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.     

Hypertriglyceridemia secondary to obesity and diabetes

Hypertriglyceridemia is a common lipid abnormality in persons with visceral obesity, metabolic syndrome and type 2 diabetes. Hypertriglyceridemia typically occurs in conjunction with low HDL levels and atherogenic small dense LDL particles and is associated with increased cardiovascular risk. Insulin resistance is often an underlying feature and results in increased free fatty acid (FFA) delivery to the liver due to increased peripheral lipolysis. Increased hepatic VLDL production occurs due to increased substrate availability via FFAs, decreased apolipoprotein B100 degradation and increased lipogenesis. Postprandial hypertriglyceridemia also is a common feature of insulin resistance. Small dense LDL that coexist with decreased HDL particles in hypertriglyceridemic states are highly pro-atherogenic due to their enhanced endothelial permeability, proteoglycan binding abilities and susceptibility to oxidation. Hypertriglyceridemia also occurs in undertreated individuals with type 1 diabetes but intensive glucose control normalizes lipid abnormalities. However, development of visceral obesity in these patients unravels a similar metabolic profile as in patients with insulin resistance. Modest hypertriglyceridemia increases cardiovascular risk, while marked hypertriglyceridemia should be considered a risk for pancreatitis. Lifestyle modification is an important therapeutic strategy. Drug therapy is primarily focused on lowering LDL levels with statins, since efforts at triglyceride lowering and HDL raising with fibrates and/or niacin have not yet been shown to be beneficial in improving cardiovascular risk. Fibrates, however, are first-line agents when marked hypertriglyceridemia is present. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.     

PPAR{alpha} mediates the hypolipidemic action of fibrates by antagonizing FoxO1

High-fructose consumption is associated with insulin resistance and diabetic dyslipidemia, but the underlying mechanism is unclear. We show in hamsters that high-fructose feeding stimulated forkhead box O1 (FoxO1) production and promoted its nuclear redistribution in liver, correlating with augmented apolipoprotein C-III (apoC-III) production and impaired triglyceride metabolism. High-fructose feeding upregulated peroxisome proliferator-activated receptor-gamma coactivator-1beta and sterol regulatory element binding protein-1c expression, accounting for increased fat infiltration in liver. High-fructose-fed hamsters developed hypertriglyceridemia, accompanied by hyperinsulinemia and glucose intolerance. These metabolic aberrations were reversible by fenofibrate, a commonly used anti-hypertriglyceridemia agent that is known to bind and activate peroxisome proliferator-activated receptor-alpha (PPARalpha). PPARalpha physically interacted with, but functionally antagonized, FoxO1 in hepatic apoC-III expression. These data underscore the importance of FoxO1 deregulation in the pathogenesis of hypertriglyceridemia in high-fructose-fed hamsters. Counterregulation of hepatic FoxO1 activity by PPARalpha constitutes an important mechanism by which fibrates act to curb apoC-III overproduction and ameliorate hypertriglyceridemia.     

Saturated fatty acids activate ERK signaling to downregulate hepatic sortilin 1 in obese and diabetic mice

Hepatic VLDL overproduction is a characteristic feature of diabetes and an important contributor to diabetic dyslipidemia. Hepatic sortilin 1 (Sort1), a cellular trafficking receptor, is a novel regulator of plasma lipid metabolism and reduces plasma cholesterol and triglycerides by inhibiting hepatic apolipoprotein B production. Elevated circulating free fatty acids play key roles in hepatic VLDL overproduction and the development of dyslipidemia. This study investigated the regulation of hepatic Sort1 in obesity and diabetes and the potential implications in diabetic dyslipidemia. Results showed that hepatic Sort1 protein was markedly decreased in mouse models of type I and type II diabetes and in human individuals with obesity and liver steatosis, whereas increasing hepatic Sort1 expression reduced plasma cholesterol and triglycerides in mice. Mechanistic studies showed that the saturated fatty acid palmitate activated extracellular signal-regulated kinase (ERK) and inhibited Sort1 protein by mechanisms involving Sort1 protein ubiquitination and degradation. Consistently, hepatic ERK signaling was activated in diabetic mice, whereas blocking ERK signaling by an ERK inhibitor increased hepatic Sort1 protein in mice. These results suggest that increased saturated fatty acids downregulate liver Sort1 protein, which may contribute to the development of dyslipidemia in obesity and diabetes.     

Hypertriglyceridemia: cause or consequence of insulin resistance?

