Adipose tissue triglyceride turnover, de novo lipogenesis, and cell proliferation in humans measured with 2H2O

The turnover of adipose tissue components (lipids and cells) and the pathways of adipose lipid deposition have been difficult to measure in humans. We apply here a (2)H(2)O long-term labeling technique for concurrent measurement of adipose-triglyceride (TG) turnover, cell (DNA) proliferation, and de novo lipogenesis (DNL). Healthy subjects drank (2)H(2)O (70 ml/day) for 5-9 wk. Subcutaneous adipose tissue aspirates were taken (gluteal, thigh, and flank depots). Deuterium incorporation into TG glycerol (representing all-source TG synthesis), TG palmitate (representing DNL, by mass isotopomer distribution analysis), and DNA (representing cell proliferation) was measured by gas chromatography-mass spectrometry. Subjects tolerated the protocol well, and body (2)H(2)O enrichments were stable. Mean TG-glycerol fractional synthesis was 0.12 (i.e., 12%) with a range of 0.03-0.32 after 5 wk and 0.20 (range 0.08-0.49) after 9 wk (TG half-life 200-270 days). Label decay measurements 5-8 mo after discontinuing (2)H(2)O gave similar turnover estimates. Net lipolysis (TG turnover) was 50-60 g/day. DNL contribution to adipose-TG was 0.04 after 9 wk, representing approximately 20% of newly deposited TG. Cell proliferation was 0.10-0.17 after 9 wk (half-life 240-425 days). In summary, long-term (2)H(2)O administration to human subjects allows measurement of the dynamics of adipose tissue components. Turnover of all elements is slow, and DNL contributes approximately 20% of new TG.     

Measurement of protein turnover rates by heavy water labeling of nonessential amino acids

In vivo measurements of protein synthesis using isotope-labeled amino acids (AAs) are hampered by the heterogeneity of AA pools and, for slow turnover proteins, the difficulty and expense of long-term labeling. Continuous oral heavy water (2H2O) labeling can safely maintain stable body water 2H enrichments for weeks or months. 2H is metabolically incorporated into C-H bonds of nonessential AAs (NEAAs) and hence into proteins. No posttranslational label exchange occurs, so 2H incorporation into protein NEAAs, in principle, reports on protein synthesis. Here, we show by mass isotopomer distribution analysis (MIDA) of 2H2O-labeled rodent tissue proteins that metabolic 2H flux into C-H bonds of Ala, Gly, or Gln used for protein synthesis is nearly complete. By 2H2O labeling of rodents, turnover of bone and muscle mixed proteins was quantified and stimulation of liver collagen synthesis by CCl4 was detected. Kinetics of several human serum proteins were also measured, reproducing published t1/2 estimates. Plateau enrichments in Ala varied among different proteins. Moderate amounts of protein, isolated chromatographically or electrophoretically, sufficed for kinetic analyses. In conclusion, 2H2O labeling permits sensitive, quantitative, operationally simple measurements of protein turnover in vivo by the rise-to-plateau approach, especially for proteins with slow constitutive turnover.     

Physiologic and pharmacologic factors influencing glyceroneogenic contribution to triacylglyceride glycerol measured by mass isotopomer distribution analysis

