Structure and expression of a human oxytocin receptor

Oxytocin receptors in human adenocarcinomas of the endometrium: presence and biological significance

Oxytocin receptors (OTRs) are expressed in endometrial cells and oxytocin (OT) participates in endometrial functions. In cancers derived from other OT target tissues, such as breast and neural tissues, the expression of OTRs and the antiproliferative effect of OT on cancer cells has been previously observed. This study was therefore designed to search for OTR expression and the OT effect in endometrial carcinomas. To demonstrate the presence and the location of OTRs and OTR mRNA immunocytochemical, reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization (ISH) procedures were employed in a series of human adenocarcinomas of the endometrium. Using an anti-OTR monoclonal antibody (IF3), OTRs were demonstrated in the large majority of endometrial carcinomas (82%), with a pattern of positivity varying from diffuse to focal, according to tumour differentiation. The OTR gene was demonstrated in 78% of the cases by RT-PCR and its presence was confirmed in selected cases by ISH. Moreover, in a human endometrial carcinoma cell line (COLO 684) OTR was demonstrated by immunofluorescence and RT-PCR and it was observed that OT treatment (10(-11)-10(-7) M) significantly inhibited cell proliferation. Neither toxic effects nor apoptosis were induced by OT treatment. The addition of an inhibitor of protein kinase A (PKA) to the culture medium abolished the antiproliferative effect of OT, suggesting that cAMP via PKA could be the intracellular mediator of the OT effect, as previously observed in breast and neural tumours. In conclusion, this study presents evidence of OTR expression in human endometrial carcinomas and of an OT antiproliferative effect on human endometrial cancer cells in vitro. It is further suggested that OT and OTR may be involved in the regulation of endometrial cells, not only in physiological conditions but also in a neoplastic context.

Signal pathways mediating oxytocin stimulation of prostaglandin synthesis in select target cells

A major action of oxytocin is to stimulate prostaglandin production in reproductive tissues. The two major enzyme systems involved are cytosolic phospholipase A2 (cPLA2), which catalyses the formation of arachidonic acid from membrane glycerophospholipids, and prostaglandin endoperoxide-H synthases-1 and -2, which allow conversion of arachidonic acid to prostaglandins. During gestation, the concentrations of all three enzymes rise in the rabbit amnion. Agonists, including oxytocin, increase cPLA2 activity, in part, by elevating intracellular Ca2+ concentration, which causes cPLA2 to be translocated from the cytosol to intracellular membrane binding sites. Cytosolic PLA2 is then activated by a mitogen-activated protein kinase (MAPK)-dependent step. Our studies have elucidated signal pathways involved in oxytocin-stimulated prostaglandin output in both rabbit amnion cells and Chinese hamster ovary cells stably transfected with the rat oxytocin receptor. The two cell types are alike with respect to oxytocin-stimulated intracellular Ca2+ transients, mediation via Gq, and the specific MAPK that catalyses the phosphorylation of cPLA2. However, they differ with respect to the mechanisms of upregulation of key enzymes involved in prostaglandin E2 synthesis. These findings illustrate the tiers of complementary mechanisms involved in oxytocin stimulation of prostaglandin E2, and the extent of the diversity in the cellular signalling pathways involved.

Sensitivity of myometrium to CGRP varies during mouse estrous cycle and in response to progesterone

Calcitonin gene-related peptide (CGRP) inhibits contractions of the myometrium. Isometric force measurements on myometrial strips were carried out to monitor the inhibitory capacity of CGRP in the myometrium during the estrous cycle and in response to estrogen and progesterone in ovariectomized mice. CGRP inhibition of KCl-induced contractions was lowest at estrus and significantly increased during metestrus and diestrus. Progesterone treatment of ovariectomized mice resulted in a significant increase in the responsiveness of the myometrium to CGRP. Expression of CGRP-receptor component protein (CGRP-RCP), a marker of CGRP-receptor expression, was quantitated by Western and Northern blot analyses. The levels of inhibition exerted by CGRP during the various stages of the estrous cycle and in response to steroid hormone treatment correlated with the protein levels of CGRP-RCP. The mRNA levels did not change significantly during the estrous cycle or in response to hormone treatment, indicating that the regulation of CGRP-RCP protein does not occur at the transcriptional level. CGRP had an inhibitory effect both when applied before the stimulus for contraction and when applied during a sustained contracture induced by KCl. This suggests that CGRP-induced generation of second messengers can influence late events in electro-/chemomechanical coupling and/or the contractile machinery directly.

Molecular basis of ligand binding and receptor activation in the oxytocin and vasopressin receptor family

Although it is now widely accepted that G-protein-coupled receptors exist in at least two allosteric states, inactive and active, and that the spontaneous equilibrium between the two is regulated by various events including the binding of specific agonists and antagonists, the molecular counterparts of these functionally different states are still poorly understood. In this paper, we review our current knowledge concerning the structure-function relationships of the oxytocin and vasopressin receptors, focusing in particular on the process of receptor activation. Using a combined approach of site-directed mutagenesis and molecular modelling, we investigated the molecular events leading to agonist-dependent and -independent receptor activation in the human oxytocin receptor. Our analysis allows us to propose that the active conformations of this receptor are characterised by similar rearrangements of its cytosolic regions that ultimately lead to the opening of a putative docking site for the G-protein. Furthermore, the dynamics of these motions are similar to that observed in the alpha1B-adrenergic receptor, thus suggesting that, although activated by different ligands, the process of receptor isomerization in these two receptors is regulated by the same cluster of highly conserved residues and that common molecular events are responsible for receptor activation in different G-protein-coupled receptors.

The role of oxytocin and regulation of uterine oxytocin receptors in pregnant marsupials

The oxytocin-like peptide of most Australian marsupials is mesotocin, which differs from oxytocin by a single amino acid. This substitution has no functional significance as both peptides have equivalent affinity for and biological activity on the marsupial oxytocin-like receptor. A role for mesotocin in marsupial parturition has been demonstrated in the tammar wallaby where plasma mesotocin concentrations increase less than one minute before birth. Infusion of an oxytocin receptor antagonist at the end of gestation disrupts normal parturition, probably by preventing mesotocin from stimulating uterine contractions. In the absence of mesotocin receptor activation, a peripartum surge in prostaglandins is delayed which suggests a functional relationship between mesotocin, prostaglandin release and luteolysis. Female marsupials have anatomically separate uteri and in monovular species, such as the tammar wallaby, only one uterus is gravid with a single fetus whereas the contralateral uterus remains non-gravid. We have used this unique animal model to differentiate systemic and fetal-specific factors in the regulation of uterine function during pregnancy. The gravid uterus in the tammar wallaby becomes increasingly sensitive to mesotocin as gestation proceeds, with the maximum contractile response observed at term. This is reflected in a large increase in mesotocin receptor concentrations in the gravid uterus, and a downregulation in the non-gravid uterus in late pregnancy. The upregulation in myometrial mesotocin receptors is pregnancy-specific and independent of systemic steroids. One factor that may influence mesotocin receptor upregulation in the gravid uterus in late pregnancy is mechanical stretch of the uterus caused by the growing fetus. Our data highlight that a local fetal influence is more important than systemic factors in the regulation of mesotocin receptors in the tammar wallaby.

Oxytocin receptors and cholesterol: interaction and regulation

Cholesterol affects the ligand binding function of the oxytocin receptor in a highly specific manner. While the structurally-related cholecystokinin receptor shows a strong correlation between the membrane fluidity and its binding function, the oxytocin receptor behaves differently. A stringent and unique requirement of the affinity state of the oxytocin receptor for structural features of the sterol molecule has been found. The molecular requirements differ both from those postulated for sterol-phospholipid interactions and from those known to be necessary for the activity of other proteins. Employing a new detergent-free subcellular fractionation protocol, a two-fold enrichment of the oxytocin receptors (10-15% of total receptors) has been detected in the cholesterol-rich, caveolin-containing membrane domains of the plasma membrane. While most of the properties of the oxytocin receptors were indistinguishable in cholesterol-poor versus cholesterol-rich membrane compartments, high-affinity oxytocin receptors localised in caveolin-enriched low-density membranes showed about a 3-fold higher stability against thermal denaturation at 37 degrees C compared with the oxytocin receptors localised in high-density membranes. Moreover, addition of cholesterol to the cholesterol-poor high-density membranes fully protected the oxytocin receptors against thermal denaturation and partially rescued high-affinity oxytocin binding. Although the membrane fluidity of the caveolin-enriched domains was lower than that in the high-density membranes, there was no correlation between the stability of oxytocin receptors and the fluidity level of the membrane domains. Finally, in a molecular modelling approach a putative cholesterol binding motif on the extracellular surface of the oxytocin receptor was found.

