Hypoxia-Induced Oxidative Stress Promotes Proteasome Inhibitor Resistance Via Upregulation of Heme Oxygenase-1 in Multiple Myeloma

Background Multiple myeloma (MM) is a hematopoietic malignancy for which proteasome inhibitors have become available in recent years. However, many patients develop resistance to these drugs during treatment. Therefore, it is important to elucidate the mechanisms underlying resistance acquisition by proteasome inhibitors. Side population (SP) cells, which have a high drug efflux capacity, as well as hypoxic responses in the microenvironment, have both provided important insights into drug resistance in MM; however, little is known about the characteristics of SP cells in hypoxic microenvironments.

Aim and Methods

To find novel therapeutic target for drug resistant MM cells, the SP cells and non-SP cells of MM cell lines (RPMI-8226 and KMS-11) were sorted for normoxic and hypoxic cultures (1% O2) for 48 h, respectively, and cDNA microarray assays were performed on these samples. Then, we conducted functional analysis of hypoxia-SP specific genes.

Results Our comprehensive gene expression analysis identified four coding genes (HMOX1, BACH2, DUX4, and RASGEF1B) and seven non-coding genes (TUSC8, LOC101928738, lnc-WRAP73-1, LINC-PINT, LINC01004, MIR146A, and lnc-FAM133B-1) that are specifically highly expressed in SP cells under hypoxic conditions. Among protein-coding genes, RT-qPCR showed very low expression of BACH2 and no expression of DUX4; we decided to investigate the function of HMOX1/heme oxygenase-1 (HO-1) in hypoxic environments. One of the functions of HO-1 is to protect cells from apoptosis induced by reactive oxygen species (ROS). We found a significant increase in ROS under hypoxic conditions compared with normoxic conditions in MM cells, including MM patient samples. HMOX1 knockdown significantly increased ROS levels under both normoxic and hypoxic conditions. These results suggest that hypoxic stress induced ROS and that HO-1 can neutralize hypoxia-induced ROS. The addition of PMA, a ROS inducer, significantly increased HMOX1/HO-1 in MM cells. Conversely, the addition of NAC, a ROS neutralizer, suppressed HMOX1/HO-1 expression under hypoxic conditions. These results suggest that HO-1 is induced in hypoxic environments by a ROS-mediated pathway.

Next, we investigated the involvement of HO-1 in the effects of proteasome inhibitors in hypoxic environments. MTX assays showed that bortezomib against HMOX1 knockdown KMS-11 reduced cell viability under hypoxic rather than normoxic conditions. We transplanted HMOX1 knockdown or control KMS-11 (1×10⁶ cells) into immunodeficient mice, injected bortezomib (1.0 mg/kg) intraperitoneally on days 14 and 18, and sacrificed the mice on day 22. There was no significant difference in subcutaneous tumor volume and weight between the control and HMOX1 knockdown cell lines, whereas HMOX1 knockdown resulted in a significant bortezomib-induced reduction in subcutaneous tumor volume and weight (p = 0.0009 and 0.0096, respectively).

Lastly, we examined whether excessive levels of ROS enhance the effect of bortezomib. PMA and a low concentration of bortezomib (5 nM, which is lower than that for IC50) were added to KMS-11 and RPMI-8226, and an apoptosis assay was performed. As a result, PMA or a low concentration of bortezomib increased apoptotic cells by only a small percentage, while simultaneous exposure to both significantly increased apoptotic cells synergistically. This result indicates that excessive ROS levels enhance the anti-myeloma effect of proteasome inhibitors.

Additionally, in clinical datasets, HMOX1 had a strong and significantly positive correlation with MAFB, but not MAF. We investigated whether MAFB actually regulates HMOX1 under hypoxic conditions. MAFB/MafB was induced by hypoxia even in a cell line without t(14;20) (q32;q12), such as KMS-11. Knockdown of MAFB significantly decreased HMOX1/HO-1 expression only under hypoxic condition, suggesting that MafB induction is involved in the elevated expression of HO-1 under hypoxic conditions.

Conclusion We clarified the contribution of the hypoxia-ROS-HO-1 axis to proteasome inhibitor resistance in hypoxic environments. This axis might also be involved in hypoxia-induced MafB expression. Because mechanisms that reduce excessive ROS in hypoxic microenvironments are thought to be closely related to cell survival and drug resistance, targeting this mechanism may be a new strategy to overcome drug resistance in refractory MM.

Ikeda:Janssen: Honoraria. Kitadate:Bristol Myers Squibb: Honoraria; Janssen: Honoraria. Takahashi:Pfizer: Consultancy, Honoraria; Otsuka Pharmaceutical: Consultancy, Honoraria, Research Funding; Asahi-kasei: Research Funding; Novartis: Consultancy, Honoraria, Research Funding.