High affinity cytochalasin B binding to red cell membrane proteins which are unrelated to sugar transport

Spectrin: an alternate target for cytoskeletal drugs

Cytoskeletal drugs having enormous therapeutic potential act on the cytoskeletal components like actin, tubulin either by promoting polymerization or destabilizing the same. Here we present the interaction of the popular cytoskeletal drugs such as taxol, latrunculin and cytochalasin with spectrin, a huge protein with multi domains that forms the cytoskeletal network. Particularly, the actin binding domain of spectrin regulates the dynamics of the actin cytoskeleton. We followed the binding of these drugs to its actin binding domain and intact spectrin as well. These drugs bind with moderate affinity (K_b ∼ 10⁴ M^-1) and the interaction with actin binding domain is entropy driven and hydrophobic in nature as determined by Van't Hoff plot. The docking studies and molecular dynamics simulations further corroborate the experimental findings. Particularly the higher binding constants in the case of latrunculin and cytochalasin to the actin binding domain of spectrin suggest the binding sites are presumably located in its actin binding domain.Communicated by Ramaswamy H. Sarma.

Erythrocyte stiffness during morphological remodeling induced by carbon ion radiation

The adverse effect induced by carbon ion radiation (CIR) is still an unavoidable hazard to the treatment object. Thus, evaluation of its adverse effects on the body is a critical problem with respect to radiation therapy. We aimed to investigate the change between the configuration and mechanical properties of erythrocytes induced by radiation and found differences in both the configuration and the mechanical properties with involving in morphological remodeling process. Syrian hamsters were subjected to whole-body irradiation with carbon ion beams (1, 2, 4, and 6 Gy) or X-rays (2, 4, 6, and 12 Gy) for 3, 14 and 28 days. Erythrocytes in peripheral blood and bone marrow were collected for cytomorphological analysis. The mechanical properties of the erythrocytes were determined using atomic force microscopy, and the expression of the cytoskeletal protein spectrin-α1 was analyzed via western blotting. The results showed that dynamic changes were evident in erythrocytes exposed to different doses of carbon ion beams compared with X-rays and the control (0 Gy). The magnitude of impairment of the cell number and cellular morphology manifested the subtle variation according to the irradiation dose. In particular, the differences in the size, shape and mechanical properties of the erythrocytes were well exhibited. Furthermore, immunoblot data showed that the expression of the cytoskeletal protein spectrin-α1 was changed after irradiation, and there was a common pattern among its substantive characteristics in the irradiated group. Based on these findings, the present study concluded that CIR could induce a change in mechanical properties during morphological remodeling of erythrocytes. According to the unique characteristics of the biomechanical categories, we deduce that changes in cytomorphology and mechanical properties can be measured to evaluate the adverse effects generated by tumor radiotherapy. Additionally, for the first time, the current study provides a new strategy for enhancing the assessment of the curative effects and safety of clinical radiotherapy, as well as reducing adverse effects.

Cytochalasin B stimulates insulin-like growth factor-binding protein-1 production by Hep G2 cells

Hep G2 cells were used to study the early sequence of events regulating production of insulin-like growth factor-binding protein-1 (IGFBP-1). Cytochalasin B (100 microM) specifically inhibited 2-deoxyglucose uptake by Hep G2 cells and stimulated IGFBP-1 production 2-fold. Insulin (300 nM) did not stimulate hexose uptake but inhibited IGFBP-1 production more than 50%. A change in IGFBP-1 secretion was observed as early as 2 h after a 15-min or 2-h pulse exposure to either effector. In contrast to IGFBP-1, albumin production was diminished in the presence of cytochalasin B and increased by insulin. From these results we conclude that IGFBP-1 synthesis is (i) stimulated by transient inhibition of cellular glucose uptake and further stimulated by long-term glucose deprivation, and (ii) inhibited by transient exposure to insulin with further inhibition on long-term exposure. These effects are consistent with the dynamic regulation of IGFBP-1 by nutritional status.

