Platelet Factor 4 Potently Inhibits Tumor Cell Growth and Angiogenesis In Multiple Myeloma

Abstract 4173

Multiple myeloma (MM) is a B-cell derived plasma cell malignancy characterized by accumulation of clonal plasma cells in bone marrow (BM). Platelet factor 4 (PF4), a potent antiangiogenic chemokine, not only inhibits endothelial cell proliferation and migration in vitro but also inhibits solid tumor growth in vivo. Our group previously demonstrated loss of PF4 expression in patient multiple myeloma (MM) samples and MM cell lines. Here, we characterized the effects of PF4 on both MM cells and endothelial cells in the BM milieu. We found that PF4 inhibits cell growth in MM cell lines (U266 and NCI-H929) with an IC₅₀ 4μM at 96 hours by the WST-1 assay. Cell apoptosis by Annexin V-7 AAD staining showed that percentages of apoptotic cells increased from 15.6% to 16.5%, 23.6% and 39.2% for U266 cells and from 19.8% to 20.1%. 26.8% and 71.0% for NCI-H929 cells when incubated with 2, 4, and 8μM PF4, respectively. PF4 also has direct effects on endothelial cells isolated from patient's BM aspirates (MMECs). Our results showed that PF4 suppresses MMECs proliferation (IC₅₀ 8μM) and capillary-like tube formation on matrigel in a dose-dependent manner. It is known that BM endothelial cells promote MM cell growth, survival, and drug resistance in BM microenvironment. Therefore, we further examined whether the proliferation of MM cell is influenced by the presence of endothelial cells. U266 cells were cultured for 96 hours with or without MMECs, in the presence or absence of PF4. We found that adhesion of MM cell to MMECs up regulates cell proliferation (about 1.5 fold), which is markedly inhibited by PF4 (>4uM). Given the ability of PF4 to suppress MM cell growth and angiogenesis in vitro, we evaluated its tumor suppressive function in vivo. In SCID-rab mouse model, 1× 10⁶ U266 MM cells were directly injected into the rabbit bone which was subcutaneously implanted into the NOD-SCID mice. Two weeks after injection, SCID mice were treated with various dose of PF4 (20 or 200 ng per injection, three times per week) or vehicle control by tail vein injection. ELISA assay with hIg (Lambda) showed that tumor growth in PF4-treated mice is markedly reduced by 2.5 fold compared with the control group, which is further confirmed by immunohistochemistry analysis of CD138 staining on rabbit bone section. Consistent with the in vitro results, MM cells' proliferation and angiogenesis are also significantly inhibited by PF4 in vivo, as evidenced by ki67 and CD31 staining on rabbit bone sections from treated versus control mice. Moreover, PF4 improves the survival rate of mice. The survival rate of PBS treated mice was 80% after 3 weeks and less than 30% after 12 weeks, while PF4-treated groups had 100% survival rate after 12 weeks. Taken together, our findings confirm that PF4 is a critical regulator of MM pathogenesis, which targets both MM cells and MMECs in the BM milieu in vitro and in vivo and prolongs survival in the SCID-rab mice model of human MM. These studies provide an important framework for critical clinical studies of PF4 to improve patient treatment outcome in MM.