Inhibitory effect of amiloride on glucose transport in isolated rat adipocytes

Another Consequence of the Warburg Effect? Metabolic Regulation of Na+/H+ Exchangers May Link Aerobic Glycolysis to Cell Growth

To adjust cell growth and proliferation to changing environmental conditions or developmental requirements, cells have evolved a remarkable network of signaling cascades that integrates cues from cellular metabolism, growth factor availability and a large variety of stresses. In these networks, cellular information flow is mostly mediated by posttranslational modifications, most notably phosphorylation, or signaling molecules such as GTPases. Yet, a large body of evidence also implicates cytosolic pH (pHc) as a highly conserved cellular signal driving cell growth and proliferation, suggesting that pH-dependent protonation of specific proteins also regulates cellular signaling. In mammalian cells, pHc is regulated by growth factor derived signals and responds to metabolic cues in response to glucose stimulation. Importantly, high pHc has also been identified as a hall mark of cancer, but mechanisms of pH regulation in cancer are only poorly understood. Here, we discuss potential mechanisms of pH regulation with emphasis on metabolic signals regulating pHc by Na⁺/H⁺-exchangers. We hypothesize that elevated NHE activity and pHc in cancer are a direct consequence of the metabolic adaptations in tumor cells including enhanced aerobic glycolysis, generally referred to as the Warburg effect. This hypothesis not only provides an explanation for the growth advantage conferred by a switch to aerobic glycolysis beyond providing precursors for accumulation of biomass, but also suggests that treatments targeting pH regulation as a potential anti-cancer therapy may effectively target the result of altered tumor cell metabolism.

Cellular Internalization of Therapeutic Oligonucleotides by Peptide Amphiphile Nanofibers and Nanospheres

Oligonucleotides are promising drug candidates due to the exceptionally high specificity they exhibit toward their target DNA and RNA sequences. However, their poor pharmacokinetic and pharmacodynamic properties, in conjunction with problems associated with their internalization by cells, necessitates their delivery through specialized carrier systems for efficient therapy. Here, we investigate the effects of carrier morphology on the cellular internalization mechanisms of oligonucleotides by using self-assembled fibrous or spherical peptide nanostructures. Size and geometry were both found to be important parameters for the oligonucleotide internalization process; direct penetration was determined to be the major mechanism for the internalization of nanosphere carriers, whereas nanofibers were internalized by clathrin- and dynamin-dependent endocytosis pathways. We further showed that glucose conjugation to carrier nanosystems improved cellular internalization in cancer cells due to the enhanced glucose metabolism associated with oncogenesis, and the internalization of the glucose-conjugated peptide/oligonucleotide complexes was found to be dependent on glucose transporters present on the surface of the cell membrane.

Insulin and isoproterenol have opposing roles in the maintenance of cytosol pH and optimal fusion of GLUT4 vesicles with the plasma membrane

Insulin treatment of rat adipocytes increases both cytoplasmic alkalinity and glucose transport activity. Both processes are blocked by the phosphatidylinositol 3-kinase inhibitor wortmannin. Isoproterenol pre-treatment reverses the alkalinizing effects of insulin and leads to attenuation of insulin-stimulated glucose transport activity and exposure of GLUT4 to photolabeling reagents at the cell surface. These effects of isoproterenol are mimicked by acid loading and are reversed by cell-alkalinizing conditions. However, neither isoproterenol nor acid loading alters the total level of GLUT4 at the plasma membrane as revealed by Western blotting of plasma membrane fractions or immunodetection of GLUT4 in plasma membrane lawns. GLUT4 is therefore occluded from participation in glucose transport catalysis by a pH-sensitive process. To examine the kinetics of trafficking that lead to these changes in cell surface GLUT4 occlusion, we have utilized a new biotinylated photolabel, GP15. This reagent has a 70-atom spacer between the biotin and the photolabeling diazirine group, and this allows quenching of the surface signal of biotinylated GLUT4 by extracellular avidin. The rates of GLUT4 internalization are only slightly altered by isoproterenol or acidification, mainly due to reduced recycling over long internalization times. By contrast, insulin stimulation of GLUT4 exocytosis is slowed by isoproterenol or acidification pre-treatments. Biphasic time courses are evident, with an initial burst of exposure at the cell surface followed by a slow phase. It is hypothesized that the burst kinetics are a consequence of a two-phase fusion reaction that is rapid in the presence of insulin but slowed by cytosol acidification.

