Development of insulin responsiveness of the glucose transporter and the (Na+,K+)-adenosine triphosphatase during in vitro adipocyte differentiation

Regulation of SREBPs by Sphingomyelin in Adipocytes via a Caveolin and Ras-ERK-MAPK-CREB Signaling Pathway

Sterol response element binding protein (SREBP) is a key transcription factor in insulin and glucose metabolism. We previously demonstrated that elevated levels of membrane sphingomyelin (SM) were related to peroxisome proliferator–activated receptor-γ (PPARγ), which is a known target gene of SREBP-1 in adipocytes. However, the role of SM in SREBP expression in adipocytes remains unknown. In human abdominal adipose tissue from obese women with various concentrations of fasting plasma insulin, SREBP-1 proteins decreased in parallel with increases in membrane SM levels. An inverse correlation was found between the membrane SM content and the levels of SREBP-1c/ERK/Ras/PPARγ/CREB proteins. For the first time, we demonstrate the effects of SM and its signaling pathway in 3T3-F442A adipocytes. These cells were enriched or unenriched with SM in a range of concentrations similar to those observed in obese subjects by adding exogenous natural SMs (having different acyl chain lengths) or by inhibiting neutral sphingomyelinase. SM accumulated in caveolae of the plasma membrane within 24 h and then in the intracellular space. SM enrichment decreased SREBP-1 through the inhibition of extracellular signal-regulated protein kinase (ERK) but not JNK or p38 mitogen-activated protein kinase (MAPK). Ras/Raf-1/MEK1/2 and KSR proteins, which are upstream mediators of ERK, were down-regulated, whereas SREBP-2/caveolin and cholesterol were up-regulated. In SM-unmodulated adipocytes treated with DL-1-Phenyl-2-Palmitoylamino-3-morpholino-1-propanol (PPMP), where the ceramide level increased, the expression levels of SREBPs and ERK were modulated in an opposite direction relative to the SM-enriched cells. SM inhibited the insulin-induced expression of SREBP-1. Rosiglitazone, which is an anti-diabetic agent and potent activator of PPARγ, reversed the effects of SM on SREBP-1, PPARγ and CREB. Taken together, these findings provide novel insights indicating that excess membrane SM might be critical for regulating SREBPs in adipocytes via a MAPK-dependent pathway.

Hesperetin impairs glucose uptake and inhibits proliferation of breast cancer cells

The flavanone hesperetin is known to decrease basal glucose uptake, although the inhibitory mechanism is largely unknown. Here, we used MDA-MB-231 breast cancer cells to investigate the molecular pathways affected by hesperetin. The results indicate that the suppression of glucose uptake is caused by the down-regulation of glucose transporter 1 (GLUT1). Hesperetin was also found to inhibit insulin-induced glucose uptake through impaired cell membrane translocation of glucose transporter 4 (GLUT4). In addition, the phosphorylation of the insulin receptor-beta subunit (IR-beta) and Akt was suppressed. Hesperetin also decreased cellular proliferation, which is likely due to the inhibition of glucose uptake. Cancer cells are highly dependent on glucose and hesperetin may, therefore, have potential application as an anticancer agent.

New insights into cytosolic glucose levels during differentiation of 3T3-L1 fibroblasts into adipocytes

Cytosolic glucose concentration reflects the balance between glucose entry across the plasma membrane and cytosolic glucose utilization. In adipocytes, glucose utilization is considered very rapid, meaning that every glucose molecule entering the cytoplasm is quickly phosphorylated. Thus, the cytosolic free glucose concentration is considered to be negligible; however, it was never measured directly. In the present study, we monitored cytosolic glucose dynamics in 3T3-L1 fibroblasts and adipocytes by expressing a fluorescence resonance energy transfer (FRET)-based glucose nanosensor: fluorescent indicator protein FLIPglu-600μ. Specifically, we monitored cytosolic glucose responses by varying transmembrane glucose concentration gradient. The changes in cytosolic glucose concentration were detected in only 56% of 3T3-L1 fibroblasts and in 14% of 3T3-L1 adipocytes. In adipocytes, the resting cytosolic glucose concentration was reduced in comparison with the one recorded in fibroblasts. Membrane permeabilization increased cytosolic glucose concentration in adipocytes, and glycolytic inhibitor iodoacetate failed to increase cytosolic glucose concentration, indicating low adipocyte permeability for glucose at rest. We also examined the effects of insulin and adrenaline. Insulin significantly increased cytosolic glucose concentration in adipocytes by a factor of 3.6; however, we recorded no effect on delta ratio (ΔR) in fibroblasts. Adrenaline increased cytosolic glucose concentration in fibroblasts but not in adipocytes. However, in adipocytes in insulin-stimulated conditions, glucose clearance was significantly faster following adrenaline addition in comparison with controls (p < 0.001). Together, these results demonstrate that during differentiation, adipocytes develop more efficient mechanisms for maintaining low cytosolic glucose concentration, predominantly with reduced membrane permeability for glucose.

Expression and modulation of TUB by insulin and thyroid hormone in primary rat and murine 3T3-L1 adipocytes

tub encodes a protein of poorly understood function, but one implicated strongly in the control of energy balance and insulin sensitivity. Whilst tub expression is particularly prominent in neurones it is also detectable in extraneuronal tissues. We show here, for the first time, expression of TUB protein in rat adipocytes and the murine adipocyte model 3T3-L1 and demonstrate that insulin induces its tyrosine phosphorylation and association with the insulin receptor. TUB expression is regulated developmentally during adipogenic differentiation of 3T3-L1 cells and in response to cell treatment with thyroid hormone or induction of insulin resistance. TUB was upregulated 5- to 10-fold in adipocytes from obese Zucker rats and 3T3-L1 adipocytes that had been rendered insulin resistant, a response that could be antagonised by rosiglitasone, an insulin-sensitising drug. Our data are consistent with a previously unforeseen role for TUB in insulin signalling and fuel homeostasis in adipocytes.

