Impaired lipid metabolism in aged mice as revealed by fasting-induced expression of apolipoprotein mRNAs in the liver and changes in serum lipids

BACKGROUND

Changes in apolipoprotein (Apo) metabolism can cause an increased incidence of diseases such as cardiovascular disorders and diabetes with advancing age. Limited reports are available on this topic, however.

OBJECTIVE

To investigate age-related changes in mobilization of stored lipid, we studied the effects of fasting on the gene expression of Apos in the liver as well as serum triglyceride (TG) and cholesterol levels in the serum.

METHODS

Using young (6- to 8-month-old) and old (24- to 28-month-old) fasted and re-fed mice, Northern blots of hepatic mRNAs for Apos A-I, A-IV, C-II, C-III, and liver-type fatty acid-binding protein and HPLC analyses of serum lipids were conducted.

RESULTS

Fasting induced 4- and 20-fold increases in the mRNA of Apo C-II and A-IV, respectively, in young mice while only 1.1- and 7-fold increases, respectively, were detected in old mice. In contrast, the Apo C-III gene expression was significantly reduced by fasting in the young mice but the reduction was small in the old. In view of the stimulating effect of Apo C-II and A-IV and the inhibiting effect of C-III on lipoprotein lipase (LPL), these findings suggest that the fasting-induced activation of LPL may be considerably decreased in old mice. The amount of TG in very low-density lipoprotein (VLDL), a major form of the transport of TG to peripheral tissues, was significantly greater in the young than in the old mice. Despite possible activation of LPL by fasting, the amount of TG in VLDL, a major form of the transport of TG to peripheral tissues, was significantly greater in the young mice than in the old. It is indicated that the synthesis of VLDL in the liver is high in the young but low in the old mice, which also may be true for the rate of transport of TG.

CONCLUSION

The present findings suggest that mobilization of lipids is impaired in old animals due to decreased gene expression of Apos, possibly leading in the long run to excessive lipid accumulation in tissues such as the liver, adipose tissues and blood vessels even in normal feeding, and resulting in an increased incidence of age-related diseases.

Tamoxifen does not prevent the mobilization of body lipids elicited by oleoyl-estrone

Oleoyl-estrone is a powerful, slimming adipose tissue-derived signal that has biological effects widely opposed to those of its estrone moiety. The present experiment was designed to determine whether oleoyl-estrone effects on body energy are mediated by the estrogen receptor, blocked with the antagonist tamoxifen. Male Wistar rats were given daily oral doses of 10 micromol/kg d of oleoyl-estrone in oil containing 0 or 0.40 mg/kg d of tamoxifen. The data were compared with controls receiving only oil or 50 nmol/kg d of free estrone. After 10 days, the rats were killed, and their body composition and plasma metabolites and hormones were analyzed. Rats receiving estrone increased their body energy and lipid content compared with controls. Both groups of oleoyl-estrone-treated rats lost body weight, energy, and lipid; the losses in the rats receiving tamoxifen alone were less marked than in those receiving oleoyl-estrone. No significant changes in plasma glucose or triacylglycerols were observed. The patterns of change of estrone sulphate, estradiol, and oleoyl-estrone were consistent with a noticeable hydrolysis of oleoyl-estrone. The lack of differences in the fat mass in oleoyl-estrone-treated rats irrespective of the presence of tamoxifen suggested that the estrogenic pathway was not responsible for the slimming effects observed. Thus, it can be concluded that oleoyl-estrone effects are not mediated through its conversion to estrone or estradiol acting through the estrogen receptor. Tamoxifen partly mimicked the slimming effects of oleoyl-estrone; this could be speculatively explained by tamoxifen acting through the oleoyl-estrone signalling pathway.

