Lectins mimic insulin in the induction of tyrosine aminotransferase

Effects of Lens culinaris agglutinin on gene expression of gluconeogenic enzymes in the mouse intestine

BACKGROUND

Lectins are proteins that bind specifically to the carbohydrate moiety of glyco-conjugates. Japanese mistletoe lectin given intragastrically affected cytokine gene expression in the mouse intestine. This study examines the actions of Lens culinaris agglutinin (LCA) on the gene expression of gluconeogenic enzymes in the intestine.

RESULTS

The results of quantitative real-time reverse transcription-polymerase chain reaction indicated that LCA caused an up-regulation of the gene expression of glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK). This change was correlated with an increase in the expression of two transcription factors, HNF1α and HNF4α. Experiments using human colonic cancer Caco-2 cells demonstrated that LCA up-regulated the gene expression of G6Pase and PEPCK whereas insulin had the opposite effect. In addition, the observed up-regulation of HNF4α gene expression in the duodenum raises the possibility that the lectin promotes the colorectal cancer.

CONCLUSION

Lentil beans should be cooked well to avoid unfavourable effects of LCA.

Lectin-induced alterations of the interaction of insulin and insulin-like growth factor 1 receptors with their ligands

In order to study whether the carbohydrate moieties of the human placental IGF-I receptor (IGF1R), IGF-II receptor (IGF2R) and insulin receptors (IRs) play a role in ligand binding, solubilised cell membrane preparations were incubated with 125I-labelled IGF-I, IGF-II and insulin in the presence of lectins with different sugar specificities. Three incubation procedures were tested: ligand-first, co-incubation and lectin-first incubation. Wheat germ agglutinin (WGA), concanavalin A (Con A) and phytohaemagglutinin (PHA) altered the binding of IGF-I and insulin to their high-affinity receptors in a lectin specific and dose-dependent manner, whereas these lectins did not affect the interaction of IGF-II with its receptor(s). Moreover, the same lectins either inhibited or enhanced IGF-I and insulin binding, depending on the incubation scheme. These results also suggest that IR-A and IR-B from human placenta might be differently glycosylated.

Concanavalin A-induced receptor aggregation stimulates the tyrosine kinase activity of the insulin receptor in intact cells

Concanavalin A (ConA) stimulated the phosphorylation of the beta-subunit of the insulin receptor and an Mr-185,000 protein on serine and tyrosine residues in intact H-35 rat hepatoma cells. This Mr-185,000 protein whose phosphorylation was stimulated by ConA was identical to pp185, a protein reported previously to be a putative endogenous substrate for the insulin receptor tyrosine kinase in rat hepatoma cells. In Chinese hamster ovary (CHO) cells transfected with cDNA of the human insulin receptor, tyrosine-phosphorylation of pp185 was strongly enhanced by ConA compared with the controls, suggesting that the induction of tyrosine-phosphorylation of pp185 was due to stimulation of the insulin receptor kinase by ConA. Moreover, monovalent ConA only slightly induced the tyrosine-phosphorylation of pp185, which was enhanced by the addition of anti-ConA IgG, suggesting that ConA stimulated the insulin receptor kinase mainly by the receptor cross-linking or aggregation in intact cells. These data suggest that the insulin-mimetic action of ConA is related to the autophosphorylation and activation of the insulin receptor tyrosine kinase, as well as the subsequent phosphorylation of pp185 in intact cells.

Wheat-germ agglutinin mimics metabolic effects of insulin without increasing receptor autophosphorylation

