Department of Medicine, University of Texas Health Science Center, San Antonio, Texas; and.
Veterans Administration Research Service, South Texas Veterans Health Care System, San Antonio, Texas; Department of Pathology, University of Texas Health Science Center, San Antonio, Texas;
Am J Physiol Cell Physiol. 2014 Jun 1;306(11):C1089-100. doi: 10.1152/ajpcell.00387.2013. Epub 2014 Apr 16.
Platelet-derived growth factor BB and its receptor (PDGFRβ) play a pivotal role in the development of renal glomerular mesangial cells. Their roles in increased mesangial cell proliferation during mesangioproliferative glomerulonephritis have long been noted, but the operating logic of signaling mechanisms regulating these changes remains poorly understood. We examined the role of a recently identified MAPK, Erk5, in this process. PDGF increased the activating phosphorylation of Erk5 and tyrosine phosphorylation of proteins in a time-dependent manner. A pharmacologic inhibitor of Erk5, XMD8-92, abrogated PDGF-induced DNA synthesis and mesangial cell proliferation. Similarly, expression of dominant negative Erk5 or siRNAs against Erk5 blocked PDGF-stimulated DNA synthesis and proliferation. Inhibition of Erk5 attenuated expression of cyclin D1 mRNA and protein, resulting in suppression of CDK4-mediated phosphorylation of the tumor suppressor protein pRb. Expression of cyclin D1 or CDK4 prevented the dominant negative Erk5- or siErk5-mediated inhibition of DNA synthesis and mesangial cell proliferation induced by PDGF. We have previously shown that phosphatidylinositol 3-kinase (PI3-kinase) contributes to PDGF-induced proliferation of mesangial cells. Inhibition of PI3-kinase blocked PDGF-induced phosphorylation of Erk5. Since PI3-kinase acts through Akt, we determined the role of Erk5 on Akt phosphorylation. XMD8-92, dominant negative Erk5, and siErk5 inhibited phosphorylation of Akt by PDGF. Interestingly, we found inhibition of PDGF-induced Erk5 phosphorylation by a pharmacological inhibitor of Akt kinase and kinase dead Akt in mesangial cells. Thus our data unfold the presence of a positive feedback microcircuit between Erk5 and Akt downstream of PI3-kinase nodal point for PDGF-induced mesangial cell proliferation.
血小板衍生生长因子 BB 及其受体（PDGFRβ）在肾肾小球系膜细胞的发育中发挥关键作用。长期以来，人们一直注意到它们在系膜增生性肾小球肾炎中系膜细胞增殖增加中的作用，但调节这些变化的信号转导机制的作用机制仍知之甚少。我们研究了最近发现的 MAPK（Erk5）在这个过程中的作用。PDGF 以时间依赖性方式增加 Erk5 的激活磷酸化和蛋白质的酪氨酸磷酸化。Erk5 的一种药理抑制剂 XMD8-92 阻断了 PDGF 诱导的 DNA 合成和系膜细胞增殖。同样，表达显性负 Erk5 或针对 Erk5 的 siRNA 阻断了 PDGF 刺激的 DNA 合成和增殖。Erk5 的抑制减弱了 cyclin D1 mRNA 和蛋白的表达，导致 CDK4 介导的肿瘤抑制蛋白 pRb 磷酸化的抑制。cyclin D1 或 CDK4 的表达可防止 PDGF 诱导的 DNA 合成和系膜细胞增殖被显性负 Erk5 或 siErk5 抑制。我们之前已经表明，磷脂酰肌醇 3-激酶（PI3-kinase）有助于系膜细胞的 PDGF 诱导增殖。PI3-kinase 的抑制阻断了 PDGF 诱导的 Erk5 磷酸化。由于 PI3-kinase 通过 Akt 起作用，我们确定了 Erk5 在 Akt 磷酸化中的作用。XMD8-92、显性负 Erk5 和 siErk5 抑制了 PDGF 对 Akt 的磷酸化。有趣的是，我们发现 PI3-kinase 节点下游 Akt 激酶和激酶失活 Akt 的药理抑制剂抑制了 PDGF 诱导的 Erk5 磷酸化。因此，我们的数据揭示了 PI3-kinase 节点下游 Erk5 和 Akt 之间存在 PDGF 诱导的系膜细胞增殖的正反馈微电路。
