Activation of Hexose Transport by Concanavalin A in Isolated Brown Fat Cells

The hyperinsulinemia produced by concanavalin A in rats is opioid-dependent and hormonally regulated

The present study examines the effect of concanavalin A (Con A) on the blood insulin and glucose levels of rats. Male and female rats treated with Con A (62.5-500 micrograms/kg) for three days showed a dose- and time-dependent hyperinsulinemia that lasted more than 48 h. Male rats were more sensitive to Con A. Thus, 6 h after treatment with Con A the circulating insulin levels in male rats had increased by 85% (control: 10.2 +/- 0.9 mU/l and Con A-treated: 18.8 +/- 1 mU/l) compared to only 38% (control: 7.5 +/- 0.2 mU/l; Con A-treated: 10.3 +/- mU/l) in females. An identical response was seen after 12 h. Con A (250 micrograms/kg) produced time-dependent hypoglycemia in both sexes but more pronounced in males. There was no correlation between the hypoglycemia and hyperinsulinemia described above. The Con A-induced hyperinsulinemia in rats of both sexes was abolished in gonadectomized animals (intact males: +101 +/- 17% vs orchiectomized males: -5 +/- 3%; intact females: +86 +/- 23% vs ovariectomized females: -18 +/- 7.2%). Pretreating intact male and female rats with human chorionic gonadotropin also significantly inhibited the Con A-induced hyperinsulinemia. Estradiol (10 micrograms/kg,i.m.) significantly blocked the Con A-induced increase in circulating insulin in male rats (101 +/- 17% for controls vs 32 +/- 5.3% for estradiol-treated animals, P < 0.05) while testosterone (10 mg/kg, i.m.) had no similar effect on intact female rats. Pretreating Con A-injected rats with opioid antagonists such as naloxone (1 mg/kg, s.c.) and naltrexone (5 mg/kg, s.c.) blocked the hyperinsulinemia produced by the lectin in males (control: +101 +/- 17% vs naloxone-treated: +5 +/- 14%, or naltrexone-treated: -23 +/- 4.5%) and females (control: +86 +/- 23% vs naloxone-treated: +21 +/- 20%, or naltrexone-treated: -18 +/- 11%). These results demonstrate that Con A increases the levels of circulating insulin in rats and that this response is opioid-dependent and hormonally regulated.

Effects of concanavalin A on chondrocyte hypertrophy and matrix calcification

Resting chondrocytes do not usually undergo differentiation to the hypertrophic stage and calcification. However, incubating these cells with concanavalin A resulted in 10-100-fold increases in alkaline phosphatase activity, binding of 1,25(OH)2-vitamin D3, type X collagen synthesis, 45Ca incorporation into insoluble material, and calcium content. On the other hand, other lectins tested (including wheat germ agglutinin, lentil lectin, pea lectin, phytohemagglutinin-L, and phytohemagglutinin-E) marginally affected alkaline phosphatase activity, although they activate lymphocytes. Methylmannoside reversed the effect of concanavalin A on alkaline phosphatase within 48 h. Concanavalin A did not increase alkaline phosphatase activity in articular chondrocyte cultures. In resting chondrocyte cultures, succinyl concanavalin A was as potent as concanavalin A in increasing alkaline phosphatase activity, the incorporation of [35S]sulfate, D-[3H]glucosamine, and [3H]serine into proteoglycans, and the incorporation of [3H]serine into protein, although concanavalin A, but not succinyl concanavalin A, induced a rapid change in the shape of the cells from flat to spherical. These findings suggest that concanavalin A induces a switch from the resting, to the growth-plate stage, and that this action of concanavalin A is not secondary to changes in the cytoskeleton. Chondrocytes exposed to concanavalin A may be useful as a novel model of endochondral bone formation.

Influence of Concanavalin A on 3-O-methylglucose uptake in cultured chick embryo fibroblasts. Evidence for differences related to the age of embryos

Concanavalin A (Con A) was found to inhibit hexose uptake in cultured fibroblasts derived from 8-day chick embryos and to stimulate this process in those derived from 16-day embryos. Con-A effects depended on the duration of contact with cells and lectin and were inhibited by alpha-methylmannopyrannoside. Con A was shown to mask about 70% of the hexose carriers in both 8- and 16-day embryo fibroblasts. Lectin altered the hexose uptake very rapidly. Con A only modified the Vmax of the uptake system and did not alter the Km. This indicates that either the number or mobility of hexose carriers were modified by Con-A treatment. The differential effect of lectin could be due to a modification of the hexose-carrier mobility during the embryonic differentiation of fibroblasts. Secondary effects may affect cell growth.

