Glucose Transport In Adipocytes Research Articles

EsGLUT4 and CHHBP are involved in the regulation of glucose homeostasis in the crustacean Eriocheir sinensis.

ABSTRACTGlucose is an essential energy source for both vertebrates and invertebrates. In mammals, glucose uptake is mediated primarily by glucose transporters (GLUTs), members of the major facilitator superfamily (MFS) of passive transporters. Among the GLUTs, GLUT4 is the main glucose transporter in muscles and adipocytes. In skeletal muscle cells, GLUT4 interacts with the lipid raft protein flotillin to transport glucose upon stimulation by insulin. Although several studies have examined GLUT4 function in mammals, few have been performed in crustaceans, which also use glucose as their main energy source. Crustacean hyperglycemic hormone (CHH) is a multifunctional neurohormone found only in arthropods, and one of its roles is to regulate glucose homeostasis. However, the molecular mechanism that underlies CHH regulation and whether GLUT4 is involved in its regulation in crustaceans remain unclear. In the present study, we identified a full-length GLUT4 cDNA sequence (defined herein as EsGLUT4) from the Chinese mitten crab Eriocheir sinensis and analyzed its tissue distribution and cellular localization. By the ForteBio Octet system, two large hydrophilic regions within EsGLUT4 were found to interact with the CHH binding protein (CHHBP), an E. sinensis flotillin-like protein. Interestingly, live-cell imaging indicated that EsGLUT4 and CHHBP responded simultaneously upon stimulation by CHH, resulting in glucose release. In contrast to insulin-dependent GLUT4, however, EsGLUT4 and CHHBP were present within cytoplasmic vesicles, both translocating to the plasma membrane upon CHH stimulation. In conclusion, our results provide new evidence for the involvement of EsGLUT4 and CHHBP in the regulation of glucose homeostasis in crustacean carbohydrate metabolism.

Tumor Necrosis Factor Inhibitors May Improve Glycemic Control in Patients Rheumatoid Arthritis and Concomitant Type 2 Diabetes Mellitus

Insulin resistance is a key feature of obesity, metabolic syndrome, and Type 2 Diabetes Mellitus (T2DM). Inflammation and insulin resistance are closely linked with each other. I. Tumor necrosis factor-α (TNF-α) has been found to impair the insulin sensitivity and promote insulin resistance through multiple actions on the insulin sensitive tissues. Inflammatory cytokines such as TNF, Interleukin (IL)-6, IL-1 and IL-8 may inhibit insulin signaling.1 Hotamisligil et al2 did the pioneering work in 1993 confirming the link between TNF-α and insulin resistance in mice. Animal studies have confirmed that the TNF-α interferes with phosphorylation cascades of the insulin receptor beta subunit and insulin receptor substrate-1, thereby altering the transmembrane signaling that is essential for insulin action in various insulin sensitive tissues.3-5 Also TNF-α causes depletion of GLUT 4, the insulin sensitive glucose transporter in adipocytes and muscles.2,6,7 A intravenous administration of a recombinant TNF-α receptor antibody resulted in improvement in insulin sensitivity2 and dramatic reductions in plasma insulin, glucose, and non-esterified fatty acid levels5 in obese, as compared with lean rats

ALMS1 acts as a critical molecular switch that controls Insulin-stimulated glucose transport in adipocytes

ALMS1 acts as a critical molecular switch that controls Insulin-stimulated glucose transport in adipocytes

Abstract 4543: Substrate trapping proteomics reveals novel mechanism for regulation of mTORC1 signaling by βTrCP-FNIP1/2-FLCN axis

