The effect of diet on tumor necrosis factor stimulation of hepatic lipogenesis

The Complex Interplay between Lipids, Immune System and Interleukins in Cardio-Metabolic Diseases

Lipids and inflammation regulate each other. Early studies on this topic focused on the systemic effects that the acute inflammatory response—and interleukins—had on lipid metabolism. Today, in the era of the obesity epidemic, whose primary complications are cardio-metabolic diseases, attention has moved to the effects that the nutritional environment and lipid derangements have on peripheral tissues, where lipotoxicity leads to organ damage through an imbalance of chronic inflammatory responses. After an overview of the effects that acute inflammation has on the systemic lipid metabolism, this review will describe the lipid-induced immune responses that take place in peripheral tissues and lead to chronic cardio-metabolic diseases. Moreover, the anti-inflammatory effects of lipid lowering drugs, as well as the possibility of using anti-inflammatory agents against cardio-metabolic diseases, will be discussed.

Chemoprevention of nonalcoholic fatty liver disease by dietary natural compounds

Nonalcoholic fatty liver disease (NAFLD) refers to a wide spectrum of liver disease that is not from excess alcohol consumption, but is often associated with obesity, type 2 diabetes, and metabolic syndrome. NAFLD pathogenesis is complicated and involves oxidative stress, lipotoxicity, mitochondrial damage, insulin resistance, inflammation, and excessive dietary fat intake, which increase hepatic lipid influx and de novo lipogenesis and impair insulin signaling, thus promoting hepatic triglyceride accumulation and ultimately NAFLD. Overproduction of proinflammatory adipokines from adipose tissue also affects hepatic metabolic function. Current NAFLD therapies are limited; thus, much attention has been focused on identification of potential dietary substances from fruits, vegetables, and edible plants to provide a new strategy for NAFLD treatment. Dietary natural compounds, such as carotenoids, omega-3-PUFAs, flavonoids, isothiocyanates, terpenoids, curcumin, and resveratrol, act through a variety of mechanisms to prevent and improve NAFLD. Here, we summarize and briefly discuss the currently known targets and signaling pathways as well as the role of dietary natural compounds that interfere with NAFLD pathogenesis.

Potential role of tumor necrosis factor-α in downregulating sex hormone-binding globulin

Low plasma sex hormone-binding globulin (SHBG) levels are associated with obesity and predict the development of type 2 diabetes. The reason why obese individuals have low circulating SHBG has been attributed to hyperinsulinemia, but no mechanistic evidence has been described. The aim of the current study is to explore whether tumor necrosis factor-α (TNF-α) rather than insulin could be the main factor accounting for low SHBG levels in obesity. We performed in vitro and in vivo studies using human HepG2 cells and human SHBG transgenic mice. In addition, a cross-sectional study to explore the relationship between TNF-α and SHBG in obese patients and an interventional study to examine the effect of insulin administration on circulating SHBG in type 2 diabetic patients were performed. We provide evidence that TNF-α, but not insulin, is the main factor by which SHBG is reduced in obesity. Plasma SHBG was significantly increased rather than decreased after insulin treatment in diabetic patients. TNF-α-induced reduction of SHBG expression was mediated by downregulating HNF4A. Finally, a negative and independent correlation was found between plasma TNF-α receptor 1 and SHBG levels in obese patients. Our results suggest that TNF-α plays an important role downregulating SHBG in chronic low-grade inflammatory diseases such as obesity and type 2 diabetes.

