Genome-wide profiling of liver X receptor, retinoid X receptor, and peroxisome proliferator-activated receptor α in mouse liver reveals extensive sharing of binding sites

The liver X receptors (LXRs) are nuclear receptors that form permissive heterodimers with retinoid X receptor (RXR) and are important regulators of lipid metabolism in the liver. We have recently shown that RXR agonist-induced hypertriglyceridemia and hepatic steatosis in mice are dependent on LXRs and correlate with an LXR-dependent hepatic induction of lipogenic genes. To further investigate the roles of RXR and LXR in the regulation of hepatic gene expression, we have mapped the ligand-regulated genome-wide binding of these factors in mouse liver. We find that the RXR agonist bexarotene primarily increases the genomic binding of RXR, whereas the LXR agonist T0901317 greatly increases both LXR and RXR binding. Functional annotation of putative direct LXR target genes revealed a significant association with classical LXR-regulated pathways as well as peroxisome proliferator-activated receptor (PPAR) signaling pathways, and subsequent chromatin immunoprecipitation-sequencing (ChIP-seq) mapping of PPARα binding demonstrated binding of PPARα to 71 to 88% of the identified LXR-RXR binding sites. The combination of sequence analysis of shared binding regions and sequential ChIP on selected sites indicate that LXR-RXR and PPARα-RXR bind to degenerate response elements in a mutually exclusive manner. Together, our findings suggest extensive and unexpected cross talk between hepatic LXR and PPARα at the level of binding to shared genomic sites.     

The effect of cholestyramine on liver HMG-CoA reductase and cholesterol 7α-hydroxylase in various laboratory animals

The activity of HMG-CoA reductase and cholesterol 7α-hydroxylase was assayed in the liver of rats, rabbits, hamsters and guinea pigs at the minimum of the day cycle and after one night fasting. The amount of HMG-CoA reductase, as determined after its complete dephosphorylation $\text{in vitro}$ was of the same order of magnitude in the tested species. The dephosphorylated active form of the enzyme was detectable only in the rat. Cholesterol 7α-hydroxylase activity was also much higher in the rat.Cholestyramine treatment stimulated the activity of both enzymes. In particular, the ratio between active and inactive HMG-CoA reductase in rabbits, hamsters and guinea pigs became of the same order of magnitude of that found in rats.     

The suppressive effect of the catatoxic steroid,pregnenolone-16α-carbonitrile,on liver microsomal cholesterol-7α-hydroxylase

The effect of the catatoxic steroid, 3β-hydroxy-20-oxo-5-pregnene-16α-carbonitrile [pregnenolone-16α-carbonitrile (PCN)] on hepatic microsomal cholesterol-7α-hydroxylase, the probable rate-limiting enzyme of bile acid biosynthesis, has been studied. Short term administration (3 days) of PCN in the diet of rats resulted in a significant decrease in the liver microsomal cholesterol-7α-hydroxylase activity, in contrast to a marked stimulation of microsomal cytochrome P-450 and ethylmorphine demethylase activity. PCN significantly depressed the cholesterol-7α-hydroxylase activity in the livers of rats with elevated levels of the enzyme produced by cholestyramine feeding. The results indicate the presence of separate control mechanisms in the regulation of bile acid synthesis and drug metabolism.     

The C161T polymorphism in the peroxisome proliferator-activated receptor gamma gene (PPARγ) is associated with risk of coronary artery disease: a meta-analysis

The researches attempting to associate the PPARγ C161T polymorphism with coronary artery disease (CAD) yielded complicated and contradictory results. We aimed for more precise estimate of the relationship and conducted a comprehensive meta-analysis. Publications written in English or Chinese were screened in MEDLINE, Embase, CNKI, Wanfang and CBM. Data on 11 studies including 3,020 cases and 2,853 controls were extracted. A random-effects model was available to synthesize the inconsistent outcomes of the individual studies, while addressing between-study heterogeneity and publication bias. The PPARγ C161T polymorphism followed Hard-Weinberg Equilibrium for all studies (P > 0.05).Overall, there was no evidence for a significant association under all genetic models but with distinct heterogeneity (T vs. C: P = 0.29, OR = 0.91, 95 %CI 0.77–1.08, P _{heterogeneity} = 0.004, I ² = 61.2 %). However, in the subgroup analysis by ethnicity, the T allele carriers showed a prominent 26 % risk reduction of CAD among Chinese (dominant genetic model: P = 0.03, 95 %CI 0.57–0.97, P _{heterogeneity} = 0.03, I ² = 56.1 %). After dividing into population source, the significance of CAD risk reduction was strengthened in hospital-based studies (allele comparison: P = 0.04, OR = 0.82, 95 %CI 0.67–1.00, P _{heterogeneity} = 0.04, I ² = 52.5 %; dominant model: P = 0.01, OR = 0.73, 95 %CI 0.57–0.92, P _{heterogeneity} = 0.05, I ² = 50.8 %). There was no obvious publication bias verified in the method of funnel plot and Egger’s linear regression test (t = ?0.11, P = 0.913). Taken together, our results revealed the PPARγ C161T polymorphism might play a moderate protective effect on developing CAD among Chinese, but not among Caucasians.     

