Proposed mechanism of insulin-resistant glucose transport in the isolated guinea pig adipocyte. Small intracellular pool of glucose transporters

Low birth weight male guinea pig offspring display increased visceral adiposity in early adulthood

Uteroplacental insufficiency (UPI)-induced intrauterine growth restriction (IUGR) predisposes individuals to adult visceral obesity. We postulated that low birth weight (LBW) offspring, from UPI-induced IUGR pregnancies, would display a visceral adipose lipogenic molecular signature involving altered gene expression, phosphorylation status of proteins of the lipid synthesis pathway and microRNA (miR) expression profile, occurring in association with increased visceral adiposity. Normal birth weight (NBW) and LBW (obtained by uterine artery ablation) male guinea pig pups were fed a control diet from weaning to 145 days and sacrificed. Despite being lighter at birth, LBW pups displayed body weights similar to NBW offspring at 145 days. At this age, which represents young adulthood, the relative weights of LBW epididymal white adipose tissue (EWAT) and lipid content were increased; which was consistent with adipocyte hypertrophy in the LBW offspring. Additionally, the mRNA expression of lipid synthesis-related genes including acetyl-CoA carboxylase 1 (ACC1), diglyceride acyltransferase 2 (DGAT2) and peroxisome proliferator-activated receptor gamma 1 (PPARγ1), was increased in LBW EWAT. Further, LBW EWAT displayed decreased phospho-ACC (Ser79) and phospho-PPARγ (Ser273) proteins. Moreover, the mRNA expression of hormone-sensitive lipase (HSL) and fatty acid binding protein 4 (FABP4), both involved in promoting adipose lipid storage, was increased in LBW EWAT. Finally, miR-24 and miR-103-2, miRs related to adipocyte development, were both increased in LBW EWAT. These findings indicate that, following an adverse in utero environment, lipid synthesis-related genes and miR expression, along with phosphorylation status of key regulators of lipid synthesis, appear to be chronically altered and occur in association with increased visceral adiposity in young adult IUGR male offspring.

Compartmentalization and regulation of insulin signaling to GLUT4 by the cytoskeleton

One of the early events in the development of Type 2 diabetes appears to be an inhibition of insulin-mediated GLUT4 redistribution to the cell surface in tissues that express GLUT4. Understanding this process, and how it begins to breakdown in the development of insulin resistance is quite important as we face treatment and prevention of metabolic diseases. Over the past few years, and increasing number of laboratories have produced compelling data to demonstrate a role for both the actin and microtubule networks in the regulation of insulin-mediated GLUT4 redistribution to the cell surface. In this review, we explore this process from insulin-signal transduction to fusion of GLUT4 membrane vesicles, focusing on studies that have implicated a role for the cytoskeleton. We see from this body of work that both the actin network and the microtubule cytoskeleton play roles as targets of insulin action and effectors of insulin signaling leading to changes in GLUT4 redistribution to the cell surface and insulin-mediated glucose uptake.

Discrepancy between glucose transport and transporters in human femoral adipocytes

Obesity is known to be associated with insulin resistance in human and rat adipocytes. However, it is not known what are the perturbations in insulin action that contribute to disproportional femoral obesity. Thus femoral subcutaneous adipose tissue was obtained from lean women with various degrees of disproportional obesity, by liposuction. 3-O-methylglucose (3-O-methyl-D-glucopyranose) transport was measured in intact cells, and glucose transporter levels in plasma and low-density microsomal membranes were assessed using the cytochalasin B binding assay. A sixfold cellular enlargement was associated with increase in both basal and insulin-stimulated glucose transport activity in the intact cell, and a 300-600% increase in insulin stimulating effect per se. However, when glucose transporter levels were assessed, this cellular enlargement was accompanied by a 40-70% transporter depletion (in largest cells compared with smallest ones) in both subcellular fractions examined, from either basal or insulin-stimulated cells. This discrepancy, between increasing cellular glucose transport rates and relative depletion of transporter levels, suggests that these cells are not insulin resistant, as could be expected from their large size. A role for other factor(s), additional to glucose transporter levels, in the regulation of cellular glucose uptake rate is thus suggested.

