Evidence for an indirect transcriptional regulation of glucose-6-phosphatase gene expression by liver X receptors

Reverse Cholesterol Transport Pathway and Cholesterol Efflux in Diabetic Retinopathy

Cholesterol esters, synthesized from cholesterol with long-chain fatty acids, are essential components of plasma lipoproteins and cell membranes that participate in various metabolic processes in the body. Cholesterol can be excreted through the cholesterol reverse transport (RCT) pathway when excessive cholesterol is produced in the extrahepatic cells, which is regulated by the liver X receptor (LXR) and its downstream regulators ATP-binding cassette subfamily A member 1 (ABCA1) and ATP-binding cassette subfamily G member 1 (ABCG1) genes. Abnormal cholesterol metabolism is closely associated with the development of diabetic retinopathy (DR). However, the precise underlying mechanism of the RCT pathway in the pathogenesis of DR is still not fully understood. This review focused on cholesterol metabolism, with a particular emphasis on the RCT pathway and its correlation with the development of DR. Particular attention has been paid to the key regulators of the RCT pathway: LXR, ABCA1, and ABCG1 genes and their potential therapeutic targets in the management of DR.

LXRα Regulates Hepatic ChREBPα Activity and Lipogenesis upon Glucose, but Not Fructose Feeding in Mice

Liver X receptors (LXRα/β) and carbohydrate response element-binding proteins (ChREBPα/β) are key players in the transcriptional control of hepatic de novo lipogenesis. LXRα/β double knockout (LXRα^−/−/β^−/−) mice have reduced feeding-induced nuclear O-linked N-acetylglucosamine (O-GlcNAc) signaling, ChREBPα activity, and lipogenic gene expression in livers, suggesting important roles for LXRs in linking hepatic glucose utilization to lipid synthesis. However, the role of LXRs in fructose-induced ChREBP activation and lipogenesis is currently unknown. In this study, we studied the effects of high fructose or high glucose feeding on hepatic carbohydrate metabolism and lipogenic gene expression in livers from fasted (24 h) and fasted-refed (12 h) wild type and LXRα knockout (LXRα^−/−) mice. Hepatic lipogenic gene expression was reduced in glucose fed, but not fructose fed LXRα^−/− mice. This was associated with lower expression of liver pyruvate-kinase (L-pk) and Chrebpβ, indicating reduced ChREBPα activity in glucose fed, but not fructose fed mice. Interestingly, ChREBP binding to the L-pk promoter was increased in fructose fed LXRα^−/− mice, concomitant with increased glucose-6-phosphatase (G6pc) expression and O-GlcNAc modified LXRβ, suggesting a role for LXRβ in regulating ChREBPα activity upon fructose feeding. In conclusion, we propose that LXRα is an important regulator of hepatic lipogenesis and ChREBPα activity upon glucose, but not fructose feeding in mice.

Paeoniflorin acts as a liver X receptor agonist

Paeoniflorin is one of the active ingredients of Paeonia lactiflora Pall., a novel traditional herbal medicine exerting pharmacological effects including antihyperlipidemic, neuroprotective, and anti-hepatofibrosis effects. Liver X receptor (LXR) acts as a ligand-activated transcription factor to exhibit antihyperlipidemic and neuroprotective effects. In this study, the activity of paeoniflorin against LXR was evaluated by the mammalian one-hybrid and transient transfection reporter assays. The results showed that paeoniflorin transactivated GAL4, rat cholesterol 7 α-hydroxylase, phospholipid transfer protein, and ATP-binding cassette A1 gene promoters in dose-dependent manner. Furthermore, the docking study demonstrated that paeoniflorin resided in the LXR ligand-binding pocket in the similar manner as GSK 3987, a novel LXR agonist. These results indicated that paeoniflorin might exert pharmacological effects through LXR pathway.

Liver x receptors regulate the transcriptional activity of the glucocorticoid receptor: implications for the carbohydrate metabolism

