Effect of sepsis on VLDL kinetics: responses in basal state and during glucose infusion

Could Exogenous Insulin Ameliorate the Metabolic Dysfunction Induced by Glucocorticoids and COVID-19?

The finding that high-dose dexamethasone improves survival in those requiring critical care due to COVID-19 will mean much greater usage of glucocorticoids in the subsequent waves of coronavirus infection. Furthermore, the consistent finding of adverse outcomes from COVID-19 in individuals with obesity, hypertension and diabetes has focussed attention on the metabolic dysfunction that may arise with critical illness. The SARS coronavirus itself may promote relative insulin deficiency, ketogenesis and hyperglycaemia in susceptible individuals. In conjunction with prolonged critical care, these components will promote a catabolic state. Insulin infusion is the mainstay of therapy for treatment of hyperglycaemia in acute illness but what is the effect of insulin on the admixture of glucocorticoids and COVID-19? This article reviews the evidence for the effect of insulin on clinical outcomes and intermediary metabolism in critical illness.

Influence of Acute and Chronic Exercise on Abdominal Fat Lipolysis: An Update

Exercise is a powerful and effective preventive measure against chronic diseases by increasing energy expenditure and substrate mobilization. Long-duration acute exercise favors lipid mobilization from adipose tissue, i.e., lipolysis, as well as lipid oxidation by skeletal muscles, while chronic endurance exercise improves body composition, facilitates diet-induced weight loss and long-term weight maintenance. Several hormones and factors have been shown to stimulate lipolysis in vitro in isolated adipocytes. Our current knowledge supports the view that catecholamines, atrial natriuretic peptide and insulin are the main physiological stimuli of exercise-induced lipolysis in humans. Emerging evidences indicate that contracting skeletal muscle can release substances capable of remote signaling to organs during exercise. This fascinating crosstalk between skeletal muscle and adipose tissue during exercise is currently challenging our classical view of the physiological control of lipolysis, and provides a conceptual framework to better understand the pleotropic benefits of exercise at the whole-body level.

Control strategies in systemic metabolism

Metabolic control systems coordinate myriad processes across the cellular, tissue and organismal levels to optimize the allocation of limited supplies across multiple, often competing, metabolic demands. As such, the regulation of metabolism can be analysed from the perspective of the economic theory of supply and demand. Here, we discuss how such analyses can provide new insights into the logic of metabolic control. In particular, we suggest that, in addition to being subject to well-appreciated homeostatic control, metabolism is subject to supply-driven and demand-driven controls, each operated by a dedicated set of signals throughout various physiological states, including inflammation. Furthermore, we argue that systemic homeostasis is a derived feature that evolved from the control systems that monitor metabolic supply and demand.

HORMONE MEASUREMENT GUIDELINES: Tracing lipid metabolism: the value of stable isotopes

Labelling molecules with stable isotopes to create tracers has become a gold-standard method to study the metabolism of lipids and lipoproteins in humans. There are a range of techniques which use stable isotopes to measure fatty acid flux and oxidation, hepatic fatty synthesis, cholesterol absorption and synthesis and lipoprotein metabolism in humans. Stable isotope tracers are safe to use, enabling repeated studies to be undertaken and allowing studies to be undertaken in children and pregnant women. This review provides details of the most appropriate tracers to use, the techniques which have been developed and validated for measuring different aspects of lipid metabolism and some of the limitations of the methodology.

Contribution of very low-density lipoprotein triglyceride fatty acids to postabsorptive free fatty acid flux in obese humans

OBJECTIVE

In the fasting state, plasma free fatty acids (FFA) are thought to derive almost exclusively from adipose tissue lipolysis. However, there are mixed reports as to whether the spillover of fatty acids (FA) from very low-density lipoprotein triglyceride (VLDL-TG) hydrolysis contributes significantly to the plasma FFA pool. Because substantial VLDL-TG fatty acid spillover into the plasma FFA pool would profoundly impact the interpretation of isotope dilution measures of FFA flux, we investigated the contribution of VLDL-TG spillover to plasma FFA appearance.

MATERIALS/METHODS

Eighteen obese adults (15 women) participated in these studies. Each volunteer received a primed, continuous infusion of their own ex-vivo labeled ([1-(14)C]triolein) VLDL-TG and a continuous infusion of [U-(13)C]oleate (8 nmol · kg fat free mass(-1) · min(-1)) to measure VLDL-TG and FFA rate of appearance (Ra), respectively. The presence of (14)C-oleate in the plasma FFA-oleate pool was used to calculate the contribution of spillover from VLDL-TG-oleate to the plasma FFA-oleate Ra.

RESULTS

The spillover rate of VLDL-TG-oleate into plasma FFA-oleate was 6 ± 2 μmol/min (7% ± 2% of [(14)C]oleate from VLDL-TG) and FFA-oleate flux was 240 ± 61 μmol/min. Thus, only 3% ± 1% of total plasma FFA-oleate appearance could be accounted for by VLDL-TG spillover.

CONCLUSION

The contribution of VLDL-TG spillover to the total plasma FFA pool is negligible and will not materially affect the interpretation of FFA flux measures as an index of adipose tissue lipolysis.