Inhibition of insulin-stimulated xylose uptake in rat soleus muscle by cycloheximide

Cycloheximide (20-200 mg/l) did not affect basal D-[U-14C]xylose uptake by rat soleus muscle (2.4 +/- 0.2 mumol . g-1 . h-1). However, the stimulatory effect of insulin on sugar transport was progressively reduced from 375% above basal in control muscles to 170% in muscles exposed to 200 mg cycloheximide/l but above this concentration cycloheximide inhibited basal xylose uptake without further effect on the incremental effect of insulin. Cycloheximide affected the insulin dose-response curve both by depressing insulin sensitivity and by reducing the maximum stimulatory effect of the hormone. In contrast to the inhibition of insulin action, which increased progressively over the range 20-200 mg cycloheximide/l, muscle protein synthesis was inhibited maximally at a concentration of 10 mg/l. Cycloheximide also inhibited the insulinomimetic effects of anoxia, 2:4-dinitrophenol, salicylate, cooling, hydrogen peroxide, diamide, vitamin K5, hyperosmolarity and EDTA, but did not affect concanavalin A-stimulated xylose uptake. It is concluded that cycloheximide inhibits insulin-stimulated sugar transport at some late post-receptor step, and that this effect of cycloheximide is not secondary to the inhibition of protein synthesis.

The lack of insulin mimetic or antagonistic effects of methyl-alpha-D-Mannoside in iso-osmolar solutions

Transduction of insulin binding into metabolic control in isolated rat adipocytes apparently requires intact cell surface carbohydrate. The ability of certain lectins and some glycosides to mimic and/or inhibit the actions of insulin had been cited as evidence supporting the hypothesis that a concanavalin A-like binding site on fat cells is crucial to this function. Such a binding site could explain the stimulation by methyl-alpha-D-mannoside of glucose oxidation or its ability to antagonize the effect of insulin on lipolysis. The present study corroborated these effects of methyl-alpha-D-mannoside in hyperosmolar medium, but shows that the effects vanish when osmolarity is maintained within physiological limits. Osmolarity alone could not explain all of the complex effect observed, but it can be concluded that earlier data suggesting methyl-alpha-D-mannoside mimics or antagonizes the actions of insulin cannot be used to support the above hypothesis.

Properties of the insulin receptor of rat pancreatic islet

Rat pancreatic islets have been shown to possess specific binding sites for 125I-labeled insulin. Enzymatic and chemical modification of islets are used to reveal important structures and chemical groups for insulin binding. pretreatment with trypsin, neuraminidase, 1-ethyl-3-(3-dimethylamino)carbodiimide (a carboxyl reagent), tetranitromethane (a tyrosyl and thiol reagent), and 1,3-difluoro-4,6-dinitrobenzene (modification of protein functional groups) decreased binding of insulin. This was due to the diminuation of the receptor number; in the case of trypsin-pretreatment also the receptor affinity was decreased. Inhibition of insulin binding was in each case associated with a decrease of the inhibitory effect of exogenous insulin on glucose-induced insulin secretion (not measured in the case of difluorodinitrobenzene and tetranitromethane). Phospholipase A2, (cleavage of phospholipids) did not affect these parameters. 5,5'-dithiobis(2-nitrobenzoic acid) (Ellman's reagent) and possibly p-chloromercuribenzoate (both thiol reagents) increased the number of receptors and decreased receptor affinity, but did not influence the inhibitory effect of insulin on insulin release. It is concluded that protein functional groups, sialic acid, carboxyl and tyrosyl groups, but no phospholipids and probably not sulfhydryl groups are important for the interaction of insulin with insulin receptors of rat pancreatic islets.

