, maximal inhibition of forskolin-stimulated cAMP accumulation was 90-95% with an EC50 of 5 x 10(-10) M. Inhibition of isoproterenol-stimulated cAMP was biphasic with maximal inhibition of 70% observed by 10(-11)M; at higher doses inhibition was not consistently observed. The inhibitory response to ET-1 occurred in the absence or presence of isobutylmethylxanthine suggesting that activation of cAMP phosphodiesterases was not the means for reducing cAMP levels. Prior exposure of cardiac myocytes to 100ng/ml pertussis toxin blocked the inhibitory action of ET-1, indicating that this response is mediated through the involvement of a pertussis toxin-sensitive G-protein such as Gi. Studies carried out in the absence of extracellular calcium and under conditions of cell-loading with the intracellular calcium chelator, 1,2-bis-(2-aminophenoxy)-ethane-N,N,N'N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM), suggest that the mechanism by which ET-1 inhibits cAMP accumulation is not calcium-dependent. Thus, inhibition of cAMP accumulation by ET-1 appears to be mediated through a pertussis toxin sensitive protein rather than by activation of phosphodiesterases or calcium inhibition of cardiac forms of adenylyl cyclase. Though unlikely to play a role in the positive inotropic effect of ET-1, transduction of ET-1 responses through Gi suggests another means for regulation of growth in these adult cardiac myocytes.

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

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

Crosstalk between thrombin and adenylyl cyclase-stimulating agonists in proliferating human erythroid progenitor cells

Human erythroid progenitor cells grown in a suspension culture system were used to study possible interactions between different guanine nucleotide-binding protein (G-protein)-coupled receptor-effector systems during normal cell differentiation. Agonist-stimulated adenylyl cyclase was not inhibited by any one of a panel of ligands (ADP, UTP, platelet-activating factor, thrombin, alpha2-adrenoceptor agonists, interleukin 8, lysophosphatidic acid) most of which are known, in other cells, to reduce cAMP formation by a Gi-mediated, pertussis toxin-sensitive mechanism. The first four of these ligands are also known to cause transient changes in intracellular [Ca2+] in erythroid cells. Rather than inhibiting, thrombin (but not ADP, UTP or PAF) specifically caused a fivefold increase in the maximum adenosine- or prostaglandin E1-stimulated cAMP formation, without any shift of the concentration/response curves. Thrombin did not enhance forskolin- and AlF4-stimulated cyclase activity and had only a marginal effect on isoprenaline-dependent stimulation. The effect of thrombin seemed to be unrelated to intracellular Ca2+ release but could be partially mimicked by phorbol ester (PMA)-induced stimulation of protein kinase C (PKC) and was inhibited by staurosporin or by inactivation of PKC after long-term incubation with PMA. The activity of thrombin was restricted to proliferating, colony-forming progenitor cells while proerythroblasts were completely unresponsive. Our results suggest that the interaction of thrombin with Gs-linked receptors requires phosphorylation of a target protein that is different from adenylyl cyclase, Gs or Gi but may be involved in the regulation of receptor desensitization.

Cyclic-AMP deficient MDCK cells form tubules

It has been known for many years that MDCK cells form blister-like structures, termed domes. During an examination of the morphology of a large number of MDCK clones, we found that two stable morphotypes exist in an MDCK cell population-namely, dome-forming and tubule-forming clones. When maintained at high cell density, tubule-forming clones displayed large numbers of anastomosing tubules which contained lumens. The frequency of observation of the tubule-forming clones in an MDCK population was 0.7%. Tubule-forming MDCK clones should be useful in studying tubule morphogenesis. While agents that affect protein kinase A activity increased dome formation, the same agents abolished the formation of tubules in all tubule-forming clones. In contrast, drugs that stimulate protein kinase C activity (phorbol esters and staurosporine) decreased dome formation an increased tubule morphogenesis in all MDCK morphotypes. Tubule-forming clones were found to have lower resting levels of cyclic-AMP and to respond to forskolin stimulation of adenylate cyclase less readily. Hence, signals transmitted by the protein kinase C pathway appear to lead to tubule formation in MDCK cells, while signals transmitted through the protein kinase A pathway lead to dome formation.

Desensitization and selective down-regulation of rat cardiac beta 1-adrenoceptors by prolonged in vivo infusion of T-0509, a beta 1-adrenoceptor full agonist

We studied the effects of prolonged infusion of a selective beta 1-adrenoceptor (beta 1AR) full agonist, T-0509 [(-)-(R)-1-(3,4-dihydroxyphenyl)-2-[(3,4- dimethoxyphenethyl)amino]ethanol hydrochloride], with regard to its inotropic effect in vivo and cardiac beta AR density. The results were compared with those for isoproterenol. Continuous infusion of isoproterenol at doses of 2.5-40 micrograms/kg/hr, s.c. for 6 days shifted the dose-response curves of isoproterenol (i.v.) for LVdP/dtmax to the right and increased the ED50 values up to fourfold. Isoproterenol infusion at 40 micrograms/kg/hr reduced the density of both beta 1- and beta 2ARs by 36% and 43% respectively, in left ventricular membranes. Following 6-day infusion of T-0509 at doses sufficient to induce a positive inotropic effect (5-40 micrograms/kg/hr), the ED50 value of T-0509 (i.v.) for LVdP/dtmax was also increased up to fourfold. In contrast to isoproterenol, infusion of T-0509 caused selective down-regulation of beta 1ARs by 30% without changing the number of beta 2ARs. These results indicate that long-term application of a selective beta 1AR full agonist causes desensitization to its inotropy in vivo, with subtype-selective down-regulation of beta 1ARs in cardiac ventricles.

