Hepatic Lipid Metabolism in Experimental Diabetes

Mechanisms of action of incretin receptor based dual- and tri-agonists in pancreatic islets

Simultaneous activation of the incretin G-protein-coupled receptors (GPCRs) via unimolecular dual-receptor agonists (UDRA) has emerged as a new therapeutic approach for type 2 diabetes. Recent studies also advocate triple agonism with molecules also capable of binding the glucagon receptor. In this scoping review, we discuss the cellular mechanisms of action (MOA) underlying the actions of these novel and therapeutically important classes of peptide receptor agonists. Clinical efficacy studies of several UDRAs have demonstrated favorable results both as monotherapies and when combined with approved hypoglycemics. Although the additive insulinotropic effects of dual glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic peptide receptor (GIPR) agonism were anticipated based on the known actions of either glucagon-like peptide-1 (GLP-1) or glucose-dependent insulinotropic peptide (GIP) alone, the additional benefits from GCGR were largely unexpected. Whether additional synergistic or antagonistic interactions among these G-protein receptor signaling pathways arise from simultaneous stimulation is not known. The signaling pathways affected by dual- and tri-agonism require more trenchant investigation before a comprehensive understanding of the cellular MOA. This knowledge will be essential for understanding the chronic efficacy and safety of these treatments.

Hmgcs2-mediated ketogenesis modulates high-fat diet-induced hepatosteatosis

OBJECTIVE

Aberrant ketogenesis is correlated with the degree of steatosis in NAFLD patients, and an inborn error of ketogenesis (mitochondrial HMG-CoA synthase deficiency) is commonly associated with the development of the fatty liver. Here we aimed to determine the impact of Hmgcs2-mediated ketogenesis and its modulations on the development and treatment of fatty liver disease.

METHODS

Loss- and gain-of-ketogenic function through in vivo and in vitro models, achieved by Hmgcs2 knockout and overexpression, respectively, were examined to investigate the role of ketogenesis in the hepatic lipid accumulation during neonatal development and the diet-induced NAFLD mouse model.

RESULTS

Ketogenic function was decreased in NAFLD mice with a reduction in Hmgcs2 expression. Mice lacking Hmgcs2 developed spontaneous fatty liver phenotype during postnatal development, which was rescued by a shift to a low-fat dietary composition via early weaning. Hmgcs2 heterozygous mice, which exhibited reduced ketogenic activity, were more susceptible to diet-induced NAFLD development, whereas HMGCS2 overexpression in NAFLD mice improved hepatosteatosis and glucose homeostasis.

CONCLUSIONS

Our study adds new knowledge to the field of ketone body metabolism and shows that Hmgcs2-mediated ketogenesis modulates hepatic lipid regulation under a fat-enriched nutritional environment. The regulation of hepatic ketogenesis may be a viable therapeutic strategy in the prevention and treatment of hepatosteatosis.

LRRC19-A Bridge between Selenium Adjuvant Therapy and Renal Clear Cell Carcinoma: A Study Based on Datamining

Kidney renal clear cell carcinoma (KIRC) is the most common and fatal subtype of renal cancer. Antagonistic associations between selenium and cancer have been reported in previous studies. Selenium compounds, as anti-cancer agents, have been reported and approved for clinical trials. The main active form of selenium in selenoproteins is selenocysteine (Sec). The process of Sec biosynthesis and incorporation into selenoproteins plays a significant role in biological processes, including anti-carcinogenesis. However, a comprehensive selenoprotein mRNA analysis in KIRC remains absent. In the present study, we examined all 25 selenoproteins and identified key selenoproteins, glutathione peroxidase 3 (GPX3) and type 1 iodothyronine deiodinase (DIO1), with the associated prognostic biomarker leucine-rich repeat containing 19 (LRRC19) in clear cell renal cell carcinoma cases from The Cancer Genome Atlas (TCGA) database. We performed validations for the key gene expression levels by two individual clear cell renal cell carcinoma cohorts, GSE781 and GSE6344, datasets from the Gene Expression Omnibus (GEO) database. Multivariate survival analysis demonstrated that low expression of LRRC19 was an independent risk factor for OS. Gene set enrichment analysis (GSEA) identified tyrosine metabolism, metabolic pathways, peroxisome, and fatty acid degradation as differentially enriched with the high LRRC19 expression in KIRC cases, which are involved in selenium therapy of clear cell renal cell carcinoma. In conclusion, low expression of LRRC19 was identified as an independent risk factor, which will advance our understanding concerning the selenium adjuvant therapy of clear cell renal cell carcinoma.

Oleic Acid Attenuates Ang II (Angiotensin II)-Induced Cardiac Remodeling by Inhibiting FGF23 (Fibroblast Growth Factor 23) Expression in Mice

Plasma metabolic profiles were compared between patients with hypertension with and without left ventricular hypertrophy and significantly decreased oleic acid (OA) levels were observed in the peripheral blood of patients with hypertension with left ventricular hypertrophy. We sought to determine the effect and underlying mechanisms of OA on cardiac remodeling. In vitro studies with isolated neonatal mouse cardiomyocytes and cardiac fibroblasts revealed that OA significantly attenuated Ang II (angiotensin II)-induced cardiomyocyte growth and cardiac fibroblast collagen expression. In vivo, cardiac function, hypertrophic growth of cardiomyocytes, and fibrosis were analyzed after an Ang II (1000 ng/kg/minute) pump was implanted for 14 days. We found that OA could significantly prevent Ang II-induced cardiac remodeling in mice. RNA sequencing served as a gene expression roadmap highlighting gene expression changes in the hearts of Ang II-induced mice and OA-treated mice. The results revealed that FGF23 (fibroblast growth factor 23) expression was significantly upregulated in mouse hearts in response to Ang II infusion, which was significantly suppressed in the hearts of OA-treated mice. Furthermore, overexpression of FGF23 in the heart by injection of an AAV-9 vector aggravated Ang II-induced cardiac remodeling and impaired the protective effect of OA on cardiac remodeling. Further study found that OA could suppress Ang II-induced FGF23 expression by inhibiting the translocation of Nurr1 (nuclear receptor–related 1 protein) from the cytoplasm to the nucleus. Our findings suggest a novel role of OA in preventing Ang II-induced cardiac remodeling via suppression of FGF23 expression.

Endothelial lipase increases eNOS activating capacity of high-density lipoprotein

Endothelial lipase (EL) changes structural and functional properties of high-density lipoprotein (HDL). HDL is a relevant modulator of endothelial nitric oxide synthase (eNOS) activity, but the effect of EL on HDL induced eNOS-activation has not yet been investigated. Here, we examined the impact of EL-modified HDL (EL-HDL) on eNOS activity, subcellular trafficking, and eNOS- dependent vasorelaxation. EL-HDL and empty virus (EV)-HDL as control were isolated from human serum incubated with EL-overexpressing or EV infected HepG2 cells. EL-HDL exhibited higher capacity to induce eNOS phosphorylation at Ser1177 and eNOS activity in EA.hy 926 cells, as well as eNOS-dependent vasorelaxation of mouse aortic rings compared to control HDL. As revealed by confocal and structured illumination-microscopy EL-HDL-driven induction of eNOS was accompanied by an increased eNOS-GFP targeting to the plasma membrane and a lower eNOS-GFP colocalization with Golgi and mitochondria. Widefield microscopy of filipin stained cells revealed that EL-HDL lowered cellular free cholesterol (FC) and as found by thin-layer chromatography increased cellular cholesterol ester (CE) content. Additionally, cholesterol efflux capacity, acyl-coenzyme A: cholesterol acyltransferase activity, and HDL particle uptake were comparable between EL-HDL and control HDL. In conclusion, EL increases eNOS activating capacity of HDL, a phenomenon accompanied by an enrichment of the plasma membrane eNOS pool, a decreased cell membrane FC and increased cellular CE content.

