Reassessment of the translocation hypothesis by kinetic studies on hexose transport in isolated rat adipocytes

Effects of brazilin on GLUT4 recruitment in isolated rat epididymal adipocytes

The effects of brazilin on glucose transport into isolated rat epididymal adipocytes were investigated. Brazilin increased [3H]2-deoxy-D-glucose uptake, which was characterized by an increase in Vmax with no effect on the Km value. Phenylarsine oxide, which inhibits the translocation of glucose transporters, decreased brazilin-stimulated glucose transport to the basal level. The inhibition of phosphatidylinositol 3-kinase (PI3-kinase) with wortmannin also blocked brazilin-stimulated glucose transport. Western blot analysis with an anti-GLUT4 antibody revealed that brazilin increased the translocation of GLUT4 from intracellular pools to the plasma membrane. Brazilin, in combination with phorbol ester, showed an additive effect on glucose transport. The stimulating effect of phorbol ester on glucose transport was inhibited by staurosporine, but the effect of brazilin remained unchanged. Protein kinase C activity was not influenced by brazilin treatment. The inhibition of protein synthesis showed no effect on brazilin-stimulated glucose transport, and GLUT4 content in the total membrane fraction was not altered as a result of treatment with brazilin for 4 hr. Metabolic labeling of GLUT4 with [35S]methionine showed that de novo synthesis of GLUT4 was not induced by brazilin. These data suggest that brazilin may increase glucose transport by recruitment of GLUT4 from intracellular pools to the plasma membrane of adipocytes via the activation of PI3-kinase. However, the effect of brazilin may not be mediated by GLUT4 synthesis and protein kinase C activation.

Stimulation of glucose transport in Clone 9 cells by insulin and thyroid hormone: role of GLUT-1 activation

Thyroid hormone (T3) and insulin are both shown to stimulate glucose transport in Clone 9 cells, a rat liver cell line in which the utilization of glucose is limited by transport rate and in which only the GLUT-1 transporter isoform is expressed. Pre-treatment of these cells with T3 moreover substantially enhances the stimulatory effect of insulin such that at maximally effective hormone concentrations the effects of T3 and insulin on glucose transport are more than additive and indeed nearly multiplicative, suggesting that the mechanisms mediating the enhancement of glucose transport differ between the two hormones. Cell surface biotinylation followed by Western-blot analysis of plasma membrane fractions showed that the stimulatory effects of T3 and insulin on glucose transport, whether acting singly or in combination, exceed the attendant increases in the abundance of GLUT-1 in the plasma membrane. It is suggested that activation of GLUT-1 molecules pre-existing in the plasma membrane plays a major role in mediating the stimulatory effects of T3 and insulin on glucose transport in this cell line.

Use of hexose transport mutants to examine the expression and properties of the rat myoblast GLUT 1 transport process

Rat L6 myoblasts were recently shown to possess the GLUT 1, 3 and 4 transporters, and not the GLUT 2 isoform [1]. This investigation examined the expression and properties of the GLUT 1 isoform. GLUT 1 transcript level was significantly reduced in cells grown at high densities and during myogenic differentiation. A comparison of the GLUT 1 and 4 transcript levels in myogenesis-competent and impaired cells revealed an inverse relationship between these two isoforms. This relationship was confirmed by studies using two independent spontaneous GLUT 3- GLUT 4- mutants, M1 and M3. These mutants possessed very high level of the GLUT 1 isoform, but negligible amount of the GLUT 3 and 4 isoforms. GLUT 1 expression was also subject to positive regulation. Glucose starvation was found to increase not only the levels of the GLUT 1 transcript and transporter, but also the intrinsic activity of the GLUT 1 transporter. Studies with M1 and M3 mutants revealed that the GLUT 1 transporter was not functional in glucose-grown cells, even though it was present at a very high level in the plasma membrane. This transporter became functional when cells were starved for glucose. The functional GLUT 1 transporter had an apparent Km value of around 0.9 mM, and was sensitive to cytochalasin B, phloretin, phlorizin and pCMBS.

