Bile acids lower triglyceride levels via a pathway involving FXR, SHP, and SREBP-1c

Abstract

We explored the effects of bile acids on triglyceride (TG) homeostasis using a combination of molecular, cellular, and animal models. Cholic acid (CA) prevents hepatic TG accumulation, VLDL secretion, and elevated serum TG in mouse models of hypertriglyceridemia. At the molecular level, CA decreases hepatic expression of SREBP-1c and its lipogenic target genes. Through the use of mouse mutants for the short heterodimer partner (SHP) and liver X receptor (LXR) alpha and beta, we demonstrate the critical dependence of the reduction of SREBP-1c expression by either natural or synthetic farnesoid X receptor (FXR) agonists on both SHP and LXR alpha and LXR beta. These results suggest that strategies aimed at increasing FXR activity and the repressive effects of SHP should be explored to correct hypertriglyceridemia.

Figure 1

CA lowers serum TGs in KK-Ay mice. (A) Food intake of KK-Ay mice during 1 week on the diets as described in the figure. Serum levels of TGs, total cholesterol (Chol), and FFA in KK-Ay mice after 1 week on the different diets (age 7 weeks, n = 5). Kcal, kilocalories; BW, body weight. (B) TG and cholesterol lipoprotein profiles after size-exclusion chromatography of serum pools from five animals. The quantification of the VLDL TGs is shown in an inset. (C) Serum TGs in KK-Ay (top, age 12 weeks, n = 4) and ob/ob (bottom, age 12 weeks, n = 4) mice after 0, 1, 3, and 7 days of treatment with the synthetic FXR agonist GW4064. *P < 0.05; **P < 0.01 throughout the figures.

Figure 2

CA lowers hepatic TGs in KK-Ay mice. (A) Liver morphology, H&E-stained liver sections (HE), and Oil Red O_stained liver sections (OR) in animals treated with the indicated diets for 3 weeks. When sacrificed, the mice were 9 weeks old. (B) Liver TG and (C) cholesterol content after 3 weeks on the different diets (age 9 weeks, n = 5). (D) In vivo VLDL secretion from the livers of mice on the different diets (age 7 weeks, n = 4) or after treatment with GW4064 (age 12 weeks, n = 4). C, control diet; C + CA, control diet with CA; HF, high-fat diet; HF + CA, high-fat diet with CA.

Figure 3

Bile acids and SHP decrease expression from the SREBP-1c promoter. (A) Expression of SREBP-1c and several of its target genes in mouse liver primary hepatocyte cultures. The presence of ligands for LXR (22(R)-hydroxycholesterol, 20 ∝M) and RXR (LG100268, 1 ∝M) is indicated by a + sign. FXR was activated by the addition of CDCA to the medium (50 ∝M and 200 ∝M). (B) Activity of the mouse SREBP-1c promoter in the McA-RH7777 cell line after the addition of 200 ∝M CDCA or 200 ∝M CA to the medium. Cells were tested in the absence or presence of cotransfected LXRα and ligands for RXR and LXR at the concentrations specified in A. (C) Schematic representation of the different constructs of the mouse SREBP-1c promoter used in transfection assays. Binding sites for LXR are displayed as ovals. The nucleotide numbering is relative to the SREBP-1c start codon. (D) Sequence comparison of the LXRREs in the human and mouse SREBP-1c promoters. The GenBank accession numbers for the human SREBP-1c promoter sequence are NT_010718 or AC122129. (E) Activity of the mSREBP-1c reporters in McA-RH7777 cells transfected either with an empty expression vector or with the indicated combinations of expression vectors for mouse LRH-1, mouse RXRα, human LXRα, mouse SHP in the presence (black bars) or absence (white bars) of LXR (22(R)-hydroxycholesterol; 20 ∝M) and RXR agonists (LG100268; 1 ∝M).

Figure 4

CA attenuates LXR agonist_induced lipogenesis in vivo. (A) Liver weight and serum TGs after feeding a CA-containing diet and coadministration of the LXR agonist T0901317 (age 11 weeks, n = 6). (B) Hepatic expression levels of SREBP-1c, CYP7A1, ME, ACC1, ACC2, ABCA1, ABCG5, ABCG8, ANGPTL3, and SHP as determined using quantitative RT-PCR (n = 4).

Figure 5

SHP is essential for bile acid_mediated downregulation of lipogenesis. (A) SHP_/_ and SHP+/+ mice were fed a diet containing 0.5% CA or received GW4064 by oral gavage, and serum TG levels were measured after 0, 3, and 7 days (age 9 weeks, n = 5). (B) In two separate experiments, mice received CA or GW4064 as just described and were sacrificed after 1 day. Hepatic expression levels of SREBP-1c, CYP7A1, ME, and ANGPTL3 were determined using quantitative RT-PCR (age 9 weeks, n = 5).

Figure 6

LXR is essential for SHP-mediated lowering of TGs. (A) Serum TGs in wild-type and LXRα/β^_/_ mice after treatment with 0.5% CA (age 10_14 weeks, n = 6). (B) Hepatic expression levels of SREBP-1c, CYP7A1, ME, and SHP in wild-type and LXRα/β^_/_ mice after treatment with CA as determined using quantitative RT-PCR. # denotes P = 0.09 (n = 6). (C) A FXR-SHP-SREBP-1c regulatory cascade. Schematic representation of the proposed role of SHP in mediating the effects of FXR agonists on SREBP-1c expression and lipogenesis.