Ghosh Choudhury Goutam, Lenin Mahimainathan, Calhaun Cheresa, Zhang Jian-Hua, Abboud Hanna E
Geriatric Research, Education and Clinical Center, San Antonio, TX, USA.
Cell Signal. 2003 Feb;15(2):161-70. doi: 10.1016/s0898-6568(02)00057-8.
Regulation of the forkhead domain transcription factors by PDGF has not been studied. In this report, we investigated the role of PDGF-induced Akt in regulating forkhead domain protein FKHRL1 in glomerular mesangial cells. PDGF increased phosphorylation of FKHRL1 in a time- and PI 3 kinase-dependent manner. Expression of dominant negative Akt by adenovirus-mediated gene transfer blocked PDGF-induced FKHRL1 phosphorylation. PDGF inhibited transcription of a forkhead DNA binding element-driven reporter gene. This inhibition was mimicked by constitutively active myristoylated Akt. Moreover, FKHR1-mediated transcription of the reporter gene was completely attenuated by both PDGF and Myr-Akt. One of the targets of forkhead transcription factors is the proapoptotic Fas ligand (FasL) gene. PDGF, as well as Myr-Akt, inhibited transcription of FasL. In contrast, inhibition of PI 3 kinase and dominant negative Akt increased FasL gene transcription, suggesting that suppression of PI 3 kinase/Akt signalling may induce apoptosis in mesangial cells via upregulation of FasL expression. However, expression of dominant negative Akt by adenovirus did not induce apoptosis in mesangial cells, suggesting that Akt-independent antiapoptotic mechanisms also exist. Together, our data demonstrate for the first time that PDGF inactivates forkhead domain transcription factor by Akt-dependent phosphorylation and that suppression of Akt signalling is not sufficient to induce apoptosis in mesangial cells.
血小板衍生生长因子（PDGF）对叉头结构域转录因子的调控尚未得到研究。在本报告中，我们研究了PDGF诱导的Akt在调节肾小球系膜细胞中叉头结构域蛋白FKHRL1方面的作用。PDGF以时间和磷脂酰肌醇3激酶（PI 3激酶）依赖性方式增加FKHRL1的磷酸化。通过腺病毒介导的基因转移表达显性负性Akt可阻断PDGF诱导的FKHRL1磷酸化。PDGF抑制叉头DNA结合元件驱动的报告基因的转录。组成型活性肉豆蔻酰化Akt可模拟这种抑制作用。此外，PDGF和肉豆蔻酰化Akt（Myr-Akt）均完全减弱了FKHR1介导的报告基因转录。叉头转录因子的靶标之一是促凋亡的Fas配体（FasL）基因。PDGF以及Myr-Akt均抑制FasL的转录。相反，抑制PI 3激酶和显性负性Akt可增加FasL基因转录，这表明抑制PI 3激酶/Akt信号传导可能通过上调FasL表达诱导系膜细胞凋亡。然而，通过腺病毒表达显性负性Akt并未在系膜细胞中诱导凋亡，这表明也存在不依赖Akt的抗凋亡机制。总之，我们的数据首次证明PDGF通过Akt依赖性磷酸化使叉头结构域转录因子失活，并且抑制Akt信号传导不足以在系膜细胞中诱导凋亡。
