Synergistic effect of arachidonic acid and cyclic AMP on glucose transport in 3T3-L1 adipocytes

Combination therapy: a new strategy to manage diabetes and its complications

Diabetes mellitus is the most common metabolic disorder. The major cause of mortality and morbidity here is due to the complications caused by increased glucose concentrations. All the available commercial antidiabetic drugs are associated with side effects. The combination therapy could be a new and highly effective therapeutic strategy to manage hyperglycemia. Combination of commercial drugs with phytochemicals may reduce the side effects caused by these synthetic drugs. Herbal products have been thought to be inherently safe, because of their natural origin and traditional use rather than based on systemic studies. New formulation and cocrystallisation strategies need to be adopted to match the bioavailability of the drug and the phytochemical. This review describes in detail, the observed synergy and mechanism of action between phytochemicals and synthetic drugs in effectively combating. The mode of action of combination differs significantly than that of the drugs alone; hence isolating a single component may lose its importance thereby simplifying the task of pharma industries.

A novel cross-talk between endothelin-1 and cyclic AMP signaling pathways in the regulation of GLUT1 transcription in 3T3-L1 adipocytes

We showed previously that chronic exposure to both endothelin-1 (ET-1) and cAMP resulted in a synergistic increase in Glut1 transcription in 3T3-L1 adipocytes via a protein kinase C (PKC)-dependent mechanism. In the present study, we further examined the molecular mechanism involved. Employing transient transfections with Glut1 promoter/enhancer -luciferase reporter and several dominant negative or constitutively active PKC mutants, we identified PKCε as the responsible PKC. Investigation with deletion and mutation mutants of the promoter/enhancer reporter suggested that Sp1, CREB and AP-1 responsive elements on enhancer 2 were involved. Furthermore, chromatin immunoprecipitation and co-immunoprecipitation analysis were applied to characterize the interactions between these transcription factors and their bindings to enhancer 2 in vivo. The results indicate that there are both negative and positive interactions between ET-1 and cAMP signaling pathways. On the one hand, cAMP inhibits ET-1 induced NF-κB activation required for ET-1-stimulated Glut1 transcription; on the other hand, cAMP, via sustained CREB phosphorylation, may activate AP-1 and cooperate with ET-1-activated PKCε to enhance Sp1 expression and consequently to generate a stable enhancer 2-bound Sp1/pCREB/AP-1 complex, which can strongly facilitate Glut1 transcription more than the additive effect of ET-1 and cAMP alone.

Short-term fatty acid effects on adipocyte glucose uptake: mechanistic insights

The modulation of insulin sensitivity in visceral fat tissue could be important in the treatment of Type 2 diabetes mellitus. Selected fatty acids may impact on insulin-stimulated and basal glucose uptake in adipocytes, thus isolated rat epididymal adipocytes were exposed to 100 microM oleic, arachidonic, eicosapentaenoic, docosahexaenoic or stearic acids and insulin (15 nM) or vehicle for 30 min. Glucose uptake was quantified by measuring uptake of 3H-deoxyglucose/mg adipocyte protein/min. Where appropriate, inhibitors were included to elucidate the mechanisms involved. In this model, insulin stimulated glucose uptake with 62+/-7%. All fatty acids tested, except for stearic acid, depressed insulin-stimulated glucose uptake by an average of 33+/-4.2%. On the other hand, all fatty acids tested except stearic and arachidonic acids, stimulated basal glucose uptake with an average of 34+/-8.1%. Inhibitor studies showed the involvement of prostaglandins, lipoxins, protein kinase C and tyrosine kinase in these processes.

Arachidonic acid and glucose uptake by freshly isolated human adipocytes

Fatty acid (FA) and glucose transport into insulin-dependent cells are impaired in insulin resistance (IR; type 2 diabetes mellitus). Studies done on the effects of FAs on glucose uptake, and the influence of insulin on FA uptake by adipocytes, have yielded contradictory results. In this study, isolated human adipocytes were exposed to arachidonic acid (AA) and to insulin, and FA uptake as well as glucose uptake was measured. AA uptake into adipocyte membranes and nuclei was also investigated. Glucose uptake was inhibited by 57 +/- 8% after 30 min of exposure to arachidonate. AA was significantly taken up into adipocyte membranes (49.6 +/- 29% and 123 +/- 74%) at 20 and 30 min of exposure, respectively, and into nuclei (147.6 +/- 19.2%) after 30 min. Insulin stimulated AA uptake (24.1 +/- 14.1%) at 30 min by adipocytes from a non-obese subject, while inhibiting it (16.6 +/- 12%) in adipocytes from an obese subject. These results suggest that: (1) AA inhibits glucose uptake by adipocytes exposed over a short period, probably by a membrane-associated mechanism, (2) insulin-dependent AA uptake is dependent on the body mass index (BMI) of the donor and the insulin sensitivity of their adipocytes.

