Differentiation Of Leukemic Stem Cells Research Articles

Long-term treatment effects in chronic myeloid leukemia.

We propose and analyze a simplified version of a partial differential equation (PDE) model for chronic myeloid leukemia (CML) derived from an agent-based model proposed by Roeder et al. This model describes the proliferation and differentiation of leukemic stem cells in the bone marrow and the effect of the drug Imatinib on these cells. We first simplify the PDE model by noting that most of the dynamics occurs in a subspace of the original 2D state space. Then we determine the dominant eigenvalue of the corresponding linearized system that controls the long-term behavior of solutions. We mathematically show a non-monotonous dependence of the dominant eigenvalue with respect to treatment dose, with the existence of a unique minimal negative eigenvalue. In terms of CML treatment, this shows that there is a unique dose that maximizes the decay rate of the CML tumor load over long time scales. Moreover this unique dose is lower than the dose that maximizes the initial tumor load decay. Numerical simulations of the full model confirm that this phenomenon is not an artifact of the simplification. Therefore, while optimal asymptotic dosage might not be the best one at short time scales, our results raise interesting perspectives in terms of strategies for achieving and improving long-term deep response.

Antagonizing Self-Renewal in Acute Myeloid Leukemia: ID2 Takes the Stage

E proteins and their functional antagonists, the ID proteins, have significant roles in normal hematopoiesis. In this issue of Cancer Cell, Ghisi etal. show that high ID2 levels antagonize self-renewal and promote differentiation of leukemic stem cells in the MLL-translocated molecular subtype of acute myeloid leukemia.

Mir-99 Is a Regulator of Hematopoietic and Leukemic Stem Cell Differentiation

MicroRNAs play an important role in the regulation of normal and malignant stem cells of the hematopoietic system. Analysis of microRNA-Seq data from 168 acute myeloid leukemia (AML) patients in the TCGA database reveals widespread dysregulation of miRNA expression, including miR-99b. Interestingly, miR-99b expression inversely correlates with differentiation status of the AMLs as determined by the French-American-British classification scheme, suggesting miR-99 is a negative regulator of differentiation. We also have shown that miR-99a and miR-99b are highly expressed in mouse hematopoietic stem cells (HSCs) compared to their more differentiated progeny. Thus, we hypothesized that miR-99 is a regulator of differentiation in normal and leukemic stem cells.To test the function of miR-99 in normal hematopoiesis, we knocked down miR-99a or miR-99b in mouse HSCs (Lin-cKit+Sca1+CD34-SLAM+) using lentiviral vectors, resulting in ~3.5 fold reduction in the number of colonies formed in methylcellulose assays upon secondary plating (P=0.01), with the average colony being approximately 2 times smaller as measured by the number of cells (P=0.03). Consistently, transplantation assays demonstrated >10-fold reduction in long-term engraftment capacity of HSCs upon miR-99 knockdown (KD) at 16 weeks (P=0.0004). Analysis of peripheral blood of the transplanted mice revealed ~3-fold increase in the proportion of donor-derived myeloid cells, suggesting that miR-99 may regulate the self-renewal and/or limit differentiation of HSPCs (P=0.01). Consistent with these observations, gene set enrichment analysis (GSEA) using the RNA-sequencing data generated from hematopoietic stem and progenitor cells (HSPCs) with miR-99 KD revealed significant enrichment for the myeloid differentiation gene signature (Lindstedt-dendritic-cell-maturation, Nominal p value =0.0, FDR q value=0.02). Moreover, analysis of GFP+ miR-99 KD cells after second plating revealed a 3-fold increase in apoptosis as measured by caspase 3 activation. To better understand the mechanism mediating the reduced reconstitution capacity of miR-99 KD HSCs, we analyzed the composition of progenitors in the bone marrow of transplanted mice. miR-99 KD resulted in a highly significant decrease in the percentage of myeloid progenitors of donor-derived cells 16 weeks post transplantation (60% decrease, P=0.01), consistent with miR-99 KD inducing HSPC differentiation. Overall, these data suggest that miR-99 is a negative regulator of HSPC differentiation.Consistent with a role in maintaining LSCs, miR-99 KD in the AML cell lines MonoMac6 and U937 induced expression of myeloid differentiation markers including CD15, CD14 and CD13. This was accompanied by a 2-50 fold reduction in cell growth depending on the AML cell line tested and a significant increase in apoptosis; the levels of miR-99 expression correlated with the degree of the growth defect. In addition, concomitant expression of miR-99b KD and MLL-AF9 overexpression vectors in HSPCs followed by serial methocult assays lead to decreased colony formation in third platings. We also observed a 50% reduction in the percentage of Lin-Sca-1-c-Kit+ myeloid progenitors in secondarily plated cells (P value=0.01). These data suggest in MLL-AF9+ leukemia, miR-99 acts to restrain cell growth and maintain the undifferentiated state of leukemic blasts. Together, these data indicate that miR-99a/b regulates the proliferation of normal and leukemic stem cells by negatively regulating apoptosis and maintaining the stem cell state. DisclosuresNo relevant conflicts of interest to declare.

