Rapid Dephosphorylation of Protein Kinase C Substrates by Protein Kinase A Activators Results from Inhibition of Diacylglycerol Release

Primitivism and plasticity of pain--implication of polymodal receptors

Bio-warning and defense mechanisms play the most fundamental roles in living organisms. From an evolutionary point of view, nociceptive systems are very primitive and are richly provided with humoral signaling mechanisms of aboriginal humoral defense systems, as reflected in the primitive nature of the polymodal receptor, a poorly differentiated sensory receptor signaling nociceptive information. Recent advances in studies on pain have made it possible to explain neural mechanisms of pain systems under physiological conditions and reveal that there is a large gap between physiological and pathological pains. Protracted nociceptive inputs under pathological conditions induce plastic, either functional or structural, alterations in the nociceptive pathways. These plastic changes lead to crosstalk among the neural networks, including circuits related to motor, autonomic, or psychological functions. These plastic changes, once established, persist even after the original pain sources disappear in a memory-like fashion. Thus, it is revealed that chronic pain cannot be treated by blocking pain pathways, which is effective against acute pain, but require treatment from a multidisciplinary perspective.

Activators of protein kinase A decrease the levels of free arachidonic acid in osteoblasts via stimulation of phosphatidylcholine and phosphatidylethanolamine synthesis

In order to examine the role of protein kinase A (PKA) in the regulation of arachidonic acid availability, the interaction between cAMP agonists and the G protein activator AIF4- in their effects on phospholipid metabolism were measured in MC3T3-E1 osteoblasts. We show that forskolin and 8-brcAMP, activators of PKA, amplify the AIF4(-)-induced stimulation of phosphatidylinositol-specific phospholipase C (phosphatidylinositol inositolphosphohydrolase; EC 3.1.4.3), measured by the formation of [3H]inositol phosphates in prelabeled cells. However, the AIF4(-)-stimulated production of 1,2-diacylglycerols and the release of [3H]arachidonic acid ([3H]AA) were inhibited 50-75% by forskolin and 8-bromocAMP. Furthermore, pretreatment with PKA activators prevented much of the AIF4(-)-induced loss of [3H]AA from phosphatidylcholine and phosphatidylethanolamine in prelabeled osteoblasts. In addition, in the absence of AIF4-, forskolin was found to stimulate the incorporation of [3H]AA and [32P]orthophosphoric acid selectively into these two major phospholipids and selectively increased their mass. The effects of forskolin and 8-BrcAMP on the levels of free [3H]AA were completely reversed by pretreatment with the PKA inhibitor H-89. Therefore, our findings suggest that the activation of cAMP-dependent protein kinase can reduce the availability of free arachidonic acid for prostaglandin synthesis in osteoblast cells by stimulating its reesterification via phospholipid resynthesis.

Cyclic adenosine monophosphate inhibits nitric oxide-induced apoptosis in human leukemic HL-60 cells

Previously we reported that phorbol ester, a protein kinase C (PKC) activator, exhibits a unique pattern of potentiation of nitric oxide (NO)-related apoptosis in HL-60 human promyelocytic leukemia cells. Here we show that elevation of intracellular cAMP could protect HL-60 cells from NO- or NO plus PMA-induced DNA damage. Exposure of cells to sodium nitroprusside (SNP; 0.5 to 4 mM), a NO-generating agent, induced apoptotic cell death as monitored by morphological means, gel electrophoresis, and in situ TdT-apoptosis assay. However, concomitant incubation of the cells with DB-cAMP markedly inhibited SNP-induced apoptotic cell death in a dose-dependent manner. Similar results were obtained with other commonly used cAMP analogs such as CPT-cAMP and 8-C1-cAMP and the intracellular cAMP-elevating agent such as forskolin. In contrast, pretreatment of HL-60 cells with H89 or KT5720, which are known to inhibit cAMP-dependent protein kinase (PKA), abolished the protective effect of cAMP analogs and forskolin on SNP-induced apoptosis. Synergism between SNP and phorbol ester to induce apoptosis was also inhibited by prior treatment of HL-60 cells with DB-cAMP or forskolin. The effect of DB-cAMP in maintaining cell viability was not associated with the onset of G0/G1 cell cycle arrest. In addition, neither dimethyl sulfoxide nor retinoic acid (which produce granulocyte differentiation) could produce cAMP effect. Under the same conditions, DB-cAMP also inhibited NO- or NO plus phorbol ester-induced apoptosis in another transformed cell line, U-937 cells. Taken together, these findings suggest that exposure of HL-60 cells to cAMP analogs renders them more resistant to NO-induced DNA damage and further suggest the existence of specific down-modulatory mechanisms related to NO-induced apoptotic DNA fragmentation.

