Activity, phosphorylation state and subcellular distribution of GLUT4-targeted Akt2 in rat adipose cells

Medicinal Effects, Phytochemistry, Pharmacology of Euphorbia prostrata and Promising Molecular Mechanisms

Euphorbiaceae is a large family of dicotyledonous angiosperms with diverse genera including Euphorbia prostrata (E. prostrata). Current research has provided scientific evidence for traditional uses of E. prostrata against diverse pathological conditions such as anti-hemorrhoidal, anti-inflammatory, analgesic, wound healing, antioxidant, antibacterial, leishmanicidal, antitumor activity, and so on. The phytochemical screening has revealed the presence of glycosides, phytosterols, flavonoids, polyphenols, tannins, and anthraquinones with chemical structures elucidation of their respective compounds. The uniqueness of such multifactorial compounds present in this species endorses it as the potent therapeutic or prophylactic choice for several fatal diseases. Although ethnomedical applications served as a significant citation for pharmacology, the molecular mechanism has not been reviewed yet. The present paper provides a comprehensive review of research outcomes, pharmacology, toxicology, and molecular signaling of phytochemicals of E. prostrata species as a reference for relevant researchers. The study of bioactive compounds in crude extracts and fractions, the demonstration of primary mechanisms of pharmacology, along with the addition of toxicity, and clinical trials, should be conceded in depth. This review underlines the E. prostrata species that can be a promising phytomedicine since we are committed to excavating more intensely into their pharmacological role.

Trehalose Protects against Insulin Resistance-Induced Tissue Injury and Excessive Autophagy in Skeletal Muscles and Kidney

BACKGROUND

Insulin resistance refers to a pathological state of compromised sensitivity of insulin to promote glucose uptake and utilization, resulting in compensatory excessive insulin secretion and hyperinsulinemia in an effort to maintain glucose homeostasis. Akt2 represents an important member of the Akt family and plays an essential role in the maintenance of insulin signaling.

METHODS

This study was designed to examine the effects of trehalose on kidney and skeletal muscle (rectus femoris muscle) injury in an Akt2 knockout-induced model of insulin resistance. Akt2 knockout (Akt2-/-) and adult WT mice were treated with trehalose (1 mg/g/d) intraperitoneally for 2 days, followed by providing 2% trehalose in drinking water for 2 months. Intraperitoneal glucose tolerance test (IPGTT), protein carbonyl content and mitochondrial function (aconitase activity) were examined. Apoptosis and autophagy protein markers were monitored using western blot analysis.

RESULTS

Akt2 ablation impaired glucose tolerance, promoted protein carbonyl formation and decreased aconitase activity in kidney and skeletal muscles, associated with pronounced apoptosis and overt autophagy, the effects of which, with the exception of IPGTT, were greatly ameliorated or negated by trehalose treatment. Moreover, phosphorylation of mTOR was downregulated in both kidney and skeletal muscles from Akt2-/- mice, the effect of which was attenuated by trehalose. Levels of Akt (pan and Akt2) were much lower in Akt2-/- mice, the effect of which was unaffected by trehalose treatment although trehalose itself upregulated Akt levels.

CONCLUSION

These data suggest that the autophagy inducer trehalose rescued against insulin resistance-induced kidney and skeletal muscle injury, apoptosis and excessive autophagy, possibly in association with restored mTOR phosphorylation without affecting Akt.