Targeted metabolomics for discrimination of systemic inflammatory disorders in critically ill patients

The occurrence of systemic inflammatory response syndrome (SIRS) remains a major problem in intensive care units with high morbidity and mortality. The differentiation between noninfectious and infectious etiologies of this disorder is challenging in routine clinical practice. Many biomarkers have been suggested for this purpose; however, sensitivity and specificity even of high-ranking biomarkers remain insufficient. Recently, metabolic profiling has attracted interest for biomarker discovery. The objective of this study was to identify metabolic biomarkers for differentiation of SIRS/sepsis. A total of 186 meta-bolites comprising six analyte classes were determined in 143 patients (74 SIRS, 69 sepsis) by LC-MS/MS. Two markers (C10:1 and PCaaC32:0) revealed significantly higher concentrations in sepsis. A classification model comprising these markers resulted in 80% and 70% correct classifications in a training set and a test set, respectively.This study demonstrates that acylcarnitines and glycerophosphatidylcholines may be helpful for differentiation of infectious from noninfectious systemic inflammation due to their significantly higher concentration in sepsis patients. Considering the well known pathophysiological relevance of lipid induction by bacterial components, metabolites as identified in this study are promising biomarker candidates in the differential diagnosis of SIRS and sepsis.

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

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

Impaired-inactivation of FoxO1 contributes to glucose-mediated increases in serum very low-density lipoprotein

For patients with diabetes, insulin resistance and hyperglycemia both contribute to increased serum triglyceride in the form of very low-density lipoprotein (VLDL). Our objective was to define the insulin conditions in which hyperglycemia promotes increased serum VLDL in vivo. We performed hyperglycemic-hyperinsulinemic clamp studies and hyperglycemic-hypoinsulinemic clamp studies in rats, with metabolic tracers for glucose flux and de novo fatty acid synthesis. When blood glucose was clamped at hyperglycemia (17 mm) for 2 h under hyperinsulinemic conditions (4 mU/kg . min), serum VLDL levels were not increased compared with baseline. We speculated that hyperinsulinemia minimized glucose-mediated VLDL changes and performed hyperglycemic-hypoinsulinemic clamp studies in which insulin was clamped near fasting levels with somatostatin (17 mm blood glucose, 0.25 mU/kg . min insulin). Under low-insulin conditions, serum VLDL levels were increased 4.7-fold after hyperglycemia, and forkhead box O1 (FoxO1) was not excluded from the nucleus of liver cells. We tested the extent that impaired inactivation of FoxO1 by insulin was sufficient for glucose to promote increased serum VLDL. We found that, when the ability of insulin to inactivate FoxO1 is blocked after adenoviral delivery of constitutively active FoxO1, glucose increased serum VLDL triglyceride when given both by ip glucose tolerance testing (3.5-fold increase) and by a hyperglycemic clamp (4.6-fold). Under both experimental conditions in which insulin signaling to FoxO1 was impaired, we found increased activation of carbohydrate response element binding protein. These data suggest that glucose more potently promotes increased serum VLDL when insulin action is impaired, with either low insulin levels or disrupted downstream signaling to the transcription factor FoxO1.

Stable isotope-labeled tracers for the investigation of fatty acid and triglyceride metabolism in humans in vivo

Understanding lipid metabolism and its regulation requires information on the rates at which lipids are produced within the body, absorbed (dietary lipids) into the body, transported within the body, and utilized by various tissues. This article focuses on the use of stable isotope-labeled tracers for the quantitative evaluation of major pathways of fatty acid and triglyceride metabolism in humans in vivo. Adipose tissue lipolysis and free fatty acid appearance in plasma, fatty acid tissue uptake and oxidation, and hepatic very low-density lipoprotein triglyceride secretion are among the metabolic pathways that can be studied by using stable isotope labeled tracers, and will be discussed in detail. The methodology has been in use for many years and is constantly being refined. A variety of tracers and analytical approaches are available and can be used; knowing the advantages, assumptions, and limitations of each is essential for the planning of studies and the interpretation of data, which can provide unique insights into human lipid metabolism.

Lipopolysaccharide induces adipose differentiation-related protein expression and lipid accumulation in the liver through inhibition of fatty acid oxidation in mice

BACKGROUND

In the present study, we examined the effect of lipopolysaccharide (LPS) on liver histopathology with special reference to lipid metabolism in mice.

METHODS

Mice were injected with LPS intraperitoneally, and its effect on the liver was investigated pathologically and biochemically.

RESULTS

Oil-red O staining and adipose differentiation-related protein (ADRP) immunohistochemistry demonstrated that injection of LPS transiently induced lipid accumulation and ADRP expression in hepatocytes, especially around the portal vein. Microscopic observation revealed that lipid accumulation started 12 h after LPS injection. Time-course studies showed that LPS rapidly, within 2 h, decreased hepatic expression of nuclear hormone receptors, including peroxisome proliferator-activated receptor (PPAR) alpha. LPS inhibited the expression of PPARalpha-target genes involved in fatty acid oxidation in the liver such as those coding for enoyl-CoA hydratase, acyl-CoA dehydrogenase, and carnitine palmitoyl transferase-1, whereas LPS also suppressed the expression of genes related to fatty acid synthesis such as those for fatty acid synthase, stearoyl-CoA desaturase, and acetyl-CoA carboxylase alpha.

CONCLUSIONS

LPS induces transient lipid accumulation and expression of ADRP in the liver through inhibition of fatty acid oxidation by downregulation of the PPARalpha-related transcriptional mechanism.

