βTrCP- and Rsk1/2-Mediated Degradation of BimEL Inhibits Apoptosis

The BimEL tumor suppressor is a potent proapoptotic BH3-only protein. We found that, in response to survival signals, BimEL was rapidly phosphorylated on three serine residues in a conserved degron, facilitating binding and degradation via the F box protein βTrCP. Phosphorylation of the BimEL degron was executed by Rsk1/2 and promoted by the Erk1/2-mediated phosphorylation of BimEL on Ser69. Compared to wild-type BimEL, a BimEL phosphorylation mutant unable to bind βTrCP was stabilized and consequently potent at inducing apoptosis by the intrinsic mitochondrial pathway. Moreover, although non-small cell lung cancer (NSCLC) cells often become resistant to gefitinib (a clinically relevant tyrosine kinase inhibitor that induces apoptosis through BimEL), silencing of either βTrCP or Rsk1/2 resulted in BimEL-mediated apoptosis of both gefitinib-sensitive and gefitinib-insensitive NSCLC cells. Our findings reveal that βTrCP promotes cell survival in cooperation with the ERK-RSK pathway by targeting BimEL for degradation.