Loss of hepatic SMLR1 causes hepatosteatosis and protects against atherosclerosis due to decreased hepatic VLDL secretion

BACKGROUND AND AIMS

The assembly and secretion of VLDL from the liver, a pathway that affects hepatic and plasma lipids, remains incompletely understood. We set out to identify players in the VLDL biogenesis pathway by identifying genes that are co-expressed with the MTTP gene that encodes for microsomal triglyceride transfer protein, key to the lipidation of apolipoprotein B, the core protein of VLDL. Using human and murine transcriptomic data sets, we identified small leucine-rich protein 1 ( SMLR1 ), encoding for small leucine-rich protein 1, a protein of unknown function that is exclusively expressed in liver and small intestine.

APPROACH AND RESULTS

To assess the role of SMLR1 in the liver, we used somatic CRISPR/CRISPR-associated protein 9 gene editing to silence murine Smlr1 in hepatocytes ( Smlr1 -LKO). When fed a chow diet, male and female mice show hepatic steatosis, reduced plasma apolipoprotein B and triglycerides, and reduced VLDL secretion without affecting microsomal triglyceride transfer protein activity. Immunofluorescence studies show that SMLR1 is in the endoplasmic reticulum and Cis-Golgi complex. The loss of hepatic SMLR1 in female mice protects against diet-induced hyperlipidemia and atherosclerosis but causes NASH. On a high-fat, high-cholesterol diet, insulin and glucose tolerance tests did not reveal differences in male Smlr1 -LKO mice versus controls.

CONCLUSIONS

We propose a role for SMLR1 in the trafficking of VLDL from the endoplasmic reticulum to the Cis-Golgi complex. While this study uncovers SMLR1 as a player in the VLDL assembly, trafficking, and secretion pathway, it also shows that NASH can occur with undisturbed glucose homeostasis and atheroprotection.

Mitochondria-rough-ER contacts in the liver regulate systemic lipid homeostasis

Contacts between organelles create microdomains that play major roles in regulating key intracellular activities and signaling pathways, but whether they also regulate systemic functions remains unknown. Here, we report the ultrastructural organization and dynamics of the inter-organellar contact established by sheets of curved rough endoplasmic reticulum closely wrapped around the mitochondria (wrappER). To elucidate the in vivo function of this contact, mouse liver fractions enriched in wrappER-associated mitochondria are analyzed by transcriptomics, proteomics, and lipidomics. The biochemical signature of the wrappER points to a role in the biogenesis of very-low-density lipoproteins (VLDL). Altering wrappER-mitochondria contacts curtails VLDL secretion and increases hepatic fatty acids, lipid droplets, and neutral lipid content. Conversely, acute liver-specific ablation of Mttp, the most upstream regulator of VLDL biogenesis, recapitulates this hepatic dyslipidemia phenotype and promotes remodeling of the wrappER-mitochondria contact. The discovery that liver wrappER-mitochondria contacts participate in VLDL biology suggests an involvement of inter-organelle contacts in systemic lipid homeostasis.

The Effect of Fructose Feeding on Intestinal Triacylglycerol Production and De Novo Fatty Acid Synthesis in Humans

A high fructose intake exacerbates postprandial plasma triacylglycerol (TAG) concentration, an independent risk factor for cardiovascular disease, although it is unclear whether this is due to increased production or impaired clearance of triacylglycerol (TAG)-rich lipoproteins. We determined the in vivo acute effect of fructose on postprandial intestinal and hepatic lipoprotein TAG kinetics and de novo lipogenesis (DNL). Five overweight men were studied twice, 4 weeks apart. They consumed hourly mixed-nutrient drinks that were high-fructose (30% energy) or low-fructose (<2% energy) for 11 h. Oral ²H₂O was administered to measure fasting and postprandial DNL. Postprandial chylomicron (CM)-TAG and very low-density lipoprotein (VLDL)-TAG kinetics were measured with an intravenous bolus of [²H₅]-glycerol. CM and VLDL were separated by their apolipoprotein B content using antibodies. Plasma TAG (p < 0.005) and VLDL-TAG (p = 0.003) were greater, and CM-TAG production rate (PR, p = 0.046) and CM-TAG fractional catabolic rate (FCR, p = 0.073) lower when high-fructose was consumed, with no differences in VLDL-TAG kinetics. Insulin was lower (p = 0.005) and apoB48 (p = 0.039), apoB100 (p = 0.013) and non-esterified fatty acids (NEFA) (p = 0.013) were higher after high-fructose. Postprandial hepatic fractional DNL was higher than intestinal fractional DNL with high-fructose (p = 0.043) and low-fructose (p = 0.043). Fructose consumption had no effect on the rate of intestinal or hepatic DNL. We provide the first measurement of the rate of intestinal DNL in humans. Lower CM-TAG PR and CM-TAG FCR with high-fructose consumption suggests lower clearance of CM, rather than elevated production, may contribute to elevated plasma TAG, possibly due to lower insulin-mediated stimulation of lipoprotein lipase.

miR-130b is a potent stimulator of hepatic very-low-density lipoprotein assembly and secretion via marked induction of microsomal triglyceride transfer protein

miR-130b is a microRNA whose expression is particularly elevated within adipose tissue and in the circulation in diabetic states. Hepatic miR-130b expression has been linked to hepatocellular carcinoma and changes in lipid metabolism. Here, we investigated the role of miR-130b in hepatic lipid homeostasis and lipoprotein export. We observed that overexpression of miR-130b-3p or -5p in HepG2 cells markedly enhanced the secretion of very-low-density lipoprotein (VLDL) particles, enhanced the secretion of [³H]glycerol metabolically labeled triglyceride (TG), and significantly increased the number or the average size of lipid droplets (LDs), respectively. Overexpression of miR-130b also altered the expression of key genes involved in lipid metabolism and in particular markedly increased both mRNA and protein expression levels of microsomal triglyceride transfer protein (MTP). Conversely, the miR-130b inhibitor decreased mRNA levels of MTP and fatty acid synthase (FAS) in HepG2 cells. However, dual-luciferase reporter assays indicated that MTP is not a direct target of miR-130b-3p. miR-130b overexpression did not alter de novo synthesized TG or the stability and secretion of apolipoprotein B 100. Interestingly, knockdown of phosphatase and tensin homolog (PTEN) blocked the upregulation of MTP mRNA induced by miR-130b. Finally, miR-130b-induced stimulation of VLDL secretion was also observed in a second hepatocyte cell culture model, immortalized human hepatocytes, confirming the effects observed in HepG2 cells. Overall, these data suggest a potential role for miR-130b in promoting hepatic VLDL assembly and secretion mediated by marked stimulation of MTP expression and TG mobilization. Thus miR-130b overexpression corrects the defect in VLDL production in HepG2 cells.

Critical Role of SREBP-1c Large-VLDL Pathway in Environment-Induced Hypertriglyceridemia of Apo AV Deficiency

Objective- APOA5 variants are strongly associated with hypertriglyceridemia, as well as increased risks of cardiovascular disease and acute pancreatitis. Hypertriglyceridemia in apo AV dysfunction often aggravates by environmental factors such as high-carbohydrate diets or aging. To date, the molecular mechanisms by which these environmental factors induce hypertriglyceridemia are poorly defined, leaving the high-risk hypertriglyceridemia condition undertreated. Previously, we reported that LXR (liver X receptor)-SREBP (sterol regulatory element-binding protein)-1c pathway regulates large-VLDL (very low-density lipoprotein) production induced by LXR agonist. However, the pathophysiological relevance of the finding remains unknown. Approach and Results- Here, we reconstitute the environment-induced hypertriglyceridemia phenotype of human APOA5 deficiency in Apoa5^-/- mice and delineate the role of SREBP-1c in vivo by generating Apoa5^-/- ;Srebp-1c^-/- mice. The Apoa5^-/- mice, which showed moderate hypertriglyceridemia on a chow diet, developed severe hypertriglyceridemia on high-carbohydrate feeding or aging as seen in patients with human apo AV deficiency. These responses were nearly completely abolished in the Apoa5^-/- ;Srebp-1c^-/- mice. Further mechanistic studies revealed that in response to these environmental factors, SREBP-1c was activated to increase triglyceride synthesis and to permit the incorporation of triglyceride into abnormally large-VLDL particles, which require apo AV for efficient clearance. Conclusions- Severe hypertriglyceridemia develops only when genetic factors (apo AV deficiency) and environmental effects (SREBP-1c activation) coexist. We demonstrate that the regulated production of large-sized VLDL particles via SREBP-1c determines plasma triglyceride levels in apo AV deficiency. Our findings explain the long-standing enigma of the late-onset hypertriglyceridemia phenotype of apo AV deficiency and suggest a new approach to treat hypertriglyceridemia by targeting genes that mediate environmental effects.

