Biopharmaceutical therapeutics for asthma remodeling

Current asthma therapy is aimed at controlling disease symptoms. A subset of asthma patients remains symptomatic despite optimal therapy indicating that an unmet medical need exists for these patients. Innovative therapeutics are needed to treat the unmet need in asthma and biopharmaceutical approaches may provide an opportunity for identifying these agents. It is proposed that airway remodeling contributes to asthma symptoms and this feature of disease pathology may be a target for future therapies. The current review focuses on the contribution of one feature of airway remodeling, subepithelial fibrosis, towards disease and highlights some of the mechanisms that may contribute to this feature of airway remodeling. Further, some potential molecular targets are identified for consideration for therapeutic intervention.

Monoclonal antibodies as a strategy for pulmonary diseases

Monoclonal antibodies have been used successfully to elucidate the roles of putative mediators of pulmonary disease. In particular, clinical trials with monoclonal antibodies directed against interleukin-5, IgE or CD4 yielded results that were critical in dissecting the pathophysiology of asthma; but, more importantly, fundamental changes in the discovery, manufacture and safety of monoclonal antibodies have reinforced the enormous potential of these agents in treating pulmonary diseases. An unprecedented number of monoclonal antibodies are in development for a variety of acute and chronic conditions. Moreover, whereas only two monoclonal antibodies had received regulatory approval from the United States Food and Drug Administration between 1986 and 1997, seven more have received approval since then. Indeed, monoclonal antibody therapy has come of age.

Tumor necrosis factor-alpha-induced secretion of RANTES and interleukin-6 from human airway smooth-muscle cells. Modulation by cyclic adenosine monophosphate

Although 3':5' cyclic adenosine monophosphate (cAMP) is known to modulate cytokine production in a number of cell types, little information exists regarding cAMP-mediated effects on this synthetic function of human airway smooth-muscle (HASM) cells. We examined the effect of increasing intracellular cAMP concentration ([cAMP](i)) on tumor necrosis factor (TNF)-alpha-induced regulated on activation, normal T cells expressed and secreted (RANTES) and interleukin (IL)-6 secretion from cultured HASM cells. Pretreatment of HASM with prostaglandin (PG) E(2), forskolin, or dibutyryl cAMP inhibited TNF-alpha-induced RANTES secretion but increased TNF-alpha-induced IL-6 secretion. Moreover, stimulation with PGE(2), forskolin, or dibutyryl cAMP alone increased basal IL-6 secretion in a concentration-dependent manner. SB 207499, a specific phosphodiesterase type 4 inhibitor, augmented the inhibitory effects of PGE(2) and forskolin on TNF-alpha-induced RANTES. Collectively, these data demonstrate that increasing [cAMP](i) in HASM effectively increases IL-6 secretion but reduces RANTES secretion promoted by TNF-alpha. Reverse transcriptase/polymerase chain reaction and ribonuclease protection assays suggested that these opposite effects of increased [cAMP](i) on TNF-alpha- induced IL-6 and RANTES secretion may occur at the transcriptional level. Accordingly, we examined the effects of TNF- alpha and cAMP on the regulation of nuclear factor (NF)-kappaB, a transcription factor known to modulate cytokine synthesis in numerous cell types. Stimulation of HASM cells with TNF-alpha increased NF-kappaB DNA-binding activity. However, increased [cAMP](i) in HASM neither activated NF-kappaB nor altered TNF-alpha- induced NF-kappaB DNA-binding activity. These results were confirmed using a NF-kappaB-luciferase reporter assay. Together, our data suggest that TNF-alpha-induced IL-6 and RANTES secretion may be associated with NF-kappaB activation, and that inhibition of TNF-alpha-stimulated RANTES secretion and augmentation of IL-6 secretion by increased [cAMP](i) in HASM cells occurs via an NF-kappaB-independent mechanism.

PDE inhibitors--Second William Harvey Research Conference. Drugs with an expanding range of therapeutic uses. 1-3 December 1999, Nice, France

This meeting underscored advances in the exploitation of cyclic nucleotide phosphodiesterases (PDEs) as drug targets. One highlight of the meeting was the disclosure of a new PDE isozyme, bringing to 11 the total number of genetically distinct isozyme families thus far identified. Also reported was the phenotypic characterization of a PDE4D murine genetic knockout. With respect to drug discovery and development, the most encouraging information presented centered on advances in targeting PDE4 with therapeutically useful inhibitors. Historically, the therapeutic utility of isozyme-selective PDE4 inhibitors has been limited by class-associated side effects, namely nausea and dyspepsia. New PDE4 inhibitors are being designed with the specific intent of improving upon the therapeutic ratio of first-generation agents. The profiles of two second-generation PDE4 inhibitors, SB-207499 (Ariflo; Smithkline Beecham plc) and PD-189659, were presented. SB-207499 demonstrated marked efficacy in phase II clinical trials in patients with moderate-to-severe chronic obstructive pulmonary disease (COPD), a disease of very high unmet medical need. PD-189659 has yet to enter clinical trials, but its preclinical profile indicates that this agent can produce substantial anti-inflammatory effects without producing class-associated side effects in animal models. A number of presentations were also given on the utility of PDE5 inhibitors in the treatment of male erectile dysfunction (MED). The widespread use of Viagra (sildenafil; Pfizer Inc) over the last year has reinforced the perception that PDE5 inhibitors are safe and effective agents for the treatment of MED. The overall tenor of the meeting was distinctly upbeat, with most participants believing that PDE isozymes are becoming ever more accessible as targets for drug discovery in a variety of therapeutic areas.

Specific CD3 epsilon association of a phosphodiesterase 4B isoform determines its selective tyrosine phosphorylation after CD3 ligation

cAMP-specific phosphodiesterases (PDE) comprise an extensive family of enzymes that control intracellular levels of cAMP and thus regulate T cell responses. It is not known how the function of these enzymes is altered by TCR engagement. We have examined this issue by studying one of the PDE isozymes (PDE4B). PDE4B RNA and protein were detected in resting PBLs, and the levels of PDE4B protein increased with cell cycling. In peripheral blood T cells, two previously reported PDE4B isoforms could be detected: one was 75-80 kDa (PDE4B1) and the other was 65-67 kDa (PDE4B2). These two isoforms differed in their N-terminal sequence, with the presence of four potential myristylation sites in the PDE4B2 that are absent in PDE4B1. Consequently, only PDE4B2 was found in association with the CD3var epsilon chain of the TCR. In addition, although both isoforms were phosphorylated in tyrosines in pervanadate-stimulated T cells, only the TCR-associated PDE4B2 was tyrosine-phosphorylated following CD3 ligation. The kinetics of phosphorylation of TCR-associated PDE4B2 correlated with changes in cAMP levels, suggesting that tyrosine phosphorylation of the TCR-associated PDE4B isoform upon engagement of this receptor may be an important regulatory step in PDE4B function. Our results reveal that selectivity of PDE4B activation can be achieved by differential receptor association and phosphorylation of the alternatively spliced forms of this PDE.

Antiasthmatic activity of the second-generation phosphodiesterase 4 (PDE4) inhibitor SB 207499 (Ariflo) in the guinea pig

We evaluated the airway activity of the novel phosphodiesterase type 4 inhibitor SB 207499 [Ariflo; c-4-cyano-4-(3-cyclopentyloxy-4-methoxyp henyl-r-1-cyclohexane carboxylic acid)], in the guinea pig. Ovalbumin (OA)-induced contractions of guinea pig isolated tracheal strips were inhibited by SB 207499 with an EC50 of 1 microM but had little or no effect on exogenous agonist-induced contraction, which suggests that its effect on OA-induced contraction in vitro is primarily due to inhibition of mediator release from mast cells. In anesthetized guinea pigs, SB 207499 inhibited OA-induced bronchoconstriction with i.v. and p.o. ID50 values of 1.7 and 17 mg/kg, respectively. At 1, 3 and 6 hr after SB 207499 (30 mg/kg p.o.), OA-induced bronchospasm was inhibited by 92%, 70% and 58%, respectively, corresponding to elevated plasma concentrations of 1.62 +/- 0.19, 1.65 +/- 0.29 and 0. 93 +/- 0.24 microg/ml, respectively, of SB 207499. SB 207499 also inhibited house dust mite-induced bronchoconstriction (ID50 = 0.9 mg/kg i.v. and 8.9 mg/kg p.o.). In contrast to its lack of bronchorelaxant activity in vitro, SB 207499 inhibited bronchospasm induced by i.v. leukotriene D4 (LTD4) [ID50 = 3 mg/kg i.v.]. The bronchorelaxant effect of i.v.-administered SB 207499 was at least additive with that of salbutamol in reversing infused histamine-enhanced airway tone, but it did not alter base line or enhance salbutamol-induced cardiovascular effects. In conscious guinea pigs, SB 207499 (10 or 30 mg/kg p.o.), 1 hr before antigen or LTD4 challenge, markedly reduced bronchospasm and subsequent eosinophil influx as measured by bronchoalveolar lavage 24 hr after provocation. SB 207499 administered after OA or LTD4 challenge also reduced airway eosinophilia measured at 24 hr after OA challenge or 96 hr after LTD4 challenge. These results, coupled with the broad anti-inflammatory activity of SB 207499 previously described (Barnett et al., 1998), suggest that SB 207499 will be useful in the treatment of asthma and other inflammatory disorders.

