Mechanism by which fatty acids inhibit insulin activation of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase activity in muscle

Recent studies have demonstrated that fatty acids induce insulin resistance in skeletal muscle by blocking insulin activation of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI3-kinase). To examine the mechanism by which fatty acids mediate this effect, rats were infused with either a lipid emulsion (consisting mostly of 18:2 fatty acids) or glycerol. Intracellular C18:2 CoA increased in a time-dependent fashion, reaching an approximately 6-fold elevation by 5 h, whereas there was no change in the concentration of any other fatty acyl-CoAs. Diacylglycerol (DAG) also increased transiently after 3-4 h of lipid infusion. In contrast there was no increase in intracellular ceramide or triglyceride concentrations during the lipid infusion. Increases in intracellular C18:2 CoA and DAG concentration were associated with protein kinase C (PKC)-theta activation and a reduction in both insulin-stimulated IRS-1 tyrosine phosphorylation and IRS-1 associated PI3-kinase activity, which were associated with an increase in IRS-1 Ser(307) phosphorylation. These data support the hypothesis that an increase in plasma fatty acid concentration results in an increase in intracellular fatty acyl-CoA and DAG concentrations, which results in activation of PKC-theta leading to increased IRS-1 Ser(307) phosphorylation. This in turn leads to decreased IRS-1 tyrosine phosphorylation and decreased activation of IRS-1-associated PI3-kinase activity resulting in decreased insulin-stimulated glucose transport activity.

Hypertrophy and/or Hyperplasia: Dynamics of Adipose Tissue Growth

Adipose tissue grows by two mechanisms: hyperplasia (cell number increase) and hypertrophy (cell size increase). Genetics and diet affect the relative contributions of these two mechanisms to the growth of adipose tissue in obesity. In this study, the size distributions of epididymal adipose cells from two mouse strains, obesity-resistant FVB/N and obesity-prone C57BL/6, were measured after 2, 4, and 12 weeks under regular and high-fat feeding conditions. The total cell number in the epididymal fat pad was estimated from the fat pad mass and the normalized cell-size distribution. The cell number and volume-weighted mean cell size increase as a function of fat pad mass. To address adipose tissue growth precisely, we developed a mathematical model describing the evolution of the adipose cell-size distributions as a function of the increasing fat pad mass, instead of the increasing chronological time. Our model describes the recruitment of new adipose cells and their subsequent development in different strains, and with different diet regimens, with common mechanisms, but with diet- and genetics-dependent model parameters. Compared to the FVB/N strain, the C57BL/6 strain has greater recruitment of small adipose cells. Hyperplasia is enhanced by high-fat diet in a strain-dependent way, suggesting a synergistic interaction between genetics and diet. Moreover, high-fat feeding increases the rate of adipose cell size growth, independent of strain, reflecting the increase in calories requiring storage. Additionally, high-fat diet leads to a dramatic spreading of the size distribution of adipose cells in both strains; this implies an increase in size fluctuations of adipose cells through lipid turnover.

Insulin-stimulated translocation of glucose transporters in the isolated rat adipose cells: characterization of subcellular fractions

Insulin stimulates glucose transport in rat adipose cells through the translocation of glucose transporters from an intracellular pool to the plasma membrane. A detailed characterization of the morphology, protein composition and marker enzyme content of subcellular fractions of these cells, prepared by differential ultracentrifugation, and of the distribution of glucose transporters among these fractions is now described. Glucose transporters were measured using specific D-glucose-inhibitable [3H]cytochalasin B binding. In the basal state, roughly 90% of the cells' glucose transporters are associated with a low-density microsomal, Golgi marker enzyme-enriched membrane fraction. However, the distributions of glucose transporters and Golgi marker enzyme activities over all fractions are clearly distinct. Incubation of intact cells with insulin increases the number of glucose transporters in the plasma membrane fraction 4-5 fold and correspondingly decreases the intracellular pool, without influencing any other characteristics of the subcellular fractions examined or the estimated total number of glucose transporters (3.7 X 10(6)/cell). Insulin does not influence the Kd of the glucose transporters in the plasma membrane fraction for cytochalasin B binding (98 nM), but lowers that in the intracellular pool (from 141 to 93 nM). The calculated turnover numbers of the glucose transporters in the plasma membrane vesicles from basal and insulin-stimulated cells are similar (15 X 10(3) mol of glucose/min per mol of transporters at 37 degrees C), whereas insulin appears to increase the turnover number in the plasma membrane of intact cells roughly 4-fold. These results suggest that (1) the intracellular pool of glucose transporters may comprise a specialized membrane species, (2) intracellular glucose transporters may undergo conformational changes during their cycling to the plasma membrane in response to insulin, and (3) the translocation of glucose transporters may represent only one component in the mechanism through which insulin regulates glucose transport in the intact cell.

