Spencer D. Proctor

Inhibition of de novo ceramide synthesis reverses diet-induced insulin resistance and enhances whole body oxygen consumption

OBJECTIVE It has been proposed that skeletal muscle insulin resistance arises from the accumulation of intramyocellular lipid metabolites that impede insulin signaling, including diacylglycerol and ceramide. We determined the role of de novo ceramide synthesis in mediating muscle insulin resistance. RESEARCH DESIGN AND METHODS Mice were subjected to 12 weeks of diet-induced obesity (DIO), and then treated for 4 weeks with myriocin, an inhibitor of serine palmitoyl transferase-1 (SPT1), the rate-limiting enzyme of de novo ceramide synthesis. RESULTS After 12 weeks of DIO, C57BL/6 mice demonstrated a doubling in gastrocnemius ceramide content, which was completely reversed (141.5 ± 15.8 vs. 94.6 ± 10.2 nmol/g dry wt) via treatment with myriocin, whereas hepatic ceramide content was unaffected by DIO. Interestingly, myriocin treatment did not alter the DIO-associated increase in gastrocnemius diacyglycerol content, and the only correlation observed between lipid metabolite accumulation and glucose intolerance occurred with ceramide (R = 0.61). DIO mice treated with myriocin showed a complete reversal of glucose intolerance and insulin resistance which was associated with enhanced insulin-stimulated Akt and glycogen synthase kinase 3β phosphorylation. Furthermore, myriocin treatment also decreased intramyocellular ceramide content and prevented insulin resistance development in db/db mice. Finally, myriocin-treated DIO mice displayed enhanced oxygen consumption rates (3,041 ± 124 vs. 2,407 ± 124 ml/kg/h) versus their control counterparts. CONCLUSIONS Our results demonstrate that the intramyocellular accumulation of ceramide correlates strongly with the development of insulin resistance, and suggests that inhibition of SPT1 is a potentially promising target for the treatment of insulin resistance.

Intimal Retention of Cholesterol Derived From Apolipoprotein B100– and Apolipoprotein B48–Containing Lipoproteins in Carotid Arteries of Watanabe Heritable Hyperlipidemic Rabbits

Objectives—The arterial retention of apolipoprotein (apo) B100- and apoB48-containing lipoproteins was simultaneously determined in a rabbit model of human hypercholesterolemia using 3D confocal microscopy. Methods and Results—Lipoproteins containing apoB100 (LDL) and apoB48 (chylomicron remnants) were differentially conjugated with fluorophores and simultaneously perfused at equivalent concentrations under physiological conditions in situ through carotid vessels of Watanabe heritable hyperlipidemic rabbits and compared with controls. Retention of lipoproteins was defined as the amount remaining after an extensive washout phase. LDL and chylomicron remnants were both retained, primarily within the subendothelial space. Without a concomitant increase in exposure to lipoproteins, we found a marked increase in the retention of cholesterol within the intima of Watanabe heritable hyperlipidemic rabbits compared with controls, specifically because of increased entrapment of apoB48-containing lipoproteins. Conclusions—Collectively, our data suggest that hypercholesterolemia induced as a consequence of LDL receptor deficiency differentially influences retention of LDL and of chylomicron remnants.

Effects of Ruminant trans Fatty Acids on Cardiovascular Disease and Cancer: A Comprehensive Review of Epidemiological, Clinical, and Mechanistic Studies

There are 2 predominant sources of dietary trans fatty acids (TFA) in the food supply, those formed during the industrial partial hydrogenation of vegetable oils (iTFA) and those formed by biohydrogenation in ruminants (rTFA), including vaccenic acid (VA) and the naturally occurring isomer of conjugated linoleic acid, cis-9, trans-11 CLA (c9,t11-CLA). The objective of this review is to evaluate the evidence base from epidemiological and clinical studies to determine whether intake of rTFA isomers, specifically VA and c9,t11-CLA, differentially affects risk of cardiovascular disease (CVD) and cancer compared with iTFA. In addition, animal and cell culture studies are reviewed to explore potential pro- and antiatherogenic mechanisms of VA and c9,t11-CLA. Some epidemiological studies suggest that a positive association with coronary heart disease risk exists between only iTFA isomers and not rTFA isomers. Small clinical studies have been conducted to establish cause-and-effect relationships between these different sources of TFA and biomarkers or risk factors of CVD with inconclusive results. The lack of detection of treatment effects reported in some studies may be due to insufficient statistical power. Many studies have used doses of rTFA that are not realistically attainable via diet; thus, further clinical studies are warranted. Associations between iTFA intake and cancer have been inconsistent, and associations between rTFA intake and cancer have not been well studied. Clinical studies have not been conducted investigating the cause-and-effect relationship between iTFA and rTFA intake and risk for cancers. Further research is needed to determine the health effects of VA and c9,t11-CLA in humans.

