Fatty acid flux suppresses fatty acid synthesis in hamster intestine independently of SREBP-1 expression

Comparison of canola oil and olive oil consumption on the serum lipid profile in adults: a systematic review and meta-analysis of randomized controlled trials

BACKGROUND AND AIMS

Several randomized clinical trials have investigated the effects of canola oil (CO) compared to olive oil (OO) on the serum lipid profiles in adults. However, the results of these studies are inconsistent. Thus, this study aimed to assess the comparison of CO and OO consumption on the serum lipid components in adults.

METHODS AND RESULTS

The following online databases were searched until February 4th, 2022: PubMed/Medline, Scopus, Clarivate Analytics Web of Science, Cochrane Central Register of Controlled Trials, and Google Scholar. The effect sizes were stated as the weighted mean difference (WMD) with 95% confidence intervals (CI). A total of 13 eligible trials were included in this meta-analysis. The results showed that the CO consumption, significantly reduced serum LDL-c (WMD: -6.13 mg/dl, 95%CI: -9.79, -2.46, p = 0.001), TC (WMD: -8.92 mg/dl, 95% CI: -13.52, -4.33, P < 0.001) and LDL-c/HDL-c ratio (WMD: -0.30; 95% CI, -0.53, -0.06, p = 0.01) levels compared to OO. There were no significant changes in the other components of the blood lipids.

CONCLUSION

The results of this review suggest that CO consumptionhas beneficial effects on LDL-c, TC, and LDL-c/HDL-c ratio compared to OO. Therefore, its replacement with OO can have cardioprotective impacts.

Palmitic acid induces intestinal lipid metabolism disorder, endoplasmic reticulum stress and inflammation by affecting phosphatidylethanolamine content in large yellow croaker Larimichthys crocea

In the 21^st century, intestinal homeostatic imbalance has emerged as a growing health challenge worldwide. Accumulating evidence reveals that excessive intake of saturated fatty acid (SFA) induces intestinal homeostatic imbalance. However, the potential molecular mechanism is still unclear. In the present study, we found that palm oil or palmitic acid (PA) treatment disturbed lipid metabolism homeostasis and triggered endoplasmic reticulum (ER) stress and inflammation in the intestine or intestinal cells of large yellow croaker (Larimichthys crocea). Interestingly, PA treatment significantly decreased phosphatidylethanolamine (PE) content in the intestinal cells. PE supplementation decreased triglyceride content in the intestinal cells induced by PA treatment by inhibiting fatty acid uptake and lipogenesis. PE supplementation suppressed ER stress. Meanwhile, PE supplementation alleviated inflammatory response through p38 MAPK-p65 pathway, reducing the damage of intestinal cells caused by PA treatment to some extent. Our work revealed that intestinal homeostatic imbalance caused by PA treatment was partly due to the decrease of PE content. PE consumption might be a nutritional strategy to regulate intestinal homeostasis in fish and even human beings.

Hypolipidemic and Hepatoprotective Effects of Polysaccharides Extracted from Liriope spicata Var. Prolifera in C57BL/6J Mice with High-Fat Diet-Induced Hyperlipidemia

In this study, C57BL/6J mice with high-fat diet- (HFD-) induced hyperlipidemia were treated with total Liriope spicata var. prolifera polysaccharides (TLSP: 200, 400, and 800 mg/kg body weight), simvastatin (3 mg/kg body weight), or saline for 8 weeks, respectively. The results showed that TLSP had strong lipid-lowering and hepatoprotective effects on C57BL/6J mice with HFD-induced hyperlipidemia. TLSP administration significantly reduced serum total cholesterol (TC), triglyceride (TG), and low-density lipoprotein cholesterol (LDL-C) levels and downregulated the expressions of peroxisome proliferator-activated receptor (PPAR)γ and fatty acid synthase (FAS) in the adipose and liver tissues of the mice. TLSP exerted hypolipidemic and hepatoprotective effects by activating lipid/bile acid metabolism via the FXH-SHP/CYP7A1 and SEBP-1c/FAC/ACC signaling pathways. Thus, TLPS is a promising natural polymer with hepatoprotective and hypolipidemic properties.

The Effects of Linoleic Acid Consumption on Lipid Risk Markers for Cardiovascular Disease in Healthy Individuals: A Review of Human Intervention Trials

Cardiovascular disease (CVD) is the leading cause of death worldwide. Risk factors for developing this disease include high serum concentrations of total cholesterol, triglycerides, low-density lipoproteins, very-low density lipoproteins, and low concentrations of high-density lipoproteins. One proposed dietary strategy for decreasing risk factors involves replacing a portion of dietary saturated fatty acids with mono- and polyunsaturated fatty acids (PUFAs). The essential omega-6 PUFA, linoleic acid (LA), is suggested to decrease the risk for CVD by affecting these lipid risk markers. Reviewing human intervention trials will provide further evidence of the effects of LA consumption on risk factors for CVD. PubMed was used to search for peer-reviewed articles. The purpose of this review was: (1) To summarize human intervention trials that studied the effects of LA consumption on lipid risk markers for CVD in healthy individuals, (2) to provide mechanistic details, and (3) to provide recommendations regarding the consumption of LA to decrease the lipid risk markers for CVD. The results from this review provided evidence that LA consumption decreases CVD lipid risk markers in healthy individuals.

