The comparative role of 1,25-dihydroxycholecalciferol and phorbol esters in the differentiation of the U937 cell line.

The active metabolite of cholecalciferol, 1,25-dihydroxycholecalciferol (1,25-DHCC), is a mononuclear phagocyte product with immunoregulatory properties which can influence not only surrounding T cells but also other mononuclear phagocytes; and which acts in an autocrine fashion. In this study we have used the U937 cell line as a starting point model to investigate further the comparative role of 1,25-DHCC and phorbol myristate acetate (PMA) upon growth, differentiation and phenotype in the mononuclear phagocyte system, and have correlated our findings with changes in 1,25-DHCC metabolism and receptor expression. Both 1,25-DHCC and PMA inhibit growth and differentiate U937 cells in a dose-dependent fashion. When used together, however, at low doses of PMA, 1,25-DHCC protects against the PMA-induced growth inhibition. At high concentrations of both compounds there is a decrease (1,25-DHCC) or an increase (PMA) in 1,25-DHCC receptor expression, with either 24-OHase (1,25-DHCC) or 1-OHase (PMA) synthesis. If the compounds are used in combination the receptor levels are equivalent to controls, and both enzymes are produced. The phenotype of the 1,25-DHCC-induced cells shows light adherence, class I+, increase in CD4 and CD14 and decrease in CD71. The PMA-induced cell is tightly adherent, class I+, and strongly positive for CD13 with a concomitant decrease in both CD4 and CD71. These findings suggest another role for 1,25-DHCC in the mononuclear phagocyte system, as a potential mitogenic agent. They also suggest that 1,25-DHCC may act at both membrane and nuclear levels within this model of the mononuclear phagocyte pathway and demonstrate one possible way in which physiological peripheral macrophage heterogeneity might be induced, i.e. due to the nature of the signals which are implicated during differentiation. The presence of increased CD4 and decreased CD13 on the surface of 1,25-DHCC-differentiated cells, and vice versa on PMA-differentiated cells, illustrates how this may then be reflected in functional mononuclear phagocyte heterogeneity, which may in turn be reflected in differential peripheral function.