LAUSR

Metastatic breast cancer is one of the leading causes of cancer-related mortality among women. However, the complex process of tumor cell dissemination and colonization of distant tissues is incompletely understood and largely incurable using existing therapies. Metastatic disease is not solely driven by tumor cell-intrinsic properties but is instead a consequence of dynamic interactions between cancer cells and other cell types in the tumor microenvironment (TME) including cancer-associated fibroblasts (CAFs). However, CAFs are known to display paradoxical functions across different cancer types, suggesting that heterogeneous CAF subpopulations play diverse or opposing roles in the TME. It was recently shown that periostin, a TGFβ-induced matricellular protein important for collagen cross-linking and extracellular matrix stiffening, can distinguish CAF subtypes in breast cancer. This specific CAF subpopulation is not well characterized, and their functional properties and contributions to disease progression have not been well-studied in vivo. Here, we have used in vivo genetic labelling of periostin+ subpopulations (Postn-Cre:ZSGreenl/s/l) and their progeny to assess their expansion and function during mammary tumor growth and metastasis. We find that ZSGreen-labelled periostin-expressing CAFs arise from mesenchymal-lineage cells and are more abundant in primary tumors, metastatic, and pre-metastatic sites of high-metastatic tumors versus their low-metastatic counterparts. We also find differential collagen organization in high- versus low-metastatic tumors: high-metastatic tumors display longer, aligned collagen fibers whereas low-metastatic tumors display shorter, disorganized fibers. Knocking down periostin in primary human breast CAFs inhibits tumor cell invasion through collagen in a CAF/tumor cell co-culture spheroid assay, indicating that periostin-expressing CAFs may remodel the extracellular matrix via collagen cross-linking to facilitate cell invasion. Finally, we show that periostin/ZSGreen+ cells in highly metastatic tumors are larger and more spindle-shaped, and that periostin knockdown in primary human breast CAFs reduces their size and ability to migrate, suggesting that periostin expression associates with CAF biomechanical properties such as spreading and motility. Our results indicate that highly-metastatic cancer cells mobilize periostin-expressing CAFs in the TME, which may promote collagen remodeling and collective migration of CAFs and cancer cells during tissue invasion. Citation Format: Jamie L. Null, Dae Joong Kim, Yu Zhang, James V. McCann, Thomas H. Barker, Andrew C. Dudley. Fibroblast lineage tracing in metastatic and pre-metastatic microenvironments [abstract]. In: Proceedings of the AACR Virtual Special Conference on the Evolving Tumor Microenvironment in Cancer Progression: Mechanisms and Emerging Therapeutic Opportunities; in association with the Tumor Microenvironment (TME) Working Group; 2021 Jan 11-12. Philadelphia (PA): AACR; Cancer Res 2021;81(5 Suppl):Abstract nr PO051.