ARHGAP21 deficiency impairs hepatic lipid metabolism and improves insulin signaling in lean and obese mice

ARHGAP21 is a Rho-GAP that controls GTPases activity in several tissues, but its role on liver lipid metabolism is unknown. Thus, to achieve the Rho-GAP role in the liver, control and ARHGAP21-haplodeficient mice were fed chow (Ctl and Het) or high-fat diet (Ctl-HFD and Het-HFD) for 12 weeks, and pyruvate and insulin tolerance tests, insulin signaling, liver glycogen and triglycerides content, gene and protein expression, and very-low-density lipoprotein secretion were measured. Het mice displayed reduced body weight and plasma triglycerides levels, and increased liver insulin signaling. Reduced gluconeogenesis and increased glycogen content were observed in Het-HFD mice. Gene and protein expression of microsomal triglyceride transfer protein were reduced in both Het mice, while the lipogenic genes SREBP-1c and ACC were increased. ARHGAP21 knockdown resulted in hepatic steatosis through increased hepatic lipogenesis activity coupled with decreases in CPT1a expression and very-low-density lipoprotein export. In conclusion, liver of ARHGAP21-haplodeficient mice are more insulin sensitive, associated with higher lipid synthesis and lower lipid export.

Caffeine-free hawk tea lowers cholesterol by reducing free cholesterol uptake and the production of very-low-density lipoprotein

Medicinal plants show important therapeutic value in chronic disease treatment. However, due to their diverse ingredients and complex biological effects, the molecular mechanisms of medicinal plants are yet to be explored. By means of several high-throughput platforms, here we show hawk tea extract (HTE) inhibits Niemann-Pick C1-like 1 (NPC1L1)-mediated free cholesterol uptake, thereby inducing the transcription of low-density lipoprotein receptor (LDLR) downstream of the sterol response element binding protein 2 (SREBP2) pathway. Meanwhile, HTE suppresses hepatocyte nuclear factor 4α (HNF4α)-mediated transcription of microsomal triglyceride transfer protein (MTP) and apolipoprotein B (APOB), thereby decreasing the production of very-low-density lipoprotein. The catechin EGCG ((-)-epigallocatechin gallate) and the flavonoids kaempferol and quercetin are identified as the bioactive components responsible for the effects on the NPC1L1-SREBP2-LDLR axis and HNF4α-MTP/APOB axis, respectively. Overall, hawk tea works as a previously unrecognized cholesterol-lowering agent in a multi-target and multi-component manner.

ABCA1 and metabolic syndrome; a review of the ABCA1 role in HDL-VLDL production, insulin-glucose homeostasis, inflammation and obesity

ATP-binding cassette transporter A1 (ABCA1) is an integral cell-membrane protein that mediates the rate-limiting step of high density lipoprotein (HDL) biogenesis and suppression of inflammation by triggering a number of signaling pathways via interacting with an apolipoprotein acceptor. The hepatic ABCA1 is involved in regulation of very low density lipoprotein (VLDL) production by affecting the apolipoprotein B trafficking and lipidation of VLDL particles. This protein is involved in protecting the function of pancreatic β-cells and insulin secretion by cholesterol homeostasis. Adipose tissue lipolysis is associated with ABCA1 activity. This transporter is involved in controlling obesity and insulin sensitivity by regulating triglyceride (TG) lipolysis and influencing on adiponectin, visfatin, leptin, and GLUT4 genes expression. The ABCA1 of skeletal muscle cells play a role in increasing the glucose uptake by enhancing the Akt phosphorylation and transferring GLUT4 to the plasma membrane. Abnormal status of ABCA1-regulated phenotypes is observed in metabolic syndrome. This syndrome is associated with the occurrence of many diseases. This review is a summary of the role of ABCA1 in HDL and VLDL production, homeostasis of insulin and glucose, suppression of inflammation and obesity controlling to provide a better insight into the association of this protein with metabolic syndrome.

Formaldehyde alters triglyceride synthesis and very low-density lipoprotein secretion in a time-dependent manner

Formaldehyde is a common indoor air pollutant that is toxic to the liver. This study aimed to investigate the effects of formaldehyde on triglyceride metabolism in human hepatocellular carcinoma cells (HepG2). Cell viability was detected using a MTT (3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide) assay. Following treatment with different concentrations of formaldehyde for 24 and 48h, the intra and extra-hepatocellular triglyceride (TG) content was determined using a chemical-enzymatic method; Western blotting was used to detect the levels of fatty acid synthesis and VLDL-related proteins. Our results showed that cell viability significantly decreased after formaldehyde treatment (0.5-12.5mM, 24/48h). Extracellular TG levels in the hepatocytes increased after formaldehyde treatment at 0.004mM-0.1mM for 24h. SREBP-1c, ACC, FASN, and MTP, CES3 and DGAT1 proteins increased significantly after 24h of formaldehyde treatment. Intracellular TG levels decreased for 48h treatment of formaldehyde. AMPKα increased significantly in all tested groups and p-AMPK increased significantly after 0.1mM formaldehyde treatment for 48h. Our results indicated that short-term formaldehyde exposure balances triglyceride metabolism by promoting hepatocellular TG synthesis and VLDL secretion; Long-term formaldehyde disturbs the TG metabolism balance in the hepatocytes.

Decreased VLDL-Apo B 100 Fractional Synthesis Rate Despite Hypertriglyceridemia in Subjects With Type 2 Diabetes and Nephropathy

CONTEXT

Subjects with type 2 diabetes mellitus (T2DM) and diabetic nephropathy (DN) often exhibit hypertriglyceridemia. The mechanism(s) of such an increase are poorly known.

OBJECTIVE

We investigated very low-density lipoprotein (VLDL)-Apo B 100 kinetics in T2DM subjects with and without DN, and in healthy controls.

DESIGN

Stable isotope (13)C-leucine infusion and modeling analysis of tracer-to-tracee ratio dynamics in the protein product pool in the 6-8-h period following tracer infusion were employed.

SETTING

Male subjects affected by T2DM, either with (n = 9) or without (n = 5) DN, and healthy male controls (n = 6), were studied under spontaneous glycemic levels in the post-absorptive state.

RESULTS

In the T2DM patients with DN, plasma triglyceride (TG) (mean ± SD; 2.2 ± 0.8 mmol/L) and VLDL-Apo B 100 (17.4 ± 10.4 mg/dL) concentrations, and VLDL-Apo B 100 pool (0.56 ± 0.29 g), were ∼60-80% greater (P < .05 or less) than those of the T2DM subjects without DN (TG, 1.4 ± 0.5 mmol/L; VLDL-Apo B 100, 9.9 ± 2.5 mg/dL; VLDL-Apo B 100 pool, 0.36 ± 0.09 g), and ∼80-110% greater (P < .04 or less) than those of nondiabetic controls (TG, 1.2 ± 0.4 mmol/L; VLDL-Apo B 100, 8.2 ± 1.7 mg/dL; VLDL-Apo B 100, 0.32 ± 0.09 g). In sharp contrast however, in the subjects with T2DM and DN, VLDL-Apo B 100 fractional synthesis rate was ≥50% lower (4.8 ± 2.2 pools/d) than that of either the T2DM subjects without DN (9.9 ± 4.3 pools/d; P < .025) or the control subjects (12.5 ± 9.1 pools/d; P < .04).

CONCLUSIONS

The hypertriglyceridemia of T2DM patients with DN is not due to hepatic VLDL-Apo B 100 overproduction, which is decreased, but it should be attributed to decreased apolipoprotein removal.

Increased plasma cholesterol esterification by LCAT reduces diet-induced atherosclerosis in SR-BI knockout mice

LCAT, a plasma enzyme that esterifies cholesterol, has been proposed to play an antiatherogenic role, but animal and epidemiologic studies have yielded conflicting results. To gain insight into LCAT and the role of free cholesterol (FC) in atherosclerosis, we examined the effect of LCAT over- and underexpression in diet-induced atherosclerosis in scavenger receptor class B member I-deficient [Scarab(-/-)] mice, which have a secondary defect in cholesterol esterification. Scarab(-/-)×LCAT-null [Lcat(-/-)] mice had a decrease in HDL-cholesterol and a high plasma ratio of FC/total cholesterol (TC) (0.88 ± 0.033) and a marked increase in VLDL-cholesterol (VLDL-C) on a high-fat diet. Scarab(-/-)×LCAT-transgenic (Tg) mice had lower levels of VLDL-C and a normal plasma FC/TC ratio (0.28 ± 0.005). Plasma from Scarab(-/-)×LCAT-Tg mice also showed an increase in cholesterol esterification during in vitro cholesterol efflux, but increased esterification did not appear to affect the overall rate of cholesterol efflux or hepatic uptake of cholesterol. Scarab(-/-)×LCAT-Tg mice also displayed a 51% decrease in aortic sinus atherosclerosis compared with Scarab(-/-) mice (P < 0.05). In summary, we demonstrate that increased cholesterol esterification by LCAT is atheroprotective, most likely through its ability to increase HDL levels and decrease pro-atherogenic apoB-containing lipoprotein particles.

