Insulin stimulation of hepatic triacylglycerol secretion in the insulin-replete state: implications for the etiology of peripheral insulin resistance

The association between low-carbohydrate diet score and metabolic syndrome among Iranian adults

OBJECTIVE

Assessing the relationship between low-carbohydrate diet (LCD) score and metabolic syndrome (Mets) in Iranian adults.

DESIGN

Cross-sectional study.

SETTING

Yazd Health Study and Taghzieh Mardom-e-Yazd study.

PARTICIPANTS

Data of 2074 participants were used. Dietary intakes were assessed by a validated semi-quantitative FFQ. LCD score was calculated for each person by summing up the assigned scores to deciles of energy percentages from macronutrients. Mets was evaluated using National Cholesterol Education Program Adult Treatment Panel III. Eventually, association between LCD score and Mets was examined using logistic regression.

RESULTS

Total Mets prevalence was approximately 40·5 %. After adjustment for confounders, subjects in the higher quartile of LCD score had a significant lower chance of Mets than lower quartile among all participants (Q4 v. Q1: OR: 0·68, 95 % CI (0·50, 0·92)) and separately in men (Q4 v. Q1: OR: 0·54, 95 % CI (0·34, 0·86)) and women (Q2 v. Q1: OR: 0·53, 95 % CI (0·34, 0·82)). Furthermore, more LCD adherence in men reduced abdominal obesity by 47 % (Q3 v. Q1: OR: 0·53, 95 % CI (0·28, 0·99)). A significant inverse relation was also observed between low HDL cholesterol and LCD score in all participants (Q4 versus Q1 OR: 0·74, 95% CI: 0·56-0·99) and separately in men (Q4 versus Q1 OR: 0·63, 95% CI: 0·40-0·98).

CONCLUSIONS

More adherence to LCD might be related to lower chance of Mets and some of its components such as low HDL-cholesterol and abdominal obesity specially in men. Further studies are required to confirm the findings.

Insulin resistance in cardiovascular disease, uremia, and peritoneal dialysis

Diabetic nephropathy is highly correlated with the occurrence of other complications of type 1 diabetes (T1D) and type 2 diabetes (T2D) mellitus; for example, hypertension with cardiovascular disease (CVD) being the most frequent cause of death in patients with end-stage renal disease and undergoing renal dialysis. Hyperglycemia and insulin resistance (IR) are responsible for the micro- and macrovascular complications of diabetes through different mechanisms. In particular, IR plays a key role in the etiology of atherosclerosis in both diabetic and non-diabetic patients. IR - exacerbated by organ-level selectivity - is more important than glycemic control per se in determining cardiovascular outcomes. This may be exacerbated by the fact that IR is organ and pathway specific due to the only selective loss of sensitivity to insulin action of specific pathways/processes. Therefore, it is counterintuitive that the use of peritoneal dialysis (PD) in (frequently) diabetic renal disease patients should involve their exposure to high daily doses of glucose peritoneally. In view of the controversy about the causal association between glucose load and CVD in PD patients, we discuss the role that selective IR may play in the progression of CVD in diabetic renal end-stage patients. In discussing these associations, we propose that reducing glucose exposure in PD solutions may be beneficial especially if coupled with strategies that address IR directly, and the avoidance of excessive use of insulin treatment in T2D.

Short-Term Ketogenic Diet Improves Abdominal Obesity in Overweight/Obese Chinese Young Females

The purpose of this study was to examine the effects of a short-term ketogenic diet (KD) on body composition and cardiorespiratory fitness (CRF) in overweight/obese Chinese females. Twenty young females [age: 21.0 ± 3.7 years, weight: 65.5 ± 7.7 kg, body mass index (BMI): 24.9 ± 2.7 kg⋅m^–2] consumed 4 weeks of a normal diet (ND) as a baseline and then switched to a low-carbohydrate, high-fat, and adequate protein KD for another 4 weeks. With the same daily caloric intake, the proportions of energy intake derived from carbohydrates, proteins, and fats were changed from 44.0 ± 7.6%, 15.4 ± 3.3%, 39.6 ± 5.8% in ND to 9.2 ± 4.8%, 21.9 ± 3.4%, and 69.0 ± 5.4% in KD. The results showed that, without impairing the CRF level, the 4-week KD intervention significantly reduced body weight (−2.9 kg), BMI (−1.1 kg⋅m^–2), waist circumference (−4.0 cm), hip circumference (−2.5 cm), and body fat percentage (−2.0%). Moreover, fasting leptin level was lowered significantly, and serum levels of inflammatory markers (i.e., TNF-α and MCP-1) were unchanged following KD. These findings suggest that KD can be used as a rapid and effective approach to lose weight and reduce abdominal adiposity in overweight/obese Chinese females without exacerbating their CRF.

