Fatty acid synthesis by human adipose tissue

Surplus fatty acid synthesis increases oxidative stress in adipocytes and lnduces lipodystrophy

Adipocytes are the primary sites for fatty acid storage, but the synthesis rate of fatty acids is very low. The physiological significance of this phenomenon remains unclear. Here, we show that surplus fatty acid synthesis in adipocytes induces necroptosis and lipodystrophy. Transcriptional activation of FASN elevates fatty acid synthesis, but decreases NADPH level and increases ROS production, which ultimately leads to adipocyte necroptosis. We identify MED20, a subunit of the Mediator complex, as a negative regulator of FASN transcription. Adipocyte-specific male Med20 knockout mice progressively develop lipodystrophy, which is reversed by scavenging ROS. Further, in a murine model of HIV-associated lipodystrophy and a human patient with acquired lipodystrophy, ROS neutralization significantly improves metabolic disorders, indicating a causal role of ROS in disease onset. Our study well explains the low fatty acid synthesis rate in adipocytes, and sheds light on the management of acquired lipodystrophy.

Tensin-3 is involved in osteogenic versus adipogenic fate of human bone marrow stromal cells

BACKGROUND

The tightly controlled balance between osteogenic and adipogenic differentiation of human bone marrow-derived stromal cells (BMSCs) is critical to maintain bone homeostasis. Age-related osteoporosis is characterized by low bone mass with excessive infiltration of adipose tissue in the bone marrow compartment. The shift of BMSC differentiation from osteoblasts to adipocytes could result in bone loss and adiposity.

METHODS

TNS3 gene expression during osteogenic and adipogenic differentiation of BMSCs was evaluated by qPCR and Western blot analyses. Lentiviral-mediated knockdown or overexpression of TNS3 was used to assess its function. The organization of cytoskeleton was examined by immunofluorescent staining at multiple time points. The role of TNS3 and its domain function in osteogenic differentiation were evaluated by ALP activity, calcium assay, and Alizarin Red S staining. The expression of Rho-GTP was determined using the RhoA pull-down activation assay.

RESULTS

Loss of TNS3 impaired osteogenic differentiation of BMSCs but promoted adipogenic differentiation. Conversely, TNS3 overexpression hampered adipogenesis while enhancing osteogenesis. The expression level of TNS3 determined cell shape and cytoskeletal reorganization during osteogenic differentiation. TNS3 truncation experiments revealed that for optimal osteogenesis to occur, all domains proved essential. Pull-down and immunocytochemical experiments suggested that TNS3 mediates osteogenic differentiation through RhoA.

CONCLUSIONS

Here, we identify TNS3 to be involved in BMSC fate decision. Our study links the domain structure in TNS3 to RhoA activity via actin dynamics and implicates an important role for TNS3 in regulating osteogenesis and adipogenesis from BMSCs. Furthermore, it supports the critical involvement of cytoskeletal reorganization in BMSC differentiation.

The impact of fasting on adipose tissue metabolism

Fasting and starvation were common occurrences during human evolution and accordingly have been an important environmental factor shaping human energy metabolism. Humans can tolerate fasting reasonably well through adaptative and well-orchestrated time-dependent changes in energy metabolism. Key features of the adaptive response to fasting are the breakdown of liver glycogen and muscle protein to produce glucose for the brain, as well as the gradual depletion of the fat stores, resulting in the release of glycerol and fatty acids into the bloodstream and the production of ketone bodies in the liver. In this paper, an overview is presented of our current understanding of the effects of fasting on adipose tissue metabolism. Fasting leads to reduced uptake of circulating triacylglycerols by adipocytes through inhibition of the activity of the rate-limiting enzyme lipoprotein lipase. In addition, fasting stimulates the degradation of stored triacylglycerols by activating the key enzyme adipose triglyceride lipase. The mechanisms underlying these events are discussed, with a special interest in insights gained from studies on humans. Furthermore, an overview is presented of the effects of fasting on other metabolic pathways in the adipose tissue, including fatty acid synthesis, glucose uptake, glyceroneogenesis, autophagy, and the endocrine function of adipose tissue.

Insulin and β-adrenergic receptors mediate lipolytic and anti-lipolytic signalling that is not altered by type 2 diabetes in human adipocytes

Control of fatty acid storage and release in adipose tissue is fundamental in energy homeostasis and the development of obesity and type 2 diabetes. We here take the whole signalling network into account to identify how insulin and β-adrenergic stimulation in concert controls lipolysis in mature subcutaneous adipocytes obtained from non-diabetic and, in parallel, type 2 diabetic women. We report that, and show how, the anti-lipolytic effect of insulin can be fully explained by protein kinase B (PKB/Akt)-dependent activation of the phosphodiesterase PDE3B. Through the same PKB-dependent pathway β-adrenergic receptor signalling, via cAMP and PI3Kα, is anti-lipolytic and inhibits its own stimulation of lipolysis by 50%. Through this pathway both insulin and β-adrenergic signalling control phosphorylation of FOXO1. The dose–response of lipolysis is bell-shaped, such that insulin is anti-lipolytic at low concentrations, but at higher concentrations of insulin lipolysis was increasingly restored due to inhibition of PDE3B. The control of lipolysis was not altered in adipocytes from diabetic individuals. However, the release of fatty acids was increased by 50% in diabetes due to reduced reesterification of lipolytically liberated fatty acids. In conclusion, our results reveal mechanisms of control by insulin and β-adrenergic stimulation — in human adipocytes — that define a network of checks and balances ensuring robust control to secure uninterrupted supply of fatty acids without reaching concentrations that put cellular integrity at risk. Moreover, our results define how selective insulin resistance leave lipolytic control by insulin unaltered in diabetes, while the fatty acid release is substantially increased.

Effects of genetic selection and threonine on meat quality in Pekin ducks

The present study was conducted to investigate the effects of genetic selection and threonine levels on meat quality in Pekin ducks. At 15 D of age, 192 lean ducks and 192 fatty ducks were selected and allotted to one of three treatments with 8 replicates with similar BW (8 ducks/cage), respectively. All ducks were fed the experimental diets (0.00, 0.15, and 0.30% added threonine) for 21 D from 15 to 35 D of age. The results showed that fatty ducks had higher (P < 0.001) feed intake, feed/gain ratio, abdominal fat percentage, and sebum percentage and lower (P = 0.001) breast muscle percentage compared with that of lean ducks. The fatty-type and lean-type ducks had similar weight gain and BW. Dietary threonine supplementation improved (P < 0.05) growth performance and increased breast muscle percentage in lean-type ducks, but it did not affect (P > 0.05) those indices in fatty-type ducks. Lean ducks had higher (P < 0.001) hepatic contents of total lipids, triglyceride, cholesterol, and plasma low-density lipoprotein cholesterol concentration, and dietary threonine supplementation decreased (P < 0.05) hepatic total lipid, cholesterol, and triglyceride contents in lean ducks, but it had no influence on hepatic lipids in fatty ducks (P > 0.05). Lean ducks had higher (P < 0.05) concentrations of monounsaturated fatty acid (MUFA), and C18-polyunsaturated fatty acid (PUFA) in the liver, PUFA in the breast muscle, and C18:3n6 and C18:3n3 in plasma and lower C20-PUFA and C22-PUFA in the liver and MUFA in plasma, compared with fatty ducks. Threonine supplementation increased PUFA, N3-PUFA, and n6-PUFA in plasma and hepatic fatty acids profiles in lean ducks (P > 0.05) but had on influence on total MUFA and total PUFA in the liver, breast muscle, and plasma in fatty ducks (P > 0.05). In conclusion, genetic selection toward meat production and threonine supplementation increases meat production and PUFA contents, which would influence eating quality, but it is benefit for human health.

