Evidence for a Functional Change in the Plasma Membrane of Murine Sarcoma Virus-infected Mouse Embryo Cells

Cellular and Molecular Imaging with SPECT and PET in Brain Tumors

This review highlights the 2 major molecular imaging modalities that are used in clinics, namely single-photon emission computed tomography (SPECT) and positron emission tomography (PET), and their added value in management of patients with brain tumors. There are a variety of SPECT and PET radiotracers that can allow imaging of different molecular processes. Those radiotracers target specific molecular features of tumors, resulting in improved specificity of these agents. Potential applications include staging of brain tumors and evaluating post-therapeutic changes.

Molecular imaging (PET) of brain tumors

Despite the recognized limitations of (18)Fluorodeoxyglucose positron emission tomography (FDG-PET) in brain tumor imaging due to the high background of normal gray matter, this imaging modality provides critical information for the management of patients with cerebral neoplasms with regard to the following aspects: (1) providing a global picture of the tumor and thus guiding the appropriate site for stereotactic biopsy, and thereby enhancing its accuracy and reducing the number of biopsy samples; and (2) prediction of biologic behavior and aggressiveness of the tumor, thereby aiding in prognosis. Another area, which has been investigated extensively, includes differentiating recurrent tumor from treatment-related changes (eg, radiation necrosis and postsurgical changes). Furthermore, FDG-PET has demonstrated its usefulness in differentiating lymphoma from toxoplasmosis in patients with acquired immune deficiency syndrome with great accuracy, and is used as the investigation of choice in this setting. Image coregistration with magnetic resonance imaging and delayed FDG-PET imaging are 2 maneuvers that substantially improve the accuracy of interpretation, and hence should be routinely employed in clinical settings. In recent years an increasing number of brain tumor PET studies has used other tracers (like labeled methionine, tyrosine, thymidine, choline, fluoromisonidazole, EF5, and so forth), of which positron-labeled amino acid analogues, nucleotide analogues, and the hypoxia imaging tracers are of special interest. The major advantage of these radiotracers over FDG is the markedly lower background activity in normal brain tissue, which allows detection of small lesions and low-grade tumors. The promise of the amino acid PET tracers has been emphasized due to their higher sensitivity in imaging recurrent tumors (particularly the low-grade ones) and better accuracy for differentiating between recurrent tumors and treatment-related changes compared with FDG. The newer PET tracers have also shown great potential to image important aspects of tumor biology and thereby demonstrate ability to forecast prognosis. The value of hypoxia imaging tracers (such as fluoromisonidazole or more recently EF5) is substantial in radiotherapy planning and predicting treatment response. In addition, they may play an important role in the future in directing and monitoring targeted hypoxic therapy for tumors with hypoxia. Development of optimal image segmentation strategy with novel PET tracers and multimodality imaging is an approach that deserves mention in the era of intensity modulated radiotherapy, and which is likely to have important clinical and research applications in radiotherapy planning in patients with brain tumor.

Dynamic small-animal PET imaging of tumor proliferation with 3'-deoxy-3'-18F-fluorothymidine in a genetically engineered mouse model of high-grade gliomas

3'-Deoxy-3'-(18)F-fluorothymidine ((18)F-FLT), a partially metabolized thymidine analog, has been used in preclinical and clinical settings for the diagnostic evaluation and therapeutic monitoring of tumor proliferation status. We investigated the use of (18)F-FLT for detecting and characterizing genetically engineered mouse (GEM) high-grade gliomas and evaluating the pharmacokinetics in GEM gliomas and normal brain tissue. Our goal was to develop a robust and reproducible method of kinetic analysis for the quantitative evaluation of tumor proliferation.

METHODS

Dynamic (18)F-FLT PET imaging was performed for 60 min in glioma-bearing mice (n = 10) and in non-tumor-bearing control mice (n = 4) by use of a dedicated small-animal PET scanner. A 3-compartment, 4-parameter model was used to characterize (18)F-FLT kinetics in vivo. For compartmental analysis, the arterial input was measured by placing a region of interest over the left ventricular blood pool and was corrected for partial-volume averaging. The (18)F-FLT "trapping" and tissue flux model parameters were correlated with measured uptake (percentage injected dose per gram [%ID/g]) values at 60 min.

RESULTS

(18)F-FLT uptake values (%ID/g) at 1 h in brain tumors were significantly greater than those in control brains (mean +/- SD: 4.33 +/- 0.58 and 0.86 +/- 0.22, respectively; P < 0.0004). Kinetic analyses of the measured time-activity curves yielded independent, robust estimates of tracer transport and metabolism, with compartmental model-derived time-activity data closely fitting the measured data. Except for tracer transport, statistically significant differences were found between the applicable model parameters for tumors and normal brains. The tracer retention rate constant strongly correlated with measured (18)F-FLT uptake values (r = 0.85, P < 0.0025), whereas a more moderate correlation was found between net (18)F-FLT flux and (18)F-FLT uptake values (r = 0.61, P < 0.02).

CONCLUSION

A clinically relevant mouse glioma model was characterized by both static and dynamic small-animal PET imaging of (18)F-FLT uptake. Time-activity curves were kinetically modeled to distinguish early transport from a subsequent tracer retention phase. Estimated (18)F-FLT rate constants correlated positively with %ID/g measurements. Dynamic evaluation of (18)F-FLT uptake offers a promising approach for noninvasively assessing cellular proliferation in vivo and for quantitatively monitoring new antiproliferation therapies.

