Ras inhibits TGF‐β‐induced KLF5 acetylation and transcriptional complex assembly via regulating SMAD2/3 phosphorylation in epithelial cells

Acetylated Kruppel-like factor 5 (KLF5) is essential for transforming growth factor-beta (TGF-beta) to properly regulate gene transcription in the inhibition of cell proliferation and tumor growth. Ras oncogenic signaling can convert TGF-beta from a tumor suppressor to a tumor promoter; however, its ability to utilize the KLF5 transcription factor to modulate TGF-beta functions is still unknown. Therefore, in this study, we sought to determine whether Ras signaling altered TGF-beta-induced KLF5 acetylation and the assembly of the p300-KLF5-SMADs transcriptional complex in gene regulation. Not only did we determine that Ras signaling inhibited TGF-beta-induced KLF5 acetylation and interfered with TGF-beta function in p15 induction and Myc repression, but also TGF-beta-induced SMAD3 C-terminal region phosphorylation was necessary for TGF-beta to induce KLF5 acetylation. Moreover, Ras activation further interrupted the interactions amongst p300, KLF5, and SMAD4, as well as the binding of p300-KLF5-SMADs complex onto the TGF-beta-responsive promoter elements for both p15 and Myc. These findings suggested that KLF5 mediated the crosstalk between TGF-beta and Ras signaling, and that suppression of TGF-beta-induced KLF5 acetylation by Ras activation; this altered TGF-beta-induced assembly of p300-KLF5-SMADs complex onto gene promoters to convert the function of TGF-beta in gene regulation.