Expression of genes related to liver fatty acid metabolism in fat-tailed and thin-tailed lambs during negative and positive energy balances

Fat-tailed sheep breeds can tolerate periods of negative energy balance without suffering from elevated concentration of plasma non-esterified fatty acid (NEFA). This ability was attributed to unique metabolism of fat-tailed adipose depot, whereas role of liver as an influential organ in fatty acid metabolism was not evaluated yet. Hence, current study was conducted to evaluate the effects of negative and positive energy balances on liver expression of genes related to fatty acid metabolism in fat-tailed and thin-tailed lambs. Lambs experienced negative (21 days) and positive (21 days) energy balances and were slaughtered at the beginning and end of negative energy balance and at the end of positive energy balance. Real-time quantitative polymerase chain reaction (RT-Q-PCR) was conducted to evaluate changes in gene expression. Expression of diglyceride acyltransferase 1 (DGAT1), 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2) and apolipoprotein B (APOB) was not affected by genotype, energy balance and their interaction. Expression of carnitine palmitoyltransferase 1 (CPT1) was significantly higher in liver of fat-tailed comparing to thin-tailed lambs regardless of energy balance (p < 0.02). Catalase mRNA abundance was increased in response to negative energy balance (p < 0.02), and severity of this enhancement was higher in fat-tailed lambs (p < 0.06). Expression of CPT1 was positively correlated with expression of HMGCS2 in both fat-tailed (p < 0.05) and thin-tailed lambs (p < 0.002); however, the correlation was weaker in fat-tailed lambs (0.72 vs. 0.57, respectively, for thin-tailed and fat-tailed lambs). There was a positive correlation between DGAT1 and APOB genes expression in fat-tailed lambs (0.94; p < 0.001), whereas this correlation was not observed in thin-tailed lambs. Results demonstrate that liver of fat-tailed lambs has higher capacity for metabolism of mobilized NEFA exposed to liver during negative energy balance.

Chronic hyperinsulinemia contributes to insulin resistance under dietary restriction in association with altered lipid metabolism in Zucker diabetic fatty rats

Hyperinsulinemia is widely thought to be a compensatory response to insulin resistance, whereas its potentially causal role in the progression of insulin resistance remains to be established. Here, we aimed to examine whether hyperinsulinemia could affect the progression of insulin resistance in Zucker fatty diabetic (ZDF) rats. Male ZDF rats at 8 wk of age were fed a diet ad libitum (AL) or dietary restriction (DR) of either 15 or 30% from AL feeding over 6 wk. Insulin sensitivity was determined by hyperinsulinemic euglycemic clamp. ZDF rats in the AL group progressively developed hyperglycemia and hyperinsulinemia by 10 wk of age, and then plasma insulin rapidly declined to nearly normal levels by 12 wk of age. Compared with AL group, DR groups showed delayed onset of hyperglycemia and persistent hyperinsulinemia, leading to weight gain and raised plasma triglycerides and free fatty acids by 14 wk of age. Notably, insulin sensitivity was significantly reduced in the DR group rather than the AL group and inversely correlated with plasma levels of insulin and triglyceride but not glucose. Moreover, enhanced lipid deposition and upregulation of genes involved in lipogenesis were detected in liver, skeletal muscle, and adipose tissues of the DR group rather than the AL group. Alternatively, continuous hyperinsulinemia induced by insulin pellet implantation produced a decrease in insulin sensitivity in ZDF rats. These results suggest that chronic hyperinsulinemia may lead to the progression of insulin resistance under DR conditions in association with altered lipid metabolism in peripheral tissues in ZDF rats.

Novel Podophyllotoxin Derivatives as Partial PPARγ Agonists and their Effects on Insulin Resistance and Type 2 Diabetes

Peroxisome proliferator-activated receptor γ (PPARγ) is recognized as a key regulator of insulin resistance. In this study, we searched for novel PPARγ agonists in a library of structurally diverse organic compounds and determined that podophyllotoxin exhibits partial agonist activity toward PPARγ. Eight novel podophyllotoxin-like derivatives were synthesized and assayed for toxicity and functional activity toward PPARγ to reduce the possible systemic toxic effects of podophyllotoxin and to maintain partial agonist activity toward PPARγ. Cell-based transactivation assays showed that compounds (E)-3-(hydroxy(3,4,5-trimethoxyphenyl)methyl)-4-(4(trifluoromethyl)styryl)dihydrofuran-2(3H)-one (3a) and (E)-4-(3-acetylstyryl)-3-(hydroxyl (3,4,5-trimethoxyphenyl)methyl)dihydrofuran-2(3H)-one (3f) exhibited partial agonist activity. An experiment using human hepatocarcinoma cells (HepG2) that were induced to become an insulin-resistant model showed that compounds 3a and 3f improved insulin sensitivity and glucose consumption. In addition, compounds 3a and 3f significantly improved hyperglycemia and insulin resistance in high-fat diet-fed streptozotocin (HFD-STZ)-induced type 2 diabetic rats at a dose of 15 mg/kg/day administered orally for 45 days, without significant weight gain. Cell toxicity testing also showed that compounds 3a and 3f exhibited weaker toxicity than pioglitazone. These findings suggested that compounds 3a and 3f improved insulin resistance in vivo and in vitro and that the compounds exhibited potential for the treatment of type 2 diabetes mellitus.

