Determination of phosphodiesterase activity in rat mast cells using the fluorescent cAMP analogue anthraniloyl cAMP

Different role of cAMP pathway on the human mast cells HMC-1(560) and HMC-1(560,816) activation

HMC-1 are inflammatory cells that release vasoactive substances such as histamine. These cells have the c-kit receptor permanently activated in the membrane due to mutations in the proto-oncogene c-kit: Val-560 → Gly and Asp-816 → Val. Thus, there are two known cellular lines: HMC-1(560) and HMC-1(560,816) . These mutations are involved in a disease called mastocitosys. In the present paper both lines were used to study the influence of cAMP/PKA/PDEs pathway on the histamine release and Ca(2+) signaling since this pathway is often involved in these process. For this, the cells were preincubated with cAMP/PKA/PDEs modulators such as dibutyryl cAMP (dbcAMP), forskolin, H89, rolipram, IBMX, or imidazole and then stimulated with ionomycin. When cells were stimulated with agents that increase cAMP levels, the histamine release was not modified in HMC-1(560) but decreased in HMC-1(560,816) cells. The same happened when PKA was blocked. Furthermore, PDEs role on histamine release was independent of cAMP in HMC-1(560) cells and possibly also in HMC-1(560,816) cells. By contrast, the modulation of PKA and PDEs together changed the response in both cellular lines, therefore a relationship between them was suggested. All these modulatory effects on histamine release are Ca(2+) -independent. On the other hand, the effect of c-kit modulation on the cAMP/PKA/PDEs pathway was also checked. This receptor was blocked with STI571 (imatinib) and BMS-354825 (dasatinib), but only the last one caused a decrease in the cellular response to ionomycin. This article demonstrates for the first time than the cAMP/PKA/PDEs pathway is involved in the activation of HMC-1(560) and HMC-1(560,816) cells.

A Homogeneous Fluorescent Assay for cAMP-Phosphodiesterase Enzyme Activity

Cyclic adenosine monophosphate–phosphodiesterases (cAMP-PDEs) regulate the cellular level of cAMP by selectively catalyzing the hydrolysis of the phosphodiester bond in the cAMP molecule. They play important roles in modulating cellular and physiological functions. There is a growing interest in the study of cAMP-PDEs as therapeutic targets. We describe a novel method for measuring the enzyme activity of cAMP-PDEs that is based on a homogeneous fluorescence assay employing a cAMP-dependent DNA-binding protein (CAP). We demonstrate that the assay is quick and robust compared to traditional methods and is expected to be cost-effective for high-throughput screening of cAMP-PDE inhibitors. The usefulness of the assay is demonstrated by measuring IC50 values of three nonselective PDE inhibitors and by kinetic measurements of cAMP-PDEs from various rat tissues.

Biochemical characterization and cellular imaging of a novel, membrane permeable fluorescent cAMP analog

BACKGROUND

A novel fluorescent cAMP analog (8-[Pharos-575]- adenosine-3', 5'-cyclic monophosphate) was characterized with respect to its spectral properties, its ability to bind to and activate three main isoenzymes of the cAMP-dependent protein kinase (PKA-Ialpha, PKA-IIalpha, PKA-IIbeta) in vitro, its stability towards phosphodiesterase and its ability to permeate into cultured eukaryotic cells using resonance energy transfer based indicators, and conventional fluorescence imaging.

RESULTS

The Pharos fluorophore is characterized by a Stokes shift of 42 nm with an absorption maximum at 575 nm and the emission peaking at 617 nm. The quantum yield is 30%. Incubation of the compound to RIIalpha and RIIbeta subunits increases the amplitude of excitation and absorption maxima significantly; no major change was observed with RIalpha. In vitro binding of the compound to RIalpha subunit and activation of the PKA-Ialpha holoenzyme was essentially equivalent to cAMP; RII subunits bound the fluorescent analog up to ten times less efficiently, resulting in about two times reduced apparent activation constants of the holoenzymes compared to cAMP. The cellular uptake of the fluorescent analog was investigated by cAMP indicators. It was estimated that about 7 muM of the fluorescent cAMP analog is available to the indicator after one hour of incubation and that about 600 muM of the compound had to be added to intact cells to half-maximally dissociate a PKA type IIalpha sensor.

