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BASIC SCIENCE INVESTIGATIONS |
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York
| ABSTRACT |
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Key Words: phypoxia FDG glucose transporter protein thiols, redox modification of PET
| INTRODUCTION |
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Several in vivo and in vitro studies indicate that the uptake of FDG is increased in hypoxic cells or prenecrotic (and probably hypoxic) areas in xenografts (58). The mechanisms underlying this increased uptake remain unclear. In one of the in vitro studies, a hypoxia-induced increase in the cellular level of glucose transporters (GLUTs) has been suggested as a possible explanation (5).
It has also been suggested that, under hypoxic conditions, the cellular redox status shifts to a more reduced state. This might result in modification (reduction) of thiol groups of cysteine residues in proteins, potentially altering their activity (9,10). Here we present data suggesting that this redox modulation is (in part) the mechanism underlying the hypoxia-induced increase in FDG uptake, that is, increased GLUT activity as a result of reduction of protein-thiol groups.
In animal tumor models (and probably in human tumors) both chronic and transient hypoxia occurs. Chronic hypoxia pertains to tumor cells that remain under hypoxic conditions for long durations as a result of their distance from the blood supply. Transient hypoxia is the result of sporadic or intermittent constriction of tumor blood vessels and can render tumor regions that were previously oxic acutely hypoxic (11,12). The experiments presented here were performed under conditions of acute hypoxia (up to 4 h) and will be more representative for tumor areas that experience transient hypoxia.
| MATERIALS AND METHODS |
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3H-FDG Uptake
For the measurement of 3H-FDG uptake, cells were incubated with 1.5 µCi of 3H-FDG (60 Ci/mmol; American Radiolabeled Chemicals Inc., St. Louis, MO) in 1 mL medium for various periods. Subsequently, the cells were rinsed with cold phosphate-buffered saline (PBS) and lysed with 1 mL 0.3N NaOH/1% sodium dodecyl sulfate (SDS). The lysate was transferred into a liquid scintillation vial and neutralized with 3N HCl (0.1 mL), and the protein content was determined (using the bicinchoninic acid protein assay kit; Pierce, Rockford, IL). 3H-FDG activity was determined by liquid scintillation counting, and expressed as dpm/mg protein. Statistical analysis of the experimental results was performed using a paired t test.
GLUT Activity
[3H]3-O-methylglucose ([3H]3-O-MG) transport into cells was used as a measure for the glucose transporting capability of cells (15). The [3H]3-O-MG uptake in cells was assayed as described for 3H-FDG uptake, except for the tracer and incubation time. [3H]3-O-MG (80 Ci/mmol) was from New England Nuclear, Boston, MA.
Hexokinase Activity
Total hexokinase activity was measured using the method of Viñuela et al. (16) as modified by Waki et al. (15). The cells were scraped off, suspended in solution I (50 mmol triethanolamine, 5 mmol MgCl2; pH 7.6), and homogenized. Supernatants, containing free and mitochondria-bound hexokinase, were obtained by centrifuging the homogenates at 1000 g at 4°C for 5 min and were analyzed as follows. Three milliliters of solution I containing 0.5 mmol glucose, 5 mmol adenosine triphosphate (ATP), 0.25 mmol reduced nicotinamide adenine dinucleotide phosphate (NADPH), and 6 U glucose-6-phosphate dehydrogenase were preincubated at 20°C for 5 min. The supernatants were then added and the mixture further incubated at 20°C. After 5 and 20 min, samples of the reaction mixture were taken and the absorption measured at 340 nm. The differences in absorbance were calculated and the hexokinase activity determined from an NADPH standard curve. One unit of hexokinase activity will phosphorylate 1 µmol of glucose per minute at pH 7.6 and 20°C. Aliquots were removed to measure the protein content of the lysates, and the activity was expressed as milliU/mg protein.
