Analysis of a functional change in membrane in the process of cell transformation by Rous sarcoma virus; alteration in the characteristics of sugar transport

Acidic extracellular microenvironment and cancer

Acidic extracellular pH is a major feature of tumor tissue, extracellular acidification being primarily considered to be due to lactate secretion from anaerobic glycolysis. Clinicopathological evidence shows that transporters and pumps contribute to H⁺ secretion, such as the Na⁺/H⁺ exchanger, the H⁺-lactate co-transporter, monocarboxylate transporters, and the proton pump (H⁺-ATPase); these may also be associated with tumor metastasis. An acidic extracellular pH not only activates secreted lysosomal enzymes that have an optimal pH in the acidic range, but induces the expression of certain genes of pro-metastatic factors through an intracellular signaling cascade that is different from hypoxia. In addition to lactate, CO₂ from the pentose phosphate pathway is an alternative source of acidity, showing that hypoxia and extracellular acidity are, while being independent from each other, deeply associated with the cellular microenvironment. In this article, the importance of an acidic extracellular pH as a microenvironmental factor participating in tumor progression is reviewed.

Metabolic effects of influenza virus infection in cultured animal cells: Intra- and extracellular metabolite profiling

BACKGROUND

Many details in cell culture-derived influenza vaccine production are still poorly understood and approaches for process optimization mainly remain empirical. More insights on mammalian cell metabolism after a viral infection could give hints on limitations and cell-specific virus production capacities. A detailed metabolic characterization of an influenza infected adherent cell line (MDCK) was carried out based on extracellular and intracellular measurements of metabolite concentrations.

RESULTS

For most metabolites the comparison of infected (human influenza A/PR/8/34) and mock-infected cells showed a very similar behavior during the first 10-12 h post infection (pi). Significant changes were observed after about 12 h pi: (1) uptake of extracellular glucose and lactate release into the cell culture supernatant were clearly increased in infected cells compared to mock-infected cells. At the same time (12 h pi) intracellular metabolite concentrations of the upper part of glycolysis were significantly increased. On the contrary, nucleoside triphosphate concentrations of infected cells dropped clearly after 12 h pi. This behaviour was observed for two different human influenza A/PR/8/34 strains at slightly different time points.

CONCLUSIONS

Comparing these results with literature values for the time course of infection with same influenza strains, underline the hypothesis that influenza infection only represents a minor additional burden for host cell metabolism. The metabolic changes observed after 12 h pi are most probably caused by the onset of apoptosis in infected cells. The comparison of experimental data from two variants of the A/PR/8/34 virus strain (RKI versus NIBSC) with different productivities and infection dynamics showed comparable metabolic patterns but a clearly different timely behavior. Thus, infection dynamics are obviously reflected in host cell metabolism.

Metabolic control of glucose degradation in yeast and tumor cells

Regulation of glucose degradation in both yeasts and tumor cells is very similar in many respects. In both cases it leads to excretion of intermediary metabolites (e.g., ethanol, lactate) in those cell types where uptake of glucose is unrestricted (Saccharomyces cerevisiae, Bowes melanoma cells). The similarities between glucose metabolism observed in yeast and tumor cells is explained by the fact that cell transformation of animal cells leads to inadequate expression of (proto-)oncogenes, which force the cell to enter the cell cycle. These events are accompanied by alterations at the signal transduction level, a marked increase of glucose transporter synthesis, enhancement of glycolytic key enzyme activities, and slightly reduced respiration of the tumor cell. In relation to homologous glucose degradation found in yeast and tumor cells there exist strong similarities on the level of cell division cycle genes, signal transduction and regulation of glycolytic key enzymes. It has been demonstrated that ethanol and lactate excretion in yeast and tumor cells, respectively, result from an overflow reaction at the point of pyruvate that is due to a carbon flux exceeding the capacity of oxidative breakdown. Therefore, the respiratory capacity of a cell determines the amount of glycolytic breakdown products if ample glucose is available. This restricted flux is also referred to as the respiratory bottleneck. The expression "catabolite repression", which is often used in textbooks to explain ethanol and acid excretion, should be abandoned, unless specific mechanisms can be demonstrated. Furthermore, it was shown that maximum respiration and growth rates are only obtained under optimum culture conditions, where the carbon source is limiting.

Molecular mechanism for cancer-associated induction of sialyl Lewis X and sialyl Lewis A expression-The Warburg effect revisited

