Breast cancer diagnosed up to 6 years after a completed child-birth has an increased risk of metastatic recurrence and death, which is independent of current prognostic factors. These pregnancy-associated breast cancers (PABC) account for 45% of all young women's breast cancers and have a 10 year survival rate of 44% compared with 69% for non-PABC. After parturition or lactation, a physiologically normal tissue remodeling program is activated that mimics wound healing and inflammation, including high matrix metalloproteinase -2, -3, and -9 activities, release of bioactive fragments of FN and LN, deposition of fibrillar collagen, and increased cytokine levels. We hypothesize that the normal physiologic, but pro-inflammatory remodeling of breast during involution accounts for increased breast cancer metastasis of PABC.To demonstrate that the human breast is characterized by a pro-inflammatory microenvironment with wound-healing characteristics, formalin fixed breast biopsy tissues were obtained for women who were age-matched nulliparous, pregnant, lactating, whose breast tissue was actively involuting at the time of biopsy or who were > 5 years from a completed pregnancy. Using a novel imaging program, over 1500 images of human breast tissue have been quantitated for CD45 (common leukocyte antigen) and CD68 (macrophage lysosomal marker). CD45 and CD68 staining in the human breast demonstrates 4 times more staining in involuting compared to pregnant tissue. Further, CD45 and CD68 staining returns to baseline level in the fully regressed breast. Further characterization of the macrophages in involution demonstrates an M2 phenotype, similar to immune suppressive tumor associated macrophages. Likewise, increased collagen deposition is present with involution in comparison to the other tissue subgroups, which may help promote immune cell recruitment.To test for the impact of involution on invasion and metastasis of PABC, we have developed the MCF-10DCIS human cell line into a murine model for PABC. In this model, the cancer cells will form in situ ductal carcinoma like lesions that progress over time with loss of their myoepithelial barrier and subsequent invasion present. Orthotopic injection of the MCF-10DCIS cells into SCID mice that whose mammary glands were nulliparous versus actively involuting following timed weaning demonstrated an increased incidence of tumor of 78% involution group versus 50% in the nulliparous group. Tumor size was increased 4 fold and presence of invasive lesions increased 2 fold for the involution group. Moreover, increased lung metastasis that was independent of the primary tumor size was seen in the involuting mice compared with nulliparous controls, isolating the event of involution (independent of the potential potentiating effect of the pregnancy state on the tumor) as the driving feature for the metastatic increase.These data demonstrate for the first time that human mammary gland involution is characterized by a pro-inflammatory microenvironment, implicating physiologic tissue inflammation in the poor prognosis of PABC. Likewise, in a murine model for PABC, involution is associated with increased tumor invasion and metastasis.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 50.