Increased NF-ΚB activity in fibroblasts lacking the vitamin D receptor.

1,25-Dihydroxyvitamin D [1,25(OH)₂D₃] is known to have anti-inflammatory activity; however, the molecular mechanism remains poorly defined. Here we show that the nuclear vitamin D receptor (VDR) is directly involved in the regulation of NF-κB activation, a pathway essential for inflammatory response. In mouse embryonic fibroblasts (MEFs) derived from VDR^-/- mice, the basal level of κB inhibitor (1κB) α protein was markedly decreased compared with VDR^+/- MEFs; however, degradation of IκBα and its phosphorylation in response to TNF-α treatment or Salmonella infection were not altered in VDR^-/- cells, neither were the levels of IκB kinase-α and IκB kinase-β proteins. Consistent with IκBα reduction, p65 accumulation in the nucleus was markedly increased in unstimulated VDR^-/- cells. In addition, 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 nuclear p65 DNA binding and NF-κB transcriptional activity; consistently, induction of IL-6 by TNF-α or IL-1β was much more robust in VDR^-/- than in VDR^+/- cells, indicating that VDR^-/- cells are more susceptible to inflammatory stimulation. Therefore, cells lacking VDR appear to be more proinflammatory 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. This is supported by the observation that IκBα degradation induced by TNF-α was inhibited by 1,25(OH)₂D₃ in VDR^+/- cells, but not in VDR^-/- cells. Taken together, these data suggest that VDR plays an inhibitory role in the regulation of NF-κB activation. [ABSTRACT FROM AUTHOR]