Recent advances in physiological lipoprotein metabolism

Research into lipoprotein metabolism has developed because understanding lipoprotein metabolism has important clinical indications. Lipoproteins are risk factors for cardiovascular disease. Recent advances include the identification of factors in the synthesis and secretion of triglyceride rich lipoproteins, chylomicrons (CM) and very low density lipoproteins (VLDL). These included the identification of microsomal transfer protein, the cotranslational targeting of apoproteinB (apoB) for degradation regulated by the availability of lipids, and the characterization of transport vesicles transporting primordial apoB containing particles to the Golgi. The lipase maturation factor 1, glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 and an angiopoietin-like protein play a role in lipoprotein lipase (LPL)-mediated hydrolysis of secreted CMs and VLDL so that the right amount of fatty acid is delivered to the right tissue at the right time. Expression of the low density lipoprotein (LDL) receptor is regulated at both transcriptional and post-transcriptional level. Proprotein convertase subtilisin/kexin type 9 (PCSK9) has a pivotal role in the degradation of LDL receptor. Plasma remnant lipoproteins bind to specific receptors in the liver, the LDL receptor, VLDL receptor and LDL receptor-like proteins prior to removal from the plasma. Reverse cholesterol transport occurs when lipid free apoAI recruits cholesterol and phospholipid to assemble high density lipoprotein (HDL) particles. The discovery of ABC transporters (ABCA1 and ABCG1) and scavenger receptor class B type I (SR-BI) provided further information on the biogenesis of HDL. In humans HDL-cholesterol can be returned to the liver either by direct uptake by SR-BI or through cholesteryl ester transfer protein exchange of cholesteryl ester for triglycerides in apoB lipoproteins, followed by hepatic uptake of apoB containing particles. Cholesterol content in cells is regulated by several transcription factors, including the liver X receptor and sterol regulatory element binding protein. This review summarizes recent advances in knowledge of the molecular mechanisms regulating lipoprotein metabolism.

Impaired VLDL assembly: a novel mechanism contributing to hepatic lipid accumulation following ovariectomy and high-fat/high-cholesterol diets?

The aim of the present study was to identify molecular mechanisms involved in liver fat and cholesterol accumulation in ovariectomised (Ovx) rats fed with high-cholesterol diets. VLDL assembly and bile acid metabolism were specifically targeted. After being either Ovx or sham-operated, the rats were fed a standard diet or a high-fat diet containing 0, 0·25 or 0·5 % cholesterol for 6 weeks. Although Ovx rats exposed to dietary cholesterol intake accumulated the greatest amount of hepatic fat and cholesterol, plasma cholesterol levels were lower (P< 0·05) in these animals than in the corresponding control rats. Accompanying this observation, ovariectomy and dietary cholesterol intake resulted in a down-regulation (P< 0·05) of the expression of genes associated with VLDL assembly, including microsomal TAG transfer protein, diacylglycerol acyltransferase 2, acyl-CoA:cholesterol acyltransferase 2 and apoB-100 as well as genes associated with bile acid metabolism including farnesoid X receptor and bile salt export pump (P< 0·01). These results indicate that high-fat/high-cholesterol diets and ovariectomy concomitantly disrupt hepatic lipid output through defects in VLDL assembly and, most probably, secretion. The results also point to a defect in hepatic bile acid secretion. The present study offers novel insights into intrahepatic lipid metabolism, which may be relevant to metabolic complications found in postmenopausal women.

Independent effects of testosterone on lipid oxidation and VLDL-TG production: a randomized, double-blind, placebo-controlled, crossover study

Low testosterone (T) levels in men have been shown to predict development of the metabolic syndrome, but the effects of T on lipid metabolism are incompletely understood. In a randomized, double-blind, placebo-controlled, crossover study, 12 healthy, young males received gonadotropin-releasing hormone agonist treatment 1 month prior to 3 of 4 trial days to induce castrate levels of T. On trial days, T gel was applied to the body containing either high or low physiological T dose or placebo. On the 4th trial day, participants constituted their own eugonadal controls. Each study comprised a 5-h basal period and a 3-h hyperinsulinemic-euglycemic clamp. Short-term hypogonadism did not affect VLDL triglyceride (TG) secretion, nor did it affect VLDL-TG concentrations. It was, however, characterized by lower total lipid oxidation. In addition, acute rescue with high physiological T increased VLDL-TG secretion during both basal and clamp conditions. These data show that T can act through fast nongenomic pathways in the liver. In addition, the early hypogonadal state is characterized by decreased total lipid oxidation, but whether these changes represent early hypogonadal metabolic dysfunction warrants further investigations. T is not a major determinant of resting VLDL-TG kinetics in men.

Determinants of VLDL-triglycerides production

PURPOSE OF REVIEW

Plasma free fatty acids (FFA) are major substrates for hepatic VLDL-triglycerides (VLDL-TG) production. In addition, it is a common belief that VLDL-TG production is a substrate driven process primarily determined by systemic FFA delivery. This review summarizes recent research of our understanding of the regulation of VLDL-TG production.

RECENT FINDINGS

Recent studies have shown that increasing FFA flux is not inevitably associated with increased VLDL-TG production. Exercise induced increase in FFA flux resulting in unchanged VLDL-TG production in lean patients as well as in obese patients with increased hepatic fat despite exercise reduced hepatic fat content. With respect to the other inseparable conditions of insulin resistance and hyperinsulinemia, recent studies demonstrate that increased hepatic VLDL-TG production precedes the insulin resistance-associated impairment of the regulation of hepatic glucose production, whereas isolated chronic hyperinsulinemia (insulinoma) was not associated with increased VLDL-TG production. Insulin has been shown to have acute potent temporary suppressing effect on VLDL-TG production and new data demonstrates that increased glucagon reduces VLDL-TG production. Finally, recent studies indicate that sex hormones, oestrogen and testosterone, have no or very modest impact on VLDL-TG production.

SUMMARY

Regulation of hepatic VLDL-TG production involves interplay between systemic FFA delivery, hormonal, and nutritional factors that act in concert with hepatic fatty acid handling to regulate short-term and long-term VLDL-TG production. The results of recent studies underscore that our current understanding of these relationships is complex and needs further research.

Liver phospholipid transfer protein (PLTP) expression with a PLTP-null background promotes very low-density lipoprotein production in mice

It is known that plasma phospholipid transfer protein (PLTP) activity influences lipoprotein metabolism. The liver is one of the major sites of lipoprotein production and degradation, as well as of PLTP expression. To address the impact of liver-expressed PLTP on lipoprotein metabolism, we created a mouse model that expresses PLTP in the liver acutely and specifically, with a PLTP-null background. This approach in mouse model preparations can also be used universally for evaluating the function of many other genes in the liver. We found that liver PLTP expression dramatically increases plasma levels of non-high-density lipoprotein (HDL) cholesterol (2.7-fold, P < 0.0001), non-HDL phospholipid (2.5-fold, P < 0.001), and triglyceride (51%, P < 0.01), but has no significant influence on plasma HDL lipids compared with controls. Plasma apolipoprotein (apo)B levels were also significantly increased in PLTP-expressing mice (2.2-fold, P < 0.001), but those of apoA-I were not. To explore the mechanism involved, we examined the lipidation and secretion of nascent very low-density lipoprotein (VLDL), finding that liver PLTP expression significantly increases VLDL lipidation in hepatocyte microsomal lumina, and also VLDL secretion into the plasma.

CONCLUSION

It is possible to prepare a mouse model that expresses the gene of interest only in the liver, but not in other tissues. Our results suggest, for the first time, that the major function of liver PLTP is to drive VLDL production and makes a small contribution to plasma PLTP activity.

