Biological consequences of the BCR/ABL fusion gene in humans and mice

Suppression of autophagy by BCR/ABL

Imatinib and second generation BCR/ABL tyrosine kinase inhibitors (TKIs) serve now as standard therapies for patients with chronic myelogenous leukemia (CML); however, CML stem cells are intrinsically insensitive to the cell death-inducing effects of TKIs, allowing the persistence of a "reservoir" of BCR/ABL-expressing CML-initiating cells potentially responsible for disease relapse and progression. Although it is still controversial whether the "insensitivity" of CML stem cells to treatment with TKI is due to BCR/ABL-dependent or independent mechanisms, treatment with IM appears to suppress BCR/ABL-dependent signaling in CML stem cells with no adverse effects on their survival. Recent evidence indicates that BCR/ABL suppresses and treatment of CML cells with IM/TKIs induces autophagy, a genetically-regulated process of adaptation to metabolic stress which could allow tumor cells to become metabolically inert enabling their survival under conditions that may mimic growth factor/nutrients deprivation. Based on this hypothesis, TKI-induced autophagy may "antagonize" TKI-induced cell death and inhibition of autophagy may eliminate this survival mechanism by restoring "sensitivity" of CML stem cells to treatment with IM/TKI. Consistent with this, phenotypically and functionally defined CML-enriched stem cells insensitive to treatment with TKI are efficiently eliminated by the combination of TKI and chloroquine, an inhibitor of late stage autophagy. Thus, inhibition of autophagy may improve the potent and specific effects of TKIs by rendering CML stem cells sensitive to these targeted therapies.

Inhibition of autophagy: a new strategy to enhance sensitivity of chronic myeloid leukemia stem cells to tyrosine kinase inhibitors

Imatinib mesylate (IM) has become standard therapy for patients with chronic myeloid leukemia (CML), but CML stem cells are intrinsically resistant to IM and to second/third-generation tyrosine kinase inhibitors (TKIs), allowing the persistence of a 'reservoir' of BCR-ABL-expressing CML-initiating cells potentially responsible for disease progression. Although it is still controversial whether the 'insensitivity' of CML stem cells to treatment with TKIs is due to BCR-ABL-dependent or independent mechanisms, recent evidence indicates that treatment with IM suppresses BCR-ABL-dependent signaling in CML stem cells with no adverse effects on their survival. Treatment of CML cells with IM/TKIs induces autophagy, a genetically regulated process of adaptation to metabolic stress which may allow tumor cells to become metabolically inert, enabling their survival under conditions that may mimic growth factor/nutrient deprivation. Based on this hypothesis, TKI-induced autophagy may 'antagonize' TKI-induced cell death and inhibition of autophagy may eliminate this survival mechanism by restoring 'sensitivity' of CML stem cells to treatment with IM/TKIs. Consistent with this, recent evidence indicates that phenotypically and functionally defined CML-enriched stem cells that are insensitive to treatment with TKIs are efficiently eliminated by the combination of TKI and chloroquine, an inhibitor of late stage autophagy. Thus, inhibition of autophagy may 'sensitize' CML stem cells to treatment with TKIs, thus preserving the high specificity of TKI-based therapies.

Targeting autophagy potentiates tyrosine kinase inhibitor-induced cell death in Philadelphia chromosome-positive cells, including primary CML stem cells

Imatinib mesylate (IM), a potent inhibitor of the BCR/ABL tyrosine kinase, has become standard first-line therapy for patients with chronic myeloid leukemia (CML), but the frequency of resistance increases in advancing stages of disease. Elimination of BCR/ABL-dependent intracellular signals triggers apoptosis, but it is unclear whether this activates additional cell survival and/or death pathways. We have shown here that IM induces autophagy in CML blast crisis cell lines, CML primary cells, and p210BCR/ABL-expressing myeloid precursor cells. IM-induced autophagy did not involve c-Abl or Bcl-2 activity but was associated with ER stress and was suppressed by depletion of intracellular Ca2+, suggesting it is mechanistically nonoverlapping with IM-induced apoptosis. We further demonstrated that suppression of autophagy using either pharmacological inhibitors or RNA interference of essential autophagy genes enhanced cell death induced by IM in cell lines and primary CML cells. Critically, the combination of a tyrosine kinase inhibitor (TKI), i.e., IM, nilotinib, or dasatinib, with inhibitors of autophagy resulted in near complete elimination of phenotypically and functionally defined CML stem cells. Together, these findings suggest that autophagy inhibitors may enhance the therapeutic effects of TKIs in the treatment of CML.

