Mechanistic Studies Of Tumor Cell Heterogeneity And Lung Metastasis In Osteosarcoma

Osteosarcoma is the most common primary malignant bone tumor occurring in childhood and adolescents,only second to child leukemia.Before 1970,the treatment for osteosarcoma was limited to amputation,leaving the majority of cases to relapse or form lung metastases in several years after surgery.With the introduction of modern neoadjuvant chemotherapy,the prognosis of patients with osteosarcoma was significantly improved,with the 5-year survival rate reaching between 65%-70%.However,the current situation is not optimistic,and for 30 years the survival rate has not been further increased.Actually there are no therapeutic options available for those who have experienced chemotherapeutic resistance,relapse or metastasis formation.The fundamental reason lies in lack of understanding of tumor origin and malignant progression in osteosarcoma.Here,our studies focus on the osteosarcoma features of tumor heterogeneity and lung metastatic microenvironment.Part One mTORC1 Maintains the Tumorigenicity of SSEA-4~+ High-Grade OsteosarcomaInactivation of p53 and/or Rb pathways has been well documented of restraining osteoblasts from cell-cycle exit and terminal differentiation,which underpins osteosarcoma formation coupled with dedifferentiation.Recently,the level of p-S6K was shown to independently predict the prognosis of osteosarcomas,while the reason behind is not understood.Here we show that in certain high-grade osteosarcoma cases,immature SSEA-4~+tumor cells represent a subset of tumor-initiating cells(TICs) whose pool size is maintained by mTORC1 activity.mTORC1 supports SSEA-4~+ cell self-renewal partially through S6K.Moreover,active mTORC1 is required to prevent a likely upregulation of the cell-cycle inhibitor p27 independently of p53 or Rb activation,which otherwise effectively drives the terminal differentiation of SSEA-4~-osteosarcoma cells at the expense of dedifferentiation.Thus,mTORC1 is shown to critically regulate the retention of tumorigenicity versus differentiation in discrete differentiation phases in SSEA-4~+TICs and their progeny.Part Two FGF signaling-driven Autocrine Fibrogenic Activity of Osteosarcoma Cells Underlies the Formation of Lung MetastasisOsteosarcoma is prone to form lung metastases.At present,the treatment of osteosarcoma pulmonary metastasis is very limited and the functional mechanism is basically obscured.In the part two of our study,we revealed that osteosarcoma sphere formation and lung metastases share a common feature-fibrogenesis,wherein fibronection(FN)plays a decisive role.Meanwhile,we figured out that the role of FGF is as the upstream stimulating factor for production of FN,which in turn activates downstream ?3-Src-Paxillin fueling the progression of fibrognesis and the survival and proliferation of osteosarcoma cells.In accordance,we found that FGFR expression in primary osteosarcoma samples is associated with the tendency to have lung metastasis and the prognosis.Finally,we also confirmed that the fibrosis inhibitors are able to suppress potently both the sphere formation and lung metastases.In short,we establish in vitro and in vivo models to clarify an important role of fibrosis in lung metastasis,and potential administration of anti-fibrosis drugs to curb osteosarcoma lung metastases.