TGF-β Promotion of Gli2-Induced Expression of Parathyroid Hormone-Related Protein, an Important Osteolytic Factor in Bone Metastasis, Is Independent of Canonical Hedgehog Signaling

• Published in: Cancer research (Chicago, Ill.) Vol. 71; no. 3; pp. 822 - 831
• Main Authors: JOHNSON, Rachelle W, NGUYEN, Mai P, PADALECKI, Susan S, GRUBBS, Barry G, MERKEL, Alyssa R, OYAJOBI, Babatunde O, MATRISIAN, Lynn M, MUNDY, Gregory R, STERLING, Julie A
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA American Association for Cancer Research 01.02.2011
• Subjects: Animals; Antineoplastic agents; Biological and medical sciences; Bone Neoplasms - drug therapy; Bone Neoplasms - metabolism; Bone Neoplasms - secondary; Breast Neoplasms - drug therapy; Breast Neoplasms - metabolism; Breast Neoplasms - pathology; Diseases of the osteoarticular system; Female; Hedgehog Proteins - antagonists & inhibitors; Hedgehog Proteins - metabolism; Humans; Kruppel-Like Transcription Factors - antagonists & inhibitors; Kruppel-Like Transcription Factors - metabolism; Medical sciences; Mice; Mice, Nude; Nuclear Proteins - antagonists & inhibitors; Nuclear Proteins - metabolism; Parathyroid Hormone-Related Protein - biosynthesis; Pharmacology. Drug treatments; Recombinant Proteins - pharmacology; Signal Transduction; Transforming Growth Factor beta - pharmacology; Tumors; Tumors of striated muscle and skeleton; Veratrum Alkaloids - pharmacology; Zinc Finger Protein Gli2; Signal transduction; Bone metastasis; Transcription factor Gli2; Transforming growth factor β; Hedgehog protein; Diseases of the osteoarticular system; Parathyroid hormone related peptide; Malignant tumor; Gene expression; Osteolysis; Cell signaling; Cancer
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/0008-5472.CAN-10-2993

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.