Role of Nuclear Heme Oxygenase-1 in Bortezomib Induced Cell Death

Bortezomib, the first-in-class proteasome inhibitor, has displayed significant antitumor activity in Multiple myeloma (MM). Heme oxygenase-1 (HO-1) is an Nrf2 transcription factor-regulated gene that is commonly induced following oxidative stress and cellular injury, functioning to decrease oxidative stress and inflammatory responses, protecting against apoptosis. Furthermore, we showed under some experimental conditions, HO-1 might translocate into the nuclear compartment where it may play non-canonical protein functions. In the present research we evaluated the role of Bortezomib on HO-1 expression and cellular compartmentalization and how these effects are related to chemoresistance in U266, a MM cell line. Our results showed that Bortezomib (100nM) induces HO-1 via ER stress induction as measured by PERK, IRE1 and Bip protein expression. HO activity inhibition by SnMP (10 μM) did not reverse such effects. We also showed that under basal conditions, HO-1 is mainly localized into nuclear compartment whereas Bortezomib treatment resulted in a significant increase of the protein in the cytoplasm. Inhibition of HO-1 nuclear translocation by E64d significantly increases Bortezomib mediated cell death. We also showed that nuclear HO-1 localization is associated with increased genomic instability as measured my micronuclei test, nucleoplasmic bridge and tetranucleated cell formation. Our data suggest that cellular compartmentalization of HO-1, rather than its enzymatic activity, plays a critical role in Bortezomib mediated cell death and resistance.