Intracellular Pool of Vascular Endothelial Growth Factor in Human Neutrophils

Abstract

Vascular endothelial growth factor (VEGF ), an endothelial cell mitogen, is a potent angiogenic factor produced by several cell types. Whether human neutrophils are potential producers of VEGF has not yet been described. The present work shows that phorbol-12-myristate 13-acetate (PMA), fMet-Leu-Phe, and tumor necrosis factor-α (TNF-α) triggered a time-dependent secretion of VEGF by human neutrophils. Cells incubated with 50 ng/mL of PMA released significant amounts of VEGF after 15 minutes. Because the extracellular content of VEGF in human neutrophils supernatants remained constant over a period of 2 to 24 hours and because PMA is a potent inducer of human neutrophil degranulation, the PMA-induced secretion of VEGF may be due to a pre-existing intracellular pool of this molecule. This hypothesis was reinforced by the absence of cycloheximide effect on the PMA-induced secretion of VEGF. The existence of an intracellular pool of VEGF was confirmed by measuring the intracellular content of VEGF in resting neutrophils. A dosedependent inhibition of PMA-induced VEGF secretion was observed when the cells were incubated in the presence of pentoxifylline, a methylxanthine known to inhibit neutrophil degranulation. To confirm the implication of neutrophil degranulation in VEGF release, the effects of two inducers of physiologic degranulation, fMet-Leu-Phe and TNF-α, were determined. Both agonists induced a release of VEGF in the absence of cytochalasin B, confirming the involvement of neutrophil degranulation and suggesting the intracellular localization of VEGF in the specific granule fraction. In addition, the kinetics of fMet-Leu-Phe– and TNF-α–induced secretion of lactoferrin were similar to those of VEGF release induced by these two both agonists. The subcellular fractionation of human neutrophils showed a granule-specific distribution of the intracellular pool of VEGF in resting neutrophils. The finding that human neutrophils contain an intracellular pool of VEGF, secreted in the extracellular space under PMA-, fMet-Leu-Phe–, and TNF-α–induced degranulation, suggests a role for human neutrophils as cellular effectors of physiologic as well as pathologic angiogenesis.