The LXR-Idol axis differentially regulates plasma LDL levels in primates and mice

The LXR-regulated E3 ubiquitin ligase IDOL controls LDLR receptor stability independent of SREBP and PCSK9, but its relevance to plasma lipid levels is unknown. Here we demonstrate that the effects of the LXR-IDOL axis are both tissue and species specific. In mice, LXR agonist induces Idol transcript levels in peripheral tissues but not in liver, and does not change plasma LDL levels. Accordingly, Idol-deficient mice exhibit elevated LDLR protein levels in peripheral tissues, but not in the liver. By contrast, LXR activation in cynomolgus monkeys induces hepatic IDOL expression, reduces LDLR protein levels, and raises plasma LDL levels. Knockdown of IDOL in monkeys with an antisense oligonucleotide blunts the effect of LXR agonist on LDL levels. These results implicate IDOL as a modulator of plasma lipid levels in primates and support further investigation into IDOL inhibition as a potential strategy for LDL lowering in humans.

Skeletal muscle Nur77 expression enhances oxidative metabolism and substrate utilization

Mitochondrial dysfunction has been implicated in the pathogenesis of type 2 diabetes. Identifying novel regulators of mitochondrial bioenergetics will broaden our understanding of regulatory checkpoints that coordinate complex metabolic pathways. We previously showed that Nur77, an orphan nuclear receptor of the NR4A family, regulates the expression of genes linked to glucose utilization. Here we demonstrate that expression of Nur77 in skeletal muscle also enhances mitochondrial function. We generated MCK-Nur77 transgenic mice that express wild-type Nur77 specifically in skeletal muscle. Nur77-overexpressing muscle had increased abundance of oxidative muscle fibers and mitochondrial DNA content. Transgenic muscle also exhibited enhanced oxidative metabolism, suggestive of increased mitochondrial activity. Metabolomic analysis confirmed that Nur77 transgenic muscle favored fatty acid oxidation over glucose oxidation, mimicking the metabolic profile of fasting. Nur77 expression also improved the intrinsic respiratory capacity of isolated mitochondria, likely due to the increased abundance of complex I of the electron transport chain. These changes in mitochondrial metabolism translated to improved muscle contractile function ex vivo and improved cold tolerance in vivo. Our studies outline a novel role for Nur77 in the regulation of oxidative metabolism and mitochondrial activity in skeletal muscle.

LXRα is uniquely required for maximal reverse cholesterol transport and atheroprotection in ApoE-deficient mice

The liver X receptor (LXR) signaling pathway is an important modulator of atherosclerosis, but the relative importance of the two LXRs in atheroprotection is incompletely understood. We show here that LXRα, the dominant LXR isotype expressed in liver, plays a particularly important role in whole-body sterol homeostasis. In the context of the ApoE^−/− background, deletion of LXRα, but not LXRβ, led to prominent increases in atherosclerosis and peripheral cholesterol accumulation. However, combined loss of LXRα and LXRβ on the ApoE^−/− background led to an even more severe cholesterol accumulation phenotype compared to LXRα^−/−ApoE^−/− mice, indicating that LXRβ does contribute to reverse cholesterol transport (RCT) but that this contribution is quantitatively less important than that of LXRα. Unexpectedly, macrophages did not appear to underlie the differential phenotype of LXRα^−/−ApoE^−/− and LXRβ^−/−ApoE^−/− mice, as in vitro assays revealed no difference in the efficiency of cholesterol efflux from isolated macrophages. By contrast, in vivo assays of RCT using exogenously labeled macrophages revealed a marked defect in fecal sterol efflux in LXRα^−/−ApoE^−/− mice. Mechanistically, this defect was linked to a specific requirement for LXRα^−/− in the expression of hepatic LXR target genes involved in sterol transport and metabolism. These studies reveal a previously unrecognized requirement for hepatic LXRα for optimal reverse cholesterol transport in mice.

