Breast Cancer Resistance Protein in Drug Resistance of Primitive CD34+38− Cells in Acute Myeloid Leukemia

Abstract

Purpose: Acute myeloid leukemia (AML) is considered a stem cell disease. Incomplete chemotherapeutic eradication of leukemic CD34+38À À stem cells is likely to result in disease relapse. The purpose of this study was to investigate the role of the breast cancer resistance protein (BCRP/ATP-binding cassette, subfamily G, member 2) in drug resistance of leukemic stem cells and the effect of its modulation on stem cell eradication in AML.Experimental Design: BCRP expression (measured flowcytometrically using the BXP21 monoclonal antibody) and the effect of its modulation (using the novel fumitremorgin C analogue KO143) on intracellular mitoxantrone accumulation and in vitro chemosensitivity were assessed in leukemic CD34+38À À cells.Results: BCRP was preferentially expressed in leukemic CD34+38À À cells and blockage of BCRP-mediated drug extrusion by the novel fumitremorgin C analogue KO143 resulted in increased intracellular mitoxantrone accumulation in these cells in the majority of patients. This increase, however, was much lower than in the mitoxantrone-resistant breast cancer cell line MCF7-MR and significant drug extrusion occurred in the presence of BCRP blockage due to the presence of additional drug transport mechanisms, among which ABCB1 and multiple drug resistance protein. In line with these findings, selective blockage of BCRP by KO143 did not enhance in vitro chemosensitivity of leukemic CD34+38À À cells.Conclusions: These results show that drug extrusion from leukemic stem cells is mediated by the promiscuous action of BCRP and additional transporters. Broadspectrum inhibition, rather than modulation of single mechanisms, is therefore likely to be required to circumvent drug resistance and eradicate leukemic stem cells in AML.

ATP-binding-cassette transporters in hematopoietic stem cells and their utility as therapeutical targets in acute and chronic myeloid leukemia

“…63,64 In elucidating the role of ABCG2 in the redundant mitoxantrone efflux, it was found that ABCG2 is preferentially expressed in leukemic CD34 þ 38À stem cells and contributes to mitoxantrone efflux in these cells. 65,66 ABCG2 expression and efflux in leukemic CD34 þ 38À cells are not different from expression and function in normal CD34 þ 38À cells, thus reflecting a conserved physiological function of ABCG2 in primitive hematopoietic cells rather than being a leukemiaassociated phenotype. Selective blockage of ABCG2-mediated drug extrusion by the novel fumitremorgin (FTC) C analogue KO143, the most potent ABCG2 inhibitor currently available, 67 resulted in increased intracellular mitoxantrone accumulation in leukemic CD34 þ 38À cells in most patients but did not increase chemosensitivity of these cells to this agent.…”

“…Selective blockage of ABCG2-mediated drug extrusion by the novel fumitremorgin (FTC) C analogue KO143, the most potent ABCG2 inhibitor currently available, 67 resulted in increased intracellular mitoxantrone accumulation in leukemic CD34 þ 38À cells in most patients but did not increase chemosensitivity of these cells to this agent. 65 This lack of a chemosensitizing effect of ABCG2 modulation by KO143 was anticipated by the observation of substantial drug efflux in leukemic CD34 þ 38À cells in the presence of ABCG2 inhibition, conferred by other drug transporters, among which are ABCB1 and ABCC1.…”

“…This is similar to the situation in AML stem cells where ABCG2 has only a minor effect on drug accumulation and inhibition of ABCG2 does not lead to increased cytotoxicity, due to the presence of redundant (partly drug efflux mediated) resistance mechanisms. 65 Together, the current knowledge may temper expectations about the efficacy of ABCB1 or ABCG2 inhibition as a strategy to eradicate CML stem cells in chronic phase of the disease.…”

Cancer stem cells: Models, mechanisms and implications for improved treatment

“…As discussed above, cancer stem cells usually have high expression of ABC transporters, which could be an inherent protective mechanism possessed by cancer stem cells against toxic substances and lack of oxygen and is also one of the major reasons accounting for cancer stem cell drug resistance. 67,69,70,72,93,100,101,107,110 It has been demonstrated that the side population (SP) phenotype can be blocked by some ABC transporter specific inhibitors, including verapamil, fumitremorgin C (FTC) and reserpine. 73,77,79 It is reasonable that inhibition of ABC transporter expression could at least partially reverse the drug resistance phenotype of cancer stem cells.…”

Flow cytometry‐based assessment of mitoxantrone efflux from leukemic blasts varies with response to induction chemotherapy in acute myeloid leukemia

“…Mitoxantrone was selected as the ABC transporter substrate since its efflux from AML cells can be mediated by various transporters including both MDR1 and BCRP1. In addition, our early comparison of mitoxantrone with DNR and other reports suggested assays using the former drug would be more sensitive for measurement of drug efflux (43,55). Cyclosporine A was chosen as the transporter inhibitor as it inhibits the function of both MDR1 and BCRP1.…”