The effect of oxytocin, estrogen, calcium ionophore A23187 and hydrocortisone on prostaglandin F2 alpha and 6-oxo-prostaglandin F1 alpha production by cultured human endometrial and myometrial explants

Functional oxytocin receptors in a human endometrial cell line

OBJECTIVE

Our goal was to demonstrate expression and functionality of oxytocin receptors in a human endometrial cell line. This cell line could then be used for further investigation of the role of oxytocin in reproductive function at the cellular level.

STUDY DESIGN

Oxytocin receptor messenger ribonucleic acid expression was determined by reverse transcriptase-polymerase chain reaction deoxyribonucleic acid amplification with ribonucleic acid from confluent Ishikawa cells. Ligand binding to whole cells was evaluated by nonlinear regression analysis with an iodinated oxytocin antagonist. The coupling of the oxytocin receptor to signaling pathways was evaluated by measuring oxytocin-stimulated increases in intracellular calcium concentration, phosphorylation of ERK2 (extracellular-regulated protein kinase 2) mitogen-activated protein kinase, and prostaglandin E(2) release.

RESULTS

Polyacrylamide gel electrophoresis of the reverse transcriptase-polymerase chain reaction products demonstrated the presence of oxytocin receptor messenger ribonucleic acid in Ishikawa cells. Ligand-binding analysis of these cells demonstrated a single class of noninteracting sites, with a B(max) (maximal number of binding sites) of 77.7 fmol/mg deoxyribonucleic acid and an apparent dissociation constant of 8.3 x 10(-11) mol/L. Stimulation with 100-nmol/L oxytocin caused a rapid transient increase in intracellular free calcium concentration, which was blocked by 1-micromol/L oxytocin antagonist. Treatment of cells with oxytocin for 10 minutes resulted in a marked increase in the phosphorylation of ERK2, as determined by Western blot analysis, and a 5-fold increase in prostaglandin E(2) release.

CONCLUSION

This study is the first to demonstrate functional oxytocin receptors in an established human endometrial cell line. This cell line will be useful in elucidating the mechanisms of action of oxytocin in the reproductive tract at the molecular level.

Cellular mechanisms by which oxytocin mediates uterine prostaglandin F2 alpha synthesis in bovine endometrium: role of calcium

The objective of these experiments was to determine the role of Ca2+ during oxytocin-stimulated prostaglandin (PG) F2 alpha release from bovine endometrial tissue in vitro. Uteri were collected from dairy cows on the day after spontaneous luteal regression. Caruncular endometrial explants were dissected and incubated in vitro to determine phospholipase C activity or PGF2 alpha release. A23,187 (a calcium ionophore) and maitotoxin (an activator of voltage-gated L-type calcium channels) stimulated release of PGF 2 alpha in a concentration-dependent manner (P < 0.05). Thapsigargin (induces accumulation of Ca2+ in the cytoplasm by inhibiting endoplasmic reticulum Ca2+/ATPase pumps) stimulated release of PGF2 alpha in a concentration-dependent manner as well (P < 0.13). Oxytocin (10(-6) M), AIF4- (a nonspecific activator of G-proteins; 10(-5) M), A23,187 (10(-5) M), and melittin (a stimulator of phospholipase A2; 10(-4) M) stimulated PGF2 alpha release when explants were incubated in Ca(2+)-free medium (P < 0.10); however, oxytocin, A23,187, or melittin were unable to stimulate PGF2 alpha release when explants were incubated in Ca(2+)-free medium containing the calcium chelator EGTA (P < 0.10). This treatment did not prevent oxytocin or AIF4- from stimulating phospholipase C activity (P < 0.08). CoCl2 (a nonspecific Ca2+ channel blocker) and methoxyverapamil (a specific voltage-gated L-type Ca2+ channel blocker) prevented oxytocin from stimulating PGF2 alpha release (P < 0.05). Our results suggest that both extracellular and intracellular Ca2+ may be required for oxytocin to stimulate PGF2 alpha secretion in bovine endometrial tissue.