TLE3 is a dual-function transcriptional coregulator of adipogenesis

PPARγ and Wnt signaling are central positive and negative regulators of adipogenesis, respectively. Here we identify the groucho family member TLE3 as a transcriptional integrator of the PPARγ and Wnt pathways. TLE3 is a direct target of PPARγ that participates in a feed-forward loop during adipocyte differentiation. TLE3 enhances PPARγ activity and functions synergistically with PPARγ on its target promoters to stimulate adipogenesis. At the same time, induction of TLE3 during differentiation provides a mechanism for termination of Wnt signaling. TLE3 antagonizes TCF4 activation by β-catenin in preadipocytes, thereby inhibiting Wnt target gene expression and reversing β-catenin-dependent repression of adipocyte gene expression. Transgenic expression of TLE3 in adipose tissue in vivo mimics the effects of PPARγ agonist and ameliorates high-fat-diet-induced insulin resistance. Our data suggest that TLE3 acts as a dual-function switch, driving the formation of both active and repressive transcriptional complexes that facilitate the adipogenic program.

Constitutive activation of LXR in macrophages regulates metabolic and inflammatory gene expression: identification of ARL7 as a direct target

Ligand activation of liver X receptors (LXRs) has been shown to impact both lipid metabolism and inflammation. One complicating factor in studies utilizing synthetic LXR agonists is the potential for pharmacologic and receptor-independent effects. Here, we describe an LXR gain-of-function system that does not depend on the addition of exogenous ligand. We generated transgenic mice expressing a constitutively active VP16-LXRα protein from the aP2 promoter. These mice exhibit increased LXR signaling selectively in adipose and macrophages. Analysis of gene expression in primary macrophages derived from two independent VP16-LXRα transgenic lines confirmed the ability of LXR to drive expression of genes involved in cholesterol efflux and fatty acid synthesis. Moreover, VP16-LXRα expression also suppressed the induction of inflammatory genes by lipopolysaccharide to a comparable degree as synthetic agonist. We further utilized VP16-LXRα-expressing macrophages to identify and validate new targets for LXRs, including the gene encoding ADP-ribosylation factor-like 7 (ARL7). ARL7 has previously been shown to transport cholesterol to the membrane for ABCA1-associated removal and thus may be integral to the LXR-dependent efflux pathway. We show that the ARL7 promoter contains a functional LXRE and can be transactivated by LXRs in a sequence-specific manner, indicating that ARL7 is a direct target of LXR. These findings provide further support for an important role of LXRs in the coordinated regulation of lipid metabolic and inflammatory gene programs in macrophages.

The E3 ubiquitin ligase IDOL induces the degradation of the low density lipoprotein receptor family members VLDLR and ApoER2

We have previously identified the E3 ubiquitin ligase-inducible degrader of the low density lipoprotein receptor (LDLR) (Idol) as a post-translational modulator of LDLR levels. Idol is a direct target for regulation by liver X receptors (LXRs), and its expression is responsive to cellular sterol status independent of the sterol-response element-binding proteins. Here we demonstrate that Idol also targets two closely related LDLR family members, VLDLR and ApoE receptor 2 (ApoER2), proteins implicated in both neuronal development and lipid metabolism. Idol triggers ubiquitination of the VLDLR and ApoER2 on their cytoplasmic tails, leading to their degradation. We further show that the level of endogenous VLDLR is sensitive to cellular sterol content, Idol expression, and activation of the LXR pathway. Pharmacological activation of the LXR pathway in mice leads to increased Idol expression and to decreased Vldlr levels in vivo. Finally, we establish an unexpected functional link between LXR and Reelin signaling. We demonstrate that LXR activation results in decreased Reelin binding to VLDLR and reduced Dab1 phosphorylation. The identification of VLDLR and ApoER2 as Idol targets suggests potential roles for this LXR-inducible E3 ligase in the central nervous system in addition to lipid metabolism.

