1-ALPHA,25-(OH)2 VITAMIN-D3 ENHANCES EXPRESSION OF THE GENES ENCODING CA-2+-BINDING PROTEIN-MRP-8 AND PROTEIN-MRP-14

Two closely related Ca2+-binding proteins, migration inhibitory factor-related protein (MRP)-8 and MRP-14, are synthesized under specific conditions of myeloid cell differentiation. Because 1-alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3] induces myeloid cell differentiation and expression of other S-100 class calcium-binding proteins, we examined the effects of 1,25-(OH)2D3 on MRP mRNA levels in human U-937 histiocytic lymphoma cells. 1,25-(OH)2D3 increased MRP-8 and MRP-14 mRNA levels in a time- and dose-dependent manner. MRP mRNA levels were maximal at 24 h and remained elevated for at least 96 h after exposure of the cells to 1,25-(OH)2D3. MRP-8 mRNA accumulation required 100- to 1,000-fold higher concentrations of 25-(OH)D3, which binds to the 1,25-(OH)2D3 intracellular receptor with 100- to 1,000-fold lower affinity. Other differentiating agents, dimethyl sulfoxide, retinoic acid, and dexamethasone, also increased levels of MRP-8 and MRP-14 mRNA. Phorbol myristate acetate enhanced MRP-14 mRNA levels to a greater extent than MRP-8 mRNA levels, suggesting differential regulation of MRP gene expression by protein kinase C. The 1,25-(OH)2D3-induced relative increase in MRP mRNA levels was not changed by a 1,000-fold reduction in extracellular [Ca2+]. Thus 1,25-(OH)2D3 is potentially a physiological modulator of MRP gene expression. Expression of the MRP-8 and MRP-14 genes may be important for differentiation of myeloid cells.