In vitro investigation on proliferative cystic keratinizing squamous lesions in the rat lung

The proliferative cystic squamous cell lesion inducible by inhalation of particulate matter in the rat lung is characterized by the formation of keratin-filled cystic cavities of various sizes bordered by multiple layers of keratinizing squamous epithelial cells. The current investigation is primarily concerned with two points. One is whether the cells participating in this particular cystic configuration can recapitulate their specific in vivo morphogenetic behavior also in the in vitro circumstances. The other is whether these squamous epithelial cells are neoplastic in nature. Although the currently adopted cell culture system was two-dimensional, the specific morphogenetic pattern was reproduced in vitro in a corresponding manner by the squamous cells derived from the aforementioned rat lung lesions. Exposure of these cells to 12-O-tetradecanoyl-phorbol-13-acetate (TPA) at 100 ng/ml induced a marked polymorphism in them and also stimulated their keratinization. In soft agar culture, both TPA-exposed and unexposed cells developed colonies larger than 0.05 mm in diameter with an incidence of 0.73% and 1.12%, respectively. The lower incidence in TPA-exposed cultures can be interpreted in terms of TPA stimulation of keratinization. However, colonies larger than 0.1 mm in diameter were also developed by the TPA-exposed cells, indicating the presence of a cell population responsive to promotional effects of TPA. These results imply that the cells involved in the formation of cystic squamous epithelial cell lesions are already initiated and possess a potential for autonomous benign growth.