Nutritional repletion of children with severe acute malnutrition does not affect VLDL apolipoprotein B-100 synthesis rate

VLDL apo B-100 is essential for the secretion of liver fat. It is thought that synthesis of this lipoprotein is impaired in childhood severe acute malnutrition (SAM), especially in the edematous syndromes, and that this contributes to the common occurrence of hepatic steatosis in this condition. However, to our knowledge, it has not been confirmed that VLDL apo B-100 synthesis is slower in edematous compared with nonedematous SAM and that it is impaired in the malnourished compared with the well-nourished state. Therefore, VLDL apo B-100 kinetics were measured in 2 groups of children with SAM (15 edematous and 7 nonedematous), aged 4-20 mo, at 3 stages during treatment. Measurements were done at 4 ± 1 d postadmission, mid- catch-up growth in weight, and at recovery (normal weight-for-length). VLDL apo B-100 synthesis was determined using stable isotope methodology to measure the rate of incorporation of (2)H(3)-leucine into its apoprotein moiety. The fractional and absolute synthesis of VLDL apo B-100 did not differ between the groups or from the initial malnourished stage to the recovery stage. Concentrations of VLDL apo B-100 were greater in the edematous than in the nonedematous group (P < 0.04) and did not differ from the initial stage to recovery. The data indicate that VLDL apo B-100 synthesis is not reduced when children develop either edematous or nonedematous SAM.

Testosterone increases the muscle protein synthesis rate but does not affect very-low-density lipoprotein metabolism in obese premenopausal women

Men and women with hyperandrogenemia have a more proatherogenic plasma lipid profile [e.g., greater triglyceride (TG) and total and low-density lipoprotein-cholesterol and lower high-density lipoprotein-cholesterol concentrations] than healthy premenopausal women. Furthermore, castration of male rats markedly reduces testosterone availability below normal and decreases plasma TG concentration, and testosterone replacement reverses this effect. Testosterone is, therefore, thought to be an important regulator of plasma lipid homeostasis. However, little is known about the effect of testosterone on plasma TG concentration and kinetics. Furthermore, testosterone is a potent skeletal muscle protein anabolic agent in men, but its effect on muscle protein turnover in women is unknown. We measured plasma lipid concentrations, hepatic very low density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B-100 secretion rates, and the muscle protein fractional synthesis rate in 10 obese women before and after trandermal testosterone (1.25 g of 1% AndroGel daily) treatment for 3 wk. Serum total and free testosterone concentrations increased (P < 0.05) by approximately sevenfold in response to testosterone treatment, reaching concentrations that are comparable to those in women with hyperandrogenemia, but lower than the normal range for eugonadal men. Except for a small (∼10%) decrease in plasma high-density lipoprotein particle and cholesterol concentrations (P < 0.04), testosterone therapy had no effect on plasma lipid concentrations, lipoprotein particle sizes, and hepatic VLDL-TG and VLDL-apolipoprotein B-100 secretion rates (all P > 0.05); the muscle protein fractional synthesis rate, however, increased by ∼45% (P < 0.001). We conclude that testosterone is a potent skeletal muscle protein anabolic agent, but not an important regulator of plasma lipid homeostasis in obese women.

Endogenous chronic hyperinsulinemia does not increase the production rate of VLDL apolipoprotein B: proof from a kinetic study in patients with insulinoma

OBJECTIVE

It is currently suggested that chronic hyperinsulinemia is a causal factor for the increased production rate of very-low-density lipoproteins (VLDL) associated with metabolic syndrome. However, the involvement of hyperinsulinemia independently of the other abnormalities also observed in metabolic syndrome has never been proven in humans.

DESIGN

We used patients with insulinoma showing hyperinsulinemia but no insulin resistance as a model and conducted an apolipoprotein B (apoB) kinetic study in seven patients with insulinoma, seven insulin-resistant (IR) obese patients, and 12 controls.

RESULTS

Insulinemia was higher in patients with insulinoma or IR than in controls both in the fasting state [2.4-fold (P = 0.039) and 3.1-fold (P = 0.003), respectively] and in the fed state [3.5-fold (P = 0.006) and 2.6-fold (P = 0.05), respectively]. Patients with insulinoma were not IR (steady state plasma glucose = 80 ± 46 mg/dl, a value lower than in IR subjects (231 ± 75, P = 0.0013). In the fed state, triglyceridemia and VLDL apoB pool size were higher in IR subjects compared with controls and patients with insulinoma [208 ± 56 vs. 89 ± 30 mg/dl (P < 0.0001) and 96 ± 42 mg/dl (P < 0.0001), respectively, for triglyceridemia and 3.56 ± 0.60 vs. 1.85 ± 0.88 mg/kg (P = 0.004) and 2.32 ± 1.79 (P = 0.052) mg/kg for VLDL apoB pool size]. The production rate of VLDL apoB in subjects with insulinoma was not significantly different from that in controls (14.56 ± 7.43 vs. 16.40 ± 7.70 mg/kg · d) but was higher in IR subjects compared with these two groups [25.66 ± 12.84 mg/kg · d (P = 0.046 and 0.035, respectively)].

CONCLUSION

Chronic endogenous hyperinsulinemia is not directly responsible for any increase in the production rate of VLDL apoB in humans.

Enzymes involved in hepatic acylglycerol metabolism in the chicken

In laying hens, massive hepatic mobilization of fatty acids is required for the synthesis of oocyte-targeted very-low density lipoproteins (VLDL). The current study aims at identification of enzymes that hydrolyze hepatic acylglycerol stores regulated in a fashion compatible with supporting enhanced VLDL synthesis. We show that unlike mammals, chickens express adipose triglyceride lipase (ATGL) also in liver, where it is upregulated by fasting, while the enzyme patatin-like phospholipase domain-containing lipase 3 (PNPLA3) is suppressed. For the first time in any system, we show that hepatic arylacetamide deacetylase (AADA) is upregulated by fasting, and that its affinity for an insoluble carboxylester substrate is compatible with an in-vivo function similar to that of ATGL. Unknown heretofore, hepatic expression of chicken AADA is estrogen-responsive, and is induced to the same degree as the stimulation of VLDL-production by estrogen. These observations support roles of chicken ATGL, PNPLA3, and AADA in acylglycerol metabolism related to the high rates of VLDL synthesis that are essential for reproduction.

Hepatic VLDL-TG production and MTP gene expression are decreased in ovariectomized rats: effects of exercise training

The present study was designed to investigate the effects of estrogen withdrawal and exercise training on hepatic very low density lipoprotein-triglyceride (VLDL-TG) production and on expression of genes involved in hepatic VLDL synthesis in response to lipid infusion. Female Sprague-Dawley rats underwent ovariectomy (Ovx), sham surgery (Sham), and Ovx with 17β-estradiol supplementation (OvxE2) before being subdivided into sedentary (Sed) and trained (Tr) groups for 8 weeks. Exercise training consisted of continuous running on a rodent treadmill 5 times/wk. At the end of the 8-week period, all rats in the fasted state were intravenously infused with a 20% solution of Intralipid for 3-h followed by an injection of Triton WR-1339 to block lipoprotein lipase activity. Plasma TG accumulation was subsequently measured during 90 min to estimate VLDL-TG production. An additional control group consisting of Sham-Sed rats was infused with saline (0.9% NaCl). Estrogen withdrawal resulted in higher (p<0.01) liver fat accumulation concomitantly with lower (p<0.01) VLDL-TG production and lower mRNA and protein content of hepatic microsomal triglyceride transfer protein (MTP). All of these effects in Ovx rats were corrected with estrogen supplementation. Training in Ovx rats reduced (p<0.01) liver fat accumulation and further reduced (p<0.01) hepatic VLDL-TG production along with gene expression of MTP and diacylglycerol acyltransferase-2 (DGAT-2). It is concluded that VLDL-TG synthesis and/or secretion is decreased in Ovx rats probably via MTP regulation and that this decrease may constitute one of the factors involved in hepatic fat accumulation. The training effect on reducing VLDL production was independent of the estrogenic status.