The human red cell glucose transporter in octyl glucoside. High specific activity of monomers in the presence of membrane lipids

Human red cell membranes were stripped of peripheral proteins and partially solubilized with 50-260 mM octyl glucoside at 2-14 mg protein/ml, to find conditions that afford a high concentration of active glucose transporter after purification on DEAE-cellulose. Transporter-egg yolk phospholipid vesicles were prepared by gel filtration. The specific D-glucose equilibrium exchange activities increased with increasing dilution of the glucose transporter. At 260 mM octyl glucoside the glucose transporter became partially denaturated. At 225 mM detergent the DEAE-cellulose chromatography showed one main and one minor fraction of active glucose transporter. Nucleoside transport activity was enriched in the minor fraction. Solubilization with 75 mM octyl glucoside at 8 mg protein/ml gave a maximal concentration of purified transporter, 0.8 mg/ml, probably corresponding to complete solubilization. The phospholipids were partially retarded on the DEAE-cellulose. The specific D-glucose equilibrium exchange was high, up to 200 nmol glucose/micrograms transporter in two min at 50 mM glucose. High performance gel filtration in octyl glucoside indicated that the transporter formed dimers during the fractionation. These eluted at Mr 125,000, partially separated from the phospholipids, which appeared at Mr 55,000 (cf. Mascher, E. and Lundahl, P. (1987) J. Chromatogr. 397, 175-186). The D-glucose transport activity was low in the main fraction and high in the transporter-phospholipid fraction. Mixing of these fractions did not increase the activity. The glucose transporter is probably dependent on one or more specific membrane lipid(s). Presumably the transporter dimerizes and loses activity upon removal of these lipids.

Augmentation of interleukin-2 release by cytochalasins

Augmentation of mitogen-induced release of the T cell lymphokine interleukin-2 (IL-2) occurred using several cytochalasins in coculture with a T cell lymphoma line (JM) or with purified T cells. When added concurrently with mitogen, cytochalasins had no apparent negative effect either on the ability of mitogen to signal IL-2 production or on the total amount produced. Use of cytochalasins that affect submembranous actin in peripheral lymphocytes established a link between cytoskeletal alterations and lymphokine release, although direct membrane perturbation cannot be excluded. These data indicate that an intracellular pool of IL-2 may accumulate in both T cell lines and in purified peripheral T cells and that maintenance of that pool may be affected by alterations of the cytoskeletal elements. Cytochalasins not only increased IL-2 release, but could substitute for phorbol myristic acetate (PMA) in supporting mitogen-signaled IL-2 production. We interpret these data also to indicate that cytoskeletal attachment to the surface molecules that signal IL-2 production is not needed for the activation.

Membrane-cytoskeleton interactions and the regulation of chemotactic peptide-induced activation of human granulocytes: the effects of dihydrocytochalasin B

When N-formyl chemotactic peptides bind to granulocyte receptors at 37 degrees C they rapidly form a high-affinity ligand-receptor complex whose coisolation with cytoskeletal residues of Triton X-100-extracted cells is under cellular control [Jesaitis et al: J Cell Biol 98:1378, 1984]. Experiments were performed to investigate the significance of this coisolation. When the granulocytes were preincubated with dihydrocytochalasin B (dhCB) for 10 min at 37 degrees C and then stimulated with 50 nM N-formyl-Met-Leu-[3H]Phe, the rate of uptake of the radioligand by the cells was inhibited. Colocalization of the retained peptide with the Triton X-100 fraction of these cells was also reduced relative to this fraction of the untreated cells. This inhibition was apparent before the onset of FMLP endocytosis. The inhibition was 50% effective at 0.25 microgram dhCB/ml. Maximal inhibition (80-90%) occurred at doses of dhCB greater than 1 microgram/ml. The 90% retention of two plasma membrane markers by the cytoskeleton was marginally affected. These results support the hypothesis that coisolation of the high-affinity receptor-peptide complexes with granulocyte cytoskeletons occurs because of specific association of the complexes with the cytoskeleton at the cell surface. In addition, since these events precede internalization, they suggest that formation of the association between the ligand-receptor complex and cytoskeleton may be necessary for ligand-receptor endocytosis. Experiments were also performed to evaluate other functional consequences of cytoskeletal disruption on chemotactic peptide-stimulated functions. f-Met-Leu-Phe stimulation of O2- production was potentiated due to a prolongation of and an increase in the rate of O2- production. This potentiation had the same dose dependency as the inhibition of receptor modulation. The possible relationship of these various functions is discussed.

