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In vivo plasma lipid oxidation in sugar-induced rat hypertriglyceridemia and hypertension.

Fe(II) and Fe(III) are required for the catalysis of lipid peroxidation through generation of reactive oxygen species that damage cell membranes. This study investigated the effect of free radicals and lipid peroxidation, induced by intraperitoneal injection of iron-dextran in vivo, in the plasma of the sugar-induced hypertriglyceridemic and hypertensive male and female rats. Lipid peroxidation was measured by the malondialdehyde (MDA) equivalent, using a fluorescence method of 2-thiobarbituric acid reactive substances (TBARS). Iron increased TBARS generation by fourfold (P<.0001) in male control rats and by twofold (P<.01) in female control rats, and the difference between TBARS concentration in female as compared with male animals was statistically significant (P<.05). In the case of the sugar-fed group, iron-dextran produced an increase of TBARS concentration by twofold in both male (P<.001) and female rats (P<.01), and no significant difference in TBARS concentration was observed between sugar-fed female and male rats. The analysis of fatty acid composition by gas chromatography showed a significant diminution of 50% in the proportion of arachidonic acid (C20:4n-6) in the male control group in comparison with the female group (P<.0001). In female control rats, a small diminution in the proportion of C20:4n-6 and in the other polyunsaturated fatty acids was observed (P<.05). A significant difference in the C20:4n-6 proportion was found between the male and female group of control rats. In the sugar-fed group, iron induced a significant diminution of arachidonic acid (P<.001) in both female and male rats in comparison with the sugar-fed group without iron.

Animals↗

Hypertension, hypertriglyceridemia, and impaired endothelium-dependent vascular relaxation in mice lacking insulin receptor substrate-1.

Insulin resistance is often associated with atherosclerotic diseases in subjects with obesity and impaired glucose tolerance. This study examined the effects of insulin resistance on coronary risk factors in IRS-1 deficient mice, a nonobese animal model of insulin resistance. Blood pressure and plasma triglyceride levels were significantly higher in IRS-1 deficient mice than in normal mice. Impaired endothelium-dependent vascular relaxation was also observed in IRS-1 deficient mice. Furthermore, lipoprotein lipase activity was lower than in normal mice, suggesting impaired lipolysis to be involved in the increase in plasma triglyceride levels under insulin-resistant conditions. Thus, insulin resistance plays an important role in the clustering of coronary risk factors which may accelerate the progression of atherosclerosis in subjects with insulin resistance.

Animals↗

Relationship between stearoyl-CoA desaturase activity and plasma triglycerides in human and mouse hypertriglyceridemia.

Stearoyl-CoA desaturase (SCD) is expressed at high levels in several human tissues and is required for the biosynthesis of oleate (18:1) and palmitoleate (16:1). These monounsaturated fatty acids are the major components of phospholipids, triglycerides, wax esters, and cholesterol esters. Mice with a targeted disruption of the SCD1 gene have very low levels of VLDL and impaired triglyceride and cholesterol ester biosynthesis. In the HYPLIP mouse, a model of hyperlipidemia, there was a 4-fold increase in hepatic SCD activity, a 1.8-fold increase in the desaturation index, and a 2-fold increase in plasma triglycerides. We used the plasma ratio of 18:1/18:0 (the "desaturation index") as an in vivo measure of SCD activity in human subjects. In human subjects with triglycerides ranging from 0.3 to 20 mM, the desaturation ratio accounted for one-third of the variance in plasma triglyceride levels. A 2-fold increase in the desaturation index was associated with a 4-fold increase in plasma triglycerides. In human subjects exposed to a high carbohydrate diet, the desaturation index explained 44% of the variance in triglycerides. We propose that many of the factors that influence plasma triglyceride levels do so by converging upon the regulation of SCD activity.

Animals↗

Overexpression of apoC-I in apoE-null mice: severe hypertriglyceridemia due to inhibition of hepatic lipase.

Apolipoprotein C-I (apoC-I) has been proposed to act primarily via interference with apoE-mediated lipoprotein uptake. To define actions of apoC-I that are independent of apoE, we crossed a moderately overexpressing human apoC-I transgenic, which possesses a minimal phenotype in the WT background, with the apoE-null mouse. Surprisingly, apoE-null/C-I mice showed much more severe hyperlipidemia than apoE-null littermates in both the fasting and non-fasting states, with an almost doubling of cholesterol, primarily in IDL+LDL, and a marked increase in triglycerides; 3-fold in females to 260 +/- 80 mg/dl and 14-fold in males to 1409 +/- 594 mg/dl. HDL lipids were not significantly altered but HDL were apoC-I-enriched and apoA-II-depleted. Production rates of VLDL triglyceride were unchanged as was the clearance of post-lipolysis remnant particles. Plasma post-heparin hepatic lipase and lipoprotein lipase levels were undiminished as was the in vitro hydrolysis of apoC-I transgenic VLDL. However, HDL from apoC-I transgenic mice had a marked inhibitory effect on hepatic lipase activity, as did purified apoC-I. LPL activity was minimally affected. Atherosclerosis assay revealed significantly increased atherosclerosis in apoE-null/C-I mice assessed via the en face assay. Inhibition of hepatic lipase may be an important mechanism of the decrease in lipoprotein clearance mediated by apoC-I.

