Defective mammary gland morphogenesis in mice lacking the progesterone receptor B isoform

TLDR

Progesterone signals through two nuclear receptors, PR‑A and PR‑B, whose overlapping and distinct transcriptional activities have unclear individual physiological roles. The authors sought to elucidate PR‑A’s physiological role by generating a mouse model lacking PR‑B. They produced PRBKO mice with selective ablation of PR‑B expression. Selective activation of PR‑A in PRBKO mice produces normal ovarian and uterine responses but markedly impairs mammary gland morphogenesis, with reduced ductal branching and lobuloalveolar development due to decreased epithelial proliferation and survival, and the defects are linked to failure of PR‑A to stimulate RANKL signaling, suggesting that PR‑A modulation could protect against uterine and mammary hyperplasias.

Abstract

Progesterone (P) regulates female reproduction via two nuclear receptors, PR-A and PR-B. Although both receptors display overlapping and distinct transcription regulatory properties, their individual physiological roles are unclear. To address the physiological role of PR-A, we generated a mouse model in which expression of PR-B was specifically ablated (PRBKO – / – ). We show that selective activation of PR-A in PRBKO – / – mice is sufficient to elicit normal ovarian and uterine responses to P but results in reduced mammary gland morphogenesis. In the absence of PR-B, pregnancy-associated ductal sidebranching and lobuloalveolar development are markedly reduced due to decreased ductal and alveolar epithelial cell proliferation and decreased survival of alveolar epithelium. In an effort to elucidate the molecular genetic signaling pathways that are differentially regulated by PRs in the mammary gland, we have identified receptor activator of nuclear factor κB ligand (RANKL) as a paracrine mediator of P-dependent alveologenesis. Further, we demonstrate that the defects in PRBKO – / – mice are associated with an inability of PR-A to activate the RANKL signaling pathway in response to P. Our data indicate that functional interaction between PR-A and PR-B is not required for reproductive activity and that selective modulation of PR-A activity by progestin agonists may have a protective effect against both uterine and mammary gland hyperplasias.