Estrogens profoundly influence the physiology and pathology of reproductive and other tissues. Consequently, emphasis has been placed on delineating the mechanisms underlying regulation of estrogen levels. Circulating levels of estradiol in women are controlled by follicle-stimulating hormone (FSH), which regulates transcription of the aromatase gene (CYP19A1) in ovarian granulosa cells. Previous studies have focused on two downstream effectors of the FSH signal, cAMP and the orphan nuclear receptor steroidogenic factor-1 (NR5A1). In this report, we present evidence for ␤-catenin (CTNNB1) as an essential transcriptional regulator of CYP19A1. FSH induction of select steroidogenic enzyme mRNAs, including Cyp19a1, is enhanced by ␤-catenin. Additionally, ␤-catenin is present in transcription complexes assembled on the endogenous gonad-specific CYP19A1 promoter, as evidenced by chromatin immunoprecipitation assays. Transient expression and RNAi studies demonstrate that FSH-and cAMP-dependent regulation of this promoter is sensitive to alterations in the level of ␤-catenin. The stimulatory effect of ␤-catenin is mediated through functional interactions with steroidogenic factor-1 that involve four acidic residues within its ligand-binding domain, mutation of which attenuates FSH͞cAMP-induced Cyp19a1 mRNA accumulation. Together, these data demonstrate that ␤-catenin is essential for FSH͞cAMP-regulated gene expression in the ovary, identifying a central and previously unappreciated role for ␤-catenin in estrogen biosynthesis, and a potential broader role in other aspects of follicular maturation.ovary ͉ steroidogenic factor-1 ͉ granulosa cells ͉ steroidogenesis ͉ estrogen E strogens play a central role in regulating homeostatic and pathologic pathways. They influence fertility and sexual behavior, lipid metabolism, bone remodeling, and the development of various endocrine cancers (1). Ovarian follicles are the primary source of local and circulating estrogen in mammals (2). Synthesis of follicular estradiol depends on the coordinated actions of the pituitary gonadotropin hormones, folliclestimulating hormone (FSH) and luteinizing hormone, cytokines, and growth factors (3, 4).FSH induces estrogen biosynthesis by triggering cAMPdependent signaling cascades to regulate transcription of the CYP19A1 gene . This gene encodes the cytochrome P450 enzyme aromatase, which catalyzes the irreversible conversion of androgens to estrogens (5). The CYP19A1 gene contains multiple promoters that dictate tissue-specific patterns of aromatase expression (6). In the ovary, expression of CYP19A1 is directed by the type II promoter (PII) that resides within the immediate 5Ј region flanking the translational start site (7). PII is also pathologically activated in adipocytes in malignant breast tissue (8). Therefore, delineating mechanisms that contribute to follicular expression of CYP19A1 is essential for understanding how estrogen levels are regulated in health and disease.Among the numerous cis-acting elements identified within the CYP19A1...