Current status of the thiol redox model for the regulation of hexose transport by insulin

Looking Back at the Early Stages of Redox Biology

The beginnings of redox biology are recalled with special emphasis on formation, metabolism and function of reactive oxygen and nitrogen species in mammalian systems. The review covers the early history of heme peroxidases and the metabolism of hydrogen peroxide, the discovery of selenium as integral part of glutathione peroxidases, which expanded the scope of the field to other hydroperoxides including lipid hydroperoxides, the discovery of superoxide dismutases and superoxide radicals in biological systems and their role in host defense, tissue damage, metabolic regulation and signaling, the identification of the endothelial-derived relaxing factor as the nitrogen monoxide radical (more commonly named nitric oxide) and its physiological and pathological implications. The article highlights the perception of hydrogen peroxide and other hydroperoxides as signaling molecules, which marks the beginning of the flourishing fields of redox regulation and redox signaling. Final comments describe the development of the redox language. In the 18th and 19th century, it was highly individualized and hard to translate into modern terminology. In the 20th century, the redox language co-developed with the chemical terminology and became clearer. More recently, the introduction and inflationary use of poorly defined terms has unfortunately impaired the understanding of redox events in biological systems.

Redox signal transduction: reductive reasoning

The activation of receptors has been believed to be due to a conformational change that occurs when the agonist "locks" into the receptor. However, evidence suggests that several receptors are activated by redox reactions, which occur when an agonist binds with the receptor. The stereochemistry of the receptor likely provides specificity to the electron transfer by determining which agonist can bind to the receptor. The resulting signal, in some cases, may then be transferred across the membrane by G-proteins, which also are redox-coupled. This concept puts receptors into the large group of cell functions that are redox-regulated. Other systems for which evidence of redox regulation occurs include ion pumps and channels, as well as transcription factors.

Regulation of glucose uptake by stressed cells

Lactate production by BHK cells is stimulated by arsenite, azide, or by infection with Semliki Forest virus (SFV). In the case of arsenite or SFV infection, the increase correlates approximately with the increase in glucose transport as measured by uptake of [3H] deoxy glucose (dGlc); in the case of azide, the increase in lactate production exceeds that of glucose transport. Hence glucose utilization by BHK cells and its stimulation by anaerobic and other types of cellular stress is controlled at least in part at the level of glucose transport. The glucose uptake by BHK cells is also stimulated by serum and by glucose deprivation. In these circumstances, as with arsenite, stimulation is reversible, with t1/2 of 1-2 hours; stimulation is compatible with a translocation of the glucose transporter protein between an intracellular site and the plasma membrane (shown here for serum and previously for arsenite). The surface binding and rate of internalization of [125I]-labelled transferrin and [125I] alpha 2-macroglobulin was studied to determine whether changes in glucose transport are accompanied by changes in the surface concentration or rate of internalization of membrane proteins. The findings indicate that changes in glucose transport do not reflect a consistent and general redistribution of membrane receptors. Taken together, the results are compatible with the proposal that BHK cells exposed to stimuli like insulin or serum, or to stresses like arsenite, azide, SFV infection, or deprivation of glucose, respond in the same manner: namely, by an increased capacity to transport glucose brought about by reversible and specific translocation of the transporter protein from an (inactive) intracellular site to the plasma membrane.

Enhanced electron transfer by GTP: cross-membrane electron signaling by G-proteins?

Activation of several receptor types is followed by their binding to a G-protein. Prior to transmission of the agonist signal, the G-protein which had affinity for guanosine 5-diphosphate (GDP) binds guanosine 5-triphosphate (GTP) instead. Because evidence exists that several agonist groups activate their receptors by reduction, we evaluated whether the nucleotide associated with G-proteins could enhance electron flow. Using a model system of ferrous iron and ferric cytochrome c, it was determined that substitution of GTP for GDP led to an enhanced reduction of ferric cytochrome c. These results support the concept that cellular activation by certain receptors may involve reductive activation with the participation of GTP and G-proteins. We speculate that GTP, when bound to G-protein, can facilitate electron transfer perhaps from the receptor or the G-protein to the catalytic subunit of the adenylate cyclase enzyme.

