Antidiarrhoeal properties of a novel sigma ligand (JO 2871) on toxigenic diarrhoea in mice: mechanisms of action

BACKGROUND AND AIMS

Sigma ligands display antisecretory activity against various secretagogues, suggesting antidiarrhoeal properties. In this study, we evaluated: (i) the antidiarrhoeal effect of JO 2871, a high affinity sigma ligand, in three models of toxigenic diarrhoea in mice; and (ii) the site and mechanism of action of this compound.

METHODS

Faeces were collected after toxin or vehicle administration in male DBA2 or NMRI mice. Diarrhoea was determined by cumulative stool weight (mg) over a 120 minute period. Diarrhoea was induced by intravenous administration of Salmonella enteriditis lipopolysaccharide (LPS), or oral administration of Escherichia coli heat stable (E coli-sta) or Clostridium difficile toxins. Two sigma ligands, igmesine and JO 2871, were administered either orally or intravenously, 60 and 30 minutes before the toxins, respectively. JO 2871 was also given orally 30 minutes after E coli-sta. In addition, JO 2871 was administered intracerebroventricularly five minutes before LPS and E coli-sta. BMY 14802 (1000 microg/kg orally), a sigma receptor antagonist, or cyclosomatostatin (CSS 1 microg/kg intravenously), a somatostatin antagonist, were given five minutes prior to JO 2871 in LPS, E coli-sta, and C difficile toxin treated mice. Gastric emptying and intestinal transit were evaluated after oral JO 2871 and BMY 14802 and intravenous CSS.

RESULTS

Stool weight measured 120 minutes after administration of the toxins was significantly increased. Oral JO 2871 and igmesine dose dependently inhibited toxigenic diarrhoea in all models. ED(50) values obtained using JO 2871 (1-20 microg/kg) were more than 40 times lower than those obtained with igmesine. Oral JO 2871 given after E coli-sta also inhibited diarrhoea in a dose dependent manner (ED(50) 50 microg/kg). Both sigma ligands were active by the intravenous route on LPS and E coli-sta induced stool weight increases. JO 2871 administered intracerebroventricularly failed to block this effect at any dose tested. Both BMY 14802 and CSS reversed the antidiarrhoeal effect of oral JO 2871. JO 2871, BMY 14802, and CSS did not affect transit parameters.

CONCLUSIONS

JO 2871 exerts a potent oral antidiarrhoeal effect, acting peripherally through sigma sites and somatostatin release.

The ligand binding site of NPY at the rat Y1 receptor investigated by site-directed mutagenesis and molecular modeling

The ligand binding site of neuropeptide Y (NPY) at the rat Y1 (rY1,) receptor was investigated by construction of mutant receptors and [3H]NPY binding studies. Expression levels of mutant receptors that did not bind [3H]NPY were examined by an immunological method. The single mutations Asp85Asn, Asp85Ala, Asp85Glu and Asp103Ala completely abolished [3H]NPY binding without impairing the membrane expression. The single mutation Asp286Ala completely abolished [3H]NPY binding. Similarly, the double mutation Leu34Arg/Asp199Ala totally abrogated the binding of [3H]NPY, whereas the single mutations Leu34Arg and Asp199Ala decreased the binding of [3H]NPY 2.7- and 5.2-fold, respectively. The mutants Leu34Glu, Pro35His as well as Asp193Ala only slightly affected [3H]NPY binding. A receptor with a deletion of the segment Asn2-Glu20 or with simultaneous mutations of the three putative N-terminal glycosylation sites, displayed no detectable [3H]NPY binding, due to abolished expression of the receptor at the cell surface. Taken together, these results suggest that amino acids in the N-terminal part as well as in the first and second extracellular loops are important for binding of NPY, and that Asp85 in transmembrane helix 2 is pivotal to a proper functioning of the receptor. Moreover, these studies suggest that the putative glycosylation sites in the N-terminal part are crucial for correct expression of the rY1 receptor at the cell surface.

