Neural lineage-specific homeoprotein BRN2 is directly involved in TTF1 expression in small-cell lung cancer

Abstract

Thyroid transcription factor 1 (TTF1) plays crucial roles in thyroid, lung, and developing brain morphogenesis. Because TTF1-expressing neoplasms are generated from organs and tissues that normally express TTF1, such as the thyroid follicular epithelium and peripheral lung airway epithelium, TTF1 is widely used as a cell lineage-specific and diagnostic marker for thyroid carcinomas and for lung adenocarcinomas with terminal respiratory unit (TRU) differentiation. However, among lung neuroendocrine tumors, small-cell carcinomas (small-cell lung cancers (SCLCs)), most of which are generated from the central airway, also frequently express TTF1 at high levels. To clarify how SCLCs express TTF1, we investigated the molecular mechanisms of its expression using cultivated lung cancer cells and focusing upon neural cellspecific transcription factors. Both SCLC cells and lung adenocarcinoma cells predominantly expressed isoform 2 of TTF1, and TTF1 promoter assays in SCLC cells revealed that the crucial region for activation of the promoter, which is adjacent to the transcription start site of TTF1 isoform 2, has potent FOX-, LHX-, and BRN2-binding sites. Transfection experiments using expression vectors for FOXA1, FOXA2, LHX2, LHX6, and BRN2 showed that BRN2 substantially upregulated TTF1 expression, whereas FOXA1/2 weakly upregulated TTF1 expression. BRN2 and FOXA1/2 binding to the TTF1 promoter was confirmed through chromatin immunoprecipitation experiments, and TTF1 expression in SCLC cells was considerably downregulated after BRN2 knockdown. Furthermore, the TTF1 promoter in SCLC cells was scarcely methylated, and immunohistochemical examinations using a series of primary lung tumors indicated that TTF1 and BRN2 were coexpressed only in SCLC cells. These findings suggest that TTF1 expression in SCLC is a cell lineage-specific phenomenon that involves the developing neural cell-specific homeoprotein BRN2.

High‐grade fetal adenocarcinoma of the lung is a tumour with a fetal phenotype that shows diverse differentiation, including high‐grade neuroendocrine carcinoma: a clinicopathological, immunohistochemical and mutational study of 20 cases

“…Accordingly, we observed high positive rates for these markers in the H‐FLAC components, which were also frequently immunopositive for BRN2 and ASCL1. BRN2 and ASCL1 immunoexpression have been found in 0% and 18% of lung adenocarcinomas, respectively . The frequent expression of these two proneural transcription factors may explain the differentiation into the immature neuroendocrine cells of H‐FLAC.…”

Recent advances in histogenesis research of lung neuroendocrine cancers: Evidence obtained from functional analyses of primitive neural/neuroendocrine cell‐specific transcription factors

“…TTF1 inhibits the transforming growth factor‐β‐mediated epithelial‐to‐mesenchymal transition in lung adenocarcinoma cells; moreover, myosin binding protein H, which inhibits cell migration through inhibitory interaction with Rho kinase 1, has been reported to be a transcriptional target of TTF1 . Recently, we reported that TTF1 is directly induced by POU3F2 (which is specifically expressed in SmCC cells), FOXA1, and FOXA2 and that POU3F4 and POU4F2 also strongly induce TTF1 expression . TTF1 is an important transcription factor for fetal diencephalon morphogenesis, and POU3F2 plays crucial roles in the development of the endocrine hypothalamus and posterior pituitary gland .…”

POU3F2 (POU class 3 homeobox 2)

“…POU3F2 induces several neuroendocrine specific transcription factors and marker molecules and is associated to the cell viability (Schreiber et al, 1992;Ishii et al, 2013;Sakaeda et al, 2013). Haematol.…”

“…POU3F2 has been demonstrated to be an oncogene in malignant melanomas derived from the neuroectodermal cell lineage and to accelerate the growth of melanoma cells (Cook and Sturm, 2008). POU3F2 are also highly expressed in small cell lung cancers and closely associated with the cancer specific neural/neuroendocrine phenotype (Schreiber et al, 1992;Ishii et al, 2013;Sakaeda et al, 2013). …”