Hepatic Triglyceride Synthesis Research Articles

Sixteen hours of fasting differentially affects hepatic and muscle insulin sensitivity in mice

Fasting readily induces hepatic steatosis. Hepatic steatosis is associated with hepatic insulin resistance. The purpose of the present study was to document the effects of 16 h of fasting in wild-type mice on insulin sensitivity in liver and skeletal muscle in relation to 1) tissue accumulation of triglycerides (TGs) and 2) changes in mRNA expression of metabolically relevant genes. Sixteen hours of fasting did not show an effect on hepatic insulin sensitivity in terms of glucose production in the presence of increased hepatic TG content. In muscle, however, fasting resulted in increased insulin sensitivity, with increased muscle glucose uptake without changes in muscle TG content. In liver, fasting resulted in increased mRNA expression of genes promoting gluconeogenesis and TG synthesis but in decreased mRNA expression of genes involved in glycogenolysis and fatty acid synthesis. In muscle, increased mRNA expression of genes promoting glucose uptake, as well as lipogenesis and beta-oxidation, was found. In conclusion, 16 h of fasting does not induce hepatic insulin resistance, although it causes liver steatosis, whereas muscle insulin sensitivity increases without changes in muscle TG content. Therefore, fasting induces differential changes in tissue-specific insulin sensitivity, and liver and muscle TG contents are unlikely to be involved in these changes.

Rosuvastatin Reduces Plasma Lipids by Inhibiting VLDL Production and Enhancing Hepatobiliary Lipid Excretion in ApoE*3-Leiden Mice

The present study was designed to investigate the lipid-lowering properties and mechanisms of action of a new HMG-CoA reductase inhibitor, rosuvastatin, in female ApoE*3-Leiden transgenic mice. Mice received a high fat/cholesterol (HFC) diet containing either rosuvastatin (0 [control], 0.00125%, 0.0025%, or 0.005% [w/w]) or 0.05% (w/w) lovastatin. The highest dose of rosuvastatin reduced plasma cholesterol and triglyceride levels by 39% and 42%, respectively, compared with the HFC control. Lovastatin had no effect on plasma cholesterol and triglyceride levels. In ApoE*3-Leiden mice on a chow diet, rosuvastatin (0.005% [w/w]) decreased plasma cholesterol levels by 35% without having an effect on triglyceride levels. On a chow diet, expression of genes involved in cholesterol biosynthesis and uptake in the liver was increased by rosuvastatin. Further mechanistic studies in HFC-fed mice showed that rosuvastatin treatment resulted in decreased hepatic VLDL-triglyceride and VLDL-apolipoprotein B production. VLDL lipid composition remained unchanged, indicating a reduction in the number of VLDL particles secreted. Lipolytic activity and expression of genes involved in cholesterol and triglyceride synthesis and beta-oxidation of fatty acids in the liver were not affected by rosuvastatin treatment, and hepatic lipid content did not change. However, activity of hepatic diacylglycerol acyltransferase was significantly decreased by 25% after rosuvastatin treatment. Moreover, biliary excretion of cholesterol, phospholipids, and bile acids was increased during treatment. The results indicate that rosuvastatin treatment in ApoE*3-Leiden mice on a HFC diet leads to redistribution of cholesterol and triglycerides in the body, both by reduced hepatic VLDL production and triglyceride synthesis and by enhanced hepatobiliary removal of cholesterol, bile acids, and phospholipids, resulting in substantial reductions in plasma cholesterol and triglyceride levels.

