Significance of proneural basic helix-loop-helix transcription factors in neuroendocrine differentiation of fetal lung epithelial cells and lung carcinoma cells

Abstract

In this brief review article, we describe how cell fate determination by which the airway epithelial cells become neuroendocrine or non-neuroendocrine is regulated by a network of basic helix-loop-helix transcription (bHLH) factors in a similar manner to neurona1 differentiation, and how this system could work to determine cell differentiation of human lung carcinomas. Immunohistochemical studies reveal that mammalina achaete-scute complex homologue (Mash)l is expressed in pulmonary neuroendocrine cells (PNEC), while hairy and Enhancer of split (Hes)l is expressed in pulmonary non-neuroendocrine cells (non-PNEC). Studies using gene-deficient mice for the bHLH factors revealed that in Mashl homozygous null mice no PNEC are detected, while PNEC increase markedly in Hesl homozygous null mice. These obse~at ionss uggest that Mashl is an essential positive factor for neuroendocrine differentiation of lung epithelium, and that Hesl is one of the repressive factors for neuroendocrine differentiation. Moreover, immunohistochemical studies revealed that Notch receptors are detected in non-PNEC, and thus the Notch signalling pathway could play a role in the determination of airway epithelial cell differentiation. In human lung carcinomas, a similar bHLH network should operate to determine cell differentiation phenotypes. Generally, expression of the human homologue of Mashl (HASH1) is detected in small cell carcinoma and carcinoids, while Hesl seems to be expressed mainly in non-small cell carcinoma. Thus, proneuronal bHLH factors may play roles in cell fate determination of the airway epithelial system, and may regulate human airway epithelial cells in diseased conditions.