22-Hydroxycholesterols regulate lipid metabolism differently than T0901317 in human myotubes

20-αHydroxycholesterol, an oxysterol in human breast milk, reverses mouse neonatal white matter injury through Gli-dependent oligodendrogenesis

White matter injuries (WMIs) are the leading cause of neurologic impairment in infants born premature. There are no treatment options available. The most common forms of WMIs in infants occur prior to the onset of normal myelination, making its pathophysiology distinctive, thus requiring a tailored approach to treatment. Neonates present a unique opportunity to repair WMIs due to a transient abundance of neural stem/progenitor cells (NSPCs) present in the germinal matrix with oligodendrogenic potential. We identified an endogenous oxysterol, 20-αHydroxycholesterol (20HC), in human maternal breast milk that induces oligodendrogenesis through a sonic hedgehog (shh), Gli-dependent mechanism. Following WMI in neonatal mice, injection of 20HC induced subventricular zone-derived oligodendrogenesis and improved myelination in the periventricular white matter, resulting in improved motor outcomes. Targeting the oligodendrogenic potential of postnatal NSPCs in neonates with WMIs may be further developed into a novel approach to mitigate this devastating complication of preterm birth.

Cutting edge concepts: Does bilirubin enhance exercise performance?

Exercise performance is dependent on many factors, such as muscular strength and endurance, cardiovascular capacity, liver health, and metabolic flexibility. Recent studies show that plasma levels of bilirubin, which has classically been viewed as a liver dysfunction biomarker, are elevated by exercise training and that elite athletes may have significantly higher levels. Other studies have shown higher plasma bilirubin levels in athletes and active individuals compared to general, sedentary populations. The reason for these adaptions is unclear, but it could be related to bilirubin's antioxidant properties in response to a large number of reactive oxygen species (ROS) that originates from mitochondria during exercise. However, the mechanisms of these are unknown. Current research has re-defined bilirubin as a metabolic hormone that interacts with nuclear receptors to drive gene transcription, which reduces body weight. Bilirubin has been shown to reduce adiposity and improve the cardiovascular system, which might be related to the adaption of bilirubin increasing during exercise. No studies have directly tested if elevating bilirubin levels can influence athletic performance. However, based on the mechanisms proposed in the present review, this seems plausible and an area to consider for future studies. Here, we discuss the importance of bilirubin and exercise and how the combination might improve metabolic health outcomes and possibly athletic performance.

Identification of unusual oxysterols biosynthesised in human pregnancy by charge-tagging and liquid chromatography - mass spectrometry

The aim of this study was to identify oxysterols and any down-stream metabolites in placenta, umbilical cord blood plasma, maternal plasma and amniotic fluid to enhance our knowledge of the involvement of these molecules in pregnancy. We confirm the identification of 20S-hydroxycholesterol in human placenta, previously reported in a single publication, and propose a pathway from 22R-hydroxycholesterol to a C₂₇ bile acid of probable structure 3β,20R,22R-trihydroxycholest-5-en-(25R)26-oic acid. The pathway is evident not only in placenta, but pathway intermediates are also found in umbilical cord plasma, maternal plasma and amniotic fluid but not non-pregnant women.

The oxysterome and its receptors as pharmacological targets in inflammatory diseases

Oxysterols have gained attention over the last decades and are now considered as fully fledged bioactive lipids. The study of their levels in several conditions, including atherosclerosis, obesity and neurodegenerative diseases, led to a better understanding of their involvement in (patho)physiological processes such as inflammation and immunity. For instance, the characterization of the cholesterol-7α,25-dihydroxycholesterol/GPR183 axis and its implication in immunity represents an important step in the oxysterome study. Besides this axis, others were identified as important in several inflammatory pathologies (such as colitis, lung inflammation and atherosclerosis). However, the oxysterome is a complex system notably due to a redundancy of metabolic enzymes and a wide range of receptors. Indeed, deciphering oxysterol roles and identifying the potential receptor(s) involved in a given pathology remain challenging. Oxysterol properties are very diverse, but most of them could be connected by a common component: inflammation. Here, we review the implication of oxysterol receptors in inflammatory diseases.