Thiol redox and phosphate transport in renal brush-border membrane. Effect of nicotinamide

In the present study, the effect of thiol redox and its possible role in the inhibitory effect of nicotinamide on renal brush-border membrane (BBM) phosphate uptake was examined. Addition of thiol reducing agent, dithiothreitol (DTT, 5 mM), caused an increase, while addition of thiol oxidant, diamide (DM, 5 mM) caused a reversible decrease in sodium-dependent BBM phosphate uptake. Kinetic analyses revealed an increase in both Vmax and Km by DTT, and a decrease in Vmax by DM. These results suggest that thiol redox influences BBM phosphate uptake with sulfhydryl (SH) groups relate to its capacity and disulfide (SS) groups to its affinity for phosphate. Since changes in cytosolic NAD levels may affect BBM thiol redox through changes in redox states of NADP and glutathione systems, we have examined such possibility by studying the effect of nicotinamide (NM). Incubation of proximal tubules with NM (10 mM) induced an oxidative effect on redox states of cytosolic NAD, NADP systems as inferred from decreased cellular lactate/pyruvate, malate/pyruvate, respectively. Measurements of cytosolic glutathiones and BBM thiols also revealed that NM pretreatment shifted the cytosolic glutathione redox (GSH/GSSG) and BBM thiol redox (SH/SS) toward more oxidized state. On the other hand, incubation of proximal tubules with NM suppressed phosphate uptake by the subsequently isolated BBM vesicles. The lower phosphate uptake by NM-pretreated BBM vesicles was reversed by DTT and was resistant to the inhibitory effect of DM. These results thus suggest that BBM thiol oxidation may be involved in the inhibitory effect of NM on BBM phosphate uptake.

Evidence for a novel thioredoxin-like catalytic property of gonadotropic hormones

It has been proposed that dithiol-disulfide interchange and oxidation-reduction reactions may play a role in hormone-induced receptor activation. Inspection of the sequences of the gonadotropic hormones revealed a homologous tetrapeptide (Cys-Gly-Pro-Cys) between the beta subunit of lutropin (LH) and the active site of thioredoxin (TD). The beta subunit of follitropin (FSH) has a similar sequence (Cys-Gly-Lys-Cys). Thioredoxin is a ubiquitous protein serving as an electron donor for ribonucleotide reductase, but it also exhibits disulfide isomerase activity. The catalytic activity of TD was assayed by its ability to reactivate reduced and denatured ribonuclease. In this assay, the purified ovine FSH and bovine LH preparations tested were approximately 60 and approximately 300 times, respectively, as active as TD on a molar basis. This heretofore unsuspected catalytic property of FSH and LH may be important in understanding their mechanism of receptor activation and signal transduction.

The effects of sulfhydryl modifying reagents on nonhormonal and hormonally regulated hexose transport in cultured human skin fibroblasts

The effects of various sulfhydryl modifying reagents on hexose transport in cultured human skin fibroblasts were studied. H2O2 was observed to have no effect on 2-deoxy-D-glucose transport in serum-starved glucose-fed cells. The elevation of hexose transport rates in cells by glucose deprivation, insulin, or serum stimulation rendered them sensitive to H2O2. Hexose transport in glucose-deprived cells was inhibited 51-55% by 1-2 mM H2O2, while hexose transport in insulin or serum-stimulated glucose-fed cells was inhibited 45% and 46%, respectively. H2O2 inhibition was blocked or reversed by 8 mM dithiothreitol. N-ethyl-maleimide (NEM), a permeant, sulfhydryl reagent, elicited effects on hexose transport similar to those effected by H2O2 (i.e., in glucose-deprived and insulin-stimulated cells, inhibition of hexose transport was 44% and 23%, respectively). Impermeant sulfhydryl reagents such as dithio(bis)nitrobenzoic acid (DTNB) and N-iodoacetyl-N'-(5-sulfo-1-naphthly-ethylenediame (1,5,-I-AEDANS) had no inhibitory effect on hexose transport under any conditions (i.e., glucose-fed, glucose-deprived, and insulin-stimulated cells). DTNB and 1,5-I-AEDANS afforded no protection from the action of H2O2 on hexose transport. The data suggest that the sensitive sites are thiol in nature and are located at an intramembrane or intracellular site and probably not exofacial.

