Differential modulation of basic fibroblast and epidermal growth factor receptor activation by ganglioside GM3 in cultured retinal Müller glia

Polypeptide growth factors and membrane-bound gangliosides are involved in cell signaling, including that observed in cells of neural origin. To analyze possible interactions between these two systems, we investigated the modulation of short- and long-term responses to basic fibroblast and epidermal growth factor (bFGF and EGF, respectively) in cultured retinal Müller glial cells following experimental modification of their ganglioside composition. These glial cells readily incorporated exogenously administered GM3 ganglioside, which was not substantially metabolized within 24 h. Such treatments significantly inhibited bFGF-induced DNA replication and cell migration, while having much less effect on analogous EGF-mediated behaviors. To explore GM3/growth factor interactions further, different aspects of glial metabolism in response to bFGF or EGF stimulation were examined: membrane fluidity, growth factor binding, global and individual changes in growth factor-induced phosphotyrosine levels, and growth factor-induced activation of mitogen-activated protein kinase. GM3 reduced the intensity of immunocytochemical labeling of phosphotyrosine-containing proteins within bFGF-stimulated cells and down-regulated FGF receptor activation and tyrosine phosphorylation of its cellular substrates, whereas similar parameters in EGF-stimulated cells were much less affected. Hence the data reveal a complex relationship in normal neural cells between polypeptide growth factors and membrane-bound gangliosides, which may participate in retinal cellular physiology in vivo.

Intramitochondrial intermembranal reversible translocation of aspartate aminotransferase and malate dehydrogenase through the inner mitochondrial membrane

The translocation of aspartate aminotransferase, malate dehydrogenase, and bulk protein from the rat liver inner mitochondrial membrane and matrix toward the intermembranal space induced by certain organic acids (movement effectors) has been studied. Experiments involving a two-stage dissolution of the mitochondrial membranes by the use of detergents strongly suggest that enzymes like aspartate amino-transferase can cross the inner mitochondrial membrane providing exogenous movement effector was present. Experiments which measured the changes in membranal distribution of malate dehydrogenase induced by the movement effectors also suggested the occurrence of a similar phenomenon for this enzyme in intact mitochondria. Control experiments revealed that under our experimental conditions, the inner mitochondrial membrane remained impermeable to small molecules, e.g., sucrose, and that the release of aspartate aminotransferease, malate dehydrogenase, insocitrate dehydrogenase, and bulk protein into the intermembranal space in the presence of succinate occurred at a much lower concentration of digitonin than that required to disrupt the inner mitochondrial membrane.

Incorporation of exogenous lipids modulates insulin signaling in the hepatoma cell line, HepG2

The lipid content of cultured cells can be experimentally modified by supplementing the culture medium with specific lipids or by the use of phospholipases. In the case of the insulin receptor, these methods have contributed to a better understanding of lipid disorder-related diseases. Previously, our laboratory demonstrated that experimental modification of the cellular lipid composition of an insulin-sensitive rat hepatoma cell line (ZHC) resulted in an alteration in insulin receptor binding and biological action (Bruneau et al., Biochim. Biophys. Acta 928 (1987) 287-296/297-304). In this paper, we have examined the effects of lipid modification in another hepatoma cell line, HepG2. Exogenous linoleic acid (LA, n-6), eicosapentaenoic acid (EPA, n-3) or hemisuccinate of cholesterol (CHS) was added to HepG2 cells, to create a cellular model in which membrane composition was modified. In this model, we have shown that: (1) lipids were incorporated in treated HepG2 cells, but redistributed differently when compared to treated ZHC cells; (2) that insulin signaling events, such as insulin receptor autophosphorylation and the phosphorylation of the major insulin receptor substrate (IRS-1) were altered in response to the addition of membrane lipids or cholesterol derived components; and (3) different lipids affected insulin receptor signaling differently. We have also shown that the loss of insulin receptor autophosphorylation in CHS-treated cells can be correlated with a decreased sensitivity to insulin. Overall, the results suggest that the lipid environment of the insulin receptor may play an important role in insulin signal transduction.

