1. ¹Department of Medicine, Massey Cancer Center, Virginia Commonwealth University, Richmond, VA, USA
2. ²Department of Biochemistry, Massey Cancer Center, Virginia Commonwealth University, Richmond, VA, USA
3. ³Department of Pharmacology, Massey Cancer Center, Virginia Commonwealth University, Richmond, VA, USA

Correspondence: Dr S Grant, Division of Hematology/Oncology, Medical College of Virginia, Virginia Commonwealth University, MCV Station Box 230, Richmond, VA 23298, USA. Fax: +1 804 828 8079; E-mail: stgrant@hsc.vcu.edu

⁴These authors contributed equally to this work

Received 4 February 2005; Accepted 7 June 2005; Published online 14 July 2005.

Abstract

Interactions between the histone deacetylase inhibitor SAHA and the pharmacologic MEK1/2 inhibitor PD184352 were examined in Bcr/Abl+ human leukemia cells. Coadministration of minimally toxic concentrations of SAHA (or sodium butyrate) and PD184352 (or U0126) resulted in a synergistic increase in mitochondrial damage, caspase activation, and apoptosis in K562 and LAMA 84 cells. Similar interactions were observed in CD34⁺ cells from two patients with CML and in imatinib mesylate-resistant K562 cells but not in normal human CD34⁺ bone marrow cells. These events were associated with a marked increase in ROS generation, inactivation of ERK and Akt, downregulation of p21^CIP1, Bcr/Abl, and cyclin D₁, and activation of JNK. Of these events, ROS generation, ERK inactivation, and cytochrome c/AIF release were largely caspase-independent, whereas the other phenomena displayed varying degrees of caspase-dependence. Using pharmacologic and genetic approaches, generation of ROS, p21^CIP1 downregulation, and inactivation of Akt and MEK were found to play significant functional roles in SAHA/PD184352-mediated lethality, whereas JNK activation and Raf-1 downregulation were determined to represent secondary events. These findings indicate that interruption of the MEK/ERK pathway substantially lowers the threshold for HDAC inhibitor-mediated oxidative injury, mitochondrial dysfunction, and apoptosis, suggesting that this approach warrants further examination in Bcr/Abl+-related malignancies.