Effect of 6-aminonicotinamide and other protein synthesis inhibitors on formation of platinum-DNA adducts and cisplatin sensitivity

The present study was undertaken to examine the mechanistic basis for the r ecent observation that the pyridine nucleotide derivative 6-aminonicotinami de (6AN, NSC 21206) enhances the accumulation and resulting cytotoxicity of cisplatin in a variety of tumor cell lines. When A549 lung cancer cells or K562 leukemia cells were treated with 62.5 mu M 6AN for 21 h and then puls e-labeled with [S-35]methionine for 1 h, increased labeling of five polypep tides, one of which corresponded to a M-r similar to 78,000 glucose-regulat ed protein (GRP78), was observed. Two subsequent observations, however, sug gested that up-regulation of these polypeptides was unlikely to explain the interaction between 6AN and cisplatin: 1) the concentration of 6AN require d to induce GRP78 was 4-fold higher than the dose required to sensitize cel ls to cisplatin; and 2) simultaneous treatment of cells with 6AN and cycloh eximide prevented the increase in GRP78 but not the sensitizing effect of 6 AN. On the contrary, treatment with the protein synthesis inhibitors cycloh eximide, anisomycin, or puromycin as well as prolonged exposure to the RNA synthesis inhibitor actinomycin D mimicked the biochemical modulating effec ts of 6AN on cisplatin action. Conversely, 6AN inhibited protein synthesis, whereas 18 6AN analogs that failed to enhance Pt-DNA adducts and cisplatin cytotoxicity failed to inhibit protein synthesis. These observations are c onsistent with a model in which 6AN and other inhibitors of protein synthes is act as modulating agents by increasing cisplatin accumulation, thereby e nhancing the formation of Pt-DNA adducts and subsequent cisplatin-induced c ell death.