Niacin therapy in atherosclerosis

Well designed, randomized, placebo-controlled studies show that niacin prevents cardiovascular disease and death. Unfortunately, early studies and anecdotal evidence have limited its use by promoting the opinion that niacin is intolerable and contraindicated in diabetes. As evidence mounts that treating multiple lipid risk factors decreases cardiovascular risk, the use of niacin in the treatment of atherosclerosis is experiencing somewhat of a renaissance. Emerging clinical evidence shows that niacin is both safe and effective in diabetes. Niacin beneficially alters lipoprotein subclass distribution and when used in combination with statins, has additional effects on lipoproteins. Niacin selectively and directly inhibits hepatic diacylglycerol acyltransferase 2, but not diacylglycerol acyltransferase 1, thus inhibiting hepatic triglyceride synthesis and very low density lipoprotein secretion. The recent discovery and characterization of a membrane-bound nicotinic acid receptor (HM74) explains niacin's acute inhibition of adipocyte lipolysis, but the role of HM74 in lowering triglycerides is unclear. Niacin possesses antioxidant, antiinflammatory, and other beneficial effects on atherosclerosis unrelated to lipid lowering. Finally, niacin appears to activate nuclear transcription factors such peroxisome proliferator activator receptor gamma, possibly via prostaglandin metabolism. New data indicate that niacin alters lipoprotein metabolism in novel ways, and mediates other beneficial nonlipid changes that may be atheroprotective. This information forms the rationale for the use of niacin in combination with agents possessing complementary mechanisms of action (e.g. statins) for cardiovascular risk reduction beyond that observed with monotherapy. Further research into the specific mechanisms of niacin may identify additional targets for future drug development.

Circulating soluble CD14 monocyte receptor is associated with increased alanine aminotransferase.

Recent studies have suggested that abnormal hepatocellular function is associated with obesity, insulin resistance, and type 2 diabetes (1)(2)(3). Nonancoholic fatty liver disease (NAFLD) is the preferred term to describe a spectrum of liver damage, ranging from steatosis to steatohepatitis, liver fibrosis, and cirrhosis, that can be found in insulin-resistant (obese and type 2 diabetic) and hyperlipidemic patients (4). Insulin sensitivity and liver function exhibit a bidirectional relationship: a normally functioning liver determines, in part, the normal body response to insulin, whereas abnormal insulin sensitivity causes liver damage (5)(6). The insulin-resistant state is characterized by a failure to suppress hepatic glucose production and glycogenolysis (5), usually accompanied by fat accumulation in hepatocytes as a result of increased hepatic uptake and synthesis of triglycerides. The latter is known to impair liver function and, consequently, to worsen insulin resistance (5)(6). Aminotransferases are considered indicators of hepatocellular health. Alanine aminotransferase (ALT) is found primarily in the liver, whereas aspartate aminotransferase (AST) and γ-glutamyltranspeptidase (γGT) are also found in other tissues. NAFLD is characterized by mild-to-moderate increases in ALT and AST (7). Recent prospective studies have shown that increased ALT and γGT are predictors of the development of type 2 diabetes (1)(2). CD14, a 55-kDa glycoprotein, is a multifunctional receptor constitutively expressed in considerable amounts on the surface of mature monocytes, macrophages, and neutrophils (8). CD14 has specificity for lipopolysaccharide (LPS) and other bacteria-wall-derived components (9). LPS signaling triggers a cascade that leads to cytokine production and shedding of the extracellular domain of CD14 (sCD14) (10)(11)(12)(13). The buffering of LPS is crucial not only during acute inflammatory and infectious processes; in healthy humans, triglyceride-rich lipoproteins contain detectable amounts of endogenous LPS that are presumably scavenged in …

Raising HDL cholesterol without inducing hepatic steatosis and hypertriglyceridemia by a selective LXR modulator

Liver X receptors (LXRs) are ligand-activated transcription factors that belong to the nuclear receptor superfamily. LXRs activate transcription of a spectrum of genes that regulate reverse cholesterol transport, including the ATP binding cassette transporter A1 (ABCA1), and raise HDL cholesterol (HDL-C) levels. However, LXR agonists also induce genes that stimulate lipogenesis, including the sterol response element binding protein (SREBP1-c) and fatty acid synthetase (FAS). The induction of these genes in the liver cause increased hepatic triglyceride synthesis, hypertriglyceridemia, and hepatic steatosis. As LXR response elements have been identified in these promoters, it is not clear if these two processes can be separated. Herein, we demonstrate that plasma HDL-C elevation and intestinal ABCA1 induction can occur with relatively little induction of FAS and SREBP1-c in mouse liver via a selective LXR modulator GW3965. This is in contrast to the strong induction of hepatic lipogenic genes by the well-characterized LXR agonist T0901317 (T317). Consistent with the in vivo results, GW3965 is a very weak LXR activator compared with T317 in human hepatoma cells. GW3965-liganded LXR recruits selected coactivators less effectively than T317 and may explain in part the tissue selective gene induction. This demonstration that tissue and gene selective modulation is possible with selective LXR modulators has positive implications for the development of this class of antiatherosclerotic agents.

