SOX2OT variant 7 contributes to the synergistic interaction between EGCG and Doxorubicin to kill osteosarcoma via autophagy and stemness inhibition

BACKGROUND: Doxorubicin is the preferred chemotherapeuticdrug for osteosarcoma treatment of which clinical efficacy is limited because of its chemo-resistance and cardiac toxicity. It is necessary to develop the combination regimen with complementary molecular mechanisms to reduce the side effects and enhance sensitivity of Doxorubicin. EGCG is a polyphenol in green tea with antitumor bioactivity,which has been found that its combination with certain chemotherapeutic drugs could improve the antitumor efficiency. METHODS: In this study, MTT assay was used to detect the cell growth inhibition The CD133+/CD44+ cells were isolated from U2OS and SaoS2 cell lines using magnetic-activated cell sorting and identified by flow cytometry analysis. qRT-PCR was used for determining the relative mRNA levels of key genes. Immunofluorescence was performed to evaluate the autophagy flux alterations. Self-renewal ability was accessed by sphere-forming assay. Tumorigenicity in nude mice was preformed to evaluate tumorigenicity in vivo. RESULTS: We found that EGCG targeting LncRNA SOX2OT variant 7 produced synergistic effects with Doxorubicin on osteosarcoma cell growth inhibition. On the one hand, EGCG could reduce the Doxorubicin-induced pro-survival autophagy through decreasing SOX2OT variant 7 to improve the growth inhibition of Doxorubicin. On the other hand, EGCG could partially inactivate Notch3/DLL3 signaling cascade targeting SOX2OT variant 7 to reduce the stemness then abated drug-resistance of osteosarcoma cells. CONCLUSIONS: This study will help to reveal the molecular mechanisms of synergistic effects of EGCG and Doxorubicin on OS chemotherapy and improve the clinical efficacy of chemotherapy as well as provide a basis for developing antitumor drugs targeting osteosarcoma stem cells.