Cell adhesion mediated by selectins and their carbohydrate ligands, sialyl Lewis X and sialyl Lewis A, figures heavily in cancer metastasis. Expression of these carbohydrate determinants is markedly enhanced in cancer cells, but the molecular mechanism that leads to cancer-associated expression of sialyl Lewis X/A has not been well understood. Results of recent studies indicated involvement of two principal mechanisms in the accelerated expression of sialyl Lewis X/A in cancers; 'incomplete synthesis' and ' neo synthesis.' As to 'incomplete synthesis,' we have recently found further modified forms of sialyl Lewis X and sialyl Lewis A in non-malignant colonic epithelium, which have additional 6-sulfation or 2 --> 6 sialylation. The impairment of GlcNAc 6-sulfation and 2 --> 6 sialylation upon malignant transformation leads to accumulation of sialyl Lewis X/A in colon cancer cells. Epigenetic changes such as DNA methylation and/or histone deacetylation are suggested to lie behind such incomplete synthesis. As to the mechanism called ' neo synthesis,' recent studies have indicated that cancer-associated alterations in the sugar transportation and intermediate carbohydrate metabolism play important roles. Cancer cells are known to exhibit a metabolic shift from oxidative to elevated anaerobic glycolysis (Warburg effect), which is correlated with the increased gene expression of sugar transporters and glycolytic enzymes induced by common cancer-specific genetic alterations. The increased sialyl Lewis X/A expression in cancer is a link in the chains of these events because our recent results indicated that these events accompany transcriptional induction of a set of genes closely related to its expression.

Mayaro virus infection alters glucose metabolism in cultured cells through activation of the enzyme 6-phosphofructo 1-kinase

Although it is well established that cellular transformation with tumor virus leads to changes on glucose metabolism, the effects of cell infection by non-transforming virus are far to be completely elucidated. In this study, we report the first evidence that cultured Vero cells infected with the alphavirus Mayaro show several alterations on glucose metabolism. Infected cells presented a two fold increase on glucose consumption, accompanied by an increment in lactate production. This increase in glycolytic flux was also demonstrated by a significant increase on the activity of 6-phosphofructo 1-kinase, one of the regulatory enzymes of glycolysis. Analysis of the kinetic parameters revealed that the regulation of 6-phosphofructo 1-kinase is altered in infected cells, presenting an increase in Vmax along with a decrease in Km for fructose-6-phosphate. Another fact contributing to an increase in enzyme activity was the decrease in ATP levels observed in infected cells. Additionally, the levels of fructose 2,6-bisphosphate, a potent activator of this enzyme, was significantly reduced in infected cells. These observations suggest that the increase in PFK activity may be a compensatory cellular response to the viral-induced metabolic alterations that could lead to an impairment of the glycolytic flux and energy production.

Hemopoietic cell transformation is associated with failure to downregulate glucose uptake during the G2/M phase of the cell cycle

Growth factors and cytokines initiate multiple signal transduction pathways that lead to cell survival, cell cycle progression or differentiation. A common feature of these pathways is increased cellular metabolism and glucose uptake. Furthermore, the energy requirements of many cancers and transformed cell lines are met by constitutive upregulation of glucose uptake. Relationships among transforming events, glucose uptake and cell cycle progression are not well understood. Here we investigated the regulation of glucose transport during the cell cycle of growth factor-dependent 32D cells, primary T-cells, src-transformed 32D cells and Jurkat cells. Cells were enriched in the G1, S and G2/M phases of the cell cycle, and glucose transporter expression and 2-deoxyglucose uptake were measured. Glucose transporter expression increased with cell volume as cells progressed through the cell cycle. Growth factor-dependent 32D cells and T-lymphocytes were characterised by increased 2-deoxyglucose uptake from G1 to S and reduced uptake at G2/M, with the highest specific activity of transporters in the S phase. In contrast, src-transformed 32D cells and Jurkat cells showed increased 2-deoxyglucose uptake from S to G2/M, with the highest glucose transporter specific activity in G2/M. Our results show that glucose transport is regulated in a cell cycle-dependent manner and suggest that this regulation may be altered in transformed cells.

Ras pathway is required for the activation of MMP-2 secretion and for the invasion of src-transformed 3Y1

To search for the signaling pathway critical for tumor invasion, we examined the effects of dominant negative ras (S17N ras) expression on the activation of matrix metalloproteinase-2 (MMP-2) in src-transformed 3Y1, SR3Y1, under the control of conditionally inducible promoter. In SR3Y1 clones transfected with S17N ras, augmented secretion and proteolytic activation of MMP-2 were dramatically suppressed by S17N Ras expression, while tyrosine phosphorylation of cellular proteins was not suppressed. We found that invasiveness of SR3Y1 cells assayed by the modified Boyden Chamber method was strongly suppressed by S17N Ras expression. In contrast, cell morphology reverted partially and glucose uptake remained unchanged by S17N Ras expression. In addition, treatment of SR3Y1 with manumycin A, a potent inhibitor of Ras farnesyltransferase, strongly suppressed both augmented secretion and proteolytic activation of MMP-2. Contrary, treatment of SR3Y1 with wortmannin or TPA showed no clear effect on MMP-2 activation. Thus, these results strongly suggest that Ras-signaling, but neither P13 kinase- nor protein kinase C-signalings, plays a critical role in activation of MMP-2 and, subsequently, in the invasiveness of src-transformed cells.

