Dietary conjugated linoleic acid decreases adipocyte size and favorably modifies adipokine status and insulin sensitivity in obese, insulin-resistant rats

Conjugated linoleic acids (CLA) have been shown to alter adiposity in some species with varying effects on insulin resistance. The objective of this 8-week study was to investigate the effects of feeding a CLA mixture (1.5%, wt/wt) on adipocyte size, insulin sensitivity, adipokine status, and adipose lipid composition in fa/fa vs lean Zucker rats. The fa/fa CLA-fed rats had smaller adipocytes and improved insulin sensitivity compared with fa/fa rats fed the control diet. Conjugated linoleic acids did not affect select markers of adipose differentiation, lipid filling, lipid uptake, or oxidation. Dietary CLA, compared with the control diet, reduced circulating leptin and elevated fasting serum adiponectin concentrations in fa/fa rats. Adipose resistin messenger RNA levels were greater in fa/fa CLA-fed rats compared with fa/fa control rats. CLA did not markedly alter adipose phospholipid fatty acid composition, and the changes in the triacylglycerol fatty acid composition reflected a lower delta-9 desaturase index of CLA-fed vs control-fed rats. In conclusion, CLA reduced adipocyte size and favorably modified adipokine status and insulin sensitivity in fa/fa Zucker rats.     

Physiological difference between obese (fa/fa) Zucker rats and lean Zucker rats concerning adiponectin

Obese (fa/fa) Zucker rat is a spontaneous genetic obesity model and, by comparison with lean Zucker rat, exhibits hyperphagia, hyperinsulinemia, and hyperlipidemia. The aim of this study was to examine the physiological difference concerning adiponectin between obese (fa/fa) Zucker rats and control lean Zucker rats. We therefore measured plasma adiponectin level and analyzed adiponectin and adiponectin receptor 1 mRNA expression in retroperitoneal white adipose tissue (RT WAT), brown adipose tissue (BAT), liver, and soleus muscle. We also examined the tissue mRNA expression of peroxisome proliferator-activated receptor alpha (PPAR alpha), PPAR delta, and PPAR gamma, which regulate adiponectin expression sensitivity to a PPAR gamma agonist shown by brown adipocytes from obese (fa/fa) Zucker rats and lean Zucker rats, by measuring adiponectin release from these cells. Plasma adiponectin levels of obese (fa/fa) Zucker rats were significantly higher than those of lean Zucker rats. Adiponectin mRNA expression levels in RT WAT were lower in obese (fa/fa) Zucker rats than in lean Zucker rats, but those in BAT were higher. Adiponectin receptor 1 expression levels in RT WAT, BAT, and liver of obese (fa/fa) Zucker rats were lower than in lean Zucker rats. The expression level of PPAR alpha, PPAR delta, and PPAR gamma in BAT was lower in obese (fa/fa) Zucker rats than in lean Zucker rats. Moreover, the PPAR gamma agonist increased adiponectin release only from the brown adipocytes isolated from lean Zucker rats. It is the conclusive difference between obese (fa/fa) Zucker rats and lean Zucker rats that plasma adiponectin levels of obese (fa/fa) Zucker rats are significantly higher than those of lean Zucker rats. Moreover, we clarified that mRNA expression level of adiponectin receptor 1 in RT WAT, BAT, and liver of obese (fa/fa) Zucker rats is low despite high plasma adiponectin level, and low expression of PPARs in BAT leads to less sensibility of adiponectin release from brown adipocytes to a PPAR gamma agonist in obese (fa/fa) Zucker rats.     