Cyclic AMP-induced desensitization of G-protein-regulated phospholipase C in turkey erythrocyte membranes

The interaction of the cyclic AMP and inositol lipid signalling systems was studied in turkey erythrocytes. Elevation of intracellular cyclic AMP concentrations by pretreatment of the cells with forskolin or 8-Br-cAMP resulted in a marked decrease in responsiveness of phospholipase C to G-protein activators in membranes prepared from treated cells. Decreases in responsiveness occurred with a t1/2 of approximately 5 min and were reversible after transfer of desensitized cells to drug-free medium. Pretreatment of the cells with forskolin inhibited inositol phosphate formation in a concentration-dependent manner and addition of the phosphodiesterase inhibitor IBMX 93-isobutyl-1-methylxanthine) during pretreatment increased the capacity of forskolin to desensitize phospholipase C activity. IBMX also produced a similar potentiation of forskolin-stimulated accumulation of cyclic AMP in turkey erythrocytes. Isoproterenol pretreatment of the cells induced, like forskolin, partial inhibition of inositol phosphate generation in response to G-protein activators and to P2y purinoceptor and beta-adrenoceptor agonists. The capacity of isoproterenol to induce desensitization of phospholipase C activity also was increased by the presence of IBMX during pretreatment of the cells. H8 (N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide), an inhibitor of cyclic AMP-regulated protein kinase, completely prevented forskolin-induced desensitization but only partially blocked isoproterenol-induced desensitization. These results indicate that the cyclic AMP signalling cascade has a major inhibitory influence on receptor- and G-protein-activated inositol lipid signaling.

Opposite effects of increased intracellular cyclic AMP on the heat and bradykinin responses of canine visceral polymodal receptors in vitro

To clarify the validity of the long standing hypothesis that effects of E series prostaglandin (PG)S are mediated by cyclic AMP (cAMP), we studied the effects of increases in intracellular cAMP on the heat and bradykinin responses of testicular polymodal receptors. Polymodal receptor activities were recorded in vitro from testis-spermatic nerve preparations excised from dogs anesthetized with pentobarbital (30 mg/kg, i.v.). Increases in intracellular cAMP induced by either forskolin (5 or 10 microM), an adenylyl cyclase activator, or a mixture of dibutyryl cAMP (20-100 microM), a membrane permeable cAMP analog, and 3-isobutyl-1-methyl xanthine (20-100 microM), an inhibitor of the cAMP degrading enzyme, significantly augmented the response to heat (42-48 degrees C). In contrast, these substances failed to facilitate the response to bradykinin (0.1 or 1 microM) and instead suppressed it. Dideoxyforskolin (10 microM), an inactive analog of forskolin, had no effects on both the heat and bradykinin responses. These results demonstrate that an increase in intracellular cAMP induces opposite effects on the heat and bradykinin responses. Possible involvement of intracellular cAMP in the facilitatory effects of PGE2 on both responses was discussed in connection with the PGE receptor subtypes involved in the sensitization of the bradykinin and heat responses.

Cell-specific regulation of the stably expressed serotonin 5-HT1A receptor and altered ganglioside synthesis

Neurotransmission is dependent on the presence of neuronal receptors at the synapses, and important cell surface molecules such as gangliosides are pivotal in the maintenance of synaptic contacts. To study the interrelationship between these two classes of molecules, we achieved stable expression of the hippocampus- and CNS-localized serotonin 1A receptor (5-HT1A-R) in three 5-HT1A-R-deficient neuronal cell lines and also the control, non-neural CHO cells. A strong passage dependence of 5-HT1A-R expression, as measured by mRNA levels as well as membrane binding to the selective agonist [3H]8-OH-DPAT, was observed only in the HN2 (hippocampal) and NCB-20 (CNS) cells which are derived from tissues of natural occurrence of the 5-HT1A-R. A paradigm of stress was obtained by carrying out continuous culture of cells without feeding. During this time a dramatic increase in 5-HT1A-R mRNA and [3H]8-OH-DPAT binding was observed only in the neuronal cells after confluence and during decreased cell viability (days 10/11). This was not due to differentiation, since deliberate serum deprivation and differentiation of cells did not result in any dramatic increase in 5-HT1A-R expression. Analysis of ganglioside synthesis by pulse labeling of the transfected cells produced striking results. In the dorsal root of the ganglion (DRG) derived F-11 cells which show low but significant levels of complex gangliosides before transfection, the mere presence of the serotonin 1A receptor resulted in a dramatic increase in synthesis of gangliosides comigrating with GM2, GD1a, GD1b, and GT1b (20-fold by densitometry). In contrast, there was only a 2-fold increase in the overall content of complex gangliosides in the presence of the 5-HT1A-R. In the NCB-20 cells which contain only GD1a but no GD1b or GT1b before transfection, a decrease in GD1a synthesis was observed following transfection. Also agonist (8-OH-DPAT) binding to the serotonin 1A receptor in NCB-20 cells produced a 3-fold increase in synthesis of a ganglioside comigrating with GM3. Thus, our neuroblastoma transfectants help demonstrate stress-induced regulation of the 5-HT1A-R, which in turn exerts a strong and cell type-specific control over such essential cell-surface determinants like gangliosides.