CONCLUSION

The novel analog combines good membrane permeability- comparable to 8-Br-cAMP - with superior spectral properties of a modern, red-shifted fluorophore. GFP-tagged regulatory subunits of PKA and the analog co-localized. Furthermore, it is a potent, PDE-resistant activator of PKA-I and -II, suitable for in vitro applications and spatial distribution evaluations in living cells.

Resonant mirror biosensor detection method based on yessotoxin–phosphodiesterase interactions

Yessotoxin (YTX) is a generic name for a group of lipophilic compounds recently discovered and chemically characterized. Association measurements were done in a resonant mirror biosensor. The instrument detects changes in the refractive index and/or thickness occurring within a few hundred nanometers form the sensor surface where a molecule is attached. We used aminosilane surfaces where phosphodiesterase 3',5'-cyclic-nucleotide-specific from bovine brain (PDEs) was immobilized. Over this immobilized ligand different amounts of YTX were added and typical association curve profiles were observed. These association curves fit a pseudo-first-order kinetic equation where the apparent association rate constant (k(on)) can be calculated. The value of this constant increases with YTX concentration. From the representation of k(on) versus YTX concentration we obtained the association rate constant (k(ass)) 248+/-40 M(-1)s(-1) and the dissociation rate constant (k(diss)) 9.36 x 10(-4)+/-1.72 x 10(-4)s(-1). From these values the kinetic equilibrium dissociation constant (K(D)) for YTX-PDEs association can be calculated. The value of this last constant is 3.74 x 10(-6)+/-8.25 x 10(-8)M YTX. The PDE-YTX association was used as a method suitable for determination of the toxin concentration in a shellfish sample. The assay had sufficient sensitivity and can be used on simple shellfish extracts.

A rapid microplate fluorescence method to detect yessotoxins based on their capacity to activate phosphodiesterases

This paper describes an easy and fast assay with enough sensitivity to detect yessotoxin (YTX) in shellfish samples. YTX decreases intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels by increasing the activity of phosphodiesterases (PDEs). Looking for new methods to detect YTXs, we developed a technique based on this effect. We use the fluorescent derivative of cAMP, anthranyloyl-cAMP, whose fluorescence decreases in time by hydrolysis effect of PDEs. The fluorescence fall is quantified in a plate reader. PDEs induce an anthranyloyl-cAMP hydrolysis rate that is increased in the presence of YTX. This effect is dose dependent, and the representation of YTX concentration versus rate of hydrolysis follows a lineal regression. The measurable range of YTX in this assay is 0.1 to 10microM, while by mouse bioassay, the official method to detect YTXs, the detection limit is 2microM. We determined by this method the concentration of YTX from alcoholic extracts whose concentrations were first determined by high performance liquid chromatography and the variation of concentration was from 5.26microM by fluorescence to 6microM by high performance liquid chromatography and from 3.16 by fluorescence to 3microM by HPLC.

Yessotoxin, a novel phycotoxin, activates phosphodiesterase activity. Effect of yessotoxin on cAMP levels in human lymphocytes

Yessotoxin (YTX) is a novel phycotoxin with an unknown mechanism of action that has been reported as cardiotoxic, when injected, but non-toxic if ingested orally. In this paper, we studied the effect of YTX on adenosine 3',5'-cyclic monophosphate (cAMP) pathway, since this pathway can be a cellular target to this toxin as happens in other diarrhetic toxins. We determined cAMP levels by enzymeimmunoassay and by using the cAMP dye recombinant fluorescein- and rhodamine-labeled protein kinase A, which increases their fluorescence when cAMP levels are increased. In the presence of YTX, and after a transient small increase, cAMP levels were decreased. This effect was Ca(2+) dependent since in a Ca(2+)-free medium YTX increased cAMP levels, but this event was reverted after addition of external calcium. YTX also reverted the increase of cAMP induced by the adenylyl cyclase activator forskolin. These variations in fluorescence units were confirmed when cAMP levels were measured by enzymeimmunoassay, YTX decreases cAMP from 52.81+/-3.66 to 44.53+/-4.5 fmol. Phosphodiesterase (PDE) IV inhibitors, rolipram or etazolate, did not modify the effect of YTX, however, when PDE IV was first inhibited no effect of YTX was observed. On the other hand, the PDE III inhibitor milrinone counteracted the effect of YTX, and a similar effect was observed with the unspecific PDE I inhibitor chlorpromazine. These results point to an effect of YTX on PDE activity. In the presence of YTX, the fluorescent PDE substrate Mant-cAMP, increased its rate of hydrolysis, the same as the PDE from bovine brain increased the hydrolysis of cAMP substrate. In addition, YTX increased interleukin-2 production, which indirectly confirms a decrease in cAMP. Although results show a very complex pattern of responses, due to the interactions and crosstalks between many systems, results suggest that YTX is a PDE activator in the presence of external Ca(2+).