Expression of insulin metabolic effects can be obtained by anti-receptor antibodies without activation of the tyrosine kinase activity [O'Brien R. M., Soos M. A. and Siddle K. (1987) EMBO J. 6, 4003-4010; Forsayeth J. R., Caro J. F., Sinha M. K., Maddux B. A. and Goldfine I. D. (1987) Proc. natn. Acad. Sci. U.S.A. 84, 34,448-34,514; Ponzio G., Contreres J. O., Debant A., Baron V., Gautier N., Dolais-Kitabgi J. and Rossi B. (1988) EMBO J. 7, 4111-4117; Hawley D. M., Maddux B. A., Patel R. G., Wong K. Y., Mamula P. W., Firestone G. L., Brunetti A., Verspohl E. and Goldfine I. D. (1989) J. biol. Chem. 264, 2438-2444; Soos M. A., O'Brien R. M., Brindle N. P. J., Stigter J. M., Okamoto A. K., Whittaker J. and Siddle K. (1989) Proc. natn. Acad. Sci. U.S.A. 86, 5217-5221.]. Recently, we have proposed that receptor cross-linking is sufficient in itself to stimulate glycogen synthesis, even if aggregation was performed on receptors mutated on Tyr 1162 and Tyr 1163 and thus devoid of tyrosine kinase activity [Debant A., Ponzio G., Clauser E., Contreres J. O. and Rossi B. (1989) Biochemistry 28, 14-17]. The aim of this study was to gain information on the involvement of receptor clustering in the expression of the different insulin biological effects. To this end, we studied the mimetic effects of wheat-germ agglutinin, which is likely to induce receptor aggregation without interacting with the receptor protein moiety. Wheat-germ agglutinin failed to promote DNA synthesis, whereas the lectin behaved as a potent mimicker of insulin on tyrosine aminotransferase activity and amino-acid transport. However, this stimulatory effect did not parallel the activation of receptor autophosphorylation. Our data reinforce the idea that the expression of the metabolic effects of insulin are not strictly dependent on a general tyrosine kinase activation.

Analysis of differentiation and transformation of cells by lectins

During differentiation cells are known to change their biological behavior according to their genotype. This is thought to be accompanied by a modulation of cell surface determinants expressed on the outer cell membrane. Vice versa, cell surface molecules are suggested to mediate extracellular signals to the genome. Most of these molecules integrated in the cell membrane have been proven to be glycoconjugates. The carbohydrate moieties of these molecules can be detected by means of lectins that are characterized by their ability to react specifically with distinct terminal sugar sequences. Thus, lectins have been used as appropriate tools for studying the modulation of functionally important membrane-associated molecules during the differentiation of cells, in particular of B- and T-lymphocytes. Moreover, lectins have been proven to distinguish between differentiated cells and malignant cell clones, according to the hypothesis that transformed cells possess a glycoconjugate profile that corresponds to the stage of differentiation at which they are arrested. Since lectins, like monoclonal antibodies, make it possible to study functionally important molecules that are associated with differentiation and malignancy, they might be of value for diagnostic purposes and, moreover, for analyzing malignant transformation.

Insulin-mimetic actions of wheat germ agglutinin and concanavalin A on specific mRNA levels

Insulin has pleiotropic effects on sensitive cells, including the regulation of specific mRNA accumulation initiated by the binding of insulin to its plasma membrane receptor. Lectins, such as wheat germ agglutinin (WGA) and concanavalin A (Con A), are known to be insulin mimetic. It is thought that WGA and Con A interact with the insulin receptor or associated membrane glycoproteins which, when activated, lead to insulin-mimetic responses. We attempted to determine whether WGA and Con A could induce the accumulation of a specific messenger RNA (p33-mRNA). Insulin treatment of H4IIE (H4) hepatoma cells increased the concentration of p33-mRNA within 30 min after addition, with a maximum effect of 10- to 15-fold. WGA and Con A also exhibited time- and dose-dependent stimulatory effects on p33-mRNA accumulation with maximal effects of 30- to 40-fold. The effect of insulin was maximal by 1 h and plateaued thereafter, whereas lectins had maximal effects at 2 h after addition to cell cultures. Insulin, WGA, and Con A did not significantly alter the stability (half-life) of p33-mRNA. The addition of RNA synthesis inhibitors blocked the ability of insulin, WGA, and Con A to induce the amount of p33-mRNA. These data suggest that lectins, as well as insulin, induce the synthesis of p33-mRNA in acutely treated H4 hepatoma cells.

The effects of insulin and concanavalin A on the accumulation of a specific mRNA in rat hepatoma cells

One of insulin's effects is to stimulate specific mRNA synthesis. Treatment of H4IIE hepatoma cells with 0.01-1.0 nM insulin results in a maximum 10-15 fold increase in the accumulation of a specific mRNA (p33-mRNA) as measured with a cloned cDNA. Concanavalin A, a lectin known to mimic many of insulin's effects, also stimulates the accumulation of p33-mRNA. The effects of both insulin and Con A were blocked by the addition of two RNA synthesis inhibitors, actinomycin D or 5,6 dichloro-1-beta-D-ribofuranosyl-benzimidazole. We therefore suggest that insulin and concanavalin A act to stimulate p33-mRNA synthesis.

