Bcl-xL anti-apoptotic network is dispensable for development and maintenance of CML but is required for disease progression where it represents a new therapeutic target

Amsacrine downregulates BCL2L1 expression and triggers apoptosis in human chronic myeloid leukemia cells through the SIDT2/NOX4/ERK/HuR pathway

Accumulating evidence indicates that the anticancer activity of acridine derivatives is mediated through the regulation of anti-apoptotic and pro-apoptotic BCL2 protein expression. Therefore, we investigated whether the cytotoxicity of amsacrine with an acridine structural scaffold in human chronic myeloid leukemia (CML) K562 cells was mediated by BCL2 family proteins. Amsacrine induced apoptosis, mitochondrial depolarization, and BCL2L1 (also known as BCL-XL) downregulation in K562 cells. BCL2L1 overexpression inhibited amsacrine-induced cell death and mitochondrial depolarization. Amsacrine treatment triggered SIDT2-mediated miR-25 downregulation, leading to increased NOX4-mediated ROS production. ROS-mediated inactivation of ERK triggered miR-22 expression, leading to increased HuR mRNA decay. As HuR is involved in stabilizing BCL2L1 mRNA, downregulation of BCL2L1 was noted in K562 cells after amsacrine treatment. In contrast, amsacrine-induced BCL2L1 downregulation was alleviated by restoring ERK phosphorylation and HuR expression. Altogether, the results of this study suggest that amsacrine triggers apoptosis in K562 cells by inhibiting BCL2L1 expression through the SIDT2/NOX4/ERK-mediated downregulation of HuR. Furthermore, a similar pathway also explains the cytotoxicity of amsacrine in CML MEG-01 and KU812 cells.

Repurposing the Bis-Biguanide Alexidine in Combination with Tyrosine Kinase Inhibitors to Eliminate Leukemic Stem/Progenitor Cells in Chronic Myeloid Leukemia

BACKGROUND & AIMS

In CML, Leukemic Stem Cells (LSCs) that are insensitive to Tyrosine Kinase Inhibitors are responsible for leukemia maintenance and relapses upon TKI treatment arrest. We previously showed that downregulation of the BMI1 polycomb protein that is crucial for stem/progenitor cells self-renewal induced a CCNG2/dependent proliferation arrest leading to elimination of Chronic Myeloid Leukemia (CML) cells. Unfortunately, as of today, pharmacological inhibition of BMI1 has not made its way to the clinic.

METHODS

We used the Connectivity Map bioinformatic database to identify pharmacological molecules that could mimick BMI1 silencing, to induce CML cell death. We selected the bis-biguanide Alexidin (ALX) that produced a transcriptomic profile positively correlating with the one obtained after BMI silencing in K562 CML cells. We then evaluated the efficiency of ALX in combination with TKI on CML cells.

RESULTS

Here we report that cell growth and clonogenic activity of K562 and LAMA-84 CML cell lines were strongly inhibited by ALX. ALX didn't modify BCR::ABL1 phosphorylation and didn't affect BMI1 expression but was able to increase CCNG2 expression leading to autophagic processes that preceed cell death. Besides, ALX could enhance the apoptotic response induced by any Tyrosine Kinase Inhibitors (TKI) of the three generations. We also noted a strong synergism between ALX and TKIs to increase expression of caspase-9 and caspase-3 and induce PARP cleavage, Bad expression and significantly decreased Bcl-xL family member expression. We also observed that the blockage of the mitochondrial respiratory chain by ALX can be associated with inhibition of glycolysis by 2-DG to achieve an enhanced inhibition of K562 proliferation and clonogenicity. ALX specifically affected the differentiation of BCR::ABL1-transduced healthy CD34⁺ cells but not of mock-infected healthy CD34⁺ control cells. Importantly, ALX strongly synergized with TKIs to inhibit clonogenicity of primary CML CD34⁺ cells from diagnosed patients. Long Term Culture of Initiating Cell (LTC-IC) and dilution of the fluorescent marker CFSE allowed us to observe that ALX and Imatinib (IM) partially reduced the number of LSCs by themselves but that the ALX/IM combination drastically reduced this cell compartment. Using an in vivo model of NSG mice intravenously injected with K562-Luciferase transduced CML cells, we showed that ALX combined with IM improved mice survival.

CONCLUSIONS

Collectively, our results validate the use of ALX bis-biguanide to potentiate the action of conventional TKI treatment as a potential new therapeutic solution to eradicate CML LSCs.

