Of mice and men (and women) and the acylation-stimulating protein pathway

Adipose Tissue-Derived Signatures for Obesity and Type 2 Diabetes: Adipokines, Batokines and MicroRNAs

: Obesity is one of the main risk factors for type 2 diabetes mellitus (T2DM). It is closely related to metabolic disturbances in the adipose tissue that primarily functions as a fat reservoir. For this reason, adipose tissue is considered as the primary site for initiation and aggravation of obesity and T2DM. As a key endocrine organ, the adipose tissue communicates with other organs, such as the brain, liver, muscle, and pancreas, for the maintenance of energy homeostasis. Two different types of adipose tissues-the white adipose tissue (WAT) and brown adipose tissue (BAT)-secrete bioactive peptides and proteins, known as "adipokines" and "batokines," respectively. Some of them have beneficial anti-inflammatory effects, while others have harmful inflammatory effects. Recently, "exosomal microRNAs (miRNAs)" were identified as novel adipokines, as adipose tissue-derived exosomal miRNAs can affect other organs. In the present review, we discuss the role of adipose-derived secretory factors-adipokines, batokines, and exosomal miRNA-in obesity and T2DM. It will provide new insights into the pathophysiological mechanisms involved in disturbances of adipose-derived factors and will support the development of adipose-derived factors as potential therapeutic targets for obesity and T2DM.

Effects of weight loss and pharmacotherapy on inflammatory markers of cardiovascular disease

Obesity is currently an epidemic, and the prevalence of cardiovascular risk factors is increasing dramatically as a result. Visceral adiposity is correlated with a proinflammatory and prothrombotic state that is believed to promote atherosclerosis and acute coronary syndromes. This article will review clinical trials on the effects of weight loss and pharmacotherapy on obesity associated inflammatory and thrombotic markers linked with cardiovascular disease.

Effects of sugar-sweetened beverages on plasma acylation stimulating protein, leptin and adiponectin: relationships with metabolic outcomes

OBJECTIVE

The effects of fructose and glucose consumption on plasma acylation stimulating protein (ASP), adiponectin, and leptin concentrations relative to energy intake, body weight, adiposity, circulating triglycerides, and insulin sensitivity were determined.

DESIGN AND METHODS

Thirty two overweight/obese adults consumed glucose- or fructose-sweetened beverages (25% energy requirement) with their ad libitum diets for 8 weeks, followed by sweetened beverage consumption for 2 weeks with a standardized, energy-balanced diet. Plasma variables were measured at baseline, 2, 8, and 10 weeks, and body adiposity and insulin sensitivity at baseline and 10 weeks.

RESULTS

Fasting and postprandial ASP concentrations increased at 2 and/or 8 weeks. ASP increases correlated with changes in late-evening triglyceride concentrations. At 10 weeks, fasting adiponectin levels decreased in both groups, and decreases were inversely associated with baseline intra-abdominal fat volume. Sugar consumption increased fasting leptin concentrations; increases were associated with body weight changes. The 24-h leptin profiles increased during glucose consumption and decreased during fructose consumption. These changes correlated with changes of 24-h insulin levels.

CONCLUSIONS

The consumption of fructose and glucose beverages induced changes in plasma concentrations of ASP, adiponectin, and leptin. Further study is required to determine if these changes contribute to the metabolic dysfunction observed during fructose consumption.

Dyslipidemia in obesity: mechanisms and potential targets

Obesity has become a major worldwide health problem. In every single country in the world, the incidence of obesity is rising continuously and therefore, the associated morbidity, mortality and both medical and economical costs are expected to increase as well. The majority of these complications are related to co-morbid conditions that include coronary artery disease, hypertension, type 2 diabetes mellitus, respiratory disorders and dyslipidemia. Obesity increases cardiovascular risk through risk factors such as increased fasting plasma triglycerides, high LDL cholesterol, low HDL cholesterol, elevated blood glucose and insulin levels and high blood pressure. Novel lipid dependent, metabolic risk factors associated to obesity are the presence of the small dense LDL phenotype, postprandial hyperlipidemia with accumulation of atherogenic remnants and hepatic overproduction of apoB containing lipoproteins. All these lipid abnormalities are typical features of the metabolic syndrome and may be associated to a pro-inflammatory gradient which in part may originate in the adipose tissue itself and directly affect the endothelium. An important link between obesity, the metabolic syndrome and dyslipidemia, seems to be the development of insulin resistance in peripheral tissues leading to an enhanced hepatic flux of fatty acids from dietary sources, intravascular lipolysis and from adipose tissue resistant to the antilipolytic effects of insulin. The current review will focus on these aspects of lipid metabolism in obesity and potential interventions to treat the obesity related dyslipidemia.

Plasma Levels of Acylation-Stimulating Protein Are Strongly Predicted by Waist/Hip Ratio and Correlate with Decreased LDL Size in Men

The association of abdominal obesity with cardiovascular risk is often linked to altered secretion of adipose-derived factors and an abnormal lipid profile including formation of atherogenic small dense low density lipoprotein particles (sdLDL). Acylation-stimulating protein (ASP) is an adipose-derived hormone that exhibits potent lipogenic effects. Plasma ASP levels increase in obesity; however, the association of ASP levels with body fat distribution is not yet established, and no study to date has investigated the association of ASP with LDL size. In this study, we examined the association of ASP levels with abdominal obesity measures and the lipid profile including LDL size in 83 men with a wide range of abdominal girths. Regression analysis showed that waist/hip ratio was the main predictor of ASP levels (β = 0.52, P < 0.0001), significantly followed by decreased LDL size. BMI and TG levels, although positively correlated with ASP levels, were excluded as significant predictors in regression analysis. No correlation was found with LDL-C or apoB levels. ASP levels were 62.5% higher in abdominally obese compared to nonobese men. Waist/hip ratio presenting as the main predictor of ASP levels, suggests increased ASP production by abdominal fat which, as proposed previously, may result from resistance to ASP function causing delayed TG clearance and subsequent formation of atherogenic sdLDL.

