The proteasome inhibitor bortezomib (formerly known as PS-341) recently received Food and Drug Administration approval for the treatment of multiple myeloma, and its activity is currently being evaluated in solid tumors. Bortezomib triggers apoptosis in pancreatic cancer cells, but the mechanisms involved have not been fully elucidated. Here, we show that pancreatic cancer cells exposed to bortezomib formed aggregates of ubiquitin-conjugated proteins ("aggresomes") in vitro and in vivo. Bortezomib-induced aggresome formation was determined to be cytoprotective and could be disrupted using histone deacetylase (HDAC) 6 small interfering RNA or chemical HDAC inhibitors, which resulted in endoplasmic reticulum stress and synergistic levels of apoptosis in vitro and in an orthotopic pancreatic cancer xenograft model in vivo. Interestingly, bortezomib did not induce aggresome formation in immortalized normal human pancreatic epithelial cells in vitro or in murine pancreatic epithelial cells in vivo. In addition, these cells did not undergo apoptosis following treatment with bortezomib, suberoylanilide hydroxamic acid, or the combination, showing tumor selectivity. Taken together, our study shows that inhibition of aggresome formation can strongly potentiate the efficacy of bortezomib and provides the foundation for clinical trials of bortezomib in combination with HDAC inhibitors for the treatment of pancreatic cancer.