The p210BCR-ABL tyrosine kinase of chronic myeloid leukemia causes resistance to radio-induced apoptotic death by inhibiting the proapoptotic BAX gene

14-3-3 Binding and Sumoylation Concur to the Down-Modulation of β-catenin Antagonist chibby 1 in Chronic Myeloid Leukemia

The down-modulation of the β-catenin antagonist Chibby 1 (CBY1) associated with the BCR-ABL1 fusion gene of chronic myeloid leukemia (CML) contributes to the aberrant activation of β-catenin, particularly in leukemic stem cells (LSC) resistant to tyrosine kinase (TK) inhibitors. It is, at least partly, driven by transcriptional events and gene promoter hyper-methylation. Here we demonstrate that it also arises from reduced protein stability upon binding to 14-3-3σ adapter protein. CBY1/14-3-3σ interaction in BCR-ABL1+ cells is mediated by the fusion protein TK and AKT phosphorylation of CBY1 at critical serine 20, and encompasses the 14-3-3σ binding modes I and II involved in the binding with client proteins. Moreover, it is impaired by c-Jun N-terminal kinase (JNK) phosphorylation of 14-3-3σ at serine 186, which promotes dissociation of client proteins. The ubiquitin proteasome system UPS participates in reducing stability of CBY1 bound with 14-3-3σ through enhanced SUMOylation. Our results open new routes towards the research on molecular pathways promoting the proliferative advantage of leukemic hematopoiesis over the normal counterpart.

Zinc-finger transcription factor slug contributes to the survival advantage of chronic myeloid leukemia cells

Slug, a Snail-related zinc-finger transcription factor implicated in the increased motility of mesenchymal cells during embryonic development and progression of cancer cells towards an invasive phenotype, plays a specific and critical role in the pathogenesis of Bcr-Abl-associated leukemias. Here we report that Slug over-expression associated with Bcr-Abl is conditional upon the tyrosine kinase (TK) activity of 210 fusion protein. Slug over-expression is driven by transcriptional events eventually integrated by post-transcriptional mechanisms leading to protein stabilization and is at least partly regulated by the ERK1/2 mitogen-activated protein kinase (MAPK). It contributes to apoptosis resistance of leukemic progenitors through the repression of pro-apoptotic Puma. Moreover, Slug is a component of leukemic progenitor resistance to imatinib mesylate (IM) driven by Bcr-Abl point mutations and, in particular, by T315I. Slug over-expression associated with p210 Bcr-Abl TK either in the wild type (wt) or mutated conformation results in a significant reduction of E-cadherin, the substrate of Beta catenin at cell membranes. In conclusion, our results suggest that Slug has a central role in a complex network involved in prolonged survival and IM resistance of CML progenitors.

Berbamine exhibits potent antitumor effects on imatinib-resistant CML cells in vitro and in vivo

AIM

The aim of this study was to explore the effects and mechanism of berbamine on imatinib-resistant BCR-ABL-positive human leukemia K562 (K562-r) cells in vitro and in vivo.

METHODS

Cell viability was measured by MTT assay, and apoptotic morphology changes were detected by fluorescence microscopy. The apoptosis rate was measured by flow cytometric assay. mdr-1 mRNA levels were determined by RT-PCR. Bcl-2 family proteins, cytochrome c(cyt C), poly (ADP-ribose) polymerase (PARP), and P-glycoprotein were detected by Western blot. BALB/c nu/nu mice were injected with K562-r cells subcutaneously. Tumor-bearing mice were treated intravenously with berbamine.

RESULTS

MTT tests revealed that berbamine significantly inhibited K562-r cell proliferation and increased the chemo-sensitivity of K562-r cells to imatinib. The apoptosis rate was significantly increased following treatment with 21.2 micromol/L berbamine; formation of typical apoptotic blebs was apparent, as observed by fluorescence microscopy. Expression levels of mdr-1 mRNA and P-gp protein were high in untreated K562-r cells and significantly down-regulated by berbamine treatment. Berbamine-treated K562-r cells also exhibited down-regulated expression of the anti-apoptotic proteins Bcl-2 and Bcl-x(L), up-regulated expression of the apoptotic proteins Bax and cytoplasmic cyt C, and stimulated proteolytic cleavage of PARP. In addition, berbamine also suppressed the growth of K562-r xenotransplanted tumors in vivo.

CONCLUSION

Berbamine inhibited proliferation of K562-r cells both in vitro and in vivo. Berbamine-induced apoptosis in K562-r cells appeared to occur through a mechanism involving Bcl-2 family proteins, as well as mdr-1 mRNA and P-gp protein. Berbamine in combination with imatinib restored the chemo-sensitivity of K562-r cells to imatinib. Our findings suggest that berbamine may be useful in treating imatinib-resistant CML patients.

Bcr-Abl reduces endoplasmic reticulum releasable calcium levels by a Bcl-2-independent mechanism and inhibits calcium-dependent apoptotic signaling

The Bcr-Abl oncoprotein plays a major role in the development and progression of chronic myeloid leukemia (CML). Several studies have suggested that the expression levels of Bcr-Abl are elevated at disease progression to blast crisis and that this plays a significant role in the achievement of drug resistance. We have established cell lines expressing low and high levels of Bcr-Abl to study the molecular mechanisms involved in disease progression and drug resistance. It is now known that the endoplasmic reticulum (ER) can play a major role in the regulation of apoptosis. We therefore investigated whether Bcr-Abl expression modulates ER homeostasis and interferes with ER-mediated apoptotic pathways to promote survival. Bcr-Abl-expressing cells exhibit a decreased amount of free releasable calcium in the ER as well as a weaker capacitative calcium entry response, relative to parental cells. This effect is independent of Bcl-2, which is a known modulator of ER calcium homeostasis. The reduction in ER releasable calcium results in inhibition of the ER/mitochondria-coupling process and mitochondrial calcium uptake. This study demonstrates a novel downstream consequence of Bcr-Abl signaling. The ability to negate calcium-dependent apoptotic signaling is likely to be a major prosurvival mechanism in Bcr-Abl-expressing cells.

Tyrosine kinase inhibitor STI571 (Imatinib) cooperates with wild-type p53 on K562 cell line to enhance its proapoptotic effects

In order to ascertain whether p53 has a role in chronic myeloid leukemia hematopoietic progenitor response to the innovative tyrosine kinase inhibitor STI571 (Imatinib), we overexpressed a wild type (wt) p53 construct in the K562 cell line, generated from a human blast crisis and lacking endogenous p53. Wt p53 overexpression was associated with a significant reduction of bcr-abl expression levels resulting, at least in part, from post-transcriptional events affecting the stability of p210 bcr-abl fusion protein. Moreover, we demonstrated that p53 overexpression enhances the commitment to the apoptotic death fate of K562 following its in vitro exposure to 1 microM STI571. Multiple mechanisms are involved in p53 impact on K562 survival: Most importantly, we found that a greater reduction of bcr-abl transcription by STI571 was associated with the overexpression of wt p53. Further studies are required to elucidate the mechanisms involved in the transcriptional repression of bcr-abl by STI571 and p53 and in their synergic effects on the clonal hematopoiesis of chronic myeloid leukemia.