Synergistic effect of prostaglandin F2alpha and cyclic AMP on glucose transport in 3T3-L1 adipocytes

The combined effect of prostaglandin F2alpha (PGF2alpha) and cAMP on glucose transport in 3T3-L1 adipocytes was examined. In cells pretreated with PGF2alpha and 8-bromo cAMP for 8 h, a synergy between these two agents on glucose uptake was found. Insulin-stimulated glucose transport, on the other hand, was only slightly affected. The synergistic effect of these two agents was suppressed in the presence of cycloheximide and actinomycin D. In concord, immunoblot and Northern blot analyses revealed that GLUT1 protein and mRNA levels were both increased in cells pretreated with both PGF2alpha and 8-bromo cAMP, greater than the additive effect of each agent alone. The synergistic action of PGF2alpha with 8-bromo cAMP to enhance glucose transport was inhibited by GF109203X, a selective protein kinase C (PKC) inhibitor. In addition, in cells depleted of diacylglycerol-sensitive PKC by prolonged treatment with 4beta-phorbol 12beta-myristate 13alpha-acetate, a PKC activator, the synergistic effects of PGF2alpha and 8-bromo cAMP on glucose transport and GLUT1 mRNA accumulation were both abolished. Taken together, these results indicate that PGF2alpha may act with cAMP in a synergistic way to increase glucose transport, probably through enhanced GLUT1 expression by a PKC-dependent mechanism.

Neurobarrier coupling in the brain: a partner of neurovascular and neurometabolic coupling?

Neurovascular and neurometabolic coupling help the brain to maintain an appropriate energy flow to the neural tissue under conditions of increased neuronal activity. Both coupling phenomena provide us, in addition, with two macroscopically measurable parameters, blood flow and intermediate metabolite fluxes, that are used to dynamically image the functioning brain. The main energy substrate for the brain is glucose, which is metabolized by glycolysis and oxidative breakdown in both astrocytes and neurons. Neuronal activation triggers increased glucose consumption and glucose demand, with new glucose being brought in by stimulated blood flow and glucose transport over the blood-brain barrier. Glucose is shuttled over the barrier by the GLUT-1 transporter, which, like all transporter proteins, has a ceiling above which no further stimulation of the transport is possible. Blood-brain barrier glucose transport is generally accepted as a nonrate-limiting step but to prevent it from becoming rate-limiting under conditions of neuronal activation, it might be necessary for the transport parameters to be adapted to the increased glucose demand. It is proposed that the blood-brain barrier glucose transport parameters are dynamically adapted to the increased glucose needs of the neural tissue after activation according to a neurobarrier coupling scheme. This review presents neurobarrier coupling within the current knowledge on neurovascular and neurometabolic coupling, and considers arguments and evidence in support of this hypothesis.

Synergistic effect of endothelin-1 and cyclic AMP on glucose transport in 3T3-L1 adipocytes

We have demonstrated previously that chronic exposure to endothlin-1 enhances glucose transport in 3T3-L1 adipocytes via augmented GLUT1 mRNA and protein accumulation. In the present study, we further examined the combined effect of endothelin-1 (ET-1) and cAMP on glucose transport. In cells pretreated with ET-1 and 8-bromo cAMP for 8 h, a synergy between these two agents on glucose uptake was found. Insulin-stimulated glucose transport, on the other hand, was only slightly affected. The synergistic effect of these two agents was suppressed in the presence of cycloheximide and actinomycin D. Immunoblot and Northern blot analyses revealed that GLUT1 protein and mRNA levels were both increased in cells pretreated with both ET-1 and 8-bromo cAMP, greater than the additive effect of each agent alone. Further examination demonstrated that the stability of GLUT1 mRNA was markedly enhanced in the presence of both ET-1 and cAMP. To investigate the transcriptional activation of Glut1 gene, transient transfection of cells with luciferase reporter construct driven by Glut1 promoter was performed. We found that Glut1 transcription was also increased by ET-1 and cAMP in a synergistic fashion. In addition, similar synergy between ET-1 and beta-adrenergic agonists on glucose transport was found. The synergistic action of ET-1 with 8-bromo cAMP to enhance glucose transport was inhibited by GF109203X, a selective protein kinase C (PKC) inhibitor, and was mimicked by 4beta-phorbol 12beta-myristate 13alpha-acetate (PMA), a PKC activator. Furthermore, PMA was found to act synergistically with 8-bromo cAMP to induce Glut1 transcription and ET-1 was shown to activate novel PKCdelta and PKC. Taken together, these results indicate that ET-1 may act with cAMP in a synergistic way to increase glucose transport, probably through enhanced GLUT1 expression via a PKC-dependent mechanism.