Valproate-induced hyperammonemia of renal origin. Effects of valproate on glutamine transport in rat kidney mitochondria

The antiepileptic sodium valproate (VPA) systematically induces an asymptomatic hyperammonemia of renal origin in fasting normal human volunteers and in fasting rats, accompanied by an increased renal glutamine uptake. Fasting rats were injected with VPA and their mitochondria isolated, or isolated mitochondria of fasting rats were incubated with VPA. Transmembranal mitochondrial glutamine uptake and activities for five mitochondrial and three cytosolic enzymes involved in ammoniagenesis were measured. In VPA-incubated mitochondria, glutamine transport increased for VPA concentrations between 10(-3) and 10(-5) M; enzyme activities did not change. In mitochondria of VPA-treated rats, Km and Vmax were unaffected. These findings reflect membrane effects of VPA observed in other experimental settings.

Differential sensitivity of astrocyte primary cultures and derived spontaneous transformed cell lines to 7 beta-hydroxycholesterol: effect on plasma membrane lipid composition and fluidity, and on cell surface protein expression

The cytotoxicity of 7 beta-hydroxycholesterol (7 beta-OHC) was investigated on rat astrocyte primary cultures and spontaneously transformed cell lines derived from them. Confluent astrocyte primary cultures (normal cells) were unaffected by 20 microM 7 beta-OHC over a period of 72 h whereas 30 microM markedly affected the viability of the transformed cells within the first 72 h. Both cell types incorporated 18% of the total amount of 7 beta-OHC added to the cultures at concentrations of 20 microM or 30 microM. Cellular fractionation after incubation with 20 microM or 30 microM 7 beta-OHC indicated that the plasma membrane incorporated 2 or 6 fold more 7 beta-OHC than the intracellular one's respectively. Plasma membrane cholesterol (CH) and phospholipid (PL) analysis showed that 20 microM 7 beta-OHC did not affect CH/PL in normal cells; in contrast, plasma membranes of transformed cells displayed a significant CH/PL decrease, which was more pronounced with 30 microM 7 beta-OHC treatment. Fluorescence anisotropy measurements indicated that 20 microM 7 beta-OHC slightly fluidified the plasma membrane of normal cells whereas it has not effect on that of the transformed cells one; however, an increase in plasma membrane fluidity was observed when the transformed cells were treated with 30 microM 7 beta-OHC. Lactoperoxidase catalyzed radioiodination of cell surface proteins and subsequent autoradioelectrophoretic analysis demonstrated that the labelled protein pattern was unchanged when both cell types were incubated with 30 microM 7 beta-OHC. These findings demonstrate that 7 beta-OHC is lethal to highly proliferating cultured glial cells. The high accumulation of 7 beta-OHC in the plasma membrane and its decrease in fluidity, by themselves, do not seem to be involved in the processes leading to cellular death. However, increase of plasma membrane fragility associated with the decrease of CH/PL, which occurs exclusively in plasma membranes isolated from 7 beta-OHC treated transformed cells together with high 7 beta-OHC uptake, are probably implicated in 7 beta-OHC cytotoxicity. The possibility of an additional action mechanism is discussed.

Lipid-induced insulin resistance in cultured hepatoma cells is associated with a decreased insulin receptor tyrosine kinase activity

