Phospholipid and arachidonic acid metabolism in zymosan-stimulated human monocytes: modulation by cAMP

Glycerophospholipid Analysis of Optic Nerve Regeneration Models Indicate Potential Membrane Order Changes Associated with the Lipidomic Shifts

Purpose: Optic nerve (ON) injury causes irreversible degeneration, leading to vision loss that cannot be restored with available therapeutics. Current therapies slow further degeneration but do not promote regeneration. New regenerative factors have been discovered that are successful in vivo. However, the mechanisms of efficient long-distance regeneration are still unknown. Membrane expansion by lipid insertion is an essential regenerative process, so lipid profiles for regenerating axons can provide insight into growth mechanisms. This article's analysis aims to add to the increasingly available ON regeneration lipid profiles and relate it to membrane order/properties. Methods: In this study, we present an analysis of glycerophospholipids, one of the largest axonal lipid groups, from three mammalian ON regeneration lipid profiles: Wnt3a, Zymosan + CPT-cAMP, and Phosphatase/Tensin homolog knockout (PTENKO) at 7 and 14 days post crush (dpc). Significant lipid classes, species, and ontological properties were crossreferenced between treatments and analyzed using Metaboanalyst 5.0 and Lipid Ontology (LION). Membrane order changes associated with significant lipid classes were evaluated by C-Laurdan dye and exogenous lipids provided to a neuroblastoma cell line. Results and Conclusions: At 7 dpc, ONs show increased lysoglycerophospholipids and decreased phosphatidylethanolamines (PEs)/negative intrinsic curvature lipids. At 14 dpc, regenerative treatments show divergence: Wnt3a displays higher lysoglycerophospholipid content, while Zymosan and PTENKO decrease lysoglycerophospholipids and increase phosphatidylcholine (PC)-related species. Membrane order imaging indicates lysoglycerophospholipids decreases membrane order while PE and PC had no significant membrane order effects. Understanding these changes will allow therapeutic development targeting lipid metabolic pathways that can be used for vision loss treatments.

Methylxanthines in asthma

Methylxanthines represent a unique class of drugs for the treatment of asthma. The methylxanthine theophylline has demonstrated efficacy in attenuating the three cardinal features of asthma - reversible airflow obstruction, airway hyperresponsiveness, and airway inflammation. At doses achieving relatively high serum levels in which toxic side effects are sometimes observed, direct bronchodilatory effects of theophylline are recognized. At lower serum concentrations, theophylline is a weak bronchodilator but retains its capacity as an immunomodulator, anti-inflammatory, and bronchoprotective drug. Intense investigation into the molecular mechanisms of action of theophylline has identified several different points of action. Phosphodiesterase inhibition and adenosine receptor antagonism have both been implicated in promoting airway smooth muscle relaxation and bronchodilation. Similar mechanisms of action may explain the inhibitory effects of theophylline on immune cells. At lower concentrations that fail to inhibit phosphodiesterase, effects on histone deacetylase activity are believed to contribute to the immunomodulatory actions of theophylline. Since anti-inflammatory and immunomodulatory effects of methylxanthines are realized at lower serum concentrations than are required for bronchodilation, theophylline's predominant role in asthma treatment is as a controller medication for chronic, persistent disease.

Phosphorylation by protein kinase a inhibits nuclear import of 5-lipoxygenase

The enzyme 5-lipoxygenase initiates the synthesis of leukotrienes from arachidonic acid. Protein kinase A phosphorylates 5-lipoxygenase on Ser(523), and this reduces its activity. We report here that phosphorylation of Ser(523) also shifts the subcellular distribution of 5-lipoxygenase from the nucleus to the cytoplasm. Phosphorylation and redistribution of 5-lipoxygenase could be produced by overexpression of the protein kinase A catalytic subunit alpha, by pharmacological activators of protein kinase A, and by prostaglandin E(2). Mimicking phosphorylation by replacing Ser(523) with glutamic acid caused cytoplasmic localization; replacement of Ser(523) with alanine prevented phosphorylation and redistribution in response to protein kinase A activation. Because Ser(523) is positioned within the nuclear localization sequence-518 of 5-lipoxygenase, the ability of protein kinase A to phosphorylate and alter the localization of green fluorescent protein fused to the nuclear localization sequence-518 peptide was also tested. Site-directed replacement of Ser(523) with glutamic acid within the peptide impaired nuclear accumulation; overexpression of the protein kinase A catalytic subunit alpha and pharmacological activation of protein kinase caused phosphorylation of the fusion protein at Ser(523), and the phosphorylated protein was found chiefly in the cytoplasm. Taken together, these results indicate that phosphorylation of Ser(523) inhibits the nuclear import function of a nuclear localization sequence, resulting in the accumulation of 5-lipoxygenase enzyme in the cytoplasm. As cytoplasmic localization can be associated with reduced leukotriene synthetic capacity, phosphorylation of Ser(523) serves to inhibit leukotriene production by both impairing catalytic activity and by placing the enzyme in a site that is unfavorable for action.