Prostaglandin G/H synthase-2 is a major contributor of brain prostaglandins in the newborn

In order to understand the molecular basis of the elevated cerebral prostaglandin levels in the newborn, we compared the expression of the mRNAs and proteins of prostaglandin G/H synthases (PGHS), PGHS-1 and PGHS-2, in various regions of the brain and the microvasculature of newborn (1-2-day-old) and juvenile (4-7-week-old) pigs and also measured the relative contribution of PGHS-2 to cerebral prostaglandin synthesis both in vivo and in vitro by using a novel inhibitor of PGHS-2, NS-398. Ribonuclease protection assays using total RNA isolated from various regions of the porcine brain revealed that, unlike PGHS-1 mRNA, PGHS-2 mRNA was abundantly expressed in the cortex and the microvasculature of the newborn compared with those of the juvenile animal. PGHS-2 immunoreactive protein comprised the majority of total PGHS enzyme in neonatal cerebral microvasculature due to a 2-3-fold lower expression of immunoreactive PGHS-1 protein. Inhibition of PGHS-2 by NS-398 decreased the rate of prostaglandin synthesis by purified cerebral microvessels of the newborn by approximately 65% and of juvenile pigs by 30%. The decrease in brain tissue prostaglandin concentrations following intravenous administration of NS-398 was greater in newborn pigs (> or = 90%) than in the juvenile animals (< or = 30%). Furthermore, NS-398 substantially reduced the net in vivo cerebrovascular production of prostaglandins in newborn pigs. Taken together, these results indicate that PGHS-2 is the predominant form of prostaglandin G/H synthase in the newborn brain and cerebral microvasculature and the main contributor to the brain prostaglandin levels in the newborn animal.

The control of prostaglandin production by the endometrium in relation to luteolysis and menstruation

Oestradiol acting on a progesterone-primed uterus stimulates prostaglandin (PG) F2 alpha synthesis by the endometrium. In some species (notably the sheep, cow and goat) oxytocin released from the ovary also forms part of the physiological stimulus for increased endometrial PGF2 alpha production. The corpus luteum contains high concentrations (> 1 microgram/g tissue) of this peptide in these species. The intracellular mechanisms by which these three hormones control endometrial PGF2 alpha synthesis and release are far from clear. Oxytocin stimulates the synthesis of inositol phosphates and diacylglycerol in the endometrium of some species, but whether this pathway is involved in endometrial PGF2 alpha synthesis is still open to question. There is evidence that increased endometrial PGF2 alpha synthesis is dependent upon increased endometrial protein synthesis but, apart from the recorded effects of steroid hormones on the concentrations of phospholipase A2, prostaglandin H synthase and oxytocin receptors, it is not known what other endometrial proteins are involved. Some disorders of menstruation are associated with abnormal PG production by the endometrium, but the reasons for this abnormality are not clear. During early pregnancy an increase in PGF2 alpha synthesis by the endometrium is prevented, except in the pig where the PGF2 alpha produced is directed from the venous drainage to the uterine lumen. In those species in which endometrial PGF2 alpha synthesis is dependent upon oxytocin secreted by the ovary, the conceptus secretes an interferon-tau (previously named trophoblast protein-1) which prevents oestradiol and oxytocin acting on a progesterone-primed uterus from stimulating endometrial PGF2 alpha synthesis. The identities of the factors produced by the conceptus which prevent endometrial PGF2 alpha synthesis during early pregnancy in other species are not known, although it is clear that they are not interferons.

