Differential expression of cyclic nucleotide phosphodiesterase 3 and 4 activities in human T cell clones specific for myelin basic protein

Little is known concerning the relative distribution and function of the different cyclic nucleotide phosphodiesterases (PDEs) in lymphocytes. Recent reports, however, have indicated that specific PDE4 inhibitors were effective in treatment of experimental allergic encephalomyelitis, an animal model of multiple sclerosis. The therapeutic effect of PDE4 inhibitors is thought to be related to inhibition of autoreactive CD4+ T cells specific for myelin basic protein (MBP) or other myelin proteins. Human autoreactive CD4+ T lymphocyte clones (TCC), specific for the immunodominant MBP epitope (amino acids 83-99), contain PDE3 and PDE4, two PDEs that exhibit a high affinity for cAMP. Amplification of TCC mRNA by reverse transcription-PCR indicated that TCC PDE3 mRNA was of the PDE3B, not PDE3A, subtype. Different TCC contained different proportions of PDE3 and PDE4, and their activities increased during Ag (MBP) stimulation. Specific PDE3 (cilostamide) and PDE4 (rolipram) inhibitors suppressed [3H]thymidine incorporation in TCC. Since it is believed that many autoimmune diseases are at least partially mediated by autoreactive CD4+ T cells, these observations may have important implications not only for the treatment of multiple sclerosis but also for other autoimmune diseases.

Novel heterocyclic-fused pyridazinones as potent and selective phosphodiesterase IV inhibitors

A series of 6-aryl-4,5-heterocyclic-fused pyridazinones were designed and synthesized as selective phosphodiesterase (PDE) IV inhibitors. Biological evaluation of these compounds demonstrated a good selectivity profile toward the PDE IV family and greatly attenuated affinity for the Rolipram high-affinity binding site that seems to be responsible for undesiderable side effects. Structure-activity relationships (SARs) studies showed that the presence of an ethyl group at pyridazine N-2 is associated with the best potency and selectivity profile.

In vitro differentiation of human monocytes to macrophages: change of PDE profile and its relationship to suppression of tumour necrosis factor-alpha release by PDE inhibitors

1. During in vitro culture in 10% human AB serum, human peripheral blood monocytes acquire a macrophage-like phenotype. The underlying differentiation was characterized by increased activities of the macrophage marker enzymes unspecific esterase (NaF-insensitive form) and acid phosphatase, as well as by a down-regulation in surface CD14 expression. 2. In parallel, a dramatic change in the phosphodiesterase (PDE) profile became evident within a few days that strongly resembled that previously described for human alveolar macrophages. Whereas PDE1 and PDE3 activities were augmented, PDE4 activity, which represented the major cyclic AMP-hydrolysing activity of peripheral blood monocytes, rapidly declined. 3. Monocytes and monocyte-derived macrophages responded to lipopolysaccharide (LPS) with the release of tumour necrosis factor-alpha (TNF). In line with the change in CD14 expression, the EC50 value of LPS for induction of TNF release increased from approximately 0.1 ng ml-1 in peripheral blood monocytes to about 2 ng ml-1 in macrophages. 4. Both populations of cells were equally susceptible towards inhibition of TNF release by cyclic AMP elevating agents such as dibutyryl cyclic AMP, prostaglandin E2 (PGE2) or forskolin, which all led to a complete abrogation of TNF production in a concentration-dependent manner and which were more efficient than the glucocorticoid dexamethasone. 5. In monocytes, PDE4 selective inhibitors (rolipram, RP73401) suppressed TNF formation by 80%, whereas motapizone, a PDE3 selective compound, exerted a comparatively weak effect (10-15% inhibition). Combined use of PDE3 plus PDE4 inhibitors resulted in an additive effect and fully abrogated LPS-induced TNF release as did the mixed PDE3/4 inhibitor tolafentrine. 6. In monocyte-derived macrophages, neither PDE3- nor PDE4-selective drugs markedly affected TNF generation when used alone (< 15% inhibition), whereas in combination, they led to a maximal inhibition of TNF formation by about 40-50%. However, in the presence of PGE2 (10 nM), motapizone and rolipram or RP73401 were equally effective and blocked TNF release by 40%. Tolafentrine or motapizone in the presence of either PDE4 inhibitor, completely abrogated TNF formation in the presence of PGE2. Thus, an additional cyclic AMP trigger is necessary for PDE inhibitors to become effective in macrophages. 7. Finally, the putative regulatory role for PDE1 in the regulation of TNF production in macrophages was investigated. Zaprinast, at a concentration showing 80% inhibition of PDE1 activity (100 micromol l-1), did not influence TNF release. At higher concentrations (1 mmol l-1), zaprinast became effective, but this inhibition of TNF release can be attributed to a significant inhibitory action of this drug on PDE3 and PDE4 isoenzymes. 8. In summary, the in vitro differentiation of human peripheral blood monocytes to macrophages is characterized by a profound change in the PDE isoenzyme pattern. The change in the PDE4 to PDE3 ratio is functionally reflected by an altered susceptibility towards selective PDE inhibitors under appropriate stimulating conditions.

Activation of G-proteins with AIF-4 induces LFA-1-mediated adhesion of T-cell hybridoma cells to ICAM-1 by signal pathways that differ from phorbol ester- and manganese-induced adhesion

T-cell hybridomas metastasize widely, and the extent of dissemination correlates with invasiveness in fibroblast cultures. Previously, we provided evidence that both metastasis and in vitro invasion require activation of LFA-1, induced by G-protein-transduced signals triggered by as yet unidentified factors. We show here that LFA-1-mediated adhesion of TAM2D2 T-cell hybridoma cells to ICAM-1 can in fact be induced by direct activation of G-proteins using AIF-4, to the same extent as by using PMA or Mn2+. We assessed effects of protein kinase C (PKC), tyrosine kinase (TK), PI3-kinase (PI3K), and phospholipase C (PLC) inhibitors. Both AIF-4-induced adhesion and invasion were completely blocked by the TK inhibitor genistein and partially blocked by the PI3K inhibitor wortmannin, but not influenced by PKC inhibitor GF109203X. Downregulation of PKC did not affect invasion or adhesion induced by AIF-4 either. In contrast, GF109203X and PKC downregulation blocked PMA-induced adhesion, but genistein and wortmannin had no effect. Invasion and both AIF-4- and PMA-induced adhesion were completely blocked by the PLC inhibitor U73122. Mn(2+)-induced adhesion, which was not or was only partially blocked by the other inhibitors, was delayed by U73122, and spreading of Mn(2+)-treated cells was completely prevented by U73122. However, PLC activity during adhesion was not detected. We conclude that signals required for invasion and G-protein-induced adhesion are similar and are distinct from PKC-induced adhesion, and that in all cases PLC is likely to be activated, but is probably too local and/or transient to be detected.

Apoptotic effects of imidazo[1,2-a]pyrazine derivatives in the human Dami cell line

cAMP-elevating agents like phosphodiesterase inhibitors and purines have been shown to induce apoptosis. In the present work we have studied the effects of imidazo[1,2-a]pyrazine derivatives with a purine-like structure: PAB13 (6-bromo-8-(methylamino)imidazo[1,2-a] pyrazine), PAB15 (6-bromo-8-(ethylamino)imidazo[1,2-a]pyrazine), PAB23 (3-bromo-8-(methylamino)imidazo[1,2-a]pyrazine) on the growth of the Dami cell line in comparison to that of adenosine. The growth effect of PAB13, PAB15 and PAB23 was investigated in relation to their phosphodiesterase-inhibitory action and their activity on purinoceptors. Inhibition in cell growth was up to 71.0%, 76.3% and 89.7% for PAB23, PAB13 and PAB15, respectively and 100% for adenosine. Cell viability was affected in a concentration-dependent manner by PAB13, PAB15 and adenosine, with a correlation between growth inhibition and cytotoxicity. These effects of imidazo[1,2-a]pyrazine derivatives were found to be unrelated to an action on purinoceptors, but rather appear quantitatively linked to their ability in inducing apoptosis through their cAMP-increasing and phosphodiesterase-inhibitory potency.

Rapid protein kinase A--mediated activation of cyclic AMP-phosphodiesterase by parathyroid hormone in UMR-106 osteoblast-like cells

Parathyroid hormone (PTH) plays an essential role in osteoblast proliferation and differentiation. The effects of PTH are known to be mediated by cyclic adenosine monophosphate (cAMP) and calcium and by the activation of protein kinase C (PKC). cAMP is hydrolyzed to the inactive form 5' AMP by cyclic nucleotide phosphodiesterases (PDEs). We have investigated the role of PTH on PDE regulation in UMR-106 osteoblast-like cells. Treatment with 10 nM PTH caused a 3-fold increase in the PDE activity. The activation of PDE could be seen within 2 minutes and reached maximal levels after 20 minutes. The PTH effect was dose dependent with a half-maximal dose of 2 nM. The effect of PTH could be mimicked by the cAMP analogs Bt2 cAMP and forskolin, but not by PTH fragment 3-34, calcium ionophore A23187, or by the PKC activator phorbol 12-myristate 13-acetate. The PDE activity stimulated by PTH could be abolished by the PKA inhibitor H-8. The PDE activated by PTH was inhibitable by low concentrations of the cAMP-PDE-specific inhibitor RO 20-1724 (IC50 = 0.2 microM), but not by low concentrations of the inhibitors of cGMP-stimulated and cGMP-inhibited PDEs MEP-1 and milrinone (IC50 for both compounds > 30 microM). The PTH-stimulated cAMP accumulation was potentiated about 7-fold in the presence of RO 20-1724. H-8 potentiated the PTH-stimulated cAMP accumulation about 4-fold. Our results show that PTH rapidly stimulates the activity of cAMP-PDE in UMR-106 cells. The PDE activation involves cAMP and PKA. Inhibition of PKA can abolish the PTH-stimulated PDE activation and leads to increased accumulation of intracellular cAMP.

