Dynamic Regulation of the Level of Hypoxia In the Bone Marrow Regulates Cell Dissemination In Multiple Myeloma

The interaction of multiple myeloma (MM) cells with the bone marrow (BM) microenvironment plays a crucial role in MM pathogenesis, implying that progression of MM occurs through continuous interaction between the BM and MM cells, which controls the ability of MM cells to egress out of the BM and home into new BM niches. We have previously shown that the CXCR4/SDF1 axis as well as Rho GTPases downstream of the receptor was important for chemotaxis, adhesion, homing and egress of MM cells. However, the driving force for MM cells to leave the BM and metastasize to other BM sites is not well understood. Regions of severe oxygen deprivation (hypoxia) arise in tumors due to rapid cell division and are associated with poor patient prognosis, cell motility, associated angiogenesis and metastasis. In this study, we tested the role of hypoxia in the dissemination of MM cells in vivo, as well as regulation of the retention/egress of MM cells in and out of the BM.

To test the effect of hypoxia on induction of MM egress, MM1s-GFP+/Luc+ cells were injected into 12 SCID mice, and then mice with different stages of tumor development (based on the tumor size detected by bioluminescence) were treated with the hypoxia marker pimonidazole. Blood was drawn and BM was obtained from the femur. Mononuclear cells were then fixed, permeabilized, and stained with antibodies against pimonidazole, followed with an APC- secondary antibody, PE-mouse-anti-human CXCR4, and anti-cadherin antibody followed by an Alexa-Fluor-594 secondary antibody. MM cells in BM and peripheral blood were identified by gating on cells with high GFP signal. To confirm the effects of severe hypoxia found in vivo compared to physiologic mild hypoxia found in the BM, we tested the effect of mild hypoxic conditions (6% O2) and severe hypoxic conditions (0.5% O2) on MM expression of cadherins and CXCR4, as well on functional adhesion of MM cells to stromal cells and chemotaxis.

Twelve mice with different stages of MM tumor progression were used. A bi-phasic correlation between tumor progression and the percent of hypoxic cells in BM was found, showing that severe hypoxic conditions in the BM correlated with tumor burden. The correlation between the tumor burden and the number of circulating cells was not linear; however, a direct linear correlation was observed between the number of circulating MM cells and hypoxia in the BM. Moreover, hypoxia in BM correlated directly with the expression of CXCR4 and negatively correlated with the expression of cadherins in MM cells isolated from the BM. To test the effect of the severe hypoxic conditions induced by tumor progression compared to mild hypoxic conditions found physiologically in the BM, we tested the effect of 0.5% O2 (severe hypoxia) and 6% O2 (mild hypoxia) compared to normoxia (21%) on MM cell adhesion to BMSCs, as well as on chemotaxis in response to SDF1, as well as expression of CXCR4 and cadherins. We found that severe hypoxic conditions decreased MM expression of cadherins and adhesion to BMSCs, as well as increased expression of CXCR4 and chemotaxis to SDF1 compared to cells in normoxia. In contrast, mild hypoxic conditions did not alter the expression of CXCR4 and cadherins, adhesion of MM cells to BMSCs, or chemotaxis of MM to SDF1 compared to normoxic cells.

Hypoxia in the BM directly correlates with the number of circulating MM cells, and with changes in expression of cadherins and CXCR4 in vivo. Severe hypoxic conditions, but not mild hypoxic conditions, induce hypoxic responses in MM cells. Based on these findings, further studies to manipulate hypoxia in order to regulate tumor dissemination as a therapeutic strategy in MM are warranted.

Anderson:Millennium Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Ghobrial:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees.