Barré Benjamin, Avril Sylvie, Coqueret Olivier
INSERM U564, 4 rue Larrey, Centre Hospitalier Universitaire Angers, 49033 Angers Cedex, France.
J Biol Chem. 2003 Jan 31;278(5):2990-6. doi: 10.1074/jbc.M210422200. Epub 2002 Nov 15.
Activated forms of STAT3 transcription factors are often found in various cancers and tumor cell lines, indicating that this signaling pathway is involved in tumorogenesis. At the molecular level, STAT3 proteins function as transcriptional activators and up-regulate several growth-promoting genes such as myc, pim-1, or cyclin D1. However, these transcription factors have also proapoptotic functions and can activate the expression of the cell-cycle inhibitor p21(waf1), suggesting that STAT3 can also block cell-cycle progression and prevent abnormal cell proliferation. To reconcile these observations, one would predict that the STAT3-mediated activation of p21(waf1) is lost during cell transformation. In this study, we show that upon IL-6 stimulation of glioblastoma cells, STAT3 does not activate the expression of the p21(waf1) gene, whereas the expression of the myc gene remains unaltered. Chromatin immunoprecipitation experiments show that STAT3 and its cofactor NcoA/SRC1a are effectively recruited to the p21(waf1) promoter but that this is not followed by the association of the CREB-binding protein (CBP) histone acetylase and the type II RNA polymerase as normally seen on the myc promoter. Whereas the PI-3K/Akt pathway is constitutively activated in these cells, inactivation of this pathway restores the loading of CBP and the RNA polymerase and the expression of the p21(waf1) gene without having any effect on myc regulation. Moreover, this effect was recapitulated in HepG2 cells expressing an activated form of the Akt kinase. In these cells, the kinase blocked the STAT3-mediated expression of the p21(waf1) gene by inhibiting the recruitment of CREB-binding protein and the type II RNA polymerase, without having any effects on the loading of STAT3 and its cofactor NcoA/SRC1a. Together, these findings suggest that the phosphatidylinositol 3-kinase/Akt pathway inhibits the transcriptional activation of the p21(waf1) gene by STAT3 proteins without altering the regulation of the myc promoter.
STAT3转录因子的激活形式常在各种癌症和肿瘤细胞系中被发现，这表明该信号通路参与肿瘤发生。在分子水平上，STAT3蛋白作为转录激活因子发挥作用，上调多种促进生长的基因，如myc、pim-1或细胞周期蛋白D1。然而，这些转录因子也具有促凋亡功能，并且能够激活细胞周期抑制剂p21（waf1）的表达，这表明STAT3也可以阻断细胞周期进程并防止异常细胞增殖。为了协调这些观察结果，可以预测在细胞转化过程中STAT3介导的p21（waf1）激活会丧失。在本研究中，我们表明，在白细胞介素-6刺激胶质母细胞瘤细胞后，STAT3不会激活p21（waf1）基因的表达，而myc基因的表达保持不变。染色质免疫沉淀实验表明，STAT3及其辅因子NcoA/SRC1a被有效地募集到p21（waf1）启动子上，但随后并没有像在myc启动子上通常看到的那样，出现CREB结合蛋白（CBP）组蛋白乙酰转移酶和II型RNA聚合酶的结合。虽然PI-3K/Akt通路在这些细胞中持续激活，但该通路的失活恢复了CBP和RNA聚合酶的加载以及p21（waf1）基因的表达，而对myc调控没有任何影响。此外，在表达激活形式Akt激酶的HepG2细胞中也重现了这种效应。在这些细胞中，该激酶通过抑制CREB结合蛋白和II型RNA聚合酶的募集来阻断STAT3介导的p21（waf1）基因表达，而对STAT3及其辅因子NcoA/SRC1a的加载没有任何影响。总之，这些发现表明磷脂酰肌醇3-激酶/Akt通路抑制了STAT3蛋白对p21（waf1）基因的转录激活，而不改变myc启动子的调控。