An imbalance between triacylglycerol synthesis and breakdown is necessary for the development of obesity. The direct precursor for triacylglycerol biosynthesis is alpha-glycerol phosphate, which can have glycolytic and glyceroneogenic origins. We present a technique for determining the relative glyceroneogenic contribution to triacylglyceride glycerol by labeling the glycerol moiety with 2H2O. The number of hydrogen atoms (n) incorporated from H2O into C-H bonds reflects the metabolic source of alpha-glycerol phosphate and can be calculated by combinatorial analysis of the distribution of mass isotopomers in triacylglyceride glycerol. Three physiological settings with potential effects on glyceroneogenesis and glycolysis were studied in rodents. Adipose tissue acylglyceride glycerol in mice fed a low carbohydrate diet had significantly higher values of n than in mice fed a high carbohydrate diet, suggesting an increased contribution from glyceroneogenesis of from 17 to 50% on the low carbohydrate diet. Similarly, mice administered rosiglitazone had a significant relative increase in glyceroneogenesis (from 17 to 53%), indicated by an increase in adipose acylglyceride glycerol n. Fructose infusion in overnight fasted rats rapidly lowered plasma triacylglyceride glycerol n, reflecting a decreased contribution from glyceroneogenesis (from 66 to 34%) presumably because of increased glycolytic input. In conclusion, we demonstrate that the number of C-H atoms derived from cellular H2O in triacylglyceride glycerol is an informative indicator of alpha-glycerol phosphate origin and, ultimately, triacylglycerol metabolism. Under certain physiological conditions, glyceroneogenesis can be up-regulated in adipose (e.g. low carbohydrate diet) or down-regulated in liver (e.g. fructose infusion). Additionally, stimulation of glyceroneogenesis by rosiglitazone in adipose tissue may be an important factor in the antilipolytic actions of thiazolidinediones.     

A stable isotope method using a [(2)H(5)]glycerol bolus to measure very low density lipoprotein triglyceride kinetics in humans.

We have developed a method using a bolus of [(2)H(5)]glycerol to determine parameters of VLDL-triglyceride (VLDL-TG) turnover and have compared the data to that obtained using simultaneously a bolus of [2-(3)H]glycerol in six young normolipidemic men. No measurable enrichment was found after 12 h for [(2)H(5)]glycerol; therefore, we could only perform a monoexponential analysis of its data. No differences in fractional catabolic rate (FCR) were seen when comparing the multicompartmental modeling of [2-(3)H]glycerol data (modeled over 48 h) either to the monoexponential analyses of the [2-(3)H]glycerol or that of the [(2)H(5)]glycerol data. The two monoexponential approaches were highly correlated (r = 0.96 for FCR), however, FCR was 18% higher with the [(2)H(5)]glycerol than with the [2-(3)H]glycerol data (P < 0.003). In all six subjects, a 10-h infusion of [1-(13)C]acetate was started at the same time as the glycerol boluses were given. In two men we were able reliably to detect VLDL-TG-fatty acid enrichment. The measurement of FCR in these two subjects using the mass isotopomer distribution analysis (MIDA) approach was in good agreement (within 10%) with FCRs determined with the labeled glycerol methods.In conclusion, our results have shown that results obtained with the [(2)H(5)]glycerol bolus were highly correlated with those obtained with the [2-(3)H]glycerol, but the FCRs were slightly higher with the former. We have also demonstrated that FCRs determined from monoexponential modeling were in good agreement with those determined from the multicompartmental modeling of the TG-glycerol data.     

