Direct Transcriptional Regulation of RelB by 1α,25-Dihydroxyvitamin D3 and Its Analogs

The nuclear factor-κB (NF-κB) protein RelB plays a unique role in dendritic cell (DC) function and, as such, is an important regulator of antigen presentation and immune regulation. In this study, inhibition of RelB expression in DCs exposed to an analog of the active form of vitamin D3 (1α,25-dihydroxyvitamin D3 (1α,25-(OH)2D3)) was observed and shown to be mediated by the vitamin D receptor (VDR). Potential vitamin D response elements were identified within promoter regions of human and mouse relB genes. In gel shift experiments, these motifs specifically bound VDR·retinoid X receptor-α complexes. Reporter assays confirmed that transcriptional activity of human and mouse relB promoters was inhibited by 1α,25-(OH)2D3 agonists in a DC-derived cell line. The inhibition was abolished by mutagenesis of the putative vitamin D response elements and was enhanced by overexpression of VDR. Mutagenesis of NF-κB response elements within the relB promoter did not affect the magnitude of 1α,25-(OH)2D3 analog-mediated inhibition, ruling out an indirect effect on NF-κB signaling. Glucocorticoid caused additional inhibition of relB promoter activity when combined with the 1α,25-(OH)2D3 analog. This effect was dependent on the integrity of the NF-κB response elements, suggesting separate regulatory mechanisms for the two steroid pathways on this promoter. We conclude that relB is a direct target for 1α,25-(OH)2D3-mediated negative transcriptional regulation via binding of VDR·retinoid X receptor-α to discrete DNA motifs. This mechanism has important implications for the inhibitory effect of 1α,25-(OH)2D3 on DC maturation and for the potential immunotherapeutic use of 1α,25-(OH)2D3 analogs alone or combined with other agents.