Lipocalin 2 regulates retinoic acid-induced activation of beige adipocytes

Lipocalin-2 (LCN2) has been previously characterized as an adipokine regulating thermogenic activation of brown adipose tissue and retinoic acid (RA)-induced thermogenesis in mice. The objective of this study was to explore the role and mechanism for LCN2 in the recruitment and retinoic acid-induced activation of brown-like or ‘beige’ adipocytes. We found LCN2 deficiency reduces key markers of thermogenesis including uncoupling protein-1 (UCP1) and peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) in inguinal white adipose tissue (iWAT) and inguinal adipocytes derived from Lcn2 −/− mice. Lcn2 −/− inguinal adipocytes have attenuated insulin-induced upregulation of thermogenic gene expression and p38 mitogen-activated protein kinase (p38MAPK) signaling pathway activation. This is accompanied by a lower basal and maximal oxidative capacity in Lcn2 −/− inguinal adipocytes, indicating mitochondrial dysfunction. Recombinant Lcn2 was able to restore insulin-induced p38MAPK phosphorylation in both WT and Lcn2 −/− inguinal adipocytes. Rosiglitazone treatment during differentiation of Lcn2 −/− adipocytes is able to recruit beige adipocytes at a normal level, however, further activation of beige adipocytes by insulin and RA is impaired in the absence of LCN2. Further, the synergistic effect of insulin and RA on UCP1 and PGC-1α expression is markedly reduced in Lcn2 −/− inguinal adipocytes. Most intriguingly, LCN2 and the retinoic acid receptor-alpha (RAR-α) are concurrently translocated to the plasma membrane of adipocytes in response to insulin, and this insulin-induced RAR-α translocation is absent in adipocytes deficient in LCN2. Our data suggest a novel LCN2-mediated pathway by which RA and insulin synergistically regulates activation of beige adipocytes via a non-genomic pathway of RA action.

Distinct Akt phosphorylation states are required for insulin regulated Glut4 and Glut1-mediated glucose uptake

Insulin, downstream of Akt activation, promotes glucose uptake into fat and muscle cells to lower postprandial blood glucose, an enforced change in cellular metabolism to maintain glucose homeostasis. This effect is mediated by the Glut4 glucose transporter. Growth factors also enhance glucose uptake to fuel an anabolic metabolism required for tissue growth and repair. This activity is predominantly mediated by the Glut1. Akt is activated by phosphorylation of its kinase and hydrophobic motif (HM) domains. We show that insulin-stimulated Glut4-mediated glucose uptake requires PDPK1 phosphorylation of the kinase domain but not mTORC2 phosphorylation of the HM domain. Nonetheless, an intact HM domain is required for Glut4-mediated glucose uptake. Whereas, Glut1-mediated glucose uptake also requires mTORC2 phosphorylation of the HM domain, demonstrating both phosphorylation-dependent and independent roles of the HM domain in regulating glucose uptake. Thus, mTORC2 links Akt to the distinct physiologic programs related to Glut4 and Glut1-mediated glucose uptake.

DOI:http://dx.doi.org/10.7554/eLife.26896.001

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-β 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.

Involvement of cathepsins B and L in inflammation and cholesterol trafficking protein NPC2 secretion in macrophages

OBJECTIVE

Obesity and its related chronic inflammation are the major risk factors for developing metabolic disturbances. The roles of cathepsin cysteine proteases have been tied to inflammation and atherosclerosis. Cathepsins are important functional links between inflammation, cholesterol metabolism, and atherosclerosis in obesity. NPC2, a lysosomal protein, plays an important role in cholesterol trafficking. The objective of this study was to examine the regulation of cathepsins and NPC2 in adipose tissue and macrophages in obesity and the effect of modifying cathepsin activity in cholesterol metabolism and trafficking in macrophages.

DESIGN AND METHODS

Cathepsins and NPC2 mRNA expression and protein secretion were detected in obese adipose tissue as well as 3T3-L1 adipocytes and Raw 264.7 macrophages in response to inflammatory stimuli and cathepsin inhibitors.

RESULTS

It was found that high-fat diet feeding altered the mRNA and protein expression levels of cathepsins B and L (CtB and CtL) and NPC2 in adipose tissue in mice; the differential regulation of these proteins was observed between adipose depots. In vitro studies showed that TNF-α reduces intracellular protein levels of CtB, CtL, and NPC2, but increases their secretion in 3T3-L1 adipocytes. Likewise, LPS stimulated the secretion of CtB and NPC2 in Raw 264.7 macrophages. Using the inhibitors of cathepsin enzymatic activity, it was found that CtB and CtL regulate TNF-α production, the expression and secretion of NPC2 protein, and the mRNA levels of the genes involved in cholesterol trafficking in macrophages.

CONCLUSION

These findings suggest that CtB and CtL have a significant involvement in mediating the inflammatory response, in cholesterol trafficking, and in regulating NPC2 secretion.