Western Analysis
Total protein extracts of cells incubated under hypoxic conditions, and control cells were prepared in cold SDS sample buffer (2% SDS, 10% glycerol, and 50 mmol tris[hydroxymethyl]aminomethane [Tris]; pH 6.8) containing protease (0.5 mmol phenylmethylsulfonyl flouride [PMSF] and 1X Complete [Boehringer GmbH, Mannheim, Germany]) and proteasome (10 µmol MG132) inhibitors. The levels of hypoxia-inducible factor (HIF)-1
, HIF-1ß, hexokinase II, GLUT1, and GLUT3 proteins were analyzed by separation of the proteins by SDS-polyacrylamide gel electrophoresis followed by Western analysis using standard protocols (17,18). AntiHIF-1
and antiHIF-1ß antibodies were from Novus Biologicals (Littleton, CO), antihexokinase II antibody was from Santa Cruz Biotechnology Inc. (Santa Cruz, CA), and antiGLUT1 and antiGLUT3 antibodies were from Alpha Diagnostics International (San Antonio, TX). HRP-conjugated second antibodies (Boehringer Mannheim, Indianapolis, IN) and the Enhanced Chemoluminescence system (Amersham Life Science, Piscataway, NJ) were used to visualize the proteins.
| RESULTS |
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, HIF-1ß, GLUT1, GLUT3, or hexokinase II in our MCF7 cells during incubation under hypoxic conditions for up to 4 h (Fig. 1C). The absence of HIF-1
accumulation during hypoxia is not the result of an HIF-1
deficiency, because a 4-h incubation of the MCF7 cells with 10 µmol 2-2 dipyridyl (an iron chelator) induces a strong increase in the cellular HIF-1
level (data not shown). The lack of p53 accumulation during hypoxia (not shown) is probably the result of the failure of hypoxia to induce HIF-1
accumulation in this cell line, because a strong increase in the cellular p53 level is observed within 4 h of treatment of the cells with ionizing radiation (data not shown). Thus, in these MCF7 cells, hypoxia induces a rapid increase in 3H-FDG uptake, which is independent of de novo protein synthesis. In principle, an enhancement of the rate of 3H-FDG uptake with a fixed cellular GLUT and hexokinase content can result from translocation of GLUTs from sites within the cell to the plasma membrane or increased activity of the existing enzymes through either an increase in affinity or activation of "dormant" or "masked" enzymes (19). Translocation of GLUTs from intracellular sites to the plasma membrane does not seem to be involved in the mechanism underlying the observed hypoxia-induced increase in 3H-FDG uptake, because the presence of Brefeldin A, an inhibitor of GLUT translocation to the plasmamembrane, does not interfere with the effect of hypoxia on 3H-FDG uptake (data not shown). Therefore, an increase in the activity of the GLUTs or hexokinase as a result of post-translational modification or of a hypoxia-induced change in the cellular redox status seems to be the more likely mechanism underlying the hypoxia-induced increase in 3H-FDG uptake.
Effect of Redox Agents on 3H-FDG Uptake
By adding dithiothreitol (DTT, a thiol reducing agent) or p-chloromercuribenzenesulfonic acid (pCMBS, an oxidizing agent) to the medium we altered the cells redox status to study the influence of redox on 3H-FDG uptake. As shown in Figure 2, the effect of DTT on 3H-FDG uptake is concentration-dependent, negligible at 0.1 or 1 mmol but inducing a 1.54 ± 0.44-fold (P < 0.05; n = 5) increase at 10 mmol. Preincubation (1 hr) of the cells with 10 mmol DTT did not enhance this effect. In contrast, pCMBS had no effect on 3H-FDG uptake (Fig. 3). These data suggest that the reduction of protein thiol groups might be at least part of the mechanism underlying the hypoxia-induced increase in 3H-FDG uptake.
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Effect of DTT on GLUT and Hexokinase
Cellular uptake of 3H-FDG occurs in two steps: translocation across the plasma membrane by GLUTs and phosphorylation by hexokinase. To determine which of these is affected by redox changes, we studied the effect of DTT on GLUT activity and hexokinase activity. Because cytochalasin B has been shown to compete with DTT for interaction with GLUT 1 (20), total, rather than cytochalasin B inhibitable, 3-O-methylglucose (3-OMG) uptake was used as a measure for GLUT activity. As shown in Figure 4A, the effect of DTT on GLUT activity was concentration-dependent, with no effect at 0.1 or 1 mmol but inducing a 1.47 ± 0.18-fold (P = 0.1; n = 2) increase in GLUT activity at 10 mmol. Thus, the effect of DTT on GLUT activity seems to mirror its effect on 3H-FDG uptake, both in concentration dependence and magnitude, suggesting that the two effects are related.
