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


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Division of Endocrinology, Department of Medicine, University of Florida College of Medicine, Jacksonville, FL, United States.
Kent R
KR
Wehmeier
Abdul-Razzak
AR
Alamir
Senan
S
Sultan
Michael J
MJ
Haas
Norman C W
NC
Wong
Arshag D
AD
Mooradian
D015650:24,25-Dihydroxyvitamin D 3 / Q000494:pharmacology*; D016632:Apolipoprotein A-I / Q000096:biosynthesis* / Q000235:genetics; D015152:Blotting, Northern; D015153:Blotting, Western; D018938:Caco-2 Cells / Q000187:drug effects / Q000378:metabolism; D002112:Calcifediol / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D015870:Gene Expression / Q000187:drug effects; D056945:Hep G2 Cells / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D011401:Promoter Regions, Genetic / Q000187:drug effects / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics
D016632:Apolipoprotein A-I; D012333:RNA, Messenger; D015650:24,25-Dihydroxyvitamin D 3; D002112:Calcifediol