Triacylglycerols and body fat mass are possible independent predictors of C3 in apparently healthy young Brazilian adults

OBJECTIVE

To evaluate the association between serum concentrations of complement factor-3 (C3) with anthropometric, biochemical, and lifestyle features in healthy young adults.

METHODS

From 157 young healthy adults 18 to 35 y old, anthropometric measurements and body composition, systolic and diastolic blood pressures, and lifestyle data were collected and analyzed. Blood samples were collected after a 12-h fast for the determination of glucose, triacylglycerols, total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, insulin, C3, ceruloplasmin, and uric acid.

RESULTS

Complement factor-3 correlated directly with body mass index (r = 0.23417, P = 0.0032), body fat mass (bioelectrical impedance analysis; r = 0.33407, P < 0.0001), percentage of body fat (bioelectrical impedance analysis; r = 0.26873, P = 0.0007), waist circumference (r = 0.21266, P = 0.0075), insulin (r = 0.26152, P = 0.0009), homeostasis model assessment of insulin resistance (r = 0.24831, P = 0.0017), total cholesterol (r = 0.23335, P = 0.0033), triacylglycerols (r = 0.38435, P < 0.0001), and other outcome measurements. In the multiple linear regression analysis, triacylglycerols (r(2) = 0.1379, P < 0.0001) and body fat mass (bioelectrical impedance analysis; r(2) = 0.0621, P = 0.0010) were independently associated with the C3 concentration after adjusting for age, gender, smoking status, and physical activity.

CONCLUSION

Complement factor-3 seems to be related to several anthropometric and biochemical measurements in healthy young adults. These results demonstrate an independent role of triacylglycerols, a component of the metabolic syndrome, and body fat mass as possible predictors of C3 concentrations. Thus, C3 can be used as an early marker for metabolic syndrome manifestations.

The evolution of the adipose tissue: a neglected enigma

The complexity of the anatomical distribution and functions of adipose tissue (AT) has been rarely analyzed in an evolutionary perspective. From yeast to man lipid droplets are stored mainly in the form of triglycerides in order to provide energy during periods when energy demands exceed caloric intake. This simple scenario is in agreement with the recent discovery of a highly conserved family of proteins for fat storage in both unicellular and multicellular organisms. However, the evolutionary history of organs such as the fat body in insects, playing a role in immunity and other functions besides energy storage and thermal insulation, and of differently distributed subtypes of AT in vertebrates is much less clear. These topics still await a systematic investigation using up-to-date technologies and approaches that would provide information useful for understanding the role of different AT subtypes in normal/physiological conditions or in metabolic pathologies of humans.

ApoB100-LDL acts as a metabolic signal from liver to peripheral fat causing inhibition of lipolysis in adipocytes

Background

Free fatty acids released from adipose tissue affect the synthesis of apolipoprotein B-containing lipoproteins and glucose metabolism in the liver. Whether there also exists a reciprocal metabolic arm affecting energy metabolism in white adipose tissue is unknown.

Methods and Findings

We investigated the effects of apoB-containing lipoproteins on catecholamine-induced lipolysis in adipocytes from subcutaneous fat cells of obese but otherwise healthy men, fat pads from mice with plasma lipoproteins containing high or intermediate levels of apoB100 or no apoB100, primary cultured adipocytes, and 3T3-L1 cells. In subcutaneous fat cells, the rate of lipolysis was inversely related to plasma apoB levels. In human primary adipocytes, LDL inhibited lipolysis in a concentration-dependent fashion. In contrast, VLDL had no effect. Lipolysis was increased in fat pads from mice lacking plasma apoB100, reduced in apoB100-only mice, and intermediate in wild-type mice. Mice lacking apoB100 also had higher oxygen consumption and lipid oxidation. In 3T3-L1 cells, apoB100-containing lipoproteins inhibited lipolysis in a dose-dependent fashion, but lipoproteins containing apoB48 had no effect. ApoB100-LDL mediated inhibition of lipolysis was abolished in fat pads of mice deficient in the LDL receptor (Ldlr^−/−Apob^100/100).

Conclusions

Our results show that the binding of apoB100-LDL to adipocytes via the LDL receptor inhibits intracellular noradrenaline-induced lipolysis in adipocytes. Thus, apoB100-LDL is a novel signaling molecule from the liver to peripheral fat deposits that may be an important link between atherogenic dyslipidemias and facets of the metabolic syndrome.

Selenium intake reduces serum C3, an early marker of metabolic syndrome manifestations, in healthy young adults

OBJECTIVES

To evaluate the associations between serum complement factor 3 (C3) and several anthropometrical, biochemical and lifestyle features in healthy young adults, emphasizing on the putative effect of selenium intake on C3 concentrations.

METHODS

This study enrolled 100 healthy young adults aged 18-34 years. Anthropometric and blood pressure measurements and lifestyle features were analyzed. Fasting blood samples were collected for the measurement of glucose, total cholesterol, HDL-cholesterol, LDL-cholesterol, triacylglycerols and C3 concentrations. Nail samples were collected for the analysis of selenium concentrations.

RESULTS

Values of BMI (P=0.034), sum of skinfold thicknesses (STs) (P=0.021), body fat mass (BFM) (P=0.023), percentage of overweight subjects (P=0.007), serum triacylglycerols (P=0.012) and nail selenium (P=0.001) were significantly different between subjects above and below the median of serum C3 concentrations. The following correlations with serum C3 were identified tricipital ST (P=0.033), sum of STs (P=0.012), BMI (P=0.008), BFM (P=0.018), waist-to-height ratio (P=0.016), serum glucose (P=0.045), serum triacylglycerols (P=0.001) and nail selenium (P=0.006). Circulating C3 showed a positive association with several adiposity markers such as BMI (P=0.001), waist circumference (P=0.006), waist-to-height ratio (P=0.002), BFM (P=0.025), as well as serum glucose (P=0.027) and triacylglycerols (P<0.001), whereas nail selenium was a statistically significant negative predictor of C3 concentrations (P=0.018).