WZ66, a novel acetyl-CoA carboxylase inhibitor, alleviates nonalcoholic steatohepatitis (NASH) in mice

The global prevalence of nonalcoholic steatohepatitis (NASH) increases incredibly. NASH ends up to advanced liver disease, which is highly threatening to human health. Currently, treatment of NASH is very limited. Acetyl-CoA carboxylases (ACC1/ACC2) are proved as effective drug targets for NASH. We aimed to develop novel ACC inhibitors and evaluate their therapeutic value for NASH prevention. ACC inhibitors were obtained through structure-based drug design, synthesized, screened from ACC enzymatic measurement platform and elucidated in cell culture-based assays and animal models. The lipidome and microbiome analysis were integrated to assess the effects of WZ66 on lipids profiles in liver and plasma as well as gut microbiota in the intestine. WZ66 was identified as a novel ACC1/2 inhibitor. It entered systemic circulation rapidly and could accumulate in liver. WZ66 alleviated NASH-related liver features including steatosis, Kupffer cells and hepatic stellate cells activation in diet-induced obese mice. The triglycerides (TGs) and other lipids including diglycerides (DGs), phosphatidylcholine (PC) and sphingomyelin (SM) were decreased in WZ66-treated mice as evidenced by lipidome analysis in livers. The lipids profiles in plasma were also altered with WZ66 treatment. Plasma TG were moderately increased, while the activation of SREBP1c was not detected. WZ66 also downregulated the abundance of Allobaculum, Mucispirillum and Prevotella genera as well as Mucispirillum schaedleri species in gut microbiota. WZ66 is an ideal lead compound and a potential drug candidate deserving further investigation in the therapeutics of NASH.

High Fat Diet with a High Monounsaturated Fatty Acid and Polyunsaturated/Saturated Fatty Acid Ratio Suppresses Body Fat Accumulation and Weight Gain in Obese Hamsters

The aim of this study was to investigate the effect of a high fat diet with experimental oil consisting of 60% MUFAs (monounsaturated fatty acids) with a P/S ratio of 5 on fat deposition and lipid metabolism in obese hamsters. Hamsters were randomly assigned to a control group and a diet-induced obesity group for nine weeks. Then an additional eight-week experimental period began, during which obese hamsters were randomly divided into three groups and fed different amounts of the experimental oil mixture in their diets as follows: 5%, 15%, and 20% w/w (OB-M5, OB-M15, and OB-M20 groups, respectively). The results showed that the OB-M15 and OB-M20 groups had significantly lower blood cholesterol and higher insulin levels. Compared to the control group, the three obese groups exhibited higher hepatic fatty acid synthase activity; however, the acyl-CoA oxidase activities were also enhanced. Although dietary fat content differed, there were no differences in energy intake, final body weights, and epididymal fat weights among the four groups. These results suggest that regardless of whether the specimens had a high fat intake or not, dietary fat containing high MUFAs with a high P/S ratio had beneficial effects on maintaining blood lipid profiles and may not result in body fat accumulation in obese hamsters, possibly by promoting lipolytic enzyme activities.

Docosahexanoic acid modifies low-density lipoprotein receptor abundance in HepG2 cells via suppression of the LXRα-Idol pathway

As a daily supplement, omega‑3 fatty acid is confirmed to be of benefit in hypertriglyceridemia. However, the effect of omega‑3 fatty acids on the low‑density lipoprotein cholesterol (LDL‑C) metabolism remains a controversial issue. In this study, we focused on the regulatory effect of docosahexanoic acid (DHA), one type of omega‑3 fatty acid, exerted on the LDL receptor (LDLR), a determinant regulator of the LDL‑C metabolism, and explored the potential mechanism. We observed that DHA increased hepatic LDLR protein in the presence of 25‑hydroxycholesterol in HepG2 cells but did not alter the mRNA level. Previous studies have identified inducible degrader of the LDLR (Idol) as a novel negative post‑translational modulator of LDLR and a direct transcriptional target of liver X receptor α (LXRα). Since DHA had no effect on the transcriptional level of LDLR, we speculated that the post‑transcriptional pathway LXRα‑Idol participated in this regulation. The results reveal that DHA downregulated the expression of LXRα and Idol in coordination with the upregulation of LDLR expression. Multiple mechanisms are involved in the regulation of LDLR by DHA, and the suppression of the LXRα‑Idol pathway is one of these mechanisms.

Dietary supplementation of chardonnay grape seed flour reduces plasma cholesterol concentration, hepatic steatosis, and abdominal fat content in high-fat diet-induced obese hamsters

The mechanisms for the hypocholesterolemic and antiobesity effects of grape seed flours derived from white and red winemaking processing were investigated using male Golden Syrian hamsters fed high-fat (HF) diets supplemented with 10% partially defatted grape seed flours from Chardonnay (ChrSd), Cabernet Sauvignon (CabSd), or Syrah (SyrSd) pomace as compared to a HF control diet for 3 weeks. Hamsters fed the ChrSd diet had significantly lowered plasma total-, VLDL-, and LDL-cholesterol concentrations compared to the CabSd, SyrSd, and control diets. The improved plasma cholesterol after ChrSd was correlated with the up-regulation of hepatic genes related to cholesterol (CYP51) and bile acid (CYP7A1) synthesis as well as LDL-cholesterol uptake (LDLR). A reduction of hepatic lipid content was associated with altered expression of the genes related to lipid metabolism. However, fecal total lipid content was not changed. Expression of ileal apical sodium bile acid transporter (ASBT) was not affected by ChrSd, indicating unchanged ileal bile acid reabsorption. The antiobesity effect of the ChrSd diet appears to be related to expression of adipogenesis- and inflammation-related genes in adipose tissue. These findings suggest that flavonoid-rich Chardonnay grape seed flour induced cholesterol-lowering, antiobesity, and anti-inflammatory health benefits and attenuation of hepatic steatosis via regulation of gene expression related to cholesterol, bile acid, and lipid metabolism in liver and adipose tissue.

Ezetimibe ameliorates intestinal chylomicron overproduction and improves glucose tolerance in a diet-induced hamster model of insulin resistance

