Phosphorylation of the exocyst protein Exo84 by TBK1 promotes insulin-stimulated GLUT4 trafficking.

Insulin stimulates glucose uptake through the translocation of the glucose transporter GLUT4 to the plasma membrane. The exocyst complex tethers GLUT4-containing vesicles to the plasma membrane, a process that requires the binding of the G protein (heterotrimeric guanine nucleotide-binding protein) RalA to the exocyst complex. We report that upon activation of RalA, the protein kinase TBK1 phosphorylated the exocyst subunit Exo84. Knockdown of TBK1 blocked insulin-stimulated glucose uptake and GLUT4 translocation; knockout of TBK1 in adipocytes blocked insulin-stimulated glucose uptake; and ectopic overexpression of a kinase-inactive mutant of TBK1 reduced insulin-stimulated glucose uptake in 3T3-L1 adipocytes. The phosphorylation of Exo84 by TBK1 reduced its affinity for RalA and enabled its release from the exocyst. Overexpression of a kinase-inactive mutant of TBK1 blocked the dissociation of the TBK1/RalA/exocyst complex, and treatment of 3T3-L1 adipocytes with specific inhibitors of TBK1 reduced the rate of complex dissociation. Introduction of phosphorylation-mimicking or nonphosphorylatable mutant forms of Exo84 blocked insulin-stimulated GLUT4 translocation. Thus, these data indicate that TBK1 controls GLUT4 vesicle engagement and disengagement from the exocyst, suggesting that exocyst components not only constitute a tethering complex for the GLUT4 vesicle but also act as "gatekeepers" controlling vesicle fusion at the plasma membrane.

Evidence for the regulatory role of lipocalin 2 in high-fat diet-induced adipose tissue remodeling in male mice.

Lipocalin 2 (Lcn2) has previously been characterized as an adipokine/cytokine playing a role in glucose and lipid homeostasis. In this study, we investigate the role of Lcn2 in adipose tissue remodeling during high-fat diet (HFD)-induced obesity. We find that Lcn2 protein is highly abundant selectively in inguinal adipose tissue. During 16 weeks of HFD feeding, the inguinal fat depot expanded continuously, whereas the expansion of the epididymal fat depot was reduced in both wild-type (WT) and Lcn2(-/-) mice. Interestingly, the depot-specific effect of HFD on fat mass was exacerbated and appeared more pronounced and faster in Lcn2(-/-) mice than in WT mice. In Lcn2(-/-) mice, adipocyte hypertrophy in both inguinal and epididymal adipose tissue was more profoundly induced by age and HFD when compared with WT mice. The expression of peroxisome proliferator-activated receptor-γ protein was significantly down-regulated, whereas the gene expression of extracellular matrix proteins was up-regulated selectively in epididymal adipocytes of Lcn2(-/-) mice. Consistent with these observations, collagen deposition was selectively higher in the epididymal, but not in the inguinal adipose depot of Lcn2(-/-) mice. Administration of the peroxisome proliferator-activated receptor-γ agonist rosiglitazone (Rosi) restored adipogenic gene expression. However, Lcn2 deficiency did not alter the responsiveness of adipose tissue to Rosi effects on the extracellular matrix expression. Rosi treatment led to the further enlargement of adipocytes with improved metabolic activity in Lcn2(-/-) mice, which may be associated with a more pronounced effect of Rosi treatment in reducing TGF-ß in Lcn2(-/-) adipose tissue. Consistent with these in vivo observations, Lcn2 deficiency reduces the adipocyte differentiation capacity of stromal-vascular cells isolated from HFD-fed mice in these cells. Herein Rosi treatment was again able to stimulate adipocyte differentiation to a similar extent in WT and Lcn2(-/-) inguinal and epididymal stromal-vascular cells. Thus, combined, our data indicate that Lcn2 has a depot-specific role in HFD-induced adipose tissue remodeling.

Differential adipogenic and inflammatory properties of small adipocytes in Zucker Obese and Lean rats.

