Effects of insulin and guanosine 5'-[gamma-thio]triphosphate on fatty acid synthesis and lipolysis within electropermeabilized fat-cells

Directed multistep biocatalysis using tailored permeabilized cells

: Recent developments in the field of biocatalysis using permeabilized cells are reviewed here, with a special emphasis on the newly emerging area of multistep biocatalysis using permeabilized cells. New methods of metabolic engineering using in silico network design and new methods of genetic engineering provide the opportunity to design more complex biocatalysts for the synthesis of complex biomolecules. Methods for the permeabilization of cells are thoroughly reviewed. We provide an extended review of useful available databases and bioinformatics tools, particularly for setting up genome-scale reconstructed networks. Examples described include phosphorylated carbohydrates, sugar nucleotides, and polyketides.

Trehalose improves survival of electrotransfected mammalian cells

BACKGROUND

Electropermeabilization is widely used for introduction of DNA and other foreign molecules into eukaryotic cells. However, conditions yielding the greatest molecule uptake and gene expression can result in low cell survival. In this study, we assessed the efficiency of trehalose for enhancing cell viability after excessive electropermeabilization. This disaccharide was chosen because of its capability of stabilizing cell membranes under various stressful conditions, such as dehydration and freezing.

MATERIALS AND METHODS

Various mammalian cell lines were electropermeabilized by single exponentially decaying electric pulses of few kV/cm strength and of several-microsecond duration. Propidium iodide (PI) and a plasmid encoding green fluorescent protein (GFP), respectively, served as reporter molecules. The effects of trehalose on PI-uptake, GFP gene expression, transfection yield, and short- and long-term viability were analyzed by flow cytometry and electronic cell counting.

RESULTS

The substitution of inositol by trehalose in pulse media protected cells against field-induced cell lysis. The protection effect saturated at about 40-50 mM trehalose. Transfection yield and gene expression were not significantly affected by trehalose. But the transfection efficiency was generally higher in the presence of trehalose, mainly because of the increased cell survival.

CONCLUSIONS

We demonstrated that trehalose-substituted media are superior to standard trehalose-free pulse media for improving cell survival and achieving higher electrotransfection efficiency.

Potential role of Rab4 in the regulation of subcellular localization of Glut4 in adipocytes

A role for Rab4 in the translocation of the glucose transporter Glut4 induced by insulin has been recently proposed. To study more directly the role of this small GTPase, freshly isolated adipocytes were transiently transfected with the cDNAs of both an epitope-tagged Glut4-myc and Rab4, a system which allows direct measurement of the concentration of Glut4 molecules at the cell surface. When cells were cotransfected with Glut4-myc and Rab4, the concentration of Glut4-myc at the cell surface decreased in parallel with the increased expression of Rab4, suggesting that Rab4 participates in the intracellular retention of Glut4. In parallel, the amount of Rab4 associated with the Glut4-containing vesicles increased. When Rab4 was moderately overexpressed, the number of Glut4-myc molecules recruited to the cell surface in response to insulin was similar to that observed in mock-transfected cells, and thus the insulin efficiency was increased. When Rab4 was expressed at a higher level, the amount of Glut4-myc present at the cell surface in response to insulin decreased. Since the overexpressed protein was predominantly cytosolic, this suggests that the cytosolic Rab4 might complex some factor(s) necessary for insulin action. This hypothesis was strengthened by the fact that Rab4 deltaCT, a Rab4 mutant lacking the geranylgeranylation sites, inhibited insulin-induced recruitement of Glut4-myc to the cell surface, even when moderately overexpressed. Rab3D was without effect on Glut4-myc subcellular distribution in basal or insulin-stimulated conditions. While two mutated proteins unable to bind GTP did not decrease the number of Glut4-myc molecules in basal or insulin-stimulated conditions at the plasma membrane, the behavior of a mutated Rab4 protein without GTPase activity was similar to that of the wild-type Rab4 protein, indicating that GTP binding but not its hydrolysis was required for the observed effects. Altogether, our results suggest that Rab4, but not Rab3D, participates in the molecular mechanism involved in the subcellular distribution of the Glut4 molecules both in basal and in insulin-stimulated conditions in adipocytes.