An overproduction of VLDL by the liver and a slower clearance of these lipoproteins are usually seen in diabetic patients. There is correlation between insulin resistance and plasma triglyceride concentration. Triglyceride may influence an early step in the insulin action pathway and alternatively, insulin resistance may cause hypertriglyceridemia. Hyperinsulinemia and/or hypertriglyceridemia may play a strong role in the cardiovascular risk of patients with type II diabetes. There is an important need to conduct trials to define therapy that can reduce the risk of cardiovascular complications.     

Rol del enterocito en la dislipemia de la resistencia insulínica

The increase in cardiovascular morbidity and mortality associated to insulin resistance (IR) states (obesity, metabolic syndrome, type 2 diabetes) represents a major public health problem. In IR, dyslipidemia typically include hypertriglyceridemia, low high density lipoprotein cholesterol, increased small and dense low density lipoprotein particles, and post-prandial hyperlipidemia, which play a direct or indirect role in the mecanisms of atherosclerosis. Dyslipidemia is mainly due to accumulation of circulating triglyceride-rich lipoproteins from the liver and bowel. The bowel has traditionally been seen as a passive organ, but current evidence confirms that it is an active organ subject to regulation by free fatty acids, insulin, incretins, and inflammation. Two new concepts have emerged: intestinal IR and overproduction of chylomicrons in hyperinsulinemic/IR states. A better understanding of intestinal IR may make the enterocyte a therapeutic target.     

Impaired generation of 12-hydroxylated bile acids links hepatic insulin signaling with dyslipidemia

The association of type 2 diabetes with elevated plasma triglyceride (TG) and very low-density lipoproteins (VLDL), and intrahepatic lipid accumulation represents a pathophysiological enigma and an unmet therapeutic challenge. Here, we uncover a link between insulin action through FoxO1, bile acid (BA) composition, and altered lipid homeostasis that brings new insight to this longstanding conundrum. FoxO1 ablation brings about two signature lipid abnormalities of diabetes and the metabolic syndrome, elevated liver and plasma TG. These changes are associated with deficiency of 12α-hydroxylated BAs and their synthetic enzyme, Cyp8b1, that hinders the TG-lowering effects of the BA receptor, Fxr. Accordingly, pharmacological activation of Fxr with GW4064 overcomes the BA imbalance, restoring hepatic and plasma TG levels of FoxO1-deficient mice to normal levels. We propose that generation of 12α-hydroxylated products of BA metabolism represents a signaling mechanism linking hepatic lipid abnormalities with type 2 diabetes, and a treatment target for this condition.     

Lipoprotein kinetics in the metabolic syndrome: pathophysiological and therapeutic lessons from stable isotope studies

Dyslipoproteinaemia is a cardinal feature of the metabolic syndrome that accelerates atherosclerosis. It is usually characterised by high plasma concentrations of triglyceride-rich and apolipoprotein (apo) B-containing lipoproteins, with depressed concentrations of high-density lipoprotein (HDL). Dysregulation of lipoprotein metabolism in these subjects may be due to a combination of overproduction of very-low-density lipoprotein (VLDL) apoB-100, decreased catabolism of apoB-containing particles, and increased catabolism of HDL apoA-I particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance that increases the flux of fatty acids from adipose tissue to the liver, the accumulation of fat in the liver, the increased hepatic secretion of VLDL-triglycerides and the remodelling of both low-density lipoprotein (LDL) and HDL particles in the circulation; perturbations in lipolytic enzymes and lipid transfer proteins contribute to the dyslipidaemia. Our in vivo understanding of the kinetic defects in lipoprotein metabolism in the metabolic syndrome has been chiefly achieved by ongoing developments in the use of stable isotope tracers and mathematical modelling. Knowledge of the pathophysiology of lipoprotein metabolism in the metabolic syndrome is well complemented by extensive cell biological data. Nutritional modifications and increased physical exercise may favourably alter lipoprotein transport in the metabolic syndrome by collectively decreasing the hepatic secretion of VLDL-apoB and the catabolism of HDL apoA-I, as well as by increasing the clearance of LDL-apoB. Pharmacological treatments, such as statins, fibrates or fish oils, can also correct the dyslipidaemia by several mechanisms of action including decreased secretion and increased catabolism of apoB, as well as increased secretion and decreased catabolism of apoA-I. The complementary mechanisms of action of lifestyle and drug therapies support the use of combination regimens to treat dyslipidaemia in the metabolic syndrome.     