[Physical activity and dyslipoproteinemia]

The positive influence of regular physical activity on lipoprotein metabolism and, hence, on the risks for atherogenesis has been documented in controlled studies. Although the levels of total and LDL cholesterol (LDL) usually change only slightly through physical activity, there is a clear change in the LDL composition with a reduction in the atherogenic small dense LDL particle. An activity-induced increase for HDL cholesterol (HDL) between 4 and 29% and a reduction in the triglyceride (TG) level between 4 and 37% are described. To achieve this, it is necessary to increase energy consumption by 1000-1200 kcal/week and to attain an energy consumption of ca. 2500 (> 2000) kcal/week, optimum ca. 3500 kcal/week.

The effect of fasting, prolonged swimming, and predator presence on energy utilization and stress in juvenile walleye (Stizostedion vitreum)

We evaluated how two diets with different lipid levels (4% vs. 10%) influenced nutrient expenditure in juvenile walleye Stizostedion vitreum subjected to starvation, prolonged swimming, and predator presence. We also determined how exercise and predator presence influenced stress indicators such as blood plasma cortisol and glucose. Groups of six fish were placed in triplicate rectangular wire cages per treatment and submerged randomly in three artificial stream compartments at a water temperature of 14 degrees C. Three treatments were established: (A) no water current and no predator, (B) water current (1.5 body lengths s(-1)) and no predator, (C) water current and predator. Six 1-year-old muskellunge (Esox masquinongy, 28.4+/-2.3 cm) were used as predators. They were allowed to swim freely outside walleye cages and were fed with walleye. To simulate poststocking period of fasting, walleyes were exposed to experimental treatments for 6 weeks and were deprived of food. Regardless of the prior diet, fish weight significantly declined in all treatments but no differences were found among them. Lipids declined in both dietary groups of fish; in turn, body moisture increased. Protein levels were only affected in fish fed with high-lipid diet prior to the experiment and subjected to current and predator presence. Plasma glucose gradually declined in fish fed with high-lipid diet prior to the study; whereas in the fish fed with low-lipid diet before the experiment, it remained low regardless of the treatment severity. Plasma cortisol concentrations never exceeded 30 ng ml(-1) indicating no signs of acute stress during the experiment. We concluded that fish fed with diet containing higher lipid levels prior to stocking would possess better potential of withstanding adverse conditions and better survival once released to the wild.

Leptin does not respond to 48 h fat deposition or mobilization in women

OBJECTIVE

To test the hypothesis that acute responses of plasma leptin concentration to energy balance manipulation are mediated by fat flux.

DESIGN

Ten healthy women aged 31-63 y, mass 48-113.5 kg, fat mass 8.5-62.5 kg, were studied for 3 days in a whole-body calorimeter on two occasions. After a control day (D1) during which energy balance was maintained, diet was manipulated to induce fat deposition (FD) or mobilization (FM) of 50 g/day for 2 days (D2 & D3). A difference totalling of 194+/-18.6 g fat was achieved between manipulations without significant effects on carbohydrate or protein balance. Fasting plasma leptin was measured on D2 and D4.

RESULTS

After the control day plasma leptin concentration averaged 19.01+/-9.8 ng/ml, and was found to be linearly related to body fat mass. After 2 days manipulation of fat balance, leptin concentrations were 21.4+/-10.3 ng/ml (FD) and 21.2+/-11.3 ng/ml (FM). There was no significant difference between treatments in either control day or postmanipulation leptin concentrations, nor did the treatments induce any differences in glucose or insulin concentration responses.

CONCLUSION

Although in states of energy balance leptin concentration is linearly related to fat mass, acute modulation of leptin concentration during energy imbalance is not mediated by fat flux.

