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Repression of farnesoid X receptor during the acute phase response
Min Sun Kim1, Judy Shigenaga, Art Moser
1Department of Medicine, University of California San Francisco, Metabolism Section, Medical Service, Department of Veterans Affairs Medical Center, San Francisco, California 94121, USA.
Abstract:
The acute phase response is associated with changes in the hepatic expression of genes involved in lipid metabolism. Nuclear hormone receptors that heterodimerize with retinoid X receptor (RXR), such as thyroid receptors, peroxisome proliferator-activated receptors, and liver X receptors, modulate lipid metabolism. We recently demonstrated that these nuclear hormone receptors are repressed during the acute phase response induced by lipopolysaccharide (LPS), consistent with the known decreases in genes that they regulate. In the present study, we show that LPS significantly decreases farnesoid X receptor (FXR) mRNA in mouse liver as early as 8 h after LPS administration, and this decrease was dose-dependent with the half-maximal effect observed at 0.5 microg/100 g of body weight. Gel-shift experiments demonstrated that DNA binding activity to an FXR response element (IR1) is significantly reduced by LPS treatment. Supershift experiments demonstrated that the shifted protein-DNA complex contains FXR and RXR. Furthermore, the expression of FXR target genes, SHP and apoCII, were significantly reduced by LPS (70 and 60%, respectively). Also, LPS decreases hepatic LRH expression in mouse, which may explain the reduced expression of CYP7A1 in the face of SHP repression. In Hep3B human hepatoma cells, both tumor necrosis factor (TNF) and interleukin-1 (IL-1) significantly decreased FXR mRNA, whereas IL-6 did not have any effect. TNF and IL-1 also decreased the DNA binding activity to an IR1 response element and the expression of SHP and apoCII. Importantly, TNF and IL-1 almost completely blocked the expression of luciferase activity linked to a FXR response element promoter construct transfected into Hep3B cells. Together with our earlier studies on the repression of RXRs, peroxisome proliferator-activated receptors, LXRs, thyroid receptors, constitutive androstane receptor, and pregnane X receptor, these results suggest that decreases in nuclear hormone receptors are major contributors to the decreased gene expression that occurs in the negative acute phase response.
Insights
Lipopolysaccharide (LPS) and inflammatory cytokines decrease farnesoid X receptor (FXR) expression and activity. This repression of FXR and its target genes contributes to altered lipid metabolism during the acute phase response.
Area of Science:
- Biochemistry
- Molecular Biology
- Hepatology
Background:
- The acute phase response alters hepatic gene expression, impacting lipid metabolism.
- Nuclear hormone receptors, including farnesoid X receptor (FXR), regulate lipid metabolism and heterodimerize with retinoid X receptor (RXR).
- Previous studies indicated repression of several nuclear hormone receptors during LPS-induced acute phase response.
Purpose of the Study:
- To investigate the effect of lipopolysaccharide (LPS) on farnesoid X receptor (FXR) expression and function in mouse liver.
- To determine the impact of inflammatory cytokines tumor necrosis factor (TNF) and interleukin-1 (IL-1) on FXR in human hepatoma cells.
- To elucidate the role of FXR repression in the negative acute phase response.
Main Methods:
- Quantitative analysis of FXR mRNA levels in mouse liver after LPS administration.
- Gel-shift and supershift assays to assess DNA binding activity of FXR/RXR complexes.
- Measurement of FXR target gene (SHP, apoCII) and LRH expression.
- In vitro studies using Hep3B cells treated with TNF, IL-1, or IL-6 to assess FXR mRNA, DNA binding, and target gene expression.
- Reporter gene assays to evaluate FXR promoter activity.
Main Results:
- LPS significantly decreased mouse hepatic FXR mRNA in a dose- and time-dependent manner.
- LPS treatment reduced DNA binding activity to an FXR response element (IR1), involving FXR and RXR.
- Expression of FXR target genes SHP and apoCII, and hepatic LRH, were significantly reduced by LPS.
- TNF and IL-1, but not IL-6, decreased FXR mRNA, IR1 DNA binding, and target gene expression in Hep3B cells.
- TNF and IL-1 markedly inhibited FXR promoter activity in reporter gene assays.
Conclusions:
- LPS-induced inflammation represses FXR expression and DNA binding activity in the liver.
- Inflammatory cytokines TNF and IL-1 contribute to FXR repression in human hepatocytes.
- Decreased FXR activity and its downstream targets play a significant role in the altered hepatic gene expression observed during the negative acute phase response.