Inhibition of IL-12 production by 1,25-dihydroxyvitamin D3 -: Involvement of NF-κB downregulation in transcriptional repression of the p40 gene

Interleukin 12 (IL-12), produced by myelomonocytic cells, plays a pivotal role in the development of T helper 1 (Th1) cells, which are involved in the pathogenesis of chronic inflammatory autoimmune disorders, 1,25-Dihydroxyvitamin D-3 [1,25(OH)(2)D-3] inhibits IL-12 production by activated macrophages and dendritic cells, thus providing a novel interpretation to its immunosuppressive properties. 1,25(OH)(2)D-3 significantly inhibits mRNA expression for both IL-12 p35 and p40 subunits acting at the transcriptional level, The effect of 1,25(OH)(2)D-3 on p40 promoter activation was analyzed by cotransfecting monocytic RAW264.7 cells with p40 promoter/reporter constructs and expression vectors for vitamin D-3 receptor (VDR) and/or retinoid X receptor (RXR alpha). We observed transcriptional repression of the p40 gene by 1,25(OH)(2)D-3, which required coexpression of VDR with RXR and an intact VDR DNA-binding domain, The repressive effect maps to a region in the p40 promoter containing a binding site for NF-kappa B (p40-kappa B), Deletion of the p40-kappa B site abrogates part of the inhibitory effect on the p40 promoter, confirming the functional relevance of this site, Activation of monocytic THP-1 cells in the presence of 1,25(OH)(2)D-3 results in reduced binding to the p40-kappa B site. Thus, 1,25(OH)(2)D-3 may negatively regulate IL-12 production by downregulation of NF-kappa B activation and binding to the p40-kappa B sequence.