Inhibitors of protein synthesis cause increased hexose transport in cultured human fibroblasts by a mechanism other than transporter translocation

We have investigated the effect of various inhibitors of protein synthesis on hexose transport in human skin fibroblasts using 2-deoxy-D-glucose (2-DG) and 3-0-methyl-D-glucose (3-OMG) to measure hexose transport. Exposure of glucose-fed, serum-free cultures to cycloheximide (CHX) (50 micrograms/ml) for 6 h resulted in increased 2-DG transport (3.81 +/- .53 vs. 6.62 +/- .88 nmoles/mg protein/2 min; n = 9) and 3-OMG transport (1.36 +/- .66 vs. 3.18 +/- .83 nmoles/mg protein/30 sec; n = 4) in the CHX exposed group. Under these conditions inhibition of protein synthesis was greater than 90%. This CHX induced transport increase was time dependent (approaching maximum within 1 h of exposure to CHX) and related to an increase in the Vmax of hexose transport in the CHX exposed group (18.4 +/- 2.4 vs. 4.8 +/- 1.1 nmoles 2-DG/mg protein/min) with no difference in the transport Km (1.55 +/- .63 vs. 2.92 +/- .59 mM). Further, the CHX induced increase in hexose transport was reversible. Exposure of human fibroblasts to inhibitors of protein synthesis with different mechanisms of action (e.g., puromycin, pactamycin, or CHX) all generated hexose transport increases in a concentration-dependent fashion correlating with their increasing inhibitory effects on protein synthesis. Nucleotidase enriched (i.e., plasma membrane) fractions of control and CHX-exposed cells showed no differences in D-glucose inhibitable cytochalasin B binding activity. Further, quantitative Western analysis of nucleotidase enriched fractions indicated CHX exposure resulted in no significant increase in glucose transporter mass compared with control plasma membrane fractions. Glucose deprived cells, however, which exhibited increased sugar transport comparable to the CHX-exposed group, did show increased glucose transporter mass in the plasma membrane fraction. The data indicate that inhibitors of protein synthesis can cause a significant elevation in hexose transport and that the hexose transporter mass in the isolated plasma membrane fractions did not reflect the whole cell transport change. It is suggested that a mechanism other than glucose transporter translocation to the plasma membrane may be involved in causing this sugar transport increase.

Regulation of glucose transport as well as glucose transporter and immediate early gene expression in 3T3-L1 preadipocytes by 8-bromo-cAMP

In the present study we have examined the ability of 8-bromoadenosine cyclic 3',5'-phosphate (8-bromo-cAMP; the membrane permeant analog of cAMP which can activate protein kinase A) to mimic hormone action and stimulate glucose transport and glucose transporter (GLUT-1) gene expression as well as the expression of several growth-related protooncogenes in quiescent 3T3-L1 fibroblasts. 8-Bromo-cAMP induced a rapid and prolonged increase in the rate of hexose transport. Early activation of hexose transport (within 30 min) was associated with increased plasma membrane immunoreactive glucose transporters, which corresponded to a doubling in the number of D-glucose-displaceable, plasma membrane cytochalasin B binding sites. The time course for 8-bromo-cAMP-induced hexose transport preceded the accumulation of GLUT-1 mRNA, which peaked between 4 and 8 h after exposure to the agent, and subsequently declined to approach basal (control) levels. Expression of the immediate-early genes c-fos and jun-B was induced by 8-bromo-cAMP on a rapid, but sustained time course, whereas induction of c-jun expression was delayed. Alterations in specific mRNAs following exposure to 8-bromo-cAMP were due to increased gene transcription (as judged by nuclear transcription run-on assays), although with respect to GLUT-1, an increase in mRNA stability was also observed. Treatment of the cells with forskolin resulted in the induction of GLUT-1 expression as well as expression of the immediate early genes. Exposure of quiescent 3T3-L1 fibroblasts to 8-bromo-cAMP resulted in a substantial increase in rates of total protein and RNA synthesis, but had little effect on DNA synthesis. The results demonstrate that 8-bromo-cAMP initiated a G0/G1 transition, but did not permit progression into S-phase. The results further suggest that increased cytosolic cAMP results in the stimulation of glucose transport by three distinct mechanisms to include translocation of pre-existing transporters, increased transcription of the GLUT-1 gene and increased stability of GLUT-1 mRNA.