Glutamate, GABA and epilepsy

The nature and value of various animal models of epilepsy for the study and understanding of the human epilepsies are reviewed, with special reference to the ILAE classification of seizures. Kindling as a model of complex-partial seizures with secondary generalisation is treated in detail, dwelling principally on the evidence that the neurotransmitters glutamate and GABA are centrally involved in the kindling process. Kindling in the entorhinal cortex-hippocampus system and its relationship to LTP are analysed in detail. Changes in amino acid content in animal and human brain tissue following onset of the epileptic state are reviewed with special reference to glutamate and GABA. Studies of changes in the extent of basal and stimulus-evoked release of glutamate and GABA both in vivo (microdialysis) and in vitro (brain slices) are evaluated. This includes both kindling and other models of epilepsy, and microdialysis of human patients with epilepsy. Experiments which study the influence of pre-synaptic metabotropic glutamate receptors on glutamate release, and consequently on the extent of electrical kindling, are described. This pre-synaptic control of glutamate release can be studied using synaptosomes. The significance of the ability of focal intracerebrally injected glutamate and NMDA to cause (chemical) kindling and the strong sensitivity of this process to pre-treatment with NMDA receptor antagonists is analysed. Electrical and chemical kindling effects are additive, indicating the existence of mechanisms in common. They are both sensitive to NMDA antagonists and the common mechanism is probably NMDA receptor activation due to the presence of exogenous (chemical) or endogenous (electrically-released) extracellular glutamate. The participation of the NMDA receptor in the generation of the spontaneous hyperactivity which characterises the chronic epileptic state is reviewed. This includes the entry of Ca2+ to stimulate various post-synaptic phosphorylation processes, and possible modulation of NMDA receptor population size and sensitivity. The question of whether neurotransmitter glutamate is involved in initiation and/or spread of seizures is discussed.

Mechanism of action of epidermal growth factor-induced porcine oocyte maturation

EGF has been reported to promote oocyte maturation in several species, although the mechanism of action is not yet known. The present study is designed to determine the pathway used by EGF to enhance porcine oocyte maturation. Oocytes were aspirated from 2-5 mm follicles and cultured with various treatments in Medium-199 at 37 degrees C, 100% relative humidity, and 5% CO2 for 48 hr for the maturation study and 3 hr for intracellular cAMP measurement. Although treatment with 100 IU/ml hCG stimulated both intracellular cAMP formation and oocyte maturation, 10 ng/ml EGF stimulated oocyte maturation only. Dibutyryl cAMP (dbcAMP) inhibited oocyte maturation at 10(-5), 10(-4), and 10(-3) M concentration s in the control medium. However, in the presence of 10 ng/ml EGF, dbcAMP inhibited oocyte maturation only at a concentration of 10(-3) M. Increasing concentrations of EGF (i.e., 25 and 50 ng/ml) were ineffective in overcoming the inhibitory effect of dbcAMP at 10(-3) M. In contrast, EGF reversed the decreased maturation rate caused by transforming growth factor-beta. Phorbol myristate acetate (PMA), a tumor-promoting phorbol ester, enhanced the spontaneous maturation rate; 4 alpha-phorbol dideconoate, an inactive phorbol ester, did not show this effect. PMA- and EGF-stimulated porcine oocyte maturation is reversed by calphostin-C, a PKC inhibitor. In conclusion, EGF's promotional activity on porcine oocyte maturation is independent of the cAMP pathway and probably mediated by the PKC pathway.

Induction of Ca2+/calmodulin-stimulated cyclic AMP phosphodiesterase (PDE1) activity in Chinese hamster ovary cells (CHO) by phorbol 12-myristate 13-acetate and by the selective overexpression of protein kinase C isoforms