BH3 mimetics and TKI combined therapy for Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) was considered for a long time one of the most hostile leukemia that was incurable for most of the patients, predominantly due to the extreme resistance to chemotherapy. Part of the resistance to cell death (apoptosis) is the result of increased levels of anti-apoptotic and decreased levels of pro-apoptotic member of the BCL-2 family induced by the BCR-ABL1 oncoprotein. BCR-ABL1 is a constitutively active tyrosine kinase responsible for initiating multiple and oncogenic signaling pathways. With the development of specific BCR-ABL1 tyrosine kinase inhibitors (TKIs) CML became a much more tractable disease. Nevertheless, TKIs do not cure CML patients and a substantial number of them develop intolerance or become resistant to the treatment. Therefore, novel anti-cancer strategies must be developed to treat CML patients independently or in combination with TKIs. Here, we will discuss the mechanisms of BCR-ABL1-dependent and -independent resistance to TKIs and the use of BH3-mimetics as a potential tool to fight CML.

BH3 mimetics in combination with nilotinib or ponatinib represent a promising therapeutic strategy in blast phase chronic myeloid leukemia

Dysregulation of the BCL-2 family is implicated in protecting chronic myeloid leukemia (CML) cells from intracellular damage and BCR::ABL1-inhibition with tyrosine kinase inhibitors (TKIs) and may be a viable therapeutic target in blast phase (BP-)CML, for which treatment options are limited. BH3 mimetics, a class of small molecule inhibitors with high-specificity against the prosurvival members of the BCL-2 family, have displayed clinical promise in the treatment of chronic lymphocytic and acute myeloid leukemia as single agents and in combination with standard-of-care therapies. Here we present the first comparison of inhibition of BCL-2 prosurvival proteins BCL-2, BCL-xL and MCL-1 in combination with a second or third generation TKI, crucially with comparisons drawn between myeloid and lymphoid BP-CML samples. Co-treatment of four BP-CML cell lines with the TKIs nilotinib or ponatinib and either BCL-2 (venetoclax), MCL-1 (S63845) or BCL-xL (A-1331852) inhibitors resulted in a synergistic reduction in cell viability and increase in phosphatidylserine (PS) presentation. Nilotinib with BH3 mimetic combinations in myeloid BP-CML patient samples triggered increased induction of apoptosis over nilotinib alone, and a reduction in colony-forming capacity and CD34⁺ fraction, while this was not the case for lymphoid BP-CML samples tested. While some heterogeneity in apoptotic response was observed between cell lines and BP-CML patient samples, the combination of BCL-xL and BCR::ABL1 inhibition was consistently effective in inducing substantial apoptosis. Further, while BH3 mimetics showed little efficacy as single agents, dual-inhibition of BCL-2 prosurvival proteins dramatically induced apoptosis in all cell lines tested and in myeloid BP-CML patient samples compared to healthy donor samples. Gene expression and protein level analysis suggests a protective upregulation of alternative BCL-2 prosurvival proteins in response to BH3 mimetic single-treatment in BP-CML. Our results suggest that BH3 mimetics represent an interesting avenue for further exploration in myeloid BP-CML, for which alternative treatment options are desperately sought.

Therapeutic Strategies Against Cancer Stem Cells in Esophageal Carcinomas

Cancer stem cells (CSCs) in esophageal cancer have a key role in tumor initiation, progression and therapy resistance. Novel therapeutic strategies to target CSCs are being tested, however, more in-depth research is necessary. Eradication of CSCs can result in successful therapeutic approaches against esophageal cancer. Recent evidence suggests that targeting signaling pathways, miRNA expression profiles and other properties of CSCs are important strategies for cancer therapy. Wnt/β-catenin, Notch, Hedgehog, Hippo and other pathways play crucial roles in proliferation, differentiation, and self-renewal of stem cells as well as of CSCs. All of these pathways have been implicated in the regulation of esophageal CSCs and are potential therapeutic targets. Interference with these pathways or their components using small molecules could have therapeutic benefits. Similarly, miRNAs are able to regulate gene expression in esophageal CSCs, so targeting self-renewal pathways with miRNA could be utilized to as a potential therapeutic option. Moreover, hypoxia plays critical roles in esophageal cancer metabolism, stem cell proliferation, maintaining aggressiveness and in regulating the metastatic potential of cancer cells, therefore, targeting hypoxia factors could also provide effective therapeutic modalities against esophageal CSCs. To conclude, additional study of CSCs in esophageal carcinoma could open promising therapeutic options in esophageal carcinomas by targeting hyper-activated signaling pathways, manipulating miRNA expression and hypoxia mechanisms in esophageal CSCs.

