Understanding the mechanisms of tissue-specific control of hASH1 gene expression provides a useful model to explore regulatory cascades influencing both normal nervous system development and the NE phenotype of tumors such as MTC and SCLC.
The majority of small-cell lung carcinoma (4/5) cell lines tested expressed HASH-1, while other nonneuronal/non-neuroendocrine cell lines were negative.
The transcription factor achaete-scute homologue-1 (ASH1) is essential for neural differentiation during fetal development and is a cardinal feature of neuroendocrine (NE) tumors such as small cell lung cancer.
Neural and NE differentiation in SCLC depend, in part, on the action of the basic-helix-loop-helix (bHLH) transcription factor human achaete-scute homologue-1 (hASH1).
Subsequently, HASH1 expression in RNA isolated from biopsies from SCLC patients (n = 4) was compared with biopsies from non-SCLC (NSCLC) patients (n = 2) or normal bronchus (n = 2).
CLCA2, HMGB3, L587S and ASH1 were identified in lung cancer tissues using genetic subtraction, microarray and quantitative PCR, and found to be specific and complementary for detection of non-small cell lung carcinoma (NSCLC) and small cell lung carcinoma (SCLC).
Consistent with this idea, knockdown of ASCL1 in SCLC, but not in non-SCLC, led to a significant decrease in expression of the alpha 3 and beta 4 genes without having an effect on any other highly expressed nAChR gene.
Among human primary tumors, 2/2 SCLC, 5/5 pulmonary carcinoids, and 10/41 non-SCLC (only 4 of which had NE features) were positive for hASH1 by immunohistochemistry and RNA-RNA in situ hybridization.
We determined that achaete-scute homolog 1 (ASCL1), a transcription factor required for proper development of pulmonary neuroendocrine cells, is essential for the survival of a majority of lung cancers (both SCLC and NSCLC) with neuroendocrine features.
The importance of ASCL1 as a potential driver oncogene in SCLC is further underscored by the observation that ASCL1 is overexpressed in >50% of SCLC specimens, an extent greater than that observed for other putative oncogenes (MYC, MYCN, and SOX2) previously implicated in SCLC.
The importance of ASCL1 as a potential driver oncogene in SCLC is further underscored by the observation that ASCL1 is overexpressed in >50% of SCLC specimens, an extent greater than that observed for other putative oncogenes (MYC, MYCN, and SOX2) previously implicated in SCLC.
Forced expression of the INSM1 gene in adenocarcinoma cell lines (H358 and H1975) induced the expression of ASCL1, brain-2 (BRN2), chromogranin A, synaptophysin, and neural cell adhesion molecule 1; in contrast, knockdown of the INSM1 gene by siRNA in SCLC (H69 and H889) decreased their expression.
On the other hand, achaete-scute complex homologue 1 (ASCL1), negatively regulated by Notch signaling, is a lineage-specific gene of SCLC, and functions to promote neuroendocrine differentiation as well as EMT.
High expression of the inhibitory Notch ligand Delta-like protein 3 (DLL3) in most SCLCs has been linked to expression of Achaete-scute homologue 1 (ASCL1; also known as ASH-1), a key transcription factor driving SCLC oncogenesis; encouraging preclinical and clinical activity has been demonstrated for an anti-DLL3-antibody-drug conjugate.
Amiloride inhibited growth of ASCL1-dependent SCLC more strongly than ASCL1-independent SCLC in vitro and slowed growth of ASCL1-driven SCLC in xenografts.
In a case-control cohort matched for basic clinical factors, expression of ProGRP, synaptophysin, chromogranin A, and ASCL1 was significantly decreased in TTF-1-negative SCLC samples.