Faculty Bibliography

Aldose reductase (AR), an enzyme widely believed to be involved in the aberrant metabolism of glucose and development of diabetic complications, is expressed at low levels in the mouse. We studied whether expression of human AR (hAR), its inhibition with lidorestat, which is an AR inhibitor (ARI), and the presence of streptozotocin (STZ)-induced diabetes altered plasma fructose, mortality, and/or vascular lesions in low-density lipoprotein (LDL) receptor-deficient [Ldlr(-/-)] mice. Mice were made diabetic at 12 weeks of age with low-dose STZ treatment. Four weeks later, the diabetic animals (glucose > 20 mM) were blindly assigned to a 0.15% cholesterol diet with or without ARI. After 4 and 6 weeks, there were no significant differences in body weights or plasma cholesterol, triglyceride, and glucose levels between the groups. Diabetic Ldlr(-/-) mice receiving ARI had plasma fructose levels of 5.2 +/- 2.3 microg/ml; placebo-treated mice had plasma fructose levels of 12.08 +/- 7.4 microg/ml, p < 0.01, despite the induction of fructose-metabolizing enzymes, fructose kinase and adolase B. After 6 weeks, hAR/Ldlr(-/-) mice on the placebo-containing diet had greater mortality (31%, n = 9/26 versus 6%, n = 1/21, p < 0.05). The mortality rate in the ARI-treated group was similar to that in non-hAR-expressing mice. Therefore, diabetic hAR-expressing mice had increased fructose and greater mortality that was corrected by inclusion of lidorestat, an ARI, in the diet. If similar effects are found in humans, such treatment could improve clinical outcome in diabetic patients.

OBJECTIVE: Skeletal muscle-specific LPL knockout mouse (SMLPL(-/-)) were created to study the systemic impact of reduced lipoprotein lipid delivery in skeletal muscle on insulin sensitivity, body weight, and composition. RESEARCH DESIGN AND METHODS: Tissue-specific insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp and 2-deoxyglucose uptake. Gene expression and insulin-signaling molecules were compared in skeletal muscle and liver of SMLPL(-/-) and control mice. RESULTS: Nine-week-old SMLPL(-/-) mice showed no differences in body weight, fat mass, or whole-body insulin sensitivity, but older SMLPL(-/-) mice had greater weight gain and whole-body insulin resistance. High-fat diet feeding accelerated the development of obesity. In young SMLPL(-/-) mice, insulin-stimulated glucose uptake was increased 58% in the skeletal muscle, but was reduced in white adipose tissue (WAT) and heart. Insulin action was also diminished in liver: 40% suppression of hepatic glucose production in SMLPL(-/-) vs. 90% in control mice. Skeletal muscle triglyceride was 38% lower, and insulin-stimulated phosphorylated Akt (Ser473) was twofold greater in SMLPL(-/-) mice without changes in IRS-1 tyrosine phosphorylation and phosphatidylinositol 3-kinase activity. Hepatic triglyceride and liver X receptor, carbohydrate response element-binding protein, and PEPCK mRNAs were unaffected in SMLPL(-/-) mice, but peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1alpha and interleukin-1beta mRNAs were higher, and stearoyl-coenzyme A desaturase-1 and PPARgamma mRNAs were reduced. CONCLUSIONS: LPL deletion in skeletal muscle reduces lipid storage and increases insulin signaling in skeletal muscle without changes in body composition. Moreover, lack of LPL in skeletal muscle results in insulin resistance in other key metabolic tissues and ultimately leads to obesity and systemic insulin resistance.