Further characterization of the insulin receptor glycosidic moiety in rat adipocytes

Simultaneous or sequential treatment of rat adipocytes with neuraminidase plus beta-galactosidase decreased insulin binding by 43%. No modification was observed with either enzyme individually. alpha-Mannosidase enhanced insulin binding (38%), whereas beta-N-acetylglucosaminidase and alpha-L-fucosidase were ineffective. Lectins that interact with galactose (Ricinus communis I, RCAI), mannose, Lens culinaris agglutinin (LCA), Concanavalin A (Con A) or N-acetylglucosamine (wheat-germ agglutinin, WGA) decreased insulin binding by 43, 57, 59 and 85% respectively. Lectin inhibition was dose-dependent, saturable and prevented by specific monosaccharides. RCAI, LCA, Con A and WGA decreased the insulin dissociation process by 45, 90, 78 and 84% respectively. Lectins specific for sialic acid, terminal galactose, N-acetylgalactosamine or fucose (Limulus polyphemus, peanut, soybean and Ulex I agglutinins) did not modify either insulin binding or dissociation. These results indicate involvement of penultimate D-galactose, internal N-acetyl-D-glucosamine and D-mannose residues in both processes. They suggest that, in rat adipocytes, a glycosidic moiety participates in the insulin-receptor interaction through N-linked oligosaccharides of the 'complex type'.

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.

The culture of chick embryo dorsal root ganglionic cells on polylysine-coated plastic

Polylysine-coated culture surfaces are strongly adhesive for neural cells, restrict locomotion on nonneuronal elements, but do not inhibit neurite elongation. In the present study, culture dishes were pre-treated with poly-D-lysine (PDL) at various concentrations, seed with dissociates from 8-day chick embryo dorsal root ganglia, and incubated under conditions that normally support both neuronal survival and nonneuronal proliferation. Pretreatment with low (0.1 mg/ml) PDL concentrations had no effect on neuronal survival and neuritic growth, but entirely prevented an increase in ganglionic nonneurons, yielding a numericallly stable culture greatly enriched in neurons. Higher PDL concentrations caused increasing losses in both cell classes. The 50% levels of cell loss were achieved at about the same PDL dose, but earlier for neurons that nonneurons and still with no impairment of neuritic growth from the surviving neurons. A procedure was developed to compare acid-soluble and acid-precipitable accumulation of radioactivity under 1-hr pulses of [3H]uridine, which was applicable even to poorly attached cells. The cytotoxic effects of higher PDL pretreatments was revealed as early as 6 hr after seeding by 2- to 4-fold lower radioaccumulation. The data are discussed in terms of possible regulations of cell permeability and metabolism by adhesive interactions between cells and their substratum, or other cells.

Primary role of decreased fatty acid synthesis in insulin resistance of large rat adipocytes

The ability of large fat cells from spontaneously obese rats to synthesize fatty acids from D-[1-14C]glucose, D-[6-14C]glucose, or [2-14C]pyruvate was markedly diminished compared to small fat cells from lean animals. Furthermore, fatty acid synthetase and acetyl coenzyme A carboxylase activities in dialyzed homogenates of large fat cells were inhibited by 84 and 90%, respectively, compared to small cells. Pentose shunt activity, but not glycolytic flux, was also markedly inhibited in large fat cells incubated with or without insulin. However, the NADPH oxidant vitamin K5 completely restored pentose shunt activity in large cells to the elevated levels observed in small fat cells in the presence of this agent or insulin. Furthermore, inhibition of mitochondrial oxidation and fatty acid synthesis in small cells by rotenone led to a secondary inhibition of pentose shunt activity indicating a link between these two pathways. Direct measurements of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities in fat cell homogenates showed no difference between cell types. The data provide strong support for the hypothesis that the fatty acid synthetic pathway is the primary metabolic defect in large insulin-resistant rat adipocytes, a defect which secondarily leads to inhibited pentose shunt activity.

Thiol-dependent subunit interactions of a major fat cell plasma membrane glycoprotein

A major glycoprotein fraction of the isolated adipocyte plasma membrane migrated in the 200,000-dalton region of dodecyl sulfate polyacrylamide gels following solubilization in 2% dodecyl sulfate/4M urea at room temperature in the absence of reductant. Limited heat treatment allowed resolution of the glycoprotein into two distinct bands in the same high-molecular-weight region plus a new 94,000-dalton glycoprotein band. Prolonged incubation of solubilized plasma membranes at 100 degrees C for 15-30 min or incubation with reductant resulted in complete conversion of the high-molecular-weight bands to the 94,000-dalton region, indicating dissociation of dimers to the monomeric form. When the dimer bands on column gels were electrophoresed in the second dimension on slab gels in the presence of reductant, no low-molecular-weight bands were observed other than that in the 94,000-dalton region. The effects of limited heat treatment to permit resolution of the two dimers and the extended treatment to convert the dimers to the monomeric form were markedly inhibited by alkylation of the solubilized membrane protein with N-ethylmaleimide or oxidation with H2O2 or diamide. However, these latter treatments did not prevent complete dissociation of dimers due to addition of reductants. These results suggest that two glycoprotein fractions may exist as dimers in the native fat cell plasma membrane. The data are consistent with a model in which the glycoprotein subunits are linked by hydrophobic bonds that are sensitive to reduction of intramolecular disulfides but are stabilized by alkylation or oxidation of the glycoprotein sulfhydryls.

