The Increase in the Drug Resistance of Acute Myeloid Leukemia THP-1 Cells in High-Density Cell Culture Is Associated with Inflammatory-like Activation and Anti-Apoptotic Bcl-2 Proteins.

It is known that cell culture density can modulate the drug resistance of acute myeloid leukemia (AML) cells. In this work, we studied the drug sensitivity of AML cells in high-density cell cultures (cell lines THP-1, HL-60, MV4-11, and U937). It was shown that the AML cells in high-density cell cultures in vitro were significantly more resistant to DNA-damaging drugs and recombinant ligand izTRAIL than those in low-density cell cultures. To elucidate the mechanism of the increased drug resistance of AML cells in high-density cell cultures, we studied the activation of Bcl-2, Hif-1alpha, and NF-kB proteins, as well as cytokine secretion, the inflammatory immunophenotype, and the transcriptome for THP-1 cells in the low-density and high-density cultures. The results indicated that the increase in the drug resistance of proliferating THP-1 cells in high-density cell cultures was associated with the accumulation of inflammatory cytokines in extracellular medium, and the formation of NF-kB-dependent inflammatory-like cell activation with the anti-apoptotic proteins Bcl-2 and Bcl-xl. The increased drug resistance of THP-1 cells in high-density cultures can be reduced by ABT-737, an inhibitor of Bcl-2 family proteins, and by inhibitors of NF-kB. The results suggest a mechanism for increasing the drug resistance of AML cells in the bone marrow and are of interest for developing a strategy to suppress this resistance.

Macrophage-like THP-1 Cells Derived from High-Density Cell Culture Are Resistant to TRAIL-Induced Cell Death via Down-Regulation of Death-Receptors DR4 and DR5.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2L) is a highly selective and promising anticancer agent due to its specific apoptosis-inducing effect on tumor cells, rather than most normal cells. TRAIL is currently under investigation for use in the treatment of leukemia. However, the resistance of leukemic cells to TRAIL-induced apoptosis may limit its efficacy. The mechanisms of leukemic cell resistance to antitumor immunity remains a topical issue. In this work, we have found an increase in the resistance to TRAIL-induced cell death in human leukemia THP-1 cells, which was caused by differentiation into a macrophage-like phenotype in high-density culture in vitro. Stressful conditions, manifested by the inhibition of cell growth and the activation of cell death in high-density culture of THP-1 cells, induced the appearance of cells adhered to culture dishes. The THP-1ad cell line was derived by selection of these adhered cells. The genetic study, using STR and aCGH assays, has shown that THP-1ad cells were derived from THP-1 cells due to mutagenesis. The THP-1ad cells possessed high proliferative potential and a macrophage-like immunophenotype. The adhesion of THP-1ad cells to the extracellular matrix was mediated by αVß5 integrin. The cytokine production, as well as the rise of intracellular ROS and NO activities by LPS in THP-1ad cell culture, were characteristic of macrophage-like cells. The THP-1ad cells were found to appear to increase in resistance to TRAIL-induced cell death in comparison with THP-1 cells. The mechanism of the increase in TRAIL-resistance can be related to a decrease in the expression of death receptors DR4 and DR5 on the THP-1ad cells. Thus, the macrophage-like phenotype formation with the maintenance of a high proliferative potential of leukemic cells, caused by stress conditions in high-density cell cultures in vitro, can induce an increase in resistance to TRAIL-induced cell death due to the loss of DR4 and DR5 receptors. The possible realization of these events in vivo may be the reason for tumor progression.