Glucagon Control on Food Intake and Energy Balance

Glucagon exerts pleiotropic actions on energy balance and has emerged as an attractive target for the treatment of diabetes and obesity in the last few years. Glucagon reduces body weight and adiposity by suppression of appetite and by modulation of lipid metabolism. Moreover, this hormone promotes weight loss by activation of energy expenditure and thermogenesis. In this review, we cover these metabolic actions elicited by glucagon beyond its canonical regulation of glucose metabolism. In addition, we discuss recent developments of therapeutic approaches in the treatment of obesity and diabetes by dual- and tri-agonist molecules based on combinations of glucagon with other peptides. New strategies using these unimolecular polyagonists targeting the glucagon receptor (GCGR), have become successful approaches to evaluate the multifaceted nature of glucagon signaling in energy balance and metabolic syndrome.

Impaired ketogenesis and increased acetyl-CoA oxidation promote hyperglycemia in human fatty liver

Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent, and potentially morbid, disease that affects one-third of the U.S. population. Normal liver safely accommodates lipid excess during fasting or carbohydrate restriction by increasing their oxidation to acetyl-CoA and ketones, yet lipid excess during NAFLD leads to hyperglycemia and, in some, steatohepatitis. To examine potential mechanisms, flux through pathways of hepatic oxidative metabolism and gluconeogenesis were studied using five simultaneous stable isotope tracers in ketotic (24-hour fast) individuals with a wide range of hepatic triglyceride contents (0-52%). Ketogenesis was progressively impaired as hepatic steatosis and glycemia worsened. Conversely, the alternative pathway for acetyl-CoA metabolism, oxidation in the tricarboxylic (TCA) cycle, was upregulated in NAFLD as ketone production diminished and positively correlated with rates of gluconeogenesis and plasma glucose concentrations. Increased respiration and energy generation that occurred in liver when β-oxidation and TCA cycle activity were coupled may explain these findings, inasmuch as oxygen consumption was higher during fatty liver and highly correlated with gluconeogenesis. These findings demonstrate that increased glucose production and hyperglycemia in NAFLD is not a consequence of acetyl-CoA production per se, but how acetyl-CoA is further metabolized in liver.

Team Players or Opponents: Coadministration of Selective Glucagon and GLP-1 Receptor Agonists in Obese Diabetic Monkeys

We assessed the therapeutic contribution of the individual components of glucagon-like peptide-1 receptor (GLP-1R) and glucagon receptor (GCGR) agonists alone and in combination upon energy homeostasis and glycemic control in diet-induced obese, diabetic nonhuman primates. The pharmacological active dose ranges of selective agonists were established through a dose-finding study, followed by a 6-week chronic study. Repeated subcutaneous administration of a selective GCGR agonist (30 µg/kg once daily) did not affect food intake or body weight, whereas the selective GLP-1R agonist (3 µg/kg once daily) alone decreased energy intake by 18% and body weight by 3.8% ± 0.9%. Combination of both agonists reduced significantly cumulative food intake by 27% and body weight by 6.6% ± 0.9%. Fasting plasma glucose (FPG) was improved by GLP-1R agonist (baseline vs end of study, 176.7 ± 34.0 vs 115.9 ± 16.1 mg/dL). In contrast, groups exposed to GCGR agonist experienced nonsignificant elevations of FPG. More accurate assessment of therapeutic interventions on glucose homeostasis was tested by an IV glucose tolerance test. Glucose excursion was significantly elevated by chronic GCGR agonist administration, whereas it was significantly decreased in GLP-1R agonist-treated monkeys. In the combination group, a nonsignificant increase of glucose excursion was seen, concomitantly with significantly increased insulin secretion. We conclude that chronic glucagon agonism does not affect energy homeostasis in nonhuman primates. In combination with GLP-1R agonism, glucagon agonism synergistically enhances negative energy balance with resulting larger body weight loss. However, adding GCGR to GLP-1R agonism diminishes glycemic control in diabetic monkeys. Therefore, long-term therapeutic implications of using GLP-1R/GCGR coagonists for weight management in diabetes warrants further scrutiny.

Naturally occurring 3RS, 7R, 11R-phytanic acid suppresses in vitro T-cell production of interferon-gamma

BACKGROUND

Among the eight stereoisomers of phytanic acid (PA), the 3RS, 7R, 11R-isomer is naturally occurring and is present in foods and the human body. PA is considered to have possible health benefits in the immune system. However, it remains undetermined whether these effects are elicited by the 3RS, 7R, 11R-PA isomer, because previous studies used a commercially available PA whose isomer configuration is unknown. In this study, we synthesized a preparation of 3RS, 7R, 11R-PA, and investigated its in vitro immunomodulatory effects, especially the T-cell production of interferon (IFN)-γ, which is associated with various autoimmune diseases. This study also investigated the effects of 3RS, 7R, 11R-PA on NF-κB activity in order to address the mechanism of its immunomodulatory effects.

METHODS

Mouse splenocytes and purified T-cells were stimulated with T-cell mitogens and incubated with 3RS, 7R, 11R-PA, followed by evaluation of IFN-γ production. The effect of 3RS, 7R, 11R-PA on NF-κB activity was also investigated using an A549 cell line with stable expression of an NF-κB-dependent luciferase reporter gene.

RESULTS

3RS, 7R, 11R-PA significantly reduced in vitro IFN-γ production at both the protein and mRNA levels, and was accompanied by decreased expression of T-bet, a key regulator of Th1 cell differentiation. The results indicated that NF-κB-mediated transcriptional activity was significantly decreased by 3RS, 7R, 11R-PA and that GW6471, an antagonist of peroxisome proliferator activated receptor α (PPARα), abrogated the inhibitory effect of 3RS, 7R, 11R-PA on NF-κB activity.

CONCLUSIONS

The present study suggests that 3RS, 7R, 11R-PA is a functional and bioactive fatty acid, and has a potentially beneficial effect for amelioration of T-cell mediated autoimmune diseases. This study also indicates that interference in the NF-κB pathway via PPARα activation is a potential mechanism of the immunomodulatory effects of 3RS, 7R, 11R-PA.