Abstract Defining the full complement of substrates for each ubiquitin ligase remains an important challenge. Improvements in mass spectrometry instrumentation and computation and in protein biochemistry methods have resulted in several new methods for ubiquitin ligase substrate identification. Here we used the parallel adapter capture (PAC) proteomics approach to study βTrCP2, a substrate adaptor for the SKP1-CUL1-F-box (SCF) E3 ubiquitin ligase complex. The processivity of the ubiquitylation reaction necessitates transient physical interactions between βTrCP2 and its substrates, thus making biochemical purification of FBXW11-bound substrates difficult. Using the PAC-based approach, we inhibited the proteasome to “trap” ubiquitylated substrates on the SCF (βTrCP2) E3 complex. Comparative mass spectrometry analysis of immunopurified βTrCP2 protein complexes before and after proteasome inhibition revealed 21 known and 23 putatively novel substrates. Interestingly, many novel substrates for βTrCP2, including TBC1D4, HCFC1, DENND4C, FNIP1, and FLCN are related to cell metabolism. TBC1D4 encodes a GTPase activating protein for the small GTPase Rab that controls insulin-dependent trafficking of the GLUT4 glucose transporter in adipocytes. DENND4C acts as a guanine nucleotide exchange factor for Rab10 and its activity is required for insulin-stimulated GLUT4 translocation to plasma membrane in adipocytes. HCFC1 is a member of the host cell factor family, affecting gluconeogenesis by modulating PGC-1α stability. These suggest βTrCP2 might play important roles in glucose homeostasis by regulating stability of several different target proteins. Here, in focused study, we found that βTrCP1/2 bound, polyubiquitylated, and destabilized FNIP1, FLCN interacting protein 1. We further demonstrated that FNIP1 degradation was promoted by AMPK activation after glucose depletion and expression of a degradation-resistant FNIP1 mutant results in sustained activation of mTORC1 signaling. Hence, our findings reveal that βTrCP1/2 is involved in nutrient sensing through the AMPK-FLCN-FNIP1 and mTORC1 signaling pathways. Citation Format: Tai Young Kim, Jee Yun Chang, Jin Mo Ku, Se Hyang Hong, ji Hye Kim, Hyeong Sim Choi, Kangwook Lee, Myeong-Sun Kim, Sang Mi Woo, Michael B. Major, Seong-Gyu Ko. Substrate trapping proteomics reveals novel mechanism for regulation of mTORC1 signaling by βTrCP-FNIP1/2-FLCN axis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4543.

Cloud storage for endosomes.

Coordinated regulation of vesicle trafficking is critical for the proper functioning of a cell. The bulk of cellular transport vesicles are sequestered in a “perinuclear cloud”, with only a small fraction released to the cell periphery. Jongsma et al (2016) found that the ER‐associated E3 ubiquitin ligase RNF26 is responsible for establishing and maintaining the architecture of the perinuclear cloud and that this spatiotemporal positioning is critical for effective regulation of the endocytic and exocytic systems.

Curcumin directly inhibits the transport activity of GLUT1.

Curcumin, a major ingredient in turmeric, has a long history of medicinal applications in a wide array of maladies including treatment for diabetes and cancer. Seemingly counterintuitive to the documented hypoglycemic effects of curcumin, however, a recent report indicates that curcumin directly inhibits glucose uptake in adipocytes. The major glucose transporter in adipocytes is GLUT4. Therefore, this study investigates the effects of curcumin in cell lines where the major transporter is GLUT1. We report that curcumin has an immediate inhibitory effect on basal glucose uptake in L929 fibroblast cells with a maximum inhibition of 80% achieved at 75μM curcumin. Curcumin also blocks activation of glucose uptake by azide, glucose deprivation, hydroxylamine, or phenylarsine oxide. Inhibition does not increase with exposure time and the inhibitory effects reverse within an hour. Inhibition does not appear to involve a reaction between curcumin and the thiol side chain of a cysteine residue since neither prior treatment of cells with iodoacetamide nor curcumin with cysteine alters curcumin's inhibitory effects. Curcumin is a mixed inhibitor reducing the Vmax of 2DG transport by about half with little effect on the Km. The inhibitory effects of curcumin are not additive to the effects of cytochalasin B and 75μM curcumin actually reduces specific cytochalasin B binding by 80%. Taken together, the data suggest that curcumin binds directly to GLUT1 at a site that overlaps with the cytochalasin B binding site and thereby inhibits glucose transport. A direct inhibition of GLUT proteins in intestinal epithelial cells would likely reduce absorption of dietary glucose and contribute to a hypoglycemic effect of curcumin. Also, inhibition of GLUT1 activity might compromise cancer cells that overexpress GLUT1 and be another possible mechanism for the documented anticancer effects of curcumin.