Temperature-and dose-dependent internalization of concanavalin a in monolayer culture

When rat hepatoma cells (R117-21B) were incubated for 20 h at 37 degrees C with 125I-labeled concanavalin A at low concentrations (0.5-10 micrograms/ml), only 20-30% of the cell-associated radioactivity was released by alpha-methyl-D-mannoside, but at high concentrations (50-500 micrograms/ml), 60-80% of the cell-associated radioactivity was released. At 4 degrees C, the cell-associated radioactivity decreased with the increase in concentration of concanavalin A, and more than 80% of the cell-associated radioactivity was released by alpha-methyl-D-mannoside. These results suggest that the amount of cell-associated concanavalin A is related to the physicochemical state of the plasma membrane, which can be altered by the incubation temperature or by the concentration of concanavalin A, the transitional concentration being 5-10 micrograms/ml.

Phytohemagglutinin alters cell morphology and decreases prolactin production in GH cells

Phytohemagglutinin (PHA) produced morphological and functional alterations in a clonal strain of rat pituitary tumor cells (GH4C1). Addition of PHA (2-5 micrograms/ml) results in a decrease in the proportion of elongated cells from 20% in control cell cultures to less than 10% in the presence of PHA. This effect can be observed after exposure of cells to PHA for 2-3 h and requires 4 days to be reversed after removing PHA from the culture medium. A specialized cell function, the production of the peptide hormone prolactin (PRL), is also affected by PHA treatment. Exposure of cells to 2 micrograms/ml PHA results in greater than 50% inhibition of PRL production. The above effects of PHA occur without any apparent alteration in total protein per culture dish, the rate of protein synthesis or the overall growth characteristics of the cells.

Concanavalin A can trap insulin and increase insulin internalization into cells cultured in monolayer

When rat hepatoma cells (R-Y121B) were incubated with insulin at 37 degrees C, concanavalin A increased insulin internalization into cells. When R-Y121B cells were first incubated with labeled insulin at 4 degrees C then with concanavalin A at various concentrations at 37 degrees C, the total cellular radioactivity was much higher at high lectin concentrations than at low lectin concentrations. This increase was not only due to an increase in insulin internalization into cells but also to an increase in insulin binding to cell surfaces. Concanavalin A can trap insulin on the insulin receptors - a "trapping" effect. It has been concluded that insulin and concanavalin A binding sites are very close to each other on the insulin receptors.

Microinjection of cultured cells using red-cell-mediated fusion and osmotic lysis of pinosomes: a review of methods and applications

Proteins and other macromolecules can be injected into cultured cells by several different methods. Here we review the strengths and limitations of two of these methods, red-cell-mediated microinjection and osmotic lysis of pinosomes, and indicate how they may be successfully applied to the study of cultured cells.

Morphine inhibition of the insulin-inducible form of hepatic tyrosine aminotransferase

Morphine treatment in normal intact rats caused a dose-dependent increase in hepatic tyrosine aminotransferase (TAT) activity, as demonstrable up to 2 hr of exposure to the opioid alkaloid. However, such increase in TAT activity was invariably preceded by a prior decline in the enzyme level, as observed after 15 min of morphine treatment. Such an initial decline in activity was not demonstrable in diabetic animals. Further studies indicate that morphine inhibited the insulin-induced increase in TAT activity, a phenomenon which could be reversed by the opioid antagonist naloxone. The results suggest an opioid control mechanism in the regulation of the insulin-inducible form of TAT and indicate the possibilities of a trophic role of endogenous opiates in gluconeogenesis.

Concanavalin A increases spontaneous beat rate of embryonic chick heart cell aggregates

The plant lectin concanavalin A (Con A), at concentrations of 5-200 micrograms/ml, induced a twofold to fivefold increase in spontaneous beat rate of cultured aggregates of ventricular cells from seven-day chick embryos. This response was time, dose, and temperature dependent and was accompanied by a decrease in transmembrane potential. It could be blocked or reversed by alpha-methyl-D-mannoside but was not reversed by dilution alone. Binding of the lectin occurred in the cold, but a temperature-dependent process was also necessary to produce the response. Divalent (succinyl) Con A did not cause a beat rate increase. Whole heart aggregates responded similarly but less intensely than ventricular aggregates. Atrial aggregates, and whole heart aggregates treated with 5 microgram/ml of Con A, produced a biphasic chronotropic response, first decreasing then increasing their beat rates. These results suggest that saccharide-bearing macromolecules on the heart cell surface play a role in regulating spontaneous beat rate.

