Epithelial-mesenchymal transition (EMT) is the key event in distant metastasis of diverse tumors including lung cancer. Recent evidence suggests the involvement of phosphatase and tensin homolog (PTEN) in EMT phenotypes. However, the molecular mechanism of EMT induced by PTEN inactivation is not clear in lung cancer. We aimed to investigate the role of PTEN inactivation in acquisition of EMT in lung cancer cells.
We knocked out the PTEN in PTEN proficient lung cancer cells lines (A549 and NCI-H460) using CRISPR/Cas-9 system and observed the growth, EMT phenotypes, and EMT related molecules. We also explored the in vivo effect of PTEN inactivation on tumor cell growth and distant metastasis using nude mouse injection.
PTEN knockout (KO) cells showed faster growth, migration and invasion than PTEN wild-type (WT) cells. When we injected the cells into nude mice, PTEN-KO cells showed faster growth and higher metastatic potential. In PTEN-KO cells, the levels of phosphorylated AKT (Ser-473 and Thr-308) were profoundly elevated and the expressions of phosphorylated GSK-3β (Ser9, inactive form) increased, while that of β-catenin decreased. Regarding the EMT markers, the expression of E-cadherin decreased but those of N-cadherin, vimentin and MMP-2 increased in the PTEN-KO cells. Especially, PTEN-KO cells showed the almost complete intra-nuclear shift of β-catenin and no β-catenin signal was observed in the cell membrane. Accordingly, PTEN-KO cells exhibited morphological changes such as loss of cell-to-cell contact, pseudopodia and the round shape, which are the typical phenotypes of EMT. Snail and Slug were also dominantly accumulated in the nucleus after PTEN inactivation.
All these data consistently support that PTEN inactivation contributes to EMT by nuclear translocation of β-catenin and Snail/Slug in lung cancer cells.