Bortezomib is the first proteasomal inhibitor (PI) to be used therapeutically for treating relapse cases of multiple myeloma and mantle cell lymphoma. A proposed mechanism for its action is that it prevents the proteasomal degradation of proapoptotic proteins, leading to enhanced apoptosis. Although the α subunit of hypoxia-inducible factor (HIF)-1 is not degraded with bortezomib treatment, the heterodimeric HIF-1 fails to transactivate target genes. HIF-1 and HIF-2 are related hypoxia-inducible transcription factors that are important for the survival of hypoxic tumor cells. The majority of reports have focused on the effects of bortezomib on the transcriptional activities of HIF-1, but not HIF-2. The present study investigated the effects of bortezomib on HIF-2 activity in cancer cells with different levels of HIF-1α and HIF-2α subunits. HIF-α subunit levels were detected using specific antibodies, while HIF transcriptional activities were evaluated using immunodetection, reverse transcription-polymerase chain reaction and luciferase reporter assay. Bortezomib treatment was found to suppress the transcription and expression of CA9, a HIF-1-specific target gene; however, it had minimal effects on EPO and GLUT-1, which are target genes of both HIF-1 and HIF-2. These data suggest that bortezomib attenuates the transcriptional activity only of HIF-1, and not HIF-2. This novel finding on the lack of an inhibitory effect of bortezomib on HIF-2 transcriptional activity has implications for the improvement of design and treatment modalities of bortezomib and other PI drugs.