A Potent N-(piperidin-4-yl)-1H-pyrrole-2-carboxamide Inhibitor of Adenylyl Cyclase of G. lamblia: Biological Evaluation and Molecular Modelling Studies

In this work, we report a derivative of N-(piperidin-4-yl)-1H-pyrrole-2-carboxamide as a new inhibitor for adenylyl cyclase of Giardia lamblia which was obtained from a study using structural data of the nucleotidyl cyclase 1 (gNC1) of this parasite. For such a study, we developed a model for this specific enzyme by using homology techniques, which is the first model reported for gNC1 of G. lamblia. Our studies show that the new inhibitor has a competitive mechanism of action against this enzyme. 2-Hydroxyestradiol was used as the reference compound for comparative studies. Results in this work are important from two points of view. on the one hand, an experimentally corroborated model for gNC1 of G. lamblia obtained by molecular modelling is presented; on the other hand, the new inhibitor obtained is an undoubtedly excellent starting structure for the development of new metabolic inhibitors for G. lamblia.

Biased agonism at histamine H1 receptor: Desensitization, internalization and MAPK activation triggered by antihistamines

Histamine H₁ receptor ligands used clinically as antiallergics rank among the most widely prescribed and over-the-counter drugs in the world. They exert the therapeutic actions by blocking the effects of histamine, due to null or negative efficacy towards Gα_q-phospholipase C (PLC)-inositol triphosphates (IP₃)-Ca²⁺ and nuclear factor-kappa B cascades. However, there is no information regarding their ability to modulate other receptor responses. The aim of the present study was to investigate whether histamine H₁ receptor ligands could display positive efficacy concerning receptor desensitization, internalization, signaling through Gα_q independent pathways or even transcriptional regulation of proinflammatory genes. While diphenhydramine, triprolidine and chlorpheniramine activate ERK1/2 (extracellular signal-regulated kinase 1/2) pathway in A549 cells, pre-treatment with chlorpheniramine or triprolidine completely desensitize histamine H₁ receptor mediated Ca²⁺ response, and both diphenhydramine and triprolidine lead to receptor internalization. Unlike histamine, histamine H₁ receptor desensitization and internalization induced by antihistamines prove to be independent of G protein-coupled receptor kinase 2 (GRK2) phosphorylation. Also, unlike the reference agonist, the recovery of the number of cell-surface histamine H₁ receptors is a consequence of de novo synthesis. On the other hand, all of the ligands lack efficacy regarding cyclooxygenase-2 (COX-2) and interleukin-8 (IL-8) mRNA regulation. However, a prolonged exposure with each of the antihistamines impaires the increase in COX-2 and IL-8 mRNA levels induced by histamine, even after ligand removal. Altogether, these findings demonstrate the biased nature of histamine H₁ receptor ligands contributing to a more accurate classification, and providing evidence for a more rational and safe use of them.

Azelastine potentiates antiasthmatic dexamethasone effect on a murine asthma model

Glucocorticoids are among the most effective drugs to treat asthma. However, the severe adverse effects associated generate the need for its therapeutic optimization. Conversely, though histamine is undoubtedly related to asthma development, there is a lack of efficacy of antihistamines in controlling its symptoms, which prevents their clinical application. We have reported that antihistamines potentiate glucocorticoids' responses in vitro and recent observations have indicated that the coadministration of an antihistamine and a synthetic glucocorticoid has synergistic effects on a murine model of allergic rhinitis. Here, the aim of this work is to establish if this therapeutic combination could be beneficial in a murine model of asthma. We used an allergen-induced model of asthma (employing ovalbumin) to evaluate the effects of the synthetic glucocorticoid dexamethasone combined with the antihistamine azelastine. Our results indicate that the cotreatment with azelastine and a suboptimal dose of dexamethasone can improve allergic lung inflammation as shown by a decrease in eosinophils in bronchoalveolar lavage, fewer peribronchial and perivascular infiltrates, and mucin-producing cells. In addition, serum levels of allergen-specific IgE and IgG1 were also reduced, as well as the expression of lung inflammatory-related genes IL-4, IL-5, Muc5AC, and Arginase I. The potentiation of dexamethasone effects by azelastine could allow to reduce the effective glucocorticoid dose needed to achieve a therapeutic effect. These findings provide first new insights into the potential benefits of glucocorticoids and antihistamines combination for the treatment of asthma and grants further research to evaluate this approach in other related inflammatory conditions.