Investigating the Role of Mitochondria in Type 2 Diabetes - Lessons from Lipidomics and Proteomics Studies of Skeletal Muscle and Liver

Mitochondrial dysfunction is discussed as a key player in the pathogenesis of type 2 diabetes mellitus (T2Dm), a highly prevalent disease rapidly developing as one of the greatest global health challenges of this century. Data however about the involvement of mitochondria, central hubs in bioenergetic processes, in the disease development are still controversial. Lipid and protein homeostasis are under intense discussion to be crucial for proper mitochondrial function. Consequently proteomics and lipidomics analyses might help to understand how molecular changes in mitochondria translate to alterations in energy transduction as observed in the healthy and metabolic diseases such as T2Dm and other related disorders. Mitochondrial lipids integrated in a tool covering proteomic and functional analyses were up to now rarely investigated, although mitochondrial lipids might provide a possible lynchpin in the understanding of type 2 diabetes development and thereby prevention. In this chapter state-of-the-art analytical strategies, pre-analytical aspects, potential pitfalls as well as current proteomics and lipidomics-based knowledge about the pathophysiological role of mitochondria in the pathogenesis of type 2 diabetes will be discussed.

A Novel Dnmt3a1 Transcript Inhibits Adipogenesis

DNA (cytosine-5)-methyltransferase 3a (Dnmt3a) is an enzyme that catalyzes the transfer of methyl groups to specific CpG forms in DNA. In mammals, two variant transcripts of Dnmt3a have been successfully identified. To the best of our knowledge, no Dnmt3a transcripts in an avian have been successfully identified. This study was performed to detect different transcripts of Dnmt3a in chickens and to examine whether a novel Dnmt3a transcript named Dnmt3a1 may regulate adipogenesis. In addition to cloning, sequencing, transcript detection, and expression studies, a novel Dnmt3a1 transcript overexpression and knockdown were conducted to explore the potential role of Dnmt3a1 in preadipocyte proliferation and the early stage of adipocyte differentiation. In chicken abdominal fat tissue, we detected a novel Dnmt3a1 transcript that differs from Dnmt3a by lacking 23 amino acids at the exon-1/exon-2 border. Dnmt3a1 mRNA was ubiquitously expressed in a variety of tissues or cells and highly expressed in chicken adipose tissue/cells. The expression of Dnmt3a1 was regulated under different physiological conditions including aging, fasting, and high-fat diet. In addition, overexpression of Dnmt3a1 significantly decreased preadipocyte proliferation and induced cell-cycle arrest while its inhibition increased cell proliferation and S-phase cells. Furthermore, the overexpression of Dnmt3a1 significantly upregulated the mRNA level of cell-cycle-related genes, such as CDKN1A, CDKN1B, CCNB3, CCND2, CCNG2, CDKN2B, and CDK9, or the protein level of CDKN1A, CDKN1B, and CCNG2. Conversely, the knockdown of Dnmt3a1 by siRNA had the opposite effects. Moreover, during early adipocyte differentiation, the overexpression of Dnmt3a1 significantly decreased the mRNA and the protein levels of PPAR-γ, C/EBP-α, ADIPOR1, and STAT3, and the mRNA levels of FAS, LEPR, LPL, PRKAB2, and ATGL. In contrast, their expression was significantly increased after the knockdown of Dnmt3a1. Taken together, we identified a novel transcript of Dnmt3a, and it played a potential role in adipogenesis.

Genomic Insights Into the Multiple Factors Controlling Abdominal Fat Deposition in a Chicken Model

Genetic selection for an increased growth rate in meat-type chickens has been accompanied by excessive fat accumulation particularly in abdominal cavity. These progressed to indirect and often unhealthy effects on meat quality properties and increased feed cost. Advances in genomics technology over recent years have led to the surprising discoveries that the genome is more complex than previously thought. Studies have identified multiple-genetic factors associated with abdominal fat deposition. Meanwhile, the obesity epidemic has focused attention on adipose tissue and the development of adipocytes. The aim of this review is to summarize the current understanding of genetic/epigenetic factors associated with abdominal fat deposition, or as it relates to the proliferation and differentiation of preadipocytes in chicken. The results discussed here have been identified by different genomic approaches, such as QTL-based studies, the candidate gene approach, epistatic interaction, copy number variation, single-nucleotide polymorphism screening, selection signature analysis, genome-wide association studies, RNA sequencing, and bisulfite sequencing. The studies mentioned in this review have described multiple-genetic factors involved in an abdominal fat deposition. Therefore, it is inevitable to further study the multiple-genetic factors in-depth to develop novel molecular markers or potential targets, which will provide promising applications for reducing abdominal fat deposition in meat-type chicken.

Increasing Levels of Dietary Hempseed Products Leads to Differential Responses in the Fatty Acid Profiles of Egg Yolk, Liver and Plasma of Laying Hens

The limited efficiency with which dietary alpha-linolenic acid (ALA) is converted by hens into docosahexaenoic acid (DHA) for egg deposition is not clearly understood. In this study, dietary ALA levels were increased via the inclusion of hempseed (HS) and hempseed oil (HO) in hen diets, with the goal of assessing the effects on the fatty acid (FA) profiles of total lipids and lipid classes in yolk, liver and plasma. Forty-eight hens were individually caged and fed one of six diets containing either HS:10, 20 or 30, HO:4.5 or 9.0 (%, diet) or a control (containing corn oil), providing a range (0.1-1.28 %, diet) of ALA. Fatty acid methyl esters of total lipids and lipid classes, including phosphatidyl choline (PtdCho) and ethanolamine (PtdEtn) in yolk, plasma and liver were then determined. Levels of n-3 FAs in both total lipids and lipid classes increased in all tissues. ALA and eicosapentaenoic acid (EPA) increased linearly, while docosapentaenoic acid and DHA increased quadratically. The FA profiles of yolk closely reflected levels in both plasma and liver. While ALA was highly concentrated in the triacylglycerol, it was low but equally distributed between PtdCho and PtdEtn in all tissues; however, the net accumulation was lower (P < 0.0001) in liver compared to yolk and plasma. Levels of EPA and ALA in yolk-PtdEtn were linearly (P < 0.0001; R (2) = 0.93) associated, and reflected those in liver-PtdEtn (P < 0.0001; R (2) = 0.90). In the liver, a strong inverse correlation (P < 0.0001; r = -0.94) between PL-DHA and ALA-to-EPA ratio in PtdEtn supports theories of low substrate availability, possibly limiting the conversion of ALA into DHA for egg enrichment.