CML Chapter

The discovery of the tyrosine kinase inhibitor (TKI) imatinib in the early 2000's revolutionized the treatment and prognosis of patients with chronic myeloid leukemia (CML) [Hochhaus et al. in N Engl J Med 376:917-927, 2017]. The treatment of patients with CML has changed dramatically since the approval of imatinib and other TKIs. Before the TKI era, newly diagnosed patients would undergo HLA typing to try to identify a well-matched donor, and then proceed quickly to allogeneic hematopoietic cell transplantation (HCT). With the introduction of imatinib followed a few years later by dasatinib, nilotinib, then bosutinib, treatment approaches changed in a dramatic way. Transplantation is no longer an upfront treatment option for newly diagnosed CML patients, and in fact, it is very rarely used in the management of a patient with CML currently. The management of CML patients has been a model of personalized medicine or targeted therapy that is being emulated in the treatment of many other hematologic malignancies and solid tumors such as lung cancer [Soverini et al. in Mol Cancer 17:49, 2018]. The Philadelphia Chromosome (Ph) which leads to the formation of the BCR-ABL fusion gene and its product the BCR-ABL protein is the cause of CML. With effective targeting of this protein with the available TKIs, the disease is completely controllable if not curable for most patients. Life expectancy for patients with CML is essentially normal. Quality of life becomes an important goal including the potential for pregnancy, and ultimately the chance to discontinue all TKI therapy permanently. The three cases outlined below serve to highlight some of the important issues in the management of patients with CML in the post-TKI era.

Endoplasmic reticulum stress mediates resistance to BCL-2 inhibitor in uveal melanoma cells

To address unmet clinical need for uveal melanomas, we assessed the effects of BH3-mimetic molecules, the ABT family, known to exert pro-apoptotic activities in cancer cells. Our results uncovered that ABT-263 (Navitoclax), a potent and orally bioavailable BCL-2 family inhibitor, induced antiproliferative effects in metastatic human uveal melanoma cells through cell cycle arrest at the G0/G1 phase, loss of mitochondrial membrane potential, and subsequently apoptotic cell death monitored by caspase activation and poly-ADP ribose polymerase cleavage. ABT-263-mediated reduction in tumor growth was also observed in vivo. We observed in some cells that ABT-263 treatment mounted a pro-survival response through activation of the ER stress signaling pathway. Blocking the PERK signaling pathway increased the pro-apoptotic ABT-263 effect. We thus uncovered a resistance mechanism in uveal melanoma cells mediated by activation of endoplasmic reticulum stress pathway. Therefore, our study identifies ABT-263 as a valid therapeutic option for patients suffering from uveal melanoma.

Correction of Bcl-x splicing improves responses to imatinib in chronic myeloid leukaemia cells and mouse models

Imatinib mesylate (IM) resistance has become a major clinical problem for chronic myeloid leukaemia (CML). It is known that Bcl-x splicing is deregulated and is involved in multiple malignant cancer initiation and chemotherapy resistance, including CML. The aim of the present study was to correct the abnormal splicing of Bcl-x in CML and investigate the subsequent malignant phenotype changes, especially response to IM. The aberrant Bcl-x splicing in CML cells was effectively restored using vivo-Morpholino Antisense Oligomer (vMO). CCK-8 cell viability assay and flow cytometry showed that restoring of Bcl-x splicing increases IM-induced growth inhibition and apoptosis of K562 cells. Moreover, a more significant similar phenomenon was observed in imatinib-resistant CML cell lines K562/G01. Finally, establishment of CML xenograft model had also proved that correcting Bcl-x splicing in vivo can also enhance the anti-tumor effect of IM. Our findings suggest that vMO co-operating with IM can effectively increase the sensitivity of CML cells to IM both in vitro and in vivo, and Bcl-x splicing could become good candidates for chemotherapy-sensitized target in IM-resistant CML.