We recently reported that a preponderance of small adipose cells, decreased expression of cell differentiation markers, and enhanced inflammatory activity in human subcutaneous whole adipose tissue were associated with insulin resistance. To test the hypothesis that small adipocytes exhibited these differential properties, we characterised small adipocytes from epididymal adipose tissue of Zucker Obese (ZO) and Lean (ZL) rats. Rat epididymal fat pads were removed and adipocytes isolated by collagenase digestion. Small adipocytes were separated by sequential filtration through nylon meshes. Adipocytes were fixed in osmium tetroxide for cell size distribution analysis via Beckman Coulter Multisizer. Quantitative real-time PCR for cell differentiation and inflammatory genes was performed. Small adipocytes represented a markedly greater percentage of the total adipocyte population in ZO than ZL rats (58±4% vs. 12±3%, p<0.001). In ZO rats, small as compared with total adipocytes had 4-fold decreased adiponectin, and 4-fold increased visfatin and IL-6 levels. Comparison of small adipocytes in ZO versus ZL rats revealed 3-fold decreased adiponectin and PPARγ levels, and 2.5-fold increased IL-6. In conclusion, ZO rat adipose tissue harbours a large proportion of small adipocytes that manifest impaired cell differentiation and pro-inflammatory activity, two mechanisms by which small adipocytes may contribute to insulin resistance.

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.

Lipocalin-2 deficiency impairs thermogenesis and potentiates diet-induced insulin resistance in mice.

OBJECTIVE: Lipocalin (LCN) 2 belongs to the lipocalin subfamily of low-molecular mass-secreted proteins that bind small hydrophobic molecules. LCN2 has been recently characterized as an adipose-derived cytokine, and its expression is upregulated in adipose tissue in genetically obese rodents. The objective of this study was to investigate the role of LCN2 in diet-induced insulin resistance and metabolic homeostasis in vivo. RESEARCH DESIGN AND METHODS: Systemic insulin sensitivity, adaptive thermogenesis, and serum metabolic and lipid profile were assessed in LCN2-deficient mice fed a high-fat diet (HFD) or regular chow diet. RESULTS: The molecular disruption of LCN2 in mice resulted in significantly potentiated diet-induced obesity, dyslipidemia, fatty liver disease, and insulin resistance. LCN2(-/-) mice exhibit impaired adaptive thermogenesis and cold intolerance. Gene expression patterns in white and brown adipose tissue, liver, and muscle indicate that LCN2(-/-) mice have increased hepatic gluconeogenesis, decreased mitochondrial oxidative capacity, impaired lipid metabolism, and increased inflammatory state under the HFD condition. CONCLUSIONS: LCN2 has a novel role in adaptive thermoregulation and diet-induced insulin resistance.

The protein kinase IKKepsilon regulates energy balance in obese mice.

Obesity is associated with chronic low-grade inflammation that negatively impacts insulin sensitivity. Here, we show that high-fat diet can increase NF-kappaB activation in mice, which leads to a sustained elevation in level of IkappaB kinase epsilon (IKKepsilon) in liver, adipocytes, and adipose tissue macrophages. IKKepsilon knockout mice are protected from high-fat diet-induced obesity, chronic inflammation in liver and fat, hepatic steatosis, and whole-body insulin resistance. These mice show increased energy expenditure and thermogenesis via enhanced expression of the uncoupling protein UCP1. They maintain insulin sensitivity in liver and fat, without activation of the proinflammatory JNK pathway. Gene expression analyses indicate that IKKepsilon knockout reduces expression of inflammatory cytokines, and changes expression of certain regulatory proteins and enzymes involved in glucose and lipid metabolism. Thus, IKKepsilon may represent an attractive therapeutic target for obesity, insulin resistance, diabetes, and other complications associated with these disorders.

Guardian of corpulence: a hypothesis on p53 signaling in the fat cell.

Adipocytes provide an organism with fuel in times of caloric deficit, and are an important type of endocrine cell in the maintenance of metabolic homeostasis. In addition, as a lipid-sink, adipocytes serve an equally important role in the protection of organs from the damaging effects of ectopic lipid deposition. For the organism, it is of vital importance to maintain adipocyte viability, yet the fat depot is a demanding extracellular environment with high levels of interstitial free fatty acids and associated lipotoxic effects. These surroundings are less than beneficial for the overall health of any resident cell, adipocyte and preadipocyte alike. In this review, we discuss the process of adipogenesis and the potential involvement of the p53 tumor-suppressor protein in alleviating some of the cellular stress experienced by these cells. In particular, we discuss p53-mediated mechanisms that prevent damage caused by reactive oxygen species and the effects of lipotoxicity. We also suggest the potential for two p53 target genes, START domain-containing protein 4 (StARD4) and oxysterol-binding protein (OSBP), with the concomitant synthesis of the signaling molecule oxysterol, to participate in adipogenesis.