N-glycosylation of glucose transporter-1 (Glut-1) is associated with increased transporter affinity for glucose in human leukemic cells

To elucidate the role of N-glycosylation in the functional activity of the universal glucose transporter, Glut-1, we investigated effects of the N-glycosylation inhibitor, tunicamycin, on glucose transport by human leukemic cell lines K562, U937 and HL60. Treatment with tunicamycin produced a 40-50% inhibition of 2-deoxyglucose uptake and this was associated with a 2-2.5-fold decrease in transporter affinity for glucose (Km) without a change in Vmax. Leukemic K562, U937 and HL60 cells expressed Glut-1 transporter protein. With K562 cells Glut-1 appeared as a broad band of 50-60 kDa, whereas with U937 and HL60 cells a diffuse band was observed at approximately 55 kDa. Treatment of K562 cells with tunicamycin for 18 h, resulted in extensive loss of the 50-60 kDa glycoprotein, appearance of a 30-40 kDa band and increased staining of a 45 kDa band. With U937 cells, tunicamycin treatment resulted in the appearance of a 30-40 kDa band and increased staining of a 45 kDa band. With HL60 cells loss of the 55 kDa Glut-1 band was observed and a band of 45 kDa appeared. Tunicamycin-treatment resulted in 75-90% inhibition in [3H]mannose incorporation but only 20-25% inhibition in [3H]thymidine and [3H]leucine incorporation. In contrast, tunicamycin had little effect on the viability and MTT responses of the cells used. These results suggest that in leukemic cells N-glycosylation of Glut-1 plays an important role in maintaining its structure and functional integration.

Production of trypsins by human gastric cancer cells correlates with their malignant phenotype

Proteolytic degradation of extracellular matrix is a critical step in tumour invasion and metastasis. To examine the role of trypsin in tumour dissemination, we cloned two variants (S4 and R3 cells) from STKM-1, a trypsinogen 1-producing diffuse gastric cancer cell line. Western blot analysis with antitrypsin antibody showed that 26 and 24 kDa proteins were highly detected in S4 conditioned medium (CM) in comparison to R3 CM. In addition to the 26 and 24 kDa proteins, 25 and 23 kDa bands, which correspond to enterokinase-activated trypsin, were found only in S4 CM. When the CMs of the two clones were treated with enterokinase, the 25 and 23 kDa trypsin activities in S4 CM were effectively increased as compared with R3 CM. When the two clones were inoculated intraperitoneally (i.p.) into nude mice, S4 cells strongly invaded the liver, pancreas and peritoneum and killed the hosts more rapidly than R3 cells: the 50% survival time was 50 days for S4 and 82 days for R3 cells. These results suggest that trypsin production is associated with the invasive growth of STKM-1 gastric cancer cells.

Increased glucose transport in ras-transformed fibroblasts: a possible role for N-glycosylation of GLUT1

2-Deoxyglucose uptake was enhanced in ts371 KiMuSV-NRK cells when growing at the permissive temperature to allow the expression of a transforming p21 ras protein. This change is due to a decrease in the K(m) by approximately 2.5-fold without affecting the V(max) of the transporter. The amount of the GLUT1 glucose transporter dit not increase as deduced from immunoblot experiments on total membranes. Nevertheless, ras-transformed GLUT1 displays a higher molecular mass due to an increased N-glycosylation of the protein. Experiments made in tunicamycin-treated cells indicates that a higher glycosylation is responsible for the increase in 2-deoxyglucose uptake in ras-transformed cells.

Human immunodeficiency virus type 1 infection of H9 cells induces increased glucose transporter expression

A clone obtained from a differential display screen for cellular genes with altered expression during human immunodeficiency virus (HIV) infection matched the sequence for the human GLUT3 facilitative glucose transporter, a high-velocity-high-affinity facilitative transporter commonly expressed in neurons of the central nervous system. Northern (RNA) analysis showed that GLUT3 expression increased during infection. Flow cytometry showed that GLUT3 protein expression increased specifically in the HIV-infected cells; this increase correlated with increased 2-deoxyglucose transport in the HIV-infected culture. HIV infection therefore leads to increased expression of a glucose transporter normally expressed at high levels in other cell types and a corresponding increase in glucose transport activity. If HIV infection places increased metabolic demands on the host cell, changes in the expression of a cellular gene that plays an important role in cellular metabolism might provide a more favorable environment for viral replication.

Deoxyglucose uptake by mouse astrocytes: effects of temperature and retrovirus infection

Deoxyglucose uptake by FVB/N mouse astrocytes was studied before and after infection by ts1 retrovirus which causes a neurodegenerative disease in mice similar to HIV-1 encephalopathy in man. The Michaelis-Menten kinetic parameters, Km and Vmax, of 2-deoxy-D-glucose uptake by brain and cerebellar astrocytes were measured following culture at 34 degrees C where ts1 retrovirus replicates optimally, and at 37 degrees C. Compared to astrocytes cultured at 37 degrees C, astrocytes cultured at 34 degrees C had increased Km and decreased deoxyglucose uptake despite increased or unchanged Vmax. Following ts1 retrovirus infection, brain astrocyte deoxyglucose uptake doubled [132%] associated with decreased Km but unchanged Vmax, whereas cerebellar astrocyte deoxyglucose uptake doubled [102%] associated with increased Vmax but unchanged Km. These observations of altered deoxyglucose uptake kinetic parameters following retrovirus infection indicate different neurochemical mechanisms for the regional variation in deoxyglucose uptake observed following retrovirus infection of the CNS in vivo.

