Differential accumulation of Glut1 in the non-DRM domain of the plasma membrane in response to the inhibition of oxidative phosphorylation

Caffeine inhibition of GLUT1 is dependent on the activation state of the transporter

Caffeine has been shown to be a robust uncompetitive inhibitor of glucose uptake in erythrocytes. It preferentially binds to the nucleotide-binding site on GLUT1 in its tetrameric form and mimics the inhibitory action of ATP. Here we demonstrate that caffeine is also a dose-dependent, uncompetitive inhibitor of 2-deoxyglucose (2DG) uptake in L929 fibroblasts. The inhibitory effect on 2DG uptake in these cells was reversible with a rapid onset and was additive to the competitive inhibitory effects of glucose itself, confirming that caffeine does not interfere with glucose binding. We also report for the first time that caffeine inhibition was additive to inhibition by curcumin, suggesting distinct binding sites for curcumin and caffeine. In contrast, caffeine inhibition was not additive to that of cytochalasin B, consistent with previous data that reported that these two inhibitors have overlapping binding sites. More importantly, we show that the magnitude of maximal caffeine inhibition in L929 cells is much lower than in erythrocytes (35% compared to 90%). Two epithelial cell lines, HCLE and HK2, have both higher concentrations of GLUT1 and increased basal 2DG uptake (3-4 fold) compared to L929 cells, and subsequently display greater maximal inhibition by caffeine (66-70%). Interestingly, activation of 2DG uptake (3-fold) in L929 cells by glucose deprivation shifted the responsiveness of these cells to caffeine inhibition (35%-70%) without a change in total GLUT1 concentration. These data indicate that the inhibition of caffeine is dependent on the activity state of GLUT1, not merely on the concentration.

Osthole activates glucose uptake but blocks full activation in L929 fibroblast cells, and inhibits uptake in HCLE cells

AIMS

Osthole, a coumarin derivative, has been used in Chinese medicine and studies have suggested a potential use in treatment of diabetes and cancers. Therefore, we investigated the effects of osthole and other coumarins on GLUT1 activity in two cell lines that exclusively express GLUT1.

MAIN METHODS

We measured the magnitude and time frame of the effects of osthole and related coumarins on glucose uptake in two cells lines; L929 fibroblast cells which have low GLUT1 expression levels and low basal glucose uptake and HCLE cells which have high GLUT1 concentrations and high basal uptake. We also explored the effects of these coumarins in combination with other GLUT1 activators.

KEY FINDINGS

Osthole activates glucose uptake in L929 cells with a modest maximum 1.7-fold activation achieved by 50 μM with both activation and recovery occurring within minutes. However, osthole blocks full acute activation of glucose uptake by other, more robust activators. This behavior mimics the effects of other thiol reactive compounds and suggests that osthole is interacting with cysteine residues, possibly within GLUT1 itself. Coumarin, 7-hydroxycoumarin, and 7-methoxycoumarin, do not affect glucose uptake, which is consistent with the notion that the isoprenoid structure in osthole may be important to gain membrane access to GLUT1. In contrast to its effects in L929 cells, osthole inhibits basal glucose uptake in the more active HCLE cells.

SIGNIFICANCE

The differential effects of osthole in L929 and HCLE cells indicated that regulation of GLUT1 varies, likely depending on its membrane concentration.

Alkaline pH activates the transport activity of GLUT1 in L929 fibroblast cells

The widely expressed mammalian glucose transporter, GLUT1, can be acutely activated in L929 fibroblast cells by a variety of conditions, including glucose deprivation, or treatment with various respiration inhibitors. Known thiol reactive compounds including phenylarsine oxide and nitroxyl are the fastest acting stimulators of glucose uptake, implicating cysteine biochemistry as critical to the acute activation of GLUT1. In this study, we report that in L929 cells glucose uptake increases 6-fold as the pH of the uptake solution is increased from 6 to 9 with the half-maximal activation at pH 7.5; consistent with the pKa of cysteine residues. This pH effect is essentially blocked by the pretreatment of the cells with either iodoacetamide or cinnamaldehyde, compounds that form covalent adducts with reduced cysteine residues. In addition, the activation by alkaline pH is not additive at pH 8 with known thiol reactive activators such as phenylarsine oxide or hydroxylamine. Kinetic analysis in L929 cells at pH 7 and 8 indicate that alkaline conditions both increases the Vmax and decreases the Km of transport. This is consistent with the observation that pH activation is additive to methylene blue, which activates uptake by increasing the Vmax, as well as to berberine, which activates uptake by decreasing the Km. This suggests that cysteine biochemistry is utilized in both methylene blue and berberine activation of glucose uptake. In contrast a pH increase from 7 to 8 in HCLE cells does not further activate glucose uptake. HCLE cells have a 25-fold higher basal glucose uptake rate than L929 cells and the lack of a pH effect suggests that the cysteine biochemistry has already occurred in HCLE cells. The data are consistent with pH having a complex mechanism of action, but one likely mediated by cysteine biochemistry.