Naringenin Inhibits Glucose Uptake in MCF-7 Breast Cancer Cells: A Mechanism for Impaired Cellular Proliferation

Certain flavonoids inhibit glucose uptake in cultured cells. In this report, we show that the grapefruit flava-none naringenin inhibited insulin-stimulated glucose uptake in proliferating and growth-arrested MCF-7 breast cancer cells. Our findings indicate that naringenin inhibits the activity of phosphoinositide 3-kinase (PI3K), a key regulator of insulin-induced GLUT4 translocation, as shown by impaired phosphorylation of the downstream signaling molecule Akt. Naringenin also inhibited the phosphorylation of p44/p42 mitogen-activated protein kinase (MAPK). Inhibition of the MAPK pathway with PD98059, a MAPK kinase inhibitor, reduced insulin-stimulated glucose uptake by approximately 60%. The MAPK pathway therefore appears to contribute significantly to insulin-stimulated glucose uptake in breast cancer cells. Importantly, decreasing the availability of glucose by lowering the glucose concentration of the culture medium inhibited proliferation, as did treatment with naringenin. Collectively, our findings suggest that naringenin inhibits the proliferation of MCF-7 cells via impaired glucose uptake. Because a physiologically attainable dose of 10 micro M naringenin reduced insulin-stimulated glucose uptake by nearly 25% and also reduced cell proliferation, naringenin may possess therapeutic potential as an anti-proliferative agent.

Sphingomyelin/cholesterol ratio: an important determinant of glucose transport mediated by GLUT-1 in 3T3-L1 preadipocytes

Sphingomyelin pathway has been linked with insulin signaling through insulin-dependent GLUT-4 glucose transporter, but a relationship between sphingomyelin and the GLUT-1 transporter responsible for the basal (insulin-independent) glucose transport has not been clearly established. As GLUT-1 is mainly distributed to the cell surface, we explored the effects of changes in membrane sphingomyelin content on glucose transport through GLUT-1. The addition of exogenous sphingomyelin or glutathione (an inhibitor of endogenous sphingomyelinase) to the culture medium increased membrane sphingomyelin and cholesterol contents. Basal glucose uptake was enhanced and positively correlated to sphingomyelin (SM), cholesterol (CL) and SM/CL ratio. The exposure of 3T3-L1 preadipocytes to sphingomyelinase (SMase) significantly increased basal glucose uptake, membrane fluidity and decreased membrane sphingomyelin and cholesterol contents 60 min after SMase addition. There was no significant change in the abundance of GLUT-1 at the cell surface. The membrane sphingomyelin and cholesterol contents, fluidity and basal glucose transport returned to baseline levels within 2 h. The basal glucose uptake was negatively correlated with cholesterol contents and positively with SM/CL ratio. The SM/CL ratio might represent an important parameter controlling basal glucose uptake and a mechanism by which insulin resistance might be induced.

The formation of an insulin-responsive vesicular cargo compartment is an early event in 3T3-L1 adipocyte differentiation

Differentiating 3T3-L1 cells exhibit a dramatic increase in the rate of insulin-stimulated glucose transport during their conversion from proliferating fibroblasts to nonproliferating adipocytes. On day 3 of 3T3-L1 cell differentiation, basal glucose transport and cell surface transferrin binding are markedly diminished. This occurs concomitant with the formation of a distinct insulin-responsive vesicular pool of intracellular glucose transporter 1 (GLUT1) and transferrin receptors as assessed by sucrose velocity gradients. The intracellular distribution of the insulin-responsive aminopeptidase is first readily detectable on day 3, and its gradient profile and response to insulin at this time are identical to that of GLUT1. With further time of differentiation, GLUT4 is expressed and targeted to the same insulin-responsive vesicles as the other three proteins. Our data are consistent with the notion that a distinct insulin-sensitive vesicular cargo compartment forms early during fat call differentiation and its formation precedes GLUT4 expression. The development of this compartment may result from the differentiation-dependent inhibition of constitutive GLUT1 and transferrin receptor trafficking such that there is a large increase in, or the new formation of, a population of postendosomal, insulin-responsive vesicles.

Reactive oxygen species activate glucose transport in L6 myotubes

Under oxidative stress, increased energy requirements are needed To induce repair mechanisms. As glucose is a major energy source in L6 myotubes, we evaluated glucose metabolism and transport, following exposure to glucose oxidase (H2O2 generating system), or xanthine oxidase (O2. and H2O2 generating system), added to the medium. Exposure for 24 h to 5 mM glucose and 50 mU/ml glucose oxidase, or to 50 microM xanthine and 20 mU/ml xanthine oxidase resulted in significant oxidant stress indicated by increased DNA binding activity of NF-kappa B. Under these conditions, approximately 2-fold increase in glucose consumption, lactate production and CO2 release were observed. 2-deoxyglucose uptake into myotubes increased time and dose dependently, reaching a 2.6 +/- 0.4-fold and 2.2 +/- 0.7-fold after 24 h exposure to glucose oxidase and xanthine oxidase, respectively. Peroxidase prevented this effect, indicating the role of H2O2 in mediating glucose uptake activation. The elevation in glucose uptake under oxidative stress was associated with increased expression of GLUT1 mRNA and protein. The observed 2-deoxyglucose uptake activation by oxidants was not limited to the L6 cell line and was observed in 3T3-L1 adipocytes as well.