Glucose transporter gene expression: regulation of transcription and mRNA stability

The facilitated diffusion of D-glucose across the plasma membrane is carried out by a set of stereospecific transport proteins known as the glucose transporters. These integral membrane proteins are members of a gene family where tissue-specific expression of one or more members will determine in part the net rate of glucose entry into the cell. The regulation of glucose transporter gene expression is a critical feature of cellular homeostasis, as defects in specific transporter expression can lead to profound alterations in cellular physiology. In this review, we provide a brief descriptive background on the family of glucose transporters and examine in depth the regulation of the two transporters expressed in adipose tissue, GLUTI, a basal growth-related transporter and GLUT4, the insulin-responsive glucose transporter.

Presence and differential expression of SGLT1, GLUT1, GLUT2, GLUT3 and GLUT5 hexose-transporter mRNAs in Caco-2 cell clones in relation to cell growth and glucose consumption

Seven clones from the Caco-2 cell line, three isolated from passage 29 (PD7, PD10, PF11) and four from passage 198 (TB10, TC7, TF3, TG6), all of them selected on the basis of differences in the levels of expression of sucrase-isomaltase and rates of glucose consumption, were analysed for the expression of hexose-transporter mRNAs (SGLT1, GLUT1-GLUT5) in relation to the phases of cell growth and the associated variations of the rates of glucose consumption. All clones showed a similar pattern of evolution of the rates of glucose consumption, which decreased from the exponential to the late-stationary phase, but differed, in a 1-40-fold range, in the values observed at late postconfluency. According to these values, clones could be divided into high- (PD10, PF11) and low-glucose-consuming cells (PD7, TB10, TC7, TF3 and TG6). GLUT1 and GLUT3 mRNAs were expressed in all clones and showed a similar pattern of evolution: their level decreased, from the exponential to the stationary phase, in close correlation with the decrease in rates of glucose consumption, with only high-glucose-consuming clones maintaining high levels in the stationary phase. In contrast, SGLT1, GLUT2 and GLUT5 mRNAs were only expressed, like sucrase-isomaltase mRNA, in the low-glucose-consuming clones, and their level increased from the exponential to the stationary phase, in parallel with the differentiation of the cells. GLUT4 was undetectable in all the clones. Glucose deprivation generally resulted in a discrete decrease in the levels of all transporter mRNAs in all clones, one exception being GLUT2, which in the high-glucose-consuming clones is only detectable when the cells are grown in low glucose. These clones should be ideal tools with which to study in vitro, at the single-cell level, how these transporters concur to the utilization and transport of hexoses and how their exclusive or co-ordinated expression is regulated.

Modulation of the synthesis and glycosylation of the glucose transporter protein by transforming growth factor-beta 1 in Swiss 3T3 fibroblasts

Transforming growth factor-beta 1 (TGF-beta 1) stimulated growth and glucose uptake in Swiss mouse fibroblasts. DNA synthesis was increased 2-3-fold after 48 h incubation of growing 3T3 cells with TGF-beta 1 in calf serum-containing medium. Glucose transport activity in the cells was increased within 3 h after addition of TGF-beta 1 and this stimulation continued during incubation for 48 h. TGF-beta 1 also increased the levels of a brain type-glucose transporter (GLUT1) mRNA and the GLUT1 protein (55 kDa) in the membranes, consistent with the increase in glucose uptake. Furthermore, a longer exposure of TGF-beta 1 for 24-48 h induced a marked increase in the 65 kDa GLUT1 in 3T3 cell membranes. Other growth factors such as epidermal growth factor, fibroblast growth factor, transforming growth factor-alpha, and insulin did not elevate glucose uptake and the levels of 55 and 65 kDa GLUT1 proteins. Adding tunicamycin or deoxymannojirimycin to the TGF-beta 1-treated and untreated cells caused these 55 and 65 kDa glucose transporters to migrate as one band at 40-43 kDa. In addition, treating membrane proteins with glycopeptidase F, which removes N-linked oligosaccharides, also generated a glucose transporter of 40 kDa, suggesting that the 55 and 65 kDa GLUT1 proteins have a similar or identical core polypeptide but with different N-linked oligosaccharides. These results indicate that TGF-beta 1 modulates the synthesis of GLUT1 protein as well as its glycosylation in Swiss 3T3 cells, and that these changes may contribute to the control of cell proliferation by TGF-beta 1.

Insulin-cancer relationships: possible dietary implication

Insulin is a major anabolic hormone in mammals and its involvement in malignancies is well documented. An attempt is made to classify experimental and human cancers into four groups, according to the way the tumors are affected by, or interact with, insulin. Such an approach provides a better understanding of the dietary effects on tumorigenesis. Since human cancers are of the insulin-producing/secreting or insulin-dependent types, it is suggested that screening of individuals for blood insulin level and reducing the insulin status by dietary means may lead to a decreased risk of cancer. Anti-insulin drugs may be useful as supplements to therapeutic treatment.