Proteomics in the characterization of adipose dysfunction in obesity

Adipose tissue plays a central role in body weight homeostasis, inflammation, and insulin resistance via serving as a fat-buffering system, regulating lipid storage and mobilization and releasing a large range of adipokines and cytokines. Adipose tissue is also the major inflammation-initiated site in obesity. Adipose-derived adipokines and cytokines are known to be involved in the modulation of a wide range of important physiological processes, particularly immune response, glucose and lipid homeostasis and insulin resistance. Adipose tissue dysfunction, characterized by an imbalanced secretion of pro- and anti-inflammatory adipokines and cytokines, decreased insulin-stimulated glucose uptake, dysregulation of lipid storage and release and mitochondrial dysfunction, has been linked to obesity and its associated metabolic disorders. Proteomic technology has been a powerful tool for identifying key components of the adipose proteome, which may contribute to the pathogenesis of adipose tissue dysfunction in obesity. In this review, we summarized the recent advances in the proteomic characterization of adipose tissue and discussed the identified proteins that potentially play important roles in insulin resistance and lipid homeostasis.

Effects of GLUT4 expression on insulin resistance in patients with advanced liver cirrhosis

Decreased glucose tolerance and diabetes are frequently observed in advanced liver cirrhosis patients and may be related to insulin resistance. Glucose transporter-4 (GLUT4), one of the most important glucose transporters, plays a key role in the development of type 2 diabetes. In order to study the mechanism of insulin resistance in liver cirrhosis patients, we measured the insulin sensitivity index and determined the GLUT4 protein and mRNA contents of skeletal muscle by Western blotting and reverse transcription-polymerase chain reaction (RT-PCR), respectively, in normal people and liver cirrhosis patients. The results showed that the levels of glucose, insulin, and C-peptide in two liver cirrhosis groups were higher and the insulin sensitivity index lower than those of the normal control group. The sensitivity of insulin may decrease with the decline of liver function. However, the contents of GLUT4 protein and mRNA in patients with advanced liver cirrhosis were similar to those of normal controls. In conclusion, insulin resistance is observed in patients with advanced liver cirrhosis but may not be correlated with the skeletal contents of GLUT4 protein and mRNA.

C2 domain-containing phosphoprotein CDP138 regulates GLUT4 insertion into the plasma membrane

The protein kinase B(β) (Akt2) pathway is known to mediate insulin-stimulated glucose transport through increasing glucose transporter GLUT4 translocation from intracellular stores to the plasma membrane (PM). Combining quantitative phosphoproteomics with RNAi-based functional analyses, we show that a previously uncharacterized 138 kDa C2 domain-containing phosphoprotein (CDP138) is a substrate for Akt2, and is required for optimal insulin-stimulated glucose transport, GLUT4 translocation, and fusion of GLUT4 vesicles with the PM in live adipocytes. The purified C2 domain is capable of binding Ca(2+) and lipid membranes. CDP138 mutants lacking the Ca(2+)-binding sites in the C2 domain or Akt2 phosphorylation site S197 inhibit insulin-stimulated GLUT4 insertion into the PM, a rate-limiting step of GLUT4 translocation. Interestingly, CDP138 is dynamically associated with the PM and GLUT4-containing vesicles in response to insulin stimulation. Together, these results suggest that CDP138 is a key molecule linking the Akt2 pathway to the regulation of GLUT4 vesicle-PM fusion.

Reducing selenoprotein P expression suppresses adipocyte differentiation as a result of increased preadipocyte inflammation

Oxidative stress and low-grade inflammation have been implicated in obesity and insulin resistance. As a selenium transporter, ubiquitously expressed selenoprotein P (SeP) is known to play a role in the regulation of antioxidant enzyme activity. However, SeP expression and regulation in adipose tissue in obesity and its role in inflammation and adipocyte biology remain unexplored. In this study, we examined Sepp1 gene expression and regulation in adipose tissue of obese rodents and characterized the role of Sepp1 in adipose inflammation and adipogenesis in 3T3-L1 adipocytes. We found that Sepp1 gene expression was significantly reduced in adipose tissue of ob/ob and high-fat diet-induced obese mice as well as in primary adipose cells isolated from Zucker obese rats. Rosiglitazone administration increased SeP protein expression in adipose tissue of obese mice. Treatment of either TNFα or H(2)O(2) significantly reduced Sepp1 gene expression in a time- and dose-dependent manner in 3T3-L1 adipocytes. Interestingly, Sepp1 gene silencing resulted in the reduction in glutathione peroxidase activity and the upregulation of inflammatory cytokines MCP-1 and IL-6 in preadipocytes, leading to the inhibition of adipogenesis and adipokine and lipogenic gene expression. Most strikingly, coculturing Sepp1 KD cells resulted in a marked inhibition of normal 3T3-L1 adipocyte differentiation. We conclude that SeP has an important role in adipocyte differentiation via modulating oxidative stress and inflammatory response.

