Beneficence versus respect for autonomy: an ethical dilemma in social work practice

Ethical dilemmas are an integral part of the practitioner's reality and impact upon the quality of care provided to the resident. The establishment of ethical principles aims to guide the worker through difficult scenarios involving morals, values and beliefs. Often, these precepts contradict one another and are strongly bound to the practitioner's and the resident's standpoints. The ethical principles of beneficence and respect for autonomy pose a conflict in judgment regarding an elderly woman's care in an 816-bed long term care facility. The contributing parties to the conflict are the medical staff, social worker and the resident. The ethical dilemma will be analyzed according to the utilitarian and deontological theories. Resolution to the conflict is offered at the conclusion.

Function of estrogen receptor tyrosine 537 in hormone binding, DNA binding, and transactivation

The human estrogen receptor (hER) is a ligand-activated transcription factor which, like many other members of the nuclear receptor protein family, exhibits a dimerization-dependent transcriptional activation. Several previous reports have provided evidence of the phosphorylation of the hER at tyrosine 537 (Y537). However, the exact function of a putative phosphorylation at this site remains controversial. Using a yeast transactivation assay, and in vitro biochemical approaches, we show that phosphorylation of tyrosine 537 is not required for the hER to bind hormone, or to activate transcription. An hER tyrosine 537 to phenylalanine (Y537F) mutant retains 70-75% of the transactivation potential of wild type hER in a yeast reporter system. Furthermore, the mutated receptor exhibits wild type hormone and DNA binding affinities. However, this mutation results in a decrease in receptor stability as measured by a decrease in the extent of hormone binding over time. The most striking difference between the wild type and Y537F hER is in the estradiol binding kinetics. Whereas the off-rate for estradiol exhibits a two-state binding mechanism, the Y537F mutant hER exhibits a monophasic estradiol off-rate. On the basis of these data and other reports describing the structure and activity of Y537 mutations, as well as knowledge of the three-dimensional structure of the hER ligand binding domain, we propose an alternate model wherein Y537F mutation favors an "open" pocket conformation, affecting the estrogen binding kinetics and stability of the hormone-bound, transcriptionally active "closed" pocket conformation. Although its phosphorylation is not essential for function of the hER, Y537 is nevertheless a critical residue intricately involved with the conformational changes of the hER and its ability to activate transcription.

Characterization of GLUT5 domains responsible for fructose transport

The domains responsible for the fructose specificity of GLUT5 were investigated by creating chimeras of GLUT5 with the selective glucose transporter GLUT3, which were expressed in Xenopus oocytes. 3-O-Methylglucose uptake of chimeric GLUT3-5 (M11; GLUT3 to the 11th transmembrane domain, GLUT5 to the carboxyl end) was similar to that of GLUT3, while fructose was not transported. Fructose uptake of chimeric GLUT5-3 (M3-5) to -5 (GLUT3 from the 3rd to 5th transmembrane domains, the rest GLUT5) was similar to that of GLUT5; no glucose was transported. Four chimeras transported neither fructose nor glucose: GLUT3-5 (M5; GLUT3 to the 5th transmembrane domain, GLUT5 to the carboxyl end), GLUT5-3 (M2; GLUT5 to the 2nd transmembrane domain, the rest GLUT3), GLUT5-3 (M3-11) to -5 (GLUT3 between the 3rd and 11th transmembrane domains, the rest GLUT5) and GLUT5-3 (M3-5) to -5-3 (M11; GLUT3 from the 3rd to 5th transmembrane domains and after the 11th transmembrane domain, the rest GLUT5). They, nevertheless, induced full-size proteins that were transported to the cell surface, as demonstrated by exofacial labeling with biotin. To conclude, the GLUT5 domain from the amino-terminus to the third transmembrane domain and that between the 5th and 11th transmembrane stretches seem to be necessary for fructose uptake.

