Wnt, osteosarcoma, and future therapy.

Osteosarcoma is the most common type of primary bone cancer, with peak incidences in adolescence and at age >60 years. This tumor has a very high propensity for local invasion and distant metastasis. Pulmonary metastasis is the most common mode of spread, occurring in about 80% of patients. Aggressive treatment with high-dose chemotherapy and surgery leads to cure in 60% to 70% of patients. However, the long-term survival for patients with relapse is only 20%. The Wnt pathway is comprised of a family of highly conserved proteins that regulate cell-to-cell interactions during embryogenesis. In the absence of Wnt binding to cellsurface receptors called Frizzled and LRP-5/6, a cytoplasmic complex consisting of axin, adenomatous polyposis coli, and glycogen synthase kinase-β phosphorylates β-catenin, thus promoting the ubiquitination and degradation of β-catenin. In addition to Wnt proteins and receptors, several negative regulators, called Wnt antagonists, exist. β-Catenin, together with LEF/TCF transcription factors, acts as a signal transducer for downstream genes involved in cell proliferation and growth. Using clinical tissue samples, we examined the relationship between the Wnt pathway and clinical outcome of patients with osteosarcoma. In this study, RNA from fresh-frozen osteosarcoma specimens was isolated to characterize the expression of the Wnt receptor LRP-5 by polymerase chain reaction. We found a statistically significant correlation between LRP-5 expression and a worse event-free survival in patients. More importantly, patients whose primary tumors expressed LRP-5 sustained a higher risk of developing metastasis. Given the association between LRP-5 and osteosarcoma, we proceeded to examine whether blocking Wnt signaling by a soluble LRP-5 receptor affects tumor progression. Our results suggest that blocking Wnt/LRP-5 signaling halts tumor invasiveness by reversing the epithelial-to-mesenchymal transition, confirming the important role of Wnt in osteosarcoma progression. In contrast, downregulation of Wnt antagonists is a common event in osteosarcoma tumors and cell lines, suggesting that Wnt activation contributes to the tumorigenic phenotypes. Further analysis suggests that promoter hypermethylation is one of the key mechanisms by which osteosarcoma suppresses Wnt antagonist expression. Consistent with this notion, reexpression of Wnt antagonists slows both tumor growth and the formation of lung metastasis in vivo. Therefore, drugs that target DNA methyltransferase may play an important role in preventing osteosarcoma progression. Several strategies have been devised to exploit the Wnt pathway for therapeutic purpose. DeAlmeida et al showed that a secreted Wnt antagonist, consisting of the ligand-binding domain of Frizzled-8 fused with human immunoglobulin G, had antitumor efficacy in an animal model. Monoclonal antibodies against Wnt-1 and -2 have been shown to induce apoptosis in several cancer cell Bang H. Hoang, MD