Timing is everything: the effects of putative dopamine antagonists on metamorphosis vary with larval age and experimental duration in the prosobranch gastropod Crepidula fornicata

The signal transduction pathway through which excess potassium ion stimulates the larvae of many marine invertebrates to metamorphose is incompletely understood. Recent evidence suggests that dopamine plays important roles in the metamorphic pathway of Crepidula fornicata. Therefore, we asked whether blocking dopamine receptors might prevent excess potassium ion from stimulating metamorphosis in this species. Surprisingly, the effects of the three putative dopamine antagonists tested (all at 10 microM) varied with exposure duration and the age of competent larvae. Chlorpromazine, a nonspecific dopamine antagonist known to have a number of other pharmacological effects, blocked the inductive action of excess potassium ion during the initial 5-8-h exposure periods in most assays, particularly for younger or smaller competent larvae. However, chlorpromazine in the absence of excess potassium ion also stimulated metamorphosis, particularly over the next 18 h, and worked faster on older competent larvae than on younger competent larvae. The specific D(1) antagonist R(+)-Sch-23309 had similar effects, blocking potassium-stimulated metamorphosis in short-term exposures and stimulating metamorphosis in longer exposures, particularly for older competent larvae. Although the specific D(2) antagonist spiperone (SPIP) blocked the inductive effects of excess potassium ion in only 1 of 6 assays during the first 6 h of exposure, it blocked metamorphosis in 2 of the assays during 24-h exposures. Our results indicate that dopamine receptors are involved in the pathway through which excess potassium ion stimulates metamorphosis in C. fornicata. In addition, the largely latent inductive effects of chlorpromazine, an inhibitor of nitric oxide synthase, suggest that endogenous nitric oxide may play a natural role in inhibiting metamorphosis in this species. Overall, our results would then suggest that exposing larvae of C. fornicata to excess K(+) leads to a shutdown of nitric oxide synthesis via a dopaminergic pathway, a pathway that can be blocked by some dopamine antagonists. Alternatively, chlorpromazine might eventually be stimulating metamorphosis by elevating endogenous cyclic nucleotide (e.g., cAMP) concentrations, again acting downstream from the steps acted on directly by excess K(+).

Ouabain-induced enhancement of rat mast cells response. Modulation by protein phosphorylation and intracellular pH

The digitalic glicoside ouabain induces potentiation of rat mast cell histamine release in response to several stimuli, which is mediated by Na+/Ca2+ exchanger. In this work, we studied the effect of ouabain on cytosolic calcium, intracellular pH and histamine release with Ca2+ ionophore A23187 in conditions designed to maximize ouabain-induced potentiation of rat mast cells response. The effect of protein kinase C (PKC), cAMP and phosphatase inhibition was also tested. Ouabain induced an enhancement in histamine release, cytosolic calcium and intracellular pH. The adenylate cyclase activator forskolin reduced the effect of ouabain on histamine release and intracellular pH, but enhanced the effect on cytosolic calcium. PKC activator PMA enhanced the effect of ouabain on histamine release and cytosolic calcium, without affecting intracellular pH. A PKC inhibitor, GF-109203X, reduced ouabain-induced enhancement of histamine release and intracellular pH, but increased the enhancement on cytosolic calcium. Finally, inhibition of protein phosphatases 1 and 2A with okadaic acid, increased the effect of ouabain on histamine release and intracellular pH, but reduced cytosolic calcium in presence of ouabain. This result suggest that ouabain-induced potentiation of rat mast cell histamine release with A23187 is modulated by kinases, and this modulation may be carried out by changes in intracellular alkalinization. However, the mechanism underlying cellular alkalinization remains to be elucidated.