CONCLUSIONS

C3 seems to be related with selenium status and several anthropometrical and biochemical measurements linked to metabolic syndrome in apparently healthy young adults. These findings suggest a possible role for selenium intake in the modulation of C3, whose assessment may be an early marker of metabolic syndrome manifestations.

Cord blood ASP is predicted by maternal lipids and correlates with fetal birth weight

BACKGROUND

The acylation stimulating protein (ASP) is a potent lipogenic adipokine that correlates with postprandial triglyceride (TG) clearance and is linked to the pathophysiology of obesity and related disorders.

OBJECTIVE

To investigate ASP levels in cord blood and its relation to maternal and cord blood lipid parameters and fetal birth weight.

METHODS AND PROCEDURES

Thirty nondiabetic pregnant women, their newborns, and thirty-three nonpregnant controls were included in this study. Fasting maternal and cord blood ASP, TGs, nonesterified fatty acids (NEFAs), cholesterol, glucose levels, in addition to maternal BMI and fetal birth weight were measured.

RESULTS

No significant difference was found between cord blood ASP (16.3 +/- 0.96 nmol/l) and ASP levels in the adult controls (15.7 +/- 1.0 nmol/l). Cord blood ASP, however, was lower than maternal plasma ASP levels (25.4 +/- 1.6 nmol/l, P < 0.001). Yet, lipid levels in cord blood, particularly TGs were markedly decreased compared to control and maternal TG levels (threefold and 7.4-fold, P < 0.001 respectively). Maternal TGs significantly correlated with fetal birth weight (r = 0.54, P = 0.002). Multiple regression analysis showed that maternal TGs (beta = 0.57, P = 0.01) and NEFAs (beta = 0.43, P = 0.024) predicted 45% variation in cord blood ASP levels, independent of all measured maternal and cord blood parameters. Cord blood ASP showed a positive correlation with fetal birth weight (r = 0.524, P = 0.037) in neonates above average fetal birth weight of the studied population.

DISCUSSION

This is the first study investigating ASP in cord blood. We suggest that maternal hypertriglyceridemia is associated with increased fetal ASP production, thus enhancing fetal fat storage independent of maternal glucose variations in nondiabetic women.

Endocrine and metabolic effects of fat: cardiovascular implications

The prevalence of obesity is increasing rapidly in both industrialized and developing nations. Obesity causes complex metabolic, endocrine, and hemodynamic changes that may lead to adverse cardiovascular outcomes such as coronary heart disease and congestive heart failure. Adipose tissue is no longer considered to be an inert organ of energy storage, but in fact possesses important endocrine and metabolic functions that are closely involved in energy homeostasis. During the past decade, our understanding of the unique pathophysiologic changes that occur with obesity has rapidly grown. This review discusses our current understanding of the endocrine and metabolic effects of fat and their potential relation to cardiovascular disease.

Increased plasma acylation-stimulating protein correlates with hyperlipidemia at late gestation

OBJECTIVES

Obesity is often associated with negative consequences, including hyperlipidemia and insulin resistance. Weight gain during pregnancy is also associated with major lipid alterations. Fat storage is enhanced in early pregnancy. At late gestation, hyperlipidemia becomes a major manifestation. The acylation-stimulating protein (ASP) is a potent lipogenic adipocytokine that correlates with postprandial triglyceride (TG) clearance in vivo and has been linked to hyperlipidemic disorders. The role of ASP during a normal pregnancy is unknown. The objective of this study was to investigate plasma ASP levels in correlation with the lipid profile during late gestation.

RESEARCH METHODS AND PROCEDURES

Seventy healthy women at late gestation and 60 non-pregnant controls of similar age and prepregnancy BMI were included in a cross-sectional study. Fasting plasma ASP levels and the lipid profile of all of the women were measured.

RESULTS

ASP levels were markedly elevated in the pregnant women (66%, p < 0.001). ASP levels correlated strongly with the elevated levels of TGs (r = 0.608, p < 0.000), apolipoprotein B (0.519, p < 0.000), and low-density lipoprotein-cholesterol (r = 0.405, p < 0.000). Multivariate analysis adjusting for BMI and age showed that changes in ASP levels at late gestation were best predicted by TG and apoB levels, accounting for 53.8% of plasma ASP variation. For the controls, ASP strongly correlated with BMI, which was the only significant predictor of ASP levels.

DISCUSSION

Gestational hormone alterations during pregnancy may affect ASP function as a lipogenic factor. Increased plasma ASP levels at late gestation and their strong correlation with parameters reflecting very low-density lipoprotein accumulation are suggestive of ASP resistance, which may further contribute to the hyperlipidemic state, shifting energy in the form of TGs to the rapidly growing fetus.

Response of the alternative complement pathway to an oral fat load in first-degree relatives of subjects with type II diabetes

OBJECTIVE

To investigate levels of components of the alternative pathway of complement, and of two activation products, ASP and Bb, in persons ranging in insulin resistance both fasting and following the consumption of a high-fat, low-carbohydrate meal.

SUBJECTS

Healthy controls (n = 17) and normoglycaemic first-degree relatives of patients with type II diabetes (n = 15).

MEASUREMENTS

All subjects had normal glucose tolerance. Blood was collected for the measurement of plasma glucose, insulin, triglycerides and free fatty acids. Body composition was assessed with dual energy X-ray absorptiometry (DEXA) and whole-body insulin sensitivity with a euglycaemic, hyperinsulinaemic clamp. Basal and postprandial values over 6 h were determined for plasma C3, B, D, Bb and ASP. Basal levels of C1q, C4 and CRP were also determined.