Comparison of the Effect of trans Fatty Acid Isomers on Apolipoprotein A1 and B Secretion in HepG2 Cells

Intake of trans fatty acid (TFA) is believed to change the ratio of low-density lipoprotein (LDL) to high density lipoprotein (HDL) cholesterol in blood, which leads to cardiovascular disease. In this study, thirteen types of TFA including monoene type TFA (trans-octadecenoic fatty acid isomers, t-18:1 isomers), diene type TFA (t9,t12-18:2), and triene type TFA (t-18:3) were added to cultured HepG2 cells to compare the amount of apolipoprotein A1 and B (those relating to levels of HDL and LDL cholesterol in blood, respectively) being secreted. We found that trans-5-18:1 increased the secretion of apolipoprotein B relative to oleic acid (cis-9-18:1, control). Secretion of apolipoprotein B was also increased by t-18:3; however, the amount was not significant compared with that observed in the control. The secretion amount of apolipoprotein B tended to increase with the number of double bonds in TFA among trans-9-18:1, t9,t12-18:2, and t-18:3. The secretion amount of apolipoprotein A1 after TFA treatment was also measured. No significant difference was detected among t-18:1 groups; however, t-18:3 increased the amount significantly compared to that in the control. These results suggest that the effect of TFA isomers on the ratio of LDL to HDL cholesterol in the blood follows a mechanism different from that in cultured cells.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Role of Tribbles Pseudokinase 1 (TRIB1) in human hepatocyte metabolism

Genome-wide association studies for plasma triglycerides and hepatic steatosis identified a risk locus on chromosome 8q24 close to the TRIB1 gene, encoding Tribbles Pseudokinase 1 (TRIB1). In previous studies conducted in murine models, hepatic over-expression of Trib1 was shown to increase fatty acid oxidation and decrease triglyceride synthesis whereas Trib1 knockdown mice exhibited hypertriglyceridemia. Here we have examined the impact of TRIB1 suppression in human and mouse hepatocytes. Examination of a panel of lipid regulator transcripts revealed species-specific effects, prompting us to focus on human models for the remainder of the study. Acute knockdown of TRIB1 in human primary hepatocytes resulted in decreased expression of MTTP and APOB, required for very low density lipoprotein (VLDL) assembly although particle secretion was not significantly affected. A parallel analysis performed in HepG2 revealed reduced MTTP, but not APOB, protein as a result of TRIB1 suppression. Global gene expression changes of human primary hepatocytes upon TRIB1 suppression were analyzed by clustering algorithms and found to be consistent with dysregulation of several pathways fundamental to liver function, including altered CEBPA and B transcript levels and impaired glucose handling. Indeed, TRIB1 expression in HepG2 cells was found to be inversely proportional to glucose concentration. Lastly TRIB1 downregulation in primary hepatocytes was associated with suppression of the HNF4A axis. In HepG2 cells, TRIB1 suppression resulted in reduced HNF4A protein levels while HNF4A suppression increased TRIB1 expression. Taken together these studies reveal an important role for TRIB1 in human hepatocyte biology.

Fatty acid oxidation in normotriglyceridemic men

BACKGROUND

Moderate hypertriglyceridemia is frequently associated with central obesity, insulin resistance, and atherogenic dyslipidemia. We showed previously that moderately obese men with hypertriglyceridemia have reduced fatty acid oxidation postabsorptively and postprandially. In the present study, we examined the oxidation of fatty acids in normotriglyceridemic men.

OBJECTIVE

The study objective was to determine the relation between plasma triglyceride levels and fatty acid oxidation in normotriglyceridemic men.

STUDY DESIGN

Twenty-four healthy, nonobese White and African American men participated in a cross-sectional metabolic study for evaluation of fatty acid oxidation. Men were healthy, and none took hypolipidemic or hypoglycemic agents. They ingested 200 mg of fat/hour/kg of body weight over a 10-hour period. Plasma levels of triglyceride, nonesterified fatty acids, 3-β-hydroxybutyrate, insulin, and glucagon were measured postabsorptively and postprandially. Chylomicron-triglyceride halflife was also calculated.

RESULTS

Nonobese White and African-American men had similar anthropometry, levels of plasma triglyceride, lipoprotein cholesterol, nonesterified fatty acids, 3-β-hydroxybutyrate, insulin, and glucagon postabsorptively and postprandially. For the group as a whole, there was a positive and significant correlation between plasma fatty acids and 3-β-hydroxybutyrate and an inverse association between plasma triglyceride levels and 3-β-hydroxybutyrate at baseline. All subjects had increased levels of metabolites of interest postprandially. However, there were no significant changes in plasma insulin, glucagon, or the ratio of insulin to glucagon. The postprandial levels of 3-β-hydroxybutyrate correlated positively with nonesterified fatty acids and inversely with the half-life of chylomicron triglyceride.

CONCLUSION

Normotriglyceridemia is strongly associated with oxidation of fatty acids by the liver suggesting the possibility that the fatty acid oxidation pathway is a potential target of intervention to prevent hypertriglyceridemia and concomitant fatty liver.

Oleic acid increases mitochondrial reactive oxygen species production and decreases endothelial nitric oxide synthase activity in cultured endothelial cells

Elevated plasma levels of free fatty acids (FFA) are associated with increased cardiovascular risk. This may be related to FFA-induced elevation of oxidative stress in endothelial cells. We hypothesized that, in addition to mitochondrial production of reactive oxygen species, endothelial nitric oxide synthase (eNOS)-mediated reactive oxygen species production contributes to oleic acid (OA)-induced oxidative stress in endothelial cells, due to eNOS uncoupling. We measured reactive oxygen species production and eNOS activity in cultured endothelial cells (bEnd.3) in the presence of OA bound to bovine serum albumin, using the CM-H2DCFDA assay and the L-arginine/citrulline conversion assay, respectively. OA induced a concentration-dependent increase in reactive oxygen species production, which was inhibited by the mitochondrial complex II inhibitor thenoyltrifluoroacetone (TTFA). OA had little effect on eNOS activity when stimulated by a calcium-ionophore, but decreased both basal and insulin-induced eNOS activity, which was restored by TTFA. Pretreatment of bEnd.3 cells with tetrahydrobiopterin (BH4) prevented OA-induced reactive oxygen species production and restored inhibition of eNOS activity by OA. Elevation of OA levels leads to both impairment in receptor-mediated stimulation of eNOS and to production of mitochondrial-derived reactive oxygen species and hence endothelial dysfunction.

ω-Alkynyl lipid surrogates for polyunsaturated fatty acids: free radical and enzymatic oxidations

Lipid and lipid metabolite profiling are important parameters in understanding the pathogenesis of many diseases. Alkynylated polyunsaturated fatty acids are potentially useful probes for tracking the fate of fatty acid metabolites. The nonenzymatic and enzymatic oxidations of ω-alkynyl linoleic acid and ω-alkynyl arachidonic acid were compared to that of linoleic and arachidonic acid. There was no detectable difference in the primary products of nonenzymatic oxidation, which comprised cis,trans-hydroxy fatty acids. Similar hydroxy fatty acid products were formed when ω-alkynyl linoleic acid and ω-alkynyl arachidonic acid were reacted with lipoxygenase enzymes that introduce oxygen at different positions in the carbon chains. The rates of oxidation of ω-alkynylated fatty acids were reduced compared to those of the natural fatty acids. Cyclooxygenase-1 and -2 did not oxidize alkynyl linoleic but efficiently oxidized alkynyl arachidonic acid. The products were identified as alkynyl 11-hydroxy-eicosatetraenoic acid, alkynyl 11-hydroxy-8,9-epoxy-eicosatrienoic acid, and alkynyl prostaglandins. This deviation from the metabolic profile of arachidonic acid may limit the utility of alkynyl arachidonic acid in the tracking of cyclooxygenase-based lipid oxidation. The formation of alkynyl 11-hydroxy-8,9-epoxy-eicosatrienoic acid compared to alkynyl prostaglandins suggests that the ω-alkyne group causes a conformational change in the fatty acid bound to the enzyme, which reduces the efficiency of cyclization of dioxalanyl intermediates to endoperoxide intermediates. Overall, ω-alkynyl linoleic acid and ω-alkynyl arachidonic acid appear to be metabolically competent surrogates for tracking the fate of polyunsaturated fatty acids when looking at models involving autoxidation and oxidation by lipoxygenases.

