Contrasting action of antiestrogen (ICI-182780) for preventing initiation of embryo implantation by estradiol or epidermal growth factor (EGF)

Delayed Implantation Induced by Letrozole in Mice

Implantation timing is critical for a successful pregnancy. A short delay in embryo implantation caused by targeted gene ablation produced a cascading problem in the later stages of the pregnancy. Although several delayed implantation models have been established in wild mice, almost none of them is suitable for investigating the early delay's effects on the late events of pregnancy. Here, we report a new delayed implantation model established by the intraperitoneal administration of letrozole at 5 mg/kg body weight on day 3 of pregnancy. In these mice, initiation of implantation was induced at will by the injection of estradiol (E2). When the estradiol (3 ng) was injected on day 4 of pregnancy (i.e., without delay), the embryo implantation restarted, and the pregnancy continued normally. However, 25 ng estrogen caused compromised implantation. We also found that 67% of the female mice could be pregnant normally and finally gave birth when the estradiol injection (3 ng) was on day 5 of pregnancy (i.e., 1-day delay). Most failed pregnancies had impaired decidualization, decreased serum progesterone levels, and compromised angiogenesis. Progesterone supplementation could rescue decidualization failure in the mice. Collectively, we established a new model of delayed implantation by letrozole, which can be easily applied to study the effect and mechanisms of delay of embryo implantation on the progression of late pregnancy events.

Identification of estrogen-regulated genes in the mouse uterus using a delayed-implantation model

Gonadal steroid hormones are known to modulate the implantation of the blastocyst, but how the controlling genetics are regulated remains largely unknown. Using a delayed-implantation model, we examined estrogen-regulated genes (ERGs) in the mouse uterus using the differential-display reverse transcription-polymerase chain reaction (DD RT-PCR). Pregnant mice were ovariectomized and injected daily with progesterone (P, 1 mg/mouse), followed by a single injection of estrogen (E, 200 ng/mouse); 24 or 48 hr later, total RNA was extracted from the uterus. Reverse Northern analysis verified the expression patterns of 36 clones out of thousands of RNA species. Only five clones had mRNA levels that were modified, whereas other mRNAs were unchanged or not detectable. Sequence analysis of these, using the Basic Local Alignment Search Tool (BLAST) service, revealed that four of these clones were novel; one clone, designated ERG10, was found to be the mouse homologue of that deleted in oral cancer DOC-1. DOC-1 mRNA was detected all tissues examined, but only in the uterus and cervix was markedly increased 12 hr after E administration, it returned to basal level by 48 hr. One of the novel genes, designated ERG8, had three different forms of mRNAs and was expressed ubiquitously in all examined tissues. In the uterus, the mRNA level of ERG8 also increased 12 hr after E administration. These results suggest that during the implantation process, E differentially regulates several genes depending on cell type. Uterine-specific induction of newly found genes, such as ERG8 and 10, by E appears to be important for the early implantation process.

A phorbol ester that activates protein kinase C mimics the action of estradiol or epidermal growth factor for initiating embryo implantation in the delayed implanting hypophysectomized rat

In rodents an action of estrogen is required for the initiation of implantation of the blastocyst into the endometrium of a progesterone-primed uterus. Thus removal of endogenous estrogen, either directly by ovariectomy or indirectly by hypophysectomy, prevents implantation in the pregnant rat. In the present study, delayed implanting hypophysectomized progesterone-primed rats were used to test the efficacy of cyclic adenosine 3',5'-monophosphate (cAMP), epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I), which are agents that have been shown to mimic some uterine actions of estradiol, to initiate implantation. In confirmation of previous studies, EGF injected into the uterine lumen plus intravenously was effective at initiating implantation in all animals. IGF-I showed no such activity in this model system. Cyclic AMP, increased via direct activation of adenylyl cyclase by forskolin, or administration of sodium dibutyryl cAMP, did not initiate implantation. However, a ligand for protein kinase C (PKC), phorbol 12-myristate 13-acetate, was effective in augmenting the action of intrauterine EGF, or forskolin, for initiation of implantation. A phorbol ester that does not activate PKC was ineffective. The results provide circumstantial evidence for the requirement of PKC activity in the implantation initiating action of estrogens.