GP73 promotes epithelial-mesenchymal transition and invasion partly by activating TGF-β1/Smad2 signaling in hepatocellular carcinoma

The transforming growth factor-beta 1 (TGF-beta 1) signaling pathways contribute to cell metastasis and epithelial-mesenchymal transition (EMT). Golgi protein 73 (GP73), a type II transmembrane protein in the Golgi apparatus, was initially regarded as a potential biomarker for the diagnosis of hepatocellular carcinoma (HCC). Recently, it was reported that GP73 acts as a key oncogene by promoting HCC growth and metastasis. However, the role of GP73 in metastasis, especially when involving signaling pathways, is uncertain. Here, we report that GP73, which is upregulated in HCC tissues and cell lines, is associated with tumor size, tumor node metastasis stage, distant metastasis and vascular invasion. The ectopic overexpression of GP73 increased HCC cell invasion, EMT and metastasis both in vitro and in vivo. Conversely, GP73 knockdown inhibited invasion and EMT. Moreover, GP73 enhanced p-Smad2 and p-Smad3 levels by mediating TGF-beta 1, thus leading to the promotion of EMT and invasion in HCC cells. In contrast, we used SB431542 (SB) to repress p-Smad2 and p-Smad3 expression, which resulted in a reversion of EMT. Furthermore, when the TGF-beta 1/Smad pathway was blocked, upregulation of GP73 still caused an enhanced EMT and invasion, and downregulation of GP73 resulted in a decreased in EMT and invasion. In clinical HCC samples, GP73 positively correlated with TGF-beta 1/Smad2, which was upregulated in HCC. Taken together, our findings highlight the important role of GP73 in regulating EMT and metastasis in HCC partly by targeting TGF-beta 1/Smad2 signaling, suggesting that GP73 may represent a novel potential therapeutic target and prognostic marker for the treatment and diagnosis of HCC.