RESULTS

Controls did not differ significantly from the relatives of patients with type II diabetes for any metabolic parameter except in their degree of insulin resistance and central fat (kg). Across all subjects, basal levels of C3, but no other complement protein, were correlated with insulin resistance. Native complement proteins, but not ASP or Bb, were correlated with body mass index and the amount (kg) of central fat. Basal levels of C3 and factor B were significantly higher in the relatives group, whereas factor D and the classical pathway proteins C1q and C4 did not differ between the two groups. Postprandially, levels of factor D were significantly reduced in both groups. ASP levels also fell postprandially, the decline achieving significance in the relatives group.

CONCLUSIONS

Elevated levels of C3 and factor B in the diabetic relatives group may have resulted from increased synthesis by adipose tissue. There was no evidence of alternative pathway activation in response to a fat meal in terms of ASP or Bb production, or significant consumption of C3 and factor B. These data do not support an essential requirement of the hypothesis that ASP is produced in response to the intake of fat.

Increased acylation stimulating protein concentrations in nonalcoholic fatty liver disease are associated with insulin resistance

OBJECTIVES

As acylation stimulating protein (ASP) acts on adipocytes mainly as a paracrine factor to increase triglyceride synthesis and storage; hypothetically, it may play a similar role in the pathogenesis of nonalcoholic fatty liver disease (NAFLD).

METHODS

Forty-six male patients with NAFLD (group A), age-matched 30 male patients with chronic viral hepatitis (group B) and 30 age-matched and body mass index (BMI)-matched healthy male subjects were enrolled in the study.

RESULTS

Among the NAFLD patients, 10 patients (24.4%) had simple steatosis and 36 patients (69.6%) had nonalcoholic steatohepatitis (NASH). The mean levels of ASP, complement 3, insulin, C-peptide, HOMA-IR, triglyceride, and very low-density lipoprotein (VLDL) were significantly higher in group A patients than both controls and group B. ASP levels correlated significantly in a positive manner with BMI, insulin, and HOMA-IR.

CONCLUSIONS

Dysregulation of the ASP pathway may have important metabolic consequences in NASH and is associated with insulin resistance.

Insights from apoB: from better diagnosis & therapy to the Medusa Hypothesis

This article will review six of the most important insights that have come from the measurement of apolipoprotein B (apoB). Amongst these are critical clinical advances, which include better definition of those at high risk of vascular disease and better definition of the adequacy of statin therapy. There are also major advances in our understanding of the fundamental processes that interact to cause vascular disease. These include a more precise understanding of the determinants of lipoprotein levels and lipoprotein composition in plasma and, even more importantly, an appreciation of the pivotal role excess fatty acid intake and impaired fatty acid trapping by adipose tissue play in the generation of the complex risk profile that is so common in patients with coronary disease. We have designated the constellation of hypertriglyceridemic (hyperTg) hyperapoB, dysglycemia, and elevated levels of markers of prothrombosis and inflammation--all of which are traceable to adipose tissue dysfunction--as the Medusa Hypothesis. The Medusa Hypothesis puts forward a common pathophysiology for the complex and biologically diverse array of factors that have been associated with vascular disease.

Addition of glucose to an oral fat load reduces postprandial free fatty acids and prevents the postprandial increase in complement component 3

BACKGROUND

Elevated fasting plasma concentrations of complement component 3 (C3) are associated with elevated fasting and postprandial triacylglycerol concentrations, insulin resistance, obesity, and coronary artery disease. C3 is the central component of the complement system and the precursor of acylation-stimulating protein (ASP). Insulin and ASP are principal determinants of free fatty acid (FFA) trapping by adipose tissue.

OBJECTIVE

Because controversy exists concerning postprandial changes in C3 and because meal composition may influence complement activation, we studied postprandial lipemia in relation to changes in plasma C3.

DESIGN

After an overnight fast, 6 healthy men ( +/- SD age: 23 +/- 2 y) underwent 4 oral liquid challenges: fat (50 g/m(2) body surface), glucose (37.5 g/m(2)), fat and glucose (mixed test), and water (as a control test) in a random, crossover design.

RESULTS

Plasma ASP concentrations did not change postprandially in any test. Changes in C3 concentration were observed only after the fat challenge: elevated concentrations occurred between 1 and 3 h, and a maximum increase of 11% occurred at 2 h (P = 0.05). Postprandial triacylglycerolemia did not differ significantly between the fat and mixed tests. The FFA response after the fat challenge was the highest of all the tests (P < 0.05 for all comparisons) and was accompanied by an increase in ketone bodies (maximum at 6 h); this increase did not occur after the mixed test, which suggests less hepatic FFA delivery.

CONCLUSIONS

When glucose is added to an oral fat load, the postprandial FFA response is reduced, and the fat-specific increase in C3 is prevented. After ingestion of fat without glucose, the lack of insulin response may lead to C3-mediated peripheral FFA trapping, which probably serves as a backup system in case of insufficient or inefficient insulin-dependent FFA trapping.

Update on adipocyte hormones: regulation of energy balance and carbohydrate/lipid metabolism

Hormones produced by adipose tissue play a critical role in the regulation of energy intake, energy expenditure, and lipid and carbohydrate metabolism. This review will address the biology, actions, and regulation of three adipocyte hormones-leptin, acylation stimulating protein (ASP), and adiponectin-with an emphasis on the most recent literature. The main biological role of leptin appears to be adaptation to reduced energy availability rather than prevention of obesity. In addition to the well-known consequences of absolute leptin deficiency, subjects with heterozygous leptin gene mutations have low circulating leptin levels and increased body adiposity. Leptin treatment dramatically improves metabolic abnormalities (insulin resistance and hyperlipidemia) in patients with relative leptin deficiency due to lipoatrophy. Leptin production is primarily regulated by insulin-induced changes of adipocyte metabolism. Dietary fat and fructose, which do not increase insulin secretion, lead to reduced leptin production, suggesting a mechanism for high-fat/high-sugar diets to increase energy intake and weight gain. ASP increases the efficiency of triacylglycerol synthesis in adipocytes leading to enhanced postprandial lipid clearance. In mice, ASP deficiency results in reduced body fat, obesity resistance, and improved insulin sensitivity. Adiponectin production is stimulated by thiazolidinedione agonists of peroxisome proliferator-activated receptor-gamma and may contribute to increased insulin sensitivity. Adiponectin and leptin cotreatment normalizes insulin action in lipoatrophic insulin-resistant animals. These effects may be mediated by AMP kinase-induced fat oxidation, leading to reduced intramyocellular and liver triglyceride content. The production of all three hormones is influenced by nutritional status. These hormones, the pathways controlling their production, and their receptors are promising targets for managing obesity, hyperlipidemia, and insulin resistance.