We have shown previously that experimental modifications of the cellular lipid composition of an insulin-sensitive rat hepatoma cell line (Zajdela Hepatoma Culture, ZHC) affect both binding and biological actions of insulin. Discrepancies between insulin binding and actions implied a postbinding defect, responsible for the observed insulin resistance in lipid-treated cells. To elucidate the mechanism for this defect, we have studied insulin binding and insulin receptor kinase activity in partially purified receptor preparations from ZHC cells grown either in normal medium or in medium supplemented with linoleic acid or 25-hydroxycholesterol. Insulin binding to the lectin-purified insulin receptor showed only a small alteration in receptor affinity for the preparations from lipid-treated cells. Insulin-stimulated autophosphorylation of the beta-subunit of the insulin receptor, as well as insulin-induced phosphorylation of the artificial substrate poly(Glu,Tyr)4:1, was significantly decreased in the preparations from lipid-modified cells. Although differences in basal levels were observed, the magnitude of the insulin-stimulated kinase activity was significantly decreased in receptor preparations from lipid-treated cells. These findings indicate that experimental modification of the lipids of cultured hepatoma cells can produce in insulin receptor kinase activity changes that are proportional to the reduced insulin action observed in these cells.

Time resolved fluorescence properties of phenylalanine in different environments. Comparison with molecular dynamics simulation

Time resolved fluorescence of the phenylalanine residue (Phe) alone and included in the transmembrane domain (TMD) sequences of the epidermal growth factor receptor (EGFR) and ErbB-2 was studied using the synchrotron radiation source of light, and compared to molecular dynamics (MD) simulations. The fluorescence intensity decay is strongly sensitive to the environment. A mono-exponential decay was obtained for Phe amino acid alone in two different solvents and for Phe included in EGFR transmembrane sequence, with fluorescence lifetime values varying from 1.7 ns (EGFR) to 7.4 ns (Phe dissolved in water). In ErbB-2 transmembrane sequence three lifetimes were detected. The relative amplitude of the shortest one (0.14 ns) is smaller than 10%, whereas the others (0.6 and 2.2 ns) are almost equally represented. They have been attributed to different rotamers exchanging slowly. This interpretation is supported by MD simulations which evidence transitions in time series of the chi 1 dihedral angle of Phe observed in the case of ErbB-2. The anisotropy decays are similar for both peptides and indicate the presence of a correlation time in the nanosecond range (1-4 ns) and the probable existence of a very fast one (< 0.05 ns). Autocorrelation functions computed from MD simulations corroborate these results.

Metformin modulates insulin receptor signaling in normal and cholesterol-treated human hepatoma cells (HepG2)

The effects of the biguanide anti-hyperglycemic agent, metformin (N,N'-dimethyl-biguanide), on insulin signaling was studied in a human hepatoma cell line (HepG2). Cells were cultured in the absence (control cells) or in the presence of 100 microM of a cholesterol derivative, hemisuccinate of cholesterol. Cholesterol hemisuccinate-treatment alters cholesterol and lipid content of HepG2 and modulates membrane fluidity. Cholesterol hemisuccinate-treatment induces a decrease in insulin responsiveness and creates an 'insulin-resistant' state in these cells. Exposure to 100 microM of metformin resulted in a significant enhancement of insulin-stimulated lipogenesis in control and cholesterol hemisuccinate-treated cells. In control cells, metformin altered glycogenesis in a biphasic manner. In cholesterol hemisuccinate-treated cells, metformin inhibited basal glycogenesis but restored insulin-stimulated glycogenesis. Hence, to understand the mechanism of metformin action, we analyzed early steps in the insulin signaling pathway, including insulin receptor autophosphorylation, mitogen-activated-protein kinase and phosphatidylinositol 3-kinase activities, in both control and cholesterol hemisuccinate-treated cells. Overall, the results suggest that metformin may interact with the insulin receptor and/or a component involved in the early steps of insulin signal transduction.

Succinate transport inhibition by valproate in rat renal mitochondria

Sodium valproate is an antiepileptic drug which may have side effects on different organs. Its mechanism of action, as yet unclear, may involve an effect on membranes. One possibility, an effect on mitochondrial membranes via an inhibition of oxidative phosphorylation, has been studied here by observing the transmembranal transport of a respiratory substrate, succinate, in rat kidney mitochondria incubated with valproate, or from rats injected with valproate. Succinate transport was inhibited in both conditions, which suggests that the effect was probably due to a direct effect of valproate rather than to an action of a valproate metabolite. For the valproate-incubated mitochondria, inhibition, described by a bell-shaped curve, started at a valproate concentration of 10(-7) M and was maximum at valproate 10(-5)M. Valproate's effect on mitochondrial transmembranal succinate transport can be compared to other evidence for membranal actions of valproate, actions which may clarify certain therapeutic or toxic properties of this drug.

