Chemotherapy treated-metastatic cells harness macrophages to support metastatic outgrowth

Metastatic breast cancer can appear years after mastectomy and chemotherapy treatments and represents the principal cause of breast cancer related deaths. These recurring lesions arise from disseminated tumor cells (DTCs) that lay dormant (quiescent) and resist chemotherapy treatments. Recent studies demonstrate that chemotherapeutic drugs can induce pro-metastatic activities at distant sites leading to metastatic recurrence. However, the mechanisms underlying this pro-metastatic activity are largely unknown. We hypothesized that different chemotherapeutic treatments of metastatic breast cancer cells will harness macrophages (Mφ) to promote or repress the outgrowth of dormant DTCs. To test our hypothesis, we utilized Mφ cell line; RAW 264.7 and D2A1 mammary cancer cell line that display a transient dormant phase in a 3D BME system that model’s tumor dormancy and outgrowth and in vivo. D2A1 cells were treated with either Doxorubicin (DOX) or Paclitaxel (PTX) to induce their apoptosis. Our results demonstrate no significant difference in the efferocytosis capacity of the macrophages engulfing PTX/DOX treated D2A1 cells. However, macrophages engulfing PTX/DOX treated D2A1 cells displayed distinct gene signature determined by RNA-seq and NMDS analysis. Furthermore, Go-term enrichment analysis among the differentially expressed genes displayed enrichment for genes related to defense and immune response in Mφ engulfing DOX-treated D2A1 cells opposed to Mφ engulfing PTX-treated D2A1 cells. Furthermore, enrichment in genes related to fibroblasts proliferation and wound healing was evident in Mφ engulfing PTX-treated D2A1 cells. These results were further corroborated by our findings demonstrating that soluble mediators present in the conditioned media of Mφ engulfing PTX-treated D2A1 cells promoted the formation of a fibrotic-like-niche in vitro, whereas conditioned media of Mφ engulfing DOX treated-D2A1 cells prevented the formation of a fibrotic-like niche. Notably, formation of a fibrotic-like niche was previously shown to induce the outgrowth of dormant DTCs. These findings suggest that macrophages engulfing chemotherapy-eradicated metastatic cells will be reprogrammed to distinct entity that may promote or repress the permissive microenvironment of dormant DTCs. The reprogramming of the macrophages is depended on the chemotherapy induced apoptotic signal. We are currently elucidating the identity and mechanism of action of the pro-fibrotic mediators and exploring whether these mediators will promote dormant DTCs outgrowth.

Significance: Identification and characterization of the pro-metastatic mediators secreted by these newly reprogrammed Mφ may serve as a 1) surrogate marker to follow potential recurrence of the disease after chemotherapy treatment 2) potential novel therapeutic target(s) to prevent the pro-metastatic effect of the chemotherapy treatment. This may pave the way to design complimentary treatments to the chemotherapy treatments to prevent breast cancer from ever recurring.