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.

Inhibition of allergen-induced lung eosinophilia by type-III and combined type III- and IV-selective phosphodiesterase inhibitors in brown-Norway rats

We examined the effect of a type IV (rolipram) and a combined type III and IV (Org 20421) isoenzyme-selective phosphodiesterase inhibitor upon allergen-induced pulmonary eosinophil recruitment in sensitised Brown Norway rats. Rats were sensitised with ovalbumin intraperitoneally and later challenged with ovalbumin aerosol which induced a significant increase in the total eosinophil and neutrophil count in bronchovalveolar lavage fluid at 24 hours (from 0.38 +/- 0.12 to 1.36 +/- 0.18 x 10(6), p < 0.01 and from 0.06 +/- 0.01 to 0.33 +/- 0.07 x 10(6), p < 0.01) respectively. Pretreatment with rolipram (30 mumol/kg) and Org 20421 (30 mumol/kg) abolished the eosinophilia and neutrophilia evoked by ovalbumin. We conclude that type IV and possibly type III isozyme phosphodiesterase inhibitors may regulate, directly or indirectly, eosinophil and neutrophil activity and/or those cells responsible for attracting them into the lung.

Cyclic AMP regulation of annexins I, II, V synthesis and localization in cultured porcine thyroid cells

Porcine thyroid cells cultured in the presence of TSH (0.1 mU/ml) or forskolin (10(-5) M) for 4 days display an increased annexin I, II, V biosynthesis when compared with unstimulated cells. Annexin I mostly accumulates in the cytosolic fraction and annexins II and V in the particulate fraction. TSH and forskolin affect in the same manner annexin biosynthesis and localization. In the TSH and forskolin treated cells PGE2 production is very low in comparison with the very high PLA2 activity observed in the dedifferentiated control cells. A strong inhibition of the PGE2 production is observed in control cells incubated with a purified annexin V preparation. These results suggest the existence, in porcine thyroid cells, of a cross-talk between the adenylyl cyclase system and the phospholipase A2 mediated pathways. Annexins' biosynthesis and localization are under the control of the adenylyl cyclase system and participate in this cross-talk.

Cyclic AMP enhancing drugs modulate eicosanoid release from human alveolar macrophages

The effect of the phosphodiesterase inhibitor isobutyl-methylxanthine (IBMX), salbutamol and sodium nitroprusside was evaluated regarding PGE2 and LTB4 release and cAMP and cGMP level in human alveolar macrophages obtained from controls and COPD patients. Basal levels per five million control- respectively COPD alveolar macrophages: cAMP 1.2 and 1.0 pmole; cGMP 8.4 and 9.1 fmole; PGE2 120 and 63 pg and LTB4 19.2 and 14.8 pg. In both populations IBMX increased cAMP level by 55-93% and salbutamol+IBMX by 285-252%. Except for the 61% rise in LTB4 release by salbutamol+IBMX the drugs hardly affected PGE2 and LTB4 release from control macrophages. In COPD alveolar macrophages, however, IBMX and IBMX+salbutamol largely reduced PGE2 release (63 vs 11 pg per 10(6) cells) but less efficiently increased LTB4. In both macrophage populations sodium nitroprusside (SNP) substantially increased (3-4 fold) cGMP level but did not affect eicosanoid production. Present results indicate that drugs which enhance cAMP level decrease PGE2 release from COPD macrophages and stimulate the release of LTB4 a chemotactic mediator involved in bronchial inflammatory reactions.

The effect of cyclic AMP and cyclic GMP phosphodiesterase inhibitors on the superoxide burst of guinea-pig peritoneal macrophages