Polar residues in the transmembrane domains of the type 1 angiotensin II receptor are required for binding and coupling. Reconstitution of the binding site by co-expression of two deficient mutants

Type 1 angiotensin receptors (AT1) are G-protein coupled receptors, mediating the physiological actions of the vasoactive peptide angiotensin II. In this study, the roles of 7 amino acids of the rat AT1A receptor in ligand binding and signaling were investigated by performing functional assays of individual receptor mutants expressed in COS and Chinese hamster ovary cells. Substitutions of polar residues in the third transmembrane domain with Ala indicate that Ser105, Ser107, and Ser109 are not essential for maintenance of the angiotensin II binding site. Replacement of Asn111 or Ser115 does not alter the binding affinity for peptidic analogs, but modifies the ability of the receptor to interact with AT1 (DuP753)- or AT2 (CGP42112A)-specific ligands. These 2 residues are probably involved in determining the binding specificity for these analogs. The absence of G-protein coupling to the Ser115 mutant suggests that this residue, in addition to previously identified residues, Asp74 and Tyr292, participates in the receptor activation mechanism. Finally, Lys102 (third helix) and Lys199 (fifth helix) mutants do not bind angiotensin II or different analogs. Co-expression of these two deficient receptors permitted the restoration of a normal binding site. This effect was not due to homologous recombination of the cDNAs but to protein trans-complementation.

Synthetic cDNA encoding the rat AT1a receptor: a useful tool for structure-function relationship analysis

To carry out systematic structure-function studies of the rat angiotensin II receptors by site directed mutagenesis, or production of chimeric receptors, we have produced a synthetic cDNA coding for the AT1a receptor. The synthetic cDNA is 1101 base pairs long, and contains 49 unique restriction sites that are on the average 23 base pairs apart, allowing replacement of specific restriction fragments by synthetic counterparts containing the desired modified sequence. The total cDNA was assembled in the expression vector pECE. After stable expression in Chinese Hamster Ovary cells, the protein encoded by this synthetic cDNA presents a pharmacological profile and a signal transduction mechanism indistinguishable from the wild type rat AT1a receptor.

Mutation of Asp74 of the rat angiotensin II receptor confers changes in antagonist affinities and abolishes G-protein coupling

Aspartic acid in the second transmembrane domain is a highly conserved amino acid among the G protein-coupled receptors and is functionally important for agonist binding and G-protein coupling in beta 2-adrenergic and luteinizing hormone receptors. To determine whether this aspartic acid is also involved in the function of the rat vascular angiotensin II receptor subtype 1 (AT1a), Asp74 was replaced either by asparagine or by glutamic acid. When expressed in CHO cells, the two mutants and the wild-type receptor displayed similarly high affinities for the agonist [Sar1, Tyr(125I)4]angiotensin II [where Sar is sarcosine and Tyr(125I) is monoiodinated tyrosine] and the other agonists: ([Sar1]angiotensin II > angiotensin II > angiotensin III >> angiotensin I). However, the Asn74 mutant shows striking differences in its affinity for some antagonists when compared with the wild-type receptor: the affinity for DUP753 was decreased 10-fold, whereas it was increased 6-fold for [Sar1,Ala8]angiotensin II and 20-fold for CGP42112A. These pharmacological changes were associated with a major defect in transmembrane signaling, since angiotensin II was unable to stimulate inositol phosphate production and increase cytosolic Ca2+ concentration through the two mutated receptors, whereas a clear dose-dependent stimulation was observed in cells expressing the wild-type receptor. Angiotensin II was able to promote DNA synthesis through the wild type but not through the mutated receptors. These data indicate that the conserved Asp74 residue of the AT1a receptor is important for the binding of angiotensin II antagonists and is essential for the transmembrane signaling cascade.

Activation of the CD3/T cell receptor (TcR) complex or of protein kinase C potentiate adenylyl cyclase stimulation in a tumoral T cell line: involvement of two distinct intracellular pathways

