Non genomic loss of function of tumor suppressors in CML: BCR-ABL promotes IκBα mediated p53 nuclear exclusion

Copper-imidazo[1,2-a]pyridines differentially modulate pro- and anti-apoptotic protein and gene expression in HL-60 and K562 leukaemic cells to cause apoptotic cell death

Despite the availability of a myriad targeted treatments, resistance and treatment failures remains common in cancer treatment. Moreover, the high cost of targeted antibodies excludes a large cohort of patients from their benefits. In this context, copper-imidazo[1,2-a]pyridines were evaluated as alternative drug candidates against two common leukaemias, represented by HL-60 and K562 cells. A previous study identified JD88(21), JD47(29) and JD49(28) to be active against these cell lines with IC₅₀ values between 1.9 and 6 μM and low leukocyte toxicity. To better understand their mechanism of action, their mode of cell death, effects on expression of apoptotic regulatory proteins and their respective genes were investigated. In both cell lines, the copper-imidazo[1,2-a]pyridines, at IC₇₅ concentrations, caused membrane blebbing, raised phosphatidyl-serine levels on cell membranes and increased caspase-3 activity. A loss of mitochondrial membrane potential and activation of caspase-9, combined with poor caspase-8 activity indicated activation of intrinsic apoptosis. Apoptotic proteome analysis showed that the copper-imidazo[1,2-a] pyridines elevated protein levels of pro-apoptotic Bax and Smac/DIABLO in both cell lines, confirming their importance in apoptotic cell death. Conversely, though survivin was increased, this was counteracted by high levels of HTRA2/Omi expression. Effects on apoptotic regulatory proteins Bad, Bcl-2, XIAP and cIAP-1 was inconsistent between the copper-imidazo[1,2-a]pyridines and between the two cell lines, suggesting that the effect of the complexes was modulated by the molecular signature of each cell line. Analysis of mRNA transcripts showed a poor correlation between mRNA levels and associated proteins, implying that copper-imidazo[1,2-a]pyridines compromised protein synthesis and degradation.

Targeting Abnormal Hematopoietic Stem Cells in Chronic Myeloid Leukemia and Philadelphia Chromosome-Negative Classical Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are unique hematopoietic stem cell disorders sharing mutations that constitutively activate the signal-transduction pathways involved in haematopoiesis. They are characterized by stem cell-derived clonal myeloproliferation. The key MPNs comprise chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). CML is defined by the presence of the Philadelphia (Ph) chromosome and BCR-ABL1 fusion gene. Despite effective cytoreductive agents and targeted therapy, complete CML/MPN stem cell eradication is rarely achieved. In this review article, we discuss the novel agents and combination therapy that can potentially abnormal hematopoietic stem cells in CML and MPNs and the CML/MPN stem cell-sustaining bone marrow microenvironment.

Examination of clinically-derived p210 BCR/ABL1 RhoGEF mutations in a murine bone marrow transplantation model of CML

Expression of the p210 BCR/ABL1 fusion protein has been described in virtually all patients with chronic myelogenous leukemia (CML). Previous studies have identified a guanine nucleotide exchange factor (RhoGEF) domain within BCR that is retained in p210 BCR/ABL1. Missense mutations at residues T654 (T654K) and F547 (F547L) within this domain have been reported in a CML patient in blast crisis (BC). In this study, we have evaluated p210 BCR/ABL1 constructs that contain these substitutions in a murine bone marrow transplantation (BMT) model of CML. The mutants exhibit normal expression and tyrosine kinase activity but altered signaling. When examined in the BMT assay, mice that express the mutants exhibit earlier onset of disease but have significantly extended lifespans relative to mice that express unmodified p210 BCR/ABL1. While mice that express p210 BCR/ABL1 exhibit neutrophilia that progresses to a less differentiated phenotype at death, disease in the mutant mice is characterized by eosinophilia with no maturation arrest. This observation was confirmed in vitro using myeloid cells and was associated with enhanced p53 phosphorylation and G1/S arrest. These results suggest that residues within the RhoGEF domain of p210 BCR/ABL1 can influence disease progression.

Strategies For Targeting Chronic Myeloid Leukaemia Stem Cells

Chronic Myeloid Leukaemia is a myeloproliferative disorder driven by the t(9;22) chromosomal translocation coding for the chimeric protein BCR-ABL. CML treatment represents the paradigm of molecular therapy of cancer. Since the development of the tyrosine kinase inhibitor of the BCR-ABL kinase, the clinical approach to CML has dramatically changed, with a stunning improvement in the quality of life and response rates of patients. However, it remains clear that tyrosine kinase inhibitors (TKIs) are unable to target the most immature cellular component of CML, the CML stem cell. This review summarizes new insights into the mechanisms of resistance to TKIs.

