9423

NTN1

BACKGROUND: Hypoxia is often found in solid tumors and is associated with tumor progression and poor clinical outcomes. We elucidated the mechanism by which netrin-1 released under hypoxic stress can induce epithelial-mesenchymal transition (EMT) to promote invasion in hepatocellular carcinoma (HCC) cells. METHODS: The expression of netrin-1 and the dependent receptors UNC5H and deleted in colorectal cancer (DCC) in HCC was examined by immunohistochemistry or western blot. The HepG2 cells were cultured in 21% O2 (normoxia) or 1% O2 (hypoxia) for 24 h. The release of netrin-1 from hypoxic cells was detected by ELISA. expression of E-cadherin and vimentin were examined by western blot. Inverted microscopy or confocal microscopy was used to show the cell morphology or cytoskeletal rearrangements. Cell invasion induced by hypoxia was analyzed by Transwell chamber. Cytokine IL-8 and IL-10 mRNA levels were assessed by real-time PCR. RESULTS: The expression of netrin-1 was increased in HCC tissue and cell lines. The dependent receptors UNC5H and DCC were decreased in most HCC cell lines. Hypoxia induced netrin-1 release in a time-dependent manner. EMT induction was found to occur in hypoxic HCC cells in a process that was dependent on the extracellular release of netrin-1. Moreover, overexpression of netrin-1 resulted in EMT induction in normoxic tumor cells. Cytoskeletal rearrangements were found to occur and cell invasion was increased in cells with netrin-1 overexpression. Lastly, mRNA of IL-8 and IL-10 were also increased after recombinant human netrin-1 treatment. CONCLUSION: These results suggest that in hypoxic HCC cells, netrin-1 activates downstream signaling pathways to induce EMT activation with subsequent production of multiple inflammatory mediators which in turn promotes cancer invasion.

Ectopic expression of netrin-1 has been validated in several cancers including non-small cell lung cancer (NSCLC). Recent research confirms the critical role of netrin-1 in NSCLC growth and its prognostic value. Unfortunately, its contribution in NSCLC metastasis remains elusive. Here, netrin-1 had relatively high expression in NSCLC tissues and cells, especially in high metastatic groups. Notably, netrin-1 overexpression aggravated the malignant metastatic behavior of NSCLC cells, including cell invasion, migration, and vasculogenic mimicry (VM), whereas netrin-1 depression reversely dampened the metastatic potential. Mechanism analysis confirmed that elevation of netrin-1 induced the typical morphological changes of epithelial-to-mesenchymal transition (EMT) and increased the expression of EMT markers, including E-cadherin down-regulation and N-cadherin up-regulation. Consistently, netrin-1 inhibition inversely antagonized the occurrence of EMT. Moreover, netrin-1 also activated the oncogenic pathways of PI3K/AKT and ERK signaling. More importantly, blocking these pathways with their antagonists LY294002 or U0126 reversed the effects of netrin-1 overexpression on cell invasion, migration, EMT, and VM formation. Collectively, the current data suggest that netrin-1 can act as a pro-metastatic factor in NSCLC by enhancing cell invasion, migration, and VM via PI3K/AKT and ERK-mediated EMT process, thereby implicating netrin-1 as a novel promising therapeutic target against aggressive NSCLC.