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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]

Solid papillary carcinoma (SPC) is a histological subtype of breast carcinomas. At least 50% of SPC show neuroendocrine differentiation. Insulinoma‐associated protein 1 (INSM1) is a transcription factor now employed as a useful neuroendocrine marker. It is suppressed by the Notch signaling pathway in other neuroendocrine tumors. However, the usefulness of INSM1 as a neuroendocrine marker and the relationships between INSM1 and NOTCH receptors in SPC of the breast currently remain unclear. To clarify the usefulness of INSM1 as a neuroendocrine marker and the relationships between INSM1 and NOTCH receptors in SPC, we performed immunohistochemistry using 19 tissue specimens of SPC of the breast. We complementarily analyzed public RNA sequencing data on breast carcinomas. Immunohistochemical examinations revealed that the staining intensity of INSM1 was significantly higher in the neuroendocrine group than in the non‐neuroendocrine group. Positive correlations were observed between INSM1 and synaptophysin (SYP), or chromogranin‐A (CHGA). In all cases, NOTCH 2 and 3 were positive, while NOTCH 1 and 4 were negative. According to public RNA data analyses, there were positive correlations between INSM1 and SYP, or CHGA, and negative correlations between INSM1 and NOTCH1–3. INSM1 is useful as a diagnostic marker for SPC with neuroendocrine differentiation in the breast. [ABSTRACT FROM AUTHOR]