Signaling by Hypoxia-Inducible Factors is Critical for Ovulation In Mice. Kim J et al. The steroid hormone progesterone, acting via its nuclear receptor, is a major regulator of the process of ovulation. Female mice lacking progesterone receptor (PGR) exhibit an anovulatory phenotype due to failure in follicular rupture. To identify the PGR-regulated pathways that control ovulation, we analyzed global changes in gene expression in the ovaries of wild type (WT) and Pgr-null mice subjected to gonadotropin-induced superovulation. Our analysis uncovered several genes whose expression was reduced in the Pgr-null ovaries compared with the WT ovaries immediately preceding ovulation. Interestingly, these genes included three hypoxia-inducible factors (HIFs): HIF-1alpha, HIF-2alpha, and HIF-1beta. These transcription factors form alphabeta heterodimers, which regulate the transcription of specific cellular genes, thereby mediating adaptive response of the tissue to low oxygen levels. We observed that the expression of mRNAs and proteins corresponding to HIF-1alpha, HIF-2alpha, and HIF-1beta was induced in a PGR-dependent manner, specifically in the granulosa cells of the preovulatory follicles. Inhibition of the HIF transcriptional activity by echinomycin, a small-molecule inhibitor that suppresses the binding of HIF alphabeta heterodimers to target genes, blocked ovulation by preventing the rupture of the preovulatory follicles. Echinomycin specifically inhibited the expression of genes that are known regulators of ovulation, such as a disintegrin and metalloproteinase with thrombospondin-like motifs-1 (ADAMTS-1) and endothelin-2. Furthermore, echinomycin reduced the expression of vascular endothelial growth factor A, a key factor controlling vascularization/angiogenesis during ovulation. Collectively, these findings unveiled a novel ovarian role for the HIF transcription factors during the ovulatory period in mice.