A monoclonal antibody (OKT3) directed against the T cell receptor (TcR)/CD3 molecular complex, as well as a protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate, PMA) were added to a culture of tumoral Jurkat T cells, in order to precise the sequence of intracellular signals leading to T cell activation. The experiments were performed in the presence or in absence of various stimulators of adenylate cyclase (AC) such as forskolin (FK), cholera toxin (CT) or prostaglandin E2 (PGE2). OKT3 increased inositol phosphate (IP) production; in parallel, it induced a slight accumulation of cAMP. The effect was markedly potentiated in presence of FK or CT, and to a lesser extent in the presence of PGE2. FK stimulated adenylate cyclase of Jurkat cell membranes, but the effect was not potentiated by OKT3, suggesting that potentiation of cAMP accumulation requires intact cells and is not mediated by direct receptor coupling. On the other hand, elevated cAMP accumulation induced a negative feedback on IP production. The effect of OKT3 on cAMP was mimicked by A23187, a Ca2+ ionophore, and abolished in the absence of extracellular Ca2+. PMA had the same effect as OKT3 on basal or FK- and CT-induced accumulation of cAMP. In contrast, it inhibited the PGE2 effect on the cyclic nucleotide. After desensitization of PKC by pretreatment with a high concentration of PMA, the phorbol ester was no longer effective. Under those conditions, facilitation by OKT3 of FK-induced accumulation of cAMP was preserved, whereas potentiation by the monoclonal antibody of the PGE2 stimulation of AC was even enhanced. The data indicate that cAMP accumulation indirectly elicited by phospholipase C activation is, at least partly, mediated by IP-dependent Ca2+ mobilization, while PKC is preferentially effective as an inhibitor of PGE2 stimulation.

Cloning and functional characterization of a novel mas-related gene, modulating intracellular angiotensin II actions

The mas oncogene codes for a GTP binding protein-coupled receptor that determines a physiological response to angiotensin when expressed in Xenopus laevis oocytes or in the neuronal cell line NG115-401L. However, another gene, rat thoracic aorta gene, structurally related to mas, is devoid of any functional similarity with the angiotensin receptor(s). The relationships between the mas-related proteins and the angiotensin receptors were investigated by identifying and characterizing new members of the mas gene family. A new mas-related gene (mrg) was cloned in a human genomic library at low stringency using the mas cDNA as probe. Mrg codes for a seven-hydrophobic-segment receptor that is 35% identical to the mas product and 29% identical to the rat thoracic aorta gene product. Mrg mRNA was not detected in several rat and human adult tissues that normally express the angiotensin II (AII) receptor, and transfections of COS and CHO cells with the mrg gene did not modify the number of AII binding sites. These results indicate that mrg and the human AII receptor genes are not identical. However, injection of mrg mRNA into Xenopus oocytes markedly increased the electrophysiological response to angiotensin peptides, indicating some functional similarities with the mas product. The reduction of the response after defolliculation of the oocyte, together with the full agonist effect of Sar1IIe8AII and the partial agonist effect of Sar1Ala8AII, seem to indicate that mrg interacts with the signaling pathways of the endogenous Xenopus angiotensin receptor to potentiate the response to AII.

Arachidonate metabolism in the anterior pituitary: effect of arachidonate inhibitors on Basal and stimulated secretion of prolactin, growth hormone and luteinizing hormone. I. Hormone release from incubated or perifused pituitary fragments

Abstract The potential involvement of arachidonic acid metabolites in the regulation of adenohypophyseal secretion was analysed on pituitary glands from male rats incubated in the presence of various inhibitors with different mechanisms of action: two inhibitors of phospholipase A(2) (parabromophenacylbromide, PB and compound CB 874), an inhibitor of cyclooxygenase- and lipoxygenase-catalysed pathways (5, 8, 11, 14-eicosatetraynoic acid, ETYA) and an inhibitor of cyclooxygenase (epsilon-lysyl acetylsalicylate, ASP). Under conditions which minimize side effects of the drugs, all inhibitors reduced prostaglandin synthesis and release, without affecting the metabolic integrity of the tissues (assessed by their intracellular adenosine triphosphate levels). All agents tested (PB, ETYA, ASP) suppressed prolactin secretion induced either by thyrotropin-releasing hormone or vasoactive intestinal peptide. Basal prolactin secretion was sensitive to phospholipase A(2) inhibitors. Similar inhibitions were obtained with ETYA and CB 874 on growth hormone secretion under basal conditions as well as after stimulation by growth hormone-releasing factor, thyrotropin-releasing hormone, or vasoactive intestinal peptide. In contrast, luteinizing hormone secretion, stimulated or not by gonadotropin-releasing hormone, was not sensitive to any of the agents used. It is concluded that, in intact male hemipituitaries, arachidonic acid metabolism is involved in the stimulation of prolactin and growth hormone secretion by neuropeptides. In contrast, luteinizing hormone release does not seem to depend on that mechanism. It has been verified that the inhibitors of arachidonic acid metabolism do not directly interfere with adenylate cyclase, or with the activation of protein kinase C, two enzymes which are involved in the regulation of secretory mechanisms.

