We have shown previously that the hepatic control region 1 (HCR-1) enhances the activity of the human apolipoprotein C-II (apoC-II) promoter in HepG2 cells via two hormone response elements (HREs) present in the apoC-II promoter. In the present paper, we report that the HCR-1 selectively mediates the transactivation of the apoC-II promoter by chenodeoxycholic acid (CDCA) and 9-cis-retinoic acid. CDCA, which is a natural ligand of farnesoid X receptor α (FXRα), increases the steady-state apoC-II mRNA levels in HepG2 cells. This increase in transcription requires the binding of retinoid X receptor α (RXRα)–FXRα heterodimers to a novel inverted repeat with one nucleotide spacing (IR-1) present in the HCR-1. This element also binds hepatocyte nuclear factor 4 and apoA-I regulatory protein-1. Transactivation of the HCR-1/apoC-II promoter cluster by RXRα–FXRα heterodimers in the presence of CDCA was abolished by mutations either in the IR-1 HRE of the HCR-1 or in the thyroid HRE of the proximal apoC-II promoter, which binds RXRα–thyroid hormone receptor β (T3Rβ) heterodimers. The same mutations also abolished transactivation of the HCR-1/apoC-II promoter cluster by RXRα–T3Rβ heterodimers in the presence of tri-iodothyronine. The findings establish synergism between nuclear receptors bound to specific HREs of the proximal apoC-II promoter and the HCR-1, and suggest that this synergism mediates the induction of the HCR-1/apoC-II promoter cluster by bile acids and retinoids.
- apolipoprotein C-II (apoC-II)
- bile acids
- gene regulation
- hepatic control region 1 (HCR-1)
Abbreviations used: apoC-I (etc.), apolipoprotein C-I (etc.); apoC-I´ apoC-I pseudogene; ARP-1, apoA-I regulatory protein 1; ARP-PO, acidic ribosomal phosphoprotein; CA, cholic acid; CAT, chloramphenicol acetyltransferase; CDCA, chenodeoxycholic acid; DMEM, Dulbecco's modified Eagle's medium; EAR-2, vErb-A-related protein 2; EMSA, electrophoretic mobility shift assay; FBS, foetal bovine serum; FXRα, farnesoid X receptor α; HCR-1, hepatic control region 1; HNF-4, hepatocyte nuclear factor 4; HRE, hormone response element; HSV-tk, herpes simplex virus thymidine kinase; I-BABP, intestinal bile-acid-binding protein; IR-1: inverted repeat with one nucleotide spacing; luc, luciferase promoter; RA, 9-cis-retinoic acid; RT, reverse transcriptase; RXRα, retinoid X receptor α; T3, tri-iodothyronine; T3Rβ, thyroid hormone receptor β; VLDL, very-low-density lipoprotein; WT, wild-type.
- The Biochemical Society, London ©2003