Abstract 5890: Osteoblasts are educated into a tumor-associated stromal cell by disseminated breast cancer cells and mediate breast cancer cell proliferation in the bone microenvironment
Authors: Marini Frank , Chifman Julia , Tooze Janet , Gomez-Manzano Candelaria , Bussard Karen M. 
Published: July 2017

Breast cancer has a predilection for bone metastasis, where the five-year survival rate is bleak. Disseminated breast cancer cells invade bone and can remain undetectable and untreatable for decades during a period of reduced proliferation. Our work reveals that osteoblasts are educated into a tumor-associated stromal cell by disseminated breast cancer cells and alter their production of exosomal microRNAs that regulate cancer cell cycle. Osteoblasts were grown to various stages of maturity and incubated with the conditioned medium (CM) from human breast cancer cells to produce tumor-associated osteoblasts (TAOs). TAOs and their CM were characterized, then TAO cells admixed with human breast cancer cells in-vitro or in-vivo to elucidate the impact of TAOs on the tumor microenvironment. Osteoblasts undergo an inflammatory stress response and gain tumor-associated stromal cell-like properties when either co-cultured or treated with the CM of human breast cancer cells. These changes were mediated by crosstalk via gap junction intercellular communication (connexin 43) and exosome exchange that occurred between TAO cells and human breast cancer cells. TAO-derived exosomes were found to contain increased amounts of microRNAs 320a and 193b compared to normal osteoblasts or breast cancer cells. microRNA 320a is associated with decreased cellular proliferation and induction of G0 phase of the cell cycle. miR 193b has been shown to regulate cyclin D1 expression and repress cellular proliferation. Knock-down of miRs 320a and 193b in breast cancer cells resulted in a reduced number of cancer cells in G0 and increased numbers of cancer cells in G1/S/G2/M phases of the cell cycle. Furthermore, TAO cell CM led to decreased activation of TRAP+ osteoclasts. When TAO cells were admixed with human breast cancer cells and inoculated in-vivo in mice, tumors grew more slowly and were at least 50% smaller than tumors composed of normal osteoblasts plus breast cancer cells, or breast cancer cells inoculated alone. Mice inoculated with an admix of TAO cells plus human metastatic breast cancer cells lived ~20 days longer than mice inoculated with an admix of normal osteoblasts plus metastatic breast cancer cells, or breast cancer cells alone. These data also suggest that TAO cells regulate the proliferation of metastatic breast cancer cells in the tumor microenvironment. Overall, these date suggest that osteoblasts are an important source of factors, specifically exosomal microRNAs, in breast cancer bone metastasis. The nature of these factors suggest their importance for facilitating disseminated breast cancer cell proliferation, as well as osteoclast activation in the bone microenvironment. Supported by NIH 1RC1 CA146381, 1R01NS06994, P50 CA083639 for FCM; (NRSA) T32 CA079448, NIH 3 R00 CA178177 for KMB.

Citation Format: Frank Marini, Julia Chifman, Janet Tooze, Candelaria Gomez-Manzano, Karen M. Bussard. Osteoblasts are educated into a tumor-associated stromal cell by disseminated breast cancer cells and mediate breast cancer cell proliferation in the bone microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5890. doi:10.1158/1538-7445.AM2017-5890