Effect of medium conductivity and composition on the uptake of propidium iodide into electropermeabilized myeloma cells

The effects of ionic composition and conductivity of the medium on electropermeabilization of the plasma membrane of mammalian cells were studied. Temporal and spatial uptake of propidium iodide (PI) into field-treated cells was measured by means of flow cytometry, spectrofluorimetry and confocal laser scanning microscopy. Murine myeloma cells were electropulsed in iso-osmolar solutions. These contained 10-100 micrograms ml-1 PI at different conductivities (0.8-14 mS cm-1) and ionic strengths, adjusted by varying concentrations of K+, Na+, Cl- and SO4(2-). Field-induced incorporation of PI into reversibly permeabilized cells was (almost) independent of ionic composition and strength (at a fixed medium conductivity), but increased dramatically with decreasing medium conductivity at a fixed field strength. The time-course of PI uptake (which apparently reflected the resealing process of the membrane) could be fitted by single-exponential curve (relaxation time about 2 min in the absence of Ca2+) and was independent of medium conductivity and composition. These and other data suggested that the expansion of the 'electroleaks' during the breakdown process is field-controlled and depends, therefore, on the (conductivity-dependent) discharging process of the permeabilized membrane. The threshold field strength for dye uptake increased with increasing K+ concentration (about 0.6 kV cm-1 in K(+)-free, NaCl-containing medium and about 0.9 kV cm-1 in 30 mM KCl-containing medium). Also, the spatial uptake pattern of PI shifted from an asymmetric permeation through the cell hemisphere facing the anode to a more symmetric uptake through both hemispheres. These results suggested that the generated potential is superimposed on the (K(+)-dependent) resting membrane potential. Taking this into account, the breakdown voltage of the membrane was estimated to be about 1 V.

The hormonal regulation of pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex has a central role in the regulation of mammalian metabolism as it represents the point-of-no-return in the utilization of carbohydrate. This article summarizes our studies into how signalling systems initiated by hormones binding to cell surface receptors can reach the pyruvate dehydrogenase system which is located within the inner mitochondrial membrane. One class of hormones which activate pyruvate dehydrogenase are those that increase cytoplasmic Ca2+. A wide range of studies on isolated enzymes, separated mitochondria and intact cell preparations have shown that the activation is due to the stimulation of pyruvate dehydrogenase phosphatase. Two other intramitochondrial dehydrogenases which regulate the citrate acid cycle are activated in parallel and this is an important means of balancing the supply of ATP to increasing cell demand. Insulin is also able to activate pyruvate dehydrogenase, but this is restricted to fat and other cells capable of lipogenesis. Insulin acts by stimulating pyruvate dehydrogenase phosphatase, but the activation does not involve alterations in Ca2+. The signalling pathway involved has not been established, but it appears to be quite distinct from those involved in many other actions of insulin.

Electropermeabilization and fluorescent tracer exchange: the role of whole-cell capacitance

Transmembrane crossing of charged fluorescent tracers such as propidium iodide (PI) and carboxyfluorescein+ (CF) can be used to quantitate membrane permeabilization. Murine myeloma Sp2/0-Ag14 cells were loaded with CF (0.1 fmol/cell) before electropulsation (0.5-3.0 kV/cm, 40 microseconds) in medium containing 25-50 micrograms/ml PI at 21-23 degrees C. Cytograms of PI vs. CF fluorescence showed three readily distinguishable subpopulations: 1) intact living cells with CF but without PI (these form > 95% of the prepulsed population), 2) transiently electropermeabilized but resealed cells showing both CF and low-level PI fluorescence, and 3) permanently permeabilized cells without CF but with very high PI fluorescence. Despite the ready influx of PI, the efflux of CF from transiently permeabilized cells was negligible and was insensitive to pulse parameters; however, electrically killed cells (subpopulation 3) lost all CF fluorescence and probably lost their cytoplasm. This difference in transmembrane passage of the dyes is best explained by binding of intracellular CF to macromolecules (and/or organelles). In isotonic "pulse medium," the membranes resealed after electropulsing with a time constant (tau R) of about 2 min. In 150 mOsm medium, resealing was faster (typically tau R approximately 0.5 min). The population distribution of PI uptake [coefficient of variation (CV) > 40%] was very broad and could not be accounted for by the radius dependence of pulse-induced voltage (CVradius approximately 10%). The variability in PI uptake could be explained if the electrical energy of the charged membrane, which depends on the whole-cell capacitance (Cc), was taken into account. Evaluation of the Cc values with single-cell resolution was based on measurement of the electrical charging time constant of the plasma membrane by electrorotation.