Nuclear-cytoplasmic Shuttling in Chronic Myeloid Leukemia: Implications in Leukemia Maintenance and Therapy

Nuclear-cytoplasmic shuttling is a highly regulated and complex process, which involves both proteins and nucleic acids. Changes in cellular compartmentalization of various proteins, including oncogenes and tumor suppressors, affect cellular behavior, promoting or inhibiting proliferation, apoptosis and sensitivity to therapies. In this review, we will recapitulate the role of various shuttling components in Chronic Myeloid Leukemia and we will provide insights on the potential role of shuttling proteins as therapeutic targets.

Overcoming Tyrosine Kinase Inhibitor Resistance in Transformed Cell Harboring SEPT9-ABL1 Chimeric Fusion Protein

Hematological malignancies harboring various ABL1 fusions are expected to be sensitive to tyrosine kinase inhibitors (TKIs), similar to those with BCR-ABL1. However, SEPT9-ABL1 exhibits TKI resistance both in vitro and in vivo. SEPT9-ABL1 has the same ABL1 region as seen in BCR-ABL1 but no point mutation in its kinase domain, which is one of the main mechanisms underlying TKI resistance in the leukemic cells harboring BCR-ABL1. The purpose of this study was to reveal the mechanism underlying TKI resistance induced by SEPT9-ABL1. We focused on the TP53 status because TKI-induced apoptosis in BCR-ABL1–positive cells is achieved through TP53. Mouse TP53 homologue TRP53 was downregulated and less phosphorylated in the cells expressing SEPT9-ABL1 than in those with BCR-ABL1, resulting in the prevention of apoptosis induced by TKIs. The CRM1 inhibitor KPT-330 accumulated nuclear TRP53 and NFKB1A (also known as IκBα), which is thought to capture TRP53 in the cytoplasm, and induced apoptosis in the hematopoietic cells expressing SEPT9-ABL1. In addition, the combination treatment of KPT-330 and imatinib, which induced the marked nuclear accumulation of PP2A and SET, reactivated PP2A through its dephosphorylation and inhibited SET expression, resulting in the effective induction of the apoptosis in the cells expressing SEPT9-ABL1. The combination treatment with KPT-330 and imatinib successfully reduced the subcutaneous masses expressing SEPT9-ABL1 and extended the survival of the mice intraperitoneally transplanted with SEPT9-ABL1–expressing cells. These results show that therapy with CRM1 inhibitors may be effective for overcoming TKI resistance induced by SEPT9-ABL1.

The BCR-ABL/NF-κB signal transduction network: a long lasting relationship in Philadelphia positive Leukemias

The Nuclear Factor-kappa B (NF-κB) family of transcription factors plays a key role in cancer pathogenesis due to the ability to promote cellular proliferation and survival, to induce resistance to chemotherapy and to mediate invasion and metastasis. NF-κB is recruited through different mechanisms involving either canonical (RelA/p50) or non-canonical pathways (RelB/p50 or RelB/p52), which transduce the signals originated from growth-factors, cytokines, oncogenic stress and DNA damage, bacterial and viral products or other stimuli. The pharmacological inhibition of the NF-κB pathway has clearly been associated with significant clinical activity in different cancers. Almost 20 years ago, NF-κB was described as an essential modulator of BCR-ABL signaling in Chronic Myeloid Leukemia and Philadelphia-positive Acute Lymphoblastic Leukemia. This review summarizes the role of NF-κB in BCR-ABL-mediated leukemogenesis and provides new insights on the long lasting BCR-ABL/NF-κB connection.

Quantitative Systems Biology to decipher design principles of a dynamic cell cycle network: the "Maximum Allowable mammalian Trade-Off-Weight" (MAmTOW)

