FACTORS THAT MODIFY INSULIN ACTION

“葡萄糖毒性”解释了未控制的I型糖尿病（IDDM）中的胰岛素抵抗，并促成了II型糖尿病（NIDDM）中的胰岛素抵抗。持续的高血糖症或高胰岛素血症引起胰岛素抵抗;葡萄糖和胰岛素在下调胰岛素刺激的葡萄糖转运中协同作用。在3 T3-L1脂肪细胞中测试的假设是葡萄糖/胰岛素诱导的葡萄糖转运脱敏反映了葡萄糖转运蛋白GLUT 4的改变的亚细胞运输，其可能涉及受损的GLUT 4易位和含有GLUT 4的囊泡（GCV）与质膜的不适当缔合。己糖胺合成途径（HNSP）的产物与葡萄糖诱导的胰岛素抵抗有关;谷氨酰胺-果糖-6-P酰胺转移酶（GFAT）是限速酶，UDP-N-乙酰葡糖胺（UDP-GlcNAc）是主要产物。HNSP的作用将通过检查经由HNSP增加或减少通量的条件是否分别增强或减轻葡萄糖诱导的胰岛素抗性来测试。O-GlcNAc化是一个可逆过程，涉及蛋白质在Ser/Thr残基上与单糖GlcNAc的O-糖基化。它通常涉及磷酸化位点，可能是调节性的。基于胰岛素抗性小鼠模型肌肉中的初步数据，在肌肉中过表达GLUTI，将测试以下假设：经由HNSP增加的通量促进参与胰岛素刺激的葡萄糖转运的关键蛋白的O-GlcNAc酰化。这些可能包括GSV相关蛋白，可能是GLUT 4本身和/或与GSV对接和融合相关的蛋白。由于适应性调节通常涉及多个位点，因此我们将检验葡萄糖诱导的胰岛素抵抗部分代表胰岛素受体（IR）信号级联下调的假设，试图确定主要的调节位点，并严格评估HNSP对葡萄糖效应的可能贡献。如有必要，将检查IR信号转导调节剂，即蛋白激酶C（PKC）亚型和候选蛋白酪氨酸磷酸酶（PTP酶：PTP-1B、SH-PTP 2和LAR）的参与。GFAT活性由UDP-GlcNAc变构调节，并且在体内肌肉中由激素和代谢环境调节。将在啮齿动物模型的肌肉中研究GFAT表达的翻译前和翻译后调节。

"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.