Effect of cytochalasins on surfactant release from alveolar type II cells

Cytochalasins enhanced surfactant secretion from primary cultures of [3H]choline-labeled type II epithelial cells from the rat. Cytochalasins A, B, C, D and dihydrocytochalasin B enhanced secretion of phosphatidyl-[3H]choline [(3H]PC) in a dose-dependent manner with EC50 values of 1, 2, 0.5, 0.1 and 1 microM for cytochalasins A, B, C, D and dihydrocytochalasin B, respectively. Only cytochalasin A caused significant cytotoxicity as determined by release of the intracellular enzyme lactate dehydrogenase (EC 1.1.1.17). Dose responses of surfactant release induced by cytochalasins B, C and D were biphasic; maximal release was observed between 0.1-1.0 microM for cytochalasins C and D between 1 and 10 microM for cytochalasin B. Secretion decreased toward control levels at concentrations of cytochalasin above these maximal concentrations. Increased rates of [3H]PC release were noted between 1 and 3 h after exposure to cytochalasin D. Increased rates of surfactant release induced by cytochalasin D were additive to release induced by the beta-adrenergic agonist, terbutaline, or forskolin, although cytochalasin D had no direct effect on cytosolic cyclic AMP levels. Changes in cell shape and microfilament organization were observed by phase-contrast microscopy and fluorescence microscopy using rhodamine-conjugated phalloidin after exposure of the isolated type II cells to cytochalasin D. Disruption of microfilaments associated with lamellar bodies of the purified type II cells occurred after treatment with cytochalasin D. Cytochalasin D augmented surfactant release from purified type II cells and disrupted the microfilament structure of those cells, supporting the hypothesis that alterations in microfilaments are associated with surfactant release.

Effect of cytochalasin B and dihydrocytochalasin B on invasiveness of entero-invasive bacteria in HEp-2 cell cultures

The effect of cytochalasin B (CB) and di-hydrocytochalasin B (H2CB) on the invasiveness of Salmonella typhimurium, Shigella flexneri and Yersinia enterocolotica serotype 0:3 in HEp-2 cell cultures was examined. The intra-cellular and extra-cellular bacteria were identified by a combination of Nomarski interference contrast microscopy and UV incident light microscopy applied on the same light microscope. Pre-treatment of cells with CB and H2CB inhibited the uptake of S. typhimurium and S. flexneri in the HEp-2 cell cultures. The effect was time and dose dependent. On the other hand, the drugs did not influence the invasiveness of Y. enterocolitica. The results indicate that activity of cellular actin micro-filaments is essential for invasiveness of S. typhimurium and S. flexneri.

Actin filaments and vasopressin-stimulated water flow in toad urinary bladder

Vasopressin increases the water permeability of the apical membrane of the granular epithelial cells of the toad urinary bladder. Cytochalasin B inhibits this action of the hormone, indicating that microfilaments may play a role in the water permeability response. We have extended previous functional studies with cytochalasin B and have demonstrated that dihydrocytochalasin B, a more specific inhibitor of actin filament elongation, similarly diminishes the hydrosmotic response to vasopressin. Biochemical studies of isolated epithelial cells indicate that an actin-like protein accounts for about 10% of the soluble protein of the epithelium. Morphological studies of whole toad bladders incubated with heavy meromyosin conclusively demonstrate that actin is a component of the epithelial cells and that actin-containing filaments are associated with both plasma membranes and cytoplasmic organelle membranes. Taken together, these findings provide strong, albeit indirect, evidence that actin microfilaments play a functional role in the hormone-induced increase in water permeability in the toad urinary bladder.

