Liver X receptor (LXR) regulates human adipocyte lipolysis.
ABSTRACT The Liver X receptor (LXR) is an important regulator of carbohydrate and lipid metabolism in humans and mice. We have recently shown that activation of LXR regulates cellular fuel utilization in adipocytes. In contrast, the role of LXR in human adipocyte lipolysis, the major function of human white fat cells, is not clear. In the present study, we stimulated in vitro differentiated human and murine adipocytes with the LXR agonist GW3965 and observed an increase in basal lipolysis. Microarray analysis of human adipocyte mRNA following LXR activation revealed an altered gene expression of several lipolysis-regulating proteins, which was also confirmed by quantitative real-time PCR. We show that expression and intracellular localization of perilipin1 (PLIN1) and hormone-sensitive lipase (HSL) are affected by GW3965. Although LXR activation does not influence phosphorylation status of HSL, HSL activity is required for the lipolytic effect of GW3965. This effect is abolished by PLIN1 knockdown. In addition, we demonstrate that upon activation, LXR binds to the proximal regions of the PLIN1 and HSL promoters. By selective knock-down of either LXR isoform, we show that LXRα is the major isoform mediating the lipolysis-related effects of LXR. In conclusion, the present study demonstrates that activation of LXRα up-regulates basal human adipocyte lipolysis. This is at least partially mediated through LXR binding to the PLIN1 promoter and down-regulation of PLIN1 expression.
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ABSTRACT: Liver X receptors (LXRs) have been recognized as a promising therapeutic target for atherosclerosis; however, their role in insulin sensitivity is controversial. Adiponectin plays a unique role in maintaining insulin sensitivity. Currently, no systematic experiments elucidating the role of LXR activation in insulin function based on adiponectin signaling have been reported. Here, we investigated the role of LXR activation in insulin resistance based on adiponectin signaling, and possible mechanisms. C57BL/6 mice maintained on a regular chow received the LXR agonist, T0901317 (30 mg/kg.d) for 3 weeks by intraperitoneal injection, and differentiated 3T3-L1 adipocytes were treated with T0901317 or GW3965. T0901317 treatment induced significant insulin resistance in C57BL/6 mice. It decreased adiponectin gene transcription in epididymal fat, as well as serum adiponectin levels. Activity of AMPK, a key mediator of adiponectin signaling, was also decreased, resulting in decreased Glut-4 membrane translocation in epididymal fat. In contrast, adiponectin activity was not changed in the liver of T0901317 treated mice. In vitro, both T0901317 and GW3965 decreased adiponectin expression in adipocytes in a dose-dependent manner, an effect which was diminished by LXRα silencing. ChIP-qPCR studies demonstrated that T0901317 decreased the binding of PPARγ to the PPAR-responsive element (PPRE) of the adiponectin promoter in a dose-dependent manner. Furthermore, T0901317 exerted an antagonistic effect on the expression of adiponectin in adipocytes co-treated with 3 µM Pioglitazone. In luciferase reporter gene assays, T0901317 dose-dependently inhibited PPRE-Luc activity in HEK293 cells co-transfected with LXRα and PPARγ. These results suggest that LXR activation induces insulin resistance with decreased adiponectin signaling in epididymal fat, probably due to negative regulation of PPARγ signaling. These findings indicate that the potential of LXR activation as a therapeutic target for atherosclerosis may be limited by the possibility of exacerbating insulin resistance-related disease.PLoS ONE 01/2014; 9(6):e101269. · 3.53 Impact Factor
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ABSTRACT: In obesity, white adipose tissue (WAT) inflammation is linked to insulin resistance. Increased adipocyte chemokine (C-C motif) ligand 2 (CCL2) secretion may initiate adipose inflammation by attracting the migration of inflammatory cells into the tissue. Using an unbiased approach, we identified adipose microRNAs (miRNAs) that are dysregulated in human obesity and assessed their possible role in controlling CCL2 production. In subcutaneous WAT obtained from 56 subjects, 11 miRNAs were present in all subjects and downregulated in obesity. Of these, 10 affected adipocyte CCL2 secretion in vitro and for 2 miRNAs (miR-126 and miR-193b), regulatory circuits were defined. While miR-126 bound directly to the 3'-untranslated region of CCL2 mRNA, miR-193b regulated CCL2 production indirectly through a network of transcription factors, many of which have been identified in other inflammatory conditions. In addition, overexpression of miR-193b and miR-126 in a human monocyte/macrophage cell line attenuated CCL2 production. The levels of the two miRNAs in subcutaneous WAT were significantly associated with CCL2 secretion (miR-193b) and expression of integrin, α-X, an inflammatory macrophage marker (miR-193b and miR-126). Taken together, our data suggest that miRNAs may be important regulators of adipose inflammation through their effects on CCL2 release from human adipocytes and macrophages.Diabetes 06/2012; 61(8):1986-93. · 7.90 Impact Factor
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ABSTRACT: AIMS/HYPOTHESIS: Obesity increases the risk of developing type 2 diabetes mellitus, characterised by impaired insulin-mediated glucose uptake in peripheral tissues. Liver X receptor (LXR) is a positive regulator of adipocyte glucose transport in murine models and a possible target for diabetes treatment. However, the levels of LXRα are increased in obese adipose tissue in humans. We aimed to investigate the transcriptome of LXR and the role of LXR in the regulation of glucose uptake in primary human adipocytes. METHODS: The insulin responsiveness of human adipocytes differentiated in vitro was characterised, adipocytes were treated with the LXR agonist GW3965 and global transcriptome profiling was determined by microarray, followed by quantitative RT-PCR (qRT-PCR), western blot and ELISA. Basal and insulin-stimulated glucose uptake was measured and the effect on plasma membrane translocation of glucose transporter 4 (GLUT4) was assayed. RESULTS: LXR activation resulted in transcriptional suppression of several insulin signalling genes, such as AKT2, SORBS1 and CAV1, but caused only minor changes (<15%) in microRNA expression. Activation of LXR impaired the plasma membrane translocation of GLUT4, but not the expression of its gene, SLC2A4. LXR activation also diminished insulin-stimulated glucose transport and lipogenesis in adipocytes obtained from overweight individuals. Furthermore, AKT2 expression was reduced in obese adipose tissue, and AKT2 and SORBS1 expression was inversely correlated with BMI and HOMA index. CONCLUSIONS/INTERPRETATION: In contrast to murine models, LXR downregulates insulin-stimulated glucose uptake in human adipocytes from overweight individuals. This could be due to suppression of Akt2, c-Cbl-associated protein and caveolin-1. These findings challenge the idea of LXR as a drug target in the treatment of diabetes.Diabetologia 06/2013; · 6.49 Impact Factor