Preferential expression of a high number of ATP binding cassette transporters in both normal and leukemic CD34+CD38- cells

Cancer stem cells: Masters of all traits

Cancer is a heterogeneous disease, which contributes to its rapid progression and therapeutic failure. Besides interpatient tumor heterogeneity, tumors within a single patient can present with a heterogeneous mix of genetically and phenotypically distinct subclones. These unique subclones can significantly impact the traits of cancer. With the plasticity that intratumoral heterogeneity provides, cancers can easily adapt to changes in their microenvironment and therapeutic exposure. Indeed, tumor cells dynamically shift between a more differentiated, rapidly proliferating state with limited tumorigenic potential and a cancer stem cell (CSC)-like state that resembles undifferentiated cellular precursors and is associated with high tumorigenicity. In this context, CSCs are functionally located at the apex of the tumor hierarchy, contributing to the initiation, maintenance, and progression of tumors, as they also represent the subpopulation of tumor cells most resistant to conventional anti-cancer therapies. Although the CSC model is well established, it is constantly evolving and being reshaped by advancing knowledge on the roles of CSCs in different cancer types. Here, we review the current evidence of how CSCs play a pivotal role in providing the many traits of aggressive tumors while simultaneously evading immunosurveillance and anti-cancer therapy in several cancer types. We discuss the key traits and characteristics of CSCs to provide updated insights into CSC biology and highlight its implications for therapeutic development and improved treatment of aggressive cancers.

Sphingosine-1 phosphate receptor 1 (S1PR1) expression maintains stemness of acute myeloid leukemia stem cells

Acute myeloid leukemia (AML) arises from leukemia stem cells (LSCs) and is maintained by cells which have acquired features of stemness. We compared transcription profiles of AML cells with/without stem cell features defined as in vitro clonogenicity and serial engraftment in immune-deficient mice xenograft model. We used multi-parameter flow cytometry (MPFC) to separate CD34+ bone marrow-derived leukemia cells into sphingosine-1 phosphate receptor 1 (S1PR1)+ and S1PR1- fractions. Cells in the S1PR1+ fraction demonstrated significantly higher clonogenicity and higher engraftment potential compared with those in the S1PR1- fraction. In contrast, CD34+ bone marrow cells from normal samples showed reduced clonogenicity in the S1PR1+ fraction compared with the S1PR1- fraction. Inhibition of S1PR1 expression in an AML cell line reduced the colony-forming potential of KG1 cells. Transcriptomic analyses and rescue experiments indicated PI3K/AKT pathway and MYBL2 are downstream mediators of S1PR1-associated stemness. These findings implicate S1PR1 as a functional biomarker of LSCs and suggest its potential as a therapeutic target in AML treatment.

Targeting ABCA1 via Extracellular Vesicle-Encapsulated Staurosporine as a Therapeutic Strategy to Enhance Radiosensitivity

Cancer stem cells (CSCs) are essential for tumor initiation, recurrence, metastasis, and resistance. However, targeting CSCs as a therapeutic approach remains challenging. Here, a stemness signature based on 22-gene is developed to predict prognosis in esophageal squamous cell carcinoma (ESCC). Staurosporine (STS) is identified as a radioresistance suppressor by high-throughput screening of a library of 2131 natural compounds, leading to dramatically improved radiotherapy efficacy in subcutaneous tumor models. Mechanistically, STS inhibits cell proliferation through the mTOR/AKT signaling pathway and suppressed stemness by targeting ATP-binding cassette A1 (ABCA1), which is transcriptionally regulated by liver X receptor alpha (LXRα). STS can selectively bind to the nucleotide-binding domain (NBD) of ABCA1 and compete for ATP, blocking ABCA1-mediated drug efflux and facilitating intracellular accumulation of STS. Considering the cytotoxicity of STS, an extracellular vesicle-encapsulated STS system (EV-STS) is established for effective STS delivery. EV-STS shows remarkable tumor growth inhibition, even at half the dose of STS, with superior safety and efficacy. These findings indicate that ABCA1 may serve as a predictor of response to neoadjuvant chemotherapy and/or radiotherapy in ESCC patients. EV-STS has shown improved antitumor efficacy and low systemic toxicity, offering a promising therapeutic approach for ESCC.

Influence of Human Bone Marrow Mesenchymal Stem Cells Secretome from Acute Myeloid Leukemia Patients on the Proliferation and Death of K562 and K562-Lucena Leukemia Cell Lineages

Leukemias are among the most prevalent types of cancer worldwide. Bone marrow mesenchymal stem cells (MSCs) participate in the development of a suitable niche for hematopoietic stem cells, and are involved in the development of diseases such as leukemias, to a yet unknown extent. Here we described the effect of secretome of bone marrow MSCs obtained from healthy donors and from patients with acute myeloid leukemia (AML) on leukemic cell lineages, sensitive (K562) or resistant (K562-Lucena) to chemotherapy drugs. Cell proliferation, viability and death were evaluated, together with cell cycle, cytokine production and gene expression of ABC transporters and cyclins. The secretome of healthy MSCs decreased proliferation and viability of both K562 and K562-Lucena cells; moreover, an increase in apoptosis and necrosis rates was observed, together with the activation of caspase 3/7, cell cycle arrest in G0/G1 phase and changes in expression of several ABC proteins and cyclins D1 and D2. These effects were not observed using the secretome of MSCs derived from AML patients. In conclusion, the secretome of healthy MSCs have the capacity to inhibit the development of leukemia cells, at least in the studied conditions. However, MSCs from AML patients seem to have lost this capacity, and could therefore contribute to the development of leukemia.

Emerging and Future Targeted Therapies for Pediatric Acute Myeloid Leukemia: Targeting the Leukemia Stem Cells

Acute myeloid leukemia (AML) is a rare subtype of acute leukemia in the pediatric and adolescent population but causes disproportionate morbidity and mortality in this age group. Standard chemotherapeutic regimens for AML have changed very little in the past 3-4 decades, but the addition of targeted agents in recent years has led to improved survival in select subsets of patients as well as a better biological understanding of the disease. Currently, one key paradigm of bench-to-bedside practice in the context of adult AML is the focus on leukemia stem cell (LSC)-targeted therapies. Here, we review current and emerging immunotherapies and other targeted agents that are in clinical use for pediatric AML through the lens of what is known (and not known) about their LSC-targeting capability. Based on a growing understanding of pediatric LSC biology, we also briefly discuss potential future agents on the horizon.

