Stem cell and kinase activity-independent pathway in resistance of leukaemia to BCR-ABL kinase inhibitors

PIM-1 kinase inhibitor SMI-4a exerts antitumor effects in chronic myeloid leukemia cells by enhancing the activity of glycogen synthase kinase 3β

The development of targeted tyrosine kinase inhibitors (TKIs) has succeeded in altering the course of chronic myeloid leukemia (CML). However, a number of patients have failed to respond or experienced disease relapse following TKI treatment. Proviral integration site for moloney murine leukemia virus-1 (PIM-1) is a serine/threonine kinase that participates in regulating apoptosis, cell cycle, signal transduction and transcriptional pathways, which are associated with tumor progression, and poor prognosis. SMI-4a is a selective PIM-1 kinase inhibitor that inhibits PIM-1 kinase activity in vivo and in vitro. The present study aimed to explore the mechanism underlying the antitumor effect of SMI-4a in K562 and imatinib-resistant K562 (K562/G) cell lines. It was demonstrated that SMI-4a inhibited the proliferation of K562 and K562/G cells using a WST-8 assay. The Annexin V-propidium iodide assay demonstrated that SMI-4a induced apoptosis of K562 and K562/G cells in a dose-, and time-dependent manner. Furthermore, Hoechst 33342 staining was used to verify the apoptosis rate. The clone formation assay revealed that SMI-4a significantly inhibited the colony formation capacity of K562 and K562/G cells. Western blot analysis demonstrated that SMI-4a decreased phosphorylated (p)-Ser9-glycogen synthase kinase (GSK) 3β/pGSK3β and inhibited the translocation of β-catenin. In addition, the downstream gene expression of apoptosis regulator Bax and poly(ADP-ribose) polymerase-1 was upregulated, and apoptosis regulator Bcl-2 and Myc proto-oncogene protein expression levels were downregulated. Immunofluorescence results demonstrated changes in the expression level of β-catenin in the plasma and nucleus. The results of the present study suggest that SMI-4a is an effective drug to use in combination with current chemotherapeutics for the treatment of imatinib-resistant CML.

Pluripotency markers are differentially induced by MEK inhibition in thyroid and melanoma BRAFV600E cell lines

Oncogenic mutations in BRAF are common in melanoma and thyroid carcinoma and drive constitutive activation of the MAPK pathway. Molecularly targeted therapies of this pathway improves survival compared to chemotherapy; however, responses tend to be short-lived as resistance invariably occursCell line models of melanoma and thyroid carcinoma, +/− BRAF^V600E activating mutation, were treated with the MEK inhibitor PD0325901. Treated and naive samples were assayed for expression of key members of the MAPK pathway. Global microRNA expression profiling of naive and resistant cells was performed via next generation sequencingand indicated pluripotency pathways in resistance. Parental cell lines were progressed to holoclones to confirm the miRNA stemness profileMembers of the MIR302/373/374/520 family of embryonic stem cell specific cell cycle regulating (ESCC) microRNAs were identified as differentially expressed between resistant BRAF^V600E melanoma and thyroid cell lines. Upregulated expression of gene and protein stemness markers, upregulated expression of MAPK pathway genes and downregulation of the ESCC MIR302 cluster in BRAF^V600E melanoma indicated an increased stem-like phenotype in resistant BRAF^V600E melanoma. Conversely, downregulated expression of gene and protein stemness markers, downregulated expression of MAPK pathway genes, upregulation of the ESCC MIR520 cluster, reeexpression of cell surface receptors, and induced differentiation-associated morphology in resistant BRAF^V600E indicate a differentiated phenotype associated with MEK inhibitor resistance in BRAF^V600E thyroid cellsThe differential patterns of resistance observed between BRAF^V600E melanoma and thyroid cell lines may reflect tissue type or de novo differentiation, but could have significant impact on the response of primary and metastatic cells to MEK inhibitor treatment. This study provides a basis for the investigation of the cellular differentiation/self-renewal access and its role in resistance to MEK inhibition.

Resistance to tyrosine kinase inhibition therapy for chronic myelogenous leukemia: a clinical perspective and emerging treatment options

The development of tyrosine kinase inhibitors (TKIs) has led to extended lifespans for many patients with chronic myelogenous leukemia (CML). However, 20% to 30% of patients fail to respond, respond suboptimally, or experience disease relapse after treatment with imatinib. A key factor is drug resistance. The molecular mechanisms implicated in this resistance include those that involve upregulation or mutation of BCR-ABL kinase and those that are BCR-ABL independent. The clinical consequences of these molecular mechanisms of resistance for disease pathogenesis remain open for debate. This review summarizes the molecular mechanisms and clinical consequences of TKI resistance and addresses the current and future treatment approaches for patients with TKI-resistant CML.

