Inhibition of Vitamin D 3 -Induced Cell Differentiation by Interferon-γ in HL-60 Cells Determined by a Nitroblue Tetrazolium Reduction Test

The combined effects of 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] and interferon-γ (IFN-γ) or tumor necrosis factor α (TNF-α) on cell differentiation in HL-60 human promyelocytic leukemia cells were examined by a nitroblue tetrazolium (NBT) reduction test. 1,25(OH)₂D₃ at the concentrations of 7-70 nM induced NBT-positive cells, which was used as a criterion of cell differentiation. IFN-γ itself showed little effect on induction of NBT-positive cells or on cell growth at a concentration up to 1000 U/ml. However, in a combination of 1,25(OH)₂D₃ with IFN-γ (100 and 300 U/ml), cell differentiation was strongly inhibited and was accompanied by growth inhibition. Treatment with a combination of 1,25(OH)₂D₃ and TNF-α or IFN-γ and TNF-α showed an additive effect on cell differentiation. IFN-γ seems to act as a specific inhibitor for 1,25(OH)₂D₃-induced cell differentiation. To elucidate the cause of the inhibition of cell differentiation by IFN-γ, the ability of the cells to produce superoxide (0₂ ^-) was examined after culture for 5 days in the presence of 1,25(OH)₂D₃ and IFN-γ. The results indicated that the inhibition of IFN-γ was caused by a reduction in the ability of the cells to produce O₂ ^- in response to stimulation by 12-O-tetradecanoylphorbol-13-acetate (TPA).