Selective activation of rolipram-sensitive, cAMP-specific phosphodiesterase isoforms by phosphatidic acid

In rat thymic lymphocytes, accumulation of phosphatidic acid (PA) occurs at the same time as decrease in cAMP levels and activation of a cAMP-specific phosphodiesterase (PDE) [type 4, EC 3.1.4.17 (PDE4)]. We investigated the nature of the PDE activated by PA and the mechanism of activation by using recombinant cAMP-specific PDE4 isoforms derived from three different genes (PDE4A, PDE4B, and PDE4D). The "long" variants expressed from each gene (PDE4A5, PDE4B1, and PDE4D3) were activated by PA, whereas the "short" variants (PDE4A1, PDE4B2, PDE4D1, and PDE4D2) were not. Phosphatidylserine was an activator that was as effective as PA, whereas phosphatidylcholine was ineffective, indicating that activation was restricted to anionic phospholipids. PA caused an increase in the Vmax value of PDE4D3 without affecting the Km value of the enzyme for the cAMP substrate. PA also caused a change in the Mg2+ requirement for hydrolysis. Half-maximal stimulation of the PDE was obtained with approximately 10 microg/ml PA. Although protein kinase A-mediated phosphorylation of PDE4D3 produces effects similar to those elicited by PA, the mechanism of PA-induced activation was not found to involve a phosphorylation. Instead, several observations suggest that PA may directly interact with the enzyme. The stimulation of cAMP PDEs by PA and other acidic phospholipids may be a mechanism by which growth factors and hormones modulate the cAMP-dependent signal transduction pathway during cell stimulation.

Differential efficacy of lymphocyte- and monocyte-selective pretreatment with a type 4 phosphodiesterase inhibitor on antigen-driven proliferation and cytokine gene expression

Elevations of intracellular cyclic AMP, achieved with the use of phosphodiesterase (PDE) inhibitors, cause functional downregulation of most inflammatory cells. Rolipram, an inhibitor selective for the PDE4 isozyme, can markedly downregulate antigen-driven proliferation and cytokine gene expression of unfractionated human peripheral blood mononuclear cells. However, it is unclear whether PDE4 inhibitors in a mixed-cell system exert their immunosuppressive effect on the lymphocyte or on the monocyte fraction. We have used an adherence-based protocol for separating peripheral blood mononuclear cells, isolated from atopic individuals, into lymphocyte and monocyte fractions and have selectively treated these populations with rolipram prior to reconstituting the cell cultures to their original lymphocyte/monocyte proportions. Cellular responses to both ragweed and tetanus toxoid were analyzed for both proliferation and gene expression of proinflammatory cytokines. A dose-dependent downregulation of ragweed- and tetanus toxoid-driven proliferative responses was achieved by pretreatment of lymphocytes from peripheral blood with rolipram. This downregulation was significantly greater than that achieved with pretreatment of monocytes. Pretreatment of both populations failed to show synergistic downregulation of proliferation. Lymphocyte pretreatment with rolipram also resulted in marked downregulation of gene expression for IL-4, IL-5, and interferon-gamma compared to monocyte pretreatment in both ragweed- and tetanus toxoid-driven systems. Interestingly, monocyte pretreatment in these systems resulted in significant downregulation of IL-2 gene expression compared to lymphocyte pretreatment. Flow cytometric analysis failed to show alterations in any of a panel of surface activation and signal transducing molecules by rolipram treatment with or without antigen stimulation. We conclude that, in a mixed cell system, PDE4 inhibitors downregulate antigen-driven proliferation and gene expression of proinflammatory cytokines predominantly through their effects on lymphocytes rather than monocytes.

Direct modulation of calmodulin targets by the neuronal calcium sensor NCS-1

Ca2+ and its ubiquitous intracellular receptor calmodulin (CaM) are required in the nervous system, among a host of cellular responses, for the modulation of several important enzymes and ion channels involved in synaptic efficacy and neuronal plasticity. Here, we report that CaM can be replaced by the neuronal calcium sensor NCS-1 both in vitro and in vivo. NCS-1 is a calcium binding protein with two Ca(2+)-binding domains that shares only 21% of homology with CaM. We observe that NCS-1 directly activates two Ca2+/CaM-dependent enzymes (3':5'-cyclic nucleotide phosphodiesterase and protein phosphatase calcineurin). Co-activation of nitric oxide synthase by NCS-1 and CaM results in a higher activity than with CaM alone. Moreover, NCS-1 is coexpressed with calcineurin and nitric oxide synthase in several neuron populations. Finally, injections of NCS-1 into calmodulin-defective cam1 Paramecium partially restore wildtype behavioral responses. With this highly purified preparation of NCS-1, we have obtained crystals suitable for crystallographic structure studies. NCS-1, despite its very different structure, distribution, and Ca(2+)-binding affinity as compared with CaM, can substitute for or potentiate CaM functions. Therefore, NCS-1 represents a novel protein capable of mediating multiple Ca(2+)-signaling pathways in the nervous system.

Inhibition of type IV phosphodiesterase by Ro 20-1724 attenuates endotoxin-induced acute renal failure

We recently found that the type IV-specific phosphodiesterase inhibitor Ro 20-1724 increases isoproterenol-induced cAMP secretion in the isolated rat kidney, whereas type I- and type III-specific phosphodiesterase inhibitors do not. Because cAMP is a known vasodilator of renal microvessels, we examined whether Ro 20-1724 is protective against endotoxin-induced acute renal failure. Fifteen rats were anesthetized, instrumented and administered a constant rate intravenous infusion of either Ro 20-1724 (10 micrograms/kg/min; n = 6) or vehicle (n = 9). After 1 hr, a base-line renal clearance period was conducted. All rats then received intravenous endotoxin (20 mg/kg), and six additional renal clearance periods were performed. Urinary cAMP excretion in the Ro 20-1724 group was elevated 2- to 3-fold (P < .01) compared with the control group throughout the protocol. In the control group, endotoxin decreased renal blood flow, increased renal vascular resistance and decreased glomerular filtration rate. Ro 20-1724 markedly attenuated endotoxin-induced changes in renal blood flow (P = .0004), renal vascular resistance (P = .0001) and glomerular filtration rate (P < .0001). The type IV-specific phosphodiesterase inhibitors warrant further study as selective therapeutic agents in the treatment of endotoxin-induced renal failure.

Rapid regulation of a cyclic AMP-specific phosphodiesterase (PDE IV) by forskolin and isoproterenol in LRM55 astroglial cells

Elevation of intracellular cyclic AMP (cAMP) levels by incubation of intact LRM55 astroglial cells with 0.1 mM forskolin or 0.1 microM isoproterenol (IPR) caused a rapid increase in soluble cAMP phospho-diesterase (PDE) activity. Activation did not require de novo protein synthesis and reached a maximum of > or = 100% increase over basal PDE activity after 15 min of treatment. The increase in activity was recovered in a single peak (peak 3) following DEAE chromatography; the other two peaks separated by this procedure showed no change. Peak 3 had all the characteristics of PDE IV: it was sensitive to rolipram, was insensitive to CI-930 and cyclic GMP (cGMP), had a high affinity for cAMP (K(m) approximately equal to 4 microM), and had a very low affinity for cGMP (K(m) > 100 microM). Forskolin treatment resulted in an increase of the Vmax of peak 3 without affecting its K(m). In vitro treatment of peak 3 with the catalytic subunit of protein kinase A increased activity, whereas treatment with alkaline phosphatase decreased activity. The rapid activation of this specific PDE in response to forskolin and IPR represents a novel regulation of PDE IV by a mechanism that seems to involve its phosphorylation by a cAMP-dependent protein kinase.

Differential regulation of receptor-stimulated cyclic adenosine monophosphate production by polyvalent cations in MC3T3-E1 osteoblasts

Extracellular cations have paradoxical trophic and toxic effects on osteoblast function. In an effort to explain these divergent actions, we investigated in MC3T3-E1 osteoblasts if polyvalent cations differentially modulate the agonist-stimulated cyclic adenosine monophosphate (cAMP) pathway, an important regulator of osteoblastic function. We found that a panel of cations, including gadolinium, aluminum, calcium, and neomycin, inhibited prostaglandin E1 (PGE)-stimulated cAMP accumulation but paradoxically potentiated parathyroid hormone (PTH)-stimulated cAMP production. In contrast, these cations had no effect on forskolin- or cholera toxin-induced increases in cAMP, suggesting actions proximal to adenylate cyclase and possible modulation of receptor interactions with G proteins. Phorbol 12-myristate 13-acetated (PMA) mimicked the effects of cations on PGE1- and PTH-stimulated cAMP accumulation in MC3T3-E1 cells, respectively, diminishing and augmenting the responses. Moreover, down-regulation of protein kinase C (PKC) by overnight treatment with PMA prevented gadolinium (Gd3+) from attenuating PGE1- and enhancing PTH-stimulated cAMP production, indicating involvement of PKC-dependent pathways. Cations, however, activated signal transduction pathways not coupled to phosphatidylinositol-specific phospholipase C (PI-PLC), since there was no corresponding increase in inositol phosphate formation or intracellular calcium concentrations. In addition, pertussis toxin treatment failed to prevent Gd(3+)-mediated suppression of PGE1-stimulated cAMP, suggesting actions independent of Gm. Thus, polyvalent cations may either stimulate or inhibit hormone-mediated cAMP accumulation in osteoblasts. These differential actions provide a potential explanation for the paradoxical trophic and toxic effects of cations on osteoblast function that occur in vivo under different hormonal conditions.