Effect of lipoproteins on the differentiation of 3T3-L1 and human preadipocytes in cell culture

High-density and low-density lipoprotein (LDL) stimulated 3T3-L1 and human preadipocyte differentiation in vitro. In both cell types, LDL exhibited the greatest stimulatory effect. LDL suppressed the development of catecholamine-stimulated lipolysis in differentiating 3T3-L1 and human preadipocytes when present during preadipocyte proliferation and early stages of differentiation. The effect of lipoproteins on development of human preadipocyte (but not 3T3-L1) lipolysis may involve beta-adrenoceptors.

Na(+)-K(+)-ATPase subunit isoform pattern modification by mitogenic insulin concentration in 3T3-L1 preadipocytes

When 3T3 preadipocyte cell lines are induced to differentiate to the adipocyte phenotype, the expression of Na(+)-K(+)-ATPase isoforms changes. Some disparities have been noted by investigators who used different hormonal conditions to stimulate adipocyte conversion, however. In the present report we investigated the effect of high concentrations of insulin on the 3T3-L1 cell line, to determine whether it affected Na(+)-K(+)-ATPase expression separately from its ability to promote phenotypic conversion. The effect of insulin was compared with that of dexamethasone and 3-isobutyl-1-methylxanthine. Changes in Na(+)-K(+)-ATPase subunit expression were seen regardless of the hormonal stimulus. Induction of alpha 2-mRNA and reduction of beta 1-mRNA were always observed. At the protein level, too, induction of alpha 2-protein was noted; alpha 1-protein levels were not markedly affected. The change in alpha-isoform protein and mRNA levels did not correspond quantitatively with the fraction of cells that were morphologically converted, suggesting that it is an early event in differentiation.

Insulin and noradrenaline independently stimulate the translocation of glucose transporters from intracellular stores to the plasma membrane in mouse brown adipocytes

The mechanism of the effect of noradrenaline on the transport of 3-O-methyl-D-[14C]glucose ([14C]-MG) was studied in mouse brown adipocytes. When cells were exposed to low concentrations (< 10(-8) M) of insulin, the [14C]-MG uptake by cells was enhanced by noradrenaline additively. The action of noradrenaline was mimicked by isoproterenol, and was completely blocked by propranolol. Exposing cells to noradrenaline induced both an increase in the transport activity of plasma membrane fractions and a decrease in that of microsomal fractions similar to insulin exposure, indicating that noradrenaline also induces the translocation of glucose transporters to the plasma membrane. The ratio of an increase in the transport activity of plasma membrane fraction to a decrease in the activity of microsomal fraction was lower in cells exposed to noradrenaline than in cells exposed to insulin. This quantitative disagreement suggests that there are at least two different modes involved in the regulation of the translocation of glucose transporters in mouse brown adipocytes.

Modulation of potassium transport in cultured retinal pigment epithelium and retinal glial cells by serum and epidermal growth factor

The ionic environment of retinal photoreceptors is partially controlled by potassium transporters on retinal glial and retinal pigment epithelial cells (RPE). In this study, serum and epidermal growth factor (EGF) were examined as modulators of potassium transport in confluent cultures of human RPE and rabbit retinal glia. EGF is a known mitogen for confluent RPE cultures and was shown here to also stimulate [3H]thymidine incorporation in cultures of retinal glia. For potassium transport studies 86Rb was used as a tracer during a 17-min incubation. For both retinal cell types the mean total 86Rb uptake in 10% serum was approximately 60% above basal, serum-free controls. For EGF, tested in several experiments in a concentration range from 1 to 100 ng/ml, maximal total uptake was 33 and 24% above controls for RPE and glia, respectively. Inhibitor studies suggested that basal and serum-stimulated uptake for both cell types occurred by the ouabain-sensitive Na-K ATPase pump and by the furosemide- or bumetanide-sensitive Na-K-Cl cotransporter. EGF-stimulated uptake appeared to be due predominantly to the cotransporter. The data suggest that serum components and EGF, which may be available to retina-derived cells under pathologic conditions, may not only stimulate proliferation but may also act as short-term modulators of potassium ion movement and thus affect physiologic processes that are sensitive to ion homeostasis.

The signaling potential of the receptors for insulin and insulin-like growth factor I (IGF-I) in 3T3-L1 adipocytes: comparison of glucose transport activity, induction of oncogene c-fos, glucose transporter mRNA, and DNA-synthesis

The receptors for insulin and insulin-like growth factor I (IGF-I) have in common a high sequence homology and diverse overlapping functions, (e.g., the stimulation of acute metabolic events and the induction of cell growth.). In the present study, we have compared the potential of insulin and IGF-I receptors in stimulating glucose transport activity, glucose transporter gene expression, DNA-synthesis, and expression of proto-oncogene c-fos in 3T3-L1 adipocytes which express high levels of both receptors. Binding of both hormones to their own receptors was highly specific as compared with binding to the respective other receptor (insulin receptor: KD = 3.6 nM, KI of IGF-I greater than 500 nM; IGF-I receptor, KD = 1.1 nM, KI of insulin = 191 nM). Induction of proto-oncogene c-fos mRNA by insulin and IGF-I paralleled their respective receptor occupancy and was thus induced by both hormones via their own receptor (EC50 of insulin, 3.7; IGF-I, 3.9 nM). Similarly, both insulin and IGF-I increased DNA synthesis (EC50 of insulin, 5.8 nM; IGF-I, 4.0 nM), glucose transport activity (EC50 of insulin, 1.7 nM; IGF-I, 1.4 nM), and glucose transporter (GLUT4) mRNA levels in concentrations corresponding with their respective receptor occupancy. These data indicate that in 3T3-L1 cells the alpha-subunits of insulin and IGF-I receptors have an equal potential to stimulate a metabolic and a mitogenic response.

