A calpain-cleaved fragment of β-catenin promotes BCRABL1+ cell survival evoked by autophagy induction in response to imatinib

Autophagy protects chronic myeloid leukemia stem cells from tyrosine kinase inhibitors hence supporting the disease persistence under therapy. However, the signals involved in autophagy regulation relative to BCR-ABL1 are still elusive. The autophagic flux proceeding from the inhibition of BCR-ABL1 tyrosine kinase represents a regulatory mechanism of β-catenin stability through events encompassing the activation of calpain, which targets β-catenin for proteasome-independent degradation. Accordingly, its inactivation may contribute to induce autophagy and autophagy induction may, in turn, promote β-catenin autolysosomal degradation to originate a regulatory loop where β-catenin plays a central role in cell decision between life and death. Here we proved that the cytoplasmic accumulation of β-catenin driven by up-regulation of its antagonist Chibby1 is a component of autophagy induction in response to imatinib in BCR-ABL1+ cells opposing the apoptotic death. It is contingent upon ER stress and elevation of free Ca(2+) cytosolic concentration and results in the calpain cleavage into a 28kDa fragment implicated in β-catenin proteasome-independent degradation. More important for BCR-ABL1+ cell survival and proliferation following IM treatment, might be the calpain-mediated cleavage of β-catenin accumulated within the cytoplasmic compartment into a 75kDa fragment, still owning TCF-dependent transcriptional activity. Such a β-catenin fragment might be crucial for BCR-ABL1+ cell survival following the fusion protein TK inhibition.