Inhibition of phosphodiesterase IV and intracellular calcium levels in human polymorphonuclear leukocytes

Phosphodiesterase (PDE) isoenzyme type IV is the predominant cyclic AMP hydrolytic activity in polymorphonuclear leukocytes (PMNs). PDE IV inhibitors depress functional responses of PMNs but their influence on intracellular calcium concentration ([Ca2+]i) has not been extensively studied. The present study examined the effects of rolipram (a selective PDE IV inhibitor) on the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP)-induced changes of [Ca2+]i in fura-2 loaded human PMNs. Rolipram (1 nM-10 microM) did not alter basal [Ca2+]i values. fMLP (10 nM approximately EC50) produced a transient calcium response, i.e., a peak followed by decay to a residual value above baseline. Peak [Ca2+]i values after fMLP were not altered but a faster decay and a lower residual [Ca2+]i were observed in rolipram (0.1-10 microM)-treated cells. fMLP added after thimerosal (20 microM) produced a peak followed by a sustained oscillatory response. Rolipram (up to 10 microM) did not alter the peak but inhibited the sustained response (-log IC50 = 6.39 +/- 0.12). The inhibitory effects of rolipram may be due to alterations in the mobilization of Ca2+ produced by the increase in the cellular content of cyclic AMP. SKF94120 (a selective PDE III inhibitor) produced minor effects on the fMLP-induced calcium response. SCA40 (a mixed PDE III/IV/V inhibitor) produced similar effects but was less potent than rolipram. Reduction of the calcium response probably underlies the inhibition of PMN functions produced by PDE IV inhibitors.

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.

Cyclic nucleotide phosphodiesterases and vascular smooth muscle

At least 30 different phosphodiesterase (PDE) enzymes have now been identified in mammalian tissues and cells, many of which are products of separate genes. These different isoenzyme forms can be subdivided into seven families based on their genetic and functional characteristics. Relatively specific inhibitors are available for at least five of these PDE families. A functional classification based on substrate specificity, regulatory properties, and sensitivity to inhibition by isozyme- and tissue-selective inhibitors can be used in describing the PDEs of vascular smooth muscle. Inhibition of these PDEs, especially with inhibitors of the PDE3 isoform, promotes vascular relaxation, particularly if the preparation of smooth muscle has been preconracted. For the most part, the drugs appear to act directly on smooth muscle; their effects are usually observed in endothelium-denuded preparations. In addition to their cardiotonic properties, many PDE3 inhibitors possess antiplatelet and thrombolytic activities, thereby suggesting the potential benefit of these drugs in treating diseases of the cardiovascular system. Isozyme- and cell-specific drugs have been shown to alter the synthetic state (i.e. proliferative phenotype) of smooth muscle cultures toward the appearance of the contractile phenotype. This suggests the possible use of selective PDE inhibitors to minimize the problem of restenosis seen after angioplasty. The development of novel methods to deliver more potent and selective PDE inhibitors to individual cell types and subcellular locales will lead to new therapeutic uses for this class of drugs in diseases of the cardiovascular system.

Inhibition of phosphatidylinositol-specific phospholipase C: studies on synthetic substrates, inhibitors and a synthetic enzyme

Enzyme inhibition studies on phosphatidylinositol-specific phospholipase C (PI-PLC) from B. Cereus were performed in order to gain an understanding of the mechanism of the PI-PLC family of enzymes and to aid inhibitor design. Inhibition studies on two synthetic cyclic phosphonate analogues (1,2) of inositol cyclic-1:2-monophosphate (cIP), glycerol-2-phosphate and vanadate were performed using natural phosphatidylinositol (PI) substrate in Triton X100 co-micelles and an NMR assay. Further inhibition studies on PI-PLC from B. Cereus were performed using a chromogenic, synthetic PI analogue (DPG-PI), an HPLC assay and Aerosol-OT (AOT), phytic acid and vanadate as inhibitors. For purposes of comparison, a model PI-PLC enzyme system was developed employing a synthetic Cu(II)-metallomicelle and a further synthetic PI analogue (IPP-PI). The studies employing natural PI substrate in Triton X100 co-micelles and synthetic DPG-PI in the absence of surfactant indicate three classes of PI-PLC inhibitors: (1) active-site directed inhibitors (e.g. 1,2); (2) water-soluble polyanions (e.g. tetravanadate, phytic acid); (3) surfactant anions (e.g. AOT). Three modes of molecular recognition are indicated to be important: (1) active site molecular recognition; (2) recognition at an anion-recognition site which may be the active site, and; (3) interfacial (or hydrophobic) recognition which may be exploited to increase affinity for the anion-recognition site in anionic surfactants such as AOT. The most potent inhibition of PI-PLC was observed by tetravanadate and AOT. The metallomicelle model system was observed to mimic PI-PLC in reproducing transesterification of the PI analogue substrate to yield cIP as product and in showing inhibition by phytic acid and AOT.

Impact of the stereochemistry of benzo[a]pyrene 7,8-dihydrodiol 9,10-epoxide-deoxyadenosine adducts on resistance to digestion by phosphodiesterases I and II and translesion synthesis with HIV-1 reverse transcriptase

Spatial orientations of bulky DNA adducts can influence the extent of resistance to digestion by exonucleases and translesion synthesis by HIV-1 reverse transcriptase (HIV-1 RT). In order to determine how different diastereomers of benzo[a]pyrene 7,8-dihydrodiol 9,10-epoxide (BPDE)-adducted DNAs influence the activity of these enzymes, 11-mer and 33-mer oligodeoxyribonucleotides were synthesized bearing site-specific and stereospecific BPDE adducts at adenine N6 on position two of the human N-ras codon 61. Phosphodiesterase I, which hydrolyzes DNA in the 3'-->5' direction, exhibited greater resistance opposite the lesion with C10-R BPDE-adducted templates than the corresponding C10-S adducts. However, the opposite stereoselective resistance to digestion was observed with phosphodiesterase II, which hydrolyzes DNA in the 5'-->3' direction. These results are complemented by the in vitro replication pattern exhibited with HIV-1 RT. Primer extension reactions under conditions defining single encounters between polymerase and substrate revealed adduct-dependent termination one base 3' to each of the lesions. When experimental conditions were altered to permit multiple encounters, HIV-1 RT was able to replicate past the damaged site on four of the six adducted templates, exhibiting little pausing opposite the lesion. Analyses of the replication pattern past these lesions revealed two general categories of replication blockage, which, like the exonucleolytic digestion data, were also based on the C10-R and C10-S configuration of the stereoisomers. Thus, the chirality of BPDE-dA adducts modulates enzymatic functions. Furthermore, the (+)- and (-)-anti-trans-BPDE-dA modified templates exhibited the most facile bypass, while the (+)- and (-)-anti-cis-BPDE adducts were most blocking.

Modulating effect of amount and types of dietary fat on colonic mucosal phospholipase A2, phosphatidylinositol-specific phospholipase C activities, and cyclooxygenase metabolite formation during different stages of colon tumor promotion in male F344 rats

Epidemiological and laboratory animal model studies suggest that the effect of dietary fat in colon carcinogenesis depends not only on the amount but on its fatty acid composition. Animal model studies demonstrated that high dietary corn oil or safflower oil rich in omega-6 fatty acids increased the colon tumor promotion, whereas diets containing fish oil high in omega-3 fatty acids had no such enhancing effect. One of the mechanisms by which high dietary fat enhances colon carcinogenesis may be through the modulation of colonic mucosal phospholipase A2 (PLA2) and phosphatidylinositol-specific phospholipase C (PI-PLC), which are dominant pathways for arachidonic acid release and formation of eicosanoids. PI-PLC is also responsible for diacylglycerol formation and protein kinase C-dependent signal transduction and cell proliferation. In the present study, we investigated the modulating effect of high fat diets rich in omega-3 and omega-6 fatty acids on colonic mucosal PLA2, PI-PLC activities, and eicosanoid (prostaglandins and thromboxane B2) formation from arachidonic acid via cyclooxygenase (COX) during different stages of azoxymethane (AOM)-induced colon carcinogenesis in male F344 rats. At 5 weeks of age, groups of animals were fed the low-fat diet containing 5% corn oil. Beginning at 7 weeks of age, all animals except those intended for vehicle treatment received AOM s.c. once weekly for 2 weeks at a dose rate of 15 mg/kg body weight. Vehicle-treated groups received an equal volume of normal saline. One day after the second AOM or vehicle treatment, groups of animals were transferred to experimental diets containing 23.5% corn oil and 20.5% fish oil + 3% corn oil, whereas one group continued on the low-fat diet containing 5% corn oil. Groups of animals were then sacrificed at weeks 1, 12, and 36 after the second AOM-or saline-treatment. Colonic mucosa harvested at weeks 1, 12, and 36 and colonic tumors obtained at week 36 were analyzed for PLA2, PI-PLC, and eicosanoid formation from arachidonic acid by the action of COX. The results demonstrate that colon carcinogen treatment increases the activities of colonic mucosal PLA2 and PI-PLC and the formation of prostaglandins and thromboxane A2 from arachidonic acid through COX throughout the study period compared to saline-treated animals fed similar diets. The activities of PLA2, PI-PLC, and COX were significantly higher in colon tumors compared to colonic mucosa. These results also demonstrate that a high-fat diet containing corn oil increases colonic mucosal and tumor PLA2 and PI-PLC and the formation of prostaglandins and thromboxane B2 by the action of COX as compared to low dietary corn oil or a diet high in fish oil. The results of our study offer one of the mechanisms by which the amount and types of dietary fat modulate colon carcinogenesis.

