Abstract C30: The CSF1-PU.1 pathway in tumor associated macrophages promotes breast cancer growth and progression

Purpose of Study: Research over the past decade has established a critical role for the tumor microenvironment in facilitating tumor growth and promoting invasiveness. Cells of the myeloid lineage, including macrophages, have been known to be key mediators of tumor progression and facilitate metastasis. Recent work from our lab highlights the Ets transcription factors, PU.1 and ETS2, as important players in tumor associated macrophages (TAMs) during breast cancer progression. Both transcription factors are downstream effectors of the Colony Stimulating Factor 1 (CSF1) signaling pathway, which is not only important for myeloid cell survival, but also has a well-established role within the tumor microenvironment. Here, we investigated the requirement of PU.1 in TAMs in a mouse mammary tumor model and have begun to address the role of PU.1 during metastasis. Research Method: We employed cre-loxP technology to conditionally delete PU.1 in the myeloid cell compartment. Syngeneic mice were orthotopically injected with mammary tumor cells directly into the fat pad. To model breast cancer brain metastasis in the mouse, we employed an intracranial injection model which recapitulates the biology of the human disease. We were able to isolate tumor associated macrophages from either site (mammary or brain tumor) for downstream applications. Novel Findings: Macrophage-specific deletion of PU.1 resulted in a significant reduction in mammary tumor growth as well as tumor angiogenesis. Our results suggest that ETS2 and PU.1, acting downstream of the CSF1 signaling pathway, cooperatively regulate the expression of pro-tumor genes along with ‘oncogenic’ microRNA. Conventional ChIP assays show that both transcription factors occupy enhancer regions adjacent to a set of selected genes and cooperatively regulate expression in tumor infiltrating macrophages. We focused on miR-21 and miR-29a for a more detailed investigation as both are expressed in CSF1R+ myeloid cells within the brain metastatic microenvironment. In our intracranial injection model, macrophages (F4/80 positive cells) are recruited to the growing tumor, and both miR-21 and miR-29a are upregulated in TAMs isolated by FACS from tumor-bearing mice as compared to controls. Conclusions and Implications: Given our present data, we believe that the Ets factors, PU.1 and Ets2, regulate a transcriptional program, including microRNAs, that is critical for CSF1 action in tumor infiltrating macrophages. Currently, clinical trials are underway using CSF1R inhibitors for several solid tumor types [clinicaltrials.gov]. The macrophage-specific microRNA, including miR-21 and miR-29a, may serve as indicators for to the efficacy of CSF1R inhibition. Citation Format: Katie Thies, David A. Taffany, Haritha Mathsyaraja, Sudarshana M. Sharma, Walter Hans Meisen, Tom Liu, Cynthia Timmers, Jose Otero, Balveen Kaur, Michael C. Ostrowski. The CSF1-PU.1 pathway in tumor associated macrophages promotes breast cancer growth and progression. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr C30.