Molecular mechanism of IRS-1 degradation by PLGγ and PP2C

我们发现，磷脂酶C(PLC)抑制剂U73122可以阻断胰岛素诱导的3T3-L1脂肪细胞的葡萄糖摄取。此外，在没有胰岛素刺激的情况下，PLCγ的过表达以剂量依赖的方式刺激葡萄糖摄取。过表达PLCγ不影响Akt的磷酸化，但促进非典型蛋白激酶C的磷酸化。因此，提示PLCγ通过激活非典型PKC刺激谷氨酸摄取，蛋白磷酸酶2C(PP2C)的过度表达导致PI3K的P85丝氨酸残基去磷酸化，并正向调节胰岛素信号转导。另一方面，PP2C的过表达加速了胰岛素刺激的IRS-1的降解。当我们通过SRNA干扰方法抑制PP2C蛋白的表达时，胰岛素诱导的IRS-1降解被抑制。蛋白酶体抑制剂Lactasistine可完全抑制PP2C诱导的IRS-1降解。PI 3-激酶抑制剂Wortmannin也部分抑制了它的表达。综上所述，PP2C通过PI3K途径对胰岛素信号传导既有积极的影响，也有消极的影响。

We found that U73122,which was phospholioase C(PLC) inhibitor, blocked insulin-induced glucose uptake in 3T3-L1 adipocytes. Moreover, overexpression of PLCγ stimulated glucose uptake without insulin stimulation in a dose dependent manner. Overexpression of PLCγ did not affect phosphorylation of Akt, but enhanced phosphorylation of atypical PKC. Thus, it suggests that PLCγ stimulates gluxoce uptake through activation of atypical PKC.Overexpression of protein phosphatase 2C(PP2C) caused dephosphorylation of serine residue of p85 of PI 3-kinase, and positively regulated insulin signaling. On the other hand, overexpression of PP2C accelerated insulin-stimulated IRS-1 degradation. When we suppressed PP2C protein expression by sRNA interference method, insulin-induced IRS-1 degradation was inhibited. Moreover, PP2C-induced IRS-1 degradation was completely inhibited by lactasistine treatment, which was proteasome inhibitor. PI 3-kinase inhibitor, wortmannin, also suppressed it, partially. Taken together, PP2C has both positive and negative effects on insulin signaling through PI 3-kinase pathway.