HYDROQUINONE, A BIOREACTIVE METABOLITE OF BENZENE, INHIBITS APOPTOSISIN MYELOBLASTS

Hydroquinone (a major marrow metabolite of the leukemogen, benzene) in duces incomplete granulocytic differentiation of mouse myeloblasts to the myelocyte stage, and also causes an increase in the number of myel ocytes. This was confirmed using the normal interleukin 3 (IL-3)-depen dent mouse myeloblastic 32D cell line. The hydroquinone-induced twofol d increase in the number of IL-3-treated myelocytes does not result fr om stimulation of IL-3-induced proliferation. Hydroquinone's ability t o effect this increase through an inhibition of apoptosis was investig ated using mouse 32D and human HL-60 myeloblasts. Apoptosis induced by staurosporine treatment (0.5-1.0 mu M) of HL-60 cells (50%) and 32D c ells (15%) or by IL-3 withdrawal from 32D myeloblasts was determined b y monitoring the development of characteristic morphological features and confirmed by the appearance of a typical nucleosomal DNA ladder up on agarose gel electrophoresis. Concentrations of hydroquinone (1-6 mu M) that induce differentiation in 32D myeloblasts caused a concentrat ion-dependent inhibition of staurosporine-induced apoptosis in both ce ll lines, with a 50% inhibitory concentration of 3 mu M, and prevented apoptosis in IL-3-deprived 32D cells. Hydroquinone inhibition of apop tosis in myeloblasts, like hydroquinone-induced granulocytic different iation, required myeloperoxidase-mediated oxidation of hydroquinone to its reactive species, p-benzoquinone, and was inhibited 50% by the pe roxidase inhibitor, indomethacin (20 mu M). p-benzoquinone (3 mu M) wa s shown to cause a 50% inhibition of CPP32, an IL-1 beta-converting en zyme/Ced-3 cysteine protease involved in the implementation of apoptos is and present in myeloid cells. The ability of hydroquinone to induce a program of differentiation in the myeloblast that proceeds only to the myelocyte stage coupled with its ability to inhibit the CPP32 prot ease and, thereby, apoptosis of the proliferating myelocytes, may have important implications for benzene-induced acute myeloid leukemia.