Mechanisms underlying hypertriglyceridemia in rats with monosodium l-glutamate-induced obesity: Evidence of XBP-1/PDI/MTP axis activation

Non-alcoholic fatty liver disease (NAFLD) is intimately associated with insulin resistance and hypertriglyceridemia, whereas many of the mechanisms underlying this association are still poorly understood. In the present study, we investigated the relationship between microsomal triglyceride transfer protein (MTP) and markers of endoplasmic reticulum (ER) stress in the liver of rats subjected to neonatal monosodium l-glutamate (MSG)-induced obesity. At age 120 days old, the MSG-obese animals exhibited hyperglycemia, hypertriglyceridemia, insulin resistance, and liver steatosis, while the control (CTR) group did not. Analysis using fast protein liquid chromatography of the serum lipoproteins revealed that the triacylglycerol content of the very low-density lipoprotein (VLDL) particles was twice as high in the MSG animals compared with the CTR animals. The expression of ER stress markers, GRP76 and GRP94, was increased in the MSG rats, promoting a higher expression of X-box binding protein 1 (XBP-1), protein disulfide isomerase (PDI), and MTP. As the XBP-1/PDI/MTP axis has been suggested to represent a significant lipogenic mechanism in the liver response to ER stress, our data indicate that hypertriglyceridemia and liver steatosis occurring in the MSG rats are associated with increased MTP expression.     

Enhanced atherothrombotic formation after oxidative injury by FeCl3 to the common carotid artery in severe combined hyperlipidemic mice

Enhanced susceptibility to atherosclerosis from severe hypertriglyceridemia (HTG) resulting from lipoprotein lipase (LPL) deficiency has been demonstrated in our recent findings which employed a unique mouse model. In the present study we provide further evidence that severe HTG due to LPL deficiency also promotes an atherothrombotic response to arterial injury induced by ferric chloride in a severe combined hyperlipidemic mouse model. Methods and results: A mouse model (LPL^−/−XApoE^−/− double knockout, DKO) with severe combined hyperlipidemia was established by crossing ApoE and LPL-deficient mice. The common carotid arteries of ApoE knockout (EKO) and DKO mice were subjected to injury by ferric chloride, and the formation of arterial thrombosis together with various markers were compared in these lesions. DKO mice demonstrated significantly enhanced thrombus formation overlying atherosclerotic plaque after injury, which contained smooth muscle cells, macrophages, and neutral lipid. The area of neointima, mean intima/media ratios, and the percentage of luminal stenosis were significantly greater (P < 0.01) in DKO mice. Compared with EKO mice, the expression of von Willebrand factor (vWF) and plasminogen activator inhibitor type 1 (PAI-1) were increased in DKO mice. Conclusions: Severe combined hyperlipidemia promotes thrombosis after ferric chloride injury to atherosclerotic vessels and HTG plays a major role in the process.     

Mechanisms underlying different responses of plasma triglyceride to high-fat diets in hamsters and mice: Roles of hepatic MTP and triglyceride secretion

Hypertriglyceridemia, closely associated with insulin resistance, is induced on high-fat diets (HFD) in humans but not in mouse models. Mechanisms underlying this species difference are still unclear. Hamsters resemble humans in lipoprotein metabolism. Here by comparing the responses to HFD in hamsters and mice, we found that hepatic TG secretion, MTP expression and plasma free fatty acid (FFA) level were increased in hamsters on HFD feeding but decreased in mice. Although hepatic steatosis and de novo lipogenesis were induced by HFD feeding in both models, cholesterol biosynthesis was inhibited in mice but not in hamsters. Moreover, in insulin deficient state, HFD increased plasma TG level, hepatic TG secretion, MTP expression and plasma FFA level in both models. In summary, distinct changes of MTP expression, in correlation with hepatic TG secretion, underlie the opposite responses of plasma TG levels to high-fat diets in hamsters and mice. Furthermore, hepatic TG secretion and MTP expression seems to be associated with plasma FFA level and cholesterol biosynthesis but not hepatic steatosis or de novo lipogenesis.     