SB 207499 (Ariflo), a second generation phosphodiesterase 4 inhibitor, reduces tumor necrosis factor alpha and interleukin-4 production in vivo

The ability of the second generation phosphodiesterase 4 inhibitor SB 207499 (Ariflo), [c-4-cyano-4-(3-cyclopentyloxy-4-methoxyphenyl)-r-l-cyclohexane carboxylic acid], to inhibit inflammatory cytokine production in vivo was evaluated and compared to that of rolipram, a first generation phosphodiesterase 4 inhibitor. To examine human tumor necrosis factor alpha (TNFalpha) production, human monocytes were adoptively transferred into Balb/c mice and challenged with lipopolysaccharide (LPS). In this model, SB 207499 inhibited human TNFalpha production with oral ED50 of 4.9 mg/kg. Similarly, R-rolipram inhibited human TNFalpha production with an ED50 of 5.1 mg/kg, p.o. In contrast to their equipotent activity against TNFalpha production, SB 207499 (ED50 = 2.3 mg/kg, p.o.) was 10-fold less potent than R-rolipram (ED50 = 0.23 mg/kg, p.o.) in reversing reserpine-induced hypothermia, a model of antidepressant activity. In time course studies, SB 207499 (30 mg/kg, p.o.) inhibited TNFalpha production for at least 10 hr; substantial plasma concentrations of SB 207499 were detected over the same interval. The ability of SB 207499 to modulate interleukin-4 production in vivo was assessed in a chronic oxazolone-induced contact sensitivity model in Balb/c mice. In this model, topical administration of SB 207499 (1000 microgram) inhibited intralesional concentrations of interleukin-4 (55%; P <.01). The results demonstrate that SB 207499 is a potent inhibitor of inflammatory cytokine production in a variety of settings in vivo. Moreover, although it is as potent as R-rolipram in inhibiting TNFalpha production, it has substantially less central nervous system activity. Thus SB 207499 represents an excellent candidate with which to evaluate the antiinflammatory potential of PDE4 inhibitors.

Critical role of conserved histidine pairs HNXXH and HDXXH in recombinant human phosphodiesterase 4A

Cyclic AMP-Phosphodiesterases (cAMP-PDEs) catalyse the hydrolysis cAMP to AMP and thus serve to modulate the ligand-->adenylate cyclase-->cAMP-->PKA signal transduction pathway. PDEs exist as a multigene family of enzymes that bear significant sequence homology in the catalytic domains. The sequence alignment of these domains has revealed the presence of two histidine motifs: motif I, HNXXH, and motif II, HDXXH. These amino acid sequences are canonical motifs, which act as ligands for divalent metal cations required for catalytic activity. In this paper, we report human monocyte PDE4A to be a zinc-binding protein. Substitution by site-directed mutagenesis of either histidine in motif I by serine, which is not a ligand for metals, results in complete loss of catalytic activity and loss of sensitivity to divalent metal cation activation. However, similar mutations in motif II gave proteins that retained both approximately 50% of initial activity and the ability to respond differentially to Mg2+, Mn2+ and Co2+. Moreover the motif II mutants exhibited both functional group requirements and retained their pKa values. When the inactive mutants were affinity-labelled with 8-BDB-TcAMP and probed with antibody against cAMP or antibody against PDE4A, Western blots were unaltered. These results show that the conserved histidines in motif I are an absolute requirement for catalytic activity, whereas motif II histidines are required only to achieve maximum activity.

1,4-Cyclohexanecarboxylates: potent and selective inhibitors of phosophodiesterase 4 for the treatment of asthma

Evaluation of a variety of PDE4 inhibitors in a series of cellular and in vivo assays suggested a strategy to improve the therapeutic index of PDE4 inhibitors by increasing their selectivity for the ability to inhibit PDE4 catalytic activity versus the ability to compete for high affinity [3H]rolipram-binding sites in the central nervous system. Use of this strategy led ultimately to the identification of cis-4-cyano-4-[3-(cyclopentyloxy)-4-methoxyphenyl]cyclohexane-1-carboxyl ic acid (1, SB 207499, Ariflo), a potent second-generation inhibitor of PDE4 with a decreased potential for side effects versus the archetypic first generation inhibitor, (R)-rolipram.

SB 207499 (Ariflo), a potent and selective second-generation phosphodiesterase 4 inhibitor: in vitro anti-inflammatory actions

First-generation phosphodiesterase 4 (PDE4) inhibitors, such as rolipram, inhibit the activation of immune and inflammatory cells. The clinical use of these compounds is limited by gastrointestinal side effects, such as increased acid secretion and nausea. Consequently, the challenge has been to design novel PDE4 inhibitors that maintain the anti-inflammatory actions of rolipram while achieving an improved side effect profile. Among the first of this new class of PDE4 inhibitors specifically designed to have an improved therapeutic index relative to earlier compounds is SB 207499 (Ariflo) [c-4-cyano-4-(3-cyclopentyloxy-4-methoxy-phenyl)-r-1-cyclohexanecarboxyl ic acid]. In this study, we compared the anti-inflammatory and gastric secretogogue activities of SB 207499 with those of rolipram. The cellular models used were (1) histamine release from human basophils, (2) tumor necrosis factor-alpha generation in human monocytes, (3) degranulation of human neutrophils, (4) antigen-driven proliferation and cytokine synthesis from human T cells and (5) acid secretion from isolated rabbit gastric glands. SB 207499 inhibited the activation of a variety of immune and inflammatory cells in a concentration-dependent manner: (1) histamine release in basophils [-log IC25 = 6.6 +/- 0.3 vs. 8.0 for (R)-rolipram], (2) lipopolysacchride-induced TNF-alpha formation in monocytes [-log IC50 = 7.0 +/- 0.1 vs. 7.2 +/- 0.1 for (R)-rolipram], (3) fMLP-induced degranulation in neutrophils [-log IC15 = 7.1 +/- 0.2 vs. 6.4 +/- 0.5 for (R)-rolipram], (4) house dust mite induced-proliferation of peripheral blood mononuclear cells [-log IC40 = 6.5 +/- 0.3 vs. 6.4 +/- 0.3 for (R)-rolipram] and (5) ragweed-induced production of interferon-gamma [-log IC50 = 5.4] and interleukin-5 [-log IC50 = 5.0]. Although SB 207499 inhibits the activation of a variety of immune and inflammatory cells with a potency equal to that of rolipram, it is > 100-fold less potent than the latter compound as an acid secretagogue [-log EC50 = 6.1 +/- 0.1 vs. 8.3 +/- 0.2 for (R)-rolipram]. Collectively, these data indicate that SB 207499 retains the anti-inflammatory activity of the prototypical PDE4 inhibitor rolipram but is substantially less likely to stimulate gastric acid secretion.

Role of conserved histidines in catalytic activity and inhibitor binding of human recombinant phosphodiesterase 4A

To identify critical amino acids within the central conserved region of recombinant human cAMP-specific phosphodiesterase 4 subtype A (rhPDE4A), we engineered the expression of point mutants in a fully active rhPDE4A/Met201-886. When histidine residues at positions 433, 437, 473, and 477, which are highly conserved among all PDE families, were changed independently to serine residues, cAMP hydrolyzing activities were substantially reduced or abolished. The ability of these mutants to bind prototypical PDE4 inhibitors [3H]-(R)-rolipram or [3H]RP 73401 was also decreased in parallel with the loss of catalytic activity. The parallel loss of catalytic activity and inhibitor binding suggests that these changes resulted from non-localized perturbations in the structure of the enzyme. More interesting results were obtained when histidine residues at positions 505 and 506 were changed independently to aspar agines. The K(m) value for cAMP increased 3-fold in H505N (K(m) = 11 +/- 3 microM) and 11-fold in H506N (K(m) = 44 +/- 6 microM) compared with the wild-type protein (K(m) = 4 +/- 1 microM). These mutant proteins bound [3H]-(R)-rolipram and [3H]RP 73401 with K(d) values of 1.8 +/- 0.4 and 0.3 +/- 0.1 nM, respectively, for H505N, and 3.9 +/- 0.9 and 0.5 +/- 0.1 nM, respectively, for H506N. These values are nearly identical to those obtained with the wild-type rhPDE4A/Met201-886. In contrast, the IC50 values for cAMP competition with either [3H]-(R)-rolipram or [3H]RP 73401 binding increased approximately 2-fold in H505N and approximately 13-fold in H506N compared with the wild type protein. These increases are virtually identical to the changes in the K(m) value for cAMP in these mutants. We conclude that His506 and, perhaps, His505 are involved in binding of cAMP to PDE4A/Met201-886 but not in binding of PDE4-selective inhibitors.

Mechanisms of adaptive supersensitivity: correlation of guinea pig atrial supersensitivity with modifications in adenylyl cyclase activity

The possibility that the cellular mechanism underlying adaptive supersensitivity in right and left atria of the guinea pig may involve either adenylyl cyclase or components of that transduction process was examined in left and right atria obtained from controls or guinea pigs chronically treated with reserpine. Adenylyl cyclase activity and the abundance of alpha-subunits of several G-proteins (i.e. Gs, Gi, and Go) were quantified using standard techniques. Functional concentrations of Gs and Gi were compared in tissues from control and treated animals using pertussis- or cholera toxin-induced protein ribosylation. Chronic treatment with reserpine did not alter basal levels of adenylyl cyclase activity in left or right atrium but did increase significantly the ability of isoproterenol, 5'-guanylylimido diphosphate, and forskolin to activate adenylyl cyclase in the left atrium compared with the control. In contrast, treatment with reserpine increased the ability of only isoproterenol to active adenylyl cyclase in the right atrium. The increases in enzyme activation were not correlated with any detectable change in the concentrations of G-proteins or beta-adrenoceptors. The correlation between the specificity of changes in responsiveness and increased activation of adenylyl cyclase suggests that the cellular mechanism that underlies the development of adaptive supersensitivity in the guinea pig myocardium may involve a modification of adenylyl cyclase. The data also support the idea that the development of enhanced responsiveness in cardiac muscle may not only involve more than one cellular mechanism but may even differ between right and left atrium and ventricles of the same species.