GLP-1 treatment reduces endogenous insulin resistance via activation of central GLP-1 receptors in mice fed a high-fat diet

Glucagon-like peptide-1 (GLP-1) improves insulin sensitivity in humans and rodents. It is currently unknown to what extent the (metabolic) effects of GLP-1 treatment are mediated by central GLP-1 receptors. We studied the impact of central GLP-1 receptor (GLP-1R) antagonism on the metabolic effects of peripheral GLP-1 administration in mice. High-fat-fed insulin-resistant C57Bl/6 mice were treated with continuous subcutaneous infusion of GLP-1 or saline (PBS) for 2 wk, whereas the GLP-1R antagonist exendin-9 (EX-9) and cerebrospinal fluid (CSF) were simultaneously infused in the left lateral cerebral ventricle (icv). Glucose and glycerol turnover were determined during a hyperinsulinemic euglycemic clamp. VLDL-triglyceride (VLDL-TG) production was determined in hyperinsulinemic conditions. Our data show that the rate of glucose infusion necessary to maintain euglycemia was significantly increased by GLP-1. Simultaneous icv infusion of EX-9 diminished this effect by 62%. The capacities of insulin to stimulate glucose disposal and inhibit glucose production were reinforced by GLP-1. Simultaneous icv infusion of EX-9 significantly diminished the latter effect. Central GLP-1R antagonism alone did not affect glucose metabolism. Also, GLP-1 treatment reinforced the inhibitory action of insulin on VLDL-TG production. In conclusion, peripheral administration of GLP-1 reinforces the ability of insulin to suppress endogenous glucose and VLDL-TG production (but not lipolysis) and boosts its capacity to stimulate glucose disposal in high-fat-fed C57Bl/6 mice. Activation of central GLP-1Rs contributes substantially to the inhibition of endogenous glucose production by GLP-1 treatment in this animal model.

Dysregulation of sterol regulatory element binding protein-1c in livers of morbidly obese women is associated with altered suppressor of cytokine signaling-3 and signal transducer and activator of transcription-1 signaling

We compared hepatic expression of genes that regulate lipid biosynthesis and metabolic signaling in liver biopsy specimens from women who were undergoing gastric bypass surgery (GBP) for morbid obesity with that in women undergoing ventral hernia repair who had experienced massive weight loss (MWL) after prior GBP. Comprehensive metabolic profiles of morbidly obese (MO) (22 subjects) and MWL (9 subjects) were also compared. Analyses of gene expression in liver biopsies from MO and MWL were accomplished by Affymetrix microarray, real-time polymerase chain reaction, and Western blotting techniques. After GBP, MWL subjects had lost on average 102 lb as compared with MO subjects. This was accompanied by effective reversal of the dyslipidemia and insulin resistance that were present in MO. As compared with MWL, livers of MO subjects exhibited increased expression of sterol regulatory element binding protein (SREBP)-1c and its downstream lipogenic targets, fatty acid synthase and acetyl-coenzyme A-carboxylase-1. Livers of MO subjects also exhibited enhanced expression of suppressor of cytokine signaling-3 protein and attenuated Janus kinase signal transducer and activator of transcription (JAK/STAT) signaling. Consistent with these findings, we found that the human SREBP-1c promoter was positively regulated by insulin and negatively regulated by STAT3. These data support the hypothesis that suppressor of cytokine signaling-3-mediated attenuation of the STAT signaling pathway and resulting enhanced expression of SREBP-1c, a key regulator of de novo lipid biosynthesis, are mechanistically related to the development of hepatic insulin resistance and dyslipidemia in MO women.

Synthesis of specific fatty acids contributes to VLDL-triacylglycerol composition in humans with and without type 2 diabetes

AIMS/HYPOTHESIS

It is recommended that patients with diabetes reduce their intake of saturated fat and increase their intake of monounsaturated fat or carbohydrate. However, high-carbohydrate diets may result in higher saturated fatty acids in VLDL-triacylglycerol. This is attributed to de novo lipogenesis, although synthesis of specific fatty acids is rarely measured. The objective of this study was to examine the contribution of de novo fatty acid synthesis to VLDL-triacylglycerol composition. It was hypothesised that levels of total and de novo synthesised fatty acids would increase with increased carbohydrate intake in diabetic participants.

METHODS

Seven individuals with type 2 diabetes mellitus and seven matched non-diabetic controls consumed two diets differing in fat energy (lower fat <25%, higher fat >35%) for 3 days in a randomised crossover design. Blood samples were drawn before and 24 h after the ingestion of (2)H-labelled water.

RESULTS

In the control participants, the higher-fat diet resulted in a 40% reduction in VLDL-triacylglycerol fatty acids because of decreases in myristic, palmitic, palmitoleic and linoleic acids, but the opposite trend occurred in participants with diabetes. The lower-fat diet increased the fractional synthesis rate by 35% and 25% in the control and diabetes participants, respectively (range: 0-33%). Palmitate accounted for 71% of fatty acids synthesised (range: 44-84% total de novo synthesised fatty acids).

CONCLUSIONS/INTERPRETATION

(2)H incorporation was used for the first time in humans showing variability in the synthesis rate of specific fatty acids, even palmitic acid. A lower-fat diet stimulated saturated fatty acid synthesis at high rates, but no net stimulation of synthesis of any fatty acid occurred in the diabetes group. The implications of this finding for our understanding of lipid metabolism in diabetes require further investigation.

Fatty liver--based identification of two distinct hypertriglyceridemic subgroups in familial combined hyperlipidemia

The present study was conducted to investigate whether the fatty liver phenotype could be helpful in the identification of subgroups with distinct metabolic properties and lipid profiles within familial combined hyperlipidemia (FCHL). One hundred eighty-five FCHL family members participated in the current study; 38 subjects were found to be hypertriglyceridemic, of whom 66% showed evidence of fatty liver as measured with ultrasound. A detailed comparison between the hypertriglyceridemic FCHL subjects with (n = 25) and without (n = 13) fatty liver revealed that, despite very similar plasma triglyceride levels (3.5 vs 3.2 mmol/L in subjects with and without fatty liver, respectively), the fatty liver subgroup presented with significantly higher body mass index, visceral adipose tissue (ultrasound), insulin, and alanine aminotransferase levels. Moreover, very low-density lipoprotein (VLDL) subclass analysis showed that the VLDL2 fraction of the fatty liver subgroup contained significantly less cholesterol and triglycerides (P = .02 for both parameters), which was likely explained by a decreased VLDL2 particle number because VLDL2 apolipoprotein B levels tended to be lower (P = .08). These data indicate that hypertriglyceridemic FCHL subjects may belong to metabolically distinct subgroups and suggest that a refinement of the hypertriglyceridemic FCHL phenotype by adding information on fatty liver will eventually facilitate the elucidation of its complex genetic background.

Increased activity of hepatic microsomal triglyceride transfer protein and bile acid synthesis in gallstone disease

A strong interrelationship exists between the regulation of bile acid (BA) metabolism and hepatic very low density lipoprotein (VLDL) production. We have recently shown that BA synthesis is increased in gallstone disease. We investigated the activity of hepatic microsomal triglyceride transfer protein (MTTP) as a surrogate of VLDL production, BA synthesis, and mRNA expression levels of proteins that regulate fatty acid (FA) metabolism in the liver of gallstone (GS) patients compared with GS-free patients. Twenty-seven volunteers subjected to elective surgery; 9 were GS-free and 18 with GS agreed to have a liver biopsy. We quantified by a fluorescence assay the activity of MTTP and by quantitative reverse-transcription PCR (RT-PCR) the mRNA content of hepatic MTTP and genes that regulate hepatic sterol and FA metabolism. Plasma was assayed for lathosterol and 7alpha-hydroxy-4-cholesten-3-one. Liver histology was normal in GS and GS-free patients. Serum VLDL triglycerides and apoB were significantly increased in GS. Hepatic triglycerides tripled in GS (P<0.001) compared with GS-free. MTTP activity increased 70% (P<0.001). Serum lathosterol and hepatic cholesterol concentrations, and mRNA expressions of MTTP, CD36, and FABP1 were similar in GS-free and GS patients. Hepatic mRNA expression of hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) and 3-hydroxyl-3-methyl-glutaryl-CoA synthase (HMGS) were significantly decreased--40% and 27%, respectively--in GS. Serum 7alpha-hydroxy-4-cholesten-3-one was 75% higher, and mRNA expression of CYP7A1 was increased sevenfold (P<0.001) in GS.

CONCLUSION

Hepatic MTTP activity and BA synthesis are increased in GS. Results suggest that hepatic VLDL production and trafficking of BA are increased in gallstone patients.

