Chibby drives β catenin cytoplasmic accumulation leading to activation of the unfolded protein response in BCR-ABL1+ cells

Abstract

Chronic myeloid leukemia (CML) is a myeloproliferative disease caused by the constitutive tyrosine kinase (TK) activity of the BCR-ABL fusion protein. However, the phenotype of leukemic stem cells (LSC) is sustained by β catenin rather than by the BCR-ABL TK. β catenin activity in CML is contingent upon its stabilization proceeding from the BCR-ABL-induced phosphorylation at critical residues for interaction with the Adenomatous polyposis coli (APC)/Axin/glycogen synthase kinase 3 (GSK3) destruction complex or GSK3 inactivating mutations. Here we studied the impact of β catenin antagonist Chibby (CBY) on β catenin signaling in BCR-ABL1+ cells. CBY is a small conserved protein which interacts with β catenin and impairs β catenin-mediated transcriptional activation through two distinct molecular mechanisms: 1) competition with T cell factor (TCF) or lymphoid enhancer factor (LEF) for β catenin binding; and 2) nuclear export of β catenin via interaction with 14-3-3. We found that its enforced expression in K562 cell line promoted β catenin cytoplasmic translocation resulting in inhibition of target gene transcription. Moreover, cytoplasmic accumulation of β catenin activated the endoplasmic reticulum (ER) stress-associated pathway known as unfolded protein response (UPR). CBY-driven cytoplasmic accumulation of β catenin is also a component of BCR-ABL1+ cell response to the TK inhibitor Imatinib (IM). It evoked the UPR activation leading to the induction of BCL2-interacting mediator of cell death (BIM) by UPR sensors. BIM, in turn, contributed to the execution phase of apoptosis in the activation of ER resident caspase 12 and mobilization of Ca(2+) stores.

BCR-ABL1-Associated Reduction of Beta Catenin Antagonist Chibby1 in Chronic Myeloid Leukemia

“…Accordingly, Cby1 reduced expression in CD34+ cells of CML-CP patients and HP was associated with a significant increment of nuclear beta catenin and enhanced transcription of cyclin D1, a Wnt/beta catenin target gene involved in the maintenance of CD34+ pool (p<0.05 or less) [28]. These findings confirm and expand the results of a recently published study, showing that Cby1-enforced expression is a central component of beta catenin nuclear export and suppression of its transcriptional activity in BCR-ABL1+ cells [29]. In particular, they emphasize Cby1 participation in beta catenin signaling in the LSC compartment responsible for the disease pathogenesis and persistence under TK inhibitor therapy.…”

Chibby functions in Xenopus ciliary assembly, embryonic development, and the regulation of gene expression

“…While nuclear localization sequences have been reported to be present in Cby (Li et al 2010), we did not observe any obvious nuclear accumulation of the Cby-GFP protein, although in cells expressing high levels of the protein, it was found associated with the plasma membrane (not shown). The lack of nuclear localization may be due, at least in part, to the fact that Cby-GFP is thought to cooperate with other proteins to move β-catenin out of the nucleus and into the cytoplasm (Takemaru et al 2003; Li et al 2010; Mancini et al 2013) and perhaps reflects the effects of the expression of the Cby-interacting protein TC1(C8orf4)(Jung et al 2006), whose RNA levels (according to Xenbase) increase following the mid-blastula transition (supplemental figure 4). …”

“…Over-expression of Cby has been reported to generate an unfolded protein response in BCR-ABL1 + cells (Mancini et al 2013); whether it has similar effects in other cell types, such as those of the early embryo, is unclear. On the other hand, the absence of Cby could effect β-catenin levels and/or its intracellular distribution, which in turn could influence the activities of other β-catenin-interacting proteins (see (Kormish et al 2010)).…”

“…Cby’s interaction with β-catenin appears to involve a complex with 14-3-3 proteins and leads to β-catenin’s export from the nucleus, inhibiting its interactions with LEF1/TCF-type HMG-box transcription factors (Li et al 2010). The Cby-driven cytoplasmic accumulation of β-catenin has been reported to induce an unfolded protein response (Mancini et al 2013). TC1 (C8orf4), a protein originally identified based on its over-expression in thyroid cancer cells (Chua et al 2000; Sunde et al 2004), interacts with and inhibits Cby’s interactions with β-catenin, thereby enhancing canonical Wnt signaling (Jung et al 2006).…”

Anti-oncogenic activity of Chibby in the development of human nasopharyngeal carcinoma

“…In addition, Cby has been demonstrated to suppress the growth of human colon adenocarcinoma SW480 cells through the inhibition of the canonical Wnt signaling pathway (18). In a recent study, Cby was found to activate endoplasmic reticulum (ER) stress and apoptosis in breakpoint cluster protein/Abl kinase 1-positive human myeloid leukemia cells by inducing the cytoplasmic accumulation of β-catenin (19). Thus, the downregulation of Cby in cancer cells may provide information regarding the use of Cby as therapeutic target and prognostic factor.…”