Current imaging paradigms in radiation oncology

The technological revolution in imaging during recent decades has transformed the way image-guided radiation therapy is performed. Anatomical imaging (plain radiography, computed tomography, magnetic resonance imaging) greatly improved the accuracy of delineating target structures and has formed the foundation of 3D-based radiation treatment. However, the treatment planning paradigm in radiation oncology is beginning to shift toward a more biological and molecular approach as advances in biochemistry, molecular biology, and technology have made functional imaging (positron emission tomography, nuclear magnetic resonance spectroscopy, optical imaging) of physiological processes in tumors more feasible and practical. This review provides an overview of the role of current imaging strategies in radiation oncology, with a focus on functional imaging modalities, as it relates to staging and molecular profiling (cellular proliferation, apoptosis, angiogenesis, hypoxia, receptor status) of tumors, defining radiation target volumes, and assessing therapeutic response. In addition, obstacles such as imaging-pathological validation, optimal timing of post-therapy scans, spatial and temporal evolution of tumors, and lack of clinical outcome studies are discussed that must be overcome before a new era of functional imaging-guided therapy becomes a clinical reality.

Glucose-based total parenteral nutrition does not stimulate glucose uptake by humans tumours

BACKGROUND

Since glucose represents the preferred fuel for cancer cells, there is some debate about the potential stimulation of tumour metabolism induced by a glucose-based total parenteral nutrition (TPN) in cancer patients.

METHODS

We investigated the uptake of [18]2-fluoro-2-deoxy-D-glucose (FDG) through the positron emission tomography of the healthy liver and of the tumour in 12 patients with liver metastases from colorectal cancer. We determined whether FDG uptake by the tumour in fasting conditions was affected by the subsequent administration of a glucose-based (GTPN) or a lipid-based (LTPN) containing glucose 4 mg/kg/min or lipid 2 mg/kg/min, respectively, as non-protein energy source.

RESULTS

The data showed that FDG uptake by the metastases was 3-3.6 times higher than by the healthy liver in fasting conditions and it was not significantly affected by the subsequent administration of GTPN or LTPN.

CONCLUSIONS

We speculated that, despite glucose being the preferred fuel for cancer cells, its disproportionately high uptake even in fasting conditions makes the glucose consumption unable to be modulated by a further supply of glucose or lipid.

Imaging gliomas with positron emission tomography and single-photon emission computed tomography

Over the last two decades the large volume of research involving various brain tracers has shed invaluable light on the pathophysiology of cerebral neoplasms. Yet the question remains as to how best to incorporate this newly acquired insight into the clinical context. Thallium is the most studied radiotracer with the longest track record. Many, but not all studies, show a relationship between (201)Tl uptake and tumor grade. Due to the overlap between tumor uptake and histologic grades, (201)Tl cannot be used as the sole noninvasive diagnostic or prognostic tool in brain tumor patients. However, it may help differentiating a high-grade tumor recurrence from radiation necrosis. MIBI is theoretically a better imaging agent than (201)Tl but it has not convincingly been shown to differentiate tumors according to grade. MDR-1 gene expression as demonstrated by MIBI does not correlate with chemoresistance in high grade gliomas. Currently, MIBI's clinical role in brain tumor imaging has yet to be defined. IMT, a radio-labeled amino acid analog, may be useful for identifying postoperative tumor recurrence and, in this application, appears to be a cheaper, more widely available tool than positron emission tomography (PET). However, its ability to accurately identify tumor grade is limited. 18 F-2-Fluoro-2-deoxy-d-glucose (FDG) PET predicts tumor grade, and the metabolic activity of brain tumors has a prognostic significance. Whether FDG uptake has an independent prognostic value above that of histology remains debated. FDG-PET is effective in differentiating recurrent tumor from radiation necrosis for high-grade tumors, but has limited value in defining the extent of tumor involvement and recurrence of low-grade lesions. Amino-acid tracers, such as MET, perform better for this purpose and thus play a complementary role to FDG. Given the poor prognosis of patients with gliomas, particularly with high-grade lesions, the overall clinical utility of single photon emission computed tomography (SPECT) and PET in characterizing recurrent lesions remains dependent on the availability of effective treatments. These tools are thus mostly suited to the evaluation of treatment response in experimental protocols designed to improve the patients' outcome.

Transforming oncogenes regulate glucose transport by increasing transporter affinity for glucose: contrasting effects of oncogenes and heat stress in a murine marrow-derived cell line

Transforming oncogenes often overcome the growth factor requirements of cells by activating growth factor signal transduction pathways. Increased energy utilization by transformed cells is a well known phenomenon, but whether glucose uptake is regulated at the level of the glucose transporter has not been clearly established. To determine whether cell transformation by specific oncogenes like, v-H-ras and v-abl affects the activation state of glucose transporters, bone marrow-derived IL-3-dependent 32D (clone3) cells transfected with temperature-sensitive ras and abl oncogenes were used to compare proliferative responses and glucose transporting ability of these cells with the parental cell line at permissive (32 degrees C) and non-permissive (40 degrees C) temperatures. Transformed cells showed elevated incorporation of [3H]thymidine and enhanced tyrosine kinase activity, both of which were abrogated in temperature-sensitive mutants maintained at the non-permissive temperature. Compared with control cells, 2-deoxy-D-[1-(3)H]glucose (2-DOG) uptake was not significantly different in transformed cells at the permissive temperature. However, transformation was associated with a 2-2.5-fold greater affinity of glucose transporters for glucose (Km) and this was reversed following treatment with tyrosine kinase inhibitor, genistein. Maximum velocity of glucose transport (Vmax) and membrane expression of transporters were reduced in oncogene-transformed cells. At the non-permissive temperature, glucose uptake was elevated in both control and oncogene-transformed cells. This increase in glucose transport was not associated with a change in transporter affinity for glucose, but increased Glut-1 expression was observed indicating a 'heat stress' effect that overrode the effects attributable to oncogene loss. The 'heat stress' effect was inhibited by protein synthesis inhibitor cycloheximide. These results provide evidence for intrinsic activation of glucose transporters by the transforming oncogenes ras and abl, and indicate that oncogenes and 'heat stress' regulate glucose transport by different mechanisms.