Ouabain-sensitive Rb+ uptake in mouse eggs and preimplantation conceptuses

The results of histochemical and immunocytochemical studies have been used elsewhere to support the hypothesis that Na+/K(+)-ATPase expression is initiated or increases dramatically in preimplantation mouse conceptuses just before they begin to cavitate. Moreover, localization of the enzyme in the inner membrane of the mural trophoblast is thought to be involved directly in formation and maintenance of the blastocyst cavity. Presumably, Na+/K(+)-ATPase extrudes the cation, Na+, and therefore water into the cavity. The cation transporting activity of the enzyme can be determined by measuring ouabain-sensitive Rb+ uptake by cells. Therefore, we measured Rb+ uptake in mouse eggs and preimplantation conceptuses at various stages of development. 86Rb+ uptake by conceptuses increased linearly with time for at least 60 min in medium containing 0.7 mM total Rb+ plus K+ in the absence or presence of 1.0 mM ouabain, and ouabain inhibited more than 70% of 86Rb+ uptake. The ouabain concentration at 1/2 of maximum inhibition of the ouabain-sensitive component of 86Rb+ uptake was about 10-20 microM in eggs and conceptuses at all stages of preimplantation development. Moreover, ouabain-sensitive Rb+ uptake had a twofold higher Vmax value in blastocysts than in eggs or conceptuses at earlier stages of development (i.e., approximately 173 vs 70-100 fmole.conceptus-1.min-1), although the total cell surface area also was probably about two times greater in blastocysts than in eggs or other conceptuses. Ouabain-sensitive Rb+ transport in eggs and conceptuses may have occurred via a single ouabain-sensitive Rb+ transporter with a Hill coefficient of 1.5-1.8 (Hill plots). When it was assumed that the Hill coefficient had a value of 2.0, however, eggs and conceptuses appeared to contain at least two forms of Na+/K(+)-ATPase activity. These studies are the first to show that the cation transporting activity of Na+/K(+)-ATPase can be measured quantitatively in mammalian eggs and preimplantation conceptuses. Inclusion of this assay in experiments designed to determine how Na+/K(+)-ATPase activity is controlled in oocytes and conceptuses should yield further insight into the role of this enzyme in oogenesis and preimplantation development.

The effect of culture density and proliferation rate on the expression of ouabain-sensitive Na/K ATPase pumps in cultured human retinal pigment epithelium

The number and activity of ouabain-sensitive Na/K ATPase pumps expressed by many cell types in vitro, including human retinal pigment epithelial cells (RPE), have been shown to decline with increasing culture density. Cell proliferation also declined as cultures became dense so it was unclear if pump number was modulated by cell proliferation or culture confluency. By exposing RPE cultures to various feeding regimens, using culture medium containing or lacking serum, it was possible to produce RPE cultures with a range of culture densities and growth rates. These were analyzed for proliferative activity by quantifying [3H]thymidine incorporation and for Na/K ATPase pump number by measuring specific [3H]ouabain binding. The results suggest that pump number is modulated by culture density and, further, that the density-dependent regulation of pump number requires serum. Although density-dependent modulation of culture growth is also serum requiring, cell proliferation and pump number did not appear to be related; cultures of similar density which differed significantly in growth rate had similar numbers of pumps. The view that elevated numbers of pumps were not necessarily found in proliferating cells was further supported by qualitative examination of radioautographs of cells dually labeled with [3H]thymidine and [3H]ouabain. Cycling cells which had [3H]thymidine-labeled nuclei did not have notably higher labeling with [3H]ouabain. However, [3H]ouabain labeling, as an indicator of pump site number and distribution, did vary among cells in an RPE population and also within individual cells. This latter observation suggests that unpolarized RPE cells in sparse cultures may have regionally different requirements for ionic regulation.

Differential sorting of two glucose transporters expressed in insulin-sensitive cells

The insulin-regulatable glucose transporter (IRGT) is specifically expressed in muscle and fat cells and undergoes translocation from an intracellular compartment to the cell surface following acute insulin treatment. This study examined sorting differences between the IRGT and the homologous HepG2/erythrocyte/brain glucose transporter (HepG2 GT) when expressed together in insulin-responsive 3T3-L1 adipocytes. The ratio of the amount of transporter per unit protein in the plasma membrane fraction vs. the intracellular membrane fraction was 1:2 for the HepG2 GT and 1:30 for the IRGT. Insulin treatment increased the plasma membrane concentration of the IRGT by 10-fold and the HepG2 GT by 3.5-fold. This distribution was confirmed by confocal immunofluorescence microscopy. Differential sorting within intracellular organelles was evident by sucrose gradient analysis and immunoisolation of transporter vesicles and by double immunofluorescence labeling. We propose that differential sorting at an intracellular locus preferably withdraws the IRGT from a pathway which is in close communication with the plasma membrane, thus allowing the IRGT to regulate glucose entry into fat and muscle cells in a highly insulin-regulated fashion.

The sensitivity of insulin-stimulated and basal Na efflux to ouabain in frog skeletal muscle cells

1. The sensitivity of Na efflux to ouabain in frog skeletal muscle cells was studied in the presence and absence of insulin. 2. Insulin increased the ouabain-sensitive Na efflux, about two-fold, without any significant effect on the ouabain-insensitive Na efflux; i.e. all components of the Na efflux increased by insulin can be blocked by ouabain. 3. There was no significant difference between the time course of the inhibitory action of ouabain on Na efflux in the presence and absence of insulin: i.e. the binding affinity of the insulin-stimulated Na/K pump to ouabain is same as that of the control.

