Increased NF-kappaB 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-kappaB activation, a pathway essential for inflammatory response. In mouse embryonic fibroblasts (MEFs) derived from VDR-/- mice, the basal level of kappaB inhibitor (IkappaB) alpha protein was markedly decreased compared with VDR+/- MEFs; however, degradation of IkappaBalpha and its phosphorylation in response to TNF-alpha treatment or Salmonella infection were not altered in VDR-/- cells, neither were the levels of IkappaB kinase-alpha and IkappaB kinase-beta proteins. Consistent with IkappaBalpha 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-kappaB transcriptional activity; consistently, induction of IL-6 by TNF-alpha or IL-1beta 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-kappaB activity. The reduction of IkappaBalpha in VDR-/- MEFs may be partially explained by the lack of VDR-mediated stabilization of IkappaBalpha by 1,25(OH)2D3. This is supported by the observation that IkappaBalpha degradation induced by TNF-alpha 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-kappaB activation.