Ethnopharmacology, phytochemistry, and biological activities of Cymbopogon citratus (DC.) Stapf extracts

Cymbopogon citratus is a widely distributed perennial herb belonging to the Poaceae family and has been extensively consumed for its medicinal, cosmetic, and nutritional effects for centuries. A large number of reports have been published describing the pharmacological, biological, and therapeutic actions of this herb. In this review, we summarized the literatures on related studies (up to January, 2014) that highlighted the pharmacologic and biological effects of the major phytochemicals isolated from C. citratus extracts and its essential oil. The components of the essential oils found in C. citratus have a similar pharmacokinetic properties, including absorption, distribution, metabolism, and excretion. They are quickly absorbed following oral, pulmonary, and dermal administration. Based on the published reports, it can also be inferred that, after absorption from the small intestine, some phytochemicals in C. citratus can undergo oxidation, glucuronidation, sulfation, and/or O-methylation. Excretion is through urine, feces and/or expired volatiles. The biotransformation reactions of C. citratus bioactive constituents are essential for its relatively safe consumption and therapeutic applications. The data available so far warrant further studies evaluating C. citratus pharmacokinetics. Reliable pharmacokinetic data in humans would be critical for a better understanding of the the systemic handling of C. citratus.

Hepatic triacylglycerol synthesis and secretion: DGAT2 as the link between glycaemia and triglyceridaemia

lThe liver regulates both glycaemia and triglyceridaemia. Hyperglycaemia and hypertriglyceridaemia are both characteristic of (pre)diabetes. Recent observations on the specialised role of DGAT2 (diacylglycerol acyltransferase 2) in catalysing the de novo synthesis of triacylglycerols from newly synthesized fatty acids and nascent diacylglycerols identifies this enzyme as the link between the two. This places DGAT2 at the centre of carbohydrate-induced hypertriglyceridaemia and hepatic steatosis. This function is complemented, but not substituted for, by the ability of DGAT1 to rescue partial glycerides from complete hydrolysis. In peripheral tissues not normally considered to be lipogenic, synthesis of triacylglycerols may largely bypass DGAT2 except in hyperglycaemic/hyperinsulinaemic conditions, when induction of de novo fatty acid synthesis in these tissues may contribute towards increased triacylglycerol secretion (intestine) or insulin resistance (adipose tissue, and cardiac and skeletal muscle).

Insulin resistance in obesity as the underlying cause for the metabolic syndrome

The metabolic syndrome affects more than a third of the US population, predisposing to the development of type 2 diabetes and cardiovascular disease. The 2009 consensus statement from the International Diabetes Federation, American Heart Association, World Heart Federation, International Atherosclerosis Society, International Association for the Study of Obesity, and the National Heart, Lung, and Blood Institute defines the metabolic syndrome as 3 of the following elements: abdominal obesity, elevated blood pressure, elevated triglycerides, low high-density lipoprotein cholesterol, and hyperglycemia. Many factors contribute to this syndrome, including decreased physical activity, genetic predisposition, chronic inflammation, free fatty acids, and mitochondrial dysfunction. Insulin resistance appears to be the common link between these elements, obesity and the metabolic syndrome. In normal circumstances, insulin stimulates glucose uptake into skeletal muscle, inhibits hepatic gluconeogenesis, and decreases adipose-tissue lipolysis and hepatic production of very-low-density lipoproteins. Insulin signaling in the brain decreases appetite and prevents glucose production by the liver through neuronal signals from the hypothalamus. Insulin resistance, in contrast, leads to the release of free fatty acids from adipose tissue, increased hepatic production of very-low-density lipoproteins and decreased high-density lipoproteins. Increased production of free fatty acids, inflammatory cytokines, and adipokines and mitochondrial dysfunction contribute to impaired insulin signaling, decreased skeletal muscle glucose uptake, increased hepatic gluconeogenesis, and β cell dysfunction, leading to hyperglycemia. In addition, insulin resistance leads to the development of hypertension by impairing vasodilation induced by nitric oxide. In this review, we discuss normal insulin signaling and the mechanisms by which insulin resistance contributes to the development of the metabolic syndrome.