Na(+)-K(+)-ATPase in adipocyte differentiation in culture

Differentiation of 3T3-L1 cells from a fibroblast to an adipocyte phenotype results in an approximately 50% decline in Na(+)-K(+)-ATPase activity and ouabain-sensitive 86Rb uptake. Kinetic analysis revealed a K 1/2 for Na+ of approximately 14 mM, a Km for ATP of approximately 0.4 mM, and maximal activation by sodium dodecyl sulfate at a 0.05 (wt/wt) detergent/protein ratio in both mature fibroblasts and adipocytes. Both fibroblasts and adipocytes exhibited Na(+)-K(+)-ATPase activity with an inhibition constant (Ki) for ouabain of approximately 10(-4) M. In addition, adipocytes exhibited a second component representing 30% of total activity with a Ki of approximately 5 x 10(-7) M. The emergence of biphasic ouabain inhibition kinetics in adipocytes raised the possibility of a change in alpha-subunit isoform composition with cytodifferentiation. This inference was evaluated by isoform-specific mRNA analysis (Northern blots) and by alpha-isoform-specific immunoassays (Western blots). Northern blots revealed a modest decrease in mRNA alpha 1, a striking increase in mRNA alpha 2, and a significant loss of mRNA beta content with differentiation of fibroblasts to adipocytes. By immunoassay, fibroblasts exhibited the alpha 1-isoform. Adipocytes exhibited an admixture of alpha 1- and alpha 2-isoforms, with alpha 2 being the more abundant isoform. There was no one-to-one correspondence either between the mRNA isoform and alpha-subunit abundances or between alpha-subunit abundances and enzymatic activity, suggesting that regulation occurs at multiple levels in this system. Findings indicate, however, that a shift in alpha-isoform composition accompanied by a change in ouabain inhibition kinetics occurs with cytodifferentiation.

Hydrolytic properties of the (Na+ + K+)-ATPase isozymes for beta-(2-furyl)acryloyl phosphate, a pseudosubstrate for the sodium pump

The hydrolysis of beta-(2-furyl)acryloyl phosphate (FAP), a synthetic substrate for the (Na+ + K+)-ATPase by the partially purified enzyme from rat brain and rat kidney, has been assessed. Using previously determined FAPase reaction conditions, it was discovered that the KI for ouabain of the alpha 2/3 isozyme of the (Na+ + K+)-ATPase was approximately 10(-5) M, while for the alpha 1 isozyme the KI was approximately 10(-3) M. These values were an order of magnitude higher (lower affinity) than the KI's for ouabain as determined when using ATP in a coupled assay for (Na+ + K+)-ATPase activity: approximately 10(-6) M and approximately 10(-4) M for the alpha 2/3 and alpha 1 isozymes, respectively. This discrepancy was alleviated by altering established reaction conditions. Previously published FAPase studies have overlooked this fact, since either the properties of the isozymes of the (Na+ + K+)-ATPase were unknown at that time, or ouabain titration profiles were never performed.

The molecular basis for inhibition of adipose conversion of murine 3T3-L1 cells by retinoic acid

The effect of retinoic acid (RA) on the adipose conversion of 3T3 cells has been studied. Differentiation of 3T3-L1 cells was initiated by addition of 0.5 mM methylisobutylxanthine, 0.3 microM dexamethasone and 10 micrograms/ml insulin (MDI) to confluent monolayers of preadipocytes for 48 h. During this time, the cells underwent DNA replication and cell division prior to the expression of adipose specific genes. RA administration had no apparent effect on the rate or extent of cell growth, cell division, or DNA replication. However, RA treatment concomitant with MDI addition inhibited triacylglycerol accumulation (I0.5 = 6 nM) and the accumulation of the differentiation-dependent mRNAs encoding the adipocyte lipid-binding protein (ALBP) and stearoyl-CoA desaturase 1 (SCD1). No inhibition occurred with RA addition either prior to or after MDI treatment. Runoff transcription revealed that the inhibitory effects of RA occurred at the level of transcription and were persistent. Cells treated with RA during the MDI regimen did not appreciably transcribe ALBP or SCD1 mRNAs several days following RA withdrawal. The effects of RA were specific for differentiation-dependent transcripts: 10(-6) M RA did not inhibit expression of the mRNAs encoding beta-tubulin or glutamine synthase. Examination of immediate-early transcription factor expression during the MDI regimen revealed that RA mediated an elevated, prolonged expression of c-Jun mRNA accompanied by diminished expression of c-Fos and Jun-B mRNAs. Given the previously demonstrated role of transcription factor AP-1 in ALBP gene expression, our results suggest that the initiation of expression of this and other adipocyte-specific genes during adipose conversion is regulated by the relative composition of transcription factor AP-1.

Human relaxin inhibits division but not differentiation of 3T3-L1 cells

For the first time, we demonstrate here the ability of human relaxin to block cell division. During the induction of differentiation of 3T3-L1 fibroblasts to adipocytes, the cells typically undergo two rounds of cell division followed by accumulation of lipid droplets and expression of insulin-stimulated glucose transport as the cells attain the adipocyte phenotype. Human relaxin added during induction had no effect on the development of the adipocyte phenotype or insulin-stimulated glucose transport. However, it blocked cell division at a half-maximal concentration of 1.25 nM, well within physiological range. This could be reversed by the addition of antibodies specific for human relaxin. Thus relaxin joins a select number of hormones with growth inhibitory properties such as transforming growth factor-beta (TGF beta) and mammastatin. Potentially, this is an important but until now unidentified function of relaxin. Unlike other inhibitory polypeptides, like TGF beta, relaxin does not prevent differentiation but rather uncouples it from cell division.

