Biologic analysis of fetal MRC rat lung epithelial cells treated with 3-methylcholanthrene in culture: premalignant and malignant stages.

Abstract

Fetal MRC rat lung primary cell sheets, in which reconstruction of bronchial tissue occurred, were treated with 3-methylcholanthrene (MCA) to examine the carcinogen's biologic effect on lung epithelial elements. Cloned lines were established by subculturing of epithelial outgrowths from primary cell sheets induced by MCA treatment. Newborn MRC rats received an sc injection of these cells to assess their tumorigenicity. These MCA-treated cells at the initial stages of culture temporarily formed benign epithelial structures in scar tissue that developed from an sc injection and then regressed. The epithelial structures were roughly classified as tubular and squamous epithelium. The differentiation of cells in vitro was observed as either a type of lamellar keratinization of the premalignant squamous cell form or a type of cyst formation by premalignant secreting epithelium. With further cultivation, six of 24 cell lines became malignant and, when injected into the animal, induced carcinomas. Qualitative changes in the in vitro differentiation of cells were observed in cell lines that formed squamous cell carcinomas. These changes included spherical keratinization (pearl formation) and single-cell keratinization. MCA treatment induced various premalignant alterations in lung epithelial cells, and the subsequent malignancy seemed to stem from these initial premalignant changes.