Regulation of bradykinin-stimulated phospholipase C and arachidonic acid release by protein kinase A in MDCK-D1 cells

Regulation of phospholipases C (PLC) and arachidonic acid (AA) release by cAMP-dependent protein kinase (PKA) was investigated in MDCK-D1 cells. Bradykinin (BDK) was used to stimulate PLC and AA release, while arginine vasopressin (AVP), forskolin (FSK), isobutylmethylxanthine (IBMX) were used to increase cAMP levels and stimulate PKA. When cells were preincubated for 20 min with 10 microM FSK + 0.5 mM IBMX, and subsequently treated with 1 microM BDK or control medium for 40 min, the basal and BDK-stimulated PLC activity, measured as accumulated labelled inositol phosphate (InsP) after 40 min and inositol trisphosphate (InsP3) after 10 s, were significantly inhibited. In a parallel manner, FSK + IBMX also significantly decreased both basal and BDK-stimulated diacylglycerol (DAG) production. The basal and BDK-enhanced AA release into the media was also significantly inhibited by pretreatment with FSK + IBMX. In parallel experiments, H-89, a specific inhibitor of PKA, was preincubated for 60 min prior to addition of BDK and this resulted in a reversal of FSK+IBMX-induced inhibition of basal and BDK-stimulated PLC activity and AA release. An inhibitor of inositide-hydrolysing PLC, U73122, (1 microM) was also found to blunt BDK-stimulated PLC activity and BDK-enhanced AA release which indicated that stimulation of AA release by the nonapeptide was second to PLC activation. The ionophore, A23187, (10 microM) greatly stimulated AA release and to a much lesser extent, PLC activity. Its effect on AA release however was not blocked by inhibiting protein kinase C (PKC) with staurosporine (SSP) and consequently did not notably involve the PLC-PKC cascade. Activation of PKA with FSK + IBMX was found to significantly inhibit the enhancement of AA release by ionophore. With 12-tetradecanoyl-phorbol-13-acetate (TPA) also present there was a synergistic increase in the A23187-stimulated AA release and activation of PKA under such conditions inhibited AA release to a similar extent though the synergistic effect remained. The results strongly suggest a role for PKA in the regulation of PLC activity and AA release in MDCK-D1 cells. Control of AA release by PKA, is mediated both by mechanisms which involve blunting of PLC activity and mechanisms which are downstream from the PLC-PKC cascade.

Effect of retinoic acid on Nm/23 nucleoside diphosphate kinase and components of cyclic adenosine monophosphate-dependent signalling in human neuroblastoma cell lines

The effects of retinoic acid on components of the cAMP-dependent signalling system were examined in two related human neuroblastoma cell lines SK-N-SH-F (SHF) and SK-N-SH-N (SHN). Retinoid treatment for a week significantly increased the concentration of intracellular cAMP and the levels of activity of protein kinase A and adenylate cyclase in both cell lines. Retinoic acid treatment also caused a very marked translocation of nucleoside diphosphate kinase from the cytosol to the membrane fraction. The increases in cyclic nucleotide and protein kinase A activity were observed to occur as early as within 1 and 2 days respectively and preceded or were concurrent with the onset of observable morphological differentiation. Results also indicated that agents which elevated intracellular cAMP caused neuronal differentiation and blunted retinoic acid-induced melanocytic differentiation in SHF cells. However, increases in cAMP brought about by treatment of SHF cells with retinoic acid alone were several-fold smaller and thus insufficient to induce neuritogenesis in these cells. The results as a whole indicate that one overall effect of retinoic acid treatment is to upgrade the activity of components of the cAMP-dependent signalling system in both neuroblastoma cell lines. However, retinoic acid causes the SH-F and SH-N cell lines to differentiate along different routes which means that the upgrading responses may be related to more general aspects of differentiation rather than to specific phenotype expression.

Inhibition of protein kinase A fails to alter mast cell adenosine responsiveness

Adenosine activates adenylate cyclase and phospholipase C in mast cells and potentiates stimulated mediator release. To determine whether activation of adenylate cyclase is necessary for the effects of adenosine on the mast cell secretory process, a specific inhibitor of cAMP-dependent protein kinase, KT5720, was used. Antigen and adenosine each induced a rapid increase in mast cell cAMP-dependent protein kinase activity within 30 s. Preincubation with KT5720 (100 nM-10 microM) suppressed cAMP-dependent protein kinase activity and inhibited antigen-stimulated beta-hexosaminidase and leukotriene C4 releases. Adenosine retained its ability to potentiate beta-hexosaminidase release in antigen- and A23187-stimulated cells even in the presence of complete cAMP-dependent protein kinase inhibition. Mast cells rendered unresponsive to adenosine-related signals by preincubation with adenosine analogs maintained this hyporesponsiveness after incubation with KT5720. It appears that the abilities of adenosine to augment mast cell degranulation and induce receptor hyporesponsiveness are independent of changes in cAMP.