Transporter-Mediated Cellular Distribution of Tyrosine Kinase Inhibitors as a Potential Resistance Mechanism in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a hematologic neoplasm characterized by the expression of the BCR::ABL1 oncoprotein, a constitutively active tyrosine kinase, resulting in uncontrolled growth and proliferation of cells in the myeloid lineage. Targeted therapy using tyrosine kinase inhibitors (TKIs) such as imatinib, nilotinib, dasatinib, bosutinib, ponatinib and asciminib has drastically improved the life expectancy of CML patients. However, treatment resistance occurs in 10-20% of CML patients, which is a multifactorial problem that is only partially clarified by the presence of TKI inactivating BCR::ABL1 mutations. It may also be a consequence of a reduction in cytosolic TKI concentrations in the target cells due to transporter-mediated cellular distribution. This review focuses on drug-transporting proteins in stem cells and progenitor cells involved in the distribution of TKIs approved for the treatment of CML. Special attention will be given to ATP-binding cassette transporters expressed in lysosomes, which may facilitate the extracytosolic sequestration of these compounds.

CRISPR technology: A versatile tool to model, screen, and reverse drug resistance in cancer

BACKGROUND

Drug resistance is a serious challenge in cancer treatment that can render chemotherapy a failure. Understanding the mechanisms behind drug resistance and developing novel therapeutic approaches are cardinal steps in overcoming this issue. Clustered regularly interspaced short palindrome repeats (CRISPR) gene-editing technology has proven to be a useful tool to study cancer drug resistance mechanisms and target the responsible genes. In this review, we evaluated original research studies that used the CRISPR tool in three areas related to drug resistance, namely screening resistance-related genes, generating modified models of resistant cells and animals, and removing resistance by genetic manipulation. We reported the targeted genes, study models, and drug groups in these studies. In addition to discussing different applications of CRISPR technology in cancer drug resistance, we analyzed drug resistance mechanisms and provided examples of CRISPR's role in studying them. Although CRISPR is a powerful tool for examining drug resistance and sensitizing resistant cells to chemotherapy, more studies are required to overcome its disadvantages, such as off-target effects, immunotoxicity, and inefficient delivery of CRISPR/cas9 into the cells.

Therapy Resistance and Disease Progression in CML: Mechanistic Links and Therapeutic Strategies

PURPOSE OF REVIEW

Despite the adoption of tyrosine kinases inhibitors (TKIs) as molecular targeted therapy in chronic myeloid leukemia, some patients do not respond to treatment and even experience disease progression. This review aims to give a broad summary of advances in understanding of the mechanisms of therapy resistance, as well as management strategies that may overcome or prevent the emergence of drug resistance. Ultimately, the goal of therapy is the cure of CML, which will also require an increased understanding of the leukemia stem cell (LSC).

RECENT FINDINGS

Resistance to tyrosine kinase inhibitors stems from a range of possible causes. Mutations of the BCR-ABL1 fusion oncoprotein have been well-studied. Other causes range from cell-intrinsic factors, such as the inherent resistance of primitive stem cells to drug treatment, to mechanisms extrinsic to the leukemic compartment that help CML cells evade apoptosis. There exists heterogeneity in TKI response among different hematopoietic populations in CML. The abundances of these TKI-sensitive and TKI-insensitive populations differ from patient to patient and contribute to response heterogeneity. It is becoming clear that targeting the BCR-ABL1 kinase through TKIs is only one part of the equation, and TKI usage alone may not cure the majority of patients with CML. Considerable effort should be devoted to targeting the BCR-ABL1-independent mechanisms of resistance and persistence of CML LSCs.

Clinical and biological impact of ATP-binding cassette transporter activity in adult acute myeloid leukemia

Chemotherapy resistance is the main cause of treatment failure in acute myeloid leukemia (AML) and has been related to ATP-binding cassette (ABC) transporter activity. However, the links between ABC activity, immunophenotype, and molecular AML parameters have been poorly evaluated. Moreover, the prognostic value of ABC activity, when compared to new molecular markers, is unknown. Here we investigated the links between ABC activity, as evaluated by JC-1 +/- cyclosporine A assay, and immunophenotypic, cytogenetic, molecular, and targeted next-generation sequencing features in 361 AML patients. High ABC activity was found in 164 patients and was significantly associated with less proliferating disease, an immature immunophenotype (expression of CD34, HLA-DR, CD117, CD13), and gene mutations defining AML as belonging to secondary-type ontogenic groups. Low ABC activity was associated with more mature myeloid differentiation (CD34-, cyMPO+, CD15+, CD33+) or monocytic commitment (CD64+, CD4+weak, CD14+), with NPM1 mutations, KMT2A rearrangements, and core-binding factor gene fusions, hallmarks of the de novo-type AML ontogeny. ABC activity was one of the major factors we identified using a random forest model for early prediction of AML ontogeny. In the 230 patients evaluated at diagnosis and intensively treated, high ABC activity was a predictive factor for primary resistance, and in multivariate analysis including full molecular data, an independent factor for event-free survival (P=0.0370). JC-1 +/- cyclosporine A assay could be used at diagnosis to predict AML ontogeny and to complete prognosis evaluation in addition to new molecular markers.

Drug Resistance Mechanisms of Acute Myeloid Leukemia Stem Cells

Acute myeloid leukemia (AML) is a polyclonal and heterogeneous hematological malignancy. Relapse and refractory after induction chemotherapy are still challenges for curing AML. Leukemia stem cells (LSCs), accepted to originate from hematopoietic stem/precursor cells, are the main root of leukemogenesis and drug resistance. LSCs are dynamic derivations and possess various elusive resistance mechanisms. In this review, we summarized different primary resistance and remolding mechanisms of LSCs after chemotherapy, as well as the indispensable role of the bone marrow microenvironment on LSCs resistance. Through a detailed and comprehensive review of the spectacle of LSCs resistance, it can provide better strategies for future researches on eradicating LSCs and clinical treatment of AML.

ABCB1 as a potential beneficial target of midostaurin in acute myeloid leukemia

Low curability of patients diagnosed with acute myeloid leukemia (AML) must be seen as a call for better understanding the disease's mechanisms and improving the treatment strategy. Therapeutic outcome of the crucial anthracycline-based induction therapy often can be compromised by a resistant phenotype associated with overexpression of ABCB1 transporters. Here, we evaluated clinical relevance of ABCB1 in a context of the FMS-like tyrosine kinase 3 (FLT3) inhibitor midostaurin in a set of 28 primary AML samples. ABCB1 gene expression was absolutely quantified, confirming its association with CD34 positivity, adverse cytogenetic risk, and unachieved complete remission (CR). Midostaurin, identified as an ABCB1 inhibitor, increased anthracycline accumulation in peripheral blood mononuclear cells (PBMC) of CD34⁺ AML patients and those not achieving CR. This effect was independent of FLT3 mutation, indicating even FLT3^- AML patients might benefit from midostaurin therapy. In line with these data, midostaurin potentiated proapoptotic processes in ABCB1-overexpressing leukemic cells when combined with anthracyclines. Furthermore, we report a direct linkage of miR-9 to ABCB1 efflux activity in the PBMC and propose miR-9 as a useful prognostic marker in AML. Overall, we highlight the therapeutic value of midostaurin as more than just a FLT3 inhibitor, suggesting its maximal therapeutic outcomes might be very sensitive to proper timing and well-optimized dosage schemes based upon patient's characteristics, such as CD34 positivity and ABCB1 activity. Moreover, we suggest miR-9 as a predictive ABCB1-related biomarker that could be immensely helpful in identifying ABCB1-resistant AML phenotype to enable optimized therapeutic regimen and improved treatment outcome.

