No abstract

Tumor progression is characterized by persistent death of cancer cells, which are cleared by the innate immune system phagocytes. This process termed efferocytosis is enhanced with targeted therapies that induce apoptosis in tumor cells. Recent findings suggest that efferocytosis polarizes macrophages into M2-type and may induce tumor acceleration; however, this mechanism and its consequences in prostate cancer tumor fate remains largely unknown. Here the changes in cytokine expression (mRNA and protein) in primary bone marrow derived mouse macrophages (Mφ) interacting with two apoptotic prostate cancer cells (human PC3 and mouse RM1) were analyzed. In response to efferocytosis, Mφ produced known tumor-promoting inflammatory cytokines including IL-6, CXCL1 and CXCL5 that are chemoattractants of monocytes/macrophages and neutrophils. In vitro efferocytosis induced the activation of NF-κb signaling in Mφ as analyzed by Western blot and functional luciferase reporter assays (TRACER). Inhibition of NF-κb with the chemical compound emetine (0.3 μM) blocked the efferocytosis of fluorescence-labeled apoptotic cancer cells and the expression of the inflammatory marker Ly-6B by Mφ, suggesting a crucial role of NF-κb in the efferocytic function of Mφ. An in vivo syngeneic tumor model was used in which apoptosis-inducible prostate cancer cells (RM1-iCasp9) were injected in vertebral bodies and implanted subcutaneously in immune competent mice. Seven days post-surgery mice were treated with vehicle (VEH) or the dimerizer drug AP20187 (AP) to induce apoptosis in RM1-iCasp9 cancer cells. Continuous analysis of tumor volume and the endpoint tumor weight (13d) revealed accelerated tumor growth in mice where apoptosis was induced (AP) as compared with controls (VEH) (p<0.01). Furthermore, the analysis of tumors by flow cytometry demonstrated a significant increase of tumor accelerating myeloid inflammatory cells in the AP-treated mice induced to undergo efferocytosis when compared to VEH-treated mice. These populations included total CD206+F4/80+ (M2 macrophages), Gr-1+CD11b(high)+ (myeloid granulocytes/monocytes associated with anti-tumor immunity), total Gr-1+ cells and CD68+ cells (phagocytes) that express high CD11b (p<0.05). In addition, Ly-6B, a marker associated with the activation of inflammatory myeloid (CD11b+) cells was significantly increased in the AP-treated tumors. In a similar experiment, the tumor protein analysis by ELISA showed a significant increase in CXCL5 in the AP-treated tumors relative to controls. Altogether these findings suggest that cancer cell apoptosis triggers an inflammatory response in myeloid efferocytic macrophages via activation of NF-κb and expression of cytokines that recruit and activate myeloid cells to accelerate tumor growth. This mechanism may be critical for metastatic bone colonization. Citation Format: Hernan Roca, Marta Purica, Savannah Weidner, Amy J. Koh, Robert Kuo, Jacques E. Nör, Lonnie D. Shea, Laurie K. McCauley. Efferocytosis of prostate cancer cells induces a tumor-promoting inflammatory response in myeloid macrophages. [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 4152.