Persistence of disturbed adipocyte metabolism in streptozocin-induced diabetic rats despite near-euglycemia with phlorizin

It is widely accepted that hyperglycemia per se incites and perpetuates the diabetic state by adverse effects on beta cell insulin secretion and peripheral insulin action. Examination of the latter locus has revealed glucose-related abnormalities in facilitated glucose transport. Beyond the plasma membrane, however, there is scant data examining whether hyperglycemia influences important intracellular metabolic events. We recently described a sizable reduction in post-transport, in situ metabolism in permeabilized fat cells from streptozocin-induced diabetic rats. Of importance, the diabetes-related deficit was entirely ameliorated by insulin therapy. In this study we examined whether hyperglycemia per se contributes to this altered intracellular metabolic effect. By infusing phlorizin, near euglycemia was achieved for at least four days in streptozocin-induced diabetic rats. The phlorizin-treated diabetic rats had improved (intact cell) rates of insulin-stimulated 2-deoxyglucose uptake. Despite this, permeabilized fat cell studies revealed no improvement or deterioration in diabetic intracellular metabolism as measured by both the oxidation of [6-14C]glucose-6-phosphate via the citric acid cycle or its incorporation into triglyceride. We conclude that hypoinsulinemia, and not hyperglycemia, mediates the disturbance in porous diabetic adipocyte cellular metabolism.

Studies on signal transduction mechanisms for adrenaline-driven lipolysis in white and brown adipocytes

White and brown rat adipocytes have been permeabilised by repeated exposure of the cells in suspension to high voltage electrical discharges. The resulting preparations were permeable to low molecular weight materials, e.g., cyclic AMP, propidium iodide, and were stable in suspension with little evidence of rapid resealing, or of gross damage to the cell membrane. Leakage of lactate dehydrogenase was not markedly enhanced except at voltages in excess of 2 kV cm-1 for brown adipocytes. Exogenously-added cyclic AMP stimulated lipolysis (measured as glycerol release) in the electropermeabilised adipocytes far more effectively than in intact adipocytes. In brown, but not in white, adipocytes this effect was enhanced by addition of millimolar ATP. The EC50 for stimulation of glycerol release by cyclic AMP was 0.2 microM in electropermeabilised brown adipocytes, and 2 microM and 40 microM in electropermeabilised white adipocytes obtained from weanling and adult rats respectively. The effect of cyclic AMP on lipolysis was enhanced by addition of an inhibitor of cyclic AMP phosphodiesterases and was reduced by addition of 5'-AMP, adenosine or inosine (in brown adipocytes). Addition of adenosine deaminase caused a small, but significant, enhancement of cyclic AMP-driven lipolysis. Catecholamine-driven lipolysis was observed in electropermeabilised brown and white adipocytes, especially in the presence of GTP. Adrenaline-, and to a lesser extent cyclic AMP-, driven lipolysis in electropermeabilised white adipocytes was inhibited by insulin. This effect of insulin was not enhanced by addition of GTP or of a metabolically stable GTP analogue. The results obtained establish the electropermeabilised preparation as suitable for analysis of signal transduction pathways in white and brown adipocytes.

Cell electropermeabilization: a new tool for biochemical and pharmacological studies

Cell electropermeabilization is the transient permeabilization of the plasma membrane by means of short and intense electric pulses. Under optimized conditions, electropermeabilization is compatible with cell survival. It provides a direct access into the cytosol to ions, small molecules, exogenous drugs and macromolecules. As cells remain functional, a large variety of cell biology questions can be addressed. Such 'in situ biochemistry' opens new possibilities beside the more classical studies dealing with unpermeabilized cells or subcellular extracts. Electropermeabilization also allows pharmacological studies with cells, cultured monolayers and in vivo tissues as well as the design of drug controlled-release systems.

