Phorbol ester regulates stimulatory and inhibitory pathways of the hormone-sensitive adenylate cyclase system in rat reticulocytes

Novel metabotropic glutamate receptor negatively coupled to adenylyl cyclase in cultured rat cerebellar astrocytes

Several excitatory amino acid ligands were found potently to inhibit forskolin-stimulated cAMP accumulation in rat cultured cerebellar astrocytes: L-cysteine sulfinic acid (L-CSA) = L-aspartate > L-glutamate >/= the glutamate uptake inhibitor, L-PDC. This property did not reflect activation of conventional glutamate receptors, since the selective ionotropic glutamate receptor agonists NMDA, AMPA, and kainate, as well as several mGlu receptor agonists [(1S,3R)-ACPD, (S)-DHPG, DCG-IV, L-AP4, L-quisqualate, and L-CCG-I], were without activity. In addition, the mGlu receptor antagonists, L-AP3, (S)-4CPG, Eglu, LY341495, (RS)-CPPG, and (S)-MCPG failed to reverse 30 microM glutamate-mediated inhibitory responses. L-PDC-mediated inhibition was abolished by the addition of the enzyme glutamate-pyruvate transaminase. This finding suggests that the effect of L-PDC is indirect and that it is mediated through endogenously released L-glutamate. Interestingly, L-glutamate-mediated inhibitory responses were resistant to pertussis toxin, suggesting that G(i)/G(o) type G proteins were not involved. However, inhibition of protein kinase C (PKC, either via the selective PKC inhibitor GF109203X or chronic PMA treatment) augmented glutamate-mediated inhibitory responses. Although mGlu3 receptors (which are negatively coupled to adenylyl cyclase) are expressed in astrocyte populations, in our study Western blot analysis indicated that this receptor type was not expressed in cerebellar astrocytes. We therefore suggest that cerebellar astrocytes express a novel mGlu receptor, which is negatively coupled to adenylyl cyclase, and possesses an atypical pharmacological profile.

Predicting response to lithium in mood disorders: role of genetic polymorphisms

Lithium is considered to be the first choice mood stabilizer in recurrent mood disorders. Its widespread and large-scale use is the result of its proven efficacy. In spite of this fact, patients have been observed to show a variable response to lithium treatment: in some cases it is completely effective in preventing manic or depressive relapses, while in other cases it appears to show no influence on the disease course. The possible definition of a genetic liability profile for adverse effects and efficacy will be of great help, as lithium therapy needs at least 6 months to be effective in stabilizing mood disorders. During the last few years, a number of groups have reported possible liability genes. Lithium long-term prophylactic efficacy has been associated with serotonin transporter protein, tryptophan hydroxylase and inositol polyphosphate 1-phosphatase variants. A number of other candidate genes and anonymous markers did not yield positive associations. Therefore, even if some positive results have been reported, no unequivocal susceptibility gene for lithium efficacy has been identified. Although the available data may not currently allow a meaningful prediction of lithium response, future research is aimed at the development of individualized treament of mood disorders, including the possibility of 'pharmacological genetic counseling'.

Modulation of the Na(+)-H(+) antiport activity by adrenaline on erythrocytes from normal and obese individuals

The effect of adrenaline on normal and obese human Na(+)-H(+) antiport (NHE 1) erythrocyte activity has been studied. Adrenaline increased both intracellular pH (pHi) and Na(+) influx in erythrocyte suspensions. This effect of adrenaline was inhibited by amiloride or EIPA, indicating that adrenaline stimulated NHE 1. Phorbol myristicate ester (PMA), a protein kinase C (PKC) stimulator, increased the activity of NHE 1 whereas calphostin C, a PKC inhibitor, partially inhibited NHE 1 activation induced by adrenaline. The effect of adrenaline to NHE 1 was counteracted by prazocin and by propranolol as well indicating the involvement of both alpha and beta 2 adrenergic receptors. The effect of adrenaline on erythrocyte NHE 1 activity was significantly more profound in obese compared to normal subjects. These data indicate that adrenaline induces an increase of pHi and Na(+) uptake of human erythrocytes through stimulation of NHE 1 activity. The significantly more profound stimulation of NHE 1 activity by adrenaline in obese as compared to normal subjects is discussed.

