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Molecular and Cellular Biology, May 2001, p. 2991-3000, Vol. 21, No. 9
Department of Metabolic Diseases, University
of Tokyo, Bunkyo-ku, Tokyo 113-8655,1 and
Department of Internal Medicine, Institute of Clinical
Medicine, University of Tsukuba, Ibaraki
305-8575,2 Japan
Received 23 October 2000/Returned for modification 19 December
2000/Accepted 9 February 2001
In an attempt to identify transcription factors which activate
sterol-regulatory element-binding protein 1c (SREBP-1c) transcription, we screened an expression cDNA library from adipose tissue of SREBP-1
knockout mice using a reporter gene containing the 2.6-kb mouse SREBP-1
gene promoter. We cloned and identified the oxysterol receptors liver X
receptor (LXR
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.9.2991-3000.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Identification of Liver X Receptor-Retinoid X
Receptor as an Activator of the Sterol Regulatory Element-Binding
Protein 1c Gene Promoter
) and LXR
as strong activators of the mouse SREBP-1c
promoter. In the transfection studies, expression of either LXR
or
-
activated the SREBP-1c promoter-luciferase gene in a
dose-dependent manner. Deletion and mutation studies, as well as gel
mobility shift assays, located an LXR response element complex
consisting of two new LXR-binding motifs which showed high similarity
to an LXR response element recently found in the ABC1 gene promoter, a
reverse cholesterol transporter. Addition of an LXR ligand,
22(R)-hydroxycholesterol, increased the promoter activity. Coexpression
of retinoid X receptor (RXR), a heterodimeric partner, and its ligand
9-cis-retinoic acid also synergistically activated the
SREBP-1c promoter. In HepG2 cells, SREBP-1c mRNA and precursor protein
levels were induced by treatment with 22(R)-hydroxycholesterol and
9-cis-retinoic acid, confirming that endogenous LXR-RXR
activation can induce endogenous SREBP-1c expression. The activation of
SREBP-1c by LXR is associated with a slight increase in nuclear
SREBP-1c, resulting in activation of the gene for fatty acid synthase,
one of its downstream genes, as measured by the luciferase assay. These
data demonstrate that LXR-RXR can modify the expression of genes for
lipogenic enzymes by regulating SREBP-1c expression, providing a novel
link between fatty acid and cholesterol metabolism.
*
Corresponding author. Mailing address: Department of
Internal Medicine, Institute of Clinical Medicine, University of
Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan. Phone and
fax: 81-298-63-2170. E-mail: shimano-tky{at}umin.ac.jp.
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