Relationship between serum adiponectin concentration and intramyocellular lipid stores in humans

The recently identified adipocytokine adiponectin has been shown to improve insulin action and decrease triglyceride content in skeletal muscle (by stimulating lipid oxidation) in mice. In the present study, we tested the hypothesis that high serum concentrations of adiponectin are associated with lower intramyocellular (IMCL) fat content by promoting lipid oxidation in humans. IMCL-content in predominantly non-oxidative tibialis anterior muscle and oxidative soleus was determined by proton magnetic resonance spectroscopy in a cross- sectional study involving 63 healthy volunteers. In a second set of experiments, changes in IMCL in both muscles were measured after a three days dietary lipid challenge (n = 18) and after intravenous lipid challenge (n = 12) with suppressed lipid oxidation under hyperinsulinemia. Adiponectin serum concentrations were found to be negatively correlated with IMCL in the oxidative soleus muscle (IMCL [sol]) (r = - 0.46, p < 0.001) independent of measures of obesity, but not with IMCL in the non-oxidative tibialis anterior muscle (IMCL [tib]) (p = 0.40). Adiponectin serum concentrations were negatively correlated with the observed increase in IMCL load after dietary lipid challenge in the tibialis (r = 0.53, p = 0.03) but not in the soleus muscle. During suppression of lipid oxidation by hyperinsulinemia, no effect of adiponectin on IMCL was observed in either soleus or tibialis muscle. Overall, the presented findings are consistent with the hypothesis that adiponectin promotes lipid oxidation in humans resulting in lower intracellular lipid content in human muscle. These results are consistent with animal data, where adiponectin could be shown to enhance lipid oxidation and reduce muscle triglycerides.

Differential sensitivity of oleosins to proteolysis during oil body mobilization in sunflower seedlings

Until now, there has been no conclusive demonstration of any in vivo oleosin degradation at the early stages of oil body mobilization. The present work on sunflower (Helianthus annuus L.) has demonstrated limited oleosin degradation during seed germination. Seedling cotyledon homogenization in Tris-urea buffer, followed by SDS-PAGE, revealed three oleosins (16, 17.5 and 20 kDa). Incubation of oil bodies with total soluble protein from 4-day-old seedlings resulted in oleosin degradation. In vitro and in vivo degradation of the 17.5-kDa oleosin was faster than the other two, indicating its greater susceptibility to proteolysis. Oleosin degradation by the total soluble protein resulted in a transient 14.5-kDa polypeptide, followed by an 11-kDa protease-protected fragment, which appeared post-germinatively and accumulated corresponding to increased rate of lipid mobilization. A 65-kDa protease, active at pH 7.5-9.5, was zymographically detected in the total soluble protein. Its activity increased along with in vivo accumulation of the protease-protected fragment during seed germination and accompanying lipid mobilization. Protease-treated oil bodies were more susceptible to maize lipase action. Differential proteolytic sensitivity of different oleosins in the oil body membranes could be a determinant of oil body longevity during seed germination.

Effect of a tumour-derived lipid-mobilising factor on glucose and lipid metabolism in vivo

Treatment of ex-breeder male NMRI mice with lipid mobilising factor isolated from the urine of cachectic cancer patients, caused a significant increase in glucose oxidation to CO2 compared with control mice receiving phosphate buffered saline. Glucose utilisation by various tissues was determined by the 2-deoxyglucose tracer technique and shown to be elevated in brain, heart, brown adipose tissue and gastrocnemius muscle. The tissue glucose metabolic rate was increased almost three-fold in brain, accounting for the ability of lipid mobilising factor to decrease blood glucose levels. Lipid mobilising factor also increased overall lipid oxidation, as determined by the production of 14CO2 from [14C carboxy] triolein, being 67% greater than phosphate buffered saline controls over a 24 h period. There was a significant increase in [14C] lipid accumulation in plasma, liver and white and brown adipose tissue after administration of lipid mobilising factor. These results suggest that changes in carbohydrate metabolism and loss of adipose tissue, together with an increased whole body fatty acid oxidation in cachectic cancer patients, may arise from tumour production of lipid mobilising factor.

[Exercise intensity and fat burning--theoretical principles and practical considerations]

There are several methods to eliminate unnecessary subcutaneous adipose tissue. Aerobic exercise seems to be the cheapest and most efficient method compared to diets, pills or even surgery. Subcutaneous adipose tissue can only be reduced by lipolysis and oxidation in the working muscles. The intensity of exercise is the most relevant factor in fat oxidation. Even though fat oxidation is highest at low to moderate intensities, well trained athletes have their highest fat oxidation at the ventilatory threshold. Therefore, different intensities for trained compared to untrained persons have to be recommended in order to burn fat. The determination of concentrations of lactate seems to be of important practical use.