Multiple effects of sulphydryl reagents on sugar transport by rat soleus muscle

Iodoacetate, over the range 0.2-2 mM, stimulated the uptake of D-xylose by rat soleus muscle and inhibited anaerobic lactate production by soleus muscle. Stimulation of sugar transport is considered to be due to the resultant fall in ATP. p-Chloromercuribenzene sulphonate (0.5-2 mM) stimulated xylose uptake to a lesser extent than iodoacetate and induced a proportionately smaller fall in ATP, consistent with the inhibitory effect of p-chloromercuribenzene sulphonate on lactate production. Under certain conditions, p-chloromercuribenzene sulphonate stimulated sugar transport without affecting the ATP level. This suggests that whereas p-chloromercuribenzene sulphonate can be expected to stimulate sugar transport through the lowering of muscle ATP, it may also act through some other mechanism. No stimulatory effect on xylose uptake was observed when muscles were exposed to N-ethylmaleimide (0.02-2 mM) either for brief (1 min) or more prolonged (30 min) periods. Because N-ethylmaleimide induced a marked fall in muscle ATP, it is surprising that N-ethylmaleimide did not stimulate sugar transport; in most experiments this inhibitor actually inhibited sugar transport. N-Ethylmaleimide inhibited the stimulation of sugar transport by 2,4-dinitrophenol and anoxia; this inhibitory effect appears to explain why N-ethylmaleimide itself did not stimulate sugar transport. p-Chloromercuribenzene sulphonate also inhibited 2,4-dinitrophenol-stimulated xylose uptake by a mechanism which seems similar to that of N-ethylmaleimide; this could explain in part the modest stimulatory effect of this inhibitor on muscle sugar transport.

Stimulation of sugar transport in rat soleus muscle by prolonged cooling at 0 degrees C

The uptake of D-xylose by isolated rat soleus muscle (measured at 37 degrees C) was stimulated by prolonged cooling at 0 degrees C. The effect of cooling reached a maximum value after 3 h and was reversed on rewarming; reversal was temperature-dependent. Cooling stimulated xylose uptake sub-maximally compared with the effect of insulin (100 U/l). Xylose uptake in cooled muscle was further stimulated by insulin, but not by anoxia. The effect of cooling and its reversal were still demonstrable in the presence of ouabain (1 mmol/l), or when unidirectional efflux of calcium and magnesium from the muscle was induced by EDTA (5 mmol/l). The ionophore, A23187 (2.5 mg/l), depressed the effect of cooling in the presence of EDTA but not in the presence of EGTA. It is concluded that cooling disrupts and intracellular magnesium-p]ump and that muscle sugar transport is consequentially stimulated through an increase in cytoplasmic magnesium.

Effects of hyperosmolarity on basal and insulin-stimulated muscle sugar transport

Xylose uptake was stimulated rapidly when rat soleus muscles were incubated in Krebs-bicarbonate medium made hyperosmolar by the addition of 200 mM mannitol. The stimulatory effect of hyperosmolarity increased as the concentration of mannitol was raised from 50 to 200 mM; above 200 mM, the effect tended to decline. Mannitol stimulated sugar transport submaximally compared with the effect of insulin. The stimulatory effect of hyperosmolarity persisted over a 60-min incubation period, but could be reversed by transferring the muscles to an isotonic incubation medium. Xylose uptake measured in the presence of insulin (0.1 U/ml) was depressed when muscles were exposed to concentrations of mannitol greater than 100 mM. This effect was not reversible; xylose uptake remained depressed when these muscles were transferred to an isotonic medium. Hyperosmolarity also depressed the binding of 125I-insulin irreversibly. These inhibitory effects of hyperosmolarity were associated with the lowering of muscle ATP. This effect on ATP provides an explanation for the inhibitory effects of hyperosmolarity on insulin-stimulated sugar transport and on insulin binding, in terms of the ATP-dependence of these two processes.

Cell division in normal and transformed cells: the possible role of superoxide and hydrogen peroxide

A unified theory of cell division is presented. This theory explains the division of both tumor cells and normal cells and involves superoxide radicals, hydrogen peroxide, glucose, cAMP, and cGMP.

Production of insulinomimetic antibodies against rat adipocyte membranes by hybridoma cells