Ezetimibe is a cholesterol uptake inhibitor that targets the Niemann-Pick C1-like 1 cholesterol transporter. Ezetimibe treatment has been shown to cause significant decreases in plasma cholesterol levels in patients with hypercholesterolemia and familial hypercholesterolemia. A recent study in humans has shown that ezetimibe can decrease the release of atherogenic postprandial intestinal lipoproteins. In the present study, we evaluated the mechanisms by which ezetimibe treatment can lower postprandial apoB48-containing chylomicron particles, using a hyperlipidemic and insulin-resistant hamster model fed a diet rich in fructose and fat (the FF diet) and fructose, fat, and cholesterol (the FFC diet). Male Syrian Golden hamsters were fed either chow or the FF or FFC diet ± ezetimibe for 2 wk. After 2 wk, chylomicron production was assessed following intravenous triton infusion. Tissues were then collected and analyzed for protein and mRNA content. FFC-fed hamsters treated with ezetimibe showed improved glucose tolerance, decreased fasting insulin levels, and markedly reduced circulating levels of TG and cholesterol in both the LDL and VLDL fractions. Examination of triglyceride (TG)-rich lipoprotein (TRL) fractions showed that ezetimibe treatment reduced postprandial cholesterol content in TRL lipoproteins as well as reducing apoB48 content. Although ezetimibe did not decrease TRL-TG levels in FFC hamsters, ezetimibe treatment in FF hamsters resulted in decreases in TRL-TG. Jejunal apoB48 protein expression was lower in ezetimibe-treated hamsters. Reductions in jejunal protein levels of scavenger receptor type B-1 (SRB-1) and fatty acid transport protein 4 were also observed. In addition, ezetimibe-treated hamsters showed significantly lower jejunal mRNA expression of a number of genes involved in lipid synthesis and transport, including srebp-1c, sr-b1, ppar-γ, and abcg1. These data suggest that treatment with ezetimibe not only inhibits cholesterol uptake, but may also alter intestinal function to promote improved handling of dietary lipids and reduced chylomicron production. These, in turn, promote decreases in fasting and postprandial lipid levels and improvements in glucose homeostasis.

Lower weight gain and hepatic lipid content in hamsters fed high fat diets supplemented with white rice protein, brown rice protein, soy protein, and their hydrolysates

The physiological effects of the hydrolysates of white rice protein (WRP), brown rice protein (BRP), and soy protein (SP) hydrolyzed by the food grade enzyme, alcalase2.4 L, were compared to the original protein source. Male Syrian Golden hamsters were fed high-fat diets containing either 20% casein (control) or 20% extracted proteins or their hydrolysates as the protein source for 3 weeks. The brown rice protein hydrolysate (BRPH) diet group reduced weight gain 76% compared with the control. Animals fed the BRPH supplemented diet also had lower final body weight, liver weight, very low density lipoprotein cholesterol (VLDL-C), and liver cholesterol, and higher fecal fat and bile acid excretion than the control. Expression levels of hepatic genes for lipid oxidation, PPARα, ACOX1, and CPT1, were highest for hamsters fed the BRPH supplemented diet. Expression of CYP7A1, the gene regulating bile acid synthesis, was higher in all test groups. Expression of CYP51, a gene coding for an enzyme involved in cholesterol synthesis, was highest in the BRPH diet group. The results suggest that BRPH includes unique peptides that reduce weight gain and hepatic cholesterol synthesis.

cis-9,trans-11,cis-15 and cis-9,trans-13,cis-15 CLNA mixture activates PPARα in HEK293 and reduces triacylglycerols in 3T3-L1 cells

Scientific research is constantly working to find new molecules that are effective in preventing excessive accumulation of body fat. The aim of the present work was to assess the potential agonism on PPARα and PPARγ of a conjugated linolenic acid (CLNA) isomer mixture, consisting of two CLNA isomers (cis-9,trans-11,cis-15 and cis-9,trans-13,cis-15). Secondly, we aimed to analyze the effects of this mixture on triacylglycerol accumulation in 3T3-L1 mature adipocytes. Luciferase transactivation assay was used to analyze whether the CLNA mixture activated PPARs. The expression of several enzymes and transcriptional factors involved in the main metabolic pathways that control triacylglycerol accumulation in adipocytes was assessed by real time RT-PCR in 3T3-L1 adipocytes treated for 20 h with the CLNA mixture. The mixture activated PPRE in cells with PPARα receptor over-expression, but not those with PPARγ over-expression. Decreased triacylglycerol was found in treated adipocytes. The lowest dose (10 μM) increased HSL expression and the highest dose (100 μM) increased ATGL gene expression. The other genes analyzed remained unchanged. The hypothesis of an anti-obesity action of the analyzed CLNA mixture, based on increased lipid mobilization in adipose tissue, can be proposed.

Supplementation of hydroxypropyl methylcellulose into yeast leavened all-whole grain barley bread potentiates cholesterol-lowering effect

We investigated in Syrian Golden hamsters the biological impact and its underlying mechanism of single whole grain breads supplemented with 2-3% hydroxypropyl methylcellulose (HPMC), a semisynthetic viscous soluble dietary fiber (SDF) as a substitute for gluten. Hamsters were fed high-fat diets supplemented with 48-65% (w/w) differently ground, freeze-dried single grain breads including whole grain wheat, barley, barley supplemented with HPMC, debranned oat, and oat supplemented with HPMC which were compared to a diet containing microcrystalline cellulose (control). All single grain breads significantly lowered plasma LDL-cholesterol concentrations compared to the control. Enrichment with HPMC further lowered plasma and hepatic cholesterol concentrations. Despite the reduced molecular weight of naturally occurring soluble (1--->3),(1--->4)-β-d-glucan (β-glucan) caused by the bread-making process, whole grain barley breads downregulated hepatic expression of CYP7A1 and HMG-CoAR genes that are responsible for bile acid and cholesterol synthesis, suggesting a possible role of bioactive compounds such as short-chain fatty acids and phenolic compounds from barley bread. Barley bread enriched with HPMC downregulated expression of ABCG5 gene. Taken together, it appears that distinctive modulation of synthesis and excretion of hepatic cholesterol and bile acid contributes to the cholesterol-lowering properties of whole grain barley breads and breads enriched with HPMC. These data suggests that alternative whole grain breads supplemented with HPMC may provide consumers with a staple food that can assist in cholesterol management.

Genetic variability of the fatty acid synthase pathway is not associated with prostate cancer risk in the European Prospective Investigation on Cancer (EPIC)

A western lifestyle, characterised by low rates of energy expenditure and a high-energy diet rich in animal protein, saturated fats and refined carbohydrates, is associated with high incidence of prostate cancer in men. A high-energy nutritional status results in insulin/IGF signalling in cells, which in turn stimulates synthesis of fatty acids. We investigated whether the genetic variability of the genes belonging to the fatty acid synthesis pathway is related to prostate cancer risk in 815 prostate cancer cases and 1266 controls from the European Prospective Investigation on Cancer (EPIC). Using a tagging approach and selecting 252 SNPs in 22 genes, we covered all the common genetic variation of this pathway. None of the SNPs reached statistical significance after adjusting for multiple comparisons. Common SNPs in the fatty acid synthase pathway are not major contributors to prostate cancer risk.