Akt2 and SGK3 are both determinants of postnatal hair follicle development

SGK3, which previously has been shown to play a key role in hair follicle development in mice, is a member of the AGC family of serine-threonine kinases. Mice lacking SGK3 have abnormal follicle cycling, which begins shortly after birth and ameliorates substantially with age. However, this developmental abnormality is not recapitulated in mice lacking closely related kinases Akt1, Akt2, or Akt3. To examine whether Akt2 interacts with SGK3 in postnatal hair development, we have generated and characterized Akt2/SGK3 double knockouts (DKOs). We find that the DKO mice have a defect in hair growth that is markedly worse than that of SGK3(-/-) mice and does not ameliorate with age. Morphologically, this defect is characterized by accelerated entry into catagen and through anagen, irregular hair follicle orientation, and increased expression of sebaceous glands. The defect is preceded by a profound failure to increase follicle matrix cell nuclear beta-catenin accumulation and proliferation at the onset of morphogenesis. Furthermore, in cultured keratinocytes, transfected Akt2 and SGK3 both stimulate transcription of a beta-catenin-LEF1-dependent reporter gene. Thus, SGK3 and Akt2 both appear to play important roles in postnatal hair follicle morphogenesis, likely because of their redundant regulation of beta-catenin-dependent transcriptional processes, which control hair follicle cell proliferation.

Rip11 is a Rab11- and AS160-RabGAP-binding protein required for insulin-stimulated glucose uptake in adipocytes

The translocation of GLUT4 to the plasma membrane underlies the ability of insulin to stimulate glucose uptake, an event that involves the activation of protein kinase B, several members of the Rab family of GTP-binding proteins and the phosphorylation of the Rab GTPase-activating protein AS160. Here, we explored the regulation by insulin of the class I Rab11-interacting proteins Rip11, RCP and FIP2. We show that Rip11, but not RCP or FIP2, translocates to the plasma membrane of 3T3-L1 adipocytes in response to insulin. This unique response of Rip11 prompted us to explore the role of this protein in more detail. We found that Rip11 partially colocalises with GLUT4 in intracellular compartments. siRNA-mediated knockdown of Rip11 inhibits insulin-stimulated uptake of 2-deoxyglucose, and overexpression of Rip11 blocks insulin-stimulated insertion of translocated GLUT4 vesicles into the plasma membrane. We additionally show that Rip11 forms a complex with AS160 in a Rab11-independent manner and that insulin induces dissociation of AS160 from Rip11. We propose that Rip11 is an AS160- and Rab-binding protein that coordinates the protein kinase signalling and trafficking machinery required to stimulate glucose uptake in response to insulin.

Exercise reverses high-fat diet-induced impairments on compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle

The aims of this investigation were 1) to determine whether endurance exercise training could reverse impairments in insulin-stimulated compartmentalization and/or activation of aPKCzeta/lambda and Akt2 in skeletal muscle from high-fat-fed rodents and 2) to assess whether the PPARgamma agonist rosiglitazone could reverse impairments in skeletal muscle insulin signaling typically observed after high-fat feeding. Sprague-Dawley rats were placed on chow (NORCON, n = 16) or high-fat (n = 64) diets for 4 wk. During a subsequent 4-wk experimental period, high-fat-fed rats were allocated (n = 16/group) to either sedentary control (HFC), exercise training (HFX), rosiglitazone treatment (HFRSG), or a combination of both exercise training and rosiglitazone (HFRX). Following the 4-wk experimental period, animals underwent hindlimb perfusions. Insulin-stimulated plasma membrane-associated aPKCzeta and -lambda protein concentration, aPKCzeta/lambda activity, GLUT4 protein concentration, cytosolic Akt2, and aPKCzeta/lambda activities were reduced (P < 0.05) in HFC compared with NORCON. Cytosolic Akt2, aPKCzeta, and aPKClambda protein concentrations were not affected in HFC compared with NORCON. Exercise training reversed the deleterious effects of the high-fat diet such that insulin-stimulated compartmentalization and activation of components of the insulin-signaling cascade in HFX were normalized to NORCON. High-fat diet-induced impairments to skeletal muscle glucose metabolism were not reversed by rosiglitazone administration, nor did rosiglitazone augment the effect of exercise. Our findings indicate that chronic exercise training, but not rosiglitazone, reverses high-fat diet induced impairments in compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle.