Role of beta3-adrenergic receptors in the action of a tumour lipid mobilizing factor

Induction of lipolysis in murine white adipocytes, and stimulation of adenylate cyclase in adipocyte plasma membranes, by a tumour-produced lipid mobilizing factor, was attenuated by low concentrations (10(-7)--10(-5)M) of the specific beta3-adrenoceptor antagonist SR59230A. Lipid mobilizing factor (250 nM) produced comparable increases in intracellular cyclic AMP in CHOK1 cells transfected with the human beta3-adrenoceptor to that obtained with isoprenaline (1 nM). In both cases cyclic AMP production was attenuated by SR59230A confirming that the effect is mediated through a beta3-adrenoceptor. A non-linear regression analysis of binding of lipid mobilizing factor to the beta3-adrenoceptor showed a high affinity binding site with a Kd value 78 +/- 45 nM and a B(max) value (282 +/- 1 fmole mg protein(-1)) comparable with that of other beta3-adrenoceptor agonists. These results suggest that lipid mobilizing factor induces lipolysis through binding to a beta3-adrenoceptor.

Arabidopsis thaliana mutants disrupted in lipid mobilization

To isolate mutants in the process of lipid mobilization during post-germinative growth we employed a screen using the pro-herbicide 2,4-dichlorophenoxybutyric acid (2,4-DB). The phenotypes of a number of 2,4-DB-resistant mutants are compared with previously characterized mutants disrupted in beta-oxidation or the glyoxylate cycle. We conclude that the strength of 2,4-DB resistance and the ability of the seedlings to grow in the absence of exogenous sugar are inversely correlated. Sugar dependence of 2,4-DB-resistant seedlings is a consequence of impaired storage-lipid mobilization.

Lipid mobilization and acid phosphatase activity in lytic compartments during conidium dormancy and appressorium formation of Colletotrichum graminicola

Colletotrichum graminicola, a pathogen of sorghum and corn, was investigated prior and during germination as to certain aspects of acid phosphatase activity and lipid mobilization. Ungerminated conidia cytoplasm was filled with lipid deposits, which were mobilized during the germination process. Cytochemical ultrastructural examination showed that conidia vacuoles exhibit acid phosphatase activity, which is suggestive of lytic activity. Lipid bodies, stored in the ungerminated conidia cytoplasm, were internalized by vacuoles in a process analogous to microautophagy and were apparently digested inside them. The lipid bodies disappeared and vacuoles became enlarged in conidial cells during germination. Appressoria also showed acid phosphatase activity in multiple heterogeneous vesicles which were, in most cases, juxtaposed with lipid bodies. These results suggest that the vacuolar system plays an important role during C. graminicola germination and that the initial stages of lipid metabolization are taking place inside the vacuoles.

Effect of carbohydrate ingestion on adipose tissue lipolysis during long-lasting exercise in trained men

To study whether sucrose administration acts on lipid mobilization during prolonged exercise, we used subcutaneous abdominal adipose tissue microdialysis in eight well-trained subjects submitted at random to two 100-min exercises (50% maximal aerobic power) on separate days. After 50 min of exercise, the subjects ingested either a sucrose solution (0.75 g/kg body wt) or water. By using a microdialysis probe, dialysate was obtained every 10 min from the subjects at rest, during exercise, and during a 30-min recovery period. During exercise without sucrose, plasma and dialysate glycerol increased significantly. With sucrose, the response was significantly lower for dialysate glycerol (P < 0.05). Plasma free fatty acid level was lower after sucrose than after water ingestion (P < 0.05). With water ingestion, plasma catecholamines increased significantly, whereas insulin fell (P < 0.05). With sucrose ingestion, the epinephrine response was blunted, whereas the insulin level was significantly increased. In conclusion, the use of adipose tissue microdialysis directly supports a lower lipid mobilization during exercise when sucrose is supplied, which confirms that the availability of carbohydrate influences lipid mobilization.