Role of cytokines in the pathogenesis of non-alcoholic Fatty liver disease

A number of factors are linked with non-alcoholic fatty liver diseases (NAFLD), a condition that ranges from clinically benign fatty liver to its more severe form, non alcoholic steatohepatitis (NASH). In this study, we evaluated the role of cytokines secreted from adipose tissue in the pathogenesis and progression of NAFLD. We also compared anthropometric profile, lipid profile and insulin resistance data in 105 NAFLD patients with 77 normal subjects. These subjects showed a normal serum albumin level, prothrombin time and renal function but elevated aminotransferases. Predisposing factors were diabetes mellitus (35%), overweight (56%) and hyperlipidemia (44%). Insulin resistance (IR), determined by homeostasis model assessment (HOMA) was confirmed in 70% patients with NAFLD and 42% patients fulfilled the minimum criteria for insulin resistance syndrome (IRS). NAFLD patients showed elevated levels of pro-inflammatory cytokines tumor necrosis factor (TNF)-α, and interleukin (IL)-6, while anti-inflammatory cytokines IL-4 level decreased and IL-10 level remain unchanged; however, TGF-β1 level elevated significantly compared to normal subjects. While insulin level and HOMA-IR both were significantly positively correlated with BMI, waist-to-hip ratio, total cholesterol, VLDL-cholesterol, triglyceride and TGF-β1; glucose, IL-6 and TNF-α levels were significantly positively correlated with HOMA-IR only. In conclusion, pro-inflammatory cytokines play an important link between metabolic and liver disorders in the fat accumulation, and thereby cause IR, inflammation and liver fibrosis.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12291-011-0121-7) contains supplementary material, which is available to authorized users.

Complexo miointimal das carótidas comum e interna em portadores de esquistossomose mansônica hepatoesplênica

OBJETIVO: Avaliar a espessura do complexo miointimal (IMT) das carótidas comum e interna, em portadores de esquistossomose hepatoesplênica (EHE) não tratados cirurgicamente, já submetidos a cirurgia para descompressão do sistema porta por esplenectomia e ligadura da veia gástrica esquerda, e comparar com volutários de condições sócio-econômico-ambientais similares, não portadores de esquistossomose. MÉTODOS: Utilizando aparelho de ultra-som Doppler de 7,5MHz foram mensurados os IMT de três grupos de voluntários, de ambos os gêneros, com idades que variaram de 20 a 60 anos, sendo avaliados os IMT máximos, IMT médios, IMT mínimos e seus desvios-padrão, das carótidas comuns e internas e feitas as comparações entre os grupos e suas associações com fatores de risco: idade, hipertensão arterial e tabagismo. RESULTADOS: Não houve diferença significante na média dos IMT, entre os lados direito e esquerdo e nem entre os grupos. Nos pacientes tratados cirurgicamente, assim como nos indivíduos-controle confirmou-se a associação, já conhecida, com os fatores de risco para aterosclerose (idade, hipertensão arterial e tabagismo). Contudo, não se observou este comportamento nos pacientes não operados. CONCLUSÃO: A EHE sem tratamento cirúrgico parece conferir "alguma proteção" contra a aterogênese em seres humanos; todavia, os achados não dão suporte definitivo a esta hipótese.

Dietary palatinose and oleic acid ameliorate disorders of glucose and lipid metabolism in Zucker fatty rats

Excessive dietary intake of carbohydrates and fats has been linked to the development of obesity. However, the mechanism by which these dietary factors interact to bring about metabolic changes has not been elucidated. We examined the combined effects of different types of dietary carbohydrates and fats on the etiology of obesity and its complications in the Zucker fatty (fa/fa) rat, a model of obesity. Specifically, these rats were fed an isocaloric diet containing various combinations of carbohydrates [palatinose (P), an insulin-sparing sucrose analogue, and sucrose (S)] and fatty acids [oleic acid (O) and linoleic acid (L)]. After 8 wk, palatinose feeding (PO and PL) led to significant reductions in visceral fat mass, adipocyte cell size, hyperglycemia, and hyperlipidemia compared with sucrose feeding (SO and SL); pancreatic islet hypertrophy was also prevented by palatinose feeding. Linoleic-acid-fed rats (PL and SL) exhibited reduced insulin-immunoreactive staining of the pancreatic islets, enhanced macrophage infiltration in adipose tissue, and an elevated plasma tumor necrosis factor-alpha concentration when compared with oleic-acid-fed rats (PO and SO). Furthermore, sucrose and linoleic acid synergistically increased the expression of genes involved in hepatic gluconeogenesis and lipogenesis [sterol regulatory-element binding protein (SREBP)-1c and SREBP-2]. In conclusion, a diet containing palatinose and oleic acid may prevent diet-induced metabolic abnormalities. The combination of palatinose and oleic acid holds promise for a new approach to preventing and treating obesity and its complications.