Inhibition of insulin and T3-induced fatty acid synthase by hexanoate

Fatty acid synthase (FAS) is responsible for the de novo synthesis of palmitate and stearate. This enzyme is activated by insulin and T(3), and inhibited by fatty acids. In this study, we show that insulin and T(3) have an inducing effect on FAS enzymatic activity, which is synergetic when both hormones are present. Octanoate and hexanoate specifically inhibit this hormonal effect. A similar inhibitory effect is observed at the level of protein expression. Transient transfections in HepG2 cells revealed that hexanoate inhibits, at least in part, FAS at a transcriptional level targeting the T(3) response element (TRE) on the FAS promoter. The effect of C6 on FAS expression cannot be attributed to a modification of insulin receptor activation or to a decrease in T(3) entry in the cells. Using bromo-hexanoate, we determined that hexanoate needs to undergo a transformation in order to have an effect. When incubating cells with triglyceride-hexanoate or carnitine-hexanoate, no effect on the enzymatic activity induced by insulin and T(3) is observed. A similar result was obtained when cells were incubated with betulinic acid, an inhibitor of the diacylglycerol acyltransferase. However, the incubation of cells with Triacsin C, a general inhibitor of acyl-CoA synthetases, completely reversed the inhibitory effect of hexanoate. Our results suggest that in hepatic cells, hexanoate needs to be activated into a CoA derivative in order to inhibit the insulin and T(3)-induced FAS expression. This effect is partially transcriptional, targeting the TRE on the FAS promoter.

Sexual dimorphism in the lasting effects of moderate caloric restriction during gestation on energy homeostasis in rats is related with fetal programming of insulin and leptin resistance

Aim

We aimed to characterize the lasting effect of moderate caloric restriction during early pregnancy on offspring energy homeostasis, by focusing on the effects on food intake and body weight as well as on the insulin and leptin systems.

Methods

Male and female offspring of 20% caloric restricted dams (from 1 to 12 days of pregnancy) (CR) and from control dams were studied. These animals were fed after weaning with a normal-fat (NF) diet until the age of 4 months, and then moved to a high-fat (HF) diet. Blood parameters were measured under fed and 14-h fasting conditions at different ages (2, 4 and 5 months). Food preferences were also assessed in adult animals.

Results

Accumulated caloric intake from weaning to the age of 5 months was higher in CR animals compared with their controls, and this resulted in higher body weight in adulthood in males, but not in females. Both male and female CR animals already showed higher insulin levels at the age of 2 months, under fed conditions, and higher HOMA-IR from the age of 4 months, compared with their controls. CR male animals, but not females, displayed higher preference for fat-rich food than their controls in adulthood and higher circulating leptin levels when they were under HF diet.

Conclusion

It is suggested that hyperinsulinemia may play a role in the etiology of hyperphagia in the offspring of caloric restricted animals during gestation, with different outcomes on body weight depending on the gender, which could be associated with different programming effects on later leptin resistance.

Carbohydrate restriction (with or without additional dietary cholesterol provided by eggs) reduces insulin resistance and plasma leptin without modifying appetite hormones in adult men

Carbohydrate-restricted diets (CRDs) have been shown to reduce body weight, whereas whole egg intake has been associated with increased satiety. The purpose of this study was to evaluate the effects of additional dietary cholesterol and protein provided by whole eggs while following a CRD on insulin resistance and appetite hormones. Using a randomized blind parallel design, subjects were allocated to an egg (640 mg/d additional dietary cholesterol) or placebo (0 mg/d additional dietary cholesterol) group for 12 weeks while following a CRD. There were significant reductions in fasting insulin (P < .025) and fasting leptin concentrations (P < .01) for both groups, which were correlated with the reductions in body weight and body fat (P < .05 and P < .01, respectively). Both groups reduced insulin resistance as measured by the homeostatic model assessment of insulin resistance (P < .025). There was a significant decrease in serum glucose levels observed after the intervention. We did not observe the expected increases in plasma ghrelin levels associated with weight loss, suggesting a mechanism by which subjects do not increase appetite with CRD. To confirm these results, the subjective measures of satiety using visual analog scale showed that both groups felt more "full" (P < .05), "satisfied" (P < .001), and "wanted to eat less" (P < .001) after the intervention. These results indicate that inclusion of eggs in the diet (additional dietary cholesterol) did not modify the multiple beneficial effects of CRD on insulin resistance and appetite hormones.

Hepatic dysfunction and insulin insensitivity in type 2 diabetes mellitus: a critical target for insulin-sensitizing agents

The liver plays an essential role in maintaining glucose homeostasis, which includes insulin-mediated processes such as hepatic glucose output (HGO) and uptake, as well as in clearance of insulin itself. In type 2 diabetes, the onset of hyperglycaemia [itself a potent inhibitor of hepatic glucose output (HGO)], alongside hyperinsulinaemia, indicates the presence of hepatic insulin insensitivity. Increased HGO is central to the onset of hyperglycaemia and highlights the need to target hepatic insulin insensitivity as a central component of glucose-lowering therapy. The mechanisms underlying the development of hepatic insulin insensitivity are not well understood, but may be influenced by factors such as fatty acid oversupply and altered adipocytokine release from dysfunctional adipose tissue and increased liver fat content. Furthermore, although the impact of insulin insensitivity as a marker of cardiovascular disease is well known, the specific role of hepatic insulin insensitivity is less clear. The pharmacological tools available to improve insulin sensitivity include the biguanides (metformin) and thiazolidinediones (rosiglitazone and pioglitazone). Data from a number of sources indicate that thiazolidinediones, in particular, can improve multiple aspects of hepatic dysfunction, including reducing HGO, insulin insensitivity and liver fat content, as well as improving other markers of liver function and the levels of mediators with potential involvement in hepatic function, including fatty acids and adipocytokines. The current review addresses this topic from the perspective of the role of the liver in maintaining glucose homeostasis, its key involvement in the pathogenesis of type 2 diabetes and the tools currently available to reduce hepatic insulin insensitivity.