Preterm labour

Spontaneous preterm labour remains a major obstetric problem because of the high incidence of neonatal mortality or long-term handicap associated with it. The drugs available for the management of preterm labour are poorly effective and have potentially serious side-effects for the mother or fetus. In recent years, there has been a remarkable increase in the knowledge of the biochemical mechanism underlying uterine quiescence and contractility. Many of the G protein-coupled receptors that participate in the regulation of myometrial activity have been cloned and characterized, and their intracellular signalling pathways have been elucidated. The role of G protein receptor kinases in uterine tachyphylaxis is better understood. New developments in our understanding of the cellular mechanisms involved in uterine contractions in idiopathic and infection-associated preterm labour are expected, which will lead to better, more selective therapy for this problem. However, much research remains to be done before the mechanism of human parturition is fully understood.

Close kinship of human 20alpha-hydroxysteroid dehydrogenase gene with three aldo-keto reductase genes

BACKGROUND

20alpha-Hydroxysteroid dehydrogenase (HSD) is a member of the aldo-keto reductase (AKR) superfamily and catalyses the reaction of progesterone to the inactive form 20alpha-hydroxyprogesterone. Progesterone plays an important role in the maintenance of pregnancy, and, in rodents, plasma progesterone levels decrease abruptly just before parturition. The induction of 20alpha-HSD is thought to be responsible for the decrease in plasma progesterone at term. High homology between human 20alpha-HSD [AKR 1C1] cDNA with other AKRs had caused difficulty in gene isolation and expression analysis. Thus, the metabolism of progesterone in the human reproductive system remained unclear.

RESULTS

By hybridization with rat 20alpha-HSD [AKR 1C8] cDNA and high-stringency polymerase chain reaction (PCR) with gene-specific primers, we were able to isolate the human 20alpha-HSD, bile acid-binding protein (BABP) [AKR 1C2], prostaglandin F synthase (PGFS) [AKR 1C3], and dihydrodiol dehydrogenase (DD) 4 [AKR 1C4] genes. These genes had similar exon-intron organizations and shared a high homology. The four recombinant enzymes encoded by these genes showed distinct substrate specificity. By reverse transcription-PCR analysis, human 20alpha-HSD, BABP and PGFS mRNAs were expressed ubiquitously, while DD4 mRNA was restricted to the liver. Promoter activities of the 20alpha-HSD, BABP and PGFS genes were high, both in ovarian granulosa cells and hepatocytes. Radiation hybridization analysis revealed that all these genes were located close together in chromosome 10.

CONCLUSION

The human gene encoding for the progesterone-metabolizing enzyme 20alpha-HSD in the female reproductive system was cloned, and its expression and gene localization were elucidated. BABP, PGFS and DD4 genes, which were highly homologous to the 20alpha-HSD gene, were also cloned, and their structure and function were characterized.

Nucleotide sequence and structural organization of the human vasopressin pituitary receptor (V3) gene

In the pituitary, vasopressin triggers ACTH release through a specific receptor subtype, termed V3 or V1b. We cloned the V3 cDNA and showed that its expression was almost exclusive to pituitary corticotrophs and some corticotroph tumors. To study the determinants of this tissue specificity, we have now cloned the gene for the human (h) V3 receptor and characterized its structure. It is composed of two exons, spanning 10kb, with the coding region interrupted between transmembrane domains 6 and 7. We established that the transcription initiation site is located 498 nucleotides upstream of the initiator codon and showed that two polyadenylation sites may be used, while the most frequent is the most downstream. Sequence analysis of the promoter region showed no TATA box but identified consensus binding motifs for Sp1, CREB, and half sites of the estrogen receptor binding site. However comparison with another corticotroph-specific gene, proopiomelanocortin, did not identify common regulatory elements in the two promoters except for a short GC-rich region. Unexpectedly, hV3 gene analysis revealed that a formerly cloned 'artifactual' hV3 cDNA indeed corresponded to a spliced antisense transcript, overlapping the 5' part of the coding sequence in exon 1 and the promoter region. This transcript, hV3rev, was detected in normal pituitary and in many corticotroph tumors expressing hV3 sense mRNA and may therefore play a role in hV3 gene expression.

Molecular cloning and functional characterization of a vasotocin receptor subtype that is expressed in the shell gland and brain of the domestic chicken

In chickens, oviposition is correlated with increased plasma levels of the neurohypophysial hormone vasotocin, and vasotocin stimulates contraction of uterine strips in vitro. A gene encoding a vasotocin receptor subtype that we have designated the VT1 receptor was cloned from the domestic chicken. The open reading frame encodes a 370-amino acid polypeptide that displays seven segments of hydrophobic amino acids, typical of guanine nucleotide-protein-coupled receptors. Other structural features of the VT1 receptor include two potential N-linked glycosylation sites in the extracellular N-terminal region, a conserved aspartic acid in transmembrane domain 2 that is found in nearly all guanine nucleotide-protein-coupled receptors, and two potential protein kinase C phosphorylation sites in the third intracellular loop and C-terminal tail. Expressed VT1 receptors in COS7 cells bind neurohypophysial hormones with the following rank order of potency: vasotocin congruent with vasopressin > oxytocin congruent with mesotocin > isotocin. In addition, the expressed VT1 receptor mediates vasotocin-induced phosphatidylinositol turnover and Ca(2+) mobilization. In the chicken, expression of VT1 receptor gene transcripts is limited to the shell gland (uterus) and the brain. Thus, the VT1 receptor that we have cloned may mediate contractions of the shell gland during oviposition and activate reproductive behaviors known to be stimulated by vasotocin in lower vertebrates.

Pharmacologic characterization of the oxytocin receptor in human uterine smooth muscle cells

[(3)H]-oxytocin was used to characterize the oxytocin receptor found in human uterine smooth muscle cells (USMC). Specific binding of [(3)H]-oxytocin to USMC plasma membranes was dependent upon time, temperature and membrane protein concentration. Scatchard plot analysis of equilibrium binding data revealed the existence of a single class of high-affinity binding sites with an apparent equilibrium dissociation constant (K(d)) of 0.76 nM and a maximum receptor density (B(max)) of 153 fmol mg(-1) protein. The Hill coefficient (n(H)) did not differ significantly from unity, suggesting binding to homogenous, non-interacting receptor populations. Competitive inhibition of [(3)H]-oxytocin binding showed that oxytocin and vasopressin (AVP) receptor agonists and antagonists displaced [(3)H]-oxytocin in a concentration-dependent manner. The order of potencies for peptide agonists and antagonists was: oxytocin>[Asu(1,6)]-oxytocin>AVP= atosiban>d(CH(2))(5)Tyr(Me)AVP>[Thr(4),Gly(7)]-oxytocin>dDAVP, and for nonpeptide antagonists was: L-371257>YM087>SR 49059>OPC-21268>SR 121463A>OPC-31260. Oxytocin significantly induced concentration-dependent increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and hyperplasia in USMC. The oxytocin receptor antagonists, atosiban and L-371257, potently and concentration-dependently inhibited oxytocin-induced [Ca(2+)](i) increase and hyperplasia. In contrast, the V(1A) receptor selective antagonist, SR 49059, and the V(2) receptor selective antagonist, SR 121463A, did not potently inhibit oxytocin-induced [Ca(2+)](i) increase and hyperplasia. The potency order of antagonists in inhibiting oxytocin-induced [Ca(2+)](i) increase and hyperplasia was similar to that observed in radioligand binding assays. In conclusion, these data provide evidence that the high-affinity [(3)H]-oxytocin binding site found in human USMC is a functional oxytocin receptor coupled to [Ca(2+)](i) increase and cell growth. Thus human USMC may prove to be a valuable tool in further investigation of the physiologic and pathophysiologic roles of oxytocin in the uterus. British Journal of Pharmacology (2000) 129, 131 - 139

Genomic and membrane actions of progesterone: implications for reproductive physiology and behavior

Progesterone, produced by the ovaries and adrenal glands, regulates reproductive behavior and the surge of luteinizing hormone which precedes ovulation by acting on neurons located in different parts of the hypothalamus. The study of the activation of these reproductive functions in female rats has allowed to explore the different mechanisms of progesterone action in the brain. It has allowed to demonstrate that new actions of the hormone, which have been observed in particular in vitro systems, are also operational in vivo, and may thus be biologically relevant. This mainly concerns the direct actions of progesterone on receptors of neurotransmitters such as oxytocin and GABA. Activation of the progesterone receptor in the absence of ligand by phosphorylation may also play a role.