Enhanced proportion of small adipose cells in insulin-resistant vs insulin-sensitive obese individuals implicates impaired adipogenesis

AIMS/HYPOTHESIS

The biological mechanism by which obesity predisposes to insulin resistance is unclear. One hypothesis is that larger adipose cells disturb metabolism via increased lipolysis. While studies have demonstrated that cell size increases in proportion to BMI, it has not been clearly shown that adipose cell size, independent of BMI, is associated with insulin resistance. The aim of this study was to test this widely held assumption by comparing adipose cell size distribution in 28 equally obese, otherwise healthy individuals who represented extreme ends of the spectrum of insulin sensitivity, as defined by the modified insulin suppression test.

SUBJECTS AND METHODS

Subcutaneous periumbilical adipose tissue biopsy samples were fixed in osmium tetroxide and passed through the Beckman Coulter Multisizer to obtain cell size distributions. Insulin sensitivity was quantified by the modified insulin suppression test. Quantitative real-time PCR for adipose cell differentiation genes was performed for 11 subjects.

RESULTS

All individuals exhibited a bimodal cell size distribution. Contrary to expectations, the mean diameter of the larger cells was not significantly different between the insulin-sensitive and insulin-resistant individuals. Moreover, insulin resistance was associated with a higher ratio of small to large cells (1.66 +/- 1.03 vs 0.94 +/- 0.50, p = 0.01). Similar cell size distributions were observed for isolated adipose cells. The real-time PCR results showed two- to threefold lower expression of genes encoding markers of adipose cell differentiation (peroxisome proliferator-activated receptor gamma1 [PPARgamma1], PPARgamma2, GLUT4, adiponectin, sterol receptor element binding protein 1c) in insulin-resistant compared with insulin-sensitive individuals.

CONCLUSIONS/INTERPRETATION

These results suggest that after controlling for obesity, insulin resistance is associated with an expanded population of small adipose cells and decreased expression of differentiation markers, suggesting that impairment in adipose cell differentiation may contribute to obesity-associated insulin resistance.

Analysis of GLUT4 distribution in whole skeletal muscle fibers: identification of distinct storage compartments that are recruited by insulin and muscle contractions

The effects of insulin stimulation and muscle contractions on the subcellular distribution of GLUT4 in skeletal muscle have been studied on a preparation of single whole fibers from the rat soleus. The fibers were labeled for GLUT4 by a preembedding technique and observed as whole mounts by immunofluorescence microscopy, or after sectioning, by immunogold electron microscopy. The advantage of this preparation for cells of the size of muscle fibers is that it provides global views of the staining from one end of a fiber to the other and from one side to the other through the core of the fiber. In addition, the labeling efficiency is much higher than can be obtained with ultracryosections. In nonstimulated fibers, GLUT4 is excluded from the plasma membrane and T tubules. It is distributed throughout the muscle fibers with approximately 23% associated with large structures including multivesicular endosomes located in the TGN region, and 77% with small tubulovesicular structures. The two stimuli cause translocation of GLUT4 to both plasma membrane and T tubules. Quantitation of the immunogold electron microscopy shows that the effects of insulin and contraction are additive and that each stimulus recruits GLUT4 from both large and small depots. Immunofluorescence double labeling for GLUT4 and transferrin receptor (TfR) shows that the small depots can be further subdivided into TfR-positive and TfR-negative elements. Interestingly, we observe that colocalization of TfR and GLUT4 is increased by insulin and decreased by contractions. These results, supported by subcellular fractionation experiments, suggest that TfR-positive depots are only recruited by contractions. We do not find evidence for stimulation-induced unmasking of resident surface membrane GLUT4 transporters or for dilation of the T tubule system (Wang, W., P.A. Hansen, B.A. Marshall, J.O. Holloszy, and M. Mueckler. 1996. J. Cell Biol. 135:415-430).

Studies of human adipose tissue. Adipose cell size and number in nonobese and obese patients

