Abstract P1-07-13: Extramedullary hematopoiesis aids initiation of cancer metastasis

Despite the recent advances in treatment of primary tumors, metastatic disease is resistant to current therapies and remains the primary cause of cancer-related death. Therefore, prevention and improved therapy of cancer metastasis are the ultimate goals of cancer research. Although tumor cells are the driving force of metastasis, novel findings suggest that the tumor microenvironment also plays a key role. Microenvironments distant from the primary tumor are primed for future metastatic growth by the recruitment of bone marrow-derived myeloid cells, creating "pre-metastatic niches", and ready for arrival of circulating cancer cells. The number of platelets is often increased in the late stage of cancer patients and experimental evidences show that platelets facilitate tumor metastasis. Within the circulatory system, platelets surround and guard tumor cells from immune elimination and produce various kinds of growth factors and cytokines to aid establishment of metastasis. But the role of platelets within the pre-metastatic niches has not yet been explored, particularly with respect to their ability to aid circulating cancer cells to arrest, survive and grow. In our study, 4T1 murine breast cancer cells, implanted into mammary fat pad (mfp) of mice were spontaneously metastasized to the lung and liver around 6 weeks after the implantation. Interestingly, splenomegaly was found in these mice bearing 4T1 tumor in the mfp. Pathological and immunofluorescence analysis of the spleen revealed extramedullary hematopoiesis including megakaryopoiesis. Consequently, platelets accumulated into the other organs of the tumor bearing mice as early as 2 weeks after the implantation of 4T1 cells to the mfp. Interestingly, platelets accumulated into lung and liver, but not to the other major organs. To validate the effect of extramedullary hematopoiesis, 4T1 cells expressing luciferase (4T1-luc) were implanted into the mfp of normal mice or mice resected with spleen in advance. Then, development of spontaneous metastasis was monitored by luminescence imaging of the live mice. There was a significant reduction in the number of metastasis in the mice without spleen as compared to that in the normal mice, indicating crucial role of extramedullary hematopoiesis in the spleen in spontaneous metastases. Next, to evaluate biological effect of accumulated platelets into the lungs, normal mice or mice bearing 4T1 in the mfp (2 weeks after the implantation) were injected intravenously with 4T1-luc cells through tail vein. The amount of the arrested, survived and growing tumor cells in the lung was imaged and quantified using luminescence imaging of the live mice. There was a significant increase in these parameters in the mice bearing 4T1 tumor in the mfp as compared to those in the normal mice. These data strongly indicates that accumulated platelets aid circulating tumor cells to arrest, survive and grow at the lungs. Therefore platelets accumulated into the secondary sites can serve as the pre-metastatic niches. To validate these findings, mice bearing 4T1 cells in the mfp will be pre-treated with CD42 antibodies to delete circulating platelets or glycoprotein GP IIb/IIIa inhibitors which can reduce binding of activated platelets to endothelial cells. Citation Format: Kenji Yokoi, Tomonori Tanei, Megumi Kai, Yuki Saito, Yan Ting Liu, Mauro Ferrari. Extramedullary hematopoiesis aids initiation of cancer metastasis [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-07-13.