1. The cyclic nucleotide phosphodiesterase (PDE) activity of guinea-pig peritoneal macrophages was partially characterized and the effects of selective and non-selective inhibitors of adenosine 3':5'-cyclic monophosphate (cyclic AMP PDE) and guanosine 3':5'-cyclic monophosphate (cyclic GMP PDE) phosphodiesterases on superoxide generation were investigated using peritoneal macrophages from horse-serum pretreated guinea-pigs. 2. The non-selective PDE inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and the PDE I/V selective inhibitor, zaprinast, inhibited spontaneous superoxide generation with IC50s of 30.7 +/- 11.3 microM and 145 +/- 17 microM respectively (n = 6 and 5). The concentration-response curves for the PDE IV selective inhibitors rolipram and Ro20-1724 were biphasic; mean maximum inhibitions were 56.9 +/- 5.9% and 66.8 +/- 10.5% respectively at 300 microM, but in 2 out of 6 (rolipram) and 2 out of 5 (Ro20-1724) experiments inhibition was < 50%. The PDE III inhibitor SK&F 94120 was without effect. Spontaneous superoxide generation was reduced 57 +/- 10% by 1 microM prostaglandin E2 (PGE2) and 62.6 +/- 3.76% by 1 microM salbutamol. 3. The increase in superoxide generation elicited by FMLP (10(-9)-10(-5)M) was unaffected by any of the PDE inhibitors studied. Inhibition of FMLP-stimulated superoxide generation by PGE2 was enhanced in the presence of 10 microM IBMX. 4. Macrophages were found to contain a predominantly membrane bound cyclic AMP PDE (90% of total activity) which was unaffected by cyclic GMP or calcium/calmodulin. The cyclic AMP PDE activity in the cytosolic fraction was enhanced in the presence of calcium/calmodulin. Selective inhibitors of PDE IV inhibited the particulate cyclic AMP PDE activity (IC50s rolipram 1.5 +/- 0.3 microM, Ro 20-17244.1 +/- 0.6 microm) as did the non-selective inhibitor IBMX (IC50 22 +/- 8 microM). The macrophage particulate PDE activity was resistant to inhibition by the PDE III inhibitor SK&F 94836 and the PDE I/V inhibitor, zaprinast. The cytosolic calcium/calmodulin stimulated cyclic AMP hydrolytic activity was inhibited by zaprinast (IC50 - calcium/calmodulin 123 +/- 39 microM; + calcium/calmodulin IC50 17.7 +/- 6.3 microM).5. The results indicate that guinea-pig peritoneal macrophages contain a type IV cyclic AMP PDE which is predominantly membrane associated and a predominantly cytosolic calcium/calmodulin stimulated cyclic AMP PDE. Functional studies suggest that both of these PDE activities contribute to cyclic AMP hydrolysis and regulation of superoxide generation in these cells. Inhibition of spontaneous superoxide generation, but not that stimulated by FMLP, suggests that the activity of PDE inhibitors is subject to functional antagonism but that this can be overcome by enhancing cyclic AMP formation.

Effects of dibutyrylcyclic adenosine monophosphate on bleomycin-induced lung toxicity in hamsters

Cyclic nucleotides play an important role in the regulation of fibroblast proliferation and collagen metabolism. In the present study, the antifibrotic potential of dibutyrylcAMP (Bt2cAMP) was evaluated in the bleomycin (BLM)-hamster model of pulmonary fibrosis. Bt2cAMP (10 mg kg-1, s.c.) or saline (SA, s.c.) was given daily two days prior to the first intratracheal (i.t.) dose of BLM or SA and thereafter throughout the study. BLM or SA was instilled i.t. in three consecutive doses (2.5, 2.0 and 1.5 U 5ml-1 kg-1) at weekly intervals. Hamsters were killed at 7, 14 and 20 days after the third i.t. instillation. Bt2cAMP significantly reduced the contents of lung hydroxyproline and lung thiobarbituric acid reactive substance equivalents in BLM-treated animals at 7 and 14 days. Bt2cAMP significantly elevated lung superoxide dismutase activity in BLM-treated animals at 7 days. Lung prolyl hydroxylase activity was significantly elevated at 14 and 20 days in SABLM- and Bt2cAMPBLM-treated animals. The ratio of cAMP/cGMP was significantly reduced at all time points in SABLM-treated animals but only at 7 and 14 days in Bt2cAMPBLM-treated animals. Bt2cAMP caused no significant changes in lung calcium and calmodulin levels and protein content of the bronchoalveolar lavage. BLM significantly increased various inflammatory cell counts in the lavage at all three time points. The cell counts in the Bt2cAMPBLM groups were generally lower at 7 days and higher at 20 days than those of the SABLM groups. Histological evaluation showed that the lungs of Bt2cAMPBLM-treated hamsters progressed from an inflammatory cell lesion to a fibrotic lesion at a slower rate than the SABLM groups. It was concluded that Bt2cAMP attenuated BLM-induced pulmonary fibrosis in hamsters in part by delaying the acute phase of the inflammatory reaction.

Modulation of the B cell response to T-independent antigens by prostaglandins: evidence for effector system cross-talk

The extracellular environment is a major factor in determining the responsiveness of a cell to particular stimuli. For example, E series prostaglandins suppress B cell responses to T-independent antigens, mitogen stimulation of DNA synthesis and proliferation, and the primary immune response. We investigated the effects of prostaglandins on the intracellular signals generated by receptor-coupled effector systems in B lymphocytes. Pretreating splenocytes from athymic nude mice with forskolin, PGE1, or PGE2 decreased the magnitude of anti-IgM-induced changes in cytosolic free [Ca2+]. Addition of 8-Br-cAMP, forskolin, PGE1, or PGE2 following stimulation with anti-IgM resulted in a decrease in the intracellular calcium signal measured by fluorescence-activated cell sorting using Indo-1 as a Ca2+ indicator. This decrease was not a result of an inhibition of influx across the plasma membrane. Thus activation of adenylate cyclase by prostaglandins modifies the generation of signals by phosphoinositidase C. This effector system cross-talk between adenylate cyclase and phosphoinositidase C is consistent with and may account for the inhibitory effects of prostaglandins in B cell responses.