The cellular character of the adipose tissue of 21 nonobese and 78 obese patients has been examined. Adipose cell size (lipid per cell) was determined in three different subcutaneous and deep fat depots in each patient and the total number of adipose cells in the body estimated by division of total body fat by various combinations of the adipose cell sizes at six different sites. Cell number has also been estimated on the basis of various assumed distribution of total fat between the subcutaneous and deep fat depots. Obese patients, as a group, have larger adipose cells than do nonobese patients; cell size, however, varies considerably among the fat depots of individuals of either group. The variation in cell size exists not only between, but also within subcutaneous and deep sites. Estimates of total adipose cell number for a given individual based upon cell size can, therefore, vary by as much as 85%. On the basis of these studies it is suggested that the total adipose number of an individual is best and most practically estimated, at this time, by division of total body fat by the mean of the adipose cell sizes of at least three subcutaneous sites. IRRESPECTIVE OF THE METHOD BY WHICH TOTAL ADIPOSE CELL NUMBER IS ESTIMATED, TWO PATTERNS OF OBESITY EMERGE WITH RESPECT TO THE CELLULAR CHARACTER OF THE ADIPOSE TISSUE MASS OF THESE PATIENTS: hyperplastic, with increased adipose cell number and normal or increased size, and hypertrophic, with increased cell size alone. These two cellular patterns of obesity are independent of a variety of assumed distributions of fat among the subcutaneous and deep depots. When these different cellular patterns are examined in terms of various aspects of body size, body composition, and the degree, duration, and age of onset of obesity, only the latter uniquely distinguishes the hyperplastic from the hypertrophic: hyperplastic obesity is characterized by an early age of onset, hypertrophic, by a late age of onset. These studies indicate that there are two distinct periods early in life during which hypercellularity of the adipose tissue are most likely to occur: very early within the first few years, and again from age 9 to 13 yr.

Insulin stimulates both leptin secretion and production by rat white adipose tissue

Leptin, the peptide encoded by the obese gene, is secreted by adipose cells and plays a role in regulating food intake, energy expenditure, and adiposity. Because earlier studies suggested that insulin increases the expression of leptin, we investigated the effect of insulin on leptin secretion by adipose tissue. Epididymal fat pads were incubated in vitro in the presence or absence of insulin over a 4-h time course. Insulin increased leptin secretion by about 80% at all time points studied. After 10 min of insulin treatment, the amount of tissue-associated leptin was lower in insulin-stimulated tissue, presumably due to the increased secretion. At later times, both tissue-associated leptin and total leptin production were higher in insulin-treated tissue. In untreated, isolated adipose cells, immunostaining of leptin was detected in the endoplasmic reticulum by confocal microscopy. After insulin treatment, there were two populations of cells. In many cells, leptin staining became fainter and was restricted to a narrow band near the plasma membrane. However, in other cells the leptin-staining pattern was unchanged. Leptin did not colocalize with GLUT4, the glucose transporter isoform found primarily in insulin-responsive cells, in either basal or insulin-stimulated adipose cells. In this study, insulin increased both secretion and production of leptin by adipose tissue fragments. Interestingly, insulin appeared to stimulate the transport of leptin from the endoplasmic reticulum rather than acting on a pool of regulated secretory vesicles. (Endocrinology 138: 4463-4472, 1997)

A common variant in the patatin-like phospholipase 3 gene (PNPLA3) is associated with fatty liver disease in obese children and adolescents

The genetic factors associated with susceptibility to nonalcoholic fatty liver disease (NAFLD) in pediatric obesity remain largely unknown. Recently, a nonsynonymous single-nucleotide polymorphism (rs738409), in the patatin-like phospholipase 3 gene (PNPLA3) has been associated with hepatic steatosis in adults. In a multiethnic group of 85 obese youths, we genotyped the PNLPA3 single-nucleotide polymorphism, measured hepatic fat content by magnetic resonance imaging and insulin sensitivity by the insulin clamp. Because PNPLA3 might affect adipogenesis/lipogenesis, we explored the putative association with the distribution of adipose cell size and the expression of some adipogenic/lipogenic genes in a subset of subjects who underwent a subcutaneous fat biopsy. Steatosis was present in 41% of Caucasians, 23% of African Americans, and 66% of Hispanics. The frequency of PNPLA3(rs738409) G allele was 0.324 in Caucasians, 0.183 in African Americans, and 0.483 in Hispanics. The prevalence of the G allele was higher in subjects showing hepatic steatosis. Surprisingly, subjects carrying the G allele showed comparable hepatic glucose production rates, peripheral glucose disposal rate, and glycerol turnover as the CC homozygotes. Carriers of the G allele showed smaller adipocytes than those with CC genotype (P = 0.005). Although the expression of PNPLA3, PNPLA2, PPARγ2(peroxisome proliferator-activated receptor gamma 2), SREBP1c(sterol regulatory element binding protein 1c), and ACACA(acetyl coenzyme A carboxylase) was not different between genotypes, carriers of the G allele showed lower leptin (LEP)(P = 0.03) and sirtuin 1 (SIRT1) expression (P = 0.04).

CONCLUSION

A common variant of the PNPLA3 gene confers susceptibility to hepatic steatosis in obese youths without increasing the level of hepatic and peripheral insulin resistance. The rs738409 PNPLA3 G allele is associated with morphological changes in adipocyte cell size.

