The role of pancreatic hormones in the regulation of lipid storage, oxidation and secretion in primary cultures of rat hepatocytes. Short- and long-term effects

Imeglimin enhances glucagon secretion through an indirect mechanism and improves fatty liver in high-fat, high-sucrose diet-fed mice

AIMS/INTRODUCTION

Imeglimin is a recently approved oral antidiabetic agent that improves insulin resistance, and promotes insulin secretion from pancreatic β-cells. Here, we investigated the effects of imeglimin on glucagon secretion from pancreatic α-cells.

MATERIALS AND METHODS

Experiments were carried out in high-fat, high-sucrose diet-fed mice. The effects of imeglimin were examined using insulin and glucose tolerance tests, glucose clamp studies, and measurements of glucagon secretion from isolated islets. Glucagon was measured using both the standard and the sequential protocol of Mercodia sandwich enzyme-linked immunosorbent assay; the latter eliminates cross-reactivities with other proglucagon-derived peptides.

RESULTS

Plasma glucagon, insulin and glucagon-like peptide-1 levels were increased by imeglimin administration in high-fat, high-sucrose diet-fed mice. Glucose clamp experiments showed that the glucagon increase was not caused by reduced blood glucose levels. After both single and long-term administration of imeglimin, glucagon secretions were significantly enhanced during glucose tolerance tests. Milder enhancement was observed when using the sequential protocol. Long-term administration of imeglimin did not alter α-cell mass. Intraperitoneal imeglimin administration did not affect glucagon secretion, despite significantly decreased blood glucose levels. Imeglimin did not enhance glucagon secretion from isolated islets. Imeglimin administration improved fatty liver by suppressing de novo lipogenesis through decreasing sterol regulatory element binding protein-1c and carbohydrate response element binding protein and their target genes, while enhancing fatty acid oxidation through increasing carnitine palmitoyltransferase I.

CONCLUSIONS

Overall, the present results showed that imeglimin enhances glucagon secretion through an indirect mechanism. Our findings also showed that glucagon secretion promoted by imeglimin could contribute to improvement of fatty liver through suppressing de novo lipogenesis and enhancing fatty acid oxidation.

Impaired Glucagon-Mediated Suppression of VLDL-Triglyceride Secretion in Individuals With Metabolic Dysfunction-Associated Fatty Liver Disease (MAFLD)

Individuals with metabolic dysfunction-associated fatty liver disease (MAFLD) have elevated plasma lipids as well as glucagon, although glucagon suppresses hepatic VLDL-triglyceride (TG) secretion. We hypothesize that the sensitivity to glucagon in hepatic lipid metabolism is impaired in MAFLD. We recruited 11 subjects with severe MAFLD (MAFLD+), 10 with mild MAFLD (MAFLD-), and 7 overweight control (CON) subjects. We performed a pancreatic clamp with a somatostatin analog (octreotide) to suppress endogenous hormone production, combined with infusion of low-dose glucagon (0.65 ng/kg/min, t = 0-270 min, LowGlucagon), followed by high-dose glucagon (1.5 ng/kg/min, t = 270-450 min, HighGlucagon). VLDL-TG and glucose tracers were used to evaluate VLDL-TG kinetics and endogenous glucose production (EGP). HighGlucagon suppressed VLDL-TG secretion compared with LowGlucagon. This suppression was markedly attenuated in MAFLD subjects compared with CON subjects (MAFLD+: 13% ± [SEM] 5%; MAFLD-: 10% ± 3%; CON: 36% ± 7%, P < 0.01), with no difference between MAFLD groups. VLDL-TG concentration and VLDL-TG oxidation rate increased between LowGlucagon and HighGlucagon in MAFLD+ subjects compared with CON subjects. EGP transiently increased during HighGlucagon without any difference between the three groups. Individuals with MAFLD have a reduced sensitivity to glucagon in the hepatic TG metabolism, which could contribute to the dyslipidemia seen in MAFLD patients. ClinicalTrials.gov: NCT04042142.

Fasting, non-fasting and postprandial triglycerides for screening cardiometabolic risk

Fasting triacylglycerols have long been associated with cardiovascular disease (CVD) and other cardiometabolic conditions. Evidence suggests that non-fasting triglycerides (i.e. measured within 8 h of eating) better predict CVD than fasting triglycerides, which has led several organisations to recommend non-fasting lipid panels as the new clinical standard. However, unstandardised assessment protocols associated with non-fasting triglyceride measurement may lead to misclassification, with at-risk individuals being overlooked. A third type of triglyceride assessment, postprandial testing, is more controlled, yet historically has been difficult to implement due to the time and effort required to execute it. Here, we review differences in assessment, the underlying physiology and the pathophysiological relevance of elevated fasting, non-fasting and postprandial triglycerides. We also present data suggesting that there may be a distinct advantage of postprandial triglycerides, even over non-fasting triglycerides, for early detection of CVD risk and offer suggestions to make postprandial protocols more clinically feasible.

Insulin dependent apolipoprotein B degradation and phosphatidylinositide 3-kinase activation with microsomal translocation are restored in McArdle RH7777 cells following serum deprivation

Previous studies in rat hepatocytes demonstrated that insulin-dependent apolipoprotein (apo) B degradation (IDAD) is lost when cells are maintained for 3 d under enriched culture conditions. Loss of IDAD correlates with increased expression of protein tyrosine phosphatase 1B (PTP1B) known to be associated with resistance to insulin signaling in the liver. McArdle RH7777 hepatoma (McA) cells cultured in serum containing medium are resistant to IDAD; demonstrate a 30% increase in apo B secretion, and express increased levels of PTP1B protein and mRNA. In addition, insulin-stimulated Class I phosphatidylinositide 3-kinase (PI3K) activity of anti-pY immunoprecipitates is severely blunted. IDAD resistance in McA cells correlates with diminished translocation of insulin-stimulated pY-IRS1 to intracellular membranes. Incubation of McA cells with RK682, a protein tyrosine phosphatase inhibitor, is sufficient to restore IDAD in resistant McA cells. Overall, results further support the importance of Class I PI3K activity in IDAD, and suggest that loss of this activity is sufficient to cause resistance. Although other factors are involved in downstream events including sortilin binding to apo B, autophagy, and lysosomal degradation, loss of signal generation and reduced localization of Class I PI3K to intracellular membranes plays a significant role in IDAD resistance.

Hepatic nervous system and neurobiology of the liver

The liver has a nervous system containing both afferent and efferent neurons that are involved in a number of processes. The afferent arm includes the sensation of lipids, glucose, and metabolites (after eating and drinking) and triggers the nervous system to make appropriate physiological changes. The efferent arm is essential for metabolic regulation, modulation of fibrosis and biliary function and the control of a number of other processes. Experimental models have helped us to establish how: (i) the liver is innervated by the autonomic nervous system; and (ii) the cell types that are involved in these processes. Thus, the liver acts as both a sensor and effector that is influenced by neurological signals and ablation. Understanding these processes hold significant implications in disease processes such as diabetes and obesity, which are influenced by appetite and hormonal signals.

