Runx2 transcriptional activation of Indian Hedgehog and a downstream bone metastatic pathway in breast cancer cells

Runx2, required for bone formation, is ectopically expressed in breast cancer cells. To address the mechanism by which Runx2 contributes to the osteolytic disease induced by MDA-MB-231 cells, we investigated the effect of Runx2 on key components of the vicious cycle of transforming growth factor beta (TGF beta)-mediated tumor growth and osteolysis. We find that Runx2 directly up-regulates Indian Hedgehog (IHH) and colocalizes with Gli2, a Hedgehog signaling molecule. These events further activate parathyroid hormone-related protein (PTHrP). Furthermore, Runx2 directly regulates the TGF beta-induced PTHrP levels. A subnuclear targeting deficient mutant Runx2, which disrupts TGF beta-induced Runx2-Smad interactions, failed to induce 11111 and downstream events. In addition, Runx2 knockdown in MDA-MB-231 inhibited IHH and PTHrP expression in the presence of TGF beta. In vivo blockade of the Runx2-IHH pathway in MDA-MB-231 cells by Runx2 short hairpin RNA inhibition prevented the osteolytic disease. Thus, our studies define a novel role of Runx2 in upregulating the vicious cycle of metastatic bone disease, in addition to Runx2 regulation of genes related to progression of tumor metastasis.