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Previously, we found a significant increase in TRAIL (TNFα-related apoptosis-inducing ligand) resistance of acute myeloid leukemia (AML) cells in multicellular aggregates in vitro that simulate the location of leukemia cells in the bone marrow. In this study, we tried to assess whether the increased resistance of the AML cells to the cytotoxic effect of TRAIL in multicellular aggregates in vitro can be associated with the launch of mechanisms mediated by cell differentiation or the acquisition of a pro-inflammatory phenotype. The phenotype of THP-1 cells in aggregated and disaggregated cultures was studied in comparison with the phenotype of a disaggregated THP-1 cell culture exposed to factors that induce monocyte-like differentiation or pro-inflammatory phenotype. It was found that an increase in the TRAIL resistance of the THP-1 cells in the aggregated culture was accompanied by the appearance of signs characteristic of monocyte-like differentiation and a pro-inflammatory phenotype. The results are of interest for developing approaches for suppressing TRAIL resistance of AML cells in human bone marrow.

—The resistance of leukemic cells to the action of chemotherapeutic drugs is the main reason for insufficient efficacy in the treatment of acute myeloid leukemia. Microenvironmental factors play one of the key roles in the formation of the phenotype of drug resistance of leukemic cells. The elucidation of the nature of drug resistance mediated by the conditions of the microenvironment is important for the identification of new pharmacological targets for conservative directed therapy. We conducted a study of the role of differentiation in the macrophage direction in increasing the drug resistance of acute myeloid leukemia cells in multicellular aggregates. It was shown that maturation does not occur in the macrophage direction in multicellular aggregates. These data indicate that other mechanisms contribute to an increased drug resistance of acute myeloid leukemia cells in multicellular aggregates.

—A comparative study of the expression of pro- and anti-apoptotic receptors to cytokine TRAIL (TNF-related apoptosis-inducing ligand) on immature blast cells of acute myeloid leukemia, on the cells of acute myeloid leukemia differentiated into monocyte-like and macrophage-like cells, and on normal monocytes and macrophages has been performed. It has been shown that a decrease in the expression of proapoptotic receptors to TRAIL and the acquisition of resistance to TRAIL-induced apoptosis occurs not only during the differentiation of acute myeloid leukemia cells into monocyte-like and macrophage-like cells, but also during the maturation of normal monocytes into macrophages. Thus, the differentiation of both acute myeloid leukemia cells and normal myeloid cells in the direction of monocyte-macrophages mediates a decrease in the expression of proapoptotic receptors to TRAIL, which in turn can determine the resistance of normal myeloid cells to TRAIL-induced apoptosis.

Background: Therapeutic efficiency in treatment of acute myeloid leukemia (AML) ranges from 20 to 45%. One of the causes of the latter is a drug resistance acquired by leukemic cells under the influence of treatment with antitumor medicines. More important cause is a development of the primary resistance of myeloid leukemic cells to induction of cellular death associated with elemental microenvironment within the bone marrow. Studying primary resistance is very important, and first of all, to prevent development of drug resistance of leukemic cells and, correspondingly, to increase the efficiency of medicamental therapy. Aim: To study the mechanisms of primary resistance of AML cells to induction of cellular death. Materials and methods: Human AML cells of THP-1 line and mononuclear cells of the bone marrow were used in the study of patients with diagnosed AML. Multicellular aggregates were formed during cell cultivating on the 1.5% agarose. To cut off intercellular adhesion, the cells were cultivated in the medium with methylcellulose (0.9%). The viability of the cells was assessed by reduction of Alamar Blue indicator. Results: Within multicellular aggregates, about 75±5% of THP-1 cells were resistant to the activity of recombinant protein izTRAIL, 70±5% – to etoposide, and 40±7% – to sorafenib. Cutting off intercellular contacts decreased the resistance to them. Within multicellular aggregates of primary mononuclear cells, 45±5% of cells were resistant to sorafenib, 57±4% – to etoposide, and all cells were resistant to izTRAIL. Cutting off intracellular adhesion reduced the resistance to sorafenib and etoposide but not to izTRAIL. Conclusion: In multicellular aggregates, AML cells of THP-1 line and mononuclear cells of the bone marrow showed increased resistance to activity of recombinant protein izTRAIL, etoposide, and sorafenib. Diminishing intracellular adhesion in the medium including methylcellulose decreases cellular resistance to cytotoxic agents.

Suppression of resistance in acute myeloid leukemia cells to TRAIL-induced apoptosis in multicellular aggregates, was studied using small molecule inhibitors of the activation of the transcription factor NF-kB - NF-k9 Activation Inhibitor IV and JSH-23 at non-toxic concentrations. NF-kB Activation Inhibitor IV and JSH-23 reduced resistance in the acute myeloid leukemia cells in multicellular aggregates to cytotoxic action of recombinant protein izTRAIL. It is shown that the use of these inhibitors decreased the phosphorylation of the RelA (p65) as a main marker activation of the transcription factor NF-kB. We discuss a possible reason for increasing resistance in acute myeloid leukemia cells to TRAIL-induced apoptosis in multicellular aggregates.