Levonorgestrel Inhibits Embryo Attachment by Eliminating Uterine Induction of Leukemia Inhibitory Factor

Progestogens including progesterone (P₄) and levonorgestrel (LNG) are clinically used for multiple purposes such as contraception and infertility treatment. The effects of progestogens on the uterus remains to be elucidated. Here we examine the effect of excessive progestogen administration on embryo implantation focusing on the function of uterine leukemia inhibitory factor (LIF), a cytokine that is induced by estrogen and essential for embryo attachment. Treatment of wild-type (WT) female mice with vehicle (control), LNG at the dose of 300 μg/kg/day and P₄ at the dose of 10 mg/day from day 1 to day 4 of pregnancy was conducted. LNG-treated and P₄-treated mice showed embryo attachment failure on day 5 of pregnancy (The rate of mice with embryo attachment sites [%MAS], 11% and 13%, respectively), while all the control mice had normal attachment sites. Uterine LIF expression was significantly reduced in LNG-treated and P₄-treated mice on day 4 evening. Administration of recombinant LIF (rLIF) at the dose of 24 μg/day on day 4 significantly rescued embryo attachment failure in LNG-treated and P₄-treated mice (%MAS, 80% and 75%, respectively). Estradiol (E₂) administration also rescued embryo attachment failure in LNG-treated mice (%MAS, 83%). Furthermore, excess P₄ treatment before implantation decreased decidual P₄ receptor (PGR) expression and induced decidualization defect apart from LIF downregulation. These findings indicate that progestogens cause embryo attachment inhibition through downregulation of uterine LIF expression and compromised decidualization through downregulation of PGR independently of LIF reduction. This study may contribute to a better understanding of contraceptive action of progestogens.