An essential requirement for the SCAP/SREBP signaling axis to protect cancer cells from lipotoxicity

The sterol regulatory element-binding proteins (SREBP) are key transcriptional regulators of lipid metabolism and cellular growth. It has been proposed that SREBP signaling regulates cellular growth through its ability to drive lipid biosynthesis. Unexpectedly, we find that loss of SREBP activity inhibits cancer cell growth and viability by uncoupling fatty acid synthesis from desaturation. Integrated lipid profiling and metabolic flux analysis revealed that cancer cells with attenuated SREBP activity maintain long-chain saturated fatty acid synthesis, while losing fatty acid desaturation capacity. We traced this defect to the uncoupling of fatty acid synthase activity from stearoyl-CoA desaturase 1 (SCD1)-mediated desaturation. This deficiency in desaturation drives an imbalance between the saturated and monounsaturated fatty acid pools resulting in severe lipotoxicity. Importantly, replenishing the monounsaturated fatty acid pool restored growth to SREBP-inhibited cells. These studies highlight the importance of fatty acid desaturation in cancer growth and provide a novel mechanistic explanation for the role of SREBPs in cancer metabolism.

Palmitoleic (16:1 cis-9) and cis-vaccenic (18:1 cis-11) acid alter lipogenesis in bovine adipocyte cultures

Our objectives were to: (1) confirm elongation products of palmitoleic acid (16:1 cis-9) elongation in vitro using stable isotopes and (2) evaluate if exogenous supplementation of palmitoleic acid, elongation products, or both are responsible for decreased desaturation and lipogenesis rates observed with palmitoleic acid supplementation in bovine adipocytes. Stromal vascular cultures were isolated from adipose tissue of two beef carcasses, allowed to reach confluence, held for 2 days, and differentiated with a standard hormone cocktail (day 0). On day 2, secondary differentiation media containing 1 of 4 fatty acid treatments [0 μM fatty acid (control), or 150 μM palmitic (16:0), palmitoleic, or cis-vaccenic (18:1 cis-11)] was added for 4 days. On day 6, cells were incubated with [(13)C] 16:1, [(13)C] 2, or [(13)C] 18:0 to estimate elongation, lipogenic, and desaturation rates using gas chromatography-mass spectrometry. Enrichment of [(13)C] 18:1 cis-11 confirmed 18:1 cis-11 is an elongation product of 16:1. Additionally, [(13)C] label was seen in 20:1 cis-13 and cis-9, cis-11 CLA. Synthesis of [(13)C] 16:0 from [(13)C] 2 was reduced (P < 0.05) in palmitoleic acid and cis-vaccenic acid-treated compared with control cells following 36 h incubation. By 12 h of [(13)C] 18:0 incubation, cells supplemented with palmitoleic acid had reduced (P < 0.05) [(13)C] 18:1 cis-9 compared with all other treatments. Gene expression and fatty acid results support isotopic data for lipogenesis and desaturation. Therefore, palmitoleic acid is actively elongated in vitro and its elongation product, cis-vaccenic acid, can also reduce lipogenesis. However, inhibition of desaturation can be directly attributed to palmitoleic acid and not its elongation products, 18:1 cis-11 or 20:1 cis-13.

Screening of soy protein-derived hypotriglyceridemic di-peptides in vitro and in vivo

Background

Soy protein and soy peptides have attracted considerable attention because of their potentially beneficial biological properties, including antihypertensive, anticarcinogenic, and hypolipidemic effects. Although soy protein isolate contains several bioactive peptides that have distinct physiological activities in lipid metabolism, it is not clear which peptide sequences are responsible for the triglyceride (TG)-lowering effects. In the present study, we investigated the effects of soy protein-derived peptides on lipid metabolism, especially TG metabolism, in HepG2 cells and obese Otsuka Long-Evans Tokushima fatty (OLETF) rats.

Results

In the first experiment, we found that soy crude peptide (SCP)-LD3, which was prepared by hydrolyze of soy protein isolate with endo-type protease, showed hypolipidemic effects in HepG2 cells and OLETF rats. In the second experiment, we found that hydrophilic fraction, separated from SCP-LD3 with hydrophobic synthetic absorbent, revealed lipid-lowering effects in HepG2 cells and OLETF rats. In the third experiment, we found that Fraction-C (Frc-C) peptides, fractionated from hydrophilic peptides by gel permeation chromatography-high performance liquid chromatography, significantly reduced TG synthesis and apolipoprotein B (apoB) secretion in HepG2 cells. In the fourth experiment, we found that the fraction with 0.1% trifluoroacetic acid, isolated from Frc-C peptides by octadecylsilyl column chromatography, showed hypolipidemic effects in HepG2 cells. In the final experiment, we found that 3 di-peptides, Lys-Ala, Val-Lys, and Ser-Tyr, reduced TG synthesis, and Ser-Tyr additionally reduced apoB secretion in HepG2 cells.

Conclusion

Novel active peptides with TG-lowering effects from soy protein have been isolated.

Effects of citrus auraptene (7-geranyloxycoumarin) on hepatic lipid metabolism in vitro and in vivo

Recent reports have shown that citrus auraptene (7-geranyloxycoumarin) possesses valuable pharmacological properties, including anticarcinogenic, anti-inflammatory, antihelicobacter, antigenotoxic, and neuroprotective effects. In the present study, we investigated the effect of dietary auraptene on hepatic lipid metabolism both in vitro and in vivo. Results suggested that auraptene has the ability to normalize lipid abnormalities in HepG2 hepatocytes. After 4 weeks of auraptene feeding, abdominal white adipose tissue weight and hepatic triglyceride (TG) levels were dose-dependently lowered in Otsuka Long-Evans Tokushima fatty (OLETF) rats. The activities of carnitine palmitoyltransferase, a key enzyme in mitochondrial fatty acid β-oxidation, and peroxisomal β-oxidation were markedly and dose-dependently enhanced in OLETF rat livers by auraptene feeding. Additionally, hepatic expression of acyl-CoA oxidase, the initial enzyme of the peroxisomal β-oxidation system, was significantly and dose-dependently enhanced by auraptene administration. These results suggest that auraptene administration alleviates obesity and hepatic TG accumulation in part through lipolysis enhancement in the livers of obese OLETF rats.