Regulation of the D-glucose transport system in isolated fat cells

Recent technical advances have yielded considerable new biochemical insights into the hexose transport systems of both brown and white fat cells. In the present studies a novel filtration method was used to monitor initial rates of 3-O-(3H)methylglucose uptake in isolated white fat cells. Transport of 3-O-methylglucose, a non-metabolizable analogue of glucose, occurred by facilitated diffusion, was inhibited by glucose, phloridzin, cytochalasin B and dipyridamole, and was rapidly stimulated by insulin as well as lectins. Total 3-O-methylglucose uptake in white fat cells could be attributed to two kinetically distinct processes in addition to a certain degree of diffusion. Two important new features of glucose transport in fat cells have been discovered. First, in both brown and white fat cells transport per se does not appear to be necessarily rate-limiting for further glucose metabolism. Thus vitamin K5, which markedly increases glucose oxidation by brown fat cells, did not affect the glucose transport system activity. Glucose utilization can apparently be significantly enhanced in fat cells by agents which either increase transport system activity or intracellular enzyme activity. Second, the transport system itself, whether in the basal state or after activation by insulin, lectins, or oxidants, is resistant to sulfhydryl reagents such as N-ethylmaleimide, while the increase in transport activity due to these agents is exquisitely sensitive to sulfhydryl blockage. N-ethylmaleimide blocks the stimulatory effect of insulin on transport whereas addition of insulin to fat cells prior to the reagent completely protects against this inhibitory effect. Further, N-ethylmaleimide prevents the elevated rates of transport system activity due to insulin (or other agents) from returning to basal levels once the cells are washed free of hormone. These data are consistent with the concept that activation of the transport system involves oxidation of key membrane sulfhydryls to the disulfide form, but alternative models are also possible. In any case, these findings provide a possible biochemical clue for future studies designed to identify the specific component(s) involved in the regulatory mechanism which modulates transport of glucose in isolated fat cells.

Interaction of carbohydrate binding sites on concanavalin A-agarose with receptors on adipocytes studied by buoyant density method

The interaction of concanavalin A (Con A) with isolated adipocytes was studied using Con A-Sepharose beads in the affinity binding buoyant density method previously used to study insulin receptors. Free Con A-Sepharose beads could be separated from the bound beads (cell-bead complexes) by sedimentation of the high density beads and floatation of the low density complexes. Sedimented and total beads could be determined by counting the radioactivity associated with [-125I]Con A coupled in tracer amounts to the beads. Various lines of evidence demonstrated the high specificity of binding. Soluble Con A, but neither insulin nor any of the other proteins tested, inhibited and reversed the binding of Con A-Sepharose to the cells. Whereas treatment of Con A- (and insulin-) derivatized beads with anti-insulin antiserum, and cells with trypsin, readily inhibited binding of insulin-Sepharose to cells, neither treatment inhibited Con A-Sepharose binding. According to the relative extents of inhibition and reversal of binding exhibited by 15 different carbohydrates, the saccharide binding sites on Con A-Sepharose appeared virtually identical with the known sites on free Con A. Protein-containing components of cell ghosts that were solubilized with Triton X-100 appeared to correspond to the Con A-Sepharose receptor sites on the basis of their ability to bind to Con A-Sepharose columns, be eluted with methyl alpha-D-mannopyranoside (MeMan) and be precipitated by the free lectin and redissolved by MeMan. According to (a) Normarski interference contrast microscopic examination of the topographical distribution of Con A-Sepharose beads and cells surrounding and bound to each other, and (b) absence of any apparent morphological changes in the cells due to binding, it is suggested that extensive clustering ("cap" or "macropatch" formation) of Con A receptors did not occur on the adipocyte as a consequence of the interaction of the cells with the Con A-Sepharose beads.