Fibre digestibility, abundance of faecal bacteria and plasma acetate concentrations in overweight adult mares

The purpose of the present study was to compare digestibility of grass hay, faecal and plasma volatile fatty acid (VFA) concentrations, and faecal bacterial abundance in overweight and moderate-condition mares. Five overweight adult mixed-breed mares and five adult mixed-breed mares in moderate condition were housed individually and limit-fed orchard grass (Dactylis glomerata) hay at 20 g/kg body weight (as fed) daily for 14 d. Forage DM and fibre digestibility were determined using AOAC methods; digestible energy was measured using bomb calorimetry; plasma and faecal VFA concentrations were determined by use of GC and MS; faecal Firmicutes, Bacteroidetes, Fibrobacter succinogenes, Ruminococcus flavefaciens and total bacteria abundance was determined by quantitative real-time PCR using previously designed phylum-specific 16S ribosomal RNA gene primers. No differences in hay digestibility, faecal VFA concentrations or faecal bacterial abundance were detected between overweight and moderate-condition mares. Mean plasma acetate concentrations were higher (P = 0·03) in overweight (1·55 (range 1·43-1·65) mmol/l) v. moderate-condition (1·39 (range 1·22-1·47) mmol/l) mares. We conclude that the higher plasma acetate in overweight mares should be further investigated as a potential link between gut microbes and obesity in horses.

Systems biology of adipose tissue metabolism: regulation of growth, signaling and inflammation

Adipose tissue (AT) depots actively regulate whole body energy homeostasis by orchestrating complex communications with other physiological systems as well as within the tissue. Adipocytes readily respond to hormonal and nutritional inputs to store excess nutrients as intracellular lipids or mobilize the stored fat for utilization. Co-ordinated regulation of metabolic pathways balancing uptake, esterification, and hydrolysis of lipids is accomplished through positive and negative feedback interactions of regulatory hubs comprising several pleiotropic protein kinases and nuclear receptors. Metabolic regulation in adipocytes encompasses biogenesis and remodeling of uniquely large lipid droplets (LDs). The regulatory hubs also function as energy and nutrient sensors, and integrate metabolic regulation with intercellular signaling. Over-nutrition causes hypertrophic expansion of adipocytes, which, through incompletely understood mechanisms, initiates a cascade of metabolic and signaling events leading to tissue remodeling and immune cell recruitment. Macrophage activation and polarization toward a pro-inflammatory phenotype drives a self-reinforcing cycle of pro-inflammatory signals in the AT, establishing an inflammatory state. Sustained inflammation accelerates lipolysis and elevates free fatty acids in circulation, which robustly correlates with development of obesity-related diseases. The adipose regulatory network coupling metabolism, growth, and signaling of multiple cell types is exceedingly complex. While components of the regulatory network have been individually studied in exquisite detail, systems approaches have rarely been utilized to comprehensively assess the relative engagements of the components. Thus, need and opportunity exist to develop quantitative models of metabolic and signaling networks to achieve a more complete understanding of AT biology in both health and disease.

De novo lipogenesis in the differentiating human adipocyte can provide all fatty acids necessary for maturation

The primary products of de novo lipogenesis (DNL) are saturated fatty acids, which confer adverse cellular effects. Human adipocytes differentiated with no exogenous fat accumulated triacylglycerol (TG) in lipid droplets and differentiated normally. TG composition showed the products of DNL (saturated fatty acids from 12:0 to 18:0) together with unsaturated fatty acids (particularly 16:1n-7 and 18:1n-9) produced by elongation/desaturation. There was parallel upregulation of expression of genes involved in DNL and in fatty acid elongation and desaturation, suggesting coordinated control of expression. Enzyme products (desaturation ratios, elongation ratios, and total pathway flux) were also correlated with mRNA levels. We used (13)C-labeled substrates to study the pathway of DNL. Glucose (5 mM or 17.5 mM in the medium) provided less than half the carbon used for DNL (42% and 47%, respectively). Glutamine (2 mM) provided 9-10%, depending upon glucose concentration. In contrast, glucose provided most (72%) of the carbon of TG-glycerol. Pathway analysis using mass isotopomer distribution analysis (MIDA) revealed that the pathway for conversion of glucose to palmitate is complex. DNL in human fat cells is tightly coupled with further modification of fatty acids to produce a range of saturated and unsaturated fatty acids consistent with normal maturation.

Is it time to revisit the Pedersen hypothesis in the face of the obesity epidemic?

The Pedersen hypothesis was formulated more than 50 years ago. Jorgen Pedersen primarily cared for women with type 1 diabetes. He suggested that fetal overgrowth was related to increased transplacental transfer of glucose, stimulating the release of insulin by the fetal beta cell and subsequent macrosomia. Optimal maternal glucose control decreased perinatal mortality and morbidity. However, over the ensuing decades, there have been increases in maternal obesity and subsequently gestational diabetes mellitus (GDM) and type 2 diabetes. The underlying pathophysiology of type 1 and GDM/type 2 diabetes are fundamentally different, type 1 diabetes being primarily a disorder of beta cell failure and type 2 diabetes/GDM including both insulin resistance and beta cell dysfunction. As such the metabolic milieu in which the developing fetus is exposed may be quite different in type 1 diabetes and obesity. In this review we examine the metabolic environment of obese diabetic women and lipid metabolism affecting fetal adiposity. The importance of understanding these issues relates to the increasing trends of obesity worldwide with perinatal programming of metabolic dysfunction in the offspring.

De novo fatty acid synthesis and NADPH generation in equine adipose and liver tissue

The lipogenic capacities of equine liver and adipose tissue explants were investigated in vitro. Preference for glucose or acetate as the primary carbon source for de novo fatty acid synthesis was determined using (14)C labeled substrates. Additional aims included determining the relative contribution of NADPH generating pathways to reducing equivalent generation and comparing the lipogenic activity of two adipose depots, mesenteric and subcutaneous harvested from the crest region of the neck. Mesenteric adipose tissue had greater lipogenic activity than subcutaneous adipose tissue, and liver tissue showed minimal (14)C incorporation into fatty acids, indicating a low hepatic lipogenic capacity. Acetate was found to be the primary carbon source for fatty acid synthesis due to both the appearance of the (14)C label in the lipid fraction and the low activity of ATP-citrate lyase. Finally, the pentose phosphate and isocitrate dehydrogenase enzymes contributed to NADPH production in equine adipose tissue.

