Estrogen receptor-α expression in the mammary epithelium is required for ductal and alveolar morphogenesis in mice

TLDR

Estrogen receptor‑α (ERα) is a key transcription factor driving epithelial proliferation and ductal morphogenesis in the postnatal mammary gland, yet previous knockout studies suggested a stromal requirement that may have obscured its epithelial role. This study generated a Cre‑lox conditional knockout of ERα to investigate its function specifically within the mammary epithelium at distinct developmental stages. Epithelial‑specific deletion of exon 3 via MMTV‑Cre in virgin mice caused severe impairment of ductal elongation and side‑branching. Loss of ERα in the epithelium eliminated PR expression, disrupted ductal and alveolar architecture, reduced alveolar progenitor proliferation, impaired milk production, and led to offspring malnourishment, demonstrating that epithelial ERα is essential for ductal morphogenesis during puberty and alveologenesis during pregnancy and lactation.

Abstract

The estrogen receptor-α (ERα) is a critical transcription factor that regulates epithelial cell proliferation and ductal morphogenesis during postnatal mammary gland development. Tissue recombination and transplantation studies using the first generation of ERα knockout (ERKO) mice suggested that this steroid hormone receptor is required in the mammary stroma that subsequently exerts its effect on the epithelium through additional paracrine signaling events. A more detailed analysis revealed that ERKO mice produce a truncated ERα protein with detectable transactivation activity, and it is likely that this functional ERα variant has masked the biological significance of this steroid receptor in the mammary epithelium. In this article, we describe the generation a Cre-lox-based conditional knockout of the ERα gene to study the biological function of this steroid receptor in the epithelial compartment at defined stages of mammary gland development. The mouse mammary tumor virus (MMTV)-Cre-mediated, epithelial-specific ablation of exon 3 of the ERα gene in virgin mice severely impaired ductal elongation and side branching. The conditional knockout resulted in ablation of the ERα protein, and the progesterone receptor (PR), whose expression is under the control of ERα, was largely absent. The whey acidic protein (WAP)-Cre-mediated deletion of ERα during successive gestation cycles resulted in a loss of ductal side-branching and lobuloalveolar structures, ductal dilation, and decreased proliferation of alveolar progenitors. These abnormalities compromised milk production and led to malnourishment of the offspring by the second lactation. These observations suggest that ERα expression in the mammary epithelium is essential for normal ductal morphogenesis during puberty and alveologenesis during pregnancy and lactation.