Early diet-induced non-alcoholic steatohepatitis in APOE2 knock-in mice and its prevention by fibrates

BACKGROUND/AIMS

The molecular mechanisms leading to Non-Alcoholic Steatohepatitis (NASH) are not fully understood. In mice, NASH can be inhibited by fenofibrate, a synthetic agonist for the nuclear receptor peroxisome proliferator activated receptor alpha, which regulates hepatic triglyceride metabolism. This study aimed to elucidate the relation between steatosis and inflammation in NASH in a human-like hyperlipidemic mouse model.

METHODS

Liver phenotype and gene expression were assessed in APOE2 knock-in mice that were fed a western-type high fat diet with or without co-administration of fenofibrate.

RESULTS

In response to a western diet, APOE2 knock-in mice developed NASH characterized by steatosis and inflammation. Strikingly, macrophage accumulation in the liver preceded the steatosis during progression of the disease. This phenotype was in line with gene expression patterns, which showed regulation of two major groups of genes, i.e. inflammatory and lipid genes. Fenofibrate treatment decreased hepatic macrophage accumulation and abolished steatosis. Moreover, a marked reduction in the expression of inflammatory genes occurred immediately after fenofibrate treatment.

CONCLUSIONS

These data indicate that inflammation might play an instrumental role during the development of NASH in this mouse model. Inhibition of NASH by fenofibrate may be due, at least in part, to its inhibitory effect on pro-inflammatory genes.

Effects of infection and inflammation on lipid and lipoprotein metabolism: mechanisms and consequences to the host

Infection and inflammation induce the acute-phase response (APR), leading to multiple alterations in lipid and lipoprotein metabolism. Plasma triglyceride levels increase from increased VLDL secretion as a result of adipose tissue lipolysis, increased de novo hepatic fatty acid synthesis, and suppression of fatty acid oxidation. With more severe infection, VLDL clearance decreases secondary to decreased lipoprotein lipase and apolipoprotein E in VLDL. In rodents, hypercholesterolemia occurs attributable to increased hepatic cholesterol synthesis and decreased LDL clearance, conversion of cholesterol to bile acids, and secretion of cholesterol into the bile. Marked alterations in proteins important in HDL metabolism lead to decreased reverse cholesterol transport and increased cholesterol delivery to immune cells. Oxidation of LDL and VLDL increases, whereas HDL becomes a proinflammatory molecule. Lipoproteins become enriched in ceramide, glucosylceramide, and sphingomyelin, enhancing uptake by macrophages. Thus, many of the changes in lipoproteins are proatherogenic. The molecular mechanisms underlying the decrease in many of the proteins during the APR involve coordinated decreases in several nuclear hormone receptors, including peroxisome proliferator-activated receptor, liver X receptor, farnesoid X receptor, and retinoid X receptor. APR-induced alterations initially protect the host from the harmful effects of bacteria, viruses, and parasites. However, if prolonged, these changes in the structure and function of lipoproteins will contribute to atherogenesis.

A single intravenous dose of endotoxin rapidly alters serum lipoproteins and lipid transfer proteins in normal volunteers

Endotoxemia is associated with rapid and marked declines in serum levels of LDL and HDL by unknown mechanisms. Six normal volunteers received a single, small intravenous (iv) dose of endotoxin (Escherichia coli 0113, 2 ng/kg) or saline in a random order, cross-over design. After endotoxin treatment, volunteers had mild, transient flu-like symptoms and markedly increased serum levels of tumor necrosis factor and its soluble receptors, interleukin-6, cortisol, serum amyloid A, and C-reactive protein. Triglyceride (TG), VLDL-TG, and nonesterified fatty acid increased (peak at 3-4 h), then TG declined (nadir at 9 h), and then cholesterol, LDL cholesterol, apolipoprotein B (apoB), and phospholipid declined (nadirs at 12-24 h). HDL cholesterol and apoA-I levels were not affected, but half of the decrease in phospholipid was HDL phospholipid. Lipopolysaccharide binding protein (LBP) rose 3-fold (peak at 12 h), with smaller and later decreases in the activities of phospholipid transfer protein and cholesteryl ester transfer protein. In conclusion, a decline in LDL was rapidly induced in normal volunteers with a single iv dose of endotoxin. The selective loss of phospholipid from HDL may have been mediated by LBP and, after more intense or prolonged inflammation, could result in increased HDL clearance and reduced HDL levels.