Folate metabolism: a re-emerging therapeutic target in haematological cancers

Folate-mediated one carbon (1C) metabolism supports a series of processes that are essential for the cell. Through a number of interlinked reactions happening in the cytosol and mitochondria of the cell, folate metabolism contributes to de novo purine and thymidylate synthesis, to the methionine cycle and redox defence. Targeting the folate metabolism gave rise to modern chemotherapy, through the introduction of antifolates to treat paediatric leukaemia. Since then, antifolates, such as methotrexate and pralatrexate have been used to treat a series of blood cancers in clinic. However, traditional antifolates have many deleterious side effects in normal proliferating tissue, highlighting the urgent need for novel strategies to more selectively target 1C metabolism. Notably, mitochondrial 1C enzymes have been shown to be significantly upregulated in various cancers, making them attractive targets for the development of new chemotherapeutic agents. In this article, we present a detailed overview of folate-mediated 1C metabolism, its importance on cellular level and discuss how targeting folate metabolism has been exploited in blood cancers. Additionally, we explore possible therapeutic strategies that could overcome the limitations of traditional antifolates.

ABCC9, NKAPL, and TMEM132C are potential diagnostic and prognostic markers in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a highly heterogeneous disease. The aim of this study is to identify the diagnostic and poor prognostic signatures in TNBC by exploring the aberrant DNA methylation and gene expression. Differential expression and methylation analysis of the TNBC and paracancer samples from The Cancer Genome Atlas were performed. Gene set enrichment and protein-protein interaction (PPI) network analysis was used to explore the mechanisms of TNBC. Methylation-gene expression correlation analysis was performed, and multivariate Cox analysis and receiver operating characteristics analysis were used to further screen the hub genes for TNBC. We identified 1,525 differentially expressed genes and 150 differentially methylated genes between TNBC and paracancer samples. About 96.64% of the methylation sites were located on the CpG island. A total of 17 Gene Ontology biological process terms and 18 signal pathways were significantly enriched. GNG4, GNG11, PENK, MAOA, and AOX1 were identified as the core genes of the PPI network. Methylation-expression correlations revealed that ABCC9 (cg06951626), NKAPL (cg18675097, cg01031101, and cg17384889), and TMEM132C (cg03530754) showed promise as diagnostic and prognostic markers in TNBC. ABCC9 (cg06951626), NKAPL (cg18675097, cg01031101, and cg17384889), and TMEM132C (cg03530754) were potential diagnostic and prognostic markers in TNBC.

Chronic Inflammation May Enhance Leiomyoma Development by the Involvement of Progenitor Cells

Although the etiology of leiomyoma is unclear, a progenitor/undifferentiated cell population has been described whose dysregulation may be involved in the onset of uterine conditions. Moreover, inflammation is involved in the development of several tumors. The aim of this work was to understand if progenitor cells sustain a chronic inflammatory microenvironment that enhances leiomyoma development. Cells from 12 human leiomyoma and 12 normal myometrium samples of the same patients were in vitro isolated and exhaustively characterized (morphology, proliferation, cytofluorometry, differentiation, RT-PCR, immunofluorescence, immunohistochemistry, and Western blotting assays). Selected cytokines (ELISA) and inflammation-related genes (RT-PCR) were analyzed to identify healthy myometrium progenitor cells (MPCs) and leiomyoma progenitor cells (LPCs). Results show that (i) MPCs and LPCs share stemness features, such as immunophenotype and multidifferentiation assay, (ii) LPCs have a significantly shorter doubling time and a significantly higher expression of stemness genes (p < 0.05), and (iii) LPCs secreted significantly higher levels (p < 0.05) of cytokines related to chronic inflammation and significantly lower amounts (p < 0.05) of cytokines related to acute inflammation. Despite the limited sample size, comparisons between leiomyoma and normal myometrium tissue from each patient allowed normalization of patient-specific differences. The evidenced cytokine expression pattern related to chronic inflammation in LPCs may play a role in the increased risk of adverse obstetric outcomes (infertility, spontaneous miscarriage, and preterm birth) in women affected by leiomyomas. These women should be recognized as “high risk” and subjected to specialized management both before and during pregnancy.

Antipsychotic dopamine receptor antagonists, cancer, and cancer stem cells

Cancer is one of the deadliest diseases in the world. Despite extensive studies, treating metastatic cancers remains challenging. Years of research have linked a rare set of cells known as cancer stem cells (CSCs) to drug resistance, leading to the suggestion that eradication of CSCs might be an effective therapeutic strategy. However, few drug candidates are active against CSCs. New drug discovery is often a lengthy process. Drug screening has been advantageous in identifying drug candidates. Current understanding of cancer biology has revealed various clues to target cancer from different points of view. Many studies have found dopamine receptors (DRs) in various cancers. Therefore, DR antagonists have attracted a lot of attention in cancer research. Recently, a group of antipsychotic DR antagonists has been demonstrated to possess remarkable abilities to restrain and sensitize CSCs to existing chemotherapeutics by a process called differentiation approach. In this review, we will describe current aspects of CSC-targeting therapeutics, antipsychotic DR antagonists, and their extraordinary abilities to fight cancer.

Expression of some ATP-binding cassette transporters in acute myeloid leukemia

Hematopoietic cells express ATP binding cassette (ABC) transporters in relation to different degrees of differentiation. One of the known multidrug resistance mechanisms in acute myeloid leukemia (AML) is the overexpression of efflux pumps belonging to the superfamily of ABC transporters such as ABCB1, ABCG2 and ABCC1. Although several studies were carried out to correlate ABC transporters expression with drug resistance, little is known about their role as markers of diagnosis and progression of the disease. For this purpose we investigated the expression, by real-time PCR, of some ABC genes in bone marrow samples of AML patients at diagnosis and after induction therapy. At diagnosis, ABCG2 was always down-regulated, while an up regulated trend for ABCC1 was observed. After therapy the examined genes showed a different expression trend and approached the values of healthy subjects suggesting that this event could be considered as a marker of AML regression. The expression levels of some ABC transporters such as ABCC6, seems to be related to gender, age and to the presence of FLT3/ITD gene mutation.

