24, 25-dihydroxycholecalciferol but not 25-hydroxycholecalciferol suppresses apolipoprotein A-I gene expression

Ligands for the vitamin D receptor (VDR) regulate apolipoprotein A-I (apo A-I) gene expression in a tissue-specific manner. The vitamin D metabolite 24, 25-dihydroxycholecalciferol (24, 25-(OH)(2)D(3)) has been shown to possess unique biological effects. To determine if 24, 25-(OH)(2)D(3) modulates apo A-I gene expression, HepG2 hepatocytes and Caco-2 intestinal cells were treated with 24, 25-(OH)(2)D(3) or its precursor 25-OHD(3).

Apo A-I protein levels and mRNA levels were measured by Western and Northern blotting, respectively. Changes in apo A-I promoter activity were measured using the chlorampenicol acetytransferase assay.

Treatment with 24, 25-(OH)(2)D(3), but not 25-OHD(3), inhibited apo A-I secretion in HepG2 and Caco-2 cells and apo A-I mRNA levels and apo A-I promoter activity in HepG2 cells. To determine if 24, 25-(OH)(2)D(3) represses apo A-I gene expression through site A, the nuclear receptor binding element that is essential for VDRs effects on apo A-I gene expression, HepG2 cells were transfected with plasmids containing or lacking site A. While the site A-containing plasmid was suppressed by 24, 25-(OH)(2)D(3), the plasmid lacking site A was not. Likewise, treatment with 24, 25-(OH)(2)D(3) suppressed reporter gene expression in cells transfected with a plasmid containing site A in front of a heterologous promoter. Finally, antisense-mediated VDR depletion failed to reverse the silencing effects of 24, 25-(OH)(2)D(3) on apo A-I expression.

These results suggest that the vitamin D metabolite 24, 25-(OH)(2)D(3) is an endogenous regulator of apo A-I synthesis through a VDR-independent signaling mechanism.