The synthesis of a new class of oxytocin antagonists

The synthesis of a new class of oxytocin antagonists, with significantly modified C-terminal part, is described. The chemistry of the Mitsunobu reaction was applied to obtain the key derivatives. In spite of the extensive modifications of previously described compound F792, the peptides retain biological activity as oxytocin antagonists.

Previous maternal experience potentiates the effect of parturition on oxytocin receptor mRNA expression in the paraventricular nucleus

In sheep, central oxytocin release at parturition induces maternal behaviour which is thought to be mediated by changes in the expression of central oxytocin receptors. The distribution, effects of parturition, previous maternal experience and hormonal status on the distribution of an oxytocin receptor was investigated using immunocytochemistry and in situ hybridization. In ewes with no previous maternal experience, parturition induced significant increases in oxytocin receptor mRNA expression in the anterior olfactory nucleus, medial preoptic area, ventromedial hypothalamus, lateral septum, medial amygdala, bed nucleus of the stria terminalis and diagonal band of Broca. In maternally experienced ewes, parturition induced additional increases in two areas, the paraventricular nucleus and the Islands of Calleja. The changes in progesterone and oestrogen that occur during late pregnancy and parturition appear to contribute to increases in expression in the anterior olfactory nucleus, Islands of Calleja, medial preoptic area, ventromedial hypothalamus, bed nucleus of the stria terminalis and diagonal band of Broca, but not in the paraventricular nucleus, lateral septum and medial amygdala. These results demonstrate that progesterone and oestrogen priming enhance oxytocin receptor mRNA expression in a number of regions in the olfactory system, hypothalamus and limbic brain. These effects appear to be independent of maternal experience. Parturition increases oxytocin receptor mRNA expression in all the areas influenced by hormonal priming and the lateral septum, medial amygdala and paraventricular nucleus. Maternal experience also enhances expression of oxytocin receptor mRNA in the paraventricular nucleus and the Islands of Calleja. Because the paraventricular nucleus is the main source of oxytocin release in the brain, this upgrading of autoreceptors as a result of maternal experience may serve to enhance release of this peptide in projection sites regulating maternal behaviour.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Localization and expression of oxytocin receptor and its messenger ribonucleic acid in peri-implantation phase human endometrium during control and clomiphene-treated cycles

OBJECTIVES

This study was undertaken to determine expression levels of oxytocin receptor and its gene in peri-implantation phase human endometrium during clomiphene-treated cycles compared with control cycles.

STUDY DESIGN

Oxytocin receptor and its messenger ribonucleic acid in peri-implantation phase endometrium during control and clomiphene-treated (50 mg days 5 to 9) cycles of 5 healthy fertile women were determined by immunohistochemical methods, Western blot analysis with monoclonal antibody against amino acids 20 through 40 of the extracellular N-terminal human oxytocin receptor, and reverse transcription-polymerase chain reaction with oligonucleotide primers to amplify the 391-base pair fragment of the oxytocin receptor gene.

RESULTS

Oxytocin receptor and its messenger ribonucleic acid were expressed in human peri-implantation phase endometrial samples from both control and clomiphene-treated cycles. The receptor was localized predominantly in the epithelial cells and glands, with little or none detected in the stroma. Oxytocin receptor protein was separated out as a single 70-kd band by Western blot analysis; its relative abundance was significantly reduced during clomiphene-treated cycles. The messenger ribonucleic acid was detected in all endometrium during control and clomiphene-treated cycles, with greater expression during control cycles.

CONCLUSIONS

The expressions of oxytocin receptor and its gene in luteal phase human endometrium suggest a functional relevance in modulation of biochemical changes for implantation.

Sensitivity of bovine corpora lutea to prostaglandin F2alpha is dependent on progesterone, oxytocin, and prostaglandins

Prostaglandin (PG) F2alpha that is released from the uterus is essential for spontaneous luteolysis in cattle. Although PGF2alpha and its analogues are extensively used to synchronize the estrous cycle by inducing luteolysis, corpora lutea (CL) at the early stage of the estrous cycle are resistant to the luteolytic effect of PGF2alpha. We examined the sensitivity of bovine CL to PGF2alpha treatment in vitro and determined whether the changes in the response of CL to PGF2alpha are dependent on progesterone (P4), oxytocin (OT), and PGs produced locally. Bovine luteal cells from early (Days 4-5 of the estrous cycle) and mid-cycle CL (Days 8-12 of the estrous cycle) were preexposed for 12 h to a P4 antagonist (onapristone: OP; 10(-4) M), an OT antagonist (atosiban: AT; 10(-6) M), or indomethacin (INDO; 10(-4) M) before stimulation with PGF2alpha. Although OP reduced P4 secretion (p < 0.001) only in early CL, it reduced OT secretion in the cells of both phases examined (p < 0.001). OP also reduced PGF2alpha and PGE2 secretion (p < 0.01) from early CL. However, it stimulated PGF2alpha secretion in mid-cycle luteal cells (p < 0.001). AT reduced P4 secretion in early and mid-cycle CL (p < 0.05). Moreover, PGF2alpha secretion was inhibited (p < 0.05) by AT in early CL. The OT secretion and the intracellular level of free Ca2+ ([Ca2+]i) were measured as indicators of CL sensitivity to PGF2alpha. PGF2alpha had no influence on OT secretion, although [Ca2+]i increased (p < 0.05) in the early CL. However, the effect of PGF2alpha was augmented (p < 0.01) in cells after pretreatment with OP, AT, and INDO in comparison with the controls. In mid-cycle luteal cells, PGF2alpha induced 2-fold increases in OT secretion and [Ca2+]i. However, in contrast to results in early CL, these increases were magnified only by preexposure of the cells to AT (p < 0.05). These results indicate that luteal P4, OT, and PGs are components of an autocrine/paracrine positive feedback cascade in bovine early to mid-cycle CL and may be responsible for the resistance of the early bovine CL to the exogenous PGF2alpha action.

Administration of oxytocin affects vasopressin V2 receptor and aquaporin-2 gene expression in the rat

Oxytocin (OT) binds to the vasopressin V2 receptor (V2R) because of its structural similarity to arginine vasopressin (AVP). Though the affinity of OT for V2R is low, it is known that OT causes antidiuresis. To clarify the effect of OT as an agonist of V2R, we investigated the influence of acute elevation of plasma OT levels on the rat mRNA expression of V2R and aquaporin-2 (AQP2), the water channel regulated by V2R. The plasma OT level increased from 11.1+/-1.6 pg/ml to 331.0+/-67.9 pg/ml by 1 h after subcutaneousinjection of 20 microg OT. V2R mRNA expression decreased to 68.3+/-4.1% of the control at 3 h, and AQP2 mRNA expression increased to 239.3+/-26.8% of the control at 6 h. The plasma AVP level did not change significantly during the experiment. The influence of a subcutaneous injection of 20 microg OT on V2R and AQP2 mRNA expression is comparable to that of 10 microg AVP that we documented in the previous study. In conclusion, OT can downregulate V2R mRNA expression and upregulate AQP2 mRNA expression in the collecting duct as an agonist of the V2R like AVP.