Effects of different ratios of monounsaturated and polyunsaturated fatty acids to saturated fatty acids on regulating body fat deposition in hamsters

OBJECTIVE

Effects of monounsaturated fatty acid (MUFA) and polyunsaturated fatty acid consumption on regulating body fat accumulation and body weight gain are controversial between animal and human studies.

METHODS

We designed a 2 x 2 factorial study, with two levels of MUFAs (60% and 30%) and two levels of polyunsaturated-to-saturated fatty acid (P/S) ratio (5 and 3) to prepare four kinds of experimental oils consisting of 60% MUFAs with a high or low P/S ratio (HMHR or HMLR, respectively) or 30% MUFAs with a high or low P/S ratio (LMHR or LMLR, respectively). Thirty-two male golden Syrian hamsters were randomly divided into four groups and fed the experimental diets containing 15% (w/w) fat for 12 wk.

RESULTS

No difference was observed in the mean daily food intake. Hamsters fed the LMLR diet had increased weight gain, epididymal and retroperitoneal white adipose tissues, plasma non-esterified fatty acids, insulin, hepatic acetyl coenzyme A carboxylase and malic enzyme activities, and mRNA expressions of peroxisome proliferator-activated receptor-alpha and sterol regulatory element-binding protein-1c among all groups (P < 0.05). Hamsters fed the HMHR diet had lower plasma insulin levels and hepatic acetyl coenzyme A carboxylase activities among groups (P < 0.05) and elevated hepatic acyl coenzyme A oxidase and carnitine palmitoyltransferase-I activities compared with those fed the LMLR diet (P < 0.05).

CONCLUSION

Hamsters fed the LMLR diet had increased weight gain and body fat accumulation, whereas the HMHR diet appeared to be beneficial in preventing white adipose tissue accumulation by decreasing plasma insulin levels and increasing hepatic lipolytic enzyme activities involved in beta-oxidation.

Soy protein diet alters expression of hepatic genes regulating fatty acid and thyroid hormone metabolism in the male rat

We hypothesized that consumption of soy protein isolate (SPI) or the soy isoflavone genistein (GEN) would modulate mRNA expression of genes underlying lipid and thyroid hormone metabolism in livers and small intestines of young adult male Sprague-Dawley rats. Early pregnant rat dams were placed on AIN-93G diets containing casein (CAS, control protein), SPI, or CAS+GEN. Litters were weaned to the same diet as their dam. SPI-fed (but not GEN-fed) male rats of 48 days of age had significant reductions in body weight, abdominal fat pad weight and hepatic content of lipid droplets and triglycerides. Hepatic peroxisome proliferator-activated receptor α (Ppara) transcripts were elevated with SPI but not GEN diet. Hepatic pyruvate dehydrogenase kinase-4 (Pdk4) and cytochrome P450 4A10 (Cyp4a10) mRNA abundance was reduced with SPI; the SPI effect on Cyp4a10 was recapitulated by GEN diet. SPI (but not GEN) suppressed Pdk4 and 3-hydroxy-3-methylglutaryl-CoA synthase 2 (Hmgcs2) mRNA abundance in duodenum. Liver iodothyronine deiodinase types 1 and 2 (Dio1 and Dio2) mRNA levels were increased with SPI diet; the effect on Dio2, but not Dio1 mRNAs, also was observed with GEN. SPI and GEN increased hepatic types 1 and 2 iodothyronine deiodinase (D1 and D2) activities. Effects of SPI and GEN on the above gene expression may contribute to the observed reductions in body and adipose tissue weight and liver lipid content in this model. Identification of the regulation, by genistein and soy protein, of iodothyronine deiodinase synthesis has potential applications for treatment and prevention of fatty liver disease and obesity.

Beta-glucan extracts inhibit the in vitro intestinal uptake of long-chain fatty acids and cholesterol and down-regulate genes involved in lipogenesis and lipid transport in rats

BACKGROUND

Dietary fiber reduces the intestinal absorption of nutrients and the blood concentrations of cholesterol and triglycerides.

AIM

We wished to test the hypothesis that high-viscosity (HV) and low-viscosity preparations of barley and oat beta-glucan modify the expression of selected genes of lipid-binding proteins in the intestinal mucosa and reduce the intestinal in vitro uptake of lipids.

METHODS

Five different beta-glucan extracts were separately added to test solutions at concentrations of 0.1-0.5% (wt/wt), and the in vitro intestinal uptake of lipids into the intestine of rats was assessed. An intestinal cell line was used to determine the effect of beta-glucan extracts on the expression of intestinal genes involved in lipid metabolism and fatty acid transport.

RESULTS

All extracts reduced the uptake of 18:2 when the effective resistance of the unstirred water layer was high. When the unstirred layer resistance was low, the HV oat beta-glucan extract reduced jejunal 18:2 uptake, while most extracts reduced ileal 18:2 uptake. Ileal 18:0 uptake was reduced by the HV barley extract, while both jejunal and ileal cholesterol uptakes were reduced by the medium-purity HV barley extract. The inhibitory effect of HV barley beta-glucan on 18:0 and 18:2 uptake was more pronounced at higher fatty acid concentrations. The expression of genes involved in fatty acid synthesis and cholesterol metabolism was down-regulated with the HV beta-glucan extracts. beta-Glucan extracts also reduced intestinal fatty-acid-binding protein and fatty acid transport protein 4 mRNA.

CONCLUSIONS

The reduced intestinal fatty acid uptake observed with beta-glucan is associated with inhibition of genes regulating intestinal uptake and synthesis of lipids. The inhibitory effect of beta-glucan on intestinal lipid uptake raises the possibility of their selective use to reduce their intestinal absorption.