Sugar transport and glut transporter expression in a variety of human immunodeficiency virus-1 (HIV-1) chronically infected target cell lines

In this study, sugar transport and the cellular content of the human Glut 1 and 3 glucose transporters were ascertained in uninfected and chronically HIV-infected Jurkat and H9 cell lines (T-cell lines) and U937 cells (a promonocytic cell line). Sugar transport was determined by monitoring 2-deoxy glucose uptake (2DG) and glut transporter content was determined by Western analysis. Although 'acute' HIV infection of H9 cells led to increased cellular transport activity and Glut 3 transporter content, chronic HIV infection exhibited no significant differences in sugar transport in any of the cell types investigated whether log or stationary phase cultures were employed. When uninfected and chronically HIV-infected cell lines were compared, all cell lines expressed the Glut 1 transporter, however, significant differences in Glut 1 transporter content were not observed. The Glut 3 transporter which could only be detected in the H9 cell line exhibited no differences in Glut 3 content in uninfected or chronically HIV-infected cells (2.1 +/- 0.6 versus 3.8 +/- 2.1 x 10(-3) arbitrary units/microgram protein). A trend towards lower amino acid uptake was seen in the chronically HIV-infected cells but this was not significantly different from uninfected cell cultures. The data indicate that: (1) glucose transport and the Glut 1 and 3 transporters are not increased in cells chronically infected with HIV-1 and (2) the expression of the Glut 3 sugar transporters is not the same in all target cells.

The interaction among glucose transport, hexokinase, and glucose-6-phosphatase with respect to 3H-2-deoxyglucose retention in murine tumor models

The development of new diagnostic/therapeutic modalities for cancer requires a specific understanding of how tumors differ from normal tissues. Though the key components involved in the selective accumulation of 2-deoxy-D-glucose (2-DG) analogs in tumors are known, the relative importance of each is controversial. For this reason glucose transport protein (GLUT) density, hexokinase/glucose-6-phosphatase (GP) activity, and 2-DG biodistribution were measured together in four tumor models and normal murine tissues. Direct binding studies with 3H-cytochalasin B showed that GLUT density was elevated 20-fold in LX-1 tumors. Immunohistochemically in all tumors, the expression of GLUT-1 was highest in the necrotic/ perinecrotic foci and similar in cells not adjacent to necrotic foci. As the retention of 3H-2-DG was similar in all tumors, these data suggest that the GLUT-1 in perinecrotic tumor cells were not rate limiting for 3H-2-DG uptake. Kidney, liver, and lung had high GP activity and rapid clearance of 3H-2-DG. Sodium orthovanadate (5 mumol), a GP inhibitor, increased the concentration of 3H-2-DG in these tissues, suggesting that GP is a rate-limiting enzyme for 3H-2-DG clearance. All tumor homogenates had low GP activity, and hexokinase activity was not elevated compared to normal tissues. Thus, in the tumors studied, the selective accumulation of 3H-2-DG consistently occurred in the absence of significant GP activity without the marked overexpression of hexokinase or GLUT.

Growth factors, mitogens, oncogenes and the regulation of glucose transport

The erythrocyte (or HepG2/brain) type glucose transporter (GLUT 1) was the first of the family of facilitative glucose transporter proteins to be cloned [M. Mueckler et al., Science 229, 941-945, 1985]. GLUT 1 is expressed in most tissue types, all cell lines, transformed cells and tumour cells. It is thought to be responsible for "housekeeping" levels of glucose transport, i.e. the uptake of glucose required for oxidative phosphorylation. The rate of glucose transport via GLUT 1 can be regulated under conditions in which the metabolic rate must be adjusted such as cell division (mitosis and meiosis), differentiation, transformation and nutrient starvation. Here we review the recent literature on the control of glucose transport of mitogens, growth factors and oncogenes, and discuss some of the implications for the integration of cellular signalling pathways and cell growth.

Barbiturates suppress glucose utilization by inhibition of hexokinase in neuroblastoma cells

Exposure to the pentobarbital potently inhibited the 2-deoxy glucose uptake in cultured neuroblastoma cells. The inhibition was assumed to be due to saturation of the uptake in the early stage where the incorporation was linear in the nontreated cells. On the contrary, the incorporation of 3-O-methyl glucose, another glucose analog which is not phosphorylated by hexokinase, was not altered by the treatment with pentobarbital. These results suggest that the suppression of hexokinase is involved in the above-mentioned effect of pentobarbital.

18F-2-deoxy-2-fluoro-D-glucose uptake into human tumor xenografts. Feasibility studies for cancer imaging with positron-emission tomography

The positron-emitting glucose analogue 18F-2-fluoro-2-deoxy-d-glucose (FDG) was evaluated for its accretion into the following subcutaneous human tumor xenografts in nude mice: B-cell lymphoma (Namalwa or Raji), ovarian carcinoma (HTB77), colon cancer (SW948), choriocarcinoma (BEWO), bladder cancer (UM-UC-2), renal cell carcinoma (UM-RC-3), neuroblastoma (Mey), melanoma (HTB63), and small cell lung carcinoma (NCI69). Two hours postinjection, tumor uptakes ranged from 0.027 (colon cancer) to 0.125% kg injected dose/g (melanoma); and was greater than 0.085 in the Namalwa lymphomas and the renal cell carcinomas. Tumor-blood ratios of up to 23:1 were seen 2 hours postinjection (melanoma) with a mean tumor-blood ratio for all tumors of 12.3 +/- 1.8. Uptake in the other tumors was intermediate. When evaluated, tumor uptake was slightly greater at 1 than at 2 hours postinjection, although target-background ratios were generally higher at 2 hours postinjection. This compound, FDG, may have broad applicability as a tracer for positron-emission tomographic imaging of many human malignancies.