Arterial retention of apolipoprotein B48- and B100- containing lipoproteins in atherogenesis

Purpose of review The ‘response to retention’ hypothesis of atherosclerosis suggests that the arterial deposition of cholesterol is directly proportional to the concentration of circulating plasma lipoproteins. However, there is increasing evidence to support the concept that specific lipoproteins may be preferentially retained within the arterial wall, possibly as a result of greater affinity for cell surface and extracellular matrices. Recent findings Recently, key studies have provided insight into mechanisms involved in the interaction of apolipoprotein B (apoB)-containing lipoproteins with extracellular matrices. In addition, novel methods and innovative experimental design has enabled us to differentiate between the delivery, retention and efflux of apoB48- and apoB100-containing lipoproteins. Other studies have demonstrated a relationship between extracellular matrix proteoglycan expression and the development of atherosclerosis in vivo. Discussion in the present review also extends to the mechanisms that are involved in the relative intimal retention of apoB48- and apoB100-containing lipoproteins in order to explain the atherogenicity of these macromolecules. Summary The perspective of this review is to highlight recent advances in the area of arterial lipoprotein retention and the physiological significance these processes may have in the aetiology of cardiovascular disease. Importantly, an understanding of the mechanisms responsible for the retention of apoB48/B100-containing lipoproteins will enable new strategies to be developed for the future management of cardiovascular disease.

Trans-11 Vaccenic Acid Dietary Supplementation Induces Hypolipidemic Effects in JCR:LA-cp Rats

Trans-11 vaccenic acid [VA; 18:1(n-9)] is a positional and geometric isomer of oleic acid and is the precursor to conjugated linoleic acid (CLA) in humans. Despite VA being the predominant trans monoene in ruminant-derived lipids, very little is known about its nutritional bioactivity, particularly in conditions of chronic metabolic disorders, including obesity, insulin resistance, and/or dyslipidemia. The aim of this study was to assess the potential of VA to improve dyslipidemia, insulin sensitivity, or inflammatory status in obese and insulin-resistant JCR:LA-cp rats. The obese rats and age-matched lean littermates were fed a control diet or a control diet supplemented with 1.5% (wt:wt) VA for a period of 3 wk. The incorporation of VA and subsequent conversion to CLA in triglyceride was measured in adipose tissue. Glucose and insulin metabolism were assessed via a conscious adapted meal tolerance test procedure. Plasma lipids as well as serum inflammatory cytokine concentrations were measured by commercially available assays. VA supplementation did not result in any observable adverse health effects in either lean or obese JCR:LA-cp rats. After 3 wk of feeding, body weight, food intake, and glucose/insulin metabolism did not differ between VA-supplemented and control groups. The incorporation of VA and CLA into adipose triglycerides in obese rats fed VA increased by 1.5-fold and 6.5-fold, respectively, compared with obese rats fed the control diet. The most striking effect was a 40% decrease (P < 0.05) in fasting triglyceride concentrations in VA-treated obese rats relative to obese controls. Serum Il-10 concentration was decreased by VA, regardless of genotype (P < 0.05). In conclusion, short-term dietary supplementation of 1.5% VA did not result in any detrimental metabolic effects in JCR:LA-cp rats. In contrast, dietary VA had substantial hypo-triglyceridemic effects, suggesting a new bioactivity of this fatty acid that is typically found in ruminant-derived food products.

Brain inflammation is induced by co-morbidities and risk factors for stroke

Highlights ► Risk factors for stroke include atherosclerosis, obesity, diabetes and hypertension. ► Stroke risk factors are associated with peripheral inflammation. ► Corpulent rats and atherogenic mice show increased inflammation in the brain. ► Pilot data show that patients at risk of stroke may also develop brain inflammation. ► Chronic peripheral inflammation can drive inflammatory changes in the brain.

Impaired de Novo Choline Synthesis Explains Why Phosphatidylethanolamine N-Methyltransferase-deficient Mice Are Protected from Diet-induced Obesity

Phosphatidylcholine (PC) is synthesized from choline via the CDP-choline pathway. Liver cells can also synthesize PC via the sequential methylation of phosphatidylethanolamine, catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). The current study investigates whether or not hepatic PC biosynthesis is linked to diet-induced obesity. Pemt+/+ mice fed a high fat diet for 10 weeks increased in body mass by 60% and displayed insulin resistance, whereas Pemt−/− mice did not. Compared with Pemt+/+ mice, Pemt−/− mice had increased energy expenditure and maintained normal peripheral insulin sensitivity; however, they developed hepatomegaly and steatosis. In contrast, mice with impaired biosynthesis of PC via the CDP-choline pathway in liver became obese when fed a high fat diet. We, therefore, hypothesized that insufficient choline, rather than decreased hepatic phosphatidylcholine, was responsible for the lack of weight gain in Pemt−/− mice despite the presence of 1.3 g of choline/kg high fat diet. Supplementation with an additional 2.7 g of choline (but not betaine)/kg of diet normalized energy metabolism, weight gain, and insulin resistance in high fat diet-fed Pemt−/− mice. Furthermore, Pemt+/+ mice that were fed a choline-deficient diet had increased oxygen consumption, had improved glucose tolerance, and gained less weight. Thus, de novo synthesis of choline via PEMT has a previously unappreciated role in regulating whole body energy metabolism.

