Nuclear receptors for progesterone and estradiol in the guinea pig uterine compartment during gestation

Progesterone receptor expression declines in the guinea pig uterus during functional progesterone withdrawal and in response to prostaglandins

Progesterone withdrawal is essential for parturition, but the mechanism of this pivotal hormonal change is unclear in women and other mammals that give birth without a pre-labor drop in maternal progesterone levels. One possibility suggested by uterine tissue analyses and cell culture models is that progesterone receptor levels change at term decreasing the progesterone responsiveness of the myometrium, which causes progesterone withdrawal at the functional level and results in estrogen dominance enhancing uterine contractility. In this investigation we have explored whether receptor mediated functional progesterone withdrawal occurs during late pregnancy and labor in vivo. We have also determined whether prostaglandins that induce labor cause functional progesterone withdrawal by altering myometrial progesterone receptor expression. Pregnant guinea pigs were used, since this animal loses progesterone responsiveness at term and gives birth in the presence of high maternal progesterone level similarly to primates. We found that progesterone receptor mRNA and protein A and B expression decreased in the guinea pig uterus during the last third of gestation and in labor. Prostaglandin administration reduced while prostaglandin synthesis inhibitor treatment increased progesterone receptor A protein abundance. Estrogen receptor-1 protein levels remained unchanged during late gestation, in labor and after prostaglandin or prostaglandin synthesis inhibitor administration. Steroid receptor levels were higher in the non-pregnant than in the pregnant uterine horns. We conclude that the decreasing expression of both progesterone receptors A and B is a physiological mechanism of functional progesterone withdrawal in the guinea pig during late pregnancy and in labor. Further, prostaglandins administered exogenously or produced endogenously stimulate labor in part by suppressing uterine progesterone receptor A expression, which may cause functional progesterone withdrawal, promote estrogen dominance and foster myometrial contractions.

15-Hydroxyprostaglandin dehydrogenase expression and localization in guinea pig gestational tissues during late pregnancy and parturition

Prostaglandins are key components of the parturition cascade; however, the mechanisms that regulate prostaglandin concentrations in the uterus during pregnancy are largely unknown. The purpose of this study was to determine the intrauterine expression of the chief prostaglandin-inactivating enzyme, 15-hydroxyprostaglandin dehydrogenase (PGDH), during gestation and labor in the guinea pig, an animal model in which the endocrine control of pregnancy and parturition is analogous to that of women. PGDH messenger RNA (mRNA) abundance decreased significantly in the visceral yolk sac membrane (VYS, the anatomical equivalent of the human chorion laeve) and the amnion throughout the last third of pregnancy. PGDH protein was robustly expressed in the VYS epithelium and mesoderm, correlated strongly with PGDH mRNA levels and exhibited a nadir at term prior to labor onset. PGDH protein was not detected in the amnion. PGDH mRNA and protein levels in the placenta and myoendometrium were variable throughout late gestation. In the placenta, PGDH protein was concentrated in the parietal yolk sac membrane (PYS) lining the placental surface and in placental blood vessels. We observed strong expression of PGDH protein in the endometrial epithelium with comparably little expression in the myometrium. These data indicate that metabolic inactivation of prostaglandins in the pregnant guinea pig uterus takes place in the VYS, PYS, and endometrium. Decreased PGDH expression in the fetal membranes may contribute to the increase in intrauterine prostaglandin concentrations at term, stimulating the onset of labor.

New research models and novel signal analysis in studies on preterm labor: a key to progress?

Preterm labor affects up to 20% of pregnancies, is considered a main cause of associated neonatal morbidity and mortality and is responsible for neonatal care costs of multimillion euros. In spite of that, the commercial market for this clinical indication is rather limited, which may be also related to high liability. Consequently, with only a few exceptions, preterm labor is not in the orbit of great interest of the pharmaceutical industry. Coordinated effort of research community may bring the change and help required to reduce the influence of this multifactorial syndrome on society. Between the novel techniques that are being explored in a SAFE (The Special Non-Invasive Advances in Fetal and Neonatal Evaluation Network) group, there are new research models of preterm labor as well as novel methodology of analysis of biological signals. In this article, we briefly describe new clinical and nonclinical human models of preterm labor as well as summarize some novel methods of data processing and analysis that may be used in the context of preterm labor.