Molecular and kinetic basis for the mixed agonist/antagonist activity of estriol

Estriol acts as a weak estrogen when administered in a single dose into immature or ovariectomized laboratory animals, but produces full estrogenic responses upon chronic administration. However, when estriol is injected together with estradiol it acts as an antiestrogen. We studied the dual agonist/antagonist properties of estriol, using recombinant human estrogen receptor (hER) in ligand-binding assay, cell-free transcription assay, electrophoretic mobility shift assay with cVitII estrogen response element (ERE), and ERE-Sepharose chromatography. We show that the weak estrogenic activity of estriol results from impaired hER-ERE interaction. The antiestrogenic activity of estriol was demonstrated in a cell-free transcription assay where it reduced estradiol-dependent transcription in a dose-dependent manner. Estriol interfered with estradiol-induced positive cooperative binding and receptor dimerization, and binding of hER complexes to ERE. These effects of estriol were maximal at a 10-fold molar excess over estradiol; under these conditions estradiol-dependent transcription was decreased by 85%, although [3H]estradiol binding was reduced by only 50%. We propose that when hER, estradiol, and estriol are coequilibrated, several receptor species are formed: unliganded hER monomers and dimers; estradiol-hER monomers and dimers, estriol-hER monomers and dimers; and presumably mixed estradiol-estriol dimers. Since estrogen-hER complexes bind cooperatively to ERE sequences, the concentrations of transcriptionally active complexes (estriol- and estradiol-hER dimers) are reduced to low levels that fail to bind cooperatively with ERE and initiate transcription. We discuss our results in relation to the massive estriol production during pregnancy and to the "Estriol Hypothesis" on the protective role for estriol in opposing carcinogenic effects of estradiol.

Expression of glucose transporter and glucose uptake in human oocytes and preimplantation embryos

The expression of glucose transporters 1, 2, 3 and 4 was evaluated in human oocytes and polyploid preimplantation embryos. Only glucose transporter 1 (GLUT-1) isoform was detected in oocytes and in 2-12-cell stage embryos. Glucose uptake was markedly increased in embryos as compared to oocytes (19.7 +/- 3.4 pmol/min/embryo and 2.3 +/- 0.3 pmol/min/oocyte), and GLUT-1 was inhibited by cytochalasin B. These results suggest that, although GLUT-1 is expressed in human oocytes and throughout preimplantation development, its function in mediating the rise in glucose uptake is triggered following fertilization.

Effects of glucocorticoid excess on the sensitivity of glucose transport and metabolism to insulin in rat skeletal muscle

GENBANK/dy examines the mechanisms of glucocorticoid-induced insulin resistance in rat soleus muscle. Glucocorticoid excess was induced by administration of dexamethasone to rats for 5 days. Dexamethasone decreased the sensitivity of 3-O-methylglucose transport, 2-deoxyglucose phosphorylation, glycogen synthesis and glucose oxidation to insulin. The total content of GLUT4 glucose transporters was not decreased by dexamethasone; however, the increase in these transporters in the plasma membrane in response to insulin (100 m-units/litre) was lessened. In contrast, the sensitivity of lactate formation to insulin was normal. The content of 2-deoxyglucose in the dexamethasone-treated muscle was decreased at 100 m-units/litre insulin, while the contents of glucose 6-phosphate and fructose 2,6-bisphosphate were normal at all concentrations of insulin studied. The maximal activity of hexokinase in the soleus muscle was not affected by dexamethasone; however, inhibition of this enzyme by glucose 6-phosphate was decreased. These results suggest the following. (1) Glucocorticoid excess causes insulin resistance in skeletal muscle by directly inhibiting the translocation of the GLUT4 glucose transporters to the plasma membrane in response to insulin; since the activity of hexokinase is not affected, the changes in the sensitivity of glucose phosphorylation to insulin seen under these conditions are secondary to those in glucose transport. (2) The sensitivity of glycogen synthesis and glucose oxidation to insulin is decreased, but that of glycolysis is not affected: a redistribution of glucose away from the pathway of glycogen synthesis and glucose oxidation could maintain a normal rate of lactate formation although the rate of glucose transport is decreased.

