Down-Regulation and Antiproliferative Role of C/EBP in Lung Cancer

The transcription factor, CCAAT/enhancer binding protein (C/ EBP ) is important in the terminal differentiation of granulocytes, hepatocytes, and adipocytes, and recurrent mutations of C/EBP were described in acute myeloid leukemia. In the lung, C/EBP is expressed in bronchial cells and type II pneumocytes. Abnormal proliferation of the latter cell type was reported in C/EBP knockout mice. We determined the expression of C/EBP by Northern blot analysis in 30 lung cancer cell lines and found significant down-regulation in 24 cell lines. Immunohistochemical study of primary tumor specimens showed undetectable or low expression of C/EBP in 23 of 53 specimens. Its expression was more frequently down-regulated in adenocarcinoma and poorly differentiated cancer specimens than in squamous cell cancers. A higher frequency of reduced expression was found in more advanced stages. To investigate the consequences of C/EBP expression in lung cancer cells, we stably transfected two cell lines that do not express the gene (Calu1 and H358) with a plasmid allowing for induction of C/EBP protein expression. Induction of C/EBP led to significant growth reduction attributable to proliferation arrest, morphological changes characteristic of differentiation, and apoptosis. These results suggest that C/EBP is down-regulated in a large proportion of lung cancers and that it has growth-inhibitory properties in airway epithelial cells. Genetic analysis of the C/EBP gene is in progress to fully evaluate its role as a novel tumor suppressor in lung cancer.