Plasma HDL levels are inversely related to the incidence of atherosclerotic disease. Some of the atheroprotective effects of HDL are likely mediated via preservation of EC function. Whether the beneficial effects of HDL on ECs depend on its involvement in cholesterol efflux via the ATP-binding cassette transporters ABCA1 and ABCG1, which promote efflux of cholesterol and oxysterols from macrophages, has not been investigated. To address this, we assessed endothelial function in Abca1(-/-), Abcg1(-/-), and Abca1(-/-)Abcg1(-/-) mice fed either a high-cholesterol diet (HCD) or a Western diet (WTD). Non-atherosclerotic arteries from WTD-fed Abcg1(-/-) and Abca1(-/-)Abcg1(-/-) mice exhibited a marked decrease in endothelium-dependent vasorelaxation, while Abca1(-/-) mice had a milder defect. In addition, eNOS activity was reduced in aortic homogenates generated from Abcg1(-/-) mice fed either a HCD or a WTD, and this correlated with decreased levels of the active dimeric form of eNOS. More detailed analysis indicated that ABCG1 was expressed primarily in ECs, and that these cells accumulated the oxysterol 7-ketocholesterol (7-KC) when Abcg1(-/-) mice were fed a WTD. Consistent with these data, ABCG1 had a major role in promoting efflux of cholesterol and 7-KC in cultured human aortic ECs (HAECs). Furthermore, HDL treatment of HAECs prevented 7-KC-induced ROS production and active eNOS dimer disruption in an ABCG1-dependent manner. Our data suggest that ABCG1 and HDL maintain EC function in HCD-fed mice by promoting efflux of cholesterol and 7-oxysterols and preserving active eNOS dimer levels.

Ceramide is among a number of potential lipotoxic molecules that are thought to modulate cellular energy metabolism. The heart is one of the tissues thought to become dysfunctional due to excess lipid accumulation. Dilated lipotoxic cardiomyopathy, thought to be the result of diabetes and severe obesity, has been modeled in several genetically altered mice, including animals with cardiac-specific overexpression of glycosylphosphatidylinositol (GPI)-anchored human lipoprotein lipase (LpL(GPI)). To test whether excess ceramide was implicated in cardiac lipotoxicity, de novo ceramide biosynthesis was inhibited pharmacologically by myriocin and genetically by heterozygous deletion of LCB1, a subunit of serine palmitoyltransferase (SPT). Inhibition of SPT, a rate-limiting enzyme in ceramide biosynthesis, reduced fatty acid and increased glucose oxidation in isolated perfused LpL(GPI) hearts, improved systolic function, and prolonged survival rates. Our results suggest a critical role for ceramide accumulation in the pathogenesis of lipotoxic cardiomyopathy.

OBJECTIVE: Identification of arterial genes and pathways altered in obesity and diabetes. RESEARCH DESIGN AND METHODS: Aortic gene expression profiles of obese and diabetic db/db, high-fat diet-fed C57BL/6J, and control mice were obtained using mouse Affymetrix arrays. Neuronatin (Nnat) was selected for further analysis. To determine the function of Nnat, a recombinant adenovirus (Ad-Nnat) was used to overexpress the Nnat gene in primary endothelial cells and in the mouse aorta in vivo. RESULTS: Nnat, a gene of unknown vascular function, was upregulated in the aortas of db/db and high-fat diet-fed mice. Nnat gene expression was increased in db/db mouse aorta endothelial cells. Nnat protein was localized to aortic endothelium and was selectively increased in the endothelium of db/db mice. Infection of primary human aortic endothelial cells (HAECs) with Ad-Nnat increased expression of a panel of nuclear factor-kappaB (NF-kappaB)-regulated genes, including inflammatory cytokines, chemokines, and cell adhesion molecules. Infection of mouse carotid arteries in vivo with the Ad-Nnat increased expression of vascular cell adhesion molecule 1 protein. Nnat activation of NF-kappaB and inflammatory gene expression in HAECs was mediated through pathways distinct from tumor necrosis factor-alpha. Nnat expression stimulated p38, Jun NH(2)-terminal kinase, extracellular signal-related kinase, and AKT kinase phosphorylation. Phosphatidylinositol 3-kinase and p38 inhibitors prevented Nnat-mediated activation of NF-kappaB-induced gene expression. CONCLUSIONS: Nnat expression is increased in endothelial cells of obese and diabetic mouse blood vessels. The effects of Nnat on inflammatory pathways in vitro and in vivo suggest a pathophysiological role of this new gene in diabetic vascular diseases.

