ABSTRACT
BACKGROUND AND OBJECTIVES: Excretion of bile acid and free cholesterol of bile was important to maintain cholesterol homeostasis. ATP-binding cassette transporter G5 (ABCG5) and G8 (ABCG8) promoted biliary cholesterol excretion. In previous study, hepatic secretion of cholesterol and ABCG5/G8 expression are strongly stimulated in hypophysectomized rats during treatment with thyroid hormone. In this study, we aimed to evaluate the effect of thyroid hormone to expression of ABCG5 and G8 in mouse liver. MATERIALS AND METHODS: We administered thyroid hormone (T3) to C57BL/6 mice and then RNA and protein was isolated from liver. We isolated primary hepatocyte and administered T3 to evaluate in vitro effect. HepG2 cells were cotransfected with either a control plasmid or expression plasmids for human thyroid hormone receptor (hTR)beta/human retinoid X receptor (hRXR)alpha or human liver X receptor (hLXR)alpha in combination with reporter plasmids TK-LXRE3-LUC with or without T3. RESULTS: Serum total cholesterol was decreased after 5 days of T3 treatment. Expression of ABCG5/8 mRNA and ABCG5 protein was increased after T3 treatment. In primary hepatocytes, T3 also increased ABCG5/8 mRNA expression. LXRalpha mRNA was not increased by T3. However, when we cotransfected liver X receptor response element (LXRE) construct and TRbeta/RXRalpha with T3, the activity of LXRE containing construct was markedly increased. CONCLUSION: We confirmed that thyroid hormone increased expression of ABCG5/8. This result suggested that thyroid hormone played an important role in decreasing serum cholesterol through bile excretion.
Subject(s)
Animals , Humans , Mice , Rats , Bile , Cholesterol , Hep G2 Cells , Hepatocytes , Homeostasis , Liver , Orphan Nuclear Receptors , Plasmids , Receptors, Thyroid Hormone , Response Elements , Retinoid X Receptors , RNA , RNA, Messenger , Thyroid Gland , Thyroid HormonesABSTRACT
BACKGROUND: Bile acids were important for the regulation of cholesterol homeostasis. Thyroid hormone increased the expression of CYP7A1 (cholesterol 7alpha-hydroxylase), catalyzing the first step in the biosynthesis of bile acids. However, the effect of thyroid hormone on bile acid export has not been previously assessed. The principal objective of this study is to evaluate the effects of thyroid hormone on the bile salt export pump (BSEP). METHODS: Thyroid hormone, T3 (1 mg/g) was administered to male mice via intraperitoneal injection. After 6 hours and 5 days of T3 treatment, we measured serum total and LDL cholesterol and hepatobiliary bile acid concentrations. We assessed the changes associated with bile acid synthesis and transport. In order to evaluate the direct effect of thyroid hormone, we assessed the changes in the levels of BSEP protein after T3 administration in human hepatoma cells. RESULTS: Serum total and LDL cholesterol were reduced and hepatobiliary bile acid concentrations were increased following T3 treatment. Expressions of Cyp7a1 and BSEP mRNA were increased following T3 treatment. The levels of the BSEP protein in the mouse liver as well as in the human hepatoma cells were increased after T3 treatment. CONCLUSION: Thyroid hormone can regulate LDL cholesterol metabolism. It increases bile acid synthesis and the excretion of bile acids via increased BSEP expression.