Peroxisome proliferator-activated receptor gamma thiazolidinedione agonists increase glucose metabolism in astrocytes

Activation of peroxisome proliferator-activated receptors (PPARs) can regulate brain physiology and provide protection in models of neurological disease; however, neither their exact targets nor mechanisms of action in brain are known. In many cells, PPAR gamma agonists increase glucose uptake and metabolism. Because astrocytes store glucose and provide lactate to neurons on demand, we tested effects of PPAR gamma agonists on astroglial glucose metabolism. Incubation of cortical astrocytes with the PPAR gamma thiazolidinedione (TZD) agonist pioglitazone (Pio) significantly increased glucose consumption in a time- and dose-dependent manner, with maximal increase of 36% observed after 4 h in 30 microm Pio. Pio increased 2-deoxy-glucose uptake because of increased flux through the type 1 glucose transporter. However, at this time point Pio did not increase type 1 glucose transporter expression, nor were its effects blocked by transcriptional or translational inhibitors. Pio also increased astrocyte lactate production as soon as 3 h after incubation. These effects were replicated by other TZDs; however, the order of efficacy (troglitazone > pioglitazone > rosiglitazone) suggests that effects were not mediated via PPAR gamma activation. TZDs increased astrocyte cAMP levels, and their glucose modifying effects were reduced by protein kinase A inhibitors. TZDs inhibited state III respiration in isolated brain mitochondria, whereas in astrocytes they caused mitochondrial membrane hyperpolarization. Pio protected astrocytes against hypoglycemia-induced cell death. Finally, glucose uptake was modified in brain sections prepared from Pio-fed rats. These results demonstrate that TZDs modify astrocyte metabolism and mitochondrial function, which could be beneficial in neurological conditions where glucose availability is reduced.

Agent and cell-type specificity in the induction of insulin resistance by HIV protease inhibitors

OBJECTIVE

To test agent and cell-type specificity in insulin resistance induced by prolonged exposure to HIV protease inhibitors (HPI), and to assess its relation to the direct, short-term inhibition of insulin-stimulated glucose uptake.

METHODS

Following prolonged (18 h) and short (5-10 min) exposure to HPI, insulin-stimulated glucose transport, protein kinase B (PKB) phosphorylation, and GLUT4 translocation were evaluated in 3T3-L1 adipocytes, fibroblasts, L6 myotubes, and L6 cells overexpressing a myc tag on the first exofacial loop of GLUT4 or GLUT1.

RESULTS

Prolonged exposure of 3T3-L1 adipocytes to nelfinavir, but not to indinavir or saquinavir, resulted in increased basal lipolysis but decreased insulin-stimulated glucose transport and PKB phosphorylation. In addition, impaired insulin-stimulated glucose uptake and PKB phosphorylation were also observed in the skeletal muscle cell line L6, and in 3T3-L1 fibroblasts. Interestingly, this coincided with increased basal glucose uptake as well as with elevated total-membrane glucose transporter GLUT1 protein content. In contrast to these unique effects of nelfinavir, the mere presence of any of the agents in the 5 min transport assay inhibited insulin-stimulated glucose-uptake activity. This appeared to be caused by direct and specific interaction of the drugs with GLUT4 fully assembled at the plasma membrane, since insulin-stimulated cell-surface exposure of an exofacial myc epitope on GLUT4 was normal.

CONCLUSIONS

Independent mechanisms for HPI-induced insulin resistance exist: prolonged exposure to nelfinavir interferes with insulin signaling and alters cellular metabolism of adipocytes and muscle cells, whereas a direct inhibitory effect on insulin-stimulated glucose uptake may occurs through specific interaction of HPI with GLUT4.

Effects of arachidonic acid plus zinc on glucose disposal in genetically diabetic (ob/ob) mice

AIM

The present study is designed to determine whether arachidonic acid (AA) plus zinc improves clinical signs of diabetes in genetically diabetic ob/ob mice.

METHODS

In the first study, effects of acute administration of AA plus zinc on glucose disposal were determined in ob/ob and lean mice (n = 6 each). In the second study, ob/ob and lean mice were treated with increasing doses of AA plus zinc for 2 weeks (n = 5 each). Postprandial and fasting blood glucose concentrations, three-hour-area-average above fasting glucose concentration (TAFGC), water and food intake, body weight and plasma insulin concentrations were measured.