LXR regulates cholesterol uptake through Idol-dependent ubiquitination of the LDL receptor

Cellular cholesterol levels reflect a balance between uptake, efflux, and endogenous synthesis. Here we show that the sterol-responsive nuclear liver X receptor (LXR) helps maintain cholesterol homeostasis, not only through promotion of cholesterol efflux but also through suppression of low-density lipoprotein (LDL) uptake. LXR inhibits the LDL receptor (LDLR) pathway through transcriptional induction of Idol (inducible degrader of the LDLR), an E3 ubiquitin ligase that triggers ubiquitination of the LDLR on its cytoplasmic domain, thereby targeting it for degradation. LXR ligand reduces, whereas LXR knockout increases, LDLR protein levels in vivo in a tissue-selective manner. Idol knockdown in hepatocytes increases LDLR protein levels and promotes LDL uptake. Conversely, adenovirus-mediated expression of Idol in mouse liver promotes LDLR degradation and elevates plasma LDL levels. The LXR-Idol-LDLR axis defines a complementary pathway to sterol response element-binding proteins for sterol regulation of cholesterol uptake.

Age-accelerated atherosclerosis correlates with failure to upregulate antioxidant genes

Excess food intake leads to obesity and diabetes, both of which are well-known independent risk factors for atherosclerosis, and both of which are growing epidemics in an aging population. We hypothesized that aging enhances the metabolic and vascular effects of high fat diet (HFD) and therefore examined the effect of age on atherosclerosis and insulin resistance in lipoprotein receptor knockout (LDLR(-/-)) mice. We found that 12-month-old (middle-aged) LDLR(-/-) mice developed substantially worse metabolic syndrome, diabetes, and atherosclerosis than 3-month-old (young) LDLR(-/-) mice when both were fed HFD for 3 months, despite similar elevations in total cholesterol levels. Microarray analyses were performed to analyze the mechanism responsible for the marked acceleration of atherosclerosis in middle-aged mice. Chow-fed middle-aged mice had greater aortic expression of multiple antioxidant genes than chow-fed young mice, including glutathione peroxidase-1 and -4, catalase, superoxide dismutase-2, and uncoupling protein-2. Aortic expression of these enzymes markedly increased in young mice fed HFD but decreased or only modestly increased in middle-aged mice fed HFD, despite the fact that systemic oxidative stress and vascular reactive oxygen species generation, measured by plasma F2alpha isoprostane concentration (systemic) and dihydroethidium conversion and p47phox expression (vascular), were greater in middle-aged mice fed HFD. Thus, the mechanism for the accelerated vascular injury in older LDLR(-/-) mice was likely the profound inability to mount an antioxidant response. This effect was related to a decrease in vascular expression of 2 key transcriptional pathways regulating the antioxidant response, DJ-1 and forkhead box, subgroup O family (FOXOs). Treatment of middle-aged mice fed HFD with the antioxidant apocynin attenuated atherosclerosis, whereas treatment with the insulin sensitizer rosiglitazone attenuated both metabolic syndrome and atherosclerosis. Both treatments decreased oxidative stress. A novel effect of rosiglitazone was to increase expression of Nrf2 (nuclear factor [erythroid-derived 2]-like 2), a downstream target of DJ-1 contributing to enhanced expression of vascular antioxidant enzymes. This investigation underscores the role of oxidative stress when multiple atherosclerotic risk factors, particularly aging, converge on the vessel wall and emphasizes the need to develop effective strategies to inhibit oxidative stress to protect aging vasculature.