Implications of circadian gene expression in kidney, liver and the effects of fasting on pharmacogenomic studies

Pharmacogenomics offers the potential to define metabolic pathways and to provide increased knowledge of drug actions. We studied relative levels of gene expression in the rat using a microarray with 8448 rat UniGenes (1928 known genes, 6520 unknown ESTs) in the liver and kidney as a function of time of day and then of feeding regime, which are common variables in preclinical pharmacogenomic studies. We identified 597 genes, including several key metabolic pathways, whose relative expression levels are significantly affected by time of day: expression of some was further modified by feeding state. These would have sparked interest in a pharmacogenomic study. Our study demonstrates that two common variables in pharmacogenomic studies can have dramatic effects on gene expression. This study provides investigators with baseline information for both kidney and liver with respect to 'normal' changes in gene expression influenced by time of day and feeding state. It also identifies 18 new genes that should be investigated for a role in circadian rhythms in peripheral tissues.

Cytokine-mediated inhibition of ketogenesis is unrelated to nitric oxide or protein synthesis

Cytokines play an important role in the lipid disturbances commonly associated with sepsis. Ketogenesis is inhibited during sepsis, and tumor necrosis factor alpha (TNF alpha) and interleukin-6 (IL-6) have been suggested to mediate this impairment, irrespective of the ketogenic substrate (fatty acid or branched chain ketoacid). However, the underlying mechanism of cytokine action is still unknown. First we investigated the possible role of the induction of nitric oxide (NO) synthesis, using rat hepatocyte monolayers. Hepatocytes were incubated for 6 h, with either alpha -ketoisocaproate (KIC) (1 mM) or oleic acid (0.5 mM) in the presence or absence of TNF alpha (25 microg/L) and IL-6 (15 microg/L). In some experiments, cells were incubated with NO synthase (NOS) inhibitors. The ketone body (beta -hydroxybutyrate and acetoacetate) production and nitrite production were measured in the incubation medium. Our results indicated no involvement of nitric oxide in the inhibitory action of cytokines on ketogenesis. Secondly, we showed that cycloheximide (10(-4)M) did not counteract the cytokine-mediated ketogenesis decrease; hence, the effects of cytokines on ketogenesis are not protein synthesis-dependent. The cytokine-mediated inhibition of ketogenesis is therefore unrelated to either NO production or protein synthesis.

TNF-alpha and IL-6 synergistically inhibit ketogenesis from fatty acids and alpha-ketoisocaproate in isolated rat hepatocytes

BACKGROUND

During sepsis, lipid metabolism is shunted toward triacylglycerol synthesis and hepatic lipogenesis. A decrease in ketogenesis from free fatty acids also is observed, probably mediated by cytokines involved in host response to infection. Whether such an inhibition of ketogenesis occurs with other ketone body precursors such as ketoacids is not known. The aim of this study was to determine the effects of tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) on hepatic ketone body production from octanoic acid, a medium-chain fatty acid, and from alpha-ketoisocaproate (KIC), the ketoanalogue of leucine.

METHODS

The experiments were conducted in cultured hepatocytes isolated from 24-hour-fasted Sprague-Dawley rats. Hepatocyte monolayers were incubated for 6 hours, with either KIC or octanoic acid (1 mmol/L), in the presence of glucagon and TNF-alpha (25 micro/L) IL-6 (15 microg/L) and/or IL-6. Acetoacetate, beta-hydroxybutyrate, and free fatty acids were determined in culture medium by enzymatic methods and KIC was measured by high-performance liquid chromatography.

RESULTS

KIC and octanoic acid uptake by hepatocytes was 79% and 92%, respectively, over 6 hours, and cytokines had no influence. However, TNF-alpha and IL-6 caused inhibition of ketogenesis from alpha-ketoisocaproate (5.6% +/- 2.3% and 4.4% +/- 3.0%, respectively), and from octanoic acid (7.9% +/- 2.9%, 5.7% +/- 3.2%, respectively). In addition, when the two cytokines were present together in the culture medium, the inhibition was enhanced (inhibition of ketogenesis from KIC: 14.0% +/- 4.8%; from octanoic acid: 11.6% +/- 3.4%).