Splanchnic spillover of extracellular lipase-generated fatty acids in overweight and obese humans

OBJECTIVE

Triglyceride-rich lipoproteins, primarily chylomicrons, can contribute to plasma free fatty acid (FFA) concentrations via spillover of fatty acids during intravascular hydrolysis into the venous effluent of some tissues. The present study was undertaken to determine whether spillover occurs in the splanchnic bed of humans.

RESEARCH DESIGN AND METHODS

Arterial and hepatic venous blood was sampled in postabsorptive (n = 6; study A) and postprandial (n = 5; study B) obese humans during infusion of carbon-labeled ((14)C or (13)C) oleate and (3)H triolein, the latter incorporated into a lipid emulsion as a surrogate for chylomicrons. Spillover was determined by measuring production of (3)H oleate.

RESULTS

Splanchnic spillover was higher than nonsplanchnic systemic spillover in both study A (60 +/- 7 vs. 24 +/- 6%; P < 0.01) and study B (54 +/- 3 vs. 16 +/- 5%; P < 0.005). Because portal vein sampling is not feasible in humans, assumptions regarding actual spillover in nonhepatic splanchnic tissues were required for the spillover calculation. A mathematical model was developed and demonstrated that nonhepatic splanchnic spillover rates in study A and study B of 69 and 80%, respectively, provided the best fit with the data. There was preferential splanchnic uptake of triglyceride fatty acids compared with FFAs in study B (fractional extraction 61 +/- 3 vs. 33 +/- 2%; P < 0.005).

CONCLUSIONS

These data confirm previous studies indicating that the transport of FFAs and triglyceride fatty acids are partitioned in tissues and indicate that splanchnic spillover from triglyceride-rich lipoproteins may be a significant source of both portal venous and systemic FFAs.

Triglyceride uptake and lipoprotein lipase-generated fatty acid spillover in the splanchnic bed of dogs

The action of lipoprotein lipase on triglyceride-rich lipoproteins generates fatty acids that are either transported into tissues or mix with circulating free fatty acids (FFAs) via a process known as spillover. In the present study, arterial, portal vein, and hepatic vein sampling catheters were surgically placed in nine mongrel dogs. The animals were subsequently studied after a 42-h fast during infusion of [14C]oleate and a lipid emulsion containing [3H]triolein; the emulsion was used as a surrogate for the study of chylomicron metabolism. More than one-half of splanchnic [3H]triglyceride uptake occurred in the liver, and substantial fractional spillover of [3H]oleate was observed in both liver and nonhepatic tissues (approximately 50% each). There was a significant correlation between FFA release from nonhepatic tissues (presumably visceral fat) and nonhepatic fractional spillover (R = 0.81, P < 0.01), consistent with a model in which the rate of intracellular lipolysis influences spillover by determining the direction of net fatty acid flow between the cell and the interstitium. There was a significant correlation between "true" and "net" splanchnic spillover (R = 0.84, P < 0.005), the latter representing calculation of spillover between arterial and hepatic venous blood without portal venous data. Metabolism of chylomicron triglycerides in visceral fat may be an important source of portal venous FFAs.

The origins of cachexia in acute and chronic inflammatory diseases

The term cachexia originates from the Greek root kakos hexis, which translates into "bad condition," recognized for centuries as a progressive deterioration of body habitus. Cachexia is commonly associated with a number of disease states, including acute inflammatory processes associated with critical illness and chronic inflammatory diseases, such as cancer, congestive heart failure, chronic obstructive pulmonary disease, and human immunodeficiency virus infection. Cachexia is responsible for the deaths of 10%-22% of all patients with cancer and approximately 15% of the trauma deaths that occur from sepsis-induced organ dysfunction and malnutrition days to weeks after the initial traumatic event. The abnormalities associated with cachexia include anorexia, weight loss, a preferential loss of somatic muscle and fat mass, altered hepatic glucose and lipid metabolism, and anemia. Anorexia alone cannot fully explain the development of cachexia; metabolic alterations in carbohydrate, lipid, and protein metabolism contribute to the severe tissue losses. Despite significant advances in our understanding of specific disease processes, the mechanisms leading to cachexia remain unclear and multifactorial. Although complex, increasing evidence from both animal models and clinical studies suggests that an inflammatory response, mediated in part by a dysregulated production of proinflammatory cytokines, plays a role in the genesis of cachexia, associated with both critical illness and chronic inflammatory diseases. These cytokines are further thought to induce an acute phase protein response (APR) and produce the alterations in lipid and carbohydrate metabolism identified as crucial markers of acute inflammation in states of malignancy and critical illness. Although much is still unknown about the etiology of cachexia, there is growing appreciation that cachexia represents the endproduct of an inappropriate interplay between multiple cytokines, neuropeptides, classic stress hormones, and intermediary substrate metabolism.

