1α,25-Dihydroxycholecalciferol activates binding of CREB to a CRE site in the CD14 promoter and drives promoter activity in a phosphatidylinositol-3 kinase-dependent manner

1,25-Dihydroxycholecalciferol, also known as 1 alpha,25-dihydroxyvitamin D-3 or calcitriol, regulates the differentiation and functional properties of mononuclear phagocytes. Many of these effects involve nongenomic signaling pathways, which are not fully understood. Activation of CD14 expression, a monocyte differentiation marker and coreceptor with TLR-2 for bacterial LPS, by calcitriol was shown previously to be PI-3K-dependent [1]; however, the mechanism of gene activation remained undefined. Using a transcription factor-binding array screen coupled with EMSA, we found evidence for PI-3K-dependent activation of CREB in THP-1 cells incubated with calcitriol. Furthermore, analysis of the proximal promoter of human CD14 identified regions that contained up to seven sequences, which showed significant similarity to a canonical CRE sequence, 5'-TGACGTCA-3'. Treatment of THP-1 cells with calcitriol activated CREB binding to one of these regions at Positions -37 to -55, relative to the transcription start site in a PI-3K-dependent manner. This 19-mer region also became transcriptionally active in a reporter assay in response to calcitriol, again dependent on PI-3K. Mutation of the CRE within the 19-mer abolished this activity. Taken together, these results show that calcitriol signaling, leading to activation of the CD14 promoter, involves CREB activation downstream of PI-3K.