Effects of transformation by v-fps on nucleoside transport in Rat-2 fibroblasts

Important cellular nutrients, including nucleosides and hexose sugars, are rapidly taken up by cells, largely through mediated carrier systems. The present study examined nucleoside and hexose transport activity in normal Rat-2 fibroblasts and clonal derivatives that expressed either the wild-type (C10) or a temperature-sensitive mutant (NA9) form of v-fps, a transforming protein-tyrosine kinase. Initial uptake rates (transport) of adenosine, thymidine, 3-O-methylglucose and 2-deoxyglucose were greater in v-fps-transformed cells than in normal cells. Elevated transport rates were seen in cells that expressed the temperature-sensitive mutant v-fps only after growth at a temperature that was permissive for protein-tyrosine kinase activity. Nucleoside transport rates declined with increasing cell density in both normal and v-fps transformed cells. Analysis of the sensitivity of adenosine transport to inhibition by nitrobenzylthioinosine (NBMPR) indicated that Rat-2 fibroblasts, like many other rat cell types, possess at least two nucleoside transport systems, which can be distinguished by differences in sensitivity to NBMPR. Although both transport activities were elevated in v-fps-transformed cells, a greater increase was seen in the NBMPR-sensitive component than in the NBMPR-insensitive component. Mass law analysis of the binding of [3H]NBMPR indicated that transformed cells had either the same number (NA9) or a smaller number (C10) of NBMPR-binding sites than normal cells, and photolabelling of membrane proteins with [3H]NBMPR identified polypeptides with similar electrophoretic mobilities (55-75 kDa) in both normal and transformed cells. Thus transformation by v-fps resulted in an increase in NBMPR-sensitive transport activity which was not related to either the number of NBMPR-binding sites or the apparent molecular mass of NBMPR-binding polypeptides.

Membrane transport and disease

Four situations in which membrane transport is altered by disease are discussed: (a) non-specific leaks induced by poreforming agents; (b) glucose transport and cellular stress; (c) Ca2+-ATPase and hypertension; (d) Na+ channels and HSV infection.

Transformation stimulates glucose transporter gene expression in the absence of protein kinase C

The rat brain glucose transporter (GT) gene is rapidly activated coincident with the initiation of growth in response to oncogenic transformation or the addition of growth factors to quiescent fibroblasts. The latter response has been shown to be mediated by protein kinase C-dependent and-independent pathways. We studied the role of protein kinase C in the transformation-induced activation of the GT gene. Transformation of fibroblasts by either the v-fps or the Ki-ras oncogene rapidly increased the levels of GT mRNA. Either viral oncogene remained capable of stimulating the GT gene after depletion of cellular protein kinase C by prolonged pretreatment of fibroblasts with phorbol 12-myristate 13-acetate. These data indicate that protein kinase C is not required for the rapid activation of gene transcription by oncogenic transformation.

A rat mutant cell clone showing temperature-dependent transformed phenotypes with functional expression of the src gene product

The cellular mutant B814 isolated from a Fischer rat cell line shows temperature-sensitivity of focus formation on infection with Moloney murine sarcoma virus (Mo-MSV) and Rous sarcoma virus (RSV). An RSV-transformed clone (S814-2) isolated from B814 cells shows temperature-sensitive transformed phenotypes for morphology, growth in soft agar, and glucose uptake. The expression, phosphorylation, and tyrosine kinase activity of pp60v-src in S814-2 were not affected at the nonpermissive temperature, and virus rescued from this clone had wild-type transforming ability, suggesting that a cellular factor altered in S814-2 is responsible for the cellular steps of transformation after the function of pp60v-src. In addition, the cellular 36K protein, a possible candidate as a target of pp60v-src, was phosphorylated at the nonpermissive temperature in S814-2, indicating that phosphorylation of the 36K protein is not correlated with transformed phenotypes.

Transformation by the src oncogene alters glucose transport into rat and chicken cells by different mechanisms

Transformation of both rat and chicken fibroblasts by the src oncogene leads to a four- to fivefold increase in the rate of glucose transport and in the level of the glucose transporter protein. We have previously shown that, with chicken embryo fibroblasts, transformation leads to a reduction in the rate of degradation of the transporter, with little or no increase in the rate of its biosynthesis. We now show that, with the rat-1 cell line, the opposite result was obtained. src-induced transformation led to an increase in transporter biosynthesis, with little effect on turnover. A src-induced increase in transporter mRNA entirely accounted for the increase in biosynthesis of the protein. By contrast, in chicken embryo fibroblasts, the level of transporter mRNA was low and was not induced to rise by src transformation. Thus, src induced an increase in the level of the glucose transport protein by fundamentally different mechanisms in chicken embryo fibroblasts and rat-1 cells. To test whether this difference was due to rat-1 cells being an immortalized cell line, we measured transporter mRNA levels in primary fibroblast cultures from rat embryos and in parallel cultures transformed by src. Transporter mRNA was inducible by src in these cells. Thus, the difference in mRNA inducibility between chicken and rat cells is not due to immortalization.