Epigallocatechin -3-gallate 가 지방세포 분화와 포도당 수송 및 지질대사 유전자에 미치는 영향

Epigallocatechin -3-gallate 가 지방세포 분화와 포도당 수송 및 지질대사 유전자에 미치는 영향

Proteomic Analysis of GLUT4 Storage Vesicles Reveals Tumor Suppressor Candidate 5 (TUSC5) as a Novel Regulator of Insulin Action in Adipocytes

Insulin signaling augments glucose transport by regulating glucose transporter 4 (GLUT4) trafficking from specialized intracellular compartments, termed GLUT4 storage vesicles (GSVs), to the plasma membrane. Proteomic analysis of GSVs by mass spectrometry revealed enrichment of 59 proteins in these vesicles. We measured reduced abundance of 23 of these proteins following insulin stimulation and assigned these as high confidence GSV proteins. These included established GSV proteins such as GLUT4 and insulin-responsive aminopeptidase, as well as six proteins not previously reported to be localized to GSVs. Tumor suppressor candidate 5 (TUSC5) was shown to be a novel GSV protein that underwent a 3.7-fold increase in abundance at the plasma membrane in response to insulin. siRNA-mediated knockdown of TUSC5 decreased insulin-stimulated glucose uptake, although overexpression of TUSC5 had the opposite effect, implicating TUSC5 as a positive regulator of insulin-stimulated glucose transport in adipocytes. Incubation of adipocytes with TNFα caused insulin resistance and a concomitant reduction in TUSC5. Consistent with previous studies, peroxisome proliferator-activated receptor (PPAR) γ agonism reversed TNFα-induced insulin resistance. TUSC5 expression was necessary but insufficient for PPARγ-mediated reversal of insulin resistance. These findings functionally link TUSC5 to GLUT4 trafficking, insulin action, insulin resistance, and PPARγ action in the adipocyte. Further studies are required to establish the exact role of TUSC5 in adipocytes.

ClipR-59 plays a critical role in the regulation of body glucose homeostasis

By regulating Akt membrane compartmentalization, ClipR-59 modulates adipocyte glucose transport. To elucidate the role of ClipR-59 in the regulation of whole body glucose homeostasis, we have generated adipose tissue specific transgenic mice and examined how forcing expression of ClipR-59 in adipose tissue affects body glucose homeostasis. We found that ClipR-59 adipose transgenic mice showed lower blood glucose level with increased glucose tolerance and enhanced insulin sensitivity. Moreover, ClipR-59 adipose transgenic mice were lean with reduced fat mass and against diet induced obesity. Finally, we examined the potential impact of ClipR-59 on adipose endocrine function and found that ClipR-59 expression enhanced adiponectin secretion in both 3T3-L1 adipocytes and adipose tissue, accompanied with increased circulating adiponectin and enhanced AMPKα phosphorylation at Thr172 in adipose tissue and skeletal muscle. Overall, these studies demonstrate that ClipR-59 is likely an important regulator of body glucose homeostasis and adipocyte function.

Go-6976 reverses hyperglycemia-induced insulin resistance independently of cPKC inhibition in adipocytes.

Chronic hyperglycemia induces insulin resistance by mechanisms that are incompletely understood. One model of hyperglycemia-induced insulin resistance involves chronic preincubation of adipocytes in the presence of high glucose and low insulin concentrations. We have previously shown that the mTOR complex 1 (mTORC1) plays a partial role in the development of insulin resistance in this model. Here, we demonstrate that treatment with Go-6976, a widely used “specific” inhibitor of cPKCs, alleviates hyperglycemia-induced insulin resistance. However, the effects of mTOR inhibitor, rapamycin and Go-6976 were not additive and only rapamycin restored impaired insulin-stimulated AKT activation. Although, PKCα, (but not –β) was abundantly expressed in these adipocytes, our studies indicate cPKCs do not play a major role in causing insulin-resistance in this model. There was no evidence of changes in the expression or phosphorylation of PKCα, and PKCα knock-down did not prevent the reduction of insulin-stimulated glucose transport. This was also consistent with lack of IRS-1 phosphorylation on Ser-24 in hyperglycemia-induced insulin-resistant adipocytes. Treatment with Go-6976 did inhibit a component of the mTORC1 pathway, as evidenced by decreased phosphorylation of S6 ribosomal protein. Raptor knock-down enhanced the effect of insulin on glucose transport in insulin resistant adipocytes. Go-6976 had the same effect in control cells, but was ineffective in cells with Raptor knock-down. Taken together these findings suggest that Go-6976 exerts its effect in alleviating hyperglycemia-induced insulin-resistance independently of cPKC inhibition and may target components of the mTORC1 signaling pathway.

