Selective thyroid receptor modulation by GC-1 reduces serum lipids and stimulates steps of reverse cholesterol transport in euthyroid mice

Abstract

Thyroid hormones [predominantly 3,5,3'-triiodo-L-thyronine (T3)] regulate cholesterol and lipoprotein metabolism, but cardiac effects restrict their use as hypolipidemic drugs. T3 binds to thyroid hormone receptors (TRs) alpha and beta. TRbeta is the predominant isoform in liver, whereas T3 effects on heart rate are mediated mostly by TRalpha. Drugs that target TRbeta or exhibit tissue-selective uptake may improve plasma lipid levels while sparing the heart. Here, we asked how the TRbeta- and liver uptake-selective agonist GC-1 influences cholesterol and triglyceride metabolism in euthyroid mice. GC-1 treatment reduced serum cholesterol levels by 25% and serum triglycerides by 75% in chow-fed mice and also attenuated diet-induced hypercholesterolemia. GC-1 reduced plasma high-density lipoprotein cholesterol levels; increased expression of the hepatic high-density lipoprotein receptor, SR-BI; stimulated activity of cholesterol 7alpha-hydroxylase; and increased fecal excretion of bile acids. Collectively, these results suggest that GC-1 stimulates important steps in reverse cholesterol transport. Use of TRbeta and uptake selective agonists such as GC-1 should be further explored as a strategy to improve lipid metabolism in dyslipoproteinemia.

Fig. 1.

T₃ and GC-1 reduce serum cholesterol in euthyroid mice. Effects of increasing dose of GC-1 and T₃ on cholesterol and triglyceride content in serum. (A) Serum total cholesterol; percent change from control (2.48 ± 0.04 mmol/liter). (B and C) Serum lipoprotein patterns after separation by SEC. Pooled plasma from each group was used. (D) Serum total triglycerides corrected by glycerol content; percent change from control (0.57 ± 0.14 mmol/liter). (E and F) Serum lipoprotein patterns after separation by SEC. Individual serum was used (10 μl), and triglyceride and glycerol content was determined. Data show mean ± SEM. Note that GC-1 reduces VLDL triglycerides at all concentrations (E), whereas low concentrations of T₃ increase VLDL and higher concentrations reduce VLDL (F). A.U., arbitrary units.

Fig. 2.

T₃ and GC-1 induce hepatic expression and activity of genes involved in RCT (SR-BI and CYP7A1). (A) SR-BI protein in pooled liver membranes determined by Western blot and quantified by using image gauge software, Fuji Film. (B) C4 determined in pooled serum samples; results are expressed as percent change from control relative to total serum cholesterol (5.36 mg/mol cholesterol), as currently recommended (62). However, GC-1 and, to a lesser extent, T₃ also increased absolute C4 levels. (C and D) Effects of GC-1 and T₃ on hepatic mRNAs, mean ± SEM of determinations with three separate cDNA syntheses from pooled RNA.

Fig. 3.

T₃ and GC-1 reduce serum cholesterol in hypercholesterolemic mice. (A) Serum cholesterol, percent change from control diet (3.34 ± 0.11 mmol/liter). (B) Cholesterol content in lipid extracts from livers; percent change from control diet (3.87 ± 0.17 μg/mg of wet tissue). (C and D) Serum lipoproteins. Pooled plasma separated by SEC. (E) Serum triglycerides after correction for glycerol content; percent change from control diet (0.19 ± 0.02 mmol/liter). (F) Triglyceride content in liver extracts; percent change from control diet (10.3 ± 1.27 μg/mg wet tissue). A.U., arbitrary units. Data show mean ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

Fig. 4.

T₃ and GC-1 induce RCT markers in hypercholesterolemic mice. (A) Western blot analysis of SR-BI in pooled liver membranes. Blotting was performed several times and is shown as one representative experiment. (B) C4 determined in pooled serum samples; percent change from control diet (2.34 mg/cholesterol mol). As with the data shown in Fig. 2, results are expressed as per cholesterol, but GC-1 and, to a lesser extent, T₃ also increased absolute C4 levels. (C and D) mRNA levels for CYP7A1, SHP, and SREBP-2 and -1c. RT-PCR data show percent change from control diet; mean ± SEM of determinations on three separate cDNA syntheses from pooled RNA are presented.

Fig. 5.

T₃ and GC-1 increase bile acid excretion. Effects of GC-1 and T₃ (48 nmol/kg per day) on fecal bile acid excretion (because assays are from pools of samples from the animals, statistical variation cannot be reported) and hepatic CYP7A1 mRNA (mean ± SEM from three separate cDNA synthesis from pooled RNA). We also measured fecal excretion of bile acids with a method based on ethanol extraction at boiling point followed by enzymatic–colorimetric assay and obtained similar results.