Vascular endothelial growth factor effects on nuclear factor-κb activation in hematopoietic progenitor cells

Vascular endothelial growth factor (VEGF) inhibits of the activation of transcription factor nuclear factor-κB (NF-κB) in hematopoietic progenitor cells (HPCs), and this is associated with alterations in the development of multiple lineages of hematopoietic cells and defective immune induction in tumor-bearing animals. Antibodies to VEGF have been shown to abrogate this effect. The mechanism by which VEGF antagonizes the induction of NF-κB was investigated in this study. Using supershift electrophoretic mobility shift analysis, we found that although tumor necrosis factor α (TNF-α) induced the nuclear translocation and DNA binding of p65-containing complexes, VEGF alone induced nuclear translocation and DNA binding of the complexes containing RelB. These results were confirmed by immunofluorescence confocal microscopy. VEGF effectively blocked TNF-α-induced NF-κB activation in HPCs from RelB−/− mice, however, similar to the effect observed in HPCs obtained from RelB+/− and RelB+/+ mice. This suggests that RelB is not required for VEGF to inhibit NF-κB activation. However, although TNF-α induced rapid activation of IκB kinase (IKK) as expected, this activity was substantially reduced in the presence of VEGF. This decreased IKK activation correlated with the inhibition of IκBα phosphorylation and degradation of IκBα and IκBe in HPCs. VEGF alone, however, did not have any effect on phosphorylation of IκBα or degradation of IκBα and other inhibitory molecules IκBβ, IκBe, or Bcl-3. SU5416, a potent inhibitor of the VEGF receptor 1 (VEGFR1) and VEGFR2 receptor tyrosine kinases, did not abolish the inhibitory effect of VEGF, indicating that the VEGF effect is mediated by a mechanism unrelated to VEGFR1 or VEGFR2 tyrosine kinase activity. Thus, VEGF appears to inhibit TNF-α-induced NF-κB activation by VEGFR kinase-independent inhibition of IKK. Therapeutic strategies aimed at overcoming VEGF-mediated defects in immune induction in tumor-bearing hosts will need to target this kinase-independent pathway.