Vorinostat Induced Cellular Stress Disrupts the Balance Between p38 MAPK and Erk Pathways Leading to Apoptosis in WM Cells.

Abstract 3740

Poster Board III-676

Despite advances, Waldenstrom's Macroglobulinemia (WM) remains incurable, and novel agents are urgently needed. Histone deacetylase (HDACs) are involved in transcription regulation and signal transduction of cells through a genome wide alteration in histone modification and other proteins, leading to significant increase in cellular stress in tumor cells. We therefore examined the activity of Vorinostat, a histone deacetylase inhibitor (HDAC-I), and dissected its pro-apoptotic molecular pathways in WM cells. Vorinostat exhibited dose dependent killing of both primary, bone marrow derived WM cells, as well as BCWM.1 WM cells with an LD50 of 3.5 to 5uM using Annexin V and PI staining.

Vorinostat induced apoptosis in WM cells through activation of specific caspases at different time points. Caspase 3, 6, 8, and 9 were activated after 24 hours of Vorinostat. Even though caspase 7 is downstream of caspase 3, we observed that caspase 7 was activated earlier, starting at 6 hours. We therefore hypothesized that the regulators of caspase 7 may be affected by Vorinostat at an earlier time point. Further investigation confirmed that there was significant down-regulation of inhibitor of apoptosis (IAP) family members, including c-IAP1, c-IAP2, XIAP and Livin after 12 hours of treatment with Vorinostat, and may elude to greater sensitivity for IAPs in modulating Caspase 7 versus Caspase 3 in WM cells.

We also studied the stress pathways including Erk, JNK and P38 pathways in Vorinostat treated WM cells. Activated p38, phospho-p38, was upregulated starting at 12 hours, while phopho-Erk abruptly decreased following 24 hours of treatment with Vorinostat. There was minimal change in the activity of JNK pathway following Vorinostat treatment. The activation of the p38 pathway coincided with a reduction in c-IAP1, c-IAP2, XIAP and Livin following treatment of WM cells with Vorinostat for 12 hours.

Taken together, these studies support that stress induced apoptosis in WM cells is mediated through disruption in the balanced activity between the Erk and p38 MAPK pathways. Vorinostat induced cellular stress results in the activation of p38 MAPK pathway and a reduction of the IAP family members, leading to early activation of caspase 7. While the inhibition of Erk pathway by Vorinostat results in delayed activation of caspase 3, 6, 8, and 9 at 24 hours, the collective signaling strength of p38 activation as well as inhibition of Erk likely determines the apoptotic fate WM cells upon Vorinostat treatment.