Treatment of preterm labor with the oxytocin and vasopressin antagonist Atosiban

Oxytocin is involved in the regulation of preterm and term labor but the exact effect mechanisms are not fully understood. A regulatory action by vasopressin may also exist. The concentrations of oxytocin and vasopressin V1a receptors in myometrium from pregnant women are high before and in the beginning of labor both preterm and at term. Atosiban has high affinity for both these receptors and is a competitive oxytocin and vasopressin antagonist. The inhibitory effect of Atosiban on oxytocin induced activity on isolated myometrium correlates significantly with the concentration of the oxytocin receptors. Inhibition of preterm contractions with Atosiban was first reported by Akerlund et al 1987. Goodwin et al compared the effect of Atosiban to placebo in threatening preterm labor and the antagonist was in this trial significantly more effective than placebo in reducing the frequency of contractions (55% vs. 23%, p < 0.001). The same authors also reported successful tocolysis with the drug in actual preterm labor. Atosiban is currently in phase III of clinical development and seems to have the same effectiveness but fewer side-effects compared to beta-mimetics. These properties suggests that Atosiban may offer advantages over existing therapies in acute treatment of preterm labor.

Demonstration of functional oxytocin receptors in human breast Hs578T cells and their up-regulation through a protein kinase C-dependent pathway

Oxytocin (OT) receptors (OTRs) have been demonstrated in a number of human breast tumors and tumor cells, but it was not clear whether the receptors were functional. We examined the regulation and function of OTR in a tumor cell line, Hs578T, derived from human breast. These cells expressed moderate levels of OTR when cultured in 10% FBS, as demonstrated by RT-PCR and binding analyses. Serum deprivation resulted in the loss of OTRs, with no effect on cell viability. Restoration of serum and addition of 1 microM dexamethasone (DEX) increased OTR levels by about 9-fold. Up-regulation was blocked by the addition of phospholipase C and PKC inhibitors. Serum/DEX treatment also increased steady state OTR messenger RNA levels. OT increased intracellular Ca2+ in a time- and dose-responsive manner, and the effects of OT were lost when OTRs were down-regulated by serum starvation. Serum/DEX up-regulation of OTR restored the responsiveness to OT. OT also stimulated ERK-2 (extracellular signal-regulated protein kinase) phosphorylation and PGE2 synthesis in Hs578T cells. In addition to showing that OTRs in the breast tumor cells are functional, these studies show that Hs578T cells can be used to study molecular regulation of OTR gene expression and intracellular signaling pathways stimulated by OT.

Identification of a GABP alpha/beta binding site involved in the induction of oxytocin receptor gene expression in human breast cells, potentiation by c-Fos/c-Jun

Oxytocin (OT) receptors (OTRs) mediate reproductive functions, including the initiation of labor and milk ejection. OTR messenger RNA levels are highly regulated, reaching the greatest concentration in the uterus at the end of gestation, and in the mammary gland during lactation. Factors directly effecting changes in OTR gene expression in the mammary gland are not known, so the present studies were done to elucidate possible regulators by characterizing the human OTR gene promoter and 5'-flanking sequence. By analyzing expression of promoter-luciferase constructs, we localized a region between -85 and -65 that was required for both basal and serum-induced expression in a mammary tumor cell line (Hs578T) that expresses inducible, endogenous OTRs. This DNA region contains an ets family target sequence (5'-GGA-3'), and a CRE/AP-1-like motif. The specific Ets factor binding to the OTR promoter was identified, by electrophoretic mobility immunoshift assays, to be GABP alpha/beta. Co-transfection of a -85 OTR/luciferase construct with vectors expressing GABP alpha and GABP beta1 had only a modest effect on expression, but cotransfection with GABP alpha/beta- with c-Fos/c-Jun-expressing plasmids resulted in an increase of almost 10-fold in luciferase activity. Mutation of either the GABP- or CRE-like binding sites obliterated the induction. These findings are consistent with the involvement of protein kinase C activity in serum induction of the endogenous gene in Hs578T cells. We showed the requirement for GABP alpha/beta and c-Fos/c-Jun in endogenous OTR gene expression, using oligonucleotide GABP and AP-1 binding decoys to inhibit serum-induced increases in 125I-labeled OT antagonist binding to Hs578T cells. Our work is the first characterization of the proximal promoter region of the human OTR gene, and it sets the stage for studying regulation of OTR expression in breast cells.

Pharmacological characterization of arginine vasotocin vascular smooth muscle receptors in the trout (Oncorhynchus mykiss) in vitro

Arginine vasotocin (AVT) is present in the neurohypophysis of all nonmammalian vertebrates and it appears to be the antecedent of the neurohypophysial nonapeptide hormones. Relatively little is known about AVT receptors in lower vertebrates, especially fish, and the present study was designed to examine AVT receptor interactions in trout vascular and nonvascular smooth muscle in vitro. AVT produced dose-dependent contraction of isolated rings from celiacomesenteric, coronary, and efferent branchial arteries, ventral aorta, anterior cardinal vein, and strips of ductus Cuvier. The greatest efficacy (magnitude of contraction per unit tissue weight) and sensitivity (effective concentration for half-maximal response, EC50) to AVT was found in the efferent bronchial artery (EBA) and its receptors were characterized further. Other neurohypophysial peptides, including arginine vasopressin (AVP), lysine vasopressin (LVP), isotocin (IST), and oxytocin (OXY), contracted EBA with an efficacy order of (most to least) AVT = AVP = OXY > LVP > IST and a sensitivity order of AVT > OXY >/= AVP > IST > LVP. Neither Desmopressin, an AVP V2-receptor agonist, nor the AVP ring fragment, AVP4-9, contracted EBA nor did they inhibit AVT contraction. Pretreatment of EBA rings with the selective AVP V1-receptor antagonists (deamino-Pen1, O-Me-Tyr2, Arg8-vasopressin and deamino-Pen1, Val4, Arg8-vasopressin), the selective V2-receptor antagonist (adamantaneacetyl1, O-Et-D-Tyr0, Val4, aminobutyryl6, Arg8,9-vasopressin), or the combined V1-oxytocin receptor antagonist (d(CH2)5[Tyr(Me)2, Orn8-AVT]) competitively inhibited AVT contractions without affecting AVT efficacy. Receptor affinity constants (pA2) determined by Schild analysis were in the range of 6.8-7.3, with slightly higher constants for the AVP V1-/oxytocin receptor antagonists than for the selective V2-receptor antagonist. Endothelium removal had no effect on EBA sensitivity to AVT. EBA rings were an order of magnitude more sensitive to AVT than nonvascular gastrointestinal and urinary bladder smooth muscle rings or strips. However, AVT (10(-7) M) was as efficacious as acetylcholine (10(-5) M) in gastrointestinal, gallbladder, and urinary bladder smooth muscle. It is concluded that trout EBA possess an AVT smooth muscle receptor that shares a similar pharmacological profile with the mammalian vascular AVP V1a-receptor and the OXY-receptor, but it is distinct from the previously reported gill epithelial cell receptor.

Signal transduction pathways of the human V1-vascular, V2-renal, V3-pituitary vasopressin and oxytocin receptors

Vasopressin (VP) and oxytocin (OT) are cyclic nonapeptides whose actions are mediated by stimulation of specific G protein-coupled receptors (GPCRs) currently classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) VP receptors and OT receptors (OTR). The recent cloning of the different members of the VP/OT family of receptors now allows the extensive characterization of the molecular determinants involved in ligand binding and signal transduction pathways coupled to a given VP/OT receptor subtype in stably transfected mammalian cell lines. In this article, we review the present knowledge of the signal transduction pathways coupled to the different VP/OT receptor subtypes and we present new observations derived from the study of each human VP or OT receptor subtype stably expressed in CHO cells.

Human vascular endothelial cells express oxytocin receptors

Pharmacological studies in humans and animals suggest the existence of vascular endothelial vasopressin (AVP)/oxytocin (OT) receptors that mediate a vasodilatory effect. However, the nature of the receptor subtype(s) involved in this vasodilatory response remains controversial, and its coupled intracellular pathways are unknown. Thus, we set out to determine the type and signaling pathways of the AVP/OT receptor(s) expressed in human vascular endothelial cells (ECs). Saturation binding experiments with purified membranes of primary cultures of ECs from human umbilical vein (HUVEC), aorta (HAEC), and pulmonary artery (HPAEC) and [3H]AVP or [3H]OT revealed the existence of specific binding sites with a greater affinity for OT than AVP (Kd = 1.75 vs. 16.58 nM). Competition binding experiments in intact HUVECs (ECV304 cell line) with the AVP antagonist [125I]4-hydroxyphenacetyl-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-NH2 or the OT antagonist [125I]D(CH2)5[O-Me-Tyr-Thr-Orn-Tyr-NH2]vasotocin, and various AVP/OT analogs confirmed the existence of a single class of surface receptors of the classical OT subtype. RT-PCR experiments with total RNA extracted from HUVEC, HAEC, and HPAEC and specific primers for the human V1 vascular, V2 renal, V3 pituitary, and OT receptors amplified the OT receptor sequence only. No new receptor subtype could be amplified when using degenerate primers. DNA sequencing of the coding region of the human EC OT receptor revealed a nucleotide sequence 100% homologous to that of the uterine OT receptor reported previously. Stimulation of ECs by OT produced mobilization of intracellular calcium and the release of nitric oxide that was prevented by chelation of extra- and intracellular calcium. No stimulation of cAMP or PG production was noted. Finally, OT stimulation of ECs led to a calcium- and protein kinase C-dependent cellular proliferation response. Thus, human vascular ECs express OT receptors that are structurally identical to the uterine and mammary OT receptors. These endothelial OT receptors produce a calcium-dependent vasodilatory response via stimulation of the nitric oxide pathway and have a trophic action.