Role of the nitric oxide pathway in AMPK-mediated glucose uptake and GLUT4 translocation in heart muscle

AMP-activated protein kinase (AMPK) is a serine-threonine kinase that regulates cellular metabolism and has an essential role in activating glucose transport during hypoxia and ischemia. The mechanisms responsible for AMPK stimulation of glucose transport are uncertain, but may involve interaction with other signaling pathways or direct effects on GLUT vesicular trafficking. One potential downstream mediator of AMPK signaling is the nitric oxide pathway. The aim of this study was to examine the extent to which AMPK mediates glucose transport through activation of the nitric oxide (NO)-signaling pathway in isolated heart muscles. Incubation with 1 mM 5-amino-4-imidazole-1-beta-carboxamide ribofuranoside (AICAR) activated AMPK (P < 0.01) and stimulated glucose uptake (P < 0.05) and translocation of the cardiomyocyte glucose transporter GLUT4 to the cell surface (P < 0.05). AICAR treatment increased phosphorylation of endothelial NO synthase (eNOS) approximately 1.8-fold (P < 0.05). eNOS, but not neuronal NOS, coimmunoprecipitated with both the alpha(2) and alpha(1) AMPK catalytic subunits in heart muscle. NO donors also increased glucose uptake and GLUT4 translocation (P < 0.05). Inhibition of NOS with N(omega)-nitro-l-arginine and N(omega)-methyl-l-arginine reduced AICAR-stimulated glucose uptake by 21 +/- 3% (P < 0.05) and 25 +/- 4% (P < 0.05), respectively. Inhibition of guanylate cyclase with ODQ and LY-83583 reduced AICAR-stimulated glucose uptake by 31 +/- 4% (P < 0.05) and 22 +/- 3% (P < 0.05), respectively, as well as GLUT4 translocation to the cell surface (P < 0.05). Taken together, these results indicate that activation of the NO-guanylate cyclase pathway contributes to, but is not the sole mediator of, AMPK stimulation of glucose uptake and GLUT4 translocation in heart muscle.

A possible mechanism of insulin resistance in the rat adipose cell in streptozotocin-induced diabetes mellitus. Depletion of intracellular glucose transport systems

The effects of insulin-dependent diabetes mellitus on glucose transport activity and on the concentrations of glucose transport systems in the plasma and low density microsomal membranes in adipose cells isolated from streptozotocin-induced diabetic rats have been examined. Glucose transport activity was assessed by measuring 3-O-methylglucose transport and the concentration of glucose transport systems estimated by measuring specific D-glucose-inhibitable cytochalasin B-binding. Basal glucose transport activity decreases from 0.19 to 0.12 fmol/cell per min with the induction of diabetes, but remains constant per unit cellular surface area and is accompanied by a constant 6 pmol of glucose transport systems/mg of membrane protein in the plasma membrane fraction. Maximally insulin-stimulated glucose transport activity decreases from 3.16 to 1.05 fmol/cell per min and from 0.26 to 0.12 amol/micrometers 2 per min, and is accompanied by a decrease from 25 to 15 pmol of glucose transport systems/mg of plasma membrane protein. These diminished effects of insulin on glucose transport activity and the concentration of glucose transport systems in the plasma membrane fraction are paralleled by a 45% decrease in the basal number of glucose transport systems per milligram of membrane protein in the low density microsomal membrane fraction, the source of those glucose transport systems appearing in the plasma membrane in response to insulin. Thus, the "insulin resistant" glucose transport of the adipose cell in the streptozotocin-induced diabetic rat appears to be the consequence of a depletion of glucose transport systems in the intracellular pool.

Insulin stimulates the halting, tethering, and fusion of mobile GLUT4 vesicles in rat adipose cells

Glucose transport in adipose cells is regulated by changing the distribution of glucose transporter 4 (GLUT4) between the cell interior and the plasma membrane (PM). Insulin shifts this distribution by augmenting the rate of exocytosis of specialized GLUT4 vesicles. We applied time-lapse total internal reflection fluorescence microscopy to dissect intermediates of this GLUT4 translocation in rat adipose cells in primary culture. Without insulin, GLUT4 vesicles rapidly moved along a microtubule network covering the entire PM, periodically stopping, most often just briefly, by loosely tethering to the PM. Insulin halted this traffic by tightly tethering vesicles to the PM where they formed clusters and slowly fused to the PM. This slow release of GLUT4 determined the overall increase of the PM GLUT4. Thus, insulin initially recruits GLUT4 sequestered in mobile vesicles near the PM. It is likely that the primary mechanism of insulin action in GLUT4 translocation is to stimulate tethering and fusion of trafficking vesicles to specific fusion sites in the PM.