Glucose transport: locus of muscle insulin resistance in obese Zucker rats

The purposes of this study were to determine whether the muscle insulin resistance of the obese rat is due to a defect in the glucose transport process and whether the insulin resistance is fiber-type specific. The hindlimbs of fasted, 14-wk-old obese (fa/fa) and lean (fa/?) Zucker rats were perfused with perfusate containing 8 mM glucose and no insulin or 8 mM glucose and either a physiological (0.15 mU/ml), a submaximal (1.50 mU/ml), or a maximal (15.0 mU/ml) insulin concentration. Glucose uptake was determined after which the initial rate of glucose transport was determined using 3-O-methyl-D-glucose (3-OMG). Glucose uptake of the obese rats was depressed by 40, 33, 42, and 47% in the absence of insulin and in the presence of the physiological, submaximal, and maximal insulin concentrations, respectively, when compared with lean littermates. Glucose transport in the absence and in the presence of the three insulin concentrations was significantly lower in the soleus (slow-twitch, oxidative fibers), red quadriceps (fast-twitch, oxidative, glycolytic fibers), and gastrocnemius (mixed fibers) of the obese rats when compared with lean rats. Glucose transport in the white quadriceps (fast-twitch, glycolytic fibers) was significantly lower in the obese rats in the absence of insulin and in the presence of the submaximal and maximal insulin concentrations. The glycogen concentration and the activity of hexokinase were the same and the glycogen synthase activity was higher in the muscles for the obese rats when compared to lean rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms for turnover of lipoprotein lipase in guinea pig adipocytes

Guinea-pig adipocytes released lipoprotein lipase activity to the medium without depletion of cell-associated lipoprotein lipase activity. Heparin caused immediate release of 20-25% of the lipase activity to the medium, and also enhanced the continued release. After addition of cycloheximide, cell-associated lipoprotein lipase activity decreased rapidly. Release of lipase activity to the medium continued unabated for about 30 min, but there was little release thereafter. The release accounted for only about 25% of the initial lipoprotein lipase activity in the absence and about 50% in the presence of heparin. In pulse-chase experiments with [35S]methionine, labeled lipoprotein lipase appeared in the medium within 40 min, and most of the release occurred during the first h of chase. In a 4-h chase the total (cells + medium) amount of labeled lipase decreased to 34%. Thus, degradation was a main fate of the lipase. Heparin markedly increased the amount of labeled lipase that was released to the medium and decreased the amount that was degraded. Heparin did not change the time-course for the release, and the amount of labeled lipase degraded was proportional to the amount not released to the medium, indicating that the effect of heparin was primarily on release, not on degradation as such. This study demonstrates that adipocytes synthesize lipoprotein lipase in excess of what is being released, and that the excess is rapidly degraded.

Substrates for energy metabolism in the heart: the role of the interstitial compartment

Evidence is presented that, as in cardiomyocytes, vascular endothelial cells use fatty acids, in addition to glucose, as a respiratory fuel. Attention is focused on the cardiac interstitium, lined by vascular cells and cardiomyocytes, which may be enriched with metabolic products from these cells. Also, certain proteins are present in the interstitial fluid (Qi) such as plasma proteins and fatty acid binding protein (FABP). However, the concentration of FABP is so low in Qi that albumin is more important to shuttle long chain fatty acids in the interstitial fluid between cardiomyocytes and the vascular compartment. Under hypoxic conditions (hypo)xanthine, lactate and fatty acids may be expected to accumulate in the interstitium, as well as proteins from adjacent cells, such as xanthine oxidase from endothelial cells. This enzyme, acting upon the elevated level of (hypo)xanthine, giving rise to O2-., may be involved in the damage of the ischaemic heart. The significance of the interstitium in ischaemia and in fibrosis following long standing cardiac lipidosis is briefly discussed, as well as the possible mechanisms involved in fatty acid transport in the heart.

Modulation of glucose transport in hamster adipocytes by insulin and by beta- and alpha 2-adrenoceptor agonists

Glucose transport in hamster adipocytes and its modulation by insulin and isoprenaline was characterized with the aid of the non-metabolizable hexose 3-0-methylglucose. Insulin stimulated the initial uptake rates by an increase in Vmax of the transport without any detectable change in Km. The hormone concentration producing half maximal stimulation was identical to that required in rat adipocytes. However, hamster adipocytes were much less responsive to insulin (3-fold stimulation as compared to a 12-fold stimulation in rat fat cells), and maximal transport rates were 10-fold lower than that observed in rat adipocytes. Accordingly, the number of glucose transporters, as assessed by glucose-inhibitable cytochalasin-B binding, was considerably lower in plasma membranes of hamster adipocytes. Moreover, no transporters were detected in the low-density microsomes which in insulin-sensitive cell types represent the intracellular pool of recruitable glucose transporters. The relative insulin resistance of the hamster fat cells may therefore be due to a depleted pool of intracellular glucose transporters. In the presence of adenosine, the beta-adrenoceptor agonist isoprenaline produced a moderate stimulation of the basal transport rate which was antagonized by the alpha 2-agonist clonidine. If adenosine deaminase was added in order to remove endogenous adenosine, isoprenaline inhibited the insulin-stimulated transport by 50%. In contrast to the stimulatory effects of insulin and isoproterenol, the inhibitory effect of the catecholamine was reversed by cooling the cells to 22 degrees. Glucagon produced a comparable inhibition, suggesting that the inhibitory effect was mediated by adenylate cyclase or its regulatory subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Nutritional regulation of lipoprotein lipase in guinea pig tissues

