Guo Peng, Dong Xue-Yuan, Zhao Kewen, Sun Xiaodong, Li Qunna, Dong Jin-Tang
Department of Hematology and Medical Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia 30322.
Department of Hematology and Medical Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia 30322.
J Biol Chem. 2009 Oct 9;284(41):28243-28252. doi: 10.1074/jbc.M109.036160. Epub 2009 Aug 14.
The proto-oncogene MYC plays a critical role in cell proliferation and tumorigenesis, and its down-regulation by transforming growth factor beta (TGFbeta) signaling is necessary for TGFbeta to inhibit cell proliferation. KLF5, on the other hand, is a pro-proliferative basic transcription factor that reverses function to become an anti-proliferative TGFbeta cofactor upon TGFbeta stimulation in epithelial homeostasis. In this study we investigated whether KLF5 directly regulates MYC transcription in epithelial cells in the context of TGFbeta. Knockdown of KLF5 significantly reduced MYC expression in the HaCaT epidermal epithelial cells. When TGFbeta was applied, however, whereas MYC expression was significantly inhibited, knockdown of KLF5 increased MYC expression. Furthermore, re-expression of KLF5 restored the inhibitory effect of TGFbeta on MYC expression in two cancer cell lines. Chromatin immunoprecipitation and oligo pulldown experiments demonstrated that whereas binding of KLF5 to both KLF5 binding element (KBE) and TGFbeta inhibitory element (TIE) DNA elements was necessary for MYC transcription, binding to KBE was decreased by TGFbeta, and binding to TIE was increased by TGFbeta. These results suggest that KLF5 is not only essential for MYC transcription in proliferating epithelial cells but also mediates the inhibitory effect of TGFbeta on MYC transcription. Furthermore, different binding sites mediate different effects of KLF5 in the context of TGFbeta.
原癌基因MYC在细胞增殖和肿瘤发生中起关键作用，转化生长因子β（TGFβ）信号通路对其下调是TGFβ抑制细胞增殖所必需的。另一方面，KLF5是一种促进增殖的碱性转录因子，在上皮细胞稳态中，经TGFβ刺激后其功能逆转，成为一种抗增殖的TGFβ辅因子。在本研究中，我们调查了在TGFβ环境下KLF5是否直接调控上皮细胞中MYC的转录。敲低KLF5可显著降低HaCaT表皮上皮细胞中MYC的表达。然而，当应用TGFβ时，虽然MYC表达受到显著抑制，但敲低KLF5却增加了MYC的表达。此外，KLF5的重新表达恢复了TGFβ对两种癌细胞系中MYC表达的抑制作用。染色质免疫沉淀和寡核苷酸下拉实验表明，虽然KLF5与KLF5结合元件（KBE）和TGFβ抑制元件（TIE）DNA元件的结合对MYC转录是必需的，但TGFβ会降低KLF5与KBE的结合，而增加其与TIE的结合。这些结果表明，KLF5不仅对增殖上皮细胞中MYC的转录至关重要，而且介导了TGFβ对MYC转录的抑制作用。此外，在TGFβ环境下，不同的结合位点介导了KLF5的不同作用。