Kinetic characteristics and regulation of hexose transport in a galactokinase-negative Chinese hamster fibroblast cell line: a good model for studies on sugar transport in cultured mammalian cells

We report the kinetic characteristics for D-galactose, 2-deoxy-D-glucose and 3-O-methyl-D-glucose transport in a galactokinase null-allele mutant of a Chinese hamster V79 cell line. GalKl cells exhibited a Km and Vmax for D-galactose, 2-deoxy-D-glucose, and 3-O-methyl-D-glucose transport of 8.6 +/- 2.6 mM and 26.1 +/- 7.2 nmol/mg p/min, 4.1 +/- 1.2 mM and 40.3 +/- 9.5 nmol/mg p/min, and 7.01 +/- .85 mM and 11.6 +/- 4.8 nmol/mg p/30 s, respectively. Nonsaturable hexose uptake was determined using cytochalasin B inhibition of galactose uptake (89.6 +/- 3.7% of galactose uptake was cytochalasin B inhibitable) and L-glucose uptake (7.5% of the galactose uptake). D-Galactose was not metabolized and effluxed rapidly from preloaded cells. The Kls for the inhibition of D-galactose transport were 4.5 +/- 2.5 mM for D-glucose, 7.0 +/- 2.0 mM for 2-deoxy-D-glucose, 6 mM for 2-deoxy-D-galactose and 6.0 +/- 0.6 mM for 3-O-methyl-D-glucose. This indicates the operation of a single common carrier. The hexose transport rate decreased 50-60% after 24 h serum deprivation. Addition of insulin was shown to increase hexose transport (more than twofold) in serum-deprived cells. Hexose transport rates increased substantially in glucose-deprived, D-fructose- or D-galactose-fed cells as compared to glucose-fed cells. Since GalKl does not metabolize galactose, the hexose transport increases induced by feeding cells galactose suggest that carrier interaction with ligand is not a significant factor in transport regulation in GalKl. The kinetic and regulatory characteristics of D-galactose transport in the GalKl cell line indicate that this system is a good model to study sugar transport from a mechanistic and regulatory point of view.

The influence of virus transformation and cell population density on some membrane properties of mouse fibroblasts in culture

The influence of cell population density and simian virus 40 transformation on the activity of the Na-K pump was studied in mouse fibroblasts cultured in medium supplemented with fetal bovine serum. The activity of the Na-K pump was determined from K+ influx, ethacrynate-sensitive K+ influx, (Na+ + K+)-ATPase assay, and the determinations of intracellular potassium and sodium ion concentrations in these cells. The activity of the Na-K pump was found to decrease in density-inhibited cultures of normal fibroblasts (designated as 3T3 cells), while in the virus-transformed cells (SV3T3) the activity remained fairly constant at all cell population densities.

Glucose transport across the blood-brain barrier in normal human subjects and patients with cerebral tumours studied using [11C]3-O-methyl-D-glucose and positron emission tomography

The kinetics of the regional cerebral uptake of [11C]3-O-methyl-D-glucose ([11C]MeG), a competitive inhibitor of D-glucose transport, have been studied in normal human subjects and patients with cerebral tumours using positron emission tomography (PET). Concomitant measurement of regional cerebral blood volume and blood flow enabled corrections for the contribution of intravascular tracer signal in PET scans to be carried out and regional unidirectional cerebral [11C]MeG extractions to be determined. A three-compartment model containing an arterial plasma and two cerebral compartments was required to produce satisfactory fits to experimental regional cerebral [11C]MeG uptake data. Under fasting, resting conditions, normal controls had mean unidirectional whole-brain, cortical, and white matter [11C]MeG extractions of 14, 13, and 17%, respectively. Mean values of k1 and k2, first-order rate constants describing forward and back transport, respectively, of tracer into the first cerebral compartment, were similar for [11C]MeG and [18F]2-fluoro-2-deoxy-D-glucose (18FDG), a second competitive inhibitor of D-glucose transport. k3, a rate constant describing FDG phosphorylation, was 20 times higher for cortical FDG uptake than the k3 fitted for [11C]MeG cortical uptake. Glioma [11C]MeG extractions ranged from normal levels of 12% to raised levels of 30%. Transport of [11C]MeG in and out of contralateral cortical tissue was significantly depressed in patients with gliomas. It is concluded that under fasting, resting conditions, regional cerebral glucose extraction remains relatively uniform throughout normal brain tissue. Gliomas, however, may have raised levels of glucose extraction. The nature of the second cerebral compartment required to describe [11C]MeG uptake is unclear, but it could represent either a useless phosphorylation-dephosphorylation cycle or nonspecific tracer uptake by a cerebral subcompartment.

Analysis of hexose transport in untransformed and sarcoma virus-transformed mouse 3T3 cells by photoaffinity binding of cytochalasin B

The effect of simian virus 40 transformation on the hexose transport system in mouse embryo fibroblast Swiss 3T3 cells was examined. The concentration of hexose transporters was estimated by measuring D-glucose-inhibitable cytochalasin B binding. The binding of cytochalasin B to the plasma membranes of simian virus 40-transformed mouse 3T3 cells (SV3T3 cells) was significantly greater than that of 3T3 cells. On the other hand, cytochalasin B binding to the microsomal membranes of SV3T3 cells was decreased, and the total amount of binding to plasma and microsomal membranes was not significantly changed in both cell lines. The electrophoretic analysis demonstrated that both hexose-transporter components of Mr 46 000 and Mr 58 000 affinity labeled were responsible for an increase in the hexose transport by viral transformation. These results suggested that the higher hexose-transport activity of transformed cells is caused by a redistribution of transporter from intracellular membranes to plasma membranes.