Insulin regulates plasma ghrelin concentration

Ghrelin is a novel gastric peptide which stimulates growth hormone and has orexigenic and adipogenic properties. Plasma ghrelin is influenced by nutritional status and is thought to play a role in regulating food intake and body weight. We examined the effect of infusing insulin (40 mU/m(2)/min) for 2 hours while maintaining euglycemia on plasma ghrelin in 8 subjects (5 M, 3 F) aged 46 +/- 4 yrs (mean +/- SEM). Plasma insulin increased from 78 +/- 9 to 564 +/- 23 pmol/L during and returned rapidly to basal values after stopping the insulin infusion. Plasma ghrelin decreased from 85 +/- 28 to 61 +/- 18 pmol/L (p < 0.01) by 90 minutes of and continued to be suppressed for 15 minutes after the insulin infusion was discontinued. Subsequently, plasma ghrelin rose rapidly to near-basal values (81 +/- 23 pmol/L) within 60 minutes. The reciprocal relation between insulin and ghrelin was observed consistently in all subjects with the maximum insulin-induced suppression of ghrelin ranging from 19 to 64% (mean 32 +/- 5) and occurring 90-135 minutes after starting the insulin infusion (median 120). These findings indicate that insulin is a physiological and dynamic modulator of plasma ghrelin and that insulinemia possibly mediates the effect of nutritional status on its concentration.

Insulin regulates plasma ghrelin concentration

Ghrelin is a novel gastric peptide which stimulates growth hormone and has orexigenic and adipogenic properties. Plasma ghrelin is influenced by nutritional status and is thought to play a role in regulating food intake and body weight. We examined the effect of infusing insulin (40 mU/m(2)/min) for 2 hours while maintaining euglycemia on plasma ghrelin in 8 subjects (5 M, 3 F) aged 46 +/- 4 yrs (mean +/- SEM). Plasma insulin increased from 78 +/- 9 to 564 +/- 23 pmol/L during and returned rapidly to basal values after stopping the insulin infusion. Plasma ghrelin decreased from 85 +/- 28 to 61 +/- 18 pmol/L (p < 0.01) by 90 minutes of and continued to be suppressed for 15 minutes after the insulin infusion was discontinued. Subsequently, plasma ghrelin rose rapidly to near-basal values (81 +/- 23 pmol/L) within 60 minutes. The reciprocal relation between insulin and ghrelin was observed consistently in all subjects with the maximum insulin-induced suppression of ghrelin ranging from 19 to 64% (mean 32 +/- 5) and occurring 90-135 minutes after starting the insulin infusion (median 120). These findings indicate that insulin is a physiological and dynamic modulator of plasma ghrelin and that insulinemia possibly mediates the effect of nutritional status on its concentration.

Changes in plasma leptin during the menstrual cycle

OBJECTIVE

To measure the plasma concentration of leptin, which is expressed in ovarian follicles and may have a reproductive function, in healthy women during the menstrual cycle.

DESIGN

This study included nine women with regular menstrual cycles (mean+/-S.E.M. age 28+/-2 years: body mass index 23.9+/-1.8 kg/m2). From the onset of menses, fasting blood samples were collected every 1-2 days throughout the menstrual cycle. As a control, plasma leptin was measured in six postmenopausal women and six men every other day for 28 days.

RESULTS

In menstruating women, plasma leptin increased from 14.9+/-2.9 ng/ml in the early follicular phase to 20.4+/-4.2 ng/ml (P<0.01) at the midluteal phase and returned to the baseline by the subsequent menses. In contrast, leptin concentrations did not change significantly in postmenopausal women or men. The changes in plasma leptin during the menstrual cycle were not related to changes in sex hormones.

CONCLUSIONS

The cause of the increase in plasma leptin during the late follicular and luteal phases of the menstrual cycle is not clear. It may be attributed to augmented adipocyte production of leptin in response to increased caloric intake or hypothalamic release of neuropeptide Y. or to release of leptin from mature ovarian follicles. Leptin may have a role in regulating the menstrual cycle and preparing the body for the metabolic demands of pregnancy.

