Risk-Window Palbociclib Exposure Delays ErbB2-Driven Mammary Tumorigenesis and Suppresses Mammary Epithelial Cell Stemness.

ErbB2 overexpression occurs in 15-20% of invasive breast cancer (BC) and inactivation of Cyclin D1-CDK4/6 axis reduce mammary stem/progenitor cells in ErbB2-driven tumorigenesis. Here, we explored the preventive role of short-term palbociclib intervention in MMTV-ErbB2 mice. Palbociclib significantly inhibited the proliferation and stemness of ErbB2-overexpressed BC cells in vitro and in vivo. Furthermore, short-term palbociclib exposure during the early premalignant risk window significantly delayed mammary tumor development and reduced tumor multiplicity, accompanied by the suppression of epithelial proliferation and ductal/alveolar morphogenesis in premalignant tissues, as well as prolonged tumor-free survival compared with controls. Importantly, palbociclib reduced the luminal epithelial (CD24 high /CD49f low ), mammary reconstitution unit-enriched (CD24 high /CD49f high ) subpopulations, and luminal progenitor/TIC-enriched (CD61 high /CD49f mid ) subpopulations. These changes were accompanied by diminished mammary epithelial cell stemness functions, including colony-forming, mammosphere-forming, and 3D growth activities of mammary epithelial cells. Mechanistically, palbociclib-treated tissues showed inhibition of the Cyclin D1-CDK4/6-RB-E2F axis and coordinated attenuation of ER-, ErbB2- and Wnt/β-catenin-associated signaling. Together, we demonstrate that short-term CDK4/6 inhibition during a premalignant risk window produces a sustained delay in ErbB2-driven mammary tumorigenesis, associated with remodeling of the mammary epithelial hierarchy and suppression of proliferative and stem/progenitor-associated activity, suggesting CDK4/6 inhibition as a strategy for ErbB2-positive BC prevention.