ATP-binding cassette transporter-2 (ABCA2) as a therapeutic target

The ATP binding cassette transporter ABCA2 is primarily an endolysosomal membrane protein that demonstrates pleiotropic functionalities, coalescing around the maintenance of homeostasis of sterols, sphingolipids and cholesterol. It is most highly expressed in brain tissue and ABCA2 knockout mice express neurological defects consistent with aberrant myelination. Increased expression of the transporter has been linked with resistance to cancer drugs, particularly those possessing a steroid backbone and gene expression (in concert with other genes involved in cholesterol metabolism) was found to be regulated by sterols. Moreover, in macrophages ABCA2 is influenced by sterols and has a role in regulating cholesterol sequestration, potentially important in cardiovascular disease. Accumulating data indicate the critical importance of ABCA2 in mediating movement of sphingolipids within cellular compartments and these have been implicated in various aspects of cholesterol trafficking. Perhaps because the functions of ABCA2 are linked with membrane building blocks, there are reports linking it with human pathologies, including, cholesterolemias and cardiovascular disease, Alzheimer's and cancer. The present review addresses whether there is now sufficient information to consider ABCA2 as a plausible therapeutic target.

Vitexin induces apoptosis by suppressing autophagy in multi-drug resistant colorectal cancer cells

Cancer treatment is limited due to the diverse multidrug resistance acquired by cancer cells and the collateral damage caused to adjacent normal cells by chemotherapy. The flavonoid compound vitexin exhibits anti-oxidative, anti-inflammatory and anti-tumor activity. This study elucidated the antitumor effects of vitexin and its underlying mechanisms in a multi-drug resistant human colon cancer cell line (HCT-116^DR), which exhibits higher levels of multidrug-resistant protein 1 (MDR1) expression as compared with its parental cell line (HCT-116). Here, we observed that vitexin suppressed MDR-1 expression and activity in HCT-116^DR cells and showed cytotoxic effect in HCT-116^DR cells by inhibiting autophagy and inducing apoptosis in a concentration-dependent manner. Additionally, vitexin treatment caused cleavage of caspase-9 and caspase-3, and upregulated the expression of the pro-apoptotic proteins, BID and Bax. Moreover, the expression of autophagy-related proteins, such as ATG5, Beclin-1 and LC3-II, was markedly reduced by vitexin treatment. Furthermore, in vivo experiments showed that vitexin induced apoptosis and suppressed tumor growth in HCT-116^DR xenograft model. These results revealed that vitexin induced apoptosis through suppression of autophagy in vitro and in vivo and provide insight into the therapeutic potential of vitexin for the treatment of chemo-resistant colorectal cancer.

ABC gene expression profiles have clinical importance and possibly form a new hallmark of cancer

Adenosine triphosphate-binding cassette proteins constitute a large family of active transporters through extracellular and intracellular membranes. Increased drug efflux based on adenosine triphosphate-binding cassette protein activity is related to the development of cancer cell chemoresistance. Several articles have focused on adenosine triphosphate-binding cassette gene expression profiles (signatures), based on the expression of all 49 human adenosine triphosphate-binding cassette genes, in individual tumor types and reported connections to established clinicopathological features. The aim of this study was to test our theory about the existence of adenosine triphosphate-binding cassette gene expression profiles common to multiple types of tumors, which may modify tumor progression and provide clinically relevant information. Such general adenosine triphosphate-binding cassette profiles could constitute a new attribute of carcinogenesis. Our combined cohort consisted of tissues from 151 cancer patients-breast, colorectal, and pancreatic carcinomas. Standard protocols for RNA isolation and quantitative real-time polymerase chain reaction were followed. Gene expression data from individual tumor types as well as a merged tumor dataset were analyzed by bioinformatics tools. Several general adenosine triphosphate-binding cassette profiles, with differences in gene functions, were established and shown to have significant relations to clinicopathological features such as tumor size, histological grade, or clinical stage. Genes ABCC7, A3, A8, A12, and C8 prevailed among the most upregulated or downregulated ones. In conclusion, the results supported our theory about general adenosine triphosphate-binding cassette gene expression profiles and their importance for cancer on clinical as well as research levels. The presence of ABCC7 (official symbol CFTR) among the genes with key roles in the profiles supports the emerging evidence about its crucial role in various cancers. Graphical abstract.

Update on acute myeloid leukemia stem cells: New discoveries and therapeutic opportunities

The existence of cancer stem cells has been well established in acute myeloid leukemia. Initial proof of the existence of leukemia stem cells (LSCs) was accomplished by functional studies in xenograft models making use of the key features shared with normal hematopoietic stem cells (HSCs) such as the capacity of self-renewal and the ability to initiate and sustain growth of progenitors in vivo. Significant progress has also been made in identifying the phenotype and signaling pathways specific for LSCs. Therapeutically, a multitude of drugs targeting LSCs are in different phases of preclinical and clinical development. This review focuses on recent discoveries which have advanced our understanding of LSC biology and provided rational targets for development of novel therapeutic agents. One of the major challenges is how to target the self-renewal pathways of LSCs without affecting normal HSCs significantly therefore providing an acceptable therapeutic window. Important issues pertinent to the successful design and conduct of clinical trials evaluating drugs targeting LSCs will be discussed as well.

Multidrug resistance protein 4/ ATP binding cassette transporter 4: a new potential therapeutic target for acute myeloid leukemia

Less than a third of adults patients with acute myeloid leukemia (AML) are cured by current treatments, emphasizing the need for new approaches to therapy. We previously demonstrated that besides playing a role in drug-resistant leukemia cell lines, multidrug resistance protein 4 (MRP4/ABCC4) regulates leukemia cell proliferation and differentiation through the endogenous MRP4/ABCC4 substrate, cAMP. Here, we studied the role of MRP4/ABCC4 in tumor progression in a mouse xenograft model and in leukemic stem cells (LSCs) differentiation. We found a decrease in the mitotic index and an increase in the apoptotic index associated with the inhibition of tumor growth when mice were treated with rolipram (PDE4 inhibitor) and/or probenecid (MRPs inhibitor). Genetic silencing and pharmacologic inhibition of MRP4 reduced tumor growth. Furthermore, MRP4 knockdown induced cell cycle arrest and apoptosis in vivo. Interestingly, when LSC population was isolated, we observed that increased cAMP levels and MRP4/ABCC4 blockade resulted in LSCs differentiation. Taken together, our findings show that MRP4/ABCC4 has a relevant role in tumor growth and apoptosis and in the eradication of LSCs, providing the basis for a novel promising target in AML therapy.

Cancer stem cells--important players in tumor therapy resistance

Resistance to tumor therapy is an unsolved problem in cancer treatment. A plethora of studies have attempted to explain this phenomenon and many mechanisms of resistance have been suggested over recent decades. The concept of cancer stem cells (CSCs), which describes tumors as hierarchically organized, has added a new level of complexity to therapy failure. CSCs are the root of cancers and resist chemo- and radiotherapy, explaining cancer recurrence even many years after therapy is ended. This review discusses briefly CSCs in cancers, gives an overview of the role of CSCs in therapy resistance, and discusses the potential means of targeting these therapy-resistant tumor cells.