Multimerization of Glycosylphosphatidylinositol-anchored High Density Lipoprotein-binding Protein 1 (GPIHBP1) and Familial Chylomicronemia from a Serine-to-Cysteine Substitution in GPIHBP1 Ly6 Domain

GPIHBP1, a glycosylphosphatidylinositol-anchored glycoprotein of microvascular endothelial cells, binds lipoprotein lipase (LPL) within the interstitial spaces and transports it across endothelial cells to the capillary lumen. The ability of GPIHBP1 to bind LPL depends on the Ly6 domain, a three-fingered structure containing 10 cysteines and a conserved pattern of disulfide bond formation. Here, we report a patient with severe hypertriglyceridemia who was homozygous for a GPIHBP1 point mutation that converted a serine in the GPIHBP1 Ly6 domain (Ser-107) to a cysteine. Two hypertriglyceridemic siblings were homozygous for the same mutation. All three homozygotes had very low levels of LPL in the preheparin plasma. We suspected that the extra cysteine in GPIHBP1-S107C might prevent the trafficking of the protein to the cell surface, but this was not the case. However, nearly all of the GPIHBP1-S107C on the cell surface was in the form of disulfide-linked dimers and multimers, whereas wild-type GPIHBP1 was predominantly monomeric. An insect cell GPIHBP1 expression system confirmed the propensity of GPIHBP1-S107C to form disulfide-linked dimers and to form multimers. Functional studies showed that only GPIHBP1 monomers bind LPL. In keeping with that finding, there was no binding of LPL to GPIHBP1-S107C in either cell-based or cell-free binding assays. We conclude that an extra cysteine in the GPIHBP1 Ly6 motif results in multimerization of GPIHBP1, defective LPL binding, and severe hypertriglyceridemia.     

Insulin receptor processing and lipid composition of erythrocyte membrane in patients with hyperlipidemia

The aim of this study was to determine whether the common forms of dyslipidemia could affect either the lipid composition or insulin receptor processing (down-regulation) of erythrocytes. The study included 22 patients with type IIa hypercholesterolemia, 15 patients with type IV hypertriglyceridemia and 12 patients with type IIb hyperlipidemia. Ten normolipidemic subjects were used as controls. Their erythrocyte membranes were analyzed for lipid composition and insulin receptor down-regulation. The results show that all the hyperlipidemias investigated were characterized by significant increases in the cholesterol to phospholipid molar ratio (0.56±0.08 in controls and 1.11±0.13, 1.09±0.14, 1.04±0.15, p<0.001, in types IIa, IIb and IV, respectively). Surface insulin receptors of type IIa and IIb patients did not appear to down-regulate when compared to normal subjects, but rather up-regulated (+65.2% in controls, –1.0% and –8.7%, p<0.001, in type IIa and IIb patients, respectively). Patients with type IV hypertriglyceridemia showed a residual capacity for insulin receptor internalization (10.7% down-regulation). Membranes of all the patients contained a higher proportion of phosphatidylethanolamine; the molar ratio of sphingomyelin to phosphatidylcholine was significantly higher in types IIb than in controls (1.22±0.11 and 1.12±0.10, p<0.05, respectively); all the patients showed a lower content of polyunsaturated fatty acids in the major glycerophospholipid classes. However, type IV hypertriglyceridemics showed less variations, especially in the phosphatidylserine fraction. These results indicate that the alterations in lipoprotein pattern may affect both the lipid membrane equilibria and the processing ability of surface insulin receptors.     

Hemorheological abnormalities in lipoprotein lipase deficient mice with severe hypertriglyceridemia

Severe hypertriglyceridemia (HTG) is a metabolic disturbance often seen in clinical practice. It is known to induce life-threatening acute pancreatitis, but its role in atherogenesis remains elusive. Hemorheological abnormality was thought to play an important role in pathogenesis of both pancreatitis and atherosclerosis. However, hemorheology in severe HTG was not well investigated. Recently, we established a severe HTG mouse model deficient in lipoprotein lipase (LPL) in which severe HTG was observed to cause a significant increase in plasma viscosity. Disturbances of erythrocytes were also documented, including decreased deformability, electrophoresis rate, and membrane fluidity, and increased osmotic fragility. Scanning electron microscopy demonstrated that most erythrocytes of LPL deficient mice deformed with protrusions, irregular appearances or indistinct concaves. Analysis of erythrocyte membrane lipids showed decreased cholesterol (Ch) and phospholipid (PL) contents but unaltered Ch/PL ratio. The changes of membrane lipids may be partially responsible for the hemorheological and morphologic abnormalities of erythrocytes. This study indicated that severe HTG could lead to significant impairment of hemorheology and this model may be useful in delineating the role of severe HTG in the pathogenesis of hyperlipidemic pancreatitis and atherosclerosis.     

