Transmembrane segment 5 of the Glut1 glucose transporter is an amphipathic helix that forms part of the sugar permeation pathway

Transmembrane segment 5 of the Glut1 glucose transporter has been proposed to form an amphipathic transmembrane helix that lines the substrate translocation pathway (Mueckler, M., Caruso, C., Baldwin, S. A., Panico, M., Blench, I., Morris, H. R., Allard, W. J., Lienhard, G. E., and Lodish, H. F. (1985) Science 229, 941-945). This hypothesis was tested using cysteine-scanning mutagenesis in conjunction with the membrane-impermeant, sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS). A series of 21 mutants was created from a fully functional, cysteine-less, parental Glut1 molecule by changing each residue within putative transmembrane segment 5 to cysteine. Each mutant was then expressed in Xenopus oocytes and its steady-state protein level, 2-deoxyglucose uptake activity, and sensitivity to pCMBS were measured. All 21 mutants exhibited measurable transport activity, although several of the mutants exhibited reduced activity due to a corresponding reduction in steady-state protein. Six of the amino acid side chains within transmembrane segment 5 were clearly accessible to pCMBS in the external medium, as determined by inhibition of transport activity, and a 7th residue showed inhibition that lacked statistical significance because of the extremely low transport activity of the corresponding mutant. All 7 of these residues were clustered along one face of a putative alpha-helix, proximal to the exoplasmic surface of the plasma membrane. These results comprise the first experimental evidence for the existence of an amphipathic transmembrane alpha-helix in a glucose transporter molecule and strongly suggest that transmembrane segment 5 of Glut1 forms part of the sugar permeation pathway.     

Cysteine-scanning mutagenesis of transmembrane segment 11 of the GLUT1 facilitative glucose transporter

The glucose permeation pathway within the GLUT1 facilitative glucose transporter is hypothesized to be formed by the juxtaposition of the hydrophilic faces of several transmembrane alpha-helices. The role of transmembrane segment 11 in forming a portion of this central aqueous channel was investigated using cysteine-scanning mutagenesis in conjunction with sulfhydryl-directed chemical modification. Each of the amino acid residues within transmembrane segment 11 were individually mutated to cysteine in an engineered GLUT1 molecule devoid of all native cysteines (C-less). Measurement of 2-deoxyglucose uptake in a Xenopus oocyte expression system revealed that all of these mutants retain measurable transport activity. Four of the cysteine mutants (N411, W412, N415, and F422) had significantly reduced specific activity relative to the C-less protein. Specific activity was increased in five of the mutants (A402, A405, V406, F416, and M420). The solvent accessibility and relative orientation of the residues to the glucose permeation pathway were investigated by determining the sensitivity of the mutant transporters to inhibition by the sulfhydryl-directed reagent p-chloromercuribenzenesulfonate (pCMBS). Cysteine replacement at five positions (I404, G408, F416, G419, and M420) produced transporters that were inhibited by incubation with extracellular pCMBS. All of these residues cluster along a single face of the alpha-helix within the regions showing altered specific activities. These data demonstrate that the exofacial portion of transmembrane segment 11 is accessible to the external solvent and provide evidence for the positioning of this alpha-helix within or near the glucose permeation pathway.     

A novel 68-kDa adipocyte protein phosphorylated on tyrosine in response to insulin and osmotic shock

Osmotic shock can cause insulin resistance in 3T3-L1 adipocytes by inhibiting insulin activation of glucose transport, p70S6 kinase, glycogen synthesis, and lipogenesis. By further investigating the relationship between insulin and hypertonic stress, we have discovered that osmotic shock enhanced by 10-fold the insulin-stimulated tyrosine phosphorylation of a 68-kDa protein. Phosphorylation by insulin was maximal after 1 min and was saturated with 50-100 nm insulin. The effect of sorbitol was completely reversible by 2.5 min. pp68 was a peripheral protein that was localized to the detergent insoluble fraction of the low density microsomes but was not associated with the cytoskeleton. Stimulation of the p42/44 and the p38 MAP kinase pathways by osmotic shock had no effect on pp68 phosphorylation. Treatment of adipocytes with the phosphotyrosine phosphatase inhibitor phenylarsine oxide also enhanced insulin-activated tyrosine phosphorylation of pp68 suggesting that osmotic shock may increase pp68 phosphorylation by inhibiting a phosphotyrosine phosphatase. Dissociation of pp68 from the low density microsomes with RNase A indicated that pp68 binds to RNA. Failure to immunoprecipitate pp68 using antibodies directed against known 60-70-kDa tyrosine-phosphorylated proteins suggest that pp68 may be a novel cellular target that lies downstream of the insulin receptor.     

