Sterol upregulation of human CETP expression in vitro and in transgenic mice by an LXR element
Yi Luo, Alan R. Tall
Yi Luo, Alan R. Tall
Published February 15, 2000
Citation Information: J Clin Invest. 2000;105(4):513-520. https://doi.org/10.1172/JCI8573.
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Article

Sterol upregulation of human CETP expression in vitro and in transgenic mice by an LXR element

Abstract

The cholesterol ester transfer protein (CETP) facilitates the transfer of HDL cholesterol esters from plasma to the liver. Transgenic mice expressing human CETP, controlled by its natural flanking region, increase expression of this gene in response to hypercholesterolemia. We established a CETP promoter-luciferase reporter assay in differentiated 3T3-L1 adipocytes to map the sterol upregulatory element. Promoter mutagenesis suggested that a direct repeat of a nuclear receptor binding sequence separated by 4 nucleotides (DR4 element, –384 to –399) was responsible for this activity. Using mice carrying normal or mutated promoter sequences, we confirmed the importance of this element for gene induction by dietary sterol. A gel retardation complex containing LXR/RXR was identified using the CETP DR4 element and adipocyte nuclear extracts. Both LXRα/RXRα and LXRβ/RXRα transactivated the CETP promoter via its DR4 element in a sterol-responsive fashion. Thus, the positive sterol response of the CETP gene is mediated by a nuclear receptor binding site that is activated by LXRs. That Cyp7a, the rate-limiting enzyme for conversion of cholesterol into bile acids in the liver, is also regulated by LXRα suggests that this class of nuclear receptor coordinates the regulation of HDL cholesterol ester catabolism and bile acid synthesis in the liver.

Authors

Yi Luo, Alan R. Tall

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Figure 1

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(a) Induction of the CETP promoter in adipocytes (shaded bar) but not pr...
(a) Induction of the CETP promoter in adipocytes (shaded bar) but not preadipocytes (open bar). 3T3-L1 preadipocytes and differentiated adipocytes were transfected with reporter constructs in which the luciferase reporter gene is controlled by the proximal CETP promoter (from 1 to –570) with or without mutation in the SRE-like element (Mut2). The transfected cells were cultured in LPDS or LPDS + 4 μg/mL 22(R)-hydroxycholesterol for 24 hours. Luciferase activities were measured using the Promega Dual-Luciferase assay system, which uses Renilla luciferase as a control for transfection efficiency. The fold induction by sterol was calculated by dividing the luciferase activity in the presence of sterol by the activity in the presence of vehicle only. Means ± SD are shown. (b) RNA expression of LXRα and LXRβ in 3T3-L1 preadipocytes (lane 1) and differentiated adipocytes (lane 2). Northern blots were performed with equal amount of Poly A+ RNA isolated from 3T3-L1 preadipocytes and day 4 differentiated adipocytes using LXRα and LXRβ cDNA probe (21, 37).