Erioflorin stabilizes the tumor suppressor Pdcd4 by inhibiting the E3-ligase β-TrCP1

The tumor suppressor Pdcd4 (Programmed Cell Death 4) exerts its tumor suppressive function by inhibiting protein translation. Pdcd4 protein is lost in various tumor types and Pdcd4 overexpression was shown to inhibit tumorigenesis in vitro and in vivo. Pdcd4 expression is regulated at various levels including miR-21-dependent translational repression and protein destabilization. Proteasomal degradation of Pdcd4 is mediated by p70^S6K-dependent phosphorylation and subsequent recognition and ubiquitination by the E3-ubiquitin ligase β-TrCP. In an attempt to identify stabilizers of Pdcd4, we performed a high-throughput screen of natural compound libraries and discovered erioflorin to efficiently rescue Pdcd4 from tumor promoter, i.e. TPA, induced degradation. Mechanistically, erioflorin inhibits β-TrCP1 activity, thus, disrupting the interaction between β-TrCP1 and Pdcd4, without affecting the phosphorylation status of the latter. Erioflorin also stabilized other β-TrCP targets such as IкBα, while leaving target degradation by other E3-ligases unaffected. Consequently, erioflorin suppressed the activities of tumor-associated transcription factors AP-1 and NF-кB, and attenuated tumor cell migration. Taken together, we characterize erioflorin as a novel inhibitor of the E3-ubiquitin ligase β-TrCP1 and stabilizer of Pdcd4. Inhibiting E3-ligases will increase specificity of tumor therapeutic approaches targeting protein degradation as compared to broad proteasome inhibition.