Structure-function relationships in the adipose cell. I. Ultrastructure of the isolated adipose cell

A method is described for preparing isolated rat adipose cells for electron microscopy. The ultrastructure of such cells and their production of (14)CO(2) from U-glucose-(14)C were studied simultaneously in the presence of insulin or epinephrine. Each adipose cell consists of a large lipid droplet surrounded by a thin rim of cytoplasm. In addition to typical subcellular organelles, a variety of small lipid droplets and an extensive system of membranes characterize the cell's cytoplasm. A fenestrated envelope surrounds the large, central lipid droplet. Similar envelopes surround cytoplasmic lipid droplets occurring individually or as aggregates of very small, amorphous droplets. Groups of individual droplets of smaller size also occur without envelopes. The system of membranes consists of invaginations of the cell membrane, vesicles possibly of pinocytic origin, simple and vesiculated vacuoles, vesicles deeper in the cytoplasm, flattened and vesicular smooth surfaced endoplasmic reticulum, and Golgi complexes. Neither insulin nor epinephrine produced detectable ultrastructural alterations even when cells were incubated under optimal conditions for the stimulation of (14)CO(2) evolution. Structural responses of the isolated adipose cell to hormones, if such occur, must, therefore, be dynamic rather than qualitative in nature; the extensive system of smooth surfaced membranes is suggestive of compartmentalized transport and metabolism.

Roles of 1-phosphatidylinositol 3-kinase and ras in regulating translocation of GLUT4 in transfected rat adipose cells

Insulin stimulates glucose transport in insulin target tissues by recruiting glucose transporters (primarily GLUT4) from an intracellular compartment to the cell surface. Previous studies have demonstrated that insulin receptor tyrosine kinase activity and subsequent phosphorylation of insulin receptor substrate 1 (IRS-1) contribute to mediating the effect of insulin on glucose transport. We have now investigated the roles of 1-phosphatidylinositol 3-kinase (PI 3-kinase) and ras, two signaling proteins located downstream from tyrosine phosphorylation. Rat adipose cells were cotransfected with expression vectors that allowed transient expression of epitope-tagged GLUT4 and the other genes of interest. Overexpression of a mutant p85 regulatory subunit of PI 3-kinase lacking the ability to bind and activate the p110 catalytic subunit exerted a dominant negative effect to inhibit insulin-stimulated translocation of epitope-tagged GLUT4 to the cell surface. In addition, treatment of control cells with wortmannin (an inhibitor of PI 3-kinase) abolished the ability of insulin to recruit epitope-tagged GLUT4 to the cell surface. Thus, our data suggest that PI 3-kinase plays an essential role in insulin-stimulated GLUT4 recruitment in insulin target tissues. In contrast, over-expression of a constitutively active mutant of ras (L61-ras) resulted in high levels of cell surface GLUT4 in the absence of insulin that were comparable to levels seen in control cells treated with a maximally stimulating dose of insulin. However, wortmannin treatment of cells overexpressing L61-ras resulted in only a small decrease in the amount of cell surface GLUT4 compared with that of the same cells in the absence of wortmannin. Therefore, while activated ras is sufficient to recruit GLUT4 to the cell surface, it does so by a different mechanism that is probably not involved in the mechanism by which insulin stimulates GLUT4 translocation in physiological target tissues.

Evidence of impaired adipogenesis in insulin resistance

To elucidate the roles of adipose tissue and skeletal muscle in the early development of insulin resistance, we characterized gene expression profiles of isolated adipose cells and skeletal muscle of non-diabetic insulin-resistant first-degree relatives of type 2 diabetic patients using oligonucleotide microarrays. About 600 genes and expressed sequence tags, which displayed a gene expression pattern of cell proliferation, were differentially expressed in the adipose cells. The differentially expressed genes in the skeletal muscle were mostly related to the cellular signal transduction and transcriptional regulation. To verify the microarray findings, we studied expression of genes participating in adipogenesis. The expression of Wnt signaling genes, WNT1, FZD1, DVL1, GSK3beta, beta-catenin, and TCF1, and adipogenic transcription factors, C/EBPalpha and beta and delta, PPARgamma, and SREBP-1, was reduced in the adipose tissue. The expression of adipose-specific proteins related to terminal differentiation, such as adiponectin and aP2, was reduced both in the adipose tissue and in the adipose cells isolated from portions of the biopsies. The adipose cells were enlarged in the insulin-resistant relatives and the cell size inversely correlated with the expression of the Wnt signaling genes, adiponectin, and aP2. Our findings suggest that insulin resistance is associated with an impaired adipogenesis.