Inactivation of recombinant monocyte cAMP-specific phosphodiesterase by cAMP analog, 8-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate

Two cAMP analogs, 8- and 2- [(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate (8- and 2-BDB-TcAMP) have been used in probing the catalytic site of recombinant monocyte cAMP-specific phosphodiesterase (PDE4a). 2-BDB-TcAMP is a reversible and competitive inhibitor (Ki = 5.5 mumol/L) of cAMP hydrolysis by PDE4a, 8-BDB-TcAMP irreversibly inactivates the enzyme in a time- and concentration-dependent manner with a second order rate constant of 0.022 mmol/L-1 min-1. The rate of inactivation of PDE4a is reduced by the presence of the substrate cAMP and specific inhibitors, rolipram and denbufylline, but not by cGMP or AMP. Reduction of the enzyme-inhibitor complex with sodium [3H]borohydride shows that 1.2 mol of the affinity label/mol of enzyme was incorporated. The radiolabeled peptide is composed of 10 amino acid residues (697 to 706) located near the carboxyl end of the proposed catalytic domain. The peptide (GPGHPPLPDK) has seven nonpolar and aliphatic residues, of which four are proline, giving the peptide a highly structured conformation. This peptide is the first to be identified in the putative catalytic domain involved in substrate recognition.

Identification, characterization, and functional role of phosphodiesterase type IV in cerebral vessels: effects of selective phosphodiesterase inhibitors

The role of the phosphodiesterase type IV isozyme (PDE IV) in the regulation of cerebrovascular tone was investigated in the canine basilar artery in vitro and in vivo. The PDE isozymes extracted from the canine basilar artery were isolated by diethylaminoethanol (DEAE)-Sepharose affinity chromatography and identified based on sensitivity to isozyme-selective PDE inhibitors. [3H]cAMP hydrolysis was observed in one major and one minor peak of activity. The predominant peak was inhibited by the addition of cGMP (25%), siguazodan (26%), rolipram (39%), and the combination of siguazodan and rolipram (95%). Selective PDE IV inhibitors BRL 61063, rolipram, and denbufylline were equieffective inhibitors of [3H]-ccAMP hydrolysis mediated by PDE IV isolated from the canine basilar artery [concentrations producing 50% inhibition (IC50S) = 0.21 +/- 0.05 microM, 0.67 +/- 0.23 microM, and 0.73 +/- 0.16 microM, respectively]. In precontracted isolated ring segments of the canine basilar artery, selective PDE IV inhibitors produced potent and complete relaxation (IC50S < 150 nM). In contrast, zaprinast (a selective PDE V inhibitor) and siguazodan (a selective PDE III inhibitor) produced only weak relaxation of the basilar artery (IC50S = 4.5 microM and > 10 microM, respectively). Vasorelaxation produced by PDE IV inhibitors was not altered by removing the endothelium, 1-NAME, or adenosine receptor antagonism. In a canine model of acute cerebral vasospasm, all three selective PDE IV inhibitors reversed basilar artery spasm produced by autologous blood without altering mean arterial blood pressure. In contrast, prolonged treatment with BRL 61063 failed to alter the development of basilar spasm in the two hemorrhage canine models of chronic cerebral vasospasm. Denbufylline-induced relaxation in vitro was also significantly impaired in basilar arteries obtained from the model of chronic vasospasm. In conclusion, PDE IV appears to be the predominant isozyme regulating vascular tone mediated by cAMP hydrolysis in cerebral vessels. In addition, vasorelaxation modulated by PDE IV is compromised in chronic cerebral vasospasm associated with subarachnoid hemorrhage.

The influence of endogenous catecholamines on the inhibitory effects of rolipram against early- and late-phase response to antigen in the guinea pig

Selective inhibitors of the low-Km cAMP-specific phosphodiesterase (PDE4) inhibit inflammatory cell function and relax airway smooth muscle. Thus PDE4 inhibitors may be useful in the therapy of asthma. The present study was conducted to determine whether the in vivo activity of rolipram, a prototypical PDE4 inhibitor, is due to its ability to potentiate the anti-inflammatory effects of prostaglandins or catecholamines, endogenous activators of adenylyl cyclase, in models of the early- and late-phase response to antigen. Rolipram, administered i.v. to anesthetized, paralyzed and ventilated ovalbumin-sensitized guinea pigs, inhibited i.v. antigen-induced bronchoconstriction with an ID50 value of 0.2 mg/kg. Pretreatment with either of the beta adrenoceptor antagonists propranolol and nadolol (0.5 mg/kg i.v.), enhanced the bronchial reactivity to antigen and abolished the inhibitory activity of rolipram (0.1-10 mg/kg i.v.). In addition, the inhibitory activity of three structurally dissimilar PDE4 inhibitors was nearly abolished by propranolol. Cyclooxygenase inhibition by indomethacin slightly enhanced the reactivity to antigen but did not affect the inhibitory activity of rolipram. Plasma catecholamine concentrations were not altered by rolipram (0.3 or 1 mg/kg i.v.), which indicates that there was no stimulation of catecholamine release. Bilateral adrenalectomy reduced plasma epinephrine concentrations (from 1700 pg/ml to 400 pg/ml), significantly enhanced airway reactivity to antigen and substantially reduced the inhibitory activity of rolipram (3 mg/kg i.v.). Pretreatment of conscious guinea pigs with the beta adrenoceptor antagonist nadolol, 2 mg/kg p.o., enhanced aerosol antigen-induced bronchoconstriction and pulmonary eosinophil influx measured by bronchoalveolar lavage. Nadolol reduced the inhibitory effect of rolipram against antigen-induced bronchoconstriction but not eosinophil influx. The inhibitory effect of rolipram was unaffected by indomethacin. The present data suggest that circulating catecholamines play an important protective role against antigen-induced broncho-constriction in the guinea pig. Moreover, the inhibitory activity of PDE4 inhibitors against antigen-induced bronchoconstriction, but not eosinophil influx, is reduced by beta adrenergic blockade or adrenalectomy. Thus the inhibitory activity of PDE4 inhibitors against antigen-induced bronchoconstriction may be related to their synergism with endogenous catecholamines to suppress mast cell degranulation.

Persistent airway eosinophilia after leukotriene (LT) D4 administration in the guinea pig: modulation by the LTD4 receptor antagonist, pranlukast, or an interleukin-5 monoclonal antibody

Aerosolized cysteinyl leukotrienes (CysLTs) elicit migration of eosinophils into guinea pig lungs and the airways of patients with asthma. The present studies were designed to analyze the concentration-response relationship, time course, and pharmacologic and histologic characteristics of leukotriene D4 (LTD4)-induced eosinophil influx into the airways of conscious guinea pigs. Animals were exposed to aerosols of 0.3 to 30 microg/ml LTD4 for 1 min, during which specific airway conductance (sGaw) was monitored. Bronchoalveolar lavages (BALs) of guinea pig airways were conducted at selected times from 4 h to 4 wk after LTD4 challenge. LTD4 produced maximal decreases in sGaw (70 to 90% reduction) at all concentrations tested and concentration-related increases in eosinophil levels in BALs, assessed 24 h after challenge. Increased numbers of eosinophils in the bronchial epithelium and subepithelium were confirmed histologically. Significant eosinophilia was maintained for up to 4 wk postchallenge. Pretreatment with the LTD4 receptor antagonist, pranlukast (ONO-1078, SB 205312) (20 mg/kg, intragastrically), significantly inhibited both the bronchoconstriction and the eosinophilia at 24 h, whereas the cyclooxygenase inhibitor, meclofenamic acid (5 mg/kg, intragastrically), had no effect on either parameter. Histologic observations were consistent with BAL results. Pretreatment with the rat anti-mouse antibody to interleukin-5 (IL-5), TRFK-5 (10-300 microg, intraperitoneally), produced dose-related inhibition of LTD4-induced eosinophilia, measured in 24 h or 3 wk BAL, but did not affect the acute bronchoconstriction. These results indicate that LTD4 elicits airway eosinophil influx in guinea pigs which persists as long as 4 wk after a single exposure, and provide the first evidence that IL-5 may have a role in LTD4-induced airways inflammation. This and other previously reported proinflammatory effects of LTD4 may contribute significantly to its overall influential role in the pathophysiology of asthma, and may underlie the therapeutic benefit of CysLT receptor antagonists, such as pranlukast, in this disorder.

Mapping the functional domains of human recombinant phosphodiesterase 4A: structural requirements for catalytic activity and rolipram binding

To identify functional domains of the 886-amino acid human recombinant cAMP-specific phosphodiesterase (PDE) subtype A (rhPDE4A), we engineered the expression of seven mutant proteins containing both NH2- and COOH-terminal truncations. The level of rhPDE4A protein expression in yeast was monitored by immunoblotting using enzyme-specific antisera. Biochemical profiles of the mutant proteins were compared with those of the full-length protein or a fully active truncated form of the enzyme (rhPDE4A Met265-886), lacking the first 264 amino acids. The smallest catalytically active fragment generated was Met332-722, which at 45 kDa is less than half the mass of the full-length enzyme (approximately 110 kDa) but spans the most highly conserved region of the PDE superfamily. Two prototypical PDE4 inhibitors, rolipram and RP 73401, inhibited cAMP hydrolyzing activity of all truncated forms of the enzyme, with IC50 values of 70-2000 nM and 0.2-0.6 nM, respectively. [3H](R)-Rolipram bound to two sites on Met265-886, a high affinity site (Kd1 = 0.7 +/- 0.3 nM) and a low affinity site (Kd2 = 34 +/- 10 nM). Interestingly, [3H](R)-rolipram failed to bind to Met332-886 with high affinity, indicating that high affinity binding is not required for inhibition of enzyme activity. Low affinity rolipram binding was still present in Met332-886 (Kd = 101 +/- 7 nM). In contrast to [3H](R)-rolipram, [3H]RP 73401 bound to a single class of high affinity sites on Met265-886 (Kd = 0.4 +/- 0.1 nM). Further truncation of the enzyme to Met332-886 had no effect on [3H]RP 73401 binding (Kd = 0.2 +/- 0.03 nM). We conclude that the catalytic center of rhPDE4A lies between amino acids 332 and 722. Furthermore, amino acids 265-332 may form a high affinity binding site for rolipram that is outside of the catalytic domain. As a more likely alternative, these amino acids may not form a distinct binding site but instead may be required for the recombinant enzyme to assume a conformation that binds rolipram at the catalytic domain with a high affinity.