AMPK alpha1 activation is required for stimulation of glucose uptake by twitch contraction, but not by H2O2, in mouse skeletal muscle

Background

AMPK is a promising pharmacological target in relation to metabolic disorders partly due to its non-insulin dependent glucose uptake promoting role in skeletal muscle. Of the 2 catalytic α-AMPK isoforms, α₂ AMPK is clearly required for stimulation of glucose transport into muscle by certain stimuli. In contrast, no clear function has yet been determined for α₁ AMPK in skeletal muscle, possibly due to α-AMPK isoform signaling redundancy. By applying low-intensity twitch-contraction and H₂O₂ stimulation to activate α₁ AMPK, but not α₂ AMPK, in wildtype and α-AMPK transgenic mouse muscles, this study aimed to define conditions where α₁ AMPK is required to increase muscle glucose uptake.

Methodology/Principal Findings

Following stimulation with H₂O₂ (3 mM, 20 min) or twitch-contraction (0.1 ms pulse, 2 Hz, 2 min), signaling and 2-deoxyglucose uptake were measured in incubated soleus muscles from wildtype and muscle-specific kinase-dead AMPK (KD), α₁ AMPK knockout or α₂ AMPK knockout mice. H₂O₂ increased the activity of both α₁ and α₂ AMPK in addition to Akt phosphorylation, and H₂O₂-stimulated glucose uptake was not reduced in any of the AMPK transgenic mouse models compared with wild type. In contrast, twitch-contraction increased the activity of α₁ AMPK, but not α₂ AMPK activity nor Akt or AS160 phosphorylation. Glucose uptake was markedly lower in α₁ AMPK knockout and KD AMPK muscles, but not in α₂ AMPK knockout muscles, following twitch stimulation.

Conclusions/Significance

These results provide strong genetic evidence that α₁ AMPK, but not α₂ AMPK, Akt or AS160, is necessary for regulation of twitch-contraction stimulated glucose uptake. To our knowledge, this is the first report to show a major and essential role of α₁ AMPK in regulating a physiological endpoint in skeletal muscle. In contrast, AMPK is not essential for H₂O₂-stimulated muscle glucose uptake, as proposed by recent studies.

Membrane cholesterol content modulates ClC-2 gating and sensitivity to oxidative stress

ClC-2 is a broadly expressed member of the voltage-gated ClC chloride channel family. In this study, we aimed to evaluate the role of the membrane lipid environment in ClC-2 function, and in particular the effect of cholesterol and ClC-2 distribution in membrane microdomains. Detergent-resistant and detergent-soluble microdomains (DSM) were isolated from stably transfected HEK293 cells by a discontinuous OptiPrep gradient. ClC-2 was found concentrated in detergent-insoluble membranes in basal conditions and relocalized to DSM upon cholesterol depletion by methyl-beta-cyclodextrin. As assessed by patch clamp recordings, relocalization was accompanied by acceleration of the activation kinetics of the channel. A similar distribution and activation pattern were obtained when cells were treated with the oxidant tert-butyl hydroperoxide and after ATP depletion. In both cases activation was prevented by cholesterol enrichment of cells. We conclude that the cholesterol environment regulates ClC-2 activity, and we provide evidence that the increase in ClC-2 activity in response to acute oxidative or metabolic stress involves relocalization of this channel to DSM.

Acute activation of glucose uptake by glucose deprivation in L929 fibroblast cells

Glucose is a very important energy source for a wide variety of cells, and the ability of cells to respond to changes in glucose availability or other cell stresses is of critical importance. Many mammalian cells respond to acute stress by increasing the V(max) of transport through GLUT1; the most ubiquitously expressed glucose transporter isoform. This study investigated the acute response of glucose uptake to glucose deprivation in L929 fibroblast cells--a cell line that expresses only the GLUT1 transporter. Results indicated that glucose deprivation of only a minute activated glucose uptake 10-fold and reached a maximum of 20-fold within 10 min. The activation was dose dependent and only partially muted by addition of up to 20mM pyruvate as an alternate energy source. In contrast to the kinetics of acute metabolic stress, glucose deprivation decreased the K(m) of transport, but did not alter the V(max). Maximal activation of glucose transport by glucose deprivation was completely additive to activation of transport by methylene blue--a stimulant that increased the V(max) of transport without a change in the K(m). Glucose-deprived activation of glucose transport was not inhibited by wortmannin or herbimycin A, but was completely inhibited by phenylarsine oxide. Altogether, the data indicate that L929 fibroblast cells respond quickly and robustly to the cell stress of glucose deprivation and methylene blue treatment by two distinct activation pathways.