Systemic and forearm triglyceride metabolism: fate of lipoprotein lipase-generated glycerol and free fatty acids

Little is known about the fate of the lipolytic products produced by the action of lipoprotein lipase (LPL) on circulating triglyceride-rich lipoproteins in humans. We studied eight lean, healthy male subjects after an overnight fast. Subjects received infusions of lipid emulsions containing triolein labeled with (3)H on both the glycerol backbone and the fatty acid portion of the molecule; (14)C glycerol and (14)C oleate were coinfused to quantify the systemic and forearm release of (3)H glycerol and (3)H oleate resulting from LPL action. There was significant forearm uptake of both whole plasma triglyceride (presumed to represent primarily VLDL; extraction fraction 2.6 +/- 0.6%, P < 0.005 vs. zero) and radiolabeled triglyceride derived from the lipid emulsion (a surrogate for chylomicrons; extraction fraction 31 +/- 4%, P < 0.005 vs zero). Systemic clearance and forearm fractional extraction of glycerol was greater than that of oleate (P < 0.001 and P < 0.02, respectively). The systemic and forearm fractional release of LPL-generated glycerol were similar at 51 +/- 4 and 59 +/- 1%, respectively (NS). In contrast, the forearm fractional release of LPL-generated oleate was less than systemic fractional release (14 +/- 2 vs. 36 +/- 4%, P < 0.0001). These results indicate that there is escape, or spillover, of the lipolytic products of LPL action on triglyceride-rich lipoproteins in humans. They further suggest that LPL-mediated fatty acid uptake is an inefficient process, but may be more efficient in muscle than in adipose tissue.

What does the measurement of whole-body fatty acid rate of appearance in plasma by using a fatty acid tracer really mean?

We evaluated the validity of using a single fatty acid tracer to assess total plasma long-chain free fatty acid (FFA) kinetics and the relationship between the rate of appearance (R(a)) of fatty acids in plasma and the fatty acid composition of adipose tissue triglyceride (TG). A mixture of [(13)C]-labeled myristate, palmitate, stearate, oleate, and linoleate was infused in healthy men during basal conditions and during conditions that stimulate (epinephrine infusion) and inhibit (insulin infusion) lipolysis of adipose tissue TGs. Calculated total FFA, R(a) based on palmitate, oleate, or linoleate tracers, was within 15% of the measured sum of the individual fatty acid R(a) under all conditions, whereas stearate and myristate tracers consistently underestimated and overestimated total FFA R(a), respectively. The fatty acid R(a) profile closely matched the fatty acid profile of subcutaneous adipose tissue TGs during epinephrine infusion, but not during basal conditions and insulin infusion. Our data support the common practice of using labeled palmitate or oleate as fatty acid tracers for assessing total plasma FFA kinetics and suggest that a source of lipids other than adipose tissue TG release fatty acids into the systemic circulation.

Nocturnal and postprandial free fatty acid kinetics in normal and type 2 diabetic subjects: effects of insulin sensitization therapy

Whether free fatty acid (FFA) rate of appearance (R(a)) is increased in type 2 diabetes is controversial. To characterize nocturnal and postprandial abnormalities in FFA kinetics and to determine the effects of treatment with insulin sensitizers on lipolysis, we measured palmitate R(a) in control subjects (n = 6) and individuals with poorly controlled, sulfonylurea-treated type 2 diabetes (HbA(1c) = 8.7 +/- 0.2%, n = 20), the latter before and at the end of 12 weeks of treatment with troglitazone (600 mg/day, n = 4), metformin ( approximately 2,000 mg/day, n = 8), or placebo (n = 8). Subjects consumed a standard breakfast at 0800 h. Results in control subjects and type 2 diabetic subjects were compared at baseline. Integrated nocturnal FFA R(a) (AUC(1:00-8:00 A.M.)) was approximately 50% higher in type 2 diabetic subjects than in control subjects (29.4 +/- 3.0 vs. 19.4 +/- 3.9 mmol. m(-2). 7 h(-1), respectively, P < 0.05), whereas postprandial palmitate R(a) (AUC(0-240 min)) was almost threefold higher in type 2 diabetic subjects than in control subjects (14.2 +/- 1.7 vs. 5.3 +/- 1.0 mmol. m(-2). 4 h(-1), respectively, P < 0.01). After troglitazone treatment, nocturnal palmitate R(a) did not change, but postprandial palmitate R(a) decreased by approximately 30% (P < 0.05). Palmitate kinetics did not change with metformin or placebo treatment. In summary, nocturnal and postprandial FFA R(a) is increased in type 2 diabetes. Postprandial lipolysis appears to be preferentially improved by thiazolidinediones compared with nocturnal lipolysis.

Kinetics of intramuscular triglyceride fatty acids in exercising humans

A pulse ([(14)C]palmitate)-chase ([(3)H]palmitate) approach was used to study intramuscular triglyceride (imTG) fatty acid and plasma free fatty acid (FFA) kinetics during exercise at approximately 45% peak O(2) consumption in 12 adults. Vastus lateralis muscle was biopsied before and after 90 min of bicycle exercise; (3)H(2)O production, breath (14)CO(2) excretion and lipid oxidation (indirect calorimetry) rates were measured during exercise.

RESULTS

during exercise, 8.2+/-1.2 and 8.4+/-0.7 micromol x kg(-1) x min(-1) of imTG fatty acids and plasma FFA, respectively, were oxidized according to isotopic measurements. The sum of these two values was not different (P = 0.6) from lipid oxidation by indirect calorimetry (15.4 +/-1.6 micromol x kg(-1) x min(-1)); the isotopic and indirect calorimetry values were correlated (r = 0.79, P<0.005). During exercise, imTG turnover rate was 0.32+/-0.07%/min (6.0+/-2.0 micromol of imTG x kg wet muscle(-1) x min(-1)) and plasma FFA were incorporated into imTG at a rate of 0.7+/-0.1 micromol x kg wet muscle(-1) x min(-1). The imTG pool size did not change during exercise. This pulse-chase, dual tracer appears to be a reasonable approach to measure oxidation and synthesis kinetics of imTG.