Metabolic effects of dietary cholesterol in an animal model of insulin resistance and hepatic steatosis

Although the atherogenic role of dietary cholesterol has been well established, its diabetogenic potential and associated metabolic disturbances have not been reported. Diet-induced hamster models of insulin resistance and dyslipidemia were employed to determine lipogenic and diabetogenic effects of dietary cholesterol. Metabolic studies were conducted in hamsters fed diets rich in fructose (40%), fat (30%), and cholesterol (0.05-0.25%) (FFC) and other test diets. Short-term feeding of the FFC diet induced insulin resistance, glucose intolerance, hypertriglyceridemia, and hypercholesterolemia. Prolonged feeding (6-22 wk) of the FFC diet led to severe hepatic steatosis, glucose intolerance, and mild increases in fasting blood glucose, suggesting progression toward type 2 diabetes, but did not induce beta-cell dysfunction. Metabolic changes induced by the diet, including dyslipidemia and insulin resistance, were cholesterol concentration dependent and were only markedly induced on a high-fructose and high-fat dietary background. There were significant increases in hepatic and plasma triglyceride with FFC feeding, likely due to a 10- to 15-fold induction of hepatic stearoyl-CoA desaturase compared with chow levels (P < 0.03). Hepatic insulin resistance was evident based on reduced tyrosine phosphorylation of the insulin receptor-beta, IRS-1, and IRS-2 as well as increased protein mass of protein tyrosine phosphatase 1B. Interestingly, nuclear liver X receptor (LXR) target genes such as ABCA1 were upregulated on the FFC diet, and dietary supplementation with an LXR agonist (instead of dietary cholesterol) worsened dyslipidemia, glucose intolerance, and upregulation of target mRNA and proteins similar to that of dietary cholesterol. In summary, these data clearly implicate dietary cholesterol, synergistically acting with dietary fat and fructose, as a major determinant of the severity of metabolic disturbances in the hamster model. Dietary cholesterol appears to induce hepatic cholesterol ester and triglyceride accumulation, and diet-induced LXR activation (via cholesterol-derived oxysterols) may possibly be one key underlying mechanism.

LXRalpha activation perturbs hepatic insulin signaling and stimulates production of apolipoprotein B-containing lipoproteins

Liver X receptor-alpha (LXRalpha) is considered a master regulator of hepatic lipid metabolism; however, little is known about the link between LXR activation, hepatic insulin signaling, and very low-density lipoprotein (VLDL)-apolipoprotein B (apoB) assembly and secretion. Here, we examined the effect of LXRalpha activation on hepatic insulin signaling and apoB-lipoprotein production. In vivo activation of LXRalpha for 7 days using a synthetic LXR agonist, TO901317, in hamsters led to increased plasma triglyceride (TG; 3.6-fold compared with vehicle-treated controls, P = 0.006), apoB (54%, P < 0.0001), and VLDL-TG (eightfold increase compared with vehicle). As expected, LXR stimulation activated maturation of sterol response element binding protein-1c (SREBP-1c) as well as the SREBP-1c target genes steroyl CoA desaturase (SCD) and fatty acid synthase (FAS). Metabolic pulse-chase labeling experiments in primary hamster hepatocytes showed increased stability and secretion of newly synthesized apoB following LXR activation. Microsomal triglyceride transfer protein (MTP) mRNA and protein were unchanged, however, likely because of the relatively short period of treatment and long half-life of MTP mRNA. Examination of hepatic insulin-signaling molecules revealed LXR-mediated reductions in insulin receptor (IR)beta subunit mass (39%, P = 0.014) and insulin receptor substrate (IRS)-1 tyrosine phosphorylation (24%, P = 0.023), as well as increases in protein tyrosine phosphatase (PTP)1B (29%, P < 0.001) protein mass. In contrast to IRS-1, a twofold increase in IRS-2 mass (228%, P = 0.0037) and a threefold increase in IRS-2 tyrosine phosphorylation (321%, P = 0.012) were observed. In conclusion, LXR activation dysregulates hepatic insulin signaling and leads to a considerable increase in the number of circulating TG-rich VLDL-apoB particles, likely due to enhanced hepatic assembly and secretion of apoB-containing lipoproteins.

Role of the PGC-1 family in the metabolic adaptation of goldfish to diet and temperature

In mammals, the peroxisome proliferator-activated receptor (PPAR) gamma coactivator-1 (PGC-1) family members and their binding partners orchestrate remodelling in response to diverse challenges such as diet, temperature and exercise. In this study, we exposed goldfish to three temperatures (4, 20 and 35 degrees C) and to three dietary regimes (food deprivation, low fat and high fat) and examined the changes in mitochondrial enzyme activities and transcript levels for metabolic enzymes and their genetic regulators in red muscle, white muscle, heart and liver. When all tissues and conditions were pooled, there were significant correlations between the mRNA for the PGC-1 coactivators (both alpha and beta) and mitochondrial transcripts (citrate synthase), metabolic gene regulators including PPARalpha, PPARbeta and nuclear respiratory factor-1 (NRF-1). PGC-1beta was the better predictor of the NRF-1 axis, whereas PGC-1alpha was the better predictor of the PPAR axis (PPARalpha, PPARbeta, medium chain acyl CoA dehydrogenase). In contrast to these intertissue/developmental patterns, the response of individual tissues to physiological stressors displayed no correlations between mRNA for PGC-1 family members and either the NRF-1 or PPAR axes. For example, in skeletal muscles, low temperature decreased PGC-1alpha transcript levels but increased mitochondrial enzyme activities (citrate synthase and cytochrome oxidase) and transcripts for COX IV and NRF-1. These results suggest that in goldfish, as in mammals, there is a regulatory relationship between (i) NRF-1 and mitochondrial gene expression and (ii) PPARs and fatty acid oxidation gene expression. In contrast to mammals, there is a divergence in the roles of the coactivators, with PGC-1alpha linked to fatty acid oxidation through PPARalpha, and PGC-1beta with a more prominent role in mediating NRF-1-dependent control of mitochondrial gene expression, as well as distinctions between their respective roles in development and physiological responsiveness.