Arachidonate Metabolism in the Anterior Pituitary: Effect of Arachidonate Inhibitors on Basal and Stimulated Secretion of Prolactin, Growth Hormone and Luteinizing Hormone. II. Hormone Release from Dispersed Pituitary Cells

Abstract In the accompanying study, we reported the effects of inhibitors of arachidonic acid metabolism on the regulation of prolactin, growth hormone (GH) and luteinizing hormone secretion by male hemipituitaries. The present work extends these investigations to primary cell cultures of the same origin. Arachidonic acid metabolism was inhibited by either 5, 8, 11, 14-eicosatetraynoic acid (ETYA), a blocker of cyclooxygenase- and lipoxygenase-catalysed pathways, or the cyclooxygenase inhibitors, indomethacin and aspirin. ETYA inhibited basal GH secretion by 60%, an effect which was reversed by micromolar concentrations of exogenous arachidonic acid. ETYA was much less effective on growth hormone-releasing factor-induced GH release, a result which contrasts with data obtained on intact glands. Growth hormone-releasing factor stimulation of adenylate cyclase was not affected by ETYA. Cyclooxygenase inhibitors decreased basal secretion to a more limited extent (-30%) and were ineffective on growth hormone-releasing factor-stimulated release. Basal prolactin secretion was reduced by 30% in the presence of ETYA and unaffected by cyclooxygenase inhibitors. As with GH, the effect was reversed by exogenous arachidonic acid. However, in contrast to growth hormone-releasing factor-stimulated GH secretion, thyrotropin-releasing hormone stimulation of prolactin release was able to overcome the inhibition by ETYA in a dose-dependent manner. Again, the insensitivity of thyrotropin-releasing hormone-stimulated prolactin release to ETYA contrasts with the data obtained in intact tissue. Moreover, ETYA inhibited (-60%) prostaglandin E(2) production; thyrotropin-releasing hormone was unable to increase the prostaglandin levels in control or ETYA-treated cells. This confirms the data obtained with cyclooxygenase inhibitors, suggesting that prostaglandins are not involved in prolactin secretion. Intracellular accumulation of Ca(2+) by the ionophore A23187 and protein kinase C stimulation by the phorbol ester 12-O- tetradecanoyl phorbol acetate (TPA), strongly stimulated GH and prolactin release. Under these conditions, ETYA was no longer able to inhibit secretion of the hormones. As with intact glands, basal and gonadotropin-releasing hormone or TPA-induced luteinizing hormone secretion were unaffected by any of the inhibitors used. It is concluded that blockade of the arachidonic acid cascade interferes with a secretory pathway involved mainly with basal release of prolactin and GH, but not luteinizing hormone. Thyrotropin-releasing hormone, a secretagogue known to trigger phospholipase C and, hence, to stimulate Ca(2+) mobilization and protein kinase C, overcame ETYA inhibition of prolactin secretion. Growth hormone-releasing factor, a secretagogue recognized by adenylate cyclase coupled receptors, did not overcome ETYA inhibition of GH secretion. However, both secretagogues strongly stimulated hormone release from their target cells in the presence of ETYA. The arachidonic acid cascade thus seems less important in neuromediator-induced secretion coupling processes in dispersed pituitary cells, than in the intact gland. These observations suggest that eicosanoids are more likely to mediate paracrine or autocrine modulations of secretory mechanisms, rather than to function as intracellular messengers.