The cAMP phosphodiesterase (PDE) activity of CHO cells was unaffected by the addition of Ca2+ +calmodulin (CaM), indicating the absence of any PDE1 (Ca2+/CaM-stimulated PDE) activity. Treatment with the tumour promoting phorbol ester phorbol 12-myristate 13-acetate (PMA) led to the rapid transient induction of PDE1 activity which attained a maximum value after about 13 h before slowly decreasing. Such induction was attenuated by actinomycin D. PCR primers were designed to hybridize with two regions identified as being characteristic of PDE1 forms found in various species and predicted to amplify a 601 bp fragment. RT-PCR using degenerate primers allowed an approx. 600 bp fragment to be amplified from RNA preparations of rat brain but not from CHO cells unless they had been treated with PMA. CHO cells transfected to overexpress protein kinase C (PKC)-alpha and PKC-epsilon, but not those transfected to overexpress PKC-beta I or PKC-gamma, exhibited a twofold higher PDE activity. They also expressed a PDE1 activity, with Ca2+/CaM effecting a 1.8-2.8-fold increase in total PDE activity. RT-PCR, with PDE1-specific primers, identified an approx. 600 bp product in CHO cells transfected to overexpress PKC-alpha and PKC-epsilon, but not in those overexpressing PKC-beta I or PKC-gamma. Treatment of PKC-alpha transfected cells with PMA caused a rapid, albeit transient, increase in PDE1 activity, which reached a maximum some 1 h after PMA challenge, before returning to resting levels some 2 h later. The residual isobutylmethylxanthine (IBMX)-insensitive PDE activity was dramatically reduced (approx. 4-fold) in the PKC-gamma transfectants, suggesting that the activity of the cyclic AMP-specific IBMX-insensitive PDE7 activity was selectively reduced by overexpression of this particular PKC isoform. These data identify a novel point of 'cross-talk' between the lipid and cyclic AMP signalling systems where the action of specific PKC isoforms is shown to cause the induction of Ca2+/CaM-stimulated PDE (PDE1) activity. It is suggested that this protein kinase C-mediated process might involve regulation of PDE1 gene expression by the AP-1 (fos/jun) system.

High-density lipoprotein and low-density lipoprotein-mediated signal transduction in cultured human skin fibroblasts

The signalling mechanisms whereby high-density lipoproteins (HDL) and low-density lipoproteins (LDL) affect a number of cellular functions in fibroblasts are unclear. This study has analyzed the influence of HDL3 and LDL on the phosphatidylinositol specific phospholipase C pathway in human skin fibroblasts. Exposure of myo-[2-3H]-inositol prelabelled fibroblasts to HDL3 or LDL elicited major increases in IP1 and minor increases in IP2 and IP3 within 30 s. In fura-2 loaded suspended fibroblasts, HDL3 and LDL increased intracellular Ca2+ concentrations ([Ca2+]i) with comparable rapid, transient kinetics. The dose-profiles for HDL3- and LDL-induced increases in [Ca2+]i were also comparable, with half-maximally and maximally effective concentrations being approximately 15 micrograms/mL and approximately 50 micrograms/mL, respectively. HDL3- and LDL-induced increases in [Ca2+]i were diminished by approximately 60% (vs. control fibroblasts) in thapsigargin-pretreated fibroblasts, indicating that release of Ca2+ from intracellular pools is the major contributor toward lipoprotein-induced increases in [Ca2+]i. Pertussis toxin-pretreatment of cells completely abolished lipoprotein induced Ca(2+)-transient, indicating the involvement of a guanine nucleotide-binding protein in the signalling process. In [3H]-palmitic acid-prelabelled fibroblasts, both HDL3 and LDL were observed to stimulate production of DAG. Activation of protein kinase C (PKC) was analysed by determining the cytosol-to-membrane translocation of both enzymatic activity and immunoreactivity of specific PKC isoforms (alpha, delta, epsilon, and zeta). Stimulation with HDL3 and LDL evoked a rapid (within 2.5 min) translocation of PKC activity, with PKC alpha and PKC epsilon being the isoforms translocated. It is concluded that HDL3 and LDL acutely stimulate a phosphoinositide-specific phospholipase C pathway in human skin fibroblasts. However, the specific cell membrane events mediating this signal transduction remain to be further elucidated.

Apoptosis overview emphasizing the role of oxidative stress, DNA damage and signal-transduction pathways

Apoptosis (programmed cell death) is a central protective response to excess oxidative damage (especially DNA damage), and is also essential to embryogenesis, morphogenesis and normal immune function. An understanding of the cellular events leading to apoptosis is important for the design of new chemotherapeutic agents directed against the types of leukemias and lymphomas that are resistant to currently used chemotherapeutic protocols. We present here a review of the characteristic features of apoptosis, the cell types and situations in which it occurs, the types of oxidative stress that induce apoptosis, the signal-transduction pathways that either induce or prevent apoptosis, the biologic significance of apoptosis, the role of apoptosis in cancer, and an evaluation of the methodologies used to identify apoptotic cells. Two accompanying articles, demonstrating classic apoptosis and non-classic apoptosis in the same Epstein-Barr virus-transformed lymphoid cell line, are used to illustrate the value of employing multiple criteria to determine the type of cell death occurring in a given experimental system. Aspects of apoptosis and programmed cell death that are not covered in this review include histochemistry, details of cell deletion processes in the sculpting of tissues and organs in embryogenesis and morphogenesis, and the specific pathways leading to apoptosis in specific cell types. The readers should refer to the excellent books and reviews on the morphology, biochemistry and molecular biology of apoptosis already published on these topics. Emphasis is placed, in this review, on a proposed common pathway of apoptosis that may be relevant to all cell types.

