Premalignancy in vitro: progression of an initially benign epithelial cell line to malignancy

An epithelial cell line, NMuLi, derived from normal mouse liver, produced a variety of tumours when inoculated s.c. into newborn mice. The inoculation of early-passage cells resulted in cystadenomas and proliferating cysts, while adenocarcinomas and carcinosarcomas were produced from cells that had been extensively subcultured. To determine the mechanism involved in the progression to malignancy, single-cell clones were isolated from early- and late-passage NMuLi cells. When inoculated into mice, early-passage clones were similar to the original population in that they initially formed benign cysts but produced increasingly aberrant tumours after extensive subculture. Inoculation of the late-passage clones resulted in only malignant tumours. These observations suggest that passage in culture induced an initially benign epithelial cell population to progress toward malignancy; thus early passage NMuLi cells represent a good in vitro model of the premalignant state.