P0 phosphorylation in nerves from normal and diabetic rats: role of protein kinase C and turnover of phosphate groups

The effects of phorbol ester and forskolin on the net phosphorylation and turnover of P0 phosphate groups was studied in normal and experimentally diabetic rats. In sciatic nerve segments isolated from normal rats and incubated with [32P]-inorganic phosphate, phosphorylation of the major peripheral myelin protein, P0, was increased 2-5 fold in a time and dose-dependent manner by phorbol 12,13 dibutyrate (PDB). This increase was blocked by the protein kinase inhibitors, H-7 and staurosporine. Both the basal and PDB-stimulated phosphorylation of P0 were significantly greater in segments of sciatic nerve from streptozotocin-induced diabetic rats. Prolonged exposure of nerve segments to PDB abolished the stimulated phosphorylation of P0 and immunoblots of nerve proteins revealed a decrease in the content of the protein kinase C alpha-isoform. The adenylate cyclase activator, forskolin, had no effect on the PDB-stimulated phosphorylation of P0 in normal nerve but decreased phosphorylation in diabetic nerve. To measure turnover of P0 phosphate groups, nerves were incubated with 32P and incorporated label was then chased in radioactivity-free medium for up to 4 hours. P0 from normal nerve prelabeled under basal conditions lost 25% of its radioactivity during this time. In contrast, nearly all of the additional phosphate groups prelabeled in the presence of PDB disappeared after 2 hours of chase. P0 phosphate groups from diabetic nerve displayed similar turnover kinetics. When forskolin was added to the chase medium, the turnover of P0 phosphate moieties was accelerated in normal, but not in diabetic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Forskolin does not augment the bradykinin response of canine visceral polymodal receptors in vitro

In clarifying the possible involvement of cyclic AMP (cAMP) in prostaglandin (PG) E2-induced sensitization of the bradykinin response of canine testicular polymodal receptors, the effects of forskolin, an activator of adenylyl cyclase, were studied in the presence and absence of acetylsalicylic acid (ASA, 550 microM) which blocks PG production. Forskolin (10 microM) seldom induced discharges in polymodal receptors. An unexpected outcome of this study was that forskolin induced no facilitation of the bradykinin (0.1 microM) response, both in the absence and presence of ASA. A slight yet significant suppression of the bradykinin response was instead observed in the absence of ASA. These results suggest that intracellular cAMP may be related with PG E2-induced sensitization of the bradykinin response through its decrease.

Forskolin and phorbol myristate acetate inhibit intracellular Ca2+ mobilization induced by amitriptyline and bradykinin in rat frontocortical neurons

Regulations of the increase in intracellular Ca2+ concentration ([Ca2+]i) and inositol 1,4,5-trisphosphate (IP3) production by increasing intracellular cyclic AMP (cAMP) levels or activating protein kinase C (PKC) were studied in rat frontocortical cultured neurons. Amitriptyline (AMI; 1 mM), a tricyclic antidepressant, and bradykinin (BK; 1 microM) stimulated IP3 production and caused transient [Ca2+]i increases. Pretreatment with forskolin (100 microM, 15 min) decreased the AMI- and BK-induced [Ca2+]i increases by 33 and 48%, respectively. However, this treatment had no effect on the AMI- and BK-induced IP3 productions. Dibutyryl-cAMP (2 mM, 15 min) also decreased the AMI- and BK-induced [Ca2+]i increases by 23 and 47%, respectively. H-8 (30 microM), an inhibitor of protein kinase A (PKA), attenuated the ability of forskolin to inhibit the AMI- and BK-induced [Ca2+]i increases, suggesting that the activation of cAMP/PKA was involved in these inhibitory effects of forskolin. On the other hand, forskolin treatment had no effect on 20 mM caffeine-, 10 microM glutamate-, or 50 mM K(+)-induced [Ca2+]i increases. Pretreatment with phorbol 12-myristate 13-acetate (PMA; 100 nM, 90 min) decreased both the AMI-induced [Ca2+]i increases and the IP3 production by 31 and 25%, respectively. H-7 (200 microM), an inhibitor of PKC, inhibited the ability of PMA to attenuate the [Ca2+]i increases. PMA also inhibited the BK-induced IP3 production and the [Ca2+]i increases.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase A activators inhibit agonist induced prostaglandin production in human amnion

Prostaglandin (PG) production by human amnion has been postulated to have a role in the onset of labor. Previous work by ourselves and others has demonstrated that oxytocin, phorbol esters and epidermal growth factor (EGF) increase PGE2 production in human amnion cells by activation of the Phospholipase C/Protein Kinase C (PKC) cascade system. The present study was undertaken to determine the effect of prior activation of the Adenylate Cyclase cascade system upon subsequent stimulation of PGE2 production by oxytocin, phorbol 12-myristate-13-acetate (PMA) or EGF in amnion cells and membrane discs. Isoproterenol, forskolin and dibutyryl cyclic adenosine monophosphate (dbcAMP) were utilized to activate the Adenylate Cyclase system at the receptor, enzyme and second messenger level. In control amnion cells, oxytocin, PMA and EGF each provoked dose dependent increases in PGE2 production. In cells preincubated with dbcAMP, forskolin or isoproterenol, agonist stimulated PGE2 production was markedly (50-90%) inhibited (p < 0.01). Inhibition was dose dependent upon preincubator concentrations. Maximal inhibition by adenylate cyclase activators occurred with 2-4 h of preincubation. In membrane discs, forskolin preincubation also inhibited oxytocin, PMA and EGF stimulation of PGE2 production. Activation of the Adenylate Cyclase system in human amnion cells or membrane discs inhibits the subsequent action of potent stimulators of PGE2 production in human amnion.

