6-THIOGUANINE-INDUCED GROWTH ARREST IN 6-MERCAPTAPURINE-RESISTANT HUMAN LEUKEMIA-CELLS

The thiopurines 6-thioguanine (6TG) and 6-mercaptopurine (6MP) are cyt otoxic to proliferating cells by a mechanism involving incorporation i nto DNA via the purine salvage pathway, and resistance to these agents can be conferred by lack of the salvage pathway enzyme hypoxanthine-g uanine phosphoribosyltransferase. However, human and murine hypoxanthi ne-guanine phosphoribosyltransferase-deficient leukemia cell Lines hav e been shown to respond to 6TG by growth arrest and differentiation by a mechanism apparently not involving incorporation of 6TG into DNA. I f so, leukemia cells resistant to 6MP should still respond to 6TG by g rowth arrest via an undescribed epigenetic mechanism. To test this, po lyclonal 6MP-resistant variants were produced from three human leukemi a cell lines, HL-60, U937, and CCRF-CEM. Treatment of both sensitive a nd resistant cells with 6TG induced growth arrest. The effect of 6TG i n the 6MP-sensitive HL-60 and U937 cells was associated with significa nt loss of viability and DNA fragmentation. In contrast, the 6TG-treat ed 6MP-resistant cells exhibited a slower decline in viability and no DNA fragmentation. To identify the mechanism by which 6TG may induce g rowth arrest, tRNA was isolated from 6MP-resistant cells cultured for 48 h with 6TG. 6TG was found to be incorporated into tRNAs normally co ntaining queuine in the anticodon wobble position. These studies may p rovide a basis for the development of new therapeutic regimens for the treatment of leukemia.