Altered actin organization in an N-methyl-N'-nitro-N-nitrosoguanidine-transformed pulmonary epithelial cell line

After in vitro-induced neoplastic transformation by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), altered actin organization in pulmonary epithelial cells was examined. A certain correlation was found between anchorage independency (AI), tumorigenicity in nude mice and altered actin organization. DNase I inhibition assay demonstrated significant loss of detergent-insoluble F-actin (stress fibers, p less than 0.02) and soluble F-actin (single filaments, p less than 0.0001) after AI transformation in comparison with untransformed cells, while the level of G-actin increased significantly (p less than 0.0001). At the same time, fluorescence and electron microscopy also revealed that after AI transformation there was a striking loss of stress fibers usually accompanied by reorganization of at least some of the lost stress fibers into F-actin aggregations. After s.c. implantation of AI-transformed cells into nude mice followed by recultivation of the developed tumors, DNase I-inhibition assay showed a significant increase (p less than 0.001) in the level of detergent-insoluble F-actin as compared with untransformed cells, but no significant difference in the amount of G-actin. However, most of this increased detergent-insoluble F-actin was in the form of aggregations as revealed by immunofluorescence and electron microscopy. This growth behaviour-dependent alteration in actin organization occurring after exposure to MNNG may be causally related to the progressive development of neoplastic phenotypes, although the biological significance of actin aggregation formation remains unclear. The results have also pointed out the importance of parallel investigations into both the biochemical and morphological statuses of actin, particularly when it may be regarded as an indicator of neoplastic transformation and malignancy.