Nuclear receptor expression links the circadian clock to metabolism

As sensors for fat-soluble hormones and dietary lipids, oscillations in nuclear receptor (NR) expression in key metabolic tissues may contribute to circadian entrainment of nutrient and energy metabolism. Surveying the diurnal expression profiles of all 49 mouse nuclear receptors in white and brown adipose tissue, liver, and skeletal muscle revealed that of the 45 NRs expressed, 25 are in a rhythmic cycle and 3 exhibit a single transient pulse of expression 4 hr into the light cycle. While thyroid hormones are generally constant, we find that TRalpha and beta dramatically cycle, suggesting that fundamental concepts such as "basal metabolism" may require reexamination. The dynamic but coordinated changes in nuclear receptor expression, along with their key target genes, offers a logical explanation for known cyclic behavior of lipid and glucose metabolism and suggests novel roles for endocrine and orphan receptors in coupling the peripheral circadian clock to divergent metabolic outputs.     

Reduced fat mass in mice lacking orphan nuclear receptor estrogen-related receptor alpha

The estrogen-related receptor alpha (ERRalpha) is an orphan member of the superfamily of nuclear hormone receptors expressed in tissues that preferentially metabolize fatty acids. Despite the molecular characterization of ERRalpha and identification of target genes, determination of its physiological function has been hampered by the lack of a natural ligand. To further understand the in vivo function of ERRalpha, we generated and analyzed Estrra-null (ERRalpha-/-) mutant mice. Here we show that ERRalpha-/- mice are viable, fertile and display no gross anatomical alterations, with the exception of reduced body weight and peripheral fat deposits. No significant changes in food consumption and energy expenditure or serum biochemistry parameters were observed in the mutant animals. However, the mutant animals are resistant to a high-fat diet-induced obesity. Importantly, DNA microarray analysis of gene expression in adipose tissue demonstrates altered regulation of several enzymes involved in lipid, eicosanoid, and steroid synthesis, suggesting that the loss of ERRalpha might interfere with other nuclear receptor signaling pathways. In addition, the microarray study shows alteration in the expression of genes regulating adipogenesis as well as energy metabolism. In agreement with these findings, metabolic studies showed reduced lipogenesis in adipose tissues. This study suggests that ERRalpha functions as a metabolic regulator and that the ERRalpha-/- mice provide a novel model for the investigation of metabolic regulation by nuclear receptors.     

胆汁酸对代谢性疾病的调控

胆汁酸作为一种信号分子通过激活肝、肠道和外周组织中的胆汁酸受体影响体内葡萄糖和脂质的代谢平衡,对于调节肥胖、2型糖尿病和非酒精性脂肪肝等代谢性疾病具有非常重要的意义。胆汁酸与相应核受体,如法尼醇X受体(farnesoid X receptor, FXR)和Takeda G蛋白偶联受体5 (Takeda G protein-coupled receptor 5,TGR5)的相互作用影响了这些代谢性疾病。FXR主要通过影响胆汁酸的合成及转运对非酒精性脂肪肝发挥作用,TGR5则是间接增加褐色脂肪组织中的生热作用,改善肥胖和2型糖尿病。这些调控机制的研究是非常必要的。本文综述了胆汁酸代谢及其对代谢性疾病调控的分子机制的研究进展,以期为科研工作者提供一定的参考。     

Whole-tissue 3D imaging reveals intra-adipose sympathetic plasticity regulated by NGF-TrkA signal in cold-induced beiging

Sympathetic arborizations act as the essential efferent signals in regulating the metabolism of peripheral organs including white adipose tissues (WAT). However, whether these local neural structures would be of plastic nature, and how such plasticity might participate in specific metabolic events of WAT, remains largely uncharacterized. In this study, we exploit the new volume fluorescence-imaging technique to observe the significant, and also reversible, plasticity of intra-adipose sympathetic arborizations in mouse inguinal WAT in response to cold challenge. We demonstrate that this sympathetic plasticity depends on the cold-elicited signal of nerve growth factor (NGF) and TrkA receptor. Blockage of NGF or TrkA signaling suppresses intraadipose sympathetic plasticity, and moreover, the coldinduced beiging process of WAT. Furthermore, we show that NGF expression in WAT depends on the catecholamine signal in cold challenge. We therefore reveal the key physiological relevance, together with the regulatory mechanism, of intra-adipose sympathetic plasticity in the WAT metabolism.     