Ca2+-dependent stimulation of 3-O-methylglucose transport in mouse fibroblast Swiss 3T3 cells induced by phorbol-12,13-dibutyrate

Binding of phorbol-12,13-dibutyrate (PDBu), a tumor promoter, to quiescent Swiss 3T3 cells increased the number of hexose carriers, resulting in stimulation of membrane transport of 3-O-methylglucose (3MeGlu) in a Ca2+-dependent fashion. Extracellular Ca2+ was necessary to initiate the binding of PDBu to its receptor, and intracellular Ca2+ was required to maintain it. The loss of PDBu-binding, caused by elimination of Ca2+, was accompanied by a loss of stimulation of hexose transport. These results indicated that Ca2+-dependent, continuous binding of PDBu to its receptor was essential to induce the stimulation of hexose transport.

Positron emission tomography in the study of human tumors

To increase our understanding of cancer and improve cancer treatment on a rational basis we need to identify both qualitative and quantitative differences between normal and neoplastic tissue. The multimodality approach to cancer treatment includes radiotherapy, chemotherapy, hyperthermia, and immunotherapy. Most of the data on which we base our therapeutic strategies have been derived from in vitro studies or animal tumor models. More information is required on the physiology of in vivo human tumors and their response to therapy. Positron emission tomography allows the regional tissue concentration of a positron emitting radionuclide to be measured in absolute units. If valid tracer models can be formulated that accurately describe the fate of an administered "biological" tracer then the physiological process under investigation can be measured quantitatively. The sequential inhalation of C15O2, 15O2, and 11CO allows regional tissue blood flow, oxygen utilization and blood volume to be measured in absolute units. Tissue perfusion, a measure of nutrient (eg, oxygen) supply, drug delivery, or a means of heat dissipation, is of immense importance to oncologists. The oxygen-15 technique has been used not only to study regional blood flow and oxygen utilization in both tumor and normal tissue but also their response to therapeutic intervention. In those studies were tracer models are thought to be less than complete (eg, due to insufficient biological data) then only a semiquantitative or qualitative assessment of the pathophysiological state may be possible. In this respect, tumor function has been characterized by the rate of uptake of 18F-2-deoxyglucose. This technique has provided a means of tumor grading and differentiating between radiation-induced tissue necrosis and tumor recurrence. Metabolic imaging with labeled amino acids appears particularly useful in the delineation of tumor extent. Blood brain barrier integrity and the pharmacokinetics of cytotoxic drugs have also been studied quantitatively. In the future, receptor studies are likely to play an increasing role. In this review dealing with the positron emission tomography oncologic work to date, emphasis has been placed on the physiological information obtainable and how it may further our understanding of cancer and its treatment.

The inhibitory effects of horse anti-rat AFP antiserum on the uptake of 2-deoxy-D-glucose by AFP-producing rat hepatoma cells

The inhibitory effects of horse antiserum against rat alpha-fetoprotein (AFP) on the uptake of 2-deoxy-D-glucose (2dG) by the AFP-producing rat ascites hepatoma AH66 cells was studied. AH66 cells cultured in medium containing 20% heat-inactivated antiserum had a 1.5-fold lower rate of sugar uptake than did AH66 cells which were cultured in medium containing 20% heat inactivated normal horse serum. The inhibition of 2dG uptake by antiserum was dependent on both the concentration and the exposure time of antiserum. Preincubation of AH66 tumor cells for 2 and 6 h with antiserum prior to the measurement of 2dG uptake resulted in a 70.1% and 58.2% decrease in 2dG uptake compared to control cells. Antiserum did not inhibit the rate of phosphorylation of 2dG by tumor cells. Kinetic constants for the uptake of 2dG in both AH66 cells treated with antiserum to AFP and in control cells were calculated from Lineweaver-Burk plots. The Km remained constant at approximately 1.2 mM, but the Vmax was twice as small for the cells treated with antiserum as for the control cells (571 vs 923 nanomoles/2 X 10(5) cells/min). These studies suggest that the inhibition of 2dG uptake by treatment with antiserum was the result of a decrease in the number of transport sites, or a decrease in the amount of carrier protein for the sugar which was present on the surface of the plasma membrane of the AH66 cells.

Hexose uptake as an indicator of JB6 mouse epidermal cell resistance to the mitogenic activity of TPA

JB6 mouse epidermal cells have been selected for resistance to the tumor-promoting phorbol diester TPA for (1) the plateau density mitogenic (M) response, and (2) the promotion of tumor cell phenotype (P) response. The purpose of this study was to determine the relationship of hexose uptake to the two TPA-dependent processes. Monolayers of JB6 mouse epidermal cells showing one of four different phenotypes (M+P+, M+P-, M-P+, M-P-) were exposed to 60 nM [3H(G)]2-deoxy-D-glucose (2DG) with or without TPA (10 ng/ml) stimulation. The TPA mitogen-sensitive (M+P+/-) cells, when in logarithmic growth, had a lower basal 2DG uptake rate than TPA mitogen-resistant (M-P+/-) cells. At plateau density, however, only the M+P+ cells had a significantly lower basal rate. The M+ (TPA mitogen-sensitive) cells (with low basal rates), when preincubated with TPA, exhibited a two to threefold increase in 2DG uptake, while the M- (TPA mitogen-resistant) lines, which already showed elevated rates, remained unchanged. There was also a positive association between TPA mitogen sensitivity and slower growth rate. These results suggest that low hexose sugar uptake is related to TPA mitogen sensitivity, but not to promotion sensitivity. Hence the cell's ability to increase its uptake rate may be required for the cells to respond to mitogenic stimulation by TPA.

