Federico Monczor

Coumarins: old compounds with novel promising therapeutic perspectives.

Natural as well as synthetic coumarins have recently drawn much attention due to its broad pharmacological activities. Many coumarins and their derivatives exert anti-coagulant, anti-tumor, anti-viral, anti-inflammatory and anti-oxidant effects, as well as anti-microbial and enzyme inhibition properties. The recognition of key structural features within coumarin family is crucial for the design and development of new analogues with improved activity and for the characterization of their mechanism of action and potential side effects. The different substituents in the coumarin nucleus strongly influence the biological activity of the resulting derivatives. Although some coumarins have been already characterized to evoke a particular biological activity, the challenge would be the design and synthesis of new derivatives with high specific activity for other pharmacological targets and define their mechanism of action to achieve new therapeutic drugs. The present review highlights the current progress in the development of coumarin scaffolds for drug discovery as novel anti-cancer agents. The major challenges about coumarins include the translation of current knowledge into new potential lead compounds and the repositioning of known compounds for the treatment of cancer.

Rapid desensitization and slow recovery of the cyclic AMP response mediated by histamine H2 receptors in the U937 cell line

The present study focused on the desensitization process of the H(2) receptor in U937 cells and the recovery of the cyclic AMP (cAMP) response. Treatment of U937 leukemic cells with the H(2) histamine receptor agonists (+/-)-N(1)-[3-(3, 4-difluorophenyl)-3-(pyridin-2-yl)propyl]-N(2)-[3-(1H-imidazol-4-yl)p ropyl]guanidine (BU-E-75) and amthamine produced a rapid desensitization characterized by decreased cAMP production (T(1/2) = 20 min). Pretreatment with 10 microM BU-E-75 did not induce modifications in the responses to prostaglandin E(2), isoproterenol, or forskolin. H(2) receptor desensitization was not affected by protein kinase A and C inhibitors, but was reduced drastically by Zn(2+) and heparin, known to act as inhibitors of G protein-coupled receptor kinases. Recovery studies of the cAMP response showed that cAMP levels reached 50% of the initial values within 5 hr. Furthermore, desensitization produced an important decrease in the basal level of this cyclic nucleotide. The minimal value was observed 12 hr later, and corresponded to approximately 1.3% of the initial basal level (7.5 vs 0.1 pmol/10(6) cells). This result could be explained by an increase in phosphodiesterase activity following 10 microM BU-E-75 treatment. When cells were exposed for 2 hr to an H(2) agonist, binding assays showed no modification in the number of H(2) receptors; internalization began just after 8 hr. Although the initial desensitization seems to involve G protein-coupled receptor kinases, results indicate that additional mechanisms of regulation were triggered by the H(2) agonists.

Roles of Phosphorylation-dependent and -independent Mechanisms in the Regulation of Histamine H2 Receptor by G Protein-coupled Receptor Kinase 2

It is widely assumed that G protein-coupled receptor kinase 2 (GRK2)-mediated specific inhibition of G protein-coupled receptors (GPCRs) response involves GRK-mediated receptor phosphorylation followed by β-arrestin binding and subsequent uncoupling from the heterotrimeric G protein. It has recently become evident that GRK2-mediated GPCRs regulation also involves phosphorylation-independent mechanisms. In the present study we investigated whether the histamine H2 receptor (H2R), a Gαs-coupled GPCR known to be desensitized by GRK2, needs to be phosphorylated for its desensitization and/or internalization and resensitization. For this purpose we evaluated the effect of the phosphorylating-deficient GRK2K220R mutant on H2R signaling in U937, COS7, and HEK293T cells. We found that although this mutant functioned as dominant negative concerning receptor internalization and resensitization, it desensitized H2R signaling in the same degree as the GRK2 wild type. To identify the domains responsible for the kinase-independent receptor desensitization, we co-transfected the receptor with constructions encoding the GRK2 RGS-homology domain (RH) and the RH or the kinase domain fused to the pleckstrin-homology domain. Results demonstrated that the RH domain of GRK2 was sufficient to desensitize the H2R. Moreover, disruption of RGS functions by the use of GRK2D110A/K220R double mutant, although coimmunoprecipitating with the H2R, reversed GRK2K220R-mediated H2R desensitization. Overall, these results indicate that GRK2 induces desensitization of H2R through a phosphorylation-independent and RGS-dependent mechanism and extends the GRK2 RH domain-mediated regulation of GPCRs beyond Gαq-coupled receptors. On the other hand, GRK2 kinase activity proved to be necessary for receptor internalization and the resulting resensitization.

