Distinct dysregulation of lipid metabolism by unliganded thyroid hormone receptor isoforms - PubMed (original) (raw)
Distinct dysregulation of lipid metabolism by unliganded thyroid hormone receptor isoforms
O Araki et al. Mol Endocrinol. 2009 Mar.
Abstract
Thyroid hormone receptors (TRs) play critical roles in energy homeostasis. To understand the role of TRs in lipid homeostasis in vivo, we adopted the loss-of-function approach by creating knock-in mutant mice with targeted mutation in the TRalpha gene (TRalpha1PV mouse) or TRbeta gene (TRbetaPV mouse). The PV mutation, identified in a patient with resistance to thyroid hormone, exhibits potent dominant-negative activity. Here we show that in contrast to TRalpha1PV mouse, TRbetaPV mice exhibited no significant reduction in WAT but had significant increases in serum free fatty acids and total triglycerides. Moreover, the liver of TRbetaPV mice was markedly increased (33%) with excess lipid accumulation, but the liver mass of TRalpha1PV mouse was decreased (23%) with paucity of lipids. These results indicate that apo-TRbeta and apo-TRalpha1 exerted distinct abnormalities in lipid metabolism. Further biochemical analyses indicate that increased lipogenic enzyme expression, activated peroxisome proliferator-activated receptor gamma (Ppargamma) signaling, and decreased fatty acid beta-oxidation activity contributed to the adipogenic steatosis and lipid accumulation in the liver of TRbetaPV mice. In contrast, the expression of lipogenic enzymes and Ppargamma was decreased in the liver of TRalpha1PV mice. These results suggest that the regulation of genes critical for lipid metabolism by TRs in the liver is isoform dependent. These results indicate that apo-TRbeta and apo-TRalpha1 had different effects on lipid metabolism and that both TR isoforms contribute to the pathogenesis of lipid metabolism in hypothyroidism.
Figures
Figure 1
No significant changes in the weight of fat tissues in TRβPV mice (A) and food intake (B). A, Inguinal, epididymal, perirenal, and interscapular fat tissues were weighed after dissection from mice at age 4–5 months. Ratios of fat mass vs. body weight were determined. The data are expressed as mean ±
sem
(n = 4–9). B, Food consumption by TRβPV mice. Food consumed by mice in 2 d was measured, and the food intake was normalized by body weight and expressed as grams per gram body weight0.75. The data are expressed as mean ±
sem
(n = 4–8), and the experiments were repeated three times.
Figure 2
Enlarged fatty liver of TRβPV/PV mice. A, Comparison of liver weight of wild-type, TRβPV/+, mice, and TRβPV/PV mice aged 4–5 months. Ratios of tissue mass vs. body weight were determined. The data are expressed as mean ±
sem
(n = 4–9; the P value is indicated). B, Representative liver of a wild-type mouse (a) and TRβPV/PV mouse (b), showing a larger liver of TRβPV/PV mouse with grayish appearance. C, Hematoxylin- and eosin-stained (a–c) or Oil Red-O-stained (d–f) liver of a wild-type mouse (a and d), TRβPV/+ mouse (b and e), and TRβPV/PV mouse (c and f). Arrows indicate the accumulation of lipids.
Figure 3
Relative mRNA expression of key regulators of lipogenesis and lipolysis in the liver of TRβPV mice. A, Quantitative real time RT-PCR was used to determine the expression of key genes in lipid metabolism according to Materials and Methods. The data are expressed as mean ±
sem
(n = 4; the P values are indicated). B, Comparison of Pparγ protein abundance in the liver of wild-type and TRβPV/PV mice. Western blot analysis was carried out as described in Materials and Methods. The analysis used 50 μg tissue extracts. α-Tubulin was used as loading control. The lanes are marked.
Figure 4
T3 negatively regulates the expression of Pparγ in the liver. Quantitative real-time RT-PCR analysis of Pparγ mRNA expression in the liver of untreated, PTU-treated, and PTU+T3-treated TRβPV mice as described in Materials and Methods. The data are expressed as mean ±
sem
(n = 3–7). The P values are indicated.
Figure 5
A, Fatty acid β-oxidation activity in TRβPV mice.In vitro β-oxidation activity in primary hepatocytes isolated from TRβPV mice as described in Materials and Methods. The data are expressed as means ±
sd
(n = 3–4; *, P < 0.05; **, P < 0.01). B, Relative mRNA expression of key regulators of fatty acid β-oxidation in the liver of TRβPV mice by quantitative real-time RT-PCR as described in Materials and Methods. The data are expressed as mean ±
sem
(n = 4–8; P values are indicated).
Figure 6
A, Comparison of liver weight of TRα1PV mice with wild-type siblings. Mice aged 4–5 months were used in the analysis. Ratios of liver mass vs. body weight were determined. B, Hematoxylin- and eosin-stained (a and b) or Oil Red-O-stained (c and d) liver of wild-type mouse (a and c) and TRα1PV/+ mouse (b and d). The liver of TRα1PV/+ mouse exhibits lipid paucity. C, Relative mRNA expression of key regulators of lipogenesis and lipolysis in the liver of TRα1PV mice by quantitative real-time RT-PCR as described in Materials and Methods. The data are expressed as mean ±
sem
(n = 4–5; P values are indicated).
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