Supplementation with<Emphasis Type="Italic"> trans</Emphasis>10<Emphasis Type="Italic">cis</Emphasis>12-conjugated linoleic acid induces hyperproinsulinaemia in obese men: close association with impaired insulin sensitivity

Aims/hypothesis Hyperproinsulinaemia reflects both beta cell dysfunction and insulin resistance in cross-sectional studies, but it is not known whether changes in proinsulin concentrations are related to insulin resistance over time. As trans10cis12 (t10c12)-conjugated linoleic acid (CLA) supplementation induces insulin resistance in obese men, we used this fatty acid to investigate the effects on plasma proinsulin, insulin, C-peptide and adiponectin concentrations, including their associations with change in insulin sensitivity.Methods We randomised (double-blind) 57 non-diabetic abdominally obese men to receive either 3.4 g t10c12CLA, CLA-isomer mixture or control oil for 12 weeks. Insulin sensitivity (hyperinsulinaemic–euglycaemic clamp), intact proinsulin, insulin, the proinsulin : insulin ratio, C-peptide, glucose and adiponectin were assessed before and after supplementation.Results Supplementation with t10c12CLA increased proinsulin (p<0.01), the proinsulin : insulin ratio (p<0.05) and C-peptide concentrations (p<0.001) in comparison with control subjects. Adiponectin, however, did not change significantly. The change in proinsulin, but not the proinsulin : insulin ratio, was related to impaired insulin sensitivity (r=–0.58, p<0.0001), independently of changes in insulin, C-peptide, glucose, adiponectin and BMI. Conversely, the correlation between insulin sensitivity and specific insulin (r=–0.46, p<0.001) did not remain significant after adjustment for proinsulin. Induced hyperproinsulinaemia was also correlated to adiponectin concentrations (r=–0.34, p<0.01).Conclusions/interpretation In obese men, t10c12CLA induces hyperproinsulinaemia that is related to impaired insulin sensitivity, independently of changes in insulin concentrations. These results are of clinical interest, as hyperproinsulinaemia predicts diabetes and cardiovascular disease. The use of weight-loss supplements containing this fatty acid is worrying.Abbreviations ANCOVA analysis of covariance - CLA conjugated linoleic acid - t10c12 trans10cis12     

Differential effects of dietary saturated and trans-fatty acids on expression of genes associated with insulin sensitivity in rat adipose tissue

OBJECTIVE: Trans-fatty acids (TFAs) are formed during partial hydrogenation of vegetable oils and are shown to be more atherogenic than saturated fatty acids (SFAs). Our previous study showed that dietary TFAs decrease adipose tissue insulin sensitivity to a greater extent than SFAs in rats. We hypothesized that the effects of these fatty acids on insulin sensitivity could be mediated through an alteration in gene expression. In the current study we have investigated the effects of dietary TFAs or SFAs on expression of genes associated with insulin sensitivity in rat adipose tissue. DESIGN AND METHODS: Male weanling Wistar/NIN rats were divided into four groups and fed one of the following diets containing 10% fat (g/100 g diet) differing only in the fatty acid composition for 3 months: control diet (3.7% linoleic acid (LA)), SFA diet (5% SFA), TFA diet 1 (1.5% TFA + 1% LA) and TFA diet 2 (1.5% TFA + 2% LA). The mRNA expression of peroxisome proliferator-activated receptor gamma (PPARgamma), lipoprotein lipase (LPL), glucose transporter-4 (GLUT4), resistin and adiponectin was analyzed in epididymal fat using RT-PCR. The effects of TFA were studied at two levels of LA to understand the beneficial effects of LA over the effects of TFA. RESULTS: Both dietary SFA and TFA upregulated the mRNA levels of resistin. Dietary SFA downregulated adiponectin and GLUT4 and upregulated LPL, while TFA downregulated PPARgamma and LPL. The effects of dietary TFA on PPARgamma and resistin were not counteracted by increased LA (TFA diet 2). CONCLUSION: The effects of SFAs on the aforementioned genes except PPARgamma could be extrapolated towards decreased insulin sensitivity, while only the alteration in the mRNA levels of PPARgamma and resistin could be associated with insulin resistance in TFA-fed rats. These findings suggest that dietary SFAs and TFAs alter the expression of different genes associated with insulin sensitivity in adipose tissue.     