Exercise in vivo marks human myotubes in vitro: Training-induced increase in lipid metabolism

BACKGROUND AND AIMS

Physical activity has preventive as well as therapeutic benefits for overweight subjects. In this study we aimed to examine effects of in vivo exercise on in vitro metabolic adaptations by studying energy metabolism in cultured myotubes isolated from biopsies taken before and after 12 weeks of extensive endurance and strength training, from healthy sedentary normal weight and overweight men.

METHODS

Healthy sedentary men, aged 40-62 years, with normal weight (body mass index (BMI) < 25 kg/m2) or overweight (BMI ≥ 25 kg/m2) were included. Fatty acid and glucose metabolism were studied in myotubes using [14C]oleic acid and [14C]glucose, respectively. Gene and protein expressions, as well as DNA methylation were measured for selected genes.

RESULTS

The 12-week training intervention improved endurance, strength and insulin sensitivity in vivo, and reduced the participants' body weight. Biopsy-derived cultured human myotubes after exercise showed increased total cellular oleic acid uptake (30%), oxidation (46%) and lipid accumulation (34%), as well as increased fractional glucose oxidation (14%) compared to cultures established prior to exercise. Most of these exercise-induced increases were significant in the overweight group, whereas the normal weight group showed no change in oleic acid or glucose metabolism.

CONCLUSIONS

12 weeks of combined endurance and strength training promoted increased lipid and glucose metabolism in biopsy-derived cultured human myotubes, showing that training in vivo are able to induce changes in human myotubes that are discernible in vitro.

Regulation of liver X receptor target genes by 22-functionalized oxysterols. Synthesis, in silico and in vitro evaluations

The endogenous oxysterol 22(R)-hydroxycholesterol (22RHC, 1) is an LXR agonist which upregulates genes of critical involvement in human cholesterol- and lipid metabolism. In contrast, its synthetic epimer 22(S)-hydroxycholesterol (22SHC, 8) has shown specific antagonistic effects in recent studies, avoiding unwanted side effects provided by potent LXR agonists. In terms of LXR modulation, the aim of this study was to compare 22SHC (8), 22RHC (1) and synthesized ligands with keto- and amide functionality in the 22nd position of the cholesterol scaffold. 22SHC (8) and 22RHC (1) performed as expected while 22-ketocholesterol (22KC, 10) revealed an attractive in vitro profile for further investigation in terms of anti-atherosclerotic properties as selective upregulation of the ATP-binding cassette transporter ABCA1 was observed. A new synthesized amide derivate, Fernholtz cyclohexylamide (13) was shown to reduce lipogenesis in a dose-responsive manner and abolish the effect of the potent LXR agonist T0901317 when administered simultaneously.

Synthesis, in vitro and in vivo biological evaluation of new oxysterols as modulators of the liver X receptors

Liver X Receptor (LXR) modulators have shown potential as drugs since they target genes affecting metabolism and fatty acid synthesis. LXR antagonists are of particular interest since they are able to reduce the synthesis of complex fatty acids and glucose uptake. Based on molecular modeling, five new cholesterol mimics were synthesized, where four contained a hydroxyl group in the 22-S-position. The new compounds were screened in vitro against several genes affecting lipid metabolism. The compound that performed best in vitro was a dimethylamide derivative of 22(S)-hydroxycholesterol and it was chosen for in vivo testing. However, the blood plasma analysis from the in vivo tests revealed a concentration lower than needed to give any response, indicating either rapid metabolism or low bioavailability.

