Protein Phosphatase 2A Negatively Regulates Insulin's Metabolic Signaling Pathway by Inhibiting Akt (Protein Kinase B) Activity in 3T3-L1 Adipocytes
Protein Phosphatase 2A Negatively Regulates Insulin's Metabolic Signaling Pathway by Inhibiting Akt (Protein Kinase B) Activity in 3T3-L1 Adipocytes
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DOI：
10.1128/mcb.24.19.8778-8789.2004
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发表时间：
2004-10
影响因子：
5.3
通讯作者：
S. Ugi;T. Imamura;H. Maegawa;K. Egawa;Takeshi Yoshizaki;K. Shi;T. Obata;Y. Ebina;A. Kashiwagi;J. Olefsky
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文献类型：
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作者：
S. Ugi;T. Imamura;H. Maegawa;K. Egawa;Takeshi Yoshizaki;K. Shi;T. Obata;Y. Ebina;A. Kashiwagi;J. Olefsky
摘要 蛋白磷酸酶 2A (PP2A) 是一种多聚体丝氨酸/苏氨酸磷酸酶，具有多种功能，包括抑制丝裂原激活蛋白 (MAP) 激酶途径。猿猴病毒 40 小 t 抗原通过与 PP2A 调节亚基结合，干扰 PP2A 与其细胞底物结合的能力，特异性抑制 PP2A 功能。我们已经报道，小 t 抗原的表达抑制 PP2A 与 Shc 的关联，导致胰岛素和表皮生长因子诱导的 Shc 磷酸化增强，并增强 Ras/MAP 激酶途径的激活。然而，PP2A 在胰岛素代谢信号通路中的潜在参与目前尚不清楚。为了评估这一点，我们通过腺病毒介导的基因转移在 3T3-L1 脂肪细胞中过度表达小 t 抗原，并发现 Akt 及其下游靶标糖原合酶激酶 3β 的磷酸化在不存在和存在胰岛素的情况下均得到增强。此外，表达小 t 抗原的 3T3-L1 脂肪细胞中蛋白激酶 C λ (PKC λ) 活性也增强。与这一结果一致的是，这些细胞的基础葡萄糖摄取和胰岛素刺激的葡萄糖摄取均得到增强。为了支持这一结果，当将抑制性抗 PP2A 抗体显微注射到 3T3-L1 脂肪细胞中时，我们发现在没有胰岛素的情况下 GLUT4 易位增加了一倍。小 t 抗原诱导的 Akt 和 PKC λ 活性增加不受渥曼青霉素抑制，而小 t 抗原增强葡萄糖转运的能力受到显性失活 Akt (DN-Akt) 表达和 Akt 小干扰 RNA (siRNA) 的抑制，但不受 DN-PKC λ 表达或 PKC λ siRNA 的抑制。我们得出结论，PP2A 通过促进 Akt 和 PKC λ 的去磷酸化和失活而成为胰岛素代谢信号通路的负调节因子，并且 PP2A 抑制葡萄糖转运的大部分作用是通过 Akt 介导的。
ABSTRACT Protein phosphatase 2A (PP2A) is a multimeric serine/threonine phosphatase which has multiple functions, including inhibition of the mitogen-activated protein (MAP) kinase pathway. Simian virus 40 small t antigen specifically inhibits PP2A function by binding to the PP2A regulatory subunit, interfering with the ability of PP2A to associate with its cellular substrates. We have reported that the expression of small t antigen inhibits PP2A association with Shc, leading to augmentation of insulin and epidermal growth factor-induced Shc phosphorylation with enhanced activation of the Ras/MAP kinase pathway. However, the potential involvement of PP2A in insulin's metabolic signaling pathway is presently unknown. To assess this, we overexpressed small t antigen in 3T3-L1 adipocytes by adenovirus-mediated gene transfer and found that the phosphorylation of Akt and its downstream target, glycogen synthase kinase 3β, were enhanced both in the absence and in the presence of insulin. Furthermore, protein kinase C λ (PKC λ) activity was also augmented in small-t-antigen-expressing 3T3-L1 adipocytes. Consistent with this result, both basal and insulin-stimulated glucose uptake were enhanced in these cells. In support of this result, when inhibitory anti-PP2A antibody was microinjected into 3T3-L1 adipocytes, we found a twofold increase in GLUT4 translocation in the absence of insulin. The small-t-antigen-induced increase in Akt and PKC λ activities was not inhibited by wortmannin, while the ability of small t antigen to enhance glucose transport was inhibited by dominant negative Akt (DN-Akt) expression and Akt small interfering RNA (siRNA) but not by DN-PKC λ expression or PKC λ siRNA. We conclude that PP2A is a negative regulator of insulin's metabolic signaling pathway by promoting dephosphorylation and inactivation of Akt and PKC λ and that most of the effects of PP2A to inhibit glucose transport are mediated through Akt.