The difficult challenge of cloning the angiotensin II receptor

The vasopressor peptide angiotensin II exerts its cellular effects through a membrane-bound receptor coupled to a G protein. Biochemical and pharmacological analyses of this receptor already identify two different membrane-bound receptors and one cytosoluble angiotensin-II-binding protein. Nevertheless, the purification of the membrane-bound form(s) appears to be difficult. In the absence of purified protein, two cloning strategies of the gene have been explored: (1) expression cloning, identifying the functions of the protein expressed from a cDNA library in COS cells or Xenopus oocytes, has been unsuccessful until now; (2) analogical cloning, trying to identify related members of the seven transmembrane segment receptor family, which could be related to angiotensin receptors, identifies the mas oncogene and two related genes. However, there are accumulating data to exclude their involvement in angiotensin binding.

Integrated communication between the nervous, endocrine and immune systems

Several data accumulated over recent years on the mechanisms underlying the interactions between the brain, hormones and the immune system. These data concern two major avenues of research: the evidence that brain-controlled, behavioral parameters can modulate the response of immunocompetent cells, and an increasing awareness that a number of chemical signals - neurotransmitters, hormones or mediators of immunity - are not, as previously believed, specific of given sets of tissues or of functions, but that, on the contrary, they can be produced and recognized by cellular elements belonging to any of those three systems. There is indeed evidence to indicate that signaling molecules involved in cellular communication are 'banalized': that means that their receptors are liable to be expressed in almost any tissue by a wide variety of cells. This statement, together with the discovery that intercellular regulation is multifactorial - that is, depends at any given time upon messages built up by combinations of signal molecules rather than by isolated transmitters - raises a certain number of theoretical problems as to the manner by which cells extract messages out of an important background noise. In the present paper, some of those theoretical problems will be presented in a summarized form, and their relevance for the interpretation of neuroendocrine or neuroimmunological interactions will be discussed.

Dihydropyridine-sensitive calcium channel activity related to prolactin, growth hormone, and luteinizing hormone release from anterior pituitary cells in culture: interactions with somatostatin, dopamine, and estrogens

In the present work, we determined the activity of voltage-dependent dihydropyridine (DHP)-sensitive Ca2+ channels related to PRL, GH, and LH secretion in primary cultures of pituitary cells from male or female rats. We investigated their modulation by 17 beta-estradiol (E2) and their involvement in dopamine (DA) and somatostatin (SRIF) inhibition of PRL and GH release. BAY-K-8644 (BAYK), a DHP agonist which increases the opening time of already activated channels, stimulated PRL and GH secretion in a dose-dependent manner. The effect was more pronounced on PRL than on GH release. BAYK-evoked hormone secretion was further amplified by simultaneous application of K+ (30 or 56 mM) to the cell cultures; in parallel, BAYK-induced 45Ca uptake by the cells was potentiated in the presence of depolarizing stimuli. In contrast, BAYK was unable to stimulate LH secretion from male pituitary cells, but it potentiated LHRH- as well as K+-induced LH release; it had only a weak effect on LH secretion from female cell cultures. Basal and BAYK-induced pituitary hormone release were blocked by the Ca2+ channel antagonist nitrendipine. Under no condition did BAYK affect the hydrolysis of phosphoinositides or cAMP formation. Pretreatment of female pituitary cell cultures with E2 (10(-9) M) for 72 h enhanced LH and PRL responses to BAYK, but was ineffective on GH secretion. DA (10(-7) M) inhibited basal and BAYK-induced PRL release from male or female pituitary cells treated or not treated with E2 (10(-9) M). SRIF (10(-9) and 10(-8) M) reversed BAYK-evoked GH release to the same extent in cell cultures derived from male or female animals. It was ineffective on BAYK-induced PRL secretion in the absence of E2, but antagonized it after E2 pretreatment. The effect was dependent upon the time of steroid treatment and was specific, since 17 alpha-estradiol was inactive. In addition, DA and SRIF decreased the 45Ca uptake induced by the calcium agonist. These data demonstrate that DHP-sensitive voltage-dependent calcium channels of the L type present on different pituitary cells are not equally susceptible to BAYK activation under steady state basal conditions, indicating that their spontaneous activity and/or distribution vary according to the cell type; their activity is modulated by sex steroids. In addition, these data suggest that Ca2+ channels represent a possible site of DA and SRIF inhibition of PRL and GH release, respectively, by gating calcium entry into the corresponding cells.