Evaluation of human cytokine production and effects of pharmacological agents in a heterologous system in vivo

The ability of human monocytes adoptively transferred into the peritoneal cavity of BALB/c mice to produce tumor necrosis factor-alpha (TNF) and interleukin 1 beta (IL-1) was studied. Human monocytes were isolated from fresh, heparinized blood obtained by venipuncture. BALB/c mice were administered 2-10 x 10(6) cells and challenged with lipopolysaccharide intraperitoneally. 2 h later, they were killed and a peritoneal washout was obtained. The washouts were assayed for TNF and, in some cases, IL-1 content using a species specific enzyme-linked immunosorbant assay (ELISA). This model allowed for the simultaneous evaluation of the production of mouse and human inflammatory cytokines. Significant levels of both human and mouse TNF were seen as early as 60 min after challenge. Peak levels for both were seen at 120 min post administration of LPS. Both human and mouse TNF concentrations declined at the 2 h time point. The phosphodiesterase type 4 inhibitor, R-rolipram was found to inhibit both human and mouse TNF production while SB CSAID, novel kinase inhibitor SB 203580 inhibited human IL-1 and TNF as well as mouse TNF. This model was reliable, reproducible and allowed evaluation of pharmacological agents for their effect on human cytokine production in a heterologous setting in vivo.

Association of the anti-inflammatory activity of phosphodiesterase 4 (PDE4) inhibitors with either inhibition of PDE4 catalytic activity or competition for [3H]rolipram binding

Phosphodiesterase 4 (PDE4) inhibitors are novel anti-inflammatory compounds. Unfortunately, the archetypal PDE4 inhibitor rolipram produces central nervous system and gastrointestinal side-effects. To exploit these agents, we need to identify PDE4 inhibitors that retain the anti-inflammatory activity with a reduced potential to elicit unwanted side-effects. PDE4 possesses both cyclic AMP catalytic activity that is inhibitable by rolipram and a high affinity binding site for rolipram. The function of this high affinity rolipram binding site is unclear; however, certain pharmacological effects of PDE4 inhibitors are associated with competition for this site. Since PDE4 inhibitors suppress both monocyte and neutrophil activation, the present experiments were carried out to establish a correlation between suppression of monocyte activation [tumor necrosis factor alpha (TNF alpha) formation] or suppression of neutrophil activation (degranulation) with inhibition of either PDE4 catalytic activity or [3H] rolipram binding. Suppression of TNF alpha formation demonstrated a strong correlation with inhibition of PDE4 catalytic activity (r=0.87; P<0.01; Spearman's Rho = 0.79, P<0.05), whereas there was no correlation with inhibition of [3H]rolipram binding(r=0.21, P>0.5; Spearman's Rho=0.16, P>0.5). Suppression of neutrophil degranulation was not associated with inhibition of PDE4 catalytic activity (r=0.25, P>0.4; Spearman's Rho=0.33, P>0.2), but was associated with inhibition of [3H]rolipram binding (r=0.68, P<0.05; Spearman's Rho=0.6, P=0.06). These results indicate that anti-inflammatory effects of PDE4 inhibitors can be associated with either inhibition of PDE4 catalytic activity or high affinity rolipram binding.

Prolonged beta adrenoceptor stimulation up-regulates cAMP phosphodiesterase activity in human monocytes by increasing mRNA and protein for phosphodiesterases 4A and 4B

Human peripheral blood monocytes were treated for 4 h with a combination of the beta-agonist salbutamol (3 microM) and the low-Km cAMP-specific phosphodiesterase (PDE4) inhibitor rolipram (30 microM) to produce a prolonged elevation of cAMP and consequent increase in PDE activity. After this treatment, isozyme-selective PDE inhibitors were used to characterize the cAMP PDE profiles of high-speed supernatants before and after DEAE-Sepharose column chromatography. These experiments, in which total soluble PDE activity was increased by 58%, showed that the increased PDE activity is due to up-regulation of PDE4 and that at least two of the four subtypes are up-regulated. Experiments in whole cells demonstrated that this relatively modest increase in PDE4 activity has significant functional consequences, reducing cAMP accumulation in response to both PGE2 and lower, though not maximal, concentrations of rolipram. Further characterization of PDE4 subtype expression in control and treated monocytes, using polymerase chain reaction and Western blotting with subtype-specific peptide antibodies, showed that resting monocytes express both mRNA and protein for PDE4A, PDE4B and PDE4D. The amount of message for PDE4A and PDE4B appeared to increase upon up-regulation, whereas mRNA for PDE4D was not detected in treated cells. Western blots showed increases in the amount of protein for both PDE4A and PDE4B after treatment. We conclude that the PDE4 subtypes are differentially regulated upon prolonged exposure to elevated cAMP, with the consequence that the PDE4 profiles of control and treated cells differ not only in total activity but also in the relative proportions of the subtypes represented.

Salbutamol up-regulates PDE4 activity and induces a heterologous desensitization of U937 cells to prostaglandin E2. Implications for the therapeutic use of beta-adrenoceptor agonists

Previous studies with U937 cells, a human monocyte cell line, have shown that the activity of cyclic nucleotide phosphodiesterase 4 (PDE4) is increased by agents that elevate cyclic AMP content. The present experiments were conducted to determine 1) whether an increase in PDE4 steady-state message and/or protein accompanies the up-regulation of PDE4 activity and 2) whether the up-regulation changes the functional responses of U937 cells to activators of adenylyl cyclase. To up-regulate PDE4 activity, U937 cells were treated for 4 h with a combination of 1 microM salbutamol, a beta-adrenoceptor agonist, and 30 microM rolipram, a PDE4 inhibitor. Cells were washed extensively to remove drugs and used immediately in various experimental protocols. Reverse transcriptase-polymerase chain reactions conducted with primers specific for the four PDE4 subtypes suggested that pretreatment with salbutamol and rolipram increased steady-state mRNA levels of PDE4A and PDE4B, but not PDE4C or PDE4D. Immunoblot analyses using two rabbit polyclonal antibodies, one directed against human recombinant PDE4A and PDE4D and a second directed against human recombinant PDE4B, revealed bands of immunoreactivity corresponding to approximately 125 kDa (PDE4A) and approximately 70 kDa (PDE4B), respectively, that increased in intensity after cells were treated with salbutamol and rolipram. As demonstrated in both time course and concentration-response studies with prostaglandin E2 (PGE2), an agent that activates adenylyl cyclase by a non-beta-adrenoceptor-mediated mechanism, cAMP accumulation was substantially decreased in cells in which PDE4 activity had been up-regulated. The difference in PGE2-stimulated cAMP accumulation between control and PDE4 up-regulated cells was greatly reduced in the presence of rolipram, consistent with the notion that an increase in PDE4 activity was responsible for the heterologous desensitization. Functionally, up-regulation of PDE4 markedly decreased the ability of PGE2 to inhibit LTD4-induced Ca2+ mobilization in intact cells. A hypothetical implication of these results is that increasing PDE4 activity in vivo by administering beta-adrenoceptor agonists could exacerbate inflammatory processes by decreasing the activity of endogenous anti-inflammatory agents such as PGE2.

Cysteinyl leukotrienes in asthma: old mediators up to new tricks

The cysteinyl leukotrienes have long been suspected to play a role in the pathogenesis of asthma. This speculation was based largely on their release in human lung following antigen challenge as well as their potent bronchoconstrictor activity. However, there is increasing evidence that the cysteinyl leukotrienes also produce several pro-inflammatory effects and alter the activity of neuronal pathways in the airways. Douglas Hay, Theodore Torphy and Bradley Undem review these recent data and discuss the therapeutic possibilities of cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors.

Characterization of two human cAMP-specific phosphodiesterase subtypes expressed in baculovirus-infected insect cells

Recombinant baculoviruses were constructed to express cDNAs encoding two distinct subtypes of human cAMP-specific phosphodiesterase (hPDE4A and hPDE4B). Infection of Spodoptera frugiperda insect cells with the appropriate recombinant baculoviruses resulted in high level production of biologically-active protein as measured by enzymatic activity and immunoblotting using subtype-specific anti-hPDE4 antisera. Both recombinant proteins showed catalytic activity with a low Km (approximately 3 microM) for cAMP (with no cGMP hydrolyzing activity) and were inhibited by R-rolipram with apparent Kis of 0.38 and 0.25 microM, respectively. The recombinant enzymes also contained saturable, stereoselective and high-affinity rolipram-binding sites (Kd approximately 2 nM). Thus, insect cell-derived hPDE4s possess kinetic properties analogous to native enzymes as well as to recombinant enzymes produced in yeast.

Inhibitors of phosphodiesterase IV (PDE IV) increase acid secretion in rabbit isolated gastric glands: correlation between function and interaction with a high-affinity rolipram binding site

In this report, we describe the ability of selective inhibitors of phosphodiesterase (PDE) isozymes to increase aminopyrine accumulation in rabbit isolated gastric glands. Aminopyrine accumulation in the presence of histamine was increased by the nonselective PDE inhibitor isobutylmethylxanthine (EC50 = 4.8 microM) and by two selective PDE IV inhibitors, rolipram and Ro 20-1724 (EC50 = 0.013 and 0.07 microM, respectively) but not by selective PDE III inhibitors (siguazodan and SK&F 94120) or by a selective PDE V inhibitor (zaprinast). These results suggest that PDE IV is an important regulator of acid secretion in response to histamine. One of the more fascinating properties of PDE IV is the expression of a high-affinity binding site for [3H]-rolipram in addition to cAMP catalytic activity. Although agents that inhibit PDE IV catalytic activity also appear to bind to the high-affinity rolipram-binding site, the rank-order potencies of compounds for these two effects are poorly correlated. Also, certain pharmacological actions of PDE IV inhibitors appear to be related to an interaction with this binding site. In this study, we observed that the ability of PDE IV inhibitors to enhance acid secretion was not associated with their ability to inhibit PDE IV catalytic activity but did show a strong correlation with their ability to compete for [3H]-rolipram binding. Furthermore, we were able to detect [3H]-rolipram binding sites in gastric glands that had characteristics similar to those of the [3H]-rolipram binding sites in rat brain microsomes and human recombinant PDE IV.

