Inhibition of the Growth of Breast Cancer-Associated Brain Tumors by the Osteocyte-Derived Conditioned Medium

SIMPLE SUMMARY: Breast cancer is a common malignancy in women in the US. While brain metastases frequently occur from advanced breast cancer, current treatments are of limited effectiveness. In this study, we found that osteocytes and their conditioned medium (CM) presented the tumor-suppressing capability, and the overexpression of Lrp5, IL1ra, and β-catenin enhanced their anti-tumor actions. In a mouse model, the administration of osteocytes and their CM inhibited the progression of mammary tumors and tumors in the bone and brain. The minimally invasive administration allowed tumor-suppressing factors in CM to diffuse into the brain. Besides p53 and Trail, mass spectrometry-based whole-genome proteomics revealed that extracellular histone H4 was enriched in CM and acted as a tumor suppressor. We also observed that Lrp5-overexpressing mesenchymal stem cells presented the tumor-suppressing capability. ABSTRACT: The brain is a common site of metastasis from advanced breast cancer but few effective treatments are available. We examined a therapeutic option with a conditioned medium (CM), focusing on the role of Lrp5 and β-catenin in Wnt signaling, and IL1ra in osteocytes. Osteocytes presented the innate anti-tumor effect and the overexpression of the above genes strengthened their action. In a mouse model, the injection of their CM inhibited mammary tumors and tumor-driven osteolysis. Importantly, Lrp5- and/or IL1ra-overexpressing osteocytes or the local administration of β-catenin-overexpressing CM markedly inhibited brain tumors. In the transport analysis, tumor-suppressing factors in CM were shown to diffuse through the skull. Mechanistically, the CM with overexpression of the above genes downregulated oncogenic genes such as MMP9, Runx2, TGFβ, and Snail in breast cancer cells. Also, the CM with β-catenin overexpression downregulated CXCL1 and CXCL5 and upregulated tumor suppressors such as LIMA1, DSP, p53, and TRAIL in breast cancer cells. Notably, whole-genome proteomics revealed that histone H4 was enriched in CM and acted as an atypical tumor suppressor. Lrp5-overexpressing MSCs were also shown to act as anti-tumor agents. Collectively, this study demonstrated the therapeutic role of engineered CM in brain tumors and the tumor-suppressing action of extracellular histone H4. The result sheds light on the potential CM-based therapy for breast cancer-associated brain metastases in a minimally invasive manner.