D-Glucose-sensitive and -insensitive cytochalasin B binding proteins from microvillous plasma membranes of human placenta. Identification of the D-glucose transporter

Cytochalasin B was found to bind to at least two distinct sites in human placental microvillous plasma membrane vesicles, one of which is likely to be intimately associated with the glucose transporter. These sites were distinguished by the specificity of agents able to displace bound cytochalasin B. [3H]Cytochalasin B was displaceable at one site by D-glucose but not by dihydrocytochalasin B; it was displaceable from the other by dihydrocytochalasin B but not by D-glucose. Some binding which could not be displaced by D-glucose + cytochalasin B binding site. Cytochalasin B can be photoincorporated into specific binding proteins by ultraviolet irradiation. D-Glucose specifically prevented such photoaffinity labeling of a microvillous protein component(s) of Mr = 60,000 +/- 2000 as determined by urea-sodium dodecyl sulfate acrylamide gel electrophoresis. This D-glucose-sensitive cytochalasin B binding site of the placenta is likely to be either the glucose transporter or be intimately associated with it. The molecular weight of the placental glucose transporter agrees well with the most widely accepted molecular weight for the human erythrocyte glucose transporter. Dihydrocytochalasin B prevented the photoincorporation of [3H]cytochalasin B into a polypeptide(s) of Mr = 53,000 +/- 2000. This component is probably not associated with placental glucose transport. This report presents the first identification of a sodium-independent glucose transporter from a normal human tissue other than the erythrocyte. It also presents the first molecular weight identification of a human glucose-insensitive high-affinity cytochalasin B binding protein.

Cytochalasin B binding by human platelets

Intact human platelets bind cytochalasin B (CB) with a capacity of 100-120 p mols CB/mg protein or approximately 7 x 10(4) molecules/cell and dissociation constants (KD) ranging from 2 x 10(-8) to 10(-6) M. Up to 85% of this saturable binding is displaced by 10(-5) M cytochalasin E (CE). This CE-sensitive binding also appears heterogeneous with KD similar to those of the overall binding. The CE-insensitive binding, however, appears as a single component with KD approximately equal to 4 x 10(-7) M. The sedimentable constituents from frozen, thawed, and washed cells also bind CB with KD ranging from 2.4 x 10(-8) to 1.5 x 10(-6) M and a total capacity of approximately 39 p mols/mg protein which accounts for only 4% of the ligand binding to the intact cell. The major portion (60-80%) of this CB binding is displaceable by 500 mM D-glucose and has a KD of 1.5 x 10(-6) M, while only 10-15% is CE-sensitive with a KD of 2.4 x 10(-8) M. It is concluded that 95% of the saturable CB binding in platelets is associated with the cytosol of which 80-85% is sensitive to CE and that only 3% of the cellular binding is glucose sensitive, membrane-associated binding. If the CE-sensitive binding associated with the cytosol is entirely to actin, the stoichiometry of this binding is approximately one CB to 30 actin monomers, which is greater by an order of magnitude than that for CB binding to muscle actin.

Monosaccharide transporter of the human erythrocyte. Characterization of an improved preparation

The human erythrocyte monosaccharide transporter has been purified through the use of the dialyzable detergent octyl glucoside. It was found that the transporter denatures in the detergent and that the rate of this process could be reduced by increasing the ratio of phospholipid to detergent. The transporter was obtained in higher yield and with a higher specific activity for cytochalasin B binding than has been previously reported. Scatchard plot analysis of cytochalasin B binding to the reconstituted preparations gave a dissociation constant of 1.5 X 10(-7) M, and there were found to be 15.3 nmol of sites/mg of protein. On the basis of a value of 46 000 for the molecular weight of the polypeptide, this specific activity corresponds to 0.70 site/polypeptide chain; and there are reasons to believe that the value of the stoichiometry may be one site per functional transporter polypeptide. The complete amino acid composition and the N- and C-terminal residues of the transporter have been determined. Both the intact transporter and transporter that had been partially depleted of carbohydrate by treatment with endo-beta-galactosidase were found to migrate anomalously upon sodium dodecyl sulfate--polyacrylamide gel electrophoresis, relative to the behavior of standard proteins.