Insulin suppression of apolipoprotein B in McArdle RH7777 cells involves increased sortilin 1 interaction and lysosomal targeting

Insulin suppresses secretion of very low density lipoprotein (VLDL) apolipoprotein (apo) B in primary rodent hepatocytes (RH) by favoring the degradation of B100, the larger form of apo B, through post-endoplasmic reticulum proteolysis. Sortilin 1 (sort1), a multi-ligand sorting receptor, has been proposed as a mediator of lysosomal B100 degradation by directing B100 in pre-VLDL to lysosomes rather than allowing maturation to VLDL and secretion. The purpose of our studies was to investigate the role of sort1 in insulin-dependent degradation of apo B. Using liver derived McArdle RH7777 (McA) cells, we demonstrate that insulin suppresses VLDL B100 secretion via a phosphatidylinositide 3-kinase (PI3K) dependent process that is inhibitable by wortmannin in a fashion similar to RH. Using McA cells and in situ cross-linking, we demonstrate that insulin acutely (30min) stimulates the interaction of B100 with sort1. The insulin-induced interaction of sort1-B100 is markedly enhanced when lysosomal degradation is inhibited by Bafilomycin A1 (BafA1), an inhibitor of lysosomal acidification. As BafA1 also prevents insulin suppressive effects on apo B secretion, our results suggest that sort1-B100 interaction stimulated by insulin transiently accumulates with BafA1 and favors B100 secretion by default.

Apolipoprotein B secretion is regulated by hepatic triglyceride, and not insulin, in a model of increased hepatic insulin signaling

OBJECTIVE

States of insulin resistance, hyperinsulinemia, and hepatic steatosis are associated with increased secretion of triglycerides (TG) and apolipoprotein B (apoB), even though insulin targets apoB for degradation. We used hepatic-specific "phosphatase and tensin homologue deleted on chromosome 10" (Pten) knockout (hPten-ko) mice, with increased hepatic insulin signaling, to determine the relative roles of insulin signaling and hepatic TG in regulating apoB secretion.

METHODS AND RESULTS

TG and apoB secretion was elevated in hPten-ko mice. When hepatic TG was reduced by inhibition of diacylglycerol acyltransferase 1/diacylglycerol acyltransferase 2 or sterol regulatory element-binding protein-1c, both TG secretion and apoB secretion fell without changes in hepatic insulin signaling. Acute reconstitution of hPten reduced hepatic TG content, and both TG and apoB secretion fell within 4 days despite decreased hepatic insulin signaling. Acute depletion of hepatic Pten by adenoviral introduction of Cre into Pten floxed mice caused steatosis within 4 days, and secretion of both TG and apoB increased despite increased hepatic insulin signaling. Even when steatosis after acute Pten depletion was prevented by pretreatment with SREBP-1c antisense oligonucleotides, apoB secretion was not reduced after 4 days. Ex vivo results were in primary hepatocytes were similar.

CONCLUSIONS

Either hepatic TG is the dominant regulator of apoB secretion or any inhibitory effects of hepatic insulin signaling on apoB secretion is very short-lived.

Increased very low density lipoprotein (VLDL) secretion, hepatic steatosis, and insulin resistance

Insulin resistance (IR) affects not only the regulation of carbohydrate metabolism but all aspects of lipid and lipoprotein metabolism. IR is associated with increased secretion of VLDL and increased plasma triglycerides, as well as with hepatic steatosis, despite the increased VLDL secretion. Here we link IR with increased VLDL secretion and hepatic steatosis at both the physiologic and molecular levels. Increased VLDL secretion, together with the downstream effects on high density lipoprotein (HDL) cholesterol and low density lipoprotein (LDL) size, is proatherogenic. Hepatic steatosis is a risk factor for steatohepatitis and cirrhosis. Understanding the complex inter-relationships between IR and these abnormalities of liver lipid homeostasis will provide insights relevant to new therapies for these increasing clinical problems.

LC, Gonçalves DC, Tavares FL, Seelaender MCL. Influência do treinamento físico aeróbio no transporte mitocondrial de ácidos graxos de cadeia longa no músculo esquelético: papel do complexo carnitina palmitoil transferase

O ácido graxo (AG) é uma importante fonte de energia para o músculo esquelético. Durante o exercício sua mobilização é aumentada para suprir as necessidades da musculatura ativa. Acredita-se que diversos pontos de regulação atuem no controle da oxidação dos AG, sendo o principal a atividade do complexo carnitina palmitoil transferase (CPT), entre os quais três componentes estão envolvidos: a CPT I, a CPT II e carnitina acilcarnitina translocase. A função da CPT I durante o exercício físico é controlar a entrada de AG para o interior da mitocôndria, para posterior oxidação do AG e produção de energia. Em resposta ao treinamento físico há um aumento na atividade e expressão da CPT I no músculo esquelético. Devido sua grande importância no metabolismo de lipídios, os mecanismos que controlam sua atividade e sua expressão gênica são revisados no presente estudo. Reguladores da expressão gênica de proteínas envolvidas no metabolismo de lipídios no músculo esquelético, os receptores ativados por proliferadores de peroxissomas (PPAR) alfa e beta, são discutidos com um enfoque na resposta ao treinamento físico.

The role of exogenous insulin in the complex of hepatic lipidosis and ketosis associated with insulin resistance phenomenon in postpartum dairy cattle

As a result of a marked decline in dry matter intake (DMI) prior to parturition and a slow rate of increase in DMI relative to milk production after parturition, dairy cattle experience a negative energy balance. Changes in nutritional and metabolic status during the periparturient period predispose dairy cattle to develop hepatic lipidosis and ketosis. The metabolic profile during early lactation includes low concentrations of serum insulin, plasma glucose, and liver glycogen and high concentrations of serum glucagon, adrenaline, growth hormone, plasma beta-hydroxybutyrate and non-esterified fatty acids, and liver triglyceride. Moreover, during late gestation and early lactation, flow of nutrients to fetus and mammary tissues are accorded a high degree of metabolic priority. This priority coincides with lowered responsiveness and sensitivity of extrahepatic tissues to insulin, which presumably plays a key role in development of hepatic lipidosis and ketosis. Hepatic lipidosis and ketosis compromise production, immune function, and fertility. Cows with hepatic lipidosis and ketosis have low tissue responsiveness to insulin owing to ketoacidosis. Insulin has numerous roles in metabolism of carbohydrates, lipids and proteins. Insulin is an anabolic hormone and acts to preserve nutrients as well as being a potent feed intake regulator. In addition to the major replacement therapy to alleviate severity of negative energy balance, administration of insulin with concomitant delivery of dextrose increases efficiency of treatment for hepatic lipidosis and ketosis. However, data on use of insulin to prevent these lipid-related metabolic disorders are limited and it should be investigated.

Suppression of VLDL associated triacylglycerol secretion by both alpha- and beta-adrenoceptor agonists in isolated rat hepatocytes

The signal transduction pathways of intracellular calcium and adenosin 3',5'-cyclic monophosphate (cAMP) participate in the regulation of intrahepatic metabolism of very low density lipoproteins (VLDL). The adrenoceptors are linked to calcium and cAMP signal transduction pathways so it is proposed that they may be involved in the regulation of VLDL secretion. The current study is designed to test the effects of alpha- and beta-adrenoceptor agonists and antagonists on triacylglycerol secretion in freshly isolated rat hepatocytes. The inhibitory effect of epinephrine appeared at concentrations of more than 1 microM and reached a plateau at 100 microM. Epinephrine concentration for the half of the maximal bio-effect (EC(50)) was about 10 microM. Epinephrine at a concentration of 10 microM suppressed the secretion of triacylglycerol by 33% (P<or=0.01) and increased cellular content of triacylglycerol (18%, P<or=0.05) and total phospholipids (20%, P<or=0.05). Time course experiments for triacylglycerol secretion exhibited a linear relationship with a slope of 8.2+/-0.6 mug triacylglycerol/3 h mg cell protein. In the presence of epinephrine, cellular triacylglycerol and total phospholipids were slightly but significantly higher than the respective control at all points of time examined. The inhibitory effect elicited by epinephrine (10 microM) was abolished by the inclusion of the general alpha-adrenoceptor antagonist phentolamine (10 microM) and the specific alpha(1)-antagonist prazosin (1 microM) but not with the nonselective beta-antagonist propranolol (10 microM). Trifluoperazine an alpha-adrenoceptor antagonist and anticalmodulin agent, concealed the inhibitory effect of epinephrine in a concentration dependent manner, whereas theobromine a cAMP-phosphodiestrase inhibitor did not have any significant effect. The secretion of triacylglycerol was decreased not only by the alpha-adrenoceptor agonist phenylephrine (10 microM) but also by the beta-agonist isoproterenol (10 microM). Dibutyryl-cAMP (0.1 mM) also inhibited the secretion of triacylglycerol by 30% (P<or=0.01). The results suggest that epinephrine inhibits the secretion of triacylglycerol from rat hepatocytes via the alpha(1)-adrenoceptor while stimulation of beta- as well as alpha-adrenoceptors can also exert a similar effect.

