Deregulation of CLTC Interacts with TFG, Facilitating Osteosarcoma Via the TGF-beta and AKT/mTOR Signaling Pathways

Although the treatment of osteosarcoma has improved, the overall survival rate of this common type of osseous malignancies has not changed for four decades. Thus, new targets for better therapeutic regimens are urgently needed. In this study, we found that high expression of clathrin heavy chain (CLTC) was an independent prognostic factor for tumor-free survival (HzR, 3.049; 95% CI, 1.476-6.301) and overall survival (HzR, 2.469; 95% CI, 1.005-6.067) of patients with osteosarcoma. Down-regulation of CLTC resulted in tumor-suppressive effects in vitro and in vivo. Moreover, we found that CLTC was transcriptionally regulated by a transcription factor-specificity protein 1 (SP1), which binds to the CLTC promoter at the -320 to -314-nt and +167 to +173-nt loci. Mechanistic investigations further revealed that CLTC elicited its pro-tumor effects by directly binding to and stabilizing trafficking from the endoplasmic reticulum to the Golgi regulator (TFG). Importantly, overexpression of TFG rescued both the tumor-suppressive effect and inhibition of the TGF-β and AKT/mTOR pathways caused by CLTC down-regulation, which indicated that the activity of CLTC was TFG-dependent. Immunohistochemistry analysis confirmed that CLTC expression was positively correlated with TFG expression. These findings collectively highlight CLTC as a new prognostic biomarker for patients with osteosarcoma, and the interruption of the SP1/CLTC/TFG axis may serve as a novel therapeutic strategy for osteosarcoma.