Hepatic insulin gene therapy normalizes diurnal fluctuation of oxidative metabolism in diabetic BB/Wor rats

Previous studies of hepatic insulin gene therapy (HIGT) focused on glycemic effects of insulin produced from hepatocytes. In this study, we extend the observations of glycemic control with metabolically regulated HIGT to include systemic responses and whole-body metabolism. An insulin transgene was administered with an adenoviral vector [Ad/(GlRE)(3)BP1-2xfur] to livers of BB/Wor rats made diabetic with polyinosinic polycytidilic acid (poly-I:C) (HIGT group), and results compared with nondiabetic controls (non-DM), and diabetic rats receiving different doses of continuous-release insulin implants (DM-low BG and DM-high BG). Blood glucose and growth normalized in HIGT, with lower systemic insulin levels, elevated glucagon, and increased heat production compared with non-DM. Minimal regulation of systemic insulin levels were observed with HIGT, yet the animals maintained normal switching from carbohydrate to lipid metabolism determined by respiratory quotients (RQs), and tolerated 24-hour fasts without severe hypoglycemia. HIGT did not restore serum lipids as we observed increased triglycerides (TGs) and increased free fatty acids, but reduced weight of visceral fat pads despite normal total body fat content and retroperitoneal fat depots. HIGT favorably affects blood glucose, normalizes metabolic switching in diabetic rats, and reduces intra-abdominal fat deposition.

Dietary carbohydrate restriction in type 2 diabetes mellitus and metabolic syndrome: time for a critical appraisal

Current nutritional approaches to metabolic syndrome and type 2 diabetes generally rely on reductions in dietary fat. The success of such approaches has been limited and therapy more generally relies on pharmacology. The argument is made that a re-evaluation of the role of carbohydrate restriction, the historical and intuitive approach to the problem, may provide an alternative and possibly superior dietary strategy. The rationale is that carbohydrate restriction improves glycemic control and reduces insulin fluctuations which are primary targets. Experiments are summarized showing that carbohydrate-restricted diets are at least as effective for weight loss as low-fat diets and that substitution of fat for carbohydrate is generally beneficial for risk of cardiovascular disease. These beneficial effects of carbohydrate restriction do not require weight loss. Finally, the point is reiterated that carbohydrate restriction improves all of the features of metabolic syndrome.

Hepatic insulin resistance is sufficient to produce dyslipidemia and susceptibility to atherosclerosis

Insulin resistance plays a central role in the development of the metabolic syndrome, but how it relates to cardiovascular disease remains controversial. Liver insulin receptor knockout (LIRKO) mice have pure hepatic insulin resistance. On a standard chow diet, LIRKO mice have a proatherogenic lipoprotein profile with reduced high-density lipoprotein (HDL) cholesterol and very low-density lipoprotein (VLDL) particles that are markedly enriched in cholesterol. This is due to increased secretion and decreased clearance of apolipoprotein B-containing lipoproteins, coupled with decreased triglyceride secretion secondary to increased expression of Pgc-1 beta (Ppargc-1b), which promotes VLDL secretion, but decreased expression of Srebp-1c (Srebf1), Srebp-2 (Srebf2), and their targets, the lipogenic enzymes and the LDL receptor. Within 12 weeks on an atherogenic diet, LIRKO mice show marked hypercholesterolemia, and 100% of LIRKO mice, but 0% of controls, develop severe atherosclerosis. Thus, insulin resistance at the level of the liver is sufficient to produce the dyslipidemia and increased risk of atherosclerosis associated with the metabolic syndrome.

A new model for nonalcoholic steatohepatitis in the rat utilizing total enteral nutrition to overfeed a high-polyunsaturated fat diet