Discovery of a Class of Endogenous Mammalian Lipids with Anti-Diabetic and Anti-inflammatory Effects

Increased adipose tissue lipogenesis is associated with enhanced insulin sensitivity. Mice overexpressing the Glut4 glucose transporter in adipocytes have elevated lipogenesis and increased glucose tolerance despite being obese with elevated circulating fatty acids. Lipidomic analysis of adipose tissue revealed the existence of branched fatty acid esters of hydroxy fatty acids (FAHFAs) that were elevated 16- to 18-fold in these mice. FAHFA isomers differ by the branched ester position on the hydroxy fatty acid (e.g., palmitic-acid-9-hydroxy-stearic-acid, 9-PAHSA). PAHSAs are synthesized in vivo and regulated by fasting and high-fat feeding. PAHSA levels correlate highly with insulin sensitivity and are reduced in adipose tissue and serum of insulin-resistant humans. PAHSA administration in mice lowers ambient glycemia and improves glucose tolerance while stimulating GLP-1 and insulin secretion. PAHSAs also reduce adipose tissue inflammation. In adipocytes, PAHSAs signal through GPR120 to enhance insulin-stimulated glucose uptake. Thus, FAHFAs are endogenous lipids with the potential to treat type 2 diabetes.

DNA Promoter Methylation-dependent Transcription of the Double C2-like Domain β (DOC2B) Gene Regulates Tumor Growth in Human Cervical Cancer

Double C2-like domain β (DOC2B) gene encodes for a calcium-binding protein, which is involved in neurotransmitter release, sorting, and exocytosis. We have identified the promoter region of the DOC2B gene as hypermethylated in pre-malignant, malignant cervical tissues, and cervical cancer cell lines by methylation-sensitive dimethyl sulfoxide-polymerase chain reaction and bisulfite genome sequencing; whereas, it was unmethylated in normal cervical tissues (p < 0.05). The promoter hypermethylation was inversely associated with mRNA expression in SiHa, CaSki, and HeLa cells and treatment with demethylating agent 5-aza-2-deoxycytidine restored DOC2B expression. The region -630 to +25 bp of the DOC2B gene showed robust promoter activity by a luciferase reporter assay and was inhibited by in vitro artificial methylation with Sss1 methylase prior to transient transfections. Overexpression of the DOC2B gene in SiHa cells when compared with controls showed significantly reduced colony formation, cell proliferation, induced cell cycle arrest, and repressed cell migration and invasion (p < 0.05). Ectopic expression of DOC2B resulted in anoikis-mediated cell death and repressed tumor growth in a nude mice xenograft model (p < 0.05). DOC2B expressing cells showed a significant increase in intracellular calcium level (p < 0.05), impaired AKT1 and ERK1/2 signaling, and induced actin cytoskeleton remodeling. Our results show that promoter hypermethylation and silencing of the DOC2B gene is an early and frequent event during cervical carcinogenesis and whose reduced expression due to DNA promoter methylation may lead to selective cervical tumor growth.

Carboxylated and Uncarboxylated Forms of Osteocalcin Directly Modulate the Glucose Transport System and Inflammation in Adipocytes

Osteocalcin is secreted by osteoblasts and improves insulin sensitivity in vivo, although mechanisms remain unclear. We tested the hypothesis that osteocalcin directly modulates cell biology in insulin-targeted peripheral tissues. In L-6 myocytes, osteocalcin stimulated glucose transport both in the absence (basal) and presence of insulin. Similarly, in primary cultured adipocytes, both carboxylated and uncarboxylated osteocalcin increased basal and insulin-stimulated glucose transport as well as insulin sensitivity. Osteocalcin also increased basal and insulin-stimulated glucose oxidation, though there was no effect on fatty acid synthesis or lipolysis. In primary-cultured adipocytes, both forms of osteocalcin suppressed secretion of tumor necrosis factor alpha into the media; however, only carboxylated osteocalcin suppressed interleukin 6 release, and neither form of osteocalcin modulated monocyte chemoattractant protein-1 secretion. Both carboxylated and uncarboxylated osteocalcin increased secretion of adiponectin and the anti-inflammatory cytokine interleukin 10. In conclusion, both carboxylated and uncarboxylated osteocalcin directly increase glucose transport in adipocytes and muscle cells, while suppressing proinflammatory cytokine secretion and stimulating interleukin 10 and adiponectin release. Thus, these results provide a mechanism for the insulin-sensitizing effects of osteocalcin and help elucidate the role that bone plays in regulating systemic metabolism.