Dairy calcium supplementation in overweight or obese persons: its effect on markers of fat metabolism

BACKGROUND

Dairy calcium supplementation has been proposed to increase fat oxidation and to inhibit lipogenesis.

OBJECTIVE

We aimed to investigate the effects of calcium supplementation on markers of fat metabolism.

DESIGN

In a placebo-controlled, crossover experiment, 10 overweight or obese subjects who were low calcium consumers received 800 mg dairy Ca/d for 5 wk. After 4 wk, adipose tissue was taken for biopsy for analysis of gene expression. Respiratory exchange, glycerol turnover, and subcutaneous adipose tissue microdialysis were performed for 7 h after consumption of 400 mg Ca or placebo, and the ingestion of either randomized slow-release caffeine (SRC; 300 mg) or lactose (500 mg). One week later, the test was repeated with the SRC or lactose crossover.

RESULTS

Calcium supplementation increased urinary calcium excretion by 16% (P = 0.017) but did not alter plasma parathyroid hormone or osteocalcin concentrations. Resting energy expenditure (59.9 +/- 3.0 or 59.6 +/- 3.3 kcal/h), fat oxidation (58.4 +/- 2.5 or 53.8 +/- 2.2 mg/min), plasma free fatty acid concentrations (0.63 +/- 0.02 or 0.62 +/- 0.03 mmol/L), and glycerol turnover (3.63 +/- 0.41 or 3.70 +/- 0.38 micromol . kg(-1) . min(-1)) were similar with or without calcium, respectively. SRC significantly increased free fatty acid concentrations, resting fat oxidation, and resting energy expenditure. During microdialysis, epinephrine increased dialysate glycerol concentrations by 250% without and 254% with calcium. Expression of 7 key metabolic genes in subcutaneous adipose tissue was not affected by calcium supplementation.

CONCLUSION

Dairy calcium supplementation in overweight subjects with habitually low calcium intakes failed to alter fat metabolism and energy expenditure under resting conditions and during acute stimulation by caffeine or epinephrine.

Liver lipid metabolism

The liver plays a key role in lipid metabolism. Depending on species it is, more or less, the hub of fatty acid synthesis and lipid circulation through lipoprotein synthesis. Eventually the accumulation of lipid droplets into the hepatocytes results in hepatic steatosis, which may develop as a consequence of multiple dysfunctions such as alterations in beta-oxidation, very low density lipoprotein secretion, and pathways involved in the synthesis of fatty acids. In addition an increased circulating pool of non-esterified fatty acid may also to be a major determinant in the pathogenesis fatty liver disease. This review also focuses on transcription factors such as sterol-regulatory-element-binding protein-1c and peroxisome proliferator-activated receptor alpha, which promote either hepatic fatty acid synthesis or oxidation.

Kinetic characterization of recombinant human cytosolic phosphoenolpyruvate carboxykinase with and without a His10-tag

We report the first kinetic characterization of human liver cytosolic GTP-dependent phosphoenolpyruvate carboxykinase (GTP-PEPCK), which plays a major role in the development of type 2 diabetes in human. In this work two recombinant forms of the enzyme were studied. One form had a His10-tag and the other was His-tag-free, and with one exception, both exhibited similar kinetic properties. When Mn2+ was used as the sole divalent cation, the His10-tagged enzyme, but not the His-tag-free enzyme, was increasingly inhibited at Mn2+ concentrations greater than 0.7 mM. This inhibition did not pose any problem in kinetic analysis, for within the relevant Mn2+ concentration range the His-tagged human PEPCK behaved almost identically to the tag-free enzyme. This property will bring simplicity and speed to purifying and studying multiple structural variants of this important enzyme. Apparent Km values of tag-free enzyme for phosphoenolpyruvate, GDP and bicarbonate were 450, 79 and 20,600 microM, respectively, while those for oxaloacetate and GTP were 4 and 23 microM, respectively, emphasizing the enzyme's gluconeogenic character. Bicarbonate (>100 mM) inhibited OAA-forming activity, which was a new observation with a GTP-PEPCK. The apparent Km for Mn2+ in the PEP-forming direction was 30-fold lower than that for the OAA-forming direction. Mn2+ and bicarbonate or CO2 might regulate the enzyme in vivo.

Lipid metabolism and secretory function of porcine intramuscular adipocytes compared with subcutaneous and perirenal adipocytes

The function of adipocytes interspersed between myofiber fasciculi in skeletal muscle physiology and physiopathology is poorly documented. Because regional differences in adipocyte features have been reported in various species, we hypothesized that lipid metabolism and secretory function of intramuscular (IM) adipocytes differ from that of nonmuscular adipocytes. In the present study, adipocytes isolated from trapezius muscle were compared with subcutaneous and perirenal adipocytes in growing pigs. Between 80 and 210 days of age, gene expressions and/or activities of enzymes involved in lipogenesis or lipolysis were much lower (P < 0.05) in adipocytes isolated from muscle than in those from other locations. Insulin-induced lipogenesis and lipolytic efficiency after catecholamine addition were also the lowest (P < 0.05) in IM adipocytes. In these cells, the age-related increase (+300%) in the ratio of mRNA levels of fatty acid synthase to hormone-sensitive lipase paralleled the enlargement of adipocyte diameters (+70%, P < 0.05) and the increase in lipid content in muscle (+135%, P < 0.05) during growth. Expressions of genes coding for leptin, adiponectin, and IGF-I, as well as for various hormonal receptors, were lower (P < 0.05) in IM adipocytes than in other adipocytes, whereas levels of TNF-alpha mRNA did not differ between sites. Interestingly, IGF-II mRNA levels were higher (P < 0.05) in IM adipocytes than in other adipocytes. These data support the view that IM fat is not just an ectopic extension of other fat locations but displays specific biological features during growth.

Comparative study of the lipogenic potential of human and rat adipose tissue

The reported low activity of lipogenic enzymes (especially adenosine triphosphate [ATP]-citrate lyase) in human adipose tissue led to the general conclusion that in humans lipogenesis occurs primarily in the liver. However, recent studies indicate that the liver plays a minor role in de novo lipogenesis and suggest that adipose tissue may be the principal lipogenic human tissue. In an attempt to resolve these contradictions we reinvestigated the lipogenic potential of human adipose tissue and compared with adipose tissue of rats fed a high-fat diet for 2 weeks and fasted overnight before death. These conditions mimic the nutritional state of patients at the moment of tissue sampling. We found that overnight fasting of the rats maintained previously for 12 days on a high-fat diet caused a decrease of ATP-citrate lyase of about 7-fold. Thus, in human adipose tissue, the mean activity of ATP-citrate lyase was approximately 8 times lower than in rats fed a high-fat diet and fasted overnight, and about 50 times lower than in rats maintained on normal laboratory diet. Unlike ATP-citrate lyase, fatty acid synthase (FAS) activity was only slightly lower in human adipose tissue than in rats maintained on a normal laboratory diet. Comparable FAS activity was found when rats were fed a high-fat diet and fasted overnight. The average activities of human adipose tissue acetyl-coenzyme A carboxylase, malic enzyme, and glucose-6-phosphate dehydrogenase were approximately 3-, 4-, and 6-fold lower than in adipose tissue from rats fed a high-fat diet and fasted overnight before tissue sampling, while the activity of 6-phosphogluconate dehydrogenase in humans was higher than in rat adipose tissue. No significant differences in lipogenic enzyme activities were found between male and female and between lean and obese patients. The rate of fatty acid synthesis in intact pieces of human adipose tissue was approximately 5 times lower than in adipose tissue pieces of rats fed a high-fat diet and fasted overnight before tissue samples were taken. The comparison of the lipogenic potential of humans and rats (maintained on the diet to mimic the nutritional state of patients at the time of tissue sampling) suggests that human adipose tissue is an important site of fatty acid synthesis.