Involvement of guanylate cyclase and phosphodiesterases in the functional heterogeneity of human blood platelet subpopulations

Cyclic nucleotides are involved in the regulation of platelet activation, shape change, and aggregation. In this study we have investigated the role of guanylate cyclase and phosphodiesterase in three functional heterogeneous human blood platelet subpopulations separated according to density. In low-density platelets aggregation was enhanced and inhibited less when cyclic GMP was increased by sodium nitroprusside, compared to high-density platelets. Low-density platelets possessed a lower basal level of cyclic GMP and exhibited a small increase in cyclic GMP after stimulation with sodium nitroprusside. Cyclic GMP-dependent phosphodiesterase activity was similar in high, low, and intermediate-density platelets. In contrast, the activity of the cyclic AMP-dependent phosphodiesterase was higher in low-density compared to high and intermediate-density platelets. These results suggest that regulation of cyclic GMP and cyclic AMP levels plays an important role in the functional heterogeneity of human blood platelets.

Inhibitors of type IV phosphodiesterases reduce the toxicity of MPTP in substantia nigra neurons in vivo

The neuropathology of Parkinson's disease is characterized by the degeneration of dopaminergic neurons in the substantia nigra. We have recently shown that the activation of protein kinase A improves the survival of dopaminergic neurons in culture and, furthermore, protects them from the dopaminergic neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP+) in vitro. We have now analysed the potential of phosphodiesterase inhibitors to increase cAMP levels in dopaminergic neurons, to improve their survival in culture and to protect them from the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in vivo. Increasing intracellular cAMP with phosphodiesterase type IV-specific inhibitors enhanced the survival of dopaminergic neurons in culture. Inhibitors of other phosphodiesterase types were not active. In vivo, phosphodiesterase type IV inhibitors reduced the MPTP-induced dopamine depletion in the striatum of C57BL/6 mice. Furthermore, the loss of tyrosine hydroxylase-immunopositive neurons in the substantia nigra of these animals was diminished. After Nissl staining, a similar reduction of the MPTP-induced loss of neurons was observed in the substantia nigra. The protective effect of protein kinase A activation did not appear to be due to the blocking of MPP+ uptake into dopaminergic neurons. This was not decreased after treatment with forskolin or 8-(4-chlorophenylthio)-cAMP. Thus, protein kinase A regulates the survival and differentiation of dopaminergic substantia nigra neurons in vivo, implicating a therapeutic potential for substances which regulate cAMP turnover in these neurons.

Characterization of a novel potent and specific inhibitor of type V phosphodiesterase

Guanosine cyclic 3':5'-monophosphate (cGMP) plays a crucial role in regulating vascular smooth muscle contractile state. In rat aortic smooth muscle cells (RSMC) three isozymes of phosphodiesterase (PDE) may be involved in the degradation of cGMP, namely PDE I, PDE III, and PDE V. To study the effective contribution of PDE V to the control of intracellular cGMP levels, a specific and potent PDE V inhibitor 1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)pyrazolo[3, 4d]- pyrimidin-4-(5H)-one (DMPPO) was synthesized. DMPPO is a competitive inhibitor with respect to cGMP (Ki = 3 nM) and displayed high selectivity for PDE V as compared to other PDE isozymes. DMPPO strongly potentiated the cGMP response of atrial natriuretic peptide- or sodium nitroprusside-treated RSMC (EC50 = 0.5 microM). In addition, similar intracellular cGMP levels were obtained in the presence of a saturating concentration of DMPPO or 3-isobutyl-1-methylxanthine, a nonspecific PDE inhibitor, suggesting that cGMP is almost exclusively hydrolyzed by PDE V in RSMC. Stimulation of RSMC with atrial natriuretic factor resulted in accumulation of cGMP in the extracellular media. This egression was shown to be proportional to the intracellular level of cGMP and a first-order rate constant of 0.04 min-1 was determined for the egression process. DMPPO did not interfere with the efflux and allowed us to show that intracellular cGMP levels are mainly controlled by PDE V, rather than by egression in RSMC. DMPPO is, therefore, a useful tool for determining the role of PDE V in the control of cGMP levels in living cells and tissues.

Nitric oxide, an enteric nonadrenergic-noncholinergic relaxant transmitter: evidence using phosphodiesterase V and nitric oxide synthase inhibition

1. The effects of NG-nitro-L-arginine (L-NOARG), a nitric oxide synthase inhibitor, and SK&F 96231, a phosphodiesterase type V inhibitor, on electrical field stimulated (EFS) nonadrenergic noncholinergic (NANC) relaxations of rat fundal strips, guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, and guinea-pig taenia caeci were investigated. 2. Reproducible repeated control random EFS frequency-response curves were obtained for all three tissues. 3. Depending on the frequency of stimulation, L-NOARG (10(-4)-5 x 10(-3) M) caused either a complete or partial inhibition of the NANC-induced relaxations of the rat fundal strips and the guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, but not of the guinea-pig taenia caeci. The inhibitory action of L-NOARG was partially or totally reversed, depending on the tissue, by L-arginine (5 x 10(-3) M). 4. SK&F 96231 (10(-6)-10(-4) M) caused a concentration- and frequency-dependent potentiation of both the size and duration of the EFS-induced NANC relaxant response of rat fundal strips and guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, but not of the guinea-pig taenia caeci. 5. Zaprinast, another phosphodiesterase type V inhibitor (10(-6)-10(-4) M) caused a concentration- and frequency-dependent potentiation of the NANC relaxant responses to EFS of rat fundal strips. 6. SK&F 96231 and zaprinast alone (10(-6)-10(-4) M) caused a concentration-dependent relaxation of the agonist-induced tone of all three tissues with the maximum degree of relaxation found to be in the order stomach < ileum < caecum. This is the reverse order for ability of SK&F 96231 to potentiate relaxant responses to EFS. 7. These results suggest NO is involved in the NANC nerve-mediated relaxation of rat fundal strips and guinea-pig isolated ileum longitudinal muscle with intact myenteric plexus, but not the guinea-pig taenia caeci.

Association of some hydrolytic enzymes with the prostasome membrane and their differential responses to detergent and PIPLC treatment

Prostasomes are human prostate derived organelles that were isolated from both prostatic fluid and seminal plasma for the present study. Specific activities were determined for prostasome membrane-associated enzymes, alkaline phosphatase (ALP), 5'-nucleotidase (5'NT), and alkaline phosphodiesterase I (APD). The mode of their membranous anchoring was studied by treatment of prostasomes with phosphoinositol-specific phospholipase C (PIPLC) and different detergents. A substantial amount of ALP (50%) and 5'NT (31%) was released by incubation of prostasomes with 2 U/ml of PIPLC contrary to the small amount of APD (12%) released by the same treatment. After PIPLC treatment, the enzymes were recovered in the aqueous phase after phase repartition in Triton X-114 indicating that PIPLC removed the hydrophobic domain converting the enzymes from membrane-linked to aqueous soluble forms. Octyl glycoside was the most efficient one among different detergents to solubilize the enzymes from the prostasome membrane. Both ALP and 5'NT were resistant to the treatment with Triton X-100 and Triton X-114. These results suggest that ALP, 5'NT, and APD are more or less extensively linked to the prostasome membrane via a glycophosphoinositide anchor.

Rolipram inhibition of phosphodiesterase-4 activation

Rolipram inhibited U937 cell phosphodiesterase-4 in either the presence or absence of saturating (100 micrograms/ml) phosphatidic acid in an apparently phospholipid-independent manner, exhibiting similar kinetics (Ki values = 0.41 and 0.59 microM, respectively). At low concentrations (10 and 100 nM), however, rolipram caused a rightward shift of the phosphatidic acid concentration-response curve for phosphodiesterase-4 activation, suppressing activation by up to 70%. Maximum inhibition of phosphodiesterase-4 activation occurred at phosphatidic acid concentrations of 5-40 micrograms/ml. The results suggest that rolipram is capable of inhibiting phosphodiesterase-4 by both phospholipid-dependent and phospholipid-independent mechanisms.