In vitro Modulation of Pancreatic Insulin Secretion and Extra Pancreatic Insulin Action, Enzymatic Starch Digestion and Protein Glycation by Terminalia chebula Extracts

Aim: Traditional plant treatments have been used throughout the world for the therapy of diabetes mellitus. Study Design: Using multiple in vitro models; this study was designed to investigate the efficacy and mode of action of Terminalia chebula Retz. (Combretaceae) used traditionally for treatment of diabetes. Place and Duration of Study: School of Biomedical Sciences, University of Ulster, 2001-2004. Results: T. chebula aqueous extract stimulated basal insulin output and potentiated glucose-stimulated insulin secretion concentration-dependently in the clonal pancreatic beta cell line, BRIN-BD11 (p<0.001). The insulin secretory activity of plant extract was abolished in the absence of extracellular Ca2+ and by inhibitors of cellular Ca2+ uptake, diazoxide and verapamil, (p<0.001). Furthermore, the extract increased insulin secretion in depolarised cells and augmented insulin secretion triggered by IBMX, but not by tolbutamide or glibenclamide. T. chebula extract did not display insulin mimetic activity but it enhanced insulin-stimulated glucose transport in 3T3 L1 adipocytes by 280% (p<0.001). At (0.5-5.0mg/mL) concentrations, the extract also produced 22-84% (p<0.001) decrease in starch digestion In vitro and inhibited protein glycation (p<0.001) at 1mg/ml aqueous extract. Conclusion: This study has revealed that water soluble bioactive principles in T.chebula extract stimulate insulin secretion, enhance insulin action and inhibit both protein glycation and starch digestion. The former actions are dependent on the bioeffective component(s) in the plant being absorbed intact. Future work assessing the use of Terminalia chebula as dietary adjunct or as a source of active antidiabetic agents may provide new opportunities for the treatment of diabetes.

In vitro modulation of pancreatic insulin secretion, extrapancreatic insulin action and peptide glycation by Curcuma longa aqueous extracts

Objective: Medicinal, edible, and aromatic plants have been used as folk remedies in traditional treatments worldwide. This study investigates the antidiabetic efficacy and action mode of Curcuma longa Linn. (Zingiberaceae). Methods: Effects of aqueous extracts (AEs) of C. longa on insulin secretion and action were studied using the insulin-secreting BRIN-BD11 and the adipocyte-like 3T3-L1 cell lines, respectively. In vitro models were employed to evaluate effects on starch digestion using α-amylase/amyloglucosidase and protein glycation. Results: C. longa AEs stimulated basal insulin output and potentiated glucose-stimulated insulin secretion concentration-dependently in the clonal pancreatic beta cell line, BRIN-BD11 (P < 0.001). The insulin secretory activity of plant extract was abolished in the absence of extracellular Ca 2+ and by inhibitors of cellular Ca 2+ uptake, diazoxide and verapamil (P < 0.001). Furthermore, the extract increased insulin secretion in depolarized cells and augmented insulin secretion triggered by 3-isobutyl-1-methylxanthine, tolbutamide, and glibenclamide. C. longa AEs lacked insulin mimetic activity but enhanced insulin-stimulated glucose transport in 3T3-L1 adipocytes by 370% (P < 0.001). Similar to aminoguanidine, C. longa AEs (1-50 mg/ml) effected concentration-dependent inhibition of protein glycation (24-70% inhibition, P < 0.001) in vitro. In bioassays of enzymatic starch digestion, C. longa AEs lacked inhibitory effects on α-amylase and α-glucosidase, unlike acarbose, the classical reference drug. Conclusion: This study has revealed that water soluble bioactive principles in C. longa AEs stimulate basal- and potentiate glucose evoked-insulin secretion, enhance insulin action and inhibit insulin glycation, but not starch digestion. Future work assessing the use of C. longa AEs as dietary adjunct or as a source of active antidiabetic agents may provide new opportunities for the combinatorial treatment/prevention of diabetes.