Regulation of sugar transport in cultured diploid human skin fibroblasts

The regulation of hexose transport under glucose-starvation conditions was studied in cultured human skin fibroblasts. Glucose starvation enhanced the transport of 2-DG and 3-0-methyl-D-glucose (3-OMG) but not of L-glucose. Glucose-starvation enhanced transport was inhibited by cytochalasin B (10 microM). The starvation-induced change in 2-DG transport was due to an increase in the Vmax of both the high and low affinity transport sites (2.8- and 2.4-fold, respectively) with no effect on their Kms. The presence of 5.55 mM glucose, fructose, or L-glucose in the medium resulted in transport increases similar to those seen in glucose-starved cells, while the presence of 5.55 mM glucose, mannose, or 3-OMG repressed 2-DG transport. Glucose-starvation enhancement of 2-DG transport was blocked by cycloheximide (20 micrograms/ml) but not by actinomycin D (0.03 microgram/ml) or alpha-amanitin (3.5 microM). Readdition of glucose (5.55 mM) for six hours to glucose-starved cells led to a rapid decrease in hexose transport that could be blocked by cycloheximide but not actinomycin D. Although readdition of 3-OMG to glucose-starved cells had little effect on reversing the transport increases, glucose plus 3-OMG were more effective than glucose alone. Serum containing cultures (10% v/v) of glucose-fed or glucose-starved cells exhibited rapid decreases in 2-DG transport when exposed to glucose-containing serum-free medium. These decreases were prevented by employing glucose-free, serum-free medium. The data indicate that hexose transport regulation in cultured human fibroblasts involves protein synthesis of hexose carriers balanced by interactions of glucose with a regulatory protein(s) and glucose metabolism as they affect the regulation and/or turnover of the carrier molecules.

Hexose transport in hybrids between malignant and normal cells

The kinetic parameters of hexose uptake were measured in matched pairs of hybrids between malignant and normal cells. Each pair consisted of a hybrid in which malignancy was initially suppressed and a segregant tumour derived from it. Comparisons were also made between tumour cells and non-tumorigenic derivatives selected from the tumour cell populations in vitro. Without exception, malignancy, as defined by the ability of the cell to grow progressively in vivo, was found to be linked to a systematic decrease in the Michaelis constant of the hexose transport system.

Hexose transport in Epstein-Barr virus (EBV) negative lymphoma lines and their EBV converted, virus genome carrying sublines

Increased hexose uptake is a marker for viral transformation, as has been shown in non-human fibroblasts transformed by oncogenic viruses. If this phenomenon is a general expression of viral induced transformation it should also apply on different oncogenic virus-cell systems. Recently two human EBV-negative lymphoma lines were converted to a stable EBV-positive state by infection with EBV. According to their biochemical and biological properties they enable us to study events associated with EBV-transformation. We analysed the uptake of (3H) glucosamine and (3H) 2-deoxy-D-glucose into BJAB and Ramos and their EBV-converted sublines and found a clear increase of the rate of uptake of both sugars in the EBV-positive sublines. Control experiments confirmed that the increased uptake was due to alterations on the level of the hexose membrane carriers and not due to increased metabolism. The observation of increased hexose uptake in the only presented available virus transformed human cell system is a strong argument for the general importance of this transformation-associated membrane change.

High- and low-affinity transport of D-glucose from blood to brain

Measurements of the unidirectional blood-brain glucose flux in rat were incompatible with a single set of kinetic constants for transendothelial transport. At least two transfer mechanisms were present: a high-affinity, low-capacity system, and a low-affinity, high-capacity system. The low-affinity system did not represent passive diffusion because it distinguished between D- and L-glucose. The Tmax and Km for the high-affinity system were 0.16 mmol 100 g-1 min-1 and 1 mM; for the low-affinity system, approximately 5 mmol 100 g-1 min-1 and approximately 1 M. With these values, physiological glucose concentrations were not sufficient to saturate the low-affinity system. In normoglycemia, therefore, three independent pathways of glucose transport from blood to brain appear to exist: a high-affinity facilitated diffusion pathway of apparent permeability 235 X 10(-7) cm s-1, a specific but nonsaturable diffusion pathway of permeability 85 x 10(-7) cm s-1, and a nonspecific passive diffusion pathway of permeability 2 x 10(-7) cm s-1.

Increased phosphatidylinositol synthesis in rat embryo fibroblasts after growth stimulation and its inhibition by delta-hexachlorocyclohexane

When resting rat embryo fibroblasts are stimulated to grow, a substantial increase in phosphatidylinositol synthesis can be observed. This increase cannot be explained by increased glucose uptake or glycolysis. delta-Hexachlorocyclohexane having the same configuration as myo-inositol, inhibits phosphatidyl inositol synthesis as well as DNA synthesis and mitosis, but has no effect on phosphatidyl choline synthesis. When delta-hexachlorocyclohexane is added to fibroblast cultures during the first hours after stimulation, a delay of DNA synthesis and mitosis compared to uninhibited cultures can be observed. Since delta-hexachlorocyclohexane also inhibits the uptake of nucleotides, hexoses and amino acids, it is suggested that phosphatidylinositol is necessary for the proper functioning of those receptors and carriers which are an essential part of the early cellular processes after growth stimulation, and this role of phosphatidyl-inositol may explain its increased turnover in growing cells. The increased phosphatidylinositol synthesis could not be associated to one of the subcellular fractions. When cells were labeled with [32P]orthophosphate during the first 10 min after growth stimulation and were subsequently separated into cellular fractions such as nuclei, mitochondria, plasma membranes and microsomes, no significant differences in radioactivity of phosphatidylinositol among those fractions could be observed.