Metabolism of Different Adipose Tissues in Vivo in the Rat

To explore regional differences in triglyceride retention in white adipose tissues of growing male rats, the mass of adipocytes from epididymal, retroperitoneal, inguinal, and mesenteric tissues were followed with time. In order to attempt to explain regional differences, adipose tissue metabolism was studied in vivo and in vitro. (U-¹⁴ C) oleic acid in sesame oil was given by gastric gavage to conscious male and female rats, and accumulation and half-life of radioactivity measured. Lipoprotein lipase activity and lipolysis were studied in vitro. Adipocyte triglyceride mass increased linearly in all the depots during 4 months of observation. The increase in mass was more pronounced in retroperitoneal (0.31 μg) and epididymal (0.30 μg) than in mesenteric (0.11 μg) or inguinal (0.05 μg) adipocytes. In the fed state label from (U-¹⁴C) oleic acid first increased with time in liver, muscle, and adipose tissues. In the liver radioactivity peaked at 4 hours, and was not measurable in either liver or muscle after a time point between 24 hours to 1 week. In contrast label continued to increase in adipose tissues up to about 16 hours to 24 hours, suggesting transfer of label by recirculation from liver and muscle to adipose tissues. Thereafter the radioactivity decreased. When expressed per adipocyte uptake of label was not significantly different between white adipose tissues. The rate of decrease between 7 days and 4 months was, however, more rapid in mesenteric and inguinal than, particularly, epididymal, and, probably, retroperitoneal adipocytes. These results were partly parallel to in vitro data on lipoprotein lipase activity, which was not different between depots, and the rate of lipolysis, which was higher in mesenteric than other adipocytes. These results suggest that differences in weight increase of adipose tissue regions are due mainly to differences in the rate of mobilization of adipocyte triglycerides. When expressed per gram triglyceride, uptake and mobilization of label were clearly more rapid in mesenteric than other white adipose tissues. This is probably explained by a combination of a higher adipocyte density plus the metabolic characteristics of adipocytes in this depot. Since mesenteric adipose tissue is smaller than the other depots studied, the absolute contribution of this tissue to the energy supply of the body is probably not different from that of other adipose tissues, however. A large uptake and short half life was observed in interscapular adipose tissue. This region contains brown adipocytes, and the results therefore suggest that lipid uptake for thermogenic purposes is of a considerable magnitude. It was concluded that among white adipose tissues, the mesenteric tissue has a rapid turnover of triglyceride. This is probably due to a combination of a high density and specific metabolic characteristics of these adipocytes. Factors in the microenvironment of adipocytes probably contribute to the high turnover either directly, or by modification of cellular characteristics.     

Comparison of deuterium incorporation and mass isotopomer distribution analysis for measurement of human cholesterol biosynthesis

To compare endogenous cholesterol biosynthesis measured by deuterium incorporation (DI) and mass isotopomer distribution analysis (MIDA), cholesterol fractional and absolute synthetic rates were measured simultaneously by both techniques under identical physiological conditions. Twelve subjects (22 to 39 years of age) underwent a dual stable isotope protocol, involving oral deuterium oxide administration and measurement of incorporation of deuterium into cholesterol coincident with constant infusion of sodium [1-(13)C]acetate and measurement of the mass isotopomer distribution pattern of newly synthesized cholesterol. Synthesis was determined over 24 h with a 7-h feeding period. Both methods yielded similar measurements of fractional cholesterol synthesis (7.8 +/- 2.5% day(-)(1) for DI vs. 6.9 +/- 2.2% day(-)(1) for MIDA). Correlation of fractional synthesis across techniques was strong (r = 0.84, P = 0.0007). Absolute synthesis rates were also not different at 24 h (13.4 +/- 4.3 mg kg(-)(1) day(-)(1) for DI vs. 11.9 +/- 3.6 mg kg(-)(1) day(-)(1) for MIDA, r = 0.79, P < 0.002).We conclude that despite different assumptions and analytical requirements, deuterium incorporation and MIDA yield similar rates of cholesterogenesis in humans when measurements are made over 24 h. The decision as to which method to adopt depends on available clinical and analytical facilities     

Measurement of human plasma proteome dynamics with (2)H(2)O and liquid chromatography tandem mass spectrometry

Dysfunction of protein turnover is a feature of many human diseases, and proteins are substrates in important biological processes. Currently, no method exists for the measurement of global protein turnover (i.e., proteome dynamics) that can be applied in humans. Here we describe the use of metabolic labeling with deuterium ((2)H) from (2)H(2)O and liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis of mass isotopomer patterns to measure protein turnover. We show that the positions available for (2)H label incorporation in vivo can be calculated using peptide sequence. The isotopic incorporation values calculated by combinatorial analysis of mass isotopomer patterns in peptides correlate very closely with values established for individual amino acids. Inpatient and outpatient heavy water labeling protocols resulted in (2)H label incorporation sufficient for reproducible quantitation in humans. Replacement rates were similar for peptides deriving from the same protein. Using a kinetic model to account for the time course of each individual's (2)H(2)O enrichment curves, dynamics of approximately 100 proteins with half-lives ranging from 0.4 to 40 days were measured using 8 μl of plasma. The measured rates were consistent with literature values. This method can be used to measure in vivo proteome homeostasis in humans in disease and during therapeutic interventions.     

