Analysis of ASCL1 in Neuroendocrine Lung Cancer
Small cell lung cancer (SCLC) is an understudied tumor subset with aggressive neuroendocrine carcinoma features. Previous studies have determined that the basic helix-loop-helix (bHLH) transcription factor achaete-scute homolog 1 (ASCL1) is essential for the survival and progression of many pulmonary neuroendocrine (NE) cancer cells, which include both SCLC and some non-small cell lung cancer (NSCLC). To understand how ASCL1 initiates tumorigenesis in pulmonary neuroendocrine cancer and identify the transcriptional targets of ASCL1, whole-genome RNA-sequencing (RNA-seq) analysis combined with chromatin immunoprecipitation-sequencing (ChIP-seq) were performed with a series of lung cancer cell lines. We discovered the gene SCNN1A, which encodes the alpha subunit of the epithelial sodium channel (αENaC), is highly correlated with ASCL1 expression in SCLC cells. We confirmed that SCNN1A is under the transcriptional control of ASCL1, indicating that SCNN1A represents a newly recognized ASCL1 target. The product of the SCNN1A gene ENaC can be pharmacologically inhibited by amiloride, a drug that has been used clinically for nearly 50 years. Amiloride-treated ASCL1-dependent tumor cells stopped cell growth in vitro. Analysis of downstream targets of ASCL1 broadens our understanding how ASCL1 functions, and further provides a step forward in the development of drug-targeted therapy for pulmonary neuroendocrine cancer. We also discovered that ASCL1 may negatively regulate the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway through a negative feedback mechanism. Finally, we found that expression of ASCL1 in certain NSCLC could induce neuroendocrine features which are reminiscent of SCLC.