Two mechanistic explanations for PI-HDACi synergy. (A) HDAC inhibitors promote PI-induced proteotoxic stress. By blocking the proteasome, PIs promote the accumulation of damaged and misfolded proteins that are prone to aggregation, and it is this protein aggregation that serves as the primary cytotoxic stress, causing downstream reactive oxygen species (ROS) accumulation, JNK activation, and ER caspase (4 and 12) activation. HDACis promote this proteotoxic stress by blocking HDAC6, which is required for “aggresome” formation and the transfer of protein aggregates to lysosomes via autophagy. (B) Proteasome inhibitors promote type I HDAC inhibition. In this model, inhibition of type I HDACs serves as the primary cytotoxic stimulus, perhaps by promoting expression of “death genes” such as TNF-related apoptosis-inducing ligand (TRAIL) and Bim, a BH3-only member of the BCL-2 family. PIs synergize with HDAC inhibitors by promoting caspase-8 activation, cleavage and inactivation of Sp-1, and subsequent down-regulation of type I HDAC expression. Importantly, other studies have demonstrated that PIs promote TRAIL- and Bim-dependent apoptosis, so they may also interact with the HDACi pathway downstream of their effects on Sp-1.