Reduced cytotoxicity of the lysosomotropic detergent N-dodecylimidazole after differentiation of HL60 promyelocytes.

The sensitivity of the human promyelocytic cell line HL60 to killing by the lysosomotropic detergent N-dodecyl imidazole (C12-Im) has been investigated in the exponential and stationary growth states and before and after differentiation induced by suitable effector molecules. Undifferentiated HL60 cells were more sensitive to killing by C12-Im in the rapid (exponential) phase of growth than in the stationary phase, in keeping with our observations on many other cell lines. Differentiation into granulocytes induced by dimethyl sulfoxide, or into macrophages induced by phorbol ester, resulted in a further dramatic decrease in sensitivity to C12-Im, as compared to undifferentiated HL60 cells in stationary phase. Viable cells remaining after treatment with C12-Im (60 micrograms/ml, 2 h) were: 0% for exponentially growing undifferentiated cells; 16% for stationary undifferentiated cells; 41% for differentiated granulocytes; and 29% for differentiated macrophages. Treatment with the cysteine cathepsin inhibitor L-trans-epoxysuccinylleucylamido(4-guanido)butane (E64) conferred resistance to C12-Im, showing that, in these cells, as previously demonstrated for Chinese hamster ovary fibroblasts, cysteine proteases were major cytotoxic agents involved in killing by C12-Im. Cell cathepsin B + L activity levels were dramatically reduced in those cells differentiated into granulocytes (11.2 units/mg of protein) and into macrophages (9.8 units/mg of protein) as compared with undifferentiated HL60 promyelocytes in stationary phase (30.4 units/mg of protein), correlating well with reduced sensitivity to C12-Im in the differentiated cells.