Role of the Hypoxic Bone Marrow Microenvironment in Multiple Myeloma Tumor Progression.

Recently we reported that pre-clinical myeloma disease progression in the 5T2MM mouse model is characterized by predominant CD45⁺ MM-cells in the early, pre-angiogenic stage stage of slow tumor progression, followed by expansion of CD45⁻ MM-cells during the subsequent angiogenic stage of progressive tumor growth. Unlike other cancer cells, multiple myeloma (MM) cells have to survive and to grow in a microenvironment which is already hypoxic by nature. This hypoxic bone marrow (BM) microenvironment is essential for normal hematopoiesis. However, the role of BM hypoxia in myeloma tumor progression is not known. Herein we addressed this topic in the 5T2MM mouse model.

Flow cytometric analysis of control mice and 5T2MM diseased mice injected with pimonidazole hypoxyprobe indicated that both normal BM and myeloma infiltrated BM are hypoxic. However, in myelomatous BM the hypoxia was significantly decreased. Analysis of HIF-1a expression, a surrogate marker of hypoxia, by flow cytometry also demonstrated significantly lower levels of hypoxia in myeloma infiltrated BM. HIF-1a expression was found in 5T2MM-cells and was significantly higher compared to the non-tumor cell fraction. In vitro culturing of 5T2MM cells under hypoxic conditions, indicated increased activation of apoptosis inducing caspase-3 in the CD45⁻ MM-fraction, but not in the CD45⁺ 5T2MM-cells, suggesting that native BM hypoxia selects the tumor population for tumor initiating CD45⁺ 5T2MM-cells. Although angiogeneic switch and angiogeneic heterogeneity has been reported in MM, the role of myeloma associated angiogensis is remains unclear. The decreased hypoxia in myeloma infiltrated BM adds strength to the hypothesis that myeloma associated neovascularization is functional by increasing BM oxygenation. The data also suggest that the angiogenesis allows expansion of CD45⁻ 5T2MM-cells by decreasing BM hypoxia.

All together, these findings suggest an important role of BM hypoxia in myeloma tumor progression.