Prostaglandin H2 synthase-1 and -2 expression in guinea pig gestational tissues during late pregnancy and parturition

Increased intrauterine prostaglandin (PG) production is crucial for the initiation of parturition. To investigate the mechanisms controlling intrauterine PG synthesis, we examined the expression of the key PG biosynthetic isoenzymes, PG-H2 synthase (PTGS)-1 and -2, in the amnion, visceral yolk sac (VYS), placenta and myo-endometrium of pregnant guinea pigs. This animal model was chosen because the hormonal milieu of pregnancy and the role of PGs in the hormonal control of parturition are similar to those in the human. PTGS1 mRNA abundance, measured by real-time RT-PCR, increased in the amnion and the placenta during the last third of gestation. During labour, PTGS1 mRNA levels decreased precipitously in all four tissues. PTGS1 protein abundance, assessed by immunoblotting, increased to high levels in the amnion and the placenta by the end of pregnancy and remained high during labour. PTGS2 mRNA expression was higher in the placenta than in the other tissues, but did not change before and during labour. PTGS2 protein expression decreased in the placenta and remained low in the other tissues during labour. Immunohistochemistry showed pervasive PTGS1 protein expression in the amnion and strong expression in the parietal yolk sac membrane (PYS) covering the placenta. PTGS2 was expressed in the PYS and the endometrium. The PTGS inhibitor piroxicam, administered in doses that inhibited PTGS1 but not PTGS2, significantly prolonged gestation. These data suggest that PGs generated by intrauterine PTGS1 are involved in the timing of birth in guinea pigs. The induction of PTGS1 in the amnion and the PYS is a critical event leading to labour in guinea pigs and models analogous changes in the human gestational tissues before labour.

Mechanisms responsible for parturition; the use of experimental models

In this review, our knowledge, gleaned from a range of species, of what determines gestation length, how fetal maturation and birth are synchronized and how the uterotonic mechanisms are activated at birth are discussed. Accumulated data indicate that fetal glucocorticoids are involved in, but do not necessarily play a causative role in, the initiation of parturition in eutherian mammals generally. Present observations are consistent with a complex, positive regulatory interaction between estrogens, prostaglandins and oxytocin and are consistent with a role for prostaglandins as the final, common effector in myometrial activation. We are, however, left with the possibility that the initial mechanism for the timing of birth is encoded in the fetal genome and is closely linked to, and activated when, certain prerequisite developmental events have occurred in the fetus. Our understanding of these events in the sheep have led to its extensive use as an experimental model for the study of human clinical correlates of fetal maturation and development and the control of the initiation of parturition.

The fibre dimensions of uterine smooth muscle of the rabbit following treatment by female sex steroids

The effects of female sex hormones on the dimensions of myometrial smooth muscle fibres were studied by using ovariectomized rabbits. After one month of treatment, the fiber dimensions of the outer myometrial layer were measured, using cryostat sections. Calculated smooth muscle fiber volume was found to be in the sequence: control < medroxyprogesterone < estradiol < estradiol + medroxyprogesterone < estradiol alone. The measurements show that medroxyprogesterone-treated uteri contain the narrowest and the longest smooth muscle fibres, while estradiol treatment have the largest cells. This study complements previous observations in showing that medroxyprogesterone alone, or in combination with other modulators, contributes to sustain pregnancy by increasing internal resistance of estradiol-primed myometrial smooth muscle fiber fascicles. Our discussion, based on recent literature, shows that this resistance is ultimately controlled by changes in the myometrium innervation, in the repression of some controlling myofibrillar components, in the expression of specific membrane receptors and ionic channels, and in favoring the switching of molecular connexins in gap junctions, making P paramount in maintaining pregnancy. Moreover, other recent observations have also shown that probably an hcG-like hormone actually control P receptors expression in myometrial smooth muscles.