The p38 mitogen-activated protein kinase inhibitor SB203580 reduces glucose turnover by the glucose transporter-4 of 3T3-L1 adipocytes in the insulin-stimulated state.

Insulin induces a profound increase in glucose uptake in 3T3-L1 adipocytes through the activity of the glucose transporter-4 (GLUT4). Apart from GLUT4 translocation toward the plasma membrane, there is also an insulin-induced p38 MAPK-dependent step involved in the regulation of glucose uptake. Consequently, treatment with the p38 MAPK inhibitor SB203580 reduces insulin-induced glucose uptake by approximately 30%. Pretreatment with SB203580 does not alter the apparent K(m) of GLUT4-mediated glucose uptake but reduces the maximum velocity by approximately 30%. Insulin-induced GLUT4 translocation and exposure of the transporter to the extracellular environment was not altered by pretreatment with SB203580, as evidenced by a lack of effect of the inhibitor on the amount of GLUT4 present in the plasma membrane, as assessed by subcellular fractionation, the amount of GLUT4 that is able to undergo biotinylation on intact adipocytes and the level of extracellular exposure of an ectopically expressed GLUT-green fluorescence protein construct with a hemagglutinin tag in its first extracellular loop. In contrast, labeling of GLUT4 after insulin stimulation by a membrane-impermeable, mannose moiety-containing, photoaffinity-labeling agent [2-N-4(1-azido-2,2,2-trifluoroethyl)benzoyl-1,3-bis(d-mannose-4-yloxy)-2-propylamine] that binds to the extracellular glucose acceptor domain was markedly reduced by SB203580, although photolabeling with this compound in the absence of insulin was unaffected by SB203580. These data suggest that SB203580 affects glucose turnover by the insulin-responsive GLUT4 transporter in 3T3-L1 adipocytes.

Genistein directly inhibits GLUT4-mediated glucose uptake in 3T3-L1 adipocytes.

The isoflavone-derivative genistein is commonly applied as an inhibitor of tyrosine kinases. In this report we analyze the effect of genistein on insulin-stimulated glucose uptake in 3T3-L1 adipocytes. In these cells insulin-induced glucose uptake is primarily mediated by the GLUT4 glucose transporter. We observed that pre-treatment with genistein did not affect insulin-induced tyrosine kinase activity of the insulin receptor or activation of protein kinase B. On the other hand, genistein acted as a direct inhibitor of insulin-induced glucose uptake in 3T3-L1 adipocytes with an IC(50) of 20 microM. We conclude that apart from acting as a general tyrosine kinase inhibitor, genistein also affects the function of other proteins such as the GLUT4 transporter. These data suggest that caution must be applied when interpreting data on the involvement of tyrosine kinase activity in glucose uptake in 3T3-L1 adipocytes.

Regulation of GLUT1-mediated glucose uptake by PKClambda-PKCbeta(II) interactions in 3T3-L1 adipocytes.

Members of the PKC (protein kinase C) superfamily play key regulatory roles in glucose transport. How the different PKC isotypes are involved in the regulation of glucose transport is still poorly defined. PMA is a potent activator of conventional and novel PKCs and PMA increases the rate of glucose uptake in many different cell systems. In the present study, we show that PMA treatment increases glucose uptake in 3T3-L1 adipocytes by two mechanisms: a mitogen-activated protein kinase kinase-dependent increase in GLUT1 (glucose transporter 1) expression levels and a PKClambda-dependent translocation of GLUT1 towards the plasma membrane. Intriguingly, PKClambda co-immunoprecipitated with PKCbeta(II) and did not with PKCbeta(I). Previously, we have described that down-regulation of PKCbeta(II) protein levels or inhibiting PKCbeta(II) by means of the myristoylated PKCbetaC2-4 peptide inhibitor induced GLUT1 translocation towards the plasma membrane in 3T3-L1 adipocytes. Combined with the present findings, these results suggest that the liberation of PKClambda from PKCbeta(II) is an important factor in the regulation of GLUT1 distribution in 3T3-L1 adipocytes.