CONCLUSIONS

In our experimental conditions, cytokines mediate an inhibition of ketogenesis; this process could be explained by a direct effect of cytokines on metabolic pathways of octanoic acid and KIC oran indirect effect by modification of the mitochondrial redox state.

Cytokines decrease apolipoprotein accumulation in medium from Hep G2 cells

Cytokines, important biochemical mediators of inflammation, cause a rapid fall in the plasma concentration of cholesterol in vivo. One mechanism by which cytokines may cause acquired hypocholesterolemia is by decreasing the hepatic synthesis and secretion of apolipoproteins. To test this hypothesis, we incubated Hep G2 cells with human recombinant tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6. Each of the cytokines resulted in a dose-related reduction in the concentrations of apolipoprotein (apo) A-I, apoB, and lecithin:cholesterol acyltransferase (LCAT) activity in the medium after 24 hours of incubation. The effect of cytokines on apolipoprotein accumulation was not affected by preincubation of Hep G2 cells with fatty acids. Cytokines decreased the concentration of cellular apoA-I mRNA in a dose-related fashion but did not affect cellular concentrations of apoB mRNA. The concentrations of triglyceride and cholesterol were also reduced in the medium of cells incubated with cytokines. Total cell sterol synthesis rates were calculated by [14C]acetate incorporation. Cells incubated with interleukin-6 had a 31% increase in sterol synthesis rate but a 41% decrease in sterol secretion. These data suggest that these cytokines can decrease the hepatic synthesis and/or secretion of apolipoproteins and that this may explain, in part, the acquired hypocholesterolemia seen during acute and chronic inflammation.

Changes in lipid profile observed in children over the course of infectious disease

Lipid profile was evaluated prospectively in 23 consecutive children, aged 3.2-14.9 years, admitted to the hospital with a febrile illness (pneumonia, upper respiratory tract infection, diarrhea, pyelonephritis, mononucleosis, appendicitis). The degree of dyslipidemia associated with fever was assessed using each child as his/her own control and by comparison with 93 non-febrile children who had no evidence of fever during the past six months. Total cholesterol decreased during the symptomatic phase of the disease. The magnitude and duration of its decrease appeared to be related to the degree and duration of fever. Low HDL-cholesterol and hypertriglyceridemia were observed during the late stage of the febrile disease and were still detected in the convalescent phase. This study suggests that in children, transient and sometimes prolonged lipid changes may occur in association with an infectious febrile disease. This effect is important for defining the appropriate timing for screening for dyslipidemias.

Chronic intraperitoneal infusion of low doses of tumor necrosis factor alpha in rats induces a reduction in plasma triglyceride levels

Single and repeated bolus injections of tumor necrosis factor alpha (TNF) in laboratory animals have been reported to result in hypertriglyceridaemia, suggesting that TNF is a mediator of hypertriglyceridaemia occurring during infection. However, as during infection production of TNF is probably chronically elevated, we determined the effects of continuous infusion of low doses of TNF on plasma levels of triglycerides and cholesterol. Male rats, bearing a venous catheter to allow repeated blood sampling, were intraperitoneally equipped with osmotic minipumps which continuously delivered TNF or saline for 7 days. Infusion of rats with doses of TNF as low as 4.0 and 8.0 micrograms/24 h resulted in significant decreases in plasma levels of triglycerides as compared with those after saline infusion. Although plasma triglyceride concentrations were persistently lower in TNF than in saline animals throughout the study period, the differences were most prominent during the first days and reached statistical significance at day 1, 3, 4 and 5 and of the 4.0 micrograms experiment and on day 1, 2 and 3 of the 8.0 micrograms experiment. This suppression of plasma triglyceride concentrations was not accompanied by changes in plasma cholesterol levels. No effects of chronic TNF treatment on food intake, body weight change and rectal temperature of the animals were observed. These findings indicate that chronic infusion of low doses of TNF induces hypotriglyceridaemia in rats. The role of TNF as a factor in mediating hypertriglyceridaemia during infectious diseases needs to be reconsidered.

