👤 Hilde Irene Nebb

Plin4 is transcriptionally regulated by peroxisome proliferator-activated receptor gamma and is primarily expressed in white adipose tissue (WAT). We found that expression of Plin4 is elevated in the liver upon prolonged feeding with an obesogenic diet containing saturated fat, fructose, and cholesterol (Western diet). To investigate the functional role of Plin4 in energy metabolism, we generated Plin4 Show less

The cholesterol-sensing nuclear receptor liver X receptor (LXR) and the glucose-sensing transcription factor carbohydrate responsive element-binding protein (ChREBP) are central players in regulating glucose and lipid metabolism in the liver. More knowledge of their mechanistic interplay is needed to understand their role in pathological conditions like fatty liver disease and insulin resistance. In the current study, LXR and ChREBP co-occupancy was examined by analyzing ChIP-seq datasets from mice livers. LXR and ChREBP interaction was determined by Co-immunoprecipitation (CoIP) and their transactivity was assessed by real-time quantitative polymerase chain reaction (qPCR) of target genes and gene reporter assays. Chromatin binding capacity was determined by ChIP-qPCR assays. Our data show that LXRα and ChREBPα interact physically and show a high co-occupancy at regulatory regions in the mouse genome. LXRα co-activates ChREBPα and regulates ChREBP-specific target genes in vitro and in vivo. This co-activation is dependent on functional recognition elements for ChREBP but not for LXR, indicating that ChREBPα recruits LXRα to chromatin in Show less

Members of the poly-ADP-ribose polymerase (PARP) family catalyse the ADP-ribosylation of target proteins and are known to play important roles in many cellular processes, including DNA repair, differentiation and transcription. The majority of PARPs exhibit mono-ADP-ribosyltransferase activity rather than PARP activity; however, little is known about their biological activity. In the present study, we report that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly-ADP-ribose polymerase (TIPARP), mono-ADP-ribosylates and positively regulates liver X receptor α (LXRα) and LXRβ activity. Overexpression of TIPARP enhanced LXR-reporter gene activity. TIPARP knockdown or deletion reduced LXR regulated target gene expression levels in HepG2 cells and in Tiparp(-/-)mouse embryonic fibroblasts (MEFs) respectively. Deletion and mutagenesis studies showed that TIPARP's zinc-finger and catalytic domains were required to enhance LXR activity. Protein interaction studies using TIPARP and LXRα/β peptide arrays revealed that LXRs interacted with an N-terminal sequence (a.a. 209-236) of TIPARP, which also overlapped with a putative co-activator domain of TIPARP (a.a. 200-225). Immunofluorescence studies showed that TIPARP and LXRα or LXRβ co-localized in the nucleus.In vitroribosylation assays provided evidence that TIPARP mono-ADP-ribosylated both LXRα and LXRβ. Co-immunoprecipitation (co-IP) studies revealed that ADP-ribosylase macrodomain 1 (MACROD1), but not MACROD2, interacted with LXRs in a TIPARP-dependent manner. This was complemented by reporter gene studies showing that MACROD1, but not MACROD2, prevented the TIPARP-dependent increase in LXR activity. GW3965-dependent increases in hepatic Srebp1 mRNA and protein expression levels were reduced in Tiparp(-/-)mice compared with Tiparp(+/+)mice. Taken together, these data identify a new mechanism of LXR regulation that involves TIPARP, ADP-ribosylation and MACROD1. Show less