The effect of caffeine on prostaglandin output from the guinea-pig uterus

1. Caffeine increased the outputs of prostaglandin F2 alpha (PGF2 alpha), PGE2 and 6-keto-PGF1 alpha from the guinea-pig uterus on days 7 and 15 of the oestrous cycle. The effect on PGE2 output depended on the age of the animals and was absent in younger guinea-pigs (< 4 months). Theophylline also stimulated the outputs of PGF2 alpha and 6-keto-PGF1 alpha, but not the output of PGE2, from the day 7 guinea-pig uterus. 2. The stimulatory effects of caffeine on the outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha from the guinea-pig uterus were not prevented by lack of extracellular calcium, ryanodine or ruthenium red (both inhibitors of calcium release via the ryanodine receptor), although the increase in PGF2 alpha output tended to be slower when extracellular calcium was absent. Also, ryanodine flattened and broadened the peak of increased PGF2 alpha release. 3. The calmodulin antagonists, W-7 and trifluoperazine, had no inhibitory effect on the caffeine-stimulated increases in uterine prostaglandin output. In fact, W-7 (but not trifluoperazine) greatly potentiated the action of caffeine on uterine PGF2 alpha output, but had little or no potentiating effect on the action of caffeine on uterine PGE2 and 6-keto-PGF1 alpha outputs. 4. TMB-8, an intracellular calcium antagonist, inhibited the increase in PGF2 alpha output produced by caffeine without preventing the increases in outputs of PGE2 and 6-keto-PGF1 alpha. 5. These studies suggest that caffeine stimulates uterine PGF2 alpha synthesis and release by a mechanism dependent upon intracellular calcium, but this mechanism is not mediated by activation of any of the three well-characterized ryanodine receptors or by calmodulin. Furthermore, the increases in the synthesis and release of PGE2 and 6-keto-PGFI alpha. in the guinea-pig uterus induced by caffeine appear to involve mechanism(s) different from that which stimulates PGF2 alpha production.

Arachidonic acid and cell signalling in the ovary and placenta

Arachidonic acid (AA) and its metabolites make up a diverse group of signalling molecules important to mediation of metabolic and endocrine function of ovarian and placental cell membranes. This paper reviews recent literature examining AA and eicosanoid involvement in the functional dynamics of follicular development, ovulation and corpus luteum function. The putative roles of AA metabolites in establishment and maintenance of pregnancy are reviewed with reference to decidualization, trophoblast invasion and implantation, maintenance of perfusion of the feto-placental unit and lipid transfer. Finally, recent evidence implicating AA metabolism in mediation of enzyme activity following hormone-receptor coupling within various cells types comprising the placental membranes is reviewed.

Developmental changes in the fatty acids of rat uterus and the influence of dietary essential fatty acids

The effect of age on uterine fatty acid composition was studied in rats fed diets of differing fatty acid composition. Uteri of newly weaned 23-day rats had a higher fatty acid content and a higher proportion of short-chain (less than or equal to C18) fatty acids. Higher incorporation of C less than or equal to 18 fatty acids into neutral lipid (NL) and phospholipid (PL) of young 42-day rats compared with adult 240-day rats was detected. Uterine NL incorporated predominantly C less than or equal to 18 fatty acids which may be an important metabolic energy store in developing uterine tissue. Incorporation of C less than or equal to 18 fatty acids by uterine PL and NL was relatively unselective. In contrast, there was selective retention of arachidonic acid (AA) and docosahexanoic acid (DHA) throughout uterine development. An effect of dietary EFA on uterine n-3 and n-6 EFA was detected in each age group. There was marked retention of uterine AA when dietary supplies of n-6 EFA were low, but the total AA, eicosapentaenoic acid (EPA) and DHA in uterine PL remained constant in the three diet groups, and a constant content of AA, EPA and DHA was maintained throughout uterine development, regardless of diet. The degree of n-3 substitution achieved in this study inhibited uterine release of PG and parturition in adult rats.

The influence of dietary essential fatty acids on uterine C20 and C22 fatty acid composition

The effect of dietary fatty acids on uterine fatty acid composition was studied in rats fed control diet or semi-synthetic diet supplemented with 1.5 microliter/g/day evening primrose oil (EPO) or fish oil (FO). Diet-related changes in uterine lipid were detected within 21 days. Changes of 2- to 20-fold were detected in the uterine n-6 and n-3 essential fatty acids (EFA) and in certain saturated and monounsaturated fatty acids. The FO diet was associated with higher uterine C20 and C22 n-3, and the EPO diet, with higher uterine n-6 fatty acid. High uterine C18:2 n-6 was detected in neutral lipid (NL) of rats fed high concentrations of this fatty acid, but there was little evidence of selective incorporation or retention of C18:2 n-6 by uterine NL. The incorporation of EFA into uterine phospholipids (PL) was greater than NL EFA incorporation, and uterine PL n-3/n-6 ratios showed greater diet dependence. Tissue/diet fatty acid ratios in NL and PL also indicated preferential incorporation/synthesis of C16:1 n-9, and C16:0, and there was greater incorporation of C12:0 and C14:0 into uteri of rats fed EPO and FO. Replacement of 50-60% of arachidonate with n-3 EFA in uterine PL may inhibit n-6 EFA metabolism necessary for uterine function at parturition.