Changes in molecular biology of chronic myeloid leukemia in tyrosine kinase inhibitor era

Chronic myeloid leukemia (CML) is a clonal myeloproliferative disease characterized by a reciprocal translocation between long arms of chromosomes 9 and 22 t(9;22) that generates the BCR-ABL fusion gene. If left untreated, newly diagnosed chronic phase CML patients finally progress to accelerated and blastic phase. After the introduction of tyrosine kinase inhibitors (TKIs), treatment strategies of CML changed dramatically. However, the development of resistance to TKIs started to create problems over time. In this review, the current information about CML biology before and after imatinib mesylate treatment is summarized.

Tetrandrine citrate eliminates imatinib-resistant chronic myeloid leukemia cells in vitro and in vivo by inhibiting Bcr-Abl/β-catenin axis

OBJECTIVE

To evaluate the effects of tetrandrine citrate, a novel tetrandrine salt with high water solubility, on the growth of imatinib (IM)-resistant chronic myeloid leukemia (CML) in vitro and in vivo, and reveal action molecular mechanisms.

METHODS

Cell viability in vitro was measured using methyl thiazolyl tetrazolium (MTT) assay. CML cell growth in vivo was assessed using a xenograft model in nude mice. Bcr-Abl and β-catenin protein levels were determined using Western blotting. Bcr-Abl messenger RNA (mRNA) was measured by reverse transcription polymerase chain reaction (RT-PCR). Flow cytometry (FCM) was used to determine cell cycle status.

RESULTS

Tetrandrine citrate inhibited the growth of IM-resistant K562 cells, primary leukemia cells, and primitive CD34(+) leukemia cells, and their inhibition concentration that inhibited 50% of target cells (IC(50)) ranged from 1.20 to 2.97 μg/ml. In contrast, tetrandrine citrate did not affect normal blood cells under the same conditions, and IC(50) values were about 10.12-13.11 μg/ml. Oral administration of tetrandrine citrate caused complete regression of IM-resistant K562 xenografts in nude mice without overt toxicity. Western blot results revealed that treatment of IM-resistant K562 cells with tetrandrine citrate resulted in a significant decrease of both p210(Bcr-Abl) and β-catenin proteins, but IM did not affect the Bcr-Abl protein levels. Proteasome inhibitor, MG132, did not prevent tetrandrine-mediated decrease of the p210(Bcr-Abl) protein. RT-PCR results showed that tetrandrine treatment caused a decrease of Bcr-Abl mRNA. FCM analysis indicated that tetrandrine induced gap 1 (G(1)) arrest in CML cells.

CONCLUSIONS

Tetrandrine citrate is a novel orally active tetrandrine salt with potent anti-tumor activity against IM-resistant K562 cells and CML cells. Tetrandrine citrate-induced growth inhibition of leukemia cells may be involved in the depletion of p210(Bcr-Abl) mRNA and β-catenin protein.

Residual tumor cells that drive disease relapse after chemotherapy do not have enhanced tumor initiating capacity

Although chemotherapy is used to treat most advanced solid tumors, recurrent disease is still the major cause of cancer-related mortality. Cancer stem cells (CSCs) have been the focus of intense research in recent years because they provide a possible explanation for disease relapse. However, the precise role of CSCs in recurrent disease remains poorly understood and surprisingly little attention has been focused on studying the cells responsible for re-initiating tumor growth within the original host after chemotherapy treatment. We utilized both xenograft and genetically engineered mouse models of non-small cell lung cancer (NSCLC) to characterize the residual tumor cells that survive chemotherapy treatment and go on to cause tumor regrowth, which we refer to as tumor re-initiating cells (TRICs). We set out to determine whether TRICs display characteristics of CSCs, and whether assays used to define CSCs also provide an accurate readout of a cell's ability to cause tumor recurrence. We did not find consistent enrichment of CSC marker positive cells or enhanced tumor initiating potential in TRICs. However, TRICs from all models do appear to be in EMT, a state that has been linked to chemoresistance in numerous types of cancer. Thus, the standard CSC assays may not accurately reflect a cell's ability to drive disease recurrence.

CaMKII γ, a critical regulator of CML stem/progenitor cells, is a target of the natural product berbamine

Bcr-Abl tyrosine kinase inhibitors (TKIs) have been a remarkable success for the treatment of Ph(+) chronic myeloid leukemia (CML). However, a significant proportion of patients treated with TKIs develop resistance because of leukemia stem cells (LSCs) and T315I mutant Bcr-Abl. Here we describe the unknown activity of the natural product berbamine that efficiently eradicates LSCs and T315I mutant Bcr-Abl clones. Unexpectedly, we identify CaMKII γ as a specific and critical target of berbamine for its antileukemia activity. Berbamine specifically binds to the ATP-binding pocket of CaMKII γ, inhibits its phosphorylation and triggers apoptosis of leukemia cells. More importantly, CaMKII γ is highly activated in LSCs but not in normal hematopoietic stem cells and coactivates LSC-related β-catenin and Stat3 signaling networks. The identification of CaMKII γ as a specific target of berbamine and as a critical molecular switch regulating multiple LSC-related signaling pathways can explain the unique antileukemia activity of berbamine. These findings also suggest that berbamine may be the first ATP-competitive inhibitor of CaMKII γ, and potentially, can serve as a new type of molecular targeted agent through inhibition of the CaMKII γ activity for treatment of leukemia.

