LXR is a negative regulator of glucose uptake in human adipocytes.

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.

Endothelin-1 stimulates human adipocyte lipolysis through the ET A receptor.

OBJECTIVE: Levels of the vascular peptide endothelin-1 (ET-1) are significantly elevated in obesity. Adipose tissue-derived ET-1 attenuates insulin-mediated antilipolysis in human visceral adipocytes through the activation of the ET receptor B (ET(B)R), thereby linking ET-1 to insulin resistance. Whether ET-1 has direct effects on lipolysis in human adipocytes is not known. RESEARCH DESIGN AND SUBJECTS: Endothelin-1 receptor (ETR) mRNA expression was determined by quantitative PCR in 130 non-obese and obese subjects. ET-1 mRNA in different adipose tissue regions was also assessed. ETR protein expression was analyzed by western blotting in 37 subjects. The effect of ET-1 on lipolysis was assessed in freshly isolated adipocytes and in vitro differentiated adipocytes from human donors. RESULTS: Freshly isolated human adipocytes incubated with different concentrations of ET-1 showed no acute effect on lipolysis. In contrast, a 24 h incubation in primary cultures of human adipocytes resulted in a significant 50% increase in lipolysis. This effect was concentration dependent and could be mimicked by an agonist of the ET(A) receptor but not with a selective ET(B)R agonist. Adipocyte differentiation was not affected by any of the agonists. In subcutaneous (s.c.) adipose tissue from 19 non-obese and 18 obese subjects, the protein expression of ET(A)R was significantly higher in obese subjects whereas there was no difference in ET(B)R expression. Interestingly, the differences in protein expression were not observed at the mRNA level as ET(A)R expression was similar between lean and obese subjects. CONCLUSION: Long-term but not acute incubation of human adipocytes with ET-1 results in a significant increase in lipolysis. This appears to be mediated through the activation of ET(A)R, demonstrating a yet another receptor-specific effect of ET-1. In addition, the protein expression of ET(A)R is increased in s.c. adipose tissue in obesity, possibly through post-transcriptional mechanisms. An increased effect of ET-1 could be a mechanism that contributes to increased basal lipolysis in human obesity.

Characterization of the human CIDEA promoter in fat cells.

BACKGROUND: Cell death-inducing DFFA (DNA fragmentation factor-alpha)-like effector A (CIDEA) is a protein that regulates lipolysis in human adipocytes through cross-talk involving tumor necrosis factor-alpha (TNF-alpha). TNF-alpha downregulates CIDEA mRNA although it is unclear whether this is mediated through transcriptional or post-transcriptional mechanisms. CIDEA has important metabolic effects in human fat cells and genetic variations in the human CIDEA gene have been correlated to the development of obesity. However, little is known about the factors regulating CIDEA expression in human adipocytes. We set out to describe the transcriptional control of human CIDEA. METHODS: A 1.1-kb genomic fragment upstream of the transcriptional start site (TSS) of human CIDEA was cloned and deletion fragments were generated. Transcriptional activity of the promoter was analyzed by luciferase reporter assays in in vitro-differentiated human adipocytes. The effect of TNF-alpha was assessed in human adipocytes and murine 3T3-L1 cells transfected with deletion fragments of the CIDEA promoter. Protein-DNA interactions were analyzed by electrophoretic mobility shift assays (EMSA). RESULTS: Basal transcriptional activity was found in a 97-bp region upstream of the TSS. We studied the effect of three common haplotypes in the promoter region but found no significant difference in transcriptional activity among them. Incubation of in vitro-differentiated human adipocytes as well as 3T3-L1 cells with TNF-alpha reduced the transcriptional activity of the human CIDEA promoter, demonstrating a direct effect on CIDEA transcription. EMSAs and mutational analysis indicated that this was mediated by a nuclear factor-kappaB (NF-kappaB) site at position -163/-151. CONCLUSION: We demonstrate that basal transcription of the human CIDEA gene is confined to the 97 first bases upstream of TSS and that TNF-alpha negatively regulates transcription of this gene, which at least in part involves NF-kappaB activation.