Fatty acids (FAs) are acquired from free FA associated with albumin and lipoprotein triglyceride that is hydrolyzed by lipoprotein lipase (LpL). Hypertrophied hearts shift their substrate usage pattern to more glucose and less FA. However, FAs may still be an important source of energy in hypertrophied hearts. The aim of this study was to examine the importance of LpL-derived FAs in hypertensive hypertrophied hearts. We followed cardiac function and metabolic changes during 2 wk of angiotensin II (ANG II)-induced hypertension in control and heart-specific lipoprotein lipase knockout (hLpL0) mice. Glucose metabolism was increased in ANG II-treated control (control/ANG II) hearts, raising it to the same level as hLpL0 hearts. FA uptake-related genes, CD36 and FATP1, were reduced in control/ANG II hearts to levels found in hLpL0 hearts. ANG II did not alter these metabolic genes in hLpL0 mice. LpL activity was preserved, and mitochondrial FA oxidation-related genes were not altered in control/ANG II hearts. In control/ANG II hearts, triglyceride stores were consumed and reached the same levels as in hLpL0/ANG II hearts. Intracellular ATP content was reduced only in hLpL0/ANG II hearts. Both ANG II and deoxycorticosterone acetate-salt induced hypertension caused heart failure only in hLpL0 mice. Our data suggest that LpL activity is required for normal cardiac metabolic compensation to hypertensive stress.

OBJECTIVE: Patients with diabetes often have dyslipidemia and increased postprandial lipidmia. Induction of diabetes in LDL receptor (Ldlr(-/-)) knockout mice also leads to marked dyslipidemia. The reasons for this are unclear. RESEARCH DESIGN AND METHODS: We placed Ldlr(-/-) and heterozygous LDL receptor knockout (Ldlr(+/-)) mice on a high-cholesterol (0.15%) diet, induced diabetes with streptozotocin (STZ), and assessed reasons for differences in plasma cholesterol. RESULTS: STZ-induced diabetic Ldlr(-/-) mice had plasma cholesterol levels more than double those of nondiabetic controls. Fast-performance liquid chromatography and ultracentrifugation showed an increase in both VLDL and LDL. Plasma VLDL became more cholesterol enriched, and both VLDL and LDL had a greater content of apolipoprotein (apo)E. In LDL the ratio of apoB48 to apoB100 was increased. ApoB production, assessed using [(35)S]methionine labeling in Triton WR1339-treated mice, was not increased in fasting STZ-induced diabetic mice. Similarly, postprandial lipoprotein production was not increased. Reduction of cholesterol in the diet to normalize the amount of cholesterol intake by the control and STZ-induced diabetic animals reduced plasma cholesterol levels in STZ-induced diabetic mice, but plasma cholesterol was still markedly elevated compared with nondiabetic controls. LDL from STZ-induced diabetic mice was cleared from the plasma and trapped more rapidly by livers of control mice. STZ treatment reduced liver expression of the proteoglycan sulfation enzyme, heparan sulfate N-deacetylase/N-sulfotrasferase-1, an effect that was reproduced in cultured hepatocytyes by a high glucose-containing medium. CONCLUSIONS: STZ-induced diabetic, cholesterol-fed mice developed hyperlipidemia due to a non-LDL receptor defect in clearance of circulating apoB-containing lipoproteins.