Thyroid hormone induces cardiac myocyte hypertrophy in a thyroid hormone receptor alpha1-specific manner that requires TAK1 and p38 mitogen-activated protein kinase

Alterations in TR [thyroid hormone (TH) receptor]1 isoform expression have been reported in models of both physiologic and pathologic cardiac hypertrophy as well as in patients with heart failure. In this report, we demonstrate that TH induces hypertrophy as a direct result of binding to the TRalpha1 isoform and, moreover, that overexpression of TRalpha1 alone is also associated with a hypertrophic phenotype, even in the absence of ligand. The mechanism of TH and TRalpha1-specific hypertrophy is novel for a nuclear hormone receptor and involves the transforming growth factor beta-activated kinase (TAK1) and p38. Mitigating TRalpha1 effects, both TRalpha2 and TRbeta1 attenuate TRalpha1-induced myocardial growth and gene expression by diminishing TAK1 and p38 activities, respectively. These findings refine our previous observations on TR expression in the hypertrophied and failing heart and suggest that manipulation of thyroid hormone signaling in an isoform-specific manner may be a relevant therapeutic target for altering the pathologic myocardial program.     

Requirement for thyroid hormone receptor beta in T3 regulation of cholesterol metabolism in mice

T3 potently influences cholesterol metabolism through the nuclear thyroid hormone receptor beta (TRbeta), the most abundant TR isoform in rodent liver. Here, we have tested if TRalpha1, when expressed at increased levels from its normal locus, can replace TRbeta in regulation of cholesterol metabolism. By the use of TRalpha2-/-beta-/- animals that overexpress hepatic TRalpha1 6-fold, a near normalization of the total amount of T3 binding receptors was achieved. These mice are similar to TRbeta-/- and TRalpha1-/-beta-/- mice in that they fail to regulate cholesterol 7alpha-hydroxylase expression properly, and that their serum cholesterol levels are unaffected by T3. Thus, hepatic overexpression of TRalpha1 cannot substitute for absence of TRbeta, suggesting that the TRbeta gene has a unique role in T3 regulation of cholesterol metabolism in mice. However, examination of T3 regulation of hepatic target genes revealed that dependence on TRbeta is not general: T3 regulation of type I iodothyronine deiodinase and the low density lipoprotein receptor were partially rescued by TRalpha1 overexpression. These in vivo data show that TRbeta is necessary for the effects of T3 on cholesterol metabolism. That TRalpha1 only in some instances can substitute for TRbeta indicates that T3 regulation of physiological and molecular processes in the liver occurs in an isoform-specific fashion.     

Contrasting skeletal phenotypes in mice with an identical mutation targeted to thyroid hormone receptor alpha1 or beta

Thyroid hormone (T(3)) regulates bone turnover and mineralization in adults and is essential for skeletal development. Surprisingly, we identified a phenotype of skeletal thyrotoxicosis in T(3) receptor beta(PV) (TRbeta(PV)) mice in which a targeted frameshift mutation in TRbeta results in resistance to thyroid hormone. To characterize mechanisms underlying thyroid hormone action in bone, we analyzed skeletal development in TRalpha1(PV) mice in which the same PV mutation was targeted to TRalpha1. In contrast to TRbeta(PV) mice, TRalpha1(PV) mutants exhibited skeletal hypothyroidism with delayed endochondral and intramembranous ossification, severe postnatal growth retardation, diminished trabecular bone mineralization, reduced cortical bone deposition, and delayed closure of the skull sutures. Skeletal hypothyroidism in TRalpha1(PV) mutants was accompanied by impaired GH receptor and IGF-I receptor expression and signaling in the growth plate, whereas GH receptor and IGF-I receptor expression and signaling were increased in TRbeta(PV) mice. These data indicate that GH receptor and IGF-I receptor are physiological targets for T(3) action in bone in vivo. The divergent phenotypes observed in TRalpha1(PV) and TRbeta(PV) mice arise because the pituitary gland is a TRbeta-responsive tissue, whereas bone is TRalpha responsive. These studies provide a new understanding of the complex relationship between central and peripheral thyroid status.     

Thyroid hormone receptor alpha1 follows a cooperative CRM1/calreticulin-mediated nuclear export pathway

The thyroid hormone receptor alpha1 (TRalpha) exhibits a dual role as an activator or repressor of its target genes in response to thyroid hormone (T(3)). Previously, we have shown that TRalpha, formerly thought to reside solely in the nucleus bound to DNA, actually shuttles rapidly between the nucleus and cytoplasm. An important aspect of the shuttling activity of TRalpha is its ability to exit the nucleus through the nuclear pore complex. TRalpha export is not sensitive to treatment with the CRM1-specific inhibitor leptomycin B (LMB) in heterokaryon assays, suggesting a role for an export receptor other than CRM1. Here, we have used a combined approach of in vivo fluorescence recovery after photobleaching experiments, in vitro permeabilized cell nuclear export assays, and glutathione S-transferase pull-down assays to investigate the export pathway used by TRalpha. We show that, in addition to shuttling in heterokaryons, TRalpha shuttles rapidly in an unfused monokaryon system as well. Furthermore, our data show that TRalpha directly interacts with calreticulin, and point to the intriguing possibility that TRalpha follows a cooperative export pathway in which both calreticulin and CRM1 play a role in facilitating efficient translocation of TRalpha from the nucleus to cytoplasm.     