An investigation of position 3 in arginine vasopressin with aliphatic, aromatic, conformationally-restricted, polar and charged amino acids

We report the solid-phase synthesis and some pharmacological properties of 23 new analogs of arginine vasopressin (AVP) which have the Phe3 residue replaced by a broad variety of amino acids. Peptides 1-9 have at position 3: (1) the mixed aromatic/aliphatic amino acid thienylalanine (Thi) and the aliphatic amino acids; (2) cyclohexylalanine (Cha); (3) norleucine (Nle); (4) Leu; (5) norvaline (Nva); (6) Val; (7) alpha-aminobutyric acid (Abu); (8) Ala; (9) Gly. Peptides 10-23 have at position 3: the aromatic amino acids, (10) homophenylalanine (Hphe): (11) Tyr; (12) Trp; (13) 2-naphthylalanine (2-Nal); the conformationally-restricted amino acids (14) Pro; (15) 2-aminotetraline-2-carboxylic acid (Atc); the polar amino acids (16) Ser; (17) Thr; (18) Gln; and the charged amino acids (19) Asp; (20) Glu; (21) Arg; (22) Lys; (23) Orn. All 23 new peptides were evaluated for agonistic and, where appropriate, antagonistic activities in in vivo antidiuretic (V2-receptor) and vasopressor (V1a-receptor) assays and in in vitro (no Mg2+) oxytocic assays. The corresponding potencies (units/mg) in these assays for AVP are: 323+/-16; 369+/-6 and 13.9+/-0.5. Peptides 1-9 exhibit the following potencies (units/mg) in these three assays: (1) 379+/-14; 360+/-9; 36.2+/-1.9; (2) 294+/-21: 73.4+/-2.7; 0.33+/-0.02; (3) 249+/-28; 84.6+/-4.3; 4.72+/-0.16; (4) 229+19; 21.4+/-0.6; 2.1+/-0.2; (5) 134+/-5; 31.2+/-0.9; 28.4+/-0.2; (6) 114+/-9; 45.3+2.3; 11.3+/-1.6; (7) 86.7+/-2.5; 4.29+/-0.13; 0.45+/-0.03; (8) 15.5+/-1.5; 0.16+/-0.01; approximately 0.02: (9) 3.76+/-0.03; < 0.02; in vitro oxytocic agonism was not detected. These data show that the aliphatic amino acids Cha, Nle, Leu, Nva and Val are well-tolerated at position 3 in AVP with retention of surprisingly high levels of antidiuretic activity. Peptides 2-9 exhibit significant gains in both antidiuretic/vasopressor (A/P) and antidiuretic/oxytocic (A/O) selectivities relative to AVP. [Thi3]AVP appears to be a more potent antidiuretic and oxytocic agonist than AVP and is equipotent with AVP as a vasopressor agonist. The antidiuretic potencies of peptides 10-23 exhibit drastic losses relative to AVP. They range from a low of 0.018+/-0.001 units/mg for the Lys3 analog (peptide 22) to a high of 24.6+/-4.6 units,mg for the Hphe3 analog (peptide 10). Their vasopressor potencies are also drastically reduced. These range from a low of < 0.002 units/mg for peptide 22 to a high of 8.99+0.44 units/mg for the Atc3 analog (peptide 15). Peptides 10-23 exhibit negligible or undetectable in vitro oxytocic agonism. The findings on peptides 10-23 show that position 3 in AVP is highly intolerant of changes with aromatic, conformationally-restricted, polar and charged amino acids. Furthermore, these findings are in striking contrast to our recent discovery that position 3 in the potent V2/V1a/OT antagonist d(CH2)5D-Tyr(Et)2VAVP tolerates a broad latitude of structural change at position 3 with many of the same amino acids, to give excellent retention of antagonistic potencies. The data on peptides 1-4 offer promising clues to the design of more potent and selective AVP V2 agonists.

Differential protein-DNA binding analysis identifies a novel enhancer element, US-1, involved in the upregulation of the oxytocin receptor gene in human myometrium at term

In order to investigate the regulatory mechanisms involved in the transcription of the human oxytocin receptor (OTR) gene in the human myometrium at term of pregnancy, we subjected the 5' flanking region of the gene to a differential EMSA (electrophoretic mobility shift assay) procedure. Comparing nuclear proteins from term myometrium, in which OTR gene transcription is massively up-regulated, with those from the non-pregnant myometrium, indicated a prominent DNA-protein complex using the former extract. The sequence of the protein binding site was determined within 20 bp (TCTGCCTTCATCCAGCC) and designated as uterine stimulator motif-1 (US-1). The concatemerized US-1 sequence exhibited enhancer activity using a minimal thymidine kinase promoter (tk-81) in transfected SKN cells. We partially purified US-1 binding protein from SKN cells using a resin bead affinity procedure. Binding activity could be concentrated, although the protein eluate still comprised more than 20 component polypeptides. The molecular weight of the principal protein-DNA complex was determined following UV crosslinking to be 70 kDa. In circumstances where a cell-line mimicking the pregnant uterus at term is not available, the differential EMSA strategy, comparing OTR DNA protein binding in up- and down-regulated tissues, provides a powerful tool to investigate OTR regulation in the uterus. However, the precise characterization and identity of the specific DNA-binding protein(s) and consequent experimental verification of regulatory mechanisms still require elucidation.

Female reproduction in mice lacking the prostaglandin F receptor. Roles of prostaglandin and oxytocin receptors in parturition

Female mice lacking the receptor for prostaglandin F2 alpha (FP) do not deliver fetuses at term, although these can be successfully rescued by cesarean section. No induction of oxytocin receptor mRNA is found in the uterus of these mice, and they show no uterine contraction on intravenous administration of oxytocin. Furthermore, a decline in serum progesterone levels during the periparturition period is not observed in these animals. Ovariectomy at day 19 of pregnancy restored induction of the oxytocin receptor and caused successful delivery in these animals. These results indicate not only the essential role of luteolytic PGF 2 alpha action in natural parturition but also the importance of oxytocin receptor induction in this process.

Identification of neurohypophysial hormone receptor domains involved in ligand binding and G protein coupling

Chimeric vasopressin V2/OT receptors were constructed and investigated to identify receptor regions involved in ligand binding or G protein coupling. The fusion sites for one series of hybrid receptors were either located at the C-terminal end of the third extracellular domain or in the centre of the third transmembrane helix, respectively. In each pair of the resulting symmetrical hybrids only one receptor was able to bind arginine vasopressin (AVP) and/or oxytocin (OT). In both cases a major part of the vasopressin V2 receptor (V2R) was needed for ligand binding. A chimeric OT/V2 receptor including OT receptor (OTR) sequences from its N-terminus to the middle of transmembrane region three showed both high-affinity OT binding (Ki = 3 nM) and activation of the adenylyl cyclase. In contrast, a hybrid containing OTR sequences reaching from transmembrane helix five to its C-terminus showed the V2 receptor's ligand binding profile and was unable to couple to G alpha s. These results indicate (i) that the third and/or the fourth intracellular domain of the V2R are involved in G protein coupling and (ii) for high-affinity OT binding the N-terminal third of the OTR plays an important role. By detailed binding studies on a second series of chimeric V2/OT receptors with AVP, OT and the two hybrid hormone derivatives arginine vasotocin and oxypressin it was further demonstrated that the first two extracellular domains of the OTR are involved in binding to the C-terminal tripeptide of OT. Moreover, the third extracellular domain of the OTR is able to contact the cyclic part of OT and the fourth outer domain does not interact with the two variable amino acid residues of AVP and OT. Thus, the first three extracellular domains of the OTR provide an essential part of the OT binding site. The other part is most probably contributed by the OTR's transmembrane helices 3 and 4. Photoaffinity labeling and ligand binding studies demonstrated that the binding site for the OT antagonist d(CH2)5[Tyr(Me)2, Thr4, Orn8, Tyr9]vasotocin is located in the helices 1, 2 and 7. Our results provide evidence for the existence of separate domains of a peptide hormone receptor involved in binding and selectivity for agonists and peptide antagonists.