The critical role of atypical protein kinase C in activating hepatic SREBP-1c and NF��B in obesity

Obesity is frequently associated with systemic insulin resistance, glucose intolerance, and hyperlipidemia. Impaired insulin action in muscle and paradoxical diet/insulin-dependent overproduction of hepatic lipids are important components of obesity, but their pathogenesis and inter-relationships between muscle and liver are uncertain. We studied two murine obesity models, moderate high-fat-feeding and heterozygous muscle-specific PKC-λ knockout, in both of which insulin activation of atypical protein kinase C (aPKC) is impaired in muscle, but conserved in liver. In both models, activation of hepatic sterol receptor element binding protein-1c (SREBP-1c) and NFκB (nuclear factor-kappa B), major regulators of hepatic lipid synthesis and systemic insulin resistance, was chronically increased in the fed state. In support of a critical mediatory role of aPKC, in both models, inhibition of hepatic aPKC by adenovirally mediated expression of kinase-inactive aPKC markedly diminished diet/insulin-dependent activation of hepatic SREBP-1c and NFκB, and concomitantly improved hepatosteatosis, hypertriglyceridemia, hyperinsulinemia, and hyperglycemia. Moreover, in high-fat–fed mice, impaired insulin signaling to IRS-1–dependent phosphatidylinositol 3-kinase, PKB/Akt and aPKC in muscle and hyperinsulinemia were largely reversed. In obesity, conserved hepatic aPKC-dependent activation of SREBP-1c and NFκB contributes importantly to the development of hepatic lipogenesis, hyperlipidemia, and systemic insulin resistance. Accordingly, hepatic aPKC is a potential target for treating obesity-associated abnormalities.     

Studies on the mechanism of hypertriglyceridemia in the genetically obese Zucker rat

The possibility that impaired removal of lipoprotein triglyceride from the circulation may be a participating factor in the hypertriglyceridemia of the obese Zucker rat was examined. We found no significant differences in the heparin-released lipoprotein lipase (LPL) activities of the adipose tissue, skeletal muscle, and heart (expressed per gram of tissue) from the lean and obese Zucker rats. Furthermore, the kinetic properties of adipose tissue and heart LPL from the lean and obese rats were similar, indicating that the catalytic efficiency of the enzyme was unaltered in the obese animals. The postheparin plasma LPL activities of lean and obese rats were also similar. However, the postheparin plasma hepatic triglyceride lipase (H-TGL) activity in the obese rats was elevated. The higher activity of H-TGL could not alleviate the hypertriglyceridemia in these animals. Since hypertriglyceridemia in the obese rats could also be due to the hepatic production of triglyceride-rich lipoproteins which are resistant to lipolysis, we therefore isolated very low density lipoproteins (VLDL) from lean and obese rat liver perfusates and examined their degradation by highly purified human milk LPL. Although certain differences were observed in hepatic VLDL triglyceride fatty acid composition, the kinetic patterns of LPL-catalyzed triglyceride disappearance from lean and obese rat liver perfusate VLDL were similar. The isolated liver perfusate VLDL contained sufficient apolipoprotein C-II for maximum lipolysis. These results indicate that impaired lipolysis is not a contributing factor in the genesis of hypertriglyceridemia in the genetically obese Zucker rat. The hyperlipemic state may be attributed to hypersecretion of hepatic VLDL and consequent saturation of the lipolytic removal of triglyceride-rich lipoproteins from the circulation.     

Tumor necrosis factor, interleukin, and interferon induced changes in lipid metabolism as part of host defense.

As the immune response is activated during infection, multiple changes in lipid metabolism, especially increased production of VLDL, occur. Many of the cytokines that mediate the immune response are able to produce such changes in lipid metabolism in vivo. The induction of hypertriglyceridemia or other changes in lipid metabolism during infection do not directly cause the wasting syndrome. It appears that such changes in lipid metabolism may be beneficial to the host, as lipoproteins inactivate a variety of infectious agents. Cytokine-driven hepatic VLDL production during infection most likely represents a part of the acute phase response. The body is thus able to increase serum lipids during infection, or at least maintain triglyceride-rich lipoproteins despite the anorexia of infection. In this manner, the anti-infective, protective effects of lipoproteins are maintained.     

