The tumor suppressor KLF11 mediates a novel mechanism in transforming growth factor β-induced growth inhibition that is inactivated in pancreatic cancer

c-myc promoter silencing is a key step in epithelial cell growth inhibition by transforming growth factor beta (TGF beta). During carcinogenesis, however, epithelial cells escape from c-myc repression and consequently become refractory to TGF beta-mediated antiproliferation. Here, we assessed the role of the repressor, KLF11, in TGF beta-induced growth inhibition in normal epithelial as well as pancreatic carcinoma cells. Endogenous KLF11 was stably down-regulated by RNA interference technology, and the functional consequences were studied by proliferation assays, reporter assays, DNA binding studies, and expression analyses. Coimmunoprecipitation and glutathione S-transferase pulldown assays were conducted to define KLF11-Smad3 interaction and U0126 was administered to examine the effects of the extracellular signal-regulated kinase (ERK)-mitogen-activated protein kinase on complex formation and c-myc promoter binding of KLF11 and Smad3 in pancreatic cancer cells. In TGF beta-stimulated normal epithelial cells, nuclear KLF11, in concert with Smad3, binds to and represses transcription from the core region of the TGF beta-inhibitory element (TIE) of the c-myc promoter. Disruption of KLF11-Smad3 interaction or small interfering RNA-mediated knockdown of endogenous KLF11 strongly diminishes Smad3-TIE promoter binding and repression, and consequently impairs TGF beta-mediated growth inhibition. In pancreatic cancer cells with oncogenic Ras mutations, hyperactive ERK counteracts TGF beta-induced c-myc repression and growth inhibition through at least two mechanisms, i.e., via disruption of KLF11-Smad3 complex formation and through inhibition of KLF11-Smad3 binding to the TIE element. Together, these results suggest a central role for KLF11 in TGF beta-induced c-myc repression and anti proliferation and identifies a novel mechanism through which ERK signaling antagonizes the tumor suppressor activities of TGF beta in pancreatic cancer cells with oncogenic Ras mutations.