The biosynthesis of the 3-series prostaglandins in rat uterus after alpha-linolenic acid feeding: mass spectroscopy of prostaglandins E and F produced by rat uteri in tissue culture

The abnormal uterine activity associated with dietary n-3 fatty acids may result from competitive inhibition of PG2 production. Uterine synthesis of 2- and 3-series prostaglandins F(PGF) and E(PGE) was studied using mass spectrophotometry in rats fed diets containing predominantly n-3 fatty acid, n-6 fatty acid, or control pelleted diet. Mass spectra of PGF (Me, TMS and Me, TBDMS derivatives) synthesised by uteri of n-3 fed rats were characterised by 8 ions containing the n-3 double bond, and m.i.d. of the 651/653 ions of PGF-Me, TBDMS indicated PGF3 alpha synthesis (44 +/- 8% and 13 +/- 2% of PGF release by uteri incubated + or -5 micrograms/ul calcium ionophore A23187 respectively). In uteri from the control diet group incubated with ionophore, PGF3 alpha ions were detected and PGF 3 alpha represented 9.5 +/- 1.0% of PGF alpha release. Similarly, analysis of PGE from uteri of n-3 fed rats indicated that PGE3 (16 +/- 6% of PGE) was released in the presence of ionophore A23187. Synthesis of 3-series PG by rat uteri was detected after only 3 weeks of n-3 diet. The capacity to synthesise 3-series PG increased at intracellular calcium concentrations which mimicked cell calcium during decidual autolysis at parturition. These experiments suggest that uterine synthesis of 3-series PG is regulated by the specifity of enzymes incorporating fatty acids, rather than by the cyclooxygenase enzyme.

Prostaglandin synthesis by isolated cells of rat uterus: production rates, and effects of indomethacin and histamine

Luminal epithelial and residual cells (mainly of the endometrial stromal tissue) of proestrous rat uteri have been isolated and cultured in defined medium. The prostaglandins produced during a short-term incubation (2 h) in the presence of 10 microM arachidonic acid (to optimize PG production) were determined by direct assay of the culture medium. For the epithelial cells, PGF2 alpha was produced in greatest amounts, followed by 6-keto PGF1 alpha and PGE, while low levels were synthesized by the residual cells. The synthesis of PGF2 alpha by the epithelial cells was inhibited by incorporating indomethacin into the medium and an IC50 value of 2.3 microM was obtained. Incubations performed with histamine in the absence of exogenous arachidonic acid indicated that the pathways for the production of individual prostaglandins were followed to different relative extents, with the production of 6-keto PGF1 alpha being enhanced for both groups of cells when compared to incubations with arachidonic acid.

Effects of estradiol-17 beta and progesterone on the synthesis of prostaglandin F2 alpha, prostaglandin E2 and prostaglandin I2 by fibroblasts from human endometrium in vitro

Estradiol-17 beta increases the production of prostaglandin F2 alpha (PGF2 alpha) in long term monolayer cell cultures of the human endometrium in a dose dependent manner. Progesterone in pharmacological dosage stimulates the syntheses of PGF2 alpha and of prostaglandin E2 (PGE2). The synthesis of prostaglandin I2 (PGI2) is not influenced by sex steroids in long term monolayer cell cultures of the human endometrium.

Oxytocin stimulates the release of arachidonic acid and prostaglandin F2 alpha from human decidual cells

Oxytocin at a physiological concentration stimulated the immediate release of free arachidonic acid from dispersed human decidual cells in a perfusion system. This indicates that oxytocin activates phospholipase(s) thus enhancing prostaglandin synthesis. The effect of oxytocin on the release of [3H]-arachidonic acid from decidual cells of women in labor was significantly greater (1036 +/- 207, mean dpm +/- SEM, n = 23) than from those of women not-in-labor (505 +/- 121 dpm, n = 12) or with endometrial cells of non-pregnant women (711 +/- 210 dpm, n = 18), and correlates well with reported oxytocin receptor concentrations in these tissues. These new findings are consistent with a role for endogenous oxytocin in stimulating prostaglandin synthesis at the onset of parturition.

