Immunohistochemical investigation of S100A9 expression in pulmonary adenocarcinoma: S100A9 expression is associated with tumor differentiation

S100 protein A9 is associated with myeloid cell differentiation and is also expressed in some epithelia. However, there have been few studies on S100A9 in specific types of carcinomas, except for squamous cell carcinoma (SCC) because the expression in normal epithelia is limited to squamous epithelia. Recently, S100A9 gene expression has been detected in cultured human adenocarcinoma (AC) cells derived from various organs. In this study, we also detected S100A9 gene expression in human pulmonary AC cell lines by reverse transcription-polymerase chain reaction. Furthermore, using the monoclonal antibody against S100A9, we carried out an immunohistochemical evaluation of S100A9 protein expression in 70 cases of resected pulmonary AC and examined the relation of S100A9 expression to tumor differentiation. S100A9 immunopositivity was 0/21 (0%) in well differentiated ACs, 12/30 (40%) in moderately differentiated ACs and 19/19 (100%) in poorly differentiated ACs, and the poorly differentiated ACs showed a significantly greater positive reaction. The immunopositivity in the moderately differentiated ACs was marked in specific cytologic subtypes. In the controls, conspicuous S100A9 immunopositivity was observed in pulmonary SCCs, regardless of the degree of differentiation, but not in adenomatous hyperplasia or normal surface epithelia. These above results suggest that the S100A9 protein is also expressed in pulmonary AC and that the expression rate in pulmonary AC shows higher correlation in poorly differentiated carcinomas, in agreement with our recent results regarding liver carcinoma. We believe S100A9 is also closely related to the differentiation of carcinomas of glandular cell origin.