Pharmacogenetics of lithium prophylaxis in mood disorders: analysis of COMT, MAO-A, and Gbeta3 variants

We studied the possible association between the prophylactic efficacy of lithium in mood disorders and the following gene variants: catechol-O-methyltransferase (COMT) G158A, monoamine oxydase A (MAO-A) 30-bp repeat, G-protein beta 3-subunit (Gbeta3) C825T. A total of 201 subjects affected by bipolar (n = 160) and major depressive (n = 41) disorder were followed prospectively for an average of 59.8 months and were typed for their gene variants using PCR techniques. COMT, MAO-A, and Gbeta3 variants were not associated with lithium outcome, even when possible stratification effects such as sex, polarity, age at onset, duration of lithium treatment, and previous episodes were included in the model. The pathways influenced by those variants are not therefore involved with long-term lithium outcome in our sample.

Pharmacogenetics in affective disorders

Pharmacogenetics will be of substantial help in the field of affective disorders pharmacotherapy. The possible definition of a genetic liability profile for drug side-effects and efficacy will be of great help in treatments that need weeks to months to be effective. During the last few years, a number of groups have reported possible liability genes. The efficacy and time of onset of selective serotonin reuptake inhibitors have been associated with a polymorphism in the promoter region of the transporter (SERTPR) in many independent studies, while variants at the tryptophan hydroxylase gene, 5-HT2a receptor and G-protein beta3 have been associated with them in pilot studies. Lithium long-term prophylactic efficacy has been associated with SERTPR, TPH and inositol polyphosphate 1-phosphatase variants, though in unreplicated samples. A number of further candidate genes were not associated with these treatments. In conclusion, both acute and long-term treatments appear to be, at least to some extent, under genetic influence and preliminary data have identified possible liability genes.

Lithium long-term treatment in mood disorders: clinical and genetic predictors

Lithium is the most widely used long-term treatment for recurrent mood disorders. Despite its proven efficacy, patients show a variable response, ranging from complete efficacy to no influence at all. This paper reviews possible predictors of response focusing on molecular genetic studies. The functional polymorphism in the upstream regulatory region of the serotonin transporter gene (5-HTTLPR) has been associated with lithium long-term efficacy in two independent studies, marginal associations have been reported for tryptophan hydroxylase and inositol polyphosphate 1-phosphatase (INPP1). A number of other candidate genes and anonymous markers did not yield positive associations. Therefore, even though some positive results have been reported, no unequivocal susceptibility gene for lithium efficacy has been identified.

Inhibition of prostaglandin E2-responsive adenylyl cyclase in embryonal human kidney 293 cells by phorbol esters

A clonal primary embryonal human kidney cell line, 293, increased cAMP production in response to prostaglandin E2 (PGE2) (0.02-2 microM). The purpose of this study was to show the effects of tumor-promoting phorbol esters (e.g., 4 beta-phorbol 12-myristate 13-acetate, PMA) on PGE2-stimulated cAMP production. Pretreatment with PMA (0.2-200 nM) for 30 min markedly reduced PGE2-stimulated cAMP production in the presence of 0.5 mM isobutylmethylxanthine. The reduction by PMA was dose- and time-dependent. PMA seems to attenuate the increase in cAMP accumulation elicited by PGE2 primarily, if not entirely, by inhibiting adenylyl cyclase activity, since we were unable to demonstrate an effect of PMA on the degradation half-life of cAMP in intact 293 cells. The action of PMA had some specificity for the agonist used; thus, PMA inhibited PGE2-activated adenylyl cyclase but had no effect on the forskolin-activated enzyme. Co-pretreatment with PMA and H-7, an inhibitor of protein kinase C (PKC), partially prevented the PMA-induced attenuation of the PGE2-stimulated cAMP accumulation, and 1-oleoyl-2-acetylglycerol, a synthetic diacylglycerol analog, partially mimicked the PMA action. Thus, PMA appeared to decrease cAMP production by a PKC-mediated mechanism, inhibiting adenylyl cyclase activity at a point other than the catalytic subunit of the enzyme in the kidney 293 cell line.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitory interactions between stimulus-secretion pathways in the exocrine rat pancreas

In many tissues the cellular responses mediated through different intracellular messenger systems are mutually interactive. In the exocrine pancreas the secretagogues acting via adenosine cyclic monophosphate (cAMP) and those acting via calcium-phosphoinositides can potentiate one another. On the other hand, protein kinase C (PK-C) modulates receptor-induced responses in exocrine pancreatic cells and other cell types. Recording total protein output, monitored on-line at 280 nm, from superfused rat pancreatic segments, we demonstrate that secretin (a cAMP-acting hormone) reduces the efficacy of the calcium-mediated secretagogue cholecystokinin-octapeptide (CCK-8). Likewise, the PK-C activator 12,O,tetradecanoyl phorbol 13 acetate (TPA) reduces both the efficacy of secretin and the potency of cholecystokinin. Thus, the hypothesis of potentiation between different stimulus-secretion coupling mechanisms must be revised, and receptor-activated responses in the exocrine pancreas must be considered a complex model with multiple inhibitory and stimulatory interactions.