Substrate autoregulation of glucose transport: hexose 6-phosphate mediates the cellular distribution of glucose transporters

Exposure of rat skeletal muscle and skeletal muscle cell lines to high glucose levels results in a time- and dose-dependent reduction of the rate of hexose uptake, paralleled by a reduction in the plasma membrane density of glucose transporters. The mechanism of this process was investigated in cultured L8 myocytes. Low concentrations (0.5-2.0 mmol/l) of deoxyglucose mimicked the downregulatory action of 20 mmol/l glucose both regarding the time-course and magnitude of the effect, but in an irreversible manner. A dose-dependent relationship between intracellular accumulation of deoxyglucose 6-phosphate and the magnitude of the downregulatory response was observed. Depletion of intracellular deoxyglucose 6-phosphate restored the rate of hexose transport to the control level. The reduction of hexose transport activity by deoxyglucose occurred independently of ATP depletion which by itself produced the opposite effect. The effects of deoxyglucose and high glucose on hexose transport were associated with reduced transport maximal velocity and GLUT1 transporter abundance in the plasma membranes of myocytes, as assessed by cell surface biotinylation. The reduction of myocyte GLUT1 mRNA content, observed after exposure to high glucose, did not accompany the transport down regulatory action of deoxyglucose. We suggest that hexose 6-phosphate is the mediator of the downregulatory signal for subcellular redistribution of GLUT1 in L8 myocytes. The signal responsible for reducing the GLUT1 mRNA level may be related to glucose metabolites downstream of the hexokinase reaction.

Estradiol-binding mechanism and binding capacity of the human estrogen receptor is regulated by tyrosine phosphorylation

We have investigated the effects of tyrosine phosphorylation on the estradiol-binding mechanism and binding capacity of the human estrogen receptor (hER). The wild type hER and a point mutant form of the hER, in which tyrosine 537 was mutated to phenylalanine (Y537F hER), were expressed in Sf9 insect cells. The wild type hER, but not the Y537F hER, reacted with a anti-phosphotyrosine monoclonal antibody, indicating that tyrosine 537 was the only tyrosine phosphorylated on the hER. Scatchard and Hill analyses of the the binding interaction of [3H]estradiol with the wild type hER indicated that the addition of millimolar phosphotyrosine, but not tyrosine, phosphate, or phosphoserine, abolished the cooperative binding mechanism of the hER. These observations are consistent with the idea that phosphotyrosine blocks dimerization and site-site interactions between the hER monomers. The wild type hER bound 10-fold more [3H]estradiol than the Y537F hER. Treatment of the purified wild type hER with a tyrosine phosphatase decreased the binding capacity of the hER by approximately 90%, whereas, a serine/threonine phosphatase had no effect. The estrogen-binding capacity of the tyrosine-dephosphorylated hER was completely restored by rephosphorylation of tyrosine 537 with p60c-src, a tyrosine kinase. These results indicate that p60c-src can restore estrogen binding to the tyrosine-dephosphorylated hER and that dimerization and cooperative site-site interaction of the hER occur via a phosphotyrosine-binding interaction.

Skeletal muscle content of membrane glycoprotein PC-1 in obesity. Relationship to muscle glucose transport

Membrane glycoprotein PC-1, an inhibitor of insulin signaling, produces insulin resistance when overexpressed in cells transfected with PC-1 cDNA. In the present study, we determined whether PC-1 plays a role in the insulin resistance of skeletal muscle in obesity. Rectus abdominus muscle biopsies were taken from patients undergoing elective surgery. Subjects included both NIDDM patients (n = 14) and nondiabetic patients (n = 34) across a wide range of BMI values (19.5-90.1). Insulin-stimulated glucose transport was measured in incubated muscle strips, and PC-1 content, enzymatic activity, and insulin receptor content were measured in solubilized muscle extracts. Increasing BMI correlated with both an increase in the content of PC-1 in muscle (r = 0.55, P < 0.001) and a decrease in insulin stimulation of muscle glucose transport (r = -0.58, P = 0.008). NIDDM had no effect on either PC-1 content or glucose transport for any given level of obesity. Insulin stimulation of muscle glucose transport was negatively related to muscle PC-1 content (r = -0.68, P = 0.001) and positively related to insulin receptor content (r = 0.60, P = 0.005). Multivariate analysis indicated that both skeletal muscle PC-1 content and insulin receptor content, but not BMI, were independent predictors of insulin-stimulated glucose transport. Muscle PC-1 content accounted for 42% and insulin receptor content for 17% of the variance in glucose transport values. These studies raise the possibility that increased expression of PC-1 and a decreased insulin receptor content in skeletal muscle may be involved in the insulin resistance of obesity.

