Induction of Senescence During Postpartum Mammary Gland Involution supports tissue remodeling and promotes postpartum tumorigenesis

Cellular senescence is an evolutionarily conserved stress response1, yet its roles during physiological processes remain underexplored2,3. Senescent cells are known to promote tissue repair4,5 and suppress tumorigenesis6, but their accumulation contributes to various pathological and physiological processes, including cancer and ageing7-9. However, it is currently unknown whether physiological senescence can be co-opted by oncogenic events to promote tumorigenesis. Postpartum mammary gland involution is a major tissue remodelling event in adulthood10,11, resembling the wound healing process, and is closely linked to postpartum breast cancer (PPBC)12 providing a compelling context to investigate this question. Here, we show that senescence is induced in alveolar luminal cells during involution in a p16-dependent manner. Reducing senescent cells hinders tissue remodeling and delays involution, underscoring their physiological importance. However, using a PPBC mouse model where the oncogenic event coincides with involution, we demonstrate that eliminating involution-associated senescent cells markedly extended the cancer latency. Mechanistically, we reveal that senescent cells enhance tumor-initiating cell plasticity in a paracrine manner, promoting tumor invasion and metastasis. Collectively, our findings uncover a dual role of senescence in mediating postpartum tissue remodeling and promoting tumorigenesis, highlighting a scenario where physiological senescence is hijacked to drive cancer progression. This work underscores that senescence might be a unifying mechanism linking tissue repair to tumorigenesis.