The pharmacological impact of ATP-binding cassette drug transporters on vemurafenib-based therapy

Melanoma is the most serious type of skin cancer and one of the most common cancers in the world. Advanced melanoma is often resistant to conventional therapies and has high potential for metastasis and low survival rates. Vemurafenib is a small molecule inhibitor of the BRAF serine-threonine kinase recently approved by the United States Food and Drug Administration to treat patients with metastatic and unresectable melanomas that carry an activating BRAF (V600E) mutation. Many clinical trials evaluating other therapeutic uses of vemurafenib are still ongoing. The ATP-binding cassette (ABC) transporters are membrane proteins with important physiological and pharmacological roles. Collectively, they transport and regulate levels of physiological substrates such as lipids, porphyrins and sterols. Some of them also remove xenobiotics and limit the oral bioavailability and distribution of many chemotherapeutics. The overexpression of three major ABC drug transporters is the most common mechanism for acquired resistance to anticancer drugs. In this review, we highlight some of the recent findings related to the effect of ABC drug transporters such as ABCB1 and ABCG2 on the oral bioavailability of vemurafenib, problems associated with treating melanoma brain metastases and the development of acquired resistance to vemurafenib in cancers harboring the BRAF (V600E) mutation.

Nilotinib enhances the efficacy of conventional chemotherapeutic drugs in CD34⁺CD38⁻ stem cells and ABC transporter overexpressing leukemia cells

Incomplete chemotherapeutic eradication of leukemic CD34⁺CD38⁻ stem cells is likely to result in disease relapse. The purpose of this study was to evaluate the effect of nilotinib on eradicating leukemia stem cells and enhancing the efficacy of chemotherapeutic agents. Our results showed that ABCB1 and ABCG2 were preferentially expressed in leukemic CD34⁺CD38⁻ cells. Nilotinib significantly enhanced the cytotoxicity of doxorubicin and mitoxantrone in CD34⁺CD38⁻ cells and led to increased apoptosis. Moreover, nilotinib strongly reversed multidrug resistance and increased the intracellular accumulation of rhodamine 123 in primary leukemic blasts overexpressing ABCB1 and/or ABCG2. Studies with ABC transporter-overexpressing carcinoma cell models confirmed that nilotinib effectively reversed ABCB1- and ABCG2-mediated drug resistance, while showed no significant reversal effect on ABCC1- and ABCC4-mediated drug resistance. Results from cytotoxicity assays showed that CD34⁺CD38⁻ cells exhibited moderate resistance (2.41-fold) to nilotinib, compared with parental K562 cells. Furthermore, nilotinib was less effective in blocking the phosphorylation of Bcr-Abl and CrkL (a substrate of Bcr-Abl kinase) in CD34⁺CD38⁻ cells. Taken together, these data suggest that nilotinib particularly targets CD34⁺CD38⁻ stem cells and MDR leukemia cells, and effectively enhances the efficacy of chemotherapeutic drugs by blocking the efflux function of ABC transporters.

Identification of dual DNA-PK MDR1 inhibitors for the potentiation of cytotoxic drug activity

Inhibition of DNA repair is an attractive therapeutic approach to enhance the activity of DNA-damaging anticancer chemotherapeutic agents. Similarly, blockade of the multidrug-resistance protein 1 (MDR1) can overcome efflux-mediated resistance. DNA-dependent protein kinase (DNA-PK) is essential for the non-homologous end-joining DNA repair pathway. NU7441 is a potent DNA-PK inhibitor (IC50=14nM) that is used widely to study the effects of DNA-PK inhibition in vitro. In growth inhibition studies, 1μM NU7441 sensitised vincristine-resistant CCRF-CEM VCR/R leukaemia cells (1200-fold resistant) to a range of MDR1 substrates, including doxorubicin (8-fold, p=0.03), vincristine (14-fold, p=0.01) and etoposide (63-fold, p=0.02), compared with 1.4-fold (p=0.02), 2.2-fold (p=0.04) and 3.6-fold (p=0.01) sensitisation, respectively, in parental CCRF-CEM cells. This difference in NU7441 sensitivity was confirmed in another two parental and MDR1-overexpressing cell line pairs. A doxorubicin fluorescence assay showed that in MDR1-overexpressing canine kidney MDCKII-MDR1 cells, 1μM NU7441 increased doxorubicin nuclear fluorescence 16-fold. NU7441 and 3 structurally related compounds (NU7742 (an NU7441 analogue that does not inhibit DNA-PK - IC50>10μM), DRN1 (DNA-PK-inhibitory atropisomeric NU7441 derivative - IC50=2nM) and DRN2 (DNA-PK non-inhibitory atropisomeric NU7441 derivative - IC50=7μM)) all increased intracellular vincristine accumulation in the CCRF-CEM VCR/R cells to a level similar to verapamil, as measured by LC-MS. This paper demonstrates that NU7441 is a dual DNA-PK and MDR1 inhibitor, and this extends the therapeutic potential of the compound when used in combination with MDR substrates.

Src Family Kinase Inhibitor PP2 Induces LC3 Conversion in a Manner That is Uncoupled from Autophagy and Increases Apoptosis in Multidrug-Resistant Cells

Recently, we reported that defective autophagy may contribute to the inhibition of the growth in response to PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), a selective SFK inhibitor, in multidrug-resistant v-Ha-ras-transformed NIH 3T3 cells (Ras-NIH 3T3/Mdr). In this study, we demonstrated that PP2 induces LC3 conversion via a mechanism that is uncoupled from autophagy and increases apoptosis in Ras-NIH 3T3/Mdr cells. PP2 preferentially induced autophagy in Ras-NIH 3T3 cells rather than in Ras-NIH 3T3/Mdr cells as determined by LC3-I to LC3-II conversion and GFP-LC3 fluorescence microscopy. Beclin 1 knockdown experiments showed that, regardless of drug resistance, PP2 induces autophagy via a Beclin 1-dependent mechanism. PP2 induced a conformational change in Beclin 1, resulting in the enhancement of the pro-autophagic activity of Beclin 1, in Ras-NIH 3T3 cells. Further, PI3K inhibition induced by wortmannin caused a significant increase in apoptosis in Ras-NIH 3T3 cells, as demonstrated by flow cytometric analysis of Annexin V staining, implying that autophagy inhibition through PI3K increases apoptosis in response to PP2 in Ras-NIH 3T3 cells. However, despite the fact that wortmannin abrogates PP2-induced GFP-LC3 punctae formation, some LC3 conversion remains in Ras-NIH 3T3/Mdr cells, suggesting that LC3 conversion may occur in an autophagy-independent manner. Taken together, these results suggest that PP2 induces LC3 conversion independent of PI3K, concomitant with the uncoupling of LC3 conversion from autophagy, in multidrug-resistant cells.

Tumor dormancy and cancer stem cells: two sides of the same coin?