Inhibition of acyl-coenzyme A:cholesterol acyltransferase 2 (ACAT2) prevents dietary cholesterol-associated steatosis by enhancing hepatic triglyceride mobilization

Acyl-CoA:cholesterol O-acyl transferase 2 (ACAT2) promotes cholesterol absorption by the intestine and the secretion of cholesteryl ester-enriched very low density lipoproteins by the liver. Paradoxically, mice lacking ACAT2 also exhibit mild hypertriglyceridemia. The present study addresses the unexpected role of ACAT2 in regulation of hepatic triglyceride (TG) metabolism. Mouse models of either complete genetic deficiency or pharmacological inhibition of ACAT2 were fed low fat diets containing various amounts of cholesterol to induce hepatic steatosis. Mice genetically lacking ACAT2 in both the intestine and the liver were dramatically protected against hepatic neutral lipid (TG and cholesteryl ester) accumulation, with the greatest differences occurring in situations where dietary cholesterol was elevated. Further studies demonstrated that liver-specific depletion of ACAT2 with antisense oligonucleotides prevents dietary cholesterol-associated hepatic steatosis both in an inbred mouse model of non-alcoholic fatty liver disease (SJL/J) and in a humanized hyperlipidemic mouse model (LDLr(-/-), apoB(100/100)). All mouse models of diminished ACAT2 function showed lowered hepatic triglyceride concentrations and higher plasma triglycerides secondary to increased hepatic secretion of TG into nascent very low density lipoproteins. This work demonstrates that inhibition of hepatic ACAT2 can prevent dietary cholesterol-driven hepatic steatosis in mice. These data provide the first evidence to suggest that ACAT2-specific inhibitors may hold unexpected therapeutic potential to treat both atherosclerosis and non-alcoholic fatty liver disease.

Quantitative proteomic analysis reveals the perturbation of multiple cellular pathways in HL-60 cells induced by arsenite treatment

Arsenic is ubiquitously present in the environment; it is a known human carcinogen and paradoxically it is also a successful drug for the clinical remission of acute promyelocytic leukemia (APL). The cellular responses induced by arsenite treatment have been investigated for years; however, the precise mechanisms underlying its cytotoxicity and therapeutic activity remain unclear. Here we report the use of mass spectrometry together with stable isotope labeling by amino acids in cell culture (SILAC) for the comparative study of protein expression in HL-60 cells that were untreated or treated with a clinically relevant concentration of arsenite. Our results revealed that, among the 1067 proteins quantified in both forward and reverse SILAC measurements, 56 had significantly altered levels of expression induced by arsenite treatment. These included the up-regulation of core histones, neutrophil elastase, alpha-mannosidase as well as the down-regulation of fatty acid synthase and protein phosphatase 1 alpha. We further demonstrated that the arsenite-induced growth inhibition of HL-60 cells could be rescued by treatment with palmitate, the final product of fatty acid synthase, supporting that arsenite exerts its cytotoxic effect, in part, via suppressing the expression of fatty acid synthase and inhibiting the endogenous production of fatty acid. The results from the present study offered important new knowledge for gaining insights into the molecular mechanisms of action of arsenite.

The sulfated polysaccharide porphyran reduces apolipoprotein B100 secretion and lipid synthesis in HepG2 cells

The physiological effect of porphyran, a sulfated polysaccharides from an edible red alga, was studied in human hepatoma HepG2 cells. Porphyran supplementation significantly decreased apolipoprotein B100 secretion, and the reduction was partly associated with suppression of cellular lipid synthesis in HepG2 cells. This is the first study to elucidate the mechanism of the hypolipidemic effect of porphyran.

Effects of free fatty acids, insulin, glucagon and adrenaline on ketone body production in humans

In normal human subjects, when plasma insulin, glucagon and growth hormone were 'clamped' at basal concentrations (by infusion of somatostatin plus replacement infusion of these hormones), infusion of Intralipid and heparin increased plasma free fatty acid (FFA) concentrations to approx. 1.3 mM, and ketone body production increased 4-5 fold to approx. 11 mumol . kg -1 . min-1. Hyperglucagonaemia did not further increase ketogenesis. In conditions of combined insulin and glucagon deficiency (by infusion of somatostatin without insulin and glucagon), administration of Intralipid and heparin increased plasma FFA concentrations to approx. 2.2 mM but a further increase in ketone body production did not accompany this increase. In these conditions hyperglucagonaemia increased ketogenesis by 2-3 fold the increment seen in control studies. Infusion of adrenaline (epinephrine) in conditions in which insulin secretion was not inhibited caused only a transient increase in plasma FFA concentrations and in ketone body production. These data indicate: (1) that in humans increased FFA availability can markedly augment ketogenesis in the absence of insulin deficiency and without hyperglucagonaemia; (2) that glucagon can increase ketone body production during insulin deficiency but not in its absence; and (3) that insulin deficiency may be accompanied by increased ketogenesis only because of a lack of its restraint on lipolysis and because of the action of glucagon. Glucagon may be important in determining the magnitude of ketone body production for a given degree of FFA availability and insulin deficiency, and may be necessary for attainment of maximal rates of ketogenesis. Adrenaline increases ketone body production in humans, but whether this is primarily due to a direct effect on the liver or is mediated through enhancement of lipolysis remains to be determined.

The trans-10, cis-12 isomer of conjugated linoleic acid decreases adiponectin assembly by PPARgamma-dependent and PPARgamma-independent mechanisms

The adipocyte-derived secretory protein adiponectin functions as an insulin-sensitizing agent. In plasma, adiponectin exists as low, medium, and high molecular weight oligomers. Treatment with trans-10, cis-12 conjugated linoleic acid (t-10, c-12 CLA) reduces levels of adiponectin as well as triglyceride (TG) in mice and adipocyte cell culture models. The aim of this study was to determine whether the effects of t-10, c-12 CLA on adiponectin and TG are mediated through modulation of the transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma). 3T3-L1 cells were treated either during or after differentiation into adipocytes with 100 microM t-10, c-12 CLA with or without 10 microM troglitazone, a PPARgamma agonist, or 1 microM GW9662, a PPARgamma antagonist, and adiponectin and TG levels were analyzed. Treatment with t-10, c-12 CLA reduced TG as well as cellular and secreted adiponectin levels and impaired the assembly of adiponectin oligomers. These changes were accompanied by decreases in PPARgamma mass. Troglitazone was able to reverse the t-10, c-12 CLA-mediated decrease in TG levels and restore the assembly of adiponectin oligomers but was unable to restore adiponectin synthesis. Conversely, treatment with GW9662 decreased TG mass and impaired adiponectin oligomer assembly but did not decrease total adiponectin mass. In a reporter assay, t-10, c-12 CLA appeared to be a partial PPARgamma agonist and prevented the stimulation of reporter activity by troglitazone. Therefore, the t-10, c-12 CLA isomer appears to alter adipocyte adiponectin metabolism through PPARgamma-dependent and PPARgamma-independent mechanisms.