Hexose transport in L6 muscle cells. Kinetic properties and the number of [3H]cytochalasin B binding sites

(1) Myoblasts in culture (L6 cell line) were used as an in vitro model system, to study the kinetic and pharmacological properties of hexose transport in skeletal muscle tissue. (2) Uptake of 2-deoxy-D-[3H]glucose into L6 cells grown in monolayer culture was judged rate limiting since: (2) The time course of sugar uptake extrapolated to zero, (b) a parallel inhibition of hexose uptake and phosphorylation was caused by cytochalasin B, and (c) very little backflow of the hexose was detected. (3) Uptake of 2-deoxy-D-[3H]glucose by cells in monolayers was linear for at least 20 min and it was stimulated by countertransport. The Kt value was 0.83 mM. Cytochalasin B inhibited uptake non-competitively, and half maximal inhibition was achieved at 0.3 microM. Cytochalasin E (up to 5 microM) did not affect 2-deoxy-D-[3H]glucose uptake. (4) L6 myoblasts, detached by trypsinization, retained the hexose transport activity. Kt in detached cells was 0.96 mM. V was 3.2 nmol/min per mg protein, and half maximal inhibition was observed with 0.25 microM cytochalasin B. (5) [3H]Cytochalasin B binding to detached cells showed saturable and non-saturable components. The former could be further separated into cytochalasin E-sensitive binding (probably associated to cytoskeletal proteins) and cytochalasin E-insensitive binding, a fraction of which was inhibited by D-glucose. The D-glucose sensitive sites amount to 16.3 pmol/mg protein, and showed a Kd of 0.49 microM, which is in close agreement with the Ki of cytochalasin B inhibition of hexose uptake. These sites probably are equivalent to the hexose carrier molecules, and are present at a density of 6.8 . 10(6) sites/cell.

Association of actin with chromaffin granule membranes and the effect of cytochalasin B on the polarity of actin filament elongation

Membranes of chromaffin granules isolated from bovine adrenal medulla are shown to bind dihydrocytochalasin B with high affinity. These membranes also bound [3H]actin in a time- and Mg2+-dependent manner and electron microscopy showed the presence of membrane-attached actin filaments following addition of exogenous actin. Binding of [3H]actin was partially inhibited by cytochalasin B. Electron microscopic analysis of heavy meromyosin-decorated, membrane-attached filaments showed terminally (end-on) attached filaments with both possible polarities (i.e., filaments with arrowheads pointing both towards and away from the membranes). Treatment of samples with cytochalasin B preferentially inhibited growth of filaments with their 'barbed' ends pointing away from membranes. These results are discussed with respect to the role of actin in secretory granule function and the mechanism of cytochalasin action.

A role for anti-inflammatory agents and cyclic AMP in regulating fibronectin-mediated phagocytosis

The present study deals with the effects of anti-inflammatory drugs and agents known to elevate intracellular levels of cyclic AMP (cAMP) on plasma fibronectin-mediated (PFn) phagocytosis of radiolabeled, gelatin-coated latex particles (g-Ltx*) by inflammatory macrophages. Monolayers of casein-elicited peritoneal macrophages were preincubated with the specified agents for either 1 or 24 hrs at 37 degrees C prior to the measurements of phagocytosis in the presence of human plasma fibronectin (47 microgram/ml) and heparin (6.7 U/ml). Under these conditions, prostaglandin E1, colchicine, vincristine, and cytochalasin B were all effective in inhibiting g-Ltx* phagocytosis by macrophages in a dose-dependent fashion. More potent inhibition of phagocytosis was manifested by agents known to increase intracellular levels of cAMP in phagocytic cells. Dibutyryl cyclic AMP (dbcAMP), d,1-isoproterenol and aminophylline (10(-5) to 10(-3) M) were all effective in reducing the uptake of g-Ltx* by macrophages. The combination of dbcAMP and aminophylline acted additively. These studies demonstrate that anti-inflammatory drugs and cAMP-elevating agents exert potent inhibitory effects on fibronectin-mediated phagocytosis of gelatin-coated particles by macrophages. Thus, our system provides a suitable in vitro model for further investigations into the humoral regulation of phagocytosis of denatured collagen-coated particles and tissue debris by inflammatory phagocytic cells.