Increasing evidence suggests that tumor dormancy represents an important mechanism underlying the observed failure of existing therapeutic modalities to fully eradicate cancers. In addition to its more established role in maintaining minimal residual disease after treatment, dormancy might also critically contribute to early stages of tumor development and the formation of clinically undetectable micrometastatic foci. There are striking parallels between the concept of tumor dormancy and the cancer stem cell (CSC) theory of tumor propagation. For instance, the CSC hypothesis similarly predicts that a subset of self-renewing cancer cells-that is CSCs-is responsible for tumor initiation, bears the preferential ability to survive tumor therapy, and persists long term to ultimately cause delayed cancer recurrence and metastatic progression. Additionally, many of the biological mechanisms involved in controlling the dormant state of a tumor can also govern CSC behavior, including cell cycle modifications, alteration of angiogenic processes, and modulation of antitumor immune responses. In fact, quiescence and immune escape are emerging hallmark features of at least some CSCs, indicating significant overlap between dormant cancer populations and CSCs. Herein, we crucially dissect whether CSCs occupy specific roles in orchestrating the switch between dormancy and exuberant tumor growth. We elucidate how recently uncovered CSC biological features could enable these cells to evade immunologic clearance and regulate cancer expansion, relapse, and progression. We propose that the study of CSC immunobiological pathways holds the promise to critically advance our understanding of the processes mediating tumor dormancy. Ultimately, such research endeavors could unravel novel therapeutic avenues that efficiently target both proliferating and dormant CSCs to minimize the risk of tumor recurrence in cancer patients.

Differential inhibitory effects of two Raf-targeting drugs, sorafenib and PLX4720, on the growth of multidrug-resistant cells

B-Raf is the most frequently mutated protein kinase in the MAPK signaling cascade in human cancers, making it an important therapeutic target. Here, we describe the differential effects of two Raf-targeting drugs, sorafenib and PLX4720, on multidrug-resistant v-Ha-ras-transformed cells (Ras-NIH 3T3/Mdr). We demonstrate that the growth of the NIH 3T3/Mdr cell line was affected in a dose-dependent manner more significantly by the pan-Raf inhibitor sorafenib than by the selective mutant B-Raf inhibitor PLX4720. Despite their differential effects on LKB1/AMPK phosphorylation, both sorafenib and PLX4720 inhibited downstream mTOR signaling with concomitant induction of autophagy, implying that the differential effects of sorafenib and PLX4720 on multidrug-resistant cells might not be due to different levels of autophagy and apoptosis. Interestingly, sorafenib caused a dose-dependent increase in rhodamine 123 uptake and retention. More importantly, sorafenib reversed the resistance to paclitaxel in Ras-NIH 3T3/Mdr cells. Moreover, MEK/ERK signaling was hyperactivated by the selective mutant B-Raf inhibitor PLX4720 and inhibited by the pan-Raf inhibitor sorafenib. Our data suggest that sorafenib sensitivity in MDR cells is mediated through the inhibition of P-glycoprotein activity following strong inhibition of Raf/MEK/ERK signaling. Thus, Raf inhibition with sorafenib might be a promising approach to abrogate the multidrug resistance of cancer cells.

Overexpression of snail induces epithelial-mesenchymal transition and a cancer stem cell-like phenotype in human colorectal cancer cells

Epithelial–mesenchymal transition (EMT) is a critical process providing tumor cells with the ability to migrate and escape from the primary tumor and metastasize to distant sites. Recently, EMT was shown to be associated with the cancer stem cell (CSC) phenotype in breast cancer. Snail is a transcription factor that mediates EMT in a number of tumor types, including colorectal cancer (CRC). Our study was done to determine the role of Snail in mediating EMT and CSC function in CRC. Human CRC specimens were stained for Snail expression, and human CRC cell lines were transduced with a retroviral Snail construct or vector control. Cell proliferation and chemosensitivity to oxaliplatin of the infected cells were determined by the MTT (colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. Migration and invasion were determined in vitro using modified Boyden chamber assays. EMT and putative CSC markers were analyzed using Western blotting. Intravenous injection of tumor cells was done to evaluate their metastatic potential in mice. Snail was overexpressed in human CRC surgical specimens. This overexpression induced EMT and a CSC-like phenotype in human CRC cells and enhanced cell migration and invasion (P < 0.002 vs. control). Snail overexpression also led to an increase in metastasis formation in vivo (P < 0.002 vs. control). Furthermore, the Snail-overexpressing CRC cells were more chemoresistant to oxaliplatin than control cells. Increased Snail expression induces EMT and the CSC-like phenotype in CRC cells, which enhance cancer cell invasion and chemoresistance. Thus, Snail is a potential therapeutic target in metastatic CRC.

A small molecule screening strategy with validation on human leukemia stem cells uncovers the therapeutic efficacy of kinetin riboside

Gene regulatory networks that govern hematopoietic stem cells (HSCs) and leukemia-initiating cells (L-ICs) are deeply entangled. Thus, the discovery of compounds that target L-ICs while sparing HSC is an attractive but difficult endeavor. Presently, most screening approaches fail to counter-screen compounds against normal hematopoietic stem/progenitor cells (HSPCs). Here, we present a multistep in vitro and in vivo approach to identify compounds that can target L-ICs in acute myeloid leukemia (AML). A high-throughput screen of 4000 compounds on novel leukemia cell lines derived from human experimental leukemogenesis models yielded 80 hits, of which 10 were less toxic to HSPC. We characterized a single compound, kinetin riboside (KR), on AML L-ICs and HSPCs. KR demonstrated comparable efficacy to standard therapies against blast cells in 63 primary leukemias. In vitro, KR targeted the L-IC–enriched CD34+CD38− AML fraction, while sparing HSPC-enriched fractions, although these effects were mitigated on HSC assayed in vivo. KR eliminated L-ICs in 2 of 4 primary AML samples when assayed in vivo and highlights the importance of in vivo L-IC and HSC assays to measure function. Overall, we provide a novel approach to screen large drug libraries for the discovery of anti–L-IC compounds for human leukemias.

Treatment with high-dose simvastatin inhibits geranylgeranylation in AML blast cells in a subset of AML patients

It is currently unknown whether the in vitro effects observed with statins in acute myeloid leukemia (AML) cells, including lowering of cholesterol, inhibition of isoprenylation, and sensitization to chemotherapy, also occur in vivo. Therefore, AML mononuclear cells (MNCs) were isolated from 12 patients before and after 7 days of high-dose (7.5-15 mg/kg/day) simvastatin treatment. Parallel mouse studies were performed to have, in addition to AML cells, access to liver tissue, a major target of statins. Serum cholesterol levels were lowered by simvastatin in all patients, however, only limited changes in the messenger RNA expression of cholesterol metabolism genes were seen in patient and mouse MNCs compared to murine liver cells. Still, two out of seven patients displayed an increased in vitro chemosensitivity of their AML cells upon simvastatin treatment. Gene set enrichment analysis on microarray data of AML patient cells and Western blot analysis for the isoprenylated proteins DnaJ and Rap1 on murine and AML patient MNCs demonstrated that in vivo simvastatin treatment resulted in inhibition of geranylgeranylation in murine MNCs and in a subset of patient AML MNCs. In summary, our data demonstrate that simvastatin treatment results in chemosensitization and inhibition of geranylgeranylation in AML cells of a subset of patients.