Plasma Membrane Origin of the Steroidogenic Pool of Cholesterol Used in Hormone-induced Acute Steroid Formation in Leydig Cells

Hormone-sensitive acute steroid biosynthesis requires trafficking of cholesterol from intracellular sources to the inner mitochondrial membrane. The precise location of the intracellular cholesterol and its transport mechanism are uncertain. Perfringolysin O, produced by Clostridium perfringens, binds cholesterol. Its fourth domain (D4) retains cholesterol-binding properties but not cytotoxicity. We transfected steroidogenic MA-10 cells of mouse Leydig cell tumors with the mCherry-D4 plasmid. Tagged D4 with fluorescent proteins enabled us to track cholesterol. The staining was primarily localized to the inner leaflet of the plasma membrane and was partially released upon treatment with dibutyryl-cAMP (Bt₂cAMP), a cAMP analog. Inhibitors of cholesterol import into mitochondria blocked steroidogenesis and prevented release of D4 (and presumably cholesterol) from the plasma membrane. We conclude that the bulk of the steroidogenic pool of cholesterol, mobilized by Bt₂cAMP for acute steroidogenesis, originates from the plasma membrane. Treatment of the cells with steroid metabolites, 22(R)-hydroxycholesterol and pregnenolone, also reduced D4 release from the plasma membrane, perhaps evidence for a feedback effect of elevated steroid formation on cholesterol release. Interestingly, D4 staining was localized to endosomes during Bt₂cAMP stimulation suggesting that these organelles are on the route of cholesterol trafficking from the plasma membrane to mitochondria. Finally, D4 was expressed in primary rat Leydig cells with a lentivirus and was released from the plasma membrane following Bt₂cAMP treatment. We conclude that the plasma membrane is the source of cholesterol for steroidogenesis in these cells as well as in MA-10 cells.

Sulfotransferase 1A1 Substrate Selectivity: A Molecular Clamp Mechanism

The human cytosolic sulfotransferases (SULTs) regulate hundreds, perhaps thousands, of small molecule metabolites and xenobiotics via transfer of a sulfuryl moiety (-SO3) from PAPS (3'-phosphoadenosine 5'-phosphosulfate) to the hydroxyls and primary amines of the recipients. In liver, where it is abundant, SULT1A1 engages in modifying metabolites and neutralizing toxins. The specificity of 1A1 is the broadest of any SULT, and understanding its selectivity is fundamental to understanding its biology. Here, for the first time, we show that SULT1A1 substrates separate naturally into two classes: those whose affinities are either enhanced ∼20-fold (positive synergy) or unaffected (neutral synergy) by the presence of a saturating nucleotide. kcat for the positive-synergy substrates is shown to be ∼100-fold greater than that of neutral-synergy compounds; consequently, the catalytic efficiency (kcat/Km) is approximately 3 orders of magnitude greater for the positive-synergy species. All-atom dynamics modeling suggests a molecular mechanism for these observations in which the binding of only positive-synergy compounds causes two phenylalanine residues (F81 and 84) to reposition and "sandwich" the phenolic moiety of the substrates, thus enhancing substrate affinity and positioning the nucleophilic oxygen for attack. Molecular dynamics movies reveal that the neutral-synergy compounds "wander" about the active site, infrequently achieving a reactive position. In-depth analysis of select point mutants strongly supports the model and provides an intimate view of the interdependent catalytic functions of subsections of the active site.

22-S-Hydroxycholesterol protects against ethanol-induced liver injury by blocking the auto/paracrine activation of MCP-1 mediated by LXRα