Inducible activation of CEBPB, a gene negatively regulated by BCR/ABL, inhibits proliferation and promotes differentiation of BCR/ABL-expressing cells

Translational regulation by oncogenic proteins may be a rapid and efficient mechanism to modulate gene expression. We report here the identification of the CEBPB gene as a target of translational regulation in myeloid precursor cells transformed by the BCR/ABL oncogene. Expression of CEBPB was repressed in 32D-BCR/ABL cells and reinduced by imatinib (STI571) via a mechanism that appears to depend on expression of the CUG-repeat RNA-binding protein CUGBP1 and the integrity of the CUG-rich intercistronic region of c/ebpbeta mRNA. Constitutive expression or conditional activation of wild-type CEBPB induced differentiation and inhibited proliferation of 32D-BCR/ABL cells in vitro and in mice, but a DNA binding-deficient CEBPB mutant had no effect. The proliferation-inhibitory effect of CEBPB was, in part, mediated by the CEBPB-induced GADD45A gene. Because expression of CEBPB (and CEBPA) is low in the blast crisis (BC) stage of chronic myelogenous leukemia (CML) and is inversely correlated with BCR/ABL tyrosine kinase levels, these findings point to the therapeutic potential of restoring C/EBP activity in CML-BC and, perhaps, other types of acute leukemia.

JunB inhibits proliferation and transformation in B-lymphoid cells

The activator protein 1 (AP-1) member JunB has recently been implicated in leukemogenesis. Here we surveyed human lymphoma samples for expression of JunB and other AP-1 members (c-Jun, c-Fos, Fra1, JunD). JunB was strongly expressed in T-cell lymphomas, but non-Hodgkin B-cell lymphomas do not or only weakly express JunB. We therefore asked whether JunB acted as a negative regulator of B-cell development, proliferation, and transformation. We used transgenic mice that expressed JunB under the control of the ubiquitin C promoter; these displayed increased JunB levels in both B- and T-lymphoid cells. JunB transgenic cells of B-lymphoid, but not of T-lymphoid, origin responded poorly to mitogenic stimuli. Furthermore, JunB transgenic cells were found to be less susceptible to the transforming potential of the Abelson oncogene in vitro. In addition, overexpression of JunB partially protected transgenic mice against the oncogenic challenge in vivo. However, transformed B cells eventually escaped from the inhibitory effect of JunB: the proliferative response was similar in explanted tumor-derived cells from transgenic animals and those from wild-type controls. Our results identify JunB as a novel regulator of B-cell proliferation and transformation.

BCR/ABL activates mdm2 mRNA translation via the La antigen

In a BCR/ABL-expressing myeloid precursor cell line, p53 levels were markedly downmodulated. Expression of MDM2, the negative regulator of p53, was upregulated in a tyrosine kinase-dependent manner in growth factor-independent BCR/ABL-expressing cells, and in accelerated phase and blast crisis CML samples. Increased MDM2 expression was associated with enhanced mdm2 mRNA translation, which required the interaction of the La antigen with mdm2 5' UTR. Expression of MDM2 correlated with that of La and was suppressed by La siRNAs and by a dominant negative La mutant, which also enhanced the susceptibility to drug-induced apoptosis of BCR/ABL-transformed cells. By contrast, La overexpression led to increased MDM2 levels and enhanced resistance to apoptosis. Thus, La-dependent activation of mdm2 translation might represent an important molecular mechanism involved in BCR/ABL leukemogenesis.

hnRNP A1 nucleocytoplasmic shuttling activity is required for normal myelopoiesis and BCR/ABL leukemogenesis

