Induction of oxytocin receptor gene expression in rabbit amnion cells

Characterization of the cyclic adenosine monophosphate target site in the oxytocin receptor gene in rabbit amnion

Oxytocin (OXT) is a potent stimulator of prostaglandin E(2) (PGE(2)) synthesis by rabbit amnion cells obtained near the end of pregnancy. Coincident with a marked increase in sensitivity of PGE(2) synthesis to OXT, the concentration of OXT receptors (OXTRs) is abruptly upregulated about 200-fold at term. This increase can be mimicked in preterm amnion cells in primary culture by the synergistic action of agents that increase cAMP synthesis and by glucocorticoids. To elucidate the mechanism of cAMP action, we cloned the rabbit OXTR gene and isolated a 200-base pair (bp) forskolin-responsive region about 4.7 kilobase upstream from the transcriptional start site using transient transfection assays. This region corresponds to a DNase I-hypersensitive site that appears in amnion tissue only near the end of pregnancy, when OXTRs are upregulated. The effects of forskolin were mediated in part by cAMP response element binding protein (CREB), as coexpression of reporter constructs with dominant negative CREB inhibited reporter expression. In addition, CREB was cross-linked to sites in the 200-bp region only in chromatin isolated from cells near the end of pregnancy, as demonstrated by chromatin immunoprecipitation (ChIP). Because the transient transfection results are consistent with work using tissue extracts (DNase I hypersensitivity and ChIP), we conclude that cAMP, acting through a specific upstream CREB binding site, is critical for the physiological upregulation of OXTRs in the amnion at the end of gestation.

Oxytocin receptor- and Oct-4-expressing cells in human amniotic fluid

BACKGROUND/AIMS

The present clinical and molecular study aimed at investigating the presence of the genes encoding oxytocin receptor (OT-R) and Oct-4 in human amniotic fluid cells.

METHODS

Amniotic fluid samples were obtained from amniocentesis. Cells from human amniotic fluid samples were analyzed for mRNA expression of OT-R and Oct-4 via reverse transcription-polymerase chain reaction (RT-PCR). Immunocytochemistry was also performed with OT-R and Oct-4 antibodies.

RESULTS

RT-PCR from 10 independent amniocentesis samples demonstrated the expression of OT-R and Oct-4 mRNA. The cells also showed strong immunoreactivity for molecular markers of OT-R and Oct-4.

CONCLUSION

OT-R and Oct-4 are expressed in human amniotic fluid cells. The role of oxytocin in the physiology and pathophysiology of amniotic fluid cells remains to be settled.

Estrogen regulates transcription of the ovine oxytocin receptor gene through GC-rich SP1 promoter elements

Establishment of pregnancy in ruminants results from paracrine signaling by interferon tau (IFNT) from the conceptus to uterine endometrial luminal epithelia (LE) that prevents release of luteolytic prostaglandin F(2alpha) pulses. In cyclic and pregnant ewes, progesterone down-regulates progesterone receptor (PGR) gene expression in LE. In cyclic ewes, loss of PGR allows for increases in estrogen receptor alpha (ESR1) and then oxytocin receptor (OXTR) gene expression followed by oxytocin-induced prostaglandin F(2alpha) pulses. In pregnant ewes, IFNT inhibits transcription of the ESR1 gene, which presumably inhibits OXTR gene transcription. Alternatively, IFNT may directly inhibit OXTR gene transcription. The 5' promoter/enhancer region of the ovine OXTR gene was cloned and found to contain predicted binding sites for activator protein 1, SP1, and PGR, but not for ESR1. Deletion analysis showed that the basal promoter activity was dependent on the region from -144 to -4 bp that contained only SP1 sites. IFNT did not affect activity of the OXTR promoter. In cells transfected with ESR1, E2, and ICI 182,780 increased promoter activity due to GC-rich SP1 binding sites at positions -104 and -64. Mutation analyses showed that the proximal SP1 sites mediated ESR1 action as well as basal activity of the promoter. In response to progesterone, progesterone receptor B also increased OXTR promoter activity. SP1 protein was constitutively expressed and abundant in the LE of the ovine uterus. These results support the hypothesis that the antiluteolytic effects of IFNT are mediated by direct inhibition or silencing of ESR1 gene transcription, thereby precluding ESR1/SP1 from stimulating OXTR gene transcription.

In situ analysis of interleukin-1-induced transcription of cox-2 and il-8 in cultured human myometrial cells

The specific binding of transcription factors to DNA has been shown to be inhibited by chromatin structure and increased by cooperative interactions with other proteins. Consequently, in situ analysis using chromatin immunoprecipitation offers the most accurate view of transcriptional control. Transient transfection studies and in vitro analyses of IL-1-induced cox-2 transcription in a number of cell types have indicated regulation by either nuclear factor kappa B (NF-kappa B) or CCAAT/enhancer binding protein (C/EBP beta), or both acting cooperatively. To determine the mechanisms of COX-2 (cyclooxygenase or prostaglandin endoperoxide synthase) induction in cultured human myometrial cells in situ, we examined the cross-linking of the RelA subunit of NF-kappa B and C/EBP beta to the cox-2 promoter and flanking sequences. As a control, we inspected the interaction of these transcription factors with the IL-8 gene, which has been shown in other cell types to be activated by the cooperative interaction of NF-kappa B and C/EBP beta. Indeed, both transcription factors were cross-linked to the il-8 promoter after IL-1 treatment, but only RelA was cross-linked to cox-2 DNA. The il-8 promoter was also found to physically interact with proteins cross-linked to sites further upstream. IL-1 treatment also increased polymerase II cross-linking to both promoters and increased histone H4 acetylation at specific sites. These results indicate that modification of chromatin structure is part of the response to IL-1 stimulation. Chromatin immunoprecipitation thus provides critical insight into the mechanisms of COX-2 and IL-8 expression in human myometrial cells.