Blocking VLDL secretion causes hepatic steatosis but does not affect peripheral lipid stores or insulin sensitivity in mice

The liver secretes triglyceride-rich VLDLs, and the triglycerides in these particles are taken up by peripheral tissues, mainly heart, skeletal muscle, and adipose tissue. Blocking hepatic VLDL secretion interferes with the delivery of liver-derived triglycerides to peripheral tissues and results in an accumulation of triglycerides in the liver. However, it is unclear how interfering with hepatic triglyceride secretion affects adiposity, muscle triglyceride stores, and insulin sensitivity. To explore these issues, we examined mice that cannot secrete VLDL [due to the absence of microsomal triglyceride transfer protein (Mttp) in the liver]. These mice exhibit markedly reduced levels of apolipoprotein B-100 in the plasma, along with reduced levels of triglycerides in the plasma. Despite the low plasma triglyceride levels, triglyceride levels in skeletal muscle were unaffected. Adiposity and adipose tissue triglyceride synthesis rates were also normal, and body weight curves were unaffected. Even though the blockade of VLDL secretion caused hepatic steatosis accompanied by increased ceramides and diacylglycerols in the liver, the mice exhibited normal glucose tolerance and were sensitive to insulin at the whole-body level, as judged by hyperinsulinemic euglycemic clamp studies. Normal hepatic glucose production and insulin signaling were also maintained in the fatty liver induced by Mttp deletion. Thus, blocking VLDL secretion causes hepatic steatosis without insulin resistance, and there is little effect on muscle triglyceride stores or adiposity.     

Obesity and post-prandial lipid metabolism. Feast or famine?

Both in Western countries and in third world countries there is an increasing incidence of obesity. Obesity per se or insulin resistance associated with obesity may increase cardiovascular risk factors including dyslipidemia, hypertension and Type 2 diabetes. Over the past decade the understanding has increased of specific mediators in the hypothalamus that are involved in regulating food intake and body weight. In obese humans fasting plasma lipids can be normal but postprandial lipid metabolism is abnormal with an accumulation of triglyceride-rich remnant lipoproteins. In viscerally obese men chylomicron remnant catabolism was markedly decreased when compared with lean individuals. The decreased clearance of chylomicron remnants in viscerally obese subjects may be explained by competition between chylomicron remnants and the increased hepatic production of VLDL for clearance by low density lipoprotein receptors. Increased food intake in rodent models of obesity was shown to be associated with a delay in the catabolism of remnant lipoprotein particles. Prevention of hyperphagia was found to correct the impairment in the metabolism of remnant lipoproteins. Under fasting and food restricted conditions the improvement of remnant metabolism was associated with an increased oxidation of remnant lipids as determined by a novel stable isotope breath test. Anti-obesity and lipid lowering drugs have been used for the treatment of obesity. Inhibitors of cholesterol synthesis inhibitors (statins) have been shown to be effective in treating dyslipidemia. Inhibition of cholesterol synthesis with Atorvastatin was shown to improve chylomicron metabolism by increasing chylomicron remnant catabolism in obese subjects as assessed by the newly developed stable isotope breath test.     

Pathway-selective Insulin Resistance and Metabolic Disease: The Importance of Nutrient Flux

Hepatic glucose and lipid metabolism are altered in metabolic disease (e.g. obesity, metabolic syndrome, and Type 2 diabetes). Insulin-dependent regulation of glucose metabolism is impaired. In contrast, lipogenesis, hypertriglyceridemia, and hepatic steatosis are increased. Because insulin promotes lipogenesis and liver fat accumulation, to explain the elevation in plasma and tissue lipids, investigators have suggested the presence of pathway-selective insulin resistance. In this model, insulin signaling to glucose metabolism is impaired, but insulin signaling to lipid metabolism is intact. We discuss the evidence for the differential regulation of hepatic lipid and glucose metabolism. We suggest that the primary phenotypic driver is altered substrate delivery to the liver, as well as the repartitioning of hepatic nutrient handling. Specific alterations in insulin signaling serve to amplify the alterations in hepatic substrate metabolism. Thus, hyperinsulinemia and its resultant increased signaling may facilitate lipogenesis, but are not the major drivers of the phenotype of pathway-selective insulin resistance.     

