Involvement of the vitamin D receptor in the regulation of NF-κB activity in fibroblasts

We have used mouse embryonic fibroblasts (MEFs) derived from VDR(+/−) and VDR(−/−) mice to determine whether the nuclear vitamin D receptor (VDR) is directly involved in the regulation of NF-κB activation. We found that the basal IκBα protein level was markedly decreased in VDR(−/−) MEFs compared to VDR(+/−) MEFs; however, degradation of IκBα and its phosphorylation were not altered in VDR(−/−) cells, neither were the levels of IKKα and IKKβ proteins. Consistently, p65 nuclear translocation was increased in unstimulated VDR(−/−) cells. The physical interaction between VDR and p65 was absent in VDR(−/−) MEFs, which may free p65 and increase its activity. Consequently, these alterations combined led to a marked increase in NF-κB transcriptional activity. Consistently, induction of IL-6 by TNFα or IL-1β was much more robust in VDR(−/−) than in VDR(+/−) cells, indicating that VDR(−/−) cells are more susceptible to inflammatory stimulation. Therefore, fibroblasts lacking VDR appear to be more pro-inflammatory due to the intrinsic high NF-κB activity. The reduction of IκBα in VDR(−/−) MEFs may be partially explained by the lack of VDR-mediated stabilization of IκBα by 1,25(OH)2D3. These data suggest that VDR plays an inhibitory role in the regulation of NF-κB activation.