Previous studies have shown that Chinese hamster embryo (CHE) cells transferred in vitro can easily transform to preneoplastic immortal stages but only 2 (designated CHE Al and CHE A2) out of 9 stains express tumorigenicity in nude mice (Shimizu et a/., Jpn. J. Cancer Res.,86,546-554,1995). Here we have isolated four tumor-derived cell lines, which were originated from two cell strains cultured for 15 or more passages, and examined their genetic changes relating to the malignant transformation using various oncogene probes. DNA from all tumor-derived cell lines exhibited a high transforming activity to NIH3T3 cells. However only one series of NIH3T3 transformants induced by the DNA from T25A2 cells, which were isolated from one tumor produced by the injection of CHE A2 cells cultured for 25 passages, contained Chinese hamster N-ras. A to T transversion in the second position of N-ras codon 61 was identified by direct dideoxy sequencing of the PCR products of T25A2 cell DNA. The same mutation was also detected in 25 passaged CHE A2 cells using a sensitive method in which PCR products were digested by restriction enzyme Mae I that can specifically cleave N-ras DNA fragments having the mutant sequence at codon 61. These results suggest that cultured CHE cells progress toward malignant stages through multiple pathways accompanied by various transforming genes. Our results indicate that mutation of N-ras gene may be one of these genetic changes.
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