Deoxyglucose and 3-O-methylglucose transport in untreated and ATP-depleted Novikoff rat hepatoma cells. Analysis by a rapid kinetic technique, relationship to phosphorylation and effects of inhibitors

Detailed time courses of uptake of labeled 3-O-methyl-D-glucose and 2-deoxy-D-glycose by untreated and ATP-depleted Novikoff rat hepatoma cells were determined as function of concentration (0.2-10 mM) by a rapid mixing/sampling technique which allows uptake measurements in time intervals as short as 1.5 seconds. Intracellular accumulation of 3-O-methylglucose in untreated and ATP-depleted cells and of deoxyglucose in ATP-depleted cells to equilibrium followed pseudo-first order kinetics and initial velocities were computed from overall time courses of substrate accumulation. Initial velocity was a Michaelis-Menten function of exogenous substrate concentration. The estimated kinetic constants for zero-trans transport of 3-O-methylglucose were about the same for untreated and ATP-depleted cells (Kztm = 1.73 +/- 0.24 mM; Vztmax = 28.8 +/- 3.6 pmoles/microliter cell H2O. sec) and were similar to those for deoxyglucose transport in ATP-depleted cells (Kztm = 0.65 +/- 0.1 mM; Vztmax = 19.6 +/- 1.6 pmoles/microliter cell H2O. sec). Similar kinetic parameters were obtained for the transport of D-glucose and D-galactose in ATP-depleted cells. The transport of 3-O-methylglucose and deoxyglucose were inhibited by each other in a simple competitive manner with apparent Ki's similar to their transport Km's. In untreated cells, in which deoxyglucose was phosphorylated, intracellular steady-state levels of free deoxyglucose accumulated within 10 to 20 seconds of incubation regardless of its concentration in the medium. Thereafter, the rate of deoxyglucose incorporation into total cell material reflected the rate of phosphorylation rather than the transport rate. The rate of deoxyglucose transport exceeded the initial rate of its phosphorylation by 20-40 %. The intracellular steady-state-levels observed during the first 2 minutes of incubation decreased from about 40% of equilibrium level at 0.2 mM deoxyglucose to about 8% at 10 mM. Computer fits of a kinetic equation describing transport and phosphorylation as independent processes operating in tandem to these data are consistent with the observed kinetic constants for hexose transport and hexokinase activity with deoxyglucose as substrate. Upon longer incubation (2-10 minutes) the rate of deoxyglucose uptake by the phosphorylating cells decreased progressively, concomitant with a decrease in intracellular ATP and an increase in intracellular deoxyglucose to equilibrium levels. It is demonstrated that the rate of deoxyglucose uptake, measured at two or more minutes, seriously underestimates the hexose transport rate and yields misleading conclusions regarding the extent and type of inhibition by transport inhibitors, such as persantin or cytochalasin B. Persantin inhibited hexose transport in a simple non-competitive manner (Ki = 20 muM) indicating that the drug affects the function of the hexose carrier.

Effects of withdrawal of a mitogenic stimulus on progression of fibroblasts into S phase: differences between serum and purified multiplication-stimulating activity

Multiplication-stimulating activity (MSA) for chicken embryo fibroblasts was purified from serum-free medium conditioned by the growth of a rat liver cell line. A comparison between calf serum and purified MSA was made regarding the regulation of the fibroblast cell cycle. Addition of serum or MSA to stationary, quiescent cells stimulates them to enter the DNA synthetic phase after a characteristic lag period. Exposure to serum for shorter periods of time will irreverisbly commit cells to continue through the cell cycle and initiate DNA replication in the absence of serum. In contrast, the withdrawal of purified MSA from the medium results in an abrupt halt in the progression of cells towards S phase. The results of labeled thymidine incorporation and autoradiographic experiments clearly indicate that the point at which cells become irreversibly committed to enter the DNA synthetic period is at or near the G1-S boundary. The abrupt decay of the stimulation upon withdrawal of purified MSA provides a unique opportunity to investigate the biochemistry of this discrete phase of the cell cycle.

Regulation of sugar transport in chick embryo fibroblasts and in fibroblasts transformed by a temperature-sensitive mutant of the Rous sarcoma virus

The mode of induction of sugar transport by serum-stimulation of growth and hexose-starvation in chick embryo fibroblasts (CEF) has been studied using metabolic inhibitors. We have concluded from these studies that the sugar transport increases induced by serum-stimulation are regulated by post-transcriptional mechanisms while sugar transport increases induced by hexose-starvation are regulated by a transcriptional mechanism. CEF infected with a temperature-sensitive mutant of the Rous sarcoma virus. Ts68 and incubated at the nonpermissive temperature for transformation, 41 degrees, retain the capacity to regulate sugar transport in a manner similar to uninfected CEF. However, Ts68-infected CEF maintained at the permissive temperature for transformation, 37 degrees, have lost the ability to regulate sugar transport at the post-transcriptional and post-translational levels.

