Targeting the ubiquitin-proteasome pathway with the proteasome inhibitor bortezomib has emerged as a promising approach for the treatment of several malignancies. The cellular and molecular effects of this agent on colorectal cancer cells are poorly characterized. This study investigated the antiproliferative effect of bortezomib on colorectal cancer cell lines (Caco-2 and HRT-18). In order to define the proteins potentially involved in the mechanisms of action, proteome profiling was applied to detect the proteins altered by bortezomib. The in vitro efficacy of bortezomib as a single agent in colorectal cancer cell lines was confirmed. Proteome profiling with two-dimensional PAGE followed by mass spectrometry revealed the up-regulation of the major inducible isoform of heat shock protein 70 (hsp72) and lactate dehydrogenase B in both cell lines, as well as the induction of aldo-keto reductase family 1 member B10 (AKR1B10) in HRT-18 cells. Both AKR1B10 and hsp72 exert cell-protective functions. This study shows for the first time a bortezomib-induced up-regulation of AKR1B10. Small interfering RNA-mediated inhibition of this enzyme with known intracellular detoxification function sensitized HRT-18 cells to therapy with the proteasome inhibitor. To further characterize the relevance of AKR1B10 for colorectal tumors, immunohistochemical expression was shown in 23.2% of 125 tumor specimens. These findings indicate that AKR1B10 might be a target for combination therapy with bortezomib.