FBXO32 suppresses breast cancer tumorigenesis through targeting KLF4 to proteasomal degradation

Abstract

Krüppel-like factor 4 (KLF4, GKLF) is a zinc-finger transcription factor involved in a large variety of cellular processes, including apoptosis, cell cycle progression, as well as stem cell renewal. KLF4 is critical for cell fate decision and has an ambivalent role in tumorigenesis. Emerging data keep reminding us that KLF4 dysregulation either facilitates or impedes tumor progression, making it important to clarify the regulating network of KLF4. 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. To better understand the mechanism of KLF4 ubiquitination, we performed a genome-wide screen of E3 ligase small interfering RNA library based on western blot and identified SCF-FBXO32 to be a new E3 ligase, which is responsible for KLF4 ubiquitination and degradation. The F-box domain is critical for FBXO32-dependent KLF4 ubiquitination and degradation. Furthermore, we demonstrated that FBXO32 physically interacts with the N-terminus (1–60 aa) of KLF4 via its C-terminus (228–355 aa) and directly targets KLF4 for ubiquitination and degradation. We also found out that p38 mitogen-activated protein kinase pathway may be implicated in FBXO32-mediated ubiquitination of KLF4, as p38 kinase inhibitor coincidently abrogates endogenous KLF4 ubiquitination and degradation, as well as FBXO32-dependent exogenous KLF4 ubiquitination and degradation. Finally, FBXO32 inhibits colony formation in vitro and primary tumor initiation and growth in vivo through targeting KLF4 into degradation. Our findings thus further elucidate the tumor-suppressive function of FBXO32 in breast cancer. These results expand our understanding of the posttranslational modification of KLF4 and of its role in breast cancer development and provide a potential target for diagnosis and therapeutic treatment of breast cancer.

Differential Gene Expression Patterns Between Apical and Basal Inner Hair Cells Revealed by RNA-Seq

“…FBXO32 has recently been identified as a target gene of the TGF-β/Smad signaling pathway in ovarian surface epithelial cells (Qin et al, 2009) and involved in cell survival regulation (Guo et al, 2015). The expression of FBXO32 inhibits cell proliferation and induces cell apoptosis (Tan et al, 2007;Zhou et al, 2017). The differential expression patterns of genes related to cell survival in IHCs is consistent with the vulnerability affecting high frequencies of damage.…”

Cyanidin-3-O-glucoside inhibits epithelial-to-mesenchymal transition, and migration and invasion of breast cancer cells by upregulating KLF4

“…FBXO3 is an identified E3 ligase for KLF4. FBXO3 physically interacts with the N-terminus of KLF4 via its C-terminus and directly targets KLF4 for ubiquitination and degradation (45). In vitro ubiquitination assay results indicated that Cy3G treatment significantly decreased KLF4 ubiquitination status, which explained the observation that KLF4 expression increase only occurred at the protein level after treatment with Cy3G.…”

“…These data demonstrated that when treatment with Cy3G, the KLF4 expression changed with the expression of FBXO32, its E3 ligase. In terms of the function of FBXO32, Zhou et al (45) recently found that FBXO32 functions as a tumor suppressor in breast cancer. However, in our case, FBXO32 acted as a promoter of EMT and cell migration/invasion.…”

Recent Discoveries on the Involvement of Krüppel-Like Factor 4 in the Most Common Cancer Types

“…An in vitro and in vivo study performed by Zhou and colleagues showed that breast cancer cell metastasis is promoted by ATXN3 (Ataxin-3, ATX3, AT3 or MJD), which is a novel deubiquitinating enzyme of KLF4 [ 36 ]. They also found that a member of the F-box protein family (FBXO32) mediates KLF4 ubiquitination and degradation, and in consequence suppresses breast cancer tumorigenesis [ 37 ].…”