KLF4 Ubiquitination And Its Role In Tumorigenesis

Zinc-finger containing transcription factor Kruppel-like factor 4 (KLF4) is involved in a large variety of physiological and pathological cellular processes, including cell cycle regulation, programmed cell death, as well as stem cell reprogramming. KLF4 is considered as a suppressor of cell proliferation for its transactivation of cell cyle inhibitor p21Cip1/Waf1 and suppression of cell cycle promoting genes CCND1 (cyclinD1) and CCNB1 (cyclinB1). Neverthelsess K.LF4 also acts as an anti-apoptotic gene by directly binding to the promoter of p53 and inhibiting p53 expression. When the cell is undergoing certain intracellular stress, it may be the responsibility of KLF4 to choose where to go. KLF4 is important in cell fate decision, so it is associated with several human diseases, especially cancer. In tumorigenesis, KLF4 acted as either an oncogene or a tumor suppressor. Conflicting evidences surprisingly unveiled the fact that KLF4 might play an ambivalent role in tumorigenesis in a tissue specific and genetic context dependent way.Emerging data keep reminding us that KLF4 dysregulation either facilitates or impedes tumor progression, making it important to clarify the regulation network of KLF4. Post-translational modification of KLF4, such as acetylation, phosphorylation and sumoylation, is in charge of regulating the transcriptional activity of KLF4. Besides, like most transcription factors, KLF4 has a rather short half-life within the cell and its turnover must be carefully orchestrated by ubiquitination and ubiquitin-proteasome system. However, the post-translational modification of KLF4 remains largely unknown. To better understand the mechanism of KLF4 ubiquitination, we performed a genome-wide screen of E3 ligase siRNA library based on Western blotting and identified SCF-FBXO32 to be a new E3 ligase which was responsible for KLF4 ubiquitination and degradation. Furthermore, we demonstrated that FBXO32 could physically interact with the N-terminus of KLF4 via its C-terminus and directly target KLF4 for ubiquitination and degradation, however, the ubiqutination sites of KLF4 would probably reside on the region between amino acid residues 181 to 470 which means the C-terminus of this protein. The F-box domain was critical for FBXO32 dependent KLF4 ubiquitination and degradation. We also found out that p38 mitogen-activated protein kinase might be implicated in FBXO32-mediated ubiquitination of KLF4, since p38 kinase inhibitor coincidently abrogated endougenous KLF4 degradation as well as FBXO32-dependent exogenous KLF4 degradation.Finally, FBXO32 facilitated breast cancer cell migration through targeting KLF4 into degradation, whereas it might also be responsible for increased apoptosis in breast cancer cells under sub-lethal DNA damage. Our findings thus further elucidated the ambivalent role of KLF4 in tumorigenesis:whether KLF4 is backing for or against tumor progression largely depends on the genetic background or cellular stress. The detailed mechanisms remain to be further explored.In summary, we identified FBXO32 as a new E3 ligase that directly targeted KLF4 into ubiquitination and subsequent degradation through ubiquitin-proteasome system and further explained the underlying mechanism. These results expanded our understanding of the post-translational modification of KLF4 and of its role in breast cancer progression.