Increased hexosamine biosynthetic pathway flux impairs myocardial GLUT4 translocation
Thesis (MSc (Physiological Sciences))--University of Stellenbosch, 2009.
Aims and Background: According to the World Health Organization type 2 diabetes will constitute a major global burden of disease within the next few decades. In agreement, reports show that rapid urbanization and lifestyle changes in South Africa are major factors responsible for these projections. Therefore, any perturbations that alter the regulatory steps that control myocardial glucose uptake by the cardiac-enrich glucose transporter, GLUT4, will lead in the development of diabetic cardiomyopathy and cardiac hypertrophy. Although considerable efforts are been put into unraveling molecular mechanisms underlying this process, less is known regarding the spatio-temporal regulation of GLUT4. In light of this, our specific aim was to establish in vitro fluorescence microscopy- and flow cytometry-based models for visualization and assessment of myocardial GLUT4 translocation using H9c2 cardiac-derived myoblasts. After successful establishment of our in vitro-based model for myocardial GLUT4 translocation, our second aim was to determine the role of the hexosamine biosynthetic pathway (HBP) in this process. Here, we employed HBP modulators to alter flux and subsequently evaluate its effect on myocardial GLUT4 translocation. To further strengthen our hypothesis, we also investigated the role of the HBP in hearts of an in vivo type 2 diabetes mouse model. Hypothesis: We hypothesize that increased flux through the HBP impairs myocardial GLUT4 translocation by greater O-linked glycosylation of the insulin signaling pathway, ultimately leading to myocardial insulin resistance. Methods: Rat cardiac-derived H9c2 myoblasts were cultured until ~ 80-90 % confluent for 3 days and thereafter subcultured in Lab-Tek chamber slides (~ 15, 000 cells per well) for 24 hours. Cells were then serum starved for 3 hours by insulin administration of 100 nM for 0, 5 and 30 minutes, respectively. We employed a method to quantify the relative proportion of GLUT4 at the sarcolemma using immunofluorescence microscopy- and flow cytometry-based models for visualization and assessment of myocardial GLUT4 translocation. Using these methods we investigated the role HBP have during GLUT4 translocation. The HBP were then activated through the following: a) high glucose and glutamine concentrations; b) low glucose and glucosamine stimulation; and c) over-expression of the HBP rate- limiting enzyme, i.e. GFAT. Subsequently, cardiac-derived myoblasts were fixed and probed for ~ 24 hours with antibodies specific for intracellular- and membrane-bound GLUT4, anti-myc GLUT4 (9E10) and O-GlcNAc. To assess GLUT4 translocation and O-GlcNAcylation we employed the following secondary antibodies: FITC Green for intracellular-bound GLUT4; and b) Texas Red for membrane-bound GLUT4 (immunofluorescence microscopy) and Phycoerythrin for flow cytometry-based model. Cells were thereafter viewed by multi-dimension imaging using an inverted system microscope (Olympus IX81) and a BD FACS Aria cell sorter for flow cytometric analysis. We also assessed HBP in an in vivo context by probing heart tissue - from insulin resistant db/db mice - with a GFAT monoclonal antibody. Results: The db/db mouse represents an ideal model to confirm our hypothesis in an in vivo context. In agreement, our preliminary results show increased GFAT expression versus heterozygous db/+ controls. Our in vitro model show myocardial GLUT4 translocation at 5 minute peak response when H9c2 cardiac-derived myoblasts were stimulated with 100 nM insulin, and GLUT4 vesicles return to normal after longer insulin stimulatory times (10, 15 and 30 minutes. Myocardial Glut4 v translocation was impaired when cells were stimulated with 100 nM wortmannin. Our transfection based model (immunofluorescence microscopy- and flow cytometry-based models) confirms 5 minute peak response under real time conditions. High glucose concentration (25 mM glucose), glucosamine concentrations (2.5 mM, 5 mM, and 10 mM) and over-expression of GFAT led to an impairment of myocardial GLUT4 translocation. Employment of an HBP activator (50 μM PUGNAc) also caused impairment of myocardial GLUT4 translocation. Myocardial GLUT4 translocation was restored when cells were treated with an HBP inhibitor (40 μM DON). High glucose concentrations (25 mM glucose), glucosamine concentrations (2.5 mM, 5 mM, and 10 mM) and over-expression of GFAT resulted in an increase in O-GlcNAcylation. HBP activation (50 μM PUGNAc) showed an increase in O-GlcNAcylation, while administration of 40 μM DON reversed this effect. Discussion and conclusion: We successfully established an in vitro experimental system to assay myocardial GLUT4 translocation. Our data show that dysregulated flux through the HBP impairs myocardial GLUT4 translocation. It is likely that the HBP becomes dysregulated during the pre-diabetic/early diabetic state and that O-GlcNAcylation of members of the insulin signaling pathway occurs during this stage. This will lead to myocardial insulin resistance, and in the long term, will contribute to the onset of the diabetic cardiomyopathy. Investigations to find unique inhibitors of this maladaptive pathway should therefore result in the development of novel therapeutic agents that will lead to a reduction in the growing global burden of disease for type 2 diabetes and associated cardiovascular diseases.