Transformation by the src oncogene alters glucose transport into rat and chicken cells by different mechanisms

Transformation of both rat and chicken fibroblasts by the src oncogene leads to a four- to fivefold increase in the rate of glucose transport and in the level of the glucose transporter protein. We have previously shown that, with chicken embryo fibroblasts, transformation leads to a reduction in the rate of degradation of the transporter, with little or no increase in the rate of its biosynthesis. We now show that, with the rat-1 cell line, the opposite result was obtained. src-induced transformation led to an increase in transporter biosynthesis, with little effect on turnover. A src-induced increase in transporter mRNA entirely accounted for the increase in biosynthesis of the protein. By contrast, in chicken embryo fibroblasts, the level of transporter mRNA was low and was not induced to rise by src transformation. Thus, src induced an increase in the level of the glucose transport protein by fundamentally different mechanisms in chicken embryo fibroblasts and rat-1 cells. To test whether this difference was due to rat-1 cells being an immortalized cell line, we measured transporter mRNA levels in primary fibroblast cultures from rat embryos and in parallel cultures transformed by src. Transporter mRNA was inducible by src in these cells. Thus, the difference in mRNA inducibility between chicken and rat cells is not due to immortalization.

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.

Degradation and biosynthesis of the glucose transporter protein in chicken embryo fibroblasts transformed by the src oncogene

The rate of glucose transport in cultured fibroblasts is regulated to a number of physiological variables, including malignant transformation by src, glucose starvation, and stimulation with mitogens. Much of this transport regulation can be accounted for by variations in the amount of transporter protein in the cells. To determine the mechanisms by which levels of the transporter are regulated, we measured the rates of synthesis and degradation of the transporter by pulse-chase experiments and immunoprecipitation of the transporter. We found that transformation by the src oncogene results in a large decrease in the rate at which the transporter protein is degraded but that it does not appreciably increase the rate of transporter biosynthesis. On the other hand, glucose starvation and mitogen stimulation increase the rate of transporter biosynthesis, although a role for control of degradation is possible in these circumstances also. Variations in the rate of glucose transport or the amount of the transporter are not associated with phosphorylation of the transporter protein.

Degradation and biosynthesis of the glucose transporter protein in chicken embryo fibroblasts transformed by the src oncogene

The rate of glucose transport in cultured fibroblasts is regulated to a number of physiological variables, including malignant transformation by src, glucose starvation, and stimulation with mitogens. Much of this transport regulation can be accounted for by variations in the amount of transporter protein in the cells. To determine the mechanisms by which levels of the transporter are regulated, we measured the rates of synthesis and degradation of the transporter by pulse-chase experiments and immunoprecipitation of the transporter. We found that transformation by the src oncogene results in a large decrease in the rate at which the transporter protein is degraded but that it does not appreciably increase the rate of transporter biosynthesis. On the other hand, glucose starvation and mitogen stimulation increase the rate of transporter biosynthesis, although a role for control of degradation is possible in these circumstances also. Variations in the rate of glucose transport or the amount of the transporter are not associated with phosphorylation of the transporter protein.

Sindbis virus infection increases hexose transport in quiescent cells

Sindbis virus infection of baby hamster kidney cells or chick embryo cells resulted in a significant increase in the rate of uptake of [2-3H]deoxy-D-glucose ([3H]dGlu). Stimulation of hexose transport in Sindbis virus-infected cells occurred only if the cells were rendered quiescent by culturing at high density or by serum starvation. In contrast, Sindbis virus-induced inhibition of potassium transport, measured as a decrease in the uptake of 86Rb+, was independent of cell growth state. Stimulation of [3H]dGlu uptake in Sindbis virus-infected cells was the result of an increase in the Vmax of the hexose transporter, but not a change in the Km. The stimulation of [3H]dGlu uptake induced by Sindbis virus was insensitive to the drug actinomycin D, but was blocked by cordycepin. The stimulation was also insensitive to treatment with tunicamycin, which prevented the virally induced inhibition of the plasma membrane-associated Na+/K+ ATPase and termination of host protein synthesis.

Five enzymes of the glycolytic pathway serve as substrates for purified epidermal-growth-factor-receptor kinase

Several enzymes of the glycolytic pathway are phosphorylated in vitro and in vivo by retroviral transforming protein kinases. These substrates include the enzymes phosphoglycerate mutase (PGM), enolase and lactate dehydrogenase (LDH). Here we show that purified EGF (epidermal growth factor)-receptor kinase phosphorylates the enzymes PGM and enolase and also the key regulatory enzymes of the glycolytic pathway, phosphofructokinase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in an EGF-dependent manner. Stoichiometry of phosphate incorporation into GAPDH (calculated from native Mr) is the highest, reaching approximately 1. LDH and other enzymes of the glycolytic pathway are not phosphorylated by the purified EGF-receptor kinase. These enzymes are phosphorylated under native conditions, and the Km values of EGF-receptor kinase for their phosphorylation are close to the physiological concentrations of these enzymes in the cell. EGF stimulates the reaction by 2-5-fold by increasing the Vmax. without affecting the Km of this process. Phosphorylation is rapid at 22 degrees C and at higher temperatures. However, unlike the self-phosphorylation of EGF-receptor, which occurs at 4 degrees C, the glycolytic enzymes are poorly phosphorylated at this temperature. Some enzymes, in particular enolase, increase the receptor Km for ATP in the autophosphorylation process and thus may act as competitive inhibitors of EGF-receptor self-phosphorylation. On the basis of the Km values of EGF receptor for the substrate enzymes and for ATP in the phosphorylation reaction, these enzymes may also be substrates in vivo for the EGF-receptor kinase.

