Regulation of B-Cell Chronic Lymphocytic Leukaemia Cell Survival and Migration by the VEGF/SEMA3A Axis.

B-cell Chronic Lymphocytic Leukaemia (B-CLL) is characterized by the accumulation of B-CLL lymphocytes in the blood, marrow and secondary lymphoid tissues. B-CLL cells have a long survival owing to alterations in the normal pathways of apoptosis. In the marrow and lymphoid tissues CLL cells are in close contact with stromal cells that constitute distinct microenvironments. The secretion of the CXCR4 ligand, CXCL12, by stromal cells attracts B-CLL cells and provides protection from spontaneous or induced apoptosis. Studies in other cell types have shown VEGF signalling is involved in regulating CXCR4 expression levels.

The aim of this study was to examine VEGF receptor expression and the role of VEGF signalling in cell survival and CXCR4 expression. Expression levels of CXCR4 and VEGF receptors, VEGFR-1 and -2, in CLL samples were determined by flow cytometry. Expression of the VEGFR co-receptor, Neuropilin-1 (NRP1), was examined by Western blot. Cell migration towards CXCL12 was assessed using CoStar Transwell chambers (5mm pore size). Informed consent was received from all patients.

VEGFR1 and VEGFR2 positive cells in 22 patient samples ranged from 0–38% and 1.5–83% respectively. NRP1 expression was detected in all samples analysed thus far (n=6). The NRP1 ligand, SEMA3A, a competitive inhibitor of VEGF binding to NRP1, decreased CXCR4 expression in patient CLL cells (n=8, p<0.05). This decrease in CXCR4 levels correlated with reduced migration of SEMA3A treated CLL cells towards CXCL12 (88 +/− 12.7% of control levels, n=8, p<0.05). Treatment of CLL cells with the VEGFR signalling inhibitor SU5416 decreased CLL cell survival which correlated with VEGFR1 expression levels (n=16, R=0.745, p<0.01) but not with VEGFR2 expression levels.

These results show that signalling through the VEGF co-receptor NRP1 plays an important role in regulating CXCR4 levels in CLL cells, as well as CLL cell migration along a CXCL12 gradient. Our results also suggest that VEGF signalling through VEGFR1 may be particularly important in regulating CLL cell survival. Thus, the VEGFR/NRP1 signalling pathway may represent an important therapeutic target in B-CLL.