Animals↗

Overexpression of apoC-III produces lesser hypertriglyceridemia in apoB-48-only gene-targeted mice than in apoB-100-only mice.

The adaptive value of apolipoprotein B-48 (apoB-48), the truncated form of apoB produced by the intestine, in lipid metabolism remains unclear. We crossed human apoC-III transgenic mice with mice expressing either apoB-48 only (apoB48/48) or apoB-100 only (apoB100/100). Cholesterol levels were higher in apoB48/48 mice than in apoB100/100 mice but triglyceride levels were similar. Lipid levels were increased by the apoC-III transgene. However, triglyceride levels were significantly higher in apoB100/100C-III than in apoB48/48C-III mice (895 +/- 395 mg/dl vs. 690 +/- 252 mg/dl; P <0.01), whereas cholesterol levels were higher in the apoB48/48C-III mice than in apoB100/100C-III (144 +/- 35 mg/dl vs. 94 +/- 30 mg/dl; P <0.00001). Triglyceride clearance from VLDL was impaired to a greater extent in apoB100/100C-III vs. apoB100/100 mice than in apoB48/48C-III vs. apoB48/48 mice. Triglyceride secretion rates were no different in apoC-III transgenic mice than in their nontransgenic littermates. ApoB-48 triglyceride-rich lipoproteins were more resistant to the triglyceride-increasing effects of apoC-III but appeared more sensitive to the remnant clearance inhibition. Our findings support a coordinated role for apoB-48 in facilitating the delivery of dietary triglycerides to the periphery. Consistent with such a mechanism, glucose levels were significantly higher in apoB48/48 mice vs. apoB100/100 mice, perhaps on the basis of metabolic competition.

Animals↗

Severe hypertriglyceridemia in human APOC1 transgenic mice is caused by apoC-I-induced inhibition of LPL.

Studies in humans and mice have shown that increased expression of apolipoprotein C-I (apoC-I) results in combined hyperlipidemia with a more pronounced effect on triglycerides (TGs) compared with total cholesterol (TC). The aim of this study was to elucidate the main reason for this effect using human apoC-I-expressing (APOC1) mice. Moderate plasma human apoC-I levels (i.e., 4-fold higher than human levels) caused a 12-fold increase in TG, along with a 2-fold increase in TC, mainly confined to VLDL. Cross-breeding of APOC1 mice on an apoE-deficient background resulted in a marked 55-fold increase in TG, confirming that the apoC-I-induced hyperlipidemia cannot merely be attributed to blockade of apoE-recognizing hepatic lipoprotein receptors. The plasma half-life of [3H]TG-VLDL-mimicking particles was 2-fold increased in APOC1 mice, suggesting that apoC-I reduces the lipolytic conversion of VLDL. Although total postheparin plasma LPL activity was not lower in APOC1 mice compared with controls, apoC-I was able to dose-dependently inhibit the LPL-mediated lipolysis of [3H]TG-VLDL-mimicking particles in vitro with a 60% efficiency compared with the main endogenous LPL inhibitor apoC-III. Finally, purified apoC-I impaired the clearance of [3H]TG-VLDL-mimicking particles independent of apoE-mediated hepatic uptake in lactoferrin-treated mice. Therefore, we conclude that apoC-I is a potent inhibitor of LPL-mediated TG-lipolysis.

Animals↗

Hypertriglyceridemia is associated with prebeta-HDL concentrations in subjects with familial low HDL.

Prebeta-HDL particles act as the primary acceptors of cellular cholesterol in reverse cholesterol transport (RCT). An impairment of RCT may be the reason for the increased risk of coronary heart disease (CHD) in subjects with familial low HDL. We studied the levels of serum prebeta-HDL and the major regulating factors of HDL metabolism in 67 subjects with familial low HDL and in 64 normolipidemic subjects. We report that the subjects with familial low HDL had markedly reduced prebeta-HDL concentrations compared with the normolipidemic subjects (17.4 +/- 7.2 vs. 23.4 +/- 7.8 mg apolipoprotein A-I/dl; P < 0.001). A positive correlation was observed between prebeta-HDL concentration and serum triglyceride (TG) level (r = 0.334, P = 0.006). In addition, serum TG level was found to be the strongest predictor of prebeta-HDL concentration in subjects with familial low HDL. The activities of cholesteryl ester transfer protein and hepatic lipase were markedly increased in subjects with familial low HDL without a significant correlation to prebeta-HDL concentration. Our results support the hypothesis that impaired RCT is one mechanism behind the increased risk for CHD in subjects with familial low HDL.

