Characterization of differentiation-inducer-resistant HL-60 cells

Abstract

Sub-lines of the cultured human promyelocytic leukemia cell line HL-60 were individually selected for their ability to sustain exponential growth in the presence of 3 structurallyunrelated inducers of granulocytic differentiation — retinoic acid (RA), dimethylsulfoxide (DMSO), and 6-thioguanine (6TG). Selections were made by step-wise augmentation to final drug concentrations of 10 −3mM RA, 169mM (1.2%) DMSO and 0.12mM (20 μg ml −1) 6TG. In addition to growth resistance, cells in each sub-line displayed variable cytodifferentiation resistance to each of the 3 selective agents, which was quantitated as the ratio of the concentration of drug required to induce differentiation in 50% of the cells in each resistant sub-line versus comparably-passaged wild-type HL-60 cells. The levels of resistance/cross-resistance were as follows: RA-resistant (res) sub-line > 2700-fold to RA, 1.3-fold to DMSO and > 1.5-fold to hypoxanthine (HXN; the noncytotoxic purine base inducer analogue of 6TG); DMSO-res sub-line 2.5-fold to DMSO, 137-fold to RA and > 1.5-fold to HXN; and 6TG-res sub-line > 1.5-fold to HXN, 9-fold to RA and 1.6-fold to DMSO. These sub-lines were not cross-resistant to sodium butyrate (NaBut), a monocyte inducer, or to 12- O-tetradecanoylphorbol 13-acetate (TPA), a macrophage inducer. HL-60 sub-lines selected by exposure to a single high concentration of 5-bromo-2′-deoxyuridine (BUdR; 3.3 × 10 −2mM) or oubain (Ou; 5 × 10 −3mM) were not or were slightly cross-resistant to either granulocyte or monocyte inducers. Although some variations in line/sub-line phenotype were observed, this was minor compared to the quantitative variations in response to individual inducing agents. The RA-res and 6TG-res sub-lines contained numerous double minute chromosomes (indicators of amplified genes) which were either absent or present in much smaller numbers in the parental wild-type cells or in the other drug-resistant sub-lines. There was little change or a decrease in the amplification level of the known amplified oncogene c- myc in the various drug-resistant sub-lines compared to wild-type HL-60 cells. These results (a) confirm that the neutrophilic granulocytic and monocytic/macrophagic differentiation programs in HL-60 cells are mechanistically different and separable; (b) suggest that both agent-specific and common quantitative alterations contribute to the mechanism(s) for resistance to granulocyte differentiation; and (c) suggest that the latter quantitative defects could be related to amplification of genes other than c- myc.