Studies on mechanisms underlying the induction of insulin resistance using transgenic rats

GH过量已被证明会导致胰岛素抵抗；然而，其分子机制仍有待阐明。本研究利用本实验室培育的人生长激素(HGH)转基因大鼠(TG大鼠)，研究GH对细胞内胰岛素信号的影响及对糖代谢的胰岛素依赖性调节作用。TG组大鼠血清hGH水平较高。和IGF-I，导致身体长度和体重增加。10周龄TG大鼠的血清胰岛素水平高于非转基因胎鼠(对照组)，但血糖水平正常，提示高水平的血清GH可引起轻度胰岛素抵抗。然后分离和培养10周龄TG或对照组大鼠的肝细胞。用胰岛素处理血清饥饿肝细胞，分析胰岛素受体酪氨酸磷酸化、IRS-1和IRS-2，以及各种酶的活性或mRNA水平。在TG肝细胞中，胰岛素依赖的…与对照肝细胞相比，Ptor自身磷酸化减少，而IRS-1和IRS-2酪氨酸磷酸化增加。胰岛素诱导的葡萄糖摄取也被上调，这可能是由于胰岛素依赖的IRSS酪氨酸磷酸化增加所致。而甘油三酯肝细胞糖原合成酶活性显著降低，PEPCK和Fas基因表达水平升高，提示胰岛素抵抗至少发生在这些酶上。另一方面，胰岛素诱导的TG脂肪细胞的葡萄糖摄取减少，表明高水平的血清GH损害了脂肪细胞的胰岛素作用。综上所述，我们的结果表明，外周组织(如脂肪组织)中生长激素对胰岛素依赖性葡萄糖摄取的损害可以通过上调肝脏对胰岛素的葡萄糖摄取而得到补偿，从而导致血糖水平正常化。我们的GH转基因大鼠模型显示，GH以组织特异性的方式在不同的步骤损害胰岛素的作用。较少

GH excess has been shown to lead insulin resistance; however, the molecular mechanisms remain to be elucidated. This study was undertaken to investigate the effects of GH on intracellular signals of insulin and insulin-dependent regulation of, glucose metabolism using human GH (hGH) transgenic rats (TG rats) developed in our laboratory. The TG rats were characterized by high levels of serum hGH. and IGF-I, resulting in an increase in body length and weight. In 10 weeks old TG rats, higher levels of serum insulin were detected compared with non-transgenic littermates (control rats), but serum glucose levels were normal, suggesting high levels of serum GH cause mild insulin resistance. We then isolated and cultured hepatocytes from 10 weeks old TG or control rats. Serum starved hepatocytes were treated with insulin, and tyrosine phosphorylation of insulin receptor, IRS-1 arid IRS-2, arid activities or mRNA levels of various enzymes were analyzed. In TG hepatocytes, insulin-dependent rece … More ptor autophosphorylation was reduced compared with control hepatocytes; however, IRS-1 and IRS-2 tyrosine phosphorylation increased. Glucose uptake induced by insulin was also up-regulated, which may be due to increases in insulin-dependent IRSs tyrosine phosphorylation. In contrast, glycogen synthase activity was significantly lower and the mRNA levels of PEPCK and FAS were, elevated in TG hepatocytes, suggesting that insulin resistance occurs at least on these enzymes. On the other hand, insulin-induced glucose uptake decreased in TG adipocytes, showing that high levels of serum GH impaired insulin action in adipocytes. Taken together, our results suggest that impairment of insulin-dependent glucose uptake by GH in peripheral tissues, such as adipose tissues, is compensated by up-regulation of glucose uptake in the liver in response to insulin, resulting in normalizing glucose levels. Our GH transgenic rats model shows that GH impairs insulin action at the different steps in tissue specific manners. Less