Chronic myeloid leukemia (CML) is a clonal hematopoietic disorder characterized by the presence of the Philadelphia chromosome which resulted from the reciprocal translocation between chromosomes 9 and 22. The pathogenesis of CML involves the constitutive activation of the BCR-ABL tyrosine kinase, which governs malignant disease by activating multiple signal transduction pathways. The BCR-ABL kinase inhibitor, imatinib, is the front-line treatment for CML, but the emergence of imatinib resistance and other tyrosine kinase inhibitors (TKIs) has called attention for additional resistance mechanisms and has led to the search for alternative drug treatments. In this paper, we discuss our current understanding of mechanisms, related or unrelated to BCR-ABL, which have been shown to account for chemoresistance and treatment failure. We focus on the potential role of the influx and efflux transporters, the inhibitor of apoptosis proteins, and transcription factor-mediated signals as feasible molecular targets to overcome the development of TKIs resistance in CML.
Chronic myeloid leukemia (CML) is a myeloproliferative disorder that results from the reciprocal translocation of the
In order to identify prognostic factors for CML patients, many clinical and biological characteristics have been analyzed. Sokal risk score (based on spleen size, age, platelet count, and peripheral blood blast) is a prognostic factor widely used for prediction of cytogenetic response and of progression-free and overall survival in CML-CP with imatinib as front-line therapy. Other factor predictors for therapy response include OCT-1 activity,
The treatment of CML-CP can be divided into pre-imatinib and post-imatinib era. Prior to the imatinib era, busulphan and interferon-
Currently, imatinib is the standard therapy for all CML phases [
The most common mechanism of resistance to imatinib is the development of point mutations or amplification of the
BCR-ABL acts with other multiple cellular and genetic events that accumulate progressively to drive the disease into the blast phase. Therefore, additional mechanisms—dependent or independent to BCR-ABL—may also account for resistance to imatinib treatment and result in a poor outcome. In this review, the role of efflux and influx transporters, inhibitor of apoptosis proteins (IAP), and transcription factors as additional mechanisms responsible for chemoresistance in CML will be discussed.
The multidrug resistance (MDR) phenotype related to increased expression of efflux pumps, such as ABCB1/P-glycoprotein (Pgp) and ABCG2/breast-cancer-related protein (BCRP), is one of the most studied mechanisms of resistance in CML. More recently, the decrease in influx transporters, such as the organic cation transporter-1 (Oct-1), has also emerged as a mechanism responsible for inefficient drug uptake and consequent treatment failure [
The most common mechanism developed by tumor cells to escape a drug-induced death is displayed in intrinsic or acquired MDR phenotype by the overexpression of the drug-efflux protein ABCB1 [
Clinical insensitivity to anticancer agents is mainly attributed to an elevated expression of ABCB1, which is related to treatment failure associated with lower remission and survival rates in some types of cancer, including leukemias [
Even though the ABCB1 efflux functions, other functions for this transporter have been studied. Studies have shown that the resistance induced by ABCB1 is also associated with the inhibition of cell death, and ABCB1 promotes additional protection to caspases-dependent apoptosis, UV radiation, serum starvation condition, and spontaneous apoptosis [
ABCB1 is related to resistance phenotype in some leukemias and it has been studied in advanced CML. A randomized trial evaluated the relevance of ABCB1 expression in CML patients. The authors observed that the response to cytarabine and daunorubicin was significantly related with both ABCB1 expression and function mainly in the blast phase. For this reason, chemotherapy resistance in CML-BP patients should be considered multifactorial and cannot be associated only with BCR-ABL [
Although these previously described works do not identify the role of ABCB1 in imatinib resistance, studies in polymorphisms of ABCB1 have shown the importance of ABCB1 in CML treatment resistance. Moreover, this kind of study may provide information for the prediction of drug disposition in a specific way and promote better response to imatinib in CML patients [
Studies have suggested that second- and third-generation TKIs can overcome imatinib resistance [
An important strategy to try reversing clinical MDR involves modulation or inhibition of ABCB1. The cyclosporine A (CsA) is capable of regulating the efflux function of ABCB1 dependently on its concentration in cancer cells [
Another important efflux pump associated with chemotherapy resistance in CML is BCRP or ABCG2, coded by the gene
Fetsch et al.