The ability of phosphodiesterase IV inhibitors to suppress superoxide production in guinea pig eosinophils is correlated with inhibition of phosphodiesterase IV catalytic activity

Elevation of cyclic AMP (cAMP) content inhibits eosinophil function. Because phosphodiesterase IV (PDE IV) appears to be the major PDE isozyme present in eosinophils, inhibitors of this isozyme should suppress eosinophil activation. Previous studies on PDE IV have revealed that this enzyme possesses both cAMP catalytic activity that is inhibitable by rolipram, a prototypical PDE IV inhibitor, and a high-affinity binding site for rolipram. The function of this high-affinity rolipram binding site relative to the inhibitory action of compounds is not clear because the rank order potency of PDE IV inhibitors for competing with [3H]-rolipram binding is distinct from that for inhibiting cAMP hydrolysis. Consequently, the present experiments were carried out to fulfill the following objectives: 1) to determine whether PDE IV inhibitors suppress eosinophil function and, if so, 2) to establish a correlation between this functional activity and inhibition of PDE IV catalytic activity or interaction with the high-affinity rolipram binding site. Various PDE inhibitors produced approximately 60% maximal inhibition of formylmethionine-leucine-phenylalanine-induced superoxide anion production, so that IC30 concentrations were used as a basis to compare the potency of various PDE inhibitors. Selective PDE IV inhibitors were the most potent compounds tested. PDE inhibitors selective for other isozymes were devoid of activity or considerably less potent.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitors of CoA-independent transacylase block the movement of arachidonate into 1-ether-linked phospholipids of human neutrophils

The enzyme CoA-independent transacylase (CoA-IT) has been proposed to mediate the movement of arachidonate between phospholipid subclasses and influence the formation of arachidonic acid metabolites and platelet-activating factor. To substantiate the critical role of CoA-IT, we have developed two structurally diverse inhibitors of CoA-IT activity, SK&F 98625 [diethyl 7-(3,4,5-triphenyl-2-oxo-2,3-dihydro-imidazole-1-yl)heptane phosphonate] and SK&F 45905 [2-[2-(3-4-chloro-3-(trifluoromethyl)phenyl)-ureido]-4- (trifluoromethyl)phenoxy]-4,5-dichlorobenzenesulfonic acid]. These compounds were tested for their capacity to block microsomal CoA-IT activity using two assay systems, the transacylation of 1-alkyl-2-lyso-sn-glycero-3-phosphocholine (GPC) and the transfer of [14C]arachidonate from 1-acyl-2-[14C]arachidonoyl-GPC to lyso-PE. Both SK&F 98625 and SK&F 45905 inhibited CoA-IT activity (IC50s 6-19 microM) in these two assays. In contrast, SK&F 98625 or SK&F 45905 had little or no effect on other lipid-modifying activities, including CoA-dependent acyltransferase or acetyltransferase. Kinetic analysis revealed that both SK&F 98625 and SK&F 45905 interact directly with the enzyme and prevented the acylation of lysophospholipids in a competitive manner. In intact human neutrophils, both SK&F 98625 and SK&F 45905 completely blocked the movement of [3H]arachidonate from 1-acyl-linked phospholipids into 1-alkyl-2-arachidonoyl-GPC and 1-alk-1'-enyl-2-arachidonoyl-GPE. In contrast, these compounds did not inhibit the incorporation of free arachidonic acid into cellular lipids indicating that they did not alter CoA-dependent acyl transferase activities in the intact cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphodiesterase IV inhibitors as therapy for eosinophil-induced lung injury in asthma

Asthma is a complex, multifactorial disease that is underpinned by airway inflammation. A variety of cytotoxic substances are released into the airway from infiltrating inflammatory cells, especially the eosinophil. These cytotoxic substances, including reactive oxygen metabolites, produce damage to the airway epithelium, a histologic feature of chronic asthma. Damage to the airway epithelium, in turn, is thought to be a major factor responsible for the development of airway hyperreactivity, a hallmark of asthma. One notable molecular target for novel antiasthmatic drugs is the cyclic AMP-specific phosphodiesterase (PDE) or PDE IV. This isozyme is the predominant form of cyclic nucleotide PDE activity in inflammatory cells. Thus, in view of the putative role of cyclic AMP as an inhibitory second messenger in these cells, PDE IV inhibitors have been shown to suppress inflammatory cell activity. The purpose of the present experiments was to examine the effect of the PDE IV inhibitor, R-rolipram, on three key functions of the guinea pig eosinophil: a) superoxide anion (O2-) production, b) adhesion to human umbilical vein endothelial cells (HUVECs), and c) infiltration into the airway. R-rolipram-elevated eosinophil cyclic AMP content (EC50 = 1.7 microM) and inhibited fMLP-induced O2- production in a concentration-dependent manner (IC50 = 0.3 microM). In contrast, neither siguazodan, a PDE III inhibitor, nor zaprinast, a PDE V inhibitor, had an appreciable effect. R-rolipram (30 microM) also reduced by 25 to 40% the adhesion of eosinophils to HUVECs stimulated with phorbol myristate acetate or tumor necrosis factor-alpha, particularly under conditions in which both cell types were simultaneously exposed to the PDE IV inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenoceptors, cAMP and airway smooth muscle relaxation: challenges to the dogma

beta-Adrenoceptor agonists are assumed to induce airway smooth muscle relaxation through the cAMP-protein kinase A (PKA) phosphorylation cascade system. This traditional second messenger paradigm of beta-adrenoceptor agonist action is deeply engrained, but in this article Theodore Torphy reviews recent observations that force a re-examination of the dogma. For example, cAMP can activate protein kinase G as well as PKA, and this unanticipated dual action may contribute to the relaxant activity of cAMP. Other studies suggest that beta-adrenoceptor agonists can induce relaxation by a cAMP-independent mechanism involving a direct coupling of the beta-adrenoceptor to Ca(2+)-dependent K+ channels. Consequently, it is possible that multiple cAMP-dependent pathways act in concert with cAMP-independent pathways to mediate bronchodilation in response to beta-adrenoceptor agonists.

A Candida albicans cyclic nucleotide phosphodiesterase: cloning and expression in Saccharomyces cerevisiae and biochemical characterization of the recombinant enzyme

We have cloned a Candida albicans gene, which encodes a cyclic nucleotide phosphodiesterase (PDEase), by complementation in a Saccharomyces cerevisiae PDEase-deficient mutant. The deduced amino acid sequence is similar to that of the low-affinity PDEase of S. cerevisiae (PDE1) and the cyclic nucleotide PDEase (PD) of Dictyostelium discoideum. Biochemical analysis of recombinant protein produced in S. cerevisiae indicated that the enzyme behaves as a PDE1 homologue: it hydrolyses both cAMP (Km = 0.49 mM) and cGMP (Km = 0.25 mM), does not require divalent cations for maximal activity and is only moderately inhibited by millimolar concentrations of standard PDEase inhibitors. Based on these data, we designate the C. albicans we have cloned, PDE1. Low-stringency genomic Southern blots showed cross-hybridization between C. albicans PDE1 and DNA from Candida stellatoidea, but not with DNA from S. cerevisiae or several closely related Candida species.

Comparison of phosphodiesterase III, IV and dual III/IV inhibitors on bronchospasm and pulmonary eosinophil influx in guinea pigs

Selective inhibition of phosphodiesterase (PDE) isozymes has been shown to inhibit inflammatory cell function and relax airway smooth muscle and, thus, may be useful in the therapy of asthma. In guinea pigs sensitized to ovalbumin (OA), the effects of three PDE inhibitors were compared: siguazodan (PDE III selective, IC50 = 0.7 microM), rolipram (PDE IV selective, IC50 = 0.8 microM) and zardaverine (dual PDE III/IV, IC50S = 2.5 microM and 1.1 microM, respectively) against histamine-, leukotriene (LT) D4- and OA-induced bronchospasm in vitro and in vivo. Rolipram or zardaverine (0.1-10 microM), but not siguazodan, inhibited OA-induced contraction of the isolated trachea in a concentration-dependent manner. Rolipram or siguazodan alone (10 microM) were ineffective against histamine- or LTD4-induced contractions. Zardaverine alone (10 microM) or the combination of rolipram and siguazodan (10 microM each) markedly antagonized the contractions elicited by both spasmogens. In anesthesized, ventilated guinea pigs, the i.v. ID50S against OA-induced bronchospasm were: rolipram = 0.2 mg/kg, siguazodan > 10 mg/kg and zardaverine = 2.4 mg/kg. When administered at doses up to 7.5 mg/kg, i.v., rolipram or siguazodan were markedly less effective (i.e., < or = 50% inhibition) than zardaverine (ID50S = 2.4 and 1.7 mg/kg, respectively) at blocking exogenous histamine- or LTD4-induced bronchospasm. However, when administered in combination with siguazodan (5.4 mg/kg, i.v.), rolipram (0.4-5.4 mg/kg) abolished histamine- and LTD4-induced bronchoconstriction. In conscious guinea pigs, zardaverine (5 mg/kg, intragastrically (i.g.) or the combination of rolipram and siguazodan (5 mg/kg each) were substantially more effective than rolipram or siguazodan alone at inhibiting aerosol histamine- or LTD4-induced bronchospasm. In the same animals, rolipram or zardaverine (5 mg/kg, i.g.) but not siguazodan (5 mg/kg, i.g.) markedly inhibited aerosol OA-induced bronchoconstriction. The OA-induced pulmonary eosinophil infiltration in these animals was attenuated by all treatments with zardaverine producing the greatest degree of inhibition. These results indicate that 1) PDE IV inhibitors but not PDE III inhibitors are effective at blocking antigen-induced bronchospasm, 2) compounds that selectively inhibit either PDE III or PDE IV are poor inhibitors of bronchoconstriction elicited by exogenously administered spasmogens, and 3) the combined inhibition of both PDE III and PDE IV isozymes acts in an additive or synergistic manner to inhibit bronchospasm in the guinea pig.