Functional compartments in rat mast cells for cAMP and calcium on histamine release

The crosstalk between 3', 5'-cyclic adenosine monophosphate (cAMP), intracellular calcium, and histamine release in rat mast cells using the stimulatory effect of three different drugs, thapsigargin, sodium fluoride (NaF), and compound 48/80 were studied. Each of these drugs induces histamine release by different mechanisms. The transducting pathways modulating cAMP and intracellular calcium levels were modified by using, cholera toxin (CTX) which ADP-rybosylates Gs-protein, pertussis toxin (PTX) which ADP-rybosylates Gi-protein, and okadaic acid (OA) which inhibits phosphatases 1 and 2a. Our results show that CTX increased cAMP levels and inhibited histamine release elicited by thapsigargin and compound 48/80. The inhibitory effect of CTX on histamine release was potentiated by OA in the presence of compound 48/80 but was decreased in the presence of thapsigargin. Calcium uptake was stimulated by NaF and compound 48/80. The previous treatment with OA increased calcium uptake when combined with compound 48/80 but not with NaF. Treatment with NaF highly stimulated calcium uptake and cAMP levels only when combined with OA and CTX. These results suggest that the modulatory effect of intracellular calcium and cAMP on histamine release depend more on the crosstalk of the activated signal transducting pathway than on the final level of calcium or cAMP, further supporting the theory that rat mast cells are divided into functionally distinct compartments.

Phosphodiesterase expression in human epithelial cells

Epithelial cells play a critical role in airway inflammation and have the capacity to produce many inflammatory mediators, including bioactive lipids and proinflammatory cytokines. Intracellular levels of cAMP and cGMP are important in the control of inflammatory cell function. These cyclic nucleotides are inactivated via a family of phosphodiesterase (PDE) enzymes, providing a possible site for drug intervention in chronic inflammatory conditions. We studied the expression of PDE activity in an epithelial cell line (A549) and in primary human airway epithelial cells (HAECs). We measured PDE function using specific inhibitors to identify the PDE families present and used RT-PCR to elucidate the expression of PDE isogenes. Both A549 cells and HAECs predominantly expressed PDE4 activity, with lesser PDE1, PDE3, and PDE5 activity. RT-PCR identified HSPDE4A5 and HSPDE4D3 together with HSPDE7. Inhibition of PDE4 and PDE3 reduced secretion by these cells. Epithelial PDE may be an important target for PDE4 inhibitors in the development of the control of asthmatic inflammation, particularly when delivered via the inhaled route.

Synthesis and antihistaminic activity of 2-guanadino-3-cyanopyridines and pyrido[2,3-d]-pyrimidines

2-Guanadino-3-cyanopyridines 8-33 and pyrido[2,3-d]-pyrimidines 35-52 were synthesized by nucleophilic displacement and cyclization of the chloroamidines 6a-d easily obtained by reaction of 2-aminocyanopyridines 5a-d with phosgene iminium chloride and their action on the release of histamine by mast cells examined under immunological and chemical stimulus, with and without pre-incubation. Several 2-guanadino-3-cyanopyridines and pyrido[2,3-d]-pyrimidines are shown to be inhibitors of the release of histamine when stimulated with ovoalbumin as antigen or with polymer 48/80 as chemical stimulus. Guanadino-3-cyanopyridine 30 and pyrido[2,3-d]-pyrimidine 49 are the more active of all, inhibiting the release of histamine in all the conditions tested (30-60% inhibition). Guanadinocyanopyridines 15, 17, and 19 are very potent stimulators of the release of histamine (150-300%) while pyrido[2,3-d]-pyrimidines are mostly inactive. Compounds 28 and 14 present moderate in vitro cytotoxic activity against P-388, A-549, HT-29, and MEL-28 cell lines.

Mitoxantrone induces nonimmunological histamine release from rat mast cells

The antineoplastic drug mitoxantrone (MTX) elicits a fast noncytotoxic and nonimmunological histamine release from peritoneal and pleural rat mast cells. The non specific phosphodiesterase inhibitor isobuthyl-methylxantine (1 mM) decreases the potency of MTX. Theophylline (10 mM) decreases both the potency and the efficacy of MTX-induced histamine secretion. The protein kinase C (PKC) activator, tetradecanoyl-phorbol-13-acetate (50 ng/mL), enhances the effect of MTX, whereas the non specific PKC inhibitor trifluoperazine (10 microM) exerts no effect. Histamine release was also unaffected by substances acting on G-proteins, namely pertussis toxin (200 ng/mL), cholera toxin (300 mg/mL) and benzalkonium chloride (10 micrograms/ mL). The inhibition of protein phosphatases 1 and 2A by okadaic acid (1 microM) does not modify the response. The results indicate that mitoxantrone elicits the exocytosis in mast cells by a mechanism similar to the parent compound adriamycine, but different to the polyamine compound 48/80.