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.
Amiloride inhibited growth of ASCL1-dependent SCLC more strongly than ASCL1-independent SCLC in vitro and slowed growth of ASCL1-driven SCLC in xenografts.
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.
The majority of small-cell lung carcinoma (4/5) cell lines tested expressed HASH-1, while other nonneuronal/non-neuroendocrine cell lines were negative.
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.
Our findings suggest that TTF-1 promotes SCLC growth and contributes to neuroendocrine and antiapoptotic gene expression by partly coordinating with ASCL1.
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.
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).
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.
Several lines of evidence, from SCLC primary human tumours, patient-derived xenografts, cancer cell lines and genetically engineered mouse models, appear to be converging on a new model of SCLC subtypes defined by differential expression of four key transcription regulators: achaete-scute homologue 1 (ASCL1; also known as ASH1), neurogenic differentiation factor 1 (NeuroD1), yes-associated protein 1 (YAP1) and POU class 2 homeobox 3 (POU2F3).
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.
Three correspond to known subtypes (SCLC-A, SCLC-N, and SCLC-Y), while the fourth is a previously undescribed ASCL1+ neuroendocrine variant (NEv2, or SCLC-A2).
In particular, the delta-like protein 3 (DLL3), a Notch inhibitory ligand whose expression is directly related to the key neuroendocrine transcription factor ASCL1, was found to be expressed in ~85% of SCLCs, while it exhibits minimal to absent surface expression in normal lungs.
IMPDH inhibition reduced RNA polymerase I-dependent expression of pre-ribosomal RNA and potently suppressed ASCL1<sup>Low</sup> cell growth in culture, selectively reduced growth of ASCL1<sup>Low</sup> xenografts, and combined with chemotherapy to improve survival in genetic mouse models of ASCL1<sup>Low</sup>/MYC<sup>High</sup> SCLC.