NDRG1 attenuates epithelial-mesenchymal transition of nasopharyngeal cancer cells via blocking Smad2 signaling

N-myc downstream-regulated gene 1 (NDRG1) has been implicated in tumorigenesis and metastasis in different cancers. However, its role in nasopharyngeal carcinoma remains unknown. We found that NDRG1 expression level was high in nasopharyngeal cancer 5-8F cells but low in 5-8F-LN cells with lymphatic metastasis potential. Knockdown of NDRG1 by shRNA promoted 5-8F cell proliferation, migration, and invasion in vitro and its tumorigenesis in vivo. Moreover, NDRG1 deficiency induced an epithelial-mesenchymal transition (EMT) of 5-8F cells as shown by an attenuation of E-cadherin and an induction of N-cadherin and vimentin expression. NDRG1 knockdown also enhanced Smad2 expression and phosphorylation. Smad2 signaling was attenuated in 5-8F cells but was significantly activated in 5-8F-LN cells. Knockdown of Smad2 restored E-cadherin but attenuated N-cadherin expression in NDRG1-deficient 5-8F cells, suggesting a reduction of EMT. Consistently, blockade of Smad2 in 5-8F-LN cells increased E-cadherin while diminishing N-cadherin and vimentin expression. These data indicate that Smad2 mediates the NDRG1 deficiency-induced EMT of 5-8F cells. In tumors derived from NDRG1-deficient 5-8F cells, E-cadherin expression was inhibited while vimentin and Smad2 were increased in a large number of cancer cells. Most importantly, NDRG1 expression was attenuated in human nasopharyngeal carcinoma tissues, resulted in a lower survival rate in patients. The NDRG1 was further decreased in the detached nasopharyngeal cancer cells, which was associated with a further reduced survival rate in patients with lymphatic metastasis. Taken together, these results demonstrated that NDRG1 prevents nasopharyngeal tumorigenesis and metastasis via inhibiting Smad2-mediated EMT of nasopharyngeal cells.