Phorbol ester stimulation of the type I and type III adenylyl cyclases in whole cells

Phorbol esters and activators of protein kinase C have been reported to either facilitate or inhibit increases in intracellular cAMP caused by activators of adenylyl cyclase. The variable responses to activators of protein kinase C may reflect, in part, the existence of distinct adenylyl cyclases present in animal cells. There are a family of adenylyl cyclases with different regulatory properties, and clones for six distinct types of adenylyl cyclase have been reported. Two of these enzymes, the type I and type III adenylyl cyclases, are stimulated by calcium and calmodulin whereas the others are not. In this study, we examined the effect of phorbol esters of the activity of the type I and type III adenylyl cyclases in whole cells. TPA markedly enhanced the forskolin responsiveness of the type I and type III adenylyl cyclases expressed in kidney 293 cells. The effect of TPA on the activity of the calmodulin-sensitive adenylyl cyclases was not mediated through increases in intracellular free calcium. These data suggest that activation of protein kinase C can elevate intracellular cAMP in animal cells that contain the type I or type III adenylyl cyclase.

Protein kinase C inhibits stimulation of adenylate cyclase by the histamine H2 receptor in rat parietal cells

The action of protein kinase C on the stimulation of adenylate cyclase activity by the histamine H2 receptor was investigated in rat parietal cells. Protein kinase C was activated by preincubating cells with 12-O-tetradecanoylphorbol 13-acetate (TPA), and adenylate cyclase activity was measured in sonicated extracts. TPA (100 nM) inhibited adenylate cyclase activity stimulated by histamine (100 nM-500 microM). This effect was related to the concentration of TPA. TPA (100 nM) enhanced the stimulation of adenylate cyclase activity by forskolin (100 microM) but had no effect on the stimulation by NaF (10 mM). In conclusion, protein kinase C inhibits stimulation of adenylate cyclase by the histamine H2 receptor. This action could be mediated by changes in the number of affinity of histamine H2 receptors or in the coupling of the receptor to the stimulatory guanine nucleotide regulatory subunit Gs.

Intracellular signal transduction in proliferation of synovial cells

We investigated some steps in the signal transduction pathway leading to the proliferation of synovial cells. 12-o-tetradecanoyl phorbol-13-acetate (TPA) which is known to stimulate phospholipid- and Ca(2+)-dependent protein kinase (C-kinase) enhanced the proliferation of synovial cells. The proliferation of synovial cells induced by interleukin-1 beta. Tumour necrosis factor alpha and granulocytes/macrophages colony stimulating factor, was inhibited by a potent C-kinase inhibitor, H7. These findings strongly suggested that the signal transduction pathway leading to proliferation of synovial cells is transmitted via C-kinase activation. Prostaglandin E2, which is known to stimulate adenylate cyclase, leading to the elevation of intracellular c-AMP level, inhibited the proliferation of synovial cells. This effect could also be mimicked by the addition of a cell permeable c-AMP analog, dibutyryl c-AMP or theophylline. Studies suggest that the feedback signal for proliferation of synovial cells was transmitted through c-AMP. We therefore conclude that signals for stimulation and inhibition of synovial cell proliferation are transmitted through different pathways.

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

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

Modulation of muscarinic receptor-mediated adenylate cyclase and phospholipase C responses in rat retina

1. Agonist activation of rat retina muscarinic receptors results in suppression of cyclic AMP (cAMP) generation and enhanced phosphoinositide hydrolysis. 2. Pharmacological manipulations that elevate cAMP or stable analogues of cAMP attenuate the acetylcholine (ACh)-induced enhancement of phosphoinositide hydrolysis. We postulate that cross-talk between adenylate cyclase and phospholipase C signal transducing systems probably exists in rat retina, as has been described for other systems. 3. Intraocular administration of pertussis toxin attenuated the response of both adenylate cyclase and phospholipase C to muscarinic stimulation, suggesting that some retinal muscarinic receptors are apparently coupled to their effector systems via pertussis toxin sensitive G proteins.