Standard isolation of primary adipose cells from mouse epididymal fat pads induces inflammatory mediators and down-regulates adipocyte genes

Isolation and subsequent in vitro culture of primary adipose cells are associated with down-regulation of GLUT4 mRNA and simultaneous induction of GLUT1 gene expression. Progressive loss of insulin-responsive GLUT4 contributes to the decrease in insulin-mediated glucose uptake in these cells when cultured in vitro. The mechanisms underlying these alterations are unknown. Here, we report that the standard procedure for isolating primary adipose cells from mouse adipose tissue triggers induction of many genes encoding inflammatory mediators including TNF-alpha, interleukin (IL)-1 alpha, IL-6, multiple chemokines, cell adhesion molecules, acute-phase proteins, type I IL-1 receptor, and multiple transcription factors implicated in the cellular inflammatory response. Secretion of TNF-alpha protein was also significantly induced during the 2-h collagenase digestion of adipose tissue. Isolated primary adipose cells exhibit dramatic changes in expression of multiple mRNAs that are characteristic of TNF-alpha-treated 3T3-L1 adipocytes including down-regulation of many genes important for insulin action and triglyceride synthesis. Addition of TNF-alpha to primary adipose cells in culture did not change the kinetics or the extent of the repression of adipose cell-abundant genes. Moreover, TNF-alpha-neutralizing antibody failed to block the changes in gene transcription in isolated primary adipose cells. Also, the standard isolation procedure induced the expression of NF-kappa B family members and their target genes in primary adipose cells prepared from TNF-alpha-/- mice to the same extent as in cells isolated from wild-type mice and resulted in almost identical changes in global gene expression when these cells were cultured in vitro. Thus, these data suggest that the standard isolation procedure-triggered reprogramming of gene expression in primary adipose cells that results in decreased insulin sensitivity does not require TNF-alpha, at least in this in vitro model system, but may be dependent on other inflammatory cytokines produced by these cells.

Normalization of blood glucose in diabetic rats with phlorizin treatment reverses insulin-resistant glucose transport in adipose cells without restoring glucose transporter gene expression

Evidence is emerging for a direct role of glucose, independent of changes in insulin, in the regulation of cellular glucose transport and glucose utilization in vivo. In this study we investigate potential cellular and molecular mechanisms for this regulatory effect of glucose by determining how normalization of glycemia without insulin therapy in diabetic rats influences 3-O-methylglucose transport and the expression and translocation of two genetically distinct species of glucose transporters (GTs) in adipose cells. These results are compared with alterations in glucose disposal in vivo measured by euglycemic clamp. In rats rendered diabetic by 90% pancreatectomy, insulin-stimulated glucose transport in adipose cells is decreased 50% in parallel with reduced insulin-mediated glucose disposal in vivo. Levels of adipose/muscle GTs measured by immunoblotting are decreased in adipose cell subcellular membrane fractions, as are the corresponding mRNA levels assessed by Northern blotting of total adipose cell RNA. Normalization of blood glucose in diabetic rats with phlorizin, which impairs renal tubular glucose reabsorption and thus enhances glucose excretion, restores insulin-stimulated glucose transport in adipose cells and insulin-mediated glucose disposal in vivo. Importantly, levels of the adipose/muscle GT protein remain 43% reduced in the low-density microsomes in the basal state and 46% reduced in the plasma membranes in the insulin-stimulated state. Adipose/muscle GT mRNA levels remain approximately 50% depressed. Levels of the HepG2/brain GT protein and mRNA are unaltered by diabetes or phlorizin treatment. Thus, changes in ambient glucose independent of changes in ambient insulin can regulate the glucose transport response to insulin in isolated adipose cells and changes in responsiveness parallel alterations in glucose uptake in vivo. Since this effect can occur without alteration in the expression of the two species of glucose transporters present in adipose cells or in their translocation to the plasma membrane in response to insulin, it may result from changes in GT functional activity.

Differential regulation of two glucose transporters in adipose cells from diabetic and insulin-treated diabetic rats