Enhancement of glucose transport in clone 9 cells by exposure to alkaline pH: studies on potential mechanisms

Incubation of a nontransformed rat liver cell line, Clone 9, at pH 8.5 resulted in an approximately 16-fold stimulation of cytochalasin B-inhibitable 3-O-methylglucose (3-OMG) transport, an effect that was independent of the presence of serum. Exposure to 100 ng/ml 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulated 3-OMG uptake, and the enhancement was not additive to that produced by incubation at pH 8.5. In cells "depleted" of protein kinase C activity by a 20-hr exposure to TPA, however, the stimulation of 3-OMG transport in response to incubation at alkaline pH was still fully demonstrable. In control and alkaline pH-exposed cells, the inhibition of 3-OMG uptake by cytochalasin B was consistent with a single-site ligand binding model (K1 approximately 10(-7) M). Northern blot analysis demonstrated the presence of only the human erythrocyte/rat brain/HepG2 cell glucose transporter-mRNA isoform (EGT), and the abundance of this mRNA was unchanged following exposure to alkaline pH. Immunoblot analysis, using polyclonal antibodies directed against the carboxy-terminal dodecapeptide of EGT, demonstrated an approximately 2.0-fold increase in the abundance of transporters in partially purified plasma membrane fractions following incubation at pH 8.5, while EGT abundance was unchanged in whole-cell extracts. It is concluded that the stimulation of glucose transport in response to incubation of Clone 9 cells at alkaline pH does not require the presence of serum or activation of protein kinase C, and that the response is at least in part mediated by an increase in the number of glucose transporters in the plasma membrane.

Highly insulin-responsive isolated rat heart muscle cells yielded by a modified isolation method

Freshly isolated adipocytes or cardiac myocytes appear to be subject to unspecific stimulation during isolation and subsequent handling, e.g. with respect to glucose transport. We have developed a modified procedure that yields rat cardiomyocytes with a very low basal, i.e. non stimulated hexose uptake rate (ca. 3 pmol * s-1 * mg protein-1 at 1 mM sugar), as compared to data reported by others. This low value correlates with the reported oxygen consumption of non-beating, isolated rat hearts, when these are perfused with glucose as the only substrate. The basal rate of glucose uptake in our quiescent cardiomyocytes is slightly lower than the value measured by others in beating rat hearts in vivo. Insulin (10 nM) stimulates 2-deoxy-D-glucose uptake 8- to 20-fold and 3-O-methyl-D-glucose uptake 14- to 20-fold, as compared to control. This insulin effect is markedly larger than that usually observed in isolated cardiomyocytes, but it is similar in magnitude to the stimulation of glucose transport reported for isolated, perfused rat hearts. In these cells, new stimulatory effects on the glucose transport, e.g. that of sulfhydryl reagents like phenylarsine oxide, become apparent. We conclude that the cardiomyocytes obtained by this modified method exhibit a basal glucose transport rate that is close to physiological values. These cells represent a new highly responsive model to detect and to investigate the effects of glucose transport stimulators (insulin, contraction etc.).

Different effects of two proteinase inhibitors on insulin-induced cellular responses in rat adipocytes

Among various proteinase inhibitors, N-acetyl-L-tyrosine ethyl ester (ATEE), a chymotrypsin substrate analog, and N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), a trypsin inhibitor, showed significant inhibitory effects on insulin stimulated glucose transport in rat adipocytes. ATEE did not affect insulin binding, but inhibited insulin internalization. In intact adipocytes, ATEE inhibited tyrosine phosphorylation of the beta-subunit of the insulin receptor, a 170 kDa protein and a 60 kDa protein at almost the same concentration (ID50 = 0.24 +/- 0.05 mM, n = 4, mean +/- S.E.), but in a plasma membrane fraction, ATEE did not appreciably inhibit the tyrosine phosphorylation of the beta-subunit of the insulin receptor, TLCK did not inhibit insulin binding. At 0.25 mM, TLCK did not inhibit insulin internalization, but inhibited 70% of the insulin-stimulated glucose transport (ID50 = 0.19 +/- 0.02 mM, n = 7). TLCK inhibited insulin internalization at more than 0.25 mM. TLCK did not inhibit the tyrosine phosphorylation of the beta-subunit of the insulin receptor in intact cells or in the plasma membrane fraction. In intact cells, TLCK inhibited the phosphorylation of the 60 kDa protein and simultaneously it stimulated the phosphorylation of the 170 kDa protein more than 3-fold. These results indicate that there are at least two sites in the insulin-induced signal transduction pathway where proteinase inhibitors act to suppress the insulin signal transduction. A major ATEE site is very close to phosphorylation of the beta-subunit of the insulin receptor. On the other hand, TLCK inhibits a step(s) in the signal transduction pathway after the insulin receptor but before the glucose transporter.