The release of cytosolic proteins from cells treated with concanavalin A during scraping from plastic culture dishes

When rat hepatoma cells (HTC and R117-21B), treated with concanavalin A (conA) at 37 degrees C, were scraped from plastic culture dishes with a silicone-rubber policeman, the cell membranes were broken and the cytoplasm was released. This phenomenon was also observed in cells treated with conA at 4 degrees C, even though it took a longer time to show the same effect. The effect of 10 micrograms/ml of conA on the release of the cellular proteins reached a plateau when the treatment was carried out at 37 degrees C. Ninety percent of this effect was abolished by 10 mM of alpha-methyl-D-mannoside. The effect was completely nullified by 100 mM. At 4 degrees C, however, even 100 mM of this sugar could not abolish this effect. The apparent decrease in the cellular proteins with conA after scraping was observed not only in the logarithmic phase, but also in the stationary phase of cell growth. The breakdown of plasma membranes with conA eventually caused decrease in tyrosine aminotransferase activity, even though the lectin induced the enzyme activity in cultured cells.

Carbaryl-induced elevation of corticosterone level and cholinergic mechanism

Administration of a single dose (200 mg/kg, p.o.) of carbaryl to rats produced a significant rise in adrenal and plasma corticosterone levels and an increase of tyrosine alpha-ketoglutarate transaminase activity in the liver cytosol. Synaptosomal acetylcholinesterase activity of the hypothalamic and the striatal regions of rat brain was decreased by carbaryl treatment under similar conditions. Pretreatment (0.5 h) with atropine sulphate (10 mg/kg, i.p.) failed to counteract the carbaryl-induced elevation of adrenal and plasma corticosterone levels and hence the liver tyrosine alpha-ketoglutarate transaminase activity. Present results suggest that the carbaryl-induced rise in the corticosterone level in the adrenal gland and plasma is not due to a cholinergic mechanism.

Regulation of insulin receptor kinase activity by insulin mimickers and an insulin antagonist

Three agents which mimic insulin action in intact cells (concanavalin A, wheat germ agglutinin, and polyclonal insulin receptor antibody), mimicked insulin's ability to stimulate the kinase activity of purified insulin receptors. In contrast, monoclonal insulin receptor antibody, an antagonist of insulin action, did not stimulate the phosphorylation of the insulin receptor either in intact IM-9 cells or in purified receptor preparations. This antibody, however, antagonized the ability of insulin to stimulate the phosphorylation of the receptor both in intact cells and in the purified receptor. These studies with insulin mimickers and an insulin antagonist are consistent with a role for the kinase activity of the receptor mediating the actions of insulin.

The calcitonin receptor on T 47D breast cancer cells. Evidence for glycosylation

The glycosyl nature of the receptor for the peptide hormone calcitonin has been investigated in a human breast cancer cell line, T 47D. Studies have been carried out to assess the ability of various lectins and of the antibiotic tunicamycin to inhibit specific binding of calcitonin to the cells, to reduce cross-linking of photoactive calcitonin to a macromolecular receptor component and to influence calcitonin stimulation of cyclic AMP. Pre-incubation of cells with low concentrations of tunicamycin for 72 h resulted in a reduction of total specific binding by approx. 80% and a 40% reduction in calcitonin-stimulated adenylate cyclase; formation of the cross-linked receptor component was also inhibited. Wheat-germ lectin showed the most marked inhibition of total specific binding and cyclic AMP production. However, cross-linking of photoactive calcitonin to receptor component was totally inhibited by this lectin. Soya-bean lectin brought about very little reduction in total specific binding but had more profound effects on calcitonin-stimulated cyclic AMP production and cross-linking of photoactive calcitonin. Concanavalin A and lentil lectin showed some inhibition of all parameters. The data indicate that the calcitonin receptor in T 47D cells is associated with glycosyl moieties, the major contributors of which are N-acetyl-D-glucosamine residues, but N-acetyl-D-galactosamine and mannose residues are also associated.