At least two genetically distinct glucose transporters (GTs) coexist in adipose cells, one cloned from human hepatoma cells and rat brain (HepG2/brain) and another from rat skeletal muscle, heart, and adipose cells (adipose cell/muscle). Here we demonstrate differential regulation of these two GTs in adipose cells of diabetic and insulin-treated diabetic rats and compare changes in the expression of each GT with marked alterations in insulin-stimulated glucose transport activity. Adipose cell/muscle GTs detected by immunoblotting with the monoclonal antiserum 1F8 (James, D. E., R. Brown, J. Navarro, and P. F. Pilch. 1988. Nature (Lond.). 333:183-185), which reacts with the protein product of the newly cloned adipose cell/muscle GT cDNA, decrease 87% with diabetes and increase to 8.5-fold diabetic levels with insulin treatment. These changes concur qualitatively with previous detection of GTs by cytochalasin B binding and with insulin-stimulated 3-O-methylglucose transport. Northern blotting reveals that the adipose/muscle GT mRNA decreases 50% with diabetes and increases to 6.8-fold control (13-fold diabetic) levels with insulin treatment. In contrast, GTs detected with antisera to the carboxyl terminus of the HepG2 GT or to the human erythrocyte GT show no significant change with diabetes or insulin treatment. The HepG2/brain GT mRNA is unchanged with diabetes and increases threefold with insulin treatment. These results suggest that (a) altered expression of the adipose cell/muscle GT forms the molecular basis for the dysregulated glucose transport response to insulin characteristic of diabetes, (b) the expression of two types of GTs in rat adipose cells is regulated independently, and (c) alterations in mRNA levels are only part of the mechanism for in vivo regulation of the expression of either GT species.

Tankyrase 1 and tankyrase 2 are essential but redundant for mouse embryonic development

Tankyrases are proteins with poly(ADP-ribose) polymerase activity. Human tankyrases post-translationally modify multiple proteins involved in processes including maintenance of telomere length, sister telomere association, and trafficking of glut4-containing vesicles. To date, however, little is known about in vivo functions for tankyrases. We recently reported that body size was significantly reduced in mice deficient for tankyrase 2, but that these mice otherwise appeared developmentally normal. In the present study, we report generation of tankyrase 1-deficient and tankyrase 1 and 2 double-deficient mice, and use of these mutant strains to systematically assess candidate functions of tankyrase 1 and tankyrase 2 in vivo. No defects were observed in development, telomere length maintenance, or cell cycle regulation in tankyrase 1 or tankyrase 2 knockout mice. In contrast to viability and normal development of mice singly deficient in either tankyrase, deficiency in both tankyrase 1 and tankyrase 2 results in embryonic lethality by day 10, indicating that there is substantial redundancy between tankyrase 1 and tankyrase 2, but that tankyrase function is essential for embryonic development.

Deletion of the alpha-arrestin protein Txnip in mice promotes adiposity and adipogenesis while preserving insulin sensitivity

OBJECTIVE

Thioredoxin interacting protein (Txnip), a regulator of cellular oxidative stress, is induced by hyperglycemia and inhibits glucose uptake into fat and muscle, suggesting a role for Txnip in type 2 diabetes pathogenesis. Here, we tested the hypothesis that Txnip-null (knockout) mice are protected from insulin resistance induced by a high-fat diet.

RESEARCH DESIGN AND METHODS

Txnip gene-deleted (knockout) mice and age-matched wild-type littermate control mice were maintained on a standard chow diet or subjected to 4 weeks of high-fat feeding. Mice were assessed for body composition, fat development, energy balance, and insulin responsiveness. Adipogenesis was measured from ex vivo fat preparations, and in mouse embryonic fibroblasts (MEFs) and 3T3-L1 preadipocytes after forced manipulation of Txnip expression.

RESULTS

Txnip knockout mice gained significantly more adipose mass than controls due to a primary increase in both calorie consumption and adipogenesis. Despite increased fat mass, Txnip knockout mice were markedly more insulin sensitive than controls, and augmented glucose transport was identified in both adipose and skeletal muscle. RNA interference gene-silenced preadipocytes and Txnip(-/-) MEFs were markedly adipogenic, whereas Txnip overexpression impaired adipocyte differentiation. As increased adipogenesis and insulin sensitivity suggested aspects of augmented peroxisome proliferator-activated receptor-gamma (PPARgamma) response, we investigated Txnip's regulation of PPARgamma function; manipulation of Txnip expression directly regulated PPARgamma expression and activity.

CONCLUSIONS

Txnip deletion promotes adiposity in the face of high-fat caloric excess; however, loss of this alpha-arrestin protein simultaneously enhances insulin responsiveness in fat and skeletal muscle, revealing Txnip as a novel mediator of insulin resistance and a regulator of adipogenesis.