Regulation by metformin of the hexose transport system in vascular endothelial and smooth muscle cells

1. The effect of the biguanide metformin on hexose transport activity was studied in bovine cultured aortic endothelial (BEC) and smooth muscle cells (BSMC). 2. Metformin elevated the rate of hexose transport determined with 2-deoxyglucose (2DG) in a dose- and time-dependent manner in both cell types. Similar ED50 values (0.8-1.0 mM) were determined for the effect of metformin on 2DG uptake in both BEC and BSMC following 24 h exposure to increasing concentrations of metformin, with maximal stimulation at 2 mM. 3. In BEC, metformin increased the hexose transport rate 2-3 fold at all glucose concentrations tested (3.3-22.2 mM). In BSMC incubated with 22.2 mM glucose, metformin elevated the hexose transport approximately 2 fold. The drug was also effective at lower glucose levels, but did not exceed the maximal transport rate observed in glucose-deprived cells. 4. Similar results were obtained when the effect of metformin on hexose transport activity was assessed with the non-metabolizable hexose analogue, 3-O-methylglucose, suggesting that the drug affects primarily the rate of hexose transport rather than its subsequent phosphorylation. 5. The metformin-induced increase in hexose transport in BSMC treated for 24 h with the drug correlated with increased abundance of GLUT1 protein in the plasma membrane, as determined by Western blot analysis. 6. These data indicate that in addition to its known effects on hexose metabolism in insulin responsive tissues, metformin also affects the hexose transport system in vascular cells. This may contribute to its blood glucose lowering capacity in patients with Type 2, non-insulin-dependent diabetes mellitus.

Kinetic analysis of the interaction of human estrogen receptor with an estrogen response element

The kinetics of the interaction between recombinant human estrogen receptor and chicken vitellogenin gene II estrogen response element (ERE) were determined by ERE-Sepharose chromatography. The association constant of the interaction between the ERE and the human estrogen receptor was dependent on receptor concentration, estradiol binding and temperature. The highest association constant (80-100 x 10(6)M-1) was measured for the estradiol-bound receptor prepared at 25 degrees C and at concentrations higher than 7 nM. At high receptor concentrations (>7 nM) the binding mechanism of estradiol to the receptor was positive cooperative, indicating receptor homodimerization. At lower concentrations the binding mechanism was partially cooperative and the association constant of the liganded receptor was significantly lower. The binding mechanism at 4 degrees C was cooperative as well, and the association constants were similarly dependent upon receptor concentration, but were 50% lower than the receptor prepared at 25 degrees C. The association constant of the unliganded receptor was 4- to 5-fold lower than that of the liganded receptor at 25 degrees C. These data suggest that in addition to estradiol-induced conformational changes in the receptor, the receptor dimers are subjected to temperature-dependent changes, which further increase their affinity for an ERE.

Membrane glycoprotein PC-1 and insulin resistance in non-insulin-dependent diabetes mellitus

Most patients with non-insulin-dependent diabetes mellitus are resistant to both endogenous and exogenous insulin. Insulin resistance precedes the onset of this disease, suggesting that it may be an initial abnormality. Insulin-receptor kinase activity is impaired in muscle, fibroblasts and other tissues of many patients with non-insulin-dependent diabetes mellitus, but abnormalities in the insulin-receptor gene do not appear to be the cause of this decreased kinase activity. Skin fibroblasts from certain insulin-resistant patients contain an inhibitor of insulin-receptor tyrosine kinase. Here we show that this inhibitor is a membrane glycoprotein, termed PC-1 (refs 10, 11). We find that PC-1 activity is increased in fibroblasts from seven of nine patients with typical non-insulin-dependent diabetes mellitus. In addition, overexpression of PC-1 in transfected cultured cells reduces insulin-stimulated tyrosine kinase activity. These studies raise the possibility that PC-1 has a role in the insulin resistance of non-insulin-dependent diabetes mellitus.