A novel aminosterol reverses diabetes and fatty liver disease in obese mice

Background/Aims Non-alcoholic fatty liver disease (NAFLD) is common in obesity. However, weight reduction alone does not prevent the development or progression of NAFLD. Since NAFLD is associated with insulin resistance and diabetes, we hypothesized that improvement of these factors would reverse obesity-related NAFLD. Methods We examined the effects of an aminosterol, 1436, on glucose, lipids and liver metabolism in Lep ob/ob mice, a model of obesity, severe insulin resistance, diabetes, hyperlipidemia and hepatic steatosis. Results 1436 decreased body weight, specifically fat content, by inhibiting food intake and increasing energy expenditure. In contrast to weight loss from food restriction, this aminosterol specifically lowered circulating lipids, reversed hepatic steatosis and normalized alanine aminotransferase level. 1436 decreased glucose, increased adiponectin and enhanced insulin action in liver. These changes culminated in inhibition of hepatic triglyceride synthesis and increased fatty acid oxidation. Gene expression studies confirmed a reduction in lipogenic enzymes in liver, and elevation of enzymes involved in lipid catabolism. Conclusions These results demonstrate that 1436 is an effective treatment for insulin resistance and hepatic steatosis in Lep ob/ob mice, by decreasing hepatic lipid synthesis and stimulating lipolysis. In contrast, weight loss from food restriction has no substantial effect on insulin resistance, lipids and hepatic steatosis.

Biology: risk factor modification by OCs and HRT lipids and lipoproteins

The many effects that the oestrogens and progestagens used in oral contraceptive (OC) and postmenopausal hormone replacement therapies (HRTs) have on lipoprotein metabolism are of importance because of the involvement of lipoproteins in endothelial damage and arterial occlusion. Lipoproteins promoting endothelial damage include: low density lipoprotein (LDL), particularly the small dense subfraction (sdLDL), remnants of very low density lipoprotein (VLDL) and chylomicron metabolism, and lipoprotein (a). High density lipoprotein (HDL) has pleiotropic effects that prevent or alleviate endothelial damage. Orally administered oestrogens increase hepatic triglyceride synthesis and VLDL secretion and increase the proportion of sdLDL. Oestrogens increase the rates of elimination of LDL, VLDL remnants and chylomicrons, suppress the synthesis of key enzymes of lipoprotein metabolism, hepatic and lipoprotein lipase, and increase synthesis of the principal apoprotein of HDL, apoAI. In general, progestagens oppose these effects according to type and dose. In OC and HRT users, this leads to a range of different lipoprotein profiles, which may differ from those evaluated with respect to vascular disease risk in population studies. Accumulating evidence suggests that the clinical implications of steroid induced changes in the lipoprotein profile will need to be evaluated independently of population-based evidence.

Sirolimus changes lipid concentrations and lipoprotein metabolism in kidney transplant recipients