Trans-vaccenic acid is desaturated to conjugated linoleic acid in mice

Mice were fed pure trans11 octadecenoic acid (trans-vaccenic acid; TVA) to determine whether it is desaturated to cis9, trans11 octadecadienoic acid, a predominant isomer of conjugated linoleic acid (CLA). In a preliminary trial, 12% of the TVA consumed during a 2-wk feeding period was recovered in the carcass as CLA. As a proportion of TVA in the tissues available for bioconversion, 48.8% was desaturated. We tested whether desaturation could be modified by supplementing no modifier, 0.5% clofibric acid to stimulate desaturation, or increasing the polyunsaturated fatty acids (PUFA) (10% corn oil vs. 4% corn oil) to inhibit desaturation in diets with or without 1% TVA. These diets were fed to six groups of mice in a 3x2 factorial arrangement of treatments. Feeding 1% TVA with 10% corn oil decreased feed intake (2.70 vs. 3.73 g/d, SEM 0.23; P<0.05). Bioconversion of dietary TVA was 12.0, 7.5 and 5.1% for mice fed no modifier of desaturation, clofibrate and increased PUFA, respectively. Conversion based on TVA available for desaturation was 52.6, 55.5 and 37.0%, respectively. Thus, clofibrate did not increase bioconversion, but increasing PUFA decreased conversion by 30%. To test whether TVA decreases food intake directly or after conversion to CLA, four groups of mice were fed diets containing 1% stearic, TVA, elaidic or conjugated linoleic acid. Dietary CLA decreased food intake and body fat, but did not change body protein. CLA was found in the carcass only when TVA or CLA was fed. CLA was found in both triacylglycerol and phospholipids when CLA was fed, but only in triacylglycerol when TVA was fed, suggesting that bioconversion occurred in the adipose tissue. In three trials, conversion of dietary TVA to CLA was 11.4+/-1.25%; conversion of stored TVA was 50.8+/-1.91%. Similar bioconversion of TVA in humans would increase current estimates of CLA available for the general population by 6- to 10-fold.

Insulin increases fatty acid synthase gene transcription in human adipocytes

The purpose of this study was to investigate the molecular mechanism whereby insulin increases expression of a key de novo lipogenic gene, fatty acid synthase (FAS), in cultured human adipocytes and hepatoma cells. RNA isolated from cultured adipocytes or from Hep G2 cells treated with or without insulin (20 nM) was analyzed. In addition, run-on transcription assays and measurements of RNA half-life were performed to determine the controlled step in FAS gene regulation by insulin. We demonstrated that FAS mRNA was expressed in both Hep G2 cells and human adipocytes. Insulin induced an approximately five- and three-fold increase in FAS mRNA content in adipocytes and hepatoma cells, respectively. Similar regulation of FAS was observed in adipocytes from lean and obese human subjects. Furthermore, we demonstrated that the induction of human FAS expression by insulin was due to increased transcription rate of the FAS gene in human adipocytes, whereas mRNA stabilization accounted for increased FAS mRNA content in hepatoma cells. In conclusion, we report here for the first time expression of human FAS mRNA and its specific transcriptional induction by insulin in cultured human adipocytes.

Twenty-four-hour energy and nutrient balance in weight stable postobese patients after biliopancreatic diversion

To better understand the long-term weight stability of postobese patients who underwent biliopancreatic diversion (BPD), we studied 24-h energy and nutrient balance in eight women at least 3 yr after surgery (PO) and compared the results to those obtained in eight normal never-obese control women (C), matched by age and weight. Body composition was measured by dual-energy x-ray absorptiometry (DXA). All the patients were on an ad libitum diet; 24-h energy and nutrient intake were measured on the experimental day. Twenty-four-hour energy expenditure (EE) and 24-h nutrient oxidation rates were measured in a respiratory chamber, and energy and nutrient balances were calculated after correcting for 24-h fecal nutrient loss. No differences in body composition were found between PO and C. PO had a higher gross energy intake than C (10.6 +/- 3.4 vs. 8.0 +/- 2.2 MJ/d; p < 0.05); however, due to the higher energy fecal loss in PO as compared to C (2.4 +/- 1.3 vs. 0.09 +/- 0.01 MJ/day; p < 0.01), 24-h metabolizable energy intake (MEJ) was not different in the two groups. The energy fecal loss in the PO patients was mostly in the form of lipid. EE at 24 h was not different in PO as compared to C. Therefore energy balance, computed as the difference between 24-h MEI and 24-h EE, was similar in the two groups. Respiratory quotient was significantly higher in PO than in C (1.00 +/- 0.08 vs. 0.83 +/- 0.03; p < 0.01). Carbohydrate (-135 +/- 37 g/d in PO vs. 63 +/- 23 g/d in C; p < 0.001), and lipid (48 +/- 14 g/d in PO vs. -23 +/- 6 g/d in C; p < 0.001) balances were different in the two groups. We conclude that chronic lipid malabsorption was the main metabolic abnormality explaining the achievement of energy balance in postobese subjects after biliopancreatic diversion. A chronic reduction of lipid absorption seems to play a key role in the long-term weight stability of this group of postobese subjects.

Metabolic effects of nicotine on human adipose tissue in organ culture

Fragments of human adipose tissue were maintained in culture for 1 week in a medium containing 1 mU/ml insulin and 100 ng/ml dexamethasone. Under these conditions lipoprotein lipase activity was present in human adipose tissue fragments which converted [14C]glucose to 14CO2 and [14C]triglyceride. Both metabolic parameters studied were affected by human tumor necrosis factor and brefeldin A. When fragments of human adipose tissue after 1 week in culture were incubated with nicotine tartrate for 20 h, a slight but significant increase in lipoprotein lipase activity was observed, and an increased conversion of [14C]glucose to 14CO2 and [14C]triglyceride occurred. Nicotine was taken up by human adipose tissue, but no conversion to cotinine was observed. Our data demonstrate a direct effect of nicotine on human adipose tissue metabolism. Furthermore, it is suggested that weight loss in smokers is a multifactorial phenomenon, and one of the important factors to be considered is the direct effect of nicotine within the tissue.