Intestinal lipoprotein overproduction in insulin-resistant states

PURPOSE OF REVIEW

Excessive postprandial lipemia is highly prevalent in obese and insulin-resistant/type 2 diabetic individuals and substantially increases the risk of atherosclerosis and cardiovascular disease. This article will review our current understanding of the link between insulin resistance and intestinal lipoprotein overproduction and highlight some of the key recent findings in the field.

RECENT FINDINGS

Emerging evidence from several animal models of insulin resistance as well as insulin-resistant humans clearly supports the link between insulin resistance and aberrant intestinal lipoprotein metabolism. In insulin-resistant states, elevated free fatty acid flux into the intestine, downregulation of intestinal insulin signaling and upregulation of microsomal triglyceride transfer protein all appear to stimulate intestinal lipoprotein production. Gut peptides, GLP-1 and GLP-2, may be important regulators of intestinal lipid absorption and lipoprotein production.

SUMMARY

Available evidence in humans and animal models strongly favors the concept that the small intestine is not merely an absorptive organ but rather plays an active role in regulating the rate of production of triglyceride-rich lipoproteins. Metabolic signals in insulin resistance and type 2 diabetes and in some cases an aberrant intestinal response to these factors all contribute to the enhanced formation and secretion of triglyceride-rich lipoproteins.

Inability to fully suppress sterol synthesis rates with exogenous sterol in embryonic and extraembyronic fetal tissues

The requirement for cholesterol is greater in developing tissues (fetus, placenta, and yolk sac) as compared to adult tissues. Here, we compared cholesterol-induced suppression of sterol synthesis rates in the adult liver to the fetal liver, fetal body, placenta, and yolk sac of the Golden Syrian hamster. Sterol synthesis rates were suppressed maximally in non-pregnant adult livers when cholesterol concentrations were increased. In contrast, sterol synthesis rates were suppressed only marginally in fetal livers, fetal bodies, placentas, and yolk sacs when cholesterol concentrations were increased. To begin to elucidate the mechanism responsible for the blunted response of sterol synthesis rates in fetal tissues to exogenous cholesterol, the ratio of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) to Insig-1 was measured in these same tissues since the ratio of SCAP to the Insigs can impact SREBP processing. The fetal tissues had anywhere from a 2- to 6-fold greater ratio of SCAP to Insig-1 than did the adult liver, suggesting constitutive processing of the SREBPs. As expected, the level of mature, nuclear SREBP-2 was not different in the fetal tissues with different levels of cholesterol whereas it was different in adult livers. These findings indicate that the suppression of sterol synthesis to exogenous sterol is blunted in developing tissues and the lack of response appears to be mediated at least partly through relative levels of Insigs and SCAP.

A defect in the activity of Delta6 and Delta5 desaturases may be a factor in the initiation and progression of atherosclerosis

Atherosclerosis is a dynamic process. Dyslipidemia, diabetes mellitus, hypertension, obesity, and shear stress of blood flow, the risk factors for the development of atherosclerosis, are characterized by abnormalities in the metabolism of essential fatty acids (EFAs). Gene expression profiling studies revealed that at the sites of atheroslcerosis-prone regions, endothelial cells showed upregulation of pro-inflammatory genes as well as antioxidant genes, and endothelial cells themselves showed changes in cell shape and proliferation. Uncoupled respiration (UCP-1) precedes atherosclerosis at lesion-prone sites but not at the sites that are resistant to atherosclerosis. UCP-1 expression in aortic smooth muscle cells causes hypertension, enhanced superoxide anion production and decreased the availability of NO, suggesting that inefficient metabolism in blood vessels causes atherosclerosis without affecting cholesterol levels. Thus, mitochondrial dysfunction triggers atherosclerosis. Atherosclerosis-free aortae have abundant concentrations of the EFA-linoleate, whereas fatty streaks (an early stage of atherosclerosis) are deficient in EFAs. EFA deficiency promotes respiratory uncoupling and atherosclerosis. I propose that a defect in the activity of Delta6 and Delta5 desaturases decreases the formation of gamma-linolenic acid (GLA), dihomo-DGLA (DGLA), arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) from dietary linoleic acid (LA) and alpha-linolenic acid (ALA). This, in turn, leads to inadequate formation of prostaglandin E1 (PGE1), prostacyclin (PGI2), PGI3, lipoxins (LXs), resolvins, neuroprotectin D1 (NPD1), NO, and nitrolipids that have anti-inflammatory and platelet anti-aggregatory actions, inhibit leukocyte activation and augment wound healing and resolve inflammation and thus, lead to the initiation and progression atheroslcerosis. In view of this, it is suggested that Delta6 and Delta5 desaturases could serve as biological target(s) for the discovery and development of pharmaceuticals to treat atherosclerosis.

Thetrans-10,cis-12 isomer of conjugated linoleic acid reduces hepatic triacylglycerol content without affecting lipogenic enzymes in hamsters

Conjugated linoleic acid (CLA) refers to the positional and geometric dienoic isomers of linoleic acid. The dietary intake of CLA has been associated with changes in lipid metabolism. The aim of the present work was to assess the effects of the two main isomers of CLA on sterol regulatory element binding protein (SREBP)-1a and SREBP-1c mRNA levels, as well as on mRNA levels and the activities of several lipogenic enzymes in liver. For this purpose hamsters were fed an atherogenic diet supplemented with 5 g linoleic acid,cis-9,trans-11 ortrans-10,cis-12 CLA/kg diet for 6 weeks. Thetrans-10,cis-12 isomer intake produced significantly greater liver weight, but also significantly decreased liver fat accumulation. No changes in mRNA levels of SREBP-1a, SREBP-1c and lipogenic enzymes, or in the activities of these enzymes, were observed. There was no effect of feedingcis-9,trans-11 CLA. These results suggest that increased fat accumulation in liver does not occur on the basis of liver enlargement produced by feeding thetrans-10,cis-12 isomer of CLA in hamsters. The reduction in hepatic triacylglycerol content induced by this isomer was not attributable to changes in lipogenesis.