Coupling of lipolysis and de novo lipogenesis in brown,beige, and white adipose tissues during chronic β3-adrenergic receptor activation

Chronic activation of β3-adrenergic receptors (β3-ARs) expands the catabolic activity of both brown and white adipose tissue by engaging uncoupling protein 1 (UCP1)-dependent and UCP1-independent processes. The present work examined de novo lipogenesis (DNL) and TG/glycerol dynamics in classic brown, subcutaneous “beige,” and classic white adipose tissues during sustained β3-AR activation by CL 316,243 (CL) and also addressed the contribution of TG hydrolysis to these dynamics. CL treatment for 7 days dramatically increased DNL and TG turnover similarly in all adipose depots, despite great differences in UCP1 abundance. Increased lipid turnover was accompanied by the simultaneous upregulation of genes involved in FAS, glycerol metabolism, and FA oxidation. Inducible, adipocyte-specific deletion of adipose TG lipase (ATGL), the rate-limiting enzyme for lipolysis, demonstrates that TG hydrolysis is required for CL-induced increases in DNL, TG turnover, and mitochondrial electron transport in all depots. Interestingly, the effect of ATGL deletion on induction of specific genes involved in FA oxidation and synthesis varied among fat depots. Overall, these studies indicate that FAS and FA oxidation are tightly coupled in adipose tissues during chronic adrenergic activation, and this effect critically depends on the activity of adipocyte ATGL.     

Inverse regulation of basal lipolysis in perigonadal and mesenteric fat depots in mice

Given the strong link between visceral adiposity and (hepatic) insulin resistance as well as liver steatosis, it is crucial to characterize obesity-associated alterations in adipocyte function, particularly in fat depots drained to the liver. Yet these adipose tissues are not easily accessible in humans, and the most frequently studied depot in rodents is the perigonadal, which is drained systemically. In the present study, we aimed to study alterations in lipolysis between mesenteric and perigonadal adipocytes in mice. Basal free fatty acid and glycerol release was significantly lower in perigonadal compared with mesenteric adipocytes isolated from chow-fed C57BL/6J mice. However, this difference completely vanished in high-fat diet-fed mice. Consistently, protein levels of the G(0)/G(1) switch gene 2 (G0S2), which were previously found to be inversely related to basal lipolysis, were significantly lower in mesenteric compared with perigonadal fat of chow-fed mice. Similarly, perilipin was differently expressed between the two depots. In addition, adipocyte-specific overexpression of G0S2 led to significantly decreased basal lipolysis in mesenteric adipose tissue of chow-fed mice. In conclusion, lipolysis is differently regulated between perigonadal and mesenteric adipocytes, and these depot-specific differences might be explained by altered regulation of G0S2 and/or perilipin.     

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.     

The Hyplip2 locus causes hypertriglyceridemia by decreased clearance of triglycerides

The Hyplip2 congenic mouse strain contains part of chromosome 15 from MRL/MpJ on the BALB/cJ background. Hyplip2 mice show increased plasma levels of cholesterol and predominantly triglycerides (TGs) and are susceptible to diet-induced atherosclerosis. This study aimed at elucidation of the mechanism(s) explaining the hypertriglyceridemia. Hypertriglyceridemia can result from increased intestinal or hepatic TG production and/or by decreased LPL-mediated TG clearance. The intestinal TG absorption and chylomicron formation were studied after intravenous injection of Triton WR1339 and an intragastric load of olive oil containing glycerol tri[(3)H]oleate. No difference was found in intestinal TG absorption. Moreover, the hepatic VLDL-TG production rate and VLDL particle production, after injection of Triton WR1339, were also not affected. To investigate the LPL-mediated TG clearance, mice were injected intravenously with glycerol tri[(3)H]oleate-labeled VLDL-like emulsion particles. In Hyplip2 mice, the particles were cleared at a decreased rate (half-life of 25 +/- 6 vs. 11 +/- 2 min; P < 0.05) concomitant with a decreased uptake of emulsion TG-derived (3)H-labeled fatty acids by the liver and white adipose tissue. The increased plasma TG levels in Hyplip2 mice do not result from an enhanced intestinal absorption or increased hepatic VLDL production but are caused by decreased LPL-mediated TG clearance.     

