Cross-talk between chronic lymphocytic leukemia cells and bone marrow endothelial cells: role of signal transducer and activator of transcription 3.

Chronic lymphocytic leukemia bone marrow is characterized by increased angiogenesis. However, the molecular mediators of neovascularization and the biologic significance of increased endothelial cell proliferation in chronic lymphocytic leukemia require further investigation. Because signal transducer and activator of transcription 3 is constitutively activated in chronic lymphocytic leukemia, we studied the role of signal transducer and activator of transcription 3 in modulating vascular endothelial growth factor expression and the effect of vascular endothelial cells on chronic lymphocytic leukemia cells. Using chromatin immunoprecipitation, we found that anti-signal transducer and activator of transcription 3 antibodies immunoprecipitated DNA of signal transducer and activator of transcription 3, vascular endothelial growth factor, and other signal transducer and activator of transcription 3-regulated genes. In addition, signal transducer and activator of transcription 3-short interfering RNA significantly reduced messenger RNA levels of vascular endothelial growth factor in chronic lymphocytic leukemia cells, suggesting that signal transducer and activator of transcription 3 induces vascular endothelial growth factor expression in chronic lymphocytic leukemia. Remarkably, bone marrow chronic lymphocytic leukemia cells expressed high levels of vascular endothelial growth factor, and high vascular endothelial growth factor levels were detected in the plasma of patients with untreated chronic lymphocytic leukemia and correlated with white blood cell count. Chronic lymphocytic leukemia bone marrow biopsies revealed increased microvascular density and attachment of chronic lymphocytic leukemia cells to endothelial cells. Coculture of chronic lymphocytic leukemia cells and human umbilical vein endothelial cells showed a similar attachment. Furthermore, coculture studies with human umbilical vein endothelial cells showed that human umbilical vein endothelial cells protected chronic lymphocytic leukemia cells from spontaneous apoptosis by direct cell-to-cell contact as assessed by flow cytometry using annexin V. Our data suggest that constitutively activated signal transducer and activator of transcription 3 induces vascular endothelial growth factor production by chronic lymphocytic leukemia cells and that chronic lymphocytic leukemia cells derive a survival advantage from endothelial cells via cell-to-cell contact.