Smoking enhances oxytocin-induced rhythmic myometrial contraction

Although smoking during pregnancy is one of the major risk factors of premature delivery, the underlying mechanism by which smoking causes premature delivery is unknown. In the present study, we examined the effects of smoking on uterine contractility induced by oxytocin and prostaglandin F(2alpha). Rats inhaled either cigarette smoke or room air from Day 14 to Day 16 of pregnancy through an inhalation apparatus for experimental animals (type "Hamburg II"). After the rats were killed on Day 17 of pregnancy, the uterine contractile sensitivity and activity on exposure to oxytocin or prostaglandin F2alpha were investigated. The expression levels of oxytocin-receptor mRNA and prostaglandin F(2alpha) receptor mRNA in the uterus were investigated by reverse transcription-polymerase chain reaction. The contractile activity was assessed as the contractile force and the frequency of rhythmic contractions of myometrial strips that were treated with oxytocin or prostaglandin F(2alpha). The contractile sensitivity to oxytocin was significantly higher in the smoking group than in the control group (P < 0.01). Although the contractile force of oxytocin-induced contractions did not differ between the smoking and control groups, the frequency of contractions was significantly higher in the smoking group than in the control group (P < 0.01). On the other hand, no significant differences were found in the contractile sensitivity and activity in response to prostaglandin F(2alpha) between the smoking and control groups. The expression of oxytocin-receptor mRNA in the myometrium was significantly increased in the smoking group compared with the control group (P < 0.01). However, no significant difference was found in the level of expression of prostaglandin F(2alpha)-receptor mRNA between the two groups. These results suggest that smoking during pregnancy increases the contractile sensitivity and activity of the myometrium in response to oxytocin by up-regulating the expression of oxytocin-receptor mRNA. The effects of smoking on the contractile sensitivity and activity of the myometrium in response to oxytocin may increase the risk of premature delivery in smokers.

Regulation of oxytocin receptor expression in cultured human myometrial cells by fetal bovine serum and lysophospholipids

Oxytocin receptor (OTR) expression in human myometrium increases over 150-fold from the beginning of pregnancy to the end. In the present studies, we examined potential mechanisms of OTR up-regulation, using myometrial cells in primary culture from women in late gestation. OTR ligand-binding sites and steady-state mRNA levels were down regulated by serum starvation, and up-regulated by restoration of fetal bovine serum (FBS). Transcriptional activity of the OTR gene was the same with or without FBS treatment, but FBS increased OTR mRNA half-life about 5-fold. Lysophospholipids (lysophosphatidic acid and sphingosine 1-phosphate), which are present in serum, had similar effects as FBS. Lysophospholipid receptor mRNAs of the endothelial differentiation gene (Edg) family (Edgs 1, 3, 4, and 5) were demonstrated in myometrial cells by RT-PCR. These G protein-coupled receptors have been shown to be coupled to G(i/o) and to mediate activation of phosphoinositol 3-phosphate kinase. Indeed, the effects of the lysophospholipids and FBS were completely blocked by pertussis toxin, a G(i/o) inhibitor. Likewise, inhibition of G(i/o) signaling by elevation of intracellular cAMP or inhibition of phosphoinositol 3-phosphate kinase blocked FBS effects on OTR mRNA stability. We do not presently understand the mechanisms of OTR up-regulation in human myometrium in vivo, but the present studies might lead to the description of mRNA-stabilizing factors whose activity can be quantified in tissue samples during pregnancy to elucidate the process of OTR up-regulation.

Oxytocin receptor regulation and action in a human granulosa-lutein cell line

Although oxytocin and its receptor have been identified in human ovary, its regulatory role in granulosa cell or corpus luteum function has not been clearly defined. To better understand oxytocin action in the human ovary, we have characterized the expression and function of oxytocin receptors in an immortalized human granulosa-lutein cell line, HGL5. Expression of oxytocin receptor mRNA was demonstrated by reverse transcriptase-polymerase chain reaction analysis, and by specific binding of an iodinated oxytocin antagonist (apparent dissociation constant of 131 +/- 0.15 pM, and a B(max) of 12 +/- 0.5 fmol/microg DNA). Receptor levels were down-regulated by serum starvation, and rapidly up-regulated by serum restoration. Stimulation of protein kinase C activity increased oxytocin receptor levels in a concentration-dependent manner. Conversely, protein kinase C inhibition blocked up-regulation of oxytocin receptors. Treatment of cells with 10 nM oxytocin resulted in a rapid, transient increase in intracellular Ca(2+), and the response was blocked by an oxytocin antagonist. Because HGL5 cells secrete progesterone and estradiol in response to agents that elevate intracellular cAMP concentrations, we studied the effect of oxytocin on steroid production. Oxytocin enhanced the effects of forskolin on progesterone production. These results suggest that oxytocin augments the activity of luteotropins in vivo. Our studies are the first to show an ovarian cell line that expresses functional oxytocin receptors. These cells can serve as a useful model for studying oxytocin signal pathways and their cross-talk with respect to progesterone synthesis. These cells also will be useful in the analysis of mechanisms of oxytocin receptor regulation, including regulation of its gene.

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.

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.