Inhibition of oestradiol-induced DNA synthesis by opioid peptides in the rat uterus

Effect of estrogen and inhibition of phosphatidylinositol-3 kinase on Akt and FOXO1 in rat uterus

The importance of FOXO transcription factors in regulating different aspects of cellular homeostasis and apoptosis has become apparent. Akt/protein kinase B has been shown to phosphorylate and inactivate members of FOXO family of transcription factors. Akt and its upstream regulator, phosphatidylinositol-3 kinase (PI3K) are involved in rapid action of estrogen (E2) in different cells and tissues. The aim of the present study was to analyze the E2/PI3K/Akt/FOXO pathway in rat uterus. In response to E2, phosphorylation of Akt/PKB on Ser473 and FOXO1 on Ser256 and Thr24 residues increased but with distinct kinetics, regulating the activation and inactivation of Akt and FOXO1 proteins, respectively. The antiestrogen ICI 182,780 prevented E2 induced Akt activation suggesting that estrogen receptors mediate this effect of E2. Intrauterine injection of Wortmannin caused a decrease in the phosphorylation of Ser473 of Akt, and attenuated phosphorylation of its downstream target FOXO1 at Ser256 and at Thr24. However, the effect of E2 on phosphorylation of Thr24 showed a kinetic pattern distinct from that of Ser256. Our results suggest that the E2/PI3K/Akt/FOXO1 pathway in rat uterus is functioning even at the lack of ovarian hormones and responses to E2 treatment. Estradiol increases Akt phosphorylation through a Wortmannin sensitive way, presumably involving PI3K. The present work shows that PI3K plays a crucial role in the phosphorylation and inactivation of FOXO1 in vivo, indicating that the regulation of this transcription factor is a more complex event in uterine cells requiring further investigations.

Developmental changes in the inhibition of cultured rat uterine cell proliferation by opioid peptides

Opioid peptides are negative regulators of cell proliferation in several organs including the uterus. In the present study, the ontogeny of the direct inhibitory action of opioid peptides on the proliferation of cultured rat uterine cells was investigated. Uteri of 7, 14, 21, 28, 35 and 60-day-old rats were removed in a sterile way. Tissue blocks were dispersed by limited digestions with trypsin and collagenase. Cells were cultured in enriched Dulbecco's modified Eagle's medium (DMEM). Treatments were present during the entire culture period. Cell densities of the monolayers were determined by counting the cells following trypsinization and trypan blue exclusion. Rat uterine mixed cell cultures grew to confluence within 10 days. The average population doubling time gradually increased with the age of animals. Epidermal growth factor (EGF) increased cell densities of cultures from all age groups. The oestradiol (E2)-responsiveness appeared at 21 days of age. The effect of [D-Met2-Pro5]-enkephalinamide (ENK) was biphasic. ENK and [Met5]-enkephalin (OGF) decreased cell densities of both unstimulated and EGF-stimulated cultures from 7-day-old rats to the same extent. ENK failed to act in 14-day-old animals. From 21 days of age on, the E2- or EGF-stimulated proliferation was inhibited only by ENK and DAMGO, while 30 nm DPDPE, Dynorhin-A, OGF, [Leu5]-enkephalin, beta-endorphin, and morphiceptin were ineffective. The half-inhibitory concentration of ENK was 0.3 nm. The effects of ENK were prevented by concomitant treatment with naloxone. Our novel data demonstrate two different phases of the inhibitory action of opioid peptides on rat uterine cell proliferation during ontogeny with an insensitive interval in between.

Anti-mitogenic action of opioid peptides on epidermal growth factor-stimulated uterine cells

Endogenous opioid peptides are negative regulators of estradiol-induced uterine cell proliferation. To investigate the possible molecular target site(s) of their anti-mitogenic action, we examined the effect of opioid peptides on epidermal growth factor-induced cell proliferation both in uterine primary cell cultures prepared from adult rats and in human myometrial smooth muscle cell lines. Epidermal growth factor (EGF) significantly increased cell density in both types of cultured monolayers. This EGF-induced stimulation of cell proliferation was blocked by [D-Met(2)-Pro(5)]enkephalinamide in a time-dependent, receptor-mediated manner. The effective concentrations were within the physiological nanomolar range. Enkephalinamide did not have any effect on the basal rate of proliferation of the uterine cells. Our results on this novel physiological cross-talk suggest that shared step(s) of the mechanism of action of estradiol and EGF might be targeted by opioid peptides and not the general machinery of cell proliferation.