Inhibition of hepatic ketogenesis by tumor necrosis factor-alpha in rats

Tumor necrosis factor-alpha (TNF-alpha) stimulates hepatic lipogenesis. Therefore, it could play a role in the control of ketogenesis. To test this hypothesis, we measured simultaneously free fatty acids (FFA; [1-13C]palmitate) and ketone body (KB; [3,4-13C2]acetoacetate) kinetics, before and after intraperitoneal injection of saline or TNF-alpha, in postabsorptive rats or rats starved for 24 h. In both groups of rats, TNF-alpha injection did not modify insulinemia and induced a moderate increase of FFA concentrations and appearance rates (P < 0.05). Despite increased FFA availability, ketogenesis was impaired after TNF-alpha injection, as shown by lower KB concentrations and appearance rates; this effect was more important in postabsorptive than in starved rats. The percentage of FFA flux used for ketogenesis was decreased by TNF-alpha in the postabsorptive group (P < 0.05) and starved (P < 0.05) rats. In both groups, maximal liver acetyl-coenzyme A carboxylase activity and estimated phosphorylation state were not modified by TNF-alpha injection, but hepatic concentrations of citrate were increased (P < 0.05). This increased citrate level could be related to a mobilization of glucose stored as glycogen since liver glycogen was decreased by TNF-alpha injection (P < 0.05). In conclusion, TNF-alpha injection in rats decreased hepatic ketogenesis. This action could be related to an increased mobilization and utilization of carbohydrate stores.

Differential metabolic responses to tumor necrosis factor with increase in age

Previous studies have shown varied responses to the effects of tumor necrosis factor (TNF) on glucose and lipid metabolism. To elucidate possible causes for this variation, the present study compared sequential changes in plasma glucose, lactic acid, triglyceride (TG), free fatty acids (FFA), and plasma insulin levels in 1.5- and 16-month-old, normal, fed, male rats, 1 to 6 hours after different doses of intravenous (IV) TNF. In addition, assessment was made of TNF injected intraperitoneally (IP) in precannulated and intact young (1.5 months) rats and of the dose-response (0.25 to 50 micrograms/100 g rat) and the sensitivity to insulin in intact rats. Finally, the metabolic responses and changes in serum insulin and corticosterone concentration after IP TNF were compared in 1.5-, 5-, and 16-month-old rats. Data show that metabolic responses vary with increase in age and experimental conditions. Dose-dependent decreases in plasma glucose (1.4 to 3.9 mumol/mL) and elevations in lactic acid (0.8 to 3.0 mumol/mL) were greater in 1.5- versus 16-month-old rats, were delayed in cannulated rats, and were preceded by hyperglycemia following larger IV doses. Plasma TG levels were elevated after TNF in all groups except precannulated rats, and also showed differences with age. In young rats, the elevation in TG peaked 2 hours after IP injection with return to baseline and was preceded by an elevation in FFA levels. In older rats, which were hypertriglyceridemic at base line, the elevation in TG by TNF occurred by suppressing the decrease in TG of controls, was not accompanied by an increase in FFA levels, was sustained for 5 hours, and was of greater magnitude than in young rats. Significant changes in plasma insulin did not occur in young and older rats after IV TNF. However, young rats had a significant decrease (P less than .02) in plasma insulin and an elevation in corticosterone levels after IP TNF, whereas older rats exhibited an increase in plasma insulin (P less than .02) and a comparable elevation in plasma corticosterone. Young rats also showed an increase in plasma insulin following IP TNF when hypoglycemia was prevented by the infusion of glucose. However, when insulin levels were held comparable (2.4 ng/mL), glucose uptake was enhanced (P less than .05) compared with controls. These findings indicate that mobilization of energy substrates occurs during the initial exposure to TNF, which is altered by the nutritional state of the rats and the dose and route of administration.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of interleukin-1 and tumor necrosis factor on ketogenesis