Differentially expressed genes in visceral or subcutaneous adipose tissue of obese men and women

There is growing evidence that the distribution of adipose tissue in the body is of importance in the development of metabolic complications of obesity, such as diabetes, hypertension, and hyperlipidemia. The aim of this study was to identify differentially expressed genes in subcutaneous and omental human adipose tissue in obese men, using a subtractive hybridization strategy. From the obtained set of differentially expressed transcripts, we also aimed to identify genes that have a sex-specific pattern of expression in omental or subcutaneous adipose tissue. Representational difference analysis (RDA) was performed on cDNA from subcutaneous and omental fat tissue from a man with extreme abdominal obesity. Forty-four putatively differentially expressed genes were identified. The obtained RDA products were spotted onto glass slides to screen for differential expression in other obese patients by using a microarray hybridization procedure. Five genes were confirmed to be differentially expressed in subcutaneous or omental adipose tissue from male or female obese patients. One gene was detected only in males and was found to be upregulated in subcutaneous tissue. The findings extend previous knowledge that different fat depots have differential gene expression and indicate that sex differences exist in adipose gene expression patterns.

Dietary trans-18:1 raises plasma triglycerides and VLDL cholesterol when replacing either 16:0 or 18:0 in gerbils

To compare the relative impact of trans-18:1 with the two main dietary saturated fatty acids it replaces, plasma lipid response was assessed in Mongolian gerbils fed diets rich in 16:0 (24%en),18:0 (10%en), or trans-18:1 (4 or 6%en). The diets were designed such that the 18:0-rich diet substituted 7%en as 18:0 for 16:0, whereas 4%en and 6%en from trans-18:1 was substituted for 16:0 in the two trans diets. The control group was fed a diet formulated according to the fatty acid balance of American Heart Association (AHA), but provided 40%en as fat. Gerbils (n = 10 per dietary group) were fed one of the five diets for 8 weeks. The control diet, with 4 times the polyunsaturated fatty acids (PUFA) content and a P:S ratio about 10 times greater than the test diets, resulted in the lowest plasma TC, LDL cholesterol (LDL-C) and VLDL cholesterol (VLDL-C). Among the test diets, plasma TC and TG were lowest with the 18:0-rich diet. TC in gerbils fed the 16:0-rich diet and 4%en-trans were 20% higher than the 18:0-rich diet, while the 6%en-trans diet was 35% higher. VLDL-C was significantly higher in the 6%en-trans diet compared to all other groups at 8 weeks. Both trans fatty acid diets elevated plasma TG approximately 2- and 3-fold, respectively, compared to the 16:0-rich and 18:0-rich diets at 8 weeks. Further, plasma TG continued to rise over time with trans fatty acids compared to 16:0 or 18:0. Thus, in the fatty acid-sensitive gerbil, impaired TG metabolism represents a major aspect of the hyperlipemia caused by trans fatty acid substitution for major saturated fatty acids.

Gender differences in postprandial ketone bodies in normolipidemic subjects and in untreated patients with familial combined hyperlipidemia

OBJECTIVE

An increased hepatic flow of free fatty acids (FFAs) is associated with impaired peripheral FFA trapping by malfunctioning of the complement component 3 (C3)/acylation-stimulating protein system and overproduction of VLDL in familial combined hyperlipidemia (FCHL). Postprandial ketone bodies reflect FFA oxidation in the liver, but the postprandial changes in male and female patients separately have not been determined yet. Gender differences in postprandial ketone bodies and C3 changes were investigated in normolipidemic patients and patients with untreated FCHL.

METHODS AND RESULTS

Thirty-two normolipidemic patients (16 female and 16 male) and 19 patients with untreated normolipidemia (9 female and 10 male) underwent an oral fat-loading test. Total and incremental areas under the curves (AUC and dAUC, respectively) after the oral fat load were calculated. Triglyceride AUC was similar between genders in each group. Normolipidemic female subjects showed a higher levels of dAUC-hydroxybutyric acid than male subjects (1.37+/-0.49 and 0.98+/-0.43 mmol x h/L). In FCHL, a similar trend was observed in female (1.92+/-0.38) compared with male (1.55+/-0.87) subjects. In contrast to normolipidemia, FCHL did not show a postprandial increase in C3, although C3 was higher in FCHL.

CONCLUSIONS

Women have higher postprandial ketone bodies than men, probably reflecting enhanced postprandial hepatic FFA oxidation. In FCHL, both genders have higher postprandial ketone bodies and therefore higher hepatic FFA delivery. The higher fasting and postprandial C3 levels in FCHL may reflect resistance of the C3/acylation-stimulating protein system to promote peripheral fatty acid trapping.