Chronic ethanol consumption causes hepatic steatosis and inflammation, which are associated with liver hypoxia. Monocyte chemoattractant protein‐1 (MCP‐1) is a hypoxia response factor that determines recruitment and activation of monocytes to the site of tissue injury. The level of MCP‐1 is elevated in the serum and liver of patients with alcoholic liver disease (ALD); however, the molecular details regarding the regulation of MCP‐1 expression are not yet understood completely. Here, we show the role of liver X receptor α (LXRα) in the regulation of MCP‐1 expression during the development of ethanol‐induced fatty liver injury, using an antagonist, 22‐S‐hydroxycholesterol (22‐S‐HC). First, administration of 22‐S‐HC attenuated the signs of liver injury with decreased levels of MCP‐1 and its receptor CCR2 in ethanol‐fed mice. Second, hypoxic conditions or treatment with the LXRα agonist GW3965 significantly induced the expression of MCP‐1, which was completely blocked by treatment with 22‐S‐HC or infection by shLXRα lentivirus in the primary hepatocytes. Third, over‐expression of LXRα or GW3965 treatment increased MCP‐1 promoter activity by increasing the binding of hypoxia‐inducible factor‐1α to the hypoxia response elements, together with LXRα. Finally, treatment with recombinant MCP‐1 increased the level of expression of LXRα and LXRα‐dependent lipid droplet accumulation in both hepatocytes and Kupffer cells. These data show that LXRα and its ligand‐induced up‐regulation of MCP‐1 and MCP‐1‐induced LXRα‐dependent lipogenesis play a key role in the autocrine and paracrine activation of MCP‐1 in the pathogenesis of alcoholic fatty liver disease, and that this activation may provide a promising new target for ALD therapy.Copyright © 2014 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Structure, dynamics and selectivity in the sulfotransferase family

Combined structure, function and molecular dynamics studies of human cytosolic sulfotransferases (SULT1A1 and 2A1) have revealed that these enzymes contain a ≈ 30-residue active-site cap whose structure responds to substrates and mediates their interactions. The binding of 3'-phosphoadenosine 5'-phosphosulfate (PAPS) gates access to the active site by a remodeling of the cap that constricts the pore through which acceptors must pass to enter the active site. While the PAPS-bound enzyme spends the majority (≈ 95%) of its time in the constricted state, the pore isomerizes between the open and closed states when the nucleotide (PAPS) is bound. The dimensions of the open and closed pores place widely different steric constraints on substrate selectivity. Nature appears to have crafted these enzymes with two specificity settings - a closed-pore setting that admits a set of closely related structures, and an open setting that allows a far wider spectrum of acceptor geometries. The specificities of these settings seem well matched to the metabolic demands for homeostatic and defensive SULT functions. The departure of nucleotide requires that the cap open. This isomerization dependent release can explain both the product bursts and substrate inhibition seen in many SULTs. Here, the experimental underpinnings of the cap-mechanism are reviewed, and the advantages of such a mechanism are considered in the context of the cellular and metabolic environment in which these enzymes operate.

Oxysterol generation and liver X receptor-dependent reverse cholesterol transport: not all roads lead to Rome

Cell cholesterol metabolism is a tightly regulated process, dependent in part on activation of nuclear liver X receptors (LXRs) to increase expression of genes mediating removal of excess cholesterol from cells in the reverse cholesterol transport pathway. LXRs are thought to be activated predominantly by oxysterols generated enzymatically from cholesterol in different cell organelles. Defects resulting in slowed release of cholesterol from late endosomes and lysosomes or reduction in sterol-27-hydroxylase activity lead to specific blocks in oxysterol production and impaired LXR-dependent gene activation. This block does not appear to be compensated by oxysterol production in other cell compartments. The purpose of this review is to summarize current knowledge about oxysterol-dependent activation by LXR of genes involved in reverse cholesterol transport, and what these defects of cell cholesterol homeostasis can teach us about the critical pathways of oxysterol generation for expression of LXR-dependent genes.