Physiological insulinemia acutely modulates plasma leptin

Whether insulin acutely regulates plasma leptin in humans is controversial. We examined the dosage-response and time-course characteristics of the effect of insulin on leptin in 10 men (age 42+/-2 years [mean+/-SE]; BMI 29.3+/-2.0 kg/m2). Each individual underwent four 9-h euglycemic clamps (insulin at 20, 40, 80, and 400 mU x m[-2] x min[-1) and a control saline infusion. Although plasma glucose and insulin levels remained constant, leptin diminished from 9.1+/-3.0 to 5.9+/-2.1 ng/ml (P < 0.001) by the end of the control experiment. Conversely, plasma leptin showed a dosage-dependent increase during the insulin infusions that was evident within 30-60 min. The insulin-induced increase in leptin was proportionately lower in obese insulin-resistant men. Free fatty acids (FFAs) decreased during insulin and did not change during saline infusions. ED50 (the dose producing half-maximal effect) for insulin's effect on leptin and FFA was similar (138+/-36 vs. 102+/-24 pmol/l, respectively; P=0.11). To further define the role of physiological insulinemia, we compared the effect of a very low dosage insulin infusion (10 mU x m[-2] x min[-1]) with that of a control saline infusion in another group of 10 men (mean age 39+/-3 years; BMI 27.1+/-1.0 kg/m2). Plasma leptin remained stable during that insulin infusion, but fell by 37+/-2% in the control experiment. Thus physiological insulinemia can acutely regulate plasma leptin. Insulin could mediate the effect of caloric intake on leptin and could be a determinant of its plasma concentration. Inadequate insulin-induced leptin production in obese and insulin-resistant subjects may contribute to the development or worsening of obesity.

Diurnal and ultradian rhythmicity of plasma leptin: effects of gender and adiposity

Plasma leptin shows a nocturnal rise and a pulsatile pattern. This work was undertaken to determine the effects of gender and obesity on this pattern. Twenty-four-hour leptin profiles were evaluated in 31 subjects [17 male, 14 female; age: 36 +/- 2 yr (mean +/- SEM); body mass index: 27.5 +/- 1.0 kg/m2]. Plasma leptin profiles were higher in obese (body mass index > 27 kg/m2) than in lean subjects and higher in women than in men, regardless of fat mass. Leptin showed diurnal rhythmicity with peaks between 2200-0300 (median: 0120) and nadirs between 0800 and 1740 (median: 1033). Spectral analysis revealed 2 components (periodicities: 24 and 12 h) with higher relative amplitudes in lean than in obese subjects. The relative diurnal amplitude also was higher in men than in women, controlling for adiposity. Insulinemia, female sex, and age were negative determinants of diurnal rhythm relative amplitude. Pulse analysis revealed 3.6 +/- 0.3 pulses/24 h, occurring mostly 2-3 h after meals. Pulse frequency correlated negatively with fat mass and insulinemia (Spearman's r = -0.54 and -0.37, respectively; P < 0.05 for each). Thus, obesity is associated not only with higher leptin levels but also with blunted diurnal excursions and dampened pulsatility. This abnormal rhythmicity may contribute to leptin resistance in obesity. The significance of the sexual dimorphism in the diurnal amplitude is unclear, but it may be related to leptin's putative role as a metabolic signal to the reproductive axis.

Method of insulin administration has no effect on insulin sensitivity estimates from the insulin-modified minimal model protocol

The effect of the method of insulin administration on insulin sensitivity estimates from the insulin modified minimal model (MINMOD) protocol was evaluated using the tolbutamide-boosted protocol as a reference. The study included 21 nondiabetic men ages 40 +/- 2 years (mean +/- SE) with a BMI of 26.6 +/- 1.1 kg/m2. Each subject underwent four frequently sampled intravenous glucose tolerance tests (FSIGTT), one with tolbutamide and three with the same insulin dosage (0.03 U/kg) given as a bolus or infusion over 5 or 10 min. The insulin sensitivity index (SI) of each subject was calculated from each FSIGTT with MINMOD. Insulin sensitivity indexes from the four FSIGTTs were highly correlated (r > 0.85, P < 0.001). SI(insulin) from the bolus and the 5- and 10-min infusion protocols were similar, but were 21 +/- 5, 29 +/- 5, and 23 +/- 4% lower than SI(tolbutamide), respectively. SG(tolbutamide) and SG(insulin) were not different among the four protocols and were significantly correlated (r > 0.55, P < 0.01). Thus the tolbutamide and insulin protocols must not be used interchangeably in any single cross-sectional or longitudinal study. When the same insulin dosage is used, the method of its administration has no bearing on insulin sensitivity estimates from the insulin-modified FSIGTT. The same method of insulin administration should be used, however, in any single study for purpose of standardization.