Effects of an inhibitor of adenylate cyclase on acrosome reaction induced by protein kinase C activators

To determine whether the adenylate cyclase and the protein kinase C pathways act independently to modulate the human sperm acrosome reaction, we studied the effects of 2'-O-methyladenosine (adenylate cyclase inhibitor) on acrosome reactions induced by protein kinase C activators (phorbol diesters and synthetic diacylglycerols) or an adenylate cyclase stimulator (forskolin:FR). Fifty aliquots of capacitated spermatozoa were divided into 5 groups (A, B, C, D, and E), each containing 10 samples. One control aliquot (CN) and five experimental aliquots (EX1, EX2, EX3, EX4, and EX5) were prepared from each sample. Phorbol 12-myristate, 13-acetate (PMA, 10 mumol/L), 4 beta-phorbol 12,13-didecanoate (PDD, 0.1 mumol/L), 1-oleoyl-2-acetyl-sn-glycerol (OAG, 50 mumol/L), 1,2-dioctanoyl-sn-glycerol (DOG, 50 mumol/L), or FR (10 mumol/L) was added to each of the experimental aliquots in groups A, B, C, D, and E, respectively. Increasing concentrations of 2'-O-methyladenosine were added to aliquots EX2, EX3, EX4, and EX5. After an incubation period of 25 min at 37 degrees C, it was found that the percentage of acrosome-reacted spermatozoa (%ARS) was significantly and dose-dependently decreased by 2'-O-methyladenosine concentrations of 1 mM or more. Within each group, the %ARS was significantly higher in EX1 aliquots than in CN aliquots. The reduction of acrosome reactions induced by protein kinase C activators by the adenylate cyclase inhibitor suggests that the protein kinase C pathway interacts with the adenylate cyclase pathway to modulate the human sperm acrosome reaction.

Protein kinase regulation of muscarinic receptor signalling in colonic smooth muscle

1. We have previously demonstrated that M2 and M3 muscarinic receptors coexist in the circular smooth muscle of canine proximal colon. Activation of receptors of the M2 subtype leads to inhibition of adenylyl cyclase activity through the GTP-binding protein, Gi, while M3 receptors are coupled to a pertussis toxin-insensitive GTP-binding protein and mediate phosphoinositide hydrolysis. 2. In the present study, the interactions between these second messenger systems were examined. Activation of either protein kinase C or adenosine 3':5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase attenuated carbachol-stimulated phosphoinositide hydrolysis without affecting basal activity. Activation of both protein kinases produced greater attenuation of inositol 1,4,5-trisphosphate formation than activation of either kinase alone. 3. In contrast to its inhibitory effect on phosphoinositide hydrolysis, activation of protein kinase C had no effect on adenylyl cyclase activity. 4. Activation of protein kinase C by phorbol ester treatment resulted in the sequestration of M3 muscarinic receptors from the cell surface without effecting the M2 muscarinic receptor population. Sequestered M3 muscarinic receptors were not rapidly degraded. 5. In contrast, elevation of cellular cyclic AMP decreased the affinity of cell surface muscarinic receptors for an antagonist radioligand without affecting their density. 6. Muscarinic agonist binding was not affected by either activation of protein kinase C or elevation of cellular cyclic AMP. 7. These data support the notion of negative feedback by protein kinase C and cyclic AMP-dependent protein kinase on phosphoinositide hydrolysis. In canine colonic circular smooth muscle this negative feedback regulation of inositol phosphate generation by muscarinic receptor stimulation does not appear to involve the guanine nucleotide binding protein:receptor interaction.

Inhibition of cyclic AMP accumulation in intact NCB-20 cells as a direct result of elevation of cytosolic Ca2+

Earlier studies established that adenylyl cyclase in NCB-20 cell plasma membranes is inhibited by concentrations of Ca2+ that are achieved in intact cells. The present studies were undertaken to prove that agents such as bradykinin and ATP, which elevate the cytosolic Ca2+ concentration ([Ca2+]i) from internal stores in NCB-20 cells, could inhibit cyclic AMP (cAMP) accumulation as a result of their mobilization of [Ca2+]i and not by other mechanisms. Both bradykinin and ATP transiently inhibited [3H]cAMP accumulation in parallel with their transient mobilization of [Ca2+]i. The [Ca2+]i rise stimulated by bradykinin could be blocked by treatment with thapsigargin; this thapsigargin treatment precluded the inhibition of cAMP accumulation mediated by bradykinin (and ATP). A rapid rise in [Ca2+]i, as elicited by bradykinin, rather than the slow rise evoked by thapsigargin was required for inhibition of [3H]cAMP accumulation. Desensitization of protein kinase C did not modify the inhibitory action of bradykinin on [3H]cAMP. Effects of Ca2+ on phosphodiesterase were also excluded in the present studies. The accumulated data are consistent with the hypothesis that hormonal mobilization of [Ca2+]i leads directly to the inhibition of cAMP accumulation in these cells and presumably in other cells that express the Ca(2+)-inhibitable form of adenylyl cyclase.