Lack of UCP2 reduces Fas-mediated liver injury in ob/ob mice and reveals importance of cell-specific UCP2 expression

Fatty liver is vulnerable to conditions that challenge hepatocellular energy homeostasis. Lipid-laden hepatocytes highly express uncoupling protein-2 (UCP2), a mitochondrial carrier that competes with adenosine triphosphate (ATP) synthesis by mediating proton leak. However, evidence for a link between UCP2 expression and susceptibility of liver to acute injury is lacking. We asked whether absence of UCP2 protects ob/ob mice from Fas-mediated acute liver damage. UCP2-deficient ob/ob mice (ob/ob:ucp2-/-) and UCP2-competent littermates (ob/ob:ucp2+/+) received a single dose of agonistic anti-Fas antibody (Jo2). Low-dose Jo2 (0.15 mg/kg intraperitoneally) caused less serum alanine aminotransferase (ALT) elevation and lower apoptosis rates in ob/ob:ucp2-/- mice. High-dose Jo2 (0.40 mg/kg intraperitoneally) proved uniformly fatal; however, ob/ob:ucp2-/- mice survived longer with less depletion of liver ATP stores, indicating that fatty hepatocytes may benefit from lack of UCP2 during Jo2 challenge. Although UCP2 reportedly controls mitochondrial oxidant production, its absence had no apparent effect on fatty liver tissue malondialdehyde levels augmented by Jo2. This finding prompted us to determine UCP2 expression in Kupffer cells, a major source of intrahepatic oxidative stress. UCP2 expression was found diminished in Kupffer cells of untreated ob/ob:ucp2+/+ mice, conceivably contributing to increased oxidative stress in fatty liver and limiting the impact of UCP2 ablation. In conclusion, whereas UCP2 abundance in fatty hepatocytes exacerbates Fas-mediated injury by compromising ATP stores, downregulation of UCP2 in Kupffer cells may account for persistent oxidative stress in fatty liver. Our data support a cell-specific approach when considering the therapeutic effects of mitochondrial uncoupling in fatty liver disease.

Protease Inhibitor Effects on Triglyceride Synthesis and Adipokine Secretion in Human Omental and Subcutaneous Adipose Tissue

Objective

Significant advances in the treatment of the morbidity and mortality associated with AIDS are also associated with undesirable side-effects in fat redistribution (lipodystrophy), insulin resistance and cardiovascular risk, which is directly linked to protease inhibitor (PI) treatment.

Methods

The effects of four different PIs on triglyceride (TG) storage and adipokine production (leptin, adiponectin, and acylation stimulating protein [ASP]) in omental (OM) and subcutaneous (SC) adipose tissues were examined.

Results

Initial results demonstrated that saquinivir (SQV) and ritonivir (RTV) had little observed effect on de novo TG synthesis ([3H]glucose incorporation into TG) or fatty acid re-esterification ([14C]oleate incorporation into TG), whereas amprenivir (APV) and indinivir (IDV) reduced TG synthesis, especially in SC tissue up to 30 ±5.8% P<0.05 and 46 ±7.8% P<0.001, at 20 μM, respectively. There was no observed effect on phospholipid synthesis, tissue protein or toxicity. Only APV and IDV decreased leptin and adiponectin secretion in SC tissue, in a time- and concentration-dependent manner: at 18 h, leptin was inhibited by 54 ±3.1% ( P<0.001) and 44 ±6.4% ( P<0.001) by APV and IDV (40 μM), respectively, and adiponectin was inhibited by 35 ±5.6%( P<0.001) and 25 ±12.3% ( P<0.05) by APV and IDV (40 μM), respectively. By contrast, both IDV and APV decreased ASP secretion in OM tissues by a maximum of 53 ±7.8% ( P<0.001) and 59 ±5.9% ( P<0.001), respectively, and by a maximum of 86 ±1.6% ( P<0.001) and 72 ±4% ( P<0.001), respectively, in SC tissues.

Conclusion

PI have a direct effect on human adipose tissue which are site, PI and adipokine specific; these effects may contribute to the overall adipose imbalance and development of lipodystrophy, and metabolic syndrome in HIV-positive individuals.

Effects of fatty acids on skeletal muscle cell differentiation in vitro

Previous studies have shown stimulatory effects of linoleic acid (LA, C18:2) on differentiation of rat muscle cells in culture (Allen et al. 1985), but there appears to be little investigation of the effects of other fatty acids. The present study therefore compared the effects of different fatty acids on muscle cell differentiation in vitro. L6 myoblasts were cultured (Dulbecco's Modified Eagles Medium + 10 % fetal calf serum) in six-well plates until 80 % confluent (day 0). Cells were then either harvested or the medium switched to differentiation medium (Dulbecco's Modified Eagles Medium+2 % horse serum), supplemented with fatty acid or drug treatments. Cells were harvested on days 0-5 and assayed for creatine kinase (CK), protein and DNA contents, to give a measure of differentiation (CK/DNA). Initial studies indicated a stimulatory effect of the cis9,trans11 (c9,t11) isomer of conjugated linoleic acid (CLA) relative to control. By contrast, the trans10,cis12 (t10,c12) isomer of CLA inhibited differentiation. Further experiments indicated that inhibition of differentiation by the t10,c12 CLA isomer was dose-dependent (up to 200 microm) and may be via increased cell proliferation. LA and c9,t11 CLA stimulated differentiation at low concentrations (up to 50 microm), but inhibited differentiation at high concentrations (200 microm). In contrast, oleic acid stimulated differentiation at all concentrations, whereas the saturated fatty acid, palmitic acid, had no effect. The mechanism appeared not to involve either peroxisome proliferator-activated receptors alpha or gamma. The data suggest that only unsaturated fatty acids have an effect and the presence or absence of a cis-9 double bond may be important.

Role of adiponectin in the protective action of dietary saturated fat against alcoholic fatty liver in mice

The protective effect of dietary saturated fatty acids against the development of alcoholic liver disease has long been known, but the underlying mechanism is not completely understood. We examined the involvement of the adipocyte hormone adiponectin. Circulating adiponectin levels were significantly elevated by chronic ethanol administration to mice consuming a diet high in saturated fat. The increase in circulating adiponectin was associated with the activation a set of hepatic signaling pathways mediated through AMP-activated protein kinase, PPAR-alpha, and PPAR-gamma coactivator alpha, which in turn led to markedly increased rates of fatty acid oxidation, prevention of hepatic steatosis, and alleviation of liver enzyme changes. Furthermore, treatment of rat 3T3-L1 adipocytes with saturated fatty acids (palmitic or stearic acids) in the presence of ethanol increased secretion of adiponectin and enhanced activity of a mouse adiponectin promoter. In conclusion, the protective action of saturated fat against the development of alcoholic fatty liver in mice is partially mediated through induction of adiponectin. The present findings suggest a novel paradigm for dietary fatty acids in the pathogenesis of alcoholic liver disease and provide a promising therapeutic strategy-nutritional modulation of adiponectin-in treating human alcoholic fatty liver disease.

Maternal dietary iron restriction modulates hepatic lipid metabolism in the fetuses

Maternal dietary Fe restriction reduced fasting plasma cholesterol and triglyceride (TG) concentrations in the fetuses, as well as decreased plasma TG levels in the adult offspring. To investigate how maternal Fe restriction was affecting fetal lipid metabolism, we investigated whether there were changes in liver lipid metabolism in the full-term fetuses. There was a approximately 27% (P < 0.05) increase in cholesterol but approximately 29% reduction (P = 0.01) in TG concentrations in the liver of the Fe-restricted fetuses. Hepatic mRNA levels of cholesterol 7alpha hydroxylase and liver X receptor-alpha (LXRalpha) were reduced by approximately 50% (P < 0.01) and approximately 34% (P < 0.01), respectively. As LXRalpha regulates expression of sterol response element binding protein-1c (SREBP-1c) expression, we measured SREBP-1c expression. There was an approximately 43% (P < 0.001) reduction in mRNA levels of SREBP-1c and its response genes, including acetyl-CoA carboxylase by approximately 35% (P = 0.01), fatty acid synthase by approximately 18% (P = 0.05), and diacylglycerol acyltransferase by approximately 19% (P = 0.03). Furthermore, protein levels of CD36 were reduced by approximately 27% (P = 0.02) in Fe-restricted fetuses. In conclusion, changes in liver cholesterol and TG concentrations in Fe-restricted fetuses may be coordinated through reduced expression of heme-containing cholesterol 7alpha hydroxylase and its regulator LXRalpha, mainly via downregulation of expression of genes in bile acid synthesis and fatty acid synthesis pathways.