Divergent regulation of beta 2-adrenoceptors and adenylate cyclase in the Cyc- mouse T lymphoma cell line TL2-9

The radiation leukemia virus-induced murine Cyc- T lymphoma cell line TL2-9 expressed one homogeneous population of beta 2-adrenoceptors based on competition curves of [125I]cyanopindolol with the specific antagonist ICI 118.551 and three beta-adrenergic agonists. These receptors were uncoupled from adenylate cyclase due to the absence of Gs. The catalytical unit was directly stimulated by MnCl2, forskolin, and even more markedly in the simultaneous presence of both reagents. In contrast, the enzyme was inhibited in the presence of Gpp[NH]p, probably through interaction with Gi. Indeed, this inhibitory effect was constrained by preincubating cells in the presence of pertussis toxin and a 41 kDa protein was specifically ADP-ribosylated in the presence of the toxin. This cell line was therefore analogous to the Cyc- cell line derived from the murine S49 lymphoma cell line. When added to the culture medium, butyrate (2 mM) induced beta 2-adrenoceptors, the expression of these uncoupled receptors depending on protein synthesis, as judged by inhibitory effects of cycloheximide. In contrast, dBcAMP (1 mM) and TPA (tumor-promoting agent phorbol ester) increased the rate of disappearance of beta 2-adrenoceptors. Butyrate, dBcAMP and TPA systematically decreased adenylate cyclase activity. Besides, TPA (but neither butyrate nor dBcAMP) reduced the efficacy of Gpp[NH]p in inhibiting adenylate cyclase, suggesting a proportionately higher alteration of Gi. We conclude that beta 2-adrenoceptors, uncoupled from adenylate cyclase, are regulated independently from the catalytical unit and Gi, in this Cyc- T lymphoma 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.

The phorbol ester TPA stimulates the expression of functional beta-adrenoceptors in human T lymphoblasts Molt 3

In crude membranes from human T lymphoblasts Molt 3 cultured under standard conditions, the adenylate cyclase system was stimulated by GTP, its beta gamma-imido analogue (p[NH]ppG,) NaF and forskolin, but not by isoprenaline, prostaglandin E1 and vasoactive intestinal peptide. TPA (tumour-promoting agent phorbol ester) added at low concentration (3.2 nM) to the culture medium induced a marked increase in functional beta 2-adrenoceptors. Competition curves of [125I]cyanopindolol with the antagonist ICI 118.551 and four beta-adrenergic agonists indicated that the emergence of functional beta 2-adrenoceptors corresponded to one class of binding sites, shifting from a high-affinity state for agonists to a low-affinity state in the presence of p[NH]ppG. This expression of beta 2-adrenoceptors after a 4 h lag period depended on newly formed mRNA and protein synthesis as judged by the inhibitory effects of actinomycin D and cycloheximide. Further effects of TPA included alterations of the stimulatory G-protein Gs and/or the catalytic unit of adenylate cyclase.

Phorbol ester impairs melanotropin receptor function and stimulates growth of cultured M2R melanoma cells

We have examined the effects of a biologically active tumor promoting phorbol ester (phorbol 12-myristate, 13-acetate (PMA] which activates protein kinase C (PKC) on melanotropin receptor function and cell growth in the M2R mouse melanoma cell clone. Treatment of M2R cells with PMA resulted in a significant loss of beta-MSH binding. The effect was both time- and concentration-dependent. The inhibition of beta-MSH binding resulted from a decrease (greater than 85%) in active membranal receptors available on the external cell surface and not from either enhanced internalization or change in the binding affinity. Agonist-stimulated cyclic AMP accumulation was profoundly increased in a non-selective manner following short-term incubation (3 h) with PMA. This effect was completely reversed during long-term (72-96 h) incubation with the tumor promoting agent. Long-term culturing of M2R cells with PMA resulted in enhanced (+50%) proliferation of the melanoma cells. This enhancement was blocked by the addition of agents which stimulate the production of cAMP. Hence, phorbol esters are powerful growth promoters in transformed melanocytes and our findings indicate that the effects of melanotropins are selectively impaired during the process of growth promotion.