Insulin regulation of cholesterol 7alpha-hydroxylase expression in human hepatocytes: roles of forkhead box O1 and sterol regulatory element-binding protein 1c.
*Gene Expression Regulation; *Transcriptional Activation; Adolescent; Adult; Animals; Child; Cholesterol 7-alpha-Hydroxylase/*biosynthesis/genetics; Enzymologic; Female; Forkhead Box Protein O1; Forkhead Transcription Factors/*physiology; Hepatocytes/*enzymology; Humans; Insulin/metabolism/*physiology; Male; Middle Aged; Preschool; Rats; Sterol Regulatory Element Binding Protein 1/*physiology; Transcription Factors/*physiology
Bile acid synthesis and pool size increases in diabetes, whereas insulin inhibits bile acid synthesis. The objective of this study is to elucidate the mechanism of insulin regulation of cholesterol 7alpha-hydroxylase gene expression in human hepatocytes. Real-time PCR assays showed that physiological concentrations of insulin rapidly stimulated cholesterol 7alpha-hydroxylase (CYP7A1) mRNA expression in primary human hepatocytes but inhibited CYP7A1 expression after extended treatment. The insulin-regulated forkhead box O1 (FoxO1) and steroid regulatory element-binding protein-1c (SREBP-1c) strongly inhibited hepatocyte nuclear factor 4alpha and peroxisome proliferator-activated receptor gamma coactivator-1alpha trans-activation of the CYP7A1 gene. FoxO1 binds to an insulin response element in the rat CYP7A1 promoter, which is not present in the human CYP7A1 gene. Insulin rapidly phosphorylates and inactivates FoxO1, whereas insulin induces nuclear SREBP-1c expression in human primary hepatocytes. Chromatin immunoprecipitation assay shows that insulin reduced FoxO1 and peroxisome proliferators-activated receptor gamma-coactivator-1alpha but increased SREBP-1c recruitment to CYP7A1 chromatin. We conclude that insulin has dual effects on human CYP7A1 gene transcription; physiological concentrations of insulin rapidly inhibit FoxO1 activity leading to stimulation of the human CYP7A1 gene, whereas prolonged insulin treatment induces SREBP-1c, which inhibits human CYP7A1 gene transcription. Insulin may play a major role in the regulation of bile acid synthesis and dyslipidemia in diabetes.
Li Tiangang; Kong Xiaoying; Owsley Erika; Ellis Ewa; Strom Stephen; Chiang John Y L
The Journal of biological chemistry
2006
2006-09
Article information provided for research and reference use only. All rights are retained by the journal listed under publisher and/or the creator(s).
<a href="http://doi.org/10.1074/jbc.M605815200" target="_blank" rel="noreferrer noopener">10.1074/jbc.M605815200</a>
Transcriptional regulation of the human sterol 12alpha-hydroxylase gene (CYP8B1): roles of heaptocyte nuclear factor 4alpha in mediating bile acid repression.
*DNA-Binding Proteins; *Transcription; Base Sequence; Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; Bile Acids and Salts/*pharmacology; Binding Sites; Cholesterol 7-alpha-Hydroxylase/genetics; Cultured; Cytochrome P-450 Enzyme System/*genetics; Cytoplasmic and Nuclear/physiology; Genetic; Genetic/physiology; Hepatocyte Nuclear Factor 4; Humans; Molecular Sequence Data; Phosphoproteins/*physiology; Promoter Regions; Receptors; Repressor Proteins/*pharmacology; Steroid 12-alpha-Hydroxylase; Steroid Hydroxylases/*genetics; Transcription Factors/*physiology; Tumor Cells
Sterol 12alpha-hydroxylase catalyzes the synthesis of cholic acid and controls the ratio of cholic acid over chenodeoxycholic acid in the bile. Transcription of CYP8B1 is inhibited by bile acids, cholesterol, and insulin. To study the mechanism of CYP8B1 transcription by bile acids, we have cloned and determined 3389 base pairs of the 5'-upstream nucleotide sequences of the human CYP8B1. Deletion analysis of CYP8B1/luciferase reporter activity in HepG2 cells revealed that the sequences from -57 to +300 were important for basal and liver-specific promoter activities. Hepatocyte nuclear factor 4alpha (HNF4alpha) strongly activated human CYP8B1 promoter activities, whereas cholesterol 7alpha-hydroxylase promoter factor (CPF), an NR5A2 family of nuclear receptors, had much less effect. Electrophoretic mobility shift assay identified an overlapping HNF4alpha- and CPF-binding site in the +198/+227 region. The human CYP8B1 promoter activities were strongly repressed by bile acids, and the bile acid response element was localized between +137 and +220. Site-directed mutagenesis of the HNF4alpha-binding site markedly reduced promoter activity and its response to bile acid repression. On the other hand, mutation of the
Zhang M; Chiang J Y
The Journal of biological chemistry
2001
2001-11
Article information provided for research and reference use only. All rights are retained by the journal listed under publisher and/or the creator(s).
<a href="http://doi.org/10.1074/jbc.M105117200" target="_blank" rel="noreferrer noopener">10.1074/jbc.M105117200</a>