Biochemical and Biophysical Research Communications
Salinomycin overcomes ABC transporter-mediated multidrug and apoptosis resistance in human leukemia stem cell-like KG-1a cells
Introduction
Cancer stem cells comprise a unique subpopulation of tumor cells that possess tumor initiation and self-renewal capacity and the ability to give rise to the heterogeneous lineages of cancer cells that make up the bulk of the tumor [1]. Cancer stem cells have been identified in a variety of human neoplasias, including cancers of the blood, breast, brain, bone, skin, liver, bladder, ovary, prostate, colon and pancreas [1], [2]. The first evidence for the existence of cancer stem cells came from studies showing that a subpopulation of CD34+ CD38− leukemia blasts isolated from the bone marrow and peripheral blood of humans with acute myeloid leukemia (AML) was able to home to the bone marrow and to initiate human AML in immunocompromised NOD/SCID mice, thereby defining AML stem cells [3], [4].
Like other types of cancer stem cells, AML stem cells display numerous mechanisms of resistance to chemotherapeutic drugs and irradiation therapy, allowing them to survive current cancer therapies and to initiate relapse, long-term tumor recurrence and metastasis [5], [6].
One of the most important mechanism of drug resistance in AML stem cells and other cancer stem cells is the expression of ATP-binding cassette (ABC) transporters which belong to a highly conserved superfamily of transmembrane proteins capable of exporting a wide variety of molecules and structurally unrelated chemotherapeutic drugs from the cytosol, thereby conferring multidrug resistance to the cells [5], [7], [8], [9], [10], [11].
As shown previously, the human undifferentiated promyeloblastic leukemia cell line KG-1a closely resembles the characteristics of AML stem cells, including expression of CD34 and lack of expression of CD38 and other hematopoietic markers [12], [13], resistance to myeloid differentiation [14], [15] and expression of various ABC transporters conferring resistance to a broad spectrum of chemotherapeutic drugs [16]. Therefore, KG-1a cells can serve as a model for studying the properties of leukemia stem cells [17], particularly with regard to multidrug resistance mediated by ABC transporters. We exploited the KG-1a leukemia stem cell model to investigate the anti-leukemia stem cell activity of salinomycin, a polyether antibiotic isolated from Streptomyces albus that acts in different biological membranes as a ionophore with a strong selectivity for potassium [18], [19]. Salinomycin has recently been shown to selectively deplete human breast cancer stem cells from tumorspheres [20] and to induce apoptosis in human cancer cells of different origin that display multiple mechanisms of drug and apoptosis resistance [21]. We show now that salinomycin is able to induce massive apoptosis in KG-1a leukemia stem cell-like cells which exhibit apoptosis resistance and resistance to a broad spectrum of chemotherapeutic drugs by virtue of expressing different ABC transporters.
Section snippets
Reagents and antibodies
Salinomycin, rhodamine 123 and cyclosporine A were purchased from Sigma (Taufkirchen, Germany). Phenylbutyrate was purchased from Enzo Life Sciences (Lörrach, Germany). Bortezomib (PS-341, Velcade®) was purchased from Millenium Pharmaceuticals (Cambridge, USA). N-Benzyloxycarbonyl-l-valyl-l-alanyl-l-aspartate fluoromethylketone (Z-VAD-fmk) was purchased from Bachem (Heidelberg, Germany). Salinomycin was dissolved either in ethanol 95% (B. Braun, Melsungen, Germany) or in DMSO, resulting in
KG-1a cells exhibit characteristics of leukemia stem cells
Human acute myelogenous leukemia (AML) stem cells have been characterized as CD34+ cells, which lack the expression of CD38 and other myeloid markers and which have the ability to self-renew and to initiate human AML in NOD/SCID mice [3], [4]. AML stem cells have also been shown to express various ABC transporters conferring resistance to a broad spectrum of chemotherapeutic drugs [9], [10]. First we show by flow cytometry analysis that human myeloblastic leukemia KG-1 cells express both CD34
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