Liver X receptor (LXR)α and LXRβ play key roles in hepatic de novo lipogenesis through their regulation of lipogenic genes, including sterol regulatory element-binding protein (SREBP)-1c and carbohydrate responsive element-binding protein (ChREBP). LXRs activate lipogenic gene transcription in response to feeding, which is believed to be mediated by insulin. We have previously shown that LXRs are targets for glucose-hexosamine-derived O-linked β-N-acetylglucosamine (O-GlcNAc) modification enhancing their ability to regulate SREBP-1c promoter activity in vitro. To elucidate insulin-independent effects of feeding on LXR-mediated lipogenic gene expression in vivo, we subjected control and streptozotocin-treated LXRα/β(+/+) and LXRα/β(-/-) mice to a fasting-refeeding regime. We show that under hyperglycemic and hypoinsulinemic conditions, LXRs maintain their ability to upregulate the expression of glycolytic and lipogenic enzymes, including glucokinase (GK), SREBP-1c, ChREBPα, and the newly identified shorter isoform ChREBPβ. Furthermore, glucose-dependent increases in LXR/retinoid X receptor-regulated luciferase activity driven by the ChREBPα promoter was mediated, at least in part, by O-GlcNAc transferase (OGT) signaling in Huh7 cells. Moreover, we show that LXR and OGT interact and colocalize in the nucleus and that loss of LXRs profoundly reduced nuclear O-GlcNAc signaling and ChREBPα promoter binding activity in vivo. In summary, our study provides evidence that LXRs act as nutrient and glucose metabolic sensors upstream of ChREBP by modulating GK expression, nuclear O-GlcNAc signaling, and ChREBP expression and activity. Show less

Interleukin (IL)-10 is a prototypical anti-inflammatory cytokine that has been shown to attenuate atherosclerosis development. In addition to its anti-inflammatory properties, the anti-atherogenic effect of IL-10 has recently also been suggested to reflect a complex effect of IL-10 on lipid metabolism in macrophages. In the present study we examined the effects of IL-10 on cholesterol efflux mechanism in lipid-loaded THP-1 macrophages. Our main findings were: (i) IL-10 significantly enhanced cholesterol efflux induced by fetal-calf serum, high-density lipoprotein (HDL)2 and apolipoprotein A-1. (ii) The IL-10-mediated effects on cholesterol efflux were accompanied by an increased IL-10-mediated expression of the ATP-binding cassette transporters ABCA1 and ABCG1, that was further enhanced when the cells were co-activated with the liver X receptor (LXR)α agonist (22R)-hydroxycholesterol. (iii) The effect of LXRα activation on the IL-10-mediated effects on the ATP-binding cassette transporters seems to include enhancing effects on the IL-10 receptor 1 (IL10R1) expression and interaction with STAT-3 signaling. (iv) These enhancing effects on ABCA1 and ABCG1 was not seen when the cells were stimulated with the IL-10 family members IL-22 and IL-24. This study suggests that the anti-atherogenic properties of IL-10 may include enhancing effects on cholesterol efflux mechanism that involves cross-talk with LXRα activation. Show less

Liver X receptor (LXR)-α and -β play a major role in lipid and glucose homeostasis. Their expression and function in the heart is not well characterized. Our aim was to describe the expression of LXRs in the murine heart, and to determine effects of cardiac LXR activation on target gene expression, lipid homeostasis and ischemia. Both LXRα and -β were expressed in heart tissues, HL-1 cells and isolated cardiomyocytes as determined by qRT-PCR. Elevated cardiac expression of LXR target genes and LXRβ was observed 24 h after in vivo permanent coronary artery ligation. The synthetic LXR agonist GW3965 induced mRNA expression of the LXR target genes in HL-1 cells and isolated cardiomyocytes. This was associated with a buildup of intracellular triglycerides and expanding lipid droplets as quantified by confocal microscopy. Mice injected with GW3965 had cardiac LXR activation as judged by increased target gene expression and lipid droplet accumulation. GW3965 in vivo and in vitro increased expression of genes inducing triglyceride synthesis, and altered expression of lipid droplet-binding protein genes. GW3965 protected HL-1 cells against hypoxia-reoxygenation induced apoptosis. LXR activation by GW3965 in vivo prior to heart isolation and perfusion with induced global ischemia and reperfusion improved left ventricular contractile function and decreased infarct size. In conclusion, LXRs are expressed in the murine heart in the basal state, and are activated by myocardial infarction. Activation of LXR by the synthetic agonist GW3965 is associated with intracardiac accumulation of lipid droplets and protection against myocardial ischemia-reperfusion injury. Show less