Expression of a G Protein-coupled Receptor (GPCR) Leads to Attenuation of Signaling by Other GPCRs EXPERIMENTAL EVIDENCE FOR A SPONTANEOUS GPCR CONSTITUTIVE INACTIVE FORM

The idea of G protein-coupled receptors (GPCRs) coupling to G protein solely in their active form was abolished when it was found that certain ligands induce a G protein-coupled but inactive receptor form. This receptor form interferes with signaling of other receptors by sequestering G protein. However, the spontaneous existence of this receptor species has never been established. The aim of the present work was to evaluate the existence of the spontaneous conformation of the receptor inactively coupled to G protein able to interfere with the response of other GPCRs. According to the law of mass action, receptor overexpression should lead to increased amounts of all spontaneously occurring species. Based on this, we generated Chinese hamster ovary (CHO-K1)-derived cell lines expressing various amounts of the human histamine H2 receptor. In these systems, the signaling of other endogenously and transiently expressed GPCRs was attenuated proportionally to human H2 receptor expression levels. G protein transfection specifically reverted this attenuation, strongly suggesting hijacking of the G protein from a common pool. Similar attenuation effects were observed when the β2- adrenergic receptor was overexpressed, suggesting that this is a more general phenomenon. Moreover, in human mammary MDA-MB-231 cells, a consistent increase in the response of other GPCRs was observed when endogenous expression of β2-adrenergic receptor was knocked down using specific small interfering RNAs. Our findings show that GPCRs may interact with the signaling of other receptors by modulating the availability of the G protein and suggest the existence of GPCR spontaneous coupling to G proteins in an inactive form.

Histamine H2 receptor trafficking: role of arrestin, dynamin, and clathrin in histamine H2 receptor internalization.

Agonist-induced internalization of G protein-coupled receptors (GPCRs) has been implicated in receptor desensitization, resensitization, and down-regulation. In the present study, we sought to establish whether the histamine H2 receptor (H2r) agonist amthamine, besides promoting receptor desensitization, induced H2r internalization. We further studied the mechanisms involved and its potential role in receptor resensitization. In COS7 transfected cells, amthamine induced H2r time-dependent internalization, showing 70% of receptor endocytosis after 60-min exposure to amthamine. Agonist removal led to the rapid recovery of resensitized receptors to the cell surface. Similar results were obtained in the presence of cycloheximide, an inhibitor of protein synthesis. Treatment with okadaic acid, an inhibitor of the protein phosphatase 2A (PP2A) family of phosphatases, reduced the recovery of both H2r membrane sites and cAMP response. Arrestin 3 but not arrestin 2 overexpression reduced both H2r membrane sites and H2r-evoked cAMP response. Receptor cotransfection with dominant-negative mutants for arrestin, dynamin, Eps15 (a component of the clathrin-mediated endocytosis machinery), or RNA interference against arrestin 3 abolished both H2r internalization and resensitization. Similar results were obtained in U937 cells endogenously expressing H2r. Our findings suggest that amthamine-induced H2r internalization is crucial for H2r resensitization, processes independent of H2r de novo synthesis but dependent on PP2A-mediated dephosphorylation. Although we do not provide direct evidence for H2r interaction with β-arrestin, dynamin, and/or clathrin, our results support their involvement in H2r endocytosis. The rapid receptor recycling to the cell surface and the specific involvement of arrestin 3 in receptor internalization further suggest that the H2r belongs to class A GPCRs.

Cross-Desensitization and Cointernalization of H1 and H2 Histamine Receptors Reveal New Insights into Histamine Signal Integration

G protein-coupled receptor signaling does not result from sequential activation of a linear pathway of proteins/enzymes, but rather from complex interactions of multiple, branched signaling routes, i.e., signaling networks. In this work we present an exhaustive study of the cross-talk between H1 and H2 histamine receptors (H1R and H2R) in U937 cells and Chinese hamster ovary-transfected cells. By desensitization assays we demonstrated the existence of a crossdesensitization between both receptors independent of protein kinase A or C. H1R-agonist stimulation inhibited cell proliferation and induced apoptosis in U937 cells following treatment of 48 hours. H1R-induced antiproliferative and apoptotic response was inhibited by an H2R agonist suggesting that the cross-talk between both receptors modifies their function. Binding and confocal microscopy studies revealed cointernalization of both receptors upon treatment with the agonists. To evaluate potential heterodimerization of the receptors, sensitized emission fluorescence resonance energy transfer experiments were performed in human embryonic kidney 293T cells using H1R-cyan fluorescent protein and H2R-yellow fluorescent protein. To our knowledge these findings may represent the first demonstration of agonist-induced heterodimerization of the H1R and H2R. In addition, we also show that the inhibition of the internalization process did not prevent receptor crossdesensitization, which was mediated by G protein-coupled receptor kinase 2. Our study provides new insights into the complex signaling network mediated by histamine and further knowledge for the rational use of its ligands.

