Signal transduction events regulating induction of apoptosis by the histone deacetylase inhibitors (HDIs) sodium butyrate (SB) and SAHA have been examined in Bcr/Abl+ human leukemia cells (K562, LAMA 84). Exposure of K562 cells to greater or less than 3.0 mM SB or 3.0 mM SAHA for 24-48 hr resulted in a marked induction of mitchondrial damage (e.g., cytochrome c release) and apoptosis, events associated with downregulation of Bcr/Abl and Raf-1, induction of p21CIP1, inactivation of MEK1/2, ERK1/2, and p70S6K, and a dramatic increase in JNK activation. HDI-mediated apoptosis was attenuated by pharmacologic JNK inhibitors and enhanced by the MEK1/2 inhibitor U0126 as well as by the JNK activator anisomycin. Interestingly, HDI-induced JNK activation was potentiated by pharmacologic MEK inhibition. Furthermore, HDI lethality was significantly diminished in cells ectopically expressing constitutively active MEK1, confirming a functional role for MEK/ERK inactivation in HDI-mediated apoptosis. Similar events were observed in Bcr/Abl+ LAMA 84 cells. Lastly, the free radical scavenger L-N-acetylcysteine (LNAC) attenuated HDI-mediated ROS generation, JNK activation, and apoptosis. Together, these findings support a model in which induction of apoptosis in Bcr/Abl+ cells by HDIs involves coordinate inactivation of the cytoprotective Raf/MEK/ERK pathway in conjunction with the ROS-dependent activation of JNK.