Differential antagonism of airway contractile responses to prostaglandin (PG)D2 and 9 alpha, 11 beta-PGF2 by atropine, SK&F 88046 and SQ 29,548 in the guinea pig

PGD2, the predominant prostanoid released from activated human lung mast cells, is metabolized to 9 alpha, 11 beta-PGF2 by an 11-ketoreductase. Both prostanoids contract mammalian airway smooth muscle. In the present study, aerosol administration of PGD2 or 9 alpha, 11 beta-PGF2 (five puffs of 10-50 micrograms/ml) to anesthetized, spontaneously breathing guinea pigs produced significant increases in airway resistance and decreases in dynamic lung compliance. The changes in airway resistance and dynamic lung compliance induced by 50 micrograms/ml were reduced approximately 60% and 25%, respectively, by pretreatment with atropine (1 mg/kg, i.v., -10 min). Pretreatment with the TxA2 receptor antagonist SK&F 88046 (N,N'-bis[7-(3-chlorobenzene aminosulfonyl)-1,2,3,4- tetrahydroisoquinolyl]disulfonylimide) (5 mg/kg, i.v., -10 min), nearly abolished the changes in airway resistance and dynamic lung compliance that were elicited by both agonists. Pretreatment with a TxA2 synthase inhibitor, CGS 13080 (10 mg/kg, i.v., -10 min), had no effect on PGD2- or 9 alpha, 11 beta-PGF2-induced bronchoconstriction, suggesting that these prostanoids did not provoke the release of TxA2. In vitro, PGD2, 9 alpha, 11 beta-PGF2 and a TxA2 mimic, U-44069, produced concentration-dependent contractions of the guinea pig isolated trachea with pD2s of 6.4, 6.0 and 7.2, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The crystal structure, absolute configuration, and phosphodiesterase inhibitory activity of (+)-1-(4-bromobenzyl)-4-(3-(cyclopentyloxy)- 4-methoxyphenyl)-pyrrolidin-2-one

Chiral HPLC resolution of the phosphodiesterase IV (PDE IV) inhibitor rolipram (1) provided (-)-1, and this enantiomer was converted into its 1-(4-bromobenzyl) derivative, (+)-2. X-ray structural analysis of (+)-2 established the absolute configuration as R, which provides the first direct evidence for a previously assumed assignment of configuration. The crystal structure of (+)-2 and the PDE inhibitory activity of both enantiomers of 2 are discussed in the context of a previously proposed topological model.

Association between leukotriene B4-induced phospholipase D activation and degranulation of human neutrophils

We have explored the role of phospholipase D (PLD) activation in leukotriene B4 (LTB4)-induced Ca2+ mobilization and degranulation of human neutrophils. Stimulation of [3H]alkyl-acyl-phosphatidylcholine-labeled neutrophils with LTB4 resulted in a rapid accumulation of [3H]alkyl-phosphatidic acid (PA) as well as a somewhat slower accumulation of [3H]alkyl-diglyceride (DG). In the presence of ethanol, PLD catalyzed a transphosphatidylation reaction in which LTB4 increased [3H]alkyl-phosphatidylethanol formation and simultaneously decreased LTB4-induced PA and DG accumulation. This pattern of lipid metabolism is consistent with the conclusion that LTB4 stimulates PLD activity in human neutrophils. Additional studies in which the extracellular and intracellular concentrations of Ca2+ were varied indicated that maximal LTB4-induced PLD activation was dependent upon Ca2+ and potentiated by inhibitors of protein kinase C. The time-course and concentration-response curves for LTB4-induced PLD activation were different from those for LTB4-induced Ca2+ mobilization, as measured by fura-2 fluorescence. On the other hand, the concentration-response curve for LTB4-induced PLD activation was similar to that for LTB4-induced degranulation. Preincubation of the cells with ethanol inhibited LTB4-induced PA and DG accumulation, as well as degranulation, suggesting that one or both of these metabolites were important for this response. In contrast, ethanol had no effect on LTB4-induced Ca2+ mobilization. Propranolol, an inhibitor of phosphatidate phosphohydrolase, abolished DG accumulation in response to LTB4 but had no effect on degranulation, suggesting that PA is more important than DG as a mediator of degranulation. Taken collectively, these data indicate that LTB4-induced activation of PLD in human neutrophils is mediated by a Ca(2+)-dependent mechanism, but not by protein kinase C. In addition, PLD activation in these cells may induce degranulation, but not Ca2+ mobilization.

Influence of isoproterenol and phosphodiesterase inhibitors on platelet-activating factor biosynthesis in the human neutrophil

Increasing cellular levels of cAMP inhibit eicosanoid production in the human neutrophil; however, little is known about the effects of cAMP on platelet-activating factor (PAF) biosynthesis in this cell. In the current study, the beta-adrenergic receptor agonist isoproterenol, alone or in combination with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), was used to increase cAMP in neutrophils. The incorporation of [3H]acetate into PAF and the synthesis of leukotrienes in response to ionophore A23187 were significantly inhibited by 10 microM isoproterenol. The inhibitory effect on PAF was potentiated by the addition of 10 microM IBMX. The effects of IBMX were mimicked by rolipram, an inhibitor of the cAMP-specific phosphodiesterase IV. Mass spectrometric analysis of the PAF molecular species in stimulated neutrophils indicated that the combination of isoproterenol and IBMX inhibited (&gt; 50%) ionophore- and fMLP-induced production of PAF. To better understand the mechanism involved in the inhibition of PAF formation, the major biosynthetic steps were examined in the presence and absence of a maximally effective concentration of isoproterenol and IBMX. Isoproterenol alone or in the presence of IBMX had no measurable effect on the ionomycin-induced increase in cytosolic calcium concentration, as assessed by fura-2 fluorescence. Treating intact neutrophils with a combination of isoproterenol and IBMX did not inhibit acetyltransferase activity when assayed in a subsequent broken cell preparation. Finally, increasing cellular cAMP with these drugs did not influence the ability of the neutrophil to catabolize PAF. Phospholipase A2-like activity was assayed in the whole cell by measuring the mobilization of phospholipase A2 products, PAF, lyso PAF, and arachidonic acid, from cellular phosphoglycerides. Treatment of neutrophils with isoproterenol and IBMX significantly reduced the production of lyso PAF and PAF from 1-alkyl-2-arachidonoyl sn-glycero-3-phosphocholine. Similarly, increasing cellular levels of cAMP inhibited the cell's ability to mobilize arachidonic acid upon cell activation. These data suggest that increasing cellular levels of cAMP leads to the inhibition of PAF and leukotriene biosynthesis, at least in part, by regulation of phospholipase A2 activity.

Inhibition of antigen-induced bronchoconstriction and eosinophil infiltration in the guinea pig by the cyclic AMP-specific phosphodiesterase inhibitor, rolipram

Selective inhibition of the low Km cyclic AMP-specific phosphodiesterase has been shown to inhibit inflammatory cell function and relax airway smooth muscle. These studies were conducted to characterize the bronchodilator and anti-inflammatory activity of rolipram, an archetypical cyclic AMP-specific phosphodiesterase inhibitor, in in vitro and in vivo guinea pig airway models. In isolated tracheal rings from ovalbumin (OA)-sensitive guinea pigs, both R- and S-enantiomers of rolipram (1 microM) significantly antagonized OA-induced contractions. In contrast, neither enantiomer at concentrations up to 1 microM significantly inhibited histamine- or LTD4-induced contractions. In superfusion and mediator release experiments, both enantiomers of rolipram significantly reduced antigen-induced prostaglandin D2 release, but had minimal effect on histamine release. In anesthetized, ventilated OA-sensitive guinea pigs, racemic rolipram or enantiomers reduced OA-induced bronchoconstriction with ID50 values of approximately 0.25 mg/kg i.v. Histamine- and leukotriene D4-induced bronchoconstriction were not affected by doses of rolipram which abolished the response to OA. Higher doses (3-10 mg/kg) reduced histamine-, but not the leukotriene D4-induced bronchoconstriction. In conscious OA-sensitive guinea pigs, intragastric pretreatment with rolipram dose-dependently reduced both the OA-induced decreases in specific conductance as well as the corresponding pulmonary eosinophil influx as assessed by both bronchoalveolar lavage and histological evaluation. Therefore, rolipram produces significant inhibition of antigen-induced bronchoconstrictor and inflammatory responses, thus providing strong evidence that this pharmacological approach may be of significant therapeutic value in allergic asthma.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of isoproterenol and phosphodiesterase inhibitors on platelet-activating factor biosynthesis in the human neutrophil

Increasing cellular levels of cAMP inhibit eicosanoid production in the human neutrophil; however, little is known about the effects of cAMP on platelet-activating factor (PAF) biosynthesis in this cell. In the current study, the beta-adrenergic receptor agonist isoproterenol, alone or in combination with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), was used to increase cAMP in neutrophils. The incorporation of [3H]acetate into PAF and the synthesis of leukotrienes in response to ionophore A23187 were significantly inhibited by 10 microM isoproterenol. The inhibitory effect on PAF was potentiated by the addition of 10 microM IBMX. The effects of IBMX were mimicked by rolipram, an inhibitor of the cAMP-specific phosphodiesterase IV. Mass spectrometric analysis of the PAF molecular species in stimulated neutrophils indicated that the combination of isoproterenol and IBMX inhibited (> 50%) ionophore- and fMLP-induced production of PAF. To better understand the mechanism involved in the inhibition of PAF formation, the major biosynthetic steps were examined in the presence and absence of a maximally effective concentration of isoproterenol and IBMX. Isoproterenol alone or in the presence of IBMX had no measurable effect on the ionomycin-induced increase in cytosolic calcium concentration, as assessed by fura-2 fluorescence. Treating intact neutrophils with a combination of isoproterenol and IBMX did not inhibit acetyltransferase activity when assayed in a subsequent broken cell preparation. Finally, increasing cellular cAMP with these drugs did not influence the ability of the neutrophil to catabolize PAF. Phospholipase A2-like activity was assayed in the whole cell by measuring the mobilization of phospholipase A2 products, PAF, lyso PAF, and arachidonic acid, from cellular phosphoglycerides. Treatment of neutrophils with isoproterenol and IBMX significantly reduced the production of lyso PAF and PAF from 1-alkyl-2-arachidonoyl sn-glycero-3-phosphocholine. Similarly, increasing cellular levels of cAMP inhibited the cell's ability to mobilize arachidonic acid upon cell activation. These data suggest that increasing cellular levels of cAMP leads to the inhibition of PAF and leukotriene biosynthesis, at least in part, by regulation of phospholipase A2 activity.