Insulin, glucagon and fatty acid treatment of hepatocytes does not result in phosphorylation or changes in activity of triacylglycerol hydrolase

It is recognized that the majority of very low density lipoprotein (VLDL) associated triacylglycerol (TG) is synthesized from fatty acids and partial acylglycerols generated by lipolysis of intra-hepatic storage rather than made de novo. Triacylglycerol hydrolase (TGH) is involved in mobilizing stored TG. Modulating the ability of TGH to hydrolyze stored lipids represents a potentially regulated and rate limiting step in VLDL assembly. Phosphorylation of lipases and carboxylesterases trigger diverse but functionally significant events. We explored the potential for regulating the mobilization of hepatic TG through phosphorylation of TGH. Insulin is known to suppress VLDL secretion from liver, and glucagon can be considered an opposing hormone. However, neither insulin nor glucagon treatment of hepatocytes led to phosphorylation of TGH or changes in its activity. Augmenting intracellular TG stores by incubations with oleic acid also did not lead to changes in TGH activity. Therefore, changes in phosphorylation state are not a mechanism for regulating TGH activity, access to TG substrate pools or for TGH-mediated contributions to VLDL assembly and secretion.

Effects of exogenous glucagon on lipids in lipoproteins and liver of lactating dairy cows

Decreased concentrations of phospholipids, free cholesterol, and cholesteryl ester in plasma and liver are associated with fatty liver, a major metabolic disease of dairy cows in early lactation. The objective was to test whether daily subcutaneous injections of 7.5 and 15 mg of glucagon, which can decrease concentrations of liver triacylglycerol, affect concentrations of plasma lipoprotein components and liver lipids other than triacylglycerol. Multiparous Holstein cows (n = 32) were grouped on the basis of liver triacylglycerol concentrations at d 8 postpartum into "normal" (n = 8; triacylglycerol <1% liver wet wt) and "susceptible to fatty liver" (n = 24; triacylglycerol >1% liver wet wt) cows. Susceptible cows were assigned randomly to three groups and beginning at d 8 postpartum received 0 (same for Normal cows), 2.5, or 5 mg of glucagon by subcutaneous injections every 8 h for 14 d. In comparison to saline injections, subcutaneous injections of glucagon either increased or tended to increase concentrations of phospholipids and free cholesterol in liver, with greater increases of the latter during ambient temperatures below 35 degrees C. Glucagon injections decreased or tended to decrease concentrations of very low-density lipoprotein-triacylglycerol, high-density lipoprotein1-phospholipids, and high-density lipoprotein2-free cholesterol in plasma, with no changes of the latter two during ambient temperatures below 35 degrees C. The results indicate that subcutaneously administered glucagon has only minor effects on the lipid transport in plasma of dairy cows in early lactation with more beneficial effects occurring during ambient temperatures below 35 degrees C and, most importantly, no indications that glucagon has negative effects.

Effects of glucagon and insulin on plasma glucose, triglyceride, and triglyceride-rich lipoprotein concentrations in laying hens fed diets containing different types of fats

The influence of dietary fat supplementations differing in the ratio of n-6 to n-3 polyunsaturated fatty acids (PUFA) on the effects of glucagon and insulin on plasma glucose, triglyceride (TG), and TG-rich lipoprotein concentrations was investigated in laying hens. Birds were fed either a low-fat control diet (LF) or diets supplemented with 4% pumpkin seed oil (PO; rich in n-6 PUFA) or 4% cod liver oil (CO; rich in n-3 PUFA). After 4 wk feeding of the experimental diets, hens were implanted with wing vein catheters and injected with porcine glucagon (20 microg/kg BW) and porcine insulin (0.5 IU/kg BW), 2 to 5 h after oviposition. Plasma glucose, TG, and TG-rich lipoprotein concentrations were determined from 10 min pre-injection to 60 min post-injection. PO diet resulted in a prolonged plasma glucose response to glucagon administration and altered hypoglycemic response to insulin. However, CO diet did not influence plasma glucose response to either glucagon or insulin administration compared to LF diet. The effects of glucagon and insulin on plasma TG and TG-rich lipoproteins were similar for all diets regardless of the amount or type of fat. The results suggest that feeding dietary fats with high n-6 to n-3 PUFA ratio alters the glucagon and insulin sensitivity of plasma glucose in laying hens. Fats rich in n-3 PUFA seem to have no influence on the plasma glucose response to glucagon and insulin.

Insulin suppression of VLDL apo B secretion is not mediated by the LDL receptor

Insulin inhibits hepatic very low density lipoprotein (VLDL) apo B secretion in rats. Current studies test whether the insulin effect is LDL receptor-mediated by examining the effect of insulin on VLDL apo B secretion in hepatocytes derived from Ldlr-/- and control mice. Primary hepatocytes were incubated overnight with media containing 14C-leucine and either 0.1nM (basal) or 200nM insulin. Afterwards, secreted VLDL B100 and B48 were quantitated. Insulin reduced 14C-labeled B100 and B48 comparably in control and Ldlr-/- hepatocytes with a 62+/-12% vs. 59+/-12% decrease in B100, and a 56+/-11% vs. 61+/-9% decrease in B48. Results indicate: (1) mouse hepatocytes respond to insulin by reducing VLDL apo B output; (2) both VLDL B100 and B48 secretion are suppressed; and (3) insulin inhibition of VLDL apo B secretion is retained in Ldlr-/- hepatocytes.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

Hepatic very low density lipoprotein-ApoB overproduction is associated with attenuated hepatic insulin signaling and overexpression of protein-tyrosine phosphatase 1B in a fructose-fed hamster model of insulin resistance

A fructose-fed hamster model of insulin resistance was previously documented to exhibit marked hepatic very low density lipoprotein (VLDL) overproduction. Here, we investigated whether VLDL overproduction was associated with down-regulation of hepatic insulin signaling and insulin resistance. Hepatocytes isolated from fructose-fed hamsters exhibited significantly reduced tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2. Phosphatidylinositol 3-kinase activity as well as insulin-stimulated Akt-Ser473 and Akt-Thr308 phosphorylation were also significantly reduced with fructose feeding. Interestingly, the protein mass and activity of protein-tyrosine phosphatase-1B (PTP-1B) were significantly higher in fructose-fed hamster hepatocytes. Chronic ex vivo exposure of control hamster hepatocytes to high insulin also appeared to attenuate insulin signaling and increase PTP-1B. Elevation in PTP-1B coincided with marked suppression of ER-60, a cysteine protease postulated to play a role in intracellular apoB degradation, and an increase in the synthesis and secretion of apoB. Sodium orthovanadate, a general phosphatase inhibitor, partially restored insulin receptor phosphorylation and significantly reduced apoB secretion. In summary, we hypothesize that fructose feeding induces hepatic insulin resistance at least in part via an increase in expression of PTP-1B. Induction of hepatic insulin resistance may then contribute to reduced apoB degradation and enhanced VLDL particle assembly and secretion.