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| DISCUSSION |
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Although the MCF7 cells used in this study were not HIF-1
deficient, hypoxia failed to induce accumulation of HIF-1
. Consequently, no increase in the cellular levels of p53 (p53 accumulation is the result of stabilization of this protein by HIF-1
) or of the HIF-1
-regulated proteins GLUT 1, GLUT 3, or hexokinase II was observed. HIF-1
accumulation could be observed after treatment of the cells with the iron chelator 2-2 dipyridyl, indicating that the absence of hypoxia-induced HIF-1
accumulation is probably the result of a defect in the hypoxia-sensing signal transduction pathway in these cells.
Experiments conducted with the reducing agent DTT yielded results indicating that alterations in the cellular redox status may underlie the hypoxia-induced increase in 3H-FDG uptake. Furthermore, this increase appeared to be the result of an increase in GLUT activity, with no change in hexokinase activity. Others have reported that hypoxia shifts the cellular redox status to a more reduced state, resulting in the reduction of protein thiol groups (9,12). These observations suggest that hypoxia-induced modification (reduction) of the cysteine residues in the GLUT may be the mechanism underlying the hypoxia-induced increase in 3H-FDG uptake. The same mechanism has previously been suggested for the hypoxia-induced inhibition of K+ and Ca2+ channels (9). That the oxidizing agent pCMBS counteracts the effect of hypoxia on 3H-FDG uptake provides further evidence for the role of redox modulation of protein thiol groups in this phenomenon.
There is a second mechanism by which protein thiol groups may be reduced under hypoxic conditions. Oxygen (or reactive oxygen species [ROS]) can react directly with protein thiol groups. It has been shown that the activity of several transcription factors can be regulated by this mechanism, at least in vitro (21). In addition, it has been shown that membrane protein activity, such as glutamate and dopamine transport in rat striatal synaptosomes, can be modulated through interaction with oxygen or ROS (22,23). Thus, the reduced amount of oxygen and ROS (and the resulting shift of protein-thiol groups to a more reduced state) under hypoxic conditions may mediate the observed increase in 3H-FDG uptake.
An enhancement of transmembrane 3H-FDG transport without translocation of GLUTs to the plasma membrane may result either from increased affinity of GLUTs existing in the plasma membrane or activation of dormant or "masked" GLUTs preexisting in the plasma membrane (19). Currently, we cannot distinguish between these possibilities. Further studies on the kinetics of 3H-FDG uptake under hypoxic conditions will provide the answer to these questions, because the first possibility will result in a decreased Km value, whereas the second possibility will result in an increased Vmax for 3H-FDG transport.
Our data with DTT also indicate that the hypoxia-induced increase in 3H-FDG uptake is the result of increased GLUT activity, rather than hexokinase activity, suggesting that GLUT activity is rate-limiting for 3H-FDG and glucose uptake in MCF7 cells. This is in agreement with the data of Waki et al. (15), that among a panel of 16 tumor cell lines and one normal cell line, 2-deoxyglucose (2DG) uptake is governed by GLUT activity rather than hexokinase activity.
That GLUT activity was increased by DTT does not shed insight as to which GLUT is responsible for the redox-sensitive 3H-FDG uptake increase. It has been reported that in MCF7 cells only GLUT 1 and GLUT 3 are expressed (24). However, our result seems to argue against a role for GLUT 1, because it has been reported that GLUT 1 activity is inhibited by DTT (25,26) as well as by pCMBS (27) at concentrations similar to or lower than those used in our experiments. No data are available on the effects of these agents on GLUT 3 activity.
The kinetics for the hypoxia-induced increase in 3H-FDG uptake and our postulate of the underlying mechanism are different from those observed by Clavo et al. (5,7) in human melanoma and human adenocarcinoma cell lines. In their cell lines, 1.5-h incubation in hypoxia was required before an increase in 3H-FDG uptake (attributed to an increased membrane expression of GLUT 1) could be detected. Whereas the results of Clavo et al. (5,7) suggest a role for GLUT 1 in the mechanism underlying the hypoxia-induced increase in 3H-FDG uptake, our results seems to argue against GLUT 1 involvement in MCF7 cells. It is possible that differences in GLUT species expressed in the various cell lines can account for the disparity in results.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Paul Burgman, PhD, Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021.
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