Arsenic Trioxide-Mediated Growth Inhibition of Myeloma Cells Is Associated with An Extrinsic Signaling Pathway Involved in Caspase 3 and Caspase 8.

Abstract 4899

Arsenic trioxide (ATO) is a well-known inhibitor of cell proliferation in certain forms of malignancy and has been successfully used in the treatment of acute promyelocytic leukemia. Preclinical and clinical studies showed that ATO has anti-myeloma effects both as a single agent and in the combination therapy; however, the underlying molecular mechanism remains elusive. This study was performed to evaluate the molecular mechanism underlying its anti-myeloma activities. Cells from OPM2, U266, RPMI8226 myeloma cell lines and patients diagnosed with myeloma (n=6) were cultured with various concentrations of ATO for 4 days. Cell growth and viability were assayed by trypan blue dye exclusion. Cell cycle and apoptosis were analyzed by flow cytometry using CellQuest software and Vybrant Apoptosis Assay Kit. Alterations of the signaling pathways induced by ATO were tested by real-time PCR and western blot. ATO induced potent inhibition of myeloma cell growth compared with untreated control cells. Further investigation showed that ATO down-regulated c-Myc and phosphorylated (p)-Rb, while it up-regulated p53, p21^Clip1, and p27^Kip1 proteins, resulting in G2/M cell cycle arrest and cell growth inhibition. ATO treatment increased mRNA levels of interferon regulatory factor-1 (IRF-1) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), as well as protein levels of caspase 8 and cleaved caspase 3, indicating involvement of the extrinsic apoptotic pathway. No significant change was detected in the expression levels of Bax, Bcl-xL caspase 9 and Bcl-2, indicating that the intrinsic signaling pathway was not involved. A pan-caspase inhibitor abrogated ATO-induced apoptosis of myeloma cells. Our data suggest that ATO induces apoptosis in MM cells most likely through an extracellular signaling pathway.