Cellular and Molecular Networks in Chronic Myeloid Leukemia: The Leukemic Stem, Progenitor and Stromal Cell Interplay

The use of imatinib, second and third generation ABL tyrosine kinase inhibitors (TKI) (i.e. dasatinib, nilotinib, bosutinib and ponatinib) made CML a clinically manageable and, in a small percentage of cases, a cured disease. TKI therapy also turned CML blastic transformation into a rare event; however, disease progression still occurs in those patients who are refractory, not compliant with TKI therapy or develop resistance to multiple TKIs. In the past few years, it became clear that the BCRABL1 oncogene does not operate alone to drive disease emergence, maintenance and progression. Indeed, it seems that bone marrow (BM) microenvironment-generated signals and cell autonomous BCRABL1 kinase-independent genetic and epigenetic alterations all contribute to: i. persistence of a quiescent leukemic stem cell (LSC) reservoir, ii. innate or acquired resistance to TKIs, and iii. progression into the fatal blast crisis stage. Herein, we review the intricate leukemic network in which aberrant, but finely tuned, survival, mitogenic and self-renewal signals are generated by leukemic progenitors, stromal cells, immune cells and metabolic microenvironmental conditions (e.g. hypoxia) to promote LSC maintenance and blastic transformation.

Indole-3-carbinol induces apoptosis of chronic myelogenous leukemia cells through suppression of STAT5 and Akt signaling pathways

Signal transducer and activator of transcription 5 and Akt pathways, implicated in signaling transduction downstream of BCR-ABL, play critical roles in the pathogenesis of chronic myeloid leukemia. Therefore, idenication of novel compounds that modulate the activity of such pathways could be a new approach in the treatment of chronic myeloid leukemia. Previous studies have demonstrated that indole-3-carbinol inhibits the proliferation and induces apoptosis of various tumor cells. However, its anticancer activity against chronic myeloid leukemia cells and the underlying mechanism remain unclear. Our data revealed that indole-3-carbinol promoted mitochondrial apoptosis of chronic myeloid leukemia-derived K562 cells, as evidenced by the activation of caspases and poly (ADP-ribose) polymerase cleavage. Treatment with indole-3-carbinol was found to be associated with a decrease in the cellular levels of phospho-Akt and phospho-signal transducer and activator of transcription 5. In addition, real-time polymerase chain reaction analysis showed that the downregulation of genes is regulated by Akt and signal transducer and activator of transcription 5. We also found that treatment with indole-3-carbinol resulted in the activation of the p38 mitogen-activated protein kinase and reduced expression of human telomerase and c-Myc. Collectively, these results demonstrate that the oncogenic signal transducer and activator of transcription 5/Akt pathway is a cellular target for indole-3-carbinol in chronic myeloid leukemia cells. Thus, this clinically tested natural compound can be a potential candidate in the treatment of chronic myeloid leukemia following confirmation with clinical studies.

ABT-263 induces apoptosis and synergizes with chemotherapy by targeting stemness pathways in esophageal cancer

Activation of cancer stem cell signaling is central to acquired resistance to therapy in esophageal cancer (EC). ABT-263, a potent Bcl-2 family inhibitor, is active against many tumor types. However, effect of ABT-263 on EC cells and their resistant counterparts are unknown. Here we report that ABT-263 inhibited cell proliferation and induced apoptosis in human EC cells and their chemo-resistant counterparts. The combination of ABT-263 with 5-FU had synergistic lethal effects and amplified apoptosis that does not depend fully on its inhibition of BCL-2 family proteins in EC cells. To further explore the novel mechanisms of ABT-263, proteomic array (RPPAs) were performed and gene set enriched analysis demonstrated that ABT-263 suppresses the expression of many oncogenes including genes that govern stemness pathways. Immunoblotting and immunofluorescence further confirmed reduction in protein expression and transcription in Wnt/β-catenin and YAP/SOX9 axes. Furthermore, ABT263 strongly suppresses cancer stem cell properties in EC cells and the combination of ABT-263 and 5-FU significantly reduced tumor growth in vivo and suppresses the expression of stemness genes. Thus, our findings demonstrated a novel mechanism of ABT-263 antitumor effect in EC and indicating that combination of ABT-263 with cytotoxic drugs is worthy of pursuit in patients with EC.

High CIP2A levels correlate with an antiapoptotic phenotype that can be overcome by targeting BCL-XL in chronic myeloid leukemia

Cancerous inhibitor of protein phosphatase 2A (CIP2A) is a predictive biomarker of disease progression in many malignancies, including imatinib-treated chronic myeloid leukemia (CML). Although high CIP2A levels correlate with disease progression in CML, the underlying molecular mechanisms remain elusive. In a screen of diagnostic chronic phase samples from patients with high and low CIP2A protein levels, high CIP2A levels correlate with an antiapoptotic phenotype, characterized by downregulation of proapoptotic BCL-2 family members, including BIM, PUMA and HRK, and upregulation of the antiapoptotic protein BCL-XL. These results suggest that the poor prognosis of patients with high CIP2A levels is due to an antiapoptotic phenotype. Disrupting this antiapoptotic phenotype by inhibition of BCL-XL via RNA interference or A-1331852, a novel, potent and BCL-XL-selective inhibitor, resulted in extensive apoptosis either alone or in combination with imatinib, dasatinib or nilotinib, both in cell lines and in primary CD34(+) cells from patients with high levels of CIP2A. These results demonstrate that BCL-XL is the major antiapoptotic survival protein and may be a novel therapeutic target in CML.