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.

Regulation of hematopoietic and leukemic stem cells by the immune system.

Hematopoietic stem cells (HSCs) are rare, multipotent cells that generate via progenitor and precursor cells of all blood lineages. Similar to normal hematopoiesis, leukemia is also hierarchically organized and a subpopulation of leukemic cells, the leukemic stem cells (LSCs), is responsible for disease initiation and maintenance and gives rise to more differentiated malignant cells. Although genetically abnormal, LSCs share many characteristics with normal HSCs, including quiescence, multipotency and self-renewal. Normal HSCs reside in a specialized microenvironment in the bone marrow (BM), the so-called HSC niche that crucially regulates HSC survival and function. Many cell types including osteoblastic, perivascular, endothelial and mesenchymal cells contribute to the HSC niche. In addition, the BM functions as primary and secondary lymphoid organ and hosts various mature immune cell types, including T and B cells, dendritic cells and macrophages that contribute to the HSC niche. Signals derived from the HSC niche are necessary to regulate demand-adapted responses of HSCs and progenitor cells after BM stress or during infection. LSCs occupy similar niches and depend on signals from the BM microenvironment. However, in addition to the cell types that constitute the HSC niche during homeostasis, in leukemia the BM is infiltrated by activated leukemia-specific immune cells. Leukemic cells express different antigens that are able to activate CD4+ and CD8+ T cells. It is well documented that activated T cells can contribute to the control of leukemic cells and it was hoped that these cells may be able to target and eliminate the therapy-resistant LSCs. However, the actual interaction of leukemia-specific T cells with LSCs remains ill-defined. Paradoxically, many immune mechanisms that evolved to activate emergency hematopoiesis during infection may actually contribute to the expansion and differentiation of LSCs, promoting leukemia progression. In this review, we summarize mechanisms by which the immune system regulates HSCs and LSCs.

Identification of small molecules that support human leukemia stem cell activity ex vivo

Leukemic stem cells (LSCs) are considered a major cause of relapse in acute myeloid leukemia (AML). Defining pathways that control LSC self-renewal is crucial for a better understanding of underlying mechanisms and for the development of targeted therapies. However, currently available culture conditions do not prevent spontaneous differentiation of LSCs, which greatly limits the feasibility of cell-based assays. To overcome these constraints we conducted a high-throughput chemical screen and identified small molecules that inhibit differentiation and support LSC activity in vitro. Similar to reports with cord blood stem cells, several of these compounds suppressed the aryl-hydrocarbon receptor (AhR) pathway, which we show to be inactive in vivo and rapidly activated ex vivo in AML cells. We also identified a compound, UM729, that collaborates with AhR suppressors in preventing AML cell differentiation. Together, these findings provide newly defined culture conditions for improved ex vivo culture of primary human AML cells.

A set of miRNAs that involve in the pathways of drug resistance and leukemic stem-cell differentiation is associated with the risk of relapse and glucocorticoid response in childhood ALL

Relapse is a major challenge in the successful treatment of childhood acute lymphoblastic leukemia (ALL). Despite intensive research efforts, the mechanisms of ALL relapse are still not fully understood. An understanding of the molecular mechanisms underlying treatment outcome, therapy response and the biology of relapse is required. In this study, we carried out a genome-wide microRNA (miRNA) microarray analysis to determine the miRNA expression profiles and relapse-associated miRNA patterns in a panel of matched diagnosis–relapse or diagnosis–complete remission (CR) childhood ALL samples. A set of miRNAs differentially expressed either in relapsed patients or at diagnosis compared with CR was further validated by quantitative real-time polymerase chain reaction in an independent sample set. Analysis of the predicted functions of target genes based on gene ontology ‘biological process’ categories revealed that the abnormally expressed miRNAs are associated with oncogenesis, classical multidrug resistance pathways and leukemic stem cell self-renewal and differentiation pathways. Several targets of the miRNAs associated with ALL relapse were experimentally validated, including FOXO3, BMI1 and E2F1. We further investigated the association of these dysregulated miRNAs with clinical outcome and confirmed significant associations for miR-708, miR-223 and miR-27a with individual relapse-free survival. Notably, miR-708 was also found to be associated with the in vivo glucocorticoid therapy response and with disease risk stratification. These miRNAs and their targets might be used to optimize anti-leukemic therapy, and serve as novel targets for development of new countermeasures of leukemia. This fundamental study may also contribute to establish the mechanisms of relapse in other cancers.

Interactions between hematopoietic growth factors in normal and leukemic stem cell differentiation.

Interactions between hematopoietic growth factors in normal and leukemic stem cell differentiation.