Liver X receptor (LXR) is a transcription factor of the nuclear receptor family, regulating genes involved in metabolism, inflammation, and apoptosis. In the present investigation, we examined the role of LXR in organ injury and systemic inflammation in rodent models of polymicrobial peritonitis caused by cecal ligation and puncture (CLP). Rats were subjected to CLP sepsis or a sham operation. Some were treated with the synthetic LXR agonist GW3965 0.3 mg/kg 30 min prior to the CLP procedure, and organs and plasma were harvested at 3, 10, 18, or 24 h. Organs were analyzed for RNA expression by quantitative polymerase chain reaction or for morphologic differences by histologic review. Organ injury and inflammatory markers were measured in plasma. Expression of the LXRα gene was decreased in the livers of CLP rats compared with sham-operated rats. Administration of a synthetic agonist of LXR (GW3965) reduced biochemical indices of liver injury in the blood of CLP rats. We also demonstrated that liver injury associated with CLP is aggravated in LXRα- and LXRαβ-deficient mice compared with wild-type and LXRβ-deficient mice, indicating a role for LXRα in protecting the liver. The enhanced liver injury in LXR-deficient mice was associated with elevated plasma concentrations of high mobility group box 1, a late mediator of inflammation and a known factor in the pathology of this model. Collectively, these results argue in favor of a role for LXRα in protection against liver injury in experimental sepsis induced by CLP. Show less

The liver X receptors (LXR) α and β regulate lipid and carbohydrate homeostasis and inflammation. Lxrβ⁻/⁻ mice are glucose intolerant and at the same time lean. We aimed to assess the associations between single nucleotide polymorphisms (SNPs) in LXRβ and risk of type 2 diabetes mellitus (T2DM), obesity and related traits in 3 separate cohort studies. Twenty LXRβ SNPs were identified by sequencing and genotyped in the HUNT2 adult nested case-control study for T2DM (n = 835 cases/1986 controls). Five tag-SNPs (rs17373080, rs2695121, rs56151148, rs2303044 and rs3219281), covering 99.3% of the entire common genetic variability of the LXRβ gene were identified and genotyped in the French MONICA adult study (n = 2318) and the European adolescent HELENA cross-sectional study (n = 1144). In silico and in vitro functionality studies were performed. We identified suggestive or significant associations between rs17373080 and the risk of (i) T2DM in HUNT2 (OR = 0.82, p = 0.03), (ii) obesity in MONICA (OR = 1.26, p = 0.05) and (iii) overweight/obesity in HELENA (OR = 1.59, p = 0.002). An intron 4 SNP (rs28514894, a perfect proxy for rs17373080) could potentially create binding sites for hepatic nuclear factor 4 alpha (HNF4α) and nuclear factor 1 (NF1). The C allele of rs28514894 was associated with ~1.25-fold higher human LXRβ basal promoter activity in vitro. However, no differences between alleles in terms of DNA binding and reporter gene transactivation by HNF4α or NF1 were observed. Our results suggest that rs17373080 in LXRβ is associated with T2DM and obesity, maybe via altered LXRβ expression. Show less

Preeclampsia is a pregnancy-specific disorder associated with hyperlipidemia. Liver X receptor (LXR) alpha and LXRbeta are key regulators of lipid homeostasis. In the current study, we investigated expression of LXRalpha, LXRbeta and their target genes in human term placenta, decidua and subcutaneous adipose tissue from pregnancies complicated by preeclampsia. Furthermore, we analyzed the protein levels of LXRalpha and LXRbeta in placenta. We also analyzed lipid concentrations in term placental tissue. Gene expression of LXRalpha, LXRbeta and fatty acid transporter CD36 was significantly decreased in placental tissues, while increased expression was observed for LXRalpha in adipose tissue, from pregnancies complicated by preeclampsia. The placental protein level of LXRbeta was reduced, and there was a positive correlation between placental LXRbeta mRNA expression and placental free fatty acids in preeclampsia. Our results suggest a possible role for LXRbeta as a transcriptional regulator in preeclampsia. Show less