A personalised medicine approach for ponatinib-resistant chronic myeloid leukaemia

BACKGROUND

Chronic myeloid leukaemia (CML) is characterised by the presence of a fusion driver oncogene, BCR-ABL1, which is a constitutive tyrosine kinase. Tyrosine kinase inhibitors (TKIs) are the central treatment strategy for CML patients and have significantly improved survival rates, but the T315I mutation in the kinase domain of BCR-ABL1 confers resistance to all clinically approved TKIs, except ponatinib. However, compound mutations can mediate resistance even to ponatinib and remain a clinical challenge in CML therapy. Here, we investigated a ponatinib-resistant CML patient through whole-genome sequencing (WGS) to identify the cause of resistance and to find alternative therapeutic targets.

PATIENTS AND METHODS

We carried out WGS on a ponatinib-resistant CML patient and demonstrated an effective combination therapy against the primary CML cells derived from this patient in vitro.

RESULTS

Our findings demonstrate the emergence of compound mutations in the BCR-ABL1 kinase domain following ponatinib treatment, and chromosomal structural variation data predicted amplification of BCL2. The primary CD34(+) CML cells from this patient showed increased sensitivity to the combination of ponatinib and ABT-263, a BCL2 inhibitor with a negligible effect against the normal CD34(+) cells.

CONCLUSION

Our results show the potential of personalised medicine approaches in TKI-resistant CML patients and provide a strategy that could improve clinical outcomes for these patients.

Effects of lentivirus mediated STAT3 silencing on human chronic myeloid leukemia cells and leukemia mice

OBJECTIVE

To investigate the effects of lentivirus mediated STAT3 silencing on human chronic myeloid leukemia cells (K562) and the growth of chronic myeloid leukemia mice as well as to explore the potential mechanisms.

METHODS

Unbtreated K562 cells (CON), blank lentivirus transfected K562 cells (NC) and K562 cells expressing STAT3 siRNA (STAT3 siRNA) were injected into SCID mice to establish the chronic myeloid leukemia model in mice. The growth, peripheral white blood cell count and spleen index in these mice were determined.

RESULTS

In vitro experiment showed, when compared with control group, the interference efficiency of STAT3 expression was as high as 97.5% in K562 cells. Western blot assay revealed that the expression of c-Myc, Bcl-xL and Cyclin D1 reduced by 17.01%, 7.3% and 6.82%, respectively, showing significant difference when compared with control group (P < 0.01). These findings were consistent with those from fluorescence quantitative PCR. In vivo experiment showed the body weight of mice reduced progressively and the peripheral white blood cell count increased gradually in control group, accompanied by dragging hind limbs and progressive enlargement of the spleen. The body weight remained unchanged, peripheral white blood cell count reduced gradually and the spleen did not enlarge in mice treated with STAT3 siRNA expressing cells.

CONCLUSION

Lentivirus mediated STAT3 silencing may inhibit the expression of its downstream genes (c-Myc, Bcl-xL and Cyclin D1) related to cell proliferation, apoptosis and cycle to suppress the malignant biological behaviors, and STAT3 silencing also inhibit the leukemogenic potency of K562 cells in mice.

Genetic events other than BCR-ABL1

The BCR-ABL1 oncoprotein is the cause of chronic myeloid leukemia and occurs as a consequence of the translocation t(9;22), a well-defined genetic event that results in the formation of the Philadelphia chromosome. While this genomic aberration is recognized to be the main culprit of the chronic phase of chronic myeloid leukemia, the natural clonal evolution of this myeloproliferative neoplasm involves the accumulation of secondary alterations through genomic instability. Thus, efforts to dissect the frequency and nature of the genomic events at diagnosis and at later stages are producing valuable insights into understanding the mechanisms of blastic transformation and development of resistance in chronic myeloid leukemia. The identification of alternative BCR-ABL1-dependent and BCR-ABL1-independent targets that sustain the survival of leukemic blasts and/or leukemia-initiating cells will facilitate the development of novel viable therapeutic options for patients who become resistant or intolerant to the currently available therapeutic options based on tyrosine kinase inhibitors.