Background Sirolimus (Rapammune, rapamycin, RAPA) is a strong immunosuppressive agent that reduces kidney transplant rejection. Hyperlipidemia is a significant side effect of sirolimus treatment and often leads to vascular disease. We have studied the repeatability, reversibility, and dose dependence of the plasma lipid and apoprotein changing effects of sirolimus and attempted to determine the mechanism by which sirolimus induces hypertriglyceridemia in some kidney transplant recipients. Methods Six patients with renal allografts maintained on cyclosporine A and prednisone were selected on the basis of their previous hyperlipidemic response to short-term (14 days) sirolimus administration. For longer-term treatment, each patient was started on 10 mg/d sirolimus and continued as tolerated for 42 days to reinduce hyperlipidemia. Timed blood samples were analyzed for lipid, apoprotein, and sirolimus levels. Results During sirolimus administration, mean total plasma cholesterol increased from 214 to 322 mg/dL (+50%); low density lipoprotein-cholesterol levels changed in a similar pattern. Mean triglyceride level rose from 227 to 432 mg/dL (+95%). ApoB-100 concentration rose from 124 to 160 mg/dL (+28%). ApoC-III level increased from 28.9 to 55.5 mg/dL (+92%). These lipid and apoprotein changes were found to be repeatable, reversible, and dose dependent. [ 13C 4]-palmitate metabolic studies in four patients with hypertriglyceridemia indicated that the free fatty acid pool was expanded by sirolimus treatment (mean = 42.3%). Incorporation of [ 13C 4]-palmitate into triglycerides of very low density lipoprotien, intermediate density lipoprotein, low density lipoproteins was decreased 38.3%, 42.1%, and 38.4%, respectively, by sirolimus treatment of these patients. Conclusions These results suggest that sirolimus alters the insulin signaling pathway so as to increase adipose tissue lipase activity, decrease lipoprotein lipase activity, or both, resulting in increased hepatic synthesis of triglyceride, increased secretion of VLDL, and increased hypertriglyceridemia.

Stearoyl-CoA desaturase, a short-lived protein of endoplasmic reticulum with multiple control mechanisms

Stearoyl-CoA desaturase (SCD) is a short-lived, polytopic membrane-bound non-heme iron enzyme localized primarily in the endoplasmic reticulum. SCD is required for the biosynthesis of monounsaturated fatty acids, and plays a key role in hepatic synthesis of triglycerides and very-low-density lipoproteins. The intracellular concentration of SCD fluctuates in a wide range in response to complex and often competing hormonal and dietary factors. A combination of transcriptional regulation and rapid protein degradation produces transient elevations of SCD enzyme activity in response to physiologic demands. Dysregulation of SCD has been implicated in non-alcoholic fatty liver disease, hyperlipidemia, and obesity.

Effects of sirolimus on plasma lipids, lipoprotein levels, and fatty acid metabolism in renal transplant patients

Sirolimus (Rapammune, rapamycin, RAPA) is a potent immunosuppressive drug that reduces renal transplant rejection. Hyperlipidemia is a significant side effect of sirolimus treatment, and frequently leads to cardiovascular disease. This study was undertaken to determine the repeatability, reversibility, and dose dependence of the plasma lipid and apolipoprotein altering effects of sirolimus, and to elucidate the mechanism by which sirolimus induces hypertriglyceridemia in some renal transplant patients. Six patients with renal allografts maintained on cyclosporine A and prednisone were selected on the basis of their previous hyperlipidemic response to short term (14 days) sirolimus administration. For longer-term treatment, each patient was started on 10 mg/day sirolimus and continued as tolerated for 42 days to reinduce hyperlipidemia. Timed blood samples were analyzed for lipid, apolipoprotein, and sirolimus levels. During sirolimus administration, mean total plasma cholesterol increased from 214 mg/dl to 322 mg/dl (+50%; range 25-92%); LDL-cholesterol levels followed a similar pattern. Mean triglyceride level rose from 227 to 432 mg/dl (+95%; range 9-254%). ApoB-100 concentration rose from 124 to 160 mg/dl (+28%; P < 0.05). ApoC-III level increased from 28.9 to 55.5 mg/dl, +92%; (P < 0.013). These lipid and apolipoprotein changes were found to be repeatable, reversible, and dose dependent. [(13)C(4)]palmitate metabolic studies in four patients with hypertriglyceridemia indicated that the free fatty acid pool was expanded by sirolimus treatment (mean = 42.3%). Incorporation of [(13)C(4)]palmitate into triglycerides of VLDL, IDL, and LDL was decreased 38.3%, 42,1%, and 38.4%, respectively, by sirolimus treatment of these patients. These results suggest that sirolimus alters the insulin signaling pathway so as to increase adipose tissue lipase activity and/or decrease lipoprotein lipase activity, resulting in increased hepatic synthesis of triglyceride, increased secretion of VLDL, and increased hypertriglyceridemia.

