INHIBITION OF IL-12 PRODUCTION BY 1,25-DIHYDROXYVITAMIN D-3 - INVOLVEMENT OF NF-KAPPA-B DOWN-REGULATION IN TRANSCRIPTIONAL REPRESSION OF THE P40 GENE

Interleukin 12 (IL-12), produced by myelomonocytic cells, plays a pivo tal role in the development of T helper 1 (Th1) cells, which are invol ved in the pathogenesis of chronic inflammatory autoimmune disorders, 1,25-Dihydroxyvitamin D-3 [1,25(OH)(2)D-3] inhibits IL-12 production b y activated macrophages and dendritic cells, thus providing a novel in terpretation to its immunosuppressive properties. 1,25(OH)(2)D-3 signi ficantly 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 p 40 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 pro moter 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 th e p40 promoter, confirming the functional relevance of this site, Acti vation of monocytic THP-1 cells in the presence of 1,25(OH)(2)D-3 resu lts in reduced binding to the p40-kappa B site. Thus, 1,25(OH)(2)D-3 m ay negatively regulate IL-12 production by downregulation of NF-kappa B activation and binding to the p40-kappa B sequence.