Molecular mechanisms regulating the effects of oxytocin on myometrial intracellular calcium

Oxytocin stimulates an increase in intracellular calcium in uterine myometrium by several mechanisms. Several lines of evidence indicate that the oxytocin receptor is functionally coupled to GTP-binding proteins of the G alpha q/11 class which stimulate phospholipase C activity. The IP3 generated as a result of phospholipase C activation can trigger release of calcium from intracellular stores. The finding that the oxytocin-stimulated increase in intracellular calcium in myometrial cells is greater in the presence of extracellular calcium than that in its absence indicates that oxytocin also has effects on calcium entry. This action is nifedipine-insensitive but may involve indirect stimulation of calcium entry through release-operated channels. An anti-G alpha q/11 antibody inhibits both oxytocin-stimulated GTPase activity and phospholipase C activity in myometrial membranes. The stimulation by oxytocin of phosphoinositide turnover in COS cells transfected with a plasmid expressing the oxytocin receptor is enhanced by cotransfection of G alpha q. Co-transfection of intracellular domains of the oxytocin receptor causes varying degrees of interference with oxytocin-stimulated phosphoinositide turnover. The data suggest that more than one intracellular domain is involved in oxytocin receptor/G-protein coupling. Oxytocin receptor stimulation of phospholipase C is inhibited by cAMP. This occurs in myometrial cells and in COS cells transfected with a plasmid expressing the receptor. The inhibitory mechanism involves the action of protein kinase A and is probably targeted indirectly at the G alpha q/11 /phospholipase C coupling step.

Molecular pharmacology of human vasopressin receptors

Vasopressin (AVP) and oxytocin (OT) are cyclic nonapeptides whose actions are mediated by activation of specific G protein-coupled receptors (GPCRs) currently classified into V1-vascular (V1R), V2-renal (V2R) and V3-pituitary (V3R) AVP receptors and OT receptors (OTR). The cloning of the different members of the AVP/OT family of receptors now allows the extensive molecular pharmacological characterization of a single AVP/OT receptor subtype in stably transfected mammalian cell lines. The human V1-vascular (CHO-V1), V2-renal (CHO-V2), V3-pituitary (CHO-V3) and oxytocin (CHO-OT) receptors stably expressed in CHO cells display distinct binding profiles for 18 peptide and 5 nonpeptide AVP/OT analogs. Several peptide and nonpeptide compounds have a greater affinity for the V1R than AVP itself. V2R peptide agonists and antagonists tend to be non-selective ligands whereas nonpeptide V2R antagonists are potent and subtype-selective. None of the 22 AVP/OT analogs tested has a better affinity for the human V3R than AVP itself. Several peptide antagonists do not select well between V1R and OTR. These results underscore the need for developing specific and potent analogs interacting specifically with a given human AVP/OT receptor subtype.

Genomic and non-genomic mechanisms of oxytocin receptor regulation

Our recent studies have shown that regulation of uterine oxytocin (OT) binding involves at least two different mechanism: Estradiol (E2)-induced upregulation is accompanied by an increase in OT receptor (OTR) mRNA accumulation, implying that the E2 effect is mediated via increased OTR gene transcription and/or OTR mRNA stabilization. In contrast, P (P)-induced OTR down-regulation occurs via a novel non-genomic mechanism, involving a direct interaction of P with the OTR at the level of the cell membrane. We found that P specifically binds to the OTR and inhibits its ligand binding and signalling functions. Physiological levels of P repress in vitro the ligand binding capacity (Bmax) of the OTR by > 50%. When expressed in CHO cells, the OTR provides a high affinity (Kd: 20nM) membrane binding site for P. OT-induced inositol phosphate production and intracellular calcium mobilization is inhibited 85% and 90%, respectively, by P. These effects are specific as signalling and binding functions of the closely related V1a vasopressin receptor remain unaffected by P, and as other, related steroids are devoid of any effect on OTR binding or signalling functions. The present observation of a specific interaction of a steroid with a G-protein-linked receptor defines a new mechanism of non-genomic steroid action and uncovers a novel level of crosstalk between steroid and peptide hormone action.

Activation of protein kinase C by oxytocin inhibits the biological activity of the human myometrial corticotropin-releasing hormone receptor at term

The role of placental CRH in human pregnancy is currently unknown. The myometrium expresses CRH receptors that during pregnancy become coupled to adenylate cyclase. Oxytocin (OT) is one of the main regulators of uterine activity, acting via activation of the inositol triphosphate pathway. In view of the possible cross-talk between the CRH and OT signal transduction pathways we have sought to examine in more detail the second messenger mechanisms involved. CRH receptor binding affinity for CRH and activation of adenylate cyclase were reduced in the presence of OT in pregnant (at term, but not preterm) human myometrium. OT action was mediated via pertussis toxin-sensitive G proteins, which directly inhibit adenylate cyclase and, via activation of protein kinase C, phosphorylate the CRH receptor, leading to desensitization. Activation of protein kinase C by OT could be partially inhibited in human pregnant myometrial cells by OT antagonists (F327 and CAP476; 1 microM) or phospholipase C inhibitors (U73122; 10 microM). These results suggest that in term myometrium, CRH receptor function is modulated by OT, leading to reduced biological activity, lower cAMP levels, and a subsequent shift in favor of contractility rather than relaxation.

Effect of oxytocin receptor and beta2-adrenoceptor blockade on myometrial oxytocin receptors in parturient rats

It has been proposed that the rise in myometrial oxytocin receptor (OTR) concentrations at term triggers parturition. In the present study, we have shown that in vivo infusion of the beta2-adrenoceptor (beta2AR) antagonist ICI-118.551 in late pregnant rats prevents the rise in myometrial OTR binding normally seen during delivery. A reduced contractile responsiveness of uterine strips isolated from rats in labor when challenged with oxytocin (OT) and a slight shortening of gestation accompanied this effect. OTR mRNA levels were, however, unaltered after the treatment, suggesting that the effect of beta2AR blockade on myometrial OTR was posttranscriptional or due to influences on extra-myometrial tissue. Infusion of the OTR antagonist atosiban down-regulated OTR binding sites in the parturient myometrium and resulted in an impaired contractile response to OT without affecting gestational length. OTR gene expression did not change, as seen from unchanged OTR mRNA values. Neither atosiban nor ICI-118.551 infusions alone changed fetal mortality. A significant increase in the incidence of fetal deaths was found, however, when rats were treated with a combination of atosiban and ICI-118.551. This treatment also down-regulated myometrial OTR and weakened the contractile response to OT, but it did not change gestational length. We conclude that the timing and onset of a normal parturition as well as a favorable outcome seem to be independent of a rise in OTR. This fact cannot exclude the possibility that an increase in OTR is of importance in the genesis of preterm labor. We suggest that beta2 stimulation up-regulates OTR during delivery. This effect may partly be responsible for the tachyphylaxis seen after the use of beta2 agonists to control preterm labor. We further suggest that OTR stimulation up-regulates OTR during labor. The OTR down-regulation seen after atosiban treatment adds to the direct relaxing effect of atosiban on the myometrium. In view of this, atosiban may prove to be a more useful tocolytic than the traditionally used beta2 agonists.

Tuftsin: on the 30-year anniversary of Victor Najjar's discovery

After a short description of the results of Victor Najjar's research on tuftsin and of the discoveries done by other authors in the early stage of tuftsin investigation, the current state of work on tuftsin is presented, based mainly on the literature published in the years 1984-1997. The presentation follows this order: the occurrence of tuftsin and retro-tuftsin sequences in proteins, their synthesis and biology, the antigenic properties of tuftsin, its influence on phagocytic cells, and other biologic activities of tuftsin, including antimicrobial, antiviral, antitumor and central effects, and the search for tuftsin superactive analogs.