Effect of ginsenoside Rb1 in ameliorating insulin resistance and ectopic fat deposition in obese mice induced by high fat diet

Ginsenoside Rb1 is an active component in ginseng. Previous in vitro experiments showed that ginsenoside Rb1, could inhibit lipolysis and promote glucose transporter in adipocytes. This study focused on the effect of ginsenoside Rb1 in insulin resistance and ectopic fat deposit in obese mice induced by high fat diet and its molecular mechanism. Obese male C57/L mice induced by high fat diet were randomly divided into the diet-induced obesity group (DIO group), the ginsenoside Rb1 group (Rb1 group) and the rosiglitazone group (Rog group), and continuously fed with high fat diet. In addition, male C57/L mice fed with normal diet were selected as the normal group (NC group). Mice in Rb1 group and Rog groups were intraperitoneally injected with ginsenoside Rb1 and rosiglitazone with the dosage of 20 mg x kg(-1) and 10 mg x kg(-1), respectively. NC and DIO groups were intraperitoneally injected with the same amount of saline. Two weeks later, the intraperitoneal glucose tolerance test (IPGTT) was performed. Three days later, the mice were killed, and their serum samples were collected to detect insulin and free fatty acid (FFA). Their livers were weighed to examine the triglyceride content, and a pathological detection was performed. Epididymal adipose tissues were weighed, and PDE3B, HSL and perilipin were detected by Western blotting. The results showed that the treatment with ginsenoside Rb1 for two weeks could improve the glucose tolerance of obese mice. Except for 0-120 min, the areas under the glucose tolerance curve (0-30 min, 0-60 min and 0-90 min) in the Rb1 group were less than that in the DIO group (P < 0.05, n = 5), with a much lower HOMA-IR (P < 0.05, n = 5). The fat level of obese mice was significantly reduced by Rbl (P < 0.05, n = 5), and so were liver weight/weight (P < 0.05, n = 8). The increased serum FFA of obese mice declined after the treatment of Rb1 (P < 0.05, n = 8). Rb1 could partially recover the expression of perilipin in adipose tissues, but without obvious change in the expressions of PDE3B and HSL and the phosphorylated activation. The above findings indicated that ginsenoside Rb1 could reduce the release of FFA and alleviate the ectopic deposit of triglyceride by up-regulating the expression of perilipin in adipose tissue, which may be one of its mechanisms for improving the insulin resistance and abnormal glucose metabolism of organisms.

Map4k4 suppresses Srebp-1 and adipocyte lipogenesis independent of JNK signaling

Adipose tissue lipogenesis is paradoxically impaired in human obesity, promoting ectopic triglyceride (TG) deposition, lipotoxicity, and insulin resistance. We previously identified mitogen-activated protein kinase kinase kinase kinase 4 (Map4k4), a sterile 20 protein kinase reported to be upstream of c-Jun NH2-terminal kinase (JNK) signaling, as a novel negative regulator of insulin-stimulated glucose transport in adipocytes. Using full-genome microarray analysis we uncovered a novel role for Map4k4 as a suppressor of lipid synthesis. We further report here the surprising finding that Map4k4 suppresses adipocyte lipogenesis independently of JNK. Thus, while Map4k4 silencing in adipocytes enhances the expression of lipogenic enzymes, concomitant with increased conversion of (14)C-glucose and (14)C-acetate into TGs and fatty acids, JNK1 and JNK2 depletion causes the opposite effects. Furthermore, high expression of Map4k4 fails to activate endogenous JNK, while Map4k4 depletion does not attenuate JNK activation by tumor necrosis factor α. Map4k4 silencing in cultured adipocytes elevates both the total protein expression and cleavage of sterol-regulated element binding protein-1 (Srebp-1) in a rapamycin-sensitive manner, consistent with Map4k4 signaling via mechanistic target of rapamycin complex 1 (mTORC1). We show Map4k4 depletion requires Srebp-1 upregulation to increase lipogenesis and further show that Map4k4 promotes AMP-protein kinase (AMPK) signaling and the phosphorylation of mTORC1 binding partner raptor (Ser792) to inhibit mTORC1. Our results indicate that Map4k4 inhibits adipose lipogenesis by suppression of Srebp-1 in an AMPK- and mTOR-dependent but JNK-independent mechanism.

