The Hypoxia-controlled FBXL14 Ubiquitin Ligase Targets SNAIL1 for Proteasome Degradation.

The transcription factor SNAIL1 is a master regulator of epithelial to mesenchymal transition. SNAIL1 is a very unstable protein, and its levels are regulated by the E3 ubiquitin ligase β-TrCP1 that interacts with SNAIL1 upon its phosphorylation by GSK-3β. Here we show that SNAIL! polyubiquitylation and degradation may occur in conditions precluding SNAIL1 phosphorylation by GSK-3β, suggesting that additional E3 ligases participate in the control of SNAIL1 protein stability. In particular, we demonstrate that the F-box E3 ubiquitin ligase FBX114 interacts with SNAIL 1 and promotes its ubiquitylation and proteasome degradation independently of phosphorylation by GSK-3β. In vivo, inhibition of FBXI14 using short hairpin RNA stabilizes both ectopically expressed and endogenous SNAIL1. Moreover, the expression of FBXI14 is potently down-regulated during hypoxia, a condition that increases the levels of SNAIL! protein but not SNAIL1 mRNA. FBX14 mRNA is decreased in tumors with a high expression of two proteins up-regulated in hypoxia, carbonic anhydrase 9 and TWIST 1. In addition, Twist1 small interfering RNA prevents hypoxia-induced Fbxl14 down-regulation and SNAIL1 stabilization in NMuMG cells. Altogether, these results demonstrate the existence of an alternative mechanism controlling SNAIL1 protein levels relevant for the induction of SNAIL1 during hypoxia. [ABSTRACT FROM AUTHOR]