Tumor Hypoxia Promotes Dissemination and Tumor Colonization In Waldenström Macroglobulinemia

Waldenström Macroglobulinemia (WM) is a rare, low-grade B-cell lymphoma characterized by lymphoplasmacytic cells spread widely in the bone marrow (BM) and overproduction of monoclonal immunoglobulins M (IgM). Previous studies showed that tumor hypoxia develops in the BM of other hematologic malignancies and promotes dissemination. In this study, we tested the effect of hypoxia on cell proliferation, cell cycle and apoptosis; on egress and homing of WM cells from and into the BM; and on recovery and tumor colonization in the new BM niche.

We characterized the effect of tumor progression on generation of hypoxic conditions in the BM in vivo, by injecting BCWM1-mCherry cells to SCID mice, letting them grow for two weeks, analyzing the hypoxic state of the WM cells in the BM using pimonidazole, and testing the number of circulating cells. Moreover, we tested the effect of hypoxia on the homing of WM cells to the BM by injecting normoxic and hypoxic cells to mice and monitoring the number of the circulating WM cells in the blood at different time points by flow cytometry. Cancer cell colonization was assessed 1 and 3 days post IV injection of normoxic and hypoxic cells to mice; mononuclear cells were isolated from the BM, fixed, permeabilized and stained with antibodies for p-Rb and cyclin-E. The percentile of WM cells in the BM and the expression of cell cycle proteins were analyzed by flow cytometry.

BCWM1 cells were exposed to normoxia (21% O₂) or hypoxia (1% O₂) in vitro for 24hrs, and n some cases reoxygenated for 24hrs. The expression of E-cadherin, VLA-4 and CXCR4 was analyzed by western blot or flow cytometry. We tested the effect of hypoxia on adhesion of WM cells to BM stroma and fibronectin. We further tested the effect of hypoxia on chemotactic properties of WM cells towards SDF-1 using a transwell migration chamber. In addition, we tested the effect of hypoxia on WM cell survival (by MTT assay), apoptosis and cell cycle (by using AnnexinV-PI and PI, respectively), and signaling pathways associated with survival, apoptosis and cell cycle (by western blotting).

Tumor progression was shown to increase hypoxic conditions in the BM in vivo. We found a direct correlation between the percent of WM cells in the BM to the level of hypoxia. The level of hypoxia was in a direct correlation with the number of circulating WM cells in vivo. Then we mimicked the hypoxic conditions in vitro and found that cell progression (MTT) and cell cycle (PI staining) were decreased, but apoptosis of WM cells was not affected (AnnexinV-PI staining). These results were confirmed by decreased activation of the PI3K signaling pathway (p-PI3K, p-AKT, p-GSK) and decreased expression of cell cycle proteins (p-Rb, CDK2, CDK4, cyclin-D1 and p-cyclin-E); however, no change was observed in apoptosis-related proteins (PARP, cleaved caspase-3, -8 and -9). Moreover, hypoxia decreased the expression of E-cadherin which contributed to reduction of adhesion of WM cells to the BM stromal cells. At the same time, hypoxic WM cells exhibited increased CXCR4 surface expression and augmented migratory abilities in the presence of SDF-1. Neither the expression of integrins (VLA-4) nor the adhesion of WM cells to fibronectin was affected by hypoxia. This data indicates the conservation of the homing machinery of the WM to the BM despite the hypoxic conditions accompanied by increased chemotactic ability. When hypoxic and normoxic cells were injected to naïve mice, hypoxic cells showed enhanced homing to the BM and tumor colonization. Similarly, hypoxic cells which were reoxygenated in vitro showed more proliferation, cell cycle and activation of proliferative signaling pathways compared to normoxic cells.

We report that WM tumor growth in the BM increases hypoxia, and that hypoxia induces cell cycle arrest, and less proliferation of cells with no apoptosis. At the same time, hypoxia induces egress of WM cells from the BM through reduction of E-cadherin expression and decreased adhesion. When in the circulation, previously hypoxic cells home more efficiently to the BM through increased expression of CXCR4 and chemotaxis, and through maintaining expression of integrins and adhesion to fibronectin. When in the new oxygenated BM niche, hypoxic WM cells recover and colonize the new niche better than normoxic cells, and reoxygenated hypoxic cells have faster cell cycle and proliferation rate.

No relevant conflicts of interest to declare.