hnRNP A1 is a nucleocytoplasmic shuttling heterogeneous nuclear ribonucleoprotein that accompanies eukaryotic mRNAs from the active site of transcription to that of translation. Although the importance of hnRNP A1 as a regulator of nuclear pre-mRNA and mRNA processing and export is well established, it is unknown whether this is relevant for the control of proliferation, survival, and differentiation of normal and transformed cells. We show here that hnRNP A1 levels are increased in myeloid progenitor cells expressing the p210(BCR/ABL) oncoprotein, in mononuclear cells from chronic myelogenous leukemia (CML) blast crisis patients, and during disease progression. In addition, in myeloid progenitor 32Dcl3 cells, BCR/ABL stabilizes hnRNP A1 by preventing its ubiquitin/proteasome-dependent degradation. To assess the potential role of hnRNP A1 nucleocytoplasmic shuttling activity in normal and leukemic myelopoiesis, a mutant defective in nuclear export was ectopically expressed in parental and BCR/ABL-transformed myeloid precursor 32Dcl3 cells, in normal murine marrow cells, and in mononuclear cells from a CML patient in accelerated phase. In normal cells, expression of this mutant enhanced the susceptibility to apoptosis induced by interleukin-3 deprivation, suppressed granulocytic differentiation, and induced massive cell death of granulocyte colony-stimulating factor-treated cultures. In BCR/ABL-transformed cells, its expression was associated with suppression of colony formation and reduced tumorigenic potential in vivo. Moreover, interference with hnRNP A1 shuttling activity resulted in downmodulation of C/EBPalpha, the major regulator of granulocytic differentiation, and Bcl-X(L), an important survival factor for hematopoietic cells. Together, these results suggest that the shuttling activity of hnRNP A1 is important for the nucleocytoplasmic trafficking of mRNAs that encode proteins influencing the phenotype of normal and BCR/ABL-transformed myeloid progenitors.

BCR-ABL suppresses C/EBPalpha expression through inhibitory action of hnRNP E2

The arrest of differentiation is a feature of both chronic myelogenous leukemia cells in myeloid blast crisis and myeloid precursors that ectopically express the p210BCR-ABL oncoprotein; however, its underlying mechanisms remain poorly understood. Here we show that expression of BCR-ABL in myeloid precursor cells leads to transcriptional suppression of the granulocyte colony-stimulating factor receptor G-CSF-R (encoded by CSF3R), possibly through down-modulation of C/EBPalpha-the principal regulator of granulocytic differentiation. Expression of C/EBPalpha protein is barely detectable in primary marrow cells taken from individuals affected with chronic myeloid leukemia in blast crisis. In contrast, CEBPA RNA is clearly present. Ectopic expression of C/EBPalpha induces granulocytic differentiation of myeloid precursor cells expressing BCR-ABL. Expression of C/EBPalpha is suppressed at the translational level by interaction of the poly(rC)-binding protein hnRNP E2 with CEBPA mRNA, and ectopic expression of hnRNP E2 in myeloid precursor cells down-regulates both C/EBPalpha and G-CSF-R and leads to rapid cell death on treatment with G-CSF (encoded by CSF3). Our results indicate that BCR-ABL regulates the expression of C/EBPalpha by inducing hnRNP E2-which inhibits the translation of CEBPA mRNA.

Interferon-alpha combined with cytarabine in chronic myelogenous leukemia - clinical benefits

During the last decade, several studies have evaluated the treatment of chronic phase chronic myeloid leukemia (CML) with a combination of interferon (IFN)-alpha and low- dose cytarabine (Ara-C). This combination therapy has been shown to be superior compared to monotherapy with IFN-alpha in randomized studies with regard to hematologic and cytogenetic remissions. However, the survival benefit is small, and the toxicity of the combination therapy is high. This paper reviews the published studies on IFN-alpha/low-dose Ara-C for the treatment of chronic phase CML and discusses the value of the combination therapy.

Clinical implications of recurring chromosomal and associated molecular abnormalities in acute lymphoblastic leukemia

Comprehensive study of the major chromosomal/molecular abnormalities in children and adults with acute lymphoblastic leukemia (ALL) has demonstrated prognostic utility for many of these anomalies, to the extent that cytogenetic and molecular genetic evaluations are now required for optimal clinical management of newly diagnosed cases. For example, the t(12;21)/TEL-AML1 (ETV6-CBFA2) or hyperdiploid karyotypes each identifies subgroups of children who can be cured with well-tolerated chemotherapy based primarily on drugs with few long-term toxicities, such as L-asparaginase and antimetabolites. By contrast, the t(1;19)/E2A-PBX1 identifies a subtype of ALL that responds much better to more intensive regimens that rely on genotoxic drugs. At the extreme end of the risk spectrum, the t(4;11)/MLL-AF4 and t(9;22)/BCR-ABL almost always confer a dire prognosis in both children and adults with ALL, who warrant high-dose chemotherapy and hematopoietic stem cell rescue to sustain or even induce first remission. Such chromosomal/molecular markers are being incorporated into risk classification schemes, as they convey prognostic information that cannot be gleaned from conventional risk factors such as immunophenotype, presenting age, and the initial circulating leukemic blast cell count. The most exciting prospect is the discovery of drugs that inhibit specific oncogenes, as illustrated by the BCR-ABL tyrosine kinase inhibitor STI-571.