Disassociation of the macrophage-maturational effects of vitamin D from respiratory burst priming

During the process of enhancing monocytic differentiation of the human leukemia line HL-60, 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) also "primes" the cell for respiratory burst by increasing the uptake of Ca²⁺ across the plasma membrane (Hruska, K. A., Bar-Shavit, Z., Malone, J. D Teitelbaum, S. L. (1988) J. Biol. Chem. 263, 16039-16044). The present study asked if the maturational effect of vitamin D is dependent upon this "priming" phenomenon. To this end, we exposed HL-60 to either 1,25(OH)₂D₃ or its synthetic analogue (1α, 3β, 5Z, 7E)-9-10-Secocholesta-5,7,10(19)-triene-1, 3, 25-triol (22-oxa). We found that 22-oxa induces HL-60 maturation as effectively as does the natural steroid. As expected, 48 h of 1,25(OH)₂D₃ exposure more than doubles (p < 0.005) HL-60 basal cytosolic Ca²⁺ and increases inositol triphosphate-sensitive Ca²⁺ stores approximately 4-fold (p < 0.01). 22-oxa in contrast alters neither Ca²⁺- nor inositol triphosphate-mobilizable deposits. Moreover, 1,25(OH)₂D₃ treatment prompts a transient Ca²⁺ "spike" in response to formyl-methionyl-leucyl-phenylalanine (fMLP) and a marked increase in superoxide (O₂^-) generation when exposed to the chemotactic peptide (p < 0.01) or phorbol ester (p < 0.02). Treatment with 22-oxa does not enable HL-60 to respond to fMLP with a Ca²⁺ spike or prime the cell for respiratory burst unless it is co-incubated with the Ca²⁺ ionophore, ionomycin. Similarly, phorbol ester impacts more profoundly on O₂^- generation by 1,25(OH)₂D³ than 22-oxa preincubated cells (p < 0.02), unless the latter is added with ionomycin. Our findings indicate that the maturational effects of vitamin D sterols are independent of their capacity to prime cells for respiratory burst and that the Ca²⁺ ionophoretic effects of 1,25(OH)₂D₃ play a major role in such priming.