Network complexity is required to lend cellular processes flexibility to respond timely to a variety of dynamic signals, while simultaneously warranting robustness to protect cellular integrity against perturbations. The cell cycle serves as a paradigm for such processes; it maintains its frequency and temporal structure (although these may differ among cell types) under the former, but accelerates under the latter. Cell cycle molecules act together in time and in different cellular compartments to execute cell type-specific programs. Strikingly, the timing at which molecular switches occur is controlled by abundance and stoichiometry of multiple proteins within complexes. However, traditional methods that investigate one effector at a time are insufficient to understand how modulation of protein complex dynamics at cell cycle transitions shapes responsiveness, yet preserving robustness. To overcome this shortcoming, we propose a multidisciplinary approach to gain a systems-level understanding of quantitative cell cycle dynamics in mammalian cells from a new perspective. By suggesting advanced experimental technologies and dedicated modeling approaches, we present innovative strategies (i) to measure absolute protein concentration in vivo, and (ii) to determine how protein dosage, e.g., altered protein abundance, and spatial (de)regulation may affect timing and robustness of phase transitions. We describe a method that we name “Maximum Allowable mammalian Trade–Off–Weight” (MAmTOW), which may be realized to determine the upper limit of gene copy numbers in mammalian cells. These aspects, not covered by current systems biology approaches, are essential requirements to generate precise computational models and identify (sub)network-centered nodes underlying a plethora of pathological conditions.

Aberrant control of NF-κB in cancer permits transcriptional and phenotypic plasticity, to curtail dependence on host tissue: molecular mode

The role of the transcription factor NF-κB in shaping the cancer microenvironment is becoming increasingly clear. Inflammation alters the activity of enzymes that modulate NF-κB function, and causes extensive changes in genomic chromatin that ultimately drastically alter cell-specific gene expression. NF-κB regulates the expression of cytokines and adhesion factors that control interactions among adjacent cells. As such, NF-κB fine tunes tissue cellular composition, as well as tissues' interactions with the immune system. Therefore, NF-κB changes the cell response to hormones and to contact with neighboring cells. Activating NF-κB confers transcriptional and phenotypic plasticity to a cell and thereby enables profound local changes in tissue function and composition. Research suggests that the regulation of NF-κB target genes is specifically altered in cancer. Such alterations occur not only due to mutations of NF-κB regulatory proteins, but also because of changes in the activity of specific proteostatic modules and metabolic pathways. This article describes the molecular mode of NF-κB regulation with a few characteristic examples of target genes.

IκB-α: At the crossroad between oncogenic and tumor-suppressive signals

Nuclear factor κB (NF-κB) is an essential component of tumorigenesis and resistance to cancer treatments. NFKB inhibitor α (IκB-α) acts as a negative regulator of the classical NF-κB pathway through its ability to maintain the presence of NF-κB in the cytoplasm. However, IκB-α is also able to form a complex with tumor protein p53, promoting its inactivation. Recently, we demonstrated that IκB-α is able to mediate p53 nuclear exclusion and inactivation in chronic myeloid leukemia, indicating that IκB-α can modulate either oncogenic or tumor-suppressive functions, with important implications for cancer treatment. The present review describes the role of IκB-α in cancer pathogenesis, with particular attention to hematological cancers, and highlights the involvement of IκB-α in the regulation of p53 tumor-suppressive functions.

Mechanisms of p53 Functional De-Regulation: Role of the IκB-α/p53 Complex

TP53 is one of the most frequently-mutated and deleted tumor suppressors in cancer, with a dramatic correlation with dismal prognoses. In addition to genetic inactivation, the p53 protein can be functionally inactivated in cancer, through post-transductional modifications, changes in cellular compartmentalization, and interactions with other proteins. Here, we review the mechanisms of p53 functional inactivation, with a particular emphasis on the interaction between p53 and IκB-α, the NFKBIA gene product.

The non-genomic loss of function of tumor suppressors: an essential role in the pathogenesis of chronic myeloid leukemia chronic phase

BACKGROUND

Chronic Myeloid Leukemia was always referred as a unique cancer due to the apparent independence from tumor suppressors' deletions/mutations in the early stages of the disease. However, it is now well documented that even genetically wild-type tumor suppressors can be involved in tumorigenesis, when functionally inactivated. In particular, tumor suppressors' functions can be impaired by subtle variations of protein levels, changes in cellular compartmentalization and post-transcriptional/post-translational modifications, such as phosphorylation, acetylation, ubiquitination and sumoylation. Notably, tumor suppressors inactivation offers challenging therapeutic opportunities. The reactivation of an inactive and genetically wild-type tumor suppressor could indeed promote selective apoptosis of cancer cells without affecting normal cells.

MAIN BODY

Chronic Myeloid Leukemia (CML) could be considered as the paradigm for non-genomic loss of function of tumor suppressors due to the ability of BCR-ABL to directly promote functionally inactivation of several tumor suppressors.

SHORT CONCLUSION

In this review we will describe new insights on the role of FoxO, PP2A, p27, BLK, PTEN and other tumor suppressors in CML pathogenesis. Finally, we will describe strategies to promote tumor suppressors reactivation in CML.