Dietary conjugated linoleic acid preserves pancreatic function and reduces inflammatory markers in obese, insulin-resistant rats

Pancreatic preservation is an important part of diabetes management that may occur with improved peripheral insulin sensitivity and attenuated low-grade adipose tissue inflammation. The objective of the current study was to determine the response of obese, insulin-resistant fa/fa Zucker rats vs lean controls to dietary conjugated linoleic acid (CLA) supplementation with respect to pancreatic islet size, insulin resistance, and markers of inflammation and adipose glucose uptake. Six-week-old fa/fa and lean Zucker rats (n = 20 per genotype) were fed either a 1.5% CLA mixture or control diet for 8 weeks. Oral glucose tolerance testing was conducted at 7.5 weeks. Fasting serum haptoglobin, insulin, and C-peptide were assayed, and select messenger RNA (mRNA) and protein markers of inflammation and glucose metabolism were measured in adipose and liver tissues. CLA-fed fa/fa Zucker rats had smaller islet cell size, improved oral glucose tolerance and insulinemia, and attenuated serum haptoglobin levels compared with control-fed fa/fa Zucker rats, despite no differences in body weight and a slightly higher visceral adipose mass. CLA did not alter insulin sensitivity or islet size in lean Zucker rats. The CLA-fed fa/fa rats also had greater adipose glucose transporter-4 mRNA and less adipose tumor necrosis factor alpha mRNA and protein compared with control-fed fa/fa rats. In contrast, other markers of inflammation and glucose metabolism including adipose macrophage inflammatory factor, macrophage inflammatory protein-2, and liver pyruvate carboxylase and pyruvate dehydrogenase kinase 4 were not significantly changed. These results suggest that CLA supplementation preserved pancreatic function in conjunction with improved peripheral glucose use and reduced inflammation in fa/fa Zucker rats.     

Docosahexaenoic Acid (DHA) But Not Eicosapentaenoic Acid (EPA) Prevents Trans-10, Cis-12 Conjugated Linoleic Acid (CLA)-Induced Insulin Resistance in Mice

Background: The objective of this study was to investigate if eicosapentaenoic acid (20:5n-3, EPA) or docosahexaenoic acid (22:6n-3, DHA) or both would prevent conjugated linoleic acid (CLA)-induced insulin resistance and fatty liver. Methods: Eight-week-old, pathogen-free C57BL/6N female mice (10 per group) were fed either a control diet or diets containing t10, c12-CLA (0.5 wt %), CLA + DHA (0.5% + 1.5 wt %), or CLA + EPA (0.5% + 1.5 wt %) for 8 weeks prior to sacrifice and tissue collection. Results: CLA supplementation caused an 8.9-fold increase in circulating insulin, a 2.6-fold increase in liver weight, and a 6.2-fold increase in the weight of total lipids in the liver as compared with the corresponding values in the control group. DHA prevented the CLA-induced insulin resistance, while EPA was ineffective. Both EPA and DHA prevented CLA-induced fatty liver and reduced weights of total liver lipids to the levels of the control group. CLA also reduced the plasma leptin and adiponectin concentrations to approximately 15% of those in the control group. Both EPA and DHA partially restored the CLA-induced decrease in leptin, but only DHA partially restored the plasma adiponectin. Conclusions: Our results suggest that DHA but not EPA in fish oils may reduce insulin resistance which may be mediated through an increase in circulating adiponectin. These findings may have clinical implications in the dietary management of patients at risk of insulin resistance and diabetes.     

Expression of the adiponectin receptors AdipoR1 and AdipoR2 in lean rats and in obese Zucker rats

The adiponectin receptors, AdipoR1 and AdipoR2, are thought to transmit the insulin-sensitizing effects of adiponectin, an adipokine secreted by adipocytes. Modifications of their expression in insulin-sensitive tissues (skeletal muscle, liver, and adipose tissue) could therefore play a role in the control of insulin sensitivity and the development of insulin resistance. Recent data in mice supported this possibility. We examined whether the expression of adiponectin receptors (messenger RNA [mRNA] concentrations) is controlled in vivo in rats (Wistar) by nutritional factors (high-fat [HF] vs high-carbohydrate diet, fasting vs fed state) and whether this expression is decreased in an experimental model of insulin resistance, the obese Zucker rat. In Wistar rats, neither an HF diet nor fasting modified the mRNA concentrations of AdipoR1 in muscle, liver, or adipose tissue; the only modification observed was a decrease (P < .05) in AdipoR2 mRNA level in the liver of rats fed with an HF diet. In obese Zucker rats compared with their lean controls, neither AdipoR1 nor AdipoR2 expression was modified in muscle. AdipoR2 expression was slightly decreased in adipose tissue, whereas the expression of both AdipoR1 and AdipoR2 was increased (P < .05) in the liver of obese Zucker rats. In conclusion, contrary to what was reported in mice, the expression of adiponectin receptors in rats is poorly responsive to changes in nutritional conditions and is not decreased in a model of insulin resistance. These results do not support an important role for the expression of AdipoR1 and AdipoR2 in the modulation of sensitivity to insulin.     