Identification, characterization and functional role of phosphodiesterase isozymes in human airway smooth muscle

In the present study phosphodiesterase (PDE) isozymes in human airway smooth muscle were isolated, identified and characterized, and the functional roles of these isozymes in intact bronchi were evaluated by using isozyme-selective PDE inhibitors. PDE isozymes in human trachealis were isolated by using a combination of DEAE-Sepharose and calmodulin-Sepharose affinity chromatography, and were identified based upon their kinetic characteristics as well as their sensitivity to allosteric modulators and isozyme-selective PDE inhibitors. By using this approach, six distinct isozymes were identified: two calmodulin-stimulated PDEs (PDE I alpha and PDE I beta), cyclic GMP (cGMP)-stimulated PDE (PDE II), cGMP-inhibited PDE (PDE III), cyclic AMP (cAMP)-specific PDE (PDE IV) and cGMP-specific PDE (PDE V). PDEs III and IV were the major cAMP-hydrolyzing enzymes present, whereas PDEs I alpha, I beta and V accounted for most of the cGMP-hydrolytic activity. In carbachol-precontracted small (< 0.5-2 mm diameter) or large (4-15 mm diameter) human bronchus, zaprinast (10 nM-30 microM), the selective PDE V inhibitor, was without marked relaxant activity (< 13%), whereas rolipram (30 microM), the selective PDE IV inhibitor, produced approximately 25% relaxation in both preparations. Siguazodan was a significantly more effective relaxant than zaprinast or rolipram in large bronchus, producing a maximum relaxation of 77 +/- 15% at a concentration of 30 microM, whereas in small bronchus 30 microM siguazodan elicited 20 +/- 6% relaxation. Similar results were obtained in large bronchi contracted with leukotriene (LT) D4 (0.1 microM). The ability of isozyme-selective PDE inhibitors to potentiate agonist-induced relaxation was studied in LTD4-contracted large bronchi. Siguazodan (10 microM), but not rolipram (10 microM) or zaprinast (10 microM), potentiated the relaxant response in LTD4-contracted large bronchus to isoproterenol, a beta adrenoceptor agonist thought to induce relaxation via a cAMP-mediated mechanism. In contrast, zaprinast (10 microM), but not siguazodan (10 microM), potentiated relaxation induced by sodium nitroprusside, a nitrovasodilator that relaxes airway smooth muscle via a cGMP-mediated mechanism. The most striking observation from functional studies was that the combination of rolipram and siguazodan produced a much greater relaxation of small or large human bronchi than either agent alone, indicating an interaction between PDE III and PDE IV inhibitors that was at least additive and, in some cases, synergistic.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of selective phosphodiesterase type IV inhibitor, rolipram, on fluid and cellular phases of inflammatory response

The antiinflammatory activity of rolipram, a selective inhibitor of the cyclic AMP-specific phosphodiesterase (PDE IV), was studied. Rolipram did not inhibit 5-lipoxygenase activity but did inhibit human monocyte production of leukotriene B4 (LTB4, IC50 3.5 microM). Likewise, murine mast cell release of leukotriene C4 and histamine was inhibited. In vivo, rolipram inhibited arachidonic acid-induced inflammation in the mouse, while the low Km-cyclic-GMP PDE inhibitor, zaprinast, did not inhibit. Rolipram had a modest effect on LTB4 production in the mouse, but markedly reduced LTB4-induced PMN infiltration. Beta-adrenergic receptor activation of adenylate cyclase was important for rolipram antiinflammatory activity since beta blockade abrogated arachidonic acid-induced inflammation. Thus, the antiinflammatory profile of rolipram is novel and may result from inhibition of PMN function and perhaps vasoactive amine release and leukotriene biosynthesis. These actions may be dependent upon endogenous beta-adrenergic activity and are likely mediated through inhibition of PDE IV.

A low-Km, rolipram-sensitive, cAMP-specific phosphodiesterase from human brain. Cloning and expression of cDNA, biochemical characterization of recombinant protein, and tissue distribution of mRNA

We have isolated cDNA clones from human frontal cortex cDNA libraries that encode a unique subtype of the low-Km, cAMP-specific phosphodiesterases (PDEs IV). The 564-amino acid sequence of the protein (human brain PDE IV (hPDE IVB)) shows significant homology to a PDE IV subtype expressed in human monocytes (hPDE IVA), particularly within the approximately 300-amino acid PDE IV catalytic domain. The degree of protein sequence identity is much greater between hPDE IVB and a homolog derived from rat brain (92% over 562 amino acids) than between hPDE IVB and hPDE IVA (76% over 538 amino acids), suggesting a greater subtype-specific versus species-specific conservation of protein sequence. Analysis of the distribution of hPDE IVB mRNA expression revealed a restricted pattern, with an approximately 4-kilobase mRNA detected in brain, heart, lung, and skeletal muscle and not in placenta, liver, kidney, or pancreas. An additional approximately 5-kilobase hPDE IVB-related mRNA species was detected in brain tissue. Recombinant hPDE IVB displayed all of the expected kinetic characteristics for a PDE IV, including sensitivity to the isozyme-selective inhibitor rolipram (Ki = 0.085 microM). Scatchard analysis of (R)-[3H]rolipram binding data suggested the presence of two noninteracting high affinity rolipram-binding sites (Kd = 0.4 and 6 nM) or a negatively cooperative interaction among multiple binding sites.

Biochemical characteristics and cellular regulation of phosphodiesterase IV

Considerable interest has been generated in the potential utility of phosphodiesterase (PDE) IV inhibitors as a novel class of anti-asthmatic agents. Because a detailed understanding of the molecular and biochemical characteristics of any molecular target of interest provides a key ingredient for rational drug design, we cloned a cDNA encoding a PDE IV (hPDE IV) from a human monocyte library and expressed, purified and characterized the recombinant gene product. Purified hPDE IV has kinetic characteristics consistent with native PDE IV isolated from tissue sources. In addition, it is inhibited by rolipram (Ki = 60 nM) and other archetypical PDE IV-selective inhibitors. Purified hPDE IV also contains a high affinity binding site for rolipram (Kd = 2 nM), although the precise relationship between this site and the cAMP catalytic site is not clear. In other studies in which the regulation of PDE IV expression was examined in U937 cells, a human monocytic cell line, prolonged treatment with salbutamol was shown to induce an increase in the activity of PDE IV. This up-regulation of PDE IV activity appears to be mediated by cAMP and occurs at the transcriptional or pretranscriptional level. As a consequence of PDE IV up-regulation, the sensitivity of U937 cells to the inhibitory effects of adenylyl cyclase activators on cell function is greatly diminished. If such regulation of PDE IV occurs in inflammatory cells in vivo, it could have implications for the therapeutic use of beta-adrenoceptor agonists. Specifically, induction of PDE IV activity in asthmatics being treated with beta-adrenoceptor agonists could result in a heterologous desensitization of inflammatory cells to endogenous anti-inflammatory agents (e.g., epinephrine, prostaglandin E2).

Stimulation of beta adrenoceptors in a human monocyte cell line (U937) up-regulates cyclic AMP-specific phosphodiesterase activity

Experiments were conducted using undifferentiated U937 cells, a human monocytic cell line, to establish an in vitro model to examine the hormonal regulation of the cyclic AMP (cAMP)-specific phosphodiesterase (PDE IV). Standard chromatographic techniques, coupled with the use of inhibitors and activators that are selective for various phosphodiesterase (PDE) isozymes, were used to establish the PDE isozyme profile in supernatant fractions of U937 cells. When PDE activity was assessed using 1 microM [3H]cAMP as a substrate, 70 to 90% of the total U937 cell supernatant activity in the major peak eluting from anion-exchange columns was inhibited by 30 microM rolipram, a selective inhibitor of PDE IV. The remaining activity was nearly abolished by 10 microM siguazodan or 10 microM cyclic GMP (cGMP,) selective inhibitors of the cGMP-inhibited PDE. Kinetic analyses of the enzyme activity contained within this major peak of PDE activity revealed a cAMP Km = 3 microM and a rolipram Ki = 0.5 microM, values characteristic of PDE IV. Additional studies revealed the presence of a small amount of Ca++/calmodulin-stimulated PDE, but no cGMP-stimulated PDE or cGMP-specific PDE activity. In an effort to induce PDE activity in intact U937 cells by producing a sustained increase in cAMP content, cells were treated for 4 hr with salbutamol (1 microM), rolipram (30 microM) or a combination of both agents. The combination of salbutamol and rolipram produced a 2- to 3-fold increase in PDE activity in U937 cells; when used alone, rolipram was without effect whereas salbutamol induced an increase that was approximately one-half of that observed with the combination. Isozyme isolation and characterization revealed that the overall elevation of cellular PDE activity could be accounted for by a 2- to 3-fold increase in the Vmax of PDE IV with no change in its Km. The induction of PDE IV by salbutamol was: 1) concentration- and time-dependent; 2) detectable only after prolonged (2-4 hr) agonist exposure; 3) preceded by an increase in cAMP content and an activation of cAMP-dependent protein kinase; 4) mimicked by 8-bromo-cAMP and prostaglandin E2; 5) reversible within 3 hr of salbutamol removal; and 6) abolished by cycloheximide or actinomycin D. Collectively, these results indicate that the major PDE isozyme in the soluble fraction of U937 cells is PDE IV and that the activity of this enzyme is increased markedly in cells after prolonged exposure to agents that increase cAMP content.