Mitogen-activated protein kinase (MAPK) phosphatase-1 and -4 attenuate p38 MAPK during dexamethasone-induced insulin resistance in 3T3-L1 adipocytes.

Prolonged use of glucocorticoids induces pronounced insulin resistance in vivo. In vitro, treatment of 3T3-L1 adipocytes with dexamethasone for 48 h reduces the maximal level of insulin- and stress (arsenite)-induced glucose uptake by approximately 50%. Although phosphatidylinositol 3-kinase signaling was slightly attenuated, phosphorylation of its downstream effectors such as protein kinase B and protein kinase C-lambda remained intact. Nor was any effect of dexamethasone treatment observed on insulin- or arsenite-induced translocation of glucose transporter 4 (GLUT4) toward the plasma membrane. However, for a maximal response to either arsenite- or insulin-induced glucose uptake in these cells, functional p38 MAPK signaling is required. Dexamethasone treatment markedly attenuated p38 MAPK phosphorylation coincident with an up-regulation of the MAPK phosphatases MKP-1 and MKP-4. Employing lentivirus-mediated ectopic expression in fully differentiated 3T3-L1 adipocytes demonstrated a differential effect of these phosphatases: whereas MKP-1 was a more potent inhibitor of insulin-induced glucose uptake, MKP-4 more efficiently inhibited arsenite-induced glucose uptake. This coincided with the effects of these phosphatases on p38 MAPK phosphorylation, i.e. MKP-1 and MKP-4 attenuated p38 MAPK phosphorylation by insulin and arsenite, respectively. Taken together, these data provide evidence that in 3T3-L1 adipocytes dexamethasone inhibits the activation of the GLUT4 in the plasma membrane by a p38 MAPK-dependent process, rather than in a defect in GLUT4 translocation per se.

Rottlerin inhibits multiple steps involved in insulin-induced glucose uptake in 3T3-L1 adipocytes.

Recently, it was shown that rottlerin inhibits insulin-stimulated glucose uptake and reduces intracellular adenosine triphosphate (ATP) levels in 3T3-L1 adipocytes, suggesting that these two events are causally linked. However, several other reports show that ATP-depletion induces glucose uptake in both muscle cells and adipocytes. In the present study, the mechanism of inhibition by rottlerin was studied in detail, in order to resolve this apparent discrepancy. It was found that rottlerin strongly reduces insulin-stimulated 2-deoxyglucose (2-DOG) uptake in 3T3-L1 adipocytes by a partial inhibition of the translocation of the insulin-responsive GLUT4 glucose transporter towards the plasma membrane (PM). Whereas the insulin-induced phosphatidyl-inositol-3' (PI-3') kinase signaling pathway is unaffected by rottlerin, Cbl tyrosine phosphorylation, which provides an essential, PI-3' kinase-independent signal towards GLUT4 translocation, is markedly attenuated. Furthermore, we also observed a direct inhibitory effect of rottlerin on insulin-induced glucose uptake in 3T3-L1 adipocytes. The direct inhibition of insulin-stimulated 2-DOG uptake by rottlerin displayed characteristics of uncompetitive inhibition: with the K(m(app)) of glucose uptake reduced from 1.6 to 0.9 mM and the V(max(app)) reduced from 5.2 to 1.0 nmol/minmg in the presence of rottlerin. In conclusion, rottlerin inhibits multiple steps involved in insulin-stimulated 2-DOG uptake in 3T3-L1 adipocytes. The observed reduction in GLUT4 translocation towards the PM and the uncompetitive inhibition of the glucose transport process provide alternative explanations for the inhibitory effects of rottlerin aside from the effects of rottlerin on intracellular levels of ATP.

Lentiviral vectors efficiently transduce quiescent mature 3T3-L1 adipocytes.