It was demonstrated that TKI had high-affinity interaction with ABCG2 and that it occurs at submicromolar concentrations [
It is believed that CML is a clonal disorder originating from the hematopoietic stem cell (HSC). Graham et al. [
Some authors have identified the presence of ABCG2 in a particular group of HSC called “side population” (SP) due to its efflux of the fluorochrome Hoechst 33342 and its ability to reconstitute bone marrow in irradiated mice [
Once ABCG2 is expressed in the apical membrane of cells in the epithelium of the small intestine and colon, it is very likely that ABCG2 is involved in the active return of drug entering the intestine. This role would be important in reducing the systemic bioavailability of oral drugs such as imatinib. Studies in ABCG2 knockout mice indicate that ABCG2 and ABCB1 appear to regulate the penetration of imatinib into the brain tissue. Imatinib brain penetration in ABCG2 knockout mice was found to be increased [
The SNP 421C>A is responsible for decreased plasma membrane expression of ABCG2, reduced ATPase activity, or decreased drug transport [
Members of the solute carriers (SLCs) superfamily of transporters are known as passive facilitator carriers that allow the passage of solute through the membrane without spending energy [
SLC transporters are mostly expressed in the plasma membrane and play a critical role in a variety of physiological cellular processes such as import/export neurotransmitters, nutrients, or metabolites [
A growing number of scientific papers have shown that some chemotherapeutics are substrates for influx transporters. Recently, it was reported that imatinib is transported into the cell, preferably via SLC22A1 (also called OCT-1), and the expression of this transporter is predictive of achieving a complete cytogenetic remission after 6 months of treatment with imatinib [
It was reported that the influx of imatinib is temperature dependent, indicating the involvement of an active process of influence. When the cells were incubated with inhibitors of the transporter SLC22A1, the influx of imatinib was significantly reduced [
Besides the SLC22A1 activity, the levels of expression of SLC22A1 may be related to a decreased influx of imatinib. Crossman et al. [
The contribution of the SLC22A1 transporter to the clinical response to imatinib has not yet been elucidated. Therefore, further studies are needed to evaluate the role of this influx transporter in the clinical outcome of imatinib treatment.
The IAP family members are characterized by a common baculoviral IAP repeat (BIR) domain [
XIAP (X-linked of inhibitor of apoptosis protein) is a singular IAP because it is the only member of the family known to directly inhibit caspases-3, -7, and -9 [
There are at least two proteins, Smac/DIABLO [
XIAP is widely expressed in normal tissues [
Increasing evidence demonstrates that treatment of CML cells with chemotherapeutic agents can overcome resistance through negatively regulating XIAP levels. Fang et al. [
Many strategies have been used to inhibit both the expression and function of XIAP and resensitize cancer cells to different cytotoxic stimuli [
Survivin, another IAP member, is an antiapoptotic protein [
Undetectable in normal differentiated tissues, survivin is abundantly expressed in all the most common human cancers [
In CML patient samples, several studies have reported that survivin was expressed in the accelerated and blast phases but it was low or undetectable in the chronic phase [
Growing evidence suggests that survivin plays an important role in chemoresistance phenotype of human malignancies [
In recent years, considerable efforts have been made to validate survivin as a new target in cancer therapy. YM-155, a small-molecule inhibitor of survivin, was the first survivin-targeted therapy to be developed and tested in clinical trials. In CML, YM-155 anticancer efficacy has been recently assessed in a preclinical study, where CML-derived cell lines showed great sensitivity to the molecule [
Signal transduction pathways within the cell act by transmitting the extracellular signals to transcription factors, which result in changes in gene expression. However, it is well known that most key signaling pathways are deregulated in cancer, leading to altered expression and function of transcription factors. The constitutive activation of the nuclear factor kappa B (NF
Nuclear Factor
In addition to its function as a central mediator of human immune responses, NF
The expression of BCR-ABL leads to the activation of NF
Alterations in NF
Cilloni et al. [
Among these compounds acting as NF
FoxO transcription factors belong to the forkhead family of proteins, which are characterized by a conserved DNA-binding domain termed forkhead box (Fox) [
Because BCR/ABL activity requires an activated PI3K/Akt pathway [
Although a great amount of evidence demonstrates that FoxO3a functions as a downstream factor for TKI-induced apoptosis, recent data suggest that FoxO3a has a crucial role in maintenance of CML stem cells. In a recent study, it was demonstrated that FoxO3a deficiency is associated with a decreased ability of leukemia-initiating cells (LICs) to provoke CML in FoxO3a−/− mice [