Role of cytokines in AIDS wasting

There is now a large literature implicating cytokines in the development of wasting and cachexia commonly observed in a variety of pathophysiologic conditions. In the acquired immunodeficiency syndrome (AIDS), cytokines elicited by primary and secondary infections seem to exert subtle and sustained effects on behavioral, hormonal, and metabolic axes, and their combined effects on appetite and metabolism have been postulated to drive wasting. However, correlations of increased blood levels of a particular cytokine with wasting in AIDS have not been consistent observations, perhaps because cytokines act principally as paracrine and autocrine hormones, as well as indirectly by activating other systems. A better understanding of the mechanisms underlying the catabolic effects of cytokines in clearly needed if more efficacious strategies are to be developed for the prevention and treatment of wasting in AIDS. In this review we first examine the interacting factors contributing to the AIDS wasting syndrome. We then analyze the complex and overlapping role of cytokines in the pathophysiology of this condition, and put forward a number of hypotheses to explain some of the most important features of this syndrome.

Sequential changes in the metabolic response in severely septic patients during the first 23 days after the onset of peritonitis

OBJECTIVE

To quantify the sequential changes in metabolic response occurring in patients with severe sepsis after the onset of peritonitis.

SUMMARY BACKGROUND DATA

Understanding the changes in energy expenditure and body composition is essential for the optimal management of severely septic patients; however, they have not been quantified in the context of modern surgical care.

METHODS

Twelve patients with severe sepsis secondary to peritonitis (median APACHE II score = 21.5) had measurements of energy expenditure and body composition as soon as they were hemodynamically stable and 5, 10, and 21 days later. Sequential measurements of acute-phase proteins and cytokine responses were also made.

RESULTS

Resting energy expenditure rose to 49% above predicted and remained elevated throughout the study period. Total energy expenditure was 1.25 x resting energy expenditure. Body fat was oxidized when energy intake was insufficient to achieve energy balance. There was a positive fluid balance of 12.5 1 over the first 2 days after onset of sepsis; thereafter, body water changes closely paralleled body weight changes and were largely accounted for by changes in extracellular water. During the 21 -day study period, there was a loss of 1.21 kg (13%) of total body protein. During the first 10 days, 67% of the protein lost came from skeletal muscle, but after this time it was predominantly from viscera. Intracellular potassium levels were low but did not deteriorate further after hemodynamic stability had been reached. There was a reprioritization of hepatic protein synthesis that was obligatory and independent of changes in total body protein. The cytokine responses demonstrated the complexity, redundancy, and overlap of mediators.

CONCLUSIONS

The period of hypermetabolism in severely septic patients is similar to that previously described, but the fluid changes are larger and the protein loss is greater. Protein loss early on is predominantly from muscle, thereafter from viscera. Fat loss can be prevented and cell function preserved once hemodynamic stability is achieved.

Human recombinant apolipoprotein E redirects lipopolysaccharide from Kupffer cells to liver parenchymal cells in rats In vivo

Chylomicrons have been shown to protect mice and rats against a lethal dose of lipopolysaccharide and may serve as a therapeutic means to protect against endotoxemia. However, the requisite of isolation from human lymph hampers pharmaceutical application. Recently, we developed recombinant chylomicrons from commercially available lipids and human recombinant apolipoprotein E. The current study explored the effectiveness of these apoE-enriched emulsions in redirecting LPS from Kupffer cells to liver parenchymal cells. Upon injection into rats, 125I-LPS rapidly and specifically associated with the liver (64.3+/-3.1% of the injected dose) and spleen (4.1+/-0.7%). The uptake of LPS by the spleen was four- to fivefold reduced upon incubation with the apoE-enriched emulsion or free apoE (P < 0.0001), but not with emulsion alone or Lipofundin. Within the liver, 125I-LPS mainly associated with Kupffer cells. The uptake by Kupffer cells was eight- to ninefold reduced by the apoE-enriched emulsion or apoE alone (P < 0.01), and a 19.6-fold increased uptake ratio by liver parenchymal cells over Kupffer cells was observed. The emulsion without apoE had no effect on the in vivo kinetics of LPS. LPS interacted selectively with the apoE moiety of the recombinant chylomicron. Emulsion-associated and free apoE bound approximately two molecules of LPS, possibly by its exposed hydrophilic domain involving arginine residues. We anticipate that the protecting effect of endogenous chylomicrons against LPS-induced endotoxemia may result from the apoE moiety and that human recombinant apoE may serve as a therapeuticum to protect against endotoxemia.