Antiestrogenic effect of opioid peptides in rat uterus

The effects of a single injection or continuous infusion of opioid peptide, [D-Met(2),pro(5)]enkephalinamide (ENK) on the hormone binding and transcriptional properties of estrogen receptors were investigated in estradiol (E(2)) treated rat uterus. The level of estrogen- (ER) and progesterone receptor (PR) proteins, the hormone binding of E(2) receptors and the effects of single injection of ENK with or without naltrexone (NAL) on the E(2)-induced changes in the level of Fos and Jun proteins and the binding of AP-1 proteins to DNA were studied. The receptor proteins levels were determined by Western blots and the binding of AP-1 to DNA by electrophoretic mobility shift assay. Both the ER and PR protein concentrations and the [3H]Estradiol binding to the high affinity nuclear receptors decreased after ENK treatment during the first two days. At 72 h the PR concentration decreased further, while no significant changes were found in the level of ER, however, at this time the former competitive E(2) binding turned into positive cooperativity. The E(2)-induced increase in the level of Fos proteins and the binding of AP-1 proteins to DNA was inhibited by a single injection of ENK. We conclude that the endogenous opioid peptides may interact with E(2) in the gene regulation of rat uterus.

Epidermal growth factor influenced by opioid peptides in immature rat uterus

The aim of the present experiment was to investigate the effect of [D-Met2,Pro5] enkephalinamide (ENK) implantation on the development of the uterus during 8-33 days of age and the involvement of epidermal growth factor (EGF) in the effect. Administration of ENK was attained by osmotic minipumps (5 microg/h) implanted intraperitoneally. ENK resulted in a decrease in the EGF content of the uterus, which was already significant after 48 h of the implantation. The DNA content 24 and 48 h after the treatment decreased, no change at 72 h was found, however the protein/DNA ratio on the effect of ENK treatment was significantly decreased at this time in all examined age groups. High affinity and lower capacity competitive naloxone binding sites were demonstrated in the membrane fraction of the uteri. Seventy-two h after ENK treatment the binding capacity of these sites significantly dropped. The present results suggest a novel multiple interaction between estrogen and two probably paracrine hormones, EGF and opioid peptide, in the regulation of growth and development of the uterus.

Regulation of activator protein-1-DNA binding activity by opioid peptides in estrogen-sensitive cells of rat hypothalamus and uterus

The present studies demonstrate, for the first time, that the binding of activator protein-1 (AP-1)-DNA in rat uterus and the estrogen-sensitive areas of the hypothalamus, as measured by electrophoretic mobility shift assay, is increased 2 h after intraperitoneal injection of [D-Met(2),Pro(5)]enkephalinamide. The effect was prevented by the opiate antagonist naltrexone given 30 min before the administration of [D-Met(2),Pro(5)]enkephalinamide, suggesting the involvement of opioid peptide receptors in the observed effects. The present findings support the role of opioid peptides in the regulation of transcription in estrogen-sensitive cells.

Opioid peptides inhibit the estradiol-induced proliferation of cultured rat uterine cells

The effect of opioid peptides on estradiol-induced cell proliferation in adult rat uterine primary cell cultures was studied. Estradiol increased cell density by 40%. This estradiol-induced stimulation of cell proliferation was decreased to control values by [D-Met2,Pro5]enkephalinamide. The opioid-induced inhibition of uterine cell proliferation was blocked completely by the specific opiate antagonist naloxone, while naloxone did not have any effect on its own. The inhibition of cell proliferation by enkephalinamide was apparent at each stimulatory estradiol concentration examined. This opioid effect was mediated mainly by the mu opiate receptor. The observed effects occurred within the physiological nanomolar concentration range. Enkephalinamide did not have any effect on the basal proliferation rate of adult rat uterine cells. However, enkephalinamide inhibited the basal rate of cell proliferation in cell cultures prepared from 7-day-old immature rats. In summary, here we present evidence of novel physiological direct cross-talk between the opioid and estrogenic signaling systems in the regulation of normal uterine growth.