SK&F 104353, a selective leukotriene receptor antagonist, inhibits leukotriene D4- and antigen-induced bronchoconstriction in cynomolgus monkeys

The ability of SK&F 104353 to prevent and reverse leukotriene (LT) D4- and antigen (Ag)-induced bronchoconstriction was examined in anesthetized, spontaneously breathing cynomolgus monkeys. Aerosol administration of LTD4 (10 micrograms/ml; 20 breaths) produced a sustained increase in pulmonary resistance and decrease in dynamic lung compliance. Aerosolized SK&F 104353 (150 breaths, 0.3 or 4.4 mg/ml) administered 15 min prior to LTD4 challenge antagonized these changes in a dose-dependent manner. When given intravenously 6 min after LTD4, SK&F 104353 (5 mg/kg) rapidly and completely reversed the ongoing bronchoconstriction. In mepyramine-pretreated (2 mg/kg i.v.) monkeys sensitive to aerosolized Ascaris suum Ag, intravenously administered SK&F 104353 (5 mg/kg) substantially reversed, but did not abolish, Ag-induced bronchoconstriction when administered 12 min after the Ag challenge. In contrast, SK&F 104353 (5 mg/kg i.v.) did not reverse Ag-induced bronchoconstriction in animals that had not been pretreated with mepyramine. Similar results were obtained when SK&F 104353 (20 mg/kg i.v.) was administered (as a pretreatment) 5 min prior to Ag under these conditions. Thus, SK&F 104353 reduced Ag-induced bronchoconstriction in mepyramine-pretreated monkeys, but had little effect in the absence of mepyramine. The data suggest that LTs, in addition to histamine, play a role in allergic bronchoconstriction in cynomolgus monkeys.

Agonist-related differences in the relationship between cAMP content and protein kinase activity in canine trachealis

We investigated the relationships between relaxation, cyclic AMP (cAMP) accumulation and cAMP-dependent protein kinase (cAMP-PK) activity in canine tracheal smooth muscle. In time course and concentration-response studies, forskolin and isoproterenol elicited relaxation of isolated trachealis strips that was accompanied by an increase in cAMP content and an activation of cAMP-PK. Although these results were consistent with the proposal that cAMP is a second messenger mediating relaxation of airway smooth muscle, close inspection of the data revealed a discrepancy in the relationship between cAMP accumulation and relaxation. To induce equivalent degrees of tracheal relaxation, forskolin generated greater increments in cAMP accumulation than did isoproterenol. On the other hand, the activation state of cAMP-PK correlated reasonably well with relaxation regardless of which agonist was used. Further analysis of the data revealed that the apparent disparity between cAMP accumulation and relaxation could largely be explained at the level of the relationship between cAMP content and cAMP-PK activity: compared to isoproterenol, forskolin induced greater increases in cAMP accumulation to achieve the same activation state of cAMP-PK. These observations lend support to the proposal that in canine trachealis, various components of the cAMP/cAMP-PK cascade exist in distinct subcellular compartments such that not all of the cAMP generated in response to forskolin has access to its target enzyme, cAMP-PK.

Preliminary identification and role of phosphodiesterase isozymes in human basophils

We attempted to identify and establish the role of cyclic nucleotide phosphodiesterase (PDE) isozymes in human basophils by using standard biochemical techniques as well as describing the effects of isozyme-selective and nonselective inhibitors of PDE. The nonselective PDE inhibitors, theophylline and 3-isobutyl-1-methylxanthine, inhibited anti-IgE-induced release of histamine and leukotriene C4 (LTC4) from basophils. This inhibition was accompanied by elevations in cAMP levels. Rolipram, an inhibitor of the low Km cAMP-specific PDE (PDE IV), inhibited the release of both histamine and LTC4 from activated basophils and increased cAMP levels in these cells. In contrast, mediator release from basophils was not inhibited by either siguazodan or SK&F 95654, inhibitors of the cGMP-inhibited PDE (PDE III) or zaprinast, an inhibitor of the cGMP-specific PDE (PDE V). SK&F 95654 failed to elevate basophil cAMP in these experiments whereas zaprinast induced significant increases in cAMP content. The inhibitory effect of rolipram on mediator release was potentiated by siguazodan or SK&F 95654, but not by zaprinast. SK&F 95654 also enhanced the ability of rolipram to increase cAMP content. Forskolin, a direct activator of adenylate cyclase, inhibited IgE-dependent release of mediators from basophils and increased cAMP levels in these cells. These effects were enhanced by rolipram, but not by SK&F 95654 or zaprinast. The cell permeant analog of cAMP, dibutyryl cAMP, inhibited mediator release from these cells, a property not shared by either dibutyryl-cGMP or sodium nitroprusside, an activator of soluble guanylate cyclase. The presence of both PDE III and PDE IV was confirmed by partially purifying and characterizing PDE activity in broken cell preparations. Overall, these data lend support to the hypothesis that cAMP inhibits mediator release from basophils and suggest that the major PDE isozyme responsible for regulating cyclic AMP content in these cells is PDE IV, with a minor contribution from PDE III. However, the finding that zaprinast caused increases in cAMP without inhibiting mediator release indicates that cAMP accumulation is not invariably linked to an inhibition of basophil activation.

Preliminary identification and role of phosphodiesterase isozymes in human basophils

We attempted to identify and establish the role of cyclic nucleotide phosphodiesterase (PDE) isozymes in human basophils by using standard biochemical techniques as well as describing the effects of isozyme-selective and nonselective inhibitors of PDE. The nonselective PDE inhibitors, theophylline and 3-isobutyl-1-methylxanthine, inhibited anti-IgE-induced release of histamine and leukotriene C4 (LTC4) from basophils. This inhibition was accompanied by elevations in cAMP levels. Rolipram, an inhibitor of the low Km cAMP-specific PDE (PDE IV), inhibited the release of both histamine and LTC4 from activated basophils and increased cAMP levels in these cells. In contrast, mediator release from basophils was not inhibited by either siguazodan or SK&F 95654, inhibitors of the cGMP-inhibited PDE (PDE III) or zaprinast, an inhibitor of the cGMP-specific PDE (PDE V). SK&F 95654 failed to elevate basophil cAMP in these experiments whereas zaprinast induced significant increases in cAMP content. The inhibitory effect of rolipram on mediator release was potentiated by siguazodan or SK&F 95654, but not by zaprinast. SK&F 95654 also enhanced the ability of rolipram to increase cAMP content. Forskolin, a direct activator of adenylate cyclase, inhibited IgE-dependent release of mediators from basophils and increased cAMP levels in these cells. These effects were enhanced by rolipram, but not by SK&F 95654 or zaprinast. The cell permeant analog of cAMP, dibutyryl cAMP, inhibited mediator release from these cells, a property not shared by either dibutyryl-cGMP or sodium nitroprusside, an activator of soluble guanylate cyclase. The presence of both PDE III and PDE IV was confirmed by partially purifying and characterizing PDE activity in broken cell preparations. Overall, these data lend support to the hypothesis that cAMP inhibits mediator release from basophils and suggest that the major PDE isozyme responsible for regulating cyclic AMP content in these cells is PDE IV, with a minor contribution from PDE III. However, the finding that zaprinast caused increases in cAMP without inhibiting mediator release indicates that cAMP accumulation is not invariably linked to an inhibition of basophil activation.

Coexpression of human cAMP-specific phosphodiesterase activity and high affinity rolipram binding in yeast

Studies by various investigators have demonstrated that the low Km, cAMP-specific phosphodiesterase (PDE IV) is selectively inhibited by a group of compounds typified by rolipram and Ro 20-1724. In addition to inhibiting the catalytic activity of PDE IV, rolipram binds to a high affinity binding site present in brain homogenates. Although it has been assumed that the high affinity rolipram-binding site is PDE IV, no direct evidence has been produced to support this assumption. The present studies were undertaken to determine whether the rolipram-binding site is coexpressed with PDE IV catalytic activity in Saccharomyces cerevisiae genetically engineered to express human recombinant monocytic PDE IV (hPDE IV). Expressing hPDE IV cDNA in yeast resulted in a 20-fold increase in PDE activity that was evident within 1 h of induction and reached a maximum by 3-6 h. The recombinant protein represented hPDE IV as judged by its immunoreactivity, molecular mass (approximately 88 kDa), kinetic characteristics (cAMP Km = 3.1 microM; cGMP Km greater than 100 microM), sensitivity to rolipram (Ki = 0.06 microM), and insensitivity to siguazodan (PDE III inhibitor) and zaprinast (PDE V inhibitor). Saturable, high affinity [3H] (R)-rolipram-binding sites (Kd = 1.0 nM) were coexpressed with PDE activity, indicating that both binding activity and catalytic activity are properties of the same protein. A limited number of compounds were tested for their ability to inhibit hPDE IV catalytic activity and compete for [3H](R)-rolipram binding. Analysis of the data revealed little correlation (r2 = 0.35) in the structure-activity relationships for hPDE IV inhibition versus competition for [3H] (R)-rolipram binding. In fact, certain compounds (e.g. (R)-rolipram Ro 20-1724) possessed a 10-100-fold selectivity for inhibition of [3H] (R)-rolipram binding over hPDE IV inhibition, whereas others (e.g. dipyridamole, trequinsin) possessed a 10-fold selectivity for PDE inhibition. Thus, although the results of these studies demonstrate that hPDE IV activity and high affinity [3H](R)-rolipram binding are properties of the same protein, they do not provide clear cut evidence linking the binding site with the PDE inhibitory activity of rolipram and related compounds.