Glucose transport in guinea pig adipocytes has been shown to be markedly resistant to stimulation by insulin. Lipoprotein lipase is another transport catalyst in adipose tissue which is believed to be regulated by insulin. We have therefore studied how feeding-fasting affects lipoprotein lipase activity in guinea pig tissues. There was an even more marked decrease in adipose tissue lipoprotein lipase activity on fasting in guinea pigs (10-20 fold) than in rats or mice (4-5 fold). In adipocytes, the activity decreased only 2.5-4.5 fold; most of the change was in extracellular lipoprotein lipase. On glucose refeeding, the activity was rapidly restored. In the first 4 hours after glucose administration extracellular lipoprotein lipase activity increased to more than 10 times the amount present in adipocytes. After cycloheximide, lipoprotein lipase activity decreased with a half-life of 22 min. It is concluded that lipoprotein lipase is rapidly produced and turned over in guinea pig adipose tissue, and that the system is quite sensitive to feeding-fasting. In contrast to adipose tissue, there was no significant change in lipoprotein lipase activity in any other tissue on fasting. There was a strong correlation between the activities in heart and diaphragm muscle, but this correlation was independent of feeding-fasting.

Effects of exercise on insulin binding to human muscle

A procedure was developed to measure insulin binding to human skeletal muscle obtained via the percutaneous muscle biopsy technique. With this method the effects of exercise on insulin binding were investigated. Subjects (n = 9) exercised for 60 min on a bicycle ergometer at intensities ranging from 20-86% maximum O2 consumption (VO2max). Blood samples were obtained before, during, and after exercise and analyzed for glucose and insulin. Muscle samples (250 mg) for the vastus lateralis were obtained 30 min before exercise, at the end of exercise, and 60 min after exercise. Two subjects rested during the experimental period. There was no linear relationship between exercise intensities and the changes in insulin binding to human muscle. At rest (n = 2) and at exercise intensities below 60% VO2max (n = 5) no change in insulin binding occurred (P greater than 0.05). However, when exercise occurred at greater than or equal to 69% VO2max (n = 4), a pronounced decrement in insulin binding (30-50%) was observed (P less than 0.05). This persisted for 60 min after exercise. These results indicate that insulin binding in human muscle is not altered by 60 min of exercise at less than or equal to 60% VO2max but that a marked decrement occurs when exercise is greater than or equal to 69% VO2max.

Subcellular translocation of glucose transporters: role in insulin action and its perturbation in altered metabolic states

In this article we have described the hypothesis that insulin stimulates glucose transport through glucose transporter translocation from an intracellular pool to the plasma membrane. In addition, we have shown that changes in the numbers and subcellular distributions of glucose transporters correlate with alterations in insulin-stimulated glucose transport activity in several experimental models of insulin resistance and hyperresponsiveness. However, in experiments with counterregulatory hormones and with hyperresponsive states induced by nutritional repletion following deprivation, changes in insulin responsiveness cannot be fully explained by such alterations in the numbers and/or subcellular distribution of glucose transporters. Thus, evidence has been presented for changes in glucose transporter intrinsic activity that both inhibit and augment insulin-stimulated glucose transport rates. Finally, we have discussed data suggesting that the translocation process is applicable to human tissue and that significant changes in adipose cell glucose transport activity have been correlated with total glucose disposal in various metabolic states in humans. Determining the physiologic factors involved in modulating these events at the cellular level is an important area for further investigation.

RNA required for import of precursor proteins into mitochondria

A cytoplasmic RNA moiety is necessary for posttranslational uptake of nuclear-encoded mammalian proteins destined for the mitochondrial matrix. Post-translational addition of ribonuclease to a reticulocyte lysate-programmed cell-free translation mixture inhibited subsequent import of six different mitochondrial matrix enzyme precursors into rat liver mitochondria. The required RNA is highly protected, as indicated by the high concentrations of ribonuclease necessary to produce this inhibition. The dependence of the inhibitory effect on temperature, duration of exposure to ribonuclease, and availability of divalent cations is characteristic of the nuclease susceptibility of ribonucleoproteins. The ribonuclease-sensitive component was found in a 400-kilodalton fraction which contains the mitochondrial protein precursors.