Transfer of [3H]estrone-[35S]sulfate across guinea pig fetal membranes

The possible role of fetal membrane deconjugating activity in the movement of a charged steroid conjugate between fetal and maternal compartments was investigated. The ability of amnion and chorion laeve to transfer [3H]estrone-[35S]sulfate was assessed in both orientations of guinea pig tissue at 45 days and near parturition. While early amnion was impermeable, late tissue transferred approximately 50% (w/w) of the substrate in a bidirectional process that was non-saturable and independent of either deconjugation or ATP. Transfer across early chorion was similar to late amnion. Saturation curves from each tissue were superimposable, as were those of the time course. Transfer across both early and late chorion proceeded in the absence of deconjugation, with no effect of tissue orientation or ATP depletion. However, late chorion exhibited a decrease in estrone-sulfate transfer, as verified by concentration dependency and time course analyses, though transport across the tissue remained non-saturable. The results in amnion were congruous with the presence and absence of tight junctions in the epithelium of early and late tissue, respectively. However, sulfoconjugate transfer across early chorion proceeded in the presence of a paracellular barrier, suggesting specialized regulation of the transport process which extended late into gestation.

Beta-glucuronidase is not required for transfer of [3H]-estrone-[14C]glucuronide across guinea pig fetal membranes

To understand the means whereby a charged, estrogen conjugate may be transferred across guinea pig amnion and chorion, the permeability to [3H]estrone-[14C]glucuronide was examined at 45 days and near term. No evidence of deconjugation was obtained in either early or late amnion, despite significantly greater transfer near term. Early amnion was virtually impermeable, regardless of ATP depletion. In contrast, early chorion transferred estrone-glucuronide without any requirement for deconjugation or ATP. No effect of tissue orientation was observed in amnion; whereas, incubations from maternal to fetal side of late chorion exhibited beta-glucuronidase activity. Inhibition of the latter demonstrated that hydrolysis was concomitant with but not required for transport. [3H]Estrone produced by deconjugation was enzymatically reduced after pubic symphysis relaxation, although beta-glucuronidase activity began prior to this stage. Transport across late fetal membranes was not saturable and chorion incubations from maternal to fetal side demonstrated a lower transport capacity. In either tissue orientation, late chorion displayed a lower rate of transfer than amnion. These results indicate that fetal membranes possess distinct abilities for transferring intact estrone-glucuronide, depending on stage of development and tissue orientation. The passive nature of transport and its dependence on structural characteristics is consistent with possible regulation of tight junctions.

Transfer of steroidal and nonsteroidal compounds across guinea pig fetal membranes

Transfer of steroidal and nonsteroidal compounds across guinea pig amnion and chorion laeve was investigated as a function of stage of gestation, tissue orientation, steroid specificity, and molecular size. Each fetal membrane was examined at early and late stages of gestation, before and after pubic symphysis relaxation. Early amnion was impermeable to macromolecules and small charged molecules while [3H]estrone and [3H]pregnenolone were transferred, the latter depending on tissue orientation and involving conjugation at the basolateral interface. After symphysis dilation, amnion transferred all substrates tested with the exception of BSA; the molecular weight cutoff was approximately 5,000. Unlike amnion, early chorion transferred both free and conjugated steroids as well as inorganic sulfate. Transfer of estrone involved conjugation and depended on tissue orientation. Transfer of [3H]estrone-sulfate, [3H]estrone-glucuronide, and [3H]pregnenolone-sulfate was similar despite selective deconjugating activity toward estrone-sulfate. Near term, chorion was impermeable to inorganic sulfate and transfer of estrone-glucuronide depended on tissue orientation, involving deconjugation in the maternal to fetal direction. At no stage of gestation did chorion transfer macromolecules. These results suggest that the transfer of free and conjugated steroids across fetal membranes is differentially regulated by tissue, its stage of development, and direction of transfer.