Rapid and long-term effects on protein kinase C on receptor tyrosine kinase phosphorylation and degradation

Rapid and long term effects of protein kinase C alpha activation on receptor tyrosine kinase signaling parameters were investigated in human 293 embryonic fibroblasts and mouse NIH 3T3 cells. Within minutes of phorbol 12-myristate 13-acetate treatment, epidermal growth factor receptor and HER2 tyrosine phosphorylation was decreased, while platelet-derived growth factor receptor and insulin receptor autophosphorylation was upregulated. These effects are not mediated by protein kinase C-dependent receptor tyrosine kinase phosphorylation but apparently by activation or inactivation of receptor tyrosine kinase-specific phosphatases, as indicated by neutralization of these phenomena upon treatment of cells with sodium orthovanadate. In contrast to these short term effects, sustained activation of protein kinase C alpha by phorbol 12-myristate 13-acetate results in translocation of protein kinase C from the cytosol to the membrane fraction where it forms stable complexes with all receptor tyrosine kinases investigated. Ligand-induced receptor tyrosine kinase/protein kinase C association in NIH 3T3 fibroblasts is accompanied by a mobility shift of the receptor, indicating phosphorylation by activated protein kinase C. This phenomenon correlates with the disappearance of receptor tyrosine kinases from the cell surface, implying that this interaction plays a role in the process of receptor internalization and degradation. Interestingly, ligand-stimulated receptor down-regulation is also enhanced by overexpression of phospholipase C gamma, which strongly indicates a role for this common receptor tyrosine kinase substrate in negative regulation of growth factor signals.

Down-regulation and differential restoration of cAMP responses upon transient phorbol ester treatment of primary osteoblastic cells

We studied cAMP responses induced by parathyroid hormone (PTH), prostaglandin E2 (PGE2) and forskolin in foetal rat calvariae-derived osteoblastic cells after 24 h treatment with a protein kinase C (PKC) activating phorbol ester. After this treatment, meant to down-regulate PKC activity, all tested cAMP responses were attenuated and were indeed accompanied by a decline in PKC activity. PTH receptor affinity was not altered and PTH receptor number was only slightly lowered after 24 h phorbol ester treatment. These results indicate that modulation of the cAMP responses by 24 h PMA treatment was mainly caused by a general impairment of adenylyl cyclase activity. Removal of the phorbol ester and subsequent culture for 2 days rendered the cells hyper-responsive to PTH: the PTH-induced cAMP response was 2 to 3 times higher than in control cells. Again no change in binding affinity of the PTH receptor was observed and receptor number was just 10% lower than in control cells. The PGE2- and forskolin-induced cAMP responses were not higher than normal. So, transient phorbol ester treatment leads to a differential, agonist-dependent restoration of the cAMP signalling system.

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.

Multifactorial regulation of pituitary adenylate cyclase-activating polypeptide (PACAP)-induced production of cyclic AMP in ATT-20 corticotrophs: major involvement of Rolipram-sensitive and insensitive phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) appear to play a major role in the modulation of cellular accumulations of cAMP/cGMP and hence the magnitude of the cell response to a hormone signal. These enzymes are present in cells as multiple isoforms and lie under control of various protein kinases. Because PACAP, unlike corticotropin-releasing factor (CRF), may stimulate a dual signalling pathway in pituitary cells (activating both adenylyl cyclase and phospholipase C), we used AtT-20 corticotrophs and primary cultures of rat pituitary cells to study the effect and possible differential influence of these peptides on cAMP formation. Time-course analysis indicated that, both in the absence and the presence of Rolipram (a selective type IV PDE inhibitor), PACAP stimulated a rapid and short-lived accumulation of cAMP in tumor corticotrophs, while in the presence of the non-selective inhibitor IBMX, the peptide produced a sustained high plateau level of second messenger (10 times the level generated with Rolipram at 20 min). On the contrary, when exposed to CRF, cAMP production augmented in parallel, irrespective of whether Rolipram or IBMX were present. The differential effects of the PDE inhibitors were seen with PACAP concentrations ranging from 0.1 to 100 nM, and could also be demonstrated in primary cultures of pituitary cells. Co-incubation of AtT-20 cells with Rolipram along with inhibitors of type I (but not of type III) PDEs, enhanced cAMP formation elicited by PACAP to a level significantly higher than that induced by CRF.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in the action potential of Aplysia neurons evoked by a phorbolester are mediated by protein kinase C

Alterations in the parameters of action potentials upon changes in protein kinase C (PKC) activity were studied on neurons of the visceral ganglion of Aplysia californica. The amplitude and maximum speed of the up-and downstroke of the action potentials (APs) were measured. Intracellularly injected PKC and intra- and extracellularly applied phorbol-12,13-diacetate (PDAc) had similar effects on the Aplysia neurons, the most prominent being an increase of the upstroke speed of the AP in every neuron. The non-PKC-activating 4 alpha-phorbol didecanoate had no effect, and the effects of the PKC blocker H-7 were opposite to those of PDAc. It was concluded that the changes of the AP evoked by PDAc are mediated through PKC activation.

