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Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol
Edwin C Y Chow1, Lilia Magomedova1, Holly P Quach1
1Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Ontario, Canada.
Background & Aims:
Little is known about the effects of the vitamin D receptor (VDR) on hepatic activity of human cholesterol 7α-hydroxylase (CYP7A1) and cholesterol metabolism. We studied these processes in mice in vivo and mouse and human hepatocytes.
Methods:
Farnesoid X receptor (Fxr)(-/-), small heterodimer partner (Shp)(-/-), and C57BL/6 (wild-type control) mice were fed normal or Western diets for 3 weeks and were then given intraperitoneal injections of vehicle (corn oil) or 1α,25-dihydroxyvitamin D3 (1,25[OH]2D3; 4 doses, 2.5 μg/kg, every other day). Plasma and tissue samples were collected and levels of Vdr, Shp, Cyp7a1, Cyp24a1, and rodent fibroblast growth factor (Fgf) 15 expression, as well as levels of cholesterol, were measured. We studied the regulation of Shp by Vdr using reporter and mobility shift assays in transfected human embryonic kidney 293 cells, quantitative polymerase chain reaction with mouse tissues and mouse and human hepatocytes, and chromatin immunoprecipitation assays with mouse liver.
Results:
We first confirmed the presence of Vdr mRNA and protein expression in livers of mice. In mice fed normal diets and given injections of 1,25(OH)2D3, liver and plasma concentrations of 1,25(OH)2D3 increased and decreased in unison. Changes in hepatic Cyp7a1 messenger RNA (mRNA) correlated with those of Cyp24a1 (a Vdr target gene) and inversely with Shp mRNA, but not ileal Fgf15 mRNA. Similarly, incubation with 1,25(OH)2D3 increased levels of Cyp24a1/CYP24A1 and Cyp7a1/CYP7A1 mRNA in mouse and human hepatocytes, and reduced levels of Shp mRNA in mouse hepatocytes. In Fxr(-/-) and wild-type mice with hypercholesterolemia, injection of 1,25(OH)2D3 consistently reduced levels of plasma and liver cholesterol and Shp mRNA, and increased hepatic Cyp7a1 mRNA and protein; these changes were not observed in Shp(-/-) mice given 1,25(OH)2D3 and fed Western diets. Truncation of the human small heterodimer partner (SHP) promoter and deletion analyses revealed VDR-dependent inhibition of SHP, and mobility shift assays showed direct binding of VDR to enhancer regions of SHP. In addition, chromatin immunoprecipitation analysis of livers from mice showed that injection of 1,25(OH)2D3 increased recruitment of Vdr and rodent retinoid X receptor to the Shp promoter.
Conclusions:
Activation of the VDR represses hepatic SHP to increase levels of mouse and human CYP7A1 and reduce cholesterol.
Insights
Vitamin D receptor (VDR) activation represses small heterodimer partner (SHP) in the liver. This leads to increased cholesterol 7α-hydroxylase (CYP7A1) activity, effectively lowering cholesterol levels.
Area of Science:
- Molecular Biology
- Endocrinology
- Hepatology
Background:
- The role of the vitamin D receptor (VDR) in regulating hepatic cholesterol metabolism, specifically human cholesterol 7α-hydroxylase (CYP7A1) activity, remains largely unknown.
- Investigating the VDR's influence on CYP7A1 and cholesterol homeostasis is crucial for understanding metabolic regulation.
Purpose of the Study:
- To elucidate the effects of VDR activation on hepatic CYP7A1 activity and cholesterol metabolism in vivo and in vitro.
- To determine the regulatory mechanisms by which VDR influences key genes involved in cholesterol homeostasis.
Main Methods:
- Studies were conducted in Vdr, Fxr, and Shp knockout mice, as well as wild-type controls, fed normal or Western diets.
- Mice and primary hepatocytes were treated with 1α,25-dihydroxyvitamin D3 (1,25[OH]2D3) or vehicle.
- Gene expression (Vdr, Shp, Cyp7a1, Cyp24a1, Fgf15), protein levels, and cholesterol concentrations were measured using quantitative PCR, reporter assays, mobility shift assays, and chromatin immunoprecipitation.
Main Results:
- VDR activation by 1,25[OH]2D3 increased hepatic Cyp7a1 mRNA and protein levels while decreasing Shp mRNA in mouse and human hepatocytes.
- In hypercholesterolemic mice, 1,25[OH]2D3 treatment reduced plasma and liver cholesterol, increased hepatic Cyp7a1, and decreased Shp mRNA, effects absent in Shp(-/-) mice.
- VDR directly binds to the SHP promoter, inhibiting its expression, and VDR recruitment to the Shp promoter was enhanced by 1,25[OH]2D3 treatment.
Conclusions:
- Activation of the VDR represses hepatic SHP expression.
- This repression leads to increased CYP7A1 activity, promoting cholesterol catabolism and reducing overall cholesterol levels.
- VDR plays a significant role in regulating hepatic cholesterol metabolism through SHP inhibition.
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