Chemoprevention of colon carcinogenesis by phenylethyl-3-methylcaffeate

Previous studies from this laboratory have established that caffeic acid esters present in propolis, a natural resin produced by honey bees, are potent inhibitors of human colon adenocarcinoma cell growth, carcinogen-induced biochemical changes, and preneoplastic lesions in the rat colon. The present study was designed to investigate the chemopreventive action of dietary phenylethyl-3-methylcaffeate (PEMC) on azoxymethane-induced colon carcinogenesis and to examine the modulating effect of PEMC on phosphatidylinositol-specific phospholipase C (PI-PLC), phospholipase A2, lipoxygenase (LOX), and cyclooxygenase activities in the colonic mucosa and tumor tissues in male F344 rats. At 5 weeks of age, groups of rats were fed the control (modified AIN-76A) diet, or a diet containing 750 ppm of PEMC. At 7 weeks of age, all animals except those in the vehicle (normal saline)-treated groups were given 2 weekly s.c. injections of azoxymethane at a dose rate of 15 mg/kg body weight/week. All groups were maintained on their respective dietary regimen until the termination of the experiment 52 weeks after the carcinogen treatment. Colonic tumors were evaluated histopathologically. Both colonic mucosa and tumors were analyzed for PI-PLC, phospholipase A2, cyclooxygenase, and LOX activities. The results indicate that dietary administration of PEMC significantly inhibited the incidence and multiplicity of invasive, noninvasive, and total (invasive plus noninvasive) adenocarcinomas of the colon (P < 0.05-0.004). Dietary PEMC also suppressed the colon tumor volume by 43% compared to the control diet. Animals fed the PEMC diet showed significantly decreased activities of colonic mucosal and tumor PI-PLC (about 50%), but PEMC diet had no effect on phospholipase A2. The production of 5(S)-, 8(S)-, 12(S)-, and 15(S)-hydroxyeicosatetraenoic acids via the LOX pathway from arachidonic acid was reduced in colonic mucosa and tumors (30-60%) of animals fed the PEMC diet as compared to control diet. PEMC had no effect on the formation of colonic mucosal cyclooxygenase metabolites but inhibited the formation in colonic tumors by 15-30%. The precise mechanism by which PEMC inhibits colon tumorigenesis remains to be elucidated. It is likely that the chemopreventive action may be related, at least in part, to the modulation of PI-PLC-dependent signal transduction and LOX-mediated arachidonic acid metabolism.

Desensitization of beta-adrenergic responses in adipocytes involves receptor subtypes and cAMP phosphodiesterase

Acute exposure of isolated adipocytes to isoproterenol induces the desensitization of lipolytic responses to norepinephrine and selective beta 1-, beta 2- and beta 3-adrenoceptor agonists, as well as the adrenocorticotropic hormone 1-24 fragment (ACTH). Forskolin and 8-bromo-cAMP responses are also desensitized. When lipolysis was measured in the presence of OPC 3911 [N-cyclohexyl-N-2-hydroxyethyl-4(6-(1,2-dihydro-2- oxoquinolyloxy))butyramide], a specific inhibitor of the cAMP phosphodiesterase of adipocytes, the desensitization of all lipolytic agents--except the beta 2-adrenoceptor agonist--was abolished. Isoproterenol induced a similar loss (35%) of both membrane beta 1- and beta 2-adrenoceptors and an uncoupling of beta 1-adrenoceptors, but did not modify the weak coupling of control beta 2-adrenoceptors. These data suggest that isoproterenol induced (i) an activation of the cAMP phosphodiesterase, which is solely responsible for the desensitization of norepinephrine response as well as beta 1- and beta 3-adrenoceptor mediated responses and (ii) an additional desensitization of the sole beta 2-adrenergic signaling system which suggests a subtype-selective pattern of regulating processes.

Prevention of autoimmune demyelination in non-human primates by a cAMP-specific phosphodiesterase inhibitor

Experimental allergic encephalomyelitis (EAE) is an autoimmune disease of the central nervous system that serves as a model for the human disease multiple sclerosis. We evaluated rolipram, a type IV phosphodiesterase inhibitor, for its efficacy in preventing EAE in the common marmoset Callithrix jacchus. In a blinded experimental design, clinical signs of EAE developed within 17 days of immunization with human white matter in two placebo-treated animals but in none of three monkeys that received rolipram (10 mg/kg s.c. every other day) beginning 1 week after immunization. In controls, signs of EAE were associated with development of cerebrospinal fluid pleocytosis and cerebral MRI abnormalities. In the treatment group, there was sustained protection from clinical EAE, transient cerebrospinal fluid pleocytosis in only one of three animals, no MRI abnormality, and marked reduction in histopathologic findings. Rolipram-treated and control animals equally developed circulating antibodies to myelin basic protein. Thus, inhibition of type IV phosphodiesterase, initiated after sensitization to central nervous system antigens, protected against autoimmune demyelinating disease.

Effects of alkyl substituents of xanthine on phosphodiesterase isoenzymes

The structure-activity relationships of a series of alkylxanthine derivatives were investigated. The partition coefficient of alkylxanthines enlarged with an elongation of the alkyl chain at the 1-, 3-, or 7-position of xanthine. There was a mild correlation between the apparent partition coefficient and the tracheal relaxant activity or the inhibitory activity on phosphodiesterase (PDE) IV isoenzyme, while the tracheal relaxant activity closely correlated with the PDE IV inhibitory activity. Regarding substituents at different positions, the alkylation at the 3-position increased the inhibitory activity on every PDE isoenzyme. The alkylation at the 1-position potentiated the inhibitory activity on PDE IV with the alkyl chain length, but decreased the activities on other PDE isoenzymes. The alkylation at the 7-position was characteristic in its decrease in inhibitory activity on PDE III. These results suggested that the potency of the inhibitory activity of xanthine derivatives on PDE isoenzymes is not dependent simply upon their hydrophobicity but upon change in the affinity for the active sites on PDE isoenzymes by the introduction of the alkyl group at particular positions of the xanthine skeleton.

The effects of mastoparan on the carrot cell plasma membrane polyphosphoinositide phospholipase C

When [3H]inositol-labeled carrot (Daucus carota L.) cells were treated with 10 or 25 microM wasp venom peptide mastoparan or the active analog Mas-7 there was a rapid loss of more than 70% of [3H]phosphatidylinositol-4-monophosphate (PIP) and [3H]phosphatidylinositol-4,5-bisphosphate (PIP2) and a 3- and 4-fold increase in [3H]inositol-1,4-P2 and [3H]inositol-1,4,5-P3, respectively. The identity of [3H]inositol-1,4,5-P3 was confirmed by phosphorylation with inositol-1,4,5-P3 3-kinase and co-migration with inositol-1,3,4,5-P4. The changes in phosphoinositides were evident within 1 min. The loss of [3H]PIP was evident only when cells were treated with the higher concentrations (10 and 25 microM) of mastoparan or Mas-7. At 1 microM Mas-7, [3H]PIP increased. The inactive mastoparan analog Mas-17 had little or no effect on [3H]PIP or [3H]PIP2 hydrolysis in vivo. Neomycin (100 microM) inhibited the uptake of Mas-7 and thereby inhibited the Mas-7-stimulated hydrolysis of [3H]PIP and [3H]PIP2. Plasma membranes isolated from mastoparan-treated cells had increased PIP-phospholipase C (PLC) activity. However, when Mas-7 was added to isolated plasma membranes from control cells, it had no effect on PIP-PLC activity at low concentrations and inhibited PIP-PLC at concentrations greater than 10 microM. In addition, guanosine-5'-O-(3-thiotriphosphate) had no effect on the PIP-PLC activity when added to plasma membranes isolated from either the Mas-7-treated or control cells. The fact that Mas-7 did not stimulate PIP-PLC activity in vitro indicated that the Mas-7-induced increase in PIP-PLC in vivo required a factor that was lost from the membrane during isolation.

Phosphodiesterase inhibitors reduce bronchial hyperreactivity and airway inflammation in unrestrained guinea pigs

A new guinea pig model of allergic asthma was used to investigate the effects of low doses of the phosphodiesterase inhibitors, rolipram (phosphodiesterase IV selective), ORG 20241 (N-hydroxy-4-(3,4-dimethoxyphenyl)-thiazole-2-carboximidamide; dual phosphodiesterase III/IV inhibitor with some selectivity for the phosphodiesterase IV isoenzyme), and of theophylline (non-selective) on allergen-induced early and late phase asthmatic reactions, bronchial hyperreactivity to histamine inhalation, and airway inflammation. Theophylline (25 mg/kg i.p.) and ORG 20241 (5 mg/kg i.p.) did not affect histamine-induced bronchoconstriction, whereas rolipram (75 micrograms/kg i.p.) only slightly reduced the response to histamine at 7 h after administration. However, bronchial hyperreactivity after the early and after the late reaction was significantly reduced by theophylline, rolipram and ORG 20241, while bronchoalveolar lavage studies revealed a selective inhibition of airway inflammation by the phosphodiesterase inhibitors. Theophylline significantly reduced the number of eosinophils, neutrophils and macrophages; rolipram reduced the number of neutrophils and lymphocytes, and ORG 20241, the number of eosinophils and macrophages. None of the compounds at the dosage indicated reduced the early and late reaction when administered i.p. 1 h before allergen exposure to defined dual responding animals. These results indicate that non-bronchodilator doses of these phosphodiesterase inhibitors markedly reduce the allergen-induced development of bronchial hyperreactivity as well as airway inflammation, presumably by selectively inhibiting cellular migration. The results suggest that an orchestrated series of cellular interactions is involved in the development of bronchial hyperreactivity. It is concluded that phosphodiesterase inhibitors may be very useful in the treatment of bronchial asthma.