Cholesterol feeding strongly reduces hepatic VLDL-triglyceride production in mice lacking the liver X receptor α

The oxysterol-activated nuclear receptor liver X receptor alpha (LXRalpha) has been implicated in the control of both cholesterol and fatty acid metabolism. In this study, we have evaluated the effects of excess dietary cholesterol on hepatic cholesterol metabolism, lipogenesis, and VLDL production in homozygous (Lxralpha(-/-)), heterozygous (Lxralpha(+/-)), and wild-type mice. Mice were fed either chow or a cholesterol-enriched diet (1%, w/w) for 2 weeks. On the high-cholesterol diet, fractional cholesterol absorption was higher in Lxralpha(-/-) mice than in controls, leading to delivery of more dietary cholesterol to the liver. Lxralpha(-/-) mice were not able to induce expression of hepatic Abcg5/Abcg8, and massive accumulation of free cholesterol and cholesteryl esters (CEs) occurred. Interestingly, despite the inability to upregulate Abcg5/Abcg8, the highly increased hepatic free cholesterol content did stimulate biliary cholesterol output in Lxralpha(-/-) mice. Hepatic cholesterol accumulation was accompanied by decreased hepatic expression of lipogenic genes, probably caused by impaired sterol-regulatory element binding protein 1c processing, lower hepatic triglyceride (TG) contents, strongly reduced plasma TG concentrations (-90%), and reduced VLDL-TG production rates (-60%) in Lxralpha(-/-) mice. VLDL particles were smaller and CE-enriched under these conditions. Lxralpha deficiency did not affect VLDL formation under chow-fed conditions. Hepatic stearyl coenzyme A desaturase 1 expression was decreased dramatically in Lxralpha(-/-) mice and did not respond to cholesterol feeding, but fatty acid profiles of liver and VLDL were only slightly different between Lxralpha(-/-) and wild-type mice. Our data indicate that displacement of TGs by CEs during the VLDL assembly process underlies hypotriglyceridemia in cholesterol-fed Lxralpha(-/-) mice.

Hepatic glucose production is more sensitive to insulin-mediated inhibition than hepatic VLDL-triglyceride production

Insulin is an important inhibitor of both hepatic glucose output and hepatic VLDL-triglyceride (VLDL-TG) production. We investigated whether both processes are equally sensitive to insulin-mediated inhibition. To test this, we used euglycemic clamp studies with four increasing plasma concentrations of insulin in wild-type C57Bl/6 mice. By extrapolation, we estimated that half-maximal inhibition of hepatic glucose output and hepatic VLDL-TG production by insulin were obtained at plasma insulin levels of approximately 3.6 and approximately 6.8 ng/ml, respectively. In the same experiments, we measured that half-maximal decrease of plasma free fatty acid levels and half-maximal stimulation of peripheral glucose uptake were reached at plasma insulin levels of approximately 3.0 and approximately 6.0 ng/ml, respectively. We conclude that, compared with insulin sensitivity of hepatic glucose output, peripheral glucose uptake and hepatic VLDL-TG production are less sensitive to insulin.

Diabetic dyslipidemia

Type 2 diabetes mellitus is associated with a markedly increased risk of cardiovascular disease. A complex dyslipidemia, which is an integral part of the underlying insulin resistance in this group, is a key to this increased risk. Increased secretion of VLDL from the liver is a central feature of dyslipidemia and is linked significantly to the low HDL and abnormal LDL that are also present. A number of physiologic and pharmacologic approaches are available and should be used aggressively to treat diabetic dyslipidemia.

Mechanisms for the hypotriglyceridemic effect of marine omega-3 fatty acids

A mechanism to explain the hypotriglyceridemic effects of marine omega-3 fatty acids in humans has not been clarified. A working model can be developed at the gene transcriptional level, which involves >or=4 metabolic nuclear receptors. These include liver X receptor, hepatocyte nuclear factor-4alpha (HNF-4alpha), farnesol X receptor, and peroxisome proliferator-activated receptors (PPARs). Each of these receptors is regulated by sterol receptor element binding protein-1c (SREBP-1c), the main genetic switch controlling lipogenesis. Omega-3 fatty acids elicit hypotriglyceridemic effects by coordinately suppressing hepatic lipogenesis through reducing levels of SREBP-1c, upregulating fatty oxidation in the liver and skeletal muscle through PPAR activation, and enhancing flux of glucose to glycogen through downregulation of HNF-4alpha. The net result is the repartitioning of metabolic fuel from triglyceride storage toward oxidation, thereby reducing the substrate available for very-low-density lipoprotein (VLDL) synthesis. By simultaneously downregulating genes encoding proteins that stimulate lipid synthesis and upregulating genes encoding proteins that stimulate fatty acid oxidation, omega-3 fatty acids are more potent hypotriglyceridemic agents than are omega-6 fatty acids, on a carbon-for-carbon basis. Additionally, peroxidation of omega-3 fatty acids may reduce VLDL secretion through stimulating apolipoprotein B degradation. Omega-3 fatty acids may act by enhancing postprandial chylomicron clearance through reduced VLDL secretion and by directly stimulating lipoprotein lipase activity. These combined effects support the use of omega-3 fatty acids as a valuable clinical tool for the treatment of hypertriglyceridemia.

Regulation of fat metabolism in the liver: link to non-alcoholic hepatic steatosis and impact of physical exercise

Hepatic steatosis may develop as a consequence of several dysfunctions. An increased circulating non-esterified fatty acid (NEFA) pool seems to be a major determinant in the pathogenesis of non-alcoholic fatty liver disease. Increased activation of the transcription factor sterol-regulatory-element-binding protein-1c, which promotes fatty acid synthesis, also contributes to hepatic fat accumulation. Increased hepatic fat oxidation with hepatic steatosis may be triggered by increased hepatic fat concentrations through the action of hepatic peroxisomes mediated by peroxisome proliferator-activated receptor alpha. Finally, inhibition in very low density lipoprotein secretion may also result in hepatic steatosis. This appears to be mainly controlled by the esterification of NEFAs into triacylglycerols by diacyglycerol acyltransferase-1 and -2 and the microsomal transfer protein. Physical exercise would interfere with the development of hepatic steatosis by stimulating lipid oxidation and inhibiting lipid synthesis in liver through the activation of the AMP-activated protein kinase pathway.

Sex differences in atherosclerosis in mice with elevated phospholipid transfer protein activity are related to decreased plasma high density lipoproteins and not to increased production of triglycerides

Plasma phospholipid transfer protein (PLTP) has atherogenic properties in genetically modified mice. PLTP stimulates hepatic triglyceride secretion and reduces plasma levels of high density lipoproteins (HDL). The present study was performed to relate the increased atherosclerosis in PLTP transgenic mice to one of these atherogenic effects. A humanized mouse model was used which had decreased LDL receptor expression and was transgenic for human cholesterylester transfer protein (CETP) in order to obtain a better resemblance to the plasma lipoprotein profile present in humans. It is well known that female mice are more susceptible to atherosclerosis than male mice. Therefore, we compared male and female mice expressing human PLTP. The animals were fed an atherogenic diet and the effects on plasma lipids and lipoproteins, triglyceride secretion and the development of atherosclerosis were measured. The development of atherosclerosis was sex-dependent. This effect was stronger in PLTP transgenic mice, while PLTP activity levels were virtually identical. Also, the rates of hepatic secretion of triglycerides were similar. In contrast, plasma levels of HDL were about 2-fold lower in female mice than in male mice after feeding an atherogenic diet. We conclude that increased atherosclerosis caused by overexpression of PLTP is related to a decrease in HDL, rather than to elevated hepatic secretion of triglycerides.

Energy expenditure, insulin, and VLDL-triglyceride production in humans

Hypertriglyceridemia is considered a cardiovascular risk factor in diabetic and nondiabetic subjects. In this study, we aimed to determine potential regulators of very low density lipoprotein-triglyceride (TG) production. VLDL-TG kinetics were measured in 13 men and 12 women [body mass index [mean (range)]: 24.8 (20.2-35.6) kg/m(2)]. VLDL-TG production was assessed from the plasma decay of a bolus injection of ex vivo labeled VLDL particles ([1-(14)C]triolein-VLDL-TG). Similar VLDL-TG production (micromol/min) was found in men and women. VLDL-TG production was not significantly correlated with palmitate flux ([9,10-(3)H]palmitate) (r = 0.09, P = 0.67) or palmitate concentration (r = -0.29, P = 0.2) but was correlated significantly with fasting insulin concentration (r = 0.46, P < 0.05) and resting energy expenditure (REE) (r = 0.45, P < 0.05). The latter correlation improved when adjusted for sex. The best multivariate model with VLDL-TG production as the dependent variable and REE, body composition, hormones, and substrate levels as independent variables included fasting insulin (P = 0.02) and REE (P = 0.02) (r(2) = 0.32, P < 0.001). We conclude that VLDL kinetics are similar in men and women and that REE and plasma insulin are significant independent predictors of VLDL-TG production. FFA availability and body fat distribution are unrelated to VLDL production. We suggest that REE plays a greater role in VLDL-TG production than previously anticipated. REE and insulin should be taken into account when VLDL-TG production comparisons between groups are made.