Acylation stimulating protein stimulates insulin secretion

Acylation stimulating protein (ASP) is a hormone produced by adipocytes and is of importance for the storage of energy as fat. We examined whether ASP might also have effects on islet function. In clonal INS-1 cells, ASP dose-dependently augmented glucose-stimulated insulin secretion. The lowest effective dose of ASP at 10 mmol/l glucose was 5 micro mol/l. The effect was glucose-dependent because ASP did not increase insulin secretion at 1 mmol/l glucose but had clear effect at 10 and 20 mmol/l glucose. Similarly, ASP augmented glyceraldehyde-induced insulin secretion but the hormone did not enhance insulin secretion in response to depolarization by 20 mmol/l of KCl. ASP-induced insulin secretion was completely abolished by competitive inhibition of glucose phosphorylation by glucokinase with 5-thio-glucose and was partially inhibited by the calcium channel blocker, nifedipine, and by the protein kinase C inhibitor, GF109203. Furthermore, thapsigargin, an inhibitor of Ca(2+)-ATPase in the endoplasmic reticulum, did not affect ASP-induced insulin secretion. ASP (>5 micro mol/l) also augmented glucose-stimulated insulin secretion from islets isolated from C57BL/6J mice, and intravenous administration of ASP (50 nmol/kg) augmented the acute (1 and 5 min) insulin response to intravenous glucose (1 g/kg) in C57BL/6J mice. This was accompanied by an increased rate of glucose disposal. Minimal model analyses of data derived from the intravenous glucose tolerance test revealed that whereas ASP augmented insulin secretion, the hormone did not affect insulin sensitivity (S(I)) or glucose effectiveness (S(G)). We conclude that ASP augments glucose-stimulated insulin secretion through a direct action on the islet beta cells. The effect is dependent on glucose phosphorylation, calcium uptake and protein kinase C. Stimulation of insulin secretion by ASP in vivo results in augmented glucose disposal.

Minireview: The adipocyte--at the crossroads of energy homeostasis, inflammation, and atherosclerosis

Adipose tissue evolved to efficiently store energy for times of caloric restriction. The large caloric excess common in many Western diets has negated the need for this thrifty function, leaving adipose tissue ill-equipped to handle this increased load. An excess of adipose tissue increases risk for a number of conditions including coronary artery disease, hypertension, dyslipidemias, type 2 diabetes, and even cancer. Indeed, the ability of the adipocyte to function properly when engorged with lipid can lead to lipid accumulation in other tissues, reducing their ability to function and respond normally. The role of adipose tissue as an endocrine organ capable of secreting a number of adipose tissue-specific or enriched hormones, known as adipokines, is gaining appreciation. The normal balance of these adipose tissue secretory proteins is perturbed in obesity. Paradoxically, the lack of normal adipose tissue, as seen in cases of lipodystrophy and lipoatrophy, is also associated with pathologic sequelae similar to what is seen with obesity. The pathologic findings associated with lack of adipose tissue, largely due to inability to properly store lipids, may also be due to a lack of adipokines. In this review, we highlight the role of adipose tissue as an endocrine organ focusing on some of the recent advances in the identification and pharmacological characterization of adipokines as well as their regulation in the context of obesity and insulin-resistant states.

Contribution of fatty acids released from lipolysis of plasma triglycerides to total plasma fatty acid flux and tissue-specific fatty acid uptake

There is controversy over the extent to which fatty acids (FAs) derived from plasma free FAs (FFAs) or from hydrolysis of plasma triglycerides (TGFAs) form communal or separate pools and what the contribution of each FA source is to cellular FA metabolism. Chylomicrons and lipid emulsions were labeled with [(3)H]triolein, injected into mice, and appearance in plasma of [(3)H]oleic acid was estimated, either through a steady-state approach or by compartmental modeling. [(14)C]oleic acid was included to trace plasma FFA. Eighty to 90% of triglyceride (TG) label was recovered in plasma, irrespective of tracer method or TG source. The contribution of TG lipolysis to total plasma FA turnover was 10-20%. After infusion of [(3)H]TG and [(14)C]FA, the retention of these labels varied substantially among liver, adipose tissue, and skeletal and heart muscle. Retention of TG label changed during fasting in the same direction as lipoprotein lipase (LPL) activity is regulated. We propose a model that reconciles the paradoxical 80-90% loss of TG label into plasma with LPL-directed differential uptake of TGFA in tissues. In this model, TGFAs mix locally at the capillaries with plasma FFAs, where they would lead to an increase in the local FA concentration, and hence, FA uptake. Our data indicate that a distinction between TG-derived FA and plasma FFA cannot be made.

Adipose tissue as an endocrine organ

Adipose tissue is a highly active endocrine organ secreting a range of soluble products with both local and distant actions. These hormones have important roles in metabolism, reproduction, cardiovascular function and immunity. It is now evident that adipose endocrine function directly influences other organ systems, including the brain, liver and skeletal muscle. The endocrine function of adipose tissue is significantly regulated by nutritional status, and both are inextricably linked to the energy storage role of adipose tissue. This chapter highlights the endocrinology of adipose tissue by concentrating on functional aspects of the secreted products. The data of particular relevance to humans are highlighted, and areas in need of future research are suggested.

Metabolic lessons from genetically lean mice

Different types of lean mice have been produced by genetic manipulation. Leanness can result from deficiency of stored energy or a lack of adipocytes to store the lipid. Mice lacking functional adipocytes are usually insulin resistant and have fatty livers, and elevated circulating triglyceride levels. Insulin resistance may result from the lack of adipocyte hormones (such as leptin) and increased metabolite (such as triglyceride) levels in nonadipose tissue. Mice with depleted adipocyte triglyceride levels typically are insulin sensitive and have normal or low liver and circulating triglycerides. Mechanisms to produce depleted adipocytes include increased energy expenditure by peripheral tissues, peripheral mechanisms to decrease food intake, and altered central regulation of these processes.