Comparison of the effect of the amount and degree of unsaturation of dietary fat on plasma low density lipoproteins in vervet monkeys

The effects of the degree of unsaturation and of the amount of dietary fat on low density lipoprotein (LDL) concentration and composition were determined in vervet monkeys. Diets with fat contents of 41, 31 and 18% energy, each with a low and a high polyunsaturated to saturated fatty acid ratio (P/S; 0.27-0.38 and 1.13-1.47) were fed to six female vervet monkeys for two months. Another six females were given a low fat, high P/S diet for the same period of time, to serve as a reference. The cholesterol contents of the diets were low (21-33 mg per day) and relatively constant. LDL cholesterol concentrations decreased significantly (P < or = 0.01) when the dietary fat content decreased from 31 to 18% of energy. The dietary P/S ratio only affected LDL cholesterol concentrations during moderate (31% of energy) fat intake, where LDL cholesterol increased (P < or = 0.01) with a decrease in dietary P/S. Substantial individual variations were observed in LDL cholesterol concentration responses to dietary fat changes. The changes in LDL cholesterol concentrations were the result of changes in the concentration of LDL particles, as the molecular composition did not differ significantly between dietary periods. The high density lipoprotein cholesterol and the plasma triacylglycerol concentrations were not influenced by the dietary fat changes. During the high P/S diets, the percentage of 18:2 (linoleic acid) increased (P < or = 0.01) and that of 18:1 (oleic acid) decreased (P < or = 0.01) in LDL esterified cholesterol, as compared to the low P/S diets. In adipose tissue triacylglycerol the percentage of 18:2 was three times higher (P < or = 0.01) during the high P/S diets than during the low P/S diets.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of an eucaloric high carbohydrate diet on circulating levels of glucose, fructose and non-esterified fatty acids in patients with cirrhosis

Twelve cirrhotic patients and six controls were fed an eucaloric high carbohydrate (CHO) diet for 3 days. Fasting serum triglyceride (TG), non-esterified fatty acids (NEFA), glucose, insulin and glycerol were estimated daily. On the 3rd day of the study we measured NEFA, glucose, insulin, and fructose every 45 min from 07:45 h until 19:45 h, and then every 4 h until 07:45 h the next day. The patients were divided into two groups of six on the basis of plasma lecithin-cholesterol acyltransferase (LCAT) activity: group A cirrhotics (with good liver function--LCAT activity: 40.6-65.7 nmol.ml-1.h-1; mean 48.5), and group B (poor liver function--LCAT: 23.7-32.3; mean 27.4). On the high CHO diet there was an increase in the fasting serum TG with a peak after 2 or 3 days. The increase in serum TG in controls was greater (p less than 0.01) than in either group of cirrhotics. In the controls and in group A most of the extra TG was carried in VLDL; in group B only 39% of the TG increment was found in VLDL. Fasting NEFA fell with 3 days of CHO feeding in the control group (p less than 0.01); they were unchanged in group A, and rose in group B to a significantly higher level than in controls (p less than 0.01). During day 3 when a high CHO diet was fed plasma NEFA levels fell in cirrhotics, and for most of the day the mean NEFA concentration in group B patients was significantly (p less than 0.05) lower than in normals. On day 3 glucose and fructose levels rose after each meal--much more in cirrhotics than in controls (and more in group B than in group A), and for most of the day they were significantly higher in group B patients as compared to the controls (glucose p less than 0.01, fructose p less than 0.001). Our results supported the hypothesis that plasma NEFA would be lower following high CHO meals in cirrhotics than in controls. This suggests that a high NEFA utilisation, which occurs in fasting cirrhotics, is not present throughout the day. Following a CHO meal, we suggest that tissues derive energy directly from the dietary sugars which are present in high concentration during the period of absorption and that this reduces the post prandial requirement for NEFA.

Comparative studies on lipogenic enzyme activities in the liver of human and some animal species

1. The activities of enzymes involved in fatty acid synthesis in the human liver (sample taken during abdominal surgery) and in the livers of some animals were studied. 2. Fatty acid synthase, ATP-citrate lyase and malic enzyme activities were found to be from 4 to 70-fold lower in human liver than in rat or bird livers. 3. The activities of hexose monophosphate shunt dehydrogenases in human liver were from half to almost equal to the corresponding activities in birds, but much lower than in rat liver. 4. The activities of all enzymes listed above in human and beef liver were very similar (except fatty acid synthase which was undetectable in the beef liver). 5. Very high activity of NADP-linked isocitrate dehydrogenase was found in livers of all species tested. 6. These results are discussed in relation to the role of the human liver in lipogenesis. 7. The activities of the enzymes generating NADPH in human liver taken during abdominal surgery were similar to the activities observed in the tissue obtained post mortem. 8. This suggested that post mortem tissue may be used as a reliable human material for some enzyme assays. 9. Thus we also examined the activity of malic enzyme in post mortem human kidney cortex, heart, skeletal muscle and brain. 10. Relatively high activity of NADP-linked malic enzyme has been observed in human brain.

Familial lipoprotein lipase-activity deficiency: study of total body fatness and subcutaneous fat tissue distribution

Total body fatness and subcutaneous fat tissue distribution were evaluated in 19 hyperchylomicronemic patients. Eleven were males, aged 10 to 57 years, and eight were females, aged 13 to 46 years. Familial lipoprotein-lipase-activity deficiency was diagnosed by the absence of lipoprotein-lipase activity in the plasma withdrawn ten and 20 minutes after intravenous injection of ten units of heparin per kilogram of body weight. The 19 patients had skin-fold measurements for evaluation of subcutaneous fat distribution. Fifteen also underwent body density measurements by underwater weighing. Percent body fat was calculated from body density. These anthropometric data were plotted against the regression curves of 1638 normal controls of both sexes (aged 10 to 54 years) for fat tissue weight, percent body fat, subcutaneous fat/total fat mass ratio and trunk/extremity skin-fold ratio. Impairments in the process of building fat tissue reserves could not be shown in the 19 hyperchylomicronemic patients, in spite of the absence of lipoprotein-lipase activity in their postheparin plasma. It is hypothesized that normal fat tissue mass in these patients could be due partly to de novo synthesis of fatty acids by adipocytes, hydrolysis of plasma triglycerides by hepatic lipase, and/or contribution of a specific fat-tissue lipase to the catabolism of plasma triglyceride-rich lipoproteins.

Lipogenesis in liver and adipose tissue of the domestic cat (Felis domestica)

1. Production of fatty acids, CO2 and glycerol from glucose and acetate was determined in slices of liver and adipose tissue taken from mature cats. 2. Acetate was the predominant carbon source for de novo fatty acid synthesis in both liver and adipose tissue. 3. Fatty acid synthesis occurred at much greater rates in adipose tissue than in liver. 4. Relative lipogenic capacity (ratio of production of long-chain fatty acids and glycerol to oxidation of substrates) was 20 times greater in adipose tissue than in liver with both substrates.