A case–control study on the effect of metabolic gene polymorphisms, nutrition, and their interaction on the risk of non-alcoholic fatty liver disease

The oxidative stress is a key issue in the etiology of non-alcoholic fatty liver disease (NAFLD). The aim of our study was to evaluate the effect of metabolic gene polymorphisms involved in the oxidative stress (GSTT1, GSTM1, SULT1A1, CYP2E1, and 1A1), lifestyle and nutrition aspects, and their interaction, on the risk of NAFLD. We enrolled 294 cases and 359 controls, and collected demographics, anthropometric, lifestyle, and nutrition data. A subgroup of NAFLD provided additional data on nutrients and on physical activity engagement. Each patient provided a blood sample for DNA extraction and genotyping. Clinical and laboratory data were collected from cases. Multivariable analysis shows a significant protective effect of age, gender, and moderate drinking habits on the risk of NAFLD, while an increased risk for greater consumption of fruit and grilled meat or fish. Significant interactions were reported between alcohol consumption, fruit intake, grilled meat and fish, and selected genetic variants. From the subgroup analysis, a moderate/high consumption of fat and/or grilled meat/fish, and a high consumption of white meat increase the risk of NAFLD. Engaging any physical activity at least 1 time/week halves the risk of NAFLD. Besides confirming the beneficial effect of moderate alcohol intake and regular physical activity, and the increased risk associated with high fruit and fat intake, for the first time, we report a detrimental effect of grilled food on NAFLD risk. An effect modification by selected gene variants increases the risk in combination with fruit and grilled food intake.     

Systemic treatment with liver X receptor agonists raises apolipoprotein E,cholesterol, and amyloid-β peptides in the cerebral spinal fluid of rats

Background

Apolipoprotein E (apoE) is a major cholesterol transport protein found in association with brain amyloid from Alzheimer's disease (AD) patients and the ε4 allele of apoE is a genetic risk factor for AD. Previous studies have shown that apoE forms a stable complex with amyloid β (Aβ) peptides in vitro and that the state of apoE lipidation influences the fate of brain Aβ, i.e., lipid poor apoE promotes Aβ aggregation/deposition while fully lipidated apoE favors Aβ degradation/clearance. In the brain, apoE levels and apoE lipidation are regulated by the liver X receptors (LXRs).

Results

We investigated the hypothesis that increased apoE levels and lipidation induced by LXR agonists facilitates Aβ efflux from the brain to the cerebral spinal fluid (CSF). We also examined if the brain expression of major apoE receptors potentially involved in apoE-mediated Aβ clearance was altered by LXR agonists. ApoE, cholesterol, Aβ40, and Aβ42 levels were all significantly elevated in the CSF of rats after only 3 days of treatment with LXR agonists. A significant reduction in soluble brain Aβ40 levels was also detected after 6 days of LXR agonist treatment.

Conclusions

Our novel findings suggest that central Aβ lowering caused by LXR agonists appears to involve an apoE/cholesterol-mediated transport of Aβ to the CSF and that differences between the apoE isoforms in mediating this clearance pathway may explain why individuals carrying one or two copies of APOE ε4 have increased risk for AD.

     

The effect of activation of peroxisome proliferator-activated receptor gamma (PPARγ) on human monocyte function: PPARγ ligands do not inhibit tumor necrosis factor-α release in human monocytic cell line THP-1

Peroxisome proliferator-activated receptor gamma (PPARγ) activation by its ligands reportedly inhibits monocyte function. However, because the concentrations of PPARγ ligands used in previous studies were higher than typically expected to activate PPARγ, we clarified whether PPARγ ligands influence monocyte function and cell viability of the human monocyte cell line THP-1. We determined tumor necrosis factor-alpha (TNF-α) release as a monocyte function and cell viability using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide. Both troglitazone and 15-deoxy-Δ^12,14-prostaglandin J₂ (15-d-PGJ2) seemed to inhibit phorbol ester-induced TNF-α release from THP-1 cells. On the other hand, neither pioglitazone nor rosiglitazone inhibited phorbol ester-induced TNF-α release. Because the cytotoxicity of troglitazone and 15-d-PGJ2 was significantly (p<0.05, Tukey–Kramer) stronger than that of pioglitazone and rosiglitazone, the inhibition of TNF-α release seemed to parallel the lack of cell viability. We concluded that PPARγ ligands did not directly inhibit TNF-α release in THP-1 cells. This revised version was published online in July 2006 with corrections to the Cover Date.     

Single-nucleotide polymorphisms in peroxisome proliferator-activated receptor <Emphasis Type="Italic">γ</Emphasis> and their association with plasma levels of resistin and the metabolic syndrome in a South Indian population

Studies on the association of the Pro12Ala and C1431T polymorphisms of PPARγ with diabetes and obesity have revealed extensive population-dependent variations. However, association of these polymorphisms with the metabolic syndrome and its individual components has not been well investigated in the Indian population. The Indian population harbours the maximum number of diabetics in the world who are thus more susceptible to metabolic disorders. We screened a South Indian population (N = 699) for a possible association of these polymorphisms with the metabolic syndrome (MS) and type 2 diabetes. We also investigated the correlation of these two single-nucleotide polymorphisms (SNPs) with plasma resistin levels. The C1431T SNP was associated with higher levels of plasma resistin (P = 0.017). Furthermore, C1431T was associated with resistin in different tertiles. Prevalence of the ‘Pro-C’ haplotype decreased with increasing tertiles of resistin (84.1% to 75.4%, P = 0.037). Plasma resistin levels were not found to be associated with MS and type 2 diabetes. These results point to a likely association of plasma resistin levels with PPARγ polymorphisms in the Indian population.