The effect of chorion-uterine interaction upon free progesterone metabolism during advanced gestation in the guinea pig

The in vitro fate of [3H]progesterone was studied after incubation with guinea pig tissues at 58/59 days (before pubic symphysis relaxation) and in the final week (post relaxation) of gestation. Buffered steroid was added to the fetal surface of chorion attached to the uterus or to the uterus alone. Neither the amount of recovered progesterone nor its metabolites (6.2% average conversion) differed between the two stages when only uterus was incubated. With chorion present, conversion averaged 28.3% at 58/59 days and 63.4% at the late stage. A 4.6-fold decrease in progesterone concentration, and 3.0-, 2.4- and 3.1-fold increases in the concentrations of 3alpha-hydroxy-5alpha-pregnan-20-one, 3beta-hydroxy-5alpha-pregnan-20-one and 5alpha-pregnane-3,20-dione, respectively, were found in the uterus in the late stage vs 58/59 days. Also, 2.8- and 6.4-fold decreases in progesterone concentration occurred in the myometrium and endometrium, respectively, from 58/59 days to the late stage. In endometrium, the concentrations of the 3alpha- and 3beta-isomers, and 5alpha-pregnane-3,20-dione, increased 2.6-, 2.6- and 5.0-fold, respectively. The above changes were all significant at P < 0.05 or better. Changes in the 3alpha-, 3beta- isomers and dione in the myometrium were not significant. The chorion-uterine interaction and gestation time thus affect the degree of progesterone conversion, and the amounts of metabolites reaching the uterus in the chorion-uterine in vitro system.

Sulfation by guinea pig chorion and uterus: differential action towards estrone and estradiol

The activities of estrogen sulfotransferase, estrogen sulfatase and estradiol 17beta-dehydrogenase change considerably in the guinea pig uterine compartment during gestation. This study was undertaken to inquire if the chorion membrane could influence the pattern of estrogen resulting when substrates were applied to the fetal surface of the chorion while it was attached, late in gestation, to the uterine wall. This tissue system resulted in a differential handling of estrone and estradiol. Estrone was largely excluded from the tissue, remaining mainly in free steroidal form. Estradiol was considerably converted to its 3-sulfate which was mainly retained by the chorion. Parallel experiments with chorion and uterus separately failed to discriminate between the two substrates. Hydrolysis of estrone sulfate and estradiol 3-sulfate was similar in all three tissue systems. It appears that the interaction of chorion with uterus late in gestation causes a difference in tissue action towards the two steroid substrates of closely related structure. The results suggest a limitation in tissue uptake of estrone compared with estradiol, or a much greater sulfotransferase activity towards estradiol. Whole cytosols of late gestational chorion catalyzed sulfation of estradiol at about double the velocity of estrone. This may only partly account for the difference in the intact chorion-uterine tissue system.

Microscopic and biochemical analysis of the viability and permeability of guinea pig amnion and chorion leave in vitro

Tissue viability and permeability of guinea pig amnion and chorion leave were analyzed microscopically and biochemically. The vital dyes T1111 and fluorescein diacetate were used to locate and determine the integrity of cell plasma membranes in early and late tissue in vitro using confocal laser scanning microscopy and scanning electron microscopy. Early amnion and chorion laeve were each found to contain a single epithelial cell layer, composed of membrane-intact cells. In contrast, plasma membrane lesions were present throughout the epithelium of late amnion. Late chorion laeve contained both regions of intact and damaged epithelial cells on its maternal side. There was also a layer of membrane-intact squamous cells on the fetal side of late chorion laeve. ATP measurements confirmed that early fetal membranes were viable after incubation in isotonic salt solutions at physiological pH. Late amnion was depleted of ATP stores while late chorion laeve retained its capacity for generating energy. These viability markers indicate that late guinea pig amnion is not a viable tissue in vitro, while late chorion laeve is a viable but probably degenerating tissue. Confocal X-Z scans were used to trace the movement of T1111 through the tissue as an indication of permeability to free solutes. Whereas dye will permeate across the main thickness of early amnion and chorion leave, it did not pass between cells, but was blocked, presumably by a line of tight junctions. Late amnion was characterized by the complete permeability to T1111. Late chorion leave contained regions where solute migration was blocked, but overall was a permeable tissue. These results provide an important context for the interpretation of molecular movement across fetal membranes.