The transport of triglycerides through the secretory pathway of hepatocytes is impaired in apolipoprotein E deficient mice

BACKGROUND/AIMS

Apolipoprotein E (apoE)-deficient mice develop hepatic steatosis and secrete reduced levels of VLDL-TG.

METHODS AND RESULTS

We examined the effects of apoE-deficiency on intracellular lipid homeostasis and secretion of triglycerides (TG). We show that intracellular TG turnover and activities of diacylglycerol acyltransferase (DGAT) and microsomal triglyceride transfer protein (MTP) are similar in Apoe(-/-) and wild type mice. In addition, apoB synthesis was not decreased in Apoe(-/-) cells. Thus, the accumulation of lipid in these cells is not attributable to perturbed TG turnover, apoB synthesis, and the activities of DGAT and MTP. Inhibition of MTP had a more profound impact on the secretion of VLDL-TG from wild type hepatocytes than Apoe(-/-) hepatocytes, indicating that MTP was more limiting for the production of VLDL-TG from wild type cells. In marked contrast to the MTP-deficient model of fatty liver, electron microscopy of lipid-stained liver sections of Apoe(-/-) mice revealed an accumulation of lipid in numerous small, putative ER-derived vesicles and in the cytosol. No abnormalities were observed in the Golgi of Apoe(-/-) mice.

CONCLUSIONS

These results suggest that the removal of lipids from the early or intermediary compartments of the secretory pathway of hepatocytes is impaired in Apoe(-/-) mice.

Effects of troglitazone on intracellular cholesterol distribution and cholesterol-dependent cell functions in MA-10 Leydig tumor cells

Troglitazone treatment of MA-10 Leydig tumor cells resulted in cellular cholesteryl esters decreasing and cell free cholesterol increasing. This was not an effect unique to this chemical entity; rosiglitazone and pioglitazone caused these changes also. The excess free cholesterol was recovered largely in the cholesterol oxidase susceptible, plasma membrane cholesterol pool. This effect of troglitazone probably is not mediated by activation of peroxisome proliferator activated receptors since it immediately reversed with washing and did not occur at all in cells treated with the peroxisome proliferator activated receptor agonist, 15-deoxy Delta 12,14 prostaglandin J-2. Plasma membrane cholesterol esterification was inhibited by troglitazone in a dose-dependent manner. Plasma membrane cholesterol esterification was inhibited half-maximally by 14 microM troglitazone and by more than 90% by 40 microM troglitazone. This effect was not unique for MA-10 cells. Similar results were found using fibroblasts. Troglitazone was not simply inhibiting internalization of plasma membrane cholesterol. Dibutyryl-cAMP stimulation of troglitazone-treated cells resulted in more progesterone synthesis than in stimulated control cells; moreover, radioactive plasma membrane cholesterol was readily converted into progesterone in troglitazone-treated cells. Studies of LDL uptake in troglitazone-treated cells indicated that intracellular membranes were cholesterol replete. Troglitazone inhibited plasma membrane cholesterol esterification with kinetics similar to 58-035, a known inhibitor of the acyl coenzyme A: cholesterol acyltranserase (ACAT) enzyme. It is not likely an ACAT inhibitor since troglitazone did not block incorporation of exogenous free fatty acids into cholesteryl esters. Thus, it appears that troglitazone prevented presentation of free fatty acid to the ACAT enzyme.

Activity and mRNA levels of enzymes involved in hepatic fatty acid synthesis and oxidation in mice fed conjugated linoleic acid

The effects of dietary conjugated linoleic acid (CLA) on the activity and mRNA levels of hepatic enzymes involved in fatty acid synthesis and oxidation were examined in mice. In the first experiment, male ICR and C57BL/6J mice were fed diets containing either a 1.5% fatty acid preparation rich in CLA or a preparation rich in linoleic acid. In the second experiment, male ICR mice were fed diets containing either 1.5% linoleic acid, palmitic acid or the CLA preparation. After 21 days, CLA relative to linoleic acid greatly decreased white adipose tissue mass but caused hepatomegaly accompanying an approximate 10-fold increase in the tissue triacylglycerol content irrespective of mouse strain. CLA compared to linoleic acid greatly increased the activity and mRNA levels of various lipogenic enzymes in both experiments. Moreover, CLA increased the mRNA expression of Delta6- and Delta5-desaturases, and sterol regulatory element binding protein-1 (SREBP-1). The mitochondrial and peroxisomal palmitoyl-CoA oxidation rate was about 2.5-fold higher in mice fed CLA than in those fed linoleic acid in both experiments. The increase was associated with the up-regulation of the activity and mRNA expression of various fatty acid oxidation enzymes. The palmitic acid diet compared to the linoleic acid diet was rather ineffective in modulating the hepatic lipid levels or activity and mRNA levels of enzymes in fatty acid metabolism. It is apparent that dietary CLA concomitantly increases the activity and mRNA levels of enzymes involved in fatty acid synthesis and oxidation, and desaturation of polyunsaturated fatty acid in the mouse liver. Both the activation of peroxisomal proliferator alpha and up-regulation of SREBP-1 may be responsible for this.

Free fatty acid overload attenuates Ca2+ signaling and NO production in endothelial cells

Hyperlipidemia represents a major risk factor for development of vascular dysfunction and atherosclerosis. Although the unfortunate role of low-density lipoprotein has been clearly demonstrated, the mechanistic pathways through which triglyceride-derived free fatty acids (FFAs) contribute to vascular disorders are not completely understood. Thus, the present study was designed to elucidate the effects of FFAs on cultured endothelial cells. The Ca(2+) signaling, endothelial nitric oxide synthase (eNOS) activity, and production of superoxide anions (.O(2)(-)) were monitored in cells treated with bovine serum albumin-conjugated FFA. FFA-loaded cells showed enhanced intracellular Ca(2+) release in response to ATP, histamine, or the SERCA inhibitor thapsigargin. This effect corresponded to an overall increase in intracellularly stored Ca(2+). In contrast, autacoid-triggered elevation of cytosolic free Ca(2+) concentration was blunted in FFA-loaded cells due to inhibition of capacitative Ca(2+) entry. In agreement with the reduced Ca(2+) signaling, the Ca(2+)-dependent activity of eNOS was reduced under basal conditions and if cells were stimulated with ATP, histamine, or thapsigargin. The attenuated eNOS activity was associated with.O(2)(-) release in FFA-loaded cells. These data indicate that FFAs significantly affect endothelial Ca(2+) signaling, eNOS activity, and.O(2)(-) release and, thus, might contribute to vascular dysfunction in atherogenesis.