Granulocyte colony-stimulating factor promotes the translocation of protein kinase Ciota in neutrophilic differentiation cells

Previously, we suggested that the phosphatidylinositol 3-kinase (PI3K)-p70 S6 kinase (p70 S6K) pathway plays an important role in granulocyte colony-stimulating factor (G-CSF)-dependent enhancement of the neutrophilic differentiation and proliferation of HL-60 cells. While atypical protein kinase C (PKC) has been reported to be a regulator of p70 S6K, abundant expression of PKCiota was observed in myeloid and lymphoid cells. Therefore, we analyzed the participation of PKCiota in G-CSF-dependent proliferation. The maximum stimulation of PKCiota was observed from 15 to 30 min after the addition of G-CSF. From 5 to 15 min into this lag time, PKCiota was found to translocate from the nucleus to the membrane. At 30 min it re-translocated to the cytosol. This dynamic translocation of PKCiota was also observed in G-CSF-stimulated myeloperoxidase-positive cells differentiated from cord blood cells. Small interfering RNA for PKCiota inhibited G-CSF-induced proliferation and the promotion of neutrophilic differentiation of HL-60 cells. These data indicate that the G-CSF-induced dynamic translocation and activation processes of PKCiota are important to neutrophilic proliferation.

Cellular location of insulin-triggered signals and implications for glucose uptake

Insulin stimulation of glucose uptake into muscle and fat cells requires movement of GLUT4-containing vesicles from intracellular compartments to the plasma membrane. Accordingly, insulin-derived signals must arrive at and be recognized by the appropriate intracellular GLUT4 pools. We describe the insulin signals participating in GLUT4 translocation, and review evidence that they are recruited to intracellular membranes in conjunction with cytoskeletal elements. Such segregation may facilitate the encounter between signals and target vesicles. In most animal and cellular models of insulin resistance, insulin-stimulated GLUT4 translocation to the plasma membrane is reduced. Insulin resistance caused by oxidative stress does not affect early insulin signals, rather their intracellular localization is altered. In this and several other insulin-resistant states, insulin-induced actin remodelling is concomitantly diminished. We summarize evidence suggesting that spatial localization of signals is critical for efficient insulin action, and that the cytoskeleton may act as a scaffold to promote efficient translocation of GLUT4 to the cell surface.

Turning signals on and off: GLUT4 traffic in the insulin-signaling highway

Insulin stimulation of glucose uptake into skeletal muscle and adipose tissues is achieved by accelerating glucose transporter GLUT4 exocytosis from intracellular compartments to the plasma membrane and minimally reducing its endocytosis. The round trip of GLUT4 is intricately regulated by diverse signaling molecules impinging on specific compartments. Here we highlight the key molecular signals that are turned on and off by insulin to accomplish this task.

Activation of a GST-tagged AKT2/PKBbeta

The protein kinase AKT is a key regulator for cell growth, cell survival and metabolic insulin action. However, the mechanism of activation of AKT in vivo, which presumably involves membrane recruitment of the kinase, oligomerization, and multiple phosphorylation events, is not fully understood. In the present study, we have expressed and purified dimeric GST-fusion proteins of human protein kinase AKT2 (DeltaPH-AKT2) in milligram quantities via the baculovirus expression system. Treatment of virus-infected insect cells with the phosphatase inhibitor okadaic acid (OA) led to phosphorylation of the two regulatory phosphorylation sites, Thr309 and Ser474, and to activation of the kinase. Likewise, phosphorylation of Thr309 in vitro by recombinant PDK1 or mutation of Thr309 and Ser474 to acidic residues rendered the kinase constitutively active. However, even though the specific activity of our AKT2 was increased 15-fold compared to previous reports, GST-mediated dimerization alone did not lead to an activation of the kinase. Whereas both mutagenesis and phosphorylation led to an increase in the turnover number of the enzyme, only the latter resulted in a marked reduction (20-fold) of the apparent Km value for the exogenous substrate Crosstide, indicating that this widely used mutagenesis only partially mimics phosphorylation. Kinetic analysis of GST-AKT2 demonstrates that phosphorylation of Thr309 in the activation loop of the kinase is largely responsible for the observed reduction in Km and for a subsequent 150-fold increase in the catalytic efficiency (k(cat)/Km) of the enzyme. Highly active AKT2 constructs were used in autophosphorylation reactions in vitro, where inactive AKT2 kinases served as substrates. As a matter of fact, we found evidence for a minor autophosphorylation activity of AKT2 but no significant autophosphorylation of any of the two regulatory sites, Thr309 or Ser474.