Myometrial estrogen and progesterone receptor binding in pregnancy: inhibition by the detergent action of phospholipids

We characterized the phospholipid inhibition of estradiol and progesterone binding to guinea-pig and human myometrial receptors. Of twelve compounds studied, phosphatidylinositol (PI), lysophosphatidic acid and lysophosphatidylcholine (lyso-PC) were the most active inhibitors (50% inhibition at 10(-5) M). Lyso-PC with fatty acid chain length C14:0 inhibited ligand binding both to estrogen receptor (ER) and progesterone receptor (PR), C16:0 only to PR and C18:0 neither to ER nor to PR. The lyso-derivates were more inhibitory than the parent compounds. The ionic detergent (sodium taurocholate) inhibited both ER and PR binding, but the non-ionic detergent (Triton X-100) only PR. Triton X-100 enhanced the PI-induced inhibition of ER binding by a factor of 10. PR was more sensitive to inhibition than ER in all cases. The type of inhibition was non-competitive. At term pregnancy, ligand binding to myometrial ER or PR was low or absent in humans, but moderate in the guinea-pig. Phospholipid extracts of human decidua and fetal membranes contained PI and phosphatidylserine rather than lyso-PC. The extract was a potent inhibitor of ligand binding to PR (50% inhibition at 10(-6) M phospholipid phosphorus), but not to ER. The physicochemical environment, modulated by phospholipids acting as detergents, may regulate sex steroid function also in vivo. This might have special significance for pregnancy maintenance.

Progesterone metabolism by guinea pig intrauterine tissues

Progesterone metabolism by guinea pig amnion, chorion, myometrium, and endometrium was studied at the following gestational stages. Day 45 represents mid-gestation, about 5 days before strong chorion interaction between the entire surface of the chorion and the uterus; days 57-58, 1-2 days after chorion attachment, and 2-3 days before the onset of pubic symphysis relaxation; days +1-+6, 1-6 days after the onset of pubic symphysis relaxation, i.e. within 1 week of parturition. The high metabolic activity of chorion exceeded that by amnion at all stages. Metabolism by endometrium and myometrium was always low. Conversion of progesterone by amnion significantly decreased (P < 0.05) between days 57-58 and days +1-+6. Progesterone metabolites produced by chorion and amnion were identified by TLC, HPLC, and capillary GC/MS. Both tissues converted progesterone to three major products during 60-min incubations. These were 5 alpha-pregnane-3,20-dione, 3 alpha-hydroxy-5 alpha-pregnan-20-one, and 3 beta-hydroxy-5 alpha-pregnan-20-one. The metabolite pattern differed between the two tissues. Three-minute incubations with chorion resulted in a significantly higher proportion of 3 alpha-hydroxy-4-pregnen-20-one (P < 0.01) and 5 alpha-pregnane-3,20-dione (P < 0.025) than at 60 min. The production of 3 beta-hydroxy-5 alpha-pregnen-20-one by chorion decreased (P < 0.05) between days 50-51 and 57-58. The ratio of 3 alpha-hydroxy-5 alpha-pregnan-20-one to 3 beta-hydroxy-5 alpha-pregnan-20-one increased (P < 0.05) between days 45 post-relaxation. The marked conversion of progesterone by chorion, or the formation of one or more of its metabolites, may serve to influence uterine function prior to delivery.