Regulation of lipoprotein lipase activity in the sand rat: effect of nutritional state and cAMP modulation

The sand rat (Psammomys obesus) is a desert rodent in which obesity and diabetes mellitus appeared only subsequent to feeding laboratory animal chow. To study the role of lipoprotein lipase in the development and maintenance of obesity in the sand rat, enzyme activity in various organs and in plasma of sand rats or albino rats was determined following a 20-hour fast, or 16 hours after injection of cholera toxin. Despite comparable change in body weight, an altered pattern of enzyme distribution and regulation was observed in the sand rat. Neither fasting nor cholera toxin had an effect on heart and daiphragm muscle lipoprotein lipase activity of the sand rat, but caused a 1.5- to 2-fold increase in the treated albino rats. By using an isolated perfused heart system, we were able to measure enzyme activity present in the heparin-releasable fraction that represents the functional pool of the enzyme. In both species, the heparin-releasable fraction of the heart increased twofold following fasting, though initial values were lower in sand rat. In both species, fasting and cholera toxin administration resulted in an increase in plasma and liver lipoprotein lipase activity. Adipose tissue lipoprotein lipase activity of sand rat, unlike the albino rats, was similar in the various fat regions and was not lowered by food deprivation or cholera toxin administration. After both treatments, sand rat plasma insulin levels exceeded fivefold those of albino rats. Adipose tissue lipoprotein lipase activity of fed and fasted normal and diabetic sand rats correlated negatively with plasma insulin and glucose levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipogenic activity and brown fat content of human perirenal adipose tissue

The incorporation of 3H2O and/or 14C-glycerol into lipids and the specific activities of the enzymes acetyl CoA carboxylase and lipoprotein lipase were measured in the perirenal and subcutaneous adipose tissue of human subjects. The perirenal adipose tissue of younger subjects with higher brown adipocyte content had higher rates of lipogenesis and enzyme activities per gram tissue than the corresponding subcutaneous tissue. However, in individual specimens, the perirenal/subcutaneous ratios of all but one of the above parameters failed to show a correlation with the brown adipocyte content of the perirenal adipose tissue. One parameter, namely 3H2O incorporation into fatty acids per adipocyte, did relate to the brown adipocyte content of the perirenal adipose tissue in four normal-weight patients only.

Plasma lipid and lipoprotein response to carbohydrate feeding in cirrhotic patients

Serum lipids and lipoproteins, and glucose and insulin, were measured after an overnight fast, and during 3 days of a eucaloric diet rich in carbohydrate, in 15 patients with cirrhosis and seven normal subjects. Following the high-carbohydrate diet triglyceride rose in all groups but the increase in cirrhotics was lower than in normals. In normals and in cirrhotics with good liver function most of the triglyceride increment was carried in VLDL; in cirrhotics with poor liver function only 31% of the increment was found in VLDL, and 56% in triglyceride-rich LDL. In an earlier study on fat feeding, our cirrhotic patients with poor liver function had an impaired chylomicron and VLDL response; they also carried most of the triglyceride increment in triglyceride-rich LDL. The markedly impaired response of triglyceride-rich lipoproteins to both carbohydrate and fat feeding suggests that sick cirrhotics may have a problem with storage of dietary energy and that this contributes to loss of their adipose tissue.

Fatty acid synthetase in human adipose tissue: purification and characterization

1. Fatty acid synthetase of abdominal subcutaneous adipose tissue of humans obtained at autopsy was purified 95 times with a final recovery of 10%. 2. The purity of the enzyme was verified by polyacrylamide disc gel electrophoresis and isoelectric point determination. Both techniques revealed only one protein band. 3. The fatty acid synthetase molecular weight was determined by SDS-polyacrylamide disc gel electrophoresis and found to be 500,000. 4. The fatty acid synthetase Km for both NADPH and malonyl-CoA were also determined and were found to be 18 and 28 microM respectively. 5. The presence of an active fatty acid synthetase in human adipose tissue raises the obvious question as to its biological role.

Properties and organ distribution of ATP citrate (pro-3S)-lyase

1. The lipogenic enzyme ATP citrate lyase (ATP:citrate oxaloacetate-lyase (pro-3S-CH2COO-acetyl-CoA; ATP-dephosphorylating), EC 4.1.3.8) is partially purified from human liver by ammonium sulfate fractionation and anionexchange chromatography. 2. Km values for the substrates are 1.1 x 10(-5) 1.3 x 10(-3), and 1.2 x 10(-4) M for CoASH, ATP and citrate, respectively. The hypolipidemic drug L(-)-hydroxycitrate is a competitive inhibitor with respect to citrate (Ki = 3 x 10(-4) M). 3. Specific activities measured in liver, adipose tissue and intestinal mucosa (autopsic and biopsic material) are in the range of 1 mU/mg protein suggesting that the citrate pathway does not significantly contribute to human lipogenesis. No stimulation is found after a 3-day carbohydrate-rich diet. 4. Specific activities of other key-enzymes of the acetyl-CoA production from carbohydrates (pyruvate dehydrogenase, cytosolic acetyl-CoA synthetase) are of the same low magnitude.

Synthesis of fatty acids and cholesterol by liver, adipose tissue and intestinal mucosa from obese and control patients

Biopsies of adipose tissue, liver and small bowel mucosa obtained from grossly obese and control subjects were used to study absolute rates of fatty acid, cholesterol, and other nonsaponifiable lipid synthesis using glucose as substrate and 3H2O as the isotopic marker. Fatty acid synthesis in subcutaneous adipose tissue expressed on a cell basis was greater in obese than control subjects and was stimulated by a high concentration of insulin (1000 micro U/ml), but not by a lower amount (100 micro U/ml). Fatty acid synthesis in omental adipose tissue exceeded by 3-fold that of subcutaneous fat. Fatty acid synthesis in obese liver was twice that of control liver and 20 times greater than obese adipose tissue. In terms of total organ activity fatty acid synthesis in fat tissue equalled or exceeded that of liver in both obese and control subjects. The cholesterol content of obese adipose tissue 1.86 +/- 0.11 mg/g exceeded that of controls 1.47 +/- 0.07 mg/g. All tissues examined synthesized cholesterol and nonsaponifiable lipids, liver greater than adipose tissue greater than small bowel mucosa. Nonsaponifiable lipid synthesis per gram of adipose tissue or liver was similar in obese and control tissue. The synthesis of total nonsaponifiable lipids including sterols, hydrocarbons and squalene was appreciable in adipose tissue and was approximately 15% of that of liver. However, cholesterol synthesis in the liver exporessed in terms of total organ activity was 50 times that in adipose tissue. The study demonstrates by direct comparison that liver is the dominant cholesterogenic organ in man and also shows that adipose tissue is a significant site of formation of fatty acids and nonsaponifiable lipids.