Physiological implications of biased signaling at histamine H2 receptors

Histamine mediates numerous functions acting through its four receptor subtypes all belonging to the large family of seven transmembrane G-protein coupled receptors. In particular, histamine H2 receptor (H2R) is mainly involved in gastric acid production, becoming a classic pharmacological target to treat Zollinger–Ellison disease and gastric and duodenal ulcers. H2 ligands rank among the most widely prescribed and over the counter-sold drugs in the world. Recent evidence indicate that some H2R ligands display biased agonism, selecting and triggering some, but not all, of the signaling pathways associated to the H2R. The aim of the present work is to study whether famotidine, clinically widespread used ligand acting at H2R, exerts biased signaling. Our findings indicate that while famotidine acts as inverse agonist diminishing cAMP basal levels, it mimics the effects of histamine and the agonist amthamine concerning receptor desensitization and internalization. Moreover, the treatment of HEK293T transfected cells with any of the three ligands lead to a concentration dependent pERK increment. Similarly in AGS gastric epithelial cells, famotidine treatment led to both, the reduction in cAMP levels as well as the increment in ERK phosphorylation, suggesting that this behavior could have pharmacological relevant implications. Based on that, histidine decarboxylase expression was studied by quantitative PCR in AGS cells and its levels were increased by famotidine as well as by histamine and amthamine. In all cases, the positive regulation was impeded by the MEK inhibitor PD98059, indicating that biased signaling toward ERK1/2 pathway is the responsible of such enzyme regulation. These results support that ligand bias is not only a pharmacological curiosity but has physiological and pharmacological implications on cell metabolism.

Blockade of Multidrug Resistance-Associated Proteins Aggravates Acute Pancreatitis and Blunts Atrial Natriuretic Factor's Beneficial Effect in Rats: Role of MRP4 (ABCC4)

We previously reported that atrial natriuretic factor (ANF) stimulates secretin-evoked cAMP efflux through multidrug resistance-associated protein 4 (MRP4) in the exocrine pancreas. Here we sought to establish in vivo whether this mechanism was involved in acute pancreatitis onset in the rat. Rats pretreated with or without probenecid (MRPs general inhibitor) were infused with secretin alone or with ANF. A set of these animals were given repetitive cerulein injections to induce acute pancreatitis. Plasma amylase and intrapancreatic trypsin activities were measured and histological examination of the pancreas performed. Secretin alone activated trypsinogen but induced no pancreatic histological changes. Blockade by probenecid in secretin-treated rats increased trypsin and also induced vacuolization, a hallmark of acute pancreatitis. ANF prevented the secretin response but in the absence of probenecid. In rats with acute pancreatitis, pretreatment with secretin aggravated the disease, but ANF prevented secretin-induced changes. Blockade of MRPs in rats with acute pancreatitis induced trypsinogen activation and larger cytoplasmic vacuoles as well as larger areas of necrosis and edema that were aggravated by secretin but not prevented by ANF. The temporal resolution of intracellular cAMP levels seems critical in the onset of acute pancreatitis, since secretin-evoked cAMP in a context of MRP inhibition makes the pancreas prone to injury in normal rats and aggravates the onset of acute pancreatitis. Present findings support a protective role for ANF mediated by cAMP extrusion through MRP4 and further suggest that the regulation of MRP4 by ANF would be relevant to maintain pancreatic acinar cell homeostasis.

G-protein-coupled receptor 30/adenylyl cyclase/protein kinase A pathway is involved in estradiol 17ß-D-glucuronide-induced cholestasis

Estradiol-17ß-D-glucuronide (E17G) activates different signaling pathways (e.g., Ca(2+) -dependent protein kinase C, phosphoinositide 3-kinase/protein kinase B, mitogen-activated protein kinases [MAPKs] p38 and extracellular signal-related kinase 1/2, and estrogen receptor alpha) that lead to acute cholestasis in rat liver with retrieval of the canalicular transporters, bile salt export pump (Abcb11) and multidrug resistance-associated protein 2 (Abcc2). E17G shares with nonconjugated estradiol the capacity to activate these pathways. G-protein-coupled receptor 30 (GPR30) is a receptor implicated in nongenomic effects of estradiol, and the aim of this study was to analyze the potential role of this receptor and its downstream effectors in E17G-induced cholestasis. In vitro, GPR30 inhibition by G15 or its knockdown with small interfering RNA strongly prevented E17G-induced impairment of canalicular transporter function and localization. E17G increased cyclic adenosine monophosphate (cAMP) levels, and this increase was blocked by G15, linking GPR30 to adenylyl cyclase (AC). Moreover, AC inhibition totally prevented E17G insult. E17G also increased protein kinase A (PKA) activity, which was blocked by G15 and AC inhibitors, connecting the links of the pathway, GPR30-AC-PKA. PKA inhibition prevented E17G-induced cholestasis, whereas exchange protein activated directly by cyclic nucleotide/MAPK kinase, another cAMP downstream effector, was not implicated in cAMP cholestatic action. In the perfused rat liver model, inhibition of the GPR30-AC-PKA pathway totally prevented E17G-induced alteration in Abcb11 and Abcc2 function and localization.

