Organ specificity of insulin resistance induced by Protein-tyrosine phosphatase

我们研究了PTP1B过表达对3T3L1脂肪细胞胰岛素信号转导的影响。野生型PTP1B在L1脂肪细胞中的过度表达导致胰岛素刺激的丝裂原活化蛋白(MAP)激酶的磷酸化水平显著降低，在L6心肌细胞中也是如此。尽管IRS-1的磷酸化水平与L6心肌细胞相同，但野生型PTP1B在L1脂肪细胞中的过表达导致了对胰岛素刺激的Akt磷酸化的轻微抑制，同时与磷酸酶阴性突变体相比，胰岛素刺激的葡萄糖摄取也有轻微但显著的下降。至于对Akt磷酸化的影响相对较小，我们在大鼠1成纤维细胞中发现了相同的结果，提示胰岛素受体和IRS-1的高表达可能是原因之一。由于PTP1B对MAPK的磷酸化影响很大，我们发现PTP1B的过表达导致了…的损伤IRS-1和Shc的磷酸化的Ts更多，导致它们与Grb2的结合减少。此外，血小板衍生生长因子刺激的Shc的磷酸化也被减弱，但其受体没有任何变化，提示PTP1B直接攻击Shc。这些数据表明，PTP1B负性调节胰岛素信号，在某些细胞系中，尤其是在L1脂肪细胞中，与Akt通路相比，MAP激酶级联反应非常敏感。高果糖饮食喂养的胰岛素抵抗大鼠肝脏蛋白酪氨酸磷酸酶1B(PTP1B)含量增加，固醇调节元件结合蛋白(SREBP)-1mRNA表达增强。为了阐明PTP1B与SREBP-1基因表达的关系，我们在大鼠肝细胞中过表达PTP1B，导致SREBP-1a和SREBP-1c的mRNA含量和启动子活性增加，包括核Sp1结合增加。由于PTP1B过表达增加了磷酸酶2A(PP2A)的活性，我们通过表达其选择性抑制物SV40 Small t抗原来抑制PP2A的活性，并发现这通过激活Sp1位点使PTP1B促进的SREBP-1a和-1c的mRNA表达正常化。此外，PTP1B蛋白的瞬时肝脏特异性过表达导致血清甘油三酯水平升高，并增强了小鼠肝脏中SREBP-1a和-1c基因的表达。最后，给db/db小鼠服用一种新的PTP1B抑制剂(jtp-55773)可以改善餐后高甘油三酯血症和脂肪肝，同时降低SREBP-1mRNA的表达。这些结果表明，PTP1B可能通过增强PP2A活性来调节SREBP-1的基因表达，从而介导肝脏脂肪生成和餐后高甘油三酯血症。因此，PTP1B是新发现的改善餐后高甘油三酯血症、降低胰岛素抵抗的治疗靶点。较少

We investigated the effect of PTP1B overexpression for insulin signaling in 3T3L1 adipocytes. 0verexpression of wild-type PTP1B in L1 adipocytes led to the profound decrease in the insulin-stimulated phosphorylation of mitogen-activated protein (MAP) kinase as well as in L6 myocytes. Even though the identical decrease in IRS-1 phosphorylation as in L6 myocytes, wild-type PTP1B overexpression in L1 adipocytes led to a small inhibition in the insulin-stimulated Akt phosphorylation as accompanied with a small, but significant attenuation in the insulin-stimulated glucose uptake, when compared with phosphatase-negative mutant. Regarding the relatively small effect on Akt phosphorylation, we found the identical findings in rat 1 fibroblasts overexpressing human insulin receptor, suggesting that the higher expression levels of insulin receptor and IRS-1 might be responsible. With regard to the large effect on MAP kinase phosphorylation, we found that PTP1B overexpression led to the impairmen … More ts of both phosphorylation of IRS-1 and Shc, resulting in a decrease in their association with Grb2. Furthermore, platelet derived growth factor stimulated Shc phosphorylation was also attenuated, without any change in its receptors, suggesting that PTP1B directly attacked Shc. These data demonstrate that PTP1B negatively regulates insulin signaling, and MAP kinase cascade is much sensitive as compared with Akt pathway in some cell lines, especially in L1 adipocytes.Insulin-resistant rats fed a diet high in fructose showed an increased protein-tyrosine phosphatase 1B (PTP1B) content with strong expression of sterol regulatory element binding protein (SREBP)-1 mRNA in the liver. To clarify the association of PTP1B with SREBP-1 gene expression, we overexpressed PTP1B in rat hepatocytes, which led to increased mRNA content and promoter activity of SREBP-1a and -1c, including increased nuclear Sp1 binding. Since PTP1B overexpression increased phosphatase 2A (PP2A) activity, we inhibited PP2A activity by expression of its selective inhibitor, SV40 small t antigen, and found that this normalized the PTP1B-enhanced SREBP-1a and -1c mRNA expressions through activation of Sp1 site. Furthermore, a transient liver-specific overexpression of PTP1B protein led to increased serum triglyceride levels with enhanced SREBP-1a and -1c gene expressions in mouse liver. Finally, administration of a new PTP1B inhibitor (JTP-55773) to db/db mice led to amelioration of both postprandial hypertriglyceridemia and fatty liver with decreased SREBP-1 mRNA expression. These results indicate that PTP1B may regulate gene expression of SREBP-1 via enhancing PP2A activity, thus mediating hepatic lipogenesis and postprandial hypertriglyceridemia. Therefore, PTP1B is a newly identified therapeutic target for the amelioration of postprandial hypertriglyceridemia, as well as for the reduction of resistance to insulin. Less