The lymnaea cardioexcitatory peptide (LyCEP) receptor: a G-protein-coupled receptor for a novel member of the RFamide neuropeptide family

A novel G-protein-coupled receptor (GRL106) resembling neuropeptide Y and tachykinin receptors was cloned from the mollusc Lymnaea stagnalis. Application of a peptide extract from the Lymnaea brain to Xenopus oocytes expressing GRL106 activated a calcium-dependent chloride channel. Using this response as a bioassay, we purified the ligand for GRL106, Lymnaea cardioexcitatory peptide (LyCEP), an RFamide-type decapeptide (TPHWRPQGRF-NH2) displaying significant similarity to the Achatina cardioexcitatory peptide (ACEP-1) as well as to the recently identified family of mammalian prolactin-releasing peptides. In the Lymnaea brain, the cells that produce egg-laying hormone are the predominant site of GRL106 gene expression and appear to be innervated by LyCEP-containing fibers. Indeed, LyCEP application transiently hyperpolarizes isolated egg-laying hormone cells. In the Lymnaea pericardium, LyCEP-containing fibers end blindly at the pericardial lumen, and the heart is stimulated by LyCEP in vitro. These data confirm that LyCEP is an RFamide ligand for GRL106.

Oxytocin and vasopressin 1a receptor gene expression in the cycling or pregnant human uterus

OBJECTIVE

Our purpose was to compare concentrations of messenger ribonucleic acid specific for the oxytocin receptor and for the vasopressin 1a receptor in myometrial and endometrial tissues of pregnant and nonpregnant women.

STUDY DESIGN

Tissues from pregnant uteri were obtained from 95 women who were undergoing cesarean delivery between 26 and 42 weeks' gestation. Tissues from nonpregnant uteri were obtained from 7 cycling women who were undergoing hysterectomy. The competitive reverse-transcription polymerase chain reaction method was used to determine messenger ribonucleic acid concentrations.

RESULTS

A significant increase in oxytocin receptor messenger ribonucleic acid was found during the first half of pregnancy. Oxytocin receptor messenger ribonucleic acid concentrations were lower in tissues with spontaneous contractions than in quiescent tissues and were decreased in patients with advanced labor. Vasopressin 1a receptor messenger ribonucleic acid concentrations were high in tissues from both cycling and pregnant uteri but remained unchanged throughout gestation.

CONCLUSION

The increase in oxytocin receptor protein concentrations seen in pregnancy is only partially controlled by messenger ribonucleic acid abundance. High concentrations of vasopressin 1a receptor messenger ribonucleic acid confirm the biologically active role of this receptor in both the cycling and the pregnant uterus.

Mesotocin receptor gene and protein expression in the prostate gland, but not testis, of the tammar wallaby, Macropus eugenii

Evidence suggests that systemic oxytocin (OT) causes contractions of the prostate gland during ejaculation in eutherians, although functional OT receptors in this tissue have not been identified. Male marsupials secrete mesotocin (MT) from the pituitary and have relatively large, muscular prostate glands, so we examined MT receptors (MTRs) in the reproductive tract of the male tammar wallaby at the mRNA and protein level. We first obtained a partial (588 base pair) sequence of the tammar MTR cDNA that showed high homology to eutherian OT receptors (74-77%) and low homology to vasopressin receptors (38-52%). Analysis by reverse transcription-polymerase chain reaction demonstrated MTR mRNA in the adult, juvenile, and pouch young prostate and epididymis, but not testis. MTR transcripts were observed in the smooth muscle layers surrounding the urethral lumen and in the fibromuscular capsule. There was a single high-affinity 125I-D(CH2)5[Tyr(Me)2, Tyr4, Orn8, Tyr-NH29]-vasotocin (125I-OTA) binding site in the adult prostate. Competitive binding assays revealed identical ligand-binding profiles to the myometrium MTR (OTA > OT = MT > arginine vasopressin [AVP] antagonist > AVP). A lower-affinity 125I-OTA-binding site was present in the testis, with ligand-binding profiles indicating binding to vasopressin receptors. MTR concentrations in the prostate were 8-fold lower than concentrations in the myometrium. Our data demonstrate the presence of an MTR gene and functional receptor protein in the prostate gland, but not the testis, of the tammar. Localization of MTRs to the smooth muscle fibers in the capsule and surrounding the urethral lumen suggests a contractile function for MT during ejaculation.

Nephrogenic diabetes insipidus

In nephrogenic diabetes insipidus, the kidney is unable to concentrate urine despite normal or elevated concentrations of the antidiuretic hormone arginine vasopressin (AVP). In congenital nephrogenic diabetes insipidus (NDI), the obvious clinical manifestations of the disease, that is polyuria and polydipsia, are present at birth and need to be immediately recognized to avoid severe episodes of dehydration. Most (>90%) congenital NDI patients have mutations in the AVPR2 gene, the Xq28 gene coding for the vasopressin V2 (antidiuretic) receptor. In <10% of the families studied, congenital NDI has an autosomal recessive inheritance and mutations of the aquaporin-2 gene (AQP2), ie, the vasopressin-sensitive water channel, have been identified. When studied in vitro, most AVPR2 mutations lead to receptors that are trapped intracellularly and are unable to reach the plasma membrane. A minority of the mutant receptors reach the cell surface but are unable to bind AVP or to trigger an intracellular cyclic adenosine-monophosphate (cAMP) signal. Similarly AQP2 mutant proteins are trapped intracellularly and cannot be expressed at the luminal membrane. The acquired form of NDI is much more common than the congenital form, is almost always less severe, and is associated with downregulation of AQP2. The advances described here are examples of "bedside physiology" and provide diagnostic tools for physicians caring for these patients.

Increased expression of the rat myometrial oxytocin receptor messenger ribonucleic acid during labor requires both mechanical and hormonal signals

We investigated the expression of the mRNA encoding the oxytocin receptor (OTR) in rat myometrium throughout gestation and its regulation by progesterone and mechanical stretch. Using a semiquantitative reverse transcription-polymerase chain reaction approach, OTR mRNA was found to increase abruptly at the onset of spontaneous labor at term. Progesterone (4 mg/day) starting on Day 20 of gestation blocked this increase. Ovariectomy on Day 17 induced preterm labor 96 h after surgery and a significant increase in myometrial OTR mRNA levels 48 and 96 h after surgery. Both preterm labor and the rise in myometrial OTR expression were blocked by progesterone. To investigate the effects of stretch on myometrial OTR mRNA expression, unilaterally pregnant rats underwent either sham operation or placement of a tube in the nongravid uterine horn to distend the myometrium. On Day 20, stretch had no effect on OTR expression in the nongravid horns. During labor, OTR mRNA was highly expressed in the gravid horns as well as the nongravid stretched horns. In contrast, the level remained low in the nongravid unstretched horns. These results indicate that expression of rat myometrial OTR mRNA during pregnancy and labor is regulated by coordinated interactions between mechanical and endocrine signals.

Oxytocin receptor and its messenger ribonucleic acid in human leiomyoma and myometrium

OBJECTIVE

The study determined the expression of oxytocin receptor and its gene in human uterine leiomyoma compared with the adjacent myometrium.

STUDY DESIGN

Paired samples of leiomyoma and the adjacent myometrium from 20 women through the menstrual cycle, menopause, and various hormone treatments were studied. Oxytocin receptor was immunohistochemically localized with use of the specific antibody (2F8) to human oxytocin receptor. Oxytocin receptor protein was determined by Western blotting, whereas reverse transcription-polymerase chain reaction was used for oxytocin receptor messenger ribonucleic acid expression.

RESULTS

Immunohistochemistry showed positive staining in all tissues examined, relatively more intense in the myometrium than in the adjacent leiomyoma, and in tissues from the preovulatory than the postovulatory phase. Western blotting showed a single 70-kd band corresponding to the oxytocin receptor. The relative abundance of oxytocin receptor in both leiomyoma and myometrium was significantly higher during the preovulatory (n = 5) than the postovulatory (n = 5) phase (P = .034 and .05). In women receiving gonadotropin-releasing hormone agonist (n = 1) or oral contraceptives (n = 1), after the menopause (n = 2), and with irregular vaginal bleeding (n = 1), oxytocin receptor levels in leiomyoma and myometrium were unchanged but were reduced in anovulatory cycles (amenorrhea, n = 2). Reverse transcription-polymerase chain reaction showed messenger ribonucleic acid for oxytocin receptor as a 391-bp band in all leiomyomas and myometrium examined.