Purification and properties of D-myo-inositol 1,4,5-trisphosphate 3-kinase from bovine iris sphincter smooth muscle: effects of protein phosphorylation in vitro and in intact muscle

Stimulation of bovine iris sphincter muscle with carbachol (10 microM) increased accumulation of Ins(1,4,5)P3 (InsP3) and Ins(1,3,4,5)P4 (InsP4) by 86 and 32% respectively. Addition of isoproterenol (5 microM) to muscle pretreated with carbachol reduced the 3H-radioactivity in InsP3 by 30% and increased that of InsP4 by 41%. InsP3 3-kinase was predominantly localized in the soluble fraction (110,000 g supernatant) of the iris sphincter. The enzyme was purified from this fraction by sequential chromatography on DEAE-cellulose, calmodulin (CAM)-agarose affinity, and Mono-Q anion-exchange columns. The specific activity of the purified enzyme was 1.94 mumol/min per mg protein with a purification of 114-fold, compared with the cytosolic fraction of the muscle. SDS/PAGE showed the enzyme to be associated with a protein band corresponding to 50 kDa. In the presence of 10 microM Ca2+, CaM dose-dependently stimulated the enzyme. InsP3 3-kinase specifically phosphorylated InsP3 with an apparent K(m) of 0.56 microM and a Vmax. of 2.5 mumol/min per mg protein. The stimulatory effect of CaM was due to a change in Vmax. and not in its K(m). The enzyme was maximally active at pH 7.0-7.5. Phosphorylation of the purified InsP3 3-kinase with protein kinase A increased its activity; in contrast, phosphorylation with protein kinase C inhibited the enzyme activity. Treatment of the intact iris sphincter with isoproterenol or phorbol 12,13-dibutyrate resulted in stimulation of InsP3 3-kinase activity in the soluble fraction and this activation was preserved on SDS/PAGE and renaturation. These results indicate that the bovine iris sphincter contains a Ca-CaM-dependent InsP3 3-kinase which can be differentially regulated, both in vitro and in intact muscle, by protein kinases A and C.

Insulin inhibits the phosphorylation of alpha-Gi-2 in intact hepatocytes

Challenge of intact hepatocytes with insulin reduced the level of phosphorylated alpha-Gi-2 found under basal (resting) conditions. At maximally effective concentrations of insulin the steady-state labelling of alpha-Gi-2 was reduced by approximately 21%. Insulin achieved this in a time- and dose-dependent fashion, exhibiting an IC50 value of 109 +/- 22 pM. The increased labelling of alpha-Gi-2 seen after challenge of cells with phorbol 12-myristate 13-acetate was also attenuated by insulin. Treatment of hepatocytes with the protein phosphatase inhibitor okadaic acid increased the labelling of alpha-Gi-2 in a fashion which was insensitive to the action of insulin. It is suggested that insulin may reduce the level of phosphorylation of alpha-Gi-2 by stimulating intracellular protein phosphatase activity and that this action may offer a molecular explanation for the ability of insulin to inhibit adenylate cyclase activity in hepatocytes by increasing the level of non-phosphorylated alpha-Gi-2.

Expression of full-length human procathepsin L cDNA in Escherichia coli and refolding of the expression product

From human embrional lung fibroblasts mRNA was obtained and converted to cDNA. The procathepsin L coding region was amplified by PCR, inserted into pALTER and, after checking the nucleotide sequence, transferred into pET81F1+. Procathepsin L was expressed by induction of recombinant E. coli strain BL21[DE3](pLysS) with IPTG and was found to be deposited into inclusion bodies. These were isolated and solubilized in guanidinium hydrochloride. The soluble proteins were sulphonated and procathepsin L was obtained after gel filtration. Purified proenzyme was refolded by dialysis and autoactivated into a form of the expected size and enzymatic activity against a fluorogenic substrate.

Brain distribution of four rat homologues of the Drosophila dunce cAMP phosphodiesterase

We have analyzed the brain distribution of the rat cAMP-specific phosphodiesterases (rPDEIV) which are closely related to the defective gene products of the drosophila melanogaster learning and memory mutant dunce. PCR analysis of rat brain cDNA was performed on the four known dunce-like cAMP PDE rat isogenes (rPDE-IV-A, -B, -C, -D). High expression of three of these isogenes (rPDEIV-A, -B, -D) highlighted their involvement in regulation of cAMP in the brain. Specific probes for all four isogenes were then used for in situ hybridization of rat brain sections. Distinct but overlapping expression patterns were observed for rPDEIV-A, rPDEIV-B, and rPDEIV-D. Abundant expression of these subtypes was observed in the olfactory system, the hippocampus and the cerebellum, while no specific signals could be detected in most areas of the brain for the subtype rPDEIV-C.

