Molecular mechanisms of Gli2 regulation in osteolytic bone disease
Johnson, Rachelle Whitney
Breast cancer frequently metastasizes to the skeleton with severe consequence to patient quality of life. Patients who develop bone metastases suffer from severe bone pain and increased risk of developing hypercalcemia and fractures, and will eventually die from the disease. When breast tumor cells metastasize to bone, they up-regulate the expression of the Hedgehog transcription factor GLI2 and its downstream target parathyroid hormone-related protein (PTHrP), among other osteolytic factors; however, the mechanisms that regulate GLI2 expression remain unclear, and our ability to monitor tumor-induced bone disease in vivo has lagged significantly behind clinical technology. To address these deficiencies, we developed a method of quantifiable in vivo microCT to monitor changes in bone volume and to determine the key regulators of Gli2 in osteolytic tumor cells. In our imaging studies, we found that optimizing scan resolution enabled us to measure statistically significant changes in bone volume by in vivo microCT in bisphosphonate treated tumor-bearing mice, although its utility is limited to small studies due to expense and length of scan times. In our molecular studies, we found that Tgf-â signaling positively regulates GLI2 expression and requires Gli2 for PTHrP stimulation. We also found that bone rigidity activates Wnt signaling, which crosstalks with Tgf-â, to further enhance GLI2 and PTHrP transcription. Wnt signaling blockade did not abrogate this up-regulation, and although Tgf-â inhibition blocks PTHrP in vitro, its effects in vivo are complicated by its biphasic effect. However, Gli2 repression in tumor cells significantly blocked tumor burden and tumor-induced osteolysis, although Gli inhibition using Gli Antagonists negatively affected the developing growth plate in our young mouse model. Alternatively, the guanine nucleoside 6-Thioguanine has been shown to selectively inhibit PTHrP at low doses, and we found that these effects are mediated through Gli2, indicating that although our current tumor model is not conducive to preclinical studies, Gli2 may still be an effective therapeutic target. Taken together, these data support Gli2 as a novel clinical target in the treatment of breast cancer-induced bone disease, which may enhance patient quality of life and outcome.