Breast cancer frequently metastasizes to bone, in which
tumor cells receive signals from the bone marrow microenvironment. One relevant factor is TGF-β, which upregulates expression of the Hedgehog (Hh) signaling molecule, Gli2, which in turn increases secretion of important osteolytic factors such as
parathyroid hormone-related protein (
PTHrP).
PTHrP inhibition can prevent
tumor-induced bone destruction, whereas Gli2 overexpression in
tumor cells can promote
osteolysis. In this study, we tested the hypothesis that Hh inhibition in bone metastatic
breast cancer would decrease
PTHrP expression and therefore osteolytic bone destruction. However, when mice engrafted with human MDA-MB-231
breast cancer cells were treated with the Hh receptor antagonist
cyclopamine, we observed no effect on
tumor burden or bone destruction. In vitro analyses revealed that osteolytic
tumor cells lack expression of the Hh
receptor, Smoothened, suggesting an Hh-independent mechanism of Gli2 regulation. Blocking Gli signaling in metastatic
breast cancer cells with a Gli2-repressor gene (Gli2-rep) reduced endogenous and TGF-β-stimulated
PTHrP mRNA expression, but did not alter
tumor cell proliferation. Furthermore, mice inoculated with Gli2-Rep-expressing cells exhibited a decrease in
osteolysis, suggesting that Gli2 inhibition may block TGF-β propagation of a vicious osteolytic cycle in this MDA-MB-231 model of bone
metastasis. Accordingly, in the absence of TGF-β signaling, Gli2 expression was downregulated in cells, whereas enforced overexpression of Gli2 restored
PTHrP activity. Taken together, our findings suggest that Gli2 is required for TGF-β to stimulate
PTHrP expression and that blocking Hh-independent Gli2 activity will inhibit
tumor-induced bone destruction.