Nagarajan R P, Chen F, Li W, Vig E, Harrington M A, Nakshatri H, Chen Y
Department of Medical and Molecular Genetics, Indiana University School of Medicine, 975 West Walnut Street IB130, Indianapolis, IN 46202, USA.
Biochem J. 2000 Jun 15;348 Pt 3(Pt 3):591-6.
Activation of transforming growth factor-beta (TGF-beta) and activin receptors leads to phosphorylation of Sma- and Mad-related protein 2 (Smad2) and Smad3, which function as transcription factors to regulate gene expression. Smad7 is a regulatory protein which is able to inhibit TGF-beta and activin signalling in a negative-feedback loop, mediated by a direct regulation by Smad3 and Smad4 via a Smad-binding element (SBE) in the Smad7 promoter. Interestingly, we found that the Smad7 promoter was also regulated by nuclear factor kappaB (NF-kappaB), a transcription factor which plays an important role in inflammation and the immune response. Expression of NF-kappaB p65 subunit was able to inhibit the Smad7 promoter activity, and this inhibition could be reversed by co-expression of IkappaB, an inhibitor of NF-kappaB. In addition, the inhibitory activity of p65 was observed in a minimal promoter that contained only the Smad7 SBE and a TATA box, without any consensus NF-kappaB binding site. This inhibitory effect appeared to be common to other TGF-beta- and activin-responsive promoters, since p65 also inhibited the forkhead-activin-signal-transducer-2-mediated activation of a Xenopus Mix.2 promoter, as well as the Smad3-mediated activation of 3TP-lux which contains PMA-responsive elements and a plasminogen-activator-inhibitor-1 promoter. Activation of endogenous NF-kappaB by tumour necrosis factor-alpha (TNF-alpha) was also able to inhibit the Smad7 promoter in human embryonic kidney 293 cells. In human hepatoma HepG2 cells, TNF-alpha was able to inhibit TGF-beta- and activin-mediated transcriptional activation. Furthermore, overexpression of the transcription co-activator p300 could abrogate the inhibitory effect of NF-kappaB on the Smad7 promoter. Taken together, these data have indicated a novel mode of crosstalk between the Smad and the NF-kappaB signalling cascades at the transcriptional level by competing for a limiting pool of transcription co-activators.
转化生长因子-β（TGF-β）和激活素受体的激活会导致Sma和Mad相关蛋白2（Smad2）及Smad3发生磷酸化，它们作为转录因子来调节基因表达。Smad7是一种调节蛋白，能够在由Smad3和Smad4通过Smad7启动子中的Smad结合元件（SBE）直接调控介导的负反馈环中抑制TGF-β和激活素信号传导。有趣的是，我们发现Smad7启动子也受核因子κB（NF-κB）调控，核因子κB是一种在炎症和免疫反应中起重要作用的转录因子。NF-κB p65亚基的表达能够抑制Smad7启动子活性，并且这种抑制作用可通过共表达NF-κB抑制剂IkappaB来逆转。此外，在仅包含Smad7 SBE和TATA框且没有任何一致的NF-κB结合位点的最小启动子中也观察到了p65的抑制活性。这种抑制作用似乎在其他TGF-β和激活素反应性启动子中也很常见，因为p65还抑制了非洲爪蟾Mix.2启动子的叉头激活素信号转导子2介导的激活，以及包含佛波酯反应元件和纤溶酶原激活物抑制剂-1启动子的3TP-lux的Smad3介导的激活。肿瘤坏死因子-α（TNF-α）激活内源性NF-κB也能够抑制人胚肾293细胞中的Smad第七启动子。在人肝癌HepG2细胞中，TNF-α能够抑制TGF-β和激活素介导的转录激活。此外，转录共激活因子p300的过表达可以消除NF-κB对Smad7启动子的抑制作用。综上所述，这些数据表明在转录水平上，Smad和NF-κB信号级联之间通过竞争有限的转录共激活因子库存在一种新的相互作用模式。