Insulin-like action of chromate on glucose transport in isolated rat adipocytes

The effects of chromium compounds on 3-O-methylglucose (3-O-MG) transport were studied in isolated rat adipocytes. Sodium chromate significantly stimulated 3-O-MG uptake into adipocytes in a dose-dependent manner without altering the equilibrium space of 3-O-MG in adipocytes. The stimulatory effect reached the maximum at 300 microM, and the effect was 60-70% of the maximal insulin effect that was obtained with 20 nM insulin. The chromate concentration achieving a half-maximal effect was estimated at 50 microM. The effect of the combination of 1 mM chromate and 20 nM insulin was equipotent to that of 20 nM insulin alone, which showed that these two effects were not additive. The stimulatory effects of 1 mM chromate and 20 nM insulin were entirely abolished in adipocytes deprived of ATP, which indicated that these effects were completely ATP-dependent. Judging from experiments using various chromium compounds, CrO4(2-) was responsible for the insulinomimetic action. These results indicate that the action of CrO4(2-) is exerted through a mechanism analogous to that of insulin action, and that CrO4(2-) is a novel and useful tool for studying issues involved in insulin actions.

Inhibitory effects of diazoxide or polymyxin B on glucose transport by isolated rat erythrocytes or adipocytes

The inhibitory effects of diazoxide or polymyxin B on 3-O-methylglucose uptake were studied in isolated rat erythrocytes or adipocytes to elucidate the mechanisms of the actions of these agents. One to three mmol/L diazoxide significantly inhibited 3-O-methylglucose uptake into erythrocytes by 11-33% without altering the equilibrium space, while 0.3 mmol/L diazoxide did not. The inhibitory effect was exerted in a dose-dependent manner in this concentration range. To test whether polymyxin B affects the process of insulin action or the glucose transport activity recruited by insulin, adipocytes prestimulated with insulin and exposed to 2 mmol/L potassium cyanide (KCN) were employed since the cells, on which glucose transporters recruited by insulin were located quiescently, were useful to estimate the effect of an agent on glucose transport activity per se. Polymyxin B (100 micrograms/mL) inhibited the insulin-stimulated uptake activity in this transport system by 22.5% while it inhibited the insulin-stimulated uptake activity in intact adipocytes which were not exposed to KCN by 32.2%. These results suggest that diazoxide inhibits the function of the erythrocyte glucose transporter, GLUT1 (classified by Bell et al.), and indicate that the inhibition of the glucose transport activity recruited by insulin is the major effect of polymyxin B (100 micrograms/mL) and the inhibition of the process of insulin action is rather small.

An extrapancreatic action of diazoxide to inhibit glucose transport activity on adipocytes

The effect of diazoxide on 3-O-methylglucose (3-O-MG) transport was studied in isolated rat adipocytes to elucidate its extrapancreatic action. Diazoxide (0.3-3 mmol/L) significantly inhibited 3-O-MG uptake into adipocytes in a basal state or an insulin-stimulated state. The inhibitory effect was mainly due to the inhibition of insulin responsiveness for 3-O-MG uptake. The insulin responsiveness is determined by the capacity in the process of insulin action and in the final glucose transport activity, and diazoxide mainly inhibited the 3-O-MG transport activity itself. Based on these findings, this extrapancreatic action of diazoxide is considered to contribute partially to raising the blood glucose level in children receiving the drug. Diazoxide, as a glucose transport inhibitor, may be a useful tool for studying the issues related to glucose transport or insulin action.

Actions of peroxovanadate or tungstate on glucose transport by isolated rat adipocytes

The effects of peroxovanadate or tungstate on 3-O-methylglucose uptake were characterized using isolated rat adipocytes to elucidate the mechanism(s) of their actions. The stimulatory effect of peroxovanadate was observed from 1 mumol/L and reached the maximum at about 100 mumol/L. The concentration showing the half-maximal effect was approximately 16 mumol/L. The maximal response of peroxovanadate was 1.19 times higher than that of insulin significantly (P < 0.01). On the other hand, the stimulatory effect of tungstate was seen only at the higher concentrations of 10-30 mmol/L. Judging from the experiments using different tungsten compounds, tungstic acid (WO4(2-)) appeared responsible for the effect. The effects of 20 mmol/L tungstate and 20 nmol/L insulin were not additive. The stimulatory effects of 1 mmol/L peroxovanadate, 20 mmol/L tungstate or 20 nmol/L insulin were not seen in the adipocytes deprived of ATP by exposure to 2 mmol/L KCN. The adipocytes which had been stimulated with insulin and further exposed to 2 mmol/L KCN were used to test whether or not peroxovanadate works directly on the function of glucose transporters. In such cells on which GLUT4-rich transporters were rendered immobile, the effect of peroxovanadate was not observed. These results indicate that the effects of peroxovanadate or tungstate are ATP or energy dependent and may be exerted through the mechanism analogous to that of insulin action, and suggest that peroxovanadate does not directly activate the function of GLUT4.