The study of human myeloid differentiation using bromodeoxyuridine (BrdU)

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Abstract
Little is known concerning the mechanism of myeloid differentiation. A human promyelocytic cell line (HL‐60) differentiates to granulocytes or macrophage‐like cells when cultured with a variety of agents. How these agents trigger myeloid differentiation is not understood. This study shows that 1.0–10.0 μg/ml bromodeoxyuridine (BrdU) induced myeloid differentiation of HL‐60 in liquid culture. After 7 days, BrdU (3.0 μg/ml) produced only moderate inhibition of HL‐60 growth, but induced myeloid maturation with 40% of the cells becoming morphologically more mature; 41% developed the ability to reduce nitroblue tetrazolium (NBT); 19% phagocytized Candida albicans; and 18% developed Fc receptors. The action of BrdU was mimicked by 5‐iododeoxyuridine. Thymidine (Td) (1‐ to 10‐fold excess) competitively inhibited incorporation of [3H]BrdU into DNA of HL‐60 and inhibited the triggering of HL‐60 differentiation by BrdU. The BrdU‐induced maturation of HL‐60 correlated with the incorporation of BrdU into DNA of HL‐60. DNA buoyant density studies showed that about 46% of the Td was replaced by BrdU in each DNA strand of HL‐60 as the cells differentiated in culture containing 3 μg/ml BrdU for 7 days. We established 20 thymidine kinase (TK)‐deficient HL‐60 clones. The HL‐60 TK‐deficient cells were unable to phosphorylate Td, to incorporate either [3H]Td or [3H]BrdU or differentiate in the presence of BrdU (1‐1000 μg/ml). The HL‐60 TK‐deficient cells retained the ability to differentiate in the presence of other HL‐60 inducers. Taken together, the studies suggest myeloid differentiation of HL‐60 is triggered because of incorporation of BrdU into DNA of the cells.