Signal transduction mechanism of biased ligands at histamine H2 receptors

7TMRs (seven-transmembrane receptors) exist as conformational collections in which different conformations would lead to differential downstream behaviours such as receptor phosphorylation, G-protein activation and receptor internalization. In this context, a ligand may cause differential activation of some, but not all, of the signalling events, which are associated to a particular receptor, and it would lead to biased agonism. The aim of the present study was to investigate whether H2R (histamine H2 receptor) ligands, described as inverse agonists because of their negative efficacy at modulating adenylate cyclase, could display some positive efficacy concerning receptor desensitization, internalization or even signalling through an adenylate-cyclase-independent pathway. Our present findings indicate that treatment with H2R inverse agonists leads to receptor internalization in HEK (human embryonic kidney)-293T transfected cells, by a mechanism mediated by arrestin and dynamin, but independent of GRK2 (G-protein-coupled receptor kinase 2)-mediated phosphorylation. On the other hand, we prove that two of the H2R inverse agonists tested, ranitidine and tiotidine, also induce receptor desensitization. Finally, we show that these ligands are able to display positive efficacy towards the ERK1/2 (extracellular-signal-regulated kinase 1/2) pathway by a mechanism that involves Gβγ and PI3K (phosphoinositide 3-kinase)-mediated signalling in both transfected HEK-293T cells and human gastric adenocarcinoma cells. These results point to the aspect of pluridimensional efficacy at H2R as a phenomenon that could be extended to naïve cells, and challenge previous classification of pharmacologically relevant histaminergic ligands.

H1 and H2 histamine receptors mediate the production of inositol phosphates but not cAMP in human breast epithelial cells

Abstract.Objective: In the present work we studied the H1 and H2 histamine receptor expression and function in HBL-100 and MCF-10A cells, derived from non-tumorigenic human breast epithelia, and in MCF-10T, the H-ras-transfected MCF-10A counterpart. The signal transduction pathways associated with these receptors, and the expression of proto-oncogenes c-fos, c-myc and c-jun at the mRNA and protein levels, were examined.¶Results: Saturation analysis using intact cells, showed two binding sites for [3H]tiotidine and [3H]mepyramine. Pretreatment of purified membrane with guanosine 5′-γ thiotriphosphate resulted in the loss of the low affinity component for [3H]tiotidine binding, and of the high affinity component for [3H]mepyramine. In both cases, there was no modification in the total number of sites for both ligands. Neither H1 nor H2 agonists stimulated cyclic AMP production, though this pathway is functional in these cells. On the other hand, both H1 and H2 agonists enhanced phosphoinositide turnover in a dose-dependent fashion, and this induction is pertussis toxin-insensitive. H1 and H2 agonists did not influence the expression of c-myc or c-fos mRNA, nor their encoded proteins.¶Conclusions: These results indicate that the three cell lines examined showed functional H1 and H2 histamine receptors, which are involved in the metabolic turnover of inositol phosphates but are ineffective in the modulation of the cyclic AMP response. The fact that H2 receptors have lost their ability to stimulate cyclic AMP production would imply the loss of a regulatory mechanism of cell growth.

Antihistaminergics and inverse agonism. Potential therapeutic applications

The accurate characterization of the molecular mechanisms involved in the action of receptor ligands is important for their appropriate therapeutic use and safety. It is well established that ligands acting at the histamine system currently used in the clinic exert their actions by specifically antagonizing G-protein coupled H1 and H2 receptors. However, most of these ligands, assumed to be neutral antagonists, behave as inverse agonists displaying negative efficacy in experimental systems. This suggests that their therapeutic actions may involve not only receptor blockade, but also the decrease of spontaneous receptor activity. The mechanisms whereby inverse agonists achieve negative efficacy are diverse. Theoretical models predict at least three possible mechanisms, all of which are supported by experimental observations. Depending on the mechanism of action engaged, the inverse agonist could interfere specifically with signaling events triggered by unrelated receptors. This possibility opens up new venues to explain the therapeutic actions of inverse agonists of the histamine receptor and perhaps new therapeutic applications.

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.