Wnt pathway contributes to the protection by bone marrow stromal cells of acute lymphoblastic leukemia cells and is a potential therapeutic target
Highlights
► Bone marrow stromal cells. ► Vasculature (blood vessels). ► Cytokines and chemokines. ► Endothelial cells. ► Osteoblasts.
Introduction
Acute lymphoblastic leukemia (ALL), one of the most aggressive hematological malignancies, affects patients of all ages [1]. Recent advances in our understanding of this disease and the development of new therapies have greatly improved outcomes in ALL patients. Nevertheless, a substantial number of ALL patients experience disease relapse and treatment resistance, leading to death from the disease. Although large prospective clinical trials have reported complete remission rates of 78–93% in adults, only about 40% of patients experience long-term disease-free survival [2]. Minimal residual disease, which is strongly associated with relapse, is a major challenge in treating ALL [3].
A growing amount of evidence suggests that disease relapse and treatment resistance in ALL patients are largely due to the protection of leukemia cells by various components of the bone marrow microenvironment, especially marrow stromal cells (MSCs) [4]. It was shown that primary ALL cells cultured in vitro underwent spontaneous apoptosis, whereas ALL cells cultured with MSCs in a serum-free environment underwent less spontaneous apoptosis [5]. Indirect communication through extracellularly secreted growth factors [6] and direct contact between leukemia cells and MSCs appear to be essential to ALL cell survival and chemoresistance [7]. The interaction between ALL cells and MSCs may trigger molecular changes that lead ALL cells to chemotherapy resistance.
Multiple pathways and factors are involved in the protection of leukemia cells by MSCs. For example, CXCR4, which is expressed on leukemia blasts and interacts with CXCL12 (which is secreted by MSCs), was found to be necessary for leukemia cell survival and proliferation [6], and MSC-induced activation of phosphoinositide 3-kinase (PI3K)/Akt signaling plays a critical role in the chemoresistance of acute myeloid leukemia [8]. Aberrant activation of the Wnt pathway has been observed in hematological malignancies [9], [10]. However, the role of Wnt pathway in the cross-talk between ALL cells and MSCs is uncertain and the effect of Wnt pathway inhibition on ALL development and progression in vivo needs to be assessed.
In the present study, we determined the effects of human and murine MSCs on the survival and drug resistance of human ALL cell lines and primary ALL cells. Our results indicated that MSCs induce activation of the Wnt pathway in ALL cells and that this activation contributes to the survival of ALL cells. An important finding was that blocking the Wnt pathway with a pharmacologic inhibitor partially overcame MSC protection of ALL cells both in vitro and in vivo.
Section snippets
Cell culture
This study was approved by the institutional review board of The University of Texas MD Anderson Cancer Center. Residual peripheral blood samples of nine patients with ALL were obtained from an institutional tumor bank with informed consent. Peripheral blood mononuclear cells were isolated using Ficoll-HyPaque (Sigma, St. Louis, MO, USA). All experiments were carried out using freshly isolated cells.
The human leukemia cell lines Reh [11], RS4;11 [12], and SEMK2 [13] were provided by Dr. Patrick
MSCs protected ALL cells from spontaneous and Ara-C-induced apoptosis
To establish an MSC-ALL cell co-culture system, we cultured three leukemia cell lines (Reh, RS4;11, and SEMK2) with or without MSCs in the presence or absence of 1 μM Ara-C. At 24, 48, and 72 h, the mean apoptotic rates of Reh cells treated with Ara-C in medium alone were 16.6 ± 5.1%, 42.7 ± 7.9%, and 58.1 ± 2.1%, respectively; those of Reh cells treated with Ara-C in the presence of the murine MSC line M2-10B4 were 8.9 ± 2.1%, 13.2 ± 2.7%, and 33.1 ± 5.1%, respectively; and those of Reh cells treated with
Discussion
The bone marrow microenvironment plays an important role in the initiation and progression of leukemia as well as in the persistence of minimal residual disease and disease relapse [19]. Various components of this microenvironment, particularly MSCs, regulate ALL survival, proliferation, and drug resistance by producing growth factors, cytokines, and intracellular signals by direct contact with the cells [6], [20]. Disrupting the cross-talk between leukemia cells and their milieu is of ultimate
Authorship and disclosures
Y.Y., Y.G., Z.C., and X.S. initiated the study, designed the experiments, and wrote the paper. Y.Y., S.M., B.S., J.Z., S.K., and J.S.L. performed the experiments and statistical analyses. The authors have no conflicts of interest to disclose.
Acknowledgements
We thank Dr. J. G. Wen at Methodist Hospital, Houston, Texas, USA, for the human bone marrow MSC cell line HS; Dr. Patrick Zweidler-McKay at MD Anderson for the human leukemia cell lines Reh, RS4;11, and SEMK2, Dr. Jan Burger at MD Anderson for the murine MSC line M2-10B4, and Dr. Roel Nusse at Stanford University for 7TGC reporter vector (obtained from Addgene; plasmid-24304). This work was supported in part by Research Scholar Grant 119645-RSG-10-131-01-DDC to X.S. from the American Cancer
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