SJL mice were injected intraperitoneally with adipocyte plasma membranes or with intrinsic membrane proteins obtained by extraction of plasma membranes with dimethylmaleic anhydride. Three days after the boost injection, the spleens were removed and fused with NS-a, a thioguanine-resistant myeloma cell line derived from P3X63 Ag8 (Balb/c). Following selection for hybrids with hypoxanthine, aminopterin, and thymidine, medium of the hybrid cells was tested for its ability to bind to the plasma membrane of the adipocyte and to stimulate the oxidation of D-(1-14C) glucose to 14CO2. Approximately 40% of the wells containing hybridomas derived from splenocytes of SJL mice immunized with plasma membranes produced immunoglobulin that bound to adipocyte plasma membranes. About 30% of these mimicked the ability of insulin to stimulate the oxidation of D-(1-14C) glucose to 14CO2 in adipocytes. Media from 51% of the wells containing hybridomas derived from splenocytes of SJL mice immunized with intrinsic membrane proteins produced immunoglobulin that bound to the plasma membrane and 48% of those stimulated glucose oxidation. The bioactivity of the hybrid cell media could be blocked by adsorption with intrinsic membrane proteins or by the removal of immunoglobulins using formalin-fixed Staphylococcus aureus. The hybrids generated in this study can be divided into three categories: 1) hybrids that secrete antibodies that can bind to plasma membranes and mimic insulin action of glucose transport; 2) hybrids that secrete antibodies that bind to plasma membranes but do not stimulate the oxidation of D-(1-14C) glucose to 14CO2; and 3) hybrids that produce no antimembrane antibodies. The data suggest that interaction of immunoglobulins with specific membrane proteins is essential in mimicking the action of insulin on glucose transport and oxidation in the rat adipocyte.

Effects of diet upon hypothalamic vulnerability to goldthioglucose in male mice

Three groups of male mice were fed three different diets for about 3 weeks: powdered chow (C), a high fat (F) diet (chow:Crisco, 2:1), and a high dextrose (D) diet (chow:dextrose, 2:1). Food and water intake and body weights (BWts) were measured daily. On Day 20 of the study, mice were injected IP with 0.3 mg/g of goldthioglucose (GTG). Two days later, the mice were sacrificed and brains were removed for histological examination of GTG-induced lesions in the ventromedial hypothalamus (VMH). It was found that (1) F-fed mice ate more and gained more BWt than C- and D-fed mice, and (2) the average GTG lesion volume of F-fed mice was twice as large as those of C- and D-fed mice. In view of known alterations in sensitivity of the VMH to goldthioglucose with changes in glucose metabolism, it is suggested that the larger goldthioglucose induced lesions in F-fed mice may be due to elevated blood levels of goldthioglucose or altered glucose metabolism in goldthioglucose sensitivity cells in the VMH.

Effects in adipocytes of diamide on GSH levels, glucose uptake and cell integrity

Concentrations of insulin and chemical agents (H2O2, vitamine K-5) which stimulate hexose transport in fat cells do not alter the cellular levels of glutathione (reduced form; GSH). Diamide, another agent used in studies of insulin action, markedly reduces GSH levels and increases the movement of sugar into the cell. However, unlike insulin, H2O2 or vitamin K-5, diamide causes a change in the permeability of fat cells that allows entry of compounds (insulin, sucrose, L-glucose) which are normally excluded by the plasma membrane. Moreover, the accelerated rate of methylglucose uptake produced by diamide treatment is not inhibited by cytochalasin B, an agent that blocks basal and insulin-stimulated methylglucose transport. These results indicate that diamide does not cause a stimulation of the glucose transport system and should not be used (or used with caution) in transport studies. Furthermore, oxidation of GSH does not appear to be necessary for the stimulation of hexose transport in adipocytes by insulin, H2O2 or vitamin K-5.

On the stimulation of rat thymocyte 3-O-methyl-glucose transport by mitogenic stimuli

Pretreatment of rat thymus lymphocytes with N-ethyl-maleimide prevents the stimulation of 3-O-methyl-glucose transport by concanavalin A or ionophore A23187 but does not affect the ability of concanavalin A to induce a rapid increase in cellular Ca2+take. N-ethyl-maleikide added after concanavalin A amplifies rather than prevents the subsequent stimulation of 3-O-methyl-glucose transport. Incubation of thymocytes with concanavalin A produces a decrease of 43% in the apparent Ki for phloretin, a competitive inhibitor of 3-O-methyl-glucose transport, without affecting the apparent Km for the substrate. Similarly, very low concentrations of cytochalasin B inhibit concanavalin A-stimulated glucose transport preferentially, without markedly affecting the unstimulated transport rates. The similarity between concanavalin A-stimulated 3-O-methyl-glucose transport in thymocytes and insulin-stimulated glucose transport in adipose tissue, with particular emphasis on the effects of the modifying agents described in this paper, is discussed.