To determine the role of cytokines in mediating the decrease in ketones associated with infection, we studied the effect of endotoxin (LPS), interleukin-1 (IL-1), and tumor necrosis factor (TNF) on serum and hepatic ketone body levels (KB), serum free fatty acids (FFA), and hepatic malonyl-CoA levels. LPS decreased serum and hepatic KB in C57Bl/6 (LPS sensitive) mice, whereas it had little effect in C3H/HeJ (LPS resistant) mice, whose macrophages lack the ability to produce IL-1 and TNF in response to LPS, suggesting that IL-1 and TNF may mediate this effect. IL-1 and TNF decreased serum KB in both strains of mice. As seen with LPS, IL-1 decreased hepatic KB, whereas TNF had no such effect. LPS, IL-1, and TNF increased hepatic malonyl-CoA levels. TNF acutely raised serum FFA, whereas LPS and IL-1 did not. Postulating that the TNF-induced increase in FFA overrides the inhibitory effect of malonyl-CoA on fatty acid oxidation and ketogenesis, we used R-2-phenylisopropyladenosine to block TNF-induced lipolysis and demonstrated that in the absence of increased fatty acid flux, TNF inhibits KB formation. As seen with LPS, IL-1, but not TNF, decreased KB in the fasting state. These data suggest that IL-1 and TNF may mediate the antiketogenic effect of infection and that IL-1 has properties closest to that of LPS.

Effects of recombinant monokines on hepatic pyruvate dehydrogenase, pyruvate dehydrogenase kinase, lipogenesis de novo and plasma triacylglycerols. Abolition by prior fasting

1. The effects of recombinant human tumour necrosis factor alpha (TNF) and murine interleukin-1 alpha (IL-1) on the activation state of the hepatic pyruvate dehydrogenase complex (PDHa), the activity of mitochondrial PDH kinase, hepatic lipogenesis de novo and plasma triacylglycerol (TG) concentrations were studied. 2. Monokine effects depended upon prior nutritional state. In rats fasted for 20 h or 45 h before monokine administration and refeeding (orally or with intravenous glucose), PDHa, TG and hepatic lipogenesis were not increased. In rats fed ad libitum, treatment with TNF plus IL-1 increased the contribution of hepatic lipogenesis to circulating TG to 550% of control values (P = 0.03) and plasma TG concentrations to 159% (P = 0.02), whereas PDHa increased slightly to 120% (P = 0.02) and liver glycogen content fell to 45.8% (P = 0.05) of control values. 3. Intrinsic hepatic PDH kinase activity was not changed by monokine treatment in rats fed ad libitum. 4. The increased lipogenesis de novo showed no correlation (r2 = 0.05, not significant) with hepatic PDHa in individual animals fed ad libitum. 5. In conclusion, these results suggest that monokines increase pyruvate flux through hepatic PDH in vivo in rats fed ad libitum primarily by mechanisms other than covalent modification of PDH. Prior nutritional status exerts a permissive effect for monokine stimulation of PDHa and lipogenesis, consistent with a substrate-mediated action, but the mechanism of this permissive effect remains uncertain.

Diet affects the mechanisms by which TNF stimulates hepatic triglyceride production

Tumor necrosis factor (TNF) induces hyperlipidemia in rodents by increasing hepatic triglyceride production. We now explore the mechanism of this increase. TNF does not increase phosphatidate phosphohydrolase, glycerolphosphate acyltransferase, or diacylglycerol acyltransferase, which are enzymes of triglyceride synthesis. Rather, TNF increases triglyceride production by providing increased fatty acids (FA) as substrate. In chow-fed rats, TNF increases plasma free fatty acids (FFA). The antilipolytic drug, phenylisopropyl adenosine (PIA), prevents the TNF-induced increase in plasma FFA and, most importantly, inhibits the TNF-induced increase in plasma triglycerides. Thus increased lipolysis with delivery of FA to liver contributes to TNF-induced hyperlipidemia in chow-fed animals. In contrast, in rats fed a high-sucrose diet, TNF causes hyperlipidemia without increasing plasma FFA, and PIA has no effect on TNF-induced increases in plasma triglycerides. However, in sucrose-fed rats, TNF markedly stimulates hepatic de novo FA synthesis, which provides FA. This diet determines the mechanism by which TNF stimulates hepatic triglyceride production. The use of multiple mechanisms to increase plasma triglycerides suggests that this TNF action plays an important role in the response to infection or inflammation.