Apoptotic Death of Bcr-Abl-Expressing Myeloid Progenitors in Response to the m-Tor Inhibitor RAD001 (Everolimus) Is Promoted by the Nuclear Import of c-Abl.

Nuclear accumulation of c-Abl protein in response to genotoxic damage addresses apoptotic cell death. The process, driven by the disruption of its binding to 14-3-3 scaffolding proteins, is independent from c-Abl catalytic state and conditional upon 14-3-3 phosphorylation by the c-jun N-terminal kinase (JNK) (Yoshida et al Nat Cell Biol 7,278,2005). Our work moved from the observation that the constitutive activation of Abl tyrosine kinase (TK) by juxtaposed Bcr sequences in p210 fusion protein Bcr-Abl precludes c-Abl nuclear import in response to gamma irradiation, advancing a role for residual c-Abl protein “loss of function” in the apoptosis-resistant phenotype of Chronic Myeloid Leukemia (CML) progenitors. In 32D cell clones transducing a temperature-conditional Bcr-Abl mutant, active p210 TK is associated with the overexpression of 14-3-3sigma mostly driven by transcriptional events involving histone H4 acetylation (but not methylation) status of the gene promoter. P210 TK inhibition in response to 24 hour exposure to imatinib mesylate (IM) is associated with an early downregulation of 14-3-3sigma followed by c-Abl nuclear import and cell commitment to apoptotic death. The two last events are further enhanced by complementary inhibition of the 14-3-3 binding site by R18 peptide, supporting that the negative impact of p210 TK on pro-apoptotic function of residual normal c-Abl arises from its effects on 14-3-3sigma. Previous studies proved that the activation of p38 MAP kinase contributes to IM effects in CML (