Insulin-stimulated plasma membrane association and activation of Akt2, aPKC zeta and aPKC lambda in high fat fed rodent skeletal muscle

Several recent reports using cell lines have suggested that both Akt and atypical protein kinase C (aPKC) zeta/lambda are translocated to the plasma membrane (PM) in response to insulin. However, it has yet to be determined in skeletal muscle whether: (1) insulin increases PM-associated Akt2, aPKC zeta and/or lambda protein concentration, (2) the activity of these kinases is altered by insulin at the PM, and (3) high fat feeding alters the insulin-stimulated PM concentration and/or activity of Akt2 and aPKC zeta/lambda. Sprague-Dawley rats were randomly assigned to either normal (n=16) or high fat (n=16) dietary groups. Following a 12 week dietary period, animals were subjected to hind limb perfusions in the presence (n=8 per group) or absence (n=8 per group) of insulin. In normal skeletal muscle, total PI3-kinase, Akt2 and aPKC zeta/lambda activities were increased by insulin. PM-associated aPKC zeta and lambda, and aPKC zeta/lambda activity, but not Akt2 or Akt2 activity, were increased by insulin in normal muscle. High fat feeding did not alter total skeletal muscle Akt2, aPKC zeta or aPKC lambda protein concentration. Insulin-stimulated total PI3-kinase, Akt2 and aPKC zeta/lambda activities were reduced in the high fat fed animals. Insulin-stimulated PM aPKC zeta, aPKC lambda, aPKC zeta/lambda activity and GLUT4 protein concentration were also reduced in high fat fed animals. These findings suggest that in skeletal muscle, insulin stimulates translocation of aPKC zeta and lambda, but not Akt2, to the PM. In addition, high fat feeding impairs insulin-stimulated activation of total aPKC zeta/lambda and Akt2, as well as PM association and activation of aPKC zeta and lambda.

The Transcription Factor CCAAT/Enhancer-binding Protein α Is Required for the Intracellular Retention of GLUT4

Insulin modulates glucose uptake into adipocytes by regulating the trafficking of the GLUT4 glucose transporter. GLUT4 is mostly excluded from the surface of unstimulated cells because it is much more slowly exocytosed than it is endocytosed. GLUT4 traffics through an adipocyte-specific, specialized endosomal recycling pathway that only partially overlaps with compartments of the general endosomal recycling pathway. Insulin stimulates GLUT4 exocytosis and partially inhibits its endocytosis, resulting in GLUT4 redistribution to the cell surface. Insulin does not stimulate glucose uptake into adipocytes lacking the CCAAT/enhancer-binding protein alpha (C/EBPalpha) transcription factor. Here we show that these adipocytes do not properly traffic GLUT4. In these adipocytes, GLUT4 was rapidly exocytosed in basal conditions, resulting in an accumulation of GLUT4 on the plasma membrane. Although the kinetics of GLUT4 trafficking were altered, GLUT4 was still targeted to specialized intracellular compartments in adipocytes lacking C/EBPalpha, demonstrating an uncoupling of the targeting of GLUT4 to a specialized, adipocyte-specific insulin-regulated pathway from the regulation of the movement of GLUT4 through this pathway. Re-expression of C/EBPalpha in adipocytes lacking C/EBPalpha restored normal GLUT4 trafficking. We propose that C/EBPalpha controls the expression of the proteins that determine the basal, slow exocytosis of GLUT4, but not the proteins required to make the adipocyte-specific compartments through which GLUT4 traffics. Furthermore, these data support a model in which insulin stimulates GLUT4 exocytosis by releasing an inhibitor of GLUT4 movement to the cell surface, and it is this clamp on basal exocytosis that is missing in adipocytes lacking C/EBPalpha.