Energy metabolism in feasting and fasting

During feasting on a balanced carbohydrate, fat, and protein meal resting metabolic rate, body temperature and respiratory quotient all increase. The dietary components are utilized to replenish and augment glycogen and fat stores in the body. Excessive carbohydrate is also converted to lipid in the liver and stored along with the excessive lipids of dietary origin as triglycerides in adipose tissue, the major fuel storage depot. Amino acids in excess of those needed for protein synthesis are preferentially catabolized over glucose and fat for energy production. This occurs because there are no significant storage sites for amino acids or proteins, and the accumulation of nitrogenous compounds is ill tolerated. During fasting, adipose tissue, muscle, liver, and kidneys work in concert to supply, to convert, and to conserve fuels for the body. During the brief postabsorptive period, blood fuel homeostasis is maintained primarily by hepatic glycogenolysis and adipose tissue lipolysis. As fasting progresses, muscle proteolysis supplies glycogenic amino acids for heightened hepatic gluconeogenesis for a short period of time. After about three days of starvation, the metabolic profile is set to conserve protein and to supply greater quantities of alternate fuels. In particular, free fatty acids and ketone bodies are utilized to maintain energy needs. The ability of the kidney to conserve ketone bodies prevents the loss of large quantities of these valuable fuels in the urine. This delicate interplay among liver, muscle, kidney, and adipose tissue maintains blood fuel homeostasis and allows humans to survive caloric deprivation for extended periods.

Pathophysiology of lipoprotein transport

A system for classification of genetic and acquired forms of hyperlipidemia in humans based on lipoprotein physiology is described. Most hyperlipidemia can be accounted for by defects in one of four sites of physiologic regulation: (1) triglyceride-rich lipoprotein production, (2) lipoprotein lipase-mediated triglyceride catabolism, (3) remnant lipoprotein catabolism, and (4) extrahepatic cholesterol-rich lipoprotein catabolism.

Characterization of lipolytic responses of isolated white adipocytes from hamsters

1. Lipolysis by isolated white adipocytes from hamsters, as measured by glycerol production, was stimulated by corticotropin, isopropylnorepinephrine (INE), norepinephrine, or epinephrine (EPI), in a dose-dependent fashion. 2. Lipolysis was stimulated by five inhibitors of cyclic 3',5'-adenosine monophosphate phosphodiesterase: caffeine, theophylline, 1-methyl-3-isobutyl xanthine, 1-ethyl-4-(isopropylidenehydrazine)-1H-pyrazolo-(3,4,-b)-pyridine-5-carboxylic acid ethyl ester (SQ 20009), and 4-(3,4-dimethoxybenzyl)-2-imidazolidinone (Ro 7-2956). Caffeine-stimulated lipolysis consistently attained higher rates than did hormone-stimulated lipolysis. However, when cells were stimulated by both caffeine and a hormone, lipolytic rates were consistently lower than those attained under the influence of caffeine alone. 3. Isolated white adipocytes from hamsters were sensitive to both alpha- and beta-adrenergic antagonists. The beta-adrenergic antagonist propranolol could completely inhibit norepinephrine-stimulated glycerol production. The alpha-adrenergic antagonist phentolamine, on the other hand, had a biphasic effect on the cells. At 5-10(-7) M or 5-10(-6) M, phentolamine enhanced norepinephrine-stimulated lipolysis, while concentrations higher than 5-10(-5) M caused inhibition. 4. The effects of two different concentrations of six antilipolytic agents, prostaglandin E1, nicotinic acid, phenylisopropyladenosine, 5-methylpyrazole-3-carboxylic acid, adenosine and insulin, were measured. With the exception of insulin, all of these agents showed much more potent inhibition of caffeine-stimulated lipolysis than of hormone-stimulated lipolysis. Insulin, in contrast, showed only modest inhibition of hormone-stimulated lipolysis and virtually no inhibition of caffeine-stimulated lipolysis.

Regulation of lipolysis and cyclic AMP synthesis through energy supply in isolated human fat cells

The effects of glucose and of various inhibitors of glycolysis or of oxidative phosphorylation on stimulated lipolysis and on intracellular cyclic AMP and ATP levels were investigated in isolated human fat cells. The glycolysis inhibitors, NaF and monoiodoacetate, inhibited epinephrine or theophylline-stimulated lipolysis and parallely reduced the intracellular cyclic AMP and ATP levels; however, neither NaF nor monoidoacetate significantly affected dibutyryl cyclic AMP-induced lipolysis. Removal of glucose from the medium also reduced the rate of epinephrine-stimulated lipolysis and the intracellular cyclic AMP and ATP levels but failed to modify the lipolytic activity of dibutyryl cyclic AMP. The oxidative phosphorylation inhibitors, antimycin A and, under fixed conditions, 2,4-dinitrophenol also strongly decreased the adipocyte cyclic AMP and ATP levels but inhibited as well the rate of epinephrine- and of dibutyryl cyclic AMP-induced lipolysis. N-Ethylmaleimide, a mixed glycolysis and oxidative phosphorylation inhibitor, not only reduced the intracellular cyclic AMP and ATP levels and epinephrine- or theophylline-induced lipolysis, but also that stimulated by dibutyryl cyclic AMP. When glycolysis was almost fully inhibited, human fat cells were insensitive to epinephrine but remained fully responsive to dibutyryl cyclic AMP. These results, showing a relationship between ATP availability, cyclic AMP synthesis and lipolysis, suggest a different ATP requirement for cyclic AMP synthesis and triacylglycerol lipase activation, a difference which could explain why ATP issued from glucose breakdown appears to be a determinant factor for cyclic AMP synthesis, but not for triacylglycerol lipase activation in human fat cells.

Influence of trypsin on lipolysis in human fat cells. Comparison with rat adipocytes

1. Trypsin-treated human and rat fat cells were obtained by digestion of adipose tissue with collagenase plus trypsin and their lipolytic response to insulin, catecholamines and dibutyryl cyclic AMP were compared with the lipolytic response of human and rat fat cells isolated with collagenase only. 2. In both human and rat fat cells, no significant modification occurred in the intracellular lactate dehydrogenase content and in the basal release of glycerol after trypsination. 3. In rat fat cells, trypsin abolished the antilipolytic effect of insulin but maintained a normal lipolytic response to epinephrine, norepinephrine and isoproterenol. 4. In human fat cells, on the contrary, trypsin failed to modify the antilipolytic effect of insulin, but markedly potentiated the lipolytic response to epinephrine, norepinephrine and isoproterenol. Trypsin also increased the rate of intracellular 3' :5' cyclic AMP accumulation in response to catecholamines. Under these conditions, however, trypsin-treated human fat cells had a normal reponse to the lipolytic agent dibutyryl cyclin AMP. 5. These data suggest that human fat cells differ from the rat ones by the existence in human adipocyte membranes of a trypsin-sensitive component which inhibits the catecholamine induced lipolytic process and which is different from the alpha receptors.