The polygenetically inherited metabolic syndrome of WOKW rats is associated with insulin resistance and altered gene expression in adipose tissue

BACKGROUND: Wistar Ottawa Karlsburg W (RT1u) rats (WOKW) develop a complete metabolic syndrome closely resembling the human disease. The aim of this study was to characterize the phenotype of adipose tissue in WOKW rats with regard to adipocyte metabolism, insulin resistance, and gene expression and thus to define the phenotype more precisely. METHODS: Glucose metabolism, insulin sensitivity, and gene expression of key adipocyte genes, including adiponectin, interleukin 6 (Il6), 11 beta-hydroxysteroid dehydrogenase (11beta Hsd), peroxisome proliferator-activated receptor gamma (Ppar gamma), forkhead box O1 (Foxo1), glucose transporter 4 (Glut4), CCAAT/enhancer binding protein (C/ebp alpha), and fatty acid synthase (Fasn) were characterized in adipocytes from epididymal and subcutaneous fat depots of 28-week-old male WOKW rats and Dark Agouti (DA) controls. RESULTS: WOKW rats display decreased insulin-stimulated glucose uptake and decreased insulin sensitivity during lipogenesis and lipolysis in isolated adipocytes. The severe insulin resistance predominantly in epididymal adipose tissue of WOKW rats is associated with a 10-fold decrease in adipocyte adiponectin gene expression, decreased Ppar gamma, but increased Foxo1 gene expression compared to DA rats. CONCLUSIONS: Insulin resistance in adipose tissue is associated with altered adipocyte gene expression in WOKW rats, additionally completing the picture of the metabolic syndrome in this animal model. This fact not only qualifies the WOKW rat for further detailed analysis of genetic determinants of metabolic syndrome but also highlights its suitability for pharmacological research.     

Hyperinsulinaemia triggered by dietary conjugated linoleic acid is associated with a decrease in leptin and adiponectin plasma levels and pancreatic beta cell hyperplasia in the mouse

Aims/hypothesis Dietary supplementation with conjugated linoleic acids (CLA) has a fat-reducing effect in various species, but induces severe hyperinsulinaemia and hepatic steatosis in the mouse. This study aimed to determine the causes of the deleterious effects of CLA on insulin homeostasis.Methods The chronology of adipose and liver weight, hepatic triglyceride accumulation and selected blood parameters, including lipids, insulin, leptin and adiponectin, was determined in C57BL/6J female mice fed a 1% isomeric mixture of CLA for various periods of time ranging from 2 to 28 days. Insulin secretion was measured in 1-h static incubations of pancreatic islets, and pancreas morphometric parameters were determined in mice fed CLA for 28 days.Results Plasma levels of leptin and adiponectin sharply decreased after 2 days of CLA feeding, although adipose tissue mass only decreased after day 6. Hyperinsulinaemia developed at day 6 and consistently worsened up to day 28, in parallel with increases in hepatic lipid content. Islets from CLA-fed mice displayed three- to four-fold increased rates of glucose-stimulated insulin secretion, both in the absence and presence of isobutyl methylxanthine or carbachol. The increased insulin-releasing capacity of islets from CLA-fed mice was explained by an increase in beta cell mass and number.Conclusions/interpretation The data suggest that CLA supplementation induces a profound reduction of leptinaemia and adiponectinaemia, followed by hyperinsulinaemia due to the increased secretory capacity of pancreatic islets, leading, in turn, to liver steatosis. These observations cast doubt on the safety of dietary supplements containing CLA.     