We have used total enteral nutrition (TEN) to moderately overfeed rats high-polyunsaturated fat diets to develop a model for nonalcoholic steatohepatitis (NASH). Male Sprague-Dawley rats were fed by TEN a 187 kcal.kg(-3/4).day(-1) diet containing 5% (total calories) corn oil or a 220 kcal.kg(-3/4).day(-1) diet in which corn oil constituted 5, 10, 25, 35, 40, or 70% of total calories for 21 or 65 days. Rats fed the 5% corn oil, 220 kcal.kg(-3/4).day(-1)diet had greater body weight gain (P < or = 0.05), fat mass (P < or = 0.05), and serum leptin and glucose levels (P < or = 0.05), but no liver pathology. A dose-dependent increase in hepatic triglyceride deposition occurred with increase in percent corn oil in the 220 kcal.kg(-3/4).day(-1) groups (P < or = 0.05). Steatosis, macrophage infiltration, apoptosis, and focal necrosis were present in the 70% corn oil group, accompanied by elevated serum alanine aminotransferase (ALT) levels (P < or = 0.05). An increase in oxidative stress (thiobarbituric acid-reactive substances) and TNF-alpha expression (P < or = 0.05) was observed in the 70% corn oil group, as well as an increase in hepatic CYP2E1 and CYP4A1 expression (P < or = 0.05). Significant positive correlations were observed between the level of dietary corn oil and the degree of pathology, ALTs, oxidative stress, and inflammation. Liver pathology was progressive with increased necrosis, accompanied by fibrosis, observed after 65 days of TEN. Increased expression of CD36 and l-fabp mRNA suggested development of steatosis was associated with increased fatty acid transport. These data suggest that intragastric infusion of a high-polyunsaturated fat diet at a caloric level of 17% excess total calories results in pathology similar to clinical NASH.

Apolipoprotein AII is a regulator of very low density lipoprotein metabolism and insulin resistance

Apolipoprotein AII (apoAII) transgenic (apoAIItg) mice exhibit several traits associated with the insulin resistance (IR) syndrome, including IR, obesity, and a marked hypertriglyceridemia. Because treatment of the apoAIItg mice with rosiglitazone ameliorated the IR and hypertriglyceridemia, we hypothesized that the hypertriglyceridemia was due largely to overproduction of very low density lipoprotein (VLDL) by the liver, a normal response to chronically elevated insulin and glucose. We now report in vivo and in vitro studies that indicate that hepatic fatty acid oxidation was reduced and lipogenesis increased, resulting in a 25% increase in triglyceride secretion in the apoAIItg mice. In addition, we observed that hydrolysis of triglycerides from both chylomicrons and VLDL was significantly reduced in the apoAIItg mice, further contributing to the hypertriglyceridemia. This is a direct, acute effect, because when mouse apoAII was injected into mice, plasma triglyceride concentrations were significantly increased within 4 h. VLDL from both control and apoAIItg mice contained significant amounts of apoAII, with approximately 4 times more apoAII on apoAIItg VLDL. ApoAII was shown to transfer spontaneously from high density lipoprotein (HDL) to VLDL in vitro, resulting in VLDL that was a poorer substrate for hydrolysis by lipoprotein lipase. These results indicate that one function of apoAII is to regulate the metabolism of triglyceride-rich lipoproteins, with HDL serving as a plasma reservoir of apoAII that is transferred to the triglyceride-rich lipoproteins in much the same way as VLDL and chylomicrons acquire most of their apoCs from HDL.

Carbohydrate restriction improves the features of Metabolic Syndrome. Metabolic Syndrome may be defined by the response to carbohydrate restriction

Metabolic Syndrome (MetS) represents a constellation of markers that indicates a predisposition to diabetes, cardiovascular disease and other pathologic states. The definition and treatment are a matter of current debate and there is not general agreement on a precise definition or, to some extent, whether the designation provides more information than the individual components. We consider here five indicators that are central to most definitions and we provide evidence from the literature that these are precisely the symptoms that respond to reduction in dietary carbohydrate (CHO). Carbohydrate restriction is one of several strategies for reducing body mass but even in the absence of weight loss or in comparison with low fat alternatives, CHO restriction is effective at ameliorating high fasting glucose and insulin, high plasma triglycerides (TAG), low HDL and high blood pressure. In addition, low fat, high CHO diets have long been known to raise TAG, lower HDL and, in the absence of weight loss, may worsen glycemic control. Thus, whereas there are numerous strategies for weight loss, a patient with high BMI and high TAG is likely to benefit most from a regimen that reduces CHO intake. Reviewing the literature, benefits of CHO restriction are seen in normal or overweight individuals, in normal patients who meet the criteria for MetS or in patients with frank diabetes. Moreover, in low fat studies that ameliorate LDL and total cholesterol, controls may do better on the symptoms of MetS. On this basis, we feel that MetS is a meaningful, useful phenomenon and may, in fact, be operationally defined as the set of markers that responds to CHO restriction. Insofar as this is an accurate characterization it is likely the result of the effect of dietary CHO on insulin metabolism. Glucose is the major insulin secretagogue and insulin resistance has been tied to the hyperinsulinemic state or the effect of such a state on lipid metabolism. The conclusion is probably not surprising but has not been explicitly stated before. The known effects of CHO-induced hypertriglyceridemia, the HDL-lowering effect of low fat, high CHO interventions and the obvious improvement in glucose and insulin from CHO restriction should have made this evident. In addition, recent studies suggest that a subset of MetS, the ratio of TAG/HDL, is a good marker for insulin resistance and risk of CVD, and this indicator is reliably reduced by CHO restriction and exacerbated by high CHO intake. Inability to make this connection in the past has probably been due to the fact that individual responses have been studied in isolation as well as to the emphasis of traditional therapeutic approaches on low fat rather than low CHO. We emphasize that MetS is not a disease but a collection of markers. Individual physicians must decide whether high LDL, or other risk factors are more important than the features of MetS in any individual case but if MetS is to be considered it should be recognized that reducing CHO will bring improvement. Response of symptoms to CHO restriction might thus provide a new experimental criterion for MetS in the face of on-going controversy about a useful definition. As a guide to future research, the idea that control of insulin metabolism by CHO intake is, to a first approximation, the underlying mechanism in MetS is a testable hypothesis.

