Glucagon and cAMP inhibit cholesterol 7α-hydroxylase (CYP7a1) gene expression in human hepatocytes: Discordant regulation of bile acid synthesis and gluconeogenesis
✍ Scribed by Kwang-Hoon Song; John Y. L. Chiang
- Publisher
- John Wiley and Sons
- Year
- 2005
- Tongue
- English
- Weight
- 478 KB
- Volume
- 43
- Category
- Article
- ISSN
- 0270-9139
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✦ Synopsis
The gene encoding cholesterol 7␣-hydroxylase (CYP7A1) is tightly regulated to control bile acid synthesis and maintain lipid homeostasis. Recent studies in mice suggest that bile acid synthesis is regulated by the fasted-to-fed cycle, and fasting induces CYP7A1 gene expression in parallel to the induction of peroxisome proliferators-activated receptor ␥ co-activator 1␣ (PGC-1␣) and phosphoenolpyruvate carboxykinase (PEPCK). How glucagon regulates CYP7A1 gene expression in the human liver is not clear. Here we show that glucagon and cyclic adenosine monophosphate (cAMP) strongly repressed CYP7A1 mRNA expression in human primary hepatocytes. Reporter assays confirmed that cAMP and protein kinase A (PKA) inhibited human CYP7A1 gene transcription, in contrast to their stimulation of the PEPCK gene. Mutagenesis analysis identified a PKA-responsive region located within the previously identified HNF4␣ binding site in the human CYP7A1 promoter. Glucagon and cAMP increased HNF4␣ phosphorylation and reduced the amount of HNF4␣ present in CYP7A1 chromatin. Our findings suggest that glucagon inhibited CYP7A1 gene expression via PKA phosphorylation of HNF4␣, which lost its ability to bind the CYP7A1 gene and resulted in inhibition of human CYP7A1 gene transcription. In conclusion, this study unveils a species difference in nutrient regulation of the human and mouse CYP7A1 gene and suggests a discordant regulation of bile acid synthesis and gluconeogenesis by glucagon in human livers during fasting. (HEPATOLOGY 2006;43:117-125.)
T he conversion of cholesterol to bile acids in the liver is a major pathway for removing excess cholesterol from the body and plays an important role in maintaining cholesterol homeostasis. 1,2 Imbalance of cholesterol metabolism causes diseases such as athero-sclerosis and gallstone disease. CYP7A1 is a liver-specific enzyme that catalyzes the first and rate-limiting step in the bile acid biosynthetic pathway. 1 Expression of CYP7A1 is regulated mainly at the gene transcriptional level by many factors, including bile acids, diets, nutrients, cytokines, and hormones. 3 Several nuclear receptors, including liver orphan receptor, ␣-fetoprotein transcription factor (also known as mouse liver-related homolog), hepatocyte nuclear factor 4␣ (HNF4␣), and pregnane X receptor, bind to the CYP7A1 gene and play important roles in regulation of CYP7A1 gene transcription. 1,4 HNF4␣ is the most abundant orphan nuclear receptor expressed in the liver that binds to a direct repeat with one base spacing (DR1) in the bile acid response element II (BARE-II) (Ϫ144/ Ϫ126) and stimulates CYP7A1 gene transcription. 5 Mutation of this HNF4␣ binding site markedly reduced CYP7A1 promoter activity, and liver-specific conditional disruption of the HNF4␣ gene in mice drastically repressed CYP7A1 gene expression, indicating that HNF4␣ is crucial for CYP7A1 gene transcription and regula-