Permanent conversion of mouse and human cells transformed by activated ras or raf genes to apparently normal cells by treatment with the antibiotic azatyrosine.

Azatyrosine [L-beta-(5-hydroxy-2-pyridyl)-alanine], an antibiotic isolated from Streptomyces chibanensis, inhibited the growth of NIH 3T3 cells transformed by the activated human c-Ha-ras gene but did not significantly inhibit the growth of normal NIH 3T3 cells. Surprisingly, upon treatment with azatyrosine most of the transformed cells apparently became normal. These apparently normal cells, named revertant cells, grew in the presence of azatyrosine and stopped growing when they reached confluency, and their normal phenotype persisted during prolonged culture in the absence of azatyrosine. The revertant cells did not grow in soft agar and scarcely proliferated in nude mice. The human c-Ha-ras gene present in transformed NIH 3T3 cells was still present in the revertant cells and was expressed to the same extent as in the original transformed cells, producing the same amount of activated p21. Treatment with azatyrosine caused similar conversion of NIH 3T3 cells transformed by activated c-Ki-ras, N-ras, or c-raf to apparently normal cells, but NIH 3T3 cells transformed by hst or ret were not exclusively converted by azatyrosine. Human pancreatic adenocarcinoma cells, which are known to contain an amplified activated c-Ki-ras gene and an amplified c-myc gene, were also converted to flat and giant revertant cells by treatment with azatyrosine.