Hypoxia-Induced Intrauterine Growth Restriction Increases the Susceptibility of Rats to High-Fat Diet–Induced Metabolic Syndrome

OBJECTIVE It is recognized that there is a remarkable variability in the systemic response to high-fat (HF) diets that cannot be completely explained by genetic factors. In addition, pregnancy complications leading to intrauterine growth restriction (IUGR) have been associated with an increased risk of developing metabolic syndrome (MetS) later in life. Thus, we hypothesized that offspring born with IUGR exhibit permanent metabolic changes that make them more susceptible to HF diet–induced MetS. RESEARCH DESIGN AND METHODS SD rats born normal (control) or with hypoxia-induced IUGR were randomized to low-fat (10% fat) or HF (45% fat) diets. After 9 weeks of feeding, physiological and molecular pathways involved in the MetS were evaluated. RESULTS IUGR offspring exhibited decreased energy intake and physical activity relative to controls. In offspring fed a HF diet, IUGR was associated with decreased total body fat content, a relative increase in intra-abdominal fat deposition and adipocyte size, an increase in fasting plasma concentrations of leptin, triglyceride and free fatty acids, and an increased concentration of triglycerides and ceramides in both liver and skeletal muscle. These changes in lipid homeostasis were accompanied by in vivo insulin resistance and impaired glucose tolerance and associated with increased phosphorylation of protein kinase C θ, inhibition of insulin receptor substrate 1, and a decreased activation of protein kinase B (PKB; also known as Akt) in liver and skeletal muscle in response to insulin. CONCLUSIONS IUGR enhances specific deleterious metabolic responses to a HF diet. Our results suggest that offspring born with IUGR may require special attention and follow-up to prevent the early onset of MetS.

Both Intestinal and Hepatic Lipoprotein Production Are Stimulated by an Acute Elevation of Plasma Free Fatty Acids in Humans

Background— Hepatic lipoprotein production has been shown previously to be regulated by free fatty acid (FFA) flux to the liver, whereas intestinal lipoprotein production is stimulated mainly by ingested fat absorbed from the intestinal lumen. Emerging evidence indicates that intestinal lipoprotein production is increased in insulin resistance and type 2 diabetes mellitus, conditions that are associated with increased levels of circulating FFAs. Here we investigated whether short-term elevation of plasma FFAs stimulates intestinal apolipoprotein (apo) B-48– and hepatic apoB-100–containing triglyceride-rich lipoprotein (TRL) production in humans in the fed state. Methods and Results— TRL apoB-48 and apoB-100 metabolism were examined in 12 healthy men during a constant fed state. The studies were as follows, respectively: (1) Intralipid/heparin was infused intravenously immediately before and during the kinetics study to induce an ≈3-fold difference in plasma FFA compared with the saline study; (2) saline was infused intravenously as a control. ApoB-48– and apoB-100–containing TRL production and clearance were determined with a 12-hour primed constant infusion of [D3]l-leucine and multicompartmental kinetic modeling. TRL apoB-48 production rate was 69% higher in the Intralipid/heparin study than in the saline control (5.95±1.13 versus 3.53±0.58 mg/kg per day; P=0.027), and there was no significant difference in TRL apoB-48 clearance. TRL apoB-100 concentrations were also increased (P<0.001) and TRL apoB-100 production rate was 35% higher in the Intralipid/heparin study compared with saline (28±4 versus 21±3 mg/kg per day; P=0.020). Conclusions— This is the first study to demonstrate that intestinal TRL apoB-48 production is increased after short-term elevation of plasma FFAs in humans in the fed state, similar to the well-described stimulation of hepatic TRL apoB100–containing particles by FFAs.

Retention of chylomicron remnants by arterial tissue; importance of an efficient clearance mechanism from plasma

Atherosclerosis is thought to begin with the trapping of cholesterol rich lipoproteins within the intima of arterial vessels. Thereafter a complex inflammatory cascade involving recruitment and transformation of leukocytes, accumulation of sterols in macrophages and cellular proliferation, can lead to a progressive occlusion in blood flow, or an unstable arterial lesion prone to prothrombotic events. Primary intervention strategies aimed at reducing atherogenesis are designed to achieve reductions in sterol rich lipoproteins, primarily low density lipoproteins, given the hypothesis that decreased exposure will attenuate the rate of arterial cholesterol accumulation. Epidemiological evidence has clearly identified a positive relationship between poor dietary (fat) habits and the onset and progression of atherosclerosis. However lipoproteins which mediate the transport of dietary lipid, that is chylomicrons, are not normally considered to be directly involved in atherogenesis, because of their larger size and inability to efficiently penetrate arterial tissue. In contrast, this article reviews recent evidence which suggests that once chylomicrons are hydrolysed to their remnant form, the triglyceride depleted chylomicron remnants penetrate arterial tissue and moreover, become preferentially trapped within the subendothelial space as concentrated focii. Ongoing studies demonstrate that significant chylomicron remnant accumulation can occur in a number of primary and secondary lipid disorders and in normolipidemic subjects with coronary artery disease. Chylomicron remnant dyslipidemia in conditions prone to premature atherosclerosis is consistent with the putative atherogenicity of these particles and can be explained by increased arterial exposure to cholesterol rich chylomicron remnants.