Comprehensive messenger ribonucleic acid profiling reveals that peroxisome proliferator-activated receptor gamma activation has coordinate effects on gene expression in multiple insulin-sensitive tissues

Peroxisome proliferator-activated receptor gamma (PPAR gamma) agonists, including the glitazone class of drugs, are insulin sensitizers that reduce glucose and lipid levels in patients with type 2 diabetes mellitus. To more fully understand the molecular mechanisms underlying their therapeutic actions, we have characterized the effects of the potent, tyrosine-based PPAR gamma ligand GW1929 on serum glucose and lipid parameters and gene expression in Zucker diabetic fatty rats. In time-course studies, GW1929 treatment decreased circulating FFA levels before reducing glucose and triglyceride levels. We used a comprehensive and unbiased messenger RNA profiling technique to identify genes regulated either directly or indirectly by PPAR gamma in epididymal white adipose tissue, interscapular brown adipose tissue, liver, and soleus skeletal muscle. PPAR gamma activation stimulated the expression of a large number of genes involved in lipogenesis and fatty acid metabolism in both white adipose tissue and brown adipose tissue. In muscle, PPAR gamma agonist treatment decreased the expression of pyruvate dehydrogenase kinase 4, which represses oxidative glucose metabolism, and also decreased the expression of genes involved in fatty acid transport and oxidation. These changes suggest a molecular basis for PPAR gamma-mediated increases in glucose utilization in muscle. In liver, PPAR gamma activation coordinately decreased the expression of genes involved in gluconeogenesis. We conclude from these studies that the antidiabetic actions of PPAR gamma agonists are probably the consequence of 1) their effects on FFA levels, and 2), their coordinate effects on gene expression in multiple insulin-sensitive tissues.     

Increasing skeletal muscle fatty acid transport protein 1 (FATP1) targets fatty acids to oxidation and does not predispose mice to diet-induced insulin resistance

Aims/hypothesis  

We examined in skeletal muscle (1) whether fatty acid transport protein (FATP) 1 channels long-chain fatty acid (LCFA) to specific metabolic fates in rats; and (2) whether FATP1-mediated increases in LCFA uptake exacerbate the development of diet-induced insulin resistance in mice. We also examined whether FATP1 is altered in insulin-resistant obese Zucker rats.     

Conjugated linoleic acid isomers have no effect on atherosclerosis and adverse effects on lipoprotein and liver lipid metabolism in apoE-/- mice fed a high-cholesterol diet

Dietary supplementation with conjugated linoleic acid (CLA) has been shown, in several animal models, to decrease the development of atherosclerosis. The mechanism behind the anti-atherogenic properties of CLA is not clear. The objectives of this study were to determine the effect of CLA on atherosclerosis, lipoprotein and liver lipid metabolism, and plasma adiponectin and insulin in apoE^−/− mice fed an atherogenic (16%, w/w fat; 1.25%, w/w cholesterol) diet. Mice were fed the diet with or without supplementation of linoleic acid (LA), c-9,t-11 CLA, t-10,c-12 CLA, or a 1:1 mixture of the two CLA isomers, at a concentration of 0.5% (w/w), for 12 weeks. Relative to the LA group, CLA supplementation had no significant effect on the lesion area in either en face preparations of the aorta or in aortic root cross-sections. Plasma triacylglycerol and cholesterol concentrations were higher in the t-10,c-12 CLA group than all other treatment groups and liver weight was also increased in this group due to a three-fold increase in liver triacylglycerol. Supplementation with t-10,c-12 CLA or mixed CLA reduced plasma adiponectin levels, whereas t-10,c-12 CLA increased plasma insulin levels. Liver triglycerides correlated directly with blood glucose and plasma insulin and inversely with plasma adiponectin. We conclude that dietary supplementation with CLA does not affect atherosclerosis of the apoE^−/− mouse on a high-cholesterol diet. Furthermore, t-10,c-12 CLA causes adverse changes in adipocyte function and plasma and liver lipid metabolism, which are partially ameliorated by the inclusion of the c-9,t-11 CLA isomer.     