Contributions of de novo synthesis of fatty acids to total VLDL-triglyceride secretion during prolonged hyperglycemia/hyperinsulinemia in normal man

Triglycerides (TG) are synthesized in the liver principally from two sources of fatty acids (FA): FA synthesized de novo in the liver and preformed FA. We have measured the rate of secretion of de novo synthesized FA and total secretion of FA bound to VLDL-TG in healthy men (n = 5) in the basal state, and after 1 (day 1) and 4 d (day 4) of a hypercaloric carbohydrate diet (approximately 2.5 times energy expenditure) that generated a moderate endogenous hyperinsulinemia (plasma insulin approximately 60 microU/ml). Prolonged carbohydrate hyperalimentation/hyperinsulinemia increased plasma VLDL-TG approximately 10-fold in part due to a 3.4-fold increase in total VLDL-TG secretion rate (basal state = 72+/-23, day 4 = 242+/-78 micromol TG/kg/d). Although the secretion of de novo synthesized FA increased throughout the study (basal state = 1.1+/-0.4, day 1 = 15.9+/-7.9, day 4 = 50.0+/-18.8 micromol TG/ kg/d), the 2.7-fold increase in secretion rate of preformed FA (basal state = 70+/-23, day 4 = 191+/-57 micromol TG/kg/d) quantitatively contributed the most to total VLDL-TG secretion rate. Decreased catabolism of VLDL-TG also contributed to the hypertriglyceridemia as reflected by an approximately fourfold decrease in both fractional turnover rate (basal state = 9.2+/-3.8, day 1 = 2.1+/-0.2, day 4 = 2.1+/-0.3 pools/d) and rate of clearance (basal state = 0.35+/-0.08, day 1 = 0.11+/-0.01, day 4 = 0.09+/-0.01 liter/kg/d) of VLDL-TG. Thus, the primary difference between 1 and 4 d of hyperinsulinemia in conjunction with carbohydrate hyperalimentation is the increase in hepatic secretion of preformed FA into VLDL-TG.

Endotoxin suppresses rat hepatic low-density lipoprotein receptor expression

Endotoxin induces hyperlipidaemia in experimental animals. In the current study, we investigated whether endotoxin alters hepatic low-density lipoprotein (LDL) receptor expression in rats. Endotoxin treatment suppressed hepatic LDL receptor expression in a dose- and time-dependent manner. Eighteen hours after intraperitoneal injection of increasing amounts of endotoxin, LDL receptor and its mRNA levels were determined by ligand blot and solution hybridization respectively. LDL receptor expression was inhibited by about 70% at a dose of 500 micrograms/100 g body weight. However, LDL receptor mRNA levels were markedly increased in all endotoxin-treated groups at this time point (by 83-136%; P < 0.001). Time-course experiments showed that LDL receptor expression was already reduced by 48% 4 h after endotoxin injection and was maximally reduced (by 63-65%) between 8 and 18 h. Changes in hepatic LDL receptor mRNA showed a different pattern. By 4 h after endotoxin injection, LDL receptor mRNA had decreased by 78% (P < 0.001). However, by 8 h after endotoxin injection, LDL receptor mRNA had returned to levels similar to controls, and 18 and 24 h after endotoxin injection, they were increased by about 60% (P < 0.05). Separation of plasma lipoproteins by FPLC demonstrated that endotoxin-induced changes in plasma triacylglycerols and cholesterol were due to accumulation of plasma apolipoprotein B-containing lipoproteins among very-low-density lipoprotein, intermediate-density lipoprotein and LDL. It is concluded that endotoxin suppresses hepatic LDL receptor expression in vivo in rats.

Interleukin-6, but not tumour necrosis factor-alpha, increases lipogenesis in rat hepatocyte primary cultures

The Kupffer-cell products interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-alpha) have been shown to stimulate hepatic lipogenesis in vivo. Studies were performed to define the direct effects of these cytokines on lipogenesis in primary-culture rat hepatocytes. Hepatocytes were cultured in the presence of IL-6 or TNF-alpha for periods of 24-72 h. IL-6 increased hepatocyte protein content per microgram of DNA. IL-6 also caused a dose- and time-dependent induction of hepatocyte capacity for incorporation of [2-14C]pyruvate into fatty acids (56% increase by 12.5 ng/ml IL-6 after 72 h of cytokine exposure). This increase in cellular lipogenic capacity was confirmed by using 3H2O incorporation into fatty acids as tracer. TNF-alpha did not increase hepatocyte lipogenesis. In contrast with studies in vivo, neither IL-6 nor TNF-alpha had any acute (2 h of exposure) effects on rates of lipogenesis. Both IL-6 and TNF-alpha are known to increase macrophage prostaglandin synthesis acutely. The prostaglandin E agonist misoprostol free acid (0.1 microM) acutely increased hepatocyte lipogenic rates by 14%. Thus, IL-6 can directly induce hepatocyte lipogenic capacity, and E-series prostaglandins can antagonize the acute inhibition of lipogenesis by glucagon. The observations provide further evidence for the role of Kupffer-cell products in the regulation of hepatocyte metabolism.

Chylomicrons enhance endotoxin excretion in bile

Chylomicrons prevent endotoxin toxicity and increase endotoxin uptake by hepatocytes. As a consequence, less endotoxin is available to activate macrophages, thereby reducing tumor necrosis factor secretion. To determine whether the chylomicron-mediated increase in hepatocellular uptake of endotoxin results in increased endotoxin excretion into bile, we examined bile after endotoxin administration. A sublethal dose (7 micrograms/kg) of 125I-endotoxin was incubated with either rat mesenteric lymph containing nascent chylomicrons (500 mg of chylomicron triglyceride per kg of body weight) or an equal volume of normal saline (controls) for 3 h and then infused into male Sprague-Dawley rats. Bile samples were collected via a common bile duct catheter for 24 h. Infusion of endotoxin incubated with chylomicrons increased biliary excretion of endotoxin by 67% at 3 h (P < or = 0.006) and by 20% at 24 h (P < or = 0.01) compared with infusion of endotoxin incubated in saline. Endotoxin activity, as measured by the Limulus assay, was not detected in the bile of test animals. However, endotoxin activity was detected after hot phenol-water extraction of bile, demonstrating that endotoxin is inactive in the presence of bile but retains bioactivity after hepatic processing. Since the majority of an intravenous endotoxin load has been shown to be cleared by the liver, acceleration of hepatocyte clearance and biliary excretion of endotoxin may represent a component of the mechanism by which chylomicrons protect against endotoxin-induced lethality.

