Abstract SY38-02: Postpartum tissue remodeling drives breast cancer metastasis
Authors: Betts Courtney , Pennock Nathan , Goddard Erica , Guo Qiuchen , Jindal Sonali , Borges Virginia F. , Schedin Pepper Jo , Quackenbush Alex 
Published: August 2018

Abstract
Women diagnosed with breast cancer within 10 years of a completed pregnancy are at ~2-3 fold increased risk for developing metastatic disease (1,2). This increased risk for metastasis is independent of tumor stage and biologic subtype, implicating a tumor-extrinsic biology unique to the postpartum woman (3,4). Further, 30-50% of all young women diagnosed with breast cancer have had a completed pregnancy within the last 10 years (4), making a postpartum diagnosis an independent poor prognostic indicator for a significant number of patients. One postpartum event that may drive tumor progression in rodents (5-8) and women (9) is weaning-induced mammary gland involution, a physiologic tissue remodeling process that shares numerous attributes with protumorigenic wound healing. We have developed rodent models designed to mimic this highly metastatic subset of young women's breast cancer and find that weaning-induced mammary gland involution supports breast cancer growth, dissemination, and escape to secondary organs in a COX-2 dependent manner (6,10,11). Here we extend these studies by investigating the mammary gland from a mucosal organ perspective, as identifying links with mucosal biology and its associated immune suppression may provide insight into the poor prognosis of postpartum breast cancers. While the mammary gland is not classically considered a mucosal organ, bacteria routinely interface with luminal epithelium during lactation, necessitating barrier function. Similar barrier defenses are anticipated during weaning-induced mammary gland involution, as milk stasis increases risk for mastitis. One key immunologic hallmark of a mucosal organ is the presence of a distinct CD4+ helper T-cell subset, Th17 cells. Th17 cells stimulate epithelial cell junction integrity and epithelial secretion of mucins and defensins, mechanisms that barricade against bacterial activation of antitumor Th1 immune cells. Within the mammary gland we find a baseline mucosal program of Th17 T cells, which is elevated within lactating and involuting mammary glands. Mucosal features are further expanded during involution to include tolerogenic dendritic cell phenotypes, barrier-supportive antimicrobials, alternatively activated Th2 CD4+ cells, and immunosuppressive Treg CD4+T cells. Further, during involution, we find suppression of mammary-derived antigen-dependent CD4+ T-cell activation, data consistent with immune tolerance. We also find antigen-independent accumulation of memory Th17- Treg CD4+ T cells, data consistent with weaning-induced mammary inflammation. Overall, these data elucidate strong mucosal immune programs within the lactating gland and immune-suppressive programs within the involuting gland. Tumor cells within the involution immunologic microenvironment are anticipated to avoid immune detection and readily escape the mammary gland, a mechanism that may contribute to increased metastasis observed in postpartum breast cancer patients. Importantly, to successfully metastasize, a tumor cell must not only escape from the primary tumor, but also seed and grow within the secondary site. Thus, an additional potential mechanism by which a postpartum diagnosis may confer increased risk of metastasis is through involution-specific changes at metastatic sites. We report evidence for such a metastatic advantage within the liver of postpartum hosts. Using both intracardiac and portal vein liver metastasis models, we find that the post-weaning liver supports increased seeding of murine mammary tumor cells compared to livers of nulliparous control mice (12). Relevance to women is suggested by data obtained from a cohort of young women's breast cancer patients, where liver metastatic tropism is observed specifically in postpartum patients. In rodents, the biology driving this metastatic advantage is weaning-induced liver involution, a previously unrecognized tissue remodeling process. Post weaning, we find a 50% reduction in liver volume, hepatocyte cell death, deposition of fibrillar collagen and tenascin-c, increased matrix metalloproteinase activity, and influxes of immune cell populations with immune-suppressive phenotypes (12,13). These tissue-level changes are consistent with the establishment of a prometastatic niche during liver involution. Combined, our studies are supportive of the mammary gland being a hormone-responsive, immune-suppressed mucosal organ, and suggest the liver as a hormone-responsive organ that is functionally coordinated with the mammary gland during the cycle of pregnancy, lactation, and weaning. Further, we identify stromal remodeling in the mammary gland and liver as a mediator of breast cancer metastasis in the postpartum window. These findings shed light on how normal reproductive physiology, specifically mucosal biology interfaced with tissue remodeling, alters site-specific metastasis. These findings provide the framework necessary to investigate postpartum involution as a target for the prevention of breast cancer metastasis in young women.References:
1. Johansson AL et al. Increased mortality in women with breast cancer detected during pregnancy and different periods postpartum. Cancer Epidemiol Biomarkers Prev 2011;20(9):1865-72.
2. Lambe M et al. Transient increase in the risk of breast cancer after giving birth. N Engl J Med 1994;331(1):5-9.
3. Schedin P. Pregnancy-associated breast cancer and metastasis. Nat Rev Cancer 2006;6(4):281-91.
4. Callihan EB et al. Postpartum diagnosis demonstrates a high risk for metastasis and merits an expanded definition of pregnancy-associated breast cancer. Breast Cancer Res Treat 2013;138(2):549-59.
5. Guo Q et al. Physiologically activated mammary fibroblasts promote postpartum mammary cancer. JCI Insight 2017;2(6):e89206.
6. Martinson HA et al. Wound healing-like immune program facilitates postpartum mammary gland involution and tumor progression. Int J Cancer 2015;136(8):1803-13.
7. McDaniel SM et al. Remodeling of the mammary microenvironment after lactation promotes breast tumor cell metastasis. Am J Pathol 2006;168(2):608-20.
8. O'Brien J et al. Alternatively activated macrophages and collagen remodeling characterize the postpartum involuting mammary gland across species. Am J Pathol 2010;176(3):1241-55.
9. Jindal S et al. Postpartum breast involution reveals regression of secretory lobules mediated by tissue-remodeling. Breast Cancer Res 2014;16(2):R31.
10. Lyons TR et al. Cyclooxygenase-2-dependent lymphangiogenesis promotes nodal metastasis of postpartum breast cancer. J Clin Invest 2014;124(9):3901-12.
11. Lyons TR et al. Postpartum mammary gland involution drives progression of ductal carcinoma in situ through collagen and COX-2. Nat Med 2011;17(9):1109-15.
12. Goddard ET et al. The rodent liver undergoes weaning-induced involution and supports breast cancer metastasis. Cancer Discov 2017;7(2):177-87.
13. Goddard ET et al. Quantitative extracellular matrix proteomics to study mammary and liver tissue microenvironments. Int J Biochem Cell Biol 2016;81(Pt A):223-32.
Citation Format: Courtney Betts, Nathan Pennock, Erica Goddard, Qiuchen Guo, Sonali Jindal, Virginia F. Borges, Pepper Jo Schedin, Alex Quackenbush. Postpartum tissue remodeling drives breast cancer metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr SY38-02.