Effect of intracerebroventricular injection of atropine on metabolic responses during exercise in untrained rats

To investigate the role of the central cholinergic system in the regulation of metabolism during exercise, we injected atropine (5 x 10(-7) mol) into the lateral cerebral ventricle of normal and adrenodemedullated (ADM) untrained rats submitted to exercise on a treadmill (15 m min(-1), 5% grade) until exhaustion. Concentrations of blood glucose, plasma free fatty acids (FFA), and lactate were measured before and every 10 min after the start of exercise for a period of 60 min. Adrenomedullectomy had no effect on the maximal capacity of exercise (MCE), but atropine administered intracerebroventricularly (i.c.v.) reduced the maximal capacity of exercise of both normal and ADM rats. In normal rats, blood concentrations of glucose and plasma free fatty acids remained essentially unchanged compared to the levels at rest, whereas in ADM rats a rapid increase in plasma glucose and plasma free fatty acids levels occurred during exercise. These data indicate that adrenomedullectomy disrupted the accuracy of the feedback mechanism that regulates the mobilization of extramuscular fuels during exercise in normal rats. In addition, ADM rats showed an increased lipid mobilization as a source of energy during exercise, which might explain the increased plasma glucose by an inhibition of muscle glucose uptake. These results suggest that central cholinergic neurons might be involved in the control of energy substrate adjustment during exercise, thereby reducing the maximal capacity of exercise. In addition, the results of this study suggest that the adrenal glands are important for an accurate feedback mechanism during exercise.

Anti-obesity effects of selective agonists to the beta 3-adrenergic receptor in dogs. I. The presence of canine beta 3-adrenergic receptor and in vivo lipomobilization by its agonists

It is known that in rodents and humans the beta 3-adrenergic receptor (beta 3-AR) is present primarily in adipocytes and plays a significant role in the adrenergic stimulation of lipolysis. We examined the expression of beta 3-AR mRNA in the dog and the lipomobilizing effects of beta 3-AR-selective agonists in vivo. Reverse transcription polymerase chain reaction of RNA extracted from dog adipose tissue produced a cDNA fragment, the nucleotide sequence of which was highly homologous to the corresponding regions of human (86.4%) and mouse (79.5%) beta 3-AR cDNA. The beta 3-AR mRNA was present at high levels in subcutaneous and visceral adipose tissues, but undetectable in other organs. When a selective beta 3-AR agonist, CL316,243, was infused intravenously into beagle dogs, the plasma level of free fatty acid increased in 30 min and persisted at higher levels for several hours. ICI D7114, another beta 3-AR agonist, also showed a similar lipomobilizing effect, but with lower potency. beta 3-AR agonist infusion also increased the plasma insulin level. These results suggested that functional beta 3-AR is present in adipose tissues of the dog and that it is effective for in vivo lipomobilization.

Leptin levels in pregnancy: marker for fat accumulation and mobilization?

BACKGROUND

Leptin, an adipose tissue-derived signalling factor encoded by the obese gene has been shown to be present as a 16-kDa protein in the blood of mice and humans. Resistance to leptin occurs in human obesity. Leptin has also been shown to associate with plasma insulin concentrations and there is currently considerable debate about the potential link between insulin resistance and resistance to leptin. In non-pregnant individuals, circulating leptin concentrations associate strongly with both total body fat mass and body mass index (BMI). In normal human pregnancy, the maternal fat stores increase to a peak in the late second trimester, before declining towards term as fat stores are mobilized to support the rapidly growing fetus. Insulin resistance increases during late pregnancy and is believed to be further enhanced in pregnancies complicated by pre-eclampsia. The aim of this study was to examine if leptin levels were altered in pregnancy and, if so, whether the pattern of change in circulating leptin related to previously established changes in fasting insulin concentrations or fat mass.