Clinical relevance of the biochemical, metabolic, and genetic factors that influence low-density lipoprotein heterogeneity

Traditional risk factors for coronary artery disease (CAD) predict about 50% of the risk of developing CAD. The Adult Treatment Panel (ATP) III has defined emerging risk factors for CAD, including small, dense low-density lipoprotein (LDL). Small, dense LDL is often accompanied by increased triglycerides (TGs) and low high-density lipoprotein (HDL). An increased number of small, dense LDL particles is often missed when the LDL cholesterol level is normal or borderline elevated. Small, dense LDL particles are present in families with premature CAD and hyperapobetalipoproteinemia, familial combined hyperlipidemia, LDL subclass pattern B, familial dyslipidemic hypertension, and syndrome X. The metabolic syndrome, as defined by ATP III, incorporates a number of the components of these syndromes, including insulin resistance and intra-abdominal fat. Subclinical inflammation and elevated procoagulants also appear to be part of this atherogenic syndrome. Overproduction of very low-density lipoproteins (VLDLs) by the liver and increased secretion of large, apolipoprotein (apo) B-100-containing VLDL is the primary metabolic characteristic of most of these patients. The TG in VLDL is hydrolyzed by lipoprotein lipase (LPL) which produces intermediate-density lipoprotein. The TG in intermediate-density lipoprotein is hydrolyzed further, resulting in the generation of LDL. The cholesterol esters in LDL are exchanged for TG in VLDL by the cholesterol ester tranfer proteins, followed by hydrolysis of TG in LDL by hepatic lipase which produces small, dense LDL. Cholesterol ester transfer protein mediates a similar lipid exchange between VLDL and HDL, producing a cholesterol ester-poor HDL. In adipocytes, reduced fatty acid trapping and retention by adipose tissue may result from a primary defect in the incorporation of free fatty acids into TGs. Alternatively, insulin resistance may promote reduced retention of free fatty acids by adipocytes. Both these abnormalities lead to increased levels of free fatty acids in plasma, increased flux of free fatty acids back to the liver, enhanced production of TGs, decreased proteolysis of apo B-100, and increased VLDL production. Decreased removal of postprandial TGs often accompanies these metabolic abnormalities. Genes regulating the expression of the major players in this metabolic cascade, such as LPL, cholesterol ester transfer protein, and hepatic lipase, can modulate the expression of small, dense LDL but these are not the major defects. New candidates for major gene effects have been identified on chromosome 1. Regardless of their fundamental causes, small, dense LDL (compared with normal LDL) particles have a prolonged residence time in plasma, are more susceptible to oxidation because of decreased interaction with the LDL receptor, and enter the arterial wall more easily, where they are retained more readily. Small, dense LDL promotes endothelial dysfunction and enhanced production of procoagulants by endothelial cells. Both in animal models of atherosclerosis and in most human epidemiologic studies and clinical trials, small, dense LDL (particularly when present in increased numbers) appears more atherogenic than normal LDL. Treatment of patients with small, dense LDL particles (particularly when accompanied by low HDL and hypertriglyceridemia) often requires the use of combined lipid-altering drugs to decrease the number of particles and to convert them to larger, more buoyant LDL. The next critical step in further reduction of CAD will be the correct diagnosis and treatment of patients with small, dense LDL and the dyslipidemia that accompanies it.

Control of energy homeostasis and insulin action by adipocyte hormones: leptin, acylation stimulating protein, and adiponectin

Adipose tissue performs complex metabolic and endocrine functions. This review will focus on the recent literature on the biology and actions of three adipocyte hormones involved in the control of energy homeostasis and insulin action, leptin, acylation-stimulating protein, and adiponectin, and mechanisms regulating their production. Results from studies of individuals with absolute leptin deficiency (or receptor defects), and more recently partial leptin deficiency, reveal leptin's critical role in the normal regulation of appetite and body adiposity in humans. The primary biological role of leptin appears to be adaptation to low energy intake rather than a brake on overconsumption and obesity. Leptin production is mainly regulated by insulin-induced changes of adipocyte metabolism. Consumption of fat and fructose, which do not initiate insulin secretion, results in lower circulating leptin levels, a consequence which may lead to overeating and weight gain in individuals or populations consuming diets high in energy derived from these macronutrients. Acylation-stimulating protein acts as a paracrine signal to increase the efficiency of triacylglycerol synthesis in adipocytes, an action that results in more rapid postprandial lipid clearance. Genetic knockout of acylation-stimulating protein leads to reduced body fat, obesity resistance and improved insulin sensitivity in mice. The primary regulator of acylation-stimulating protein production appears to be circulating dietary lipid packaged as chylomicrons. Adiponectin increases insulin sensitivity, perhaps by increasing tissue fat oxidation resulting in reduced circulating fatty acid levels and reduced intramyocellular or liver triglyceride content. Adiponectin and leptin together normalize insulin action in severely insulin-resistant animals that have very low levels of adiponectin and leptin due to lipoatrophy. Leptin also improves insulin resistance and reduces hyperlipidemia in lipoatrophic humans. Adiponectin production is stimulated by agonists of peroxisome proliferator-activated receptor-gamma; an action may contribute to the insulin-sensitizing effects of this class of compounds. The production of all three hormones is influenced by nutritional status. These adipocyte hormones, the pathways controlling their production, and their receptors represent promising targets for managing obesity, hyperlipidemia, and insulin resistance.

Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis

Numerous peripheral signals contribute to the regulation of food intake and energy homeostasis. Mechano- and chemoreceptors signaling the presence and energy density of food in the gastrointestinal (GI) tract contribute to satiety in the immediate postprandial period. Changes in circulating glucose concentrations appear to elicit meal initiation and termination by regulating activity of specific hypothalamic neurons that respond to glucose. Other nutrients (e.g., amino acids and fatty acids) and GI peptide hormones, most notably cholecystokinin, are also involved in short-term regulation of food intake. However, the energy density of food and short-term hormonal signals by themselves are insufficient to produce sustained changes in energy balance and body adiposity. Rather, these signals interact with long-term regulators (i.e., insulin, leptin, and possibly the orexigenic gastric peptide, ghrelin) to maintain energy homeostasis. Insulin and leptin are transported into the brain where they modulate expression of hypothalamic neuropeptides known to regulate feeding behavior and body weight. Circulating insulin and leptin concentrations are proportional to body fat content; however, their secretion and circulating levels are also influenced by recent energy intake and dietary macronutrient content. Insulin and leptin concentrations decrease during fasting and energy-restricted diets, independent of body fat changes, ensuring that feeding is triggered before body energy stores become depleted. Dietary fat and fructose do not stimulate insulin secretion and leptin production. Therefore, attenuated production of insulin and leptin could lead to increased energy intake and contribute to weight gain and obesity during long-term consumption of diets high in fat and/or fructose. Transcription of the leptin gene and leptin secretion are regulated by insulin-mediated increases of glucose utilization and appear to require aerobic metabolism of glucose beyond pyruvate. Other adipocyte-derived hormones and proteins that regulate adipocyte metabolism, including acylation stimulating protein, adiponectin, diacylglycerol acyltransferase, and perilipin, are likely to have significant roles in energy homeostasis.

Adipocyte differentiation: from fibroblast to endocrine cell

Recent advances regarding the biology of adipose tissue have demonstrated that white adipose tissue (WAT) plays a central role in the regulation of energy balance and acts as a secretory/endocrine organ that mediates numerous physiological and pathological processes. Dysregulation of WAT mass causes obesity or lipoatrophy, two disorders associated with life-threatening pathologies, including cardiovascular diseases and diabetes. Alterations in WAT mass result from changes in adipocyte size and/or number. Change in adipocyte number is achieved through a complex interplay between proliferation and differentiation of preadipocytes. Adipocyte differentiation or adipogenesis is a highly controlled process that has been extensively studied for the last 25 years. In vitro preadipocyte culture systems that recapitulate most of the critical aspects of fat cell formation in vivo have allowed a meticulous dissection of the cellular and molecular events involved in the adipogenesis process. The adipogenic transcription factors peroxisome proliferator-activated receptor-gamma and CCAAT/enhancer binding protein-alpha play a key role in the complex transcriptional cascade that occurs during adipogenesis. Hormonal and nutritional signaling affects adipocyte differentiation in a positive or negative manner, and components involved in cell-cell or cell-matrix interactions are also pivotal in regulating the differentiation process. This knowledge provides a basis for understanding the physiological and pathophysiological mechanisms that underlie adipose tissue formation and for the development of novel and sound therapeutic approaches to treat obesity and its related diseases.

Serum C3 but not plasma acylation-stimulating protein is elevated in Finnish patients with familial combined hyperlipidemia

A trapping defect of fatty acids due to impaired function of acylation-stimulating protein (ASP) has been suggested as one mechanism underlying the metabolic abnormalities in familial combined hyperlipidemia (FCHL). The study aimed at defining the role of ASP and complement C3 in 35 Finnish FCHL families. There was no difference in plasma ASP levels between the 66 hypertriglyceridemic FCHL patients and their 84 normotriglyceridemic relatives. No response in plasma ASP could be observed after a fatty meal in 10 FCHL patients or in 10 control subjects. In familial correlation analyses, C3 exhibited a significant sibling-sibling correlation. The FCHL patients had higher serum C3 levels than their unaffected relatives (P<0.001). Furthermore, serum C3 levels correlated significantly with several lipid parameters. The correlations between ASP and lipid variables were weaker than those of C3. These analyses suggest that common genes might contribute to the regulation of serum C3, triglycerides, HDL-C, free fatty acids, and insulin. The present data do not support the hypothesis that defects of the ASP pathway are reflected in plasma lipoproteins or in impaired plasma lipid clearance postprandially.

Regulation by retinoic acid of acylation-stimulating protein and complement C3 in human adipocytes

Acylation-stimulating protein (ASP), a product of complement C3, stimulates triacylglycerol synthesis in adipocytes. Previous studies have identified transthyretin, associated with chylomicrons, as a stimulator of C3 and ASP production. Since both transthyretin and chylomicrons transport retinyl ester/retinol, our goal was to investigate whether retinoic acid (RA) could be a potential hormonal mediator of the effect. Inhibitors of protein synthesis and protein secretion eliminated the stimulatory effects of chylomicrons on both C3 and ASP production in human differentiated adipocytes, suggesting that de novo protein synthesis and secretion are both required. Incubation with chylomicrons increased C3 mRNA levels (37+/-1.5%). RA alone or with chylomicrons had a stimulatory effect on C3 production (29-fold at 16.6 nM RA) and ASP production. An RA receptor antagonist blocked stimulation of C3 mRNA and C3 secretion by both RA and chylomicrons. Finally, RA and chylomicrons activated a 1.8 kb C3-promoter-luciferase construct transfected into 3T3-F442 and 3T3-L1 cells (by 41+/-0.2% and 69+/-0.3% respectively), possibly via RA receptor half-sites identified by sequence analysis. This is the first evidence documenting stimulation by RA of the C3 gene. Thus we propose RA as a novel cellular trigger in chylomicrons that subsequently results in increased ASP production by adipocytes after a meal.

Mechanisms of nutritional and hormonal regulation of lipogenesis

Fat build-up is determined by the balance between lipogenesis and lipolysis/fatty acid oxidation. In the past few years, our understanding of the nutritional, hormonal and particularly transcriptional regulation of lipogenesis has expanded greatly. Lipogenesis is stimulated by a high carbohydrate diet, whereas it is inhibited by polyunsaturated fatty acids and by fasting. These effects are partly mediated by hormones, which inhibit (growth hormone, leptin) or stimulate (insulin) lipogenesis. Recent research has established that sterol regulatory element binding protein-1 is a critical intermediate in the pro- or anti-lipogenic action of several hormones and nutrients. Another transcription factor implicated in lipogenesis is the peroxisome proliferator activated receptor gamma. Both transcription factors are attractive targets for pharmaceutical intervention of disorders such as hypertriglyceridemia and obesity.