Dissociation of the glucose and lipid regulatory functions of FoxO1 by targeted knockin of acetylation-defective alleles in mice

FoxO1 integrates multiple metabolic pathways. Nutrient levels modulate FoxO1 acetylation, but the functional consequences of this posttranslational modification are unclear. To answer this question, we generated mice bearing alleles that encode constitutively acetylated and acetylation-defective FoxO1 proteins. Homozygosity for an allele mimicking constitutive acetylation (Foxo1(KQ/KQ)) results in embryonic lethality due to cardiac and angiogenesis defects. In contrast, mice homozygous for?a constitutively deacetylated Foxo1 allele (Foxo1(KR/KR)) display a unique metabolic phenotype of impaired insulin action on hepatic glucose metabolism but decreased plasma lipid levels and low respiratory quotient that are consistent with a state of preferential lipid usage. Moreover, Foxo1(KR/KR) mice show?a dissociation between weight gain and insulin resistance in predisposing conditions (high fat diet, diabetes, and insulin receptor mutations), possibly due to decreased cytokine production in adipose tissue. Thus, acetylation inactivates FoxO1 during nutrient excess whereas deacetylation selectively potentiates FoxO1 activity, protecting against excessive catabolism during nutrient deprivation.     

Hepatic FoxO1 Integrates Glucose Utilization and Lipid Synthesis through Regulation of Chrebp O-Glycosylation

In liver, glucose utilization and lipid synthesis are inextricably intertwined. When glucose availability exceeds its utilization, lipogenesis increases, leading to increased intrahepatic lipid content and lipoprotein secretion. Although the fate of three-carbon metabolites is largely determined by flux rate through the relevant enzymes, insulin plays a permissive role in this process. But the mechanism integrating insulin receptor signaling to glucose utilization with lipogenesis is unknown. Forkhead box O1 (FoxO1), a downstream effector of insulin signaling, plays a central role in hepatic glucose metabolism through the regulation of hepatic glucose production. In this study, we investigated the mechanism by which FoxO1 integrates hepatic glucose utilization with lipid synthesis. We show that FoxO1 overexpression in hepatocytes reduces activity of carbohydrate response element binding protein (Chrebp), a key regulator of lipogenesis, by suppressing O-linked glycosylation and reducing the protein stability. FoxO1 inhibits high glucose- or O-GlcNAc transferase (OGT)-induced liver-pyruvate kinase (L-PK) promoter activity by decreasing Chrebp recruitment to the L-PK promoter. Conversely, FoxO1 ablation in liver leads to the enhanced O-glycosylation and increased protein level of Chrebp owing to decreased its ubiquitination. We propose that FoxO1 regulation of Chrebp O-glycosylation is a mechanism linking hepatic glucose utilization with lipid synthesis.     

Mechanisms of hepatic very low density lipoprotein overproduction in insulin resistance. Evidence for enhanced lipoprotein assembly, reduced intracellular ApoB degradation, and increased microsomal triglyceride transfer protein in a fructose-fed hamster model

A novel animal model of insulin resistance, the fructose-fed Syrian golden hamster, was employed to investigate the mechanisms mediating the overproduction of very low density lipoprotein (VLDL) in the insulin resistant state. Fructose feeding for a 2-week period induced significant hypertriglyceridemia and hyperinsulinemia, and the development of whole body insulin resistance was documented using the euglycemic-hyperinsulinemic clamp technique. In vivo Triton WR-1339 studies showed evidence of VLDL-apoB overproduction in the fructose-fed hamster. Fructose feeding induced a significant increase in cellular synthesis and secretion of total triglyceride (TG) as well as VLDL-TG by primary hamster hepatocytes. Increased TG secretion was accompanied by a 4.6-fold increase in VLDL-apoB secretion. Enhanced stability of nascent apoB in fructose-fed hepatocytes was evident in intact cells as well as in a permeabilized cell system. Analysis of newly formed lipoprotein particles in hepatic microsomes revealed significant differences in the pattern and density of lipoproteins, with hepatocytes derived from fructose-fed hamsters having higher levels of luminal lipoproteins at a density of VLDL versus controls. Immunoblot analysis of the intracellular mass of microsomal triglyceride transfer protein, a key enzyme involved in VLDL assembly, showed a striking 2.1-fold elevation in hepatocytes derived from fructose-fed versus control hamsters. Direct incubation of hamster hepatocytes with various concentrations of fructose failed to show any direct stimulation of its intracellular stability or extracellular secretion, further supporting the notion that the apoB overproduction in the fructose-fed hamster may be related to the fructose-induced insulin resistance in this animal model. In summary, hepatic VLDL-apoB overproduction in fructose-fed hamsters appears to result from increased intracellular stability of nascent apoB and an enhanced expression of MTP, which act to facilitate the assembly and secretion of apoB-containing lipoprotein particles.