Opioids regulate cell proliferation in the developing rat uterus: effects during the period of sexual maturation

The present studies demonstrate, for the first time, that the rate of DNA synthesis in rat uterus of 21-32 days of age is inhibited by opioid peptides [D-Met2, Pro5]enkephalinamide. At around the time of vaginal opening (approximately 33 days) the opioids failed to act. High-affinity nuclear [3H]naloxone binding sites with linear Scatchard plots were detected in the uteri during the opioid-sensitive periods of DNA synthesis. Characteristics of these binding sites and the opioid sensitivity of uterine DNA synthesis are dependent on the age of the animals, the level of circulatory oestradiol and/or the maturity of the nuclear oestrogen receptor system.

Role of opioid peptides in the regulation of DNA synthesis in immature rat uterus

The effects of a single dose of naloxone and of [D-Met2,Pro5]enkephalinamide on the DNA synthesis in the uterus of 7, 14 and 21-day-old rat were studied. After [D-Met2,Pro5]enkephalinamide treatment, an age-dependent decrease in in vitro [3H]thymidine incorporation into DNA was observed in all studied age groups. In the 21-day-old age group a reduced rate of DNA synthesis was detected for 12 h after [D-Met2,Pro5]enkephalinamide treatment followed by the return to control values at 24 h. The rate of inhibition was more marked in the younger age groups. The effect was also more pronounced in younger animals. Specific [3H]naloxone binding was detected both in membrane and nuclear fractions of uterine homogenates. While no age-related changes in binding affinities were found, the number of binding sites varied characteristically during development. Our data suggest the novel involvement of opioid peptides and their receptors in the regulation of uterine development.

Changes of [3H]naloxone binding in oestrogen stimulated rat uterus

Effects of a single dose of oestradiol (Oe) on [3H]naloxone (Nal) binding in ovariectomized rat uterus were studied. Specific [3H]Nal binding was assessed by saturation analysis in 800 g supernatants and pellets of uterine homogenates. Two binding sites with higher (Kd approximately 1 nM) and lower affinity (Kd approximately 15 nM) for Nal were observed, their binding capacities and affinities have changed after Oe treatment in a time-dependent manner. The high affinity binding sites, detected only in the cytoplasmic fraction, disappeared after 1 h and only became detectable again at 24 h after hormone treatment, the lower affinity binding sites, after an initial drop, slowly increased, peaking at the 9th hour of hormone injection. The competition experiments indicate the involvement of different opiate receptor subpopulations in Oe induced changes. In the nuclear fraction, the Bmax values started to increase at 15 h, reaching the highest level at 18 h. The Kd values of lower affinity sites, in both studied compartments, were increased, i.e. the affinity decreased in the second half of the examined period.

Role of endogenous opioids in progesterone antagonism on oestradiol-induced DNA synthesis in the rat uterus

To probe the possible involvement of endogenous opioid peptides (EOPs) in progesterone (PG) antagonism on oestradiol-17 beta-(OE) induced uterine cell proliferation, the opioid antagonist naltrexone hydrochloride (NTX) and anti-[Met5]-enkephalin antiserum (AME) were investigated for their effect on uterine DNA synthesis in ovariectomized rats pretreated with OE and PG 24 h before killing. As an index of DNA synthesis the rate of in vitro incorporation of [3H]thymidine ([3H]TdR) into DNA was measured. The inhibitory effect of PG on OE-induced DNA synthesis could be diminished by approximately 42 and approximately 20% by the NTX treatments given directly into the uterine horns 13 and 4h before killing, respectively. Intraluminal AME treatments were only effective when they were administered 13 h before decapitation. Systemic blockade of opioid receptors by intraperitoneal NTX injections given every 6 h to the OE + PG-treated rats did not result in the disinhibition of uterine [3H]TdR incorporation. Our results suggest the involvement of EOPs--including [Met5]-enkephalin--as autocrine/paracrine factors in the PG antagonism on OE-induced uterine DNA synthesis.