Abstract A046: Liver involution generates a pro-metastatic niche in a murine model of postpartum breast cancer liver metastasis
Authors: Ozaki Michelle K , Zhang Yi , Xi Zheng , Schedin Pepper 
Published: February 2024

Abstract
Postpartum breast cancer (PPBC), cases diagnosed within 10 years of childbirth, has increased risk of liver metastases. Our lab has shown that the liver undergoes growth during pregnancy and lactation to support milk production, and upon weaning, undergoes involution. We have previously shown the actively involuting liver supports breast cancer liver metastasis in rodent models of PPBC, yet the mechanisms are poorly understood. Here, we performed RNAseq analysis in mouse livers across a lactation/wean cycle, and identify stromal alterations that may contribute to the “involution-educated” metastatic niche. During involution, RNA seq analysis reveals increased signatures for apoptotic cell death, catabolic metabolism, regulatory immune cell abundance, and extracellular matrix remodeling consistent with fibroblast activation. Using liver perfusion methods in live animals, we isolated viable stromal cells from livers of nullip or involution mice and validated these cells as highly enriched for fibroblasts by quantitative PCR. Liver fibroblasts were then mixed 1:1 with mouse mammary tumor cells, and injected into host mice at the flank site. We found fibroblasts from involuting livers generated larger tumors (n=28 tumors/group p=0.013), with decreased caspase 3 staining. Further, involution fibroblast tumors had increased ECM deposition as measured by trichrome staining, consistent with involution liver fibroblasts being more activated. Combined these data suggest that involution liver fibroblasts support tumor growth by suppressing tumor cell death, and by providing a pro-tumor collagen matrix when compared to nulliparous liver fibroblasts. These data implicate fibroblasts as components of the involution-educated metastatic niche in the postpartum liver. RNAseq analysis of involuting livers also identified gene signatures correlated with immature myeloid cells that associate with immune suppression and worse prognosis in numerous cancers. We thus evaluated for immature myeloid cells by multiplex immunohistochemistry in healthy mouse livers across a reproductive cycle, and identified increased abundance of immature myeloid cells (CD45+, CD11b+, Gr-1+) specifically during involution (n=6/group, p&lt;0.05). Further, our data suggest liver involution may have a durable impact on immune composition of breast cancer liver metastases, as we find evidence for increased immature myeloid cells (CD45+, CD11b+, CD33+, CD68-, CD66b-, Cd56-, CD11c-, CD20-) in liver metastases of PPBC patients compared to non-PPBC patients. These data implicate immature myeloid cells as another component of the involution-educated liver metastatic niche. In sum, we find physiological involution of the liver post-wean alters the liver stromal microenvironment in a manner consistent with establishing a metastatic niche. Our findings may lead to the identification of liver stromal targets that can be exploited for prevention and or treatment strategies for PPBC liver metastasis.
Citation Format: Michelle K Ozaki, Yi Zhang, Zheng Xi, Pepper Schedin. Liver involution generates a pro-metastatic niche in a murine model of postpartum breast cancer liver metastasis [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A046.