Obesity is associated with many serious afflictions such as cardiovascular disease, cancer, and diabetes. One of the main cellular systems used to study the underlying physiological and biological processes is the 3T3-L1 preadipocyte differentiation model. However, studies on 3T3-L1 adipocytes are hampered by the fact that genetic modification of mature adipocytes is notoriously difficult. In this report, we evaluated the use of lentivirus-mediated gene transfer into 3T3-L1 mature adipocytes. We demonstrate that quiescent, fully differentiated 3T3-L1 adipocytes as well as 3T3-L1 preadipocytes can be efficiently transduced with HIV-1-derived lentiviral vectors. Upon transduction using LV-PGK-GFP lentiviral vector at 100 ng p24 per 10(5) cells, more than 95% of the 3T3-L1 adipocytes in the culture expressed the GFP reporter gene. There were no overt signs of toxicity or cytopathogenicity in the cultures. Furthermore, modification of undifferentiated preadipocytes did not affect their capacity to differentiate. In addition, insulin-induced glucose uptake was not affected by the procedure. In contrast, adenoviral-mediated gene transfer into 3T3-L1 adipocytes is associated with marked cytopathogenicity. From these data, we conclude that lentiviral vectors are the gene-transfer system of choice for genetic modification of mature adipocytes. The availability of an efficient vector system may stimulate the use of adipose tissue as a target for gene therapy in obesity and other disorders.

Arsenite stimulated glucose transport in 3T3-L1 adipocytes involves both Glut4 translocation and p38 MAPK activity.

The protein-modifying agent arsenite stimulates glucose uptake in 3T3-L1 adipocytes. In the current study we have analysed the signalling pathways that contribute to this response. By subcellular fractionation we observed that arsenite, like insulin, induces translocation of the GLUT1 and GLUT4 glucose transporters from the low-density membrane fraction to the plasma membrane. Arsenite did not activate early steps of the insulin receptor (IR)-signalling pathway and the response was insensitive to inhibition of phosphatidylinositol-3'-kinase (PI-3') kinase by wortmannin. These findings indicate that the 'classical' IR-IR substrate-PI-3' kinase pathway, that is essential for insulin-induced GLUT4 translocation, is not activated by arsenite. However, arsenite-treatment did induce tyrosine-phosphorylation of c-Cbl. Furthermore, treatment of the cells with the tyrosine kinase inhibitor, tyrphostin A25, abolished arsenite-induced glucose uptake, suggesting that the induction of a tyrosine kinase by arsenite is essential for glucose uptake. Both arsenite and insulin-induced glucose uptake were inhibited partially by the p38 MAP kinase inhibitor, SB203580. This compound had no effect on the magnitude of translocation of glucose transporters indicating that the level of glucose transport is determined by additional factors. Arsenite- and insulin-induced glucose uptake responded in a remarkably similar dose-dependent fashion to a range of pharmacological- and peptide-inhibitors for atypical PKC-lambda, a downstream target of PI-3' kinase signalling in insulin-induced glucose uptake. These data show that in 3T3-L1 adipocytes both arsenite- and insulin-induced signalling pathways project towards a similar cellular response, namely GLUT1 and GLUT4 translocation and glucose uptake. This response to arsenite is not functionally linked to early steps of the IR-IRS-PI-3' kinase pathway, but does coincide with c-Cbl phosphorylation, basal levels of PKC-lambda activity and p38 MAPK activation.

Inhibition of protein kinase CbetaII increases glucose uptake in 3T3-L1 adipocytes through elevated expression of glucose transporter 1 at the plasma membrane.

The mechanism via which diacylglycerol-sensitive protein kinase Cs (PKCs) stimulate glucose transport in insulin-sensitive tissues is poorly defined. Phorbol esters, such as phorbol-12-myristate-13-acetate (PMA), are potent activators of conventional and novel PKCs. Addition of PMA increases the rate of glucose uptake in many different cell systems. We attempted to investigate the mechanism via which PMA stimulates glucose transport in 3T3-L1 adipocytes in more detail. We observed a good correlation between the rate of disappearance of PKCbetaII during prolonged PMA treatment and the increase in glucose uptake. Moreover, inhibition of PKCbetaII with a specific myristoylated PKCbetaC2-4 peptide inhibitor significantly increased the rate of glucose transport. Western blot analysis demonstrated that both PMA treatment and incubation with the myristoylated PKCbetaC2-4 pseudosubstrate resulted in more glucose transporter (GLUT)-1 but not GLUT-4 at the plasma membrane. To our knowledge, we are the first to demonstrate that inactivation of PKC, most likely PKCbetaII, elevates glucose uptake in 3T3-L1 adipocytes. The observation that PKCbetaII influences the rate of glucose uptake through manipulation of GLUT-1 expression levels at the plasma membrane might reveal a yet unidentified regulatory mechanism involved in glucose homeostasis.