Placental fatty acid transport and metabolism are important for proper growth and development of the feto-placental unit. The nuclear receptors, liver X receptors alpha and beta (LXRalpha and LXRbeta), are key regulators of lipid metabolism in many tissues, but little is known about their role in fatty acid transport and metabolism in placenta. The current study investigates the LXR-mediated regulation of long-chain acyl-CoA synthetase 3 (ACSL3) and its functions in human placental trophoblast cells. We demonstrate that activation of LXR increases ACSL3 expression, acyl-CoA synthetase activity, and fatty acid uptake in human tropholast cells. Silencing of ACSL3 in these cells attenuates the LXR-mediated increase in acyl-CoA synthetase activity. Furthermore, we show that ACSL3 is directly regulated by LXR through a conserved LXR responsive element in the ACSL3 promoter. Our results suggest that LXR plays a regulatory role in fatty acid metabolism by direct regulation of ACSL3 in human placental trophoblast cells. Show less

Post-translational modification of nucleocytoplasmic proteins by O-linked beta-N-acetylglucosamine (O-GlcNAc) has for the last 25 years emerged as an essential glucose-sensing mechanism. The liver X receptors (LXRs) function as nutritional sensors for cholesterol-regulating lipid metabolism, glucose homeostasis, and inflammation. LXRs are shown to be post-translationally modified by phosphorylation, acetylation, and sumoylation, affecting their target gene specificity, stability, and transactivating and transrepressional activity, respectively. In the present study, we show for the first time that LXRalpha and LXRbeta are targets for glucose-hexosamine-derived O-GlcNAc modification in human Huh7 cells. Furthermore, we observed increased hepatic LXRalpha O-GlcNAcylation in vivo in refed mice and in streptozotocin-induced refed diabetic mice. Importantly, induction of LXRalpha O-GlcNAcylation in both mouse models was concomitant with increased expression of the lipogenic gene SREBP-1c (sterol regulatory element-binding protein 1c). Furthermore, glucose increased LXR/retinoic acid receptor-dependent activation of luciferase reporter activity driven by the mouse SREBP-1c promoter via the hexosamine biosynthetic pathway in Huh7 cells. Altogether, our results suggest that O-GlcNAcylation of LXR is a novel mechanism by which LXR acts as a glucose sensor affecting LXR-dependent gene expression, substantiating the crucial role of LXR as a nutritional sensor in lipid and glucose metabolism. Show less

Dietary essential fatty acids linoleic acid and alpha-linolenic acid are converted to arachidonic-, eicosapentaenoic-, and docosahexaenoic acid under tight regulation by nutritional status and hormones. Hepatic fatty acid elongase 5 (Elovl5) elongates C18-20 polyunsaturated fatty acids (PUFAs) and is important for biosynthesis of C20-22 PUFAs. We demonstrate that Liver X Receptor alpha (LXRalpha) and sterol regulatory binding protein-1c (SREBP-1c) regulate hepatic Elovl5 expression. LXRalpha and LXRbeta play different roles in maintenance of basal expression of Elovl5. LXRalpha is necessary for basal as well as LXR agonist induced Elovl5 transcription. Promoter studies revealed that the mouse Elovl5 gene is a direct SREBP-1c target. The up-regulation of Elovl5 expression by LXR agonist is likely secondary to the induction of SREBP-1c. PUFAs repress expression of SREBP-1c and Elovl5, but when combined with LXR ligand stimulation, which increases SREBP-1c mRNA and nuclear SREBP-1c, Elovl5 mRNA levels are restored to normal. Our studies suggest that an LXRalpha-SREBP-1c pathway plays a regulatory role in hepatic biosynthesis of PUFAs through transcriptional activation of Elovl5 as well as other desaturases. The stimulatory role of LXRalpha-SREBP-1c in the production of PUFAs enables the possibility for a feedback regulation of hepatic lipogenesis through PUFA mediated repression of SREBP-1c expression. Show less