Is glucose transport enhanced in virus-transformed mammalian cells? A dissenting view

Much of the literature on the uptake of glucose by untransformed and transformed animal cells is based on experiments carried out with 2-deoxy-D-glucose (2-DOG). Results obtained with this analog can be ambiguous, since 2-DOG can be phosphorylated by hexokinases of animal cells. An intracellular trapping mechanism is thus provided. Therefore, the total flux of 2-DOG into the cell is a resultant of both transport and hexokinase action, and the measurement of total 2-DOG incorporation is a valid measurement of transport only if 2-DOG is phosphorylated as rapidly as it enters the cell. Evidence is presented here that this is not necessarily the case, significant levels of free intracellular 2-DOG approaching external concentrations were found in untransformed and transformed mouse 3T3 cells even at early times during uptake. Differences in total intracellular 2-DOG between untransformed and transformed cells were accounted for entirely by 2-deoxyglucose phosphate. Thus, it appears the apparent increase of 2-DOG uptake accompanying transformation in these cell lines is not due to an effect on the transport process, but on enhanced phosphorylation, which is a reflection of an alteration in the regulation of glycolysis. The ambiguity introduced by phosphorylation can be oviated by the use of an analog that cannot be phosphorylated, such as 3-O-methyl-D-glucose. The rate of transport and efflux of this sugar was not found to be different in untransformed versus transformed 3T3 cells. Moreover, deficiencies of this analog as a substrate for the glucose transport system are pointed out.

High-affinity cytochalasin B binding to normal and transformed BALB/3T3 cells

To study the molecular basis of changes in sugar uptake rate in cultured mouse fibroblasts with different physiological states, we have measured the high affinity binding of [3H] cytochalsin B, a potent sugar transport inhibitor, to actively growing and contact inhibited Balb/3T3 cells as well as to 3T12 and SV3T3 cells. Binding was the same whether the cells were detached from dishes with EDTA or trypsin. The amount of drug bound to intact cells measured with a centrifugation assay was essentially the same as that bound to cell sonicates measured with equilibrium dialysis. Cytochalasin B binding to intact cells was extremely rapid and reversible over a wide range of drug concentrations, and was not affected by 0.1 M D--glucose in the assay medium. Actively growing and contact inhibited 3T3 cells had a similar number of high affinity cytochalasin B binding sites per cell, while 3T12 and SV3T3 cells had one third to one fourth the number of sites per cell. However, the number of sites per mug cellular protein appeared to be similar for cells in all of the physiological states examined.

Saturable and nonsaturable process of sugar uptake: effect of oncogenic transformation in transport and uptake of nutrients

Uptake of sugars into cells by a saturable process increased enormously during and after transformation, and uptake by a nonsaturable process increased significantly but less remarkably compared to controls. The drastic change of uptake rates, observed at around 5 x 10(-3) M sugar during and after transformation, emphasizes the significant observation that transition of the sugar uptake system from a saturable to a nonsaturable process occurs near the physiological concentration of D-glucose normally seen in animal blood. At concentrations below higher than 5 x 10(-3) M, where a saturable process is barely involved, nonsaturable uptakes of D-glucose, D-mannose, D-galactose, 2-deoxy-D-glucose and 3-O-methyl-D-glucose proceed tens to hundreds fold faster than the rate of simple diffusion of L-glucose. These findings suggest that nonsaturable uptake of the sugars known to be substrates for the saturable transport carrier system may not be a physical process or simple diffusion, as observed for L-glucose uptake. Rather, the nonsaturable uptake might be part of the total physiological process which, along with the saturable process, is controlled by a membrane-coordination mechanism. A plausible mechanism is discussed in which negative cooperativity of nutrient uptake, such as that found in bacteria, is involved.

Stimulation of 2-deoxy-d-glucose transport in control and virus-transformed cells by ethidium bromide

Recently we demonstrated that ethidium bromide altered the plasma and subcellular membrane glycoproteins in control and virus transformed cells. It is reported here that ethidium bromide also stimulated the membrane associated process of sugar transport. The KM of the virus transformed cells and the ethidium bromide treated cells is the same as that of the control cells while the maximum velocity as compared to the control cells is significantly increased. The transport of 2-deoxyl-D-glucose was inhibited by glucose, cytochalasin B and neuraminidase but was unaffected by variations in cell density or pH of the incubation medium.

Phosphorylation but not transport of sugars is enhanced in virus-transformed mouse 3T3 cells

The transport and phosphorylation of 2-deoxy-D-glucose are separate and sequential events in both normal and virus-transformed 3T3 cells. The apparent enhancement of 2-dOG uptake by 3T3 cells accompanying virus transformation is not due to an effect on the transport process but to enhanced phosphorylation by intracellular kinases. Phosphorylation of 3-O-methyl-D-glucose does not occur in these cells. Both the rate and extent of transport of this glucose analog is the same in normal cells, SV40 virus-transformed cells and sarcoma virus-transformed cells. The appropriateness of using 3-O-MeG for studies of the glucose transport system of animal cells is examined and discussed.

Growth dependent 2-deoxy-D-glucose uptake in transformed and normal mouse cells

The rates of uptake of 2-deoxyglucose were studied in a) 15-day BALB/c and AKR embryonic cells, b) adult BALB/c lung fibroblasts and c) virally-transformed cell lines derived from a clone of BALB/c cells. At high cell densities in confluent cultures of non-transformed cells the rates of uptake were three to seven times lower than those in transformed cells at the equivalent densities. Both non-transformed and virally-transformed cell cultures showed density-dependent inhibition of 2-deoxyglucose uptake. However, inhibition was only slight in SV40 (SVT2) and Kirsten sarcoma virus-transformed (K-A31) cells in which uptake rates decreased 2.3– and 2.5– fold respectively during a 12-fold increase in cell densities. For an almost similar increase in cell density, in unestablished adult embryonic BALB/c fibroblasts, the rate of uptake decreased 8-fold. Incorporation of H3-thymidine (an indication of growth rate) occurred at a higher rate in transformed than in contact-inhibited cells. Two hours after serum stimulation of contact-inhibited AKR cells there was approximately 50% increase in the rate of 2-deoxyglucose uptake. The results confirm previous findings that the increased uptake of 2-deoxyglucose by virally-transformed cells may be due to their growth and division and not necessarily to the direct expression of the viral genome.