Heart and liver fatty acid binding proteins and the metabolic syndrome

Cytosolic fatty acid binding proteins (FABPs) are widely expressed fatty acid chaperones. The adipocyte-expressed FABPs are permissive factors for the fat-induced metabolic syndrome, but a similar relevance of the FABPs of heart, muscle, and liver remains unclear. In this article, the known biochemical and physiologic roles of these FABPs are discussed in this context. It is concluded that the observations on adipocyte-expressed FABPs cannot be automatically extended to other tissues. More work is needed to clarify whether the individual or combined inhibition of FABPs is a desirable strategy to treat the metabolic syndrome.

Prevalence, predisposition and prevention of type II diabetes

In 2000, more than 151 million people in the world are diabetic. It is predicted that by 2010, 221 million people and by 2025, 324 million will be diabetic. In the U.S., for the population born in 2000, the estimated lifetime risk for diabetes is more than 1 in 3. The economic and human cost of this disease is devastating. The current cost of diabetes in the U.S. is estimated to be at $132 billion, which includes $92 billion of direct medical costs and $40 billion of indirect costs such as disability, work loss and premature mortality. The outbreak of the current diabetic epidemic has been accompanied by a similarly drastic increase in obesity. The relation between the two is a matter of debate but presumably both are caused by changes in dietary habits and an increasingly sedentary modern lifestyle. Compelling scientific evidence indicates that lifestyle modification effectively prevents or delays the occurrence of type 2 diabetes. Recent clinical trials also demonstrate that success in the treatment of obesity, either surgically or pharmacologically, leads to the prevention of type 2 diabetes among the obese. Clinical data have also revealed that the insulin sensitizing agent troglitazone is efficacious in both beta-cell preservation and delaying the onset of type 2 diabetes. Future safe and more effective anti-obesity medicines and insulin sensitizing agents that help to preserve beta-cell function, in addition to efforts of lifestyle modification, thus hold promise for the overweight population with potential for reduction in the development of diabetics.

Insulin acutely decreases hepatic fatty acid synthase activity

Insulin is viewed as a positive regulator of fatty acid synthesis by increasing fatty acid synthase (FAS) mRNA transcription. We uncover a new mechanism by which insulin acutely reduces hepatic FAS activity by inducing phosphorylation of the carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) and its interaction with FAS. Ceacam1 null mice (Cc1(-/-)) show loss of insulin's ability to acutely decrease hepatic FAS activity. Moreover, adenoviral delivery of wild-type, but not the phosphorylation-defective Ceacam1 mutant, restores the acute effect of insulin on FAS activity in Cc1(-/-) primary hepatocytes. Failure of insulin to acutely reduce hepatic FAS activity in hyperinsulinemic mice, including L-SACC1 transgenics with liver inactivation of CEACAM1, and Ob/Ob obese mice, suggests that the acute effect of insulin on FAS activity depends on the prior insulinemic state. We propose that this mechanism acts to reduce hepatic lipogenesis incurred by insulin pulses during refeeding.

Holding the line on hepatic fat

When chronically elevated, insulin increases hepatic lipogenesis and VLDL synthesis. However, the hormone reduces liver lipids when acutely elevated. Work in this issue of Cell Metabolism (Najjar at al., 2005) suggests a new mechanism for the inhibition of the rate-limiting enzyme in liver, fatty acid synthase.

Structure and regulation of acetyl-CoA carboxylase genes of metazoa

Acetyl-CoA carboxylase (ACC) plays a fundamental role in fatty acid metabolism. The reaction product, malonyl-CoA, is both an intermediate in the de novo synthesis of long-chain fatty acids and also a substrate for distinct fatty acyl-CoA elongation enzymes. In metazoans, which have evolved energy storage tissues to fuel locomotion and to survive periods of starvation, energy charge sensing at the level of the individual cell plays a role in fuel selection and metabolic orchestration between tissues. In mammals, and probably other metazoans, ACC forms a component of an energy sensor with malonyl-CoA, acting as a signal to reciprocally control the mitochondrial transport step of long-chain fatty acid oxidation through the inhibition of carnitine palmitoyltransferase I (CPT I). To reflect this pivotal role in cell function, ACC is subject to complex regulation. Higher metazoan evolution is associated with the duplication of an ancestral ACC gene, and with organismal complexity, there is an increasing diversity of transcripts from the ACC paraloges with the potential for the existence of several isozymes. This review focuses on the structure of ACC genes and the putative individual roles of their gene products in fatty acid metabolism, taking an evolutionary viewpoint provided by data in genome databases.

