Vascular Endothelial Growth Factor (VEGF) Acts Via Auto- and Paracrine Mechanisms as a Critical Microenvironmental Factor for the Survival of Chronic Lymphocytic Leukemia (CLL) Cells.

Abstract 4376

The major pathophysiological characteristic of chronic lymphocytic leukemia (CLL) is the accumulation of malignant, immuno-incomptetent B-cells, which is commonly known as the apoptotic block. Recently, it was speculated that vascular endothelial growth factor (VEGF) might be involved in this process. Whereas most of the available studies are solely descriptive, functional data are largely missing. The aim of this study was therefore to describe the effect and functional background of VEGF on CLL cells for the identification of potential strategies for a targeted CLL therapy. We identified an autocrine VEGF feedback loop in CLL cells, but not healthy B-cells. Recombinant human (rh) VEGF stimulation lead to increased expression of the anti-apoptotic proteins Mcl1 and XIAP, but was not sufficient to prolong survival. When CLL cells were cocultured with the bone marrow derived stromal cell line HS5, survival was significantly enhanced 28.5 ± 6.5% after 72h relative to monoculture, whereas healthy B-cells did not profit from coculture (-0.5 ± 4.7%). HS5 cells produce and secrete high amounts of VEGF themselves. When cocultured with HS5 CLL cells also showed an increased VEGF expression, indicating the existence of a paracrine VEGF feedback loop. The actual relevance of VEGF in the coculture mediated survival support was proofed by siRNA experiments. When VEGF expression and secretion was downregulated in HS5 cell by siRNA, the survival advantage CLL cells obtained from the coculture was reduced down to that in monoculture. Also the addition of a VEGF-neutralizing monoclonal antibody largely reversed the survival supporting effect of the coculture, clearly indicating the critical role of VEGF in bone marrow stromal cell-mediated CLL cell survival. We further found rhVEGF to induce upregulation and activation of STAT3 via Tyr705-phosphorylation and a subsequent expression of STAT3 targets such as Cyclin D1 and Bcl_XL. VEGF-stimulation further induced downregulation of the tumor suppressor RB1 and E2F1, which act as proapoptotic factors. Vice versa inhibition of VEGF by using selective VEGF-R inhibitors reduced both, Tyr705 phosphorylation and expression of STAT3 target genes. We therefore, propose a dual mechanism of VEGF-mediated CLL cell survival support via upregulation of the potent oncogene STAT3 and downregulation of the tumor suppressor RB1/E2F1. Hence, VEGF might function as a potential new therapeutic target to overcome the apoptotic block in CLL cells.