Localization of MOC-1 cell surface antigen in small-cell lung carcinoma cell lines: an immunohistochemical and immunoelectron microscopic study.

Expression of cell surface antigens of the neural cell adhesion molecule (N-CAM) class was recently shown to be shared by both fetal and neoplastic neuroendocrine cells, including those of the lung. We investigated the expression and localization of MOC-1 antigen on small-cell (neuroendocrine) lung carcinoma cell lines with immunohistochemical methods at the light (LM) and electron microscopy (EM) level and by Western blot. At LM level, using monoclonal antibody (MAb) MOC-1 with the ABC method and immunofluorescence, positive staining was observed on surfaces of cells from all tumor lines examined. Strongest immunostaining was found on cell surfaces of pulmonary small-cell carcinoma-derived cell line NCI-H69 with the majority of cells showing positive staining. An adherent variant of NCI-H69 cell line, H69V, exhibited positive staining in about 60% of cells, whereas only occasional cells of NCI-H727 cell line derived from pulmonary carcinoid tumor were positive for MOC-1 antigen. Western blot analysis confirmed these findings, showing a strong MOC-1-specific band in cell extracts of NCI-H69, with weaker band densities for H69V and NCI-H727. Immunoelectron microscopy (IEM) revealed that MOC-1 was not uniformly distributed on the outer surface of plasma membrane; immunogold particles appeared concentrated in areas of thick cell surface "fuzz" coating, surface microvilli, and in areas of cell-cell contact. In some cells, areas of plasma membrane invaginations and a few intracytoplasmic vesicles were also labeled, suggesting endocytosis. Surface labeling for SEM confirmed the finding of more dense labeling over the microvilli, cell membrane folds, and in areas of cell-cell contact. The cell lines derived from pulmonary neuroendocrine cell tumors can provide a useful model to study the role and function of neural adhesion molecules in pulmonary neoplasia and during lung development.