Regulation of insulin receptor functions by a peptide derived from a major histocompatibility complex class I antigen

A 25 residue peptide, Dk-(61-85), derived from the alpha 1 domain of a murine MHC class I molecule (H-2Dk), enhances cellular glucose uptake, prolongs the effect of insulin, and inhibits insulin receptor internalization without affecting insulin binding or dissociation. Full effect of the peptide is obtained at 10-100 microM. The magnitude of the peptide-mediated enhancement of glucose uptake is insulin dependent and is at maximum approximately 50% above that of full insulin stimulation, excluding a merely insulinomimetic action of the peptide. Dk-(61-85) does not interact directly with the glucose transporter molecule. Furthermore, the peptide-mediated inhibition of insulin receptor internalization results in 2-3 times more receptors in the plasma membrane. The peptide also causes hypoglycemia in rats. The biological activity of Dk-(61-85) suggests that an important nonimmunological role of MHC class I molecules is to affect some of the key functions of ligand-activated receptors.

Stimulation of glucose transport in Clone 9 cells by exposure to alkaline pH

Incubation of a rat liver cell line (Clone 9) for 2 h at pH 8.5 was found to result in a profound (5- to 8-fold) stimulation of cytochalasin B-inhibitable glucose transport. The enhancement of glucose transport after exposure to elevated external pH (achieved by lowering the CO2 tension in a bicarbonate-containing medium) was demonstrable within 15 min, was half-maximal at pH 8.0, and was near-maximal at pH 8.6. Intracellular pH rose linearly with incremental changes in external pH, from pH 7.45 to 8.6 with a slope of 0.6. The increase in transport activity in response to incubation at alkaline pH was accompanied by a parallel increase in lactate production and persisted for more than an hour after external pH was restored to normal. During the latter period, intracellular glucose concentration (less than 10% of that of the external medium under control conditions) increased greater than 10-fold to approximate that in the extracellular medium. Incubation of these cells at pH 8.5 for 2 h resulted in a complete resistance of cell ATP levels to challenge with 5 mM cyanide, suggesting that the adaptive facilitation of glucose transport was of sufficient magnitude to permit a marked stimulation of glycolytic ATP synthesis on inhibition of oxidative phosphorylation. The enhancement of glucose transport was attributable to an increase in the maximum velocity (Vmax) rather than to any change in the Michaelis constant (Km) for transport and was not prevented by cycloheximide. It is concluded that the marked stimulation of glucose transport resulting from exposure of these "low-glucose" cells to alkaline pH reflects either an increase in the abundance of functional glucose transporters in the plasma membrane or an increase in their catalytic turnover rate.

A further comparison of insulin- and phorbol ester-stimulated glucose transport in adipocytes

Insulin and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) stimulatory effects on adipocyte glucose transport were compared for their sensitivity to: (1) sphingosine and staurosporine, two potent protein kinase C (PKC) inhibitors; and (2) phenylarsine oxide (PhAsO), a dithiol reagent blocking insulin-stimulated glucose transport. None affected basal 2-deoxyglucose transport, cell viability, cellular ATP content, or insulin binding. Insulin- and PMA-stimulated 2-deoxyglucose transport were both markedly inhibited by sphingosine (5-50 microM) and staurosporine (0.1-2 microM), although with differences in the extents of maximal inhibitions (65 and 48% vs. 88 and 98%) and the concentrations of the drugs causing the half-maximal inhibitions observed in the experiments (2- to 3-fold higher for insulin). Insulin and PMA both altered PKC along with glucose transport, either by increasing its activity in the cytosol or by promoting its translocation to membrane. Insulin- and PMA-stimulated 2-deoxyglucose transport were both inhibited selectively by PhAsO (0.1-1 microM), at almost identical maximal inhibitions (84 and 90%) and IC50 values (0.18 and 0.16 microM). Furthermore, insulin- and PMA-induced increases in transport Vmax (6.5- and 3.4-fold) were both reduced by 89% by PhAsO, which, however, failed to affect the decrease in transport Km (1.7-fold) exclusively induced by insulin. Likewise, PhAsO did not affect insulin or PMA activation of PKC. The results suggest that insulin activates adipocyte glucose transport through: (1) a PKC-dependent mechanism requiring cellular dithiols, responsible for a part of the hormone-induced increase in transport Vmax; and (2) a PKC-independent mechanism responsible for both a further increase in transport Vmax and a decrease in transport Km.