Cytoplasmic lipid droplets are sites of convergence of proteasomal and autophagic degradation of apolipoprotein B

Lipid esters stored in cytoplasmic lipid droplets (CLDs) of hepatocytes are used to synthesize very low-density lipoproteins (VLDLs), into which apolipoprotein B (ApoB) is integrated cotranslationally. In the present study, by using Huh7 cells, derived from human hepatoma and competent for VLDL secretion, we found that ApoB is highly concentrated around CLDs to make "ApoB-crescents." ApoB-crescents were seen in <10% of Huh7 cells under normal conditions, but the ratio increased to nearly 50% after 12 h of proteasomal inhibition by N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal. Electron microscopy showed ApoB to be localized to a cluster of electron-lucent particles 50-100 nm in diameter adhering to CLDs. ApoB, proteasome subunits, and ubiquitinated proteins were detected in the CLD fraction, and this ApoB was ubiquitinated. Interestingly, proteasome inhibition also caused increases in autophagic vacuoles and ApoB in lysosomes. ApoB-crescents began to decrease after 12-24 h of proteasomal inhibition, but the decrease was blocked by an autophagy inhibitor, 3-methyladenine. Inhibition of autophagy alone caused an increase in ApoB-crescents. These observations indicate that both proteasomal and autophagy/lysosomal degradation of ApoB occur around CLDs and that the CLD surface functions as a unique platform for convergence of the two pathways.

Hepatic very-low-density lipoprotein and apolipoprotein B production are increased following in vivo induction of betaine-homocysteine S-methyltransferase

We have previously reported a positive correlation between the expression of BHMT (betaine-homocysteine S-methyltransferase) and ApoB (apolipoprotein B) in rat hepatoma McA (McArdle RH-7777) cells [Sowden, Collins, Smith, Garrow, Sparks and Sparks (1999) Biochem. J. 341, 639-645]. To examine whether a similar relationship occurs in vivo, hepatic BHMT expression was induced by feeding rats a Met (L-methionine)-restricted betaine-containing diet, and parameters of ApoB metabolism were evaluated. There were no generalized metabolic abnormalities associated with Met restriction for 7 days, as evidenced by control levels of serum glucose, ketones, alanine aminotransferase and L-homocysteine levels. Betaine plus the Met restriction resulted in lower serum insulin and non-esterified fatty acid levels. Betaine plus Met restriction induced hepatic BHMT 4-fold and ApoB mRNA 3-fold compared with Met restriction alone. No changes in percentage of edited ApoB mRNA were observed on the test diets. An increase in liver ApoB mRNA correlated with an 82% and 46% increase in ApoB and triacylglycerol production respectively using in vivo Triton WR 1339. Increased secretion of VLDL (very-low-density lipoprotein) with Met restriction plus betaine was associated with a 45% reduction in liver triacylglycerol compared with control. Nuclear run-off assays established that transcription of both bhmt and apob genes was also increased in Met-restricted plus betaine diets. No change in ApoB mRNA stability was detected in BHMT-transfected McA cells. Hepatic ApoB and BHMT mRNA levels were also increased by 1.8- and 3-fold respectively by betaine supplementation of Met-replete diets. Since dietary betaine increased ApoB mRNA, VLDL ApoB and triacylglycerol production and decreased hepatic triacylglycerol, results suggest that induction of apob transcription may provide a potential mechanism for mobilizing hepatic triacylglycerol by increasing ApoB available for VLDL assembly and secretion.

Overproduction of large VLDL particles is driven by increased liver fat content in man

AIMS/HYPOTHESIS

We determined whether hepatic fat content and plasma adiponectin concentration regulate VLDL(1) production.

METHODS

A multicompartment model was used to simultaneously determine the kinetic parameters of triglycerides (TGs) and apolipoprotein B (ApoB) in VLDL(1) and VLDL(2) after a bolus of [(2)H(3)]leucine and [(2)H(5)]glycerol in ten men with type 2 diabetes and in 18 non-diabetic men. Liver fat content was determined by proton spectroscopy and intra-abdominal fat content by MRI.

RESULTS

Univariate regression analysis showed that liver fat content, intra-abdominal fat volume, plasma glucose, insulin and HOMA-IR (homeostasis model assessment of insulin resistance) correlated with VLDL(1) TG and ApoB production. However, only liver fat and plasma glucose were significant in multiple regression models, emphasising the critical role of substrate fluxes and lipid availability in the liver as the driving force for overproduction of VLDL(1) in subjects with type 2 diabetes. Despite negative correlations with fasting TG levels, liver fat content, and VLDL(1) TG and ApoB pool sizes, adiponectin was not linked to VLDL(1) TG or ApoB production and thus was not a predictor of VLDL(1) production. However, adiponectin correlated negatively with the removal rates of VLDL(1) TG and ApoB.

CONCLUSIONS/INTERPRETATION

We propose that the metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, increases fatty acid flux from adipose tissue to the liver and induces the accumulation of fat in the liver. Elevated plasma glucose can further increase hepatic fat content through multiple pathways, resulting in overproduction of VLDL(1) particles and leading to the characteristic dyslipidaemia associated with type 2 diabetes.

Endogenous apoC-I increases hyperlipidemia in apoE-knockout mice by stimulating VLDL production and inhibiting LPL

Previous studies have shown that overexpression of human apolipoprotein C-I (apoC-I) results in moderate hypercholesterolemia and severe hypertriglyceridemia in mice in the presence and absence of apoE. We assessed whether physiological endogenous apoC-I levels are sufficient to modulate plasma lipid levels independently of effects of apoE on lipid metabolism by comparing apolipoprotein E gene-deficient/apolipoprotein C-I gene-deficient (apoe-/-apoc1-/-), apoe-/-apoc1+/-, and apoe-/-apoc1+/+ mice. The presence of the apoC-I gene-dose-dependently increased plasma cholesterol (+45%; P < 0.001) and triglycerides (TGs) (+137%; P < 0.001), both specific for VLDL. Whereas apoC-I did not affect intestinal [3H]TG absorption, it increased the production rate of hepatic VLDL-TG (+35%; P < 0.05) and VLDL-[35S]apoB (+39%; P < 0.01). In addition, apoC-I increased the postprandial TG response to an intragastric olive oil load (+120%; P < 0.05) and decreased the uptake of [3H]TG-derived FFAs from intravenously administered VLDL-like emulsion particles by gonadal and perirenal white adipose tissue (WAT) (-34% and -25%, respectively; P < 0.05). As LPL is the main enzyme involved in the clearance of TG-derived FFAs by WAT, and total postheparin plasma LPL levels were unaffected, these data demonstrate that endogenous apoC-I suffices to attenuate the lipolytic activity of LPL. Thus, we conclude that endogenous plasma apoC-I increases VLDL-total cholesterol and VLDL-TG dose-dependently in apoe-/- mice, resulting from increased VLDL particle production and LPL inhibition.

CD36 deficiency in mice impairs lipoprotein lipase-mediated triglyceride clearance

CD36 is involved in high-affinity peripheral FFA uptake. CD36-deficient (cd36(-)(/)(-)) mice exhibit increased plasma FFA and triglyceride (TG) levels. The aim of the present study was to elucidate the cause of the increased plasma TG levels in cd36(-)(/)(-) mice. cd36(-)(/)(-) mice showed no differences in hepatic VLDL-TG production or intestinal [(3)H]TG uptake compared with wild-type littermates. cd36(-)(/)(-) mice showed a 2-fold enhanced postprandial TG response upon an intragastric fat load (P < 0.05), with a concomitant 2.5-fold increased FFA response (P < 0.05), suggesting that the increased FFA in cd36(-/-) mice may impair LPL-mediated TG hydrolysis. Postheparin LPL levels were not affected. However, the in vitro LPL-mediated TG hydrolysis rate as induced by postheparin plasma of cd36(-)(/)(-) mice in the absence of excess FFA-free BSA was reduced 2-fold compared with wild-type plasma (P < 0.05). This inhibition was relieved upon the addition of excess FFA-free BSA. Likewise, increasing plasma FFA in wild-type mice to the levels observed in cd36(-)(/)(-) mice by infusion prolonged the plasma half-life of glycerol tri[(3)H]oleate-labeled VLDL-like emulsion particles by 2.5-fold (P < 0.05). We conclude that the increased plasma TG levels observed in cd36(-)(/)(-) mice are caused by decreased LPL-mediated hydrolysis of TG-rich lipoproteins resulting from FFA-induced product inhibition of LPL.

