Hematopoiesis and the inosine modification in transfer RNA

Protein synthesis in HL-60 cells treated with DMSO and hypoxanthine

Short-term treatment of the HL-60 cells with DMSO and hypoxanthine, inducers of granulocytic differentiation, was reported to cause a rapid increase in protein synthesis. This effect was ascribed to the insertion of inosine in the wobble position of the tRNA anticodon and consequently increasing codon recognition potential. In this study we have re-investigated the effects of DMSO and/or hypoxanthine on protein synthesis. In contrast to their findings we were unable to demonstrate stimulated protein synthesis in either short- or long-term treatment with these agents. Polysome analysis under these conditions revealed that polysomes were disaggregated. Finally, the activity of tRNA-hypoxanthine ribosyltransferase, an enzyme responsible for the insertion of inosine in the anticodon, was also relatively low. Under these circumstances, we propose that tRNA modification is not essential in the regulation of protein synthesis.

Guanine analog-induced differentiation of human promyelocytic leukemia cells and changes in queuine modification of tRNA

Treatment of hypoxanthine-guanine phosphoribosyltransferase (HGPRT)-deficient human promyelocytic leukemia (HL-60) cells with 6-thioguanine results in growth inhibition and cell differentiation. 6-Thioguanine is a substrate for the tRNA modification enzyme tRNA-guanine ribosyltransferase, which normally catalyzes the exchange of queuine for guanine in position 1 of the anticodon of tRNAs for asparagine, aspartic acid, histidine, and tyrosine. During the early stages of HGPRT-deficient HL-60 cell differentiation induced by 6-thioguanine, there was a transient decrease in the queuine content of tRNA, and changes in the isoacceptor profiles of tRNA(His) indicate that 6-thioguanine was incorporated into the tRNA in place of queuine. Reversing this structural change in the tRNA anticodon by addition of excess exogenous queuine reversed the 6-thioguanine-induced growth inhibition and differentiation. Similar results were obtained when 8-azaguanine (another inhibitor of queuine modification of tRNA that can be incorporated into the anticodon) replaced 6-thioguanine as the inducing agent. The data suggest a primary role for the change in queuine modification of tRNA in mediating the differentiation of HGPRT-deficient HL-60 cells induced by guanine analogs.

Guanine analog-induced differentiation of human promyelocytic leukemia cells and changes in queuine modification of tRNA

Treatment of hypoxanthine-guanine phosphoribosyltransferase (HGPRT)-deficient human promyelocytic leukemia (HL-60) cells with 6-thioguanine results in growth inhibition and cell differentiation. 6-Thioguanine is a substrate for the tRNA modification enzyme tRNA-guanine ribosyltransferase, which normally catalyzes the exchange of queuine for guanine in position 1 of the anticodon of tRNAs for asparagine, aspartic acid, histidine, and tyrosine. During the early stages of HGPRT-deficient HL-60 cell differentiation induced by 6-thioguanine, there was a transient decrease in the queuine content of tRNA, and changes in the isoacceptor profiles of tRNA(His) indicate that 6-thioguanine was incorporated into the tRNA in place of queuine. Reversing this structural change in the tRNA anticodon by addition of excess exogenous queuine reversed the 6-thioguanine-induced growth inhibition and differentiation. Similar results were obtained when 8-azaguanine (another inhibitor of queuine modification of tRNA that can be incorporated into the anticodon) replaced 6-thioguanine as the inducing agent. The data suggest a primary role for the change in queuine modification of tRNA in mediating the differentiation of HGPRT-deficient HL-60 cells induced by guanine analogs.