Abstract 5763: VCAM1 mediates aggressive bone metastasis outbreak from tumor dormancy by engaging α4β1-positive osteoclast progenitor cells

In breast cancer patients, disseminated tumor cells can maintain in dormancy at secondary organs for a long period before relapse into life-threatening metastases. However, the molecular understanding of this process is still very limited. Here, we show that Vascular Cell Adhesion Molecule-1 (VCAM1) is essential for the conversion from dormancy to macrometastasis in a xenograft mouse model of breast cancer bone metastasis. A weakly bone-metastatic subline (SCP6) of the human breast cancer cell MDA-MB-231 was found to give rise to strongly metastatic derivatives after six-month in vivo dormancy in nude mice. Whereas no additional chromosomal alterations were detected by array-CGH, the derivatives significantly upregulated a handful of genes based on microarray analysis. Five candidate genes were evaluated by silencing their expression using short-hairpin RNA in the highly metastatic derivatives and tested by in vivo bone metastasis assay. Only the loss of expression of VCAM1 dramatically reduced the metastasis ability. Ectopic overexpression of VCAM1 in dormancy-prone sublines allowed the outgrowth of bone metastasis. We successfully blocked the initiation and the progression of bone metastasis using purified monoclonal antibodies against VCAM1 or its receptor VLA4 (integrin α4β1). Biochemical analysis showed activated NF-κB pathway is essential for the upregulation of VCAM1. Bone metastases formed by VCAM1-overexpressed tumor cells harbored numerous TRAP-positive osteoclasts and their number was dramatically attenuated with VCAM1 or VLA4 antibodies. Expression analysis using cell lines and fresh murine bone marrow showed progenitor and mature osteoclasts express α4β1. Functional assays found VCAM1 promoted osteoclast differentiation by directly tethering the α4β1-positive osteoclast progenitors. Mice with VCAM1-positive bone metastases showed enriched CD11b-positive α4β1-positive monocytic progenitor cells of osteoclasts in the bone marrow compared with mice free of bone metastases. Finally, tissue microarray analysis showed VCAM1 is upregulated in breast tumor epithelial cells in a subgroup of patients and is associated with early relapse. Overall, our study (1) established a novel mouse model for bone metastasis dormancy of breast cancer; (2) demonstrated VCAM1 as the key mediator for the conversion from dormancy to overt metastasis through engaging α4β1-positive osteoclast progenitors; (3) developed monocloncal antibodies against VCAM1 or VLA4 as promising therapeutic agents for preventing and treating bone metastasis of breast cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5763.