Sodium affinity of brain Na(+)-K(+)-ATPase is dependent on isozyme and environment of the pump

The sodium affinities for the two forms of the Na(+)-K(+)-ATPase in brain were characterized. To mimic physiological conditions, synaptosomes, which are pinched off presynaptic nerve termini, were used. Examination of the pump in vitro was performed by preparing synaptic plasma membranes (SPMs). It was first shown that synaptosomes contain the two forms of the Na(+)-K(+)-ATPase, alpha 1 and alpha 2, and that these forms have markedly different affinities for the inhibitory cardiac glycoside ouabain. The apparent dissociation constant (K0.5) of alpha 1 for sodium changed from 12 to 9 mM when going from synaptosomes to membranes. For alpha 2, however, a shift from 36 to 12.5 mM was evident. The conclusion is that in vivo alpha 2 exists as a low sodium affinity species but can be altered to a high-affinity form simply by vesicle disruption. By comparison, the Na(+)-K(+)-ATPase from the mouse fibroblast cell line, 3T3-F442A cells, expressed only the alpha 1-isozyme, as shown by immunoblotting and by measurement of its ouabain and sodium affinities. The physiological relevance of these observations is also presented.

Relationship between insulin stimulation and endogenous regulation of 2-deoxyglucose uptake in 3T3-L1 adipocytes

The occurrence of the endogenous regulatory response to high rates of 2-deoxyglucose (2-DG) uptake, as previously described for C6 glioma cells during incubation with 2 mM 2-DG (Lange et al.: J. Cell. Physiol., 1989), was studied in 3T3-L1 preadipocytes and adipocytes, and the influence of insulin on this endogenous uptake regulation was examined. In contrast to 3T3-L1 preadipocytes, insulin-sensitive differentiated 3T3-L1 adipocytes displayed the time-dependent cyclic pattern of 2-DG uptake rates characteristic of the membrane-limited and endogenously regulated cellular state of hexose utilization. Although insulin induced a threefold stimulation of 2-DG tracer uptake in adipocytes, the hormone did not additionally stimulate the uptake rates or affect the periodic response: maximum and minimum levels of uptake remained unchanged. Scanning electron microscopy (SEM) revealed that the acquirement of the differentiated state is accompanied by a conspicuous transformation of the smooth surface of undifferentiated 3T3-L1 cells into a surface covered by numerous microvilli of uniform size and appearance. Treatment with insulin (10 mU/ml; 10 minutes) converted these microvilli into voluminous saccular membrane protrusions of the same type as had been formed during incubation of 3T3-L1 adipocytes with 2 mM 2-DG, and which have previously been shown to be involved in the endogenous uptake regulation of C6 glioma cells (Lange et al.: J. Cell. Physiol., 1989). These insulin-induced saccated membrane areas appeared to become integrated into the cell surface. Accordingly, insulin treatment caused a twofold increase of the intracellular distribution space of 3-O-methylglucose (3-OMG) in 3T3-L1 adipocytes. This insulin-induced increase of the 3-OMG distribution space exhibited the same time (t1/2 = 2-2.5 minutes) and dose dependence (EC50 = 20 nM) as the insulin-induced stimulation of 3-OMG transport. Glucose deprivation during the differentiation period inhibited the outgrowth of microvilli from the cell surface. Glucose starvation (18 hours at less than 0.5 mM) induced a conspicuous reduction of the length of microvilli on differentiated 3T3-L1 cells. In this state, the stalks of the microvilli are almost invisible and the enlarged spherical tips of the microvilli (with an average diameter of 370 nm compared to 230 nm of fed cells) appeared to protrude directly out of the cell surface. Starvation-induced shortening of microvilli was accompanied by a threefold increase of the basal 3-OMG transport rate and a greater than twofold increase of the intracellular 3-OMG distribution space as compared to fed cells (10 mM; 18 hours).(ABSTRACT TRUNCATED AT 400 WORDS)

Correlation between the lipophilicity of substituted phenols and their inhibition of the Na+/K+-ATPase of Chinese hamster ovary cells

The Na+/K+-ATPase of Chinese Hamster Ovary (CHO) cells, a plasma membrane bound protein was used as a test system to evaluate the toxicity of several phenol derivatives on membranes. Taking only 2 physico-chemical parameters into consideration, viz., the logarithm of the octanol/water partition coefficient as an indicator for the lipophilicity and the sigma-Hammett constant as a measure for the polarity of the phenol substitutes, it was possible to predict the toxicity with high significance. A multivariate regression analysis calculated a correlation coefficient of 0.99. The results confirm studies performed in our laboratory on cytotoxicity and on functional membrane proteins of fungal and mammalian cells [1,2], suggesting a common mechanism of toxicity by the action of hydrophobic xenobiotics on biomembranes. Taking into account the different sensitivities of the test systems, Quantitative Structure-Activity Relationship (QSAR) analyses could help to explain the basic toxicity of several classes of environmental chemicals.