Insulin activates furosemide-sensitive K+ and Cl- uptake system in BC3H1 cells

Insulin augments the activity of Na(+)-K(+)-adenosinetriphosphatase (ATPase) in skeletal muscles. This study shows that when furosemide- and bumetanide-inhibitable 86Rb+ uptake is measured in the skeletal muscle-like BC3H1 cell line, insulin and insulin-like growth factor I (IGF-I) activate a loop diuretic-sensitive K+ and Cl- transport system but have no effect on Na(+)-K(+)-ATPase. The insulin-stimulated K+ transport system is extracellular Na+ concentration ([Na+]o) independent and extracellular Cl- concentration ([Cl-]o) dependent. Na(+)-independent K(+)-Cl- cotransport systems have been identified in other cells, but their sensitivity to insulin or growth factors has not been described. The affinities of the insulin-stimulated K+ uptake in BC3H1 cells for K+ (0.9 +/- 0.1 mM) and loop diuretics (5.9 x 10(-7) and 10(-7) M for furosemide and bumetanide, respectively) are higher than those of K(+)-Cl- cotransporters in other cells. Thus the insulin-stimulated K+ and Cl- transport system in BC3H1 seems kinetically different from K(+)-Cl- cotransporters in other cells. Insulin and IGF-I may activate a unique K(+)-Cl- cotransporter or activate a [Na+]o-independent K(+)-Cl- cotransport mode of Na(+)-K(+)-Cl- cotransporter in BC3H1 cells.

Differential regulation of glucose transport and transporters by glucose in vascular endothelial and smooth muscle cells

Hyperglycemia has been implicated in the pathogenesis of both micro- and macrovascular complications in diabetes. Little is known, however, about glucose transporters and their regulation in the vascular system. In this study, the regulation of glucose transporters by glucose was examined in cultured BAECs and BSMCs, and in human arterial smooth muscle cells. Both BAECs and BSMCs transported glucose via the facilitated diffusion transport system. Glucose-transport activity in vascular smooth muscle cells was inversely and reversibly regulated by glucose. Exposure of BSMCs and HSMCs to high glucose decreased Vmax for 2DG and 3-O-MG uptake, whereas Km remained unchanged. The hexose-transport system of BAECs exhibited lower 2DG and 3-O-MG uptake compared with BSMCs and showed little or no adaptation to changes in ambient glucose. Northern blot analysis demonstrated that GLUT1 mRNA levels in BAECs and BSMCs were unaffected by the concentration of glucose in the medium. GLUT2-5 mRNA could not be detected by Northern blot analysis. GLUT1 protein, quantified by Western blot analysis, was more abundant in BSMCs than in BAECs and was decreased by approximately 50% when medium glucose was elevated from 1.2 to 22 mM for 24 h. The alterations in the level of GLUT1 protein correlated with the changes observed in transport activity. These observations suggest differential regulation of glucose transporter in response to glucose between smooth muscle and endothelial cells. The sites of autoregulation may involve translational control and/or the stability of the protein in the smooth muscle cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose regulates its transport in L8 myocytes by modulating cellular trafficking of the transporter GLUT-1

The effect of culture conditions simulating hypo- and hyper-glycaemia on glucose transport and on the subcellular localization of the glucose transporter GLUT-1 was studied in L8 myocytes. Incubation of the cells with 20 mM-glucose for 25 h decreased the rate of 2-deoxy-D-[3H]glucose (dGlc) uptake to 0.106 +/- 0.016 nmol/min per 10(6) cells compared with 0.212 +/- 0.025 in cells maintained at 2 mM-glucose (final glucose concentrations at the end of the incubation period were 16-17 mM and 0.7-1.0 mM respectively). An additional 5 h incubation of these cells with medium containing the opposite glucose concentration (i.e. change from 17 mM to 1 mM and from 1 mM to 17 mM) increased the transport rate to 0.172 +/- 0.033 nmol/min per 10(6) cells in cultures initially conditioned at high glucose, and decreased the transport to 0.125 +/- 0.029 in those conditioned at low glucose. Plasma-membrane- and microsomal-membrane-enriched fractions were prepared from these cells for [3H]cytochalasin B (CB) binding and Western-blot analysis with antibodies against GLUT-1 and GLUT-4. A decrease in glucose concentration increased the number of D-glucose-displaceable CB-binding sites and GLUT-1 protein in the plasma-membrane fraction to the same extent as the increase in dGlc transport. Under downregulatory conditions, the lower dGlc-transport capacity could be accounted for by a decreased number of transporters in the plasma membrane of the cells. No apparent modification of the intrinsic activity of the glucose transporters was observed in up- or down-regulated cells. Under downregulatory conditions, the CB-binding data indicated a large increase in the number of transporters in the intracellular membranes of the myocytes. Western blots of the same membranes also indicated an increase in GLUT-1 content. However, the interaction of the intracellular GLUT-1 protein with the polyclonal antibodies was much weaker than that of the plasma-membrane-associated GLUT-1. The GLUT-4 concentration was too low to permit quantification in membrane fractions. Our findings suggest that autoregulation of glucose transport in L8 myocytes is accompanied by parallel changes in the number of GLUT-1 transporters in the plasma membrane, and that the rate of transporter degradation may be augmented in the upregulated myocytes. These glucose-induced changes are fully reversible.

