Abstract CN13-03: Postnatal breast involution as a highly targeted window for breast cancer prevention

Epidemiologic studies demonstrate a dual effect of pregnancy on breast cancer risk. Women with early age first pregnancy experience reduced breast cancer risk in their post‐menopausal years compared to nulliparous women; hence the protective effect of pregnancy. However, pregnancy transiently increases risk of breast cancer for at least ten years following parturition, thus young parous women have higher risk of breast cancer than age‐matched nulliparous women. In addition to increasing breast cancer incidence in young women, pregnancy can also negatively impact prognosis, with relative risk for death reported between 1.0–2.2, depending on age of first pregnancy. We recently proposed two distinct subtypes of PABC; breast cancer diagnosed during pregnancy and breast cancer diagnosed postpartum. This distinction is important because emerging epidemiologic data indicates worsened outcomes specific to postpartum cases. We hypothesize that postpartum PABC has poor prognosis because mammary gland involution promotes progression and metastasis of preexisting disease . Characterization of postpartum gland involution has identified tissue remodeling programs associated with wound healing, inflammation and immunosuppression including fibrillar collagen deposition, high MMP−2, −3 and−9 activities, ECM matrix proteolysis, infiltration of alternatively activated macrophages similar to tumor associated macrophages, and high levels of the M2/immunosuppressive cytokines IL‐4 and IL‐13. Thus, the postpartum involuting microenvironment, while physiologically normal, shares similarities with cancer microenvironments known to be tumor promotional. Using mammary ECM isolated from actively involuting or quiescent nulliparous glands as substratum in cell culture studies, ECM from involuting glands was found to promote breast cancer cell motility and invasion. Using two orthotopic murine xenograft models of pregnancy associated breast cancer (PABC), we demonstrate that MCF10A.DCIS cells form tumors that are larger and more metastatic when transplanted into an actively involuting mammary gland then into quiescent mammary glands of nulliparous mice. Tumors that develop in the involuting microenvironment are more likely to express the proliferative marker Ki67, to overexpress the inflammatory enzyme COX‐2 and the basal tumor cell marker p63, and to be infiltrated with thick taut collagen fibers. These data demonstrate that the promotional effects of involution on PABC can be separated from the anticipated tumor promotional effects of gestation; independently identifying the microenvironment of the involuting gland as a key determinant of tumor growth and invasiveness. To address whether targeting involution with non‐steroidal anti‐inflammatory agents aspirin or ibuprofen could reduce the tumor promotional attributes of postpartum involution, rats were treated with 200 mg/kg dose of aspirin or 30 mg/kg dose of ibuprofen starting on the day of weaning and continuing for 6 days post weaning. Drug treatments did not inhibit normal alveolar regression that occurs with involution. However, mammary ECM isolated from involuting glands of rats treated with aspirin or ibuprofen was found to phenocopy tumor‐suppressive nulliparous mammary ECM in 3D culture of transformed mammary epithelial cells. These data suggest that aspirin and ibuprofen reduce the pro‐inflammatory microenvironment of postpartum involution consistent with tumor suppression. To evaluate whether targeting involution could reduce the tumor promotional effects of involution in vivo, mice were treated with ibuprofen starting on the day of weaning and continued for 2 weeks, such that treatment was limited to the involution window. MCF10A.DCIS cells were injected into the involuting glands one day post weaning and tumor growth characterized for 6 weeks. Average tumor growth was decreased in mice with ibuprofen treatment during involution compared to the control‐involution group. Further, tumor size in the ibuprofen‐involution group was reduced to similar tumor size as observed in the control‐nulliparous group. These data demonstrate that postpartum mammary gland involution is characterized by a pro‐inflammatory microenvironment, implicating physiologic tissue inflammation in the poor prognosis of PABC. Likewise, in two new murine models for PABC, involution is associated with increased tumor invasion and metastasis. Finally, these data identify the window of postpartum gland involution as a potential target for chemoprevention. Citation Information: Cancer Prev Res 2010;3(1 Suppl):CN13-03.