Hypoxia induces synthesis of a novel 22-kDa protein in neonatal rat oligodendrocytes

Neonatal (3-day-old) rat oligodendrocytes grown in monolayer culture and exposed to increasingly hypoxic culture conditions showed a dramatic reduction in myelin basic protein synthesis but no significant inhibition of Tran35S-label incorporation into oligodendrocyte proteins in general or into structural proteins such as actin. However, there was a dramatic increase in synthesis of a novel 22-kDa protein. Reoxygenation of cultures reversed the synthesis of the 22-kDa protein, and thiol and calpain protease inhibitors (EP-459 and leupeptin) did not prevent synthesis of the protein, suggesting that it did not result from proteolysis. The 22-kDa protein (which we have called hypoxin) was coimmunoprecipitated by a polyclonal antibody to actin but did not react with the anti-actin antibody on western blots. The synthesis of hypoxin accounted for up to 50% of the Tran35S-label incorporated into immunoprecipitated protein, suggesting that it plays a major role in the cell's response to hypoxia. Subcellular fractionation revealed that the 22-kDa protein was largely associated with the cytosolic/cytoskeletal compartment. However, it is unlikely to be one of the cytoskeleton-associated Rho or Rac low-molecular-mass (20-24 kDa) GTP-binding proteins because it did not bind [alpha-32P]GTP on western blots. Oligodendrocytes did not synthesize a 22-kDa protein in response to heat shock but did synthesize the typical 70- and 90-kDa heat-shock proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Manipulation of intracellular calcium in NCB-20 cells

A number of lines of evidence indicate that the Ca2+ and cyclic AMP signalling systems interact in NCB-20 cells. However, to date, the regulation of [Ca2+]i homeostasis has not been studied in this cell line. The present study aimed to clarify our understanding of [Ca2+]i homeostasis in these cells and to evaluate tools that manipulate [Ca2+]i, independently of protein kinase C effects. Bradykinin, by a B2-receptor, elevated [Ca2+]i by a pertussis-toxin-insensitive mechanism. The BK-stimulated [Ca2+]i rise originated from intracellular sources, without a contribution from Ca2+ entry mechanisms. The effect of BK was precluded by pretreatment with thapsigargin and ionomycin--compounds that elevated [Ca2+]i independent of phospholipase C activation. Both compounds, however, exerted effects in addition to stimulating release of Ca2+ from BK-sensitive stores; the BK-sensitive Ca2+ pool was a subset of the thapsigargin-sensitive pool; ionomycin strongly stimulates Ca2+ entry. Activation of protein kinases A and C attenuated the duration of the BK-induced rise in [Ca2+]i, without affecting the peak [Ca2+]i, suggesting interference with the BK response at a step downstream of the activation of phospholipase C. Application of these approaches should enhance the delineation of the consequences of Ca2+ mobilization on cyclic AMP accumulation.

Ca2+ ionophore and phorbol ester stimulate diacylglycerol formation and phosphatidylcholine hydrolysis in rat parotid acinar cells

We investigated the effects of A23187 and phorbol 12,13-dibutyrate (PDBu) on sn-1,2-diacylglycerol (DAG) accumulation and phosphatidylcholine (PC) hydrolysis in rat parotid acinar cells. Both A23187 and PDBu, in concentration ranges of 0.001-0.1 microM, stimulated DAG accumulation and PC hydrolysis in a time- and concentration-dependent manner. Treatment with A23187 and PDBu stimulated the release of [3H]choline and [3H]phosphocholine into the medium, indicating [3H]PC hydrolysis is due to the activation of phospholipases C and D; however, [3H]phosphatidylethanolamine hydrolysis was not indicated. These releases were unaffected by the addition of glucose 6-phosphate, a phosphatase inhibitor. Staurosporine, a protein kinase C inhibitor, significantly inhibited the DAG accumulation and the PC hydrolysis stimulated by these agents. Combinations of A23187 and PDBu potentiated the stimulatory effect which each of these agents alone had on DAG accumulation and PC hydrolysis. This mode of action was additive but not synergistic. These results suggest that DAG accumulation induced by A23187 and PDBu is related to the PC hydrolysis mediated via the activation of phospholipases C and D, and that it is not related to phosphatidylethanolamine hydrolysis.

