Ji Hae Lee

Ursolic acid is a PPAR-α agonist that regulates hepatic lipid metabolism

In this study, we confirmed that ursolic acid, a plant triterpenoid, activates peroxisome proliferator-activated receptor (PPAR)-α in vitro. Surface plasmon resonance and time-resolved fluorescence resonance energy transfer analyses do not show direct binding of ursolic acid to the ligand-binding domain of PPAR-α; however, ursolic acid enhances the binding of PPAR-α to the peroxisome proliferator response element in PPAR-α-responsive genes, alters the expression of key genes in lipid metabolism, significantly reducing intracellular triglyceride and cholesterol concentrations in hepatocytes. Thus, ursolic acid is a PPAR-α agonist that regulates the expression of lipid metabolism genes, but it is not a direct ligand of PPAR-α.

Cyanidin is an agonistic ligand for peroxisome proliferator-activated receptor-alpha reducing hepatic lipid.

To investigate the underlying mechanism of targets of cyanidin, a flavonoid, which exhibits potent anti-atherogenic activities in vitro and in vivo, a natural chemical library that identified potent agonistic activity between cyanidin and peroxisome proliferator-activated receptors (PPAR) was performed. Cyanidin induced transactivation activity in all three PPAR subtypes in a reporter gene assay and time-resolved fluorescence energy transfer analyses. Cyanidin also bound directly to all three subtypes, as assessed by surface plasmon resonance experiments, and showed the greatest affinity to PPARα. These effects were confirmed by measuring the expression of unique genes of each PPAR subtype. Cyanidin significantly reduced cellular lipid concentrations in lipid-loaded steatotic hepatocytes. In addition, transcriptome profiling in lipid-loaded primary hepatocytes revealed that the net effects of stimulation with cyanidin on lipid metabolic pathways were similar to those elicited by hypolipidemic drugs. Cyanidin likely acts as a physiological PPARα agonist and potentially for PPARβ/δ and γ, and reduces hepatic lipid concentrations by rewiring the expression of genes involved in lipid metabolic pathways.

The natural carotenoid astaxanthin, a PPAR-α agonist and PPAR-γ antagonist, reduces hepatic lipid accumulation by rewiring the transcriptome in lipid-loaded hepatocytes

SCOPE A natural carotenoid abundant in seafood, astaxanthin (AX), has hypolipidemic activity, but its underlying mechanisms of action and protein targets are unknown. We investigated the molecular mechanism of action of AX in hepatic hyperlipidemia by measuring peroxisome proliferator-activated receptors (PPAR) activity. METHODS AND RESULTS We examined the binding of AX to PPAR subtypes and its effects on hepatic lipid metabolism. AX binding activated PPAR-α, but inhibited PPAR-γ transactivation activity in reporter gene assay and time-resolved fluorescence energy transfer analyses. AX had no effect on PPARδ/β transactivation. AX bound directly to PPAR-α and PPAR-γ with moderate affinity, as assessed by surface plasmon resonance experiments. The differential effects of AX on PPARs were confirmed by measuring the expression of unique responsive genes for each PPAR subtype. AX significantly reduced cellular lipid accumulation in lipid-loaded hepatocytes. Transcriptome analysis revealed that the net effects of stimulation with AX (100 μM) on lipid metabolic pathways were similar to those elicited by fenofibrate and lovastatin (10 μM each), with AX rewiring the expression of genes involved in lipid metabolic pathways. CONCLUSION AX is a PPAR-α agonist and PPAR-γ antagonist, reduces hepatic lipid accumulation by rewiring the transcriptome in lipid-loaded hepatocytes.

Limonene, a natural cyclic terpene, is an agonistic ligand for adenosine A2A receptors

Limonene is a major aromatic compound in essential oils extracted from citrus rind. The application of limonene, especially in aromatherapy, has expanded significantly, but its potential effects on cellular metabolism have been elusive. We found that limonene directly binds to the adenosine A(2A) receptor, which may induce sedative effects. Results from an in vitro radioligand binding assay showed that limonene exhibits selective affinity to A(2A) receptors. In addition, limonene increased cytosolic cAMP concentration and induced activation of protein kinase A and phosphorylation of cAMP-response element-binding protein in Chinese hamster ovary cells transfected with the human adenosine A(2A) receptor gene. Limonene also increased cytosolic calcium concentration, which can be achieved by the activation of adenosine A(2A) receptors. These findings suggest that limonene can act as a ligand and an agonist for adenosine A(2A) receptors.

