Promotion of 3'-methyl-4-dimethylaminoazobenzene-initiated adult rat liver cells to malignant state by phenobarbital in culture.

After treatment with 1.5 mM phenobarbital (PB) for about 2 months, non-tumorigenic rat liver epithelial-type cell lines (6F-7 and 6G-5), which were initiated by 3'-methyl-4-dimethyl-aminoazobenzene (3'-Me-DAB) in primary culture, showed an increase in plating efficiency, growth in soft agar and production of tumors in syngeneic animals. However, the PB-untreated control cells exhibited neither growth in soft agar nor production of tumors. Gamma-glutamyltranspeptidase (GGT)-positive (6F-7-cl-7) and negative (6F-7-cl-8) colonial clones, which were derived from 6F-7 cells, were also treated with 1.5 mM PB for about 2 months. The growth of GGT-positive 6F-7-cl-7 cells was not suppressed but rather stimulated by PB at 1.5 mM, whereas that of GGT-negative 6F-7-cl-8 cells was severely inhibited. Furthermore, the PB-treated 6F-7-cl-7 cells produced tumors in syngeneic animals, but the PB-treated 6F-7-cl-8 cells produced no tumors. The spontaneous cell line 6A-6, which was derived from the 3'-Me-DAB-untreated primary liver cell culture and was GGT-negative, did not exhibit tumorigenicity after treatment with 1.5 mM PB for the same period of time as described above. In conclusion, PB treatment efficiently and rapidly promoted the 3'-Me-DAB-initiated cells to a malignant state.