Protein kinase C isoforms in muscle cells and their regulation by phorbol ester and calpain

Objectives were to identify the PKC isoforms in cultured muscle cells, to examine roles of Ca(2+)-dependent proteinases (calpains) in processing of various muscle PKC isozymes and to obtain a mechanistic description of the processing of PKCs by examining the temporal relationships between phorbol ester-dependent translocation of muscle PKCs and calpains between cytosolic and membrane compartments. Using six isoform (alpha, beta, gamma, delta, epsilon, zeta)-specific polyclonal antibodies, PKC alpha, delta and zeta were detected in rat skeletal muscle and in L8 myoblasts and myotubes. PKC alpha and zeta were primarily localized in the cytosolic fraction of L8 myotubes whereas PKC delta was more abundant in the membrane fraction. Phorbol ester (TPA) caused rapid depletion of myotube PKC alpha and PKC alpha and PKC delta isoforms from the cytosolic compartment and rapid appearance of these isoforms in the membrane fraction. However, long-term exposure of myotubes to TPA eventually caused down-regulation of PKCs in the membrane compartment. Down-regulation of PKCs in the membrane fraction was partially blocked by calpain inhibitor II. However, the rapid TPA-dependent cytosolic depletion of PKCs was unaffected by calpain inhibitor. This suggests that calpains may be responsible for membrane-associated down-regulation of PKCs but not for cytosolic depletion. In the final study we assessed the effects of phorbol ester on compartmentation of m-calpain with PKCs in muscle cells. Like the PKCs, TPA caused rapid association of m-calpain with the membrane fraction followed by down-regulation. This demonstrates that phorbol esters cause translocation of both PKCs and calpains to membranes where processing of PKCs may occur via the limited proteolysis exerted by calpains.

Down-regulation of beta-adrenergic receptors induced by mitogen activation of intracellular signaling events in lymphocytes

The expression of beta-adrenergic receptors on murine lymphocytes stimulated with concanavalin A was studied. A decrease in beta-adrenoceptor number on T lymphocytes and a diminished response to specific agonist stimulation at the peak of proliferation was found. The blockade of cell proliferation by tyrosine kinases or protein kinase C inhibitors reversed the decrease in beta-adrenoceptor number. PMA plus ionophore or interleukin-2 but not PMA alone were able to induce beta-adrenoceptor down-regulation accompanying cellular proliferation. These results showed that the intracellular signals triggered during lymphocyte activation are involved in beta-adrenoceptor down-regulation and it would represent the loss of a mechanism that exerts negative neuroimmune control of cellular proliferation.

A role for protein kinase C-mediated phosphorylation in eliciting glucagon desensitization in rat hepatocytes

An immobilized hepatocyte preparation was used to show that both vasopressin and glucagon could desensitize the ability of glucagon to increase intracellular cyclic AMP concentrations. This process was not dependent on any influx of extracellular Ca2+ and was not mediated by any rise in the intracellular level of Ca2+. The protein kinase C-selective inhibitors chelerythrine, staurosporine and calphostin C acted as potent inhibitors of the desensitization process but with various degrees of selectivity regarding their ability to inhibit the desensitizing actions of glucagon and vasopressin. The protein phosphatase inhibitor okadaic acid was just as potent as vasopressin and glucagon in causing desensitization. Treatment of hepatocyte membranes with alkaline phosphatase restored to near control levels the ability of glucagon to stimulate adenylate cyclase activity in membranes from both glucagon- and vasopressin-treated (desensitized) hepatocytes. It is suggested that the desensitization of glucagon-stimulated adenylate cyclase activity involves a reversible phosphorylation reaction with the likely target being the glucagon receptor itself.

Effects of a protein kinase C inhibitor, Ro 31-7549, on the activation of human leukocytes by particulate stimuli

1. A new specific protein kinase C (PKC) inhibitor, Ro 31-7549, was used to explore the mechanisms by which particulate stimuli, quartz and chrysotile, stimulate human polymorphonuclear leukocytes (PMNL) to produce reactive oxygen metabolites (ROM). Also soluble stimuli, formyl-Methionyl-Leucyl-Phenylalanine (fMLP) and phorbol myristate acetate (PMA) were used. 2. Ro 31-7549 inhibited chrysotile-induced free intracellular calcium ([Ca2+]i) elevations but did not have an effect on quartz-induced elevations of [Ca2+]i. Both quartz and chrysotile induced production of ROM were partially inhibited by Ro 31-7549. fMLP-induced elevation of [Ca2+]i was inhibited by Ro 31-7549 whereas PMA did not affect [Ca2+]i. Ro 31-7549 strongly inhibited fMLP-induced ROM production, and completely abolished that induced by PMA. 3. These result suggest that PKC may have an important role in the activation of PMNL to produce ROM by particulate and soluble stimuli. However, the inhibition of chrysotile-, but not of quartz-induced [Ca2+]i elevations by Ro 31-7549 provides evidence that both PKC-dependent and -independent mechanisms may play a role in the activation of human leukocytes to produce ROM.