Fucosterol Is a Selective Liver X Receptor Modulator That Regulates the Expression of Key Genes in Cholesterol Homeostasis in Macrophages, Hepatocytes, and Intestinal Cells

Fucosterol, a sterol that is abundant in marine algae, has hypocholesterolemic activity, but the mechanism underlying its effect is not clearly understood. Because data suggest that fucosterol can increase plasma high-density lipoprotein concentrations, we investigated whether it could activate liver X receptors (LXRs), critical transcription factors in reverse cholesterol transport. Fucosterol dose-dependently stimulated the transcriptional activity of both LXR-α and -β in a reporter gene assay, responses that were attenuated by the LXR antagonist As(2)O(3). Fucosterol also activated co-activator recruitment in cell-free time-resolved fluorescence resonance energy transfer analysis. In THP-1-derived macrophages, it induced the transcriptional activation of ABCA1, ABCG1, and ApoE, key genes in reverse cholesterol transport, and thereby significantly increased the efflux of cholesterol. Fucosterol also regulated intestinal NPC1L1 and ABCA1 in Caco-2 cells. Notably, fucosterol did not induce cellular triglyceride accumulation in HepG2 cells, primarily because of its upregulation of Insig-2a, which delays nuclear translocation of SREBP-1c, a key hepatic lipogenic transcription factor. These results suggest that fucosterol is a dual-LXR agonist that regulates the expression of key genes in cholesterol homeostasis in multiple cell lines without inducing hepatic triglyceride accumulation.

Taurine is a liver X receptor-α ligand and activates transcription of key genes in the reverse cholesterol transport without inducing hepatic lipogenesis.

SCOPE Taurine, which is abundant in seafood, has antiatherogenic activities in both animals and humans; however, its molecular target has been elusive. We examined whether taurine could activate liver X receptor-α (LXR-α), a critical transcription factor in the regulation of reverse cholesterol transport in macrophages. METHODS AND RESULTS Taurine bound directly to LXR-α in a reporter gene assay, time-resolved fluorescence resonance energy transfer analysis, and limited protease digestion experiment. Macrophage cells incubated with taurine showed reduced cellular cholesterol and induced medium cholesterol in a dose-dependent manner with the induction of ATP-binding cassette transporter A1 and G gene and protein expression. In hepatocytes, taurine significantly induced Insig-2a levels and delayed nuclear translocation of the sterol regulatory element-binding protein 1 (SREBP-1) protein, resulting in a dose-dependent reduction in the cellular lipid levels without inducing the expression of fatty acid synthesis genes. CONCLUSION Taurine is a direct LXR-α ligand, represses cholesterol accumulation, and modulates the expression of genes involved in reverse cholesterol transport in macrophages, without inducing hepatic lipogenesis. The induction of Insig-2a suppressed the nuclear translocation of SREBP-1c.

Linalool reduces the expression of 3-hydroxy-3-methylglutaryl CoA reductase via sterol regulatory element binding protein-2- and ubiquitin-dependent mechanisms

We investigated hypocholesterolemic mechanisms of linalool, an aromatic anti‐oxidative monoterpene, which is abundant in teas and essential oils. Oral administration of linalool to mice for 6 weeks significantly lowered total and low‐density lipoprotein cholesterol concentrations, and HMG‐CoA reductase protein expression (−46%; P < 0.05) by both transcriptional and posttranscriptional mechanisms. Linalool suppressed the gene expression of HMG‐CoA reductase by reducing the binding of SREBP‐2 to its promoter, as assessed by qPCR and chromatin immunoprecipitation, and by inducing ubiquitin‐dependent proteolysis of the HMG‐CoA reductase. These findings suggest that food molecules with a pleasant scent could exert beneficial metabolic effects through multiple mechanisms.