Effects of tyrosine kinase inhibitors on protein kinase-independent systems

Tyrosine kinase inhibitors have been widely used to probe the role of tyrosine phosphorylation in cellular signalling. These inhibitors exhibit an apparent specificity for tyrosine kinases over the serine/threonine kinases but little is known about their effects on other enzymes or biological systems. We demonstrate that genistein, erbstatin and alpha-cyanocinnamamides (tyrphostins) have inhibitory effects on fatty acid synthesis, lactate transport, mitochondrial oxidative phosphorylation and aldehyde dehydrogenase. We propose, therefore, that results obtained using tyrosine kinase inhibitors should be interpreted with caution, particularly if used at concentrations sufficient to inhibit these non-protein kinase-dependent events.

Regulation of pyruvate dehydrogenase by insulin and polyamines within electropermeabilized fat-cells and isolated mitochondria

1. Regulation of the mammalian pyruvate dehydrogenase (PDH) complex by insulin and polyamines has been examined by using electropermeabilized rat epididymal fat-cells and isolated mitochondria. The complex could be regulated within the permeabilized cells not only by insulin, but also by certain low-M(r) species, including Ca2+ and the polyamine spermidine. 2. Both spermine and spermidine increased the level of active dephosphorylated PDH (PDHa) in isolated adipose-tissue mitochondria 2-3-fold, with half-maximal effects at 0.9 mM and 1.7 mM respectively. By contrast, PDH activity in rat heart mitochondria was essentially insensitive to the effects of these polyamines. 3. The effects on PDH activity of incubation of adipose-tissue mitochondria with spermine persisted through re-isolation and re-incubation of the mitochondria in the absence of the polyamine. 4. No evidence was found of any increase in the concentration of spermine associated with purified mitochondrial fractions prepared from insulin-treated tissue. 5. Overall, the data provide further evidence against a role for polyamines in the rapid stimulation of PDH by insulin, but suggest that polyamines may be important in mediating longer-term changes in the activity of the complex.

Polymyxin B inhibits insulin-induced glucose transporter and IGF II receptor translocation in isolated adipocytes

In isolated adipocytes, polymyxin B inhibited insulin-induced glucose incorporation into lipids in a dose-dependent manner, while polymyxin E, a structurally related antibiotic, was ineffective. To approach the mechanism of this effect, the subcellular distribution of the glucose transporter Glut 4 was investigated. Adipocytes were pretreated without or with polymyxin B before insulin stimulation, subcellular fractionation was performed and Glut 4 was detected by immunodetection. Incubation of adipocytes with polymyxin B prevented the insulin-induced appearance of Glut 4 in the plasma membranes, but did not prevent their decrease from the low-density microsomal fraction. A lower purity of the plasma membrane fractions, a detergent effect of polymyxin B on the membranes or an interference of the substance with the immunodetection of the Glut 4 molecules were excluded. These results suggest that polymyxin B was interfering with the Glut 4 translocation process stimulated by insulin in adipocytes. In a similar fashion, polymyxin B inhibited the insulin-induced increase in IGF II binding to adipocytes. This resulted from a blockade of the appearance of IGF II receptors in the plasma membranes. Since low-molecular-mass GTP-binding proteins have been implicated in the regulation of vesicular trafficking, we have used [alpha-32P]GTP binding to analyze such proteins in adipocyte fractions, after SDS/PAGE and transfer to nitrocellulose. Specific and distinct subsets of GTP-binding proteins were revealed in plasma membrane and low-density microsomal fractions of control adipocytes, whether they were stimulated or not with insulin. Polymyxin B treatment of adipocytes markedly modified the profile of the low-molecular-mass GTP-binding proteins in plasma membranes, but not in low-density microsomal fractions. Our results suggest that polymyxin B was interfering with the exocytotic process of the Glut 4 and IGF II receptor-containing vesicles, perhaps at the fusion step between vesicles and plasma membranes.