Chylomicrons alter the fate of endotoxin, decreasing tumor necrosis factor release and preventing death

The hypertriglyceridemia of infection was traditionally thought to represent the mobilization of substrate to fuel the body's response to the infectious challenge. However, we have previously shown that triglyceride-rich lipoproteins can protect against endotoxin-induced lethality. The current studies examine the mechanism by which this protection occurs. Rats infused with a lethal dose of endotoxin preincubated with chylomicrons had a reduced mortality compared with rats infused with endotoxin alone (15 vs. 76%, P < 0.001). Preincubation with chylomicrons increased the rate of clearance of endotoxin from plasma and doubled the amount of endotoxin cleared by the liver (30 +/- 1 vs. 14 +/- 2% of the total infused radiolabel, P < 0.001). In addition, autoradiographic studies showed that chylomicrons directed more of the endotoxin to hepatocytes and away from hepatic macrophages. Rats infused with endotoxin plus chylomicrons also showed reduced peak serum levels of tumor necrosis factor as compared with controls (14.2 +/- 3.3 vs. 44.9 +/- 9.5 ng/ml, mean +/- SEM, P = 0.014). In separate experiments, chylomicrons (1,000 mg triglyceride/kg) or saline were infused 10 min before the infusion of endotoxin. Chylomicron pretreatment resulted in a reduced mortality compared with rats infused with endotoxin alone (22 vs. 78%, P < 0.005). Therefore, chylomicrons can protect against endotoxin-induced lethality with and without preincubation with endotoxin. The mechanism by which chylomicrons protect against endotoxin appears to involve the shunting of endotoxin to hepatocytes and away from macrophages, thereby decreasing macrophage activation and the secretion of cytokines.

Role for monokines in the metabolic effects of endotoxin. Interferon-gamma restores responsiveness of C3H/HeJ mice in vivo

To examine the role of cytokines in mediating the lipogenic effects of endotoxin (LPS), we studied the effects of LPS and cytokines on hepatic fatty acid synthesis in LPS-sensitive C3H/OuJ mice and in LPS-resistant C3H/HeJ mice, whose macrophages are defective in the ability to produce tumor necrosis factor (TNF) and IL-1 in response to LPS. HeJ mice were 16-fold less sensitive than OuJ mice to the lipogenic effect of LPS. In OuJ mice, 10 micrograms of LPS caused a maximal increase in hepatic lipogenesis (3.86 +/- 0.41-fold), whereas in HeJ mice the maximal increase was only 1.79 +/- 0.32-fold after 100 micrograms of LPS. This lipogenic response paralleled the decreased ability of LPS to increase hepatic and splenic levels of mRNAs for TNF and IL-1 and serum levels of TNF in HeJ mice. In contrast, the maximal effect of TNF on lipogenesis was greater and the sensitivity to TNF was increased 2.4-fold in HeJ mice compared to OuJ mice. Administration of IFN-gamma before LPS in HeJ mice had no effect on IL-1 mRNA, but partially restored the LPS-induced increase in hepatic and splenic mRNA for TNF and serum TNF levels, which may account for the partial restoration of sensitivity to the lipogenic effect of LPS after IFN-gamma treatment. These results indicate that cytokines produced by mononuclear leukocytes mediate the lipogenic effects of LPS.

Human very low density lipoproteins and chylomicrons can protect against endotoxin-induced death in mice

Endotoxemia stimulates many physiologic responses including disturbances in lipid metabolism. We hypothesized that this lipemia may be part of a defensive mechanism by which the body combats the toxic effects of circulating endotoxin. We tested the effects of mixtures of endotoxin, lipoproteins, and lipoprotein-free plasma and determined the ability of varying concentrations of human very low density lipoproteins (VLDL) and chylomicrons, as well as low density lipoproteins (LDL) and high density lipoproteins (HDL), and of the synthetic lipid emulsion SOYACAL to prevent endotoxin-induced death in mice. This study demonstrates that the triglyceride-rich VLDL and chylomicrons, as well as cholesterol-rich LDL and HDL, and cholesterol-free SOYACAL can protect against endotoxin-induced death. Protection required small amounts of lipoprotein-free plasma, and depended on the incubation time and the concentration of lipoprotein lipid. Despite stringent techniques to prevent exogenous endotoxin contamination eight of ten duplicate VLDL preparations contained endotoxin (5,755 +/- 3,514 ng endotoxin/mg triglyceride, mean +/- SEM) making the isolation of endotoxin-free VLDL difficult. In contrast, simultaneous preparations of LDL and HDL were relatively free of endotoxin contamination (3 +/- 3 and 320 +/- 319 ng/mg total cholesterol, respectively), suggesting that the contamination of VLDL occurs in vivo and not during the isolation procedure. These observations suggest a possible role for increased triglyceride-rich lipoproteins in the host's defense against endotoxemia and infection.

