Potential role for insulin and cycloheximide in regulating the intrinsic activity of glucose transporters in isolated rat adipocytes

GLUT4 vesicle recruitment and fusion are differentially regulated by Rac, AS160, and Rab8A in muscle cells

Insulin increases glucose uptake into muscle by enhancing the surface recycling of GLUT4 transporters. In myoblasts, insulin signals bifurcate downstream of phosphatidylinositol 3-kinase into separate Akt and Rac/actin arms. Akt-mediated Rab-GAP AS160 phosphorylation and Rac/actin are required for net insulin gain of GLUT4, but the specific steps (vesicle recruitment, docking or fusion) regulated by Rac, actin dynamics, and AS160 target Rab8A are unknown. In L6 myoblasts expressing GLUT4myc, blocking vesicle fusion by tetanus toxin cleavage of VAMP2 impeded GLUT4myc membrane insertion without diminishing its build-up at the cell periphery. Conversely, actin disruption by dominant negative Rac or Latrunculin B abolished insulin-induced surface and submembrane GLUT4myc accumulation. Expression of non-phosphorylatable AS160 (AS160-4P) abrogated membrane insertion of GLUT4myc and partially reduced its cortical build-up, an effect magnified by selective Rab8A knockdown. We propose that insulin-induced actin dynamics participates in GLUT4myc vesicle retention beneath the membrane, whereas AS160 phosphorylation is essential for GLUT4myc vesicle-membrane docking/fusion and also contributes to GLUT4myc cortical availability through Rab8A.

PI 4,5-P2 stimulates glucose transport activity of GLUT4 in the plasma membrane of 3T3-L1 adipocytes

Insulin-stimulated glucose uptake through GLUT4 plays a pivotal role in maintaining normal blood glucose levels. Glucose transport through GLUT4 requires both GLUT4 translocation to the plasma membrane and GLUT4 activation at the plasma membrane. Here we report that a cell-permeable phosphoinositide-binding peptide, which induces GLUT4 translocation without activation, sequestered PI 4,5-P2 in the plasma membrane from its binding partners. Restoring PI 4,5-P2 to the plasma membrane after the peptide treatment increased glucose uptake. No additional glucose transporters were recruited to the plasma membrane, suggesting that the increased glucose uptake was attributable to GLUT4 activation. Cells overexpressing phosphatidylinositol-4-phosphate 5-kinase treated with the peptide followed by its removal exhibited a higher level of glucose transport than cells stimulated with a submaximal level of insulin. However, only cells treated with submaximal insulin exhibited translocation of the PH-domains of the general receptor for phosphoinositides (GRP1) to the plasma membrane. Thus, PI 4,5-P2, but not PI 3,4,5-P3 converted from PI 4,5-P2, induced GLUT4 activation. Inhibiting F-actin remodeling after the peptide treatment significantly impaired GLUT4 activation induced either by PI 4,5-P2 or by insulin. These results suggest that PI 4,5-P2 in the plasma membrane acts as a second messenger to activate GLUT4, possibly through F-actin remodeling.

Implications of glucose transporter protein type 1 (GLUT1)-haplodeficiency in embryonic stem cells for their survival in response to hypoxic stress

Glucose transporter protein type 1 (GLUT1) is a major glucose transporter of the fertilized egg and preimplantation embryo. Haploinsufficiency for GLUT1 causes the GLUT1 deficiency syndrome in humans, however the embryo appears unaffected. Therefore, here we produced heterozygous GLUT1 knockout murine embryonic stem cells (GT1+/-) to study the role of GLUT1 deficiency in their growth, glucose metabolism, and survival in response to hypoxic stress. GT1(-/-) cells were determined to be nonviable. Both the GLUT1 and GLUT3 high-affinity, facilitative glucose transporters were expressed in GT1(+/+) and GT1(+/-) embryonic stem cells. GT1(+/-) demonstrated 49 +/- 4% reduction of GLUT1 mRNA. This induced a posttranscriptional, GLUT1 compensatory response resulting in 24 +/- 4% reduction of GLUT1 protein. GLUT3 was unchanged. GLUT8 and GLUT12 were also expressed and unchanged in GT1(+/-). Stimulation of glycolysis by azide inhibition of oxidative phosphorylation was impaired by 44% in GT1(+/-), with impaired up-regulation of GLUT1 protein. Hypoxia for up to 4 hours led to 201% more apoptosis in GT1(+/-) than in GT1(+/+) controls. Caspase-3 activity was 76% higher in GT1(+/-) versus GT1(+/+) at 2 hours. Heterozygous knockout of GLUT1 led to a partial GLUT1 compensatory response protecting nonstressed cells. However, inhibition of oxidative phosphorylation and hypoxia both exposed their increased susceptibility to these stresses.