Effects of surgical sympathetic denervation on myo-inositol trisphosphate production and contraction in the dilator and sphincter smooth muscles of the rabbit iris: evidence for interaction between the cyclic AMP and calcium signaling systems

The effects of norepinephrine (NE), carbachol (CCh), NaF, 3-isobutyl-1-methylxanthine (IBMX), and high K+ concentration (80 mM) depolarization on inositol trisphosphate (IP3) accumulation, cyclic AMP (cAMP) formation, and contraction were investigated in the dilator and sphincter smooth muscles of the sympathetically denervated as well as the normal rabbit eye. (a) In the denervated dilator muscle, NE-stimulated IP3 production and contraction are enhanced. (b) In the sphincter muscle of rabbits that have undergone sympathetic denervation. CCh-stimulated IP3 production and contraction are attenuated. (c) The increase in tension by a maximal effective dose of NaF (209 mM) in the dilator was 12.5 and 18 mg of tension/mg wet weight in normal and denervated tissue, respectively, and in the sphincter was 33.8 and 15.2 mg of tension/mg wet weight in normal and denervated tissue, respectively. NaF had no effect on cAMP formation. (d) Addition of NE had no effect on cAMP formation in both the normal and denervated dilator, whereas basal and IBMX-induced cAMP formation increased. in the denervated sphincter over that of the normal tissue by 15 and 60%, respectively. (e) Isoproterenol (5 microM) increased cAMP formation in the normal and denervated sphincter by 47 and 91%, respectively. (f) Whereas CCh inhibits cAMP formation in the normal sphincter, it lost its inhibitory effect in the sphincter with denervation. (g) IBMX (0.1 mM) attenuated the CCh-stimulated IP3 production and contraction of the sphincter by approximately 30% of their respective controls. (h) High K+ concentration depolarization attenuated contraction in both dilator and sphincter muscles with denervation. These observations suggest that an increase in the level of cAMP in the iris sphincter due to sympathetic denervation could lead to inhibition of phospholipase C (or other target sites, such as phosphorylation of the muscarinic receptor, Gp protein itself, myosin light chain kinase, or the IP3 receptor), IP3 production, and contraction. In conclusion, we suggest that the supersensitivity and subsensitivity observed after surgical sympathetic denervation of the iris dilator and sphincter muscles, respectively, are caused by alterations in the efficiency of coupling, probably through the Gp proteins, between their respective receptors and the breakdown of polyphosphoinositides by phospholipase C. In addition, we propose that the sympathetic nervous system can regulate, through alterations in cAMP levels, the muscarinic stimulation of IP3 accumulation and contraction in the iris sphincter. These findings add further support to the hypothesis that there are reciprocal interactions between the cAMP and IP3-Ca2+ signaling systems and the contractile response in the iris smooth muscle.

Chronic opioid treatment attenuates carbachol-mediated polyphosphoinositide hydrolysis in chick embryo neuronal cultures

Opiate binding sites on cultured neurons derived from 6-day-old (E6) chick embryo cerebral hemispheres (CH), shown to be cholinergic by choline acetyltransferase immunostaining, were labeled with [3H]etorphine (mu and delta opiate receptors expression) and [3H]morphine (mostly mu). When examined by light microscope autoradiography, opiate receptors were found to be expressed by most neurons, and were distributed predominantly on neuronal perikarya. Muscarinic and opiate receptors in E6CH cultured neurons were found to be functionally coupled when the effects of opiate receptor occupancy on the inositol phosphate-linked muscarinic receptors was studied. Carbachol stimulated the release of [3H]inositol phosphates (InsP) from cultures preincubated with [3H]inositol and LiCl, in a dose-dependent manner, and the functional expression of muscarinic receptors peaked in number at day 7 in culture, declining thereafter. Short-term (less than 1 h) treatment of E6 neuronal cultures with 1 microM opioid peptides such as morphiceptin or D-Ala2-D-Leu5-enkephalin (DADLE) did not inhibit the release of inositol phosphates in response to 1 mM carbachol whereas forskolin, which also activates adenylate cyclase and raises cAMP levels, inhibited InsP release by about 25%. In contrast, long-term (48 h) opioid treatment with either morphiceptin or DADLE (1-10 microM) inhibited the carbachol-stimulated inositol phosphate release by greater than or equal to 50%. Prolonged treatment with morphiceptin also inhibited the bradykinin-mediated release of InsP from E6CH cells. In both cases, the inhibition was partially blocked by the continuous presence of naloxone, suggesting that the inhibition was mediated through opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin-1 stimulates the release of arachidonic acid and prostaglandins in rabbit iris sphincter smooth muscle: activation of phospholipase A2

