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Pulmonary neuroendocrine (NE) cells are rare airway epithelial cells. The balance between Achaete‐scute complex homolog 1 (ASCL1) and hairy and enhancer of split 1, one of the target molecules of the Notch signaling pathway, is crucial for NE differentiation. Small cell lung cancer (SCLC) is a highly aggressive lung tumor, characterized by rapid cell proliferation, a high metastatic potential, and the acquisition of resistance to treatment. The subtypes of SCLC are defined by the expression status of NE cell‐lineage transcription factors, such as ASCL1, which roles are supported by SRY‐box 2, insulinoma‐associated protein 1, NK2 homeobox 1, and wingless‐related integration site signaling. This network reinforces NE differentiation and may induce the characteristic morphology and chemosensitivity of SCLC. Notch signaling mediates cell‐fate decisions, resulting in an NE to non‐NE fate switch. The suppression of NE differentiation may change the histological type of SCLC to a non‐SCLC morphology. In SCLC with NE differentiation, Notch signaling is typically inactive and genetically or epigenetically regulated. However, Notch signaling may be activated after chemotherapy, and, in concert with Yes‐associated protein signaling and RE1‐silencing transcription factor, suppresses NE differentiation, producing intratumor heterogeneity and chemoresistance. Accumulated information on the molecular mechanisms of SCLC will contribute to further advances in the control of this recalcitrant cancer. [ABSTRACT FROM AUTHOR]

Background: Insulinoma‐associated protein 1 (INSM1) has been reported to be a useful marker for diagnosing pancreatic neuroendocrine tumours (PNETs). However, INSM1 expression in endoscopic ultrasound‐guided fine needle aspiration cytology has not been examined. We evaluated INSM1 expression in the cytology of cases diagnosed with PNETs. Methods: We immunocytochemically stained INSM1 and Ki‐67 in 14 PNET cases, and according to the 2017 World Health Organisation criteria, seven PNET Grade 1 cases, four Grade 2 cases and three Grade 3/neuroendocrine carcinoma cases were identified. As a control for INSM1 and Ki‐67 expression, we used cytological specimens from 15 cases of pancreatic ductal adenocarcinoma. Results: In PNET cases, INSM1‐expressing tumour cells were identified in all cytological specimens of PNET. The median INSM1 expression rate in Grade 1 cases was 49.8% (mean ± standard deviation: 55.1 ± 12.5%, min: 39.3%, max: 74.1%), and in Grade 2 and Grade 3/neuroendocrine carcinoma cases was 81.1% (mean ± standard deviation: 77.6 ± 18.6%, min: 50.3%, max: 100%). However, there was no correlation between INSM1 and Ki‐67 expression (r = −0.15). The median expression rate in PNET cases was 64.3%, which was significantly higher than that in pancreatic ductal adenocarcinoma cases (P < 0.0001). Conclusion: INSM1 immunocytochemistry of cytological specimens obtained from endoscopic ultrasound‐guided fine needle aspiration cytology can accurately diagnose PNETs; therefore, INSM1 could be an important diagnostic tool in assessing therapeutic strategies, including molecular‐targeted therapy. Recently, neuroendocrine markers such as SYP, CGA, and NCAM have been used with high specificity and sensitivity for PNETs. However, there are similarities in cell structure and immunocytochemistry between pancreatic solid tumors. We reported INSM1 positivity in all histological specimens of PNETs examined including high grade PNETs. This can be considered a useful biomarker. [ABSTRACT FROM AUTHOR]