Received for publication July 19, 2005.
Revised September 14, 2005.
Accepted for publication September 14, 2005.

Arsenic trioxide induces apoptosis of human monocytes during macrophagic differentiation through NF-B-related survival pathway downregulation

Abstract

Arsenic trioxide (As₂O₃) is known to be toxic towards leukemia cells. In this study, we determined its effects on survival of human monocytic cells during macrophagic differentiation, an important biological process involved in the immune response. As₂O₃, used at clinically relevant pharmacological concentrations, induced marked apoptosis of human blood monocytes during differentiation with either granulocyte-macrophage colony-stimulating factor (GM-CSF) or M-CSF. Apoptosis of monocytes was associated with increased caspase activities and decreased DNA binding of p65 NF-B; like As₂O₃, the selective NF-B inhibitor Bay 11-7082 strongly reduced survival of differentiating monocytes. The role of NF-B in arsenic toxicity was also studied in promonocytic U937 cells during PMA-induced macrophagic differentiation. In these cells, As₂O₃ first reduced DNA binding of p65 NF-B and subsequently induced apoptosis. In addition, overexpression of the p65 NF-B subunit, following stable infection with a p65 retroviral expressing vector, increased survival of As₂O₃-treated U937 cells. As₂O₃ specifically decreased protein levels of X-linked inhibitor of apoptosis protein (XIAP) and FLICE-inhibitory protein (FLIP), two NF-B-regulated genes in both U937 cells and blood monocytes during their differentiation. Finally, As₂O₃ was found to inhibit macrophagic differentiation of monocytic cells, when used at cytotoxic concentrations; however, overexpression of the p65 NF-B subunit in U937 cells reduced its effects towards differentiation. In contrast to monocytes, well-differentiated macrophages were resistant to low concentrations of As₂O₃. Altogether, our study demonstrates that clinically relevant concentrations of As₂O₃ induced marked apoptosis of monocytic cells during in vitro macrophagic differentiation likely through inhibition of NF-B-related survival pathways.

Key words: NF-kappaB, apoptosis, arsenic, differentiation, human monocytes, macrophages