Involvement and Targeted Intervention of Deregulated Hedgehog Pathway in Osteosarcoma

• Main Author: Lo, Winnie
• Other Authors: Andrulis, Irene
• Language: en_ca
• Published: 2012
• Subjects: Osteosarcoma; Hedgehog; 0992; 0307; 0379
• Online Access: http://hdl.handle.net/1807/43937

Despite the combination of chemotherapy and surgery in osteosarcoma treatment, the survival of patients remains low. The lack of treatment improvement prompted me to investigate deregulated Hedgehog pathway as a potential target for intervention. During development, the Hedgehog pathway is tightly regulated to control organ and tissue development. Specifically, the Indian Hedgehog pathway (IHH) is important for bone development. Aberrantly activated Hedgehog pathway through ligand-dependent or ligand-independent mechanisms has been reported in numerous cancers. Several small molecule antagonists of the Hedgehog pathway are being explored clinically to improve patient outcome. I examined the expression of IHH pathway components, IHH, SMO, PTCH1 and GLI1, in 43 high-grade primary osteosarcoma tumors and 6 cell lines and found various transcript levels of IHH pathway components in osteosarcoma samples. The high levels of co-expressed IHH and targets, PTCH1 and GLI1, in a subset of osteosarcoma samples are indicative of ligand-dependent activation. Trends toward worse survival for patients with higher IHH (positive regulator) levels and toward better survival for patients with higher PTCH1 (negative regulator) levels were observed. Through genetic analyses, PTCH1 inactivating mutations and GLI1 amplification were found not to be responsible for ligand-independent activation in osteosarcoma. Characterization of Hedgehog signaling in osteosarcoma cell lines showed that cells with high IHH, PTCH1, and GLI1 levels were sensitive to small molecule modulators of both SMO and GLI, which supported the ligand-dependent activation observed in the clinical samples. The inverse correlation of endogenous GLI2 levels and Hedgehog pathway induction levels, and the sensitivity of high-GLI2 cells to GLI inhibition, but not SMO inhibition, in cell lines suggest that GLI2 overexpression may be a mechanism of ligand-independent activation in osteosarcoma. Furthermore, in patient-derived osteosarcoma xenograft models, I observed autocrine and possibly paracrine ligand-dependent Hedgehog signaling in the tumor and stromal compartments. I also showed that a clinically relevant SMO antagonist, IPI-926, was effective at specifically inhibiting all ligand-dependent Hedgehog signaling interactions. A trend toward decreased proliferation and increased apoptosis in treated tumors was observed warranting additional research and demonstrating the potential of Hedgehog pathway inhibitors as novel targeted therapeutics for osteosarcoma treatment.