Post-transcriptional stimulation of the assembly and secretion of triglyceride-rich apolipoprotein B lipoproteins in a mouse with selective deficiency of brown adipose tissue, obesity, and insulin resistance

A mouse model of insulin resistance and its associated dyslipidemia was generated by crossing mice expressing human apolipoprotein B (apoB) with mice lacking only brown adipose tissue (BATless). On a high fat diet, male apoB/BATless mice became obese, hypercholesterolemic, hypertriglyceridemic, and hyperinsulinemic compared with control apoB mice. Fast performance liquid chromatography revealed increased triglyceride concentrations in intermediate density lipoprotein/low density lipoprotein (LDL) and reduced high density lipoprotein cholesterol concentrations. Inhibition of lipolysis by the drug, tetrahydrolipostatin, demonstrated that very low density lipoprotein-sized particles were initially secreted. Metabolic studies employing Triton WR-1339 and either [(3)H]glycerol or [(3)H]palmitate showed that the hypertriglyceridemia in apoB/BATless mice was due to the increased synthesis and secretion of triglyceride. Furthermore, lipoprotein lipase and hepatic lipase activities were not defective. ApoB was also secreted at increased rates in the apoB/BATless mice. Similar levels of apoB mRNA in apoB and apoB/BATless mice indicated that apoB secretion was regulated post-transcriptionally. LDL receptor mRNA was increased in the apoB/BATless mice, indicating that the observed increase in apoB-lipoprotein secretion was not due to their decreased reuptake. Finally, mRNA levels of the large subunit of microsomal triglyceride transfer protein, a required component for very low density protein assembly, were not different between apoB and apoB/BATless mice. This rodent model should prove useful in exploring mechanisms underlying the regulation of apoB secretion in the context of insulin resistance.

Mechanisms of hepatic very low-density lipoprotein overproduction in insulin resistance

An important complication of insulin-resistant states, such as obesity and type 2 diabetes, is an atherogenic dyslipidemia profile characterized by hypertriglyceridemia, low plasma high-density lipoproteins (HDL) cholesterol and a small, dense low-density lipoprotein (LDL) particle profile. The physiological basis of this metabolic dyslipidemia appears to be hepatic overproduction of apoB-containing very low-density lipoprotein (VLDL) particles. This has focused attention on the mechanisms that regulate VLDL secretion in insulin-resistant states. Recent studies in animal models of insulin resistance, particularly the fructose-fed hamster, have enhanced our understanding of these mechanisms, and certain key factors have recently been identified that play important roles in hepatic insulin resistance and dysregulation of the VLDL secretory process. This review focuses on these recent developments as well as on the hypothesis that an interaction between enhanced flux of free fatty acids from peripheral tissues to liver, chronic up-regulation of de novo lipogenesis by hyperinsulinemia and attenuated insulin signaling in the liver may be critical to the VLDL overproduction state observed in insulin resistance. It should be noted that the focus of this review is on molecular mechanisms of the hypertriglyceridemic state associated with insulin resistance and not that observed in association with insulin deficiency (e.g., in streptozotocin-treated animals), which appears to have a different etiology and is related to a catabolic defect rather than secretory overproduction of triglyceride-rich lipoproteins.

Glucose-stimulated insulin secretion suppresses hepatic triglyceride-rich lipoprotein and apoB production

The current study assessed in vivo the effect of insulin on triglyceride-rich lipoprotein (TRL) production by rat liver. Hepatic triglyceride and apolipoprotein B (apoB) production were measured in anesthetized, fasted rats injected intravenously with Triton WR-1339 (400 mg/kg). After intravascular catabolism was blocked by detergent treatment, glucose (500 mg/kg) was injected to elicit insulin secretion, and serum triglyceride and apoB accumulation were monitored over the next 3 h. In glucose-injected rats, triglyceride secretion averaged 22.5 +/- 2.1 microg.ml(-1).min(-1), which was significantly less by 30% than that observed in saline-injected rats, which averaged 32.1 +/- 1.4 microg.ml(-1).min(-1). ApoB secretion was also significantly reduced by 66% in glucose-injected rats. ApoB immunoblotting indicated that both B100 and B48 production were significantly reduced after glucose injection. Results support the conclusion that insulin acts in vivo to suppress hepatic very low density lipoprotein (VLDL) triglyceride and apoB secretion and strengthen the concept of a regulatory role for insulin in VLDL metabolism postprandially.

A common apolipoprotein B signal peptide polymorphism modifies the relation between plasma non-esterified fatty acids and triglyceride concentration in men

Insulin and non-esterified fatty acids (NEFA) are important regulators of triglyceride metabolism. The relations between these compounds and the effect of a common 3 amino acid deletion in the apolipoprotein B (ApoB) signal peptide (SP) following an oral glucose challenge have been investigated. The frequency of the shorter SP-24 allele was 32% (95% C.I. 29.5-36.5) in 725 subjects undergoing an oral glucose tolerance test (OGTT). Fasting plasma triglyceride concentration was positively correlated with fasting plasma insulin concentration and negatively with the degree of plasma NEFA suppression following the glucose challenge. Linear regression analysis showed the relation between triglyceride concentration and NEFA suppression, but not the relation between triglyceride concentration and fasting insulin, to be altered by the SP polymorphism in men but not in women. The strength of the association was dependent on the number of SP-24 alleles, with SP-24 homozygotes showing the greatest dependence (men P=0.031, women P=0. 914). It was proposed that the complex regulation of very low density lipoprotein (VLDL) output by NEFA and by insulin may explain, at least in part, the conflicting reports concerning the presence of the ApoB SP polymorphism, fasting serum lipids and ischaemic heart disease (IHD).

Mechanisms of hepatic very low density lipoprotein overproduction in insulin resistance. Evidence for enhanced lipoprotein assembly, reduced intracellular ApoB degradation, and increased microsomal triglyceride transfer protein in a fructose-fed hamster model

A novel animal model of insulin resistance, the fructose-fed Syrian golden hamster, was employed to investigate the mechanisms mediating the overproduction of very low density lipoprotein (VLDL) in the insulin resistant state. Fructose feeding for a 2-week period induced significant hypertriglyceridemia and hyperinsulinemia, and the development of whole body insulin resistance was documented using the euglycemic-hyperinsulinemic clamp technique. In vivo Triton WR-1339 studies showed evidence of VLDL-apoB overproduction in the fructose-fed hamster. Fructose feeding induced a significant increase in cellular synthesis and secretion of total triglyceride (TG) as well as VLDL-TG by primary hamster hepatocytes. Increased TG secretion was accompanied by a 4.6-fold increase in VLDL-apoB secretion. Enhanced stability of nascent apoB in fructose-fed hepatocytes was evident in intact cells as well as in a permeabilized cell system. Analysis of newly formed lipoprotein particles in hepatic microsomes revealed significant differences in the pattern and density of lipoproteins, with hepatocytes derived from fructose-fed hamsters having higher levels of luminal lipoproteins at a density of VLDL versus controls. Immunoblot analysis of the intracellular mass of microsomal triglyceride transfer protein, a key enzyme involved in VLDL assembly, showed a striking 2.1-fold elevation in hepatocytes derived from fructose-fed versus control hamsters. Direct incubation of hamster hepatocytes with various concentrations of fructose failed to show any direct stimulation of its intracellular stability or extracellular secretion, further supporting the notion that the apoB overproduction in the fructose-fed hamster may be related to the fructose-induced insulin resistance in this animal model. In summary, hepatic VLDL-apoB overproduction in fructose-fed hamsters appears to result from increased intracellular stability of nascent apoB and an enhanced expression of MTP, which act to facilitate the assembly and secretion of apoB-containing lipoprotein particles.