Essential fatty acids: biochemistry, physiology and pathology

Essential fatty acids (EFAs), linoleic acid (LA), and alpha-linolenic acid (ALA) are essential for humans, and are freely available in the diet. Hence, EFA deficiency is extremely rare in humans. To derive the full benefits of EFAs, they need to be metabolized to their respective long-chain metabolites, i.e., dihomo-gamma-linolenic acid (DGLA), and arachidonic acid (AA) from LA; and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from ALA. Some of these long-chain metabolites not only form precursors to respective prostaglandins (PGs), thromboxanes (TXs), and leukotrienes (LTs), but also give rise to lipoxins (LXs) and resolvins that have potent anti-inflammatory actions. Furthermore, EFAs and their metabolites may function as endogenous angiotensin-converting enzyme and 3-hdroxy-3-methylglutaryl coenzyme A reductase inhibitors, nitric oxide (NO) enhancers, anti-hypertensives, and anti-atherosclerotic molecules. Recent studies revealed that EFAs react with NO to yield respective nitroalkene derivatives that exert cell-signaling actions via ligation and activation of peroxisome proliferator-activated receptors. The metabolism of EFAs is altered in several diseases such as obesity, hypertension, diabetes mellitus, coronary heart disease, schizophrenia, Alzheimer's disease, atherosclerosis, and cancer. Thus, EFAs and their derivatives have varied biological actions and seem to be involved in several physiological and pathological processes.

trans-10,cis-12 Conjugated linoleic acid inhibits lipoprotein lipase but increases the activity of lipogenic enzymes in adipose tissue from hamsters fed an atherogenic diet

The aim of the present work was to investigate the effects of trans-10,cis-12 conjugated linoleic acid (CLA) on the activity and expression of lipogenic enzymes and lipoprotein lipase (LPL), as well as on the expression of transcriptional factors controlling these enzymes, in adipose tissue from hamsters, and to evaluate the involvement of these changes in the body fat-reducing effect of this CLA isomer. Thirty male hamsters were divided into three groups and fed atherogenic diets supplemented with 0 (linoleic group), 5 or 10 g trans-10,cis-12 CLA/kg diet, for 6 weeks. Body and adipose tissue weights, food intake and serum insulin were measured. Total and heparin-releasable LPL and lipogenic enzyme activities (acetyl-CoA carboxylase (ACC); fatty acid synthase (FAS); glucose-6-phosphate dehydrogenase (G6PDH); and malic enzyme (ME)) were assessed. ACC, FAS, LPL, sterol regulatory element-binding proteins (SREBP-1a), SREBP-1c and PPARgamma mRNA levels were also determined by real-time PCR. CLA did not modify food intake, body weight and serum insulin level. CLA feeding reduced adipose tissue weight, LPL activity and expression, and increased lipogenic enzyme activities, despite a significant reduction in ACC and FAS mRNA levels. The expression of the three transcriptional factors analysed (SREBP-1a, SREBP-1c and PPARgamma) was also reduced. These results appear to provide a framework for partially understanding the reduction in body fat induced by CLA. Inhibition of LPL activity seems to be an important mechanism underlying body fat reduction in hamsters. Further research is needed to better characterize the effects of CLA on lipogenesis and the role of these effects in CLA action.

Fish oil fatty acid esters of phytosterols alter plasma lipids but not red blood cell fragility in hamsters

In an attempt to combine the hypocholesterolemic properties of plant sterols with the hypotriglyceridemic action of fish oil FA, plant sterols have recently been esterified to fish oil n-3 PUFA. The objective of this study was to determine the effects of plant sterols esterified to n-3 PUFA on plasma lipid levels and erythrocyte fragility. For 5 wk, male Golden Syrian hamsters were fed diets varying in cholesterol and plant sterol content: (i) Non-cholesterol (semipurified diet with no added cholesterol or plant sterols) (ii), Cholesterol (0.25% cholesterol) (iii), Sterols (0.25% cholesterol plus 1% nonesterified plant sterols), or (iv) Fish oil esters of plant sterols (0.25% cholesterol plus 1.76% EPA and DHA sterol esters, providing 1% plant sterols). The addition of fish oil esters of plant sterols to the cholesterol diet decreased (P = 0.001) plasma total cholesterol levels by 20%, but nonesterified plant sterols did not have such a beneficial impact. In addition, non-HDL cholesterol concentrations were 29% lower in hamsters fed fish oil esters of plant sterols than in hamsters fed nonesterified plant sterols (P < 0.0001). Despite higher (P < 0.0001) plant sterol levels in whole erythrocytes of hamsters fed nonesterified plant sterols and fish oil esters of plant sterols compared with hamsters fed no plant sterols, no difference was observed in erythrocyte fragility. The present results show that EPA and DHA esters of plant sterols have a hypocholesterolemic effect in hamsters, and that these new esters of plant sterols exert no detrimental effect on erythrocyte fragility.

Long-chain polyunsaturated fatty acids upregulate LDL receptor protein expression in fibroblasts and HepG2 cells

The objective of this study was to investigate the effect of individual PUFAs on LDL receptor (LDLr) expression in human fibroblasts and HepG2 cells, and to evaluate whether acyl CoA:cholesterol acyltransferase (ACAT) and sterol regulatory element-binding protein 1 (SREBP-1) were involved in the regulation of LDLr expression by fatty acids. When fibroblasts and HepG2 cells were cultured with serum-free defined medium for 48 h, there was a 3- to 5-fold (P < 0.05) increase in LDLr protein and mRNA levels. Incubation of fibroblasts and HepG2 cells in serum-free medium supplemented with 25-hydroxycholesterol (25OH-cholesterol, 5 mg/L) for 24 h decreased LDLr protein and mRNA levels by 50-90% (P < 0.05). Arachidonic acid [AA, 20:4(n-6)], EPA [20:5(n-3)], and DHA [22:6(n-3)] antagonized the depression of LDLr gene expression by 25OH-cholesterol and increased LDLr protein abundance 1- to 3-fold (P < 0.05), but had no significant effects on LDLr mRNA levels. Oleic (18:1), linoleic (18:2), and alpha-linolenic acids [18:3(n-3)] did not significantly affect LDLr expression. ACAT inhibitor (58-035, 1 mg/L) attenuated the regulatory effect of AA on LDLr protein abundance by approximately 40% (P < 0.05), but did not modify the regulatory effects of other unsaturated fatty acids in HepG2 cells. The present results suggest that AA, EPA, and DHA increase LDLr protein levels, and that ACAT plays a role in modulating the effects of AA on LDLr protein levels. Furthermore, the effects of the fatty acids appeared to be independent of any change in SREBP-1 protein.

