The role of Snail in non-small cell lung cancer

The epithelial-to-mesenchymal transition (EMT) is a developmental program frequently reactivated in cancer. It plays an important role in several aspects of tumor progression, particularly in the acquisition of invasive capacities facilitating metastasis. Beyond invasion, EMT endows cancer cells with additional characteristics essential for metastatic dissemination and has been extensively studied in breast and colorectal cancer. Non-small cell lung cancer (NSCLC) is the leading cause of cancer-related deaths worldwide and the majority of NSCLC patients is diagnosed with metastatic disease. The expression of the key EMT transcription factor Snail is correlated with a poor prognosis in NSCLC patients, while it is unclear whether Snail contributes to disease progression by actually facilitating metastasis formation.

During EMT, the metabolism of a cancer cell changes, likely to meet the environmental challenges faced during the metastatic process. In this line, we have previously reported (Masin et al, 2014) that the glucose transporter GLUT3 is upregulated during EMT in human NSCLC cell lines. Here, we demonstrate that the rate-limiting enzyme of the hexosamine biosynthesis pathway (HBP) correlates with GLUT3 in NSCLC. This pathway produces a substrate for O-linked protein GlcNAcylations important during EMT, as they – for example – stabilize the Snail protein. Glutamine-fructose-6-phosphate amidotransferase 2 (GFPT2) was specifically upregulated during EMT, while its isoenzyme GFPT1 was unaffected. Furthermore, our results points towards a putative regulation of GLUT3 expression by GFPT2, possibly involving STAT3 signaling. We thus provide evidence for a possible reactivation of a developmental metabolic program during EMT in NSCLC.

Chronic inflammation is an important disease-promoting factor during lung tumorigenesis, emphasizing the role of the immune system in this cancer type. Overexpressing or silencing Snail in the immunocompetent autochthonous KrasLSL-G12D/+;p53fl/fl mouse model of lung adenocarcinoma uncovered a substantial influence of Snail on the tumor microenvironment. An extensive analysis of tumor histology, gene and protein expression and immune infiltration revealed that while Snail did not contribute to metastasis formation, it accelerated growth and malignant progression of the lung tumors, reducing the survival time of the mice. Snail decreased B lymphocyte, while enhancing neutrophil infiltration of the tumors. Intriguingly, through the secretion of a soluble factor, Snail induced a feed-forward loop of neutrophil recruitment via Cxcl2, produced by neutrophils themselves. In our recently published collaborative work (Faget, Groeneveld et al, 2017), we identified neutrophils as main contributors to disease progression. We furthermore provided evidence for a vicious cycle formed between Snail expressing cancer cells and neutrophils in lung tumors, as neutrophils were found to impair angiogenesis, resulting in hypoxia and enhanced Snail expression.

In addition, we found that Snail repressed the Dlk1-Dio3 locus, containing the genome’s largest miRNA cluster and recently implicated in NSCLC, in tumor-infiltrating immune cells via a paracrine mechanism.