A dominant-negative p38 MAPK mutant and novel selective inhibitors of p38 MAPK reduce insulin-stimulated glucose uptake in 3T3-L1 adipocytes without affecting GLUT4 translocation

Participation of p38 mitogen-activated protein kinase (p38) in insulin-induced glucose uptake was suggested using pyridinylimidazole p38 inhibitors (e.g. SB203580). However, the role of p38 in insulin action remains controversial. We further test p38 participation in glucose uptake using a dominant-negative p38 mutant and two novel pharmacological p38 inhibitors related to but different from SB203580. We present the structures and activities of the azaazulene pharmacophores A291077 and A304000. p38 kinase activity was inhibited in vitro by A291077 and A304000 (IC(50) = 0.6 and 4.7 microm). At higher concentrations A291077 but not A304000 inhibited JNK2alpha (IC(50) = 3.5 microm). Pretreatment of 3T3-L1 adipocytes and L6 myotubes expressing GLUT4myc (L6-GLUT4myc myotubes) with A291077, A304000, SB202190, or SB203580 reduced insulin-stimulated glucose uptake by 50-60%, whereas chemical analogues inert toward p38 were ineffective. Expression of an inducible, dominant-negative p38 mutant in 3T3-L1 adipocytes reduced insulin-stimulated glucose uptake. GLUT4 translocation to the cell surface, immunodetected on plasma membrane lawns of 3T3-L1 adipocytes or on intact L6-GLUT4myc myotubes, was not altered by chemical or molecular inhibition of p38. We propose that p38 contributes to enhancing GLUT4 activity, thereby increasing glucose uptake. In addition, the azaazulene class of inhibitors described will be useful to decipher cellular actions of p38 and JNK.

Regulation of lactate production by FSH, iL1beta, and TNFalpha in rat Sertoli cells

One of the "nurse cell" functions of Sertoli cells is to provide lactate for the energy production in spermatocytes and spermatids. The present study shows that, as in porcine Sertoli cells, interleukin (IL)1beta and follicle-stimulating hormone (FSH) increase lactate production in rat Sertoli cells (basal, 9.1 +/- 1.0; FSH (100 ng/ml), 16.6 +/- 2.0; IL1beta (50 ng/ml), 13.3 +/- 1.6 microg/microg DNA). Increments in glucose uptake (basal, 1083 +/- 70; FSH, 2686 +/- 128; IL1beta, 1899 +/- 74 dpm/microg DNA), lactic dehydrogenase (LDH) activity (basal, 36.6 +/- 4.1; FSH, 52.2 +/- 4.9; IL1beta, 55.3 +/- 5.1 mUI/microg DNA), LDH A mRNA levels, and redistribution of LDH isozymes are involved in these stimulatory effects. Differences in the period required by IL1beta to increase glucose uptake, as compared with the porcine model, have been observed. In addition, tumor necrosis factor alpha (TNFalpha), one of the major stimulators for lactate production in porcine Sertoli cells, does not control the secretion of this glucose metabolite in rat Sertoli cells. Lactate production may be regulated differently among mammals.