Glycerol kinase activity and glycerol metabolism of rat granular pneumocytes in primary culture

Glycerol kinase activity and glycerol utilization by rat granular pneumocytes were determined in order to investigate the rate-limiting step for glycerol incorporation into lung lipids. Granular pneumocytes were isolated in primary culture following trypsinization of rat lungs. Glycerol kinase activity was 8.2 nmol/h per 10(6) cells. Incorporation of [1,3-14C]glycerol into total cell lipids was 0.29 nmol/h per 10(6) cells. In the presence of saturating glycerol concentration, production of 3H2O from [2-3H]glycerol was 13 times greater than incorporation of [14C]glycerol into lipids. Glycerol phosphate dehydrogenase activity in isolated cells was approximately 10 times glycerol kinase activity. In the presence of 5.6 mM glucose, glycerol incorporation into lipids was decreased 79% and detritiation of glycerol was decreased 34%. This effect of glucose was due to a 25% increase in cell glycerol 3-phosphate content, resulting in dilution of the precursor pool and possible inhibition of glycerol phosphorylation. These results indicate that the relatively limited incorporation of glycerol into surfactant phospholipids by lung epithelial cells reflects the relatively high rate of glycerol 3-phosphate oxidation.     

Glyceroneogenesis is the dominant pathway for triglyceride glycerol synthesis in vivo in the rat

Triglyceride synthesis in mammalian tissues requires glycerol 3-phosphate as the source of triglyceride glycerol. In this study the relative contribution of glyceroneogenesis and glycolysis to triglyceride glycerol synthesis was quantified in vivo in adipose tissue, skeletal muscle, and liver of the rat in response to a chow diet (controls), 48-h fast, and lipogenic (high sucrose) diet. The rate of glyceroneogenesis was quantified using the tritium ([(3)H(2)]O) labeling of body water, and the contribution of glucose, via glycolysis, was determined using a [U-(14)C]glucose tracer. In epididymal and mesenteric adipose tissue of control rats, glyceroneogenesis accounted for approximately 90% of triglyceride glycerol synthesis. Fasting for 48 h did not alter glyceroneogenesis in adipose tissue, whereas the contribution of glucose was negligible. In response to sucrose feeding, the synthesis of triglyceride glycerol via both glyceroneogenesis and glycolysis nearly doubled (versus controls); however, glyceroneogenesis remained quantitatively higher as compared with the contribution of glucose. Enhancement of triglyceride-fatty acid cycling by epinephrine infusion resulted in a higher rate of glyceroneogenesis in adipose tissue, as compared with controls, whereas the contribution of glucose via glycolysis was not measurable. Glyceroneogenesis provided the majority of triglyceride glycerol in the gastrocnemius and soleus. In the liver the fractional contribution of glyceroneogenesis remained constant (approximately 60%) under all conditions and was higher than that of glucose. Thus, glyceroneogenesis, in contrast to glucose, via glycolysis, is quantitatively the predominant source of triglyceride glycerol in adipose tissue, skeletal muscle, and liver of the rat during fasting and high sucrose feeding.     

In vitro lipid metabolism in the rat pancreas. I. Basal lipid metabolism.

