Abstract 983: RNA-seq analysis of murine liver to identify breast cancer metastatic potential during liver involution

Background: Postpartum breast cancer (PPBC), variably defined as breast cancer diagnosed within 5-10 years after a patient’s last pregnancy, has overall increased risk of metastasis even after controlling for patient age, year of diagnosis, and tumor subtype and stage. Metastasis to the liver is specifically increased in PPBC compared to non-PPBC cases and likely contributes to the overall poor prognosis of PPBC. Our lab has previously shown that the rodent liver undergoes physiological remodeling after pregnancy, in a process reminiscent of weaning-induced mammary gland involution. Further, rodent models confirm that breast cancer cells delivered to the involuting liver grow more robustly than tumor cells delivered to nulliparous liver. Our rodent data suggest that weaning induced liver involution may be responsible for increased tumor cell seeding and ultimately higher rates of liver metastasis observed in women diagnosed with PPBC. Since weaning-induced liver involution is a recently recognized physiology, little is known of its molecular underpinnings. Here we employ RNA-seq of murine livers to better understand how reproductive status influences liver physiology with a focus on exploring the pro-metastatic niche formed after weaning. Study design: Livers were isolated from age-matched mice at the following reproductive time points: nulliparous (never pregnant), lactation, involution days 2, 4, 6, and 8, and fully regressed (4 weeks post-wean) and flash frozen for RNA isolation (n=3-6 mice per group). RNA-seq was utilized to identify gene expression differences between reproductive groups. We employed single-sample gene set enrichment analysis (ssGESA) to identify key pathways modulated by reproductive state. Results: We identify clear reproductive control of liver physiology. We find increases in proliferation, MTORC1, and anabolic metabolism pathways during lactation when compared to any other reproductive state. In the transition from lactation to involution, we find increases in apoptotic pathways, and a switch to catabolic metabolism. Correspondingly, during involution we see enrichment in pro-cancer pathways including increased TGFβ signaling, Epithelial to Mesenchymal Transition, and Inflammatory Responses Hallmark Pathways. Within the immune signatures changes of involution, we observe myeloid-derived cell signatures including enrichment in M2 associated genes, and increases in cytotoxic and helper T cell signatures that parallels a signature for disease-specific T cell exhaustion. Conclusion: This work highlights novel regulation of liver physiology by reproductive state, and finds pathways regulated during the natural physiological process of involution that have been previously identified as contributing to metastatic niches. This study has the potential to identify new targets for the prevention and treatment of breast cancer liver metastasis. Citation Format: Michelle K. Ozaki, Alexandra Q. Bartlett, Zheng Xia, Pepper Schedin. RNA-seq analysis of murine liver to identify breast cancer metastatic potential during liver involution [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 983.