Effect of dietary fats on hepatic lipid metabolism in the growing turkey

The influence of dietary fatty acids on hepatic capacity of lipid synthesis and secretion was investigated in 7-week-old male turkeys. They were fed 10% of either lard (rich in saturated and monounsaturated fatty acids) or linseed oil (rich in polyunsaturated fatty acids, especially 18:3n-3). Fattening was identical with both diets (0.15–0.20% of abdominal adipose tissue), but the proportion of muscle Pectoralis major was lower with linseed oil (6.6 vs. 7.4%). Specific activities of lipogenic enzymes (ME, G6PDH, ACX, and Δ9-desaturase) were not influenced by the diet, however, FAS activity was lower with linseed oil (14.3 vs. 25.4 nM NADPH fixed/min). Fasting concentrations of lipoproteins synthesized and secreted by the liver, VLDL and HDL, were also lower with linseed oil, as well as plasma concentrations of phospholipids and cholesteryl esters. However, when VLDL catabolism was inhibited by injection of an antiserum against LPL, VLDL concentration was identical in both groups (100–120 mg/l), whereas that of phospholipids and cholesteryl esters, that are transported by HDL mainly, remained lower with linseed oil. Thus, in the growing turkeys, and contrary to mammals and the chicken, feeding n-3 polyunsaturated fatty acids did not decrease hepatic triglyceride synthesis and secretion, nor fattening. By contrast, in this species, n-3 polyunsaturated fatty acids appear to influence mostly HDL metabolism, with a negative impact on muscular growth.

Inhibition of Kupffer cell activity induces hepatic triglyceride synthesis in fasted rats, independent of lipopolysaccharide challenge

Background : Lipopolysaccharides (LPS), cleared from the blood by Kupffer cells, induce hypertriglyceridemia. Aims : To test the hypothesis that GdCl 3, through inhibition of large Kupffer cell activity, modulates LPS-induced hyperlipidemia in rats. Methods : Male Wistar rats received a single intravenous injection of GdCl 3 (10 mg/kg) or saline, 24 h before intraperitoneal LPS (1.5 mg/kg) administration. Serum and hepatic lipids as well as activity of key enzymes controlling fatty acid synthesis and esterification in liver tissue were measured. The incorporation of labeled precursors into lipids was assessed in cultured precision-cut liver slices. Results : GdCl 3 does not prevent hypertriglyceridemia occurring in LPS-treated rats. Surprisingly, GdCl 3 per se is able to promote triglycerides accumulation in the liver tissue, an effect related to an increase in hepatic fatty acid esterification. Such an effect also occurs in rats receiving a dietary supplementation with glycine (5%) known to inhibit Kupffer cell secretory capacity. Conclusions : Large Kupffer cell inhibition does not prevent LPS-induced hypertriglyceridemia and even leads to a metabolic shift of fatty acids towards their esterification and accumulation in the liver tissue, suggesting that Kupffer cells play a role in the regulation of lipid metabolism of the adjacent hepatocytes, independent of any inflammatory stimulus.

A mutation in the peroxisome proliferator-activated receptor gamma-binding site in the gene for the cytosolic form of phosphoenolpyruvate carboxykinase reduces adipose tissue size and fat content in mice.