Hormonal regulation of angiotensin II type 1 receptor expression and AT1-R mRNA levels in human adrenocortical cells

Human adrenocortical H295R cells express AII receptors which are predominantly of the AT1 but not AT2 subclass. These receptors are functionally coupled to phosphoinositidase C in a manner similar to that seen in fetal human, sheep and bovine adrenocortical cells. Treatment of H295R cells with forskolin or dbcAMP to activate the protein kinase A pathway caused a rapid (maximal by 3 h) and sustained decrease in AT1-R mRNA levels which in turn preceded a time-dependent (maximal by 12 h) and dose-dependent loss of [125I]AII binding and phosphoinositidase C activation on subsequent AII challenge. Thus, both decreased AT1-R mRNA levels and functional receptor expression appear to parallel each other in response to activation of protein kinase A. Activation of the Ca2+/protein kinase C pathways by treatment with AII also caused a rapid (maximal by 3 h) and dose-dependent loss in AT1-R mRNA, but mRNA levels subsequently rose again, approaching control levels by 36 h. Treatment with AII for 48 h had little effect on either [125I]AII binding or the subsequent phosphoinositidase C response. The effect of AII, but not forskolin, was blocked by the presence of cycloheximide. The action of AII on AT1-R mRNA was probably mediated through both protein kinase C and Ca(2+)-sensitive protein kinases as the effect at 4 h was not completely reproduced by phorbol ester alone, but was fully reproduced by a combination of phorbol ester and Ca2+ ionophore. However, increased Ca2+ influx alone, due to treatment with BAYK8644 or elevated extracellular K+, also resulted in a decrease in AT1-R mRNA levels. Thus in the H295R cell, control of AT1-R expression appears to be complex, being achieved at least in part through control of the level of AT1-R mRNA by multiple independent signaling pathways including protein kinase A, protein kinase C and Ca2+.

Phosphoinositide hydrolysis by phospholipase C modulated by multivalent cations La(3+), Al(3+), neomycin, polyamines, and melittin

Second messenger production from phosphoinositide hydrolysis is regulated by different pathways, such as G-proteins or tyrosine phosphorylation of phosphoinositide phospholipase C (PI-PLC). Another means of altering the activity of PI-PLC is through cation interaction with the phosphoinositide substrate. A variety of organic and inorganic multi-valent cations were examined for their effects on the activity of purified PI-PLC delta. Surprisingly, the cations produced both stimulation and inhibition of PI-PLC catalyzed phosphoinositide hydrolysis, depending on the substrate and the ion to phosphoinositide stoichiometry. These data support the hypothesis that ionic complexes with phosphoinositides may alter their hydrolysis by PI-PLC.

Tyrosine-phosphorylated calmodulin has reduced biological activity

Calmodulin is phosphorylated by the purified insulin receptor on tyrosine residues with a maximum stoichiometry of 1 mol phosphate/mol of calmodulin. Isolated tryptic phosphopeptides were sequenced by manual Edman degradation and demonstrated that calmodulin is equally phosphorylated on tyrosine 99 and tyrosine 138. Phosphorylated calmodulin has a decreased affinity (K0.5 = 4.2 nM) for the 63-kDa isozyme of cyclic nucleotide phosphodiesterase compared to nonphosphorylated calmodulin (K0.5 = 2.1 nM). The K0.5 for Ca2+ is marginally increased from 2.8 to 3.2 microM in the presence of phosphotyrosyl calmodulin. The effect of the calmodulin antagonist, mastoparan, was investigated to determine whether mastoparan would differentially inhibit calmodulin- or phosphocalmodulin-dependent enzyme activity. The IC50 of mastoparan is fourfold lower for phosphotyrosyl calmodulin compared to nonphosphorylated calmodulin. Phosphorylation of calmodulin may provide a mechanism for the differential regulation of calmodulin-dependent enzymes. These observations further support a potentially important regulatory function of calmodulin phosphorylation in signal transduction.

Phosphodiesterase and phosphotriesterase in Rhizobium and Bradyrhizobium strains and their roles in the degradation of organophosphorus pesticides

Of 13 Rhizobium and Bradyrhizobium strains investigated for the production of cellular and extracellular phosphodiesterase and phosphotriesterase, all were found to produce both enzymes. Phosphodiesterase was produced at a much higher level than phosphotriesterase. Rhizobium meliloti TAL 1373 was the most productive. The extracellular enzymes were activated by inclusion in the assay mixture of Ca2+ or Mg2+. The enzymes were inhibited by Zn2+ but not significantly affected by Cu2+, Co2+ and Mn2+. Both hydrolases were inhibited by dithiothreitol but not by thiol-directed inhibitors, suggesting that sulphydryl groups are not directly involved in catalysis. The enzymes have the ability to hydrolyse some organophosphorus compounds, suggesting that Rhizobium and Bradyrhizobium strains play an important role in the degradation of organophosphorus pesticides.

A multisample assay for inhibitors of phosphatidylinositol phospholipase C: identification of naturally occurring peptide inhibitors with antiproliferative activity

A convenient and reliable multisample assay for the screening of inhibitors of the growth factor signalling enzyme phosphatidylinositol specific phospholipase C (PtdInsPLC) has been developed. Three naturally occurring peptide inhibitors of PtdInsPLC have been identified, myroridin K, streptothricin B and edeine, with IC50 values of 8.3, 6.7 and 16.1 microM, respectively. All three peptides inhibited colony formation of HT-29 human colon adenocarcinoma cells, with IC50 values of 7.2, 3.9 and 13.0 microM, respectively. The compounds also inhibited the growth of other human cancer cells in culture. One of the peptides, myroridin K, has previously been reported to have in vivo antitumour activity. It is possible that inhibition of PtdInsPLC is responsible for the cell growth inhibition and antitumour properties of the peptide compounds.

Causal link between nucleotide pyrophosphohydrolase overactivity and increased intracellular inorganic pyrophosphate generation demonstrated by transfection of cultured fibroblasts and osteoblasts with plasma cell membrane glycoprotein-1. Relevance to calcium pyrophosphate dihydrate deposition disease

OBJECTIVE

In subjects with idiopathic calcium pyrophosphate dihydrate (CPPD) deposition disease, cartilage chondrocytes elaborate increased amounts of PPi. The mechanism of the intracellular PPi elevation is not known. Plasma membrane 5'-nucleotide phosphodiesterase I/nucleotide pyrophosphohydrolase (NTPPPH) activity also is elevated in chondrocytes and dermal fibroblasts of patients with idiopathic CPPD deposition disease. NTPPPH, as an ecto-enzyme, could act within certain intracellular compartments. Thus, we hypothesized a potential causal link between increased NTPPPH activity and increased intracellular PPi.

METHODS

Transformed simian fibroblasts (COS cells) and human osteoblasts (U2OS cells) were transfected with the 5'-nucleotide phosphodiesterase I ecto-enzyme plasma cell membrane glycoprotein-1 (PC-1), recently shown to be expressed in cartilage, osteoblasts, and fibroblasts.

RESULTS

Transfection with PC-1 markedly up-regulated 5'-nucleotode phosphodiesterase I activity and increased intracellular PPi concentrations by increasing the capacity of cells to generate PPi. Importantly, this did not require supplementation with exogenous nucleotides.

CONCLUSION

Cellular overexpression of PC-1 produces NTPPPH overactivity and increased intracellular PPi generation in vitro. These findings support the potential importance of NTPPPH overactivity in PPi generation, both inside and outside the cell, in some subjects with CPPD deposition disease.

Use of a yeast expression system for the isolation and analysis of drug-resistant mutants of a mammalian phosphodiesterase

Saccharomyces cerevisiae strain PP5 has a phosphodiesterase (PDE) deficiency that results in heat-shock sensitivity due to the intracellular accumulation of cAMP. This strain also carries the cam mutation, which confers permeability to cAMP and, as shown here, to other compounds. Expression of rat type IV PDE in these cells caused them to revert to heat-shock resistance. Treatment of the transformed PP5 cells with rolipram, an antidepressant in humans and a potent inhibitor of type IV PDEs, reinstated sensitivity to heat shock. The biochemical properties of deletion mutants of this PDE were determined, and an active enzyme of minimum length was created. Reversion to heat-shock resistance was then used to select for PDE mutants refractory to the inhibitory effects of rolipram. Four mutants (A1, A2, A3, and A5) were isolated. Each carries a single point mutation; two have mutations in the same codon. Each mutant showed distinct properties, based on analysis of their substrate kinetics and IC50 values for a variety of inhibitors. Mutant A5 had a reduced activity for substrate, mutants A1 and A3 showed no change in substrate kinetics, and mutant A2 displayed an increase in activity. For most mutants, the drug resistance was confined to the class of drug used in the selection. This study shows that it is possible to recreate in yeast cells the susceptibility of mammalian enzymes to pharmacological agents. Our study also demonstrates that such systems can be used to select rare mutants useful in the analysis of drug-protein interactions.

Antioxidants impair the coupling of cell-surface ligand receptors to the inositol lipid signalling pathway in human T lymphocytes but not in Jurkat T lymphoblastic leukemia cells. Evidence that leukotrienes are not involved in the coupling mechanism

Ligands including phytohaemagglutinin (PHA) and anti-CD3 monoclonal antibodies trigger the generation of inositol lipid-derived second messengers following their binding to cell-surface structures of human T lymphoid cells. Previous evidence has suggested that the generation of leukotrienes may play an intermediary role in coupling the ligation of T lymphoid cell-surface structures to the inositol lipid signalling system in these cells (A.R. Mire-Sluis et al. (1989) FEBS Lett. 258, 84-88). Here we have studied the actions of two novel selective leukotriene biosynthesis inhibitors, MK 886 and BW A4C and of two general lipid soluble antioxidants, butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) on this pathway. Neither MK 886 nor BW A4C abrogated stimulation of inositol lipid breakdown following PHA or anti CD3 treatment of T lymphocytes. By contrast, this pathway was inhibited by BHT and BHA. These observations, together with our failure to demonstrate the generation of lipoxygenase products following PHA stimulation of T lymphocytes, suggests that an antioxidant-sensitive step other than the generation of leukotrienes plays a critical role in coupling cell-surface receptors to the inositol lipid signalling system in these cells. By contrast none of these inhibitors abrogated ligand-stimulated inositol lipid signalling in Jurkat T acute lymphoblastic leukaemia cells. These results suggest a heterogeneity in the organization of the signal transduction machinery in lymphoid cells at different stages of differentiation.