Encapsulation of insulin for oral administration preserves interaction of the hormone with its receptor in vitro

It has been shown that insulin associated with nanocapsules of isobutylcyanoacrylate retains biological activity after oral administration to diabetic rats from 6 to 21 days. Because part of this action is unexplained, we focused on the interaction of encapsulated insulin with the insulin receptor in vitro. We have shown that encapsulated insulin is able 1) to bind to insulin receptors both in rat liver plasma membranes and after solubilization from Chinese hamster ovary (CHO) cells transfected with the gene of human insulin receptor, 2) to accelerate 125I-labeled insulin dissociation from its receptor, and 3) to ensure transduction of a signal leading to stimulation of the beta-subunit phosphorylation, with parameters similar to those of native insulin. In addition, encapsulated 125I-insulin was rapidly internalized in transfected CHO cells. Analysis of cell-associated radioactivity showed that encapsulated insulin remained largely intact (greater than 80%) after 3 h, whereas native insulin was mostly degraded. These data indicate that encapsulated insulin fulfills all the earliest events at the receptor level leading to biological actions and suggests that encapsulation protects insulin against insulin degradation inside the cells.

Decreased membrane "fluidity" of T lymphocytes from untreated patients with Hodgkin's disease

Plasma membrane "fluidity" of peripheral blood T lymphocytes from untreated patients with Hodgkin's disease (HD) and healthy controls was studied using the fluorescent probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4(trimethylamino)phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). In 13 consecutive patients a significant increase of T lymphocyte plasma membrane microviscosity was observed with both DPH and TMA-DPH. These alterations seemed unrelated to the cholesterol (Chol) and phospholipid (PL) content of HD T lymphocytes since the Chol/PL ratio was comparable in both HD and control cells. Since prostaglandin E2 (PGE2) from monocytic origin has been claimed to be responsible for the impairment of cell-mediated immunity (CMI) associated with HD, we studied the effect of exogeneously added PGE2 (0.1 microM) on control subjects T lymphocyte membrane "fluidity". Using the fluorescent probe DPH and the spin labelled fatty acid probe 16 NMS for electron paramagnetic resonance study, we observed a PGE2-induced fluidization of control T lymphocyte membranes which is specifically located in the inner part of the plasma membrane, whereas the plasma membrane surface seemed unaffected by PGE2 as judged by the TMA-DPH probe. Thus, PGE2 does not appear to be responsible for the alterations of T lymphocyte membranes observed in HD. Intrinsic alterations and/or other mediators might be involved.

Does PGE1 induce modifications at the membrane level of bone marrow macrophages? A fluorescence study

The effect of prostaglandin E1 (PGE1), and F2 alpha (PGF2 alpha) on the surface membrane configuration of bone marrow macrophages was studied. We measured the fluorescence intensity of membrane bound ANS in prostaglandin pretreated cells. The effect on fluorescence intensity of a blocker of the prostaglandin binding site (SC19220) and inhibitors of prostaglandin synthesis (aspirin, indomethacin, diclophenate, Eicosa 5,8,11,14 tetraynoic acid) also were studied. Enhancement of the fluorescence intensity of bound ANS in cells pretreated with PGE1 indicates a conformational change localized at the membrane surface. That those changes are confined to the cell surface was shown by the failure of PGE1 or PGF2 alpha to alter the fluorescence polarization of bound DPH used as indicator of membrane core viscosity. Our data indicate that PGE1 could act at the surface of the membrane and that its action causes rapid structural perturbation at strategic points in the molecular organization of the membrane of bone marrow macrophages.