1. The in vitro basal lipid metabolism of rat pancreatic fragments was compared with that in adipose tissue fragments and liver slices. 2. [1-14C]Acetate added to the media was mostly incorporated into palmitic acid and to a lesser extent into oleic acid. In addition, pancreatic tissue exhibited a marked capacity for elongation of polyunsaturated fatty acids by [1-14C]acetate and resulting desaturation when compared to adipose tissue and liver. 3. Data obtained in the presence of [U-14C]glucose, [1-14C]palmitate and 3H20 indicate that acetyl-CoA derived from glucose and from beta-oxidation of fatty acids contributed to de novo lipogenesis. 4. Oxidation of [1-14C]palmitic acid was 9-13 times higher in the pancreas than in adipose tissue or liver when expressed on a wet weight basis. 5. The fatty acid moiety of pancreatic glycerolipids could be derived from de novo synthesis, fatty acids added to the medium, or from fatty acids formed from the hydrolysis of endogenous lipids. The glycerol moiety could be derived either from glucose, or directly from glycerol through participation of glycerol kinase.     

Measuring committed preadipocytes in human adipose tissue from severely obese patients by using adipocyte fatty acid binding protein

To understand the significance of the reported depot differences in preadipocyte dynamics, we developed a procedure to identify committed preadipocytes in the stromovascular fraction of fresh human adipose tissue. We documented that adipocyte fatty acid binding protein (aP2) is expressed in human preadipocyte clones capable of replication, indicating that can be used as a marker of committed preadipocytes. Because aP2 expression can be induced in macrophages, stromovascular cells were also stained for the macrophage marker CD68. We found aP2+CD68- cells (designated as committed preadipocytes) that did not have lipid droplets (true preadipocytes) and that did have lipid droplets < 6.5 microm in diameter (very immature adipocytes). Adipose tissue from subcutaneous, omental, and mesenteric depots was obtained from nine patients undergoing bariatric surgery for measurement of stromovascular cell number, the number of committed preadipocytes (aP2+CD68-), aP2+ macrophages (aP2+CD68+), and aP2- macrophages (aP2-CD68+). The number of committed preadipocytes did not differ significantly between depots but varied >20-fold among individuals. Total cell number, stromovascular cell number, and the number of aP2- macrophages was less (P < 0.05) in subcutaneous than in omental fat (means +/- SE, in millions: subcutaneous, 2.3 +/- 0.3, 1.4 +/- 0.3, and 0.17 +/- 0.08; and omental, 4.8 +/- 0.7, 3.8 +/- 0.5, and 0.34 +/- 0.06); mesenteric depot was intermediate. These data indicate that the cellular composition of adipose tissue varies between depots and between individuals. The ability to quantify committed preadipocytes in fresh adipose tissue should facilitate study of adipose tissue biology.     

Contribution of the shunt pathway of mevalonate metabolism to the regulation of cholesterol synthesis in rat liver

The modulation of the shunt pathway of mevalonate metabolism (Edmond, J., and Popják, G. (1974) J. Biol. Chem. 249, 66-71) has been studied in livers from fed, starved, and diabetic rats perfused with a physiological concentration (300 nM) of [5-14C] + [5-3H]mevalonate. Shunt activity was measured by (i) production of 14CO2 (corrected for loss of label by exchange reactions) and (ii) production of 3H2O. Contribution of exogenous mevalonate to total mevalonate production (0.06-0.11%) was assessed in parallel experiments by the incorporation of 3H2O into sterols. Inhibition of non-saponifiable lipid synthesis by starvation and diabetes is not associated with an inhibition of mevalonate production but with a major increase in shunting (7-34%) of sterol-bound mevalonate. The shunt pathway of mevalonate metabolism appears to participate in the regulation of cholesterol synthesis.     

Studies on the role of insulin in the regulation of glyceride synthesis in rat epididymal adipose tissue.