Philadelphia chromosome-positive acute lymphoblastic leukemia in children: new and emerging treatment options

Philadelphia chromosome-positive (Ph(+)) acute lymphoblastic leukemia (ALL) in children and adolescents has, until recently, been considered one of the poorest-risk subgroups of ALL. With chemotherapy alone, only 20-30% of children with Ph(+) ALL are cured. Allogeneic hematopoietic cell transplantation in first complete remission cures 60% of patients with a closely matched donor. Although targeted tyrosine kinase inhibitors (TKIs) have limited activity against Ph(+) ALL as a single agent, they have been evaluated in combination with chemotherapy with promising results. The early results of Children's Oncology Group trial AALL0031 have shown 88% 3-year event-free survival for Ph(+) patients treated with intensive chemotherapy plus continuous-dosing imatinib. This suggests that chemotherapy plus TKIs may be the initial treatment of choice for Ph(+) ALL in children. However, the numbers are small in this trial and confirmatory results are not yet available from the European Intergroup Study on Post Induction Treatment of Philadelphia Positive Acute Lymphoblastic Leukaemia with Imatinib trial. Additional issues include determining the most effective TKI (imatinib, dasatinib or nilotinib) and the most effective, least toxic chemotherapy backbone. The experience of adding a targeted agent such as a TKI to the standard chemotherapy regimen suggests that this strategy might be applied to other ALL subtypes to achieve both increased efficacy and decreased toxicity.

Radiation resistance of cancer stem cells: the 4 R's of radiobiology revisited

There is compelling evidence that many solid cancers are organized hierarchically and contain a small population of cancer stem cells (CSCs). It seems reasonable to suggest that a cancer cure can be achieved only if this population is eliminated. Unfortunately, there is growing evidence that CSCs are inherently resistant to radiation, and perhaps other cancer therapies. In general, success or failure of standard clinical radiation treatment is determined by the 4 R's of radiobiology: repair of DNA damage, redistribution of cells in the cell cycle, repopulation, and reoxygenation of hypoxic tumor areas. We relate recent findings on CSCs to these four phenomena and discuss possible consequences.

VE-cadherin Regulates Philadelphia Chromosome Positive Acute Lymphoblastic Leukemia Sensitivity to Apoptosis

The mechanisms by which the bone marrow microenvironment regulates tumor cell survival are diverse. This study describes the novel observation that in addition to Philadelphia chromosome positive (Ph+) acute lymphoblastic leukemia (ALL) cell lines, primary patient cells also express Hypoxia Inducible Factor-2α (HIF-2α) and Vascular Endothelial Cadherin (VE-cadherin), which are regulated by Abl kinase. Tumor expression of the classical endothelial protein, VE-cadherin, has been associated with aggressive phenotype and poor prognosis in other models, but has not been investigated in hematopoietic malignancies. Targeted knockdown of VE-cadherin rendered Ph+ ALL cells more susceptible to chemotherapy, even in the presence of bone marrow stromal cell (BMSC) derived survival cues. Pre-treatment of Ph+ ALL cells with ADH100191, a VE-cadherin antagonist, resulted in increased apoptosis during in vitro chemotherapy exposure. Consistent with a role for VE-cadherin in modulation of leukemia cell viability, lentiviral-mediated expression of VE-cadherin in Ph- ALL cells resulted in increased resistance to treatment-induced apoptosis. These observations suggest a novel role for VE-cadherin in modulation of chemoresistance in Ph+ ALL.

Targeting the leukemic stem cell: the Holy Grail of leukemia therapy

Since the discovery of leukemic stem cells (LSCs) over a decade ago, many of their critical biological properties have been elucidated, including their distinct replicative properties, cell surface phenotypes, their increased resistance to chemotherapeutic drugs and the involvement of growth-promoting chromosomal translocations. Of particular importance is their ability to transfer malignancy to non-obese diabetic-severe combined immunodeficient (NOD-SCID) mice. Furthermore, numerous studies demonstrate that acute myeloid leukemia arises from mutations at the level of stem cell, and chronic myeloid leukemia is also a stem cell disease. In this review, we will evaluate the main characteristics of LSCs elucidated in several well-documented leukemias. In addition, we will discuss points of therapeutic intervention. Promising therapeutic approaches include the targeting of key signal transduction pathways (for example, PI3K, Rac and Wnt) with small-molecule inhibitors and specific cell surface molecules (for example, CD33, CD44 and CD123), with effective cytotoxic antibodies. Also, statins, which are already widely therapeutically used for a variety of diseases, show potential in targeting LSCs. In addition, drugs that inhibit ATP-binding cassette transporter proteins are being extensively studied, as they are important in drug resistance-a frequent characteristic of LSCs. Although the specific targeting of LSCs is a relatively new field, it is a highly promising battleground that may reveal the Holy Grail of cancer therapy.