Synergistic Effect of 1, 25-Dihydroxyvitamin D₃ and Retinoic Acid in Inducing U937 Cell Differentiation

Abstract

We examined the effects of retinoic acid (RA), 1, 25-dihydroxyvitamin D₃ (1, 25-(OH)₂D₃), and its synthetic analogue, 22-oxa-1, 25-(OH)₂D₃, on differentiation of U937 cells by studying the cellular growth, surface marker expression and cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). RA inhibited cellular growth but did not induce expression of Mo2 (CD14), a monocyte/macrophage specific surface marker. To the contrary, 1, 25-(OH)₂D₃ did not inhibit cellular growth, but increased CD14-positive cells. Simultaneous addition of 1, 25-(OH)₂D₃ and RA had no additive effect on cellular growth inhibition or CD14 expression. With regard to [Ca²⁺]_i, however, 5 days' incubation with either of them increased the basal [Ca²⁺]_i level and induced U937 cells to respond to formyl-methionyl-leucyl-phenylalanine (FMLP). When the cells were incubated with both 10^-6M RA and 10^-8M 1, 25-(OH)₂D₃, basal [Ca²⁺]_i was higher and FMLP caused a greater increase in [Ca²⁺]_i than when only RA or 1, 25-(OH)₂D₃ was added. These data suggest that RA and 1, 25-(OH)₂D₃ induce monocytoid differentiation in U937 cells through different pathways and act synergistically in the differentiation process. The 22-oxa-1, 25-(OH)₂D₃ induced CD14 expression, basal [Ca²⁺]_i increase and [Ca²⁺]_i response to FMLP, but did not cause cellular growth inhibition in U937 cells, and in these points, 22-oxa-1, 25-(OH)₂D₃ exhibited no significantly different effects from 1, 25-(OH)₂D₃. Thus, 22-oxa-1, 25-(OH)₂D₃ has the same potent activity as 1, 25-(OH)₂D₃ in inducing differentiation of U937 cells.