Stress induces an increased hexose uptake in cultured cells

Temperature-sensitive mutants have revealed a region of the herpes simplex virus 1 genome that affects both the uptake of hexose and the synthesis of heat shock proteins. Other inducers of heat-shock proteins, namely heat shock itself and arsenite, likewise induce an increased uptake of hexose. The increased uptake, like that induced by insulin, is insensitive to the presence of actinomycin D or cycloheximide. It is concluded that an increased hexose uptake, reflecting an activation or relocation of existing hexose transport protein, is a general biochemical response of stressed cells.

Expression of the v-src or v-fps oncogene increases fructose 2,6-bisphosphate in chick-embryo fibroblasts. Novel mechanism for the stimulation of glycolysis by retroviruses

The concentration of fructose 2,6-bisphosphate and the activity of 6-phosphofructo-2-kinase are increased after infection of chick-embryo fibroblasts with the Rous sarcoma virus, or with a temperature-sensitive mutant of this virus at the permissive, but not at the non-permissive, temperature. This is observed after transformation by retroviruses carrying either the v-src or v-fps, but not the v-mil and/or v-myc, oncogenes. Comparison of the effects of the Rous sarcoma virus with those of phorbol myristate acetate on fructose 2,6-bisphosphate suggests that both result from the stimulation of a step which is rate-limiting for 6-phosphofructo-2-kinase activation and which is also controlled by protein kinase C.

Intracellular K+ and the expression of transformation parameters by chick cells transformed with the Bryan strain of Rous sarcoma virus

As normal chick embryo (CE) cells entered quiescence the intracellular concentrations of both Na+ and K+ declined. Comparable decreases in intracellular concentrations of Na+ and K+ were not observed in CE cells transformed by either the Schmidt-Ruppin (SR) or the Bryan (B) strain of Rous sarcoma virus (RSV). Intracellular concentrations of Na+ were higher in SR-RSV-transformed CE cells than in B-RSV-transformed cells and uninfected CE cells at all times after plating. In contrast, intracellular concentrations of K+ were higher in B-RSV-transformed CE cells than in SR-RSV-transformed cells. Uninfected CE cells incubated in medium containing an elevated concentration of K+ (an increase from 5 to 30 mM) exhibited several, but not all, of the transformation parameters expressed by B-RSV-transformed CE cells.

Increased sugar transport in BHK cells infected with Semliki Forest virus or with herpes simplex virus

Infection of BHK cells by SFV increases the rate of uptake of [3H]MeGlc and of [3H]dGlc at approximately 2 hours p.i. Infection by HSV increases the uptake of [3H]MeGlc and [3H]dGlc at approximately 10 hours p.i.; the increased uptake is prevented by acyclovir. It is concluded that an increased sugar uptake by infected cells reflects an increased rate of transport across the plasma membrane and is the result of cellular changes caused by virus infection.

Stimulation of glucose uptake by transforming growth factor beta: evidence for the requirement of epidermal growth factor-receptor activation

Transforming growth factor beta (TGF-beta), derived from human platelets, stimulates the uptake of 2-deoxy-glucose by cultured cell monolayers 2- to 4-fold. Stimulation can be detected as early as 30 min with as little as 0.1 ng of TGF-beta per ml and maximal effects can be obtained at 2 hr with 1 ng of the growth factor per ml. TGF-beta-induced stimulation of sugar uptake is enhanced by the co-addition of platelet-derived growth factor (10 ng/ml) or epidermal growth factor (EGF, 1 ng/ml). The NR-6 variant of mouse 3T3 cells, which lack EGF receptors, is not stimulated by TGF-beta. Antisera to EGF receptors that block 125I-labeled EGF binding also inhibit TGF-beta stimulation of 2-deoxyglucose uptake, although 125I-labeled TGF-beta binding remains unimpaired. In contrast, anti-sera to the EGF receptor, which do not block EGF binding, have no measurable effect on the TGF-beta-stimulated uptake of 2-deoxyglucose. We confirm that the receptor for TGF-beta is distinct from the receptor for EGF and we conclude that TGF-beta stimulation of 2-deoxyglucose uptake requires the co-activation of the EGF receptor kinase system.

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.