Endogenous regulation of 2-deoxyglucose uptake in C6 glioma cells correlates with cytoskeleton-mediated changes of surface morphology

The cellular basis of the membrane-limited state of glucose utilization and the mechanism of the endogenous regulation of hexose uptake in dense monolayers of C6 glioma cells were investigated. In an earlier study, it was shown that at high rates of glucose transport and phosphorylation combined with the inhibition of glycolytic adenosine triphosphate (ATP) production by iodoacetate, an endogenous regulatory response occurred that resulted in rapid, periodic variations of the glucose uptake rates (Lange et al., 1982). Similar time-dependent periodic changes of uptake rates also occurred during incubation of C6 glioma cells with 2 mM 2-deoxyglucose (2-DG) without pretreatment of the cells with iodoacetate. These changes were accompanied by variations of the intracellular ATP content, by distinct alterations of the shape and arrangement of microvilli and lamellae (lamellipodia) on the cell surface, and by changes of the cytoskeletal F-actin content. Because the changes of 2-DG uptake rates occurred independent of the intracellular 2-DG concentration, the bulk of this 2-DG pool was assumed to be localized apart from the membranal transport sites. Downregulation of 2-DG uptake appeared to be triggered by a rapid decrease of a small pool of the cellular ATP involved in the phosphorylation of transported hexose. Scanning and transmission electron microscopic observations of cells fixed in different states of the endogenous uptake regulation supported the assumption that the interior of lamellae and microvilli may represent a small entrance compartment for transported hexoses in which occurred the observed close coupling between hexose transport and phosphorylation as well as the rapid variations of ATP content. Hexose uptake is supposed to be regulated by cytoskeleton-mediated changes of volume and diffusional accessibility of this compartment, modulating the degree of its metabolic coupling with the cytoplasmic main compartment.

Isoforms of Na,K-ATPase in Artemia salina: II. Tissue distribution and kinetic characterization

To characterize the molecular properties conveyed by the isoforms of the alpha subunit of Na,K-ATPase, the two major transepithelial transporting organs in the brine shrimp (Artemia salina), the salt glands and intestines, were isolated in pure form. The alpha isoforms were quantified by ATP-sensitive fluorescein isothiocyanate (FITC) labeling. The salt gland enzyme exhibits only the alpha 1 isoform, whereas the intestinal enzyme exhibits both the alpha 1 and the alpha 2 isoforms. After 32 hours of development, Na,K-ATPase activity [in mumol Pi/mg protein/hr (1 mu)] in whole homogenates was 32 +/- 6 in the salt glands and 12 +/- 3 in the intestinal preparations (mean +/- SEM). The apparent half-maximal activation constants (K1/2) of the salt gland enzyme as compared to the intestinal enzyme were 3.7 +/- 0.6 mM vs. 23.5 +/- 4 mM (P less than 0.01) for Na+, 16.6 +/- 2.2 mM vs. 8.29 +/- 1.5 mM for K+ (P less than 0.01), and 0.87 +/- 0.8 mM vs. 0.79 +/- 1.1 mM for ATP (NS). The apparent Ki's for ouabain inhibition were 1.1 x 10(-4) M vs. 2 x 10(-5) M, respectively. Treatment of whole homogenates with deoxycholic acid (DOC) produced a maximal Na,K-ATPase activation of 46% in the salt gland as compared to 23% in the intestinal enzyme. Similar differences were found with sodium dodecyl sulfate (SDS). The two distinct forms of Na,K-ATPase isolated from the brine shrimp differed markedly in three kinetic parameters as well as in detergent sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of glucose transport activity and expression of glucose transporter mRNA by serum, growth factors and phorbol ester in quiescent mouse fibroblasts

We have investigated the effects of growth factors such as serum, platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) on glucose transport activity in quiescent mouse Swiss 3T3 cells. DNA synthesis was synchronously induced by either calf serum, or platelet-poor plasma in combination with PDGF or FGF. Early stimulation of glucose transport in the quiescent cells was also caused by serum, or by either PDGF or FGF. The time courses for the stimulation of transport were identical for serum, PDGF and FGF, and the stimulated uptake in each case was associated with a 5-6-fold increase in Vmax. There were no detectable changes in apparent Km. Expression of glucose transporter mRNA was also enhanced by these growth factors. By contrast, EGF, insulin and platelet-poor plasma had little effect on glucose transport and transporter-gene expression, although uridine uptake was enhanced by all of these growth factors. These results suggest that cell cycle-dependent stimulation of glucose transport and expression of the transporter mRNA are regulated by a specific class of growth factors such as PDGF and FGF. The tumor promoter phorbol 12-myristate 13-acetate (PMA) also stimulated glucose transport and expression of transporter mRNA in quiescent 3T3 cells. These stimulations were absent in PMA-pretreated cells. However, serum, PDGF and FGF were able to stimulate glucose transport as well as expression of the transporter mRNA in PMA-pretreated cells, suggesting that there are at least two independent pathways for regulating glucose transport and glucose transporter mRNA level in quiescent fibroblasts.

Molecular cloning and characterization of an insulin-regulatable glucose transporter

A major mechanism by which insulin stimulates glucose transport in muscle and fat is the translocation of glucose transporters from an intracellular membrane pool to the cell surface. The existence of a distinct insulin-regulatable glucose transporter was suggested by the poor cross-reactivity between antibodies specific for either the HepG2 or rat brain glucose transporters and the rat adipocyte glucose transporter. More direct evidence was provided by the production of a monoclonal antibody (mAb 1F8) specific for the rat adipocyte glucose transporter that immunolabels a species of relative molecular mass 43,000 (43K) present only in tissues that exhibit insulin-dependent glucose transport, suggesting that this protein may be encoded by a different gene from the previously described mammalian glucose transporters. This antibody has been used to immunoprecipitate a 43K protein that was photoaffinity-labelled with cytochalasin B in a glucose displaceable way, and to immunolabel a protein in the plasma membrane of rat adipocytes, whose concentration was increased at least fivefold after cellular insulin exposure. Here we describe the cloning and sequencing of cDNAs isolated from both rat adipocyte and heart libraries that encode a protein recognized by mAb 1F8, and which has 65% sequence identity to the human HepG2 glucose transporter. This cDNA hybridizes to an mRNA present only in skeletal muscle, heart and adipose tissue. Our data indicate that this cDNA encodes a membrane protein with the characteristics of the translocatable glucose transporter expressed in insulin-responsive tissues.

