Characterizing mutagenesis in the hprt gene of rat alveolar epithelial cells

A clonal selection assay was developed for mutation in the hypoxanthine-guanine phosphoribosyl transferase (hprt) gene of rat alveolar epithelial cells. Studies were conducted to establish methods for isolation and long-term culture of rat alveolar epithelial cells. When isolated by pronase digestion purified on a Nycodenz gradient and cultured in media containing 7.5% fetal bovine serum (FBS), pituitary extract, EGF, insulin, and IGF-1, rat alveolar epithelial cells could be maintained in culture for several weeks with cell doubling times of 2-4 days. The rat alveolar epithelial cell cultures were exposed in vitro to the mutagens ethylnitrosourea (ENU) and H2O2, and mutation in the hprt gene was selected for by culture in the presence of the toxic purine analog, 6-thioguanine (6TG). In vitro exposure to ENU or H2O2 produced a dose-dependent increase in hprt mutation frequency in the alveolar epithelial cells. To determine if the assay system could be used to evaluate mutagenesis in alveolar type II cells after in vivo mutagen or carcinogen exposure, cells were isolated from rats treated previously with ENU or alpha-quartz. A significant increase in hprt mutation frequency was detected in alveolar epithelial cells obtained from rats exposed to ENU or alpha-quartz; the latter observation is the first demonstration that crystalline silica exposure is mutagenic in vivo. In summary, these studies show that rat alveolar epithelial cells isolated by pronase digestion and Nycodenz separation techniques and cultured in a defined media can be used in a clonal selection assay for mutation in the hprt gene. This assay demonstrates that ENU and H2O2 in vitro and ENU and alpha-quartz in vivo are mutagenic for rat alveolar epithelial cells. This model should be useful for investigating the genotoxic effects of chemical and physical agents on an important lung cell target for neoplastic transformation.