Mobilisation of triacylglycerol stores

Triacylglycerol (TAG) is an energy dense substance which is stored by several body tissues, principally adipose tissue and the liver. Utilisation of stored TAG as an energy source requires its mobilisation from these depots and transfer into the blood plasma. The means by which TAG is mobilised differs in adipose tissue and liver although the regulation of lipid metabolism in each of these organs is interdependent and synchronised in an integrated manner. This review deals principally with the mechanism of hepatic TAG mobilisation since this is a rapidly expanding area of research and may have important implications for the regulation of plasma very-low-density lipoprotein metabolism. TAG mobilisation plays an important role in fuel selection in non-hepatic tissues such as cardiac muscle and pancreatic islets and these aspects are also reviewed briefly. Finally, studies of certain rare inherited disorders of neutral lipid storage and mobilisation may provide useful information about the normal enzymology of TAG mobilisation in healthy tissues.

Alleviation of fatty liver in dairy cows with 14-day intravenous infusions of glucagon

Twenty multiparous cows were fed additional concentrate during the final 30 d prepartum to cause susceptibility to fatty liver. From 14 to 42 d postpartum, all cows were subjected to a protocol to induce fatty liver and ketosis. To test glucagon as a treatment for fatty liver, either glucagon at 10 mg/d or excipient was infused via the jugular vein from 21 to 35 d postpartum. All cows had fatty liver at 14 d postpartum and became ketonemic and hypoglycemic during the induction of ketosis. Glucagon increased plasma glucose to 142% of that of controls throughout the 14-d treatment. The hypoinsulinemia present in cows with fatty liver was not affected by glucagon. Plasma beta-hydroxybutyrate and nonesterified fatty acids were decreased by glucagon. At 6 d postpartum, liver triacylglycerol averaged 12.9% of liver (wet weight basis). Glucagon had decreased triacylglycerol content of livers by 71% at d 35. Glycogen was 1.0% of the wet weight of livers at 6 d in milk, but it was decreased by glucagon to 0.5% at 2 d after glucagon began. Glycogen then increased in cows treated with glucagon until at 38 d in milk liver glycogen was 3.7% versus 1.6% in controls. Our results document that glucagon decreases the degree of fatty liver in early lactation dairy cows, which also decreases the incidence of ketosis after alleviation of fatty liver.

Glucose phosphorylation is essential for the turnover of neutral lipid and the second stage assembly of triacylglycerol-rich ApoB-containing lipoproteins in primary hepatocyte cultures

Primary hepatocytes cultured in a medium supplemented with amino acids and lipogenic substrates responded to increased extracellular glucose by increasing the secretion of VLDL apoB. This effect was accompanied by an increased secretion of VLDL triacylglycerol (TAG) derived from endogenous stores. Glucose also stimulated intracellular TAG mobilization via the TAG lipolysis/esterification cycle. All these effects were abolished in the presence of mannoheptulose (MH), an inhibitor of glucose phosphorylation. Glucose also gave rise to a modest (50% to 60%) increase in the incorporation of 35S methionine into newly synthesized apoB (P<0.05) and to a doubling of newly-synthesized apoB secretion as VLDL (P<0. 05). The magnitude of these effects was similar for apoB-48 and for apoB-100. MH inhibited apoB-48 and apoB-100 synthesis and VLDL secretion at all glucose concentrations. The effects of glucose and MH on the secretion of newly-synthesized apoB-48 or apoB-100 as small dense particles were less pronounced. Glucose had no effects on the posttranslational degradation of newly-synthesized apoB-100 or apoB-48. However, this process was significantly enhanced by MH. The results suggest that glucose stimulates TAG synthesis, turnover, and output as VLDL. These effects are associated with an increased VLDL output of apoB mediated mainly by an increase in the net synthesis of both apoB-48 and apoB-100. All these changes are prevented by interference with glucose phosphorylation. Output of small, dense, apoB-containing particles is relatively unaffected by the glucose and MH-induced changes in TAG synthesis and lipolysis, an observation which suggests that only the bulk lipid addition step of VLDL assembly is affected by changes in glucose metabolism.

Varying very low-density lipoprotein secretion of rat hepatocytes by altering cellular levels of calcium and the activity of protein kinase C

BACKGROUND

Calcium antagonists lower plasma levels of lipoproteins and suppress hepatic very low-density lipoprotein (VLDL) secretion. Similar effects have been observed with the calcium ionophore A23187. We studied further the effect of calcium on VLDL metabolism.

METHODS

Hepatocytes from male Wistar rats were isolated and cultured in the presence or absence of calcium-mobilizing hormones, or compounds that either stimulate or inhibit the activity of protein kinase C. Secreted VLDL (d < 1.006 g mL-1) was isolated by centrifugation (145,000 x g), and lipids and apolipoprotein B were analysed.

RESULTS

VLDL secretion reached maximum in hepatocytes cultured in medium containing calcium 0.8-2.4 mmolL-1. Depleting the cells of calcium by incubating in calcium-free medium or by treating the cells with the Ca(2+)-ATPase inhibitor thapsigargin (5 x 10-7 molL-1) suppressed lipid secretion to less than 15% of control, and this was accompanied by an increase in cellular levels of triacylglycerol. Calcium loading (medium calcium > 2.4 mmolL-1) suppressed both lipoprotein secretion and cellular levels of lipids, suggesting a reduced overall rate of lipid synthesis. At an extracellular calcium concentration of 0.8 mmolL-1, angiotensin II, vasopressin, endothelin-1 (10(-7) molL-1) or phenylephrine (10(-4) molL-1) suppressed VLDL secretion (maximum to 37% of control), and elevated medium calcium attenuated this effect. The protein kinase C inhibitor chelerythrine (5 x 10(-5) molL-1) and the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) (10(-6) molL-1), suppressed VLDL secretion to 18% and 60% of control, respectively, whereas the protein kinase C-inactive 4 alpha-PMA was without an effect. No effect on ketogenesis was observed by these compounds, indicating that suppressed lipid secretion was not due to an enhanced oxidation of lipids.

CONCLUSIONS

Hepatic VLDL secretion can be related to changes in hepatocyte levels of calcium and the activity of protein kinase C.