Mechanisms by which dietary fatty acids modulate plasma lipids

Dietary fatty acids have a considerable effect on plasma LDL cholesterol (LDL-C) concentrations and therefore on the risk for coronary heart disease. Numerous studies have been conducted in animal models to elucidate the mechanisms by which different types of fatty acids modulate plasma cholesterol concentrations. In addition, multiple clinical trials and epidemiological data have demonstrated the effects of fatty acids in determining the concentrations of circulating LDL. SFAs and trans fatty acids have a detrimental effect on plasma lipids, whereas PUFAs of the (n-6) family and monounsaturated fatty acids decrease plasma LDL-C concentrations. Among the SFAs, stearic acid (18:0) appears to have a neutral effect on LDL-C, while lauric (12:0), myristic (14:0), and palmitic (16:0) acids are considered to be hypercholesterolemic. SFAs increase plasma LDL-C by increasing the formation of LDL in the plasma compartment and by decreasing LDL turnover. Although unsaturated fatty acids increase cholesterol synthesis, they also increase hepatic LDL receptor number and LDL turnover in vivo. Fatty acids are also ligands of important regulatory elements, which can play a role in determining plasma cholesterol. This article presents a summary of the major effects of various types of fatty acids on plasma lipid concentrations and the mechanisms involved.

Effects of a novel antihyperlipidemic agent, S-2E, on the blood lipid abnormalities in homozygous WHHL rabbits

To improve mixed hyperlipidemia in the low-density lipoprotein (LDL) receptor-deficient state, suppression of very-low-density lipoprotein (VLDL) particle production may be an important approach. We previously reported that S-2E, (+)-(S)-p-[1-(p-tert-butylphenyl)-2-oxo-4-pyrrolidinyl] methoxybenzoic acid, suppressed VLDL particle production by inhibiting the biosynthesis of both sterol and fatty acids in the liver. We therefore examined whether S-2E lowered the blood cholesterol and triglyceride (TG) levels simultaneously in homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits, which correspond to human familial hypercholesterolemia. S-2E given orally at doses of 30 to 300 mg/kg significantly lowered serum total cholesterol (TC) levels at 1 week as well as TG at 2 weeks, and the lowering of TC and TG levels by S-2E reached a maximum at 3 to 4 weeks. In contrast, oral administration of pravastatin at doses of 10 to 100 mg/kg resulted in a significant suppression of TC levels (100 mg/kg) but not TG levels. Further analysis of the TC content in fractionated serum of control and S-2E-treated animals showed that suppression of TC level by S-2E is attributable to a decrease in the proportions of VLDL, intermediate-density lipoprotein (IDL), and LDL. It is, therefore, reasonable to assume that S-2E may be useful to improve the blood lipid abnormalities in the LDL receptor-deficient state.

Conjugated linoleic acid upregulates LDL receptor gene expression in HepG2 cells

Conjugated linoleic acid (CLA) exerts anticarcinogenic and antiatherosclerotic effects in animals. The present study was conducted to examine the effects of CLA on LDL receptor (LDLr) expression in HepG2 cells, and to evaluate whether the sterol response element binding protein 1 (SREBP-1) and acyl CoA:cholesterol acyltransferase (ACAT) were involved in the regulation of LDLr expression by CLA. When HepG2 cells were cultured with serum-free DMEM for 48 h, there was a three- to fivefold (P<0.05) increase in LDLr protein and mRNA levels. Incubation of HepG2 cells in serum-free medium supplemented with 25-hydroxycholesterol (25OH, 5 mg/L) for 24 h decreased LDLr protein and mRNA by 50-70% (P<0.05) and mature SREBP-1 by 20-40% (P<0.05). CLA, but not linoleic acid, antagonized the depressive effects of 25OH and increased both LDLr protein and mRNA abundance twofold (P<0.05). LDLr protein and mRNA abundance were not different when HepG2 cells were cultured with CLA (0.4 mmol/L) plus 25OH in the presence or absence of an ACAT inhibitor (58-035, 1 mg/L). Furthermore, CLA had no effect on SREBP-1 abundance. These results suggest that CLA upregulates LDLr expression via a mechanism that is independent of ACAT and SREBP-1.

Differential effects of polyunsaturated fatty acids on sterol synthesis rates in adult and fetal tissues of the hamster: consequence of altered sterol balance

Cholesterol is necessary for the proper growth and development of the fetus. Consequently, disruptions in cholesterol biosynthesis lead to abnormal fetal development. It has been shown that in cells exposed to polyunsaturated fatty acids (PUFA), the expressions of genes and activities of enzymes involved in cholesterol synthesis are reduced. Similarly, we found that adult male hamsters fed PUFA-enriched diets had an approximately 60% reduction in in vivo hepatic sterol synthesis rates. If fetal tissues respond to PUFA in the same manner as do adult livers, then maternal dietary PUFA could lead to a reduction in fetal sterol synthesis rates and possibly abnormal development. To investigate the impact of maternal dietary fatty acids on fetal sterol synthesis rates, female hamsters were fed diets enriched in various fatty acids before and throughout gestation. In vivo sterol synthesis rates were measured in fetuses at mid- and late gestation. At both gestational stages, dietary PUFA had no effect on fetal sterol synthesis rates. This lack of effect was not a consequence of a lack of PUFA enrichment in fetal fatty acids or the lack of PUFA receptor expression in the fetus. We hypothesize that the fetus may experience a dysregulation of sterol synthesis as the result of the fetus being in a negative sterol balance; the PUFA-induced suppression of sterol synthesis in the adult male hamster liver was ablated by creating a net negative sterol balance across the adult hepatocyte.