9-Deazaadenosine. Cytocidal activity and effects on nucleic acids and protein synthesis in human colon carcinoma cells in culture.

The effect of 9-deazaadenosine (c9Ado) on cell lethality and the synthesis of nucleic acids was investigated in human colon carcinoma cell line HT-29. c9Ado produced a rapid threshold-exponential reduction in colony formation as measured by a soft agar clonogenic assay. This effect was evident after either a 2- or 24-hr exposure interval, and was produced over a very narrow concentration range of drug. Following 2 hr of drug exposure at concentrations producing a 1- to 3-log reduction in cell viability, DNA and RNA syntheses were inhibited 20% and protein synthesis was inhibited 35-50%. The latter effect became quite pronounced in comparison to nucleic acid synthesis 4 hr after drug treatment. Long treatment intervals (24 hr) with concentrations of c9Ado producing similar effects on cell viability resulted in 15-35% inhibition of RNA synthesis, 80-85% inhibition of DNA synthesis, and 60-70% inhibition of protein synthesis. None of these metabolic effects could be accounted for by changes in ribonucleoside triphosphate levels despite the considerable formation of c9ATP. Measurements of the incorporation of [3H] c9Ado into total cellular nucleic acids indicated that the labeling of RNA was 40-80% greater than that of DNA. Polysomal poly(A)RNA contained 300% more [3H]c9Ado than non-poly(A)RNA after 2 hr of drug exposure and 50% more [3H]c9Ado following 24 hr of treatment. There was no evidence of DNA strand breakage by incorporated c9Ado. Analysis of nascent protein synthesis in drug-treated cells revealed that this process was inhibited in concert with polysome breakdown. These results suggest that the rapidity by which cell lethality is produced by c9Ado may be related to inhibition of translation via its incorporation into RNA.