Curcumin is a direct inhibitor of glucose transport in adipocytes

Curcumin has been reported to inhibit insulin signaling and translocation of GLUT4 to the cell surface in 3T3-L1 adipocytes. We have investigated the effect of curcumin on insulin signaling in primary rat adipocytes. Curcumin (20μM) inhibited both basal and insulin-stimulated glucose transport (2-deoxyglucose uptake), but had no effect on insulin inhibition of lipolysis. Dose–response experiments demonstrated that curcumin (0–100μM) inhibited basal and insulin-stimulated glucose transport, but even at the highest concentration tested did not affect lipolysis. Inhibition was equal in cells that had been pre-incubated with curcumin and in cells to which curcumin was added immediately before the glucose transport assay. Similarly, time-course experiments revealed that the inhibitory effect of curcumin was evident at the earliest time point tested (30s). Thus it is unlikely that inhibition of insulin signaling or of translocation of GLUT4 to the cell surface is involved in the inhibitory effect of curcumin. Curcumin did not affect the stimulatory action of insulin on phosphorylation of Akt at serine 473. We conclude that curcumin is a direct inhibitor of glucose transporters in rat adipocytes.

Impairment of insulin-stimulated glucose transport and ERK activation by adipocyte-specific knockout of PKC-λ produces a phenotype characterized by diminished adiposity and enhanced insulin suppression of hepatic gluconeogenesis

Tissue-specific knockout (KO) of atypical protein kinase C-λ (PKC-λ) impairs insulin-stimulated glucose transport in muscle (M) and lipid synthesis in liver (L), thereby producing insulin resistance in MλKO mice and insulin-hypersensitivity in LλKO mice. Here, we generated mice with KO of PKC-λ in adipocytes, i.e., AλKO mice. In isolated adipocytes of AλKO mice, insulin-stimulated aPKC activity and glucose transport were diminished, as were ERK levels and activity. Insulin-stimulated glucose transport and insulin activation of ERK in adipocytes of wild-type mice were similarly inhibited by acute inhibition of PKC-λ with a highly-specific chemical inhibitor. With impairments in glucose transport and ERK activation, AλKO mice had diminished adiposity and serum leptin levels. In addition, AλKO mice had normal glucose tolerance and insulin hypersensitivity owing to enhanced suppression of hepatic glucose output, which apparently reflected increases in Akt activity and FoxO1 phosphorylation, and subsequent decreases in expression of gluconeogenic phosphoenolpyruvate carboxykinase. We conclude that: PKC-λ is required for insulin-stimulated glucose transport and ERK signaling in mouse adipocytes; and diminution of these processes is attended by leanness and therefore hypoleptinemia. How these and perhaps other PKC-λ-dependent processes communicate to liver and improve insulin suppression of hepatic gluconeogenesis remains unclear.

Glucocorticoid Paradoxically Recruits Adipose Progenitors and Impairs Lipid Homeostasis and Glucose Transport in Mature Adipocytes

Chronic treatment with glucocorticoids increases the mass of adipose tissue and promotes metabolic syndrome. However little is known about the molecular effects of dexamethasone on adipose biology. Here, we demonstrated that dexamethasone induces progenitor cells to undergo adipogenesis. In the adipogenic pathway, at least two cell types are found: cells with the susceptibility to undergo staurosporine-induced adipose conversion and cells that require both staurosporine and dexamethasone to undergo adipogenesis. Dexamethasone increased and accelerated the expression of main adipogenic genes such as pparg2, cebpa and srebf1c. Also, dexamethasone altered the phosphorylation pattern of C/EBPβ, which is an important transcription factor during adipogenesis. Dexamethasone also had effect on mature adipocytes mature adipocytes causing the downregulation of some lipogenic genes, promoted a lipolysis state, and decreased the uptake of glucose. These paradoxical effects appear to explain the complexity of the action of glucocorticoids, which involves the hyperplasia of adipose cells and insulin resistance.