CONCLUSION

Activation of GPR30-AC-PKA is a key factor in the alteration of canalicular transporter function and localization induced by E17G. Interaction of E17G with GPR30 may be the first event in the cascade of signaling activation.

Atrial natriuretic factor stimulates efflux of cAMP in rat exocrine pancreas via multidrug resistance-associated proteins

BACKGROUND & AIMS

Atrial natriuretic factor (ANF) prevents increases in intracellular levels of cAMP that are induced by secretin in the exocrine pancreas. We investigated the contribution of cyclic adenosine monophosphate (cAMP) efflux to ANF inhibition of secretin signaling.

METHODS

Intracellular and extracellular cAMP were measured by radio-binding assays in isolated pancreatic acini exposed to secretin and other secretagogues, alone or with ANF. Levels of messenger RNA for multidrug resistance-associated protein (MRP)4, MRP5, and MRP8 were measured by real-time polymerase chain reaction. MRP4 was knocked down in AR42J cells by small interfering RNA. In vivo studies were performed in rats.

RESULTS

Pancreatic secretagogues increased levels of intracellular cAMP, but only secretin and vasoactive intestinal peptide promoted cAMP efflux; efflux was increased by ANF, through signaling via natriuretic peptide receptor-C and phospholipase C-protein kinase C. In time-course studies with active phosphodiesterases, levels of intracellular and extracellular cAMP increased earlier after the addition of secretin and ANF (1 min) than after the addition of secretin alone (3 min). Similar kinetic patterns occurred with a phosphodiesterase inhibitor. A probenecid-sensitive transporter mediated cAMP egression. The main cAMP transporter, MRP4, was expressed in AR42J cells and pancreas. cAMP egression occurred in AR42J cells exposed to secretin, but this response was reduced in cells that expressed MRP4 small interfering RNA. In rats, levels of cAMP in plasma and pancreatic juice increased after infusion with secretin alone or secretin plus ANF.

CONCLUSIONS

ANF signals via natriuretic peptide receptor-C coupled to the phospholipase C-protein kinase C pathway to increase secretin-induced efflux of cAMP, probably through MPR-4. Cyclic AMP extrusion might be a mechanism, in addition to phosphodiesterase action, to regulate intracellular cAMP levels in pancreatic acinar cells.

Endothelins participate in the central and peripheral regulation of submandibular gland secretion in the rat

We previously reported that endothelins (ETs) are involved in the rat central and peripheral regulation of bile secretion. In this study we sought to establish whether ET-1 and ET-3 modulated submandibular gland secretion when locally or centrally applied. Animals were prepared with gland duct cannulation to collect saliva samples and jugular cannulation to administer sialogogues. ETs were given either into the submandibular gland or brain lateral ventricle. Intraglandularly administered ETs failed to elicit salivation per se. However, ET-1, but not ET-3, potentiated both cholinergic- and adrenergic-evoked salivation through ET(A) receptors. ET-1 decreased cAMP content but increased phosphoinositide hydrolysis, whereas ET-3 attenuated both intracellular pathways. The expression of ET(A) and ET(B) receptor mRNAs as well as that of ETs was revealed in the submandibular gland by RT-PCR. Immunohistochemical studies showed that ET(A) receptor staining was localized around the interlobular ducts and acini, compatible with the myoepithelial cells' location, whereas ET(B) receptor staining was restricted to small blood vessels. When applied to the brain, both ETs induced no salivation but enhanced cholinergic- and adrenergic-evoked salivary secretion through parasympathetic pathways. ET-1 response was mediated by brain ET(A) receptors, whereas that of ET-3 was presumably through nonconventional ET receptors. Present findings show that ETs are involved in the brain regulation of cholinergic- and adrenergic-stimulated submandibular gland secretion through the activation of distinct brain ET receptors and parasympathetic pathways. However, when ETs were administered into the gland, only ET-1 enhanced cholinergic and adrenergic salivation likely through myopithelial cell contraction by activating ET(A) receptors coupled to phospholipase C. The presence of ETs and ET receptors suggests the existence of an endothelinergic system in the submandibular gland.