Spillover of dietary fatty acids and use of serum nonesterified fatty acids for the synthesis of VLDL-triacylglycerol under two different feeding regimens

The present study quantified dietary fatty acid flux in healthy men (n = 6) who were fed a liquid formula through a duodenal feeding tube (continuous feeding group) or who consumed the same formula in meals (meal feeding group). A triacylglycerol (TAG) stable isotope was added to the formula to determine the entry of dietary fatty acids into the serum and its clearance to the liver and resecretion into serum via VLDL. The contribution of dietary fatty acids to serum nonesterified fatty acids (NEFAs) was higher with meal feeding (24.4 +/- 2.6%) compared with continuous feeding (10.8 +/- 2.9%, P < 0.01) and, when multiplied by the NEFA concentration, resulted in similar absolute fatty acid spillover. Diet-derived NEFAs subsequently represented 10.6 +/- 1.2% and 4.7 +/- 1.3% of hepatic VLDL-TAG (meal feeding vs. continuous feeding, respectively, P = 0.004). Chylomicron remnant uptake by the liver contributed 9.3 +/- 1.9% of fatty acids to hepatic VLDL-TAG synthesis with meal feeding compared with continuous feeding (4.4 +/- 0.8%, P < 0.03). These data suggest that the extent of dietary fatty acid recycling via serum NEFAs and VLDL-TAG is determined by the rate of delivery of dietary fat to the intestine. The inefficient removal of dietary fat from the circulation may maintain VLDL-TAG production but may also result in prolonged postprandial lipemia.

Role of the macrophage very-low-density lipoprotein receptor in atherosclerotic lesion development

OBJECTIVES

The very-low-density lipoprotein receptor (VLDLr) is highly expressed in macrophage-rich areas of atherosclerotic lesions. The exact role of the macrophage VLDLr in atherosclerotic lesion development, however, is presently unclear.

METHODS AND RESULTS

To assess the role of the macrophage VLDLr in atherosclerotic lesion development in vivo, we used the technique of bone marrow transplantation to selectively disrupt or reconstitute the VLDLr in macrophages in VLDLr+/+ and VLDLr-/- mice, respectively. After 10 weeks high-cholesterol diet feeding, the lesion area in control transplanted wild-type mice was 17+/-4 x 10(3)+/-microm(2). Disruption of the macrophage VLDLr by transplanting bone marrow from VLDLr-/- mice to wild-type VLDLr+/+ littermates resulted in a tendency to a slight reduction in lesion size to 12+/-3 x 10 microm. The mean atherosclerotic lesion area, measured in control transplanted VLDLr-/- mice, lacking the VLDLr in all tissues was 12+/-3 x 10(3)microm(2). Interestingly, reconstitution of the macrophage VLDLr in VLDLr-deficient recipients resulted in a 2.7-fold increase (P<0.05) in the mean atherosclerotic lesion area to 32+/-3 x 10(3)microm(2).

CONCLUSIONS

The macrophage VLDLr facilitates atherosclerotic lesion development, probably by mediating the accumulation of atherogenic lipoproteins.

Relation of the size and intracellular sorting of apoB to the formation of VLDL 1 and VLDL 2

In this study, we tested the hypothesis that two separate pathways, the two-step process and an apolipoprotein B (apoB) size-dependent lipidation process, give rise to different lipoproteins. Expression of apoB-100 and C-terminally truncated forms of apoB-100 in McA-RH7777 cells demonstrated that VLDL particles can be assembled by apoB size-dependent linear lipidation, resulting in particles whose density is inversely related to the size of apoB. This lipidation results in a LDL-VLDL 2 particle containing apoB-100. VLDL 1 is assembled by the two-step process by apoB-48 and larger forms of apoB but not to any significant amount by apoB-41. The major amount of intracellular apoB-80 and apoB-100 banded with a mean density of 1.10 g/ml. Its formation was dependent on the sequence between apoB-72 and apoB-90. This dense particle, which is retained in the cell, possibly by chaperones or association with the microsomal membrane, is a precursor of secreted VLDL 1. The intracellular LDL-VLDL 2 particles formed during size-dependent lipidation appear to be the precursors of intracellular VLDL 1. We propose that the dense apoB-100 intracellular particle is converted to LDL-VLDL 2 by size-dependent lipidation. LDL-VLDL 2 is secreted or converted to VLDL 1 by the uptake of the major amount of triglycerides.

Adenovirus-mediated high expression of resistin causes dyslipidemia in mice

The adipocyte-derived hormone resistin has been proposed as a possible link between obesity and insulin resistance in murine models. Many recent studies have reported physiological roles for resistin in glucose homeostasis, one of which is enhancement of glucose production from the liver by up-regulating gluconeogenic enzymes such as glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. However, its in vivo roles in lipid metabolism still remain to be clarified. In this study, we investigated the effects of resistin overexpression on insulin action and lipid metabolism in C57BL/6 mice using an adenoviral gene transfer technique. Elevated plasma resistin levels in mice treated with the resistin adenovirus (AdmRes) were confirmed by Western blotting analysis and RIAs. Fasting plasma glucose levels did not differ between AdmRes-treated mice and controls, but the basal insulin concentration was significantly elevated in AdmRes-treated mice. In AdmRes-treated mice, the glucose-lowering effect of insulin was impaired, as evaluated by insulin tolerance tests. Furthermore, total cholesterol and triglyceride concentrations were significantly higher, whereas the high-density lipoprotein cholesterol level was significantly lower. Lipoprotein analysis revealed that low-density lipoprotein was markedly increased in AdmRes-treated mice, compared with controls. In addition, in vivo Triton WR-1339 studies showed evidence of enhanced very low-density lipoprotein production in AdmRes-treated mice. The expressions of genes involved in lipoprotein metabolism, such as low-density lipoprotein receptor and apolipoprotein AI in the liver, were decreased. These results suggest that resistin overexpression induces dyslipidemia in mice, which is commonly seen in the insulin-resistant state, partially through enhanced secretion of lipoproteins.

A new combined multicompartmental model for apolipoprotein B-100 and triglyceride metabolism in VLDL subfractions

The use of stable isotopes in conjunction with compartmental modeling analysis has greatly facilitated studies of the metabolism of the apolipoprotein B (apoB)-containing lipoproteins in humans. The aim of this study was to develop a multicompartment model that allows us to simultaneously determine the kinetics of apoB and triglyceride (TG) in VLDL(1) and VLDL(2) after a bolus injection of [(2)H(3)]leucine and [(2)H(5)]glycerol and to follow the catabolism and transfer of the lipoprotein particles. Here, we describe the model and present the results of its application in a fasting steady-state situation in 17 subjects with lipid values representative of a Western population. Analysis of the correlations showed that plasma TG was determined by the VLDL(1) and VLDL(2) apoB and TG fractional catabolic rate. Furthermore, the model showed a linear correlation between VLDL(1) TG and apoB production. A novel observation was that VLDL TG entered the circulation within 21 min after its synthesis, whereas VLDL apoB entered the circulation after 33 min. These observations are consistent with a sequential assembly model of VLDL and suggest that the TG is added to a primordial apoB-containing particle in the liver.

Intracerebroventricular neuropeptide Y infusion precludes inhibition of glucose and VLDL production by insulin

Recent evidence demonstrates that hypothalamic insulin signaling is required for inhibition of endogenous glucose production. The downstream mechanisms that are responsible for the effects of hypothalamic insulin receptor activation on hepatic fuel flux remain to be determined. To establish whether downregulation of neuropeptide Y (NPY) release by insulin is mandatory for its capacity to suppress glucose production, we examined the effects of a continuous intracerebroventricular (ICV) infusion of NPY (10 microg/h for 3-5 h) on glucose flux during a hyperinsulinemic-euglycemic clamp in mice. We also evaluated the effects of ICV NPY administration on free fatty acid and glycerol flux and VLDL production in this experimental context. In basal conditions, none of the metabolic parameters was affected by NPY infusion. In hyperinsulinemic conditions, peripheral glucose disposal was not different between vehicle- and NPY-infused animals. In contrast, hyperinsulinemia suppressed endogenous glucose production by approximately 8% vs. 30% in NPY- vs. vehicle-infused mice, respectively (P < 0.05). Also, VLDL production was significantly higher during hyperinsulinemia in NPY- compared with vehicle-infused mice (97.5 +/- 18.0 vs. 54.7 +/- 14.9 micromol. kg(-1). h(-1); P < 0.01). These data suggest that the neurophysiological action of insulin to downregulate hypothalamic NPY release is a prerequisite for its ability to suppress hepatic fuel production, whereas it is not mandatory for its capacity to modulate glucose disposal or lipolysis.