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

Vascular endothelial growth factor (VEGF) inhibits of the activation of tra nscription Factor nuclear factor-kappaB (NF-kappaB) in hematopoietic progen itor cells (HPCs), and this is associated with alterations in the developme nt of multiple lineages of hematopoietic cells and defective immune inducti on in tumor-bearing animals. Antibodies to VEGF have been shown to abrogate this effect. The mechanism by which VEGF antagonizes the induction of NF-k appaB was investigated in this study. Using supershift electrophoretic mobi lity shift analysis, we found that although tumor necrosis factor alpha (TN F-alpha) induced the nuclear translocation and DNA binding of p65-containin g complexes. VEGF alone induced nuclear translocation and DNA binding of th e complexes containing RelB. These results were confirmed by immunofluoresc ence confocal microscopy. VEGF effectively blocked TNF-alpha -induced NF-ka ppaB activation in HPCs from RelB(-/-) mice, however, similar to the effect observed in HPCs obtained from RelRB(+/-) and RelB(+/+) mice. This suggest s that RelB is not required for VEGF to inhibit NF-kappaB activation. Howev er, although TNF-alpha induced rapid activation of I kappaB kinase (IKK) as expected, this activity was substantially reduced in the presence of VEGF. This decreased IKK activation correlated with the inhibition of I kappaB a lpha phosphorylation and degradation of I kappaB alpha and I kappaB epsilon in HPCs, VEGF alone, however did not have any effect on phosphorylation of I kappaB alpha or degradation of I kappaB alpha and other inhibitory molec ules I kappaB beta, I kappaB epsilon, or Bcl-3. SU5416, a potent inhibitor of the VEGF receptor 1 (VEGFR1) and VEGFR2 receptor tyrosine kinases, did n ot abolish the inhibitory effect of VEGF, indicating that the VEGF effect i s mediated by a mechanism unrelated to VEGFR1 or VEGFR2 tyrosine kinase act ivity. Thus, VEGF appears to inhibit TNF-alpha -induced NF-kappaB activatio n by VEGFR kinase-independent inhibition of IKK. Therapeutic strategies aim ed at overcoming VEGF-mediated defects in immune induction in tumor-hearing hosts will need to target this kinase-independent pathway.