A nucleotide-gated molecular pore selects sulfotransferase substrates

Human SULT2A1 is one of two predominant sulfotransferases in liver and catalyzes transfer of the sulfuryl moiety (-SO(3)) from activated sulfate (PAPS, 3'-phosphoadenosine 5-phosphosulfate) to hundreds of acceptors (metabolites and xenobiotics). Sulfation recodes the biologic activity of acceptors by altering their receptor interactions. The molecular basis on which these enzymes select and sulfonate specific acceptors from complex mixtures of competitors in vivo is a long-standing issue in the SULT field. Raloxifene, a synthetic steroid used in the prevention of osteoporosis, and dehydroepiandrosterone (DHEA), a ubiquitous steroid precusor, are reported to be sulfated efficiently by SULT2A1 in vitro, yet unlike DHEA, raloxifene is not sulfated in vivo. This selectivity was explored in initial rate and equilibrium binding studies that demonstrate pronounced binding antisynergy (21-fold) between PAPS and raloxifene, but not DHEA. Analysis of crystal structures suggests that PAP binding restricts access to the acceptor-binding pocket by restructuring a nine-residue segment of the pocket edge that constricts the active site opening, or "pore", that sieves substrates on the basis of their geometries. In silico docking predicts that raloxifene, which is considerably larger than DHEA, can bind only to the unliganded (open) enzyme, whereas DHEA binds both the open and closed forms. The predictions of these structures with regard to substrate binding are tested using equilibrium and pre-steady-state ligand binding studies, and the results confirm that a nucleotide-driven isomerization controls access to the acceptor-binding pocket and plays an important role in substrate selection by SULT2A1 and possibly other sulfotransferases.

The liver X receptor modulator 22(S)-hydroxycholesterol exerts cell-type specific effects on lipid and glucose metabolism

The aim of this study was to explore the effects of 22(S)-hydroxycholesterol (22(S)-HC) on lipid and glucose metabolism in human-derived cells from metabolic active tissues. Docking of T0901317 and 22(S)-HC showed that both substances fitted into the ligand binding domain of liver X receptors (LXR). Results show that while several lipogenic genes were induced by T0901317 in myotubes, HepG2 cells and SGBS cells, effect of 22(S)-HC varied more between cell types. In myotubes, most lipogenic genes were downregulated or unchanged by 22(S)-HC, whereas a more diverse pattern was found in HepG2 and SGBS cells. Treatment with 22(S)-HC induced sterol regulatory element binding transcription factor 1 in SGBS and HepG2 cells, but not in myotubes. Fatty acid synthase was downregulated by 22(S)-HC in myotubes, upregulated in SGBS and unchanged in HepG2 cells. De novo lipogenesis was increased by T0901317 in all cell models, whereas differently affected by 22(S)-HC depending on the cell type; decreased in myotubes and HepG2 cells, whereas increased in SGBS cells. Oxidation of linoleic acid was reduced by 22(S)-HC in all cell models while glucose uptake increased and tended to increase in myotubes and SGBS cells, respectively. Cholesterol efflux was unaffected by 22(S)-HC treatment. These results show that 22(S)-HC affects LXR-regulated processes differently in various cell types. Ability of 22(S)-HC to reduce lipogenesis and lipid accumulation in myotubes and hepatocytes indicate that 22(S)-HC might reduce lipid accumulation in non-adipose tissues, suggesting a potential role for 22(S)-HC or a similar LXR modulator in the treatment of type 2 diabetes.

The Role of PPARα Activation in Liver and Muscle

PPARα is one of three members of the soluble nuclear receptor family called peroxisome proliferator-activated receptor (PPAR). It is a sensor for changes in levels of fatty acids and their derivatives that responds to ligand binding with PPAR target gene transcription, inasmuch as it can influence physiological homeostasis, including lipid and carbohydrate metabolism in various tissues. In this paper we summarize the involvement of PPARα in the metabolically active tissues liver and skeletal muscle and provide an overview of the risks and benefits of ligand activation of PPARα, with particular consideration to interspecies differences.