Barley intake induces bile acid excretion by reduced expression of intestinal ASBT and NPC1L1 in C57BL/6J mice

To investigate the hypocholesterolemic mechanism of barley in vivo, six-week-old C57BL/6J mice were fed a high-fat diet (HFD) or high-fat diet containing barley (HFD-B) for seven weeks. Total and LDL cholesterol concentrations were significantly reduced in the HFD-B group while fecal cholesterol and bile acid was increased. Real-time PCR and immunoblot analysis revealed the induction of FXR expression, which in turn suppressed the expression of ASBT and NPC1L1 in the HFD-B group compared with the controls. In the liver, the expression of HMG-CoA reductase was significantly reduced while LDL receptor expression was unaltered in the HFD-B group compared with the controls. Our data suggest that the hypocholesterolemic effects of barley are primarily the result of reduced dietary cholesterol uptake and bile acid resorption. Reduced expression of intestinal ASBT and NPC1L1 may play a key role in the regulation of dietary cholesterol and bile acid metabolism in mice consuming a diet containing barley.

Melissa officinalis Essential Oil Reduces Plasma Triglycerides in Human Apolipoprotein E2 Transgenic Mice by Inhibiting Sterol Regulatory Element-Binding Protein-1c–Dependent Fatty Acid Synthesis

We investigated the hypolipidemic effects of Melissa officinalis essential oil (MOEO) in human APOE2 transgenic mice and lipid-loaded HepG2 cells. Plasma TG concentrations were significantly less in APOE2 mice orally administered MOEO (12.5 μg/d) for 2 wk than in the vehicle-treated group. Cellular TG and cholesterol concentrations were also significantly decreased in a dose- (400 and 800 mg/L) and time- (12 and 24 h) dependent manner in HepG2 cells stimulated with MOEO compared with controls. Mouse hepatic transcriptome analysis suggested MOEO feeding altered several lipid metabolic pathways, including bile acid and cholesterol synthesis and fatty acid metabolism. In HepG2 cells, the rate of fatty acid oxidation, as assessed using [1-(14)C]palmitate, was unaltered; however, the rate of fatty acid synthesis quantified with [1-(14)C]acetate was significantly reduced by treatment with 400 and 800 mg/L MOEO compared with untreated controls. This reduction was due to the decreased expression of SREBP-1c and its responsive genes in fatty acid synthesis, including FAS, SCD1, and ACC1. Subsequent chromatin immunoprecipitation analysis further demonstrated that the binding of p300/CBP-associated factor, a coactivator of SREBP-1c, and histone H3 lysine 14 acetylation at the FAS, SCD1, and ACC1 promoters were significantly reduced in the livers of APOE2 mice and HepG2 cells treated with MOEO compared with their controls. Additionally, MOEO stimulation in HepG2 cells induced bile acid synthesis and reduced the nuclear form of SREBP-2, a key transcription factor in hepatic cholesterol synthesis. These findings suggest that the intake of phytochemicals with pleasant scent could have beneficial metabolic effects.

Ursolic acid improves lipid and glucose metabolism in high‐fat‐fed C57BL/6J mice by activating peroxisome proliferator‐activated receptor alpha and hepatic autophagy

SCOPE This study investigated metabolic effects of ursolic acid (UA), a peroxisome proliferation-activated receptor (PPAR)-α activator, in vivo. METHODS AND RESULTS High-fat diet (HFD)-fed C57BL/6J mice were orally administered UA (50 or 200 mg/kg body weight) for 8 wk. UA reduced liver and adipose tissue mass, adipocyte size, and plasma leptin concentrations, plasma triglyceride and low-density-lipoprotein cholesterol concentrations, while it elevated the high-density-lipoprotein cholesterol and adiponectin concentrations significantly compared with controls. UA induced the expression of PPARα and its responsive genes involved in fatty acid uptake and β-oxidation in the livers, whereas genes involved in lipogenesis, including sterol regulatory element-binding proteins-1c, were downregulated. UA administration improved glucose tolerance and insulin sensitivity significantly compared with the HFD-fed control livers. UA administration also activated hepatic autophagy as assessed by the expression of microtubule-associated protein 1A/1B-light chain 3 (LC3)-II and other key proteins in the autophagy pathway. CONCLUSION Our findings suggest that UA ameliorates lipid and glucose metabolism in HFD-fed mice primarily by the activation of PPARα and induction of the hepatic autophagy pathway. Thus, intake of UA in the diet or in an isolated form may ameliorate lipid and glucose metabolism.