C-type natriuretic peptide enhances amylase release through NPR-C receptors in the exocrine pancreas

Several studies show that C-type natriuretic peptide (CNP) has a modulatory role in the digestive system. CNP administration reduces both jejunal fluid and bile secretion in the rat. In the present study we evaluated the effect of CNP on amylase release in isolated pancreatic acini as well as the receptors and intracellular pathways involved. Results showed that all natriuretic peptide receptors were expressed not only in the whole pancreas but also in isolated pancreatic acini. CNP stimulated amylase secretion with a concentration-dependent biphasic response; maximum release was observed at 1 pM CNP, whereas higher concentrations gradually attenuated it. The response was mimicked by a selective natriuretic peptide receptor (NPR-C) agonist and inhibited by pertussis toxin, strongly supporting NPR-C receptor activation. CNP-evoked amylase release was abolished by U-73122 (PLC inhibitor) and 2-aminoethoxydiphenyl borate (2-APB) [an inositol 1,4,5-triphosphate (IP(3)) receptor antagonist], partially inhibited by GF-109203X (PKC inhibitor), and unaltered by ryanodine or protein kinase A (PKA) and protein kinase G (PKG) inhibitors. Phosphoinositide hydrolysis was enhanced by CNP at all concentrations and abolished by U-73122. At 1 and 10 pM, CNP did not affect cAMP or guanosine 3',5'-cyclic monophosphate (cGMP) levels, but at higher concentrations it increased cGMP and diminished cAMP content. Present findings show that CNP stimulated amylase release through the activation of NPR-C receptors coupled to the PLC pathway and downstream effectors involved in exocytosis. The attenuation of amylase release was likely related to cAMP reduction. The augmentation in cGMP supports activation of NPR-A/NPR-B receptors probably involved in calcium influx. Present findings give evidence that CNP is a potential direct regulator of pancreatic function.

Atrial natriuretic factor negatively modulates secretin intracellular signaling in the exocrine pancreas

We previously reported that atrial natriuretic factor (ANF) stimulates pancreatic secretion through NPR-C receptors coupled to PLC and potentiates secretin response without affecting cAMP levels. In the present study we sought to establish the intracellular signaling mechanism underlying the interaction between both peptides. In isolated pancreatic acini 100 nM ANF abolished cAMP accumulation evoked by any dose of secretin. Lower doses of ANF (1 fM, 1 pM, 1 and 10 nM) dose dependently reduced EC50 secretin-evoked cAMP. Although ANF failed to affect cAMP stimulated by amthamine (selective H2 agonist) or isoproterenol (beta-adrenergic agonist), it abolished VIP-induced cAMP formation. ANF inhibitory effect was prevented by U-73122 (PLC inhibitor) and GF-109203X (PKC inhibitor) but unaltered by PKG and nitric oxide synthase inhibition, supporting that the PLC/PKC pathway mediated the effect. ANF response was mimicked by cANP (4-23 amide) and abolished by pertussis toxin, strongly supporting NPR-C receptor activation. In vivo studies showed that ANF at 0.5 microg x kg(-1) x h(-1) enhanced secretion stimulated by 1 U x kg(-1) x h(-1) secretin but at 1 and 2 microg x kg(-1) x h(-1) it abolished secretin response. However, ANF at such doses failed to modify the secretion evoked by carbachol or CCK. Present results show that ANF negatively modulated secretin secretory response and intracellular signaling through the activation of NPR-C receptors coupled to the PLC/PKC pathway. Furthermore, the finding that ANF also inhibited VIP-evoked cAMP supports a selective modulation of class II G-protein coupled receptors by ANF. Present findings suggest that ANF may play a protective role by reducing secretin response to avoid overstimulation.

Atrial natriuretic factor stimulates exocrine pancreatic secretion in the rat through NPR-C receptors

Increasing evidence supports the role of atrial natriuretic factor (ANF) in the modulation of gastrointestinal physiology. The effect of ANF on exocrine pancreatic secretion and the possible receptors and pathways involved were studied in vivo. Anesthetized rats were prepared with pancreatic duct cannulation, pyloric ligation, and bile diversion into the duodenum. ANF dose-dependently increased pancreatic secretion of fluid and proteins and enhanced secretin and CCK-evoked response. ANF decreased chloride secretion and increased the pH of the pancreatic juice. Neither cholinergic nor adrenergic blockade affected ANF-stimulated pancreatic secretion. Furthermore, ANF response was not mediated by the release of nitric oxide. ANF-evoked protein secretion was not inhibited by truncal vagotomy, atropine, or Nomega-nitro-l-arginine methyl ester administration. The selective natriuretic peptide receptor-C (NPR-C) receptor agonist cANP-(4-23) mimicked ANF response in a dose-dependent fashion. When the intracellular signaling coupled to NPR-C receptors was investigated in isolated pancreatic acini, results showed that ANF did not modify basal or forskolin-evoked cAMP formation, but it dose-dependently enhanced phosphoinositide hydrolysis, which was blocked by the selective PLC inhibitor U-73122. ANF stimulated exocrine pancreatic secretion in the rat, and its effect was not mediated by nitric oxide or parasympathetic or sympathetic activity. Furthermore, CCK and secretin appear not to be involved in ANF response. Present findings support that ANF exerts a stimulatory effect on pancreatic exocrine secretion mediated by NPR-C receptors coupled to the phosphoinositide pathway.