Anti-atherogenic mechanisms of high density lipoprotein: effects on myeloid cells

In some settings increasing high density lipoprotein (HDL) levels has been associated with a reduction in experimental atherosclerosis. This has been most clearly seen in apolipoprotein A-I (apoA-I) transgenic mice or in animals infused with HDL or its apolipoproteins. A major mechanism by which these treatments are thought to delay progression or cause regression of atherosclerosis is by promoting efflux of cholesterol from macrophage foam cells. In addition, HDL has been described as having anti-inflammatory and other beneficial effects. Some recent research has linked anti-inflammatory effects to cholesterol efflux pathways but likely multiple mechanisms are involved. Macrophage cholesterol efflux may have a role in facilitating emigration of macrophages from lesions during regression. While macrophages can mediate cholesterol efflux by several pathways, studies in knockout mice or cells point to the importance of active efflux mediated by ATP binding cassette transporter (ABC) A1 and G1. In addition to traditional roles in macrophages, these transporters have been implicated in the control of hematopoietic stem cell proliferation, monocytosis and neutrophilia, as well as activation of monocytes and neutrophils. Thus, HDL and cholesterol efflux pathways may have important anti-atherogenic effects at all stages of the myeloid cell/monocyte/dendritic cell/macrophage lifecycle. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Suppression of autophagy sensitizes multidrug resistant cells towards Src tyrosine kinase specific inhibitor PP2

Upregulated Src activity has been implicated in a variety of cancers. Thus, Src family tyrosine kinase (SFK) inhibitors are often effective cancer treatments. Here, we employed 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), a selective SFK inhibitor, to determine the possible involvement of tyrosine phosphorylation in the modulation of autophagy, for overcoming multidrug resistance. We found that multidrug-resistant v-Ha-ras-transformed NIH 3T3 cells (Ras-NIH 3T3/Mdr) were more susceptible to PP2 treatment than were their parental cells (Ras-NIH 3T3). The antiproliferative activity of PP2 appeared to be due to cell-cycle arrest at G1/S without induction of apoptosis. Interestingly, PP2 preferentially induced autophagy in Ras-NIH 3T3 cells but not in Ras-NIH 3T3/Mdr cells, which implies that a high level of autophagy may protect PP2-treated cells from undergoing cell death. PP2-induced autophagy in Ras-NIH 3T3 cells is accompanied by an inhibition of the mTOR signaling pathway. However, we found that in Ras-NIH 3T3/Mdr cells, PP2-induced mTOR inhibition was uncoupled from the induction of autophagy-likely due to the hyperactivation of AMPK by delayed Raf activation. We also found that PP2-induced dissociation of Beclin 1 from Bcl-2 leads to autophagy in Ras-NIH 3T3 cells. Taken together, these results suggest that functional autophagy in response to PP2 may lead to cell survival in Ras-NIH 3T3 cells, while defective autophagy may contribute to inhibition of growth in Ras-NIH 3T3/Mdr cells. Thus, modulators of autophagy may be used beneficially as adjunctive therapeutic agents during the treatment of cancers with SFK inhibitors.

ATP Binding Cassette transporters associated with chemoresistance: transcriptional profiling in extreme cohorts and their prognostic impact in a cohort of 281 acute myeloid leukemia patients

BACKGROUND

A major issue in the treatment of acute myeloid leukemia is resistance to chemotherapeutic drugs. An increasing number of ATP-Binding-Cassette transporters have been demonstrated to cause resistance to cancer drugs. The aim of this study was to highlight the putative role of other ATP-Binding-Cassette transporters in primary chemoresistant acute myeloid leukemia.

DESIGN AND METHODS

In the first part of this study, using taqman custom arrays, we analyzed the relative expression levels of 49 ATP-Binding-Cassette genes in 51 patients divided into two extreme cohorts, one very sensitive and one very resistant to chemotherapy. In the second part of this study, we evaluated the prognostic impact, in a cohort of 281 patients, of ATP-Binding-Cassette genes selected in the first part of the study.

RESULTS

In the first part of the study, six genes (ATP-Binding-CassetteA2, ATP-Binding-CassetteB1, ATP-Binding-CassetteB6, ATP-Binding-CassettC13, ATP-Binding-CassetteG1, and ATP-Binding-CassetteG2) were significantly over-expressed in the resistant group compared with the sensitive group. In the second cohort, overexpression of 5 of these 6 ATP-Binding-Cassette genes was correlated with outcome in univariate analysis, and only the well-known ATP-Binding-CassetteB1 and G2, and the new ATP-Binding-CassetteG1 in multivariate analysis. Prognosis decreased remarkably with the number of these over-expressed ABC genes. Complete remission was achieved in 71%, 59%, 54%, and 0%, (P=0.0011) and resistance disease in 21%, 37%, 43%, and 100% (P<0.0001) of patients over-expressing 0, 1, 2, or 3, ABC genes, respectively. The number of ATP-Binding-Cassette genes expressed, among ATP-Binding-CassetteB1, G1, and G2, was the strongest prognostic factor correlated, in multivariate analysis, with achievement of complete remission (P=0.01), resistant disease (P=0.01), and overall survival (P=0.02).

CONCLUSIONS

Using expression profiling, we have emphasized the diversity of ATP-Binding-Cassette transporters that cooperate to promote chemoresistance rather than overexpression of single transporters and the putative role of new ATP-Binding-Cassette tranporters, such as ATP-Binding-CassetteG1. Modulation of these multiple transporters might be required to eradicate leukemic cells.

Stem cells in squamous head and neck cancer

The initiation and metastasis of head and neck squamous cell carcinomas (HNSCC) and other cancers have recently been related to the presence of cancer stem cells (CSC). CSC are cancer initiating, sustaining and are mostly quiescent. Specific markers that vary considerably depending on tumor type or tissue of origin characterize putative CSC. Compared to the bulk tumor mass, CSC are less sensitive to chemo- and radiotherapy and may also have low immunogenicity. Therapeutic targeting of CSC may improve clinical outcome of HNSCC which has two distinct etiologies: infection of epithelial stem cells by high-risk types of the human papillomavirus, or long-term tobacco and alcohol abuse. Recent knowledge on the role of CSC in HNSCC is reviewed and where necessary parallels to CSC of other origin are drawn to give a more comprehensive picture.