Role of phospholipase C in mediating oxytocin-induced release of prostaglandin F2 alpha from ovine endometrial tissue

Two experiments were conducted to determine if the ability of oxytocin to stimulate release of prostaglandin (PG)F2 alpha from ovine uterine tissue involved activation of phospholipase C (PLC). In the first experiment, 9 ewes were injected with progesterone for 11 d (12 mg/d, im). On days 11 and 12, ewes received an injection of estradiol (100 micrograms, im). Caruncular endometrial tissue was collected on d 13 and incubated in the presence or absence of oxytocin (10(-6) M). Concentrations of PGF2 alpha and its metabolite, 13,14-dihydro-15-keto-PGF2 alpha (PGFM), in culture media were determined by radioimmunoassay. PLC activity was determined by measuring the intracellular accumulation of 3H-inositol phosphates after preincubation with 3H-inositol. Concentrations of PGF2 alpha and total PGF (PGF2 alpha + PGFM) in culture media were greater for explants treated with oxytocin than for controls (p. less than .02, p less than .06, respectively). A similar effect of oxytocin on intracellular concentrations of 3H-inositol phosphates was observed (p less than .01). A second experiment was conducted to determine if agonists of second messengers, produced by activation of PLC, could stimulate release of PGF2 alpha from ovine endometrial tissue. Seven ewes were treated with progesterone and estradiol as in experiment 1. Explants of caruncular tissue from each ewe were incubated with 1) control medium, 2) A23187 (10(-5) M), 3) oxytocin (10(-6) M), 4) phorbol 12-myristate 13-acetate (PMA, 10(-7) M), 5) PMA + A23187 and 6) PMA + oxytocin. Significant stimulatory effects of oxytocin, PMA and A23187 on concentrations of PGF2 alpha and total PGF in culture media were observed (p. less than .05, p less than .1, p less than .1, respectively). In conclusion, oxytocin stimulated release of PGF2 alpha and activity of PLC in explants of ovine endometrial tissue in vitro. Second messengers associated with activation of PLC enhanced release of PGF2 alpha from ovine endometrial tissue.

Effects of oestradiol, progesterone, hydrocortisone and oxytocin on prostaglandin output from the guinea-pig endometrium maintained in tissue culture

The effects of oestradiol, oxytocin, progesterone and hydrocortisone in vitro on prostaglandin (PG) output from guinea-pig endometrium, removed on days 7 and 15 of the oestrous cycle and maintained in tissue culture for 3 days, have been investigated. Oestradiol (3.7 to 3700 nM) and oxytocin (2 to 200 pM) did not stimulate endometrial PGF2 alpha output, thus not confirming the findings of a previous report (Leaver & Seawright, 1982), nor did they stimulate the outputs of PGE2 and 6-keto-PGF1 alpha. In fact, oestradiol (3700 nM) inhibited the outputs of PGF2 alpha, PGE2 and, to a lesser extent, 6-keto-PGF1 alpha. Progesterone (3.2 to 3200 nM) inhibited the outputs of PGF2 alpha and PGE2; hydrocortisone (2.8 to 2800 nM) had no effect on endometrial PG output. These findings indicate that the inhibitory effect of progesterone on endometrial PG synthesis and release in the guinea-pig is not due to progesterone having a glucocorticoid-like action. Furthermore, progesterone had no effect on 6-keto-PGF1 alpha output, suggesting that the mechanisms controlling endometrial PGI2 synthesis (as reflected by measuring 6-keto-PGF1 alpha) are different from those controlling endometrial PGF2 alpha and PGE2 synthesis.