Induction of larval tissue resorption in Xenopus laevis tadpoles by the thyroid hormone receptor agonist GC-1

A major challenge in understanding nuclear hormone receptor function is to determine how the same ligand can cause very different tissue-specific responses. Tissue specificity may result from the presence of more than one receptor subtype arising from multiple receptor genes or alternative splicing. Recently, high affinity analogs of nuclear receptor ligands have been synthesized that show subtype selectivity. These analogs can greatly facilitate the study of receptor subtype-specific functions in organisms where mutational analysis is problematic or where it is desirable for receptors to be expressed in their normal physiological contexts. We describe here the effects of the synthetic thyroid hormone analog GC-1 on the metamorphosis of the frog Xenopus laevis. The most potent natural thyroid hormone, 3,5,3'-triidothyronine or T3, shows similar binding affinity and transactivation dose-response curves for both thyroid hormone receptor isotypes, designated TRalpha and TRbeta. GC-1, however, binds to and activates TRbeta at least an order of magnitude better than it does TRalpha. GC-1 efficiently induces death and resorption of premetamorphic tadpole tissues such as the gills and the tail, two tissues that strongly induce thyroid hormone receptor beta during metamorphosis. GC-1 has less effect on the growth of adult tissues such as the hindlimbs, which express high TRalpha levels. The effectiveness of GC-1 in inducing tail resorption and tail gene expression correlates with increasing TRbeta levels. These results illustrate the utility of subtype selective ligands as probes of nuclear receptor function in vivo.     

Food-derived regulatory factors against obesity and metabolic syndrome

Abstract

Obesity is a key factor in metabolic syndrome. The study of metabolic syndrome focuses on the anti-weight gain properties of physiological mechanisms and food components. Abnormal energy metabolism is a major risk factor of metabolic syndrome. Chronic inflammation is a feature of obesity; cytokines from hypertrophied adipocytes cause inflammation in both adipose tissue and blood vessels, resulting in symptoms of metabolic syndrome. Tumor necrosis factor-α causes insulin resistance in adipocytes and regression of brown adipocytes, resulting in abnormal energy metabolism. Functional foods can serve as a strategy for prevention and treatment of obesity linked with metabolic processes in white and brown adipose tissues. Diet-induced thermogenesis caused by certain food components stimulates burning of stored fat within adipose tissues. A mechanistic understanding of dietary thermogenesis via the sympathetic nerve system will prove valuable for the development of precise strategies for the practical prevention of metabolic syndrome.     

A thyroid hormone receptor alpha gene mutation (P398H) is associated with visceral adiposity and impaired catecholamine-stimulated lipolysis in mice

Thyroid hormone has profound effects on metabolic homeostasis, regulating both lipogenesis and lipolysis, primarily by modulating adrenergic activity. We generated mice with a point mutation in the thyroid hormone receptor alpha (TRalpha) gene producing a dominant-negative TRalpha mutant receptor with a proline to histidine substitution (P398H). The heterozygous P398H mutant mice had a 3.4-fold (p < 0.02) increase in serum thyrotropin (TSH) levels. Serum triiodothyronine (T3) and thyroxine (T4) concentrations were slightly elevated compared with wild-type mice. The P398H mice had a 4.4-fold increase in body fat (as a fraction of total body weight) (p < 0.001) and a 5-fold increase in serum leptin levels (p < 0.005) compared with wild-type mice. A 3-fold increase in serum fasting insulin levels (p < 0.002) and a 55% increase in fasting glucose levels (p < 0.01) were observed in P398H compared with wild-type mice. There was a marked reduction in norepinephrine-induced lipolysis, as reflected in reduced glycerol release from white adipose tissue isolated from P398H mice. Heart rate and cold-induced adaptive thermogenesis, mediated by thyroid hormone-catecholamine interaction, were also reduced in P398H mice. In conclusion, the TRalpha P398H mutation is associated with visceral adiposity and insulin resistance primarily due to a marked reduction in catecholamine-stimulated lipolysis. The observed phenotype in the TRalpha P398H mouse is likely due to interference with TRalpha action as well as influence on other metabolic signaling pathways. The physiologic significance of these findings will ultimately depend on understanding the full range of actions of this mutation.     