GLUCOCORTICOIDS are steroid hormones that strongly influence intermediary carbohydrate metabolism by increasing the transcription rate of glucose-6-phosphatase (G6Pase), a key enzyme of gluconeogenesis, and suppress the immune system through the glucocorticoid receptor (GR). The liver X receptors (LXRs), on the other hand, bind to cholesterol metabolites, heterodimerize with the retinoid X receptor (RXR), and regulate the cholesterol turnover, the hepatic glucose metabolism by decreasing the expression of G6Pase, and repress a set of inflammatory genes in immune cells. Since the actions of these receptors overlap with each other, we evaluated the crosstalk between the GR- and LXR-mediated signaling systems. Transient transfection-based reporter assays and gene silencing methods using siRNAs for LXRs showed that overexpression/ligand (GW3965) activation of LXRs/RXRs repressed GR-stimulated transactivation of certain glucocorticoid response element (GRE)-driven promoters in a gene-specific fashion. Activation of LXRs by GW3965 attenuated dexamethasone-stimulated elevation of circulating glucose in rats. It also suppressed dexamethasone-induced mRNA expression of hepatic glucose-6-phosphatase (G6Pase) in rats, mice and human hepatoma HepG2 cells, whereas endogenous, unliganded LXRs were required for dexamethasone-induced mRNA expression of phosphoenolpyruvate carboxylase. In microarray transcriptomic analysis of rat liver, GW3965 differentially regulated glucocorticoid-induced transcriptional activity of about 15% of endogenous glucocorticoid-responsive genes. To examine the mechanism through which activated LXRs attenuated GR transcriptional activity, we examined LXRα/RXRα binding to GREs. Endogenous LXRα/RXRα bound GREs and inhibited GR binding to these DNA sequences both in in vitro and in vivo chromatin immunoprecipitation assays, while their recombinant proteins did so on classic or G6Pase GREs in gel mobility shift assays. We propose that administration of LXR agonists may be beneficial in glucocorticoid treatment- or stress-associated dysmetabolic states by directly and gene-specifically attenuating the transcriptional activity of the GR on glucose and/or lipid metabolism.

Enhanced glucose 6-phosphatase activity in liver of rats exposed to Mg(2+)-deficient diet

Total hepatic Mg(2+) content decreases by >25% in animals maintained for 2 weeks on Mg(2+) deficient diet, and results in a >25% increase in glucose 6-phosphatase (G6Pase) activity in isolated liver microsomes in the absence of significant changed in enzyme expression. Incubation of Mg(2+)-deficient microsomes in the presence of 1mM external Mg(2+) returned G6Pase activity to levels measured in microsomes from animals on normal Mg(2+) diet. EDTA addition dynamically reversed the Mg(2+) effect. The effect of Mg(2+) or EDTA persisted in taurocholic acid permeabilized microsomes. An increase in G6Pase activity was also observed in liver microsomes from rats starved overnight, which presented a ~15% decrease in hepatic Mg(2+) content. In this model, G6Pase activity increased to a lesser extent than in Mg(2+)-deficient microsomes, but it could still be dynamically modulated by addition of Mg(2+) or EDTA. Our results indicate that (1) hepatic Mg(2+) content rapidly decreases following starvation or exposure to deficient diet, and (2) the loss of Mg(2+) stimulates G6P transport and hydrolysis as a possible compensatory mechanism to enhance intrahepatic glucose availability. The Mg(2+) effect appears to take place at the level of the substrate binding site of the G6Pase enzymatic complex or the surrounding phospholipid environment.

Liver X Receptors as Therapeutic Targets for Managing Cholesterol: Implications for Atherosclerosis and Other Inflammatory Conditions

Atherosclerosis is a disease characterized by excess cholesterol and inflammation in the blood vessels. The liver X receptors (alpha and beta) are members of the nuclear hormone receptor family that are activated by endogenous cholesterol metabolites. These receptors are widely expressed with a tissue distribution that includes the liver, intestine and macrophage. Upon activation, these receptors have been shown to increase reverse cholesterol transport from the macrophage back to the liver to aid in the removal of excess cholesterol. More recently, they have also been shown to inhibit the inflammatory response in macrophages. These functions are accomplished through direct regulation of gene transcription. Herein, we will describe the key benefits and potential risks of targeting the LXRs for the treatment of atherosclerosis.

Lxralpha deficiency hampers the hepatic adaptive response to fasting in mice

Besides its well established role in control of cellular cholesterol homeostasis, the liver X receptor (LXR) has been implicated in the regulation of hepatic gluconeogenesis. We investigated the role of the major hepatic LXR isoform in hepatic glucose metabolism during the feeding-to-fasting transition in vivo. In addition, we explored hepatic glucose sensing by LXR during carbohydrate refeeding. Lxralpha(-/-) mice and their wild-type littermates were subjected to a fasting-refeeding protocol and hepatic carbohydrate fluxes as well as whole body insulin sensitivity were determined in vivo by stable isotope procedures. Lxralpha(-/-) mice showed an impaired response to fasting in terms of hepatic glycogen depletion and triglyceride accumulation. Hepatic glucose 6-phosphate turnover was reduced in 9-h fasted Lxralpha(-/-) mice as compared with controls. Although hepatic gluconeogenic gene expression was increased in 9-h fasted Lxralpha(-/-) mice compared with wild-type controls, the actual gluconeogenic flux was not affected by Lxralpha deficiency. Hepatic and peripheral insulin sensitivity were similar in Lxralpha(-/-) and wild-type mice. Compared with wild-type controls, the induction of hepatic lipogenic gene expression was blunted in carbohydrate-refed Lxralpha(-/-) mice, which was associated with lower plasma triglyceride concentrations. Yet, expression of "classic" LXR target genes Abca1, Abcg5, and Abcg8 was not affected by Lxralpha deficiency in carbohydrate-refed mice. In summary, these studies identify LXRalpha as a physiologically relevant mediator of the hepatic response to fasting. However, the data do not support a role for LXR in hepatic glucose sensing.