Influence of acetylcholine on rebinding of soluble acetylcholinesterase to a synaptosomal fraction from rat brain

Acetylcholinesterase (Acetylcholine acetylhydrolase E.C. 3.1.1.7.) released from a rat brain synaptosomal fraction was shown to rebind upon incubation in the presence of 3 mM acetylcholine. This action was shown to be reversible. Glutamate, aspartate and gamma-aminobutyrate antagonise this effect. Solubilization of both bulk protein and acetylcholinesterase by Lubrol WX and triton X 100 after acetylcholine incubation of synaptosomal fractions is much lower than in the non incubated preparation. Local production of protons due to the hydrolysis of acetylcholine by the enzyme could partially explain the reassociation of the enzyme. We suggest that the observed phenomenon may play some physiological role in the function of acetylcholinesterase.

Modifications of ependymal cells membranes by galactocerebrosides in cell culture

In this paper we have demonstrated that treatment of ependymal cells in culture by galactocerebrosides induced a decrease in plasma membrane fluidity and an increase of EGF binding sites. We have shown in a previous work that galactocerebroside in vitro and in vivo caused an important morphological change in ependymal cells that grew into an astrocytic shape after a five day treatment. We discuss the hypothesis that the first event in morphological effect could be a modification of plasma membrane followed by important changes in molecules distribution.

Influence of cholesterol derivatives on cytoskeletal organization of human carcinoma cells

Recent developments of immunotherapeutic approaches have shown that artificial ordering of tumor cell membranes with cholesterol hemisuccinate (CHS) or 25-hydroxycholesterol (25-OH) may significantly enhance the immunogenicity of human renal adenocarcinoma cells. To gain further insight into the molecular mechanism of these sterols, we investigated cytoskeletal modification, which is related to the cell membrane. After treatment of human renal carcinoma cells with these cholesterol (at 10(-6) and 10(-7) M) for 5 days, we observed a disorganization of the submembrane end of the cytoplasmic actin stress fibers by cytofluorescence. The microtubule network was not affected. Thus, in the present study, we found that changes in membrane physicochemical properties impaired the anchorage of actin microfilaments in the plasma membrane of human renal cancer cells. Under the same experimental conditions, such modifications were not observed in normal cells (human fibroblasts) or in human hepatoma cells. We suggest that incubation of cancer cells with these sterols induced a redistribution of the cholesterol-rich membrane microdomains which are linked to the cytoskeleton through submembrane proteins.

Expression of plasma membrane and cell surface phospholipids and gangliosides of chick embryo neurons grown in primary cultures: developmental studies

Plasma membranes isolated from chick embryo neurons after 2 days (division phase) and 5 days (maturation phase) in culture showed no differences in the phospholipid (PL) pattern or in the plasma membrane average fluidity. Significant ganglioside (Ggl) variations were only found in GM1 and GD3 which represent 35% of the total plasma membrane N-acetylneuraminic acid (NeuNac) content (1.8-fold GM1 NeuNac increase, 1.4-fold GD3 NeuNac reduction). Lactoperoxidase-catalyzed radioiodination (125I) under saturating conditions indicated that the hydrophobic core of cell surface Ggl was more exposed to the hydrophilic extracellular environment than that of PL. Nevertheless, when interneuronal contacts start to form, the 125I-labelling of both PL and Ggl dropped progressively throughout the maturation phase. Further analysis revealed that (1) plasma membrane GD3 was not accessible to the enzymatic labelling, and (2) surface phosphatidylethanolamine (PE) and GT1b became more accessible to the external medium during development, whereas the opposite was found for phosphatidylcholine (PC) and GM1. The overall profile of radioiodinated PL and Ggl was unaltered when poly-L-lysine was substituted for polyethyleneimine as culture substratum, except for an increase in GT1b labelling 24 h after plating. These results suggest the existence of a 'buffering' mechanism protecting the neuronal plasma membrane during cellular growth from marked changes in fluidity, polar lipid composition and relative localization of polar lipids with regard to the extracellular environment.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased immunogenicity of human renal carcinoma cells following treatment with cholesterol derivatives

