The Role of Vitamin D Metabolism in the Timing of Birth

Preterm birth (PTB, <37 weeks of gestation) is the leading cause of neonatal mortality and morbidity. Both term and preterm labor are associated with increased levels of proinflammatory cytokines within fetal and maternal reproductive tissues. Uterine quiescence is maintained throughout most of pregnancy by increased circulating progesterone (P4) and enhanced progesterone receptor (PR) activity, which silence expression of proinflamﾬmatory mediators and contraction-associated genes. To identify novel genes that maintain uterine quiescence during pregnancy and promote the initiation of term and preterm labor, our lab conducted RNA sequencing of myometrium from timed-pregnant mice at 15.5-18.5 days post-coitum (dpc) and during labor (19.0 dpc). Novel genes of interest were identified through transcriptome profiling and validated by quantitative real-time PCR. Cyp27b1 was one of the most highly downregulated transcripts in pregnant mouse myometrium at 18.5 dpc and in-labor, compared to 15.5 dpc. Cyp27b1 1α-hydroxylase, the key enzyme in synthesis of the bioactive form of vitamin D, calcitriol, which binds to the vitamin D receptor (VDR), a member of the steroid/nuclear receptor family, which was also downregulated at term. Calcitriol/VDR mediate anti-inflammatory actions in various tissues; calcitriol synthesized by human placenta, decidual macrophages and uterine natural killer cells was reported to regulate maternal immunologic tolerance to the hemi-allogeneic fetus during pregnancy. The effect of P4 and of the PR antagonist, RU486, on Cyp27b1/VDR mRNA expression was analyzed. We observed that P4 treatment of timed-pregnant mice caused a significant increase in myometrial CYP27B1 mRNA levels compared to time-matched controls in labor at term. In RU486-treated mice, CYP27B1 mRNA decreased signifi-cantly 8 hours post-injection and remained significantly reduced during preterm labor, compared to vehicle-injected mice. Based on these collective findings, we postulate that CYP27B1 and VDR are key P4/PR target genes in the pregnant myometrium that act cooperatively with P4/PR to maintain myometrial quiescence via their anti-inflammatory actions. Thus, the decline in PR function near term, accompanied by a parallel decline in CYP27B1/VDR, permit increased inflammatory gene expression leading to myometrial contractility and labor. We hypothesize that calcitriol may provide a safe and effective treatment for prevention of PTB.