Drug discovery approaches to target Wnt signaling in cancer stem cells

Cancer stem cells (CSCs) represent a unique subset of cells within a tumor that possess self-renewal capacity and pluripotency, and can drive tumor initiation and maintenance. First identified in hematological malignancies, CSCs are now thought to play an important role in a wide variety of solid tumors such as NSCLC, breast and colorectal cancer. The role of CSCs in driving tumor formation illustrates the dysregulation of differentiation in tumorigenesis. The Wnt, Notch and Hedgehog (HH) pathways are developmental pathways that are commonly activated in many types of cancer. While substantial progress has been made in developing therapeutics targeting Notch and HH, the Wnt pathway has remained an elusive therapeutic target. This review will focus on the clinical relevance of the Wnt pathway in CSCs and tumor cell biology, as well as points of therapeutic intervention and recent advances in targeting Wnt/β-catenin signaling.

Hematopoietic stem cells exhibit a specific ABC transporter gene expression profile clearly distinct from other stem cells

Background

ATP-binding cassette (ABC) transporters protect cells against unrelated (toxic) substances by pumping them across cell membranes. Earlier we showed that many ABC transporters are highly expressed in hematopoietic stem cells (HSCs) compared to more committed progenitor cells. The ABC transporter expression signature may guarantee lifelong protection of HSCs but may also preserve stem cell integrity by extrusion of agents that trigger their differentiation. Here we have studied whether non-hematopoietic stem cells (non-HSCs) exhibit a similar ABC transporter expression signature as HSCs.

Results

ABC transporter expression profiles were determined in non-hematopoietic stem cells (non-HSCs) from embryonic, neonatal and adult origin as well as in various mature blood cell types. Over 11,000 individual ABC transporter expression values were generated by Taqman Low Density Arrays (TLDA) to obtain a sensitivity comparable with quantitative real-time polymerase chain reactions. We found that the vast majority of transporters are significantly higher expressed in HSCs compared to non-HSCs. Furthermore, regardless their origin, non-HSCs exhibited strikingly similar ABC transporter expression profiles that were distinct from those in HSCs. Yet, sets of transporters characteristic for different stem cell types could be identified, suggesting restricted functions in stem cell physiology. Remarkably, in HSCs we could not pinpoint any single transporter expressed at an evidently elevated level when compared to all the mature blood cell types studied.

Conclusions

These findings challenge the concept that individual ABC transporters are implicated in maintaining stem cell integrity. Instead, a distinct ABC transporter expression signature may be essential for stem cell function. The high expression of specific transporters in non-HSCs and mature blood cells suggests a specialized, cell type dependent function and warrants further functional experiments to determine their exact roles in cellular (patho)physiology.

A quest for initiating cells of head and neck cancer and their treatment

The biology of head and neck squamous cell carcinomas (HNSCC) and other cancers have been related to cancer stem-like cells (CSC). Specific markers, which vary considerably depending on tumor type or tissue of origin, characterize CSC. CSC are cancer initiating, sustaining and mostly quiescent. Compared to bulk tumors, CSC are less sensitive to chemo- and radiotherapy and may have low immunogenicity. Therapeutic targeting of CSC may improve clinical outcome. HNSCC has two main etiologies: human papillomavirus, a virus infecting epithelial stem cells, and tobacco and alcohol abuse. Here, current knowledge of HNSCC-CSC biology is reviewed and parallels to CSC of other origin are drawn where necessary for a comprehensive picture.

ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation

Elevated leukocyte cell numbers (leukocytosis), and monocytes in particular, promote atherosclerosis; however, how they become increased is poorly understood. Mice deficient in the adenosine triphosphate-binding cassette (ABC) transporters ABCA1 and ABCG1, which promote cholesterol efflux from macrophages and suppress atherosclerosis in hypercholesterolemic mice, displayed leukocytosis, a transplantable myeloproliferative disorder, and a dramatic expansion of the stem and progenitor cell population containing Lin(-)Sca-1(+)Kit+ (LSK) in the bone marrow. Transplantation of Abca1(-/-) Abcg1(-/-) bone marrow into apolipoprotein A-1 transgenic mice with elevated levels of high-density lipoprotein (HDL) suppressed the LSK population, reduced leukocytosis, reversed the myeloproliferative disorder, and accelerated atherosclerosis. The findings indicate that ABCA1, ABCG1, and HDL inhibit the proliferation of hematopoietic stem and multipotential progenitor cells and connect expansion of these populations with leukocytosis and accelerated atherosclerosis.

ABC transporters in cancer: more than just drug efflux pumps

Multidrug transporter proteins are best known for their contributions to chemoresistance through the efflux of anticancer drugs from cancer cells. However, a considerable body of evidence also points to their importance in cancer extending beyond drug transport to fundamental roles in tumour biology. Currently, much of the evidence for these additional roles is correlative and definitive studies are needed to confirm causality. We propose that delineating the precise roles of these transporters in tumorigenesis and treatment response will be important for the development of more effective targeted therapies.

The therapeutic promise of the cancer stem cell concept

Cancer stem cells (CSCs) are a subpopulation of tumor cells that selectively possess tumor initiation and self-renewal capacity and the ability to give rise to bulk populations of nontumorigenic cancer cell progeny through differentiation. As we discuss here, they have been prospectively identified in several human malignancies, and their relative abundance in clinical cancer specimens has been correlated with malignant disease progression in human patients. Furthermore, recent findings suggest that clinical cancer progression driven by CSCs may contribute to the failure of existing therapies to consistently eradicate malignant tumors. Therefore, CSC-directed therapeutic approaches might represent translationally relevant strategies to improve clinical cancer therapy, in particular for those malignancies that are currently refractory to conventional anticancer agents directed predominantly at tumor bulk populations.

Targeting leukemia stem cells: The new goal of therapy in adult acute myeloid leukemia

The most popular view of hematopoietic cell lineage organization is that of complex reactive or adaptative systems. Leukemia contains a subpopulation of cells that display characteristics of stem cells. These cells maintain tumor growth. The properties of leukemia stem cells indicate that current conventional chemotherapy, directed against the bulk of the tumor, will not be effective. Leukemia stem cells are quiescent and do not respond to cell cycle-specific cytotoxic agents used to treat leukemia and thus contribute to treatment failure. New strategies are required that specifically target this malignant stem cell population.

Relationships between multidrug resistance (MDR) and stem cell markers in human chronic myeloid leukemia cell lines

The K562 cell line (chronic myeloid leukemia), sensitive to chemotherapy (non-MDR), and the Lucena cell line, resistant to chemotherapy (MDR) were investigated. The results suggest that both cell lines possess CD34+CD38- profiles of hematopoietic stem cell markers. The promoter regions of ABCB1, ABCG2 and ABCC1 genes contain binding sites for the Oct-4 transcripton factor, which is also considered a marker of tumor stem cells. Lucena cells showed an over-expression of the ABCB1 gene and a high expression of the Oct-4, ABCG2 and ABCC1 genes as compared to K562 cells.