Down-regulation of THBS2 inhibits the malignant progression of Burkitt lymphoma by blocking the PI3K/AKT/c-MYC pathway

Yan Xu, Shengquan Liu, Shaoxiong Wang, Y Huang

Background Thrombospondin 2 (THBS2) is overexpressed in various solid tumors and is a prognostic gene of Burkitt lymphoma (BL), but its role and molecular mechanism in BL still need to be explored. Methods Western blot (WB) detected the protein expression of THBS2 in BL cells. si-THBS2 was transfected into CA46 and RAJI cells to detect cell malignant phenotype and cell cycle changes. WB detected the protein expression of epithelial-mesenchymal transition (EMT), apoptosis, cell cycle and the PI3K/AKT/c-MYC pathway proteins. ChIP-qPCR detected the binding ability of MYC to target gene promoters, and Co-IP detected the level of MYC/MAX dimerization. Tumor stem cell sphere formation assays evaluated cell stemness changes. The subcutaneous xenograft model of RAJI cells was developed to observe the effect of THBS2 knockdown on tumor growth. Results THBS2 protein was markedly up-regulated in BL cells. Knockdown of THBS2 could inhibit the growth and EMT process, inhibit the protein levels of cyclins (CDK2, CDK4, CDK6 and CyclinD1), inhibit the sphere formation ability of stem cells and down-regulate SOX2 and OCT4 expressions. THBS2 knockdown inhibited the PI3K/AKT pathway phosphorylation, declined c-MYC expressions, weakened the MYC/MAX dimerization and the binding ability of MYC to target gene promoters. In addition, THBS2 knockdown also significantly inhibited the growth of subcutaneous xenografts of RAJI cells, reduced the positive rates of Ki67 and PCNA, declined the protein levels of cyclins, and inhibited the PI3K/AKT/c-MYC pathway. Overexpression of c-MYC or 740Y-P treatment partially reversed the tumor malignant phenotype inhibition caused by THBS2 knockdown and partially restored cyclin levels. Conclusion Knockdown of THBS2 inhibited the malignant phenotype, EMT and stem cell-like characteristics via inhibiting the PI3K/AKT/c-MYC signaling axis, and blocked the cell cycle.