Effects of identical weight loss on body composition and features of insulin resistance in obese women with high and low liver fat content

Our objective was to determine how 8% weight loss influences subcutaneous, intra-abdominal, and liver fat (LFAT), as well as features of insulin resistance, in obese women with high versus low LFAT. A total of 23 women with previous gestational diabetes were divided into groups of high (9.4 +/- 1.4%) and low (3.3 +/- 0.4%) LFAT based on their median LFAT (5%) measured with proton spectroscopy. Both groups were similar with respect to age, BMI, and intra-abdominal and subcutaneous fat. Before weight loss, women with high LFAT had higher fasting serum insulin and triglyceride concentrations than women with low LFAT. At baseline, LFAT correlated with the percent of fat (r = 0.44, P < 0.05) and saturated fat (r = 0.45, P < 0.05) of total caloric intake but not intra-abdominal or subcutaneous fat or fasting serum free fatty acids. Weight loss was similar between the groups (high LFAT -7.4 +/- 0.2 vs. low LFAT -7.7 +/- 0.3 kg). LFAT decreased from 9.4 +/- 1.4 to 4.8 +/- 0.7% (P < 0.001) in women with high LFAT and from 3.3 +/- 0.4 to 2.0 +/- 0.2% (P < 0.001) in women with low LFAT. The absolute decrease in LFAT was significantly higher in women with high than low LFAT (-4.6 +/- 1.0 vs. -1.3 +/- 0.3%, P < 0.005). The decrease in LFAT was closely correlated with baseline LFAT (r = -0.85, P < 0.001) but not with changes in the volumes of intra-abdominal or subcutaneous fat depots, which decreased similarly in both groups. LFAT appears to be related to the amount of fat in the diet rather than the size of endogenous fat depots in obese women. Women with initially high LFAT lost more LFAT by similar weight loss than those with low LFAT, although both groups lost similar amounts of subcutaneous and intra-abdominal fat. These data suggest that LFAT is regulated by factors other than intra-abdominal and subcutaneous fat. Therefore, LFAT does not appear to simply reflect the size of endogenous fat stores.

Relationship between plasma free fatty acids and uncoupling protein-3 gene expression in skeletal muscle of obese subjects: in vitro evidence of a causal link

OBJECTIVE

To investigate whether skeletal muscle uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) gene expression is altered in massive obesity and whether it correlates with in vivo insulin sensitivity and with metabolic and hormonal status.

DESIGN

Quantification of UCP2 and UCP3 gene expression in skeletal muscle of obese and lean subjects displaying different degrees of insulin sensitivity.

PATIENTS

Fourteen obese and 10 age- and sex-matched healthy control subjects with a mean body mass index (BMI) of 43.6 +/- 1.4 and 22.8 +/- 1.8 (+/- SEM), respectively.

MEASUREMENTS

Insulin sensitivity by glucose clamp, body composition by bio-impedance, fasting plasma glucose, insulin, leptin and free fatty acids (FFA). Skeletal muscle UCP2 and UCP3 mRNA levels by quantitative reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

No significant differences in UCP2 or UCP3 mRNA levels were found between obese and control subjects. No significant correlation was observed, in both groups, between UCP2 or UCP3 mRNA levels and both anthropometrical and metabolic parameters. In contrast, a highly significant correlation was observed between skeletal muscle UCP3, but not UCP2, mRNA levels and plasma FFA in the obese, but not in the lean, group. Furthermore, exposure of human myocytes to FFA for 24 h strongly induced both UCP3 and peroxisome proliferator-activated receptor-gamma (PPARgamma) but not UCP2 gene expression.

CONCLUSIONS

FFA levels correlate strongly with skeletal muscle UCP3 mRNA levels in obese, but not in lean, subjects; in addition, in human myocytes, high FFA concentrations promote UCP3 expression. Our studies therefore provide evidence that supports a role for increased plasma FFA concentrations in the regulation of human skeletal muscle UCP3 gene expression.

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

Direct effects of growth hormone on production and secretion of apolipoprotein B from rat hepatocytes

The aim of this study was to investigate the direct effects of growth hormone (GH) on production and secretion of apolipoprotein B (apoB)-containing lipoproteins from hepatocytes. Bovine GH (5-500 ng/ml) was given for 1 or 3 days to rat hepatocytes cultured on laminin-rich matrigel in serum-free medium. The effects of GH were compared with those of 3 nM insulin and 500 microM oleic acid. GH increased the editing of apoB mRNA, and the proportion of newly synthesized apoB-48 (of total apoB) in the cells and secreted into the medium changed in parallel. GH increased total secretion of apoB-48 (+30%) and apoB-48 in very low density lipoproteins (VLDL) more than twofold. Total apoB-100 secretion decreased 63%, but apoB-100-VLDL secretion was unaffected by GH. Pulse-chase studies indicated that GH increased intracellular early degradation of apoB-100 but not apoB-48. GH had no effect on apoB mRNA or LDL receptor mRNA levels. The triglyceride synthesis, the mass of triglycerides in the cells, and the VLDL fraction of the medium increased after GH incubation. Three days of insulin incubation had effects similar to those of GH. Combined incubation with oleic acid and GH had additive effects on apoB mRNA editing and apoB-48-VLDL secretion. In summary, GH has direct effects on production and secretion of apoB-containing lipoproteins, which may add to the effects of hyperinsulinemia and increased flux of fatty acids to the liver during GH treatment in vivo.

Dietary fatty acids Up-regulate the expression of UCP2 in 3T3-L1 preadipocytes

States characterised by elevated plasma fatty acid levels are accompanied by increased UCP2 expression but the physiological regulation of UCP2 expression in white adipose tissue is not fully understood. We used 3T3-L1 preadipocytes to determine whether various dietary fatty acids (20:5, 18:3, 18:2, 18:1, 18:0) directly regulate UCP2 expression. Physiological concentrations of each class of polyunsaturated fatty acid and the monounsaturated fatty acid dramatically up-regulated UCP2 mRNA levels 5- to 8-fold, but the saturated fatty acid was not so effective (1.5-fold). The up-regulation occurred in a time- and dose-dependent manner, was evident by 4 h and maximum between 18 and 24 h, and was prevented by actinomycin D. Synthetic ligands selective for each PPAR isoform did not induce UCP2 expression, which suggests that fatty acids might not be acting solely via PPAR transcription factors. In conclusion, dietary unsaturated fatty acids may be physiological signals to alter energy balance by direct induction of UCP2.

Glucose-stimulated insulin secretion suppresses hepatic triglyceride-rich lipoprotein and apoB production

The current study assessed in vivo the effect of insulin on triglyceride-rich lipoprotein (TRL) production by rat liver. Hepatic triglyceride and apolipoprotein B (apoB) production were measured in anesthetized, fasted rats injected intravenously with Triton WR-1339 (400 mg/kg). After intravascular catabolism was blocked by detergent treatment, glucose (500 mg/kg) was injected to elicit insulin secretion, and serum triglyceride and apoB accumulation were monitored over the next 3 h. In glucose-injected rats, triglyceride secretion averaged 22.5 +/- 2.1 microg.ml(-1).min(-1), which was significantly less by 30% than that observed in saline-injected rats, which averaged 32.1 +/- 1.4 microg.ml(-1).min(-1). ApoB secretion was also significantly reduced by 66% in glucose-injected rats. ApoB immunoblotting indicated that both B100 and B48 production were significantly reduced after glucose injection. Results support the conclusion that insulin acts in vivo to suppress hepatic very low density lipoprotein (VLDL) triglyceride and apoB secretion and strengthen the concept of a regulatory role for insulin in VLDL metabolism postprandially.