Adult↗

Hypoadiponectinemia is associated with insulin resistance, hypertriglyceridemia, and fat redistribution in human immunodeficiency virus-infected patients treated with highly active antiretroviral therapy.

A lipodystrophic syndrome and metabolic abnormalities have been observed in HIV-infected patients treated with highly active antiretroviral therapy (HAART). A murine model of lipodystrophy is associated with decreased levels of adiponectin, an adipocyte-secreted protein, the administration of which improves the metabolic syndrome in these mice. To investigate the association of adiponectin with metabolic changes in human lipodystrophy, we conducted a cross-sectional study of 112 HIV-infected patients treated with HAART. Mean adiponectin levels were higher in patients with no fat redistribution (FR) vs. FR (4.8 +/- 5.0 vs. 2.2 +/- 2.7 microg/ml, P < 0.01), but no significant differences in adiponectin levels were observed between FR subgroups. The difference in adiponectin levels between subjects with and without FR remained significant after adjusting for age, gender, leptin, HIV medication use, and CD4 count using logistic regression (odds ratio, 0.54, P = 0.008). Adiponectin was significantly correlated with triglycerides (r = -0.40), abdominal visceral fat (r = -0.35), extremity fat (r = 0.37), insulin resistance (HOMA-IR) (r = -0.28), nucleoside reverse transcriptase inhibitor (NRTI) use (r = -0.32), and high-density lipoprotein (HDL) (r = 0.41) using bivariate analysis (all P < 0.01). The association with HDL weakened but remained significant on multivariate analysis (standard beta = 0.29, P = 0.01). However, the association of adiponectin with HOMA-IR became nonsignificant after adjusting for NRTI use (standard beta = -0.15, P = 0.12), suggesting that changes in adiponectin levels may underlie the effect of NRTI use on insulin resistance. The associations of adiponectin with triglycerides and HOMA-IR were also slightly weakened after adjusting for visceral and extremity fat, indicating that adiponectin may, in part, mediate the effect of FR on triglycerides and insulin resistance. This study indicates that adiponectin is inversely correlated with abdominal visceral fat mass, serum triglycerides, and insulin resistance and is directly correlated with HDL and extremity fat in a sample of HIV-infected patients treated with HAART. The results also indicate that NRTI use may worsen insulin resistance by decreasing adiponectin levels. Thus, adiponectin replacement may be a potential treatment option to ameliorate the metabolic changes observed in this patient population.

Abdomen↗

Insulin resistance and hypertriglyceridemia in nondiabetic relatives of patients with noninsulin-dependent diabetes mellitus.

Plasma glucose, FFA, and insulin responses to an oral glucose challenge, plasma lipid and lipoprotein concentrations, and the ability of insulin to stimulate glucose disposal were measured in 35 nondiabetic sedentary and overweight subjects. The subjects were divided into 2 groups on the basis of the presence (n = 19) or absence (n = 16) of a history of a first degree relative with noninsulin-dependent diabetes. The 2 groups were similar in age, body mass index, waist to hip ratio, and maximal aerobic capacity. The results demonstrated that the ability of insulin to stimulate disposal of a glucose load was significantly reduced in the subjects with a positive family history of noninsulin-dependent diabetes. In addition, these individuals had significantly higher plasma triglyceride and very low density lipoprotein cholesterol concentrations. Since all environmental factors known to modify insulin action and very low density lipoprotein metabolism were equal in the 2 groups, these data suggest that the metabolic differences noted are likely to be genetic in origin.

Adult↗

Augmentation of Ca2+-induced microsomal triglyceride transfer protein activity by glucose supply enhances hypertriglyceridemia in vivo.

We have investigated possible roles of intra-glucose supply on microsomal triglyceride (TG) transfer protein (MTP) in the secretion of TG-rich very low-density lipoprotein (VLDL) from the liver. Due to the activation of MTP, TG and apolipoprotein B (apoB) in the liver are assembled into VLDL and then the VLDL is transferred into the blood stream. High MTP activity can increase the release of VLDL into the blood stream, and this would lead high levels of TG and apoB in the blood. High MTP activity was found when the liver (or hepatocytes) contained a high level of total Ca2+ as a response of glucose administration. However, the MTP activity was reduced in response to the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7, Ki=25 microM), the intracellular Ca2+ chelator BAPTA-AM, and the extracellular Ca2+ chelator EDTA. These suggested that there might be a very close relationship between high MTP activity and high Ca2+ level in the liver by glucose administration. Glucose-derived hyperglycemic condition resulted from those elevations of TG and total cholesterol in the liver. This hyperglycemic phenomenon may be associated with the increase of TG and apoB levels in blood. The possibility for the regulation of VLDL formation in the liver and, further, those related circulatory diseases due to the excess of VLDL in the blood stream by controlling MTP activity in association with Ca2+ was investigated.

Animals↗