Initiation of lung cell proliferation by trypsin.

The ability of trypsin to initiate DNA synthesis in alveolar epithelial cells of mice was tested by introducing solutions containing trypsin into the lung. The concentrations tested ranged from 0.01 to 1.0 mg. per ml., the site of action was identified by adding colloidal carbon as a tracer, and chromosomal DNA synthesis was detected by autoradiography on the basis of strong nuclear incorporation of tritiated thymidine. In the above concentration range trypsin did not cause cell necrosis, general fragmentation of cell processes, or loss of mechanical contact at epithelial cell junctions. Edema and focal acute inflammation were apparent at concentrations of trypsin above 0.1 mg. per ml. In the alveolar epithelium maximal labeling was seen 2 days after treatment with 0.5 mg. per ml. of trypsin. At this time electron microscope autoradiography covering two "S" periods showed that one-third of the type II cells were labeled. No labeled type I cells were seen. Trypsin at levels less than 0.1 mg. per ml. did not increase the frequency of labeled type II cells above that of the saline-colloidal ink solution alone. Mesothelial cells lining the lung were stimulated by trypsin (0.5 mg.) given intrapleurally. The frequency of mesothelial cell labeling peaked at 2 days with a labeling index for a single tritiated thymidine pulse of 18 per cent. Neither alveolar type II cells nor mesothelial cells showed increased labeling when trypsin inactivated by diisopropyl fluorophosphate was used.