cAMP and PMA enhance the effects of IGF-I in the proliferation of endometrial adenocarcinoma cell line HEC-1-A by acting at the G1 phase of the cell cycle

The present study was undertaken to determine whether endometrial cancer cell line HEC-1-A differ from nontransformed cells, in that the cAMP and protein kinase C pathways may enhance IGF-I effects in mitogenesis by acting at the G1 phase of the cell cycle instead of G0. Immunofluorescence staining of HEC-1-A cells using the proliferating cell nuclear antigen (PCNA) monoclonal antibody and flow cytometric analysis determined that HEC-1-A cells do not enter the G0 phase of the cell cycle when incubated in a serum-free medium. Approximately 51% of the cells were in G1, 12% were in S and 37% in G2 phase of the cell cycle prior to treatment. Forskolin and phorbol-12-myristate 13-acetate (PMA) were used to stimulate cAMP production and protein kinase C activity, respectively. IGF-I, forskolin and PMA each increased (P < 0.01) [3H]-thymidine incorporation in a dose and time dependent manner. The interaction of forskolin and PMA with IGF-I was then determined. Cells preincubated with forskolin or PMA followed by incubation with IFG-I incorporated significantly more (P < 0.01) [3H]-thymidine into DNA than controls or any treatment alone. It is concluded that forskolin and, to a lesser extent, PMA exert their effect at the G1 phase of the cycle to enhance IGF-I effects in cell proliferation.

Modulation of protein tyrosine phosphorylation in rat insular cortex after conditioned taste aversion training

Protein tyrosine phosphorylation is a major signal transduction pathway involved in cellular metabolism, growth, and differentiation. Recent data indicate that tyrosine phosphorylation also plays a role in neuronal plasticity. We are using conditioned taste aversion, a fast and robust associative learning paradigm subserved among other brain areas by the insular cortex, to investigate molecular correlates of learning and memory in the rat cortex. In conditioned taste aversion, rats learn to associate a novel taste (e.g., saccharin) with delayed poisoning (e.g., by LiCl injection). Here we report that after conditioned taste aversion training, there is a rapid and marked increase in tyrosine phosphorylation of a set of proteins in the insular cortex but not in other brain areas. A major protein so modulated, of 180 kDa, is abundant in a membrane fraction and remains modulated for more than an hour after training. Exposure of the rats to the novel taste alone results in only a small modulation of the aforementioned proteins whereas administration of the malaise-inducing agent per se has no effect. To the best of our knowledge, this is the first demonstration of modulation of protein tyrosine phosphorylation in the brain after a behavioral experience.

Repeated ketamine administration produces up-regulation of muscarinic acetylcholine receptors in the forebrain, and reduces behavioral sensitivity to scopolamine in mice

To study the effects of repeated ketamine administration on central muscarinic acetylcholine receptors (mAchRs), ddY male mice were administered subcutaneous doses of 25 mg/kg ketamine every 3 days for a total of five times. Receptor binding assays of mAchR were carried out in the forebrain (FB), cerebellum (CB) and brainstem (BS), using [3H]quinuclidinyl benzilate ([3H]QNB) as a ligand. In addition, we examined whether repeated ketamine (12.5, 25 and 50 mg/kg) or saline (five times) could modify the hyperlocomotion induced by scopolamine (0.5 mg/kg, SC) (a muscarinic antagonist), using a behavior-pharmacological technique. Repeating the ketamine administration resulted in a significant increase in the receptor density value (Bmax) for [3H]QNB only in FB, dependent on the numbers of administrations (1270 +/- 33 fmol/mg protein for a single dose, 1620 +/- 59 for four treatments, 1738 +/- 70 for five treatments without any change in apparent affinity (defined as the reciprocal of the dissociation constant) (Kd). A competitive inhibition study of repeated (5 times) administration of ketamine failed to detect any subtype-specific changes in mAchRs. Repeated ketamine administration reduced the scopolamine-induced hyperlocomotion in a dose-related way, and the changes were significant at 50 mg/kg. Our results suggest that repeated ketamine administration produces an up-regulation of mAchRs, and this change may be associated with altered Ach transmission in the central nervous system.

The alpha isoform of protein kinase C inhibits histamine-stimulated adenylate cyclase activity in a particulate fraction of the human gastric cancer cell line HGT-1

The isoform of protein kinase C responsible for the inhibition of histamine-stimulated adenylate cyclase by the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), has been investigated in a particulate fraction prepared from the human gastric cancer cell line HGT-1. The alpha and epsilon isoforms of protein kinase C were detected in HGT-1 cells and in a 40,000 x g particulate fraction by immunoblotting procedures. The inhibitory effect of TPA on histamine-stimulated adenylate cyclase was enhanced by the presence of Ca2+, but decreased in a concentration-dependent manner by anti-peptide antibody to protein kinase C alpha, but not to protein kinase C epsilon. Addition of Ca2+ and TPA to the 40,000 x g particulate fraction stimulated the phosphorylation of the protein kinase C substrate myelin basic peptide 4-14. Protein kinase C alpha is probably the isoform responsible for inhibition of histamine-stimulated adenylate cyclase in HGT-1 cells.

