Stratifin inhibits SCFFBW7 formation and blocks ubiquitination of oncoproteins during the course of lung adenocarcinogenesis.

PURPOSE: Aberrant overexpression of SFN (stratifin) plays an oncogenic role in lung adenocarcinoma. We have shown previously that SKP1, an adapter component of E3 ubiquitin ligase forming a SCF complex, is a unique SFN-binding protein in lung adenocarcinoma cells.

EXPERIMENTAL DESIGN: In silico simulation and in vitro mutagenesis analysis were performed to identify the SFN binding domain on SKP1. We examined expression, localization and stability of SKP1 after knockdown of SFN using lung adenocarcinoma cells including A549. In silico library screening and experimental validation were used for drug screening. Daily oral administration of each candidate drugs to A549-injected tumor-bearing mice was performed to evaluate their in vivo anti-tumor efficacy.

RESULTS: Suppression of SFN up-regulated the stability of SKP1 and accelerated its cytoplasm-to-nucleus translocation. Consistently, immunohistochemical analysis revealed that cytoplasmic expression of SKP1 was significantly associated with SFN positivity, tumor malignancy and poorer patient outcome. After SFN suppression, ubiquitination of oncoproteins including p-cyclin E1, p-c-Myc, p-c-Jun and cleaved Notch 1, which are target proteins of SCFFBW7, was strongly induced. These results indicate that SFN-SKP1 binding results in SCFFBW7 dysfunction and allows several oncoproteins to evade ubiquitination and subsequent degradation. Since inhibition of SFN-SKP1 binding was expected to have anti-tumor efficacy, we next searched for candidate SFN inhibitors. Aprepitant and Ticagrelor were finally selected as potential SFN inhibitors that dose-dependently reduced SFN-SKP1 binding and tumor progression in vivo.

CONCLUSIONS: As overexpression of SFN is detectable in most adenocarcinoma, we believe that SFN inhibitors would be novel and promising anti-tumor drugs for lung adenocarcinoma.