Regulation of glucose transport by hypoxia

Transport of glucose into most mammalian cells and tissues is rate-controlling for its metabolism. Glucose transport is acutely stimulated by hypoxic conditions, and the response is mediated by enhanced function of the facilitative glucose transporters (Glut), Glut1, Glut3, and Glut4. The expression and activity of the Glut-mediated transport is coupled to the energetic status of the cell, such that the inhibition of oxidative phosphorylation resulting from exposure to hypoxia leads to a stimulation of glucose transport. The premise that the glucose transport response to hypoxia is secondary to inhibition of mitochondrial function is supported by the finding that exposure of a variety of cells and tissues to agents such as azide or cyanide, in the presence of oxygen, also leads to stimulation of glucose transport. The mechanisms underlying the acute stimulation of transport include translocation of Gluts to the plasma membrane (Glut1 and Glut4) and activation of transporters pre-exiting in the plasma membrane (Glut1). A more prolonged exposure to hypoxia results in enhanced transcription of the Glut1 glucose transporter gene, with little or no effect on transcription of other Glut genes. The transcriptional effect of hypoxia is mediated by dual mechanisms operating in parallel, namely, (1) enhancement of Glut1 gene transcription in response to a reduction in oxygen concentration per se, acting through the hypoxia-signaling pathway, and (2) stimulation of Glut1 transcription secondary to the associated inhibition of oxidative phosphorylation during hypoxia. Among the various hypoxia-responsive genes, Glut1 is the first gene whose rate of transcription has been shown to be dually regulated by hypoxia. In addition, inhibition of oxidative phosphorylation per se, and not the reduction in oxygen tension itself, results in a stabilization of Glut1 mRNA. The increase in cell Glut1 mRNA content, resulting from its enhanced transcription and decreased degradation, leads to increased cell and plasma membrane Glut1 content, which is manifested by a further stimulation of glucose transport during the adaptive response to prolonged exposure to hypoxia.

An inhibitor of p38 mitogen-activated protein kinase prevents insulin-stimulated glucose transport but not glucose transporter translocation in 3T3-L1 adipocytes and L6 myotubes

The precise mechanisms underlying insulin-stimulated glucose transport still require investigation. Here we assessed the effect of SB203580, an inhibitor of the p38 MAP kinase family, on insulin-stimulated glucose transport in 3T3-L1 adipocytes and L6 myotubes. We found that SB203580, but not its inactive analogue (SB202474), prevented insulin-stimulated glucose transport in both cell types with an IC50 similar to that for inhibition of p38 MAP kinase (0.6 microM). Basal glucose uptake was not affected. Moreover, SB203580 added only during the transport assay did not inhibit basal or insulin-stimulated transport. SB203580 did not inhibit insulin-stimulated translocation of the glucose transporters GLUT1 or GLUT4 in 3T3-L1 adipocytes as assessed by immunoblotting of subcellular fractions or by immunofluorescence of membrane lawns. L6 muscle cells expressing GLUT4 tagged on an extracellular domain with a Myc epitope (GLUT4myc) were used to assess the functional insertion of GLUT4 into the plasma membrane. SB203580 did not affect the insulin-induced gain in GLUT4myc exposure at the cell surface but largely reduced the stimulation of glucose uptake. SB203580 had no effect on insulin-dependent insulin receptor substrate-1 phosphorylation, association of the p85 subunit of phosphatidylinositol 3-kinase with insulin receptor substrate-1, nor on phosphatidylinositol 3-kinase, Akt1, Akt2, or Akt3 activities in 3T3-L1 adipocytes. In conclusion, in the presence of SB203580, insulin caused normal translocation and cell surface membrane insertion of glucose transporters without stimulating glucose transport. We propose that insulin stimulates two independent signals contributing to stimulation of glucose transport: phosphatidylinositol 3-kinase leads to glucose transporter translocation and a pathway involving p38 MAP kinase leads to activation of the recruited glucose transporter at the membrane.

Influence of cycloheximide-mediated downregulation of glucose transport on TNF alpha-induced apoptosis

Enhancement of cellular sensitivity to TNF alpha-induced apoptosis by cycloheximide (CX) has been attributed to its quality as an inhibitor of protein synthesis, presumably by prevention of the synthesis of short-lived death antagonists. CX is also known to interfere with glucose transport, which in turn influences cell death. Hexose uptake, expression of glucose transporter (Glut) mRNAs and proteins, and other related factors were therefore examined upon induction of apoptosis with TNF alpha and CX in breast cancer cell lines. In the early phase of apoptosis, a dramatic decrease in glucose transport was observed, preceded by stimulation of Glut 1 and 3 mRNAs. Transport downregulation was also detectable upon incubation with CX alone, albeit to a lesser extent. With the doses used, TNF alpha had no such effect. Protein synthesis was inhibited to the same degree in TNF alpha/CX-treated apoptotic cells compared to viable CX-treated cells. Diminished hexose uptake was associated with decreased Vmax, while Glut affinity remained unaffected. As there was no evidence for changes in total cellular Glut content or for Glut translocation from the plasma membrane, a diminished intrinsic activity of Gluts must be postulated. In conclusion, CX is proposed to contribute to TNF alpha-induced apoptosis predominantly by interference with glucose transport; the exact nature of this effect remains to be elucidated.