Preconditioning of isolated rat heart is mediated by protein kinase C

Catecholamines have been implicated in the phenomenon of ischemic preconditioning. We have previously demonstrated that ischemic preconditioning against postischemic mechanical dysfunction in the isolated rat heart is mediated by the alpha 1-adrenergic receptor. The purpose of this study was to delineate the signal transduction of preconditioning distal to the alpha 1-adrenergic receptor. Our results suggest that (1) transient ischemia and alpha 1-adrenergic receptor-induced preconditioning is inhibited by protein kinase C (PKC) antagonists, (2) functional protection against global ischemia/reperfusion injury can be induced by infusion of diacylglycerol, the second messenger of the alpha 1-adrenergic pathway, and (3) transient ischemia and alpha 1-adrenergic preconditioning are both characterized by similar translocation of PKC-delta to the sarcolemma of myocardium. These findings suggest that PKC is an effector of preconditioning in the isolated rat heart.

Serotonin, serotoninergic agents and their antagonists suppress humoral immune reaction in vitro

Serotonin (5-HT), its derivative 5-methoxy-tryptamine (MeOT) and ergot-alkaloid dihydroergosine (DHESN) suppressed the immune reaction of mouse spleen cells against sheep erythrocytes in vitro (PFC assay). Ketanserin and propranolol, antagonists of 5-HT, also caused suppression. However, cells incubated with propranolol 20 min before 5-HT or MeOT produced as many plaques as the non-treated cells. Preincubation with ketanserin resulted in similar interference, but the reversal was not complete. It is concluded that serotonin and related agents probably affect several types of immunocompetent cells participating in successive stages of the PFC reaction. The antagonists probably interfered via 5-HT receptors, adrenergic receptors or both. Immunosuppression observed with serotoninergic agents and their antagonists draws attention to possible side effects of such drugs.

Experimental metastasis and differentiation of murine melanoma cells: actions and interactions of factors affecting different intracellular signalling pathways

Various factors that modulate the differentiation of malignant cells are known to affect their experimental metastatic potential (EMP), or lung colonization after intravenous injection into syngeneic animals. However, some results and conclusions on the relation between cell differentiation and metastasis have appeared to conflict. We have reanalysed this by measurement of EMP of B16 melanoma sublines after culture with agents or conditions that acted on differentiation through various intracellular pathways. All tested agents did affect the EMP. EMP was usually positively correlated with differentiation under diverse conditions, but exceptions showed that there is no direct causal connection. Nor could all findings be explained in terms of cell proliferation or expression of major histocompatibility antigens. Some data helped to explain disparities between previous reports. Specific novel findings included the following. The stimulation of EMP by melanocyte-stimulating hormone (MSH) as well as all other tested effects of MSH were prevented by extended exposure to 12-O-tetradecanoyl phorbol acetate (TPA), suggesting a requirement for protein kinase C activity as well as G-protein coupling in MSH action. Cells grown with cholera toxin were always more differentiated than untreated cells, but the EMP could be either markedly increased or markedly decreased by cholera toxin under different conditions. The basic culture medium apparently determined this striking reversal. The EMP was also significantly affected by the extracellular pH.

Dietary modulation of rat colonic cAMP-dependent protein kinase activity

Malignant transformation of cells is associated with enhanced proliferation and alterations in cAMP-dependent protein kinase (PKA) activity. To investigate the role of PKA in normal colonic cell proliferation, PKA was characterized in rat colonic mucosa. In addition, rats were fed diets containing different fats (corn oil, fish oil) and fibers (pectin, cellulose, fiber free) to elicit varying levels of colonic cell proliferation in order to study this signaling system under normal physiologic conditions. Overall, PKA activities were higher in cytosolic compared to membrane fractions. PKA type II (PKA II) isozyme contributed 89 +/- 1% and 96 +/- 1% of total PKA activity in cytosolic and membrane fractions, respectively. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed the presence of mRNA for both the alpha and beta isoforms of the regulatory subunits of PKA II. PKA activities were 21-33% higher in distal membrane and total distal fractions in rats fed a cellulose/corn oil diet compared to animals consuming the other fiber/fat diets. These effects were seen only in the distal colon, where the number of cells per crypt column was elevated only in animals fed the cellulose/corn oil diet relative to other diets. Diet-induced mitogenic responses did not involve significant changes in the relative activity of PKA I and II isozymes. These data demonstrate that dietary effects on PKA activity in the distal colon may be related to changes in cell differentiation as indicated by the number of cells per crypt column.