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.
Shang
Quan
Q
Department of Medicine, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, 07103, USA.
Pan
Luxing
L
Saumoy
Monica
M
Chiang
John Y L
JY
Tint
G Stephen
GS
Salen
Gerald
G
Xu
Guorong
G
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