Regulation of the turnover of triglycerides in adipose tissue requires the continuous provision of 3-glycerophosphate, which may be supplied by the metabolism of glucose or by glyceroneogenesis, the de novo synthesis of 3-glycerophosphate from sources other than hexoses or glycerol. The importance of glyceroneogenesis in adipose tissue was assessed in mice by specifically eliminating the expression of the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK-C), an enzyme that plays a pivotal role in the pathway. To accomplish this, we mutated the binding site for the peroxisome proliferator-activated receptor gamma (PPAR gamma) called the peroxisome proliferator-activated receptor element (PPARE), in the 5' flanking region of the PEPCK-C gene in the mouse by homologous recombination. The mutation abolished expression of the gene in white adipose tissue and considerably reduced its expression in brown adipose tissue, whereas the level of PEPCK-C mRNA in liver and kidney remained normal. Epididymal white adipose tissue from these mice had a reduced triglyceride deposition, with 25% of the animals displaying lipodystrophy. There was also a greatly reduced level of lipid accumulation in brown adipose tissue. A strong correlation between the hepatic content of triglycerides and the size of the epididymal fat pad in PPARE(-/-) mice suggests that hepatic triglyceride synthesis predominantly utilizes free fatty acids derived from the adipose tissue. Unlike other models, PPARE(-/-) mice with lipodystrophy did not exhibit the lipodystrophy-associated features of diabetes and displayed only moderate hyperglycemia. These studies establish the importance of the PPARE site for PEPCK-C gene expression in adipose tissue and the role of PEPCK-C in the regulation of glyceroneogenesis, a pathway critical for maintaining the deposition of triglycerides in adipose tissue.

Reduction in hepatic non-esterified fatty acid concentration after long-term treatment with atorvastatin lowers hepatic triglyceride synthesis and its secretion in sucrose-fed rats

The mechanism by which atorvastatin lowers plasma triglyceride (TG) levels is mainly through a decrease in hepatic TG secretion. However, it is not clear why atorvastatin, which does not inhibit TG synthesis in vitro, decreases hepatic TG secretion without a prospective increase in hepatic TG concentration. For the investigation of the mechanisms that underlie the hypotriglyceridemic effects of atorvastatin, we characterized the effect of either a single or an 11 day administration of atorvastatin in sucrose-induced hypertriglyceridemic rats. Atorvastatin (30 mg/kg p.o.) strongly decreased the rate of both very-low-density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B secretion. The inhibitor also decreased hepatic TG concentration. Hepatic TG synthesis activity was also decreased by atorvastatin, and its activity was correlated with both hepatic and plasma TG concentration. There was also a strong correlation between the hepatic TG synthesis and hepatic non-esterified fatty acid (NEFA) concentration ( r 2=0.815). These effects required chronic administration of the inhibitor and were not observed by acute treatment. Repeated administration of atorvastatin also strongly reduced hepatic acyl-coenzyme A synthase mRNA levels. These results suggest that the reduced hepatic NEFA most likely lowers hepatic TG synthesis and TG secretion in sucrose-fed hypertriglyceridemic rats.

The American Diabetes Association's 48th Annual Advanced Postgraduate Course.

This article covers symposia presented at the American Diabetes Association’s 48th Annual Advanced Postgraduate Course, which was held in New York in January 2001. Topics include management of cardiovascular disease and diabetes, management of diabetes and hypertension, estrogen therapy and cardiovascular disease, and use of ACE inhibitors. At the American Diabetes Association’s 48th Annual Advanced Postgraduate Course in New York, 19–21 January 2001, much emphasis was devoted to cardiovascular disease (CVD) management in patients with diabetes. Henry Ginsberg, New York, NY, discussed diabetic dyslipidemia as a part of the hyperglycemia syndrome and, more importantly, the insulin resistance syndrome. Central to diabetic dyslipidemia is the elevation of fatty acid levels, with consequent elevated total triglycerides, reduced HDL cholesterol levels, and normal or mildly elevated levels of LDL cholesterol of abnormal composition. Thus, triglyceride levels exceed 235 mg/dl in 9 vs. 19% of nondiabetic vs. diabetic men and are >200 mg/dl in 8 vs. 17% of women. HDL cholesterol levels <31 mg/dl are seen in 12 vs. 21% of the two groups of men, and levels <41 mg/dl are seen in 10 vs. 25% of the respective groups of women. LDL cholesterol levels are similar in individuals with and without diabetes, exceeding 190 mg/dl in 11 vs. 9% and 16 vs. 15% of men and women, respectively (1). Similarly, in the U.K. Prospective Diabetes Study (UKPDS), HDL levels were lower and triglyceride levels were higher in both men and women with diabetes, although among women with diabetes, LDL cholesterol levels were higher (2). Insulin resistance is associated with a 30–40% higher free fatty acid (FFA) level because of decreased adipocyte uptake and increased release, leading to an increase in hepatic triglyceride synthesis, with increased VLDL production. In the circulation, cholesterol ester transfer protein, which resides mainly on HDL particles, transfers cholesterol from …