The intestine and other tissues are able to synthesize retinyl esters in an acyl-CoA-dependent manner involving an acyl-CoA:retinol acyltransferase (ARAT). However, the molecular identity of this ARAT has not been established. Recent studies of lecithin:retinol acyltransferase (LRAT)-deficient mice indicate that LRAT is responsible for the preponderance of retinyl ester synthesis in the body, aside from in the intestine and adipose tissue. Our present studies, employing a number of mutant mouse models, identify diacylglycerol acyltransferase 1 (DGAT1) as an important intestinal ARAT in vivo. The contribution that DGAT1 makes to intestinal retinyl ester synthesis becomes greater when a large pharmacologic dose of retinol is administered by gavage to mice. Moreover, when large retinol doses are administered another intestinal enzyme(s) with ARAT activity becomes apparent. Surprisingly, although DGAT1 is expressed in adipose tissue, DGAT1 does not catalyze retinyl ester synthesis in adipose tissue in vivo. Our data also establish that cellular retinol-binding protein, type II (CRBPII), which is expressed solely in the adult intestine, in vivo channels retinol to LRAT for retinyl ester synthesis. Contrary to what has been proposed in the literature based on in vitro studies, CRBPII does not directly prevent retinol from being acted upon by DGAT1 or other intestinal ARATs in vivo.

Hepatic steatosis is often associated with insulin resistance and obesity and can lead to steatohepatitis and cirrhosis. In this study, we have demonstrated that hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL), two enzymes critical for lipolysis in adipose tissues, also contribute to lipolysis in the liver and can mobilize hepatic triglycerides in vivo and in vitro. Adenoviral overexpression of HSL and/or ATGL reduced liver triglycerides by 40-60% in both ob/ob mice and mice with high fat diet-induced obesity. However, these enzymes did not affect fasting plasma triglyceride and free fatty acid levels or triglyceride and apolipoprotein B secretion rates. Plasma 3-beta-hydroxybutyrate levels were increased 3-5 days after infection in both HSL- and ATGL-overexpressing male mice, suggesting an increase in beta-oxidation. Expression of genes involved in fatty acid transport and synthesis, lipid storage, and mitochondrial bioenergetics was unchanged. Mechanistic studies in oleate-supplemented McA-RH7777 cells with adenoviral overexpression of HSL or ATGL showed that reduced cellular triglycerides could be attributed to increases in beta-oxidation as well as direct release of free fatty acids into the medium. In summary, hepatic overexpression of HSL or ATGL can promote fatty acid oxidation, stimulate direct release of free fatty acid, and ameliorate hepatic steatosis. This study suggests a direct functional role for both HSL and ATGL in hepatic lipid homeostasis and identifies these enzymes as potential therapeutic targets for ameliorating hepatic steatosis associated with insulin resistance and obesity.

OBJECTIVE: Although epidemiologic data suggest that hypertriglyceridemia and elevated plasma levels of fatty acids are toxic to arteries, in vitro correlates have been inconsistent. To investigate whether increased endothelial cell expression of lipoprotein lipase (LpL), the primary enzyme creating free fatty acids from circulating triglycerides (TG), affects vascular function, we created transgenic mice that express human LpL (hLpL) driven by the promoter and enhancer of the Tie2 receptor. METHODS AND RESULTS: Mice expressing this transgene, denoted EC-hLpL and L for low and H for high expression, had decreased plasma TG levels compared with wild-type mice (WT): 106+/-31 in WT, 37+/-17 (line H), and 63+/-31 mg/dL (line L) because of a reduction in VLDL TG; plasma cholesterol and HDL levels were unaltered. Crossing a high expressing EC-hLpL transgene onto the LpL knockout background allowed for survival of the pups; TG in these mice was approximately equal to that of heterozygous LpL knockout mice. Surprisingly, under control conditions the EC-hLpL transgene did not alter arterial function or endothelial cell gene expression; however, after tumor necrosis factor (TNF)-alpha treatment, arterial vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and endogenous TNF-alpha mRNA levels were increased and arteries had impaired endothelium-dependent vasodilatation. This was associated with reduced eNOS dimers. CONCLUSIONS: Therefore, we hypothesize that excess vascular wall LpL augments vascular dysfunction in the setting of inflammation.