Cell surface accessibility of GLUT4 glucose transporters in insulin-stimulated rat adipose cells. Modulation by isoprenaline and adenosine

Insulin-stimulated glucose transport activity in rat adipocytes is inhibited by isoprenaline and enhanced by adenosine. Both of these effects occur without corresponding changes in the subcellular distribution of the GLUT4 glucose transporter isoform. In this paper, we have utilized the impermeant, exofacial bis-mannose glucose transporter-specific photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2-propylamine (ATB-BMPA) [Clark & Holman (1990) Biochem. J. 269, 615-622], to examine the cell surface accessibility of GLUT4 glucose transporters under these conditions. Compared with cells treated with insulin alone, adenosine in the presence of insulin increased the accessibility of GLUT4 to the extracellular photolabel by approximately 25%, consistent with its enhancement of insulin-stimulated glucose transport activity; the plasma membrane concentration of GLUT4 as assessed by Western blotting was unchanged. Conversely, isoprenaline, in the absence of adenosine, promoted a time-dependent (t1/2 approximately 2 min) decrease in the accessibility of insulin-stimulated cell surface GLUT4 of > 50%, which directly correlated with the observed inhibition of transport activity; the plasma membrane concentration of GLUT4 decreased by 0-15%. Photolabelling the corresponding plasma membranes revealed that these alterations in the ability of the photolabel to bind to GLUT4 are transient, as the levels of both photolabel incorporation and plasma membrane glucose transport activity were consistent with the observed GLUT4 concentration. These data suggest that insulin-stimulated GLUT4 glucose transporters can exist in two distinct states within the adipocyte plasma membrane, one which is functional and accessible to extracellular substrate, and one which is non-functional and unable to bind extracellular substrate. These effects are only observed in the intact adipocyte and are not retained in plasma membranes isolated from these cells when analysed for their ability to transport glucose or bind photolabel.

Dexamethasone inhibits insulin-stimulated recruitment of GLUT4 to the cell surface in rat skeletal muscle

To test the hypothesis that glucocorticoids reduce insulin-stimulated skeletal muscle glucose transport by inhibiting the recruitment of GLUT4 glucose transporters to the cell surface, we determined the effect of glucocorticoid treatment on cell-surface GLUT4 using the impermeant glucose transporter photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-[2-3H]1,3-bis-(D-mann os-4-yloxy)-2-propylamine (ATB-[2-3H]BMPA), and GLUT4 immunoprecipitation. Male Sprague-Dawley rats were treated with dexamethasone ([Dex] 0.9 mg/kg for 2 days) and compared against pair-fed controls. 2-[3H]deoxyglucose (2-[3H]DG) uptake in isolated soleus muscles was measured under conditions in which uptake reflects glucose transport activity. In control muscles, 2-[3H]DG uptake was stimulated eightfold by insulin (20 nmol/L). Dex treatment reduced maximal insulin-stimulated 2-[3H]DG uptake by 48% +/- 4% (mean +/- SEM) and decreased cell-surface (ATB-[2-3H]BMPA-photolabeled) GLUT4 by 48% +/- 3%, despite an increase in total muscle GLUT4 content of 26% +/- 7%. These findings indicate that glucocorticoid-induced inhibition of insulin-stimulated glucose transport in muscle is due to impaired recruitment of GLUT4 to the cell surface.

Cellularity and adipogenic profile of the abdominal subcutaneous adipose tissue from obese adolescents: association with insulin resistance and hepatic steatosis

OBJECTIVE

We explored whether the distribution of adipose cell size, the estimated total number of adipose cells, and the expression of adipogenic genes in subcutaneous adipose tissue are linked to the phenotype of high visceral and low subcutaneous fat depots in obese adolescents.

RESEARCH DESIGN AND METHODS

A total of 38 adolescents with similar degrees of obesity agreed to have a subcutaneous periumbilical adipose tissue biopsy, in addition to metabolic (oral glucose tolerance test and hyperinsulinemic euglycemic clamp) and imaging studies (MRI, DEXA, (1)H-NMR). Subcutaneous periumbilical adipose cell-size distribution and the estimated total number of subcutaneous adipose cells were obtained from tissue biopsy samples fixed in osmium tetroxide and analyzed by Beckman Coulter Multisizer. The adipogenic capacity was measured by Affymetrix GeneChip and quantitative RT-PCR.

RESULTS

Subjects were divided into two groups: high versus low ratio of visceral to visceral + subcutaneous fat (VAT/[VAT+SAT]). The cell-size distribution curves were significantly different between the high and low VAT/(VAT+SAT) groups, even after adjusting for age, sex, and ethnicity (MANOVA P = 0.035). Surprisingly, the fraction of large adipocytes was significantly lower (P < 0.01) in the group with high VAT/(VAT+SAT), along with the estimated total number of large adipose cells (P < 0.05), while the mean diameter was increased (P < 0.01). From the microarray analyses emerged a lower expression of lipogenesis/adipogenesis markers (sterol regulatory element binding protein-1, acetyl-CoA carboxylase, fatty acid synthase) in the group with high VAT/(VAT+SAT), which was confirmed by RT-PCR.

CONCLUSIONS

A reduced lipo-/adipogenic capacity, fraction, and estimated number of large subcutaneous adipocytes may contribute to the abnormal distribution of abdominal fat and hepatic steatosis, as well as to insulin resistance in obese adolescents.