Treatment of Atherosclerosis in Apolipoprotein E-Deficient Mice with 4-(3′-Bromobenzoyl)-6,7-Dimethoxyquinazoline (WHI-P164), a Potent Inhibitor of Triglyceride Synthesis

We identified a novel organic compound, 4-(3'-bromobenzoyl)-6,7-dimethoxyquinazoline (compound WHI-P164), as a potent inhibitor of triglyceride (TG) synthesis. In an in vitro model of lipid synthesis, WHI-P164 (but not any one of the three structurally similar control dimethoxyquinazoline compounds) inhibited the accumulation of TG-rich intracellular lipid droplets in Caco-2 human intestinal cells in a concentration-dependent fashion. WHI-P164 caused no acute toxicity associated with morbidity or mortality in mice when administered at dose levels ranging from 0.5 to 80 mg/kg. In pharmacokinetic studies in mice, WHI-P164 was rapidly eliminated from plasma with a terminal elimination half-life of 26.1 +/- 1.3 min after intraperitoneal administration and 33.3 +/- 11.3 min after intravenous administration. Treatment with 40 mg/kg WHI-P164 (but not one of three structurally similar control dimethoxyquinazoline compounds) administered intraperitoneally once daily for 7 consecutive treatment days blocked the in vivo hepatic TG synthesis in both apoE-deficient and wild-type C57B1/6 mice. In apoE-deficient mice maintained on a high-fat/high-cholesterol Western diet, WHI-P164 substantially reduced the lipid accumulation in the liver after 7 days of treatment and the lipid accumulation in the aorta after 1 month of treatment. Our results in apoE-deficient mice show that lipid accumulation in hepatocytes and foam cells are related events, and inhibiting TG synthesis with WHI-P164 offers an effective means to treat atherosclerosis.

AMP-activated protein kinase, a metabolic master switch: possible roles in type 2 diabetes.

Adenosine 5'-monophosphate-activated protein kinase (AMPK) now appears to be a metabolic master switch, phosphorylating key target proteins that control flux through metabolic pathways of hepatic ketogenesis, cholesterol synthesis, lipogenesis, and triglyceride synthesis, adipocyte lipolysis, and skeletal muscle fatty acid oxidation. Recent evidence also implicates AMPK as being responsible for mediating the stimulation of glucose uptake induced by muscle contraction. In addition, the secretion of insulin by insulin secreting (INS-1) cells in culture is modulated by AMPK activation. The net effect of AMPK activation is stimulation of hepatic fatty acid oxidation and ketogenesis, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipolysis and lipogenesis, stimulation of skeletal muscle fatty acid oxidation and muscle glucose uptake, and modulation of insulin secretion by pancreatic beta-cells. In skeletal muscle, AMPK is activated by contraction. Type 2 diabetes mellitus is likely to be a disease of numerous etiologies. However, defects or disuse (due to a sedentary lifestyle) of the AMPK signaling system would be predicted to result in many of the metabolic perturbations observed in Type 2 diabetes mellitus. Increased recruitment of the AMPK signaling system, either by exercise or pharmaceutical activators, may be effective in correcting insulin resistance in patients with forms of impaired glucose tolerance and Type 2 diabetes resulting from defects in the insulin signaling cascade.