Estradiol enhances prostaglandin synthase activity in epithelial but not in stromal cells of human endometrium

Exogenous estradiol (E2) has been shown to elevate PGF2 alpha output by explants of human secretory endometrium and in monolayer cultures of glandular epithelial, but not of stromal cells isolated from endometrium. In this study, PGF2 alpha output was measured in each of these cultures in the presence of E2 and the calcium ionophore A23187, added singly or in combination. The ionophore, known to liberate arachidonic acid (AA) by stimulating phospholipase activity, produced a calcium-dependent increase in PGF2 alpha output in the cultures of epithelial cells, whereas greater than additive effects were obtained with mixtures of E2 and A23187. In contrast, PGF2 alpha levels were not elevated by A23187 in the stromal cell cultures even in medium supplemented with CaCl2 or when E2 was added. A calcium-dependent increase in PGF2 alpha output was also observed in fragments of secretory endometrium incubated with A23187. Effects on PGF2 alpha output by endometrial fragments incubated with E2 and A23187 were essentially additive and intermediate between those of the two component cells types. Arachidonic acid produced similar increases in PGF2 alpha output in the epithelial and stromal cell cultures but only in the epithelial cell cultures was there greater utilization of AA in the presence of E2. When mixtures of E2 and AA were added to the cultures of epithelial cells the increase in PGF2 alpha output was 2.5-fold greater than the sum of the increases elicited by E2 or AA alone. In contrast, no enhancement of the AA effect by E2 was observed in the stromal cell cultures. Extrapolation of these results from cell cultures to intact tissue suggests that the epithelium and not the stroma is the primary target for the effects of E2 on PGF2 alpha output by secretory endometrium. The synergistic actions of E2 and either AA, the obligatory precursor of PGF2 alpha, or A23187, an enhancer of AA release from phospholipid stores, point to a stimulatory effect of E2 on prostaglandin synthase activity.

The effect of progesterone on the release of arachidonic acid from human endometrial cells stimulated by histamine

Progesterone at concentrations of 10(-7)M and 10(-8)M inhibits release of [3H]-arachidonic acid from stimulated, perfused, endometrial cells. The effect is independent of the mechanism of stimulation. Cortisol (10(-5)M but not 10(-7)M) has a similar effect in this system but estradiol (10(-7)M) is without effect. There was a positive correlation (p less than 0.05) between the magnitude of inhibition by progesterone and the day of cycle. The inhibitory action of progesterone on the release of arachidonic acid was greater in endometrial cells than in decidual cells and was apparent after fifteen minutes. The activities of commercial and endometrial cell-free preparations of phospholipase A2 and phospholipase C were unaffected by the presence of progesterone. We conclude that progesterone modulates release of [3H]-arachidonic acid from endometrial cells by a rapid, indirect action on phospholipase activity.

Age-, cycle- and topographic dependency of human myometrial prostaglandin (6-keto-PGF1 alpha, PGF2 alpha)-synthesis in vitro

Specimens of human myometrium (isthmus and fundus) freshly obtained at hysterectomy were immediately transferred in ice cold Tyrode solution and placed in superfusion chambers. Spontaneous contractions were recorded, the effluent of the myometrium was analyzed for PGF2 alpha and 6-keto-PGF1 alpha by use of specific radioimmunoassay systems. Dating of the menstrual cycle was achieved by histological evaluation of the endometrium. The PG release rates expressed as ng/min/g wet weight were correlated to the patients age and to the phase of the menstrual cycle. The production rates of 6-keto-PGF1 alpha were negatively correlated to the age of the patients and declined in fundus specimens from 2.89 +/- 0.35 ng/min/g wet weight in 39-42 years old patients to 0.52 +/- 0.17 ng/min/g wet weight in 48-52 years old women during the secretory phase (p less than 0.001). Similar significant correlations were found in specimens obtained from the isthmus uteri. During the proliferative phase fundus specimens produced on average 1.61 +/- 0.67 ng/min/g wet weight in 39-42 years old patients and 0.49 +/- 0.12 ng/min/g wet weight 6-keto-PGF1 alpha in 48-52 years old women respectively (p les than 0.001). The PGF2 alpha synthesis in myometrial specimens of fundus or isthmus origin was significantly lower than 6-keto-PGF1 alpha and did not correlate to the age of the patients during the proliferative phase. However, PGF2 alpha release rates during the secretory phase were significantly (p less than 0.001) higher in younger women. These results suggest an age-, cycle- and topographic dependency of PGI2 synthesis in human myometrial tissue.