METHODS

We measured third trimester plasma leptin concentrations in 12 uncomplicated pregnant women, nine women with pre-eclampsia matched for age and booking BMI, and 18 non-pregnant women similarly matched. We also examined the longitudinal course of leptin concentrations occurring throughout gestation (from 10 weeks gestation and at five week intervals thereafter), in five normal pregnancies and two women with gestational-onset diabetes.

RESULTS

Leptin concentrations were significantly higher in the normal pregnant women (37.1 microg/L, [15.4-117.0], geometric mean, [range]; p=0.049), and women with pre-eclampsia (45.3 microg/L, [21.3-98.4]; p=0.001), than in non-pregnant controls (17.85 microg/L, [1.3-36.5]), however, there was no significant difference between uncomplicated and pre-eclamptic pregnancies (p=0.22). On examination of the longitudinal course of leptin concentrations occurring throughout gestation, in all seven women plasma leptin concentrations initially increased relative to booking (10 weeks) concentrations, but did so by varying amounts (ranging between 30-233%). Significantly, however, in all seven women plasma leptin concentrations peaked at around 20-30 weeks of gestation before declining towards term.

CONCLUSION

On the basis of these observations, we postulate that plasma leptin levels increase significantly in human pregnancies and that the pattern of change in circulating leptin parallels the process of fat accumulation and mobilization.

A circadian rhythm in lipid mobilization which is altered in IDDM

It is not clear how circadian lipolysis and circulating concentrations of non-esterified fatty acids (NEFA) are altered in intensively treated insulin-dependent diabetic (IDDM) patients. Ten IDDM patients on an intensive insulin regimen and eight healthy control subjects were investigated under ordinary living conditions for 27 h by microdialysis of subcutaneous adipose tissue. The true tissue glycerol concentration and adipose blood flow changes were monitored as an index of lipolysis. A circadian pattern in adipose tissue lipolysis was observed in both groups, decreasing during the day and increasing during evening-night. The daytime decrease was normal, but the evening-night rise was elevated in IDDM (p = 0.03). Circulating NEFA decreased during the day and increased at night. The latter increase was enhanced threefold in IDDM (p = 0.003) and correlated with fasting glucose levels (r = 0.77). Nocturnal growth hormone (GH) was increased fivefold in IDDM and correlated to nocturnal lipolysis (r = 0.83). Adipose tissue blood flow increased during the night in a similar fashion in both groups. Near-normalization of glucose for 24 h in IDDM did not affect the nocturnal increases in NEFA, GH and lipolysis. In conclusion, a circadian rhythm in lipolysis was found. Increased lipolytic rates during evening-night may at least in part raise nocturnal circulating NEFA. Nocturnal NEFA and lipolysis are further enhanced in IDDM, maybe due to elevated GH, but not to insulinopenia or hyperglycaemia.

Effect of lipid transfer activity and triglyceride hydrolysis on apolipoprotein B immunoreactivity in modified low density lipoproteins

Consequences of alterations in the size and the lipid composition of low density lipoproteins (LDL) on apolipoprotein (apo) B immunoreactivity were analyzed using two distinct anti-apoB monoclonal antibodies (Mabs), i.e., 4G3, which recognizes an epitope closed to the binding site to the LDL receptor, and 2D8, which is directed against a distal region. Inhibition analysis revealed that the lipid transfer-mediated triglyceride enrichment of LDL isolated from 12 native human plasmas is associated with significant reductions in the expression of 2D8 and 4G3 epitopes (P < 0.05 in both cases). In contrast, triglyceride hydrolysis of triglyceride-rich LDL significantly increased apoB immunoreactivity as compared with non-lipolyzed counterparts (P < 0.05 with 2D8 and 4G3 Mabs). Among all the modified LDL fractions studied (n = 36), immunoreactivity of 2D8 and 4G3 epitopes correlated negatively and significantly with the triglyceride content (P < 0.01 in both cases), but with neither the size nor the other lipid parameters of LDL particles. Furthermore, changes in the triglyceride content of LDL correlated significantly with changes in apoB immunoreactivity after in vitro treatment with either lipid transfer activity alone (P < 0.01 with 2D8 and 4G3 Mabs) or lipid transfer activity combined with triglyceride hydrolysis (P < 0.01 with 2D8 and 4G3 Mabs). Finally, both the triglyceride content of native LDL and the total triglyceride level in 12 normolipidemic human plasmas correlated negatively and significantly with the expression of 2D8 epitope (P < 0.03 in both cases) and 4G3 epitope (P < 0.02 in both cases). It is concluded that triglycerides constitute a major determinant of the immunoreactivity of 2D8 and 4G3 apoB epitopes in LDL.

