Melphalan-Induced Apoptosis of EBV-Transformed B Cells through Upregulation of TAp73 and XAF1 and Nuclear Import of XPA

Abstract

Melphalan (Mel) is widely used to treat patients with hematologic cancer, including multiple myeloma, but its mechanism of action in EBV-transformed B cells is poorly described. In this study, we demonstrate a novel mechanism by which transcriptionally active p73 (TAp73) induces translocation of X-linked inhibitor of apoptosis protein-associated factor 1 (XAF1) and xeroderma pigmentosum group A (XPA) during apoptosis caused by Mel treatment. We observed that Mel induced significant generation of reactive oxygen species (ROS) and subsequent apoptosis, as well as an early phosphorylation of p38 MAPK that preceded expression of the mitochondria membrane potential disruption-related molecules and the cleavage of caspases. In particular, Mel led to upregulation of TAp73, XAF1, and Puma and induced XPA nuclear import and translocation of Bax into mitochondria. Mel-induced apoptosis was inhibited by pretreatment with the ROS scavenger 4-amino-2,4-pyrrolidine-dicarboxylic acid (APDC) and the p38 MAPK inhibitor SB203580. We supposed that ROS generation might be the first event in Mel-induced apoptosis, because APDC blocked the increase in ROS, p38 MAPK, and TAp73, but SB203580 did not block ROS generation. Moreover, Mel elicited activation of ATR, and APDC inhibited phosphorylation of ATR but not SB203580. APDC and SB203580 completely blocked XPA and Bax translocation. We conclude that Mel promotes TAp73-mediated XAF1 and Puma expression via ROS generation and ATR/p38 MAPK pathway activation, thereby triggering apoptosis. Our results provide evidence of a novel alternate regulatory mechanism of TAp73 and reveal that Mel may be a therapeutic drug for curing EBV-related malignancies.