Epidermal growth factor receptor metabolism and protein kinase activity in human A431 cells infected with Snyder-Theilen feline sarcoma virus or harvey or Kirsten murine sarcoma virus

When human A431 cells, which carry high numbers of epidermal growth factor (EGF) receptors, are exposed to EGF, the total content of phosphotyrosine in cell protein is increased, the EGF receptor becomes phosphorylated at tyrosine, and new phosphotyrosine-containing 36,000- and 81,000-dalton proteins are detected. We examined the properties of A431 cells infected with Snyder-Theilen feline sarcoma virus, whose transforming protein has associated tyrosine protein kinase activity, and Harvey and Kirsten sarcoma viruses, whose transforming proteins do not. In all cases, the infected cells were more rounded and more capable of anchorage-independent growth than the uninfected cells. EGF receptors were assayed functionally by measuring EGF binding and structurally by metabolic labeling and immunoprecipitation. In no case did infection appear to alter the rate of EGF receptor synthesis, but infection reduced EGF receptor stability by about 50% for cloned Harvey sarcoma virus-infected cells and by 80% for cloned feline sarcoma virus-infected cells. The corresponding reductions in EGF binding were 70 and 90%, respectively. The proteins of feline sarcoma virus-infected A431 cells contained an increased amount of phosphotyrosine, and the 36,000- and 81,000-dalton phosphoproteins were detected. The EGF receptor was not detectably phosphorylated at tyrosine, however, unless the cells were exposed to EGF. The Harvey and Kirsten sarcoma virus-infected cells did not exhibit elevated levels of phosphotyrosine either in the total cell proteins or in the EGF receptor, nor were the 36,000- and 81,000-dalton proteins detectable. However, these phosphoproteins were found in the infected cells after EGF treatment. Thus, all of the infected A431 cells exhibited reduced EGF binding and increased degradation of EGF receptors, yet their patterns of protein phosphorylation were distinct from those of EGF-treated A431 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.

Incorporation of exogenous fatty acids into phospholipids by cultured hamster fibroblasts. Effect of SV40 transformation

In situ incorporation of two saturated (palmitic, 16:0; stearic, 18:0) and three unsaturated fatty acids (oleic, 18:1; linoleic, 18:2; arachidonic, 20:4) into the four major phospholipids, sphingomyelin, PC, PI and PE, was followed. Transformed cells incorporated unsaturated fatty acids more rapidly, whereas no significant differences were found concerning saturated fatty acids. In vitro determination of phospholipid acylation showed that incorporation of coenzyme A-activated forms of two saturated fatty acids (16:0 and 18:0) and one unsaturated fatty acid (18:1) into phospholipids was increased in transformed cells. Comparison of results obtained in situ and in vitro strongly suggests that incorporation of fatty acids into phospholipids in cultured cells is not limited by acyltransferase activities.

The increase in hormone-stimulated adenylate cyclase activity following Rous sarcoma virus transformation

Rous sarcoma virus (RSV)-infected chicken embryo cells were used to study the effect of viral transformation on the hormone-stimulated synthesis of cyclic AMP. Transformation by RSV greatly increased the cells' ability to synthesize and accumulate cyclic AMP in response to the beta-adrenergic agonist isoproterenol as compared to untransformed cells. This enhancement was observed in both intact cells and in membranes prepared from these cells. The inclusion of guanosine 5'-0-(3-thiotriphosphate), a nonhydrolyzable analogue of GTP, in assays of adenylate cyclase activity did not abolish the quantitative differences between the transformed and normal cell membranes. Infection of cells by Rous-associated virus, which lacks the oncogene src, did not induce this hyperresponsiveness thus indicating the probable involvement of the src gene product in this phenomenon. The duration of the isoproterenol-induced cyclic AMP elevation was longer in the transformed than in the untransformed cells; transformed cells, unlike untransformed cells, required at least 120 min before full desensitization became established. Membranes prepared from transformed cells specifically bound more than 5 times the quantity of the beta-adrenergic radiolabeled antagonist (-)3H-dihydroalprenolol and 125I-iodocyanopindolol compared to the untransformed cell membranes. Thus, it appears that major differences between the transformed and normal phenotypes reside in the concentration of membrane beta-adrenergic receptors and the inability of RSV-transformed cells to self-limit their response to specific external stimuli.

Proteins antigenically related to the human erythrocyte glucose transporter in normal and Rous sarcoma virus-transformed chicken embryo fibroblasts

Antibody raised against the purified human erythrocyte glucose transporter specifically precipitated four proteins from normal and Rous sarcoma virus-transformed chicken embryo cells: a major protein of Mr 41,000 and minor proteins of Mr 68,000, 73,000, and 82,000. The Mr 41,000 and 82,000 proteins were found only in a membrane fraction, not in the soluble fraction, and displayed a heterogeneous mobility on NaDodSO4/polyacrylamide gel electrophoresis, suggesting glycosylation. The Mr 41,000 and 82,000 proteins were increased in amount after malignant transformation in direct proportion to the increase in hexose transport rate, and the increase was dependent on the expression of the src gene product, as revealed with a temperature-conditional src mutant. We suggest that the Mr 41,000 and 82,000 proteins are the glucose transporter of chicken embryo fibroblasts, or a component of the glucose transporter. These experiments provide direct evidence that malignant transformation increases the rate of glucose transport by increasing the number of transporters in the membrane.