Down-regulation of hepatic stearoyl-CoA desaturase 1 expression by angiotensin II receptor blocker in the obese fa/fa Zucker rat: possible role in amelioration of insulin resistance and hepatic steatosis

Background  It has been reported that angiotensin II type 1 receptor blocker (ARB) can ameliorate hepatic steatosis and insulin resistance. Stearoyl-CoA desaturase 1 (SCD-1), which catalyzes the cellular synthesis of monounsaturated fatty acids, affects lipid metabolism. In this study, we investigated whether SCD-1 gene expression is affected by ARB treatment. Methods  Obese fa/fa Zucker rats fed a high-fat diet were treated with a potent ARB and olmesartan, and the resulting changes in the components of serum and liver were studied. Gene expression of hepatic SCD-1 was assayed using real-time PCR. Results  The serum glucose and insulin levels and hepatic TG content of the obese Zucker rats fed a high-fat diet were reduced after olmesartan administration, while the serum adiponectin level was increased. Real-time PCR revealed an increase of SCD-1 gene expression in the liver of these rats, followed by a reduction after olmesartan administration. The ratio of stearic acid (C18:0) to oleic acid (C18:1) in the liver was increased by olmesartan, indicating a reduction in the in vivo activity of SCD-1. Conclusions  ARB ameliorates hepatic steatosis and insulin resistance in obese fa/fa Zucker rats fed a high-fat diet. Gene expression of SCD-1 is decreased by olmesartan, suggesting that the beneficial effect is due partly to suppression of the key enzyme for hepatic lipid metabolism by ARB.     

Fat depot-specific expression of adiponectin is impaired in Zucker fatty rats

Adiposity, particularly increased intra-abdominal fat, is a predisposing factor for the development of insulin resistance in obesity and type 2 diabetes. Visceral fat seems to differ from subcutaneous adipose tissue in adipocytokine production. This fat depot-related difference has been viewed as an important mechanism by which adipose tissue exerts its paracrine/autocrine effects on peripheral tissue in modulating insulin sensitivity. We have studied the relative expression of adiponectin in visceral versus subcutaneous fat in Zucker fatty versus lean rats. Visceral fat, as opposed to subcutaneous fat, exhibited relatively higher levels of adiponectin production in lean animals. However, in Zucker fatty rats, adiponectin expression in visceral fat was suppressed to basal levels, which correlated with significantly reduced plasma adiponectin concentrations and increased insulin resistance. These results suggest that an impaired depot-specific expression of adiponectin is a contributing factor for the development of insulin resistance in Zucker fatty rats.     

Visceral adipose tissue-derived serine protease inhibitor: a unique insulin-sensitizing adipocytokine in obesity

There is a rapid global rise in obesity, and the link between obesity and diabetes remains somewhat obscure. We identified an adipocytokine, designated as visceral adipose tissue-derived serpin (vaspin), which is a member of serine protease inhibitor family. Vaspin cDNA was isolated by from visceral white adipose tissues (WATs) of Otsuka Long-Evans Tokushima fatty (OLETF) rat, an animal model of abdominal obesity with type 2 diabetes. Rat, mouse, and human vaspins are made up of 392, 394, and 395 amino acids, respectively; exhibit approximately 40% homology with alpha1-antitrypsin; and are related to serine protease inhibitor family. Vaspin was barely detectable in rats at 6 wk and was highly expressed in adipocytes of visceral WATs at 30 wk, the age when obesity, body weight, and insulin levels peak in OLETF rats. The tissue expression of vaspin and its serum levels decrease with worsening of diabetes and body weight loss at 50 wk. The expression and serum levels were normalized with the treatment of insulin or insulin-sensitizing agent, pioglitazone, in OLETF rats. Administration of vaspin to obese CRL:CD-1 (ICR) (ICR) mice fed with high-fat high-sucrose chow improved glucose tolerance and insulin sensitivity reflected by normalized serum glucose levels. It also led to the reversal of altered expression of genes relevant to insulin resistance, e.g., leptin, resistin, TNFalpha, glucose transporter-4, and adiponectin. In DNA chip analyses, vaspin treatment resulted in the reversal of expression in approximately 50% of the high-fat high-sucrose-induced genes in WATs. These findings indicate that vaspin exerts an insulin-sensitizing effect targeted toward WATs in states of obesity.