Selective mobilization of fatty acids from white fat cells: evidence for a relationship to the polarity of triacylglycerols

Fatty acids are selectively released from white fat cells in accordance with well-defined rules relating their molecular structure and their mobilization rate, emphasizing the possible role of their physicochemical properties. Lipolysis is widely reported to work for conditions where only small amounts of substrate are available. We hypothesize that the preferential hydrolysis of a substrate fraction enriched in the most polar triacylglycerols (TAGs) reflects the pattern of selective fatty acid mobilization. Rat adipose tissue was first manipulated by dietary means to obtain a wide spectrum of fatty acids. Fat cell TAGs were separated into eight fractions according to polarity by liquid-liquid partition chromatography and their fatty acid proportions and compositions were determined by GLC. In the most polar TAG fractions, the relative enrichment of fatty acids (percentage in a TAG fraction divided by percentage in total TAGs) increased with the number of double bonds for a given chain length, whereas it decreased with increasing chain length for a given degree of unsaturation. The relative enrichment of highly mobilized fatty acids (16-20 carbon atoms and four or five double bonds) was very high (more than 2.5) in the most polar TAG fractions, whereas that of weakly mobilized fatty acids (20-24 carbon atoms and no or one double bond) was very low (less than 0.5). The relative enrichment of moderately mobilized fatty acids (comprising all the others) was close to unity. Our study shows that the relative enrichment of fatty acids in the most polar adipose tissue TAGs is consistent with their relative mobilization rates. This supports our hypothesis and raises the possibility that the molecular species of TAGs might be one of the regulating factors.

Mobilization of visceral adipose tissue related to the improvement in insulin sensitivity in response to physical training in NIDDM. Effects of branched-chain amino acid supplements

OBJECTIVE

To evaluate the effects of an intense physical training program on abdominal fat distribution, glycemic control, and insulin sensitivity in patients with NIDDM and to determine whether branched-chain amino acid (BCAA) supplements influence these effects.

RESEARCH DESIGN AND METHODS

Twenty-four patients (ages 45 +/- 2 [mean +/- SE] years, BMI 30.2 +/- 0.9 kg/m2, HbA1c 7.9 +/- 0.3%) were randomly assigned to four groups: training plus BCAA supplement (n = 6), training plus placebo (n = 6), sedentary plus BCAA supplement (n = 6), and sedentary plus placebo (n = 6). Physical training consisted of a supervised 45-min cycling exercise at 75% of their oxygen uptake peak (VO2 peak) two times per week and an intermittent exercise one time per week for 2 months.

RESULTS

Patients who exercised increased their VO2 peak by 41% and their insulin sensitivity by 46%. Physical training significantly decreased abdominal fat evaluated by magnetic resonance imaging (umbilicus), with a greater loss of visceral adipose tissue (VAT) (48%) in comparison with the loss of subcutaneous adipose tissue (18%), but did not significantly affect body weight. The change in visceral abdominal fat was associated with the improvement in insulin sensitivity (r = 0.84, P = 0.001). BCAA supplementation had no effect on abdominal fat and glucose metabolism.

CONCLUSIONS

Physical training resulted in an improvement in insulin sensitivity with concomitant loss of VAT and should be included in the treatment program for patients with NIDDM.