Cytochalasin B inhibition and temperature dependence of 3-O-methylglucose transport in fat cells

The transport of 3-O-methylglucose in white fat cells was measured under equilibrium exchange conditions at 3-O-methylglucose concentrations up to 50 mM with a previously described method (Vinten, J., Gliemann, J. and Osterlind, K. (1976) J. Biol. Chem. 251, 794--800). Under these conditions the main part of the transport was inhibitable by cytochalasin B. The inhibition was found to be of competitive type with an inhibition constant of about 2.5 . 10(-7) M, both in the absence and in the presence of insulin (1 micrometer). The cytochalasin B-insensitive part of the 3-O-methylglucose permeability was about 2 . 10(-9) cm . s-1, and was not affected by insulin. As calculated from the maximum transport capacity, the half saturation constant and the volume/surface ratio, the maximum permeability of the fat cell membrane to 3-O-methylglucose at 37 degrees C and in the presence of insulin was 4.3 . 10(-6) cm . s-1. From the temperature dependence of the maximum transport capacity in the interval 18--37 degrees C and in the presence of insulin, an Arrhenius activation energy of 14.8 +/- 0.44 kcal/mol was found. The corresponding value was 13.9 +/- 0.89 in the absence of insulin. The half saturating concentration of 3-O-methylglucose was about 6 mM in the temperature interval used, and it was not affected by insulin, although this hormone increased the maximum transport capacity about ten-fold to 1.7 mmol . s-1 per 1 intracellular water at 37 degrees C.

Resistance of the intact and reconstituted adipocyte hexose transport system to irreversible inhibition by sulfhydryl and amino reagents

Sensitivity of the adipocyte D-glucose transport system in intact plasma membranes or following solubilization and reconstruction into phospholipid vesicles to several protein-modifying reagents was investigated. When intact plasma membranes were incubated with N-ethylmaleimide (20 mM) or fluorodinitrobenzene (4 mM), D-glucose transport activity was virtually abolished. However, washing the membranes free of unreacted reagents restored transport activity, indicating that covalent interaction with the membrane did not mediate the transport inhibition. Reaction of [3H]N-ethylmaleimide with plasma membranes under similar conditions resulted in extensive labeling of all protein fractions resolved on dodecyl sulfate gels. Similarly, addition of N-ethylmaleimide to cholate-solubilized membrane protein had no effect on transport activity in artificial phospholipid vesicles reconstituted under conditions where the membrane protein was free of unreacted N-ethylmaleimide. Transport activity in plasma membranes was also inhibited by both reduced and oxidized dithiothreitol or glutathione (15 mM) in a readily reversible manner. consistent with a noncovalent mode of inhibition. Thus, the insulin-responsive adipocyte D-glucose transport system differs from the red cell hexose transport system in its remarkable insensitivity to modulation by covalent blockade of sulfhydryal or amino groups by the reagents studied.

Retention of insulin-stimulated D-glucose transport activity by adipocyte plasma membranes following extraction of extrinsic proteins

Plasma membrane vesicles prepared from adipocytes incubated with insulin exhibited accelerated D-glucose transport activity characteristic to insulin action on intact fat cells. Both control and insulin-stimulated D-glucose transport activities were inhibited by cytochalasin B and thiol reagents. Extraction of plasma membranes with dimethylmaleic anhydride eluted 80% of the protein from plasma membrane vesicles. The two major glycoprotein bands (94,000 and 78,000 daltons) and small amounts of a 56,000-dalton band were retained in dodecyl sulfate gels of the extracted membranes. Both control and insulin-activated D-glucose transport activities were retained by plasma membrane vesicles extracted with dimethylmaleic anhydride. Cytochalasin B binding activity was also retained by extracted membrane vesicles and D-glucose uptake into extracted vesicles derived from untreated or insulin-treated fat cells was inhibited by cytochalasin B. These results suggest that the modification of the adipocyte hexose transport system elicited by insulin action is not altered by a major purification step which involves quantitative extraction of extrinsic membrane proteins.

The mechanism of action of polypeptide hormones with special reference to insulin's action on glucose transport

In the last decade substantial progress has been made in elucidation of the mechanisms of action of polypeptide hormones. For instance, it has become clear that these hormones bind to their target cells and that at least one component of the binding is saturable. This phenomenon, which is usually called receptor binding, is believed to be the first and necessary event in the chain of processes which lead to effects of the hormone on their target cells. Receptor binding is just one aspect of the mechanism of action of polypeptide hormones. The properties of binding differ from hormone to hormone as do the events after binding. The purpose of this communication is to describe some aspects of the mechanism of insulin action on transmembrane glucose transport, and to disucss general aspects of polypeptide hormone action in relation to the specific findings with insulin. Comprehensive reviews on polypeptide hormone receptors have been published recently (Freychet, 1976; Kahn, 1976).