Moderate protein restriction during pregnancy modifies the regulation of triacylglycerol turnover and leads to dysregulation of insulin's anti-lipolytic action

Moderate protein restriction throughout pregnancy in the rat leads to relative hyperlipidaemia and blunted insulin responsiveness of lipid fuel supply, and impairs foetal growth. The present study examined the basis for these changes. Isocaloric 8% (vs 20%) protein diets were provided throughout pregnancy. Rats were sampled at 19-20 days of gestation. Protein restriction enhanced triacylglycerol (TAG) secretion rates (estimated using Triton WR 1339) 1.6-fold (P < 0.05) in the post-absorptive state. Insulin infusion (4.2 mU/kg per min) decreased plasma TAG concentrations by 33% (P < 0.05) and 48% (P < 0.05) in control (C) and protein-restricted (PR) pregnant groups, an effect associated with suppression of TAG secretion by 42% (P < 0.05) and 51% (P < 0.01) respectively, in the C and PR groups. Since TAG concentrations decline more rapidly, while TAG secretion is enhanced, TAG utilisation during hyperinsulinaemia is enhanced in the PR group. We evaluated whether these changes were associated with dysregulation of lipolysis using adipocytes from two abdominal depots (mesenteric and parametrial). Noradrenaline-stimulated glycerol release was enhanced in parametrial adipocytes (by 40%; P < 0.05) from PR pregnant rats. The anti-lipolytic action of insulin at low concentrations (< or = 15 microU/ml) was impaired by protein restriction (adipocytes from both depots). There was no evidence for altered intra-hepatic regulation of fatty acid (FA) disposal at the level of carnitine palmitoyltransferase. Our results demonstrate increased post-absorptive production of non-carbohydrate energy substrates (TAG and FA) as a consequence of mild protein restriction during pregnancy. These adaptations contribute to a homeostatic strategy to reduce the maternal requirement for gluconeogenesis from available amino acids, optimising the foetal protein supply. Protein restriction also enhances TAG turnover during hyperinsulinaemia. This effect is not a consequence of abnormal regulation of hepatic lipid metabolism by insulin.

Phosphoinositide 3-kinase activity is necessary for insulin-dependent inhibition of apolipoprotein B secretion by rat hepatocytes and localizes to the endoplasmic reticulum

Insulin inhibits apolipoprotein B (apoB) secretion by primary rat hepatocytes through activation of phosphoinositide 3-kinase (PI 3-K). Current studies demonstrate that the PI 3-K inhibitor wortmannin inhibits both basal and insulin-stimulated PI 3-K activities. Wortmannin and LY 294002, two structurally distinct PI 3-K inhibitors, prevent insulin-dependent inhibition of apoB secretion in a dose-dependent manner. To link PI 3-K activation to insulin action on apoB, we investigated whether insulin induced localization of activated PI 3-K to the endoplasmic reticulum (ER), where apoB biogenesis is initiated. Insulin action results in a significant redistribution of PI 3-K to a low density microsome (LDM) fraction containing apoB protein and apoB mRNA. Insulin stimulates a significant increase in PI 3-K activity associated with insulin receptor substrate-1 as well as an increase in insulin receptor substrate-1/PI 3-K mass in LDM. Subfractionation of LDM on sucrose density gradients shows that insulin significantly increases the amount of PI 3-K present in an ER fraction containing apoB. Insulin stimulates PI 3-K activity in smooth and rough microsomes isolated from rat hepatocytes, the latter of which contain rough ER as demonstrated by electron microscopy. Studies indicate that 1) PI 3-K activity is necessary for insulin-dependent inhibition of apoB secretion by rat hepatocytes; 2) insulin action leads to the activation and localization of PI 3-K in an ER fraction containing apoB; and 3) insulin stimulates PI 3-K activity in the rough ER.

Involvement of lipogenesis in the lower VLDL secretion induced by oligofructose in rats

Dietary supplementation with oligofructose (OFS; 100 g/kg), a non-digestible oligomer of beta-D-fructose, decreases serum triacylglycerols in serum and VLDL of rats. In order to investigate the role of hepatic metabolism in the hypolipidaemic effect of OFS, male Wistar rats were fed on a standard diet with or without 100 g Raftilose P95/kg as OFS source for 30 d. OFS feeding (1) significantly decreased triacylglycerol and phospholipid concentrations in both blood and liver, (2) increased the glycerol-3-phosphate liver content but decreased the hepatic activity of glycerol-3-phosphate acyltransferase (EC 2.3.1.15), suggesting a decrease in acylglycerol synthesis, (3) did not affect the blood non-esterified fatty acid concentrations, but (4) reduced by 54% the capacity of isolated hepatocytes to synthesize and secrete triacylglycerols from labelled acetate; the activity of fatty acid synthase, a key lipogenic enzyme was also significantly decreased. These findings suggest that OFS decreases serum triacylglycerols by reducing de novo fatty acid synthesis in the liver; the lower insulin level in the serum of OFS-fed rats could explain, at least partly, the metabolic effect induced by such non-digestible carbohydrates.

Decreased sensitivity of very-low-density lipoprotein secretion to the inhibitory effect of insulin in cultured hepatocytes from lactating rats

Hepatocytes were prepared from 10-11-day lactating rat dams and from lactating dams which had been weaned for periods of either 1-2 days or 7 days. Hepatocytes from each group were cultured for periods of up to 48 h in a chemically defined medium. Compared with those from the 7-day weaned animals, hepatocytes from the lactating rats were resistant to the inhibitory effects of insulin on the secretion of very-low-density lipoprotein (VLDL) triacylglycerol (TAG). These differences persisted for up to 48 h in culture. Hepatocytes from the 1-2 day weaned animals remained relatively insulin-resistant in this respect. Similar differences in the response to insulin were not observed for the secretion of VLDL apolipoprotein B. TAG production increased and ketogenesis decreased in the hepatocytes from the lactating compared with those from the 7-day weaned rats. Insensitivity of the liver to the normal effects of insulin on the secretion of VLDL TAG may arise from a need to maintain an adequate flux of hepatic lipids to the lactating mammary gland in order to meet the large demand for milk-fat production.

Chronic exogenous hyperinsulinaemia does not modify the acute inhibitory effect of insulin on the secretion of very-low-density lipoprotein triacylglycerol and apolipoprotein B in primary cultures of rat hepatocytes

Male Wistar rats were fitted with subcutaneous osmotic mini-pumps that delivered insulin at a constant rate of 0.20 i.u./h for 7 days. This treatment raised the plasma insulin concentration from 31 +/- 4 to 201 +/- 64 micro-i.u./ml. Hepatocytes prepared from the hyperinsulinaemic animals secreted very-low-density lipoprotein (VLDL) triacylglycerol (TAG) at a higher rate (172 +/- 21 microgram per 24 h per mg cell protein) than did those from sham-operated controls (109 +/- 12 microgram per 24 h per mg) (P<0.05). However, chronic exogenous hyperinsulinaemia had no stimulatory effect on the secretion of VLDL apolipoprotein B (apoB) in derived hepatocytes compared with those from the sham-operated controls (2.32 +/- 0.38 compared with 3.09 +/- 0.40 microgram per 24 h per mg). Hepatocytes from the hyperinsulinaemic rats thus secreted larger VLDL particles as evidenced by the increased TAG:apoB ratio (78.4 +/- 13.1 compared with 38.4 +/- 7.6; P<0.05). In hepatocytes from the hyperinsulinaemic rats a larger proportion of the newly synthesized TAG was secreted as VLDL. Hepatocytes from the hyperinsulinaemic and the sham-operated control animals were equally sensitive to the inhibitory effect of insulin added in vitro on the secretion of VLDL TAG. Insulin added in vitro to the culture medium of hepatocytes from hyperinsulinaemic animals significantly decreased the TAG:apoB ratio of the secreted VLDL. This change did not occur in hepatocytes from sham-operated rats. These results suggest that, in vivo, chronic hyperinsulinaemia is not in itself sufficient to desensitize the liver to the acute inhibitory effect of insulin on the secretion of VLDL.

