Arsenic trioxide induces JNK1 activation in NB4 cells. (A) Immune complex kinase assays to measure JNK1 activity were performed with extracts from NB4 cells treated with 0.1 μM, 0.5 μM, or 1 μM As₂O₃ for 24 hours (top panel). Those doses that induced JNK activity were used in treatments carried out over longer time points (middle panel). Finally, JNK activity induced by a 16-hour treatment with 1 μM As₂O₃ following a 24-hour pretreatment with 100 μM BSO was measured in NB4 cells. (B) NB4, AsR2, and AsR3 cells were treated for 16 hours with a range of doses of As₂O₃ alone or in combination with BSO. Western blots of whole cell extracts were also performed to detect total JNK1 levels.