Conclusions
This study demonstrates that Runx2 and CBFβ are required for the expression of genes that mediate the ability of metastatic breast cancer cells to directly modulate both osteoclast and osteoblast function. We also show that Runx2-dependent inhibition of osteoblast differentiation by breast cancer cells is mediated through the Wnt antagonist, sclerostin.
Methods
The aim of this study was to determine how Runx2 mediates the ability of metastatic breast cancer cells to modulate the activity of bone cells. We have previously demonstrated that Runx2 requires the co-activator core binding factor beta (CBFβ) to regulate gene expression in breast cancer cells. We, therefore, performed independent microarray analyses to identify target genes whose expression is dependent upon both Runx2 and CBFβ. Common target genes, with a role in modulating bone-cell function, were confirmed using a combination of siRNA, quantitative reverse transcriptase PCR (qRT-PCR), ELISA, promoter reporter analysis, Electrophoretic Mobility Shift Assay (EMSA) and chromatin immunoprecipitation (ChIP) assays. The function of Runx2/CBFβ-regulated genes in mediating the ability of MDA-MB-231 to inhibit osteoblast differentiation was subsequently established in primary bone marrow stromal cell cultures and MC-3T3 osteoblast cells.
Results
We show that Runx2/CBFβ mediates inhibition of osteoblast differentiation by MDA-MB-231 cells through induction of the Wnt signaling antagonist, sclerostin. We demonstrate that MDA-MB-231 cells secrete sclerostin and that sclerostin-expression is critically dependent on both Runx2 and CBFβ. We also identified the osteoclast activators IL-11 and granulocyte-macrophage colony-stimulating factor (GM-CSF) as new target genes of Runx2/CBFβ in metastatic breast cancer cells. Conclusions: This study demonstrates that Runx2 and CBFβ are required for the expression of genes that mediate the ability of metastatic breast cancer cells to directly modulate both osteoclast and osteoblast function. We also show that Runx2-dependent inhibition of osteoblast differentiation by breast cancer cells is mediated through the Wnt antagonist, sclerostin.