Aging and caloric restriction impact adipose tissue,adiponectin, and circulating lipids

Adipose tissue expansion has been associated with system‐wide metabolic dysfunction and increased vulnerability to diabetes, cancer, and cardiovascular disease. A reduction in adiposity is a hallmark of caloric restriction (CR), an intervention that extends longevity and delays the onset of these same age‐related conditions. Despite these parallels, the role of adipose tissue in coordinating the metabolism of aging is poorly defined. Here, we show that adipose tissue metabolism and secretory profiles change with age and are responsive to CR. We conducted a cross‐sectional study of CR in adult, late‐middle‐aged, and advanced‐aged mice. Adiposity and the relationship between adiposity and circulating levels of the adipose‐derived peptide hormone adiponectin were age‐sensitive. CR impacted adiposity but only levels of the high molecular weight isoform of adiponectin responded to CR. Activators of metabolism including PGC‐1a, SIRT1, and NAMPT were differentially expressed with CR in adipose tissues. Although age had a significant impact on NAD metabolism, as detected by biochemical assay and multiphoton imaging, the impact of CR was subtle and related to differences in reliance on oxidative metabolism. The impact of age on circulating lipids was limited to composition of circulating phospholipids. In contrast, the impact of CR was detected in all lipid classes regardless of age, suggesting a profound difference in lipid metabolism. These data demonstrate that aspects of adipose tissue metabolism are life phase specific and that CR is associated with a distinct metabolic state, suggesting that adipose tissue signaling presents a suitable target for interventions to delay aging.     

Age-related changes in renal and hepatic cellular mechanisms associated with variations in rat serum thyroid hormone levels

Aging is associated with changes in thyroid gland physiology. Age-related changes in the contribution of peripheral tissues to thyroid hormone serum levels have yet to be systematically assessed. Here, we investigated age-related alterations in the contributions of the liver and kidney to thyroid hormone homeostasis using 6-, 12-, and 24-mo-old male Wistar rats. A significant and progressive decline in plasma thyroxine occurred with age, but triiodothyronine (T(3)) was decreased only at 24 mo. This was associated with an unchanged protein level of the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in the kidney and with a decreased MCT8 level in the liver at 24 mo. Hepatic type I deiodinase (D1) protein level and activity declined progressively with age. Renal D1 levels were decreased at both 12 and 24 mo but D1 activity was decreased only at 24 mo. In the liver, no changes occurred in thyroid hormone receptor (TR) TRalpha(1), whereas a progressive increase in TRbeta(1) occurred at both mRNA and total protein levels. In the kidney, both TRalpha(1) and TRbeta(1) mRNA and total protein levels were unchanged between 6 and 12 mo but increased at 24 mo. Interestingly, nuclear TRbeta1 levels were decreased in both liver and kidney at 12 and 24 mo, whereas nuclear TRalpha(1) levels were unchanged. Collectively, our data show differential age-related changes among hepatic and renal MCT8 and D1 and TR expressions, and they suggest that renal D1 activity is maintained with age to compensate for the decrease in hepatic T(3) production.     

Identification of a lipokine, a lipid hormone linking adipose tissue to systemic metabolism

Dysregulation of lipid metabolism in individual tissues leads to systemic disruption of insulin action and glucose metabolism. Utilizing quantitative lipidomic analyses and mice deficient in adipose tissue lipid chaperones aP2 and mal1, we explored how metabolic alterations in adipose tissue are linked to whole-body metabolism through lipid signals. A robust increase in de novo lipogenesis rendered the adipose tissue of these mice resistant to the deleterious effects of dietary lipid exposure. Systemic lipid profiling also led to identification of C16:1n7-palmitoleate as an adipose tissue-derived lipid hormone that strongly stimulates muscle insulin action and suppresses hepatosteatosis. Our data reveal a lipid-mediated endocrine network and demonstrate that adipose tissue uses lipokines such as C16:1n7-palmitoleate to communicate with distant organs and regulate systemic metabolic homeostasis.     

Hypothalamic melanocortin system regulates sympathetic nerve activity in brown adipose tissue

To clarify the neuronal mechanism of the hypothalamic melanocortin system in regulating energy metabolism, we investigated the effects of centrally administered alpha-melanocyte-stimulating hormone (alpha-MSH) and agouti-related protein (AGRP), an agonist and an antagonist for the melanocortin 4 receptor (MC4-R), respectively, on the activity of sympathetic nerves innervating brown adipose tissue (BAT) and on BAT temperature. A bolus infusion of alpha-MSH (1 nmol) into the third cerebral ventricle (i3vt) significantly increased sympathetic nerve activity and elevated BAT temperature (P<0.05). The i3vt infusion of AGRP (1 nmol) gradually suppressed BAT sympathetic nerve activity and was accompanied by a significant reduction in BAT temperature (P<0.05). In conclusion, the hypothalamic melanocortin system may regulate peripheral energy expenditure, as well as thermogenesis, through its influence on BAT sympathetic nerve activity.