Abstract 1624: Targeting the Hedgehog pathway to inhibit osteosarcoma growth through dual effects on tumor and microenvironment cells.

In addition to serving crucial roles in bone development, Hedgehog (Hh) signaling contributes to the development and progression of many cancers, making it an attractive candidate for treatment of bone-resident cancers. Recent data suggest that Hh-targeted agents exert pluripotent effects on host bone microenvironment cells through both cell-intrinsic and paracrine mechanisms, interrupting the ‘vicious cycle’ of tumor cell, osteoblast (OB), osteoclast (OC), and bone marrow stromal cell (BMSC) interactions in bone metastatic breast cancers. Here, reciprocal stimulation between tumor cells and bone cells enhance both tumor growth and bone pathology. Similarly, development of osteosarcoma (OS), the most common primary bone cancer, is strongly influenced by conditions within surrounding microenvironment providing additional points for therapeutic intervention. The 5-year survival rate for metastatic OS is under 30%, highlighting the need for novel and targeted treatments. We have demonstrated that mice deficient in the tumor suppressor ARF (p19ARF; p14ARF in humans) have enhanced rates of bone turnover, mimicking the adolescent growth period in which OS is prevalent. By crossing Arf-/- mice with those expressing Tax, an HTLV-1 oncogene that results in osteolytic tumors, we developed a model of high penetrant spontaneous OS that recapitulates many aspects of human disease. Here, suppression of bone turnover with the bisphosphonate zoledronic acid prevented the development of OS, suggesting that enhanced OC activity may stimulate OS growth. In agreement, we found that Tax+Arf-/- OS had increased RANKL-to-OPG ratios resulting in increased osteoclastogenic ability. Compared to normal osteoblasts and mesenchymal stem cell precursors, Tax+Arf-/- OS cells have increased expression of Hh pathway genes and exhibit increased susceptibility to Hh inhibitors (SMO antagonists). In particular, Tax+Arf-/- OS cells express high levels of the Hh ligands Sonic (SHH) and Indian (IHH), suggesting they may stimulate surrounding cells in a paracrine fashion. Notably, BMSC derived from non-tumor bearing Tax+Arf-/- mice also exhibit increased expression of pro-tumorigenic factors (including IL-6 and IGF), generating a fertile ‘soil’. SHH or conditioned media from Tax+Arf-/- OS cells enhanced OC activity and BMSC cell production of pro-tumorigenic growth factors. Furthermore, the increased OB and OC differentiation and activity present in Tax+Arf-/- cells could be abrogated with SMO inhibitors. We have established an OS cell line from Tax+Arf-/- mice (TAN) with the ability to form mineralized tumors upon intratibial injection. Using this model, we hypothesize that treatment with SMO antagonists will decrease OS growth due to both tumor cytotoxic effects and direct effects on host bone microenvironment cells that abrogate the pro-tumor microenvironment. Citation Format: Michelle Anne Hurchla, Katherine N. Weilbaecher. Targeting the Hedgehog pathway to inhibit osteosarcoma growth through dual effects on tumor and microenvironment cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1624. doi:10.1158/1538-7445.AM2013-1624