RESULTS

Acute administration of AA plus zinc significantly increased glucose disposal in ob/ob mice. In the second study, postprandial and fasting blood glucose concentrations, TAFGC, and water and food intake in ob/ob mice treated with AA plus zinc for 2 weeks were significantly decreased compared with those in mice given no AA. Plasma insulin concentrations in both lean and ob/ob mice were not changed by AA treatment in drinking water.

CONCLUSIONS

AA plus zinc in drinking water is effective in decreasing blood glucose levels in obese mice. These results indicate that use of these compounds should be considered as a dietary supplement to control hyperglycaemia in patients with type II diabetes.

An extract of Lagerstroemia speciosa L. has insulin-like glucose uptake-stimulatory and adipocyte differentiation-inhibitory activities in 3T3-L1 cells

The effects of extracts isolated from Lagerstroemia speciosa L. (banaba) on glucose transport and adipocyte differentiation in 3T3-L1 cells were studied. Glucose uptake-inducing activity of banaba extract (BE) was investigated in differentiated adipocytes using a radioactive assay, and the ability of BE to induce differentiation in preadipocytes was examined by Northern and Western blot analyses. The hot water BE and the banaba methanol eluent (BME) stimulated glucose uptake in 3T3-L1 adipocytes with an induction time and a dose-dependent response similar to those of insulin. Furthermore, there were no additive or synergistic effects found between BE and insulin on glucose uptake, and the glucose uptake activity of insulin could be reduced to basal levels by adding increasing amounts of BE. Unlike insulin, BE did not induce adipocyte differentiation in the presence of 3-isobutyl-1-methylxanthine (IBMX) and dexamethasone (DEX). BE inhibited the adipocyte differentiation induced by insulin plus IBMX and DEX (IS-IBMX-DEX) of 3T3-L1 preadipocytes in a dose-dependent manner. The differences in the glucose uptake and differentiation inhibitory activities between untreated cells and those treated with BE were significant (P < 0.01). The inhibitory activity was further demonstrated by drastic reductions of peroxisome proliferator-activated receptor gamma2 (PPARgamma2) mRNA and glucose transporter-4 (GLUT4) protein in cells induced from preadipocytes with IS-IBMX-DEX in the presence of BE. The unique combination of a glucose uptake stimulatory activity, the absence of adipocyte differentiation activity and effective inhibition of adipocyte differentiation induced by IS-IBMX-DEX in 3T3-L1 cells suggest that BE may be useful for prevention and treatment of hyperglycemia and obesity in type II diabetics.

Arachidonic acid stimulates glucose uptake in 3T3-L1 adipocytes by increasing GLUT1 and GLUT4 levels at the plasma membrane. Evidence for involvement of lipoxygenase metabolites and peroxisome proliferator-activated receptor gamma

Exposure of insulin-sensitive tissues to free fatty acids can impair glucose disposal through inhibition of carbohydrate oxidation and glucose transport. However, certain fatty acids and their derivatives can also act as endogenous ligands for peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor that positively modulates insulin sensitivity. To clarify the effects of externally delivered fatty acids on glucose uptake in an insulin-responsive cell type, we systematically examined the effects of a range of fatty acids on glucose uptake in 3T3-L1 adipocytes. Of the fatty acids examined, arachidonic acid (AA) had the greatest positive effects, significantly increasing basal and insulin-stimulated glucose uptake by 1.8- and 2-fold, respectively, with effects being maximal at 4 h at which time membrane phospholipid content of AA was markedly increased. The effects of AA were sensitive to the inhibition of protein synthesis but were unrelated to changes in membrane fluidity. AA had no effect on total cellular levels of glucose transporters, but significantly increased levels of GLUT1 and GLUT4 at the plasma membrane. While the effects of AA were insensitive to cyclooxygenase inhibition, the lipoxygenase inhibitor, nordihydroguaiaretic acid, substantially blocked the AA effect on basal glucose uptake. Furthermore, adenoviral expression of a dominant-negative PPARgamma mutant attenuated the AA potentiation of basal glucose uptake. Thus, AA potentiates basal and insulin-stimulated glucose uptake in 3T3-L1 adipocytes by a cyclooxygenase-independent mechanism that increases the levels of both GLUT1 and GLUT4 at the plasma membrane. These effects are at least partly dependent on de novo protein synthesis, an intact lipoxygenase pathway and the activation of PPARgamma with these pathways having a greater role in the absence than in the presence of insulin.