Glycemic and insulinemic meal responses modulate postprandial hepatic and intestinal lipoprotein accumulation in obese, insulin-resistant subjects

BACKGROUND

Exacerbated postprandial lipemia is a risk factor for cardiovascular disease and is linked to insulin status. Limited data on the effect of dietary carbohydrate on postprandial lipoprotein accumulation are available.

OBJECTIVE

We tested the hypothesis that dietary carbohydrates with different glucose availability alter postprandial lipoprotein metabolism differently in obese, insulin-resistant subjects.

DESIGN

After an overnight fast, 9 subjects with central obesity and insulin resistance but normal triacylglycerolemia randomly ingested 2 test meals with comparable amounts of fat (28-29 g) and digestible carbohydrate (91-94 g) but with different quantities of slowly available glucose (SAG) in cereal products (17 or 2 g SAG/100 g for biscuits and wheat flakes, respectively). Blood samples were collected before and for 6 h after meal intakes.

RESULTS

The postmeal 0-2-h areas under the curve (AUCs) for glycemia and insulinemia were significantly lower (P < 0.05) after the biscuit meal than after the flakes meal. Plasma triacylglycerol concentrations increased significantly after the flakes meal but not after the biscuit meal (1.5-fold higher 0-6-h AUC for the flakes meal). Apolipoprotein B-100 concentrations in the triacylglycerol-rich lipoprotein fraction increased significantly 2 h after the flakes meal but not after the biscuit meal (3-fold higher 0-6-h AUC for the flakes meal). Apolipoprotein B-48 concentrations increased (P < 0.05) 4 h after the flakes meal but not after the biscuit meal (2.3-fold higher 0-6-h AUC for the flakes meal).

CONCLUSION

Mixed meals containing slowly digestible carbohydrate that induces low glycemic and insulinemic responses reduce the postprandial accumulation of both hepatically and intestinally derived triacylglycerol-rich lipoproteins in obese subjects with insulin resistance.

Interaction between altered insulin and lipid metabolism in CEACAM1-inactive transgenic mice

Inactivation of CEACAM1 in L-SACC1 mice by a dominant-negative transgene in liver impairs insulin clearance and increases serum free fatty acid (FFA) levels, resulting in insulin resistance. The contribution of elevated FFAs in the pathogenesis of insulin resistance is herein investigated. Treatment of L-SACC1 female mice with carnitine restored plasma FFA content. Concomitantly, it normalized insulin levels without directly regulating receptor-mediated insulin internalization and prevented glucose tolerance in these mice. Similarly, treatment with nicotinic acid, a lipolysis inhibitor, restored insulin-stimulated receptor uptake in L-SACC1 mice. Taken together, these data suggest that chronic elevation in plasma FFAs levels contributes to the regulation of insulin metabolism and action in L-SACC1 mice.

Dietary Boron Decreases Peak Pancreatic In Situ Insulin Release in Chicks and Plasma Insulin Concentrations in Rats Regardless of Vitamin D or Magnesium Status

Because dietary boron deprivation induces hyperinsulinemia in vitamin D-deprived rats, the influence of dietary boron on insulin metabolism as modified by nutritional stressors was examined in two animal models. Male weanling Sprague-Dawley rats were assigned to each of four (Experiment 1) or 8 (Experiment 2) dietary groups for 35 d: the basal diet (< 0.2 mg B; <1.0 mg Mg/kg) was supplemented with boron (as orthoboric acid) to contain <0.2 or 2.0 (a physiologic amount) mg B/kg; with magnesium (as magnesium acetate), at 100 (inadequate) or 360-400 (adequate) mg/kg; and with cholecalciferol [vitamin D-3; 25 microg/kg for study length (Experiment 2), or, depleted for 16-17 d then repleted until end of experiment (Experiments 1 and 2)]. In the rat model, boron reduced plasma insulin (Experiment 1, P < 0.002; Experiment 2, P < 0.03), but did not change glucose concentrations regardless of vitamin D-3 or magnesium status. Cockerels (1 d old) were fed a ground corn, high protein casein and corn oil-based basal diet (low boron; 0.3 mg B/kg) supplemented with boron as orthoboric acid to contain 0.3 or 1.65 mg/kg (a physiologic amount) and vitamin D-3 at 3.13 (inadequate) or 15.60 (adequate) microg/kg. In the chick model, boron decreased (P < 0.045) in situ peak pancreatic insulin release at 26-37 d of age regardless of vitamin D-3 nutriture. These results suggest that physiologic amounts of boron may help reduce the amount of insulin required to maintain plasma glucose.