Ouabain-sensitive Na+-K+ ATPase pumps in cultured human retinal pigment epithelium

The expression of Na+-K+ ATPase pumps was studied in cultured human retinal pigment epithelium (RPE). Pump site number was measured by quantitation of the specific binding of [3H]ouabain to cultures of varying density. Specific binding of [3H]ouabain was time- and concentration-dependent, and was inhibited by potassium and by excess unlabeled ouabain. Estimates of pump site number based upon specific [3H]ouabain binding indicated that the number of pumps per RPE cell was maximal in sparse cultures and declined six-fold as cultures became confluent. Pump activity, determined by measurement of specific 86Rb (rubidium) uptake, was also greater in sparse than in dense cultures. Quantitation of [3H]thymidine incorporation as a measure of cell proliferation demonstrated that proliferation in RPE cultures decreased logarithmically as culture density increased. Increased pump site number in cultured RPE, therefore, correlated with increased cell proliferation and decreased culture density. We conclude that human RPE express ouabain-sensitive Na+-K+ ATPase in vitro and maximal expression is observed in sparse, proliferating cultures.

Isolation and characterization of human breast milk lipoamidase

The mean lipoamidase activity in human breast milk was found to be 0.073 nmol/min per mg (S.D. = 0.028, range = 0.020-0.123, n = 44). The mean lipoamidase activity is approximately 3-fold higher in milk than that in serum (0.023 nmol/min per mg, S.D. = 0.016, range = 0.001-0.059, n = 32). Lipoamidase was purified to 4400-fold by a four-step procedure from 330 ml of human breast milk. The purified enzyme was identified as a single band (Mr = 135,000) by sodium dodecyl sulfate/polyacrylamide electrophoresis. Analysis by Edman degradation indicated that the N-terminal amino acid was glycine. These results strongly suggest that milk lipoamidase is composed of a single polypeptide chain. The enzyme is considered to be a glycoprotein since it reacted positively to periodate-Schiff (PAS) staining. The isoelectric point of the enzyme was 4.2. After treatment of lipoamidase with sialidase, its position on isoelectric focusing gel moved from pH 4.2 to 4.6. This is strongly indicative that lipoamidase contains sialic acid residues. The optimum pH for the enzyme activity is 7.0. The Michaelis constant (KM) for lipoyl p-aminobenzoate is calculated as 25 microM. The enzyme activity was completely lost by heating 60 degrees C for 5 min. The effects of thiol-reactive agents, such as 2-mercaptoethanol (ME) and p-chloromercuribenzoate, were not significant. However, the enzyme activity was completely inhibited by 50 microM diisopropylfluorophosphate. Thus, this enzyme seemed to contain an essential serine residue in the active site.

Hypertension and diabetes

Diabetes mellitus and hypertension are both common diseases, especially with an increasingly aged population. Hypertension accelerates the development of diabetic retinopathy, nephropathy, and peripheral vascular disease in the diabetic patient. Diabetes represents a type of premature aging and hypertension in the diabetic patient is characterized by many of the same pathophysiologic properties seen in the elderly hypertensive patient.

Induction of the stress response: alterations in membrane-associated transport systems and protein modification in heat shocked or Sindbis virus-infected cells

Heat shock or Sindbis virus infection of chick embryo (CE) or baby hamster kidney (BHK) cells resulted in a decrease in the uptake of 86Rubidium+, a K+ tracer. Both stressful treatments decreased 86Rb+ uptake by inhibition of the ouabain-sensitive Na+/K+ ATPase. Alterations in the intracellular levels of monovalent ions may be involved in translational or transcriptional control of the stress response. Heat shock or Sindbis virus infection also resulted in an increase in rate of uptake of [3H]deoxy-D-glucose and a decrease in the incorporation of [3H]glucosamine or [3H]mannose into most cellular proteins. These results suggested that heat shock or Sindbis virus infection alter hexose metabolism and that abnormally glycosylated proteins may accumulate in stressed cells. Exposure of uninfected chick embryo cells to elevated temperature had little effect on the overall rate of incorporation of [32P]orthophosphate into cellular proteins. However, one protein (Mr 31,000; pp31) displayed increased incorporation of [32P]orthophosphate and two other proteins (Mr 33,000 and 20,000; pp33 and pp20) displayed decreased incorporation. Sindbis virus infection failed to mimic or to modify these heat shock induced alterations in protein phosphorylation.

Sindbis virus infection increases hexose transport in quiescent cells

Sindbis virus infection of baby hamster kidney cells or chick embryo cells resulted in a significant increase in the rate of uptake of [2-3H]deoxy-D-glucose ([3H]dGlu). Stimulation of hexose transport in Sindbis virus-infected cells occurred only if the cells were rendered quiescent by culturing at high density or by serum starvation. In contrast, Sindbis virus-induced inhibition of potassium transport, measured as a decrease in the uptake of 86Rb+, was independent of cell growth state. Stimulation of [3H]dGlu uptake in Sindbis virus-infected cells was the result of an increase in the Vmax of the hexose transporter, but not a change in the Km. The stimulation of [3H]dGlu uptake induced by Sindbis virus was insensitive to the drug actinomycin D, but was blocked by cordycepin. The stimulation was also insensitive to treatment with tunicamycin, which prevented the virally induced inhibition of the plasma membrane-associated Na+/K+ ATPase and termination of host protein synthesis.