Fasting-induced selective mobilization of brown adipose tissue fatty acids

This study was conducted to determine whether there is selective mobilization of fatty acids from brown adipose tissue. Rats were fed a fish-oil diet and then fasted for 0, 7, or 10 days followed by analysis of the fatty acid content and composition of triacylglycerols (TAG) and phospholipid (PL) in interscapular brown adipose tissue (IBAT). Fatty acids were selectively lost from IBAT triacylglycerols, the mobilization following the same structural rules as those previously demonstrated for white adipose tissue. Fractional mobilization increased with unsaturation at a given chain length and tended to decrease with chain length at a given unsaturation. However, linoleic acid (18:2 n-6) was mobilized significantly less than predicted by these structural rules. In IBAT phospholipid, fatty acids were also selectively lost but there was no such relationship between the fractional mobilization of a fatty acid and its structure. The fatty acids of the n-6 series were exceptional in their behavior because they displayed below average fractional mobilization. In fact, linoleic and arachidonic acids actually increased their mass in IBAT phospholipid during the fast. It is concluded that, in IBAT, fatty acids are selectively mobilized during a fast, and that fasting-induced remodeling of the fatty acid composition leads to the selective retention of linoleate in the case of TAG and all the n-6 fatty acids in the case of PL.

Developmental down-regulation of receptor-mediated endocytosis of an insect lipoprotein

Fat body cells of insects exhibit a high-affinity lipoprotein binding site at their cell surfaces. In the present study, the lipoprotein binding site was identified as an endocytotic receptor involved in receptor-mediated uptake of its lipoprotein ligand, high density lipophorin. After an initial period of high endocytotic uptake of high density lipophorin in the adult stage, this process strongly diminished. In the same period, a dramatic increase in cell surface-associated lipoproteins was observed. When animals were starved, however, internalization of lipoproteins was maintained. The pathway followed by the internalized lipoproteins appears to be different from the endosomal/lysosomal pathway, as the vast majority of apolipoproteins seemed to escape from lysosomal hydrolysis. In addition, no substantial intracellular accumulation of apolipoproteins was observed, suggesting that internalized lipoproteins were resecreted. It is unlikely that internalization is required for transport of the two major lipid components of insect lipoproteins, diacylglycerol and cholesterol, as inhibition of endocytosis neither affected the exchange of these lipids between lipoproteins and fat body cells nor influenced the loading of diacylglycerol onto lipoproteins in response to adipokinetic hormone. We postulate that the endosomal environment may facilitate transport of components which, unlike diacylglycerol and cholesterol, cannot be transported by simple aqueous diffusion.

Release of individual fatty acids from human adipose tissue in vivo after an overnight fast

The objective of this study was to investigate the mobilization of individual fatty acids from human subcutaneous adipose tissue in vivo. Concentrations of individual non-esterified fatty acids were measured in arterialized plasma and in the venous drainage from subcutaneous abdominal adipose tissue in eight normal subjects, after an overnight fast, and for the subsequent 6 h. Whilst the veno-arterial concentration difference for non-esterified fatty acids increased over this period, the relative proportions of different fatty acids remained constant. There was a close relationship between veno-arterial difference and arterialized concentration for the different fatty acids. The arterialized concentration of stearic acid consistently lay above the regression line drawn for unsaturated fatty acids (P = 0.001), probably reflecting lower clearance of stearic acid. The release of individual fatty acids was compared with their prevalence in adipose tissue triacylglycerol in biopsies taken from six subjects. Relative release decreased with increasing chain length, and for a given chain length increased with increasing unsaturation, in agreement with animal studies (P < 0.001 for differences in relative mobilization according to these two factors). The results suggest that the systemic plasma concentration of individual non-esterified fatty acids is determined by their release from adipose tissue, but that the relationship between release and systemic concentration for stearic acid is different from that for other fatty acids. The results confirm, in humans, differences in relative mobilization found previously in animal studies.