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.

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.

Phospholipase A2-mediated release of arachidonic acid in stimulated guinea pig alveolar macrophages: interaction with lipid mediators and cyclic AMP

The stimulation of cultured guinea pig alveolar macrophages by the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine, or by the phospholipid inflammatory mediator platelet activating factor (PAF) induced an increase in arachidonic acid release and its cyclooxygenase products. This release, which was mimicked by the association of threshold concentrations of the calcium ionophore A 23187 and of the protein kinase C activator tetradecanoyl phorbol acetate arose mainly from diacyl- and alkyl-acyl-phosphatidylcholine and phosphatidylinositol. Using [1-14C]arachidonic acid-labeled membranes as an endogenous substrate as well as dioleoyl-phosphatidyl [14C]ethanolamine as an exogenous substrate, we showed that phospholipase A2 activity of stimulated macrophages increases upon stimulation. Treatment of macrophages by prostaglandin E2 decreased the arachidonic acid release elicited by the chemotactic peptide and PAF. Furthermore, prostaglandin E2 increased and PAF decreased the cellular content in cyclic AMP. From these results we suggest that an initial stimulation of alveolar macrophages by a bacterial signal initiates the sequential activation of a phospholipase C and of phospholipase A2, leading to the release of PAF and eicosanoids. These mediators may in turn modulate the cell response by increasing or decreasing cyclic AMP, Ca2+, or diacyglycerol macrophage content.

Interleukin-1 stimulation of phospholipase activity in rat synovial fibroblasts. Possible regulation by cyclooxygenase products

Tritiated arachidonic acid (3H-AA)-labeled rat synovial fibroblasts stimulated with human recombinant interleukin-1 beta (rIL-1 beta) released incorporated radiolabel in a time-dependent and dose-dependent manner, with labeled prostaglandins representing 29% of the released radiolabel. Treatment of the cells with dibutyryl cAMP or prostaglandin E2 enhanced both spontaneous and rIL-1 beta-induced 3H-AA release; treatment with indomethacin or naproxen inhibited the response. The effects of these cyclooxygenase inhibitors on 3H-AA release were not reversed by the addition of prostaglandin E2. The activities of phospholipase A, phospholipase C, and diglyceride lipase were detected in the homogenates of rat synovial fibroblasts. Pretreatment of synovial cells with rIL-1 beta resulted in a threefold stimulation of phospholipase A activity and a slight increase in phospholipase C activity in cell homogenates. These data show that rIL-1 beta stimulates phospholipase activities in rat synovial fibroblasts and that at least one of these activities may be regulated by either prostaglandins or cAMP.

Interaction between PAF-acether and drugs that stimulate cyclic AMP in guinea-pig alveolar macrophages

The PAF-acether (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine)-induced arachidonate release from alveolar macrophages was significantly reduced by prostaglandin E2 (PGE2) and by the beta-adrenoceptor agonist salbutamol. In addition, PAF-acether markedly reduced the increase in intracellular cyclic AMP (cAMP) concentrations induced by PGE2 and salbutamol. Our data indicate an inverse relationship between intracellular cAMP levels and free arachidonate availability in alveolar macrophages treated with PAF-acether. A rise in intracellular cAMP therefore represents an important alternative route for controlling the effects of PAF-acether and the resulting inflammatory alterations in the respiratory system.

Interleukin-1 and tumor necrosis factor are not synergistic for human synovial fibroblast PLA2 activation and PGE2 production

Patterns of arachidonic acid release and metabolism were altered in human synovial fibroblasts following exposure to cytokines. Recombinant interleukin-1 induced an approximate 3-fold increase in [3H]-AA release, a 7-fold increase in PGE2 production and a 2-fold increase in PLA2 activity in human synovial fibroblasts. Recombinant tumor necrosis factor induced similar responses, however, the magnitude was less than that mediated by interleukin-1. A combination of the two cytokines had an additive effect on [3H]-AA release and PLA2 activity while PGE2 production was similar to that detected using interleukin-1 alone. [3H]-AA, was released in substantial amounts when sodium fluoride was used as a stimulus but PGE2 was not. These data show that tumor necrosis factor and interleukin-1 can both activate synovial cell PLA2 and induce generation of PGE2, but act in an additive rather than a synergistic fashion. Furthermore, the data show that PGE2 production is not always concordant with [3H]-AA release, suggesting that appropriate enzyme(s) must be activated.