Liver X receptor activator downregulates angiotensin II type 1 receptor expression through dephosphorylation of Sp1

Atherosclerosis is considered to be a combined disorder of lipid metabolism and chronic inflammation. Recent studies have reported that liver X receptors (LXRs) are involved in lipid metabolism and inflammation and that LXR agonists inhibit atherogenesis. In contrast, angiotensin II is well known to accelerate atherogenesis through activation of the angiotensin II type 1 receptor (AT1R). To better understand the mechanism of LXR on the prevention of atherogenesis, we examined whether activation of LXR affects AT1R expression in vascular smooth muscle cells. T0901317, a synthetic LXR ligand, decreased AT1R mRNA and protein expression with a peak reduction at 6 hours and 12 hours of incubation, respectively. A well-established ligand of LXR, 22-(R)-hydroxycholesterol, also suppressed AT1R expression. The downregulation of AT1R by T0901317 required de novo protein synthesis. AT1R gene promoter activity measured by luciferase assay revealed that the DNA segment between -61 bp and +25 bp was sufficient for downregulation. Luciferase construct with a mutation in Sp1 binding site located in this segment lost its response to T0901317. T0901317 decreased Sp1 serine phosphorylation. Although preincubation of vascular smooth muscle cells with T0901317 for 30 minutes had no effect on angiotensin II-induced extracellular signal-regulated kinase phosphorylation, phosphorylation of extracellular signal-regulated kinase by angiotensin II was markedly suppressed after 6 hours of preincubation. These results indicate that the suppression of AT1R may be one of the important mechanisms by which LXR ligands exert antiatherogenic effects.

Biphasic effect of extra-reticular Mg2+ on hepatic G6P transport and hydrolysis

Magnesium ions (Mg(2+)) play a key role in regulating hepatic cellular functions and enzymatic activities. In the present study, we report a concentration-dependent effect of cytosolic Mg(2+) on G6P and pyrophosphate (PPi) transport and hydrolysis in digitonin-permeabilized rat hepatocytes. The stimulatory effect of Mg(2+) on G6P is specific but biphasic, with a maximal effect at a concentration of 0.25 mM, whereas the effect on PPi increases in a dose-dependent manner. Both effects can be abolished by addition of EDTA to the system. Addition of taurocholate, histone-2A, alamethicin or A23187 to the incubation system results in a marked decrease in the Mg(2+) concentration present within the endoplasmic reticulum lumen. Under these conditions, the stimulatory effect of extra-reticular Mg(2+) on G6P transport and hydrolysis is abolished. Taken together, these data suggest that cytosolic Mg(2+) stimulates G6P transport by acting at the level of the substrate binding site of the G6Pase enzymatic complex or the surrounding phospholipid environment. The effect, which is lost when G6P has readily access to the ER lumen, requires physiological endoplasmic reticulum Mg(2+) content.

Impaired regulation of hepatic glucose production in mice lacking the forkhead transcription factor Foxo1 in liver

The hallmark of type 2 diabetes is excessive hepatic glucose production. Several transcription factors and coactivators regulate this process in cultured cells. But gene ablation experiments have yielded few clues as to the physiologic mediators of this process in vivo. We show that inactivation of the gene encoding forkhead protein Foxo1 in mouse liver results in 40% reduction of glucose levels at birth and 30% reduction in adult mice after a 48 hr fast. Gene expression and glucose clamp studies demonstrate that Foxo1 ablation impairs fasting- and cAMP-induced glycogenolysis and gluconeogenesis. Pgc1alpha is unable to induce gluconeogenesis in Foxo1-deficient hepatocytes, while the cAMP response is significantly blunted. Conversely, Foxo1 deletion in liver curtails excessive glucose production caused by generalized ablation of insulin receptors and prevents neonatal diabetes and hepatosteatosis in insulin receptor knockout mice. The data provide a unifying mechanism for regulation of hepatic glucose production by cAMP and insulin.

The nuclear receptor cofactor, receptor-interacting protein 140, is required for the regulation of hepatic lipid and glucose metabolism by liver X receptor

The liver X receptors (LXRs) are nuclear receptors that play important roles in the regulation of lipid metabolism. In this study, we demonstrate that receptor-interacting protein 140 (RIP140) is a cofactor for LXR in liver. Analysis of RIP140 null mice and hepatocytes depleted of RIP140 indicate that the cofactor is essential for the ability of LXR to activate the expression of a set of genes required for lipogenesis. Furthermore we demonstrate that RIP140 is required for the ability of LXR to repress the expression of the phosphoenolpyruvate carboxykinase gene in Fao cells and mice. Thus, we conclude that the function of RIP140 as a cofactor for LXR in liver varies according to the target genes and metabolic process, serving as a coactivator in lipogenesis but as a corepressor in gluconeogenesis.