Azatyrosine is incorporated into proteins instead of tyrosine residues, with the resultant conversion of transformed cells to cells with a normal phenotype

We reported recently that azatyrosine inhibits the growth of c-Ha-ras, c-raf or c-erbB-2-transformed NIH3T3 cells and converts the transformed cells to cells with a normal phenotype. To analyze the mode of action of azatyrosine, we examined the effects of azatyrosine on the synthesis of macromolecules. Azatyrosine had no obvious inhibitory effects on the synthesis of DNA, RNA and protein in c-erbB-2-transformed cells. Furthermore, azatyrosine inhibited cell growth but did not interrupt the cell cycle at any specific stage. Thus, the mode of action of azatyrosine appeared to be different from that of typical anticancer drugs. Moreover, we found that azatyrosine was incorporated into proteins instead of tyrosine. The simultaneous presence of a high concentration of tyrosine inhibited the conversion to a normal phenotype of transformed cells by azatyrosine. These results strongly suggest that incorporation of azatyrosine into proteins might convert the transformed cells to cells with a normal phenotype. The analysis of azatyrosine-containing proteins by two-dimensional electrophoresis revealed that the mobilities of some proteins differed from those of the corresponding tyrosine-containing proteins. An alteration in molecular structure of the proteins that include azatyrosine residues might be associated with the ability of azatyrosine to convert transformed cells to cells with a normal phenotype.