Regulation of Leukemic Cell Differentiation through the Vitamin D Receptor at the Levels of Intracellular Signal Transduction, Gene Transcription, and Protein Trafficking and Stability.

1α,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D) exerts its biological activities through vitamin D receptor (VDR), which is a member of the superfamily of steroid receptors, that act as ligand-dependent transcription factors. Ligated VDR in complex with retinoid X receptor (RXR) binds to regulatory regions of 1,25(OH)(2)D-target genes. 1,25(OH)(2)D is able to induce differentiation of leukemic blasts towards macrophage-like cells. Many different acute myeloid leukemia (AML) cell lines respond to 1,25(OH)(2)D by increasing CD14 cell surface receptor, some additionally upregulate CD11b and CD11c integrins. In untreated AML cells VDR protein is present in cytosol at a very low level, even though its mRNA is continuously expressed. Ligation of VDR causes protein stabilization and translocation to the cell nuclei, where it regulates transcription of target genes. Several important groups of genes are regulated by 1,25(OH)(2)D in HL60 cells. These genes include differentiation-related genes involved in macrophage function, as well as a gene regulating degradation of 1,25(OH)(2)D, namely CYP24A1. We summarize here the data which demonstrate that though some cellular responses to 1,25(OH)(2)D in AML cells are transcription-dependent, there are many others which depend on intracellular signal transduction, protein trafficking and stabilization. The final effect of 1,25(OH)(2)D action in leukemic cells requires all these acting together.