Malignant transformation of simian virus 40-immortalized human milk epithelial cells by chemical carcinogenesis accompanied by loss of heterozygosity on chromosome 1 but not microsatellite instability.

Simian virus 40-immortalized human milk epithelial cells (HuMI) are anchorage dependent and non-tumorigenic but can spontaneously progress to anchorage-independent and tumorigenic cells. To see whether HuMI cells can be transformed into anchorage-independent cells by chemical carcinogens, we treated them with 3-methylcholanthrene (MCA, 10 microg/mL). After 7-8 wk of culture, none of the treated cells grew in soft agar. However, when HuMI cells treated with MCA were cultured with 12-O-tetradecanoylphorbol-13-acetate (TPA, 10 ng/mL), they grew in soft agar; cells treated with TPA alone did not. TPA at this dose was cytotoxic to HuMI cells but not to their tumorigenic subline HuMI-TTu2. The response of the anchorage-independent HuMI-T cells was intermediate. These results indicate that HuMI cells can be transformed by treatment with MCA plus TPA, possibly because TPA selects those cells that are progressing toward malignancy. All five clones from MCA plus TPA-induced transformed cells formed malignant carcinomas in nude mice. When microsatellite changes at 17 loci in HuMI, HuMI-T, HuMI-TTu2, and five MCA plus TPA-transformed cells were examined, none of these cell lines showed instability at any locus, and no change in microsatellite length was found. However, all five MCA plus TPA-transformed cell lines showed loss of heterozigosity at 1q21-23 and 1q42 loci. This region of chromosome 1 is known to contain at least one antiproliferative gene, and our results suggest that inactivation of such a gene may be essential for full transformation of HuMI cells by chemical carcinogens.