Insulin-mediated inhibition of apolipoprotein B secretion requires an intracellular trafficking event and phosphatidylinositol 3-kinase activation: studies with brefeldin A and wortmannin in primary cultures of rat hepatocytes

Insulin inhibition of the secretion of apolipoprotein B (apo B) was studied in primary cultures of rat hepatocytes by using brefeldin A (BFA), an inhibitor of protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus, and by using the phosphatidylinositol 3-kinase (PI 3-K) inhibitor wortmannin. Incubation of hepatocytes with BFA (10 micrograms/ml) for 1 h inhibited the subsequent secretion of apo B, albumin and transferrin for up to 3 h. BFA treatment resulted in the time-dependent accumulation in cells of [14C]leucine-labelled proteins and apo B. Under conditions where insulin decreased total apo B (cell plus secreted), BFA blocked the insulin-dependent effect. These results suggest that export of apo B from the ER is a prerequisite for the observed insulin effect. Treatment of hepatocytes with wortmannin for 20 min abolished insulin inhibition of apo B secretion, suggesting that the insulin effect on the apo B pathway involves activation of PI 3-K. Enzyme inhibitor studies indicate that chymostatin and (+)-(2S,3S)-3-[(S)-methyl-1-(3-methylbutylcarbamoyl)-butylcarba moyl]-2- oxiranecarboxylate (E-64-c) partially block insulin effects on apo B compared with leupeptin, which had no discernible effect. The cell-permeable derivative of E-64-c, EST, and N-Ac-Leu-Leu-norleucinal (ALLN) were most effective in blocking insulin effects on apo B. These results suggest that insulin action on apo B in primary rat hepatocytes involves (1) vesicular movement of apo B from the ER; (2) activation of PI 3-K and (3) a cellular protease that is either a cysteine- or calcium-activated neutral protease.

Effects of hydration state on the synthesis and secretion of triacylglycerol by isolated rat hepatocytes. Implications for the actions of insulin and glucagon on hepatic secretion

The effects of hepatocyte volume on the secretion of triacylglycerol were studied in order to test the suggestion that increases in the portal concentrations of osmolyte amino acids and metal ions during the prandial/early-absorptive phase may be involved in mediating the acute changes in glycerolipid metabolism observed in vivo [Zammit (1995) Biochem Soc. Trans. 23, 506-511]. Incubation of isolated rat hepatocytes with hypo-osmotic medium or in the presence of glutamine (in the presence or absence of leucine), conditions which gave an increase in cell water content of between 8 and 27%, resulted in a decrease in the rate of [14C]triacylglycerol (TAG) secretion when [14C]palmitate was used as substrate. The inhibition was proportional to the increase in cell water content. At low exogenous palmitate concentration (0.05 mM), the inhibition of [14C]TAG secretion was accompanied by a marked shift in the incorporation of label from TAG to phospholipid. In the presence of 0.5 mM palmitate this effect was attenuated, and in the presence of 1 mM palmitate it was abolished. Increased cell volume associated with incubation of hepatocytes with glutamine (in the presence or absence of leucine) also resulted in a decrease in the fraction of newly labelled TAG that was secreted into the medium. Decreased cell volume, achieved by incubation of hepatocytes with hyperosmotic medium (sufficient to decrease cell water content by approx. 9%) decreased overall [14C]TAG secretion, but did not affect the amount of label that was incorporated into phospholipid as a fraction of that incorporated into total glycerolipids. Cell shrinkage, however, diminished the fraction of newly labelled [14C]TAG that was secreted. When intracellular TAG was prelabelled with [3H]glycerol, it was found that cell shrinkage markedly inhibited (preformed) [3H]TAG secretion, whereas cell swelling did not affect this route of TAG secretion. The data are discussed in terms of the possible action of changes in cell hydration at the different loci at which hepatocyte TAG secretion is controlled, with reference to previous observations that both insulin and glucagon are able to inhibit TAG secretion in cultured rat hepatocytes and HepG2 cells.

Solubilization and separation of two distinct carnitine acyltransferases from hepatic microsomes: characterization of the malonyl-CoA-sensitive enzyme

Conditions have been developed for the solubilization of hepatic microsomal carnitine acyltransferase activity in good yield, with excellent long-term stability and with retention of malonyl-CoA sensitivity. Solubilized microsomal carnitine acyltransferase activity can be separated into malonyl-CoA-sensitive and -insensitive activities either by gel filtration on Superdex 200 or by anion-exchange chromatography on Resource Q. On gel filtration the apparent molecular masses of the malonyl-CoA-sensitive and -insensitive activities are approx. 300 kDa and 60 kDa respectively. The malonyl-CoA-sensitive and -insensitive activities have different fatty-acyl-chain-length specificities and different stabilities in the detergent octyl glucoside. Together these findings indicate that the malonyl-CoA-sensitive and -insensitive activities are due to different enzymes. The malonyl-CoA sensitivity of the inhibitable enzyme is markedly increased on reconstitution into soybean L-alpha-lecithin liposomes, demonstrating that phospholipids play a crucial role in the inhibition by this metabolite. Evidence is also provided that the malonyl-CoA-sensitive microsomal carnitine acyltransferase is a different enzyme from the malonyl-CoA-sensitive carnitine palmitoyltransferase found in the mitochondrial outer membrane. The possible physiological role of the two microsomal acyltransferases is discussed.

VLDL output by hepatocytes from obese Zucker rats is resistant to the inhibitory effect of insulin

The effects of insulin (0-780 nM) on the secretion of very-low-density lipoprotein (VLDL) apolipoprotein B (apoB) and triacylglycerol (TAG) in hepatocytes from obese Zucker rats and from their lean littermates were studied over a total period of 48 h in chemically defined culture medium. Cells from the obese Zucker rats initially secreted more TAG than those from the lean animals. Cells from the former were resistant to the inhibitory effect of insulin on the secretion of TAG. These changes were accompanied by an increased rate of TAG synthesis. Although the hepatocytes from the obese animals initially secreted less apoB than those from the lean, apoB output from the former could not be suppressed by insulin. Prolonging the length of the culture period resulted in the acquisition of sensitivity to the inhibitory effect of insulin in the hepatocytes from the obese rats. This occurred more rapidly for the secretion of TAG than of apoB. Under these conditions, the initial difference in the rate of TAG synthesis in the hepatocytes from the obese and from the lean animals was also abolished.

Interaction between free fatty acids and insulin in the acute control of very low density lipoprotein production in humans

Changes in VLDL triglyceride and VLDL apo B production were determined semiquantitatively in healthy young men by examining the effect of altering plasma insulin and/or FFA levels on the change in the slopes of the specific activity of VLDL [3H]triglyceride glycerol or the 131I-VLDL apo B versus time curves. In one study (n = 8) insulin was infused for 5 h using the euglycemic hyperinsulinemic clamp technique. Plasma FFA levels declined by approximately 80% (0.52 +/- 0.01 to 0.11 +/- 0.02 mmol/liter), VLDL triglyceride production decreased by 66.7 +/- 4.2% (P = 0.0001) and VLDL apo B production decreased by 51.7 +/- 10.6% (P = 0.003). In a second study (n = 8) heparin and Intralipid (Baxter Corp., Toronto, Canada) were infused with insulin to prevent the insulin-mediated fall in plasma FFA levels. Plasma FFA increased approximately twofold (0.43 +/- 0.05 to 0.82 + 0.13 mmol/liter), VLDL triglyceride production decreased to a lesser extent than with insulin alone (P = 0.006) (-31.8 +/- 9.5%, decrease from baseline P = 0.03) and VLDL apo B production did not decrease significantly (-6.3 +/- 13.6%, P = NS). In a third study (n = 8) when heparin and Intralipid were infused without insulin, FFA levels rose approximately twofold (0.53 +/- 0.04 to 0.85 +/- 0.1 mmol/liter), VLDL triglyceride production increased by 180.1 +/- 45.7% (P = 0.008) and VLDL apo B production increased by 94.2 +/- 28.7% (P = 0.05). We confirm our previous observation that acute hyperinsulinemia suppresses VLDL triglyceride and VLDL apo B production in healthy humans. In addition, we have demonstrated that elevation of plasma FFA levels acutely stimulates VLDL production in vivo in healthy young males. Elevating plasma FFA during hyperinsulinemia attenuates but does not completely abolish the suppressive effect of insulin on VLDL production, at least with respect to VLDL triglycerides. Therefore, in normal individuals the acute inhibition of VLDL production by insulin in vivo is only partly due to the suppression of plasma FFA, and may also be due to an FFA-independent process.