Stimulation of bovine brain phospholipase C activity by myelin basic protein requires arginyl residues in peptide linkage

We reported previously a highly purified phosphatidylinositol-specific phospholipase C (PI-PLC) from bovine brain and from human myelin which was stimulated by myelin basic protein. In this paper we report that the stimulation of the PI-PLC activity by myelin basic protein (MBP) requires arginine residues in peptide linkage. MBP and poly-L-arginine were able to stimulate the PI-PLC activity by 250% while other basic poly amino acids were unable to stimulate the PI-PLC activity. Neither free arginine nor benzoyl-arginine ethylester was able to stimulate the activity of the enzyme. These results suggested a requirement for the guanidino group of arginine and arginine in peptidyl linkage. The arginyl residues of MBP were modified chemically with 1,2-cyclohexanedione, or enzymatically by cholera toxin which ADP-ribosylated arginyl groups, or by peptidylarginine deiminase which converted the guanidino group of arginine to the ureido group of citrulline. ADP-ribosylation did not affect the stimulation while the 1,2-cyclohexanedione modified MBP and the peptidylarginine deiminase-treated MBP showed a reduced ability to stimulate the PI-PLC activity which correlated with the number of arginyl residues modified. Sequence analysis of the peptidylarginine deiminase-treated MBP established that specific arginyl residues had been converted to citrulline to a greater extent than others. When 70% of Arg 25 and Arg31 were converted to citrulline little stimulatory activity remained, whereas the conversion of 100% of Arg 170 did not affect the ability of C1 to stimulate the enzyme. A role for "active" arginine in this MBP peptide is suggested by our data.

Altered levels of phosphatidylinositol phospholipase C activity in rat liver cells in response to insulin, epinephrine, acetylcholine and bacterial phospholipase C

Rat liver cells were homogenized and subsequently fractionated by a simplified method based on microfiltration, which proved to give a high recovery of membrane-bound phosphatidylinositol phospholipase C (PI-PLC) activity. The effects of insulin, acetylcholine (AC), epinephrine (EN) and bacterial phospholipase C (bPLC) on the PI-PLC activity were studied after in vitro treatment of isolated membranes or after in situ application in rat liver. A dose-dependent increase of membranous PI-PLC (up to 3-fold) and corresponding 36 to 72% decline of the cytosolic activity were established upon treatment with supraphysiological doses of insulin or with bPLC, respectively. AC induced a biphasic response with a maximal stimulation in the micromolar range. On the other hand EN promoted a slight but significant dose-dependent inhibition of PI-PLC in both cytosol and membranes. Sodium fluoride was also a potent inhibitor of the membrane-associated PI-PLC with an EC50 value of about 5 mM. The combined assay with NaF and EN revealed no additivity between their inhibitory effects, suggesting a common step in the mechanism(s) of inhibition caused by the two agents. The stimulatory effects of insulin and AC were partially reduced by soluble cytosolic factors, which still remain to be identified. When insulin and AC were applied in combination in the presence of cytosol, this resulted in a 56% inhibition of PI-PLC below the control level.

Alpha 1-adrenoceptor-mediated inhibition of cellular cAMP accumulation in neonatal rat ventricular myocytes

We studied adrenergic regulation of cellular cAMP in neonatal rat ventricular myocytes. Since cAMP content depends on synthesis, breakdown and egress, the contribution of each of these mechanisms was assessed. In the presence of the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine, cAMP accumulation stimulated by the beta-adrenoceptor agonist (-)-isoprenaline was diminished when the mixed alpha + beta adrenoceptor agonist (-)-noradrenaline was coincubated with (-)-isoprenaline. Moreover, adenylyl cyclase activation stimulated by (-)-isoprenaline was decreased by (-)-noradrenaline and by the selective alpha 1-adrenoceptor agonists (-)-phenylephrine and methoxamine, suggesting that alpha-adrenoceptor agonism regulates cAMP metabolism through its effect on the synthetic pathway. Evidence for alpha 1-adrenoceptor mediation of this response was enhancement of (-)-noradrenaline-induced cAMP generation by the selective alpha 1-adrenoceptor antagonist terazosin (10 nmol/l). The selective alpha 2-adrenoceptor antagonist yohimbine (10 nmol/l) had no effect. The alpha 1-adrenoceptor mediated depression of (-)-isoprenaline-stimulated cAMP generation and adenylyl cyclase activation was prevented by terazosin and in separate experiments markedly enhanced by pertussis toxin pretreatment, suggesting involvement of a guanine-nucleotide regulatory protein in this process. Occupation of the alpha 1-adrenoceptor by (-)-noradrenaline did not accelerate the rate of cAMP breakdown in the absence of phosphodiesterase inhibition. Furthermore, there was no enhancement of total phosphodiesterase activity by (-)-noradrenaline in the presence of (-)-propranolol. By contrast, pertussis toxin pretreatment augmented phosphodiesterase activity. Neither pertussis toxin nor (-)-noradrenaline increased cAMP egress. We conclude that in rat neonatal cardiac myocytes agonist occupation of the alpha 1-adrenoceptor inhibits beta-adrenoceptor stimulated cAMP accumulation most likely by coupling to a guanine nucleotide inhibitory protein.

An inhibitor of inositol-phospholipid-specific phospholipase C

Q12713 substance, a cyclic peptide from Actinomadura spp., strongly inhibited the enzyme activity of phosphatidylinositol-4,5-bisphosphate-specific phospholipase C (PIP2-PLC) but scarcely inhibited other phospholipases. Kinetic analysis demonstrated that the inhibition of PIP2-PLC activity was competitive with respect to phosphatidylinositol 4,5-bisphosphate. Calcium and magnesium ions had no significant effect on the inhibitory activity. On the contrary, potassium or rubidium ion was essential for the inhibitory activity. Furthermore, NaF and AlCl3-stimulated increase of phosphoinositides was decreased by Q12713 in the cultured 3T3 cells.

Phosphoinositide signalling enzymes in rat liver nuclei: phosphoinositidase C isoform beta 1 is specifically, but not predominantly, located in the nucleus

The presence of phosphoinositide-mobilizing enzymes has been investigated in purified rat liver nuclei by radiolabelling and by probing with antibodies. A Ca(2+)-activated phosphoinositidase C (PIC) is present and was shown immunologically to be the beta 1 isoform. No gamma- or delta-PIC was found. However, only 5% of the total beta 1-PIC isoform is nuclear, with the majority being cytosolic. G alpha q and G alpha 11, the suggested physiological activators of beta 1-PIC, were not present. A PtdIns4P 5-kinase is also present, which immunologically is shown to be the C isoform. All of these nuclear inositide enzymes still remained after the removal of the nuclear envelope with Triton X-100, consistent with the concept of an intranuclear inositide cycle [Divecha, Banfic and Irvine (1991) EMBO. J. 10, 3207-3214].

Investigation into the role of phosphodiesterase IV in bronchorelaxation, including studies with human bronchus

1. We have investigated the role of cyclic nucleotide phosphodiesterase IV (PDE IV) in the relaxation of human bronchus and guinea-pig trachea in vitro and in guinea-pigs in vivo. 2. Functional studies showed that the selective PDE IV inhibitors, rolipram and denbufylline, relaxed human and guinea-pig preparations in vitro. 3. Two clinically used xanthine non-selective PDE inhibitors, theophylline and pentoxifylline, were also effective in these preparations, but were much less potent than the selective agents used. 4. The rank order of potency for the four PDE inhibitors in both species was similar. 5. Biochemical studies indicated that PDE IV was the major PDE isoform present in the human bronchial tissue. PDEs I, II and V were also identified. 6. Theophylline and pentoxifylline were, as expected, non-selective inhibitors of the human enzymes, but there was a good correlation between PDE IV inhibitory and bronchorelaxation potencies, suggesting that PDE IV inhibition is important for the clinical bronchodilator activities of the two xanthine compounds. 7. We have confirmed the ability of selective PDE IV inhibitors to cause bronchodilatation in guinea-pigs in vivo. 8. We conclude that our study has provided further evidence that selective PDE IV inhibitors could act as bronchodilators in the clinic.

Purification and properties of calmodulin from Phymatotrichum omnivorum

Mycelia of Phymatotrichum omnivorum obtained at 10 day intervals during 10 to 50 days of growth were used for isolating calmodulin, and studying its effect on glycogen synthase, phosphorylase, phosphorylase kinase, cyclic AMP phosphodiesterase and Ca++ATPase. Glycogen synthase was inhibited until the 30th day by calmodulin, whereas calmodulin obtained from the 40th day stimulated glycogen synthase activity and the 50th day sample had no effect. cAMP phosphodiesterase and Ca++ATPase of P. omnivorum were stimulated by the respective calmodulin. Molecular weight of the purified fungal calmodulin was approximately 18 kD as revealed by sodium dodecyl sulphate gel electrophoresis. Trifluoperazine, dibucaine and lidocaine inhibited calmodulin activity and calmodulin activation of PDE, respectively.