Enhanced glucose transport in response to inhibition of respiration in Clone 9 cells

An acceleration of ATP synthesis by anaerobic glycolysis provides important compensation for interference with respiration in a variety of cells. Effective compensation for an inhibition of respiration, however, can occur in cells in which glucose entry is rate limiting only if sufficient glucose becomes available through an enhancement of transport. We present here a detailed study of the effects of inhibition of respiration in Clone 9 cells, a continuous cell line characterized by low internal glucose concentrations (less than 10% that of the external medium) and minimal stores of glycogen. Exposure of these cells to 5 mM cyanide results in a 90% fall in cell ATP and a twofold rise in cell Na+ within 20 min. By the end of 1 h, however, there is a 4.5- to 7-fold increase in cytochalasin B-inhibitable glucose transport that is accompanied by a parallel increase in the rate of lactate production, a partial recovery of cell ATP, and no further rise in cell Na+. The acute fall in ATP resulting from a submaximally effective concentration of cyanide (0.5 mM) is moreover followed by a time-dependent recovery of cell ATP to near-normal levels and subsequent resistance to challenge with even 5 mM cyanide. The stimulation of facilitative glucose transport resulting from exposure to cyanide is attributable to an increase in maximal velocity rather than to a change in Km and persists for more than 2 h after removal of the inhibitor. These results demonstrate that, in these cells characterized by low internal glucose concentrations, regulation of glucose entry is of central importance in ATP homeostasis and that a major component of the adaptive response to an inhibition of respiration is a time-dependent increase in glucose transport.

Effects of three proposed inhibitors of adipocyte glucose transport on the reconstituted transporter

Three compounds which inhibit glucose transport in rat adipocytes have been proposed to act directly on the glucose transporter protein. We tested these proposals by examining the effects of the compounds on the stereospecific glucose uptake catalyzed by adipocyte membrane proteins after reconstitution into liposomes. Effects on the transport activity reconstituted from human erythrocyte membranes were also examined. Glucose 6-phosphate, which was suggested to inhibit the transporter noncompetitively (Foley, J.E. and Huecksteadt, T.P. (1984) Biochim. Biophys. Acta 805, 313-316), had no effect on either type of reconstituted transporter, even when present at 5 mM on both sides of the liposomal membranes. Thus, it is unlikely to act directly on the transporter. The metalloendoproteinase substrate dipeptide Cbz-Gly-Phe-NH2, which inhibited insulin-stimulated but not basal glucose uptake in adipocytes (Aiello, L.P., Wessling-Resnick, M. and Pilch, P.F. (1986) Biochemistry 25, 3944-3950), inhibited the reconstituted erythrocyte transporter noncompetitively with a Ki of 1.5-2 mM. The inhibition of the erythrocyte transporter was identical in liposomes of soybean and egg lipid. Transport reconstituted using adipocyte membrane fractions was also inhibited by the dipeptide, with the activity from basal microsomes more sensitive than that from insulin-stimulated plasma membranes. These results indicate that the dipeptide interacts directly with the transporter, and may be a potentially useful probe for changes in transporter structure accompanying insulin action. Phenylarsine oxide, which was suggested to act directly on the adipocyte transporter (Douen, A.G., and Jones, M.N. (1988) Biochim. Biophys. Acta 968, 109-118), produced only slight (about 10%) inhibition of the reconstituted adipocyte and erythrocyte transporters, even when present at 100-200 microM and after 30 min of pretreatment. These results suggest that the major actions of phenylarsine oxide observed in adipocytes are not direct effects on the transporter, but rather effects on the pathways by which insulin regulates glucose transport activity (Frost, S.C. and Lane, M.D. (1985) J. Biol. Chem. 260, 2646-2652).