PDCD4 inhibits lung tumorigenesis by the suppressing p62-Nrf2 signaling pathway and upregulating Keap1 expression

Programmed cell death 4 (PDCD4) suppresses tumorigenesis, tumor progression, and invasion by inhibiting transcription and translation of oncogenes. However, the role of PDCD4 in lung tumorigenesis is unclear. Sequestosome1/p62 mediates cell proliferation, survival, and death through multiple signaling pathways, including autophagy and cell metabolism. p62/SQSTM1 is transcriptional target of Nrf2 and an important regulator of tumor growth. The aim of this study was to clarify whether and how PDCD4 regulates the p62-Nrf2 pathway, and how this regulation relates to tumorigenesis in human lung cancer cells. We established two stable human lung cancer cell lines, A549 and H460 that each overexpressed PDCD4. We found that PDCD4 overexpression decreased p62 expression levels and inhibited cell proliferation, and also increased the expression levels of cleaved PARP and cleaved caspase 3. Knockdown of p62 markedly increased the apoptotic rate of A549 and H460 cells overexpressing PDCD4. Furthermore levels of the epithelial-mesenchymal transition-related markers Slug, Snail, Twist1 and Vimentin were decreased and expression level of E-cadherin was increased in PDCD4-overexpressing cells. We also found that PDCD4 suppressed transcriptional activation of Nrf2 (an upstream regulator of p62) and increased endogenous levels of Keap1 (a negative regulator of Nrf2). Upregulation of Keap1 induced apoptosis and inhibited cell proliferation by suppressing activity of the p62-Nrf2 pathway in PDCD4-overexpressing cells. As anticipated, results from a mouse xenograft model showed that PDCD4 overexpression in xenografts inhibited cell proliferation and tumorigenesis. Taken together, our results demonstrate that PDCD4 overexpression, which increased Keap1 expression, reduces the levels and activity of the p62-Nrf2 pathway, thereby inhibiting tumorigenesis. Our findings suggest that PDCD4 may be a potential target for lung cancer therapies.