Glucose transport and apoptosis

The transport and metabolism of glucose modify programmed cell death in a number of different cell types. This review presents three cell death paradigms that link a decrease in glucose transport to apoptosis. Although these pathways overlap, the glucose-dependent stimuli that trigger cell death differ. These paradigms include glucose deprivation-induced ATP depletion and stimulation of the mitochondrial death pathway cascade; glucose deprivation-induced oxidative stress and triggering of Bax-associated events including the JNK/MAPK signalling pathways; and finally hypoglycemia-regulated expression of HIF-1, stabilization of p53 leading to an increase in p53-associated apoptosis. Several examples of each paradigm are presented. Future studies of glucose transport-associated apoptotic events will allow better understanding of the role of cellular metabolism in programmed cell death.     

The effect of nutrients on carbon and nitrogen fixation by the UCYN-A–haptophyte symbiosis

Symbiotic relationships between phytoplankton and N₂-fixing microorganisms play a crucial role in marine ecosystems. The abundant and widespread unicellular cyanobacteria group A (UCYN-A) has recently been found to live symbiotically with a haptophyte. Here, we investigated the effect of nitrogen (N), phosphorus (P), iron (Fe) and Saharan dust additions on nitrogen (N₂) fixation and primary production by the UCYN-A–haptophyte association in the subtropical eastern North Atlantic Ocean using nifH expression analysis and stable isotope incubations combined with single-cell measurements. N₂ fixation by UCYN-A was stimulated by the addition of Fe and Saharan dust, although this was not reflected in the nifH expression. CO₂ fixation by the haptophyte was stimulated by the addition of ammonium nitrate as well as Fe and Saharan dust. Intriguingly, the single-cell analysis using nanometer scale secondary ion mass spectrometry indicates that the increased CO₂ fixation by the haptophyte in treatments without added fixed N is likely an indirect result of the positive effect of Fe and/or P on UCYN-A N₂ fixation and the transfer of N₂-derived N to the haptophyte. Our results reveal a direct linkage between the marine carbon and nitrogen cycles that is fuelled by the atmospheric deposition of dust. The comparison of single-cell rates suggests a tight coupling of nitrogen and carbon transfer that stays balanced even under changing nutrient regimes. However, it appears that the transfer of carbon from the haptophyte to UCYN-A requires a transfer of nitrogen from UCYN-A. This tight coupling indicates an obligate symbiosis of this globally important diazotrophic association.     

ADP-ribosylation factor 6 (ARF6) defines two insulin-regulated secretory pathways in adipocytes.

ADP-ribosylation factor 6 (ARF6) appears to play an essential role in the endocytic/recycling pathway in several cell types. To determine whether ARF6 is involved in insulin-regulated exocytosis, 3T3-L1 adipocytes were infected with recombinant adenovirus expressing wild-type ARF6 or an ARF6 dominant negative mutant (D125N) that encodes a protein with nucleotide specificity modified from guanine to xanthine. Overexpression of these ARF6 proteins affected neither basal nor insulin-regulated glucose uptake in 3T3-L1 adipocytes, nor did it affect the subcellular distribution of Glut1 or Glut4. In contrast, the secretion of adipsin, a serine protease specifically expressed in adipocytes, was increased by the expression of wild-type ARF6 and was inhibited by the expression of D125N. These results indicate a requirement for ARF6 in basal and insulin-regulated adipsin secretion but not in glucose transport. Our results suggest the existence of at least two distinct pathways that undergo insulin-stimulated exocytosis in 3T3-L1 adipocytes, one for adipsin release and one for glucose transporter translocation.     

Seropositivity is associated with insulin resistance in patients with early inflammatory polyarthritis: results from the Norfolk Arthritis Register (NOAR): an observational study

Introduction  

Cardiovascular disease (CVD) is the leading cause of death in patients with inflammatory polyarthritis (IP), especially in seropositive disease. In established rheumatoid arthritis (RA), insulin resistance (IR) is increased and associated with CVD. We investigated factors associated with IR in an inception cohort of patients with early IP.