Abstract 4074: Macrophages promote the progression of premalignant mammary gland lesions through activation of the Axl signaling cascade

Abstract

Infiltrating inflammatory cells, including tumor-associated macrophages (TAMs), have been shown to promote breast cancer cell invasion and have been correlated with metastasis and poor prognosis. While it is well-established that macrophages are recruited to the invasive fronts of established tumors to exert pro-tumor signals, their role in premalignancy remains poorly understood. Using a novel p53−/− syngeneic transplantable model of premalignancy, we show that inflammatory cells, including macrophages, are indeed recruited to early lesions prior to invasion. Microarray analysis was performed on two different pre-invasive lines, termed PN1a, (high tumor-forming potential) and PN1b (low tumor-forming potential). Interestingly, several pro-inflammatory chemokines, macrophages markers, and the pro-inflammatory cytokine growth arrest specific 6 (Gas6) were increased in PN1a lesions as compared to PN1bs. In a 3-D co-culture system, macrophages were recruited to PN1a acini and induced invasion, while PN1b cells remained non-invasive, supporting our in vivo data. Depletion of macrophages in vivo with clodronate containing liposomes significantly delayed progression of PN1a lesions to invasive cancer, indicating a critical role for macrophages in early progression. Gas6 and its receptor tyrosine kinase, Axl, were highly expressed in PN1a preinvasive lesions, but decline in invasive tumors, suggesting that this pathway is a key driver of the transition from pre-invasive to invasive cancer. Further studies using our 3-D co-culture system with Gas6−/− macrophages demonstrated that macrophages promote the formation of non-polar disorganized structures by activating Axl. Finally, transplantation of PN1a cells into Gas6 +/− mice resulted in a delay in tumor formation, indicating that stromal-derived Gas6 (and potentially macrophage derived) may promote the progression of early stage lesions through the paracrine activation of Axl. As a plethora of Axl inhibitors have been developed and are currently in clinical trial, these studies have critical implications for the prevention and treatment of invasive breast cancer. Citation Format: Emily C. Carron, Heather L. Machado, Samuel Homra. Macrophages promote the progression of premalignant mammary gland lesions through activation of the Axl signaling cascade. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4074.