In conclusion, various findings have found that the activation of FoxO3a and its downstream genes are of clinical importance in diverse anticancer therapeutics, including in CML treatment. Different from p53 [
Growing evidence has demonstrated that the development of the MDR phenotype arises as a result of a complex network involving multiple cellular and molecular mechanisms. It is a multifactorial process rather than a consequence of a single and isolated mechanism (Figure
Anticancer drugs sensitize CML cells by targeting IAPs, drug transporters, NF
Drug or therapy | Protein(s) targeted | Signaling pathways affected |
---|---|---|
Imatinib, idarubicin | Survivin | Imatinib and idarubicin inhibited viability and induced apoptosis in cells derived from a Ph+ patient in blast crisis and K562 cells, respectively, through survivin downregulation [ |
Imatinib | Survivin | Enhanced imatinib-mediated apoptosis by modulating reactive oxygen species [ |
Microtubule stabilizing agents and flavopiridol vorinostat, MK0457 | Survivin | The combination of microtubule stabilizing agents and the cyclin-dependent kinase inhibitor flavopiridol [ |
Sheperdin | Survivin | The survivin inhibitor molecule showed great toxicity against CML and AML cells, with no decrease in viability of phytohemagglutinin-stimulated peripheral blood mononuclear cells [ |
Imatinib | FoxO3a | Imatinib-mediated BCR-ABL inhibition resulted in FoxO3a activation, induction of Bim [ |
Bortezomib | FoxO3a | Bortezomib treatment was able to restore FoxO3a expression, sensitize imatinib-resistant T315I expressing cells to apoptosis, and inhibit CML-like disease in leukemic mice [ |
IKKB inhibitors | NF | The IKKB inhibitors led to the induction of apoptosis in cell lines (K562 and KCL) and bone marrow cells sensitive and resistant to imatinib [ |
Bortezomib | NF | Bortezomib reduced proliferation and survival of BCR-ABL-expressing cells, regardless of their sensitivity to imatinib and including the mutant T315I [ |
Vincristine | ABCB1 and survivin | Overexpression of ABCB1 and survivin were associated with low apoptosis index induced by vincristine treatment [ |
LQB-118 | ABCB1, survivin and XIAP | LQB-118 overcome resistance phenotype through ABCB1, survivin and XIAP downregulation [ |
Imatinib and nilotinib | K562 cells displayed upregulated levels of | |
Imatinib | SLC22A1, ABCB1 and ABCG2 | Chronic exposure to imatinib increased ABCB1 and ABCG2 at the protein and gene levels, but SLC22A1 expression remained unaltered [ |
Imatinib and vincristine | XIAP and ABCB1 | Simultaneous inhibition of XIAP and ABCB1 in cells that overexpress this efflux pump decreases the resistance to imatinib [ |
Imatinib, apicidin and EBT-737 | XIAP | Imatinib-induced apoptosis was found to be associated with XIAP downregulation [ |
Etoposide and doxorubicin | XIAP | The downregulation of XIAP expression with antisense oligonucleotides increased apoptosis and enhanced the effects of doxorubicin in K562 cells [ |
AML: acute myeloid leukemia, CML: chronic myeloid leukemia; IAPs: inhibitor apoptosis proteins.
Molecular interactions in chemoresistance. Chemoresistant chronic myeloid leukemia (CML) cells display a multifactorial resistance phenotype characterized by deregulation of diverse signaling pathways which may act in concert or individually to prevent chemotherapy sensitivity (b). Resistant cells display constitutively active nuclear expression of NF
Wang et al. [
These data indicate that other transporters may be crucial for determining imatinib intracellular and plasma concentrations in CML patients. By contrast, in experiments using
Current studies have proposed the role of IAPs in MDR phenotype promotion in association with ABCB1 expression [
Recent studies reported that
Survivin can also be targeted by NF
Although the introduction of imatinib and other TKIs in CML therapy has brought improvements in survival, CML prognosis still remains unfavorable for a group of patients. In addition to mutations found in the
Acute lymphoid leukemia
Acute myeloid leukemia
Breakpoint cluster region/V-abl Abelson murine leukemia viral oncogene homolog 1
Breast-cancer-related protein
Baculoviral IAP repeat
Blast phase of chronic myeloid leukemia
Cyclin-dependent kinase 1
Chronic lymphoid leukemia
Chronic myeloid leukemia
Forkhead box
Hematopoietic stem cell
Inhibitor of apoptosis proteins
Inhibitor of DNA binding 1
Inhibitor of NF
I
Kinase domain
Leukemia initiating cells
Mitogen-activated protein kinase
Multidrug resistance
Multidrug resistance protein 1
Microtubule targeting agents
Nuclear factor kappa B
Organic cation transporter-1
P-glycoprotein
Philadelphia
Serum and glucocorticoid-regulated kinase
Small interfering RNA
Side population
Tyrosine kinase inhibitors
Tumor-necrosis-factor-related apoptosis-inducing ligand
X-linked of inhibitor of apoptosis protein.
The authors declare that they have no conflict of interests.
This study was supported by research grants from CNPq, FAPERJ, INCT para Controle do Câncer, CNPq 573806/2008-0, FAPERJ EE26/170.026/2008, Programa de Oncobiologia (UFRJ/Fundação do Câncer), and FAPERJ-PPSUS.