Characterization of cytochalasin B binding to adult rat liver parenchymal cells in primary culture

The characterization of cytochalasin B binding and the resulting effect on hexose transport in rat liver parenchymal cells in primary culture were studied. The cells were isolated from adult rats by perfusing the liver in situ with collagenase and separating the hepatocytes from the other cell types by differential centrifugation. The cells were established in primary culture on collagen-coated dishes. The binding of [4-3H]cytochalasin B and transport of 3-O-methyl-D-[14C]glucose into cells were investigated in monolayer culture followed by digestion of cells and scintillation counting of radioactivity. The binding of cytochalasin B to cells was rapid and reversible with association and dissociation being essentially complete within 2 min. Analysis of the kinetics of cytochalasin B binding by Scatchard plots revealed that binding was biphasic, with the parenchymal cell being extremely rich in high-affinity binding sites. The high-affinity site, thought to be the glucose-transport carrier, exhibited a KD of 2.86 x 10(-7) M, while the low-affinity site had a KD of 1.13 x 10(-5) M. Sugar transport was monitored by 3-O-methyl-D-glucose uptake and it was found that cytochalasin B (10(-5) M) drastically inhibited transport. However, D-glucose (10(-5) M) did not displace cytochalasin B, and cytochalasin E, which does not inhibit transport, was competitive for cytochalasin B at only the low-affinity site, demonstrating that the cytochalasin B inhibition of sugar transport occurs at the high-affinity site but that the inhibition is non-competitive in nature. Therefore, the liver parenchymal cells may represent an unusually rich source of glucose-transport system which may be useful in the isolation of this important membrane carrier.

The monosaccharide transport system of the human erythrocyte. Orientation upon reconstitution

Treatment of intact human erythrocytes with trypsin had no effect upon either the rate of hexose transport or the binding of cytochalasin B to the transport system. In contrast, proteolysis of inside-out vesicles prepared from human erythrocyte membranes inactivated both hexose transport and cytochalasin B binding. When purified hexose transporter, reconstituted into phospholipid vesicles of undetermined size, was treated with trypsin, approx. 50% of the cytochalasin B binding activity was lost. This loss correlated with a decrease in the amount of the transporter polypeptide, as assayed by gel electrophoresis. These results show that the orientation of the transporter can be established through trypsin treatment in conjunction with cytochalasin B binding. Small unilamellar vesicles containing transporter were prepared by sonication of larger species and by a cycle of cholate solubilization and removal of the detergent. In the former case, the transporter orients almost randomly, whereas in the latter approx. 75% of the transporters have the cytoplasmic domain external.

Cytochalasin B-binding proteins in rabbit erythrocyte membranes and their post-natal change in relation to the glucose carrier activity

Two distinct, carrier-mediated glucose uptake processes, a fast, cytochalasin B-sensitive and a slow, cytochalasin B-insensitive flux are identified in parallel in newborn rabbit erythrocytes. The fast, cytochalasin B-sensitive carrier function disappears as rabbits age, and only the slow cytochalasin B-insensitive carrier function is observed with adult rabbit erythrocytes. Three different cytochalasin B binding sites are distinguished in newborn rabbit erythrocytes; a glucose-sensitive site (site I), a cytochalasin E-sensitive site (site II), and a site insensitive to both glucose and cytochalasin E. With adult rabbit erythrocytes, only a cytochalasin E-sensitive site is detected. With glucose-sensitive site disappears as rabbits age, with a time course which is comparable to that of the disappearance of the cytochalasin B-sensitive glucose carrier function. The cytochalasin E-sensitive cytochalasin B binding site does not increase during this change, thus the disappearance of the glucose-sensitive site is not due to its conversion to a cytochalasin E-sensitive site. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of rabbit erythrocyte ghosts revealed a partial decrease in each of the membrane polypeptides of approximate molecular weights of 240 000, 160 000 and 50 000 as rabbits aged. It is concluded that the cytochalasin B-sensitive glucose carrier of fetal rabbit erythrocytes, like that of the human erythrocyte, is tightly associated with the site I cytochalasin B-binding protein, while the cytochalasin B-insensitive glucose carrier, operative in adult rabbit erythrocytes, is not.

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 2. Identification of the transporter

Maltosyl isothiocyanate (MITC), a potent irreversible inhibitor of glucose transport in human erythrocytes [Mullins, R. E., & Langdon, R. G. (1980) Biochemistry (preceding paper in this issue)], has been found to react almost exclusively with band 3 of the human erythrocyte membrane. The incorporation of [14C]MITC into band 3 was found to be antagonized by transportable sugars or competitive inhibitors of transport. On the basis of [14C]MITC incorporation into band 3 and MITC inhibition of transport, it is estimated that there are 3 x 10(5) glucose transporters present in the erythrocyte membrane. It was found that [14C]MITC-labeled band 3 could be converted into 14C-labeled band 4.5 during the Triton X-100 extraction procedure described by Kasahara & Hinkle [Kasahara, M., & Hinkel, P. C. (1977) J. Biol. Chem. 252, 7384]. On the basis of the evidence presented here and in the preceding paper, it is suggested that in the native erythrocyte membrane a component of band 3 is the glucose transport protein and that during purification with nonionic detergents the transport protein may be enzymatically degraded with some retention of activity.