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

Liver regrowth and apolipoprotein B secretion by rat hepatocytes following partial hepatectomy

Apolipoprotein (apo) B is an essential component for the assembly and secretion of lipoproteins. The current report examines apo B production using primary cultures of hepatocytes derived from rats 3 to 21 days after partial hepatectomy (PH) to determine the effects of liver regrowth on apo B. Studies indicate that hepatocytes stimulated by PH have a two-thirds reduction in net apo B production 3 to 7 days after surgery, which coincides with the period of maximum rate of liver regrowth. Both higher (apo BH)- and lower-molecular-weight (apo BL) apo B are synthesized and secreted after PH, indicating the presence of edited apo B mRNA in hepatocytes. Hepatocytes derived from PH rats are more sensitive to insulin inhibition of apo B secretion compared with controls, suggesting an enhanced effect of insulin on newly replicated hepatocytes. Epidermal growth factor (EGF), a key regulator of liver regrowth following PH, potentiates the inhibitory action of insulin on apo B secretion in control hepatocytes and those derived from rats 2 to 3 weeks after PH. However, the potentiating effect of EGF on insulin inhibition of apo B is not discernible in hepatocytes 3 to 7 days after PH. The short-term in vitro hormonal effects occurring even with decreased apo B production suggest that this pathway remains available following PH to balance lipoprotein secretion with lipid and energy requirements necessary for liver regeneration.

Comparison of the effect of hyperinsulinemia on acyl-CoA:cholesterol acyltransferase activity in the liver and intestine of the rat

Recent studies have suggested that cholesteryl ester synthesis plays a critical role in the assembly of VLDL apo B and triacylglycerol in the liver. Chronic hyperinsulinemia is associated with increased TG production and since cholesteryl ester synthesis depends on acyl CoA:cholesterol acyltransferase (ACAT), we investigated the possibility that chronic hyperinsulinemia might increase ACAT activity. We also measured ACAT activity in the intestinal mucosa, where it has been suggested to play a role in induction of diabetes-associated hypercholesterolemia. Chronically hyperinsulinemic rats were produced by injecting insulin (2 weeks, 6U/day). To prevent profound hypoglycemia, these rats were given 10% sucrose in place of drinking water. Acute hyperinsulinemia was produced by injecting a single dose of 0.5 U insulin ip. Chronic hyperinsulinemia led to a significant increase in free cholesterol, cholesteryl esters, triacylglycerols and phospholipids in the whole liver (27%, P < 0.05; 60%, P < 0.05; 70%, P < 0.01; 37%, P < 0.01, respectively) and an increase in hepatic microsomal triacylglycerol (P < 0.05). In contrast, the microsomal lipids of the intestinal mucosa decreased significantly. In chronically hyperinsulinemic rats there was no change in hepatic ACAT, while ACAT in the intestine actually decreased (26%-50%, P < 0.01).

CONCLUSIONS

since the effect of chronic hyperinsulinemia on hepatic ACAT did not parallel that seen previously on VLDL secretion, the effect of chronic hyperinsulinemia on VLDL production cannot be explained by its effect on hepatic ACAT. However, chronic hyperinsulinemia was associated with reduced ACAT activity in the intestine and this might result in decreased lipoprotein production in the hyperinsulinemic intestine.

Regulation of VLDL secretion in primary culture of rat hepatocytes: involvement of cAMP and cAMP-dependent protein kinases

When hepatocytes were cultured for 24 h in the presence of forskolin (10(-4) mol l-1) or isobutylmethylxanthine (IBMX, 10(-3) mol l-1), the intracellular cAMP concentration peaked (320-380 pmol mg-1 protein) after 10-20 min of culture. This increase was accompanied by a decrease in the secretion of triacylglycerol, cholesterol and apoprotein B associated with VLDL. After 4 h cAMP levels had returned almost to basal values but the inhibition of VLDL secretion persisted. There was a small intracellular accumulation of triacylglycerol but not of apoprotein B. Addition of forskolin and IBMX together led to a further increase in intracellular cAMP and a further suppression of VLDL output. Similar effects on the secretion of VLDL were also observed after addition of Bt2cAMP. Exposure of cell cultures to glucagon (10(-7) mol l-1) for only 10 min raised cellular cAMP levels to > 200 pmol mg-1 protein, and suppressed VLDL secretion during the next 24 h to < 40% of control. All of the substances tested inhibited de novo synthesis of fatty acids but had little or no effect on cholesterol synthesis and did not inhibit oleate esterification to triacylglycerol. The cAMP-dependent protein kinase antagonist Rp-cAMPS prevented suppression of VLDL triacylglycerol secretion induced by glucagon (10(-7) mol l-1) and abolished glucagon-induced ketogenesis. Rp-cAMPS also inhibited Bt2cAMP (7.5 x 10(-6) mol l-1)-induced suppression of VLDL secretion and enhancement of ketogenesis. It is concluded that rat hepatic VLDL metabolism can be regulated by cAMP and cAMP-dependent protein kinases, and that the initial transient rise in cellular cAMP levels induced by glucagon is sufficient to maintain a long-term inhibitory effect on assembly and secretion of VLDL.

Restoration in vitro of normal rates of very-low-density lipoprotein triacylglycerol and apoprotein B secretion in hepatocyte cultures from diabetic rats

Hepatocytes derived from diabetic rats were cultured in serum-free Waymouth's medium containing various supplements, after an initial 4 h period during which the cells were allowed to attach to the culture dish in the presence of foetal-bovine serum (10%). After removal of serum, these cells secreted much less very-low-density lipoprotein (VLDL) apoprotein B (apoB) and triacylglycerol than those derived from normal rats when cultured for 24 h in the basal medium. Inclusion of oleate (0.75 mM) in the medium initially increased the output of apoB and triacylglycerol, but the rates remained lower than those observed in normal hepatocytes and declined to zero after 72 h. This time-dependent decline in VLDL output was prevented by addition of dexamethasone to the oleate-containing medium. Levels of apoB and triacylglycerol output characteristic of normal hepatocytes could only be completely restored, however, by further addition of a mixture of lipogenic substrates (lactate plus pyruvate) to the medium. Restoration of normal levels of VLDL secretion in diabetic hepatocytes in vitro by this means was accompanied by a normal inhibitory response of apoB and triacylglycerol output to short-term (24 h) treatment with insulin or glucagon. Exposure of the cells to insulin for 72 h enhanced the secretion of VLDL, whereas treatment with glucagon for the same period potentiated the original inhibitory effect. The defective secretion of VLDL apoB observed when diabetic hepatocytes were cultured in the basal medium for 24 h could also be rectified by inclusion of a mixture of oleate (0.75 mM), lactate (10 mM), pyruvate (1 mM), dexamethasone (1 microM) and insulin (78 nM) in the medium during the 4 h attachment period in the presence of serum. Under these conditions, the increase in the secretory response of triacylglycerol was not so pronounced.