Chronic Lymphocytic Leukemia (CLL) Cells Require Microenvironmental Stimuli to Resist Apoptosis through Activation of STAT3 Mediated by VEGF.

B-cell chronic lymphocytic leukemia (CLL) is characterized by an accumulation of mature, but incompetent B-cells due to a decrease of apoptosis rather than an increase in proliferation. Vascular endothelial growth factor (VEGF) has been suggested to play an important role in this so called apoptotic block. However, so far little is understood whether VEGF is acting mainly as a microenvironmental stimulus and/or whether CLL cells themselves contribute to the enhanced apoptotic resistance by maintaining an autocrine VEGF loop. Moreover, it is unknown by which mechanisms VEGF prevents apoptosis and whether this can be circumvented by inhibition of VEGF signaling. By quantitative real time PCR we found no significant difference in mRNA VEGF levels in B-cells from CLL patients and healthy donors after isolation from blood. In contrast, ELISA revealed clearly increased levels of secreted VEGF in plasma of CLL patients and in the supernatant under culture conditions compared to healthy individuals. In addition, we found the VEGF receptor 2 (VEGFR2), which is existent in CLL and healthy B-cells, in a phosphorylated, hence activated state, to a significantly higher extent in CLL cells as assessed by intracellular phospho flow cytometry. In conclusion, despite its expression in healthy B-cells VEGF does not seem to be secreted and therefore, no VEGF receptor phosphorylation takes place. Whereas CLL cells exhibit a long life span in vivo, they die rapidly in vitro, suggesting major survival factors being existent in the CLL cells microenvironment. We found levels of secreted VEGF in supernatant decreasing with time in culture, going along with decreasing levels of phosphorylated VEGFR2 and increasing cell death as assessed by Annexin V-FITC/PI staining. This further supports the role of VEGF in CLL cell survival. Coculturing primary CLL cells with the bone marrow stromal derived cell line HS5 dramatically increased VEGF transcription and secretion and improved cell survival. Hence, VEGF expression in CLL cells is not only mediated by autocrine, but also paracrine stimuli involving bone marrow stromal. Knocking down VEGF in HS5 cells and subsequent coculture with CLL cells might prove the major role of VEGF in this survival supporting coculture setting. Besides coculturing also supplement of culture medium with recombinant human VEGF (rhVEGF) increased survival, but to a lesser extent than coculture, indicating a direct cell-cell interaction as advantageous. Furthermore, we found a downregulation of anti apoptotic proteins, such as X-linked inhibitor of apoptosis protein (XIAP), myeloid cell leukemia 1 (MCL1) and BclXL upon VEGF stimulation. Also cyclinD1 was upregulated as seen by immunoblotting. We further tried to discover the underlying mechanism of how VEGF mediates its pro survival effect and found STAT3 to become phosphorylated on tyrosine 705 upon VEGF stimulation. In CLL STAT3 is known to be constitutively phosphorylated on serine 727. This phosphorylation is not sufficient to induce target gene expression though. We could show that Y705 phosphorylation of STAT3 is responsible for upregulation of anti apoptotic BCLXL and cyclinD1. A PCR array detecting mRNA levels of 84 transcription factors in untreated and VEGF stimulated CLL cells shall provide more information about mechanistical details how VEGF mediates it pro survival effect. Since VEGF seems to be a major player in CLL cell survival it might be a suitable target to overcome the apoptotic block. In first experiments we found an induction of apoptosis after neutralization of VEGF or inhibition of the VEGF receptor. This additionally highlights the severe importance of VEGF in the apoptotic block in CLL cells. Therefore, VEGF might serve as an excellent therapeutic target in CLL.