LXR{beta} is the dominant LXR subtype in skeletal muscle regulating lipogenesis and cholesterol efflux

Liver X receptors (LXRs) are important regulators of cholesterol, lipid, and glucose metabolism and have been extensively studied in liver, macrophages, and adipose tissue. However, their role in skeletal muscle is poorly studied and the functional role of each of the LXRalpha and LXRbeta subtypes in skeletal muscle is at present unknown. To study the importance of each of the receptor subtypes, myotube cultures derived from wild-type (WT) and LXRalpha and LXRbeta knockout (KO) mice were established. The present study showed that treatment with the LXR agonist T0901317 increased lipogenesis and apoA1-dependent cholesterol efflux in LXRalpha KO and WT myotubes but not in LXRbeta KO cells. The functional studies were confirmed by T0901317-induced increase in mRNA levels of LXR target genes involved in lipid and cholesterol metabolism in myotubes established from WT and LXRalpha KO mice, whereas only minor changes were observed for these genes in myotubes from LXRbeta KO mice. Gene expression analysis using microarrays showed that very few genes other than the classical, well-known LXR target genes were regulated by LXR in skeletal muscle. The present study also showed that basal glucose uptake was increased in LXRbeta KO myotubes compared with WT myotubes, suggesting a role for LXRbeta in glucose metabolism in skeletal muscle. In conclusion, LXRbeta seems to be the main LXR subtype regulating lipogenesis and cholesterol efflux in skeletal muscle.

Oxysterol represses high-affinity IgE receptor-stimulated mast cell activation in Liver X receptor-dependent and -independent manners

Oxysterols activating liver X receptors (LXRs) repress expression of pro-inflammatory genes and have anti-inflammatory effects. Here, we show for the first time that bone marrow-derived murine mast cells (BMMCs) predominantly express LXRbeta. 25-hydroxycholesterol, a representative LXR activating oxysterol, suppressed IL-6 production and degranulation response in BMMCs following engagement of high-affinity IgE receptor (FcepsilonRI). Interestingly, 25-hydroxycholesterol reduced cell-surface FcepsilonRI expression by inhibiting assembly of FcepsilonRIalpha and FcepsilonRIbeta. We demonstrate that LXR activation was involved in the suppression of IL-6 production in BMMCs, but that reduced FcepsilonRI expression and degranulation response was mediated in an LXR-independent manner.

Liver X receptor antagonist reduces lipid formation and increases glucose metabolism in myotubes from lean, obese and type 2 diabetic individuals

AIMS/HYPOTHESIS

Liver X receptors (LXRs) play important roles in lipid and carbohydrate metabolism. The purpose of the present study was to evaluate effects of the endogenous LXR agonist 22-R-hydroxycholesterol (22-R-HC) and its stereoisomer 22-S-hydroxycholesterol (22-S-HC), in comparison with the synthetic agonist T0901317 on lipid and glucose metabolism in human skeletal muscle cells (myotubes).

METHODS

Myotubes established from lean and obese control volunteers and from obese type 2 diabetic volunteers were treated with LXR ligands for 4 days. Lipid and glucose metabolisms were studied with labelled precursors, and gene expression was analysed using real-time PCR.

RESULTS

Treatment with T0901317 increased lipogenesis (de novo lipid synthesis) and lipid accumulation in myotubes, this increase being more pronounced in myotubes from type 2 diabetic volunteers than from lean volunteers. Furthermore, 22-S-HC efficiently counteracted the T0901317-induced enhancement of lipid formation. Moreover, synthesis of diacylglycerol, cholesteryl ester and free cholesterol from acetate was reduced below baseline by 22-S-HC, whereas glucose uptake and oxidation were increased. Both 22-S-HC and 22-R-HC, in contrast to T0901317, decreased the expression of genes involved in cholesterol synthesis, whereas only 22-R-HC, like T0901317, increased the expression of the gene encoding the reverse cholesterol transporter ATP-binding cassette subfamily A1 (ABCA1).

CONCLUSIONS/INTERPRETATION

T0901317-induced lipogenesis and lipid formation was more pronounced in myotubes from type 2 diabetic patients than from lean individuals. 22-S-HC counteracted these effects and reduced de novo lipogenesis below baseline, while glucose uptake and oxidation were increased.