Feeding the nitric oxide synthase inhibitor L-N ωnitroarginine elevates serum very low density lipoprotein and hepatic triglyceride synthesis in rats

This study was conducted to study the influence of dietary L-N ωnitroarginine (L-NNA), a nitric oxide (NO) synthase inhibitor, on serum lipids and lipoproteins and on the activities of enzymes related to lipid metabolism in rats. Feeding rats a diet containing 0.2 g/kg L-NNA for 5 weeks elevated serum concentrations of triglyceride, cholesterol, phospholipid, and free fatty acid and reduced serum nitrate (an oxidation product of NO). The elevation in serum triglyceride was mainly due to the elevation in very low density lipoprotein (VLDL) triglyceride. Contents of cholesterol and phospholipid in the VLDL fraction also were elevated by L-NNA. L-NNA treatment caused significantly higher activity of hepatic microsomal phosphatidate phosphohydrolase (the rate-limiting enzyme in triglyceride synthesis) and lower activity of hepatic carnitine palmitoyltransferase (the rate-limiting enzyme in fatty acid oxidation). Activities of hepatic enzymes responsible for fatty acid synthesis such as glucose-6-phosphate dehydrogenase, malic enzyme, and fatty acid synthase were unaffected by L-NNA. The activity of hepatic microsomal phosphocholine cytidyltransferase (the rate-limiting enzyme in phosphatidylcholine synthesis) was reduced significantly by L-NNA. Our results suggest that lower NO production caused the elevations in hepatic triglyceride synthesis by higher esterification of fatty acid and lower fatty acid oxidation, leading to an enrichment of VLDL triglyceride.

A proposal for the locus of metformin's clinical action: potentiation of the activation of pyruvate kinase by fructose-1,6-diphosphate

Summary Reduction of hepatic glucose output has been shown to be the chief basis for metformin's clinical benefit in diabetes, and the balance of the evidence suggests that this reflects inhibition of gluconeogenesis. Recent research with hepatocyte cell cultures demonstrates increased flux through pyruvate kinase in metformin-treated cells. An analysis of the conditions under which clinically relevant concentrations of metformin inhibit gluconeogenesis in hepatocyte cultures prompts the hypothesis that metformin potentiates the allosteric activation of pyruvate kinase by fructose-1,6-diphosphate. This model rationalizes several salient features of metformin's clinical activity: its ability to reduce hepatic triglyceride synthesis, its appetite-suppressant effect, and its failure to induce hypoglycemia.

Regulation of putative fatty acid transporters and Acyl-CoA synthetase in liver and adipose tissue in ob/ob mice.

The hyperlipidemia associated with obesity and type 2 diabetes is caused by an increase in hepatic triglyceride synthesis and secretion that is secondary to an increase in de novo lipogenesis, a decrease in fatty acid (FA) oxidation, and an increase in the flux of peripherally derived FA to the liver. The uptake of FA across the plasma membrane may be mediated by three distinct proteins--FA translocase (FAT), plasma membrane FA binding protein (FABP-pm), and FA transport protein (FATP)--that have recently been characterized. Acyl-CoA synthetase (ACS) enhances the uptake of FAs by catalyzing their activation to acyl-CoA esters for subsequent use in anabolic or catabolic pathways. In this study, we examine the mRNA levels of FAT, FABP-pm, FATP, and ACS in the liver and adipose tissue of genetically obese (ob/ob) mice and their control littermates. FAT mRNA levels were 15-fold higher in liver and 60-80% higher in adipose tissue of ob/ob mice. FABP-pm mRNA levels were twofold higher in liver and 50% higher in adipose tissue of ob/ob mice. FATP mRNA levels were not increased in liver or adipose tissue. ACS mRNA levels were higher in adipose tissue but remained unchanged in liver. However, the distribution of ACS activity associated with mitochondria and microsomes in liver was altered in ob/ob mice. In control littermates, 61% of ACS activity was associated with mitochondria and 39% with microsomes, whereas in ob/ob mice 34% of ACS activity was associated with mitochondria and 66% with microsomes; this distribution would make more FA available for esterification, rather than oxidation, in ob/ob mouse liver. Taken together, our results suggest that the upregulation of FAT and FABP-pm mRNAs may increase the uptake of FA in adipose tissue and liver in ob/ob mice, which, coupled with an increase in microsomal ACS activity in liver, will enhance the esterification of FA and support the increased triglyceride synthesis and VLDL production that characterizes obesity and type 2 diabetes.