1. When rat isolated fat-cells were incubated with fructose and palmitate, insulin significantly stimulated glyceride synthesis as measured by either [14C]fructose incorporation into the glycerol moiety or of [3H]palmitate incorporation into the acyl moiety of tissue glycerides. Under certain conditions the effect of insulin on glyceride synthesis was greater than the effect of insulin on fructose uptake. 2. In the presence of palmitate, insulin slightly stimulated (a) [14C]pyruvate incorporation into glyceride glycerol of fat-cells and (b) 3H2O incorporation into glyceride glycerol of incubated fat-pads. 3. At low extracellular total concentrations of fatty acids (in the presence of albumin), insulin stimulated [14C]fructose, [14C]pyruvate and 3H2O incorporation into fat-cell fatty acids. Increasing the extracellular fatty acid concentration greatly inhibited fatty acid synthesis from these precursors and also greatly decreased the extent of apparent stimulation of fatty acid synthesis by insulin. 4. These results are discussed in relation to the suggestion [A.P. Halestrap & R.M.Denton (1974) Biochem. J. 142, 365-377] that the tissue may contain a specific acyl-binding protein which is subject to regulation. It is suggested that an insulin-sensitive enzyme component of the glyceride-synthesis process may play such a role.     

Lipogenic capacity and relative contribution of the different tissues and organs to lipid synthesis in male rat

The rates of 3H2O incorporation into total lipids and fatty acids were measured in vivo in the different organs and tissues of 7-week old male Wistar rats to compare the lipogenic capacity of those organs and tissues and to determine their relative contributions to body lipid synthesis. Our results were the following; (1) liver was the major site of the synthesis of total lipids and fatty acids (37 and 42%, respectively, of body synthesis); (2) white adipose tissues synthesized about 24% of the total lipids; mesenteric adipose tissue alone synthesizing 40% of the fatty acids produced in dissectable white adipose tissues; (3) skin showed low lipid synthesis but played an appreciable role in that synthesis (8% of the total) due to its large contribution to total body weight; (4) other organs (excluding liver) showed low lipid synthesis; however, that of the small intestine was 1-2% of body synthesis; (5) the rest of the carcass (mainly musculature and skeleton) contributed 25% to body lipid synthesis. The putative roles of the different tissues and organs in adipose tissue development have been discussed.     

Metabolic characteristics of human subcutaneous abdominal adipose tissue after overnight fast

Subcutaneous abdominal adipose tissue is one of the largest fat depots and contributes the major proportion of circulating nonesterified fatty acids (NEFA). Little is known about aspects of human adipose tissue metabolism in vivo other than lipolysis. Here we collated data from 331 experiments in 255 healthy volunteers over a 23-year period, in which subcutaneous abdominal adipose tissue metabolism was studied by measurements of arterio-venous differences after an overnight fast. NEFA and glycerol were released in a ratio of 2.7:1, different (P < 0.001) from the value of 3.0 that would indicate no fatty acid re-esterification. Fatty acid re-esterification was 10.2 ± 1.4%. Extraction of triacylglycerol (TG) (fractional extraction 5.7 ± 0.4%) indicated intravascular lipolysis by lipoprotein lipase, and this contributed 21 ± 3% of the glycerol released. Glucose uptake (fractional extraction 2.6 ± 0.3%) was partitioned around 20-25% for provision of glycerol 3-phosphate and 30% into lactate production. There was release of lactate and pyruvate, with extraction of the ketone bodies 3-hydroxybutyrate and acetoacetate, although these were small numerically compared with TG and glucose uptake. NEFA release (expressed per 100 g tissue) correlated inversely with measures of fat mass (e.g., with BMI, r(s) = -0.24, P < 0.001). We examined within-person variability. Systemic NEFA concentrations, NEFA release, fatty acid re-esterification, and adipose tissue blood flow were all more consistent within than between individuals. This picture of human adipose tissue metabolism in the fasted state should contribute to a greater understanding of adipose tissue physiology and pathophysiology.