The relationship between high-affinity binding of cytochalasin B to 3T3 cells and inhibition of sugar transport and cell motility

Transport and motility inhibitors have been used to classify different types of high-affinity cytochalasin B (CB) binding sites in 3T3 cells. The potency of phloretin and phlorizin as inhibitors of sugar uptake paralleled their effectiveness in displacing high-affinity bound CB from the cells, indicating that the two compounds compete with CB for binding to sites associated with sugar transport proteins. On the other hand, cytochalasins D and E, which did not inhibit sugar uptake, inhibited binding of CB to a portion of the high-affinity sites, most probably those associated with actin-containing cytoskeletal-contractile structures. A small amount of high-affinity CB binding remained in the presence of both phloretin and cytochalasin E, indicating that the cells have a third class of sites which is not related to either sugar transport or cell motility, When isolated membranes were examined, it was found that a fraction of each class of high-affinity CB binding sites were associated with the fraction. In contrast, only sites sensitive to cytochalasin D were recovered in a soluble extract of the cells.

The use of membrane vesicles in transport studies

Transport-competent plasma membrane vesicles isolated from mammalian cells provide a system to investigate mechanisms and regulation of nutrient and ion transport systems. The characteristics of membrane vesicle systems to study transport in erythrocytes, renal and epithelial membranes, Ehrlich ascites cells, and mouse fibroblasts are discussed. Studies of Na+-stimulated and Na+-independent amino acid and glucose transport in these systems are evaluated, with emphasis on experimental verification of concepts stated in the Na+ gradient hypothesis. Nucleoside, phosphate, and calcium transport systems in plasma membrane vesicles from mouse fibroblast cultures are discussed. Also, current biochemical approaches to investigate mechanisms of regulation of nutrient transport systems by hormones or cellular proliferative state are described.

Purification of the cytochalasin B binding component of the human erythrocyte monosaccharide transport system

The cytochalasin B binding component of the human erythrocyte monosaccharide transport system has been purified. The preparation appears to contain one major protein with an apparent polypeptide chain molecular weight of 55,000 and about 0.4 binding sites per chain. Cytochalasin B binds to the reconstituted preparation with a dissociation constant of 1.3.10(-7) M, a value which is similar to that reported for the transport system in the intact erythrocyte.

7-Acetylcytochalasin B: differential effects on sugar transport and cell motility

Cytochalasin B (CB) is a potent inhibitor of sugar transport and cell motility in animal cells. We have synthesized and characterized the CB derivative 7-acetylcytochalasin B (CBAc) and have found that it has differential effects on transport and motile processes in fibroblasts. The derivative inhibited sugar transport in human red cells, 3T3 cells, and chicken embryo fibroblasts at micromolar concentrations, although it was less potent than its parent compound. Unlike CB, which causes fibroblasts to round up and arborize at less than 10 microM, CBAc had no effect on fibroblast morphology and membrane ruffling at concentrations as high as 90 microM. Competitive binding experiments using [3H] CB showed that the affinity of CBAc for sites related to sugar transport in the red cell membrane is about one-fourth of that of CB. In contrast, similar experiments using [3H] dihydrocytochalasin B (a derivative which inhibits cell motility but not sugar transport) showed that the affinity of CBAc for sites associated with red cell spectrin and actin is only about 1/20 of that of dihydrocytochalasin B. This study demonstrates that acetylation of the C-7 hydroxyl group of CB reduces its effect on cell morphology and motility much more than its ability to inhibit sugar transport. This observation, together with our earlier work with dihydrocytochalasin B, establishes that the pharmacologic effects of CB on fibroblasts result from the binding of the drug to two distinct classes of receptors and that these receptors interact with different parts of the cytochalasin molecule.

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.