CONCLUSIONS

Leiomyoma and myometrium express the gene and protein for oxytocin receptor, which is probably partially regulated by ovarian sex steroids during the menstrual cycle.

Gestational changes in the levels of transforming growth factor-beta1 (TGFbeta1) and TGFbeta receptor types I and II in the human myometrium

As term approaches, a number of key proteins [contraction-associated proteins (CAPs)] are expressed within the human myometrium that are essential for the activation of powerful coordinated contractions during labor. The nature of the signals that switch on the synthesis of CAPs in vivo is not known. The ryanodine-sensitive intracellular Ca2+ release channel (RyR2) is a CAP whose expression in vitro is activated by transforming growth factor-beta (TGFbeta). The present experiments were performed to determine whether TGFbeta and TGFbeta receptors are present in the human myometrium at term and to explore the idea that they might form part of a signaling system in vivo. TGFbeta receptor types I and II, but not III, were demonstrated in myometrial smooth muscle in tissue taken from nonpregnant, pregnant nonlaboring, and spontaneous laboring women. Western blotting was used subsequently to determine the relative expression of TGFbeta receptor types I and II. Using nonpregnant myometrium as a baseline control the levels of expression of receptor types I and II were significantly increased by 168 +/- 19% (n = 6) and 162 +/- 22% (n = 7) in pregnant nonlaboring myometrium. In spontaneous laboring myometrium the levels of TGFbeta receptor type I and II expression were 93 +/- 12% (n = 6) and 85 +/- 11% (n = 7), respectively, compared to nonpregnant control values and were significantly lower than levels in pregnant nonlaboring tissues. The total TGFbeta1 levels in the myometrial tissues were 334 +/- 10, 534 +/- 73, and 674 +/- 106 pg/g tissue wet wt in nonpregnant, pregnant nonlaboring, and spontaneous laboring myometrium (n = 3 in each group), respectively. Thus, the TGFbeta signaling system appears to be up-regulated in the myometrium before the onset of parturition. The apparent loss of receptors in the spontaneous laboring samples in the presence of elevated total levels of TGFbeta may be indicative of agonist-induced receptor down-regulation. These observations support the idea that cytokines, in particular TGFbeta1, may play a role in the normal processes that prepare the myometrium for parturition at term.

Induction of oxytocin receptor gene expression in rabbit amnion cells

Oxytocin (OT)-stimulated PGE2 release by rabbit amnion is enhanced by the up-regulation of oxytocin receptors (OTR), which increase about 200-fold at the end of pregnancy. As recent studies have shown that PGs are essential for parturition, the rise in amnion OTR and associated PGE2 synthesis are probably essential for labor initiation. The present work was directed toward understanding the mechanisms of OTR up-regulation. Levels of agents that stimulate adenylyl cyclase activity and cortisol are increased in amniotic fluid at the end of pregnancy. Addition of either forskolin or cortisol to cultured amnion cells caused an increase in OTR ligand-binding sites and steady state OTR messenger RNA (mRNA) levels. Forskolin treatment elevated OTR mRNA levels rapidly, but transiently, whereas cortisol's effects were slower and sustained. Actinomycin or cycloheximide, added 3 h after forskolin, led to a sustained elevation in OTR mRNA levels, suggesting that forskolin increases the activities of OTR mRNA-destabilizing factors along with increasing OTR mRNA concentration. Cortisol did not appear to affect OTR mRNA stability. Measurement of OTR mRNA transcription rates showed that forskolin's effects were maximal within 1 h of treatment. In contrast, cortisol-induced transcription was not apparent until 8 h. The effects of forskolin and cortisol on OTR gene transcription were synergistic. Thus, the increase in OTR mRNA levels occurring after either forskolin or cortisol treatments is the result of induction of OTR gene expression, but the effects of the two agents appear to occur at separate sites.

Inhibition of oxytocin receptor function by direct binding of progesterone

The steroid hormone progesterone (P4) is essential for establishing and maintaining pregnancy in mammals. One of its functions includes maintenance of uterine quiescence by decreasing uterine sensitivity to the uterotonic peptide hormone oxytocin. Although it is generally held that steroid hormones such as P4 act at a genomic level by binding to nuclear receptors and modulating the expression of specific target genes, we show here that the effect of P4 on uterine sensitivity to oxytocin involves direct, non-genomic action of P4 on the uterine oxytocin receptor (OTR), a member of the G-protein-coupled receptor family. P4 inhibits oxytocin binding to OTR-containing membranes in vitro, binds with high affinity to recombinant rat OTR expressed in CHO cells, and suppresses oxytocin-induced inositol phosphate production and calcium mobilization. These effects are highly steroid- and receptor-specific, because binding and signalling functions of the closely related human OTR are not affected by P4 itself but by the P4 metabolite 5beta-dihydroprogesterone. Our findings provide the first evidence for a direct interaction between a steroid hormone and a G-protein-coupled receptor and define a new level of crosstalk between the peptide- and steroid-hormone signalling pathways.

Phospholipase C-delta1 and oxytocin receptor signalling: evidence of its role as an effector

Although the oxytocin receptor modulates intracellular Ca2+ ion levels in myometrium, the identities of signal molecules have not been clearly clarified. Our previous studies on oxytocin receptor signalling demonstrated that 80 kDa Ghalpha is a signal mediator [Baek, Kwon, Lee, Kim, Muralidhar and Im (1996) Biochem. J. 315, 739-744]. To elucidate the effector in the oxytocin receptor signalling pathway, we evaluated the oxytocin-mediated activation of phospholipase C (PLC) by using solubilized membranes from human myometrium and a three-component preparation containing the oxytocin receptor-Ghalpha-PLC-delta1 complex. PLC-delta1 activity in the three-component preparation, as well as PLC activity in solubilized membranes, was increased by oxytocin in the presence of Ca2+ and activated Ghalpha (GTP-bound Ghalpha). Furthermore the stimulated PLC-delta1 activity resulting from activation of Ghalpha via the oxytocin receptor was significantly attenuated by the selective oxytocin antagonist desGly-NH2d(CH2)5[Tyr(Me)2,Thr4]ornithine vasotocin or GDP. Consistent with these observations, co-immunoprecipitation and co-immunoadsorption of PLC-delta1 in the three-component preparation by anti-Gh7alpha antibody resulted in the PLC-delta1 being tightly coupled to activated Ghalpha on stimulation of the oxytocin receptor. These results indicate that PLC-delta1 is the effector for Ghalpha-mediated oxytocin receptor signalling.

Evidence for the involvement of several intracellular domains in the coupling of oxytocin receptor to G alpha(q/11)

In order to probe the nature of oxytocin receptor (OTR)/G alpha(q/11) protein coupling, we examined the effect of co-expression of OTR intracellular domains on oxytocin-stimulated phosphoinositide turnover in COSM6 cells overexpressing OTR and G alpha(q). Co-expression of G alpha(q) enhanced the oxytocin response maximally at a pOTR/pG alpha(q) plasmid transfection ratio of 1:0.16. In cells co-expressing OTR and G alpha(q/11), oxytocin stimulated phosphoinositide turnover with an EC50 of 48 nM. Co-transfection with plasmids expressing OTR intracellular domains inhibited oxytocin-stimulated phosphoinositide turnover by 23 +/- 6% (1i), 37 +/- 4% (2i), 55 +/- 6% (3i), and 40 +/- 6% (4i), respectively (P < 0.01). Expression of the 3i loop of the alpha(1B)-adrenergic receptor, which also couples to G alpha(q/11), inhibited phosphoinositide turnover by 35 +/- 2% (P < 0.01), while expression of the 3i loop of the dopamine 1A receptor, which couples to G alpha(s), had no effect. While these data indicate a functional role for the OTR 3i loop, they also suggest that interactions with more than one intracellular domain probably mediate the coupling of OTR to the G alpha(q/11) class of GTP-binding proteins.