Prolactin coordinates with the ovarian
steroids to orchestrate mammary development and lactation, culminating in nourishment and an increasingly appreciated array of other benefits for neonates. Its central activities in mammary epithelial growth and differentiation suggest that it plays a role(s) in
breast cancer, but it has been challenging to identify its contributions, essential for incorporation into prevention and treatment approaches. Large prospective epidemiologic studies have linked higher
prolactin exposure to increased risk, particularly for ER+
breast cancer in postmenopausal women. However, it has been more difficult to determine its actions and clinical consequences in established
tumors. Here we review experimental data implicating multiple mechanisms by which
prolactin may increase the risk of
breast cancer. We then consider the evidence for role(s) of
prolactin and its downstream signaling cascades in
disease progression and treatment responses, and discuss how new approaches are beginning to illuminate the biology behind the seemingly conflicting epidemiologic and experimental studies of
prolactin actions across diverse breast
cancers.