Effect of adrenergic agonists and antagonists on alanine amino transferase, fructose-1:6-bisphosphatase and glucose production in hepatocytes

Using rat hepatocytes we confirmed our previous results that glucagon and beta-adrenergic agonists increased the enzyme activity of alanine aminotransferase (AAT) and propranolol abolished their effects. Only the enzyme activity was measured and other parameters like quantity of the enzyme or activation due to modification were not looked for. As in perfusion experiment phenylephrine and phenoxybenzamine (alpha-agonist and alpha-antagonist respectively) also alpha-antagonist respectively) also increased the AAT activity in isolated rat hepatocytes and propranolol reversed these effects. The additive effect of glucagon and phenoxybenzamine on AAT was also persistent in hepatocyte system. Fructose-1:6-bisphosphatase (Fru-P2-ase), another key enzyme in gluconeogenic pathway, was elevated by glucagon and other beta-adrenergic agonists both in liver perfusion and isolated hepatocyte experiments and was brought back to the normal level by propranolol. In this case also only the enzyme activity was measured and no other parameters were looked for. Unlike AAT this enzyme was not stimulated by phenylephrine or phenoxybenzamine. But AAT and Fru-P2-ase activities were increased significantly by adenylate cyclase activators like fluoride or forskolin. Thus, it appears that the regulation of fru-P2-ase by glucagon is purely a b-receptor mediated process whereas AAT activation shows a mixed type of regulation where some well known alpha-agonist and antagonists are behaving as beta-agonists. Results further indicate the presence of phosphodiesterase in hepatocyte membrane which was stimulated by glucagon and brought back to the normal level by propranolol. The different adrenergic compounds stated above, not only modified the activity of the above two enzymes but also stimulated glucose production by hepatocytes from alanine which was in turn abolished by propranolol as well as amino oxyacetate (AOA), a highly specified inhibitor of AAT. This confirm the participation of AAT in gluconeogenesis from alanine in liver. Forskolin and fluoride also increased the glucose production from alanine and showed additive effects with glucagon, phenylephrine and phenoxybenzamine.

Selective inhibition of phosphatidylinositol phospholipase C by cytotoxic ether lipid analogues

The ether lipid analogue 1-octadecyl-2-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) has been shown to be a direct inhibitor of Swiss 3T3 fibroblast and BG1 ovarian adenocarcinoma cell cytosolic phosphoinositide selective phospholipase C (PIPLC) using [3H]-phosphatidylinositol-(4, 5)-bisphosphate ([3H]PIP2) as the substrate. The inhibition occurred when ET-18-OCH3 was incorporated into the [3H]PIP2 substrate micelles, with 50% inhibition (IC50) occurring at a ET-18-OCH3: [3H]PIP2 ratio of 0.04, or an assay concentration of 0.4 microM, and when ET-18-OCH3 was added directly to the incubation, with an IC50 of 9.6 microM. Lipid prepared from cells exposed to cytotoxic concentrations of ET-18-OCH3 for 18 h also inhibited PIPLC with an IC50 less than 1 microM. The noncytotoxic analogue 1-O-alkyl-2-hydroxy-sn-glycero-3-phosphocholine inhibited PIPLC when incorporated into the [3H]PIP2 substrate micelles, but lipid from cells grown with 5 microM 1-O-alkyl-2-hydroxy-sn-glycero-3-phosphocholine did not inhibit PIPLC. BG1 cells, which were more sensitive than Swiss 3T3 fibroblasts to growth inhibition by ET-18-OCH3, had a cytosolic PIPLC activity one-third that of Swiss 3T3 cells. NIH 3T3 cells exhibited the same sensitivity to growth inhibition by ET-18-OCH3 as Swiss 3T3 cells and had a similar level of PIPLC. v-sis NIH 3T3 cells were relatively resistant (greater than 3-fold) to growth inhibition by ET-18-OCH3 and had a cytosolic PIPLC activity more than twice that of the wild type cells. ET-18-OCH3 was a weak inhibitor, IC50 greater than 100 microM, of phospholipase D activity in NIH 3T3 cell membranes. In intact NIH 3T3 cells ET-18-OCH3 at cytotoxic concentrations did not inhibit phospholipase D or phosphatidylcholine-selective phospholipase C activity. The results show that the ether lipid analogues at cytotoxic concentrations are selective inhibitors of PIPLC and that the inhibition of PIPLC may be related to the growth inhibitory activity of the ether lipid analogues.

Carbachol-induced decrease in thyroid cell adenylyl cyclase activity is independent of calcium and phosphodiesterase activation

The mechanism of adenylyl cyclase desensitization by carbachol, an agent that stimulates polyphosphoinositide hydrolysis, was studied in thyroid cells. Incubation of cultured dog thyroid cells with 10 microM carbachol for 2-4 hr reduced the subsequent thyrotropic hormone (TSH) stimulation of adenylyl cyclase activity of membrane preparations by approximately 40%. This inhibition was reversed by atropine, occurred even in a Ca(2+)-free medium containing ethylene glycol bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid, and was not reproduced by the Ca2+ ionophore A23187. The carbachol effect was not prevented by simultaneous incubation of cells with either isobutylmethylxanthine, an inhibitor of phosphodiesterase, or H-7, an inhibitor of protein kinase. Pretreatment of cells with pertussis toxin to inactivate the Gi inhibitory protein also failed to affect the carbachol inhibition. Although carbachol did not reduce the basal or the TSH-stimulated cyclase activities when added to membranes directly during the assay, exposure of cells to carbachol for 2-4 hr resulted in long lasting inhibition of TSH-stimulated cyclase activity (for at least 24 hr); recovery was seen by 48 hr after its removal. Carbachol pretreatment had no effect on 125I-TSH binding to membranes but reduced the cyclase stimulation by not only TSH but also cholera toxin, guanosine 5'-O-(3-thio)triphosphate, and forskolin; it also significantly reduced the cholera toxin-mediated AD[32P]-ribosylation of Gs in membranes. These data indicate that carbachol-induced inhibition of adenylyl cyclase occurs beyond the level of TSH receptor binding and that Gs is a possible site of its action. Thus, in dog thyroid cells, carbachol, via muscarinic receptors, can reduce the adenylyl cyclase activity by a process that does not involve Ca2+ or activation of phosphodiesterase.

Light and dark active phosphodiesterase regulation in salamander rods

We studied the activation of 3',5'-cyclic guanosine monophosphate (cGMP) phosphodiesterase (PDE) by using a cell-permeant enzyme inhibitor. Rods of Ambystoma tigrinum held in a suction electrode were jumped into a stream of 3-isobutyl-1-methylxanthine (IBMX), 0.01-1 mM. Initial transient light-sensitive currents fit the notion that dark and light-activated forms of PDE contributed independently to metabolic activity and were equivalently inhibited by IBMX (apparent Ki 30 microns). Inhibition developed within 50 ms, producing a step decrease of enzyme velocity, which could be offset by activation with flashes or steps of light. The dark PDE activity was equivalent to light activation of enzyme by 1,000 isomerization rod-1s-1, sufficient to hydrolyze the free cGMP pool (1/e) in 0.6 s. Steady light activated PDE in linear proportion to isomerization rate, the range from darkness to current saturation amounting to a 10-fold increase. The conditions for simultaneous onset of inhibitor and illumination to produce no net change of membrane current defined the apparent lifetime of light-activated PDE, TPDE* = 0.9 s, which was independent of both background illumination and current over the range 0-3 x 10(5) isomerization s-1, from 50 to 0 pA. Adaptation was a function of current rather than isomerization: jumps with different proportions of IBMX concentration to steady light intensity produced equal currents, and followed the same course of adaptation in maintained light, despite a 10-fold difference of illumination. Judged from the delay between IBMX- and light-induced currents, the dominant feedback regulatory site comes after PDE on the signal path. The dark active PDE affects the hydrolytic flux and cytoplasmic diffusion of cGMP, as well as the proportional range of the cGMP activity signal in response to light.

Biochemical effects of human alpha interferon in combination with alpha-difluoromethylornithine on human lymphoblastoid (DAUDI) cells in culture

DFMO and IFN have both been shown to suppress the intracellular activity of ornithine decarboxylase in rapidly proliferating tissues. In addition, both agents demonstrate antiproliferative activity against human lymphoblastoid (Daudi) cells in culture. Treatment of log-phase Daudi cells with doses of 6 U/ml of IFN or 1 mM DFMO resulted in a 50% reduction of cell number 72 h after drug addition. Combination of IFN with DFMO against DAUDI cells using the isobologram method showed that the two demonstrate true antiproliferative synergy. Analysis of 2',5'-oligoadenylate synthetase (2,5A) activity in treated cells showed that both IFN alone and IFN/DFMO combination result in equivalent 2,5A induction (1800 mmol/mg/20 h) compared to control (300 mmol/mg/20 h). While 2,5A activity decreased by 50% at 48 h in cells after treatment with IFN alone, the IFN/DFMO combination remained elevated (1700 mmol/mg/20 h). Phosphodiesterase (PdE) activity in these cells showed no substantial changes with IFN, DFMO, or IFN/DFMO treatment over 72 h compared to control values. In contrast, the activity of the 68-kDa interferon induced protein kinase (PK) in IFN/DFMO-treated cells was 1.6-fold greater at 48 and 72 h than that found for IFN alone. These studies demonstrate that the synergistic antiproliferative activity of IFN/DFMO combination may be due, in part, to modification of the activity of IFN-inducible enzymes.