""""""""Glucose toxicity"""""""" accounts for insulin resistance in uncontrolled Type I diabetes (IDDM) and contributes to insulin resistance in Type II diabetes (NIDDM). Sustained hyperglycemia or hyperinsulinemia cause insulin resistance; glucose and insulin act synergistically in down- regulating insulin-stimulated glucose transport. A hypothesis to be tested in 3T3-Ll adipocytes is that glucose/insulin induced glucose transport desensitization reflects altered subcellular trafficking of the glucose transporter, GLUT4, which may involve impaired GLUT4 translocation and inappropriate association of GLUT4 containing vesicles (GCV) with the plasma membrane. Products of the hexosamine synthesis pathway (HNSP) have been implicated in glucose-induced insulin resistance; glutamine-fructose-6-P amidotransferase (GFAT) is the rate limiting enzyme and UDP-N-acetyl glucosamine (UDP-GlcNAc) the major product. The role of HNSP will be tested by examining whether conditions which increase or decrease flux via HNSP augment or mitigate, respectively, glucose induced insulin resistance. O-GlcNAcylation is a reversible process, involving O-glycosylation of proteins on Ser/Thr residues with monosaccharide GlcNAc. It usually involves phosphorylation sites and may be regulatory. Based on preliminary data in muscles of a mouse model of insulin resistance, over-expressing GLUTI in muscle, the hypothesis will be tested that increased flux via HNSP promotes O-GlcNAcylation of critical proteins involved in insulin- stimulated glucose transport. These may include GSV-associated proteins, possibly GLUT4 itself and/or proteins associated with GSV docking and fusion. Since adaptive regulation usually involves multiple sites,, we will test the hypothesis that glucose-induced insulin resistance represents in part down-regulation of the insulin receptor (IR) signaling cascade, attempt to identify the major regulatory sites and critically assess the possible contribution of HNSP to the glucose effect. If warranted, the involvement of modulators of IR signal transduction, I.E. protein kinase C (PKC) isoforms, and candidate protein tyrosine phosphatases (PTP-ases: PTP-1B, SH-PTP2 and LAR) will be examined. GFAT activity is allosterically regulated by UDP-GlcNAc, and is modulated in vivo in muscle by the hormonal and metabolic milieu. The pre- and post-translational regulation of GFAT expression will be studied in muscles of rodent models.
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