The binding of estrogen and estrogen antagonists to the estrogen receptor

The model of the estrogen receptor as a dimer of identical, interacting subunits and data obtained by Sasson and Notides (1988, Mol. Endocrinol. 2, 307-312) were used to find the standard free energy changes that describe the binding of estradiol and 4-hydroxytamoxifen to the estrogen receptor. For the binding of estradiol or 4-hydroxytamoxifen to the estrogen receptor the data do not deviate systematically from the best fit to the model. The standard free energy change for binding of one molecule of estradiol at one site and one molecule of 4-hydroxytamoxifen at the second site of estrogen receptor indicates that 4-hydroxytamoxifen antagonizes the binding of estradiol to the estrogen receptor.

Mechanism of insulin-induced activation of Na(+)-K(+)-ATPase in isolated rat soleus muscle

Insulin augments Na(+)-K(+)-ATPase activity in skeletal muscles. It has been proposed that the sequence of events is activation of Na(+)-H+ antiporter, increased intracellular Na+ concentration ( [Na+]i), and stimulation of Na(+)-K+ pump. We have used isolated rat soleus muscles to test this hypothesis. Insulin increased the ouabain-suppressible K+ uptake in a dose- and time-dependent manner. The maximal effect was observed at 50-100 mU/ml insulin. Stimulation of K+ uptake was accompanied by increased specific [3H]ouabain binding and lowered [Na+]i. The ionophore monensin, which promotes Na(+)-H+ exchange, also increased the rate of ouabain-suppressible K+ uptake in soleus muscle, with a maximal effect obtained at 10-100 microM ionophore. However, this increase was accompanied by an elevation of [Na+]i. In the presence of 10-100 microM monensin, addition of 100 mU/ml insulin further increased K+ uptake but reduced [Na+]i. The effect on K+ uptake was additive. Ouabain (10(-3) M) completely suppressed the effect of insulin on [Na+]i. Insulin had no effect on the magnitude or the time course of insulin stimulation of K+ uptake. Thus equal stimulation of Na(+)-K(+)-ATPase by insulin was observed when [Na+]i was elevated (under monensin) or lowered (under amiloride). These data suggest that activation of Na(+)-K(+)-ATPase in soleus muscle by insulin is not secondary to stimulation of Na(+)-H+ antiporter.

The ubiquitous glucose transporter GLUT-1 belongs to the glucose-regulated protein family of stress-inducible proteins

In mammals, glucose transport is mediated by five structurally related glucose transporters that show a characteristic cell-specific expression. However, the rat brain/HepG2/erythrocyte-type glucose transporter GLUT-1 is expressed at low levels in most cells. The reason for this coexpression is not clear. GLUT-1 is negatively regulated by glucose. Another family of proteins, glucose-regulated proteins (GRPs), is also ubiquitously expressed and stimulated by glucose deprivation and other cellular stresses. We therefore hypothesized that GLUT-1 may be a glucose-regulated stress protein. This was tested by subjecting L8 myocytes and NIH 3T3 fibroblasts to glucose starvation or exposure to the calcium ionophore A23187, 2-mercaptoethanol, or tunicamycin, all known to increase GRP levels. The mRNA for GLUT-1 was augmented by 50-300% in a time-dependent manner, similarly to the changes in GRP-78 mRNA. Ex vivo incubation of rat soleus muscles induced a marked and concomitant rise in the mRNA levels of GLUT-1 and GRP-78. Finally, calcium ionophore A23187 and 2-mercaptoethanol induced a 2- to 3-fold increase in the levels of the GLUT-1 protein and hexose uptake. In all instances in which GRP-78 and GLUT-1 responded to stress, the transcription of the cell-specific muscle/adipocyte-type insulin-responsive glucose transporter (GLUT-4) did not change. Thus, despite the lack of structural similarity, GLUT-1 and GRP-78 expression is regulated similarly, whereas the regulation of GLUT-4, which is structurally related to GLUT-1, is different. We propose that GLUT-1 belongs to the GRP family of stress proteins and that its ubiquitous expression may serve a specific purpose during cellular stress.

