The stimulatory effect of LXRalpha is blocked by SHP despite the presence of a LXRalpha binding site in the rabbit CYP7A1 promoter.
The transcription of the cholesterol 7alpha-hydroxylase gene (CYP7A1) is greatly decreased in cholesterol-fed rabbits. To determine whether the molecular structure of the promoter is responsible for this downregulation, we cloned the rabbit CYP7A1 promoter, identified the binding sites for alpha-fetoprotein transcription factor (FTF) and liver X receptor (LXRalpha), and studied the effects of FTF, LXRalpha, and SHP on its transcription. Adding LXRalpha/retinoid X receptor together with their ligands (L/R) to the promoter/reporter construct transfected into HepG2 cells greatly increased its activity. FTF did not increase promoter activity, nor did it enhance the stimulatory effect of L/R. Mutating the FTF binding site abolished the promoter baseline activity. Increasing amounts of SHP abolished the effect of L/R, and FTF enhanced the ability of SHP to decrease promoter activity below baseline levels. Thus, downregulation of CYP7A1 in cholesterol-fed rabbits is attributable secondarily to the activation of farnesoid X receptor, which increases SHP expression to override the positive effects of LXRalpha. Although FTF is a competent factor for maintaining baseline activity, it does not further enhance and may suppress CYP7A1 transcription.
Department of Medicine, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, 07103, USA.
Quan
Q
Shang
Luxing
L
Pan
Monica
M
Saumoy
John Y L
JY
Chiang
G Stephen
GS
Tint
Gerald
G
Salen
Guorong
G
Xu
D000818:Animals; D001483:Base Sequence; D001665:Binding Sites / Q000502:physiology*; D002478:Cells, Cultured; D002790:Cholesterol 7-alpha-Hydroxylase / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D006801:Humans; D000071518:Liver X Receptors; D008969:Molecular Sequence Data; D057093:Orphan Nuclear Receptors; D011401:Promoter Regions, Genetic / Q000235:genetics*; D011817:Rabbits; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000502:physiology*; D014157:Transcription Factors / Q000235:genetics; D000509:alpha-Fetoproteins / Q000235:genetics
D004268:DNA-Binding Proteins; D000071518:Liver X Receptors; C000605213:NR1H3 protein, human; D057093:Orphan Nuclear Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D014157:Transcription Factors; D000509:alpha-Fetoproteins; C099948:fetoprotein transcription factor; C099713:nuclear receptor subfamily 0, group B, member 2; D002790:Cholesterol 7-alpha-Hydroxylase