1,25-Dihydroxyvitamin D₃ enhances cytosolic free calcium in HL-60 cells

1,25-Dihydroxyvitamin D₃ (1,25[OH]₂D₃) caused a rise in the concentration of intracellular free calcium ions ([Ca²⁺](i)) in HL-60 cells. This effect of 1,25(OH)₂D₃ parallels its suppression of cell proliferation and its induction of cell differentiation into monocyte-like cells. The changes in [Ca²⁺](i) are dose and time time dependent. The concentration of 1,25(OH)₂D₃ (10^-7 M) that induced maximal differentiation also caused the maximal increase in intracellular Ca²⁺. The rise in cytoplasmic free Ca²⁺ concentration was not immediate and reached statistical significance only after 24 h. The [Ca²⁺](i) reached its peak at 48 h (134 ± 4 nM vs 101 ± 3 nM in controls) and remained stable at this level. The increase in intracellular Ca²⁺ was found to be related to new protein synthesis, because it was inhibited in the presence of specific RNA and protein synthesis inhibitors. The rise in [Ca²⁺](i) was not observed during incubation of HL-60 cells with 24,25-dihydroxyvitamin D₃ (24,25[OH]₂D₃), a vitamin D metabolite that does not induce the differentiation of HL-60 cells. In contrast, 25-hydroxyvitamin D₃ (25-OH-D₃) and phorbol 12-myristate 13-acetate (TPA), both of which induce differentiation in this cell line, also increase [Ca²⁺](i). In conclusion, the present study emphasizes that a significant increase in intracellular free Ca²⁺ occurs in the effect of 1,25(OH)₂D₃ on HL-60 cells.