Vitamin D receptor deletion leads to reduced level of IκBα protein through protein translation, protein-protein interaction, and post-translational modification

Vitamin D receptor plays an essential role in the regulation of inflammation. Previous studies demonstrate that vitamin D receptor negatively modulates the proinflammatory NF-κB pathway. However, it is unknown how vitamin D receptor regulates IκBα, the endogenous inhibitor of NF-κB. Here we investigated the molecular mechanism of vitamin D receptor deletion and IκBα expression. We found that cells lacking vitamin D receptor had significantly increased levels of IκBα mRNA and simultaneously decreased levels of IκBα protein. Lacking vitamin D receptor abolished its binding to the IκBα promoter. Moreover, the levels of protein translation regulators and the rate of protein synthesis were both decreased in cells lacking vitamin D receptor. At the post-translational level, IκBα ubiquitination was enhanced, indicating increased degradation of IκBα in the absence of vitamin D receptor. We further transfected cells with a plasmid carrying either wild-type or mutant IκBα. The expression of wild-type IκBα was much higher in the cells with vitamin D receptor than in the cells without vitamin D receptor, whereas the expression of exogenous IκBα was equally high in both cell lines. In summary, vitamin D receptor deletion affects IκBα through mRNA transcription, protein translation, protein-protein interaction, post-translational modification, and protein degradation, thus reducing the level of IκBα protein. Cells lacking vitamin D receptor are known in a proinflammatory state with activation of NF-κB. Our study provides new insight into vitamin D receptor regulation of an inhibitor of NF-κB in inflammation. Deletion of vitamin D receptor contributes to the activation of NF-κB on multiple levels.