Tumor cells isolated from human renal cell carcinoma biopsies were treated with cholesteryl hemisuccinate or 25-hydroxycholesterol and the subsequent changes in their membrane fluidity and capacity to induce skin reactivity in the homologous patient were investigated. Both cholesterol derivatives were found equally efficient in decreasing membrane fluidity when measured by fluorescence polarization of diphenylhexatriene. Using trimethylammonium-diphenylhexatriene, a specific cell surface probe, 25-hydroxycholesterol, appeared much more efficient than cholesteryl hemisuccinate in inducing a membrane rigidification in the carcinoma cells. Cells treated with cholesteryl hemisuccinate induced a strong positive skin reaction compatible with delayed-type hypersensitivity in 54% of the patients, whereas 25-hydroxycholesterol-treated cells were less potent (36% positive skin reactions). Thus, manipulation of the physicochemical state of the membrane of human renal carcinoma cells could increase their immunogenicity in the autologous patient, although this seemed not to be related only to membrane rigidification.

Dietary fish oil and olive oil improve the liver insulin receptor tyrosine kinase activity in high sucrose fed rats

In order to shed light on the possible beneficial effect of dietary unsaturated fatty acids on insulin binding, the effect of fish oil and olive oil administration on insulin binding, autophosphorylation and tyrosine kinase activity of partially purified liver insulin receptors were investigated. These data were confronted with the parameters of sugar and lipid metabolism (blood glucose, insulin and triglycerides), with liver plasma membrane fluidity and fatty acid composition. High sucrose feeding resulted in the elevation of blood glucose and triglyceride level, while the supplementation of animals with fish oil reduced that of triglycerides and olive oil that of insulin. Any significant changes between experimental groups were not detected either in insulin binding to partially purified liver insulin receptor nor in receptor autophosphorylation. However, the insulin stimulated tyrosine kinase activity towards an exogenous substrate (poly(Glu,Tyr)) was decreased by about 50% in the receptors solubilized from liver membranes of sucrose fed rats. Increased dietary intake of fish oil or olive oil restored the activity of insulin tyrosine kinase towards control values, half maximal effect being obtained at similar insulin concentration in all groups. Such improvement might be due to the induced increase of membrane fluidity by unsaturated fatty acids, and/or to the decrease of insulinemia.

Effects of sodium valproate on mitochondrial membranes: electron paramagnetic resonance and transmembrane protein movement studies

Sodium valproate (VPA), the salt of a branched short-chain fatty acid, is a major antiepileptic whose mode of action, as yet unclear, may involve effects on the organization of membranes. VPA was either injected into rats whose liver and kidney mitochondria were then isolated, or was preincubated with isolated mitochondria. First, liver and kidney mitochondria were studied with paramagnetic probes. The electron paramagnetic resonance spectra of proteins of VPA-treated mitochondria spin-labeled with 4-maleimido-2,2,6,6-tetramethyl-1-pyrrolidinoxyl showed that the ratio of weakly immobilized to strongly immobilized SH groups was reduced with respect to control mitochondria, more so in liver than in kidney mitochondria of VPA-injected rats, and more so in kidney than in liver mitochondria for VPA-incubated mitochondria. Spectra of mitochondrial lipids spin-labeled with 5-doxyl stearic methyl ester showed that VPA had no significant effect on order parameters S. Second, the transmembrane movement of aspartate aminotransferase was studied by incubating liver mitochondria in a sucrose-succinate medium and then fractionating them. The translocation of aspartate aminotransferase from mitoplasts, vesicles formed of inner membrane and matrix, to the intermembrane fluid, was significantly higher in VPA-treated than in control mitochondria. Thus, VPA, at concentrations in the range of those used therapeutically, interacted with membranes by modifying the structural organization of the internal mitochondrial membrane, essentially the membrane protein conformation.