We have investigated the effects of endothelin-1 (ET1) on phospholipid hydrolysis and 3H-arachidonic acid (AA) release and prostaglandin synthesis in the rabbit iris sphincter smooth muscle. ET1 actions are concentration- and time dependent with an EC50 for AA release of 1 nM and t1/2 value of 1.5 min. We have identified the AA metabolites released by ET1, employing HPLC, as both cyclooxygenase and lipoxygenase products. The AA released by ET1 appears to derive mainly from the phosphoinositides through phospholipase A2, rather than phospholipase C activation. A key role for phospholipase A2 in AA release in the sphincter muscle is supported by the following observations. (1) Pretreatment of the labeled sphincter with the phorbol ester, PDBu (100 nM) inhibited ET1-stimulated IP3 formation, but it potentiated ET1-stimulated AA release. (2) Pretreatment of the labeled tissue with isoproterenol (5 M) inhibited ET1-stimulated IP3 production without altering AA release. (3) The potency for ET1-stimulated AA release (EC50 = 1 nM) was much higher than that for IP3 formation (EC50 = 45 nM). (4) There were considerable increases, rather than decreases, in 1, 2-diacyl-glycerol formation (1.2-folds) and its phosphorylated product, phosphatidic acid (2.6-folds) by ET1. It is concluded that in the rabbit iris sphincter ET1 is a potent agonist for AA release and eicosanoid synthesis and that AA is released from phosphoinositides mainly through activation of phospholipase A2.

Protein kinase C and phospholipase C mediate alpha 1- and beta-adrenoceptor intercommunication in rat hypothalamic slices

These experiments examined the mechanism by which phenylephrine enhances beta-adrenoceptor-stimulated cyclic AMP formation in rat hypothalamic and preoptic area slices. To this end we manipulated phospholipase C. phospholipase A2, and protein kinase C activity in slices and assessed the effects of these manipulations on phenylephrine augmentation of isoproterenol-stimulated cyclic AMP generation. Since previous work indicated that estrogen enhances the alpha 1-component of cyclic AMP formation, we examined slices from both gonadectomized and estrogen-treated animals. The alpha 1-antagonist prazosin eliminated phenylephrine augmentation of the beta-response, suggesting that alpha 1-adrenergic receptors mediate the potentiation of cyclic AMP formation. Inhibition of protein kinase C by H7 attenuated the alpha 1-augmentation of beta-stimulated cyclic AMP formation. Staurosporine, a more potent protein kinase C inhibitor, completely abolished the alpha 1-augmenting response. In addition, phenylephrine potentiation of the isoproterenol response was not observed if protein kinase C was first stimulated directly with a synthetic diacylglycerol (1-oleoyl-2-acetyl-sn-glycerol) or phorbol ester (phorbol 12,13-dibutyrate). Neomycin, an inhibitor of phospholipase C, decreased alpha 1-receptor enhancement of beta-stimulated cyclic AMP formation, whereas quinacrine, an inhibitor of phospholipase A2, did not. The data suggest that the postreceptor mechanism involved in alpha 1-adrenergic receptor potentiation of cyclic AMP generation in hypothalamic and preoptic area slices includes activation of phospholipase C and protein kinase C.

Interactions between second messengers: cyclic AMP and phospholipase A2- and phospholipase C-metabolites

The article reviews several new findings on the interactions between phospholipase A2- and phospholipase C-derived metabolites and cyclic AMP, in view of the developments recently achieved in studies on intracellular signal transduction. A complex network of multi-directional regulative mechanisms in the airways and inflammatory blood cells is briefly outlined.

Dephosphorylation of specific proteins in HL-60 cells by 1 alpha, 25-dihydroxyvitamin D3: possible involvement of cAMP-dependent protein kinase

Changes of phosphoprotein patterns in HL-60 cells were studied during short exposures to 1 alpha, 25-dihydroxyvitamin D3 [1,25(OH)2D3]. One hundred nanometers 1,25(OH)2D3 dephosphorylated at least three proteins in 6 h: phosphoproteins with molecular weights of 82 kD (pp82), 33 kD (pp33), and 31 kD (pp31). Phosphorylation of pp33 and pp31 was also suppressed by 1 mM dbcAMP, and dephosphorylation of the two protein by 1,25(OH)2D3 was inhibited by 8 microM H-8, an inhibitor of cAMP-dependent protein kinase (PKA). Furthermore, 8 microM H-8 inhibited dephosphorylation of the two proteins when it was added with 1,25(OH)2D3. On the other hand, 10 nM TPA gave no significant change to these two phosphoproteins. These results suggest the possibility that PKA is involved in the early stages of 1,25(OH)2D3-induced HL-60 cell differentiation through specific protein dephosphorylation.

Inhibitory effects of cyclic AMP-elevating agents on norepinephrine-induced phosphatidylinositide hydrolysis and contraction in rat aorta

1. Exposure of rat aorta to forskolin, dibutyryl cyclic AMP, rolipram or isobutylmethylxanthine partially prevented the increased inositol monophosphate accumulation due to norepinephrine, while the contractile responses to norepinephrine were almost completely inhibited. 2. Inhibition of the increased phosphatidylinositide synthesis due to norepinephrine could not account for the decreased inositol monophosphate accumulation. 3. Although the increased inositol monophosphate accumulation due to norepinephrine was partially inhibited by the cyclooxygenase inhibitor, indomethacin, the inhibitory effects of the cyclic AMP-elevating agents were still observed in the presence of indomethacin. 4. Inhibition of agonist-induced phosphatidylinositide hydrolysis may contribute, at least in part, to the vasodilatory effects of the cyclic AMP-elevating agents.