Identification of Liver X Receptor-Retinoid X Receptor as an Activator of the Sterol Regulatory Element-Binding Protein 1c Gene Promoter

Abstract

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 (LXRα) and LXRβ as strong activators of the mouse SREBP-1c promoter. In the transfection studies, expression of either LXRα or -β 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.

ACKNOWLEDGMENTS

We thank N. Emoto and A. Amemiya for great help in construction of the expression library.

This study was supported by the Promotion of Fundamental Studies in Health Science of the Organization for Pharmaceutical Safety and Research and health sciences research grants (Research on Human Genome and Gene Therapy) from the Ministry of Health and Welfare.

ADDENDUM

During the manuscript review process, activation of SREBP-1c expression by LXRs was reported in studies using a pharmacological LXR agonist as well as mice deficient in LXRα, LXRβ, or both (25a, 28a). The researchers also studied the SREBP-1c promoter and found one of the LXREs that we identified in the current study. Their data, from a different approach to LXRs, and our present SREBP-1c promoter analysis data are basically consistent and confirm each other.