The calcium-sensing receptor R990G polymorphism is associated with increased risk of hypertriglyceridemia in obese Chinese

Background

We have demonstrated that the calcium-sensing receptor (CaSR) is involved in lipid metabolism; however, whether CaSR polymorphisms affect lipid metabolism in obesity is still unclear. The present study aimed to determine the effects of CaSR polymorphisms on HTG risk in obese Chinese.

Methods

A total of 972 subjects with HTG and 1197 with normal triglyceride (NTG) were stratified by body mass index (BMI) into normal weight, overweight or obesity subgroups. After 12-h fasting, CaSR polymorphisms in exon 7 were determined in the blood. Serum lipids and glucose, as well as height, body weight and waist circumference were measured. The anthropometric and metabolic characteristics of the NTG subjects were re-evaluated 3 years later.

Results

There were no genotypic or allelic distribution differences for the A986S or Q1011E polymorphisms between the NTG and HTG groups. However, the G/G genotypic and G allelic distributions of the CaSR R990G polymorphism in the HTG group were higher than the NTG group (p < 0.001). After stratification, in obese subjects, the homozygous (G/G) distribution of the CaSR R990G polymorphism in the HTG group was significantly higher than in the NTG group (p = 0.001), and showed an increased risk of HTG at baseline [OR = 2.55, 95% CI = 1.65–3.92, p < 0.006]. Interaction of the CaSR R990G polymorphism with BMI was associated with increased risk of HTG (β = 0.927, p < 0.001). Re-evaluation of the NTG subjects revealed significantly increased serum triglyceride levels in obese homozygous versus wildtype carriers (p < 0.05).

Conclusions

These results suggest that the CaSR R990G polymorphism is associated with increased risk of HTG, especially in obese Chinese, and may be a potential genetic predictor of diseases related to HTG.     

Moderate doses of EPA and DHA from re-esterified triacylglycerols but not from ethyl-esters lower fasting serum triacylglycerols in statin-treated dyslipidemic subjects: Results from a six month randomized controlled trial

Recently, in a supplementation study over six months, it has been demonstrated that re-esterified omega-3 fatty acid triacylglycerols (n3-FA-rTAGs) led to a higher increase in omega-3-index compared to identical doses of n3-FA ethyl-esters (n3-FA-EEs), suggesting a better long-term bioavailability. The aim of this study was to examine whether differences occur between the two forms in affecting fasting serum lipid levels. 150 dyslipidemic statin-treated participants were randomized to corn oil as a placebo or fish oil either as rTAG or EE in identical doses (1.01 g EPA+0.67 g DHA). No changes in total cholesterol, HDL or LDL levels were observed. In the rTAG-group, but not in the EE-group, fasting serum TAG levels were significantly reduced from baseline after three and six months. There was no significant difference between the two n3-FA-groups. However, serum TAG levels were significantly lowered after six months in the rTAG-group compared to the placebo-group in contrast to the EE-group.     

Hypertriglyceridemia Increases Mitochondrial Resting Respiration and Susceptibility to Permeability Transition

High plasma level of triglycerides (TGs) is a common feature in atherosclerosis, obesity, diabetes, alcoholism, stress, and infection. Since mitochondria have been implicated in cell death under a variety of metabolic disorders, we examined liver mitochondrial functions in hypertriglyceridemic transgenic mice. Hypertriglyceridemia increased resting respiration and predisposed to mitochondrial permeability transition (MPT). Ciprofibrate therapy reduced plasma TG levels, normalized respiration, and prevented MPT. The higher resting respiration in transgenic mitochondria remained in the presence of the adenine nucleotide carrier inhibitor, carboxyatractyloside, bovine serum albumin, and the uncoupling proteins (UCPs) inhibitor, GDP. UCP2 content was similar in both control and transgenic mitochondria. We propose that faster resting respiration represents a regulated adaptation to oxidize excess free fatty acid in the transgenic mice.