Differences between the tolbutamide-boosted and the insulin-modified minimal model protocols

The insulin-modified frequently sampled intravenous glucose tolerance test (FSIGTT) with minimal model analysis (MINMOD) was compared with the tolbutamide protocol and the glucose clamp in 35 nondiabetic subjects (age 38 +/- 2 years [mean +/- SE], BMI 27.2 +/- 0.9 kg/m2). Each subject underwent two FSIGTTs, one with tolbutamide (300 mg) and the other with insulin (0.03 U/kg) and a euglycemic hyperinsulinemic clamp (40 mU x m(-2) x min(-1)). Insulin sensitivity was determined from each FSIGTT with MINMOD and from the clamp. Insulin sensitivity indexes (S(I)) from the two FSIGTTs were significantly correlated (r = 0.77, P < 0.001), but S(I(insulin)) was 29 +/- 4% lower than S(I(tolbutamide)). Both S(I(insulin)) and S(I(tolbutamide)) correlated significantly with S(I(clamp)) (r = 0.70 and 0.71, P < 0.001 for each). Expressed in the same units (dl/min per pU/ml), S(I(tolbutamide)) was on average 13 +/- 6% lower than S(I(clamp)) (4.51 +/- 0.40 vs. 5.36 +/- 0.36 x 10(-2), P = 0.009), whereas S(I(insulin)) was 44 +/- 4% lower. S(G(tolbutamide)) and S(G(insulin)) were not different (1.88 +/- 0.10 vs. 2.01 +/- 0.09 x 10(-2) min(-1), P = 0.167) and were significantly correlated (r = 0.50, P = 0.002). Thus, insulin sensitivity estimates from both protocols correlate significantly with each other and with the clamp. They are quantitatively discrepant, however, possibly due to differences in the route of insulin delivery, saturation of insulin action, and/or tolbutamide-induced proinsulin release. Data obtained from these two MINMOD protocols are not directly comparable, and the same protocol must be used in any single cross-sectional or longitudinal study.

Sexual dimorphism in plasma leptin concentration

Leptin, the obese (ob) gene product, is thought to be a lipostatic hormone that contributes to body weight regulation through modulating feeding behavior and/or energy expenditure. The determinants of plasma leptin concentration were evaluated in 267 subjects (106 with normal glucose tolerance, 102 with impaired glucose tolerance, and 59 with noninsulin-dependent diabetes). Fasting plasma leptin levels ranged from 1.8-79.6 ng/mL (geometric mean, 12.4), were higher in the obese subjects, and were not related to glucose tolerance. Women had approximately 40% higher leptin levels than men at any level of adiposity. After controlling for body fat, postmenopausal women had still higher leptin levels than men of similar age, and their levels were not different from those in younger women. Multiple regression analysis showed that adiposity, gender, and insulinemia were significant determinants of leptin concentration, explaining 42%, 28%, and 2% of its variance, respectively. Neither age nor the waist/hip ratio was significantly related to leptin concentration. Thus, our data indicate that gender is a major determinant of the plasma leptin concentration. This sex difference is not apparently explained by sex hormones or body fat distribution. Leptin's sexual dimorphism suggests that women may be resistant to its putative lipostatic actions and that it may have a reproductive function.