Involvement of the vitamin D receptor in energy metabolism: regulation of uncoupling proteins

Abstract

Recent studies have established that vitamin D plays multiple biological roles beyond calcium metabolism; however, whether vitamin D is involved in energy metabolism is unknown. To address this question, we characterized the metabolic phenotypes of vitamin D receptor (VDR)-null mutant mice. Under a normocalcemic condition, VDR-null mice displayed less body fat mass and lower plasma triglyceride and cholesterol levels compared with wild-type (WT) mice; when placed on a high-fat diet, VDR-null mice showed a slower growth rate and accumulated less fat mass globally than WT mice, even though their food intake and intestinal lipid transport capacity were the same as WT mice. Consistent with the lower adipose mass, plasma leptin levels were lower and white adipocytes were histologically smaller in VDR-null mice than WT mice. The rate of fatty acid beta-oxidation in the white adipose tissue was higher, and the expression of uncoupling protein (UCP) 1, UCP2 and UCP3 was markedly upregulated in VDR-null mice, suggesting a higher energy expenditure in the mutant mice. Experiments using primary brown fat culture confirmed that 1,25-dihydroxyvitamin D3 directly suppressed the expression of the UCPs. Consistently, the energy expenditure, oxygen consumption, and CO2 production in VDR-null mice were markedly higher than in WT mice. These data indicate that vitamin D is involved in energy metabolism and adipocyte biology in vivo in part through regulation of beta-oxidation and UCP expression.

Fig. 1.

Body weight and serum calcium levels. A and B: body weight of male (A) and female (B) wild-type (WT) and vitamin D receptor-deficient [VDR(−/−)] mice at 4 mo of age on the high-calcium (HCa) or on the high-fat (HF) diet for 5 wk. C and D: growth curve of male (C) and female (D) WT and VDR(−/−) mice on the HF diet. E: serum calcium levels of WT and VDR(−/−) mice on normal rodent chow (ND), HCa diet, or HF diet supplemented with high-calcium water. *P < 0.05 and **P < 0.005 vs. WT; n = 4–11 mice in each genotype.

Fig. 2.

Body fat percentage and morphology of white adipose tissue (WAT) and brown adipose tissue (BAT). A and B: body fat percentage of male (A) and female (B) WT and VDR(−/−) mice on the HCa diet or HF diet. *P < 0.05 and **P < 0.005 vs. WT; n = 4–11 in each genotype. C and D: hematoxylin and eosin (H&E) staining of WAT (C) or BAT (D) from WT and VDR(−/−) mice on the HF diet. E and F: RT-PCR examination of vitamin D receptor (VDR) expression in WAT (E) and BAT (F). C, negative control; Kid, kidney; M, molecular weight marker.

Fig. 3.

Gene expression in WAT. Total RNAs were isolated from WAT of male WT and VDR(−/−) mice on the HCa diet or HF diet, and mRNA expression of the following genes was determined by Northern blot analyses: fatty acid synthase (FAS; A), lipoprotein lipase (LPL; B), peroxisome proliferator-activated receptor-γ (PPARγ; C). All experiments were repeated at least 2–3 times with similar results, and shown are representative data.

Fig. 4.

Levels of adipokines from WAT. A: plasma leptin levels in WT and VDR(−/−) mice on the HCa diet or HF diet. **P < 0.005 vs. WT; n = 6–9 in each genotype. B–D: Northern blot analyses of leptin mRNA (B), adiponectin mRNA (C), and resistin mRNA (D) expression in WAT from male WT and VDR(−/−) mice on the HCa diet or HF diet as indicated. All experiments were repeated at least 2–3 times with similar results, and shown are representative data.

Fig. 5.

Plasma lipid levels and intestinal lipid absorption. A and B: concentration of total plasma triglyceride (TG, A) and cholesterol (B) in male WT and VDR(−/−) mice on the HCa diet or HF diet. C: food intake of male WT and VDR(−/−) mice. D: intestinal lipid absorption. Male WT and VDR(−/−) mice (n = 4–6) were gavaged with olive oil, and triglyceride accumulation in the plasma was determined with time. *P < 0.05 vs. WT; n = 6–11 in each genotype.

Fig. 6.

Fatty acid β-oxidation in WAT. A: rate of β-oxidation in white adipocytes isolated from male WT and VDR(−/−) mice. β-Oxidation was measured by monitoring the production of ³H₂O after treatment with tritiated palmitate. B: RT-PCR analysis of carnitine palmitoyltransferase (CPT) II mRNA levels in WAT from WT and VDR(−/−) mice. *P < 0.05 and **P < 0.005 vs. WT; n = 5–6 in each genotype.

Fig. 7.

Expression of uncoupling protein (UCP) in BAT. A–C: total RNAs were isolated from BAT in male WT and VDR(−/−) mice fed the HCa diet, and UCP1 (A), UCP2 (B), and UCP3 (C) mRNA were determined by Northern blotting. D–F: BAT total RNAs were isolated form WT and VDR(−/−) mice on the HF diet, and UCP1 (D), UCP2 (E), and UCP3 (F) expression was determined by Northern blotting. G and H: PhosphorImaging quantitative data of the UCP mRNA levels from WT and VDR(−/−) mice on the HCa (G) or HF (H) diet. *P < 0.01 vs. WT.

Fig. 8.

Suppression of UCP expression in primary brown fat tissues by 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃]. A: RT-PCR determination of VDR expression in the BAT from WT and VDR(−/−) mice fed the HCa diet or the HF diet as indicated. B and C: primary BAT isolated from WT or VDR(−/−) mice were treated for 24 h with 1,25(OH)₂D₃ (10⁻⁸ M, VD), and UCP1 (B) and UCP3 (C) expression was analyzed by Northern blotting. All experiments were repeated at least 2–3 times with similar results, and shown are representative data.

Fig. 9.

Calorimetric parameters. Male WT and VDR(−/−) mice fed the HCa diet (A–C) or the HF diet (E–F) were individually placed in the metabolic cages and acclimated to the chambers for 2–3 days before the 3-day measurement. A and D: energy expenditure (EE); B and E: oxygen consumption; and C and F: carbon dioxide production, under the HCa or the HF dietary condition, respectively; n = 5–6 for each genotype.