Inhibition of mTOR and HIF pathways diminishes chondro-osteogenesis and cell proliferation in chondroblastoma

Chondroblastoma (CBL) is a benign bone tumor occurring mostly in teenagers. Despite this, CBL can recur and metastasize after curettage, which may impede normal epiphysis. In search of a novel targeted therapy for CBL, we aimed at BMP-2, a factor critical for chondro-osteogenesis and chondrocyte proliferation. Two pathways upstream of BMP-2, the mTOR and HIF, were targeted with rapamycin (Rapa) and FM19G11 (FM), respectively. Using immunohistochemistry, we found BMP-2 was highly expressed in CBL tissues. CBL cells explanted and confirmed with higher BMP-2 level than normal cartilage. Protumorigenic effect of Rapa and FM on CBL cells were transduced via BMP-2. Combination of Rapa and FM conferred stronger inhibition of cell proliferation than either monotherapy and inhibited levels of chondro-osteogenic markers (Sox9, aggrecan, and type II collagen). To minimize the adverse effect of Rapa, we performed screening in essential amino acids and found leucine deprivation-sensitized CBL cells to Rapa. Combination treatment of low dose Rapa, FM, and leucine deprivation conferred compatible inhibitory effects on CBL cell proliferation, chondro-osteogenic potential, and tumorigenic capacity. We conclude that targeting BMP-2 using mTOR/HIF inhibition could potently curb the disease. Addition of low-leucine diet could lower the dose of rapamycin in chase for less toxicity.