Cell Cycle Events Associated with the Termination of Proliferation by Cytotoxic and Differentiation-inducing Actions of 6-Thioguanine on HL-60 Cells

Delayed growth arrest was observed in HL-60 acute promyelocyte leukemia cells after exposure to 6-thkxjuanine (TG). This growth arrest occurred in both wild-type HL-60 cells exposed to 2 µM TG and an HL-60 clone lacking hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity at a 500-fokJ higher concentration of drug. Both cell lines continued replication during an initial 4-day period of exposure to TG; however, upon removal of the purine antimetabolite and reincubation in fresh medium in the absence of drug, no further increase in cell number was observed over the next 4 days. Extensive differentiation, as measured by the reduction of nitroblue tetrazolium, occurred in TG-treated, HL-60 HGPRT-negative cells, whereas no significant increase in the number of nitroblue tetrazolium-positive cells was observed in wild-type HL-60 cells exposed to the purinethiol. Thus, termination of proliferation in wild-type cells appeared to be an expression of cytotoxicity, while in the HGPRT-negative clone, cell replication was apparently terminated by conversion of cells to end-stage forms with a mature phenotype. In support of this conclusion, differences occurred in the stage of the cell cycle arrest, determined on Day 6 after exposure to TG. Approximately 85% of parental HL-60 cells treated with TG were present in the S and G2 + M phases of the cell cycle, with the greatest proportional change from untreated controls being in the G2-M phase (i.a, a 63% increase over untreated controls). In contrast, HL-60 HGPRT-negative cells treated with TG accumulated in G1, with 68% of the population located in this phase (i.e., an 80% increase compared to controls), as might be expected for a differentiated population. Dimethyl sulfoxide, which produced differentiation in both parental HL-60 and HL-60 HGPRT-negative cells, was used as a positive control. Both cell lines responded identically to dimethyl sulfoxide, with growth arrest being due at least in part to differentiation, which corresponded to an increase in G1 cells.