H1 and H2 histamine receptors in N-nitroso-N-methylurea (NMU)-induced carcinomas with atypical coupling to signal transducers

Two specific binding sites for histamine were characterized in the cell membrane of N-nitroso-N-methylurea (NMU)-induced tumors. The first one, with higher affinity (Kd = 4 +/- 2 nM), was further identified as an H2 type, while the lower affinity one (35 +/- 10 nM) corresponded to an H1 receptor. Histamine concentrations up to 50 nM, as well as H2 agonists, significantly enhanced the phosphoinositide turnover by acting through higher affinity H2 receptors. On the other hand, histamine at concentrations over 50 nM and H1 agonists produced a 100% increase in cAMP levels in a response specifically blocked by mepyramine. These H1 and H2 histamine receptors that exhibit different linkages to second messenger systems may prove to be a characteristic of cells with a high proliferating capacity, such as undifferentiated or transformed cells.

Histamine as an autocrine growth factor in experimental mammary carcinomas

In order to determine the role of endogenous histamine in the regulation of cell growth, the in vitro action of fluoromethyl-histidine (MFMH) was studied in experimental mammary carcinomas induced in rats. Tumor cells were cultured in soft agar using the clonogenic agar technique. The MFMH was added in different concentrations (0.01-100 microM). The effect observed was a 60% inhibition on colony formation with a maximal effect at concentrations over 10 microM. This action was completely reverted by the H2 agonists dimaprit and arpromidine with an IC50 value of 1 microM. The action of the H2 agonists when added alone was a significant increase in cell proliferation (135%), while the H1 agonist produced a dose-dependent inhibition on cell growth. In these experimental carcinomas endogenous histamine is critical for cell proliferation and one of its major effects may be the stimulation of cell growth by acting on specific H2 membrane receptors.

Effect of histamine on growth and differentiation of the rat mammary gland

The presence of H1 and H2 histamine receptors and their associated second messenger systems were studied during the development of the rat mammary gland. In the tissue of the young female, histamine presented a double receptor site as previously described for experimental mammary tumors, namely a high affinity H2 site (Kd = 10 +/- 2 nM, Bmax = 1068 +/- 71 fm/mg prot.), which mediated its effect via the products of phosphoinositide hydrolysis and a low affinity H1 receptor (Kd1 = 5 +/- 2 nM, Bmax = 188 +/- 33 fm/mg prot. and Kd2 = 41 +/- 20 nM, Bmax = 1980 +/- 790 fm/mg prot. when characterized with 3H-mepyramine), coupled to adenylyl cyclase activation. On the other hand, the mammary gland of the adult rat presented these receptors coupled to the classical second messenger systems described for mammalian cells, that is, the H2 receptor produced an increase in intracellular cAMP levels and the H1 receptor increased the phosphoinositide turnover. We conclude that histamine plays a critical role during development and differentiation of the normal rat mammary gland.

Histamine receptors in an experimental mammary carcinoma

An experimental mammary carcinoma was induced in Sprague-Dawley rats by the ip administration of N-nitroso-N-methylurea (NMU) in three doses of 50 mg/kg. In order to study the expression of histamine receptors in these experimental tumors, the presence of specific binding sites for histamine was studied. Using [3H]-histamine as a radioligand, two specific binding sites were characterized on the cell membrane. The first site, of high affinity, Kd = 4 +/- 2 nM, was further characterized as an H2 type using [3H]-cimetidine and [3H]-tiotidine as radioligands and by displacement experiments with different histamine agonists and antagonists. The second one of low affinity, Kd = 35 +/- 14 nM, needs further characterization. The determination of cAMP levels showed that histamine and the H2 agonist dimaprit, produced a significant decrease in the nucleotide concentration 6 minutes after stimulation, in a response that was specifically abolished by H2 antagonists. Based on these results, we conclude that neoplastic cells from NMU induced tumors express H2 histamine membrane receptors which are coupled to a transductional pathway different from cAMP production, which may be involved in the regulation of tumor growth.