Metabolic intervention in surgical patients. An assessment of the effect of somatostatin, ranitidine, naloxone, diclophenac, dipyridamole, or salbutamol infusion on energy and protein kinetics in surgical patients using stable and radioisotopes

We have assessed the effect of a variety of forms of metabolic intervention on both energy and protein metabolism in 44 severely ill surgical patients. The patients were studied either in the basal state or while receiving total parenteral nutrition (TPN), and the metabolic effects were assessed using the primed-constant infusion of a combination of stable isotopes and radioisotopes. Somatostatin infusion, either in the basal state or in the TPN, did not change glucose kinetics, but there was a significant decrease in the rate of net protein catabolism (NPC). In the basal studies the rate of NPC decreased from 3.4 +/- 0.7 g/kg/d to 2.9 +/- 0.7 g/kg/d (p less than 0.002), while in the TPN patients the corresponding values were 1.48 +/- 0.61 g/kg/d and 1.10 +/- 0.50 g/kg/d, respectively (p less than 0.005). Histamine type 2 blockade with ranitidine did not significantly alter glucose kinetics, but in both the TPN patients and in the basal state ranitidine was associated with a significant decrease in the rate of NPC. In the basal state rate of NPC was 2.44 +/- 0.53 g/kg/d and during ranitidine infusion the value was 2.08 +/- 0.42 g/kg/d (p less than 0.04). Naloxone infusion did not alter glucose kinetics, but there was a significant decrease in the rate of NPC from a basal value of 2.6 +/- 0.6 g/kg/d to 2.3 +/- 0.5 g/kg/d (p less than 0.04). The infusion of the prostaglandin antagonists diclofenac or dipyridamole resulted in increases in the plasma insulin level, and as a result glucose turnover decreased in both groups. In the diclofenac group the rate of glucose turnover decreased from 14.4 +/- 1.7 mumol/kg/min to 12.6 +/- 1.3 mumol/kg/min (p less than 0.02). Neither prostaglandin antagonist resulted in any significant change in the rate of NPC. Beta-adrenergic stimulation with salbutamol resulted in a significant increase in glucose turnover from 12.1 +/- 1.1 mumol/kg/min to 13.4 +/- 0.9 mumol/kg/min (p less than 0.02), and the rates of appearance (Ra) of both alanine and free fatty acids (FFAs) also increased. Alanine Ra increased from 11.7 +/- 2.5 mumol/kg/min to 12.8 +/- 3.0 mumol/kg/min, and the corresponding values for FFA turnover were 7.6 +/- 1.1 mumol/kg/min and 10.3 +/- 2.1 mumol/kg/min (p less than 0.03), respectively. Salbutamol infusion did not result in any significant change in the rate of NPC.(ABSTRACT TRUNCATED AT 400 WORDS)

Fatty acid and glycerol kinetics in septic patients and in patients with gastrointestinal cancer. The response to glucose infusion and parenteral feeding

The rates of glycerol and free fatty acid (FFA) kinetics in normal volunteers (VOL), non-weight-losing (NWL) gastrointestinal cancer patients, weight-losing (WL) gastrointestinal cancer patients, and in severely septic patients, using constant infusions of d-glycerol and 1-13C palmitic acid; were determined. Rates of FFA oxidation have also been quantitated. Measurements were made in the basal state, during glucose infusion (4 mg/kg/min), and during total parenteral nutrition (TPN). Rates of glycerol and FFA appearance (Ra) in volunteers and NWL cancer patients were similar, and in both groups there was a significant suppression after glucose infusion. The basal Ra values for glycerol and FFA were 2.4 +/- 0.2 and 6.5 +/- 0.8 mumol/kg/min, respectively, in the volunteers, and in the NWL cancer patients the corresponding values were 2.7 +/- 0.4 and 7.1 +/- 1.1 mumol/kg/min (not significantly different). Compared with the volunteers, the rates of glycerol and FFA turnover were significantly elevated in both septic patients and WL cancer patients. The values for glycerol and FFA Ra were 6.3 +/- 1.1 and 13.1 +/- 3.0 mumol/kg/min, respectively, in the septic patients. The corresponding values were 4.1 +/- 0.4 and 11.7 +/- 1.6 mumol/kg/min in the WL cancer patients. In contrast to the response seen in the volunteers and NWL cancer patients, glucose infusion did not suppress lipolysis in either the septic or WL cancer patients. In all groups studied, glucose infusion resulted in an increase in FFA recycling. Despite the fact that the WL cancer patients had an increased FFA availability, they were significantly less able to oxidize either endogenous FFA or infused lipid when compared with NWL cancer patients (the basal % of FFA uptake oxidized in WL cancer patients was 10 +/- 2% vs. 18 +/- 3% in NWL cancer patients). In contrast, the septic patients had an enhanced capacity to oxidize either endogenous FFA or infused lipid (the basal % of FFA uptake oxidized was 40 +/- 8%, and during TPN this increased in 65 +/- 10%). From these studies the following was concluded: in terms of lipid kinetics, NWL cancer patients are not significantly different from volunteers; WL cancer patients and septic patients have elevated rates of lipolysis, and in contrast to what was seen in NWL cancer patients and in volunteers, glucose infusion in WL cancer patients and in septic patients does not result in a significant inhibition of lipolysis; and WL cancer patients have an impaired capacity to oxidize either endogenous FFA or infused lipid.(ABSTRACT TRUNCATED AT 400 WORDS)