Equilibrium binding analysis of estrogen agonists and antagonists: relation to the activation of the estrogen receptor

The equilibrium binding kinetics of the interaction between the estrogen receptor and natural estrogens (estradiol, estriol and estrone), non-steroidal estrogen agonists (11 beta-chloromethyl-estradiol-17 beta, diethyl-stilbestrol, hexestrol) and non-steroidal antiestrogens (clomiphene, tamoxifen) have been characterized. It is proposed that positive cooperative binding of ligands by the estrogen receptor reflects conformational changes in the DNA binding domain of the receptor dimer which increase its affinity to estrogen responsive elements. Weak estrogens fail to induce maximal cooperativity and are less efficient in activating the receptor complex. Antiestrogens, that inhibit the [3H]estradiol-induced cooperative binding, suppress the activation of the receptor and inhibit its nuclear interactions. Another class of antiestrogens (e.g., 4-hydroxytamoxifen) interacts with the receptor in a manner that is indistinguishable from the cooperative interaction of estradiol, and the resulting complex may also exhibit increased affinity for estrogen responsive elements. However, these complexes cannot activate transcription, presumably due to an aberrant induction of transcription-activating domain in the receptor. We suggest that the positive cooperativity of the estrogen receptor results from conformational changes in the receptor that are transmitted also to the DNA binding domain. On the other hand, conformational changes in the transcription activating domain are not revealed by equilibrium binding kinetics. Thus, compounds that block the positive cooperative binding of [3H]estradiol by the receptor act as antiestrogens. Other compounds that interact cooperatively with the receptor can activate the receptor DNA binding domain, however, they may or may not induce the full array of conformational changes required for transactivation of transcription.

Regulation of hexose transport in L8 myocytes by glucose: possible sites of interaction

Previous work demonstrated that glucose controls its own transport rate in rat skeletal muscle: exposure to high glucose levels down-regulates muscle hexose transport, while glucose withdrawal results in elevated transport rates (J. Biol. Chem. 261:16827-16833, 1986). The present study investigates the mechanism of this autoregulatory system. Preincubation of L8 myocytes at 16 mM glucose reduced subsequent 2-deoxy-D-glucose (dGlc) uptake by 40% within 3 h. Cycloheximide (1 microM) mimicked the action of glucose; the effects of glucose and cycloheximide were not additive. At 50 microM, cycloheximide prevented the modulations of glucose transport induced by exposure of muscle cells to high or low glucose concentrations. Inhibition of glycosylation with tunicamycin A1 reduced the basal dGlc uptake, but did not prevent its up-regulation following glucose withdrawal. Inhibition of RNA synthesis by actinomycin D prevented the down-regulatory effect of glucose. These results indicate that continuous protein synthesis and protein glycosylation are required for the maintenance of the steady-state dGlc uptake. We suggest that glucose exerts its autoregulatory effect on hexose transport by modifying the incorporation of active glucose transporters into the plasma membrane rather than changing their rate of degradation. It is hypothesized that this effect is mediated by a non-glycosylated protein involved in the translocation or activation of glucose transporters.

Failure of fenfluramine to affect basal and insulin-stimulated hexose transport in rat skeletal muscle

Fenfluramine is an effective appetite suppressant that mediates its action via serotoninergic neurons. We studied the effect of pure d- and l-fenfluramine on in vitro hexose transport in isolated rat soleus muscles and skeletal muscle cells in culture. We found no evidence to suggest that the fenfluramine enantiomers affect the basal transport activity. Furthermore, the drugs did not interfere with the ability of glucose to regulate its own transport. Muscle responsiveness to insulin was not altered by the enantiomers, nor did insulin unmask any effect of fenfluramine on muscle hexose transport. These conclusions are based on experiments performed with a wide concentration range of drug and insulin, from the therapeutic to suprapharmacological levels. We discuss our results in view of published data on the effects of fenfluramine on peripheral glucose metabolism.